Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Z1: Quasiparticles and non-Abelian Statistics in the 5/2 State
Sponsoring Units: DCMPChair: Gregory Fiete, California Institute of Technology
Room: Spirit of Pittsburgh Ballrom A
Friday, March 20, 2009 11:15AM - 11:51AM |
Z1.00001: Status of Experiments on the 5/2 Quantized Hall State: A Theorist's View Invited Speaker: A number of experiments have been proposed, which, at least in principle, should clarify the nature of the fractional quantized Hall state at filling fraction 5/2. For example, experiments might measure the spin polarization of the ground state and the charge of the elementary quasiparticles, and could test whether the quasiparticles indeed obey non-Abelian statistics, as predicted by the Moore-Read model. These experiments are difficult, however, and their interpretation may be complicated by non-uniformities in the electron density and other problems, which also pose difficulties at simpler filling fractions. We shall discuss the current experimental situation, with these issues in mind. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:27PM |
Z1.00002: Fractionally charged quasiparticles at filling fraction 5/2 Invited Speaker: |
Friday, March 20, 2009 12:27PM - 1:03PM |
Z1.00003: Quasiparticle Tunneling in the Fractional Quantum Hall effect at filling fraction $\nu$=5/2 Invited Speaker: In a two-dimensional electron gas (2DEG), in the fractional quantum Hall regime, the quasiparticles are predicted to have fractional charge and statistics, as well as modified Coulomb interactions. The state at filling fraction $\nu$=5/2 is predicted by some theories to have non-abelian statistics, a property that might be exploited for topological quantum computing. However, alternative models with abelian properties have been proposed as well. Weak quasiparticle tunneling between counter-propagating edges is one of the methods that can be used to learn about the properties of the state and potentially distinguish between models describing it. We employ an electrostatically defined quantum point contact (QPC) fabricated on a high mobility GaAs/AlGaAs 2DEG to create a constriction where quasiparticles can tunnel between counter-propagating edges. We study the temperature and dc bias dependence of the tunneling conductance, while preserving the same filling fraction in the constriction and the bulk of the sample. The data show scaling of the bias-dependent tunneling over a range of temperatures, in agreement with the theory of weak quasiparticle tunneling, and we extract values for the effective charge and interaction parameter of the quasiparticles. The ranges of values obtained are consistent with those predicted by certain models describing the 5/2 state, indicating as more probable a non-abelian state. This work was done in collaboration with J. B. Miller, C. M. Marcus, M. A. Kastner, L. N. Pfeiffer and K. W. West. This work was supported in part by the Army Research Office (W911NF-05-1-0062), the Nanoscale Science and Engineering Center program of NSF (PHY-0117795), NSF (DMR-0701386), the Center for Materials Science and Engineering program of NSF (DMR-0213282) at MIT, the Microsoft Corporation Project Q, and the Center for Nanoscale Systems at Harvard University. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:39PM |
Z1.00004: Measurement of filling factor 5/2 quasiparticle interference Invited Speaker: A standing problem in low dimensional electron systems is the nature of the 5/2 fractional quantum Hall state: its elementary excitations are a focus for both elucidating the state's properties and as candidates in methods to perform topological quantum computation. Interferometric devices may be employed to manipulate and measure quantum Hall edge excitations. Here we use a small area edge state interferometer designed to observe quasiparticle interference effects. Oscillations in transmission consistent in detail with the Aharanov-Bohm effect are observed for integer and fractional quantum Hall states (filling factors 2, 5/3 and 7/3) with periods corresponding to their respective charges and magnetic field positions. With these charge calibrations, at filling factor 5/2 and at lowest temperatures periodic transmission through the device consistent with quasiparticle charge e/4 is observed. The principal finding of this work is that in addition to these e/4 oscillations, periodic structures corresponding to e/2 are also observed at filling factor 5/2 and at lowest temperatures. Properties of the e/4 and e/2 oscillations at 5/2 are examined with the device sensitivity sufficient to observe the relative prevalence of e/4 and e/2 oscillations, transitions between the periods, and temperature evolution of the 5/2 quasiparticle interference. Among possible etiologies, this presence of an effective e/2 period may empirically reflect an e/2 quasiparticle charge, or may reflect multiple passes of the e/4 quasiparticle around the interferometer. These results are discussed within a potential picture of e/4 quasiparticle excitations possessing non-Abelian statistics. Some critical consistencies are met between the experimental results and properties of non-Abelian e/4 quasiparticles. These studies demonstrate the capacity to perform interferometry on 5/2 excitations and reveal properties important for understanding this state and its excitations. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 2:15PM |
Z1.00005: The Anti-Pfaffian and anti-Read-Rezayi States Invited Speaker: |
Session Z2: Detection of Non-Gaussian Noise in Mesoscopic Systems
Sponsoring Units: DCMPChair: Norman Birge, Michigan State University
Room: Spirit of Pittsburgh Ballrom BC
Friday, March 20, 2009 11:15AM - 11:51AM |
Z2.00001: Detection of the third moment of shot noise by a Josephson junction Invited Speaker: We use a hysteretic Josephson junction as an on-chip detector of shot noise of a tunnel junction. The detectable bandwidth is determined by the plasma frequency of the detector, which is about 50 GHz in the experiments that we report. The second moment of shot noise manifests itself as increased effective temperature of junction switching. The third moment results in a measurable change of the switching rate when reversing polarity of the current through the noise source. We have successfully analyzed the observed asymmetry using a phenomenological model. We compare our results to the more quantitative theories as well. Experiments on quantum point contacts and further work on tunnel junctions are in progress. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:27PM |
Z2.00002: Asymmetric noise probed with a Josephson junction Invited Speaker: Using a Josephson junction, we have measured the fluctuations of the current through a tunnel junction. The current noise adds to the bias current of the Josephson junction and affects its switching out of the supercurrent branch. The experiment is carried out in a regime where switching is determined by thermal activation. The variance of the noise results in an elevated effective temperature, whereas the third moment, related to its asymmetric character, leads to a difference in the switching rates observed for opposite signs of the current through the tunnel junction. Measurements are compared quantitatively with recent theoretical predictions. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 1:03PM |
Z2.00003: Fluctuation-induced switching and the switching path distribution. Invited Speaker: Fluctuation-induced switching is at the root of diverse phenomena currently studied in Josephson junctions, nano-mechanical systems, nano-magnets, and optically trapped atoms. In a fluctuation leading to switching the system must overcome an effective barrier, making switching events rare, for low fluctuation intensity. We will provide an overview of the methods of finding the switching barrier for systems away from thermal equilibrium. Generic features of the barrier, such as scaling with the system parameters, will be discussed. We will also discuss the motion of the system in switching and the ways of controlling it. Two major classes of systems will be considered: dynamical systems, where fluctuations are induced by noise, and birth-death systems. Even though the motion during switching is random, the paths followed in switching form a narrow tube in phase space of the system centered at the most probable path. The paths distribution is generally Gaussian and has specific features, which have been seen in the experiment [1]. Finding the most probable path itself can be reduced to solving a problem of Hamiltonian dynamics of an auxiliary noise-free system. The solution also gives the switching barrier. The barrier can be found explicitly close to parameter values where the number of stable states of the system changes and the dynamics is controlled by a slow variable. The scaling of the barrier height depends on the type of the corresponding bifurcation. We show that, both for birth-death and for Gaussian noise driven systems, the presence of even weak non-Gaussian noise can strongly modify the switching rate. The effect is described in a simple explicit form [2,3]. Weak deviations of the noise statistics from Gaussian can be sensitively detected using balanced dynamical bridge, where this deviation makes the populations of coexisting stable states different from each other; a realization of such a bridge will be discussed. We will also discuss the sharp anisotropy of fluctuations induced by Poisson noise in overdamped systems and how it is changed with decreasing damping. \\[4pt] [1] H. B. Chan, M. I. Dykman, and C. Stambaugh , Phys. Rev. Lett. \textbf{100}, 130602 (2008). \\[0pt] [2] M. I. Dykman, I. B. Schwartz, A. S. Landsman, Phys. Rev. Letts. \textbf{101, }078101 (2008). \\[0pt] [3] L. Billings, M. I. Dykman, and I. B. Schwartz, Phys. Rev. E \textbf{78} (2008). [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:39PM |
Z2.00004: Theory of Mesoscopic Threshold Detectors of non-Gaussian Noise Invited Speaker: Recently, measurements of current fluctuations arising from the charge discreteness (shot noise) have become an invaluable tool in mesoscopic physics, the most noticeable achievement being the measurement of quasi-particle charge in the fractional quantum Hall state. Typically, shot noise experiments report measurements of the zero-frequency noise power, which is a characteristic of the Gaussian component of current fluctuations. A natural generalization of the noise power, the counting statistics of charge transmitted through a system, is interesting in itself, because it contains complete information about the electron transport on a long time scale. However, the measurement of non-Gaussian noise effects presents an experimental challenge because of the limitations imposed by the central limit theorem. This difficulty can be partly overcome by placing an auxiliary mesoscopic system (detector) very close to the noise source and arranging strong coupling to the noise. This leads to the idea of a threshold detector, which is able to measure rare current fluctuations. Its basic principle is analogous to a pole vault: A detection event occurs when the measured system variable exceeds a given threshold value. A natural candidate for such a threshold detector is a metastable system operating on an activation principle. By measuring the rate of switching out of the metastable state, information about the statistical properties of the noise driving the system may be extracted.~ This requires solving the Kramers' problem of noise-activated escape from a metastable state beyond the Gaussian noise approximation and investigating how the measurement circuit affects threshold detection. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 2:15PM |
Z2.00005: Theory of Josephson junction detectors of higher order noise cumulants Invited Speaker: A promising strategy pursued at various laboratories to measure higher order cumulants of the electrical current of nanoscopic devices employs on-chip Josephson junction detectors. The non-Gaussian nature of the noise generated by electronic nanostructures modifies the switching rate of the Josephson junction out of the zero voltage state, and the noise cumulants can be extracted from this modification. When the decay of the metastable zero voltage state occurs by noise activation to the top of the barrier of the Josephson potential, the third noise cumulant gives rise to an asymmetry of the rate when the bias current is inverted. In the range of decay by macroscopic quantum tunneling (MQT) through the barrier potential, the forth noise cumulant leads to an enhancement of the MQT rate. The theoretical methods to describe a Josephson junction noise detector in these parameter regimes are outlined and associated experimental strategies are discussed. [Preview Abstract] |
Session Z3: Non-viral Based Gene Delivery Systems: Opportunities, Obstacles and Challenges
Sponsoring Units: DPOLY DBPChair: Yongmei Wang, Memphis
Room: 301/302
Friday, March 20, 2009 11:15AM - 11:51AM |
Z3.00001: Small Bioactive Lipoplex (SBL) Nanoparticles Self-Assembled at Elevated Temperature and Pressure Invited Speaker: Conventional lipoplex (cationic liposome/DNA complex) serves well for gene transfer in cultured cells. However, their \textit{in vivo} gene delivery activity is limited due to its relatively large size (\underline {$>$}100 nm). This is due to incomplete charge neutralization as a result of the steric hindrance during the complexation between DNA and liposomes. Behr et al hypothesized that monomolecular DNA condensate can be prepared if the DNA sees the cationic lipid as monomers. Indeed, small nanoparticles ($\sim $30 nm) were prepared by using a single-chain cationic amphiphile which has a high solubility at the physiological condition. To stabilize the monomolecular condensate, Behr has included a SH group in the cationic amphiphile which could be oxidized to form a dimer. Unfortunately, the stabilized nanoparticles showed no transfection activity when delivered into cells. We hypothesized that similar small lipoplex can be prepared by using a double-chain cationic amphiphile if both DNA and the amphiphile can be soluble in the same solvent. A hydrofluorocarbon HFC-152a is an excellent solvent for the cationic lipid DOTAP at an elevated temperature ($\sim $35 $^{\circ}$C) and pressure ($\sim $300 atm). Since the solvent can accommodate small amounts of water, DNA or siRNA could be introduced into the system to allow lipoplex formation. The resulting Small Bioactive Lipoplex (SBL) is 30-50 nm in diameter and can transfect cultured cells. Freeze-fracture electron microscopy showed that SBL are solid nanoparticles without any lipid bilayer structure. Since plasmid DNA is fragile at elevated temperature and pressure, we have concentrated our effort in siRNA which is stable under the same conditions. The new formulation shows great promise as an \textit{in vivo} delivery vector when small particles are required for efficient penetration into the tissues. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:27PM |
Z3.00002: New Developments in Non Viral Gene Delivery Invited Speaker: |
Friday, March 20, 2009 12:27PM - 1:03PM |
Z3.00003: Image-Guided Hydrodynamic Gene Delivery Invited Speaker: Gene delivery by rapid injection of a large volume of DNA solution into a blood vessel, commonly called hydrodynamic gene delivery, has become a common method for gene therapy studies in rodents. In this presentation, I will focus on our recent work aiming at establishment of an image-guided hydrodynamic procedure for gene delivery in humans. Our study employed swine as an animal model and the procedure developed includes image-guided insertion of a balloon catheter into the selected blood vessel of the targeted organ from the jugular vein and hydrodynamic injection of plasmid DNA in saline. The talk will cover the rationale of our approach, the effectiveness of procedure for gene delivery to liver and muscle, and the impact of the procedure on physiological functions and serum chemistry of the animals. The results will be discussed with respect to potential applications of the hydrodynamic gene delivery to human gene therapy. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:39PM |
Z3.00004: Revisit an old problem -- Complexation between DNA and PEI Invited Speaker: After revisiting the captioned problem by using a combination of chemical synthesis and physical methods, we studied the dynamics of the complexation between branched polyethyleneimine ($b$PEI) and plasmid DNA ($p$DNA) and characterized the structure, size and surface charge of the resultant DNA/PEI complexes (polyplexes). As expected, in order to reach a high efficiency in gene transfection into cells it is necessary to use a higher N:P ratio and make the polyplexes positively charged. Our results reveal that it is those uncomplexed $b$PEI chains free in the solution mixture that plays a vitally important role in enhancing the transfection efficiency, inspiring new thinking of how to correlate in vitro and in vivo studies so that we can improve the in vivo transfection efficiency. Increasing the N:P ratio normally results in a higher cytotoxicity, which is a catch-22 problem. Recently, we found that a proper modification of $b$PEI can greatly reduce its cytotoxicity without any suffering in the transfection efficiency. In this lecture, we will show that our properly modified $b$PEI is even much more effective and less cytotoxic in the gene transfection than those commercially available lipoflexes. Our recent breakthrough leads to a complete new direction in the development of non-viral vectors for molecular medicines, including gene transfection. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 2:15PM |
Z3.00005: Recent Developments in Non-Viral Gene Delivery Invited Speaker: |
Session Z4: Biological Polyelectrolytes
Sponsoring Units: DPOLY DBPChair: Garegin Papoian, University of North Carolina
Room: 306/307
Friday, March 20, 2009 11:15AM - 11:51AM |
Z4.00001: How mechanical properties of DNA relate to chromosome structure Invited Speaker: |
Friday, March 20, 2009 11:51AM - 12:27PM |
Z4.00002: Electrostatic Rigidity of Biological Polyelectrolytes Invited Speaker: Electrostatic persistence length is one of the most controversial subjects in polymer physics. In this talk I will present an overview of the history of the problem and a new results showing that the bending rigidity of the biological polyelectrolytes (semiflexible charged polymers) is a multiscale process. The existence of the different length scales in the bending process is manifested in change of the form of the correlation function, describing bond-bond orientational correlations, with salt concentration. At high salt concentrations when the electrostatic interactions are significantly screened these correlations may be approximated by a single exponential function indicating the existence of a dominant length scale. However, when the Debye screening length exceeds a critical value the orientational correlations between chain's bond vectors undergo a qualitative change resulting in appearance of two different bending rigidities (persistence lengths). One increases quadratically and the other changes logarithmically with the Debye screening length. This transition occurs when the chain's bare persistence length becomes on the order of so-called OSF electrostatic persistence length. Simulation results and theoretical model demonstrate good qualitative agreement. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 1:03PM |
Z4.00003: Deafness and espin-actin self-organization in stereocilia Invited Speaker: Espins are F-actin-bundling proteins associated with large parallel actin bundles found in hair cell stereocilia in the ear, as well as brush border microvilli and Sertoli cell junctions. We examine actin bundle structures formed by different wild-type espin isoforms, fragments, and naturally-occurring human espin mutants linked to deafness and/or vestibular dysfunction. The espin-actin bundle structure consisted of a hexagonal arrangement of parallel actin filaments in a non-native twist state. We delineate the structural consequences caused by mutations in espin's actin-bundling module. For espin mutation with a severely damaged actin-bundling module, which are implicated in deafness in mice and humans, oriented nematic-like actin filament structures, which strongly impinges on bundle mechanical stiffness. Finally, we examine what makes espin different, via a comparative study of bundles formed by espin and those formed by fascin, a prototypical bundling protein found in functionally different regions of the cell, such as filopodia. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:39PM |
Z4.00004: Mesoscale Modeling of Chromatin Folding Invited Speaker: Eukaryotic chromatin is the fundamental protein/nucleic acid unit that stores the genetic material. Understanding how chromatin fibers fold and unfold in physiological conditions is important for interpreting fundamental biological processes like DNA replication and transcription regulation. Using a mesoscopic model of oligonucleosome chains and tailored sampling protocols, we elucidate the energetics of oligonucleosome folding/unfolding and the role of each histone tail, linker histones, and divalent ions in regulating chromatin structure. The resulting compact topologies reconcile features of the zigzag model with straight linker DNAs with the solenoid model with bent linker DNAs for optimal fiber organization and reveal dynamic and energetic aspects involved. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 2:15PM |
Z4.00005: Complexation of oppositely charged polyelectrolytes in gene delivery and biology Invited Speaker: Charge inversion of a DNA double helix by a positively charged flexible polymer (polyelectrolyte) is widely used to facilitate DNA contact with negative cell membranes for gene delivery. Motivated by this application in the first part of the talk I study the phase diagram a solution of long polyanions (PA) with a shorter polycations (PC) as a function the ratio of total charges of PC and PA in the solution, $x$, and the concentration of monovalent salt. Each PA attracts many PCs to form a complex. When $x= 1$, the complexes are neutral and condense in a macroscopic drop. When $x$ is far away from 1, complexes are strongly charged and stable. PA are overcharged by PC at $x > 1$ and undercharged by PC at $x < 1$. As $x$ approaches 1, PCs attached to PA disproportionate between complexes. Some complexes become neutral and condensed in a macroscopic drop while others become even stronger charged and stay free. The second part of the talk deals with biological example of PA -PC complexes namely self-assembly of vegetable viruses from long ss-RNA molecule paying role of scaffold and identical capsid proteins with long positive tails. I show that optimization Coulomb energy of the virus leads to the charge of RNA twice larger than the total charge of the capsid, in agreement with the experimental data. Then I discuss kinetics of the Coulomb complexation driven virus self-assembly. Capsid proteins stick to unassembled chain of ss RNA (which we call ``antenna'') and slide on it towards the assembly site. I show that at excess of capsid proteins such one-dimensional diffusion accelerates self-assembly more than ten times. On the other hand at excess of ss-RNA, antenna slows self-assembly down. Several experiments are proposed to verify the role of ss-RNA antenna in self-assembly. [Preview Abstract] |
Session Z5: Physics in the Aerospace Industry
Sponsoring Units: FIAPChair: Jeffrey Hunt, Boeing
Room: 401/402
Friday, March 20, 2009 11:15AM - 11:51AM |
Z5.00001: Quantum Key Distribution in the private sector Invited Speaker: |
Friday, March 20, 2009 11:51AM - 12:27PM |
Z5.00002: INFORMATION ARCHITECTURES FOR SMALL PLATFORM Invited Speaker: |
Friday, March 20, 2009 12:27PM - 1:03PM |
Z5.00003: Advances in High-Resolution Imaging using Fourier Telescopy Invited Speaker: Fourier Telescopy is an essentially the reverse of long-baseline interferometry, wherein light is projected onto a distant (but non-astronomical) object by multiple small apertures and the reflected light is received by one or more large apertures. As in long-baseline interferometry, the principle of phase closure is applied to reconstruct the image. The projected light is coherent so that fringes are formed on the object of interest, and the light reflected from these fringe patterns then contains information about the corresponding Fourier components of the object. The use of phase closure allows recovery of the phase of these Fourier components, within an overall tip and tilt of the phase in the Fourier plane. The key benefits of the approach are (a) the realization of very high spatial resolution using sparse arrays, despite aberrations due to an intervening medium, and (b) high sensitivity due to the fact that large light-bucket receivers may be used to capture the time-encoded information. Potential applications are discussed, and recent verifications, advances, and extensions of the basic concept are presented. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:39PM |
Z5.00004: Recent progress in optical communications Invited Speaker: |
Friday, March 20, 2009 1:39PM - 2:15PM |
Z5.00005: High precision low light level imaging Invited Speaker: |
Session Z6: Impact of NSF's TeraGrid on Physics Research
Sponsoring Units: DCOMPChair: Michael Levine, Pittsburgh Supercomputing Center
Room: 406
Friday, March 20, 2009 11:15AM - 11:51AM |
Z6.00001: Teragrid and Physics Research Invited Speaker: TeraGrid is the program that includes and ties together the high end computational resources (supercomputing, storage, visualization, data collections, science gateways) provided by NSF for the nation's researchers, and supported by computing and technology experts. World-class facilities, on a much larger scale than ever before, present major new opportunities for physics researchers to carry out computations that would have been infeasible just a few years ago. I will briefly review the available resources, how they can be used (together, if needed), explain how easy it is to get access to them, and present some examples of outstanding physics research that they have already enabled. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:27PM |
Z6.00002: Impact of the TeraGrid on Materials Science Invited Speaker: The TeraGrid cyberinfrastructure has enabled many scientific successes. This talk will highlight - through a set of examples using a variety of computational methods - how the TeraGrid has in particular affected research in materials science. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 1:03PM |
Z6.00003: Impact of TeraGrid on Cosmic Simulations Invited Speaker: Many of the advances in our understanding of cosmic structure have come from direct computer modeling. In cosmology, we need to develop computer simulations that cover this vast dynamic range of spatial and time scales: we need to include the effect of gravitational fields generated by (dark matter in) superclusters of galaxies on the formation of galaxies, which in turn harbor gas that cools and makes stars and is being funneled into supermassive blackholes the size of the solar system. Computational cosmology, simulating the entire universe, represents perhaps one of most challenging application for the TeraGrid. I will present recent and upcoming work on computational cosmology using the TeraGrid systems. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:39PM |
Z6.00004: Virtual photons in imaginary time: Computing Casimir forces in new geometries Invited Speaker: One of the most dramatic manifestations of the quantum nature of light in the past half-century has been the Casimir force: a force between neutral objects at close separations caused by quantum vacuum fluctuations in the electromagnetic fields. In classical photonics, wavelength-scale structures can be designed to dramatically alter the behavior of light, so it is natural to consider whether analogous geometry-based effects occur for Casimir forces. However, this problem turns out to be surprisingly difficult for all but the simplest planar geometries. (The deceptively simple case of an infinite plate and infinite cylinder, for perfect metals, was first solved in 2006.) Many formulations of the Casimir force, indeed, correspond to impossibly hard numerical problems. We will describe how the availability of large-scale computing resources in NSF's Teragrid, combined with reformulations of the Casimir-force problem oriented towards numerical computation, are enabling the exploration of Casimir forces in new regimes of geometry and materials. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 2:15PM |
Z6.00005: Molecular mechanics of DNA stretching, electrophoresis and condensation Invited Speaker: Without a doubt, DNA is the most celebrated macromolecule as it carries the genetic blueprint of a living organism. In addition to its fundament role in biology, DNA exhibits a variety of unusual physical properties. For example, the force-extension dependence of double-stranded DNA has a well-defined plasticity plateau that is associated with melting of its two strands. Being highly negatively charged, DNA molecules can attract one another and form a condensed state. The direction of the DNA motion in free electrophoresis can reverse upon changing the concentration of the surrounding electrolyte. Despite a number of theoretical and experimental studies, the nature of the microscopic processes that give rise to the above phenomena remain highly debated. With the advent of massively parallel supercomputers it became possible to characterize these processes directly, through all-atom molecular dynamics simulations. In this talk I will describe the results of multiple sub-microsecond simulations of various DNA systems that provide insights into the microscopic origin of the plasticity plateau in stretched DNA and into the mechanisms of DNA-DNA attraction and electrophoresis. [Preview Abstract] |
Session Z7: Nanoprobes of Molecules and Cells
Sponsoring Units: DBPChair: Adam Cohen, Harvard University
Room: 407
Friday, March 20, 2009 11:15AM - 11:51AM |
Z7.00001: Real time imaging of mRNA expression dynamics in live cells using protein complementation methods Invited Speaker: Traditional methods for mRNA quantification in cells, such as northern blots, quantitative PCR or microarrays assays, require cell lysis and therefore do not preserve its dynamics. These methods cannot be used to probe the spatio-temporal localization of mRNA in cells, which provide useful information for a wide range biomolecular process, including RNA metabolizim, expression kinetics and RNA interference. To probe mRNA dynamics in live prokaryotic and eukaryotic cells, we develop a method, which exploit the strong affinity of the eukaryotic initiation factor 4A (eIF4A) to specific RNA aptamers. Two parts of the eIF4A are fused to a split Green Fluorescence Protein (GFP), and are expressed in the cells at high abundance. However, only when the RNA apatmer is also present, the two protein parts complement and become fluorescent. Thus, the fluorescent background remains low, allowing us to directly image the expression of mRNA molecules in live \textit{e-coli} cells from its early onset, over hours. We find that the expression kinetics can be classified in one out of at least three forms, which also display distinct spatial distributions. I will discuss the possible biological origin for these distributions and their time evolution. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:27PM |
Z7.00002: Nanopores and nanofluidics for single DNA studies Invited Speaker: Lab-on-a-chip fluidic technology takes inspiration from electronic integrated circuits, from which its name, its fabrication methods, and its ``smaller, cheaper, faster'' paradigm are derived. For silicon-based electronics, miniaturization eventually gave rise to qualitatively different behavior, as quantum mechanical phenomena grew increasingly important. As we shrink fluidic devices down to the nanoscale to probe samples as minute as a single molecule, what physical phenomena will dominate in this new regime, and how might we take advantage of them? This talk will focus on our studies of single DNA molecules using nanofluidic devices and solid-state nanopores. We are studying how nanofluidic structures, whose critical dimensions are tens to hundreds of nanometers, can manipulate long DNA molecules by a variety of nanoscale phenomena, including electrokinetics, hydrodynamics, Coulomb interactions, and the statistical properties of polymers. Our work also focuses on solid-state nanopores, single-nanometer-scale devices that can not only manipulate single molecules, but also detect them electronically. The basic principle behind this is that when DNA is electrophoretically driven through a nanopore, it blocks a measureable fraction of the ionic current that is transmitted through the pore. Thanks to its size, the nanopore also forces each base along the DNA to pass through in sequence, suggesting intriguing possibilities for genetic analysis. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 1:03PM |
Z7.00003: Toward Single-Molecule Nanomechanical Mass Spectrometry Invited Speaker: Mass spectrometry (MS) has become a preeminent methodology of proteomics since it provides rapid and quantitative identification of protein species with relatively low sample consumption. Yet with the trend toward biological analysis at increasingly smaller scales, ultimately down to the volume of an individual cell, MS with few-to-single molecule resolution will be required. We report the first realization of MS based on single-biological-molecule detection with nanoelectromechanical systems (NEMS). NEMS provide unparalleled mass resolution, now sufficient for detection of individual molecular species in real time. However, high sensitivity is only one of several components required for MS. We demonstrate a first complete prototype NEMS-MS system for single-molecule mass spectrometry providing proof-of-principle for this new technique. Nanoparticles and protein species are introduced by electrospray injection from the fluid phase in ambient conditions into vacuum and subsequently delivered to the NEMS detector by hexapole ion optics . Mass measurements are then recorded in real-time as analytes adsorb, one-by-one, onto a phase-locked, ultrahigh frequency (UHF) NEMS resonator. These first NEMS-MS spectra, obtained with modest resolution from only several hundred mass adsorption events, presage the future capabilities of this methodology. We outline the substantial improvements feasible in near term, through recent advances and technological avenues that are unique to NEMS-MS. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:39PM |
Z7.00004: Using Photoactivation Light Microscopy (PALM) to construct comprehensive, nanometer precision atlases of signaling complexes Invited Speaker: The E. coli chemotaxis network is a model system for biological signal processing. In E. coli, transmembrane receptors responsible for signal transduction assemble into large clusters containing several thousand proteins. These sensory clusters have been observed at cell poles and future division sites. Despite extensive study, it remains unclear how chemotaxis clusters form, what controls cluster size and density, and how the cellular location of clusters is robustly maintained in growing and dividing cells. Here we use photoactivated localization microscopy (PALM) to map the cellular locations of three proteins central to bacterial chemotaxis (the Tar receptor, CheY, and CheW) with a precision of 15 nanometers. We find that cluster sizes are approximately exponentially distributed, with no characteristic cluster size. One third of Tar receptors are part of smaller lateral clusters and not the large polar clusters. Analysis of the relative cellular locations of 1.1 million individual proteins (from 326 cells) suggests that clusters form via stochastic self-assembly. The super-resolution PALM maps of E. coli receptors support the notion that stochastic self-assembly can create and maintain approximately periodic structures in biological membranes, without direct cytoskeletal involvement or active transport. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 2:15PM |
Z7.00005: Single-molecule dynamics in nanofabricated traps Invited Speaker: The Anti-Brownian Electrokinetic trap (ABEL trap) provides a means to immobilize a single fluorescent molecule in solution, without surface attachment chemistry. The ABEL trap works by tracking the Brownian motion of a single molecule, and applying feedback electric fields to induce an electrokinetic motion that approximately cancels the Brownian motion. We present a new design for the ABEL trap that allows smaller molecules to be trapped and more information to be extracted from the dynamics of a single molecule than was previously possible. In particular, we present strategies for extracting dynamically fluctuating mobilities and diffusion coefficients, as a means to probe dynamic changes in molecular charge and shape. If one trapped molecule is good, many trapped molecules are better. An array of single molecules in solution, each immobilized without surface attachment chemistry, provides an ideal test-bed for single-molecule analyses of intramolecular dynamics and intermolecular interactions. We present a technology for creating such an array, using a fused silica plate with nanofabricated dimples and a removable cover for sealing single molecules within the dimples. With this device one can watch the shape fluctuations of single molecules of DNA or study cooperative interactions in weakly associating protein complexes. [Preview Abstract] |
Session Z8: Statistical Physics in Biology
Sponsoring Units: DBPChair: Marek Cieplak, Johns Hopkins University
Room: 414/415
Friday, March 20, 2009 11:15AM - 11:51AM |
Z8.00001: Allometric Scaling in Biology Invited Speaker: The unity of life is expressed not only in the universal basis of inheritance and energetics at the molecular level, but also in the pervasive scaling of traits with body size at the whole-organism level. More than 75 years ago, Kleiber and Brody and Proctor independently showed that the metabolic rates, B, of mammals and birds scale as the three-quarter power of their mass, M. Subsequent studies showed that most biological rates and times scale as $M^{-1/4}$ and $M^{1/4}$ respectively, and that these so called quarter-power scaling relations hold for a variety of organisms, from unicellular prokaryotes and eukaryotes to trees and mammals. The wide applicability of Kleiber's law, across the 22 orders of magnitude of body mass from minute bacteria to giant whales and sequoias, raises the hope that there is some simple general explanation that underlies the incredible diversity of form and function. We will present a general theoretical framework for understanding the relationship between metabolic rate, B, and body mass, M. We show how the pervasive quarter-power biological scaling relations arise naturally from optimal directed resource supply systems. This framework robustly predicts that: 1) whole organism power and resource supply rate, B, scale as $M^{3/4}$; 2) most other rates, such as heart rate and maximal population growth rate scale as $M^{-1/4}$; 3) most biological times, such as blood circulation time and lifespan, scale as $M^{1/4}$; and 4) the average velocity of flow through the network, $\bar {v}$, such as the speed of blood and oxygen delivery, scales as $M^{1/12}$. Our framework is valid even when there is no underlying network. Our theory is applicable to unicellular organisms as well as to large animals and plants. This work was carried out in collaboration with Amos Maritan along with Jim Brown, John Damuth, Melanie Moses, Andrea Rinaldo, and Geoff West. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:27PM |
Z8.00002: From gene expressions to genetic networks Invited Speaker: A method based on the principle of entropy maximization is used to identify the gene interaction network with the highest probability of giving rise to experimentally observed transcript profiles [1]. In its simplest form, the method yields the pairwise gene interaction network, but it can also be extended to deduce higher order correlations. Analysis of microarray data from genes in Saccharomyces cerevisiae chemostat cultures exhibiting energy metabollic oscillations identifies a gene interaction network that reflects the intracellular communication pathways. These pathways adjust cellular metabolic activity and cell division to the limiting nutrient conditions that trigger metabolic oscillations. The success of the present approach in extracting meaningful genetic connections suggests that the maximum entropy principle is a useful concept for understanding living systems, as it is for other complex, nonequilibrium systems. The time-dependent behavior of the genetic network is found to involve only a few fundamental modes [2,3]. \\[4pt] REFERENCES:\\[0pt] [1] T. R. Lezon, J. R. Banavar, M. Cieplak, A. Maritan, and N. Fedoroff, Using the principle of entropy maximization to infer genetic interaction networks from gene expression patterns, Proc. Natl. Acad. Sci. (USA) 103, 19033-19038 (2006) \\[0pt] [2] N. S. Holter, M. Mitra, A. Maritan, M. Cieplak, J. R. Banavar, and N. V. Fedoroff, Fundamental patterns underlying gene expression profiles: simplicity from complexity, Proc. Natl. Acad. Sci. USA 97, 8409-8414 (2000) \\[0pt] [3] N. S. Holter, A. Maritan, M. Cieplak, N. V. Fedoroff, and J. R. Banavar, Dynamic modeling of gene expression data, Proc. Natl. Acad. Sci. USA 98, 1693-1698 (2001) [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 1:03PM |
Z8.00003: Non-equilibrium thermodynamic effects during cell division Invited Speaker: A mitotic spindle is a regular structure within a cell consisting of oriented microtubule fibers. It plays a fundamental role in chromosome separation during cell division. Forming a spindle pattern is a major structural step towards mitosis. We have developed biophysical non-equilibrium thermodynamic models to describe in vitro chromosome driven spindle formation experiments in Xenopus extracts. Our first 2D model calculations [1] successfully described the order of events seen in some of the Xenopus extracts experiments, where the chromosomes are replaced by chromatin-covered micrometer magnetic beads. I will describe more realistic 3D improvements in our modeling analysis, which include microtubule contact forces and excluded volume [2, 3]. There are, however, a number of challenges that must be addressed for spindle modeling to continue to be a useful tool for understanding this fundamental biological process, in particular the biophysical simulation times. In this talk I will describe some important problems needing better biological data and hypothesis. I will also discuss our most recent numerical algorithmic improvements that are expected to greatly increase the simulations speed and thus allowing a more realistic representation of the experimental situation in Xenopus extracts. [1] S. C. Schaffner and J. V. Jose, PNAS, 103, 11166 (2006), [2] ibid in ``Methods in Cell Biology'' (Elsevier- Academic Press)(2008)and [3]ibid(to be published). [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:39PM |
Z8.00004: New Proposed Mechanism for Actin-Polymerization-Driven Motility Invited Speaker: When a cells crawls, its shape re-organizes via polymerization and depolymerization of a network of actin filaments. The growing ends of the filaments are localized near the leading edge of the crawling cell, and their polymerization, regulated by a host of proteins, pushes the cell membrane forwards in a biological model known as the dendritic nucleation model. We have performed Brownian dynamics simulations to see how the dendritic nucleation model leads to motion. Our results are not consistent with previous models of motility, and suggest a new picture for the physical mechanism underlying this form of motility. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 2:15PM |
Z8.00005: to be determined by you Invited Speaker: |
Session Z9: Nonlinear Dynamics and Chaotic Systems
Sponsoring Units: GSNPChair: Robert Behringer, Duke University,
Room: 303
Friday, March 20, 2009 11:15AM - 11:27AM |
