Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V1: New Developments in Supersolids
Sponsoring Units: DCMPChair: William Halperin, Northwestern University
Room: Colorado Convention Center Four Seasons 2-3
Thursday, March 8, 2007 11:15AM - 11:51AM |
V1.00001: Squeezing superfluid from a stone: Coupling superfluidity and elasticity in a supersolid Invited Speaker: Superfluidity---the ability of liquid $^4$He, when cooled below 2.176 K, to flow without resistance through narrow pores---has long served as a paradigm for the phenomenon of ``off-diagonal long-range order'' (ODLRO) in quantum liquids and superconductors. Supersolidity---the coexistence of ODLRO with the crystalline order of a solid---was proposed theoretically over 35 years ago as an even more exotic phase of solid $^4$He, but it has eluded detection. Recently, Kim and Chan [1,2] have reported an anomalous decoupling transition of solid $^4$He in a torsional oscillator measurement, and interpret their results as evidence for non-classical rotational inertia and a possible supersolid phase of $^4$He. In this talk I will give brief historical review of the theory of and experimental searches for supersolidity. I will then discuss a phenomenological Landau theory of the normal solid to supersolid (NS-SS) transition in which superfluidity is coupled to the elasticity of the crystalline $^4$He lattice, and underscore the implications of this theory for experimental searches for supersolidity [3]. I will also discuss a hydrodynamic model for supersolids, in which the additional broken gauge symmetry in the supersolid phase produces a collective mode that is analogous to second sound in superfluid helium. \newline \newline [1] E. Kim and M. H. W. Chan, Nature (London) \textbf{427}, 225 (2004). \newline [2] E. Kim and M. H. W. Chan, Science \textbf{305}, 1941 (2004). \newline [3] A. T. Dorsey, P. M. Goldbart, and J. Toner, ``Squeezing superfluid from a stone: Coupling superfluidity and elasticity in a supersolid,'' Phys. Rev. Lett. \textbf{96}, 055301 (2006). [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:27PM |
V1.00002: Elimination of the Supersolid State Through Crystal Annealing Invited Speaker: We have employed the torsional oscillator technique in the study of the supersolid state of solid 4He. We find that the supersolid state is not a universal property of solid helium, but in certain cases can be reduced or even eliminated through an annealing of the sample. We have also studied the supersolid in a number of cells with differing geometries, including cylindrical, cubic, and annular geometries, in an attempt to examine the possible influence of geometry on the stability of the supersolid state. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 1:03PM |
V1.00003: Superfluidity of grain boundaries and supersolid behavior Invited Speaker: We have found that, at the liquid-solid equilibrium pressure $P_m$, supersolid behavior is due to the superfluidity of grain boundaries in solid helium [1]. After describing this experiment and reviewing some of the related theoretical work [2], we discuss the possibility that , at larger pressure ($P > P_m$), grain boundaries could also explain the supersolid behavior which was observed with torsional oscillators [3-6]. \newline \newline [1] S. Sasaki, R. Ishiguro, F. Caupin, H.J. Maris, and S. Balibar, Science 313, 1098 (2006)\newline [2] E. Burovski, E. Kozik, A. Kuklov, N. Prokof'ev, and B. Svistunov, Phys. Rev. Lett. 94, 165301 (2005)\newline [3] E. Kim and M.H. Chan, Nature 427, 225 (2004)\newline [4] E. Kim and M.H. Chan, Science 305, 1941 (2004)\newline [5] A.S.C. Rittner and J.D. Reppy, Phys. Rev. Lett. 97, 165301 (2006)\newline [6] K. Shirahama, Bull. Am. Phys. Soc. 51, 302 (2006) [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:39PM |
V1.00004: Supersolids? Invited Speaker: A brief, biased and selective review will be given of various theoretical and experimental results (recent and not so recent) addressing questions of supersolidity and related properties of quantum solids, especially helium. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 2:15PM |
V1.00005: Measurements on the melting curve of $^{4}$He down to 10 mK Invited Speaker: Recent discovery of a nonclassical rotational inertia in solid $^{4}$He below about 0.2 K by Kim and Chan has initiated an intensive study on the properties of solid $^{4}$He. As Kim and Chan have interpreted their observation as the evidence of supersolid behavior, we have decided to measure very accurately the melting curve of $^{4}$He because, as the slope of the melting curve is proportional to the difference in the entropy of the liquid and solid phases, there should be an anomaly at the possible supersolid transition. We have measured the melting curve of $^{4}$He with the accuracy of about 0.5 microbar with $^{4}$He crystals which had various concentration of defects. All our samples showed only the expected $T^{4}$ dependence due to phonons without any sign of the supersolid transition in the temperature range of 80{\ldots}400 mK. Below 80 mK we observed a small deviation from $T^{4}$ dependence which, however, cannot be attributed to the supersolid transition because our recent measurements with the cell containing liquid sample only suggest that this deviation is by the temperature variation of properties of BeCu membrane of our capacitive pressure gauge. In addition to our published data with $^{4}$He of natural purity [Phys. Rev. Lett. 97, 165302 (2006)] we also report our recent measurements of the melting curve with ultra pure $^{4}$He (0.3 ppb of $^{3}$He impurities). In these measurements we followed optically the shape of the sample crystals which had very good quality. [Preview Abstract] |
Session V2: Kavli Foundation Symposium on Nanowire Quantum Devices
Sponsoring Units: DCMPChair: Lars Samuelson, Lund University
Room: Colorado Convention Center Four Seasons 4
Thursday, March 8, 2007 11:15AM - 11:51AM |
V2.00001: Engineering the electronic properties of nanowires for device applications Invited Speaker: Semiconductor nanowires have recently been recognized as a possible add-on technology to silicon CMOS. Successful integration of nanowires may push the miniaturization of components further and could also bring improved, and completely new, device functions to a chip. In particular, nanowires composed of III-V materials are of interest for applications as they benefit from a small and/or direct bandgap. We will present results from electrical measurements on InAs/InP nanowires grown by chemical beam epitaxy. Changes in the precursors fed to the growth chamber can be made to control the electronic properties of the grown material. In this way it is possible to create atomically sharp heterostructure interfaces, as well as to change the carrier concentration along the wire. The latter can be achieved by controlling the carbon incorporation from the In precursor. It will be shown that heterostructure nanowires can be used in memory cells, and also as single-electron transistors for electrostatic read-out of such cells. Finally, we will discuss the design and application of InAs nanowire-based field-effect transistors, where issues related to lateral and vertical processing of nanowires will be addressed. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:27PM |
V2.00002: Kinetic measurements during the vapor-liquid-solid growth of Si and Ge nanowires Invited Speaker: Growth of nanowires using vapour-liquid-solid (VLS) process has been successfully demonstrated over the past 40 years, but the exact mechanisms are not well understood. In this talk, we will present \textit{in situ} transmission electron microscopy studies of Si and Ge nanowire growth kinetics as a means to develop a fundamental understanding of the mechanisms governing their shape and structure. From the images of the wires, collected at video rates as a function of growth pressure, temperature, and gas environment, we identify several novel aspects of wire growth: Ostwald ripening of catalyst droplets on top of the wires, effect of oxygen on Si wire morphology, and VLS growth of Ge wires at temperatures below the bulk Au-Ge alloy eutectic temperature. We will consider the generality and applicability of these results for the growth of nanowires of other materials. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 1:03PM |
V2.00003: Spin properties of strongly interacting quantum wires Invited Speaker: A number of recent experiments on quantum wires report deviations from perfect conductance quantization at low densities. These conductance anomalies manifest themselves as quasi-plateaus in the conductance as a function of gate voltage at about 0.5 to 0.7 of the conductance quantum $G_0=2e^2/h$, depending on the device. Most commonly the experimental findings are attributed to non-trivial spin properties of quantum wires. In particular, spontaneous spin polarization of the ground state has been proposed as a possible origin of the conductance anomalies. The issue has generated a lot of interest in the community as this interpretation is in apparent contradiction with the Lieb-Mattis theorem, which forbids spontaneous spin polarization in one dimension. However, the spin properties may change dramatically when the system becomes quasi-one-dimensional. We show [1] that sufficiently strong interactions between electrons induce deviations from the strictly one-dimensional geometry and indeed give rise to a ferromagnetic ground state in a certain range of electron densities. A novel phase with more complicated spin interactions generated by four-particle ring exchanges is identified at higher densities. \smallskip [1] A.D. Klironomos, J.S. Meyer, and K.A. Matveev, Europhys. Lett. {\bf 74}, 679 (2006). [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:39PM |
V2.00004: Mesoscopic Josephson Junctions Employing Ge/Si Core/Shell Nanowires Invited Speaker: Semiconductor nanowires are finding increased importance in nanoelectronics due to their controlled growth and reduced dimensions. Band structure engineering of heterostructure nanowires is proving to be instrumental in creating low-dimensional carrier gases with enhanced mobility, low scattering and reproducible contacts. We present low temperature transport measurements of one-dimensional hole gases formed in (undoped) germanium/silicon (Ge/Si) core/shell heterostructure nanowires. The Ge core diameter of the nanowires is 15 nm with a 2 nm Si shell. The length of the nanowire between the contacts is typically 150 nm. Transparent contacts to the nanowires allow observation of transport through one-dimensional subbands arising from radial confinement. When connected to superconducting aluminum leads, a dissipationless supercurrent flows through the semiconductor nanowire due to proximity-induced superconductivity. By using a Au top gate, which modulates the carrier density of the nanowire and the number of one-dimensional subbands populated, the critical current of these mesoscopic Josephson junctions can be tuned from zero to greater than 100 nA. Resonant multiple Andreev reflections in the superconductor-nanowire-superconductor system is also observed. Finally, we investigate the interplay between one-dimensional quantum confinement and superconductivity. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 2:15PM |
V2.00005: Imaging Few-Electron Quantum Dots in InAs/InP Nanowires Invited Speaker: Heterostructure semiconducting nanowires provide an excellent system to make high quality, ultra-small quantum dots for future applications in nano-electronics, spintronics, and quantum information processing. We use a liquid helium cooled scanning probe microscope (SPM) as a movable gate to image electrical conduction through an InAs quantum dot grown inside an InAs/InP heterostructure nanowire. Electrical transport measurements in the few-electron Coulomb-blockade regime exhibit the shell structure of quantum dot states down to the last electron. SPM images are formed by recording nanowire conductance as the charged SPM tip is scanned above the nanowire. The images display rings of peaked conductance centered on the quantum dot; the rings correspond to Coulomb-blockade oscillations of the quantum dot. In this way the tip locates the quantum dot and can be used as a movable gate to change the induced charge on a single dot in a spatially dependent way, down to zero electrons. We have also imaged homogeneous InAs nanowires. At 4K, the wires exhibit Coulomb blockade oscillations in conductance versus backgate voltage that are indicative of multiple quantum dots in series. The images reveal the location of the quantum dots along the wire and the tip voltage can tune their charge state. [Preview Abstract] |
Session V3: Detection & Applications of NMR and MRI at Microtesla Magnetic Fields
Sponsoring Units: DCMPChair: John Clarke, University of California, Berkeley
Room: Colorado Convention Center Korbel 2A-3A
Thursday, March 8, 2007 11:15AM - 11:51AM |
V3.00001: SQUID-detected microtesla MRI Invited Speaker: We have developed a system to detect nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) signals in magnetic fields of 1-100 microtesla. At such low fields, the very small nuclear polarization and the frequency dependence of conventional Faraday detection would lead to extremely weak signals. To overcome these problems we use a combination of prepolarization and frequency-independent detection with an untuned superconducting gradiometer coupled to a Superconducting Quantum Interference Device (SQUID). We demonstrate narrow linewidths in NMR spectra of nuclei in liquids and in spectra of J-coupled nuclei in molecules. Our MRI system operates at 132 $\mu $T (proton Larmor frequency 5.6 kHz), uses a prepolarizing field up to 150 mT and has a magnetic field noise below 1 fT/Hz$^{1/2}$. This system demonstrates submillimeter in-plane resolution of phantoms, and can acquire \textit{in vivo} images of the human forearm, wrist and fingers. In high-field MRI, the susceptibility difference between tissue and, for example, a medical implant, can cause severe image distortion. We show that such artifacts are absent at microtesla fields, so that this technique could enable distortion-free MRI of patients with medical implants. Furthermore, microtesla MRI displays a greatly enhanced T$_{1}$-weighted contrast between different concentrations of agarose gel (T$_{1}$ is the longitudinal relaxation time). Preliminary experiments on \textit{ex vivo} prostate specimens containing normal and cancerous tissue demonstrate similarly enhanced contrast, suggesting that this technique could be used to image tumors. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:27PM |
V3.00002: Low Field Nuclear Magnetic Resonance (NMR) using SQUIDs Invited Speaker: Using a high resolution SQUID system in a magnetically highly shielded room, we measured the precession of 1H nuclei of liquid benzene, distilled water, and chloroform in magnetic fields around a microTesla. We found that the NMR lines of these liquids are in the range of a few hundred milliHertz and increase linearly with the detection field over a Larmor frequency range of two orders of magnitude. The slope is attributed to the inhomogeneity of the detection field and enables the extrapolation of the natural line width to zero magnetic field. For this limit, where any molecular motion is fast with respect to the Larmor frequency, the natural resonance line widths of benzene, chloroform and distilled water were determined to be 120 mHz, 150 mHz, and 170 mHz, respectively. In low magnetic fields, chemical shift and homonuclear coupling become negligible. All that remains as a source of a spectral structure is pure J-coupling between nuclei of different gyromagnetic ratio. We studied pure J-coupling between methylene protons and fluorine nuclei of trifluorethanol and between methyl protons and phosphorus in trimethylphosphate at detection fields from 0.5 microTesla to 4 microTesla. This corresponds to a variation of d=J(H,F)/(f(H)-f(F)) from 8 to 1 and of d=J(H,P)/(f(H)-f(P)) from 0.8 to 0.08, respectively. At very low fields, i.e. at d=8, the spectra of trifluorethanol exhibited only one single resonance line with an irregular structure. With increasing field, more and more individual lines were revealed. For trimethylphosphate, d=0.08 represents the transition to the weak coupling regime. In addition, we employed a 304 SQUID vector magnetometer system for the recording of the magnetic field generated by water protons in two adjacent sample tubes precessing about a magnetic field of a microTesla. From the spatially resolved data, positions and moments of the samples were calculated, yielding a reconstructed moving image of the two precessing magnetic dipoles. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 1:03PM |
V3.00003: High resolution NMR spectroscopy in the Earth's magnetic field Invited Speaker: High resolution nuclear magnetic resonance (NMR) spectroscopy at high magnetic fields has developed into a most powerful tool for the determination of molecular structures. The dream is a mobile molecular low field NMR scanner which allows the determination of molecular structures. Until to now at low magnetic fields NMR spectroscopy suffers from the low signal to noise ratio (S/N) and from the lack of access to chemical information in terms of chemical shifts and homo-nuclear $J$-couplings. We demonstrate that chemical analysis of liquids is possible by mobile ultrahigh-resolution $^{1}$H, $^{19}$F and $^{129}$Xe NMR spectroscopy in the Earth's magnetic field. The $^{129}$Xe chemical shift in liquids is determined in the Earth's magnetic field with a precision comparable to that obtained by superconducting magnets. The $^{1}$H and $^{19}$F NMR spectra allow the determination of hetero-nuclear $J$-coupling constants with an accuracy of a few mHz. Very fine details of the molecular structure which are not observable with conventional superconducting magnets can be discriminated. For molecules where a rare spin such as carbon $^{13}$C is present the high-resolution low-field $^{1}$H NMR spectrum indeed reveal all hetero- and homo-nuclear $J$-couplings. All these results open the door for the mobile study of molecular structures as well as for the online monitoring of chemical reactions at ultra-low magnetic fields. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:39PM |
V3.00004: Simultaneous Measurement of Magnetic Resonance and Neuronal Signals Invited Speaker: Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) at ultra low magnetic fields (ULF, $\sim $ microT) have advantages over their counterparts at higher magnetic fields, despite the reduction in signal strength. Among these advantages are that the instrumentation uses superconducting quantum interference devices (SQUIDs), and is now compatible with simultaneous measurements of biomagnetic signals, such as magnetoencephalography (MEG). This presents a new opportunity for noninvasive simultaneous functional and anatomical brain imaging. We present here the physical basis and experimental evidence for a variety of ULF-MRI techniques being developed at Los Alamos to enable simultaneous anatomical and functional imaging of the human brain. We conclude by presenting a novel technique, based on the resonant interaction between the magnetic fields such as those that arise from neural activity and the spin population in ULF-MRI experiments, that may enable direct tomographic imaging of the consequences of neural activity. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 2:15PM |
V3.00005: Optical methods for detection of nuclear magnetic resonance. Invited Speaker: Nuclear magnetic resonance is commonly detected with inductive pick-up coils or, less commonly, with SQUID magnetometers. I will discuss recent work in our group on optical detection of NMR using two separate techniques. In one approach, optically-pumped alkali-metal atoms are used to detect the magnetic fields generated by nuclear magnetic moments. Such atomic magnetometers reach sensitivity similar to low-$T_{c}$ SQUID magnetometers without requiring cryogenic cooling. We recently demonstrated atomic magnetometer detection of NMR and NQR signals at frequencies ranging from 20 Hz to 423 kHz. In the other approach, NMR signals from a transparent substance are obtained by direct optical detection. In this technique the plane of polarization of a linearly polarized light transmitted through the sample is rotated by interaction with nuclear spins. We detected NMR signals from water and liquid $^{129}$Xe using this method. Such nuclear spin optical rotation (NSOR) signals do not rely on measurement of long range dipolar fields and allow new modalities of imaging and spectroscopy. [Preview Abstract] |
Session V4: Dynamics in Polymeric Systems
Sponsoring Units: DPOLYChair: Ophelia Tsui, Boston University
Room: Colorado Convention Center Korbel 2B-3B
Thursday, March 8, 2007 11:15AM - 11:51AM |
V4.00001: Dynamics of polymer glasses under active deformation. Invited Speaker: Polymer glasses can often be deformed significantly without breaking. What microscopic mechanism allows this ``flow'' under conditions where mobility is otherwise absent? We utilize an optical photobleaching technique to measure the segmental mobility of polymer glasses and nanocomposites during active deformation. It has been previously established that the reorientation of dilute dye molecules (on the time scale of thousands of seconds) can monitor the segmental dynamics of a polymer melt. Here we utilize this method to measure mobility during tensile deformation of a free-standing poly(methyl methacrylate) glass. We have observed increases in mobility during deformation from Tg-10 K to Tg -30 K, with larger changes at lower temperatures. At Tg-18 K, with a strain rate of 0.00001/s, segmental mobility increases slowly at first and then dramatically, so that the increase in mobility during deformation reaches a factor of about 200. After removing the stress, we observe that the enhanced mobility disappears slowly. These measurements are compared to continuum and mesoscopic models of polymer glass dynamics and rheology. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:27PM |
V4.00002: Dynamics of Polymer Blends: Beyond Self-Concentration. Invited Speaker: The self-concentration model has been shown to describe the component dynamics of a wide variety of miscible polymer blends remarkably well. However, there are some systems, such as PEO/PMMA and polymer/solvent systems, where the model is clearly inadequate. We will present new experimental results on such systems, and explore the possible origins of the discrepancies. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 1:03PM |
V4.00003: Looking inside the tube: what molecular dynamics simulations are revealing about polymer entanglements Invited Speaker: Using concepts developed over the years by de Gennes, Doi, Edwards, Marrucci, Rubinstein, McLeish, Milner, and others, a kind of ``standard model'' for entangled polymer relaxation and rheology has been developed, which, like the ``standard model'' of high-energy physics, has a number of ad hoc assumptions and fitting parameters. The ``standard model'' of polymer relaxation is based on a phenomenological ``tube'' surrounding each polymer chain that represents the effect on that chain of non-crossability constraints imposed by surrounding chains. As a result of its confinement to the tube, the chain relaxes by reptation -- or sliding along the tube, accordion-like fluctuations of the chain within the tube, and movement of, or dilation of, the tube due to motion of the surrounding chains creating the tube-like region. Increasing computer speed and advanced simulation methods are now making possible the direct molecular dynamics simulations of entangled polymers resolved at the monomer scale, over time scales sufficient to test the underlying assumptions of the tube model and allow direct calculation of some of the phenomenological parameters. Here we illustrate how these simulations allow us to estimate the distribution of tube lengths, the average diameter of the tube, and the mobility of the branch point in a simple ``star'' branched polymer. These findings confirm the validity of the tube ansatz, but suggest that some corrections to the ``standard model'' may be needed. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:39PM |
V4.00004: Glass Transition Temperature Reductions in Freely-Standing Films of Different Polymers Invited Speaker: The effects of confinement and free surfaces on the dynamics of polymers in thin films have been studied extensively since the original observation of reductions in $T_{g}$ with decreasing film thickness $h$ in thin polystyrene (PS) films [1]. One particularly striking result, which is yet to be understood in detail, is the observation of very large, molecular weight (\textit{MW}) dependent reductions of $T_{g}$ in very thin, freely-standing PS films using Brillouin light scattering and ellipsometry [2]. We have recently measured $T_{g}$ ($h$, \textit{MW}) for freely-standing PMMA films [3] and we find that the results are in qualitative agreement with those obtained for freely-standing PS films. However, the overall magnitude of the $T_{g}$ reduction is much less (by roughly a factor of three) for the high-\textit{MW} freely-standing PMMA films than for freely-standing PS films of comparable \textit{MW} and $h$. The observed differences between the freely-standing PMMA and PS film data suggest that differences in chemical structure determine the magnitude of the $T_{g}$ reduction and we discuss the possible origins of these differences. Our analysis of the \textit{MW}-dependence of the $T_{g}$ reductions suggests that the mechanism responsible for the \textit{MW}-dependent $T_{g}$ reductions observed in the high-\textit{MW} freely-standing films is different than that responsible for the \textit{MW}-independent $T_{g}$ reductions observed in low-\textit{MW} freely-standing and supported films. [1] Keddie et al., Europhys. Lett. \textbf{27}, 59 (1994); [2] Dalnoki-Veress et al., Phys. Rev. E \textbf{63}, 031801 (2001); [3] Roth and Dutcher, Eur. Phys. J. E \textbf{20}, 441 (2006). [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 2:15PM |
V4.00005: Dynamics in Confined Systems:~ Polymer Thin Films and Surfaces Invited Speaker: Here we visit the problem of dynamics of polymers confined in ultrathin films and at surfaces. We present results obtained on ultrathin polymer films using a novel nanobubble inflation device to measure the biaxial creep of films as thin as 13 nm. The results show that the viscoelastic response of PVAc in the glass transition regime is similar to that of the bulk material, while polystyrene shows considerable acceleration of the molecular dynamics that corresponds to a 40 K or more reduction in the glass transition. We will discuss the breadth of the transition, as this is related to the presence of a liquid layer at the surface as well as to the thickness of such a layer. We also discuss the effects of film thickness on the rubbery plateau response of these films. An observed stiffening of the material is found that gives an apparent plateau compliance that is over 100 times smaller than that of the bulk material. In the thinnest films a significant fraction of the deformation energy comes from surface tensions. However, in films above approximately 30 nm, this contribution is less than 30{\%} of the total energy of deformation and suggests that much of the stiffening is due to a thin film effect. This is discussed in detail. Finally, we show results for nanoparticle embedment experiments and argue that the mobility increases observed on the polymer surface are insufficient to explain a reduction of 40 K or more of the glass transition in ultrathin polymer films. [Preview Abstract] |
Session V5: Unusual Transport Phenomena in Chalcogenides
Sponsoring Units: DCMPChair: Marie-Louise Saboungi, CNRS
Room: Colorado Convention Center Korbel 1A-1B
Thursday, March 8, 2007 11:15AM - 11:51AM |
V5.00001: Quantum Linear Magnetoresistance: Solution of an Old Mystery Invited Speaker: In the paper the history of the discovery of the linear magnetoresistance in metals by P. L. Kapitza in 1928 - 1929 and its explanations are presented. Actually, Kapitza discovered two different phenomena. One of them -- the linear magnetoresistance at classically large magnetic fields in polycrystalline samples of metals, having open Fermi surfaces, was explained by I. Lifshits and V. Peschansky in 1958. The other phenomenon is the quantum linear magnetoresistance, appearing in metals, or semimetals, with a small concentration of carriers and a small effective mass, when only the lowest Landau band participates in the conductivity. Manifestations of this unusual phenomenon in different materials are described. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:27PM |
V5.00002: The quest for imperfection Invited Speaker: The stoichiometric compounds Ag$_2$S, Ag$_2$Se and Ag$_2$Te are superionic conductors at higher temperature. Below 400 K, ion migration is effectively frozen and the compounds are non-magnetic semiconductors that exhibit no appreciable magnetoresistance. We showed that slightly altering the stoichiometry can lead to a marked increase in the magnetic response; up to 200 $\%$ at room temperature and in a magnetic field of 5.5T in Ag$_{2+\delta}$Se and Ag$_{2+\delta}$Te ($\delta$ about 10$^{-4}$) reaching a maximum of about 350 $\%$ at low temperature. But more importantly, the response can be almost linear in magnetic field even at low magnetic fields. Not only do these silver chalcogenides show linear magnetoresistance, this response also is still unsaturated up to 55 T showing no signs of saturation. M. Parish and P. Littlewood identified inhomogeneities as a key factor. Their theoretical model has initiated a project on artificially created structures in semiconductors to mimic transport in real materials with inhomogeneities. We processed a number of different geometric realizations in collaboration with L. Cohen's group at Imperial College, London, on their InSb epilayers on GaAs (001). However, the films themselves have a very large and almost linear and non-saturating (up to applied fields of 13 T) magnetoresistance intrinsically which made the interpretation of the results somewhat difficult. We then went back to the origin of this intrinsic magnetoresistance by comparing, at room temperature, undoped InSb epilayers grown on GaAs(001) by molecular-beam epitaxy with varying thickness from 100 to 2000nm. The question is whether these films and the silver chalcogenides share a similar physical origin for their magnetoresistance. Experiments to very high fields in InSb films are on the way. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 1:03PM |
V5.00003: Non-saturating magnetoresistance in heavily disordered semiconductors Invited Speaker: We present a classical model of the magnetotransport of strongly inhomogeneous semiconductors based on an array of coupled four-terminal elements. We show that this model generically yields non-saturating, quasi-linear magnetoresistance at large magnetic fields, in contrast to the resistance of a homogeneous semiconductor, which increases quadratically with magnetic field at low fields and, except in very special cases, saturates at fields much larger than the inverse of the carrier mobility. We argue that our model provides an explanation for the observed non-saturating magnetoresistance in doped silver chalcogenides and potentially in other macroscopically disordered conductors. Finally, our method may be used to design the magnetoresistive response of a microfabricated array and thus pave the way to the construction of magnetic field sensors with a controllable response. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:39PM |
V5.00004: The Evolution from CDW to Superconductivity in Cu$_{x}$TiSe$_{2}$ Invited Speaker: Charge density waves (CDWs) are periodic modulations of the conduction electron density in solids: collective states that arise due to intrinsic instabilities often present in low dimensional electronic systems. The layered dichalcogenides are the most well known examples of CDW-bearing systems, and TiSe$_{2}$ was one of the first CDW-bearing materials known. The competition between CDW superconducting states at low temperatures has often been characterized and discussed, and yet no chemical system has been previously reported where finely controlled chemical tuning allows for this competition to be studied in detail. This talk will describe our work [1] reporting how, upon controlled intercalation of TiSe$_{2}$ with Cu to yield Cu$_{x}$TiSe$_{2}$, the CDW transition is continuously suppressed, and a new superconducting state emerges near x = 0.04, with a maximum Tc of 4.15 K found at x = 0.08. The anisotropic superconducting properties, obtained by characterization of the resistivity and magnetization of single crystals of Cu$_{0.07}$TiSe$_{2}$, will also be described. \newline \newline [1] E. Morosan, H. W. Zandbergen, B. S. Dennis, J. W. G. Bos, Y. Onose, T. Klimczuk, A.P. Ramirez, N. P. Ong, and R. J. Cava \textit{Nature Physics}\textbf{2,} 544 (2006). [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 2:15PM |
V5.00005: Current Jets and Non-saturating Magnetoresistance in Disordered Semiconductors Invited Speaker: The transverse, positive magnetoresistance of doped silver telluride and silver selenide changes linearly with field by thousands of percent, with no sign of saturation up to MegaGauss. The inhomogeneous distribution of excess/deficient silver atoms lies behind this anomalous magnetoresistive response, introducing spatial conductivity fluctuations with length scales independent of the cyclotron radius. Theoretical simulations of two and three-dimensional random resistor networks reveal distorted current flows that provide a linear contribution to the transverse magnetoresistance, but a pronounced negative longitudinal magnetoresistance. We show that a systematic investigation of the resistivity tensor in longitudinal field could be used to identify the spatial inhomogeneities in the silver chalcogenides and determine the associated length scale of the current distortion. The incorporation of macroscopic inhomogeneities to other semiconductors, such as InSb, opens the gate to artificial fabrication of conducting networks with micron scale unit size for enhanced magnetoresistive sensitivity. [Preview Abstract] |
Session V6: Cell Motility
Sponsoring Units: DBPChair: Henrik Flyvbjerg, Riso National Laboratory
Room: Colorado Convention Center 207
Thursday, March 8, 2007 11:15AM - 11:51AM |
V6.00001: Extremes in motility: actin acrobatics, spasmin spasms and jellyfish jabs Invited Speaker: Fast movements in biology are functionally relevant in the context of avoidance and capture. I will talk about some of the adaptations in biology that lead to speed at the cellular level in a variety of organisms, and then discuss three in some detail: the explosive motility of jellyfish stings, the fast contraction of some pond weeds, and the extrusion of an actin spring. In each case, the morphology and mechano-chemistry come together in unusual ways that are adapted for functionality. This leads to questions of both a comparative and an evolutionary nature, and serve to perhaps move these questions from the realm of stamp collecting to physiology and physics. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:27PM |
V6.00002: Natural descriptions of motor behavior: examples from \textit{E. coli} and \textit{C. elegans}. Invited Speaker: \textit{E. coli} has a natural behavioral variable - the direction of rotation of its flagellar rotorary motor. Monitoring this one-dimensional behavioral response in reaction to chemical perturbation has been instrumental in the understanding of how \textit{E. coli} performs chemotaxis at the genetic, physiological, and computational level. Here we apply this experimental strategy to the study of bacterial thermotaxis - a sensory mode that is less well understood. We investigate bacterial thermosensation by studying the motor response of single cells subjected to impulses of heat produced by an IR laser. A simple temperature dependent modification to an existing chemotaxis model can explain the observed temperature response. Higher organisms may have a more complicated behavioral response due to the simple fact that their motions have more degrees of freedom. Here we provide a principled analysis of motor behavior of such an organism -- the roundworm \textit{C. elegans}. Using tracking video-microscopy we capture a worm's image and extract the skeleton of the shape as a head-to-tail ordered collection of tangent angles sampled along the curve. Applying principal components analysis we show that the space of shapes is remarkably low dimensional, with four dimensions accounting for $>$ 95{\%} of the shape variance. We also show that these dimensions align with behaviorally relevant states. As an application of this analysis we study the thermal response of worms stimulated by laser heating. Our quantitative description of \textit{C. elegans} movement should prove useful in a wide variety of contexts, from the linking of motor output with neural circuitry to the genetic basis of adaptive behavior. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 1:03PM |
V6.00003: How Molecular Motors Shape the Flagellar Beat Invited Speaker: Cilia and eukaryotic flagella are slender cellular appendages whose regular beating drives fluid flows across epithelia and propels cells and microorganisms through aqueous media. The beat is an oscillating pattern of propagating bends generated by dynein motor proteins that induce sliding between adjacent axonemal microtubules. A key open question is how the activity of the motors is coordinated in space and time to produce the observed regular oscillatory beat pattern. We have developed a physical description of Flagellar dynamics based on the interplay of collective action of dynein motors and relative sliding of microtubules in two and three dimensions. To elucidate the nature of motor coordination, we have inferred the mechanical properties of the motors by analyzing the shape of beating sperm. Steadily beating bull sperm were imaged at a high frame rate and their shapes were measured with high precision using a Fourier averaging technique. We compared our experimental data with theoretical waveforms and found that the observed flagellar beats were in accordance with a model based on sliding controlled motor activity, but not with curvature controlled motor activity. Furthermore, good agreement between observed and calculated waveforms was obtained only if significant sliding between microtubules occurred at the base. This highlights the role of basal sliding in shaping the flagellar waveform. Thus we conclude, that the flagellar beat patterns are determined by an interplay of the basal properties of the axoneme and the collective behavior of sliding controlled dynein motors that are coordinated mechanically via the sliding of adjacent microtubules. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:39PM |
V6.00004: Physical Aspects of Evolutionary Transitions to Multicellularity Invited Speaker: An important issue in evolutionary biology is the emergence of multicellular organisms from unicellular individuals. The accompanying differentiation from motile totipotent unicellular organisms to multicellular ones having cells specialized into reproductive (germ) and vegetative (soma) functions, such as motility, implies both costs and benefits, the analysis of which involves the physics of buoyancy, diffusion, and mixing. In this talk, I discuss recent results on this transition in a model lineage: the volvocine green algae. Particle Imaging Velocimetry of fluid flows generated by these organisms show that they exist in the regime of very large Peclet numbers, where the scaling of nutrient uptake rates with organism size is highly nontrivial. In concert with metabolic studies of deflagellated colonies, investigations of phenotypic plasticity under nutrient-deprived conditions, and theoretical studies of transport in the high-Peclet number regime, we find that flagella-generated fluid flows enhance the nutrient uptake rate per cell, and thereby provide a driving force for evolutionary transitions to multicellularity. Thus, there is a link between motility, mixing, and multicellularity. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 2:15PM |
V6.00005: Mechanics of actin-based motility Invited Speaker: The ability of cells to move is critical for organism evelopment, maintenance, and repair. Growth of actin filament networks drives a variety of cellular and intracellular motions and contributes to the mechanical rigidity of the cell's cytoskeleton. During motility, eukaryotic cells and intracellular pathogens are propelled by dendritic actin networks oriented in the direction of motion and characterized by a branched architecture. Nucleation-promoting factors activated near the cell membrane trigger the formation of nascent filaments from the side of existing filaments in the network. Here we use laser tracking and atomic force microscopy to test models of actin-based motility and actin network elasticity. A Brownian ratchet mechanism has been proposed to couple actin polymerization to cellular movements, whereby thermal motions are rectified by the addition of actin monomers at the end of elongating filaments. By following actin-propelled microspheres using three-dimensional laser tracking, we find that the movement of beads adhered to growing actin networks is consistent with an object-fluctuating Brownian ratchet. Elasticity of actin networks has been shown to arise in part from the resistance of filaments under extension. Using atomic force microscopy, we find that dendritic actin networks exhibit nonlinear stress softening behavior that points to an important role for filaments under compression. Together, these results raise new questions about how actin network architecture is involved in the propulsion and guidance of crawling cells. [Preview Abstract] |
Session V7: Current Issues in Climate Change
Sponsoring Units: FPSChair: Barbara Levi, Physics Today, American Institute of Physics
Room: Colorado Convention Center Korbel 4A-4B
Thursday, March 8, 2007 11:15AM - 11:51AM |
V7.00001: Overview of Climate Change 2007: IPCC WG1 Key Findings Invited Speaker: The Intergovernmental Panel on Climate Change (IPCC) was jointly established by the World Meteorological Organization and the United Nations Environment Programme (UNEP) in 1988. The purpose of IPCC is to assess available information on the science of climate change and to provide policy-relevant but not policy-prescriptive assessments of interest to policymakers, scientists, and the public. IPCC will release its fourth comprehensive Working Group 1 (WG1) assessment report on the state of understanding of the physical science basis of climate change in early 2007. This talk will summarize the key scientific findings of that report, including the understanding of observations of changes in the atmosphere, ocean, and ice, forcing agents such as carbon dioxide and aerosol, feedbacks in the climate system, paleoclimatic observations and theory, and projections of future changes in coming decades and centuries. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:27PM |
V7.00002: The uncertain hockey stick: a statistical perspective on the reconstruction of past temperatures Invited Speaker: A reconstruction of past temperatures based on proxies is inherently a statistical process and a deliberate statistical model for the reconstruction can also provide companion measures of uncertainty. This view is often missed in the heat of debating the merits of different analyses and interpretations of paleoclimate data. Although statistical error is acknowledged to be just one component of the total uncertainty in a reconstruction, it can provide a valuable yardstick for comparing different reconstructions or drawing inferences about features. In this talk we suggest a framework where the reconstruction is expressed as a conditional distribution of the temperatures given the proxies. Random draws from this distribution provide an ensemble of reconstructions where the spread among ensemble members is a valid statistical measure of uncertainty. This approach is illustrated for Northern Hemisphere temperatures and the multi-proxy data used by Mann, Bradley and Hughes (1999). Here we explore the scope of the statistical assumptions needed to carry through a rigorous analysis and use Monte Carlo sampling to determine the uncertainty in maxima or other complicated statistics in the reconstructed series. The principles behind this simple example for the Northern Hemisphere can be extended to regional reconstructions, incorporation of additional types proxies and the use of statistics from numerical models. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 1:03PM |
V7.00003: Counting the Clouds Invited Speaker: Cloud processes are very important for the global circulation of the atmosphere. It is now possible, though very expensive, to simulate the global circulation of the atmosphere using a model with resolution fine enough to explicitly represent the larger individual clouds. An impressive preliminary calculation of this type has already been performed by Japanese scientists, using the Earth Simulator. Within the next few years, such global cloud-resolving models (GCRMs) will be applied to weather prediction, and later they will be used in climate-change simulations. A ``multi-scale modeling framework'' can be used as a bridge between current low-resolution climate models and future GCRMs. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:39PM |
V7.00004: Simulating Earth's Past and Future Greenhouse Climates Invited Speaker: Earth's climate has experienced dramatic changes through geologic time. Specific intervals in the geologic record indicate climates that were much warmer than the past few thousand years. These warm climates, or greenhouse climate, are associated with times of elevated levels of carbon dioxide relative to current levels. For a number of these greenhouse climates major perturbations to ocean circulation occurred, which had significant impacts on marine life. Our ability to simulate these past climates with global climate models provide tests for models that are used to project Earth's future climate. I will present simulations of greenhouse climates of the past using a comprehensive global climate model, the Community Climate System Model. I will then present simulations of Earth's potential climate for the end of the 21$^{st}$ century. I will show how our understanding of simulations of past greenhouse climates can provide information on where Earth's climate is heading for the end of this century and beyond. I will also indicate how Earth's greenhouse effect has evolved over Earth's history and how these past changes provide a context for future changes in Earth's greenhouse effect. [Preview Abstract] |
Session V8: Superconductivity: Spin Excitations and Ordering in Cuprates
Sponsoring Units: DCMPChair: Penchang Dai, University of Tennessee
Room: Colorado Convention Center Korbel 1C
Thursday, March 8, 2007 11:15AM - 11:27AM |
V8.00001: Antiferromagnetism and superconductivity do not seem to coexist in Nd$_{2-x}$Ce$_x$CuO$_{4\pm\delta}$ Eugene Motoyama, Guichuan Yu, Inna Vishik, Owen Vajk, Patrick Mang, Martin Greven High transition-temperature ($T_{\mathrm{c}}$) superconductivity develops near antiferromagnetic (AF) phases, and it is possible that magnetic excitations contribute to the superconducting (SC) pairing mechanism. In order to assess the role of antiferromagnetism, it is essential to understand the doping and temperature dependence of the two-dimensional AF spin correlations. The phase diagram is asymmetric with respect to electron and hole doping, and for the comparatively less-studied electron-doped materials, the AF phase extends much further with doping and it appears to overlap with the SC phase: the archetypical compound Nd$_{2-x}$Ce$_x$CuO$_{4\pm\delta}$ shows bulk superconductivity above $x \approx 0.13$, while evidence for AF order has been found up to $x\approx 0.17$. However, our inelastic magnetic neutron scattering measurements point to the distinct possibility that genuine long-range antiferromagnetism and superconductivity do not co-exist. The data reveal a magnetic quantum critical point where superconductivity first appears, consistent with an exotic quantum phase transition between the two phases. Our measurements also demonstrate that the pseudogap phenomenon in the electron-doped materials arises from a build-up of spin correlations, in agreement with recent theoretical proposals. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V8.00002: New phase diagram of ideally flat CuO$_{2}$ plane: Cu-NMR study in Five-layered Cuprates Hidekazu Mukuda, Machiko Abe, Sunao Shimizu, Yoshio Kitaoka, Akira Iyo, Hijiri Kito, Yasumoto Tanaka, Yasuharu Kodama, Kazuyasu Tokiwa, Tsuneo Watanabe We report a systematic Cu-NMR study on Hg-, Tl-,Cu-based five-layered high-Tc cuprates. In underdoped HgBa$_{2}$Ca$_{4}$Cu$_{5}$O$_{y}$ (Hg-1245) with a Tc=72 K, the AFM order is detected with 0.1$\mu _{B}$ even at two outer planes (OP's) that are responsible for the onset of superconductivity (SC). This is the first microscopic evidence for the uniform mixed phase of AF and SC on a single CuO$_{2}$ [1]. Recently we found AFM insulating state at low temperatures in disordered fivelayered cuprate Cu-1245, whereas the carrier densities are similar to Hg-1245(OPT) where the AFM metallic state are realized in IP's[2]. This finding reinforces the phase diagram in which the AFM metallic phase exists between AFM insulator and SC states for the case of ideally-flat CuO$_{2}$ plane without disorder. [1] Mukuda et al., Phys. Rev. Lett. 96,087001 (2006) [2] Mukuda et al., J. Phys. Soc. Jpn., 75, No.12 (2006) [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V8.00003: High-Tc superconductivity and antiferromagnetism on self-doped high-Tc cuprate Ba$_{2}$Ca$_{3}$Cu$_{4}$O$_{8}$F$_{2}$ Sunao Shimizu, Hidekazu Mukuda, Yoshio Kitaoka, Akira Iyo, Yasuharu Kodama, Hijiri Kito, Kazuyasu Tokiwa, Tsuneo Watanabe We report on the antiferromagnetism and high-Tc superconductivity in a F-substituted four-layered cuprate, composed of two outer and inner CuO$_{2 }$planes in a unit cell, Ba$_{2}$Ca$_{3}$Cu$_{4}$O$_{8}$F$_{2}$. Although a formal Cu valence is expected to be just +2.0 in the nominal composition, this is not a half-filled Mott insulator but a superconductor with Tc = 55K. Recently, it has been suggested that the origin of the superconductivity in this compound is self-doping by ARPES measurement [1] and band calculation [2], which means either outer or inner CuO$_{2}$ planes are hole-doped, and the others are electron-doped. From F-NMR study, we have confirmed magnetic order with T$_{N}$ = 100K, concluding the uniform mixing of superconductivity and magnetic order in a single CuO$_{2}$ plane. In addition, we have compared a three-layered compound Ba$_{2}$Ca$_{2}$Cu$_{3}$O$_{6}$F$_{2}$, which is also superconductor with Tc = 76K. We will introduce the unique magntic and superconducting phenomena in F-substituted cuprates from microscopic points of view. [1]Y. Chen, \textit{et al}., cond-mat/0611291 (2006) [2] W. Xie, et al., cond-mat/0607198 (2006) [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V8.00004: Effect of oxgyen ordering on the spin dynamics of YBa$_2$Cu$_3$O$_{6.5}$ Shiliang Li, Z. Yamani, H.J. Kang, Feng Ye, C. Bircher, K. Segawa, Y. Ando, Xin Yao, H. A. Mook, Pengcheng Dai We use inelastic neutron scattering to study the oxgyen disorder effect in electronic properties of YBCO 6.5. Previous work have shown when copper oxgyen chains in the YBCO 6.5 form ortho-II order, magnetic excitations in YBCO6.5 form a resonance around 33 meV and incommensurate spin flucutations below the resonance. In oder to study oxygen disorder effect on spin excitations of YBCO 6.5, we measured spin dynamics of detwinned Ortho-I YBCO (x=6.5) ($T_c$=48K), whose Cu-O chains are not well ordered. We find that oxygen disorder can have dramatic effect on spin dynamics of YBCO6.5. We discuss similarities and differences in spin dynamics in these two materials and their possible interpretation. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V8.00005: Evolution of hourglass like magnetic excitation in underdoped La$_{1.90}$Sr$_{0.10}$CuO$_{4}$. Maiko Kofu, Tetsuya Yokoo, Kazuyoshi Yamada, Frans Trouw Recent neutron scattering studies have revealed similar ``hourglass-lik'' magnetic excitations in mono-layered La$_{1.875}$Ba$_{0.125}$CuO$_{4}$ and bi-layered YBa$_{2}$Cu$_{3}$O$_{6.6}$. These results indicate that such hourglass-like dispersive excitations might be common to high-T$_{c}$ cuprates. To elucidate the interplay between the excitation and the superconductivity, we investigated the hole doping dependence of excitations. We performed inelastic neutron experiments for underdoped La$_{1.90}$Sr$_{0.10}$CuO$_{4 }$(T$_{c}$=29K) using chopper spectrometer PHAROS installed at Los Alamos National Laboratory. Clear four incommensurate peaks were observed at $\omega $=8meV and the incommensurability corresponds to $\sim $0.1 r.l.u., which is consistent with previous study. Around $\omega $=30meV, a single peak was observed at the antiferromagnetic magnetic zone center. This indicates that the hourglass-like excitations also exist in underdoped La$_{1.90}$Sr$_{0.10}$CuO$_{4}$ and that the saddle point becomes $\sim $30meV. For optimally doped La$_{1.84}$Sr$_{0.16}$CuO$_{4 }$(T$_{c}$ =38K), the saddle point is expected to correspond to 40meV or more, suggesting that the saddle point goes up with increasing hole doping. Moreover, we found that the slope of dispersion is almost same for La$_{1.90}$Sr$_{0.10}$CuO$_{4}$ and La$_{1.84}$Sr$_{0.16}$CuO$_{4}$. The reduction of energy at saddle point is recognized as a consequence of difference of incommensurability. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V8.00006: Neutron Scattering in High-T$_c$ Cuprates: Two Component Spin-Fermion Model Yunkyu Bang Recent neutron scattering experiments reveal that the generic form of the magnetic excitations in the high-Tc cuprates has the so-called ``hour-glass'' form, in which both the quasi-elastic incommensurate (IC) excitations and the commensurate resonance peak show up at different excitation energies. We propose the two-component spin-fermion model having the local spin degrees of freedom and itinerant fermions. Our calculations of the dynamic spin correlation function both at normal and superconducting states explain the essential features of the hour-glass form of the neutron experiments. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V8.00007: Inelastic neutron scattering study on spin excitations of Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta}$ ($T_c$=27.5K) Jun Zhao, Shiliang Li, Stephen Wilson, Hye Jung Kang, Jeff Lynn, Pengcheng Dai We use neutron scattering to study the evolution of spin excitations in electron doped Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta}$(PLCCO). For $T_c$ = 24 K PLCCO, Wilson et al. [Nature 442, 59 (2006)] have reported the presence of a resonance mode, a localized magnetic excitations coupled directly to the superconductivity in high-$T_c$ superconductors, similar to hole-doped superconductors such as YBa$_2$Cu$_3$O$_{6+\delta}$, Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and Tl$_2$Ba$_2$CuO$_ {6+\delta}$. Below the resonance are continuing of magnetic scattering with little temperature dependence. We show here our studies of the magnetic excitations on PLCCO ($T_c$=27.5K) with the highest $T_c$ reported in the literature. We confirm the presence of a resonance mode in this sample, and demonstrate that the low-energy magnetic scattering here is much different from the $T_c$=24 K PLCCO. Our results thus shed new light to the understanding of spin excitations in electron-doped copper oxides. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V8.00008: Spin wave excitations in Nd2CuO4 Chad Bircher, Hyunjge Woo, Shiliang Li, Pengcheng Dai In this talk we will present the inelastic neutron scattering results of spin wave excitations of the electron-doped superconductor parent compound Nd2CuO4 (NCO). We carried out our experiments on the HET time of flight spectrometer at the ISIS facility in England. Our work will complement previous studies on the parent compound of hole-doped superconductors, La2CuO4 (LCO). We are studying whether there is a difference between the magnetic exchange coupling of hole-doped and electron-doped superconductors. We have probed energies from 50 meV to above 300 meV in order to detemine the magnetic exchange coupling. We will present the results and discuss the similarities and differences between LCO and NCO. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V8.00009: Incommensurate Spin Ordering and Excitations in Underdoped La$_{2-x}$Ba$_x$CuO$_4$ S.R. Dunsiger, Y. Zhao, B.D. Gaulin, Y. Qiu, J.R.D. Copley, H.A. Dabkowska, Z. Yamani, W.J.L. Buyers The diverse magnetic properties of the La$_{2-x}$(Sr,Ba)$_x$CuO$_4$ transition metal oxides may be tuned in a controllable way by doping with mobile holes. In one interpretation, the holes are believed to organise into correlated static or dynamic stripes. We report the first observation of static {\it incommensurate} spin ordering in underdoped La$_{2-x}$Ba$_x$CuO$_4$ (x$\sim $0.05, x=0.08) using neutron diffraction. Elastic collinear incommensurate peaks are observed below the superconducting transition (T$_C \sim $27 K) in La$_{2-x}$(Sr,Ba)$_x$CuO$_4$ (x=0.08). In marked contrast, diagonal satellite peaks have been observed at low temperature in positions rotated by 45$^\circ $ within the (HK0) plane for La$_{2-x}$(Sr,Ba)$_x$CuO$_4$ (x$\sim $0.05). Our neutron scattering results are compared with analogous studies on La$_{2-x}$Sr$_x$CuO$_4$ which indicate that such a rotation of the spin structure may be a generic feature of the underdoped La-214 cuprates. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V8.00010: Spin and Charge Excitations in Checkerboards based on a Superstructure David K. Campbell, Daoxin Yao, Erica W. Carlson Checkerboard patterns have been proposed to explain the real space structure observed in STM experiments on BSCCO and Na-CCOC. However, simple checkerboard patterns always give incommsensurate (IC) spin peaks rotated 45 degrees from the direction of the charge IC peaks, contrary to what is seen in neutron scattering. [1] Here, we study a more complicated checkerboard pattern which can resolve the low frequency inconsistency. Using spin wave theory, we explore the finite frequency response of this superstructure and find that the high energy response is inconsistent with neutron scattering results. In particular, there is no spin resonance peak. \newline \newline [1] D.X.Yao, E.W. Carlson and D.K.Campbell, Phys. Rev. B 73, 224525 (2006) [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V8.00011: Connecting spin and charge response in electron-doped cuprates Andreas Schnyder, Dirk Manske We perform a detailed analysis of angle-resolved photoemission data from electron-doped cuprates [1,2] in order to extract both the self-energy and the bare dispersion of the quasiparticles. The self-energy contains important information about the interactions among the quasiparticles. Taking the extracted bare quasiparticle dispersion as an input parameter we compute dynamical response functions employing a spin-fluctuation-based theory. In particular we estimate the dynamical spin susceptibility, which we then compare to recent inelastic neutron scattering data [3]. We obtain a resonance at the anti-ferromagnetic ordering wavevector $(\pi, \pi)$, whereas incommensurate spin excitations are mostly suppressed. Our approach provides a consistent theoretical description of both the spin and charge response in electron-doped cuprates. \newline \newline [1] H.~Matsui \textit{et al.}, Phys.~Rev.~Lett. \textbf{94}, 047005 (2005). \newline [2] H.~Matsui and T.~Takahashi, private communication. \newline [3] S.~D.~Wilson \textit{et al.}, Phys.~Rev.~B \textbf{74}, 144514 (2006). [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V8.00012: Local moment and inhomogeneous hyperfine interaction in the CuO$_2$ plane of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) single crystal by ${}^{17}$O NMR Bo Chen, Sutirtha Mukhopadhyay, William Halperin The ${}^{17}$O NMR spectra of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) single crystals were measured in the magnetic field of 8 T from 4 K to 200 K. The linewidth of the oxygen in CuO$_2$ plane, O(1), was found to follow a Curie temperature dependence in the normal state, where the Curie coefficient decreases with the increase of $\delta$ oxygen in the crystal. In the superconductive state, it decreases with deceasing temperature, proportional to the decreasing Knight shift. This temperature dependence of the linewidth identifies the existence of local moment and inhomogeneous hyperfine interaction in the CuO$_2$ plane. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V8.00013: Low-energy spin dynamics in La$_{1.95}$Sr$_{0.05}$CuO$_4$ Wei Bao, Ying Gasparovic, Kasu Yamada, Emilio Lorenzo, J-H. Chung The low energy part of spin fluctuation spectra $S({\bf q},E)$ of La$_{1.95}$Sr$_{0.05}$CuO$_4$ were measured using the cold neutron triple-axis spectrometer SPINS from 1.5 to 80 K. The incommensurate doublets of magnetic peaks show a tendency to move towards the commensurate (pi,pi) point with increasing energy, like the incommensurate quartets in (La,Ba)$_2$CuO$_4$ and other related cuprates. There is an energy-resolution limited ``central peak'' at $E=0$ below ~20K in the spectra $S({\bf q},E)$, likely due to the spin-glass transition [1]. The inelastic component has a different temperature dependence from the ``central peak'' and a detailed quantitative description of the data becomes available. The {\em local} spectra $S(E)$ behave in a manner closely resembling those of Li-doped La$_{2}$CuO$_4$ [2].\\ \,[1] Y. Chen et al., Phys. Rev. B {\bf 72}, 184401 (2005).\\ \,[2] W. Bao et al., Phys. Rev. Lett. {\bf 91}, 127005 (2003); Y. Chen et al., cond-mat/0408547. [Preview Abstract] |
Session V9: Superconductivity: Vortex States
Sponsoring Units: DCMPChair: Allen Hermann, Center for Nanoscale Science and Engineering, University of Kentucky
Room: Colorado Convention Center Korbel 1D
Thursday, March 8, 2007 11:15AM - 11:27AM |
