Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session P1: Recent Advances in Magnetization Dynamics
Sponsoring Units: DCMPChair: Jonathan Sun, IBM T.J. Watson Research Center
Room: Colorado Convention Center Four Seasons 2-3
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P1.00001: Imaging fast spin dynamics at the nanoscale with soft x-ray microscopy Invited Speaker: Nanoscale and multicomponent magnetic systems are attracting both fundamental interest and are widely used in technological applications such as high density magnetic storage and sensor devices. The challenge to modern magnetic microscopies is to image magnetic microstructures in such specimens with high spatial and time resolution and elemental specificity. Magnetic soft X-ray microscopy is a novel technique combining a spatial resolution down to currently 15nm, elemental sensitivity due to X-ray magnetic circular dichroism used as huge magnetic contrast mechanism and a sub-ns time resolution limited by the current time structure of the synchrotron radiation used as source for circularly polarized soft X-rays. We report on recent results and achievements in magnetic soft X-ray microscopy obtained at the full-field soft X-ray microscopy beamline 6.1.2 (XM-1) located at the Advanced Light Source in Berkeley CA. Magnetization reversal processes at the grain level in a nanogranular CoCrPt system were studied with 15nm spatial resolution to obtain insight into spin fluctuations on a fundamental length scale. The inherent elemental sensitivity of XMCD contrast allows e.g. in (coupled) multilayered magnetic systems to explore their microscopic magnetization reversal process with layer resolution. Spin dynamics in magnetic nanostructures can be addressed by a stroboscopic pump and probe scheme utilizing the inherent time structure of synchrotron radiation, where the pump is a fast electronic pulse launched into a waveguide structure to excite the spin dynamics of a magnetic nanoelement. Varying the delay time between the pump and the probing x-ray flash one can follow the time development of e.g local spin and vortex dynamics and relaxation phenomena, but also spin-torque driven domain wall displacements with sub-ns time resolution. Current developments of X-ray optics aim to achieve better than 10nm spatial resolution. At upcoming high brillant ultrast X-ray sources snapshots of spin dynamics with fs time resolution recorded with magnetic soft X-ray microscopy can be foreseen. Many thanks to D.-H. Kim, B. Mesler, W. Chao, R. Oort, E. Anderson, G. Meier, R. Eiselt, M. Bolte, M.-Y. Im, S.-C. Shin, S. Mangin, E. Fullerton. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:27PM |
P1.00002: The Spin Transfer Torque Critical Current in Magnetic Nanopillars Invited Speaker: Spin transfer in magnetic nanopillar has become a major focus of experimental research since Slonczewski and Berger's seminal theoretical work in 1996. A spin current has been demonstrated to switch the magnetization direction of a small magnet at a specific current density, as well as to induce microwave excitations. However, there are basic questions about the factors that control the critical current density for magnetization dynamics. For instance, in Slonczewski's model, spin angular momentum transfer occurs at ferromagnetic/non-magnetic interfaces and competes with bulk magnetization damping. This model predicts a critical current that scales linearly with ferromagnet layer thickness and extrapolates to zero in the limit of zero thickness. In this talk I will present experiments on Co (10 nm) /Cu (10 nm) /Co ($t$) nanopillars in which the Co free-layer thickness, $t$, has been varied from $2$ to $5$ nm. The critical current has been studied at low-temperature as a function of applied magnetic field perpendicular to the plane of the layers. The critical current decreases linearly with decreasing free-layer thickness, but extrapolates to a finite critical current in the limit of zero thickness, while the junction magnetoresistance is independent of thickness [1]. The limiting current is in agreement with that expected due to a spin-pumping contribution to the magnetization damping. It is also consistent with our FMR studies of Co films, which indicate an enhancement of the magnetization damping in ultra-thin ($<4$ nm thick) Co layers due to spin-pumping [2]. Finally, I will discuss more recent studies of nanopillars with Ni/Co multilayer free layers. In these experiments, the role of the magnetic easy plane anisotropy can be explored, as this anisotropy varies with the number of Ni/Co interfaces within a fixed film thickness [3]. \\ 1. W. Chen, et al., Phys. Rev. B 74, 144408 (2006) \\ 2. J-M. Beaujour et al., Phys. Rev. B 74, 214405 (2006) \\ 3. J-M. Beaujour et al., cond-mat/0611027 [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 1:03PM |
P1.00003: Very low damping in epitaxial Fe and Fe1-xVx films Invited Speaker: |
Wednesday, March 7, 2007 1:03PM - 1:39PM |
P1.00004: Spin Momentum Transfer and Oersted Field Induce a Vortex Nano-Oscillator in Thin Ferromagnetic Film Devices Invited Speaker: A nonlinear model of spin-wave excitation involving a point contact in a thin ferromagnetic film that includes the Oersted magnetic field contribution is presented. We consider the case of an external dc field applied perpendicular to the film plane. The two-dimensional vectorial model reduces to an exact one-dimensional equation of motion. Large-amplitude vortex modes are computed, which represent a fundamental shift in the geometrical understanding of spin transfer nano-oscillators. Odd symmetry forces the magnetization to be pinned in the center of the point contact. Using the spin transfer efficiency as a single fitting parameter, the calculated dependence of frequency on current and contact size is in good agreement with recent experimental data. These vortex states are geometrically very different from previously computed cylindrical modes that exhibit even symmetry when the Oersted field is ignored. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 2:15PM |
P1.00005: Fingerprinting Magnetic Nanostructures by First Order Reversal Curves Invited Speaker: Realistic systems of magnetic nanostructures inevitably have \textit{inhomogeneities}, which are manifested in distributions of magnetic properties, mixed magnetic phases, different magnetization reversal mechanisms, etc. The first order reversal curve (FORC) method [1-3] is ideally suited for ``fingerprinting'' such systems, both qualitatively and quantitatively. Here we present recent FORC studies on a few technologically important systems. In arrays of Fe nanodots [4], as the dot size decreases from 67 to 52nm, we have observed a vortex state to single-domain transition. Despite subtle changes in the major hysteresis loops, striking differences are seen in the FORC diagrams. The FORC method also gives quantitative measures of the magnetic phase fractions and vortex nucleation and annihilation fields. Furthermore, with decreasing temperature, it is more difficult to nucleate vortices within the dots and the single domain phase fraction increases. In exchange spring magnets [3], we have investigated the reversibility of the soft and hard layers and the interlayer exchange coupling. In FeNi/polycrystalline-FePt films, the FeNi and FePt layers reverse in a continuous process via a vertical spiral. In Fe/epitaxial-SmCo films, the reversal proceeds by a reversible rotation of the Fe soft layer, followed by an irreversible switching of the SmCo hard layer. As the SmCo partially demagnetizes, the Fe layer still remains reversible, as revealed by second order reversal curves (SORC). The exchange coupling between the two layers can be extracted as a function of the SmCo demagnetization state. These results demonstrate that FORC is a powerful method for magnetization reversal studies, due to its capability of capturing magnetic inhomogeneities, sensitivity to irreversible switching, and the quantitative phase information it can extract. Work done in collaboration with J. E. Davies, R. K. Dumas, J. Olamit, C. P. Li, I. V. Roshchin, I. K. Schuller, O. Hellwig, E. E. Fullerton, J. S. Jiang, S. D. Bader, J. Wu, C. Leighton, H. G. Katzgraber, C. R. Pike, R. T. Scalettar, G. T. Zimanyi, and K. L. Verosub. \newline \newline [1] C. R. Pike, et al, JAP \textbf{85}, 6660 (1999). \newline [2] H. G. Katzgraber, et al. PRL \textbf{89}, 257202 (2002). \newline [3] J. E. Davies, et al, PRB \textbf{70,} 224434 (2004); APL \textbf{86,} 262503 (2005); PRB \textbf{72}, 134419 (2005). \newline [4] K. Liu, et al., APL. \textbf{81}, 4434 (2002). [Preview Abstract] |
Session P2: Nanopore World: from Single-Molecules to Bionanotechnology Prospects
Sponsoring Units: DBPChair: Liviu Movileanu, Syracuse University
Room: Colorado Convention Center Four Seasons 4
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P2.00001: Towards DNA Sequencing using Solid-State Nanopores Invited Speaker: 10 Years ago John Kasianowicz and coworkers invented the concept of using ionic conductance as a mechanism for scanning a DNA molecule for genetic information. Their proposal has since led to the creation of an exciting field of nanopore biophysics. I will discuss our current effort in combining the SBH (sequencing-by-hybridization) concept and solid-state nanopores for fast DNA sequencing. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:27PM |
P2.00002: Entropic springs in single-molecule polymer partitioning into protein nano-pores Invited Speaker: The capture and release of single polyethylene glycol molecules by the alpha-Hemolysin pore are observed as time-resolved reversible steps in ion conductance. The capture on-rate, inferred from the step frequency, decreases monotonically with polymer size. However, the polymer residence time shows a cross-over behavior, first increasing and then decreasing with molecular weight (\textit{Phys. Rev. Lett.}, 2006, \textbf{97}:018301). Our interpretation is that in case of polymers which are too large to be accommodated within the pore, the out-of-the-pore part of the molecule pulls on the trapped part thus acting as an entropic spring. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 1:03PM |
P2.00003: Forces on DNA in a solid-state nanopore Invited Speaker: Amongst the variety of roles for nanopores in biology, an important one is enabling polymer transport, for example in gene transfer between bacteria and transport of RNA through the nuclear membrane. Recently, this has inspired the use of protein and solid-state nanopores as single-molecule sensors for the detection and structural analysis of DNA and RNA by voltage-driven translocation. The magnitude of the force involved is of fundamental importance in understanding and exploiting this translocation mechanism. Furthermore, solid-state nanopores can be seen as a model system for biological nanopores. We will discuss the forces acting on single DNA strands electrophoretically driven through a solid-state nanopore. The force was directly measured using optical tweezers [1]. The force depends linearly on the applied voltage for a wide range of salt concentrations (0.02M -- 1M KCl) and nanopore diameters (6 nm -- 80 nm). Interestingly, we find for small nanopores with a diameter less than 15 nm that the force on the DNA is independent of the salt concentrations. However, the force decreases significantly in the larger nanopores. We will qualitatively discuss our results using the Poisson-Boltzmann and Navier-Stokes equations for a simple geometry. The influence of hydrodynamic coupling between the nanopore walls and the DNA molecule is of crucial importance to understand the force on a DNA molecule in nanopores. [1] U. F. Keyser et al. Nature Physics 2, 473 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:39PM |
P2.00004: Molecular Tweezers: Using the Electric Field in a Synthetic Nanopore to Disrupt Biomolecular Binding Forces Invited Speaker: The forces binding proteins to DNA in an aqueous solution are vital to biology, but inadequately understood. In particular, restriction enzymes like EcoRI are extraordinarily sequence-specific and yet the complex with DNA is very stable. To stringently test these forces, we use the electric field inside a synthetic nanometer-diameter pore in a thin membrane to pull on double-stranded DNA bound to EcoRI and BamHI, introducing a shear between the enzyme and their respective cognate sites in DNA. We observe a sharp threshold near 1nN in the force required to disrupt the binding in the complex, which is in stark contrast with previous measurements of the force (10pN) accomplished by unzipping the DNA molecule at a constant loading rates (1nN/sec). This force, acting over a distance corresponding to the separation between bases, coincidentally corresponds to the free energy of formation for the EcoRI-DNA complex. Using molecular dynamics, we interpret the measurements and elucidate the binding with atomic precision. [Preview Abstract] |
Session P3: Advances in Nanostructured Materials for Electronics
Sponsoring Units: DCMPChair: John Wilkins, The Ohio State University
Room: Colorado Convention Center Korbel 2A-3A
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P3.00001: From nano to micro: hierarchical ordering at the nanoscale Invited Speaker: The overall goal of controlling structural and electronic materials properties at nanometer length scales can be thought of as the intersection of two distinct but correlated challenges. The first is the synthesis/fabrication of individual nanoscale structures and the second is the arrangement of those structures into tailored nano- and micro-scale assemblies. Motivated by these twin challenges, the development of the superlattice nanowire pattern transfer (SNAP) technique has enabled the fabrication of highly ordered arrays of hundreds of nanowires (both metallic and semiconducting) at pitches down to 16 nm and aspect ratios up to 10$^{6}$. As a result of the hierarchical ordering of these assemblies (ranging from nanometer to micrometer length scales), it is possible to achieve electronic point-addressability within the arrays using traditional lithography. Further, iterative use of this technique to generate orthogonal nanowire arrays yields extremely dense crossbar circuits; with a bit density of $\sim $ 0.5 TBit/in$^{2}$ (10$^{11}$ Bits/cm$^{2})$ these structures approach crystallographic density. Both realized and potential applications of these structures ranging from ultra-dense electronic circuits to optical and electronic meta-materials will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:27PM |
P3.00002: Controlled formation of epitaxial III-V nanowires for device applications Invited Speaker: For the realization of devices with dimensions on the 10 nm scale, there is today a great interest in the possible use of self-assembly as a tool. In this talk will be described the state-of-the-art in growth of epitaxially nucleated, vertically standing semiconductor nanowires made from III-V semiconductors, with high level of control of dimensions, position and structural properties. Such wires hold great promise for use in future electronics and photonics applications. Three key aspects will be specifically addressed, namely: \textbf{(1) The combination of top-down and bottom-up processes in lithographically aided formation of nanowires.} A concern from industry is that bottom up techniques should suffer from ``fundamental placement problem[s], i.e. there is no practical and reliable way to precisely align and position them.'' (Chau R., et al. Opportunities and challenges of III-V nanoelectronics for future high-speed, low-power logic applications. (2005)). One way to resolve this issue is lithography where individual nanowire site control with high precision can be achieved. Electron beam lithography has the advantage of being a flexible high-resolution method, whereas nanoimprint lithography offers great opportunities for up-scaling and high-throughput processing. \textbf{(2) The successful growth of III-V nanowires on silicon, including designed heterostructures.} The special nanowire geometry with tens of nanometer radius and very small nanowire / substrate interface, enables monolithic integration of high-performance III-V materials on Silicon substrates. As an example, GaAsP heterostructure nanowires for photonic applications are discussed. Also the formation of InAs nanowires for high-speed and low-power-electronics directly on Si will be described. In the latter process, the use of foreign metal particles for wire growth is completely avoided, greatly reducing compatibility concerns between CMOS and nanowire technology. \textbf{(3) Nanowire devices}, such as field-effect transistors and light-emitting diodes will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 1:03PM |
P3.00003: Electron confinement and long-range interactions in 1-D atom chains Invited Speaker: In nanostructures, when electrons are confined to reduced dimensions, the geometry of the confinement leads to the formation of quantized electronic states. In turn, these quantized states determine the energetic stability of a particular geometry. For systems that are self assembled, where thermodynamics and the cohesive energy can play a key role in the formation process, this interplay between the geometry of the confinement and the electronic states leads to the formation of nanostructures with ``magical sizes.'' Gold deposited on Si(553) leads to self-assembly of 1-D atomic chains, which are broken into finite segments by defects. Scanning tunneling spectroscopy measurements of the differential conductance along the chains revealed quantized states in isolated segments with differentiated states forming over the end atoms. These ``end states'' are the zero-dimensional analogs of the two-dimensional states that occur at a crystal surface[1]. Scanning tunneling microscopy was used to investigate the distribution of chain lengths and the correlation between defects separating the chains. The length distribution is not that for random defects, but exhibits oscillations that indicate changes in the cohesive energy as a function of chain length. We observe two separate components of the interaction and suggest a possible interpretation in terms of the electronic scattering vectors at the Fermi surface. The correlation function shows long-range correlations that extend beyond nearest-neighbor defects, indicating coupling between chains[2]. \begin{enumerate} \item J. N. Crain and D. T. Pierce, Science \textbf{307}, 703 (2005). \item J. N. Crain, M. D. Stiles, J. A. Stroscio, and D. T. Pierce, Phys. Rev. Lett. \textbf{96}, 156801 (2006). \end{enumerate} [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:39PM |
P3.00004: Biologically templated synthesis of Co3O4/Au nanowires for flexible Li-ion batteries Invited Speaker: |
Wednesday, March 7, 2007 1:39PM - 2:15PM |
P3.00005: Superconductivity in DNA templated metal nanowires Invited Speaker: |
Session P4: Magnetic Bose-Einstein Condensation
Sponsoring Units: GMAGChair: Stephen Hill, University of Florida
Room: Colorado Convention Center Korbel 2B-3B
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P4.00001: High field behavior of the spin-dimer compound BaCuSi$_2$O$_6$: magnon BEC and the role of crystal structure Invited Speaker: BaCuSi$_2$O$_6$ is a quasi-2D compound comprising layers of well separated vertical Cu$^{2+}$ dimers. The material has a singlet ground state in zero magnetic field, with a gap of 3.14 meV to the lowest triplon mode. A structural distortion at 100 K leads to multiple triplon bands, separated by less than their band width. Magnetic fields in excess of $H_{c1}$ $\sim$ 23 T close the spin gap, resulting in a state characterized by long- range XY antiferromagnetic order at low temperature. Critical exponents describing the phase boundary approaching the QCP at $H_{c1}$ are consistent with BEC universal scaling, and EPR measurements confirm the absence of terms in the spin Hamiltonian that explicitly break axial symmetry down to an energy scale of 11 mK. A cross-over from 3D to 2D BEC scaling is observed below $\sim$ 0.5 K, which can be attributed to geometric frustration in the body centered crystal structure. \newline \newline *Work performed in collaboration with S. E. Sebastian, E. Samulon, N. Harrison, M. Jaime, C. D. Batista, Z. Islam, S. Hill, N. Kawashima, R. Stern, Ch. Ruegg, H. Ronnow and D. McMorrow. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:27PM |
P4.00002: Geometric Frustration and Dimensional Reduction at a Quantum Critical Point Invited Speaker: The universality class of a critical point depends on a few properties such as the symmetry of the underlying model, the range of the interactions, the number of components of the order parameter, and the space dimensionality. Usually these properties do not change when the system approaches its critical point. However, this is not true in general. For instance, it is natural to ask what happens when the coupling along one of the spatial dimensions vanishes right at the critical point. This is the case of a Bose Einstein Condensation quantum phase transition whenever the underlying lattice consists of layers coupled by a geometrically frustrated interaction. We will show in this talk that this results in a dimensional reduction at the quantum critical point as manifested by its critical exponents. Our theoretical predictions for the critical temperature as a function of the chemical potential correspond very well with recent measurements in BaCuSi$_{2}$O$_{6}$ [S. E. Sebastian \textit{et al}, Nature \textbf{411}, 617 (2006)]. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 1:03PM |
P4.00003: Quantum Magnetism and possible BEC in an organic Nickel compound Invited Speaker: I will review recent experimental and theoretical work on the S=1 quantum magnet, NiCl$_{2}$-4SC(NH$_{2})_{2}$. [1] This compound exhibits field-induced XY antiferromagnetism for magnetic fields along the tetragonal c-axis between H$_{c1}$ = 2.1 and H$_{c2}$ = 12.6 T. The axial symmetry of the spin environment allows us to understand the quantum phase transitions at H$_{c1}$ and H$_{c2}$ in terms of Bose-Einstein condensation (BEC) of spin levels. Here the tuning parameter for BEC transition is the magnetic field and not the temperature. Specific heat, magnetocaloric effect, and magnetization data at low temperatures confirm the predicted behavior for a BEC: Hc-H$_{c1} \quad \sim $ T$^{\alpha }$ and M(H$_{c1}) \quad \sim $ T$^{\alpha }$ where $\alpha $ = 3/2. I will also present magnetostriction data [2] taken at dilution refrigerator temperatures that show significant magnetoelastic coupling and magnetic-order-induced modifications of the lattice parameters in this soft organic compound. The data are well-described by Quantum Monte Carlo calculations, allowing us to make a quantitative determination of the magnetoelastic coupling, and also extract the spin-spin correlation function from the magnetostriction data. \newline \textbf{ \newline }[1] V. S. Zapf, D. Zocco, B. R. Hansen, M. Jaime, N. Harrison, C. D. Batista, M. Kenzelmann, C. Niedermayer, A. Lacerda, and A. Paduan-Filho, Phys. Rev. Lett. 96, 077204 (2006).\newline [2] V. S. Zapf, V. Correa, C. D. Batista, T. Murphy, E. D. Palm, M. Jaime, S. Tozer, A. Lacerda, A. Paduan-Filho, ``Magnetostriction in the Bose-Einstein Condensate quantum magnet NiCl$_{2}$-4SC(NH$_{2})_{2}$,'' cond-mat/0611229. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:39PM |
P4.00004: Magnetic phase diagram of F$_{2}$PNNNO Invited Speaker: 2-[2',6',-difluoro-4'-($N$-\textit{tert}-butyl-$N$-oxyamino)phenyl]-4,4,5,5-tetramethyl-4,5- dihydro-1$H$-imidazol-1-oxyl 3-oxide, F$_{2}$PNNNO for short, is an organic molecule containing two unpaired electrons. Residing in the $N$-\textit{tert}-butyl nitroxide and nitronyl nitroxide groups, the two $S$=1/2 spins of these electrons are ferromagnetically coupled with an exchange constant of 407 K. In a crystal, two neighboring F$_{2}$PNNNO molecules form a pair in which the nitronyl-nitroxide spins are coupled antiferromagnetically with an exchange constant of 67 K. The magnetism of F$_{2}$PNNNO is that of the spin tetramers of these molecular pairs, which in turn are antiferromagnetically coupled with an exchange constant of 7.4 K [1]. Specific-heat and magnetocaloric-effect measurements reveal a highly symmetric boundary of the ordered phase in the phase diagram, with a lower critical field of $H_{c1}$=9.46 T and an upper critical field of $H_{c2}$=15.37 T. The ordering temperature $T_{c}$ obeys a power law $T_{c }\sim (H-H_{c1})^{\alpha }$ near $H_{c1}$, with the exponent $\alpha $ approaching 2/3 in the low-temperature limit, indicative of a Bose-Einstein condensation (BEC) of $\vert S$, $S_{z}>=\vert $1,1$>$ tetramers. Near the upper critical field $H_{c2}$, where one expects a BEC of singlet $\vert $0,0$>$ tetramers in the ``vacuum'' comprising $\vert $1,1$>$ tetramers, the corresponding power-law exponent remains around 0.4. Remarkably, the temperature dependence of the specific heat indicates that the magnon dispersion is independent of magnetic field between the two critical fields. This work is in collaboration with H. Tsujii, B. Andraka, Y. Hosokoshi, and K. Inoue. [1] Y. Hosokoshi \textit{et al.}, \textit{Phys. Rev. B }\textbf{60}, 12924 (1999). [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 2:15PM |
P4.00005: Quasiparticle condensation and breakdown in a quantum spin liquid Invited Speaker: Piperazinium hexachlorodicuprate (PHCC) is a frustrated bilayer antiferromagnet with a disordered quantum spin-liquid (QSL) ground state at zero field [1] and a diverse magnetic field versus temperature phase diagram which includes two field- induced quantum critical points [2]. The spin excitations in PHCC have a spectral gap of $\Delta \approx 1$~meV above which they follow a nearly 2D-isotropic dispersion with a bandwidth slightly larger than $\Delta$. Field dependent neutron scattering and thermodynamic measurements reveal a lower critical field, $H_{c1}=7.5$~T, separating the QSL phase from a three dimensional spin-ordered state and an upper critical field, $H_{c2}=37$~T, marking the onset of a saturated ferromagnetic phase. The two-dimensional antiferromagnet supports a field induced long range ordered phase well described as a Bose-Einstein condensate (BEC) embedded within a gapless quasi-two-dimensional paramagnetic regime. Inelastic neutron scattering experiments also reveal a peculiar type of hybridization of magnetic excitations in PHCC with their two- particle continuum [3], similar to the post-roton regime in superfluid helium. The excitations at this point become broadened and diffuse, no longer describable as quasiparticles. Although such effects are expected to be strongest in one- dimensional systems with gapped spectra [4], such as Haldane chains, direct observation therein is difficult due to a weak scattering structure factor in the vicinity of the quasiparticle breakdown point [5,6]. The dimer-dominated magnetism in PHCC, on the other hand, is favorable for investigating changes in quasiparticle spectra in the vicinity of their breakdown point. Our results have implications for a variety of condensed matter systems, in particular for other QSLs, where spin excitations have a bandwidth greater than the gap energy. \newline [1] M. B. Stone, {\it et al}. Phys. Rev. B {\bf 64}, 144405 (2001).\newline [2] M. B. Stone, {\it et al}. Phys. Rev. Lett. {\bf 96}, 257203 (2006).\newline [3] M. B. Stone, {\it et al}. Nature, {\bf 440}, 187 (2006).\newline [4] T. Giamarchi, Quantum Physics in One Dimension, Oxford University Press (2005).\newline [5] S. Ma, {\it et al}. Phys. Rev. Lett. {\bf 69}, 3571 (1992).\newline [6] I. A. Zaliznyak, {\it et al}. Phys. Rev. Lett. {\bf 87}, 017202 (2001). [Preview Abstract] |
Session P5: Entanglement Entropy in Condensed Matter Physics
Sponsoring Units: DCMPChair: Eduardo Fradkin, University of Illinois at Urbana-Champaign
Room: Colorado Convention Center Korbel 1A-1B
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P5.00001: Probing order beyond the Landau paradigm Invited Speaker: For many years, it was thought that Landau's theory of symmetry breaking could describe essentially all phases and phase transitions. Then, in 1982, the limitations of Landau theory were exposed in a dramatic way with the discovery of the fractional quantum Hall (FQH) effect. The FQH states contain a new kind of order - known as ``topological order'' - that is fundamentally beyond the Landau paradigm. Topological order cannot be understood using symmetry breaking, order parameters, or long range order. This poses an interesting theoretical problem: these states must contain some kind of structure that is responsible for their unusual physical properties. But what is this structure and how can we probe it without order parameters? In my talk, I will describe recent progress in answering this question. I will show that topological order is intimately connected with nonlocal quantum entanglement. I will introduce a new quantity - called ``topological entropy'' - that measures precisely this nonlocal entanglement. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:27PM |
P5.00002: Universal contributions to entanglement entropy at critical points in two spatial dimensions Invited Speaker: The entanglement entropy of a pure quantum state of a bipartite system $A \cup B$ is defined as the von Neumann entropy of the reduced density matrix obtained by tracing over one of the two parts. Critical ground states of local Hamiltonians in one dimension have entanglement that diverges logarithmically in the subsystem size, with a universal coefficient that for conformally invariant critical points is related to the central charge of the conformal field theory. We find the entanglement entropy for a standard class of $z=2$ quantum critical points in two spatial dimensions with scale invariant ground state wave functions: in addition to a nonuniversal ``area law'' contribution proportional to the size of the $AB$ boundary, there is generically a universal logarithmically divergent correction. This logarithmic term is completely determined by the geometry of the partition into subsystems and the central charge of the field theory that describes the equal-time correlations of the critical wavefunction. Taken together with results on entanglement entropy in gapped, topologically ordered phases, these results indicate that even when the ``area law'' correctly predicts the leading behavior of entanglement, universal subleading terms can reflect important properties of a quantum many-body system. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 1:03PM |
P5.00003: Entanglement entropy of quantum-critical spin chains with strong randomness Invited Speaker: For disorder-free critical quantum spin chains, the entanglement of a segment of $N\gg 1$ spins with the remainder scales as $log_2 N$, with a prefactor fixed by the central charge of the associated conformal field theory. The mean entanglement entropy of quantum spin chains with randomness follows the same logarithmic scaling, and provides a universal critical entropy, which is equivalent to the central charge in the pure case. In my talk I will explore the origin and derivation of the universal entanglment entropy of the random spin-1/2 Heiseneberg model in the random-singlet phase, as well as that of the random spin-1 Heisenberg chain at the breakdown of its Haldane phase. The entanglement of these and related infinite-randomness fixed points makes it possible to speculate on possible extensions of the c-theorem of CFTs to the realm of systems with strong randomness. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:39PM |
P5.00004: Entanglement and efficient simulation of many-body quantum systems Invited Speaker: |
Session P6: Condensed Matter Physics and Future Accelerator-based Light Sources
Sponsoring Units: FIAP DPBChair: Alan Todd, Advanced Energy Systems
Room: Colorado Convention Center 207
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P6.00001: New x-ray sources illuminate hidden corners of space and time -- revolutionary solid-state science and how we'll get there through the Advanced Photon Source upgrade Invited Speaker: Third-generation x-ray storage ring sources have major impact in condensed-matter and materials physics. Many cutting-edge experiments, including nanoscale imaging and studies of fast dynamics, demand shorter and more spatially coherent x-ray pulses. Unfortunately, these parameters are intrinsically limited by the physics of storage rings. Many experiments at sources such as the Advanced Photon Source (APS) could benefit from revolutionary performance that is promised by fourth-generation capabilities. The key element which makes fourth-generation sources better in these respects is that the electron beam does not travel long enough (is not ``stored'') for the pulse to come to equilibrium. We are proposing to introduce such a beam into the APS storage ring through an Energy-Recovery LINAC concept, as part of a major upgrade. I will describe some of the research in condensed matter and materials physics which demand this upgrade, and outline the technical performance and our plans to realize the upgrade. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:27PM |
P6.00002: Energy Recovery LINAC: Experimental Challenges Invited Speaker: ERL projects are ongoing at JLab, Daresbury, KEK and Cornell. Here, we describe the experimental challenges of using high-coherence and ultra-fast pulses from the Cornell ERL and illustrate some potential opportunities. The Cornell ERL is designed to run in several different modes. In the hi-flux mode, the ERL runs at 5 GeV and 100 mA. Many experiments, such as inelastic x-ray scattering are photon-starved. In the high-coherence mode the ERL runs at 25 mA and the transverse emittances could be as low as 8 pm. The beam size will be at its smallest under this operating condition and an average spectral brightness as high as 10$^{23}$ (standard units) is predicted. We expect to produce a round (3 micron diameter) source for imaging and coherence experiments on individual biological cells. In the ultra-fast mode, the repetition rate is reduced from 1.3 GHz to 1 MHz, the bunch charge is increased to 1 nC per pulse, and the natural 2 ps bunch length is compressed to less than 100 femtoseconds. We will present opportunities for x-ray experiments on a single atom as well as the challenges in x ray optics, other experiments, and beam control issues when making a 1 nm focused x-ray beam size. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 1:03PM |
P6.00003: Seeded Free Electron Lasers: The Technical Challenges and the Scientific Impact Invited Speaker: The fields of laser and accelerator technology stand now at a point of remarkable opportunity---the creation of fully coherent and powerful pulses of x-ray radiation ranging in wavelength from 100 nm to 0.1 nm. Radiation in this essential wavelength range is unlikely to be produced with substantial power, particularly below wavelengths of 10nm, by table-top laser sources. Sources based solely on circular accelerators, such as synchrotrons and energy-recovery linacs, cannot achieve full coherence. Even the current generation of free-electron lasers (FELs) based on self-amplified spontaneous emission (SASE) expect to produce less than 1{\%} of the power in a single mode in their baseline designs. The key to achieving full transform-limited coherence is to imprint the electron bunch with a fully coherent seed pulse generated by either an external source or by clever manipulations of the FEL radiation itself. We will review the technological challenges presented by this approach in the context of efforts such as the Trieste FERMI project, and a major new soft x-ray/VUV FEL user facility being studied for the University of Wisconsin. Further, we will summarize the impact of such a source in various areas of science [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:39PM |
P6.00004: Studies of Dynamics Using Coherent X-rays Invited Speaker: X-ray photon correlation spectroscopy (XPCS) has emerged over the last decade as a new technique for studies of fluctuation dynamics at small length scales. Such dynamics is ubiquitous in countless processes in condensed matter systems, such as viscoelastic flow of glasses, polymer diffusion, phase transitions, or domain switching. Like the dynamic light scattering techniques originally developed using visible wavelengths, XPCS is based on scattering a coherent beam from structural fluctuations in a material to produce a speckle pattern. The speckle pattern reflects the exact arrangement of the disorder, and thus evolves in time in concert with fluctuations. The time correlations of the speckle intensity provide a direct measure of fluctuation dynamics at the length scale corresponding to the scattering wavenumber. The extension of this technique to x-ray wavelengths provides access to atomic-scale dynamics. In practice, however, XPCS studies using 3rd-generation synchrotrons have been limited by the available coherent flux. Experiments to date have been most successful using small-angle scattering to study dynamics of $\sim $100 nm structures, which have sufficiently high scattering efficiency and relatively long time constants (e.g. milliseconds). Future accelerator-based x-ray sources such as free-electron lasers and energy recovery linacs will provide significantly increased coherent x-ray flux, which will greatly expand the applicability of XPCS to shorter length scales, faster time scales, and more weakly scattering systems. In particular, the ultrashort pulse structure of the new x-ray sources will allow observation of dynamics into the femtosecond range. I will discuss potential experiments as examples of the anticipated capabilities. Work supported by the U.S. Dept. of Energy contract DE-AC02-06CH11357. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 2:15PM |
P6.00005: Research at DESY's VUV-FEL user facility FLASH Invited Speaker: FLASH is currently the only operating free electron laser providing lateral coherent radiation in the wavelength range from 13 to 48 nm in the fundamental in flashes of 10 femtoseconds duration. One obtains 10$^{12}$ photons per flash, i.e. as many as we get today from the best storage ring facilities per second. The maximum peak pulse energy for 13 nm is 120 $\mu $J, the peak power is larger than 4 GW, the average power goes up to 30 mW. The peak brilliance reaches 5x10E29. The intensity of the third harmonic at 4.6 nm (270 eV) is on the 0.5{\%} level of the fundamental. Since August 2005 FLASH operates as a user facility and experiments have been performed successfully on atoms, highly charged ions and clusters. First photoelectron spectra have been taken, materials damage problems have been studied and first pump and probe experiments with an additional optical laser beam have been performed successfully. With respect to single particle imaging it was demonstrated that a fully interpretable diffraction pattern can be obtained by one flash of 25 femtoseconds duration before the sample heats up to about 60 000 K and evaporates. The experiments show the importance of the combination of extremely high peak brilliance with very high average brilliance for future experiments at X-ray free electron lasers. [Preview Abstract] |
Session P7: Science Meets Policy: Careers and Advocacy
Sponsoring Units: FGSAChair: Gavi Begtrup, University of California, Berkeley
Room: Colorado Convention Center Korbel 4A-4B
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P7.00001: Becoming the Citizen Scientist: Opportunities and Challenges in Science Policy Invited Speaker: The methodologies, creativity and intellectual capacity of today's physicists are becoming more and more relevant in the world of policy and politics. Some issues such as climate change, alternative energy and avian influenza clearly reveal the relevance of scientific knowledge and research in policy. However, the connection between science and issues such as electronic voting, government earmarks and international cooperation are not as obvious, but the role of scientists in these topics and their effects on science itself are critical. As the world becomes increasingly technological and global, the need for the involvement of scientists in the political process grows. The traditional scientific training of physicists emphasizes intense scrutiny of specific physical phenomena in the natural world but often misses the opportunity to utilize trained scientific minds on some of society's greatest problems. I will discuss the many ways in which scientists can contribute to society far beyond the academic community and the unique opportunities science policy work offers to the socially conscious scientist or even those just looking to get more grant money. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:27PM |
P7.00002: Fixing a Flat at the Noisy Intersection of Science and Politics Invited Speaker: Science is rarely a talent of Kings and governments. When King Solomon built his temple, he declared that pi = 3. Three thousand years later, in 1897, the Indiana State Legislature unanimously passed Public Law 246, making pi = 3.2. This talk will review some of the key factors that influence government decisions and then examine some of the issues that the American Physical Society has been engaged in, including confronting the Intelligent Design movement. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 1:03PM |
P7.00003: Advising Students about Participating in the Science and Policy Interface Invited Speaker: This talk will first address general principles that I have found useful in guiding physics graduate students and post-docs in choosing options for their future careers. In this context, we discuss careers in science that are connected to society and careers in policy that affect science. Finally we discuss options for careers that involve both the pursuit of science and the involvement of science policy. The presentation will draw from my experience in mentoring graduate students and postdocs on these topics and from my own rewarding career which combined science and science policy. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:39PM |
P7.00004: Connecting Science and Society: Basic Research in the Service of Social Objectives Invited Speaker: A flawed dichotomy of basic versus applied science (or of ``curiosity-driven'' vs. ``mission-oriented'' science) pervades today's thinking about science policy. This talk argues for the addition of a third mode of scientific research, called Jeffersonian science. Whereas basic science, as traditionally understood, is a quest for the unknown regardless of societal needs, and applied science is known science applied to known needs, Jeffersonian science is the quest for the unknown in the service of a known social need. It is research in an identified area of basic scientific ignorance that lies at the heart of a social problem. The talk discusses the conceptual foundations and then provides some case examples of Jeffersonian-type science initiatives, such as the Lewis and Clark Expedition, initiated by Thomas Jefferson (which led us to call this mode of research Jeffersonian), research conducted under the auspices of the National Institutes of Health, and a science policy project by President Jimmy Carter and his Science Adviser, Frank Press, in the late 1970s. Because the concept of Jeffersonian science explicitly ties basic research to the social good, one of the potential benefits of adding a Jeffersonian dimension to our thinking about science is that it might make science careers more attractive to women and underrepresented minorities. [Preview Abstract] |
Session P8: Focus Session: Novel Superconductors:Miscellaneous Materials
Sponsoring Units: DMPChair: Amy Liu, Georgetown University
Room: Colorado Convention Center Korbel 1C
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P8.00001: Recent progress in applications of the superconducting density functional theory Invited Speaker: One of the great challenges of condensed-matter theory is the prediction of material specific properties of superconductors (SC) such as the critical temperature $T_c$ or the gap at zero temperature. Recently, based on a seminal work by Oliveira, Gross and Kohn(1), an extention of density functional theory to the superconducting state (SCDFT) was introduced and applied to elemental superconductors (2). Later work showed how the method is able to describe the properties of real materials ranging from weak to strong coupling. Unique feature of the method is the ab-initio inclusion of the Coulomb interaction which, recently combined with a fully anisotropic treatment of the electron-phonon coupling, allows for a detailed description of the most important material specific properties, including the relevance of multiple gaps, in good agreement with the available experiments. \\ The discovery of novel electron-phonon SC provided new challenges to the method. We will report on the most recent applications, including MgB$_2$, alkali metals under pressure, Ca intercalated graphite and other new and traditional SC. The subtle interplay between e-ph mediated attraction and Coulomb repulsion, normally hidden by the use of the pseudopotential $\mu^*$, will show its material-specific importance in the resulting $T_c$. (1) L. N. Oliveira, E. K. U. Gross, and W. Kohn, Phys. Rev. Lett. 60, 2430 (1988) (2) Marques et al., Phys Rev. B 72, 024545 (2005); M. Lueders et al., $ibid$ 024546 (2005) [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P8.00002: First-Principles Investigation of Superconductivity in Transition Metal Carbides Jesse Noffsinger, Feliciano Giustino, Steven G. Louie, Marvin L. Cohen We investigate the origin of superconductivity in the transition metal carbides TaC and HfC by a first-principles approach. The electronic structure is described within density functional theory in the local density approximation, and the lattice dynamical properties are determined through density functional perturbation theory. We calculate the average electron-phonon coupling strength through the isotropic approximation to the Migdal-Eliashberg theory, and the superconducting transition temperature through the McMillan formula. The calculated transition temperatures are found to be in excellent agreement with experiment. The relatively high transition temperature of TaC (10.3 K) is associated with a Kohn anomaly in the phonon dispersions, and arises from significant Fermi surface nesting. In contrast, the absence of nesting in HfC results in a limited phase-space availability for electron-phonon scattering. Correspondingly, HfC exhibits a negligible transition temperature ($<0.1$ K). [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P8.00003: Strong Electron-Phonon Coupling in Elemental Metals Under Pressure Zhiping Yin, Warren Pickett The superconductivity of yttrium with T$_c$=20K at 115 GPa has been confirmed by strong electron-phonon coupling obtained using linear response methods. The increase of T$_c$ under pressure mainly comes from the increasingly strong coupling to the transverse modes at all high-symmetry zone boundary points X, K, and L. Evaluation of the electron-phonon spectral function shows a very strong increase with pressure of coupling strength in the 2-8 meV range, but with an accompanying steady increase in the 8-20 meV range.The superconductivity of Ca under pressure, however, is a challenge. While other elemental superconductors are usually close-packed, Ca is simple cubic (SC) at pressure between 30 GPa and 109 GPa, and its T$_c$ increases significantly in this pressure range, and goes to 23 K at 109 GPa (25 K at 161 GPa), making Ca the highest T$_c$ superconductor among elements. From linear response calculations we find the harmonic frequencies are unstable over a large portion of the zone for a wide range of pressure in the SC phase. We present calculational results and discuss possibilities, which include the likely stabilization of the SC structure by large anharmonic contributions to the lattice dynamics. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P8.00004: Calculations of Superconducting Properties in Yttrium and Calcium under High Pressure Dimitrios Papaconstantopoulos, Lei Shi, Michael Mehl We have used first-principles electronic structure calculations to generate the bulk modulus as a function of volume as well as the densities of states and scattering phase shifts at the Fermi level. These quantities were used in conjunction with the rigid-muffin-tin theory of Gaspari and Gyorffy and the McMillan theory to determine the electron-phonon coupling and the superconducting transition temperature for Yttrium and Calcium under high pressures. Our results provide a good interpretation of the measured increase of $T_{c}$ in these metals. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P8.00005: Assessment of the importance of correlation effects in Li$_x$NbO$_2$ K.-W. Lee, R. T. Scalettar, W. E. Pickett, J. Kunes About 15 years ago Geselbract {et al.} reported superconductivity
with $T_c$=5K for $x \approx$0.5 in Li$_x$NbO$_2$.