Z9.00001: Rotating Space Elevator: Classical and Statistical Mechanics of cosmic scale spinning strings Steven Knudsen, Leonardo Golubovic We introduce a novel and unique nonlinear dynamical system, the Rotating Space Elevator (RSE). The RSE is a multiply rotating system of cables (strings) reaching beyond the Earth geo-synchronous satellite orbit. Strikingly, objects sliding along the RSE cable do not require internal engines or propulsion to be transported far away from the Earth's surface. The RSE action employs, in a very fundamental way, basic natural phenomena -- gravitation and inertial forces. The RSE exhibits interesting nonlinear dynamics and statistical physics phenomena. Its kinetic phase diagram involves both chaotic and quasi-periodic states of motion separated by a morphological phase transition that occurs with changing the RSE angular frequency. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z9.00002: Numerical investigation on the chaos assisted tunneling for a coupled microwave cavity Hoshik Lee, Louis Pecora, Dong Ho Wu It is known that chaos-assisted dynamical tunneling may occur in nonintegrable (chaotic) systems. Recently we investigate wave chaotic systems to see if the system may promote the chaos-assisted spatial tunneling in addition to the dynamic tunneling. Our previous experiments suggest some enhancement of the spatial tunneling rate in a coupled, wave-chaotic 2D microwave double cavity, indicating that the presence of chaotic modes changes not only the dynamical tunneling rate but also the spatial tunneling rate. To understand the underlying physics we carry out numerical simulations on nonintegrable 2D cavities as well as on integrable 2D cavities. We will present details about the experiments and numerical simulation results. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z9.00003: Scaling properties of delay times in one-dimensional random media Joshua Bodyfelt, Antonio Mendez-Bermudez, Andrey Chabanov, Tsampikos Kottos The scaling properties of the inverse moments of Wigner delay times are investigated in finite one-dimensional (1D) random media with one channel attached to the boundary of the sample. We find that they follow a simple scaling law which is independent of the microscopic details of the random potential. Our theoretical considerations are confirmed numerically for systems as diverse as 1D disordered wires and optical lattices to microwave waveguides with correlated scatterers. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z9.00004: Lyapunov exponent calculation via reconstruction of the invariant density function from iterative Chebyshev maximum entropy approach Nagendra Dhakal, Hiro Shimoyama, Parthapratim Biswas We apply a maximum entropy approach (MaxEnt) to compute invariant density functions to obtain the Lyapunov exponents. The method gives the solution by iteratively calculating the Lagrange multipliers within the maximum entropy method from moment constraints. We illustrate our method by reproducing known invariant densities for several cases of discrete maps (in both chaotic and non-chaotic regime). The global convergence of invariant density function is studied with particular emphasis on Lynapunov exponent of the maps for varying number of moments. We demonstrate that Lyapunov exponent of a chaotic map can be computed with a high degree of precision from this approach. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z9.00005: Time-Shifted Synchronization of Chaotic Oscillator Chains without Explicit Coupling Delays Jonathan Blakely, Mark Stahl, Ned Corron It has recently been reported that time-shifted synchronization (i.e., lag or anticipation) of chaotic oscillators can result from forms of coupling that do not contain explicit delay terms. Identical time-shifted synchronization is not a solution in these systems so the dynamics are a form of generalized synchronization where trajectories are similar but not exactly identical. Here we examine chains of uni-directionally coupled oscillators in which time-shifted synchronization occurs without delays in the coupling. We observe the distortion of the waveforms of the response oscillators located far from the drive oscillator. Under weak coupling, we see much less distortion occurs over chains with significant total time shift than predicted by a recently introduced theoretical estimate. Under stronger coupling, we find better agreement with the theoretical prediction and, despite sometimes severe attenuation, generalized synchronization is maintained over long chain lengths. We report results from numerical models as well as from an experimental system of electronic circuits. Such oscillator chains may prove useful in applications requiring a variable delay such as chaotic radar or beam forming. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z9.00006: Combining Wave Chaos and the Loschmidt Echo to Extend the Concept of Fidelity to Classical Waves Biniyam Taddese, James Hart, Thomas Antonsen, Edward Ott, Steven Anlage We propose and demonstrate a new remote sensor scheme by applying the wave mechanical concept of fidelity loss to classical waves. The sensor makes explicit use of time- reversal invariance and spatial reciprocity in a wave chaotic system to sensitively and remotely measure the presence of small perturbations to the system. The loss of fidelity is measured through a classical wave-analog of the Loschmidt echo by employing a single-channel time-reversal mirror to rebroadcast a probe signal into the perturbed system. We also introduce the use of exponential amplification of the probe signal to partially overcome the effects of propagation losses. It is demonstrated that exponential amplification can be used to vary the spatial range of sensitivity to perturbations, thereby actively modifying the range of operation of the sensor. Experimental results are presented for both electromagnetic and acoustic versions of the Loschmidt echo based sensor. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z9.00007: Synchronization and Competition in a Double-Bump-on-Tail Instability Dmitry V. Dylov, Jason W. Fleischer We experimentally and theoretically consider a double-bump-on-tail instability by mapping the general wave-kinetic problem to a multiple beam propagation problem using statistical light. More specifically, we consider the nonlinear interaction of three spatially-incoherent beams in a self-focusing photorefractive crystal. For weak nonlinearity, we observe instability competition and sequential flattening of the bumps in momentum space, with no observable variations in position-space intensity. This joint dynamics resembles the phase synchronization of a ``classical'' laser system (relaxation from a ``non- equilibrium'' state to a lower-energy one), with the corresponding gain rates following from the optical equivalent of inverse Landau damping. For strong nonlinearity, intensity modulations appear and the triple-hump spectrum merges into a single-peaked profile with an algebraic $k^{-2}$ inertial range. This spectrum, with its associated modulations, is a definitive observation of soliton, or Langmuir, turbulence. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z9.00008: Stratospheric Ozone and Dynamical Systems Francisco J. Uribe, Rosa Maria Velasco, Ernesto Perez-Chavela We consider the Chapman mechanism for stratospheric ozone dynamics. The resulting nonlinear differential equations are studied from the point of view of the theory of dynamical systems. In particular we calculate and analyze the nature of the critical points and show that the region in which the concentrations are non-negative is a positively invariant set, meaning that initial conditions with non-negative concentrations always give non-negative concentrations. Poincar\'e compactification is used to ellucidate the global flow. Comments about the inclusion of nitrogen oxides are also given. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z9.00009: Chaotic Escape of Rays from a Vase-shaped Billiard: Simulations and Experiment Jaison Novick, Matthew Len Keeler, John Delos We study the escape of rays from a two dimensional, specularly-reflecting open vase-shaped cavity. The narrowest point of the vase's neck defines a dividing surface between rays that escape without return and those turned back into the cavity. Our simulations show that a point burst of rays emitted in all directions can contain both regular and chaotic scattering trajectories. The chaotic trajectories leak out in an infinitely long pulse train organized by a fractal. For escaping trajectories, we record the propagation time to escape and find that the fractal manifests itself in the escape time versus the launch angle. We have experimentally verified the early fractal structure. A two dimensional aluminum vase with reflective Teflon walls was constructed with an ultrasound transmitter as the point source. A microphone was placed at points along the vase's mouth. We find good agreement between measurements and classical simulations. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z9.00010: Scattering a pulse from a chaotic cavity: Transitioning from algebraic to exponential decay James Hart, Thomas Antonsen, Edward Ott The ensemble averaged power scattered in and out of lossless chaotic cavities (such as microwave resonators, acoustic cavities and quantum dots) decays as a power law in time for large times. In the case of a pulse with a finite duration, the power scattered from a single realization of a cavity closely tracks the power law ensemble decay initially, but eventually transitions to an exponential decay. In this presentation, we explore the nature of this transition in the case of coupling to a single port. We find that for a given pulse shape, the properties of the transition are universal if time is properly normalized. We define the crossover time to be the time at which the deviations from the mean of the reflected power in individual realizations become comparable to the mean reflected power. We demonstrate numerically that, for randomly chosen cavity realizations and given pulse shapes, the probability distribution function of reflected power depends only on time, normalized to this crossover time. Paper: http://arxiv.org/abs/0810.1664 [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z9.00011: Stability of large complex systems Harold Hastings We use a random matrix model to study the stability of large, complex systems. Our approach was motivated by a long-standing dilemma concerning stability of large systems. MacArthur (1955) and Hutchinson (1959) argued that more ``complex'' natural systems tended to be more stable than less complex systems based upon energy flow. May (1972) argued the opposite, using random matrix models. In prior work we showed that in some sense both are right: under reasonable scaling assumptions on interaction strength, Lyapunov stability increases but structural stability decreases as complexity is increased (c.f. Harrison, 1979; Hastings, 1984). We now apply these methods to a variety of complex systems. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z9.00012: The Initiation of Optical Breakdown in Simple Liquids Kevin Cissner The probability of breakdown in 40 simple HPLC hydrocarbons and water from a Q-switched laser at 1064 (6ns) and 532 nm (5 ns) was measured using a variety of lenses and cell path length. In each instance a plot of the cumulative distribution function vs. the input laser fluence fits an error-function well, except at low probabilities. Care was taken to measure the light distribution \textit{in sit}u across the entire focal plane. The transmission within the HOMO{\_}LUMO gap was also measure using the long-path-length cells. Trends in the breakdown data with the optical/electronic properties of the target liquids are confounded by spherical aberration. However, the data suggest a connection to the chemical group of the liquid and especially to the C-X bond. In all cases the threshold at 1064 nm is actually less than that at 532 nm. No evidence was found for a mechanism involving dissolved air. A comparison is made to the behavior for static breakdown in gases. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z9.00013: Analysis of Laser Breakdown Data Roger Becker Experiments on laser breakdown for ns pulses of 532 nm or 1064 nm light in water and dozens of simple hydrocarbon liquids are analyzed and compared to widely-used models and other laser breakdown experiments reported in the literature. Particular attention is given to the curve for the probability of breakdown as a function of the laser fluence at the beam focus. Criticism is made of the na\"{\i}ve forms of both ``avalanche'' breakdown and multi-photon breakdown. It appears that the process is complex and is intimately tied to the chemical group of the material. Difficulties with developing an accurate model of laser breakdown in liquids are outlined. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z9.00014: Fidelity Gap in Dynamical Systems with Critical Chaos Carl T. West, Tomaz Prosen, Tsampikos Kottos We analyze the fidelity decay for a class of dynamical systems showing {\it critical chaos}, using a Kicked Rotor with singular kicking potential as a prototype model. We found that the classical fidelity shows a gap $F_g$ (initial drop of fidelity) which scales as $F_g(\alpha, \epsilon, \eta ) = f(\chi\equiv\frac{\eta^{3-\alpha}}{\epsilon})$ where $\alpha$ is the order of singularity of the non-analytical potential, $\eta$ is the characteristic spread of the initial phase space density and $\epsilon$ is the perturbation strength. Instead, the corresponding quantum fidelity gap is insensitive to $\alpha$ due to strong diffraction effects that dominate the quantum dynamics. [Preview Abstract] |
Friday, March 20, 2009 2:03PM - 2:15PM |
Z9.00015: Irreversibility, Poincare Recurrence and Stochasticity in Statistical Mechanics Puru Gujrati We will show that deterministic dynamics always leads to the conservation of entropy and Poincare recurrence.\footnote{P.D. Gujrati, Poincare Recurrence, Zermelo's Second Law Paradox, and Probabilistic Origin in Statistical Mechanics, http://arxiv.org/abs/0803.0983 (arXiv:0803.0983)} Thus, recurrence is incompatible with entropy change. The law of increase of entropy can only occur for systems with stochastic dynamics, and the irreversibility emerges out of their indeterminate evolution,\footnote{P.D. Gujrati, Irreversibility, Molecular Chaos, and A Simple Proof of the Second Law, http://arxiv.org/abs/0803.1099 (arXiv:0803.1099)} as we will discuss. This stochasticity requires some weak but uncontrollable interaction of the system with outside or the walls of the container. Boltzmann infuses this stochasticity in his deterministic approach by invoking the assumption of molecular chaos. The molecular chaos cannot emerge out of deterministic dynamics, as shown elsewhere in this meeting.\footnote{Pradeep Fernando and P.D. Gujrati (poster)} [Preview Abstract] |
Session Z10: Insulators and Dielectrics: Dielectric, Thermal, and Transport Properties
Sponsoring Units: DCMPChair: Jivtesh Garg, Massachusetts Institute of Technology
Room: 304
Friday, March 20, 2009 11:15AM - 11:27AM |
Z10.00001: Dielectric properties of cluster-deposited TiO$_{2}$ nanocomposites B. Balamurugan, Kristin Kraemer, X. Wei, Stephen Ducharme, D.J. Sellmyer TiO$_{2}$-polymer nanocomposites are expected to have a high dielectric permittivity of TiO$_{2}$ and large breakdown strength of the polymer, resulting in high energy density suitable for energy storage devices. Since chemically prepared nanocomposites tend to have poor film quality and inhomogeneities due to agglomeration, cluster deposition technique was used to prepare monodispersed TiO$_{2}$--paraffin nanocomposite films. TiO$_{2}$ clusters were coated in-flight with paraffin wax having comparatively better dielectric strength (7.9 -- 11.8 MV/m) using a thermal evaporation source in prior to deposition. Bare TiO$_{2}$ clusters with average particle size ranging from 8 to 12 nm having a maximum dielectric permittivity of 54 were obtained. The structural and dielectric properties of these nanocomposites with varying volume fractions will be discussed. This research is supported by ONR and NCMN. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z10.00002: Lattice Dielectric and Thermodynamic Properties of Yttria Stabilized Zirconia (YSZ) Solids Kah Chun Lau, Brett I. Dunlap A study of lattice dielectric and thermodynamic properties of Yttria Stabilized Zirconia (YSZ) solids as a function of yttria concentration is reported. Within the local density approximation (LDA) and the harmonic approximation, we find excellent agreement between calculated and experimental specific heat and dielectric constants. From the variation of the specific heat of YSZ with yttria composition, we propose a simple additivity rule that estimates the dependence of the specific heat of YSZ on yttria concentration. Whereas for the dielectric constants of YSZ, the values are bounded by the dielectric constants of the cubic and amorphous zirconia. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z10.00003: Space-Charge-Limited Conduction Under Trap Density Gradient Exhibiting Bulk-Limited Diode Yukio Watanabe Space-charge-limited (SCL) conduction in the presence of a trap density gradient is studied theoretically, which is compared with experiments in detail. Under this condition, the current ($J)$ -- voltage ($V)$ characteristics at low voltage are ohmic and symmetric with respect to the bias polarity. At high voltage, \textit{JV} characteristics follow the $J \propto \quad V^{m}$ (1 $\le \quad m \quad \le $ 2) law at both polarities and are asymmetric with respect to the bias polarity. These characteristics have not reported in the previous studies and agree well with experiments. This agreements verify that a bulk-limited conduction can exhibit rectifying, i.e., diode-like \textit{JV} characteristics without relying on Schottky barriers and diffusion currents. The theory is presented in easily tractable algebraic recurrence formulae and reproduces experimental \textit{JV }characteristics excellently using three free parameters, only one more than that used in standard SCL theory. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z10.00004: Optical second-harmonic generation measurements of porous low-k dielectric materials Joanna Atkin, Thomas Shaw, Robert Laibowitz, Tony Heinz Low-$k$ dielectric materials based on porous carbon-doped oxides, with relative dielectric constants as low as 2.1, are widely used as thin insulating films in the microelectronics industry. Knowledge of these materials' basic electronic properties, such as energy gaps, barrier heights, and trap states, is essential for modeling their electrical leakage and stability characteristics. We use femtosecond laser pulses to probe the dynamics of charge-carrier transfer processes across Si/LKD interfacial barriers by optical second harmonic generation (SHG). Larger electric fields from multiphoton injection can be developed in Si/LKD systems compared to Si/SiO$_{2}$, indicating a significantly higher density of traps in the LKD. This is consistent with previously reported measurements of trap density by photoinjection techniques$^{\ast }$. We will also discuss results on the dynamics of discharging and on the dependence of charging phenomena on layer thickness. $^{\ast }$J. M. Atkin, D. Song, T. M. Shaw, E. Cartier, R. B. Laibowitz, and T. F. Heinz, \textit{J. Appl. Phys.} \textbf{103}, 094104 (2008). [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z10.00005: Energy-band parameters of atomic-layer-deposited Al$_{2}$O$_{3}$ and HfO$_{2}$ on InxGa$_{1-x}$As M.L. Huang, Y.C. Chang, Y.H. Chang, T.D. Lin, M. Hong, J. Kwo X-ray photoelectron spectroscopy (XPS) combined with reflection electron energy loss spectroscopy (REELS) were used to determine the energy-band parameters, valence-band offsets $\Delta $E$_{V}$, conduction-band offsets $\Delta $E$_{C}$, and energy-band gaps E$_{g}$, of the atomic layer deposited (ALD) high k dielectrics of Al$_{2}$O$_{3}$ and HfO$_{2}$ on InxGa$_{1-x}$As (x=0, 0.15, 0.25, and 0.53). Using REELS, Eg values of the ALD-Al$_{2}$O$_{3}$ and -HfO$_{2}$ were estimated to be 6.77 and 5.56 $\pm$0.05 eV, respectively. The $\Delta $E$_{C}$'s and $\Delta $E$_{V}$'s are larger than 1.5 and 2.5 eV, respectively, for all the ALD-oxide/In$_{x}$Ga$_{1-x}$As samples. The $\Delta $Ec values obtained from the HR-XPS and REELS analyses are in good agreement with those estimated from the electrical measurement according to Fowler-Nordheim tunneling. The results are valuable to the understanding and modeling of the III-V high k MOS devices. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z10.00006: Resonant photo-ionization of charged oxygen vacancy defects in Si/SiO2/HfO2 film stacks observed by second-harmonic generation. J. Price, M. C. Downer The semiconductor industry recently achieved a historic milestone with the introduction of high-k gate dielectrics. However, much effort continues to focus on characterizing defects in these materials that promote charge trapping. Using internal multi-photon photoemission (IMPE) and time-dependent electrostatic field-induced second harmonic (TD-EFISH) generation, we probe the charge trapping kinetics in Si/SiO2/HfO2 gate stacks. During IMPE charging, the TD-EFISH response of the HfO2 film uniquely increases and then decreases when irradiated at the characteristic three-photon energy of 4.71 eV. The decrease in TD-EFISH is explained by resonant three-photon excitation of HfO2-induced negatively charged oxygen vacancy defects in the SiO2 interfacial layer, and subsequent removal of its negative charge by tunneling to the Si substrate. This interpretation is supported by spectroscopic ellipsometry and electron spin resonance measurements, and ab initio calculations, which identify the 4.71 eV transition with this defect. Photo-ionization of this defect also explains hysteresis in the TD-EFISH response when charges trapped at the surface are quenched between successive IMPE charging cycles. The results demonstrate that second harmonic generation can potentially be used as an in situ, real time monitor of charge trapping kinetics prior to device fabrication. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z10.00007: Combined theoretical and experimental study of thin hafnia films Xuhui Luo, Alexander A. Demkov, Dina Triyso, Peter Fejes, Rich Gregory, Stefan Zollner Hafnia-based dielectric films have replaced silica as a gate dielectric in field effect transistors. We present a joint experimental and theoretical study of ultra thin hafnia films grown on Si (001) by atomic layer deposition. Using density functional theory we investigate the surface energy of monoclinic and tetragonal hafnia films in search for thermodynamic means of controlling the film microstructure. Our calculations of the surface phase diagram reveal that in the absence of hydrogen (111), and ($\bar {1}11)$ are the lowest energy surface terminations of monoclinic hafnia under a wide range of chemical environment. On the other hand, the structural analysis, indicates films with thickness of 4 nm or less to be polycrystalline, predominantly monoclinic with the texture axis being the normal to the ($21\bar {1})$, ($11\bar {2})$. Our calculations suggest that under oxygen rich conditions the ($11\bar {2})$ termination can be stabilized. Furthermore, we discuss the effects of the hydroxylation on the thermodynamics of the hafnia film grown by ALD and provide a new perspective into the dynamics of the film growth. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z10.00008: Resonant Localized Nanoplasma in Oblique Far Infrared Reflectivity of Transition Metal Granular Films N.E. Massa, J.C. Denardin, L.M. Socolovsky, M. Knobel, X.X. Zhang We report on near normal and angle dependent specular infrared reflectivity of transition metal and SiO$_{2}$ cosputtered nanogranular $\sim $550 nm thick films in the insulating regime. Their reflectivity is characterized by well defined vibrational bands, an overdamped Drude contribution, due to carriers denoting the existence of conducting critical paths not yet truncated, and a distinctive band at $\sim $1450cm$^{-1}$ originating in electron promotion, localization, and polaron formation. (TM) P-polarized oblique reflectivity, as from globally insulating Co$_{0.38}$(SiO$_{2})_{0.62}$, Fe$_{0.34}$(SiO$_{2})_{0.66 }$ or Ni$_{0.28}$(SiO$_{2})_{0.72}$, reveals a remarkable resonance at the $\sim $1450cm$^{-1}$ band threshold. Its maximum intensity is reached at the radiation tangential component null condition allowing for a collective electronic excitation induced as localized plasma. It is attributed to carriers that are not able to overcome the metal-dielectric rough interface. As the angle of incidence increases the longitudinal highest frequency vibrational band merges with the P-polarized resonance inducing broadening and softening reminiscent to lattice modes undergoing strong electron-phonon interactions. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z10.00009: Phase Equilibrium of Size-Dispersed Colloid Systems with soft pair interactions: A Monte Carlo Study Ioannis Bitsanis, Ioannis Economoy, Marianna Yiannourakou We have studied the solid-fluid coexistence for systems of polydisperse soft spheres that interact via power-law potentials. We employed isobaric semi-grand ensemble simulations. Gibbs-Duhem integration traced the coexistence pressure as a function of the variance of the imposed activity distribution. Fluid-solid coexistence densities were determined to be monotonically increasing functions of the breadth of particle size dispersity. Our simulations testified to the existence of terminal diameter dispersity, i.e a dispersity above which there can be no amorphous-crystalline phase coexistence. At the terminus, size-dispersity increases from 5.8\% to 6.7\% for the crystalline phase as pair interactions soften. The crystalline phases' allowable size dispersity is enhanced by potential softness, while the dispersity of the amorphous (liquid) phases follows an opposite trend. Overall, the 2 phase region shrinks monotonically and substantially as colloid-colloid interactions soften. Furthermore, amorphous phases accommodate, on average, smaller particles than those of the ordered (fcc) phase. Crystalline phases composed of size disperse particulates exhibit a higher degree of local order than their mono-disperse counterparts, admittedly at differing thermodynamic conditions. GSRT-ENTER [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z10.00010: First-principles simulations of extended phosphorus oxynitride structures in LiPON glasses Yaojun Du, N. A. W. Holzwarth The thin film electrolyte LiPON, having the composition of Li$_{3+x}$PO$_{4-y}$N$_z$ with $x=3z-2y$, was developed at Oak Ridge National Lab in the 1990's for use in solid state batteries and related applications.\footnote{ N. J. Dudney, {\em{Interface}} {\bf{17}}:3, 44 (2008) and listed references.} In an effort to understand and to optimize properties of this electrolyte material, we expanded previous studies of isolated defects in crystalline Li$_3$PO$_4$\footnote{Y. A. Du and N. A. W. Holzwarth, {\em{Phys. Rev. B}} {\bf{78}}, 174301 (2008).} to focus on more complicated phosphate structures based on combinations of tetrahedral P$-$O bonds and bridging P$-$O$-$P bonds. For example, crystalline LiPO$_3$\footnote{E. V. Murashova and N. N. Chudinova, {\em{Cryst. Rept.}} {\bf{46}}, 942 (2001).} and P$_2$O$_5$\footnote{E. H. Arbib and co-workers, {\em{J. Solid State Chem.}} {\bf{127}}, 350 (1996).} are composed of phosphate structures with linear and branched chains, respectively. Both these and related structures derived from substituting O with N and adjusting mobile Li ion concentrations approximate components found in LiPON films.$^{\rm{2}}$ In the simulated structures, we find that N is energetically more stable at bridging bond sites than at tetrahedral sites by 2-3~eV and that the Li ion migration energies are 0.5$-$0.6~eV, similar to values measured in LiPON films. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z10.00011: ABSTRACT HAS BEEN MOVED TO Z22.00011 |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z10.00012: Systematic Size-Dependence of Electrical Resistivity Profiles in Bi$_{0.9}$Sb$_{0.1}$ Crystals Dongxia Qu, J.G. Checkelsky, Y.S. Hor, R.J. Cava, N.P. Ong Recently, Fu and Kane[1] predicted that surface states with a odd-$Z_{2}$ topopological signature exist in the alloy Bi$_{1-x}$Sb$_{x}$ over a broad rangeof $x$. Hsieh \textit{et al}. [2] have confirmed by ARPES that, in Bi$_{0.9}$Sb$_{0.1}$, an odd number of surface states cross the chemical potential. To investigate the surface-state transport properties, we have examined how the resistivity-temperature (\textit{$\rho $-T}) profiles change as the transverse widths of crystals are varied from 20 to 1000 $\mu $m. In large crystals, the bulk conductance dominates, whereas in the small sample limit, surface conductance may be observable. Measurements on a large number of crystals reveal a reproducible systematic change in the $\rho $-T profiles. We find that the conductance at 4 K converges to a well-defined value when expressed as sheet conductance per square$ G$. The value is roughly 1000 times the quantum of conductance $e^{2}$/$h$. We discuss the interpretation of these results in the context of surface state conduction. 1. L. Fu and C. L. Kane, Phys. Rev. B 76, 045302 (2007). 2. D. Hsieh et al., Nature 452, 970 (2008). [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z10.00013: Low temperature C/T$^{3}$ peak in dipole disordered Bi$_{2}$Ti$_{2}$O$_{7}$ Gavin Lawes, Ron Tackett, Ambesh Dixit, Art Ramirez, Jim O'Brian, Brent Melot, Ram Seshadri The presence of a low temperature peak in C/T$^{3}$ vs T, indicating excess entropy above the Debye contribution, is practically ubiquitous among both crystalline and amorphous materials. We present specific heat measurements on crystalline Bi$_{2}$Ti$_{2}$O$_{7}$, which is known to have incoherent ionic displacements leading to dipole disorder, and other related crystalline materials, including Bi$_{2}$NbInO$_{7}$, Bi$_{4}$Ti$_{3}$O$_{12}$, Y$_{2}$Ti$_{2}$O$_{7}$, and PbTiO$_{3}$, all of which exhibit excess low temperature entropy. We find that the C/T$^{3}$ peak for these samples agree roughly with a proposed scaling relation, with the peak temperature for Bi$_{2}$Ti$_{2}$O$_{7}$ being among the lowest reported for any crystalline material. We discuss our results in the context of understanding the evolution of crystalline degrees of freedom to glassy degrees of freedom through our investigations of dipole disordered Bi$_{2}$Ti$_{2}$O$_{7}$. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z10.00014: Understanding the Material Thermodynamics of Two-Step Solar Thermochemical Water-Splitting Cycles Bryce Meredig, Chris Wolverton Metal oxide materials may be used in two-step solar thermochemical water-splitting cycles to renewably produce hydrogen: At high temperature, the oxide material is reduced, and at a lower temperature, the material re-oxidizes upon contact with water vapor producing hydrogen gas. Here, we present the first completely general analysis of the equilibrium thermodynamics of a two-step metal oxide water splitting cycle. We determine the temperature and pressure regimes in which both steps of the cycle are thermodynamically favorable in terms of the enthalpy and entropy of oxide reduction. Armed with this thermodynamic construct, we apply computational methods, such as density functional theory (DFT) and CALPHAD modeling, to assess many proposed oxide cycles. Using CALPHAD thermodynamic data, we survey a large number (more than 100) binary oxide redox couples, and show that none have both thermodynamically favorable reduction and oxidation steps. In an effort to find more thermodynamically favored redox couples, we investigate the utility of DFT calculations to screen materials' thermodynamics. In addition, we identify several driving forces which could enable more efficient two-step cycles, including a large positive solid-state entropy of reduction of the oxide. [Preview Abstract] |
Friday, March 20, 2009 2:03PM - 2:15PM |
Z10.00015: Using High-Voltage Direct Current in Removing Coke from a Zeolite Catalyst Grain Abdulaziz Aljalal Zeolite catalysts are commonly used in petroleum refining processes. Over a period of time, these catalysts lose their activity due to gradual deposition of carbonaceous materials, called coke. The coked catalysts are usually reactivated by combusting the coke at elevated temperatures in presence of an oxygen-enriched gas. But the elevated temperatures cause damages to the structure of the catalyst which result in reduced activity. Normally, the catalyst is reactivated 3 or 4 times before it must be returned to the manufacturer for reclamation of the valuable platinum and/or rhenium content. This study is an attempt to come up with a new procedure to remove coke from a zeolite catalyst grain using high-voltage direct current. It is found that the process is self-terminated due to the loss of electrical conductivity of the grain. In addition, it is found that there an optimum current range for which up to 90{\%} of the coke can be removed. Higher or lower currents result in much smaller removal of the coke. [Preview Abstract] |
Session Z12: Phase Transitions at Surfaces
Sponsoring Units: DCMP DMPChair: Robert Bartynski, Rutgers University
Room: 308
Friday, March 20, 2009 11:15AM - 11:27AM |
Z12.00001: A martensitic-like transition in a normal alkane Jeffrey Hutter, Shailesh Nene, Eric Karhu, Roberta Flemming The normal alkanes, C$_{n}$H$_{2n+2}$, with a structure consisting of a single chain, are the simplest hydrocarbons. These are an interesting class of material, both in terms of their intrinsic properties and the fact that many biological molecules contain hydrocarbon domains. Normal alkanes exhibit an unusual phase diagram with several solid phases, some of which---the ``rotator phases''---are characterized by positional order without long-range orientational order. We have found a striking pattern of twinned, striped domains that occurs in thin layers the monoclinic rotator RV phase of tricosane (C$_{23}$H$_{48}$). We have studied this structure, and its transitions to other phases, by X-ray diffraction, as well as by optical and atomic-force microscopy. Intriguingly, transitions between the RV phase and the RI orthorhombic phase lying at higher temperatures appear to be diffusionless, and preserve molecular-scale features even after multiple transitions between the phases. These properties are reminiscent of martensitic transformations, which are better-known in metal alloys, but occur here at convenient temperatures and with slow kinetics. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z12.00002: Monolayer solids of short (perfluoro)alkanes on graphite L.W. Bruch Calculations are reported for the relative stability of monolayer solid latices on graphite for C$_2$H$_6$, C$_3$H$_8$, C$_2$F$_6$, and C$_3$F$_8$. Triangular, centered rectangular and two-sublattice herringbone lattices are treated. The calculations use all-atom (AA) models and are based on non-bonding interactions formulated for three dimensional dense phases of alkanes and perfluoroalkanes. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z12.00003: Simulation of methane on Al(111) Majid Karimi, Carl LeBlond, Wahyu Setyawan, Stefano Curtarolo, Renee Diehl Classical many-body interatomic potentials for methane molecules interacting with Al(111) are developed using the embedded-atom method (EAM). The optimized EAM parameters for methane-Al(111) are obtained by fitting to the data generated from the first principles calculations. Adsorption of methane on the Al(111) substrate is studied using the Grand Canonical Monte-Carlo method. Adsorption isotherms are calculated at several temperatures near the triple point temperature of methane Tt = 90K, and structural parameters obtained from the adsorbed density profiles. The structures and the thermodynamics of film growth are compared to methane adsorption on other metal surfaces, graphite and the d-Al-Co-Ni quasicrystal. This research is supported by NSF and ACS-PRF. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z12.00004: Conformation-controlled networking of H-bonded assemblies on surfaces M. Enache, M. Matena, M. Stoehr, T.A. Jung, A. Llanes-Pallas, D. Bonifazi In order to prepare networks that could undergo phase transitions through a thermally-induced inversion of the molecular conformation leading to a variation of the intermolecular interactions, directional intermolecular forces can be regarded as promising candidates. In particular, H-bonding interactions will be exploited since their intermolecular interaction strength and geometry can be controlled by the number and arrangement of available H-bonding donor or acceptor moieties. We studied the 2D self-organization of a conjugated molecule bearing terminal 2,6-di(acylamino)pyridine moieties [1], which are well-known to form H-bonds, on a Ag(111) surface by STM. The hexagonal porous network, which is formed for room temperature deposition, is transformed into a close-packed rhombic pattern by a thermally induced trans-cis inversion of the terminal groups. This transformation can be explained by the fact that the system wants to minimize its energy: at the same time the free surface energy is minimized while the number of H-bonds per terminal group is doubled from two to four. [1]~A. Llanes-Pallas et al., Angew. Chem. Int. Ed. 2008, 47, 7726 [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z12.00005: IR enhancement and the surface potential of n-alkanethiol SAMs on GaAs(001) Gregory M. Marshall, Gregory P. Lopinski, Farid Bensebaa, Jan J. Dubowski n-Alkanethiol self-assembled monolayers (SAMs) were prepared on the GaAs(001) surface according to [1]. FTIR modal analysis of the CH$_{2}$ stretching mode region (2800-3000 cm$^{-1})$ verified SAM structural coherence, revealed evidence of an assembly threshold and allowed the absorption coefficient of the SAM phase to be directly measured. A 6x enhancement factor was observed relative to coefficients derived from the liquid and polycrystalline phases. This effect is reviewed in terms of the molecular order and is largely attributed to the chemical properties of the interface [2]. Confirmation is provided by Kelvin Probe measurement of the sheet dipole potential, interpreted in terms of the Cooperative Molecular Field Effect [3]. [1] McGuiness et al., J. Am. Chem. Soc. 128, 5231 (2006). [2] Marshall et al., submitted. [3] Cahen et al., Adv. Funct. Mater. 15, 1571 (2005). [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z12.00006: Faceting of Ru($11\bar {2}0)$ Surface: A Model System for Catalysis Quantong Shen, Wenhua Chen, Hao Wang, Robert Bartynski We have studied NO$_{2}$-induced faceting of a Ru($11\bar {2}0)$ surface by means of low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and Auger electron spectroscopy (AES). By annealing the sample at $>$ 600 K in NO$_{2}$ (10$^{-8}$ Torr), the surface becomes fully faceted as revealed by LEED although it is rather smooth, with only two layers exposed. The faceted surface remains the same at NO$_{2}$ exposure ranging from 20 L to 12000 L and is stable for substrate temperature $T \quad <$ 850 K.The STM results confirmed the LEED observations and showed that the faceted surface consists of sawtooth ridges along the [0001] direction with typical dimensions of $\sim $5 nm in width and $>$ 100nm in length. We have found that the faceted O/Ru surface is very active for NH$_{3}$ decomposition to produce H$_{2}$ with high selectivity to N$_{2}$ at room temperature. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z12.00007: Oxygen-induced nano-faceting of the Pd(112) surface Alina Vlad, Andreas Stierle, Rasmus Westerstroem, Edvin Lundgren, Helmut Dosch Extensive efforts are currently made to understand the elementary steps in heterogeneous catalytic reactions, with the ultimate goal of designing more efficient catalysts. The structure of the catalytically active particles, as well as the temperature and the gas phase pressure, play a decisive role in the behavior of these systems. We studied the interaction of oxygen with the Pd(112) surface from ultrahigh vacuum to atmospheric oxygen pressures by means of {\em in-situ\/} surface x-ray diffraction, high resolution core level spectroscopy and scanning tunneling microscopy. The rearrangement of the stepped (112) surface into different type of facets was observed and is strongly dependent on the oxygen pressure. The transition from different adsorbed-oxygen phases to the bulk oxide was also followed as a function of the oxygen pressure. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z12.00008: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z12.00009: Phase-field theory for phase transition dynamics of reconstructed semiconductor surfaces Bang-Gui Liu We propose a natural two-speed model for the phase dynamics of Si(111) 7$\times $7 phase transition to unreconstructed phase. Our simulated results show that a 7$\times $7 island decays with its shape kept unchanged, and its area decay rate is a constant increasing with its initial area. LEEM experiments concerned are explained, which confirms that the dimer chains and corner holes are broken first, and then the stacking fault is remedied slowly. We also propose a phase-field-crystal model for the (2$\times $1)-(1$\times $1) phase transitions of Si(001) and Ge(001) surfaces. Simulated periodic arrays of dimerization is consistent with STM images. Calculated temperature dependence of the dimerization indicates that normal dimers and broken ones coexist between T$_{L}$ and T$_{H}$, and a first-order phase transition takes place in between. This phase-field method is a reliable approach to phase dynamics of surface phase transitions. Phys. Rev. Lett. 100, 056103 (2008). [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z12.00010: Investigation of Structural Phase Transitions on Wurtzite Gallium Nitride Surfaces Tianjiao Chen, Abhijit Chinchore, Yinghao Liu, Kangkang Wang, Wenzhi Lin, Arthur Smith Surface structures of wurtzite gallium nitride (w-GaN) have been investigated previously,[1][2] and it is well known that above 300K there exist order-disorder phase transitions. For N-polar w-GaN (000-1) at 300K, a family of surface reconstructions occurs, including 1$\times $1, 3$\times $3, 6$\times $6, and c(6$\times $12). Not much is known, however, about what happens to these structures as they are cooled below 300K. We have recently developed a new epitaxy/analysis system, including a sample stage which can be both heated and cooled. The N-polar w-GaN surfaces are prepared using rf N-plasma-assisted molecular beam epitaxy, and monitored in-situ using reflection high energy electron diffraction (RHEED). The approach is to monitor the [11-20] and [10-10] RHEED diffractions during cryogenic cooling, starting with the 1$\times $1 or 3$\times $3 structures. A critical issue to explore is the interrelationship between surface gallium concentration and structural deformation. This study may provide the missing link to new reconstructions of w-GaN recently observed using LT scanning tunneling microscopy.[3] This work is supported by NSF (Grant No. 0730257). [1] A. R. Smith et al., Phys. Rev. Lett. \textbf{79}, 3934 (1997). [2] A. R. Smith et al., Surface Science \textbf{423}, 70 (1999). [3] D. Acharya, S.-W. Hla et al., unpublished. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z12.00011: Magnetism of bulk Cr$_{2}$O$_{3 }$and its (0001) surface: An ab initio study Siqi Shi, Aleksander Wysocki, Kirill Belashchenko Magnetic properties of bulk Cr$_{2}$O$_{3}$ and its (0001) surface are studied using the LSDA+U method. Magnetic energies are well fitted by the Heisenberg model and the Neel temperature is calculated using the quantum pair-cluster approximation. Very good agreement with experiment is found for the equilibrium volume, spectral density, local magnetic moment, band gap, and the Neel temperature. The stable (0001) surface is known to be terminated by a Cr semilayer [1], but its detailed structure is unknown. We identify two competing surface sites. The configurational surface Hamiltonian is constructed from supercells and the structural thermodynamics is studied. We find that in a wide range of temperatures about 1/3 of Cr atoms are below the oxygen layer and that there is a ordering phase transition from $\sqrt 3 \times \sqrt 3 $ to $1\times 1$ structure. Further, we find that (0001) surface has a unique feature of having an uncompensated magnetic moment that is not destroyed by surface roughness. This phenomenon makes Cr$_{2}$O$_{3}$ a promising exchange bias application. [1] M. Bender, \textit{et. al. }J. Phys.: Condens. Matter \textbf{7}, 5289 (1995). . [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z12.00012: Fingerprints of surface magnetism in Cr$_{2}$O$_{3}$ based exchange bias heterostructures Xi He, Yi Wang, Ch. Binek Magnetoelectric materials experienced a recent revival as promising components of novel spintronic devices [1, 2, 3]. Since the magnetoelectric (ME) effect is relativistically small in traditional antiferromagnetic (AF) compounds like Cr$_{2}$O$_{3}$ (max. \textit{$\alpha $}\textit{zz }\textit{$\approx $}4ps/m) and also cross-coupling between ferroic order parameters is typically small in the modern multiferroics, it is a challenge to electrically induce sufficient magnetization required for the envisioned device applications. In exchange bias systems the bias field depends critically on the AF interface magnetization. Hence, a strong relation between the latter and the surface magnetization of the free Cr$_{2}$O$_{3 }$pinning layer can be expected. Our recent research indicates that there are surface magnetic phase transitions in free Cr$_{2}$O$_{3}$ (111) films accompanying surface structural phase transitions. Well defined AF interface magnetization is initialized through ME annealing to T=20K. Subsequently, the interface magnetization is thermally driven through phase transitions at T=120 and 210K. Their effects on the exchange bias are studied in Cr$_{2}$O$_{3}$ (111)/CoPt films with the help of polar Kerr and SQUID magnetometry. [1] P. Borisov et al. Phys. Rev. Lett. 94, 117203 (2005). [2] Ch. Binek, B.Doudin, J. Phys. Condens. Matter 17, L39 (2005). [3] R. Ramesh et al. 2007 \textit{Nature Materials }6 21. Financial support by NSF through Career DMR-0547887, MRSEC DMR-0820521 and the NRI. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z12.00013: Magnetic Susceptibility $\chi$ of O$_2$ in Confined Geometries Timothy Prisk, Paul Sokol Bulk solid oxygen exhibits three distinct crystallographic and paramagnetic phases, $\alpha$, $\beta$, $\gamma$ (in order of ascending temperature). The thermodynamic behavior of some systems, including their possible phases and phase transition temperatures, are known to change in interesting and nontrivial ways when these systems are subjected to confined geometries. Recent work by Kilburn and Sokol on the phonon density of states for confined solid oxygen indicates that the first of these crystallographic phases, $\alpha $, is completely suppressed. The magnetic susceptibility $\chi$ of solid oxygen confined within various porous materials will be presented. The effect of confinement on the magnitude of the susceptibility $\chi$ and the transition temperatures will be discussed. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z12.00014: Size dependent transport of amorphous Indium Oxide films. Swati Soman, Dan Shahar Superconductivity in presence of disorder is a topic of interest among experimentalists as well as theoreticians for past several decades. Experiments performed on disordered films of various materials, elemental as well as mixture, have demonstrated Superconductor to Insulator transition (SIT) with increase in disorder or externally applied magnetic field. Disorder is difficult to quantify. However, in an experiment it is controlled by tuning film thickness and/or composition. We present experimental evidence of SIT, in disordered, amorphous Indium Oxide (a:InO$_{x})$ films, which is tuned by films' lateral dimensions. By fabricating films of same thickness and composition in Hall bar geometry and changing only the sizes of Hall bars, we observe that the sheet resistance per square, R changes with the size of the square, contrary to its definition. The systematic dependence of R on square size is observed to occur only for a critical disorder, similar to percolation model. The observations suggest an inhomogeneous nature of transport near SIT in our samples which are found to be structurally homogeneous. We postulate that such size dependent transport properties are possible to observe in disordered films of other materials that exhibit SIT. [Preview Abstract] |
Friday, March 20, 2009 2:03PM - 2:15PM |