V9.00001: Does a Bose-Glass State Exist in Commercial High-T$_c$ Wires? Jose P. Rodriguez The hypothesis that the vortex lattice induced by perpendicular magnetic field in films of PLD-YBCO is in a thermodynamic Bose glass state accounts for the inverse-square-root power law shown by the critical current density versus perpendicuar magnetic field.$^0$ We study here how robust such a state is to the addition of point pinning centers. This is done by first calculating the tilt modulus of the ``pristine'' Bose-glass state. It is found to diverge at long-wavelength along the magnetic-field/correlated-defect direction. A Larkin-Ovchinnikov analysis then yields a 2D/3D phase transition in collective pinning that is first-order. In particular, a {\it broken} Bose glass state characterized by finite Larkin domains, within which correlated pinning centers remain effective, exists at strong enough point pinning, at small enough coherence lengths. A signature of the broken glass state is found in the dependence shown by the critical current with film thickness along the c axis, which is predicted to crossover from [1] 2D to 3D collective-pinning behavior at a film thickness equal to the longitudinal Larkin scale $L_c$. \newline [1] J. Rodriguez \& M. Maley, Phys. Rev. B {\bf 73}, 094502 (2006). [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V9.00002: The low-temperature vortex liquid in $\rm La_{2-x}Sr_xCuO_4$ and $\rm Bi_2Sr_{2-y}La_yCuO_6$ Lu Li, Yayu Wang, Joseph G. Checkelsky, Seiki Komiya, Shimpei Ono, Yoichi Ando, Nai Phuan Ong Diamagnetism in lightly-doped crystals of $\rm La_{2-x}Sr_xCuO_4 $ (LSCO)with doping $x$ = 0.03 to 0.09 has been investigated by torque magnetometry, which resolves weak 2D supercurrents against a nearly isotropic paramagnetic spin response.By carefully subtracting the large paramagnetic susceptibilities of the van Vleck and spin terms, we observe the diamagnetic signal of vortices in fields up to 45 T, even for $x$ as low as 0.03. The torque results allow the $x$ dependence of both the melting field $H_{irr}$ and upper critical field $H_{c2}$ to be measured. We find that $H_{c2}$ extends smoothly across the critical doping value $x_c\simeq 0.055$. Below $x_c$, the pair condensate survives as a vortex liquid in intense fields, but long-range phase coherence is absent down to our lowest temperature $T$ = 0.35 K. We discuss the interesting differences between the vortex liquid and solid phases, and the collapse at low $T$ of phase coherence at the boundary. The close correlation of $T_{onset}$ to $H_{c2}$ in LSCO and Bi 2201 will also be reported. Resarch supported by NSF grant DMR 0213706. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V9.00003: High-frequency response of Josephson vortex lattice in layered superconductors Alexei Koshelev Magnetic field applied along the layer direction of layered superconductors generates the Josephson vortex lattice. We studied response of this state to the high-frequency c-axis electric field. Numerically solving equations for the oscillating phases, we computed the frequency dependences of the loss function, $\mathrm{Im}[1/\epsilon(\omega)]$, at different magnetic fields, including regions of both dilute and dense Josephson vortex lattices. The main feature of the response is the Josephson-plasma-resonance peak. In the dilute-lattice regime this peak is displaced to slightly lower frequency. An interesting feature of the dilute regime is the appearance of satellites at the higher-frequency part, which are caused by excitation the plasma modes with the wave vectors set by the lattice structure. In the dense-lattice limit the plasma peak moves to higher frequency and its intensity rapidly decreases, in agreement with analytical theory. An additional broad peak exists at low frequencies, and can be described by phenomenological theory of vortex oscillations. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V9.00004: Universality in the Self Organized Critical behavior of a cellular model of superconducting vortex dynamics Yudong Sun, Tegy Vadakkan, Kevin Bassler We study the universality and robustness of variants of the simple model of superconducting vortex dynamics first introduced by Bassler and Paczuski in Phys.\ Rev.\ Lett.\ {\bf 81}, 3761 (1998). The model is a coarse-grained model that captures the essential features of the plastic vortex motion. It accounts for the repulsive interaction between vortices, the pining of vortices at quenched disordered locations in the material, and the over-damped dynamics of the vortices that leads to tearing of the flux line lattice. We report the results of extensive simulations of the critical ``Bean state" dynamics of the model. We find a phase diagram containing four distinct phases of dynamical behavior, including two phases with distinct Self Organized Critical (SOC) behavior. Exponents describing the avalanche scaling behavior in the two SOC phases are determined using finite-size scaling. The exponents are found to be robust within each phase and for different variants of the model. The difference of the scaling behavior in the two phases is also observed in the morphology of the avalanches. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V9.00005: Molecular Dynamics simulations of the vortex matter two-stage melting transition in Bi$_2$Sr$_2$CaCu$_2$O$_8$ in the presence of straight and of tilted columnar defects Jin-Tao Liu, Yadin Y. Goldschmidt In this paper we use London Langevin molecular dynamics simulations to investigate the vortex matter melting transition in the highly anisotropic high-temperature superconductor material Bi$_2$Sr$_2$CaCu$_2$O$_8$ in the presence of low concentration of columnar defects (CDs). We reproduce with further details our previous results obtained by using Multilevel Monte Carlo simulations that showed that the melting of the nanocrystalyne vortex matter occurs in two stages: a first stage melting into nanoliquid vortex matter and a second stage delocalization transition into a homogeneous liquid. Furthermore, we report on new dynamical measurements in the presence of a current that identifies clearly the second stage delocalization transition. In addition to CDs aligned along the c-axis we also simulate the case of tilted CDs which are aligned at an angle with respect to the applied magnetic field. Results for a $45^0$ tilt of the CDs show that the location of the melting transition is not affected by the tilt when the ratio of flux lines to CDs remains constant. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V9.00006: Disparities in the vortex state electrodynamics of high Tc cuprates Andrew LaForge, Zhiqiang Li, Willie Padilla, Kenneth Burch, Sasa Dordevic, Kouji Segawa, Yoichi Ando, Dimitri Basov We report new far infrared measurements of underdoped YBCO in magnetic field and situate these new data within earlier work on related compounds. We show that the cuprate superconductors, including YBa$_{2}$Cu$_{3}$O$_{y}$ (YBCO), La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO), and Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (BSCCO), have revealed strikingly different vortex state electrodynamic responses between the various families. For example, in the Josephson vortex regime a strongly field dependent transverse resonance is observed in the interplane conductivity of YBCO, whereas no such feature is found in LSCO. Microwave magnetoabsorption spectra of BSCCO exhibit two resonances which are separated by a temperature gap, again at variance with the other systems. We compare and contrast these findings with several models of the electrodynamics in the vortex state and offer possible explanations for the observed discrepancies. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V9.00007: Magnetic measurement of RF-induced flux lattice annealing (RIFLA) in the electron-doped superconductor Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$. W.G. Clark, G. Gaidos, G. Wu, S.E. Brown, R.L. Greene, H. Balci A strained flux lattice (FL) in a superconductor (SC) can be annealed to a lower free energy by the RF-field used to generate an NMR spin-echo signal [W.G. Clark et al., J. Phys. IV Proceedings \textbf{9}, Pr10-49-52 (1999)]. Here, we report the change in the rf magnetic susceptibility ($\chi )$ in the SC phase of a single crystal of Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$ with a strained FL by successive RIFLA pulses. The distorted FL is prepared by rotating the sample through a small angle in a magnetic field well below the SC transition temperature, leaving the FL pinned in a non-equilibrium configuration. RIFLA pulses then reduce $\chi $, which is measured by the change in the inductance of a coil surrounding the sample using an NMR probe and spectrometer. The interpretation is that shaking the FL by the RF pulses progressively anneals it to a lower free energy configuration, which has a smaller $\chi $. These measurements provide a very sensitive measure of the RIFLA effect. Work at UCLA was supported by NSF Grants DMR-0334869 (WGC) and DMR-0520552 (SEB). [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V9.00008: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V9.00009: Puzzling magnetic behavior of non-centrosymmetric superconductor Re$_{3}$W V. Kuznetsova, J.R. Thompson, Y. Zuev, D.K. Christen, R. Jin We have studied magnetic properties of non-centrosymmetric superconductor Re$_{3}$W. Unlike ordinary BCS superconductors, annealed samples of Re$_{3}$W exhibit linear Abrikosov-like dependence of the equilibrium (M$_{eq})$ and non-equilibrium (M) magnetization on magnetic field H in an anomalously large range H$\approx $(0.1-1)H$_{c2}$. This behavior is drastically different for the quenched (``as prepared'') samples of Re$_{3}$W. Equilibrium magnetization was obtained by ``shaking'' the flux line lattice with an alternating transverse field. M$_{eq}$(H) curves show change in slope in about the same region of magnetic fields where the slopes of corresponding critical currents J$_{c}$(H) also change. In our talk, we discuss probable causes of these effects. ORNL is managed by UT-Battelle, LLC for USDOE under contract DE-AC05-00OR22725. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V9.00010: Ground state of interlayer Josephson vortex systems and the shear modulus Yoshihiko Nonomura, Xiao Hu Ground state of interlayer Josephson vortex systems is investigated on the basis of the full Lawrence-Doniach model. We find: (1) For low fields, field dependence of $c_{66}$ approximately coincides with the elastic theory, and jumps in $c_{66}$ correspond to the structural phase transitions between various rotated-lattice configurations. (2) For intermediate fields, the ground state is characterized by continuously-varying shearing angle of vortex lattices. In this region $c_{66}$ becomes smaller than those in the elastic theory, and vanishes at the shear instability field. This critical field is increased by inhomogeneity of the magnetic field. (3) For high fields, $c_{66}$ converges to a constant value, which is quite different from the exponentially-decaying $c_{66}$ in the London theory. This saturation of $c_{66}$ can be interpreted as the effect of ``effective core size" within the elastic theory, and is not affected by inhomogeneity of the superconducting amplitude for material parameters of cuprate high-$T_{\rm c}$ superconductors. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V9.00011: Thermal study of vortex states in mesoscopic superconducting disks Florian Ong, Olivier Bourgeois, Sergey Skipetrov, Jacques Chaussy We present low temperature highly sensitive heat capacity $C$ measurements [1,2] of aluminum disks with diameters close to $\xi(T)$, the superconducting coherence length. $C(T)$ scans under fixed perpendicular magnetic fields $H$ reveal a quasiperiodic modulation of the height $\Delta C$ of the $C$ jump at the superconducting to normal phase transition. This behavior is due to transitions between several arrangements of vortices in the disks. Indeed giant vortex states or multivortex states can be observed, with a vorticity (an integer equal to the number of vortices threading a single disk) depending on $H$, $T$, and on the size of the disks. Heat capacity measurements enable to study phase transitions between such states, without contacting or biasing them. Thus phase boudaries in the $H-T$ plane can be drawn in all the superconducting region. \newline \newline [1] O. Bourgeois, F. Ong, S.E. Skipetrov, and J. Chaussy, \textit{Phys. Rev. Lett.} {\bf{94}}, 057007 (2005) \newline [2] F.R. Ong, O. Bourgeois, S.E. Skipetrov, and J. Chaussy, \textit{Phys. Rev. B} {\bf{74}}, 140503(R) (2006) [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V9.00012: Vortex Lattice Structural Transitions: A generalized (thermal fluctuations and quench disorder) Ginzburg-Landau Approach Pei-Jen Lin, Golan Bel, Baruch Rosenstein Base on Guzburg-Landau model, we study the rhombic-to-square transition for tetragonal material in the magnetic fields along c axis with the account of thermal fluctuation and disorder influence. The nonperturbative method are used to approach the melthing line of vortex lattice.The result will compare with the recent neutron scattering data and second peak effect on high $T_{c} $ SC and Low $T_{c}$ SC. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V9.00013: ABSTRACT HAS BEEN MOVED TO J8.00012 |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V9.00014: Deconfinement of Vortices with Continuously Variable Fractions of the Unit Flux Quanta in Two-Gap Superconductors Jun Goryo, Singo Soma, Hiroshi Matsukawa We propose a new stage of confiment-decofinment transition, which can be observed in laboratory. In two-gap superconductors, two kinds of vortex exist by the presence of two different $U(1)$ phases. Each of them carries a continuously variable fraction of the unit flux quanta $\Phi_0=h c / 2 e$. The confined state of these two is a usual vortex and stable in the low temperature region of the system under a certain magnetic field above $H_{c1}$. We see an analogy to quarks in a charged pion. An entropy gain causes two fractional vortices to be deconfined above a certain temperature. We estimate the condition of the deconfinement by using parameters for a typical two-gap superconductor MgB$_2$. [Preview Abstract] |
Session V10: Critical and Non-equilibrium Phenomena
Sponsoring Units: DCMPChair: Thomas Vojta, University of Missouri - Rolla
Room: Colorado Convention Center Korbel 1E
Thursday, March 8, 2007 11:15AM - 11:27AM |
V10.00001: Scaling Functions and Conformal Invariance at the Quantum Critical Point of the Sub-Ohmic Bose-Fermi Kondo Model Stefan Kirchner, Qimiao Si In one approach to the quantum critical heavy fermion metals, Kondo lattice systems are studied through a self-consistent Bose-Fermi Kondo Model (BFKM) within the extended dynamical mean field theory. It has become clear in recent years that the critical behavior of the BFKMs is not captured by the classical critical theory obtained through the standard ``quantum-to-classical mapping'' of the model. In this work, we study the finite temperature scaling functions of the easy-axis BFKM using a cluster-updating Monte Carlo approach, and their counterparts of a spin-isotropic BFKM in a dynamical large-N limit. The scaling functions are found to have the form dictated by a boundary conformal field theory even though the underlying Hamiltonian lacks conformal invariance. In the large-N limit, this is established for all multiple-spin correlation functions as well. The results raise the possibility that the quantum critical point of the BFKM has an enhanced symmetry, which should be helpful to the understanding of the underlying critical field theory. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V10.00002: Zero Bias Anomaly Out of Thermal Equlibrium Dmitri Gutman, Yuval Gefen, Alexander Mirlin We consider the out-of-equilibrium tunneling density-of-states for a two-dimensional diffusive film. Starting from a Keldysh $sigma$-model formalism we have obtained an effective action capable of accounting for both real and virtual processes. The ensuing zero bias anomaly, obtained non-perturbatively in the interaction, exhibits a two-dip structure, whose singularity is rounded off by the electron-electron inelastic rate. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V10.00003: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V10.00004: Numerical renormalization group study of a dissipative quantum dot M.T. Glossop, K. Ingersent We study the quantum phase transition (QPT) induced by dissipation in a quantum dot device at the degeneracy point. We employ a Bose-Fermi numerical renormalization group approach [1] to study the simplest case of a spinless resonant-level model that couples the charge density on the dot to a dissipative bosonic bath with density of states $B(\omega)\propto \omega^s$. In anticipation of future experiments [2] and to assess further the validity of theoretical techniques in this rapidly developing area, we take the conduction-electron leads to have a pseudogap density of states: $\rho(\omega) \propto |\omega|^r$, as considered in a very recent perturbative renormalization group study [3]. We establish the conditions on $r$ and $s$ such that a QPT arises with increasing dissipation strength --- from a delocalized phase, where resonant tunneling leads to large charge fluctuations on the dot, to a localized phase where such fluctuations are frozen. We present results for the single-particle spectrum and the response of the system to a local electric field, extracting critical exponents that depend in general on $r$ and $s$ and obey hyperscaling relations. We make full comparison with results of [3] where appropriate. Supported by NSF Grant DMR-0312939. [1] M. T. Glossop and K. Ingersent, PRL {\bf 95}, 067202 (2005); PRB (2006). [2] L. G. G. V. Dias da Silva, N. P. Sandler, K. Ingersent, and S. E. Ulloa, PRL {\bf 97}, 096603 (2006). [3] C.-H. Chung, M. Kir\'{c}an, L. Fritz, and M. Vojta (2006). [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V10.00005: Anomalous Conductance Quantization in Side-Gated InAs Quantum Point Contacts Saydur Rahman, Steven Herbert, Richard Newrock, Philippe Debray Conductance measurements as function of Fermi energy of InAs quantum point contacts created by side gating on InAs/InAlAs quantum-well structure exhibit a distinct plateau at e$^{2}$/h and a less distinct one at 2e$^{2}$/h. Applied magnetic field has little or no influence on the conductance quantization pattern. We believe the conductance plateau at e$^{2}$/h indicates a spin contribution of h/e$^{2}$ to the resistance of the one-dimensional channel when the electron density is low and the electron-electron interaction is strong. This can be understood in the framework of the recently proposed spin-incoherent Luttinger liquid (SILL) state that is considered to exist when \textit{J$<<$T$<<$E}$_{F}$, where $J$ is the spin exchange constant and E$_{F}$ the Fermi energy. In the SILL regime, the spin excitations are reflected back to the leads reducing the conductance of the quantum wire to e$^{2}$/h. At higher Fermi energy, when the electron density is higher, the spin contribution to resistance vanishes and the conductance takes the well-known quantized value of 2e$^{2}$/h. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V10.00006: Phase-ordering dynamics in itinerant quantum ferromagnets R. Saha, D. Belitz, T.R. Kirkpatrick Phase ordering following a rapid quench from the disordered phase to the ordered phase occurs via growth of domains that arise from spontaneous fluctuations. The linear size $L$ of these domains grow as a power law function of time for late times: $L(t) \propto t^{1/z}$, with z a dynamical exponent[1]. Until now this description of phase ordering dynamics has been applied to classical systems only. We apply this theory to describe domain growth in both clean and dirty itinerant quantum ferromagnets. The fluctuation effects that invalidate Hertz's theory of the quantum phase transition[2] also affect the phase ordering. For a quench into the ordered phase a transient regime appears, where the dynamical exponent differs from the classical case, and for asymptotically long times the prefactor of the growth law has an anomalous magnetization dependence[3]. A quench to the quantum critical point results in a growth law which is not a power-law function of time.\newline [1] A.J. Bray, Adv. in Phys. {\bf 43}, 357 (1994). [2] D. Belitz, T.R. Kirkpatrick, and T. Vojta, Rev. Mod. Phys. {\bf 77}, 579 (2005). [3] D. Belitz, T. R. Kirkpatrick, and Ronojoy Saha, cond-mat/0610650. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V10.00007: Universal non-linear conductivity near to an itinerant-electron quantum critical point Patrick Hogan, Andrew Green Quantum critical systems display universal, power-law temperature dependence in their response functions. These universal power-laws provide an experimental window upon quantum criticality. Concentrating upon conductivity in itinerant-electron systems near to a magnetic quantum critical point, we show that universal power-law dependence upon temperature is reflected in a universal non-linear conductivity; when a strong electric field is applied, the resulting current has a universal power-law dependence upon the applied electric field. For a system with thermal equilibrium resistivity proportional to $T^\alpha$, we find a non-linear resistivity proportional to $E^{\alpha/(1+\alpha)}$. This provides a new experimental handle upon the physics of itinerant-electron quantum critical points. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V10.00008: Quantum critical behavior in itinerant electron systems: Eliashberg theory and instability of a ferromagnetic quantum-critical point Jerome Rech, Catherine Pepin, Andrey Chubukov We consider the problem of fermions interacting with gapless long-wavelength collective bosonic modes. The theory describes, among other cases, a ferromagnetic quantum-critical point (QCP). We construct a controllable expansion at the QCP in two steps: we first create a new, non Fermi-liquid ``zero-order'' Eliashberg-type theory, and then demonstrate that the residual interaction effects are small, provided we meet two conditions on the parameters of the system. For an $SU(2)$ symmetric ferromagnetic QCP, we find that the Eliashberg theory itself includes a set of singular renormalizations which can be understood as a consequence of an effective long-range dynamic interaction between quasi-particles, generated by the Landau damping term. These singular renormalizations give rise to a negative non-analytic $q^{3/2}$ correction to the static spin susceptibility, and destroy a ferromagnetic QCP. We demonstrate that this effect can be understood in the framework of the $\phi^4$ theory of quantum-criticality, and show that it is specific to the $SU(2)$ symmetric case. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V10.00009: Verification of universality in strong critical adsorption with three techniques. Matthew Brown, Bruce Law, Lyle Marschand, Laurence Lurio, Serif Uran Fischer and de Gennes [C. R. Acad. Sci., Ser. B 287, 207 (1978)] suggested that for a mixture which was critical with respect to the demixing phase transition an interfacial adsorption profile would exhibit universal (system independent) scaling behavior. Carpenter et al. [Phys. Rev. E 61, 532 (2000) and references therein] verified this theory with a model which described ellipsometry data previously taken from several critical binary liquid mixtures. Until recently this model had not been verified with other experimental techniques. We describe a successful effort to verify Carpenter's model with data from a neutron reflectometry study of a critical D2O + 3-methylpyridine mixture, as well as an analysis of data from both ellipsometry and x-ray reflectometry from a critical mixture of dodecane + tetrabromoethane. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V10.00010: Critical Behavior Near the Ferromagnetic Transition in Nanostructured Gadolinium Paul Shand, Justin Bohnet, Jared Goertzen, Jeffrey Shield, Geoffrey Shelburne, David Schmitter, Diandra Leslie-Pelecky ac susceptibility and dc magnetization have been measured near the ferromagnetic transition in melt-spun nanostructured Gd. Effective critical exponents and the critical temperature were extracted using modified Arrott plots. The values obtained were \textit{$\beta $}$_{eff}$ = 0.415 $\pm $ 0.005, \textit{$\gamma $}$_{ eff}$ = 1.36 $\pm $ 0.04, \textit{$\delta $}$_{ eff}$ = 4.24 $\pm $ 0.02, and $T_{C}$ = 290.0 $\pm $ 0.1 K. These exponent values satisfy the scaling relation \textit{$\beta \delta $ }= (\textit{$\beta $}+\textit{$\gamma $}). The experimental exponent values are also close to those for a short-range 3D Heisenberg ferromagnet; however, there is a systematic shift toward mean-field values. Such a shift has been previously seen in amorphous ferromagnets and is likely due to the presence of longer-ranged interactions, especially in Gd-based alloys. The critical exponents for nanostructured Gd are closer to the short-range 3D Heisenberg exponents than are those for amorphous Gd$_{67}$Co$_{33}$ and Gd$_{80}$Au$_{20}$. This suggests that the dominant Gd-Gd interactions are shorter-ranged in nanostructured Gd than in the amorphous alloys. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V10.00011: Spin-orbit coupling triggered Mott insulator Sr$_{2}$IrO$_{4}$. Bum Joon Kim, Hosub Jin, Soon Jae Moon, Jae-Young Kim, Choon Shik Leem, Jaejun Yu, Tae-Won Noh, Changyoung Kim, Se-Jung Oh, Jae-Hoon Park, Gang Cao Electronic structures of 5d transition-metal oxide (TMO) Sr$_{2}$IrO$_{4 }$are investigated by angle-resolved photoemission spectroscopy and density-functional electronic structure calculations. The insulating nature of this compound and its measured valence band symmetry are correctly accounted for by the calculation only when both spin-orbit coupling and electronic correlation effects are included. It is shown that the spin-orbit coupling plays a crucial role in stabilizing the Mott-insulating ground state, which is unexpected in a compound with extended 5d valence band. The spin-orbit coupling leads to symmetry-split-off bands near the Fermi level which are narrow enough to be gapped by the Coulomb repulsion of moderate strength. This results in a conduction band as narrow as $\sim $0.5 eV, defying its general character expected for 5$d$ TMO. Our finding marks an establishment of a new type of Mott insulator and suggests possibility of novel Mott-derived phenomena in 5$d$ based materials. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V10.00012: Spin-orbit coupling in quasiperiodic systems Carlos Wexler, Daijiro Tobe, Mahito Kohmoto Electrons in incommensurate systems (e.g., electrons in a lattice in presence of a perpendicular magnetic field) have a rich behavior exemplified by the beautiful Hofstadter butterfly, a self-similar spectrum which is a multifractal Cantor set. We analyze the effect of spin-orbit coupling in this system which can be described by a generalization of Harper's equation. We find that the added term significantly modifies the scaling laws and appears to induce a delocalization of the insulator. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V10.00013: Influence of Boundary Conditions on the Critical Scaling of Finite Systems Benjamin Vollmayr-Lee, Erik Luijten We investigate the influence of boundary conditions on the behavior of finite spin systems at criticality; in particular, a comparison is made between periodic boundary conditions and free boundary conditions. Via simulations, we find that the critical magnetization distribution of the short-range Ising model with free boundary conditions differs significantly from the periodic case. We also consider spin systems with long-range interactions $V\sim 1/r^{(d+\sigma)}$ where for $\sigma \leq 2$ the upper critical dimension is $d_c=2\sigma$, which allows for accurate simulations for $d>d_c$ systems. Using analytic results from Br\'ezin and Zinn-Justin for periodic boundary conditions, and generalizing the techniques developed by Rudnick, Gaspari, and Privman for free boundary conditions, we obtain explicit expressions for the scaling of the susceptibility and the shape of the magnetization and energy distributions at criticality. The numerical data exhibit excellent agreement with our analytic results, providing in most cases the first explicit test for these predictions. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V10.00014: Measurement Noise Maximum as a Signature of a Phase Transition Zhi Chen, Clare Yu Noise is ubiquitous and is being increasingly used as an experimental tool to probe condensed matter systems, but unfortunately, when studying phase transitions, the usefulness of the experimental results is diminished by the fact that little is known about what to expect in the noise spectra. We propose that an increase in the measurement noise can be used to signal the onset of a phase transition since noise arises from the fluctuations of microscopic entities which, in turn, play a key role in phase transitions. As an example, we study the noise in the 2D Ising model which undergoes a second order phase transition, and in the 5-state Potts model which undergoes a first order phase transition by using Monte Carlo simulations. We monitor these systems as the temperature drops below the critical temperature. At each temperature, after equilibration is established, we obtain the time series of quantities characterizing the properties of the system, i.e., the energy and magnetization per site for different size of systems. We apply different methods, such as the noise power spectrum and the second spectrum of the noise, to analyze the fluctuations in these quantities. We show that fluctuations produce an increase in the low frequency noise and the total noise power as first and second order phase transitions are approached. [Preview Abstract] |
Session V11: Heavy Fermions
Sponsoring Units: DMPChair: Peter Sharma, RIKEN
Room: Colorado Convention Center Korbel 1F
Thursday, March 8, 2007 11:15AM - 11:27AM |
V11.00001: Magnetic Field Effects in the Heavy Fermion Ce$_{3}$Co$_{4}$Sn$_{13}$ A.D. Christianson, F. Ronning, Y. Tokiwa, J.S. Gardner, H.J. Kang, J.-H. Chung, E.A. Goremychkin, P. Manuel, J.D. Thompson, J.L. Sarrao, A.L. Cornelius, J.M. Lawrence Ce$_{3}$Co$_{4}$Sn$_{13}$ is a heavy Fermion system with a low temperature specific heat as large as 4 J/mol K$^{2}$. Measurements on single crystal specimens of the elastic neutron scattering response and the heat capacity (C) show that Ce$_{3}$Co$_{4}$Sn$_{13}$ is extremely sensitive to applied magnetic fields. The heat capacity measurements show that the broad peak in C/T at 0.8 K at zero field initially moves downward in temperature before moving upward with a characteristic energy scale of 2 T. Neutron diffraction measurements at 0.15 K show that upon application of very small magnetic fields, the intensity on the (100) peak grows and does not saturate below 6 T. Although reduced in magnitude the effect persists to temperatures at least as high at 2 K. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V11.00002: Thermal Expansion of the Heavy Fermion Borocarbide YbNi$_{2}$B$_{2}$C G.M. Schmiedeshoff, A.W. Lounsbury, D.J. Luna, W.E. Okraku, S.J. Tracy, J.C. Cooley, S.L. Bud'ko, P.C. Canfield YbNi$_{2}$B$_{2}$C is a heavy fermion compound with a coherence temperature of about 10 K. We have measured its thermal expansion from room temperature to below 1 K. The thermal expansion is anisotropic, negative below about 16 K, and deviates from simple metallic behavior near the coherence temperature. We will present and discuss our results in the context of a Gruneisen analysis. This work was supported by the National Science Foundation under DMR-0305397. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-ENG-82.~ This work was supported by the Director for Energy Research, Office of Basic Energy Sciences. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V11.00003: Infrared and magneto-optical studies of heavy fermion skutterudites YbFe$_4$Sb$_{12}$ and CeRu$_4$Sb$_{12}$ L.W. Kohlman, S.V. Dordevic, K.S.D. Beach, D.N. Basov, R. Baumbach, M.B. Maple, R. Tung, Y.J. Wang, N. Takeda We will report infrared and magneto-optical results on two heavy fermion skutterudites YbFe$_4$Sb$_{12}$ and CeRu$_4$Sb$_{12}$. Detailed temperature dependence of infrared spectra will be presented for both compounds. In addition, magneto-transmission measurements on YbFe$_4$Sb$_{12}$ in magnetic field as high as 33 Tesla, and magneto-reflection measurements on CeRu$_4$Sb$_{12} $ in 17 Tesla field will be reported. The results reveal suppression of heavy fermion state with magnetic field, and recovery of a more conventional metallic state. In particular, the effective mass of charge carriers is gradual diminished. On the other hand, hybridization gap is much more insensitive to the application of magnetic field. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V11.00004: Kondo ground state of 6 related Yb-based intermetallic compounds E-D. Mun, S. Jia, M.S. Torikachvili, A.S. Sefat, S.L. Bud'ko, P.C. Canfield Isostructrural YbT$_2$Zn$_{20}$ (T=Fe, Co, Ru, Rh, Os, Ir) have less than 5\% concentration atomic Yb, in which the Yb atoms fully occupy one unique crystallographic site of cubic (-43m) symmetry. This series of compounds offer the opportunity to systematically study Kondo lattice systems approaching the single ion limit. With the thermodynamic and transport measurements, we show that the Kondo ground state has different degeneracies for T=Fe, Ru, Rh, Os and Ir, reflecting the competition between the different Kondo temperatures ($T_K$) and the similar crystal-electric field (CEF) splitting temperatures ($T_{CEF}$). On the other hand, the unusual thermodynamic behavior at low temperature suggests that YbCo$_2 $Zn$_{20}$ has low $T_K$ and seems to be very close to a quantum critical point. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V11.00005: Spin Dynamics in the f-electron non Fermi Liquid alloy Sc1-xUxPd3 Stephen Wilson, Pengcheng Dai, D. Adroja, Y. Qiu, N.P. Butch, M.B. Maple We will discuss our recent inelastic neutron scattering experiments probing the spin dynamics in the non Fermi-liquid alloy, Sc$_{1-x}$U$_{x}$Pd$_{3}$. The increased homogeneity of U- sites in this system allows for an investigation of magnetism without the disordering effects of U-site clustering observed in isostructural Y$_{1-x}$U$_{x}$Pd$_{3}$. Spin dynamics indicative of the influence of a spin glass (SG) quantum critical point (QCP) in Sc$_{1-x}$U$_{x}$Pd$_{3}$ have been observed previously. Our current study probes spin fluctuations now doped away from this SG QCP and into the antiferromagnetic (AF) ordered phase of the system. The evolution of the spin excitations as the system is tuned across its phase diagram away from the SG QCP will be discussed, with particular emphasis given to the evolution of the localized spin fluctuations attributed to the QCP in this system. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V11.00006: Interplay between Disorder and Quantum and Thermal Fluctuations in Ferromagnetic Alloys -- New Systems G.R. Stewart, J.S. Kim, M.B. Silva Neto, A.H. Castro Neto Previously$^{1}$ we addressed the effects of disorder on the ferromagnetic ordering temperature, T$_{C}$, in UCu$_{2}$Si$_{2-x}$Ge$_{x}$. In that work the measured non-monotonic variation of T$_{C}$ with disorder (as measured by the resistivity) could be explained within a model$^{2}$ of localized spins interacting with an electronic bath. This model predicts that, in some cases, T$_{C}$ can be \textit{enhanced} by the interplay between quantum and thermal fluctuations with disorder. We have extended this work in other ferromagnetic alloys, with both significantly larger as well as similar variations of T$_{C}$ with doping compared to the $<$10 {\%} variation of T$_{C}$ observed in UCu$_{2}$Si$_{2-x}$Ge$_{x}$. Resistivity, magnetic susceptibility, and specific heat will be presented, along with a comparison to the theory$^{2}$. $^{1}$M. B. Silva Neto, A. H. Castro Neto, D. Mixson, J. S. Kim, and G. R. Stewart, Phys. Rev. Lett. \underline {91}, 257206 (2003). $^{2}$M. B. Silva Neto and A. H. Castro Neto, Europhys. Lett. \underline {62}, 890 (2003). [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V11.00007: Effects of Disorder in the Heavy Fermion Antiferromagnet CeCu$_{6-x}$Au$_{x}$ D.J. Burnette, J.S. Kim, G.R. Stewart Using a quick quenching technique that also produces bars of known geometry for absolute resistivity measurements, we have measured the effects of this fairly rapid ($\sim $ 10$^{4}$ K/s) solidification on the high angle x-ray line width, resistivity, magnetic susceptibility, and specific heat of CeCu$_{6-x}$Au$_{x}$ for several compositions. Gradual variations of the disorder and its effect on the properties were investigated by annealing the quenched samples at 700 $^{\circ}$C over periods of days to weeks. The discussion of the effects of the quenched-in disorder on the measured properties will focus on T$_{Neel}$, the low temperature specific heat $\gamma $, and the non-Fermi liquid behavior. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V11.00008: Elastic property of a high-field ordered state observed in PrFe$_{4}$P$_{12}$ Yoshiki Nakanishi, Takuya Fujino, Peijie Sun, Mitsuteru Nakamura, Masahito Yoshizawa, Hitoshi Sugawara, Daisuke Kikuchi, Hideyuki Sato We present experimental results of elastic constants as a function of temperature and magnetic field for the Pr-based heavy fermion system PrFe$_{4}$P$_{12}$, especially in a high-field (HF) ordered phase discovered. Since the HF phase exists in a narrow temperature range below 0.7 K and for high fields above 7 T in a highly limited angular range around the [111] directions, the elastic constants C$_{L}$ = (C$_{11}$+2C$_{12}$+4C$_{44})$/3 and C$_{T}$ = (C$_{11}$-C$_{12}$+C$_{44})$/3 were measured by the longitudinal (L) one propagating along the [111] direction and by the transverse (T) one along the [111] direction polarized to the [1-11] one, respectively. A clear upturn was observed in both of the elastic constants below the HF ordered phase transition temperature. Furthermore, a remarkable elastic softening toward the transition temperature was observed in the temperature dependence of C$_{L}$, whereas no softening was observed in C$_{T}$. These results indicate that the softening is most likely to be due to the bulk modulus (C$_{11}$+2C$_{12})$/3. Thus, it is expected that the HF ordered phase seems to be accompanied by a strain fluctuation with $\Gamma _{1}$ spatial symmetry, namely a scalar phase transition with a multi-polar moment is a candidate for the HF ordered phase. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V11.00009: Disorder and quantum size effects on Kondo interactions and magnetic correlation in CePt$_{2}$ Y.Y. Chen, T.K. Lee, J.M. Lawrence, C.H. Booth Measurements of specific heat C(T) and magnetic susceptibility $\chi $(T) on a series of CePt$_{2}$ nanoparticles with size d= 3.1, 22 and 26 nm are compared to those of bulk CePt$_{2}$ to determine the size effects on Kondo interactions and magnetic correlation therein. Kondo interactions (T$_{K}$=5.6 K) and an antiferromagnetic correlation (T$_{N}$=1.6 K) coexist in CePt$_{2}$ with $\sim $60{\%} and $\sim $40{\%} of Ce magnetic ions involved in respectively. While the antiferromagnetism is diminished by size reduction, Kondo behaviour predominates, with a sommerfield constant $\gamma$ increasing from 460 mJ/f.u. K$^{2 }$ for the bulk to 3000 mJ/f.u. K$^{2 }$ for 3.1 nm. Meanwhile, a decrease of Kondo temperature T$_{K}$ from 5.6 to 0.42 K is observed with the size reduction. The consequences were explained by structural disorders, however for d $<$ 10 nm electronic quantum size effect play a more significant role. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V11.00010: Probing the Magnetic Order of UPt$_{3}$ by Magnetoresistance Thomas M. Lippman, John P. Davis, Hyoungsoon Choi, Johannes Pollanen, William J. Gannon, William P. Halperin It is believed that magnetic order has a strong influence on the unconventional superconducting state of UPt$_{3}$, both as a symmetry breaking field that splits two degenerate superconducting phases, and as an element of the pairing mechanism. There are conflicting reports of the behavior at 5 K, static antiferromagnetic order versus slow fluctuating magnetism. We present measurements of the temperature dependence of the magnetoresistance in bulk single crystals of UPt$_{3}$. Below 5 K, we observe an onset and subsequent rapid increase of a term linear in the applied field. With the field in the basal plane, this linear contribution is positive with current along the c-axis and negative with the current along the b-axis. We identify this behavior in the magnetoresistance with observation of magnetic Bragg peaks from neutron scattering that appear below 5 K. We discuss preliminary results of similar measurements at temperatures in the millikelvin range. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V11.00011: Doped Mott insulator physics in the 4f antiferromagnet CeIn3 and implications for pressure-tuned superconductivity Neil Harrison, Suchitra Sebastian, Cristian Batista, Marcelo Jaime, Stuart Trugmann, Victor Fanelli, Takao Ebihara, Eric Palm, Tim Murphy, Hisatomo Harima We report recent de Haas-van Alphen experiments performed at low temperatures ($T$ $>$ 30 mK) and high magnetic fields in CeIn$_3$ that motivate us to revise our understanding of strongly coupled antiferromagnetism in $f$-electron systems. In addition to the known light Fermi surface sheets, heavy ellipsoid pockets are observed with a symmetry consistent with them being situated at the $ \pi/2,\pi/2,\pi/2$ point in the paramagnetic Brillouin zone. Their topology is the 3D analog of the 2D hole pockets reported to exist at $\pi/2,\pi/2$ in underdoped cuprate superconductors, indicating some degree of similarity between the the electronic structures in the $d$- and $f$-electron systems. The effective masses of the ellipsoids are sufficient to account for all of the electronic specific heat of CeIn $_3$ at ambient pressure within the antiferromagnetic phase, making them strong candidates for the origin of the coexisting superconductivity under pressure. High magnetic fields cause these pockets to undergo a topological deformation, passing through a Lifshitz transition at $\sim$~40~T. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V11.00012: The superconducting pairing states in CeCu$_2$Si$_2$ H. Q. Yuan, M. B. Salamon, H. S. Jeevan, C. Geibel, F. Steglich The prototype heavy fermion superconductor CeCu$_2$Si$_2$ exhibits very rich physical properties. It has been shown that two distinct superconducting states, one around an AFM QCP at low pressures and the other one around a weak first-order valence transition at high pressures, uniquely exist in the pressurized CeCu$_2$(Si/Ge)$_2$ [1]. In this talk, I will first briefly review the recent progress on the study of these two superconducting phases. Then I will focus on the superconducting pairing state of CeCu$_2$Si$_2$ at $p=0$. Early measurements, e.g., specific heat and $\mu$ SR, revealed quite controversial behavior due to the limitations of experimental techniques and sample quality. In order to elucidate the nature of superconductivity in CeCu$_2$Si$_2$ and to investigate the interplay of AFM and superconductivity, we performed precise measurements of the magnetic penetration depth $\Delta \lambda (T)$ in high quality single crystals down to $T=80$ mK. A linear temperature dependence of $\lambda(T)$ is observed in both A/S-type and S-type CeCu$_2$Si$_2$ below $T=150$ mK, providing uncontroversial evidence for the existence of line nodes in the superconducting energy gap. [1] H. Q. Yuan et al., Science 302, 2104 (2003); Phys. Rev. Lett. 96, 047008 (2006). [Preview Abstract] |
Session V12: Focus Session: Magneto-optics and Spin Dynamics
Sponsoring Units: GMAG DMP FIAPChair: Scott Crooker, Los Alamos National Laboratory
Room: Colorado Convention Center Korbel 3C
Thursday, March 8, 2007 11:15AM - 11:51AM |
V12.00001: Determination of effective sp-d exchange integrals in wide-gap DMS Invited Speaker: This work presents a magneto-optical study of diluted magnetic semiconductors (DMS) based on ZnO and GaN and doped with manganese, iron and cobalt. Both host materials, ZnO and GaN, are wide bandgap semiconductors with a wurtzite structure, a weak spin-orbit coupling and a strong electron-hole exchange interaction within the excitons. In the presence of magnetic field, the magnetic ions induce in such materials giant Zeeman effect with no straightforward interpretation, e.g.: excitons anti-cross, and not only the transition energies, but also the oscillator strengths are strongly affected by the giant Zeeman effect. On thin epitaxial layers grown on (0001) sapphire, we observed the giant Zeeman splitting of $A$ and $B$ excitons which are optically active in the Faraday configuration when the propagating light is parallel to the $c$--axis. The Zeeman splitting decreases with the temperature and increases non-linearly with the magnetic field, demonstrating a dependence on the magnetization of the localized spins. A~quantitative analysis allows us to discuss the detailed behavior of the magnetization and to estimate the p-d exchange integral $\beta $ for the studied wide bandgap DMS. For the d$^{5}$ electronic configuration (Mn$^{2+ }$and Fe$^{3+})$ the magnetization follows a Brillouin function B$_{5/2}$, whereas, for d$^{7}$ or d$^{4}$ of Co$^{2+}$ and Mn$^{3+}$ respectively, the spin orbit coupling and the trigonal crystal field lead to an anisotropic magnetization, consistent with that deduced independently from the analysis of intra-ionic optical transitions. We find a positive sign of $\beta $ for GaN:Mn$^{3+}$ and GaN:Fe$^{3+}$. Assuming that the valence band ordering in ZnO is $\Gamma _{9}$, $\Gamma _{7}$, $\Gamma _{7}$ (this corresponds to usuall, positive sign of the spin-orbit coupling), we find $\beta $ to be negative for ZnO:Co$^{2+}$ and ZnO:Mn$^{2+}$. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V12.00002: Magneto-optical Kerr Spectroscopy of GaMnAs C. Sun, D. J. Hilton, J. Kono, H. Munekata, L. Cywinski, L. J. Sham We have performed static and dynamic magneto-optical Kerr measurements on ferromagnetic GaMnAs as a function of temperature, magnetic field, and photon energy. The static Kerr angle at zero magnetic field, which exists only below the Curie temperature, is a strong function of photon energy in the probed range (1.5-2.5 eV), exhibiting a sign change around 2.35 eV. We will present detailed comparison of the experimental spectra with theoretical spectra calculated based on an 8-band k.p model. In two-color ultrafast magneto-optical Kerr measurements, we observe ultrafast demagnetization, similar to what we recently reported for InMnAs [1]. The demagnetization signal (i.e., photoinduced decrease in Kerr angle), which decreases with increasing temperature and vanishes at Curie temperature, has two dynamic components: an ultrafast ($\sim $ 1 ps) drop in magnetization is followed by a slower ($\sim $ 100 ps) demagnetization process. The fast component strongly depends on the pump laser fluence. We will discuss how this dynamics changes with the photon energy and polarization of the pump beam, including both above and below band-gap excitation. 1. J. Wang et al., Phys. Rev. Lett. \textbf{95}, 167401 (2005). [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V12.00003: Theory of sub-picosecond light-induced demagnetization in GaMnAs and InMnAs Lukasz Cywinski, L.J. Sham When a (III,Mn)V ferromagnetic semiconductor is excited by a strong laser pulse, its magnetization decreases on a sub- picosecond time-scale [1,2]. We explain such rapid magnetization dynamics by spin-flip scattering due to the sp-d exchange interaction between the hot carriers and the localized spins. We derive the equations for the dynamics of Mn spins and phenomenologically describe the energy and spin relaxation of carriers. For efficient demagnetization a large density of states and short spin relaxation time of carriers is necessary, so that the excited holes cause magnetization quenching. The calculation of demagnetization using spin-flip transition rates derived from the 6 band Luttinger model gives results in qualitative agreement with experiments. \newline 1. J. Wang et al., Phys. Rev. Lett. {\bf 95}, 167401 (2005) \newline 2. J. Wang et al, J. Phys.: Condens. Matter {\bf 18}, R501 (2006) [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V12.00004: Mn ion spin dynamics in GaMnAs quantum wells R.C. Myers, M.H. Mikkelsen, N.P. Stern, A.C. Gossard, D.D. Awschalom We investigate the spin precession of Mn ions within highly diluted GaMnAs-AlGaAs quantum wells grown by molecular beam epitaxy at intermediate temperatures \footnote{R. C. Myers, M. Poggio, N. P. Stern, A. C. Gossard, and D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf{95}, 017204 (2005).}. An exciton bound to the neutral Mn acceptor (A$_{Mn}^{0}$) emits photons at an energy red-shifted from the free exciton emission, thus providing selective optical access to the Mn acceptors within the quantum wells. We observe that the Mn emission can be efficiently oriented using circularly polarized excitation, becoming increasingly efficient for narrow line width excitations close to the exciton absorption edge. In addition, Hanle effect measurements in the quantum wells reveal that the optically-induced polarization of the Mn emission tracks a sharp Lorentzian as a function of magnetic field in the Voigt geometry. We calculate the spin lifetime of the neutral Mn acceptor complex from the width of the Hanle curves and observe an exponential increase in the lifetime with decreasing Mn concentration. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V12.00005: Photo-induced precession of magnetization in ferromagnetic GaMnAs Yusuke Hashimoto, Hiro Munekata Precession of magnetization induced by the pure optical excitation with femto-second light pulses has been reported recently through the study of time-resolved magneto-optical (TRMO) signal in ferromagnetic GaMnAs layers [1]. The present work reports newer TRMO data which were obtained very recently with much precise experimental setups and wider time windows. Three different dynamic behaviors have been found in different time windows; (i) a relatively large TRMO oscillation within 300 ps which shows a strong excitation wavelength dependence, (ii) subsequent oscillatory behavior which lasts up to 1 ns with much weaker excitation wavelength dependence, and (iii) rather long TRMO decay signal in 3 ns without spin precession. Temperature and magnetic-field dependences of the signals indicate that these phenomena are associated with ferromagnetism of the sample. The modeling based on Landau-Lifshitz-Gilbert equation with three different magnetization components suggests that a change in magnetic anisotropy occurs immediately after the optical excitation and decays within 100ps, to which magnetization follows with the precessional motion in the sample plane. [1] H. Takechi et al., presented in PASPS-IV (2006); pss-c in print. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V12.00006: Time Resolved Magneto-Optical Studies of Ferromagnetic InMnSb Films Matthew Frazier, Rajeev Kini, Kanokwan Nontapot, Aliya Gifford, Giti Khodaparast, Tomasz Wojtowicz, Xinyu Liu, Jacek Furdyna Current research activities in the area of ferromagnetic semiconductor have been mainly focused on III-Mn-V alloys with small lattice constants and large effective masses of valence-band such as GaMnAs. Various theoretical models have been proposed to explain the actual mechanism of ferromagnetism in III-Mn-Vs but the microscopic mechanism is still a matter of controversy. It is therefore important to explore the opposite extreme of the III-Mn-V ternaries $i.e.$, InMnSb, which has the largest lattice constant in this family of materials. We report magneto-optical measurements in time domain of photo-induced spin and carrier in InMnSb and compare them to analogous measurements in InBeSb and InSb films. In this work, magneto-optical Kerr effect and standard pump-probe technique provided a direct measure of the photo-excited spin and carrier lifetimes, respectively. Our measurements provide new information on the dynamics and interactions in these materials systems. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V12.00007: Ultrafast Photo-induced Ferromagnetism in III-Mn-V Semiconductors Ingrid Cotoros, Jigang Wang, K.M. Dani, X. Liu, J.K. Furdyna, Daniel Chemla We report on ultrafast photoenhancement of hole-mediated ferromagnetism, and paramagnetic to ferromagnetic phase transition in III-Mn-V semiconductor GaMnAs via laser excited transient carriers. Our femtosecond UV pump/NIR probe vectorial MOKE spectroscopy reveals sub-picosecond demagnetization, precessional trajectory of the magnetization vector, and establishment of the ferromagnetic order on a 100-ps time scale. The dynamic enhancement of the magnetic ordering, manifesting as the photo-induced magnetization below and above the Curie temperature T$_{c}$, is well explained by a transient increase of T$_{c}$ via a population of photo-generated holes. This constitutes the first evidence for an ultrafast, nonthermal manipulation of the magnetic order in ferromagnetic semiconductors, thereby opening up fascinating opportunities for future high-speed spin-photon-carrier integrated devices, and above GHz magneto-optical recording. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V12.00008: Gate-tunability of electron spin precession in an InGaAs quantum well below an interdigitated ferromagnetic grating Gian Salis, Lorenz Meier, Christoph Ellenberger, Emilio Gini, Klaus Ensslin Time-resolved Faraday rotation is used to measure the coherent electron spin precession in a GaAs/InGaAs quantum well below an interdigitated magnetized Fe grating. We show that the electron spin precession frequency can be modified by applying a gate voltage of opposite polarity to neighboring bars. A tunability of the precession frequency of 0.5~GHz/V is observed. Modulating the gate potential at a gigahertz frequency allows the electron spin precession to be controlled on a nanosecond timescale. Besides the contribution from a spatial displacement of electrons in the inhomogeneous stray field, we also observe spin precession induced by spin-orbit coupling of the moving electrons. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V12.00009: Dynamically Decoupled Precession of Interfacial Electron Spins in Fe/AlGaAs (001) Gunter Luepke, Haibin Zhao, Diyar Talbayev, Aubrey Hanbicki, Connie Li, Berry Jonker We report on the coherent spin precession dynamics of the interface magnetization in Fe/AlGaAs (001) heterostructures using the time-resolved magnetization-induced second-harmonic generation technique, and compare these results with the bulk spin precession dynamics obtained by time-resolved magneto-optical Kerr effect. We have measured the frequency, phase and hysteretic behavior of the precession dynamics of the interface and bulk. Our results clearly show: (a) the coherent precession of the interface magnetization is decoupled from the bulk magnetization precession even at the picosecond time scale; (b) higher frequency spin precession occurs at the interface than in the bulk; (c) the phase of the interface spin precession is opposite to that of the bulk precession at low fields; and (d) the interface and bulk precession exhibit different hysteretic behavior. This is attributed to different magnetization switching processes and vanishingly small exchange coupling between the interface magnetization and the bulk Fe. The higher precessional frequencies observed at the interface for a given field indicate that higher speed performance can be realized in nanoscale magnetic devices where interface properties dominate. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V12.00010: Development of an 8-12 GHz variable frequency microwave resonant cavity for optically-detected magnetic resonance (ODMR) of GaAs-related semiconductors J.S. Colton, L.R. Wienkes, L.R. Oestreich, P.M. Schroeder In order to do spin echo measurements of the T2 spin coherence time in GaAs and related materials, one must have a resonant microwave cavity; the resonance serves mainly to increase the strength of the magnetic field at the sample. In order to probe materials with g-factors as low as $\vert $g$\vert $ = 0.1 (such as quantum-confined GaAs samples, and bulk/quantum-confined AlGaAs alloys) in moderate magnetic fields ($<$ 7 T), we have selected a resonant frequency of around 10 GHz for the resonator design. In order to fit a 10 GHz cavity into moderately-sized ($<$ 1 inch) magnet bore, however, a high dielectric material must be used, a so-called ``dielectric resonator''. We will present the design and testing of such a resonant cavity (resonant mode: TE011), that additionally allows for optical access of the sample, as well as a highly-variable resonant frequency. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V12.00011: Simple Model for the Photon Energy Dependence of Optical Nuclear Polarization in GaAs Patrick Coles, Jeffrey Reimer It was discovered forty years ago that nuclear Zeeman order can be induced by prolonged irradiation of a bulk semiconductor with near-band-gap light. The microscopic mechanism of this optical nuclear polarization process is still under debate. Yet, we are able to predict a major feature of the data, the dramatic photon-energy dependence, from a simple model, irrespective of the spin microscopics. We discuss this model in the context of semi-insulating GaAs. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V12.00012: Localization of excitons in Cd$_{1-x}$Mn$_{X}$Te semimagnetic semiconductors near temperature of phase transition : paramagnetic-spin glass Yurij Gnatenko, Petro Bukivskij Excitons in semimagnetic semiconductors become trapped by fluctuations of the crystal field as in ordinary solid solution based on II-YI semiconductors. In addition, magnetic-polaron effects are seen in these crystals. These effects stem from an exchange interaction of electrons and holes bound in excitones with d electrons of the magnetic ions. As a result, there is an additional trapping of excitons. At magnetic ion concentration X$>$0.10 the trapping of excitons not only by fluctuation of the crystal-field potential but also by fluctuations of the crystal magnetization become important. In this paper we are reporting the study of the emission of excitons trapped by magnetization fluctuations at temperatures from 1.8 K to 100 K. Analysis the form and the temperature dependence of exciton band position allowed us to determine the temperature of the phase transitions to a spin-glass state in Cd$_{1-X}$Mn$_{X}$Te (X=0.40) semimagnetic semiconductors which corresponds to T$_{SG}$=14.5 K. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V12.00013: Magnetoplasmon excitations in a quasi-two-dimensional Rashba spintronic systems: Oscillations, resonances, and energy gaps Manvir S. Kushwaha We report on the theoretical investigation of plasmon excitations in a quasi-two-dimensional electron gas (Q2DEG) in the presence of a perpendicular magnetic field and spin-orbit (SO) interaction induced by the Rashba effect. We derive and discuss the dispersion relations for charge-density excitations within the framework of Bohm-Pines' random-phase approximation (RPA). The magnetoplasmons in a 2DEG are known to be characterized by two important properties: (i) the oscillatory behavior of the dispersion curves in the short wavelength limit (SWL), and (ii) the resonance splitting at the frequency $\omega = n \omega_c$ in the long wavelength limit (LWL); $n$ ($\ge 2$) being an integer and $\omega_c$ the cyclotron frequency. Here we study the effect of the Rashba spin-orbit interactions (SOI) on these characteristics in depth. We observe that the SOI modifies drastically both the oscillatory behavior in the SWL and yields multiple resonance splittings [at $\omega = (n \pm x_0) \omega_c$] in the LWL. Such resonance splittings make the spintronic systems potential candidates for quantum-well-based new devices as spin filters. We discuss the dependence of the magnetoplasmon energy on the propagation vector, the magnetic field, the 2D charge-density, and the Rashba parameter characterizing the SOI. [Preview Abstract] |
Session V13: Focus Session: Geometrical Frustration
Sponsoring Units: DMP GMAGChair: Seunghun Lee, University of Virginia
Room: Colorado Convention Center Korbel 4C
Thursday, March 8, 2007 11:15AM - 11:27AM |
V13.00001: Field-induced spin-solid phases in the spin liquids Tb$_{2}$Ti$_{2}$O$_{7}$ and Nd$_{3}$Ga$_{5}$SiO$_{14}$ Luis Balicas, Younjung Jo, Haidong Zhou, Eun Sang Choi, Christopher Wiebe Here we report thermodynamic and magnetization measurements at zero and high fields in the pyrochlore Tb$_{2}$Ti$_{2}$O$_{7}$ and in the Kagome lattice Nd$_{3}$Ga$_{5}$SiO$_{14}$. In both compounds, previous neutron scattering studies did not reveal any form of magnetic ordering down to the lowest temperatures and have proposed these materials to display a spin-liquid ground state [1]. Here we show that heat capacity normalized by temperature down to $\sim $ 350 mK confirms the absence of ordering. Furthermore, magnetization as a function of field at the lowest temperatures reveal the existence of an intermediary phase protruded between the zero field spin-liquid and the high-field spin-polarized state. In the case Nd$_{3}$Ga$_{5}$SiO$_{14 }$it leads to a 1/2 magnetization plateau for fields along the inter-planar direction, similar to the one recently reported in the pyrochlore compound CdCr$_{2}$O$_{4}$ [2]. But for fields along the planes it displays a 1/3 plateau followed by a metamagnetic transition towards a value closer to 4/5. [1] J. S. Gardner et al. Phys. Rev. Lett. 82, 1012 (1999)~; J. Robert et al., ibid 96\textbf{, }197205 (2006) [2] H. Ueda, Phys. Rev. Lett. 94, 047202 (2005). [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V13.00002: Freezing the 2D distorted kagome spin liquid Nd$_3$Ga$_5$SiO$_{14}$ Christopher Wiebe, Haidong Zhou, Brandon Vogt, John Janik, Y.-J. Jo, Luis Balicas, Jason Gardner The distorted kagome system Nd$_3$Ga$_5$SiO$_{14}$ has been investigated with neutron scattering down to 0.046 K with no evidence of magnetic long-ranged order of the Nd$^{3+}$ moments in zero field. Substantial diffuse scattering is observed which is consistent with previous measurements of nearest neighbor correlations between the fluctuating spins. Upon the application of a magnetic field in the c-direction, the diffuse scattering is reduced in intensity while magnetic Bragg peaks grow in intensity to saturate by 1 T. The net moment along the c-axis is 1.5(1) $\mu_{B}$, only 1/2 of the value of the full moment of 3.2 $\mu_{B}$ per Nd spin, consistent with a 1/2 magnetization plateau in the DC susceptibility. A phase diagram is constructed to denote the boundary between a 2D spin liquid and spin solid phase. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V13.00003: Geometric Frustration and Chemical Tuning of Magnetic Order in the Kagome Lattice System YBaCo$_{4}$O$_{7+x}$ John Mitchell, Hong Zheng, Laurent Chapon, Paolo Radaelli, Ashfia Huq, Peter Stephens Transition metal oxides containing a Kagome lattice motif of magnetic ions form the basis for exploring geometric frustration and exotic magnetic ground states. Examples of such systems include pyrochlores, spinels, SrCr$_{9p}$Ga$_{12-9}$pO$_{19}$ (SCGO) and jarosite minerals. Joining this class of Kagome lattice antiferromagnets is the recently reported YBaCo$_{4}$O$_{7+x}$ (Y-114), a member of the more general R-114 which can be prepared with small rare-earth ions. The structure is comprised of Kagome layers of CoO$_{4}$ tetrahedra linked in the c direction by a triangular layer of CoO$_{4}$ tetrahedra. We show that appropriate control of oxygen stoichiometry so that x=0.0 yields a long-range antiferromagnet with a unique spin arrangement that seeks to satisfy the 120$^{o}$ ground state of the Kagome net in the a-b plane with a strong collinear interaction along the c-axis. This AFM ground state results because of a structural phase transition that breaks the 6-fold symmetry of the Kagome layers. We also discuss chemical approaches whose objective is to preserve the Kagome symmetry to lowest temperatures with concomitant geometric frustration. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V13.00004: Magnetic Phase Diagram of Co$_{3}$V$_{2}$O$_{8}$ Fei Yen, Bernd Lorenz, Y. Q. Wang, Y. Y. Sun, C. W. Chu Kagom\'{e}-staircase lattice structures like Ni$_{3}$V$_{2}$O$_{8}$ and Co$_{3}$V$_{2}$O$_{8}$ have recently attracted attention because of their complex magnetic phase diagrams and the magnetically induced ferroelectric (FE) phase observed in Ni$_{3}$V$_{2}$O$_{8}$. Co$_{3}$V$_{2}$O$_{8}$ at zero magnetic field exhibits five subsequent magnetic phase transition in a narrow temperature range. It has an incommensurate antiferromagnetic phase below T$_{N}$=11.4 K and weak ferromagnetic behavior along the a-axis at T$_{C}$=6.2 K. Along with three other phase transitions in between; T$_{1}$=8.9 K, T$_{2}$=7.0 K and T$_{3}$=6.9 K, the evolution of these five phase transitions under magnetic field, phase boundaries, is traced through magnetic susceptibility and dielectric constant anomalies. We resolve the complete magnetic phase diagram of Co$_{3}$V$_{2}$O$_{8}$ with the magnetic field applied along the principal crystallographic orientations. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V13.00005: Field dependence of the magnetic order in Co$_3$V$_2$O$_8$ Ying Chen, J. W. Lynn, Q. Huang, F. M. Woodward, T. Yildirim, G. Lawes, A. P. Ramirez, N. Rogado, R. J. Cava, A. Aharony, O. Entin-Wohlman, A. B. Harris Co$_{3}$V$_{2}$O$_{8}$ (CVO) has a geometrically frustrated magnetic lattice, a Kagom$\acute{e}$ staircase. In zero field [1], CVO initially orders magnetically at 11.3 K into an incommensurate phase, with wave vector $k$ = (0,$\delta$,0) with $\delta$ = 0.55. $\delta$ decreases monotonically with decreasing temperature. It locks into a commensurate antiferromagnetic value of $\frac{1}{2}$ and $\frac{1}{3}$ before the ferromagnetic ground state ($\delta~=~0$) is revealed at 6.2 K. The spin direction for all spins is along the $a$ axis. A theory based on a minimal Ising model with competing exchange interactions can explain the basic features of the magnetic ordering. The application of magnetic field along the $a$ axis strongly affects all of the phases. In particular, the ferromagnetic state is suppressed in favor of the $\delta=0.5$ antiferromagnetic state. [1] Y. Chen, J. W. Lynn, Q. Huang, F. M. Woodward, T. Yildirim, G. Lawes, A. P. Ramirez, N. Rogado, R. J. Cava, A. Aharony, O. Entin-Wohlman, and A. B. Harris, Phys. Rev. B 74, 014430 (2006). [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V13.00006: High-Energy Magneto-Dielectric Effect in Co$_{3}$V$_{2}$O$_{8}$. L. I. Vergara, R. C. Rai, J. Cao, S. Brown, J. L. Musfeldt, D. J. Singh, G. Lawes, N. Rogado, R. J. Cava, X. Wei We investigate the optical and magneto-optical properties of the Kagom\'{e} staircase compound Co$_{3}$V$_{2}$O$_{8}$ in order to explore mechanistic aspects of the high-energy magneto-dielectric effect. Co$_{3}$V$_{2}$O$_{8}$ displays a much smaller dielectric contrast compared to quasi-isostructural Ni$_{3}$V$_{2}$O$_{8}$, a result that we attribute to a high-temperature local structural distortion in Co$_{3}$V$_{2}$O$_{8}$ along the cross-tie direction. Such a distortion prevents the low temperature magnetic transitions from having a strongly coupled lattice component. This proposition is supported by vibrational studies. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V13.00007: A Microscopic Model of Magnetoelectric Interactions in Ni3V2O8 Taner Yildirim, A. B. Harris, A. Aharony, O. Entin-Wohlman We develop a microscopic magnetoelectric coupling in Ni$_3$V$_2 $O$_8$ (NVO) which gives rise to the trilinear phenomenological coupling used previously to explain the phase transition in which magnetic and ferroelectric order parameters appear simultaneously. Using combined neutron scattering measurements and first-principles calculations of the phonons in NVO, we determine eleven phonons which can induce the observed spontaneous polarization. Among these eleven phonons, we find that a few of them can actually induce a significant dipole moment. Using the calculated atomic charges, we find that the required distortion to induce the observed dipole moment is very small ($\sim$0.001 {\AA}) and therefore it would be very difficult to observe the distortion by neutron-powder diffraction. Finally, we identify the derivatives of the exchange tensor with respect to atomic displacements which are needed for a microscopic model of a spin-phonon coupling in NVO and which we hope to obtain from a fundamental quantum calculation such as LDA+U. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V13.00008: Synthesis and characterization of thin film Ni$_3$V$_2$O$_8$ G. Lawes, C. Sudakar, P. Kharel, R. Naik We have prepared thin films of multiferroic Ni$_3$V$_2$O$_8$ using sputter deposition and spin coating techniques. Raman spectroscopy and XRD confirm that the as-deposited films are amorphous, single-phase Ni$_3$V$_2 $O$_8$. These films develop increasing crystalline order on annealing at 900 $^ {\circ}$C, although they remain polycrystalline. These thin film Ni$_3$V$_2$O$_8$ samples develop a net magnetization below T=4 K; this temperature is consistent with the transition to a canted antiferromagnetic state in bulk samples. We observe an anomaly in the dielectric constant coincident with this magnetic transition. Despite being able to apply an electric field of over 6 MV/m to these samples, we are unable to observe any voltage-induced shift in this anomaly. We will discuss the implications of these results for future studies on thin film multiferroics. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V13.00009: Thermal expansion and pressure effect in the Kagome-staircase compound Ni$_{3}$V$_{2}$O$_{8}$ R. Chaudhury, F. Yen, C. R. dela Cruz, B. Lorenz, Y. Q. Wang, Y. Y. Sun, C. W. Chu Ni$_{3}$V$_{2}$O$_{8}$ has attracted attention because of the ferroelectricity (FE) induced by a helical magnetic order. Strong spin-lattice interaction is necessary to explain the ionic displacements leading to FE. To reveal the signature of lattice strain associated with the ferroelectric transitions we have conducted high-resolution thermal expansion measurements along the $a$, $b, c $axes. The strongest lattice anomalies are observed at the low-temperature (3.9 K) lock-in transition from the incommensurate helical magnetic modulation into a commensurate magnetic structure. The stability of the FE with respect to lattice strain as induced by hydrostatic pressure was investigated by measuring the dielectric constant and the ferroelectric polarization under pressures up to 2 GPa. The pressure-temperature phase diagram of Ni$_{3}$V$_{2}$O$_{8}$ is determined. The low-temperature commensurate phase in Ni$_{3}$V$_{2}$O$_{8}$ is stabilized under pressure and the ferroelectricity is completely suppressed above a critical pressure of 1.64 GPa. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V13.00010: Optical Properties and Magnetic Field-Induced Phase Transitions in the Ferroelectric State of Ni$_3$V$_2$O$_8$ R.C. Rai, J. Cao, S. Brown, J.L. Musfeldt, D. Kashinathan, D.J. Singh, G. Lawes, N. Rogado, R.J. Cava, X. Wei We present a combination of optical spectra, first principles calculations, and magneto-optical measurements to elucidate the electronic structure and to study the phase diagram of Ni$_3$V$_2$O$_8$. We find a remarkable interplay of magnetic field and optical properties that reveals additional high magnetic field phases and an unexpected electronic structure which we associate with the strong magneto-dielectric couplings in this material over a wide energy range. Specifically, we observed several prominent magneto-dielectric effects that derive from changes in crystal field environment around Ni spine and cross-tie centers. This effect is consistent with a field-induced modification of local structure. We find Ni$_3$V$_2$O$_8$ to be an intermediate gap, local moment band insulator. This electronic structure is particularly favorable for magneto-dielectric couplings, because the material is not subject to the spin charge separation characteristic of strongly correlated large gap Mott insulators, while at the same time remaining a magnetic insulator independent of the particular spin order and temperature. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V13.00011: Magnetodielectric coupling in Mn$_3$O$_4$ and MnCr$_2$O$_4$ R. Tackett, E. Toberer, R. Seshadri, G. Lawes We have investigated the temperature and magnetic field dependent dielectric constants of the ferrimagnetic insulators Mn$_3$O$_4$ and MnCr$_2$O$_4$. We have also measured the heat capacity and AC magnetic susceptibility through the multiple spin ordering transitions in these materials. At the zero field T = 42 K and T = 35 K magnetic transitions in Mn$_3$O$_4$ we observed sharp drops in the dielectric constant. In an applied field of 5 kOe, Mn$_3$O$_4$ shows a positive shift in dielectric constant at the intermediate T = 40 K transition in addition to the features observed at zero field. MnCr$_2$O$_4$ also shows features in the dielectric constant at the magnetic transitions at T = 40 K and T = 20 K, though these shifts were approximately 100 times smaller than those observed in Mn$_3$O$_4$. These results will be discussed in the framework of models for coupling the dielectric constant to non-colinear long-range magnetic order. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V13.00012: Ab-intitio studies of electronic properties of chalcogenide spinels. Mairbek Chshiev, Y.-H. A. Wang, Arunava Gupta, Joanna Bettinger, Yuri Suzuki, William H. Butler CuCr$_{2}$Se$_{4}$ is a normal chalcogenide spinel which exhibits ferromagnetic properties including a relatively high Curie temperature of 450 K [1] which makes it a promising candidate for use in spintronics devices. Another chalcogenide spinel of enhanced interest for spintronics is CdCr$_{2}$Se$_{4}$ which seems to be a promising ferromagnetic semiconductor for electrical spin injection into III-V device heterostructures [2]. We report first principles calculations of the electronic structure of substoichiometric CuCr$_{2}$Se$_{4-x}$ and Cu$_{x}$Cd$_{1-x}$CrSe$_{4}$ spinels. The calculations were performed using the Vienna ab-initio simulation program (VASP) within the Generalized Gradient Approximation (GGA) of Density Functional Theory (DFT). Our calculations indicate that both Se deficient CuCr$_{2}$Se$_{4-x}$ as well as Cu$_{x}$Cd$_{1-x}$CrSe$_{4}$ show half-metallic behavior over a wide range of $x $with a gap around the Fermi level in the minority density of states. [1] F.K. Lotgering, Solid State Commun. 2 (1964) 55 [2] G. Kioseoglou et al., Nature Materials 3 (2004) 799 [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V13.00013: Magnetic anisotropy and geometrical frustration in the Ising spin-chain system Sr$_{5}$Rh$_{4}$O$_{12}$ Gang Cao, Shalinee Chikara, Vinobalan Durairaj, Sean Parkin, Pedro Schlottmann A structural and thermodynamic study of the newly synthesized single crystal Sr$_{5}$Rh$_{4}$O$_{12}$ is reported. Sr$_{5}$Rh$_{4}$O$_{12}$ consists of a triangular lattice of spin chains running along the c-axis. It is antiferromagnetically ordered below 23 K with the intrachain and interchain coupling being ferromagnetic (FM) and antiferromagnetic (AFM), respectively. There is strong evidence for an Ising character in the interaction and geometrical frustration that causes incomplete long-range AFM order. The isothermal magnetization exhibits two step-like transitions leading to a ferrimagnetic state at 2.4 T and a FM state at 4.8 T, respectively. Sr$_{5}$Rh$_{4}$O$_{12}$ is a unique frustrated spin-chain system ever found in 4d and 5d based materials without a presence of an incomplete 3d-electron shell. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V13.00014: Antiferromagnetism and geometrical frustration in one-dimensional Ca$_{5}$Ir$_{3}$O$_{12}$ and Ca$_{4}$IrO$_{6}$ single crystals Angela Douglass, Vinobalan Durairaj, Shalinee Chikara, Gang Cao, Sean Parkin, Pedro Schlottmann We report a structural, thermodynamic and transport study of the newly synthesized single crystal Ca$_{5}$Ir$_{3}$O$_{12}$ and Ca$_{4}$IrO$_{6}$. Both materials consist of a triangular lattice of spin chains running along the c-axis. Ca$_{5}$Ir$_{3}$O$_{12 }$and Ca$_{4}$IrO$_{6}$ are antiferromagnetically ordered below 7.8 K and 12 K, respectively. The study reveals an unusually large ratio of the Curie-Weiss temperature to the Neel temperature ($>$36 for Ca$_{5}$Ir$_{3}$O$_{12})$ and a small entropy removal associated with the magnetic phase transition. In addition, the magnetic susceptibility and heat capacity show that the phase transition is essentially insensitive to the application of the magnetic field. All results suggest the presence of the geometrical frustration that causes incomplete long-range antiferromagnetic order. The results will be presented and discussed along with comparisons drawn with other related systems. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V13.00015: Configurational Electronic Entropy and the Phase Diagram of Mixed-Valence Oxides: The Case of Li$_x$FePO$_4$ Fei Zhou, Thomas Maxisch, Gerbrand Ceder We demonstrate that configurational electronic entropy, previously neglected, in {\it ab initio} thermodynamics of materials can qualitatively modify the finite-temperature phase stability of mixed-valence oxides. While transformations from low-T ordered or immiscible states are almost always driven by configurational disorder (i.e.\ random occupation of lattice sites by multiple species), in FePO$_4$--LiFePO$_4$ the formation of a solid solution is almost entirely driven by electronic, rather than ionic configurational entropy. We argue that such an electronic entropic mechanism, rather than an ionic one, may be relevant to most other mixed-valence systems. Details in Phys. Rev. Lett. {\bf 97}, 155704 (2006) [Preview Abstract] |
