Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session D1: Induced Superconductivity in Carbon Nanotubes and Graphene
Sponsoring Units: DCMPChair: Chun Ning (Jeanie) Lau, University of California, Riverside
Room: Colorado Convention Center Four Seasons 2-3
Monday, March 5, 2007 2:30PM - 3:06PM |
D1.00001: Induced Superconductivity in Nanowires and Nanotubes Invited Speaker: We study experimentally electron transport in 1 dimensional semiconductor nanowires (consisting of InAs and InP combinations) and carbon nanotubes. The wires are connected to superconducting source-drain contacts with gate electrodes in the substrate or on the surface. In the regime of weak coupling to the contacts we observe Coulomb blockade effects. We present level spectroscopy including a determination of the spin states. In the regime of strong coupling to the contacts interference effects are observed. In this regime and using superconducting contacts, we find supercurrents flowing through InAs-nanowires over micrometer length scales. The critical current is tunable by gate voltage, thus realizing so-called JOFETs (Josephson FETs) [1]. When we define quantum dots in between superconducting contacts the direction of the supercurrent is determined by the single electron spin state in the quantum dot [2,3]. \newline \newline \textbf{1.} Yong-Joo Doh, Jorden A. van Dam, Aarnoud L. Roest, Erik P. A. M. Bakkers, Leo P. Kouwenhoven, and Silvano De Franceschi, \textit{Tunable supercurrent through semiconductor nanowires, }Science \textbf{309}, 272-275 (2005) \newline \textbf{2.} P. Jarillo-Herrero, J.A. van Dam and L.P. Kouwenhoven, \textit{Quantum supercurrent transistors in carbon nanotubes, }Nature 439, 953-956 (2006) \newline \textbf{3.} Jorden A. Van Dam, Yuli V. Nazarov, Erik P.A.M. Bakkers, Silvano De Franceschi and Leo P. Kouwenhoven, \textit{Supercurrent reversal in quantum dots, }Nature \textbf{442}, 667-670 (2006) [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D1.00002: Andreev reflection in graphene Invited Speaker: Relativity and superconductivity have no common ground in ordinary matter, because the velocity of electrons is only a small fraction of the velocity of light. The unusual band structure of a single layer of carbon atoms (graphene) contains negatively and positively charged particles that move as relativistic electrons and positrons. The electron-like particles in the conduction band can be converted into positron-like particles in the valence band when they are reflected by a superconductor. (The missing charge of $2e$ enters the superconductor as a Cooper pair.) This {\em inter}band reflection process can be distinguished from the usual {\em intra}band Andreev reflection, because the reflection angle has the opposite sign. A new phenomenology of graphene--superconductor junctions is predicted, including an anomalous scaling of the supercurrent with the length of the junction and the existence of charge-neutral modes propagating along the interface. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 4:18PM |
D1.00003: Superconducting Nanotube Dots Invited Speaker: In this talk, I will focus on charge transport in carbon nanotube devices with superconducting source and drain contacts in the finite-bias non-equilibrium transport regime. As contact material, bi-layers of Au and Al were used and transport has been studied at temperatures in the 0.1 K range. Because carbon nanotubes are quantum dots (qdots), we in fact explore the physics of qdots with superconducting contacts, something which only recently became possible thanks to carbon nanotubes and most recently to semiconducting nanowires. In my talk, I will first summarize our pioneering work on multiwalled carbon nanotubes in which we could demonstrate proximity induced effects both in the weak and the strong coupling regime. In the latter an intriguing interplay between superconductivity and Kondo physics appears. Then, I will discuss the physics of multiple Andreev reflection in a situation when only one resonant state is present and compare this with experimental results. Finally, I will compare our early results with our recent measurements on single-wall carbon nanotubes. This work has been supported by the Swiss Institute on Nanoscience, the Swiss National Science Foundation, EU projects DIENOW and HYSWITCH. I gratefully acknowledge contribution of the following persons to this work (in alphabetic order): B. Babic, W. Belzig, C. Bruder, M. R. Buitelaar, J.-C. Cuevas, A. Eichler, L. Forro, J. Gobrecht, M. Gr\"{a}ber, M. Iqbal, T. Kontos, A. Levy Yeyati, A. Martin-Rodero, T. Nussbaumer, S. Oberholzer, C. Strunk, H. Scharf, J. Trbovic, E. Vecino, M. Weiss [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:54PM |
D1.00004: Josephson junctions with tuneable single wall carbon nanotubes as barriers Invited Speaker: Weak superconductivity induced in a barrier between two stronger superconductors constitutes the essence of a Josephson junction. The original prediction of Josephson assumed a thin potential barrier much higher than the energy gap of the superconductors on either side. The Josephson junction does, however, not require a potential barrier, a metallic barrier may be a sufficient to reduce the penetration of Cooper pairs. The barrier can also be of geometric origin as for superconducting microbridges smaller than the coherence length. In the last two situations the Cooper pair Josephson transmission is most conveniently pictured as a multiple Andreev reflection process at the barrier/superconductor interfaces. We have studied Josephson junctions, where the barrier region is a single wall carbon nanotube (CNT) with a long mean free path. The junction between the 1-dimensional CNT and the superconductor often constitutes an additional thin potential barrier, I. The Josephson junction has therefore the structure S/I/CNT/I/S. The CNT has a capacitance, C, to the external electrodes, which typically yields a Coulomb blockade energy larger than the energy gap of the superconductors and the finite length of the CNT (smaller than it's mean free path) gives rise to Fabry-Perot eigenenergies. The Coulomb blockade is only strongly developed if the resistance R of the barrier, I, is large (R$>$h/e$^{2})$. If R$\sim $h/4e$^{2}$ we have an almost adiabatic transmission, where the Josephson critical current may approach the quantized value 2$\Delta$e/$\hbar$. In the region where R$\sim $h/2e$^{2}$ Josephson tunnelling can be observed simultaneously with charge quantization and Kondo resonance tunneling. In this regime we study gate imposed even-odd electron number effect leading to characteristic variation of the supercurrent exhibiting an alternation between 0- and $\pi \quad -$Josephson junctions. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:30PM |
D1.00005: Gate-controlled superconductivity in diffusive multiwalled carbon nanotube Invited Speaker: We have investigated electrical transport in a diffusive, PECVD-grown multiwalled carbon nanotube contacted using superconducting leads made of Al/Ti sandwich structure. We find proximity-induced superconductivity with measured critical currents up to $I_{cm} = 1.3$ nA, tunable by gate voltage. The supercurrent branch displays a finite zero bias resistance which varies as $R_0 \propto I_{cm}^{-\alpha}$ with $\alpha =0.74$. We discuss the interpretation of these findings in terms of the RCSJ-model as well as the diffusive junction model for long SNS structures. In addition, we will compare the results with our recent data on proximity-induced supercurrents in singlewalled carbon nanotubes. [Preview Abstract] |
Session D2: Ion Traps for Scalable Quantum Computation
Sponsoring Units: GQIChair: Lorenza Viola, Dartmouth College
Room: Colorado Convention Center Four Seasons 4
Monday, March 5, 2007 2:30PM - 3:06PM |
D2.00001: Architectural Design Issues for Reliable Trapped Ion Quantum Computers Invited Speaker: Central to the design of large-scale quantum computers is the fact that reliable quantum systems must explicitly and actively deal with relatively large component failure rates during operation. A quantum computing system must thus attain reliability by encoding operations with unreliable components such that faults can be detected and corrected, despite occasional failures induced by ubiquitous quantum noise. Accomplishing this requires more than just high gate fidelities, however, as two recent interesting results highlight. First, universal quantum computation on standard stabilizer quantum codes is impossible using only transversal gates, those which limit error propagation; it turns out non-transversal steps such as teleportation are necessary. Second, the overall reliability of trapped ion and other quantum computing schemes will ultimately be limited not by gate or measurement fidelities, but very likely, by the fidelity of movement and waiting operations, due to the necessity of non-transversal gates. These results are presented, together with implications for architectural design requirements. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D2.00002: Quantum information processing and quantum-limited metrology using trapped ions at NIST. Invited Speaker: With the use of atomic ions confined in a multi-zone array, we implement simple quantum algorithms and study the problems in scaling such a device to tens of qubits [1]. Current work is devoted to better control of classical parameters such as laser intensity, suppression of heating from ambient fluctuating electric fields, and studying limitations caused by more fundamental sources of decoherence, such as spontaneous emission. Along with other groups, we are studying ways to increase the number of trap zones; in particular, we concentrate on a surface-electrode multi-zone geometry. Although a general purpose quantum computer appears to be a distant goal, simple applications of quantum information processing methods enable new techniques for spectroscopy and efficient quantum detection. [1] Current research in collaboration with D. Leibfried, J. Amini, J. C. Bergquist, R. B. Blakestad, J. J. Bollinger, J. Britton, K. Brown, R. J. Epstein, D. B. Hume, W. M. Itano, J. D. Jost, E. Knill, C. Langer, R. Ozeri, T. Rosenband, S. Seidelin, N. Shiga, and J. H. Wesenberg. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 4:18PM |
D2.00003: Ion Trap Quantum Networks Invited Speaker: Trapped atomic ions are among the most promising candidates for a future quantum information processor, with each ion storing a single quantum bit (qubit) of information.~ Trapped ion qubits enjoy an unrivaled level of quantum coherence, and small numbers of ions can be entangled through a suitable interaction with optical fields.~ The next generation experiments will transport and distribute trapped ion qubits to generate truly large-scale entangled quantum states.~ Several approaches for networking trapped ion qubits will be discussed, along with state-of-the-art experimental progress.~ This includes the use of phonons between ions in a Coulomb crystal, the physical shuttling of ions throughout complex and microfabricated ion trap structures, the coupling of remotely-located ions through a photonic coupling, and perhaps even the use of a cold (neutral) atomic gas. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:54PM |
D2.00004: Scalable Designs for Planar Ion Trap Arrays Invited Speaker: Recent progress in quantum operations with trapped ion qubits has been spectacular for qubit counts up to approximately ten ions. Two qubit quantum gates, quantum error correction, simple quantum algorithms and entanglement of up to 8 qubits have been demonstrated by groups including those at NIST, University of Michigan, University of Innsbruck and Oxford. Interesting problems in quantum information processing including quantum simulations of condensed matter systems and quantum repeaters for long distance quantum communication systems require hundreds or thousands of qubits. Initial designs for an ion trap ``Quantum CCD'' using spatially multiplexed planar ion traps\footnote{D. Kielpinski, C. Monroe, and D.J. Wineland, ``Architecture for a large-scale ion-trap quantum computer,'' Nature, Vol.417, pp.709--711, (2002).} as well as initial experiments\footnote{S. Seidelin, J. Chiaverini, R. Reicle, J. J. Bollinger, D. Leibfried, J. Briton, J. H. Wesenberg, R. B. Blakestad, R. J. Epstein, D. B. Hume, J. D. Jost, C. Langer, R. Ozeri, N. Shiga, and D. J. Wineland, ``Amicrofabricated surface-electrode ion trap for scalable quantum informtion processing,'' quant-ph/0601173, (2006).} using planar ion traps are promising routes to scaling up the number of trapped ions to more interesting levels. We describe designs\footnote{J. Kim, S. Pau, Z. Ma, H.R. McLellan, J.V. Gates, A. Kornblit, and R.E. Slusher, ``System design for large-scale ion trap quantum information processor,'' Quantum Inf. Comput., Vol 5, pp 515--537, (2005).} for planar ion traps fabricated using silicon VLSI techniques. This approach allows the control voltages required for the moving and positioning the ions in the array to be connected vertically through the silicon substrate to underlying CMOS electronics. We have developed techniques that allow the ion trap structures to be fabricated monolithically on top of the CMOS electronics. The planar traps have much weaker trapping depths than the more conventional multi-level traps. However, the trap depths are still adequate for trapping hot ions from many ion sources. The planar traps also involve more complex configurations for laser cooling and micromotion control. Initial solutions to these problems will be presented. Laser access to the ions can be provided by laser beams grazing the trap surface or by using vertical slots through the trap chip. We will also discuss limits imposed by power dissipation and ion transport through trap junctions (e.g. crosses and Ys). We have fabricated these VLSI based traps in a number of configurations. Initial fabrication and packaging challenges will be discussed. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:30PM |
D2.00005: Quantum Simulations in Ion Traps Invited Speaker: When Richard Feynman famously proposed a quantum computer, his intended application was to simulate quantum dynamical systems. This is a hard problem because as the number of elements of a quantum system linearly increases, the complexity of the equations modeling it grows exponentially. Feynman's proposed solution to this problem was to simulate one quantum mechanical system with another. Such quantum simulators can solve only a limited set of problems, but building one would represent an important milestone in the road to universal quantum computation.~ At LANL we use an array of strontium ions confined in a linear rf trap to build a multi-body quantum simulator. Each ion simulates a single spin system, while Coulomb and optical forces simulate spin-spin interactions and magnetic fields. This system can simulate the most basic models of condensed matter physics, the Ising model and the Heisenberg XY model, in addition to more complex physical systems. We have modeled the basic interactions in this system and are starting to demonstrate the interactions central to the simulations. [Preview Abstract] |
Session D3: DMP Prize Symposium
Sponsoring Units: DMPChair: David Vanderbilt, Rutgers University
Room: Colorado Convention Center Korbel 2A-3A
Monday, March 5, 2007 2:30PM - 3:06PM |
D3.00001: Organic-based Magnets - New Materials for New Physics Invited Speaker: Organic-based materials exhibiting the technologically important property of bulk magnetism, including ferro-, ferri-, and metamagnetism, have been prepared. These magnets are prepared via conventional organic chemistry methodologies, but unlike classical inorganic-based magnets do not require metallurgical processing, and are frequently soluble in conventional solvents (e.g., toluene, dichloromethane, acetonitrile, THF). They have saturation magnetizations that in some cases exceed twice that of iron metal on a mole basis as well as have coercive fields exceeding that of Co5Sm. Also magnets with critical temperatures exceeding room temperature have been prepared. In addition to an overview of the observed magnetic behaviors, numerous examples of magnets made from molecules will be discussed. These will include [M(III)(C5Me5)2][A], [Mn(III)(porphyrin)][A] (A = cyanocarbon etc. electron acceptors) as well as M[TCNE]x (M = V, Mn, Fe, Co, Ni) which for M = V is a room temperature magnet, which can be fabricated as a thin film magnet. Another new class of magnets of [Ru(II/III)2(O2CR)4]3[M(III)(CN)6] (M = Cr, Fe; R = Me, t-Bu) composition will also be discussed. This broad new family of materials have examples that exhibit most of known magnetic phenomena observed for classical inorganic transition/rare earth metal based magnets, as well as some new, unexpected phenomena and combinations of properties not previously reported. The design of examples of these organic-based magnets will be discussed setting the stage for an overview of their unusual magnetic properties and new physics that will be discussed by Arthur J. Epstein. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D3.00002: Predictable and New Physics and Potential for Applications of Organic-based Magnets Invited Speaker: As discussed by Joel S. Miller in the previous talk, magnets utilizing organic groups with essential spin have been reported since the mid-1980's. Though initial organic-based magnets had magnetic ordering temperatures (T$_{c}$'s) below 5K, organic-based magnets now have T$_{c}$'s to above 400K. In addition to magnetic phenomena already known for conventional transition metal and rare earth magnets, organic-based magnets feature unique phenomena enabled by the shape and internal electronic structure of the organic molecules. Examples are illustrated with experimental results for magnets based on tetracyanethylene, [TCNE], which as an anion has spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $. For example, chains with spin containing molecules having relatively strong exchange within a chain and weak dipolar interaction with neighboring chains can have an unusual fractal ground state with unusual dynamics leading to `coercive fields' approaching 3 tesla. In contrast to conventional magnets, the internal electronic structure of the molecules that make up a molecule-based magnet can be excited by light of the appropriate wavelength. This leads to changes of the spin state of the molecule and/or changes in the exchange interaction between molecules, opening up the concept of reversible light control of magnetism. Examples will be given from the M$^{++}$[TCNE]$^{-}_{x}$ (x$\sim $2) (M = Mn, V) materials systems. Finally, we explore the new phenomena enabled by V$^{++}$[TCNE]$^{-}_{x}$ (x$\sim $2), a material with T$_{c}$ up to 400K and for which films may be prepared using low temperature CVD. It is a semiconductor (room temperature resistivity and activation energy similar to silicon) and magnetization M(H,T) and coercive field are controlled by chemical composition. Magnetoresistance to 32 tesla supports that V[TCNE]$_{2}$ is a ``half-semiconductor'' with fully spin polarized valence and conduction bands of interest for spintronics applications. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 4:18PM |
D3.00003: A Brief History of the Harris Criterion Invited Speaker: In this talk I will briefly review the ``Harris criterion,'' which was given in a 1974 paper in J. Phys. C. This criterion indicates whether the critical exponents of a system at a phase transition are modified by the presence of locally random impurities. To frame the discussion and since the argument for the criterion is so simple, I will repeat its derivation here. Since some of those who quote the paper may not have actually read it in detail, I will discuss some of the applications given there to systems with randomness which have longer-range correlations and I will emphasize those aspects which are perhaps less well-known. Also, with the benefit of hindsight, I will slightly reinterpret some of the conclusions of the 1974 paper. To further put this work in context, I will discuss how the renormalization group indicated that this argument indeed captured the essential role of local randomneess. Later work on longer range models fits in nicely with the criterion. Finally, I will briefly mention experimental studies of this criterion. Perhaps the appropriate general conclusion from all of this is that a sound qualitative argument can have an honored place along side technically exact solutions. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:54PM |
D3.00004: Opportunities in Nanomagnetism Invited Speaker: This talk addresses the challenges and scientific problems in the emerging area of nanomagnetism. [1] Included are fabrication strategies, and experiments that explore new spin-related behavior in metallic systems, as well as efforts to understand the observed phenomena. As a subfield of nanoscience, nanomagnetism shares many of the same basic organizing principles, such as geometric confinement, physical proximity, and chemical self-organization. These principles are illustrated by means of examples drawn from the quests for ultra-strong permanent magnets, ultra-high-density magnetic recording media, and nanobiomagnetic sensing strategies. As a final example showing the synergetic relationship to other fields of science, the manipulation of viruses to fabricate magnetic nanoparticles is presented. \newline \newline [1] S. D. Bader, Rev. Mod. Phys. 78, 1 (2006). [Preview Abstract] |
Session D4: Polymer Crystallization: 50 years of Chain Folding
Sponsoring Units: DPOLYChair: Buck Crist, Northwestern University
Room: Colorado Convention Center Korbel 2B-3B
Monday, March 5, 2007 2:30PM - 3:06PM |
D4.00001: Fifty (Plus) Years of Polymer Nano-Science (Art) Invited Speaker: At least one dimension of the fundamental structure of all polymers, on the next hierarchical size scale larger than the repeat distance and unit cell, is on the order of 100 {\AA}; hence these days one uses the label ``nano.'' This includes the coil size in solution and melt, and the morphology of both crystalline and block polymers. For study of morphology, the principle techniques, until recently, have been transmission electron microscopy (TEM) and small angle x-ray diffraction, with polymer TEM being the ``art of producing interpretable artifacts.'' Having first been shown chain folded polyethylene single crystals almost exactly 51 years ago, we will summarize and represent some half century of morphology research, and the resulting art, including addressing the areas of nucleation, growth from solution and melt, and deformation of macromolecular materials. Particular emphasis will be placed on early observations with implications on various current crystallizable polymer morphology proposals including chain folding regularity, spherulite nucleation and growth, molecular mobility in the melt and thin film crystallization. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D4.00002: The Morphology of Crystallizable Polymers: Past and Present. Invited Speaker: A perspective will be presented of the evolution of current phenomenological knowledge and views regarding the crystallization habits and morphological characteristics of polymers since the early 1950's. By the mid-1950's the characteristically spherulitic crystallization of polymers from the molten state under quiescent conditions was well established. The origins of the orientation of the chains in the constituent radiating fine texture of spherulites preferentially normal to the radial direction remained obscure then. This matter was resolved as a consequence of the seminal studies reported in 1957 independently by P.H. Till Jr. [J. Polym. Sci., \underline {26}, 301 (1957)]; A. Keller, [Phil. Mag. \underline {2}, 1171 (1957), and E. W. Fischer [Z. Naturforch., \underline {12a}, 753 (1957)] on solution-grown polyethylene lamellar single crystals. Chain-folding, as proposed explicitly by Keller, resulting in typically lamellar polymer crystal habits, and the radiating lamellar texture of spherulites became generally accepted very shortly thereafter. Among the aspects which have received much attention in subsequent years are the details of chain-folding, the bulkiness of chain- folds, the nature of order/disorder at the fold surfaces in lamellae, the diversity in the lateral growth habits and 3-D conformations of lamellar crystals (hollow pyramidal, dished or bowl-shaped, scrolled, twisted),and the processes underlying the characteristic evolution of spherulites from transient precursor axialites/hedrites. Some of these aspects will be focused on in this summary which will also touch briefly on some current discussion regarding the nature of the nucleation of crystallization from the molten state, and the nature of the lateral propagation of lamellar growth from the molten state. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 4:18PM |
D4.00003: Insights provided by the build-up, structure and morphology of polymer single crystals Invited Speaker: Polymer single crystals have been essential in the discovery and widespread acceptance of the concept of chain folding and have been investigated in their own right for decades. Their impact on polymer science is however much wider, since they are essential tools for the analysis of growth mechanisms, of small scale (crystal structure) and large scale (morphology) levels of organization. Several illustrative examples are given. Single crystals have been used recently to determine the impact of secondary nucleation and extent of lateral spread in polymer crystal growth. They are invaluable in the elucidation of mechanically unstable crystal structures of polymers and biopolymers (frequently in combination with epitaxial crystallization). They help reveal otherwise out of reach details of the molecular arrangement or rearrangement in crystal structures: structural disorder, impact of chain folding on crystal structure symmetry, mechanisms of crystal-crystal transformation, etc. The lamellar shape often reveals the impact of chain folds, which can explain the three-dimensional architecture of spherulites produced in the bulk. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:54PM |
D4.00004: Laws controlling crystallization and melting in bulk polymers Invited Speaker: When the fundamentals of the structure of semi-crystalline polymers - layer-like crystallites with fold surfaces being embedded in an amorphous matrix - were revealed in the Fifties, considerations about the mechanism of formation started immediately. In the Sixties and Seventies, they became a major field of research and a focus of interest. In the years which followed the approach put forward by Hoffman, Lauritzen and their co-workers [1] gained superiority. The picture envisaged by the treatment - a crystalline lamella with an ordered fold surface and smooth lateral faces, growing layer by layer with a secondary nucleation as rate determining step - is easy to grasp and yields simple relationships. Supercooling below the equilibrium melting point $T_{\rm f}^{\infty}$ is the control parameter determining both the thickness $d_{\rm c}$ and the lateral growth rate of the crystallites $G$. Experiments carried out during the last decade provided new insights and are now completely changing the understanding. They showed in particular \\- that $d_{\rm c}$ is inversely proportional to the distance to a temperature $T_{\rm c}^{\infty}$ distinctly above $T_{\rm f}^{\infty}$\\- that the activation energy determining $G$ diverges at a temperature $T_{\rm zero}$ clearly below $T_{\rm f}^{\infty}$.\\ Further simple relationships concern\\ - recrystallization processes: $d_{\rm c}$ is again inversely proportional to the distance to $T_{\rm c}^{\infty}$\\ - the extension of ordered regions within the lamellar crystallites: it is proportional to $d_{\rm c}$. We interpret the observations as indication that the pathway followed in the growth of polymer crystallites includes an intermediate phase of mesomorphic character. A thin layer with mesomorphic inner structure forms between the lateral crystal face and the melt, stabilized by epitaxial forces. The first step in the growth process is an attachment of chain sequences from the melt onto the growth face of the mesomorphic layer. The high mobility of the chains in the layer allows a spontaneous thickening, up to a critical thickness, where the layer solidifies under formation of block-like crystallites. The last step is a perfectioning of the crystallites, leading to a further stabilization. We constructed a thermodynamic scheme dealing with the transitions between melt, mesomorphic layers and lamellar crystallites, assuming for the latter ones that they exist both in an initial \lq native\rq~and a final \lq stabilized\rq~form. $T_{\rm c}^{\infty}$ and $T_{\rm zero}$ are identified with the temperatures $T_{\rm mc}$ and $T_{\rm am}$ of the (hidden) transitions mesomorphic $\rightarrow $ crystalline and amorphous$\rightarrow $ mesomorphic, respectively. Application of the scheme in a quantitative evaluation of small angle X-ray scattering and calorimetric results yields the equilibrium transition temperatures between the various phases, latent heats of transition and surface free energies [2]. [1] J.D Hoffman, G.T Davis, and J.I. Lauritzen. \newblock In {\em Treatise on Solid State Chemistry {\rm Vol.3, N.B.Hannay Ed.}}, page 497. Plenum, 1976. [2] G.~Strobl.\newblock {\em Eur.Phys.J.E}, 18:295, 2005. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:30PM |
D4.00005: Growth kinetics and morphology of polymer crystals Invited Speaker: Originating from the nature of chain folding, polymer single crystals are quite unique in the growth kinetics and morphology. The developments of the understanding in the past 50 years are discussed and the unsolved important issues will be suggested. Polymer single crystals are thin lamellae with the thickness in the order of 10nm determined by the period of chain folding, which keeps a constant value for the isothermal crystallization. The growth of polymer single crystals is modeled by the kinetics of creation and annihilation of growth steps on a rectangular substrate with the pre-determined thickness. The growth face is therefore regarded as a one-dimensional substrate and the kinks and anti-kinks on the substrate correspond to the growth steps propagating in the opposite directions. The kinetic equations of those kinks proposed by Seto and Frank well describe the transition of growth regime as a crossover from single nucleation to multi-nucleation on the basis of the standard model of chain-folded polymer crystallization with surface nucleation proposed by Lauritzen and Hoffman. However, the analysis of the growth kinetics and morphology of single crystals having curved growth front suggests an unusual behavior of the step propagation velocity. The anomaly can be accounted for by a self-poisoning of the growth step interrupted by polymer chains with folding shorter than required. An entropic barrier of pinning proposed by Sadler and Gilmer is a possible candidate of the self-poisoning and is in accordance with recent computer simulation results suggesting the kinetics on a rugged free energy landscape having a resemblance to protein folding. Therefore, the quantitative evaluation of the kinetic barriers of surface nucleation and pinning has been an important issue. In addition, examination of the kinetics of melting will have valuable information because melting of a crystal must be free from nucleation but can still be limited by the entropic barrier. [Preview Abstract] |
Session D5: Pake Prize Symposium: Magnetic Storage and Applications
Sponsoring Units: FIAPChair: Stefan Zollner, Freescale Semiconductor
Room: Colorado Convention Center Korbel 1A-1B
Monday, March 5, 2007 2:30PM - 3:06PM |
D5.00001: Seventy Years of Magnetic Disk Drive Technology. Invited Speaker: The first hard disk drive, the IBM RAMAC, was shipped in September 1956. It was the size of a couple of refrigerators, contained fifty 24-inch diameter disks and stored information at an areal density of 2000 bits per square inch. Although ten years ago, the industry was widely perceived as facing a fundamental limit at 36 Gbit per square inch (Gbpsi) in the form of superparamagnetism, current disk drives provide areal densities in excess of 130 Gbpsi and capacities of 750 Gbytes. Although the original projections of superparamagnetism were correct, by changing the way the devices were scaled and, ultimately by changing from longitudinal to perpendicular recording, it has been possible to circumvent superparamagnetic effects. Our current understanding indicates that it may be possible to extend the areal density by yet another factor of 400 from present densities, if advanced technologies such as heat assisted magnetic recording and bit patterned media are implemented. Assuming this proceeds at the recent rate of 40 percent increase in areal density per year, we would reach roughly 50 Terabit per square inch (Tbpsi) in about 2026, 70 years after the development of the first disk drive. To achieve this, however, the industry will need higher sensitivity giant magnetoresistive sensors, high efficiency near-field transducers powered with surface plasmons and self-assembled or nano-imprinted magnetic particle arrays for media. In this presentation, the author will briefly describe the history of recording on magnetic disk drives, then describe the potential for future growth and some of the physics and materials problems that need solution in order to realize this full potential. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D5.00002: New Developments in Perpendicular Magnetic Recording Media Invited Speaker: Recording areal density in current drives is approaching 200 Gb/in$^{2}$. While compositions of individual layers do generally vary, the structure of the PMR media in today's drives is remarkably similar with synthetic antiferromagnetically coupled soft underlayer (SAF SUL) structures, Ru based nucleation layers and oxide segregated recording (storage) layers [e.g., 1,2]. As areal density increases and track width is correspondingly decreased, a serious writability problem develops limiting the usable coercivity. This limitation, combined with the need to decrease grain size to improve signal-to-noise ratio (SNR), imposes serious constrains in the design of improved PMR media. New media structures using dynamic tilting switching mechanisms such as exchange coupled composite media (ECC) [3] and exchange spring media [4] have been proposed in order to overcome these obstacles. In this paper we will discuss the merits and limitations of current PMR media structures and propose possible improvement direction for media applicable for $>$ 200 Gb/in$^{2}$ recording including results from our latest attempts to reduce exchange spring media and related conceptual structures into practice. [1] G. Choe, A. Roy, Z. Yang, B.R. Acharya, and E.N. ABarra, \textit{IEEE Trans. Magn}., vol. 42, no. 10, pp. 2327-2329, Oct. 2006 [2] U. Kwon, H.S. Jung, M. Kuo, E.M.T. Velu, S.S. Malhotra, W. Jiang, G. Bertero, R. Sinclair , \textit{IEEE Trans. Magn}., vol. 42, no. 10, pp. 2330-2332, Oct. 2006 [3] R. Victora and X. Shen, \textit{IEEE Trans. Magn}., vol. 41, no. 2, pp. 537-542, Feb. 2005 [4] D. Suess, T. Schrefl, M. Kirschner, G. Hrkac, F. Dorfbauer, O. Ertl, J. Fidler, \textit{IEEE Trans. Magn}., vol 41, no.10, pp. 3166-3168, Oct. 2005. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 4:18PM |
D5.00003: New Developments in MRAM Invited Speaker: Bringing a new technology from Research and Development to first commercialization requires overcoming numerous technical challenges while maintaining continuous business commitment. Magnetoresistive Random Access Memory (MRAM) is a recent example of this process. MRAM combines magnetic devices with standard silicon to obtain the combined attributes of non-volatility, high-speed operation, and unlimited read/write endurance not found in any other existing memory technology. The first successful commercialization of MRAM was enabled by the convergence of several solutions to underlying challenges in the technology. One of the keys to manufacturability was the invention of the Toggle Write mode that provides robust magnetic switching margin. For high-speed read, a key solution was the ability to deposit and pattern high-quality, high-TMR magnetic tunnel junctions with narrow bit-to-bit resistance variation, low defect density and long-term reliability. In this talk, I will present details of each of the above technology elements, the performance and reliability of the 4Mb product, and the outlook for extending and scaling MRAM to other markets and nodes. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:54PM |
D5.00004: High Anisotropy Magnetic Recording Media Invited Speaker: Areal densities in magnetic recording have exhibited Moore's Law like increases in the last ten years. This is partially due to improvements in the media microstructure where reduced grain sizes, tighter grain size distribution, and chemical isolation between grains to break exchange provided increased signal-to-noise from decreased transition noise. With the recent shift from longitudinal to perpendicular recording, areal densities have again continued to increase with demonstrations of over 250 Gbits/in$^{2}$. However, areal density is limited by thermal stability considerations where the ratio of stored magnetic energy K$_{u}$V (anisotropy energy times the magnetic switching volume) to the thermal energy kT must be $\sim $ 50-70. The projected limit for traditional CoPtCr(X) granular media is on the order of 500 Gbits/in$^{2}$. Further increases in the areal density will require greater reduction in the grain size (switching volume), which necessitates finding media with higher anisotropy to maintain thermal stability. Possible candidate materials systems include FePt and SmCo$_{5}$, which have bulk K$_{u}$ values 50 to 100 times greater than CoPtCr(X) media materials. High K$_{u}$ allows for thermally stable grains sizes down to $\sim $ 2.5 nm, which would permit areal densities in the Tbit/in$^{2}$ regime. Accompanying this increase in K$_{u}$ is an increase in the media switching field (H$_{0})$, which is proportional to the ratio K$_{u}$/M$_{s}$ where M$_{s}$ is the saturation magnetization. Therefore, while providing thermal stability, these high K$_{u}$ materials would potentially require writing fields greater than 50 kOe which far exceed those of available recording head materials. One possible solution is heat-assisted magnetic recording (HAMR) where a laser locally heats the media in order to reduce the coercivity so that available head fields are sufficient. Numerous challenges exist for HAMR including high cooling rates so that the heating process does not render adjacent bits thermally unstable. This paper will review recent progress in this area and concentrate on the challenges for the production of high anisotropy media for Tbit/in$^{2}$ areal densities, such as maintaining grain sizes of 2 to 4 nm with the correct crystallographic texture and sufficient grain isolation to break exchange. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:30PM |
D5.00005: Automotive Applications of Physics and Materials Research: Permanent Magnets, Thermoelectrics, and Hydrogen Storage Invited Speaker: Advances in the research and development of new materials continue to have a major impact on the automotive industry and on many other technologies today. A physics background in materials provides a research scientist with many opportunities to pursue a very satisfying and successful career doing industrial and applied physics, ranging from discovering and characterizing new and technologically useful materials to always learning something new in science. For example, the use of permanent magnets in the auto industry was revolutionized by the discovery at General Motors Research and Development Center of a new iron based magnetic material Nd$_{2}$Fe$_{14}$B. Moreover, this material rapidly became the state-of-the-art permanent magnet in a wide range of applications across many technologies beyond automotive. More recently, with growing pressure to improve efficiencies of automobiles and other energy consumers, thermoelectrics are a very exciting class of energy conversion materials that have seen dramatic advances in performance. General Motors is at the leading edge of research and development on automotive waste heat recovery using thermoelectric materials. Today, the very promising fuel cell future of the auto industry relies, in part, on advances in on-board alternative fuel storage. New materials and schemes for hydrogen storage that will help bring automotive fuel cell technology to fruition is a rapidly advancing field of physics and materials research. In this presentation, I will discuss my work in these areas during my career at General Motors Research and Development Center. [Preview Abstract] |
Session D6: Computational Challenges in Describing Mechanical Phenomena at the Nanoscale
Sponsoring Units: DCOMPChair: Jeff Grossman, University of California, Berkeley
Room: Colorado Convention Center 207
Monday, March 5, 2007 2:30PM - 3:06PM |
D6.00001: Energy Transfer and Resonance Enhancement at the Nanoscale. Invited Speaker: Classical molecular dynamics (MD) simulations of carbon nanotubes are used to elucidate important phenomena in the transfer of (lattice) vibrational energy between nanoscale objects. We study in particular the transfer of energy between specific vibrational modes. The calculations show efficient transfer of energy between modes that are in resonance and the time scale over which energy is transfered is set by the weak van der Waal's coupling between nanotubes. These observations provide the mechanistic basis for a new theoretical framework for describing energy transfer at the nanoscale. The insight gained from this theoretical picture is used to propose several novel nanomechanical devices with applications in chemical sensing and wireless communications that function by the exchange of vibrational energy. The operation and feasibility of these devices is demonstrated by further MD simulations. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D6.00002: Interfacial proximity effects on nanostructure thermal transport Invited Speaker: Thermal transport across interfaces is a key consideration in the development of advanced nanostructured materials for thermal management and thermoelectric energy conversion. Presented here is recent work from molecular dynamics simulation that illustrates how interfacial spacing can be tailored to modify thermal transport in such materials. Specifically, two examples are discussed: thermal transport between single wall carbon nanotubes and thermal conduction in superlattices. A four order of magnitude decrease in nanotube-nanotube thermal resistance is observed as nanotube spacing decreases, and a clear thermal conductivity minimum is observed in lattice matched superlattices. Mechanical phenomena emerging from the simulations include length dependence of nanotube Young's modulus and the importance of interfacial strain in maintaining coherent lattice waves in superlattices. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 4:18PM |
D6.00003: Heat flow at solid-liquid interfaces: confrontation between experiment and simulation Invited Speaker: Heat transport in nanostructures and nanostructured materials provides a novel paradigm for direct comparisons between the results of experiment and simulation. Time-resolved, pump-probe optical techniques enable measurements of the evolution of temperature on time scales from ps to ns. Our pump-probe experiments take two basic forms: measurements of heat transport across planar interfaces using time-domain thermoreflectance and measurements of heat flow from a metal or semiconductor nanostructure into its surroundings using transient absorption. The systems that we are studying are directly accessible to simulation by classical molecular dynamics on the same time and length scales that are encountered in the experiments. Working with our collaborators, P. Keblinski and his colleagues at RPI, we have made quantitative comparisons between experiment and simulation for heat transport from carbon nanotubes and fullerene molecules into a surrounding fluid; and heat transport across hydrophilic and hydrophobic interfaces with water. Any such comparison must take into account i) non-idealities in the experiments; ii) uncertainties in the potentials and atomic geometries in the computational model; and iii) the fact that classical simulations may include high frequency vibrational modes that are not thermally excited in the experiments. Despite the fact that transport at solid-liquid interfaces is more difficult to measure than more commonly studied solid-solid interfaces, we argue that solid-liquid interfaces provide a more reliable system for quantitative comparisons between experiment and simulation. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:54PM |
D6.00004: First-principles simulations of failure mechanisms, mechanical strength and electromechanical response Invited Speaker: Mechanical failure in response to external strain occurs at time scales that are usually much longer than those that are accessible to simulation. In particular, when processes leading to failure are highly activated, straight-forward molecular dynamics simulations will lead to qualitatively wrong results for both failure modes and mechanical strength. Nevertheless, judiciously chosen simulations can identify potential failure mechanisms, which can then be accurately mapped out and assessed, resulting in correct predictions of the initial modes of failure, breakage mechanisms, and strength. As an example, we will discuss simulations of the breakage of carbon nanotubes, which have been shown by first-principles calculations to be the strongest materials known. Very recently, the predictions of their stress-induced transformations and temperature-dependent failure have been spectacularly confirmed by experiments, which corroborated the key theoretical results. Nevertheless, the predicted ultimate strength is still substantially higher than the observed one, probably due to the presence of defects in as-grown samples. The simulations have also suggested avenues for forming nanotube-based electronic devices solely by mechanical transformations. In some molecular devices, e.g., those based on rotaxane, electric field and current-induced atomic transformations are the basis of device operation. For such structures, it is necessary to use open boundary conditions to account for current flow through the molecule and for the buildup of charge at the molecular interfaces. We will describe the key steps of this approach, based on linear-scaling non-equilibrium Green's function methodology, and its first applications. In collaboration with M. Buongiorno Nardelli, W. Lu, S. Wang and Q. Zhao. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:30PM |
D6.00005: Lattice thermal transport through atomically defined systems in a quantum mechanical description. Invited Speaker: There are different theoretical approaches to describe lattice thermal transport through nano-sized solid structures. From those approaches, atomistic calculations represent the smallest level of description, and provide a straight route towards fully understanding the phonon transport process across nanomaterials and interfaces. Within the atomistic descriptions themselves, there are several categories: 1-``classical,'' such as molecular dynamics, 2-``semi-classical,'' such as the Boltzmann-Peierls equation, and 3-``quantum-mechanical,'' such as Green's functions techniques. In this talk we will focus on quantum mechanical effects on nanoscale thermal transport, with specific examples in nanowires, nanotubes, and molecular junctions. Thus, we will discuss specific theoretical techniques from categories 2 and 3 above. We will start from the simplest of these approaches [1], which gives a good account of experimental measurements in semiconductor nanowires. Then we will discuss the more complex problem of thermal conduction in single walled carbon nanotubes, graphene, and graphite. We will see how the character of the 3-phonon scattering process in these systems results in long phonon mean free paths and thermal conductivities [2]. Subsequent experimental results have confirmed findings from the theoretical study [3]. Then, we will discuss a newer technique, based on non-equilibrium Green's functions, that allows to study the quantum mechanical many-body problem of interacting phonons flowing through generic, atomically described, anharmonic structures [4]. This technique is applied to investigate a simple model molecular junction. We will show some strictly quantum mechanical effects that take place in the anharmonic scattering process. Finally, we will present new results on first-principles calculations of phonon conduction across nitrogen impurities in carbon nanotubes [5]. \newline \newline [1] N. Mingo, Phys. Rev. B 68, 113308 (2003); N. Mingo and D. A. Broido, Phys. Rev. Lett. 93, 246106 (2004). \newline [2] N. Mingo and D. A. Broido, Nano Letters 5, 1221-1225 (2005); N. Mingo and D. A. Broido, Phys. Rev. Lett. 95, 096105 (2005). \newline [3] C. Yu, L. Shi, Z. Yao, D. Li, A. Majumdar, Nano. Lett., Vol. 5, 1842-1846 (2005); E. Pop, D. Mann, Q. Wang, K. E. Goodson and H. Dai, Nano Letters, 6, 96 (2006). \newline [4] N. Mingo, Phys. Rev. B, 74, 125402 (2006). \newline [5] N. Mingo, D. A. Stewart, D. A. Broido, and D. Srivastava, Nanoscale phonon transport from First-Principles (to be published). [Preview Abstract] |
Session D7: Signatures of Non-Abelian Quantum Hall States
Sponsoring Units: DCMPChair: Steven Simon, Lucent Technologies
Room: Colorado Convention Center Korbel 4A-4B
Monday, March 5, 2007 2:30PM - 3:06PM |
D7.00001: Non-Abelian quantum Hall states of fermions and bosons Invited Speaker: In a non-Abelian quantum Hall state, there are types of elementary excitations or quasiparticles that obey non-Abelian statistics. This is an extension of the idea of fractional statistics that means that when several of these quasiparticles are present in the system and are well-separated at well-defined positions, there is a degenerate space of lowest-energy states. When the quasiparticles are then exchanged adiabatically, the result is a matrix operation on this space of states. Greg Moore and the author$^1$ introduced this idea to condensed matter physics in 1991. They proposed a basic example called the Moore-Read Pfaffian state. The physics of the existence of the degenerate states for the quasiparticles in this system can be understood by viewing it as a $p_x-ip_y$ paired state of composite fermions, in which quasiparticles are $hc/2e$ vortices that carry Majorana fermion zero modes. This state is expected to be realized in the filling factor $\nu=5/2$ fractional quantum Hall (FQH) state. In later work, a series (labeled by an integer $k$) of ``parafermion'' states was proposed$^2$. This talk will review these ideas, and describe recent numerical work that strongly supports the idea that the $k=3$ member of the sequence occurs in the filling factor $12/5$ FQH state for electrons$^3$, and also$^4$ in a system of bosons, such as rotating cold atoms, at filling factor $3/2$. It will also describe recent analytical results$^5$ on the explicit quasihole trial wavefunctions of the parafermion states. \newline 1. G. Moore and N. Read, Nucl. Phys. {\bf B 360}, 362 (1991). \newline 2. N. Read and E. Rezayi, Phys. Rev. B {\bf 59}, 8084 (1999). \newline 3. E.H. Rezayi and N. Read, cond-mat/0608346. \newline 4. E.H. Rezayi, N. Read, and N.R. Cooper, Phys. Rev. Lett.{\bf 95}, 160404 (2005). \newline 5. N. Read, Phys. Rev. B {\bf 73}, 245334 (2006). [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D7.00002: Edge states and tunneling of non-Abelian quasiparticles in the nu=5/2 quantum Hall state and p+ip superconductors Invited Speaker: |
Monday, March 5, 2007 3:42PM - 4:18PM |
D7.00003: Current noise and AC conductivity as probes of non-abelian quasi-particles Invited Speaker: We consider two scenarios for probing signatures of non-abelian quasi-particles through transport and noise measurements in the $\nu=5/2$ fractional quantum Hall effect. In the first scenario we study bulk transport in the presence of a Wigner crystal of quasi-particles, which would form at filling factors close to $\nu=5/2$. For immobile quasi-particles, we find a mechanism for dissipative transport at frequencies below the gap, which is manifested in a nonzero conductivity in response to an electric field with finite wave vector ${\bf q}$ and frequency $\omega$, and reflects the exponential degeneracy of the ground state. The second scenario deals with noise measurements in a Hall bar geometry where two quantum point contacts (QPCs) introduce two interfering amplitudes for back-scattering. Thermal fluctuations of the number of quasi-particles enclosed between the two point contacts induce current noise of the telegraph type. The non-abelian $\nu=5/2$ state is characterized by a unique switching pattern of current, originating from the suppression and revival of the interference term as the parity of the number of quasi-particles between the two QPCs fluctuates. This work was done in collaboration with Ady Stern and Steve Simon. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:54PM |
D7.00004: Toplogical Quantum Compiling Invited Speaker: A quantum computer must be capable of manipulating quantum information while simultaneously protecting it from error and loss of quantum coherence due to coupling to the environment. Topological quantum computation (TQC) offers a particularly elegant way to achieve this. In TQC quantum information is stored in exotic states of matter which are intrinsically protected from decoherence, and quantum computation is carried out by dragging particle-like excitations (quasiparticles) around one another in two space dimensions. The resulting quasiparticle trajectories define world-lines in three-dimensional space-time, and the corresponding computation depends only on the topology of the braids formed by these world-lines. A variety of proposed fractional quantum Hall states are believed to possess quasiparticles that can be used for TQC -- among them the so-called ``Fibonacci anyons". These quasiparticles are conjectured to exist in the experimentally observed $\nu = 12/5$ fractional quantum Hall state. In this talk, I will review the basic ideas behind TQC, and describe our recent work showing explicitly how to translate (compile) arbitrary quantum algorithms into specific braiding patterns for Fibonacci anyons. (Work done in collaboration with N.E. Bonesteel, S.H. Simon, and G. Zikos.) [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:30PM |
D7.00005: Detecting fractional statistics with anyonic Mach-Zehnder interferometer Invited Speaker: Fractionally charged quasiparticles in the quantum Hall state with filling factor $\nu=5/2$ are expected to obey non-Abelian statistics. We demonstrate that their statistics can be probed by transport measurements in a recently fabricated device, an electronic Mach-Zehnder interferometer. The tunneling current through the interferometer exhibits a characteristic dependence on the magnetic flux and a non-analytic dependence on the tunneling amplitudes which can be controlled by gate voltages. In contrast to the case of Abelian statistics, the I-V curve is asymmetric. \vskip 3mm [1] K. T. Law, D. E. Feldman, and Y. Gefen, Phys. Rev. B 74, 045319 (2006). \vskip 1mm [2] D. E. Feldman and A. Kitaev, cond-mat/0607541. [Preview Abstract] |
Session D8: Superconductivity: STM of Cuprates
Sponsoring Units: DMPChair: Allen Goldman, University of Minnesota
Room: Colorado Convention Center Korbel 1C
Monday, March 5, 2007 2:30PM - 2:42PM |