Z12.00015: Impact of Boron Additions on the A1 to L1$_{0 }$Phase Transformation in FePt Alloy Films B. Wang, K. Barmak The combination of high magnetocrystalline anisotropy energy density and good corrosion resistance has resulted in significant interest in L1$_{0}$ ordered alloys such as FePt for ultrahigh density, heat assisted magnetic recording (HAMR) media, with areal storage densities of $\ge $1Tb/in$^{2}$. When deposited at room temperature, these FePt forms in the chemically-disordered A1 state, requiring a post-deposition anneal to form the ordered L1$_{0}$ phase. Previous work has shown that the composition of FePt films has a significant impact on the kinetics and thermodynamics of the A1 to L1$_{0}$ phase transformation. In this paper, we report on the impact of ternary additions of B to FePt on the thermodynamic and kinetic parameters of the transformation. We also compare and contrast the impact of B additions with those of ternary additions of Cu and Ni. It is shown that the introduction of B into Pt-rich FePt films lowers the activation energy and the kinetic ordering temperature for the A1 to L1$_{0}$ phase transformation. However, this impact becomes negligible when B is introduced into Fe-rich FePt films. [Preview Abstract] |
Friday, March 20, 2009 2:15PM - 2:27PM |
Z12.00016: ABSTRACT WITHDRAWN |
Session Z14: Equilibrium Self-Assembly
Sponsoring Units: DFDChair: James Gilchrist, Lehigh University
Room: 315
Friday, March 20, 2009 11:15AM - 11:27AM |
Z14.00001: Para-, ferro- and antiferro-magnetic order in beta-sheet tapes of oligopeptides Sara Jabbari-Farouji, Paul van der Schoot Beta-sheet-forming peptides give rise to self-assembled hierarchical structures such as tapes, ribbons and fibrils, which at sufficiently high concentrations form nematic liquid crystalline solutions and gels. Applications of these novel materials are found in nanotechnology, medicine and personal care products. Such aggregates not only appear in the context of desirable biomaterials but also in pathological self-assembly of mis-folded proteins, forming aggregates such as ``amyloids''. Recently a theoretical model was developed to understand the properties of these self-assembling structures [1]. The question which arises is what happens if we mix different peptide species varying e.g. in length or interaction energy. Do they mix in self-assembled structures or form separate ones? This is of crucial importance as most of industrially produced materials are not monodisperse. To model the simplest polydisperse system, we apply two-component self-assembled Ising model, in which three energy scales are involved. We show that depending upon the relative values of these energy scales and concentrations of the two components, different morphologies of tapes consisting of both components are formed exhibiting paramagnetic, ferromagnetic or antiferromagnetic order. [1] A. Aggeli, et al; PNAS \textbf{2001, }98, 11857 [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z14.00002: Self-assembly induced protein crystallization Hongjun Liu, Sanat Kumar, Jack Douglas The strongly anisotropic nature of inter-protein interactions naturally leads them to self-assemble into structures mirroring the symmetry of the inter-protein potential. Self-assembly is a thermodynamically distinct phenomenon from phase separation, and we consider whether it can play a direct role in nucleating protein crystallization. Previous simulations and measurements have established that protein clusters formed below the critical point for liquid-liquid phase separation ($T_c$) can facilitate crystal nucleation. However, recent experiments have indicated the existence of clustering-induced protein nucleation even for $T > T_c$, where phase separation does not exist. Here we simulate a minimal model of patchy particles and indeed find that transient clusters formed through self-assembly (even above $T_c$) can nucleate crystal growth. Importantly, the self-assembled clusters help to select the symmetry of the resulting crystal growth. In contrast, protein crystallization for $T < T_c$ does not have this directing influence, and polycrystalline growth forms, such as spherulites, are then prevalent. Our simulations suggest that self-assembly directed crystallization might be common in protein solutions and that this process is relevant for understanding protein crystallization polymorphism. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z14.00003: Mechanisms for semi-flexible filament self-assembly: an experimental and simulation study Lam Nguyen, Wei Yang, Steve Acquah, Harold Kroto, Linda Hirst The self-assembly of semi-flexible filaments, such as F-actin, in the presence of cross-linkers has been studied experimentally and via molecular dynamics simulation. Several imaging techniques including fluorescence and electron microscopy have been used to elucidate the structural properties of formed bundles and networks of filaments. With the help of simulation we are able to observe the dynamical process of the self-assembly and study the driving forces behind filament aggregation. The roles of different parameters such as cross-linker density and filament length have been investigated, determining the assembled system properties. We find both of these parameters to play a key role in the final structure formation. Understanding the mechanism for the self-assembly of these semi-flexible filaments will be very useful in the application of developing a new class of biological materials. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z14.00004: Computer simulations of the self-assembly of chiral superstructures from rigid achiral constituents Christopher Hixson, Fangyong Yan, David Earl We present the results of computer simulations of an achiral rigid bent-core model system at a range of temperatures and densities. We observe nematic and smectic phases, but more interestingly observe chiral micelles and columns at lower densities. The origin of these chiral features are explored using minimization techniques and parallel tempering searches. We show that chiral structures are minima of the potential energy surface. Additionally, we show that the addition of chiral dopant induces the system to order into a single twist direction.\footnote{Fangyong Yan, Christopher Adam Hixson, and David J. Earl, Phys. Rev. Lett. {\bf 101}, 157801 (2008)} [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z14.00005: Reversible pH-Induced Structural Transition in a Polyelectrolyte-Surfactant System: from Semi-flexible Rod to String of Spheres Viet Lam, Lynn Walker We have characterized a polyelectrolyte-surfactant system that forms stable rod-like aggregates in aqueous solution. While this structure is stable to most changes in solution condition, we have observed a reversible change in behavior with pH. This is due to a pH-induced structural transition from the original semi-flexible rod at neutral pH to a more flexible object at acidic conditions. A simple model of polyelectrolyte chain crossing multiple surfactant spherical micelles, or a string of spheres, has been proposed as the structure of the aggregates at low pH. This represents a novel rod-like nanoscale system that goes through a reversible gelation with pH, with possible use in oil drilling (matrix acidification aid), liquid flow control, or transport of hydrophobic materials. Here, we will present a simple model of the structural change and experimental justification. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z14.00006: Self-Assembly of Highly Segregating Diblock Copolymer in Solution Dilru Ratnaweera, Stephen Clarson, Dvora Perahia Solvent affinity drives the association of diblock co-polymers in selective solvents. The shape of the micelles is affected by the size of the blocks and their interaction with the solvent. Most experimental and theoretical studies have investigated solutions of diblocks with a relatively low incompatibility, requiring relatively large blocks to associate. The current work introduces a small angle neutron scattering study of a highly segregated diblock-copolymer, a trifluoro propylmethyl siloxane - polystyrene (PTFPMS-PS) in solutions of$ d$-toluene, a good solvent for the polystyrene. Studies were carried out over volume fractions of 0.1 to 0.5 of the fluorinated siloxane segment. The high degree of segregation results in association into star-like micelles with the fluorinated siloxane in the core and a swollen corona even at very low volume fractions of the fluorinated segments. The micelles exhibit unique temperature stability in comparison with aggregates formed by diblock-copolymers in a lower segregation regime. The detailed structure of these aggregates as a function of volume fraction and temperature will be discussed. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z14.00007: Mesophases of soft-sphere aggregates Homin Shin, Gregory Grason, Christian Santangelo Soft spheres interacting via a hard core and purely repulsive shoulder self-assemble into clusters forming a variety of mesophases. We combine a mean field theory developed from a lattice model with a level surface analysis of the periodic structures of soft-sphere aggregates to study stable morphologies for a class of interaction potentials. The mean- field solution shows that the site occupation density and interparticle potential are self-consistently related to an ``effective field'' acting on each particle. In the strong segregation limit, the space group symmetry of possible aggregate structures associated with the spatially modulated field, together with a half-filling condition at the interface of morphology, allows us to produce a phase diagram including Lamella, Hexagonal-columnar, and BCC phases, and their inverse phases in the parameter space of chemical potential and interparticle potential. Finally, we discuss the finite- temperature corrections to strong segregation theory in terms of Sommerfeld-like expansion and how these corrections affect the thermodynamic stability of bicontinuous mesophase structures, such as gyroid. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z14.00008: Breaking it up: Simulations of micelle fission in explicit solvent Mikko Karttunen, Maria Sammalkorpi, Mikko Haataja We study self-assembly in micellar systems consisting of sodium dodecyl sulfate (SDS) using detailed 200-400 ns atomic scale molecular dynamics simulations. The simulations were done with explicit solvent, counterions and salt. We focus on the role of molecular level interactions driving self-assembly [1] and, in particular, show how micelle fission can be controlled using electrostatics. As our main result, we demonstrate the existence of a new fission pathway in charged micelles [2] and provide a physical explanation for it.\\[0.2cm] 1. M. Sammalkorpi, M. Karttunen, M. Haataja, J. Phys. Chem. B {\bf 111}, 11722 (2007).\\ 2. M. Sammalkorpi, M. Karttunen, M. Haataja, J. Am. Chem. Soc., in press. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z14.00009: Neutron Scattering Analysis of the Dynamics and Structure of Semiflexible, Self-Assembled Peptide Chain Networks and WormLike Micelles N. Wagner, M. Branco, D. Pochan, J. Schneider Self assembled peptide hydrogels are formed from synthetic $\beta $-hairpin peptides that undergo triggered self assembly to form a physically crosslinked network of entangled fibrils. Upon salt addition at pH 7.4, these peptides fold into a $\beta $-hairpin self-assemble to form a rigid hydrogel stabilized by non-covalent crosslinks. A single amino acid substitution is performed to change the charge on the peptide and greatly alter the rate of assembly. As a result, faster folding and self assembly kinetics are observed leading to more rigid gels. Transmission electron microscopy (TEM) and rheology demonstrate that the resultant, rigid networks of the semiflexible fibrils are composed of a bilayer of hairpins with a cross-sectional diameter of 3 nm, corresponding to the width of a folded peptide. Neutron spin echo (NSE) measurements show that the peptides can be modeled as semiflexible chains on lengthscales shorter than the characteristic mesh size. The chain diffusivity is reduced by the peptide substitution and this can be attributed to alteration of the electrostatic interactions between peptides in the fibril. Small angle neutron scattering (SANS) measurements show a transition from a cylindrical rod-like geometry to a more branched, fractal-like network topology upon amino acid substitution. These measurements explain the large increase in gel modulus observed upon amino acid substitution. These results facilitate the rational design of self-assembling peptide materials for biomaterial applications. NSE results for semiflexible wormlike micelles will also be discussed. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z14.00010: Chiral Self-Assembly of Rodlike Viruses Edward Barry, Zvonimir Dogic, Robert Meyer, Robert Pelcovits, Rudolf Oldenbourg The self-assembly of two dimensional achiral membranes which occurs in entropic mixtures of monodisperse colloidal rods and non-adsorbing polymers will be described. The colloidal nature of the rod/polymer model system enables us to simultaneously examine the behavior of self-assembled membranes at both the molecular and continuum lengthscales. Combining observations made at the very different lengthscales, we investigate how chirality frustrates assembly of achiral 2D membranes altogether, and instead drives the formation of very complex and highly regular chiral structures. Representative structures obtained through chiral self-assembly include: twisted ribbons, double helices, two dimensional analogs of a TGB phase, and regular arrays of pores embedded within a 2D membrane. [Preview Abstract] |
Session Z16: Bosons in Optical Lattices II
Sponsoring Units: DAMOPChair: Trey Porto, National Institute of Standards and Technology, Gaithersburg
Room: 317
Friday, March 20, 2009 11:15AM - 11:27AM |
Z16.00001: Optical lattice-based addressing and control of long-lived neutral-atom qubits Nathan Lundblad, Trey Porto, Ian Spielman, Radu Chicireanu, William Phillips Many proposed quantum computational platforms are driven by competing needs: isolating the quantum system from the environment to prevent decoherence, and easily and accurately controlling the system with external fields. For example, neutral-atom optical-lattice architectures provide environmental isolation through the use of states that are robust against fluctuating external fields, yet external fields are inherently useful for qubit addressing. Here we demonstrate a technique to address qubits formed from a pair of field-insensitive states by transferring the qubit into a {\em different} pair of field-insensitive states. A spatially inhomogeneous external field allows the addressing of particular ``marked" elements of a qubit register, leaving unmarked qubits unaffected, despite the presence of crosstalk or leakage of the addressing field. We demonstrate this technique in an ensemble of $^{87}$Rb atoms and show that we can robustly perform single-qubit rotations on qubits located at addressed lattice sites. This precise coherent control is an important step forward for lattice-based neutral-atom quantum computation, and is applicable to state transfer and qubit isolation in other architectures using field-insensitive qubits. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z16.00002: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z16.00003: Vortices near the Mott phase of a trapped Bose-Einstein condensate Daniel Goldbaum, Erich Mueller We present a theoretical study of vortices within a harmonically trapped Bose-Einstein condensate in a rotating optical lattice. We find that proximity to the Mott insulating state dramatically affects the vortex structures. To illustrate we give examples in which the vortices: (i) all sit at a fixed distance from the center of the trap, forming a ring, or (ii) coalesce at the center of the trap, forming a giant vortex. We model the imaging of these structures by calculating time-of-flight column densities. As in the absence of the optical lattice, the vortices are much more easily observed in a time-of-flight image than \emph{in-situ}. \\ \\ D.~S. Goldbaum and E.~J. Mueller, \emph{Vortices near the Mott phase of a trapped Bose-Einstein condensate}, arXiv:0808.1548. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z16.00004: Boson Hubbard model with weakly coupled fermions Sumanta Tewari, Roman Lutchyn, Sankar Das Sarma Using an imaginary-time path integral approach, we develop the perturbation theory suited to the boson Hubbard model, and apply it to calculate the effects of a dilute gas of spin- polarized fermions weakly interacting with the bosons. The full theory captures both the static and the dynamic effects of the fermions on the generic superfluid-insulator phase diagram. We find that, in a homogenous system described by a single-band boson Hubbard Hamiltonian, the intrinsic perturbative effect of the fermions is to suppress the Mott insulating lobes and enhance the superfluid phase. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z16.00005: Boson Hubbard model with weakly coupled fermions: Effects of higher bands and shrinking of the superfluid phase Sankar Das Sarma, Roman Lutchyn, Sumanta Tewari We study Boson Hubbard model with weakly coupled fermions and take into account the effects of the higher boson Bloch bands. For attractive couplings between the bosons and the fermions, mixing of the higer bands results in an effective enhancement of the boson on-site repulsion. The overall shift of the boson Hubbard phase diagram due to the presence of the fermions is thus determined by two competing effects: an effective fermion- mediated interaction between the constituent bosons (which favors the superfluid phase), and the renormalization of the boson-boson interaction due to the virtual boson transitions to the higher Bloch bands (which favors the Mott insulating phase). We find that the latter is typically dominant for the cold-atom experiments, which is consistent with the observed loss of the superfluid coherence in recent experiments. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z16.00006: Scattering Properties of Bose-Hubbard Hamiltonians with Two and Three Sites Moritz Hiller, Stefan Hunn, Tsampikos Kottos, Doron Cohen, Andreas Buchleitner We consider a probe particle in a tight binding geometry with two leads and a central site that is coupled to a Bose-Hubbard system consisting of two or three wells (dimer/trimer). In the case of the dimer we find that the resonance widths undergo a sequence of bifurcations resulting from the complexity of the underlying classical phase space structure. For the trimer we show that the statistical properties of the scattering matrix are well described by the random matrix theory predictions for chaotic scattering. The origin of this agreement is due to the fact that inelastic scattering from a chaotic system (trimer) is formally equivalent to elastic scattering in a waveguide that has a chaotic mode space. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z16.00007: Quantum fluctuations and self-organization of a BEC in a multimode optical cavity Sarang Gopalakrishnan, Benjamin Lev, Paul Goldbart An ultracold bosonic gas, trapped in an optical cavity, crystallizes at either the even or the odd antinodes of the cavity mode, if the cavity is pumped transversely with a strong laser beam. Spontaneous symmetry breaking between even and odd antinodes is favored because atoms spaced one wavelength apart coherently emit the light absorbed from the laser, populate the cavity with photons, and thus trap themselves in attractive optical potential wells. For a single-mode cavity, the transition to a crystalline state has been observed [1] and is well described by mean-field theory [2]. However, in multimode cavities, either confocal or concentric, fluctuations are enhanced and change the character of the transition, resulting in a quantal version of the Brazovskii transition in layering systems. We derive a field-theoretic description of the atom- cavity system near the transition, and describe how fluctuations and defects imprint themselves on the correlations of the light leaking out of the cavity. [1] A.T. Black et al, Phys. Rev. Lett. 91, 203001 (2003). [2] J.K. Asboth et al, Phys. Rev. A 72, 053417 (2005). [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z16.00008: Role of Spatial Inhomogeneity in the Experimental Determination of the Two Dimensional Bose-Hubbard Model Critical Point Khan W. Mahmud, Valery G. Rousseau, Marcos Rigol, George G. Batrouni, Richard T. Scalettar Recent experiments at NIST on confined Rb atoms in two dimensions, combined with high precision Quantum Monte Carlo (QMC) values for the homogeneous Bose-Hubbard model critical point, represented important progress toward testing the concept of optical lattice emulator. The experimentally determined critical coupling for the superfluid-Mott transition is in quite good agreement with the QMC results $(U/t)_c = 16.74$ for the homogeneous case. We present an analysis of these results which takes into account the spatial inhomogeneity arising from the confining potential. We perform a detailed QMC calculation of the density profile, local density fluctuations, and condensate fraction along the trajectory followed experimentally. We demonstrate that for the number of atoms, optical lattice depth, curvature of the confining potential, and temperature in the NIST experiment, the critical value for the formation of Mott domains is rather close to that of the homogeneous system. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z16.00009: Superfluid to Mott-insulator transition of hardcore bosons in a superlattice Itay Hen, Marcos Rigol We present results of analytical and numerical studies of the superfluid to Mott-insulator transition of hardcore bosons in a superlattice potential in arbitrary dimensions. In this study, we use mean-field plus spin-wave corrections and the stochastic series expansion (SSE) algorithm to compute various properties of the system, such as the ground-state energy, the condensate fraction, the superfluid density, and the compressibility. We will show that in some cases the spin-wave approximation is in remarkable agreement with the exact numerical results. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z16.00010: The single-atom box: bosonic staircase and effects of parity Christoph Bruder, D.V. Averin, T. Bergeman, P.R. Hosur We have developed [1] a theory of a Josephson junction formed by two tunnel-coupled Bose-Einstein condensates in a double-well potential in the regime of strong atom-atom interaction for an arbitrary total number $N$ of bosons in the condensates. The tunnel resonances in the junction are shown to be periodically spaced by the interaction energy, forming a single-atom staircase sensitive to the parity of $N$ even for large $N$. One of the manifestations of the staircase structure is the periodic modulation with the bias energy of the visibility of the interference pattern in lattices of junctions. A different, e.g. fermionic, additional particle in the junction leads to non-trivial modifications of the staircase, that can be experimentally observed in the visibility of the interference pattern. [1] D.V. Averin, T. Bergeman, P.R. Hosur, and C. Bruder, Phys. Rev. A {\bf 78}, 031601(R) (2008). [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z16.00011: Spin field effect transistors with ultracold atoms G. Juzeliunas, J. Ruseckas, Charles W. Clark, J.Y. Vaishnav We propose a method of constructing cold atom analogs of the spintronic device known as the Datta-Das transistor (DDT), which despite its seminal conceptual role in spintronics, has never been successfully realized with electrons. We propose two alternative schemes for an atomic DDT, both of which are based on the experimental setup for tripod stimulated Raman adiabatic passage. Both setups involve atomic beams incident on a series of laser fields mimicking the relativistic spin orbit coupling for electrons that is the operating mechanism of the DDT. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z16.00012: Vortex quantum dynamics of two dimensional lattice bosons Netanel Lindner, Assa Auerbach, Daniel P. Arovas We study hard core lattice bosons in a magnetic field near half filling\footnote{See: arXiv:0810.2604}. The strong periodic potential scatters the vortices by units of reciprocal lattice momenta, enhancing their mobility and modifying their effective Magnus field. The bare vortex hopping rate on the dual lattice is extracted by exact diagonalizations of square clusters. We deduce quantum melting of the vortex lattice above vortex density of $6.5\times10^{-3}$ per lattice site. The Hall conductivity, which reflects the vortex Magnus dynamics, reverses sign abruptly at half filling. The characteristic temperature scale of the Hall conductivity vanishes at the transition point. We prove that at half filling, each vortex carries a spin half quantum number (`v-spin'). Experimental implications of these results are relevant for diverse systems of current interest, e.g. cold atoms on rotating optical lattices, arrays of Josephson junctions and underdoped cuprate superconductors. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z16.00013: Mapping out the finite temperature phase diagram of the Bose Hubbard model Qi Zhou, Yasuyuki Kato, Naoki Kawashima, Nandini Trivedi We propose a method to experimentally map out the phase diagram of Bose-Hubbard Model at finite temperatures solely based on the density distribution of trapped bosonic atoms in optical lattices. Based on Quantum Monte Carlo simulations in a trap with 10$^{5}$ bosons, we show that the phase boundary between the superfluid and normal state is directly located from kinks in the compressibility, which are extracted from the density profile itself. The temperature of bosons in the lattice is obtained from the density profile at the edge. Our method uses general aspects of critical fluctuations at a phase transition and can be extended to other systems, even when exact numerical simulations are not available. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z16.00014: Distributions of entropy and superfluid density of trapped bosons in optical lattices Nandini Trivedi, Yasuyuki Kato, Qi Zhou, Naoki Kawashima Based on a large scale quantum Monte Carlo simulations of the Bose Hubbard model using the worm algorithm[1], we calculate the inhomogeneous distribution of entropy and superfluid density of trapped bosons in optical lattices. We show that most of the entropy is concentrated in the conducting shells. As the lattice is ramped up under adiabatic conditions, we show that the temperature increases and the superfluid regions in the trap can vanish. However, by opening up the trap at fixed lattice height, the system effectively cools, the entropy gets redistributed in the trap and superfluid regions reemerge. [1] Sharp Peaks in the Momentum Distribution of Bosons in Optical Lattices in Normal State Yasuyuki Kato, Qi Zhou, Naoki Kawashima and Nandini Trivedi Nature Physics, 4, 617 (2008) [Preview Abstract] |
Friday, March 20, 2009 2:03PM - 2:15PM |
Z16.00015: Observing the Quantum Spin Hall Effect with Ultracold Atoms J.Y. Vaishnav, Tudor D. Stanescu, Charles W. Clark, Victor Galitski The quantum spin Hall (QSH) state is a topologically nontrivial state of matter proposed to exist in certain 2-D systems with spin-orbit coupling. While the electronic states of a QSH insulator are gapped in the bulk, a QSH insulator is characterized by gapless edge states of different spins which counterpropagate at a given edge; the spin is correlated with the direction of propagation. Recent proposals \footnote{T. D. Stanescu, C. Zhang, V. Galitski, {\it Physical Review Letters} {\bf 99}, 110403 (2007), J. Y. Vaishnav, Charles W. Clark, {\it Physical Review Letters} {\bf 100}, 153002 (2008).} suggest that synthetic spin-orbit couplings can be created for cold atoms moving in spatially varying light fields. Here, we identify an optical lattice setup which generates an effective QSH effect for cold, multilevel atoms. We also discuss methods for experimental detection of the atomic QSH effect. [Preview Abstract] |
Session Z17: Spin Qubit Coherence and Control
Sponsoring Units: GQIChair: Paola Cappellaro, Harvard University
Room: 318
Friday, March 20, 2009 11:15AM - 11:27AM |
Z17.00001: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z17.00002: Dephasing of exchange coupled spin qubits by electron-phonon coupling: Effect of phonon relaxation Xuedong Hu Electron-phonon interaction leads to pure dephasing between two-electron singlet and triplet states for two exchange-coupled spin qubits in a semiconductor double quantum dot because of the Coulombic nature of exchange interaction. Here we clarify the effect of phonon relaxation, whether via boundary scattering or phonon anharmonicity, on phonon-induced two-spin dephasing. Specifically, we show that within a spin-boson model, phonon relaxation leads to a complete exponential decay of the phase between the singlet and triplet two-spin states, with the decoherence rate proportional to the phonon relaxation rate. We examine the relevant parameter regimes and clarify the importance of this decoherence effect. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z17.00003: Soft-pulse refocusing in the presence of Markovian dephasing Gregory D. Quiroz, Leonid P. Pryadko We consider the effect of Markovian decoherence on the performance of refocusing sequences. This is relevant if dynamical decoupling is to be concatenated with quantum error correcting codes as the first stage of decoherence protection. The basic effect is that an asymmetric decoherence can cause a change in the direction of polarization of a quantum system. For example, dephasing of a single qubit reduces transverse components of the spin polarization vector, thus shifting it towards the $z$ axis. In this work we construct perturbation expansions of effective decoherence operators for generic shaped pulses, and for several sequences of $\pi$- and $\pi/2$-pulses. While in general the performance of soft pulses is worse that that of the ideal $\delta$-pulses, the detrimental effect of dephasing can be reduced by pulse shaping. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z17.00004: Fock-Space Coherence in Quantum Dots Eduardo Vaz, Jordan Kyriakidis We investigate the non-Markovian time evolution of the Fock-space coherence between states with different particle numbers in a multilevel quantum dot. By analyzing the off diagonal density matrix elements for a model where the dominant relaxation mechanism is through sequential tunneling transport, we observe a decoupling between the evolution of the Fock-space coherence and that of the population probabilities for the dot states. When tunneling rates to distinct orbitals differ --- a common occurrence --- the decoherence time of the Fock-space elements of the density matrix can be dramatically increased even when the Hilbert-space coherence between states with same particle number decreases. This is an example of how a many-body coherence can remain robust even in the presence of rather large single-particle noise. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z17.00005: Randomized Benchmarking of Superconducting Qubits Jerry M. Chow, Jay Gambetta, Lars Tornberg, Jens Koch, Lev Bishop, Andrew Houck, Steven Girvin, Robert Schoelkopf We present measurements of average gate errors for a superconducting qubit using randomized benchmarking [1]. The results are compared with gate errors obtained from a double $\pi$ pulse experiment and quantum process tomography. Randomized benchmarking reveals a minimum average gate error of $1.1\pm0.3\%$ and a simple exponential dependence of fidelity on the number of applied gates. It shows that the limits on gate fidelity are primarily imposed by qubit decoherence and finite gate lengths (constrained by qubit anharmonicity), in agreement with theory. \\[3pt] [1] E. Knill et al., Phys. Rev. A. \textbf{77}, 012307(2008). [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z17.00006: Simulating quantum spin systems with superconducting electrical circuits Emily Pritchett, Michael Geller There is currently great interest in the simulation of quantum spin and lattice models using cold atoms. Motivated by experimental progress in the fabrication and control of superconducting electrical circuits and their use in quantum information processing, we investigate their use as simulators of quantum spin systems, and consider a wide family of spin models that can be simulated with existing Josephson junction devices. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z17.00007: Electromagnetic radiation emanating from the molecular nanomagnet Fe$_{8}$ Amit Keren, Oren Shafir Photons emitted by transition between the discrete levels of single molecular magnets have an interesting property: their wave length can be similar to that of the sample size. This is the elementary condition for Dicke's super-radiance. In this radiative process a short intense pulse of light from a molecular system appears as a result of enhanced spontaneous emission rate due to interactions via the electro-magnetic field. Consequently, several investigators have been looking for this type of radiation in the molecular magnet Mn$_{12}$, where energy bursts were reported after magnetic avalanches. We investigate the same phenomenon in the Fe$_{8}$ molecule. Unlike in Mn$_{12}$ we found energy bursts each time there is a jump in the magnetization, confirming their quantum nature. A series of tests indicated that photons carry out the energy. These photons obey the elementary conditions for super-radiance. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z17.00008: Asymmetries in electron spin resonance signal of magnetized spin chains and quantum wires due to spin-orbital interactions Suhas Gangadharaiah, Oleg Starykh We discuss consequences of the symmetry breaking Zeeman and uniform Dzyaloshinskii-Moriya (DM) terms for the electron-spin-resonance (ESR) measurements in a spin-1/2 Heisenberg anti-ferromagnetic spin chain. At the non-interacting level, a non-orthogonal orientation of the magnetic field and DM vector leads to a sharp delta-function ESR signal for the right and left moving excitations. The peak positions and their intensities generally differ, and can serve as a possible chiral probe for the two excitations. Similar results hold for a magnetized quantum wire with spin-orbit terms. Including a momentum dependent fluctuations in the spin-orbit coupling smears the delta-function and instead results in an asymmetric square root singularity. We discuss the role of temperature and interactions in the further modification of the ESR signal. [Preview Abstract] |
Session Z18: Surfaces and Adsorption II
Sponsoring Units: DPOLYChair: Elaine Zhu, University of Notre Dame
Room: 319
Friday, March 20, 2009 11:15AM - 11:27AM |
Z18.00001: Dynamics of six-member molecular rings adsorbed onto graphite and MgO(100) John Z. Larese, Peter Yaron Molecular dynamic studies of adsorption of six-member molecular rings (cyclohexane and benzene) onto various substrates (like MgO(100) and graphite) have been undertaken using a commercial modeling package (Materials Studio by Accelyris Software Inc.) that employs central force field potentials. These studies indicate that both systems exhibit rotational translational coupling and strong signs that the translational diffusion is lattice-like on graphite. The cyclohexane rotational motion has a distinct out of plane component coupled to the translational diffusion much like the rolling motion of a wobbly wheel with three hydrogen atoms in ``contact'' with the substrate plane, while the benzene is more planar. These studies will be compared to results of neutron scattering investigations of the temperature dependance of the dynamics. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z18.00002: Heterogeneity of nitrogen multilayers adsorbed on graphite M. Golebiowska, L. Firlej, B. Kuchta, M.W. Roth The low temperature structures and phase transitions in nitrogen multilayers physisorbed on graphite are analyzed using Monte Carlo simulations. The systems that differ in number of layers and their relative packing (hexagonal or cubic) are analyzed. The nitrogen molecules are simulated as rigid, interacting via site-site potential (Etters model). The interaction with graphite includes atomic corrugation (Steele's potential). We show that the temperature and the mechanism of both orientational and melting phase transitions vary with the surface coverage and change from layer to layer. In particular, melting of the bilayer is preceded by compression of the first layer, which has not been observed before. The results are compared with simulations of two similar systems: (i) three nitrogen layers confined in slit graphite pore, and (ii) an adsorbed incommensurate structure that mimics low temperature alpha phase of bulk nitrogen. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z18.00003: Surface Hopping and Sliding of Single DNA Chains under Electric Field Benxin Jing, Jiang Zhao The motion of non-adsorbing DNA chains under electric field at solid-liquid interfaces was investigated by single molecule fluorescence microscopy at the total internal reflection geometry (TIRF). In-situ observation discovered that the motion of single non-adsorbing lamda-DNA chains was hopping-and-sliding-like along the surface. By varying the surface chemistry of the solid substrates, from the negative-charged hydroxyl group-rich surface to positive-charged amino group-rich surface, as well as hydrophobic surfaces, the dependence of DNA mobility on the surface-DNA interaction was studied. The results show that a well-defined dependence of the mobility of DNA on the surface polarity with respect to DNA itself. The study on different surfaces such as hydroxyl, amide, amino, and methyl-group rich surface show a sequence of DNA mobility of hydroxyl $>$ amide$>$ amino. The mobility of DNA on methyl terminated surface was found to be similar to that in amino surface. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z18.00004: First Principles study of the formation of molecular junctions: benzenethiolate on Au (111) Yongduo Liu, Vidvuds Ozolins We perform density functional calculations to study the formation mechanism of benzenethiolate molecular junctions on the Au (111) surface. Specifically, we investigate the geometry change and the mechanical properties of the metal-molecule contact when it is under stretching. It is found that by pulling up the thiolate molecule from Au (111), one Au surface atom can be converted to an adatom. And moreover, if the stretching is continued, another Au atom would successively be pulled up to form a two-atom bridge between the Au (111) and the sulfur end group. Based on these findings, we propose a mechanism to the formation of pyramidal molecular junctions: benzenethiolate on Au (111). [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z18.00005: Molecular Dynamics Simulation of friction in contact-mode Atomic Force Microscopy of alkane films and nanoparticles F.Y. Hansen, P. Soza, H. Taub, U. Volkmann In addition to sample topography, contact-mode Atomic Force Microscopy (AFM) can yield the lateral frictional force experienced by the AFM tip as it moves across a surface. This frictional force is measured by the torsional angle of the microscope's cantilever arm, which, in the case of a surface composed of highly anisotropic alkane molecules, can depend on the molecular orientation. We have conducted molecular dynamics simulations of an AFM tip moving over films and nanoparticles of $\rm C_{24}H_{50}$ (C24) in the contact mode. For films in which the long axis of the C24 molecules is oriented parallel to the surface, we find a smaller frictional force in a scan direction perpendicular to the long axis than parallel to it. On surfaces where the alkane molecules are oriented perpendicular to the interface, we find that in all scan directions the frictional force is less than when the long molecular axis is parallel to the interface. All of these findings are consistent with experimental observations \footnote{S. Trogisch {\it et al.}, J. Chem. Phys, {\bf 123}, 154703 (2005).} [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z18.00006: The Measurement of Surface Rheological and Surface Adhesive Properties of a PDMS Rubber using Micro- and Nano-Particle Embedment Stephen Hutcheson, Gregory McKenna In previous work, we used particle embedment data to determine the rheological response of the surfaces of a polystyrene film, a phase separated copolymer and a commercially available polydimethylsiloxane (PDMS) rubber through the application of a viscoelastic contact mechanics model. The goal of the current research is to build off this analysis and use micro- and nano-sphere embedment experiments to probe the surface rheological behavior of PDMS in the rubbery state. The work includes measurements made with different particle diameters and chemistries. An atomic force microscope (AFM) is used to measure the embedment depth as nanoparticles are pulled into the surface by the thermodynamic work of adhesion. Present results show that silica probes of different sizes (500 nm and 300 nm) give different results for the surface adhesion properties and the surface rheological properties determined from the particle embedment data and at scales much larger than the nanometer size scale where one might expect such deviations. Possible water entrapment and effects of particle surface composition on the results will be discussed. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z18.00007: Test of the universality of the scaling energy in alkanes based on the melting transition of monolayers adsorbed on graphite L. Firlej, B. Kuchta, M. Roth, C. Wexler The quality of the results of computational treatment of complex systems depends crucially on the quality of the interaction potentials used. When modeling non-rigid molecules an essential difficulty resides in the correct accounting for all energies related to internal degrees of freedom. Of particular importance is to avoid over- or under-counting non-bonded intramolecular van der Waals and electrostatic interactions between close neighbors that are chemically bonded. In many force fields (e.g. CHARMM) 1-4 non-bonded interactions are scaled down by the use of the ``scaling factor'' (SF). Typically, this SF is assumed to be universal (and $\sim 0.5$). In this talk, we study this universality hypothesis by comparing computational and experimental melting temperatures of alkane monolayers adsorbed on a solid graphite surface. Three alkanes (C6, C12, C24) of different lengths (7-30 $\AA$) have been analyzed using an all-atom representation, standard CHARMM parameters, and various SF's. We show that the scaling parameter has no universal value but depends on the molecular length. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z18.00008: The role of local domain formation in the melting of hexane adlayers on graphite M.W. Roth, L. Firlej, B. Kuchta, C. Wexler Hexane is the shortest alkane [CH$_3$-(CH$_2$)$_{n-2}$-CH$_3$] whose flexibility has any considerable impact on its dynamics. When adsorbed on graphite, a monolayer of hexane melts at a temperature of approximately 175 K. To understand the mechanisms of this transition we have performed large scale molecular dynamics simulations (several runs over 100 ns, total computation time $\sim$ 10 cpu-years), using the most realistic model of the system (a fully atomistic representation of hexane, explicit site-by-site interaction with graphite carbons and CHARMM force field with carefully chosen adjustable parameters of interactions). We show that the melting of the low temperature herringbone solid phase starts with the formation of gauche defects at the ends of neighboring molecules, followed by molecular reorientation within a lamellae, without perturbing the overall structure of the adsorbed film. The melted phase has a domain-type structure with domains' orientation that reflects the 6-fold symmetry of graphite. The size of domains decreases progressively when the temperature increases and the deformation of molecules to quasi globular shape is driven by progressive formation of gauche defects. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z18.00009: Chain-length dependence in surface stresses of alkanethiolate-covered Au(111) V. Srinivasan, G. Cicero, J. C. Grossman We have recently shown~\footnote{Phys. Rev. Lett. {\bf 101}, 185504 (2008)} that adsorption-induced stresses in alkanethiolate-covered Au(111) contribute significantly to the stress-response in nano-mechanical cantilever sensors. In particular, we proposed a local stress relief (LSR) mechanism whereby charge removal by the Au-S bond from the Au surface promotes a stress reducing rearrangement of surface Au atoms. Since LSR depends on the nature of the Au-S bond it was unclear how the contribution to the stress- response would depend on the alkanethiolate structure. We present a first-principles study of the chain-length dependence in surface stresses of alkanethiolate-covered Au(111). We find that the surface stress upon adsorption is anisotropic and tensile, increasing in magnitude with the chain-length. We analyze this trend in the context of the LSR mechanism and inter-adsorbate interactions. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z18.00010: All-atom Molecular Dynamics simulations of partial pentane and hexane films on graphite M. Kaspar, M.W. Roth, Carlos Wexler, L. Firlej, B. Kuchta We compare the self-assembly patterns of pentane (C$_{5}$H$_{12})$ and hexane (C$_{6}$H$_{14})$ adlayers physisorbed onto graphite at various coverages using the results of molecular dynamics simulations. Near monolayer coverage, the solid low temperature structure of the pentane film is nematic-like, and that of hexane-herringbone-like. At submonolayer coverages both systems exhibit three distinct topological regimes: vacancy patches at higher densities, percolating networks at intermediate densities and ultimately individual patches. The systems' orientational behavior and melting dynamics is discussed with respect to its unique density-dependent topology. The simulations explicitly include hydrogens of pentane and hexane and the graphite is modeled as a six-layer all atom structure. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z18.00011: Temperature Dependent Adsorption Dynamics of Binary Mixtures of Halomethanes on Graphite and $\alpha$-quartz Surfaces Jonathan Nehring, G. Leuty, Mesfin Tsige Using atomistic molecular dynamics simulations, we have investigated the structure and dynamics of binary mixtures of halomethanes ($CF_4$, $CF_3Cl$, and $CF_3Br$) as a function of temperature on two structurally and chemically different surfaces. The initial distribution of the binary mixtures is either they are uniformly mixed or a layer or layers of one component is placed on top of a layer or layers of the other component. As a function of temperature (below and above the melting temperature of $CF_4$) and the two surface types, we observed a marked change on the concentration, dynamics, orientation, and structure of each of the components in the first layer of the binary mixtures next to the surfaces. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z18.00012: Liquid Chromatography at Critical Conditions: Balancing size exclusion and adsorption in nanopores Asem Abdulahad, Jeffrey Amos, Chang Ryu Liquid chromatography at critical condition (LCCC) is a measure to identify thermodynamic conditions, in which polymers elute independently of molar mass during high performance liquid chromatography. Under these critical conditions the entropic exclusions that dominate size exclusion chromatography (SEC) and the enthalpic adsorption that governs adsorption-based interaction chromatography (IC) are said to negate one another resulting in simultaneous elution of the polymer of different molecular weights. Using multiple C18-bonded silica columns with different average nanopore sizes (from 5 nm to 30 nm), we will study the LCCC conditions of PS in methylene chloride/acetonitrile solvent mixture at different temperature. In addition, we will show that the separation of polystyrene can be fine tuned using a refined temperature gradient interaction chromatography (TGIC) that employs multiple columns of varying pore size in sequence. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z18.00013: Using Amphiphilic Copolymers and Nanoparticles to Organize Charged Biopolymers Jung Hyun Park, Marla McConnell, Yujie Sun, Yale Goldman, Russell Composto Nanoparticles (NPs) on amphiphilic random copolymers control filamentous actin (F-actin) attachment. 3-aminopropyltriethoxysilane (APTES) coated silica NPs are selectively bonded to acrylic acid groups on the surface of a poly(styrene-r-acrylic acid) (PS-r-PAA) film. By changing the concentration of NPs in the medium, the surface density of positively charged anchors is tuned. Using total internal reflection fluorescence (TIRF) microscopy, immobilization of F-actin is observed via electrostatic interaction with NPs at high NP coverages. Below a critical coverage, F-actin is weakly attached and undergoes thermal fluctuations near the surface. Another method to tune F-actin attachment is to use APTES to cross-link and create positive charge in PAA films. Here, the surface coverage of F-actin decreases as APTES concentration increases. This observation is attributed to an increase in surface roughness and hydrophobicity that reduces the effective surface sites that attract F-actin. In addition, in-situ G-actin polymerization to F-actin is observed on both the NP and cross-linked PAA templates. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z18.00014: Non-translational Molecular Diffusive Motion on Two Different Time Scales in Alkane Nanoparticles S.-K. Wang, M. Bai, H. Taub, E. Mamontov, K.W. Herwig, F.Y. Hansen, J.R.D. Copley, T. Jenkins, M. Tyagi, U.G. Volkmann Using quasielastic neutron scattering, we have investigated molecular diffusive motion in $n$-C$_{32}$H$_{66}$ nanoparticles whose structure and phase transitions have been studied previously.$^{2}$ The spectra reveal non-translational (dispersionless) diffusive motion occurring simultaneously on time scales of $\sim $1 ns and $\sim $40 ps. The onset of the faster motion occurs in the crystalline phase at least 15 K below the melting point and is tentatively identified with rotation about the long molecular axis. Similarly, we suggest that the slower motion involves molecular conformational changes whose onset appears to coincide with the abrupt transition to the bulk rotator phase about 3 K below melting. These two types of diffusive motion bear a strong resemblance to those observed previously in C24 monolayers adsorbed on a graphite surface.$^{3}$ $^{2}$M. Bai \textit{et al.}, Europhys. Lett. \textbf{79}, 26003 (2007). $^{3}$F. Y. Hansen \textit{et al}., Phys. Rev. Lett. \textbf{92}, 046103 (2004)]. [Preview Abstract] |