Session V14: Focus Session: High Anisotropy Magnetic Nanoparticles and Composite Materials
Sponsoring Units: GMAG FIAPChair: Andreas Berger, Hitachi GST
Room: Colorado Convention Center Korbel 4D
Thursday, March 8, 2007 11:15AM - 11:51AM |
V14.00001: Magnetic and chemical ordering properties of FePt nanoparticles Invited Speaker: The presence of anisotropic L1$_0$ type chemical order in self- assembled, monodispersed FePt nanoparticles produces an extremely high magnetic anisotropy thus making these particles potentially useful for information storage. However, it has been difficult in experiments to achieve a high degree of L1$_0$ order in 3.5-nm-diam nanoparticles. To explore the possible reasons for this observed low degree of order, we studied theoretically the equilibrium chemical ordering of L1$_0$ type for spherical FePt nanoparticles of different sizes (2.5 to 6-nm- diam) in a wide temperature range. In our study we used first- principles calculations together with the cluster expansion technique and Monte Carlo simulation. Our results indicate that the theoretical equilibrium chemical order is higher than that observed experimentally in 3.5-nm-diam nanoparticles annealed at 600 C or below. Using first-principles calculations we considered one of the possible reasons for this differentce - the surface segregation in FePt and Fe-Pt-X (X=Ag, Au, Cr, and Cu) nanoparticles. It was established that surface segregation has only a small effect on ordering. We conclude that the experimental absence of (relatively) high L1$_0$ order is primarily a problem of kinetics rather than equilibrium. The recent experimental data supporting such a conclusion are discussed. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V14.00002: Array Formation and Size Effects in Chemically Synthesized FePt Nanoparticles Levent Colak, George Hadjipanayis FePt nanoparticles with controlled size have been synthesized following a route given by Shukla et. al.[1] The effect of particle size on the magnetic properties has been investigated for nanoparticles with sizes of 3.0 and 6.0 nm. With the addition of the surfactants at a later stage of preparation, a long range self-assembled array of particles was obtained as evidenced by transmission electron microscope (TEM). By comparing bright field images of the samples with projected potential image simulations$^{ }$[2], the packing structures and stacking sequences of the arrays were identified. Well-aligned mono and multi layered hcp to bcc nanostructures are formed from 5 nm FePt nanoparticles. Subjecting the NP's to thermal processing at 800 $^{o}$C results in a transformation of the nanoparticles from the disordered fcc phase to the ordered L1$_{0}$ phase. HRTEM studies have been carried out to investigate the development of particle morphology and microstructure during the synthesis and subsequent annealing of nanoparticles. 1. N. Shukla, C. Liu, A. G. Roy, Matt. Lett. 60, 2006, 995-998. 2. S. Yamamuro, D. F. Farrell and S. A. Majetich, Phys. Rev. B 65, 224431 (2002). [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V14.00003: Role of disorder and structural inhomogeneity on the magnetic structure of FePt nanoparticles Paul Kent, Don Nicholson, Markus Eisenbach, Thomas Schulthess Recent experiments (e.g. [1]) have demonstrated a substantial increase in Fe magnetic moment on annealing of FePt nanoparticles, underscoring the likely importance of local composition variations at the nanoscale. To test these ideas, we have calculated the magnetic structure of FePt nanoparticles up to 3nm in size using density functional theory. We investigate the role of disorder and the influence of explicitly constructed Fe and Pt rich regions, both at the center and at the surface of the particles. The size and disorder dependent spin-moment distributions are related back to changes in the electronic structure of the materials relative to the bulk. Even for 3nm nanoparticles, the magnetic structure deviates significantly from the bulk due to the large fraction of near-surface atoms. Structural relaxation is shown to significantly influence the magnetic structure, particularly reducing the magnetic moments of surface atoms. This work used resources of the National Center for Computational Sciences at Oak Ridge National Laboratory (ORNL) and is supported in part by the Division of Scientific User Facilities, U. S. Department of Energy as well as the Laboratory Directed Research and Development program at Oak Ridge National Laboratory. [1] C. Antoniak \emph{et al.} Phys. Rev. Lett {\bf 97} 117201 (2006). [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V14.00004: Magnetic Properties and Chemical Synthesis of FePt and CoPt Nanoclusters Yao Zhao, Yucheng Sui, David Sellmyer Much interest has been paid to self-assembled L1$_{0}$ FePt and CoPt nanoclusters for basic science and potential applications in ultra-high density magnetic recording media. In this study, thermal decompostion of a Fe and Pt acetylacetonates mixture in trioctylamine was employed to produce FePt nanoclusters with average size about 4 nm and a narrow size distribution. The atomic ratio of Fe and Pt for the as-synthesized FePt clusters is 50:50. Using the same procedure, CoPt nanoclusters were obtained with the atomic ratio of 40:60. With the addition of oleic acid into the reaction solution the atomic ratios were adjusted to 50:50. As-synthesized FePt and CoPt nanoclusters have fcc structures, but after rapid thermal annealing in forming gas the nanoclusters transform into hard magnetic L1$_{0}$ ordered structures. The nanostructures of the clusters were characterized by TEM and XRD. The magnetic properties were measured by a SQUID magnetometer. The coercivity of FePt and CoPt clusters reached the maximum value of 23 kOe and 17kOe respectively after annealing at 750 $^{o}$C for 5mins. Beyond 750 $^{o}$C, the coercivity decreased slightly because of the sintering effect and a different magnetic reversal mechanism. This work is supported by NSF-MRSEC, DOE, INSIC, NRI and NCMN. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V14.00005: Synthesis and Magnetic Properties of L1$_{0}$FePt / Silica Core Shell Nanoparticles Andrew Heitsch, Doh Lee, Brian Korgel FePt nanocrystals were coated with silica (SiO$_{2})$ shells in an inverse micelle microemulsion by tetraethyl orthosilicate (TEOS) hydrolysis and condensation. The shell thickness can be varied (from 6 to 25 nm), along with the FePt loading (per silica shell) to a limited extent, by changing the FePt:TEOS ratio. The silica-coated FePt nanocrystals can be heated up to $\sim $850$^{o}$C without shell layer decomposition or FePt sintering. Annealing under forming gas (7{\%}H2/ 93{\%}N2) at 700\r{ }C for 2 hours transforms the as-synthesized fcc FePt nanocrystals to the L1$_{0}$ phase with at least 90{\%} conversion. Magnetic measurements of annealed FePt nanocrystals confirm their phase transformation, with blocking temperatures exceeding room temperature. However, the hysteresis loops exhibit a constriction at low fields and zero field cooled (ZFC) magnetization scans show an intermediate plateau at temperatures between 50K$\sim $200K. Temperature and time-dependent remanance relaxation measurements reveal a short and fast, $\sim $10$^{4}$ and 10$^{5}$ seconds, relaxation of the remanance, which might be due to the presence of an additional ``soft''magnetic phase in the sample. The possible origins of the soft magnetic component will be presented and discussed. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V14.00006: Surface anisotropy enhancement of Co nanoparticles by capping with Ag. Juan Bartolome, Fernando Luis, Luis Miguel Garc\'ia Vinuesa, Fernando Bartolome, Frederic Petroff, C. Deranlot, Anatoly Rogalev, P. Bencok, F. Wilhelm, Nick Brookes The present work deals with the observed enhancement of the magnetic anisotropy of Co nanoparticles by capping with Ag, and its origin. The studied samples are granular multilayers prepared by sequential sputtering deposition of Co on amorphous Al$_{2}$O$_{3}$. For low Co deposition rates, fcc Co nanoparticles with average diameter 0.7$<<$ 5 nm are formed, and self-organized in a quasi-regular spatial order. We report on the enhancement of anisotropy induced by capping with Ag (4d$^{10})$, which represents an interesting case due to its intermediate spin orbit coupling and Co-interface metal binding energy, lying between those of Cu and Au [1]. We present magnetic measurements and X-ray Magnetic Circular Dichroism (XMCD) data on the L$_{2,3 }$edges of Co, showing an increase in the mean orbital moments of Co, and a polarization of the 3d bands of Ag observed ayt the L$_{2,3}$ edges of Ag. From the XMCD K-edge spectra of Co we verify that the induced magnetic moments of the 3d and 4p bands of Co have opposite sign. We prove therefore that surface anisotropy enhancement is caused by the increase of surface Co orbital moments due to hybridization of the 3d Co and 4d Ag bands. [1] F. Luis et al., Europhys Lett. 76, 142 (2006). [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V14.00007: Synthesis and characterization of hard/soft bimagnetic nanoparticles Vikas Nandwana, Girija Shankar Chaubey, Kazuaki Yano, J. Ping Liu Bimagnetic nanoparticles are synthesized from high-temperature solution method by growing soft magnetic phases on a hard magnetic phase. The hard phase is chosen as the FePt phase and the soft phases include Fe$_{3}$O$_{4, }$CoFe$_{2}$O$_{4}$ and FeCo. The soft phases can be coated or attached to the hard phase by changing reaction conditions. When the soft phases are coated on the hard phase, core/shell structured bimagnetic nanoparticles are formed; when the soft phases are attached to the hard phase, brick-like bimagnetic nanoparticles are formed. Magnetic properties of these nanoparticles are affected by dimensions of the soft and hard components due to the exchange coupling between them. Upon reductive annealing, an assembly of the bimagnetic nanoparticles is transformed into a hard magnetic nanocomposite with enhanced energy product which is 35{\%} higher than single FePt phase. With proper choice of materials and dimension of both phases, these bimangetic nanoparticles may be used as building blocks for novel functional nanomaterials for various applications. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V14.00008: Element- and depth resolved magnetic study of Sm-Co/Fe exchange spring magnet films with enhanced effective coupling Y. Choi, J. S. Jiang, J. E. Pearson, S. D. Bader, Y. Ding, Z. L. Wang, A. Zambano, M. Murakami, I. Takeuchi, J. P. Liu Sm-Co/Fe exchange-spring bilayers with intermixed interfaces exhibit enhanced exchange coupling effectiveness [1]. We have examined the element- and depth-resolved magnetization reversal process using x-ray resonant magnetic scattering (XRMS) magnetometry. The XRMS measurements indicate significant Co diffusion into the Fe layer, and electron microscopy observations give consistent results in the composition profiles. Using model concentration profiles in combination with micromagnetic codes, we simulated demagnetization curves that are in good agreement with the XRMS results. The results reveal that the enhanced exchange coupling effectiveness is due to the intermixing in the interfacial region and that the diffused Co behaves similarly to the surrounding Fe. [1] J. S. Jiang et al., Appl. Phys. Lett. 85, 5293 (2004). [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V14.00009: Bulk nanocomposite magnets with enhanced exchange coupling Chuan-bing Rong, Vikas Nandwana, J. Ping Liu Bulk FePt/Fe$_{3}$Pt nanocomposite magnets have been prepared by compaction of chemically synthesized nanoparticles. Advanced compaction techniques, warm compaction, shock-wave compaction, spark plasma sintering and microwave sintering, have been adopted in our research. Density of the bulk samples up to 95{\%} theoretical value has been obtained while the nanostructured morphology is retained. It is found that the density increases with the compaction pressure and temperature and is also affected by the phase transition of the FePt compound from the face-centered cubic structure to the face-centered tetragonal structure. Pressure is more effective in reaching high densities. Comparison of different compaction techniques has been made. It is also found that compaction of the particles and post-annealing lead to enhanced intergrain exchange coupling. Energy products up to 16.3 MGOe of the isotropic bulk nanocomposite magnets have been achieved, which is significantly higher than the theoretical limit for fully dense single-phase FePt magnets. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V14.00010: High-throughput Investigation of Exchange Coupling Interaction in Soft/Hard Magnetic Bilayer Systems A. Zambano, H. Oguchi, I. Takeuchi, Y. Choi, J. S. Jiang, J. P. Liu, S. Lofland, D. Josell, L. Bendersky To enhance the maximum energy product of exchanged coupled hard/soft phase nanocomposite, we need to establish the parameters that govern the exchange coupling interaction (ECI). To this end, on single chips, up to a hundred thin film bilayer samples were grown by e-beam evaporation varying composition and soft layer thicknesses (t$_{s})$. Magnetic hysteresis loops were taken by magneto-optical Kerr effect measurements. The simultaneous analysis of the samples allows us to delineate subtle variations of the exchange coupling behavior. We will show examples of studies of CoPt/(Fe, Co or Ni) and SmCo/(Fe, Co or Ni) libraries. We characterized ECI by measuring the variations of the coupling length ($\lambda )$ and the nucleation field (H$_{N})$ on various magnetic parameters, t$_{s}$, and interface conditions. The trend indicates that the dominant factors determining $\lambda $ and H$_{N }$are the hard layer magnetic constants and the saturation magnetization of the soft layer. We will also discuss the role played by other magnetic parameters. The results allow us to predict the behavior of coupled hard/soft magnetic layers in general. Supported by ONR MURI N00014-05-1-0497. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V14.00011: First-principles study of interface exchange coupling in SmCo$_{5}$/Co nanocomposite materials Dangxin Wu, Qiming Zhang, J.P. Liu, Renat F. Sabirianov Understanding the interface exchange coupling is very important in designing and enhancing the performance of exchange-coupled hard/soft phase nanocomposite magnets, which were proposed to increase the maximum energy product by combining the large anisotropy of hard phase materials and the high saturation magnetization of soft phase materials. In this talk, we will present our results of our recent first-principles calculations in investigate the interface exchange coupling between hard phase SmCo$_{5}$ and soft phase Co using superlattice model. The calculations were based on Density Functional Theory. The atomic structures were optimized and the electronic ground state was obtained. Then the noncollinear magnetic order calculations were performed to study the exchange interactions. We found that the change of the total energy is a quadratic function of angle $\theta $ between the directions of magnetic moments of hard phase and middle layer of soft phase. We also performed calculations on a SmCo$_{5}$/CoFe system in which the soft phase was doped with Fe. Comparison and discussions will be made between these two systems. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V14.00012: Simulation of magnetic coupling in die-upset composite magnets George Hadjipanayis, Alexander Gabay Die-upset composite magnets fabricated from blends of melt-spun Nd-Fe-B ribbons and coarse (micron-size) soft magnetic powders (Fe, Fe-Co) exhibit uniform magnetization behavior despite the fact that the soft magnetic inclusions are too large to be magnetically coupled through inter-phase exchange interactions. In this study, we present the results of numerical simulation showing that in the case of a layered microstructure (typical of the die-upset composites) the long-range magnetostatic interactions assure smooth demagnetization curves. Still, at least partial exchange coupling is required to have an increased remanence. It is argued that the effect of magnetostatic coupling in composite magnets with a layered morphology considerably relaxes the strict requirements for the size of the soft inclusions and it may facilitate the future development of high-performance composite anisotropic permanent magnets. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V14.00013: High performance isotropic R-(Co,Fe)-C magnets by melt spinning (R= Sm, Pr) Dilara Sultana, Alexander Gabay, George Hadjipanayis In our search of high performance isotropic magnets for high temperature applications a large variety of quasiternary R$_{x}$(CoFe)$_{100}$-x-y$_{ }$C$_{y}$ alloys with x=10.5-15,y = 0-4.5 were produced by melt-spinning. The alloys were characterized by X-ray diffraction and Scanning and transmission electron microscopy. The Fe substitution for Co and Pr substitution for Sm increases magnetization, while the C addition increases the coercivity. Remanence values of more than 100 emu/g and coercivity of 13 kOe were obtained. The hard magnetic properties are associated with the disordered hexagonal 2:17 compound. The microstucture properties are studied to optimize the properties for high temperature applications. The effects of the alloying elements (Pr,Fe,C) , quenching rate and heat treatments are analyzed and coercivity mechanism are discussed. [Preview Abstract] |
Session V15: Focus Session: Nonequilibrium Thermodynamics of Small Systems
Sponsoring Units: GSNP DBPChair: R. Dean Astumian, University of Maine
Room: Colorado Convention Center Korbel 4E
Thursday, March 8, 2007 11:15AM - 11:51AM |
V15.00001: The nonequilibrium thermodynamics of small systems Invited Speaker: Nonequilibrium behavior is widespread and rich in nature. Yet our understanding of the fundamental principles underlying nonequilibrium behavior is still poor as shown by the fact that non-equilibrium theories tend to be ad-hoc and specific (1). Recently there has been a lot of interest in applying single-molecule techniques to scrutinize nonequilibrium theories (2). The use of new micromanipulation tools in the exploration of the behavior of tiny objects (such as biomolecules and motors) embedded in a thermal environment opens the possibility to investigate how these systems exchange energy with their environment. The study of such questions, nowadays referred to as ``Nonequilibrium thermodynamics of small systems,'' is becoming quite popular among statistical physicists who recognize there new aspects of thermodynamics where large Brownian fluctuations are of pivotal importance as compared to fluctuations in macroscopic (or large) systems (3). Nonequilibrium small systems are characterized by large deviations in work/heat distributions that satisfy some relations called fluctuation theorems. In this talk I will discuss single-molecule experiments where some of these fluctuation theorems have been tested (4). \newline \newline REFERENCES: \newline (1) F. Ritort, Nonequilibrium fluctuations in small sytems: From physics to biology, To be published in Advances in Chemical Physics, volume 137; \newline (2) F. Ritort, Single molecule experiments in biological physics: methods and applications, Journal of Physics C (Condensed Matter),18 (2006) R531-R583; \newline (3) C. Bustamante, J. Liphardt and F. Ritort, The nonequilibrium thermodynamics of small systems, Physics Today,58 (2005) 43-48; \newline (4) D. Collin, F. Ritort, C. Jarzynski, S. B. Smith, I. Tinoco Jr and C. Bustamante, Verification of the Crooks fluctuation theorem and recovery of RNA folding free energies, Nature, 437 (2005) 231-234. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V15.00002: Activation barrier scaling and crossover for noise-induced switching in a micromechanical parametric oscillator Corey Stambaugh, Ho Bun Chan We explore fluctuation-induced switching in a parametrically-driven micromechanical torsional oscillator, a system far from thermal equilibrium. Under sufficiently strong parametric modulation of the spring constant, the oscillator possesses one, two or three stable attractors depending on the modulation frequency. Near the bifurcation points where the number of attractors changes, the activation barrier for switching out of a stable state is predicted to display universal, system-independent scaling relationships. We induce the oscillator to switch between the coexisting states by injecting noise in the excitation. By measuring the rate of random transitions as a function of noise intensity, we deduce the activation barrier as a function of frequency. Near both bifurcation points, the activation barriers are found to depend on frequency detuning with critical exponent of 2, consistent with the predicted universal scaling in parametrically driven systems. Away from the immediate vicinity of the bifurcation point, universal scaling relationships for the activation barrier no longer hold. At large detuning, we observe a crossover to a different power law dependence with an exponent that is specific to our device. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V15.00003: Scaling crossovers in activated escape of nonequilibrium systems: a resonantly driven oscillator Oleg Kogan, Ira Schwartz, Mark Dykman The rate of metastable decay in nonequilibrium systems is expected to display scaling behavior: i.e., the logarithm of the decay rate should scale as a power of the distance to a bifurcation point where themetastable state disappears. Recently such behavior was observed and some of the earlier predicted exponents were found in experiments on several types of systems described by a model of a modulated oscillator. Here we establish the range where different scaling behavior is displayed and show how the crossover between different types of scaling occurs. The analysis is done for a nonlinear oscillator with two coexisting stable states of forced vibrations. We map out the entire parameter range. We find the regions where the scaling exponents are 1 or 3/2, depending on the damping. We also uncover new scaling behavior which extends, numerically, beyond the close vicinity of the bifurcation point. The results of the numerical calculations based on the instanton method are compared with the results of Monte Carlo simulations. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V15.00004: Spontaneous symmetry breaking in parametrically driven atomic trap and measurement of dynamic critical exponents Wonho Jhe, Myoung-Sun Heo, Yonghee Kim, Kiwhan Kim, Heung-Ryoul Noh While critical phenomena in equilibrium systems has been well established both in theory and in experiment, experimental studies in~ non-equilibrium or far-from-equilibrium systems still lack of quantitative investigation and remain as challenging subjects. Here we report on the use of laser cooled and trapped atoms can be a good candidate for such study since one can easily control its temperature and numbers. By parametrically modulating the magneto-optical trap potential we have observed several interesting phenomena such as dynamic double well, Hopf bifurcation and spontaneous symmetry-breaking (SSB). Particularly SSB is identified as the mean-field system exhibiting the Ising-like phase transition. We measured critical exponents relevant to this phase transition, with respect to the control parameter, the size of the system or the total number of atoms. We also have observed the occurrence of SSB as the temperature is changed by illuminating a resonant laser light. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V15.00005: Equilibrium theory for a particle pulled by a moving optical trap Raymond Dean Astumian The viscous drag on a colloidal particle pulled through solution by an optical trap is large enough that on experimentally relavant time scales the mechanical force exerted by the trap is equal and opposite the viscous drag force. The rapid mechanical equilibritation allows the system to be modeled using equilibrium theory where the effects of the energy dissipation ({\em thermodynamic} disequilibrium) show up only in the coordinate transformations that map the system from the laboratory frame of reference, relative to which the particle is moving, to a frame of reference in which the particle is, on average, stationary and on which the stochastic dynamics is governed by a canonical equilibrium distribution function. The simple equations in the stationary frame can be analyzed using the Onsager-Machlup theory for stochastic systems and provide generalizations of equilibrium and near equilibrium concepts such as detailed balance and fluctuation-dissipation relations applicable to a wide range of systems including molecular motors, pumps, and other nano-scale machines. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V15.00006: Energy and efficiency optimization of a Brownian heat engine Mulugeta Bekele, Yeneneh Yalew A simple Brownian heat engine is modeled as a Brownian particle moving in an external sawtooth potential (with or without) load assisted by the thermal kick it gets from alternately placed hot and cold heat reservoirs along its path. We get closed form expression for its current in terms of the parameters characterizing the model. After analyzing the way it consumes energy to do useful work, we also get closed form expressions for its efficiency as well as for its coefficient of performance when the engine performs as a refrigerator. Recently suggested optimization criteria enables us to exhaustively explore and compare the different operating conditions of the engine. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V15.00007: Relationships involving spatial transitions for Brownian particles within a potential-well. Ross Brody Using an optical tweezer apparatus we have trapped single latex spheres and analyzed their Brownian motion within a potential well. By considering transitions from various initial and final positions within the well, we experimentally show that the ratio of conditional probabilities, $P(x_f,t+\Delta t|x_i,t)/P(x_i,t+\Delta t|x_f,t)$, is independent of $\Delta t$. We also show the instanton times corresponding to last-touch-first-touch (LTFT) trajectories obey the equality, LTFT($x_i\rightarrow x_f$)=LTFT($x_f\rightarrow x_i$), shown by Bier et al. [Phys. Rev. E \bf{59}\rm, 6422 (1999)]. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V15.00008: Analytical calculation of Jarzynski free-energy estimator bias Matteo Palassini, Nikos Skantzos, Felix Ritort The Jarzynski equality connects the free-energy difference DF between two equilibrium states A and B of a system to the work done on the system in a non-equilibrium process that takes it from A to B, averaged exponentially over all possible realizations of the process. This provides an estimator for DF given N non-equilibrium experiments, which has been applied in a variety of contexts. Because of the exponential averaging, the Jarzynski estimator suffers a statistical bias for finite N, which can be substantial. Computing this bias is important for estimating correctly the free-energy, and is a notoriously difficult problem for which only results in the large-N limit are known. We propose an analytical method to estimate the bias and test it in the case of a Gaussian work distribution, for which it provides satisfactory estimates both in the large N and small N regimes. Finally, we discuss the applicability of these results to experimental studies on single biomolecules. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V15.00009: Exact equality between dissipation and irreversibility Ryoichi Kawai, Juan M. R. Parrondo, Christian Van den Broeck We show, through a reformulation of the Crooks theorem and the Jarzynski equality, that the average dissipation for a system perturbed to go from one equilibrium state to another one, is exactly given by $\langle W \rangle_{diss} = \langle W \rangle -\Delta F =kT D(\rho\|\widetilde{\rho})= kT \langle \ln (\rho/\widetilde{\rho})\rangle$, where $\rho$ and $\widetilde{\rho}$ are the phase space density of the system measured at the same but otherwise arbitrary intermediate point in time, for the forward and backward process. $D(\rho\|\widetilde{\rho})$ is the relative entropy of $\rho$ versus $\widetilde{\rho}$. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V15.00010: Free Energy Surface Reconstruction Using Jarzynski's Equality Ching-Hwa Kiang, Nolan Harris Atomic force microscope was used to manipulate and unfold individual molecules of the muscle protein titin. We reconstructed the free energy surface of stretching and unfolding of titin I27 domain using Jarzynski's equality. An exact formula that relates the nonequilibrium work fluctuations to the molecular free energy was used for the reconstruction. From the free energy surface, the unfolding free energy barrier, i.e. the activation energy, was directly obtained from experimental data for the first time. [Preview Abstract] |
Session V16: Correlated Electrons: Luttinger Liquids, Ladders, and Charge Ordered Systems
Sponsoring Units: GMAGChair: Noah Bray Ali, University of Southern California
Room: Colorado Convention Center Korbel 4F
Thursday, March 8, 2007 11:15AM - 11:27AM |
V16.00001: Much Ado about Zeros: The Luttinger Surface and Mottness Philip Phillips, Tudor Stanescu, Ting-Pong Choy We prove that the Mott insulating state is characterized by a divergence of the electron self energy at well-defined values of momenta in the first Brillouin zone. When particle-hole symmetry is present, the divergence obtains at the momenta of the Fermi surface for the corresponding non-interacting system. Such a divergence gives rise to a surface of zeros (the Luttinger surface) of the single-particle Green function and offers a single unifying principle of Mottness from which pseudogap phenomena, spectral weight transfer, and broad spectral features emerge in doped Mott insulators. We also show that only when particle-hole symmetry is present does the volume of the zero surface equal the particle density. We identify that the general breakdown of Luttinger's theorem in a Mott insulator arises from the breakdown of a perturbative expansion for the self energy in the single-particle Green function around the non-interacting limit. A modified version of Luttinger's theorem is derived for special cases. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V16.00002: Theory of STM Spectroscopy in Luttinger Liquids with a Spin Gap Akbar Jaefari, Eduardo Fradkin We discuss the problem of tunneling of electrons into a Luttinger liquid with a spin gap for a semi infinite one-dimensional system. This system is of interest for a problem of STM spectroscopy of a spin-gap Luttinger liquid with a backscattering impurity. We will discuss this problem using semi-classical and refermionization methods. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V16.00003: A finite Luttinger liquid with longer range interactions Imke Schneider, Sebastian Eggert, Alexander Struck We consider the bosonization of a finite quantum wire with longer range interactions. In order to use the Luttinger liquid formalism it is necessary to introduce a changing effective interaction parameter as a function of momentum. We show that it is possible to modify the formalism so that the electron distribution in individual states can be analyzed with help of a recursive formula. Quantitative predictions for the local density of states and STM experiments can be made. In limiting cases the well known power laws are recovered. We compare the calculations with DMRG simulations of the spin $\frac{1}{2}$ Heisenberg chain. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V16.00004: Possible Dimensional Crossover in Li$_{0.9}$Mo$_6$O$_{17}$ observed through Thermal Expansion Experiments Carlos A. M. dos Santos, B. D. White, Y.-K. Yu, J. J. Neumeier, J. A. Souza During recent years much attention has been given to Li$_{0.9}$Mo$_6$O$_{17}$ which is believed to be the best-known example of a Luttinger-liquid (LL). In order to study the thermal expansion behavior of this compound prepared single crystals via flux growth using Li$_2$MoO$_4$, MoO$_2$ and MoO$_3$. The crystals were mechanically, chemically separated and characterized by x-ray and Laue diffraction, and electrical resistance. Thermal expansion measurements performed with a high-resolution quartz dilatometer revealed high anisotropy. Dominant a-axis thermal expansion reduces the separation of the conducting zigzag chains enhancing the interchain coupling. This is argued to destabilize the LL leading to a crossover from metallic to insulating-like behavior near 28 K and a repulsion-induced CDW. Competing electron-electron interactions support superconductivity at 1.9 K. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V16.00005: Electrical Transport Properties of the Purple Bronze Li$_{0.9}$Mo$_6$O$_{17}$ Mario S. da Luz, Jamy Moreno, C. A. M. dos Santos, B. D. White, J. A. Souza, J. J. Neumeier The crossover from metallic to semiconductor-like behavior in Li$_{0.9}$Mo$_6$O$_{17}$ has been the subject of intense discussion. An important issue is whether or not it is a Luttinger Liquid. This idea has received substantial support because of the quasi-one dimensionality observed in electrical resistivity measurements performed 20 years ago. In this work the transport properties of Li$_{0.9}$Mo$_6$O$_{17}$ single crystals are revisited. Crystals were characterized by x-ray powder diffractometry and the crystallographic orientations were determined by transmitted Laue diffraction. Electrical resistance as a function of temperature, $R(T)$, performed using standard four probe and Montgomery methods are compared. The magnetic field dependence of the minimum in the $R(T)$ curves and the temperature dependence of the insulating-like regime are also presented. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V16.00006: Ground state properties of quasi-1D systems Kenneth Graham, Samuel Moukouri We use the two-step density-matrix renormalization group method to study the competition between an antiferromagnet and superconductivity. We find that in the absence of frustration the ground state is dominated by antiferromagnetic fluctuations. In the presence of frustration the antiferromagnetism is reduced but we don't find a significant pairing tendency. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V16.00007: Binding of spinon and holon in the anisotropic 1D $t-J$ model Jurij Smakov, Alexander Chernyshev, Steven White In the 1D $t-J$ and Hubbard models spin and charge dynamics are independent, leading to the well-known spin-charge separation: splitting of the physical electron (hole) into free spinon and holon excitations. Considerably less is known about the interaction between such excitations. To address this problem, we have performed a detailed theoretical and numerical study of anisotropic $XXZ$-version of the $t$-$J$ model using Bethe-Salpeter equation and exact diagonalization and DMRG methods on systems of up to 128 sites. We find that holon and spinon form a bound state for any value of anisotropy $\alpha=J_\perp/J_z$ smaller than one. A remarkable agreement between the theory and numerical results has been demonstrated. We argue that the binding is largely controlled by the spinon energy spectrum. Qualitatively, the bound state near the Ising limit $\alpha\ll 1$ corresponds to the confinement of a free holon in the vicinity of an essentially immobile, impurity-like spinon with binding energy $\Delta \sim J_z^2/t$. However, already for $\alpha\agt 0.5$ an almost relativistic spinon is only weakly bound to the holon. Such a change in the character of pairing results in a dramatic evolution of $\Delta$ as a function of $\alpha$: binding energy goes from a finite value at $\alpha=0$ to the exponential drop-off close to the isotropic case. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V16.00008: Probing the pairing symmetry and pair charge stiffness of doped $t-J$ ladders: a DMRG study Adrian E. Feiguin, S. R. White, D. J. Scalapino We perform the numerical equivalent of a phase sensitive experiment on $t-J$ ladders. We apply proximity effect fields with different complex phases at both ends of an open system and we study the transport of Cooper pairs. Measuring the response of the system and the induced Josephson current, Density Matrix Renormalization Group calculations show how, depending on the doping fraction, the rung-leg parity of the pair field changes from minus to plus as the density of holes is increased. We also study the pair charge stiffness. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V16.00009: Hole Dynamics in a 2D Striped-Ordered Quantum Antiferromagnet Satyaki Kar, Efstratios Manousakis We study the single hole motion in a $2D$ quantum antiferromagnet in a stripe background within the $t-J$ model, using an additional ferromagnetic term along the domain walls to stabilize the stripes. The spin wave approximation is used to linearize the Hamiltonian and we calculate the different flavors of spin-wave dispersions and hole propagators by means of a self consistent perturbation scheme within the non-crossing approximation. The hole spectral functions will be compared to the experimental ARPES data available for $La_{2-x}Sr_{x}CuO_{4}$ and concentration of $x=\frac{1}{8}$. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V16.00010: Charge dynamics in an incommensurate layered cuprate $Sr_{14}Cu_{24}O_{41}$ : A momentum-resolved study Lewis Wray, Dong Qian, David Hsieh, Matthew Xia, Hiroshi Eisaki, Zahid Hasan We report the first \textit{momentum resolved} charge mode spectrum of insulating $Sr_{14}Cu_{24}O_{41}$ using inelastic resonant x-ray scattering. Our results show that the intense excitation modes at the charge gap edge predominantly originate from the ladder-containing two-dimensional planes. The observed modes (E vs. Q) are found to be dispersive for momentum transfers along the ``legs'' ($\vec{Q}$ $\parallel$ $\hat{c}$) but nearly localized along the ``rungs'' ($\vec{Q}$ $\parallel$ $\hat{a}$). Dispersion and peakwidth characteristics are strongly similar to the low energy charge spectrum of quasi-one dimensional $SrCuO_2$, and we suggest a qualitative explanation in terms of a model in the strongly correlated limit ($U\gg t$). This behavior is in marked contrast to the charge spectrum observed in most two dimensional cuprates. Quite generally, our results also show that momentum-tunability of inelastic scattering can be used to resolve mode contributions in multi-component correlated systems. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V16.00011: Pressure effect on the charge-density-wave order in pure and doped (La,R)AgSb$_2$ (R = Ce, Nd) compounds M.S. Torikachvili, S.L. Bud'ko, S.A. Law, M.E. Tillman, E.D. Mun, P.C. Canfield We studied the pressure dependent resistivity, $\rho(T)$, of single-crystals of pure and doped (La,R)AgSb$_2$ (R = Ce, Nd) compounds, up to 23 kbar, for temperatures $2 \leq T \leq 300$ K. LaAgSb$_2$ displays features at $T_1 = 210$ K, and $T_2 = 185$ K, due to the formation of charge-density-wave (CDW) order along the $a-$ and $c-$ axis of the tetragonal structure, respectively. The effect of pressure in LaAgSb$_2$ is to lower $T_1$ at the rate of -4.5 K/kbar. Partial substitutions of Nd and Ce for La depress $T_1$ as well, and the effect of pressure is to lower it further. The value of $T_1 = 112$ K for La$_{0.75}$Nd$_{0.25}$AgSb$_2$ drops with pressure at the rate of -5.8 K/kbar, until the feature in $\rho(T)$ cannot be identified above 12 kbar, suggesting the closure of the CDW gap. The effect of pressure on La$_{0.9} $Ce$_{0.1}$AgSb$_2$, and La$_{0.8}$Ce$_{0.2}$AgSb$_2$ is to depress $T_1$ at the rate of $\approx -10$ K/kbar, with the suppression of CDW order for $P \approx 12$ and $\approx 8$ kbar, respectively. The $\rho(T)$ data for the Ce-doped compounds show upturns at low temperatures due to the Kondo effect, and the minimum in $\rho(T)$ is lowered with pressure. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V16.00012: Bond-stretching phonon mode in stripe ordered orthorhombic $\bf Nd_{1.67}Sr_{0.33}NiO_4$. M. H\"ucker, T. Fukuda, G. D. Gu, J. M. Tranquada, A. Q. R. Baron, J. P. Hill Inelastic x-ray scattering (IXS) experiments on $\rm Nd_{1.67} Sr_{0.33}NiO_4$ have been performed to study electron-phonon interactions in this charge stripe ordered nickelate. Resurgent interest in such phenomena has been triggered by recent results on the high temperature superconductors, where a kink in the electron dispersion as well as striking anomalies in high- energy optical phonon modes have been observed. A significant softening of the bond-stretching-phonon mode for $\rm Q||[100]$ was also observed in the tetragonal nickelate $\rm La_{1.69}Sr_ {0.31}NiO_4$ with inelastic neutron scattering. Moreover, this compound shows an apparent splitting of the bond-stretching mode along the $\rm [110]$ direction. Here we present first IXS results for $\rm Q||[110]$ on the orthorhombic compound $\rm Nd_ {1.67}Sr_{0.33}NiO_4$, which is characterized by domains with unidirectional stripe order. By probing different spots on the sample with different domain distribution, a weak contrast between the phonon spectra has been observed. We discuss these differences in terms of phonons propagating parallel and perpendicular to the stripe direction, as well as the anisotropic, i.e. orthorhombic, lattice structure. [Preview Abstract] |
Session V17: Hybrid Organic/Inorganic Nanomaterials: Synthesis, Assembly, and Applications II
Sponsoring Units: DPOLY FIAPChair: Ryan Hayward, University of Massachusetts
Room: Colorado Convention Center 102
Thursday, March 8, 2007 11:15AM - 11:27AM |
V17.00001: Photoinduced Reduction of Noble Metal Ions to Metal Nanoparticles on Tubular J-Aggregates D.M. Eisele, A. Burmistrova, H. v. Berlepsch, C. Boettcher, S. Kirstein Cyanine dye molecules are well known to serve as sensitizers for photo induced electron transfer processes. Technically, this feature is utilized in photographic films to form elementary silver specks in solid silver halide crystallites. In this contribution it is shown that the photo-induced electron transfer reaction from cyanine dyes to noble metal ions can be utilized to grow metallic nanoparticles at the surface of tubular J-aggregates in solution. The J-aggregates are formed by amphiphilic cyanine dye molecules upon aggregation in aqueous solution. The particles are grown by addition of noble metal salts (Na$_{2}$PdCl$_{4}$ or AgNO$_{3})$ to the cyanine dye aggregate solution and by illumination with visible light. The particles are observed by Cryogenic transmission electron microscopy (cryo-TEM) and are rather uniform in size with a mean diameter of a few nanometres. In case of Pd salt the aggregates are destroyed upon particle formation, while in case of Ag salt the aggregates are unaffected by the formation of silver particles. In parallel to the growth of the particles a dramatic quenching of the aggregate fluorescence is observed. The particles form spontaneously and the number and size of the particles depends on the molar Ag/dye ratio. A systematic study of this photo induced process for AgNO$_{3}$ will be presented. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V17.00002: The chemical and structural properties of PECVD polymerized ferrocene deposited by the sublimation of the precursor material Jesse Enlow, Hao Jiang, Someshwar Peri, Mark Foster, Timothy Bunning The novel deposition of metal containing precursor materials in plasma enhanced chemical vapor deposition through the sublimation of the material in its solid state is investigated. The chemical composition and structural properties of these thin films, examined through XPS, variable angle ellipsometry, FT-IR, AFM and X-ray reflectivity, are reported. Using a custom designed plasma chamber and sublimation system, pp-ferrocene films have shown high chemical and mechanical robustness, are pin-hole free, have extremely smooth surface morphologies, and are highly crosslinked through the bulk. The use of sublimation in PECVD opens up the deposition technique to a wealth of new metal containing monomers. And with PECVD you retain a high amount of control over the deposition parameters and resultant film compositions for these organo-metallic films. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V17.00003: Selective dispersion of nanofillers in PET/PC blends E. Manias, M.J. Heidecker The nanocomposite formation of immiscible PET/PC blends with organically-modified montmorillonite layered-silicates was studied as a model system to tailor thermodynamics, so as to achieve (a) selective dispersion in the PET only phase, and (b) promote physical mixing (``compatibilization'') of the PET and PC matrices. Dispersion was controlled by design of appropriate surfactant chemistries used for nanofiller modification. The desired composite structure is obtained even when the organically-modified fillers are premixed (masterbatched) in the ``unfavorable'' polymer. This behavior, i.e. the composite structure being markedly independent of processing conditions, indicates that, for these systems, the thermodynamics of dispersion overwhelmingly determine the resulting structure rather than the processing conditions. The resulting changes in PET crystal morphology afford novel new mechanical properties, that combine substantial increases in modulus with accompanying increases in ductility and toughness. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V17.00004: The thermal properties and the microstructures of organic-inorganic nano-composite materials KengChing Lin, Kuo-Hsin Chang, WeiFang Su The imaging capability of the atomic force Microscope (AFM) with thermal accessory is utilized to study the nano-scale T$_{g }$and morphological evolution of silica-polyacrylate composites. The polymer matrix is made by irradiation-inducing polymerization of the blend of TEGDA and EOBDA (Ethoxylated (3) Bisphenol A Diacrylate)acrylate. The surface area ratio of the nano-particles and the measured frame is applied to define the melting event on the composite film and make a direct comparison with the macro-scale T$_{g}$ obtained from Differential Scanning Calorimetry (DSC) and Thermo-Mechanical Analyzer (TMA). The decomposition temperature T$_{d}$ of the composites is measured by Thermo-Gravimetic Analyzer (TGA). In analog to the mechanical property transition as the nanoparticle content reaches the critical density, the measured melting temperature of these hybrid materials displays a nonlinear trend as the filler content increases. When the silica particles form a percolation network within the composite, the 2-D melting and 3-D disintegration behave differently. This discrepancy arising either from the fundamental difference of physical nature or the instrumental limits will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V17.00005: Enhanced Oxygen Barrier and Interfacial Adhesion of Polystyrene/Clay Nanocomposites via Plasma Surface Modification Patchara Tasanatanachai, Rathanawan Magaraphan Bentonite layered-silicate was the selected nanofiller to be studied and filled in polystyrene, consequently represented PS/clay nanocomposites. The plasma technique developed in our laboratory was utilized as the clay pretreatment method to provide the radical sites on the clay surface prior to styrene grafting step. Gas barrier property and impact strength, one of the most serious deficiencies of polystyrene, were examined. Oxygen permeability was found to be decreased moderately even small amount of modified clay loaded. As the result of plasma surface modification, it was found that grafting of styrene was occurred mainly on the outer clay layer since FT-IR spectra showed the characteristic peak of polystyrene with the same basel-spacing like pristine clay. The broadening of diffraction peak of PS/clay nanocomposites produced by melt intercalation was observed suggesting the structure of both intercalation and partial exfoliation; however, the impact strength was reasonably improved which can imply to the enhancement of interfacial adhesion between clay particles and polystyrene matrix. In addition, the ratio of styrene and initiator played the significant role on these properties as well. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V17.00006: Structure and Morphology of Polymer/Clay Nanocomposites formed by Chaotic Smart Blending Dilru R. Ratnaweera, Dvora Perahia, Chaitra Mahesha, Dvid Zumbrunnen , Mark A. Kampf The dispersion and orientation of the nano particles within polymeric matrices determine their properties. They depend on inherent characteristics such as miscibility, entropic barriers, structure of the polymer etc., and on processing conditions under which the polymer and the particles are mixed. Chaotic flow patterns have been used to define the structure and orientation on a micron length scale. In efforts to derive quantitative correlations, we carried out structural studies using X-ray, AFM and TEM on Nylon 6/modified clay nano composites as a function of the strength of the chaotic advection in a smart blending process. On the micron length scale, the clay particles are dispersed into multi layers, where the thickness of these sheets and the orientation and the distribution of the clay particles are controlled by the strength of the chaotic advection. On the molecular level, increasing the strength results in increase of the $\gamma $ crystal form of the Nylon, a higher internal energy state, in which hydrogen bonds are formed between parallel chains. The structure on the micron level is attributed to the chaotic mixing which o the molecular level it is may be attributed to the fractal nature of the process. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V17.00007: A Nanoparticle Self-Assembled Tactile Sensor with Sensitivity {\&} Resolution of Human Finger vivek maheshwari, Chieu Nguyen, Ravi Saraf Sensation of touch, primarily the determination of stress distribution over the area of physical contact between the sensor and the object surfaces, is critical for advancement of minimum invasive surgical procedures and for development of humanoid robots. The spatial resolution of current large-area tactile sensor ($>$ 1 cm$^{2})$ lags by over an order of magnitude compared to human finger. Using metal and semiconducting nanoparticles $\sim $100 nm thick, large area thin-film device is self assembled such that the change in current density through the film and the electroluminescent light intensity are proportional to local stress. Both lateral and height resolution of texture are comparable to that of human finger, 40$\mu $m and 2 $\mu $m respectively. The sensitivity of 9 KPa is well within the 10 to 40 KPa range that a human finger applies to sense texture and shape. Stress image can be constructed by directly imaging the electroluminescence on a CCD or by mapping the current density over the surface of the device. The device is based on the principle of electron tunneling. Being solution processed it can be easily assembled on large surfaces and complex shapes.. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V17.00008: Novel Route to Nanoparticle Dispersion Using Supercritical Carbon Dioxide Rahmi Ozisik, Kumin Yang An experimental study was carried out to determine the effects of supercritical carbon dioxide (scCO$_{2}$) on the dispersion of untreated and modified alumina nanoparticles in polystyrene. For the untreated alumina, the sudden expansion of the carbon dioxide did not alter the size of the agglomerates. This was probably caused by the weak interaction of scCO$_{2}$ with the untreated alumina that keep the agglomerate intact upon depressurization. On the other hand, the large agglomerates of the modified alumina showed signs of catastrophic fragmentation. It is speculated that the smaller agglomerates lacked the surface coating, which may have lead to their intact structure. The fluorinated modifiers used had a high degree of solubility with the scCO$_{2}$ at the processing conditions used, and therefore, the scCO$_{2}$ in the modified alumina composites was able to diffuse easily into the agglomerate compared to the untreated alumina. Large agglomerates can absorb more scCO$_{2}$ than smaller ones, and therefore, have an adequately larger bursting pressure to overcome both the hydrogen bonding between the modified alumina nanoparticles and the weak electrostatic interactions. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V17.00009: Directing self-assembly of gold nanoparticles in diblock copolymer scaffold Qifang Li, Jinbo He, Elizabeth Glogowski, Todd Emrick, Thomas Russell A versatile hierarchical approach for directing self -assembly of gold nanostructures with size 2-3nm in diblock copolymer scaffolds is found. Diblock copolymer polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) is used to form a regular scaffold of highly anisotropic, stripe-like domains, and controlled differential wetting by dichloromethane and thermal annealing guides gold nanoparticles with half hydrophilic ligand to aggregate selectively along the scaffold, producing highly organized metal nanostructures. In as-cast block-copolymer and gold nanoparticles thin films, micelle structure and gold nanoparticles random distribution on scaffold are typically observed. However, samples annealed in dichloromethane exhibit well-defined short-range ordered nanostructure with gold nanoparticles located at the interface of PS and P2VP nanoscale domain. After annealing at 170\r{ }C, the gold nanoparticles at interface migrated into the middle of P2VP phase and exhibited long-range ordered hierarchical structures. Synergistic interactions between the gold nanoparticles and the PS-b-P2VP caused an orientation of the microdomains normal to the film surface. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V17.00010: Simple Fabrication of Mesoporous Silica with Remarkable High Temperature Stability at Neutral pH and Ambient Conditions from TEOS David Hess, Radha Vippagunta, James Watkins Traditional silica synthesis processes can yield well ordered materials, but the synthesis conditions also lead to incomplete condensation of the silica network, which results in significant structural contraction upon calcination and limited thermal, hydrothermal and mechanical stability. Here we report the synthesis that, surprisingly, yields nearly complete condensation of the silica network (virtually all Q4 linkages) using cysteamine as the catalyst and polyoxyethylene surfactants as the structure directing agents in buffered solution at neutral pH and ambient temperature. Recently, small molecule bifunctional amines, including cysteamine, were evaluated by Morse and co-workers and found to produce silica from TEOS(JACS 2005, 127, 35). Our work combines the cysteamine catalyst system with structure-directing block copolymer surfactants at neutral pH and ambient temperature to produce mesoporous silica. The addition of tetraethyl orthosilicate (TEOS) to a solution of containing cysteamine, citrate buffer (pH 7.2) and 5wt Brij amphiphilic block copolymer (polyethylene oxide-polyethylene) yields mesoporous silica. The resulting mesoporous silica powder was analyzed using XRD, TGA, FTIR, TEM, and NMR. The materials were found to exhibit stability under extreme temperature calcinations (up to 800 C) in the presence of water. SAXS shows that 1.0 shrinkage upon calcination up to 800C. 29Si NMR analysis indicates a fully condensed silica network, Q4 linkages, in accordance with this observation. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V17.00011: Hydrophilic Silica-Polypeptide Composite Particles Erick Soto-Cantu, Paul Russo Composite, pH-responsive particles have been synthesized by covalently attaching a simple polypeptide to a silica core. The synthesis begins with the production of organophilic poly(benzylglutamate)-coated silica particles.\footnote{Fong,B.; Russo, P.S. Organophilic Colloidal Particles with a Synthetic Polypeptide Coating. \textit{Langmuir} \textbf{1999}, $15$, 4421-4426.} The particles are rendered hydrophilic by cleaving the benzyl side group by treatment with hydrogen bromide in benzene. The resulting poly(glutamic acid)-coated silica spheres exhibit a change in hydrodynamic radius in response to pH stimulus. The size transition is due to a change in the polypeptide conformation, as deduced from circular dichroism measurements. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V17.00012: Polymer-Graphite Nanocomposites: Comparison to Clay- and Carbon Nanotube-Based Hybrids Katsuyuki Wakabayashi, Kosmas Kasimatis, John M. Torkelson Although polymer-layered silicate and polymer-carbon nanotube nanocomposites have been widely studied in the last decade, hybrids containing nanoscale entities of graphite have been studied far less. Its structural analogy to layered silicates and chemical analogy to carbon nanotubes make graphite an attractive nanofiller in both scientific study and technological application. A common challenge of efficient dispersion of the nanofiller in the polymer matrix associated with conventional fabrication methods is overcome by processing using the solid-state shear pulverization technique. The level of dispersion and presence of graphite nanosheets are confirmed by X-ray diffraction and electron microscopy, while enhanced mechanical, thermal, and electrical properties of the resulting materials are characterized using tensile testing, dynamic mechanical testing, differential scanning calorimetry, thermogravimetric analysis and impedance spectroscopy. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V17.00013: Organic-Inorganic Photovoltaic Composite Materials Based on Polymer-Functionalized Semiconductor Nanorods Qingling Zhang, Suresh Gupta, Todd Emrick, Thomas Russell Polymer-based photovoltaics, using composite materials consisting of inorganic particles embedded within the polymer, are considered to be promising candidates for the enhancement of power conversion efficiency, due to the directionality of electron transport enabled by the nanorods. We have demonstrated that oriented nanorods phase separate into close-packed micron-scale arrays within a matrix of photoactive polymer matrix with the assistance of electric field. However, for the material to be useful in photovoltaic applications, the separation distance between elements must be about 10 nm or less due to the exciton diffusion length. Here, we describe the attachment of photoactive polymers to CdSe nanorods. The photoluminescence spectra of these photoactive polymer-functionalized CdSe nanorods exhibited photoluminescence quenching of both nanorods and polymer, indicating charge transfer between donor and acceptor, a prerequisite for a successful photovoltaic material. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V17.00014: Controlled Clustering of Oxide Nanoparticles using Block Copolymers for Coating and Biomedical Applications Jean-Francois Berret During the past years, we have investigated the complexation between nanocolloids and oppositely charged polymers. The nanocolloids examined were ionic surfactant micelles and inorganic oxide nanoparticles. For the polymers, we used homopolyelectrolytes or block copolymers with linear and comb architectures. The attractive interactions between oppositely charged species are strong and in general the simple mixing of disperse solutions yield to a precipitation, or to a phase separation. We have developed means to control the electrostatically-driven attractions and to preserve the stability of the mixed solutions. With these approaches, we designed novel core-shell nanostructures, as those obtained recently with polymers and cerium and iron oxide nanoparticles (Berret et al., J. Am. Chem. Soc. 2006, 128, 1755). In this presentation, we show that electrostatic complexation can be used to tailor new functionalized nanoparticles and provide examples related to surface coating and biomedical applications. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V17.00015: Novel Nanocomposite Materials Synthesis and Their Application YuanQiao Rao This paper focuses on our effort to develop novel nanocomposite materials. A new method, in situ nanoparticle synthesis, has been used to make a variety of polymer-metal oxide nanocomposites. This method provides a simple and versatile tool to overcome the difficulty of controlling particle dispersion during nanocomposite synthesis. The resulting synthesized nanocomposites have well-controlled morphology and enhanced optical properties. The structure-property relation of these nanocomposites will also be discussed. [Preview Abstract] |
Session V18: Chemical Dynamics & Molecular Spectroscopy
Sponsoring Units: DCPChair: Elliot Bernstein, Colorado State University
Room: Colorado Convention Center 103
Thursday, March 8, 2007 11:15AM - 11:27AM |