The critical temperature does not show significant change in the
range 0.45 $ |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P8.00006: Nearly-free electron superconductor Ag$_5$Pb$_2$O$_6$ Shingo Yonezawa, Mike Sutherland, Peter D. A. Mann, Christoph Bergemann, Yoshiteru Maeno Superconductivity in the silver lead oxide Ag$_5$Pb$_2$O$_6$ has been discovered below 52~mK [1,2]. Although its $T_c$ is one of the lowest among the known oxide superconductors, this oxide is interesting from the viewpoint that it is the first superconductor with a nearly-free-electron Fermi surface. This fact is revealed by our quantum oscillation study [3] as well as recent band-calculation studies, which concluded that the system possesses one near-spherical Fermi surface with a small electron- mass enhancement. We will present its type-I superconducting properties, as well as the properties of the normal state where the resistivity varies nearly as $T^2$ up to room temperature [1]. \newline [1] S. Yonezawa and Y. Maeno, Phys.~Rev.~B 70, 184523 (2004). [2] S. Yonezawa and Y. Maeno, Phys.~Rev.~B 72, 180504(R) (2005). [3] M. Sutherland et al., Phys.~Rev.~Lett. 96, 097008 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P8.00007: Electron-phonon interaction in the polymeric superconductor, polysulfur nitride, (SN)$_{x}$. Paul M. Grant In early 1975, superconductivity at temperatures between 0.3 -- 0.4 K was discovered in the inorganic polymer, polysulfur nitride, (SN)$_{x}$. The compound itself was originally synthesized in the first decade of the 20$^{th}$ century, but its transport properties went largely undetermined until their investigation was sparked by the emergence of the low dimensional layered organic charge transfer salts in the decade of the 1980s. The issue of why the transition temperature of (SN)$_{x}$ is so low has not been adequately addressed computationally, especially in view of the realization of superconductivity at nearly 40 K in magnesium diboride, MgB$_{2}$, in 2001, a compound whose electronic structure is remarkably similar to (SN)$_{x}$, in that both are two-band, hole-electron semimetals with low-dimensional Fermi surface topologies. In this talk, we report our results on the calculation of the electron-phonon dispersion relation,$\alpha ^2F(\omega ),$ for (SN)$_{x}$ obtained from the application of recently available DFT algorithms capable of accurately treating screening of electron-phonon interactions in metals. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P8.00008: Microscopic theories for Cubic and Tetrahedral Superconductors Shantanu Mukherjee, Daniel Agterberg We will examine the weak coupling theory for the unconventional superconducting states of cubic or tetrahedral superconductors for arbitrary order parameters and Fermi surfaces in zero applied magnetic fields. We will also look at multiple transitions where a higher symmetry is weakly broken to account for them. We will then perform a weak coupling theory where two representations of the symmetry group have accidentally nearly degenerate transition temperatures. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P8.00009: Density of states, specific heat and nuclear spin-lattice relaxation rate in PrOs$_4$Sb$_{12}$ Tayseer Abu Alrub, Stephanie Curnoe We present a theoretical study of the density of states, specific heat and nuclear spin-relaxation rate in the unconventional superconductor PrOs$_4$Sb$_{12}$. In this material, superconductivity is best described by a three component order parameter in the triplet channel. Instead of nodes, deep dips appear in the gap function producing power law temperature dependencies at higher temperatures and exponential suppression at low temperatures of the specific heat and the nuclear spin lattice relaxation rate. Various experimental observations will be discussed in this context. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P8.00010: Multiband superconductivity and penetration depth in PrOs$_{4}$Sb$_{12}$ D.E. MacLaughlin, Lei Shu, R.H. Heffner, J.E. Sonier, F.D. Callaghan, G.D. Morris, O.O. Bernal, W.M. Yuhasz, N.A. Frederick, M.B. Maple The penetration depth lambda in the filled-skutterudite heavy-fermion superconductor PrOs$_4$Sb$_{12}$ has been measured using transverse-field muon spin rotation. It is found to be temperature-independent at low temperatures, consistent with a nonzero gap for quasiparticle excitations. In contrast, zero-field radiofrequency inductive measurements yield a stronger temperature dependence of lambda, indicative of point nodes in the gap. A $\sim$10\% discrepancy is found at intermediate temperatures. This seems likely to be due to multiband superconductivity in this compound, recently found from thermal conductivity measurements. A sufficiently large difference between gaps would render the field distribution in the vortex controlled exclusively by the larger gap band, whereas all bands would participate in zero field. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P8.00011: Fully gapped $s$-wave superconductivity in KOs$_2$O$_6$ I. Bonalde, R. Ribeiro, W. Bramer-Escamilla, Z. Hiroi, Y. Yamaura The discovery of superconductivity in the $\beta$-pyrochlore oxides AOs$_2$O$_6$ (A=Cs, Rb, and K) has attracted so much attention, because the geometric spin frustration inherent to their pyrochlore crystal structures is supposed to give rise to unconventional superconductivity via magnetic spin fluctuations. Until now experimental results suggest, however, that CsOs$_2$O$_6$ ($T_c=3.3$ K) and RbOs$_2$O$_6$ ($T_c=6.3$ K) are fully gapped $s$-wave superconductors. On the other hand, the experimental data of KOs$_2$O$_6$ ($T_c=9.6$ K) show somewhat unusual behaviors, pointing out in some cases to unconventional superconductivity. In this talk we will discuss magnetic penetration depth data of single crystals of KOs$_2$O$_6$ down to 30 mK. The data clearly indicate that KOs$_2$O$_6$ is a fully single-gapped $s$-wave superconductor. This implies that all of the geometrically spin-frustrated compounds known until now respond as conventional superconductors, which would suggest that spin frustration does not lead to unconventional pairing as expected. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P8.00012: Muon spin relaxation and hyperfine-enhanced $^{141}$Pr nuclear spin dynamics in (Pr,La)Os$\rm{_4}$Sb$\rm{_{12}}$ and Pr(Os,Ru)$\rm{_{4}}$Sb$\rm{_{12}}$ Lei Shu, D.E. MacLaughlin, W. Higemoto, R.H. Heffner, K. Ohishi, T.U. Ito, Y. Aoki, Y. Tunashima, Y. Yonezawa, S. Sanada, D. Kikuchi, H. Sato, K. Ishida, R. Kadono, A. Koda, O.O. Bernal, H. Sugawara, N.A. Frederick, W.M. Yuhasz, T.A. Sayles, T. Yanagisawa, M.B. Maple The longitudinal-field muon relaxation experiments have been carried out in the Pr,La)Os$\rm{_4}$Sb$\rm{_{12}}$ and Pr(Os,Ru)$\rm{_{4}}$Sb$\rm{_{12}}$ alloy systems. At low temperatures, the dynamic fluctuations are involved in muon relaxation in addition to the contribution from a distributions of static muon local fields. The temperature and concentration dependencies of the muon damping rate $\Lambda$ indicate that this dynamic contribution is due to $^{141}$Pr nuclear magnetism, which is enhanced by hyperfine coupling to the Pr$^{3+}$ Van Vleck susceptibility. Further evidence comes from the field dependence of $\Lambda$, which is in reasonable agreement with the modified model for muon spin relaxation by dipole-coupled nuclear spins. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P8.00013: Andreev Spectroscopy Study of the Heavy-Fermion Superconductor PrOs$_{4}$Sb$_{12}$ C.S. Turel, I. Fridman, J.Y.T. Wei, W.M. Yuhasz, M.B. Maple The discovery of superconductivity in the heavy-fermion material PrOs$_{4}$Sb$_{12}$ has attracted widespread interest. In particular, there is evidence for multiple superconducting order parameters, at least one of which is believed to have nodes. We present Andreev spectroscopy data, down to 80 mK and up to 2.5 T, taken using ballistic point contacts made with Pt-Ir tips on single crystals of PrOs$_{4}$Sb$_{12}$. Pronounced zero-bias conductance peaks (ZBCP's) seen in the differential conductance spectra, show the existence of nodes in the order parameter. The magnetic field and temperature evolution of the spectra were studied to track how the pairing symmetry evolved, allowing us to map out the order parameter phase diagram. We observed that the ZBCP's vanished at a magnetic field, $H^{*}$, lower than the upper critical field, $H_{c2}$. This implies a field-driven change in the nodality of the order parameter at $H^{*}$, suggesting there are multiple superconducting phases with different pairing symmetries in PrOs$_{4}$Sb$_{12}$. [Preview Abstract] |
Session P9: Superconductivity: Synthesis
Sponsoring Units: DMPChair: Doug Kirven, Sigma-K
Room: Colorado Convention Center Korbel 1D
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P9.00001: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P9.00002: Superconducting properties of the hexagonal layered molybdenum carbide $\eta $-Mo$_{3}$C$_{2}$ K. Yamaura, Q. Huang, M. Akaishi, E. Takayama-Muromachi Superconductivity of $\eta $-Mo$_{3}$C$_{2}$ ($T_{c}$=8.5K) was reported in 1960s, while detailed superconducting and structure properties remained uncertain probably because those were complicated somewhat by carbon non-stoichiometry, partially thermal decomposition, and so on. Recently, we found the degree of problems is fairly reduced by employing high-pressure method, resulting in a distinct sample quality-improvement, which allowed us to conduct neutron diffraction, magnetic susceptibility, and specific-heat measurements on a polycrystalline form of $\eta $-Mo$_{3}$C$_{2}$ [1]. A significant layered character was found in the structure, which comprises edge-sharing CMo$_{6}$ octahedra sheets and $\sim $50{\%} carbon occupied blocks. Magnetic characterization revealed the Ginzburg-Landau parameter of $\eta $-Mo$_{3}$C$_{2}$ is $\sim $26, which is close to that for the comparable $T_{c}$ compound Li$_{2}$Pd$_{3}$B ($\sim $21), but less than a half of that for MgCNi$_{3}$ ($\sim $54). \newline \newline [1] K. Yamaura et al., PRB 74, 184510 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P9.00003: Origin of Superconductivity in the Y-Sr-Ru-O System Eduard Galstyan, Yuyi Xue, Milko Iliev, Yanyi Sun, Paul Chu The origin of the superconductivity (SC) in the perovskite-based Sr$_{2}$YRuO$_{6}$ system with minor Cu doping on Ru ion sites has attracted much interest among researchers in recent years. Addressing the question of the nature of SC, J. D. Dow and D. R. Harshman proposed a model in which SC condensation occurs in the SrO plane. While the model appears to be in disagreement with many other research works, the exact reason of SC in the Y-Sr-Ru-O system is still uncertain. In this work we provide experimental evidence that the SC behavior of these materials is attributed to a minor impurity phase with the stoichiometry of YSr$_{2}$Cu$_{3}$O$_{7-\delta }$ (YSCO). We show that both the SC and the YSCO grains appear only when the synthesis temperature is higher than a local melting temperature. In addition, the YSCO grains, which are thermodynamically unstable under ambient pressure, appear within dense surroundings, where the melting indications are obvious. Stresses during solidification seem to play roles and may also explain the earlier reports of trace SC in YSCO samples synthesized at ambient pressure. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P9.00004: Growth and superconductivity of single crystals La$_{2-x}$Ba$_{x}$CuO$_{4}$ G. D. Gu, J. S. Wen, Z. J. Xu, G. Y. Xu, J. M. Tranquada, Q. Li, A. R. Moodenbaugh, K. Yamada In 1986, Bednorz and Muller made a great discovery of the high temperature superconductivity in La$_{2-x}$Ba$_{x}$CuO$_{4}$ cuprate materials. Since the discovery of the superconductivity in high temperature superconducting oxide La$_{2-x}$Ba$_{x}$CuO$_{4 }$, a large number of groups have attempted to grow the single crystals. However, no single crystal La$_{2-x}$Ba$_{x}$CuO$_{4 }$with x$>$0.11 has been successfully grown. In this work, the effects of the growth condition and the compositions of a feed rod on the crystal growth of La$_{2-x}$Ba$_{x}$CuO$_{4 }$has been studied by an infrared image floating zone method. The experimental result shows that a planar solid-liquid growing interface tends to break down into a cellular interface when the growth velocity is more than 1 mm/h. When the planar solid-liquid growing interface break down into a cellular interface, the single crystal size decreases abruptly and the as-grown rod is not single phase. The large single crystals of La$_{2-x}$Ba$_{x}$CuO$_{4 }$with x=0 to 0.165 has been successfully grown. The single crystals of La$_{2-x}$Ba$_{x}$CuO$_{4 }$with x=0 to 0.165 up to 6 mm diameter and 150 mm length have been grown. The superconductivity transition temperature T$_{c}$ of as-grown single crystals of La$_{2-x}$Ba$_{x}$CuO$_{4 }$(x=0 to 0.165) have been measured. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P9.00005: Refined crystal growth and characterization of the high-T$_{c}$ superconductor HgBa$_{2}$CuO$_{4+\delta }$ Yuan Li, Neven Barisic, Guillaume Chabot-Couture, Yong-Chan Cho, Gertjan Koster, Guichuan Yu, Xudong Zhao, Martin Greven Among the high-T$_{c}$ superconductors, HgBa$_{2}$CuO$_{4+\delta }$ (Hg1201) is one of the most desirable systems for experimental study due to its relatively simple structure and high T$_{c}$. For quantitative experimental work, it is necessary to grow sizable, high-quality crystals, and to obtain fine oxygen/doping control. Here we report on our most recent improvements in the growth and characterization of Hg1201, leading to further improved sample quality. Our new results include charge transport, magnetic susceptibility, and x-ray photoelectron spectroscopy (XPS) measurements. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P9.00006: Carrier density control and phase diagram of Li$_x$ZrNCl superconductors Yasujiro Taguchi, Atsushi Kitora, Yoshihiro Iwasa We succeeded in synthesizing a series of Li$_x$ZrNCl samples with controlled doping level $x$ ($0 \le x \le 0.3$) which are confirmed to be of single phase by means of synchrotron x-ray diffraction measurements. We found that $T_c$ rapidly increases upon reducing Li concentration below $x$=0.12 to reach the maximum value of 15.2 K at $x$=0.06, and that a superconductor-to-insulator transition (SIT) is encountered at $x$=0.05 due to the Anderson localization effect. Such an increase in $T_c$ on the verge of SIT seems to be difficult to explain by the conventional theory, but may be indicative of the charge fluctuation contribution to superconductivity in low-carrier-density systems. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P9.00007: Strain induced morphological instability of epitaxial YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ films deposited on LAO and STO. Priya.V. Chinta, O. Lozano, P. Wadekar, Q.Y. Chen*, X.M. Wang, J.R. Liu, W.K. Chu, H.W. Seo, L.W. Tu, H.M. Huang, Y.L. Cheng, C.P. Sun, H.D. Yang Surface morphological instabilities in superconducting YBCO films have not yet been fully understood because of the intractably involved underlying driving forces. In this work we attempted to pin-point the lattice mismatch effect on the evolution of surface morphology for YBCO films on LaAlO$_{3}$ and SrTiO$_{3}$ substrates grown epitaxially by DC magnetron sputtering or laser ablation. The initial root-mean-square roughness of the samples was about 10-15 nm. This value was reduced to 1-3 nm after the samples were subjected to a 30-keV (Ar)$_{n}^{+}$ gas cluster ion beam (GCIB) sputtering at right angle to a dose of 2$\times $10$^{16}$/cm$^{2}$. Controlled annealing of these smoothened films in flowing O$_{2}$ atmosphere was then conducted at different temperatures and time periods, upon which consistent surface roughening (SR) was observed. This SR is attributed to the elastic strain of lattice-mismatch between the film and substrate. The effects of interface coherency on such phenomenon will be discussed. *Also with NSYSU [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P9.00008: Disordered superconducting films in strong magnetic fields Yonatan Dubi, Yigal Meir, Yshai Avishai Experimental studies of magneto-resistance in disordered superconducting thin films reveal an abundance of unexpected results, such as a huge peak in the magneto-resistance on the insulating side of the superconductor-insulator transition which evolves as the field is tilted, and traces of superconducting correlations that survive well above the transition. Recently, a theory that accounts for these effects was suggested, in which it is postulated that (i) well-separated SC islands are formed in the disordered film, and (ii) their size and strength diminishes with magnetic field. In this work we present extensive numerical calculations which support these conjectures, by means of a locally self-consistent solution of the BdG equations in the presence of disorder and tilted magnetic field. Simple phenomenological arguments from percolation theory are then used to explain various experimental findings, such as the non- monotonic magneto-resistance in parallel field and the relation between the critical field and the magnetic field tilt angle. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P9.00009: Investigation of tunneling density of state of FSF trilayers. Jun Hyung Kwon, Jewook Park, Seong Kook Choi, Kookrin Char, P.G. SanGiorgio, M.R. Beasley Unconventional superconductivity such as $\pi $-state and long range triplet superconductivity may arise in an FSF trilayer structure, depending on the relative directions of the magnetization of two ferromagnetic layers. In order to observe such unconventional states by measuring tunneling density of state(DOS) of FSF structure, we fabricated Al(12nm)/AlOx/CoFe(2,5nm)/Nb(20,30,40nm)/NiFe(10nm) using stencil mask method in a cross-strip geometry. In order to easily change magnetization direction of the magnetically soft NiFe layer, we further etched the last NiFe layer into a square shape by ion beam milling. We will present magnetic force microscope(MFM) image of the NiFe layer as its magnetization direction is switched to different directions. Density of state measurement of FSF structure with different thicknesses of the CoFe layer will be able to probe the unconventional superconductivity with appropriate magnetization directions of the two F layers. Our latest efforts to measure such state will be presented. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P9.00010: Magnetic Field Enhanced Insulating Behavior in Thin Films with Local Cooper Pairing K. H. Sarwa B. Tan, Kevin A. Parendo, Z. Ovadyahu, A. M. Goldman The effects of a perpendicular magnetic field on insulating amorphous indium oxide thin films exhibiting local superconductivity have been investigated. At very low temperatures the application of relatively low magnetic fields produces a giant positive magnetoresistance that increases with decreasing temperature. This suggests that the ground state in zero field may be a Cooper pair insulator. At nonzero temperatures a magnetic field enhances this insulating behavior. The low temperature I-V characteristics exhibit strong nonlinearities with threshold voltages for enhanced conduction whose magnitudes are independent of both temperature and magnetic field. This behavior indicates a connection with Cooper pairing and might be associated with the depinning of a charge structure. Although X-ray analyses characterize the films being amorphous, AFM images exhibit significant roughness that might be responsible for the formation of regions with a high local density of Cooper pairs. This work was supported in part by the National Science Foundation under grant NSF/DMR-0455121. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P9.00011: Electrical transport properties of ultrathin superconducting Pb films. R.P. Panguluri, M.M. Ozer, J.R. Thompson, H.H. Weitering, B.E. Nadgorny We present electrical transport properties of metallic ultra thin epitaxially grown Pb (111) films on Si (111) substrate. We observed a reduced superconducting transition temperature from bulk Pb using electrical resistivity measurements and deduced the temperature dependence of out-of-plane critical magnetic fields from the sheet resistance R as a function of the applied magnetic field. These results are consistent with M. M. Ozer et al.,$^{1}$ obtained by magnetic techniques. We identified the mean field critical temperature and current densities from I-V curves in zero magnetic field. We explored the possible presence of Kosterlitz-Thouless transition (T$_{KT})$ in this system. We discuss these results based on the Ginzburg-Landau Coloumb-Gas (GLCG) model for 2D vortex fluctuations. 1. M. M. Ozer, J. R. Thompson, and H. H. Weitering, Nature Physics, \textbf{2}, 173 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P9.00012: The effect of magnetic field on superconductivity in ultrathin amorphous Pb films with paramagnetic impurities. Ashwani Kumar, H. Jeffrey Gardner, Peng Xiong We report on a systematic study of the effect of applied magnetic fields on superconductivity in ultrathin amorphous Pb films containing various amounts of paramagnetic impurities. The Pb film, along with a 1 nm thick Sb buffer layer, was quench-condensed onto a Si substrate with pre-deposited Au contacts in a modified dilution refrigerator. Cr impurities were then deposited onto the Pb film by heating a NiCr wire at a fixed current. Both the Pb thickness (thus its T$_{C})$ and the Cr density can be varied, and electrical measurements can be performed at each step in perpendicular magnetic fields up to 8 T, all\textit{ in situ}. The reduction of the Pb T$_{C}$ with increasing Cr density is well described by the Abrikosov-Gorkov theory. The application of perpendicular magnetic fields did not result in any suppression of the pair-breaking effect by the Cr impurities, i.e., field enhanced superconductivity, on several samples covering a wide range of Pb thicknesses and Cr densities. The pronounced reentrant behavior found in the magnetic field-tuned transitions in pure Pb films$^{1}$ was progressively suppressed by increasing Cr impurities. $^{1}$ J.S. Paker et al., Europhys. Lett. 75, 950 (2006). [Preview Abstract] |
Session P10: URu2Si2 and Related Compounds
Sponsoring Units: DCMPChair: Filip Ronning, Los Alamos National Laboratory
Room: Colorado Convention Center Korbel 1E
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P10.00001: Effects of Rhenium doping on the high magnetic field versus temperature phase diagram of URu2Si2 Sonia Francoual, Neil Harrison, Marcello Jaime, Scott Baily, Alex Lacerda, Nicholas Butch, Brian Maple Magnetoresistance and magnetization measurements carried out in URu$_{2-x}$Re$_{x}$Si$_{2}$ at low temperatures and high magnetic fields for x values of the rhenium doping between 0.01 and 0.10 enable us to investigate the robustness of the multiple ordered phases previously identified in URu$_{2}$Si$_{2}$ near the putative metamagnetic quantum critical point at fields around 37 $\pm $ 1 T. From the transport study, rhenium doping is shown to reduce considerably the elliptical region occupied by the hidden order phase in the (H, T) phase diagram and to shift to lower fields the broad magnetoresistivity maximum observed in the high temperature phase. In addition, the upper temperature limit at which the field-induced phase transitions are observed inside the metamagnetic crossover region in the magnetization curves decreases rapidly with increasing rhenium doping. All results tend to indicate that for x $<$ 0.10 the dilute substitution of Re in place of Ru in URu$_{2}$Si$_{2}$, unlike Rh substitution, weaken the ordering in the vicinity of the putative quantum critical point. [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P10.00002: Field-induced Fermi surface reconstruction and adiabatic continuity between antiferromagnetism and hidden-order state in URu$_2$Si$_2$ Y.J. Jo, L. Balicas, C. Capan, K. Behnia, P. Lejay, J. Flouquet, J.A. Mydosh, P. Schlottmann Shubnikov-de Haas oscillations at very low temperatures and high magnetic fields reveal an abrupt reconstruction of the Fermi surface within the hidden-order phase of URu$_{2}$Si$_{2}$. Taken together with reported Hall effect results, this implies an increase in the effective carrier density and suggests spectrum of itinerant quasiparticles. While hydrostatic pressure favors antiferromagnetism in detriment to the hidden-order state, we found that it has a modest effect on the complex $H-T$ phase diagram. This suggests adiabatic continuity between the hidden-order and antiferromagnetism. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P10.00003: Interplay between Fermi surface topology and ordering in URu$_{2}$Si$_{2}$ revealed through abrupt Hall coefficient changes Yoon Seok Oh, Kee Hoon Kim, P. A. Sharma, N. Harrison, H. Amitsuka, J. A. Mydosh The Hall effect is becoming recognized as a viable alternative for understanding Fermi surface(FS) changes in $f$-electron antiferromagnets and ferromagnets tuned close to quantum criticality. Any knowledge of the extent to which the $f$- electrons contribute to the FS topology is of crucial importance for understanding the nature of the ordering and the fate of the heavy quasiparticles. We show that Hall effect measurements extended to high magnetic fields uncover an intricate level of interplay between the FS topology and the stability of the various phases of pure and 4~\% Rh-doped URu$_2$Si$_2$. At low $H$ and $T$, the enhancement of the Hall coefficient and Hall angle shows that the otherwise large FS is reconstructed into small high mobility pockets below $T_{\rm o}$ in URu$_2$Si$_2$: a finding that is ubiquitous among imperfectly-nested itinerant forms of broken translational symmetry groundstates. This groundstate is then destabilized when a magnetic field causes two of the high mobility pockets to become spin polarized, ultimately leading to its destruction at $\approx$~35~T. Intermediate larger and strongly polarized FSs appear in phases II, III and V before a fully polarized unreconstructed FS is achieved beyond $\approx$~39~T with 1 hole/U and 1.5~$\mu_{\rm B}$/U. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P10.00004: NMR Study of Line Shape Effects Caused by the Hidden Order in a Random Powder of URu$_2$Si$_2$ S. Jung, O. O. Bernal, D. E. MacLaughlin, T. J. Gortenmulder, J. A. Mydosh We present NMR data for an epoxy-potted random-powder sample of URu$_2$Si$_2$. We have followed the line shape from 280 K down to about 4 K and observed its changing features as functions of temperature and two values of the applied magnetic field strength (1 and 2 T). We will describe the effects of the hidden order (transition temperature $T_0 \sim 17.5$~K) on the line shape and compare them with previous results in c-axis oriented samples and single crystals. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P10.00005: Inelastic neutron scattering studies of itinerant spin excitations in URu2Si2 near the hidden order transition John Janik, G. MacDougall, G. Luke, Y.-J. Jo, L. Balicas, Y. Qiu, J. Copley, Z. Yamani, B. Buyers, C. Wiebe We performed extensive neutron scattering studies on the heavy fermion superconductor URu$_{2}$Si$_{2}$. Using the C5 triple axis spectrometer at Chalk River, we studied the spin excitations recently reported [C. R. Wiebe, J. A. Janik et al, Nature Physics] above and below the T$_{0}$ transition. It has become clear from our previous work that these incommensurate itinerant spin excitations account for the entropy change into the hidden order (HO) state. This severely limits the possible theoretical scenarios on the ground state of the HO phase. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P10.00006: The Evolution of the Hidden Order Phase in URu$_{2-x}$Re$_x$Si$_2$ under Pressure J.R. Jeffries, N.P. Butch, B.T. Yukich, M.B. Maple The heavy fermion compound URu$_2$Si$_2$ exhibits three distinct ordered states as a function of temperature and pressure: ``hidden order'' (HO), in which the order parameter has yet to be identified; antiferromagnetism (AFM), which seemingly develops out of the HO state at $P \leq 15$ kbar; and superconductivity (SC), which exists at ambient pressure. While URu$_2$Si$_2$,the parent compound of the URu$_{2-x}$Re$_x$Si$_2$ system, has been the subject of much scrutiny in the past several years, the nature of the HO phase is still uncertain. The evolution of the ordered phases as a function of pressure in the URu$_{2-x}$Re$_x$Si$_2$ system could provide clues to or constraints on the elusive order parameter of this HO phase. To this end, oriented single crystal samples of URu$_{2-x}$Re$_x$Si$_2$ with small values of $x$ have been synthesized and investigated under nearly hydrostatic pressure via electrical resistivity measurements. While the SC in URu$_{2-x}$Re$_x$Si$_2$ is rapidly suppressed with Re concentration, the HO phase persists up to $x~\approx~0.1$. The pressure dependence of the HO phase in this concentration range will be discussed along with possible consequences to the ordered states. This research was sponsored by the U.S. DOE under Research Grant No. DE-FG02-04ER46105 and by the U.S. NNSA under the Stewardship Science Academic Alliances program through DOE Research Grant No. DE-FG52-03NA00068. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P10.00007: The Search for Quantum Criticality in the URu$_{2-x}$Re$_{x}$Si$_{2}$ Phase Diagram N.P. Butch, J.R. Jeffries, B.T. Yukich, T.A. Sayles, J. Paglione, P.-C. Ho, M.B. Maple It has been established that as Re is doped into polycrystalline URu$_{2}$Si$_{2}$, the hidden order/antiferromagnetic and superconducting phases are suppressed, while at intermediate doping, long range ferromagnetism emerges. To further investigate observations of the persistence of non-Fermi liquid behavior well into the ferromagnetic phase, we have prepared single crystals of URu$_{2-x}$Re$_{x}$Si$_{2}$ in the Re concentration range of $0 \leq x \leq 0.6$ and performed magnetization, electrical and thermal transport, and calorimetry measurements at low temperatures down to $0.1$~K. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P10.00008: Crystal fields, magnetoresistance, and superconductivity of Pr$_{1-x}$La$_{x}$Os$_{4}$Sb$_{12}$ Bohdan Andraka, Costel R. Rotundu, Martin E. McBriarty Investigation of Pr$_{1-x}$La$_{x}$Os$_{4}$Sb$_{12}$ in strong magnetic fields implies that crystalline electric field (CEF) energies of Pr are unchanged to at least x=0.2. CEF energies for x=0.4 are approximately 20 {\%} larger than for x=0.2 and increase further for x=0.67. Specific heat discontinuity at T$_{c}$ and the upeer critical field slope at T$_{c}$ indicate that the strongest suppression of m* takes place between x=0 and x$_{cr}\sim $0.3. High accuracy specific heat data obtained on a large crystal of x=0.67 exhibit significant deviations with respect to the Schottky specific heat corresponding to singlet-triplet excitations. Similar deviations are seen for other crystals with x$>$x$_{cr}$. On the other hand, magnetoresistance of moderately and strongly dilute alloys is consistent with predictions for a singlet-triplet CEF model. Correlations between these measurements of CEF's and superconductivity of Pr$_{1-x}$La$_{x}$Os$_{4}$Sb$_{12}$ will be discussed. Also, evidences against and for homogeneous coexistence of two superconducting transitions in the specific heat of pure PrOs$_{4}$Sb$_{12}$ will be presented and discussed. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P10.00009: A comparison of the normal and superconducting state properties of Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ and Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ P.-C. Ho, T. Yanagisawa, N.P. Butch, W.M. Yuhasz, N.A. Frederick, M.B. Maple The evolution of unconventional superconductivity and the high field ordered phase (HFOP), the latter of which has been identified with antiferroquadrupolar order, in PrOs$_4$Sb$_{12}$ has been investigated in two pseudoternary systems, Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ and Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$. In the Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ system, the superconducting transition temperature $T_{c}$ is suppressed nearly linearly with $x$ to a minimum at $x = 0.6$ from both end member compounds, the upper critical field $H_{c2}$ has an approximately linear dependence on $T$ for $x > 0.4$, and the features related to the HFOP in the electrical resistivity disappear for $x \ge 0.1 $. On the other hand, in the Pr$_{1-x} $Nd$_x$Os$_4$Sb$_{12}$ system, $T_{c}$ and the Curie temperature $\Theta_{C}$ are suppressed monotonically toward $x = 0.55$ from $x=0$ and $x=1$, respectively, no linear $T$ dependence of $H_{c2}$ is observed, the HFOP persists up to at least $x \sim 0.45$, and there is an indication of the coexistence of superconductivity and ferromagnetism for $x \sim 0.45$ according to specific heat measurements. In both systems, $H_{c2}(T)$ is limited by the orbital motion of the electrons and the decrease of $T_{c}$ with $x$ from $x=0$ is nearly the same. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P10.00010: The disordered ground state of quantum critical Ce(Ru$_{1-x}$Fe$_x$)$_2$Ge$_2$ Wouter Montfrooij, Jagat Lamsal, Meigan Aronson, Marcus Bennett, Anne de Visser, Huang Ying Kai, Nguyen Thanh Huy, Mark Lumsden, Mohana Yethiraj, Yiming Qiu We present neutron scattering data that show that magnetic ordering in the vicinity of a quantum critical point is restricted to short length scales. Remarkably, the spatial extent of the magnetic correlations is independent of the inter- moment distances. We argue that our data on Ce(Ru$_{1-x}$Fe$_x$)$_2$Ge$_2$ demonstrate that quantum fluctuations disorder the system and dilute the magnetic moments to such an extent that the response of the system is disorder dominated. Our observations naturally explain how E/T- scaling is possible in systems whose apparent dimensionality is above the upper critical dimension. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P10.00011: Magnetic field tuning of the low temperature state in YbNiSi$_3$: magnetic field induced quantum criticality Marcos A. Avila, Toshiro Takabatake, Sergey L. Bud'ko, Paul C. Canfield We present detailed, low temperature, magnetoresistance and specific heat data from measurements on YbNiSi$_3$ in magnetic field applied along the easy magnetic axis, $H \| b$. Initially the antiferromagnetic ground state changes into a field-induced metamagnetic phase at $\sim 16$ kOe ($T \to 0$). On further increase of magnetic field, magnetic order is suppressed to below 0.4 K at $\sim 85$ kOe. The functional behavior of the resistivity and specific heat is discussed in comparison with that of the few other stoichiometric, heavy fermion compounds with established field-induced quantum criticality. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P10.00012: Thermal Transport in ZrZn$_2$ - Probing the Marginal Fermi Liquid State Michael Sutherland, R. Smith, K. Noriaki, G.G. Lonzarich, S. Takashima, M. Nohara, H. Takagi The electronic properties of metals on the border of magnetism are often found to exhibit unusual temperature dependencies, not easily understood within a conventional Fermi liquid picture. The weak itinerant ferromagnet ZrZn$_2$ is a notable example, where resistivity evolves at low temperatures as T$^{5/3}$ as a result of spin fluctuation scattering. Here we investigate the effects of these fluctuations on heat transport, by measuring thermal conductivity to low temperatures in high quality samples. We compare these results to expectations from spin fluctuation theory, and comment on the relative effectiveness of spin fluctuations at degrading heat and charge currents. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P10.00013: ABSTRACT HAS BEEN MOVED TO R1 |
Session P11: Correlated Electrons: DMFT
Sponsoring Units: DCMPChair: Randy Fishman, Oak Ridge National Laboratory
Room: Colorado Convention Center Korbel 1F
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P11.00001: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P11.00002: An Extended Dynamical Cluster Approximation for Local-Moment Systems Karlis Mikelsons, Thomas Maier, Randy Fishman, Mark Jarrell We introduce an Extended Dynamical Cluster Approximation for studying local moment systems, such as Heisenberg and Ising models. Below $T_c$, self-consistency is imposed both for the order parameter and the correlation function. Exact enumeration and Monte-Carlo methods are used to solve the local moment problem on a cluster with coarse-grained effective interactions. Even if the original exchange interactions are short-ranged, the effective interactions are long-ranged. For a single site cluster this method is equivalent to the EDMFT. As the size of the cluster is increased, the fluctuations are systematically included into this approximation. We apply this method to the one-, two- and three-dimensional Ising models. We develop scaling analysis by increasing the cluster size to find the $T_c$. Unlike other expansions about the mean-field theory, our technique converges quite rapidly to the exact solutions, which are known analytically in one and two dimensions and to great accuracy from numerical work in three dimensions. Long- ranged interactions can easily be included in this method, i. e., for studying glassy systems. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P11.00003: Performance analysis of continuous-time solvers for quantum impurity models Emanuel Gull, Philipp Werner, Andrew Millis, Matthias Troyer Impurity solvers play an essential role in the numerical investigation of strongly correlated electrons systems within the ``dynamical mean field" approximation. Recently, a new class of continuous-time solvers has been developed, based on a diagrammatic expansion of the partition function in either the interactions or the impurity-bath hybridization. We investigate the performance of these two complimentary approaches and compare them to the well-established Hirsch-Fye method. The results show that the continuous-time methods, and in particular the version which expands in the hybridization, provide substantial gains in computational efficiency. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P11.00004: Efficient DMFT simulation of the Holstein-Hubbard model Philipp Werner, Andrew J. Millis We show that the hybridization expansion algorithm for quantum impurity models [PRL 97, 076405 (2006)] can easily handle a Holstein coupling to phonons. Our approach, which is based on the Lang-Firsov transformation, treats the phonons without approximations and does not affect the overall scaling of the algorithm. We apply the method to the Holstein-Hubbard model in the single site dynamical mean field approximation. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P11.00005: Dynamical vertex approximation --- a step beyond dynamical mean field theory Alessandro Toschi, Andrey Katanin, Karsten Held We have developed a new diagrammatic approach[1], coined ``Dynamical Vertex Approximation'' (D$\Gamma$A), with the aim of going beyond dynamical mean field theory for strongly correlated systems, by including the effects of long-range spatial correlations. Without resorting to any finite-size cluster scheme, D$\Gamma$A allows us to compute momentum dependent self-energies (and spectra), whose expressions are diagrammatically constructed starting from the two-particle irreducible local vertex. Therefore, D$\Gamma$A naturally applies for studying effects of magnetic fluctuations with large correlation length in strongly correlated systems, such as the Hubbard model. Specifically, we analyze the interplay between antiferromagnetic fluctuations and the Mott metal-insulator transition in three dimensions and the formation of a pseudogap in two dimensions. The diagrammatic nature of D$\Gamma$A, moreover, should allow for a generalization to the more realistic case of multi-band Hamiltonians. \newline [1] A. Toschi, A. Katanin, K. Held, cond-mat/0603100. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P11.00006: Optical Spectral Weight of the Hubbard Model -- Single-site DMFT Calculation and Comparison to Experimental Data Armin Comanac, Luca de' Medici, Massimo Capone, Andrew J. Millis The single-site dynamical mean field method is used to calculate the variation of optical spectral weight with doping, interaction strength and frequency for the one band Hubbard model. Upper Hubbard band, mid-infrared and coherent quasiparticle contributions are distinguished. It is argued that mid-infrared and coherent contributions can meaningfully be compared to experimental data on transition metal oxide materials such as high-temperature supercondutors. The comparison is used to estimate the strength of correlation effects in electron- and hole-doped superconductors. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P11.00007: Strong correlations on a triangular lattice: spectral weight suppression on the cobaltates. Dimitrios Galanakis, Tudor Stanescu, Philip Phillips Two experimental puzzles of strong correlations of the cobaltates (Na$_x$ CoO$_2$) are investigated. First the experimental claims that the optical conductivity displays a pseudogap feature. Second the suppression of spectral weight near the chemical potential in the removal spectrum of the x=0.3 material. We address this questions using the 2D Hubbard model in the triangular lattice in the framework of Cluster Dynamical Mean field theory (CDMFT). For x=0.3 we find a suppression of the spectral weight slightly above the chemical potential. The integrated optical conductivity displays no significant transfer of spectral weight from low to high energy as it would be indicative of a pseudogap. Comparison is made with single site, three site and four site clusters. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P11.00008: Analysis of the Dynamical Cluster Approximation for the Triangular Lattice Hubbard Model Christopher Varney, Richard Scalettar, Mark Jarrell, Alexandru Macridin, Brian Moritz The behavior of correlated electrons on triangular lattices is attracting increasing interest driven by experimental systems such as the cobaltates. To facilitate understanding of these correlations, we study the Hubbard model using Determinant Quantum Monte Carlo and the Dynamical Cluster Approximation. The spin, charge and pairing response functions obtained with the two methods are compared as a function of spatial lattice and cluster size, respectively, and the one particle spectrum is calculated. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P11.00009: NiO - Dynamical Mean Field Study of Charge-Transfer Insulator J. Kunes, V.I. Anisimov, A.V. Lukoyanov, D. Vollhardt Charge-transfer (CT) Mott insulators present an important group of transition metal compounds which exhibit phenomena such as metal-insulator transitions or high temperature superconductivity. The location of ligand states between the interaction-split $d$ bands leads to additional complexity, which requires a description beyond a simple Hubbard model. Using a combination of {\it ab initio} bandstructure and dynamical mean field theory we study the single particle spectrum of the prototypical CT insulator NiO. Including the O-$p$ orbitals to the Hamiltonian we obtain good agreement with PES and BIS experiments. Notably we find $d$-peak at the top of the valence band, which cannot be described in static theories, but which is seen in experiment and was reproduced in many-body calculations on small clusters. Studying the effect of doping we find the added holes to occupy the ligand $p$ orbitals despite large Ni-$d$ spectral weight at the top of the valence band. Heavy hole doping leads to a significant reconstruction of the single-particle spectrum and filling of the CT gap. This is the first LDA+DMFT study of charge transfer systems, which includes the $p-d$ hybridization explicitly and is thus able to provide a full description of valence and conduction band spectra. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P11.00010: Calculation of Magnetic Exchange Interactions in Mott-Hubbard Systems Quan Yin, Xiangang Yin, Sergey Savrasov An efficient method to magnetic exchange interactions in systems with strong electronic correlations is introduced. It is based on a magnetic force theorem which evaluates linear response due to rotations of magnetic moments and uses a novel spectral density functional framework combining our exact diagonalization based LDA+DMFT method. Applications on spin waves and magnetic transition temperatures of 3d transition metal oxides and 5f actinide oxides are in good agreement with experiments. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P11.00011: Combined LDA+Exact Diagonalization Study for Actinide Compounds Alexey Gordienko, Sergey Savrasov Exact diagonalization (ED) is a most straightforward and powerful way to study problems related to strong electron correlations, but very computationally demanding for $f$-electron systems. Computational efficiency of the ED approach can be greatly increased with help of iterative methods and we shall present our recent ED implementation which makes use of Kernel Polynomial Method (KPM) to calculate temperature Green's Function and self-energy. This allows us to deal relatively easy with problems whose size is 5$\cdot$10$^5$ states that is a characteristic for impurity problems with $f$-electrons. As an application, actinide compounds PuO$_2$ and UO$_2$, will be studied self-consistently using self- energies extracted from cluster ED and combined with electronic structure LDA calculation. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P11.00012: Non-collinear magnetism of GdB$_{4}$: A DFT+$U$ study M. N. Huda, Leonard Kleinman Lanthanide-borides show antiferromagnetic behavior where the magnetocrystalline anisotropy plays a major role in their magnetic structures. A recent neutron scattering experiment showed a particular noncollinear behavior of GdB$_{4}$ at room temperature. We will present our study on the non-collinear magnetism of GdB$_{4}$ with the GGA + $U$ method with spin orbit coupling. We have found that with or without spin-orbit coupling and with $U $collinear magnetism is favorable by few meV than the experimentally found noncollinear magnetic configuration. Among the noncollinear magnetism configurations that we have studied, when a $U $parameter and spin-orbit coupling are considered, the experimentally found noncollinear configuration was found to be favorable. However, the value of $U$ parameter is not unique; a range of values were able to get this magnetic order. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P11.00013: GW+exact diagonalization approach for electronic structure calculations in Mott insulators Nikolay Zein, Serguie Savrasov, Gabriel Kotliar We combine GW and exact diagonalization approaches to calculate electronic structure both in antiferromagnetic and paramagnetic states and find parameters of the corresponding Hubbard model in several transition metal oxides. We discuss extraction of double counting terms, renormalization of one-particle spectrum and interaction, mutual influence of Hubbard-like and GW contributions. Results are compared with LDA+DMFT calculations and the importance of self-consistent approach is stressed [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P11.00014: Novel Phase Between Band and Mott Insulators in Two Dimensions Srivenkateswara Sarma Kancharla, Elbio Dagotto We investigate the ground state phase diagram of the half-filled repulsive Hubbard model in two dimensions in the presence of a staggered potential $\Delta$, the so-called ionic Hubbard model, using cluster dynamical mean field theory. We find that for large Coulomb repulsion, $U >>\Delta$, the system is a Mott insulator (MI). For weak to intermediate values of $\Delta$, on decreasing U, the Mott gap closes at a critical value $U_{c1}(\Delta)$ beyond which a correlated insulating phase suggesting bond order (BO) is found. Further, this phase undergoes a first-order transition to a band insulator (BI) at $U_{c2}(\Delta)$ with a finite charge gap at the transition. For large $\Delta$, there is a direct first-order transition from a MI to a BI with a single metallic point at the phase boundary. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P11.00015: Kernel sweeping method for exact diagonalization of spin models - numerical computation of a CSL Hamiltonian Darrell Schroeter, Eliot Kapit, Ronny Thomale, Martin Greiter We have recently constructed a Hamiltonian that singles out the chiral spin liquid on a square lattice with periodic boundary conditions as the exact and, apart from the two-fold topological degeneracy, unique ground state [1]. The talk will present a kernel-sweeping method that greatly reduces the numerical effort required to perform the exact diagonalization of the Hamiltonian. Results from the calculation of the model on a $4\times4$ lattice, including the spectrum of the model, will be presented. [1] D. F. Schroeter, E. Kapit, R. Thomale, and M. Greiter, \textit{Phys. Rev. Lett.} in review. [Preview Abstract] |
Session P12: Focus Session: Spin-Orbit Coupling
Sponsoring Units: GMAG DMP FIAPChair: Roland Winkler, Northern Illinois University
Room: Colorado Convention Center Korbel 3C
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P12.00001: Electrical manipulation of spin-orbit coupling in semiconductor heterostructures Invited Speaker: Spin-orbit coupling provides a pathway for electrically initializing and manipulating electron spins. This coupling creates momentum-dependent spin-splittings related to the inversion asymmetries of the semiconductor heterostructure. We demonstrate that we can regulate these spin-splittings in semiconductor epilayers with strain\footnote{V. Sih, H. Knotz, J. Stephens, V. R. Horowitz, A. C. Gossard and D. D. Awschalom, \textit{Phys. Rev. B} \textbf{73}, 241316(R) (2006).} and in heterostructures using quantum confinement and orbital quantization\footnote{V. Sih, W. H. Lau, R. C. Myers, A. C. Gossard, M. E. Flatt\'{e} and D. D. Awschalom, \textit{Phys. Rev. B} \textbf{70}, 161313(R) (2004).}. These spin-splittings can provide a mechanism for electrically generating spin polarization without magnetic materials or magnetic fields. Using Kerr rotation microscopy, current-induced spin polarization and the spin Hall effect have been observed in bulk semiconductors and in a two-dimensional electron gas confined in (110) AlGaAs quantum wells\footnote{V. Sih, R. C. Myers, Y. K. Kato, W. H. Lau, A. C. Gossard and D. D. Awschalom, \textit{Nature Physics} \textbf{1}, 31 (2005).}. In contrast to measurements on bulk systems, the data for the quantum wells reveal that the spin Hall profile exhibits a complex structure and that the current-induced spin polarization is out-of-plane. The current-induced spin polarization is dependent on the direction along which the electric field is applied, reflecting the anisotropy of the spin-orbit interaction. More recently, we demonstrate that the observed spin accumulation due to the spin Hall effect is due to a bulk electron spin current\footnote{V. Sih, W. H. Lau, R. C. Myers, V. R. Horowitz, A. C. Gossard and D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf{97}, 096605 (2006).}. Channels with transverse arms allow us to observe that this spin current can drive spin transport over macroscopic distances in bulk GaAs. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P12.00002: Spin generation by strong inhomogeneous electric fields Ilya Finkler, Hans-Andreas Engel, Emmanuel Rashba, Bertrand Halperin Motivated by recent experiments [1], we propose a model with extrinsic spin-orbit interaction, where an inhomogeneous electric field ${\bf E}$ in the x-y plane can give rise, through nonlinear effects, to a spin polarization with non-zero $s_z$, away from the sample boundaries. The field ${\bf E}$ induces a spin current ${\bf j}_s^z= \hat{z} \times(\alpha{\bf j}_c+\beta{\bf E})$, where ${\bf j}_c=\sigma {\bf E}$ is the charge current, and the two terms represent,respectively, the skew scattering and side-jump contributions. [2]. The coefficients $\alpha$ and $\beta$ are assumed to be $E$- independent, but conductivity $\sigma$ is field dependent. We find the spin density $s_z$ by solving the equation for spin diffusion and relaxation with a source term $\nabla \cdot {\bf j}_s^z$. For sufficiently low fields, $j_c$ is linear in $E$, and the source term vanishes, implying that $s_z=0$ away from the edges. However, for large fields, $\sigma$ varies with $E$. Solving the diffusion equation in a T-shaped geometry, where the electric current propagates along the main channel, we find spin accumulation near the entrance of the side channel, similar to experimental findings [1]. Also, we present a toy model where spin accumulation away from the boundary results from a nonlinear and anisotropic conductivity.\\{ }[1] V. Sih, et al, Phys.\ Rev.\ Lett. {\bf 97}, 096605 (2006).\\{ } [2] H.-A. Engel, B.I. Halperin, E.I.Rashba, Phys.\ Rev.\ Lett. {\bf 95}, 166605 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P12.00003: Tuning of the spin-orbit interaction and resistance in two-dimensional GaAs holes via strain Babur Habib, Javad Shabani, Etienne P. De Poortere, Mansour Shayegan, Roland Winkler We report direct measurements, via the Fourier analysis of the Shubnikov-de Hass oscillations, of the spin-orbit interaction induced spin-splitting in modulation-doped GaAs two-dimensional hole systems as a function of strain applied in the sample plane. The data reveal a remarkably strong dependence of the spin-splitting on strain, with up to about 20{\%} enhancement of the splitting upon the application of only about 2x10$^{-4}$ strain. The results are in very good agreement with our numerical calculations of the strain-induced spin-splitting. We also show a remarkable dependence of the anisotropy of the heavy hole band on strain. Its manifestation as a change of resistance with strain implies the use of GaAs 2D holes as a sensitive piezo-resistance sensor at low temperatures. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P12.00004: Electron transport in semiconductor heterostructures with strong spin orbit coupling Andrei Garcia, Dennis Lo, David Goldhaber-Gordon, Jason Stephens, Shawn Mack, David Awschalom GaAs/AlGaAs two dimensional electron gases (2DEGs) have been studied extensively in the context of mesoscopic transport through devices such as quantum point contacts and quantum dots. 2DEGs in heterostructures based on InGaAs or InAs instead of GaAs provide testbeds to study similar phenomena in systems with much larger intrinsic spin-orbit coupling. Stronger spin orbit coupling provides greater ease of control of the electron spin degree of freedom, leading to applications in spintronics as well as the possibility of observing novel quantum Hall states. We present some preliminary electronic transport data on gated InGaAs 2DEGs and discuss directions for possible further experiments on nanostructures in this material. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P12.00005: Physical factors affecting Rashba Spin-orbit coupling Chih-Piao Chuu, Ming-Che Chang, Qian Niu The Rashba Spin-orbital coupling plays a crucial role in charge and spin transport in semiconductor heterostructure. We study several physical parameters which may contribute to Rashba Spin-orbital coupling, including asymmetric potential barriers, quantum well inclination, effective mass, and band mixing. This may provide some insights in designing spintronic microdevices . [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P12.00006: Spin transport and the giant Zeeman effect in systems with spin-orbit interaction Anh Ngo, Sergio Ulloa Spin-orbit coupling in semiconductors provides a pathway for electrically initializing and manipulating electron spins for applications in spintronics and spin-based quantum information processing. This coupling can be regulated with quantum confinement, band structure engineering and applied fields. Here we investigate the spin-dependent transport properties of electrons in diluted magnetic two dimensional electron gas (2DEG) systems using a scattering matrix approach. We include the Rashba spin-orbit interaction and the role of realistic magnetic barriers produced by the deposition of ferromagnetic stripes on heterostructures [1]. We show that the quantum conductance in these systems depends on spin orientation of the incident carriers, the magnitude of spin-orbit coupling, and the giant Zeeman effect present in diluted magnetic semiconductors. We will describe how all effects can be employed in the efficient control of spin polarization via the application of moderate fields. \\ 1. A. Matulis, F. M. Peeters, P. Vasilopoulos, Phy. Rev. Lett. {\bf 72}, 1518 (1994). [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P12.00007: Spin-Orbit Coupling in AlGaN/AlN/GaN Heterostructures with a Polarization Induced Two-Dimensional Electron Gas H. Cheng, C. Kurdak, N. Biyikli, U. Ozgur, H. Morkoc, V.I. Litvinov Spin-orbit coupling is investigated by weak antilocalization and Shubnikov-de Haas measurements in wurtzite Al$_{x}$Ga$_{1-x}$N/AlN/GaN heterostructures with a polarization induced two dimensional electron gas. By employing the persistent photoconductivity effect and by using five different heterostructures with different Al compositions, we cover a carrier density range extending from $0.8\times 10^{12}\mbox{ cm}^{-2}$ to $10.6\times 10^{12}\mbox{ cm}^{-2}$. We determine electron splitting energies for different carrier densities by analyzing the weak antilocalization measurements using the Iordanskii, Lyanda-Geller, and Pikus theory. We find the spin splitting energies do not scale linearly with the Fermi wavevector $k_{F}$ at high carrier densities. By fitting the spin splitting energies to a form $E_{SS}$=2($\alpha k_{F}+\gamma k_{F}^{3})$ we extract linear and cubic spin-orbit coupling parameters $\alpha $=5.13$\times $10$^{-13}$ eV m and $\gamma $=1.2$\times $10$^{-31}$eV m$^{3}$, respectively. The cubic spin-orbit coupling parameter is purely due to the bulk inversion asymmetry of the wurtzite crystal and has not been previously measured for the GaN system. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P12.00008: A Study of Dresselhaus and Rashba Effects in InSb/InAlSb Heterostructures via Anti-Weak Localization Measurements Aruna Dedigama, Dilhani Jayathilaka, Sheena Murphy, Madhavie Edirisooriya, Niti Goel, Tetsuya Mishima, Michael Santos The InSb/InAlSb system has both the largest Dresselhaus effect (due to bulk inversion asymmetry) and Rashba effect (due to structural inversion asymmetry) of the III-V semiconductor family. Both mechanisms contribute to electronic spin splitting, even in zero applied field. While the Dresselhaus effect is purely materials specific, the Rashba interaction is less well understood with both the electric field at the interface and the discontinuity due to the barrier predicted to play significant roles. Standard measurements of the zero field spin splitting however, are usually performed at high field where Zeeman effects and higher subband occupancy become problematic. In this talk we will present our results in extremely low fields using anti-weak localization (AWL) measurements where these complications are absent. We report on systematic measurements of the Dresselhaus and Rashba interactions on a series of InSb/InAlSb heterostructures, where carrier density, dopant density and the Al concentration in the barrier have all been varied to extract the role of each in the strength of the spin-orbit coupling. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P12.00009: Spin interference effect in a triangular loop array fabricated in (001)In$_{0.53}$Ga$_{0.47}$As/In$_{0.52}$Al$_{0.48}$As quantum wells Hiroshi Okutani, Takaaki Koga, Yoshiaki Sekine We report, for the first time, the spin interference (SI) effect in a triangular loop array fabricated in (001) In$_{0.53}$Ga$_{0.47}$As/In$_{0.52}$Al$_{0.48}$As quantum wells (QW). Previously [1], we studied the SI effect in a square loop array, where the sides of the squares are either parallel or perpendicular to the $<$110$>$ crystallographic axis. For an electron with a wave vector\textbf{ k} that is parallel or perpendicular to the $<$110$>$ direction, the magnitude of the effective magnetic field due to the all spin-orbit effects,$ B_{TOT}$, is given either by the sum ($B_{R}+B_{D})$ or by the difference ($B_{R}-B_{D})$ between the Rashba and Dresselhaus fields, which makes the analysis relatively simple. Though the situation is more complicated in a triangular loop array, theory including both the Rashba and Dresselhaus terms predicts clear difference in the SI patterns between the following two situations: the bases of the triangles in the array are placed parallel/perpendicular to the $<$110$>$ crystallographic axis. This finding is being confirmed experimentally. [1] T. Koga\textit{ et al.}, Phys. Rev. B \textbf{70}, 161302(R) (2004); \textit{ibid}. \textbf{74}, 041302(R) (2006). [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P12.00010: Spin and charge optical conductivities in spin-orbit coupled systems Jesus A. Maytorena, Catalina Lopez-Bastidas, Francisco Mireles Spin-orbit interaction (SOI) in systems lacking inversion symmetry is a phenomenon with great potential in the development of spintronic-based devices. Since the celebrated proposal by Datta and Das, of a spin-FET relying on the tunability of the Rashba SOI strength through electrical gating, there has been a remarkable attention in the search for new ways of manipulating electron spins without employing ferromagnetic materials and/or external magnetic fields. In this work we study the frequency dependent spin- and charge- conductivity tensors of a two-dimensional electron gas (2DEG) with both Rashba and Dresselhaus spin-orbit interaction. We show that the spectral behavior of the spin and charge response due to the angular anisotropy of the spin-splitting energy induced by the interplay between the Rashba and Dresselhaus couplings is much richer. The new spectral structures open the possibility for control of the optical response by applying an external bias and/or by adjusting the light frequency. In addition, we show that the relative strength of the spin-orbit coupling parameters can be obtained through optical probing. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P12.00011: Spin interference effects in a 2D-hole ring with spin-orbit interaction. Alexey Kovalev, Mario Borunda, Jairo Sinova We study the quantum interference effects in one-dimensional heavy hole (HH) rings with spin-orbit interaction realizable in HgTe quantum wells. The influence of the spin-orbit interaction strength on the transport is investigated analytically and numerically. The analytical results allow us to explain the interference effects as a signature of Berry phases. We compare our results with the previous studies on the electron Rashba systems and find more rapid oscillations as a function of the spin-orbit strength. The structures with stronger signature of the spin-orbit strength can lead to more sensitive spintronic devices that enable observation of quantum interference effects and control of spin at mesoscopic scales. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P12.00012: Exchange energy and generalized polarization in the presence of spin-orbit coupling in two dimensions Stefano Chesi, Gabriele F. Giuliani We discuss the concomitant effects of the exchange energy and the spin-orbit interaction in a homogeneous system of interacting electrons in two spatial dimensions. This work extends the mean-field method originally developed in the case of Rashba spin-orbit to a more general form of spin-orbit interaction. The mean-field phase diagram and spin response for a number of representative cases are discussed. Our theory is rigorous in the high-density limit of the paramagnetic phase, where it can be expressed in terms of a generalized fractional electronic polarization. We show that in many cases, the effect of the exchange is to quench, rather than enhance, the generalized polarization induced by the spin-orbit coupling. Our results account qualitatively for the findings of recent experimental investigations. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P12.00013: High density limit of the correlation energy of a two dimensional electron liquid in the presence of Rashba spin-orbit Gabriele F. Giuliani, Stefano Chesi We obtain analytic expressions for the high density limit of the correlation energy of a two dimensional electron liquid in the presence of Rashba spin-orbit. As a byproduct we have derived an analytic expression for the dependence of the ring diagrams contribution to this quantity on the fractional spin polarization of the system in the absence of spin-orbit. We will show that the latter is not properly represented by current standard interpolation formulas obtained from Monte-Carlo calculations. [Preview Abstract] |