D8.00001: Tunneling Spectroscopy on $c$-axis Y$_{1-x}$Ca$_x$Ba$_2$Cu$_3$O$_{7-\delta}$ Thin Films: Evidence for Multiband Superconductivity J.H. Ngai, W.A. Atkinson, J.Y.T. Wei We report scanning tunneling spectroscopy measurements on \{001\} oriented Y$_{1-x}$Ca$_x$Ba$_2$Cu$_3$O$_{7-\delta}$ thin films at \emph{x}= 0, 0.05, 0.15 and 0.20 Ca-doping at 4.2K. The tunneling spectra exhibit main-gap, sub-gap and satellite features which we analyze using a generic multiband tunneling model that accounts for the separate contributions of the plane and chain bands to the tunneling conductance spectrum. Our analysis indicates the sub-gap features could arise from the chain band density of states, while the satellite features could come from the plane band for a $d_{x^2-y^2}+s$ pairing symmetry. The doping dependent evolution indicates that all three spectral features are set by a single parameter $\Delta_0$, which monotonically decreases with Ca-doping, suggesting that superconductivity in Y$_{1-x}$Ca$_x$Ba$_2$Cu$_3$O$_{7-\delta}$ involves multiple bands. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D8.00002: Direct evidence for predominantly phonon-mediated pairing in high-temperature superconductors Guo-meng Zhao The spectra of the second derivative of tunneling current d$^{2}I$/d$V^{2}$ in the high-temperature superconductors YBa$_{2}$Cu$_{3}$O$_{7-delta}$ and Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ show clear dip and peak features due to strong coupling to the bosonic modes mediating electron pairing. The energies of all the peaks in $-$d$^{2}I$/d$V^{2}$-like spectra match precisely with the energies of the peaks in the phonon density of states obtained by inelastic neutron scattering. The results clearly demonstrate that the bosonic modes mediating the electron pairing are phonons and that high-temperature superconductivity arises primarily from strong coupling to multiple phonon modes. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D8.00003: Inelastic tunneling spectroscopic imaging study of electron-lattice interactions in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$. Kazuhiro Fujita, J. Lee, K. McElroy, J. Slezak, M. Wang, Y. Aiura, H. Bando, M. Ishikado, T. Masui, J. -X. Zhu, A. Balatsky, H. Eisaki, S. Uchida, J. C. Davis We investigated impact of oxygen isotope effect on d$^{2}$I/dV$^{2}$ spectroscopy on $^{16}$O and $^{18}$O substituted Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta ,}$respectively, with the same doping level (nearly optimally doping. $^{16}$O$\to ^{18}$O: Tc=89K$\to $88K), using the newly developed inelastic spectroscopic imaging technique (Jinho Lee \textit{et al.}, \textit{Nature} \textbf{422},546 (2006)) . We found that oxygen isotope effect ($^{16}$O$\to ^{18}$O) leads to reduction of mode energy from 52meV to 49meV, while superconducting gap remained unchanged. Oxygen isotope re-substitution shifted mode energy back to the original energy ($^{18}$O$\to ^{16}$O) as well as $T_{c}$ back to 89K, completing the series of isotope effect probed by STM/S. We concluded that renormalization effect seen in dI/dV spectra is caused by strong electron-lattice interaction from a mode near 52meV. We will also discuss about relevance of this lattice vibration mode as a pairing glue in the talk. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D8.00004: Atomic Scale Investigation of the Effects of Pb doping in Bi$_{2}$Sr$_{2}$CuO$_{6+x}$ W. D. Wise, Kamalesh Chatterjee, M. C. Boyer, Ming Yi, Takeshi Kondo, E. W. Hudson In the study of the Bi-based high temperature superconducting cuprates, an incommensurate structural ``supermodulation'' that runs throughout the crystal can often lead to experimental complications, especially for scattering studies. In order to eliminate this problem, a fraction of the Bi atoms are occasionally substituted by Pb, leading to a suppression of the supermodulation. As even a relatively large substitution has only a modest effect on transition temperature, it is widely believed that this suppression has little or no direct effect on superconductivity in the crystal. We will present the results of temperature dependent scanning tunneling microscopy studies of Pb doped Bi$_{2}$Sr$_{2}$CuO$_{6+x}$ (Bi-2201) which demonstrate local spectroscopic variations associated with Pb doping. Although the exact cause of these variations is still unknown, we hypothesize that a modulated strain field due to the presence of Pb may play a role. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D8.00005: Spectroscopic imaging STM study of Bi$_{2}$Sr$_{2-x}$Ln$_{x}$CuO$_{6+\delta }$ Jhinhwan Lee, K. Fujita, K. McElroy, Jinho Lee, J.W. Alldredge, M. Wang, S. Illani, H. Eisaki, S. Uchida, J.C. Davis We will present the spectroscopic imaging STM data of near optimally doped, Ln-substituted Bi2201. The Ln (Lanthanide) atoms substituting the Sr atoms are known to cause out-of-CuO$_{2}$-plane lattice disorder and large variation of Tc (Tc(La)$\sim $34K, Tc(Gd)$\sim $13K). We will demonstrate how the coherence peaks, the gap maps, the quasi-particle interference, and the checkerboard electronic structures are affected by lattice disorder and varied Tc in these materials. We will also show, for Gd-Bi2201, how the above atomic scale spectroscopic features change at temperatures below and above Tc. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D8.00006: $d^2I/dV^2$-STS measurements of isotope effect on the electron-lattice interaction(ELI) energy and the gap energy in Bi-2212. Jinho Lee, K. Fujita, M. Wang, K. McElroy, J. Slezak, J. -X. Zhu, A. V. Balatsky, H. Eisaki, S. Uchida, J. C. Davis We measured gap energy and the electron-lattice interaction(ELI) energy on the $O^{16}$ and $O^{18}$ substituted Bi-2212 single crystals using $d^2I/dV^2$-STS(Scanning Tunnelling Spectroscopy). Bi-2212 crystals were from the same batch, and nearly optimally doped after $O^{16}$ and $O^{18}$ annealing under the same conditions. $T_c$'s were around 88K in both crystals. While the ELI energy was shifted to $\sim 4mV$ lower energy in $O^{18}$ substituted Bi-2212, we found the gap energy of $\sim 38mV$ in both crystals virtually didn't change. We also extracted gap energies from the quasi-particle(QP) dispersions using QP interferences, and will determine weather this gap energy is altered by isotope substitution. The possible identity of the lattice mode will be addressed in the context of recent results from IR spectroscopy and ARPES. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D8.00007: Local pairing and the origin of pseudogaps in Bi$_{2}$Sr$_{2}$Ca$_{2}$CuO$_{8+\delta }$ probed with high-resolution STM measurements Kenjiro K. Gomes, Abhay Pasupathy, Aakash Pushp, Shimpei Ono, Yoichi Ando, Ali Yazdani The evolution of the local electronic density of states in Bi$_{2}$Sr$_{2}$Ca$_{2}$CuO$_{8+\delta }$ has been measured as function of doping (x=0.12-0.22) and temperature (20-180K) using state-of-art variable temperature scanning tunneling microscopy (STM). These measurements allow the first characterization of spatially varying energy gaps in both the superconducting and non-superconducting states with temperature. From these observations, we are able to demonstrate that pseudogaps observed at optimal doping at temperatures as high as 50K above T$_{c}$ are due to pairing correlations. We have been able to extract a remarkably universal relation that describes the strength of local pairing to a local pairing temperature, for samples which are weakly underdoped to highly overdoped. With decreasing doping, we show that the evolution of the local electronic states with temperature cannot be captured with a single pairing energy scale, signaling the presence of another phenomenon, which is unrelated or perhaps competing with superconductivity. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D8.00008: Spatially Inhomogeneous Collapse of Superconducting Gaps on the Nanoscale: Connection to Macroscopic Measurements on Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ Ali Yazdani, Abhay Pasupathy, Kenjiro Gomes, Aakash Pushp, Shimpei Ono, Yoichi Ando Using spatially resolved STM spectroscopy, we have mapped the superconducting correlations in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ to show that these correlations collapse in a spatially inhomogeneous manner with increasing temperature. These experiments provide valuable insight for understanding the results of spatially averaged measurements such as angle-resolved photoemission, vortex Nernst and field-induced diamagnetism on the same material system. The connection between nanoscale measurements and bulk parameters such as T$_{c}$ and T* is clarified. Finally, high-resolution STM measurements provide a method to characterize the processes by which superconductivity is destroyed with increasing temperature in samples with various hole doping levels. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D8.00009: Atomic-Position Tracking and Spectroscopy of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ across T$_{c}$ Abhay Pasupathy, Kenjiro Gomes, Aakash Pushp, Genda Gu, Shimpei Ono, Yoichi Ando, Ali Yazdani The inhomogeneous nature of the pairing in the Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ system makes the precise characterization of the changes in the density of states at the superconducting transition difficult to measure. Thermal drift in a typical STM prevents the tracking of a specific area while varying the temperature. We have used a specially designed STM, as well as a controlled electrical and acoustic environment, to track the position of a single atomic site on the surface as the temperature is varied. Using this technique, we have been able to measure, for the first time, how the onset of superconductivity changes the local tunneling spectra at a specific site. These measurements enable us to extract changes to the local density of states at T$_{c}$ independent of the normal state background. We will describe this technique and our measurements on the Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ system for different doping concentrations. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D8.00010: Real Space Mapping of Electronic States in Bi$_{2}$Sr$_{2-y}$La$_{y}$CuO$_{6}$ as a function of Temperature. Aakash Pushp, Abhay Pasupathy, Kenjiro K. Gomes, Shimpei Ono, Yoichi Ando, Ali Yazdani We have studied the single layer Bi$_{2}$Sr$_{2-y}$La$_{y}$CuO$_{6}$ system using high resolution scanning tunneling microscopy and spectroscopy at various temperatures. From these measurements, we have determined the evolution of the local density of states from the superconducting to the non-superconducting state. In this talk, we will describe the evolution of the gap and other spectroscopic features as a function of doping (y=0.2 to 0.9) and temperature (20-100K). Real space maps of the electronic states that show strongly modulated patterns will also be presented. These experimental results are used to determine the connection between the superconducting gaps and the pseudo-gaps in this compound. Finally, we will discuss the similarities and differences between the measurements on the single-layer Bi$_{2}$Sr$_{2-y}$La$_{y}$CuO$_{6}$ and the double-layer Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D8.00011: Superconducting Scanning Tunneling Microscope -- Josephson effect and High-T$_{C}$ superconducting cuprate Hikari Kimura, Robert Dynes, Shimpei Ono, Yoichi Ando We have developed and characterized superconducting scanning tunneling microscope (STM) tips that consist of Pb coated Pt/Ir wires. We have observed the thermally fluctuated Josephson effects between a conventional superconductor and this superconducting STM. STM-based Josephson junctions formed between the superconducting-STM tip and superconducting samples can be a powerful tool to detect both superconducting quasiparticles and the phase of the superconducting condensate via the Josephson effect on a length scale of nanometers. This technique is especially powerful when we study spatially inhomogeneous electronic systems such as High-T$_{C}$ superconducting cuprates. In this talk we present data of the STM Josephson junctions formed between S-STM tips and both Pb/Ag films and NbSe$_{2}$ single crystals. The former experiments give us the effective noise temperature T$_{n}$ and the impedance of the environment around the junction, Z$_{env}$. The latter is a precursor to the Pb/I/HT$_{C}$ cuprate Josephson junctions. We have derived the I$_{C}$R$_{N}$ product of NbSe$_{2}$ junctions using T$_{n}$ and Z$_{env}$ obtained as described above. Preliminary results of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ single crystals by S-STM tips are also discussed. This work is supported by DOE Grant No. FDDE-FG02-05ER46194. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D8.00012: STM Spectroscopy of Electron Doped Pr$_{1-x}$LaCe$_{x}$CuO$_{4-\delta }$ Francis Niestemski, Shankar Kunwar, Vidya Madhavan We present high resolution scanning tunneling spectroscopy (STS) of electron doped Pr$_{1-x}$LaCe$_{x}$CuO$_{4-\delta }$. We focus on x=0.12 doping (T$_{c} \quad \sim $ 25K ) at various temperatures at and above 2 K. Our data reveal both small and large gap behavior in the same sample. The magnitude of the smallest measured gap is consistent with superconductive gaps observed by other probes. The larger gaps are of varying magnitudes ranging from 10meV to 100meV or greater. We will discuss our observations in light of experimental data on similar PLCCO samples from ARPES and neutron scattering. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D8.00013: Atomic Resolution Scanning Tunneling Microscopy of Electron-Doped Pr$_{1-x}$LaCe$_{x}$CuO$_{4-\delta }$ Shankar Kunwar, Francis Niestemski, Vidya Madhavan The study of electron-doped superconductors offers a new window into the still intractable problem of high temperature superconductivity. While hole-doped High T$_{C}$ superconductors have been extensively studied with scanning tunneling microscopy (STM), there has been very little atomic resolution STM data on electron doped superconductors. We present STM images of \textit{in situ} cleaved Pr$_{1-x}$LaCe$_{x}$CuO$_{4-\delta }$ (PLCCO), obtained with a low temperature (5.6K), ultra high vacuum (UHV) STM. Due to the post annealing process required for superconductivity, superconducting PLCCO is expected to contain between 0.1{\%} to 1{\%} of the oxide impurity phases, (Pr, Ce)$_{2}$O$_{3}$. Our STM data on superconducting PLCCO reveal a few different atomic scale features, some of which have a periodicity consistent with the lattice constant expected for the impurity phases. Spectroscopy performed on these areas reveal gaps in the meV energy range. Possible origins of these and the other atomic scale structures will be discussed in this talk. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D8.00014: Tunneling spectroscopy of e-doped cuprates Lei Shan, Yan Huang, Yong-Lei Wang, Shi-Liang Li, Jun Zhao, Peng-Cheng Dai, Hai-Hu Wen Point-contact tunneling spectra were measured on electron-doped high-T$_{c}$ cuprates (NCCO and PLCCO). By phenomenological analysis, we found that the superconducting gap ($\Delta _{sc})$ definitely decreases towards zero in an almost universal law with continuously increasing temperature or magnetic field. At the fields above H$_{c2}$, a clear ``pseudogap'' was opened indicated by the obvious spectral losing below a characteristic energy scale ($\Delta _{pg})$ which is much larger than $\Delta _{sc}$. All the phenomena observed here seem to be crucial to distinguish the mechanism of HTSC and need to be extensively studied on more doping levels. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D8.00015: Competing Orders: Origin of the Non-Universal Low-Energy Pseudogap Phenomena in Cuprate Superconductors Ching-Tzu Chen, A. D. Beyer, N.-C. Yeh The contrasting low-energy pseudogap phenomena and quasiparticle spectral characteristics between the electron- and hole-type cuprates remain an open issue in cuprate superconductivity. Here we review the experimental manifestation of various non-universal properties and show that a phenomenological model of coexisting density-wave orders with superconductivity can consistently explain these disparate observations. By incorporating quantum phase fluctuations and adopting realistic bandstructures, numerical simulations of the quasiparticle tunneling spectra reproduce the empirical observations for both types of cuprates. Specifically, by tuning the ratio of the density waves to superconductivity, we can account for the absence of low-energy pseudogap in electron-type cuprates and the presence of pseudogap in hole-type cuprates. We therefore conclude that competing orders play an important role in the rich phenomenology of cuprate superconductivity. [Preview Abstract] |
Session D9: Superconductivity: Charge Order and Inhomogeneity
Sponsoring Units: DMPChair: Steve Kivelson, Stanford University
Room: Colorado Convention Center Korbel 1D
Monday, March 5, 2007 2:30PM - 2:42PM |
D9.00001: What does charge order have to do with the mechanism of high temperature superconductivity? Steven Kivelson, Eduardo Fradkin Charge order clearly ``competes'' with superconductivity under many circumstances. It always tends to suppress the superfluid stiffness of the superconducting state by localizing electrons that might otherwise participate in the superconducting condensate. Thus, where the superconducting T$_{c}$ is determined by phase fluctuations, charge order suppresses T$_{c}$. However, there is suggestive experimental and theoretical evidence that charge ordering of just the right sort can enhance pairing, and hence ``assist'' superconductivity. Some of this evidence will be presented. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D9.00002: How optimal inhomogeneity produces high temperature superconductivity Eduardo Fradkin, Steven Kivelson The role of Coulomb frustrated phase separation in doped Mott insulators, and especially the consequences of the resulting local electronic structures on the ``mechanism'' of high temperature superconductivity will be discussed. The resulting perspective on superconductivity in the cuprates, and on the more general theoretical issue of what sorts of systems can support high temperature superconductivity is discussed as are some of the general, qualitative aspects of the experimental lore which should constrain any theory of the mechanism. Finally, it is show how they are accounted for within the context of the present theory. Reference: S. A. Kivelson and E. Fradkin, ``How optimal inhomogeneity produces high temperature superconductivity,'' cond-mat/0507459, to appear as a chapter in ``Treatise of High Temperature Superconductivity'' by J. Robert Schrieffer and J. Brooks, to be published (Springer, 2006) [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D9.00003: Thermodynamic properties of inhomogeneous superconductors near their transition temperature Srinivas Raghu, Reza Jamei, Steven Kivelson Recently, scanning tunneling spectroscopy (STS) experiments have revealed suggestive evidence of the existence of superconducting gap inhomogeneities at low temperatures in some families of cuprate materials. The consequences of such inhomogeneity near the superconducting transition, however, remain an important and unresolved issue. Here, we study the effect of intrinsic gap inhomogeneities on the mean-field electronic specific heat (and other thermodynamic properties) in the vicinity of the superconducting transition. We consider a spatially-varying pairing interaction in a d-wave BCS model, solve the mean-field equations self-consistently for the magnitude of the gap function, and determine the thermodynamic properties of the system. As T approaches Tc, the coherence length grows, causing the system to become effectively more homogeneous due to self-averaging; we explore the extent to which various types of inhomogeneity remain important or get washed out near Tc. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D9.00004: Global phase diagram of the checkerboard Hubbard model Hong Yao, Wei-Feng Tsai, Steven Kivelson Local electronic structure (self-organized inhomogeneity) may play an essential role for the ``mechanism'' of high-T$_{c}$ superconductivity. Moreover, in the limit of large inhomogeneity, well-controlled theoretical solutions of strongly interacting models can be obtained. We have computed the phase diagram of the checkerboard Hubbard model in the limit of small inter-cluster electron hopping, t', for all doping (x=hole density per site) and for all interaction strengths, 0$<$U/t. For O(t')$<$U$<$U$_{c}$ =4.58t, and all 0$\le $ x $\le $1/2, the existence of an effective pair attraction results in one of two d-wave superconducting ground states - either with nodal or without nodal quasiparticles. For U$_{c }<$U$<$U$_{t }$=18.6t, the ground state is a Fermi liquid of spin 1/2 fermions with two possible orbital flavors. Interestingly, around x=1/4 the ground state is a spin-1/2 antiferromagnet which also possesses alternating orbital currents on every other plaquette that spontaneously break time reversal symmetry. For U$>$U$_{t}$, the ground state is a Fermi liquid of fermions with spin-3/2, with a spin-3/2 antiferromagnet is favored near x=1/4. By including next nearest neighbor hopping, t$_{2}$, within clusters, we can study the physics of particle-hole asymmetry. Strikingly, we find that increasing t$_{2}$ increases the range of U for which hole doping leads to a superconducting state, but suppresses the range of U for electron doping. (For t$_{2}\to $--t$_{2}$, the roles of electrons and holes are interchanged.) [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D9.00005: Superconductivity in zigzag CuO chains Erez Berg, Steve Kivelson Superconductivity was recently discovered in Pr$_2$Ba$_4$Cu$_7 $O$_{15- \delta}$ with a maximum T$_c$ of about 12K [1]. This material's structure is identical to that of the high T$_c$ superconductor YBCO$-247$. However, the cupper-oxide planes in this material (which are essential for the superconductivity in YBCO) are known to be insulating. Therefore it is believed that the superconductivity originates in the array of quasi-1d CuO chains and NMR experiments appear to corroborate this belief. In this work we study a microscopic model for a CuO double-chain (zigzag chain) using a combination of bosonization and numerics (DMRG). We derive a schematic phase diagram for this model, which exhibits a narrow doping region where superconducting correlations are dominant and a broader range where CDW correlations are dominant. Unlike the situation in the two-leg Hubbard ladder, superconductivity does not arise from the formation of a spin gap. Rather, it is related to a subtle ordering driven by magnetic interactions. The implications for experiment are discussed. \newline [1] See, for example, Y. Yamada and A. Matsushita, Physica C 426-431, 213(2005). [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D9.00006: Stripes near a Quantum Critical Point Erica W. Carlson, Daoxin Yao, David K. Campbell Competing tendency in strongly correlated materials can cause spontaneous nanoscale structure, pattern formation, and even long-range spatial order. We explore the magnetic excitation spectrum in the stripe phase of high-Tc cuprates. Using a semiclassical spin wave treatment, we calculate the dynamical spin structure factor for weakly coupled stripes. We find a characteristic hourglass magnetic excitation spectrum with high-energy peaks rotated by 45 degrees compared to the incommensurate (IC) low-energy peaks in good agreement with the experimental data.The similarity at high energy between this semiclassical treatment and quantum fluctuations in spin ladders may be attributed to the proximity of a quantum critical point with a small critical exponent $\eta $. We also find that the low energy intensity is strongly peaked on the inner branches of the spin wave cones when coupling across the stripes is weak, so that the entire spin wave cone is not likely to be resolvable experimentally. (Phys. Rev. Lett. 97, 017003 (2006), Phys. Rev. B 73, 224525 (2006)) [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D9.00007: s-Wave Superconductivity Phase Diagram for the Two Dimensional Inhomogeneous Attractive Hubbard Model Karan Aryanpour, Thereza C. Paiva, Warren E. Pickett, Richard T. Scalettar We study s-wave superconductivity in the two-dimensional square lattice attractive Hubbard Hamiltonian for various inhomogeneous patterns of interacting sites at different concentration values $f$. Using the Bogoliubov-de Gennes (BdG) mean field approximation, we find the phase diagram for inhomogeneous interaction patterns in which the on-site interaction $U_i$ takes on two values, $U_i=0,U/(1-f)$ as a function of electron occupation per site $n$ and study the evolution of the phase diagram as $f$ varies. In certain regions of the phase diagram, inhomogeneity results in a larger zero temperature averaged pairing amplitude and also the superconducting phase transition temperature $T_c$, relative to a uniform system with $U_i=U$ on all sites. These effects are observed for stripe, checkerboard, and even random patterns of the attractive centers, suggesting that the pattern of inhomogeneity is unimportant. The phase diagrams also include regions where superconductivity is obliterated due to the formation of various charge ordered phases. We show that for certain regular inhomogeneous patterns, increasing temperature works against the formation of these charge ordered phases and as a result, can enhance superconductivity. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D9.00008: Competition between charge order and superconductivity in La$_{7/8}$Ba$_{1/8}$CuO$_{4}$ Jungho Kim, A. Kageda, G.D. Gu, C.S. Nelson, T. Gog, D. Casa, Young-June Kim Understanding the role of stripe physics in cuprate superconductors is believed to be essential in elucidating the superconducting mechanism of the cuprates. Despite the fundamental importance of charge ordering in the cuprates, a comprehensive examination of the relationship between charge stripes and superconductivity is still lacking. We have carried out a detailed investigation of temperature and magnetic field dependence of charge order in La$_{7/8}$Ba$_{1/8}$CuO$_{4}$ utilizing high-resolution x-ray scattering. We find that the correlation length of the charge order exhibits unusual temperature and magnetic field dependence. Specifically, at zero field the correlation length decreases as the sample is cooled below $\sim $12K, while it increases as magnetic field is applied in the superconducting phase. These observations suggest that the size of the charge ordered region seems to be inversely correlated with superconductivity. This finding clearly shows that static charge order competes with the superconducting ground state, and supports the microscopic phase separation picture discussed by the recent $\mu $SR work. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D9.00009: On the dimensionality of spin and charge modulations in 1/8 doped lanthanum cuprates Boris Fine I compare the standard one-dimensional stripe interpretation of elastic scattering experiments in 1/8 doped lanthanum cuprates with two two-dimensional interpretations. One of them is known as grid[1,2] and the other one is the lattice of magnetic vortices[3]. Both can induce a 4x4 charge modulation similar to the one detected by scanning tunneling spectroscopy. The case of magnetic vortices, however, is favored against grid by a recent spin polarized neutron scattering experiment. \newline [1] B.V. Fine, Phys. Rev. B, v. 70, p. 224508 (2004) \newline [2] B.V. Fine, cond-mat/0606300 \newline [3] B.V. Fine, cond-mat/0610748 [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D9.00010: Fluctuating Cu-O-Cu Bond model of high temperature superconductivity in cuprates D.M. Newns, C.C. Tsuei Twenty years of research have yet to produce a consensus on the origin of high temperature superconductivity (HTS). However, several generic characteristics of cuprate superconductors have emerged as the essential ingredients of and/or constraints on any viable microscopic model of HTS. Besides a $T_{c}$ of order $100$ K, they include a $d$-wave superconducting (SC) gap with Fermi liquid nodal excitations, a pseudogap with $d$-symmetry and the characteristic temperature scale $T^{\ast}$, an anomalous doping-dependent oxygen isotope shift, nanometer-scale gap inhomogeneity, etc.. The isotope shift implies a key role for oxygen vibrations, but conventional BCS single-phonon coupling is essentially forbidden by symmetry and by the on-site Coulomb interaction $U$. Hence the present work invokes nonlinear coupling of planar oxygen vibrations to the Cu-Cu hopping integral $t$. A dominant Fluctuating Bond field emerges involving oxygen vibrational square amplitudes - and associated Cu-Cu $t$'s - in a pattern of quadrupolar symmetry around a given Cu site. Such fluctuations in Cu-Cu bonds mediate $d$-wave pairing, leading to a $d$-wave SC gap, and an explanation of the anomalous isotope shift. A quadrupolar CDW generates a $d$-wave pseudogap related to $T^{\ast}$. Other salient features of HTS are also explained by our model. This work is to appear in Nature Physics. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D9.00011: Pair Binding in Small Hubbard Clusters W.-F. Tsai, H. Yao, S. Kivelson, A. Lauchli One of the key issues in high-T$_{c}$ superconductors is how (and whether) high temperature pairing can arise in an electronic system with only repulsive interactions. Here, we report the results of analytic and numerical exact diagonalization studies of small Hubbard clusters (up to 16 sites). Taking the N-electron ground-state as the ``vacuum state'' of the cluster, we define the effective interaction between two added electrons to be V$^{eff}$(N) = E(N+2)+E(N)--2E(N+1), where E(N) is the ground-state energy with N electrons on the cluster. Not surprisingly, for most clusters and most values of N, V$^{eff}$ is repulsive (V$^{eff}>$0), but there exist special clusters in which, for special N and in an appropriate range of U/t, there is an effective attraction, V$^{eff}<$0. In the weak coupling limit (U/t$<<$1), the results can be understood within perturbation theory, and the effective attraction, where it is occurs, is associated with the existence of an anomalous ``resonantly entangled'' groundstate. In the strong coupling limit, V$^{eff}$ is always positive (or zero) due to Nagaoka physics. In some sense, the optimal cluster is the Hubbard-tetrahedron, for which V$^{eff}$ is negative for all U/t. Finally, by studying the dependence of V$^{eff}$ on the patterns of inhomogeneous couplings within a single cluster, we obtain some insight into the issue of whether there exists an optimal inhomogeneity for high-T$_{c}$ superconductivity. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D9.00012: Searching for orbital currents in the pseudo-gap state of $\mathrm{La_{2-x}Sr_{x}CuO_{4}}$ G.J. MacDougall, G.M. Luke, A.A. Aczel, J. Rodriguez, Y.J. Uemura, J.P. Carlo, T. Ito, P.L. Russo, S. Wakimoto Among the many outstanding riddles involving the cuprate materials is the microscopic nature of the so-called `pseudo-gap state'. Several theories have been put forth over the years, including pre-formed pairs, superconducting fluctuations and several brands of unconventional order. An example of the latter which has been getting particular attention of late is the idea that the pseudo-gap corresponds to an ordering of orbital currents. This renewed debate is mostly due to recent polarized neutron data on $\mathrm{YBa_{2}Cu_{3}O_{7-\delta}}$, which claims to support a current ordered state which does not break translational invariance [PRL 96, 197001 (2006)]. These neutron results are not universally accepted, however, and clarifying experiments are necessary. In this spirit, we performed zero-field $\mu SR$ on $\mathrm{La_{2-x}Sr_{x}CuO_{4}}$ crystals with a wide range of $T^{*}$ values, and searched for the sponteous magnetic fields that would necessarily be associated with current order. We present the results of this search and discuss the implications our data for the interpretation of past and future experiments. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D9.00013: The effect of inhomogeneous pairing amplitude on superfluid stiffness in a $d$-wave superconductor Ming Cheng, Wu-Pei Su To explain the disparity between $T_c$ and $T^*$ in optimally doped and underdoped cuprates, we propose that $T_c$ is related to superconducting gap amplitude standard deviation ($\sigma$); while $T^*$ is related to average gap amplitude. We calculate the superfluid stiffness ($D_s$) using BdG formalism for a $d$- wave superconductor. The calculations show that $D_s$ decreases as ($\sigma$) increases, suggesting lower $T_c$ for more inhomogeneous gap distribution. The theoretic result is consistent with recent STM experiments which study the electronic inhomogeneities due to out-of-plane disorder. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D9.00014: The Ground State of the Pseudogap in Cuprates T. Valla, A. V. Fedorov, Jinho Lee, J. C. Davis, G. D. Gu In conventional superconductors, the appearance of an energy gap in the electronic spectrum indicates pairing of electrons into Cooper pairs and a simultaneous transition into a macroscopic superconducting state. In contrast, in the underdoped high temperature superconductors, an energy gap is already present in the normal state. An understanding of this normal state gap or `pseudogap' has proven elusive, because its ground state electronic structure was unknown. Here, we present studies of electronic structure in La$_{2-x}$Ba$_{x}$CuO$_{4}$, a unique system where the superconductivity is strongly suppressed and static spin and charge orders or `stripes' develop near a doping level of $x=$1/8. Using angle-resolved photoemission and scanning tunneling microscopy, we detect an energy gap at the Fermi surface with magnitude consistent with $d$-wave symmetry and with linear density of states, vanishing only at four nodal points, even when superconductivity disappears at $x=$1/8. Thus, the non-superconducting, `striped' state at $x=$1/8 is consistent with a phase incoherent $d$-wave superconductor whose Cooper pairs form spin/charge ordered structures instead of becoming superconducting. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D9.00015: The effects of local inhomogeneities on the phonon modulated DOS in Bi2212 Steven Johnston, Thomas Devereaux Recent scanning tunneling microscopy experiments on Bi2212 have reveled microscopic inhomogeneities in the local density of states and anomalous signatures of coupling to a bosonic mode. Gap referenced estimates for the mode energy are negatively correlated with the local gap size and the distribution of the mode estimates shows a clear isotope shift upon $^{18}$O substitution. Motivated by the clear isotope effect we examine electron-phonon coupling to the 55 meV apical oxygen mode in Bi2212 within the framework of Migdal-Eliashberg theory. The interplay of this interaction with local inhomogeneous broadening effects are also considered. The effects of the local dopant atoms on the electron-phonon interaction strength are examined using the Ewald summation technique. [Preview Abstract] |
Session D10: Theory and Simulations of Systems with Disorder
Sponsoring Units: DMPChair: Vadim Oganesyan, Yale University
Room: Colorado Convention Center Korbel 1E
Monday, March 5, 2007 2:30PM - 2:42PM |
D10.00001: Percolation transition and dissipation in quantum Ising magnets Jose Hoyos, Thomas Vojta We study the effects of dissipation on a randomly diluted transverse-field Ising magnet close to the percolation threshold. For weak transverse fields, a novel percolation quantum phase transition separates a superparamagnetic cluster phase from an inhomogeneously ordered ferromagnetic phase. The properties of this transition are dominated by large frozen and slowly fluctuating percolation clusters. This leads to a discontinuous magnetization-field curve and exotic hysteresis phenomena as well as highly singular behavior of magnetic susceptibility and specific heat. We compare our results to the smeared transition in generic dissipative random quantum Ising magnets. We also discuss the relation to metallic quantum magnets and other experimental realizations. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D10.00002: Nature of triplet excitations in the diluted 2D Heisenberg model Ling Wang, Anders Sandvik We study the nature of ground state excitations of the 2D S=1/2 Heisenberg model on percolating clusters. We have previously argued that they involve weakly interacting localized moments, which are formed due to local sublattice imbalance [1]. We here discuss further confirmation of this picture for clusters with singlet ground states. First, we study a hard-core classical dimer-monomer model on percolating clusters. We find that the monomers are localized in small regions of local sublattice imbalance, and these regions coincide with regions of small local gaps (large local magnetic susceptibility), thus supporting the existence of localized magnetic moments due to sublattice imbalance. Second, we use quantum Monte Carlo simulations in the valence bond basis [2], with which we can study the spatial distribution of a triplet bond in the lowest-energy excited state. We find that the triplets are localized predominantly in a subset of the regions of localized monomers, supporting the notion that the lowest excitation is the singlet-triplet excitation of a small number of interacting effective moments. Supported by NSF grant DMR-0513930. \newline \newline [1] L. Wang and A. W. Sandvik, Phys. Rev. Lett. 97, 117204 (2006). \newline [2] A. W. Sandvik, Phys. Rev. Lett. 95, 207203 (2005). [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D10.00003: Monte Carlo Study of Entanglement Scaling in Random S=1/2 Heisenberg Chains Huan Tran, Nicholas Bonesteel We present the results of a quantum Monte Carlo study of the S=1/2 Heisenberg chain with random antiferromagnetic nearest-neighbor coupling. Using the method of ground state projection in the singlet-bond basis, recently introduced by Sandvik,\footnote{A. Sandvik, PRL {\bf 95}, 207203 (2005).} we are able to directly confirm the expected freezing of the ground state into a random singlet phase at long length scales, while at the same time exactly capturing the nonuniversal (i.e. detail dependent) short-range bond fluctuations. By computing the bond-length distribution in the random singlet phase we are then able to determine the mean entanglement entropy, $S_N$, associated with a segment of $N \gg 1$ spins, both by self-averaging over segments for a particular realization of disorder, and by averaging over many distinct realizations of disorder. Our results confirm the $S_N \simeq \frac{\ln 2}{3} \log_2 N$ scaling found by Refael and Moore using real space RG,\footnote{G. Refael and J. E. Moore, PRL {\bf 93}, 260602 (2004).} showing that the ``effective central charge" of the critical random S=1/2 Heisenberg chain is $\tilde c = \ln 2$. Work supported by US DOE. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D10.00004: Quantum Monte Carlo Study of a Magnetic-Field-Driven 2D Superconductor-Insulator Transition Kwangmoo Kim, David Stroud Using quantum Monte Carlo calculations of the $(2+1)$D $XY$ model, we study the superconductor-insulator phase transition of a disordered 2D superconducting film vs. the applied magnetic field. The $XY$ coupling is assumed to be $-J\cos(\theta_i-\theta_j-A_{ij})$, where $A_{ij}$ has a standard deviation $\Delta A_{ij}$. The critical coupling constant $K_{c} = \sqrt{[J/(2U)]_c}$ and the universal conductivity $\sigma^{*}$ are found to increase monotonically with $\Delta A_{ij}$. Beyond a certain critical value of $\Delta A_{ij}$, the superfluid density vanishes for all $K$'s, but a renormalized coupling constant $g$ remains finite, suggesting a transition into a Bose glass phase. At a larger value of $\Delta A_{ij}$, the system becomes a Mott insulator. The critical values are found to be $K_{c}=0.490\pm 0.001$ and $\sigma^{*}/\sigma_{Q}=0.324\pm 0.003$ when $\Delta A_{ij}=1/2$; $K_{c}=0.532\pm 0.001$ and $\sigma^{*}/\sigma_{Q}=0.494\pm 0.011$ when $\Delta A_{ij}=1/\sqrt{2}$; $K_{c}=0.585\pm 0.004$ when $\Delta A_{ij}=0.854$; and $K_{c}=0.630\pm 0.002$ when $\Delta A_{ij}=\infty$. The last value, which represents a Bose glass to Mott insulator transition, is obtained from $g$, whereas the others represent a superconductor-to-insulator transition and are obtained from the superfluid density. We conclude that, for certain couplings, a disordered film may undergo a transition from superconductor to Bose glass to insulator with increasing field. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D10.00005: On the role of inhomogeneities for correlated d-wave superconductors. Rastko Sknepnek, Jun Liu, Joerg Schmalian We investigate the impact of inhomogeneities on pairing and off diagonal long range order in a correlated superconductor. Using a variational Monte Carlo study of the t-J model we demonstrate that the local pairing strength and superconducting long range correlations are sensitive with respect to spatial variations of external charge and pairing potentials. In addition we analyze evolution the underlying Fermi surface which is changing towards a diamond shape due to strong but local spin correlations. We analyze the robustness of this effect with respect to spatial inhomogeneities. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D10.00006: The geometrically-averaged density of states as a measure of localization Rachel Wortis, Yun Song, William Atkinson Motivated by current interest in disordered systems of interacting electrons, we examine the use of the geometrically-averaged density of states, $\rho_g(\omega)$, as an order parameter for the Anderson transition. In infinite systems, when $\rho_g(\omega)$ vanishes, while the density of states remains nonzero, the states at energy $\omega$ are localized. In the context of noninteracting finite-size systems we show that a finite energy resolution, a common feature of many-body calculations, changes the scaling of $\rho_g(\omega)$ such that the critical disorder is over-estimated. Furthermore we demonstrate that even in infinite systems a decline in $\rho_g(\omega)$ with increasing disorder strength is not uniquely associated with localization. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D10.00007: Mott and Band Insulator Transitions in the Binary Alloy Hubbard Model Andrew Baldwin, Richard Scalettar, Norman Paris We use determinant Quantum Monte Carlo simulations and exact diagonalization to explore insulating behavior in the Hubbard model with a bimodal distribution of randomly positioned local site energies. From the temperature dependence of the compressibility and conductivity, we show that gapped, incompressible Mott insulating phases exist away from half filling when the variance of the local site energies is sufficiently large. The compressible regions around this Mott phase are metallic only if the density of sites with the corresponding energy exceeds the percolation threshold, but are Anderson insulators otherwise. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D10.00008: Quantum critical behaviour of the cluster glass phase Matthew Case, Vladimir Dobrosavljevic In disordered itinerant magnets with arbitrary symmetry of the order parameter, the conventional quantum critical point between the ordered phase and the paramagnetic Fermi-liquid (PMFL) is destroyed due to the formation of the cluster glass (CG) phase. In this talk, we will discuss the quantum critical behaviour at the CG-PMFL transition. We will show that fluctuations due to quantum Griffiths anomalies induce a first-order transition from the PMFL at T=0, while at higher temperatures a conventional continuous transition is restored. This is in contrast to the behaviour of a collection of identical droplets where the second-order transition persists down to T=0. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D10.00009: Local defect in a magnet with long-range interactions Thomas Vojta, Jose Hoyos We investigate a single defect coupling to the square of the order parameter in a nearly critical magnet with long-range spatial interactions of the form $r^{-(d+\sigma)}$, focusing on magnetic droplets nucleated at the defect while the bulk system is in the paramagnetic phase. Because of the long-range interaction, the droplet develops a power-law tail which is energetically unfavorable. However, as long as $\sigma>0$, the tail contribution to the droplet free energy is subleading in the limit of large droplets; and the free energy becomes identical to the case of short-range interactions. We also study the droplet quantum dynamics with and without dissipation; and we discuss the consequences of our results for defects in itinerant quantum ferromagnets. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D10.00010: ``Exact'' algorithm for random-bond Ising models in 2D Yen Lee Loh, Erica W. Carlson For nearly 80 years the Ising model and its variants have given valuable insight into phase transitions and critical phenomena in magnets, alloys, and many other systems. Random-bond Ising models (RBIMs) in particular are often used to study frustration and spin-glass behavior, and they are closely related to neural networks and information theory. We present an algorithm for solving two-dimensional Ising models with any configuration of bond strengths [1]. The algorithm is an extension of the bond-propagation algorithm originally developed for resistor networks [2]. It calculates the partition function and correlation functions at a single temperature for any planar Ising model of linear dimension L in $O(L^3)$ time or less. The results are numerically exact (subject only to roundoff error). The method is especially efficient for dilute models near the percolation threshold, for which it executes in $O(L^2 \ln L)$ time. Moreover, it operates directly in the spin basis, without the need for mapping to fermion or dimer models, and it is massively parallelizable. It gives fresh insight on the peculiar ``hidden integrability'' of 2D Ising models and suggests new directions for tackling other problems. \\ $[1]$ Y. L. Loh and E. W. Carlson, to appear in Phys. Rev. Lett. (2006) \\ $[2]$ D. J. Frank and C. J. Lobb, Phys. Rev. B, 37, 302 (1988). [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D10.00011: Quenched disorder and structure of short-range spin correlations Igor Zaliznyak In many important cases, magnetic order existing in a crystal does not possess long-range coherence, but has short-range nature. In particular, such is the situation in a variety of doped perovskite oxides, including cuprates, nickelates and cobaltates, which have recently been extensively studied in view of their fascinating electronic properties. In the absence of macroscopic spin coherence, the Fourrier-transform of spin-spin correlation in the crystal, which determines elastic magnetic scattering measured in experiment, does not contain delta- functions giving rise to magnetic Bragg peaks. Instead, it contains broad diffuse peaks which experimenters usually describe by phenomenological profiles, such as Lorentzian, Lorentzian-squared, etc., some of which are only appropriate in the near vicinity of the peak position (e.g. in the Orstein- Zernike approximation). Here we consider a simple model of quenched disorder introduced by a system of static magnetic disclinations/stacking faults of various symmetry and dimensionality. The corresponding spin correlation function has a form of the ``lattice-Lorentzian,'' where the Lorentzian's power is determined by the dimensionality of the disorder. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D10.00012: Simulation on depinning of a magnetic domain wall based on Heisenberg spin model Katsuyoshi Matsushita, Xiao Hu Motion of magnetic-field driven magnetic domain-wall subject to random pinning centers has attracted much attention. One of the characteristic phenomena in the system is the depinning transition at non-zero depinning force. It is expected that such motion can be described by an elastic deformable interface in a disordered medium. The depinning transition of a magnetic domain wall in an Ising spin system with random pinning fields has been studied which confirmed this expectation. In the present study, by using Monte Carlo and molecular dynamics simulations, we investigate motion of a domain wall in a Heisenberg spin system. In contrast to the Ising case, we observed discontinuous jump in domain-wall velocity upon depinning. Simulation results will be presented and the physics behind the difference will be discussed. [Preview Abstract] |
Session D11: Focus Session: Correlated Electron Superlattices
Sponsoring Units: DMPChair: Jak Chakhalian, University of Arkansas
Room: Colorado Convention Center Korbel 1F
Monday, March 5, 2007 2:30PM - 2:42PM |
D11.00001: A tight-binding LDA+DMFT study of manganite superlattices Claude Ederer, Chungwei Lin, Andrew Millis The combination of {\it ab initio} density functional theory with model ``many-body'' calculations provides a very promising way for a realistic theoretical treatment of surface and interface effects of strongly correlated electron materials. Here we show in detail how the electronic structure of LaMnO$_3$ calculated within the local density approximation (LDA) can be efficiently parametrized using a physically transparent tight binding model and considering both nearest and next nearest neighbor hoppings. In particular, we address effects due to rotations of the oxygen octahedra surrounding the Mn cations. The resulting two-band model is then applied to study LaMnO$_3$-SrMnO$_3$ superlattices using dynamical mean-field theory (DMFT) and the predicted ground state phases for superlattices with a small number of individual LaMnO$_3$ and SrMnO$_3$ layers are compared with the results obtained by density functional theory. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D11.00002: A Realistic Model Calculation on Manganite Superlattice Chungwei Lin, Claude Ederer, Andrew Millis We present a realistic model calculation for (0,0,1) (LaMnO$_3$)$_m$ (SrMnO$_3$)$_n$ superlattices. In this model, the superlattice is defined by the long-range Coulomb interaction generated by different ion charges of La (3+) and Sr (2+). The electronic degree of freedom contains two Manganese-Oxygen hybridized e$_g$ bands coupled to localized Mn $t_{2g}$ spins and to Jahn-Teller phonon modes, and the most general on-site electron-electron interactions within those $e_g$ bands. We will show how charge reconstruction, structural constraints and the symmetry breaking induced by the superlattice affect the orbital and magnetic orders. This work is supported by DMR-0213574. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D11.00003: Two-Dimensional Spin-Polarized Electron Gas at the Perovskite Manganite Interface : SrMnO$_3$/LaMnO$_3$ Birabar Nanda, Sashi Satpathy Electronic structure calculations for the perovskite manganite heterostructure (SrMnO$_3$)$_{n}$/(LaMnO$_3$)$_{1}$/(SrMnO$_3$)$_{n}$ reveal the presence of a novel spin-polarized electron gas at the interface, generated from the stripped-off La (5d$^1$) electrons, which become confined in the electrostatic V-shaped potential well of the positively charged (LaO) sheet, occupying the Mn(e$_g$) states near the interface. The presence of these electrons turns the interaction between the interfacial Mn atoms to be ferromagnetic via the Anderson-Hasegawa double exchange, overcoming the original antiferromagnetic superexchange present in the SrMnO$_3$ bulk. The FM Mn atoms at the interface in turn make the electron gas spin-polarized, as confirmed by the total energy calculations, and the type G AFM of the bulk is resumed a few layers into the bulk. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D11.00004: First-principles study of LaAlO$_3$/SrTiO$_3$ thin films Kristopher Andersen, C. Stephen Hellberg Although the perovskite oxides LaAlO$_3$ (LAO) and SrTiO$_3$ (STO) are conventional band insulators, an electron gas can form at their interface. Several mechanisms have been proposed to produce the electron gas, including the electrostatic divergence within LAO that results from the growth of alternating charged (LaO)$^+$ and (AlO$_2$)$^-$ layers and an electronic reconstruction in which Ti$^{3+}$ and Ti$^{4+}$ is formed at the interface. Of practical interest, thin films of LAO on STO have been observed to have highly mobile carriers and a carrier density that is tunable via LAO thickness---in recent work, Thiel et al. observed an insulator-metal transition between 3--4 MLs. In this talk, first-principles electronic structure calculations are performed on LAO thin films grown on STO to investigate surface reconstructions and the penetration depth of the electron gas into the substrate. An insulator-metal transition is found in good agreement with experiment. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D11.00005: Theory of band alignment at the LaAlO$_{3}$/SrTiO$_{3}$ interface Jaekwang Lee, Alex Demkov A polar discontinuity at the abrupt oxide/oxide interface is one of several problems that need to be addressed before we can realize the promise of multiferroic oxide structures. To avoid the so-called polar catastrophe the interface undergoes roughening which renders the structure useless, unless the system finds a mechanism for compensating the interface charges. Recent experiments of Hwang and co-workers (\textit{Nature }\textbf{427}, 423-426 (2004) and \textit{Nature }\textbf{430}, 657-661 (2004)) suggest that in the case of perovskite oxides two quite different compensatory mechanisms are at play at the heterojunction. For the n-type LaAlO$_{3}$/SrTiO$_{3}$ interface it is purely electronic involving mixed valence Ti ions, while for the p-type it is an actual ionic reconstruction involving oxygen vacancies. We report a first-principles study of both interfaces within density functional theory. We consider the energetics and electronic structure of the interface, including the role of oxygen vacancies and band offsets. In addition we consider the interface stability with respect to inter diffusion of La and Sr across the interface. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 4:06PM |