V18.00001: Ultrafast 2D IR vibrational echo chemical exchange spectroscopy Junrong Zheng, Michael Fayer Ultrafast 2D IR vibrational echo chemical exchange spectroscopy, akin the 2D NMR methods, is applied to the study of dynamics of weakly hydrogen bonded solute-solvent complexes in liquid solutions. The strengths of the solute-solvent hydrogen bonds are adjusted by modifying the chemical structures of the solutes and solvents. For the eight samples studied, the formation enthalpies vary from -0.6 kcal/mol to -2.5 kcal/mol, and the dissociation time constants vary from 3 ps to 32 ps. The dissociation rates of the hydrogen bonds are found to be strongly correlated with their formation enthalpies. The correlation can be described with an equation similar to the Arrhenius equation. As another example of chemical exchange spectroscopy, the rate of carbon-carbon single bond rotational isomerization of an ethane derivative in room temperature liquid solution is measured. Based on the experimental results and density functional theory calculations, the time constant for the ethane internal rotational isomerization under the same conditions is about 12 ps. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V18.00002: Dynamic Nuclear Polarization (DNP) Using Nitroxide Radicals Brandon Armstrong, Evan McCarney, Songi Han The theory of the Overhauser Effect in liquids is well established, but measured DNP enhancements from nitroxide radicals depart significantly from prediction. To achieve large signal enhancements, milli-molar concentrations of radicals are needed, a regime where Heisenberg exchange of the unpaired electron is significant. Therefore, the three electron transitions resulting from hyperfine interactions with the $^{14}$N nucleus cannot be treated as independent in a DNP experiment. Furthermore, the relaxation rate of $^{14}$N can be easily on the same order of magnitude or even greater than the relaxation rate of the unpaired electron, contributing to the mixing of the hyperfine states, even in the absence of Heisenberg exchange. We present a quantitative study and a new model of $^{1}$H DNP enhancement of water by varying radical concentrations and solvent viscosities of natural abundance $^{14}$N versus $^{15}$N isotope enriched 4-Oxo-TEMPO free radical at 0.35 Tesla. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V18.00003: Comparison of Electronically Excited Photodissociation between Nitramine Energetic Materials and Model Systems Yuanqing Guo, Margo Greenfield, Atanu Bhattacharya, Elliot Bernstein Nitramine energetic materials (RDX, HMX and CL20) have broad applications as explosives and fuels. Model systems (1,4-dinitropiperazine, nitropiperidine, nitropyrrolidine and DMNA) have similar molecular structures, but they are unable to be used as fuels and explosives. To elucidate the difference between them, both nanosecond and femtosecond mass resolved excitation spectroscopy have been employed to investigate the mechanisms and dynamics of the electronically excited photodissociation of these materials. NO is a dominant dissociation product. Based upon the experimental observation and calculations of potential energy surfaces for these systems, we suggest that energetic materials dissociate from their ground electronic states after relaxing from the first excited states, and that the model systems dissociate from their excited state. In both cases a nitro-nitrite isomerization is part of the reaction mechanism. Parent ions of DMNA and nitropyrrolidine are observed in fs experiments. All the other molecules generate NO as a product even in fs time regime. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V18.00004: UV excited electronic state decomposition of energetic materials and model systems using fs laser spectroscopy Margo Greenfield, Yuanqing Guo, Atanu Bhattacharya, Elliot Bernstein Time resolved (fs) photodissociation experiments have been performed in efforts to elucidate the dynamics controlling the excited electronic state decomposition of the energetic materials RDX and HMX and their associated model systems (dimethylnitramine, nitropyrrolidine, nitropiperidine, and dinitropiperazine). The initial decomposition product of the energetic materials and model systems is the NO molecule. Femtosecond pump-probe techniques have been employed to measure the photodissociation dynamics of these systems via the initial NO product at three wavelengths (226 nm, 228 nm, 230 nm). The NO molecule has a non-resonant two-photon absorption at 228 nm and 230 nm and single photon resonant absorption for the A$^{2}\Sigma \leftarrow $X$^{2}\Pi $(0,0) transition. Both pump-probe transients at non-resonant absorption and resonant absorption wavelengths indicate the dynamics of the energetic material's decomposition from the excited electronic state is faster than the time duration of our laser pulse (180 fs) and notably different from some of the model systems. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V18.00005: Photoinduced ring-opening mechanism in several model diarylethenes Petra Sauer, Roland E. Allen Recent experimental results suggest that derivatives of diarylethenes may be viable candidates for switches in molecular devices. In these molecules, the ring-opening and ring-closing reactions are induced with laser pulses of different frequencies. We have shown that the ring-opening mechanism in the simplest of all diarylethenes, stilbene, occurs via a HOMO-LUMO avoided crossing and subsequent depopulation of the excited state with minimal involvement of other orbitals. We now show that the same photoinduced ring-opening process for both oxygen ($C_{10}H_{8}O_{2}$) and sulfur ($C_{10}H_{8}S_{2}$) containing diaryethenes involves higher order excited states. We will show simulation results in which the initial laser pulse excitation induces a transition from HOMO to LUMO. Due to both nuclear motion and symmetry changes, this laser pulse also excites some percentage of the electronic population out of orbitals lower in energy than the HOMO and into orbitals higher in energy than the LUMO. In order for the ring-opening event to occur, the electronic population in the higher excited states is first transferred, via a series of avoided crossings, into the LUMO. A subsequent avoided crossing between the HOMO and LUMO then allows ring-opening. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V18.00006: Cooling and Trapping of NH radicals L. Paul Parazzoli, Carlos Romero, Daniel Lobser, Heather Lewandowski In the past decade, cooling and trapping of atoms has allowed physicists to probe the nature of quantum mechanics on a macroscopic scale. Molecules, having a more complex structure, are considerably more difficult to cool. However, it is their complex structure, including rovibrational states and permanent dipole moments, which make them so interesting. We cool metastable NH ($^{1}\Delta )$ radicals using supersonic expansion coupled with Stark deceleration. The NH radicals are created by photolysis of HN$_{3}$ during supersonic expansion. The supersonic expansion produces a cold beam of radicals, which is loaded into a Stark decelerator. The Stark decelerator uses time varying inhomogeneous electric fields to decelerate the NH molecules. The resulting molecular sample has a temperature of 10 -100 mK. Further cooling will be explored using interactions with ultracold Rubidium atoms. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V18.00007: First Principles Simulations of THz Spectra of Acephate: Insight Into the Phonon Signatures. Yiming Zhang, Xihong Peng, Yunqing Chen, Saroj Nayak, X.-C. Zhang Acephate is an insecticide that kills insects by disrupting nervous system functions. THz spectroscopy offers a unique tool for detecting trace amount of these materials. Using a combination of solid state first principles simulations and gas phase quantum mechanical modeling we have studied phonon spectra of acephate compound. This talk will present a detailed vibrational spectra analysis over a wide range of frequency and our computational data will be compared with available experimental results. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V18.00008: Hydrophobic dependence of molecules at the liquid/solid interface as studied with infrared-visible sum frequency generation spectroscopy Bryan Hsu, Veronique Lachat, Gabor Somorjai, Mohsen Yeganeh We report IR-vis SFG spectra obtained at the interface of liquids with hydrophobic and hydrophilic solid surfaces. The hydrophilic and hydrophobic surfaces used were sapphire and a dense methyl-terminated sapphire surface from chemically bonded octadecyltrichlorosilane (OTS), respectively. Orientation calculations of SSP (IR, vis, and SF polarizations) and SPS polarizations of acetonitrile on the OTS and sapphire showed tilt angles of approximately 90 and 40 degrees, respectively. The CD3 symmetric stretch of methanol (d4) at OTS and sapphire showed a blue-shift for the latter but no shift for the former when compared to FTIR of the bulk liquid. This may be due to changing H-bonding characteristics with methanol orienting its hydroxyl end towards sapphire and away from OTS. PPP spectra of n-heptane (d16) and n-hexadecane (d34) showed weaker intensity CD3 antisymmetric stretches on sapphire compared to OTS, with a larger decrease for n-hexadecane. This can be interpreted as the hydrocarbons curling away from sapphire more so than OTS, especially with n-hexadecane. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V18.00009: Observation of the $\widetilde{A}-\widetilde{X}$ Electronic Transition of the Jet-Cooled Methyl Peroxy Radical by High Resolution CRDS Patrick Dupr\'{e}, Shenghai Wu, Patrick Rupper, Terry Miller Reactive intermediates are of crucial importance both for combustion and atmospheric chemistry. By using our new home made Fourier Transform limited (10--30~MHz) Ti:Sa laser source we have probed the vibrationless level of the first electronic state (in the near-IR range) of both ${\rm CH}_{3}{\rm OO}$ and ${\rm CD}_{3}{\rm OO}$ radical species. The radicals are formed inside a ${\rm Ne/He/O}_{2}/{\rm CH}_{3}{\rm I}$ plasma created by a DC or a RF electrical discharge. The supersonic jet expansion necessary for the rotational cooling ($\sim20\,{\rm K}$) is obtained by a pulsed slit nozzle ($\sim50\times0.5\,{\rm mm}^{2}$). The near-IR radiation, obtained by Stimulated Raman Scattering (SRS) is injected inside a high finesse cavity. A sensitivity of the order of $\sim20\times10^{-9}\,/{\rm pass}/\sqrt{{\rm Hz}}$ is currently obtained. Spectrum with a resolution $\sim350\,{\rm MHz}$ for ${\rm CD}_{3}{\rm OO}$ clearly shows rotational and spin-rotation structure with effects of the internal methyl group rotation possibly evolved. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V18.00010: Modeling the transient vibrational dynamics of photofragments Steve Young, Sara Mason, Hai-Lung Dai, Andrew Rappe Knowledge of radical spectroscopy and the structure of radicals is important in many scientific areas, such as atmospheric systems, combustion reactions, biological processes, and more. Because many radicals are transient, unstable, and generally produced in small quantities, they are often difficult to characterize spectroscopically. In this talk, we will present our synthesis of theoretical and experimental data to understand the behavior of radical photofragments. The first part of the talk outlines our approach to understanding vibrationally hot but electronically cold radical dynamics, with direct molecular dynamics and performing electronic structure calculations using DFT within the GAMESS package. We will then summarize our recent development and application of time-resolved FTIR emission spectroscopy for the study of photofragments. Finally, we will present a joint theoretical and experimental investigation of the dynamics of the vinyl radical, including characterization of the complex interaction of rotation, alpha-proton motion, and anharmonic effects, and discuss their influence on the IR spectrum. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V18.00011: Raman spectra of benzene derivatives adsorbed on metal substrates D.A. Alexson, S.C. Badescu, O.J. Glembocki, S.M. Prokes, R.W. Rendell We study the molecular orientations of several benzene derivatives on large Ag and Au clusters via first-principles calculations. We find the lowest-energy structures, several local minima and the diffusion barriers for benzene, nitrobenzene, 2,4-dinitrotoluene (DNT) and 1,4-benzenedimethanethiol (BDMT). The theoretical calculations are compared to experimental measurements of SERS for 2,4-DNT and 1,4-BDMT on Ag and Au coated dielectric nanowires. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V18.00012: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V18.00013: Single photon ionization of hydrogen bonded clusters with a desk-top size soft x-ray laser: (HCOOH)$_{n }$ and (HCOOH)$_{m}$(H$_{2}$O)$_{n}$ Scott Heinbuch, Feng Dong, Jorge Rocca, Elliot Bernstein Pure neutral (HCOOH)$_{n}$ clusters and mixed (HCOOH)$_{m}$(H$_{2}$O)$_{n}$ clusters are investigated employing time of flight mass spectroscopy and single photon ionization at 26.5 eV (from a soft x-ray laser). The distribution of pure (HCOOH)$_{n }$clusters is dependant on experimental conditions. At certain conditions a magic number is found at n = 5. During the ionization process, neutral clusters suffer little fragmentation because almost all excess energy above the vertical ionization energy is taken away by the photoelectron. Metastable dissociation rate constants of (HCOOH)$_{n}^{+}$ are measured in the range of (0.1--0.8)x10$^{4}$ s$^{-1}$ for cluster sizes of 4$<$n$<$9. The rate constants display an alternating behavior between monomer and dimer loss that can be attributed to the structure of the cluster. When small amounts of water are added to the formic acid, the predominant signals in the mass spectrum are still (HCOOH)$_{n}^{+}$ cluster ions. Also observed are the mixed cluster series (HCOOH)$_{m}$(H$_{2}$O)$_{n }$for m=1-8 and n=0-4. A magic number in the cluster series n=1 at m=5 is observed. The mechanisms and dynamics of the formation of these neutral and ionic clusters are discussed. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V18.00014: Formation and Distribution of Neutral Transition Metal Oxide Clusters: Single Photon Ionization at 26.5 eV Feng Dong, Scott Heinbuch, Jorge Rocca, Elliot Bernstein A single photon of an EUV laser (26.5 eV) has enough energy to ionize any metal oxide cluster generated in a molecular beam. Neutral vanadium, niobium, and tantalum oxide clusters are studied by single photon ionization employing a 26.5 eV EUV laser. During the ionization process, metal oxide clusters are virtually free of fragmentation. The most stable neutral metal oxide clusters under saturated oxygen conditions can be represented as (MO$_{2})_{0,1}$(M$_{2}$O$_{5})_{y}$ (M=V, Nb, Ta). Both O-rich and O-deficient clusters can be observed. Oxygen-rich metal oxide clusters with high ionization energy are detected by 26.5 eV, but not by 10.5 eV, ionization. For O-rich clusters M$_{x}$O$_{y}$H$_{z}$ species are also observed for the first time. Given these experimental capabilities, neutral cluster reactions and reactivity can be studied. We will present preliminary results of these studies. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V18.00015: UV Single Photon Dissociation of Furazan Based Energetic Materials: DAAF Atanu Bhattacharya, Yuanqing Guo, Margo Greenfield, Elliot Bernstein The new series of furazan-based energetic materials is characterized by low sensitivity to impact and friction. They have broad application as fuels and propellants; however, extra nitro functional groups attached to the furazan ring (e.g. 4,4$^{\prime} $-dinitro-3,3$^{\prime} $-azoxyfurazan) adversely impact the thermal stability of these energetic materials. In order to evaluate the effect of nitro functional groups on furazan-based energetic materials the decomposition of 4,4$^{\prime} $-diamino-3,3$^{\prime} $-azoxyfurazan (DAAF), from excited electronic states, has been investigated by UV excitation (8 ns duration) and time of flight mass spectroscopy. The NO molecule is observed as an initial product. Three vibronic transitions of NO are characterized. Simulation of the NO [A $^{2}\Sigma $ (v$^{\prime} $=0)$\leftarrow $X $^{2}\Pi $ (v$\prime $=1)] transition and fitting to the intensity ratios among NO vibronic transition yields rotational and vibrational temperatures of 30 K and 1265 K, respectively. Compared with NO gas spectra, under comparable condition, the NO from decomposition of DAAF is vibrationally hot and rotationally cold. [Preview Abstract] |
Session V19: Focus Session: Ultrafast Dynamics using X-rays and Electrons II
Sponsoring Units: DCPChair: Christoph Rose-Petruck, Brown University
Room: Colorado Convention Center 104
Thursday, March 8, 2007 11:15AM - 11:51AM |
V19.00001: Picosecond X-ray absorption spectroscopy of light-induced processes in liquids Invited Speaker: The importance of capturing the dynamics of molecular motion in real-time is crucial for our understanding of physical, chemical and biological phenomena. In addition, structural changes in molecular systems stem from ultrafast electronic structure changes, which change the field of forces within a molecule, and between a molecule and its environment. Therefore, observing both electronic and structural changes in a given system provides deeper insight into its dynamics. Ultrafast optical spectroscopy does not deliver molecular structure. Ideal structural tools in this respect are X-rays, via methods such as diffraction or spectroscopy. Here, we will present our recent results on the probing of structural changes in electronically excited solvated species, using picosecond X-ray absorption spectroscopy, in a laser pump/X-ray probe configuration. We will demonstrate the power of this new approach on three different types of processes: intramolecular electron transfer, ultrafast molecular magnetism and solvation dynamics around an atomic ion, and discuss future extensions to biological systems. We will also discuss future experiments with femtosecond time resolution at synchrotrons and 4$^{th}$ generation light sources. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:27PM |
V19.00002: Ultrafast Structural Dynamics of Photoactive Metal Complexes in Solar Energy Conversion. Invited Speaker: The photoexcited states of metal complexes are precursors for many important photochemical processes in solution phase. Using laser-initiated time-resolved x-ray absorption spectroscopy (LITR-XAS), transient metal oxidation states, coordination geometry, and atomic rearrangements that closely reflect photochemical processes can be probed, which complements with ultrafast optical laser spectroscopic studies for kinetics and coherence among different excited states as well as intra- and intermolecular energy/charge transfer processes associated with solar energy conversion. We have studied by LITR-XAS combined with transient absorption spectroscopy excited state structures, such as metalloporphyrins and platinum(II) complexes, in solution. Direct evidence of photoinduced redox reactions and coordination geometry changes have been observed. These experimental studies are combined with time-dependent density functional theory (TDDFT) calculations to rationalize the excited state structural nuclear changes with electronic configuration changes that may be responsible for the reactivity of the molecules. These studies will have a great impact in fundamental understanding of solar fuel production. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 1:03PM |
V19.00003: Watching proteins function with 150-ps time-resolved X-ray crystallography Invited Speaker: We have used time-resolved Laue crystallography to characterize ligand migration pathways and dynamics in wild-type and several mutant forms of myoglobin (Mb), a ligand-binding heme protein found in muscle tissue. In these pump-probe experiments, which were conducted on the ID09B time-resolved beamline at the European Synchrotron and Radiation Facility, a laser pulse photodissociates CO from an MbCO crystal and a suitably delayed X-ray pulse probes its structure via Laue diffraction. Single-site mutations in the vicinity of the heme pocket docking site were found to have a dramatic effect on ligand migration. To visualize this process, time-resolved electron density maps were stitched together into movies that unveil with $<$2-{\AA} spatial resolution and 150-ps time-resolution the correlated protein motions that accompany and/or mediate ligand migration. These studies help to illustrate at an atomic level relationships between protein structure, dynamics, and function. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V19.00004: The Laser-Assisted Photoelectric Effect on Surfaces Luis Miaja-Avila, Guido Saathoff, Chifong Lei, Margaret Murnane, Henry Kapteyn, Martin Aeschlimann, John Gland The laser-assisted photoelectric effect (LAPE) in atoms is widely used for the characterization of ultrashort EUV pulses and for femtosecond-to-attosecond spectroscopy. We recently observed the equivalent process in the original manifestation of the photoelectric effect i.e. photoemission from surfaces [PRL \textbf{97, }113604 (2006)]. In our experiment, ultrafast 800nm pulses are split into probe and pump beams. The probe beam is upconverted into the EUV at 30nm using high harmonic frequency conversion. The 800nm pump beam is spatially and temporally overlapped with the EUV beam on a Pt(111) sample. A time-of-flight detector measures the kinetic energy of the photoemitted electrons. In the presence of the pump beam, these electrons can either absorb or emit an IR photon, leading to sidebands in the EUV photoelectron spectrum. These sidebands are visible as modulations near the Fermi edge. Surface LAPE will extend EUV pulse measurements to higher photon energies. It also has the potential to study ultrafast, femtosecond-to-attosecond time-scale processes in solids and in surface-adsorbate systems, where complex, correlated, electron dynamics are expected. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V19.00005: Dynamics of small polaron formation in epitaxial pentacene films Matthias Muntwiler, William Tisdale, Ehua Fan, Chad Lindstrom, Xiaoyang Zhu Using time-resolved two-photon photoemission (TR-2PPE) we observe the formation of a small polaron from electrons injected into the LUMO band of thin epitaxial pentacene films. Such observation is made directly in the energy and time domains by analysing the photoelectron after excitation by pump and delayed probe pulses from a femtosecond laser system. The LUMO level of pentacene is observed in a charge transfer process that originates from an initial state of the substrate and as such does not involve exciton formation. Due to interaction with the nuclear lattice, the energy level of the LUMO-derived polaron state relaxes by about 200 meV towards lower energy over a time interval of several hundred femtoseconds. Small polarons account for one possible mechanism of charge trapping in organic semiconductors. In our experiments, pentacene films are grown in a bulk-like structure (standing phase) on a thin film Bi(111) substrate. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V19.00006: Dynamics on nanointerfaces investigated by ultrafast electron nano-crystallography Chong-Yu Ruan, Yoshie Murooka, Ramani Kalyan Raman, Ryan Murdick The correlation between the material structures and the size-dependent properties is a fundamental problem in nanoscience. Through the development of ultrafast electron nano-crystallography and spectroscopy techniques, we have looked at some important mechanisms pertaining to the nanometer scales. To accentuate the structurally correlated transformations from bulk to the molecular length scale, we size-select and deposit nanoparticles (Au, Ag) on well characterized interfaces. Using femtosecond optical pulse as the pump and femtosecond electron pulse as the probe, the electronic and temperature driven transformations of nanostructures and phases are examined at calibrated levels on the energy landscape with atomistic spatialtemporal resolutions ($\le $ 10$^{-12}$ sec, $\le $ 0.01 {\AA}). We observed the ultrafast nonthermal phase transformations of nanoparticles with transient full-scale radial distribution function accurately determined. The coherent motions of atoms driven by nonthermal energy transfer persist into the liquid phase. But the recrystallization process is more thermal-like with interesting reconstruction of lattices from the melt, nonreciprocal to that of melting. We also examined the ultrafast molecular structural responses to charge transfer that exhibits a dynamical phase transition going from conducting to insulating phases. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V19.00007: Multiple Reference Soft X-ray Fourier Transform Holography W. Schlotter, K. Chen, R. Rick, A. Scherz, J. Stohr, J. Luning We demonstrate multiple reference Fourier transform holography with soft x-rays. This technique results in enhanced image quality without increased exposure to the sample. There are two categories of experiments where this is particularly relevant: imaging with limited intensity sources and imaging radiation sensitive samples. In Fourier transform holography a unique image of the sample object is reconstructed for each reference source that is used to record the hologram. Thus with multiple reference sources, multiple images are reconstructed with the same radiation exposure to the sample necessary for a single image. When the multiple images are formed by effectively identical reference sources the images can be averaged to enhance image quality. X-ray free electron lasers are an example of sources where single shot images must be captured with one ultrafast x-ray pulse. Since the number of photons in each pulse, incident on a nanoscale sample, is finite, high efficiency imaging is essential. To mimic a finite illumination scenario we have successfully imaged a nanoscale test object by detecting fewer than 2500 soft x-ray photons. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V19.00008: X-ray Resonant Photoemission in organic thin films. Alberto Morgante, Dean Cvetko, Albano Cossaro, Luca Floreano, Gregor Bavdek Resonant photoemission (RESPES) allows to investigate the charge transfer processes in thin films at the femtosecond time scale and it has been recently applied to thin organic films on inorganic substrates in order to obtain information related to the carrier injection at the interface, a process of great importance for organic electronic applications. High resolution RESPES on monolayer and multilayer organic films on semiconductor and metal substrates at C K-edge will be presented. It will be shown that in the monolayer range RESPES makes possible to clearly identify very weak molecular valence band structures which can't be distinguished from substrate ones in normal photoemission. Moreover the charge transfer time information will be deduced from the spectroscopic data by the comparison of monolayer and multilayer RESPES spectra. It will be also demonstrated that, due to the localized nature of the resonant process, RESPES gives a clear spatial correlation between filled and empty states and that this effect should be carefully taken into account in the analysis of the resonant spectra for the charge transfer time determination. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V19.00009: Atomic Scale Force Spectroscopy Suggests Low Reorganization Entropy For Electron Transfer J.T. Sage, A. Barabanschikov, B. Barbiellini, E.D. Coulter, D.M. Kurtz, E.E. Alp, W. Sturhahn Molecular mechanics simulations of the electron transfer protein rubredoxin suggest significant delocalization of active site vibrations, but experimental vibrational data have been successfully modelled using empirical models that exclude distant atoms. We address this question using nuclear resonance vibrational spectroscopy (NRVS), an emerging synchrotron-based technique that yields the complete vibrational spectrum of $^{57}$Fe in rubredoxin, coupled with quantum chemical vibrational predictions. The measured vibrational density of states reveals that oxidation strengthens the Fe-S bonds linking iron to the polypeptide, as expected from the reported decrease in Fe-S bond lengths. Moreover, comparison with Raman isotope shift measurements provides direct experimental evidence that the Fe-S vibrations remain localized at the active site. Vibrational predictions based on DFT calculations reproduce the observed vibrational data and confirm the localized nature of the Fe-S vibrations, although lower frequency vibrations mix significantly with the polypeptide. The Fe-S vibrations are an important component of the electron transfer reaction coordinate, and we suggest that vibrational localization may facilitate electron transfer by minimizing the reorganization entropy of the reaction. [Preview Abstract] |
Session V20: Focus Session: Superlattices and Oxides on Silicon
Sponsoring Units: DMPChair: Kristopher Andersen, Naval Research Laboratory
Room: Colorado Convention Center 105
Thursday, March 8, 2007 11:15AM - 11:27AM |
V20.00001: Tailoring the properties of artificially layered ferroelectric superlattices Matthew Dawber, Nicolas Stucki, Celine Lichtensteiger, Stefano Gariglio, Jean-Marc Triscone A key attraction of artificial ferroelectric superlattices is the potential to be able to tailor the properties of the material to a particular application. Here we demonstrate that the key ferroelectric parameters, polarization and critical temperature can be tuned over a very large range in PbTiO$_{3}$/SrTiO$_{3}$ superlattices by varying the ratio of the layer thicknesses. It is shown that the polarization can be tuned from 0-60 $\mu C/cm^{2}$ and the transition temperature from room temperature to 700$^o$C while maintaining a perfect crystal structure and low leakage currents in these heterostructures. We developed a simple model based on Landau theory that would guide straightforward production of samples with ferroelectric properties designed for particular applications. We also explore the phase transition behaviour with temperature in superlattices with very thin PbTiO$_{3}$ layers where we find not only unexpected evidence of ferroelectricity but also an unusual relationship between strain and polarization. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V20.00002: Electrostatics of superlattices by first principles Xifan Wu, Oswaldo Di\'eguez, Massimiliano Stengel, Karin Rabe, David Vanderbilt A complete theory of epitaxial perovskite superlattices requires an understanding of both epitaxial strain effects and of electrostatic boundary conditions. Here, focusing on the latter issue, we have carried out first-principles calculations of the nonlinear dielectric properties of short-period BaTiO$_3$/SrTiO$_3$ and PbTiO$_3$/SrTiO$_3$ superlattices having the in-plane lattice constant of SrTiO$_3$. In particular, we have calculated the layer polarizations $p_j$ as defined using the Wannier-based method of Wu, Di\'eguez, Rabe and Vanderbilt\footnote{X. Wu, O. Di\'eguez, K. Rabe and D. Vanderbilt, Phys.\ Rev.\ Lett.\ {\bf 97}, 107602 (2006).} for each neutral BaO, SrO, PbO, or TiO$_2$ layer, and modeled $p_j$ as a function of displacement field $D$ (which is uniform throughout the superlattice), the chemical identity of the layer itself, and the chemical identity of its near neighbors. We then test our expectation that the dependence on the identity of neighboring layers should decay rapidly with distance. If we apply a cut-off to the range of this interlayer interaction, we arrive at a model description that allows us to predict $p_j(D)$ for each layer, and thus the overall $P(D)$ (and trivially, also $P$ vs. electric field and related quantities) for a superlattice of arbitrary layer sequence. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V20.00003: Interfacial Intermixing in Ferroelectric Superlattices from First Principles Valentino R. Cooper, Karen Johnston, Karin M. Rabe Ferroelectric superlattices present a unique foundation for creating novel materials for modern devices. In ideal superlattices with perfectly flat, compositionally abrupt interfaces, first- principles studies have shown how factors such as strain due to lattice mismatches, charge compensation and bonding at the interface can be controlled to enhance the ferroelectric properties of the superlattice. In real superlattices, the presence of an additional factor, cation intermixing at the interface, is suggested by high-resolution COBRA studies\footnote{D.~D.~Fong et al. PRB \textbf{{71}}, 144112 (2005)}. As the period of a superlattice decreases, the effect of this intermixing would be expected to become increasingly important. Here, we present results of a first-principles study of the effect of interfacial intermixing on short-period $x_{\rm{PT}}$/$y_{\rm{ST}}$ superlattices. We find that the effect of intermixing on the superlattice polarization can indeed be substantial, and use first-principles information about atomic and electronic properties to interpret and model the effect. Implications for other superlattice combinations and experiments will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:27PM |
V20.00004: Understanding, enhancing and fine-tuning polar properties in multicomponent perovskite superlattices Invited Speaker: Modern epitaxial thin-film techniques make it possible to synthesize artificial multicomponent perovskite-oxide superlattices (SLs) with interfaces that are atomically flat and compositionally abrupt. The behavior of such systems is dominated by strong interactions between individual SL layers, high levels of epitaxial strain and symmetry lowering relative to the bulk. All of these factors can be manipulated in order to enhance or custom-tailor the useful polar properties --- such as polarization and piezoelectric response --- for a wide variety of technological applications. First-principles computational techniques are a tool of choice to help us understand how the strain, symmetry and composition of these complex systems influence their polar properties. However, the prohibitive computational costs associated with such simulations, growing rapidly as the period of the SL increases, make it impossible to answer some broader, more interesting questions: in particular, how could the SL layer arrangement be optimized to obtain the best possible polar properties? Here we show how first-principles calculations combined with a simple model for SL polarization and a genetic-algorithm optimization allow us to find answers to the questions mentioned above and, among other things, to predictively identify the most polar perovskite-oxide SLs that can be grown on currently available substrates. This flexible modeling procedure can be applied to a wide variety of layered perovskite-oxide nanostructures, providing guidance for experimental development of nanoelectromechanical devices with substantially improved polar properties. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V20.00005: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V20.00006: Ferroelectricity and Polarization-Dependent Tunneling in BaTiO$_{3}$ Films Below 10 Layers Arthur P. Baddorf, Junsoo Shin, Sergei V. Kalinin, Von Braun Nascimento, E. Ward Plummer Predictions for the minimum critical film thickness for ferroelectricity have continuously decreased. For BaTiO$_{3}$, ferroelectricity has previously been observed experimentally down to 12 layers and predicted by first-principles calculations in 6 layer films. We have examined BaTiO$_{3}$ ultra-thin films grown on SrRuO$_{3}$/SrTiO$_{3}$ using laser-MBE in high oxygen pressures and report evidence of a ferroelectric state at room temperature by in-situ characterization of structure, using low energy electron diffraction (LEED I-V) and by scanning tunneling spectroscopy (STS). Films produce sharp (1x1) LEED patterns, indicating well-ordered tetragonal phase structure. Comparison of observed diffraction intensities for 4 and 10 layer films at 130 and 300 K with calculated intensities reveals a vertical displacement of the central Ti, corresponding to a polarization consistent with compressive strain. Reversible polarization switching was observed locally as a jump in the electron tunneling current at +/- 2.5 V using a scanning tunneling microscope. Research was sponsored by the Division of Materials Sciences and Engineering and the Center for Nanophase Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V20.00007: Surface morphology of PbTiO$_{3}$ films on SrTiO$_{3}$ (001) K. Latifi, Carol Thompson, D. D. Fong, G. B. Stephenson, P. H. Fuoss, J. A. Eastman, F. Jiang, S. K. Streiffer, R.-V. Wang Strain relaxation is a ubiquitous process in the synthesis of heteroepitaxial films. Films deposited onto a substrate with a small lattice parameter mismatch will often initially form as coherently strained (lattice matched) layers. As the film grows and exceeds a critical thickness, the stored elastic energy is released through the creation of crystal defects such as misfit dislocations. Even before dislocation introduction, epitaxial strain can lead to morphological instabilities of the growth interface, with the formation of mounds. We use atomic force microscopy to investigate surface morphology related to strain relaxation during the epitaxial growth of PbTiO$_{3}$ films ranging from 10 nm to 385 nm in thickness on SrTiO$_{3}$ (001) substrates. The ferroelectric phase transition temperature of coherently strained PbTiO$_{3}$ films is increased, but also depends on thickness. Therefore, for some range of typical growth temperatures of organo-metallic vapor phase epitaxy, it is possible for the film to undergo a paraelectric to ferroelectric phase transition during growth. This could lead to additional mechanisms of strain relaxation becoming active during growth. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V20.00008: Ferroelectricity in ultra-thin and thick ferroelectric films of~Pb$_{0.35}$Sr$_{0.65}$TiO$_{3}$ studied by second~harmonic generation Shiwei Liu, Jacques Chakhalian, Min Xiao, Chonglin Chen Second harmonic generation (SHG) measurements were performed in the reflection geometry using the femtosecond Ti:Sapphire pulse laser at the wavelength of 810 nm for a 16-nm-thick ultra-thin Pb$_{0.35}$Sr$_{0.65}$TiO$_{3}$ film and a 243-nm-thick Pb$_{0.35}$Sr$_{0.65}$TiO$_{3}$ film, which were epitaxially deposited on (001) MgO substrates by pulsed laser ablation (PLD). It is concluded that in the ultra-thin film the ferroelectric phase is still present and a remarkably sharp ferroelectric phase transition was observed. In contrast, the thick film exhibits a pronounced diffuse phase transition. Theoretical analysis based on the polarization diagrams show the compensated c-domain fraction is dominant in both films whereas the nonlinear susceptibility of the ultra-thin film has a different tensor property from the thick film. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V20.00009: Functional Interfaces of a Crystalline Oxide on Semiconductor Fred Walker, Curt Billman, Marco Buongiorno-Nardelli, Rodney McKee Interfaces play a pivotal role in the properties of complex oxides such as polarization of ferroelectrics, band offsets of gate dielectrics and the field effect in correlated electron materials. The concept of the interface phase has guided us in understanding these diverse roles of the interface. Moreover, this concept has been especially powerful in guiding the development of processes for the heteroepitaxial growth of oxides on semiconductors. Functional substitutions of elements at a crystalline-oxide and silicon interface have been accomplished using molecular beam epitaxy of alkaline earth oxides deposited on silicon. We present capacitance-voltage measurements to spectroscopically characterize the interface states induced by changing the composition of the interface from, for example, strontium to barium silicide. Research sponsored jointly by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy at Oak Ridge National Laboratory under contract DE-AC05-00OR22725 with UT-Battelle, LLC and at the University of Tennessee under contract DE-FG02-01ER45937. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V20.00010: Surprises in the Growth of SrTiO$_3$ on Silicon: a Charged Interface and Polar Film. C. Stephen Hellberg, Kristopher E. Andersen Growth of complex oxides on silicon has been of great interest for device applications. SrTiO$_3$ has served as the prototypical system, but initial optimism has faded as well ordered epitaxial films have been difficult to achieve. Recently there have been several developments that have dramatically improved our understanding of these systems. Growth of coherent lattice-matched films has finally been achieved, and the measured expansion of the out-of-plane lattice constant exceeds the prediction of the bulk elastic constants of SrTiO$_3$ by nearly 100\%. Simultaneously, growth by a different process in thermodynamic equilibrium yields islands of SrTiO$_3$. We will present first principles density functional calculations consistent with both experiments: The energetically favored interface is electrically charged, and the film grows ferroelectrically polarized, with an accompanying out-of-plane expansion. Additionally, the films are unstable to phase separation. Methods of substitutionally doping the interface to eliminate the charge are discussed. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V20.00011: Structural and electrical properties of crystalline LaAlO$_{3}$ on Silicon (100) James Reiner, Agham Posadas, Miaomiao Wang, T.P. Ma, Charles Ahn LaAlO$_{3}$ (LAO) is a promising gate insulator material for use in future generations of silicon technology because of its relatively large dielectric constant (24) and band gap (5.5 eV). The growth of crystalline LAO on silicon is impeded, however, by interface reactions. To overcome this difficulty, we have deposited epitaxial LAO on silicon (100) surfaces by using a 2 monolayer crystalline SrTiO$_{3}$ transition layer. The growth was performed by oxide molecular beam epitaxy. The crystallinity of the structure was confirmed by in situ reflection high energy electron diffraction (RHEED), and also x-ray diffraction and transmission electron microscopy (TEM). Cross-sectional TEM shows no SiO$_{2}$ at the complex oxide-silicon interface. Metal-oxide-semiconductor (MOS) capacitors have been fabricated and measured (I-V and C-V characteristics). Inelastic electron tunneling spectroscopy (IETS) measurements have been carried out on these MOS capacitors, which also indicate the absence of SiO$_{2}$ at the interface. [Preview Abstract] |
Session V21: General Theory: Computational Quantum Monte Carlo Methods
Sponsoring Units: DCOMPChair: Jim Gubernatis, Los Alamos National Laboratory
Room: Colorado Convention Center Korbel 106
Thursday, March 8, 2007 11:15AM - 11:27AM |
V21.00001: Efficiency of Rejection-Free Monte Carlo Algorithms for Particles Systems Marta Guerra, Mark Novotny We calculated the efficiency of rejection-free methods for dynamic Monte Carlo studies of off-lattice systems. Following the methodology of Watanabe et al\footnote{\label{watanabe}H. Watanabe, S. Yukawa, M.A. Novotny and N. Ito, \textit{Efficiency of Rejection-free dynamic Monte Carlo methods for homogenous spin models, hard disk systems, and hard sphere system}, Phys. Rew. E, \textbf{74}, 026707 (2006)}, we studied $d=1$ particles models including the hard rod model, and models with both harmonic and Lennard-Jones potentials. The hard-rod results are in agreement with [1], namely the efficiency near the close-packing density $\rho_{_{cp}}$ is proportional to $(\rho_{_{cp}}-\rho)^{-d}$, where $d$ is the dimension of the system and $\rho$ the system density. We also report on the algorithmic efficiency for cases with heterogenous particles. Some results in $d=2$ will also be presented. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V21.00002: Faster QMC with Lagrange and splines William Parker, John Wilkins, Richard Hennig, Cyrus Umrigar Computing the wave function can be the most time-intensive part of a quantum Monte Carlo calculation. Orbitals represented by extended basis functions scale in evaluation time as O($N^3$) while localized basis functions scale as O($N^2$). Two methods of localizing the orbital representation are: piecewise-polynomial (pp) interpolation and transformation to a localized basis. The Lagrange form of the pp-interpolant is simple but has discontinuous derivatives at sampling points. The pp-spline is continuous in certain low derivatives. The B-spline shares the pp-spline's derivative continuity but is a transformation not an interpolant. These methods have O($N^2$) scaling at all $N$ tested (up to $N=864$). While increasing the number of sampling points of the original orbital, the total QMC energy converges to the value calculated using plane waves at similar sampling point numbers for Lagrange, pp-splines and B-splines. At fixed sampling density, the three are of comparable speed. pp-splines use $8$ memory words per sampling point, Lagrange use $5$ and B-splines use $1$. Due to smaller memory usage, B-splines are the best choice. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V21.00003: Approach toward Linear Scaling QMC Bryan Clark, David Ceperley, Eric de Sturler Quantum Monte Carlo simulations of fermions are currently done for relatively small system sizes, e.g., fewer than one thousand fermions. The most time-consuming part of the code for larger systems depends critically on the speed with which the ratio of a wavefunction for two different configurations can be evaluated. Most of the time goes into calculating the ratio of two determinants; this scales naively as O($n^3$) operations. Work by Williamson, et al. (2) have improved the procedure for evaluating the elements of the Slater matrix, so it can be done in linear time. Our work involves developing methods to evaluate the ratio of these Slater determinants quickly. We compare a number of methods including work involving iterative techniques, sparse approximate inverses, and faster matrix updating.\\ (2) A. J. Williamson, R.Q. Hood and J.C. Grossman, Phys. Rev. Lett. 87, 246406 (2001) [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V21.00004: Spinor path integral Quantum Monte Carlo for fermions Daejin Shin, Hosam Yousif, John Shumway We have developed a continuous-space path integral method for spin 1/2 fermions with fixed-phase approximation. The internal spin degrees of freedom of each particle is represented by four extra dimensions. This effectively maps each spinor onto two of the excited states of a four dimensional harmonic oscillator. The phases that appear in the problem can be treated within the fixed-phase approximation. This mapping preserves rotational invariance and allows us to treat spin interactions and fermionic exchange on equal footing, which may lead to new theoretical insights. The technique is illustrated for a few simple models, including a spin in a magnetic field and interacting electrons in a quantum dot in a magnetic field at finite temperature. We will discuss possible extensions of the method to molecules and solids using variational and diffusion Quantum Monte Carlo. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V21.00005: Accuracy of the fixed-node and pseudopotential approximations in diffusion Monte Carlo Richard G. Hennig, Julien Toulouse, Cyrus J. Umrigar Diffusion Monte Carlo is one of the most accurate methods for molecules and solids. Its accuracy is controlled by the fixed-node and pseudopotential approximations. For atoms and small molecular systems, efficient energy optimization methods enable the optimization of all parameters of many-body wave functions and systematically eliminate the fixed error. This enables our pseudopotential tests. Calculations for Si and C atoms and dimers demonstrate the importance of optimized multi-determinant trial-wave functions for chemical accuracy. The fixed-node error of the two seemingly similar dimers, Si$_2$ and C$_2$, differs dramatically with values of 0.1~eV and 1~eV, respectively. Calculations of the ionization energies and electron affinities of Si and C as well as binding energies and bond lengths for the Si$_2$ and C$_2$ for relativistic LDA, PBE and HF pseudopotentials of the Troullier-Martins, Vanderbilt and Gaussian form assess their accuracy and efficiency. The results are compared to experimental and quantum chemistry data. Reduced non-locality by larger cutoff distances and the Vanderbilt construction improve the efficiency. PBE and HF pseudopotentials result in accurate energies and HF pseudopotential are the most accurate for the dimer geometries. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V21.00006: Correction of finite-size errors in many-body electronic structure Hendra Kwee, Shiwei Zhang, Henry Krakauer Quantum Monte Carlo (QMC) calculations using simulation cells with periodic boundary conditions are subject to finite-size errors. Often, such errors are corrected or reduced by extrapolation using increasingly larger simulation cells, combined with size corrections from less accurate calculations, such as from Hartree Fock (HF) or density functional theory (DFT). Direct extrapolation is computationally costly. Size-corrections from standard HF and DFT calculations introduce additional errors and are less reliable. This has led to several recent attempts at improved methods to correct for finite-size errors. Here we develop a scheme which uses modified density functionals to estimate the finite size errors. Tests on simple solids and molecules using plane-wave auxiliary-field QMC calculations show encouraging results. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V21.00007: Wang-Landau integration --- The application of Wang-Landau sampling in numerical integration Ying Wai Li, Thomas Wuest, David P. Landau, Hai Qing Lin Wang-Landau sampling was first introduced to simulate the density of states in energy space for various physical systems. This technique can be extended to numerical integrations due to certain similarities in nature of these two problems. It can be further applied to study quantum many-body systems. We report the feasibility of this application by discussing the correspondence between Wang-Landau integration and Wang-Landau sampling for Ising model. Numerical results for 1D and 2D integrations are shown. In particular, the utilization of this algorithm in the periodic lattice Anderson model is discussed as an illustrative example. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V21.00008: Quantum Monte Carlo with short directed loops Ying-Jer Kao We introduce a new type of directed loop algorithm with short-loop generation for the stochastic series expansion quantum Monte Carlo method[1]. Short-loop algorithms have been shown to greatly improve the dynamics at low temperature in studies of classical spin ice models[2]. We will discuss the framework of this algorithm and make comparisons to the conventional directed loop algorithm in a specific quantum spin model. \newline [1]O.Suljuasen and A. W. Sandvik, Phys. Rev. E66, 046701 (2002). \newline [2]R. Melko et al., Phys. Rev. Lett. 87, 067203 (2001). [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V21.00009: Optimization of quantum Monte Carlo wave functions by energy minimization Julien Toulouse, Cyrus Umrigar We present a simple, robust and highly efficient method for optimizing all parameters of many-body wave functions by energy minimization in quantum Monte Carlo calculations. Using a strong zero-variance principle, the optimal parameters are determined by diagonalizing the Hamiltonian matrix in the space spanned by the wave function and its derivatives [1-2]. We discuss the connection with previously-proposed energy minimization methods, namely the modified Newton method [3] and the perturbative energy fluctuation potential method [4]. Application of the method to electronic atomic and molecular systems show that it systematically reduces the diffusion Monte Carlo fixed-node error. [1] J. Toulouse and C. J. Umrigar, submitted to J. Chem. Phys. [2] C. J. Umrigar, J. Toulouse, C. Filippi, S. Sorella, and R. G. Hennig, cond-mat/0611094. [3] C. J. Umrigar and C. Filippi, Phys. Rev. Lett. 94, 150201 (2005). [4] A. Scemama and C. Filippi, Phys. Rev. B 73, 241101 (2006). [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V21.00010: Systematic improvement of trial wavefunctions for Constrained Path Quantum Monte Carlo R. Torsten Clay Constrained Path Monte Carlo (CPMC) provides an approximate solution to the Fermion sign problem for lattice models such as the Hubbard model. In the zero-temperature CPMC algorithm, a trial wavefunction is used to eliminate random walkers when their overlap with the trial function becomes zero. CPMC often produces surprisingly good results for ground state energy and correlation functions, even when a simple trial function is used. However, there is no reason to expect that simple wavefunctions (free electron or Hartree Fock) will have any overlap with complex correlated ground states. We therefore describe a method to improve CPMC results by optimizing the trial wavefunction. The trial function we use is a sum of Slater determinants that is optimized by the Path Integral Renormalization Group (PIRG) procedure. The wavefunction produced by PIRG is a sum of L determinants, with an energy that is variational. We show CPMC+PIRG data for a system where CPMC with a free electron trial function fails, the Hubbard model on an anisotropic triangular lattice. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V21.00011: Generalized pairing wave functions in electronic structure quantum Monte Carlo Michal Bajdich, Lubos Mitas, Kevin E. Schmidt We investigate several types of trial wave functions with pairing orbitals in fixed-node quantum Monte Carlo. Following upon our previous study[1], we explore the possibilities of expanding the wave function in linear combinations of pfaffians. We observe that molecular systems require much larger expansions than atomic systems and linear combinations of a few pfaffians lead to rather small gains in correlation energy. Further, we test the wave function based on fully-antisymmetrized product of independent pair orbitals. Despite its seemingly large variational potential, we do not observe significant gains in correlation energy. Finally, we combine these developments with the recently proposed inhomogeneous backflow transformations[2]. The trade-offs between computational efficiency and amounts correlation energy recovered will be discussed. \newline [1] M. Bajdich et al. Phys. Rev. Lett. 96, 130201 (2006). \newline [2] N. D. Drummond et. al., J. Chem. Phys. 124, 224104 (2006). [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V21.00012: Bond breaking in auxiliary-field quantum Monte Carlo Wissam A. Al-Saidi, Henry Krakauer, Shiwei Zhang Bond stretching mimics different levels of electron correlations in the system and provides a challenging testbed for all approximate many-body computational methods. Using the recently developed phaseless auxiliary-field quantum Monte Carlo (AF QMC) method, we study the potential-energy curves of several well-known molecules --- BH, N$_2$, and F$_2$ --- and of the H$_{50}$ chain. To control the sign/phase problem, the phaseless AF QMC method constrains the random walks with an approximate phase condition that depends on a trial wave function. With single-determinant unrestricted Hartree-Fock trial wave functions, the phaseless AF QMC method generally gives better overall accuracy and a more uniform behavior than the coupled cluster CCSD(T) method in mapping the potential-energy curve. In the molecules, the use of multiple-determinant trial wave functions from multi-configuration self-consistent-field calculations is also explored. The increase in computational cost versus the gains in statistical and systematic accuracy are examined. With such trial wave functions, excellent results are obtained across the entire region between equilibrium and the dissociation limit. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V21.00013: Bond Breaking of Simple Molecules in Auxiliary-Field Quantum Monte Carlo with GVB Wave Functions Wirawan Purwanto, Henry Krakauer, Shiwei Zhang, Wissam Al-Saidi Accurate potential energy curves are an essential ingredient in understanding chemical reactions. This problem spans a wide range of correlations, with correlation effects being the most important in the bond-breaking regime. We report potential energy curves of simple molecules, including water and the carbon dimer, within the framework of the auxiliary-field quantum Monte Carlo (AFQMC) method. AFQMC projects the many-body ground-state from a trial wave function, which is also used to control the sign/phase problem. A previous calculation\footnote{Al-Saidi, Zhang, Krakauer, J. Chem. Phys. \textbf{124}, 224101 (2006)} showed that AFQMC could provide a fairly uniform description of the bond stretching of a water molecule, even with a simple unrestricted Hartree-Fock (UHF) trial wave function. We investigate the use of Generalized Valence Bond (GVB). GVB gives a better description of the molecule than UHF; so it is a simple yet efficient alternative to using a single Slater determinant trial wave function. We will compare AFQMC results with other correlated methods and the exact configuration interaction calculations. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V21.00014: ABSTRACT HAS BEEN MOVED TO N21.00014 |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V21.00015: Monte-Carlo Simulations of High-q Anti-ferromagnetic Potts Models Shafiqur Rahman, Suvarshi Bhadra, Joshua Monk Using a highly efficient cluster-flip Monte-Carlo algorithm$^{1}$, we have investigated the ordering in the five- and the six-state anti-ferromagnetic Potts models on a simple cubic lattice. Using a method developed previously$^{2 }$to examine in detail the distribution of spins on sublattices, we show that the five-state case has a phase transition only at zero temperature, and that the six-state case is disordered at all temperatures. $^{1}$ R. H. Swendsen and J.-S. Wang, Phys. Rev. Lett. 58, 86 (1987). $^{2}$ S. Rahman, E. Rush and R.H. Swendsen, Phys. Rev. \underline {B58}, 9125 (1998). * present address: Dept. of Materials Science and Engineering, Virginia Tech, VA, USA [Preview Abstract] |
Session V22: Focus Session: Jamming II
Sponsoring Units: GSNPChair: David Wu, Colorado School of Mines
Room: Colorado Convention Center 108
Thursday, March 8, 2007 11:15AM - 11:51AM |
V22.00001: A Local, Geometrical Probe for Jamming Invited Speaker: When jammed disordered materials such as Lennard-Jones systems, granulates and foams are forced externally, the resulting deformation fields exhibit large scale vortical patterns and are strongly non-affine. Here we introduce the distribution P(alpha) as a local probe of the non-affine nature of this response. Alpha denotes the angle between the bonds and the local deformations of pairs of particles in contact - hence alpha = pi/2 corresponds to particles sliding past each other, while particles squeezed together or moving apart correspond to alpha = 0 or pi. We find that near jamming, P(alpha) becomes strongly peaked around pi/2, with the width and height of the peak scaling with the distance to the jamming point. Grains then predominantly slide past each other, which signals an increasingly non-affine response of the material caused by the proximity of floppy modes. We relate this local measure to the global, anomalous scaling of the elastic moduli and contact numbers near jamming, and show the first experimental determination of P(alpha) in sheared, 2D foams. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V22.00002: A critical length scale in jammed granular media Wouter Ellenbroek, Ellak Somfai, Martin van Hecke, Wim van Saarloos Granular media consist of macroscopic, athermal particles that ``jam'' into a solid-like state when subjected to a confining pressure. Recent studies of this jamming transition in systems of frictionless particles have shown, quite remarkably, that the jamming point has many features of a critical point, exhibiting power law scalings of various quantities nearby. We study the linear response of these jammed systems to a localized mechanical perturbation. The response fluctuates over a length scale that diverges at the jamming transition, providing a direct numerical observation of a critical length scale in jammed granular media. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V22.00003: Critical Scaling at the Jamming Transition for Zero and Finite Applied Shear Stress Peter Olsson, Stephen Teitel We carry out numerical simulations to study the jamming transition of a model granular material in two dimensions at zero temperature. Behavior is simulated as a function of particle density and applied shear stress. We find a collapse of our data to scaling curves that provides evidence for a sharp 2nd order jamming transition in non-equilibrium steady states at finite shear, that ends at a critical point (point ``J'') as the shear stress vanishes. We estimate the values of the critical exponents at both zero and finite shear stress. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V22.00004: The fluid-glass transition for hard spheres John Drozd, Colin Denniston A gravity-driven hard sphere simulation is used to study the phenomena of disorder-order transitions, or simply the glass transition from a granular hard sphere fluid to a jammed glass. We find a diverging length scale and a diverging viscosity at this transition and compare our simulation to experiment on the connection between local velocity fluctuations and shear rate. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V22.00005: Correlated Percolation Models of Jamming and Glass Transitions Monwhea Jeng, Jennifer Schwarz Toninelli, Biroli, and Fisher recently introduced a model of correlated percolation called the Knight model, which they claimed to prove underwent a dynamical glass transition. This transition had novel properties, with a discontinuous jump in the order parameter, but with diverging time scales and correlation lengths. We show that their proof misidentified the critical point, so that these properties are currently unproven for this model. However, we show that these novel properties can in fact be proven for suitably modified models of correlated percolation, with qualitatively similar culling rules. We discuss the features of the models necessary for a rigorous proof to be possible. We also discuss properties of models such as the force balance model and the original Knight model, which appear to undergo novel transitions despite the lack of a rigorous proof of such a transition. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V22.00006: Fluctuations in the crossover from aging to equilibrium of a structural glass Azita Parsaeian, Horacio E. Castillo We investigate the fluctuations in the aging regime, the equilibrium regime and the crossover between them, for a simple structural glass with purely repulsive Weeks-Chandler-Anderson interactions. We characterize how the fluctuations evolve by studying the probability distributions of local observables such as individual particle displacements $\Delta x$ and intermediate scattering functions $C_r$ associated with small regions. We compare the results for probability distributions in the different regimes, and we also compare with results obtained previously for the aging regime of a glass with both repulsive and attractive interactions. We discuss the fitting of the probability distributions of local intermediate scattering functions $C_r$ by generalized Gumbel distributions, and the tails of the probability distributions of particle displacements by non-linear exponential forms. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V22.00007: Colloidal Glass Transition Observed in Confinement Kazem Edmond, Carrie Nugent, Hetal N. Patel, Eric R. Weeks We study a colloidal suspension confined between two parallel walls as a model system for glass transitions in confined geometries. The suspension is a mixture of two particle sizes to prevent wall-induced crystallization. We use confocal microscopy to directly observe the motion of colloidal particles. This motion is slower in confinement, thus producing glassy behavior in a sample which is a liquid in an unconfined geometry. We present results from a range of volume fractions. Our results demonstrate that the maximum thickness at which confinement effects still occur defines a length scale for a given particle volume fraction. This length scale increases as the volume fraction increases toward the glass transition. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V22.00008: The Manhattan Model: A simple model for glassy dynamics Prasanta Pal, Corey O'Hern, Jerzy Blawzdziewicz We study the dynamics of 1d hard rods undergoing Brownian motion in an array of narrow, multiply intersecting channels. In the current version, the junction size equals the particle size and particles are prevented from making turns at each intersection. This simple model shares many of the important features of glassy systems including kinetic arrest, cooperative and heterogeneous dynamics, and aging behavior in the high packing fraction limit. One of our key results is that the structural relaxation time and other dynamical quantities increase super-exponentially with $\phi$ and diverge at $\phi_g <\phi_{cp}$ significantly below the close-packed density. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V22.00009: Simultaneous measurements of bulk moduli and particle dynamics in a sheared colloidal glass Michael V. Massa, Christoph Eisenmann, Chanjoong Kim, David A. Weitz We present a novel study of glassy colloidal systems, using a stress-controlled rheometer in conjunction with a confocal microscope. This experimental setup combines the measurement of bulk moduli, using conventional rheology, with the ability to track the motion of individual particles, through confocal microscopy techniques. We explore the response of the system to applied shear, by simultaneously monitoring the macroscopic relaxation and microscopic particle dynamics, under conditions from the quiescent glass to a shear-melted liquid. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V22.00010: Discontinuous shear thickening, or shear jamming, of dense suspensions of uniform non-spherical particles Ryan Larsen, Jin-Woong Kim, David Weitz Discontinuous shear thickening, or shear jamming, occurs when suspensions undergo a shear-induced transition from fluid-like behavior to solid-like behavior. Because jamming is associated with geometrical confinement of the particles, it is reasonable to expect particle shape to have an effect on the jamming of suspensions. To test this dependence, we synthesize uniform polystyrene particles of dumbbell and triangle shape and compare their jamming behavior to that of equivalent spheres. We show that the non-spherical particles display more dramatic viscosity increases during jamming, and they persist in the jammed state for longer periods of time. Moreover, as the spherical particles approach the jamming transition, they oscillate stochastically between the jammed and un-jammed states on milli-second time scales, whereas the non-spherical particles display no such behavior. We rationalize these qualitative differences in jamming behavior in terms of the higher packing efficiency of non-spheres at low shear rates relative to that at high shear rates. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V22.00011: Flow fields caused by local perturbations of colloidal glasses Lov Goel, Eric R. Weeks Colloidal suspensions can be used to model atomic and molecular systems. At relatively high densities, these particle suspensions form a metastable jammed phase known as a colloidal glass; we study a series of colloidal samples close to this glass transition point. We add a small number of superparamagnetic beads to our samples so that we can perturb the colloidal system locally with an external magnetic field. We use laser scanning confocal microscopy to observe the response of the colloidal particles to this perturbation. The trajectories of several thousand particles surrounding the magnetic bead are followed to high accuracy using particle tracking techniques. The perturbed motion of colloidal particles decays exponentially away from the moving magnetic bead, thus revealing a new length scale. We measure this length scale as a function of volume fraction and applied force. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V22.00012: Signatures of critical phenomena of a filled elastomer under deformation Mindaugas Rackaitis, Xiaorong Wang Fluctuations and critical phenomena have drawn much attention for many years. But, no report anticipates that an elastomer containing fillers under gentle deformations will show similar effects. In this presentation, we show that a filled rubber system under about 2{\%} strain may display feature fluctuations that could be associated with a transition of the filler from an elastic solid state to a dispersed fluid state and that is reminiscent of critical phenomena. Besides, electrical conductivity fluctuations and their link to the mechanical fluctuations will also be discussed. In addition, we show that the fluctuation of macroscopic parameters can be related to the microscopic fluctuation of filler structures in the rubber compound and in principle it can act as a probe of what is happening physically at the microscopic scale. [Preview Abstract] |
Session V23: High Pressure VI
Sponsoring Units: DCMPChair: Renata Wentzcovitch, University of Minnesota
Room: Colorado Convention Center 110
Thursday, March 8, 2007 11:15AM - 11:27AM |