Session P13: Focus Session: High-Tc Cuprates and Nickelates
Sponsoring Units: DMP GMAGChair: Michelle Johannes, Naval Research Laboratory
Room: Colorado Convention Center Korbel 4C
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P13.00001: Enhanced polaron formation, suppression of superconducting T$_c$, and the isotope effect in the Hubbard model with phonons Invited Speaker: Using a dynamical cluster quantum Monte Carlo approximation we investigate the effect of dynamical Holstein, buckling and breathing phonons on the physics of the 2D Hubbard model at small doping. For all three phonon modes the interplay of electronic correlations and the electron-phonon interaction produces two competing effects, an enhancement of the effective $d$-wave pairing interaction and a strong suppression of the single-particle quasiparticle weight. Due to the later effect we find that Holstein, buckling and breathing phonons suppress superconductivity in the region of parameter space relevant for cuprate superconductors. The renormalization of the single- particle propagator, associated with polaron formation, is significantly enhanced by the presence of antiferromagnetic correlations. Moreover, as a complementray effect, the electron-phonon scattering strongly enhances the spin correlations at finite doping, showing a synergistic interplay between the electron-phonon coupling and antiferromagnetic correlations. The suppression of superconductivity due to polaron formation can explain the isotope effect observed in cuprates. We find a positive and large isotope exponent in the underdoped region where the antiferromagnetic correlations are strong and a small positive isotope exponent in the optimally doped region, in agreement with experiment. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P13.00002: The effect of strong impurity scattering on superconductivity in the 2D Hubbard model Alexander Kemper, Thomas Maier, Mark Jarrell, Cheng Hai-Ping We study the effect of strong impurity scattering in the two-dimensional Hubbard model to model the effect of Zn substitution in the cuprates, using the dynamical cluster quantum Monte Carlo framework. The superconducting Tc is strongly suppressed by impurity doping, while the spin susceptibility indicates moment formation. We will discuss the dependence of Tc on the strength of the impurity scattering potential, and by investigating the properties of sites neighboring the impurity, the relevance to the experimental STM image of Zn impurity in cuprates. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P13.00003: Exact thermodynamics of pairing and charge-spin separation in Hubbard nanoclusters Tun Wang, Armen Kocharian, Gayanath Fernando, Kalum Palandage, Jim Davenport An exact thermal studies of charge-spin separation, pairing fluctuations and pseudogaps are carried out by exact diagonalization of 4-site, frustrated (three dimensional) tetrahedral Hubbard and planar (2x4) clusters. Our exact results for 4-site cluster strongly suggest the existence of a quantum critical points in small Hubbard clusters for particle-particle/hole pair binding, antiferromagnetism, unsaturated and saturated ferromagnetism. Exact studies of larger planar and three dimensional Hubbard clusters yield more intriguing insight supporting the analytical results obtained for the 4-site clusters. Our microscopic theory reproduces electron pairing correlations, phase separation and magnetism in clusters, small nanoparticles, and, surprisingly, in transition metal oxides and high T$_c$ doped cuprates. Theory describes also the effect of pressure on the superconducting transition temperature, the presence of a dormant magnetic state in a narrow region of doping and variation of spin pseudogap with doping level, etc. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:51PM |
P13.00004: Field Induced Suppression of the Resonance Mode in N-type High-T$_{c}$ Cuprate Pr$_{.88}$LaCe$_{.12}$CuO$_{4-\delta }$ (T$_{c}$=24K) Invited Speaker: We discuss the results of our recent inelastic neutron scattering experiments probing the magnetic field dependence of the resonance mode in an electron-doped high-T$_{c}$ cuprate. The resonance mode in the high-T$_{c}$ superconductors is a magnetic excitation widely believed to be fundamentally connected to the superconducting mechanism. The mode itself appears only below T$_{c}$ in optimally-doped cuprates, and its characteristic energy follows the universal relation E$_{Resonance}$=5.8k$_{B}$T$_{c}$ in all classes of cuprate systems. Using a c-axis aligned magnetic field, superconductivity in the electron-doped cuprate, Pr$_{.88}$LaCe$_{.12}$CuO$_{4-\delta }$ (PLCCO), can be completely suppressed with an experimentally realizable field of $\sim $9 T at 2 K. This fact combined with the recent discovery of the resonance mode in this PLCCO system, allows, for the first time, an experimental observation of the evolution of the resonance mode as a cuprate system is driven into its field-suppressed ground state. We will present such a study in a nearly optimally-doped sample of PLCCO (T$_{c}$=24K). The simultaneous emergence under field of static antiferromagnetic (AF) order at the commensurate AF ordering wavevector will also be discussed along with the influence of a c-axis field on low energy excitations in this system. Changes in magnetism coupled to the suppression of the superconducting phase in this PLCCO system will be given particular focus. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P13.00005: Local Electronic Structure of Bi2Sr2CaCu2O8 near Oxygen Dopants: A Window on the High-Tc Pairing Mechanism Yao He, Peter Hirschfeld, Hai-Ping Cheng The cuprate material Bi2Sr2CaCu2O8(BSCCO-2212) is believed to be doped by a combination of cation switching and excess oxygen. The interstitial oxygen dopants are of particular interest because scanning tunneling microscopy (STM) experiments have shown that they are positively correlated with the local value of the superconducting gap, and calculations suggest that the fundamental attraction between electrons is modulated locally. In this work, we use density functional theory to try to ascertain which locations in the crystal are energetically most favorable for the O dopant atoms, and how the surrounding cage of atoms deforms. Our results provide support for the identification of STM resonances at -1.0 eV with dopant interstitial O atoms, and show how the local electronic structure is modified nearby. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P13.00006: Metal to insulator transition and ground state electronic structure of La$_{2-x}$Sr$_x$CuO$_4$ T.C. Schulthess, W.M. Temmerman, Z. Szotek, P.R.C. Kent We use the self-interaction corrected local spin-density (SIC-LSD) method to study the ground state electronic structure of La$_{2-x}$Sr$_x$CuO$_4$ as a function of Sr doping $x$. SIC-LSD is a parameter free method based on Density Functional Theory that has proven reliable for the study of strongly correlated electron systems. Sr is introduced via the virtual crystal approximation by linearly mixing the La and Sr potentials. In our calculations, we find that the nature of Cu-$d_{x^2-y^2}$ orbital changes character with varying Sr concentration. In the under-doped regime, one of the Cu-$d_{x^2-y^2}$ orbitals per atom is fully occupied and localized on the Cu site, leading to the formation of magnetic moments on Cu aligned antiferromagnetically in the CuO2 plane. Sr doping introduces holes mainly into the O-p bands and the system is a doped charge transfer insulator. In the over-doped regime, the Cu-$d_{x^2-y^2}$ orbitals are band-like and not spin-split. The moments on the Cu atoms vanish and the system is a nonmagnetic metal. In the orthorhombic structure, the transition from localized to band-like Cu-$d_{x^2-y^2}$ states occurs at about18\% Sr doping, i.e. within the region of optimal doping for superconductivity. We find a similar behavior if the calculations are performed with the idealized tetragonal structure. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P13.00007: A pressure induced insulator-to-metal critical point in the copper-oxides Tanja Cuk, Viktor Struzhkin, Thomas Devereaux, Alexander Goncharov, Christopher Kendziora, Hiroshi Eisaki, Ho-kwang Mao, Zhi-Xun Shen The presence of a quantum critical point inside the superconducting dome is a novel ideal unifying high-Tc superconductivity in the copper-oxides with that of other unconventional superconductors in strongly correlated materials. Experimental progress, however, has been difficult since superconductivity protects it from most direct measurements. Yet, the tuning parameter of all efforts to date has been chemical doping, which varies crystal fields, electron-phonon, and electron-electron interactions with potentially very different physical metrics. We report pressure tuned Raman and x-ray scattering data revealing an insulator-to-metal critical point near 20GPa with anomalies in six physical quantities: electronic Raman background, phonon lineshape and temperature dependence, density dependent behaviour of phonon and magnon frequencies, and a subtle structural change in the c-axis. We also suggest why this critical point may be near optimal doping in the high-Tc phase diagram. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P13.00008: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P13.00009: Suppression of charge stripes in highly strained, epitaxial La$_{5/3}$Sr$_{1/3}$NiO$_{4}$ films Changkun Xie, Barrett Wells, Feizhou He, Arnold Moodenbaugh We have successfully grown epitaxial La$_{5/3}$Sr$_{1/3}$NiO$_{4}$ films with a small crystalline mosaic using pulsed laser deposition. Using synchrotron radiation, the x-ray diffraction peaks associated with charge stripes have been successfully observed for relatively thick films with little strain. Anomalies due to the charge-ordering transition have been examined using four-point probe resistivity measurement. We also have produced highly strained films with the same total thickness through the use of multilayers of La$_{5/3}$Sr$_{1/3}$NiO$_{4}$ alternating with SrTiO$_{3}$. These films remain under in-plane tension. A thorough search for the charge stripe peaks in the strained multilayers has been negative; the stripes appear to be suppressed under these conditions. This suggests that electron-lattice interactions are critical for the formation of stripe phases. This work is supported through NSF DMR-0239667. Some data was taken at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P13.00010: Study of the antiferromagnetism in electron-doped cuprate superconductors with disorder C. S. Ting, Xin-Zhong Yan, Qingshan Yuan On the basis of the Hubbard model, we study the antiferromagnetic (AF) properties in electron-doped cuprates using the fluctuation-exchange approach. Taking into account the spin fluctuations in combination with the impurity scattering effect due to the randomly distributed dopant-atoms, we formulate the theory of antiferromagnetism in the system. By self-consistently solving the integral-equations for the Green's function, the Neel's temperature is determined by the condition that the Goldstone mode from the transverse spin susceptiblility first appears as the temperature is lowered. Our numerical calculation shows that the Goldstone mode always is pinned at ($pi$,$pi$), insensitive to the doping level. We also calculate the onset temperature of the pseudogap formation which is due to the antiferomagnetic fluctuations, the single particle spectral density, the Fermi surface evolution with doping concentration, and the staggered magnetization. It is shown that the results obtained by the present approach are in very good agreement with the experiments. In the present approach, the density of states (DOS) of the antiferromagnetic phase exhibits a zero-energy peak in the under-doped region. [Preview Abstract] |
Session P14: Focus Session: Magnetic Nanostructures I
Sponsoring Units: GMAG DMPChair: Richard Kodama, University of Illinois at Chicago
Room: Colorado Convention Center Korbel 4D
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P14.00001: Coercivity of nanometer size Ni granular films as a function of temperature, grain size and dipolar interaction R. Das, A.F. Hebard, A. Gupta, D. Kumar The influence of temperature, grain size and dipolar interaction (DI) on coercive field $H_{c}$ determined from hysteretic magnetization loops has been studied in nanometer size Ni granular films embedded in an insulating AlOx host matrix. Single layer (SL) and multilayer (ML) samples were grown using pulsed laser deposition by sequential deposition from AlOx and Ni targets. The Ni film thickness $d$, and hence the average grain size, is varied over the range of 3nm to 60nm. In the ML samples, the Ni layers are separated by 3nm-thick AlOx. At low temperatures $H_{c}(d)$ exhibits a peak at a crossover thickness $d_{x}$ delineating \textit{single domain} (SD) from \textit{multi domain} (MD) behavior. The ML sample has a smaller $d_{x}$ because of the increase in magneto static energy due to an increased DI associated with a greater number of nearest neighbors. In the SD region common to both samples, the $H_{c}$`s are considerably higher for ML samples compared to those for SL samples. This effect can be understood in terms of collective dynamics of the interacting particles [1]. Surprisingly, $H_{c}(T)$ shows the well known Stoner-Wohlfarth square root temperature dependence in the MD region for both SL and ML samples. Even more surprising is the unexpected oscillatory dependence of $H_{c}(d)$ in the MD region for the SL samples. [1] C. Djurberg \textit{et al.}, Phys. Rev. Lett. \textbf{79}, 5154 (1997). [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P14.00002: Dynamic Hysteresis of Fe$_{10}$Co$_{90}$ Nanoparticle Compacts K.M. Chowdary, S.A. Majetich The time-dependent magnetic response of composites made of consolidated Fe$_{10}$Co$_{90 }$nanoparticles was measured and modeled. 200 nm particles with average grain size 20 nm synthesized by the polyol method were consolidated to 95{\%} theoretical density by plasma pressure compaction. Power loss, complex permeability, and coercivity were extracted from dynamic minor hysteresis loops measured over a range of temperatures (77 K -- 873 K) and frequencies (100 Hz -- 100 kHz) for toroidal samples. When the data were scaled relative to the peak frequency of the imaginary permeability, universal behavior was observed, with two distinct components. This behavior is explained through simulations of the N\'{e}el-Brown thermal aftereffect in which a time-dependent energy barrier in an Arrhenius-N\'{e}el law gives a rate equation for magnetization reversal. Quantitative attempts to match model and experiment indicate a distribution of energy barriers along with coupled and uncoupled regions in the compacted sample. The uncoupled regions limit the useful frequency range of the sample. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P14.00003: Cluster Beam Synthesis of Magnetic Nanoparticles R.H. Kodama, J.J. Kavich, M. Vedpathak, M.C. Peterson Highly mono-dispersed Ni and Fe nanoparticles are produced using a cluster beam source. The source chamber is isolated from a deposition chamber using a small orifice. By balancing process gas flow, orifice size, and pumping speed we can create a high-pressure sputtering environment, suitable for nanoparticle condensation. The average beam flux and a spatial beam profile are acquired using a quartz crystal monitor with linear motion control. We find that the time stability of the nanoparticle flux is very sensitive to sputtering power and temperature gradients in the cluster source. AFM and TEM measurements have shown a correlation of particle size with position in the beam. Both size distribution and time stability seem to be sensitive to small perturbations in the gas flow near the sputtering source. High-Resolution TEM images indicate that the particles are randomly oriented and nano-crystalline in nature. The magnetic properties of Ni nanoparticles are measured using a SQUID magnetometer. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P14.00004: Detection of exchange interaction in diatomic molecules by Fano resonance Jonas Fransson, Alexander Balatsky We propose a mechanism to use STM for direct measurements of the two-electron singlet-triplet exchange splitting $J$ in diatomic molecular systems, based on the coupling between the molecule and the substrate electrons. The different pathways for electrons lead to interference effects and generate kinks in the differential conductance at the energies for the singlet and triplet. These features are related to Fano resonance due to the branched electron wave functions. Since the ratio between the tunnelling through the two atoms can be modulated by spatial movements of the tip along the surface this suggests a technique for detection of the singlet-triplet exchange splitting with STM. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P14.00005: Measurement of Magnetic Anisotropy for Individual Atomic Spins on Surfaces Cyrus F. Hirjibehedin, Alexander F. Otte, Markus Ternes, Christopher P. Lutz, Andreas J. Heinrich We measure the effects of magnetic anisotropy on individual magnetic atoms on a thin-insulating surface. Using the inelastic electron tunneling spectroscopy capabilities of a scanning tunneling microscope, we probe the spin excitation spectra of Mn and Fe atoms adsorbed on a single copper nitride layer. Magnetic anisotropy is directly manifested as finite-energy spin excitations that exist even in the absence of a magnetic field. The effects of anisotropy are found to be relatively weak for Mn atoms but are substantially larger for Fe atoms, in which spin-orbit coupling is prominent. When a magnetic field is applied to the Fe atoms, the spin excitations shift in a manner that is strongly dependent on the direction of the applied field. These shifts in energy can be understood both qualitatively and quantitatively with a Hamiltonian containing in-plane and out-of-plane magnetic anisotropies. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P14.00006: Ferromagnetic multipods fabricated by solution phase synthesis and hydrogen reduction Yucheng Sui, Yao Zhao, Jun Zhang, Sitaram Jaswal, Xingzhong Li, David Sellmyer New functional materials might emerge if nanocrystals of higher complexity than those with simple geometries (spheres, rod, discs) could be produced. Branched nanostructures (called multipods) have attracted much attention owing to their potential as building blocks in the fabrication of complex, multi-terminal devices through self assembly. In this work, we demonstrate that ferromagnetic Co multi-branched nanostructures can be produced through the combination of solution-phase synthesis and hydrogen reduction. The CoO multipods were produced through the pyrolysis of cobalt-oleate in octadecane at 280$^{o}$C in the presence of oleic acid under the protection of pure nitrogen. Arm lengths and diameters of the CoO multipods are about 30 and 10 nm respectively, and the angles between the nearest arms are 90 degrees. The multipods were assembled onto Si substrates, and after reduction in flowing hydrogen gas at 290$^{o}$C, pure cobalt with hexagonal crystal structure and multi-branched structures were created. Anisotropic magnetic properties were found for cobalt multipods. The growth mechanism of CoO multipods will be presented in this work. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 1:03PM |
P14.00007: Magnetic Exchange Force Microscopy Invited Speaker: Magnetic Exchange Force Microscopy (MExFM) is a new technique that was proposed [1] to perform magnetic imaging with atomic resolution. It is based on conventional atomic force microscopy, but uses a magnetic tip, which is approached very closely to a magnetic sample in order to detect the magnetic exchange interaction. Unlike Spin-Polarized Scanning Tunneling Microscopy (SP-STM) [2], it is not limited to well conducting materials. Although theoretical calculations indicate the feasibility of MExFM and several attempts have been made to perform such an experiment, no clear evidence for successful MExFM imaging has been reported so far. To prove the detection of the magnetic exchange interaction between magnetic moments (spins) of tip and sample, we investigated the (001) surface of the prototypical antiferromagnetic insulator nickel oxide. Imaging with atomic resolution was performed in the non-contact attractive force regime using the dynamic mode with frequency modulation. Apart from the chemical contrast between nickel and oxygen atoms an additional modulation originating from the row-wise antiferromagnetic arrangement of the spins at the nickel atoms could be observed. We discuss experimental prerequisites to perform MExFM and present different tests to unambiguously assign the additional modulation to the magnetic exchange force.\newline \newline [1] R. Wiesendanger et al., J. Vac. Sci. Technol. B \bf{9} \rm, 519 (1990).\newline [2] S. Heinze et al., Science \bf{288} \rm, 1805 (2000). [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P14.00008: Micromagnetic Modeling of Ferromagnetic Resonance in Nonuniform Magnetic Field D.V. Pelekhov, I. Martin, Yu. Obukhov, J. Kim, E. Nazaretski, T. Mewes, P.E. Wigen, R. Movshovich, P.C. Hammel We compare micromagnetic modeling of Ferromagnetic Resonance (FMR) excitations in thin ferromagnetic samples in the presence of a nonuniform magnetic field to our FMR data obtained with Magnetic Resonance Force Microscopy (MRFM). MRFM is a novel scanned probe technique based on mechanical detection of magnetic resonance. Its extreme sensitivity originates partially from the high magnetic field gradient of the MRFM probe micromagnet. The presence of the high field gradient imposes unusual conditions on the FMR resonance in the sample under investigation. We will discuss their manifestations in both simulations and experimental data. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P14.00009: Local investigations of 2 micrometer permalloy dot array using Magnetic resonance force microscopy. J. Kim, Yu. Obukhov, D. Pelekhov, T. Mewes, S. Batra, P.E. Wigen, S. An, T. Gramila, P.C. Hammel Ferromagnetic resonance images of 2 micrometer diameter permalloy dots in an array with a center to center distance of 2.2 micrometer have been microscopically investigated at 4K using magnetic resonance force microscopy. Both local and global ferromagnetic resonance properties of the sample are observed due to the influence of the strong field immediately beneath the micromagnetic probe. Localized spectral changes reveal the dynamics of ferromagnetic resonance of a dot just underneath the tip and neighboring dots in proximity to the tip. The combination of spatial and spectral information is a promising new way to investigate magnetization dynamics using magnetic resonance force microscopy. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P14.00010: Self-assembly of magnetic nanoparticles JiYeon Ku, Phillip Geissler When a solution containing nanocrystals dries, the solute deposits onto the underlying substrate. The nonequilibrium nature of such a process, together with anisotropic interactions between nanoparticles, can drive the formation of intricate transitory patterns. In particular, we are investigating how magnetic nanocrystals can coalesce into faceted, mesoscopic domains that have been observed in experiments. We model the nanoparticles as dipolar spheres and use Monte Carlo methods to advance their arrangements in time from an initially dispersed configuration. Competition between short-ranged, isotropic van der Waals forces and long-ranged, anisotropic electrostatic forces generates diverse hybrid structures, which exhibit both imperfect close-packing and incomplete dipole alignment. We explore the structures obtained under various conditions and speculate on dynamical mechanisms of aggregation and pattern formation. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P14.00011: FeCo Nanoparticles by Salt-Matrix Annealing Narayan Poudyal, Girija S. Chaubey, Chuan-bing Rong, J. Ping Liu Preparation of monodisperse FeCo nanoparticles remains a challenge due to poor chemical stability of the nanoparticles during heat treatments. We report a novel route of preparation of monodisperse FeCo nanoparticles with controllable particle size and size distribution. CoFe$_{2}$O$_{4}$ nanoparticles were first prepared by chemical solution method via reduction of iron acetylacetonate and cobalt acetylacetonate. The as-synthesized CoFe$_{2}$O$_{4}$ nanoparticles were then mixed with NaCl powder particles and the mixtures were annealed in forming gas to form FeCo nanoparticles. Structural characterization showed that the FeCo nanoparticles obtained by salt-matrix annealing have been transformed to body-centered cubic (bcc) structure without sintering and agglomeration. The particle size can be well controlled by adjusting the synthetic parameters for CoFe$_{2}$O$_{4}$ nanoparticles. It is also found that the recovered bcc FeCo nanoparticles are stable under ambient condition. The magnetization of the FeCo nanoparticles is found to be size dependent. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P14.00012: Static and dynamic magnetic properties of ``dumbbell'' and ``flower'' shaped Au-Fe$_{3}$O$_{4}$ nanoparticles N.A. Frey, S. Srinath, H. Srikanth, Chao Wang, Shouheng Sun We report studies of the static (DC) and dynamic (AC, RF) magnetization of chemically synthesized Au-Fe$_{3}$O$_{4}$ nanoparticles with dumbbell and flower shaped configurations. Dumbbell particles form with Fe$_{3}$O$_{4}$ (18 nm) growing epitaxially on Au seed particles (4 -- 8 nm). Multiple Fe$_{3}$O$_{4}$ particles also can be made to grow on Au particles with flower-like cluster geometry. While measurements on dumbbell particles revealed standard signatures of superparamagnetism, the flower-like nanoparticles exhibited remarkable novel features. Two magnetic transitions are observed --one representing the blocking temperature ($\sim $88K) and the other ($\sim $48K) likely associated with freezing of surface spins. Our experiments revealed the presence of exchange bias (EB), high field irreversibility as well as training and memory effects. EB was also confirmed through RF transverse susceptibility measurements that directly probe the effective magnetic anisotropy and switching fields. Our studies demonstrate how engineering the configuration of nanoparticle clusters in a controlled manner can result in dramatically different magnetic properties. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P14.00013: Magnetite nanoparticles with almost bulk magnetic properties: the role of the surfactant Xavier Batlle, Pablo Guardia, Oscar Iglesias, Am\'ilcar Labarta, Alejandro G. Roca, M. Puerto Morales, Carlos J. Serna Uniform magnetite nanoparticles of 6, 10 and 17 nm were synthesised by thermal decomposition of an iron precursor. Oleic acid was used as surfactant. Saturation magnetization M$_{s}$ reaches the expected value for bulk magnetite at low temperature, in contrast to results in small particle systems for which M$_{s}$ is usually much smaller due to surface spin disorder. The coercive field for the 6 nm particles is also in agreement with that of bulk magnetite. Both results suggest that the oleic acid molecules covalently bonded to the nanoparticle surface yield a strong reduction in the surface spin disorder, such that the new O$^{2-}$ surface ligands partially reconstruct the crystal field of the surface Fe cations, as suggested by XPS. This may be of relevance in biomedical applications to reduce the strength of the magnetic field required to obtain a high M$_{s}$ and opens the question of whether M$_{s}$ above the bulk value may be obtained by taking advantage of the orbital contribution. Work funded by Spanish NAN2004-08805-CO4-02 and NAN2004-08805-CO4-01, and CONSOLIDER CSD2006-12 [Preview Abstract] |
Session P15: Focus Session: Current and Magnetization Driven Effects
Sponsoring Units: GMAG DCOMP DMPChair: Allan MacDonald, University of Texas at Austin
Room: Colorado Convention Center Korbel 4E
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P15.00001: Nonlinear and Nonequilibrium Spin Injection in Magnetic Tunneling Junctions Invited Speaker: Quantitative analysis of charge and spin quantum transport in spintronic devices requires an atomistic first principles approach that can handle nonlinear and nonequilibrium transport conditions. We have developed an approach for this purpose based on real space density functional theory (DFT) carried out within the Keldysh nonequilibrium Green's function formalism (NEGF). We report theoretical analysis of nonlinear and nonequilibrium spin injection and quantum transport in Fe/MgO/Fe trilayer structures as a function of external bias voltage. Devices with well relaxed atomic structures and with FeO oxidization layers are investigated as a function of external bias voltage. We also report calculations of nonequilibrium spin injection into molecular layers and graphene. Comparisons to experimental data will be presented. Work in collaborations with: Derek Waldron, Vladimir Timochevski (McGill University); Ke Xia (Institute of Physics, Chinese Academy of Science, Beijing, China); Eric Zhu, Jian Wang (University of Hong Kong); Paul Haney, and Allan MacDonald (University of Texas at Austin). [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P15.00002: Fermi-Surface Evaluation of Anomalous Hall Conductivity using Wannier Interpolation Xinjie Wang, David Vanderbilt, Jonathan Yates, Ivo Souza Recently, Haldane showed that the nonquantized part of the intrinsic anomalous Hall conductivity (AHC) can be represented as a Fermi-surface property.\footnote{F.D.M. Haldane, Phys. Rev. Lett. {\bf 93} 206602 (2004).} The time-consuming integration of the Berry curvature over the entire Brilliouin zone is thereby converted into a more efficient integral over the Fermi surface only. Here we present an ab-initio approach for computing the AHC which combines a Haldane-like strategy with Wannier interpolation of the Bloch functions. First, a conventional electronic-structure calculation is performed and maximally-localized Wannier functions are constructed by a post-processing step, in order to transform the full ab-initio problem into an ``exact'' tight-binding form. Second, the Brilliouin zone is sampled by a large number of equally spaced parallel slices oriented normal to the total magnetization. We find the intersections of each Fermi surface sheet with every slice, organize these into a set of closed loops, and compute the Berry phase of the Bloch states as they are transported around these loops. The AHC is then just proportional to the sum of the Berry phases of all the loops on all the slices. The method is used to calculate the intrinsic AHC of Fe, Co and Ni. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P15.00003: Spin Hall effect: from the ballistic to diffusive regime Roksana Golizadeh-Mojarad, Supriyo Datta We describe a model based on the Non-Equilibrium Green's function (NEFG) method that allows us to study the spin Hall effect continuously from the ballistic to the diffusive regime. Our numerical results show good agreement with recent experiments by Sih et. al. [PRL 97, 096605 (2006)]. Analytical expressions for the spin accumulation density will also be presented that describe the numerical results very well as different parameters are varied. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P15.00004: Berry-phase blockade in single-molecule magnets Gabriel Gonzalez, Michael Leuenberger We formulate the problem of electron transport through a single- molecule magnet (SMM) in the Coulomb blockade regime taking into account topological interference effects for the tunneling of the large spin of a SMM. The interference originates from spin Berry phases associated with different tunneling paths. We show that in the case of incoherent spin states it is essential to place the SMM between oppositely spin-polarized source and drain leads in order to detect the spin tunneling in the stationary current, which exhibits topological zeros as a function of the transverse magnetic field. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P15.00005: The anomalous Hall effect and Nernst effect in CuCr$_{2}Se_{4-x}Br_{x}$: First principles studies Zhong Fang, Yugui Yao The non-vanishing Berry curvature of Bloch states in ferromagnetic crystals with spin-orbit coupling (broken time reversal symmetry) can act as gauge field in the momentum space, which in turn affects the transport behavior of electrons in real space, and produces the fascinating phenomena in solid crystals. Typical example is the intrinsic anomalous Hall effect (IAHE). Recent progresses in this field not only deepen our understanding of the physics behind, but also enable quantitative evaluations of the effects from the parameter-free electronic structure calculations. In this presentation, the recent progresses in this field will be addressed with emphasis on the quantitative evaluations of IAHE and Nernst effect in ferromagnetic spinel CuCr$_{2}Se_{4-x}Br_{x}$ from the first- principles calculations. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P15.00006: Chern Number effective Hamiltonian for Mn clusters in GaAs. Tor Olof Strandberg, Carlo M. Canali, Allan H. MacDonald Small numbers of Mn atoms can be manipulated into arbitrary spatial arrangements on the $<110>$ surface of GaAs by means of a novel STM atom-by-atom substitution technique, which enables the replacement of individual Ga atoms by Mn[1]. The tunnelling differential conductance over an isolated Mn atom reveals a large and broad resonance in the GaAs energy gap. For a Mn pair placed less than 1nm apart, the resonance splits into two peaks, whose spacing is thought to be related to the exchange-energy interaction between Mn ions. We report on theoretical results for the local density of states and the Mn acceptor-level splittings for a Mn dimer, based on a tight-binding model of Mn substitutions on the $<110>$ GaAs surface. We compare our model with previous work which does not account for the surface. We then derive an effective quantum spin Hamiltonian for the Mn cluster, based on a Chern number theory developed recently, which includes Berry phase effects[2]. The influence of quantum fluctuations of the Mn spin orientations on the tunnelling differential conductance will be discussed. [1] D. Kitchen et al., Nature {\bf 442}, 436 (2006). [2] C.M. Canali, A. Cehovin and A.H. MacDonald, Phys. Rev. Lett. {\bf 91}, 046805 (2003) [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P15.00007: Electron corrected Lorentz forces in solids and molecules in magnetic field Davide Ceresoli, Riccardo Marchetti, Erio Tosatti We describe the effective Lorentz forces on the ions of a generic insulating system in a magnetic field, in the context of Born-Oppenheimer ab-initio molecular dynamics. The force on each ion includes an important contribution of electronic origin, which depends explicitly on the velocity of all other ions, and is given in terms of a Berry curvature, directly suitable for classical dynamics simulations. The formulation is valid at strong magnetic field, where a scheme for ab-initio simulations based on plane wave methods is outlined. As a simple analytical demonstration we present the dynamics of an H$_2$ molecule in a weak field, describing the electrons approximately through Slater's variational wavefunction. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P15.00008: Tunneling anisotropic magnetoresistance driven by resonant surface states Athanasios Chantis, Kirill Belashchenko, Evgeny Tsymbal, Mark van Schilfgaarde Fully-relativistic first-principles calculations of the Fe(001) surface demonstrate that resonant surface (interface) states may produce sizeable tunneling anisotropic magnetoresistance in magnetic tunnel junctions with a single magnetic electrode. The effect is driven by the spin-orbit coupling. It shifts the resonant surface band via the Rashba effect when the magnetization direction changes. We find that spin-flip scattering at the interface is controlled not only by the strength of the spin-orbit coupling, but depends strongly on the intrinsic width of the resonant surface states. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P15.00009: Fully relativistic spin torques and spin currents Peter Weinberger, Andras Vernes, Balazs L. Gyorffy In using the one-particle Dirac equation in the presence of an external electro-magnetic field an exact equation of motion for the density of the four-component Bargmann-Wigner polarization operator $T_{\mu }=(\vec{T},T_{4}) $ is presented, the various occuring terms of which can be viewed as the relativistic counterparts of \textit{ad hoc} defined non- relativistic spin-currents and spin-transfer torques. Based on the properties of the Berry phase the particle and the magnetization density can be formulated in terms of a instantanous resolvent $G(z;t)$ of the time dependent Dirac equation by means of contour integrations. The corresponding Greens function $G(\mathbf{r,r}^ {\prime }$, $z;t)$ can in turn be evaluated within a multiple scattering scheme by solving at each given time $t$ a ``quasi-stationary'' problem. In terms of this Greens function the time evolution of any single-particle density, i.e., also of $T_{\mu }=(\vec{T},T_{4})$ can be evaluated. As a first application the case of a single Fe atom is considered, for which very easily a comparison with a time- dependent first order perturbational scheme can be given. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P15.00010: First principles theory of the current-modulated exchange bias. Paul Haney, Rembert Duine, Alvaro Nunez, Olle Heinonen, Allan MacDonald Recent experiments[1] have demonstrated the influence of current on exchange-bias fields in point-contact spin-valve structures. With this motivation, we consider current induced torques in multilayer structures containing ferromagnetic, paramagnetic, and antiferromagnetic layers. Our description is based on ab initio spin-density-functional theory combined with the non-equilibrium Greens' function formalism and direct microscopic evaluation [2] of spatially resolved torques. We find that current induced torques are generically present in both ferromagnetic and antiferromagnet layers. We theoretically demonstrate that current-induced torques in an antiferromagnetic layer that is exchange coupled to a ferromagnetic layer can alter exchange bias and discuss materials combinations in which this effect can be exploited. [1] Wei et al.. cond-mat/0606462 [2] Haney et al.. cond-mat/0611534 [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P15.00011: Conventional spin current in Dirac equation Soo Yong Lee, Hyun-Woo Lee The spin current has been one of main concerns in the field of the spintronics. Recently Rashba [PRB \textbf{68}, 241315 (2003)] pointed out that in certain nonmagnetic systems with the spin-orbit coupling, the conventional definition of the spin current leads to a rather strange prediction, namely a nonzero spin current should flow even without external biases. Though the nonvanishing equilibrium spin current does not violate the time reversal symmetry, it still led many scientists to reexamine the definition of the spin current. Recalling that the spin-orbit coupling arises due to the relativistic effects, we examine in this work properties of the conventionally-defined spin current for a Dirac electron subject to an electrostatic potential V(r). Interestingly it is found that in this fully relativistic treatment, the equilibrium spin current vanishes for a wide class of V(r) including those representing the zincblende structure and the asymmetric quantum well, which is in clear contrast with the nonvanishing equilibrium spin current obtained from some effective nonrelativistic Hamiltonians. The origin of this difference is also examined. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P15.00012: Markov Chain Analysis of Stochastic Micromagnetic Simulations S. Hill Thompson, G. Brown, P.A. Rikvold Stochastic micromagnetic simulations are employed to study the magnetization dynamics of a realistic model of an iron nanopillar in an oblique applied field at nonzero temperature. The results suggest the existence of more than one reversal path, revealed by the distribution of switching times. The dynamics are further analyzed by considering the system as an absorbing Markov chain and studying the properties of the associated transition matrix. In particular, the eigenvalue spectrum provides the time to cross the free-energy saddlepoint separating the metastable well from the equilibrium configuration. Additionally, eigenvectors from individual runs are used to determine which reversal path each simulation followed, since it is likely the switching-time distributions overlap. Along with projective dynamics, this analysis shows that the evolution of the faster mode is indicative of a relatively flat free-energy landscape, while the slower-mode dynamics are dominated by a well-defined metastable well. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P15.00013: Thermoinduced Magnetization in Antiferromagnetic Heisenberg Chains Gregory Brown, Markus Eisenbach, G. Malcolm Stocks The magnetic properties of linear chains of classical three-dimensional Heisenberg spins, with antiferromagnetic nearest-neighbor exchange and uniaxial single-site anisotropy, are determined analytically and numerically to investigate the phenomenon of thermoinduced magnetization (TiM). TiM is the ferromagnetic response observed in nanoparticles of antiferromagnetic materials at low temperatures, with the ferromagnetic response increasing as temperature increases. In the strong-anisotropy limit, TiM is shown analytically to result from the relaxation of individual spins away from the anisotropy axis. In the weak-anisotropy limit, it is shown numerically that TiM occurs only at temperatures low enough for long-range ordering of the entire finite chain. In the absence of anisotropy, long-range order does not occur and TiM is not observed. Both of these results present serious challenges to current theories, which describe TiM only in terms collective motions at q=0 and in the limit of vanishing anisotropy. This work was sponsored by the Laboratory Directed Research and Development Program of ORNL (GB, ME, GMS), and by the DOE-OS through the Offices of BasicSciences, Division of Materials Sciences and Engineering (GMS). [Preview Abstract] |
Session P18: Condensed Phase Dynamics, Structure and Thermodynamics
Sponsoring Units: DCPChair: Branka Ladanyi, Colorado State University
Room: Colorado Convention Center 103
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P18.00001: First-principles study of molecular point defects in ice Ih Maurice de Koning, Alex Antonelli, Antonio J.R. da Silva, Adalberto Fazzio We present a first-principles study of the structure and energetics of molecular point defects in ice Ih [1]. Our approach is based on a DFT-GGA description, utilizing a periodic supercell containing 96 water molecules. We compute the formation free energies and corresponding thermal equilibrium concentrations as a function of temperature for the molecular vacancy and 3 different interstitial structures: the Tc, Tu and Bc configurations. The latter involves bonding to the surrounding lattice, whereas the first two do not. The results indicate that, due to its bonding to the surrounding lattice, the equilibrium concentration of the Bc interstitial is larger than that of the Tc and Tu structures, suggesting that the Bc structure is the preferred interstitial configuration in ice Ih. Comparison with the molecular vacancy, on the other hand, indicates that the vacancy is expected to be the overall dominant molecular point defect in ice Ih, at least for temperatures below T $\approx $ 200 K. Due to the elevated formation entropy of the Bc interstitial, however, a crossover scenario in which the Bc interstitial becomes favored at temperatures below the melting point, as has been suggested experimentally, is conceivable. \newline [1] M. de Koning, A. Antonelli, A.J.R. da Silva and A. Fazzio, Phys. Rev. Lett. \textbf{97}, 155501 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P18.00002: Size- and Temperature- Dependent Crystal Growth Rates Xian-Ming Bai, Mo Li Using molecular dynamics simulations and a crystal/melt coexistence model, we have calculated the size-dependent crystal growth rates of a Lennard-Jones system over a wide range of undercooling temperatures. Our results show that the growth rate or interface moving velocity decreases substantially with the increasing system size. It is found that the system fluctuations are related to the finite-size effects. By treating the atomic site acceptance ratio as a function of temperature and system size rather than a constant, we modified the collision-controlled model which fits the simulation results well. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P18.00003: ESR Studies of a Reorienting Nickel Complex Bruce Kowert Electron spin resonance spectra of the planar bis(maleonitriledithiolato)nickel anion radical (BMNT) in the intermediate motional region have been simulated in several polar solvents using axially symmetric reorientation. The rotational diffusion about the long in-plane axis is three to four times faster than that about the two axes perpendicular to it. The reorientational model needed to produce agreement with experiment is either in or close to the Brownian rotational diffusion limit. The solvents are 4-allyl-2-methoxyphenol (eugenol), dimethyl phthalate, tri-$n$-butyl phosphate, tris(2-ethyl-hexyl)phosphate, and 2-methoxyethyl ether (diglyme), ethyl alcohol, and a dimethylformamide-chloroform mixed solvent. The reorientational rates from the simulations are in general agreement with those from line width analyses carried out from the fast to the slow motional regions. The temperature dependence of the diffusion rates is discussed in terms of the Stokes-Einstein-Debye (SED) model and the Vogel-Tammann-Fulcher equation. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P18.00004: Low coverage Neon adsorption on HiPCo$^{TM}$ nanotube bundles Subramanian Ramachandran, Oscar Vilches We present heat capacity measurements of Ne adsorbed on single-walled closed-end carbon nanotube bundles (HiPCo$^{TM})$ between 2 and 20 K. Limited adsorption isotherms measurements for 17K$<$T$<$28K allow us to estimate the isosteric heat of adsorption for these films. Particular emphasis is on the results at very low coverage, between 0.02 monolayer to 0.3 monolayer where Ne may form single-line and three-lines of atoms in imperfect adsorption sites and grooves. The specific heat versus temperature at 0.02 -- 0.04 monolayer coverage shows qualitative agreement with a model of adsorption of Ne on grooves by Kostov et.al (PRB 68, 245403, (2003)). For 0.06 monolayer and higher coverages, however, the measured low temperature specific heat vs temperature has a T$^{2}$ dependence, which yields 2D Debye temperatures in the range of 44 to 47 K. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P18.00005: Structure and dynamics of levitated liquid aluminates Louis Hennet, Irina Pozdnyakova, Marie-Louise Saboungi, David L. Price We have used the aerodynamic levitation technique combined with CO$_{2}$ laser heating to study the structures of liquid CaAl$_{2}$O$_{4}$ and MgAl$_{2}$O$_{4}$ with x-ray and neutron diffraction. We determined the structure factors and corresponding pair correlation functions describing the short-range order in the liquids. The combination of the two scattering techniques makes it possible to derive information not accessible with a single measurement. We have also obtained information on the dynamics of liquid MgAl$_{2}$O$_{4}$ with inelastic x-ray scattering. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P18.00006: Thermal Fluctuations and Excitonic Interactions in Polyadenosine John Jean Interest in the electronic structure and photophysics of DNA has been sparked in recent years by increased awareness of the biological effects of non--ionizing UV radiation as well as the possibility of using short DNA oligomers as molecular conduits for efficient charge or energy transport in nanoscale devices. The optical properties of DNA in the near UV region are governed by dipole-allowed transitions localized on the aromatic bases, which have excited state lifetimes on the order of a few hundred femtoseconds. Recent time-resolved experiments on polyadenosine oligomers, however, provide clear evidence for stacking-induced long-lived, stacking-induced states and spectral shifts compared to those of the monomer bases. The origin of these states and their relation to stacking dynamics is still largely unknown. DNA is a highly dynamic structure undergoing large-amplitude structural fluctuations on timescales spanning many orders of magnitude and spatial correlation lengths, thus characterization of the excitonic states requires understanding how nearest-neighbor Coulombic and exchange interactions couple to picosecond motions of the bases. In this paper, we present results from hybrid QM/MD simulations of polyadenosine in solution that provide detailed insight into the time-dependent stacking interactions in these systems and the effects of dynamic disorder on the temporal and spatial properties of the low-lying excitons. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P18.00007: The activation of phosphoramide mustard anticancer drugs from ab initio simulations. Markus Allesch, Eric Schwegler, Mike Colvin, Francois Gygi, Giulia Galli The nitrogen mustard based DNA alkylating agents were the first nonhormonal drugs to be used effectively in the treatment of cancer and remain one of the most important drugs for the chemotherapeutic management of many common malignancies today. An understanding of the activation of these compounds is, in itself, of scientific interest, but also critical in designing improved analogs of greater selectivity and efficacy. We have investigated the activation pathways of one of the most active metabolites, phosphoramide mustard (PM), and its methylated ester (PMME). In particular, we have examined the activation barrier and reaction free energy for the intramolecular cyclization reaction using first principles molecular dynamics simulations with explicit and continuum solvation models. Structural, dynamical and electronic properties along the reaction path have been computed mainly to address the question why de-esterification is required to activate these drugs. This work was performed under the auspices of the U.S. Dept. of Energy at the University of California/Lawrence Livermore National Laboratory under contract no. W-7405-Eng-48. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P18.00008: Use of light scattering data to determine free energies of ternary mixtures George Thurston, David Ross, Carl Lutzer, Seth Fraden We demonstrate a method that uses light scattering data to determine the free energy of ternary liquid mixtures, through solving a second-order nonlinear partial differential equation appropriate for single isotropic phases. We show that forward light scattering efficiency data, together with boundary condition data, permit integration of the second derivative of the intensive free energy along curves tangent to the local dielectric coefficient gradient vector. With suitable information about phase boundaries the method also accommodates the presence of phase-separated regions next to single-phase regions in which the governing equation is an appropriate model. In the presence of composition-dependent optical dielectric dispersion, light scattering at more than one wavelength can help augment and check such free energy determination. In summary, light scattering provides a non-invasive method of determining ternary liquid mixture free energies without adopting specific free energy models in advance. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P18.00009: Proton momentum distributions in water: A path integral molecular dynamics study Varadharajan Srinivasan, Joseph A. Morrone, Daniel Sebastiani, Roberto Car Recent neutron Compton scattering experiments have detected the proton momentum distributions of water. This density in momentum space is a quantum mechanical property of the proton, due to the confining anharmonic potential from covalent and hydrogen bonds. The theoretical calculation of this property can be carried out via ``open'' path integral expressions. In this work, we present an extension of the staging path integral molecular dynamics method, which is then employed to calculate the proton momentum distributions of water in the solid, liquid, and supercritical phases. We utilize the SPC/F2 empirical force field to model the system's interactions. The calculated momentum distributions depict both agreement and discrepancies with experiment. The differences may be explained by the deviation of the force field from the true interactions. These distributions provide an abundance of information about the environment and interactions surrounding the proton. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P18.00010: Calculation of Proton Transfer Rates in Liquids Using Molecular Dynamics Simulations Yin Guo We have been investigating a computational method that incorporates WKB tunneling calculations within the framework of classical molecular dynamics (MD) simulations. The computational cost is at the same level as the usual MD simulation, thus providing a practical and efficient dynamical approach for treating quantum tunneling. Building upon the earlier gas-phase studies, we extend the method to condensed phase materials. As a test case, the method is applied to a model system that represents proton transfer AH--B$\leftrightarrow $A$^{-}$--H$^{+}$B in liquid methyl chloride, where AH--B is a linear complex with parameters chosen to model a typical phenol-amine complex. The calculated results are compared with those of earlier studies on the same system by Azzouz and Borgis and by Hammes-Schiffer and Tully using different methods. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P18.00011: Local structure and the intermediate-energy fine structure in x-ray Raman scattering from ice Ih G.T. Seidler, T.T. Fister, C. Hamner, F.D. Vila, J.O. Cross The structure of the various different equilibrium and nonequilibrium phases of water ice is a topic of considerable interest, with strong relevance for geophysics, atmospheric sciences, and space sciences.~ Recent advances in non-resonant x-ray Raman scattering (XRS) provide a new method for studying local structure of water ices in extreme environments including especially in high-pressure cells.~ Here, we investigate two pragmatic issues: the optimum choice of momentum transfer $q$ for these measurements and the usefulness of the intermediate-energy fine structure as a strong fingerprint of local atomic structure out to several coordination shells.~ To this end, we present new XRS measurements of ice Ih with greatly improved statistics over earlier work, and also present extensive full-multiple calculations of the dependence of the intermediate-energy fine structure on local structure.~ We find that XRS measurements at high $q$, where the XRS cross-section is largest but where multipole transitions can be important, show little difference from dipole-limited soft x-ray absorption studies.~ In addition, our calculations predict significant sensitivity of the XRS intermediate-energy fine structure to different ice structures. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P18.00012: Covalency in Actinide and Lanthanide Hexachloride Anions Ping Yang, Enrique Batista, Richard Martin, Christin Carlson, David Clark, Steven Conradson, Daniel Schwarz, Marinne Wilkerson Whether actinide atoms form covalent or ionic bonds is still a matter of debate after many years of study and it remains a challenge for experimentalists and theoreticians. From the experimental side, synchrotron-based ligand K-edge X-ray absorption spectroscopy appears as a promising technique for probing this issue. From the theoretical perspective, quantum chemical simulations should be able to add on a first principle understanding. To tackle this problem, we have applied these techniques on a series of octahedral uranium and lanthanide chloride salts, MCl$_{6}^{n-}$ (M= U, Ce, Pr) in various oxidation states (n=1, 2, 3). We will show hybrid density functional theory calculations that give evidence of the covalent nature of the M-Cl bond in a quantitative manner. This covalency was found to increase for higher oxidation states. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P18.00013: Site Sensitivity and local electronic symmetries in carboranes T.T. Fister, G.T. Seidler, F.D. Vila, J.O. Cross, J.C. Linehan Icosohedral carboranes containing ten boron and two carbon atoms are seeing renewed interest because of their potential use in new cancer and AIDS therapies.~ These molecules have flexible geometries which allow bonding to three types of carbon sites (e.g. \textit{ortho}-, \textit{para}-, and \textit{meta}- configurations).~ Using a new multielement spectrometer, we present the first x-ray Raman scattering (XRS) study on each isomer with excited state spectra taken from the both the carbon and boron 1$s$ states.~ The change in the electronic structure between the isomers is most pronounced in the carbon spectrum, where the position in the edge confirms prior density functional theory calculations.~ With the boron spectra, we used the unique momentum transfer dependence of XRS to extract the symmetry components of the density of unoccupied states, i.e. the $l$-DOS.~ These results give an improved picture of the local electronic properties of the carboranes. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P18.00014: The electronic structure of Co and Ni tetraazaannulenes Jing Liu, Jie Xiao, P. Jeppson, P.A. Dowben, Seok-Bong Choi, L. Jarabek, A.N. Caruso, Ya.B. Losovyj We compare two metal centered tetraazaannulene (TMTAA) macrocyclic complex molecules: 5,7,12,14- tetramethyl -2,3:9,10- dibenzo [b,i] -1,4,8,11- tetraazacyclotetradecine nickel (II) and 5,7,12,14- tetramethyl -2,3:9,10- dibenzo [b,i] -1,4,8,11- tetraazacyclotetradecine cobalt (II). The highest occupied molecular orbital to the lowest unoccupied molecular orbital gap, obtained from combined ultraviolet photoemission and inverse photoemission studies, is close to the expected value of 6.6 eV expected from simple model calculations. While both the Co(II) (s=1/2) and Ni(II) (s=0) tetramethyldibenzo-tetraazaannulene molecular electronic structures are very similar, the Ni(II) adopts a high symmetry molecular configuration upon adsorption, with a strong preferential orientation. The role of an unpaired electron upon molecular symmetry and stability is discussed. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P18.00015: Pump-Probe Photoionization Spectroscopy of penta methyl cyclopentadiene Peter Weber, Fedor Rudakov The ultrafast curve crossing from the excited electronic state to the ground state in cyclic dienes often proceeds via conical intersections.~ Time-resolved experiments were performed by exciting the first excited state of pentamethylcyclopentadiene, as well as other methylated cyclopentadiene derivatives, with femtosecond pulses at 260 nm.~ Photoionization with a time-delayed probe pulse yields delay-time dependent mass and photoelectron spectra that reveal the ultrafast character of the curve crossing dynamics.~ [Preview Abstract] |
Session P19: Focus Session: Frontiers in Electronic Structure Theory IV
Sponsoring Units: DCP DCOMPChair: Troy van Voonhis, Massachusetts Institute of Technology
Room: Colorado Convention Center 104
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P19.00001: Ab-initio DMRG and Canonical Transformation Theories of Electronic Structure Invited Speaker: I will talk about two complementary methods that are under development in our group: (1) Ab-initio Density Matrix Renormalization Group: The Density Matrix Renormalization Group (DMRG) is a natural multireference method. Recently, we have implemented a quadratic-scaling DMRG algorithm which opens up the description of multireference (strongly interacting) correlation in large quasi-one-dimensional systems [1]. I will report calculations using this technique on conjugated oligomers correlating exactly, in the sense of Full-CI, complete pi-active spaces with up to 100 electrons in 100 orbitals (100, 100). (2) Canonical Transformation Theory: We have been developing a canonical transformation method to incorporate dynamical correlation on top of a multireference starting point. Our theory, termed Canonical Transformation Theory (CT) [2] is based on an exponential ansatz and is size-consistent. It retains the accuracy of coupled cluster theory at equilibrium bond geometries, but extends this accuracy to the full potential energy surface. The cost of the calculation is the same as for single-reference coupled cluster theory. I will report calculations using this technique for bond-breaking and excited states. I will also describe our recent efforts in developing a reduced-scaling version of the theory for large molecules. [1] J. Chem. Phys. 125, 144101 (2006) [2] J. Chem. Phys. 124, 194106 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:27PM |