D11.00006: Correlation driven charge order at LaAlO$_{3}$/SrTiO$_{3}$ and LaTiO$_{3}$/SrTiO$_{3}$ Interfaces Invited Speaker: Correlated behavior at complex oxide interfaces offers additional degrees of freedom to compensate charge imbalance not available $e.g$. in polar semiconductor heterostructures. This can result in electronic, charge and magnetic phases that do not exist in the bulk and offers new possibilities for device applications. For instance, the interfaces of LaTiO3 and SrTiO3 [1] as well as LaAlO3 and SrTiO3 [2] show metallic conductivity, although the respective bulk materials are Mott (LTO) and band insulating (STO, LAO). Here we present the results of material-specific correlated band theory (LDA+U) employing the FP-LAPW code in the WIEN2k implementation for a variety of (n,m) multilayers containing $n$ LTO (or LAO) and $m$ STO-layers. To explore the relaxation length towards bulk behavior $n$ and $m$ is varied between 1 and 9. We find that charge mismatch at the LTO/STO IF is accommodated by the formation of a charge and orbitally ordered (CO/OO) layer with a checkerboard arrangement of Ti$^{3+}$ and Ti$^{4+}$ and an antiferromagnetic coupling of the Ti$^{3+}$-spins [3]. Lattice relaxations lead to the observed conducting behavior. An analogous diluted layer of Ti$^{3+ }$spins is obtained for the $n$-type LAO/STO interface, although the corresponding bulk materials are nonmagnetic. For a structurally ideal $p$-type LAO/STO IF the measured insulating behavior can only be understood by a charge disproportionation on the oxygen sublattice and the formation of a CO/OO magnetic OP$\pi $ hole. Alternatively, charge compensation by oxygen vacancies and the formation of a charge conjugate F-center is considered. [1] A. Ohtomo, and H.Y. Hwang, Nature \textbf{423}, 378 (2002). [2] A. Ohtomo, D.A. Muller, J.L. Grazul, and H.Y. Hwang, Nature \textbf{419}, 378 (2002). [3] R. Pentcheva and W.E. Pickett, cond-mat/0608212. [4] R. Pentcheva and W.E. Pickett, Phys. Rev. B \textbf{74}, 035112 (2006). [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D11.00007: Origin of the charge carriers at LaAlO$_{3}$-on-SrTiO$_{3 }$ hetero-interfaces; possibility of intrinsic doping Gertjan Koster, Wolter Siemons, Hideki Yamamoto, Walter Harrison, Gerald Lucovsky, Theodore Geballe, Dave Blank, Malcolm Beasley We have made very thin films of LaAlO$_{3}$ on TiO$_{2}$ terminated SrTiO$_{3}$. \textit{In situ} UPS, XAS, vis-VUV-SE, transport measurements and annealing experiments results indicate that oxygen vacancies play an important role in the creation of the charge carriers and that these vacancies are introduced by the pulsed laser deposition process. Our results explain for the first time the origin of the large sheet carrier densities and high mobility observed previously. Simple model calculations confirm the plausibility of having defects at the origin of charge carriers while still maintaining a high mobility. By means of annealing experiments in atomic oxygen we examine the question whether an intrinsically doped interface does indeed exist at lower carrier concentrations. Work supported by the DoE BES and EPRI. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D11.00008: Kondo effect and ferromagnetic ordering at the $n$-type SrTiO$_3$ - LaAlO$_3$ conducting interface Alexander Brinkman, Mark Huijben, Maarten van Zalk, Jeroen Huijben, Wilfred van der Wiel, Guus Rijnders, Dave Blank, Hans Hilgenkamp The intriguing phenomenon of electrical conductivity at the interface between two insulators is one of the possible consequences of electronic reconstruction of materials. In analogy with interface conduction, the question arises whether or not it is possible to induce magnetism at the contact between two nonmagnetic materials. We show how a polar discontinuity at the $n$-type conducting interface between the nonmagnetic perovskites SrTiO$_3$ and LaAlO$_3$ can induce a local magnetic moment on the Ti site. The resulting interface magnetism manifests itself in the form of ferromagnetic ordering at 0.3 K and the scattering of conduction electrons on the magnetic local moment, which provides a large negative magnetoresistance of 30\%. The scattering can be described in terms of the Kondo effect with a Kondo temperature of 50 K. Electronically reconstructed interfaces now provide another versatile class of solid state Kondo systems, next to dilute impurities in metals and artificial quantum dots. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D11.00009: Optical Conductivity of LaAlO$_{3}$/SrTiO$_{3}$ Superlattices C.L.S. Kantner, M. Huijben, J. Seidel, M. Warusawithana, D.G. Schlom, R. Ramesh, J. Orenstein Precise contactless measurements of the optical conductivity of LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO) superlattices can be achieved using time-domain terahertz spectroscopy. We report the optical conductivity in the frequency range 3-30 cm$^{-1}$ as a function of temperature and concentration of oxygen vacancies in the STO layers. Superlattices were grown by laser-MBE, enabling control of the structure on the single-unit cell level. Optical measurements were made possible through the use of Si rather than STO substrates. To identify the interface contribution to the conductivity we compared the optical transmission of structures with different numbers of interfaces, while maintaining constant the total number of unit cells. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D11.00010: Properties of epitaxial LaMnO$_{3}$/SrTiO$_{3}$ interfaces. Hans M. Christen, Dae Ho Kim, Ho Nyung Lee, Maria Varela, Leon Petit, Thomas Schulthess Electronic effects at interfaces between dissimilar oxides are known to have fundamental consequences on their transport and magnetic properties. Interfaces between the band insulator SrTiO$_{3}$ and the antiferromagnetic charge-transfer insulator LaMnO$_{3 }$ provide a particularly interesting platform to test such effects. Both perovskites are grown epitaxially by pulsed-laser deposition, and electron energy loss spectra (EELS) collected in a scanning transmission electron microscope (STEM) show interfacial valence changes on the Mn-sites, while Ti remains in a 4+ state even in direct contact with the LaO layer of the LaMnO$_{3}$. This observation is fully consistent with computational results obtained for such structures using the self-interaction corrected (SIC) local spin density (LSD) method. In this presentation, we discuss the physical origin and consequences of these valence changes in single interfaces as well as LaMnO$_{3}$/SrTiO$_{3}$ superlattices. This research was sponsored by the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory (ORNL), managed and operated by UT-Battelle, LLC, and ORNL's Laboratory Directed Research and Development Program. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D11.00011: Interface effect on the positive magnetoresistence in a heterostructure composed of two perovskite oxides Kui-juan Jin, Hui-bin Lu, Qing-li Zhou, Guo-zhen Yang, Meng He, Kun Zhao Different from the negative colossal magnetoresistance (CMR) of doped manganites, a positive CMR is discovered at low applied magnetic field and high temperature in the epitaxial $p-n$ heterostructure with Sr-doped LaMnO$_{3}$ and Nb-doped SrTiO$_{3}$ fabricated by laser molecular-beam epitaxy [1,2]. We have found that such unusual positive CMR is an interface effect which causes a charge redistribution in at the interface with different electron spin polarization at Fermi level from that in the corresponding bulk CMR materials. Self-consistent calculation was carried out the band structure around the interface of the heterostructure and confirms the unusual behavior. Other puzzling CMR features with bias voltage, temperature and even composition are well explained by the present scenario. 1. Kui-Juan Jin et al., Phys. Rev. B\textbf{ 71}, 184428 (2005). 2. Qing-li Zhou, et al., Europhys. Lett. \textbf{71}, 1-7 (2005) [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D11.00012: Band structure engineering of LaMnO3:SrTiO3 superlattices at the molecular level Xiaofang Zhai, Chandra Mohapatra, Anand Bhattacharya, Amish Shah, Bin Jiang, Jianguo Wen, Jian-Min Zuo, James Eckstein We have made single crystal short period superlattices consisting of alternating slabs of LaMnO3 and SrTiO3, using atomic layer by layer molecular beam epitaxy. The supercells consist of N-layers of each component, with N=1, 2, 3, 4. Ellipsometric measurements of the new materials show that optical absorption in the visible light range is significantly different from bulk LaMnO3 or SrTiO3, and depends on N. The new band structure is dependent on the superlattice design. This is an example of engineered coherent ``meta-materials'', and this fabrication technique can be extended to other lattice matched transition metal oxides with a wide range of conducting and magnetic properties. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D11.00013: Potential profiling of the nanometer-scale charge-depletion layer in $n$-ZnO/$p$-NiO junction using photoemission spectroscopy Yukiaki Ishida, Hiromichi Ohta, Masahiro Hirano, Atsushi Fujimori, Hideo Hosono We have performed a depth-profile analysis of an all-oxide $p-n$ junction $n$-ZnO/$p$-NiO [1] using photoemission spectroscopy combined with Ar-ion sputtering, and investigated the potential profile of the space-charge region embedded at the interface [2]. Systematic core-level shifts were observed during the gradual removal of the ZnO overlayer, and were interpreted using a model based on charge conservation. Spatial profile of the potential around the interface was deduced, including the charge-depletion width of 2.3 nm extending on the ZnO side and the built-in potential of 0.54 eV. [1] H. Ohta, \textit{et al}., APL \textbf{83}, 1029 (2003). [2] Y. Ishida, \textit{et al}., APL \textbf{89}, 153502 (2006). [Preview Abstract] |
Session D12: Focus Session: Spin Dynamics and Magnetism in Quantum Dots
Sponsoring Units: GMAG DMP FIAPChair: Leonid Rokhinson, Purdue University
Room: Colorado Convention Center Korbel 3C
Monday, March 5, 2007 2:30PM - 2:42PM |
D12.00001: Trion spectroscopy and electrical control of a Single Mn Atom in a Quantum Dot J. Fern\'andez-Rossier I present a microscopic theory for the photoluminescence (PL) of a single self-assembled CdTe quantum dot doped with a single Mn atom. The few-body problem of electrons and holes exchange-coupled to the Mn spin is diagonalized exactly. The model permits a complete understanding of the non-trivial 11 peak spectra reported recently [1] in terms of charge-dependent effective Hamiltonian for the Mn spin. Whereas in the neutral configuration the Mn in the quantum dot is paramagnetic, the electron-doped dot spin states are spin rotationally invariant and the hole-doped dot spins states are quantized along the growth direction. Preliminary results of the time resolved response of the Mn spin to suitably engineered laser pulses will be discussed, both for the case of charged [1] and neutral dots[2,3].\\ \\ $[1]$ Y. L\'eger, L. Besombes, J. Fern\`andez-Rossier, L. Maingault, and H. Mariette Phys. Rev. Lett. {\bf 97}, 107401 (2006) \\ $[2]$ J. Fern\'andez-Rossier Phys. Rev. B {\bf 73}, 045301 (2006)\\ $[3]$ A. O. Govorov and A. V. Kalameitsev, Phys. Rev. B {\bf 71}, 035338 (2005) [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D12.00002: Tailoring Magnetism in Quantum Dots Igor Zutic, Ramin Abolfath, Pawel Hawrylak We study magnetism in magnetically doped quantum dots as a function of particle numbers, temperature, confining potential, and the strength of Coulomb interaction screening. We show that magnetism can be tailored by controlling the electron-electron Coulomb interaction, even without changing the number of particles. The interplay of strong Coulomb interactions and quantum confinement leads to enhanced inhomogeneous magnetization which persists at substantially higher temperatures than in the non-interacting case or in the bulk-like dilute magnetic semiconductors. We predict a series of electronic spin transitions which arise from the competition between the many-body gap and magnetic thermal fluctuations. Cond-mat/0612489. \newline [1] R. Abolfath, P. Hawrylak, I. \v{Z}uti\'c, preprint. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D12.00003: The influence of quantum confinement on magnetism in quantum dots. Ramin Abolfath, Igor Zutic, Pawel Hawrylak Owing to its simplicity, the vast majority of theoretical studies of magnetically doped quantum dots imply parabolic shape of the quantum confinement. However, several methods of fabricating quantum dots are more appropriately described by other forms of quantum confinement that remain largely unexplored. To assess the influence of the choice of confining potential and its strength, we perform a systematic comparison of magnetic phases of quantum dots described by parabolic and Gaussian confinement. We focus on the magnetization, carrier spin polarization, and magnetic transition temperature. We clarify which of these quantities could be strongly modified by the choice of non-parabolic quantum confinement and predict related experimental implications [1]. Cond-mat/0612489. \newline [1] R. Abolfath, P. Hawrylak, I. Zutic, preprint. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D12.00004: Theory of phonon-induced spin relaxation in coupled lateral quantum dots Invited Speaker: Electron spins in lateral quantum dots at GaAs/GaAlAs interfaces relax in milliseconds. Spin relaxation here means transitions from the upper to the lower Zeeman split orbital ground state, at an applied magnetic field. Both spin-orbit and electron-phonon couplings are needed for spin flips between spectrally distinct and opposite-spin states. We have carried out realistic numerical and analytical calculations of spin relaxation and spin dynamics in single and coupled lateral quantum dots [1]. Our results agree with existing experiments on single dots, while predict interesting effects for coupled dots. Most important, spin relaxation in coupled dots is dominated by spin hot spots--anticrossings of states of opposite spins--at practical couplings (say, 0.1 meV). Spin hot spots reduce spin relaxation to nanoseconds! Fortunately, spin hot spots are strongly anisotropic and there can be (rather singular) configurations, we call them {\it easy passages}, in which spin relaxation slows down to milliseconds as in single dots. For a (001) plane, for example, an easy passage occurs if coupled dots are oriented along [110] and the in-plane magnetic field lies perpendicular, along [1$\overline{1}$0]. This configuration should be used for spin-based quantum information processing. This easy passage also protects spin qubits from electrical field disturbances which occur in ``on-chip" single electron spin resonance experiments, as will be demonstrated theoretically using density matrix formalism for electron spins in the presence of both dissipation and driving oscillating electric and magnetic field [2]. \\ \noindent [1] P. Stano and J. Fabian, Phys. Rev. Lett. 96, 186602 (2006).\\ \noindent [2] P. Stano and J. Fabian, cond-mat/0611228. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D12.00005: Control of electron spin and orbital resonance in quantum dots through spin-orbit interactions Peter Stano, Jaroslav Fabian Dynamics of a single electron in coupled lateral quantum dots in the presence of a static and oscillating electric and magnetic fields as well as phonon-induced relaxation and decoherence is investigated. Using symmetry arguments it is shown that spin and orbital resonance can be efficiently controlled by spin-orbit couplings. The so called easy passage configuration is shown to be particularly suitable for magnetic manipulation of spin qubits, ensuring long spin relaxation time and protecting the spin qubit from electric field disturbances connected with on-chip manipulation. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D12.00006: Electron Spin Decoherence via Optical Phonons in Quantum Dots Yuriy Semenov, Ki Wook Kim Electron spin decoherence caused by elastic spin-phonon processes is investigated comprehensively in a zero-dimensional environment. Specifically, a theoretical treatment is developed for the processes associated with the anharmonic vibrations of optical phonons in the semiconductor quantum dots. The optical phonons possess relatively high energy that was reasons not involving them to the problem of quantum computing decoherence to present day. This is true if we associate spin decoherence with inelastic processes of spin relaxation that needs thermal activation of optical phonons. In the case of elastic processes the uncontrolled variation of spin phase can happen without presence of thermal phonons. Zero-point optical vibrations, which survive at low enough temperatures can contribute to spin decoherence. Advantage of the optical modes is conditioned by their relatively high contributions at small phonon wave vectors as well as sufficiently short optical phonon lifetime. Calculations of decoherence time T$_{2}$ under the g-factor optical phonon modulation predict relatively weak dependence on a magnetic field, $\sim $ B$^{-2}$, that leads to estimation T$_{2} \sim$ 10$^{-5}$ s in III-V semiconductor quantum dot at B=1 T. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D12.00007: Towards Electrical Spin Injection into a Single InAs/GaAs Quantum Dot C. H. Li, G. Kioseoglou, A. T. Hanbicki, O. M. J. van 't Erve, B. T. Jonker We aim to isolate emission from a single InAs/GaAs self-assembled QD to elucidate the details of electrical spin injection from an Fe Schottky contact and consequent spin polarization in QDs. MBE growth methods have been developed to reduce the dot density to the order of 10$^{8}$/cm$^{2}$, which in turn also increases the uniformity of the dots, allowing us to resolve their atomic-like s, p, d, f{\ldots} quantum confined states. The aperture sizes of the surface-emitting LEDs are also reduced to the order of a hundred nanometers using ebeam lithography. As the density and aperture size decrease, the initially broad emission spectrum of the dot ensemble [1] breaks into distinct narrow features attributed to single dot emission at low biases. With increasing bias, the number of peaks increases and their linewidth broadens, suggesting contributions from emission from an increasing number of dots and/or from various charge states of the dot. At even higher bias, the sets of peaks merge and approach broad emissions. Progress towards electrical spin injection into a single QD, and details of the electroluminescence spectra as a function of bias and magnetic field will be discussed at the meeting. [1] C. H. Li et al. APL \textbf{86}, 132503 (2005). [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D12.00008: Quantum Point Contacts as Spin Injectors and Detectors for Studying Rasha Spin Precession in Semiconductor Quantum Wires Philippe Debray, Ivan Shorubalko, Hongqi Xu We have studied polarized spin transport in a device consisting of three quantum point contacts (QPCs) in series made on InGaAs/InP quantum-well (QW) structures. The QPCs were created by independent pairs of side gates, each pair for one QPC. By adjusting the bias voltages of the side gates, the widths of the QPCs are independently tuned to have transport in the fundamental mode. An external magnetic field of a few T causes spin splitting of the lowest one-dimensional (1D) subbands. The widths of the end QPCs are adjusted to position the Fermi level in the spin-split energy gap, while that of the central QPC is kept wide enough to populate both spin-split bands. Measurement of the conductance of the end QPCs at low temperatures ($\le $ 4.2K) showed a splitting of the first conductance quantization plateau. The end QPCs are used as spin injectors and detectors with 100{\%} efficiency to study spin-polarized transport in the central QPC. The 3-QPC device we have studied can conceivably be used to study Rashba spin precession in a 1D channel to check the concept of the Datta-Das spinFET. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D12.00009: Spin Dynamics of InAs Quantum Dots with Uniform Height. T.A. Kennedy, J. Whitaker, A.S. Bracker, D. Gammon, S.E. Economou, T.L. Reinecke Spin splittings and relaxation times were studied by Time-Resolved Faraday Rotation (TRFR) in InAs self-assembled quantum dots. Three twenty-layer samples with different dopings were grown by the Indium-flush method. This technique produces a nearly constant dot-height of 2.5 nm. The TRFR was performed using a 1.3 ps pulse Ti:sapphire laser with the sample at 5.7 K. In the undoped and lightly doped samples, signals are observed from exitons in neutral dots and from electrons and trions in negatively charged dots. Simulations for both the neutral and charged dots account for the results very well. The in-plane electron g-factor is 0.42 and shows very little variation from sample to sample or with energy in spectral studies. We ascribe this to the fixed height of the dots. The hole g-factor can be extracted cleanly from the results for the heavily doped sample. Two of the samples exhibit mode-locking of the electrons spins at 12 ns demonstrating that T$_{2}$ is much longer than T$_{2}$*. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D12.00010: Gating a ferromagnetic semiconductor A. Bove, F. Altomare, N. Kundtz, A.M. Chang, Y.J. Cho, X. Liu, J. Furdyna Ferromagnetic semiconductors have the potential of revolutionizing the way current electronic devices work: more so, because they are compatible with current fabrication lines and can easily be integrated with today's technology. Particular interest lies in III-V Diluted Magnetic Semiconductor (DMS), where the ferromagnetism is hole-mediated and the Curie temperature can therefore be tuned by changing the concentration of free carriers\footnote{T. Dietl \textit{et al.}, Phys. Rev. B \textbf{63}, 195205 (2001)}. In these systems, most of the effort is currently applied toward the fabrication of devices working at room-temperature: this implies high carrier density accompanied by low mobility and short mean free path. We will report our results for a ferromagnetic 2DHG system with low carrier density ($\sim 3.4E12$ cm$^{-2}$) and mobility ($\sim$ 1000 cm$^2/(Vs)$), and we will discuss the effects of local gating\footnote{H. Ohno \textit{et al.}, Nature \textbf{408}, 944 (2000)} in light of possible applications to the fabrication of ferromagnetic quantum dots. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D12.00011: Non-equilibrium Kondo effect in a quantum dot: Real-time density matrix formulation with non-crossing approximation Changxue Deng, Xuedong Hu We study the non-equilibrium electron transport through a quantum dot in the Kondo regime for an infinite-U Anderson model with the self-consistent non-crossing approximation (NCA). We apply the real-time density matrix (RTDM) formulation, which is appropriate for both equilibrium and non-equilibrium situations. We study the Kondo resonances by calculating the spectral function of the localized electron. Results are reported for both spin-degenerate and spin-resolved cases by applying external magnetic fields on the electron in the QD. It is well-known that NCA gives a spurious peak at the chemical potential as it neglect the vertex correction for the spin splitting case. We show that this spurious peak can be removed by using the exact result of the non-interacting Anderson model when calculating the empty state self-energy. We also discuss the differential conductance through the QD, which can be measured in a transport experiment. We find that the separation of the two Kondo peaks in the conductance for a spin resolved dot is smaller than twice of the Zeeman energy, and there exists a critical field below which the Kondo resonance does not split. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D12.00012: Nuclear Polarisation in Quantum Wires Anson Cheung, V. Tripathi We consider the intriguing possibility that current flow within a quantum wire can produce nuclear polarisation. The quantum wire is special because electrons can only move along one direction. Also, because of its heterogeneous structure, spin-orbit effects come into play. Together, this means that electrons are only permitted to have spin up or down orientations within the wire. By exploiting this and the Overhauser effect, we calculate the degree of nuclear polarisation and the electronic conductance arising from the effect of a non-equilibrium current. [Preview Abstract] |
Session D13: Focus Session: Electronic Structures of Transition-Metal Oxides
Sponsoring Units: DMP GMAGChair: Silke Biermann, Centre de Physique Theorique, Ecole Polytechnique
Room: Colorado Convention Center Korbel 4C
Monday, March 5, 2007 2:30PM - 3:06PM |
D13.00001: Probing the Electronic Structure of Metal Oxides using Resonant Inelastic Soft X-Ray Scattering and Soft X-ray Emission Spectroscopy. Invited Speaker: While photoemission spectroscopy is often the probe of choice in studying the electronic structure of solids, there are many sample and environmental constraints that must be satisfied before meaningful data can be obtained with this spectroscopy. Specifically, samples generally need to be electrically conducting single crystals, with atomically clean and ordered surfaces. Clearly, complimentary electronic structure probes applicable to non-crystalline samples, insulators, or samples with poorly controlled surfaces are highly desirable. I will discuss the application of two such techniques: synchrotron radiation-excited soft x-ray emission spectroscopy and resonant inelastic x-ray scattering. By virtue of being photon-in/ photon-out probes, these techniques can measure the electronic structure of solids in circumstances where photoemission spectroscopy is inapplicable. Soft x-ray emission spectroscopy provides a direct measure of the element- and site-specific local partial density of states, while resonant inelastic soft x-ray scattering measures element specific low energy excitations such as d-d$* $or charge transfer transitions. Recent examples of the application of these spectroscopies to a variety of metal oxide systems, including correlated and low dimensional systems, will be presented. \textit{Research supported in part by the Department of Energy under DE-FG02-98ER45680 and the AFOSR under FA9550-06-1-0157. Experiments were performed at the NSLS and the ALS, which are supported by the Department of Energy. } [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D13.00002: Hybridization of local Frenkel excitons in strongly interacting NiO Wei Ku, Chi-Cheng Lee, Hung-Chung J. Hsueh Recent experimentally observed tightly bound excitons in NiO are explained by a newly developed linear response theory within LDA+U approximation. A novel picture of local Frenkel excitons naturally emerges from a real space formulation of exciton formation using the energy- resolved Wannier functions. Systematic analysis of microscopic interacting processes reveals that the large 1 eV splitting between the excitons is due to a strong hybridization between the Frenkel excitons via strong local interactions. Our new picture can be viewed as a simplest representation of the charge excitation involving complex multiplet structure in strongly correlated systems. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D13.00003: Unusual magnetic ground state in MnO under pressure. Klaus Koepernik, Deepa Kasinathan, Warren E. Pickett A study of the phase transitions in MnO under pressure is presented. The calculations are based on density functional theory. The onsite correlations in the Mn 3d shell are treated within the framework of LSDA+U. The major result is that the first phase transition (with increasing pressure), which is characterized by an isostructural magnetic moment collapse from spin 5/2 to spin 1/2, results in a low spin solution exhibiting an unexpected intra-atomic spin polarization pattern. An analysis of the influence of the symmetry, the magnetic ordering and the LSDA+U interactions shows that this unusal spin arrangement is the result of inter-orbital exchange terms. The dependence of the results on the parameters U and J will be discussed. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D13.00004: Evolution of MnO under Pressure from Dynamical Mean Field Theory Warren E. Pickett, Kwan-woo Lee, Richard T. Scalettar, Jan Kune\v{s}, A.V. Lukoyanov, V.I. Anisimov Late transition metal oxides qualify as so called charge-transfer insulators whose description requires that the simple Hubbard interaction within the $3d$ orbitals has to be augmented by mixing with the ligand $2p$ states. MnO is a relatively simple realization (at ambient pressure) of such a system. Its pressure evolution at room temperature exhibits structural (B1--B8), magnetic (high spin -- low spin) and electronic (insulator -- metal) transitions, and correlated band theories$^1$ predict a S=5/2 to S=1/2 moment collapse. We report All-Electron + DMFT high-spin to low-spin or insulator to metal transitions, and a study of the paramagnetic fcc phase as volume is reduced, focusing on the behavior of the local magnetic moment and the metal-insulator transition. We also present single-particle excitation spectra that illuminate the character of the evolution.\\ $^1$D. Kasinathan et al., Phys. Rev. B {\bf 74}, 195110 (2006) [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D13.00005: Structural and Electronic Properties of Monoclinic TiO$_2$ (B) Polymorph Michel Posternak, Alfonso Baldereschi, Bernard Delley Three major polymorphs of TiO$_2$ are known: rutile, anatase, and brookite. A further phase, TiO$_2$ (B), which coexists with, and derives from natural anatase has recently been identified\footnote{J.F.Banfield \textit{et al.}, Am. Mineral. \textbf{76}, 343 (1991).}. It is monoclinic with $C^3_{2h}$ space group, and its conventional cell contains 8 TiO$_2$ formula units. Using the DMol$^3$ approach\footnote{B. Delley, J. Chem. Phys. \textbf{113}, 7756 (2000).}, we study the structural and electronic properties of this polymorph in terms of the OTi$_3$ complex, that we have recently shown\footnote{M. Posternak \textit{et al.}, Phys. Rev. B \textbf{74}, 125113 (2006).} to be the relevant building block for describing the electronic properties of the three major polymorphs. At variance with these latter cases, the 16 O atoms in TiO$_2$ (B) are \textit{not} all threefold coordinated: indeed, 12 O atoms belong to anataselike OTi$_3$ structural units, and the remaining 4 O atoms are twofold coordinated. The outcome of structural differences on the electronic properties of the TiO$_2$ phases is analyzed. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D13.00006: Photoinduced charge and spin dynamics in strongly correlated electron systems Hiroaki Matsueda, Sumio Ishihara It is widely recognized that the competition among multiple phases is a key issue to understand electronic properties in strongly correlated electron systems. A tiny amount of external perturbation breaks balance among these phases, and gigantic response appears. Photoirradiation by the femtosecond pulse laser is a powerful tool to induce the response. For understanding the mechanism of the response after the photoirradiation, pump-probe spectroscopy measurements on perovskite manganese oxides have been performed in recent years. The main issue is the photoinduced transition between ferromagnetic metallic and charge-ordered insulating (COI) phases. Motivated by the transition, we examine the effect of the photoirradiation on the COI phase in the extended double-exchange model. We calculate the transient optical absorption spectrum by the density matrix renormalization group method. The COI once goes to a metallic state, and the metal tends to go back to the initial COI. The antiferromagnetic spin correlation of the localized spins in the ground state is greatly suppressed. Possible scenarios for photoinduced ferromagnetism are discussed. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D13.00007: Novel electronic and magnetic properties of a new class of cupper oxides Xiao Hu, Xiangang Wan, Masanori Kohno Cuprates have not been considered seriously as candidate of useful magnetic material since the known ferromagnetic cuprates show quite low \(T_c\). The recently reported cuprate Sr\(_8\) CaRe\(_3\)Cu\(_4\)O\(_{24}\), a Mott insulator with perovskite structure, exhibits surprisingly macroscopic magnetization up to \(T_c=440 K\). Doing LSDA+U calculations, we reveal theoretically [X.-G. Wan, M. Kohno, and X. Hu, Phys. Rev. Lett. vol. 94, 087205 (2005).] that an orbital order appears in Cu atoms which results in a ferrimagnetic ground state, and that the pd\(\sigma\) bonds are responsible to the strong super exchange interactions and thus the high \(T_c\). We propose a spin model and perform quantum Monte Carlo simulations, with which we can reproduce accurately the observed magnetization curve including the critical point \(T_c\). Moreover, a half-metal (HM), which behaves as metal for one spin channel and insulator for the opposite, is predicted when replacing Re with W or Mo [X.-G. Wan, M. Kohno, and X. Hu, Phys. Rev. Lett. vol. 95, 146602 (2005)]. Hole doping into the material will also result in HM, with the net magnetic moment changing from negative to positive upon tuning the doping rate. Therefore, an antiferromagnetic HM, a material which has yet been found so far, may be achieved where net magnetization disappears. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D13.00008: Theory of thermopower in strongly correlated electron systems Wataru Koshibae, Sadamichi Maekawa We have studied the effects of spin and orbital degrees of freedom in the strongly correlated electron systems, and have derived the formula of the high-temperature thermopower: \begin{equation} Q = - (k_B/e)\ln (g_e/g_h) - (k_B/e)\ln [n_h/(1 - n_h)], \end{equation} where $n_h$ is the hole concentration, and $g_e$ ($g_h$) denotes the local degeneracy of the electronic configuration on the transition metal ion without (with) hole carrier. The local degeneracy is determined by the spin and orbital degrees of freedom. It has been established that the formula (1) gives a good estimation of the thermopower in not only the 3$d$ transition metal oxides but also the 4$d$ ones, recently. We have studied the thermopower in the oxides composed of several kinds of transition metal ions. Its high-temperature formula shows a complicated expression, however, it is expressed to be the average of the first term of the equation (1) in the case that $n_h = 0.5$, in the double perovskite system. This is because the thermopower is nothing but the entropy carried by the electric current. We will discuss the thermopower of the oxides with several kinds of transition metal ions in the light of the theory. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D13.00009: Hybrid density functional study of Mott transition in MnO Cristian V. Diaconu, Richard L. Martin, Ionut D. Prodan, Gustavo E. Scuseria The electronic structure, the magnetic moment and volume collapse of MnO under pressure is obtained from hybrid density functional theory using the recently developed screened hybrid exchange-correlation functional of Heyd, Scuseria and Ernzerhof (HSE). We study two crystal structures for MnO: cubic (rock salt) and hexagonal (nickel arsenide). We find two antiferromagnetic states for the NaCl structure: a high-spin state that couples two $S=5/2$ moments, and a low-spin state that couples two $S=1/2$ moments. At ambient pressure the high-spin state lies lowest. The low-spin phase becomes favored at a pressure of about $248\,\mathrm{GPa}$, leading to a first order volume collapse. However, this transition is pre-empted by another first-order volume collapse at $178\,\mathrm{GPa}$ from the NaCl structure to a NiAs structure. This transition is predicted to be insulator to metal and is the realization of the Mott transition. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D13.00010: Disorder driven quantum phase transitions in transition metal oxides Kohjiro Kobayashi, Nandini Trivedi We investigate the effect of disorder on a class of transition metal oxides described by a single orbital Hubbard model at half filling and away from half filling. The phases are characterized by the nature of the electronic and spin excitations. We calculate the local density of states, frequency and temperature-dependent conductivity and spin susceptibility as functions of disorder and interaction. The interplay of disorder and correlations produces ususual behavior in the correlated metal, for example, characteristic suppression of density of states at low energies, persistence of gap like features at finite frequency and the presence of local moments. Some of these puzzles can be understood in terms of an inhomogeneous system composed essentially of two-components. We compare our results with recent local scanning tunneling spectroscopy, and optical conductivity measurements. Reference: D. Heidarian and N. Trivedi, Phys. Rev. Lett. 93, 126401 (2004); K. Kobayashi, B.H. Lee, and N. Trivedi, cond-mat. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D13.00011: ABSTRACT WITHDRAWN |
Session D14: Focus Session: MRAM and Magnetic Devices
Sponsoring Units: GMAG FIAPChair: Steve Russek, National Institute of Standards and Technology, Boulder
Room: Colorado Convention Center Korbel 4D
Monday, March 5, 2007 2:30PM - 2:42PM |
D14.00001: Microstrip phase shifter using Fe as the active element Andrew Hutchison, Yuri Khivintsev, Bijoy Kuanar, Ian Harward, Zbigniew Celinski, Robert Camley In this work, we investigate the performance of microstrip phase shifters using FMR effects of an iron film. Using standard techniques, Fe films of thickness varying from 100nm to 800nm were placed between the signal line and dielectric. This geometry may be constructed in one technological step. For a microstrip line with 100 nm iron the frequencies providing the maximum performance were at 8 and 27 GHz over a magnetic field range of 0.08 to 2.77 kOe. For an Fe film of 800 nm, the best performance was found at 10 and 45 GHz. In this case, the high-frequency differential phase shift had a weaker frequency dependence suitable for broadband applications. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D14.00002: Nonlinear effects in iron based microstrip structures. Zbigniew Celinski, Bijoy K. Kuanr, Yuri V. Khivintsev, Andrew Hutchison, Robert E. Camley Nonlinear effects in magnetic films are a subject of growing interest. The onset of parametric instability translates into practical power limits for microwave devices. Nearly all high power studies were done in ferrites; recently An et al investigated Permalloy. No work has been performed on planar devices or on iron films. Here we investigate the transmission of cw-microwaves in a 6 mm x 13 micron, 200 nm iron based microstrip notch filter in the frequency domain. There are three regions in the transmission response. Up to a threshold power of P = 90 mW, the differential absorption of ferromagnetic resonance (FMR) is nearly constant as a function of input power. Above P, the sample absorption decreases significantly as the power is increased. In addition we observe a subsidiary absorption (SA) peak at a frequency above that of the FMR. In comparison to Fe, a 7.2 micron YIG film in the transducer geometry has P at 1$\sim $2 mW; for a permalloy 128 nm film it is 10 mW. This indicates that an Fe microstrip has a much higher power handling capability. Finally, our structures can also be used as a power limiter. The SA can be significantly increased at high powers, thus limiting the transmission in the frequency range where the SA occurs [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D14.00003: Injection and Propagation of Magnetic Domain Walls in Thin Film Nanowires Carl Knutson, Geoffrey Beach, James Erskine, Maxim Tsoi The fields required for injection and propagation of magnetic domain walls in thin-film nanowires were studied using high-bandwidth scanning Kerr polarimetry. This method has recently been used for studying the field- and current-driven dynamics of magnetic domain walls [1, 2]. Nanowires of various widths were fabricated from a 20 nm-thick film of Permalloy (Ni$_{80}$Fe$_{20})$ etched using a focused ion beam (FIB). These wires were joined at their ends by large-area continuous film regions. Domain walls were introduced into a wire by applying an injection field sufficient to ``inject'' a nucleated wall from the continuous film into the geometrically-constrained wire neck. After injection, a dc propagation field, typically less than the injection field, is capable of driving the wall at a constant velocity. The influence of nanowire geometry on the injection and propagation fields, and the variation of these fields induced by a dc spin-polarized electric current, will be discussed. [1] G.S.D. Beach, C. Nistor, C. Knutson, M. Tsoi, J.L. Erskine, Nat. Mater. \textbf{4}, 741 (2005). [2] G.S.D. Beach, C. Knutson, C. Nistor, M. Tsoi, J.L. Erskine, Phys. Rev. Lett. \textbf{97}, 057203 (2006). [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D14.00004: Switching of magnetic domain structure in permalloy microstructures using 2D electron gas. Andrey Belkin, Jan Fedor, Piotr Pankowski, Valentyn Novosad, Goran Karapetrov, Vladimir Cambel, Dagmar Gregusova, Robert Kudela We demonstrate the ability to monitor and change the magnetization state of microscopic permalloy element deposited on the active area of a 2DEG Hall probe. While sweeping the external magnetic field recorded Hall voltage signal provides information on local magnetization of the ferromagnetic element. Simultaneously, the exact magnetization state of permalloy element is imaged with a magnetic force microscope. Applying short, but intense current pulses through the Hall probe we can change the magnetization state of the permalloy ellipse. Such hybrid semiconductor-ferromagnet structures could offer novel direction for non-volatile memory storage elements. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D14.00005: Systematic tuning of magnetization reversal properties in Permalloy nanowires using sloped ends Oleg Petracic, Dan Read, Russell Cowburn The magnetization reversal of Permalloy (NiFe) nanowires was investigated by Magneto Optic Kerr effect (MOKE) magnetometry, where one end of the wire exhibits a slope in the thickness. Arrays of straight nanowires with a thickness of 8nm, widths of 150nm and 100um length were prepared by electron-beam lithography. The sloped ends were achieved by using penumbra shadow masks during NiFe deposition. The topography of the wires was studied by atomic force microscopy. One finds that the slope profile can be tuned by the position under the mask, mask-to-sample distance and angle of deposition. Corresponding MOKE hysteresis loops show a systematic reduction of the coercive field with increasing length of the sloped part. E.g. wires, where the slope has a length of 30um exhibit a coercive field of 11 Oe, whereas nanowires without sloped ends show 107 Oe. One can conclude that the coercivity can be controlled not only by modifying the lateral shape of magnetic nanoelements but also by their thickness profile. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D14.00006: First principles Modeling of Magnetoresistance in Magnetic Memory devices Kurt Stokbro, Morten Stilling, Karsten Flensberg We have performed first principles calculations of the zero- bias conductance and TMR for crystalline Fe-MgO-Fe MTJs, and studied the effects of different oxide layers in the Fe/MgO interface, and the effects of structural ``disorder'' in the device. We find that such ``defects'' in the atomic structure have strong effects on the conductance. We use the result of the calculations to rationalize recent experimental findings. The simulations have been done with the software package Atomistix ToolKit (ATK), which is based on density functional theory (DFT) and non-equilibrium Green's functions (NEGFs). [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 4:18PM |
D14.00007: Nanomagnetic Bit Cells for MRAM Applications Invited Speaker: Magnetoresistive Random Access Memory (MRAM) combines magnetic tunnel junction devices with standard silicon-based microelectronics to obtain the combined attributes of non-volatility, high-speed operation, and unlimited read/write endurance not found in any other existing memory technology. The first MRAM product to market, Freescale's 4Mb MR2A16A, is built on 180 nm CMOS technology with magnetic bit cells of 300 nm minimum dimensions integrated in the upper layers of metal. At these dimensions, both the magnetic switching and magnetoresistive property distributions are governed by a combination of material and patterning variations. One of the keys to controlling these distributions and insuring manufacturability was the invention of the Toggle Write mode. This mode uses a balanced synthetic antiferromagnetic free layer combined with a phased write pulse sequence to achieve robust magnetic switching margin by eliminating the half-select disturb issue found in conventional approaches. Another crucial solution was the ability to deposit and pattern high-quality, high-TMR magnetic tunnel junctions with narrow bit-to-bit resistance variation, low defect density and long-term reliability. In this talk, I will present details of each of the above technology elements, the performance and bit cell reliability, and the scaling behavior to the reduced dimensions of advanced technology nodes. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D14.00008: Permanent-Magnet Free Biasing of MR Sensors with Tunable Sensitivity Sean Halloran, Fabio daSilva, David Pappas Exchange coupling$^{1}$ has been previously observed in a trilayer structure of ferromagnet (FM)/non-magnetic/antiferromagnet (AFM) and the exchange bias was found to be a function of the thickness of the buffer layer.$^{2,3,4}$ This unique coupling is used as a stabilizing bias for the sense layer with the additional ability to tailor the magnetic gain of the sensor for various applications. The elimination of permanent magnet bias results in the elimination of one patterning and one deposition step. Ruthenium (Ru) is used as the buffer layer and is self aligned with the FM and AFM layers and the thickness is varied to change the slope of the transfer curve in the linear region. Sensor devices are fabricated with a bipolar output, a medium sensitivity, and a wide field range. The results show that this biasing scheme is well suited for barber pole and soft adjacent layer (SAL) anisotropic magnetoresistance (AMR) stripes used in magnetic field sensors with a FM layer of Permalloy (NiFe) and an AFM layer of Iridium-Manganese (IrMn). Applications include a 256 channel read head used for magnetic forensics. 1N.J. Gokemeijer, T. Ambrose, C.L. Chien, N. Wang and K.K. Fung, J. Appl. Phys. \textbf{81 }(8), 4999, 15 April 1997. 2W.H. Meiklejohn and C.P. Bean, Phys. Rev. \textbf{102}, 1413 1956; \textbf{105}, 904, 1957. 3L. Thomas, A.J. Kellock and S.S.P. Parkin, J. Appl. Phys. \textbf{87 }(9), 5061, 1 May 2000. 4D. Wang, J. Daughton, C. Nordman, P. Eames and J. Fink, J. Appl. Phys. \textbf{99}, 2006. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D14.00009: Effect of Interfacial Disorder on 1/f Noise in Magnetic Tunnel Junctions Stephen Russek, Justin Shaw, Juan Francisco Sierra Magnetic tunnel junctions (MTJs) have the potential for low field ( 1pT/Hz$^{0.5 }$@ 1 Hz) magnetic sensors. However, 1/f noise limits their performance. Here we correlate measured 1/f noise with dynamic Lorentz imaging and high-frequency ferromagnetic resonance (FMR) measurements. The measurements show that a large fraction of the 1/f noise is due to thermal fluctuations of nano-scale magnetic ripple structure which arises from a combination of disorder in the antiferromagnetic exchange bias layer and interfacial roughness in the tunnel barrier. We have changed the interfacial properties by varying growth conditions and by inserting nano-oxides. The samples show varying amounts of disorder that manifests itself as increased ripple structure, increased 1/f noise, and a broadened FMR linewidth. Time dependent Lorentz imaging has been used to directly observe nano-scale thermal fluctuations that give rise to 1/f noise. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D14.00010: IrMn pinning rings to suppress closure domains in the sense layer of magnetic tunnel junctions William Egelhoff, Cindi Dennis, John Unguris, Casey Uhlig, Robert McMichael, Mark Stiles, Cedric Powell We have investigated very soft magnetic materials as sense films for use in magnetic tunnel junctions. One problem MTJs face is that the soft layer tends to break up into domains. Flux closure at the edges seems to be the driving force. Such domains are incompatible with low noise MTJs. We have found what appears to be a solution to this problem using an IrMn ring at the edges of the sense layer. Using a soft-film circle $\sim $100 microns in diameter and an IrMn ring $\sim $10 microns in width at the edge of the circle, we have a central circle $\sim $80 microns in diameter that is nearly as soft as a large area film. The pinning of the soft layer appears to die off within a few microns of the IrMn edge, leaving ample area for an MTJ structure on a single-domain soft layer. In addition, the hard axis of the soft film (which is used to attain a sensor with linear response) has very little hysteresis. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D14.00011: Position dependence of thin layer notch filter waveguides Tim Fal, Bijoy K. Kuanr, Robert E. Camley, Zbigniew Celinski Because of the importance of processing high frequency electromagnetic waves for communication, there has been interest in developing ultra-small thin-layer magnetic notch filters. These filters operate in the 5-40 GHz range. In the past theoretical work has concentrated on a structure where the magnetic film was right next to the one of the conductive films in a waveguide. Here we present a theoretical model, which investigates the properties of a waveguide with two dielectric films and one magnetic film placed between two conductive layers. We find that this more general structure produces a deeper attenuation and a narrower peak compared to the earlier structure. The additional attenuation is on the order of 20 dB/cm for the same parameters. The comparison between experiment and theory is presented. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D14.00012: Subwavelength Magnetic Plasmon Waveguides Dentcho Genov, Hui Liu, Dongmin Wu, Yongmin Liu, Jennifer Steele, Cheng Sun, Shining Zhu, Xiang Zhang A one-dimensional magnetic plasmon propagating in a linear chain of single split ring resonators is proposed. The subwavelength size resonators interact mainly through exchange of conduction current, resulting in stronger coupling as compared to the corresponding magnetoinductive interaction. Finite-difference time-domain simulations in conjunction with a developed analytical theory show that efficient energy transfer with signal attenuation of less then 0.57dB/$\mu $m and group velocity higher than 1/4c can be achieved. The proposed novel mechanism of energy transport in the nanoscale has potential applications in subwavelength transmission lines for a wide range of integrated optical devices. [Preview Abstract] |
Session D15: Focus Session: Triangular Lattice Antiferromagnetism
Sponsoring Units: GMAGChair: Oleg Tchernyshyov, The Johns Hopkins University
Room: Colorado Convention Center Korbel 4E
Monday, March 5, 2007 2:30PM - 3:06PM |