Friday, March 20, 2009 2:03PM - 2:15PM |
Z18.00015: Synergetic binding and lateral segregation in polymer decorated micelles and nanoparticles Igal Szleifer, Rikkert Nap Nanocarriers show great potential as drug delivery devices or as imaging agents. Experimental relevant examples of nanocarriers involve micelles made of low molecular weight polyethylene glycol and phospholipids. An important feature of these 'nano' micelles is that the polymers are mobile. A fundamental question is how different polymeric coatings result in optimal nanoparticle-surface interactions. We used a molecular theory to investigate the effect of the conformational entropy, specific interactions and lateral mobility on the structure of the polymer coatings and the binding of the nanocarrier to a cell surface. In micelles that contain chains of different molecular weights, the long and short polymer chains segregate upon approaching the surface, as a result of competing entropic forces. Nanocarriers made of mixtures of weak polyelectrolytes with ligands at their free ends and neutral polymers can bind to charged surfaces or through specific ligand-receptor interactions. We show that under appropriate conditions there is a dramatic synergetic effect between electrostatic and ligand-receptor binding. The synergetic effect is due to the optimal compensation between charge regulation, specific binding and counterion release. The potential use of these carriers for cancer drug delivery will be discussed. [Preview Abstract] |
Session Z19: Semi-Crystalline Polymers
Sponsoring Units: DPOLYChair: Steve Hudson, National Institute of Standards and Technology
Room: 320
Friday, March 20, 2009 11:15AM - 11:27AM |
Z19.00001: Pseudobrush theory of amorphous interphase in semicrystalline polymers. Scott Milner The conformation of polymer chains emerging from the face of a crystalline lamella has long been a matter of dispute. Long ago, arguments pitted ``adjacent reentry'' versus the ``switchboard model'' as extreme limits of possible behavior. Later, two theoretical approaches were attempted, but one (the Gambler's Ruin model) did not properly account for the constraint of melt density, and the other (heuristic configuration counting of Flory et al.) did not account for chain connectivity. These shortcomings are resolved by a new ``pseudobrush'' theory of the amorphous interphase, which represents the reentrant chains as a polydisperse brush of loops in a self-consistent hydrostatic pressure field. This theory predicts the fraction of adjacent reentry, shows how the anisotropy of the interphase dies away with distance, and how the Gambler's Ruin model is recovered far from the interface. Extension to the case of a finite slab between two crystal-melt interfaces predicts the frequency of tie chains, a key parameter for nonlinear deformation and ductile failure of semicrystalline polymers. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z19.00002: The impact of chain folding on the structure and morphology of polymer lamellae Bernard Lotz The major structure and morphology features of polymer lamellae (and single crystals) are usually defined by the characteristics of the crystal lattice (cell geometry and symmetry, stem length). However, the fold surfaces that sandwich the crystalline core may have an impact on that crystalline core. In isotactic polyolefins, restrictions on the relative (up/down) orientation of stems linked by a fold may result in lowered unit-cell symmetry. Also, surface stresses linked with the presence of folds determine to a large extent the non-planar shape of polymer crystals. In bulk crystallization, they may induce twisted and/or scrolled lamellae. However, this impact can be inferred only for specific polymers and/or crystal structures and/or unit-cell symmetries. (Work performed with Dr. A. Thierry and J. Ruan.) [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z19.00003: Unexpected Observation of 2.5 Dimensional Growth of Polymer Spherulite Dujin Wang, Changming Wang, Ying Zhao, Jinliang Song, Buxing Han Preparation of integrated polymer spherulite from both solution and melt has been a challengeable subject. In this letter, micro-sized spherulites of ultrahigh molecular weight polyethylene (UHMWPE) have been successfully prepared from supercritical ethanol. The spherulite grows to 2.5 but not 3 dimensions with only one nucleation site on the surface. The 2.5 dimensional growth makes it possible to observe both the nucleation site and sheaf-like structure on the surface of a spherulite and to obtain one global final spherulite as a single particle. A possible mechanism for the particular morphology of spherulites is proposed based on the contributions of surface nucleation on polymer droplet, high molecular weight of UHMWPE as well as the soft confinement of supercritical fluid. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z19.00004: Structural evolution under uniaxial drawing of Poly(D, L-lactide) Films Gr\'egory Stoclet, Jean-Marc Lefebvre, Roland Seguela Aliphatic polyesters are an important class of biodegradable polymers. They have drawn particular attention in the last few years as food packaging materials because they can be derived from renewable resources. Among this family, polylactide (PLA) is considered as one of the most promising ``green'' polymer for use as a substitute to petroleum-based polymers. In the present work, we investigate the mechanical behaviour of amorphous poly(D, L-lactide) films in relation to the structural evolution upon stretching at various draw temperatures (Td) above the glass transition temperature. Examination of the drawing behaviour shows that PLA initially behaves like a rubbery material until a true strain of the order of 1. Strain hardening occurs beyond this strain level, up to film fracture. Such strain hardening is generally ascribed to a strain induced crystallization phenomenon. In the present case, it is clearly more pronounced for Td = 90\r{ }C than for Td = 70\r{ }C. The corresponding structural evolutions are investigated by means of WAXS. The diffraction patterns reveal the marked influence of draw temperature. Indeed for Td = 70\r{ }C a mesophase is induced whereas strain-induced crystallisation takes place at Td = 90\r{ }C. Further work is in progress, in order to elucidate mesophase development and mechanical response. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z19.00005: Vitrification and Devitrification of Rigid Amorphous Fraction of PET during Quasi-isothermal Cooling and Heating Peggy Cebe, Huipeng Chen Poly(ethylene terephthalate), PET, was studied by quasi-isothermal (QI) Temperature Modulated Differential Scanning Calorimetry (TMDSC). For the first time, both the temperature dependent crystalline fraction and rigid amorphous fraction (RAF) were quantitatively analyzed during QI cooling and reheating. Specific reversing heat capacity measurements show that most RAF vitrifies step by step during QI cooling after completion of crystallization. Upon subsequent QI reheating, the RAF devitrifies also step by step and only a small RAF of 0.04 remains at 470K, while melting starts above 473K. To obtain the exact temperature of the start of melting, heat capacity measurements were made using subsequent standard DSC heating, after QI cooling. By combining this method with the QI results, the temperature dependent phase fractions were obtained during standard DSC heating. We conclude that RAF completely devitrifies before the temperature reaches the crystal melting endotherm under the conditions used in this work. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z19.00006: Crystallization of Model Long Chain Branched Polyethylenes with Different Branching Architectures M. Vadlamudi, R. G. Alamo While the impact of long chain branching (LCB) of different architectures (stars, $\alpha -\omega $ H type, pom-pom, combs) on rheology has been studied extensively, the effect on crystallization is less known. This work analyses the influence of LCB architecture on crystallization from quiescent melts using models based on hydrogenated polybutadienes, all with a constant 2.1 mol{\%} of ethyl branches (LCB PEs). Crystallization rates measured by DCS, the phase structure, and morphology of the LCB PEs are studied in reference to the linear chain. At a fixed undercooling the crystallization rates of all LCB PEs are 30 to 40{\%} lower than the rate of the linear as expected from transport limitations to the nucleation rate of the LCB systems. Smaller differences in the rate are found within the various LCB architectures. The components of the phase structure are controlled by the content of short chain branching with a negligible effect from the LCB architectures. For all LCB PEs the crystalline component is $\sim $30{\%} and the interphase region $\sim $15{\%} as determined by WAXD, RAMAN and DSC. A major impact of LCB is found in the supermolecular morphology. Restrictions from the LCB melt topology to propagate long organized arrays bring about a change from spherulites (linear) to poorly organized crystallites (LCB PEs). Long range dynamics (NMR T$_{2H})$ and lamellar structures (AFM) are presently investigated. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z19.00007: An anomaly in the crystallisation rate of bimodal poly(ethylene oxide) weight distributions Jessica L. Carvalho, Sara L. Cormier, Kari Dalnoki-Veress We present results on the crystallisation of blends of poly(ethylene oxide) (PEO) of differing molecular weight, $M_{w}$. Previous studies probing PEO blends have mainly focused on blends of low $M_{w},$ for which integral chain folding is important, with very high $M_{w}$. The PEO samples used in this study consist of a blend with both $M_{w}$'s well above the integral chain folding limit. In general, one would expect that such blends should show a monotonic decrease in spherulite growth rate, $G$, as the blend contains more high-$M_{w }$component. Our results however show a clear non-monotonic $G$, with a minimum in a plot of $G$ as a function of the volume fraction. In short, blending a small amount of the low- $M_{w}$ into the High- $M_{w}$ PEO slows the growth kinetics. These results along with a possible mechanism will be discussed. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z19.00008: Effect of ppm Levels of Long Chain Branching on the Crystallization of Isotactic Poly(propylenes) from the Melt and from Solution Rufina G. Alamo, Juan P. Fernandez-Blazquez, Syed A. Abdullah, Madhavi Vadlamudi Small concentrations of long chain branching (LCB) added via copolymerization with a diene to a linear poly(propylene) chain have a dramatic effect on crystallization. LCB iPPs with diene levels between 100 and 400 ppm show greatly enhanced nucleation density, increasing with diene content. The increased nucleation observed in optical micrographs, is also evident in the crystallization half-time observed by DSC in isothermal crystallization as a function of crystallization temperature. Isothermal crystallization studies from dilute solution measured by light scattering gave the same trend with diene content and rule out any effect of gel structures affecting the nucleation rate. The linear growth rates are unchanged. The increase in nucleation rate is not linear with the increase of diene as revealed from studies of blends of LCBiPPs with the linear matrix, suggesting that length of molecules and connectivity of the LCB structure are more important than the chemical nature of the junction. The enhanced nucleation levels off at concentrations of diene that approach the critical concentration for coil overlapping (c*) as calculated for chains generated by a Monte Carlo simulation of the LCB molecules having the experimental molecular weight distribution. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z19.00009: Melting, Recrystallization and Superheating of Polymer Crystals Studied by Fast Calorimetry (1 MK/s) Christoph Schick, Alexander Minakov, Andreas Wurm, Evgeny Zhuravlev For polymers the origin of the multiple melting peaks observed in DSC curves is still controversially discussed. This is due to the difficulty to investigate the melting of the originally formed crystals exclusively. Recrystallization is a fast process and most experimental techniques applied so far do not allow fast heating in order to prevent recrystallization totally. We developed a thin-film (chip) calorimeter allowing scanning rates as high as one million Kelvin per second. The calorimeter was used to study the melting of isothermally crystallized polymers like isotactic polystyrene (iPS), isotactic polypropylene (iPP), poly(ethylene therephthalate) (PET) and others. Our results on melting at rates as high as 1,000,000~K/s support the validity of a melting-recrystallization-remelting process at low scanning rates (DSC) for all studied polymers. At isothermal conditions they form crystals, which all melt within a few dozens of K slightly above the isothermal crystallization temperature. There is no evidence for the formation of different populations of crystals with significantly different stability (melting temperatures) under isothermal conditions. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z19.00010: Nano-Scale Confinement Effects on Poly($\varepsilon $-caprolactone) Crystallization at the Air/Water Interface Qiongdan Xie, Bingbing Li, Tomonori Saito, Wen Yin, Timothy Long, Richard Gandour, Alan Esker Poly($\varepsilon $-caprolactone)s (PCL) with different mole mass were synthesized by using tri-ester primary amine as an initiator, tin(II) 2-ethylhexanoate as the catalyst, anhydrous tetrahydrofuran as a solvent at 80 \r{ }C. The three triester groups were further hydrolyzed to render PCL linear polymers with triacid end groups (PCL Triacid). The as-synthesized PCL triacids were attached to 9 nm magnetic nanoparticles (MNP) by ligand exchange with oleic acid in refluxing chloroform for 24 h. The crystallization behavior of PCL trihead and PCL MNPs at the air/water interface were compared with reported linear PCL crystallization. Regular crystalline morphologies observed for linear PCL are not observed for PCL-trihead and PCL-MNPs. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z19.00011: Microphase Separation Controlled Beta Sheet Crystallization Kinetics in Silk Fibroin Protein. Xiao Hu, Qiang Lu, David Kaplan, Peggy Cebe We investigate the mechanism of isothermal crystallization kinetics of beta-sheet crystals in silk multiblock fibrous proteins. The Avrami analysis kinetic theory, for studies of synthetic polymer crystal growth, is for the first time extended to investigate protein self-assembly in beta-sheet rich \textit{Bombyx mori} silk fibroin samples, using time-resolved Fourier transform infrared spectroscopy, differential scanning calorimetry and synchrotron real-time wide-angle X-ray scattering. Results indicate formation of beta sheet crystals in silk proteins is different from the 3-D spherulitic crystal growth found in synthetic homopolymers. Observations by scanning electron microscopy support the view that the protein structures vary during the different stages of crystal growth, and show a microphase separation pattern after chymotrypsin enzyme biodegradation. We present a model to explain the crystallization of the multiblock silk fibroin protein, by analogy to synthetic block copolymers. This model could be widely applicable in other proteins with multiblock (i.e., crystallizable and non-crystallizable) domains. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z19.00012: Correlation between Structure and Vapor Sorption in Semi-crystalline Polymers: One Dimensional Nano-Swelling Measured using iVSANS Man-Ho Kim, Glinka J. Charles Changes in the nanoscale structure of semi-crystalline polyethylene (PE) resulting from the sorption/diffusion of n-hexane vapor have been observed and quantified by the \textit{in-situ }Vapor Sorption Small-Angle Neutron Scattering (iVSANS). We found a linear correlation between vapor sorption/diffusion and nano-expansion of the amorphous phase within the lamellae confined in the spherulite domains. The diffusion coefficient measurements using (iVASNS) revealed that the molecular packing in amorphous phase is different between low and high crystalline PE. Furthermore, the one dimensional expansion is reversible during cyclic sorption and desorption processes, suggesting that the crystals were not destroyed unlike the inference of a previous study. These results are relevant to processing methods for tailoring the barrier and sorptive properties of semicrystalline polymer films. * This work utilized facilities supported in part by the National Science Foundation under Agreement No. DMR-0454672. The author, MHK, acknowledges support by the KIST (2E20844). [Preview Abstract] |
Session Z20: Polymer Blends
Sponsoring Units: DPOLYChair: Mark Foster
Room: 321
Friday, March 20, 2009 11:15AM - 11:27AM |
Z20.00001: Microemulsions in Asymmetric Polymer Blends Alisyn Nedoma, Megan Robertson, Nitash Balsara Microemulsions and lamellar phases have been observed in previous experiments wherein block copolymers are added to blends of immiscible homopolymers. To our knowledge, all of the previous studies are restricted to homopolymers of nearly identical chain lengths with critical volume fractions in the vicinity of 0.5 (symmetric systems). The present study concerns the formation of microemulsions and lamellar phases phases in blends of immiscible polymers with substantial differences in chain lengths and critical volume fractions far removed from 0.5 (asymmetric systems). The characteristics of the block copolymers that enable the creation of these phases will be discussed in the presentation. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z20.00002: Nanostructured polymer blends by addition of gradient copolymer during melt mixing: Effects of copolymer sequence distribution on morphology and crystallization behavior Robert Sandoval, Jungki Kim, John Torkelson Nanostructured blends of polystyrene and poly(ethylene oxide) (PEO) are produced via the addition of styrene/methyl methacrylate (S/MMA) gradient copolymer during conventional melt mixing, introducing dipole-dipole interactions between PEO and MMA repeat units. Upon addition of S/MMA gradient copolymer, stable PEO domains with diameters of $\sim $ 100 nm are formed in optimized cases. This results in PEO domains homogenously crystallizing at $\sim $ -20 C, well below the crystallization temperature of bulk PEO ($\sim $ 50 C). Additionally, nanostructured blends annealed at room temperature for 40 days resulted in no melting behavior upon heating, showing that the nanoconfined PEO domains remain in a rubbery state at room temperature, while bulk PEO typically remains in a crystalline state. This demonstrates that the blend properties can be easily tuned by adjusting the copolymer characteristics. This study is the first to lead to nanostructured polymer blends from non-reactive, simple melt mixing of two homopolymers and a compatibilizer. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z20.00003: A Morphological Study of Poly(Butylene Succinate)/Poly(Butylene Adipate) Blends with Different Blend Ratios and Crystallization Processes Jerold Schultz, Haijun Wang, Zhigua Gan, Shouke Yan Morphologies of blends of poly(butylene succinate) (PBS, m.p. 114\r{ }C) with poly(butylene adipate) (PBA, m.p. 60\r{ }C) varying in blend ratio and in crystallization temperature of the PBS component were studied using optical and atomic force microscopies. When PBS is crystallized at 75\r{ }C, subsequent PBA crystallization occurs only within PBS spherulites. When PBS is crystallized at 100\r{ }C, a portion of the PBA is rejected from the growing PBS. The morphological difference is also reflected in the time-dependence of the crystallization kinetics. The difference in behavior at these two temperatures reflects a large change in the diffusion length. Further, the location of PBA crystals within PBS spherulites depends on PBA concentration and on PBS crystallization temperature. Lower PBA concentrations lead to interlamellar segregation, while when PBA is the majority phase, interfibrillar crystallization crystallization dominates. Replace this text with your abstract body. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z20.00004: Phase equilibria and crystallization in mixtures of azobenzene chromophore and triacrylate Garrett O'Malley, Kenneth Milam, Namil Kim, Thein Kyu A temperature versus composition phase diagram of azobenzene chromophore/triacrylate system was established by means of differential scanning calorimetry (DSC). The isotropic liquid (I) and crystal solid + liquid (Cr$_{1}$ + I$_{2})$ coexistence regions bound by liquidus and solidus lines were tested with the theoretical curves obtained by self consistently solving the combined free energies of Flory-Huggins (FH) theory for isotropic mixing and phase field (PF) theory for crystal solidification pertaining to the compositional order parameter (\textit{$\phi $}$_{1}$\textit{, $\phi $}$_{2})$ and the crystal order parameter (\textit{$\psi $}$_{1})$, respectively. With the aid of phase diagram, various phase morphologies were mapped through thermal quenching into various coexistence gaps. Azobenzene in the blend produced multiple crystal structures, including gigantic single crystals. Real time images demonstrating the nucleation and growth of the crystallization process were captures under polarized optical microscopy. The spatiotemporal growth of such single crystal has been elucidated theoretically using the time-dependant Ginzburg Landau (TDGL) dynamics. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z20.00005: Miscibility Studies on Polymer Blends Modified with Phytochemicals Neelakandan Chandrasekaran, Thein Kyu The miscibility studies related to an amorphous poly(amide)/poly(vinyl pyrrolidone) [PA/PVP] blend with a crystalline phytochemical called ``Mangiferin'' is presented. Phytochemicals are plant derived chemicals which intrinsically possess multiple salubrious properties that are associated with prevention of diseases such as cancer, diabetes, cardiovascular disease, and hypertension. Incorporation of phytochemicals into polymers has shown to have very promising applications in wound healing, drug delivery, etc. The morphology of these materials is crucial to applications like hemodialysis, which is governed by thermodynamics and kinetics of the phase separation process. Hence, miscibility studies of PA/PVP blends with and without mangiferin have been carried out using dimethyl sulfoxide as a common solvent. Differential scanning calorimetry studies revealed that the binary PA/PVP blends were completely miscible at all compositions. However, the addition of mangiferin has led to liquid-liquid phase separation and liquid-solid phase transition in a composition dependent manner. Fourier transformed infrared spectroscopy was undertaken to determine specific interaction between the polymer constituents and the role of possible hydrogen bonding among three constituents will be discussed. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z20.00006: Phase Separation Mechanism of Polybutadiene/Polyisoprene Blends Under Oscillatory Shear Flow Charles C. Han, Ruoyu Zhang, He Cheng, Xia Dong Viscoelastic polymer blends of polybutadiene (PB)/low vinyl content polyisoprene (LPI), with a lower critical solution temperature (LCST) has been studied under oscillatory flow conditions. The phase separation mechanism has been investigated with the consideration of the nucleation mechanism, spinodal fluctuations, and also the shear induced mixing. Frequency and temperature ramping rate dependence of the apparent binodal and spinodal points will be discussed. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z20.00007: Impact of Deuterium Substitution on the Physical Properties of Polymer Melts and Blends Ronald White, Jane Lipson We examine the effects on blend properties when one of the polymeric components is deuterated. Here we use SANS-fitted model calculations to explore the underlying physical behavior, and relate our findings to subtle effects in molecular size and energetics. A special emphasis is given to the prediction and analysis of phase behavior for polymeric mixtures (e.g. liquid-liquid partial miscibility), including a comparison of phase diagrams for several related systems. We discuss effects such as pressure and molecular weight dependence and also include an analysis of calculations in which we probe the influence of key model parameters on blend miscibility. As with our earlier studies, the results featured here involve the application of a microscopically parameterized equation of state derived from an integral equation theory for lattice-based chain molecule fluids. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z20.00008: Mapping Mechanical Properties and Glass Transition Temperature in Polymer Materials with sub-100 nm Resolution Maxim Nikiforov, Stephen Jesse, Sangah Gam, Russell Composto, Louis Germinario, Sergei Kalinin Thermomechanical properties at the nanoscale are extremely important for understanding fundamental as well as technological problems. To date, Local Thermal Analysis (LTA) provides information about glass transition and melting temperature with about 1-2 $\mu$m spatial resolution. We developed LTA technique based on Scanning Probe Microscopy that allows probing not only melting and glass transition temperatures, but also elastic and loss moduli on a 100 nanometer length scale. This universal method for quantitative thermomechanical analysis was used to study the kinetics of phase separation in PMMA:SAN system. The maps of mechanical properties as function of temperature were obtained with sub-100 nm resolution. The difference of mechanical properties for two materials was determined. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z20.00009: Understanding Dynamics of Multicomponent Polymer Systems Using Homopolymer/Copolymer Blends Caleb Dyer, Dias Linton, Mark Dadmun Blending two polymers is a cost-effective method to produce new materials with tailored properties, although the effect of the presence of one component on the dynamics of the second component is not well understood. Recent studies on miscible polymer blends show that the local environment in a polymer blend has a critical impact on the dynamics of each component in the blend. To this end we have utilized neutron reflectivity (NR), quasi-elastic neutron scattering (QENS), and rheology to study the dynamics of a polystyrene-\textit{ran}-poly(methyl methacrylate) (PS-\textit{ran}-PMMA) copolymer in a PMMA matrix. The system consists of a miscible blend that is 90{\%} PMMA/10{\%} random copolymer. The copolymer composition varies from 60{\%} to 90{\%} MMA in the blend, effectively tuning the thermodynamic interactions in the system. We will present these results to provide insight into the role of thermodynamic interactions on the dynamics of this miscible polymer blend. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z20.00010: Influence of polymer chain connectivity on local composition distribution in miscible polymer blends Dmitry Bedrov, Wenjuan Liu, Ralph Colby Molecular dynamics simulations using bead-necklace model have been utilized to investigate concentration distribution of local environments in model polymer blends. Distribution of effective composition around polymer segments has been investigated for different blend scenarios and bulk concentrations. Inter- and intramolecular contributions to the effective composition have been analyzed. Our analysis indicates that chains connectivity has a significant and nontrivial effect on distribution of effective composition around polymer segment. The results of this work are compared with assumptions of several theoretical models that commonly used to describe structural and dynamical correlations in miscible polymer blends. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z20.00011: Origins of Deviations from the RPA in Polymer Blends: Simulations and Theory Jun Kyung Chung, David Morse We performed continuum Monte Carlo simulations of symmetric binary polymer blends to precisely quantify deviations from RPA predictions for composition fluctuations, including critical phenomena. This comparison is made possible by an unambiguous procedure for determining the self-consistent field (SCF) $\chi$ parameter by extrapolating thermodynamic perturbation theory to the limit of infinite chain length $N$. Corrections to the RPA are shown to be proportional to $N^{-1/2}$, and to be accurately predicted outside of the critical region by a renormalized one-loop theory. The difference between the apparent (i.e., measured) interaction parameter and the SCF value is positive (destabilizing) far from the spinodal ($\chi N \ll 1$) as the result of an $N$-dependence of the depth of the correlation hole in a melt. Near the critical point, this effect is almost exactly cancelled by the stabilizing effect of long-wavelength composition fluctuations, yielding a critical value of $\chi N$ quite close to the RPA prediction of $(\chi N) \simeq 2$. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z20.00012: Two DSC Glass Transitions in Miscible Blends of Polyisoprene / Poly(4-\textit{tert}-butyl styrene) Junshu Zhao, Ye Sun, Lian Yu, Mark Ediger Conventional and temperature modulated differential scanning calorimetry experiments have been carried out on miscible blends of polyisoprene (PI) and poly(4-\textit{tert}-butyl styrene) (P4tBS) over a broad composition range. This system is characterized by an extraordinarily large component T$_{g}$ difference ($\sim $215 K) between the two homopolymers. Two distinct calorimetric T$_{g}$s were observed in blends with an intermediate composition range (25{\%}$\sim $50{\%} PI) by both conventional and temperature modulated DSC. Good agreement was found between the T$_{g }$values measured by the two methods. Fitting of the measured T$_{g}$s to the Lodge-McLeish model gives a $\phi _{self}$ of 0.62$\sim $0.64 for PI in this blend and 0.02$\sim $0.05 for P4tBS. The extracted $\phi _{self}$ for PI$_{ }$is comparable to reported values for PEO in blends with PMMA and is significantly larger than those reported for other PI blends with smaller component T$_{g}$ differences. This observation suggests the presence of a confinement effect in PI/P4tBS blends, which results in enhanced fast component dynamics below the effective T$_{g}$ of the slow component. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z20.00013: Segmental dynamics in polymer blends: adapting the Long-Lequeux model. Gareth Royston, Paul Sotta, Didier Long In recent years several models have been proposed which attempt to describe the distribution of relaxation times observed in glass forming systems as they approach vitrification. We have adapted the Long-Lequeux model, initially proposed for van der Waals liquids, for application to polymer blends. Considering thermally induced density fluctuations, the glass transition is considered to be controlled by percolation of small domains of slow dynamics. Here we present a comparison of the model with experimental data including recently acquired data on miscible blends of poly(alpha-methylstyrene) and poly(cyclohexyl methacrylate). The model is shown to provide a good fit to the data over a range of conditions. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z20.00014: The Viscoelastic Behavior of Polymer/Oligomer Blends Wei Zheng, Gregory McKenna, Sindee Simon The dynamics in athermal blends of poly($\alpha $-methyl styrene) (PaMS) and its short chain oligomer are investigated using rheometry and differential scanning calorimetry (DSC). Master curves for the dynamic shear responses, G' and G", are successfully constructed for both the pure materials and the blends, indicating the validity of the time-temperature superposition principle. The temperature dependence of the shift factor follows the WLF (Williams-Landel-Ferry) behavior over the temperature range studied, and for the blends, the dependence is dominated by the high mobility oligomer. The discrete relaxation spectra of the materials are calculated and are found to be broader for the blends than for the pure materials. A similar domination of the dynamics by the oligomer is observed in DSC enthalpy recovery studies and in the broadened glass transition from DSC. The ability to predict the dynamic responses of the blends from the responses of the neat materials is examined, and whether this prediction needs to incorporate the self-concentration idea as described in Colmenero's model will be discussed. [Preview Abstract] |
Session Z22: Focus Session: Quantum Spin Hall Effect
Sponsoring Units: GMAG DMP FIAPChair: Ron Jansen, University of Twente
Room: 324
Friday, March 20, 2009 11:15AM - 11:51AM |
Z22.00001: Optical Control of Topological Quantum Transport in Semiconductors Invited Speaker: Spin-orbit coupling enables electrical manipulations of spins, e.g. through the spin Hall effect, but it also causes spin relaxation and thus a rapid loss of information stored in spins. We propose a solution to this dilemma by exploiting light-matter interactions in the reactive regime: light is used as a control knob to switch on/off spin-orbit coupling readily without exciting real carriers. In electron-doped semiconductors, when an off-resonant optical field virtually excites interband transitions, the large spin-orbit coupling in the valence bands can be partially transferred to the photon-dressed conduction band. The adiabatic electronic ground state can thus be reactively controlled by optical pulses, exhibiting anomalous Hall conductivity. By the control of linearly polarized light, a pure spin Hall current of electrons can be driven by an in-plane DC electric field, which results in net spin accumulations at the edges of the optical excitation area. Effectively, one has created a spin battery powered by optical pulses together with DC electric field, which allows the spatial and temporal control of spin generations. The resultant electron spin accumulations can have long lifetime when spin-orbit coupling vanishes with the adiabatic switch off of the control light. Circularly polarized light breaks the time reversal symmetry and can result in spin polarized charge Hall conductance. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z22.00002: Spin resonance and spin-orbit coupling effects in quantum Hall edge channels A.V. Stier, C.J. Meining, V.R. Whiteside, B.D. McCombe, E.I. Rashba, P. Grabs, L.W. Molenkamp We report studies of far-infrared (FIR) photo-response (E$_{FIR}$=3.15meV) of a 2D electron gas in an asymmetric 15nm InAs quantum well in a field/frequency regime where electron spin resonance is expected. Photo-induced changes in the longitudinal resistance were measured in a Hall-bar geometry in a tilted magnetic field (B) whose angle $\theta $ was varied. For $\theta \approx $40\r{ } and Landau Level (LL) filling factor $\nu \approx $7, we observe \underline {several} sharp minima with a dominant central feature. This feature vanishes for $\theta <$38.4\r{ } and splits into two sharper lines at larger angles. The center of gravity of this pair tracks approximately the center of the $\nu $ =7 Quantum Hall (QH) plateau. The appearance of the central feature coincides with the condition of complete filling of the $\nu $=7 LL at an applied B where the Zeeman spin splitting equals the energy of the FIR laser line. We attribute the sharp multiple line structure to EDSR transitions in pairs of QH edge channels whose resonance conditions are modified by Rashba effective fields. A detailed model that describes qualitatively the experimental findings will be discussed. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z22.00003: Tunneling between edge states in a quantum spin Hall system Anders Str\"om, Henrik Johannesson We analyze a quantum spin Hall (QSH) device with a point contact connecting two of its edges. The contact supports a net spin tunneling current that can be probed experimentally via a two-terminal resistance measurement. We find that the low-bias tunneling current and the differential conductance exhibit scaling with voltage and temperature that depend nonlinearly on the strength of the electron-electron interaction. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z22.00004: Kondo effect in the helical edge liquid of the quantum spin Hall state Joseph Maciejko, Chaoxing Liu, Yuval Oreg, Xiao-Liang Qi, Congjun Wu, Shou-Cheng Zhang Following the recent observation of the quantum spin Hall (QSH) effect in HgTe/CdTe quantum wells, an important question is to understand the effect of impurities on transport in the QSH regime. Using linear response and renormalization group methods, we calculate the edge conductance of a QSH insulator in the presence of a single magnetic impurity. At high temperatures, due to Kondo scattering we find a logarithmic temperature dependence consistent with current experiments. At low temperatures, for weak Coulomb interactions in the edge liquid the conductance is restored to unitarity with unusual power-laws due to the formation of the Kondo singlet, while for strong interactions transport proceeds by weak tunneling through the impurity where only half an electron charge is transferred in each tunneling event. We propose scanning gate and shot noise experiments to search for these effects. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z22.00005: Corner Junction as a Probe of Helical Edge States Chang-Yu Hou, Eun-Ah Kim, Claudio Chamon We propose and analyze inter-edge tunneling in a quantum spin Hall corner junction as a mean to probe the helical nature of the edge states. We show that electron-electron interactions in the one-dimensional helical edge states result in Luttinger parameters for spin and charge that are intertwined, and thus rather different from those for a quantum wire with spin rotation invariance. Consequently, we find that the four-terminal conductance in a corner junction has a distinctive form that could be used as evidence for the helical nature of the edge states. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z22.00006: A topological Dirac insulator in a quantum spin Hall phase David Hsieh, Dong Qian, Lewis Wray, YuQi Xia, Yew San Hor, Robert Cava, Zahid Hasan When electrons are subject to a large external magnetic field, the conventional charge quantum Hall effect dictates that an electronic excitation gap is generated in the sample bulk, but metallic conduction is permitted at the boundary. Recent theoretical models suggest that certain bulk insulators with large spin orbit interactions may also naturally support conducting topological boundary states in the quantum limit, which opens up the possibility for studying unusual quantum Hall-like phenomena in zero external magnetic fields. Bulk Bi$_ {1-x}$Sb$_x$ single crystals are predicted to be prime candidates for one such unusual Hall phase of matter known as the topological insulator. The hallmark of a topological insulator is the existence of metallic surface states that are higher-dimensional analogues of the edge states that characterize a quantum spin Hall insulator. Here, using incident-photon-energy-modulated angle-resolved photoemission spectroscopy, we report the direct observation of massive Dirac particles in the bulk of Bi$_{0.9}$Sb$_{0.1}$ and provide a comprehensive mapping of the Dirac insulators gapless surface electron bands. These findings taken together suggest that the observed surface state on the boundary of the bulk insulator is a realization of the topological metal. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z22.00007: Non-magnetic disorder effects on 3-dimensional $Z_2$ quantum spin Hall systems Ryuichi Shindou, Shuichi Murakami Motivated by the recent discovery of the $Z_2$ quantum spin Hall insulator (QSHI) in the antimony doped bismuth, we have studied the non-magnetic disorder effects onto the quantum critical point (QCP) which always exists between an ordinary insulator and the $Z_2$ QSHI. Namely, intervening the topologically distinct states of matter, such QCP should be generally stable against any perturbations (i.e. disorders), as far as the time-reversal symmetry is preserved. In this talk, I will present a possible microscopic mechanism of this stability, based on simple weak-localization calculations. Specifically, at the QCP between the topological insulator and an ordinary insulator, so-called the {\it parity} degree of freedom also becomes the conserved quantity, in addition to the usual charge. As a result of this, the diffuson near the QCP consists of the {\it two} quasi-degenerate dominant contributions having the diffusion poles; one contributes to the usual charge diffusion, while the other is ascribed to the {\it parity diffusion}. In terms of these two quasi-degenerate low-energy modes, I will construct a possible microscopic picture for the stability of the QCP against no-magnetic disorders. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z22.00008: Magnetic impurities on the surface of a topological insulator Qin Liu, Chao-Xing Liu, Cenke Xu, Xiao-Liang Qi, Shou-Cheng Zhang The surface states of a topological insulator are described by an emergent relativistic massless Dirac equation in 2+1 dimensions. In contrast to graphene, there is an odd number of Dirac points, and the electron spin is directly coupled to the momentum. We show that a magnetic impurity opens up a local gap and suppresses the local density of states. Furthermore, the Dirac electronic states mediate an RKKY interaction among the magnetic impurities which is always ferromagnetic, whenever the chemical potential lies near the Dirac point. These effects can be directly measured in STM experiments. We also study the case of quenched disorder through a renormalization group analysis. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z22.00009: Seeing the magnetic monopole through the mirror of topological surface states Xiao-Liang Qi, Run-Dong Li, Jiadong Zang, Shou-Cheng Zhang Existence of the magnetic monopole is compatible with the fundamental laws of nature, however, this illusive particle has yet to be detected experimentally. In this work, we show that an electric charge near the topological surface state induces an image magnetic monopole charge due to the topological magneto-electric effect. The magnetic field generated by the image magnetic monopole can be experimentally measured, and the inverse square law of the field dependence can be determined quantitatively. We propose that this effect can be used to experimentally realize a gas of quantum particles carrying fractional statistics, consisting of the bound states of the electric charge and the image magnetic monopole charge. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z22.00010: Theoretical study on quantum spin Hall phases in bismuth ultrathin films Shuichi Murakami, Masaki Wada, Frank Freimuth, Gustav Bihlmayer It has been proposed that the (111) 1-bilayer bismuth is in the quantum spin Hall phase [1]. This argument is based on a simple tight-binding model for the 3D bismuth, and it is desirable to examine this result by more realistic methods. In this presentation, we investigate possibilities of the quantum spin Hall phases in two of the bismuth ultrathin films by first- principle calculations and confirmed the result in Ref.[1]. Bulk bismuth is a semimetal, while some of the bismuth ultrathin films have a gap. As proposed by first-principle calculations, among various phases seen in experiments, only two cases are gapped: (i) (111) 1-bilayer film and (ii) \{012\} 2-monolayer film. These two structures are almost degenerate in energy. We calculate the $Z_2$ topological numbers for the two structures. both from the bulk Bloch wavefunctions and from band structure calculations in the geometry with edges. In the calculations we use the maximally localized Wannier orbitals constructed from first-principle calculations. We found that (i) is the quantum spin Hall phase while (ii) is an ordinary insulator. Their difference can be observed in STM/STS and other possible experiments to verify our results are discussed. [1] S. Murakami, Phys. Rev. Lett. 97, 236805 (2006). [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z22.00011: Probing Surface States of the Topological Insulator Bi$_{1-x}$Sb$_{x}$ with Scanning Tunneling Microscopy and Spectroscopy Pedram Roushan, Anthony Richardella, Colin Parker, Kenjiro Gomes, Abhay Pasupathy, Aakash Pushp, Yew San Hor, Robert Cava, Ali Yazdani There is a considerable interest in surface properties of \textit{Bi}$_{1-x}$\textit{Sb}$_{x}$ alloys, for which there is growing evidence that they are topological bulk insulators with novel surface states [1]. We have used a cryogenic scanning tunneling microscope (STM) to probe the surface of \textit{Bi}$_{1-x}$\textit{Sb}$_{x}$ directly, and confirming the presence of surface states within the bulk band gap. Energy resolved conductance mapping of these surface states reveal strong spatial modulations, similar to those observed with the STM for noble metal surface states [2]. Fourier analysis of these maps shows that the spatial modulation of the surface states can be understood within a model for scattering between various k-states of the band structure of the surface. We will present these results in connection with the angle-resolved photoemission measurements of the contours of constant energy. [1] D. Hsieh \textit{et al.}, Nature \textbf{452}, 970 (2008) [2] M. F. Crommie \textit{et al.}, Nature \textbf{363}, 524 (1993) [Preview Abstract] |