P19.00002: Real-time {\it ab initio} simulations of excited-state dynamics in nanostructures Invited Speaker: Combining time-dependent {\em ab initio} density functional calculations for electrons with molecular dynamics simulations for ions, we investigate the effect of excited-state dynamics in nanostructures. In carbon nanotubes, we find electronic excitations to last for a large fraction of a picosecond.\footnote{Yoshiyuki Miyamoto, Angel Rubio, and David Tomanek, Phys.\ Rev.\ Lett.\ {\bf 97}, 126104 (2006).} The de-excitation process is dominated by coupling to other electronic degrees of freedom during the first few hundred femtoseconds. Later, the de-excitation process becomes dominated by coupling to ionic motion. The onset point and damping rate in that regime change with initial ion velocities, a manifestation of temperature dependent electron-phonon coupling. Considering the fact that the force field in the electronically excited state differs significantly from the ground state, as reflected in the Franck-Condon effect, atomic bonds can easily be broken or restored during the relatively long lifetime of electronic excitations. This effect can be utilized in a ``photo-surgery" of nanotubes, causing structural self-healing at vacancy sites\footnote{Yoshiyuki Miyamoto, Savas Berber, Mina Yoon, Angel Rubio, and David Tomanek, Chem.\ Phys.\ Lett.\ {\bf 392}, 209 (2004).} or selective de-oxidation processes induced by photo-absorption.\footnote{Yoshiyuki Miyamoto, Noboru Jinbo, Hisashi Nakamura, Angel Rubio, and David Tomanek, Phys.\ Rev.\ B {\bf 70}, 233408 (2004).} Also, electronic excitations are a key ingredient for the understanding of sputtering processes in nanostructures, induced by energetic collisions with ions.\footnote{Yoshiyuki Miyamoto, Arkady Krasheninnikov, and David Tomanek (in preparation).} [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P19.00003: First principles spectroscopy of confined water. Manu Sharma, Giulia Galli In order to characterize the changes in hydrogen bonding in water confined at the nanoscale, and to understand the effect of the interface between water and the confining medium, we carried out a spectroscopic investigation using first principles calculations. In particular, we computed the infrared (IR) spectrum of liquid water confined between two sheets of graphite. While the far IR region of the spectrum contains features characterizing the H-bond dynamics in water, we find a significant overlap of this region with the vibrational modes of graphite. We also find modes in the near IR region $\sim $2500 cm$^{-1}$, associated to the OH stretching mode, which while present in the kinematical (power) spectrum are absent from the computed IR spectrum. We demonstrate that these modes arise due to a dynamical charge transfer between water molecules and the p -- orbitals of the graphite surface. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P19.00004: Quadratic Scaling Local Canonical Transformation Method. Debashree Ghosh, Takeshi Yanai, Garnet Kin-Lic Chan Canonical transformation theory [1] can be used to describe the detailed dynamic correlation in bonding situations where there is significant non-dynamic, i.e. multireference character. This theory uses an exponential ansatz and is size-consistent. The computational cost of this method scales as N$^{6}$ which is about the same as in a single reference coupled cluster theory. We have devised a local Canonical transformation method for large systems. For large systems, we have been able to obtain quadratic scaling with the size of the system. Reduced and linear scaling algorithms for methods like MP2 and coupled cluster are well known. However, all these reduced scaling algorithms have been primarily developed for single reference correlation calculations. By combining the local canonical transformation method with, e.g. the quadratic scaling ab-initio Density Matrix Renormalization Group theory, we can now obtain a reduced-scaling description of dynamical and non-dynamical correlation in large multireference problems. [1] Takeshi Yanai, Garnet K.L. Chan, J. Chem. Phys. \textbf{124}, 194106, 2006. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P19.00005: Representing molecules as atomic-scale electrical circuits with fluctuating-charge models Jiahao Chen, Todd Mart\'Inez Fluctuating-charge models (FCMs), also known as chemical potential equilibration models, can describe charge transfer in molecular mechanics (MM). Examples of FCMs are QEq [1], \textit{fluc}-q (FQ) [2], and our recently proposed PE-CC-QVB2 [3] and QTPIE [4]. FCMs describe the accumulation and depletion of atomic charges with electronegativities and chemical hardnesses. We show that this description of atoms maps molecular systems onto electrical circuits. Unlike other models [1, 2], our models correctly model a diatomic molecule in the dissociation limit; we explain how this is reflected in its circuit representation. FCMs hence establish a new connection between the statistical mechanics of molecular electronic structure [5] and classical circuit theory. [1] A. K. Rappe, and W. A. Goddard III, \textit{J. Phys. Chem.} \textbf{95}, 3358 (1991). [2] S. W. Rick, S. J. Stuart, and B. J. Berne, \textit{J. Chem. Phys.} \textbf{101}, 6141 (1994). [3] J. Morales, and T. J. Mart\'{\i}nez, \textit{J. Phys. Chem.} \textbf{108A}, 3076 (2004). [4] J. Chen, and T. J. Mart\'{\i}nez, \textit{Chem. Phys. Lett.} submitted (2006). [5] J. Morales, and T. J. Mart\'{\i}nez, \textit{J. Phys. Chem.} \textbf{105A}, 2842 (2001). [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P19.00006: Partition-of-unity finite element method for large, accurate electronic-structure calculations John Pask, Natarajan Sukumar Over the past few decades, the planewave pseudopotential (PW) method has established itself as the method of choice for large, accurate, density-functional calculations in condensed matter. However, due to its global Fourier basis, the PW method suffers from substantial inefficiencies in parallel implementation and problems involving localized states. Modern real-space approaches, such as finite-difference (FD) and finite-element (FE) methods, resolve these problems but have until now required much larger bases to attain the required accuracy. Here, we present a new real-space FE based method which employs modern partition-of-unity FE techniques to substantially reduce the number of basis functions required. Initial results show order-of-magnitude improvements relative to current state-of-the-art PW and adaptive-mesh FE methods for systems involving localized states such as d- and f-electron metals. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P19.00007: Anti-Hermitian Contracted Schr{\"o}dinger Equation for the Determination of Ground-state Energies and Two-electron Reduced-density-matrices without Wavefunctions David Mazziotti A recent advance in the theory of the contracted Schr{\"o}dinger equation (CSE), in which only the anti-Hermitian part of the equation is solved, permits the direct determination of ground-state two-electron reduced density matrices (2-RDMs) that yield 95-100\% of the correlation energy of atoms and molecules [Mazziotti, Phys. Rev. Lett. {\bf 97}, 143002 (2006)]. Here we discuss in detail the anti-Hermitian contracted Schr{\"o}dinger equation (ACSE) and its comparison to the CSE with regard to cumulant reconstruction of RDMs, the role of Nakatsuji's theorem, and the structure of the wavefunction. The ACSE is also formulated in the Heisenberg representation and related to canonical diagonalization. The solution of the ACSE is illustrated with a variety of molecules. The computed 2-RDMs very closely satisfy known $N$-representability conditions. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P19.00008: A Near Linear-Scaling Smooth Local Coupled Cluster Algorithm for Electronic Structure Joseph Subotnik, Alex Sodt, Martin Head-Gordon We demonstrate near linear-scaling of a new algorithm for computing smooth local coupled-cluster singles-doubles (LCCSD) correlation energies of quantum mechanical systems. Full CCSD provides an excellent, size-consistent treatment of electron correlation, but is computationally expensive, scaling formally as O($n^6$); by contrast, our LCCSD algorithm recovers more than 99\% of the CCSD correlation energy, while achieving near linear-scaling. Furthermore, previous domain-based LCCSD models had discontinuous potential-energy curves, with correspondingly infinite nuclear forces; by contrast, our domain-free algorithm's correlation energy is a rigorously differentiable function of nuclear coordinates, with correspondingly finite nuclear forces. Thus, our algorithm should allow, in the future, for the propagation of quantum dynamics on a highly correlated electron surface. We present applications to small polypeptide conformational energies, and demonstrate how one may smoothly dissociate two benchmark molecules (ethane and ketene) at the LCCSD level of electronic correlation using our algorithm. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P19.00009: Linear scaling integral fitting Alexander Sodt, Martin Head-Gordon In density (or integral) fitting methods, the density (or an orbital product) is replaced with a sum of atom-centered ``auxiliary'' functions, which are used to efficiently compute Coulomb interactions. In this work, we present a method for computing localized fit coefficients that scales linearly with system size, and introduces only extremely modest errors. We apply the algorithm to a variety of methods, including the J piece of the Fock matrix. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P19.00010: New many-body approach to photoemission and spectral functions Carl-Olof Almbladh, Claudio Verdozzi A new method for the description of photoemission and other spectra is presented. The key idea is to expand the transition amplitudes rather than the spectral function themselves. This leads to spectral intensities of a Golden-rule-like form. In the language of Keldysh path-ordered technique, contributions to ``lesser'' functions such as $G^<$ are classified into loss and no-loss diagrams, and in each diagram transition amplitudes can be identified. Conserving theories in the sense of Kadanoff and Baym exactly fulfill macroscopic conservation laws but may violate the positiveness of spectral functions. In contrast, the present scheme may violate conservation laws but it will always give positive spectra, thus being especially suitable for photoemission and other processes where spectral shapes are of primary interest. As examples, we will discuss the one-electron spectral function beyond GW theory and in presence of phonons. In both cases we find subtle interference effects between self-consistency and vertex corrections and a marked improvement of satellites. As a final example, photoemission beyond the sudden approximation will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P19.00011: Targeting individual excited states in DMRG. Jonathan Dorando, Johannes Hachmann, Garnet Kin-Lic Chan The low-lying excited states of $\pi $-conjugated molecules are important for the development of novel devices such as lasers, light-emitting diodes, photovoltaic cells, and field-effect transistors [1,2]. The \textit{ab-intio} Density Matrix Renormalization Group (DMRG) provides a powerful way to explore the electronic structure of quasi-one-dimensional systems such as conjugated organic oligomers. However, DMRG is limited to targeting only low-lying excited states through state-averaged DMRG (SDMRG). There are several drawbacks; state-averaging degrades the accuracy of the excited states and is limited to at most a few of the low-lying states [3]. In this study, we present a new method for targeting higher individual excited states. Due to progress in the field of numerical analysis presented by Van Der Horst and others [4], we are able to target individual excited states of the Hamiltonian. This is accomplished by modifying the Jacobi-Davidson algorithm via a ``Harmonic Ritz'' procedure. We will present studies of oligoacenes and polyenes that compare the accuracy of SDMRG and Harmonic Davidson DMRG. [1] Burroughes, et al. , Nature 347, 539 (1990). [2] Shirota, J. Mater. Chem. 10, 1, (2000). [3] Ramasesha, Pati, Krishnamurthy, Shuai, Bredas, Phys. Rev. B. 54, 7598, (1997). [4] Bai, Demmel, Dongarra, Ruhe, Van Der Horst, Templates for the Solution of Algebraic Eigenvalue Problems, SIAM, 2000. [Preview Abstract] |
Session P20: Focus Session: Multiferroics and other Functional Materials
Sponsoring Units: DMP GMAGChair: Marco Fornari, Central Michigan University
Room: Colorado Convention Center 105
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P20.00001: The ferroelectric to antiferroelectric transition in multiferroic BiFe$_{1-x}$Cr$_{x}$O$_{3}$ epitaxial films Dae Ho Kim, Ho Nyung Lee, Maria Varela, Hans M. Christen With the renewed interest in multiferroics, intensive investigations on BiFeO$_{3}$ films have enhanced the understanding of the nature of the ferroelectricity and the weak parasitic ferromagnetism. In contrast, despite having similar structural and chemical properties as BiFeO$_{3}$, little is know about BiCrO$_{3}$, due to the difficulty of synthesizing single-phase material. We have grown high quality BiCrO$_{3}$ epitaxial films by pulsed laser deposition and revealed that they exhibit antiferroelectricity with an electric- field induced ferroelectric phase. This antiferroelectricity is consistent with the picture of the Bi lone pair inducing polarization in bismuth-based perovskites. Furthermore, we have grown BiFe$_{1-x}$Cr$_{x}$O$_{3}$ solid-solution epitaxial films from BiFeO$_{3}$ and BiCrO$_{3}$ targets and observed a ferroelectric to antiferroelectric transition with increasing the Cr content. The interplay between the structural and (anti) ferroelectric properties and the role of the epitaxial strain will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P20.00002: Broadband Characterization of Multiferroic Thin-Films Nathan Orloff, Jordi Mateu, Makoto Murakami, Ichiro Takeuchi, James Booth The electromagnetic response of ferroelectric and multiferroic thin films at microwave frequencies is important for a fundamental understanding of these materials, as well for potential applications in electronics and communications systems. We explore the high-frequency response (to 40 GHz) of dielectric thin-film samples using a distributed measurement technique that utilizes patterned transmission line devices. We combine these measurements with measurements of lumped-element capacitors at lower frequencies (100 Hz - 100 MHz) to obtain true broadband measurements (100 Hz - 40 GHz) of the complex permittivity of thin film samples as a function of temperature, and electric- or magnetic-field bias. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P20.00003: Quantum Monte Carlo calculations of BiFeO3 Lucas K. Wagner, David Sulock, Lubos Mitas Multiferroic Bismuth Ferrite (BiFeO3) exhibits both ferroelectricity and antiferromagnetism, possibly enabling a connection between the two effects in the same material. While its antiferromagnetic character is relatively well-understood, experimental measurements of the spontaneous polarization vary significantly over two orders of magnitude, from 0.06 C/m$^2$ to 1.50 C/m$^2$. We cary out accurate quantum Monte Carlo calculations to estimate the cohesion energy and the ferroelectric distortion well depth. We discuss the mechanisms proposed to understand the variations of polarization experimental data in the light of our quantum Monte Carlo results. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P20.00004: Ab-initio investigation of ferroelectricity in asymmetrically layered magnetic perovskites Alison Hatt, Nicola Spaldin In an effort to combine magnetism and ferroelectricity in a single material we are motivated to explore creative routes to ferroelectricity that allow the coexistence of magnetism. In this talk we present results from an ab-initio study of a system of asymmetrically layered magnetic perovskite oxides in which the asymmetric layering should induce a ferroelectric polarization. We investigate this prediction in a model system of La(Al,Fe,Cr)O$_3$, and find that a large switchable ferroelectric polarization can indeed be obtained, although it does not originate from the asymmetric layering. We examine the forces driving polarization in this system, and propose two- and three-dimensional heteroepitaxy as a general route to stabilizing novel ferroelectrics and multiferroics. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P20.00005: Bond polarization induced by magnetic order Jung Hoon Han, Chenglong Jia, Shigeki Onoda, Naoto Nagaosa A number of recent experimental breakthroughs have revived interests in the phenomena of coupling of magnetic and electric (dipolar) degrees of freedom in a class of materials known as ``multiferroics". Some noteworthy observations include the development of dipole moments accompanying the collinear-to- helical spin ordering and adiabatic control of dipole moments through sweeping of applied magnetic fields, which all unambiguously point to the strong coupling of electric and magnetic degrees of freedom in these compounds. A number of phenomenological and microscopic theories has been advanced to establish the connection between noncollinear spin order and ferroelectricity. In particular the work of Katsura, Nagaosa, and Balatsky proposed a microscopic theory for the interplay between non- collinear magnetic order and the dipolar polarization of the electronic wave function induced by it. The magnetic (M) ion is modeled by three degenerate $t_{2g}$ levels experiencing some external magnetic field (to guarantee magnetic order) and subject to spin-orbit coupling. Two such magnetic ions are bridged by an intermediate oxygen (O) atom which itself has no spin-orbit interaction. Solving the model Hamiltonian perturbatively in the M-O hybridization amplitude, KNB finds an electronic polarization orthogonal to the M-O-M axis in the ground states of one and two holes. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P20.00006: Resonant soft x-ray scattering study of the multiferroicity in TbMn$_2$O$_5$ J. Okamoto, D. J. Huang, K. S. Chao, H.-J. Lin, C. T. Chen, C. Y. Mou, S. Park, S-W. Cheong TbMn$_2$O$_5$ is one of the fascinating multiferroic compounds whose spontaneous polarization can be controlled by applying magnetic field. Neutron diffraction measurements reported that incommensurate-commensurate transition of antiferromagnetic ordering is related to the appearance of ferroelectricity. In order to investigate the relationship between magnetic ordering and ferroelectricity associated with electronic structures of the Mn 3$d$ states, we measured soft x-ray resonant magnetic scattering of the single crystalline TbMn$_2$O$_5$ with photon energies around Mn $L_{2,3}$ edge. We observed that antiferromagnetic ordering of TbMn$_2$O$_5$ with incommensurate propagation vectors ($\frac12$$\pm \delta_x$, 0, $\frac14 $+$\delta_z$) coexists with antiferromagnetic ordering with a commensurate propagation vector ($\frac12$, 0, $\frac14$) in the ferroelectric phase (22 K $<$ T $<$ 37 K). Comparing the temperature dependence of resonant x-ray scattering and the arguments based on symmetry considerations, we discuss the magnetic ordering which leads to the magneto-electric effect in TbMn$_2$O$_5$. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P20.00007: Growth and Characterization of Low Loss BaM-BSTO Multilayer Films Jaydip Das, Arkajit RoyBarman, Carl Patton, Boris Kalinikos Ferrite/ferroelectric multilayer films are attractive as electronic materials because of the unique possibility of the electric field tuning of magnetic properties and vice versa. Up to now, however, it has not been possible to produce such layered structures with low microwave magnetic loss. The present work demonstrates the realization of pulse laser deposited low loss barium ferrite (BaM) in a BaM - barium strontium titanate (BSTO) layered film. The structure, from top to bottom, consists of a gold layer (30 nm), a polycrystalline BSTO layer (0.5 $\mu $m), another gold layer (30 nm), and a $c$-axis oriented BaM (0.5 $\mu $m) layer on a sapphire substrate. X-ray diffraction shows all components. Hysteresis loop and ferromagnetic resonance data show the properties of a low-loss $c$-axis oriented uniaxial BaM component. Capacitance measurements indicate a somewhat low but electric field tunable dielectric constant of the BSTO component. Supported in part by the ARO-MURI and ARO-DARPA-Seedling programs. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P20.00008: Ferroelectric switching induced magnetic anisotropy in Fe/BaTiO$_{3}$ bilayers Chun-Gang Duan, S. S. Jaswal, E. Y. Tsymbal Ferromagnetic/ferroelectric heterostructures have recently attracted significantly interest due to their potential applications in multifunctional electronic devices. We have recently predicted a magnetoelectric effect at the Fe/BaTiO$_{3}$ interface induced by ferroelectric polarization reversal [1]. In this report, calculations are being carried out on the magnetic anisotropy of Fe/BaTiO$_{3}$ films. Preliminary results show that the ferroelectric switching of the BaTiO$_{3}$ has appreciable effect on the magnetic anisotropy of magnetic Fe films. This should be of interest in multiferroic device applications. [1] Chun-gang Duan, S. S. Jaswal, E. Y. Tsymbal, Phys. Rev. Lett. 97, 047201 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P20.00009: Magneto-electric Coupling in Ferromagnetic Cobalt/Ferroelectric Copolymer Multi-layer Films A. Mardana, Mengjun Bai, A. Baruth, S. Ducharme, S. Adenwalla We report on the magnetoelectric coupling of a thin multi-layer film sandwich consisting of ferromagnetic Cobalt (10 nm)/ferroelectric polymer (PVDF-TrFE)/ferromagnetic Cobalt (10 nm ).The metallic ferromagnetic 1mm wide electrodes are deposited perpendicular to each other through a shadow mask. The ferroelectric polymer films (53nm thick) are deposited by the Langmuir-Blodgett technique. The ferromagnetic and ferroelectric layers of the samples have been characterized by the Magneto-Optical Kerr Effect (MOKE) and the pyroelectric effect, respectively. After electrical saturation, the sample is placed in a magnetic field perpendicular to the plane of the sample. A large magneto-electric coupling is observed, with the pyroelectric response decreasing by $\sim $ 30{\%} on application of a 2kG field. Our observations indicate that the polarization change occurs abruptly at the closing of the magnetic hysteresis loop, shows little hysteresis and is even with magnetic field. The change is far too large to be accounted for by the magnetostriction of Co. Possible explanations for this unexpectedly large effect are discussed. NSF Grant No MRSEC DMR-0213808 [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P20.00010: Natural off-stoichiometry and asymmetry in $p$/$n$-dopability of wide-gap oxides$^{1}$ S. Lany, J. Osorio-Guillen, H. Raebiger, A. Zunger Oxides such as In$_{2}$O$_{3}$ and ZnO can be doped $n$-type and are naturally anion deficient, while oxides such as NiO and Cu$_{2}$O are$ p$-type and tend to be naturally metal-deficient. Furthermore, they exhibit the property of transparent conductivity, unlike most oxides. To decipher these phenomena, we perform thermodynamic simulations based on first-principles calculated formation energies of many neutral and charged defects. We find that the metal-vacancies (and not O-interstitials) in NiO and Cu$_{2}$O are responsible for their simultaneous metal deficiency and $p$-type conductivity. The O-deficiency of In$_{2}$O$_{3}$ and ZnO is caused by the O-vacancy V$_{O}$ (and not the metal interstitials). Since V$_{O}$ has a \textit{deep} level in the gap, it does not provide for equilibrium stable $n$-type conductivity. We suggest, however, that a metastable state of V$_{O}$ in In$_{2}$O$_{3}$ and ZnO can cause persistent photoconductivity, and can explain the paradoxical coexistence of coloration (deep absorption level in the optical range) and conductivity (shallow level), which is observed after metal rich growth. By calculating the band offsets, we further show that the $p$-type dopability of NiO is facilitated by the relative high energy of the valence band maximum, while the $n$-type dopability of ZnO is facilitated by the relative low energy of the conduction band minimum. \newline $^{1}$Funded by DOE-BES under contract DE-AC36-99GO10337 [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P20.00011: Raman scattering studies of resistance-changing NiO films with and without IrO$_{2}$ buffer layers S. Yoon, E. Cho, H. Cheong, S. Seo, B. Schulz, M. Ruebhausen NiO films are known to exhibit resistive memory switching behavior and inserting thin IrO$_{2}$ layers between electrodes and the NiO film is claimed to minimize the dispersion of memory switching parameters, thus greatly improving the device properties. We present Raman scattering results of a NiO film, a NiO film with a 20 nm-thick IrO$_{2}$ layer, and a NiO film with a 50 nm-thick IrO$_{2}$ layer. We discuss the microscopic structural changes in the three different films and their relations to the switching behavior changes. We also discuss the role of IrO$_{2}$ buffer layers in the device structures. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P20.00012: Molecular Beam Epitaxy of MgO on Perovskite Substrates M. Snyder, J. Xu, P. Fisher, M. Skowronski, P. Salvador, O. Maksimov, V. Heydemann Rock salt oxides are promising interface layer materials for the integration of multifunctional oxides with semiconductors (Si, SiC, and GaN). Although rock salt oxides were previously grown on a wide range of semiconductor (Si and GaAs) and oxide (LaAlO$_{3}$ and SrTiO$_{3})$ substrates, the influence of lattice mismatch on the crystalline quality of the films was not studied. MgO thin films were grown by molecular beam epitaxy on LSAT, LaAlO$_{3}$ and SrTiO$_{3}$ perovskite substrates to investigate the effects of lattice mismatch on the film crystal quality. Despite a lattice mismatch of $\sim $7.9{\%} and $\sim $9{\%}, respectively, epitaxial growth of MgO was achieved on SrTiO$_{3}$ and LSAT substrates. Films grown on LaAlO$_{3}$ substrates exhibiting a lattice mismatch of $\sim $10.5{\%} were polycrystalline, yet epitaxial MgO on LaAlO$_{3}$ was deposited after the introduction of a SrTiO$_{3}$ buffer layer. The effects of deposition rate, substrate temperature, ozone flux, SrTiO$_{3 }$buffer layer thickness and stoichiometry were also investigated. This work was supported by the Office of Naval Research under grants N00014-05-1-0238 and N00014-06-1-1018. [Preview Abstract] |
Session P21: Computational Methods for Strongly Correlated Systems and Many Body Theory
Sponsoring Units: DCOMPChair: Adriana Moreo, University of Tennessee and Oak Ridge National Laboratory
Room: Colorado Convention Center 106
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P21.00001: Spectral weight of the Emery model within different computational schemes Simone Chiesa, Jan Kunes, Warren Pickett, Richard Scalettar Although the single band Hubbard model captures many important aspects of the phenomenology of the high-temperature superconductors, the three-band Emery model allow the study of additional effects associated with the transfer of charge between the copper and oxygen orbitals and the strong hole repulsion at the oxygen sites. Here we present a comparison of the integrated and angle resolved spectral weight using exact diagonalization, dynamical mean field theory (with a quantum Monte Carlo solver and MaxEnt), and determinantal quantum Monte Carlo in the hole doped regime. [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P21.00002: Equation of state in single-band Hubbard models in two- and three-dimensions Chia-Chen Chang, Shiwei Zhang We present results on ground-state energetics of the single-band Hubbard model in two- and three-dimensions with nearest-neighbor hopping and repulsive contact s-wave (on-site) interaction. Our calculations are done with the constrained-path auxiliary-field quantum Monte Carlo method. By incorporating generalized boundary conditions, we reduce finite-size effects due to open and closed shell filling and finite simulation cells. Results are obtained for the kinetic, interaction, and total energies and extrapolated to the thermodynamic limit for a range of interaction strengths ($U/t$) and electron densities. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P21.00003: Does the $t^{'}{-}t{-}J$ model catch the main features of the cuprates phase diagram? Leonardo Spanu, Massimo Lugas, Federico Becca, Sandro Sorella Using the Green's Function Monte Carlo Technique (GFMC), we investigate the effects of the $t^{'}$ interaction on the phase diagram of the $t{-}J$ model and its possible relevance for the physics of high-temperature superconductors (HTcS). In practice, we consider a very accurate guiding wave function including both magnetic and superconducting order parameters, as well as long-range Jastrow factors, in order to reproduce the correct low-energy spin and charge excitations. The $t^{'}$ interaction induces a suppression of the antiferromagnetic order parameter for hole concentration $\delta \sim 3-4\% $ (for $t^{'}=-0.2t$ and $J/t=0.2$), while the paramagnetic phase is characterized by an incommensurate peak in the spin structure factor. The inclusion of the $t^{'}$ term allows one to strongly suppress superconductivity at small doping, i.e., for $\delta < 6\% $. On the contrary, away from the antiferromagnetic phase, d-wave pairing correlations are enhanced up to the optimally doping region ($\delta \sim 20\%$) Our results then indicate that the $t^{'}{-}t{-}J$ model, though it is a very simple and crude approximation of realistic materials, is able to capture important properties of the HTcS phenomenology [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P21.00004: Fermionic functional renormalization group flows into phases with broken symmetry Roland Gersch, Carsten Honerkamp We describe how functional renormalization group flows for interacting fermions can be continued into phases with broken symmetries. A symmetry-breaking term in the initial condition for the self-energy prevents a true divergence of the interactions at the critical scale. At the same scale, the anomalous self-energy grows rapidly such that the flow can be followed down to zero scale and all modes can be integrated out. Within simple mean-field models, we demonstrate two versions of this idea: one where the initial symmetry breaking is sent to zero, and another where it is compensated by a counter-term. The latter scheme is capable of detecting symmetry-broken phases separated from the symmetric state by an energy barrier. We discuss generalizations to more realistic models. Refs.: M. Salmhofer et al., Prog. Theor. Phys. 112, 943 (2004); R. Gersch et al., Euro. Phys. J. B 48: 349 (2005); R. Gersch et al., cond-mat/0609520. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P21.00005: Variational reduced-rensity-matrix theory applied to the hubbard model Jeff Hammond, David Mazziotti The application of variational reduced-density-matrix theory to the Hubbard model will be described. Recent results [Physical Review A 73, 062505 (2006)] demonstrate that computationally efficient N-representability conditions produce accurate ground-state energies and reduced-density-matrices for a wide range of interaction strengths for the one-dimensional lattice. I will discuss various types of N-representability conditions, the relationship between symmetries and reduced-density-matrices, and application of this method to other strongly correlated models. Preliminary results for the two-dimensional Hubbard model will be presented. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P21.00006: Functional renormalization group and bosonization as a solver for 2D fermionic Hubbard models Florian Schuetz, Brad Marston The functional renormalization group (fRG) provides an unbiased framework to analyze competing instabilities in two-dimensional electron systems and has been used extensively over the past decade [1]. In order to obtain an equally unbiased tool to interprete the flow, we investigate the combination of a many-patch, one-loop calculation with higher dimensional bosonization [2] of the resulting low-energy action. Subsequently a semi-classical approximation [3] can be used to describe the resulting phases. The spinless Hubbard model on a square lattice with nearest neighbor repulsion is investigated as a test case. [1] M. Salmhofer and C. Honerkamp, Prog. Theor. Phys. 105, 1 (2001). [2] A. Houghton, H.-J. Kwon, J. B. Marston, Adv.Phys. 49, 141 (2000); P. Kopietz, Bosonization of interacting fermions in arbitrary dimensions, (Springer, Berlin, 1997). [3] H.-H. Lin, L. Balents, M. P. A. Fisher, Phys. Rev. B 56, 6569–6593 (1997); J. O. Fjaerestad, J. B. Marston, U. Schollwoeck, Ann. Phys. (N.Y.) 321, 894 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P21.00007: Effects of electron-phonon coupling on the d-wave pairing superconducting phase Ka-Ming Tam, Shan-Wen Tsai, Antonio H. Castro Neto, David K. Campbell Recent experimental evidence has shown that the electron-phonon coupling could play a role in the formation of a d-wave pairing superconducting phase. Using a multiscale functional renormalization group (MFRG) technique, we study the effects of electron-phonon coupling in the two-dimensional Hubbard model with a band structure appropriate for the cuprate superconductors. We show that a momentum-independent electron- phonon coupling does not favor d-wave pairing but instead leads to the s-wave pairing and incommensurate density wave ordering. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P21.00008: Sum-rule Conserving Spectral Functions from the Numerical Renormalization Group Andreas Weichselbaum, Jan von Delft We show how spectral functions for quantum impurity models, i.e. nanosystem embedded in fermionic or bosonic environment, can be calculated very accurately using a complete set of ``discarded'' numerical renormalization group (NRG) eigenstates, recently introduced by Anders and Schiller. The only approximation is to judiciously exploit energy scale separation. Our rigorous derivation avoids both the overcounting ambiguities and the single-shell approximation for the equilibrium density matrix prevalent in current methods including state of the art DM-NRG. The resulting procedure based on the full density matrix of the system (FDM-NRG) ensures that relevant sum rules hold rigorously and spectral features at energies below the temperature can be described accurately. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P21.00009: New diagrammatic approach to the steady-state transport: nonlinear thermoelectric effects in interacting systems Jong Han, Ryan Heary Steady-state nonequilibrium described by a Gibbsian ensemble $e^{-\beta(H-Y)}$ with the boundary condition operator $Y$ is shown to be equivalent to the Keldysh formulation, through an explicit perturbation calculation of Anderson impurity model. We also show that the diagrammatics can be significantly simplified in the steady-state problems with a single real-time contour, in contrast to the double-contour Keldysh method. We apply this method to a quantum dot system in the Anderson impurity model with finite chemical potential bias and finite temperature gradient across the source-drain leads. We discuss the nonlinear nonequilibrium behavior of the Kondo resonance caused by strong potential and temperature bias. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P21.00010: Density Matrix Renormalization Group study of magnon bound states of Heisenberg S=1 Y-junctions Haihui Guo, Steven White Systems of Y-junctions are interesting both from a fundamental viewpoint and because of their potential use in nanoscale devices. Here we present a numerical study of S=1 Heisenberg model Y-junctions using a recently developed Y-junction DMRG algorithm[1]. We will focus on the question of the existence of magnon bound states at the junction, as a function of junction geometry and interaction parameters. \newline \newline [1] Haihui Guo and Steven R. White, Phys. Rev. B 74, 060401 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P21.00011: A new warm-up procedure for the density-matrix renormalization group Masaki Tezuka A density-matrix renormalization group (DMRG) calculation starts with the infinite-system algorithm (the warm-up stage), where the system size $l$ is enlarged by adding new sites in the middle, which is then fed into the finite algorithm where the cut location is moved back and forth to enhance accuracy. Usually a considerable proportion of total calculation time has to be spent on the infinite algorithm, before the finite-size sweeps can be started. This is because at each step the target wavefunction for a different $l$ has to be calculated by some numerical diagonalization technique, and it is more difficult to give a good initial vector than in the finite-size algorithm where $l$ is constant. Here we propose a new infinite algorithm procedure where one value of $l$ is used to provide several blocks with different numbers of sites, which in fact dramatically reduces the overall computational time. This is demonstrated for various models such as the Hubbard model. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P21.00012: A DMRG study of transport properties and correlations of quantum dots Fabian Heidrich-Meisner, Khaled Al-Hassanieh, Elbio Dagotto, George Martins, Adrian Feiguin We study transport through quantum dots using the time-dependent density matrix renormalization group method (tDMRG), recently proposed as a powerful computational tool to investigate transport through interacting nanostructures [1]. Since this technique relies on the numerical solution of finite clusters, we analyze the finite-size dependence of both static properties such as spin and charge fluctuations, spin-spin correlations and the conductance in detail, focusing on the example of one quantum dot. Our study reveals a crucial influence of global quantum numbers of finite clusters such as total spin on the results of tDMRG simulations, reflected in even-odd effects. We further establish a connection between the size of charge fluctuations on the quantum dot and the convergence of tDMRG with system size. Similar substantial even-odd effects exist within the framework of another technique, the embedded cluster approximation method (ECA). For the example of three quantum dots, we show that such even-odd effects strongly affect the spin fluctuations, leading to qualitatively different results for the conductance within ECA. [1] Al-Hassanieh et al., Phys. Rev. B 73, 195304 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P21.00013: Quadratic scaling \textit{ab initio} DMRG for strong nondynamic correlation Johannes Hachmann, Wim Cardoen, Garnet Kin-Lic Chan We have devised a quadratic scaling \textit{ab initio} Density Matrix Renormalization Group (DMRG) algorithm for large, linear systems (such as unbranched polymers and long molecules) [1]. It is particularly suited for the description of strong active-space nondynamic correlation. This new local method (LDMRG) is inherently multireference, compact, variational, size-consistent and size-extensive. The reduced scaling is achieved solely through integral screening and without the artificial construction of correlation domains. Due to the multireference nature of the ansatz, we also do not require restricted localization in the occupied and virtual subspaces. Numerically exact (FCI) correlated energies (in a single-zeta 1-particle basis) up to 1-10$\mu $E$_{h}$ accuracy for systems with up to 100 electrons in 100 active orbitals (i.e. determinant spaces up to dimension 10$^{58})$ are presented. We also demonstrate the performance of the method in the study of the challenging metal-insulator transition in hydrogen-chains. We can now study nondynamic correlation in interesting classes of chemical systems, such as organic (opto-) electronic materials [2], or non-repeating chain-like molecules such as unfolded peptide backbones. [1] Hachmann, Cardoen, Chan, \textit{JCP} 125 (\textbf{2006}), 144101. [2] Hachmann, Dorando, Avil\'{e}s, Chan, \textit{in preparation}. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P21.00014: Quantum Optimization: Spin Glasses and Wavefunction Annealing Javier Rodriguez-Laguna, Giuseppe Santoro The density matrix renormalization group (DMRG) has been extended in order to analyse the quantum spin glass transition (QSGT) for a random Ising model in a transverse field --$\Gamma$-- on a random graph with fixed connectivity $K=3$. The system is solved easily for a high value of $\Gamma$, and the wavefunction is {\em annealed} decreasing it slowly until the transition is reached. This way, the QSGT has been characterized in detail. A further decrease of $\Gamma$, down to $\Gamma=0$, allows to obtain the solution of the classical minimization problem associated, thus providing a possible alternative route to quantum annealing methods. Reference: J. Rodriguez-Laguna, G.E. Santoro, {\em Quantum Spin Glass Transition: the Ising model on random graphs}, submitted to Phys. Rev. B. ArXiv: {\tt cond-mat/0610661} (2006). [Preview Abstract] |
Session P22: Focus Session: Social Dynamics and Scaling
Sponsoring Units: GSNPChair: Sidney Redner, Boston University
Room: Colorado Convention Center 108
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P22.00001: The impact of social network complexity: from collaboration teams to epidemics Invited Speaker: Recent years have witnessed a tremendous progress in the gathering of large scale social networks thanks to the development of new informatics tools and the increase in computational power. Networks which trace the activities and interactions of individuals, social patterns, transportation fluxes and population movements on a local and global scale have been analyzed and found to exhibit complex features encoded in large scale heterogeneity, self-organization and other properties typical of complex systems. We will review the complex features characterizing many of these networks and their impact on dynamical processes ranging from the establishment of collaboration teams and the emergence of consensus to the geographical behavior of large scale epidemics. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P22.00002: Modeling self-organization of communication and topology in Social Networks Kim Sneppen We introduce a model of self-organization of communication and topology in social networks with a feedback between different communication habits and the topology. To study this feedback, we let agents communicate to build a perception of a network and use this information to create strategic links. We observe a narrow distribution of links when the communication is low and a system with a broad distribution of links when the communication is high. We also analyze the outcome of chatting, cheating, and lying, as strategies to get better access to information in the network. Chatting, although only adopted by a few agents, gives a global gain in the system. Contrary, in a system with too many liars a global loss is inevitable. \newline \newline References: M. Rosvall and K. Sneppen. ``Modeling self-organization of communication and topology in social networks.'' Phys. Rev. E 74:16108 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P22.00003: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P22.00004: Large-scale Individual-based Models of Pandemic Influenza Mitigation Strategies Kai Kadau, Timothy Germann, Ira Longini, Catherine Macken We have developed a large-scale stochastic simulation model to investigate the spread of a pandemic strain of influenza virus through the U.S. population of 281 million people, to assess the likely effectiveness of various potential intervention strategies including antiviral agents, vaccines, and modified social mobility (including school closure and travel restrictions) [1]. The heterogeneous population structure and mobility is based on available Census and Department of Transportation data where available. Our simulations demonstrate that, in a highly mobile population, restricting travel after an outbreak is detected is likely to delay slightly the time course of the outbreak without impacting the eventual number ill. For large basic reproductive numbers R$_{0}$, we predict that multiple strategies in combination (involving both social and medical interventions) will be required to achieve a substantial reduction in illness rates. [1] T. C. Germann, K. Kadau, I. M. Longini, and C. A. Macken, Proc. Natl. Acad. Sci. (USA) \textbf{103}, 5935-5940 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P22.00005: Dynamics of epidemics outbreaks in heterogeneous populations Dirk Brockmann, Alejandro Morales-Gallardo, Theo Geisel The dynamics of epidemic outbreaks have been investigated in recent years within two alternative theoretical paradigms. The key parameter of mean field type of models such as the SIR model is the basic reproduction number $R_0$, the average number of secondary infections caused by one infected individual. Recently, scale free network models have received much attention as they account for the high variability in the number of social contacts involved. These models predict an infinite basic reproduction number in some cases. We investigate the impact of heterogeneities of contact rates in a generic model for epidemic outbreaks. We present a system in which both the time periods of being infectious and the time periods between transmissions are Poissonian processes. The heterogeneities are introduced by means of strongly variable contact rates. In contrast to scale free network models we observe a finite basic reproduction number and, counterintuitively a smaller overall epidemic outbreak as compared to the homogeneous system. Our study thus reveals that heterogeneities in contact rates do not necessarily facilitate the spread to infectious disease but may well attenuate it. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P22.00006: The effect of heterogeneity in infectivity and susceptibility on epidemic spread Joel Miller We consider the spread of an epidemic on a network with few short cycles. We develop analytical tools to determine the probability and final size of an epidemic when the infectiousness and/or susceptibility of individuals is heterogeneous. Using these tools, we find the distributions of infectiousness or susceptibility which maximize or minimize the size or probability of an epidemic [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P22.00007: Phase diagram of the diffusive epidemic process Ronald Dickman, Daniel Souza Maia We study the absorbing-state phase transition in the one- dimensional diffusive epidemic process via mean-field theory and Monte Carlo simulation. In this model, particles of two species (A and B) hop on a lattice and undergo reactions B $\to$ A and A + B $\to$ 2B; the total particle number is conserved. A phase transition between the (absorbing) B-free state and an active state is observed as the parameters (reaction and diffusion rates, and total particle density) are varied. Mean-field theory reveals a surprising, nonmonotonic dependence of the critical recovery rate on the diffusion rate of B particles. A computational realization of the process faithful to the master equation the model is devised. Using the quasi-stationary simulation method we determine the order parameter and the survival time in systems of up to 4000 sites. Due to strong finite-size effects, the results converge only for large system sizes. We find no evidence for a discontinuous transition. Our results are consistent with the existence of three distinct universality classes, depending on whether A particles diffusive more rapidly, less rapidly, or at the same rate as B particles. We also perform quasi-stationary simulations of the triplet creation model, which yield results consistent with a discontinuous transition at high diffusion rates. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P22.00008: Epidemics on dynamic networks with spatial structure Leah Shaw, Ira Schwartz When a population is faced with an epidemic outbreak, individuals are likely to modify their social behavior to avoid exposure to the disease. Epidemic models that assume a fixed network of contacts do not address this phenomenon. We consider an extension of the model of Gross et al (PRL 96: 208701, 2006), in which the contact network is rewired dynamically so that susceptibles avoid contact with infectives. We add a spatial structure to the network and explore both the network geometry and the dynamics of the infection. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P22.00009: Proximity Networks and Epidemics Hasan Guclu, Zolt\'an Toroczkai We presented the basis of a framework to account for the dynamics of contacts in epidemic processes, through the notion of dynamic proximity graphs. By varying the integration time-parameter $T$, which is the period of infectivity one can give a simple account for some of the differences in the observed contact networks for different diseases, such as smallpox, or AIDS. Our simplistic model also seems to shed some light on the shape of the degree distribution of the measured people-people contact network from the EPISIM data. We certainly do not claim that the simplistic graph integration model above is a good model for dynamic contact graphs. It only contains the essential ingredients for such processes to produce a qualitative agreement with some observations. We expect that further refinements and extensions to this picture, in particular deriving the link-probabilities in the dynamic proximity graph from more realistic contact dynamics should improve the agreement between models and data. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P22.00010: Adaptive networks: the example of consensus formation. Balazs Kozma, Alain Barrat It is well known that the structure of a network can significantly influence the properties of the dynamical processes on them. Though, the interplay between a process and the network topology on adaptive networks is still an open question. Adaptive rewiring of links can happen in real life systems such as acquaintance networks where two people are more likely to maintain a social connection if their views and values are similar. Similar adaptation should also be observed in biological and ecological networks. In our study, we consider various systems modeling the consensus formation of people and try to identify the quantities that are relevant in determining the behavior of adaptive networks. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P22.00011: Social network analysis based on WWW search engine Sang Hoon Lee, Pan-Jun Kim, Yong-Yeol Ahn, Hawoong Jeong Recently, massive digital records have made it possible to analyze a huge amount of data in social sciences, one of which is social network theory. We investigate social networks between people by extracting information on the World Wide Web. Using famous search engines such as Google, we construct weighted social networks where the nodes are the names of people and the weight of each link is assigned as the number of web pages including both of the names attached to the link. The weight distribution is found to be quite broad with the heavy-tail. The strength of a node, defined as the sum of weights over the node, is strongly correlated with the number of web pages including the single node. We compare networks constructed by this method with real networks to test the reliability of the method. Furthermore, we suggest the quantity, called the effective degree, characterizing the homogeneity (or heterogeneity) of weight distribution for each node in the weighted network. Another way to quantify the importance of each node, based on the effective degree, is also introduced. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P22.00012: The voter model on an adaptive network. Beate Schmittmann, Izabella Benczik, Royce K.P. Zia, Sandor Benczik In social networks, friendships emerge and fade, as individuals change their opinions. We discuss a simple model of such a network, in which the individuals are modeled by Ising spins (taking just two values: up or down) on the nodes of the network, while their connections are modeled by the presence or absence of edges. Nodes and edges evolve simultaneously. The spins are updated according to a simple majority rule (the voter model). Then, any pair of spins is then connected by an edge with probability p (q) if they are in the same (different) state. Thus, the edges also become dynamic variables, correlated with the state of the nodes, and the network is termed ``adaptive.'' Using simulations and exact solutions, we find four phases in the thermodynamic limit. There are two absorbing states in which all nodes are in the same state (all up or down). Then, there is a disordered phase in which the nodes take random values, and a phase in which the system remembers its initial magnetization. For finite systems, only the two absorbing states survive in the long-time limit. Consequences for social networks will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P22.00013: Resolution limit in community detection Marc Barthelemy, Santo Fortunato Understanding the relation between structure and function in a complex network is a fundamental issue for practical applications in many disciplines such as biology or sociology. An important step in this direction has been made with the identification of communities through a now widely used method relying on the optimization of a quantity called modularity. However, we will show here that modularity optimization fail to identify modules smaller than a scale which depends on the total size of the network and on the degree of interconnectedness of the modules, even in cases where modules are unambiguously defined. We will illustrate this with simple examples taken both in artificial and in real social, biological and technological networks for which we show that modularity optimization indeed does not resolve a large number of modules. Reference: S. Fortunato and M. Barthelemy, PNAS, in press. [Preview Abstract] |
Session P23: Structural Materials and Hydrogen
Sponsoring Units: DCMPChair: David Singh, Oak Ridge National Laboratory
Room: Colorado Convention Center 110
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P23.00001: Thin Film Synthesis and Characterization of MAX-Phase Compounds T. H. Scabarozi, W. Tambussi, J. D. Hettinger, S. E. Lofland, M. W. Barsoum We present the synthesis and characterization of thin film MAX-phase compounds. Thin film synthesis was performed by magnetron sputtering from compound and elemental target materials on 2-inch c-axis sapphire wafers. A series of experiments were carried out where parameters of temperature, gas flow, pressure, and cathode power were varied. Films were characterized by Raman spectroscopy, electron microscopy, X-ray diffraction, and atomic force microscopy. Most films were readily synthesized with multiple-phases which were hexagonal or cubic. All phases were epitaxial, with growth along the (000l) and (111) direction for hexagonal and cubic compounds, respectively. With careful control of temperature and stoichiometry, single phase films were produced. Surprisingly, we synthesized Ti$_{2}$AlC by diffusion of Al from the substrate while only sputtering Ti and C. [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P23.00002: Composition Dependence of Elastic properties in M$_{2}$AX Materials T. Scabarozi, S. E. Lofland, J. D. Hettinger, S. Amini, P. Finkel, M. Barsoum We report on correlations between thermal expansion, elastic modulii, thermal transport, specific heat, and electrical transport measurements of materials within the MAX-phase family. Elastic modulus measurements are made using an ultrasonic time of flight technique. Thermal expansion measurements are made using high-temperature x-ray diffractions. We see a clear variation in elastic properties in materials of the form M$_{2}$AC(M=Ti) with A=S having the largest elastic modulus of all M$_{2}$AX materials measured to date. It also has the largest Debye temperature as measured from specific heat. The phonon contribution to the thermal conductivity is relatively large, similar in size to the thermal transport resulting from charge carriers. The elastic modulus approaches that found in Ti$_{3}$SiC$_{2}$, a M$_{3}$AX$_{2}$ material. The overall goal of this work is to correlate measurements of these properties varying M and A to unravel the role of both lattice constituents in determining the elastic properties of this class of materials. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P23.00003: Structural phase transformations in Ti$_{3}$Al$_{2}$Nb system, a first-principles approach Mahdi Sanati, Damien West First-principles method is employed to determine the vibrational entropy and Gibbs free energy as a function of temperature of the homogenous Ti$_{3}$Al$_{2}$Nb system. Calculated energies at T=0 K show instabilities in ternary B2 Ti$_{3}$Al$_{2}$Nb alloy against the $\omega $ and B8$_{2}$ structures. We show that at high temperatures the B2 phase is stabilized by the vibrational entropy. The transition temperatures for B2 $\to \quad \omega $ and B2 $\to $ B8$_{2}$ have been calculated and are in excellent agreement with experiment. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P23.00004: Precipitation behavior of sigma phase in 304 and 430 stainless steels as hot-rolled at 800$^{\circ}$C Chih-Chun Hsieh, Dong-Yih Lin, Weite Wu The effect of various reduction ratios on the precipitation of sigma phase in 304 and 430 stainless steels as hot-rolled at 800$^{o}$C have been investigated in this study. The sigma phase showed a dendrite --like morphology in the as received materials. A hot rolling process changed the morphology of sigma phase from dendrite-like to globular, especially at higher reduction ratio. The amounts of sigma phase in the stainless steels increased gradually at 800$^{\circ}$C with the increasing the reduction ratios from 0 to 75{\%}. The XRD analyses showed that a higher reduction ratio also enhanced the conversion of the delta (110) to sigma (542). [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P23.00005: Intrinsic mechanism of the solid solution softening and hardening in bcc transition metal alloys: combined first principles calculations with atomic row modeling N. I. Medvedeva, Yu. N. Gornostyrev, A. J. Freeman The solute--dislocation interaction is of great interest since it determines one of the important strengthening mechanisms in alloys. It is still unclear why some alloying elements lead to hardening but others give rise to softening at low temperature. To reveal the intrinsic mechanisms in solution softening/hardening, the interaction of $d$ transition metal additions with dislocations in bcc metals was studied by using a combined approach including the atomic row model with {\it ab-initio} parametrization of interatomic interactions. We found opposite trends in solute-dislocation interaction for the Groups V and VI bcc metals. Additions with extra valence electrons, which enhance the double kink nucleation and result in softening in the Group VI metals, cannot lead to softening in the Group V metals and vice versa; additions with fewer electrons may give rise to softening in the Group V metals but lead to strong hardening in the Group VI metals. We demonstrate that the electronic structure, rather than atomic size or shear modulus misfits, plays an important role in the softening/hardening effects in bcc transition metal alloys. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P23.00006: Phase field model for recrystallization kinetics S. Sreekala, Mikko Haataja In the recrystallization process, dislocation-free grains grow at the expense of highly deformed matrix. We introduce a phase-field model to study the isothermal recrystallization process as a phase transformation driven by the stored elastic energy which is explicitly non-local due to the long-ranged dislocation strain fields. The dislocations are represented by a coarse-grained Burgers vector density in two spatial dimensions. We have used this model to study the influence of several spatially distinct dislocation distributions (random, cellular and power-law correlated) on the growth kinetics of the recrystallized grain. Our results show that random dislocation distribution produces isotropic growth, whereas the other two distributions show anisotropic and irregular growth as seen in experiments. Also, the growth rate for the random and power-law correlated dislocation distribution follows the JMAK theory rather closely, while the highly anisotropic cell structure shows significant deviations.~ We demonstrate that the deviations arise from the presence of non-local elastic strain fields. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P23.00007: Possible Mechanism of the Pseudogap Formation in Intermetallic Compound Al$_3$V Susumu Miyahara, Kazuo Tsumuraya The pseudogaps in compounds give a unique electronic character in the materials such as the ones in the skutterudites. The Al$_3$V compound with DO$_{22}$ structure has also a deep pseudogap that has been explained by the presence of the covalent Al-V and Al- Al bonds in the compound [1]. We propose another possibility of the formation of the pseudogap in the DO$_{22}$ compound using a density functional method. We introduce a Peierls distortions along c-axis of the double-stacked ordered unit cell, we calculate the density of states and check the gap formation. We will apply the mechanism to the other systems with pseudogap. [1] M. Kraj\v{c}\'{\i} and J. Hafner, J. Phys.: Condens. Matter, 14, 1865 (2002). [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P23.00008: Thermal expansion and magnetostriction in RAl$_3$ (R = Tm,Yb,Lu) single crystals S.L. Bud'ko, J. Frederick, P.C. Canfield, G.M. Schmiedeshoff We present temperature dependent thermal expansion and low temperature longitudinal magnetostriction measurements taken using a capacitance dilatometer [G.M. Schmiedeshoff et al., RSI, in press] in a PPMS-14 instrument for several cubic RAl$_3$ (R = rare earth) compounds. Quantum oscillations in the magnetostriction were observed in LuAl$_3$ and YbAl$_3$, including few new frequences for the latter. Data on qualitative changes in TmAl$_3$ thermal expansion in presence of the longitudinal magnetic field will be presented and discussed. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P23.00009: Theoretical Prediction of Activities in Dilute .$\gamma $-Ni(Al) Solid Solution at Elevated Temperatures Yong Jiang, John Smith, Anthony Evans For the prediction and interpretation of high-temperature diffusion and related phenomena in multiplayer systems, knowledge of thermodynamic activities is essential. For example, interfacial structures and adhesion strengths of .$\gamma $-Ni(Al)/Al2O3 at elevated temperatures strongly depend on Al activities. In this study, we present a method for predicting activity coefficients and hence activities in dilute .$\gamma $-Ni(Al) solid solutions from first-principles. Both thermal lattice vibration and electronic contributions to free energies are considered and compared. Vibrational contributions tend to dominate the temperature dependencies of the free energies: though electron thermal effects are significant. Calculations show opposing temperature trends for the formation enthalpies and entropies, leading to a partial cancellation of their role in the overall energetics. Nevertheless, their remaining temperature effects are strong. Over the temperature range, 400 K $<$ T $<$ 1700 K, the Al activity coefficient varies by 15 orders of magnitude, due to the relative strength of Al-Ni and Al-Al bonds. The Ni activity coefficient only varies less than 4{\%} over the same range. Calculational results compare well with available experimental data. The thermodynamic principles elucidated from the calculations are used to provide a fundamental interpretation. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P23.00010: Ab Initio Study of the Effect of Solute Atoms on Stacking Fault Energy in Aluminum Yue Qi, Raja Mishra The stacking fault energy (SFE) in binary and ternary alloys of Al with common alloying elements was studied using density function theory. Among these alloying elements, Fe further increases the SFE and Ge reduces the SFE of Al. The elements increasing the directional inhomogeneity in the electronic charge distribution of the FCC structure correlates with the increasing SFE. The maximum value of charge difference on the fault plane, Max($\Delta \rho )$, is used to characterize how much electron has been redistributed due to the stacking fault formation, and the SFE monotonically increases with Max($\Delta \rho )$. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P23.00011: Thermal stability and diffusion of defects on an Al(100) Francesca Baletto, Nicola Marzari Understanding and controlling homoepitaxial metal-on-metal growth is a key challenge in surface physics due to its great technological interest. We use a combination of classical and ab-initio techniques, including molecular dynamics and transition-state finding, to identify the dominant mechanisms of diffusion for adatoms and vacancies on an Al(100) surface. We find that exchange and concerted exchange mechanisms, well known in adatom diffusion, play also a central role in vacancy diffusion. In addition, they lead to a distinct phase transition localized in the first layer as temperature is raised. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P23.00012: Phonons in nickel and aluminum at elevated temperatures from neutron scattering Max Kresch, Olivier Delaire, Rebecca Stevens, Jiao Lin, Brent Fultz Measurements of the neutron scattering from elemental nickel were made at 10\,K, 300\,K, 575\,K, 875\,K and 1275\,K, and from elemental aluminum at 10\,K, 150\,K, 300\,K, 525\,K and 775\,K. From the scattering, the phonon densities-of-states (DOS) were calculated, and subsequently fit to Born von K\'{a}rm\'{a}n models of the lattice dynamics. Comparing to previous measurements of thermal expansion, and elastic moduli, we found a small, negative anharmonic contribution to the entropy in both cases. For nickel, we used this to place new bounds on the high temperature magnetic entropy. In both metals, the DOS displayed significant broadening at elevated temperatures. This anharmonic broadening was quantified, and shown to increase approximately as T$^2$. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P23.00013: A first principles calculation and statistical mechanics modeling of defects in Al-H system Min Ji, Cai-Zhuang Wang, Kai-Ming Ho The behavior of defects and hydrogen in Al was investigated by first principles calculations and statistical mechanics modeling. The formation energy of different defects in Al+H system such as Al vacancy, H in institution and multiple H in Al vacancy were calculated by first principles method. Defect concentration in thermodynamical equilibrium was studied by total free energy calculation including configuration entropy and defect-defect interaction from low concentration limit to hydride limit. In our grand canonical ensemble model, hydrogen chemical potential under different environment plays an important role in determing the defect concentration and properties in Al-H system. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P23.00014: Elastic Constants of Rare Earth and Transition Metal Di-Hydrides C. S. Snow, J. A. Knapp, J. F. Browning Determinations of the elastic constants of rare earth (RE=Er, La,) and some transition metal (TM=Sc, Ti, Zr) di-hydrides are extremely difficult. Single crystals of these di-hydrided metals can not be obtained because they break up into fine powders due to the large stresses in the materials caused by the crystallographic changes upon hydriding. However, polycrystalline thin films of these hydrided materials can be grown and are stable over a wide temperature and pressure range. In order to determine the elastic constants of thin metal di-hydride films ab-initio electronic structure calculations using the VASP code have been carried out. These calculations are then compared to bulk and shear moduli measured by a nano-indentation technique. Details and results of the calculations and measurements of the elastic constants of rare earth and transition metal di-hydride films will be presented and a discussion of future applications of this technique will be given. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P23.00015: Ab initio electronic and lattice dynamical properties of cerium dihydride Tanju Gurel, Resul Eryigit The rare-earth metal hydrides are interesting systems because of the dramatic structural and electronic changes due to the hydrogen absorption and desorption. Among them, cerium dihydride (CeH$_{2}$) is one of the less studied rare-earth metal-hydride. To have a better understanding, we have performed an ab initio study of electronic and lattice dynamical properties of CeH$_{2}$ by using pseudopotential density functional theory within local density approximation (LDA) and a plane-wave basis. Electronic band structure of CeH$_ {2}$ have been obtained within LDA and as well as GW approximation. Lattice dynamical properties are calculated using density functional perturbation theory. The phonon spectrum is found to contain a set of high-frequency ($\sim$ 850-1000 cm$^{-1}$) optical bands, mostly hydrogen related, and low frequency cerium related acoustic modes climbing to 160 cm$^ {-1}$ at the zone boundary. [Preview Abstract] |