D15.00001: Spin-1/2 Heisenberg antiferromagnet on an anisotropic triangular lattice Invited Speaker: The Triangular lattice spin-1/2 Heisenberg AntiFerromagnet (TAF) is a prototypical model of frustrated quantum magnetism. While it is believed to exhibit long-range order in the isotropic limit, changes such as spatial anisotropy can alter the delicate balance amongst competing ground states. I will describe the static and dynamic properties of the spatially anisotropic TAF, with inter-chain diagonal exchange $J'$ much weaker than the intrachain exchange $J$. Treating $J'$ as a perturbation of decoupled Heisenberg spin-$1/2$ chains, I find that the ground state is spontaneously dimerized in a four-fold degenerate zig-zag pattern. This dimerization instability is driven by quantum fluctuations, which are dramatically enhanced here by the frustrated nature of inter-chain exchange. A magnetic field partially relieves frustration, by canting the spins along the field direction, and causes a quantum phase transition into a magnetically-ordered spin-density-wave phase. This is followed by cone and, finally, fully polarized (saturated) phases, as a function of increasing magnetic field. I show that many of these features are in fact observed in experiments on the celebrated material Cs$_2$CuCl$_4$ ($J'/J =1/3$). I will also discuss the significant modification of the phase diagram by symmetry-breaking anisotropic Dzyaloshinskii-Moriya (DM) interactions, present in this interesting magnet. In addition to static and thermodynamic properties, the proposed ``one-dimensional'' approach offers a compelling explanation of the unusual experimentally measured dynamical structure factor of Cs$_2$CuCl$_4$ in terms of descendants of one-dimensional spinons. Quite generally, I find characteristic features of a momentum-dependent spinon bound state and a dispersing incoherent excitation in the structure factor, in agreement with experiments. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D15.00002: Effective field theory with a $\theta$-vacua structure for 2d spin systems Akihiro Tanaka, Xiao Hu We derive a new 2+1d nonlinear sigma (NL$\sigma$) model description for coupled spin chains with competing AF-VBS orders, incorporating methods developed recently by ourselves and by Senthil and Fisher. The resulting 2+1d $O$(4) NL$\sigma$ model contains a topological $\theta$-term whose vacuum angle $\theta$ varies continuously with $\delta$, the bond-alternation strength of the interchain interaction. This implies that the $\theta$- vacua structure for this NL$\sigma$ model can be explored by tuning $\delta$ in a suitable 2+1d spin system, as in the case of the 1+1d AF spin chains with bond-alternation. We discuss the implications for frustrated spin systems. A. Tanaka and X. Hu, Phys. Rev. B{\bf 74}, 140407 (2006). [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D15.00003: Spin Waves in Stacked Triangular Antiferromagnetic Thin Films E. Meloche, M. L. Plumer, C. M. Pinciuc A theory is presented for surface and bulk spin waves (SW) in stacked triangular antiferromagnetic thin films. The model uses a microscopic Heisenberg Hamiltonian which includes the dominant exchange interactions with either easy-plane [1] or easy-axis single-ion anisotropy. The equilibrium spin configurations in the frustrated thin films are obtained using a local field alignment approach. The effects of modified surface exchange and anisotropy parameters on the equilibrium spin configurations and the SW dispersion relations are discussed. Numerical results show that the presence of surfaces can give rise to several localized excitations that are characterized with decaying amplitudes into the bulk. The energy of the localized SW modes are found to be extremely sensitive to the boundary conditions. The spin-correlation functions are obtained using a Green's function formalism and used to evaluate the mean-squared amplitude of spin precession as a function of the distance away from the surfaces. Comparisons are made between the properties of SW excitations in frustrated magnetic systems that are quasi one- and two-dimensional in character. [1] E. Meloche, C. M. Pinciuc, M. L. Plumer, Phys. Rev. B \textbf{74} (2006) 94424. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D15.00004: Octupolar order in the multiple spin exchange model on a triangular lattice Tsutomu Momoi, Philippe Sindzingre, Nic Shannon We showed how a gapless spin liquid with hidden octupolar order arises in applied magnetic field, in a model applicable to thin films of solid $^3$He with competing ferromagnetic and antiferromagnetic (cyclic) exchange interactions. In this, dynamical effects lead to the formation of three-magnon bound states, which undergo Bose-Einstein condensation, giving rise to octupolar order. We confirmed its existence through the exact diagonalization of finite-size clusters. We also presented evidence for nematic -- i.e. quadrupolar -- correlations bordering on ferromagnetism in the absence of magnetic field. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D15.00005: Quadrupolar phases of the S=1 bilinear-biquadratic Heisenberg model on the triangular lattice Andreas L\"auchli, Fr\'ed\'eric Mila, Karlo Penc Using mean-field theory, exact diagonalizations and SU(3) flavour theory, we have precisely mapped out the phase diagram of the $S=1$ bilinear-biquadratic Heisenberg model on the triangular lattice in a magnetic field, with emphasis on the quadrupolar phases and their excitations. In particular, we show that ferroquadrupolar order can coexist with short-range helical magnetic order, and that the antiferroquadrupolar phase is characterized by a remarkable 2/3 magnetization plateau, in which one site per triangle retains quadrupolar order while the other two are polarized along the field. Implications for actual $S=1$ magnets, such as NiGa$_2$S$_4$, are discussed. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D15.00006: Flat spin wave dispersion in a triangular antiferromagnet Oleg Starykh, Andrey Chubukov, Alexander Abanov The excitation spectrum of a S=1/2 two-dimensional triangular quantum antiferromagnet is studied using 1/S expansion. Due to the non-collinearity of the classical ground state significant and non-trivial corrections to the spin wave spectrum appear already in the first order in 1/S in contrast to the square lattice antiferromagnet. The resulting magnon dispersion is almost flat in a substantial portion of the Brillouin zone. Our results are in quantitative agreement with recent series expansion studies by Zheng, Fj\ae restad, Singh, McKenzie, and Coldea [PRL {\bf 96}, 057201 (2006) and cond-mat/0608008]. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D15.00007: Frustrated antiferromagnet on generalized partial line graphs of a honeycomb lattice Shin Miyahara, Chisa Hotta, Kenn Kubo, Nobuo Furukawa Recently we have proposed generalized partial line graphs on which tight binding models of electronic energy bands realize flat bands [1]. We study Heisenberg antiferromagnets on these structures, which are frustrated and may realize novel ground states. In this report, we focus on a generalized partial line graph created on a honeycomb lattice. The model is a honeycomb lattice composed of A and B-sublattice, where a triangle cluster sits on a site of A-sublattice and a single spin exists on a site of the B-sublattice. We assume the two-types of exchange coupling: J inside the triangle cluster and J' between the cluster and B-sublattice. In the limit J $<<$ J' the model is equivalent to the spin-1 triangle model. We introduce an effective model for two limits J $>>$ J' and J $<<$ J'. The results of exact diagonalization will be reported and discussed. [1] S. Miyahara et al., J. Phys. Soc. Japan, 74 1918 (2005). [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D15.00008: Excitation spectra of the spin-$1/2$ triangular-lattice Heisenberg antiferromagnet Rajiv R. P. Singh, Weihong Zheng, John Fj{\ae}restad, Ross McKenzie, Radu Coldea We use series expansion methods to calculate the dispersion relation of the one-magnon excitations for the spin-$1/2$ triangular-lattice nearest-neighbor Heisenberg antiferromagnet above a three-sublattice ordered ground state. Several striking features are observed compared to the classical (large-$S$) spin-wave spectra. Whereas, at low energies the dispersion is only weakly renormalized by quantum fluctuations, significant anomalies are observed at high energies. In particular, we find roton-like minima at special wave-vectors and strong downward renormalization in large parts of the Brillouin zone, leading to very flat or dispersionless modes. We present detailed comparison of our calculated excitation energies in the Brillouin zone with the spin-wave dispersion to order $1/S$ calculated recently by Starykh, Chubukov, and Abanov [cond-mat/0608002]. We find many common features but also some quantitative and qualitative differences. We show that at temperatures as low as $0.1J$ the thermally excited rotons make a significant contribution to the entropy. Consequently, unlike for the square lattice model, a non-linear sigma model description of the finite-temperature properties is only applicable at temperatures $\ll 0.1J$. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D15.00009: Excitation spectrum of an ordered Heisenberg antiferromagnet on a triangular lattice Sasha Chernyshev, Michael Zhitomirsky We have shown that magnon decays must be prominent in a wide class of noncollinear antiferromagnets. We calculated the decay rate in the spin-1/2 triangular lattice antiferromagnet within the spin-wave theory. In the long-wavelength limit, the life-time of low-energy excitations is predicted to exhibit a non-trivial scaling. For the short-wavelength magnons, the decay rate is large, $2\Im{\rm m}\{\tilde\varepsilon_{\bf k}\} \sim 0.4\Re{\rm e}\{\tilde\varepsilon_{\bf k}\}$, in a substantial part of the Brillouin zone. Topological transitions of the decay surface also lead to strong singularities in the spectrum that remain essential even for large values of spin. Therefore, excitations in ordered, spin-$S$, AFs may not necessarily be well-defined for all wave-vectors.\newline \newline [1] A. L. Chernyshev and M. E. Zhitomirsky, Phys. Rev. Lett. {\bf 87}, 207202 (2006). [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D15.00010: Spinon excitation in spatially anisotropic frustrated Heisenberg models Masanori Kohno, Leon Balents, Oleg Starykh We investigate the elementary excitations of spin-1/2 antiferromagnetic Heisenberg models with spatially anisotropic frustrated couplings. By projecting the Hamiltonian into a subspace of exact spinon eigenstates of the one-dimensional chains, we obtain and solve an effective Schr\"odinger equation. We argue this weak interchain-coupling approach has a broad regime of validity for frustrated models. As a general feature, we find a bound state of spinons and incoherent excitations depending on the momentum. Various experimental features observed in Cs$_{2}$CuCl$_{4}$ such as asymmetry of the dispersion relation and large tail at $k_x$=$\pi$\footnote{R. Coldea, D. A. Tennant, and Z. Tylczynski, Phys. Rev. B 68, 134424 (2003).} are consistently explained within the framework of the present approach with few adjustable parameters. Our results suggest that the spectral features in Cs$_{2}$CuCl$_{4}$ should be interpreted as descendents of one-dimensional spinons, which persist more strongly at some momenta in spatially anisotropic frustrated systems even with finite interchain couplings. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D15.00011: Competition between charge and spin orders on a frustrated system: The triangular lattice. B. Davoudi, S.R. Hassan, A.-M.S. Tremblay We study the properties of the extended Hubbard model on a triangular lattice by means of generalized two-particle self consistent approach(GTPSC). We present the structure functions over a wide range of physical parameters. Rapid increase in the charge and spin response functions indicates crossover towards various types of charge and spin orders. The competition between the on-, off-site interaction and the frustration leads to a very rich density dependent phase diagram. We also evaluate the one-particle spectral function in various ranges of physical parameters and find that the pseudo-gap can also appear in the regime where the charge density wave is dominant. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D15.00012: Anisotropic interactions in a Quantum Antiferromagnet Cs$_2$CuCl$_4$ as revealed by NMR. Marc-Andre Vachon, Georgios Koutroulakis, Vesna F. Mitrovic, Arneil P. Reyes, Philip L. Kuhns, Radu Coldea We report nuclear magnetic resonance (NMR) measurements on the 2D frustrated quantum antiferromagnet Cs$_2$CuCl$_4$. $^{133}$Cs spectra at temperatures down to $60$\,mK and in external magnetic field up to $16$\,T are presented. We discuss the details of the destruction of the 3D long range ordered phases in this compound. The effects of temperature and the applied field are contrasted, that is, the details of the nature of the phase transition will be discussed. Furthermore, we will address the influence of anisotropic interactions on these phase transitions. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D15.00013: Spin Order by Frustration in Triangular Lattice NaCrO$_2$ : A Neutron Scattering Study D. Hsieh, D. Qian, R.F. Berger, B. Ueland, P. Schiffer, R.J. Cava, J.W. Lynn, Q. Huang, M.Z. Hasan We report high resolution neutron scattering measurements on the triangular lattice antiferromagnet NaCrO$_2$. In contrast to the host compound of the 2D cobaltate superconductor Na$_x$CoO$_2$, no magnetic long-range order is observed down to 1.5K. For $T >$ 40K purely 2D quasi-static 120$^{\circ}$ spin correlations extend to a maximum of 20 lattice constants. For $T <$ 30K, a small in-plane incommensuration develops together with short-range $c$-axis correlations. Our observation of strong spin fluctuations over an extended temperature range and a long wave-length incommensurate modulation strongly suggest an out-of-plane frustration as the mechanism for stabilizing this rare 2D correlated phase in NaCrO$_2$. [Preview Abstract] |
Session D16: Focus Session: Multiferroics II
Sponsoring Units: DMP GMAGChair: Gavin Lawes, Wayne State University
Room: Colorado Convention Center Korbel 4F
Monday, March 5, 2007 2:30PM - 2:42PM |
D16.00001: Manipulation of the ferromagnetic domains of a manganite using an electric field. Tara Dhakal, Sinan Selcuk, Arthur F. Hebard, Amlan Biswas We have measured the response of the fluid like phases$^{1}$ of ferromagnetic metal (FMM) and charge ordered insulator (COI) in thin films of the manganite (La$_{0.4}$Pr$_{0.6})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) to an external electric field. The electric field (set by applying a voltage difference across the material) alters the fluid phases and increases the conductivity of the material by about 2 orders of magnitude above a threshold voltage$^{2}$. To check if the enhanced conductivity is associated with an increase in the size of the FMM domains, we measured the magnetization of the thin films using a SQUID magnetometer with and without an applied electric field. The saturation magnetization remained the same in either case showing that the FMM domains do not increase in size, which led us to hypothesize that the domains are just reoriented by the electric field. This hypothesis was verified by measuring the transverse resistance while a voltage difference was applied longitudinally across the material. At a threshold voltage when the longitudinal resistance decreased by about 2 orders of magnitude, the transverse resistance showed a small increase. This increase in resistance was attributed to the FMM domains being stretched in the direction of the electric field. [1] P. A. Sharma et al., Phys. Rev. B 71, 224416 (2005), [2] Tara Dhakal, et. al, Cond-mat/0607502. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D16.00002: Controlling the magnetic phase of a hole-doped manganite with an out-of-plane electric field. Sung Hee Yun, Rajiv Misra, A. F. Hebard, Amlan Biswas We have measured the effect of an electric field applied perpendicular to the plane of a (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) manganite thin film. Since the thickness of the film is 30 nm, the electric field across the manganite is about an order of magnitude greater than when the voltage is applied in the plane of the film. The films are grown on an insulating substrate NdGaO$_{3 }$(NGO). Hence, we deposited a 30 nm LPCMO film on top of a 60 nm La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) film on an NGO substrate using pulsed laser deposition (PLD). The LCMO film is metallic below 250 K and is used as the bottom electrode to apply the voltage perpendicular to the plane of the LPCMO film. We deposited gold contacts on the LPCMO film as the top electrode. We then etched the LPCMO film, using ion plasma etching or chemical etching, leaving only a small LPCMO pillar beneath the gold contact. An indium contact was pressed on the exposed LCMO film. We then measured current-voltage (I-V) characteristics of this structure. Due to the small interelectrode distance of 30 nm the resultant electric field is high and we were able to measure the effect of an electric field on LPCMO at temperatures well above the insulator to metal transition temperature, for voltages less than 10 V. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D16.00003: Colossal piezoresistance in manganites. Jacob Tosado, Josymir Lopez, Tara Dhakal, Amlan Biswas We have studied the effect of the application of direct mechanical stress on thin films of the hole-doped manganese oxide (manganite) (La$_{1-y}$Pr$_{y})_{1-x}$Ca$_{x}$MnO$_{3}$ (LPCMO). The two competing phases in manganites are the pseudocubic ferromagnetic metallic (FMM) phase and the orthorhombic charge-ordered insulating (COI) phase. Due to the different structures of the FMM and COI phases, manganites are susceptible to mechanical stresses. The traditional methods of applying stress on oxide thin films result in different growth modes which makes it difficult to quantify the strain in the thin film. Using a calibrated screw we applied direct mechanical stress on the substrate and measured the change in the phase diagram of the manganite as a function of strain. Our results show that the effect of strain is the largest in the fluid phase separated region of the phase diagram$^{2}$, where we observe a strain-induced change in resistance of about 5 orders of magnitude. [2] Tara Dhakal, et. al, Cond-mat/0607502. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D16.00004: Spin filtering with magnetic oxide tunnel barriers Invited Speaker: Interesting physical phenomena and potential new devices arise when the barrier of magnetic tunnel junctions is made of a ferroic material. In the most studied case of ferromagnetic barriers, carriers are spin-polarized by the spin-filter effect, which gives rise to tunnel magnetoresistance (TMR). We will present results on the use of ultrathin ferrimagnetic and ferromagnetic layers as tunnel barriers. We have use thin films of NiFe$_{2}$O$_{4}$ (NFO), T$_{C}$=850K, to filter electrons according to their spin with an efficiency of $\sim $25{\%}, as evidenced by a TMR of up to 60{\%} in La$_{2/3}$Sr$_{1/3}$MnO$_{3}$(LSMO)/NFO/Au junctions. We will discuss these results in the frame of a model describing tunneling through epitaxial magnetic barriers. We will also show results on ferromagnetic films of BiMnO$_{3}$ and La$_{0.1}$Bi$_{0.9}$MnO$_{3}$ (LBMO) and their use as spin-filter barriers. Interestingly, LBMO films are also ferroelectric and therefore exhibit a multiferroic character, that is retained down to thicknesses of only 2 nm. Accordingly, LSMO/LBMO/Au junctions exhibit four different resistance states, instead of two with conventional spin-filters. We will discuss the origin of this behavior on the basis on the combination of the spin-filter effect and the influence of ferroelectricity on tunneling. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D16.00005: Ferroelectricity in a Collinear Magnetic Phase of Orthorhombic Perovskites Ivan Sergienko, Cengiz Sen, Elbio Dagotto Below $T_c$ = 30 K, a number of perovskite manganites (such as HoMnO$_3$) and nickelates order magnetically in the so-called E-type phase with zigzag chains of parallel spins. We demonstrate that this magnetic phase is also ferroelectric. We discuss the magnetoelectric coupling based on the symmetry arguments of the Landau theory of phase transitions. We also explore a microscopic mechanism of ferroelectricity induced by the collinear spin arrangement and address the order of magnitude of the ferroelectric polarization. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D16.00006: Ferroelectricity in perovskite HoMnO$_{3}$ and YMnO$_{3}$ induced by magnetic order. B. Lorenz, Y. Q. Wang, C. W. Chu Ferroelectricity is observed in orthorhombic HoMnO$_{3}$ and YMnO$_{3 }$at the magnetic lock-in transitions into an E-type structure or an incommensurate phase with a temperature independent wave vector, respectively. In HoMnO$_{3}$ the ferroelectric polarization strongly depends on the external magnetic field indicating the involvement of the rare earth moment order in this compound. The results are discussed within the framework of recent theoretical models, in particular the double exchange driven polar displacements predicted for E-type magnetic structures. The ferroelectricity observed in YMnO$_{3}$ cannot be explained within the current picture of the magnetic order and a refinement of the magnetic structure seems to be necessary. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D16.00007: Ferroelectricity driven by zig-zag magnetic chains: orthorhombic HoMnO3 as a multiferroic? Silvia Picozzi, Kunihiko Yamauchi, Biplab Sanyal The search for multiferroic materials showing a coexistence of magnetic and ferroelectric long-range order has recently triggered enormous interests due to their potential use in magnetic recording industry. Based on first-principles density functional calculations including electron-electron correlations explicitly in the framework of Hubbard model, we show that a peculiar arrangement of Mn spins can induce a sizeable ferroelectric polarization in distorted rare-earth manganites. In particular, our calculated ground state magnetic structure of orthorhombic HoMnO3 is AFM-E type in which the zig-zag ferromagnetic chains in the MnO2 planes are antiferromagnetically coupled to their neighboring chains. This unconventional magnetic configuration and the different values of the Mn-O-Mn bond angle for parallel and antiparallel Mn spins lead to a coherent displacement of the O center of mass with respect to the Mn center of mass giving rise to a permanent dipole i.e. ferroelectricity. A quantitative analysis of the ferroelectric polarization, as well as trends in the structural, magnetic and ferroelectric properties as a function of the strength of Coulomb correlation parameter will be presented. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D16.00008: Multiferroicity induced by phase modulated spin-density waves Jeroen Vandenbrink, Joseph Betouras, Gianluca Giovannetti Materials in which magnetic and ferroelectric order coexist--termed multiferroics--have recently become the focus of much interest. From a technological point of view the possibility to control magnetic properties by electric fields and, vice versa, ferroelectric order by magnetic fields, is very attractive. But despite the possible coexistence of ferroelectricity and magnetism, materials with a pronounced interplay between these properties are very rare. Here we report on a novel route to generate such an interdependence: we show that symmetry arguments allow a finite magnetoelectric coupling for any spin-density wave that is phase modulated and commensurate, even if the spin ordering is collinear. It is this new coupling that drives the formation of multiferroic phases at the magnetic commensurability transitions, for instance the one of YMn$_2$O$_5$ at 23 K. This example makes clear that materials with phase modulated spin-density wave ordering are a new class of multiferroics with a strong interdependence of magnetization and ferroelectric polarization. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D16.00009: High Magnetic Tunability of Dielectric Properties in Magnetically-Driven Ferroelectricity Sang-Wook Cheong, S. Park, Y. J. Choi, C. L. Zhang, S. Guha Lattice relaxation in magnetically-ordered states with broken inversion symmetry through exchange-striction can induce non- centrosymmetric lattice distortions, leading to the presence of electric polarization. In these magnetically-driven ferroelectrics, dielectric properties turn out to be highly susceptible to applied magnetic fields. Both symmetric and antisymmetric exchange coupling can be involved in the exchange- striction. Magnetically-driven ferroelectrics with the symmetric coupling are associated with acentric spin density wave (SDW) states, and the antisymmetric coupling, relevant to the Dzyaloshinskii-Moriya-type interaction, becomes active when ferroelectricity is induced by spiral magnetic orders. A few examples of magnetically-driven ferroelectrics, exhibiting high tunability of dielectric properties in magnetic fields, will be discussed. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D16.00010: A Microscopic Model of Multiferroics RMn$_2$O$_5$ Chen Fang, Jiangping Hu A microscopic model is developed to explain the phase diagram and the mechanism of magnetoelectric coupling in RMn$_2$O$_5$. We show that frustrated magnetic structure drives the system to a commensurate-incommensurate phase trasition which can be understood as a competition between a collinear order, which stems from the `order by disorder' mechanism, and a chiral symmetry order. The magnetoelectric interaction couples the collinear order to the electric polarization. The low energy excitation and the effect of external magnetic field are also analyzed. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D16.00011: Direct evidence of spin-phonon coupling in multiferroic DyMn$_2$O$_5$ via magneto-infrared measurements J. Cao, J. L. Musfeldt, Y. J. Wang, S. Park, S.-W. Cheong The infrared active phonons in multiferroic DyMn$_2$O$_5$ are investigated as a function of magnetic field and temperature. Both field-induced frequency shifts and oscillator strength redistributions are observed in three important modes (the $f$-manifold crystal field splitting of Dy$^{3+}$ near $\sim$150 cm$^{-1}$, the Mn-O bending mode at $\sim$270 cm$^{-1}$, and the Mn-O stretching mode near $\sim$630 cm$^{-1}$), indicating strong spin-phonon coupling in this material. The crystal field levels of Dy$^{3+}$ are weakly sensitive to temperature induced magnetic phase transitions, whereas the Mn-O bending and stretching modes are insensitive to these phase boundaries. These measurements provide direct proof of spin-lattice interactions in DyMn$_2$O$_5$. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D16.00012: Magnetic-field-induced quantum phase transition in multiferroic BiMn$_{2}$O$_{5}$ J.W. Kim, S.Y. Ham, Y.S. Oh, Kee Hoon Kim, S. Park, S.-W. Cheong, P. Sharma, M. Jaime, N. Harrison Multiferroic BiMn$_{2}$O$_{5}$ exhibits both antiferromagnetic and ferroelectric ordering below $\sim $40 K. We have systematically investigated the electric/magnetic phase of BiMn$_{2}$O$_{5}$ by magnetization (M), dielectric constant ($\varepsilon )$, electric polarization (P) and specific heat (C$_{p})$ measurements down to 0.6 K and magnetic field (H) up to 45 tesla. At 4 K, BiMn$_{2}$O$_{5}$ shows a single magnetic-field-induced transition near H$_{c}\sim $18 T as evidenced by a sharp increase in M. Interestingly, $\varepsilon $ vs H shows a sharp peak at H$_{c}$, of which magnitude systematically increases as critical temperature T$_{c}$ approaches proximity to 0 K. Furthermore, P changes its sign with increasing H from positive to negative near H$_{c}$ with no hysteresis. The trajectory of which above three transitions occur follows the scaling relation T$_{c}$(H)$\sim $(H-H$_{c})^{1/2}$. The shape of C$_{p}$ vs H curve indicates that this transition is 2$^{nd}$ order down to 0.6 K, consistent with the absence of hysteresis in M, $\varepsilon $, and P measurements. Temperature dependent $\varepsilon $ measurements under fixed H near H$_{c}$ reveal that $\varepsilon $ increases on cooling to 5 K and slightly decreases down to 0.6 K, as similarly observed in a quantum paraelectric SrTiO$_{3}$. All of these observations support an interesting possibility that BiMn$_{2}$O$_{5}$ can be the first system to exhibit quantum fluctuation of ferroelectricity tuned by magnetic field. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D16.00013: Local Structure of the Multiferroic System RMn$_{2}$O$_{5}$ T. A. Tyson, M. DeLeon, Q. Qian, A. Ignatov, S. Park, S.-W. Cheong In order to understand the origin of the coupled magnetic and ferroelectric properties observed in RMn$_{2}$O$_{5}$ temperature and magnetic field dependent local structure measurements have been performed. The local structure about the rare earth (R) and the Mn sites have been examined. The atomic correlations involved in the spin-lattice coupling are identified. [Preview Abstract] |
Session D19: Focus Session: New Frontiers in Imaging III
Sponsoring Units: DCPChair: Mark Conradi, Washington University in St. Louis
Room: Colorado Convention Center 104
Monday, March 5, 2007 2:30PM - 3:06PM |
D19.00001: Tissue Imaging and Multidimensional Spectroscopy Using Shaped Femtosecond Laser Pulses Invited Speaker: We use rapidly updatable, femtosecond pulse shaping and multidimensional spectroscopy to make new targets accessible by nonlinear optical imaging. For example, we observe two-photon absorption (TPA), sum frequency absorption (SFA) and self phase modulation (SPM)). Detection of TPA and related effects, such as the local quantum yield (fluorescence/absorption) permits direct observation of important endogenous molecular markers which are invisible in multiphoton fluorescence microscopy; it also permits excitation in the long-wavelength water windows which have significantly reduced scattering, but little endogenous two-photon fluorescence. The fundamental problem is that at the powers one might reasonably apply to tissue (e.g. 5 mW from a modelocked laser) typically $10^{-6}$of the light is removed by TPA, with the rest lost to scattering and linear absorption; and SPM does not broaden the spectrum in the dramatic way associated with (for example) continuum generation. A variety of solutions to these problems using femtosecond pulse shaping will be presented. The simplest solution, which uses amplitude modulation of a fs pulse train, has led to high quality microscopic images of the melanin distribution in melanotic lesions, and has led to discrimination between the different types of melanin in melanosomes. Shaping individual pulses instead of the envelope permits high sensitivity detection of both SPM and TPA via spectral hole refilling combined with heterodyne detection. We manufacture laser pulses with a narrow (ca. 3 nm) spectral hole, which can only be refilled by nonlinear processes; TPA causes refilling 180 degrees out of phase with the wings of the pulse, SPM is 90 degrees out of phase. By inserting a phase-coherent pedestal in the hole, then repeating the experiment with a different phase on a timescale rapid compared to any physiological processes, we can extract the phase of the refilling, hence the relative contributions of SPM and TPA. This method can extract excellent signatures from hemoglobin as well as melanin. We have also used it to image neurons firing in tissue, and to characterize off-diagonal peaks of contrast agents in two-dimensional spectra. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D19.00002: Laser-detected Magnetic Resonance Imaging Shoujun Xu, Marcus Donaldson, Charles Crawford, Simon Rochester, Valeriy Yashchuk, Dmitry Budker, Alexander Pines Magnetic resonance imaging is often performed in the presence of a superconducting magnet for high polarization and sensitive detection. However the cost and immobility of the system impose some restrictions on its applications. To overcome these limiting factors, we present an alternative detection technique: laser-based atomic magnetometry. This technique detects nuclear magnetization at virtually room temperature with an excellent sensitivity at low fields, eliminating the necessity of cryogenics and a homogenous high magnetic field. We show the characteristics of a gradiometer based on two atomic magnetometers and its coupling to a low-field encoding setup. Various flow images are obtained, with spatial resolution reaching sub-millimeter regime. Additional applications and future developments are discussed. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:54PM |
D19.00003: New Techniques for Signal Optimization in Harmonic and Multiphoton Absorption Fluorescence Microscopy Invited Speaker: Nonlinear imaging with ultrafast lasers continues to broaden its application base as a significant tool for exploring and understanding biological structure and function at the microscopic level. The challenge is significant - the biological community needs to be able to quantitatively visualize 100 cubic micrometer volumes with a resolution of 50 nm, and do so in a dynamic fashion -- millisecond time scales are desirable. In order to achieve these demanding imaging requirements we need to strive to achieve new levels of efficiency -- improved resolution is a function of how many photons can be extracted from ever smaller volumes. Towards this end, in this talk we discuss new methods for fiber delivery of femtosecond pulses, spatio-temporal characterization of femtosecond pulses through high-numerical aperture optics, and adaptive spatio-temporal control of these pulses. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D19.00004: Research Applications of Photoelectron Emission Microscopy Wayne Hess, Gang Xiong, Alan Joly, Kenneth Beck, Wei Wei, J. Mike White, Mingdong Cai, J. Thomas Dickinson Photoelectron emission microscopy (PEEM) is a developing technique that images electrons emitted from conductor and semiconductor surfaces under UV, X-ray, or laser irradiation. Low energy PEEM can reveal surface morphology on a 10 nm scale and is sensitive material properties such as phase, adsorbed molecules, surface electronic structure, and other physical properties that affect work function and hence the photoelectron yield. We have used PEEM to study phase transformation in shape memory alloys diffusion of Cu in Cu/Ru bilayers and laser-induced oxygen vacancy creation on TiO$_{2}$. Femtosecond laser irradiation from a frequency-doubled Ti:sapphire oscillator was used to remove bridge-bonded oxygen atoms. To further illustrate the utility of PEEM, we will discuss applications in different fields such as thermal-induced structural phase transformation of shape memory alloys and diffusion of Cu through an Ru barrier layer. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D19.00005: Medical Applications of X-Ray Phase Contrast Imaging Christoph Rose-Petruck, Christopher Laperle, Theron Hamilton, Guohua Cao, Philip Wintermeyer, Gerald Diebold, Jack Wands We report the use of an inline holographic x-ray imaging technique for medical purposes. In contrast to conventional x-ray radiography a phase-sensitive x-ray imaging method is employed. This phase-contrast x-ray imaging is fundamentally different from conventional x-ray shadowgraphy because the mechanism of image formation does not rely on differential absorption by tissues. Instead, x-ray beams undergo differential phase shifts in passing through an organ and subsequently interfere constructively or destructively at the x-ray camera. Hence, tissues are distinguished by their different indices of refraction rather than their absorptive properties. This imaging method is more than a thousand times more sensitive to density variations of tissues than conventional absorption methods and enables imaging of soft tissues with high contrast without the use of contrast agents. For example, we will present images of mouse livers yielding resolution of arterial capillaries as small as tens of micrometers. We also show the imaging technique operates in combination with ultrasound-induced, tissue-selective, differential movement of cancer tumors which highlights the tumor of interest and in some cases obviates the need for chemical contrasting agents. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D19.00006: Optical/MRI Multimodality Molecular Imaging Lixin Ma, Charles Smith, Ping Yu Multimodality molecular imaging that combines anatomical and functional information has shown promise in development of tumor-targeted pharmaceuticals for cancer detection or therapy. We present a new multimodality imaging technique that combines fluorescence molecular tomography (FMT) and magnetic resonance imaging (MRI) for in vivo molecular imaging of preclinical tumor models. Unlike other optical/MRI systems, the new molecular imaging system uses parallel phase acquisition based on heterodyne principle. The system has a higher accuracy of phase measurements, reduced noise bandwidth, and an efficient modulation of the fluorescence diffuse density waves. Fluorescent Bombesin probes were developed for targeting breast cancer cells and prostate cancer cells. Tissue phantom and small animal experiments were performed for calibration of the imaging system and validation of the targeting probes. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D19.00007: Compressed Sensing for Multispectral and Confocal Microscopy Kevin Kelly, Dharmpal Takhar, Ting Sun, Jason Laska, Marco Duarte, Richard Baraniuk Compressive sensing is an emerging field based on the revelation that a small number of random linear projections of a signal or an image contain enough information for reconstruction of a high resolution one. This technique has been applied to magnetic resonance imaging and neutron scattering. We have previously developed an optical camera based on this concept which is capable of megapixel images while utilizing a single photodiode for acquisition and implemented through the use of a digital micromirror device to randomly modulate and acquire the necessary projections of the image. In addition, this scheme allows for the rapid acquisition of multispectral information. We are now extending this scheme to imaging beyond the visible spectrum into the infrared and terahertz where high resolution image sensors are much more costly. Lastly we will present a scheme for utilizing this method in confocal microscopy similar to the flying pinhole concept except that the individual pinhole is replaced by a complex random projection and reconstructed via linear programming. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D19.00008: Source localization of auditory evoked responses from a human brain with an atomic magnetometer K. Kim, H. Xia, A. Ben-Amar Baranga, D. Hoffman, M. V. Romalis We report first measurements of auditory evoked fields (AEF) in a human brain with an atomic magnetometer system and discuss the techniques for magnetic source localization using this system. Until recent development of spin-exchange relaxation free (SERF) atomic magnetometers with a sensitivity of 0.5fT/Hz$^{1/2}$, only SQUID magnetometers had sufficient sensitivity to measure a magnetoencephalograph (MEG). With simple multi-channel operation and no cryogenic maintenance, the atomic magnetometer provides a promising alternative for brain activity measurements. A clear N100m feature in AEF was observed after averaging over 600 stimuli. Currently the intrinsic magnetic noise level is 3.5 fT/Hz$^{1/2}$ at 10 Hz. Optical detection of magnetic fields allows flexibility in magnetic mapping while in the same time imposing certain geometrical constraints. To investigate the magnetic source localization capabilities of the atomic MEG system we performed extensive numerical simulations and measurements with a brain phantom consisting of an artificial current source in a saline-filled sphere. We will discuss the results of numerical analysis and experimental implementation of magnetic source localization with atomic magnetometer. [Preview Abstract] |
Session D20: Focus Session: Ferroelectric and Other Oxides
Sponsoring Units: DMPChair: C. Stephen Hellberg, Naval Research Laboratory
Room: Colorado Convention Center 105
Monday, March 5, 2007 2:30PM - 2:42PM |
D20.00001: Electric Field Gradient Comparisons in Perovskites Dandan Mao, Eric J. Walter, Henry Krakauer Piezoelectric and dielectric properties of Pb-based complex ferroelectric alloys with the A(B'B''B''')O$_3$ perovskite structure depend on composition and the local ordering of the B-site cations. A prototypical example is Pb(Sc$_{1/2}$Ta$_{1/2}$)O$_3$ (PST), which is a normal ferroelectric when the B atoms are ordered and becomes a relaxor when they are disordered. Electric field gradients (EFG) are sensitive to variations in local structure, and they can be probed using high-field nuclear magnetic resonance experiments. Calculations of EFGs are presented using the LAPW method within the local density approximation. We examine trends in B-site EFGs as a function of composition and order in PST, Pb(Zr$_{1-x}$Ti$_{x}$)O$_3$ (PZT), Pb(Sc$_{2/3}$W$_{1/3}$)O$_3$ (PSW), and Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$ (PMN). We discuss these results in terms of B- and Pb-atom off-centerings, and B-atom ordering. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D20.00002: Comparative study of the properties of BaTiO$_3$ and PbTiO$_3$ using DFT (LDA, GGA), HF and hybrid (B3LYP) functionals D. I. Bilc, P. Hermet, Ph. Ghosez, J. Iniguez The study of ferroelectrics (FE) requires to be able to reproduce accurately not only the structure but also in some cases their energy gap $E_g$. DFT underestimates $E_g$ of a typical FE by a factor of about two, while hybrid Hartree Fock (HF)-DFT functionals such as B3LYP are known to give improved values for $E_g$. Therefore, we performed B3LYP and HF study of BaTiO$_3$ and PbTiO$_3$ and compared our results to those of LDA and GGA. For the cubic phase, B3LYP gives very good agreement with the experimental lattice constants and $E_g$. For the tetragonal FE phase, if the atomic positions are relaxed at the experimental lattice constants then LDA, GGA and B3LYP give comparable results for the atomic distortions. However, full relaxation of the tetragonal phase using B3LYP gives a supertetragonality comparable to GGA. We performed hybrid calculations for different values of the three Becke's parameters and found that the supertetragonality is introduced by the non-local part of the Becke GGA exchange. This suggests that a hybrid functional generated from a better GGA$^1$ might provide simultaneously better structural and electronic properties of FE. 1. Z. G. Wu, and R. E. Cohen, PRB {\bf 73}, 235116, (2006). [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D20.00003: Nuclear Magnetic Resonance Chemical Shielding Calculations of Bulk Oxides Daniel Pechkis, Eric J. Walter, Henry Krakauer We will present calculations of nuclear magnetic resonance (NMR) chemical shielding in oxides, modeled using embedded clusters. NMR spectroscopy is an important probe of local structure in disordered materials, such as ferroelectric perovskite solid solutions. Combined with electric field gradient (EFG) calculations, a complete interpretation of high magnetic field NMR spectra is possible in principle. Determination of NMR parameters within the embedded cluster method allows the study of both periodic and disordered systems. Moreover, this approach can take advantage of mature chemical shielding methods found in standard quantum chemistry electronic structure packages at several levels of theory, including different forms of density functionals as well as more correlated approaches. Results will be presented for several materials including ferroelectric perovskites. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D20.00004: Structure and Antiferroelectric Properties of Cesium Niobate, Cs$_{2}$Nb$_{4}$O$_{11}$ Jianjun Liu, Wai-Ning Mei, Robert W. Smith, Chunhua Hu, Kuan-Jiuh Lin We determined the crystal structure of Cs$_{2}$Nb$_{4}$O$_{11}$ by using single crystal X-ray diffraction. The structural refinements at both 100 and 297 K show it to have a centrosymmetric structure in point group mmm and orthorhombic space group Pnna. The lattice is comprised of niobium-centered tetrahedra and octahedra connected through shared vertices and edges; cesium atoms occupy channels afforded by the three-dimensional polyhedral network. We also studied the electric-field dependence of the polarization and observed double hysteresis loops which we interpret as the manifestation of antiferroelectricity. We then elucidated the origin of the antiferroelectricity by using symmetry analysis of the structural space groups. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D20.00005: First-principles study of polarization and piezoelectric properties of Zn$_{1-x}$Mg$_x$O Andrei Malashevich, David Vanderbilt Wurtzite ZnO can be substituted with up to $\sim$30\% MgO to form a metastable Zn$_{1-x}$Mg$_x$O alloy while still retaining the wurtzite structure. Because this alloy has a larger band gap than pure ZnO, Zn$_{1-x}$Mg$_x$O/ZnO quantum wells and superlattices have been much studied recently as promising candidates for applications in optoelectronic and electronic devices. Here, we report the results of an {\it ab-initio} study of the spontaneous polarization of Zn$_{1-x} $Mg$_x$O alloys as a function of their composition. We perform calculations of the crystal structure based on density-functional theory in the local-density approximation, and the polarization is calculated using the Berry-phase approach. We decompose the changes in polarization into purely electronic, lattice-displacement mediated, and strain mediated components, and quantify the relative importance of these contributions. We consider both free-stress and epitaxial-strain elastic boundary conditions, and show that our results can be fairly well reproduced by a simple model in which the piezoelectric response of pure ZnO is used to estimate the polarization change of the Zn$_{1-x}$Mg$_x$O alloy induced by epitaxial strain. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D20.00006: Ferroelectricity in CsPbF$_3$ Marco Fornari We discuss the structural and electronic features of ABX$_3$ halides with perovskitic structure. In particular we analyze the instabilities that drive CsPbF$_3$ to a rhombohedral phase and study the effect of halogen substitutions. The properties of CsPbF$_3$ are considered from the point of view of ferroelectricity and compared with prototypical oxides. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 4:18PM |
D20.00007: Enhanced Piezoelectricity from Polarization Rotation in Perovskites Invited Speaker: Theoretical and experimental studies show that the large electromechanical response of PbMg$_{1/3}$Nb$_{2/3}$O$_3$ (PMN)-PbTiO$_3$ (PT) and related perovskite materials is due to the ease of rotating the polarization with an electric field applied oblique to the polarization. The applied field rotates the polarization from a rhombohedral phase towards tetragonal through monoclinic. Underlying the large coupling is the large c/a strain (about 6\%) of ferroelectric PT. The best materials, including the most commonly used piezoelectric material, PZT (PbZrO$_3$ (PZ)-PT) tend to be solid solutions with PT. The other endmember is typically a relaxor, such as PMN or PZN. Is the relaxor behavior crucial to obtaining large coupling transducer materials? First-principles total energy and linear response computations for PT show the surprising prediction of a pressure induced morphotropic phase transition from tetragonal to monoclinic to rhombohedral, and finally cubic. In the transition regions the dielectric constant and piezoelectric constants become very large, in fact larger than those of the new single crystal piezoelectrics PMN-PT and relatives. This shows that large strain piezoelectricity in solutions with PT is due to the behavior of PT itself, and the other components simply tune the transition to zero pressure. This suggests that the key to finding new materials is in finding new pure compounds with pressure induced morphotropic phase transitions. First-principles and multiscale simulations will be discussed for relaxor ferroelectrics. This work was done in collaboration with A. Asthagiri, Y. Lei, M. Sepliarsky., Z. Wu and X. Zeng. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D20.00008: Theoretical study of ferroelectric KNO$_3$ Oswaldo Dieguez, David Vanderbilt We present a detailed study of the structural behavior and polarization reversal mechanism in phase III of KNO$_3$, an unusual ferroelectric material in which the nitrate groups rotate during polarization reversal. This work extends a preliminary study presented as an example in our earlier paper on the mapping of the energy ($E$) versus polarization ($P$) in insulators.\footnote{O.~Di\'eguez and D.~Vanderbilt, Phys.\ Rev.\ Lett.\ {\bf 96}, 056401 (2006).} Here we analyze in detail a two-parameter model in which the energy of the system is written as a low-order expansion in the polarization and in the nitrate group orientation. Apart from confirming that this model reproduces very well the first-principles results for KNO$_3$ presented in Ref.~[1], we construct its parameter-space phase diagram, and in particular we describe regions of parameter space in which the $E(P)$ curves have an unusual triple-well structure. We also present first-principles calculations of KNO$_3$ under pressure, finding that as the material is compressed its $E(P)$ curves change character, going from having continuous to having discontinuous first derivatives at zero polarization. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D20.00009: Distortions in charge-ordered LuFe$_2$O$_4$ Takeshi Nishimatsu, David Vanderbilt, Karin M. Rabe, Yoichi Horibe, Sang-Wook Cheong, Claude Ederer Experimental evidence\footnote{Nature {\bf 436}, 1136 (2005)} suggests that LuFe$_2$O$_4$ develops ferroelectricity via a novel charge-ordering mechanism in which the Fe sites (of average valence 2.5) disproportionate into Fe$^{2+}$ and Fe$^{3+}$ sublattices in such a way as to break inversion symmetry. However, the precise nature of the charge and magnetic order, and the structural distortions that accompany them, remain poorly understood. With this motivation, we have undertaken a first-principles study of LuFe$_2$O$_4$ using DFT-based methods (especially LDA+U). We search for the ground state consistent with a variety of supercell choices and symmetry constraints, and thereby investigate whether the system is unstable to several possible charge and spin orderings. Despite the limitations of a DFT-based approach, it is argued that the results provide useful guidance in the quest to develop an understanding of the novel form of ferroelectricity displayed by this material. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D20.00010: High-pressure x-ray diffraction of PbTiO$_{3 }$at low temperature Muhtar Ahart, Maddury Somayazulu, Ronald Cohen, Russell Hemley We combined the angular and energy dispersive x-ray diffraction methods to investigate the structural behaviors of PbTiO$_{3}$ (PT) in a diamond anvil cell (pressure up to 23 GPa) at 10 K. The energy dispersive x-ray diffraction results show drastic change in Bragg peak intensities at 16 and 20 GPa which indicate that lead titanate undergoes successive phase transitions with pressure. The results of angular dispersive x-ray diffraction indicate that the lattice parameters a and c decrease with pressure and crossover between 10 and 11 GPa. Pressure induced phase transitions at low temperature are reversible. The experimental results confirm theoretical calculations, including the predicted the phase diagram. This work is supported by the ONR under the contract number N000140210506 and the Carnegie/Department of Energy Alliance Center (CDAC) (DF-FC03N00144). [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D20.00011: Phase diagram of PZT solid solutions near the morphotropic phase boundary from first principles I. Kornev, L. Bellaiche, P.-E. Janolin , B. Dkhil , E. Suard A first-principles-derived scheme, that incorporates ferroelectric and antiferrodistortive degrees of freedom, is developed to study finite-temperature properties of Pb(Zr$_{1-x} $Ti$_{x}$)O$_3$ solid solutions near its morphotropic phase boundary [1]. The use of this numerical technique (i) resolves controversies about the monoclinic ground-state for some Ti compositions, (ii) leads to the discovery of an overlooked phase, and (iii) yields three multiphase points, that are each associated with four phases. Additional neutron diffraction measurements strongly support some of these predictions. [1] Igor A. Kornev, L. Bellaiche, P.-E. Janolin, B. Dkhil, and E. Suard, {\em Phys. Rev. Lett.} {\bf 97}, 157601 (2006) This work is supported by ONR grants N00014-04-1-0413, N00014-01-1-0600 and N00014-01-1-0365, by NSF grant DMR- 0404335, and by DOE grant DE-FG02-05ER46188. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D20.00012: Dynamic fluctuations and static speckle in critical X-ray scattering from SrTiO$_{3}$ Martin Holt, Mark Sutton, Paul Zschack, Hawoong Hong, T.-C. Chiang We report a study of critical x-ray scattering from SrTiO$_{3}$ near the antiferrodistortive structural phase transition at $T_C \approx 105\mbox{K}$. A lineshape analysis of the thermal diffuse scattering results in the most precise experimental determination to date of the critical exponent $\gamma =1.38\pm 0.08$. The microscopic mechanism behind the anomalous ``central peak'' critical scattering component is clarified here by the first-ever observation of a static coherent diffraction pattern (speckle pattern) within the anomalous critical scattering of SrTiO$_{3}$. This observation allows us to directly attribute the origins of the central peak to Bragg diffraction from remnant static disorder above $T_C$. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D20.00013: Theoretical investigation of phase transitions in hafnia Xuhui Luo, Alexander A. Demkov Transition metal (TM) oxides find applications in ceramics, catalysis and semiconductor technology. In particular, hafnium dioxide or hafnia will succeed silica as a gate dielectric in advanced CMOS devices. However, the thermodynamics properties of thin TM oxide films are not well understood, despite their technological importance. We study theoretically phase transitions in hafnia using density functional theory. We find that the cubic phase of hafnia transforms without a barrier into a tetragonal phase \textit{via} a soft-phonon mode. The direct calculation of the cubic phase phonon dispersion confirms the existence of a zone edge soft mode in the cubic phase. Using the nudged elastic band method (NEBM) we find a barrierless transition path between the cubic and tetragonal phases which coincides with the same soft-mode path. In addition we identify the pathway for the tetragonal to monoclinic phase transition, and find a 0.045 eV/mol barrier. We construct the effective Hamiltonian for zone center distortions correct to the fourth order including the strain renormalization. The energy surface found correctly explains the symmetry of the phase structure observed at low temperature. We find that there are two sides to the phase transition driving forces. First there are local distortion modes; secondly there is strong coupling between the local modes. The coupling determines the transition temperature. We calculate the coupling and estimate the 3-D cubic to tetragonal transition temperature in fair agreement with experiment. [Preview Abstract] |
Session D21: Undergraduate Physics Education
Sponsoring Units: FEdChair: David Haase, North Carolina State University
Room: Colorado Convention Center 106
Monday, March 5, 2007 2:30PM - 2:42PM |
D21.00001: Five Inexpensive Ways to Involve More Women in Undergraduate Physics Hillary Smith Everyone knows that not enough women study physics. Low representation of women in undergraduate physics begins a trend of decreasing participation that continues through graduate school into university faculty. There are plenty of expensive ways to study the problem and there are equal numbers of expensive solutions to reverse the trend. But there is only so much money to go around at any university. This talk proposes five ways to involve more women in undergraduate physics without breaking your budget. Each tactic focuses on taking advantage of existing resources to create an environment conducive to the recruitment and retention of women. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D21.00002: NASA's Reduced Gravity Student Flight Opportunities Program as an Effective Educational Outreach Platform for Native Americans T.M. Ritter, M.L. Grimsley We present the experiences from a microgravity research and outreach program utilizing the specially converted C-9 aircraft flown by NASA. Over the past four years several multidisciplinary groups of Native American undergraduate students from UNC Pembroke and UNC Charlotte have participated in NASA's Reduced Gravity Student Flight Opportunities Program. All of the experiments and outreach demonstrations performed have investigated the affects of microgravity and hypergravity on fluid related phenomena. The vigorous outreach portion of the project has taken our experiences across the state in order to stimulate an interest in science and math within the Native American communities. Our outreach presentations have been held at various levels of schools, government functions, local and national Native American conferences, and area powwows. Our outreach presentations include both multi media and hands-on involvement by the audience and emphasize a good understanding of the fundamental science. Together, the hands-on experience, discussion, and flight video provide a complete and portable outreach package on NASA and the Reduced Gravity Student Flight Opportunities Program. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D21.00003: Polymer-Based Nanocomposites: An Internship Program for Deaf and Hard of Hearing Students Peggy Cebe, Daniel Cherdack, B. Seyhan Ince-Gunduz, Robert Guertin, Terry Haas, Regina Valluzzi We report on our summer internship program in Polymer-Based Nanocomposites, for deaf and hard of hearing undergraduates who engage in classroom and laboratory research work in polymer physics. The unique attributes of this program are its emphasis on: 1. Teamwork; 2. Performance of a start-to-finish research project; 3. Physics of materials approach; and 4. Diversity. Students of all disability levels have participated in this program, including students who neither hear nor voice. The classroom and laboratory components address the materials chemistry and physics of polymer-based nanocomposites, crystallization and melting of polymers, the interaction of X-rays and light with polymers, mechanical properties of polymers, and the connection between thermal processing, structure, and ultimate properties of polymers. A set of Best Practices is developed for accommodating deaf and hard of hearing students into the laboratory setting. The goal is to bring deaf and hard of hearing students into the larger scientific community as professionals, by providing positive scientific experiences at a formative time in their educational lives. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D21.00004: An ``Unknown'' Magnetic Moment Measured Five Independent Ways Jonathan F. Reichert In an introductory, or even advanced student laboratory, it is difficult to find an experiment where the ``unknown'' or the physical constant can be measured in multiple ways. This talk will describe five independent methods students can use to measure the magnetic moment of a NdFeB disc imbedded in a 6.1 cm diameter snooker ball. Mechanics, as well as E\&M concepts are necessary to analyze these experiments. Expressing results in the proper units turns out to be an additional challenge for most students. Each measurement has its own unique systematic as well as precision uncertainties. I believe that no other undergraduate experiment uses such a wide range of physical phenomena and complementary measurements to zero-in on a single unknown. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D21.00005: Teaching Laboratory and Research Skills as Preparation for Careers in Science and Education Brian Thoms Recipients of bachelor's degrees in physics have identified lab skills, team work, and research skills as abilities necessary for success in their jobs. However, they also report having received less than adequate preparation in these areas during their college careers. We report on the redesign of a junior physics-major modern physics laboratory course into an inquiry-based, research-like laboratory course. The overall strategy was such as to require the students to approach the experiments in a research-like fashion. In addition, experiments which explore materials properties which can't be looked up in textbooks, e.g. Hall Effect, have been added to further emphasize a research-like approach to the investigations. Laboratory reporting requirements were written to closely reproduce current practices in scientific journals. Assessment of the redesign was performed through surveys of current and graduated students and through comparison of laboratory reports. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D21.00006: Implementing a New Undergraduate Course: Biophysics Mitra Shojania Feizabadi One of the objectives at Canisius College is to introduce students to the big picture of interdisciplinary collaboration early in their academic career. In support of this, the new course, biophysics, was developed, approved and offered by the physics department. Taking advantage of the experiences gained in introducing this course, I will discuss different aspects of the course and further steps which can be taken to heighten student retention of the information and the overall effectiveness of the course. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D21.00007: NanoLab: a Hands-On Introduction to Nanoscience for Scientists and Engineers Matthew Johnson, Lloyd Bumm We have developed a sophomore level laboratory course in nanotechnology. We have taken this hands-on approach to introduce students to the concepts used in nanotechnology much earlier than they would see them in the standard curriculum. Although sophomore level students do not generally have the background to understand the full theoretical explanation of all the phenomena, they do take with them a basic understanding that can serve as a framework for appreciating the broader issues when they encounter them in later courses. Topics we have covered are: crystal structure, x-ray diffraction, electron microscopy, electron microprobe, spectrophotometry, extinction, light scattering (Rayleigh {\&} Mie), microfluidics, scanned probe microscopy, and thin-film growth. A report of our experience will be presented. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D21.00008: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 4:06PM - 4:18PM |