V23.00001: Potentially novel ultrahigh pressure form of ABX$_3$-type compounds Koichiro Umemoto, Renata Wentzcovitch By means of first-principles computations we have identified two new dynamically stable structures that are candidate ultra-high pressure forms of ABX$_3$-type compounds. To our knowledge, they have not been experimentally observed yet. They are produced by metastable pressure-induced transformations in $Cmcm$ NaMgF$_3$, a post-perovskite phase. The first transition to a $Pmcn$ structure is related to a soft phonon mode in post-perovskite. The second one is a regular enthalpically driven transition from $Pmcn$ to a $P6_3/mmc$ structure. In NaMgF$_3$ these phases are metastable with respect to the dissociation into CsCl-type NaF and cotunnite-type MgF$_2$. However, the $Pmcn$ phase might be observed at low temperatures. We have also identified a candidate post-perovskite material that prefers the $Pmcn$ phase over the dissociation into AX- and BX$_2$-type solids. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V23.00002: Reactive Molecular Dynamics Studies of Thermal Induced Chemistry in TATB Timothy Germann, Jason Quenneville Equilibrium molecular dynamics (MD) simulation of high explosives can provide important information on their thermal decomposition by helping to characterize processes with timescales that are much longer than those attainable with non-equilibrium MD shock studies. A reactive force field is used with MD to probe the chemisty induced by intense heating (`cook-off') of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). The force field (ReaxFF) was developed by van Duin, Goddard and coworkers [1] at CalTech and has already shown promise in predicting the chemistry in small samples of RDX under either shock compression or intense heat. Large-system simulations are desired for TATB because of the high degree of carbon clustering expected in this material. We will show results of 800-particle simulations at several temperatures, and detail current capabilities for large-scale (10$^{4}$ -- 10$^{5}$ atoms) systems carried out with the massively parallel GRASP MD software developed at Sandia National Lab. Finally, we will compare the reaction timescales with those of RDX and HMX. [1] A. C. T. Van Duin, \textit{et al}, \textit{J. Phys. Chem. A}, \textbf{1005}, 9396 (2001). [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V23.00003: Long timescale dynamics of shocked nitromethane Laurence Fried, Evan Reed, Riad Manaa, Kurt Glaesemann, John Joannopoulos We present the farthest ever glimpse behind the shock front in a chemically reactive molecular dynamics simulation by applying a multi-scale shock wave simulation technique to the study of chemical reactions in shocked nitromethane (CH3NO2) represented by the DFTB tightbinding method. Shock speeds from 5.5 km/s to 8 km/s are simulated for durations up to 0.8 ns demonstrating substantial computational savings compared with the non-equilibrium molecular dynamics (NEMD)shock simulation approach. These simulations indicate that the reaction zone in detonating nitromethane is greater than 0.3 $\mu$m in length. Ionic species are found to be prevalent in the early reactions of shocked nitromethane. Results are consistent with available experimental data. As a validation of our multiscale approach, we compare spatial wave profiles computed with the multiscale technique to profiles computed using the NEMD approach. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V23.00004: Shear strain induced structural and electronic modifications of the energetic molecular crystal 1,1-diamino-2,2-dinitroethylene Sergey Rashkeev, Maija Kuklja First-principles calculations of the structural and electronic properties of the deformed energetic molecular crystal 1,1-diamino-2,2-dinitroethylene (FOX-7) under shear-strain loading are presented. The reaction of the crystal to applied shear-strain loading is found to be highly anisotropic. When the external loading is removed, the relaxation of the system is mainly defined by stretching, bending, and rotations of the NO$_{2}$ groups of neighboring molecules. In general, the deformed molecular crystal never relaxes to its initial, ideal crystalline FOX-7 structure. Instead, different planes remain shifted relatively to each other on vectors, which are typically incommensurated with any translational vector of the ideal crystal. We also found that no metallization occurs under shear-strain loading. We suggest that the considered mechanisms of the shear-strain relaxation of the structural and electronic degrees of freedom are typical for layered anisotropic molecular crystals, and that they should significantly affect their chemical reactivity, conductivity, optical properties, and initiation of detonation in energetic materials. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V23.00005: An Infrared Study of Secondary Explosives under High Pressure. Brian Yulga, Michael Pravica, Zhenxian Liu, Oliver Tschauner, Malcolm Nicol We report synchrotron FTIR and far infrared measurements on PETN, RDX, HMX and TATB at ambient temperature and high pressure, using various media for pressurization of the samples. In all cases, we have carefully studied any phase transitions in the 0 - 15GPa pressure range and have cycled pressures to interrogate sample survivability and reproducibility of the phase sequences. For PETN, we used differing pressurizing media (Ar and KBr) and have found that the onset of a prior-reported phase transition around 5GPa varies with the different media, portending the importance of sheer stress in inducing some or all of this phase transitions. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V23.00006: Vibrational spectra of solid HNFX (C$_6$H$_8$F$_8$N$_8$O$_4$): Experiments and theory Malcolm Nicol, Cedric Gobin, Eunja Kim Assignment of the vibrational spectra of molecular solids such as HNFX is very complex. We have made a combined experimental and modeling study of the vibrational spectra of solid HNFX. Crystalline HFNX consists of unit cell with 9 HNFX molecules in Ci symmetry. Vibrational modes were calculated by using the PCFF force field method and were directly compared to measured IR and Raman spectra. A complimentary calculation for molecular HNFX allows us to identify the intramolecular motions measured in experiments. Intermolecular motion by F--H bonds between HNFX molecules will be discussed in this talk. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V23.00007: High Pressure Behavior in Hydrated Metal Hexafluorosilicates M(H2O)6SiF6 Alice Acatrinei, Monika Hartl, Luke Daemen, Jianzhong Zhang, Yusheng Zhao The octahedral (6-fold) coordination is highly unusual in Si chemistry, making hexafluorosilicates (HFS) interesting from a structural standpoint. It has been observed that Si coordinates to O octahedrally at extremely high pressures deep in the Earth mantle, but no compound possessing this property is known to be thermodynamically stable at ambient conditions. We suggest that HFS could act as surrogate materials to study Si in this coordination state in hydrated materials. Transition metal HFS exhibit a variety of structural transitions and magnetic properties changes when pressure is applied. Some structural phases and phase transitions exhibited by some compounds at ambient pressure can occur in other compounds at high pressure only. We examined the behavior of Zn(II) as well as Cd(II) and Hg(II) HFS hexahydrates as a function of pressure and temperature, and their possible role in understanding structural phase transitions in HFS. Measurements were performed between 0-18 GPa at room temperature. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V23.00008: On the High-Pressure Behavior of Titanium Hydride Patricia E. Kalita, Stanislas Sinogeikin, Kristina E. Lipinska-Kalita, Thomas Hartmann, Andrew Cornelius Hydrogen storage research has recently invested a great deal of efforts into investigations of metal hydrides. Although titanium hydride is not the ideal candidate for storing hydrogen, Ti hydrides can act as active species to catalyze the reversible dehydrogenation of other hydrides and carbon nanotubes. In addition the basic science interest of this project lies in investigating the structure and especially the high-pressure behavior of TiH2. In the present study, we show the first in situ, high-pressure angle-dispersive and energy dispersive synchrotron x-ray diffraction studies of titanium hydride. We investigate the effects of hydrostatic and non-hydrostatic conditions. We also show the results of structural refinements as well as the bulk modulus of TiH2. To the best of our knowledge, this work is the first attempt to measure the equation of state of TiH2 using synchrotron x-ray diffraction and diamond anvil cells. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V23.00009: Pressure tuned phonon mode splitting in magnetic frustrated spinel ZnCr$_{2}$O$_{4}$ Tao Zhou, Zhenxian Liu, Chenglin Zhang, Sang-Wook Cheong ZnCr$_{2}$O$_{4}$ has cubic spinel structure. Below 390 K, the geometrically frustrated magnet enters a paramagnetic state. Below 12.5 K, it undergoes a first-order phase transitions, resulting into an antiferromagnetic order and a structural distortion simultaneously. An IR-active phonon related to the Cr$^{3+}$ ion's motion undergoes a splitting at 12.5 K. This transition is explained as a spin-Peierls like transition. However, the exact cause and effect in such a transition is not clear. Is it because the lattice undergoes transition first, spin just follows, or is it spins' interaction that forces the lattice to undergo changes? Pressure can provide a crucial service in clarifying this issue, since pressure can change spin and lattice interactions in different ways, it can differentiate these two scenarios. We have measured the infrared absorption spectra of ZnCr$_{2}$O$_{4}$ under pressure. Our data shows that Tc, at which the spin-Peiers like transition occurs and the phonon at about 370 cm$^{-1}$ starts to show the splitting, increases from its ambient pressure value of 12.5 K to about 15.8 K at 1 GPa. This provides an important clue for the exact nature of this transition. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V23.00010: Optical properties of CdSe semiconductor nanocrystals under high pressures Chi-Tsu Yuan, Wu-Ching Chou, Der-San Chuu In general, the physical properties of semiconductor nanocrystals are different from the bulk materials. CdSe nanoparticles are attracted much attention due to excellent fluorescence properties for potential applications in biological labels$^{1}$. In particular, the emission colors can be tuned to cover whole visible range by changing particle size with the same chemical composition. On the other hand, high pressure technique is another tool to tune the electronic states of crystalline materials. Incorporated colloidal QDs under high pressure environment can provide valuable information to study the electronic and vibrational states of nanometer size materials. In this study, the electronic and vibrational states of colloidal core/shell CdSe/ZnS quantum dots are studied at room temperatures by using high pressure optical measurements. Pressure dependent quadratic lattice behavior can be observed clearly from photoluminescence (PL) and Raman spectra up to $\sim $7 GPa. This quadratic relationship is consistent with theoretical prediction. The average pressure coefficients for PL and Raman measurements, as well as deformation potential are 32 meV/GPa, 4.2 cm$^{-1/}$GPa and -1.69 eV, respectively. [1] M. Jr Bruchez, M. Moronne, P. Gin, S. Weiss, and A. P. Alivisatos, Science \textbf{281}, 2013 (1998). [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V23.00011: High Pressure Characterization of the thermoelectric Bi2Te3 Matthew Jacobsen, Ravhi Kumar, Andrew Cornelius One of the current goals of the US Department of Energy is to find a new energy source that is non-reliant on fossil fuels for the production of energy. In an effort to address this concern, we have developed facilities to investigate the fundamental properties of thermoelectric materials utilizing the technique of pressure tuning. Pressure tuning, or application of high pressures, causes a material to undergo distinct, but controllable changes, to the physical properties. To this end, results of reference standards will be presented along with the first comprehensive set of data on the thermoelectric materials Bi2Te3. [1]Chen, G. International Materials Reviews, \textbf{48}, 45-66 (2003). [2]Jacobsen, M.K., Masters Thesis, in preparation, (2006). [3]Khvostansev, L.G. Phys.Stat.Solids A, \textbf{71}, 49-53 (1982). [4]Venkatasubramanian, R. Nature, \textbf{413}, 597-602, 2001. [5]Vereshchagin, L.F. et.al. Soviet Physics-Solid State, \textbf{13}, 2051-2053 (1972). [6]Yamashita, O. J.Mat.Sci.,textbf{40},6439-6444 (2005). [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V23.00012: Pressure dependance of the Curie temperature of TbNi$_2$Mn, investigated using designer diamond anvils Damon Jackson, Scott McCall, Samuel Weir, Dave Young, Qiu Wei, Yogesh Vohra \\ TbNi$_2$Mn is a cubic Laves structured material with a Curie temperature at ambient pressure of $T_C=151$~K. The behavior of the Curie temperature has been investigated by AC magnetic susceptibility under both hydrostatic and non-hydrostatic conditions using designer diamonds up to 29~GPa, for which it was found to decrease at $dT_C/dP$=-2.0~K GPa$^{-1}$. However, non-hydrostatic conditions result in a flattening out of $T_C$ with pressure which is non-reservable. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V23.00013: Spectroscopic evidence for pressure-induced metallization in solid silane Xiao-Jia Chen, Viktor V. Struzhkin, Alexander Goncharov, Yang Song, Zhen-Xian Liu, Ho-kwang Mao, Russell J. Hemley Infrared reflectance measurements on solid silane SiH$_{4}$ have been performed under pressure up to 70 GPa at room temperature. After passing through three phase transformations, solid SiH$_{4}$ is already black at 30 GPa. At high pressures around 60 GPa, the infrared reflectance spectra exhibit a Drude metallic behavior, signaling the pressure-induced metallization in solid silane. Angle-dispersive powder x-ray diffraction studies reveal that a structural transition is accompanying the silane transition to the metallic state. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V23.00014: Search for superconductivity in LiBC at high pressure Amy Lazicki, Choong-Shik Yoo, Hyunchae Cynn, William J. Evans, Warren E. Pickett, Justin Olamit, Kai Liu, Y. Ohishi Lithium borocarbide, which is a structural and electrical analog to high-T$_c$ superconductor MgB$_2$, remains insulating at ambient conditions due to atomic alternation in the crystal structure. We investigated experimentally and theoretically the properties of this material under pressure, including structural and bonding anisotropy and the possibility of metallization and superconductivity under high pressure. It is found to remain stable up to 60 GPa with no crystal structure change and without a previously reported lattice parameter anomaly. In this crystal structure, metallization is not predicted to occur until at least 345 GPa, at which pressure the electronic bands responsible for superconductivity in MgB$_2 $ remain unoccupied in LiBC, ruling out the possibility of a new MgB$_2$-like high pressure superconductor. [Preview Abstract] |
Session V24: Phase Transitions in Polymeric Systems II
Sponsoring Units: DPOLYChair: Kristopher Lavery, National Institute of Standards and Technology
Room: Colorado Convention Center 201
Thursday, March 8, 2007 11:15AM - 11:51AM |
V24.00001: Phase Transition of Long Chain Normal Alkanes in Confined Geometry Invited Speaker: Encapsulated normal ($n-)$ alkanes can provide well-defined model systems for studying the complex crystallization behaviors of polymers. The size controllable microcapsules with different surface morphology (SEM) have been prepared in the present work, which provide ideal confined geometry for the crystallization research of long chain $n$-alkanes. Crystallization and phase transition behaviors of long chain $n$-alkanes (from C18 to C21) in microcapsules were studied with the combination of differential scanning calorimetry (DSC), X-ray diffraction (XRD) as well as Fourier transform infrared spectroscopy (FTIR). As evident from the DSC measurement, a surface freezing monolayer, which is formed in the microcapsules before the bulk crystallization, induces a novel metastable rotator phase (RII) on the cooling process of microencapsulated $n-$nonadecane. We argue that the existence of the surface freezing monolayer provides the ideal nucleation sites, and consequently decreases the nucleating potential barrier of RII phase and turns the transient RII phase to a ``long-lived'' metastable phase. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V24.00002: Supercritical Fluid-Assisted Electrospinning of Polymers Mark McHugh, Manuel Marquez, Zhihao Shen, Jun Liu, Sanho Lee In this talk we describe the application of near-critical and supercritical CO$_{2}$ as an electrospinning processing aid to create fibers with novel morphology readily varied by adjusting the operating pressure and temperature. We demonstrate the application of CO$_{2}$ to electrospin poly(vinyl pyrrolidone) (PVP) from PVP-dichloromethane (DCM) solutions. The formation of PVP fibers is directly related to the DCM-CO$_{2}$ phase behavior since the electrospinning operating pressures are well below those needed to dissolve neat PVP in CO$_{2}$. In addition, when spinning into a CO$_{2}$-rich bath, an open-cell fiber morphology is created with features that correlate with the operating pressure. We emphasize in the talk that extreme pressures are not needed to tailor a specific morphology when using CO$_{2}$. The effective removal of the solvent from the polymer solution depends on the partitioning of the liquid solvent between the PVP-rich phase and the CO$_{2}$-rich phase so the thermodynamics of polymer-SCF solvent phase behavior plays a significant role in this process. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V24.00003: Origin of the Difference in Order-Disorder Transition \mbox{Temperature} Between Polystyrene-block-Poly(2-vinylpyridine) and Polystyrene-block-Poly(4-vinylpyridine) Copolymers Jin Kon Kim, Dong Hyun Lee, Sung Hyun Han, Weibin Zha, Chang Dae Han, Jin Ho Kang, Cheol Park The order-disorder transition temperatures (T$_{ODT}$) of polystyrene-block-poly(2-vinylpyridine) (S2VP diblock) and polystyrene-block-poly(4-vinylpyridine) (S4VP diblock) copolymers were determined using oscillatory shear rheometry and small-angle X-ray scattering (SAXS). It has been found that for comparable molecular weight and block composition, the T$_{ODT}$ of S4VP diblock copolymer is exceedingly high compared with that of S2VP diblock copolymer. This observation is attributable to the stronger polarizability of P4VP in S4VP diblock copolymer compared with the polarizability of P2VP in S2VP diblock copolymer. With the SAXS profiles obtained in the disordered state of low-molecular-weight S2VP and S4VP diblock copolymers, we determined, with the aid of the Leibler theory, the temperature-dependent segmental interaction parameter ($\alpha$) for PS/P2VP and PS/P4VP pairs. The values of $\alpha$ for PS/P2VP pair were found to be much smaller than those for PS/P4VP pair. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V24.00004: Stucture and dynamics of a microphase separating block copolymer melt. Amish Patel, Nitash Balsara, Suresh Narayanan, Alec Sandy, Simon Mochrie A block copolymer melt is quenched from the disordered state to a temperature ($T_{q})$ below the order-to-disorder transition ($T_{ODT})$. The structure and dynamics of the melt is studied as a function of quench time using Small-Angle X-ray Scattering (SAXS) and X-ray Photon Correlation Spectroscopy (XPCS) respectively. For $T_{q}$ well below $T_{ODT}$, we observe that the broad SAXS peak corresponding to the disordered state transforms into a sharp peak indicating the formation of the ordered state. The ordering process is accompanied by an increase in the XPCS relaxation time. On the other hand, for $T_{q}$ close to $T_{ODT}$, the static structure factor remains unchanged and the melt remains disordered. However, even though the structure remains unchanged, the XPCS relaxation time increases and then reaches a plateau. Finally, we have shown that microphase separation can be stimulated in the above case of $T_{q}$ close to $T_{ODT}$. In order to do so, the melt is first taken to a temperature, $T'< T_{q}$, where microphase separation would eventually occur. After holding the melt at $T'$ for a short period of time, it is taken to $T_{q}$. At this time the melt is still disordered. However, in contrast to a direct quench to $T_{q}$, this procedure eventually results to the formation of a micrphase-separated state. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V24.00005: Pressure Dependence of Block Copolymer Phase Transition in Selective Solvent. Yongsheng Liu, Rama Bansil, Milos Steinhart Synchrotron based small angle x-ray scattering (SAXS) was used to study the pressure dependence of the order-disorder transition (ODT) in a 30{\%} (w/v) solution of a diblock copolymer of poly(styrene -- isoprene) (SI 18-12, Polymer Source) in diethylphthalate (DEP), a selective solvent for the PS block. This diblock copolymer solution undergoes an order-disorder transition from the face-centered-cubic (FCC) phase to the disordered phase upon increasing the temperature. We will describe the design of the pressure instrument, sample cell and details of the experimental set up. The pressure applied to the sample is up to 0.35 GPa (3.5 kbar). The preliminary results show that the ODT temperature increases at about 20C/kbar with pressure. The spacing of the FCC lattice increases with pressure. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V24.00006: Nanoporous Materials Formed in Condensed Carbon Dioxide William Edmonds, Timothy Lodge, Marc Hillmyer We propose a strategy utilizing condensed carbon dioxide as a selective--solvent for creating nanoporous materials from block copolymer templates. Cylinder--forming polystyrene--$b$--polylactide ($f_{PLA}$ = 0.37) monoliths were annealed in carbon dioxide at constant temperature and various solvent densities. The swollen structures were then quenched at low temperature isochorically. Small-angle X-ray scattering measurements indicated the domain spacing increased with increasing CO$_{2}$ density. This result is consistent with the formation of cylindrical pores within the intact polylactide domains, a conclusion confirmed by scanning electron micrographs of the processed monoliths. This controlled, non-destructive technique allows for creating tunable pore structures from a single block copolymer. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V24.00007: Morphological transformation and mesostructure formation in diblock copolymer blends Kishore Tenneti, Xiaofang Chen, Christopher Li, Lixia Rong, Benjamin Hsiao We report the phase structures of a diblock copolymer (BCP) (PS- b-P4VP) blended with bent-core mesogen (BE3-10). High blending concentrations (1:1) resulted in phase separation. However, smaller concentrations resulted in the formation of complexes between the BE3-10 and the BCP. At moderate blending ratio of 1:5, the BCP morphology transformed from a lamellar to hexagonal cylindrical structure and the BE3-10 formed Sm layers that are oriented perpendicular to the BCP interface. At a blending ratio of 1:10, the sample showed remarkable enhancement in the lamellar ordering. In all the blends, the d- spacing was smaller than the original BCP. It is proposed that Sm layer formation and the corresponding chain stretching are responsible for this behavior. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V24.00008: Salt Complexation in Cleavable Polystyrene-b-poly (ethylene oxide) Thin Films Ling Yang, Mingfu Zhang, Serkan Yurt, Matthew Misner, E. Bryan Coughlin, D. Venkataraman, Thomas Russell, Benjamin Ocko, Xuefa Li Salt complexation in a newly developed cylinder forming Polystyrene-b-polyethylene (PS-b-PEO) with a cleavable linker between two blocks was investigated. Highly oriented, close-packed arrays of nanoscopic cylindrical domains with a high degree of long-range lateral order was obtained in the copolymer thin films during solvent annealing and solvent evaporation at relatively low humidity or extremely dry condition. The orientation and lateral ordering of cylindrical microdomains were found to strongly depend on salt concentration. With the addition of certain amount of salt, we observed that the copolymer microdomains remained ordered at high degree of swelling by in-situ grazing incidence small angle X-ray scattering. With the ability to rapidly cleave the PEO block through the designed cleavable linker under mild condition, highly ordered porous structures are readily generated. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V24.00009: Modeling of self-assembly in diblock copolymer and particle mixtures using self-consistent field theory Seung Ha Kim, Eric Cochran We use self-consistent field theory (SCFT) to investigate the spatial preference of particle position in the self-assembly of lamellae-forming diblock copolymer and spherical nanoparticle mixtures. We calculate the free energy of the system, which is dependent on the interaction parameter between B segments and particles ($\chi _{BP})$, the effective particle volume fraction ($\phi _{eff})$, and the ratio of the particle diameter to block copolymer domain spacing (d$_{p}$/d$_{BCP})$. We show that both small and large particles segregate to the intermaterial dividing surface (IMDS), whereas only for intermediate values of d$_{p}$/d$_{BCP}$ particles are located at the center of the domain. These results are interpreted as a subtle consequence of the competition between enthalpic polymer-particle interactions and the chain packing frustration imposed by the particulate inclusion. These findings are useful for understanding the mechanism of spontaneous assembly in block copolymer nanocomposites, and will assist in the rational design of systems where control of the particle placement is crucial. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V24.00010: Effect of solvent vapor type on evolution of thin film morphology of block copolymer-nanoparticle composites. Deepali Palta, David Bucknall In this study we report the effect that different solvent vapors have on the resultant phase morphology of thin films of poly(styrene-butadiene-styrene) triblock copolymer (SBS) and the effect this has on the ordering when mixed with nanoparticulate inclusions. When exposed to different saturated solvent vapors, the surface morphology of the film changes depending on both the solvent vapor type and the exposure time. When the SBS film spun cast from cyclohexanone is exposed for 48 hours to vapors of the same solvent, the as made film morphology of in-plane cylinders changes to cylinders that are perpendicular to the substrate. These cylinders possess an almost perfect degree of crystallographic ordering over lateral dimensions of several microns. By contrast solvent annealing in chloroform or toluene which are also both relatively good solvents for both the blocks, the degree of order observed in cyclohexanone is not observed. In this presentation we describe the evolution of the phase morphology of these films as a function of film thickness ($<$1000 nm), type of solvent vapor, exposure time to the saturated vapors and percentage content of 3 different types of 1-50 nm particles (gold, FePt, and C60). [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V24.00011: An SCFT Study of Nanostructuring in Epoxy Thermosets Folusho Oyerokun, Glenn Fredrickson, Ludwik Leibler Increasing fracture resistance of epoxy thermosets via self assembly of block copolymers has generated significant interest in the past decade. Nanostructuring occurs because of the selectivity of the epoxy precursors to the different blocks of the block copolymer, i.e. one block is miscible (before curing), while the other block remains immiscible. The size and geometry of the nanostructures formed depends on the copolymer composition, solvent concentration and selectivity. Understanding the conditions at which nanostructuring occurs is important for rational design of high impact thermoset materials. A self-consistent field based study of blends of polystyrene-b-polybutadiene-b-polymethylmethacrylate (SBM) triblock and the reactive solvent DGEBA has been performed. At low copolymer concentration, the theory predicts micelles of PS and PB in the PMMA/DGEBA matrix. Increasing the copolymer concentration above a threshold value leads to formation of core-shell cylindrical or ``sphere-on-sphere'' morphologies, depending on the length of PB midblock. The theoretical predictions are in reasonable agreement with experiments. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V24.00012: Fluctuation effects and stability of the Fddd phase in diblock copolymer melts Bing Miao, Robert Wickham Tyler and Morse recently found that an Fddd network phase is stable over a narrow region in the diblock copolymer phase diagram, within mean-field theory. However, in the weak segregation regime, where the Fddd phase is stable, it is well-known that composition fluctuations significantly modify the results of mean-field theory, including the phase diagram. We investigate the effect of composition fluctuations on the stability of the Fddd phase self-consistently at the one-loop level within the framework of the theory of Fredrickson and Helfand. The stability of the Fddd phase relative to the neighbouring lamellar, cylindrical, and gyroid phases will be discussed. [Preview Abstract] |
Session V25: Organic Based Magnetism and Organic Spintronics
Sponsoring Units: DPOLY DMPChair: Bin Hu, University of Tennessee
Room: Colorado Convention Center 203
Thursday, March 8, 2007 11:15AM - 11:27AM |
V25.00001: Spin Response in Organic Spin-Valves based on LSMO Electrodes Fujian Wang, Cungeng Yang, Z. Valy Vardeny, Xiaoguang Li We fabricated spin-valves made of organic semiconductor (OSEC) thin films sandwiched between ferromagnetic La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO) and cobalt electrodes, using several OSEC small molecules. We measured the temperature (T) and voltage bias (V) dependence of the spin-valve related giant magneto-resistance (GMR) effect. We found a universal GMR decrease with T, where the GMR completely diminishes at $\sim $ 250K regardless of the OSEC layer. We show evidence that the underlying mechanism for the GMR decrease with T is the decrease in the spin injection capability of the LSMO electrode. We also found that the GMR steeply decreases with V, and is asymmetric respect to the applied voltage direction. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V25.00002: Spin Valve Effects in Hybrid Organic-Inorganic Devices Yaohua Liu, Taegweon Lee, Howard E. Katz, Daniel H. Reich Magnetoelectronic devices based on organic semiconductors (OSC) hold promise due to the long spin coherence in these materials and the ability to tune relevant properties such as carrier mobility and interface barriers via organic synthesis. We have studied spin valve effects in vertical geometry organic-based devices, using Fe and Co as the bottom and top electrodes. Several different organic semiconductors, including Perylenetetracarboxylic dianhydride (PTCDA) and the previously studied Alq3, have been used as the spin transport layers. At low temperatures, up to 5\% positive hysteretic magnetoresistance (MR) has been observed at low field in devices with semiconductor thickness of 140 nm, which is much larger than the tunneling limit. The MR decreases as the bias voltage or current increases. Possible mechanisms for spin-polarized transport in these devices and prospects for synthesis of materials with improved performance will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V25.00003: Morphology Influenced Properties in Organic Semiconducting Thin Films for Spin-Valves J. Sheung, M. Teague, C.R. Hughes, S. Mitrovic, N.-C. Yeh The physical and electronic properties influenced by the morphology in organic thin films of tris(8-hydroxyquinoline) aluminum (Alq$_3$) are investigated systematically. This material is of interest for spintronics as the tunneling barrier in spin valves and for optoelectronics because it exhibits electroluminescence. In particular, in vacuo vapor deposited thin films are studied by atomic force microscopy, scanning tunneling microscopy and tunneling spectroscopy to determine the spatially resolved correlation of the electronic properties with the morphology. The contributions of various coulombic and thermodynamic parameters to achieving smooth and monolayer thick Alq$_3$ films are also investigated. Additionally, various high Curie temperature ferromagnets, including La$_{1-x}$Sr$_x$MnO$_3$, are explored as the spin-polarized electrode for optimized spin and charge transport properties in the organic/ferromagnetic heterostructures. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:27PM |
V25.00004: Dipolar-Biased Tunneling of Magnetization in Crystals of Single Molecule Magnets Invited Speaker: The molecular cluster Mn12 has attracted much interest as a single-molecule magnet (SMM) and as a multi-redox system. It has a high-spin ground state of $S$=10 and a strong uniaxial magnetic anisotropy, and the combination of the two natures makes an effective potential barrier between the up and down spin states. At low temperatures, the magnetization curve exhibited a hysteresis loop and the quantum tunneling of magnetization (QTM). In the present work, we studied the structure and magnetic properties of the mixed-metal SMM, Mn11Cr, through the analysis of Mn11Cr/Mn12 mixed crystal. High-frequency EPR spectra were well explained by assuming that Mn11Cr was in a ground spin-state of $S$=19/2 with nearly the same EPR parameter set as for Mn12. QTM in Mn11Cr was observed with the same field interval as for Mn12. The magnetization of Mn11Cr and Mn12 in the mixed crystal can be independently manipulated by utilizing the difference between their coercive fields. The resonance fields of QTM in Mn11Cr are significantly affected by the magnetization direction of Mn12, suggesting the effect of dipolar-biased tunneling. Besides SMM, we would also like to report the unusual magnetic properties of spherical hollow nanomagnets, the electrical properties of heterocyclic thiazyl radicals, and their possible applications in spintronics and organic electronics. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V25.00005: Regioregular polythiophene based spintronic devices: effect of interface Ronald Osterbacka, Sayani Majumdar, Himadri Majumdar, Reino Laiho, Pekka Laukkanen, Juhani Vayrynen Polymeric spin valves have been fabricated using regio-regular (poly 3-hexylthiophene) (RRP3HT) as the spacer layer sandwiched between La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) and Co electrodes. The devices show high spin valve magnetoresistance (MR) at 5K (80{\%}) which reduces at room temperature to 1.5{\%}. The spin valve behavior is quite similar to a magnetic tunnel junction although the non-magnetic spacer layer ($\sim $100 nm) is much thicker than the tunneling limit. We attribute this behavior to the formation of a thin spin-selective tunneling interface between LSMO and RRP3HT caused by the chemical bonding between RRP3HT and LSMO as observed by x-ray photoelectron spectroscopy measurement. Deliberate destruction of the spin injecting interface by the introduction of a monolayer of organic insulators between LSMO and RRP3HT reduces the spin injection. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V25.00006: Ferrimagnetic resonance study on photo-induced magnetism in hybrid magnetic semiconductor V(TCNE)$_x$, $x \sim 2$ film Jung-Woo Yoo, R. Shima Edelstein, D. M. Lincoln, A. J. Epstein The V(TCNE)$_{x}$, $x\sim2$ is a fully spin-polarized magnetic semiconductor, whose magnetic order exceeds room temperature ($T_ {c}$ $>$ 350 K), and electronic transport follows hopping mechanism through the Coulomb energy split $\pi^{\ast}$ subband. In addition, it was determined that this material has thermally reversible persistent change in both magnetism and conductivity driven by the optical excitation [1]. Here, we report detailed investigation on photo-induced magnetism in V(TCNE)$_x$ by employing ferrimagnetic resonance (PIFMR) study with an in-situ light illumination. Upon optical excitation ($\lambda \sim$ 457.9 nm), the FMR spectra display substantial change in their linewidth and resonance field. Angular dependence analyses of line shift indicate the increase of unixial anisotropy field in the film caused by the light irradiation. The results demonstrated that the change in overall magnetic anisotropy by the illumination plays an important role in inducing photo- induced magnetism in (TCNE) class magnet. \newline \newline [1] J.-W. Yoo, et al. to be published in Phys. Rev. Lett. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V25.00007: On the Mechanism Causing Large Room-Temperature Magnetoresistance in OLEDs Y. Sheng, T. Nguyen, G. Veeraraghavan, J. Rybicki, O. Mermer, M. Wohlgenannt We report on the experimental study of a recently discovered, large room-temperature magnetoresistance effect in sandwich devices comprised of nonmagnetic electrodes and various organic semiconductor thin films. The effect reaches up to 10{\%} in a magnetic field of 10 mT at room temperature and saturates at fields larger than several tens of milliTeslas. In materials with strong spin-orbit coupling the characteristic magnetic field scale shifts to fields that are 10-100 times larger, consistent with the spin-orbit coupling strength. Our experiments therefore show that the organic magnetorestive effect is caused by spin-dynamics, possibly induced by the hyperfine interaction. We discuss two recently proposed models to explain the organic magnetorestive effect, which are based on spin-dependent exciton formation and spin-dependent hopping, respectively. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V25.00008: Molecular Beam Epitaxy Growth of Organic Spin Valves K. Pi, W. Wang, R. Thamankar, Y. Chye, Y. F. Chiang, Y. Li, R. K. Kawakami Spin-polarized transport across organic semiconductors has recently been demonstrated in organic spin valves consisting of ferromagnet(FM)/Alq$_3$/FM trilayers [a]. Organic semiconductors are interesting for spintronics due to their low spin-orbit coupling (which could in principle be tuned via chemical synthesis) and opto-electronic coupling to spin. We are utilizing molecular beam epitaxy (MBE) deposition through shadow masks to fabricate Co/Alq$_3$/Fe devices on MgO(001) substrates. Furthermore, we have developed the capability to perform variable temperature magnetotransport measurements without removing the sample from UHV. We find that for Alq$_3$ thickness below 100 nm, the sample exhibits linear I-V curves indicating a short across the Alq$_3$ layer, consistent with previous studies [a]. This is likely due to the interdiffusion of Co as the top electrode is deposited onto the Alq$_3$. By employing cryogenic techniques during top electrode growth, we are able to reduce the effects of Co diffusion and the formation of pinholes. We will report our progress on the in situ magnetotransport measurements to investigate spin-polarized transport in our devices. \linebreak (a) Z. H. Xiong, et.al. Nature 427, 821 (2004). [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V25.00009: Boosting quantum efficiency of single layer organic light emitting device by doping CoFe magnetic nanoparticles. Chengjun Sun, Yue Wu, Zhihua Xu, Bin Hu, Jian-Ping Wang, Jian Shen The effects of doping magnetic CoFe nanoparticles on electroluminance (EL) efficiency vs. current density, and current density vs. applied voltage for single layer organic light emitting devices (OLEDs) have been investigated. The electron trap densities increased with the increase of CoFe dopants, resulting in a high trap-filled limit (TFL) threshold voltage and significant enhancement EL efficiency ($\sim $27{\%}). The EL efficiencies were further improved ($\sim $5-7{\%}) by applied magnetic field. These improvements could be attributed to the enhancements of the ratios of the formation of excitons, and singlets to triplets, respectively and simultaneously. A maximum 32{\%} enhancement combing the two effects in EL efficiency has been achieved. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V25.00010: Spin dynamics of photoexcited polarons in MEH-PPV: optically detected magnetic resonance studies Cungeng Yang, Zeev Vardeny, Eitan Ehrenfreund We studied the \textit{full dynamics }of the photoluminescence detected magnetic resonance (PLDMR), photoinduced absorption (PA), and PA detected magnetic resonance (PADMR) in MEH-PPV films, as a function of microwave power, P, and modulation frequency, f$_{M}$. We found it critically important to measure both in-phase and quadrature components; otherwise key characteristics of the dynamics are not unraveled. For example, the PLDMR in-phase component \textit{changes sign }at f$_{M }=f_{0}$ of about 30 kHz before decaying at higher frequencies. In contrast the quadrature PLDMR component retains its sign within the same experimental frequency range. We account for these peculiar dynamics by a model in which the polaron recombination is spin dependent (SDR). Specifically by solving the SDR rate equations we found that it correctly explains the PLDMR frequency dependent phase, and reproduce the obtained increase of $f_{0}$ with $P$. Also the SDR model explains equally well the PADMR and PLDMR time resolved measurements. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V25.00011: Role of triplet polaron pairs in conjugated polymer photophysics Elizabeth Wesely, Lewis Rothberg, Alfred Marchetti, Shaw Chen, Yanhou Geng, Sean Culligan We measure the decay of the long-lived fluorescence of a conjugated oligofluorene at temperatures from 300 K to 20 K. We conclude that nearly all of this emission arises from geminate recombination of photogenerated polaron pairs to reform the singlet exciton, and that charge pair recombination represents a significant contribution to the overall fluorescence quantum yield. The unusual nonmonotonic decay dynamics of the delayed fluorescence can be explained if we assume interconversion between singlet and triplet polaron pairs on the submicrosecond time scale. ($\sim $500 ns.) We are able to model the decay of the delayed fluorescence by assuming activated recombination from a Gaussian energy distribution of singlet polaron pairs centered 0.2 eV below the excited state and having a standard deviation of 0.12 eV. The model is relevant to recent work involving the measurement of singlet-triplet branching ratios and to the yields of electroluminescent devices. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V25.00012: Efficient plastic scintillators utilizing phosphorescent dopants. Ian Campbell, Brian Crone We demonstrate improved light yield from plastic scintillators utilizing a phosphorescent dopant to collect both singlet and triplet excitations created by ionizing radiation. We specifically considered poly(vinyltoluene) and poly(9-vinylcarbazole) doped with an Ir phosphor. We present the spectral, temporal, and integrated yield response as a function of dopant concentration to pulses of 10 keV electrons. Both doped plastics yield a maximum light output $\sim$ 200{\%} of anthracene with decay times $<$ 850 ns. High light yield was obtained for Ir element fractions up to $\sim $ 10 wt{\%} implying that these scintillators may be useful for gamma detection. [Preview Abstract] |
Session V26: Focus Session: Charge Transport in Nanostructures II
Sponsoring Units: DCPChair: Hongkun Park, Harvard University
Room: Colorado Convention Center 205
Thursday, March 8, 2007 11:15AM - 11:51AM |
V26.00001: Electronic transport in semiconductor nanowires: physics studies and possible device applications Invited Speaker: Semiconductor nanowires are attractive for physics as well as for applications due to the highly ideal character of their electronic and structural properties. We grow our III-V nanowires by what can be described as guided self-assembly, by which we can accurately control location as well as dimensions of epitaxially nucleated nanowires. The level of control of growth allows controlled formation of axial as well as radial heterostructures. I will describe studies of charge transport via single, double and multiple quantum dots positioned inside InAs/InP nanowires. Such studies have allowed detailed studies of the addition of electrons one-by-one, from the very first electron into an empty quantum dot to the addition of up to 50 electrons. By replacing the one-dimensional emitter by a small quantum dot in a double-dot configuration, the discrete character of the injecting state allows ever more detailed spectroscopic studies of the charge additions to the second dot. Comparisons will be made with transport through quantum dots defined by tunnel barriers induced via gating techniques. Finally, a recently developed technique for the formation vertical wrap-gate field-effect transistors around InAs nanowires will be described, suggesting interesting opportunities for the realization of high-speed and low-power transistors and circuits. The geometrical design of such nanowire wrap-gate field-effect transistors, offers exciting ways of formation of ultra-short transistor gate-lengths as well as the use of heterostructures to further enhance the performance of such devices. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V26.00002: Charge Transport in Semiconductor Nanocrystal Solids Dmitri Talapin, Elena Shevchenko, Jong Soo Lee, Jeffrey Urban, David Mitzi, Christopher Murray Self-assembly of chemically-synthesized nanocrystals can yield complex long-range ordered structures which can be used as model systems for studying transport phenomena in low-dimensional materials [1]. Treatment of close-packed PbSe nanocrystal arrays with hydrazine enhanced exchange coupling between the nanocrystals and improved conductance by more than ten orders of magnitude compared to native nanocrystal films [2]. The conductivity of PbSe nanocrystal solids can be switched between n- and p-type transports by controlling the saturation of electronic states at nanocrystal surfaces. Nanocrystal arrays form the n- and p-channels of field-effect transistors with electron and hole mobilities of 2.5 cm$^{2}$V$^{-1}$s$^{-1}$ and 0.3 cm$^{2}$V$^{-1}$s$^{-1}$, respectively, and current modulation I$_{on}$/I$_{off} \quad \sim $10$^{3}$-10$^{4}$. The field-effect mobility in PbSe nanocrystal arrays is higher than the mobility of organic transistors while the easy switch between n- and p-transport allows realization of complimentary circuits and p-n junctions for nanocrystal-based solar cells and thermoelectric devices. [1] E. V. Shevchenko, D. V. Talapin, N. A. Kotov, S. O'Brien, C. B. Murray. \textit{Nature} \textbf{439}, 55 (2006). [2] D. V. Talapin, C. B. Murray. \textit{Science} \textbf{310}, 86 (2005). [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V26.00003: Tuning the Height of the Tunnel Barrier in Colloidal Semiconductor Nanoparticle Films Venda Porter, Scott Geyer, Jonathan Halpert, Moungi Bawendi, Tamar Mentzel, Marc Kastner Much of the work in the field of charge transport through arrays of semiconductor nanoparticles has focused on improving the conductivity by tuning the organic ligand spacer between particles. We present a study in which we enhance the conductivity in nanoparticles films by instead tuning the energetic height of the tunnel barrier by removing the organic ligand spacer and tuning the inorganic shell around each particle. Experimentally, we modify the height of the tunnel barrier by depositing an array of core/shell nanoparticles and burning away all of the organic ligands. The height of the tunnel barrier is now the energy difference between the conduction band of the core and the conduction band of the shell, rather than the much larger energy difference between the conduction band of the nanoparticle and the LUMO of the organic ligand. In addition, this method may reduce the impact of surface states on conductivity as the shell may provide better passivation than organic ligands that may leave surface trap sites unbound. These unpassivated sites can trap charge carriers, lowering the mobility in nanoparticles films. The reduction of charge trapping is also critical to raising the efficiency of nanoparticle solar devices. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V26.00004: Transport measurements of semiconductor nanocrystals and nanorods using nanoscale devices. Claudia Querner, Michael D. Fischbein, Marija Drndic Efficient charge transport through nanocrystal arrays is important for many applications in electronics or optoelectronics. Various parameters can influence the transport in nanocrystals. Beside the material itself, other parameters such as shape (spherical, rod-shaped or branched structures), surface capping (insulating or electroactive surface-ligands), as well as the nanocrystal assembly may affect the observed transport phenomena. We carry out transport measurements of semiconductor nanoparticles using devices made on silicon nitride membranes. This approach enables both transport measurements and device imaging using high-resolution transmission electron microscopy, allowing a direct correlation of the measured transport phenomena with the local structure of the nanomaterial on the device surface. We will discuss the effects that we observe by varying parameters such as shape, size and surface capping of the nanoparticles, in particular CdSe, as well as temperature and photo-excitation. 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] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V26.00005: Electron Transport in Arrays of Lead Selenide Nanocrystals Tamar Mentzel, Venda Porter, Scott Geyer, Sophie Charpentier, Moungi Bawendi, Marc Kastner We report on measurements of electron transport in self-assembled arrays of PbSe nanocrystals (NCs). NCs $\sim $8 nm in diameter are colloidally synthesized and drop cast onto an inverted field effect structure. The NCs self assemble into hexagonal close-packed arrays with $\sim $1.5 nm interdot spacing after annealing. The field-effect device enables us to measure the dependence of current on gate voltage (V$_{g})$ as well as source-drain voltage (V$_{ds})$. At high temperature we find that the conductance is exponentially dependent on both V$_{ds}$ and temperature. At low temperature the conductance is still exponentially dependent on V$_{ds}$, but is independent of temperature indicating a tunneling mechanism. While the conductance is independent of V$_{g}$ at high temperatures, it decreases with V$_{g}$ at low temperature suggesting that holes are the dominant carriers. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V26.00006: Charge Transport in Magnetite Nanoparticle Arrays Seongjin Jang, Hao Zeng Charge transport properties of magnetite (Fe$_{3}$O$_{4})$ nanoparticle arrays were studied as a function of annealing conditions. These arrays were prepared by self-assembling chemically synthesized nanoparticles with micro-gaps between lateral electrodes. Annealing removed surfactant molecules and varied the interparticle spacing systematically. Arrays annealed under 200 $^{o}$C are insulating. Arrays annealed between 200 $^{o}$C to 500 $^{o}$C show thermally assisted tunneling behavior, with the tunneling barrier decreasing with increasing annealing temperatures. Above 500 $^{o}$C, a transition from tunneling to hopping mechanism is observed. Magnetoresistance decreases with increasing annealing temperature. For the hopping samples, Verway transition is observed from both the resistivity and magnetoresistance measurements. Work supported by NSF DMR 0547036 [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V26.00007: Single Step Growth and Low Resistance Interconnecting of Metallic Nanowires Bret Flanders, Birol Ozturk We present an innovative approach to nanowire growth and interconnecting with external circuitry. Depositing salt-solution over a pair of on-chip electrodes and applying an alternating voltage induces the growth of metallic nanowires between the electrode tips. The voltage-signal provides sensitive control over the metal deposition process. For example, precise specification of the nanowire-diameter is attained through the frequency $\omega $ of the alternating voltage that induces the wire-growth process. For indium wires, increasing $\omega $ from 0.5 to 3.5 MHz increases the growth velocity of the wires from 11 to 78 $\mu $m/s and reduces their diameter from 770 to 114 nm. Gold wires exhibit diameter-tunability that extends below 100 nm. By the feedback-controlled application of the alternating voltage, it becomes possible to produce electrode-nanowire-electrode assemblies with contact-resistances of less than 25 $\Omega $, which would not be possible were the voltage terminated manually. This combination of capabilities enables study of the intrinsic transport properties of metallic nanowires. An area of particular interest is the contribution of electron-surface scattering to the total resistivity, an effect that is expected to increase with decreasing diameter. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V26.00008: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V26.00009: Electronic-structure and quantum conductance of pristine and defective graphene layers and ribbons Giovanni Cantele, Young-Su Lee, Domenico Ninno, Nicola Marzari Graphene has recently emerged as a fascinating alternative to carbon nanotubes as a subject both of fundamental research and of promising technological applications. In this work, we study the electronic structure and the transport properties of graphene layers and of graphene ribbons in the presence of several defects - from vacancies to topological defects to substitutional impurities. Very large systems with random distributions of defects are treated fully from first-principles and with chemical accuracy thanks to a formulation that combines density-functional theory and maximally-localized Wannier functions \footnote{Young-Su Lee et al, Phys. Rev. Lett. 95, 076804 (2005)}. Our results are also compared with previous tight-binding calculations, when available. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 2:03PM |
V26.00010: Theory of transport through molecular magnets Invited Speaker: Quantum transport through single molecular magnets (SMM) is starting to become a new exciting field in molecular spin electronics. Recent experiments [1,2] have shown that magnetic excitations can be identified in transport measurements and that NDC effects and complete current suppression can be explained by charge dependent anisotropies. Recent theoretical investigations [3,4,5] are presented which demonstrate fingerprints of quantum tunneling of magnetization (QTM). For weak tunneling, the violation of spin-selection rules leads to the occurence of fake resonances with temperature-dependent position [3]. For strongtunneling, it is show that a pseudo spin-1/2 Kondo effect is induced by QTM. If the Kondo temperature T$_K$ is smaller than the distance to excited magnetic states, selection rules depending on spin and symmetry of the SMM are derived for the Kondo effect to occur [4]. If T$_K$ exceeds the anisotropy barrier, it is shown that a reentrant Kondo effect can be induced by application of a longitudinal magnetic field for SMM with half-integer or integer spin [5]. This effect can be used for transport spectroscopy of the various anisotropies characterizing a SMM. \newline \newline [1] H.B. Heersche et al., Phys. Rev. Lett. 96, 206801 (2006). \newline [2] Moon-Ho Jo et al., Nano Lett. 6, 2014 (2006). \newline [3] C. Romeike, M.R. Wegewijs, H. Schoeller, Phys. Rev. Lett. 96, 196805 (2006). \newline [4] C. Romeike, M.R. Wegewijs, W. Hofstetter, H. Schoeller, Phys. Rev. Lett. 96, 196601 (2006). \newline [5] C. Romeike, M.R. Wegewijs, W. Hofstetter, H. Schoeller, to be published in Phys. Rev. Lett., cond-mat/0605514. [Preview Abstract] |
Session V27: Focus Session: Computational Nanoscience VIII - Nanotransport, Contact and Conduction
Sponsoring Units: DMP DCOMPRoom: Colorado Convention Center 301
Thursday, March 8, 2007 11:15AM - 11:51AM |
V27.00001: Understanding Molecular Conduction: Old Wine in a New Bottle? Invited Speaker: Molecules provide an opportunity to test our understanding of fundamental non-equilibrium transport processes, as well as explore new device possibilities. We have developed a unified approach to nanoscale conduction, coupling bandstructure and electrostatics of the channel and contacts with a quantum kinetic theory of current flow. This allows us to describe molecular conduction at various levels of detail, -- from quantum corrected compact models, to semi-empirical models for quick physical insights, and `first-principles' calculations of current-voltage (I-V) characteristics with no adjustable parameters. Using this suite of tools, we can quantitatively explain various experimental I-Vs, including complex reconstructed silicon substrates. We find that conduction in most molecules is contact dominated, and limited by fundamental electrostatic and thermodynamic restrictions quite analogous to those faced by the silicon industry, barring a few interesting exceptions. The distinction between molecular and silicon electronics must therefore be probed at a more fundamental level. Ultra-short molecules are unique in that they possess large Coulomb energies as well as anomalous vibronic couplings with current flow -- in other words, strong non-equilibrium electron-electron and electron-phonon correlations. These effects yield prominent experimental signatures, but require a completely different modeling approach -- in fact, popular approaches to include correlation typically do not work for non-equilibrium. Molecules exhibit rich physics, including the ability to function both as weakly interacting current conduits (quantum wires) as well as strongly correlated charge storage centers (quantum dots). Theoretical treatment of the intermediate coupling regime is particularly challenging, with a large `fine structure constant' for transport that negates orthodox theories of Coulomb Blockade and phonon-assisted tunneling. It is in this regime that the scientific and technological merits of molecular conductors may need to be explored. For instance, the tunable quantum coupling of current flow in silicon transistors with engineered molecular scatterers could lead to devices that operate on completely novel principles. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V27.00002: Ab initio Green's function method and Boltzmann averaging for electrical conductance of a single molecular junction Tomofumi Tada, Arihiro Tawara, Toshiya Matsuyama, Satoshi Watanabe, Satoru Tanibayashi, Hideo Sekino The electrical conductance through benzene-dithiolate (BDT) between gold electrodes is studied using ab initio Green's function method coupled with GAUSSIAN 03 [1]. To simulate break junction experiments of BDT [2], we consider the energetic stability of transient structures of the BDT junction and possibility of the fluctuation among several structures, because these points have not been examined yet in spite that they may affect measurements results considerably. For this purpose, the most probable conductance of BDT is estimated by taking an average using Boltzmann factor [3]. The averaged conductance shows good agreement with the observed conductance and also shows a flat plateau just before the break of the junction, which is also observed by Xiao et al. [2]. We further investigate solvent effects on conductance by including solvent molecules between electrodes. The results for solvent effects will be presented at the meeting. [1] T. Tada et al., J. Chem. Phys. 121, 8050 (2004). [2] Xiao et al., Nano Lett. 4, 267 (2003). [3] S. Tanibayashi et al., Chem. Phys. Lett. 428, 367 (2006). [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V27.00003: The current-voltage characteristic of a metal-molecule-metal junction studied by an integrated and piecewise thermal equilibrium approach Y.-H Tang, T.-H Lu, M.-H. Tsai The current-voltage characteristic of a metal-molecule-metal junction has been studied by a new approach. The Au electrodes are modeled by 3-layer (111) films and the self-assembled monolayer (SAM) of Au-benzene-1,4-dithiol-molecule-Au molecules is sandwiched between them. The non-equilibrium electron distribution function is approximated by the Fermi-Dirac distribution function with a position dependent chemical potential to reflect spatial variation of the local electrostatic potential. The electronic states of the whole Au-film-SAM-Au-film system are calculated and are regarded as standing waves, which can be decomposed into +z and --z moving waves, $\Psi_{+}$ and $\Psi_{-}$, respectively, where z is the coordinate normal to the films. The current per molecule is obtained from the standard quantum mechanical current densities of the $\Psi_{+}$ and $\Psi_{-}$ states. With this approach the calculated I-V characteristic is improved substantially with respect to those obtained by the conventional transmission-probability-Green-function type approaches.. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V27.00004: First-Principles Investigation on Atomic and Electronic Transport in Ag-Ag$_{2}$S-Ag Zhongchang Wang, Takuya Kadohira, Tomofumi Tada, Satoshi Watanabe A novel atomic switch using Ag-Ag$_{2}$S-Ag heterostructure has seized a wide range of attentions recently. Its switching mechanism, however, has not been understood sufficiently. As a first step to clarify the mechanism, we investigated migration pathways of Ag ions and activation energies for the migration in Ag$_{2}$S, and then examined the interface structures, electronic states and electric properties of the Ag-Ag$_{2}$S-Ag system, using the density functional theory. The calculated activation energies for the migration are between 0.31 to 0.50 eV, which are comparable to the experimental values of 0.43 to 0.48 eV. The calculated transmission coefficient of Ag-Ag$_{2}$S-Ag at the Fermi level increases from 0.04 before atomic relaxation to 0.455G$_{0}$ after relaxation, which shows the opening of a conduction channel in the relaxed structure. Further analysis of atomic configuration in the relaxed structure shows formation of a chain-like arrangement of Ag in Ag$_{2}$S. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V27.00005: GW electronic Correlations in Quantum Transport : Renormalization and finite lifetime effects on real systems Pierre Darancet, Andrea Ferretti, Didier Mayou, Valerio Olevano We present an {\it ab initio} approach to electronic transport in nanoscale systems which includes electronic correlations through the GW approximation. With respect to Landauer approaches based on density-functional theory (DFT), we introduce a physical quasiparticle electronic-structure into a non-equilibrium Green's function theory framework. We use an equilibrium non-selfconsistent $G^0W^0$ self-energy considering both full non-hermiticity and dynamical effects. The method is applied to a real system, a gold mono-atomic chain. With respect to DFT results, the conductance profile is modified and reduced by to the introduction of diffusion and loss-of-coherence effects. The linear response conductance characteristic appear to be in agreement with experimental results. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V27.00006: Non-equilibrium Transport in Carbon based Adsorbate Systems Joachim F\"urst, Mads Brandbyge, Kurt Stokbro, Antti-Pekka Jauho We have used the Atomistix Tool Kit(ATK) and TranSIESTA[1] packages to investigate adsorption of iron atoms on a graphene sheet. The technique of both codes is based on density functional theory using local basis sets[2], and non-equilibrium Green's functions (NEGF) to calculate the charge distribution under external bias. Spin dependent electronic structure calculations are performed for different iron coverages. These reveal adsorption site dependent charge transfer from iron to graphene leading to screening effects. Transport calculations show spin dependent scattering of the transmission which is analysed obtaining the transmission eigenchannels for each spin type. The phenomena of electromigration of iron in these systems at finite bias will be discussed, estimating the so-called wind force from the reflection[3]. [1] M. Brandbyge, J.-L. Mozos, P. Ordejon, J. Taylor, and K. Stokbro. Physical Review B (Condensed Matter and Materials Physics), 65(16):165401/11-7, 2002. [2] Jose M. Soler, Emilio Artacho, Julian D. Gale, Alberto Garcia, Javier Junquera, Pablo Ordejon, and Daniel Sanchez-Portal. Journal of Physics Condensed Matter, 14(11):2745-2779, 2002. [3] Sorbello. Theory of electromigration. Solid State Physics, 1997. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V27.00007: First-Principles Analyses of Capacitance and Conductance of Atomic Point Contacts Michiko Tanaka, Shinnosuke Furuya, Satoshi Watanabe Conductance of atomic point contacts has been actively examined. On the other hand, there are only few investigations on their capacitance in the atomic scale, and thus its behavior has not been understood sufficiently yet. Here, we examine the capacitance of atomic point contacts together with their conductance. To calculate electronic states under applied bias voltages self-consistently, we adopt the boundary-matching scattering-state density functional method developed by our group. We investigate three models: (a) Two Al (100) electrodes with attached Al pyramidal clusters, (b) Al electrode with an attached Al pyramidal cluster and flat Al electrode, and (c) two flat Al electrodes. We found that the behavior of capacitance depends on structure: In (a) and (c), the capacitance first increases and then decreases with the increase in inter-electrode distance, while it shows a monotonic decrease in (b). For the conductance, our calculation reproduces the observed behavior that during stretching processes the conductance increases just before breaking of the contact in (a). [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V27.00008: Atomic dimer shuttling and two-level conductance fluctuations in Nb nanowires Robert N. Barnett, Chun Zhang, Alexei Marchenkov, Zhenting Dai, Uzi Landman We describe density-functional structural optimization and conductance calculations which were carried out to explain high-resolution conductance measurements of niobium nanowires. In particular, the observed bistability manifesting itself as telegraph noise in the measured conductance is associated with the formation of a niobium dimer between the opposing electrodes, with the dimer shuttling between symmetric, high-conductance, and asymmetric, low-conductance, configurations. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V27.00009: Electronic Structure of Metal-Semiconductor Nanocontacts Denis Demchenko, Lin-Wang Wang Future nanoelectronics will depend on the electron/hole transport in a nanostructure and across nanostructure/metal electrode interfaces. Measurements of nanoscale transport are often conducted by contacting a semiconductor nanostructure with large metallic electrodes. Theoretical interpretation of such experiments, however, is often based on electronic structure of an isolated nanostructure, ignoring the influence of the electrodes. Here we address this issue by calculating the classical electrostatic polarization potential $P(r)$, and incorporating it into the atomistic pseudopotential method, to calculate the electronic structure of experimentally-relevant sizes of nanorods. We calculate several electrode/nanorod geometries, with varying contact depths. We show that the presence of an electrode can produce localized electron and hole states near the electrode. The localization is caused by the spatial variation of the $P(r)$. We have calculated the effects of the applied bias necessary to overcome the electron/hole localization, as well as the change of the band gap and the binding energy of the localized state as functions of the nanorod-electrode separation. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V27.00010: Benchmark Quantum Monte Carlo Calculations of Optical gaps of carbon Nanotubes Fernando Reboredo, Paul Kent Optical properties of single wall carbon nanotubes SWCNT have attracted considerable experimental and theoretical attention because they are strongly dependent on the details of the atomic structure (chiral vector). In these systems electronic correlations have been shown to play a dominant role both theoretically [1] and experimentally [2] as electron-electron interactions are increased in low dimensions. In this talk we present ongoing calculations of the optical gaps and quasi-particle energies of SWCNT with an alternative ab-initio technique: Diffusion Quantum Monte Carlo (DMC). We take advantage of a novel algorithm based on non-orthogonal localized orbitals that allows almost linear scaling calculations for $\sim $1000 electrons. DMC is a complementary technique to methods based on the GW approximation and the Bethe-Salpeter equation avoiding strong approximations. While the full absorption spectra cannot be obtained with DMC, we provide accurate benchmark values for the quasiparticle energy gaps and exciton binding energies. Research sponsored by the Division of Materials Sciences and Engineering, U. S. DOE, under Contract DE-AC05-00OR22725 with UT-Battelle, LLC. and by~ the Division of Scientific User Facilities, U. S.~ DOE. This work used resources of the NCCS at ORNL. [1] C. S. Spataru, PRL \textbf{92}, 077402 (2004). [2] Z. Wang PRL 96, 047403 (2006) [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V27.00011: First-principles study of effects of metallic electrode contacts on transport properties of carbon nanotubes Nobuhiko Kobayashi, Taisuke Ozaki, Kenji Hirose Towards a development of constructing nanometer-scale devices, considerable effort has been made in experiments using carbon nanotubes for fabricating nanoscale field-effect transistors. To detect electric signals, electrodes must be connected to the conductors. Contact with the electrodes sensitively influences the transport properties. Therefore, we have studied the transport properties on the basis of the detailed electronic state calculation that includes the effect of contact with the electrodes. We have investigated quantum transport in carbon nanotubes bridged between metallic electrodes. The electronic states are calculated using a numerical atomic orbital basis set in the framework of the density functional theory, and the conductance is calculated using the Green's function method. We have analyzed transport properties of the finite size of carbon nanotubes bridged between Al, Au, Pt, Pd metallic electrodes, and discuss the contact effect of the electrodes on the transport properties. We reveal their dependency on the electrode materials. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V27.00012: Nano-helix based quantum transistor, charge pump and motor Xiaoliang Qi, Shoucheng Zhang We propose several novel device concepts based on nano-scale helical wires. Applying a static electric field transverse to the helical wire induces a metal to insulator transition, enabling the construction of a new type of transistor switch. The band gap is purely determined by the applied transverse voltage, and can be continuously tuned. The resulting light-emitting-diode can emit light with a tunable color spectrum. With a quadrupolar electrode configuration, the electric field could rotate in the transverse plane, leading to a quantized dc charge current proportional to the frequency of the rotation. Such a device could be used as a new standard for the high precession measurement of the electric current. The inverse effect implies that passing an electric current through the helical wire in the presence of a transverse static electric field leads to a mechanical rotation of the helix. This effect can be used to construct nano-scale electro-mechanical motors. Finally, our methodology also enables new ways of controlling and measuring the electronic properties of helical biological molecules such as the DNA. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V27.00013: Low temperature transport properties of semiconducting nanocrystal arrays Andreas Glatz, Igor Beloborodov, Valerii Vinokur We study the electron transport in semiconducting nanocrystal arrays at temperatures $T\ll E_c$, where $E_c$ is the charging energy for a single grain. In this temperature range the electron transport is dominated by co-tunneling processes. We discuss both elastic and inelastic co-tunneling and show that for semiconducting nanocrystal arrays the inelastic contribution is strongly suppressed at low temperatures. We also compare our results with available experimental data. [Preview Abstract] |
Session V28: Focus Session: Carbon Nanotubes: Chemistry
Sponsoring Units: DMPChair: Philip Collins, University of California, Irvine
Room: Colorado Convention Center 302
Thursday, March 8, 2007 11:15AM - 11:27AM |