Session P24: Focus Session: Organic Heterojuncture Photovoltaics
Sponsoring Units: DPOLY DMPRoom: Colorado Convention Center 201
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P24.00001: Organic Semiconductors: A Molecular Picture of the Charge-Transport and Energy-Transport Processes. Invited Speaker: Conjugated organic oligomer and polymer materials are being increasingly considered for their incorporation as the active semiconductor elements in devices such as photo-voltaic cells, light-emitting diodes, or field-effects transistors. In the operation of these devices, electron-transfer and energy-transfer processes play a key role, for instance in the form of charge transport (in the bulk or across interfaces), energy transport, charge separation, or charge recombination [1]. Here, we provide a theoretical description of electron-transfer phenomena based on electron-transfer theory, which allows us to provide a molecular, chemically-oriented understanding. In this presentation, we focus on the parameters that impact the mobility of charge carriers [2], that is the electronic coupling within chains and between adjacent chains and the reorganization energy of the chains upon ionization. Materials under study include conjugated oligomers such as oligoacenes, oligothiophene-acenes, oligothiophenes, and oligothienacenes. \newline \newline [1] J.L. Br\'{e}das, D. Beljonne, V. Coropceanu, and J. Cornil, ``Charge-Transfer and Energy-Transfer Processes in pi-Conjugated Oligomers and Polymers'', Chemical Reviews, 104, 4971-5004 (2004). \newline [2] V. Coropceanu, J. Cornil, D.A. da Silva Filho, Y. Olivier, R. Silbey, and J.L. Br\'{e}das, ``Charge Transport in Organic Semiconductors'', Chemical Reviews, 107, xxx (2007). [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P24.00002: Predicting structure/property relations in polymeric photovoltaic devices. Gavin Buxton, Nigel Clarke Plastic solar cells are attractive candidates for providing cheap, clean and renewable energy. However, such devices are critically dependent on the internal structure, or morphology, of the polymer constituents. We have developed a model that enables us to predict photovoltaic behaviour for arbitrary morphologies, which we also generate from numerical simulations. We illustrate the model by showing how diblock copolymer morphologies can be manipulated to optimise the photovoltaic effect in plastic solar cells. In this manner, we can correlate photovoltaic properties with device structure and hence guide experiments to optimise polymer morphologies to meet photovoltaic needs. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P24.00003: Synthesis and Application of Conducting Block Copolymers in Organic Photovoltaics Bryan W. Boudouris, Marc A. Hillmyer, C. Daniel Frisbie Recent advances in the fabrication and post-processing of polymer -- fullerene bulk heterojunction solar cells have allowed for devices with power conversion efficiencies up to 5{\%} to be generated. An understanding of how the internal morphology of the active layer affects device performance would facilitate optimization and ultimately lead to higher efficiencies. Block copolymers have been shown to self-assemble into well-structured, microphase-separated domains on the order of the diffusion length ($\sim $ 10 nm) of an exciton (bound electron-hole pair) in thin films. In an effort to make a nanostructured active layer morphology we have synthesized block copolymers where the conducting moiety is either poly(3-hexylthiophene) or poly(3-dodeclythiophene) and the second, etchable block is polylactide. Hydroxyl-terminated polythiophene molecules were synthesized via the McCullough method and used as macroinitiators for the ring-opening polymerization of D,L-lactide. AFM images of spin-coated block copolymer films show separation between the polythiophene and polylactide segments. After subjecting the samples to a dilute aqueous base for short periods of time, we have selectively etched the polylactide segments to create pits in the semicrystalline polythiophene matrices. In addition to these findings, preliminary device results will also be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P24.00004: Fluorescence of Dendrons based on Donors and Accepter with Different Linkages J.H. Park, Y. Wu, D.A. Modarelli, J.R. Parquette, A.J. Epstein Earlier indirect studies utilizing wavelength and bias spectra of photocurrent in simple photovoltaic cells demonstrated charge transfer (CT) in 1st generation dendritic macromolecules prepared using two different donor (tetraphenylporphyrin) groups bound to an accepter (naphthalenediimide) group. We report here fluorescence for solid-state films and solutions of these donor and dendrons. Using 460nm excitation, fluorescence (660nm, 715nm) in solution samples can be observed for both donor and dendron but fluorescence in the solid state can be observable only in donor sample due to fluorescence quenching within the dendron. This demonstrates intermolecular CT from donor to accepter. Fluorescence lifetime measurements (460nm 1.5nsec FWHM pulse excitation) of donor and dendron solutions show that it depends on length of the linkage between donor and accepter. This shows a direct relaxation path from donor to accepter (intramolecular CT). The separation of the exciton to separate electron and on the donor and acceptor portions of the dendron would open the potential for its use in photovoltaic application. Supported in part by DOE \#DE-FG02-01ER45931 [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P24.00005: The interplay of morphology and carrier recombination in dendrimer-based organic photovoltaics. Sean Shaheen, Nikos Kopidakis, William Mitchell, William Rance, Jao van de Lagemaat, Garry Rumbles Pi-conjugated dendrimers provide an alternative to polymers in organic photovoltaic devices that allow for systematic study of how the molecular structure affects the morphology of the donor and acceptor components and subsequently how the device operates. The degree of mixing and specific geometry of the donor-acceptor blend play a determining role in the rate of exciton dissociation as well as the efficacy of charge transport out of the active layer. We find that pi-conjugated dendrimers are more miscible with the fullerene-derivative acceptor than their polymeric counterparts, which leads to smaller domains than are commonly found in polymer-fullerene blends. Here we discuss how these differing morphologies affect exciton dissociation, carrier transport, and carrier recombination in the devices. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P24.00006: Below gap external quantum efficiency of organic solar cells. Alexandre Ndobe, Valy Vardeny We fabricated a variety of organic bulk hetero-junction photovoltaic (PV) solar cells based on blends of regio-regular polythiophene (RR-P3HT) and MEHPPV with the fullerene molecules C60- and C70- PCBM. We found, surprisingly that the organic devices show a photovoltaic effect even when excited with light having photon energy below the optical gap of the polymers. This implies that organic solar cells efficiencies can be improve by considering material other than PCBM that have higher infrared absorption but still can serve as a good acceptors for the polymers. To complement this finding we measured the excitation dependence of various PV parameters such as the PV fill-factor, open-circuit voltage, and external quantum efficiency. The interesting excitation spectra reveal the device structure geometry as will be discussed in detail. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P24.00007: Nanocrystalline organic solar cells. Fan Yang, Kai Sun, Stephen Forrest Donor/acceptor (DA) heterointerfaces effectively dissociate excitons into carriers in organic solar cells. Unfortunately, the low carrier mobility of amorphous DA blends limits the active layer thickness to $\sim $25 nm, resulting in low solar absorption. Solar cells made from blends of organic and inorganic semiconductor nanorods overcome the low charge mobility in disordered organic films but have disadvantages due to the mismatch between the nanorods and organic material properties. Here we demonstrate organic solar cells in which both DA materials grow into an extended nanocrystalline network. Structural analysis confirms the existence of crystalline phases of the constituent donor molecule, copper phthalocyanine (CuPc), and the acceptor, C$_{60}$. The structure has a power conversion efficiency of 6.2$\pm $0.3{\%} at 1 sun, AM1.5 simulated solar illumination. This cell shares many of the merits of all organic DA blends and organic/inorganic nanorod cells without many of their disadvantages. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P24.00008: Fabrication and characterization of photovoltaic devices based on `self corralled' CdSe nanorods functionalized with polythiophene Suresh Gupta, Qingling Zhang, Ali Cirpan, Frank Karasz, Todd Emrick, Thomas P. Russell It has been shown that the CdSe nanorods can be oriented normal to the surface by employing an electric field and a polymer matrix where nanorods phase separate. The nanorods close pack with orientation normal to surface in a thin film when the CdSe nanorods are functionalized with alkane and poly(methyl methacrylate) or poly(3-hexyl thiophene)(P3HT) is the matrix. The film is drop-cast under electric field. The phase separation of the nanorods in polymer matrix can be directed by using a patterned surface. The patterned surface was prepared by soft-lithography. Further, the nanorods functionalized with P3HT are `self corralled' under electric field by using a polymer matrix and photovoltaic devices are fabricated. The devices are characterized and the results for devices with normally oriented nanorods are compared to the devices with nanorods parallel to surface. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P24.00009: Femtosecond transient studies of photoinduced charge transfer in~polymers doped with strong acceptor molecules; applications~for organic solar cells Josh Holt, Tomer Drori, Chuanxiang Sheng, Z. Valy Vardeny Current developments in organic solar cells ($\sim $5{\%} efficiency nowadays)~require understanding and control of photoinduced charge carrier transfer and electronic state dynamics of donor-acceptor pairs. One current drawback to organic solar cell efficiency is negligible absorption in the near infrared region of the solar spectrum. We provide and compare evidence that poly(2-methoxy-5(2'-ethyl)hexoxy-phenylenevinylene) (MEH-PPV) and regio-regular poly-3-hexyl thiophene (RR-P3HT) doped with 2,7-dinitrofluorenone (DNF) or 2,4,7-trinitrofluorenone (TNF) form below-gap charge transfer complex state that can extend absorption into the near infrared. Using fs transient and CW spectroscopies we found that the photoluminescence and~mid-ir photoinduced absorption (PA)~band of excitons~are simultaneously quenched, when excited in the visible/uv or near ir. We compare our results to those of comparable systems using C$_{60}$ as acceptor molecules. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P24.00010: Photoinduced charge transfer from polymers to fullerene molecules revisited. Tomer Drori, Chuanxiang Sheng, Alex Ndobe, Cungeng Yang, Minghong Tong, Valy Vardeny We study the process of photoinduced charge transfer (PCT) between conjugated polymers and fullerene molecules as electron acceptors, using the technique of picosecond transient, and steady state photomodulation at various modulation frequencies and temperatures. The polymers studied were MEH-PPV and regio-regular P3HT [RR-P3HT], which are some of the common polymers that are used in organic photovoltaic, as well as polyfluorene [PFO] with optical gap in the blue spectral range; whereas the fullerene molecules where C$_{60}$, C$_{70}$ and their PCBM variations. In all cases we found PCT as evident by the formation of strong photoinduced absorption (PA) polaron bands in the mid ir spectral range. Surprisingly we also found PCT with photon energy below the polymer optical gap. This below-gap PCT process will be discussed and compared with the more usual PCT process with above gap excitation. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P24.00011: Nanoscale Composition and Efficiency of Conjugated Polymer Based Photovoltaic Devices Benjamin Watts, Chris McNeill, Lars Thomsen, Warwick Belcher, Harald Ade, Neil Greenham, Paul Dastoor Organic solar cells based on thin blend films of conjugated polymers and/or fullerene derivatives promise significant advantages in flexibility and low-cost fabrication over conventional, silicon based devices. However, these polymer systems tend to display complex segregation of the component materials during film formation, with the degree of segregation observed shown to depend on parameters such as spincasting spin-speed and solvent type. Many studies in recent years have demonstrated a link between film morphology and device performance and subsequent changes in fabrication methods have resulted in improved device efficiencies that now approach 5\% total power conversion. Here, we present studies providing further details on the morphology-efficiency relationship through the application of scanning transmission X-ray Microscopy (STXM) to generate quantitative composition maps of conjugated polymer blend films and comparison to the measured efficiency of photovoltaic devices incorporating corresponding blend film active layers. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P24.00012: Optimization of the Negative Electrode in Organic Photovoltaic Devices Matthew Reese, Matthew White, Garry Rumbles, David Ginley, Sean Shaheen A blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C$_{61}$-butyric acid methyl ester (PCBM) is used as the active layer in a series of bulk heterojunction organic solar cells. This polymer blend serves as a test-bed to explore the significant effects on device performance of using low work function metals and/or alkali metal halides as the top, negative electrode. Work function values reported in the literature are compared with those measured for our thin films. A series of contact materials are investigated including Al, Ca/Al, Ba/Al, LiF/Al; many devices are prepared with each contact type to validate the statistical significance of the results. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P24.00013: Time-Resolved Microwave Photoconductivity study of the Photophysics of Bulk Heterojunction Organic Photovoltaic Devices Nikos Kopidakis, Andrew Ferguson, Sean Shaheen, Garry Rumbles Bulk heterojunctions composed of a blend of the polymer poly(3-hexylthiophene) (P3HT) and the acceptor fullerene derivative [6,6]-phenyl C$_{61}$-butyric acid methyl ester (PCBM) are the prototypical organic photovoltaic devices. The photophysical processes that take place in these structures involve exciton generation and quenching, and free carrier transport, trapping and recombination. To probe these processes we have performed contactless Time-Resolved Microwave Photoconductivity measurements in pure polymer films and in bulk heterojunctions with varying PCBM concentration. We compare our results with various models for free carrier generation in the pure polymer and in the bulk heterojunction and develop a kinetic scheme to describe free carrier generation and recombination that is consistent with our experimental data. We show that exciton quenching in the presence of the acceptor (PCBM) involves first and second order processes that become prevalent at low and high light intensities, respectively. [Preview Abstract] |
Session P25: Focus Session: Dynamics and Structure in Polymer Melts and Glasses
Sponsoring Units: DPOLYChair: Ramanan Krishnamoorti, University of Houston
Room: Colorado Convention Center 203
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P25.00001: Evolution of stress and entanglements during deformation of glassy polymers Invited Speaker: Simulations of the mechanical properties of model polymer glasses have been performed over a wide range of entanglement densities, temperatures, strain rates and chain lengths. Primitive Path Analysis (PPA) is used to examine the corresponding changes in entanglement structure during deformation. Results for the initial yield stress, strain hardening and crazing will be presented. The initial yield stress is a function of strain rate and the thermal history of the sample. Strain hardening can be fit to entropic network models for the stress-strain curve. The stress shows a neo- Hookean response at low entanglement density $\rho_e$ and Langevin strain-hardening at high $\rho_e$. As expected from network models, entangled polymers deform affinely at scales larger than the entanglement length. However simulations and experiments show strain hardening decreases with increasing temperature while entropic models predict a linear increase. Our results show that strain hardening scales with the flow stress rather than temperature and that substantial strain hardening occurs for unentangled chains. Studies of craze formation show that it does not lead to entanglement loss in our systems. Instead, small scale motions concentrate entanglements at the nodes between fibrils. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P25.00002: Role of Fluctuations in Predicting the Glass Formation Line Grigori Medvedev, James Caruthers Problems with application of the Ehrenfest relations to predicting pressure dependence of the glass transition temperature are well documented in the literature. The resolutions of this problem proposed by various authors range from claiming general inapplicability of the standard phase transition theory to glasses to postulating additional thermodynamic variables to describe glassy state. In this paper we follow a different approach based on explicit acknowledgement of the dynamic heterogeneity of materials observed at a nanometer length scale in and near the glassy state. As a result, the macroscopic relaxation response of a material (defining the glass transition) emerges as an average over an ensemble of local responses which vary from one location to another due to thermodynamic fluctuations. Because of the strong non-linearity of the relaxation time as function of its variables, regions with different values of thermodynamic parameters unevenly contribute to the average effectively shifting the macroscopic relaxation time and, thus, the glass transition point. Moreover, since the magnitude of fluctuations depends on temperature, the glass formation line rotates in the volume-temperature plane. We show that the fluctuation model provides improvement in predicting the glass formation line in PMMA and a number of other polymeric glass formers for which literature data are available. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P25.00003: Entanglement Theories: Packing vs. Percolation Richard Wool There are two emergent theories of polymer entanglements, the Packing Model (Fetters, Lohse, Graessley, Milner, Whitten, $\sim $'98) and the Percolation Model (Wool $\sim $'93). The Packing model suggests that the entanglement molecular weight M$_{e}$ is determined by M$_{e}$ = K p$^{3}$, where the packing length parameter p = V/R$^{2}$ in which V is the volume of the chain (V=M/$\rho $Na), R is the end-to end vector of the chain, and K$\approx $357 $\rho $Na, is an empirical constant. The Percolation model states that an entanglement network develops when the number of chains per unit area $\Sigma $, intersecting any load bearing plane, is equal to 3 times the number of chain segments (1/a cross-section), such that when 3a$\Sigma $ =1 at the percolation threshold, M$_{e}\approx $31 M$_{j}$C$_{\infty }$, in which M$_{j}$ is the step molecular weight and C$_{\infty }$ is the characteristic ratio. There are no fitting parameters in the Percolation model. The Packing model predicts that M$_{e}$ decreases rapidly with chain stiffness, as M$_{e}\sim $1/C$_{\infty }^{3}$, while the Percolation model predicts that M$_{e}$ increases with C$_{\infty }$, as M$_{e}\sim $C$_{\infty }$. The Percolation model was found to be the correct model based on computer simulations (M. Bulacu et al) and a re-analysis of the Packing model experimental data. The Packing model can be derived from the Percolation model, but not visa versa, and reveals a surprising accidental relation between C$_{\infty }$ and M$_{j}$ in the front factor K. This result significantly impacts the interpretation of the dynamics of rheology and fracture of entangled polymers. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P25.00004: Interdiffusion at Ring-Shaped Polystyrene / Its Deuterated Counterpart Bilayer Interfaces Daisuke Kawaguhi, Atsushi Takano, Keiji Tanaka, Toshihiko Nagamura, Naoya Torikai, Robert Dalgliesh, Yushu Matsushita Time evolution of interfacial thicknesses between a cyclic polystyrene (c-hPS) / its deuterated counterpart (c-dPS) and a linear polystyrene (l-hPS) / its deuterated counterpart (l-dPS) bilayer films was investigated by neutron reflectivity as a function of molecular weight. The interfacial thickness of (c-hPS/c-dPS) film with molecular weight of 15k was almost equivalent to that of (l-hPS/l-dPS) one at any given annealing time. In contrast, the interfacial thickness of (c-hPS/c-dPS) film with molecular weight of 115k was significantly larger than that of (l-hPS/l-dPS) film at any given annealing time, indicating that the diffusion constant of c-PS ($D_{c})$ is qualitatively larger than that of l-PS ($D_{l})$. It might be explained in terms of less topological constraint of the entanglement for the cyclic PS at high molecular weight region. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P25.00005: Monte Carlo Simulation of the Glass Transition in Polyethylene Rajesh Khare, Orestis Alexiadis, Vlasis Mavrantzas, Job Beckers, Arlette Baljon The end-bridging (EB) Monte Carlo (MC) move has been used to simulate united atom models of bulk and end-grafted polyethylene both above and below the glass transition temperature (Tg). In previous work, EB move has been shown to lead to significant improvement in the relaxation of melts consisting of long polymer chains. We find that although a reasonable fraction of EB moves get accepted below glass transition in our MC simulation, the autocorrelation function of the chain end-to-end vector does not relax completely at these low temperatures. The temperature dependence of enthalpy and density is used to calculate the Tg for both bulk and thin film systems. For the film consisting of chains grafted on a hard surface, the Tg is slightly lower than that for the bulk, as expected in a system with unfavorable polymer-surface interactions. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P25.00006: Glassy Dynamics and Pressure Effects in Polymer Melts Erica J. Saltzman, Kenneth S. Schweizer A statistical mechanical theory of collective dynamic barriers, slow segmental relaxation and the glass transition in polymer melts has been developed by combining and extending methods of mode coupling, density functional and activated hopping transport theories. Previously, atmospheric pressure results were obtained for the crossover and glass transition temperatures, collective barrier and segmental relaxation time. A cooperativity parameter is introduced based on dynamic consequences of local chain stiffness, and its effects on dynamic fragility are discussed. The theory is extended to elevated pressures, which are found to broaden the deeply supercooled regime and reduce the dynamic fragility while retaining a universal Rossler-Sokolov scaling law for the temperature dependence of the alpha relaxation time. The ratio of the dynamic crossover temperature (ideal mode coupling critical temperature) and kinetic glass transition temperature plays an essential role in the dynamics at all pressures. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P25.00007: Anomalous Surface Dynamics Near Tg in Supported Polystyrene Films by XPCS Zhang Jiang, Mrinmay Mukhopadhyay, Sunil Sinha, Sanghoon Song, Hyunjung Kim, Laurence Lurio The reduction of the glass transition temperature (Tg) in supported thin polymer films is of great interest. One proposed explanation is that close to the surface region there exists a thin layer with very low viscosity. Here we report a recent XPCS (X-ray photon correlation spectroscopy) measurement from silicon supported polystyrene films. At just above Tg, we have found a relaxation mode of the surface fluctuations at least 100 times faster than the capillary wave theory predicts. Surprisingly, this mode does not show significant molecular weight dependence. At higher temperatures (T-Tg$>$=50C), the surface relaxation becomes normal, as predicted by the capillary wave theory [1]. \newline \newline [1] H. Kim, et al., Phys. Rev. Lett. 90, 68302 (2003) [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P25.00008: Theory of Segmental Relaxation and Physical Aging in Polymer Glasses Kenneth Schweizer, Kang Chen A predictive statistical mechanical theory of collective dynamic barriers and segmental relaxation of deeply supercooled polymer melts has been recently developed and widely applied [1]. The theory is based on a dynamic density functional perspective and the concept of a confining nonequilibrium free energy due to interchain forces. Dynamical constraints are primarily quantified by the temperature, pressure and material dependent dimensionless amplitude of long wavelength thermal collective density fluctuations, S0. This theory has now been generalized to the nonequilibrium glass based on the idea of a freezing in of the structural component of density fluctuations. Below Tg an apparent crossover of the segmental relaxation time to an Arrenhius form is predicted. Physical aging is addressed based on a simple first order kinetic equation for the time evolution of S0. At intermediate time scales after a quench the relaxation time generally grows with aging time as a power law with a temperature dependent exponent. The theoretical approach can be generalized to treat nonlinear mechanical properties including stress-strain response, yielding, modulus softening, strain hardening, and stress acceleration of relaxation and aging. [1] K.S.Schweizer and E.J.Saltzman, J.Chem.Phys. 121, 1984 (2004). [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P25.00009: Power Law Behavior of Dynamics in Simple Glass Formers John McCoy, Julieanne Heffernan, Joanne Budzien, Douglas Adolf Simulation results for the diffusive behavior of polymer chain/ penetrant systems are analyzed. Both freely jointed and freely rotating chains are studied. In all cases, the characteristic times, $\tau $, extracted from the diffusion constants are found to be single valued functions of the packing fraction, $\eta $. The functions $\tau (\eta )$ are found to be power-laws with exponents that are sensitive to both chain stiffness and particle type. For a specific system type, all measures of motion extrapolate to zero (or infinity) at a single $\eta _{0}$. In addition, ($\eta _{0}-\eta )$ can be interpreted as a ``scalar metric'' of the ``distance'' to the glass ``transition.'' [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P25.00010: Cure of Bisphenol M Dicyanate Ester/Polycyanurate under Nanoscale Constraint Qingxiu Li, Sindee Simon It is well known that properties are affected by constraint at the nanoscale. Although thermosetting resins have been cured in the presence of nanoparticles and nanotubes, cure of thermosetting resins under the well defined nanoscale constraints imposed by controlled pore glass or similar matrices has not been previously documented. In this work, we investigate the isothermal curing of bisphenol M dicyanate ester/polycyanurate under various nanoscale constraints, including within an aluminum oxide nanofilter, in unsilanized controlled pore glass, and in silanized controlled pore glass. Differential scanning calorimeter and Fourier transform infrared spectroscopy are used to monitor the evolution of the glass transition temperature and the conversion, respectively, as a function of pore size and pore surface chemistry. For the glass transition temperatures of the polycyanurate networks cured in the silanized controlled pore glasses, only nanoconfinement effects are observed; whereas for the material cured in the unsilanized controlled pore glasses, both a nanoconfinement and a surface effect are observed. Furthermore, curing under nanoscale constraint accelerates the cure of bisphenol M dicyanate ester. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P25.00011: Probing Chain Entanglement in Polymer Glasses in Sub-nano Level Gi Xue, Xiaoliang Wang, Dongshan Zhou, Pinchuan Sun A new approach to characterize the chain distance of polymer glasses in isotopic blends using a dipolar filter NMR experiment was developed using ultra fast magic angle spinning (MAS) to selectively enhance sensitivity and resolution of the signals corresponding to the entanglement region. When the deuterated chain entangles with the hydrogenous one within 0.8 nm, the strong 1H dipolar interaction will be diluted. Ultra fast MAS (25-kHz) is used to further average the residual dipolar interaction in the entanglement region. And a dipolar filter experiment is used to suppress the signals from strong 1H-1H dipolar interactions that exist in hydrogenated polymer chains. Based on the above three aspects, an unique 1H signal enhancement effect was found selectively in the overlapping region where the distance between two chains is shorter than 0.8 nm, which can be used as a sensitive probe to characterizing the entanglements in sub-nano level. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P25.00012: Synchrotron X-ray scattering study of structure and dynamics of thin block copolymer films Hyunjung Kim, Heeju Lee, Young Joo Lee, Sanghoon Song, Youngsuk Byun, Zhang Jiang, Sunil K. Sinha, Adrian R{\" u}hm, Suresh Narayanan We have studied the structure and the dynamics of block copolymer films of poly(styrene)-b-poly(dimethylsiloxane) in the melt using X-ray Photon Correlation Spectroscopy. Block-copolymers exhibit internal interactions and therefore an internal structure (in our case spherical micelles). This ought to have a strong influence on the physical properties of the thin films. It can be expected that the dynamics is strongly altered once the film thickness reaches the characteristic length scale in the polymer, which is in our case given by the micelle diameter. The surface tension obtained from static grazing incidence scattering data shows that a PDMS layer segregates to the free surface of the film. The dynamics results are compared with the theory of overdamped thermal capillary waves on thin films. Both the surface dynamics and the micelle dynamics, which were selectively measured by changing the incident angle, will be discussed. It was supported by Korea Science {\&} Eng. Foundation / Seoul Research {\&} Business Development Program. [Preview Abstract] |
Session P26: Focus Session: Electron & Ion Solvation in Clusters & the Condensed Phase I
Sponsoring Units: DCPChair: Peter Rossky, University of Texas at Austin
Room: Colorado Convention Center 205
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P26.00001: How do solvent structure and counterion distribution control quantum solvation in liquids? Invited Speaker: Molecular liquids differ from each other not only in their polarity or their ability to make or accept hydrogen bonds but also in their intrinsic packing. Here, we show that the way a solvent packs can have dramatic effects on the dynamics of electron transfer reactions. Using a combination of nonadiabatic mixed quantum/classical molecular dynamics simulations and ultrafast pump-probe spectroscopy, we show that the presence of intrinsic cavities in liquid THF makes charge transfer dynamics in this solvent different from that in other solvent such as water. For example, we find that photoexcitation can cause solvated electrons in THF to transfer from one cavity to another, providing a mechanism for light-induced electron relocalization. We also find that the way a solvent distributes counterions around a reacting solute can dramatically alter not only the rate but even the products of charge transfer reaction. For example, following excitation of the charge-transfer-to-solvent (CTTS) band of iodide in THF, we find that for soft counterions such as tetrabutylammonium, roughly 10{\%} of the ejected electrons form a loose complex with the counterion within a few ps of excitation. For harder counterions such as sodium, however, we find that there can be photoinduced transfer of the CTTS electron from the of I\={ } anion to the Na$^{+}$. If the sodium cations are complexed into crown ethers, however, electron transfer to Na$^{+}$ is shut off. Finally, we also investigate electron solvation and the CTTS dynamics of I\={ } in THF/water mixtures. We find that CTTS excitation leads to ejection of the electron in an initially THF-rich environment characteristic of the I\={ } solvation structure, but that the electrons subsequently become hydrated on a tens to hundreds of ps time scale. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:27PM |
P26.00002: Dynamics in the First Hydration Shell of Anions Invited Speaker: We will describe our recent efforts to elucidate theoretically the vibrational and reorientation dynamics of water molecules in the first hydration shells of anions in aqueous solution, to assist in the interpretation of recent ultrafast infrared spectroscopic experiments on this issue. In particular, we will discuss (a) OH vibrational frequency dephasing for an iodide ion dilute in a solution of HOD in D2O and (b) the reorientation dynamics for an HOD in the first hydration shell of a chloride ion dilute in a solution of HOD in D2O. This work has been performed in collaboration with Damien Laage, Suyong Re and Bruno Nigro of the Dept. de Chimie, Ecole Normale Superieure, Paris. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 1:03PM |
P26.00003: Electronic Excitation in Aqueous Anions Invited Speaker: Anions when hydrated in water exhibit new features in their electronic absorption spectrum. In the completely hydrated medium of bulk water, charge transfer bands are fully developed and valence transitions exhibited in vacuum can also lead to production of solvated electrons. Using broadband femtosecond transient absorption spectroscopy, we have recorded two-photon absorption spectra that characterize the spectrum of electronic states of aqueous organic and inorganic anions and explored the electronic relaxation dynamics occurring after excitation of valence and charge-transfer-to-solvent states. The detachment dynamics typically are strongly dependent on the excitation energy. The overall solvated electron yields can be understood in terms of competing non-adiabatic, solvation and vibrational relaxation pathways in the excited state. Understanding these electronic states and pathways provides several critical tests for solution electronic structure theories. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P26.00004: Infrared spectroscopy of hydrated sulfate dianions Jia Zhou, Gabriele Santambrogio, Mathias Brummer, David Moore, Ludger Woste, Gerard Meijer, Daniel Neumark, Knut Asmis The first infrared spectra of a multiply-charged anion in the gas phase are presented. The spectra of SO$_{4}^{2-}\cdot $(H$_{2}$O)$_{n}$, with $n$=3 to 24, show four main bands assigned to two vibrations of the dianionic core, the water bending mode, and solvent libration. The triply degenerate SO$_{4}^{2-}$ antisymmetric stretch vibration probes the local solvent symmetry, while the solvent librational band is sensitive to the solvent hydrogen bonding network. The spectra and accompanying electronic structure calculations indicate a highly symmetric structure for the $n=$6 cluster and closure of the first solvation shell at $n=$12. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P26.00005: Charge-transfer (CT) dynamics of iodide salts in tetrahydrofuran (THF) and THF-water mixtures. Arthur Bragg, Benjamin Schwartz We have used the spectral sensitivity of the solvated electron to its local environment to probe counterion and cosolvent effects on ultrafast CT dynamics in THF and THF-water mixtures following 1-photon excitation of the I\={ } CTTS band. We find that dynamics in pure THF are dramatically influenced by the presence of the counterion, such that CTTS-generated electrons associate strongly with nearby cations and recombine negligibly with the geminate iodine radical. Studies in solvent mixtures aim to examine preferential ion solvation according to its effects on CT, focusing on THF-rich mixtures, in which water is thought to preferentially solvate equilibrated electrons. Results demonstrate that electrons are initially introduced into water-\textit{deficient} regions of these solutions, subsequently hydrating over 10's-100's of picoseconds. Trends in the CT and hydration dynamics of electrons generated near various counterions and in solutions of varied water content are used to develop an understanding of the local solvent environments of these ion pairs. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P26.00006: Density Functional Theory calculations of energies of ions in water and in nanopores Kevin Leung, Martijn Marsman Accurate estimates of ion hydration and electrostatic energies are critical for predicting the permeation or rejection of ions in water-filled nanopores. Ab initio Molecular Dynamics methods (AIMD), based on Density Functional Theory (DFT), accounts for the electronic properties and polarizability of materials, water molecules, and ions, and it may appear to be the method of choice for predicting accurate ion energies in water and in nanopores. In practice, applying DFT coupled with the use of periodic boundary conditions in a charged simulation cell leads to anomalous shifts in the electrostatic potential. Using the projector augmented-wave (PAW) method, Wannier functions, and appropriate corrections, we report energies of ions in several systems that can be referenced to interfaces or unambiguous (``vacuum'') values. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P26.00007: Hydrogen bonding and coordination in normal and supercritical water from X-ray inelastic scattering P. H.-L. Sit, Christophe Bellin, Bernardo Barbiellini, D. Testemale, J.-L. Hazemann, T. Buslaps, Nicola Marzari, Abhay Shukla A direct measure of hydrogen bonding in water under conditions ranging from the normal state to the supercritical regime is derived from the Compton scattering of inelastically-scattered X-rays. First, we show that a measure of the number of electrons $n_{e}$ involved in hydrogen bonding at varying thermodynamic conditions can be directly obtained from Compton profile differences. Then, we use first-principles simulations to provide a connection between $n_{e}$ and well-defined structural measures for the number of hydrogen bonds $n_{HB}$. Our study shows that over the broad range studied the relationship between $n_{e}$ and $n_{HB}$ is linear, allowing for a direct experimental measure of bonding and coordination in water. In particular, the transition to supercritical state is characterized by a sharp increase in the number of water monomers, but also by a significant number of residual dimers and trimers. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P26.00008: Binding Energies between Guest Atoms in Clathrate II Hidekazu Tomono, Kazuo Tsumuraya The guest atom displacements in clathrates II have been reported on experimental and theoretical points of views. The recent papers on the displacements are given in the reference [1]. The displacements are found to be about 0.6 \AA \ from the center of the Si$_{28}$ cage to the hexagonal ring between the Si$_{28}$ cages. The binding energies between the guest atoms however have been unknown so far. In the present work we calculate the energies between Na atoms in clathrates II Na$_2$@Si$_{136}$ and Na$_{24}$@Si$_{136}$ with a density functional analysis. We will discuss the cohesion mechanism of the clathrates based on the binding nature between the cations in Zintl phase. [1] H. Takenaka and K. Tsumuraya, Mater. Trans., 47, 63 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P26.00009: Solvation of anions by aromatic molecules probed by infrared spectroscopy J. Mathias Weber, Holger Schneider, Kristen M. Vogelhuber We have studied the interaction of chloride ions with partially fluorinated benzenes by gas phase infrared photodissociation spectroscopy. Our studies were motivated by the fact that fluorination changes the charge distribution in a benzene molecule. While C$_{6}$H$_{6}$ has a negatively charged carbon ring and a positively charged hydrogen periphery, C$_{6}$F$_{6}$ has a positively charged carbon ring and a negatively charged fluorine periphery. If the interaction between a closed-shell anion (such as Cl$^{-})$ and the aromatic molecule were based mostly on electrostatic interaction, such an ion would bind to C$_{6}$H$_{n}$F$_{6-n}$ via the $\pi $ system for small $n$ and via H bonds to the periphery for large $n$. We have used IR spectroscopy of Ar solvated Cl$^{-}\cdot $C$_{6}$H$_{n}$F$_{6-n}$ complex anions to investigate if this paradigm holds, using the red shift and intensity increase of CH stretching modes for H-bonded CH oscillators to discern whether Cl$^{-}$ binds to the $\pi $ system rather than to the periphery at different levels of F substitution. [Preview Abstract] |
Session P27: Focus Session: Computational Nanoscience V - Nanotubes
Sponsoring Units: DMP DCOMPChair: Rajendra Zope, University of Texas at El Paso
Room: Colorado Convention Center 301
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P27.00001: Absorption Coefficient for Cylindrical Nanotubes Godfrey Gumbs, Antonios Balassis A self-consistent field theory is presented for calculating the absorption coefficient for a pair of coaxial tubules. The spatially nonlocal dynamic formalism is obtained in terms of the electrostatic potential produced by the charge density fluctuations and the external electric field. There are peaks in the absorption spectrum arising from plasma excitations corresponding either to plasmon or particle-hole modes. We calculate numerically the plasmon contribution to the absorption. The number of peaks depends on the radius of the inner as well as outer tubule. The height of each peak depends on the plasmon wavelength and energy. For a chosen wavenumber, the most energetic plasmon has the highest peak corresponding to the largest oscillator strength. Some of the less energetic plasmon modes have such weak coupling to an external electric field that they are not seen on the same scale. We plot the peak positions of the plasmon excitations on a pair of coaxial tubules. The coupled modes on the two tubules are split by the Coulomb interaction. The energies of the two highest plasmon branches increase with the radius of the outer tubule. On the contrary, the lowest modes decrease in energy as this radius is increased. [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P27.00002: Nanocables made of a transition metal wire and boron nitride Chih-Kai Yang, Jijun Zhao, Jianping Lu The boron nitride (BN) nanotube has a very wide band gap and can shield the nanowire encapsulated inside its cavity from outside interference. Our calculations indicate that transition metal wires can be inserted inside a variety of zigzag BN nanotubes exothermically. In particular a cobalt wire and the BN tube interact just like two giant molecules. The weak interaction between the BN tube and the wire ensures a low binding energy and a high magnetic moment that comes solely from the transition metals. High spin polarization at the Fermi level also indicates that the hybrid structure can be used as a nanocable for spintronic applications [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P27.00003: {\it First principles} study of Crystalline Bundles of Single-Walled Boron Nanotubes Kah Chun Lau, Roberto Orlando, Ravindra Pandey {\it First principles} calculations based on density functional theory are performed to study the structural and electronic properties of the crystalline bundles of (n,0) zigzag-type single-walled boron nanotubes (SWBNT). The results predict a substantial modification in the properties of SWBNT bundles relative to those of the isolated nanotubes. The predicted modification can be attributed to a significant interplay between intra- and inter-tubular bonds in determining the stability of SWBNT bundles, analogous to the role played by intra- and inter-icosahedral bonds in the boron crystalline solids. The result shows the SWBNTs exhibit polymorphism, which is likely to be the cause of the difficulty in growing SWBNTs experimentally. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P27.00004: Dielectric Response and Born Dynamic Charge of BN Nanotubes from \textit{Ab Initio} Finite Electric Field Calculations Guang-Yu Guo, Shoji Ishibashi, Tomoyuki Tamura, Kiyoyuki Terakura Since the discovery of carbon nanotubes (CNTs) in 1991 by Iijima, carbon and other nanotubes have attracted considerable interest worldwide because of their unusual properties and also great potentials for technological applications. Though CNTs continue to attract great interest, other nanotubes such as BN nanotubes (BN-NTs) may offer different opportunities that CNTs cannot provide. In this contribution, we present the results of our recent systematic \textit{ab initio} calculations of the static dielectric constant, electric polarizability, Born dynamical charge, electrostriction coefficient and piezoelectric constant of BN-NTs using the latest crystalline finite electric field theory [1]. [1] I. Souza, J. Iniguez, and D. Vanderbilt, Phys. Rev. Lett. 89, 117602 (2002); P. Umari and A. Pasquarello, Phys. Rev. Lett. 89, 157602 (2002). [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P27.00005: First-Principles Study of Nucleic Acid Bases Physisorbed on Graphene S. Gowtham, R. H. Scheicher, Rajeev Ahuja, Ravindra Pandey, Shashi P. Karna We report the results of our investigation on the interaction of nucleic acid bases bases with graphene, carried out within the density functional theory framework, with additional calculations utilizing Hartree--Fock plus second--order M{\o}ller--Plesset perturbation theory. The calculated binding energy of the five nucleobases shows the following hierarchy: G $>$ T $\approx$ C $\approx$ A $>$ U, though the equilibrium configuration consists of nearly the same separation between the sheet and the bases considered. The stabilizing factor in the interaction between the nucleobases and the graphene sheet appears to be dominated by the molecular polarizability that induces a weakly attractive dispersion force between them. The present study is a first step toward understanding why different DNA sequences interact differently with CNTs, as observed experimentally. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P27.00006: The work function of small radius carbon nanotubes. Wan-Sheng Su, Tsan-Chuen Leung Carbon nanotubes workfunctions can deviate from that of graphene due to geometric and structural factors. We have systematically investigated the work functions of various forms of small radius carbon nanotubes and their derivatives by density functional calculations. We considered and compared the workfunctions of tubes and tube-bundles in order to understand the effects of chirality, tube-length and capping condition. Systematic trends are unraveled and discussed. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P27.00007: Work function of functionalized single-wall carbon nanotubes Nicholas Singh-Miller, Nicola Marzari Engineering the properties of carbon nanotubes is of fundamental importance for many of their practical applications; we focus here on the work function of metallic nanotubes, and on the changes that can be induced by electropositive or electronegative functionalizations. We study with density functional theory pristine, hydrogenated, and fluorinated (5,5) and (5,0) nanotubes along with more complex organic ligands as functional moieties, paying particular attention to the additional dipole interactions that can arise in periodic boundary conditions. Control of the Fermi level alignment is especially relevant for field-effect devices and for the Schottky barrier at carbon nanotube interfaces. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P27.00008: Continuum description of defects in carbon nanotubes Elif Ertekin, Daryl Chrzan Recently, indications of plastic deformation have been observed in carbon nanotubes: strain stiffening in nanotube torsional shafts and direct observations of kink motion to assist with elongation. These observations suggest the importance of defects, both their formation and dynamics, to nanotube mechanical properties. Remarkably, defect formation energies are not well--understood: formation energies of Stone--Wales defects (dislocation--like defects) vary by $\approx$ 3 eV, depending on the environment. Further, no attempt has been made to compute the total energies of dissociated Stone--Wales defects. To address these issues, we develop a continuum theory of defect formation in nanotubes based on the idea that the distortion field associated with the presence of a defect distribution is that which minimizes the elastic and curvature energies but is consistent with the topological constraints imposed by the defects. It makes no a priori assumptions about the defect strain fields, accounts for defect--defect interactions, and accomodates changes to the curvature and out--of--plane buckling. Formation energies of Stone--Wales defects in a wide variety of configurations are computed using total energy electronic structure methods and compared with the results of the continuum theory; the agreement is excellent, irrespective of defect arrangement. The result is an accurate and transferable continuum description of defect formation energies in carbon nanotubes. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P27.00009: Anharmonic phonon lifetimes in graphene, graphite and carbon nanotubes Nicola Bonini, Nicola Marzari, Michele Lazzeri, Francesco Mauri We present a density-functional study of anharmonic phonon lifetimes in low-dimensional graphitic structures. Phonon lifetimes are evaluated from the cubic terms in the interatomic potential, using density-functional perturbation theory and the 2n+1 theorem. We show that in graphene and graphite the phonon lifetimes of the $E_{2g}$ LO mode at {\bf $\Gamma$} and the ${A'}_1$ mode at {\bf K} due to phonon-phonon scattering are larger than those due to the electron-phonon interaction. We discuss the relevance of this finding for the transport properties and the implications for carbon nanotubes. We also present the results for the temperature-dependent frequency shift of the Raman G mode, in which third- and fourth-order anharmonic terms contribute equally. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P27.00010: Electronic structures of MoS2 nanotubes. Lingyun Xu, Murray Daw, Xing Gao, Erdi Bleda The electronic structure of MoS2 nanotubes has been studied using first principles. We investigated MoS2 zigzag (n, 0) nanotubes as well as armchair (n, n) structure. We constructed MoS2 nanotube with ABA and ABC stacking. The structures have been completely optimized. We compare to previous tight-binding calculations by Seifert et al.(Phys. Rev. Lett. 85, 146 (2000)). [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P27.00011: The role of electron-phonon interactions and external strain on the~ electronic properties of semiconducting carbon nanotubes Denis Karaiskaj, Angelo Mascarenhas The electron-phonon interactions play an important role in the temperature dependent photoluminescence of semiconducting carbon nanotubes. The energy shifts and spectral narrowing of the excitonic transitions can both be attributed to the electron-phonon interaction. The thermal broadening was fitted by a theoretical expression previously used to model the thermal broadening of critical points in conventional semiconductors. Moreover, careful studies of the energy shifts induced by the external strain had revealed a (n-m) family behavior. We further conclude that using a mathematical expression that combines the theory of semiconducting carbon nanotubes under hydrostatic pressure and strain, this family behavior observed experimentally could be theoretically reproduced, providing new tools to model and predict the effect of strain on the electronic properties of carbon nanotubes. The temperature dependence of the photoluminescence decay of excitons in single walled carbon nanotubes was measured for two nanotube species, (7,6) and (7,5), representative of the two nanotube (n-m)mod3 families. The effect of temperature and external strain on the photoluminescence lifetime will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P27.00012: Pi electron plasmon modes by the method of tight binding in an array of armchair single-walled carbon nanotubes. M. E. Markes, P. F. Williams P. R. Wallace introduced the method of tight binding in the study of graphene sheets about 50 years ago [1]. More recently, it has been possible to explain the relationship between the semiconductor/metallic behavior and carbon nanotube chirality by applying periodic axial boundary conditions to Wallace's band model [2]. Several years ago one of us (P. F. Williams [3]) developed a self-consistent dielectric response model for arrays of one-dimensional metal filaments using a tight-binding approximation. At the time this model was found useful in a study of the single-particle excitations and plasmon dispersion curves of tetrathiofulvalene-tetracyano-quinodimethane (TTF-TCNQ). This paper is a report of work in progress to extend the Williams-Bloch model to arrays of single-walled carbon nanotubes modeled using the Wallace tight-binding model and axial periodic boundary conditions. The collective excitations are characterized by a frequency and wavelength dependent dielectric function obtained using a self-consistent field in the random phase approximation. The zeros of this function yield the plasmon modes. [1] P. R. Wallace, Phys. Rev., vol. 71, 1947. [2] R. Saito et. al., Appl. Phys. Lett., vol. 60, 1992. [3] P. F. Williams and A. N. Bloch, Phys. Rev. B, vol. 10, 1974. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P27.00013: Electronic structure of Defective and Deformed Single Wall Carbon Nanotubes L.M. Woods, Ya. Shtogun, T.L. Reinecke Carbon nanotube properties can be modified by the introduction of defects on their surface or by mechanical deformations. Here we present an alternative way to modify carbon nanotube characteristics by considering both types of mechanical alterations, defects and deformations, on the nanotube surface. Electronic structure calculations from first principle density functuional theory using the VASP code (Viena Ab initio Simulations Package) for metallic and semiconducting single walled carbon nanotubes with Stone-Wales defect and radial deformation are presented. The difrferent degrees of deformation and various defect locations are analyzed in terms of the density of states and bandstructures of these systems. We compare the defective and deformed nanotube electronic structure to the electronic structure of only defective or only deformed nanotubes. In this way we determine the relative importance of the two types of mechanical alterations on the defective and deformed nanotubes. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P27.00014: Electron-phonon coupling mechanism, Kohn anomalies and Peierls instabilities in two-dimensional graphite and single-wall carbon nanotubes Georgy Samsonidze, Eduardo Barros, Riichiro Saito, Hyungbin Son, Gene Dresselhaus, Mildred Dresselhaus The electron-phonon coupling in two-dimensional graphite and metallic single-wall carbon nanotubes (SWNTs) is analyzed. The $G'$-band phonon mode opens a dynamical band gap that induces a Kohn anomaly in two-dimensional graphite, while truly metallic armchair SWNTs undergo Peierls transitions driven by the $G$- and $G'$-band phonon modes. The dynamical band gap induces a non-linear dependence of the phonon frequencies on the doping level and gives rise to strong anharmonic effects. [Preview Abstract] |
Session P28: Focus Session: Graphene III
Sponsoring Units: DMPChair: Pablo Jarillo-Herrero, Columbia University
Room: Colorado Convention Center 302
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P28.00001: Band Gaps and Quasiparticle Energies of Graphene Nanoribbons Li Yang, Cheol Hwan Park, Young-Woo Son, Marvin L. Cohen, Steven G. Louie We present calculations of the quasiparticle energies and band gaps of graphene nanoribbons (GNRs) carried out using a first-principles many-electron Green's function approach. The self-energy of electrons is evaluated within the GW approximation. In our supercell calculation, due to the geometry of GNRs, a rectangular truncation of the Coulomb interaction is applied, which significantly improves the efficiency of the calculation. The quasiparticle results are compared and contrasted with results from previous studies that have been carried out either within the tight-binding or density functional formalism. [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P28.00002: Graphene ribbon electronics Zhihong Chen, Phaedon Avouris Graphene consists of a single layer of carbon atoms that are arranged in a hexagonal structure. This ideal two-dimensional system represents a gapless semiconductor with six intersecting points per Brillouin zone between the valence and conduction band. In principle, a semiconducting gap can be introduced when the width of the graphene sheet is made small enough and the carbon hexagons are orientated in certain directions. In this study, we have combined e-beam lithography and etching techniques to form graphene ribbons of different widths. Electrical properties of these ribbons were studied through gate dependent transport measurements at various temperatures. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P28.00003: Engineering The Energy Band Gap of Graphene Quantum Structures Melinda Han, Yuanbo Zhang, Barbaros Oezyilmaz, Philip Kim We report on experimental studies of electrical transport in patterned graphene ribbon structures where the lateral confinement of charge carriers creates an energy gap near the charge neutral Dirac point. Single graphene layers are contacted with metal electrodes and patterned into ribbons of varying widths (10 to 100nm) and orientations. Energy gaps of the ribbons are investigated using both stability diagrams obtained at low temperatures (1.7K) and temperature dependent conduction behavior. An understanding of ribbon dimension and orientation as control parameters for the electrical properties of graphene structures can be seen as a first step toward the development of graphene-based electronic devices. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:27PM |