D21.00009: Physics Problems Based on Up-to-Date Science and Technology. Lorcan M. Folan, Vladimir I. Tsifrinovich We observe a huge chasm between up-to-date science and undergraduate education. The result of this chasm is that current student interest in undergraduate science is low. Consequently, students who are graduating from college are often unable to take advantage of the many opportunities offered by science and technology. Cutting edge science and technology frequently use the methods learned in undergraduate courses, but up-to-date applications are not normally used as examples or for problems in undergraduate courses. There are many physics problems which contain information about the latest achievements in science and technology. But typically, the level of these problems is too advanced for undergraduates. We created physics problems for undergraduate science and engineering students, which are based on the latest achievements in science and technology. These problems have been successfully used in our courses at the Polytechnic University in New York. We believe that university faculty may suggest such problems in order to provide information about the frontiers of science and technological, demonstrate the importance of undergraduate physics in solving contemporary problems and raise the interest of talented students in science. From the other side, our approach may be considered an indirect way for advertising advanced technologies, which undergraduate students and, even more important, future college graduates could use in their working lives. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D21.00010: Computational Physics in a Classical Mechanics Text Javier Hasbun Earlier$^1$, a textbook draft (now to be published$^2$) for the junior level mechanics physics course that employs computational techniques was introduced. Here, I show more involved computations such as the interacting spring-mass system, the motion of a charged particle in 3d under both E\&M fields, and the Foucault pendulum, and simulations such as a binary system, Rutherford scattering, the symmetric top, the double pendulum and the principle of least action. The text does not intend to teach students how to program, instead it makes use of students' abilities to use programming to go beyond the analytical approach. The texbook uses MATLAB$^3$,$^4$ since its versatile and students learn it quickly. This is important to use computational physics and build on the traditional analytical approach to problem solving. While it is hoped that students have had computational physics a priori when this text is used, it is not a requirement. The textbook includes the computational code for the convinience of both, students and teachers. Experience shows that students grasp the material well and gain a deeper understanding of the subject than in the absence of the computational environment. \newline $^1$J. E. Hasbun, APS Bull. Vol. 51, No.1, 452 (2006). $^2$Jones \& Bartlett Publishers, 40 Tall Pine drive, Sudbury, MA 01776. $^3$J. E. Hasbun, APS Bull. Vol 51, No.8, 46 (2006). $^4$http://www.mathworks.com [Preview Abstract] |
Session D22: Focus Session: Fracture
Sponsoring Units: GSNP DMPChair: Elisabeth Bouchaud, CEA-SACLAY
Room: Colorado Convention Center 108
Monday, March 5, 2007 2:30PM - 3:06PM |
D22.00001: Failure of heterogeneous materials: Scaling properties of fracture surfaces and implications on models of cracks in disordered media. Invited Speaker: While there exists a unified theoretical framework - Linear Elastic Fracture Mechanics (LEFM) - to describe the failure of homogeneous materials, understanding and modelling the mechanical properties of heterogeneous media continue to raise significant fundamental challenges. These mechanical properties, observed at the macroscopic scale, result from microscopic processes occurring at the scale of the material. To include these local processes into a statistical description constitutes then a crucial step toward the setup of predictive macroscopic models. Crack surface roughness is a consequence of these local processes. Consequently, many fractography experiments have focussed on their analysis. In this context, it was recently evidenced that, in many materials, fracture surfaces exhibit anisotropic scaling properties reminiscent to interface growth problems, fully characterized by two couples of parameters: The roughness exponents and the characteristic length-scales measured along and perpendicular to the direction of crack growth respectively. While the characteristic length-scales do depend on the considered material, the exponents are surprisingly universal: Two \textit{distinct} sets of critical exponents are observed whether the surfaces are examined at scales below or above the size of the damaged zone at the crack front. Models of crack growth in disordered media are discussed at the light of these experimental observations. In particular, one can derive a model from LEFM which describe the development of crack roughness as an ``elastic'' manifold creeping in a random media. This approach captures quantitatively the experimental observations performed at length-scales above the size of the process zone. In this approach, the onset of crack propagation can be interpreted as a dynamic phase transition while sub-critical crack growth can be assimilated to thermally-assisted depinning. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D22.00002: Low self-affine exponents of fractured glass ceramics surfaces Laurent Ponson, Harold Auradou, Daniel Bonamy, Elisabeth Bouchaud, Jean-Pierre Hulin The morphology of fracture surfaces encodes the various complex damage and fracture processes occurring at the microstructure scale during crack propagation. It is now well established that fracture surfaces are self-affine characterized by a roughness exponent usually found close to $\zeta \quad \approx $ 0.75 for a wide range of materials. Recently, fracture surfaces of sandstone were found to be also self-affine but with a lower roughness exponent $\zeta \quad \approx $ 0.4-0.5. To investigate its origin, we studied fracture surfaces of glassy ceramics which are obtained by sintering glass beads. Such a material mimics the structure of sandstone with the advantage that their porosity may be tuned. They are also found to be self-affine, characterized by a roughness exponent $\zeta \quad \approx $ 0.40 $\pm $ 0.04 significantly lower than the ``universal'' roughness exponent $\zeta \quad \approx $ 0.75 widely reported in the literature. Its value is found to depend very slightly on the crack growth velocity and the microstructure (grain diameter, porosity) in the range studied. This suggests the existence of a second universality class in failure problems. Its physical origin is then discussed and a model proposed. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D22.00003: Rapid and slow self-affine fracture in glass Moises Hinojosa, Claudia Guerra, Leonardo Chavez, Edgar Reyes-Melo, Virgilio Gonzalez We discuss the self-affine properties of the fracture surfaces of soda-lime glass obtained in conditions of both rapid and slow fracture in bending. The fracture surfaces were studied by SEM and AFM. The analysis of the mirror and mist-hackle zones for the two conditions suggest the existence of two well defined self-affine regimes governed by universal or attractor values. At low-speed/fine-scales the roughness exponent$\zeta =0.5 \quad \zeta $ dominates whereas the value$\zeta =0.8$ is recovered for high-speed/large scales regimes. These values are subjected to significant deviations that give rise to a possible transitional regime at intermediate scales and speeds, where both attractor values may coexist, particularly in the case of slow fracture. In this context the transitional regime can thus be regarded as the result of the competition of these attractors at intermediate scales and velocities. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D22.00004: Roughness Exponent Measurements for the Central Force Model Jan {\O}. H. Bakke, Alex Hansen We study the roughness properties of fracture profiles from the two-dimensional central force lattice model for a wide range of disorders. The intrinsic and the extrinsic roughness exponent have been measured together with the step size distribution $p(|\Delta h|)$ and the height difference distribution $p(\Delta h,l)$. We find that the profiles are self-affine for systems with narrow disorders and that broader disorders introduces overhangs in the fracture surface leading to deviation from self-affinity for small length scales and to non-trivial finite size scaling. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D22.00005: Local waiting time fluctuations along a randomly pinned crack front Stephane Santucci, Knut Jorgen Maloy, Renaud Toussaint, Jean Schmittbulh The propagation of an interfacial crack along a heterogeneous weak plane of a transparent Plexiglas block is followed using a high resolution fast camera. We show that the fracture front dynamics is governed by local and irregular avalanches with very large size and velocity fluctuations. We characterize the intermittent dynamics observed, i.e. the local pinnings and depinnings of the crack front which trigger a rich burst activity, by measuring the local waiting time fluctuations along the crack front during its propagation. The local front line velocity distribution deduced from the waiting time analysis exhibits a power law behavior, $P(v) \propto v^{-\eta}$ with $\eta = 2.55 \pm 0.15$, for velocities $v$ larger than the average front speed $\langle v \rangle$. The burst size distribution is also a power law, $P(S)\propto S^{-\gamma}$ with $\gamma=1.7 \pm 0.1$. Above a characteristic length scale of disorder $L_d \sim 20 \mu m$, the avalanche clusters become anisotropic, and the scaling of the anisotropy ratio provides an estimate of a local roughness exponent, $H=0.6$. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D22.00006: A Dissipative Particle Dynamics Model of Fracture Da Gao, Paul Meakin The role of thermal fluctuations and dissipative physical processes in fracture initiation and propagation has not been systematically studied due to the absence of appropriate simulation models. In order to investigate this issue, we have developed a dissipative particle dynamics (DPD) model, in which the elastic interactions between adjacent nodes in a two-dimensional spring network model are supplemented by dissipative interactions and random forces related through the fluctuation-dissipation theorem. With this newly developed model, we have simulated two different scenarios: One is self-initiated spontaneous fracturing, and the other is externally forced fracturing. Our preliminary results show that the fluctuating and dissipative forces have an important influence on the propagation mode, and propagation path. Both qualitative analysis and quantitative results will be presented and discussed. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D22.00007: Sub-critical crack growth in a sheet of paper L. Vanel, S. Santucci, N. Mallick, P.-P. Cortet, S.G. Roux, S. Ciliberto We present experiments on the slow growth of a single crack in a fax paper sheet submitted to a constant force $F$. The non-averaged crack growth curves present a stepwise growth dynamics. Modelling the material as a lattice where the crack is pinned by elastic traps and grows due to thermal noise, we find that, in agreement with experiments, the distribution of step sizes follows subcritical point statistics with a power law (exponent 3$/$2) and a stress-dependent exponential cutoff diverging at the critical rupture threshold [1]. Taking into account the microstructure of cellulose fibers, the model is able to reproduce the shape of the statistically averaged crack growth curves, the dependence of the characteristic growth length on $F$ as well as the effect of temperature on the rupture time. Finally, roughness of the crack interface is shown to depend on whether the crack grows in the subcritical regime, or in the rapid regime, over the critical rupture threshold. We analyze this roughness difference using a new approach based on the cumulants of the statistical distribution of the crack front height variations. \newline [1] S. Santucci, L. Vanel and S. Ciliberto, Phys. Rev. Lett. 93, 095505 (2004). [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D22.00008: Stability and roughness of crack paths in 2D heterogeneous brittle materials Eytan Katzav, Mokhtar Adda-Bedia, Bernard Derrida We present a recent study on the stability of propagating cracks in heterogeneous two-dimensional brittle materials and on the roughness of the surfaces created by this irreversible process. We introduce a stochastic model describing the propagation of the crack tip based on an elastostatic description of crack growth in the framework of linear elastic fracture mechanics. The model recovers the stability of straight cracks and allows for the study of the roughening of fracture surfaces. We show that in a certain limit, the problem becomes exactly solvable and yields analytic predictions for the power spectrum of the paths. This result suggests a surprising alternative to the conventional power law analysis often used in the analysis of experimental data and thus calls for a revised interpretation of the experimental results. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D22.00009: Statistical properties of microcracking in polyurethane foams under tensile and creep tests: influence of temperature and density. Stephanie Deschanel, Gerard Vigier, Nathalie Godin, Loic Vanel, Sergio Ciliberto For some heterogeneous materials fracture can be described as a clustering of microcracks: global rupture being not controlled by a single event. We focus on polyurethane foams whose heterogeneities (pores) constitute the termination points where microcracks can stop. We record both the spatial and time distributions of acoustic emission emitted by a sample during mechanical tests: each microcrack nucleation corresponds to a burst of energy that can be localized on the widest face of the specimen. The probability distributions of the energy released is power-law distributed, independently of the material density, the loading mode or the mechanical behavior. On the other hand, the agreement of a power law for the time intervals between two damaging events seems to require a quasi constant stress during damaging. Moreover, we notice a behavior difference of the cumulative number of events and the cumulative energy of the localized events with temperature in the case of tensile tests and not any more for creep tests. The occurrence of a unique behavior and a power law in a restricted time interval for the cumulative number of events and the cumulative energy in creep allow us to apprehend interesting later studies of materials' lifetime prediction. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D22.00010: Do Plastic Zones form at Crack Tips in Silicate Glasses? Sheldon Wiederhorn, Theo Fett, Jean-Pierre Guin In a number of recent studies, the claim has been made that silicate glasses fracture by the formation, growth and coalescence of cavities, in the same way as in metals but at a much smaller scale. Evidence for this premise is based on the examination of side surfaces of fracture mechanics specimens, at the point where the crack intersects the free surface. Such measurements were carried out with an atomic force microscope, which demonstrated finite depressions in the regions around and in front of crack tips in silicate glasses. The height profile around crack tips supposedly differed from that obtained from a simple linear elastic fracture mechanics analysis; while, in front of the crack tip small depressions were observed which were interpreted as cavities. We used a three-dimensional finite element analysis to show that the calculated depression around the crack tip is in excellent agreement with that obtained by atomic force microscopy. In addition, we used AFM measurements on the fracture surfaces themselves to demonstrate the absence of the kind of residual damage that should be present on fracture surfaces if cavitation occurred at crack tips in glass. Our results are proof that cracks in glass propagate by brittle fracture; glass is elastic and bond snapping is the dominant feature of crack growth. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D22.00011: Fragmentation in brittle rods Nicolas Vandenberghe, Romain Vermorel, Emmanuel Villermaux When a rod made of brittle material is axially impacted it breaks into fragments of various sizes. Before the first breaking event, an axial compression wave propagates along the rod, triggering a buckling instability. The instability selects a transverse mode with a well defined wavelength. Recently, Gladden et al. have shown that the fragment size distribution exhibits two peaks corresponding to the length selected by the buckling instability. In the present work we explore in more details the dynamics of elastic waves in the rod and the different phenomena that may explain the broad distribution of fragment sizes. In particular, we will discuss the coupling between the longitudinal and the transverse displacement in the post buckling dynamics. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D22.00012: Mechanisms for Fragment Formation in Brittle Solids Artem Levandovsky, Anna Balazs The fracture process is usually analyzed in terms the fractal dimension of a crack, the crack surface roughness, or fragment size distributions. It is established that relatively simple scaling laws exist for the crack surface roughness in mode I fracture and for the power law distribution for fragment sizes in fracture by impact. These two types of fracture are usually studied separately. Consequently, much less is known about the relationship between crack roughness and fragment size distribution. In this work, we study this relationship by developing a simple model of mode I fracture, which nevertheless produces sufficiently rich behavior in terms of crack roughness and fragment formation. Using this model, we show that different roughness in local regions of the crack path leads to different mechanisms for the subsequent fracture of those regions. We observe two robust power laws for the size distribution of smaller and larger fragments. We connect measurements in fragment size distribution with the local fractal dimension of cracks in the region of fragment formation. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D22.00013: Competition between Diffusion and Fragmentation: Evolution of polycrystalline materials under stress Joachim Mathiesen, Jesper Ferkinghoff-Borg, Mogens H. Jensen, Poul Olesen We propose a dynamical model for the grain evolution in polycrystalline materials. The model is based on the competition of the common physical processes diffusion and fragmentation. Specifically, we formulate a rate equation in terms of the distribution N(x, t) of grains or crystallites of linear size x at time t. The grains either grow by boundary diffusion or shrink by deformation and subsequent fragmentation. The equation leads to a third order differential equation which we solve exactly in terms of Bessel functions. The stationary state is a universal Bessel distribution described by one parameter. Our model perfectly fits experimental data on grain evolution in sheets of ice. [Preview Abstract] |
Session D23: Focus Session: High Pressure III - Earth and Planetary Materials
Sponsoring Units: DMP DCOMPChair: Eric Schwegler, Lawrence Livermore National Laboratory
Room: Colorado Convention Center 110
Monday, March 5, 2007 2:30PM - 2:42PM |
D23.00001: First principles investigation of the ice VII-VIII (order-disorder) phase boundary Renata Wentzcovitch, Koichiro Umemoto, Stefano de Gironcoli, Stefano Baroni Phase boundaries among the various forms of ice are difficult to determine experimentally because of the large hystereses involved. Theoretical determination is also very challenging. Treatment of disorder in hydrogen sublattice is one of major problems. We present a first-principles study of order-disorder transition between ice VII and VIII. This study involves the complete statistical sampling of configurations generated within a 16 molecules supercell and includes the important effects of vibrational energy on this phase boundary. Since this transition has been well constrained experimentally, it is a good test of our treatment. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D23.00002: Freezing kinetics in overcompressed water Marina Bastea, S. Bastea, J. Reaugh, D. Reisman The transformation of water into ice is among the most common first order phase transitions occurring in nature, but it is far from being an ordinary one. Water has unusual physical properties both as a liquid and as a solid due largely to hydrogen bonding effects, which also play a major role in determining the characteristics of its freezing kinetics. We report high pressure dynamic compression experiments of liquid water along a quasi-adiabatic path leading to the formation of ice VII. We observe dynamic features resembling Van der Waals loops and find that liquid water is compacted to a metastable state close to the ice density before the onset of crystallization. By analyzing the characteristic kinetic time scale involved we estimate the nucleation barrier and conclude that liquid water has been compressed to a high pressure state close to its thermodynamic stability limit. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D23.00003: The effect of dynamic compression on phase transformation: Solidification of water and crystal growth of ice VI using dynamic diamond anvil cell Geun Woo Lee, William Evans, Choong-Shik Yoo The kinetics of phase transformation depends on how driving parameters are applied. Under high pressure, compression rate can give different paths of phase transformation. For this purpose, we have developed a new device, called dynamic diamond anvil cell (d-DAC), which can modulate a given static pressure with various compression rate and type. Using d-DAC, liquid water can be overpressurized up to 75 {\%} in ice VI phase field without crystallization, and after transforms to metastable iceVII phase in the stable ice VI pressure field. Interestingly, when fast sinusoidal compression is applied, the crystal morphology of ice VI surrounded by liquid water dramatically changes to fractal and dendritic shape. In this talk, we will describe the details of crystallization, following a brief description of the technical development of d-DAC. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D23.00004: Laser-driven shock studies on planetary ices Invited Speaker: Planetary ices such as water, methane and ammonia make up the bulk composition of planets such as Uranus and Neptune. Additionally, extra-solar planets recently discovered may also be partially composed of these ices. Due to their shear size, the interiors of these planets are at simultaneous high pressures and temperatures. Using laser-driven shock compression, experiments at these extreme conditions---up to $\sim $10 TPa pressures currently and up to $\sim $100 TPa (1 Gbar) in the near future---is possible and covers the full range of planetary pressures, including ``super-giant'' extra-solar planets. Additionally we can couple the dynamic compression with that of static compression in a diamond-anvil cell in order to decrease the temperatures along the principal Hugoniot. Laser-driven shock compression of water samples pre-compressed to 1 GPa produces high-pressure and high-temperature conditions inducing two significant changes in the optical properties of water: the onset of opacity followed by enhanced reflectivity in the initially transparent water. The onset of reflectivity at infrared wavelengths can be interpreted as a semi-conductor $\leftrightarrow $ electronic conductor transition in water, and is found at pressures above $\sim $130 GPa for single-shocked samples pre-compressed to 1 GPa in contrast to pressures above $\sim $100 GPa for water samples without precompression. Our results indicate that conductivity in the deep interior of ``icy'' giant planets is greater than realized previously because of an additional contribution from electrons. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D23.00005: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 3:54PM - 4:06PM |
D23.00006: High pressure-temperature Raman spectroscopy of H$_{2}$-H$_{2}$O clathrate. Maddury Somayazulu, Alexander Levedahl, Alexander Goncharov, Ho-Kwang Mao, Russell Hemley The melting curve of the C2 clathrate H$_{2}$-H$_{2}$O has been determined by \textit{in-situ} Raman spectroscopy measurements in an externally heated diamond anvil cell. We have determined the melting curve to a maximum pressure of 27 GPa. These are the first measurements on the melting line in this clathrate. Depending on the stoichiometry of the starting mixture of H$_{2}$ and H$_{2}$O, we are able to study either a mixture of C2 and H$_{2}$O or C2 and H$_{2}$. In either case, we were able to pinpoint the melting of the clathrate from the measurements of the molecular stretching mode (vibron) in the clathrate. In the case of C2 + Ice VII, we observe the vibron in the clathrate at a frequency higher than in pure H$_{2}$ at the same pressure. We have cross-calibrated the melting temperatures using the Stokes-anti Stokes ratio of the diamond first order and Raman active TO phonon of cubic Boron Nitride. We find that the clathrate melts well above the H$_{2}$ melting at all pressures studied indicating that the stabilization of this clathrate at high pressures is indeed due to interactions between the host and guest molecules. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D23.00007: Computational analysis of methane occupation within gas hydrates Phillip Mendonca, Philip Shemella, Saroj Nayak, Anurag Sharma Gas hydrates are considered a future energy resource that have large quantities of hydrocarbon gases (mostly methane) trapped and stabilized under moderate pressures in continental shelf and permafrost regions. The global estimate of hydrocarbon stored in these ice-like structures far exceeds all fossil fuel reserves. Methane escape from these phases, therefore, is considered a potential global warming contributor. These characteristics make the gas hydrates energy recovery a technological challenge and requires constraining the methane diffusion process within the structure. Here, we present a first principles theoretical investigation into the structure, energetics and dynamics of the `guest' molecule in gas hydrates, with the goal of building a physical model for methane diffusion. In particular, our study focuses on the sI (low pressure) methane hydrate phase by combining isolated cluster calculations and periodic structure calculations and closely guided by high pressure experimental work on methane hydrate. Based on the known crystal structure, we compare binding energies for methane and other gas molecules (e.g. Xe, Ar, CO$_{2}$) to guide the high pressure experiments. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D23.00008: Theoretical Tools for the Analysis and Prediction of Multi-component Systems at High Pressures and Densities J. F. Kenney J. F. Kenney, Gas Resources Corporation, Houston, Texas, U.S.A. To describe or predict theoretically the evolution of a multi-component system at high pressures, one must have a reliable expression for the system's partition function, or its Helmholtz free energy, or its equation of state. Such formalism must possess the following properties: The formalism must be based upon fundamental, first-principles, quantum statistical mechanics argument, and the highest level of rigor available; it cannot be \textit{ad hoc}, or use fitted expressions; the equation of state developed by the formalism must be generate accurately, not only the system's basic pressure-density relationship, but also its multi-phase transition and coexistence lines, and its complex-behavior curves; and it must include also an adequate optimization procedure capable to determine the equilibrium state of the system. Here is described a general formalism that has been used to describe high pressure systems and has resolved the previously-outstanding problems of optical activity in abiological compounds, the anomalous distribution of isomers in petroleum, and the spontaneous generation of the hydrocarbon system. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D23.00009: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 4:42PM - 4:54PM |
D23.00010: Chemical Dissociation of Cyclohexane under Shock Loading Ricky Chau, Neil C. Holmes We present a study of the chemical dissociation process in the ringed hydrocarbon cyclohexane under shock loading. Cyclohexane was subjected to shock loading in the pressure range of 12 GPa to 39 GPa. The dissociation was observed using double pass optical absorption spectroscopy. We observed the onset of dissociation as the shock pressure was increased. A strong wavelength dependence was observed in the absorption first beginning at 650 nm and eventually at 400 nm at 39 GPa. The absorption mechanism is is suggestive of Mie scattering of fine carbon particles. The kinetics of the dissociation and the formation of the carbon particles will be discussed.\\\\ This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48. [Preview Abstract] |
Session D26: Focus Session: Protein Folding: Theory and Simulations III
Sponsoring Units: DCP DBPChair: Dave Thirumalai, University of Maryland
Room: Colorado Convention Center 205
Monday, March 5, 2007 2:30PM - 3:06PM |
D26.00001: Mechanisms of Protein Assembly and Folding: Lessons from Minimalist Models Invited Speaker: Globally the energy landscape of a folding protein resembles a partially rough funnel. The local roughness of the funnel reflects transient trapping of the protein configurations in local free energy minima. The overall funnel shape of the landscape, superimposed on this roughness, arises because the interactions present in the native structure of natural proteins conflict with each other much less than expected if there were no constraints of evolutionary design to achieve reliable and relatively fast folding (minimal energetic frustration). A consequence of minimizing energetic frustration is that the topology of the native fold also plays a major role in the folding mechanism. Topological effects go beyond the structure of the TSE. The overall structure of the on-route and off-route (traps) intermediates for the folding of more complex proteins is also strongly influenced by topology. Going beyond folding, the power of reduced models to study the physics of protein assembly will be discussed. Since energetic frustration is sufficiently small, native topology-based models have shown that binding mechanisms are robust and governed primarily by the protein's native topology. These models impressively capture many of the binding characteristics found in experiments and highlight the fundamental role of flexibility in binding. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D26.00002: Direct application of a simple model to the quantitative analysis of experiments on an ultrafast folding protein Invited Speaker: A simple Ising-like statistical-mechanical model for protein folding (Henry and Eaton, \textit{Chem. Phys.} {\bf 307}, 163-185, 2004) is used to analyze a broad set of experimental data on the ultrafast folding villin subdomain. In this model each residue in the protein sequence can adopt one of two possible microscopic states corresponding to native and non-native conformations; model protein states are identified with distinct sequences of native/non-native residues. The folding properties of the protein are determined entirely by the map of inter-residue contacts in the native structure. To compute partition functions by complete enumeration of all protein states, only those states are included that contain at most two contiguous sequences of native residues. Native contacts are only permitted between residues lying in such contiguous sequences. The stability of any state of the chain is determined by the offsetting effects of the stabilizing native contacts and the destabilizing entropy losses associated with fixing residues in the native conformation and with closing loops of nonnative residues created by contacts between distinct native sequences. In a least-squares fitting analysis, the temperature-dependent populations predicted by the model for all the protein states, combined with a simple description of the spectroscopic properties of individual states, are used to model the results of spectroscopic and thermodynamic experiments. The model reproduces the temperature dependence of the excess heat capacity, tryptophan fluorescence quantum yield, circular dichroism, and relaxation rates and amplitudes, as well as the effects of site-directed mutants on the folding rates and equilibrium constants. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D26.00003: Desolvation effects and topology-dependent protein folding Allison Ferguson, Zhirong Liu, Hue Sun Chan As a protein folds, water molecules must be excluded from the hydrophobic core, and thus desolvation barriers between the protein's constituents must be crossed in order to reach the final folded state. Previous research on continuum G$\bar{o}$-like protein models has demonstrated that pairwise-additive desolvation potentials lead to more thermodynamically and kinetically cooperative folding/unfolding transitions (Z. Liu and H. S. Chan, Phys. Biol. \textbf{2}, S75-S85, \textit{2005}). The present work focuses on the role of this elementary desolvation potential in improving predictions of the well-known topology-folding rate relationship (K. W. Plaxco \textit{et al}, J. Mol. Biol. \textbf{277}, 985-994, \textit{1998}) of small single-domain proteins. Recent computational studies without desolvation barriers have shown (S. Wallin and H. S. Chan, J. Phys.: Condens. Matt. \textbf{18}, S307-S328, \textit{2006}) that the observed correlation between topological parameters and folding rates is because these parameters may be proxies for rate-determining properties of the transition state, such as the activation free energy $\Delta G^{\ddagger}$ and activation conformational entropy $\Delta S^{\ddagger}$. Including the desolvation barrier in the model results in stronger correlations between measures of topology and simulated folding rates / transition state properties, reinforcing the theory that even simple representations of the desolvation effect are important for understanding crucial features of protein folding. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D26.00004: Networks in Protein Folding Erzs\'ebet Ravasz Regan, Zolt\'an Toroczkai, G. Gnanakaran We take a networks approach to protein folding by identifying different protein conformations with nodes, while an elementary step of the system (rotation around a bond) that takes one configuration to another is defined as a link. The energies of configurations are scalar quantities associated with each node. Using this approach we can show that the scale-free nature of the observed protein conformation networks can be explained by simple results obtained on gradient networks. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D26.00005: Intramolecular Vibrational Preparation of the Unfolding Transition State of Zn$^{\rm II}$-substituted Cytochrome \emph{c}: Picosecond Time-Resolved Fluorescence and Dynamic Stokes Shift Studies Warren F. Beck, Sanela Lampa-Pastirk We show that an intramolecular vibrational excitation provided by the radiationless decay of a covalently bound electronic chromophore can be exploited to drive a protein from its native folded state to the transition state for unfolding. Using this approach, we examine the effect of the polarity and viscosity of the solvent medium on the unfolding and refolding reactions of Zn$^{\rm II}$-substituted cytochrome \emph{c} at room temperature. The dynamic Stokes shift of the S$_1$-state Zn$^{\rm II}$--porphyrin is monitored using picosecond time-resolved fluorescence spectroscopy as a probe of the protein and solvent dynamics that are associated with the crossing of the unfolding transition state and with the subsequent unfolding and refolding trajectories. The results show that the solvent polarity controls the activation energy for the unfolding and refolding reactions; the solvent viscosity further controls the rate by frictionally hindering the moving polypeptide. These findings suggest an important role for the solvent in the kinetic control of protein-folding trajectories on the energy landscape. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D26.00006: Single Mutation Effect on Lysozyme Stability and Misfolding Ruhong Zhou We propose a mechanism, based on an unprecedended 10+ microsecond molecular dynamics simulation, for the surprising misfolding of hen lysozyme caused by a single mutation (W62G). Our simulations of the wild-type and the mutant lysozyme in 8M urea solution at biological temperature (with both pH = 2 and pH = 7) reveal that the mutant structure is much less stable than the wild-type, with the mutant showing larger fluctuations and less native-like contacts. Analysis of local contacts reveals that the Trp62 residue is the key to a cooperative long-range interaction within the wild-type where it acts like a bridge between two neighboring basic residues. A native-like cluster or nucleation site can thus form near these residues in the wild-type, but not in the mutant. These findings, while supporting the general conclusions of a recent experimental study by Dobson and coworkers, provide a detailed but different molecular picture of the misfolding mechanism. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D26.00007: On the Mechanism of Protein Unfolding by Pressure A Molecular Dynamics Simulation Study J. Raul Grigera, Andres McCarthy , Carlos Ferrara Proteins are denaturized at high pressure and the mechanism of such a denaturation is still on debate. We have studied lyzozyme and apomyoglobin, under pressure up to 0.3GPa using molecular dynamics simulation. Lysozyme shows more stability, although it cannot retain the native structure. On the other hand apomyoglobin shows a continuing unfolding process during the 180 ns simulation time. The analysis of the hydrophilic and hydrophobic proteins Solvent Accessed Surface clearly shows the increment of the hydrophobic exposed area in the formation of crevices and in the appearing of hydrophobic `spikes' around the overall surface. The observation of the final state, within the simulation time, shows a clear effect on the conformational state of the proteins. Comparing the behavior of the proteins with de aggregation state of simple non-polar solutes at different pressures we have been able to conclude that the driving force of the denaturation is the change in the hydrophobic contribution to the native folding due to the changes of water structure under pressure, which have been shown both experimental and by computer simulation. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D26.00008: Free Energy Landscape - Settlements of Key Residues. Svetlana Aroutiounian FEL perspective in studies of protein folding transitions reflects notion that since there are $\sim $10$^{N}$ conformations to scan in search of lowest free energy state, random search is beyond biological timescale. Protein folding must follow certain fel pathways and folding kinetics of evolutionary selected proteins dominates kinetic traps. Good model for functional robustness of natural proteins - coarse-grained model protein is not very accurate but affords bringing simulations closer to biological realm; Go-like potential secures the fel funnel shape; biochemical contacts signify the funnel bottleneck. Boltzmann-weighted ensemble of protein conformations and histogram method are used to obtain from MC sampling of protein conformational space the approximate probability distribution. The fel is F(\textit{rmsd}) = -1/$\beta $\textbullet Ln[Hist(\textit{rmsd})], \textit{$\beta $}=k$_{B}$T and\textit{ rmsd} is root-mean-square-deviation from native conformation. The sperm whale myoglobin has rich dynamic behavior, is small and large - on computational scale, has a symmetry in architecture and unusual sextet of residue pairs. Main idea: there is a mathematical relation between protein fel and a key residues set providing stability to folding transition. Is the set evolutionary conserved also for functional reasons? Hypothesis: primary sequence determines the key residues positions conserved as stabilizers and the fel is the battlefield for the folding stability. Preliminary results: primary sequence - not the architecture, is the rule settler, indeed. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D26.00009: Interplay between secondary and tertiary structure formation in a lattice model alpha helical hairpin peptide Prem Chapagain, Bernard Gerstman We present results from Monte Carlo simulations of folding dynamics of a model alpha helical hairpin peptide. The dynamics shows that the peptide chain folds in a two step fashion that involves the formation of partial helical segments followed by the formation of a stable tertiary structure by joining these semi-stable helical segments. The interplay between the formation of secondary and tertiary structures during the folding process was investigated by calculating the heat capacity and other thermodynamic quantities at various simulation temperatures. In addition to a sharp peak in the heat capacity curve for the transition between unfolded state and folded native state, the helix-random coil transition in the unfolded state is also cooperative. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D26.00010: ABSTRACT HAS BEEN MOVED TO P35.00002 |
Monday, March 5, 2007 5:18PM - 5:30PM |
D26.00011: First Principles Study of the Reaction Mechanism for Intein C-terminal Cleavage Philip Shemella, Saroj Nayak, Brian Pereira, Shekhar Garde, Georges Belfort Protein splicing, consisting of the excision and ligation of two flanking sequences (the exteins), is auto-catalyzed by the internal sequence (the intein). It has been shown experimentally that by mutating the critical first residue of the intein, the first step of splicing is inhibited, although intein C-terminal cleavage can still occur independently. Using a tripeptide model system with QM methods, we have investigated the effect of different mutants in order to provide an atomic level understanding of this mechanism. We find that the reaction energy barrier for asparagine cyclization can be controlled by mutation of non-essential residues: specifically we found that the barrier with a methionine mutant is larger than to the barrier for cysteine, resulting in slower C-terminal cleavage in agreement with experiment. The accuracy of our model system is further confirmed by comparing results with that of a combined quantum mechanics and molecular mechanics (QM/MM) approach. These results suggest that certain mutations in inteins could be used to control the reaction rate without affecting the overall reaction mechanism and could exploited for many applications including molecular switches, sensors and controlled drug delivery. [Preview Abstract] |
Session D27: Fullerenes, Nano-membranes, and Quasicrystals
Sponsoring Units: DCMPChair: Yayu Wang, University of California, Berkeley
Room: Colorado Convention Center 301
Monday, March 5, 2007 2:30PM - 2:42PM |
D27.00001: The equilibrium and electronic structure of large icosahedral fullerenes using an all-electron fully analytic density functional theory Rajendra Zope, Brett Dunlap We have recently developed a fast, variational and fully analytic density functional theory (ADFT). Instead of numerical integration it employs analytic integration using Gaussian basis sets and the calculus of variations to express the molecular orbitals as well as the Kohn-Sham potential in linear-combination of atomic orbital form. We first parametrize the ADFT to provide the experimental geometry of C$_{60}$ fullerene. Using this parametrization, the triple zeta 6-311G(d,p) orbital basis, and density fitting with exchange-correlation bases that include up to $f$ functions, the geometries of C$_{240}$, C$_{540}$, C$_{960}$, C$_{1500}$, and C$_{2160}$ fullerenes are optimized. The equilibrium structures of these fullerenes are polyhedral in nature, confirming the previous predictions by tight-binding methods. Bond distances are converging towards those of graphene. The evolution of electron removal energies, electron affinities, and singlet excitation energies from C$_{60}$ to C$_{2160}$ is studied using the $\Delta \, SCF$ and transition-state methods. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D27.00002: Fullerene in a Metal-Organic Matrix: Design of the Electronic Structure Michel Cote, Sebastien Hamel, Vladimir Timochevski By combining C$_{60}$ fullerenes and a metal-organic framework, a novel material has been designed with enhanced electronic properties aimed at improving the superconducting transition temperature. Combining these materials within the same structure gives new possibilities to tailor the electronic properties. Higher superconducting transition temperatures have previously been achieved in the fullerene solids by intercalating with ever-larger alkali atoms into the C$_{60}$ crystal. The current study demonstrates by means of state-of-the-art calculations that MOF can be used as a placeholder to set the distance between C$_{60}$ fullerenes introduced inside their pores giving another means to increase the distance between them and further tailor their electronic properties. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D27.00003: The Smoluchowski Effect and Step-Edge Behavior of Nanocars and Azofullerenes Andrew Osgood, Yasuhiro Shirai, Takashi Sasaki, J.M. Tour, K.F. Kelly The nanocar molecule - four fullerene wheels connected by rotating alkyne axles to a central chassis - was the first molecule designed and fabricated specifically for nanoscale manipulation. We have investigated the imaging and manipulation of the nanocar molecule on Au(111) by variable-temperature STM, with specific focus on their unique dynamic step-crossing and -straddling abilities. Our static analysis of the molecules adsorbed at step edges under the influence of the Smoluchowski effect has begun to explain the complex interactions of their behavior in these regions, with an eye towards surface manipulation in three dimensions. Further manipulation studies also attempt to elucidate the fullerene-substrate interactions that make rolling manipulation possible, with special attention paid to the azofullerene dimer - one of the first specifically designed and tested molecules incorporating two simple mechanical functions -- actuation of the ``azo'' unit and rolling / rotation of the wheel-like fullerenes. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D27.00004: Cage-Core Interactions in Fullerenes Enclosing Metal Clusters with Multiple Scandium and Yttrium Atoms. Liu Dan, Frank Hagelberg Pronounced stability has been reported for metallofullerenes of the form NSc3@CN (N = 68, 78) /1/. In response of these and related findings, Density Functional Theory studies have been performed on the relation between cage-core interactions and the geometry as well as stability of endofullerenes with metal impurities containing Sc and Y. Substantial electron transfer from the metal core to the fullerene cage combines with electron backdonation, involving the interaction between the occupied orbitals of the negatively charged cage and the unoccupied d orbitals of the positively charged core. The Hueckel 4n+2 rule, well established in organic chemistry, is shown to provide a valuable heuristic tool for understanding the intramolecular electron transfer and the related stability gain /1/. The usefulness of the aromaticity concept for explaining and predicting the architecture of metallofullerenes is further exemplified by the units Sc2@C84 and Y2@C84 which were analyzed in spin triplet and singlet conditions. The Sc2 core turns out to be realized by two separated ions, while Y2 forms a bound subunit. These findings are in agreement with conclusions based on the 4n + 2 rule, assisted by Nucleus Independent Chemical Shift (NICS) calculations. /1/ Stevenson, S.; Fowler, P.W.; Heine, T.; Duchamp, J.C.; Rice, G.; Glass, T.; Harich, K.; Hadju, F.; Bible, R.; Dorn, H.C. Nature, 2000, 408, 427, /2/ S. S. Park, D. Liu, F. Hagelberg, J. Phys. Chem. A 109, 8865 (2005). [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D27.00005: Hydrogen Storage in Novel Carbon-based Nanostructured Materials Erin Whitney, Calvin Curtis, Chaiwat Engtrakul, Mark Davis, Kim Jones, Philip Parilla, Lin Simpson, Anne Dillon One of the biggest challenges facing a future hydrogen economy is that of onboard vehicular hydrogen storage, for which novel carbon-based nanostructured materials have emerged as potential candidates. Towards this end, we present the synthesis and characterization of ``bucky dumbbell,'' a new organometallic compound comprised of two buckyballs complexed to a central iron atom. This new compound has been characterized using both $^{13}$C solid-state NMR and Raman spectroscopy, and electron spin paramagnetic resonance spectroscopy reveals the presence of Fe$^{3+}$. Temperature-programmed desorption has revealed a new hydrogen binding site via the appearance of a peak centered at approximately -50 \r{ }C, indicating the hydrogen is stabilized at a temperature significantly above that expected for physisorption but still lower than that of C-H bond formation. Comparison with C$_{60}$ under the same hydrogen exposure and heating conditions shows almost no hydrogen adsorption, and the exact binding energy (or desorption activation energy, E$_{d})$ for the bucky dumbbell shows an enhanced value of $\sim $6.2 kJ/mol. Initial volumetric analyses conducted at 77K and 3 bar show a storage capacity of $\sim $0.4 wt{\%}. The synthesis and analysis of other novel fullerene-based organometallic hydrogen complexes will also be discussed. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D27.00006: Microscopic ESR study of N@C$_{60}$ using a Magnetic Resonance Force Microscope P. Banerjee, D. V. Pelekhov, K. C. Fong, I. H. Lee, P. C. Hammel, W. Harneit We report electron spin resonance studies of the endohedral fullerene N@C$_{60}$ using the novel technique of magnetic resonance force microscopy (MRFM). These studies are performed at temperatures down to 1 K on both thin films of N@C$_{60}$ and in samples where the endohedral fullerene is incorporated into a bulk crystalline matrix\footnote{B. Naydenov, C. Spudat, W. Harneit, H. I. Suss, J. Hullinger, J. Nuss, M. Jansen, Chem. Phys. Lett., 424, 327 (2006)}. Utilizing the large magnetic field gradients ($\sim$ 10$^5$ Tesla/meter) in the vicinity of our micromagnetic probe tip, we are able to selectively probe the electron spins in sub--micron volumes. Further, our schemes for spin manipulation allow us to measure the spin--lattice relaxation rate (T$_1^{-1}$) with a spatial resolution in one dimension of approximately 20 nanometers. We will also discuss our efforts to improve the sensitivity of our microscope for detecting {\em individual} electronic spins. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D27.00007: Dimensional evolution of the electronic and structural properties of K$_{x}$C$_{60}$ multilayers studied by Scanning Tunneling Microscopy Yayu Wang, Ryan Yamachika, Andre Wachowiak, Mike Grobis, Mike Crommie We investigate the effect of dimensionality on the properties of potassium doped C$_{60}$ (K$_{x}$C$_{60})$ by studying thin films with precisely controlled doping levels and layer structures using scanning tunneling microscopy and spectroscopy. We observe systematic variation in spatial and electronic structure as the films change from the 2D to the quasi-3D regime. In metallic K$_{3}$C$_{60}$, the large electronic density of states at the Fermi level (E$_{F})$ is seen to split, with a small gap opening at E$_{F}$. In the Jahn-Teller-induced K$_{4}$C$_{60}$ insulator, the energy gap around E$_{F}$ increases monotonically with increased film thickness. In K$_{5}$C$_{60}$, the spectra change from a re-entrant metal in the first layer to an insulator in the third layer. These trends can be explained by considering the increase of Coulomb repulsion in multilayers as screening from the metal substrate is reduced. These results highlight the role of strong electron correlation and dimensionality in determining the properties of doped fullerides. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D27.00008: Crystal structure of Rb$_{4}$C$_{60}$ under pressure Ashfia Huq, Peter W. Stephens We show that Rb$_{4}$C$_{60}$ transforms from its orientationally disordered tetragonal structure at ambient pressure to an orthorhombic phase in the neighborhood of 0.4 GPa. Lattice parameters, interfullerene distances, and closest Rb-C distances evolve continuously up to 2.2 GPa. Rietveld refinements establish that the high pressure phase is isostructural to Cs$_{4}$C$_{60}$. The previously observed conducting phase at 0.8 GPa is therefore structurally distinct from the ambient pressure insulator. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D27.00009: Structural characterization and molecular dynamics of fullerene or fullerene-derivative nanowhiskers Hironori Ogata, Satoru Motohashi Recently, a new type of fibrous fullerene crystals called fullerene nanowhisker has been reported by a liquid-liquid interfacial precipitation method using saturated $m$-xylene solution of fullerene and isopropyl alcohol. Considerable interests have been generated in the structure and properties of fullerene or fullerene-derivative nanowhiskers. In this study, we present the results of structural characterization and molecular dynamics of C$_{60}$, C$_{70}$ and C$_{61}$H$_{2}$ --nanowhiskers(NWs) by x-ray diffraction and solid state NMR. The XRD pattern of as-grown C$_{60}$-NWs have a hexagonal structure with lattice constants of $a$=23.732 and $c$=10.126. Both solid-state $^{13}$C-CP/MAS and wideline $^{1}$H-NMR measurement clearly shows that $m$-xylene molecules are included in NWs. Both lineshape and spin-lattice relaxation time of wideline $^{13}$C-NMR measurements clearly show that C$_{60}$-NWs$ exhibited the phase transition at 250 K. Detailed results on the molecular dynamics and the other properties for C$_{60}$-, C$_{70}$- or C$_{61}$H$_{2}$-NWs will be presented. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D27.00010: Formation of SiC Clusters with Bucky Diamond Structures M. Yu, C. Ghosh, C.S. Jayanthi, S.Y. Wu SiC clusters with bucky diamond structures have been found in a quantum-mechanical molecular dynamics study based on our recently developed self-consistent and environment dependent Hamiltonian in the framework of a linear combination of atomic orbitals [1]. Starting from a spherically truncated bulk diamond structure, stable structures of SiC clusters containing 147 atoms were studied for various compositions of Si and C atoms. In particular, the following initial configurations were considered: (i) C-rich configuration with Si-core, (ii) Si-rich configuration with C-core, and (iii) an almost equal admixture Si and C atoms. It is found that in the first case Si atoms are dragged to the exterior and a cage-like structure formed, while in the second case some C atoms remain in the interior region and some move to the exterior region forming distorted tetrahedral structures with Si atoms. Finally, in the third case, the bucky-diamond structure is obtained, where the interior has a diamond-like structure and the exterior a fullerene-like structure. The reason why (SiC)$_{147}$ clusters form different stable structures can be understood based on hybridization characteristics of Si (\textit{sp}$^{3 })$ and C atoms (\textit{sp}$^{1}$, \textit{sp}$^{2}$, and \textit{sp}$^{3 })$, respectively. [1] Leahy \textit{et al}. Phys. Rev. B74, 155408 (2006). [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D27.00011: In-situ microscopic investigations of the nucleation and growth of C$_{60}$ films on Bi(0001)/Si(111) Jerzy T. Sadowski, T. Nishihara, A. Al-Mahboob, Y. Fujikawa, K. Nakajima, T. Sakurai, T. Nagao Growth of epitaxial C$_{60}$ films on Si is of particular interest for technological reasons. However, strong interaction between the C$_{60}$ molecules and the clean Si induces film growth in the Stransky-Krastanov mode with only local ordering in the first monolayer. Passivation of the Si dangling bonds -- for example with hydrogen -- leads to van der Waals bonding of adsorbates and thus higher degree of crystallinity in C$_{60}$ film, but the true relation between surface properties, and the crystallinity of the fullerene film is not yet fully understood. In this work, C$_{60}$ thin films were grown by UHV deposition on Si(111) substrate covered with thin Bi(0001) passivation layer. Real-time, dark-field low-energy electron microscope (LEEM) investigation of the growth revealed that C$_{60}$ film nucleates in fcc(111) phase, having an epitaxial relation with the Bi(0001) surface. At a growth temperature of $\sim $400K, preferential nucleation of C$_{60}$ at Bi twin boundaries has been detected. Low-energy electron diffraction (LEED) confirmed that film had a single orientation and an excellent crystallinity. The in-plane lattice parameter in the C$_{60}$ films with thickness up to 3ML has been measured to be 10.04 $\pm $ 0.02 A, which is very close to the bulk value of 10.01 A. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D27.00012: Elastic strain-sharing as a means of fabricating strained-Si(110) nanomembranes Shelley Scott, Arrielle Opotowsky, Donald Savage, Michelle Roberts, Max Lagally Hole mobility is higher in Si(110) than it is in Si(001), and straining Si(110) produces further improvements, making strained-Si(110) desirable for p-MOS devices. We describe elastic strain sharing in Si:SiGe:Si(110) heterostructure membranes, which generates flexible, transferable, and dislocation-free strained-Si(110) nanomembranes. Membranes are grown by chemical vapor deposition on the Si template layer of (110) silicon-on-insulator (SOI) substrates. Selective etching of the buried oxide layer `releases' the epitaxial tri-layer system. X-Ray diffraction measurements show that the heterostructure elastically relaxes by transferring strain from compression in the alloy layer, into tensile strain in the Si layers, and we will discuss the achieved mobility values. The XRD line scans exhibit narrow peak widths and thickness fringes, which are both signatures of high-quality (negligible dislocation density) single-crystal strained-Si. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D27.00013: Dislocation-free, uniformly strained Si fabricated by Si nano-membrane (SiNM) technology Chanan Euaruksakul, Zhiwei Li, Donald E. Savage, Max G. Lagally It is known that the interface of a thin film with the substrate on which it is grown plays an important role in dislocation nucleation and kinetic critical thickness. A crystalline-amorphous interface reduces the line energy of dislocations and makes strained structures on SiO$_{2}$ [e.g., strained-Si-on-insulator (sSOI) or a strained SiGe film grown on SOI], susceptible to dislocation formation. We describe fabrication of elastic strain-sharing Si nanomembranes and demonstrate that these strained structures are more thermally stable than strained structures on noncompliant substrates. Our studies with low-energy electron microscopy (LEEM) and x-ray absorption spectroscopy (XAS) show that the structures have a more uniform strain than the strained Si fabricated by conventional SmartCut{\textregistered} sSOI technology. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D27.00014: Mechanism of quasicrystal nucleation and growth Sharon Glotzer, Aaron Keys On cooling, liquids ordinarily solidify into glasses or into crystalline phases with long- range periodic ordering. However, it is also possible to form quasicrystals, ordered solids with long-range aperiodicity. Although quasicrystals have been observed in many materials, their formation is poorly understood. We present the results of a molecular simulation study to elucidate the process by which quasicrystals form from supercooled liquids. We show that, as has been speculated in previous theoretical and experimental works, icosahedral clusters play a significant role in quasicrystal formation. Specifically, icosahedral clusters facilitate the formation of the so-called quasicrystal ``critical'' nucleus, and, together with phasons, facilitate the complicated mechanism that allows quasicrystals to grow aperiodic structures via local interactions. Our findings suggest that direct correlations between liquid ordering and solid structure may be a requisite property for quaiscrystal-forming systems, and is consistent with the class of systems that are known to form quasicrystals experimentally. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D27.00015: Specific heat of rhombohedral C60 polymer in the temperature range of 2-300K Min Gu, Guanglei Cui, Lihang Wang, Xiao Chen Under high temperature of 700 K and high pressure of 6 GPa, we have prepared a batch of C60 polymer. XRD data confirmed it is rhombohedral phase and solid 13C NMR showed a formation of sp3 bond between two neighbor C60 in (111) plane. We have measured the specific heat of C60 polymer and pristine C60 by PPMS in the range from 2 to 300 K. The experimental result of pristine C60 agreed well with previous report. For C60 polymer, above T=80 K it is found that temperature dependence of the specific heat is similar to that of pristine C60 besides an anomaly from order-disorder phase transition at 260K, but in range from 2 to 80K the specific heat is much less than that of pristine C60. Assuming three- (3D) and two-dimensional (2D) Debye phonon modes to contribute respectively to the specific heat in different temperature zone, the calculated values of specific heat have got a good agreement with the experimental data in the whole temperature range. These results show the 2D planar modes but not 3D modes are a dominator to the specific heat of C60 polymer, and the low-frequency intermolecular modes of C60 lattice are restrained in the case of C60 polymer by sp3 bonds from 2+2 cycloaddition reaction. [Preview Abstract] |