Session Z24: Hydrogen Storage: Dopants and Catalysts
Sponsoring Units: DMPChair: Fred Pinkerton, GM Research
Room: 326
Friday, March 20, 2009 11:15AM - 11:27AM |
Z24.00001: Effect of Ti Dopant on Surface Diffusion of Isolated Alane Species: A Comparison between Al (111) and Al (100) surfaces Altaf Karim, James Muckerman Our density functional theory-based kinetic Monte Carlo simulations show that an embedded Ti atom creates a well in the potential energy surfaces of Al(111) and Al(100) as probed by hydrogen and other isolated alane species. Hydrogen adatoms become trapped around Ti atoms on an Al(111) surface, whereas Al adatoms do not exhibit any significant effect of the potential energy well created by the Ti atoms. In contrast to the case of Al(111), Al adatoms on an Al(100) surface also become trapped around the Ti atoms for a longer period of time compared to the hydrogen adatoms on this surface. Therefore, Ti sites on Al(100) become poisoned by the presence of Al adatoms around them for long periods of time, thereby blocking further dissociative adsorption of hydrogen. The overall diffusion of Al adatoms on an Al(100) surface is significantly lower compared to the Al(111) surface. This fact suggests that the Ti-doped Al(111) surface is perhaps more conducive to the production of alane species than the Ti-doped Al(100) surface despite its higher activation barrier for the dissociation of molecular hydrogen. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z24.00002: Hydrogen adsorption on Al (100) facilitated by surface alloying with Sc Feng Zhang, Yan Wang, M. Y. Chou We report first-principles investigations of hydrogen adsorption on the Al (100) surface modified by alloying with Sc, as the first step to understand the catalytic role of scandium in the hydrogenation process in Sc-doped NaAlH4. Sc prefers to stay at subsurface sites with or without adsorbed hydrogen. The adsorption energy on the Sc-modified surface is 0.5 eV/H2 lower than that on the pure Al surface, while the dissociation barrier of H2 is similar for the two systems. The structure with two H atoms adsorbed on two nearest-neighbor bridge sites is at least 0.3 eV more stable than other structures; but no additional activation energy is required for H to diffuse among these structures. Electronic structures are also examined to explain these alloying-induced effects. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z24.00003: Hydrogen-related defects and the role of metal additives in complex hydrides Khang Hoang, Chris G. Van de Walle Complex hydrides such as NaAlH$_{4}$ and Li$_{4}$BN$_{3}$H$_{10}$ doped with selected transition metals (e.g., Ti, Ni, and Pt) are promising hydrogen storage materials. The mechanism of the enhancement in (de)hydrogenation rates caused by these metals is, however, not well understood. We have carried out first-principles studies based on density functional theory of hydrogen vacancies and interstitials, which play an important role in the (de)hydrogenation processes. We find that these defects are always charged; their formation energy therefore depends on the Fermi level. The metallic impurities can also exist in different charged states and, therefore, modify the Fermi level, thus changing the defect concentrations. Our first-principles results shed light on the role of transition-metal impurities in hydrides and lead to the design of storage materials with improved characteristics. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z24.00004: First-principles and Tight-binding Calculations in the Pd-H System A. Shabaev, D.A. Papaconstantopoulos Using the linearized augmented plane wave(LAPW) method we have generated a large database of electronic structure results that include fcc, bcc, NaCl, CsCl, Cu3Au, Fluorite crystal structures as well as supercell configurations with various hydrogen occupations for the Pd-H system. The formation energies and energy bands from this database were used to construct a tight-binding model that reproduces well the above LAPW results and, in addition, is transferable to other crystal lattices including random occupation of crystal sites as well as treating vacancies. We calculate the phonon frequencies, elastic constants, the density of states, coefficient of thermal expansion, mean-squared displacement and the energy of vacancies formation in Pd. The objective of this work is to be able to perform electronic structure calculations for systems containing up to a few thousand atoms where first- principles calculations are computationally intractable. This approach is used in both static and dynamic calculations and enables us to vary the amount of hydrogen entering into the Pd matrix. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z24.00005: Nitrogen and Hydrogen on a Palladium-covered proton conductor: a first principle study of Ammonia catalysis Lorenzo Paulatto, Stefano de Gironcoli Being liquid at ambient conditions Ammonia would be an ideal Hydrogen vector. However, the industrial Haber process for Ammonia synthesis involves high pressures ($\approx 100$~bar) and temperatures ($450-500$~$^{\circ}$C), making the process very expensive. Recently, ambient pressure Ammonia production, in the $570-750$~$^{\circ}$C temperature range, has been reported at the Palladium cathode of a proton conducting cell-reactor [1]. The rate limiting step in the Haber process is N$_2$ dissociation, while the observed limiting factor in Ref. [1] appears to be the proton transfer through the conductor and it has been proposed that Nitrogen hydrogenation may in this case precede dissociation. We use first-principles techniques to study Nitrogen, Hydrogen and Ammonia interaction with flat and stepped Pd surfaces, in presence of external electric fields. Our aim is to study the effect of electrochemically provided protons on the catalysis of the reaction. [1]{G. Marnellos and M. Stoukides, Science 282, 98 (1998); G. Marnellos, S. Zisekas, and M. Stoukides, J. of Catalysis 193, 80–87 (2000)} [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z24.00006: Synthesis and Characterization of Au and Pd Decorated ZnO Powders Paige Landry, Hangning Chen, Andi Barbour, Michael Felty, John Z. Larese We report our synthesis and characterization of ZnO nanopowders decorated with Pd and Au clusters. Ultrapure ZnO powders are readily produced. Pd and Au nanoclusters are deposited from solution and reduced using hydrogen gas. Characterization of these materials using high resolution adsorption isotherms and inelastic neutron scattering have been performed.~Particular attention has been paid to the adsorption of hydrogen and deuterium on these pure and decorated materials. Preliminary results indicate that hydrogen preferentially adsorbs to the Pd metal sites. Our results will underscore the significant promise of these combined systems for use in providing basic knowledge and for technological applications. [Preview Abstract] |
Session Z25: Focus Session: Graphene XIX: Electronic Properties
Sponsoring Units: DMPChair: Silvia Viola, Boston University
Room: 327
Friday, March 20, 2009 11:15AM - 11:27AM |
Z25.00001: Chiral structure and mixed parity anomalies in graphene-related systems Akihiro Tanaka We reanalyze the chiral symmetry structure of graphene and its variants (boron-nitride sheets, bond alternated graphene, etc) using a representation of Dirac fermions previously employed by the author in a search for topological effects in the pi- flux state of a square lattice electron system (PRL ${\bf 95}$ 036402 (2005)). We find that the electromagnetic responses of nontopological insulators to curvature-induced gauge fields mimics in an interesting way the responses of topological insulators of the Haldane/Kane-Mele category to conventional (Maxwellian) gauge fields. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z25.00002: Many-electron Effects on the Electronic Structure and Optical Spectrum of Few-layer Graphene Li Yang, Jack Deslippe, Cheol-Hwan Park, Marvin Cohen, Steven Louie We present a first-principles calculation of the optical properties of single- and few-layer graphene with many-electron effects included, employing the GW-Bethe Salpeter equation (GW- BSE) approach. We have found enhanced excitonic effects that result in significant changes in the optical absorption of few- layer graphene as compared to the independent-particle picture. Our calculated absorption spectrum is in good agreement with recent experiments. This study is of importance for understanding excitonic effects in two-dimensional semimetal systems and expected to be useful for possible optoelectronics applications of graphene. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z25.00003: Geometrical constraint on alkali and halogen adsorption on graphene ChengIng Chia A seamless sp2 graphene sheet prevents the penetration of atoms through the sheet, yet allows the penetration of electrons. Thus, a suspended single sheet graphene forms a geometrical constrained background by separating the surrounding vacuum into upper half and lower half spaces. Alkali and halogen atoms, each constrained to one of the spaces, are forced to interact electrostatically via charge transfer through the sheet. A new type of chemical interaction is formed under this constraint, which we call topologically frustration bonding. We have calculated the interaction of a K atom on the upper surface with a halogen atom on the lower surface of a pure-carbon graphene sheet using density functional theory. The system becomes ferroelectric under this new geometrical constraint. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z25.00004: Pivotal role of buffer layer in tuning electronic properties of epitaxial graphene Yufeng Guo, Wanlin Guo, Changfeng Chen We explore the response of epitaxial bilayer graphene on SiC and Ru to electric field and mechanical tuning using first- principles calculations. Our calculations reveal that, in contrast to prevailing view, the buffer layer plays an active role in the distribution of charge transfer within the epitaxial graphene layers and with the substrate. The charge distribution and electronic structure are also sensitive to the type of substrate. These results provide new insights for fundamental understanding and practical application of these fascinating materials. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z25.00005: Graphene on Silicon Dioxide: Band gap modulation via substrate surface chemistry Philip Shemella, Saroj K. Nayak We have studied the electronic structure of graphene deposited on a SiO$_2$ surface using density functional methods. The band structure of the graphene monolayer strongly depends on surface characteristics of the underlying SiO$_2$ surface: for an oxygen-terminated surface, the monolayer exhibits a finite energy band gap while the band gap is closed when the oxygen atoms on the substrate are passivated with hydrogen atoms. We find that at least a graphene bilayer is required for a near zero energy gap when deposited on a substrate without H-passivation. Our results are discussed in the light of recent experiments. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z25.00006: Current polarization in B-doped graphene nanoribbons: ab initio simulations Alexandre Rocha, Thiago Martins, Adalberto Fazzio, Ant\^onio J. R. da Silva Single layer graphene has been recently isolated and can pave the way to a number of nanoscale technologies. One interesting possibility is to use the spin of the electron - instead of its charge - as information carrier in carbon-based systems where the spin coherence length can reach hundreds of nanometers. Up until now, spintronics devices have been assembled using magnetic electrodes as a source of spin polarized electrons. In this work we use a combination of density functional theory and non-equilibrium Green's functions techniques to study the electronic transport properties of graphene nanoribbons (GNRB) up to 500 nm long containing substitutional Boron atoms. We demonstrate that in realistic systems where the B atoms are randomly distributed along the GNRB, the polarization of the current can reach up to 100\% and is independent of impurity concentration. These effects can be explained in terms of different scattering probabilities for majority and minority spins from a single B atom. This consequently leads to different Anderson localization lengths for each spin population. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z25.00007: Impact of the electron-electron correlation on phonon dispersion:Failure of LDA and GGA DFT functionals in graphene and graphite. Michele Lazzeri, Claudio Attaccalite, Ludger Wirtz, Angel Rubio, Francesco Mauri GW is nowadays the most accurate ab-initio method to determine electronic bands. So far GW has never been used to determine neither the electron-phonon coupling (EPC) nor phonon dispersions. We show that GW approach [1] can be used to compute the EPC and the phonon dispersion. In particular, in graphene and graphite, standard DFT (LDA and GGA) underestimates, by a factor of 2, the slope of the highest optical branch at the zone boundary (K) and the square of its EPC by almost 80\%. On the contrary, GW reproduces the experimental phonon dispersion near K, the value of the EPC, and the electronic band dispersion, in agreement with phonon dispersions from inelastic x-ray scattering and Raman spectroscopy. Comparing these results with other computational methods, the B3LYP hybrid functional gives phonons close to GW but overestimates the EPC at K by about 30\%. Within Hartree-Fock, the graphene structure displays an instability under a distortion following the A'1 phonon at K. [1] M. Lazzeri et al., Phys. Rev. B 78, 081406(R) (2008). [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z25.00008: First-Principles Studies of Oxidation Functional Groups on Graphene Jia-An Yan, Mei-Yin Chou Opening a band gap in monolayer graphene is of special interest for the graphene-based electronics applications. Inspired by the potential applications of graphene oxide, we have systematically investigated the effects of the oxidation functional groups (epoxy and hydroxyl) on the structural, energetics, and electronic properties of graphene by first-principles calculations. Our energetics calculations show that the OH group tends to aggregate to the neighboring carbon sites of an epoxy group, resulting in the formation of several possible building units. We find that the epoxy group strongly hybridizes with the extended $\pi$ ($\pi^*$) bands, giving rise to a shift of the Dirac point in the momentum space and a decrease in the Fermi velocity. In contrast, the adsorption of a single hydroxyl group leads to the formation of a localized state and a gap opening near the Fermi level. The oxidation concentration dependence of the energy gap is investigated. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z25.00009: Phyiscal adsorption induced band gap openning in graphene Youjian Tang, VIncent Crespi Gapping graphene is crucial for enabling its use in next-generation electronic devices. Here we show that physical adsorption of suitable aromatic molecules onto graphene can generatate a moderate band gap of approximately 0.125 eV, with an adsorption energy ~0.67 eV. The reason for such a band gap is that the Lowest unoccupied molecule orbit of adsobate is right across the fermi level of graphene and thus gennerate a big perturbation on graphene dirac point energy level. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z25.00010: First-Principles Studies of Covalent Functionalization of Graphene by Carboxyl Groups Nabil Al-Aqtash, Igor Vasiliev We study the mechanism of covalent functionalization of graphene by the carboxyl (COOH) group in the framework of density functional theory combined with the generalized gradient approximation. The structures and binding energies of the COOH group attached to the surface of graphene are examined in cases of graphene containing no defects, containing a Stone-Wales defect, and containing a vacancy. Our calculations confirm that the binding of the COOH group with graphene is significantly stronger in the presence of surface defects. We also observe substantial changes in the structure of defective graphene after the attachment of the COOH group. These results suggest that surface defects play an important role in the carboxylation of graphene. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z25.00011: First-principles calculations of electronic transport through graphene with realistic metallic leads Salvador Barraza-Lopez, M. Y. Chou We present transmission characteristics for electrons through graphene with realistic metallic contacts. The methodology relies on an in-house version of the electronic transport SMEAGOL code [1], in which the memory required to allocate for the matrices of contact leads and the graphene sheet in the Green's function solver is distributed into more than one processor, for a given electron energy. We are able to accommodate for commensurate graphene-metal supercells which have the correct atomic structure (namely, stress caused by contracting/extending the metal contacts to match the periodicity of graphene is avoided). In addition, and despite of the large size of the leads, the electronic properties and transport are computed at the density-functional theory level [2] within a double-zeta plus polarization basis[3], ensuring the accuracy of the atomic forces in the system, as well as on the final transmission characteristics. [1] A. R. Rocha et al, PRB. \textbf{73}, 085414 (2006); [2] J. M. Soler et al, J. Phys.: Condens. Matter \textbf{14}, 2745-2779 (2002); [3] J. Junquera et al, PRB \textbf{64}, 235111 (2001). [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z25.00012: Exfoliation of graphene flake from SiC substrate using hydrogen injection; a first-principle study Bora Lee, Seungwu Han, Yong-Sung Kim Recently there is an immense interest in studying graphene for investigating its unique electronic properties as well as practical applications to nanoscale devices. Up to now there are two methods to obtain graphene layers. The first one is a mechanical method in which the single graphene sheet is split off the bulk graphite crystals using adhesives. The other method is graphitization of SiC surfaces by annealing at elevated temperatures. Even though the latter approach can provide a graphene layer in a more controlled way, the exfoliation of the graphene layer still poses a big challenge. In this presentation, based on the first-principles results, we propose a novel exfoliation method using hydrogen. As a model system, the 6H-SiC(0001) 4$\times $4 cell is used, which corresponds to the 3$\times $3 graphene cell. We calculate the binding energy of single hydrogen atom in various places; above and below graphene surface and inside the first SiC layer. The binding energies of hydrogen are calculated for different coverages. It is found that at high coverages, the hydrogen atoms prefer to bind below the graphene surface, cutting the graphene-SiC bonds. This means that the graphene can be exfoliated in the hydrogen-rich environment. The detailed analysis including the electronic structures will be presented. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z25.00013: Electronic Properties of Graphene Oxide Geunsik Lee, Kyeongjae Cho Graphene has shown promising electronic properties as future device applications beyond the current CMOS (complimentary metal-oxide-semiconductor) technology based on silicon microelectronics. As a critical insulating component in all-carbon nanoelectronic devices, graphene oxides (GOs) are shown to have insulating behavior, but their electronic and atomic structures are poorly understood. We investigated electrical property of GO using density functional theory (DFT) and non-equilibrium Green's function (NEGF) method with tight binding (TB) scheme. We model the basal plane oxidation with top site (OH) and bridge site (epoxide) chemisorptions. By varying the chemisorption ratio of the hydroxyls and epoxides as well as their coverage, the conductance of GO is calculated and quantitatively compared with experimental reports. We have investigated the electronic structure of graphene and GO multilayers for pseudospin device application. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z25.00014: Lattice Monte Carlo studies of quantum critical phenomena in graphene Timo Lahde, Joaquin Drut The Lattice Monte Carlo approach is well suited to the study of strongly interacting fermionic systems, such as the quasi-relativistic charge carriers in graphene, as it is non-perturbative and takes full account of quantum fluctuations. Recent simulational results on the semimetal-insulator critical point in graphene are presented, with emphasis on the question whether the transition to an insulating phase is of second order or of infinite order. This critical point is likely to be relevant for the physics of suspended graphene, as its location determined in arXiv:0807.0834 (see abstract by J.~E.~Drut) suggests that suspended graphene should be an insulator rather than a semimetal. An observable of particular interest is the DC conductivity of graphene, as most analytical studies underpredict this by a factor $\sim 3$. It has been pointed out that a complete description of the DC conductivity of graphene should account for non-perturbative effects due to the long-range Coulomb interaction between the fermionic quasiparticles. A possible method for determining the DC conductivity of graphene using the Lattice Monte Carlo technique is presented. [Preview Abstract] |
Session Z26: Composite and Porous Media
Sponsoring Units: DCMPRoom: 328
Friday, March 20, 2009 11:15AM - 11:27AM |
Z26.00001: Flexibility of zeolite frameworks Vitaliy Kapko, Michael Treacy, Michael Thorpe Zeolites are an important class of industrial catalysts because of their large internal surfaces and molecular-sieving properties. Recent geometric simulations (1) show that almost all of the known zeolites can exist without distortion of their tetrahedra within some range of densities, which we call the flexibility window. Within this window, the framework accommodates density changes by rotations about the shared tetrahedral corners. We argue that the presence of a flexibility window can be used as a topological criterion to select potential candidates for synthesis from millions of hypothetical structures. We also investigate the exceptions to the rule, as well as the shape of the flexibility window and the symmetric properties of zeolites inside it. (1) A. Sartbaeva, S.A. Wells, M.M.J. Treacy and M.F. Thorpe The flexibility window in zeolites, Nature Materials 5, 962-965 (2006); I. Rivin, commentary 931-932. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z26.00002: Electronic and ionic conduction in oxo-vanadium arsenates Victoria Soghomonian Electrically conducting microporous zeolite-like or zeoate frameworks are largely unstudied as electronic materials, but may offer new avenues in catalysis and in electrical energy storage applications. Zeolites and zeoates are characterized by the presence on nanoscale channels and cavities delineated by their crystalline framework, and occur naturally or can be synthesized by inorganic methods. The material under discussion here, an oxo-vanadium arsenate system, combines the properties of the well known but electrically insulating microporous zeolites, with an electronically active framework. We present the structure and the experimentally measured electronic and ionic conductivities of the materials system, the temperature dependences of the conductivities, and discuss possible electronic and ionic conduction mechanisms at play in empty frameworks and ion-exchanged frameworks. We also discuss how microporous electrically conducting frameworks can find use in electrical energy storage, and compare the zeoate frameworks in such applications to other materials systems such as mesoporous carbon. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z26.00003: Synthesis of hybrid zeolite materials with TiO2 nanocrystals using solid-solid method Corina Orha, Carmen Lazau, Cornelia Ratiu, Paula Sfirloaga, Paulina Vlazan, Alexandra Ioitescu, Florica Manea, Paul Barvinschi, Ioan Muscutariu, Ioan Grozescu Zeolite seems to be a promising support for TiO2 photocatalyst because of its regular pores and good adsorption ability. TiO2 supported on zeolite integrates the photocatalytic activity of TiO2 with the adsorption properties of zeolites. The aim of this paper was the syntheses and characterizations of functionalized zeolite materials with undoped, Fe-doped and N-doped TiO2 nanocrystals. The zeolite hybrid materials impregnation with titanium dioxide was achieved through solid-solid method. TiO2 doped with metallic (Fe) and non-metallic (N) ions was obtained directly from precursors by sol-gel and hydrothermal methods. The hybrid materials were characterized by XRD, SEM with EDAX, IR and AFM. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z26.00004: Carbon dioxide and methane transport in DDR zeolite: insights from molecular simulations into carbon dioxide separations in small pore zeolites Sang eun Jee, David Sholl Zeolites are good candidates as a membranes for chemical separations because of their excellent chemical and thermal stability. Cage type zeolites are promising materials for gas separation since their narrow windows are expected to control molecular transport. DDR is one of the strongest candidates for light gas separations because of its narrow 8MR window. In our study, we examined the separation selectivity of DDR for CO$_{2}$/CH$_{4}$ separation using atomistic simulation methods. We introduced new force fields which can reproduce experimental single component adsorption and diffusion data for this material for the first time. Previously interatomic potentials that have been applied to DDR overestimate experimental diffusivities at least one order of magnitude. We characterized single-component and binary adsorption using Grand Canonical Monte Carlo, and single-component. diffusion using a combination of Molecular Dynamics and Transition State Theory. The most important observation from our calculation is that CO$_{2}$/CH$_{4}$ diffusion in DDR is very different from the usual situation in nanoporous materials, where the presence of a slowly diffusing species retards transport rates of a more rapidly diffusing species. In DDR, we show that CO$_{2}$ diffusion rates are only weakly affected by the presence of CH$_{4}$, despite the very slow diffusion of the latter species. The physical origins of this unusual behavior are explained by analyzing the adsorption sites and diffusion mechanism for each species. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z26.00005: Effects of inhomogeneous partial absorption and the geometry of the boundary on the population evolution of molecules diffusing in general porous media Seungoh Ryu, David L. Johnson We consider the diffusion-relaxation dynamics in porous media with partially absorbing boundary conditions. Spectral analysis of Helmholtz equation for the uniform boundary condition has been widely used as a probe of geometry of the medium. The NMR relaxation of the fluid magnetization, for example, is used for a variety of media such as rocks, cement, bones, and cheese. While direct relationship between their geometry and the spectrum forms the basis for such applications, little attention has been paid to the spatial variation of the boundary condition. We report on the way the geometry and such inhomogeneity become intertwined and affect the spectrum. It is often impossible to gauge how severe such interference is in the biological or geophysical experiments. We develop a perturbative theory and numerical techniques and test for cases for which exact solution is obtained. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z26.00006: Decomposing First Passage Random Walks Lawrence Schwartz, David Johnson, Sidney Redner We develop a simulation method to model the time dependence of diffusion in composite materials with a wide range of pore sizes. Here, first passage techniques are useful because they allow a walker to move efficiently through the large open regions of the pore space.\footnote{Toumelin et at. J. Mag. Res. 188, 83 (2007).} However, because one does not keep track of each intermediate position, these techniques are not well suited to calculating the time development of the effective diffusion coefficient, D(t). To address this problem we show that first passage propagation can be decomposed in terms of a sequence of intermediate probability distributions. For example, given a first passage walk from the origin to the surface of a sphere of radius R in a time t, we can evaluate the probability distributions for the particle's location at any earlier time t$'$. We will illustrate the behavior of these intermediate distributions with a series of examples in one and three dimensions. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z26.00007: Thermally Stable Mesoporous Silica Spheres synthesized under Mild Conditions Christopher Ziegler, Eunyoung You, James Watkins Thermally stable, mesoporous silica spheres were synthesized using a one-pot technique under mild conditions. As-calcined silica spheres were shown to be highly porous with surface areas greater than 1000 m$^{2}$/g and pore volumes on the order of 1 cc/g. Pore walls were found to be highly resistant to collapse as a consequence of thermal treatment at temperatures exceeding 750\r{ }C or hydrothermal treatment in boiling water at temperatures exceeding 100\r{ }C for over 100 hours. $^{29}$Si-$^{1}$H cross polarization NMR data indicate that the silica is highly condensed at the surface providing rationale for the exceptional pore wall stability observed. The mesoporous silica spheres were synthesized from tetraethyl orthosilicate (TEOS) at room temperature and near-neutral pH using cysteamine and cetyltrimethylammonium bromide (CTAB) in a mixed water and ethanol system. Sphere size was shown to be tunable by altering the relative amounts of ethanol, CTAB, or TEOS. Sphere diameters ranging from 30 nm to 560 nm were observed. The preparation method and characterization of these highly condensed, thermally stable, mesoporous silica spheres for applications including sensing, catalysis, purification, and payload encapsulation is presented. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z26.00008: Ordered and disordered pores in porous anodic alumina: Partial equilibrium results from hexagonal lattice Arief Budiman, Daniel Lo Cluster variation method is employed to find equilibrium states of pore arrangement in porous anodic alumina. Our work is motivated by a need to predict the extent of pore ordering in the anodic alumina. The pores are assumed to occupy a hexagonal two-dimensional lattice and interact with each other through their elastic deformations. Alumina-electrolyte interface energy and metal-oxide interface stress are included in the analysis. Equilibrium ordered and disordered states are obtained. Spatial pore arrangements of these states will be presented. Incorporation of ionic transports in the electrolyte and aluminum-to-alumina reactions to the model will be discussed. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z26.00009: Nanoporous Substrate with Mixed Nanoclusters for Surface Enhanced Raman Scattering. Sehoon Chang, Hyunhyub Ko, Srikanth Singamaneni, Ray Gunawidjaja, Vladimir Tsukruk Rapid detection of plastic and liquid explosives is an urgent need due to various societal and technological reasons. We employed a novel design of surface enhanced Raman scattering (SERS)-active substrate based on porous alumina membranes decorated with mixed nanoclusters of gold nanorods and nanoparticles. We demonstrated trace level detection of several important explosives such as dinitrotolene (DNT), trinitrotoluene (TNT), and hexamethylenetriperoxidediamine (HMTD) by fast, sensitive, reliable Raman spectroscopic method. We achieved near molecular-level detection (about 15$\sim $ 30 molecules) of DNT and TNT utilizing the SERS substrate. However, trace level detection is challenging due to the lack of common optical signatures (fluorescence, absorption in UV-vis range) or chemical functionality of peroxide-based explosives such as HMTD. To overcome this, we employed photochemical decomposition approach and analyzed chemical fragments using SERS. We suggest that tailored polymer coating, mixed nanoclusters, and laser-induced photocatalytic decomposition are all critical for achieving this unprecedented sensitivity level.. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z26.00010: Sorption Isotherms in Networked Nanoporous Media: Do the Parts Equal the Whole? Casey E. Chiang, Felix Casanova, Anne M. Ruminski, Michael J. Sailor, Ivan K. Schuller Sorption isotherms are a very important tool in the characterization of nanoporous materials. However, there is still controversy as to how capillary condensation and evaporation transitions along the isotherm correlate with the pore morphology (shape) and topology (within a network). We combined narrow ($<$10 nm) and wide ($>$30 nm) pores in nanoporous silicon to tailor the simplest networks: narrow above wide (ink-bottle) and vice versa (funnel). In addition, we compared these against their single-layered constituents. From sorption isotherms measured by optical interferometry, we observe that capillary \textbf{condensation} occurs identically in all pores with direct access to the gas reservoir and slightly delayed (delayed adsorption) when access is blocked, while capillary \textbf{evaporation} occurs identically in all pores with direct access to the gas reservoir and is delayed (pore-blocking) until direct access is allowed otherwise. These experimental results allow us to understand the global capillary behavior in nanoporous silicon. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z26.00011: Mechanical stability of Metal Organic Framework-5. Wei Liu, Ju Li Metal organic frameworks (MOFs) are crystalline structures of metal ions bridged by organic linkers. They have been proved to be highly useful in gas storage, separation, purification and catalysis. Mechanical stability is very important for their applications in industry. We studied the stress-strain relations of MOF-5 (the prototypical MOF) under different temperatures via Molecular Dynamics method (MD). It has been found that under normal stress MOF-5 is relatively strong; while under shear stress it is easy to collapse. Furthermore, under both normal and shear stress condition, its stability becomes weaker as temperature increases. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z26.00012: Smart nanoporous preconcentrator of explosives based on MOF5 Khorgolkhuu Odbadrakh, James Lewis We present investigations of interactions of explosive molecules RDX and TATP with metal organic framework MOF-5, using DFTY based ab-initio simulation method FIREBALL. Energetics studies in bulk shows that only one of the binding sites of RDX in MOF-5 suggested by quantum chemistry calculations confirm. The absorption site is on a linker of the framework through 2-(OH) bond. However, surface interactions are stronger, with significantly higher binding barriers. We confirmed two adsorption sites on the surface: one with the linker and the other on a connector of the framework through 3-(OH) bonds. The stronger interactions on the surface suggest importance of size, and surfaces of MOF nanoparticles in precontentrating the explosive molecules in the framework. Ab-initio Molecular Dynamics simulations show that the absorption of the RDX in MOF-5 is highly sensitive to temperatures, suggesting high diffusion rates for the explosive molecules at room temperature. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z26.00013: Optical birefringence in Compressed Aerogels P. Bhupathi, J. Hwang, R. M. Martin, L. Jaworski, D. B. Tanner, Y. Lee, J. Blankstein, N. Mulders We performed optical birefringence measurements on 98{\%} porosity silica aerogel samples subjected to various degrees of uniaxial compression up to 15{\%} strain over a wide range of wavelength, 200 to 800 nm. Silica aerogels are composed of an entangled network of 3 - 5 nm diameter SiO$_{2}$ strands and can be synthesized in a wide range of porosity, especially in the high porosity limit close to 99.9{\%}, yet maintaining good mechanical stability. Uncompressed aerogels exhibit no or minimal degree of birefringence, indicating the isotropic nature of the material. Uniaxial compression of aerogel introduces global anisotropy, which produces optical activity in the material. We observed a quasi-linear strain dependence in \textit{$\vert \Delta $n$\vert =\vert $n}$_{e} - n_{o}$\textit{$\vert $} in compressed aerogels, where $n_{e(o)}$ is the index of refraction for the extraordinary (ordinary) ray which has its polarization parallel to the compression axis. This effect has potential applications for aerogels as tunable waveplates operating in a broad spectral range. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z26.00014: Microwave Absorption in Percolating Metal-insulator Composites Darin Zimmerman, Jeremy Cardellino, Kyle Cravener, Kelly Feather, Nicholas Miskovsky, Gary Weisel We measure several electromagnetic properties of tungsten-Teflon composites as a function of metal volume concentration. The electric (E) and magnetic (H) loss tangents at 2.45 GHz and the dc conductivity each exhibit a percolation transition at a different critical value of the metal volume fraction, $p$. Moreover, the transition behavior depends on the average particle size and size distribution of the metal component. We explain the variation in each case by a schematic model derived from established percolation theory and the distinct response of conducting particles to microwave electric and magnetic fields. [Preview Abstract] |
Friday, March 20, 2009 2:03PM - 2:15PM |
Z26.00015: Anisotropy of induced polarization in the context of the generalized effective-medium theory. Vladimir Burtman, Michael S. Zhdanov, Alexander Gribenko The rock samples are examples of heterogeneous complex structure material. Modeling of electromagnetic response of this medium makes it possible to study the anisotropy of induced polarization (IP) effect. The IP effect is studied it in the context of the developed generalized effective-medium theory of induced polarization (GEMTIP). The effective-medium conductivity defined by the GEMTIP model, in a general case, is represented by a tensor function. This tensorial property of the effective-medium conductivity provides a new insight in the anisotropy phenomenon in the IP effect. As an example, we consider a multiphase composite polarized model of a rock formation with ellipsoidal inclusions. We demonstrate that the effective conductivity of this formation may be anisotropic, even if the host rock and all the grains are electrically isotropic. [Preview Abstract] |
Session Z27: Advances in Instrumentation and Measurement II
Sponsoring Units: GIMSChair: Charles Agosta, Clark University
Room: 329
Friday, March 20, 2009 11:15AM - 11:27AM |
Z27.00001: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z27.00002: Surface Coatings for Gas Detection via Porous Silicon Serdar Ozdemir, Ji-Guang Li, James Gole Nanopore covered microporous silicon interfaces have been formed via an electrochemical etch for gas sensor applications. Rapid reversible and sensitive gas sensors have been fabricated. The fabricated porous silicon (PS) gas sensors display the advantages of operation at room temperature as well as at a single, readily accessible temperature with an insensitivity to temperature drift; operation in a heat-sunk configuration, ease of coating with gas-selective materials; low cost of fabrication and operation, and the ability to rapidly assess false positives by operating the sensor in a pulsed mode. The PS surface has been modified with unique coatings on the basis of a general theory in order to achieve maximum sensitivity and selectivity. Sensing of NH$_{3}$, NO$_{x}$ and PH$_{3}$ at or below the ppm level have been observed. A typical PS nanostructure coated microstructured hybrid configuration when coated with tin oxide (NO$_{x}$, CO) and gold nanostructures (NH$_{3})$ provides a greatly increased sensitivity to the indicated gases. Al$_{2}$O$_{3}$ coating of the porous silicon using atomic layer deposition and its effect on PH$_{3}$ sensing has been investigated. 20-100 nm TiO$_{2}$ nanoparticles have been produced using sol-gel methods to coat PS surfaces and the effects on the selectivity and the sensitivity have been studied. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z27.00003: Directed-Assembly of Carbon Nanotubes on Soft Substrates for Flexible Biosensor Array Hyoung Woo Lee, Juntae Koh, Byung Yang Lee, Tae Hyun Kim, Joohyung Lee, Seunghun Hong, Mihye Yi , Young Min Jhon We developed a method to selectively assemble and align carbon nanotubes (CNTs) on soft substrates for flexible biosensors. In this strategy, thin oxide layer was deposited on soft substrates via low temperature plasma enhanced chemical vapor deposition, and linker-free assembly process was applied onto the oxide surface where the assembly of carbon nanotubes was guided by methyl-terminated molecular patterns on the oxide surface. The electrical characterization of the fabricated CNT devices exhibited typical p-type gating effect and 1/$f$ noise behavior. The bare oxide regions near CNTs were functionalized with glutamate oxidase to fabricate selective biosensors to detect two forms of glutamate substances existing in different situations: L-glutamic acid, a neuro-transmitting material, and monosodium glutamate, a food additive. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z27.00004: Mid-infrared Verdet coefficient studies in GaAs, BaF$_2$, and LaSrGaO$_4$, and ZnSe Myoung-Hwan Kim, Volker Kurz, Gheorghe Acbas, Chase Ellis, John Cerne We measure the mid-infrared (wavelength $\lambda$=11 - 0.8 $\mu$m; 0.1 - 1.5 eV) Faraday rotation and ellipticity in GaAs, BaF$_2$, LaSrGaO$_4$, and ZnSe. Since these materials are commonly used as substrates and windows in the mid-infrared, it is important to measure the Faraday signals for background subtraction and to test the accuracy of our measurement techniques. The light sources are lasers and a new custom-modified double-pass prism monochromator with a Xe lamp, which allowed continuous broadband measurements in the 0.31-1.5 eV energy range. Surprisingly, we find reproducible ellipticity signals, even though the radiation is well below the absorption edge of these materials and therefore no circular dichroism is expected. We suggest that the Faraday ellipticity is produced by the static retardance ($R_s$) of the ZnSe photoelastic modulator (PEM), which converts rotation signals into ellipticity. We determine $R_s$ experimentally from the Faraday rotation and ellipticity ratio, produced by either applying a magnetic field or rotating the polarization of light incident on the PEM. Work supported by the Research Corp. Cottrell Scholar Award, NSF-CAREER-DMR0449899, and an instrumentation award from the CAS. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z27.00005: Diffraction by a Metallic Edge Near Plasma Frequency Miguel A. Alvarez-Cabanillas The behaviors of the fields diffracted by a metallic half-plane near plasma frequency are obtained. Incident plane wave with transverse magnetic polarization TM is diffracted by a gold sheet. The size of the atoms and the interatomic distance in the metallic sheet are assumed to be smaller than the wavelength. The electromagnetic field cannot detect the inner structure of the system and thus observes a homogeneous structure of the medium. These justify the use of a semi-classic theory and quantum effects will not be included. In this limit, the permittivity and the permeability are valid concepts. Finite difference time domain FDTD approach is applied to solve the Maxwell equations, PML as absorbing boundary conditions and a conductor as gold for the half-plane diffractor. Auxiliary Differential Equation method ADE is implemented in the FDTD to consider the nonlinear dispersion behavior of the metallic sheet near the plasma frequency. As results, the diffraction behavior near the plasma frequency by the wave-electron effect is shown; as the dependence of the diffracted fields with the longitude of the thickness of the metallic half-plane. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z27.00006: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z27.00007: Fabrication of directional sound sensor by silicon micromachining Michael Touse, Jeffrey Catterlin, Jose Sinibaldi, Gamani Karunasiri A directional sound sensor based on the operational principle of the Ormia ochracea fly's hearing organism [1] was fabricated using micro-electromechanical system (MEMS) technology. The fly uses coupled bars hinged at the center to achieve directional sound sensing by monitoring the difference in their vibration amplitudes. The MEMS design employed in this work consisted of a 1x2 square millimeter polysilicon membrane hinged at the center and positioned about 1 micrometer above the substrate using a sacrificial silicon dioxide layer. Finite element analysis of the device shows two primary vibrational mode frequencies, one corresponding to a rocking mode which is highly dependent on angle of incidence, and the other to a bending motion which remains constant through all angles. Using a laser vibrometer to measure response, rocking and bending modes were observed at driving frequencies of 3.0 and 11.4 kHz, respectively, and angular dependence was in close agreement with modeling. [1] R.N. Miles, R. Robert, and R. R. Hoy, ``Mechanically coupled ears for directional hearing in the parasitoid fly Ormia ochracea,'' J. Acoust. Soc. Am., \textbf{98} (6), Dec. 1995 [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z27.00008: The OSL dating behavior of Martian sediment analogue materials exposed to a simulated Martian solar spectrum Marissa Detschel, Ken Lepper Optically stimulated luminescence dating (OSL) is a terrestrial geochronometric technique being developed for \textit{in-situ} dating of the geomorphic features on the surface of Mars. The solar spectral irradiance reaching the surface of Mars includes ionizing ultraviolet (UV) radiation that does not reach the surface of the Earth. In view of this, an optical system was constructed that closely simulates the Martian solar spectral irradiance, including seasonal variations due to the planet's orbit and attenuation by atmospheric dust. Upon exposure to this simulated environment, the OSL dating behavior of a suite of Martian sediment analogue materials were catalogued. Results suggest that the presence of the additional ionizing UV radiation on the surface of Mars will not compromise optical dating measurements of K- and Ca-feldspars, anhydrite, or hydrous Ca and Mg sulfates. However, Na-feldspar does appear to retain a trapped charge population, which could hinder optical dating of sediments containing more than trace amounts of sodic feldspars. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z27.00009: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z27.00010: Variable temperature measurements in cryogenic probe stations; Measurements with Magnetic Fields Jeffrey Lindemuth Electrical and magnetic property measurements of nanoscale materials are important for characterization and understanding of materials and devices. Equally important is to measure these properties at various temperatures. These measurements are facilitated with cryogenic probe stations that provide a variable temperature environment over a wide range of temperatures. However, until this time a major inconvenience was caused by the thermal expansion of the probe tips and probe station as the temperature changed. To prevent the tip movement from damaging the sample, the normal procedure is to lift the probe tips as the temperature changes. This prevents the implementation of totally automated variable temperature measurements. We present results using a new probe design that allows the probe tips to remain in contact to sample during temperature changes. With this new design we demonstrate, with optical microscopy, the total tip movement of less than 2 microns when the temperature of the sample changes from 4.2K to 300K. The same probes that eliminate the movement from thermal expansion also improve the isolation of the measurements to external vibrations. To show the performance of this probe design, variable temperature magnetotransport measurements were preformed. These measurements show that the probe tips do not move with changing temperature and magnetic fields. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z27.00011: GHz-Bandwidth Signal Processing for Time-Resolved Faraday/Kerr Rotation Experiments Yanjun Ma, Patrick Irvin, Jeremy Levy Faraday/Kerr rotation is a sensitive measurement of the electron spin and its dynamics in semiconductor nanostructures such as GaAs quantum dots. The Kerr rotation angle from a single spin, however, can be as small as $10^{-6}$ rad, which requires massive averaging of the Kerr signal in order to maximize the signal-to-noise ratio. By replacing the mechanical delay line typically found in time resolved Kerr rotation (TRKR) measurements with a continuous wave probe and high-speed electronics, the signal and noise can be sampled more often which results in a higher SNR. However, real-time methods for data collection are typically limited by available memory, resulting in unavoidable dead time for which data cannot be collected and averaged. The approach we have developed integrates a field-programmable gate array (FPGA) with a high-speed digitizer, thus allowing high-speed on-board averaging to overcome these technical limitations. We will demonstrate the performance of this instrument by comparing the results of this system with traditional pump-probe (sampling) techniques and discuss its applicability for a variety of dynamical spin-sensitive experiments in the solid state. [Preview Abstract] |