V28.00001: Resonance Raman spectroscopy of length fractionated single-walled carbon nanotubes A. Hight Walker, J.A. Fagan, B.J. Bauer, E.K. Hobbie, J.R. Simpson In many potential applications of single-walled carbon nanotubes (SWNTs), the difficulty of separating nanotubes by their structural properties, \textit{e.g.}, length and chirality, remains an impediment to their widespread implementation. Our studies include HiPco, CoMoCat, and arc-discharge SWNTs wrapped with 30-mer 5'-GT(GT)$_{13}$-3' single-stranded DNA and dispersed in solution. These samples display an exceptionally low degree of SWNT bundling and clustering, in the limit of nanodispersion.\footnote{J. A. Fagan \textit{et al.}, J. Phys. Chem. B (in press).} Size-exclusion chromatography collects length fractions ranging in size from $< 100\,$nm to $\approx 400\,$nm. Multi-angle light scattering, AFM, and TEM characterize the length distribution of each fraction. We measure resonance Raman spectroscopy (RRS) over a wide range of laser excitation wavelengths for vibrational modes including the radial breathing mode (RBM) and higher order graphite modes. All modes exhibit a monotonic increase of Raman scattering intensity with increasing nanotube length. We discuss these results in terms of an optical scattering model. Furthermore, we hybridize these length separated fractions with complimentary DNA sequences functionalized with nanoparticles to study the effects of DNA-wrapping on SWNT properties. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V28.00002: Modeling of the SWNT-DNA complexes in the water solution Alexey A. Tsukanov, Eugene A. Grachev, Slava V. Rotkin It is known that the single-wall nanotubes (SWNTs) may form a hybrid with a single-stranded DNA having a regular helical structure of the DNA wrap around the SWNT cylinder. Such DNA wrapping creates a periodic potential at the NT surface, which results in developing a specific modulation of the NT bands. Numerical self-consistent modeling of these effects requires knowledge of the polarization of the environment. We have shown that the result is very sensitive to what extent the exterior water (and ions in the solution) are polarized to screen the potential of the DNA. Both the NT screening and the response of the environment are important to include self-consistently to obtain quantitative results. We present the Monte-Carlo simulation of the interaction of a NT, a DNA and a solvent and provide heuristic physics interpretation of the results. We show that the NT screening is different from what one expects for a metal or insulator material due to non-local Coulomb correlations. An effective dielectric screening of the water exterior is extracted from the simulations. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V28.00003: Electronic structure of single-walled carbon nanotubes inside helical DNA wraps Stacy Snyder, Slava Rotkin Single stranded DNA can helically wrap a single-walled carbon nanotube (SWNT) leading to changes in electronic structure, which is the subject of our study. Other charged polymers may produce band gap modulation similar to that observed for DNA-SWNT complexes. For these hybrids we assume a regular helical wrap, the potential of which breaks the symmetry of the pristine SWNT. Band structure changes are modeled quantum mechanically using the tight binding method together with self-consistent electrostatics. Gap modulation and band structure symmetry-lowering effects may result in variation of the optical spectra, especially for (slightly forbidden) transverse optical transitions. The effect of environmental screening of charges is investigated. Self-consistent electrostatic calculations yield cohesion energy between a charged, regular wrap and a SWNT of the order of tenths of eV per DNA base [1]. [1] Snyder, S. E., and Rotkin, S. V., Polarization Component of Cohesion Energy in Single-Wall Carbon Nanotube-DNA Complexes, JETP Letters 84, 348 (2006). [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V28.00004: Aminoacid Functionalization and Raman Characterization of DWNT Moreno Meneghetti, Gabriele Marcolongo, Giorgio Ruaro, Vincenzo Amendola, Jessica Alfonsi, Marina Gobbo Carbon nanotubes are difficult to manipulate because of their aggregation and low reactivity. For this reason many types of functionalization have been obtained and, usually, large functionalizations are needed. However, in particular considering single wall carbon nanotubes (SWNT), a large functionalization modifies their electronic properties because it introduces a large number of defects states. To overcome this problem we have considered double wall carbon nanotubes (DWNT) which can be considered as SWNT protected by an external carbon nanotube. We have performed an oxidation and a functionalization of DWNT covalently linking charged aminoacids. From the Raman characterization of the functionalized nanotubes we find that the external nanotubes have been modified by the functionalization but not the internal ones. We think that this is an interesting approach to obtain carbon nanotubes which are easy to manipulate but with electronic properties, in this case of the internal nanotube, which are preserved. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V28.00005: Directed Linking of Carbon Nanotubes with CdSe Quantum Dots and Au Nanoparticles Kathryn Leach, Todd Krauss As circuit miniaturization continues, the demand for smaller and more efficient component parts has increased. Metallic single-walled carbon nanotubes (SWNTs) are the ideal nanometer-scale wire, as they can withstand current densities up to 2 to 3 orders of magnitude higher than copper currently used in electronic chips. These conductive nanotubes can therefore be utilized as ``nano-electrodes'' to efficiently electrically contact another nanoscale object, such as a single semiconductor quantum dot (QD) or metallic nanoparticle (NP), thus creating macroscopic integrated systems based on nanometer-scale components. Although NPs have been previously attached to NTs, the attachment scheme was uncontrolled; direct and defined attachment of NPs to SWNTs remains elusive. We have designed a strategy for directed assembly of fabricated QD--SWNT devices. NTs were grown across patterned catalyst islands on a silicon wafer followed by electrode placement. After cutting the NTs, the resulting carboxylic group moieties found at the cut NT edges were used to covalently attach CdSe QDs or Au NPs. Electrostatic force microscopy (EFM) and transport measurements were used to monitor NT conductivity before and after cutting, as well as after NP attachment. The photoelectrical transport properties of a typical hybrid QD--SWNT device will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V28.00006: Size Selective Interaction of Single Wall Carbon Nanotubes with Collagen Sanjib Bhattacharyya, Jean-Paul Salvetat, Debdulal Roy, Marie-Louise Saboungi One of the big challenges in using single-wall carbon nanotubes (SWNTs) in nanotube{\-}electronics at the present time is to produce SWNT's of specific diameters. Unfortunately, it is almost impossible to achieve this by existing synthesis procedures. All these produce SWNT's with a mixture of diameters and chiralities and, therefore, different electrical properties such as semiconducting and metallic. Here, we propose a method of functionalization that selects SWNTs of a single specific diameter from a mixture of tubes. We have shown that denaturation of collagen type-I solution in the presence of sodium dodecyl sulphate (SDS) and SWNT's leads to wrapping of carbon nanotubes of a specific diameter by collagen peptides, which are soluble in water. Separation is achieved by centrifugation of the solution at 10,000 RPM and taking the supernatant, which is rich in nanotubes having one specific diameter. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 1:03PM |
V28.00007: Optical modulation of single walled carbon nanotubes Invited Speaker: Recent advances in the spectroscopy of single walled carbon nanotubes have significantly enhanced our ability to understand and control their surface chemistry, both covalently and non-covalently. Our work has focused on modulating the optical properties of semiconducting single walled carbon nanotubes as near infrared photoluminescent sensors for chemical analysis. Molecular detection using near-infrared light between 0.9 and 1.3 eV has important biomedical applications because of greater tissue penetration and reduced auto-fluorescent background in thick tissue or whole-blood media. In one system, the transition of DNA secondary structure modulates the dielectric environment of the single-walled carbon nanotube (SWNT) around which it is adsorbed. The SWNT band-gap fluorescence undergoes a red shift when an encapsulating 30-nucleotide oligomer is exposed to counter ions that screen the charged backbone. We demonstrate the detection of the mercuric ions in whole blood, tissue, and from within living mammalian cells using this technology. Similar results are obtained for DNA hybridization and the detection of single nucleotide polymorphism. We also report the synthesis and successful testing of near-infrared $\beta$-D-glucose sensors2 that utilize a different mechanism: a photoluminescence modulation via charge transfer. The results demonstrate new opportunities for nanoparticle optical sensors that operate in strongly absorbing media of relevance to medicine or biology. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V28.00008: Targeted Damage of Carbon Nanotubes Luke Donev, Paul McEuen One possible pathway to fabricating a narrow strip of graphene would be to cut open a carbon nanotube. To that end, we present a preliminary procedure for selectively damaging carbon nanotubes. Nanotubes of diameter 3-10 nm were grown from iron nanoparticles and electrical contacts were lithographically applied to make transistor devices several microns long. To selectively damage the nanotubes a region was opened in a blanket of photoresist over the tube. A thin aluminum oxide layer (2-3 nm) was deposited at an angle in the exposed region to partially protect the nanotube. The nanotubes were then briefly exposed to an oxygen plasma. After the plasma etch and removal of the photoresist and aluminum oxide, a fraction of the nanotubes no longer conduct and others have higher resistance. For some of the nanotubes with increased resistance there is a height difference between the damaged and undamaged sections and scanned gate microscopy shows enhanced sensitivity in the etched region. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V28.00009: {\em Ab Initio} Study of Crosslinking of Functionalized Carbon Nanotubes Igor Vasiliev, Seamus A. Curran We investigate the mechanism of covalent crosslinking between carbon nanotubes functionalized with thiocarboxylic and dithiocarboxylic esters. The structures of interconnected nanotubes are modeled in the framework of density functional theory combined with the pseudopotential approximation. Our calculations reveal an important role of surface defects in the formation of chemical bonds connecting nanotubes to each other. The strength and stability of intertube bonds increases in the vicinity of defect sites. The computed binding energies and potential energy profiles of linked nanotubes are found to be sensitive to the choice of the exchange-correlation functional used within the density functional formalism. This sensitivity could be explained by a nonuniform distribution of the electronic charge density near defect sites. Our results imply that the use of gradient-corrected functionals is essential for accurate theoretical modeling of functionalized carbon nanotubes and nanotube-based composites. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V28.00010: Deformation of $sp^2$ graphitic nanostructure by irradiation with highly charged ion Yoshiyuki Miyamoto I will discuss possibility of structural deformation of graphitic $sp^2$ network by irradiation of highly charged ions. Meguro {\it et al.}, [Appl. Phys. Lett..{\bf 79}, 3866 (2001)] reported indication of nano-diamond formation on graphite surface when it is irradiated by Ar$^{+8}$ ions with incident kinetic energy of 400 eV. The nano-diamond structure was suggested from STS and IR spectrum taken after irradiation showing energy gap of 6 eV and C-C stretching with the frequency of 1360 cm$^{-1}$. This structural change was considered to be induced by injection of holes from highly charged ion (Ar$^{+8}$) while role the kinetic energy of Ar ion was thought to be marginal. Electron-ion dynamics simulation combined to the time-dependent density functional theory [O. Sugino, Y. Miyamoto. Phys Rev B{\bf 59}, 2579 (1999), {\it ibid}, Phys. Rev. B{\bf 66}, 088901(E) (2002)] has been performed to examine the mechanisms of the structural change. This simulation has found that the role of incident kinetic energy of Ar$^{+8}$ and subsequent cooling mechanisms also play crucial role in determining the structural change. More details will be presented and discussed in my talk. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V28.00011: Metallic carbon nanotubes destruction using Laser Irradiation Hisashi Kajiura, Houjin Huang, Ryuichiro Maruyama, Koji Kadono, Kazuhiro Noda We demonstrated that, using laser irradiation in air, metallic single-walled carbon nanotubes (SWNTs) in carbon nanotube thin film can be preferentially destroyed to their semiconducting counterparts if SWNTs are not heavily bundled. Although all metallic SWNTs were not destroyed using the lasers with an excitation wavelength of 514.5nm and 632.8nm due to a large distribution of SWNTs diameter, it is clear that if SWNTs with a small distribution of diameter can be produced, it should be possible to destroy all of the metallic SWNTs using one or two lasers. [Huang et al. J.Phys.Chem.B, 2006, 110, 7316-20. and 4686-90.] [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V28.00012: First-principles characterization of carbon nanotubes functionalized with [2+1] cycloadditions Young-Su Lee, Nicola Bonini, Nicola Marzari First-principles calculations predict that [2+1] cycloadditions of carbenes or nitrenes on single-wall carbon nanotubes can induce bond cleaving between adjacent sidewall carbons, recovering in the process the $sp^2$ hybridization of the pristine tubes \footnote{Y.-S. Lee and N. Marzari, Phys. Rev. Lett. 97, 116801 (2006)}. Electrical conductance is strongly affected by the local bonding environment, and the $sp^2$ re-hybridization induced by cycloadditions restores the conductance of the pristine tubes even in the presence of significant chemical or structural disorder. Phonon dispersions, Born effective charges, and polarizabilities of functionalized carbon nanotubes have been also studied, to provide a link between the local bonding structure and experimental Raman and infrared spectra. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V28.00013: First Principles Calculations for Destructive Deformation of Carbon Nanotubes by Oxygen Molecule Takazumi Kawai, Yoshiyuki Miyamoto It is well-known that carbon nanotube devices are significantly sensitive to ambient gas molecules such as oxygen molecules. Especially chemisorbed oxygen molecules have possibility to destruct carbon network and even destroy the devices. Then oxidation of carbon nanotubes is crucial for nano device application. On the other hand, the oxidation is very useful method to cut the cap structures or purify nanotubes by burning the defective nanotubes. Furthermore, some experimental results suggest that the chirality dependent oxidation of nanotubes. To understand the properties of oxidation, we focus on an initial process where the C=C bond is broken after the cyclo-addition of oxygen molecule on nanotube surface. The DFT-LDA calculations show that the diameter dependence and also the chirality dependence of reaction barriers to break the C=C bond. We will also discuss the effect of hole doping which would correspond to the oxidation process in solution. [Preview Abstract] |
Session V29: Foams and Emulsions
Sponsoring Units: DFDChair: Douglas Durian, University of Pennsylvania
Room: Colorado Convention Center 303
Thursday, March 8, 2007 11:15AM - 11:27AM |
V29.00001: Coarsening in steady-state aqueous foam Klebert Feitosa, Douglas J. Durian We perform an experiment with a column of aqueous foam maintained in steady-state by a constant gas flow rate at the bottom. In steady-state we measure the bubble velocity $u$, bubble radius $R_{32}$ and liquid fraction $\varepsilon$ in the foam as a function of height. Away from the bottom, capillary effects are negligible and the liquid fraction profile, set by the balance of viscous forces and gravity, does not change with time. Taking the liquid fraction as a given, the gas transport is investigated. We find that the bubbles rise with constant speed equals to the measured gas flux and coarsen as a function of height. We measure the coarsening rate for almost three decades in liquid fraction combining data from steady-state and free drainage experiments. The results show that the coarsening rate grows without bound proportional to $1/\sqrt{\varepsilon}$ for the entire range of liquid fractions. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V29.00002: Polymorphism in Monodisperse Foams Olivia L. Halt, Randall D. Kamien The aging of dry foams combines the local requirements of Plateau's rules and surface to volume relations with the global requirement of volume conservation. In a wet foam, the size of each spherical bubble is directly related to its radius of curvature, $R$. For dry foams, however, we must instead consider the mean curvature, $H$, which controls gas diffusion but is not directly related to the bubble size. Using a mean-field approach, our model connects distributions of mean curvature to distributions of cell size. This conversion makes use of mean field bubbles [1,2]. By considering the positive and negative curvature distributions separately, such that the cells have equal surface area, we obtain an average number of faces close to previously measured values. Also, distributions of cell sizes are obtained that are seen in real foam. \newline \newline [1] Glicksman M., Phil. Mag., 85 (2005) 3. \newline [2] Hilgenfeldt S., Kraynik A., Reinelt D., and Sullivan J. Europhys. Lett. 67 (2004) 484. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V29.00003: Rheology of wet foams of different bubble sizes and surfactant chemistry Stephan Koehler, Raenell Soller We present a new rheological technique for measuring the mechanical properties of aqueous foams at different liquid volume fractions, and of different surfactant compositions. We also cosider the influence of particulate matter on the rheology. We find that at high shear rates the liquid drainage rates are diminished. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V29.00004: Flow of soft glassy materials in confined geometry Julie Goyon, Annie Colin In this work, we address the question of the flow soft glassy materials in confined geometry. A transparent direct concentrated emulsion of micrometric size flows in a microfluidic channel under a constant applied drop of pressure. The continuous phase or the dispersed phase is seeded with some sub-micrometric fluorescent latex beads. Taking successive pictures of the flow and correlating them allows us to get the velocity profile. We use rectangular micro-channels with high ratio aspect. The experimental data are analysed in the framework of the lubrication. On one hand, the shear stress is calculated thanks to the position in the channel and the pressure. On the other hand, the shear rate is obtained thanks to the slope in the velocity profile. We point out that the flow cannot be described using a unique behaviour law. Indeed, it is perturbed by rearrangements events which induce three-dimensional flows. These events occur preferentially in the vicinity of the wall where they modify and increase locally the velocity. A comprehensive study of the statistics of the rearrangements events is presented. The role of the drop of pressure, the liquid fraction of the emulsion, the droplet size of the emulsion and the attractive forces between droplets are studied. . [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V29.00005: Novel Shear Banding in 2D Foam Probes Soap Film Friction James Glazier, Ariel Balter, Rebecca Perry A flowing 2D foam (a single layer of bubbles between two glass plates), experiences dissipation from two sources: soap flims moving against the glass plates and soap flims moving against each other. We present preliminary results showing how a new type of shear banding helps us study these various drag forces. We experimentally generate a shear band by injecting air part way along a flowing bubble field in a narrow Hele-Shaw cell. The injected air inflates bubbles as they flow by. These bubbles form an independently flowing channel down the middle of the Hele-Shaw cell. The width and velocity of this channel appear to be selected by the system minimizing the total dissipation. We propose a simple model that agrees with the experimental data. Also, numerical simulations using the Cellular Potts Model ({\it CPM}) software {\it CompuCell3D} appear to faithfully reproduce this shear band phenomenon. Agreement between our expeirment and simulations provide support for {\it CPM} methods for studying foam rheology. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V29.00006: ABSTRACT HAS BEEN MOVED TO A29 |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V29.00007: Entropically Driven Colloidal Assembly in Emulsions Keng-hui Lin, Liang-jie Lai, Hui Chen Using the techniques developed by Manoharan [1], we encapsulate small numbers of colloidal microspheres and polymers in oil-in-water emulsion droplets, remove the oil and generate colloidal clusters covered with polymers. We observe two types of arrangement in the clusters. The first kind is the same as the type reported in [1] of which the clusters are formed without polymer. The second kind is the same as the structure reported in [2] of which the clusters are formed by binary colloidal microspheres. The polymers we put in the emulsions induce depletion interactions between colloidal particles. We will show that two types of structures are from the interplay between the depletion interactions and surface tension. [1] Manoharan, Elsesser, Pine, \textit{Science} \textbf{301}, 483(2003). [2] Cho \textit{et al. JACS} \textbf{127}, 15968 (2005). [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V29.00008: Rheology of solid-stabilized emulsions Kosta Ladavac, Rodrigo Guerra, Pabitra Sen, David Weitz Concentrated emulsions can possess strong shear rigidity, in spite of being comprised solely of fluids. When stress is applied the drops deform, create additional surface area and are able to store energy. For surfactant-stabilized emulsions this elasticity is driven by surface tension alone. In case of sollid- stabilized emulsions, where droplets are protected by colloidal particles adsorbed at the interfaces, organization of particles and their rigidity leads to a different response to deformation. We study this packing of a packing -- the interplay between 3D structure of emulsion droplets and 2D structure of colloidal particles at their interfaces. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V29.00009: Flow of colloidal gels through constrictions Jacinta Conrad, Jennifer Lewis We use confocal microscopy to investigate the flow behavior of colloidal gels through constrictions of varying geometry. We flow suspensions of attractive silica colloids through microchannels containing a single constriction point. As the colloid volume fraction is increased, the colloids in the microchannels jam and form a clog. Here we investigate the flow properties and the clogging as a function of applied pressure, microchannel geometry, and the colloid volume fraction. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V29.00010: Non-affine bubble motion in a two-dimensional, linearly sheared foam Matthias Mobius, Gijs Katgert, Martin van Hecke Two-dimensional foams are an excellent model system to study the non-affine deformations of a disordered, jammed medium under shear. In our experiment we apply linear shear to a monolayer of bubbles that is confined between a soap solution and a glass plate. Through video imaging we track the motion of individual bubbles. We characterize the non-affine motion by looking at the distribution of relative displacement angles, $\alpha$, of neighboring bubbles [1]. A peak at 90 degrees emerges, which corresponds to bubbles sliding past each other. We investigate the change of the probability distribution of this angle, $P(\alpha)$, as a function of liquid fraction and shear rate. We discuss $P(\alpha)$ in the context of the jamming transition and show that near the transition the bubble motion is dominated by sliding. Moreover, we look at the relationship between the local velocity fluctuations and the shear rate. [1] W. Ellenbroek et al. , accepted for Phys.Rev.Lett. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V29.00011: Thermal conductivity measurements in a 2D Yukawa system V. Nosenko, A. Ivlev, S. Zhdanov, G. Morfill, J. Goree, A. Piel Thermal conductivity was measured for a 2D Yukawa system. First, we formed a monolayer suspension of microspheres in a plasma, i.e., a dusty plasma, which is like a colloidal suspension, but with an extremely low volume fraction and a partially-ionized rarefied gas instead of solvent. In the absence of manipulation, the suspension forms a 2D triangular lattice. To melt this lattice and form a liquid, we used a laser-heating method. Two focused laser beams were moved rapidly around in the monolayer. The kinetic temperature of the particles increased with the laser power applied, and above a threshold a melting transition occurred. We used digital video microscopy for direct imaging and particle tracking. The spatial profiles of the particle kinetic temperature were calculated. Using the heat transport equation with an additional term to account for the energy dissipation due to the gas drag, we analyzed the temperature distribution to derive the thermal conductivity. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V29.00012: Experimental microrheology of quiescent soap films Vikram Prasad, Eric R. Weeks A soap film consists of a thin water layer that is separated from two bulk air phases above and below it by surfactant monolayers. Previous experiments (Prasad, Koehler and Weeks, PRL 2006) have shown that the coupling between an interface and an infinite bulk fluid is set by a length scale, the ratio between the interfacial viscosity and the bulk viscosity (of order microns to millimeters). This length scale determines the nature of the flow field in the interface and the adjoining bulk phases. In the case of soap films, the thickness of the water layer is an additional length scale, and therefore the exact nature of the coupling between the thin water layer, the surfactant interface and the bulk air phases is unclear. In order to determine this coupling, we use polystyrene spheres as tracer particles and track their motion in the soap films, using both one-and two-particle microrheology. The experimental results are compared to theory, and the consequences for the hydrodynamics of interfaces are discussed. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V29.00013: Janus particles on colloidosomes (Pickering emulsions) and the role of added surfactant Shan Jiang, Liang Hong, Steve Granick We describe systematically the synergy between particle and surfactant in stabilizing colloidosomes. Special attention is given to what determines the inversion between O/W and W/O emulsions, the so-called `catastrophic phase inversion'. At the onset of the catastrophic phase inversion, we find an exceptional double-emulsion structure. Extending this idea, we find that when the dispersed phase is frozen by lowering the temperature below its solid-liquid phase transition, particles can be locked at the interface and further chemically modified into Janus colloidal particles. This affords an easy way to produce Janus colloidal particles with versatile chemical makeup in large quantity. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V29.00014: Observing asphaltene aggregation by NMR spectroscopy and relaxation Yi-Qiao Song, Andrew Pomerantz, Kosta Ladavac, Pabitra Sen Asphaltenes are a class of molecules commonly found in the oilfield and defined by their simultaneous solubility in toluene and insolubility in hexanes. The aggregation dynamics of asphaltenes is currently poorly understood but presents a serious problem to the oil industry because aggregation can clog flow though pipelines and the oil-bearing rocks. Recently, aggregation dynamics of asphaltenes at very low concentration was measured by nuclear magnetic resonance (NMR) of spin-spin relaxation and diffusion, and fluorescence correlation spectroscopy (FCS). Here, asphaltene aggregation at higher concentrations is observed by monitoring the NMR spectroscopy and longitudinal relaxation times (T1) of the solvent protons. These measurements shed new light on the dynamics of aggregation. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V29.00015: Towards a 2D nonperiodic Solid Xiaochao Xu, David Pine We report on an experimental study of the two-dimensional phase behavior of colloidal dumbbells (dimers) trapped at a water-oil interface. The dimers are made out of $1.0\,\mu {\rm m}$ silica microspheres that are fused together at a point. The water-oil interface is very slightly concave so that the dimers are gently compressed by gravity towards the center of interface. The spheres form a stable dense state after a few days. The pair correlation function of single spheres exhibits order on a length scale of about 10 particle diameters. We report on the translational and orientational order of the dumbbells as a function of particle density. [Preview Abstract] |
Session V30: Biologically Inspired Physics
Sponsoring Units: DFDChair: Marilyn Gunner, City College of New York
Room: Colorado Convention Center 304
Thursday, March 8, 2007 11:15AM - 11:27AM |
V30.00001: Investigating gecko setae adhesion using a dual-axis MEMS force sensor Ginel Hill, Daniel Soto, Anne Peattie, Robert Full, Thomas Kenny A dual-axis piezoresistive MEMS force sensor was used to investigate the role of orientation angle on the adhesion of gecko hairs, called setae. Made of keratin with nanoscale features, gecko setae are a spectacular, robust dry adhesive with anisotropic adhesion properties. A wealth of recent research has been devoted to synthetic mimicry of the gecko seta. However, most synthetics do not yet display anisotropic adhesion, which is critical for controllable attachment and release. Previous research using a wire gauge tested the role of the pitch angle between the stalk of natural setae and the substrate and found a dramatic cutoff angle of 30$^{\circ}$, above which setae detach from the substrate [1]. The present work details the effect of the ``roll'' angle on natural setae adhesion. [1] K. Autumn, et al. Nature, 405: 681 (2000). [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V30.00002: The venation network in leaves as anticracks? Pedro Reis, Denis Vallet, Benoit Roman Thus far, existing models of venation in leaves are entirely biochemical, involving hormonal diffusive processes. These are, however, unable to capture some crucial structural features of the vascular bundles' network such as the existence of reconnection loops. Couder et. al. [1] have recently highlighted the striking similarities between leaf venation and fracture patterns in drying gels, suggesting that a tensorial mechanism may be at play. It is known that, in the initial stages of the formation of vascular bundles, the promesophyll (leave's \emph{bulk}) is under compression since it grows at a faster rate than the protoderm (leave's \emph{skin}). Hence, to take this analogy further, we introduce the concept of an \emph{anticrack}: a localization of deformation under compressive stresses. We have developed an experimental system to develop and explore this concept as when a solid foam is compressed, either uniaxially or biaxially. We analyze the resulting anticrack networks and relate them to the fracture and venation counterparts. For this purpose we use a high resolution image correlation technique to measure the statistics of the localization zones, structural hierarchy and reconnection loops. [1] Y. Couder, L. Pauchard, C. Allain, M. Adda-Bedia and S. Douady, \emph{Eur. Phys. J. B} \textbf{28} 135 (2002). [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V30.00003: Dynamics of macromolecules in confined environments Armin Rahmanisisan, Claudio Castelnovo, Jeremy Schmit, Claudio Chamon The dynamics of a ring macro-molecule confined to a two dimensional cell is studied. A connectivity-preserving kinetically constrained lattice gas model is introduced and used to study the effects of the shrinking of the box on dynamical correlations using Monte-Carlo simulations. It is found that the monomers comprising the macro-molecule manage to diffuse around the box with a mean squared displacement of the order of the square of the box dimensions, even at densities close to the frozen fully-packed configuration where the overall geometry remains almost unchanged over long times. Reptation and fingering events are observed and appropriate correlation functions are introduced to analyze the monomer motion and polymer reshaping at different densities. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V30.00004: Piezoelectricity as possible mechanism for mechano-, and magneto-receptions Antal Jakli, John Harden, Cody Notz, Chris Bailey We have studied the piezoelectric responses of 2 dry phospholipids (L-a-Phosphatidylcholine) purchased from Avanti Inc.model membranes. The material was sandwiched between two parallel plates separated by 5$\mu $m-60$\mu $m distances and aligned with smectic layers mainly parallel to the substrates. The material has a SmA* phase which is similar to the bilayers of cell membranes. Due to its symmetry it should be piezoelectric and may produce electric current normal to the shear plane when one of the substrates is moved with respect to the other one. We have experimentally verified this statement and found the generation of electric polarization up to 300nC/cm$^{2}$ when the shear induced a director tilt of about 5 degrees. We have also measured generation of electric current in phospholipids doped with 1{\%} of ferrofluid of Fe$_{2}$O$_{3}$ nanoparticles when 100G magnetic field was applied periodically on the material. Details of the observations and the relevance of these effects in mechano-, and magneto-receptions will de discussed. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V30.00005: Planar Model of Orbital Paramagnetism in B-DNA and A-DNA at Low Temperatures. Michael J. Harrison We develop a planar model of B-DNA which exhibits orbital paramagnetic properties at low temperatures arising from a small number of conducting pi electrons that move in a parabolic potential attracting them towards the central axis of the model molecule. The paramagnetism is nonlinear in applied field and can have a maximum several times the magnitude of diamagnetism per particle in an extended 2DES. The model is consistent with recent experimental observations [1] provided that only a small number of itinerant pi band electrons of higher energy are assumed to participate, amounting to only 1{\%} of the total number of pi stack electrons from base pairs which are thought to constitute a hybridized core within the actual double helix. The model indicates that the encladding water molecules attached to wet B-DNA provide dielectric screening of the attractive parabolic potential, and can explain the presence of orbital paramagnetism, which is experimentally absent in dry A-DNA [1]. S. Nakamae, et al, PRL 94, 248102, (2005). [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V30.00006: Electromechanics: An analytic solution for graded biological cell. Kin Lok Chan, K. W. Yu Electromechanics of graded material has been established recently to study the effective response of inhomogeneous graded spherical particles under an external ac electric field.[1, 2]Such particles having a complex dielectric profile varies along the radius of the particles. The gradation in the colloidal particles is modeled by assuming both the dielectric and conductivity vary along the radius. More precisely, both the dielectric and conductivity function are assumed to be a isotopic linear function dependence on the radius variable $r$, namely, $\varepsilon (r)=\varepsilon (0)+A_1 r$, $\sigma (r)=\sigma (0)+A_2 r$.In this talk, we will present the exact analytical solutions of the dipole moment of such particle in terms of the hypergeometric functions, and the effective electric response in dilute limit. Moreover, we applied the dielectric dispersion spectral representation (DDSR) to study the Debye Behavior of the cell. Our exact results may be applied to graded biological cell suspensions, as their interior must be inhomogeneous in nature. [1] En-Bo Wei, L. Dong, K. W. Yu, Journal of Applied Physics 99, 054101(2006) [2] L. Dong, Mikko Karttunen, K. W. Yu, Phys. Rev. E, Vol. 72, art. no. 016613 (2005) [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V30.00007: TASEP with extended particles and local inhomogeneities Jiajia Dong, Beate Schmittmann, Royce K.P. Zia Though much is known about the totally asymmetric simple exclusion process (TASEP), there are still non-trivial characteristics worthy of further exploration. In particular, in TASEPs with extended particles that ``cover'' $\ell >1$ lattice sites, non-trivial correlations between the particles exist, even for a case with a trivial distribution for the microscopic configurations. Further, $\xi $, the characteristic length of these correlations can be extremely long, e.g., $O\left( 10^{2}\right) $. They set up interesting structures in the density profile behind ``bottlenecks'' (localized inhomogeneities, with hopping rates $q$ smaller than those in the rest of the lattice: $q<1$) in the system. For TASEPs with open boundaries, we study how one or more such bottlenecks affect both the profiles and the overall current. Using simulations, we present results for a range of $% q,\ell ,$ and the {\em locations }of the inhomogeneities: $x_{i}$. For example, the current is somewhat enhanced if a single bottleneck is located close to either system boundary. But it is reduced significantly if two bottlenecks are present and closely spaced, i.e., provided $\left| x_{1}-x_{2}\right| $$\leq$ $\xi .$ We also discuss the possible impact of these findings on ribosome queueing and codon optimization in protein production. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V30.00008: The Real-Time Dose Measurement Scintillating Fiber Array for Brachytherapy Procedures Lawrence Tynes Brachytherapy is a treatment modality that uses tiny radioactive sources (few mm in length) by delivering enough doses to kill cancer tumors or plaque build-up. The type of sources used in hospitals include both gamma and beta emitters. Presently, the technique suffers from not having a single detector with the capability of providing accurate dose distribution information within sub-mm accuracy. The current standard is based primarily on well chambers and film dosimetry. The Center for Advanced Medical Instrumentation (CAMI) at Hampton University is developing a Scintillating Fiber Based Beta Detector prototype in collaboration with the National Institute for Standards and Technology (NIST) to address this problem. The device is composed of an array of 1x1 \textbf{mm}$^{2}$ scintillating fibers optically coupled to photo-multiplier tubes for photon-to-current conversion. A CAMAC LabView based data acquisition system is used for real time data collection and histogramming, data analysis. A set of data were collected at the nearby Bon Secours DePaul Medical Center using a GammaMed 12i HDR after-loader housing a 6.62 mCi Ir-192 source. Preliminary comparison between our device and film dosimetry will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V30.00009: Modeling Blood Filtration in the Treatment of Septic Shock Glenn Foster, Alfred Hubler Sepsis, the overreaction of the inflammation and coagulation responses to infection, is the leading cause of death in non-coronary intensive care unit patients in the US. Anti-mediator drugs have been generally ineffective, but by considering the network of cytokine interactions, we illustrate how filtering the cytokines in the blood leads to a reduced response. We further illustrate by applying an appropriate filter to existing immune response models as well as discuss both practical and optimal filter parameters. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V30.00010: A novel flow cytometry configuration for the detection of magnetic microparticles. John Martin, Christopher Carr, Andrei Matlachov, Henrik Sandin, Michelle Espy, Robert Kraus We have developed a technique for detecting magnetic microparticles in a novel laser-based flow cytometry configuration that incorporates a giant magnetoresistive (GMR) sensor. To achieve the highest possible sensitivity, it is advantageous to minimize the distance from the GMR sensor to the microparticle. Initially, we passed ferromagnetic microparticles (diameter $<$100 microns) through polymer capillary tubing that passed directly on top of the GMR. While the capillary tubing provides a controlled flow path, it imposes a standoff between the sensor and the microparticle that is never less than 200 microns (due to the tube wall thickness). This standoff limits the range of magnetic microparticles we can detect. Another proposed technique to achieve minimum standoff is to fabricate microfluidic flow channels on top of the GMR itself. We have developed a new approach for minimum standoff, which does not require microfluidics. We will describe this technique, discuss the performance of the commercial GMR sensor and finally report on the detection of magnetic microparticles in this new flow configuration. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V30.00011: Computational Phlebology: The Simulation of a Vein Valve Gavin Buxton, Nigel Clarke We present a three-dimensional computer simulation of the dynamics of a vein valve. In particular, we couple the solid mechanics of the vein wall and valve leaflets with the fluid dynamics of the blood flow in the valve. Our model captures the unidirectional nature of blood flow in vein valves; blood is allowed to flow proximally back to the heart, while retrograde blood flow is prohibited through the occlusion of the vein by the valve cusps. Furthermore, we investigate the dynamics of the valve opening area and the blood flow rate through the valve, gaining interesting insights into the physics of vein valve operation. It is anticipated that through computer simulations we can help raise our understanding of venous hemodynamics and various forms of venous dysfunction. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V30.00012: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V30.00013: Evolution of DNA Compaction in X-Ray Compatible Microflow Foil Devices Sarah Koester, Heather M. Evans, Rolf Dootz, Bernd Struth, Thomas Pfohl Spatially resolved X-ray microdiffraction in hydrodynamic focusing microdevices provides new opportunities to study time-resolved reactions of complex fluids. A demonstration of this technique as applied to the liquid crystal 8CB was recently reported [1]. Here, we discuss the dynamics of the compaction of DNA by polyimine dendrimers, as studied using microfluidic devices. Due to the laminar flow inside the channels a highly defined, diffusion controlled compaction of DNA occurs. Different snapshots in the time of the reaction are accessible at varying spatial positions along the interaction jet. We use newly developed X-ray compatible microflow foils made from PDMS and Kapton and having dimensions ranging from 30 to 150 micrometers. The real-time evolution of a DNA-dendrimer columnar mesophase with an in-plane square symmetry is reported. These studies are also extended to include a larger library of dendrimers whose size and charge approach those of the biological histone proteins. [1] Dootz, Evans, Koester, Pfohl, accepted to Small. [Preview Abstract] |
Session V31: Focus Session: Carbon Nanotubes: Thermal, Mechanical, Adsorption
Sponsoring Units: DMPChair: Meninder Purewal, Columbia University
Room: Colorado Convention Center 401
Thursday, March 8, 2007 11:15AM - 11:27AM |
V31.00001: Thermoelectric power of Single Walled Carbon Nanotubes at the Ballistic Conduction Limit Yuri M. Zuev, Philip Kim Thermoelectric power (TEP) measurements of single walled carbon nanotubes(SWNTs) with low electrical contact resistance are reported. TEP was measured in-situ using a microfabricated heater and thermometers. High quality Ohmic contact to the SWNT was achieved with Pd electrodes. TEP measurements are sensitive to the change in conductivity, and therefore provide a complementary method for probing the electronic band structure of SWNTs. Deviations of the low temperature TEP gate dependence from the semiclassical Mott relation allows us to gain insight into the quantum transport regime in this one dimensional conductor. Modulation of TEP as a function of applied gate voltage will be discussed in connection with the shell filling effects and Fabri-Perot oscillations observed in electrical conductance. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V31.00002: Phonon-Phonon Interaction In Carbon Nanotube Assemblies. Ali Aliev, Mei Zhang, Anvar Zakhidov, Ray Baughman We present the comparative study of the anisotropic 1D thermal conductivity and the thermal diffusivity of assemblies of carbon nanotubes (CNTs) comprising an increasing number of aligned free standing carbon nanotubes (SWNT and MWNT) using two techniques: laser flash and self-heating 3$\omega $ methods. The concept of mode quenching is considered for alignment of few individual CNTs. The length dependence of thermal conductivity is studied for CNT with different number of intrinsic defects (HiPCO, Laser ablation, Arc-Charge). The extremely high surface area of CNT assemblies like highly aligned MWNT sheet [1] leads to the excessive radial radiation of the heat and dose not allow to transfer the heat energy by means of phonons to distances more than 2 mm. [1]. M. Zhang, S. Fang, A. A. Zakhidov, S. B. Lee, A. E. Aliev, C. D. Williams, K. R. Atkinson, R. H. Baughman, \textit{Science } \textbf{309}, 1215 (2005). [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V31.00003: Electromechanical Resonators from Atomically Thin Graphite Scott Bunch, Arend van der Zande, Scott Verbridge, Ian Frank, David Tanenbaum, Jeevak Parpia, Harold Craighead, Paul McEuen We fabricate nanoelectromechanical systems (NEMS) from atomically thin graphite by mechanically exfoliating thin sheets over trenches in SiO$_{2}$. Vibrations with fundamental resonant frequencies in the MHz range are actuated either optically or electrically and detected optically by interferometry. We make a detailed study of the mechanical properties of these resonators including resonance frequency, spring constant, built in tension, and quality factor. The thinnest resonator consists of a single suspended layer of atoms and represents the ultimate limit of a two dimensional NEMS. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V31.00004: Microwave Nano-abacus Electro-mechanical Oscillator Haibing Peng, C.W. Chang, S. Aloni, T.D. Yuzvinsky, A. Zettl We describe nanoscale electromechanical oscillators capable of operating in ambient-pressure air at room temperature with unprecedented fundamental resonance frequency of $\sim $4 GHz. The devices, created from suspended carbon nanotubes loaded abacus-style with inertial metal clamps yielding short effective beam lengths, open windows for immediate practical microwave frequency nanoelectromechanical systems (NEMS) applications. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V31.00005: Metal-carbon nanotube composite nanoelectromechanical torsional resonators Young Duck Kim, Seung Sae Hong, Jung Hoon Bak, Byung Yang Lee, Sung Woon Cho, Ki Sung Suh, Seunghun Hong, Yun Daniel Park Metallic based nanoelectromechanical systems (NEMS) resonator structures are of interest due to higher optical reflectivity, ductility, and conductivity compared to insulator- and semiconductor- based NEMS structures. We present NEMS torsional resonator structures fabricated from aluminum-carbon nanotube (CNT) and palladium-CNT composites. Metal and metal-CNT NEMS structures are released from III-V based substrates. The resonators are electrostatically driven and are detected at room temperatures under moderate vacuum conditions using optical modulation techniques. We note significant differences in the resonant frequencies (f$_{0})$ and the quality factors (Q) between metal and metal-CNT NEMS torsional resonators. Aluminum based structures with paddle dimensions of $\sim $5 micron x $\sim $5 micron, with support beams of $\sim $1 micron x $\sim $3 micron, show a fundamental resonant frequency corresponding to translational mode of 1.7 MHz with Q of 20, while Al-CNT based structures of same dimensions show f$_{0}$ of 3 MHz and Q of 50, as a typical example. We will further discuss the effects on the mechanical properties of metallic NEMS torsional resonators due to addition of CNT. {\dag}parkyd@phya.snu.ac.kr [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V31.00006: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V31.00007: Strain sensitivity in the photocurrent of single wall nanotubes Prasanth Gopinath, A. Mohite, H. Shah, J. Lin, B. Nagabhirava, T. Bansal, B. Alphenaar The energy spectrum of carbon nanotubes is highly sensitive to strain and mechanical deformation. Calculations predict a shift in the bandgap of single wall nanotubes (SWNT) with axial strain, which in turn affects the conductance\footnote{ Liu Yang et.al, Phys. Rev. B 60, 13874 (1999) }. We have measured the influence of strain on the photocurrent spectrum of SWNT's and observe as much as 600meV shift in the band gap energy of semiconducting nanotubes for 300 micro strains. The experiments were performed on SWNT's CVD grown on a 300$\mu $m thick quartz cantilever; the SWNT's are strained by pushing down on the free end of the cantilever. We use a capacitive photocurrent technique for detecting the photocurrent as a function of incident photon energy. The peak corresponding to the band- to- band free electron transition in the semiconducting nanotubes is observed to shift to lower energies with increasing strain. Further measurements using a fixed wavelength (488nm) Ar ion laser show as much as an order of magnitude change in the photocurrent with strain, implying a gauge factor of more than a 1000. Our measurements provide a direct probe of the influence of strain on the bandgap of SWNT's and open up the possibility of using SWNT's as optical strain sensors. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V31.00008: Nanotubes stretched in Liquid-Metal-Ion-Sources:their influence on the cluster emission and on the isotopic anomalies. Rene Jean Tarento, Pierre Joyes, Jean van de Walle The present contribution argues that an intense electric field (few V/A) provides an alternative method to stretch matter and to form nanotubes locally. The very high electric field is supplied by a Liquid Metal Ion Source (LMIS). Intriguing aspects are displayed by the LMIS mass spectra of some pure elements. The periodicity of pure Ge or Sn LMIS i.e. series of equidistant peaks such Ge$_{6n+1}^{3+}$ with n=3 to 8 or Ge$_{6n+4}^{3+}$ with n=7 to 14 or the formation of unexplained Au$_{8}^{3+}$ and Au$_{16}^{3+}$ ions for the pure Au LMIS are attributed to the existence of Ge, Sn or Au nanotubes in operating LMIS. LMIS results on eutectic Au$_{0.73}$Ge$_{0.27}$ alloy show the formation of a gold nanotube associated with the strong Au$_{8}^{3+}$ emission. The Ge$_{2}^{+}$ emitted near the gold nanotube interact with a larger electric field than in the pure Ge LMIS provoking a bond break in heteroisotope dimers and therefore isotope anomalies in dimer emission. Finally we analyse the results from Au-Si eutectic [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V31.00009: Organic-vapor-induced repeatable movements of C$_{60}$ in/from single-wall carbon nanohorns at room temperature Jin Miyawaki, Masako Yudasaka, Ryota Yuge, Sumio Iijima Incorporation of functional materials inside carbon nanotubes (CNTs) has been actively investigated with the goal of improving the physical and chemical properties of CNTs. We found intriguing phenomena of repeatable movement of materials from inside to outside of CNTs and vice versa, which resulted simply by exposure to different organic vapors at room temperature. For example, C$_{60}$ entered inside single-wall carbon nanohorn (SWNHs), a type of single-wall CNTs, when exposed to toluene vapor at room temperature, and the incorporated-C$_{60}$ exited when exposed to ethanol vapor. Here the entrance and exit were evidenced by X-ray diffraction and N$_{2}$ adsorption measurements, and microscopic observations. The back-and-forth transportations of C$_{60 }$could be repeated. We think that the C$_{60}$ movements were mediated by the layers of toluene or ethanol adsorbed on the SWNH surfaces. The present findings will help in the fabrication of various nanometer-scale hybrid systems from CNTs. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V31.00010: Resonance Raman study of polyynes inside single-walled carbon nanotubes. Marcos Pimenta, Leandro Malard, Daisuke Nishide, Ana Paula Gomes, Ado Jorio, Hisanori Shinohara We report a resonance Raman study of polyynes C$n$H2 molecules ($n $= 10 and 12) composed of linearly bonded sp-carbon atoms aligned inside the single-walled carbon nanotubes (SWNTs), using many laser lines in the range 1.9 to 2.7 eV. The C10H2@SWNT hybrid material exhibits a Raman peak at 2066 cm-1 related with the stretching vibration of the C10H2 molecules interacting with SWNTs, while the C12H2@SWNT exhibits a peak at a lower frequency, around 2020 cm-1. The intensities of these peaks are strongly dependent on the laser energy, and exhibit maxima around 2.15 and 2.10 eV for the C10H2@SWNT and C12H2@SWNT materials, respectively. However, the optical absorption spectrum of the purified C10H2 and C12H2 dispersed in n-hexane exhibits peaks in the UV, around 4.9 and 4.5 eV, respectively.This result can be associated with a two-photon Raman resonance enhancement or can reflect important changes in the electronic structure of the polyynes when they are encaged in a carbon nanotube. New experimental and theoretical works are needed to understand this result. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V31.00011: Localization of inserted species inside single walled carbon nanotubes bundles: experimental and \textit{ab initio} study. Nedjma Bendiab, A. Marco Saitta, Robert Almairac, Raymond Aznar, Jean-Louis Sauvajol, Isabelle Mirebeau The aim of this work is to understand the structural organization of inserted alkali atoms inside single walled carbon nanotube bundles. First of all, we present X-rays and neutrons diffraction results obtained on rubidium inserted carbon nanotubes ($n$-doping). The results of X-rays and neutrons diffraction experiments are surprising and in apparent contradiction, and will be discussed in connection with the debated question of the lattice expansion of the hexagonal tubes framework under insertion. The possible insertion sites of the rubidium atoms in the nanotube bundle will be discussed in terms of their effects on the diffraction spectra. The experimental results will be compared to diffraction simulations and ab initio DFT calculations. The main outcome of our combined experimental and theoretical study is that: i) up to saturation, the spectra show no lattice expansion; ii) the extinction of the (10) peak is only compatible with \textit{Rb} insertion inside the tubes; iii) DFT calculations show that at constant lattice parameter the insertion within the tubes is energetically favored with respect to insertion between the tubes. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V31.00012: Experimental Evidence for Water Intercalation into Graphite Dinko Chakarov, Hans Fredriksson, Guido Ketteler, Bengt Kasemo Using different experimental methods we follow the uptake and release of water from highly oriented pyrolytic graphite sample. We found that water can intercalate into graphite following transient binding to defect sites and accumulate in the subsurface regions with concentrations amounting up to 10{\%} of the monolayer. The process is thermally activated and could be manipulated by changing the water vapor pressure or amount of water (ice) on the surface. Photoelectron and vibration spectroscopy data indicate strong perturbance of the intercalated water molecules and lowered barrier for dissociation. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V31.00013: Dependence of single-walled carbon nanotubes' adsorption kinetics on temperature and binding energy Vaiva Krungleviciute, Dinesh Rawat, Murat Bulut, Luke Heroux, Aldo Migone We present adsorption kinetics results for hydrogen, freon and ethane on single walled carbon nanotubes. We measured the decrease of the pressure as a function of time as equilibrium is approached. Our results indicate that the equilibration time is a function of$\frac{\varepsilon }{T}$; where$\varepsilon $ is the binding energy of adsorbate and T is the isotherm temperature. We also compare the dependence of the equilibration time on the shape of the adsorbate. We found that for linear molecules the equilibration times decrease with increasing coverage at a much slower rate than those times for spherical molecules. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V31.00014: Temperature and Pressure Dependence of Hydrogen Coverage on Single-Walled Carbon Nanotubes Jun Ni, Xiaobao Yang The safe and compact storage of hydrogen is of great interest in theoretical and experimental research. Carbon nanotubes are reported to be highly efficient for gas and alkali atom storage. The process of hydrogen adsorption on the carbon nanotubes is changed under various circumstances. it is important to know how the hydrogen coverage depends on the tube diameters, temperature and pressure. We have investigated the stability of various hydrogenated single-walled carbon nanotubes. We find the storage capacity of hydrogen depends significantly on the diameters of carbon nanotubes. The full hydrogen coverage can be reached for the nanotubes with small size, while for the nanotubes with large size, the saturation coverage is lower than 1. We have calculated the variation of the hydrogen coverage with the change of temperature and pressure. In particular, we find that the nanotubes with diameters of about 1 nm can achieve the coverage of 80{\%} at ambient temperature and low pressure, which is in agreement with the experimental results. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V31.00015: Transport characteristics of a single multiwall carbon nanotube by bending in SEM and STM Suenne Kim, Jeehoon Kim, Morgann Berg, Alex de Lozanne Multiwall carbon nanotubes(MWCNTs) were grown on a W wire by chemical vapor deposition(CVD). Two homebuilt xyz-walkers were employed to manipulate individual CNTs in our scanning electron microscope (SEM). To improve the electrical and mechanical contact to a second electrode, we welded the CNT by delivering gas to the welding point while focusing the SEM beam on the same spot. The bending dependent I-V characteristics were observed in situ in the SEM at room temperature. We will measure the transport properties by bending the same MWCNT (already measured in SEM) inside our ultrahigh vacuum low temperature scanning tunneling microscope (UHV-LTSTM). We will also compare the bending properties of MWCNTs at different temperatures. [Preview Abstract] |
Session V32: Optical Lattices
Sponsoring Units: DAMOPChair: Chris Oates, National Institute of Standards and Technology, Boulder
Room: Colorado Convention Center 402
Thursday, March 8, 2007 11:15AM - 11:27AM |
V32.00001: Ion Chains in Optical Lattices as Simple Quantum Glasses Roman Schmied, Tommaso Roscilde, Diego Porras, Ignacio Cirac We propose the loading of linearly trapped ions onto an intense optical lattice. In the limit of a deep lattice, we recover a classical one-dimensional Coulomb lattice gas in a harmonic trap. This system exhibits glassiness, due to significant metastability in its translational degrees of freedom. Quantum fluctuations can be induced at will by lowering the lattice amplitude, which allows for the controlled realization of a quantum glassy system. We study the dynamics of such systems during thermal and quantum annealing, and discuss how the effects of glassiness can be observed in the currently available experimental ion-trap setups. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V32.00002: Predicted quantum stripe ordering in optical lattices Congjun Wu, W. Vincent Liu, Joel Moore, Sankar Das Sarma We predict the robust existence of a novel quantum orbital stripe order in the $p$- band Bose-Hubbard model of two-dimensional triangular optical lattices with cold bosonic atoms. An orbital angular momentum moment is formed on each site exhibiting a stripe order both in the superfluid and Mott-insulating phases. The stripe order spontaneously breaks time-reversal, lattice translation and rotation symmetries. In addition, it induces staggered plaquette bond currents in the superfluid phase. Possible signatures of this stripe order in the time of flight experiment are discussed. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V32.00003: BCS-BEC crossover on the two dimensional honeycomb lattice Erhai Zhao, Arun Paramekanti We study the attractive Hubbard model on a honeycomb lattice. At half-filling, we find a quantum critical point (QCP) separating a weakly interacting semimetal with massless Dirac fermions from a strong coupling s-wave superconducting state. Away from half-filling, this model exhibits a BCS-BEC crossover in the vicinity of this QCP. Studying this model using ultracold atoms in an optical lattice could shed light on quantum phase transitions and BCS-BEC crossovers in electronic models. We present results for the evolution of several observables through the BCS-BEC crossover at zero temperature -- the Fermi surface, the superfluid density and the collective sound and Leggett modes. We also suggest a method to observe the Leggett mode in an optical lattice. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V32.00004: Mixture of bosonic and spin-polarized fermionic atoms in an optical lattice Lode Pollet, Corinna Kollath, Ulrich Schollw\"ock, Matthias Troyer We investigate the properties of Bose-Fermi mixtures for experimentally relevant parameters in one dimension using numerical methods. The effect of the fermions on the bosons is not only to deepen the parabolic trapping potential, but also to reduce the bosonic repulsion in higher order. This reduction would theoretically lead to an increase in the bosonic visibility. The opposite was observed however in the experimental ${}^{87}$Rb - ${}^{40}$K systems, most likely due to a sharp rise in temperature. We discuss the features which could be observed experimentally if temperature remains low, such as a bosonic Mott insulator transition driven by the fermionic concentration, and the formation of various composite particles. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V32.00005: Coexistence of superfluid and Mott phases of strongly-interacting lattice bosons Courtney Lannert, Roman Barankov, Smitha Vishveshwara Recent experiments on strongly-interacting bosons in optical lattices [1,2] have revealed the co-existence of spatially-separated Mott-insulating and number-fluctuating phases in the presence of an external trapping potential. Employing a simple theoretical model [3], we obtain an effective description of the superfluid state trapped between the Mott states. We calculate the collective excitation spectrum of such a superfluid and its critical temperature, and discuss the crossover between two- and three-dimensional behavior of its thermal properties as a function of the lattice parameters. \newline [1] S.~F\"olling, A.~Widera, T.~M\"uller, F.~Gerbier, and I.~Bloch, Phys. Rev. Lett. {\bf 97}, 060403 (2006). \newline [2] G.~K.~Campbell, J.~Mun, M.~Boyd, P.~Medley, A.~E.~Leanhardt, L.~Marcassa, D.~E.~Pritchard, and W.~Ketterle, Science {\bf 313}, 649 (2006). \newline [3] R.~A.~Barankov, C.~Lannert, and S.~Vishveshwara, cond-mat/0611126. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V32.00006: Noise spectroscopy for detecting multi-atomic composite states in optical lattices Henning Moritz, Anatoly Kuklov We propose and discuss methods \footnote[1]{A.B. Kuklov, H. Moritz, cond-mat/0609531} for detecting quasi-molecular complexes which are expected to form in strongly interacting optical lattice systems. Particular emphasis is placed on the detection of composite fermions forming in Bose-Fermi mixtures. We argue that, as an indirect indication of the composite fermions and a generic consequence of strong interactions, periodic correlations must appear in the atom shot noise of bosonic absorption images, similar to the bosonic Mott insulator \footnote[2]{S. F\"olling, et al., Nature {\bf 434}, 481 (2005)}. The composites can also be detected directly and their quasi-momentum distribution measured. This method -- an extension of the technique of noise correlation interferometry \footnote[3]{E. Altman et al., Phys. Rev. A {\bf 79}, 013603 (2004)} -- relies on measuring higher order correlations between the bosonic and fermionic shot noise in the absorption images.The method is expected to work well for fermionic composites consisting of less than four atoms and for bosonic ones consisting of less than six atoms. Above these numbers, the uncorrelated noise becomes too large. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V32.00007: Dynamics of multicomponent Bose-Einstein condensates on two- and three-dimensional optical lattices R. Mark Bradley, L.D. Carr, J.E. Bernard Exact solutions to the mean field equations of motion are constructed for multicomponent Bose-Einstein condensates on square, rectangular and simple cubic optical lattices. For two condensates on a rectangular optical lattice, we find temporally-periodic solutions in which the optical lattice is divided into two sublattices, and the condensates oscillate back and forth between these sublattices. For a square optical lattice, a solution is found in which single condensate moves in a checkerboard vortex-antivortex array. We also obtain fascinating solutions for two condensates in which the square optical lattice is divided into a total of four sublattices, and the condensates move cyclically between these sublattices. Stationary solutions of high symmetry are constructed for two, three and four condensates on a simple cubic optical lattice. Finally, the stability of the solutions in two dimensions is probed thorough numerical integrations of the mean field equations of motion. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V32.00008: Analysis of the coherence time of a Bose-Einstein-condensate interferometer with optical control of dynamics James Stickney, Dana Z. Anderson, Alex Zozulya Atom interferometers using Bose-Einstein condensate that is confined in a waveguide and manipulated by optical pulses have been limited by their short coherence times. We present a theoretical model that offers a physically simple explanation for the loss of contrast and propose the method for increasing the fringe contrast by recombining the atoms at a different time. A simple, quantitatively accurate, analytical expression for the optimized recombination time is presented and used to place limits on the physical parameters for which the contrast may be recovered. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V32.00009: Pairing and density-wave phases in Fermion-Boson mixtures at fixed filling Filippos Klironomos, Shan-Wen Tsai We study a mixture of fermionic and bosonic cold atoms on a two-dimensional optical lattice, where the fermions are prepared in two hyperfine (isospin) states and the bosons have Bose-Einstein condensed (BEC). The coupling between the fermionic atoms and the bosonic fluctuations of the BEC has similarities with electron-phonon couplings in crystals. We study the phase diagram for this system at fixed fermion density of one per site (half-filling). We find that tuning of the lattice parameters and interaction strengths (for fermion-fermion, fermion-boson and boson-boson interactions) drives the system to undergo antiferromagnetic ordering, s-wave and d-wave pairing superconductivity or a charge density wave phase. We use functional renormalization group analysis where retardation effects are fully taken into account by keeping the frequency dependence of the the interaction vertices and self-energies. We calculate response functions and also provide estimates of the energy gap associated with the dominant order, and how it depends on different parameters of the problem. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V32.00010: Quantum Monte Carlo simulations of resonantly interacting ultracold atoms Valy Rousseau, Peter Denteneer A one-dimensional Hubbard-like model with a term describing conversion of two atoms into a bound state (``molecule'') is presented. This model is thought to be relevant for ultra-cold atoms in an optical lattice, interacting via a Feshbach resonance. The model is solved exactly by means of Quantum Monte Carlo simulations, which allow for the measurement of physical quantities of interest, such as the superfluid density and (quasi)condensate fraction. The calculated momentum distribution function is directly comparable with experiments. Results are also confronted with mean-field theory which may be expected to give a poorer description of quantum fluctuations, especially for one-dimensional systems. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V32.00011: Systematic Ground-state Exploration for Strongly-interacting Fermions Loaded on Optical Lattices Masahiko Machida, Yoji Ohashi, Hideki Matsumoto, Susumu Yamada We systematically investigate ground state properties and effects of an optical lattice potential in one- and two-dimensional two-component trapped Fermi gases with the same population. Using an exact diagonalization method and a density-matrix renormalization group technique, we calculate the ground state many-body wave-function, as well as the density profile, as a function of the strength of an attractive interaction. We show that fine inhomogeneous zigzag patterns universally emerge in the above models under the presence of attractive on-site interaction and trap potential Theoretical and numerical analyses suggest that these structures originate from an effective repulsive interaction between tightly-bound pairs and a breakdown of translational invariance. Furthermore, it is emphasized that the pattern obtained numerically in the 2-D model is the checkerboard type, which is very similar to results recently observed in a vortex core of High-Tc cuprate superconductor. In the presentation, we will touch imbalanced cases, too. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V32.00012: Two Critical velocities for a superfluid in a periodic potential Biao Wu, Junren Shi In contrast to a homogeneous superfluid which has only one critical velocity, there exist two critical velocities for a superfluid in a periodic potential. The first one, which we call inside critical velocity, is for a macroscopic impurity to move frictionlessly in the periodic superfluid system; the second, which is called trawler critical velocity, is the largest velocity of the lattice for the superfluidity to maintain. The result is relevant to the superfluidity observed in the Bose-Einstein condensate in an optical lattice and supersolid helium. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V32.00013: Finite-size effects and entanglement in ultracold atoms on optical lattices L. D. Carr$^{2,3}$, R. C. Brown$^{2,3}$, D. G. Schirmer$^{2,3}$, R. V. Mishmash$^2$, S. P. Santos$^2$, I. Danshita$^3$, J. E. Williams$^3$, Charles W. Clark$^3$ We study finite size effects in the phase diagrams of a number of Fermi-, Bose-, and Fermi-Bose-Hubbard Hamiltonians relevant to ultracold atoms in one dimension. Both exact numerical solutions and approximations via Vidal's algorithm (Time Evolving Block Decimation) are utilized. We characterize excited states by their entanglement, in particular comparing three entanglement measures: the entropy of entanglement, Meyer's Q-measure, and the Schmidt number. We show that the phase diagrams and the entanglement structure of excited eigenstates as a function of the Hamiltonian parameters depends strongly on the number of sites and the dimensionality of on-site Hilbert space. These results are vital for experiments on small systems, as they differ greatly from what is found in the thermodynamic limit. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V32.00014: Quantum Entangled Dark Solitons in the Bose-Hubbard Model R.V. Mishmash, L.D. Carr We investigate the existence and stability of dark quantum solitons formed by Bose-Einstein condensates in a one-dimensional optical lattice. This is done by employing a one-level Bose-Hubbard model and simulating the real time dynamics of the condensate using both exact numerical techniques and Vidal's simulation method, i.e., Time Evolving Block Decimation. For the initial condition, we take a Gutzwiller ansatz wavefunction with on-site truncated coherent states and build a direct quantum analog to the soliton solutions of the Discrete Nonlinear Schr\"{o}dinger Equation. The stability of these solutions are then analyzed in the Bose-Hubbard model for different parameter regimes. We are especially interested in the behavior of dark solitons near the Mott-superfluid border. Also, we quantitatively examine the effect of quantum entanglement on dark quantum soliton stability. [Preview Abstract] |