Session D28: Focus Session: Carbon Nanotube Optics II
Sponsoring Units: DMPChair: Vasili Perebeinos, IBM Watson
Room: Colorado Convention Center 302
Monday, March 5, 2007 2:30PM - 2:42PM |
D28.00001: Inelastic X-ray Scattering Studies of Plasmons in Carbon Nanotubes M.H. Upton, R.F. Klie, J.P. Hill, T. Gog, D. Casa, W. Ku, Y. Zhu, M.Y. Sfeir, J. Misewich, G. Eres, D. Lowndes We investigate the physical parameters controlling the low energy screening in carbon nanotubes via electron energy loss spectroscopy and inelastic x-ray scattering. Two plasmon-like features are observed, one near 9 eV (the so- called $\pi$ plasmon) and one near 20 eV (the so-called $\pi+\sigma$ plasmon). At large nanotube diameters, the $\pi+\sigma$ plasmon energies depend exclusively on the number of walls and not on the radius or chiral vector. This shift indicates a change of strength of screening and the effective interaction at inter-atomic distance, and thus suggests an alternative mechanism of tuning the properties of the nanotube in addition to the well-known control provided by chirality and tube diameter. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D28.00002: Ab-inito study of metallic and semi-conducting carbon nanotubes . Sumit Saxena, Trevor A. Tyson We present first principle calculations to study the metal -- semiconductor transitions with pressure in zigzag nanotubes using the Local density approximation. Spin restricted calculations for metallic (9, 0) and semi-conducting (10, 0) carbon nanotubes were performed using the full potential projected augmented wave (PAW) method and using ultra-soft pseudo potentials. Our calculations show qualitative agreement to the reported experimental density of states (DOS) for the semi-conducting (10, 0) nanotubes [1]. The band gap between the valence and the conduction band using the pseudo potential formalism is found to be very close to that predicted using PAW approach. We observe that the DOS obtained using pseudo potentials reproduces the essential features however the full potential approach reproduces most of the features of the experimentally reported results. The details of the calculations and other results will be presented. [1] T. W. Odom, J. L. Huang, P. Kim, C. M. Lieber, J. Phys. Chem. B 104 2794 (2000) [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D28.00003: Raman Study of Phonon Softening in Individual Metallic Single Wall Nanotubes Hootan Farhat, Hyungbin Son, Jing Kong We have studied the Breit-Wigner-Fano (BWF) lineshape and frequency of the G$^{-}$ Raman mode in individual metallic nanotubes as function of the Fermi level position. Single wall carbon nanotubes are grown from dispersed nanoparticles and are doped electrostatically by means of a polymer electrolyte gate. The frequency of the G$^{-}$ phonon in metallic tubes is very sensitive to the position of the Fermi level. As the Fermi level is tuned below and above the Fermi point, a semiconducting like G-band is recovered both in terms of frequency and linewidth. Near the Fermi point, the downshift of the G$^{-}$ frequency with respect to that of semiconducting tubes reaches a maximum of up to 50cm$^{-1}$. The doping and diameter dependence of the phonon softening are explained in terms of electron phonon coupling. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D28.00004: Controlled Screening of Excitons in Single, Suspended Carbon Nanotubes. Andrew Walsh, A. Nickolas Vamivakas, Yan Yin, Stephen Cronin, Bennett Goldberg, M. Selim Unlu, Anna Swan Recent measurements in carbon nanotubes (CNTs) have demonstrated that the optical transition energies are excitonic (e-h) in nature, with binding energies that are large fractions of an eV. The exciton energies in CNTs should be sensitive to screening by the environment, yet only small variations of the optical transition energies have been reported for widely varying dielectric environments. Here, we use resonant Raman spectroscopy to follow the change in the optical transition energy of single carbon nanotubes suspended across trenches in dry nitrogen, in high humidity, and after immersion in water. The transition energies are shown to red shift monotonically with increased screening, up to 33 meV. We develop a scaling relationship between the exciton binding energy and the external $\varepsilon $ to quantify the effect of screening on the e-e and e-h interaction energies. These energies are shown to change by hundreds of meV with screening but almost cancel, leading to the small observed shifts reported both here and in the literature. For the nanotubes measured here, the e-e energy is found to be about 25{\%} greater than the exciton binding energy in an unscreened environment. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D28.00005: Direct Measurement of Strain-induced Changes in Carbon Nanotube Bandstructure Mingyuan Huang, Yang Wu, Bhupesh Chandra, Yuyao Shan, Tony Heinz, James Hone The transition energies of single-walled carbon nanotubes under uniaxial strain were measured by Rayleigh scattering spectroscopy. The transitions display significant strain-induced shifts, as predicted by theory. In semiconducting tubes, successive transitions shift in opposite directions. In chiral metals, the split peaks merge with strain. We also observe small, but measurable shifts in the transitions of armchair tubes. The behavior is qualitatively consistent with theoretical predictions based on the trigonal warping effect in nanotube bandstructure. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D28.00006: Raman Spectroscopy of Axially Strain Carbon Nanotubes Rajay Kumar, Stephen Cronin We investigate resonant Raman scattering of carbon nanotube bundles on an elastomer substrate under axial strains as high as 15{\%}. Over the applied strain range, the $G_{+}$ band Raman frequency decreases for both metallic and semiconducting nanotubes. The $G_{-}$ band Raman spectra, however, respond differently to strain for metallic and semiconducting nanotubes, giving insight into the nature of the broad metallic $G_{-}$ band lineshape. The $G_{- }$band frequency downshifts with applied strain for semiconducting nanotubes, while the $G_{-}$ band frequency increases with strain for metallic nanotubes. The $G_{-}$ band linewidth of metallic nanotubes also becomes narrower with strain, making it appear more semiconductor-like. Surprisingly, this metal to semiconductor transition is not reversible with strain, which indicates that nanotube-nanotube coupling plays a role in the observed broad $G_{-}$ band lineshape of metallic nanotubes. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 4:18PM |
D28.00007: Optical Spectroscopy of Individual Carbon Nanotubes Invited Speaker: Single-walled carbon nanotubes (SWNTs) constitute a family of more than 100 one-dimensional structures. With properties varying significantly as a function of their precise atomic structure and environment, SWNTs provide a rich material system to study 1-dimensional physics. To unravel the wealth of different behavior in the SWNTs, which range from metallic to semiconducting, it is generally desirable, and often essential, to probe them \textit{individually}. In this talk, I will describe the development and application of three techniques for optical spectroscopy of individual SWNTs: Rayleigh scattering$^{1}$, multiphonon-Raman scattering$^{2}$, and absorption spectroscopy. We will illustrate the wide range of physical information attainable from these methods, including analysis of the excited electronic states of semiconducting and metallic nanotubes$^{1}$, nanotube-nanotube interactions$^{3}$, and electron-phonon coupling$^{2}$. In addition to their separate use, these spectroscopies can also be fruitfully combined with one another and with other complementary non-optical, single nanotube characterization methods. The correlation of Rayleigh scattering with multi-phonon Raman measurements provides, for example, direct information on the resonance enhancement of electron-phonon interaction$^{2}$. On the other hand, application of Rayleigh scattering in conjunction with single nanotube electron diffraction has permitted us to obtain electronic spectra of SWNTs of independently determined structure$^{4}$. These measurements have permitted verification of the underlying theoretical trends used in previous assignments of nanotube optical spectra. Work done in collaboration with: D. Cho, W. Liu, B. Kessler, A. Zettl, Y. R. Shen (UC Berkeley and LBNL), J. Schuck (LBNL) T. Beetz, J. A. Misewich, L. Wu, Y. Zhu, M. Y. Sfeir (Brookhaven National Lab), and Y. Wu, L. Huang, J. Hone, S. O'Brien, L. E. Brus, and T. F. Heinz (Columbia University). \newline \newline $^{1 }$M. Y. Sfeir*, F. Wang*\textit{ et al.}, Science \textbf{306}, 1540 (2004). \newline $^{2 }$F. Wang\textit{, et al.}, Phys. Rev. Lett., submitted (2006). \newline $^{3 }$F. Wang\textit{, et al.}, Phys Rev Lett \textbf{96} (2006). \newline $^{4 }$M. Y. Sfeir, T. Beetz, F. Wang \textit{ et al.}, Science \textbf{312}, 554 (2006). [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D28.00008: Direct Measurement of the Quantum Yield of Isolated Single Walled Carbon Nanotubes Lisa Carlson, Todd Krauss Owing to their unique optical properties, single walled carbon nanotubes (SWNTs) have received much recent attention. However, questions remain about whether the fluorescence quantum yield (QY) varies among SWNT ($n,m)$ structures and whether the QY for isolated nanotubes differs from the ensemble. With an ensemble QY of less than 0.1{\%}, it is surprising that single nanotube fluorescence can be detected with relatively high signal to noise. This important photophysical parameter potentially limits how SWNTs could be used for applications in biological sensing, telecommunications, displays, solar cells, and quantum optics. We will present measurements of the fluorescence QY of isolated nanotubes, measured relative to CdTe/ZnS quantum dots (QDs) using single molecule microscopy. CoMoCAT SWNTs were ultrasonically dispersed into micelles using sodium cholate surfactant in D$_{2}$O; dilute mixtures of SWNTs and QDs were then spin cast onto quartz and their fluorescence intensities were directly compared. By accounting for differences in the absorption cross sections between the systems, the SWNT QY was determined to be $\sim $2{\%}, nearly two orders of magnitude greater than the ensemble measurement. We will report on whether the measured QY represents an intrinsic nanotube property or if it depends upon other factors such as local environment, intertube interactions, and defects. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D28.00009: Low Temperature Micro-photoluminescence and Raman Spectroscopy of Single-Walled Carbon Nanotubes Ajit Srivastava, Erik Haroz, Yoichi Murakami, Junichiro Kono We report micro-photoluminescence (PL) and resonance Raman spectroscopy studies performed on single single-walled carbon nanotubes at low temperatures. At sufficiently low temperatures, where the thermal energy k$_{B}$T is smaller than the predicted dark-bright exciton splitting, PL is expected to be quenched as excitons populate only the dark ground state. However, we observe strong PL from single tubes with very sharp linewidths ($\sim $ 1 meV for 1 nm diameter tubes) even at temperatures as low as 5 K. We will discuss the origin of this emission. We also study the PL linewidth as a function of temperature in order to provide insight into the PL line-broadening mechanisms. Resonance micro-Raman spectroscopy of single tubes was also performed at cryogenic temperatures, scanning the wavelength of the excitation laser beam around the E$_{22}$ transition of the nanotubes, which revealed rich structure both in the vibrational spectrum and the excitation profile. The temperature dependence of various Raman features will be presented. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D28.00010: Photoluminescence from inter-tube carrier migration in single-walled carbon nanotube bundles O. N. Torrens, D. E. Milkie, M. Zheng, J. M. Kikkawa We detect new, dominant PL features from aqueous suspensions of single-walled carbon nanotubes (SWNTs) associated with energy transfer between semiconducting species in SWNT bundles.$^{1}$ In these bundles, excitons are resonantly photoexcited at the E22 excitonic transition of populous, large bandgap SWNTs ((6,5), (7,5), and (8,3)). Excited excitons then efficiently migrate to smaller bandgap SWNTs ((7,6), (8,4), and (9,2)) and radiatively relax by emitting photons resonant with the E11 excitonic transition of these less common species. These energy transfer (ET) emission peaks demonstrate efficient exciton coupling between different SWNT species within bundles. Aqueous SWNT solutions with low levels of metallic SWNTs prevent quenching of bundle PL, and linear dichroism measurements of SWNT magnetic alignment detect bundle formation. [1] O. N. Torrens, D. E. Milkie, M. Zheng, J. M. Kikkawa, Nano Lett. (in press). [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D28.00011: Unique Optical and Electrical Properties of Almost-Isolated Vertically Aligned Single-Walled Carbon Nanotubes Shigeo Maruyama, Erik Einarsson, Masayuki Kadowaki, Zhengyi Zhang A new insight is gained on the structure of the vertically aligned single-wall carbon nanotubes (VA-SWNTs) generated by ACCVD technique. Our recent finding of the simple removal method using hot-water enabled us to transfer this film to various flat substrates. Transferring this film on TEM grid made it possible to directly observe the morphology of nanotubes from the top. To our surprise, the average number of nanotubes of a bundle is less than about 10. Electronic properties measured by EELS revealed that nanotubes are virtually electronically isolated. Then, the characteristic resonant Raman features are reconsidered. The high resolution Raman measurements show the sharp features for the RBM peak which have been assigned to cross-polarized resonance. The isolated and cross-polarized absorption resonance in Raman will be discussed based on the recent identification of the excitonic cross-polarized absorption through photoluminescence spectroscopy. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D28.00012: Absolute potential of the Fermi level of single-walled carbon nanotubes via hydrogenase complex formation. Timothy McDonald, Drazenka Svedruzic, Yong-Hyun Kim, Jeffrey Blackburn, Shengbai Zhang, Paul King, Michael Heben The absolute potential of the Fermi level of nanotubes as a function of nanotube type is not presently understood, and is important for many nanotube applications and sorting strategies. Here, we study complexes of recombinant [FeFe] hydrogenases and single-walled carbon nanotubes. We find evidence that novel charge-transfer complexes are formed and are stable, which enables further study and application of this system. The hydrogenase functions as a hydrogen electrode sensitizing the nanotubes to the redox half-reaction for hydrogen. Thus the potential can be altered by changing the molecular hydrogen concentration, and this tunability is utilized to bleach various semiconducting nanotube transitions. By observing which are bleached and which remain emissive, we determine the alignment of the potential of the Fermi level of semiconducting single-walled carbon nanotubes. The experimentally determined Fermi level alignment is confirmed theoretically by the first-principles DFT-PBE method. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D28.00013: $\iota -$Carrageenan as a Matrix for Carbon Nanotube Spectroscopy William Rice, Yoichi Murakami, Junichiro Kono We have developed films of individualized single-walled carbon nanotubes (SWNTs) for spectroscopic studies using $\iota $-carrageenan, a polysaccharide macromolecule with a double helix structure, which is extruded from red seaweed. SWNTs produced by both the HiPco and CoMoCAT methods were separated using sodium cholate surfactants and ultracentrifugation. We found that for both HiPco and CoMoCAT tubes, the introduction of $\iota $-carrageenan did not significantly affect the interband optical absorption spectrum, indicating that separation was largely maintained. Further, we show that the optical density of the film is low in the mid-infrared ($\sim $3.5 -- 6 $\mu $m). This transparency is observed at temperatures as low as 4.2 K, making this film a good candidate for temperature-dependent spectroscopic studies of nanotubes. In addition, we confirmed that the polymer film transmits in the terahertz regime (.2 -- .9 THz). [Preview Abstract] |
Session D29: Focus Session: Colloids II
Sponsoring Units: DFDChair: Mark Bowick, Syracuse University
Room: Colorado Convention Center 303
Monday, March 5, 2007 2:30PM - 3:06PM |
D29.00001: Imaging the Dynamics of Freezing and Sublimation of Colloidal Crystals Invited Speaker: We study the kinetics of freezing and sublimating colloidal crystals with single-particle resolution. In experiments, a short-ranged depletion attraction between spheres leads to crystallites that are one to three layers thick. The spheres are tracked with optical microscopy and the sizes and bond-orientational order parameters of the crystallites are measured. The inter-particle attraction is reduced or increased by modest changes in temperature, which lead either to sublimation of crystallites or to formation of crystallites from a gas phase. The sublimation process is also investigated using Brownian Dynamics simulations. In both experiments and simulations of sublimation, we find a two-stage process: at first, large crystallites sublimate by escape of particles from the perimeter. The rate of crystallite shrinkage is then greatly enhanced as the size falls below a cross-over value that ranges between 20 and 50 in different regions of the phase diagram. Simultaneous with the enhanced sublimation rate, the crystallites transform to a dense amorphous structure, which then rapidly vaporizes. The two-step kinetics are also seen in freezing at sphere area fractions near 0.3, but not at substantially higher or lower area fractions. The two-step kinetics are attributed to a thermodynamically meta- or unstable amorphous phase (ten Wolde and Frenkel, Science \textbf{277}, 1975 (1997).). The results should be relevant in diverse systems including colloids, proteins, and atoms such as Argon. We gratefully acknowledge support from Research Corporation and from the NSF through grant DMR-0605839. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D29.00002: Dynamics and Instabilities of Defects in Curved Two-Dimensional Crystals Mark Bowick, Homin Shin, Alex Travesset Point defects play a fundamental role in determining the thermodynamic, elastic and mechanical properties of two-dimensional crystals. When such crystals are curved, finite length grain boundaries (scars) appear as basic structural features. We discuss an analytical determination of the elastic spring constants of dislocations bound within scars and compare them with existing experimental measurements from optical microscopy. We further show that vacancies and interstitials, which are stable defects in flat crystals, are generally unstable in curved geometries. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D29.00003: Two-Dimensional Melting of Microgel Colloidal Crystals Yilong Han, Na Young Ha, Ahmed Alsayed, Arjun Yodh We investigate the phase behavior of 2D colloidal crystals composed of NIPA (N-isopropyl acrylamide) microgel spheres whose diameters can be temperature-tuned. The measurement of a variety of densities of defects, order parameters and correlation functions (static and dynamic) are reported and are in agreement with KTHNY theory at least some of the time. In contrast to previous experiments we use the divergence of translational and rotational susceptibilities (i.e. fluctuations of the corresponding order parameters) to determine the phase transition points. This approach avoids some ambiguities inherent in the other analyses and clearly resolves the intermediate hexatic phase between the solid and liquid phases. Our measurements uncover a novel premelting stage in solid and suggest that traditional analysis methods can incorrectly associate the premelting stage with the hexatic phase. In separate measurements of the melting of two-layer square lattices, we also observed a 'middle' phase. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D29.00004: Angular rheology study of colloidal nanocrystals using Coherent X-ray Diffraction Mengning Liang, Ross Harder, Ian Robinson A new method using coherent x-ray diffraction provides a way to investigate the rotational motion of a colloidal suspension of crystals in real time. Coherent x-ray diffraction uses the long coherence lengths of synchrotron sources to illuminate a nanoscale particle coherently over its spatial dimensions. The penetration of high energy x-rays into various media allows for in-situ measurements making it ideal for suspensions. This technique has been used to image the structure of nanocrystals for some time but also has the capability of providing information about the orientation and dynamics of crystals. The particles are imaged in a specific diffraction condition allowing us to determine their orientation and observe how they rotate in real time with exceptional resolution. Such sensitivity allows for the study of rotational Brownian motion of nanocrystals in various suspensions and conditions. We present a study of the angular rheology of alumina and TiO2 colloidal nanocrystals in media using coherent x-ray diffraction. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D29.00005: Correlated Motion of Rods Diffusing in 3D Kenneth Desmond, Eric R. Weeks It's well known that micron size particles suspended in a fluid will undergo Brownian motion. This Brownian motion is the result of thermal fluctuations that cause the particles to exhibit both translational and rotational diffusion. Translational diffusion due to Brownian motion has been well studied in the past, but rotational diffusion has not received nearly as much investigation. In our experiments, we observe rotational diffusion using polystyrene ellipsoids suspended in a water glycerol mixture. We have developed an algorithm to detect both the center of mass and orientation of our ellipsoidal particles in 3 dimensions. We examine spatial correlations between rotational and translational motion of pairs of these particles. It's known that the spatial correlation between the translational motion of spherical particles decays as 1/r in a homogeneous solution where r is the separation distance between two particles. We are currently investigating the spatial decay of rotational correlation of the colloidal rods. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D29.00006: Two-Dimensional Phase Behavior of Colloidal Peanuts Sharon Gerbode, Angie Wolfgang, Stephanie Lee, Bettina John, Chekesha Liddell, Fernando Escobedo, Itai Cohen While the phase behavior of spherical colloidal suspensions has been well studied, the ordering of non-spherical colloidal particles remains a largely unexplored yet important problem. In this talk we will describe ongoing studies of one very simple extension of the spherical particle: the colloidal peanut. These peanuts have an aspect ratio that makes them comparable to dimer particles. Confining the colloidal peanuts to two dimensions, we find that the suspension can undergo a phase transition from a liquid to an ordered phase in which each individual peanut lobe resides on a triangular lattice site. The lobe packing is very similar to the hexagonally close packed crystalline arrangement formed by spheres in 2D. Unlike their spherical counterparts, however, the colloidal peanuts are not isotropic, and in particular, each peanut has a specific orientation, or director. In this talk we will describe the correlations between defects in the underlying triangular lattice and the local director field. We will also report on our measurements of long-range director correlations, and if time permits, we will describe ongoing work relating to phases formed by peanut particles with different aspect ratios. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D29.00007: Cage Trapping and Melting of Colloidal Suspensions under Confinement and Shear Excitation Prasad Sarangapani, Y. Elaine Zhu The details of the glass transition are still hotly debated. The unusual phenomenon where the viscosity of supercooled fluids diverges near the glass transition without marked structural change is often attributed to a growing length scale of cooperatively rearranging clusters (CRC) of molecules or particles. One way to probe the dynamics of CRC is through confinement, where a glass transition can be observed `sooner' as film thickness approaches a critical value while temperature and volume fraction remain constant. We study a hard-sphere poly(methyl methacrylate) colloidal suspension to model glassy materials. Using a home-designed micro-rheometer interfaced with a confocal microscope, we visualize the structure and dynamics of confined colloidal thin films between two surfaces at narrow gap spacing ranging from 50 $\mu $m to 1-2 $\mu $m. Recent experimental evidence has shown that the size of CRC grows dramatically as film thickness approaches an apparent critical dimension of 10-15 particle layers. In preliminary experiments by \textit{in situ} shear force measurements and microscopic characterization, we investigate the re-fluidization or `melting' of glassy colloidal thin films by applying large shear amplitude and frequency. This phenomenon consequently causes the $\alpha $ and $\beta $ relaxation regimes to occur sooner compared to un-sheared confined glassy thin films. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D29.00008: Phase transitions in charged colloidal suspensions Gregg Lois, Corey O'Hern Experiments on charged colloidal suspensions suggest that their structure and dynamics are sensitive to small variations in the amount of charge deposited on each particle. We numerically explore the phase diagram of charged colloidal suspensions for different values of temperature and charge polydispersity. For increasing charge polydispersity we find that the crystalline ground state is no longer accessible at low temperature and the system forms a glass. We compare the dynamic signatures of this state to the properties of hard-sphere colloids with size polydispersity. We also observe spatial and temporal inhomogeneities in the glassy state and examine the length and time scales over which they persist. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D29.00009: Coulomb Interactions of Colloidal Particles in Oil Sunil Sainis, Eric Dufresne We study the electrostatic interactions of microspheres (PMMA-PHSA) in solutions of surfactant (NaAOT) in oil (hexadecane). We directly measure the forces between isolated pairs of particles to extract the particle charge and solvent ionic strength. Over a wide range of surfactant concentrations, the interparticle forces are indistinguishable from unscreened Coulomb interactions. Far above the critical micelle concentration, however, the interactions assume the familiar screened Debye-Huckel form. Long-ranged interactions between micron-sized particles provide a window to study the structure and dynamics of strongly-correlated systems. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D29.00010: Computer Simulation of Colloidal Electrophoresis Burkhard Duenweg, Vladimir Lobaskin, Krishnan Seethalakshmy-Hariharan, Christian Holm We study the motion of a charged colloidal sphere surrounded by solvent, counterions, and salt ions, under the influence of an external electric field. The ions are modeled as particles which interact dissipatively with a lattice Boltzmann background, such that hydrodynamic interactions are taken into account. Similarly, the colloid is modeled as a spherical array of such point particles. Finite concentration values are taken into account by simulating the system in a box with periodic boundary conditions. In terms of dimensionless reduced parameters, the results compare favorably with experimental data. As a complementary approach, we solve the electrokinetic equations by a finite element method. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D29.00011: Structure of highly packed microgel particles Alberto Fernandez-Nieves, Johan Mattson, Hans Wyss, Lidia Rodriguez-Maldonado, Manuel Marquez, Enrique Lopez-Cabarcos, Antonio Fernandez-Barbero, David A. Weitz We study the structure of concentrated suspensions of ionic microgel particles. In the shrunken state, the particles are essentially charged hard spheres and crystallize at high enough volume fractions. When swollen, however, we find no sings of crystallization, as shown by light and neutron scattering experiments; this is the case irrespective of particle concentration. Instead, the scattered intensity is characterized by the presence of two distinct peaks at low and high scattering wave vectors. Surprisingly, we find that the shift of the peaks follow identical scaling laws with concentration both above and below random close packing. The scaling is different for both peaks indicating they have a different physical origin. While the first maximum seems to be related to the structure of the system, the second peak seems to arise from charge correlations inside the microgel particles. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D29.00012: Effective Interactions Between Like-Charged Colloids: The Role of Colloid Charge Stephen Barr, Erik Luijten We investigate the effect of colloid charge on the interactions between like-charged colloids in the presence of multivalent counterions by means of computer simulations. Because there is a large size asymmetry between the colloids and the counterions, conventional simulation methods are inefficient. In order to overcome this, we extend the generalized geometric cluster algorithm for colloidal suspensions [J. Liu and E. Luijten, Phys.\ Rev.Lett.\ \textbf{92}, 035504 (2004)] to allow for the efficient simulation of systems with electrostatic interactions. In the presence of multivalent counterions, like-charged attraction between the colloids is found to occur over a window of colloid charges. If the colloid charge is too low, the colloid-counterion attraction is too weak for like-charged attraction to occur, and if the colloid charge is too high, the direct electrostatic repulsion overwhelms the attraction induced by the counterions. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D29.00013: Diffusion of charged colloidal particles at aqueous interfaces Penger Tong, Wei Chen We report our recent experimental study of Brownian dynamics of weakly charged particles at a water-air interface. Optical microscopy and multi-particle tracking are used to measure the mean square displacement of the interfacial particles. The measured short-time self-diffusion coefficient $D^s_s$ has the form, $D^s_s /D_0 = \alpha (1 - \beta n)$, where $n$ is the area fraction occupied by the particles and $D_0$ is the Stokes- Einstein diffusion coefficient. The values of the fitting parameters $\alpha$ and $\beta$ are found to be different from those for the three dimensional (3D) colloidal suspensions, indicating that hydrodynamic interactions at the interface have interesting new features when compared with their 3D counterpart. *This work was supported by the Research Grants Council of Hong Kong SAR under Grant No. HKUST603305. [Preview Abstract] |
Session D30: Fluid Structure and Properties
Sponsoring Units: DFDChair: David Vaknin, Affiliation: Ames Laboratory, Iowa State University
Room: Colorado Convention Center 304
Monday, March 5, 2007 2:30PM - 2:42PM |
D30.00001: Crystallization-induced fluid flow in polymer melts undergoing solidification Zhigang Wang, Donghua Xu, Jack F. Douglas The formation of `plastic' polymer materials often occurs under confinement where high pressure imprinting or casting in a mold are involved. To gain insight into this highly non-equilibrium process, we examine the nature of fluid flow that occurs in the non-crystallized regions of melts during spherulitic crystallization by following the movement of tracer particles in isotactic polypropylene films using optical microscopy. We observe a relatively rapid (average particle velocity 13 $\mu $m/min at 138 $^{o }$C, compared to a spherulite growth rate of 0.86 $\mu $m/min) particle movement in the melt until the spherulites become geometrically percolated. We interpret this transient flow to arise from the buildup of local stresses under confinement. Crystallization-induced fluid flow is expected to significantly influence crystal morphology, defect formation and ultimate properties of materials forming by injection molding, pressure imprinting and other processing involving both polymeric and non-polymeric materials where crystallization occurs under confinement. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D30.00002: The reversible freezing and melting of colloidal crystal and glass Hua Guo, Gerard Wegdam, Peter Schall, T. Narayanan, Michael Sztucki We present the observation of gas-liquid and gas-solid phase transitions in a close density matched system of charge stabilized polystyrene spheres suspended in the quasi binary 3-methylpyridine /H$_{2}$O/D$_{2}$O mixture. The reversible phase transitions are induced by using the temperature as control parameter. The temperature control parameter can be varied actively and accurately and applied to the same system to study the phase behaviors. The ``aggregation'' observed by Beysens is in reality a phase transition of the colloidal system. Density matching enables us to observe stable gas-liquid and gas-solid equilibriums. Thus the phases formed could be characterized by the measurement of the structure factor with Small Angle X-ray Scattering (SAXS): dense liquid, glass and face centered cubic ( fcc) crystal. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D30.00003: The riddle of nanoconfined liquids -solid or liquid? Peter Hoffmann, George Matei, Mircea Pantea, Shiva Patil, Ashis Mukhopadhyay Using a specially designed Atomic Force Microscope (AFM), we recently found that the mechanical behavior of simple liquids can be surprisingly rich when liquids are confined to only a few molecular layers. Under nanoscale confinement, OMCTS, a model silicone oil, remains liquid at thermal equilibrium while exhibiting molecular layering. However, at the application of a very small squeeze rate of the order of 1 molecular layer/second, elastic (`solidlike') behavior can be induced. On the other hand a different silicone oil, TEHOS, which has a more open molecular structure, behaves `solidlike' even at very slow squeeze rate and there is an indication, using fluorescence correlation spectroscopy, that it may spontaneously `solidify' close to a flat solid surface. Shear measurements show that when the liquid is allowed to order between the AFM tip and the substrate, the shear stiffness is enhanced, supporting the notion that these liquids can indeed `solidify' under certain circumstances. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D30.00004: Phonons in a One-Dimensional Microfluidic Crystal at Very Low Re Tsevi Beatus, Tsvi Tlusty, Roy Bar-Ziv The development of a general theory for the behavior of a crystal driven far from equilibrium has proved difficult. Microfluidic crystals of water-in-oil droplets provide a convenient means to explore and develop models for non-equilibrium dynamics. Owing to the fact that these systems operate at low Reynolds number (Re), in which viscous dissipation dominates inertial effects, vibrations are expected to be over-damped. Against such expectations, we report the emergence of collective normal vibrational modes (equivalent to acoustic `phonons') in a 1D microfluidic crystal of droplets at Re$\sim $10-4. These phonons propagate at ultra-low sound velocity of $\sim $100$\mu $m/s and frequencies of a few Hz, exhibit unusual dispersion relations markedly different to those of harmonic crystals, and give rise to a variety of crystal instabilities that could have implications for the design of commercial microfluidic systems. First-principles theory shows that these phonons the symmetry-breaking flow field that induces long-range inter-droplet interactions, similar in nature to those observed in other systems including dusty plasma crystals, vortices in superconductors and active membranes.\newline Nature Physics 2, 743-748 (2006). [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D30.00005: Synchrotron x-ray ultrafast x-ray imaging on dynamic multiphase flow studies Yujie Wang, Kamel Fezzaa, Jin Wang, Kyoung-Su Im To overcome the long-exposure time of x-ray imaging for liquid systems. In the past year, we have developed the first ultrafast white-beam synchrotron x-ray phase-contrast imaging technique in the world. With its unprecedented temporal (0.5 $\mu $s) and spatial resolutions (1 $\mu $m), this new technique has already shown great promises in the study of complex fluid mechanical systems. It can probe complex surface morphology and transient dynamics of these interfaces of fluid mechanical systems without the nuisance of multiple scattering. This technique is a big step forward in comparison to millisecond-temporal and micrometer-spatial imaging resolutions normally achieved at various synchrotron sources. With the development of this new technique, we can already carry out research in fluid mechanical systems in competition with world-leading research groups. Our study of the primary breakup process of a coaxial air-assisted liquid jet revealed that the dynamics is dominated by a ``liquid membrane breakup'' process instead of a simple ``ligament mediated breakup'' process owing to our far superior temporal and spatial resolutions. This observation will naturally lead to a cascade idea for the unified treatment of liquid jets, droplets, and liquid membranes breakup mechanism. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D30.00006: Circular polarization memory effect in low-coherence enhanced backscattering of light Young L. Kim, Prabhakar Pradhan, Min H. Kim, Vadim Backman We experimentally study the propagation of circularly polarized light in the subdiffusion regime by exploiting enhanced backscattering [(EBS), also known as coherent backscattering] of light under low spatial coherence illumination. We demonstrate for the first time, to the best of our knowledge, that a circular polarization memory effect exists in EBS over a large range of scatterers' sizes in this regime. We show that low-coherence EBS signals from the helicity preserving and orthogonal helicity channels cross over as the mean free path length of light in media varies, and that the cross point indicates the transition from multiple to double scattering in EBS. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D30.00007: Penetration depth of low-coherence enhanced backscattering photons in the sub-diffusion regime Hariharan Subramanian, Prabhakar Pradhan, Young Kim, Vadim Backman The mechanisms of photon propagation in random media in the diffusive multiple scattering regime have been previously studied using diffusion approximations. However, similar understanding in the low-order (sub-diffusion) scattering regime is not complete due to difficulties in tracking photons that undergo very few scatterings events in the medium. Recent developments in low-coherence enhanced backscattering (LEBS) overcome these difficulties and enable us to probe photons that travel very short distances and undergo only a few scattering events. We derive the analytical expression of the probability of penetration depth and most probable penetration depth of photons due to LEBS, and also performed Monte Carlo numerical simulations to support our analytical results. Our results demonstrate that, the most probable penetration depth $z_p $of photons that undergo low-order scattering events have only weak dependence on scattering mean free path $l_s $and anisotropy factor $g$ of the medium, and strong dependence on the spatial coherence length of illumination,$L_{sc} $. For very small $L_{sc} $ ($<< \quad l_s )$, we show that the penetration depth is proportional to $1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-\nulldelimiterspace} 3$ power of the coherence volume, i.e. ${\kern 1pt}z_p \propto \;\,\left( {\,l_s \,\pi \,L_{sc}^2 } \right)^{1/3}$. Important implications of our results and its application in biological media are also discussed. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D30.00008: Dynamics of Single Polymer Chain in Colloidal Suspensions in Narrow Channels Amir Amini, Marc Robert The self-diffusion coefficient of a linear polymer in a narrow cylindrical channel is calculated. The Polymer is treated as a Gaussian chain in the external potential established by neighboring colloids considered as hard obstacles. The approach is based on the Kirkwood equation, in which the hydrodynamic interactions are taken into account approximately. Monomer-monomer correlation function is obtained via a self-consistent mean-field method. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D30.00009: In Situ Surface Enhanced Infrared Absorption Spectroscopy for the Analysis of the Adsorption and Desorption Process of Au nanoparticles on the SiO2/Si Surface D. Enders, T. Nagao, T. Nakayama The adsorption and desorption of Au nanoparticles (AuNP) in colloidal D2O suspension on the APTES treated SiO2/Si surface was investigated by in situ ATR-IR spectroscopy. With increasing surface density of AuNP the absorption of the vibrational modes of D2O and of the citrate molecules covering the AuNP increases due to surface enhanced infrared absorption (SEIRA). We show that the adsorption kinetics can be investigated by monitoring in situ the molecular vibrational modes of D2O and the citrate molecules, and furthermore we clarify that the adsorption process can be described very well by a diffusion-limited first-order Langmuir-kinetics model. When exposing a saturated AuNP submonolayer to 2-aminoethanethiol (AET)/D2O solution, the AuNP are removed from the surface and the IR absorption of the D2O vibrational modes become weaker again. Taking into account the time dependencies of the CH and the OD peaks, we propose a microscopic model, where the AET molecules quickly adsorb on the AuNP by replacing most of the precovering citrate molecules exposed to the AET solution. As this takes place, the AuNP are finally removed from the surface. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D30.00010: Opening the Pandora's box to understand flow behavior of polymeric fluids. Sham Ravindranath, Pouyan Boukany, Yangyang Wang, Shi-Qing Wang Structure-property relationship has been explored for decades in the context of flow behavior of entangled polymeric liquids. For a long time, it has been assumed that the structure of an entangled polymer, i.e., the entanglement network would experience smooth changes during flow. Using an effective particle-tracking velocimetric (PTV) method recently developed in our lab [1], we found that the nonlinear flow dynamics are associated with an elastic breakdown of the fluid structure. This cohesive failure does not necessarily occur homogeneously in a macroscopic-scale experiment, making it ambiguous to interpret traditional rheological measurements. The presentation complies a whole set of PTV observations to elucidate the physical origin of nonlinear flow phenomena in complex fluids such as polymers. [1] \textit{Phys. Rev. Lett. }\textbf{96}, 016001 (2006); \textit{ibid}. \textbf{96}, 196001; \textit{ibid.} \textbf{97}, 187801. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D30.00011: Critical stresses and cracking in thin films of colloidal dispersions Weining Man, William Russel Colloidal dispersions are often coated on a substrate to leave a uniform thin film after solvent evaporating. However, during drying, a negative capillary pressure develops as the air-water interface is pulled down into the interstices between particles, putting the drying film in tension. The film responds by collapsing normal to the surface but is constrained from shrinking laterally unless cracks open. In this study, we use a high-pressure ultra-filtration device to measure directly the pressure responsible for cracking in uniform films of latex or silica dispersions containing particles of varying radii, avoiding a drying process with edge effects that generate lateral flows and propagating fronts. The results confirm that cracking is controlled by the recovery of elastic energy with the critical pressure increasing with the modulus of the particle, decreasing with film thickness, and independent of particle size. The Griffith's criterion for equilibrium crack propagation along with the nonlinear stress-strain relation provides a necessary, but not sufficient, condition for cracking. When pressure increases beyond the critical value, additional cracks open in qualitative agreement with our elastic energy recovery model. We also find that films with randomly close packed particles crack at a higher pressure than predicted, while those with hexagonally ordered domains particles crack at the critical pressure. These observations suggest an important role for defects that nucleate cracks. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D30.00012: A Novel Approach to Extracting the Pair Distribution Function of Bulk Liquids and Liquid Surfaces David Vaknin, Wei Bu, Alex Travesset The liquid structure factor $S(Q;\alpha)$ of water was measured by synchrotron X-rays in a reflection mode using a liquid surface diffractometer up to $Q \approx 6$ {\AA}$^{-1}$ at various angles of incident beam $\alpha$. The measurements were conducted at incident beam angles above and below the critical angle for total reflection. We calculated the geometrical and penetration depth corrections to $S(Q;\alpha)$s above the critical angle that collapse them into a single bulk $S(Q)$ within experimental error. A new approach to determining the pair distribution function (PDF) from X-ray measured $S(Q)$ was used to analyze the data. The approach involves the calculation of $S(Q)$ from a model PDF, constructed by a linear combination of Error functions, and refined by non-linear least square fit procedure to the measured $S(Q)$. The advantages of this procedure is that no absolute scaling of the intensity is necessary and the PDF is determined with uncertainties. The methodology is currently implemented to determine the PDF at water and other liquid surfaces. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D30.00013: Femtosecond movies of water at sub-nanometer lengthscales Robert H. Coridan, Ghee Hwee Lai, Nathan S. Schmidt, Michael Krisch, Peter Abbamonte, Gerard C. L. Wong The nanometer-scale structure and picosecond-scale dynamics of water are relevant to a wide range of problems in physics, such as the hydrophobic interaction and ion hydration. The behavior of water at these scales has been subject of theoretical and MD studies for decades, and water dynamics has been recently accessed using femtosecond `pump-probe' optical experiments. We will show that it is possible to image dynamical sub-angstrom density fluctuations in water by extracting the density propagator from the dynamical structure factor measured via high-resolution inelastic x-ray scattering spectra at 3$^{rd}$ generation synchrotron sources. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D30.00014: Fluctuation dynamics of water-hydrophobic interface Sung Chul Bae, Adele Poynor, Steve Granick Previous x-ray reflectivity measurements of the interface between water and hydrophobic surfaces with contact angle $>$100\r{ } indicate the existence of depletion layer. However, x-ray measurements provide little information of the fluctuation dynamics. In this presentation, surface plasmon resonance imaging technique with $<$ 1ms temporal resolution and $<$ 1$\mu $m lateral resolution has been built to investigate interface between water and methyl-terminated gold surface. This technique enables to examine the fluctuation dynamics of the depletion layer with temporal and spatial correlation analysis. The characteristic time and length scales of this fluctuation are explored. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D30.00015: Anisotropic polarizability of single wall carbon nanotubes measured via the electro-optical effect Jeffrey A. Fagan, Barry J. Bauer, Erik K. Hobbie The electro-optical response of 400 nm long single wall carbon nanotubes (SWNTs) suspended in water with single stranded DNA was measured in response to high frequency electric fields. Specifically, the dichroism of the SWNTs at their chirality dependent optical transitions was recorded, allowing for calculation of the induced alignment of the SWNTs by the applied field. The anisotropic polarizability of an individual SWNT chirality can be clearly assigned from this data. Strong alignment with nematic order parameters above 0.5 was achieved at high field strengths. We find anisotropic polarizabilities a factor of five larger than that previously measured for gold colloidal rods and an order of magnitude larger than that previously measured for tobacco mosaic virus (TMV). The characterization of the anisotropic polarizability is a large step towards exploiting this property for the directed manipulation of specific nanotubes. [Preview Abstract] |
Session D31: Novel Properties in Nanomaterials
Sponsoring Units: DCMPChair: Guangyu Chai, University of Central Florida
Room: Colorado Convention Center 401
Monday, March 5, 2007 2:30PM - 2:42PM |