Session Z28: Nanotechnology II
Sponsoring Units: FIAPChair: Keith Williams, University of Virginia,
Room: 330
Friday, March 20, 2009 11:15AM - 11:27AM |
Z28.00001: Fundamental Etching and Roughening Mechanisms of Photoresist Polymers during Plasma Processing Dustin Nest, Ting-Ying Chung, David Graves, Florian Weilnboeck, Robert Bruce, Tsung Cheng Lin, Ray Phaneuf, Gottlieb Oehrlein, Eric Hudson, Deyan Wang, Cecily Andes Reducing the etching and roughening of photoresist polymers during plasma processing is required as optical lithography for integrated circuit manufacture is extended to patterning features with critical dimension control on the order of nanometers. We use a vacuum beam system to simulate plasma exposure but under well-defined conditions. Samples are exposed to well-characterized beams of ions, vacuum ultraviolet (VUV) radiation, and electrons under high vacuum conditions. Post-exposure analysis includes atomic force and scanning electron microscopy and FTIR spectroscopy. We show that VUV radiation, ion bombardment, the ion / photon flux ratio and heating all play a role in the roughening of current-generation PMMA-based 193 nm photoresists. VUV radiation breaks carbon-oxygen bonds to a depth of approximately 100 nm whereas ion bombardment forms a dehydrogenated surface layer. Qualitatively similar roughening was observed in plasmas with the same ion bombardment energy and ion and VUV fluence. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z28.00002: Understanding the Differences between Electron and Ion Guiding Susanta Das, Buddhika S. Dassanayake, John A. Tanis, Nikolaus Stolterfoht Significant differences in the transmission and guiding of slow positive ions and fast electrons through insulating PET nanocapillary foils have been observed.$^{1,2}$ While ions are transmitted without energy loss or change in charge state even when the foil is tilted with respect to the incoming ion beam, electrons undergo inelastic as well as elastic scattering leading to considerable reduction in the transmitted intensities. The underlying reasons for the differences between ions and electrons will be discussed and quantitative comparisons made. Qualitatively, slow positive ions are fully neutralized when striking a surface and consequently deposit all of their charge, whereas electrons can be elastically or inelastically scattered.$^{3}$ Additionally, secondary electron emission by incident ions increases the deposited charge, while this same emission decreases the deposited charge for electrons. *Supported by Research Corporation $^{1}$N. Stolterfoht \textit{et al}., Phys. Rev. A \textbf{77}, 032905 (2008). $^{2}$S. Das \textit{et al}., Phys. Rev. A \textbf{76}, 042716 (2007). $^{3}$B. Stix \textit{et al}., XXV ICPEAC, Freiburg, Germany, July 2007, Abstracts, MO 128. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z28.00003: Laser Manipulation of Nanostructures Dinko Chakarov This work describes a new method for controlling the pattern into which nanoparticles in a disordered metal-nanoparticle layer organize themselves by a single light pulse. [1]. The phenomena behind formation of one- and two-dimensional grating patterns are attributed to interference effects between the incident light and waveguided modes. Such self-patterning behavior could be useful for the fabrication of complex nanostructures and advanced photonic devices. \\[3pt] [1]. L. Eurenius, C. H\"{a}gglund, E. Olsson, B. Kasemo and D. Chakarov, ``Grating formation by metal nanoparticle-mediated coupling of light into waveguided modes,'' \textit{Nature Photon. }\textbf{2, }360 (2008). [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z28.00004: A new method of nano-manipulation with AFM derived from nanotribology Suenne Kim, Daniel Ratchford, Xiaoqin Li Based on principles of nanotribology, a new approach is explored for manipulating nanoparticles (ranging from 5nm to 60nm in diameter) with an Atomic Force Microscope (AFM). In this new approach, one first kicks a nanoparticle. Immediately following the kicking event, static friction is greatly reduced. One can then dribble the nanoparticle to the desired position in the tapping (imaging) mode of the AFM. The major advantage of this scheme lies in the active manipulation with simultaneous visual feedback. Our study revealed the mechanism for the nano-displacement is primarily governed by the ``stick'' events of the stick-slip process. We also found that the manipulation can be effectively controlled by adjusting the scanning speed, and the critical speed depends on the local roughness of the surface. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z28.00005: Single-Particle Placement on a large scale Pradeep Bhadrachalam, Hong-Wen Huang, Vishva Ray, Seong Jin Koh The capability of positioning single nanoparticles onto exact substrate locations holds prime technological and scientific importance. We present a novel technique to precisely place exactly one single nanoparticle onto a targeted substrate location on a large scale. This was done by defining an electrostatic guiding structure using CMOS compatible fabrication technology, which guides exactly one single 20nm gold nanoparticle onto a desired substrate location with a success rate over 90\%. The measured precision of this single-particle placement (SPP) was 12.1nm. This technique has an inherent capability of limiting one single nanoparticle for each target location. Theorectical calculations has revealed that this self-limiting capability originates from an increase of free energy barrier after a nanoparticle is placed on the target location, effectively blocking the approach of other nanoparticles. We also demonstrate size-selective placement of single nanoparticles, where individual nanoparticles of different sizes are guided to different target locations on the same substrate. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z28.00006: Magnetically Driven Swimming of Nanoscale Colloidal Assemblies Jennifer Breidenich, Jason Benkoski, Lance Baird, Ryan Deacon, H. Bruce Land, Allen Hayes, Pei Keng, Jeffrey Pyun At microscopic length scales, locomotion can only be generated through asymmetric conformation changes, such as the undulating flagellum employed by protozoa. This simple yet elegant design is optimized according to the dueling needs of miniaturization and the fluid dynamics of the low Reynolds number environment. In this study, we fabricate nanoscale colloidal assemblies that mimic the head + tail structure of flagellates. The assemblies consist of two types of magnetic colloids: 25 nm polystyrene-coated Co nanoparticles, and 250 nm polyethylene glycol coated magnetite nanoparticles. When mixed together in N-dimethylformamide, the Co nanoparticles assemble into flexible, segmented chains ranging in length from 1 - 5 $\mu$m. These chains then attach at one end to the larger magnetic beads due to magnetic attraction. This head + tail structure aligns with an external uniform magnetic field and is actuated by an oscillating transverse field. We examine the effects of Co nanoparticle concentration, magnetite bead concentration, magnetic field strength, and oscillation frequency on the formation of swimmers and the speed of locomotion. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z28.00007: Observation of Fano Interference and Field Dependence by Raman Spectroscopy of Molecularly Doped Silicon Brian Burke, Keith Williams, Jack Chan We have investigated various doping techniques on silicon to form thin, highly p-doped layers near the surface (approximately 10 nm). We are able to observe through Raman spectroscopy the signature Fano lineshape present in the zone-centre optical phonon. We have defined high resolution channels by electron beam lithography and subsequently annealed adsorbed dopant molecules into the silicon lattice by RTA. We have performed sheet resistance measurements as well as Raman mapping to characterize the doping profile. The Raman laser line of 325 nm provides a penetration depth of roughly 8 nm, ideal for studying the surface of silicon. After studying the highly doped channel, electrodes were deposited and field dependence measurements were made. Additionally, IETS and transport measurements have been conducted for various geometries to compare with Raman data. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z28.00008: Profiling surfaces with a carbon nanotube oscillator Adrian Popescu, Lilia Woods, Igor Bondarev A practical device for profiling surfaces is proposed, as an alternative to an Atomic Force Microscopy (AFM) tip. The device consists of a finite length double wall carbon nanotube oscillator with the outer tube being stationary and oriented perpendicular to the surface plane. By investigating the changes in the oscillatory behavior of the inner tube due to the proximity of the surface, the roughness of the surface can be determined. The role of the length and the initial extrusion of the moving tube, and the friction losses in the motion process are also explored. We suggest that such a device can be virtually a non-fatigue, non-wear system, and that it is possible to obtain a higher in-plane resolution as compared to the ``traditional'' AFM tip. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z28.00009: An Electrodynamics Ratchet Motor Jiufu Lim, John Sader, Paul Mulvaney Brownian ratchets are often used to generate translational motion for biological separation processes and colloidal transport. This talk will propose a Brownian ratchet motor that enables the transduction of electrical energy into rotary micro-mechanical work. This is achieved through torque generation provided by boundary shaping of equipotential surfaces. Stochastic simulations elucidate the performance characteristics of this device as a function of its geometry. Miniaturization to nanoscale dimensions yields rotational speeds in excess of 1kHz, which is comparable to biomolecular motors of similar size. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z28.00010: When a magnetized quantum wire can act as an ``active" laser medium Manvir Kushwaha We report on the theoretical investigation of magnetoplasmon excitations in a quantum wire characterized by a confining harmonic potential and in the presence of a perpendicular magnetic field. The problem involves two length scales: $l_0=\sqrt{\hbar/m^*\omega_0}$ and $l_c=\sqrt{\hbar/m^*\omega_c}$, which characterize the relative strengths in the interplay of confinement and the magnetic field. We embark on the charge-density excitations within a two-subband model in the framework of Bohm-Pines' random-phase approximation. The main focus of our study is the (intersubband) magnetoroton excitation which changes the sign of its group velocity twice before merging with the respective single-particle continuum. We analyze the terms and conditions within which the magnetoroton excitation persists in the quantum wires. It is suggested that the electronic device based on such magnetoroton modes can act as an {\it active} laser medium. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z28.00011: Suppression of electric field domains in semiconductor superlattices with side shunting layer Huidong Xu, Andreas Amann, Eckehard Sch{\"o}ll, Stephen Teitsworth We have numerically studied the electronic transport properties of a weakly-coupled semiconductor superlattice that possesses a conductive side shunting layer, using a model that includes lateral dynamics in each quantum well of the superlattice [1]. Depending on the lateral size of the superlattice quantum wells and the quality of the connection between the shunt layer and the superlattice, the shunt may inhibit the formation of electric field domains in the superlattice under conditions of negative differential resistance (NDR). We determine conditions to achieve a stable spatially-uniform electric field distribution, an important condition for practical NDR devices such as superlattice THz oscillators. For a superlattice with small lateral extent, a high quality shunt stabilizes the uniform field configuration in the entire structure, whereas a lower quality shunt leads to current oscillations and/or static field domains. We characterize the bifurcations associated with the transitions between these different behaviors. [1] A. Amann and E. Sch{\"o}ll, Phys. Rev. B {\bf 72}, 165319 (2005). [Preview Abstract] |
Session Z29: Magnetostructural Effects and General Magnetism
Sponsoring Units: GMAGChair: Sara Majetich, Carnegie Mellon University
Room: 333
Friday, March 20, 2009 11:15AM - 11:27AM |
Z29.00001: Non-cubic, coherent nanoprecipitates observed by neutron diffuse scattering in highly magnetostrictive Galfenol (Fe$_{1-x}$Ga$_{x})$ alloys. Peter Gehring, Hu Cao, Chris DeVreugd, Jose Abelardo Rodriguez, Jie Fang Li, Dwight Viehland We report neutron diffuse scattering measurements on highly magnetostrictive Fe$_{1-x}$Ga$_{x}$ alloys with different thermal treatments. Diffuse scattering is seen for compositions 0.14$<$x$<$0.20 heat-treated to increase the magnetostriction that is otherwise absent. This diffuse scattering exhibits asymmetric peaks at the (100) and (300) reciprocal lattice positions that are consistent with the coexistence of short-range ordered, coherent nanoprecipitates embedded in a long-range ordered, body-centered cubic matrix. A large peak splitting is observed at (300) for x=0.19, which suggests that the nanoprecipitates are not cubic but have a lower symmetry and a large elastic strain. The strongest diffuse scattering occurs for x=0.19 where the maximum magnetostriction is found. This suggests a structural origin for the enhanced magnetostriction in these materials. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z29.00002: Magnetic and magnetoelastic properties of Ge/Co co-substituted cobalt ferrite Naresh Ranvah, Eugene Melikhov, John Snyder, David Jiles The highly magnetostrictive material cobalt ferrite and its derivatives based on substitution of cations have been shown to have extreme sensitivity of their magnetization to stress. In order to control their properties (e.g. magnetostriction, magnetic anisotropy, strain derivative, and hysteresis) substitution of specific cations are needed. We have shown Ge$^{+4}$/Co$^{+2}$ co-substituted cobalt ferrites (Co$_{1+x}$Ge$_{x}$Fe$_{2-2x}$O$_{4})_{ }$to have very interesting combinations of magneto-elastic properties. In the present study the variation of magnetic anisotropy with composition and temperature was investigated, for the new germanium/cobalt co-substituted cobalt ferrite Co$_{1+x}$Ge$_{x}$Fe$_{2-2x}$O$_{4}$.. Hysteresis loops were measured in the range T = 10 - 400 K, with \textit{$\mu $}$_{0}H_{max}$~=~0 - 5~T. The high field regions of these loops were then fitted to the Law of Approach to Saturation (LA) for cubic materials as given by $M=M_{s}$[1-(8/105)($K_{1}$/\textit{$\mu $}$_{0}M_{s}H)^{2}$], plus a linear forced magnetisation term. Values for first order cubic anisotropy constant $K_{1}$ were calculated and it was found that anisotropy increased as temperature decreases for all compositions. At most temperatures anisotropy decreased with increase in $x$. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z29.00003: Magnetostriction of Fe-based alloy Yanning Zhang, Juexian Cao, Ruqian Wu High magnetostrictive materials are important for sensor and actuator applications. Although the rare earth-3d metal compound, such like TbFe2, have large positive magnetostriction of $\sim $2600ppm, their practical application was hindered due to larger activation external magnetic field and their high material costs. Large-scale industrial applications require soft magnetic materials with high magnetostrictive materials with low material costs. Nanocrystalline Fe-based alloys without rareearths are very interesting candidates due to the fact doping some nonmagnetic element, such as Ga, Al, Zn and Be, strongly alters the anisotropy energy and enhances the magneticostiction, which indicated substituting Fe by nonmagnetic elements might be a new method to develop cheap and soft magnetic material with high magnetostriction. With First-principle calculations, we have investigated the Young's modulus, shear modulus, anisotropy energy and magnetostriction of Ga, Al, Zn, Ge and Be doped in bulk Fe. The magneto-crystalline anisotropy energy $E_{MCA}$ and the magnetostrictive coefficients ($\lambda _{001})$ strongly depend on the compositions and atomic arrangement. A rigid band picture is proposed to estimate the trend of magnetostrictive coefficients of Fe, Ga and Zn compounds. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z29.00004: The Magnetocaloric Effect in Single Crystal and Processed Polycrystalline MnP Ryan A. Booth, Sara A. Majetich Manganese Phosphide (MnP) is a promising magnetocaloric effect (MCE) material for use in room temperature-magnetic refrigeration because it exhibits a first-order ferromagnetic to paramagnetic phase transition at a Curie temperature of 290 K, possesses no measurable magnetic hysteresis, and has a saturation field of lower than 7.5 kOe along the c-axis. The magnetic entropy change ($\Delta$S) was measured in single crystals along the c-axis to be 2.2, 3.3, and 6.0 J / kg K in applied fields of 10 kOe, 20 kOe, and 50 kOe respectively. Cold-rolling followed by a short annealing of polycrystalline MnP was shown to create preferential crystallographic alignment that mimics the favorable properties of single crystals without the expenses associated with their growth. A comparison of the magnetocaloric effect between single crystals and rolled polycrystalline MnP is presented. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z29.00005: Magnetic anisotropy dependency on structural properties in strained MnAs thin films Magnus Wikberg, Mikael Ottoson, Janusz Sadowski, Ronny Knut, Olof Karis, Peter Svedlindh High quality thin films of MnAs (between 30 and 200 {\AA}) have been grown with molecular beam epitaxy (MBE) on GaAs(111)B and (001) substrates and under different growth conditions. The magnetic anisotropy of the MnAs layers has been investigated with SQUID magnetometry and magnetic force microscopy (MFM). A clear correlation between choice of substrate, growth temperature and film thickness can be seen in the magnetic anisotropy and T$_{c}$ measurements with a rapid transition towards bulk like anisotropy constants as the film thickness is increased. From X-ray diffraction, a relationship between T$_{c}$ and lattice strain has been established, where the film with increasing film thickness rapidly exhibits a transition from a highly strained to a fully relaxed film. A complex dependence on the structural transition from the ferromagnetic to the paramagnetic phase is also seen in X-ray magnetic circular dichroism (XMCD) measurements, where the orbital moment does not strictly follow the spin moment near the ferromagnetic-paramagnetic phase transition. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z29.00006: Coercivity of Melt-Spun Gd$_{100-x}$Fe$_{x}$ Paul Shand, Andrew Meyer, David Schmitter, Geoffrey Rojas, Jeffrey Shield, Jared Goertzen, Danial Haskel, Diandra Leslie-Pelecky We have measured the coercivity of melt-spun Gd$_{100-x}$Fe$_{x}$ (0$\le x\le $40) alloys over the temperature range 2 K$\le T\le $340 K. Previously performed structural measurements revealed that the system consists of crystalline hcp-Gd grains surrounded by a non-crystalline Gd or Gd-Fe phase composed of Gd$_{100-x'}$Fe$_{x'}$, where $x$'$>x$ is the iron concentration in the amorphous region. The two-phase structure is responsible for an unusual dependence of the coercivity on temperature in which non-zero coercivity is observed above the hcp-Gd $T_{c}$ with a peak near 320 K. The coercivity decreases as the hcp-Gd grains order, then increases with decreasing temperature. This behavior is explained by the presence of Fe-rich magnetically correlated regions. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z29.00007: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z29.00008: Epitaxial growth and spin dependent states of Co$_{x}$Mn$_{y}$Si$_{z}$ (111) thin films Liang He, Brian Collins, Frank Tsui, Yong Chu Epitaxial growth of Co$_{x}$Mn$_{y}$Si$_{z}$ on Ge (111) substrates has been studied using combinatorial MBE techniques, including that of the Heusler alloy Co$_{2}$MnSi. For Si concentration of 25 at. {\%}, in-situ RHEED and ex-situ X-ray diffraction experiments indicate that the epitaxial growth is coherent for atomic ratio Co:Mn between 1 and 9, while the film is microcrystalline for Co:Mn $<$ 1 and it is rough and of poor crystalline quality for Co:Mn $>$ 9. The crystalline quality is the highest around Co:Mn = 4, whereas it exhibits a plateau around a ratio of 2, i.e. the Heusler alloy, Co$_{2}$MnSi. Within the region of coherent growth, at coverages below 100{\AA}, the growth front is smooth 2D-like. As thickness increases, the surface morphology systematically changes from quasi-2D into 3D. The morphology transition also depends sensitively on composition, i.e. Co:Mn ratio, and temperature. Spin-dependent states as a function of composition at low coverages have been examined by tunneling spectroscopy using Al$_{2}$O$_{3}$ as the tunneling barrier and Fe and Nb as the detector layers. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z29.00009: Self-energy of half-metallic ferromagnet Mn$_5$Ge$_3$ calculated from infrared spectroscopy data S.V. Dordevic, N. Stojilovic, L.W. Kohlman, C. Petrovic We will report the results of our infrared and optical spectroscopy study of a half-metallic ferromagnet Mn$_5$Ge$_3$. This compound is currently being investigated as potential injector of spin polarized currents into germanium. Infrared measurements have been performed over a broad frequency (30 - 50000~cm$^{-1}$) and temperature (10 - 300 K) range. From the complex optical conductivity $\sigma(\omega)$ we extract the electron self-energy $\Sigma(\omega)$. The calculation of $\Sigma (\omega)$ is based on novel numerical technique which uses Levenberg--Marquardt algorithm for solution of systems of non- linear equations. Obtained self-energy provides a new insight into electron correlations in Mn$_5$Ge$_3$. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z29.00010: Electronic and Magnetic properties of Transition Metal Borides. Aditi Herwadkar, Yufeng Zhao Boron has three valance electron and a small covalent radius undergoing sp$_{2 }$hybridization in many boron clusters. It is also known to form large variety of crystal structures both planer and 3D caged molecules. Our current work is to study transition metal boride clusters. This would be useful towards designing metal boride nanoclusters with tunable optical, magnetic and electron transport properties. Different stoichiometries are studied. The metal to Boron ratio varies from 0.3 to 0.75. For each of these compositions the lowest energy geometries were determined by optimizing the bond length for several initial symmetric geometries. The lowest energy structure is then chosen as the equilibrium structure. Most strikingly, all these transition-metal boride clusters are cage-like although both boron and transition metal usually favor high coordination number. We also find that some of the clusters have an extremely high magnetic moment per unit mass. This renders metal boride nanoparticles as potentially promising light-weighted magnetic materials. All the calculations are done using the spin-polarized density functional theory method implemented in the Vienna ab initio simulation package. A plane wave basis set with (400 eV cutoff) was used in combination with an all electron like projector augmented wave potential and PBE exchange correlation functional with in the generalized gradient approximation. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z29.00011: Revisiting Classical Diamagnetism: A Surprise of Physics Narendra Kumar, Vijay Kumar KrishnaMurthy The Classic Bohr-van Leeuwen (BvL) theorem states that the orbital diamagnetism of a classical system of charged particles in thermal equilibrium is identically zero. This theorem is universally accepted and has entered textbooks. Physically, the theorem derives from the exact cancellation of the orbital diamagnetic moment associated with the completed cyclotron orbits of the charged particles by the paramagnetic moment subtended by the incomplete orbits skipping the boundary cuspidally in the opposite sense. In this work we have revisited the problem of this crucial but subtle role of the boundary by considering the case of a finite but unbounded system, namely that of a charged particle moving on a sphere in the presence of an externally applied magnetic field. The orbital moment calculated on the basis of the classical Langevin equation in the infinite time limit now indeed turns out to be non-zero, and has the diamagnetic sign. This violates the BvL theorem as stated in the literature. To the best of our knowledge, this is the first report of non-zero classical diamagnetism. It is explicitly owing to the above avoided cancellation. We also present possible experimental realization of the predicted classical diamagnetism. [Preview Abstract] |
Session Z30: Focus Session: Iron-based Multiferroics
Sponsoring Units: DMP GMAGChair: Sang-Wook Cheong, Rutgers University
Room: 334
Friday, March 20, 2009 11:15AM - 11:27AM |
Z30.00001: Multiferroic states in perovskite type orthoferrites Yusuke Tokunaga, Satoshi Iguchi, Yasujiro Taguchi, Takahisa Arima, Yoshinori Tokura Versatile and gigantic magnetoelectric (ME) phenomena have been found for a single crystal of perovskite-type orthoferrite DyFeO$_3$[1]. Below the antiferromagnetic ordering temperature of Dy moments, a linear ME tensor component as large as $\alpha_{zz}\sim 2.4 \times 10^{-2}$ in dimensionless CGS unit is observed. In addition, it is revealed that the application of magnetic field along the c axis induced a ferroelectric order whose large polarization ($\geq$ 0.2 $\mu$C$/$cm$^2$ along the $c$-axis) can be directly reversed by either of magnetic field or electric field. It is noteworthy that this magnetically driven ferroelectric state is even weakly ferromagnetic, i.e., truly multiferroic, in nature. We propose here that the exchange striction working between adjacent Fe$^{3+}$ and Dy$^{3+}$ layer with the respective layered antiferromagnetic components can be the origin of the ferroelectricity with such a large polarization value. It is further argued that the reversal process of electric polarization by magnetic (electric) field is inherently related to the change of the relative phase of antiferromagnetic spin (moment) arrangement of Fe (Dy)\\[0pt] [1] Y. Tokunaga et al., Phys. Rev. Lett. 101, 097205 (2008). [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z30.00002: Composite Domain Walls in Multiferroic Orthoferrites RFeO3 Nobuo Furukawa, Hosho Katsura In order to analyze novel magneto-electric effects such as electric-field controlled magnetization-flips in multiferroic orthoferrites $R$FeO$_3$, we study an effective model which includes $d$-spins on Fe sites and $f$-spins on $R$ sites. Order parameters for $d$ and $f$ spins are coupled through Peierls distoritons. Ginzburg-Landau theory is applied to investigate domain wall structures of the model. As a result, we find various types of solitons corresponding to ferromagnetic, ferroelectric and composite ferromagnetic-ferroelectric domail walls. Dynamics of the domail walls under external fields will also be presented. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z30.00003: Multiferroic BiFeO3 Sang-Wook Cheong BiFeO3 (BFO) is a unique multiferroic in the sense that the magnitude of ferroelectric polarization is large (about 90 microC/cm) - similar with that of standard ferroelectrics such as BaTiO3 and PbTiO3. In addition, both magnetic and ferroelectric temperatures are much high than room temperature. BFO has been extensively studied, but mostly in the form of films. In order to explore the intrinsic properties of BFO and also properties that cannot be measured in film forms, we have investigated comprehensive physical properties of bulk BFO single crystals using a number of techniques such as neutron scattering, piezoelectric force microcopy and transport property measurement. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z30.00004: Polarized Neutron Investigations of BiFeO$_{3}$ William Ratcliff, Seongsu Lee, Taekjib Choi, Ross Erwin, Sang Wook Cheong, Valery Kiryukhin BiFeO$_{3}$ is a multiferroic material at room temperature. Until recently, the only studies on this material were performed on either thin films or powders. We report on the results of neutron diffraction studies performed on single crystals of BiFeO$_{3}$. Polarized neutron diffraction results unambigously reveal that the magnetic structure of this material is chiral. Furthermore, neutron diffraction experiments have shown that it is possible to control magnetic domain populations through the application of an external electric field. These results may suggest directions for future research performed in thin films. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z30.00005: Weak ferromagnetism in a high-pressure phase of FeTiO$_{3}$ with polar lattice distortion Tamas Varga, John Mitchell, Craig Fennie, Stephen Streiffer, Seungbum Hong, Moonkyu Park, Venkatraman Gopalan, Amit Kumar, Eftihia Vlahos, Takeshi Sanehira, Yanbin Wang Today's challenge in multiferroics is to identify materials in which polarization and magnetization -- normally considered contraindicated properties - are strongly coupled. Recent density functional theory calculations have predicted that the family of compounds MTiO$_{3}$ (M = Mn, Fe, Ni) are promising candidates where a polar lattice distortion can induce weak ferromagnetism. The crucial insight is that while the equilibrium one-atmosphere structure of these is ilmenite, they must be transformed to a closely related LiNbO$_{3}$-type structure. We have prepared the corresponding FeTiO$_{3}$ phase at 18 GPa and 1200 $^{\circ}$C. It shows a sharp antiferromagnetic (AF) transition at 111.5 K. FeTiO$_{3}$ also displays ferroelectric domains, and weak ferromagnetism coincident with the AF transition. Possible coupling between its polarization and weak ferromagnetism is discussed based on results of piezoelectric force microscopy (PFM), second harmonic generation (SHG), dielectric, and polarization measurements. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z30.00006: EXAFS Studies of Mulfiferroic Pb(Ti,Fe)O$_{3}$ Ying Zou, Somaditya Sen, Shishir Ray, Mark Williamsen, Tomohiro Shibata, Soma Chattopadhyay, Mali Balasubramanian, Prasenjit Guptasarma Recent suggestions [1] of multiferroic behavior in Pb(Ti,Fe)O$_{3}$ has revived an interest in this text-book ABO$_{3}$ type ferroelectric material. Here, we study the effect of Fe-substitution on the local structural and chemical environment of the parent PbTiO$_{3}$ phase. We have carried out extended x-ray absorption fine structure (EXAFS) measurements at the Fe-K, Ti-K and Pb-LIII edge at the Advanced Photon Source, on a series of single phase samples of PbTi$_{1-x}$Fe$_{x}$O$_{3}$ (0$<$ x $<$ 0.5) synthesized using a sol-gel technique. The near edge fine structure (XANES) reveals that Fe cations are trivalent. A fascinating new result is the observation of charge disproportionation of Pb into Pb2+ and Pb4+, likely a result of charge redistribution arising from Fe3+ substitution. ~Radial distribution function (RDF) study of EXAFS spectra from the Ti-K edge and the Fe-K edge confirms that Fe substitutes Ti up to x=0.5. 1. Palkar et al, Appl. Phys. Lett. 90(2007)172901. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z30.00007: Phase formation, Crystal Lattice and Microstructure Studies of sol-gel derived Pb(Ti,Fe)O$_{3}$ Somaditya Sen, David Gelting, Shishir Ray, Ying Zou, Donald Robertson, Marija Gajdardziska-Josifovska, Larry Buroker, Mark Williamsen, Prasenjit Guptasarma It has recently been suggested[1] that Fe-substituted PbTiO$_{3}$ can exhibit magnetoelectric multiferroic behavior. With an intent to examine whether Fe can fully substitute the lattice in Pb(Ti,Fe)O$_{3 }$and to study its effect on crystal structure, we have synthesized highly phase pure nanopowders from citric acid metal ion chelate complexes stabilized by glycerol in a sol gel. Using variety of probes, we demonstrate that Fe can substitute Ti up to at least 0.5 atoms per formula unit of Pb(Ti,Fe)O$_{3}$. Rietveld refinement of XRD data, from both laboratory and synchrotron sources, demonstrates that crystal structure of Fe substituted phases can be derived from the parent orthorhombic PbTiO$_{3}$ phase. Increasing concentration of Fe up to x=0.3 results in drastic change in lattice parameters and decrease in orthorhombic distortion. These results are supported by detailed studies of XRD, TEM and XAFS. \\[3pt] [1] Palkar et al, Appl. Phys. Lett. 90(2007)172901. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z30.00008: Electric polarization and magneto-dielectric effect in charge ordered system with frustrated geometry Tsutomu Watanabe Recently discovered multiferroics materials, where electric polarization and non-collinear spin structure coexist, are recognized as ferroelectric materials driven by spin ordering. There is another class of ferroelectricity where the electric polarization is attributed to the electronic charge ordering. Layered iron oxide LuFe$_{2}$O$_{4}$ belongs to this class of material. Ferroelectric transition occurs at almost the same temperature with the charge ordering one where Fe$^{2+}$ and Fe$^{3+}$ are aligned in the paired triangular lattices. Therefore, it is expected that the frustration plays important roles on breaking of the space inversion symmetry. We study theoretically a possibility of the charge-driven ferroelectric transition, and, in particularly, focus on the electron quantum transfer effects. We analyzed the V-t model, where t and V are the transfer integral and the Coulomb repulsion between nearest-neighbor sites, respectively, by using the variational Monte Carlo method. We found that the quantum fluctuation and frustration tend to enhance the stabilization of a three-fold charge ordered state and electric polarization, although the polarization is small. In addition, we studied the effect of the spin ordering. It is found that, in some spin ordered structures, the electric polarization is stabilized accompanying the three-fold charge ordered state. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z30.00009: Charge order, dynamics, and magneto-structural transition in multiferroic LuFe$_2$O$_4$ Xiaoshan Xu, Manuel Angst, Tatiana Brinzari, Raphael Hermann, Janice Musfeldt, Andy Christianson, David Mandrus, Brian Sales, Steve McGill, Jong-Woo Kim, Zahirul Islam We investigated the series of temperature and field-driven transitions in LuFe$_2$O$_4$ by optical and M\"{o}ssbauer spectroscopies, magnetization, and x-ray scattering in order to understand the interplay between charge, structure, and magnetism in this multiferroic material. We demonstrate that charge fluctuation has an onset well below the charge ordering transition, supporting the ``order by fluctuation'' mechanism for the development of charge order superstructure. Bragg splitting and large magneto optical contrast suggest a low temperature monoclinic distortion that can be driven by both temperature and magnetic field. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z30.00010: Electronic approaches on orbital anisotropy and spin configuration in Multiferroic LuFe$_{2}$O$_{4}$ K.-T. Ko, H.-J. Noh, B.-G. Park, J.-Y. Kim, J.-H. Park, A. Tanaka, Sung-Baek Kim, S.-W. Cheong The orbital anisotropy and spin configuration of the multiferroic LuFe$_{2}$O$_{4}$ are investigated by the x-ray absorption spectroscopy (XAS) at Fe L$_{2,3}$- and O K-edges, and the theoretical cluster model calculations including the configuration interactions and full multiplets. The x-ray magnetic circular dichroism (XMCD) results show that the system has a surprisingly large orbital moment as large as $m_{O} \quad \sim $ 0.8 $\mu _{B}$/f.u., which also agrees with the theoretical model calculation result. This result also well explains the observed total magnetic moment of 2.9 $\mu _{B}$/f.u.. The polarization dependent XAS enables us to identify the orbital level and occupation, which turns out to be rather different from the band structure prediction. We also found that the polar charge ordering plays an essential role for the 0.7 eV charge gap. Finally, we discuss about the local electronic structure, orbital anisotropy, and the spin configuration of LuFe$_{2}$O$_{4}$. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z30.00011: Multiple magnetic transitions and glass dynamics in LuFe2O4 Hari Srikanth, Manh-Huong Phan, Natalie Frey, Manuel Angst, Brian Sales, David Mandrus LuFe$_{2}$O$_{4}$ is a complex oxide of topical interest as ferroelectricity in this material arises from charge ordering and it also exhibits multiferroic behavior. A good understanding of the magnetic phase diagram has remained elusive primarily due to the complexity of the system as well as the sensitivity to sample quality. In this study, we report on the magnetic properties of high quality LuFe$_{2}$O$_{4}$ single crystals grown by the floating zone method. Temperature dependent DC, AC susceptibility and isothermal magnetization reveal a rich and complex phase diagram. Magnetic transitions at 240K and 175K are accompanied by strong frequency dependence of the real ($\chi $') and imaginary ($\chi $'') parts of the AC susceptibility indicative of glassy behavior. Quantitative fits to the glass model confirm cluster glass dynamics and this is consistent with the presence of ferrimagnetic domains within the Fe-O planes. Magnetocaloric effect (MCE) in these materials will also be presented. We will place our results in the context of work by other groups on this system and clarify the nature of the magnetic phase diagram that emerges from our studies. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z30.00012: Stacking the Collinear Magnetic Phases of the Geometrically-Frustrated Antiferromagnet CuFeO$_{2}$ Randy Fishman, Feng Ye, Jaime Fernandez-Baca The correct stacking of hexagonal layers is used to obtain accurate estimates for the exchange and anisotropy parameters of the geometrically-frustrated antiferromagnet CuFeO$_{2}$. Those parameters are highly constrained by the stability of a collinear metamagnetic phase between fields of 13.5 and 20 T. Constrained fits of the spin-wave frequencies of the ``up up down down'' phase below 7 T are used to identify the magnetic unit cell of the metamagnetic ``up up up down down'' phase, which contains two hexagonal layers and 10 Fe$^{3+}$ spins. The resulting exchange parameters are much smaller than those obtained from an unconstrained fit of the zero-field spin-wave data and successfully describe not only the main branch of spin-wave excitations but also the spin-wave excitations of the two twins in the (H,K,0) plane [1]. Research sponsored by the Division of Materials Sciences and Engineering, U.S. Department of Energy under contract with UT-Battelle, LLC. \\[0pt] [1] R.S. Fishman, F. Ye, J.A. Fernandez-Baca, J.T. Haraldsen, and T. Kimura, \textit{Phys. Rev. B} \textbf{78}, 140407 (2008). [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z30.00013: Site occupancy and magnetic properties of aluminum substituted barium hexaferrite Amitava Moitra, Sungho Kim, Seong-Gon Kim, Yang-Ki Hong, Steven C. Erwin Aluminum substituted barium hexaferrite has been studied using density functional theory (DFT). The substitution has been carried out for BaFe$_{12-x}$Al$_{x}$O$_{19}$ from $x$ = 1 to $x$ = 3 in steps of 0.5. With the aid of accurate DFT study, our result show that the Al$^{3+}$ ions preferentially occupy the 2a and 12k site, unlike the previously reported 4f2, 2a, 4f1, and 12k sites. Our result confirms the experimental fact that with increasing of Al substitution the total magnetic moment monotonically decreases. We also present a possible reason of the site preference of 2a and 12k. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z30.00014: Magnetoelectric Coupling and Relaxation in Yttrium Iron Garnet Yuichi Yamasaki, Yuki Kohara, Yoshinori Tokura We report the results of the magnetic and dielectric relaxation measurement on yttrium iron garnet Y$_{3}$Fe$_{5}$O$_{12}$ which shows the 2nd order magneto-electric (ME) effect under an applied electric field [1] and the 1st order ME effect by an electric field cooling procedure [2]. We found that the temperature dependence of the dielectric relaxation dynamics coincides with that of magnetic one. This suggests a strong magneto-electric coupling between dielectric and magnetic relaxation dynamics. Indeed the dielectric relaxation strengths are enhanced by an applied magnetic field; namely the amplitude of electric dipole moment can be tuned by magnetic field. This effect accounts for the magnetic field induced change of the static dielectric permittivity and thereby the gigantic 2nd order ME effect as observed. [1] T.H. O'Dell, Phil. Mag. 16, 487 (1967), [2] H. Ogawa et al., JPSJ 56, 452 (1987). [Preview Abstract] |
Friday, March 20, 2009 2:03PM - 2:15PM |
Z30.00015: Dielectric and magnetic Properties of the Non-centrosymmetric Fe-Lagasite Nara Lee, Young Jai Choi, Sang-Wook Cheong The non-centrosymetric compound Ba$_{3}$NbFe$_{3}$Si$_{2}$O$_{14}$ known as the Fe-langasite forms in a unique magnetic triangular lattice of Fe$^{3+}$ spins. The interesting magnetic and dielectric properties may arise from the spin frustration of the triangular magnetic lattice as well as the broken inversion symmetry of the crystallographic structure. In order to understand the complex structure and magnetic spin ordering, we have performed comprehensive experiments on single crystals grown by a floating zone method, including x-ray diffraction analysis and measurements of magnetic susceptibility, dielectric constant and heat capacity under variation of temperature and magnetic field. [Preview Abstract] |