P28.00004: Electronic, magnetic and transport properties of graphene nanoribbons Invited Speaker: The recent fabrication of a single graphite layer opens a new possibility in the area of nanoelectronics. These experimental findings motivated us to study a novel one dimensional nanomaterial $-$ a graphene nanoribbon (GNR). Based on a first-principles approach, we have established the scaling rules for electronic energy bandgaps as a function of ribbon width. Both armchair and zigzag edged GNRs, with homogeneous edges passivated with hydrogen, are shown to have bandgaps, differing from the results of simple tight-binding calculations or solutions of the Dirac's equation based on them. Our {\it ab initio} calculations show that the origin of energy gaps for GNRs with armchair shaped edges arises from both quantum confinement and the crucial effect of the edges. The variations in energy bandgap of GNRs with armchair shaped edges exhibit three distinct family behaviors. For GNRs with zigzag shaped edges, gaps appear because of a staggered sublattice potential on the hexagonal lattice due to edge magnetizations. Based on electronic structure calculations on GNRs, we present two novel phenomena in GNRs and GNR nano-constrictions. First, our calculations show that the magnetic properties of nanoribbons can be controlled by electric fields. In particular, half-metallicity is predicted in GNRs if in-plane homogeneous electric fields are applied across zigzag shaped edges of these systems. Such asymmetric electronic structure for each spin originates from the fact that the spatially separated spin polarized states with opposite spin orientations in the semiconducting GNRs are shifted oppositely in energy by the applied fields. This closes the gap associated with one spin orientation and widens the other. Second, in GNR nano-constrictions with armchair shaped edge, conductances are shown to depend on the family behavior of energy gap of GNRs forming nano-constrictions. Depending on the width of nano-constriction, the incoming electrons from GNR leads are shown to experience perfect transmissions or nearly complete reflection in a wide range of energy. This work has been collaborated with M. L. Cohen and S. G. Louie. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P28.00005: First-Principles Simulations of Armchair-Edge Graphene Nanostrips. Junwen Li, John W. Mintmire, Daniel Gunlycke, Carter T. White We have carried out a series of first-principles, local-density functional band structure calculations of finite-width graphene nanostrips with armchair edges. A simple nearest-neighbor tight-binding model predicts that the band structures of these materials should be directly related to those of zigzag single wall carbon nanotubes, with two-thirds of the structures being small gap semiconductors and one-third of the structures being zero gap systems. The band gap in the semiconducting strips would be expected to decrease monotonically with increasing strip width. In our first-principles results, we find that in addition to the zero gap systems becoming finite gap quasimetallic systems because of symmetry breaking (as in the single-walled nanotubes), we also find that the semiconducting strips split into two families with band gaps that deviate from the simple nearest-neighbor tight binding model. Within the framework of our computational results, we compare the band structures of graphene, single-walled nanotubes, graphene nanostrips, and other carbon nanostructures. This work was supported by the US Office of Naval Research and the DoD HPCMO CHSSI program, both directly and through the US Naval Research Laboratory. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P28.00006: Performance Limit and Scaling Behaviors of Carbon Nanoribbon Transistors Jing Guo, Yijian Ouyang Carbon-based nanostructures promise near ballistic transport and are being intensively explored for device applications. In this work, the performance limits of carbon nanoribbon (CNR) field-effect transistors (FETs) are assessed using a semiclassical model, and compare to those of carbon nanotube (CNT) FETs. The ballistic channel conductance and the quantum capacitance of the CNRFET are about a factor of 2 smaller than those of the CNTFET, because of the different valley degeneracy factors for CNTs and CNRs. The intrinsic speed of the CNRFET is faster due to a larger average carrier injection velocity. The gate capacitance plays an important role in determining which transistor delivers a larger on-current. The scaling behaviors of CNRFETs are studied using an atomistic quantum simulation. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P28.00007: Applications of Nanoribbon Devices Thushari Jayasekera, John W. Mintmire Modern experiments allow us to grow ultra-thin epitaxial graphene which shows two-dimensional electron gas (2DEG) behavior. Electron transport in these 2DEG systems can be further confined in lateral directions using micro-electronics lithography methods (nano-patterned epitaxial graphene, NPEG). We study the properties of the NPEG multi-terminal devices made at a crossing of a zig-zag and armchair nanoribbons, in particular, plus junction and T-junction devices. We investigate the effect of size, shape, and, chirality on the transport properties of the device. We also discuss the effect of defects in the junction region on the electron transport of the device. Our results find that the properties of nanoribbon junctions are highly sensitive to the details of the junction region, thus we can engineer different properties by changing those details of the device. This work was supported by the DoD HPCMO CHSSI program through the Naval Research Laboratory. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P28.00008: Ballistic transport in zigzag-edge graphene nanostrips Daniel Gunlycke, Hadley M. Lawler, Denis A. Areshkin, Carter T. White Graphene nanostrips (GNSs) constitute a class of materials where one of the two in-plane dimensions of graphene has a small finite width. We present results of zigzag-edge GNSs terminated with hydrogen atoms which suggest that ballistic transport may be possible over micrometer lengths. The single channel near the Fermi level appears to possess a natural resistance to back-scattering. Long-range disorder have a negligible back-scattering since the only allowed coupling requires a large crystal momentum change. We find that disorder on atomic scale and edge disorder have also little impact on the conductance in the single-channel window. Not only are the zigzag-edge GNSs resistent to static disorder, they may also offer longer electron-phonon mean-free paths which are longer than those in carbon nanotubes. Back-scattering in the conduction band requires a large transfer of crystal momentum from phonons which immediately eliminate long-wavelength acoustic phonon scattering. Therefore, it might be feasible to have single-channel ballistic transport in zigzag-edge GNSs at room temperature. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P28.00009: first-principles tight-binding study of band gaps in graphene ribbons Daniel Finkenstadt, Gary Pennington, Chris Ashman, Mike Mehl Graphene has recently received much attention for the many interesting physical properties that it exhibits, including light Dirac fermion characteristics of its charge carriers and some experimental evidence of a minimum conductivity, even as the carrier concentration goes to zero. From a practical standpoint, the potential for large carrier mobility in graphene provides an attractive alternative to silicon-based devices, e.g. for field-effect transistors. Theoretical efforts towards designing these devices are focused on determining the geometry and chemistry needed to open up a semiconducting gap in the otherwise semi-metal band structure of a perfect, infinite graphene sheet. Such effects may allow gate control of the electronic conductance as found in semiconducting carbon nanotube devices. Here we use the NRL tight-binding method, which is fit to first-principles calculated data, to study the possibility of opening a gap in graphene by varying strip-width, edge shape with and without termination, and by allowing Peierl's distortion of the edges for narrow ribbons. We compare the tight-binding results with calculations based on the density functional theory. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P28.00010: Effects of disorder in the biased graphene bilayer Johan Nilsson, Antonio Castro Neto We discuss the effects of disorder on the peculiar electronic properties of a biased graphene bilayer, which is a semiconductor that has the property that its band-gap can be controlled externally by the field effect. We focus on the low-energy region inside of and near the band-gap and have studied the properties of bound states as well as possible effects due to a finite density of impurities such as impurity band formation and band gap renormalization. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P28.00011: Edge disorder in armchair-edge graphene nanostrips Denis A. Areshkin, Daniel Gunlycke, Carter T. White Graphene nanostrips created using current lithography techniques will likely contain edge irregularities due to lack of atomic precision. We present tight-binding calculations which show that these edge irregularities have a strong effect on electron transport in armchair-edge graphene nanostrips. The edge disorder causes Anderson localization which effectively suppress the electronic conductance in samples which are longer than the mean-free path. We estimate the mean-free path via the localization length which is calculated by averaging over a large number of disordered nanostrips. We find that the localization length approximately decreases with the square of the width of the nanostrip and is of the order of tens of nanometers at the width $20$ nm. The localization length also depends on the concentration of edge defects and energy. Only nanostrips with low concentration of edge disorder reflect expected semiconducting gaps in the localization length. We also find that the Anderson localization extends over the entire $\pi$-electron energy range. With this result in mind, we predict that long and narrow armchair-edge graphene nanostrips are insulators. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P28.00012: Carrier transport in 2D graphene layers near the Dirac point Shaffique Adam, E.H. Hwang, S. Das Sarma In a recent work we studied carrier transport in gated 2D graphene monolayers theoretically in the presence of scattering by random charged impurity centers using a Boltzmann theory formalism (cond-mat/0610157). Comparing our results with available experimental data suggested that the low density saturation of conductivity arises from charged impurity induced inhomogeneity in the graphene carrier density. In the present work, we develop a model for carrier transport in a disorder-induced inhomogeneous potential and examine the consequences on conductivity. This work was partially supported by U.S. ONR. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P28.00013: Dislocation and pentagon-heptagon pair generation in vacancy-induced graphene layer Byoung Wook Jeong, Hoonkyung Lee, Gun-Do Lee, Jisoon Ihm We investigate the mechanism of the generation of long range order defects in graphene layer by tight binding molecular dynamics simulations and first-principles total energy methods. It is found that the vacancies are diffused and coalesced to make the dislocation defect with the two 5-7 pair defects when more than a certain number of vacancies are present. We examine the magic number of the vacancy which gives dislocation defects in a graphene layer. STM simulation results related to the graphite lattice with the period of $\sqrt 3 \times \sqrt 3 $ in an STM topograph will be discussed. [Preview Abstract] |
Session P29: Focus Session: Granular Flows II
Sponsoring Units: DFD GSNPChair: Karen Daniels, North Carolina State University
Room: Colorado Convention Center 303
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P29.00001: Beyond Navier-Stokes Order Effects in Granular Gases Invited Speaker: The vast majority of continuum theories for rapid granular flows are based on Navier-Stokes order descriptions (up to first order in spatial gradients). In this effort, a simple system is used to illustrate the presence and impact of higher-order effects in both the Knudsen boundary layer and the domain interior. Specifically, a thermally-driven, zero mean flow system is considered via molecular dynamics (MD) simulations. The Knudsen boundary layer is identified via an abrupt mismatch in the simulation data for heat flux and predictions from Navier-Stokes order theories. When access to heat flux measurements is not available, a rule-of-thumb is established to estimate the thickness of the Knudsen boundary layer based on concentration measurements. The effect of boundary layer thickness on continuum predictions is assessed via MD simulations, and further illustrated via a comparison between predictions and experimental data for a vibro-fluidized bed. Next, the presence of higher-order effects in the domain interior is explored via MD simulations. The system displays a stress anisotropy, which can be traced to Burnett order effects. Furthermore, a surprisingly large mismatch is observed between Navier-Stokes order theory and MD values for the heat flux. Because there are no Burnett-order contributions to the heat flux, the responsible mechanisms appear to be beyond Burnett order. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P29.00002: Power-law tail of the velocity distribution in granular gases Wenfeng Kang, Eli Ben-Naim, Jon Machta We use a two-dimensional event-driven molecular dynamics simulation to study the velocity distribution of a granular gas. We implement the high energy injection mechanism described in Ref. [1]. At a small rate $\gamma $ we boost randomly chosen particles to a high energy. The resulting driven steady state is found to have a power-law high-energy tail in the velocity distribution, f(v) $\sim $ v $^{- \sigma }$. The simulation results for the exponent $\sigma $ are in good agreement with the theoretical predictions of Ref. [1]. \newline \newline [1] E. Ben-Naim and J. Machta, Phys. Rev. Lett. 94, 138001 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P29.00003: Translations and Rotations are correlated in Granular Gases Annette Zippelius, Nicolai Brilliantov, Thorsten Poeschel, Till Kranz In a granular gas of rough spheres the axis of rotation is shown to be correlated with the translational velocity of the partciles. The average relative orientation of angular and linear velocities depends on the parameters which characterise the dissipative nature of the collision. We derive a simple theory for these correlations and validate it with numerical simulations for a wide range of normal and tangential restitution.The limit of smooth spheres is shown to be singular: even an arbitrarily small roughness of the particles gives rise to orientational correlations. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P29.00004: On the possibility of aeolian dunes on a laboratory scale Matthias Sperl, R.P. Behringer Recent progress in modeling aeolian sand dunes in the field has resulted in the prediction of a critical linear length scale below which no shape stable dune can form. Under typical field conditions on earth, this length scale is around 10m or larger. Using small (0.05mm) lightweight (0.2g/cc) particles with a proper surface treatment to reduce cohesion we can demonstrate how the dune problem can be scaled down to a lab-size wind tunnel. We demonstrate (a) different transport properties of the particles upon variation of the wind speed, (b) the growth of a heap, (c) the formation of a crest, and (d) ripples on a smaller scale than the heap. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P29.00005: Experiments on Washboard Road Stephen Morris, Nicolas Taberlet, James McElwaine, Stuart Dalziel Granular surfaces to develop lateral ripples (so-called ``washboard" or ``corrugated" road) under the action of rolling wheels. Similar ripples are observed on railroad tracks and many other rolling, load bearing surfaces. Our aim was to investigate this instability of the flat road surface from the point of view of driven, dissipative granular dynamics. We report the results of both laboratory experiments and soft-particle direct numerical simulations. The experiment consisted of a rotating table 60 cm in radius with a thick layer of sand forming a roadbed around the circumference. A 6 cm radius hard rubber wheel, with a support stationary in the lab frame, rolled on the sand layer. We varied the speed of the table, the details of the grains and the suspension of the wheel. The ripple pattern appears as small patches of travelling waves which eventually spread to the entire circumference. The ripples drift slowly in the driving direction. Interesting secondary dynamics of the saturated ripples were observed, as well as various ripple creation and destruction events. The wavelength of the ripples can be quantized by the finite circumference of the road. All of these effects are captured qualitatively by 2D soft particle simulations in which a disk rolls over a 2D bed of polydisperse particles in a periodic box. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P29.00006: Probing Avalanche Dynamics using Speckle-Visibility Spectroscopy. Adam Abate, Hiroaki Katsuragi, Doug Durian We apply a new light scattering technique called Speckle-Visibility Spectroscopy to the study of avalanches. By directly relating the rate of change of the scattered speckle pattern to the fluctuation dynamics of the flowing sand particles, we attain a precision of 0.1 mm/s. Running for 35 hours at 58 kHz, we simultaneously observe the microscopic short-time fluctuations of the sand particles and the long time behavior of thousands of avalanche events, and thus report avalanche frequency statistics and average shape. Interestingly, while all avalanches turn on in 0.3 s and in a similar way, there is a wide variation in how avalanches turn off. The fluctuation speed reaches a maximum just after the avalanche begins, it remains constant for a while, and then decays to zero. Power spectra of the full data set show that as avalanches slow the dynamics are self-similar ($\sim $1/f$^{ 2})$ and the normalized variance of different events diverge at the turning off time. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P29.00007: Erosion of a granular bed by laminar fluid flow Ashish Orpe, Alex Lobovsky, Ryan Molloy, Arshad Kudrolli, Daniel Rothman Motivated by examples of erosive incision of channels in sand, we investigate the motion of individual grains in a granular bed as a function of fluid flow rate to give us new insight concerning the relationship between hydrodynamic stress and surficial granular flow. A closed channel of rectangular cross section is partially filled with glass beads and a fluid and a constant flux $Q$ is circulated through the channel. The fluid has same refractive index as the glass beads and is illuminated with a laser sheet away from the sidewalls. The bed erodes quadratically in time to a height $h_c$ which depends on $Q$. The Shields criterion, which is proportional to the ratio of the viscous shear stress and gravitational normal stress, describes the observed $h_c \propto \sqrt {Q}$ when a height offset of approximately half a grain diameter is introduced. The offset can be interpreted as arising due to differences between the flow near a porous boundary and a smooth wall. Introducing this offset in the estimation of the shear stress yields a grain flux $q_x$ in the bed load regime proportional to $(\tau - \tau_c)^2$, where $\tau$ is the non-dimensional shear stress, and $\tau_c$ corresponds to the Shields criteria. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P29.00008: Segregation in horizontal rotating cylinders: radial and axial band formation, band traveling and merging studied by Magnetic Resonance Imaging. Thoa Nguyen, Andrew Sederman, Lynn Gladden Radial and axial segregations are investigated by Magnetic Resonance Imaging (MRI). For the first time, full 3D structures and real-time 2D MRI movies showing the progress of segregation over many hours are reported. Data were acquired with high temporal (74 ms) and in-plane spatial resolutions (1 mm $\times $ 1 mm), giving new insights into the underlying mechanisms. The mixture composition can be quantified throughout segregation. The cylinder to be considered is 48 mm in diameter, up to 50 cm long and filled to 50 -- 82{\%} by volume with millet and poppy seeds at a 3:1 ratio. In particular, the effects of filling fraction, cylinder length and rotational speed on segregation are addressed. Radial segregation is found to be driven by both core diffusion and the free surface. The former is dominant in the cylindrical core buried under the avalanche layer in systems over 75{\%} full while the latter is significant at lower filling levels. Axial segregation is characterized by band formation, traveling, and merging. In all cases studied, the formation of poppy-rich bands is observed, after which individual bands start to travel at $\sim$3 $\mu $m s$^{-1}$ until they are within $\sim $3 cm of a stationary band. Adjacent bands then merge into a single, enlarged poppy band as millet seeds move out of the merging region. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P29.00009: Granular Flow in a Rotating Drum: Dry vs. Submerged Flow Deniz Ertas, Hubert E. King, Arnold Kushnick, Fuping Zhou, Christopher Briscoe, Paul Chaikin We have experimentally studied granular flows in a cylindrical rotating drum, half-filled with nearly monodisperse spherical glass particles in order to investigate the effect of interstitial fluid on these flows. We have conducted two classes of experiments under otherwise identical conditions: The first with air as interstitial fluid and the second where the empty space in the cylinder was completely filled with water. For varying rotation rates, we used a particle tracking method to measure particle velocities near the side wall as a function of distance from the flow surface and the surface velocity as a function of distance from the side wall. In all cases, the velocity (relative to rigid rotational motion) initially decreases linearly from its surface value, followed by exponential decay, as a function of increasing distance from the surface. At a given rotation angle (i.e. overall flux), subaqueous flows exhibit more dissipation and therefore result in steeper surface slopes, a lower strain rate and deeper flows. The effect of the interstitial fluid weakens as rotation rate is lowered, resulting in the same slope in the limit of no rotation, i.e., angle of repose. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P29.00010: Velocity Profiles in a Rotating Drum: The Effects of Cohesion Robert C. Brewster, Leonardo E. Silbert, Gary S. Grest, Alex J. Levine The dynamics of granular media in a rotating drum is important in a wide range of applications in industry associated with mixing granular materials. The rotating drum also serves as a standard experimental geometry to observe continuous avalanching in the laboratory. We study the effect of interparticle cohesion on the velocity field of the rotating drum using large scale granular dynamics simulations. Such cohesion is easily introduced in the system by a wetting fluid that forms menisci at interparticle contacts. Previously, we have examined the effect of interparticle cohesion in gravity driven chute flows, and have shown that the cohesion has a dramatic effect on the granular rheology. For strong enough cohesion, these forces generate a coherently moving plug at the free surface. In this talk, we examine the velocity profile in the rotating drum geometry in this plug-flow regime. We compare our results for angle of the pile in the continuous flow regime to the experiments of Nowak et al. [\textit{Nature Physics}, \textbf{1} (2005)] and we examine the stress and velocity profile within the pile as well. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P29.00011: Stability of Binary Granular Mixtures Adrian Swartz, Jeremy Olson, J. Bryce Kalmbach, Rena Zieve We study stability of a binary granular mixture. The two grain types are spherical ball bearings, and hexagonal shapes created by welding seven of the spheres together. The shapes are confined to a two-dimensional drum, which rotates slowly enough for discrete avalanches to occur. On average homogeneous piles of hexagonal reach a higher angle before an avalanche than homogeneous piles of spheres, by nearly twenty degrees. As the concentration of spheres is increased in a pile of mostly hexagons, the stability angle decreases more than twice as fast as expected by a linear interpolation between the homogeneous values. The spheres also tend to clump in the middle of the drum, and this segregation appears to cause the nonlinearity in angle. This indicates that the central portion of the drum is the most important in triggering avalanches. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P29.00012: Shape and Velocity Profile of the Core in a Radially Segregated Rotating Cylinder of Granular Particles Lori Sanfratello, Eiichi Fukushima We experimentally investigate a 3D biparticulate system that segregates only radially, with no evidence of axial segregation either at or below the surface even after hours of rotation. We compare the location and shape of the core of smaller particles, as well as the location of the bottom of the flowing layer, at various rotation rates using magnetic resonance imaging (MRI) in a 5mm slice at the axial center of a 3D cylinder. MRI is used because of its ability to non-invasively measure bulk behavior as well as spatially resolve dynamic variables (e.g. velocity, diffusion) at any location within a 1-, 2- or 3D system. We also compare the velocity depth profile of the radially segregated system with that of pure small and pure large particle systems and provide an explanation for the observed differences. These investigations may help clarify not only what is occurring within a radially segregating system of particles, but also which mechanisms influence the development of axial segregation. [Preview Abstract] |
Session P30: Focus Session: Rheology and Hydrodynamics of Wormlike Micellar Fluids
Sponsoring Units: DFDChair: Andrew Belmonte, Pennsylvania State University
Room: Colorado Convention Center 304
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P30.00001: Dynamics of the Shear Banding Instability Invited Speaker: A variety of complex fluids, such as liquid crystals, polymers, and surfactant solutions (lamellae or cylindrical micelles), are easily perturbed by shear flow and exhibit apparent ``phase transitions'' and complex nonlinear dynamics. ``Shear banding,'' or separation of material into bands of different apparent viscosities, has been reliably observed in wormlike micelles and many other systems. Despite a general one dimensional (1D) theory that predicts stable bands, recent experiments suggest that the generic situation is dynamic, rather than steady bands. After an overview I will discuss recent calculations that address the possibility of rheo-chaos, and two dimensional calculations to verify or refute the previously found 1D solutions. I will also discuss the possibility of shear banding in the closely related system of entangled polymer solutions. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P30.00002: Light-Induced Gelling in a Micellar Fluid Based on a Zwitterionic Surfactant. Rakesh Kumar, Srinivasa Raghavan Fluids with photoresponsive rheological properties (i.e. photorheological or PR fluids) can be useful in a range of applications, such as in dampers, sensors, and valves for microfluidic or MEMS devices. Previously, we have demonstrated a cationic surfactant-based PR fluid whose viscosity can be rapidly decreased by UV irradiation. This viscosity decrease was not reversible. Here, we describe a different formulation based on a zwitterionic surfactant that shows a rapid increase in viscosity (gelling) upon exposure to UV radiation. The formulation consists of the zwitterionic surfactant and a photosensitive cinnamic acid derivative. Initially, the viscosity of the fluid is low indicating the presence of small micelles. Upon UV irradiation, the cinnamic acid derivative is photoisomerized from trans to cis. In turn, the small micelles transform into long wormlike micelles, thus increasing the solution viscosity by more than five orders of magnitude. Small angle neutron scattering (SANS) data confirms the dramatic increase in micelle length. Possible reasons for such changes in micelle dimensions will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P30.00003: PB-PEO wormlike micelles under oscillatory shear flow as probed by Time-resolved SANS Minne Paul Lettinga, Barbara Lonetti, Joerg Stellbrink, Joachim Kohlbrecher Polybutadiene-poly(ethylene oxide) (2.5 kd:2.5 kd) diblock copolymers form wormlike micelles, known to undergo an isotropic to nematic phase transition at 5{\%} w/w. The rheological properties of this system display similarities to surfactant wormlike micelles. The theory describing this type of `living' polymers uses the concept of reptation in combination with the kinetics of breaking and re-formation of the micelles to predict the dynamical response of such systems. In practice the dynamical characterization is limited to the determination of the crossover point between the storage and lost moduli and thus to the linear properties of the system. Here we present an \textit{in situ} study of the response of pb-peo in the vicinity of the I-N transition to an oscillatory shear field. We determine the (non-) linear response of the Kuhn-segments applying a novel approach to obtain high time-resolution Small Angle Neutron Scattering ($\Delta $t$\ge $5 ms). We interpret our data using the reptation time as determined by high-speed confocal microscopy on labeled pb-peo. Thus we obtain a microscopic understanding of the dynamics of `living' polymers. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P30.00004: A rheological study of wormlike micelles flows in microchannel Jean-Baptiste Salmon, Chlo\'e Masselon, Annie Colin Complex fluids show non linear properties under simple shear flow leading to flow induced phase transitions and instabilities. The flow curve of wormlike micelles exhibit a stress plateau separating high and low viscosity branches, corresponding to shear-banding flows. Our aim is to understand the structure/concentration/flow coupling of wormlike micelles. A microfluidic chip is easy to couple with many analytical methods; it is hence well adapted to our study. We both perform particle image velocimetry and microscopy on a microfluidic chip consisting in channels with dimensions 250 $\mu $m large and 1 mm deep. Such a canyon geometry enables us to relate the measured velocity profiles to the local rheology. We evidence shear banding and slip at the walls. Strikingly there is no single rheological law that describes the velocity profiles at different pressure drops. Using microscopy, we point out turbid bands at the walls corresponding to the highly sheared bands. At low pressure drops, these bands are stable in time and their widths increase with increasing pressure until a limit where they fluctuate in space and time. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P30.00005: Nonlinear microrheology of wormlike micelle solutions using ferromagnetic nanowire probes N. Cappallo, C. Lapointe, D. H. Reich, R. L. Leheny We describe the application of high-aspect-ratio ferromagnetic nanowires to the microrheology of wormlike micelle solutions composed of equimolar cetylpyridinium chloride/sodium salicylate (CPCl/NaSal). Employing video microscopy to track the rotation of suspended nanowires in response to external magnetic fields, we access both the linear and nonlinear rheology of the fluid. The linear viscosity at low rotation rates is strongly temperature dependent as expected from macroscopic rheometry. At high rotation rates the viscosity exhibits pronounced shear thinning. The onset of the nonlinear response is characterized by a temperature-dependent shear thickening that has no apparent counterpart in the macroscopic rheometry. Time-resolved measurements involving step changes in rotation rate reveal that, once the fluid has been prepared into a shear-induced state, it exhibits nonlinear viscosity within the expected linear regime. Further, the shear-induced state of the fluid generates an out-of-plane torque on the wire that we have characterized by time-resolved studies. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P30.00006: Disentanglement behavior of DNA and wormlike micellar solutions as probed with particle-tracking velocimetry. Pouyan Boukany, Shi-Qing Wang We study an ideal entanglement network to test a number of emerging ideas about how topological entanglement reorganizes in presence of shear flow. Aqueous DNA solutions and wormlike micellar solutions can be highly entangled at very low concentrations and thus very soft yet sluggish. A particle tracking velocimetric method, which was developed recently in our lab [1], was applied to determine the velocity profile of these solutions in simple shear under several flow conditions including large step strain, large amplitude oscillatory shear, startup continuous shear and creep. It is shown [2] that all of the nonlinear viscoelastic flow behavior is associated with development of inhomogeneous shear when nucleation of chain disentanglement takes place in reaction to imposed shear deformation. [1] \textit{Phys. Rev. Lett. }\textbf{96}, 016001 (2006); \textit{ibid}. \textbf{96}, 196001; \textit{ibid.} \textbf{97}, 187801. [2] Manuscripts to be submitted to \textit{Macromolecules} and \textit{Langmuir}. [Preview Abstract] |
Session P31: Supersolid Experiments
Sponsoring Units: DCMPChair: Norbert Mulders, University of Delaware
Room: Colorado Convention Center 401
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P31.00001: Effect of $^{3}$He impurity on the supersolid transition of $^{4}$He E. Kim, J. S. Xia, J. T. West, X. Lin, M. H. W. Chan The supersolid phase of $^{4}$He was reported by a series of torsional oscillator experiments [1]. One of the most striking features of the supersolid transition is the intriguing $^{3}$He impurity effect. The addition of an extremely small amount of $^{3}$He impurity broadens the transition and enhances the transition temperature T$_{c}$. This effect is very different from that in helium film and that in `bulk' superfluid helium. We have studied the influence of $^{3}$He impurity on the supersolid transition systematically by progressively diluting isotopically-pure $^{4}$He ($^{3}$He impurity less than 2ppb) with $^{3}$He. The transition temperature is monotonically enhanced with increasing $^{3}$He concentration and the supersolid fraction shows a broad maximum around 0.2 ppm. [1] E. Kim and M. H. W. Chan, \textit{Science} \textbf{305}, 1941 (2004); \textit{Nature} \textbf{425}, 227 (2004); \textit{J. Low Temp. Phys.} \textbf{138}, 859 (2005); \textit{Phys. Rev. Lett. }\textbf{97}, 115302 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P31.00002: NCRI in Helium Crystals Grown Under Constant Pressure A.C. Clark, M.H.W. Chan A prominent issue concerning supersolidity in $^4$He is crystal quality. Several theoretical studies have demonstrated that a perfect crystal is insulating. Apparent experimental discrepancies between different laboratories, while very interesting, have not resolved the matter. In the torsional oscillator experiments, all solid samples previously studied were grown under constant volume. A decrease in pressure occurs during growth so that crystals are forced to expand, possibly resulting in highly strained crystals. There has also been no attempt to seed a single crystal, presumably leading to polycrystallinity. It is known that crystals carefully grown under constant pressure are of high quality. We report on new torsional oscillator measurements of isotopically pure solid $^4$He grown under constant pressure. We detect non-classical rotational inertia (NCRI) in all samples grown to date. Comparisons will be made to earlier studies. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P31.00003: Characterization of $^4$He Samples Exhibiting NCRI M.J. Bowne, Z. Cheng, J.T. West, A.C. Clark, M.H.W. Chan We plan to carry out sound measurements on solid $^4$He samples contained in a torsional oscillator. We believe \textit{in situ} characterization of samples demonstrating non-classical rotational inertia (NCRI) can lead to a better understanding of the microscopic mechanism behind supersolidity. Sound pulses will be generated and detected with a single quartz transducer housed within the torsion cell. The velocity of sound and attenuation will be extracted from the pulse echoes. This information conveys the relative quality of samples, as well as the orientation of the c-axis for single crystals. Preliminary measurements are under way in a test apparatus. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P31.00004: Excess specific heat of solid $^4$He Xi Lin, A. C. Clark, M. H. W. Chan An experimental challenge on heat capacity measurement is discerning the small specific heat of solid helium from that of the metallic cells typically used to confine the high-pressure solid. We report on heat capacity measurements of solid $^4$He contained in a silicon cell, in our search of a thermodynamic signature of the supersolid phase. Data will be presented for several solid samples around 26 bar, where the heat capacity is at least 10 times larger than that of the silicon cell. Below 200mK we observe a heat capacity in excess of that predicted by Debye theory. It is unclear if our observations are directly associated with the supersolid $^4$He phase. In the hope to elucidate whether this phenomenon is connected to the supersolid phase, we are currently investigating the effect of $^3$He impurities. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P31.00005: Shear measurements of bulk solid $^{4}$He James Day, John Beamish Recent torsional oscillator experiments indicate that the non-classical rotational inertia (NCRI) fraction depends on isotopic purity and on the details of crystal growth and annealing, suggesting that defects may be involved. While solid helium does not flow in response to pressure gradients at low temperatures, plastic deformation of solid helium closer to melting creates defects and pressure gradients which are not easily eliminated by thermal annealing. Similar defects must be created during crystal growth by the blocked capillary method or by large thermal gradients. Given the theoretical arguments against supersolidity in defect-free crystals and the preliminary experimental evidence linking NCRI to annealing, it is important to control and study defects in solid helium more directly. To that effect, we have begun to study the static and low frequency shear deformation of crystals grown by different methods. This is a direct measure of the shear modulus of the crystal and should allow us to separate elastic from inertial effects. We can also compare the elastic to the plastic deformation response by increasing the magnitude of the shear stress applied to the crystal. We will describe our experimental design and present preliminary results. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P31.00006: Flow mechanisms of solid $^{4}$He near melting John Beamish, James Day In our recent experiments, we saw no evidence of pressure induced flow of solid helium, neither in the pores of Vycor nor in bulk, in the temperature range where non-classical rotational inertia (NCRI) has been observed; however, we did observe mass flow close to the melting points of our samples. Mass can be transported in crystals through vacancy movement or via the motion of extended defects like dislocations. The high temperature flow of helium confined in Vycor is quite different from that of bulk helium; different mechanisms appear to be involved. In bulk helium the flow is irreversible and is consistent with the creation of defects like dislocations during plastic flow. Plastic flow and dislocation creation cannot occur in nanometer scale channels, and so it is not surprising that we see different behavior for helium in the pores of Vycor. The thermally activated mass flow in Vycor must be due to motion of vacancies or similar point defects. In this talk we will describe the nature of observed flow and possible mechanisms, and discuss its relevance to the NCRI experiments. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P31.00007: Effect of Crystal Growth Velocity on ``Supersolidity'' of $^4$He Keiya Shirahama, Motoshi Kondo, Shunichi Takada, Yoshiyuki Shibayama One of the most important issues of supersolid studies is to elucidate the relationship between the supersolid behaviors and quality of $^4$He crystal. Recent observation of the annealing effect by Rittner and Reppy suggests that supersolidity is strongly dependent on the sample history. We have examined the effect of crystal growth velocity and crystal annealing on supersolidity of solid $^4$He formed in a cylinder torsional oscillator[1]. Solid samples at various pressures are grown by cooling liquid $^4$He under isochoric conditions (blocked capillary method). When the cooling velocity is high (0.1K/min) during crystal growth, the supersolid fraction is 3 times as large as that of slowly grown samples. This supersolid fraction decreased to 1/3 by sample annealings for 1 day near the melting point. On the other hand, no annealing effect is observed in the slowly grown samples. These behaviors strongly suggest that lattice defects formed in the crystal growth process play a crucial role on supersolidity of $^4$He. [1] M.Kondo et al., J.Low.Temp.Phys., to be published [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P31.00008: Influence of sample geometry on the supersolid signal Ann Sophie C. Rittner, John D. Reppy We have used a torsional oscillator with an annular geometry in order to study the correlation between sample volume and supersolid signals systematically. We varied the width of the annulus in the cell with cylindrical magnesium inserts of different radii. In preliminary measurements on an open cylinder cell, we have found an apparent supersolid fraction $\frac{\rho_s}{\rho}$ of 0.04 \% at 26 bar, a maximum velocity of 23 $\mu m/s$ and a sample volume of 2 cc. In an annular cell with a width of 0.635 mm we measured a supersolid fraction of about 0.33 \% at a velocity of 16 $\mu m/s$ and a pressure of 32 bar. The increase of the supersolid fraction in restricted geometries suggests that defects cause the NCRI behavior and could explain the different results of previous torsional oscillator measurements. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P31.00009: Torsional Oscillator for Studying Supersolid $^4$He at Two Resonant Frequencies Joseph Graves, Yuki Aoki, Harry Kojima In order to observe supersolid behavior in solid $^4$He at multiple frequencies while keeping all other parameters constant, an oscillator with two torsional modes has been constructed. The torsion rod is made of beryllium copper and the cylindrical sample chamber is made of Stycast 1266. The two modes have resonant frequencies of 500 and 1200 Hz. Preliminary studies have shown fairly high quality factors of 10$^4$ at 300 K and 7$\times$10$^4$ at 77 K. We plan to measure the changes, at the two frequencies in the identical solid $^4$He sample, of the resonant frequency, dissipation and critical velocity associated with the supersolid phase at temperatures below 200 mK. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P31.00010: Search for new evidence of superfluidity in solid $^4$He by phonon propagation Yuki Aoki, Harry Kojima The phonon propagation generated by heat pulse has been studied in solid $^4$He under pressure between 25 and 56 bar to search for a sign of supersolid behavior at temperatures down to 40 mK. Response to input heat pulses are detected by a titanium film superconducting edge bolometer separated by a 4.5 mm thick solid $^4$He from the heater. According to theoretical studies, a new fourth sound-like mode is expected to emerge in the supersolid state. The sensitivity of our bolometer has been improved from an earlier version by an order of magnitude by changing the film structure for the purpose of searching for a small temperature deviation signal accompanying the fourth sound-like propagation mode. The response of the bolometer to heat pulse was measured in different quality solid samples which had been grown with different cooling rates during solidification. The detected response signal has not revealed any identifiable signature of a new mode within a temperature excursion of about $\Delta T$ = 5 $\mu$K from the background signal shape. An estimated superflow velocity corresponding to the temperature excursion is greater than the critical velocity observed by Kim and Chan. Our detection sensitivity must be further increased before a definitive conclusion on the fourth sound-like mode can be made. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P31.00011: Debye-Waller factor in solid He-4 at sub-Kelvin temperatures Elizabeth Blackburn, John M. Goodkind, Sunil K. Sinha, Jacob Hudis, Collin Broholm, Joost van Duijn, Richard Down, Oleg Kirichek, Chris D. Frost The recent observation by Kim and Chan [Science 305 (2204) 1941] of a transition at low temperatures ($\sim $ 200 mK) in the hcp-phase of solid helium has re-opened interest in the old question of supersolidity. The nature of the low-temperature phase remains in question, and to investigate this in more detail, we have measured the density distribution of He-4 nuclei in crystals of He-4 with a molar volume of 21.3 cm$^{3}$ down to 140 mK. We find no evidence for any changes in the vicinity of the transition. Treating the material as a traditional crystal, we have extracted the mean square displacement for the nuclei and find anisotropy between the in- and out-of-plane motions. Our values are in agreement with previous work at higher temperatures. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P31.00012: Precise neutron diffraction study of hcp and bcc $^{4}$He Ralph Simmons, Robert Blasdell Precise lattice parameter measurements are reported for $^{4}$He in both bcc and hcp phases at low density and low temperature. The results can be used to set limits on a proposed incommensurate equilibrium state of solid $^{4}$He near $T$ = 0. ``Incommensurate" means a net difference between atomic sites and atoms. The relative difference is defined as $\epsilon$. Present measurements were made by carefully calibrated neutron diffraction. The value established at melting, by comparison with published bulk density values, is $\epsilon = 0.4\pm 0.4 \%$. Much of the uncertainty comes from uncertainties in the bulk values. These neutron results on hcp $^{4}$He are also consistent with previous precise x-ray diffraction work on bcc $^{4}$He and, at higher densities, on both $^{4}$He and $^{3}$He. Published isochoric measurements of changes in x-ray lattice parameters as $T$ is reduced from melting can be used to extrapolate $\epsilon$ toward zero $T$, where its most probable value is zero, with the same uncertainty. The present neutron work on hcp phase agrees with published high-resolution synchrotron x-ray work in showing that the $(c/a)$ ratio is slightly smaller than that corresponding to ideal close-packing. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P31.00013: New excitations in bcc $^{4}$He - an inelastic neutron scattering study Oshri Pelleg, Jacques Bossy, Emmanuel Farhi, Meni Shay, Slava Sorkin, Emil Polturak We report results of inelastic neutron scattering experiments on bcc solid $^{4}$He (Pelleg et al. \textit{Phys. Rev. B.} \textbf{73}, 180301(R)(2006)). In the experiments, we studied the excitation spectrum of the solid, including the phonon branches and the recently discovered ``optic-like'' branch ( T.Markovich et al., \textit{Phys. Rev. Lett.} \textbf{88}, 195301(2002)). We were able to determine that the new ``optic-like'' branch has an intrinsic dispersion, hence it is a propagating mode. This excitation also couples to the usual phonons. In addition, in the new experiments we discovered another ``optic-like'' branch. The second ``optic-like'' branch is dispersionless, with an energy around 1 meV ($\sim$~11K). This excitation does not seem to couple to phonons. Hence, the properties of the two ``optic-like'' branches seem different. Since one expects only 3 acoustic phonon branches in a monoatomic cubic crystal, these two new branches must represent some different type of excitations. Some potential interpretations, based on Path Integral Monte Carlo simulations, will be presented. [Preview Abstract] |
Session P32: Bosons in Optical Lattices
Sponsoring Units: DAMOPChair: Lincoln Carr, Colorado School of Mines
Room: Colorado Convention Center 402
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P32.00001: Paired phases of bosons in optical lattices Stephen Powell, Subir Sachdev We describe the conditions under which bosons in optical lattices can form paired condensates, focusing on the case of bosons with spin. We show that the ground state of such a system, with sufficiently strong spin-dependent interactions, is a \textit{spin-singlet condensate}, which preserves spin-rotation symmetry. We then consider the gapped single-particle excitations across the phase transition from the insulator, and show that they have nontrivial scaling behavior, determined by coupling to the critical pair modes. [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P32.00002: Edge States in Cold Atom Optical Lattices Vito Scarola, Sankar Das Sarma We argue that edge state response to external potentials applied to trapped insulators in cold atom optical lattices offer a unique probe of bulk physics. As an example we study the trapped Bose-Hubbard model using Gutzwiller mean-field theory. We calculate the response of Mott insulator edge states to external potentials. We show that the response leads to observables which may be extracted from time of flight measurements. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P32.00003: Superfluid to Mott Transition in the Bose Hubbard Model: Evidence for New Modes Naoki Kawashima, Yasuyuki Kato, Chiara Menotti, Nandini Trivedi Using a combination of methods (mean-field theory, fluctuations within random phase approximation, and quantum Monte Carlo simulations), we determine the nature of the phases of the Bose Hubbard model. In addition to the sound mode, we find evidence for extra gapped modes in the correlated superfluid phase from the location of the poles of the Green function. We also calculate the effect of thermal and quantum fluctuations on the condensate fraction and compare with recent experiments in optical lattices. In particular, we have obtained the superfluid density and the order parameter independently which agree with each other deep in the condensate phase but disagree in the critical region. We also calculate the Green's function as a function of the distance and the imaginary time separation, from which we estimate the excitation gap of the boson quasi particles. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P32.00004: Lattice with a Twist: A Helical Waveguide for Ultracold Matter M. Bhattacharya The behavior of matter is governed by the geometry of the potential it experiences. We consider the construction of optical potentials with helical symmetry, which can confine cold atoms and molecules. Microparticles have been experimentally confined in similar potentials [1]. Using two counter-propagating Laguerre-Gaussian beams we show that this simple chiral system realizes a superlattice of helical waveguides for ultracold matter and allows experimental control of their number, helicity, radius, pitch as well as strength and aspect ratio of confinement. In the simplest nontrivial case the potential has double-helical symmetry, similar to DNA. In general the behavior of massive particles in a helical potential is expected to be rich due to the periodic modulation of their motion along the lattice; negative group velocities and effective masses are expected. Effects such as spin squeezing and Berry's phase are also possible. A helical waveguide can provide a phase hologram for atom-waves, and perhaps support geometrically bound states. We will also address the curious possibility of simulating atom transport in carbon nanotubes. \newline [1] M. P. MacDonald \textit{et al.} Opt.Commun. \textbf{201},21(2002). [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P32.00005: Tunneling resonances and entanglement dynamics of ultracold bosons in a tilted two-well potential Dimitri Dounas-Frazer, Ann Hermundstad, Lincoln Carr We study the quantum sloshing of ultracold bosons in a tilted double-well potential via exact diagonalization of the two-mode Bose-Hubbard Hamiltonian. Tunneling is extremely sensitive to a small potential difference between wells, or tilt. However, when the barrier is high, atom-atom interactions can compensate the tilt and produce a tunneling resonance [1, 2].~ At resonance, tunneling times on the order of 10-100 ms are possible. Furthermore, tunneling resonances constitute a dynamic scheme for creating robust few-atom entangled states in the presence of many bosons. \newline [1] D. R. Dounas-Frazer and L. D. Carr, e-print: quant-ph/0610166 (2006). \newline [2] D. R. Dounas-Frazer, A. M. Hermundstad, and L. D. Carr, e-print: quant-ph/0609119 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P32.00006: Cavity QED determination of atomic number statistics in optical lattices. Wenzhou Chen, Dominic Meiser, Pierre Meystre We study the reflection of two counter-propagating modes of the light field in a ring resonator by ultracold atoms either in the Mott insulator state or in the superfluid state of an optical lattice. We obtain exact numerical results for a simple two-well model and carry out statistical calculations appropriate for the full lattice case. We find that the dynamics of the reflected light strongly depends on both the lattice spacing and the state of the matter-wave field. Depending on the lattice spacing, the light field is sensitive to various density-density correlation functions of the atoms. The light field and the atoms become strongly entangled if the latter are in a superfluid state, in which case the photon statistics typically exhibit complicated multimodal structures. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P32.00007: Bose-Einstein Condensates in Optical Lattices: Resonantly Enhanced Tunneling and Nonlinear Effects Alessandro Zenesini, Carlo Sias, Lignier Hans, Yeshpal Singh, Donatella Ciampini, Sandro Wimberger, Riccardo Mannella, Oliver Morsch, Arimondo Ennio In our experiments we study the tunneling between different sites of a periodic potential in the presence of an external force. As a consequence of Wannier-Stark localization of atomic wavefunctions inside the single lattice sites, \emph{Resonantly Enhanced Tunneling (RET)} occurs when the spacing between energy levels in a potential well is equal to the field-induced energy shift between different wells. These resonances are an important modification to the smooth Landau-Zener formula. We observed \emph{RET} using Bose-Einstein condensates in accelerated optical lattice potentials. We have perfect control over the parameters of this system: the depth of the lattice $U_0$, the recoil energy $E_{rec}$ and the peak density $n_0$ in the dipole trap. The latter determines the nonlinear interaction energy of the system, which allowed us to study the behavior of condensates in different regimes of the nonlinearity. In the linear case, as predicted in the Wannier-Stark solution, we observed \emph{RET} and we verified the dependence between the positions of the resonances and the lattice depth for tunneling between $1^{st}$, $2^{nd}$ and $3^{rd}$ neighboring sites. In the nonlinear regime, we observed a suppression of the resonances for increasing nonlinearity, in agreement with numerical simulations. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P32.00008: Phase diagram for ultracold bosons in double-well optical lattices Ippei Danshita, James E. Williams, Carlos Sa de Melo, Charles W. Clark We study the superfluid-Mott insulator transition of bosons in double-well optical lattices. Applying a mean-field approximation to the Bose-Hubbard Hamiltonian, we obtain the zero-temperature phase diagram and find that there exist the half-integer-filling and integer-filling Mott insulator domains in the phase diagram. We show that the half-integer-filling Mott insulator phase is stabilized as the intra-well hopping energy increases. We also calculate the phase diagram by employing the time evolving block decimation (TEBD) algorithm and compare the results obtained from the mean-field approximation with those from the TEBD. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P32.00009: On-site number statistics of ultracold lattice bosons Evgeny Kozik, Barbara Capogrosso-Sansone, Nikolay Prokof'ev, Boris Svistunov We study on-site occupation number fluctuations in a system of interacting bosons in an optical lattice. The ground-state distribution is obtained analytically in the limiting cases of strong and weak interaction, and by means of exact Monte Carlo simulations in the strongly correlated regime. As the interaction is increased, the distribution evolves from Poissonian in the non-interacting gas to a sharply peaked distribution in the Mott-insulator (MI) regime. In the special case of large occupation numbers, we demonstrate analytically and check numerically that there exists a wide interval of interaction strength, in which the on-site number fluctuations remain Gaussian and are gradually squeezed until they are of order unity near the superfluid (SF)-MI transition. Recently, the on-site number statistics were studied experimentally in a wide range of lattice potential depths [Phys. Rev. Lett. \textbf{96}, 090401 (2006)]. In our simulations, we are able to directly reproduce experimental conditions using temperature as the only free parameter. Pronounced temperature dependence suggests that measurements of on-site atom number fluctuations can be employed as a reliable method of thermometry in both SF and MI regimes. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P32.00010: Phases in an anisotropic two-dimensional optical lattice Sara Bergkvist, Anders Rosengren, Robert Saers, Emil Lundh, Magnus Rehn, Anders Kastberg We have studied the effects of anisotropy on a two-dimensional optical lattice using quantum Monte Carlo simulations. For finite lengths, such a system undergoes a one-dimensional quantum phase transition to a 1D Mott insulator of decoupled chains. Time of flight pictures and other measurable observables are calculated for a specified experimental setup. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P32.00011: Mach-Zehnder Interference of Boson Flavor States in the Excited Band of a 2D Optical Lattice John Challis, Steven Girvin, Leonid Levitov Bosons promoted to the first excited Bloch band of an optical lattice have two important properties: they are metastable, having lifetimes long compared to the nearest neighbor hopping rate, and they carry a ``flavor'' quantum number which controls the direction of highly anisotropic hopping in the lattice \footnote{A Isacsson and SM Girvin. Phys. Rev. A 72, 053604 (2005) }. For a 2D optical lattice where the laser beams are not quite perpendicular, there is a small energy which causes the flavors to mix. The two flavor states can be treated as a two-level system with an avoided crossing, with the relative intensity of the two laser beams serving as a tuning parameter controlling the energy difference between the two flavors. When the tuning parameter is varied sinusoidally around some nonzero offset, the avoided crossing acts like a beam splitter in a Mach-Zehnder interferometer \footnote{WD Oliver et al. Science Vol. 310, Issue 5754, pp. 1653-1657(2005)}. Since this offset is momentum dependent, the rate of flavor change varies throughout the Brillouin Zone. This fact leads to interesting time-dependent momentum distributions which should be readily observable experimentally by free expansion of the bosons. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P32.00012: Exploring the correspondence principle with spinor condensates: from quantum Bloch oscillations to classical Bogoliubov excitations Reinhold Walser, Carsten Wei{\ss}, Oliver Crasser, Wolfgang Schleich By tuning the relative strength between single and two-body energies in a spinorial F=1 Bose-Einstein condensate (e.g. $^{87}$Rb), we can effectively control the dynamics of the macroscopic Fock-state [1,2,3]. We will study the static as well as dynamic aspects of this few mode quantum system and illustrate the ``classical'' as well as quantum aspects of this system, which can be realized in deep optical lattices.\\ \noindent [1] M-S. Chang {\em et al.}, Coherent spinor dynamics in a spin-1 Bose-Einstein condensate, Nature Phys., {\bf 1}, 111 (2005). \\ \noindent [2] J. Kronj{\"a}ger {\em et al.}, Evolution of a spinor condensate: coherent dynamics, dephasing and revivals, Phys. Rev. A, {\bf 72}, 063619 (2005).\\ \noindent [3] A. Widera {\em et al.}, Precision measurements of spin-dependent interaction strength for spin-1 and spin-2 $^{87}$Rb atoms, NJP, {\bf 8}, 152, (2006). [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P32.00013: Dispersive shock waves in optical lattices Shu Jia, Wenjie Wan, Jason Fleischer We study dispersive, superfluid-like shock waves in optical lattices. Compared with the homogeneous case, the presence of a periodic potential inhibits shock propagation, both via Peierls-Nabarro trapping forces and through Bragg reflections. Photonic experiments are performed in SBN photorefractive crystals, using optical induction to create nonlinear waveguide arrays. By applying defocusing (repulsive) nonlinearity, we directly observe the nonlinear properties of shock waves as a function of the intensity of lattice, $i.e.$ the depth of the potential wells. Direct comparisons are made between a plane-wave background, suitable for homogeneous systems, and Bloch-wave backgrounds, which are more appropriate for arrays. Nonlinear coupling between transmission bands is demonstrated, with numerical simulations showing excellent agreement with experimental results. Similarities between photonic systems and cold atom systems in periodic potentials will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P32.00014: Evolution of Hard-Core Bosons in a Time-Dependent Trap Aditya Raghavan, Marcos Rigol, Stephan Haas We present a study of the time evolution of hard-core bosons (HCB) in a one-dimensional, time-varying optical trap. Previous results have shown that one-dimensional HCBs can form superfluid and Mott-insulator phases. Using numerical techniques in the Bose-Hubbard model, we explore different types of time variations, such as sinusoidally varying trap curvature, using either initial configurations (filling {\&} trap curvature) of a superfluid or a Mott-insulator. ~When the curvature of the optical trap is suddenly increased, we observe a ``melting'' of the Mott-insulator. The approximate numerical technique used to study time-varying traps is discussed. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P32.00015: Quantum phases and phase transitions in bosonic mixtures induced by non-s-wave Feschbach resonances in optical lattices Anatoly Kuklov Feschbach resonance at finite angular momentum in a mixture of distinguishable bosons in optical lattice (OL) can induce quantum phase transitions (QPTs) into states which break OL symmetries and time reversal. In particular, a two-component mixture, with one component being superfluid and the other Mott insulator, can undergo QPT into, e.g., p-wave condensate characterized by lines of zeros, spontaneous currents and by strong quantum depletion \footnote[2]{A.B.Kuklov, PRL {\bf 97}, 110405(2006)}. The ground state is sensitive to rotation of OL. Analogously, a featureless two-component Mott insulator can undergo QPT into the insulator with broken lattice symmetries. While impossible for an absolute ground state, such effect can be realized in the context of metastable phases generic for atomic traps and OLs as long as there is a large energy difference between the resonance and true molecular ground state. A condition for such transition is that the closed-open channels coupling exceeds the onsite excitation energy in the regime of weak tunneling between sites. Standard imaging techniques can be used to identify such phases. [Preview Abstract] |