D31.00001: In Situ Measurement of Noise and Resistance Reductions During the High Temperature Anneal of Single-Walled Carbon Nanotubes Alexander Kane, Brett Goldsmith, Philip Collins The use of high temperature treatments is increasingly common in the post-processing of lithographically-fabricated carbon nanotube electronic devices. Empirically, high temperature treatments decrease both the resistance and noise of as-fabricated devices through undetermined mechanisms. This work investigates the most effective processing temperatures by measuring devices in situ in a UHV environment. The measurements focus on metallic nanotubes contacted by Ti or Pd electrodes. The results clearly differentiate between the resistive effects of adsorbates and of non-ohmic nanotube-metal contacts, since the two are eliminated at different temperatures and with different degrees of reversibility. The two mechanisms also affect device noise and fluctuations differently, with the net effect that noise decreases are more than proportional to resistance decreases. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D31.00002: Characterization and Reduction of 1/f Noise in Carbon Nanotube Devices Yu-Ming Lin, Phaedon Avouris 1/f noise is a ubiquitous fluctuation in semiconductors and metals. Unlike other types of fluctuations such as the thermal noise and the shot noise, 1/f noise increases with decreasing device dimension and is highly dependent on the material quality and interface properties. Therefore, the noise characteristics in nanoscaled devices are usually dominated by the 1/f-type fluctuations. Here we perform a systematic study on the 1/f noise of carbon nanotube devices consisting of individual single-wall carbon nanotubes. We have examined the impact of the contact and the substrate to the 1/f noise in carbon nanotube devices in order to reduce the 1/f noise level. By eliminating the charge traps associated with oxide substrates, we found that the 1/f noise in carbon nanotube devices can be lowered by up to two orders of magnitude. These results reveal important factors contributing to the 1/f noise source in carbon nanotube devices, and are of great importance for applications based on carbon nanotubes. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D31.00003: Tunneling spectroscopy in carbon nanotubes Yung-Fu Chen, Gassem Al-Zoubi, Norman Birge, Nadya Mason Carbon nanotubes are one-dimensional metallic or semiconducting wires that serve as good model systems to study Luttinger liquids, in which electron-electron interaction are essential to electronic transport. Luttinger behavior has previously been measured via transport through the ends of nanotubes. We have fabricated novel nanotube devices with three-terminal configurations---two normal contacts at the ends and one non-invasive superconducting tunnel probe in the middle. This configuration is well-suited to tunnel spectroscopy studies of bias-dependence, non-equilibrium effects, and carrier interactions in nanotubes. We present results on low-temperature tunneling measurements performed using this configuration. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D31.00004: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 3:18PM - 3:30PM |
D31.00005: Experimental and Theoretical studies on Synthesis of Massively Aligned Single-Walled Carbon Nanotubes and Transistor Applications. Koungmin Ryu, Chongwu Zhou Synthesis of highly aligned single-walled carbon nanotubes with controlled diameters is an important step towards manufacturable ultra dense carbon nanotube integrated circuits. We have successfully demonstrated the synthesis of highly aligned carbon nanotube arrays on a-plane sapphire and miscut quartz substrates. Our calculation of the Lennard-Jones potential clearly reveals that a nanotube would lie normal to the c-axis of a-plane sapphire for minimized potential energy, consistent with our experimental observation. In addition, we have developed a patterned growth method to control both the orientation and position of the aligned nanotubes. This was achieved by using photolithography to deposit catalyst at desired locations on sapphire or quartz, followed by CVD growth of the aligned nanotubes. Furthermore, based on aligned nanotubes array, we have fabricated transistors combined with Pd source/drain contact and HfO2 high-k dielectric material. The transistors show on/off ratios up to 1000000 and subthreshold swings down to around 150 mV/decade. Our aligned Nanotube growth work paves the way for a better understanding of the aligned synthesis and could eventually lead to the growth of aligned nanotubes with controlled diameters and even chiralities. Moreover, transistors approach based on massively aligned Nanotube arrays may work as a platform for explorations of nanotube integrated circuits. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D31.00006: Extreme Thermal Stability of Carbon Nanotubes Gavi Begtrup, Keith G. Ray, Brian M. Kessler, Thomas D. Yuzvinsky, Henry Garcia, Alex Zettl The versatility of carbon-carbon bonding creates a wealth of extraordinary physical properties. Of the two common allotropes of carbon, diamond (sp-3 bonded) exhibits record thermal conductivity but is meta-stable and transitions to graphite at elevated temperatures. Graphite (sp-2) is electrically conducting but sublimes at temperatures as low as 2400K. Carbon nanotubes (also sp-2) capitalize on the extraordinary strength of the sp-2 hybridized carbon-carbon bond and exhibit high electrical and thermal conductivities as well as tremendous mechanical strength. Here we report a new technique to measure the thermal properties of nanosystems. We apply this technique to determine the extreme high temperature stability and thermal conductivity of multiwalled carbon nanotubes. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D31.00007: High Current All-Semiconductor Carbon Nanotube Electronics Guangyu Zhang, Pengfei Qi, Xinran Wang, Yuerui Lu, Xiaolin Li, Ryan Tu, Sarunya Bangsaruntip, David Mann, Li Zhang, Hongjie Dai Existence of both metallic and semiconducting carbon nanotubes in as-grown materials has hindered the development of nanotube electronics. A gas-phase plasma hydrocarbonation reaction is shown here to selectively etch and gasify metallic nanotubes, retain semiconducting nanotubes in near-pristine forms without covalent modification, and narrow down diameter distribution of the semiconductors. 100{\%} of purely semiconducting nanotubes are obtained and connected in parallel for high-current transistors without shorts by metallic species. The `dry' chemical approach is scalable and compatible with existing semiconductor processing technology for future integrated circuits. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D31.00008: Chemoresistance of carbon nanotube circuits incorporating electrochemically-decorated defects Vaikunth Khalap, Alexander Kane, Phillip Collins The chemical functionalization of single-walled carbon nanotubes (SWNTs) is of broad interest, since it allows SWNT properties to be widely tailored. We specifically investigate SWNT devices with single point functionalizations. Standard fabrication techniques are supplemented by an electrochemical point-oxidation process that creates insulating defects into otherwise pristine SWNTs. Selective electrochemistry subsequently deposits metal onto the insulating site(s) and restores the device conductivity. Furthermore, the resulting circuits inherit the chemical sensitivity of the metal deposits. For example, nickel deposits produce an air-sensitive reconnection which readiy oxidizes in air back to an open circuit. Palladium deposits are air stable but highly sensitive to hydrogen gas. The interaction of Pd with point defects appears to entirely reproduce the reported characteristics of SWNT-based hydrogen sensors. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D31.00009: Electron channeling through an individual multiwall carbon nanotube Guangyu Chai, Helge Heinrich, Lee Chow, Thomas Schenkel The hollow structure of the carbon nanotube (CNT) provides a significant chance to use it for the channeling of charged particles and associated channeling radiation. However, the nano size of the CNTs make them difficult to be precisely controlled the position and the orientation. We successfully prepared a monolithic multiwall CNT with a graphitic shield by chemical vapor deposition technique. The graphitic shield provides a handle which allows the manipulation of the supported CNTs. A single CNT collimator is fabricated with focused ion beam technique. The electron channeling through the single CNT collimator is demonstrated for the first time. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D31.00010: Test for superconductivity in individual end-bonded MWNTs Y. Sun, S. Chen, J.Y. Huang, Z.F. Ren, J.I. Oh, M.J. Naughton, M. Vaziri Takesue {\it{et al.}} [1] recently reported 12K superconductivity in templated arrays of $\sim 10^4$ ``end-bonded" multi-walled carbon nanotubes (MWNTs). They attributed the occurrence of superconductivity to intershell (interlayer) effects within each MWNT. We have tested this by preparing and measuring {\it{individual}} end-bonded MWNTs, which were grown by arc-discharge without catalyst. High resolution TEM showed they had typical outer (inner) diameters of 10 - 15 nm (1-2 nm), with no visible defects, values verified by AFM and electrical measurements. We also verified by TEM that, as grown, the nanotube ends were closed. We then used a novel nanolithographic approach to facilitate end-bonding ({\it{i.e.}} contacting all layers), which was subsequently verified in $I-V$ tests. Four-probe resistivity was measured for several such individual end-bonded MWNTs, to 1.4 K, including the use of current densities smaller than those used in Ref. 1. No evidence for superconductivity was found. \newline \newline [1] I. Takesue \textit{et al.}, Phys. Rev. Lett. $\bf{96}$, 057001 (2006) [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D31.00011: Electron-phonon scattering in suspended carbon nanotubes Vikram Deshpande, Adam Bushmaker, Steve Cronin, Marc Bockrath We perform variable temperature transport measurements of individual suspended single-walled carbon nanotubes varying in length from 0.5um to 3um. To interpret our data, we use a model for electronic scattering that incorporates twiston phonons as well as radial breathing mode (RBM) phonons. We estimate the electron-phonon coupling for these phonons and find that it is in qualitative agreement with theory. At low temperatures, the RBM phonons can also be observed at finite bias voltage corresponding to characteristic RBM phonon energies. At higher bias, the mean free path is strongly reduced to $\sim $30 nm, suggesting the production of a non-equilibrium population of RBM phonons analogous to the non-equilibrium optical and zone-boundary phonon population observed previously to limit transport in substrate-supported and suspended nanotube devices [1-3]. Finally, we are also conducting simultaneous Raman spectroscopy and electrical measurements on our devices to study the signatures of electron-phonon scattering in Raman data. We will report our latest findings in this regard. 1. Z Yao et al, Phys Rev Lett 84, 2941 (2000) 2. E Pop et al, Phys Rev Lett 95, 155505 (2005) 3. M Lazzeri et al, Phys Rev Lett, 95, 236802 (2005) [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D31.00012: Ge/Si nanowire mesoscopic Josephson junctions Jie Xiang, Andy Vidan, Michael Tinkham, Robert M. Westervelt, Charles M. Lieber Superconductor-normal conductor-superconductor (S-N-S) Josephson junctions have displayed rich macroscopic quantum phenomena. A novel mesoscopic regime emerges when the width of the normal conductor shrinks to become comparable to carrier Fermi wavelength and its normal conductance becomes quantized in multiples of 2e2/h due to quantum confinement. We have previously demonstrated transport through individual 1D subbands in the hole gas formed in Ge/Si core/shell nanowire (NW) heterostructures. Here we present a study of the interplay between quasi-1D transport and proximity-induced superconductivity using Ge/Si NWs contacted by superconducting leads. Transport measurements on S-NW-S devices reveal high order resonant multiple Andreev reflections, indicating that the NW channel is smooth and that transport is highly coherent. By using a top gate to modulate carrier density in the NW, the critical supercurrent Ic can be tuned from zero to $>$ 100 nA. Significantly, we found that Ic exhibits step-wise increases as a function of gate voltage, corresponding to transport through discrete 1D subbands due to radial carrier confinement. The implications of these results and possible applications of S-NW-S devices will be discussed. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D31.00013: One-dimensional electron behavior in semiconducting carbon nanotubes studied using random telegraph signal David Tobias, A. Tselev, P. Barbara, C.J. Lobb, M.S. Fuhrer We have analyzed the random telegraph signal (RTS) from a semiconducting carbon nanotube in a field effect transistor geometry. We interpret the RTS as due to a single electronic charge tunneling between the nanotube and a nearby defect. We study the tunneling rate of electrons between the nanotube and defect as a function of defect energy, controlled by gate bias, and temperature. An analysis of the tunneling rates allows us to determine an effective temperature for the electron system inside the nanotube as a function of drain bias. The change in the tunneling rates versus the energy of the defect is inconsistent with Fermi liquid theory, providing evidence for the non-Fermi liquid ground state of the one-dimensional semiconducting carbon nanotube. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D31.00014: Torsional Electromechanics of Carbon Nanotubes Ernesto Joselevich, Tzahi Cohen-Karni, Lior Segev, Onit Srur-Lavi, Sidney R. Cohen Carbon nanotubes are known to be distinctly metallic or semiconducting depending on their diameter and chirality. Here we show that continuously varying the chirality by mechanical torsion can induce conductance oscillations, which can be attributed to metal-semiconductor periodic transitions. The phenomenon is observed in multi-walled carbon nanotubes, where both the torque and the current are shown to be carried predominantly by the outermost wall. The oscillation period with torsion is consistent with the theoretical shifting of the corners of the first Brillouin zone of graphene across different subbands allowed in the nanotube. Beyond a critical torsion, the conductance irreversibly drops due to torsional failure, allowing us to determine the torsional strength of carbon nanotubes. Our experiments indicate that carbon nanotubes could be used as self-sensing torsional springs for nanoelectromechanical systems (NEMS). [1] E. Joselevich, Twisting nanotubes: From torsion to chirality, \textit{ChemPhysChem} \textbf{2006}, $7$, 1405. [2] T. Cohen-Karni, L. Segev, O. Srur-Lavi, S. R. Cohen, E. Joselevich, Torsional electromechanical quantum oscillations in carbon nanotubes, \textit{Nature Nanotechnology}, \textbf{2006}, $1$, 36. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D31.00015: Effects of Torsional Strain in Single Wall Carbon Nanotubes Hyungbin Son, Xiaojie Duan, Yingying Zhang, Georgii Samsonidze, Mildred Dresselhaus, Jing Kong, Jin Zhang Since it was predicted that the electronic properties of single wall carbon nanotubes (SWNTs) can be tuned drastically by strain, strain in SWNTs was intensively studied. Particularly, the effects of uniaxial strain on electronic and vibrational properties of SWNTs have been reported in several experimental works. However, little experimental work has been reported on other types of strain such as torsional strain. Our previous work has reported that we can induce torsional strain in SWNTs using AFM manipulation and that torsional strain has distinctive signature in various vibrational modes. In this work, we further investigate the effect of torsional strain on the vibrational modes of SWNTs in detail: frequency shift on different symmetry modes, mode splitting due to symmetry breaking, and changes in electron-phonon matrix elements. [Preview Abstract] |
Session D32: Focus Session: Fermi Gases with Unequal Spin Populations or Masses
Sponsoring Units: DAMOPChair: Leo Radzihovsky, University of Colorado
Room: Colorado Convention Center 402
Monday, March 5, 2007 2:30PM - 2:42PM |
D32.00001: Polarized Fermi gases in an axially symmetric trap: A Bogoliubov-deGennes analysis William Schneider, Rajdeep Sensarma, Mohit Randeria We study the T=0 Fermi gas with an unequal population of up and down spins in an axially symmetric three-dimensional trap. Our motivation is to understand the differences in the experimental data from the MIT and Rice groups, which might arise from the rather different asymmetries of the trapping potentials. Using a fully self-consistent numerical solution of the Bogoliubov deGennes equations, we address the question of the validity of the local density approximation (LDA) as a function of asymmetry. We will present results for the spatial variations of the up and down densities and the superfluid order parameter as a function of polarization, trap asymmetry and interaction strength in the vicinity of unitarity. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D32.00002: Single-plane-wave Larkin-Ovchinnikov-Fulde-Ferrell state in BCS--Bose-Einstein condensation crossover Yan He, Chih-Chun Chien, Qijin Chen, Kathy Levin We study the single-plane-wave Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) states for BCS--Bose-Einstein condensation (BEC) crossover at general temperatures $T$. Because we include the important effects of noncondensed pairs, our $T \neq 0$ phase diagrams are different from those reported in earlier work. We find that generalized LOFF phases may be the ground state for a wide range of (weak through moderately strong) interactions, including the unitary regime. However, these LOFF phases are readily destroyed by non-zero $T$. We also explore the competition between LOFF phases and phase separated states. In the cold gases, phase separation is generally the more stable, although in QCD and other applications, of LOFF physics, phase separation is not always a physical option. \\ cond-mat/0610274 \\ cond-mat/0608662 [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D32.00003: Pairing and superfluid properties of dilute fermions with unequal masses Chien-Hua Pao, Shin-Tza Wu, Sungkit Yip We study the pairing between Fermions of different masses in a harmonic trap potential. Within the mean field theory, we calculate the density profiles systemically for the weak coupling BCS, the unitary limit, and the strong coupling BEC regimes. For a system with spin population imbalance, we found that the system is phase separated into concentric shells with the superfluid in the core surrounded by the normal fermion gas in both the weak-coupling BCS side and at unitary limit. In the strong-coupling BEC side, the composite bosons and left-over fermions can be mixed. The density profiles for unequal mass Fermions can be drastically different from their equal-mass counterparts in the unitary limit. We will discuss some possible experiments with different mass ratios which exhibit different ground state properties compared to the equal masses cases. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D32.00004: Universal phase diagram of a strongly interacting polarized Fermi gas Invited Speaker: The recent combined theoretical and experimental breakthroughs in the field of ultra-cold Fermi systems have permitted the clarification of the ground state properties of an ensemble of attractive fermions with equal spin populations. However, many open questions remain concerning the behavior of polarized systems, where the different spin states are unequally populated. The various theoretical models imply a wide range of different scenarios and phase digram, while two recent experiments performed at Rice and MIT present contradictory results. We will present an analysis of the ground state of an ensemble of fermions with unequal spin population in the regime of infinite scattering length. To address this problem, we will make use of universality which is characteristic of this strongly interacting regime and leads to simple scaling laws for the different physical quantities describing the system. We will in particular show that this problem is closely related to the study of an impurity imbedded in a non interacting Fermi sea of polarized atoms. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D32.00005: Finite Temperature Effects in Trapped Unitary Fermi Gases with Population Imbalance Chih-Chun Chien, Qijin Chen, Yan He, Katheryn Levin We study the finite temperature $T$ behavior of trapped Fermi gases in the unitary regime and in the presence of a population imbalance with polarization $p$. We obtain a phase diagram in the $p$ - $T$ plane, which establishes various superfluid and normal phases. Our theory, which is consistent with the standard $T=0$ calculations in the literature, incorporates the important effect of non-condensed pairs. These are essential in order to arrive at physically meaningful transition temperatures $T_c(p)$. Moreover, as a result of these non-condensed pairs our $ T \leq T_c$ profiles evolve from the well documented featureless behavior at $p=0$ to behavior which shows clear indications of the presence of a condensate at $p \neq 0$. We also show profiles and central densities in different regimes of the phase diagram, and detailed comparisons with recent experiments are presented, \\ 1. C.-C. Chien, Q.J. Chen, Y. He, and K. Levin, \textit{Intermediate temperature superfluidity in an atomic Fermi gas with population imbalance}, Phys. Rev. Lett. 97, 090402 (2006) \\ 2. C.-C. Chien, Q.J. Chen, Y. He, and K. Levin, \textit{Finite temperature effects in trapped Fermi gases with population imbalance}, Phys. Rev. A 74, 021602(R) 2006. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D32.00006: Unconventional interaction between vortices in a polarized Fermi gas Vladimir M. Stojanovic, W. Vincent Liu, Yong Baek Kim Using effective field theory approach we study a homogeneous superfluid state with a single (gapless) Fermi surface, recently suggested as a possible phase for an ultracold Fermi gas with spin-population imbalance. We find an unconventional form of the interaction between vortices. The presence of gapless fermions gives rise to an attractive long-range potential oscillating in space, analogous to the RKKY magnetic interaction in metals. Our study then leads to an interesting question as to the nature of the vortex lattice in the presence of the competition between the usual repulsive logarithmic Coulomb and the fermion-induced attractive oscillatory interactions. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D32.00007: Two-component Fermi gas with unequal masses at unitarity: A diffusion Monte Carlo study D. Blume, G.E. Astrakharchik, S. Giorgini Two-component Fermi gases with varying interaction strengths have been realized in the laboratory using ultracold atoms in two different hyperfine states. In view of experimental efforts to simultaneously cool and trap two fermionic species with different masses, such as Li and K, we investigate the behavior of two-component Fermi gases with unequal masses in the strongly-interacting regime using the diffusion Monte Carlo technique. We consider mass ratios ranging from one to 100, and determine the equation of state at unitarity for a gas with identical number of ``spin up'' and ``spin down'' atoms. Furthermore, we determine the pairing gap of the system and interpret our findings. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D32.00008: The effective Bose-Fermi scattering length in spin-polarized Fermi superfluids Edward Taylor, Allan Griffin, Yoji Ohashi The analysis of experiments done on the BEC side of a Feshbach resonance for spin-polarized Fermi superfluids is greatly simplified by realizing that the system can be described by a Hamiltonian for a Bose-Fermi mixture, where the bosons are diatomic molecules and the fermions are the remaining unpaired atoms. To do this, however, one needs an expression for the effective boson-fermion scattering length $a_{BF}$ that includes many-body effects which become important close to unitarity. For two-body scattering {\it in vacuo}, Skorniakov and Ter-Martirosian (STM) showed in 1957 that the exact value is $a_{BF} = 1.18a_F$, a result also obtained recently by Brodsky and coworkers using a diagrammatic approach. We derive an expression for $a_{BF}$ in the BEC region of a spin-polarized Fermi superfluid using an alternative path-integral treatment of quartic fluctuations, which gives the essential physics of $a_{BF}$ is a simple manner and also allows us to include many-body effects. In the experimentally relevant regime outside the extreme BEC limit, we find corrections to the STM value arising from the fact that scattering occurs in a background gas of condensed Cooper pair bosons, and not in the vacuum. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D32.00009: Pairing in Asymmetrical Fermi Systems with Intra- and Inter-Species Correlations Renyuan Liao, Khandker Quader Motivated by ultracold fermions, we study pairing in two-species Fermi systems with unequal population. We include both inter-species ``singlet'' and intra-species ``triplet'' pairing interactions. Using the equation of motion method, we derive two-point correlation functions, from which various physical quantities can be extracted. We self-consistently solve the resulting coupled mean-field equations for superfluid gap functions and chemical potentials, and study the effects of ``triplet'' correlations on various quantities at T=0 and finite-T. By imposing stability conditions, we construct a phase diagram across the BEC-BCS regimes; it is dramatically different from that without triplet correlations: the BCS singlet superfluid state can sustain a finite polarization, P. For larger P, we find phase separation in BCS and BEC regimes. A superfluid phase exists for all P deep in the BEC regime. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D32.00010: FFLO phase in one-dimensional polarized Fermi gases Xia-Ji Liu, Hui Hu, Peter Drummond Based on the integrable Gaudin model and local density approximation, we discuss the phase structure of one- dimensional trapped Fermi gases with imbalanced spin populations for arbitrary attractive interactions. A phase separation state, with a polarized superfluid core immersed in an unpolarized superfluid shell, emerges below a critical spin polarization. Above it, a coexistence of polarized superfluid matter and a fully polarized normal gas is favored. These two exotic states could be realized experimentally in highly elongated atomic traps, and diagnosed by measuring the lowest density compressional mode. We identify the polarized superfluid as having an FFLO structure, and predict the resulting mode frequency as a function of the spin polarization. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D32.00011: Nonuniform mixed-parity superfluid state in Fermi gases M. S. Mar'enko, K. V. Samokhin We study the effects of dipole interaction on the superfluidity in a homogeneous Fermi gas with population imbalance. We show that the Fulde-Ferrell-Larkin-Ovchinnikov phase is replaced by another nonuniform superfluid phase, in which the order parameter has a nonzero triplet component induced by the dipole interaction. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D32.00012: Polarised Fermi condensates with equal and unequal masses Francesca Maria Marchetti, Meera Parish, Austen Lamacraft, Ben Simons We consider a two-spin atomic Fermi gas, where both the mass and population of each component are unequal. We show that the finite temperature phase diagram contains a region of phase separation between the superfluid and normal states that touches the boundary of second-order superfluid transitions at a tricritical point, reminiscent of the phase diagram of $^3$He-$^4$He mixtures. We discuss the implications of our findings for a trapped gas at finite temperature and the different topologies of phase separation by changing the mass ratio. Finally we find that the interior gap state is always unstable to phase separation, while the breached pair state with one Fermi surface for the excess fermions exhibits differences in its DoSs and pair correlation functions depending on which side of the resonance it lies. [Preview Abstract] |
Session D33: Focus Session: Quantum Foundations II
Sponsoring Units: GQIChair: Steven Flammia, University of New Mexico
Room: Colorado Convention Center 403
Monday, March 5, 2007 2:30PM - 2:42PM |
D33.00001: Adiabaticity in Open Quantum Systems Daniel Lidar, Marcelo Sarandy The adiabatic approximation is an 80+ year old pillar of quantum mechanics, which has found rich applications in a variety of physics and chemistry problems. However, in its original formulation the adiabatic theorem was derived in the context of closed quantum systems, described by unitary dynamics. We have recently introduced a generalization of the the adiabatic theorem to open quantum systems described by convolutionless master equations [1]. This version of the adiabatic theorem is naturally suited to problems in quantum information theory, and we describe applications to the adiabatic quantum computing paradigm [2], and to the problem of geometric phases (both Abelian and non-Abelian) in open quantum systems undergoing cyclic adiabatic evolution [3]. One of our main findings is that, in general, adiabaticity in an open quantum system depends on two competing timescales: the speed of the driving field and the decoherence due to the interaction with the environment. These timescales generically determine a finite interval for adiabaticity. This has implications for both adiabatic quantum computing and the robustness of geometric phases to decoherence. \newline \newline [1] M.S. Sarandy and D.A. Lidar, PRA 71, 012331 (2005). \newline [2] M.S.S. and D.A.L., PRL 95, 250503 (2005). \newline [3] M.S.S. and D.A.L., PRA 73, 062101 (2006). [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D33.00002: Quantum Erasure and Nonlocality in Electronic Mach-Zehnder Interferometers Kicheon Kang We propose a possible realization of solid-state quantum eraser in which electrons are injected into a mesoscopic conductor in the quantum Hall regime [1]. The conductor is composed of a two-path interferometer, an electronic analog of the optical Mach-Zehnder interferometer, and a quantum point contact detector electrostatically coupled to the interferometer. The Coulomb interaction between the interferometer and the detector induces a phase shift that enables the entanglement and the {\it which-path} detection. While the interference of the average output current at the interferometer is suppressed by the which-path information, the which-path information is erased and the hidden coherence reappears in the cross-correlation measurement between the interferometer and the detector output leads. We also investigate a modified setup in which the detector is replaced by a two-path interferometer. We show that the distinguishability of the path and the visibility of joint detection can be controlled in a continuous manner and satisfy a complementarity relation for the entangled electrons. Further, we show that this geometry can be used to test the Bell's inequality. [1] K. Kang, quant-ph/0607031. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D33.00003: Lyapunov Generation of Entanglement and the Correspondence Principle Cyril Petitjean, Philippe Jacquod We show how a classically vanishing interaction generates entanglement between two initially nonentangled particles, without affecting their classical dynamics. For chaotic dynamics, the rate of entanglement is shown to saturate at the Lyapunov exponent of the classical dynamics as the interaction strength increases. In the saturation regime, the one-particle Wigner function follows classical dynamics better and better as one goes deeper and deeper in the semiclassical limit. This demonstrates that quantum-classical correspondence at the microscopic level does not require coupling to a large number of external degrees of freedom. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D33.00004: Time-Symmetric Quantum Mechanics K.B. Wharton The standard Copenhagen Interpretation of quantum mechanics (QM) is not time-symmetric. For example, the collapse postulate applies only in the forward time direction. But this is merely an asymmetry of the interpretation, not QM itself, which is as time-symmetric as the boundary conditions on the system (1). This motivates the search for an interpretation that consistently explains both a forward-time and reversed-time perspective of the same physical events. I present soon-to-be-published results (2) demonstrating that it is possible to have such an interpretation of non-relativistic QM. This is accomplished by applying two consecutive boundary conditions onto solutions of a time-symmetric wave equation. The results appear to match standard QM, but do so without requiring a time-asymmetric discontinuity upon measurement, thereby realigning QM with an important fundamental symmetry. I will also discuss the application of this technique to the Klein-Gordon wave equation. \\ (1) Y. Aharonov, P. Bergmann, and J. Lebovitz, ``Time symmetry in the quantum process of measurement,'' \textit{Phys. Rev.} \textbf{134}, B1410 (1964). (2) K. B. Wharton, ``Time-Symmetric Quantum Mechanics'', accepted for publication in \textit{Foundations of Physics} \textbf{37} (2007). [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D33.00005: Decoherence and the Uncertainty Principle: Theory David Craig, Adam Lemke We investigate the relation between decoherence and the uncertainty principle, both analytically and numerically, in some simple models of quantum measurement. In consistent histories formulations of quantum theory, probabilities may be consistently assigned only in sets of histories which decohere, i.e. in sets in which the interference among the various branches vanishes. Measurements may thus in part be construed as local processes which serve to destroy interference. We study the extent to which the uncertainty principle may be interpreted as the failure of measurements of non- commuting observables to lead to decohering branches in some simple quantum measurement models in the context of a general framework for consistent histories quantum theories due to Hartle known as generalized quantum theory. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D33.00006: Decoherence and the Uncertainty Principle: Numerics Adam Lemke, David Craig We investigate the relation between decoherence and the uncertainty principle, both analytically and numerically, in some simple models of quantum measurement. In consistent histories formulations of quantum theory, probabilities may be consistently assigned only in sets of histories which decohere, i.e. in sets in which the interference among the various branches vanishes. Measurements may thus in part be construed as local processes which serve to destroy interference. We study numerically the extent to which the uncertainty principle may be interpreted as the failure of measurements of non-commuting observables to lead to decohering branches in some simple quantum measurement models. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D33.00007: Quantum Young's experiment, nonlocality and trajectories Edward R. Floyd The trajectory representation of Young's diffraction experiment is developed for a quantum particle. The double slit problem is idealized by simplifying each slit by a point source. The correlated point sources induce a self-entanglement in $\psi$ of the synthesized quantum particle. In turn, entanglement induces nonlocality. A composite reduced action (a generator of nonlocal motion) for the self entangled $\psi$ is developed. Contours of reduced action and nonlocal trajectories are generated in the region near the two point sources. The nonlocal trajectory through any point in configuration space also goes through both point sources simultaneously. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D33.00008: Phase-Covariant Cloning and EPR Correlations in Entangled Macroscopic Quantum Systems Francesco De Martini, Fabio Sciarrino Theoretical and experimental results on the Quantum Injected Optical Parametric Amplification (QI-OPA) of optical qubits in the high gain regime are reported. The large size of the gain parameter in the collinear configuration, g = 4.5, allows the generation of EPR nonlocally correlated bunches containing about 4000 photons. The entanglement of the related Schroedinger Cat-State (SCS) is demonstrated as well as the establishment of Phase-Covariant quantum cloning. The cloning ``fidelity'' has been found to match the theoretical results. According to the original 1935 definition of the SCS, the overall apparatus establishes for the first time the nonlocal correlations between a microcopic spin (qubit) and a high J angular momentum i.e. a mesoscopic multiparticle system close to the classical limit. The results of the first experimental realization of the Herbert proposal for superluminal communication via nonlocality will be presented. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D33.00009: A Local de-Broglie Bohm interpretation of entanglement Michael Clover We present a local interpretation of the de Broglie-Bohm (pilot wave) trajectory prescription for entangled singlet states of massive particles and show that by using appropriately retarded wavefunctions, this local model will exceed Bell's inequality, making no appeal to any detector inefficiencies. We then analyze a possible experimental configuration appropriate to massive two-particle singlet wavefunctions and find that as long as the particles are not ultra-relativistic, the Dirac wave(s) can propagate from Alice or Bob's changing magnetic field, through space to the other detector before the particle arrives, allowing our local interpretation of the two-particle entangled trajectories. The same analysis suggests a physical mechanism that can actually throw away events and create a detector loophole. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D33.00010: The Fabrication of de Broglie Wave J.X. Zheng-Johansson, P.-I. Johansson If an electron e$^{\mbox{-}}$ of de Broglie wavelength ${\mit {\Lambda}}_d$ is fired at $A$ into a field-free chamber of size $AB>>{\mit{\Lambda}}_d$, then, it is a classical point particle. We will be able to register its position, e.g. $B$, and time $T$. If at $B$ is a diffraction grating of spacing $\sim {\mit{\Lambda}}_d$, then the e$^{\mbox{-}}$ arriving in it is a quantum particle and produces diffraction patterns; e$^ {\mbox{-}}$ must be a train of traveling plane wave of many ${\mit{\Lambda}}_d$'s. Naturally a viable theory for the formation of basic particles, like the electron, ought to pass among others the above simple but critical test: being a particle and wave. One may illustrate the former feature by a wave packet which yet lacks periodicity in space-time of a plane wave. We recently developed [1-3] based on overall experiments a particle formation scheme. By it a basic particle like the electron is made of a massless oscillatory charge $-e$ or $+e$ of a fixed oscillatory energy, and the resulting electromagnetic waves. When the particle is in motion, so is its source charge, then owing to the resulting Doppler effect the EM wave evolves into a beat wave resembling precisely a traveling de Broglie wave. It firstly passes the above test well, it obeys de Broglie relations and Schr\"odinger equation, and it has the overall other observational particle properties. \ Refs: JXZJ \& PIJ in 1. {\it Unif. of Clas., Quant. \& Rel. Mech. \& Four Forces}, Nova Sci. 2005, Fwd R Lundin; 2. {\it Quant. Theory \& Symm. IV}, ed V Debrev, Heron Press, 2006, 763; 771; 3. {\it Prog. in Phys.} {\bf 4}, 32, 2006; refs therein. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D33.00011: Replacing The General Covariance In the SM Dirac Equation Gauge Derivatives With An Equivalent General Covariance In The Metric That This Dirac Equation is Derived From Joel Maker We replace the general covariance in the gauge derivatives in the Standard Model (SM) with a general covariance in the\textit{ original} metric that is used to start the derivation of the SM Dirac equation. This puts in the general covariance at the very beginning of the Dirac equation derivation, \textit{where it belongs}. The result is a new Dirac equation ($\surd $\textbf{\textit{g}}$_{\mu \mu }$\textit{$\gamma $}$_{\mu }$\textit{$\partial \psi $/$\partial $x}$_{\mu }$\textit{+i$\omega \psi $=0 }with\textbf{~}\textbf{g}$_{oo}$=1-2e$^{2}$/rm$_{e}$c$^{2})$ that does not require the covariant gauge derivatives anymore but yet still \textit{retains }the general covariance creating a \textbf{ONE} free parameter theory, instead of 18 of the SM. For example this new Dirac equation has a singularity-stability radius r$_{H}$ and, because of equivalence principle considerations, is allowed only \textit{one} type of charge e. Thus near r$_{H}$ the 2P$_{3/2}$ state for this new Dirac equation gives a $\psi ^{tt}\psi $ azimuthal trifolium, 3 lobe shape; so this ONE charge e (so don't need \textbf{ color} to guarantee this) spends \textbf{1/3} of its time in each lobe (\textbf{fractionally charged} lobes), the lobe structure is locked into the center of mass \textbf{(asymptotic freedom}), there are\textbf{ six} 2P states (corresponding to the 6 flavors) ;~ which are the~~\textbf{main properties of quarks}!~ Thus we end up with the experimental implications of the Standard Model (SM) by postulating just ONE particle with mass. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D33.00012: Purple bacteria and quantum Fourier transform Samir Lipovaca The LH-II of purple bacteria Rhodospirillum (Rs.) molischianum and Rhodopseudomonas (Rps.) acidophila adopts a highly symmetrical ring shape, with a radius of about 7 nm. In the case of Rps. acidophila the ring has a ninefold symmetry axis, and in LH-II from Rs. molischianum the ring has an eightfold symmetry axis. These rings are found to exibit two bands of excitons. A simplified mathematical description of the exciton states is given in Hu, X. {\&} Schulten, K. (1997) Physics Today 50, 28-34. Using this description, we will show, by suitable labeling of the lowest energy (Qy) excited states of individual BChls, that the resulting exciton states are the quantum Fourier transform of the BChls excited states. For Rs. molischianum ring exciton states will be modeled as the four qubit quantum Fourier transform and the explicit circuit will be derived. Exciton states for Rps. acidophila ring cannot be modeled with an integer number of qubits. Both quantum Fourier transforms are instances of the hidden subgroup problem and this opens up a possibility that both purple bacteria implement an efficient quantum circuit for light harvesting. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D33.00013: The Mystery of Genetic Tape Shantilal Goradia I attempt to explain information exchanges of a DNA at particle level at Planck scale in terms of the quantum computer theory. The substitution of Euclidean wormholes proposed by Hawking in hep-th/0507171, in Fig 2 of my gr-qc/0507130 [1] makes them unstable, giving them the ability to register alternate bits of information, OPEN or CLOSE, every other natural unit of Planck time with time dependent sequencing. I will present a pictorial view, of my idea introduced at Feynman Festival Aug, 2006 and DNP06. Whether the wormholes are Euclidean and therefore inherently unstable, or they are Lorentzian and destabilized by quantum fluctuations of gravity, reported degrading with time [2] is of secondary importance. The point is that the instability of wormholes explains cellular communications, in addition to running couplings explained in more detail in physics/0210040. The quantum mouths proposed in [1] can cross the cell membrane and cell wall to express their information. Nature creates the instability to generate qubits of information, for storage and expression of heredity. Nature burns no encyclopedia. [1] Goradia S. G., \textit{Indian Journal of Theoretical Physics,} \textbf{52} 143 (2004) [2] Leslie J., \textit{Physical Cosmology and Philosophy,} 90 (1990) [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D33.00014: Do Particles have Barcodes? Shantilal Goradia If an elementary particle shown in Fig 2 of gr-qc/0507130 has an UNSTABLE quantum connection to the rest of the universe calibrated by nature in terms of Planck times, as also proposed in my separate MAR07 abstract, there exists a possibility that each particle has a barcode of its own. Instability implies varying periods of connections and disconnections of particles to the universe, which would be equivalent to the varying widths of white and black strips of commercial barcodes. Considering the high order of magnitude of Planck times in a second, each particle and the universe generated by its radiations may have their unique birth times registered in their barcodes. My quest for the cause of consciousness, in MAR06 abstracts, as an additional implication of physics/0210040, leads to the inquiry if these unique parallel universes are like the ones that give rise to consciousness as proposed by some physicists. With all due respect, the attempts to explain TOE of inert matter may not be attempts to explain one step to climb up on a stairway at a time. They may be attempts to explain only half a step at a time to on a stairway made with only integer number of steps. The search for TOE assumes such a theory exists. Mathematics has no barrels to fire bullets that can shoot down a non-existent bird. A Hamiltonian knows no consciousness, a missing ingredient of biology made of particles or vice versa, and of realistic TOE. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D33.00015: More on Atonic Mechanics Alfred Phillips We have shown that crystal based experiments, such as those of Davisson and Germer, do not empirically verify de Broglie's matter-wave hypothesis. We named this theory Atonic Mechanics. This mechanics has also been used to accurately calculate the hundreds of helium atom energy levels tabulated by NIST. We have joined Atonic Mechanics with Einstein's General Relativity. Now we show how fractional values of h-bar arise for the angular momentum of the helium atom in this theory. We now also discuss energy and momentum in Atonic Mechanics. [Preview Abstract] |
Session D34: Cell Level Patterning During Embryonic Development
Sponsoring Units: DBPChair: James Glazier, Indiana University
Room: Colorado Convention Center 404
Monday, March 5, 2007 2:30PM - 3:06PM |
D34.00001: The \textit{de novo} formation of a vascular network, in warm-blooded embryos, occurs via a self-assembly process that spans multiple length and time scales Invited Speaker: Taking advantage of wide-field, time-lapse microscopy we examined the assembly of vascular polygonal networks in whole bird embryos and in explanted embryonic mouse tissue (allantois). Primary vasculogenesis assembly steps range from cellular (1-10 $\mu $m) to tissue (100$\mu $m-1mm) level events: Individual vascular endothelial cells extend protrusions and move with respect to the extracellular matrix/surrounding tissue. Consequently, long-range, tissue-level, deformations directly influence the vascular pattern. Experimental perturbation of endothelial-specific cell-cell adhesions (VE-cadherin), during mouse vasculogenesis, permitted dissection of the cellular motion required for sprout formation. In particular, cells are shown to move actively onto vascular cords \textit{that} \textit{are subject to strain via tissue deformations}. Based on the empirical data we propose a simple model of preferential migration along stretched cells. Numerical simulations reveal that the model evolves into a quasi-stationary pattern containing linear segments, which interconnect above a critical volume fraction. In the quasi-stationary state the generation of new branches offsets the coarsening driven by surface tension. In agreement with empirical data, the characteristic size of the resulting polygonal pattern is density-independent within a wide range of volume fractions. These data underscore the potential of combining physical studies with experimental embryology as a means of studying complex morphogenetic systems. \newline In collaboration with Brenda J. Rongish$^{1}$, Andr\'{a}s Czir\'{o}k$^{1,2}$, Erica D. Perryn$^{1}$, Cheng Cui$^{1}$, and Evan A. Zamir$^{1}$ \newline \newline $^{1}$Department of Anatomy and Cell Biology, the University of Kansas Medical Center, Kansas City, KS \newline $^{2}$Department of Biological Physics, E\"{o}tv\"{o}s Lor\'{a}nd University, Budapest, Hungary. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D34.00002: Physical Mechanisms of Patterning Instabilities in the Formation of Vascular Network Abbas Shirinifard, James Glazier, Shantia Yarahmadian Endothelial cells, which line the inner walls of blood vessels, self-organize into network structures in vitro and in vivo. The physical mechanisms of network formation are a current subject of debate may be important during development, wound heeling, and tumor growth. Using Glazier and Graner's Cellular Potts Model (CPM) to model chemotactically migrating cells, we studied the patterning instabilities and scaling properties of the network in two and three-dimensions. We ran our simulations in Compucell3D, an open-source software environment based on CPM (http://simtk.org/home/compucell3d). The average characteristics of the network structure are independent of the initial configuration of cells and scale with the diffusion parameters of the chemoattractant. We have also developed an analytical PDE model to study nature of patterning instabilities. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D34.00003: Physical Mechanisms of Pattern Formation in the Early Chick Embryo Ariel Balter, James Glazier, Benji Zaitlen, Mark Chaplain, Cornelis Weijer Gastrulation marks a critical step in early embryogenesis when the first recognizable patterns are laid down. Although the genome maintains ultimate responsibility for this pattern formation, it cannot actually control the organization of individual cells. The robustness of embryogenic pattern formation suggests that a few simple, physical mechanisms are unleashed and that self-organization results. We perform numerical simulations of early chick gastrulation using an agent based method in which individual cells interact via a handful of behaviors including adhesivity, secretion and chemotaxis. Through these simulations we have identified certain behaviors as being important for various stages and morphological events. For instance, experimental results on primitive streak formation are best reproduced by a model in which the Kohler's Sickle secretes a chemo repellant for streak tip cells, and cell polarization appears to be important for initiating {\it polonaise} motion during streak elongation. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D34.00004: Interface Instabilities and Fingering in a Simulated Growing Tumor Nikodem Poplawski, Maciej Swat, James Glazier, Alexander Anderson We study the physical origin of interface instabilities, which may lead to metastasis in medical contexts, during the invasion of healthy tissue by a solid tumor. We use Glazier and Graner's Cellular Potts Model (CPM), a lattice-based stochastic framework designed to simulate cell interactions and movement. This model reduces the large molecular complexity of living cells to a few basic processes: cell-cell adhesion, cell growth, division, differentiation and death, secretion and absorption of materials, chemotaxis, and cellular deformation. We run our simulations in CompuCell3D, an open-source software environment based on the CPM (https://simtk.org/home/compucell3d). We show that cells adhesivity and growth, and rate per unit nutrient consumed, determine whether the growing tumor has a flat or fingered interface. Our results differ from those reported by Anderson (A. R. A. Anderson, Math. Med. Biol. (2005) 22:163) using a continuum model. This difference shows the importance of explicit modeling of spatially extended cells to understanding the morphologies of developing tissues. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D34.00005: Pattern formation of glioma cells: effects of adhesion Evgeniy Khain, Michal O. Nowicki, E. Antonio Chiocca, Sean E. Lawler, Leonard M. Sander \emph{Glioblastoma multiforme} is a highly malignant brain tumor. We investigate the mechanism of clustering of glioma cells \emph{in vitro}; this may shed light on clustering in the brain. Recent experiments with tumor spheroids growing in a transparent gel showed that one cell line formed clusters in a region where invasion occurs, whereas a very similar cell line does not cluster significantly. Using stochastic discrete modeling of motile adhesive and proliferative cells, we identified two important mechanisms which may lead to clustering. First, there is a critical value of the strength of cell-cell adhesion; above the threshold, large clusters grow from a homogeneous suspension of cells; below it the system remains homogeneous. Second, when several single cells form a small cluster, they may switch their phenotype from ``invasive'' to ``proliferative,'' increasing their division rate. The theoretical predictions were tested in an experiment in which we followed the clustering dynamics of glioma cells on a surface. We have successfully reproduced the experimental findings and found that both mechanisms are crucial for cluster formation and growth. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D34.00006: Role of viscosity and surface tension of zebrafish embryonic tissues in tissue flows during gastrulation. E.M. Schoetz, T. Bacarian, M.S. Steinberg, R.D. Burdine, W. Bialek, C.P. Heisenberg, R.A. Foty, F. Julicher At the onset of gastrulation in zebrafish, complex flows and cell movements occur, which are not well understood. Here, we study the material properties of zebrafish embryonic tissues which are important for the tissue dynamics. We found that these tissues behave viscoelastic and exhibit liquid-like properties on long time scales. They relax internal stress caused by compressive forces or, in the absence of external forces, round up and fuse into spheres to minimize their free surface. Quantitative differences in the adhesivity between different types of tissues result in their immiscibility and sorting behavior analogous to that of ordinary immiscible liquids. When mixed, cells segregate into discrete phases, and the position adopted correlates with differences in the aggregate surface tensions for these phases. Surface tensions were measured with a tissue surface tensiometer. Aggregates were compressed and their force response and shape were recorded as a function of time. From the analysis of the force-relaxation curves, we determined the surface tensions, relaxation times, tissue viscosities and shear moduli. Furthermore, by 4D-cell tracking, we measured kinetic parameters such as cell speed, directionality and persistence of cell movement. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D34.00007: Stochastic model of cell rearrangements in convergent extension of ascidian notochord Sharon Lubkin, Tracy Backes, Russell Latterman, Stephen Small We present a discrete stochastic cell based model of convergent extension of the ascidian notochord. Our work derives from research that clarifies the coupling of invagination and convergent extension in ascidian notochord morphogenesis (Odell and Munro, 2002). We have tested the roles of cell-cell adhesion, cell-extracellular matrix adhesion, random motion, and extension of individual cells, as well as the presence or absence of various tissue types, and determined which factors are necessary and/or sufficient for convergent extension. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D34.00008: Time markers for Drosophila morphogenesis based on cell-pattern topology. Richard Zallen, Jennifer A. Zallen Recent work on convergent extension in Drosophila has shown that the accumulation of actin-myosin networks at specific cell interfaces initiates planar polarity and the formation of multicellular rosette structures that contribute to elongation of the body axis [1]. This cell-rearrangement process takes place within a one-cell-thick layer, and the changing two-dimensional cell pattern can be characterized using topological measures such as cell-shape statistics [2]. We find that the timeline for the process contains a well-defined marker corresponding to a sharp increase in the slope of the time dependence of the variance of the cell-shape (number-of-sides) distribution. A rosette in this context is a cluster of cells enclosing high-order vertices at which 4 or 5 or more cells meet. While the cell-shape variance climbs steadily during axis elongation, the frequency of high-order vertices and large rosettes plateaus after 10 and 13 minutes, respectively. These time markers calibrate the conventional timeline descriptors referred to as stages 7 and 8 of embryonic development [3]. [1] J.T. Blankenship et al., Developmental Cell 11, 459 (2006); [2] J.A. Zallen and R. Zallen, J. Phys.: Condensed Matter 16, S5073 (2004); [3] J.A. Campos-Ortega and V. Hartenstein, The embryonic development of Drosophila melanogaster (1985). [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D34.00009: Different Strategies for Aggregation in Social Amoeba Colonies Carl Franck, Ryan Monaghan, Albert Bae, Duane Loh, Eberhard Bodenschatz When confronted by starvation, collections of the amoeba Dictyostelium discoideum seek to aggregate in order to form genome-preserving stalk and spore structures. We have been interested in the means by which individual cells unite for this purpose. It has long been recognized that communication by means of diffusion of small molecules affords one such strategy: periodic chemical wave signaling can direct individual cells to an aggregation site. By employing thin layer substrates that presumably alter the propagation characteristics of such waves, we have shifted the colonial aggregation strategies to modes that rely on adhesive interactions for initial stages of multicellular assembly. Besides relentless aggregation of individual cells into large scale streams, these substrates reveal remarkable structures composed of only a few cells which we call ``squads'' that search for each other in order to achieve sufficient aggregation mass in sparse populations. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D34.00010: Optimal Foraging by Zooplankton Ricardo Garcia, Frank Moss We describe experiments with several species of the zooplankton, \textit{Daphnia, }while foraging for food. They move in sequences: \textit{hop-pause-turn-hop }etc. While we have recorded hop lengths, hop times, pause times and turning angles, our focus is on histograms representing the distributions of the turning angles. We find that different species, including adults and juveniles, move with similar turning angle distributions described by exponential functions. Random walk simulations and a theory based on active Brownian particles indicate a maximum in food gathering efficiency at an optimal width of the turning angle distribution. Foraging takes place within a fixed size food patch during a fixed time. We hypothesize that the exponential distributions were selected for survival over evolutionary time scales. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D34.00011: Precision and Reproducibility in Biological Patterning Thomas Gregor, Eric F. Wieschaus, William Bialek, David W. Tank During embryonic development, information about spatial location is represented by the concentration of various morphogen molecules. The reproducibility and precision of biological pattern formation thus is limited by the accuracy with which these concentration profiles can be established and ``read out'' by their target pathways. We consider four measures of precision for the Bicoid morphogen in the Drosophila embryo: The concentration differences that distinguish neighboring cells, the limits set by the random arrival of Bcd molecules at their targets (which depends on the absolute concentration), the noise in readout of Bcd by the activation of Hunchback, and the reproducibility of Bcd concentration at corresponding positions in multiple embryos. We show, through a combination of different experiments, that all of these quantities are $\sim$10\%. This agreement among different measures of accuracy, which depend on very different molecular mechanisms, indicates that the embryo is not faced with sloppy input signals and noisy readout mechanisms; rather we have to understand how the embryo exerts precise control over absolute concentrations and responds reliably to small changes in these concentrations, down to the limits set by basic physical principles. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D34.00012: A lattice model of parasite-host population dynamics Brian Skinner, Beate Schmittmann, Royce Zia The study of simple parasite-host population models may help us advance fundamental understanding of nonequilibrium steady-states and provide insight into potential applications for controlling epidemics. Using Monte Carlo techniques, we investigate a model of interacting parasite-host populations in which parasites must come into contact with a host in order to reproduce. We treat the parasites and hosts as random walkers on a two-dimensional lattice with reflecting boundary conditions and vary the parasite death rate and the relative diffusion rates of the two species. For low death rates and slow host diffusion, steady state populations can exist and the resulting non-trivial spatial distributions are measured. We also explore the consequences of allowing the hosts to respond to local gradients in the parasite concentration. If the hosts are biased to move away from regions of high parasite concentration, an effective repulsion between hosts emerges. Both the population levels and the spatial distributions are observed to depend sensitively on the details of this response. Some aspects of these phenomena can be understood analytically. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D34.00013: Cell Assisted Cell Growth Experiments with Dictyostelium discoideum Albert Bae, Wui Ip, Carl Franck In eukaryotic cell culture, it is routinely recommended to keep the cells above a minimum cell density to maintain vigorous growth. We are investigating the basis for this prescription by viewing cell growth as a social behavior facilitated by cell-cell communication. Employing Dictyostelium discoideum, we find good evidence for a slow-fast transition in the cell growth rate vs. density in well mixed, 25 ml, cell cultures. We also use low height microfluidic chambers (four orders of magnitude smaller in volume) to find similar behavior even though the system is not well mixed and the cells are confined to substrates. A preliminary measurement at a flow rate that should strongly perturb cell-cell communication by means of diffusing signal molecules suggests that cell communication essential for growth is not accomplished by such means but possibly via direct contacts. [Preview Abstract] |