Session Z31: Focus Session: Magnetic Nanoparticles and Nanocomposites: Characterization
Sponsoring Units: DMP GMAGChair: Manh-Huong Phan, University of South Florida
Room: 335
Friday, March 20, 2009 11:15AM - 11:27AM |
Z31.00001: Crystal Structure and Magnetic properties of Fe-substituted nanoscale Hydroxyapatite Andreas Kyriacou, Riccardo Venturelli, Korey Sorge, Theodora Leventouri Magnetic nanoscale hydroxyapatite (HAp) of chemical formula Ca$_{(5-x)}$Fe$_{x}$(PO$_{4})_{3}$OH has been prepared by a chemical precipitation method where $x$ varies from 0 to 1.26. Single phase HAp is identified in XRD patterns of samples with $x \quad \le $ 0.30 while maghemite (Fe$_{2}$O$_{3})_{ }$is formed as a secondary phase for $x \quad \ge $ 0.60. The average crystallite size as calculated by the Scherrer equation varies from 16 nm to 28 nm. Rietveld refinement reveals a decrease of the unit cell for $x \quad \le $ 0.15. Magnetic moment measurements as a function of temperature at applied field \textit{$\mu $}$_{0}H$ = 1.5 T shows a two component system: a temperature-dependent paramagnet (PM) or superparamagnet (SPM) and a roughly temperature-independent ferromagnetic (FM) component. No FM activity is shown for low $x$, followed by increased activity for higher $x$. Increasing SPM activity is observed for $x\le $0.60. Hysteresis measurements show irreversible loops for $x \quad \ge $ 0.22. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z31.00002: Structure determination of CoPt nanoparticles: Chemical ordering and its effect on magnetic properties Nils Blanc, Laurent Bardotti, Matthias Hillenkamp, Alexandre Tamion, Florent Tournus, Juliette Tuaillon-Combes, Veronique Dupuis, Edgar Bonet, Helio Tolentino, Aline Ramos, Maurizio De Santis, Philippe Ohresser, Thierry Epicier Due to the huge magnetocrystalline anisotropy of bulk CoPt crystallized in the L1$_{0}$ phase, CoPt nanoparticles have been widely studied during the last decade. In order to determine the intrinsic magnetic properties of CoPt clusters, we synthesize benchmark samples: 3 nm diameter CoPt clusters, pre-formed in the gas phase, are embedded in an amorphous carbon matrix under UHV conditions. The transition from the chemically disordered A1 to the ordered L1$_{0}$ phase is then obtained by annealing. Chemical ordering has clearly been evidenced by different techniques (HRTEM, GIXRD). In the case of nanoparticles, this phase transition goes with a magnetic anisotropy increase much lower than for the bulk. Besides, XMCD measurements have revealed a $\mu $L/$\mu $S increase for Co and Pt atoms and a strong $\mu $S enhancement for Co upon L1$_{0}$ ordering. F. Tournus et al. Phys. Rev. B 77, 144411 (2008) Thanks are due to the CLYM (Centre Lyonnais de Microscope) for the access to the tranmission electron microscope [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z31.00003: Origin of magnetic anomalies and relaxation mechanisms in ferrofluids M. B. Morales, M. H. Phan, N. A. Frey, S. Pal, H. Srikanth From a fundamental physics perspective, it is proposed that blocking of magnetic nanoparticles and freezing of a carrier fluid would affect the magnetization and relaxation processes in ferrofluids. To verify this hypothesis, we have conducted systematic DC magnetization and AC susceptibility studies in different ferrofluids composed of Fe$_{3}$O$_{4}$ and CoFe$_{2}$O$_{4}$ nanoparticles suspended in hexane and dodecane, which respectively have freezing temperatures below (178K) and above (264K) the blocking temperature of magnetic nanoparticles ($\sim $200K). Experimental results reveal that the particle blocking and carrier fluid freezing effects play key roles in the formation of glass-like relaxation peaks in ferrofluids, which remained largely unexplained in previous studies. It is also shown that the nature of these peaks is strongly affected by varying particle size and carrier fluid medium. Quantitative fits of the frequency dependent AC susceptibility to the Vogel-Fulcher model, $\tau =\tau _{o}$exp[E$_{a}$/k(T-T$_{o})$], clearly indicate that the blocking of magnetic nanoparticles in the frozen state significantly affects the interparticle dipole-dipole interaction, causing characteristic spin-glass-like dynamics. A clear correlation between the blocking and freezing temperatures emerges from our studies for the first time. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z31.00004: Competing effect of blocking and spin frustration in nanostructured gadolinium iron garnets M.H. Phan, M.B. Morales, H. Srikanth, C.N. Chinnasamy, V.G. Harris The ground state magnetic properties and relaxation mechanism in magnetically frustrated system of Gd$_{3}$Fe$_{5}$O$_{12}$ is of topical interest due to its complex magnetic structure. As a consequence of geometric and magnetic frustrations, the Gd$_{3}$Fe$_{5}$O$_{12}$ system is expected to show glassy magnetic behavior. Through a comprehensive study of DC magnetization, AC susceptibility, transverse susceptibility, and magnetocaloric effect in Gd$_{3}$Fe$_{5}$O$_{12}$ bulk and nanostructured materials, we provide physical insights into the glassy nature and magnetic relaxation mechanisms in the gadolinium iron garnet system. It is shown that bulk Gd$_{3}$Fe$_{5}$O$_{12}$ undergoes two different glassy states at temperatures below its compensation temperature with the low temperature glass properties strongly influenced by Gd ordering. However, the glassy nature is largely suppressed in Gd$_{3}$Fe$_{5}$O$_{12}$ nanoparticles in which the blocking phenomenon competes with the spin frustration effect. As particle size is decreased, the blocking effect is dominant over the spin frustration effect. As a result, the nanostructured system shows magnetic relaxation features arising mainly from superparamagnetism. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z31.00005: Thermoinduced Magnetization in NiO Nanoparticles Gregory Brown The low-temperature magnetic susceptibility for model NiO nanoparticles is calculated using the Monte Carlo method, and three different behaviors are seen. With uncompensated spins present, the susceptibility diverges as $T$$\rightarrow$$0$. For cube- shaped nanoparticles, a temperature-dependent thermoinduced magnetization is observed. For spherical and octahedral nanoparticles, a temperature-independent susceptibility associated with the spin-flop configuration is observed. Calculations for arbitrary values of the uniaxial anisotropy indicate that thermoinduced magnetization can be observed for all geometries in materials with strong enough anisotropy. This work was sponsored by the LDRD program of ORNL, by the DOE-OS through the Offices of BES, Division of MSE and ASCR, MICS Division. The $\Psi$--Mag tool set was developed as part of a BES sponsored Computational Material Science Network project. ORNL is managed by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z31.00006: Magnetic and EPR Characterization of Ni(core)/NiO(shell) Nanoparticles Saritha Nellutla, Alex Smirnov, Junwei Wang, Joseph B. Tracy Core/shell nanoparticles have interesting applications in various fields [1-4]. Among these systems, ferromagnet(core)/antiferromagnet(shell) are of particular importance because of their potential use as MRI contrasting agents, high density magnetic recording devices, etc. [3, 4]. Here, Ni(core)/NiO(shell) nanoparticles of different core sizes ranging from 8 nm to 22 nm have been synthesized and characterized by TEM, magnetic susceptibility and electron paramagnetic resonance (EPR) spectroscopy, as ``free'' (non-agglomerated) particles and agglomerated clusters. Using EPR at 9.1 GHz it is shown that the temperature dependence of the g-value and the EPR linewidth are similar for both the free particles and the agglomerated clusters. This suggests that at this magnetic field ($\sim $0.3 T) the EPR signal arises mostly from the saturated magnetic moment. EPR measurements at multiple fields/frequencies provide further insight on the microscopic magnetic structure in the free particles and the agglomeration effects. [1]. M. A. Hines, P. Guyot-Sionnest, \textit{J. Phys. Chem.}, $100$, 468 (\textbf{1996}). [2]. Z.C. Xu, Y.L. Hou, S.H. Sun, \textit{J. Am. Chem. Soc.}, $129$, 8698 (\textbf{2007}). [3]. V. Skumryev, S. Stoyanov, Y. Zhang, G. Hadjipanayis, D. Givord, J. Nogues, \textit{Nature} $423$, 850 (\textbf{2003)}. [4]. A. H\"{u}tten, D. Sudfeld, I. Ennen, G. Reiss, W. Hachmann, U. Heinzmann, K. Wojczykowski, P. Jutzi, W. Saikaly, G. Thomas\textit{ J. Biotechnology} $112$, 47-63 (\textbf{2004}). [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z31.00007: Development of Novel Biopolymer/Synthetic-Polymer/Iron Oxide Nanocomposites Marleth Mena Montoya, Sugeheidy Carranza, Mois\'es Hinojosa, Virgilio Gonz\'alez In this work we report the successful development of a family of magnetic nanocomposites based on chitosan or/and polyamide 6 matrix with dispersed iron oxide nanoparticles synthesized by chemical co-precipitation. The iron oxide contents varied from 5 up to 23 wt{\%}, the nanocomposites were studied by FTIR, UV-vis, TGA, XRD, TEM and magnetometry. The FTIR analysis demonstrates an interaction between the amide group of the polyamide 6 and the ceramic material. In formic acid, the nanocomposites absorb in the UV-Vis range, and the magnitude of the band gap (optical), calculated using the band of higher wavelength, is between 2.16 and 2.19 eV. In nanocomposites with chitosan/polyamide 6 matrix the developed morphologies are spherulites of polyamide 6 surrounded by chitosan, with the iron oxide particles presumably in the form of ferrihidryte. The measured magnetic properties revealed a superparamagnetic character on the studied specimens. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z31.00008: Characterization of ultrasonically prepared $\gamma$-Fe$_{2}$O$_{3}$-Al$_{2}$O$_{3}$ shell-core nanocomposites Matthew Vannette, Joshua Hugen, Daniel Stoecklein, Brett McCarty, Ruslan Prozorov High intensity ultrasonic irradiation (sonication) of slurries of Al$_{2}$O$_{3}$ nanopowder in an Fe(CO)$_{5}$/decane mixture produce superparamagnetic $\gamma$-Fe$_{2}$O$_{3}$ shells on non-magnetic cores. In this contribution we discuss the effect of the various adjustable parameters (sonication time and intensity, powder loading, and Fe(CO)$_{5}$:decane ratio) on the dc and ac magnetic properties of these composite materials. Effects of post production modification such as heat treating powders and cold pressing pellets is also presented for a subset of samples. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z31.00009: Phase Transformation in Silica-Coated FePt Nanoparticles Levent Colak, George Hadjipanayis The A1 to L1$_{0}$ phase transformation has been examined in silica-coated FePt particles. The nanoparticles were synthesized by reduction of platinum acetylacetonate (Pt (acac)$_{ 2})$ followed by thermal decomposition of iron pentacarbonyl (Fe(CO)$_{5})$ in the presence of oleic acid (OA) and oleyl amine (OY) as surfactants at low temperature $^{[1]}$. The monodispersed FePt nanoparticles, with a size of 5.8 nm were then coated with silica (SiO$_{2})$ shells $^{[2]}$ . The thickness of the silica shell could be controlled between 7.5-25 nm. The coated particles were subjected to thermal processing at 800\r{ }C for various amounts of times. No significant sintering was observed up to 2 hours of annealing for the shell thickness of 15.0 nm. In some silica-coated samples an increase in the particle size was observed after annealing. Selected Area Diffraction analysis and magnetic measurements showed the development of ordered L1$_{0}$ structure. Coercivity values up to 15 kOe at 7K are obtained. The phase transformation is currently being examined in other samples annealed at different times and temperatures and the results will be reported.\textbf{1}.Levent Colak and George C. Hadjipanayis, Nanotechnology 19 (2008) 235703.\textbf{2}.M. Aslam, L. Fu, S. Li, Vinayak P. Dravid, Journal of Colloid and Interface Science 290 (2005) 444--449. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z31.00010: Crystallization thermodynamics and kinetics of SmCo$_{5}$/Fe system. Chuanbing Rong, J. Ping Liu High energy ball milling is an effective and economic way to produce the hard/soft nanocomposite permanent magnetic materials which have immense potential to exhibit much higher energy products than the conventional single phase hard magnets. Intermetallic materials undertaken high energy ball milling are usually of amorphous structures. It is therefore necessary to study the grain nucleation and growth behavior of the ball-milled amorphous powders. There has not been a systematical study of thermal dynamic and kinetic behavior of the mechanically milled nanocomposite powders. In this work, powder mixtures of SmCo$_{5} \quad +$ x $\alpha $-Fe (x=0-30 wt{\%} ) were mechanically milled for 2 - 10 hours. The thermal dynamic and kinetic behavior of the powders was studied by measuring the differential scanning calorimetry (DSC) curves with different heating rate and isothermal methods. It was observed that the crystallization process of the SmCo$_{5}$ phase shifted to high temperature while that of Fe phase shifted to low temperature with increasing milling time. Kissinger analysis shows that the activation energy of SmCo$_{5}$ phase significantly decreased with increasing milling time and increasing Fe content. Isothermal analysis showed that the nucleation of SmCo$_{5}$ phase started around 300-350 $^{o}$C which is 100-150 $^{o}$C lower than the crystallization temperature (460 $^{o}$C). [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z31.00011: Magneto-structural study of phase pure $\alpha $ and $\beta $ type MnAs nanoparticles P. Kharel, Keerthi Senevirathne, Ron Tackett, Stephanie Brock, G. Lawes There is extensive interest in understanding the properties of nanoscale materials that exhibit magneto-structural phase transitions because of their possible use in magnetocaloric applications. Bulk MnAs exhibits a ferromagnetic transition in the range 313-317 K, together with a structural transition from a hexagonal (\textit{$\alpha $}-MnAs) to an orthorhombic (\textit{$\beta $}-MnAs) lattice. We have studied the structural and magnetic properties of \textit{$\alpha $}-MnAs and\textit{ $\beta $}-MnAs nanoparticles synthesized using solution-phase arrested precipitation method. XRD and TEM studies show that both the \textit{$\alpha $} and\textit{ $\beta $} phase nanoparticles are crystalline, phase pure, and stable for weeks at room temperature. Magnetic measurements show that both the \textit{$\alpha $}-MnAs and \textit{$\beta $}-MnAs phase nanoparticles undergo ferromagnetic phase transitions near 315K, but we find no evidence for the associated \textit{$\alpha $} to \textit{$\beta $} structural transition seen in bulk MnAs. We will present an experimental investigation on the connection between structural and magnetic properties in \textit{$\alpha $} and\textit{ $\beta $} type MnAs nanoparticles, and discuss relevance to studies on other nanostructured systems. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z31.00012: Magnetic Force Microscopy of Ferromagnetic MnAs Nanoparticles in GaAs. Ben Chaprut, Radhika Barua, Laura Lewis, Don Heiman The switching behavior and anisotropy of ferromagnetic MnAs nanoparticles in GaAs was investigated with variable-temperature magnetic force microscopy. Nanoparticles of MnAs were synthesized by annealing thin layers of GaMnAs, with Mn/Ga=0.1. Annealing at 640 C resulted in thin disc-shaped MnAs particles with diameters $\sim $100 nm embedded in GaAs. Magnetization measurements at room temperature show that the samples are ferromagnetic and exhibit hysteresis with a coercive field $\sim $0.1 T. In MFM images at room temperature, the majority of the nanoparticles have a single-domain magnetic dipole moment which can be oriented in one direction after applying a magnetic field $\sim $0.1 T. The dipole orientation can be reversed after applying a field in the opposite direction. After raising the temperature above the Curie point, Tc=337 K, the sample becomes demagnetized at room temperature, with equal numbers of particles aligned in opposite directions. A sharp phase transition, from the ferromagnetic hexagonal phase to the paramagnetic orthorhombic phase, was found at 340 C, nearly coincident with the Curie temperature. This transition occurs $\sim $30 C higher than in thin MnAs films and is attributed to tensile strain on the nanoparticles from the surrounding GaAs. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z31.00013: Nanoparticulate Alnico Thin Films with High Coercivity Ozan Akdogan, George C. Hadjipanayis Alnico V (Fe--8{\%} Al--14{\%} Ni--24{\%} Co--3{\%} Cu) nanoparticulate thin films have been produced by dc magnetron sputtering. The films were sputtered on Si substrates for magnetic measurements and carbon-coated copper grids for TEM measurements. The as-deposited films have a fine grained microstructure with the bcc crystal structure. The as-made films were subjected to a full heat treatment which consists of heating the sample to 900 $^{\circ}$C, then cooling it to 600 $^{\circ}$C and finally annealing it at 600 $^{\circ}$C for several hours. After the heat treatment, the thin films broke up into large nanoparticles (20-60 nm) surrounded by small nanoparticles (2 nm). Electron diffraction data showed that the annealed samples had an fcc structure. The maximum room temperature coercivity was found to be 2 kOe after 6h of annealing at 600 $^{\circ}$C. The high coercivity could be due to strain that was induced during precipitation. The evolution of crystal structure and microstructure with annealing will be monitored and related to the observed magnetic properties. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z31.00014: Magnetic Properties of As-Prepared and Annealed Nanocrystalline Fe Particles Karl Unruh, Thomas Ekiert Air stable Fe-core/oxide-shell particles with diameters between about 100 and 200 nm have been synthesized by the reduction of a ferrous Fe salt in the presence of citrate ions. Structural, chemical, and magnetic measurements indicate that the oxide shell is 2-3 nm thick and that the core consists of essentially oxide free, $\alpha $-Fe nanocrystals (about 5 nm in diameter) in addition to regions of non-crystalline, disordered Fe. The as-prepared particles evolve into a continuous porous solid structured at about the 100 nm scale after annealing in forming gas at temperatures near 750 K followed by a progressive elimination of the porosity at higher annealing temperatures. Prior to the formation of the porous solid the saturation magnetization, coercivity, and remanence ratio all increase slightly with annealing temperature due to an increase in the size of the core Fe crystallites at the expense of the disordered Fe component. The structural transformation to a porous solid, however, results in an abrupt increase in both the coercivity (by about 50{\%} at 300 K and 100 {\%} at 5 K) and remanence ratio (about 100{\%} at 300 K and 150{\%} at 5 K). [Preview Abstract] |
Friday, March 20, 2009 2:03PM - 2:15PM |
Z31.00015: Anisotropic Sm-Co(Fe) Nanoparticles Produced by Surfactant-Assisted Ball Milling Nilay Gunduz-Akdogan, George C. Hadjipanayis, David J. Sellmyer Magnetically hard SmCo$_{5}$ and Sm$_{2}$(Co$_{0.8}$Fe$_{0.2})_{17}$ nanoparticles have been produced by using surfactant assisted low- and high-energy ball milling. Surfactants prevent the re-welding of the crashed particles during the milling process and thus limit the particle growth. Oleic acid was used as the surfactant and the heptane as the milling medium. High energy ball milling experiments took place in a milling vial with carbon steel balls by using a Spex 8000M high energy ball milling machine. The coercivity was found to increase with milling time with a value of 2.9 kOe for Sm$_{2}$(Co$_{0.8}$Fe$_{0.2})_{17}$ and 19.5 kOe for SmCo$_{5}$ after 12 hrs of milling. TEM data showed that the milled powders have a narrow size distribution. The TEM grid-deposited samples showed self-assembled nanoparticles in the Sm$_{2}$(Co$_{0.8}$Fe$_{0.2})_{17 }$alloy after 4 hours of milling, which could be further aligned when subjected to a magnetic field. The evolution of structural and microstructural properties of the particles will be monitored and compared with their magnetic properties. [Preview Abstract] |
Session Z32: Focus Session: Magnetoresistance and Spin-dependent Transport
Sponsoring Units: GMAG DMP FIAPChair: Kiran Thadani, Cornell University
Room: 336
Friday, March 20, 2009 11:15AM - 11:27AM |
Z32.00001: Nitrogen doping in single-crystal MgO magnetic tunnel junctions. Justin Brockman, Cheng-han Yang, Mahesh Samant, Kevin Roche, Stuart Parkin Recent experiments have shown evidence for induced ferromagnetism in thin films of carbon and nitrogen-doped zinc oxide. We have discovered similar behavior in nitrogen-doped MgO films grown by plasma-assisted thermal evaporation. Here, we incorporate these films as tunneling barriers into single-crystal multilayer magnetic tunnel junctions and present experimental results showing the magnetoresistance and current-voltage characteristics for these structures. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z32.00002: Bias dependence of Fe-MgO-Fe magnetic tunnel junction devices within a single-band tight-binding model Tehseen Raza, Hassan Raza We have developed a transferable single-band tight-binding model benchmarked with the ab initio methods for Fe-MgO-Fe magnetic tunnel junction (MTJ) devices [1]. The computational complexity of our model is on the order of an effective mass one, but additionally it includes the bandstructure physics over the two-dimensional transverse Brillouin zone in an average manner. We study the bias dependence of the tunnel magnetoresistance (TMR) ratio in MTJ devices. At low bias, for both the 4-layer and 12-layer MgO barrier, the TMR is bias-independent. It is higher for the 12-layer device due to relatively a larger decrease in the AP current density. At high bias, our model predicts a sharp roll-off in TMR ratio, which is attributed to a rapid increase in the Delta{\_}1 band current density in the anti parallel (AP) configuration due to the bandedge states entering the conduction window. The TMR ultimately becomes negative when the AP current becomes higher than the P current due to the different k-states tunneling through the same barrier. [1] T. Z. Raza and H. Raza, arXiv:0804.2557 [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z32.00003: Magnetoresistance in Double Spin Filter Tunnel Junctions with Nonmagnetic Electrodes and Its Unconventional Bias Dependence Guo-Xing Miao, Martina Muller, Jagadeesh Moodera We demonstrate a large tunnel magnetoresistance (TMR) originating purely from the tuning of tunnel barrier heights in double barrier junctions with nonmagnetic electrodes. This is achieved by the spin filtering that occurs due to the selective tunneling probabilities for spin-up and -down electrons through a magnetic semiconductor barrier resulting in highly spin polarized tunnel currents. Combining two such barriers in a tunnel junction thus leads to a TMR without the necessity of magnetic electrodes. This is significantly different from traditional approaches for generating TMR involving two ferromagnetic electrodes and for example, using Al2O3 or MgO barriers. We demonstrate the first realization of such unconventional tunnel junctions and TMR using EuS / EuO based spin filter barriers with nonmagnetic Al electrodes. The novel non-monotonic and asymmetric bias behavior in magnetoresistance can be qualitatively modeled in the framework of WKB approximations. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z32.00004: The critical role of the barrier thickness in spin filter tunneling Casey Miller Spin filter tunneling is considered in the low bias limit as functions of the temperature dependent barrier parameters. We demonstrate the generation of spin polarized tunneling currents in relation to the magnetic order parameter, and discuss how an interfacially suppressed order parameter leads to a temperature dependent tunneling current asymmetry. Analyzing the full parameter space reveals that the often overlooked barrier thickness plays a critical role in spin filter tunneling. With all else fixed, thicker barriers yield higher spin polarization, and allow a given polarization to be achieved at higher temperatures. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z32.00005: Spin-Dependent Electronic Transport in Fe/MnAs/Fe Structures Kyung-Yeon Kim, Hyoung Joon Choi We have performed first-principles calculations of electronic structures and tunneling magnetoresistance of Fe/MnAs/Fe structures, which are junctions of two semi-infinite metals and a half-metal placed between them. The electronic structures are described by the Kohn-Sham density functional theory, with local spin density approximation, norm-conserving semicore pseudopotentials, and pseudo-atomic orbital basis set. The tunneling magnetoresistance is obtained by using a scattering- state method, considering different configurations of the magnetization. This work was supported by the KRF (KRF-2007-314- C00075) and by the KOSEF Grant No. R01-2007-000-20922-0. Computational resources have been provided by KISTI Supercomputing Center (KSC-2008-S02-0004). [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z32.00006: Spin dependent transport in FeCo$\vert $MgBO$\vert $FeCo magnetic tunnel junctions: Can boron in the oxide region be a good thing? Derek Stewart Recent experimental studies on FeCoB/MgO/FeCoB tunnel junctions have shown that boron can diffuse into the oxide region during rf-sputtering and result in the formation of crystalline MgBO regions[1,2]. These tunnel junctions still provide high tunneling magnetoresistance values as well as very low RA products[3]. Using a plane wave-pseudopotential density functional approach, I have examined potential Mg(B) oxides such as Mg$_{2}$B$_{2}$O$_{5}$ (both monoclinic and triclinic) as well at kotoite (Mg$_{3}$B$_{2}$O$_{6})$. Total energy calculations indicate that these oxides should be more stable than the formation of separate regions of MgO and B$_{2}$O$_{3}$. Kotoite (Mg$_{3}$B$_{2}$O$_{6})$ also has a boron concentration close to that found in the experimentally grown MgBO regions. In addition, kotoite provides a good lattice match with MgO and could act to template neighboring FeCo into crystalline bcc layers during annealing. This evidence suggests that kotoite is formed during the deposition process. I will also discuss the complex band structure of kotoite (Mg$_{3}$B$_{2}$O$_{6})$ and examine how this will also affect spin dependent transport from the FeCo leads. [1] J. Y. Bae \textit{et al.}, J. Appl. Phys. \textbf{99} 08T316 (2006) [2] J. C. Read \textit{et al.}, Appl. Phys. Lett. \textbf{90} 132503 (2007) [3] J. C. Read \textit{personal communication} [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z32.00007: Effect of thermal spin disorder on transport through magnetic tunnel junctions Aleksander Wysocki, Kirill Belashchenko We study the transport properties of Fe$_{1-x}$Co$_{x}$MgOFe$_{1-x}$Co$_{x}$ magnetic tunnel junction in the presence of spin disorder using the noncollinear density functional theory. For a given temperature the spin disorder in ferromagnetic leads is introduced by randomizing the directions of spin densities in atomic spheres according to the mean-field angular distribution function. For pure FeMg0Fe we found that even small spin disorder has a dramatic effect on transmission as compared to the zero temperature case due to the presence of interface states in the minority spin channel that are strongly affected by spin disorder. This results in a complicated temperature dependence of the tunneling magnetoresistance (TMR). On the other hand, in the case of Fe$_{1-x}$Co$_{x}$MgOFe$_{1-x}$Co$_{x}$ the interface states are less important and the main effect of the spin disorder is to decrease the spin polarization diminishing TMR. The temperature dependence of TMR is in agreement with Julliere model prediction with the spin polarization being proportional to magnetization. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z32.00008: Evolution of low-frequency resistance noise during annealing in CoFeB/MgO/CoFeB tunnel junctions Ryan Stearrett, Weigang Wang, Lubna Shah, Edmund Nowak, John Xiao We have studied the evolution of tunneling magnetoresistance (TMR) and resistance noise in magnetic tunnel junctions (MTJs) as a function of annealing time at 425$^{o}$C. Previously, we showed that short annealing times do lead to significant improvement in the MgO crystal structure and crystallization of the CoFeB electrodes, resulting in large TMR values up to 200{\%}. We also observe that the low-frequency resistance noise decreases significantly after annealing for only a few minutes. The resistance noise has a 1/f spectrum and is quantified by a Hooge-like parameter, $\alpha $, given in units of $\mu $m$^{2}$. In unannealed samples $\alpha $ is of order 10$^{-9} \quad \mu $m$^{2 }$and decreases with increasing voltage bias. Upon annealing, $\alpha $ drops to 10$^{-10} \quad \mu $m$^{2 }$and is less dependent on bias, particularly in the parallel configuration. We attribute the decrease in $\alpha $ and its bias dependence, $\alpha $(V), to a reduction of defects in and around the barrier due to annealing. The implications for optimizing the signal to noise ratio of MgO-based MTJ sensors will also be discussed. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z32.00009: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z32.00010: Current-Perpendicular-to-Plane Magnetoresistance of CoFe-based Exchange-Biased Spin-Valves Chiyui Ahn, Kyung-Ho Shin, Reza Loloee, Jack Bass, William Pratt Concentrated Co(50)Fe(50) and Co(70)Fe(30) alloys are of interest for spintronics applications. We have constrained the spin-transport properties of these alloys by measuring at 4.2 K the specific resistance (CPP area times resistance) and magnetoresistance of exchange-biased spin-valves (EBSVs) of the form (FeMn/CoFe/Cu/CoFe)---here CoFe indicates one of the two alloys of interest---where the magnetization of one CoFe layer is exchange bias pinned by the adjacent antiferromagnetic FeMn layer, and the magnetization of the other CoFe layer is free to switch from parallel (P) to anti-parallel (AP) to that of the pinned layer in a modest magnetic field. For each CoFe alloy, we have measured EBSVs where the thicknesses of both the fixed and free layers were held equal and varied together, and EBSVs where the thickness of the pinned CoFe layer was held fixed and that of the free layer was varied. From such measurements we have estimated two parameters: the bulk scattering asymmetry and the spin-diffusion length, for each of the two CoFe alloys. We will present both our data and the derived parameters. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z32.00011: Specific Resistance of Pd/Ir Interfaces Rakhi Acharyya, Hoang Yen Thi Nguyen, Reza Loloee, William P. Pratt Jr., Jack Bass, Shuai Wang, Ke Xia In electronic transport with current-flow perpendicular to the layer planes (CPP) of a metallic multilayer, the interface specific resistance AR (area A through which CPP-current flows times sample resistance R) is fundamental. Special interest focuses upon AR for metals M1 and M2 with the same crystal structure, and lattice parameters the same to within $\sim $ 1 percent, as AR can then be calculated with no free parameters. From measurements of the total AR of sputtered Pd/Ir multilayers, we obtain twice the interface specific resistance, 2AR$_{Pd/Ir}$ = 1.02 $\pm $ 0.06 f$\Omega$ m$^{2}$. For a single fcc structure with average lattice parameter of Pd and Ir, calculations including only spd orbitals give for perfect interfaces, 2AR$_{Pd/Ir}$(Perf) = 1.21 $\pm $ 0.1 f$\Omega $m$^{2}$, and for interfaces composed of two monolayers of a random 50{\%}-50{\%} alloy, 2AR$_{Pd/Ir }$(50/50) = 1.22 $\pm $ 0.1 f$\Omega $m$^{2}$. These values fall just outside the range of the experimental value. Upgrading to include f-orbitals gives 2AR$_{Pd/Ir}$(Perf) = 1.1 $\pm $ 0.1 f$\Omega $m$^{2}$ and 2AR$_{Pd/Ir}$(50-50) = 1.15 $\pm $ 0.1 f$\Omega $m$^{2}$. Within mutual uncertainties, these values are compatible with the experimental one. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z32.00012: Enhancement of Biquadratic Coupling in Co/Fe/MgO/Fe(001) Jared Wong, Yueh-Feng Chiang, Andrew Hoff, Xiaojing Tan, Yan Li, Keyu Pi, Wei Hau Wang, Harry Tom, Roland Kawakami One interesting aspect of the interlayer exchange coupling (IEC) across MgO is that in addition to the bilinear coupling, a biquadratic coupling favoring 90\r{ } magnetization alignment has been observed [1, 2, 3]. We investigate IEC in Co/Fe/MgO/Fe(001) and the affects of non-ideal aspects of the MgO-heterostructure, such as interface oxidation and impurities in the MgO, via molecular beam epitaxy (MBE) synthesis and magneto-optic Kerr effect (MOKE) measurements across wedged samples. By independently varying the oxygen content of the MgO film and the Fe/MgO interface, we find that the biquadratic coupling is correlated to the interfacial oxidation. Furthermore, the temperature dependence of the biquadratic coupling exhibits a strong increase at low temperatures. Our findings strongly support the loose spin mechanism as the origin of the biquadratic coupling across MgO[4]. 1. J. Faure-Vincent, C. Tiusan, C. Bellouard, et al., Phys. Rev. Lett. 89, 107206 (2002). 2. T. Katayama, S. Yuasa, J. Velev, et al., Appl. Phys. Lett. 89, 112503 (2006). 3. E. Snoeck, P. Baules, G. BenAssayag, et al., J. Phys.: Cond. Mat. 20, 055219 (2008). 4. J. C. Slonczewski, J. Appl. Phys. 73, 5957 (1993). [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z32.00013: Magnetic tunnel junctions with a ferroelectric barrier using epitaxial La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/(Ba, Sr)TiO$_{3}$/La$_{0.7}$Ca$_{0.3}$MnO$_{3 }$trilayers Shengming Guo, Ke Chen, Xiaoxing Xi, Qi Li, Yonggang Zhao We have fabricated multiferroic tunnel junctions using ferromagnetic La$_{0.7}$Ca$_{0.3}$MnO$_{3 }$as electrodes and ferroelectric (Ba, Sr)TiO$_{3}$ as the barrier. We have observed tunneling magnetoresistance as in a typical magnetic tunnel junction (MTJ). Since the ferroelectric barrier can be charge polarized in two opposite directions which alters tunneling conductance, we have observed large tunneling electroresistance ($\sim $ 50{\%}) when the charge polarization is switched. This has been observed for both magnetic parallel and antiparallel states. As a result, this type of junctions has four resistance states instead of two for a normal MTJ, corresponding to positive- and negative-polarized parallel and antiparallel states. The four states can be manipulated by the magnetic and electric fields. The dependence of the magnetoresistance and electroresistance as functions of magnetic field, electrical field, and bias voltage will be presented. [Preview Abstract] |
Session Z33: Fluctuation Phenomena
Sponsoring Units: DCMPChair: Ruslan Prozorov, Ames Laboratory
Room: 403
Friday, March 20, 2009 11:15AM - 11:27AM |
Z33.00001: Fluctuations of the superconducting order parameter as an origin of the Nernst effect Karen Michaeli, Alexander Finkel'stein We show that the strong Nernst signal observed recently in amorphous superconducting films far above the critical temperature is caused by the fluctuations of the superconducting order parameter. We demonstrate a striking agreement between our theoretical calculations and the experimental data at various temperatures and magnetic fields. Besides, the Nernst effect is interesting not only in the context of superconductivity. We discuss some subtle issues in the theoretical study of thermal phenomena that we have encountered while calculating the Nernst coefficient. In particular, we explain how the Nernst theorem (the third law of thermodynamics) imposes a strict constrain on the magnitude of the Nernst effect. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z33.00002: Interstate switching induced by non-Gaussian noise Lora Billings, Mark Dykman, Ira Schwartz We consider the rate of switching between stable states of a dynamical system driven by a non-Gaussian noise. The problem of the switching barrier is reduced to a variational problem of finding a mechanical action for an auxiliary noise-free system. The emphasis of our analysis is placed on the generic system-independent features of fluctuations induced by Poisson noise. If the system is overdamped, Poisson noise leads to switching only for a certain polarity of pulses. This is qualitatively different from the noise effect on underdamped systems. We study the transition between these types of behavior with varying damping. For systems close to a bifurcation point, the barrier height displays a scaling dependence on the control parameter and on the noise parameters. We study parametric dependence for generic types of bifurcations, such as saddle-node and pitchfork bifurcations. Analytical results are compared with the results of detailed numerical simulations. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z33.00003: Josephson current noise above Tc in superconducting tunnel junctions Alex Levchenko Tunnel junction between two superconductors is considered in the vicinity of the critical temperature. Superconductive fluctuations above Tc give rise to the noise of the ac Josephson current although the current itself is zero in average. As a result of fluctuations, current noise spectrum is peaked at the Josephson frequency, which may be considered as precursor of superconductivity in the normal state. Temperature dependence and shape of the Josephson current noise resonance line is studied for various junction configurations. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z33.00004: Superfluid density anisotropy as a probe of electronic nematic order in cuprate superconductors Benjamin Phillabaum, Yen Lee Loh, Erica Carlson, Karin Dahmen We propose that hysteretic effects in superfluid density anisotropy may be used as a probe of electronic nematic order in cuprate superconductors. Stripes, a unidirectional, nanoscale modulation of electronic charge, are strongly affected by quenched disorder in two-dimensional and quasi-two-dimensional systems. While stripe orientations tend to lock to ma jor lattice directions, dopant disorder locally breaks rotational symmetry. In a host crystal with otherwise C4 rotational symmetry, stripe orientations in the presence of quenched disorder map to the random field Ising model. We use simulations of the random field Ising model to generate ensembles of local stripe orientations, and then further simulate the effects of such a pattern on the superfluid density within the XY model. We find clear hysteretic effects in the superfluid density anisotropy as a function of applied orienting field. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z33.00005: Current-driven quantum switch M.V. Milosevic, A. Kanda, S. Hatsumi Hatsumi, F.M. Peeters, Y. Ootuka As a key component of a ballistic quantum switch proposed by Mel'nikov and Vinokur [Nature {\bf 415}, 60 (2002)], we realize the current-driven {\it giant-vortex splitting} in a mesoscopic superconducting square, in a given perpendicular magnetic field. We also demonstrate the controllable {\it current-induced transitions} between the states with different angular momenta, which provide the stepwise behavior of the up-down sample conductance as a function of applied current (not field). Theoretical simulations using time-dependent Ginzburg-Landau theory are fully corroborated by transport measurements, where vortex states are monitored using the small-tunnel-junction technique. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z33.00006: In search for the superconducting spin-switch: Magnetization induced resistance switching effects in La$_{0.67}$Sr$_{0.33}$MnO$_{3}$/Yba$_{2}$Cu$_{3}$O$_{7-\delta }$ bi- and trilayers Menno Veldhorst, Maarten van Zalk, Alexander Brinkman, Hans Hilgenkamp, Jan Aarts The influence of the magnetization on the superconducting $T_{c}$ in bi- and trilayers consisting of the half-metallic ferromagnet La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ and the high-temperature superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ is studied. Interfaces that are partly oriented within the crystallographic ab-plane are achieved by tilted epitaxial growth on SrTiO$_{3}$ (305) substrates. Sharp magnetization switching behavior is observed in the (305) oriented structures, due to the uniaxial magnetic anisotropy. At temperatures close to $T_{c}$, resistance jumps are induced by magnetization switching. Our results indicate that the switching behavior arises from magnetic stray fields from the ferromagnetic layers that penetrate into the superconductor, rather than spin-switch or spin-accumulation effects. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z33.00007: Theory of Low-Temperature Hall Effect in Stripe-Ordered Cuprates Jie Lin, Andrew Millis We investigate the effect of static anti-phase stripe order on the weak-field Hall effect of electrons with dispersion appropriate to the high T$_c$ cuprates. We first consider the cases where the magnitudes of the spin and charge stripe potentials are smaller than or of the same order as the bandwidth of electrons. In a model with only spin stripe potential, and at carrier concentrations appropriate to hole-doped cuprates, the calculated $R_H$ shows sign change as increasing the stripe potential, in semi-quantitative agreement with data. In a charge-stripe-potential-only model, $R_H$ increases as the charge stripe potential increases, with no sign change occurring. In a model with both stripe potentials, $R_H$ may be enhanced or may change sign. We also consider the case in which the magnitudes of the stripe potentials are much larger than the bandwidth, where analytical results can be obtained. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z33.00008: Emergence of Particle-Hole Symmetry near Optimal Doping in High-Temperature Copper Oxide Superconductors Shiladitya Chakraborty, Dimitrios Galanakis, Philip Phillips High-temperature copper oxide superconductors (cuprates) display unconventional physics when they are lightly doped whereas the standard theory of metals prevails in the opposite regime. For example, the thermoelectric power changes sign abruptly near optimal doping in a wide class of cuprates, a stark departure from the standard theory of metals in which the thermopower vanishes only when one electron exists per site. We show that this effect arises from proximity to a state in which particle-hole symmetry is dynamically generated. The operative mechanism is dynamical spectral weight transfer from states that lie at least 2eV away from the chemical potential. The emergence of this symmetry close to optimal doping points to pairing in the cuprates being driven by high-energy electronic states. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z33.00009: Exact two-band model of Cu-O planes with charge stripes and plaquettes Stellan \"Ostlund, Mats Granath A standard model of the Cu-O planes of high $T_c$ superconductors suggests the relevance of a three-band model of electrons with strong Coulomb repulsion. Particularly dominant is the copper site interaction and charge fluctuations are most strongly suppressed on these sites. We simplify this model further by {\it completely} suppressing the charge fluctuations on the copper sites and replacing the copper spins by a spin texture that couples to the local hopping. The resulting generic two-band model of electrons in is then studied for various spin textures on the copper atoms. For general values of the effective hopping parameters, the low energy eigenstates strongly favor charge stripe and plaquette ordering, with a complex multiply connected Fermi surface with the possibility of both pockets and open orbits coexisting. Particularly striking is the emergence of multiparticle ground states that are both delocalized and still effectively minimize nearest neighbor density correlations. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z33.00010: Two-order-parameter theory of magnetism and superconductivity Victor Galitski, Tudor Stanescu Starting with a microscopic interacting electron Hamiltonian, we derive a self-consistent two-order-parameter theory to describe a general case of competing or co-existing magnetic and superconducting instabilities. This is achieved by splitting the initial interaction in two different channels and weighting each channel with an auxiliary field with a non-linear constraint. The double Hubbard-Stratonovich transform leads to a model similar to that in gauge theories. We analyze the resulting theory and argue that generally the magnetic and superconducting fluctuations are equally important and should be treated on equal footing. We discuss the general criteria of the two transitions occurring at similar energy scales and discuss the relevance of these results to superconductivity in the heavy fermion compounds and possibly the cuprates. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z33.00011: Two band model for the cuprates Shiu Liu, Steven White We use a numerical canonical transformation approach to derive an effective two-band model for the hole-doped cuprates, which keeps both oxygen and copper orbitals but removes double occupancy from each. A similar model was considered previously by Frenkel, Gooding, Shraiman, and Siggia (PRB 41, number 1, page 350). We compare the numerically derived model with previously obtained analytical results. In addition to the usual hopping terms between oxygens $t_{pp}$ and Cu-Cu exchange terms $J_{dd}$, the model also includes a strong copper-oxygen exchange interaction $J_{pd}$ and a Kondo-like spin-flip oxygen-oxygen hopping term $K_{pdp}$. We use the density matrix renormalization group to study the charge, spin, and pairing properties of the derived model on ladder systems. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z33.00012: Classification of topological insulators and superconductors in three spatial dimensions Shinsei Ryu, Andreas Schnyder, Akira Furusaki, Andreas Ludwig We systematically study topological phases of insulators and superconductors (or superfluids) in 3D. We find that there exist 3D topologically non-trivial insulators or superconductors in five out of ten symmetry classes introduced in seminal work by Altland and Zirnbauer within the context of random matrix theory, more than a decade ago. One of these is the recently introduced $Z_2$ topological insulator in the symplectic (or spin-orbit) symmetry class. We show there exist precisely four more topological insulators. For these systems, all of which are time-reversal invariant in 3D, the space of insulating ground states satisfying certain discrete symmetry properties is partitioned into topological sectors that are separated by quantum phase transitions. Three of the above five topologically non-trivial phases can be realized as time-reversal invariant superconductors, and in these the different topological sectors are characterized by an integer winding number defined in momentum space. When such 3D topological insulators are terminated by a 2D surface, they support stable surface Dirac (Majorana) fermion modes. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z33.00013: Spin-Orbit Scattering and Quantum Metallicity in Ultra-Thin Be Films Philip Adams, Yimin Xiong, Amar Karki, David Young We compare and contrast the low temperature magnetotransport properties of ultra-thin, insulating, Be films with and without spin-orbit scattering (SOS). Beryllium films have very little intrinsic SOS, but by ``dusting'' them with sub-monolayer coverages of Au, one can introduce a well controlled SOS rate. Pure Be films with sheet resistance $R>R_Q$ exhibit a low-temperature negative magnetoresistance (MR) that saturates to the quantum resistance $R_Q=h/e^2$. This high-field {\it quantum metal} phase is believed to represent a new ground state of the system. In contrast, the corresponding negative MR in Be/Au films is greatly diminished, suggesting that, in the presence of strong SOS, the quantum metal phase can only be reached at field scales well beyond those typically available in a low temperature laboratory. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z33.00014: Quantum critical point and van Hove singularity in La$_{2-x-y}$Sr$_x$Nd$_y$CuO$_4$ Ben Mallett, Jeffery Tallon By means of Zn substitution, thermopower and magnetic measurements we locate, distinguish and track the evolution of the pseudogap critical point and the van Hove singularity as a function of Nd content and relate these to pressure dependent effects in the Nd-free compound. The results have generic implications for all HTS cuprates. [Preview Abstract] |