Session V33: Quantum Cryptography and Quantum Communication II
Sponsoring Units: GQIChair: Christopher Fuchs, Bell Labs, Alcatel-Lucent
Room: Colorado Convention Center 403
Thursday, March 8, 2007 11:15AM - 11:27AM |
V33.00001: Photonic quantum memories for quantum repeaters Christoph Simon, Nicolas Sangouard, Hugues de Riedmatten, Mikael Afzelius, Hugo Zbinden, Nicolas Gisin We analyze a quantum memory protocol for photons based on the controlled reversible broadening of absorption lines. Based on an explicit solution of the equations of motion, we have studied the dependence of the memory efficiency on the optical depth of the medium and on the characteristics of the spectral distribution of the absorbers. Our group is particularly interested in the implementation of the described protocol using crystals doped with rare-earth ions. We describe how quantum memories based on this principle could be used to realize effective quantum repeaters. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V33.00002: Investigations of an optical memory based on stimulated photon echoes Wolfgang Tittel, Matthias Staudt, Sara Simon-Hastings, Mikael Afzelius, Valerio Scarani, Nicolas Gisin By interfering photon echoes produced in a Erbium-doped LiNbO3 waveguiding structure, we investigated the preservation of information encoded into the relative phase and amplitudes of optical pulses during storage and retrieval in an optical memory based on stimulated photon echo. Our findings are of particular interest for future long-distance quantum communication protocols, which rely on the reversible transfer of quantum states between light and atoms with high fidelity. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V33.00003: Numerical Modeling and Optimization of Type-I Entangled-Photon Sources Gleb Akselrod, Joseph Altepeter, Michael Goggin, Jaime Valle, Joseph Yasi, Paul Kwiat A numerical model of double-crystal entangled-photon sources based on type-I down conversion is presented. The purity of such sources is reduced by spectral and spatial decoherence resulting from crystal birefringence, imperfect phase-matching conditions, collection optics, and pump beam bandwidth and spatial modes. By accounting for these experimental phenomena, we simulated the total two-photon quantum state for sources employing uniaxial or biaxial nonlinear crystals. To verify the model, the predicted state was compared with experimentally obtained quantum state tomography data, showing good agreement. Furthermore, the model was used to design spatial and temporal phase compensation crystals to reduce the phase decoherence and improve the brightness and purity of our sources. This code will be freely available to the quantum optics community as a resource for designing and characterizing optimized entangled-photon sources. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V33.00004: Hyperentangled Bell-state analysis Tzu-Chieh Wei, Julio Barreiro, Paul G. Kwiat It is known that it is impossible to unambiguously distinguish the four Bell states of two photons using linear optics. However, hyperentanglement, the simultaneous entanglement in more than one degree of freedom, has been shown to assist the complete Bell measurement of the four Bell states (given a fixed state of the other degrees of freedom). Yet introducing other degrees of freedom also enlarges the number of Bell-like states. We investigate the limits for unambiguously distinguishing a subset of all Bell-like states. In particular, we show that the optimal number is 7 out of the 16 Bell-like states, which are composed of polarization and one additional qubit-like degree of freedom. The implications for quantum communication are also discussed. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V33.00005: Security aspects of the authentication used in quantum key growing Jan-{\AA}ke Larsson, J\"orgen Cederl\"of Unconditionally secure message authentication is an important part of Quantum Cryptography (QC). We analyze security effects of using a key obtained from QC for authentication purposes in later rounds of QC. In particular, the eavesdropper gains partial knowledge on the key in QC that may have an effect on the security of the authentication in the later round. Our initial analysis indicates that this partial knowledge has little effect on the authentication part of the system, in agreement with previous results on the issue. However, when taking the full QC protocol into account, the picture is different. By accessing the quantum channel used in QC, the attacker can change the message to be authenticated. This together with partial knowledge of the key does incur a security weakness of the authentication. The underlying reason for this is that the authentication used, which is insensitive to such message changes when the key is unknown, becomes sensitive when used with a partially known key. We suggest a simple solution to this problem, and stress usage of this or an equivalent extra security measure in QC. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V33.00006: Unconditional security at a low cost Xiongfeng Ma, Hoi-Kwong Lo In this talk, I will discuss several post-processing schemes for quantum key distribution (QKD). I will compare QKD with and without decoy states. By simulating four QKD experiments and analyzing one decoy-state QKD experiment, we compare two data postprocessing schemes based on security against individual attacks by L\"{u}tkenhaus, and unconditional security analysis by Gottesman-Lo-L\"{u}tkenhaus-Preskill. Our results show that these two schemes yield close performances. In QKD, since unconditional security is highly sought after, we conclude that one is better off considering unconditional security, rather than restricting to individual attacks. This work is appeared in Ref.~[X. Ma, Phys.~Rev.~A \emph{74}, 052325, (2006)]. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V33.00007: Decoy State Quantum Key Distribution: Theory and Practice Yi Zhao, Hoi-Kwong Lo, Xiongfeng Ma, Bing Qi, Kai Chen, Li Qian Decoy state quantum key distribution (QKD) has been proposed as a novel approach to improve dramatically both the security and the performance of practical QKD set-ups. We proved its security, and proposed the first practical decoy state QKD protocols, including the one-decoy protocol, the weak+vacuum protocol, and the general two-decoy protocol. Our further study shows that the two-way communication can effectively improve the performance of decoy state QKD. We performed the first experiments of decoy state QKD. Two protocols -- the one-decoy protocol and the weak+vacuum protocol -- were implemented with a maximum transmission distance of 60km. We implemented the decoy state method by adding commercial acousto-optic modulator to a commercial QKD system. Our theoretical and experimental studies show explicitly the power and the feasibility of decoy method, and brings it to our real- life. Our works are published in [1-5]. [1] H. -K. Lo, X. Ma, and K. Chen, Phys. Rev. Lett. 94 230504 (2005) [2] X. Ma \emph{et. al.}, Phys. Rev. A 72, 012326 (2005) [3] Y. Zhao \emph{et. al.}, Phys. Rev. Lett., 96, 070502 (2006) [4] Y. Zhao \emph{et. al.}, in \emph{Proceedings of IEEE ISIT} (IEEE, 2006) pp. 2094-2098 [5] X. Ma \emph{et. al.}, Phys. Rev. A 74, 032330 (2006) [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V33.00008: What kind of assistance enables quantum cloning? Koji Azuma, Masato Koashi, Nobuyuki Imoto We investigate the cases where a set $S$ of states $\{ |\psi_i \rangle \}$ cannot be cloned by itself, but is clonable with the help of another system prepared in state $\hat{\rho}_i$. When $S$ is pair-wise nonorthogonal, it is known that one can generate the copy from $\hat{\rho}_i$ alone, with no interaction with the original system. Here we show that a set containing orthogonal pairs exhibits a property forming a striking contrast; For any such set, there is a choice of $\hat {\rho}_i$ that enables cloning only when the two systems are interacted in a purely quantum manner that is not achievable via classical communication. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V33.00009: Stochastic Local Distinguishability Somshubhro Bandyopadhyay, Anirban Roy, Jonathan Walgate We pose the question, ``when is globally available information is also locally available?'', formally as the problem of local state discrimination, and show that the deep qualitative link between local distinguishability and entanglement lies at the level of stochastic rather than deterministic local protocols. We restrict our attention to sets of mutually orthogonal pure quantum states. We define a set of states $\{ |\psi_{i}\rangle \}$ as being\emph{stochastically locally distinguishable} if and only if there is a LOCC protocol whereby the parties can conclusively identify a member of the set with some nonzero probability. If a set is stochastically locally distinguishable, then the complete global information is potentially locally available. If not, the physical information encoded by the system can never be completely locally exposed. Our results are proved true for all orthogonal quantum states regardless of their dimensionality or multipartiality. First, we prove that entanglement is a necessary property of any system whose total global information can never be locally accessed. Second, entangled states that form part of an orthogonal basis can never be locally singled out. Completely entangled bases are, always stochastically locally indistinguishable. Third, we prove that any set of three orthogonal states, is stochastically locally distinguishable. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V33.00010: Detector dead-time effects in high-speed quantum key distribution Daniel Rogers, Joshua Bienfang, Anastase Nakassis, Charles W. Clark Recent advances in quantum key distribution (QKD) have given rise to systems that operate at transmission periods significantly shorter than the dead times of their component single-photon detectors. As systems continue to increase to multi-gigahertz transmission rates, the effects of detectors with dead times on the order of 50 ns become progressively more important. We demonstrate that security constraints in the presence of finite dead times create the seemingly-counterintuitive situation where an increase in transmission rate results in a decrease in sifted bit rate. We present an analytic model of this condition and propose novel and efficient methods to mitigate its effects. We further show that there exists an optimal transmission rate to achieve maximum secret key production for given detectors and link loss and verify that analysis with a Monte Carlo simulation. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V33.00011: Wigner Function in Phase Coding QKD Martin Suda Phase coding systems in quantum cryptography use two largely separated fiber glass Mach-Zehnder (MZ) interferometers which belong to Alice and Bob, respectively. If both partners adjust their phase shifters accordingly, the time pulses behind the second MZ interferometer exhibit interference effects which can be applied to the encoding-decoding procedure in quantum key distribution. Using Gaussian wave packets, the energy and time spectra of those interference effects are analyzed by means of the Wigner function taking into account the wave length distribution, chromatic dispersion, absorption and the dimensions of the interferometric set-up. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V33.00012: Characteristics of the near field diffraction of Laguerre-Gaussian modes using computer generated holograms. Ayman Sweiti, Warner Miller There has been interest in quantum communication devices that utilize the higher dimensions afforded by the orbital angular momentum (OAM) of photons. We present experimental and computational results on the production and superposition of OAM states. In particular, we present here the near field diffraction of the orbital angular momentum states with gray scale diffraction gratings. These patterns exhibit different fork-like structures characteristic of OAM states. These are modeled using Fresnel-Kirchhoff approximation. Also the phase structure of the diffracted field has been identified. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V33.00013: Quantum Communication with a Twist: QKD using Orbital Angular Momentum Photons Warner Miller, Ayman Sweiti We briefly outline our progress in developing a programmable-optics QKD system that utilizes the orbital angular momentum (OAM) eigenstates of photons. A photon can be prepared in a state that exhibits both its polarization as well as OAM. A single photon with polarization can communicate one bit of information. However a single photon from an appropriate set of axial eigenstates (OAM or ``twisted photon'') can, in principle, transmit many bits. While there is no improvement in bandwidth over conventional spin-based QKD devises, the use of an OAM eigenmodes in an n-state QKD system can substantially reduce the system's optical fidelity requirements. We outline the relative strengths and weaknesses in using OAM states verses polarization states regarding the (1) state preparation, (2) state propagation and (3) state detection. An essential element of any QKD system is the generation, propagation and sorting of mutually unbiased (MUB) quantum states. We demonstrate here the diffractive optics generation of MUB states built from the superpositions in an n-dimensional Hilbert space of OAM photons. In particular, we show the generation of a MUB state utilizing a liquid crystal spatial light modulator. [Preview Abstract] |
Session V34: Focus Session: Nonlinear Dynamics of Neuronal Systems
Sponsoring Units: DBP GSNPChair: Peter Jung, Ohio University
Room: Colorado Convention Center 404
Thursday, March 8, 2007 11:15AM - 11:51AM |
V34.00001: Dynamics and pattern formation of synaptic learning: Why do we profit from slow learning? Invited Speaker: Neuronal dynamics is the dynamics of the brain. It is highly nonlinear because neurons are elements responding to a membrane potential that needs to exceed a threshold in order to generate an action potential or `spike'. Neuronal dynamics occurs on at least two different levels: that of the neurons themselves (on a millisecond timescale) and a much slower one at the synapses, where learning takes place. Synapses are situated on a neuron, receive spikes emitted by other neurons, and are located at the end of an axon transmitting spikes with a finite delay. This talk will concentrate on many fascinating questions such as: What do synaptic representations (maps) of the outside sensory world look like, how do they develop as a consequence of synaptic learning, and is their development compatible with chaos or is it governed by totally different principles? In so doing, we focus on universal principles underlying both the rich diversity of neuronal dynamics of many interacting neurons and the corresponding, adiabatic, learning dynamics at the synapses in conjunction with pattern formation in large systems of synapses. The timescale of the latter is, in general, at least five orders of magnitude slower than that of the neurons. Not only does this ``slow'' synaptic learning lead to an adiabatic principle and, hence, to analytical insight into the learning process itself but it also allows for robustness of learning as compared to the much more fragile neuronal dynamics. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V34.00002: Synchronized dynamics of cortical neurons with time-delay feedback Alexandra Landsman, Ira Schwartz The dynamics of three mutually delay coupled cortical neurons are explored. When coupled in a line, delays introduce correlations in the time series at the time-scale of the delay. The middle neuron leads the outer ones by the delay time, while the end neurons are synchronized with zero lag times. Synchronization is found to be highly dependent on the synaptic time constant, with faster synapses increasing both the degree of synchronization and the firing rate. Analysis shows that pre-synaptic input during the inter-spike interval stabilizes the synchronous state, even for arbitrarily weak coupling, and independent of the initial phase. The finding may be of significance to synchronization of large groups of cells in the cortex that are spatially distanced from each other. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V34.00003: Eye-Target Synchrony and Attention R. Contreras, R. Kolster, S. Basu, H. U. Voss, J. Ghajar, M. Suh, S. Bahar Eye-target synchrony is critical during smooth pursuit. We apply stochastic phase synchronization to human pursuit of a moving target, in both normal and mild traumatic brain injured (TBI) subjects. Smooth pursuit utilizes the same neural networks used by attention. To test whether smooth pursuit is modulated by attention, subjects tracked a target while loaded with tasks involving working memory. Preliminary results suggest that additional cognitive load increases normal subjects' performance, while the effect is reversed in TBI patients. We correlate these results with eye-target synchrony. Additionally, we correlate eye-target synchrony with frequency of target motion, and discuss how the range of frequencies for optimal synchrony depends on the shift from attentional to automatic-response time scales. Synchrony deficits in TBI patients can be correlated with specific regions of brain damage imaged with diffusion tensor imaging (DTI). [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V34.00004: Complex patterns of synchrony in networks undergoing exogenous drive Jack Waddell, Michal Zochowski It has been established that various exogenous oscillatory drives modulate neural activity (and potentially information processing) in the brain. We explore the effect of an exogenous drive on the spatio-temporal pattern formation of a network of coupled non-identical R\"{o}ssler oscillators. We investigate the formation and properties of the phase locked states, dependent on the network properties as well as those of the external drive. We have found that such drive has a complex effect on the pattern formation in the network, depending on the coupling strength between the oscillators, drive strength as well as its frequency relative to the oscillators. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V34.00005: Measurements of synchronization between interacting networks in a model of focal epilepsy S. Feldt, H. Osterhage, F. Mormann, K. Lehnertz, M. Zochowski We use a simple model of two interacting networks of neurons to explain a seemingly paradoxical result observed in epileptic patients indicating that the level of phase synchrony drops below normal levels during the preictal state. We show that the transition from the interictal to preictal and then to ictal state may be divided into separate dynamical regimes: the formation of slow oscillatory activity observed during the normal (interictal) period, structureless activity during the preictal period when the two networks have different properties, and bursting dynamics driven by the network corresponding to the focus. We thus hypothesize that the beginning of the preictal period marks the beginning of the transition of the focal network from normal activity towards seizing and compare our results to measurements of the preictal length in human patients. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V34.00006: Structural network heterogeneities and network dynamics: a possible dynamical mechanism for hippocampal memory reactivation. Piotr Jablonski, Gina Poe, Michal Zochowski The hippocampus has the capacity for reactivating recently acquired memories and it is hypothesized that one of the functions of sleep reactivation is the facilitation of consolidation of novel memory traces. The dynamic and network processes underlying such a reactivation remain, however, unknown. We show that such a reactivation characterized by local, self-sustained activity of a network region may be an inherent property of the recurrent excitatory-inhibitory network with a heterogeneous structure. The entry into the reactivation phase is mediated through a physiologically feasible regulation of global excitability and external input sources, while the reactivated component of the network is formed through induced network heterogeneities during learning. We show that structural changes needed for robust reactivation of a given network region are well within known physiological parameters. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V34.00007: Processing of odor stimuli by neuronal network models of the olfactory bulb Stuart Wick, Martin Wiechert, Hermann Riecke, Rainer Friedrich The space of perceptable odors is high-dimensional and its representation in the various brain structures is still poorly understood. We focus on the olfactory bulb, which constitutes the first processing stage for odor stimuli after they have been sensed by receptor neurons. Experimentally it is found that the correlations between the outputs of the bulb are significantly reduced relative to those of the corresponding inputs, thus enhancing the discriminability of similar odors. We have generated a firing-rate-based network model with parameters derived from experimental data that reproduces decorrelation. Here we use this model to investigate the dependence of stimulus representations on odor concentration. We address the possibility of a change in perceived odor identity with changing concentration and the dependence of odor discriminability on odor concentration. We interpret some of our results within a simple mean-field model for the neural activity. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V34.00008: The dynamics of temporal ordering in driven integrate-and-fire-neurons. Jan Engelbrecht, Renato Mirollo Spike-timing neural codes involve the development of some kind of temporal order (synchrony) between a neuron's spike times and timing features in either the stimulus, local field potentials or the average activity in a population of synchronizing neurons. In order to explore the dynamics of temporal ordering we study an integrate-and-fire neuron with a (small) oscillatory component in its input. Tuning the frustration due to the interplay between the neuron's natural firing time and the oscillatory rhythm's period, leads to a rich structure of asymptotic phase locking patterns and ordering dynamics controlled by a correlation time that diverges at phase boundaries -- quite analogous to diverging correlation lengths in equilibrium phase transitions. Our results can be understood in terms on an extension of the theory of circle maps. In addition, they address how fast synchronous behavior can emerge in biological or artificial neural networks. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V34.00009: Rocking the boat: Auditory localization of ground-borne vibrations in snakes J. Leo van Hemmen Experiments [1] have shown that sand-dwelling desert snakes can localize prey in the absence of visual, chemosensory, and infrared cues. Instead, prey-generated surface waves traveling along the substrate surface provide the necessary information for a snake to estimate the stimulus position. The snake's inner ear is mechanically coupled to the lower jaw through a lever construction. Moreover, the left and right jaws in snakes are only loosely linked, thus providing the possibility of detecting surface vibrations and locating a stimulus through interaural time differences. Using the theory of floating bodies as an approximation of a snake jaw resting on a sandy substrate, we explicitly calculate [2] the response of the lower jaw to incoming surface waves and show that the sensitivity of the snake ear suffices to allow prey localization on the basis of interaural time-of-arrival differences. Refs.: [1] B.A. Young and M. Morain, J. Exp. Biol. \textbf{205} (2002) 661; [2] P. Friedel, B.A. Young, and J.L. van Hemmen, TU Munich preprint (2007). [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V34.00010: Two-dimensional encoding and adaptation in the songbird auditory forebrain Tatyana Sharpee, Katherine Nagel, Allison Doupe Neural adaptation is crucial for many auditory tasks, such as speech recognition, where robust performance is achieved over a wide range of signal-to-noise ratios and in the presence of 1/f- type noise. While faithful high-rate sampling can work well in the presence of noise which is largely uncorrelated between successive signal samples, alternative strategies might be needed to achieve reliable performance in the presence of strongly correlated noise. We studied how neurons in songbird auditory forebrain region (field L) encode temporal modulations of the amplitude of band-limited sounds using an information- theoretic method for finding relevant stimulus dimensions [1]. We robustly found that neurons in field L perform temporal processing based on simultaneous sampling of locally smoothed values of log-amplitude and its time-derivative. Either one of the two stimulus features could play the dominant role in neural response. We conclude with a theoretical explanation for the optimality of such signal processing strategies in situations where noise and signals have comparable correlation times. [1] T. Sharpee, N.C. Rust, W. Bialek, Neural. Computation 16, 223 (2004). [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V34.00011: Astrocytes optimize synaptic fidelity Suhita Nadkarni, Peter Jung, Herbert Levine Most neuronal synapses in the central nervous system are enwrapped by an astrocytic process. This relation allows the astrocyte to listen to and feed back to the synapse and to regulate synaptic transmission. We combine a tested mathematical model for the Ca$^{2+}$ response of the synaptic astrocyte and presynaptic feedback with a detailed model for vesicle release of neurotransmitter at active zones. The predicted Ca$^{2+}$ dependence of the presynaptic synaptic vesicle release compares favorably for several types of synapses, including the Calyx of Held. We hypothesize that the feedback regulation of the astrocyte onto the presynaptic terminal {\it optimizes} the fidelity of the synapse in terms of information transmission. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V34.00012: Dynamical analysis of Bayesian inference models and its relation to connectionist neural network models for the Eriksen task Yuan Liu, Angela Yu, Philip Holmes We analyze Bayesian compatibility bias and spatial uncertainly models for the two-alternative forced choice Eriksen task, in which subjects must correctly identify a central stimulus and disregard flankers that may or may not be compatible with it. We simplify the models, deriving linear, uncoupled, discrete dynamical systems and their continuum limits: stochastic differential equations. Analytical solutions of these allow us to describe how posterior probabilities and psychometric functions depend upon parameters. We compare our results with numerical simulations of original inference models and show that agreement is good enough for them to be useful in parameterizing such models. Our analysis also reveals that Bayesian updating is closely related to a simple drift diffusion process that can be derived from neural network models. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V34.00013: Walking at stability's edge John Milton, David Nichols, Adam Coleman, Coury Clemens, Annie Nguyentat, Ami Radunskaya During self-paced human walking, the variability in inter- stride intervals exhibit fractal dynamics characterized by long--range correlations having a power-law decay with exponent $\alpha$. We used diffusion fluctuation analysis (DFA) to estimate $\alpha$ as a function of the roughness of the walking surface for eight (8) healthy subjects (1200-1400 inter- stride intervals for each walking surface). For each subject the highest $\alpha$ (mean 0.96, range 0.88- 1.10) occured for walking on a running track and $\alpha$ was $15-20\%$ lower for walking on either a relatively smoother (tennis hard court) or a rougher (dirt path) surface. These observations are captured by a stochastic discrete time cubic map: $I_{i+1}=a(\xi_i)I_i - bI^3_i + \eta_i$, where $I_i$ is the $i$--th inter--stride time, $a(\xi_i)=a_o (\xi) + \xi_i$ describes parametric, colored noise where $a_0(\xi)$ is a constant that depends on surface roughness and $\xi_i$ is colored noise with mean zero, $\eta_i$ is low--intensity additive white noise, and $b$ is a constant. As the roughness, and hence $a_0(\xi)$, of the walking surface increases, the fluctuations in the inter--stride interval are predicted to obey a power law whose exponent changes non-monotonically: the highest values of $\alpha$ determined with DFA occur when $a_0(\xi)$ is close to the deterministic stability boundary $a=1$. Thus the neural control of walking appears to involve a dynamical system tuned close to the edge of stability subjected to the effects of parametric noise. [Preview Abstract] |
Session V35: DNA/RNA in Vitro
Sponsoring Units: DBPChair: Henryk Bohr, Technical University of Denmark
Room: Colorado Convention Center 405
Thursday, March 8, 2007 11:15AM - 11:27AM |
V35.00001: UV exposed electronically activated damage and photoreactivation repair Henrik Bohr, Bary Malik An investigation of the possible physics underlying the damage caused to DNA by UV radiation and its subsequent repair via a photoreactivation mechanism is presented in this study. An electronic pathway starting from the initial damage to the final repair process is proposed. UV radiation is absorbed to create a hole-excited thymine or other pyrimidine that subsequently is responsible for the formation of the thymine dimer. The negative-ion of the cofactor riboflavin, FADH-, formed by the exposure of the photolyase protein to visible light interacts with the hole-excited electronic orbital of the thymine dimer inducing a photon-less Auger transition, which restores the two thymines to the ground state, thereby detaching the lesion and repairing the DNA. Due to energy balance, the process has to involve an electronic excited state (s). The mechanism involves the least amount of energy dissipation and is charge neutral. It also avoids radiation damage in the repair process, that is, is a radiationless process. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V35.00002: Mean-Field Analysis of Recursive Entropic Segmentation of Biological Sequences Siew-Ann Cheong, Paul Stodghill, David Schneider, Christopher Myers Horizontal gene transfer in bacteria results in genomic sequences which are mosaic in nature. An important first step in the analysis of a bacterial genome would thus be to model the statistically nonstationary nucleotide or protein sequence with a collection of $P$ stationary Markov chains, and partition the sequence of length $N$ into $M$ statistically stationary segments/domains. This can be done for Markov chains of order $K = 0$ using a recursive segmentation scheme based on the Jensen-Shannon divergence, where the unknown parameters $P$ and $M$ are estimated from a hypothesis testing/model selection process. In this talk, we describe how the Jensen-Shannon divergence can be generalized to Markov chains of order $K > 0$, as well as an algorithm optimizing the positions of a fixed number of domain walls. We then describe a mean field analysis of the generalized recursive Jensen-Shannon segmentation scheme, and show how most domain walls appear as local maxima in the divergence spectrum of the sequence, before highlighting the main problem associated with the recursive segmentation scheme, i.e. the strengths of the domain walls selected recursively do not decrease monotonically. This problem is especially severe in repetitive sequences, whose statistical signatures we will also discuss. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V35.00003: Exploring Threaded Intercalation Using Optical Tweezers Thayaparan Paramanathan, Micah J. McCauley, Fredrik Westerlund, Ioulia Rouzina, Mark C. Williams Dumbbell-shaped binuclear ruthenium complexes are of interest due to their potential for use in selective chemotherapy. In bulk experiments, these complexes exhibit extremely slow binding kinetics. In contrast, single molecule studies use optical tweezers to stretch the DNA and induce much more rapid intercalation. The observed DNA force-extension curves clearly indicate an increase in DNA melting force and elongation of the DNA molecule upon drug binding, which is evidence of stabilization of the DNA and intercalation of the binuclear ruthenium complex. Hysteresis in the stretching-relaxation curves implies very slow dissociation of these molecules due to threaded intercalation. The concentration profile suggests unusually strong DNA binding affinity for the binuclear complexes compared to simple intercalators. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V35.00004: A simple method to deposit elongated DNA onto fused-silica surfaces for single molecule studies of protein-DNA interactions Yao Zhang, Keryn Gold, Y. M. Wang In order to study facilitated diffusion of proteins along DNA using single molecule fluorescence imaging methods, it is necessary to deposit elongated DNA molecules along fused-silica surfaces [1]. Here we have developed a simple method to deposit elongated DNA molecules onto fused-silica surfaces with high yield. We attached the ends of DNA molecules to streptavidin coated quantum dots and then deposited the end-labeled DNA onto fused-silica surfaces. The flow created by a cover slip is adequate to generate arrays of elongated and suspended DNA anchored by the two ends of each molecule, ideal for protein-DNA interaction studies. Interactions of LacI with these elongated DNA molecules will also be discussed. \newline \newline [1] Y. M. Wang, E. C. Cox and R. Austin, ``Single molecule measurements of repressor proteins 1D diffusion on DNA,'' Phys. Rev. Lett., 97, 048302, (2006). [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V35.00005: Strain dependent twist-stretch elasticity in elastic filaments Moneesh Upmanyu Structural chirality (i.e. handedness) often results in large mechanical couplings which modify the conformation and expression of natural and synthetic filamentous aggregates. Twist-stretch elasticity of double stranded DNA is vital during chromatin organization, transcription regulation and protein binding. Engineering such couplings in structurally robust and multifunctional nanowires and nanotubes offers an elegant route for fabrication of nanoscale motors, oscillators and switches. In instances where the device operation relies upon mechanical coupling, twist-stretch elasticity, eliminating the need for an externally actuated rotational degree of freedom. Recent results on single-walled carbon nanotubes and DNA reveal a reversal in the sign of the twist-stretch coupling at large strains. Here, we present a simple non-linear theory that captures the behavior macroscopically. Model simulations reveal that the higher order coefficients are sensitive functions of the microscopic deformation energetics. Such dynamic couplings already exist in nature, a general design principle that remains to be exploited for mechanically coupled self-actuation in nanoscale devices and biomimetic strategies. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V35.00006: Condensation of liquid crystals of complementary nDNA duplexes from a solution of mixed oligomers Giuliano Zanchetta, Tommaso Bellini, Michi Nakata, Noel Clark We have investigated the phase behavior of concentrated mixtures of: (i) the complementary oligonucleotides CCTCAAAACTCC (``oligoA'') + GGAGTTTTGAGG (``oligoB'') and (ii) the self complementary oligomer CGCGAAAATTTTCGCG (``oligoSelf'') with mixed random 20-22bp non-complementary single stranded oligomers (``oligoMix''). We find that upon cooling from above the duplex unbinding temperature, sub-picoliter liquid crystal domains of complementary oligomers condense out from the isotropic mixture of non-complementary sequences. This phenomenon is observed in 300-600 mg/ml oligomer solutions and for mixtures with the ratio of complementary/non-complementary sequences down to [oligoA]/[oligoB] = 1/15 and [oligoSelf]/[oligoMix] = 1/5. Comparison of condensated volumes and complementary/non-complementary weight ratios indicates that the segregation is strong, as also suggested by the columnar ordering on the condensed domains. We interpret these findings in terms of depletion forces acting on mixtures of flexible+rigid solutes. The spontaneous condensation of well paired sequences into microdroplets where the duplexes face each other at their endings opens new possibilities to prebiotic scenarios for the formation of biopolymers. Work was supported by NSF Grant DMR 0606528 and NSF MRSEC Grant No. DMR 0213918. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V35.00007: Structural Analysis of D- and L-RNA by UV-Resonance Raman Spectroscopy S. Binder, S. Bolik, B. Schulz, M. Ruebhausen, M. Perbandt, M. Kramer, C. Betzel, V.E. Erdmann, S. Klussmann, N. Genov Chirality is a fundamental aspect of chemical biology. Nucleic molecules naturally only exist in D- but not in L-configuration. However, the origins of this homochirality are not understood. Here we show that there are differences between the Raman spectra of D-RNA and L-RNA at different photon energies. We have analyzed the L and the D enantiomer of an RNA molecule with the sequence (r(CUGGGCGG)\textbf{.}r(CCGCCUGG)) by Raman spectroscopy at different wavelengths. The bases of nucleic acids as well as aromatic amino acids and peptide bonds show electronic transitions in the deep UV. As the oscillation modes depend on conformation and surrounding of a protein, Raman Spectroscopy can be used for structural analysis.$^{ }$When subtracting the Raman spectra of D- and L-RNA from each other, the resulting Raman Difference Spectra indicates that both forms have slightly different Raman tensors. Differences in the D- and L-RNA spectra for different incident photon energies can be explained when assuming that the electronic states in both configurations are slightly shifted with respect to each other. Our results therefore reveal new insights into the nature of chirality in nucleic acids. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V35.00008: Anomalous small-angle x-ray scattering (ASAXS) study of multivalent ion-DNA interactions Kurt Andresen, Jessica Lamb, Xiangyun Qiu, Lisa Kwok, Hye Yoon Park, Lois Pollack Multivalent ion-DNA interactions are important for biological function. The condensation and aggregation of DNA by multivalent ions has been extensively studied theoretically and (to a lesser extent) experimentally. We report on the related, but largely unexplored, interactions between DNA and multivalent ions below the critical concentration for condensation/aggregation. Using ASAXS, a technique used for previous studies of monovalent and divalent atmospheres around DNA, we have investigated the competition of monovalent and trivalent ions around the biopolymer. These data should prove vital for modeling DNA-trivalent ion interactions and the mechanisms of DNA condensation and aggregation. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V35.00009: Study of Electronic Structures of Nucleobases and Associated Nuclear Quandrupole Interactions for $^{14}$N, $^{17}$O and $^{2}$H in A-DNA and B-DNA R.H. Scheicher, Dip N. Mahato, R.H. Pink, M.B. Huang, T.P. Das, Archana Dubey, H.P. Saha, Lee Chow As part of a research program for first-principles investigation of electronic structures of A-DNA and B-DNA systems we have previously carried out studies of the magnetic hyperfine interactions for the spin-label[1] muonium attached to A-DNA and B-DNA. The present work involves the nuclear quadrupole interactions (NQI) of $^{14}$N, $^{17}$O and $^{2}$H in these two systems. We will present the results of our investigations of the NQI properties using the Hartree-Fock-Roothaan procedure with many-electron correlations included using many-body perturbation theory. For the A-DNA and B-DNA systems we are using available structural data for the four nucleobases. For the free nucleobases, the geometry from the energy optimization procedure is being employed. Comparisons will be made with available experimental NQI data and planned future improvements will be discussed. [1] R.H. Scheicher, E. Torikai, F.L. Pratt, K Nagamine, and T.P. Das, Hyperfine Interactions,158, 53 (2004); Physica B, Physics of Condensed Matter, 374, 448 (2006). [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V35.00010: Experimental studies of the relationship between DNA structure and chemical modification, and its charge transport properties V. Soghomonian, D. E. Davis, A. A. Belak, J. F. Dowd, J. J. Heremans We experimentally investigate the influence of the physico-chemical properties of DNA molecules on its charge transport capabilities. By performing comparative rather than absolute charge transport measurements, we probe the effect of chemical modifications on the electronic properties of the molecule. Modifications include the introduction of phosphodiester bond breaks, and intercalation of metal cations, as probes to ascertain the relationship between DNA structure and electronic properties. Furthermore, we perform comparative measurements between double strand and single strand DNA molecules, to probe the importance of DNA duplex structure on its electronic properties. Our comparative current-voltage measurements yield distinct curves associated with specific modifications to the DNA molecule. We also investigate different lengths of lambda DNA. AFM images confirm the presence of DNA molecules between the lithographic measurement electrodes. (NSF DMR 0103034). [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V35.00011: Density Functional Analysis of Stabilizing Effects of Stacking Interactions in Nucleic Acid Base Pair Steps David C. Langreth, Valentino R. Cooper, Timo Thonhauser, Aaron Puzder, Elsebeth Schr\"{o}der, Bengt I. Lundqvist Base pair stacking interactions contribute significantly to the stability of DNA. In addition, numerous studies highlight the stabilizing effect of thymine within DNA. Electrostatic, van der Waals (vdW) and hydrophobic interactions all contribute to these stacking interactions, but their relative contributions are unclear. In this paper, we use the newly developed vdW density functional\footnote{Dion, Rydberg, Schr\"{o}der, Langreth, Lundqvist, PRL \textbf{{92}}, 246401 (2004)} to investigate the importance of vdW interactions to stacking interactions between Watson-Crick DNA base pairs. Our results indicate that these interactions are essential for defining both the base pair step distance and the helical twist angle of DNA. Furthermore, we show that the stability gained from the presence of thymine is due to vdW interactions between the methyl group of the thymine with neighboring bases. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V35.00012: Molecular Simulations of DNA Hybridization in Solution and in Microarrays. Juan Araque, Athanassios Panagiotopoulos, Marc Robert Nucleic acid hybridization describes a thermodynamic transition in which a single-stranded DNA molecule associates with its complementary sequence. A comprehensive understanding of the thermodynamic behavior of this process can be achieved by computer simulation. However, the collective behavior of DNA hybridization in solution and on grafted surfaces exhibits disparate time and length scales that make atomistic simulations technically unfeasible. We propose a coarse-grained model where DNA strands are described by the single-site bond-fluctuation model on a cubic lattice. Our approach incorporates physically relevant features such as the sequence and orientation dependence of base-stacking and base-pairing interactions. We perform parallel tempering Monte Carlo simulations of DNA oligomers in the canonical ensemble. We explore how chain length, interaction heterogeneity, chain stiffness, and surface density alter the location of the melting temperature and the width of the transition. The model allows the determination of the free energy change associated with the grafting of probe chains onto the array surface with respect to the free probes in solution. Overall, the thermodynamic behavior predicted is in qualitative agreement with experimental observations both in solution and in microarrays. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V35.00013: Interactions between Counterions and Brushes of ssDNA D. Y. Petrovykh, A. Opdahl, Xiaosong Liu, F. J. Himpsel, L. J. Whitman We investigate interactions between counterions and brushes of single-stranded DNA (ssDNA) using x-ray photoelectron (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopies. Monolayers of thiol-modified thymine homo-oligonucleotides on gold are convenient model systems because for these ssDNA films the interpretation of the spectroscopic data is simplified and therefore quantitative analysis of the surface density, conformation, and composition is possible. A series of experiments was designed to quantify residual counterions retained in ssDNA brushes after common rinsing procedures. We find that while the residual amount of divalent Ca cations is essentially unaffected by rinsing, the monovalent K cations can be effectively removed by a rinse under flowing deionized water. Our results demonstrate that ex situ surface spectroscopies can be effectively used to systematically investigate interactions between ssDNA and counterions. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V35.00014: Single DNA electrophoresis in Pluronic F127 in a real-time fluorescence microscopy Seungyong You, David Van Winkle Electrophoresis is the separation of bio-molecules in a sieving medium by applying an electric field. The Pluronic F127 gel was introduced as a new sieving medium for electrophoresis. The mobility of DNA in this gel is not fully explained by conventional reptation theories. Here, in our work, the migration of single DNA molecule pre-stained was studied on the gel electrophoresis by real-time fluorescence microscopy. Separations were performed on dsDNA fragments ranging in length from 200 base pairs (bp) to 2500 bp in pluronic gel in various concentrations. Evidence is presented that in some cases DNA fragments electrophorese along gel crystallite grain boundaries and in other cases directly through gel crystallites. This is direct observation of DNA migration through the pluronic gel on a microscopic scale. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V35.00015: Shear unzipping of DNA: A semi-microscopic approach Buddhapriya Chakrabarti, David R. Nelson The denaturation force of double stranded DNA in shear mode is observed to be much higher than the force required to unzip individual base pairs. We present an analysis of this problem using a nonlinear generalization of a model of shear unzipping first considered by deGennes. We find that the strain on the DNA is localized over a small region on either side of the chain. The nonlinear springs of length $\kappa^{-1}$ acting in parallel on either side of the chain make the chain stiffer. The competition between this length scale $\kappa^{-1}$ and the system size $L$ gives rise to a system size dependent rupture force. While for small systems, the force scales as $F_{c} \approx f_0 L$, where $f_0$ is the rupture force of a single bond, it saturates to a value $F_{c} \approx 2 \kappa^{-1} f_0$ for large systems. We explore the role of temperature and sequence heterogeneity on the unzipping process and discuss its implications in biology and material science. [Preview Abstract] |
Session V38: Focus Session: Instrumentation and Metrology for Fundamental Physics
Sponsoring Units: GIMSChair: J. Geremia, University of New Mexico
Room: Colorado Convention Center 501
Thursday, March 8, 2007 11:15AM - 11:51AM |
V38.00001: Mechanical effects of strong measurement: back-action noise and cooling Invited Speaker: Our recent experiments show that it is now possible to prepare and measure mechanical systems with thermal occupation factors of N$\sim $25 and perform continuous position measurements close to the limits required by the Heisenberg Uncertainty Principle (1). I will discuss our back-action measurements with nanomechanical structures strongly coupled to single electron transistors. We have been able to observe the stochastic back-action forces exerted by the SET as well as a cooling effect which has analogies to cooling in optical cavities. Furthermore, I will discuss progress using optical fields coupled to mechanical modes which show substantial cooling using the pondermotive effects of the photons impacting a flexible dielectric mirror (2). Both of these techniques pave the way to demonstrating the true quantum properties of a mechanical device: squeezed states, superposition states, and entangled states. \newline \newline (1) ``Quantum Measurement Backaction and Cooling Observed with a Nanomechanical Resonator,'' A. Naik, O. Buu, M.D. LaHaye, M.P. Blencowe, A.D. Armour, A.A. Clerk, K.C. Schwab, \textit{Nature}\textbf{ 443}, 193 (2006). \newline (2) ``Self-cooling of a micro-mirror by radiation pressure,'' S. Gigan, H.R. Boehm, M. Patemostro, F. Blaser, G. Langer, J. Hertzberg, K. Schwab, D. Baeuerle, M. Aspelmeyer, A. Zeilinger, \textit{Nature} \textbf{444}, 67 (2006). [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V38.00002: An apparatus for measuring short-range deviations from Newtonian gravity including a magnetic force calibration Andrew Geraci, David Weld, John Chiaverini, Sylvia Smullin, Aharon Kapitulnik Several recent theories suggest that new physics related to gravity may appear at short length scales. For example, light moduli or particles in ``large'' extra dimensions could mediate macroscopic forces of(super)gravitational strength at length scales below a millimeter. We have built an apparatus utilizing cryogenic micro-cantilevers capable of measuring atto-Newton forces [1], which now includes a magnetic analog for force calibration. Our most recent experimental constraints on Yukawa-type deviations from Newtonian gravity will be presented. \newline References: \newline [1] J. Chiaverini, S. J. Smullin, A. A. Geraci, D. M. Weld, A. Kapitulnik, Phys.Rev.Lett. {\bf{90}}, 151101 (2003). S. J. Smullin, A.A.Geraci, D.M.Weld, J.Chiaverini, S.Holmes, and A. Kapitulnik, Phys. Rev. D 72, 122001 (2005). [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V38.00003: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V38.00004: Feedback Control and Characterization of a Microcantilever Using Optical Radiation Pressure David Weld, Aharon Kapitulnik We describe a simple method for feedback-regulation of the response of a microcantilever using the radiation pressure of a laser. A modified fiber-optic interferometer uses one laser to read out the position of the cantilever and another laser of a different wavelength to apply a force that is a phase-shifted function of that position. The method does not require a high-finesse cavity, and the feedback force is due solely to the momentum of the photons in the second laser. The feedback phase can be adjusted to increase or decrease the microcantilever's effective quality factor $Q_{\mathrm{eff}}$ and effective temperature $T_{\mathrm{eff}}$. We demonstrate a reduction of both $Q_{\mathrm{eff}}$ and $T_{\mathrm{eff}}$ of a silicon nitride microcantilever by more than a factor of 15 using a root-mean-square optical power variation of $\sim$2~$\mu$W. This technique was developed to control the response of a cantilever used as a force sensor in a next-generation test of Newtonian gravity at length scales of 20~$\mu$m. Additionally, we suggest a method for determination of the spring constant of a cantilever using the known force exerted on it by radiation pressure. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V38.00005: SQUID-based magnetic thermometry for fundamental physics and applications below $1$ K R. Sultan, R. Rahman, F. Baset, B. L. Zink One approach to sensitive thermometry below $1$ K is to measure the temperature-dependent magnetization of a paramagnet using a SQUID. Devices based on bulk materials (such as cerium manganese nitrate) and superconducting transformers provide some of the most sensitive thermometers available for dilution-refrigerator temperatures. Microcalorimeter x-ray or $\gamma$-ray detectors can be made using the same concept, with a small erbium-doped gold paramagnet (again prepared using bulk techniques) measured with a thin-film dc SQUID magnetometer. In this talk we describe our recent work toward a SQUID-based magnetic thermometer fabricated entirely using thin-film techniques. This thermometer has potential applications not only for high spectral-resolution x-ray detectors, but also for fundamental measurements of thermal transport in thin-films and nanostructures. We will discuss optimization of a low-noise dc SQUID and its coupling to the sensor, the choice of a paramagnetic thin-film, and the performance of the thermometer for various applications. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V38.00006: Shot Noise Thermometry down to 10 mK Lafe Spietz, Robert Schoelkopf We report measurements of the Shot Noise Thermometer (SNT), a primary thermometer based on the electronic noise from a tunnel junction, in the range from 10 mK to 200 mK. We demonstrate operation of the SNT down to 10 mK with 10\% accuracy at the lowest measured temperature. At 10 mK, where for a measurement frequency of f=450 MHz, $hf=2.5k_BT$, we demonstrate that, provided that quantum corrections are taken into account, the SNT continues to be a practical thermometer. We also show that self-heating is not a measurable problem and demonstrate a simplified readout of the SNT. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V38.00007: A new approach to the measurement of relaxation heat capacity. Jonathan B. Betts, Albert Migliori, Izabela Stroe, Scott Riggs We have developed low-addenda calorimeters for the 300mK -- 380K temperature range. The low addenda produces a calorimeter relaxation response with exceptionally low scatter, typically less than one part in 1000. Such low scatter exposes small errors in the usual logarithmic variation of temperature with time, and because of the low scatter, small corrections to this function can be made consistently, increasing our heat-capacity precision by an order of magnitude. We also present a calorimeter design for measuring small micro-liter liquid samples using the relaxation method in the 300K -- 400K temperature range. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V38.00008: A new thin film nanocalorimeter for measuring the heat capacity of 50nm films from 300mK to 500K. Daniel Queen, Frances Hellman Low stress silicon nitride is used in a variety of MEMS devices as an important mechanical membrane material. The high Debye temperature of the silicon nitride makes it an ideal material for use in a MEMS based calorimeter. Microfabrication techniques provide for a high degree of reproducibility between devices on a single silicon wafer. Microcalorimeters based on low stress silicon nitride have been successfully used to measure the heat capacity of 200nm films and small single crystals ($\sim $50 micrograms) for over a decade. We report results on a scaled down calorimeter for measurements of 50nm thin films. By scaling down the dimensions of the device, we have reduced the background addenda by an order of magnitude as compared to previous designs. The resulting decrease in die size allows us to use the nanocalorimeter in confined spaces such as small bore magnets and beam lines. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V38.00009: New design of a microcalorimeter with enhanced accuracy through the consideration of thermal loss of the membrane platform K. S. Suh, H. J. Kim, J. W. Kim, Y. D. Park, Kee Hoon Kim We report on the development of a Si-N/Si based microcalorimeter for measuring specific heat of small samples in a wide temperature range. By using well-known MEMS fabrication techniques [1], the heater and sensor elements are integrated on the Si-N membrane. The fabricated calorimeter is operated by a custom-made program based on the curve fitting method [2]. By comparing measured thermal conductance ($\lambda )$ from the membrane platform to the thermal reservoir in three different designs, we find $\lambda $ can critically affect the accuracy of measurement, and that the geometry of metal lines is a key parameter to control$\lambda $. Based on those findings, we provide a new design of microcalorimeter resulting in the specific heat of Cu ($\sim $ 300 $\mu g)$ consistent with literature values within 5{\%} in a temperature range between 20 and 300 K. \newline {\dag}parkyd@phya.snu.ac.kr, khkim@phya.snu.ac.kr \newline [1] D. W. Denlinger \textit{et al.}, Rev. Sci. Instrum. 65, 946 (1994); S. L. Lai \textit{et al.}, Appl. Phys. Lett. 67, 1229 (1995). \newline [2] K. S. Suh \textit{et al.}, J. Korean Phys. Soc. 49, 1370 (2006); J. S. Hwang \textit{et al.}, Rev. Sci. Instrum. 68, 94 (1997). [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V38.00010: Nonlinearity in the effect of an inhomogeneous Hall angle Daniel W. Koon The differential equation for the electric potential in a conducting material with an inhomogeneous Hall angle is extended to the large-field limit. This equation is solved for a square specimen, using a successive over-relaxation [SOR] technique for matrices of up to 101x101 size, and the Hall weighting function $--$ the effect of local pointlike perturbations on the measured Hall angle -- is calculated as both the unperturbed Hall angle, $\tan \Theta _H $, and the perturbation, $\delta \tan \Theta _H $, exceed the linear, small angle limit. Preliminary results show that the Hall angle varies by no more than 5{\%} if both $\left| {\tan \Theta _H } \right|<1$ and $\left| {\delta \tan \Theta _H } \right|<1$. Thus, previously calculated results for the Hall weighting function can be used for most materials in all but the most extreme magnetic fields. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V38.00011: Electrical measurements of parametric resonance in silicon cantilevers Michael Requa, Kimberly Turner Micro- and nano- scale mechanical oscillators, in particular cantilevers, show great promise as highly sensitive mass sensors for their small inertial mass and high Q-factors. Sensitivity to environmental factors (force and mass) are limited by theresolution of the resonant frequency measurement in such systems. Experimental investigations to the frequency resolution of bistable nonlinear dynamics for resonators exhibiting parametric resonance have been performed. Using Lorentz interactions for excitation and detection, the all electrical measurements of such oscillations in nonlinear dynamics require nontrivial instrumentation demonstrated in this work. Frequency resolution of 100 parts in 1 billion in vacuum are demonstrated at room temperature suggesting potential enhancements over more conventional harmonic resonant techniques. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V38.00012: Detectors of the Cyrogenic Dark Matter Search: Charge Transport in the Zero-Point Regime Kyle Sundqvist The Cryogenic Dark Matter Search (CDMS) seeks to detect weakly-interacting massive particles (WIMPs) in the halo of our galaxy. WIMPs are predicted under supersymmetric particle theory, and are a favored solution to the dark matter problem in cosmology. We will describe how the CDMS experiment measures both the ionized charge and the energy in athermal phonons created by particle interactions in Ge and Si crystals at a temperature of 40 mK. Performing simultaneous measurements in this way creates a signature response for each event. The result offers an unsurpassed ability to discriminate candidate WIMP-nucleon interactions from those of radioactive background. Of particular interest to condensed matter physics is a consideration of experimental challenges that arise. Charge collection potentials must remain at only a few volts, else emitted phonons from drifted carriers will dominate the phonons of the original interaction. Also, at this low lattice temperature, we operate in a charge transport regime rarely encountered for low-bias levels; carrier scattering is dominated by the zero-point fluctuations of the lattice ions. We apply this understanding to charge-related phenomena seen in our detectors. [Preview Abstract] |
Session V39: Focus Session: Negative Differential Resistance I
Sponsoring Units: FIAPChair: Udo Pernisz, Dow Corning Corporation
Room: Colorado Convention Center 502
Thursday, March 8, 2007 11:15AM - 11:51AM |