Session P33: Focus Session: Superconducting Qubits IV
Sponsoring Units: GQIChair: Charles Bennett, IBM Yorktown Heights
Room: Colorado Convention Center 403
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P33.00001: A new type of superconducting qubit: How the transmon thwarts the $T_2$ problem Terri M. Yu, Jens Koch, Jay Gambetta, Andrew A. Houck, David I. Schuster, Johannes Majer, Robert J. Schoelkopf, Steven M. Girvin Superconducting qubits have long been dogged by small energy relaxation ($T_1$) and dephasing times ($T_2$). Here we propose a new type of superconducting qubit that we call the ``transmon.'' This device consists of a Cooper pair box shunted by a large capacitance. The two quantities crucial to the operating the transmon as a qubit are a) energy level anharmonicity and b) charge noise sensitivity. Sufficient anharmonicity is required to prevent transitions out of the qubit two-level system. Low sensitivity is desired so that fluctuations of the gate charge do not appreciably change the qubit transition frequency. Decreasing (increasing) the Josephson energy to charging energy ratio ($E_J$/$E_C$) of the transmon raises (reduces) anharmonicity and charge noise sensitivity. By operating the transmon in a radically different parameter regime $10^1 < E_J/E_C \ll 10^3$, the qubit becomes exponentially more stable to charge fluctuations compared to the Cooper pair box, yet it retains enough anharmonicity for fast qubit operations. For large enough $E_J/E_C$, dephasing due to charge noise becomes completely negligible so that greatly enhanced $T_2$ times should be achievable. [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P33.00002: Circuit QED with a new type of qubit: The transmon coupled to a transmission line resonator Jens Koch, Terri M. Yu, Jay Gambetta, Andrew A. Houck, David I. Schuster, Johannes Majer, Robert J. Schoelkopf, Steven M. Girvin The idea of coupling a superconducting qubit to a one-dimensional transmission line resonator, termed circuit QED [1], has evolved into an important paradigm in the ongoing pursuit of quantum computing. Recent experiments using Cooper pair boxes (CPBs) have revealed impressive results ranging from the realization of the strong-coupling limit [2] to the observation of the ac Stark shift and measurement-induced dephasing [3]. Here, we present theoretical and experimental results on the circuit-QED physics of a new type of qubit - the transmon. We show that the reduced anharmonicity gives rise to a generalized Jaynes-Cummings model, whose coupling strength can be even larger as compared to typical CPB qubits. In the dispersive limit, the transmon displays an intriguing regime of large positive dispersive shifts. \newline [1] A. Blais et al., Phys. Rev. A 69, 062320 (2004) \newline [2] A. Wallraff et al., Nature (London) 431, 162 (2004) \newline [3] D. I. Schuster et al., Phys. Rev. Lett. 94, 123602 (2005) [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P33.00003: Sources of Decoherence in the Transmon Qubit. Joseph Schreier, Steve Girvin, Rob Schoelkopf Here we discuss a new type of superconduting qubit known as the transmon, a Cooper Pair Box in the high E$_{j}$/E$_{c}$ limit. This qubit offers insensivity to 1/f noise in charge while maintaining sufficient anharmonicity to be treated as a two level system. In this talk we consider other experimentally important sources of dephasing and relaxation including: substrate/dielectric loss, flux coupling through the SQUID loop, radiation to parasitic modes, vortices, phonons, and quasiparticles. Order of magnitude estimates for these sources of decoherence indicate that T$_{1 }$and T$_{2}$ coherence times of at least 10 $\mu $s should be attainable. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P33.00004: Macroscopic quantum tunneling in high-T$_c$ superconducting rings with intrinsic Josephson junction stacks X.Y. Jin, J. Lisenfeld, Y. Koval, A.V. Ustinov, P. M\"uller The properties of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ superconducting rings broken by intrinsic Josephson junction stacks were studied. The stack height was in between 4 and 50 junctions. SQUID behavior was observed in all devices. The modulation depth of critical currents increased with decreasing number of junctions in the stack, and conformed to the $\beta_L$ values. Furthermore, switching current distributions were investigated as a function of magnetic field and temperature. Crossover temperatures were in the range of 300 to 600 mK. Whereas the small stacks behaved like series arrays of independent junctions, the larger stacks were uniform and showed anomalous enhancement of escape rates. An unconventional coherent retrapping was observed, i.e., the retrapping probability decayed exponentially with the trapped flux. Possible implications for the realization of high-T$_c$ phase qubits are discussed. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P33.00005: Observation of macroscopic resonant tunneling in a superconducting flux qubit Bo Mao, Wei Qiu, Siyuan Han It has been argued recently that various coherent phenomena observed in superconducting phase and flux qubits could be due to classical phase-locking between oscillators (qubits) and sinusoidal driving (microwave) [1]. We report observation of macroscopic resonant tunneling (MRT) in a weakly damped flux qubit, i.e., a radio-frequency superconducting quantum interference device (rf SQUID). Since no microwave was involved in the experiment the observation of MRT unambiguously confirms that dynamics of superconducting flux qubits are governed by quantum rather than classical physics and that superconducting flux qubits are good candidates for implementing quantum computing. The measured tunneling rate as a function of flux bias agrees with the energy level structure calculated from independently determined physical parameters of the qubit. \newline \newline [1] N. Gronbech-Jensen and M. Cirillo, Phys. Rev. Lett. 95, 067001 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P33.00006: Effects of a resonant cavity on macroscopic quantum tunneling of fluxons in long Josephson junctins Ju Kim, Ramesh Dhungana We investigate the effects of a resonant cavity on the tunneling rate of a Josephson vortex (i.e., fluxon) which is pinned by a microresistor in long Josephson junction (LJJ). In a single LJJ, we find that the tunneling rate can be enhanced significantly when the fluxon couples to the electromagnetic field of the resonant cavity. Here the main effect of the cavity is reducing the barrier potential for the trapped fluxon. In a two LJJs that are coupled by the magnetic induction effect, the tunneling rate is determined by the competition between the strength of pinning due to the microresistor which tends to break the phase-locking behavior of the fluxons and the cavity mode which tends to promote collective motion of these fluxons. We discuss the effects of the resonant cavity on the tunneling of phase-locked fluxon-fluxon and fluxon-antifluxon pairs. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P33.00007: rf-reflectrometry measurements of a Josephson junction oscillator circuit at milliKelvin temperatures R. M. Lewis, B. K. Cooper, B. Palmer, Hanhee Paik, S. K. Dutta, T. A. Palomaki, A. J. PrzyPysz, H. Kwon, J. R. Anderson, A. J. Dragt, C. J. Lobb, F. C. Wellstood We report on rf-reflectometry measurements on a Nb/AlOx/Nb Josephson junction tank circuit. The junction has nominal critical current of 5 $\mu$A and is loaded with an on chip capacitance of 50 pF to suppress the plasma frequency to $f_p \approx 2$ GHz. Measurements were performed at temperature $T \approx 100$ mK in a dilution refrigerator. Reflection data show a clear rf absorption resonance and concomitant phase change about the resonant frequency. We will discuss use of this circuit for state readout \footnote{I. Siddiqi {\it et al.}, Phys.\ Rev.\ Lett.\, {\bf 93} 207002 (2004).} of a phase qubit and as a device for measuring critical current noise in Josephson junctions. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P33.00008: For Improving Superconducting Qubits. Raymond W. Simmonds, M.S. Allman, F. Altomare, K. Cicak, K.D. Osborn, A.J. Sirois, J.A. Strong, J.D. Whittaker Josephson junction-based superconducting qubits are still a very promising platform for creating quantum computers of the future. We have created a strategy to improve the coherence of superconducting phase qubits, through the removal of unwanted two-level system defects known to be a significant source of decoherence. Through creating dielectric free fabrication techniques and vaccum gap capacitors, we can remove a considerable amount of troublesome defects in the construction of phase qubits. Here, we discuss some results and obstacles still facing the design and fabrication of phase qubits. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P33.00009: Energy Decay in Josephson Qubits from Off-Resonant Coupling to Two-Level States J. Martinis, M. Ansmann, R. Bialczek, N. Katz, E. Lucero, R. McDermott, M Neeley, A. O'Connell, M. Steffen, E. Weig, A. Cleland Decoherence of Josephson qubits is thought to be protected from dielectric loss of two-level states by using sub-micrometer tunnel junctions that statistically avoids resonant coupling. Here, we calculate that off-resonant coupling and the subsequent phonon radiation of the two-level states may produce significant energy loss even for ultra-small junctions. This theory possibly explains several key features in a variety of experimental data for phase, flux, and charge qubits, such as the magnitude of the observed energy decay time, its statistical variation, and the increased decay rate with qubit area and frequency. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P33.00010: Quantum Process Tomography Using Superconducting Qubits Radoslaw C. Bialczak, M. Ansmann, N. Katz, E. Lucero, R. McDermott, M. Neeley, A. D. O'Connell, M. Steffen, E. Weig, A. Cleland, J. Martinis Due to recent advances in device design and materials, universal quantum gates using Josephson junction phase qubits are now feasible. To measure gate performance other quantum computation architectures have utilized standard quantum process tomography (SQPT). In SQPT one obtains a process matrix with which gate operations for arbitrary input states can be predicted and performance measures such as fidelity and entangling capability can be obtained. Here we demonstrate how to implement SQPT with our Josephson junction phase qubits and use it to characterize a CNOT gate. We show how to obtain the process matrix of a CNOT gate and extract its fidelity and entangling capability. This allows us to compare our gate performance to that of quantum gates in other architectures. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P33.00011: A Josephson junction~resonator to test~the quality of superconducting qubit circuits Kevin Osborn, Josh Strong, Adam Sirois, Raymond Simmonds Superconducting Josephson junction~resonators can~probe nonlinear oscillators~such~as~qubit~readout amplifiers and~qubits themselves.~We have fabricated~weakly coupled resonators~with a~flux tunable resonant frequency of over 1 GHz.~ At high powers the Josephson junction~resonators become nonlinear and two stable oscillation states are observed which can be harnessed to readout qubits.~ At sufficiently low powers, the~resonators~can probe a qubit since they store no more than~one photon of energy.~~At these low powers we observe two-level system defects attributed to the Josephson junction,~similar to those observed in the phase qubit.~ We have fabricated and measured~resonators with different~device parameters, such as the junction area and the critical current density.~ We will report on~the quality of the~resonators as a function of the device parameters. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P33.00012: Optimal Control of a Qubit coupled to a Two-Level Fluctuator Patrick Rebentrost, Ioana Serban, Frank K. Wilhelm, Thomas Schulte-Herbr\"uggen Experimental realizations of superconducting qubits are prone to decoherence from a fluctuating environment. An important source of charge and critical current noise are two-level fluctuators with a Lorentzian noise spectrum. We apply a recent generalization of quantum optimal control in presence of decoherence, the openGRAPE algorithm [1], to a microscopic model of a qubit coupled to a single two-level fluctuator. We find pulses that decrease the error of single qubit quantum gates and quantum memory up to a magnitude compared with conventional pulses. The qubit is pulsed in a way that the effect of the fluctuator is minimized. We investigate system and optimized pulses in various parameter regimes. [1] T. Schulte-Herbr\"uggen et al., quant-ph/0609037 [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P33.00013: A two-qubit gate based on a multi-terminal double barrier Josephson Serhii Shafraniuk A multi-terminal double barrier SISIS junction (S and I denote a superconductor and an insulating barrier respectively) is suggested as a two-qubit gate with tunable intrinsic coupling. Two quantum wells are formed in vicinities of the left and right SIS subjunctions. This gives two individual qubits, which are intrinsically coupled via the middle S layer due to phase coherence. The inter-qubit coupling $J$ is tuned by two bias supercurrents $I_1$ and $I_2$ across each of the SIS subjunctions independently. Additional coupling is accomplished by transport supercurrents $I^{\mathrm{tr}}_l$ along adjacent S layers. Using a microscopic model we compute major qubit characteristics and study sources of the intrinsic decoherence. We compute the entanglement of the two qubit states, leakage and fidelity characteristics versus $J$, and discuss the readout process. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P33.00014: Non-Linear, DC-Biased, Vacuum-Gap Capacitor LC Oscillators M.S. Allman, K. Cicak, K.D. Osborn, J.A. Strong, R.W. Simmonds We have observed non-linear behavior at higher powers in vacuum- gap capacitor LC resonators. These non-linear effects are a result of electrostatic forces on the vacuum capacitors. Electrostatic forces cause the distance between the capacitor plates to contract, resulting in a power-dependent resonant frequency. In an effort to characterize these non-linear effects, we have incorporated a bias T on chip allowing us to apply a DC voltage-bias to a fabricated vacuum-gap LC resonator. We then measure the resonant frequency of the circuit as a function of applied bias-voltage or equivalently, applied force on the vacuum-gap capacitor. This information allows us to predict the non-linear effects in our LC resonators as well as gives insight into the structural integrity of the vacuum-gap capacitors. These devices can lead to future applications of DC-biased vacuum-gap capacitors as tuneable superconducting phase qubit couplers. [Preview Abstract] |
Session P34: Focus Session: Cytoskeletal Dynamics and Cell Migration I
Sponsoring Units: DBP GSNP DPOLYChair: Margaret Gardel, Scripps Institute
Room: Colorado Convention Center 404
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P34.00001: Integration of actin dynamics and adhesion in cell migration Invited Speaker: Cell migration requires transmission of motion generated in the actin cytoskeleton to the extracellular environment through a complex assembly of proteins in focal adhesions. We developed Correlational Fluorescent Speckle Microscopy to measure the coupling of focal adhesion proteins to actin filaments. Different classes of focal adhesion structural and regulatory molecules exhibited varying degrees of correlated motions with actin filaments, indicating hierarchical transmission of actin motion through focal adhesions. Interactions between vinculin, talin and actin filaments appear to constitute a slippage interface between the cytoskeleton and integrins, generating a molecular clutch that is regulated during the morphodynamic transitions of cell migration. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P34.00002: The Translation of Actin Dynamics into Traction Force via Focal Adhesions in Migrating Cells Margaret Gardel, Benedikt Sabass, Lin Ji, Ulrich Schwarz, Clare Waterman Forces are generated in the actin cytoskeleton by myosin-II motors and transmitted to the extracellular matrix (ECM) via dynamic macromolecular assemblies called focal adhesions (FA). To explore how forces are transmitted from the contractile actomyosin network to the ECM, we combine traction force microscopy and fluorescent speckle microscopy (FSM) of FAs and actin cytoskeleton in Ptk1 epithelial cells. We find that the relationship between intracellular actin flow and traction force is spatially segregated within individual focal adhesions. Near the leading edge, actin flow is inversely related to force, while towards the cell center, there is a positive correlation. This change is regulated by small GTPase signal transduction pathways and myosin II motor based contraction. Thus, the FA is a molecular clutch that exhibits regulatory switching between different coupling mechanisms. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P34.00003: Modeling and imaging the topography of nascent adhesions. Erdinc Altigan, David Entenberg, Ben Ovryn We have developed a model to explain the initiation of adhesions on the ventral surface of a cell. An analysis of the energetics of membrane bending and the effects of a composite system of freely diffusing repellers and receptors and a fixed network of ligands on the extracellular matrix demonstrates that a small bundle of actin filaments is able to push the membrane down to the extracellular matrix and nucleate a nascent adhesion. This model is consistent with experiments which demonstrate that cell motility requires cycles of actin polymerization and depolymerization at the leading edge of cell protrusions; the leading lamella adheres to the extracellular matrix and stable focal contacts form which can resist strong contractile forces. Although several of the mechanisms responsible for focal contact formation have been elucidated, the detailed processes leading to the formation of the earliest adhesions have remained elusive. Based upon the energetics of adhesion formation, our model predicts the shape of the membrane at the nucleated adhesion. We have developed a novel form of confocal interference microscopy to measure the distance between the ventral surface of a cell and the substratum with several nanometer precision and we have measured the topography of focal adhesions. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P34.00004: Actin-Filamin Networks and Cell Mechanics Karen Kasza, Fumihiko Nakamura, Thomas Stossel, Ning Wang, David Weitz We seek to elucidate the mechanisms underlying stress dependent stiffening of the cellular cytoskeleton. Filamin A (FLNa) is a protein that cross-links and bundles actin filaments into soft gels that stiffen dramatically with applied mechanical stress. Living cells show similar stiffening behavior, but the underlying physical mechanism is poorly understood. While it is known that FLNa plays an important \textit{biological} role in some very mechanical cellular processes, it is still unclear whether FLNa plays such a dominant \textit{mechanical} role in the cell as it does in simple reconstituted actin networks. Here, we work with a human melanoma cell line deficient in FLNa and a transfected subline expressing FLNa. For both cell lines, we probe cell stiffness measured by magnetic twisting cytometry as we increase the stress supported by the actin cytoskeleton to determine the contribution of FLNa to both the linear and nonlinear material properties of the cell cytoskeleton. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P34.00005: Critical state enhances cross-linker denaturation under stress in biopolymer networks Brian DiDonna, Alex J. Levine We report on the statistical behavior of cross-linker molecules containing numerous unfolding domains when they are used to bind a random semiflexible polymer network. Cross-linkers with unfolding domains are ubiquitous in the F-actin component of the cytoskeleton - examples include filamin and a-actinin. We show, through mean field calculations and simulations, that under tension the cross-linkers naturally organize into a critical state which greatly enhances their propensity to unfold. Unfolding of cross-links could play a role in stress-regulation and mechanotransduction. The critical state is characterized by an exponential or faster growth in the population of cross-linkers as a function of tension up to a characteristic unfolding tension. This critical state should occur at physiologically relevant stress levels in any open random network built with such cross-linkers. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P34.00006: Molecular motor-induced instabilities and crosslinkers determine biopolymer organization David Smith, Falko Ziebert, David Humphrey, Cynthia Duggan, Walter Zimmermann, Josef Kaes All eukaryotic cells rely on the active self-organization of protein filaments to form a responsive intracellular cytoskeleton. The need for motility and reaction to stimuli additionally requires pathways that quickly and reversibly change cytoskeletal organization. While thermally-driven order-disorder transitions are, from the viewpoint of physics, the most obvious method for controlling such organization, the timescales necessary for effective cellular dynamics would require temperatures exceeding the physiologically viable temperature range. We report a mechanism whereby myosin II can cause near-instantaneous order-disorder transitions in reconstituted cytoskeletal actin solutions. When motor-induced filament sliding diminishes, the actin network structure rapidly and reversibly self-organizes into various assemblies. Addition of stable crosslinkers was found to alter the architecture of ordered assemblies. These isothermal transitions between dynamic disorder and self-assembled ordered states illustrate that the interplay between passive crosslinking and molecular motor activity plays a substantial role in dynamic cellular organization. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P34.00007: Instabilities in filament-motor solutions with crosslinkers. Falko Ziebert, Ronny Peter, Walter Zimmermann Filament-motor systems are in nonequilibrium due to the energy consumption during motor movement (via ATP hydrolysis), and thus display pattern and structure formation. We report on simple mesoscopic modeling based on conservation laws with active filament currents. We discuss instabilities in a recent experiment on actomyosin, where ATP is depleted in the presence of a small amount of crosslinker proteins. In the limit of high density of crosslinkers, we propose a model where transported filaments are coupled to an elastic crosslinked network, leading to oscillatory behavior. \newline References: \newline D. Smith, F. Ziebert, D. Humphrey, C. Duggan, W. Zimmermann and J. Kaes, submitted to Biophys. J. ; R. Peter, F. Ziebert and W. Zimmermann, submitted to Europhys. Lett. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P34.00008: Interaction of Semi-flexible Filaments and Molecular Motors Dmitry Karpeev, Igor Aronson, Lev Tsimring, Hans Kaper We consider effects of finite flexibility on interaction of two microtubules with molecular motor. On the basis of numerical solution to nonlinear elasticity equation we show that the flexibility enhances tendency of microtubules to align, which, in turn, favors formation of large-scale structures in the multi-tubules system. Moreover, for much softer filaments, like actin, we observed that the action of the motor may result in formation of multiple loops due to buckling of the filaments. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P34.00009: Effective medium theory of semiflexible filamentous networks Moumita Das, Alex J. Levine, F.C. MacKintosh We develop an effective medium approach to the mechanics of disordered, semiflexible polymer networks such as those forming the cytoskeleton and study their response to both spatially uniform and nonuniform strain. We identify distinct elastic regimes in which the effective filament bending stiffness or stretch modulus vanishes. We also show that our effective medium theory predicts a crossover between affine and non-affine strain, consistent with both prior numerical studies and scaling theory. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P34.00010: Forced shape deformations of interfaces and biopolymer networks Wolfgang Losert, Andrew Pomerance, Cory Poole, Erin Rericha What sets the characteristic length and timescale of shape deformations of motile cells? To investigate possible contributions to these scales, we investigate shape deformations of biopolymer networks and lipid bilayers, two key components of motile cells. Controlled deformations are generated with holographic optical tweezers and detected optically. We observe that for small deformation lengths of up to 4 microns (for cage sizes less than one micron) and short time deformations of order seconds, actin networks respond mostly elastically. We see evidence of coupling between two nearby deformation fields in an actin network. Relaxations of directly forced giant unilamellar vesicles reveal that -during free relaxation- apparent membrane stresses remain localized on micron scales. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P34.00011: Viscoelasticity and rheology of a suspension of active filaments M. Cristina Marchetti, Tanniemola B. Liverpool We study the viscoelasticity of an active solution of polar biofilaments and motor proteins under an externally imposed stress. Adapting methods from polymer physics, we derive the constitutive equations for the stress tensor in the isotropic phase and in phases with liquid crystalline order (nematic and polarized). The stress relaxation in the various phases is discussed. Activity is responsible for a strong enhancement (a divergence in 2d) of the viscosity at the isotropic-nematic transition. This behavior is reminiscent of an equilibrium liquid-solid transition rather than a liquid-liquid transition, and is a direct consequence of contractile bundling. A second signature of activity is found in the nematic phase, where the stress tensor acquires a nonequilibrium contribution proportional to ATP (Adenosine Tri-Phosphate) consumption rate that remains finite in the absence of imposed mechanical deformation. The role of boundaries on these phenomena will also be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P34.00012: Dynamics and statistical mechanics of semiflexible polymer bundles Claus Heussinger, Mark Bathe, Erwin Frey Bundles formed from semiflexible polymers are ubiquitous in nature (e.g. filopodia) and many areas of technology (e.g. carbon nanotube bundles). Despite their simple structure, their mechanical and dynamical properties are only poorly understood. We set up an elastic energy functional that allows characterizing the dynamical and statistical mechanical properties of polymer bundles, in much the same way as the standard worm-like chain model (WLC) does for single polymers. The key result of our analysis is that bundles must be characterized by a wave-number dependent persistence length $l_p(q)$ instead of just a single $q$-independent value. This finding is shown to have dramatic consequences not only on the static and dynamic fluctuation spectrum of an isolated bundle but also on the scaling behaviour of their entangled solutions as well as their cross-linked networks. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P34.00013: Dynamic Control of F-actin Polymerization Using Electrical Interfaces Ian Y. Wong, Matthew J. Footer, Nicholas A. Melosh The cytoskeletal biopolymer F-actin plays a crucial role in the mechanics and motility of eukaryotic cells and is also a model system for the investigation of the physics of semiflexible polymers. Historically, the polymerization of reconstituted F-actin has been initiated in vitro by increasing the bulk ion concentration from reduced to physiological levels. In this work, nanoscale electrodes are used to achieve spatial and temporal control of F-actin polymerization. The application of a low-frequency AC voltage alternately concentrates divalent cations and negatively charged G-actin monomers at the electrode surface, promoting highly localized polymerization. Unlike bulk polymerization, the kinetics of this electronically activated polymerization are governed by two competing mechanisms: ionic activation through Mg$^{2+}$ binding and nucleation of actin trimers. Additional control can be achieved through the superposition of a high-frequency AC signal to align and trap filaments through dielectrophoresis. This combination of low and high frequency AC voltages may allow for the dynamic assembly of nanostructures with precisely controlled size and registry. [Preview Abstract] |
Session P35: Focus Session: Protein Motin Vibrations to Conformational Changes
Sponsoring Units: DBP DCPChair: Aihua Xie, Oklahoma State University
Room: Colorado Convention Center 405
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P35.00001: Dose and exposure requirements for the protein x-ray serial crystallography. Dmitri Starodub We have proposed spraying proteins (aligned by a laser) across a synchrotron beam to solve proteins which cannot be crystallized.$^{1}$ A single-file stream of ice-jacketed proteins is considered. We compute diffraction patterns for the GroEL at the incident x-ray flux predicted for a new coherent scattering beamline at the Advanced Photon Source. Using iterative phasing of the data, we determine the relationship between the count rate at a reconstructed pixel (or 3D voxel) of a given size in the real-space charge-density map and number N of proteins in the 10-$\mu $m 2 kV x-ray beam at any instant. A modulation transfer function estimates resolution for various exposure times. With the incident flux of 10$^{6}$ photons/s\textbf{/}nm$^{2}$ and N=10, over 5,000 counts/s are distributed over the entire diffraction pattern, which is sufficient for a nm resolution with 200 s exposure. We compare the results of this numerical lensless imaging experiment with a simple theoretical treatment of image formation in the dark and bright field phase contrast. Supported by ARO, NSF and co-workers.$^{1}$ $^{1}$J. Chem Phys. 123, 244304 . [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P35.00002: Impact of solvent pH on buried charge formation and protein quake of photoactive yellow protein Aihua Xie, Sandip Kaledhonkar, Lorand Kelemen, Wouter D. Hoff, Anupama Thubagere Embedding a charge group inside a protein in a low dielectric environment is energetically unfavorable. Therefore, most charged groups are solvent exposed. We have developed a hypothesis that a new buried charge transiently formed in a non-polar environment serves as an electrostatic epicenter that drives protein quake (protein conformational changes). Here we report an experimental study on the effects of solvent pH on the protonation states of buried ionizable groups, and their correlation with protein quakes. Time-resolved Fourier transfer infrared (FTIR) difference absorbance spectroscopy is the major experimental technique for simultaneous detection of the proton transfer event (to generate a new buried charge) and the protein quake event. The results are expected to provide insight into the impact of solvent pH on protein structural dynamics in general. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P35.00003: ABSTRACT HAS BEEN MOVED TO D26.00011 |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P35.00004: Conformational dependence of a protein kinase phosphate transfer reaction Montiago LaBute, Graeme Henkelman, Chang-Shung Tung, Paul Fenimore, Ben McMahon Atomic motions and energetics for a phosphate transfer reaction catalyzed by the cAMP-dependent protein kinase have been calculated using plane-wave density functional theory, starting from structures of proteins crystallized in both the reactant conformation (RC) and the transition-state conformation (TC). In TC, we calculate that the reactants and products are nearly isoenergetic with a 20-kJ/mol barrier, whereas phosphate transfer is unfavorable by 120 kJ/mol in the RC, with an even higher barrier. Our results demonstrate that the phosphate transfer reaction occurs rapidly and reversibly in a particular conformation of the protein, and that the reaction can be gated by changes of a few tenths of an angstrom in the catalytic site [1]. [1] G.H. Henkelman, M.X. LaBute, C.-S. Tung, P.W. Fenimore, B.H. McMahon, Proc. Natl. Acad. Sci. USA vol. 102, no. 43:15347-15351 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P35.00005: Interaction of Receptors and GTPase-Activating Proteins in a G Protein Signaling Module Marc Turcotte, Wei Tang, Elliott M. Ross We have developed a model of the interactions of proteins involved in G protein signaling using steady-state data from reconstituted vesicles. The model includes receptor, G protein (G), GTPase activating protein (GAP), GTP and GDP. Implementation is done using coupled ordinary differential equations. We performed a global fit to the model parameters against enzymologic and nucleotide-binding data using simulated annealing constrained by thermodynamics. Validation was done using Monte Carlo data. Fit parameters uncertainties were obtained via multiple repeats of stochastic searches. We studied fit parameter correlations near a solution by local thermal sampling of the cost manifold. The best fit parameters agree with values derived from dynamic data not used in our fit. We used our model to study signaling in familiar regimes and to predict new, testable behaviors in others. Signal output is a complex function of the inputs: receptor and GAP at physiologic and experimental concentrations of GTP and GDP. We studied the shape of the activation surface. Its complexity derives from stoichiometric relationships among protein concentrations. Our model predicts signaling pathways and dynamical response in G protein modules. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P35.00006: Fast motion of the surface alcohol molecules deduced from sum-frequency vibrational spectroscopy Jaeho Sung, Doseok Kim Sum-frequency generation (SFG) vibrational spectroscopy was used to investigate the surface of the homolog series of alcohols from methanol to octanol. It was found that SFG signal strengths from the terminal methyl group of short-chain alcohols cannot be explained by assuming the surface molecules were fixed in time. Introduction of the rotational motion with time scale comparable to the dephasing time of the vibrational mode of the terminal methyl group ($\sim $0.7 picosecond) was able to explain the reduction of the SFG signal by motional averaging effect. This timescale of motion increased with the increase in the molecule size and bulk viscosity. Our result also suggests that surface alcohol molecules move faster as compared to the same molecules in the bulk liquid. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P35.00007: Effect of molecular vibrations on charge transfer in polypeptide chains Nikolai Sergueev, Alexander Demkov We present first principles framework suitable for analyzing and understanding the effect of molecular vibrations on charge transfer in polypeptide chains. Our approach is based on density functional theory and Keldysh nonequilibrium Green's function formalism. This method allows us to treat both electrons and molecular vibrations (phonons) on equal footing in a self-consistent manner. The salient feature of our technique is that we consider the vibration of the whole polypeptide bridge. We present a numerical results for a charge transfer through alanine polypeptide chains of the various length and show that the electron tunneling is greatly affected when the interaction between electrons and molecular vibrations is taken into account. We also present a vibrational spectroscopy analysis and identify those vibrational modes of the alanine polypeptides involved into the inelastic charge transfer. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P35.00008: No Long-Lived Coherent Oscillations in Proteins at Room Temperature Robert Austin, Michael White A recent PRL (PRL 95, 253601 (2005)) suggested that proteins could have very narrow holes (Hz wide) burnt into their electronic spectra at 300K, and suggested that ``snail-paced'' light group velocity light could result. We will show that the authors mistook conformational diffusion phase shifts for narrow lines and show that there are no narrow long-lived holes in a protein spectra at 300 K nor is there any snail-paced light. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P35.00009: Slow light with bacteriorhodopsin solutions Chandra Yelleswarapu, Francisco Aranda, Reji Philip, Rao Devulapalli Slow light in gases and solids has been studied in recent years. Various applications are possible depending on the modulation frequency and the amount of delay that can be induced in the traveling wave. Recently we demonstrated ultra slow light in the biological photo-membrane bacteriorhodopsin (bR) polymer film at room temperature [Phy. Rev. Lett., \textbf{95}, 2536011, 2005]. By exploiting the photoisomerization property of bR for coherent population oscillation, the group velocity is controlled from about 0.1 mm/sec to the speed of light. But as bR is embedded in a polymer matrix, the isomerization rates are slow and hence limited to low modulation frequencies. On the other hand bacteriorhodopsin solution can be used for obtaining slow light at higher modulation frequencies. Studies in solution also offer the advantage of changing the optical density at ease resulting in longer pulse delays. Detailed results on slow light where the delay is varied with modulation frequency, optical density and all-optical control with a blue laser beam will be presented. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P35.00010: Physical basis for membrane-charge selectivity of cationic antimicrobial peptides Bae-Yeun Ha, Sattar Taheri-Araghi Antimicrobial peptides are known to selectively disrupt (highly-charged) microbial membranes by asymmetrical incorporation into the outer layers. We present a physical basis for membrane-charge selectivity of cationic antimicrobial peptides. In particular, we provide a clear picture of how peptide charge, Q, influences the asymmetrical insertion -- one salient feature is the existence of an optimal peptide charge, at which selective insertion is optimized. Our results suggest that large Q is required for antimicrobial selectivity, consistent with experiments. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:51PM |
P35.00011: Investigating Potential Surfaces with QM/MM Methods Invited Speaker: Geometry optimization of large QM/MM systems is not trivial, especially when transition states or higher order saddle points are desired. The optimization can be carried out with a macro/micro scheme, which alternates (internal coordinate) geometry steps in the QM region with full (cartesian) minimizations of the MM region. This significantly reduces the number of QM calculations, and avoids bottlenecks associated with coordinate transformation and Hessian manipulation. This standard macro/micro scheme, however, suffers from numerical instability and compromised convergence behavior. This affects particularly the optimization of transition states, which is therefore not often successful. To address these problems we present extensions to the macro/micro scheme, which have been implemented in the ONIOM framework for hybrid methods. In the standard scheme, the QM and MM regions are coupled only through first order terms. We now include second order coupling using analytical MM contributions, employing linear scaling methods. We show how this improves convergence and allows for the optimization and characterization of saddle points in very large systems. We demonstrate our methods using various examples, such as the hydrogen peroxide reduction by Selenoprotein Glutathione Peroxidase, proton transfer in H-Y zeolite, and thermal isomerization of retinal in Bacteriorhodopsin. [Preview Abstract] |
Session P38: Sensors and Signal Analysis
Sponsoring Units: GIMSChair: Charles Agosta, Clark University
Room: Colorado Convention Center 501
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P38.00001: An Ultra-Wideband Cross-Correlation Radiometer for Mesoscopic Experiments Ryan Toonen, Cyrus Haselby, Hua Qin, Mark Eriksson, Robert Blick We have designed, built and tested a cross-correlation radiometer for detecting statistical order in the quantum fluctuations of mesoscopic experiments at sub-Kelvin temperatures.~ Our system utilizes a fully analog front-end--operating over the X- and Ku-bands (8 to 18 GHz)--for computing the cross-correlation function.~ Digital signal processing techniques are used to provide robustness against instrumentation drifts and offsets.~ The economized version of our instrument can measure, with sufficient correlation efficiency, noise signals having power levels as low as 10 fW.~ We show that, if desired, we can improve this performance by including cryogenic preamplifiers which boost the signal-to-noise ratio near the signal source.~ By adding a few extra components, we can measure both the real and imaginary parts of the cross-correlation function--improving the overall signal-to-noise ratio by a factor of sqrt[2].~ We demonstrate the utility of our cross-correlator with noise power measurements from a quantum point contact. [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P38.00002: Monochromatic Photo-Field Electron Emission Sources Theodore Vecchione, Gary Hembree, Uwe Weierstall, John Spence, Nigel Browning Laser-pulsed photo-field-emission sources with high coherence and brightness are needed for time-resolved electron microscopy. Our ongoing work explores the possibility of using sharpened semiconductor electron emitters to achieve this goal. Intrinsic GaAs field-emission sources have been prepared from cleaved needles that are clipped into refractory metal holders. These needles are chemically sharpened and surface cleaned by field-desorption and electron-bombardment heating. Field emission I-V curves have been analyzed, with and without laser illumination, which demonstrate a range of metallic and semiconductor characteristics. He-Ne laser illumination has been observed to increase field emission currents by more then an order of magnitude. The band structure of a semiconductor can be used to create a lower bound on the energy of photo-excited field-emitted electrons, producing a beam whose energy width is E=Eg-h$\nu $ (bandgap Eg). Energy analysis is planned using a hemispherical analyzer, aimed at achieving an energy spread less than the 0.26 eV of conventional W tips. Calculations are underway to understand complications arising from surface effects and bulk transport. NNSA award DE-PS52-05NA funds this research. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P38.00003: Design and Fabrication of Piezoresistive Microcantilevers for Low Temperature Torque Magnetometry Dan J. Hills, Jack K. Luo, Christoph Bergemann Piezoresistive microcantilevers provide an experimentally simple and highly sensitive way of measuring the magnetization of small (\,$<\!(100\,\mu\mathrm{m})^3$\,) samples. These devices --- micromachined from crystalline silicon --- were originally designed as probes for atomic force microscopy, but were implemented by several researchers as torque sensors. Here we present newly designed and fabricated levers with properties optimized for torque measurements, including specifically those at low temperatures. In particular they may be used for de Haas- van Alphen measurements in high magnetic fields. Torque magnetometry detection of quantum oscillations is a potentially advantageous method for materials with anisotropic Fermi surfaces, existing in very small crystals or platelets. In addition, several other potential applications exist for torque measurements using levers of the same or similar design. Our designs couple high sensitivity with very small lever deflections in order to minimise torque interaction effects arising from field corrections introduced by the cantilever movement. Lever heat-sinking is also considered so as to maximise the sensing current that may be used, and hence the sensitivity, while maintaining the sample at low temperature. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P38.00004: New de Haas-van Alphen effect measurement electronics Patrick Rourke, Alix McCollam, Stephen Julian We have implemented a new data collection infrastructure for measurements of de Haas-van Alphen oscillations in metals. Traditionally, such measurements required large banks of costly lock-in amplifiers, in order to measure on several harmonics of a fundamental excitation frequency at once for a given crystal sample. By moving to a high-quality analog-to-digital-converter/software lock-in algorithm set-up we are able to realize significant improvements in parallel data collection, configurability, data quality and cost. These performance gains will be illustrated through examples of measurements we have performed on various strongly correlated electron systems. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P38.00005: Permanent Magnet with Very Low Field Gradient (0.1G/mm) for NMR Spectroscopy Ognjen Ilic, David Issadore, Tom Hunt, Robert Westervelt Nuclear Magnetic Resonance (NMR) is a powerful analytical tool for obtaining chemical, physical and structural information. To produce the uniform fields required, NMR experiments typically employ large, expensive electromagnets and shimming coils. We have developed a small permanent magnet with an iron yoke that produces a field of $\sim $10 kG with gradient $<$ 0.1G/mm across a 6 mm region for a total field homogeneity of 10 ppm. The system consists of two parallel cylindrical NdFe permanent magnets, 50mm in diameter and 25mm thick, separated by 4mm. The magnets are surrounded by hollow low-carbon steel cylinders with steel caps on each end of the yoke. By adjusting the distance between the yoke caps and the magnet we cancel first-order field strength variations, as shown in simulations. This design is an important innovation for low cost, benchtop NMR systems. *Supported by the NCI MIT-Harvard CCNE. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P38.00006: Identifying the Constituents of and Transformations in Diatomaceous Earth and Polysiloxane Foams Through the Use of Electron Paramagnetic Resonance Spectroscopy Michael Blair, Ross Muenchausen, Bryan Bennett, James Smith, Thomas Stephens, Wayne Cooke The chemical aging of polymeric materials is largely governed by the characteristics of the storage environment. For polysiloxane foams, the diatomaceous earth (DE) filler is a small component of the foam, but it plays a large role in the handling of water in the system. The DE filler can act as either a ``source'' or a ``sink'' for water via both chemical hydroxylation/ dehydroxylation and physical adsorption/ desorption processes, depending on the processing history and storage conditions. We have used electron paramagnetic resonance (EPR) spectroscopy to examine composite foam material as well as the DE filler alone. Intense, broad (400 Gauss) resonances were recorded at room temperature as a function of the microwave power at X-band frequency. The observed spectra have been assigned to the iron oxide compounds goethite, lepidocrocite, hematite, and magnetite based upon the measured EPR spectra of these minerals. As the presence or absence of free H$_{2}$O and the temperature of processing and storage also affects the interconversion of these various iron oxides, we indicate how this process can be followed by monitoring changes in the EPR spectra. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P38.00007: Time resolved measurements of single electron tunneling events Julie Love, Michel Devoret, Robert Schoelkopf We have observed time resolved single electron tunneling events in a metallic thin film circuit. Using a radio frequency single electron transistor (RF-SET) capacitively coupled to a single electron trap (a circuit consisting two small metallic islands and two tunnel junctions) we are able to measure tunneling events on the 10 microsecond time scale. In the 400 microsecond average lifetime of the charge state with one excess electron on the trap island, 80 data points with SNR=10 can be obtained. We will present these measurements along with comparisons to cotunneling (quantum tunneling) and Orthodox (thermal tunneling) theories. The dynamics of cotunneling has never before been studied in an experimental system. These time domain measurements also demonstrate the possibility of measuring the higher moments of charge noise in a metallic system. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P38.00008: Noise Characteristics of Nanocluster-Based Chemiresistors Walter Kruppa, Ronald Rendell, Arthur Snow, Edward Foos, Mario Ancona Thin films of metallic nanoclusters interspersed between interdigitated electrodes are the basis of a promising chemiresistor technology known as MIME sensors. The chemical vapor detection limit of these sensors is set by their signal-to-noise ratio at low frequencies where the noise is found to be 1/f in nature. In this work we explore the experimental dependences of the 1/f noise on various material parameters such as nanocluster core diameter, shell thickness and shell composition. Among other things, we find that the 1/f noise decreases by more than three orders of magnitude as the core diameter increases and the shell thickness decreases, observations that are expected to be important for sensor design. The data are found to fit the well-known Hooge formula and this allows the intrinsic strength of the 1/f noise to be gauged using the Hooge parameter. For the interpretation one needs to know the number of electrons participating in the transport and we discuss how this can be obtained through simulation using the orthodox theory of Coulomb blockade. This factor is then shown to be crucial for understanding the trends in our noise data. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P38.00009: Stray Light Correction as a Deblurring Problem John Hornstein The problem of correcting for stray light is shown to be a type of deblurring problem. When the optical system is linear, correcting for stray light reduces to a generalization of a deconvolution problem. As such, it is an ill-posed inverse problem, in which the goal is to estimate the true radiances incident on the instrument's entrance aperture from the signals registered by its detectors. Optical ghosts and out of field and out of band stray light are all included in this formulation. They are due to the non-ideal character of the optical impulse response function, which, in turn, is proportional to the system's point spread function. Backgrounds due to thermal emission within the optical system or from the radioactivity of its components are not included, since they are independent of the true scene. Several standard techniques of solving ill-posed inverse problems are being tested for correcting for stray light in spectral imagers. Results obtained via Backus-Gilbert estimation are reported here. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P38.00010: The Physical Origin of the Forward Character of the Electromagnetic Optical Theorem Matthew Berg, Christopher Sorensen, Amit Chakrabarti Particles or scatterers, both spherical and nonspherical in shape, are often encountered in the natural environment. Examples include atmospheric clouds and aerosols. The scattering of sun light by these particles produces radiative forcing effects that influence the Earth's climate. Additionally, electromagnetic scattering can offer an unintrusive way to study the physical properties of a scatterer including its shape, size and composition. Extinction is the process by which radiant energy is removed from an incident field due to the scattering and absorption of the field by a system of scatterers. The extinction cross section $\sigma^{ext}$ measures the total power removed from the incident light and hence, is a quantity of interest in many electromagnetic scattering applications. A well-known relationship, called the optical theorem, relates $\sigma^{ext}$ and the amplitude of the scattered field in the exact forward direction. This work investigates the physical origin of the forward character of the optical theorem using computer simulations of simple scattering systems. The conclusion is that the optical theorem derives its forward character from the interference of the incident and scattered fields. This energy flow is seen to consist of opposing directions of flow that cancel each other in all but the forward direction when integrated to yield $\sigma^{ext}$. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P38.00011: Generating, and Processing, Quadrature Signals in Interferometry David Van Baak The Michelson interferometer is well-known for its ability to produce sinusoidal signals or `fringes' in response to changes in the optical path difference between its arms. Less well known is the `other output' of a Michelson interferometer, where a second set of fringes can be observed. In the simplest case of a lossless interferometer, these standard and non standard output signals are complementary, and therefore redundant. This presentation points out that the use of a lossy metal-film beamsplitter in an interferometer renders the two output signals non-redundant; they can in practice be made to occur in phase quadrature. This immediately makes a Michelson interferometer sensitive to the direction, as well as the rate, of change of optical path difference. Remarkably simple modelling makes it possible to extract the phase shift of the beamsplitter, and the instantaneous phase difference in the interferometer, from the pair of output signals. The method is illustrated via the quantification of magnetostriction by interferometry. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P38.00012: The effect of the energy and the momentum resolution on the extraction of Eliashberg function from angle-resolved photoemission spectroscopy TeYu Chien, Hong Liu, Ward Plummer The effects of energy and momentum resolution on the extraction of Eliashberg function from angle-resolved photoemission spectroscopy(ARPES) have been examined. The advantage of ARPES is that it can obtain the information of the dispersion of quasiparticles with energy and momentum resolutions. Moreover, recently, Eliashberg function can be directly extracted from the ARPES data by means of Maximum Entropy Method (MEM). The data near the Fermi energy are very important for the extracting procedure, and, unfortunately, are severely affected by the energy resolution. The case study here is the electron phonon coupling system -- Be(0001) surface. MEM works improper when the energy resolution is larger than 10 meV. A truncation method was proposed to make MEM can work with worse energy resolution up to 30 -- 40 meV. This method reduces the needs of ultra-high energy resolution of the instrument used in ARPES experiment. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P38.00013: Spectral Weight Oracle: Model-Independent Sum Rule Analysis Based on Limited-Range Spectral Data Alexey Kuzmenko, Dirk van der Marel, Fabrizio Carbone, Frank Marsiglio Partial sum rules are widely used in physics to separate low- and high-energy degrees of freedom of complex dynamical systems. Their application, though, is challenged in practice by the always finite spectrometer bandwidth and is often performed using risky model-dependent extrapolations. We show that, given spectra of the real and imaginary parts of any causal frequency-dependent response function (for example, optical conductivity, magnetic susceptibility, acoustical impedance etc.) in a limited range, the sum-rule integral from zero to a certain cutoff frequency inside this range can be safely derived using only the Kramers-Kronig dispersion relations without any extra model assumptions. This implies that experimental techniques providing both active and reactive response components independently, such as ellipsometry in optics or simultaneous measurement of attenuation and speed of sound in acoustics, allow an extrapolation-independent determination of spectral weight 'hidden' below the lowest accessible frequency. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P38.00014: Experimental Confirmation of Backscattering Enhancement Induced by a Photonic Jet. Alexander Heifetz, Kevin Huang, Alan Sahakian, Xu Li, Allen Taflove, Vadim Backman We report experimental confirmation of backscattering enhancement induced by a photonic jet emerging from a dielectric sphere, a phenomenon recently predicted by theoretical solutions of Maxwell's equations. To permit relatively straightforward laboratory measurements at microwave frequencies rather than visible light, we appropriately scaled the original conceptual dimensions of the dielectric microsphere and its adjacent perturbing nanoparticle (located within the microsphere's photonic jet). Our experiments verified the existence of enhanced position-dependent backscattering perturbations by the adjacent particle. Our measured backscattering perturbations agreed well with prior theory and with additional finite-difference time-domain computational models of the complete microwave test geometry. [Preview Abstract] |
Session P39: Focus Session: Hydrogen Storage III
Sponsoring Units: FIAP DMPChair: Taner Yildirim, NIST Center for Neutron Research
Room: Colorado Convention Center 502