Session Z35: Focus Session: Iron Pnictides and Other Novel Superconductors XVII: Similar Materials: Selenides and Tellurides
Sponsoring Units: DMPChair: Jiangping Hu, Purdue University
Room: 405
Friday, March 20, 2009 11:15AM - 11:27AM |
Z35.00001: Poperties of iron selenide single crystals Cedomir Petrovic, Rongwei Hu Single crystals of FeSe were grown by molten metallic flux technique. Synchrotron powder X-ray diffraction confirms phase purity. Thermodynamic, magnetic and electrical transport properties will be presented. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z35.00002: Single crystal growth and anisotropy of magnetic and transport properties of FeTe and FeTe1-xSx superconductors Rongwei Hu, Cedomir Petrovic Single crystals of FeTe and FeTe1-xSx were grown by molten metallic flux technique. Synchrotron powder X-ray diffraction confirms phase purity. We will present anisotropy of magnetic and electrical transport properties in normal and superconducting state of these compounds. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z35.00003: New Fe-based superconductor S doped FeTe using nontoxic elements Yoshihiko Takano, Yoshikazu Mizuguchi Discovery of Fe-based new superconductor was received with considerable surprise. The Fe based-superconductor is expected to be a new series of high-Tc superconductors such as cuprate High-Tcs. However, one of the demerit of Fe based-superconductor is that it contains toxic element of As or Se. So, new Fe based- superconductors composed of nontoxic elements were required. Recently, we have discovered S doped FeTe as a new superconductor using nontoxic elemtns. Its superconductong transition temperature is around 10K and the upper critical fields is ~70T. This material has merit for superconducting applications. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z35.00004: Superconducting and magnetic properties of Fe-Se-Te compounds C.V. Tomy, G. Balakrishnan, M.R. Lees The discovery of a new Fe based superconductor $\alpha $-FeSe$_{x}$ with a T$_{c}$ of 8~K, hot on the heels of the discovery of superconductivity in LaOFeP/As compounds, has triggered a fresh interest in the study of Fe based superconductors. Se can be replaced with Te in FeSe$_{1-x}$Te$_{x}$ and this results in an increase in T$_{c}$ from 8~K for x = 0 to 15~K for x = 0.5 while compounds for x $>$ 0.8 are no longer superconducting. We report the synthesis and characterization of the compounds FeSe$_{1-x}$Te$_{x}$ covering the entire solid solution range. The superconducting transition in resistivity measurements does not show any broadening in magnetic fields up to 9~T, but shifts to lower temperatures linearly with a value $\sim $ -0.22~K/T. This results in extremely high upper critical fields (H$_{c2})$ of the order of 70-80~T in these compounds. The superconducting properties are also sensitive to applied pressure and exhibit a positive dT$_{c}$/dP of around 0.41~K/kbar for the x = 0.5 composition. We observe a jump in specific heat at T$_{c}$ corresponding to a superconducting gap of 1.8~meV, indicating the bulk nature of superconductivity. Detailed investigations through magnetization, transport and specific heat measurements are presented. A study of the magnetic properties of the non superconducting end compound, FeTe is also presented to gain insight into the onset of superconductivity in the doped systems. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z35.00005: High Pressure Structure and Transport Properties of the FeSe$_{0.88}$ Superconductor T. Wu, Z. Chen, T.A. Tyson, Z. Qin, T. Zhou, C. Zhang, S.-W. Cheong The structure of FeSe$_{0.88}$ was measured for pressures up to 7 GPa using diamond anvil cells in order to probe the changes in the lattice which coincide with change in the transport properties. These measurements are being complement by high pressure transport measurement over the same pressure range. The trends in structure and transport with pressure will be presented in order to under stand the origin of the strong pressure dependence of the superconducting transition temperature. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z35.00006: Superconductivity and antiferromagnetism in Fe(Se$_{1-x}$Te$_x$)$_{0.82}$ E.K. Vehstedt, M.H. Fang, B. Qian, T.J. Liu, L. Spinu, H.M. Pham, W. Bao, M.R. Fitzsimmons, M. Zhernenkov, Y. Qiu, Q. Huang, M.A. Green, P. Zajdel, J. Yang, Y. Liu, Z.Q. Mao The search for unconventional superconductors has been reenergized by the discovery of $T_c$ up to 56 K in FeAs type materials [1]. Revelation of the binary superconductor FeSe, $T_c \approx 8$ K [2], prompted our investigation of the phase diagram and the evolution of superconductivity and magnetism in the ternary Fe(Se$_{1-x}$Te$_x$)$_{0.82}$ ($0 \leq x \leq 1.0$) system. We discovered a new superconducting phase with $T_{c,max} = 14$ K for $0.3 < x < 1.0$. End member FeTe$_{0.82}$ is non-superconducting and exhibits incommensurate antiferromagnetic (AFM) order. The AFM order contains both linear and spiral components, propagating diagonally in the Fe tetragonal lattice, in contrast with the commensurate AFM order in FeAs-based superconductors. Superconductivity occurs when the long-range AFM order evolves into short-range correlations with the isovalent substitution of Se for Te. These findings strongly suggest that superconductivity in this system is associated with magnetic correlations, and thus may be unconventional in nature. \\ $\left[1\right]$ A. Cho, Science \textbf{320}, 870 (2008).\\ $\left[2\right]$ F. C. Hsu \textit{et al}., Proc. Natl. Acad. Sci. USA. \textbf{105}, 14262 (2008). [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z35.00007: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z35.00008: Growth and superconductivity of FeSe$_{x}$ crystals Zhili Xiao*, Umesh Patel*, Suhong Yu*, Helmut Claus, Vitalii Vlasko-Vlasov, Sevda Avci*, John Schlueter, Ulrich Welp, Wai-Kwong Kwok Stimulated by the recent discovery of high temperature superconductivity in ferrous pnictides, other iron-based planar compounds have been revisited to search for superconductivity. The most promising outcome of this effort is the discovery of superconductivity in \textit{ alpha}\textbf{ - }FeSe$_{x}$ which is less toxic but has a FeSe$_{4}$ tetrahedra planar crystal sublattice similar to that consisting FeAs$_{4}$ in the oxypnictides. Investigations on the superconductivity in FeSe$_{x}$ can shed light on the superconducting mechanism in oxypnictides. We report the growth of FeSe$_{x}$ crystals through a vapor self-transport approach. Both tetragonal and hexagon shaped FeSe$_{x}$ crystals with a lateral dimension of up to a few millimeters were obtained and their superconductivity was investigated with both magnetization and resistive measurements. We systematically explored the effect of synthesis parameters such as Fe/Se ratio, sintering temperature and cooling rate on the quality of the crystals. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z35.00009: Superconductivity and Antiferromagnetism In Fe(Te$_{1-x}$S$_x$)$_y$ System T.J. Liu, M.H. Fang, B. Qian, E.K. Vehstedt, J.H. Yang, H.M. Pham, L. Spinu, Z.Q. Mao The surprising discovery of superconductivity up to 56 K $\left[1\right]$ in FeAs based compounds has reinvigorated the search for unusual superconductors. The recently revealed FeSe superconductor $\left[2\right]$ has inspired the community to take a second look at other previously studied materials, such as FeTe. We have investigated properties of Fe(Te$_{1-x}$S$_{x}$)$_{y}$. Our results show that the solid solution of S in this system is limited, $< 30\%$. We observed superconductivity at $~ 9$ K in both polycrystalline samples Fe(Te$_{1-x}$S$_{x}$)$_{y}$ with $0 < x \leq 0.3$ and $0.86 \leq y \leq 1.1$, and single crystals with the composition Fe(Te$_{0.9}$S$_{0.1}$)$_{0.91}$. In addition, our results suggest that this superconducting phase coexists with antiferromagnetism and that the superconducting volume fraction depends on excess Fe at interstitial sites, and excess Fe suppresses superconductivity. This allows us a unique view into the important role of magnetic correlations in mediating superconducting pairing.\\ $\left[1\right]$ A. Cho, Science \textbf{320}, 870 (2008).\\ $\left[2\right]$ F. C. Hsu \textit{et al}., Proc. Natl. Acad. Sci. USA. \textbf{105}, 14262 (2008). [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z35.00010: Superconductivity of Iron Selenide Thin Films Yuefeng Nie, Erind Brahimi, Joseph Budnick, William Hines, Menka Jain, Barrett Wells Near stoichiometry FeSe films were successfully grown on MgO, SrTiO$_{3}$, and LaAlO$_{3}$ single crystal substrates using pulsed laser deposition (PLD). X-ray diffraction analysis showed that the FeSe films have a tetragonal structure on SrTiO$_{3}$ and LaAlO$_{3}$ substrates. A mixture of tetragonal and hexagonal structures was observed on MgO substrates due to the larger lattice constant misfit. The superconductivity of films exhibited a strong dependence on epitaxial strain and thickness. Thicker films ($\sim $ 100 nm and $\sim $ 200 nm) are fully relaxed and have a clear superconducting transition near that of the bulk FeSe. Thinner films ($\sim $ 50 nm) are strained. Films on nearly lattice-matched LaAlO$_{3}$ are superconducting, while films under tension on SrTiO$_{3}$ or MgO are metallic but not superconducting down to 5K. The onset temperature for superconductivity have a near linear magnetic field dependence with dH/dT = - 2.8 T/K for fields up to 9T. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z35.00011: Crystal growth and superconductivity of new Fe-Te base materials Genda Gu, Jinsheng Wen, Zhijun Xu, Z.W. Lin, Qiang Li, J.M. Tranquada A number of Fe-base superconducting materials with critical superconducting temperature up to 56K have been discovered recently. Because As and its oxide of the Fe-As base superconducting materials is poison, it is a serious safety issue for researchers to make the bulk materials. The new Fe-Te base superconducting materials with Se doping are less toxic and safe to handle. We have grown a number of the FeTe$_{1-x}$Se$_{x}$ single crystals (x = 0$\sim $0.5) by using a modified floating-zone growth technique. The effects of the growth condition and the composition of a feed rod on the single crystal growth of FeTe$_{1-x}$Se$_{x}$ have been studied by using a floating zone machine. The single crystals of the PbO-type tetragonal structure FeTe$_{1-y}$Fe$_{y}$ (y = 0.04 to 0.08) are not superconducting. When Se substitutes for Te in FeTe$_{1-x}$Se$_{x}$ single crystals, the superconducting transition temperature increases with increasing Se content. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z35.00012: Magnetic order and structural transition in BaFe$_2$As$_2$ and $\alpha$-Fe(Se,Te) Wei Bao, Yiming Qiu, Qingzheng Huang, Mark Green, P. Zajdel, Mike Fitzsimmons, M. Zhernenkov, Minhu Fang, B. Qian, E.K. Vehstedt, Jinhu Yang, Zhiqiang Mao, Jeff Lynn, Ying Chen, T. Wu, G. Wu, Xianhui Chen We report neutron diffraction determination of magnetic structure in BaFe$_2$As$_2$ [1] and $\alpha$-Fe(Se,Te) [2], the first in both the 122 and 11-type of the Fe-based materials. The former has the ($\pi,0$) and the later $\delta$($\pi,\pi$), $\delta= 0.346$-0.5, kind of in-plane propagation vector. In both systems, the magnetic transition is accompanied by a first-order structural transition which breaks the tetragonal symmetry of the paramagnetic phase. Antiferromagnetic wavevector as well as the moment direction were determined in relation to the associated structural distortion. The fundamentally different magnetic structures in the two types of systems would stimulate further research into the relation between the Fermi surface nesting and electronic correlation effect.\\ Reference: [1] Huang et al., arxiv:0806.2776 (Phys. Rev. Lett. accepted); [2] Bao et al., arxiv:0809.2058 [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z35.00013: Se-NMR study of superconductor FeSe under pressure Satoru Masaki, Hisashi Kotegawa, Hideki Tou, Yoshikazu Mizuguchi, Yoshihiko Takano Binary FeSe with $T_{c}$=8 K shows the simplest crystal structure in iron-based superconductors recently reported. Moreover, FeSe is reported that $T_{c}$ increases with applying pressure and the effect of pressure for $T_{c}$ is attracted. Thus the study on FeSe is quite important to investigate the role of iron-network in Fe-based layer. We have carried out Se-NMR measurements under pressure in order to clarify the symmetry of superconducting gap, the magnetic properties of normal state and the origin of pressure dependence of $T_{c}$ in FeSe. In the normal state, the nuclear spin-lattice relaxation rate 1/$T_{1}$ is in proportion to temperature (Korringa relation) at absent pressure. The Korringa relation was also observed under pressure, 0.7 GPa, However the observed value of 1/$T_{1}$ at 0.7 GPa is about twice as large as that at absent pressure. Since 1/($T_{1}T)$ is proportional to the square root of the density of state at Fermi energy $D(E_{F})$, this increase of 1/$T_{1}$ suggests the increase of $D(E_{F})$ by the applying pressure. In the superconductor, 1$/T_{1}$ abruptly decreases by superconducting transition and obeys the cube of $T$ from 7 K to 1.5 K at 2 T. the reduction of 1/$T_{1}$ was also observed below 10K under pressure. The increase of $T_{c}$ by applying pressure is conformed from microscopic viewpoint. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z35.00014: Density Functional Study of ThCr$_{2}$Si$_{2}$-type Ternary Iron Chalcogenides: TlFe$_{2}$X$_{2}$ (X = S, Se, Te) Lijun Zhang, David Singh We report density functional calculations of electronic structure, Fermi surface, and magnetism for ternary iron chalcogenides TlFe$_{2}$X$_{2}$ (X = S, Se, Te). TlFe$_{2}$S$_ {2}$ and TlFe$_{2}$Se$_{2}$ were experimentally synthesized with the ThCr$_{2}$Si$_{2}$-structure, which consist of alternated Fe-X and electron-doping Tl layers, very similar to ternary BaFe$_{2}$As$_{2}$ system. As in all the Fe-based superconductors, the electronic structure near the Fermi level is dominated by Fe 3{d} states, with a pseudogap. Tl occurs with valence Tl${^+}$, and thus provides heavy electron-doping with 0.5 additional carrier per Fe relative to Fe-X layers. This pushes the Fermi level to the upper edge of the pseudogap and results in disappearance of hole cylinders of Fermi surface at zone center. As expected, the spin density wave instability is completely suppressed and the checkboard antiferromagnetism becomes the favored magnetic order. This over-doped system may be helpful in elucidating the magnetic order, superconducting mechanism, and spin pseudogap behavior in Fe-based materials. Tl deficiency is predicted to reinstate the part of hole Fermi surface and again induce spin fluctuations corresponding to the spin density wave, which are essential for pairing states in Fe- based superconductors. [Preview Abstract] |
Session Z37: Mesoscopic Systems, Clusters, and Nanoscale Systems II
Sponsoring Units: DCPChair: Shiv Khanna, Virginia Commonwealth University
Room: 409
Friday, March 20, 2009 11:15AM - 11:27AM |
Z37.00001: Al$_{n}$Bi Clusters: A Transition from Aromatic to Jellium Stability Penee Clayborne, J.U. Reveles, S.N. Khanna, C.E. Jones, Jr., U. Gupta, J. Melko, A.W. Castleman, Jr. Through a synergetic effort using density functional theory and negative ion photodetachment studies we have investigated multiple aluminum-bismuth clusters. Our studies show that Al$_{3}$Bi and Al$_{5}$Bi are two very stable clusters. Further investigation of their electronic structure reveal that their stability can be explained using the aromatic and Jellium models, respectively. The Al$_{3}$Bi cluster has a large ionization potential of 7.1 eV, a low electron affinity of 1.4 eV and a HOMO-LUMO gap of 1.7 eV. The molecular orbitals of the cluster are reminiscent of an aromatic system and the cluster has a nucleus independent chemical shift (NICS) value of -32.19 ppm confirming its aromatic character. The Al$_{5}$Bi cluster has a HOMO-LUMO gap of 1.2 eV and a large ionization potential of 6.5 eV. The compact structure of Al$_{5}$Bi has 20 electrons, which indicates that this cluster's stability could be accounted for by the Jellium model. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z37.00002: Hydrogen Promoted Oxygen Activation by Free Gold Cluster Cations Robert N. Barnett, Bokwon Yoon, Uzi Landman, Sandra M. Lang, Thorsten M. Bernhardt In this contribution we present experiments and first-principles density functional theory calculations on gas-phase reaction of small gold clusters, aiming at elucidation of the role of hydrogen in the activation of molecular oxygen for the selective oxidation of hydrocarbons. Positively charged gold clusters. Au$_{4}^{+}$ and Au$_{6}^{+}$. were chosen because electronic factors and experimental data suggest them to be most suitable for promoting the oxidation of unsaturated hydrocarbons. Our investigations show that, although small gas phase gold cluster cations are inert toward molecular oxygen, the pre-adsorption of molecular hydrogen cooperatively activates the adsorption of O$_{2}$ on Au$_{4}^{+}$ and Au$_{6}^{+}$. Temperature and reaction time dependent investigations in an octopole ion trap under multi-collision conditions reveal that hydrogen promotes the activation and dissociation of molecular oxygen on the gold clusters at temperatures as low as 200 K. The detailed mechanism of the hydrogen induced oxygen activation, involving an intermediate hydro-peroxy-complex is revealed by the DFT calculations. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z37.00003: Large electric dipole moments and even-odd effects observed in Bi clusters. John Bowlan, Anthony Liang, Walter de Heer The electric and magnetic response of Bi$_N$ (N=1-40) produced in a cryogenic molecular beam (15K-30K) shows that these clusters have large electric dipole moments as previously observed in ferroelectric Nb, V, and Ta clusters. The electric deflection profiles are measured as a function of size, field, and source temperature. Bi clusters exhibit the characteristic even-odd oscillations with even N showing an enhanced response. The clusters are paramagnetic but the odd-N clusters have enhanced susceptibilities. These results will be discussed and compared with other work done on low-dimensional bismuth nanostructures. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z37.00004: Copper nanocluster growth at experimental conditions using temperature accelerated dynamics C. S. Dias, A. C. Cadilhe, A. F. Voter We study the dynamics of vapor phase cluster growth near experimental conditions of pressure at temperatures below $200$K. To this end, we carried out temperature accelerated dynamics (TAD) simulations at different vapor pressures to characterize the morphology of the resulting nanoparticles, which leads to a range of values of the flux of impinging atoms at fixed vapor temperature. At typical experimental pressures of $10^{-3}-10^{-4}\mbox{ bar}$ TAD provides substantial boost over regular Molecular Dynamics (MD). TAD is also advantageous over MD, regarding the sampling of the network of visited states, which provides a deeper understanding of the evolution of the system. We characterize the growth of such clusters at different vapor pressures. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z37.00005: Photoabsorption by Volume Plasmons in Metal Clusters Chunrong Yin, Chunlei Xia, Vitaly Kresin Metal clusters exhibit strong photoabsorption resonances in the visible part of the spectrum. These are collective ``surface plasmon'' excitations which have been extensively investigated. However, the UV part of the spectrum has remained rather unexplored. Some theoretical calculations have predicted that a measurable portion of the delocalized valence electrons' dipole oscillator strength should be located in this region. This predicted absorption feature has been ascribed to a volume-plasmon type of excitation, which in small particles can couple to light, in contradistinction to the situation in bulk metals. We have carried out a photodepletion cross section measurement on a pair of prototypical simple-metal nanoclusters, Na$_{20}$ and Na$_{92}$, finding that these systems indeed possess a broad volume-plasmon absorption peak centered at $\approx $4 eV and having an oscillator strength contents of $\approx $15-20{\%} of the total, in good agreement with theoretical calculations. These spectra provide the first experimental confirmation of the existence of optically active volume-type collective electronic excitations in metal nanocluster particles. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z37.00006: Atomistic Dipole Moments and Polarizabilities of Na$_{N}$ Clusters, N= 2-30, 38, and 55. Koblar Jackson, Li Ma, Mingli Yang, Julius Jellinek The response of Na$_{N}$ clusters, N = 2 - 30, 38, and 55 to a static external electric field is studied using a new method that decomposes the total cluster dipole moment and polarizability into contributions from non-overlapping atomic volumes (Jackson et al., J. Chem. Phys. \textbf{129}, 144309 (2008)). The atomic dipole moments and polarizabilities are in turn partitioned into local dipole and charge-transfer components, corresponding to dielectric and metallic responses, respectively. Analysis of the atomic polarizabilities indicates a strong dependence on the location of the atoms within the clusters and shows directly the effect of electrostatic screening in the clusters. We show that the relative importance of the charge-transfer component of the cluster polarizability increases with cluster size and approaches the bulk-limit on a per-atom basis for clusters as small as 20 atoms. The charge-transfer component is shown to be responsible for the structure/shape driven variations, and for shape-related anisotropies, in the cluster polarizabilities. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z37.00007: Magnetic Properties of Arsenic Cluster Assemblies Meichun Qian, A. C. Reber, S. N. Khanna, A. Sen, S. Mandal, N. K. Chaki Clusters have the potential to serve as building blocks of materials, enabling the tailoring of materials with novel properties. Recently, we synthesized a magnetic cluster assembled material using the As$_{7}$ cluster and cryptated K, which are the elements from main group. X-ray studies show that the As cluster is distorted to accompany two cryptated K. We have investigated the magnetic properties of As cluster assembly using density functional theory. We found the As$_{7}$ cluster has the (-2) valence state and possess one unpaired electron. The magnetic state is stabilized in the crystal and has a lower energy of 90 meV than the nonmagnetic state. The magnetic moments are located on the As$_{7}$ cluster and display antiferromagnetic order in the crystal. We also study the magnetic properties of the As cluster assemblies with transition metal as a linker, and these findings are possible to provide a new kind of magnetic materials. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z37.00008: Interpreting the magnetic and electric deflections of free metal clusters in molecular beams Anthony Liang, John Bowlan, Walter de Heer A short review of the analysis of magnetic and electric deflection data will be presented. Electric and magnetic beam deflection of several metal clusters, including Au$_{N}$, Al$_{N}$, Nb$_{N}$O, Na$_{N}$, Ho$_{N}$, Co$_{N}$, Rh$_{N}$ at various temperatures (20K-300K) and field strengths will be presented. The relation between the broadening of the deflected beam and the polarization distribution of the ensemble assuming a classical polar spherical rigid rotor model is demonstrated. Complicating factors including asymmetry effects and residual temperature effects will be discussed. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z37.00009: Comparative study of metal clusters by Quantum Monte Carlo method Yuning Wu, Hai-Ping Cheng, Paul Kent Lithium and sodium clusters have been studied by fixed-node diffusion quantum Monte Carlo method. This stochastic wave-function-based approach can provide more accurate results and serve as benchmarks against which other techniques may be compared. We studied the binding energies and investigate different geometries to decide the ground state. Our results are compared with those derived from other method such as DFT and CI methods. Our objective is to validate current quantum Monte Carlo methods for small metal clusters that undergo size dependent geometrical transitions. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z37.00010: Electronic Structure of Bi$_{3}$Ga$_{y}^-$ Semiconductor Clusters and the Special Stability of Bi$_{3}$Ga$_{2}^{-}$ - A Gas Phase Zintl Analogue Jose Ulises Reveles, Ujjwal Gupta, Joshua J. Melko, Shiv N. Khanna, A. W. Castleman, Jr. Here we present evidence that the gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO gap) can be tuned (1.12eV-1.89eV) by changing the Ga composition of Bi$_{3}$Ga$_{y}$ neutral and anionic clusters, some of which display special stability. Collaboratively, mass spectrometry, photoelectron spectroscopy and computational results show that Bi$_{3}$Ga$_{2}^{-}$ is a very stable cluster with a large calculated HOMO-LUMO gap of 1.89 eV, and can be viewed as a gas phase Zintl analogue of Sn$_{5}^{2-}$, already synthesized in the solution phase. The stability of Bi$_{3}$Ga$_{2}^{-}$ is further attributed to the fact that it has 12 valence electrons and possesses a closo structure in agreement with Wade's rules. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z37.00011: Simulation of the Adsorption on Nano/Micro-Cantilever Sensors Padet Khosathit, Phillip Choi, P.-Y. Ben Jar Recent advances in nano/micro fabrication techniques have led to the development of biosensors utilizing nano or micro-sized cantilevers. Through chemical coating of probe molecules that exhibit strong affinity to the target molecules, the cantilever would deflect when the target molecules bind with the probe molecules. Previous simulation studies on these systems often involved the use of a multi-scale approach in which molecular models are used for the probe and target molecules while the cantilever is modeled as a continuum solid beam using surface tension and strain energy. In this work, we use a molecular approach to describe the entire system. In particular, the solid beam is modeled as a collection of molecules connected by lattice springs. The intermolecular interactions between three types of molecules (i.e., probe, target and cantilever molecules) are modeled using Lennard-Jones potentials. Our results show that the cantilever deflection depend on the combination of the Lennard-Jones parameters as well as the number and positions of the probe and target molecules. Details of their effects will be presented in the paper. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z37.00012: Origin of Structural Stability in Cubic ZrO2 Nanocrystals Studied by EXAFS Y.L. Soo, S.L. Chang, C.L. Cheung, R. Sabirianov, F. Namavar, W.N. Mei, P. Chu, J.F. Lee Local environments surrounding Zr nanocrystalline powders and thin films of cubic zirconia prepared by sol-gel and ion beam assisted deposition (IBAD) methods were investigated by using extended x-ray absorption fine structure (EXAFS) technique. These materials have shown cubic long-range-order structure and high hardness without chemical stabilizers. To understand the origin of structural stability, the short-range-order local structural information obtained from EXAFS measurements is of central importance. Powder samples with different nanoparticle sizes prepared by different sol-gel processes were analyzed. Zr k-edge EXAFS, as well as N K-edge x-ray absorption near-edge structures (XANES), will also be presented to demonstrate the evolution of O vacancies and possible N impurities due to thermal annealing in the IBAD deposited films. [Preview Abstract] |
Session Z38: Surfaces, Interfaces, and Colloids II
Sponsoring Units: DCPChair: Sunil Sainis, Rowland Institute, Harvard University
Room: 410
Friday, March 20, 2009 11:15AM - 11:27AM |
Z38.00001: Understanding surface energies of transition metals with density-functional theory Aloysius Soon, Martin Fuchs, Matthias Scheffler Determining index-specific surface energies of metals is, to date, still a non-trivial task, both experimentally and theoretically. Density-functional theory (DFT) calculations within the local-density approximation (LDA) for exchange-correlation (XC) have provided understanding of qualitative trends. Yet, absolute surface energies, in particular of $d$-metals still exhibit significant uncertainties related to the description of XC: gradient-corrected functionals (GGA) which improve over the LDA for other properties often predict less accurate surface energies. This calls for a careful analysis of XC effects on surface energies, including non-local exchange and/or correlation. Here we analyze the surface energies of $4d$-metals with modern GGA functionals (PBEsol, AM05, developed to better describe bulk solids and (jellium) surfaces than the LDA and previous GGAs), using the all-electron FHI-aims code [1]. Relating the bulk cohesive energy and surface energy via a bond-cutting model we find modern GGAs can indeed correct the poorer results of the usual PBE-GGA but worsen the bulk cohesive energies of $4d$-metals. In addition, we consider hybrid XC functionals, using a cluster correction scheme [2], and discuss the effects of including exact exchange on the calculated surface energies.\\[0pt] [1] \textit{http://www.fhi-berlin.mpg.de/aims/};\\[0pt] [2] Q.-M. Hu \textit{et al.}, Phys. Rev. Lett. \textbf{98}, 176103 (2007); \textbf{99}, 169903(E). [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z38.00002: The initial stages of NaCl dissolution: Ion or ion pair solvation? Jiri Klimes, Angelos Michaelides The interaction of water with rock salt (NaCl) is important in a wide variety of natural processes and human activities. A lot is known about NaCl dissolution at the macroscopic level but we do not yet have a detailed atomic scale picture of how salt crystals dissolve. Here we report an extensive series of density functional theory, forcefield and molecular dynamics studies of water clusters at flat and defective NaCl surfaces and NaCl clusters. The focus is on answering seemingly elementary questions such as how many water molecules are needed before it becomes favorable to extract an ion or a pair of ions from the crystal or the cluster. It turns out, however, that the answers to these questions are not so straightforward: below a certain number of water molecules ($\sim$ 12) solvation of individual ions is less costly and above this number solvation of ion pairs is favored. These results reveal a hitherto unknown complexity in the NaCl dissolution process born out of a subtle interplay between water-water and water-ion interactions. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z38.00003: Accelerated Molecular Dynamics of Temperature-Programmed Desorption Kristen Fichthorn, Kelly Becker, Maria Mignogna The most widely used experimental method for quantifying thermal desorption is temperature-programmed desorption (TPD). Despite its extensive use, interpretation of this experiment can still be controversial. A significant difficulty with interpreting TPD is that this macroscopic experiment offers a limited picture of the underlying microscopic kinetic events. In this work, we use accelerated molecular-dynamics to simulate TPD of $n$-pentane from the basal plane of graphite, in the first atomistic simulations to probe TPD over laboratory time scales. Although the simulated TPD spectra agree with experiment, a detailed analysis reveals underlying kinetic phenomena that contrast the standard experimental interpretation and opens new possibilities for understanding molecular kinetics at solid surfaces. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z38.00004: Charge Interactions of Unilamellar Vesicles in Aqueous Suspensions Seongmin Park, Joseph Junio, Mahn-Won Kim, H.D. Ou-Yang This project reports the results of an experimental optical trapping study of the charge interactions between phosphor-lipid unilamellar vesicles. A 1064nm laser coupled into a high NA objective lens provided the optical trap. Using fluorescently labeled vesicles, we were able to monitor the particle number density by using a 532nm excitation beam aligned to be parfocal with the trapping beam through the same objective. Fluorescent signals from the focal region common to both beams were band-passed to a pinhole for confocal detection. Using the number density of the vesicles in the focal spot as a function of trapping intensity and a force balance model, we were able to calculate the effective trapping energy per vesicle as well as the osmotic virial coefficients for a system of lipid vesicles prepared with DOPG, cholesterol, and DiI. We measured the compressibility of these vesicle suspensions as a function of surface charge and ionic strength of the suspending medium. Compared to conventional scattering methods, this optical trapping method is advantageous, since it can be used for concentrated suspensions, yielding an in situ measurement of colloidal interactions. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z38.00005: Probing colloidal physics on the nanometer length scale Sunil Sainis, Frank Vollmer The sharp spectral features associated with ultra-high Q microresonator modes are sensitive to changes in the local environment and surface of the resonator [1]. Microresonator cavities have been used to detect the binding of single molecules [2] and viruses in an aqueous medium. We report on recent experiments that use microresonators to access colloidal physics on the nanometer length scale. We examine shifts in the resonator as a function of bulk ionic strengths and surface adsorption in a colloid. \\[3pt] [1] S. Arnold et al., Nature Methods {\bf 5}, 591 - 596 (2008)\\[0pt] [2] A. M. Armani, et al. Science {\bf 317}, 783-787 (2007). [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z38.00006: Effect of boundary condition on the viscosity of olefins: A molecular dynamics study Ling Ti Kong, Colin Denniston, Martin Muser, Yue Qi The viscosity of hexene was examined by means of non-equilibrium molecular dynamics simulations under different wall-liquid boundary conditions, namely over-smooth wall, more or less realistic wall, and over-adhesive wall. It is found that the wall-liquid interaction plays an important role in the ordering/layering of liquid, and consequently affects the behavior of olefins upon different normal pressures. With the same moving speeds of walls, the shear-rate (the slope of velocity profile) in the liquid is found to decrease with the increasing of normal pressure under over-smooth wall condition, while it is found to increase under the over-adhesive wall condition. The viscosity, in turn, shows a linear dependence on the normal pressure under over-smooth condition while exhibits an exponential dependence under the over-adhesive wall condition. The underlying mechanism of these observations will be presented and discussed in this talk. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z38.00007: Suppressing and Amplifying Depletion Attractions between Surfaces Roughened by Asperities Kun Zhao, Thomas Mason Motivated by recent experiments on roughness-controlled depletion attractions, we study the effect of roughness on depletion attractions between flat surfaces decorated using hemispherical and hemispheroidal asperities. Our calculations show that the depletion attraction between rough surfaces can be either dramatically reduced or amplified depending on the details of the surface morphologies. This model also explains the observed self-assembly of rough Janus platelets into dimers and provides quantitative predictions of roughness-controlled depletion attractions for conditions that have not yet been explored experimentally. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z38.00008: The Dynamics of Charging of Muscovite Mica: Measurement and Modeling Paul J. Sides, Danish Faruqui, Andrew J. Gellman The advent of a new method for measuring the zeta potential of planar surfaces, the rotating disk, allowed the investigation of the charging process of mica after immersion in water. The zeta potential of freshly-cleaved muscovite mica was recorded within seconds of immersion of the sample and in fractions of a second thereafter. The zeta potential of mica in water at pH = 5.6 with no added potassium changed by 40 -- 50 mV over approximately a minute. A model of adsorption and desorption of potassium ions and protons captured this behavior and provided a framework for determination of surface reaction rate constants. The charging of mica in alkaline KCl solutions of arbitrary concentration, however, was too fast for observation. The equilibrium zeta potential depended on the logarithm of salt concentration, in agreement with a model based on ion exchange reactions. The average values of the proton adsorption, proton desorption, potassium adsorption, and potassium desorption rate coefficients were 45 liter/s $\pm $ 15, 0.0014/s $\pm $ 0.0006, 58 liter/s $\pm $ 5, and 0.14/s $\pm $ 0.03, respectively. Web Page: http://zetaspin.com [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z38.00009: Effects of Embedded Dipoles on the Electrical Response of Self Assembled Monolayers P.P. Zhang, O.M. Cabarcos, T.A. Daniel, P.S. Weiss, D.L. Allara There has been recent interest in the use of polar molecules assembled at electrodes for tuning work functions and engineering charge injection barriers in organic electronic devices. With this in mind we have been investigating the electrostatic properties of simple model systems prepared from self-assembled alkanethiolate monolayers on Au{\{}111{\}} with the incorporation of an embedded ester moiety [-(CO$_{2})$- = E] in the adsorbate molecules. The intrinsic static dipole moment of the ester moiety of $\sim $1 Debye magnitude leads to the formation of a strong, highly organized, planar electric dipole layer in the SAM. From our previous X-ray photoelectron spectroscopy data we observe a consistent shift of the C 1s photoelectron kinetic energies between the top and bottom alkyl segments, defined as -(CH$_{2})_{m}-$E-(CH$_{2})_{n}$CH$_{3}$, regardless of the relative lengths $m$ and $n$. This shift correlates well with the value of the electrostatic potential across the E layer. Our recent surface potential AFM measurements, however, reveal an apparently anomalous strong dependence of surface potential on the sizes and ratios of $m$ and $n$, in contrast to the constant electrostatic potential observed in XPS measurements. Mechanisms underlying these effects will be discussed, with possible implications for the electrostatic behavior of more complicated organic and biological systems. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z38.00010: Interfacial Composition of the Ionic Aqueous Solution Studied by the Adsorption of the Cationic Molecules Jinsuk Song, Mahn Kim Knowing the interfacial composition of the ionic aqueous solution is important not only for understanding many atmospheric and environmental chemistry processes$^{1}$ but also for understanding many biological processes because the interaction between biomaterials happens often at the interfacial region such as water-vesicle interface in ionic aqueous solution. In this study, the surface anion density is estimated by measuring the surface density and adsorption angle of the cationic molecule, Malachite Green(MG) adsorbed at the air-ionic aqueous solution interface using the second harmonic generation technique. The anion number density at the interface increases with the increasing concentration of the ions and with the increasing size of the anions for spherical ions. It is consistent with other experimental measurements and simulation results$^{2,3}$. However, it seems that the anion density depends not only on the anion but also on the cation and shape and chemical composition of the ions. $^{1}$ E. Knipping et al., Science \textbf{288}, 301 (2000) $^{2}$ S. Ghosal et al., Science \textbf{307}, 563 (2005) $^{3}$ P. Jungwirth et al., J. Phys. Chem. B \textbf{105}, 10468 (2001) [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z38.00011: Coverage Dependent X-ray Photoemission on Halogenated Benzene on Graphite Keisuke Fukutani, Ning Wu, Peter Dowben We studied the adsorption of isomers of halogenated benzene on graphite. We found difference in the behavior of three different symmetry types, (1,2), (1,3), and (1,4), of diiodobenzene (C$_{6}$H$_{4}$I$_{2}$) and 1,4-bromoiodobenzene (C$_{6}$H$_{4}$IBr) adsorbed on graphite surface at 95K by X-ray photoemission spectroscopy. Although the molecules are expected to be similar in their electronic structure, the sticking coefficients and the strength of screening effects are considerably different for the different isomers. We find evidence for different intermolecular interactions both in the initial state and in the final state as well. Symmetry, not simply the the chemical constituents, play a role in adsorbate chemistry. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z38.00012: Retardation of ice crystallization by short peptides Jun Soo Kim, Arun Yethiraj The effect of short peptides on the growth of ice crystals is studied using molecular dynamics simulations. The simulations focus on two sequences (Gly-Pro-Ala-Gly and Gly-Gly-Ala-Gly) that are found in collagen hydrolysate, a substance that is known to retard crystal growth. In the absence of peptides, the growth of ice crystal in the solution with the ice/water interface is observed in at a rate comparable to the experimental data. When peptides are present in the liquid phase, the crystal growth is retarded to a significant extent compared to the pure water. It is found that Gly-Pro-Ala-Gly is more effective (crystallization is up to 5 times slower than in its absence) than Gly-Gly-Ala-Gly (up to 3 times slower) implying that the role of the proline residue is important. The mechanism can be understood in the nature of binding of the peptides to the growing crystal. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z38.00013: AC Electrowetting of Polymer Aqueous Drops on Parallel Electrodes Lu Zhang, Nishant Chetwani, Hsueh-Chia Chang, Yingxi Elaine Zhu We have recently observed the strong field dependence of AC-electrowetting of simple electrolyte aqueous drops on parallel gold electrodes, yet the detailed dynamic process of AC-field induced surface wetting remains unclear. In this work, we use fluorescence labeled DNA aqueous solution as a model system to directly visualize the wetting process of aqueous drops under varied AC electric fields by using combined fluorescence microscopy and contact angle goniometer. The electrowetting behavior of DNA aqueous drops is observed at AC-field frequency greater than the reciprocal of the RC time scale for electrode screening. And the onset of AC electrowetting is accompanied by the observed oscillation in drop contour shape and contact line. In addition, the ejection of nanodrops from the parent aqueous drop is observed when the threshold AC-field amplitude is exceeded. A scaling theory based on electrode interfacial screening is developed to quantify the AC-electrowetting behavior with the dependence of AC-field frequency, strength and medium conductivity. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z38.00014: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 2:03PM - 2:15PM |
Z38.00015: Diverging Tolman's length and asymmetric interfacial density profiles in near-critical fluids Mikhail Anisimov, Heather St. Pierre, Deepa Subramanian The surface tension of a curved surface behaves differently than that of a planar surface; however, the curvature correction to the surface tension - known as Tolman's length - is commonly ignored in practice. We show that asymmetric fluid phase equilibria result in diverging Tolman's length at the critical point with the amplitude of the divergence depending on the degree of asymmetry in fluid phase coexistence. The divergence of Tolman's length originates from the diverging critical fluctuations and does not exist in mean-field theories. However, the amplitude of this divergence (``intrinsic asymmetry'') can be obtained from an appropriate mean-field model. The asymmetry in phase equilibria is especially pronounced in ionic fluids and polymer solutions. In particular, Tolman's length in polymer solutions may become as large as the thickness of the interface, thus playing a significant role in behavior of micro droplets and confined polymer fluids. [Preview Abstract] |
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