V39.00001: Negative Differential Resistance Phenomena in Molecular Metal-Insulator-Metal Junctions Invited Speaker: Negative-differential-resistance (NDR) phenomena have been observed in metal-insulator-metal (MIM) junctions composed of various materials and are of interest because of their potential for producing bistability, oscillation, and gain in electronic circuitry. Recently, NDR has been observed by a number of different techniques in molecular MIM junctions based on self-assembled monolayers of nitro-substituted oligo(phenlyene-ethynylene) (OPE). Like NDR in other MIM systems, the mechanism behind the NDR in molecular MIM junctions is not well understood. We have recently investigated Hg-C14//OPE-Au bilayer molecular junctions that exhibit stable NDR characteristics over many bias sweeps, thereby permitting a systematic study of this effect. Our results on the variation of NDR peak with sweep rate, the charge flow during the NDR branch, the current-voltage characteristic for cyclic sweeps and other details of the characteristics suggest that the NDR is caused by charge storage (oxidation or reduction) within the junction that acts to modify the effective tunneling parameters. Quantitative estimates of the stored charge, the tunneling barrier height and effective mass also support this basic physical picture. These results will be compared with those for other MIM junction systems and discussed in the context of proposed mechanisms for this system. The physical picture developed here should be useful for understanding molecular MIM junctions at a fundamental chemical level. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V39.00002: Characterization of Electrical Devices Based Upon Organic Monolayers Directly Attached to Si. C.A. Richter, C.A. Hacker, N. Gergel-Hackett, L.J. Richter, T.C. Allison, V. Mujica, C.A. Gonzalez We present the results of electrical measurements of alkane monolayers directly attached to Si-substrates. Molecular electronic test structures consisting of thiol or alcohol terminated molecules covalently attached via UV-assembly to Si (n-type, p-type, $<$100$>$, or $<$111$>)$ in oxide defined ``wells'' were characterized by capacitance-voltage and current-voltage (IV) measurements. These results are compared to models based on a first-order expansion of the system's Green function and a new method for the Si-molecule contact that predict a negative differential peak in the IV measurements due to an alignment between the Fermi energy of the Si substrate and the molecular LUMO. The structure of the molecular monolayers under full metallization was assessed with a novel backside incident FTIR technique and depth-profiling XPS. Au fully displaces the molecular monolayers (and forms a silicide), while Ag does not react with the underlying Si and forms a Ag/molecule/Si junction. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V39.00003: Voltage-controlled non-linear characteristics of interdigitated MIM devices Bonnie Ludwig, Alex Hegyi, Christopher Kelly, Damian Khan, Mark Banaszak Holl, Bradford Orr Self-assembly was key to creating the first metal-molecule-metal (M-mol-M) junctions in Au-(HSiO$_{1.5})_{n}$-Au devices. Metal-insulator-metal (MIM) junctions are simple thin-film devices that, after application of an electroforming pulse, self-assemble into a system of conductive critical links. Electroforming causes the voltage-controlled response of the device to change from ohmic behavior to an unusual combination of nonlinearities including negative differential resistance (NDR), multiple rewriteable resistance states, and random telegraph signal (RTS) noise. The specific physical or chemical changes that cause these effects are unknown. Certain Au-(HSiO$_{1.5})_{n}$-Au devices have voltage-controlled behavior that is indicative of a reduced number of critical links. A comparison of the electronic behavior of MIM devices on two different size scales will be presented that provides further insight into the nature of individual critical links in these devices. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V39.00004: Scanning Probe Microscopic Characterization of Electronic Properties of Self-Assembled Organic Monolayers on Au Surface Govind Mallick, Shashi Karna The surface ordering and electronic properties of $\sigma $-bonded 1-dodecanethiol molecules and a series of $\pi $-bonded molecules of comparable length self-assembled on Au-on-mica surface have been studied by atomic force and scanning tunneling microscopic measurements. The molecular-level surface-probe microscopic images show a higher degree of ordering in the monolayers of $\sigma $-bonded molecule compared to that of the conjugated $\pi $ molecules. This is explained in terms of a larger molecular volume and intermolecular repulsion in the later than the former. Electronic measurements reveal, as expected, a higher tunnel current in the $\pi $-bonded molecules compared to $\sigma $-bonded molecules. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V39.00005: Absolute Negative Resistance Induced by Directional Electron-Electron Scattering in a Two-Dimensional Electron Gas Ismet I. Kaya A ballistic conductor is restricted to have positive three terminal resistance just as a Drude conductor. Intercarrier scattering does not influence the conductivity of the latter transport regime and does not exist in the former. However, as the electron energies increased, in the intermediate regime, single or few intercarrier scattering events starts to dominate the transport properties of a conductor with sufficiently small dimensions. A three-terminal device formed by two electrostatic barriers crossing an asymmetrically patterned two dimensional electron gas displays an unusual potential depression at the middle contact, yielding absolute negative resistance. The device displays momentum and current transfer ratios that far exceed unity. The observed reversal of the current or potential in the middle terminal is interpreted as the analog of Bernoulli's effect in a Fermi liquid. The results are explained by directional scattering of electrons in two dimensions. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V39.00006: Resonant tunnelling assisted electrical switching in amorphous-carbon multilayer-superlattice structures Somnath Bhattacharyya, S. R. P. Silva Negative differential resistance (NDR) in an amorphous carbon (a-C) double barrier resonant tunnel diode (DB-RTD) with an estimated cut-off frequency well into the gigahertz regime is reported [1]. Presently we extend this work in carbon multi-layer superlattice structures by showing room temperature resonant tunnelling and establish a high value of the phase coherence length of $\sim $10 nm for low-dimensional amorphous materials. By applying a high bias, these structures are modified with reversible current switching of up to four orders of magnitude with a NDR signature and multiple peaks representative of resonant tunnelling in the ON state. In addition to the formation of filamentary channels by applying high bias, all these features are also explained using concepts based on tunnelling through the interface of the carbon layers, quantum-dot heterostructures and the presence of a confined two dimensional electron gas. This switching behavior and its tunability have been tested by applying a microwave signal up to 100 GHz which suggest the potential for novel high-speed memory devices. [1] S. Bhattacharyya, S.J. Henley, E. Mendoza, L.G-Rojas, J. Allam and S.R.P. Silva, Nature Mater. \textbf{5}, 19 (2006). [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V39.00007: Negative Differential Resistance in Covalently-Bridged Carbon Nanotube Junctions K.H. Khoo, Y.-W. Son, Marvin L. Cohen, J.B. Neaton, Steven G. Louie Recently, negative differential resistance (NDR) was observed in IV characteristics of carbon atomic wires connected across multi-wall carbon nanotubes.$^{1}$ Motivated by these results, we calculated IV characteristics of carbon atomic wires covalently bridging capped single-wall armchair carbon nanotubes using an \textit{ab-initio} scattering-state formalism based on density functional theory.$^{2}$ Our calculations for carbon chains with an odd number of atoms yield currents orders of magnitude larger than that of even chains, demonstrating clear even-odd behavior. We also observe NDR for odd chains and shorter even chains, in agreement with experiment. The current drop at higher voltages is attributed to an energy mismatch between localized nanotube ``cap states'' on different leads, possibly a generic feature of carbon nanotube molecular junctions. [1] T. Yuzvinsky \textit{et al., Nano Lett.} 10.1021/nl061671j (2006). [2] H.J. Choi, M.L. Cohen and S.G. Louie, to be published. This work supported by NSF Grant No DMR04-39768 and DOE Contract No DE-AC02-05CH11231. Computational resources from NERSC and SDSC. [Preview Abstract] |
Session V40: FQHE: Nu=5/2, Topological Quantum Computing
Sponsoring Units: FIAPChair: Nick Bonesteel, Florida State University
Room: Colorado Convention Center 503
Thursday, March 8, 2007 11:15AM - 11:27AM |
V40.00001: Quantum Hall effect in a quantum point contact at Landau filling fraction $\nu=\frac{5}{2}$ Jeffrey Miller, Iuliana Radu, Dominik Zumb\"uhl, Eli Levenson-Falk, Marc Kastner, Charles Marcus, Loren Pfeiffer, Ken West We study the transport properties of quantum point contacts (QPC) fabricated on a GaAs/AlGaAs two dimensional electron gas that exhibits excellent bulk fractional quantum Hall effect, including a strong plateau in the Hall resistance at Landau level filling fraction $\nu= \frac{5}{2}$. We find that the $\nu=\frac{5}{2}$ plateau is identifiable in point contacts with lithographic separations as small as 0.8 microns, but is not present in a 0.5 micron QPC. We study the temperature and dc-current-bias dependence of the $\nu=\frac{5}{2}$ plateau---as well as neighboring fractional and integer plateaus---in the QPC. We also discuss our method to study the QPC at one filling fraction while the bulk remains at a higher filling fraction. Research supported in part by Microsoft Corporation, Project Q, and HCRP at Harvard University, and ARO (W911NF-05-1-0062), the NSEC program of the NSF (PHY-0117795) and NSF (DMR-0353209) at MIT. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V40.00002: Spin and NMR at $\nu = 5/2$ L.A. Tracy, J.P. Eisenstein, L.N. Pfeiffer, K.W. West The spin degree of freedom in a 2DES at $\nu = 5/2$ has been probed using resistively-detected NMR (RDNMR). We have measured the nuclear spin-lattice relaxation time $T_{1}$ of the host semiconductor $^{75}$As nuclei and the response of the 2DES longitudinal resistance to a NMR-induced reduction of the nuclear spin polarization. Measurements were made at temperatures ranging from 200 mK down to 75 mK, where the $\nu = 5/2$ quantum Hall state is just beginning to form. Via the hyperfine coupling, a decrease in nuclear spin polarization increases the electronic Zeeman splitting. Within this model, our RDNMR signal implies that the 2DES resistivity depends on Zeeman energy and, therefore, that the electronic spin polarization at $\nu = 5/2$ is incomplete in this temperature regime. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V40.00003: 5/2 FQHE without assuming pairing Jainendra Jain, Csaba T\H oke We investigate the fractional quantum Hall effect at filling factor 5/2 without assuming pairing at the outset; this provides an alternative starting point to the Pfaffian wave function of Moore and Read. The model of noninteracting composite fermions produces a gapless Fermi sea at 5/2. We demonstrate that the residual interaction between composite fermions opens a gap and establishes incompressibility at this filling factor. This approach has the advantage of being amenable to systematic perturbative improvements, and produces ground as well as excited states. It also shows that the 5/2 state can be obtained from the composite-fermion Fermi sea by a slight reorganization of composite fermions near the Fermi energy. We will comment on the quantitative accuracy of this approach, as well as its relation to other approaches. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V40.00004: Properties of a double point contact in the Moore-Read quantum Hall state. Eddy Ardonne, Eun-Ah Kim The double point contact is an ideal setup to probe the statistics properties of the 5/2 quantum Hall state. We provide quantitative predictions for such a setup at finite temperature and voltage, based on the assumption that the system is in the Moore- Read (a.k.a. pfaffian) quantum Hall state. We will focus on the non-abelian features in our predictions for current, and in particular, in the current-current correlation measurements. We find that the current-current correlations have qualitatively different frequency dependence, depending on the state of the device. Therefore, the current-current correlations provide an excellent signature of the non-abelian statistics, which should be accessible with current technology. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V40.00005: Nature of excitations of the 5/2 fractional quantum Hall effect Nicolas Regnault, Csaba Toke, Jainendra Jain We show, with the help of exact diagonalization studies on systems with up to sixteen electrons in the presence of up to two delta function impurities, that the Pfaffian model is not accurate for the actual quasiholes and quasiparticles of the 5/2 fractional quantum Hall effect. We discuss implications for non-Abelian statistics. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V40.00006: Explicit monodromy of Moore-Read wave functions on a torus Suk Bum Chung, Michael Stone We construct the wave functions for the Moore-Read $\nu = 5/2$ quantum Hall state on a torus in the presence of two quasiholes. These explicit wave functions allow us to compute the monodromy matrix that describes the effect of quasihole motion on the space of degenerate ground states. The result agrees with that discussed recently by Oshikawa {\it et al.} (cond-mat/067743) Our calculation provides a conformal field theory explanation of why certain transitions between ground states are forbidden. It is because taking a quasihole around a generator of the torus can change the fusion channel of the two quasiholes, and this requires a change of parity of the electron number in some of the ground states. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V40.00007: Mapping the Braiding Properties of Non-Abelian FQHE Liquids. Emil Prodan, F. D. M. Haldane Non-Abelian FQHE (NAFQHE) states have elementary excitations that cannot be individually locally-created. When widely separated, they give rise to topological (quasi-)degeneracy of the quantum states; braiding of such non-Abelian quasiparticles (NAQP's) implements unitary transformations among the degenerate states that may be useful for ``topological quantum computing'' (TQC). We have developed a new technique for explicit computation of NAQP braiding in models exhibiting ideal NAFQHE behavior (where the topological degeneracy is exact), in particular the Moore-Read $\nu$ = 5/2 state. For systems of small numbers of NAQP's on a sphere, we have computed the non-Abelian Berry curvature and Hilbert space metric, as one NAQP is moved relative to a fixed configuration of the others, showing how the topological properties develop as the system size (NAQP separation) increases. We also studied the effect of perturbations (Coulomb interaction and substrate potentials) that lift the exact degeneracy, and become the dominant corrections when NAQP's are brought together so that quantum measurements can be made; these effects are likely to be crucial in determining whether TQC is viable in NAFQHE systems. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V40.00008: Edge Excitations and Non-Abelian Statistics in the Moore-Read State: A Numerical Study in the Presence of Coulomb Interaction and Edge Confinement Kun Yang, Xin Wan, Edward Rezayi We study the ground state and low-energy excitations of fractional quantum Hall systems on a disk at filling fraction 5/2, with Coulomb interaction and background confining potential. We find the ground state that has the same angular momentum quantum number of, and subtantial overlap with the Moore-Read state is stable within a finite but narrow window in parameter space. The corresponding low-energy edge excitations contain a fermionic branch and a bosonic branch, with widely different velocities. A short-range repulsive potential can stabilize a charge +e/4 quasihole at the center, leading to a different edge excitation spectrum due to the change of boundary conditions for the edge Majorana fermion mode, clearly indicating the non-Abelian nature of the quasihole. On the other hand the stabilization of a charge +e/2 quasihole does not change the characteristics of the fermionic edge excitation spectrum qualitatively. Possible edge instabilities due to the competition between Coulomb interaction and confining potential will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V40.00009: Spin Order and Spin Textures in Paired Quantum Hall States Ivailo Dimov, Chetan Nayak We consider quantum Hall ground states and low-lying excitations at even-denominator filling fractions, especially $\nu=5/2$, in the limit of small Zeeman energy. We show that an incompressible ground state will exhibit spontaneous ferromagnetism, which implies that it is in the universality class of the Pfaffian state. Sufficiently strong spin-orbit coupling will drive the system into the $(3,3,1)$ state. The Pfaffian state has low-energy charged excitations which are spin textures. If the Zeeman energy is below a critical value, the ferromagnetic exchange energy stabilizes a bound state of two non-Abelian quasiparticles. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V40.00010: Interferometry Measurements of Anyonic Charge Parsa Bonderson, Johannes Slingerland, Kirill Shtengel We examine interferometric measurements of the topological charge of (possibly non-Abelian) anyons. Such measurements are essential to the implementations of topological quantum computation which have been proposed in the context of quantum Hall states. Anyons are placed in a Mach-Zehnder interferometer and their topological charge is determined from the effect it has on the interference of probe particles sent through the interferometer. We find that superpositions of distinct anyonic charges in the target collapse when the probe particles have nontrivial monodromy with the differences between the anyonic charges. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V40.00011: Anyons in a weakly interacting system Conan Weeks, Babak Seradjeh, Marcel Franz Anyons -- particles with fractional exchange statistics -- are known to emerge as excitations in certain strongly correlated two dimensional systems such as the fractional quantum Hall liquids. Here we describe a theoretical proposal for a system whose excitations are anyons with the exchange phase $\pi/4$, but, remarkably, can be thought of as a composite of essentially noninteracting electrons. The system consists of an artificially structured type-II superconducting film adjacent to a 2D electron gas in the integer quantum Hall regime with filling fraction 1. The proposal is based on the observation that a vacancy in an otherwise periodic vortex lattice creates a bound state in 2DEG with total charge $-e/2$. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V40.00012: Topological invariants of time-reversal-invariant band structures Joel Moore, Leon Balents The topological invariants of a time-reversal-invariant band structure in two dimensions are multiple copies of the $\boldmath{Z}_2$ invariant found by Kane and Mele. Such invariants protect the topological insulator and give rise to a spin Hall effect carried by edge states. Each pair of bands related by time reversal is described by a single $\boldmath{Z}_2$ invariant, up to one less than half the dimension of the Bloch Hamiltonians. In three dimensions, there are four such invariants per band pair. The $\boldmath{Z}_2$ invariants of a crystal determine the transitions between ordinary and topological insulators as its bands are occupied by electrons. We derive these invariants using maps from the Brillouin zone to the space of Bloch Hamiltonians and clarify the connections between $\boldmath{Z}_2$ invariants, the integer invariants that underlie the integer quantum Hall effect, and previous invariants of ${\cal T}$-invariant Fermi systems. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V40.00013: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V40.00014: Probing Non-Abelian Statistics with Quasiparticle Interferometry Johannes Slingerland, Parsa Bonderson, Kirill Shtengel We examine interferometric experiments in systems that exhibit non-Abelian braiding statistics, particularly the non-Abelian quantum Hall states that have recently been proposed as media for topologically protected quantum computation. We find a general expression for the current through a two point contact interferometer in these systems in terms of the topological S-matrix of the non-Abelian anyons. In particular, we give detailed results for the Read-Rezayi series of states, providing explicit predictions for the recently observed $nu=\frac{12}{5}$ quantum Hall plateau. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V40.00015: Abelian and non-abelian quasi-particles as domain-wall type defects Alexander Seidel, Dung-Hai Lee The traditional framework to study fractional quantum Hall states is based on Laughlin type wavefunctions and Chern-Simons field theories. Recently, a new framework has been proposed that puts stronger emphasis on the one-dimensional (1d) Hilbert space structure of Landau levels. This formalism is based on the fact that all known many-body wavefuntions describing fractional quantum Hall liquids reduce to simple one-dimensional charge-density-wave (CDW) patterns when studied on a thin torus or cylinder. These CDW states are adiabatically connected to the bulk quantum Hall liquid states when the circumference of the cylinder or torus is increased. Many general properties of fractional quantum Hall systems are rooted in these CDW states, such as degeneracies and fractional quantum numbers. In this talk, it will be shown in particular how the properties of the $\nu=1$ bosonic Pfaffian state are encoded in the corresponding CDW-patterns. It will also be explained how even braiding statistics can be addressed in a language of adiabatically evolved 1d domain-wall states, at least in the abelian case. [Preview Abstract] |
Session V42: Metal Oxides: Growth, Structure, Interfaces
Sponsoring Units: DCMPChair: Jiandi Zhang, Florida International University
Room: Colorado Convention Center 505
Thursday, March 8, 2007 11:15AM - 11:27AM |
V42.00001: Characterization of Epitaxial Ag$_{2-x}$O Thin Films Grown on Sapphire S.B. Rivers, G. Bernhardt, M.W. Wright, D.J. Frankel, M.M. Steeves, R.J. Lad We have grown silver oxide films with a range of stoichiometry near Ag$_{2}$O by e-beam evaporation of silver in an oxygen electron cyclotron resonance (ECR) plasma. Films were deposited on r-cut sapphire substrates. A quartz crystal oscillator was used to monitor the film growth and to determine ECR oxygen flux by examining the rate of oxygen uptake on a silver film. This information was used to select the silver rate (0.1 or 1.0 {\AA}/s) and the oxygen flow rate (from 2 to 10 sccm). XRD and RHEED analysis reveals films grew with one-dimensional $<$111$>$ epitaxy, true three-dimensional $<$002$>$ epitaxy, or a mixed phase depending on the deposition conditions. XRD and XPS shows the composition varies with deposition conditions and can be a mixture of AgO and Ag$_{2}$O. UV-vis spectroscopy shows that the films have a single absorption edge between 3.1 and 3.5 eV. Optical transmission from 500 to 700 nm is between 70 and 80{\%}. Four-point van der Pauw conductivity and Hall effect measurements indicate that the Ag$_{2-x}$O films are p-type with a conductivity on the order of 3$\times $10$^{-3} \quad \Omega ^{-1}$cm$^{-1}$. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V42.00002: Ultrathin CoO (100) films on Fe$_{3}$O$_{4}$ (100): a photoemission study Hui-Qiong Wang*, Eric I. Altman**, Victor E. Henrich* Using molecular beam epitaxy (MBE), 1 to 20 monolayer (ML) thick CoO (100) films were grown monolayer by monolayer on Fe$_{3}$O$_{4}$ (100) substrates. Auger measurements as a function of CoO film thickness indicated a layer-by-layer growth mode. Ultraviolet photoelectron spectroscopy (UPS) was used to monitor the evolution of the thin film electronic properties. To avoid oxidizing the Fe$_{3}$O$_{4}$ surface, Co was deposited in UHV and then oxidized for each monolayer. By comparing UPS spectra taken before and after oxidization of the Co, the separate contributions of Co and oxygen to the valence band electronic structure could be identified. Very thin ($e.g.$, 1 -- 3 ML) films exhibit valence band structures very different from those of bulk CoO. The interfacial electronic states were analyzed by comparing measured and model UPS spectra. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V42.00003: Biaxial Texture Evolution in Ion-Beam Assisted Deposition of MgO Vladimir Matias We examine the evolution of biaxial crystalline texture during ion-beam assisted deposition (IBAD) of MgO using reflection high-energy electron diffraction, \it in situ \rm ion scattering, and x-ray diffraction. The IBAD-MgO templates on metal tape are used for second generation high-temperature superconducting wire, also known as coated conductors. For MgO and some other materials with a rock salt crystalline structure, IBAD texturing can be achieved within the first few nanometers of deposited material. We find that the texture development is very sensitive to the nucleation surface conditions, both chemical species and surface morphology. In the best cases an in-plane texture of 2 degrees and an out-of-plane texture of 1 degree are attainable in a homoepitaxial MgO layer. We are utilizing a methodology of presenting data in terms of IBAD texture contour plots where we collect data as a function of ion-to-molecule ratios and film thickness. The striking conclusion from the data is that the texture development for different ion-to-molecule ratios can be scaled with the cumulative ion damage normalized to deposited MgO material. We discuss the results in terms of possible mechanisms for IBAD-MgO biaxial texturing and relationship to other IBAD texturing processes. This work is supported by the Department of Energy Office of Electricity Delivery \& Energy Reliability. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V42.00004: Phase diagram for the Ni/Al$_{2}$O$_{3}$ interface and relationships to adhesion Xiao-Gang Wang, John Smith First-principles$^{ }$calculations conducted over a broad range of atomic configurations have$^{ }$been used to determine the phase diagram and work of$^{ }$separation for Ni/Al$_{2}$O$_{3}$ interfaces[1]. Seven interfacial phases have been identified.$^{ }$The results reveal that the strongest (O-rich) phases derive their$^{ }$strength from ionic Ni-O bonds across the interface, reminiscent of$^{ }$NiO. The Al-rich phases are also strong, exhibiting a mix$^{ }$of Ni$_{3}$Al-like and Al$_{2}$O$_{3}$-like interfacial bonds. The stoichiometric interfaces are$^{ }$the weakest since they are formed from the ground-state Al$_{2}$O$_{3}$(0001)$^{ }$surface. [1] X.-G. Wang, J. Smith, A. G. Evans, Phys. Rev. B 74, 081403(2006). [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V42.00005: Interplay between structure and electronic properties in metal-oxide interfaces Matias Nunez, Marco Buongiorno Nardelli Using first principles calculations we have investigated a broad spectrum of metal-insulator interfaces, including crystalline oxides and ferroelectrics. In our study we have focussed on the role of the interface phase in determining the properties of the composite system and, in particular, the relation between interface structure, charge transfer at the interface, and the associated interface dipole. For crystalline oxides such as BaO or SrO, our results demonstrate the possibility of tuning the Schottky barrier height by manipulating the interface phase [1]. For ferroelectric materials, such as BaTiO3, we have analyzed the interplay between the interface phase, the thickness of the ferroelectric layer and the residual polarization of the thin film. The polarization of the ferroelectric has been computed using modern theory of polarization via the displacements of the centers of the Wannier functions associated with the system. [1] M. Nunez and M. Buongiorno Nardelli, Phys. Rev. B, 73, 235422 (2006). [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V42.00006: Morphology of cerium oxide surfaces in an oxidzing enviroment:a first-principles investigation Marco Fronzi, Aloysius Soon, Catherine Stampfl, Bernard Delley, Enrico Traversa A good understanding of the stability and chemistry of CeO$_2$ surfaces is crucial for a better designing of solid oxide fuel cells. As the first step, we use DFT [1] to study the structural and electronic ground state properties of bulk CeO$_2 $. various surface termination of the low-index surface of CeO$_2$ are then investigated, namely the stoichiometric, metal- and oxygen- rich terminations, and defected surfaces. Using the concept of \textquotedblleft{$ab$ $initio$ atomistic thermodynamics}\textquotedblright~[2], we calculate the surface free energy phase diagram. This allows us to identify and predict stable, and potentally catalytically important, structures. There is an evidence to suggest an interesting morphological change in the surface structures with varying oxygen concentration. Reaction pathways for methane oxidation on low energy cerium oxide surfaces are being investigated and will be reported.\newline [1] Formulated in the DMol$^3$ code; B. Delley, J. Chem. Phys. 92, 508 (1990);ibid. 113, 7756 (2000).\newline [2] K. Reuter, C. Stampfl and M. Scheffler, in Handbook of Materials Modeling, Volume 1, Fundamental Models and Methods, Sidney Yip (Ed)(2005). [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V42.00007: Stabilization mechanisms of polar ZnO surfaces revisited Shengbai Zhang, Mao-Hua Du, Steven Erwin, John Northrup The polar (0001) surfaces of ZnO exhibit a variety of different morphologies. The mechanisms underlying this diversity have not been definitively identified. Here we evaluate the role of several possible candidates. We show that electrostatics does not play a significant role. Instead, we argue that surface morphology is determined by a competition between two other mechanisms. The first is the electron counting rule, which leads to semiconducting surfaces. The second arises from the large cohesive energy of ionic crystals such as ZnO, which tends to preserve the surface stoichiometry at its bulk value, leading to metallic surfaces. First-principles calculations show a crossover in the relative stability of semiconducting and metallic surfaces as the chemical potential of oxygen is varied. This behavior accounts for the many observed surface morphologies on ZnO(0001), including triangular islands and pits. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V42.00008: Crystalline Metal Oxide Nanoparticle Films for Renewable Energy Technologies Anne Dillon, Se-Hee Lee, Rohit Deshpande, Philip Parilla, Kim Jones, Harv Mahan Hot-wire chemical vapor deposition (HWCVD) has been employed as a scalable method for the deposition of crystalline tungsten oxide nanorods and nanoparticles. Under optimal synthesis conditions, only crystalline WO$_{3}$ nanostructures with a smallest dimension of $\sim $ 10 - 50 nm are observed with extensive transmission electron microscopy (TEM) analyses. X-ray diffraction (XRD), Raman spectroscopy and electron diffraction confirm that the crystalline phases of the nanostructures may be tuned by varying the synthesis conditions such that a single phase is obtained. HWCVD has also been employed to produce crystalline molybdenum oxide nanoparticles at high density. TEM analyses show that the smallest dimension of these nanostructures is $\sim $ 5 -- 30 nm. XRD and Raman analyses reveal that the materials are highly crystalline and consist of Mo, MoO$_{2}$ and MoO$_{3}$ phases. It is also possible to fabricate large area porous films of either the tungsten or molybdenum oxide nanoparticles using a novel electrophoresis deposition technique. Furthermore, WO$_{3}$ nanoparticle films have led to profound advancement in state-of-the--art electrochromic technologies, and MoO$_{x}$ films are promising for new lithium-ion batteries. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V42.00009: Nanostructured Molybdenum Oxides for Lithium-Ion Batteries Se-Hee Lee, Rohit Deshpande, Phil Parilla, Kim Jones, Bobby To, Harv Mahan, Anne Dillon Lithium-ion batteries are the current power sources of choice for portable electronics. Although such batteries are commercially successful, they are not keeping pace with the rapid advances in computing technologies. Also, further improvement of performance and simultaneous reduction in cost as well as material toxicity remain the subject of intensive research. Here we report the synthesis and electrochemical performance of a novel molybdenum oxide nanoparticle anode that dramatically improves current Li-ion battery technologies. Crystalline MoO$_{x}$ nanoparticles have been grown by an economical hot-wire chemical-vapor-deposition (HWCVD) technique and a recently developed electrophoresis technique is employed for the fabrication of porous nanoparticle anodes. Our material exhibits a high reversible capacity of $\sim $600 mAh/g in the range 0.005-3.0 V with excellent cycling characteristics as well as high-rate capability. Both cycling stability and rate capability issues are addressed by employing these porous molybdenum oxide films that consist of nanoscale active particles. These materials will impact the next generations of rechargeable lithium batteries, not only for applications in consumer electronics, but also for clean energy storage and use in hybrid electric vehicles. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V42.00010: The Onset of Ordered Vacancy Configurations on the TiO$_{2}$ (110) Surface. Scott J. Thompson, Steven P. Lewis The (110) surface of TiO$_{2}$ is a prototypical metal-oxide surface system that has numerous important industrial applications relating to solar energy, gas sensors, and the decomposition of harmful organic compounds. Extensive experimental and theoretical studies have shown that the most common surface defects, bridging O vacancies, play an important role in the desired oxidation processes of this model photocatalyst. Additionally, experimentally observed surfaces have shown the tendency for these defects to arrange themselves in both isolated and highly ordered configurations at low and high vacancy concentrations, respectively. Through Monte Carlo simulations of a converged cluster-expansion model parameterized by density functional calculations, we have observed the onset of multiple ordered configurations of bridging oxygen vacancies at different concentrations. In this talk, we will present our results that show two long-ranged ordered configurations over a relatively wide range of chemical potentials in addition to two semi-ordered configurations, all of which are in good qualitative agreement with experimental and theoretical results. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V42.00011: One-dimensional Au chains on TiO$_{2}$(110) Adam Kiejna, Tomasz Pabisiak One-dimensional (1D) Au chains on the 1$\times $2 missing row (mr) defected, and the added row (ar) reconstructed, TiO$_{2}$(110) surfaces are calculated from first principles. Single, dimer, and triple Au rows were considered. The single Au row binds strongly to the mr (2.83 eV) but much weaker to the ar surface (1.56 eV). On the mr surface the bonding of Au is mainly to the Ti atoms below, and to the neighboring Au atoms. In the mr surface the binding energy is decreasing with the number of Au atoms per row, while in the ar surface converse is observed. Even for triple Au rows the binding per atom (2.42 eV) is by 0.17 eV stronger for the mr than for the ar, the latter being suggested as the most favored structure of the clean surface. Thus, Au forms on the 1$\times $2 missing row TiO$_{2}$(110) surface strongly adsorbed 1D chains. The charge density distribution and the increased density of occupied states around the Fermi edge suggest metallic behavior of the Au rows. The bonding to the substrate is predominantly covalent. Intra- and inter-chain spacings are determined by the substrate periodicity. The large inter-chain distance (13 {\AA}) on the more or less insulating substrate makes this system ideal for studies of 1D phenomena. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V42.00012: Time Resolved Surface Diffuse Scattering During Oxide Growth J.Z. Tischler, B.C. Larson, Gyula Eres, C.M. Rouleau, P. Zschack The time dependence of surface x-ray diffraction during pulsed laser deposition provides detailed information about transverse surface structure and interlayer transport during layer-by-layer growth. To investigate the nature of growth during homoepitaxy of SrTiO$_3$, we measured the time-dependent evolution of surface diffuse scattering around the specular crystal truncation rod during deposition from a single laser shot (0.1 monolayer/pulse) on a pristine surface of (001) SrTiO$_3$. We observed the nucleation of very small islands and measured the time dependence of ripening into larger structures for the temperature range of 600 to 760C. We will relate these measurements to measurements of time-resolved diffuse scattering from multiple pulses during layer-by-layer growth and previous measuremens of time-resolved truncation rod intensities. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V42.00013: Leakage current reduction and magneto-electric coupling studies in BiFeO$_{3}$ thin films N.M. Murari, Ashok Kumar, Ram S. Katiyar The sol-gel derived BiFeO$_{3}$ thin films were prepared on Pt/Si substrate with less than 20nm of interfacial layer of Ba$_{0.25}$Sr$_{0.75}$TiO$_{3}$ (BST). The XRD data revealed a single-phase compound having crystallite size of 25-50 nm. Surface morphology was characterized utilizing atomic force microscope and the surface roughness and the particle size were found to be reduced compared to BFO films grown without the BST sheet layer. Current voltage characteristic graphs indicated a significant reduction in leakage current of 2-3 orders of magnitude. Anomalies in the dielectric constant as a function of temperature were observed near the Neel's temperature $\sim $600K which are indicative of the so called magneto-electric coupling in this compounds. Tangent loss spectra as a function of temperature and frequencies indicated the dielectric relaxation near the Neel temperature. Micro Raman spectroscopy was carried out as a function of temperature shows the disappearance of normal modes in the range of 300-600 cm$^{-1}$ and there was a shift towards the low frequency side with increase in half width. The anomalies in Raman spectra also support the spin-phonon coupling near the Neel temperature. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V42.00014: Ab initio Study of Metal Interfaces with HfO$_{2}$ and SiO$_{2}$: Work Function Modulation Blanka Magyari-K\"ope, Yoshio Nishi, Luigi Colombo, Kyeongjae Cho For the next generation of metal-oxide-semiconductor field-effect transistors (MOSFETs), the suitable metal candidate has to be identified. The choice of a specific metal electrode on high-k gate dielectric oxide films is strongly influenced by the possible interface chemical reactions and defects. In this study, \textit{ab initio} calculations are employed to investigate and analyze a number of possible interface structures between gate dielectric oxides, HfO$_{2}$ and SiO$_{2}$, and metal electrodes. The structural stability and electronic structure of the interfaces with implications to metal work functions are discussed. The work function of metals on oxides is significantly influenced by the interface configurations and by the particular bonding pattern at the interface. For work function modulation, model interface systems of metal bi-layers are constructed based on structural and compositional heterogeneity. It is found that a few atomic layers of the underlying metal shift the work function of bi-layers to that of underlying metal. [Preview Abstract] |
Session V43: Electronic Properties in Nanostructures
Sponsoring Units: DCMPChair: Ilya Ponomarev, Naval Research Laboratory
Room: Colorado Convention Center 506
Thursday, March 8, 2007 11:15AM - 11:27AM |
V43.00001: $g$-factors and discrete energy level velocities in nanoparticles Eduardo R. Mucciolo, Caio H. Lewenkopf, Leonid I. Glazman We establish relations between the statistics of $g$ factors and the fluctuations of energy in metallic nanoparticles where spin-orbit coupling is present. These relations assume that the electron dynamics in the grain is chaotic. The expressions we provide connect the second moment of the $g$ factor to the root-mean square ``level velocity'' (the derivative of the energy with respect to magnetic field) calculated at magnetic fields larger than a characteristic correlation field. Our predictions relate readily observable quantities and allow for a parameter-free comparison with experiments. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V43.00002: Full counting statistics for a quantum nanoelectromechanical system Steven Bennett, Aashish Clerk Experiments on nanoelectromechanical systems often involve the effects of a mechanical oscillator on the current noise of a mesoscopic conductor. Coupling to the oscillator induces correlations between tunneling electrons in the conductor, leading to signatures in the shot noise. To better characterize such correlations it is useful to consider full counting statistics (FCS), which describe the complete probability distribution of tunneled charge. We study theoretically the FCS in a tunnel junction coupled to a nanomechanical oscillator. This system has been realized in experiment using an atomic point contact where one electrode is free to vibrate \footnote{N. E. Flowers-Jacobs, D. R. Schmidt, and K. W. Lehnert ({submitted}).}, and it has been predicted that the oscillator dynamics leads to large signatures in the shot noise that cannot be explained classically \footnote{A. A. Clerk and S. M. Girvin, {\it Phys. Rev. B} {\bf 70}, 121303(R) (2004).}. Thus motivated, we investigate the FCS using a reduced density matrix tracking the oscillator and the number of tunneled electrons, for which we obtain an equation of Caldeira-Leggett form with additional terms due to tunneling. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V43.00003: {\em Ab Initio} Calculations for the Surface Energy of Silver Nanoclusters Bharat Medasani, Igor Vasiliev, Young Ho Park We apply first principles computational methods to study the surface energy and the surface stress of silver nanoparticles. The structures, energies and lattice contractions of spherical Ag nanoclusters are calculated in the framework of density functional theory combined with the generalized gradient approximation. Our calculations predict the surface energies of Ag nanoclusters to be in the range of 1$-$2 J/m$^2$. These values are close to the bulk surface energy of silver, but are significantly lower than the recently reported value of 7.2 J/m$^2$ derived from the Kelvin equation for free Ag nanoparticles\footnote{ K. K. Nanda {\it et al.}, Phys. Rev. Lett. {\bf 91}, 106102 (2003)}. From the lattice contraction and the nearest neighbor interatomic distance, we estimate the surface stress of the silver nanoclusters to be in the the range of 1$-$1.45 N/m. This result suggests that a liquid droplet model can be employed to evaluate the surface energy and the surface stress of Ag nanoparticles. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V43.00004: The structures and energetics of interacting ionic nanocrystals from atomistic simulations. Paul Tangney, Steven G. Louie Self-assembled ordered aggregates of nanocrystals (NCs) of many different sizes, shapes and compositions have been synthesized in recent years. These ``supercrystals'' form a new class of material with potentially new and useful properties in which nanoparticles take the place of atoms as the fundamental building blocks of matter. However, at present neither the detailed structures of NCs themselves nor the interactions between them are well understood and it is not yet clear which forces are responsible for binding and ordering them in supercrystals. In this work, NCs of highly ionic materials are simulated using first principles molecular dynamics (MD) and MD based on accurate polarisable force-fields. Individual NCs and multiple NCs in close proximity are simulated and their structures and the electrostatic contributions to their energetics are studied in detail. From our understanding of NCs in this ionic limit we provide insight into the importance of electrostatic contributions to NC bonding in more covalent materials. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V43.00005: Fermi-edge singularity in a spin-incoherent Luttinger liquid Gregory A. Fiete We theoretically investigate the Fermi edge singularity in a spin incoherent Luttinger liquid. Both cases of finite and infinite core hole mass are explored, as well as the effect of a static external magnetic field of arbitrary strength. For a finite mass core hole the absorption edge behaves as $(\omega-\omega_{\rm th})^\alpha/ \sqrt{|\ln(\omega-\omega_{\rm th})|}$ for frequencies $\omega$ just above the threshold frequency $\omega_{\rm th}$. The exponent $\alpha$ depends on the interaction parameter $g$ of the interacting one dimensional system, the electron-hole coupling, and is independent of the magnetic field strength, the momentum, and the mass of the excited core hole (in contrast to the spin coherent case). In the infinite mass limit, the spin incoherent problem can be mapped onto an equivalent problem in a spinless Luttinger liquid for which the logarithmic factor is absent, and backscattering from the core hole leads to a universal contribution to the exponent $\alpha$. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V43.00006: Free Energy of a 1D Metal-Molecule Interface: C$_{60}$-Decorated Ag Islands T. J. Stasevich, C. Tao, E. D. Williams, T. L. Einstein We study the structural and dynamical properties of one- dimensional metal-molecule interfaces by investigating Ag monolayer islands on Ag(111) decorated by C$_{60}$. At 300K bare Ag(111) islands have hexagonal equilibrium shapes. When C$_{60}$ is deposited on the surface, it preferentially nucleates along island step edges, near the island corners, making them round.\footnote{C. Tao et al., Phys. Rev. B \textbf{73}, 125436 (2006).} We tune coverage so that a single chain of C$_{60}$ fully decorates the island, forming a closed ring, circular in shape. From a simple model for the C$_{60}$ step decoration, we derive the decorated step free energy as a function of step angle, yielding the equilibrium shape of the decorated islands via the Wulff construction. By comparing the model to experiment, we estimate the Ag-C$_{60}$ attraction. Using fast STM scanning, we also study the fluctuations of the C$_{60}$ decorated islands. By fitting the time correlation function of the fluctuation component Fourier modes, we show the decorated step dynamics are consistent with attachment-detachment (AD) kinetics, in contrast to the step-edge diffusion of the bare island. Finally, from our analysis, we extract the decorated step free energy and estimate the C$_{60}$-C$_{60}$ attraction. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V43.00007: Permanent polarization of small metallic particles Andrey Shytov, Michael Pustilnik Electric charge density in a metal fluctuates on the spatial scale of the Fermi wavelength due to various types of disorder. These fluctuations are usually compensated in the bulk due to Coulomb interaction between electrons. However, a small metallic particle may have a non-vanishing static electric dipole moment, owing to uncompensated density fluctations near the surface on the scale set by the screening length. We analyze these fluctuations statistically and find that the typical value of the dipole moment increases linearly with the particle size, and fluctuates strongly from particle to particle. Our results are applicable to small metallic clusters and nanocrystals. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V43.00008: Tuning the Conductivity of Semiconductor Nanostructures by Dielectric Engineering Aniruddha Konar, Debdeep Jena Electron transport properties in semiconductor nanoscale quantum structures grown by bottom-up techniques can be fundamentally different from those grown by epitaxial methods. Transport properties in 1D nanotubes and nanowires and 2D nanoscale thin films are strongly affected by carrier-impurity interactions mediated by the dielectric environment of the nanostructure. We show that by suitable dielectric engineering of this environment, electron mobility in 1D and 2D semiconductor nanostructures can be enhanced by 1-2 orders of magnitude. The enhancement takes place only when the smallest length scale of the nanostructure is less than the effective Bohr radius of the bulk semiconductor. The enhancement in mobility occurs predominantly due to a large damping of Coulombic scattering, which results in a reduction of electron scattering rates. When the dielectric constant of the environment is changed from 1 (air) to 100 (high-k oxide dielectric), the Coulomb scattering rate decreases from 700/ps to 4/ps for a 1D nanowire, and from 66/ps to 3/ps for a 2D sheet. When other scattering mechanisms such as surface roughness and phonon scattering are considered, we find that the total conductivity of the nanostructures can be enhanced by 1-2 orders of magnitude by coating them with high-k dielectrics. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V43.00009: Dielectrophoretic alignment of VO$_{2}$ nanowires in device geometries Irving Herman, Sarbajit Banerjee, Vladimir Blagojevic, Kellen Petersen, Manav Malhotra, Michael Steigerwald, Louis Brus Bulk VO$_{2}$ is characterized by a Mott metal---insulator phase transition at $\sim $68 $^{o}$C and has been widely studied for optical and electrical switching applications. However, nanostructured vanadium oxides have been challenging to fabricate and thus not much is known about their properties. Here, we present the AC dielectrophoretic alignment of hydrothermally grown VO$_{2}$ nanoribbons, $\sim $40 nm in width and several micrometers in length, in device geometries. The alignment process has been studied as a function of the applied voltage and frequency, gap distance, and concentration of the VO$_{2}$ dispersion. VO$_{2}$ nanowires have also been precisely positioned in different device geometries, such as across deep trench structures. The electrodes have been designed based on electric-field simulations. The nanowire devices show gate dependence at room temperature. The temperature dependence of the transport properties has also been examined. This work was supported primarily by the MRSEC Program of the National Science Foundation under Award Number DMR-0213574 and the New York State Office of Science, Technology and Academic Research (NYSTAR), and partially by the NSEC Program of the National Science Foundation under Award Number CHE-0641523. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V43.00010: Universal behavior of surface-dangling bonds in hydrogen-terminated Si, Ge, and Si/Ge nanowires. Ricardo Nunes, Ricardo Kagimura, H\'elio Chacham We report an {\it ab initio} study of the electronic properties of surface dangling bond (SDB) states in hydrogen-terminated Si, Ge, and Si/Ge nanowires with diameters between 1 and 2 nm. We find that the charge transition levels $\varepsilon(+/-)$ of SDB states are deep in the bandgap for Si wires, and shallow (near the valence band edge) for Ge wires. In both Si and Ge wires, the SDB states are localized. We also find that the SDB $\varepsilon(+/-)$ levels behave as a ``universal" energy reference level among Si, Ge, and Si/Ge wires within a precision of 0.1 eV. By computing the average bewteen the electron affinity and ionization energy in the atomi limit of several atoms from the III, IV and V columns, we conjecture that the universality is a periodic-table atomic property. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V43.00011: Superlattice nanowires via double-sided heteroepitaxy on compliant ultra-thin Si ribbons Clark S. Ritz, Frank S. Flack, Michelle M. Roberts, Douglas M. Detert, Yu Zhang, Donald E. Savage, Paul G. Evans, Feng Liu, Max G. Lagally We fabricate and characterize a novel type of Si/SiGe superlattice nanowire. Such structures, traditionally created by VLS growth, have been of great interest for thermoelectric applications for some time. We have developed a technique for creating a superlattice-like system using strained SiGe epitaxial islands. We pattern free-standing Si nanowire ribbons made from ultrathin silicon-on-insulator (SOI) substrates and use them as a substrate for the Stranski-Krastanov growth of coherent 3D islands. Interaction between islands growing on the top and bottom surfaces causes them to order laterally. The periodic strain induced in the substrate by the ordered islands affects the electronic band structure in a periodic way. The discussion will cover the fabrication and electrical properties of these strain superlattice structures. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V43.00012: Ground-state properties of quantum rings with a few electrons Yasuhiro Saiga, Dai Hirashima, Junko Usukura Quantum dots occupy an important position not only in the field of basic science, but also in the field of nanotechnology. Among various shapes of dots, a ring structure is a particularly interesting nanostructure, because the diameter and the ring width can be separately changed. In this talk, we discuss the ground-state properties of one-dimensional quantum rings with a few electrons, which interact with each other in the form of 1/r-Coulomb repulsion. By using exact diagonalization, we find that for three electrons, the fully spin-polarized ground state is uniquely realized when the diameter of the ring is sufficiently large. In contrast, for four and five electrons, the fully polarized state never becomes the unique ground state, however large the diameter is. These results can be understood in terms of multiple-spin exchanges. We also show that a magnetic field applied perpendicularly to the ring induces not only the persistent current but also the spin chirality. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V43.00013: Quantum Impurities and Persistent Currents: Decoupling through Integrability Hans-Peter Eckle, Johan Nilsson, Henrik Johannesson We consider the problem of a persistent current in a one-dimensional mesoscopic ring with the electrons coupled by a spin exchange to a magnetic impurity. We show that this problem can be mapped onto an integrable model with a quadratic dispersion (with the latter property allowing for an unambiguous definition of the persistent current). We have solved the model exactly by a Bethe ansatz and found that the current is insensitive to the presence of the impurity. From the structure of the Bethe ansatz equations we conjecture that this result holds for any integrable quantum impurity model with an electronic dispersion $\epsilon(k)$ that is an even function of $k$. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V43.00014: Energy Spectra and Oscillatory Magnetization of Two-Electron Self-Assembled InGaAs/GaAs Ring-Like Nanostructures V.M. Fomin, V.N. Gladilin, J.T. Devreese, N.A.J.M. Kleemans, H.C.M. van Genuchten, P.M. Koenraad We have analyzed the effect of the Coulomb interaction on the energy spectrum and the magnetization of two electrons in a strained In$_x$Ga$_{1-x}$As/GaAs ring-like nanostructure with realistic parameters inferred from our X-STM data. With increasing magnetic field, the lowest spin-singlet and spin- triplet states sequentially replace each other as the ground state. This is reminiscent of the Aharonov-Bohm effect for the ring-like structures. The exchange interaction leads to a more complicated oscillatory structure of the magnetic moment of the two electrons as a function of the magnetic field as compared to the magnetization pattern for a single-electron ring-like nanostructure. We discuss the relevance of the two-electron systems for the interpretation of the Aharonov-Bohm oscillations in the persistent current observed in low temperature magnetization measurements on self-assembled In$_x$Ga$_{1-x}$As/GaAs ring-like nanostructures. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V43.00015: Electric-field lithography of LaAlO$_{3}$/SrTiO$_{3}$ quasi-two-dimensional electron gas Cheng Cen, Jeremy Levy, Stefan Thiel, Jochen Mannhart Recent reports$^{2,3}$ have indicated that the existence of polar discontinuities at the interface between LaAlO$_{3}$ and SrTiO$_{3}$ is unstable to the formation of a quasi-two-dimensional electron gas. Below a critical thickness electrons can still accumulate at the interface under the influence of an applied electric field$^{3}$. We use a biased conducting atomic force microscope (AFM) probe to create conducting nanowires at the LaAlO$_{3}$/SrTiO$_{3}$ interface without physical alteration of the interface. The conducting regions written by AFM probe can be written and erased repeatedly. This form of quasi-two-dimensional lithography demonstrates the utility of the LaAlO$_{3}$/SrTiO$_{3}$ interface as a rewritable medium, with the potential for creating passive as well as active circuits such as field-effect transistors. ( $^{2}$ A. Ohtomo and H. Y. Hwang, Nature \textbf{427}, 423 (2004). $^{3}$ S. Thiel, G. Hammerl, A. Schmehl, C. W. Schneider, and J. Mannhart, Science \textbf{313}, 1942 (2006).) [Preview Abstract] |
Session V44: Focus Session: Plasmons in Nanoholes, Arrays and Structured Surfaces
Sponsoring Units: DMPChair: Fernando Reboredo, Oak Ridge National Laboratory
Room: Colorado Convention Center 507
Thursday, March 8, 2007 11:15AM - 11:27AM |
V44.00001: Nonlinear Optical Effects in Periodic Arrays of Nanoholes in Metallic Films Xiwen Wang, Stephen Gray, George Schatz Extraordinary optical transmission (EOT) by periodic subwavelength apertures in metallic films has been much studied. Recent studies have shown that different aperture shapes can give quite different transmission spectra. For circular holes, EOT is often attributed to coupling with surface plasmons. Rectangular holes, however, can show more enhanced EOT and localized modes inside the holes, as opposed to plasmons, are often the dominant features. This implies that in the process of resonant transmission, light is highly concentrated inside the holes, which opens the possibility for exploring nonlinear effects. We use the finite-difference time-domain calculations to study the transmission properties of metallic films with arrays of rectangular subwavelength holes, each hole filled with a Kerr nonlinear material. We consider both the perfect metal limit and silver that supports plasmons. We analyze the transmission spectra as a function of incident intensity. Due to the large electric fields associated with the localized modes inside the holes, moderate incoming fluxes can result in dramatic changes in the positions of the transmission peaks. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V44.00002: Theoretical Study of Optical Transmission Enhancement through Sub-Wavelength Apertures: Determining the Role of Surface Plasmon Polaritons Philip Flammer, James Martineau, Reuben Collins, Ian Schick, Michael Horowitz, Russell Hollingsworth Enhanced optical transmission (EOT) through sub-wavelength apertures in metal films has been observed from both experimental and theoretical studies of circular apertures surrounded by bulls-eye groove configurations or simpler linear apertures flanked by grooves. These studies have also generated much debate over the driving mechanisms involved. In this talk, theoretical results from a commercial finite element PDE solver will be presented with supporting experimental results for linear aperture/groove structures. This study confirms the integral role of surface plasmon polaritons in causing EOT, and also shows the importance of surface cavity resonances. Results will be presented exploring the role of the geometry of the grating structures, and how to tune the EOT resonance wavelength by changing the aperture/groove geometry. This material is based on work supported by the National Science Foundation under Grant No. DMI-052228. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V44.00003: The coupling of surface plasmons in periodic arrays of subwavelength holes Ruwen Peng, Zhaohui Tang, Zhan Wang, Yongjun Bao, Mu Wang We demonstrate here that transmission optical enhancement originates not only from surface plasmons(SPs) but also from the coupling of SPs on the silver film perforated with a periodic array of subwavelength holes. We fabricate the structured silver films by coating the film with magnetron sputtering, and then drilling holes with focused-ion-beam facility. The optical measurements are in good agreement with numerical calculations based on the full-vectorial three-dimensional finite-difference time-domain method. The peaks in measured transmission spectrum have also been analytically indexed by using effective-dielectric-constant model. It is shown that the coupling of SPs leads to blue shift of transmission peaks, and increases with decreasing the thickness of silver films. We suggest that these properties open an unique way to tune electromagnetic wave in subwavelength optics. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V44.00004: Plasmonic properties of a nanosized hole in a thin metallic film Tae-Ho Park, Peter Nordlander We investigate the optical properties of a nanosize hole in a thin metallic film. We show that the optical absorption spectrum is characterized by a plasmon resonance of an energy that depends strongly on the ratio of the hole diameter and the film thickness in qualitative agreement with experimental results. Microscopically, the nanohole plasmon is shown to consist of a collective state formed by propagating thin film plasmons. The hole surface exposes the film plasmons and introduce a dipole moment which allows the coupling to incident light. We also show that the energy of the hole plasmon resonance depends strongly on the polarization and direction of the incident light. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V44.00005: Interaction between surface plasmon and 2-dimensional nano-defects at metallic surfaces. Raul Garcia-Llamas, Jorge Gaspar-Armenta, Judith Tánori-Cordova, Manuel Leyva-Lucero A theoretical study of light diffraction and intensity of near field from two-dimensional nano-defects at metallic surface illuminated with electromagnetic plane waves is presented. Results for one or two Gaussian-shaped sub-wavelength defects at silver surface are shown. The light diffraction patterns shown minima at specific angular directions in the case of two defects separated a distance $a_{x}$. These minima are associated to the ration (\textit{$\lambda $}/2$a_{x})$ and depend on the localization of the defects, being \textit{$\lambda $} the wavelength of the illumination light. The Near-Field intensity, calculated to constant height, shown oscillations associated to the excitation of surface plasmon, which amplitude are grater for smaller width of the Gaussian defect. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V44.00006: Mapping of the optical emission in the vicinity of the surface of a subwavelength aperture flanked by periodic grooves in a gold film Ian C. Schick, James T. Martineau, Reuben T. Collins, P. David Flammer, Russell E. Hollingsworth Enhanced optical transmission through subwavelength apertures surrounded by periodic features has been a subject of great recent interest. Equally important are the emission characteristics of these structures. Transmission enhancement has typically been observed as peaks in the spectral dependence of transmission measured in the far-field. Here we use near-field scanning optical microscopy to spatially map the optical emission in the vicinity of the aperture and as emission propagates into the far field. We show the dependence of the emission pattern on the wavelength of the incidence in addition to structural parameters, such as spacing between the grooved regions and the aperture and the spatial period of the grooves. We observe that the emission pattern is governed by interference effects between emission from the aperture and scattered light from adjacent surface features. This material is based on work supported by the National Science Foundation under Grant No. DMI-0522281. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V44.00007: Dependence of Transmission Through Subwavelength Linear Apertures on Grating/Aperture Separation Michael Hurowitz, Ian Schick, Philip Flammer, James Martineau, Russell Hollingsworth, Reuben Collins Far-field transmission spectra were obtained for structures consisting of subwavelength linear apertures flanked on one or both sides by periodic grating arrays in Au films. In each set of structures, the distance from the grating arrays to the aperture was incrementally varied. Transmission spectra clearly showed enhancement and suppression relative to an isolated aperture. A systematic shift in these extremes was observed with varying cavity width while holding other structural parameters constant. Distinct bands of enhancement and suppression arise from this analysis, consistent with a predictive analytical model. Our conclusions allow for precise control over enhancement/suppression at specific wavelengths in future structures. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V44.00008: Lensless focusing and optical trapping Christopher DuFort, Bogdan Dragnea Force mapping of optical gradients associated with electromagnetic fields above subwavelength apertures in a gold thin film has been studied using scanning probe techniques. Vertical cross-sections of this resulting field demonstrate that, in certain conditions, the light emerges in the form of a tightly focused beam even when the incident beam is only weakly focused. Quantification of the near-field in the vicinity of these apertures has shown it is possible to deflect a free particle resulting in an optical trapping effect. Applications involving lensless focusing below the diffraction limit and comparisons with traditional optical tweezing are discussed. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V44.00009: Effective Non-Localities of Nano-Layered Meta-Materials Justin Elser, Viktor Podolskiy, Ildar Salakhutdinov, Ivan Avrutsky Multi-layered nano-composites have been suggested for negative index of refraction systems, photonic funnels, super- and hyper-lenses, as well as other nanophotonic structures. We analyze the electromagnetic modes in such systems and show that they are not described by conventional effective-medium theories. We demonstrate the response of a majority of realistic layered structures is strongly affected by effective non-localities. We develop the analytical description of the relevant phenomena and confirm our results with rigorous numerical solutions of the Maxwell equations. Finally, we demonstrate that multi-layered plasmonic nanostructures support high-index volume modes confined to deep subwavelength areas by using the formalism we have developed. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V44.00010: Fluorescence enhancement from silver nanostructures. Shyhauh Guo, Hung-Chih Kan, Ray Phaneuf We report on experimental investigations of the fluorescence enhancement by nanotextured silver structures and its dependence on the incident light polarization and on the lateral periodicity. We find strong enhancement for TM mode polarization at smaller periods for thin spacer layers. Thicker spacer layers instead produce strong TE mode enhancement at small spacings. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V44.00011: Influence of Local Field and Particle Plasmon on Fluorescence Enhancement from Spherical Nano-silver Particles Shu-Ju Tsai, Hung-Chih Kan, Shy-Hauh Guo, De-Hao Tsai, Michael Zachariah, Ray Phaneuf We report on investigation of fluorescence enhancement from silver nano-particles with selected diameters ranging from 50 nm to 320 nm. We measure the fluorescent intensity for two fluorophores, Cy3 and Cy5, coated on silver nano-particles deposited on silicon substrate using excitation source of an Ar ion laser at 514 nm and a HeNe laser at 633nm, respectively. We find that the optimum diameter shifts to a larger value for the larger wavelength fluorophore; this is consistent with the particle plasmon- resonance dependence on particle size. However, we find that while the fluorescence enhancement drops sharply for larger particle sizes the extinction coefficient doesn't. To understand this inconsistency, we perform numerical calculation with the discrete dipole approximation (DDA) method to calculate the extinction coefficient and the electric field strength near a single Ag particle on Si substrate. In comparison with measurement, the local field strength near the particle shows a size dependence in qualitative agreement with the fluorescence; the extinction coefficient does not. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V44.00012: Ultra-long range surface plasmon modes Charles G. Durfee, Reuben T. Collins, Thomas E. Furtak, Russell E. Hollingsworth It is well known that the propagation length of surface plasmon waves can be extended by exciting the appropriate mode of an isolated noble metal layer. The losses, however, increase substantially as the refractive index of the surrounding medium increases. Using a transfer matrix calculation, we have discovered that a thin, low-index dielectric adjacent to the metal layer can increase the intrinsic propagation length arbitrarily as the bound mode approaches cutoff. This geometry can be implemented in structures that combine metal-oxide-semiconductor (MOS) fabrication with plasmonic waveguides. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V44.00013: Suppression of transmission minima and maxima with structured metal surface Yongyuan Zhu, Qianjin Wang, Chengping Huang, Jiaqi Li Extraordinary optical transmission through perforated metal films has received much attention recently. In this paper, we propose a method for studying the transmission properties, in which the Fourier coefficient of reciprocal lattice vectors is manipulated. Especially, due to a zero Fourier coefficient, the suppression of transmission minima and maxima has been experimentally observed. The results open a new way to tailor the transmission properties of light. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V44.00014: Enhanced Optical Transmission with Coaxial Apertures Michael Haftel, Carl Schlockermann, Shannon Orbons, Ann Roberts, David Jamieson, Darren Freeman, Barry Luther-Davies Recently it has been shown that ``cylindrical'' surface plasmons (CSP's) on cylindrical interfaces of coaxial ring apertures produce a new form of extraordinary optical transmission (EOT) that extends to ever increasing wavelengths as the dielectric ring narrows.\footnote{F. I. Baida et al., Phys. Rev. B \textbf{67,} 155314 (2003); M.I Haftel et al., Appl. Phys. Lett. \textbf{88}, 193104 (2006).} Using analytic and FDTD calculations we present some of the consequences of CSP's on EOT as well as experimental confirmation of such effects. We find that EOT, even with cylindrical apertures, is aided by the increase in cutoff wavelength due to CSP's, which is a consequence of the mode structure of individual apertures. CSP effects also explain most of the long-wavelength features of transmission spectra measured for CR apertures. We also show that CSP's can be ``spoofed'' at low frequencies by coaxial apertures in metamaterials consisting of a (macroscopic) periodic dielectric structure embedded in a perfect conductor. [Preview Abstract] |
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