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P39.00001: Hydrogen Storage in Chemically Reducible Microporous Ti Oxides Invited Speaker: Micro- and mesoporous Ti oxides with controlled pore sizes from 12 {\AA} to 26 {\AA} were synthesized. The hydrogen storage capacity at 77 K was tested as a function of surface area, pore size, and reducing agent. Surprisingly, the oxidation state of the surface Ti species had a greater effect on the storage densities than surface area or pore size. The 12 {\AA} material reduced with bis(toluene) Ti possesses a surface area of less than 500 m$^{2}$/g, but absorbs over 5 wt{\%} and 40 kg/m$^{3}$ of H$_{2}$ reversibly at 77K and 100 atm. The H$_{2}$ binding enthalpies increased from less than 5 kJ/mol to over 8 kJ/mol as the surface oxidation state of the Ti decreased. The enthalpies also increased with surface coverage, which is opposite to all other cryogenic physisorption systems. These results suggest that a Kubas-type $\sigma $ H$_{2}$ complex is involved and that further tuning of the H$_{2}$ binding enthalpies through use of various chemical reagents may achieve even higher storage levels at more moderate temperatures. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P39.00002: Novel nanostructured materials with binding ``pockets" for hydrogen storage media Sungjong Woo, Young-Kyun Kwon Hydrogen storage issue is one of the key barrier to the effort to substitute the hydrogen with the conventional fossil fuel. Chemisorption using metal hybrides and physisorption using nanostructured carbon-based materials have suffered several serious problems such as low storage capacity, insufficient binding energy and poor releasing process. In order to overcome such issues, we have investigated novel nanostructured materials of low density that bear hydrogen binding ``pockets", which can significantly enhance molecular hydrogen binding -- physisorption -- compared to carbon-based materials. Using numerical simulation based on the density functional theory, the hydrogen-molecule binding-energies of different candidate materials are calculated and optimized. With the obtained binding energies, we develop nanostructures similar to metal-oxide-framework that maximize the hydrogen capacity of the storage. The statistical properties of the structure, which is necessary to understand the process and efficiency of hydrogen release, are studied. In order to enhance the capacity even further, we synthesize the nanostructure with transition metals and the result will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P39.00003: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P39.00004: Transition Metal-Ethylene Complexes as High-Capacity Hydrogen Storage Media E. Durgun, S. Ciraci, W. Zhou, Taner Yildirim From first-principles calculations, we predict that a single ethylene molecule can form a stable complex with two transition metals (TM) such as Ti. The resulting TM-ethylene complex then absorbs up to ten hydrogen molecules, reaching to gravimetric storage capacity of 14 wt\%. Dimerization, polymerizations and incorporation of the TM- ethylene complexes in nanoporous carbon materials have been also discussed. Our results are quite remarkable and open a new approach to high-capacity hydrogen storage materials discovery. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P39.00005: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P39.00006: Functionalized Carbon Nanostructures as Potential Hydrogen Storage Media Mina Yoon, Shenyuan Yang, Enge Wang, Zhenyu Zhang Nanoscaled carbon materials have attracted great attention as promising hydrogen storage media due to their light weight and high surface areas. However, a major limitation is the poor hydrogen uptake resulting from the weak interactions of hydrogen molecules with pristine carbon nanostructures. Recent theoretical studies have investigated ways to increase the binding strength of molecular hydrogen by coating and/or substitutional doping of the carbon nanostructures with transition metals, yet experimentalization of these approaches have been difficult because of metal clustering. In this talk, we study hydrogen storage in carbon nanotubes and fullerenes, by functionalizing such structures with tunable charge states. The tunability is achieved via chemical or electron doping. Our study shows that with the proper method of charge doping, the hydrogen binding strength can be substantially increased. In this way, hydrogen uptake of $>$ 6.0 wt \% at ambient conditions can be realized. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P39.00007: Theoretical study of hydrogen bonding to metal-coated carbon nanotubes Jeongnim Kim Dihydrogen transition metal complexes and carbon nanostructures are promising hydrogen storage materials~[1]. While the practical storage capacity of pure carbon nanostructures is low, calculations predict a possible hydrogen capacity of above 6~wt.\% for Ti coated nanotubes~[2]. A unique hybridization of Ti-d, H-H $\sigma^*$ and carbon $\pi$-orbitals was attributed for the bonding; light alkali and alkaline metals were excluded as alternatives to Ti [2]. This is at odd with earlier predictions of non-transition-metal complexes and synthesis of alkali-doped carbon nanotubes (CNT) [1]. Quantum Monte Carlo (QMC) methods are well suited to describe the strong correlation effects tha to the weak hydrogen binding and metal-hydrogen interactions. We present QMC study of hydrogen bonding to metal-coated CNT using correlated umbrella samplings. Specifically, we study hydrogen bonding to Ti and Mg at various doping levels on CNT. \newline \newline \noindent [1] R. C. Lochan and M. Head-Gordon, Phys. Chem. Chem. Phys. {\bf 8}, 1357 (2006). \newline \noindent [2] T. Yildirim and S. Ciraci, Phys. Rev. Lett. {\bf 94}, 175501 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P39.00008: Interaction of Transition Metals with Carbon Nanostructures Shenyuan Yang, Mina Yoon, Enge Wang, Zhenyu Zhang Recent theoretical studies have shown that transition-metal (TM) decorated carbon nanotubes and fullerenes may serve as promising media for hydrogen storage. However, one prerequisite for this functionality is that the metal atoms decorate the carbon nanostructures as a homogeneous layer. To date, no experimental evidence supports this feasibility; instead, several subsequent studies indicated strong preference of clustering by the TM atoms. In this talk, we investigate several possible ways to prevent TM clustering on the surfaces of carbon nanostructures, based on first-principles total energy calculations. First, we discuss the energetics and kinetics of various TMs as they interact with carbon nanostructures. We then explore the possibility to suppress or enhance clustering by electron or chemical doping. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P39.00009: First principles study of Interaction of H2 with doped Carbon Nanotube and Graphite Surfaces Li Chen, Yiming Zhang, Nikihil Koratkar, Puru Jena, Saroj Nayak Using first principles density functional theory based on gradient corrected approach we have studied interaction of H2 molecule with doped carbon nanotube and graphite surfaces. In agreement with earlier study we find that H2 physorbs on carbon nanotube and graphite surfaces while the binding increases dramatically when H2 binds to Li atoms decorated on carbon nanotube surfaces: the binding further enhances with Li atoms on fullerene doped nanotube pea-pod structures. The increase in binding in the latter structures arises due to charge transfer between the nanotube and dopants and the bonding is primarily in electrostatic in nature. The binding is further improved with decrease in diameter of nanotube suggesting a combination of various effects could be exploited for engineering suitable graphitic surfaces for molecular hydrogen storage. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P39.00010: Endohedral Metallofullerenes: A Smart Material for Hydrogen Storage Yufeng Zhao, Michael J. Heben, Anne C. Dillon, Lin Simpson, Jeff Blackburn, Harry C. Dorn, Shengbai B. Zhang We report a first-principle computational study on tunable hydrogenation of the fullerene C$_{60}$ and endohedral metallofullerenes M@C$_{60}$ and M$_{2}$@C$_{60}$ (M = Li, Be, Mg, Ca, Al, and Sc). The interaction between the encapsulated metal atoms and the C$_{60}$ cage leads to a smart-material behavior, which tunes the hydrogen binding in a desired manner as the hydrogenation proceeds. At lower H densities, when H atoms are too strongly bound to pure C$_{60}$, the endohedral dopants weaken the binding. The dopants also enhance the hydrogen binding energy at higher coverages, and enable the degree of hydrogenation to be substantially increased relative to that available with un-modified C$_{60}$. Overall, the encapsulated metals increase the capacity and improve the energy efficiency for hydrogen storage in hydroendofullerides. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P39.00011: Computational Study of hydrogen storage characteristics of the Covalent-Bonded Graphites. Noejung Park, Seung-Hoon Jhi, Kyubong Kim, Suklyun Hong We perform electronic structure calculations to investigate hydrogen-storage characteristics of the solid carbon structures which consist of covalent-bonded graphenes. First, we show that some regular or irregular combinations of \textbf{\textit{sp}}$^{2}$-and \textbf{\textit{sp}}$^{3}$-bonded carbon atoms lead to very stable porous carbon structures, which is designated as the covalent-bonded graphites (CBGs). Using the density-functional calculation and the M{\o}ller-Plesset perturbation method we show that the H$_{2}$ molecular bindings in CBGs are stronger than those on the isolated graphene by about 20{\%}. We also suggest the CBGs with appropriate pore sizes can be utilized as framework structures for dispersing metal atoms. Energetics show that the Ti atoms are likely to be adsorbed at vertex sites of the CBGs. The hydrogen adsorption properties on metal atoms dispersed inside the CBGs are also presented. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P39.00012: Recombination pathways for atomic hydrogen on the graphite (0001) and single-wall carbon nanotubes Zeljko Sljivancanin, Liv Hornekaer, Eva Rauls, Bjork Hammer Using density functional theory we investigated the lowest energy configurations of two H atoms on a graphite surface, and found two states with an approximately identical binding energy. These states are the dimer A state with two hydrogen atoms adsorbed on two neighbour carbon atoms and the dimer B state with two hydrogen atoms adsorbed on carbon atoms at opposite sides of a carbon hexagon. Hydrogen atoms in the dimer A state will recombine via diffusion into state B and then directly recombine from B. We also studied the corresponding pathways for molecular hydrogen formation from H atoms adsorbed at the single-wall carbon nanotubes and compared results to those obrained for the graphite surface. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P39.00013: Hydrogen generation and storage over transition metal-decorated fullerenes and related materials Liping Huang, Erik Santiso, Keith Gubbins, Marco Buongiorno Nardelli Economical ways to generate and store hydrogen are crucial steps towards the hydrogen economy and fuel-cell technologies. By using first-principles density functional theory calculations, we found out that transition metal-decorated fullerenes and related materials can simultaneously dissociate small molecules like water to produce and store hydrogen. Hydrogen production from water will allow us to have a clean hydrogen economy by using renewable source rather than fossil fuels so that we can stop releasing carbon into the atmosphere. Our studies show that the bonding between transition metal and hydrogen is of a combination of chemical and physical adsorption, which is essential for reversible hydrogen uptake/release. Car-Parrinello molecular dynamics simulations demonstrate that these systems are stable and exhibit associative desorption of H$_{2}$ upon heating without breaking the bond between carbon and transition metal. This fulfills another requirement for reversible hydrogen storage. [Preview Abstract] |
Session P40: Semiconductors: Structure, Surface, and Phase Transitions
Sponsoring Units: FIAPChair: Roy Clarke, Univeristy of Michigan
Room: Colorado Convention Center 503
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P40.00001: Interfacial Structure, Bonding and Composition of InAs and GaSb Thin Films Determined Using COBRA Codrin Cionca, Donald Alan Walko, Yizhak Yacoby, Catalina Dorin, Joanna Mirecki Millunchick, Roy Clarke We have used Bragg rod x-ray diffraction and Coherent Bragg Rod Analysis (COBRA) direct phase retrieval method to extract atomic resolution electron density maps of a complementary series of heteroepitaxial III-V semiconductor samples. From the 3D electron density maps we derived the spacing between monolayers, the chemical composition and the distribution of bond lengths for all atomic planes in the film and across the interface with the substrate. InAs films grown on GaSb (001) using different As species (dimer or tetramer form) both showed conformal roughness and mixed GaSb/InSb interfacial bonding character. The tetramer conditions favored InSb bonding at the heterointerface; the percentages corresponding to InSb and GaAs bonding were equal in the case of the dimer. The GaAs film grown on InAs (001) displayed significant In and As interdiffusion and had a significant percentage of GaAs-like bonds at the heterointerface. [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P40.00002: A RHEED study on the self-assembly of InAs quantum dots on GaAs(001) by MBE Itaru Kamiya, Kohtaro Matsuura, Tsuyoshi Higashinakagawa Control of size, density, and distribution of self-assembled (SA) quantum dots (QDs) by epitaxial growth remains to be a challenge. Reflection high-energy electron diffraction (RHEED) observation on nucleation and formation of SA InAs QD growth on GaAs(001) by MBE has been performed to shed light on this issue. RHEED specular beam, which provides us with information about the formation of QDs through rise of chevron-shape patterns, are measured \textit{in situ}. The results obtained under low InAs growth reveal that there are processes dependent and independent of growth rate. In addition, the results indicate that surface migration of In/As atoms and their incorporation into QDs, with the aid of the wetting layer, can be observed. Such information is complementary to the previously obtained results by STM or AFM, and provides us with the opportunity to understand the dynamics during QD formation. Based on these results, we propose a model on the QD formation process. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P40.00003: Molecular Dynamics Simulations of the Nanoparticle-Substrate Collisions Traian Dumitrica, Paolo Valentini Nanoparticle impact allows for the production of high quality thin films. To elucidate the microscopic details of nanoparticle-surface collisions in the low energy range (up to 1 eV/atom) of interest for the hypersonic plasma deposition technology, we have performed molecular dynamics simulations employing incident silicon particles of different sizes that are focused onto a silicon substrate. Our simulations offer a detailed microscopic picture of the dynamics of the collision process, including the energy conversion and redistribution, the local heating and melting, and the nanoparticle-surface bonding. Interestingly, beyond an impacting velocity threshold our simulations identified a soft landing regime mediated by a structural phase transition occurring in the nanoparticle. More insight into the pressure-induced structural phase transition was obtained by separate nanomechanical studies for the response of silicon nanoparticles to compression. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P40.00004: Measuring and Modeling Dopant Charging in Semiconductors using a Scanning Probe Method Stuart Tessmer, Irma Kuljanishvili, Cemil Kayis, James Harrison, Carlo Piermarocchi, Thomas Kaplan, Loren Pfeiffer, Ken West The ability to manipulate and probe small numbers of dopant atoms in semiconductors represents an emerging line of research, motivated by the continued miniaturization of semiconductor devices and potential applications where the dopants themselves form the functional part of a device. For example, the low-lying electronic states of donor atoms have been proposed as candidate systems for quantum computation. We have performed a localized measurement of the electron addition spectrum of silicon donors in a gallium-arsenide heterostructure using a scanning probe technique. The data are compared to a theoretical model based on the idea that nearest-neighbor pairs effectively form two-donor-molecules. To the best of our knowledge, this is the first low-temperature scanning probe measurement to resolve individual electrons entering a semiconductor system. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P40.00005: Self-Consistent Non-Equilibrium Green's Function Considerations in STM Height Models Applied to Si(100) K.H. Bevan, F. Zahid, D. Kienle, H. Guo In this study we report on the self-consistent non-equilibrium potential drop between Si(100) and a scanning tunneling microscope (STM) tip. It is found to play a role in the height characteristics of adsorbed hydrocarbons in conjunction with the silicon band gap. Results are reported for styrene against a hydrogen passivated Si(100) background. The potential drop is found to reduce the effective STM height of styrene by shifting molecular levels. Calculations are performed within density functional theory (DFT) under both the self-consistent and non-self consistent non-equilibrium Green's function (NEGF) formalism. Tunneling current distance dependence is captured by an ab-initio basis via free pseudo-potential eigenstates benchmarked for clean metallic surfaces. The resulting trends indicate that participation of an applied bias potential profile in measured STM surface heights should not be limited to Si(100). [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P40.00006: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P40.00007: Low energy positrons as probes of reconstructed semiconductor surfaces. Nail G. Fazleev, Alex H. Weiss Positron probes of semiconductor surfaces that play a fundamental role in modern science and technology are capable to non-destructively provide information that is both unique to the probe and complimentary to that extracted using other more standard techniques. We discuss recent progress in studies of the reconstructed Si(100), Si(111), Ge(100), and Ge(111) surfaces, clean and exposed to hydrogen and oxygen, using a surface characterization technique, Positron-Annihilation-Induced Auger-Electron Spectroscopy (PAES). Experimental PAES results are analyzed by performing first-principles calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons for the reconstructed surfaces, taking into account discrete lattice effects, the electronic reorganization due to bonding, and charge redistribution effects at the surface. Effects of the hydrogen and oxygen adsorption on semiconductor surfaces on localization of positron surface state wave functions and annihilation characteristics are also analyzed. Theoretical calculations confirm that PAES intensities, which are proportional to annihilation probabilities of the surface trapped positrons that results in a core hole, are sensitive to the crystal face, surface structure and elemental content of the semiconductors. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P40.00008: The type-C defect on the Si(001)-2$\times$1 surface Ja-Yong Koo, Sang-Yong Yu, Hanchul Kim The type-C defect is the last unresolved point defect on the Si(001)-2$\times$1 surface. Several kinds of atomic models have been suggested as the origin of the C-defect. We investigated the clean Si(001)-2$\times$1 surface by scanning tunneling microscopy to measure the statistical distribution of several kinds of point defects on the surface. We compared the results on the clean surface with the adsorption of water molecules on the Si(001)-2$\times$1 surface. We discuss the possibility of water molecules as the origin of the type-C defect on Si(001)-2$\times$1. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P40.00009: Self-Alignment of Dimer Vacancies on Si(001) Arief Budiman A formation of single dimer vacancy line on a reconstructed Si(001) terrace can be understood from their interaction elastic energy and configuration entropy. Both reconstructed terrace and dimer vacancies are represented by elastic dipoles. A grand canonical ensemble is used to analyze four possible alignments of a dimer vacancy line on such terrace. The self-organization is induced by an elastic interaction between the dimer vacancy and the reconstructed terrace and a kinematic restriction for a line to have a maximum length equal to terrace width when aligned perpendicular to the step direction. Numerical simulations using the obtained energy expression also reveal the importance of elastic interaction in inducing motion of dimer vacancies, while their self-alignment to forming a line is assisted by chemical bond between nearby dimer vacancies. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P40.00010: $\alpha -$ to $\beta $- to $\gamma $-spinel transformations in Mg$_{2}$SiO$_{4}$. Zhongqing Wu, Yonggang Yu, Renata Wentzcovitch Phase relations in Mg$_{2}$SiO$_{4}$ have been investigated by first principles quasiharmonic calculations. The $\alpha $- to $\beta $-spinel transition is believed to cause the 410-km discontinuity, while the $\beta $- to $\gamma $-spinel transformation may contribute to the 520-km discontinuity. We have obtained these phase boundaries using LDA and PBE/GGA exchange correlation functionals and results confirm the trend usually displayed by these functionals. Information on this series of transformations in Mg$_{2}$SiO$_{4}$ will help us understand the importance of other elements on this sequence of phase transformations across the Earth's transition zone. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P40.00011: DFT study of the structural properties of silver halides: LDA vs GGA calculations. G. Murrieta, R. de Coss, L.A. Palomino-Rojas, M. Lopez-Fuentes, G.H. Cocoletzi, N. Takeuchi We perform total energy calculations to investigate the atomic structure of three silver halides: AgCl, AgBr, and AgI in the sodium chloride, cesium chloride, zincblende and wurtzite structures. Calculations are done within the density functional theory. We employ the full potential LAPW method, and the exchange-correlation potential energies are treated in the LDA and GGA approximations. We find that LDA correctly predict the ground state structure of all three binary compounds: rocksalt for AgCl and AgBr, and zincblende/wurtzite for AgI, while GGA always prefer the more covalent zincblende/wurtzite configurations. We observe that the distribution of electron densities for rocksalt is more homogeneous than for zincblende. As a consequence, the energy difference between zincblende/wurtzite and rock-salt phases is enhanced in the GGA approximation, predicting the wrong ground state structure for AgCl and AgBr. Assuming the right experimental structure, the GGA approximation gives lattice parameters, bulk moduli and cohesive energies closer to experimental values. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P40.00012: The high-pressure structural phase transition in~heavier RH3 (R: rare earth metals) by ab initio theory Wei Luo, Rajeev Ahuja Rare earth hydrides are very interesting because they exhibit a reversible metal-insulator transition upon hydrogenation. In present work, we have studied the structural stabilities of heaver~rare earth~trihydrides, RH3 (R=Sm, ..., Lu), under high pressure~using ab initio calculations. Our results show the hexagonal structure with HoD3-type structure is stable~for all studied RH3 at ambient pressure.~Further~these RH3~transform to a face-center cubic structure under high pressure. In HoD3-type phase bulk modulus decreases, whereas the~transition pressure for~hexagonal to fcc structural transformation increases,~as the atomic number of rare earth~element increases. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P40.00013: Phase transition of the Ge-Sb-Te(GST) ternary alloy system for the phase-change memory Jae-Hyeon Eom, Jino Im, Jin-Woo Jung, Young-Gui Yoon, Ki-Min Park, Jisoon Ihm A theoretical investigation on the phase transition from the crystalline to the amorphous phase of the Ge-Sb-Te(GST) ternary alloy system for the phase-change memory is presented. The local structure of the amorphous phase of the GST is shown to be composed of the stibnite-like structure for the Sb$_2 $Te$_3$ and chain-like structure for the GeTe by examining the coordination number for (GeTe)$_n$(Sb$_2$Te$_3$)$_m$ homologous series. The phase transition occurs by the change of the arrangement and connection between the building blocks while the structure of the building blocks is preserved. Energy barriers, transition states and the change of the electronic states during the phase transition are obtained using ab initio electronic structure calculations. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P40.00014: Structural phase transition of GeTe under pressure Gap-Sok Do, Seung-Hoon Jhi Structural phase transition of a representative chalcogenide semiconductor, GeTe, is studied with the use of ab initio pseudopotential density functional method. The transition pressure and atomic structures are particularly investigated. By fitting calculated energy-volume data with Birch-Murnaghan equation, we obtained a transition pressure of 3.7 GPa for rhombohedral to NaCl structural transition and 42 GPa for NaCl to CsCl transition, which is in an excellent agreement with experiment [1, 2]. Other structures are also studied for possible transitions at intermediate pressures. The role of Te d orbitals is discussed in regard to the transition pressure and cohesive energy. \newline \newline [1] Onodera A, Sakamoto I, Fujii Y, Mori N and Sugai S, Phys. Rev. B 56 , 7935(1997) \newline [2] N. R. Serebryanaya, V. D. Blank, V. A. Ivdenko, Phys. Lett. A 197, 63 (1995) [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P40.00015: Development of a neural network based algorithm for multi-scale roughness~parameters of In6S7 semiconductucting compound. Raouf Bennaceur, Lilia Bennaceur Farah, Imed Riadh Farah, Houda Ben Abdallah The overall objective of this paper is to retrieve In6S7 semiconducting roughness surfaces parameters by inverting the backscattered EM waves. Because the classical description of roughness using statistical parameters like the correlation length doesn't lead to satisfactory results to predict backscattering, we used a multi-scale roughness description using the wavelet transform and the Mallat algorithm. In this description, the surface is considered as a superposition of a finite number of one-dimensional Gaussian processes each having a spatial scale. A second step in this study consisted in adapting a direct model simulating backscattering namely the small perturbation model to this multi-scale surface description. We investigated the impact of this description on backscattering through a sensitivity analysis of backscattering coefficient to the multi-scale roughness parameters. The dielectric constants are obtained from ab initio FPLAPW band structure calculation. To perform the inversion of the small perturbation multi-scale scattering model (MLS SPM) we used a multi-layer neural network architecture trained by back propagation learning rule. The inversion leads to satisfactory results with a relative uncertainty of 8 {\%}. [Preview Abstract] |
Session P42: Focus Session: Biological and Chemical Self-Assembly at Surfaces
Sponsoring Units: DMPChair: Kalyanaraman Ramki, Washington University in St. Louis
Room: Colorado Convention Center 505
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P42.00001: Directing the assembly of nanostructured films with living cells Invited Speaker: This talk describes our recent discovery of the ability of living cells to organize extended nanostructures and nano-objects in a manner that creates a unique, highly biocompatible nano//bio interface (\textit{Science} \textbf{313}, 337-340, 2006). We find that, using short chain phospholipids to direct the formation of thin film silica mesophases during evaporation-induced self-assembly, the introduction of cells (so far yeast and bacteria) alters profoundly the inorganic self-assembly pathway. Cells actively organize around themselves an ordered, multilayered lipid-membrane that interfaces coherently with a lipid-templated silica mesophase. This bio/nano interface is unique in that it withstands drying (even evacuation) without cracking or the development of tensile stresses -- yet it maintains accessibility to molecules, proteins/antibodies, plasmids, etc - introduced into the 3D silica host. Additionally cell viability is preserved for weeks to months in the absence of buffer, making these constructs useful as standalone cell-based sensors. The bio/nano interfaces we describe do not form `passively' -- rather they are a consequence of the cell's ability to sense and actively respond to external stimuli. During \textit{EISA}, solvent evaporation concentrates the extracellular environment in osmolytes. In response to this hyperosmotic stress, the cells release water, creating a gradient in pH, which is maintained within the adjoining nanostructured host and serves to localize lipids, proteins, plasmids, lipidized nanocrystals, and a variety of other components at the cellular surface. This active organization of the bio/nano interface can be accomplished during ink-jet printing or selective wetting -- processes allowing patterning of cellular arrays - and even spatially-defined genetic modification. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P42.00002: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P42.00003: STM studies of the molecular-level organization of chiral tartaric acid domains on Ag(111) Nancy Santagata, Amit Lakhani, Darryl DeWitt, Thomas Pearl The expression of chirality in molecular domains on surfaces has important implications for enantioselective catalysis and chemically tuned thin films. In this talk we will discuss the organizational structure of a chiral molecule, tartaric acid (C4H6O6), weakly bound to an achiral metal surface, Ag(111), as studied with low temperature scanning tunneling microscopy (STM). Molecularly resolved images of both (R, R)- and (S, S)- tartaric acid on Ag(111) will be presented, and the role of intermolecular hydrogen bonding in stereospecific domain and superlattice formation will be addressed. In addition, we will consider chiral domain formation and phase separation from a racemic mixture of both tartaric acid enantiomers. Finally, we will present data that indicates a proposed multilayer structure and discuss the growth mode associated with its formation. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P42.00004: Inter- and intramolecular dispersion in a highly ordered organic molecular crystal. Stephen Berkebile, Peter Puschnig, Georg Koller, Falko P. Netzer, Michael G. Ramsey The inter- and intramolecular dispersion in organic molecular crystals have been predicted by theory, but never measured to a satisfying degree. Further, organic pi-conjugated molecules, as they are intrinsically one-dimensional objects with a well-defined number of repeating units, serve as a simple model for understanding what happens to the basic electronic structure in systems of limited size and low dimensions. Here, the band structure of a highly ordered and crystalline para-sexiphenyl (6P) film has been measured using angle-resolved photoemission spectroscopy (ARUPS) in the three directions important to charge transport in organic devices. The ARUPS behavior reveals both strong intermolecular dispersion perpendicular to the molecular axis and intramolecular dispersion along the axis of the molecules. The data will be shown to be in very good agreement with calculations in terms of the extent of the band dispersion, the ARUPS intensity and the k-spread of the peaks associated with quantum size effects. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P42.00005: Kinetics-driven growth mechanism of self-organized pentacene thin films Abdullah Al-Mahboob, Jerzy T. Sadowski, Yasunori Fujikawa, Kazuo Nakajima, Toshio Sakurai The growth kinetics of self-organized, highly ordered (001)-oriented pentacene (Pn) thin films was studied in situ by low-energy electron microscopy (LEEM) and complementary density functional theory calculations. We propose a model of `molecule incorporation-controlled' growth mechanism, according to which the attachment pathway at the island edge and the attachment energy of crystallization unit, rather than step or surface energies, determine the island shape in the kinetic growth of organic molecular thin film. We have found that experimentally observed growth anisotropy can be reproduced exactly by our model, if molecule attachment at island-edge is realized in the form of herringbone pair, for all low-indexed growth directions of Pn thin film. The observed kinetic anisotropy also results in a preference in Pn domain orientations, which tend to have the $<$01$>^{\ast }$ direction of the surface lattice aligned with the direction of the gradient density of supplied molecular flux. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P42.00006: Improved Molecular Dynamics simulations of hexane on graphite near monolayer completion M.W. Roth, M.J. Connolly, Carlos Wexler, Paul A. Gray We present the results of computer simulations of hexane on graphite near monolayer completion utilizing NAMD Scalable Molecular Dynamics in parallel computing environments. We include hydrogens explicitly on the hexane molecules, and the graphite substrate is represented as six all - atom graphene sheets. Results presented for temperatures between T = 100 K and T = 200 K have features which differ from those obtained using the united Atom (UA) model, where hydrogens are suppressed. Various structural and thermodynamic quantities show that the improvement obtained from explicitly including hydrogens come not only from their interaction with the substrate but also by their manipulation of in - plane space. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P42.00007: Nucleation and post-growth relaxation of tetracene thin films on silicon oxide. Jun Shi, X. R. Qin We demonstrate that layered morphology of tetracene films on silicon oxide can be achieved at room temperature via vacuum evaporation. Island size distribution analysis shows that tetracene nucleation in a high-flux growth regime is diffusion-mediated with a critical island size $i$ =3, similar to that in pentacene growth. A pronounced post-growth relaxation has been observed on a time scale of minutes. It is suggested that the high flux rate is crucial in the growth kinetics of forming the layered morphology and also important in overcoming the effect of post-growth relaxation which is sensitive to the film coverage and substrates. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P42.00008: Designing Self-assembled Nanostructures: Metal -- Organic Molecule Coordination Networks at Surfaces Steven L. Tait, A. Langner, N. Lin, S. Stepanow, C. Rajadurai, M. Ruben, K. Kern Networks of isolated metal atoms and organic ligands can be designed to self-assemble at surfaces in desired patterns, producing regular 2D nanopore lattices, whose dimensions and properties can be controlled by selection of the organic ligand. We constructed such a network by coordination of molecules containing pyridyl groups with the inherent adatom population on a Cu(100) surface at room temperature. We produced the same network on Ag(100) and Ag(111) by depositing Cu atoms with molecules, showing the network rigidity on different substrates. Rectangular networks with higher complexity were formed from two species of organic ligands with metal atom nodes. The nanopore size and aspect ratio can be tuned by independently selecting the two ligand species. Some properties of these designed nanostructures can be `tuned' by rational selection of the organic molecule and metal components. We have especially explored the adsorption of simple molecules on these networks, which may be of interest for future chemical or catalytic applications. The ability to tailor the size and functionality of nanometer-scale arrays produced by self assembly represents a unique opportunity for molecular recognition, heterogeneous catalysis, and other fields. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P42.00009: Real-Time Monitoring of Organic Thin Film Morphology by Organic Vapor Phase Deposition Richard R. Lunt, Jay B. Benziger, Stephen R. Forrest We demonstrate the real-time monitoring of the development of crystalline structure in the growth of films by organic vapor-phase deposition (OVPD) using high-pressure reflection high-energy electron diffraction (HP-RHEED). Through control of the probe electron beam energy, sample damage from impinging electrons was avoided and beam attenuation in the 8mTorr OVPD deposition environment was minimized. The growth of copper phthalocyanine (CuPc) on highly oriented pyrolytic graphite was used to demonstrate the ability of such \textit{in-situ} organic-growth monitoring, where it was observed that the first several monolayers formed ordered films independent of the substrate temperature and deposition rate, while the evolution of thicker films was strongly affected by substrate temperature. Higher temperatures resulted in greater in-plane crystalline ordering. We thereby have shown HP-RHEED to be a powerful tool for real-time monitoring of growth morphology, ultimately leading to \textit{in-situ} control of thin film crystalline order. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P42.00010: Characterizing the copper-based catalyst for the oxygen-assisted water-gas shift reaction at a sub-nano scale. Aloysius Soon, Mira Todorova, Catherine Stampfl, Bernard Delley To obtain insight into the structure and surface stoichiometry of copper-based catalysts in commercially important chemical reactions such as the oxygen-assisted water-gas shift reaction, we perform density-functional theory calculations to investigate the stability of oxide surfaces. Taking into account the pressure and temperature through the framework of \textit{ab initio} thermodynamics [1,2] our earlier investigation found that for the conditions relevant to technical catalysis, the bulk oxide is energetically most favourable. Using the same technique, we examine the relative stability of low-index copper oxide surfaces [3], and identify two low energy surface structures that are most stable under such conditions which could be catalytically relevant. These oxide surface structures are found to be non-stoichiometric (with surface defects) and exhibit a metallic character. \\ $[1]$ A. Soon, M. Todorova, B. Delley and C. Stampfl, \textit{Phys. Rev. B} \textbf{73}, 165424 (2006) \\ $[2]$ C. Stampfl, \textit{Catal. Today} \textbf{105}, 17 (2005). \\ $[3]$ A. Soon, M. Todorova, B. Delley and C. Stampfl, submitted to \textit{Phys. Rev. B}. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P42.00011: First-principles study of the self-organization mechanism of NH$_{3}$ on Si(001) Yong-Sung Kim, Hanchul Kim We have investigated the self-organization of NH$_{3}$ molecules on the Si(001) surface using the first-principles pseudopotential calculations. In order to find out the adsorption pathways and understand the mechanism of self-organization, we have calculated the potential energy surfaces of an incoming NH$_{3}$ molecule with one pre-adsorbed NH$_{3}$ molecule. Based on the results, we propose a kinetic process model of NH$_{3}$ self-organization: (i) the incoming molecules are attracted towards the pre-adsorbed molecules due to the H-bonding interaction. (ii) By forming the H-bond with the pre-adsorbed molecule, an incoming molecule can achieve physisorption states. (iii) Subsequently, the physisorbed NH$_{3}$ molecule is attracted to adjacent ``down'' Si atoms to complete the molecular adsorption process. (iv) Finally, the adsorbed NH$_{3}$ dissociates into NH$_{2}$ and H fragments. The resultant self-organized pattern is in accordance with recent STM experiments. However, it is in stark contrast with the energetically favored pattern that is characterized by H-bond formation between the dissociated fragments. This indicates that the self-organization of NH$_{3}$ on Si(001) is governed by the kinetics rather than the energetics. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P42.00012: Structure of tetracene films on hydrogen-passivated Si(001) studied via STM, AFM and NEXAFS Andrew Tersigni, Jun Shi, D. T. Jiang, X. R. Qin Scanning tunneling microscopy (STM), atomic force microscopy (AFM) and near-edge x-ray absorption fine structure (NEXAFS) have been used to study the structure of tetracene films on hydrogen-passivated Si(001). STM imaging of the films with nominal thickness of three monolayers (3 ML) exhibits the characteristic ``herringbone'' molecular packing known from the bulk crystalline tetracene, showing standing molecules on the \textit{ab}-plane. The dimensions and orientation of the herringbone lattice indicate a commensurate structural relationship between the lattice and the crystalline substrate. The corresponding AFM images illustrate that at and above the third layer of the films, the islands are anisotropic, in contrast with the submonolayer fractals, with two preferred growth directions appearing orthogonal to each other. The polarization dependent NEXAFS measurements indicate that the average molecular tilting angle with respect to the surface first increases with the film thickness up to 3 ML, then stabilizes at a value close to the bulk tetracene case afterwards. The combined results indicate a distinct growth morphological change that occurs around a few mononalyers of thickness. [Preview Abstract] |
Session P43: Focus Session: Materials for Quantum Information Processing III
Sponsoring Units: DMPChair: Ray Simmonds, National Institute of Standards and Technology, Boulder
Room: Colorado Convention Center 506
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P43.00001: Complete stabilization and improvement of the characteristics of tunnel junctions by thermal annealing Ilari Maasilta, Panu Koppinen, Lasse Vaisto We have observed that submicron sized Al--AlO$_x$--Al tunnel junctions can be stabilized completely by annealing them in vacuum at temperatures between $350^{\circ}$C and $450^{\circ}$C. In addition, low temperature characterization of the samples after the annealing show a marked improvement of the tunneling characteristics, by disappearance of unwanted resonances in the current. Charging energy, tunneling resistance, barrier thickness and height all increase after the treatment. The superconducting gap is not affected, but supercurrent is reduced in accordance with the increase of the tunneling resistance. A useful application of the annealing is in increasing the sensitivity of Josephson junction threshold current detectors, currently used for example in superconducting quantum bit readouts. It is also expected that all other barrier dependent characteristics will also improve (e.g. critical current noise). [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P43.00002: Gating a two dimensional electron gas in silicon using a metallic single electron transistor Luyan Sun, K.R. Brown, B.E. Kane A wealth of physical phenomena has been observed in two dimensional electron systems such as the silicon metal-oxide-semiconductor field effect transistor (MOSFET). Due to impurities and interface states, a silicon MOSFET channel is usually imperfect. A single electron transistor (SET) close to the channel could provide a useful probe of these imperfections and of the channel behavior. We have incorporated an Al/AlO$_{x}$/Al SET as the top gate of a conventional MOSFET. The SET is fabricated with standard electron-beam lithography and double-angle thermal evaporation. A thermally grown SiO$_{2}$ barrier layer about 20 nm thick isolates the SET from the lightly p-doped MOSFET channel beneath. The drain and source of the MOSFET are heavily n-doped and conduct at cryogenic temperatures. A nearby surface metal gate is used to modulate the width of the channel right beneath the SET island. Near the pinch off regime we expect to see a correlation between fluctuations in the current through the SET and fluctuations in the current of the MOSFET channel. We will present preliminary data from these devices. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P43.00003: Fluctuating Potentials In Micrometer Scale Atomic Ion Traps J. Britton, S. Seidelin, J. Chiaverini, R. Reichle, J.J. Bollinger, D. Leibfried, J.H. Wesenberg, R.B. Blakestad, R.J. Epstein, N. Shiga, J.M. Amini, K.R. Brown, J.P. Home, D.B. Hume, W.M. Itano, J.D. Jost, C. Langer, R. Ozeri, D.J. Wineland Electromagnetic confinement of atomic ion qubits coupled with laser cooling has permitted observation of 10 minute coherence times [1, 2]. Recent work to miniaturize electromagnetic traps promises qubit densities attractive for large scale quantum computing [3]. However, motional heating resulting from poorly understood fluctuating trapping potentials is observed to increase as approximately dimensions$^{-4}$ [4]. We discuss efforts to suppress this heating and present experimental results for several microtrap fabrication techniques [5, 6]. [1] P. T. H. Fisk et al., IEEE Trans. Instrum. Meas. 44, 113 (1995). [2] J. J. Bollinger et al., IEEE Trans. Instrum. Measurement 40, 126 (1991). [3] A. Steane, quant-ph/0412165. [4] L. Deslauriers et al., Phys. Rev. Lett. 97, 103007 (2006). [5] S. Seidelin et al., Phys. Rev. Lett. 96, 253003 (2006). [6] J. Britton et al., quant-ph/0605170. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:27PM |
P43.00004: Oxide-Semiconductor Materials for Quantum Computation Invited Speaker: In this talk I will describe efforts to create a quantum information processor using ferroelectrically coupled electron spins in silicon. The constituent material systems are Ge quantum dots, whose size must be compatible with storage of single electrons, and and whose spacing must allow for significant spin exchange to occur. Epitaxial ferroelectric oxides must be capable of rectifying light to allow for optical gating of spin interactions. ~Progress toward these goals, pursued within the Center for Oxide-Semiconductor Materials for Quantum Computation (COSMQC), will be described. This work is supported by DARPA QuIST through ARO contract number DAAD-19-01-1-0650. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P43.00005: Ultra-efficient electron transfer above micro-channels of superfluid helium F.R. Bradbury, Guillaume Sabouret, Shyam Shankar, S.A. Lyon The spin of an electron, bound by its image charge to the surface of superfluid helium, is a promising two level system for quantum information processing. The ability to efficiently move these qubits is one of the key promises of this technology since it would allow for the large scale integration required for quantum computation. We have fabricated 60 parallel channels, 10 $\mu $m wide by 3 $\mu $m deep, which fill with helium through capillary action. The channels are reactive ion etched into a thin polymer layer. Electrons are photoemitted into the vacuum above the device, attracted to it electrostatically, and moved laterally within the channels by underlying electrostatic gates. The electrons are measured capacitively as a sequence of voltages (clocking sequence) is applied to the gates. Results show that initially there are $\sim $3 electrons per channel and the signal decays exponentially (due to transfer failures) with number of clocking cycles. At frequencies as high as 800 kHz the charge transfer efficiency is 0.99999992 $\pm $ 6\textbullet 10$^{-8}$. This ability to reliably transport electrons makes them a very attractive spin qubit. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P43.00006: Epitaxial growth of V and MgO films for Josephson junction qubits Jeffrey Kline, Seongshik Oh, David Pappas The growth of ultrathin epitaxial aluminum oxide tunnel barriers on rhenium has been proven to reduce the number of spurious resonators in Josephson phase qubits when compared to qubits fabricated with amorphous tunnel barriers. Other epitaxial tunnel barrier materials such as MgO may also improve device performance. The superconductor V is latticed matched to MgO and was studied in this work. Vanadium films were deposited on MgO(001) substrates by UHV magnetron sputtering in argon gas. Magnesium oxide tunnel barriers were deposited by reactive sublimation of magnesium in a controlled oxygen background. To achieve epitaxy, the substrate was held at elevated temperature during the deposition. Surface science characterization tools such as Auger electron spectroscopy, scanning tunneling microscopy, and reflection high energy electron diffraction were used for in-situ analysis of the films. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P43.00007: Investigating energy loss in substrates of gigahertz LC resontaors J.D. Whittaker, K.D. Osborn, A.J. Sirois, R.W. Simmonds Dissipation in superconducting qubits is a significant obstacle to the realization of a superconducting quantum computer. One source of dissipation is through coupling to two-level system defects in the substrate on which qubits are fabricated. To study this effect, loss measurements on LC resonators fabricated on bulk silicon were compared to those fabricated on thin silicon nitride membranes, where much of the substrate material has been removed. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P43.00008: Shadow Evaporated Josephson Junctions for superconducting qubits Fabio Altomare, Jos\'{e} Aumentado, Kevin Osborn, Joshua Strong, Raymond Simmonds Superconducting circuits are a promising system for the implementation of quantum computing. At present two-level system defects in junctions create a formidable obstacle for superconducting qubits. As shown previously, juctions of reduced size have fewer defects. Making high quality utra-small Josephson junctions is crucial for futher progress. In this talk we will discuss how we define and assess the quality of our shadow evaporated Josephson junctions. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P43.00009: Role of tunnel barrier crystallinity in the coherence properties of superconducting phase qubits Seongshik Oh, Jeffrey Kline, Mika Sillanpaa, Adam Sirois, Katarina Cicak, Kevin Osborn, Raymond Simmonds, David Pappas The standard amorphous AlO$_{x}$ tunnel barriers in superconducting qubits contain many nanoscopic fluctuators. These nanoscopic fluctuators destroy the coherent quantum information stored in the qubit. Recently, we eliminated 80{\%} of these fluctuators using single-crystal Al$_{2}$O$_{3}$ tunnel barriers. This clearly shows that the tunnel barrier crystallinity is important for coherent superconducting qubits. Along this line, we started investigating another well-known crystalline tunnel barrier, MgO. Unlike the aluminum oxide tunnel barrier, which requires $\sim $800 \r{ }C for crystallization, the MgO tunnel barrier grows crystalline even at room temperature. We will compare the Josephson junctions and the superconducting phase qubits made out of amorphous AlO$_{x}$, single-crytal Al$_{2}$O$_{3}$ and single-crystal MgO tunnel barriers, and discuss the effect of barrier crystallinity and electrode/tunnel-barrier interface quality on the performance of the coherent quantum-devices. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P43.00010: Generating Spin Echoes with Pi-Pulses: More is Different Dale Li, Anatoly Dementyev, Yanqun Dong, Rona Ramos, Sean Barrett NMR spin echo measurements in several solids linked through the form of the homonuclear dipolar coupling are shown to defy conventional expectations of the delta-function pulse approximation. Multiple pi-pulse echo trains may either freeze-out or accelerate the decay of the signal, depending upon pi-pulse phase. Average Hamiltonian theory, combined with exact quantum calculations, reveal an intrinsic cause for these coherent phenomena: the dipolar coupling has a many-body effect during any real, finite pulse. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P43.00011: Visualizing and Manipulating the Density Matrix to Understand Multiple Pulse NMR Yanqun Dong, Dale Li, Rona Ramos, Sean Barrett NMR spin echo experiments using Carr-Purcell-Meiboom-Gill (CPMG) sequence produce an abnormally long-lived echo train in many solid samples. Average Hamiltonian theory and exact quantum simulations reveal that the dipolar couplings during the finite pulses play an important role in this phenomenon. In this talk we will use density matrix tomography to visualize the effect of dipolar couplings during finite pulses. Using this visualization and other simulations, we explore how the complicated flow through many coherence transfer pathways can lead to a measurable signal. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P43.00012: New Spin Echo Phenomena Arising from Strong Pi-Pulses Rona Ramos, Yanqun Dong, Dale Li, Sean Barrett The application of average Hamiltonian theory to multiple pulse NMR experiments using finite pulses will be discussed. Through this analysis, we are able to explain an observed pulse phase sensitivity and develop new pulse sequences that exploit and enhance these finite pulse effects. Experiments in both the strong and weak coupling limits will be shown. [Preview Abstract] |
Session P44: Focus Session: Nanoscale Transport - Metals
Sponsoring Units: DMPChair: Doug Natelson, Rice University
Room: Colorado Convention Center 507
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P44.00001: Universal conductance fluctuations imply excess high frequency noise in mesoscopic gold wires. A. Trionfi, S. Lee, D. Natelson In cold, mesoscopic conductors, two-level fluctuators lead to time-dependent conductance fluctuations manifested as 1/f noise that are enhanced by quantum interference up to a universal limit (TDUCF). In Au nanowires, we measure the magnetic field dependence of TDUCF, weak localization (WL), and magnetic field-driven (MF) UCF before and after treatments that alter magnetic scattering and passivate surface fluctuators. Our coherence length data resolve a long-standing inconsistency between L$_{WL}$ and L$_{TDUCF}$, and may imply that fluctuators produce high frequency noise in excess of 1/f expectations. [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P44.00002: Low Resistance Interfacing of Single-Crystal Gold Nanowires Birol Ozturk, Tetsuya D. Mishima, Daniel R. Grischkowsky, Bret N. Flanders We have developed an innovative approach to growing individual, single-crystal gold nanowires between targeted points on lithographic electrodes from simple salt solutions. This approach has allowed us to address a fundamental problem in nano-device-fabrication: the interconnecting of nanowires with external circuitry. That is, we have developed a Labview program which simultaneously controls a function generator and a sourcemeter. This program modulates the growth-inducing voltage to attain low contact resistances between gold nanowires and lithographic electrodes. Four-probe measurements revealed that the contact resistances of the electrode-nanowire-electrode assemblies are consistently less than 25 $\Omega $. To our best knowledge, the sub-25 $\Omega $ contact resistances are the smallest that have been attained by any single-step nanowire growth and interfacing approach. Thus, gold nanowires grown with this method are ideal for use as conducting lines in nanoelectronic and nanobiological applications. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P44.00003: Long Range Directional Growth of Electrochemical Nanowires Prem Thapa, Bret Flanders We report on the directional growth of crystalline metallic nanowires between targeted sites in on-chip circuitry. We observed that 200 nm diameter, needle-shaped wires grow between the electrodes after the deposition of 10 $\mu $l aqueous indium acetate solution and application of a 10 MHz alternating voltage. This effect occurs even when the electrodes are separated by as much as 100 $\mu $m. Hence, this effect is indicative of a long-range interaction, which is surprising given that this occurs in an electrolytic solution where Debye screening is expected. In this talk, we will discuss the possible origin of this long-range interaction. This capability provides an innovative way to interface multiple nanowires to a single cell membrane, enabling the future study electrophysiological events in live cells. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P44.00004: Scanning Tunneling Spectroscopy of Few-Atom Nb Nanoclusters on an Ultrathin Insulating Surface. C.D. Ruggiero, T. Choi, J.A. Gupta The study of small metallic clusters offers insight into the evolution of electronic structure from atomic orbitals to bulk-like band structure. We report scanning tunneling spectroscopy on small Nb clusters ranging in size from a single atom to a few atoms. All data were collected using a low-temperature ultrahigh vacuum scanning tunneling microscope operating at 5K. Insulating islands of CuN ($\sim $5nm x 5nm) were grown on a Cu(100) surface in order to decouple deposited Nb clusters from the metal substrate. Tunneling spectra on a bare CuN island reveal an insulating gap exceeding 4eV despite a thickness of only one monolayer. The dI/dV spectra of few-atom Nb clusters on CuN islands reveal a pronounced peak that may be associated with an atomic resonance. The peak position shifts in energy by as much as 0.5eV as a function of cluster size. http://www.physics.ohio-state.edu/$\sim $jgupta [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P44.00005: Electron transport in magnetite nanoparticles Sungbae Lee, John T. Mayo, Vikki L. Colvin, Douglas Natelson Magnetite (Fe$_{3}$O$_{4})$ is an example of a strongly correlated, mixed valence oxide. Electron transport through small numbers of monodisperse magnetite nanocrystals (20nm in diameter) is measured on nanometer-scale three-terminal devices where nanoparticles are decorated on lithographically defined platinum electrodes. The abrupt development of discontinuities on IV curves around 120K and below strongly suggests the particles are going through the Verwey transition. Initial measurements of the full range of IV characteristics and magnetoresistive behaviors of these devices are presented. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P44.00006: Strong magnetic scattering from TiO$_{x}$ adhesion layers D. Natelson, A. Trionfi, S. Lee Electronic phase coherence in normal metals is incredibly sensitive to magnetic scattering. As a result, the weak localization magnetoresistance and time-dependent universal conductance fluctuations are powerful probes of magnetic impurities. We report measurements of these effects in Au and Ag nanowires, comparing samples with and without an underlying 1.5~nm thick Ti adhesion layer. Because of background oxygen, this layer is likely TiO$_{x}$, with $x < 2$. Samples with no adhesion layer show no sign of magnetic contamination. Samples with adhesion layers measured immediately after fabrication show clear evidence of strong magnetic scattering. Annealing in air reduces the concentration of scatterers, as does evaporation under conditions that encourage the formation of TiO$_{2}$. This strongly suggests that the magnetic scattering and its evolution are related to the oxidation state of the Ti, and is consistent with recent reports of ferromagnetism in oxygen-poor TiO$_{2-\delta}$. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 1:03PM |
P44.00007: Quantum Tunneling of Phase Slips in Al Nanowires. Invited Speaker: Superconductivity is a unique phenomenon which manifests itself, most strikingly, as the absence of electrical resistance at very low temperatures. While the resistance in 3-D Superconductors is exactly zero below the Normal-Superconducting transition temperature ($T_c$), in 1-Dimensional ones the proliferation of phase slips-- small regions which become normal allowing the phase of the order parameter to rapidly change by 2$\pi$-- can produce a residual resistance or destroy the superconductivity altogether. In the classical Langer, Ambegaokar, McCumber and Halperin theory, phase slips are caused by thermal excitation over free energy barrier that separates metastable states but Giordano suggested that Macroscopic Quantum Tunneling of phase slips through the barrier (Phys. Rev. Lett. 61, 2137 (1988)) could be significant at very low temperatures where thermally activated phase slips would be exponentially suppressed. However, despite intense experimental effort over the past 20 years, quantum tunneling of phase slips has remained controversial in 1-D superconductors. This talk will discuss the limiting case of 1-D superconductivity in an extremely long (100 $\mu$m) and narrow (5 nm--25 atoms) aluminum wire. In applied magnetic field, and at temperatures well below the superconducting transition, we find evidence ofm macroscopic quantum tunneling at temperatures where the classical theory of thermally activated phase slips would not be able to reproduce the experimental results (Phys. Rev. Lett. 97, 017001 (2006)). Not only are these results valid in linear regime, where most of the experimental data so far has been obtained, but they are consistent and supported by a newly proposed analysis in non-linear regime. These results help ruling out other scenario and establishing that, at temperatures much below $T_c$, the transport properties of superconducting 1-D nanowires are primarily determined by macroscopic quantum tunneling of phase slips. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P44.00008: Observation of Josephson junction behavior in an individual superconducting NbSe$_{2}$ nanowire Zhixian Zhou, R. Jin, Gyula Eres, D. Mandrus, P. Schlottmann, Y.S. Hor, Z.L. Xiao, J.F. Mitchell Resistance and current-voltage characteristics of an individual superconducting NbSe$_{2}$ nanowire of 75 nm diameter were investigated employing four-probe transport measurements. With the absence of the dissipative motion of vortices, we find that the critical current is limited by a single asymmetric Josephson junction with unequal energy gaps across the junction and that their temperature variation is in excellent agreement with the BCS theory. The difference in the magnitude of the superconducting gap can be attributed to the existence of multiple Fermi surface sheets possessing different electronic structure and electron-phonon interactions. In addition, the temperature dependence of the critical current can be well described by the Ambegaokar-Baratoff relation. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P44.00009: Electric Field Effect Modulation of Charge Transport in Atomically Thin Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ Asher Mullokandov, Solomon Endlich, Joel Chudow, Yuanbo Zhang, Philip Kim We investigate the superconducting properties of mesoscopic Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (BSCCO) crystallites of thickness $\sim $2.5-4 unit cells. The crystals are mechanically extracted from the bulk, deposited on a silicon oxide substrate, and 4-probe measurements are conducted with gold contacts. In films that are 3 to 3.5 unit cells thick, the transition from the metallic to the superconducting state is observed at a critical temperature of $\sim $95K, while for 2.5 unit cell crystals the resistance versus temperature curve indicates semiconducting behavior. We also investigate resistance (conductance) variation and critical temperature shifting due to carrier density modulation via an applied gate field at temperatures from 300K to helium temperature. Electric field effect modulation of transport properties in these thin crystallites will be discussed in different temperature ranges. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P44.00010: Electron transport studies of superconducting Pb single-electron tunneling transistors Kang Luo, Dong-Hun Chae, Zhen Yao We investigate the electronic transport properties of superconducting Pb single-electron tunneling transistors created by electromigration of Pb nanowires. In the superconducting state, the conductance is suppressed due to the Coulomb blockade effect and the absence of density of states in the superconducting gap. Within the region of suppressed conductance, fine structures are observed which can be attributed to quasiparticle tunneling processes involving singularity matching. These features exhibit strong odd-even parity effect at 2 K and become smeared out at 4.2 K. Our preliminary results of single-molecule transistors using superconducting Pb electrodes will also be presented. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P44.00011: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P44.00012: Fabrication of Variable-Height Nanostructures via Dynamic Stencil Deposition Jeffrey Wasserman, Kristin Lucas, Soo Hyung Lee, Caitlin Crowl, Anthony Ashton, Nina Markovic Shadow masks of thin low-stress silicon nitride membranes with nanopore punctures allow for direct deposition of material with features as small as 10nm, without need for resists or other chemical exposure. We have built a device to translate the shadow mask with a nanopore relative to a substrate, allowing controllable nanoscale features to be `drawn' directly onto the substrate. By modulating the speed of the shadow mask we can vary the height of the nanostructure as it is being deposited. This allows for direct fabrication of nanowires and quantum dots, as well as controllable granular nanostructures and parallel arrays of nanostructures, not feasible using other techniques. We present in this talk our method for implementing nanoscale dynamic stencil deposition, as well as a variety of nanostructures and other components we have fabricated and studied via this deposition technique. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P44.00013: Sub-10 nm Device Fabrication in a High-Resolution Transmission Electron Microscope Michael Fischbein, Marija Drndic Materials are known to be susceptible to electron-irradiation induced damage during their imaging in a TEM. Though these effects are typically undesirable, we show here that electron-irradiation by the imaging beam of a HRTEM can be used to controllably sculpt metal with single-nanometer precision, thereby enabling device fabrication at a size scale that traditional fabrication methods cannot access. We have used this technique to fabricate metal structures with sub-10 nm features on silicon nitride membrane substrates. Examples include arbitrarily curved nanowires, nanometer-wide channels and nanorings. It will be shown that these ultra-small structures can be integrated into large-scale circuitry, without contact resistance. Potential applications of this technique include nanoelectronics, nanofluidics and the study of size effects on superconductivity. This work was supported by ONR Young Investigator Award (N000140410489), NSF Career Grant (DMR-0449553), NSF NSEC Grant (DMR-0425780), and NSF-IGERT (DGE-022166). [Preview Abstract] |
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