Session D35: Focus Session: Biological Networks
Sponsoring Units: DBP GSNPChair: Eivind Almaas, Lawrence Livermore National Laboratory
Room: Colorado Convention Center 405
Monday, March 5, 2007 2:30PM - 3:06PM |
D35.00001: Synthetic Gene Networks: \textit{De novo constructs -- in numero} descriptions Invited Speaker: Uncovering the structure and function of gene regulatory networks has become one of the central challenges of the post-genomic era. Theoretical models of protein-DNA feedback loops and gene regulatory networks have long been proposed, and recently, certain qualitative features of such models have been experimentally corroborated. This talk will focus on model and experimental results that demonstrate how a naturally occurring gene network can be used as a ``parts list'' for synthetic network design. The model formulation leads to computational and analytical approaches relevant to nonlinear dynamics and statistical physics, and the utility of such a formulation will be demonstrated through the consideration of specific design criteria for several novel genetic devices. Fluctuations originating from small molecule-number effects will be discussed in the context of model predictions, and the experimental validation of these stochastic effects underscores the importance of internal noise in gene expression. The underlying methodology highlights the utility of engineering-based methods in the design of synthetic gene regulatory networks. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D35.00002: Origin of Modularity in Recombination Evolution Jun Sun, Michael Deem Modularity is a well-known phenomenon in biology. Modularity implies a hierarchical character, and is manifested in both phenotypic and genotypic levels. A module is defined, in general, as a component which operates relatively independently of other components of the system. The independence is in both the structural and functional levels. How does modularity originate? Evolvability is a selectable trait and modularity enhances evolvability. Thus, under conditions that select for evolvability, we expect to see the emergence of modularity. We used a spin-glass model to simulate the evolution of genomes. This model captures the interactions between amino acids or epistasis between genes. The evolutions include both sequence evolution and structure evolution. The environment changes and recombination plays an important role in evolution. We will present our result of the emergence of modularity, a symmetry breaking of the system. We will present the dependence of modularity on the amplitude and frequency of environment changing. The crucial role of recombination in the emergence of modularity will be discussed as well. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D35.00003: Dynamic network analysis of protein interactions Eivind Almaas, Joya Deri Network approaches have recently become a popular tool to study complex systems such as cellular metabolism and protein interactions. A substantial number of analyses of the protein interaction network (PIN) of the yeast {\it Saccharomyces cerevisiae} have considered this network as a static entity, not taking the network's dynamic nature into account. Here, we examine the time-variation of gene regulation superimposed on the PIN by defining mRNA expression profiles throughout the cell cycle as node weights. To characterize these network dynamics, we have both developed a set of novel network measures as well as studied previously published measures for weighted networks. We expect that our approach will provide a deeper understanding of protein regulation during the cell cycle. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D35.00004: Growth induced instability in metabolic networks Sidhartha Goyal, Ned S. WIngreen Networks of molecular interactions are essential for mass, energy, and information transport into and within cells. Thus, understanding the emergent physical properties of various network architectures is of fundamental interest in biology. One such architecture, product-feedback inhibition is widely used in~the regulation of biosynthetic pathways of all organisms. Importantly, these biosynthetic pathways are often coupled both by the use of a common substrate and by stoichiometric utilization of their products for cell growth. We analyze networks having~the following three essential features: all branches start from a common substrate, the product of each branch inhibits the first dedicated step towards its synthesis, and all products are essential for growth. We show that such a coupled network can have at most one steady state. However, the network~may be~unstable about this steady state, even if the branches are individually stable. In the unstable region, the network exhibits limit-cycle oscillations which arise via a Hopf bifurcation. In the oscillating regime, a two-branch coupled network can be mapped to a three-species frustrated system. Our results highlight new design principles essential for realizing robust biosynthetic pathways. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D35.00005: Emergent Criticality from Co-evolution in Random Boolean Networks Min Liu, Kevin E. Bassler The co-evolution of network topology and dynamics is studied in an evolutionary Boolean network model that is a ``coarse-grained" model of a gene regulatory network. We find that a critical state emerges spontaneously from the interplay between topology and dynamics when the network is updated by a rule that rewires its internal connections based on the activities of nodes and changes the dynamical functions. The final evolved state is shown to be critical and independent of initial conditions. The network appears to be driven to a random Boolean network with uniform in-degree of 2 in the large network limit. However, for biologically realized network sizes, significant finite-size effects are observed including a broad in-degree distribution and an average in-degree connection between 2 and 3. These results may be important for explaining the formation of heterogeneous topology in real gene regulatory networks. Detailed work is discussed in the paper Phys. Rev. E \textbf{74}, 041910 (2006). [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D35.00006: Protein-Protein interaction networks: why static MpK model works and preferential attachment does not Jingshan Zhang, Eugene Shakhnovich Various approaches have been proposed to explain the observed scale free structure $p(k) \sim k^{-\gamma}$ of protein-protein interaction networks. We argue that the preferential attachment coming from gene duplication[1] is questionable. A static ``MpK'' model produces the scale free structure via computer simulations[2] for unexplained reasons. On the other hand, it was analytically proved[3] that deterministic threshold models produce scale free networks (with $\gamma\equiv 2$) if fitness distributions are exponential. We study the static MpK model further and find the above analytical proof applicable with extensions, and $\gamma$ dependent on the threshold parameter. This work not only predicts the dependence of $\gamma$ on protein concentrations, but also provides a generic mechanism of scale free networks. The clustering coefficient distribution in the model is interpreted by a simple picture. \newline \newline [1] A.-L. Barab\'asi and Z. N. Oltvai, Nature Reviews Genetics \textbf{5}, 101 (2004). \newline [2] E. J. Deeds, O. Ashenberg, E. I. Shakhnovich, Proc. Natl. Acad. Sci. USA \textbf{103}, 311 (2006). \newline [3] G. Caldarelli, A. Capocci, P. De Los Rios, and M. A. Mu\~noz, Phys. Rev. Lett. \textbf{89}, 258702 (2002). [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D35.00007: A closer look at activity in metabolic networks Natali Gulbahce, Takashi Nishikawa, Adilson E. Motter Single-cell organisms are assumed to optimize growth under specific conditions. Using flux balance analysis, it is possible to estimate the number of reactions that are utilized (active) by the metabolism in random and optimal metabolic states. Here we investigate the mechanisms that determine the number of active reactions mathematically and compare them to those of real organisms. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D35.00008: Stochastic Chemical Kinetics in Biochemical Reaction Networks. Garegin Papoian, Yueheng Lan We used various analytic and numerical methods to elucidate complex dynamics in stochastic signal transduction. We demonstrate that the commonly used linear noise approximation to solving the chemical master equation fails when the number of proteins becomes too low. Consequently, we developed a new analytical approximation to the solution of the master equation, based on the generating function approach, which works in a much wider range of protein number fluctuations. We show that in a linear signaling pathway, a reaction rate at a node could be tuned so the node acts either as a noise amplifier or as a noise filter. For more complex cascades, we mapped the stochastic chemical kinetics master equation into a quantum field theoretical problem, which we solved using the variational principle. We demonstrate stochastic resonance in signal transmission in enzymatic cascades with and without feedback loops. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D35.00009: Noise propagation in combined cellular control motifs Cheol-Min Ghim, Eivind Almaas A cell's ability to respond robustly to noisy stimuli critically depends on the structure of its regulation and control circuitry, as well as kinetic parameters. While kinetic parameters take a wide range of values, there is markedly less variation in the basic network building blocks. We have explored the functional implications of several motif-combinations, investigating their information processing properties. Adopting a spectral-analysis approach, we study how circuit topology affects the propagation or attenuation of intrinsic and extrinsic noise. Finally, we discuss possible fitness benefits of the different circuit topologies, relating design principles to evolutionary selection. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D35.00010: Stochastic effects in reaction networks Aryeh Warmflash, Aaron Dinner Experiments that yield information about single cells make clear that intrinsic noise in reactions involving low copy numbers of molecules can have important functional consequences. Although it is typically assumed that noise introduces isotropic fluctuations about a mean, this need not be the case. Within the Langevin framework, we develop ``rules of thumb'' for understanding the impact of noise on systems of reactions. We show analytically how increases in either the magnitude or correlation time of fluctuations can give rise to amplifications and bifurcations. As an example, we consider the enzymatic cycle studied by Goldbeter and Koshland. Fluctuations in the total number of enzyme for the forward reaction have been shown to amplify the concentration of the modified substrate and can even create additional peaks in its distribution. We show how our results lead to a transparent physical interpretation of these observations, and we clarify how ultrasensitivity, amplification, bifurcation, and stochastic focusing relate to each other. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D35.00011: Universal patterns in the behavior of complex systems: from relaxation in fractal networks to distribution of income Valerica Raicu, Michael Stoneman, Russell Fung The study of relaxation is an active area of research in the fields of dielectric, mechanical and optical spectroscopy, which is insufficiently developed for the case of complex systems. It has been established that the relaxation of many systems deviates markedly from classical Debye dispersion function (in the frequency domain) or from pure exponential decay (in the time domain), but the exact ways in which these deviations occur and their significance are still debated issues. Here we propose that a fractal-tree network appropriately describes the relaxation pathway in a variety of complex systems and predicts coupled (or hierarchical) as well as uncoupled (parallel) relaxation processes. This approach has been originally introduced for description of dielectric relaxation in Cantorian trees in biology. Upon adequate generalization this approach sheds new light on a variety of processes, ranging from kinetics of protein-ligand rebinding through distribution of income in populations of humans. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D35.00012: Understanding Dynamic Patterns of NF-$\kappa $B Signaling: Derivation and Analysis of a Minimal Model through Sensitivity Analysis Jaewook Joo, Steve Plimpton, Shawn Martin, Laura Swiler, Jean-Loup Faulon Understanding the pleiotropism of NF-$\kappa $B signal transduction is a challenge of clear medical importance and systems biology. Current mathematical modeling frameworks for NF-$\kappa $B signal transduction, though limited to a small signaling module located in a downstream of IKK, heavily rely on the parameterizations and the numerical studies of ODE models and doubtless lack intuitive explanations about underlying mechanisms of the dynamic patterns of the NF-$\kappa $B signaling. Here we present a systematic way to derive a minimal model from an up-to-dated and detailed NF-$\kappa $B signaling network by means of sensitivity analysis. Using analysis of the minimal model, we predict a dose-response curve shape, existence of Hopf-bifurcation, and underlying mechanisms of all possible dynamic patterns of NF-$\kappa $B signaling. Simulating the detailed ODE model for NF-$\kappa $B signaling network with large sets of the parameter values that are sampled from the biologically feasible parameter space, we present an ensemble of all possible dynamic patterns of NF-$\kappa $B signaling and verify the predictions from the minimal model. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D35.00013: Sloppy systems biology: tight predictions with loose parameters James Sethna, Ryan Gutenkunst, Joshua Waterfall, Fergal Casey, Kevin Brown, Christopher Myers Directly measuring the parameters involved in dynamical models of cellular processes is typically very difficult, and collectively fitting such parameters to other data often yields large parameter uncertainties. Nonetheless, a collective fit which only weakly constrains model parameters may strongly constrain model \emph{predictions}, if the model is ill-conditioned: much more sensitive to some directions in parameter space than others. In the quadratic approximation, the model sensitivities are proportional to the inverse square roots of the hessian matrix eigenvalues. Using a collection of 14 models from the systems biology literature, we show that for large systems the eigenvalue spectra are universally \emph{sloppy}; they span huge ranges ($> 10^6$) and have approximately constant logarithmic spacing. Thus the models are ill-conditioned and have no well-defined cutoff between important and unimportant parameter combinations. This universal sloppiness suggests that collective fits will often poorly constrain parameters but usefully constrain many predictions. [Preview Abstract] |
Monday, March 5, 2007 5:30PM - 5:42PM |
D35.00014: A Physical Theory of the Competition that Allows HIV to Escape from the Immune System Michael Deem Competition within the immune system may degrade immune control of viral infections. We formalize the evolution that occurs in both HIV-1 and the immune system quasispecies [1]. Inclusion of competition in the immune system leads to a novel balance between the immune response and HIV-1, in which the eventual outcome is HIV-1 escape rather than control. The analytical model reproduces the three stages of HIV-1 infection. We propose a vaccine regimen that may be able to reduce competition between T cells, potentially eliminating the third stage of HIV-1. 1) G. Wang and M. W. Deem, Phys. Rev. Lett. 97 (2006) 188106. [Preview Abstract] |
Session D38: Nanoinstrumentation for Biological and Other Applications
Sponsoring Units: GIMSChair: Karen Waldrip, Sandia National Laboratories
Room: Colorado Convention Center 501
Monday, March 5, 2007 2:30PM - 2:42PM |
D38.00001: Assembly of a minimal protocell Steen Rasmussen What is minimal life, how can we make it, and how can it be useful? We present experimental and computational results towards bridging nonliving and living matter, which results in life that is different and much simpler than contemporary life. A simple yet tightly coupled catalytic cooperation between genes, metabolism, and container forms the design underpinnings of our protocell, which is a minimal self-replicating molecular machine. Experimentally, we have recently demonstrated this coupling by having an informational molecule (8-oxoguanine) catalytically control the light driven metabolic (Ru-bpy based) production of container materials (fatty acids). This is a significant milestone towards assembling a minimal self-replicating molecular machine. Recent theoretical investigations indicate that coordinated exponential component growth should naturally emerge as a result from such a catalytic coupling between the main protocellular components. A 3-D dissipative particle simulation (DPD) study of the full protocell life-cycle exposes a number of anticipated systemic issues associated with the remaining experimental challenges for the implementation of the minimal protocell. Finally we outline how more general self-replicating materials could be useful. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D38.00002: High throughput electronic cell identification techniques for microfluidic systems David Wood, Gary Braun, Jean-Luc Fraikin, Loren Swenson, Norbert Reich, Andrew Cleland We address the problem of whole-cell identification using an all-electronic microfluidic approach, with potential applications to cell sorting. We present the development of a radiofrequency microsensor, capable of detecting cells or cell labels in a microfluidic system. This device has demonstrated detection of individual cellular labels at throughputs of 30,000 labels/s in a single microfluidic channel. We also present the development of digital barcodes, which can be used to label cells for identifying individual strains in a diverse population. These barcodes were developed using fully-scalable lithographic techniques, providing a means for low-cost, large volume production. We have demonstrated biological functionalization of these barcodes as well as readout, using our radiofrequency microsensor, at throughputs greater than 1,000 labels/s. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D38.00003: Dynamic Detection of a Single Bacterium: Nonlinear Rotation Rate Shifts of Driven Magnetic Microspheres Brandon H. McNaughton, Rodney R. Agayan, Raoul Kopelman We report on a new technique which was used to detect single Escherichia coli that is based on the changes in the nonlinear rotation of a magnetic microsphere driven by a magnetic field. The presence of one Escherichia Coli bacterium on the surface of a 2.0 micron magnetic microsphere (with an aluminum ``nanocap'' that indicates the microsphere's orientation) caused an easily measurable change in the drag of the system and, therefore, in the nonlinear rotation rate. The straight-forward measurement uses standard microscopy techniques and the observed average shift in the nonlinear rotation frequency changed by a factor of $\sim$3.8 (Arxiv preprint cond-mat/0610144). Further miniaturization will allow for dynamic detection of viruses and potentially even biomolecules in fluidic environments. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D38.00004: Nanolaser spectroscopy of Normal and Genetically Defective Mitochondria: New Biostatistical Tool for Studying Disease Paul Gourley, Judy Hendricks, Robert Naviaux, Michael Yaffe We report an analysis of wild and mutant strains of mitochondria from yeast cells using nanolaser spectroscopy to measure cytochrome density and its statistical variation in the population. The first strain 110 was derived from wild-type strain 104 (Saccharomyces cerevisiae) by removal of its mitochondrial DNA (mtDNA), resulting in loss of all mtDNA-encoded proteins and RNAs, and loss of the pigmented, heme-containing cytochromes a and b that can be detected in the laser spectra. Histograms of laser wavelengths produced by wild-type mitochondria produced peaked distributions, while mutant mitochondria exhibit asymmetric, highly skewed distributions. Surprisingly, all of these distributions exhibit extended tails and can be self-consistently fit with log-normal distribution functions. In striking contrast, the mitochondrial diameters (measured separately by microscopy) exhibit normal Gaussian distributions. These results indicate that the nanolaser spectra are useful for quantifying cytochrome content in mitochondria and may have important implications for quantifying defects in mitochondria that manifest human disease. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D38.00005: Gas sensing behavior of individual carbon nanotube field effect transistors Michael Stadermann, Alexander Artyukhin, Olgica Bakajin, Aleksandr Noy Carbon nanotube field effect transistors (FETs) have been found to be gas sensors with amazing sensitivity. Thus far, however, the exact sensing mechanism of the devices remains unknown. Recent results indicate that the analytes may bind to defects in the nanotubes and change their conductance through charge transfer, but all of these measurements have been performed in networks of carbon nanotubes, in which the properties of the network rather than those of the individual tube are measured. In this work, we study the behavior of individual nanotube devices. We measure the response times of both pristine as well as damaged nanotubes to determine the effect of defects on the sensor response, and we measure devices with coated and uncoated electrodes to determine the contribution of the contacts. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D38.00006: Large-scale assembly of CNT based DNA sensor array on SiO2 substrate Joohyung Lee, Byung Yang Lee, Dong Joon Lee, Kyungeun Byun, Seunghun Hong DNA sensors based on CNTs have been attracting attention due to their possible applications such as genotyping, disease diagnosis, etc. Previous works were mostly based on CNTs functionalized with DNA molecules. However, a major bottleneck holding back their practical applications has been a lack of mass production method of such sensors. Furthermore, immobilization of DNA on CNTs using linker molecules can severely degrade their electrical properties. Herein, we report a new method to fabricate a large-scale array of CNT-based DNA sensors on SiO2 and glass substrates. In this method, non-polar molecular patterns guide the assembly of CNTs onto uncoated bare surface regions (Nature Nanotechnology 1, 66 (2006)). After fabrication of electrodes on the CNT patterns, we further functionalized the bare surface regions with single-stranded (ss) PNAand successfully demonstrated detection of target ss-DNA with high sensitivity. Since we functionalize the bare surface between CNTs, this process can be applied to virtually general nanotubes circuits on SiO2 or glass substrates to fabricate DNA sensors. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D38.00007: Calcium Induced Voltage Gating and Negative Incremental Resistance in Single Conical Nanopores. Zuzanna Siwy, Matthew Powell, Michael Sullivan, Christina Trautmann, Robert Eisenberg We will present a nanopore device working in an ionic solution that has transport characteristics similar to unijunction transistors working in electronic circuits, namely negative incremental resistance and voltage dependent ion current fluctuations. Our device consists of a single conical nanopore in solutions containing potassium chloride and sub-millimolar concentrations of calcium and cobalt ions. I will talk about importance of electrostatic and chemical interactions of translocating ions with pore walls. We explain the transport effects on the basis of transient binding of calcium ions to chemical groups on the pore walls that cause transient changes in electric potential inside a conical nanopore. Possibilities of constructing a chemical oscillator with tens of Hz operating frequency will be presented as well. We will also discuss application of this oscillating system to building a synthetic stochastic sensor. Since the system operates far from equilibrium, we expect it to be very sensitive to any changes/perturbations, e.g. presence of molecules that we want to detect. The mechanism of detection strongly suggests that the sensor will respond to a whole variety of organic molecules with little modification. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D38.00008: Protein denaturing on Nanospheres James Forrest, Jonathan Teichroeb We have used localized surface plasmon resonance (LSPR) to monitor the structural changes that accompany thermal denaturing of Bovine Serum Albumin(BSA) adsorbed onto gold nanospheres of size 5nm-60nm. The effect of the protein on the LSPR was monitored by visible extinction spectroscopy. The position of the resonance is affected by the conformation of the adsorbed protein layer, and as such can be used as a very sensitive probe of thermal denaturing that is specific to the adsorbed protein. The results are compared to detailed calculations and show that full calculations can lead to significant increases in knowledge where gold nanospheres are used as biosensors. Thermal denaturing on spheres with diameter $>$ 20 nm show strong similarity to bulk calorimetric studies of BSA in solution. BSA adsorbed on nanospheres with d$\leq$ 15 nm shows a qualitative difference in behavior, suggesting a sensitivity of denaturing characteristics on local surface curvature. Studies of isothermal denaturing kinetics were used to obtain an activatiuon barrier for thermal denaturing. This activation barrier also exhibited a strong dependence on nanoparticle size. These results may have important implications for other protein-nanoparticle interactions. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D38.00009: Nanoscale Radiofrequency Molecular Biosensing Jean-Luc Fraikin, David Wood, Mike Stanton, Andrew Cleland We are developing an all-electronic, radiofrequency, nanoscale-biosensor. We use RF reflectometry to measure impedance changes in the sensor electrodes, which should occur upon binding of the target analyte, enhanced through the subsequent attachment of gold nanospheres, using a sandwich-type assay. The sensor is embedded in a microfluidic, lab-on-a-chip configuration, allowing for in-situ sensor functionalization. We are pursuing various routes to sensor functionalization, including both oligonucleotide and peptide linking chemistries. The electrical functionality of our sensor prototype has been demonstrated, yielding sensitivity to impedance changes of order 1 part in $10^{5}$, with an active sensing volume of order a few hundred attoliters. Using this technique we expect sensitivity to single-nanosphere binding events to be attainable. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D38.00010: Nano-scale resolution full-field microscopy using tabletop extreme ultraviolet lasers Fernando Brizuela, Courtney Brewer, Georgiy Vaschenko, Yong Wang, Miguel Larotonda, Bradley Luther, Mario Marconi, Jorge Rocca, Carmen Menoni, Weilun Chao, Yanwei Liu, Erik Anderson, David Attwood, Alexander Vinogradov, Igor Artioukov, Yuri Pershyn, Viktor Kondratenko We have developed two compact full-field extreme ultraviolet (EUV) microscopes that combine short-wavelength light from high-brightness tabletop lasers with zone plate and multilayer-coated reflective optics. One of these systems uses 47 nm wavelength light from a desktop-sized capillary discharge laser with resolution down to 70nm. This microscope can image in both transmission and reflection mode, allowing for imaging of surfaces. The other microscope uses 13 nm wavelength light from a table-top optically pumped EUV laser to acquire images with spatial resolution better than 38 nm. Both of these systems have the ability to render images with typical acquisition times of 10- 30 seconds These results open a path to the development of compact and widely available extreme-ultraviolet imaging tools capable of inspecting samples in a variety of environments with a 15-20 nm spatial resolution and a picosecond time resolution. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D38.00011: Nano-Optics for Chemical and Materials Characterization Michael Beversluis, Stephan Stranick Light microscopy can provide non-destructive, real-time, three-dimensional imaging with chemically-specific contrast, but diffraction frequently limits the resolution to roughly 200 nm. Recently, structured illumination techniques have allowed fluorescence imaging to reach 50 nm resolution [1]. Since these fluorescence techniques were developed for use in microbiology, a key challenge is to take the resolution-enhancing features and apply them to contrast mechanisms like vibrational spectroscopy (e.g., Raman and CARS microscopy) that provide morphological and chemically specific imaging.. We are developing a new hybrid technique that combines the resolution enhancement of structured illumination microscopy with scanning techniques that can record hyperspectral images with 100 nm spatial resolution. We will show such superresolving images of semiconductor nanostructures and discuss the advantages and requirements for this technique. Referenence: 1. M. G. L. Gustafsson, P. Natl. Acad. Sci. USA 102, 13081-13086 (2005). [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D38.00012: Simultaneous Surface-Enhanced Raman Scattering Imaging and Spectroscopy in Confocal Mode. Denis Pristinski$^1$, Melek Erol$^2$, Henry Du$^1$, Svetlana Sukhishvili$^2$ Noble colloidal metal nanoparticles deposited on a planar substrate facilitate ultrasensitive measurements via surface-enhanced Raman scattering (SERS) spectroscopy. Due to the random nature of nanoparticle immobilization, the variation of interparticle distance and possible aggregate formation cause significant fluctuation in SERS signal intensity across the substrate. To study the nature of these intensity fluctuations we have built a microscope capable of simultaneous imaging in epi-fluorescent mode and spectroscopy of a point of interest in confocal mode. Two excitation beams from the same laser (DPSS 532 nm) are mixed to expose the imaged area and to focus on the point of interest through high N.A. objective. The scattered light collected by the same objective is filtered and split between a cooled CCD camera for imaging and a fiber-connected spectrometer for confocal mode spectroscopy. Positively charged Ag nanoparticles prepared by polyethyleneimine-assisted reduction were deposited on glass substrate and used for the assessment of uniformity of SERS signal from subsequently adsorbed anionic molecules and for the identification of proteins. \newline \newline $^1$ Department of Chemical, BioMedical, and Materials Engineering \newline $^2$ Department of Chemistry and Chemical Biology [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D38.00013: Initial Development of a sub-micron Angle Resolved Photoemission Microscope Aaron Bostwick, Jessica McChesney, Eli Rotenberg -abstract- We have begun initial development of a sub-micron angle resolved photoemmision microscope. The current test system consists of an SES-200 detector and a zone plate based focusing system operating at 180eV photon energy. We have measured angle resolved spectra using the SES-200 angle-dispersive collection mode at resolution of $\sim $500nm. We have used this to show orientational contrast on highly oriented pyrolytic graphite (HOPG). The domains on HOPG are on the order of 1-20 microns and are well orientated along the c-axis but show random azimuthal order. We are able to clearly image these domains even though they show no chemical contrast, and can measure the single crystal band structure on disordered polycrystalline sample. We believe this demonstrates the promise of such a system for the measurement of materials which cannot be found in bulk single crystals. [Preview Abstract] |
Session D39: Focus Session: Materials and Applications for Solar Energy II
Sponsoring Units: FIAP DMPChair: Dave Ginley, National Renewable Energy Laboratory
Room: Colorado Convention Center 502
Monday, March 5, 2007 2:30PM - 3:06PM |
D39.00001: Intermediate- band solar cells: future prospects and challenges Invited Speaker: The intermediate band solar cell is a novel type of solar cell with the potential of exceeding the limiting efficiency of single gap solar cells. Its principle of operation relies on the existence of a material characterized by an electronic band (intermediate band) located within the semiconductor bandgap. This intermediate band allows the absorption of two below-band gap energy photons to produce one electron-hole pair and is engineered to prevent introducing non-radiative recombination mechanisms in the cell. This basic principle of operation has been recently experimentally demonstrated in devices employing InAs/GaAs quantum dots. In this system, the intermediate band arises from the energy states associated to the confinement of the electrons in the dots. The challenges for the future are: a) to produce devices in which the intermediate band effect is enhanced, b) to identify and synthesize other intermediate band material systems, maybe different from the ones based on quantum dots and c) to produce high efficiency devices that allow the production of photovoltaic electricity at competitive costs. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D39.00002: Materials characterization and optimization in silicon heterojunction solar cells using spectroscopic ellipsometry Dean Levi, Eugene Iwaniczko, Qi Wang, Howard Branz Silicon heterojunction solar cells (SHJ) utilize very thin (3-5 nm) layers of amorphous silicon deposited on the surface of crystal silicon to produce very high efficiency solar cells with low temperature processing. Our research team has used hot wire chemical vapor deposition (HWCVD) to fabricate SHJ solar cells on p-type FZ silicon with efficiencies as high as 18.2{\%}. The best cells are deposited on textured (100) silicon substrates where the texturing process creates pyramidal facets with (111) crystal faces. One of the key factors in maximizing the efficiency of our SHJ devices is the process of optimizing the material properties of the amorphous silicon (a-Si) layers used to create the junction and back contact in these cells. Such optimization is technically challenging because of the difficulty in measuring the properties of extremely thin layers. This difficulty is compounded by the fact that the properties of such amorphous layers are substrate- and thickness-dependent. We report in this study how the substrate temperature and substrate orientation affect the structural, optical, and electronic properties of the a-Si layers used in our SHJ devices, and how these properties affect the final device performance. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D39.00003: Characterization of Grain Boundaries in Polycrystalline Photovoltaic Devices using Near-Field Scanning Optical Microscopy. J.M. Yarbrough, I.C. Schick, V. Kaydanov, T.R. Ohno, R.T. Collins Polycrystalline thin film PV devices have the potential to reduce the cost per watt for commercial photovoltaic, but, their lower efficiency compared to their counter parts and lack of stability have prevented their widespread adoption. There is a need for a more fundamental understanding of these PV devices. A near-field scanning optical microscope (NSOM) has been built to optically and electrically characterize polycrystalline thin film PV devices. The NSOM is presently being used in air and at room temperature to perform spatially resolved photocurrent measurements using a broad range of visible excitation wavelengths on planar PV devices. Results from the front side illuminated planar CdTe devices show between a 5 and 10{\%} increase in the generated photocurrent between the grains supporting the idea of charge separation at the grain boundary. Unlike previous studies, these photocurrent measurements have been decoupled from the topographical cross talk typically common to NSOM measurements. The authors gratefully acknowledge support from the National Science Foundation under Grant No. DMR-0103945 and samples provided by the University of Toledo. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D39.00004: Theory of photo-conversion in polycrystalline silicon A.I. Shkrebtii, A.V. Sachenko, A.P. Gorban, V.P. Kostylyov, I.O. Sokolovsky, A. Kazakevitch We developed a three--dimensional analytical formalism of photo-conversion in polycrystalline silicon based solar cells. Polycrystalline Si was modeled by representing the grains as parallelepipeds or cylinders, considering spatial dependence of generation and recombination of electron-hole pairs both in the bulk and at the grain boundaries. We calculated spectral dependence of the short circuit current and open circuit voltage averaged over the grain. The recombination of the photo carriers at the grain boundary was described by introducing the effective diffusion length, responsible for the attenuation of excess electron--hole pairs. The recombination dependence on the bulk diffusion length, grain size and effective recombination velocity at the boundaries were derived and discussed. The short circuit current, open circuit voltage and photo-conversion efficiency in polycrystalline Si are in good agreement with the experimental data available. The research was supported by the Centre for Materials and Manufacturing/Ontario Centres of Excellence (OCE/CMM) ``Sonus/PV Photovoltaic Highway Traffic Noise Barrier'' project. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D39.00005: Photovoltaic applications of hydrogenated amorphous silicon thin films grown by the Saddle Field Glow Discharge Method F. Gaspari, A.I. Shkrebtii, A. Kazakevitch, A.V. Sachenko, I.O. Sokolovsky, N. Kherani, T. Teatro, J. Perz Thin film hydrogenated amorphous silicon (a-Si:H) is widely used for photovoltaic solar cells. We present a combined theoretical and experimental study of the thin a-Si:H films for efficient and inexpensive solar cells, grown by the Saddle Field Glow Discharge Method. The type of solar cell studied is glass/SnO$_{2}$/p-i-n Si:H/Al. We investigated the mechanism of hydrogen diffusion inside the film, its relation to the bonding within the amorphous silicon network. Hydrogen diffusion in a-Si:H was modeled using first-principles finite temperature molecular dynamics. Optimization of the solar cells was performed based on the experimental diffusion coefficients, carrier mobilities, parameters of the p-i-n structures, and electron band structure (defect distribution inside the gap). An analytical model to optimize photo-conversion efficiency of a-Si:H based solar cells with contact grid has been developed. The research was supported by the Centre for Materials and Manufacturing/Ontario Centres of Excellence (OCE/CMM) ``Sonus/PV Photovoltaic Highway Traffic Noise Barrier'' project. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D39.00006: Simple method to examine the work function of transparent conducting oxide for traditional and organic based photovoltaics Joseph Berry, Matthew Reese, John Perkins, David Ginley Transparent conducting oxides (TCOs) are key components in both traditional and organic based optoelectronic devices. In photovoltaic applications in which TCOs are employed as transparent electrical contacts, the matching of the TCO work function to that of the active material is critical to device performance. We report the adaptation of a commercial electrostatic voltmeter to measure the work function of In-Zn-O and other TCO materials relevant to photovoltaics. The applicability of this technique to high-throughput combinatorial studies of compositionally graded TCO libraries will be presented. We will also examine correlations between the observed work function and other material properties in these TCO libraries. The relationship between device performance and the measured work function will also be assessed. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D39.00007: Combinatorial Development of Amorphous Mixed Metal Oxide Transparent Conductors J.D. Perkins, M.F.A.M. van Hest, M.I. Bertoni, C.W. Teplin, J.J. Berry, J.L. Alleman, M.S. Dabney, L.M. Gedvilas, B.M. Keyes, B. To, A. Leenheer, M.P. Taylor, Dennis Readey, R. O'Hayre, D.S. Ginley We are using combinatorial approaches to optimize both amorphous In-Zn-O (a-IZO) and amorphous Zn-Sn-O (a-ZTO) transparent conductors for photovoltaic applications. Compositionally-graded combinatorial samples (``libraries'') are deposited by co-sputtering onto 2"x2" glass substrates at temperatures ranging from room-temperature to 500 \r{ }C. Three to five libraries are generally required to cover the full composition range for a binary tie-line, such as from In2O3 to ZnO. For IZO, we have found that IZO films deposited in Ar at 100 \r{ }C are amorphous for films with 65 to 85 cation{\%} In, with a maximum conductivity of 3000 S/cm at 80 cation{\%} In and an RMS roughness of 0.4 nm. Subsequent sequential annealing experiments in both Ar and air show that a-IZO films are structurally, electrically and optically quite robust for anneals up to 500 or 600 \r{ }C. For a-ZTO, the best conductivity obtained to date for an amorphous ZTO film is 200 S/cm for films grown at 400 \r{ }C with 35 cation{\%} Zn. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D39.00008: Optimization of amorphous In-Zn-O (IZO) transparent conductor sputtered at ambient temperature Andrew Leenheer, John Perkins, Andrew Cavendor, Matthew Taylor, Maikel van Hest, David Ginley Amorphous indium zinc oxide (IZO) is an n-type transparent conducting oxide (TCO) that offers high electrical conductivity, visible-spectrum transparency, smoothness and ease of deposition, all properties of interest for photovoltaic and optoelectronic applications. Previous work has shown that magnetron-sputtered IZO is amorphous over the metals-only composition range $\sim $55 to 85 atomic {\%} indium. In this work, five different single-composition targets spanning the amorphous range were used to sputter thin films at ambient temperature with varying oxygen content in the sputter gas. In addition, highly resistive films were deposited to make field-effect thin-film transistors. The resistivity, carrier concentration, and hall mobility, as well as the optical transmission and reflection for $\lambda $=300-900 nm light were measured for each film. The conductivity was tunable from $\sim $2.5 x 10$^{3}$ S/cm to $\sim $10$^{-3}$ S/cm depending on the amount of oxygen present. Generally, increasing the oxygen or lowering the indium content lowers the carrier concentration, while increasing the indium content increases the electron mobility. For thin-film transistors, a low carrier concentration but high mobility is desired. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D39.00009: Transparent Conducting ZnO Thin Films Doped with Al and Mo Joel Duenow, Timothy Gessert, David Wood, David Young, Timothy Coutts Transparent conducting oxide (TCO) thin films are a vital part of photovoltaic cells, flat-panel displays, and electrochromic windows. ZnO-based TCOs, due to the relative abundance of Zn, may reduce production costs compared to those of the prevalent TCO In$_{2}$O$_{3}$:Sn (ITO). Undoped ZnO, ZnO:Al (0.5, 1, and 2 wt.{\%} Al$_{2}$O$_{3})$, and ZnO:Mo (2 wt.{\%}) films were deposited by RF magnetron sputtering. Optimal deposition temperature was found to be 200$^{o}$C. Controlled incorporation of H$_{2}$ in the Ar sputtering ambient increased mobility of undoped ZnO significantly to 48 cm$^{2}$V$^{-1}$s$^{-1}$. H$_{2}$ also appears to catalyze ionization of dopants. This enabled lightly doped ZnO:Al to provide comparable conductivity to the standard 2 wt.{\%}-doped ZnO:Al while demonstrating reduced infrared absorption. Mo was found to be an n-type dopant of ZnO, though material properties did not match those of ZnO:Al. Scattering mechanisms were investigated using temperature-dependent Hall measurements and the method of four coefficients. This abstract is subject to government rights. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D39.00010: A theoretical study on native point defects and dopants in cuprous oxide Weichao Wang, Dangxin Wu, Qiming Zhang, Meng Tao We have performed a first-principle study on the electronic structures, atomic configurations, and formation energies of native point defects in cuprous oxide, i.e. vacancies$(V_{Cu} ,V_O )$, interstitials $(Cu_i ,O_i )$ and antisite defects $(Cu_O ,O_{Cu} )$ by using Density Function Theory based VASP package with PAW potentials. We have carefully studied the formation of native point defects under different chemical environments and Fermi level positions. We have also calculated the electronic structures of dopants such as F, Cl, N, Ca and Mg in the cuprous oxide crystal. Their formation at different chemical environments and Fermi level positions will be presented as well. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D39.00011: Thin film preparation of the p-type transparent semiconductor Cu$_{3}$TaS$_{4}$ Paul Newhouse, Peter Hersh, Douglas Keszler, Janet Tate Thin films of a new wide band gap p-type semiconductor Cu$_{3}$TaS$_{4}$ (CTS) are prepared by PLD deposition of Cu and Ta metal multilayers and subsequent ex-situ rapid thermal processing in a sulfur environment. X-ray diffraction confirmed the presence of single phase CTS. 275 nm thick CTS films on fused SiO$_{2}$ substrates show reflection-corrected transmission $>$70{\%} over the range 400-700 nm, with an optical band gap near 2.8 eV. The electrical resistivity of undoped CTS thin films is $\sim $ 5 Ohm cm. These properties indicate that CTS thin films may find application in transparent electronics. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D39.00012: Properties of a potential transparent p-type semiconductor Cu$_{3}$TaQ$_{4}$ (Q = S or Se) Peter Hersh, Paul Newhouse, Douglas Keszler, Janet Tate Physical, optical and electrical properties of powder samples of the Cu$_{3}$TaQ$_{4}$ (Q = S or Se) series are investigated to determine the potential as a transparent semiconductor. The series crystallizes in a P-43m sulvanite structure. The sulfide has a lattice parameter of a = 5.5036(4) {\AA} and the selenide has a lattice parameter of a = 5.6535(7) {\AA}. The optical band gaps are 2.75eV for Cu$_{3}$TaS$_{4}$ and 2.36eV for Cu$_{3}$TaSe$_{4}$. Seebeck coefficients of +27$\mu $V/K for Cu$_{3}$TaS$_{4}$ and +24$\mu $V/K for Cu$_{3}$TaSe$_{4}$ confirm that both materials are p-type. FLAPW band structure calculations indicate that the band gap is indirect. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D39.00013: Transparent conductive BaCuTeF thin films by pulsed laser deposition Robert Kykyneshi, David McIntyre, Janet Tate, Cheol-Hee Park, Douglas Keszler Transparent p-type carrier conductive BaCuTeF thin films are reported. Undoped BaCuTeF films obtained \textit{in-situ} by pulsed laser deposition in UHV exhibit maximum conductivities of 50-55 S/cm on fused silica substrates. The polycrystalline films deposited at various temperatures up to 600$^{\circ}$C are single phase with optical band gap of about 3 eV and 70{\%} average transparency in the visible and near-IR optical ranges. BaCuTeF films deposited on single crystal MgO substrates are highly oriented. [Preview Abstract] |
Session D40: Semiconductors: Electronic Structure
Sponsoring Units: FIAPChair: Hai Ping Cheng, University of Florida
Room: Colorado Convention Center 503
Monday, March 5, 2007 2:30PM - 2:42PM |
D40.00001: First-Principles Dielectric Spectra of Silicon: THz through UV H.M. Lawler, S. Dalosto, Z.H. Levine, E.L. Shirley, J.J. Rehr We present an implementation of the GW-Bethe-Salpeter-equation approach to first-principles calculations of dielectric response based in part on input from the plane-wave, pseudopotential code ABINIT. This work, together with lattice dynamical calculations, aims to develop versatile codes capable of calculating dielectric spectra in insulators for the full spectral range from THz to the UV. Below the bandgap, lattice vibrations absorb light in the THz range. These spectra are generally composed of sharp infrared-active features (absent by symmetry in silicon); weak, temperature dependent continuum effects from IR-active-multiphonon state hybridization; and contributions to the macroscopic polarization directly from multiphonon states. Above the bandgap, density-functional band structures are taken as a starting point for the inclusion of many-body interactions within the GW-BSE approximation. Emphasis will be on treating the excitionic effects and non-zero-momentum application of the modern theory of polarization with ABINIT. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D40.00002: Symmetry in optical properties: Silicon temperature dependent dielectric function M.J.G. Lee, A. Shkrebtii, W. Richter, M. Drago, G. Nicholls, Z.A. Ibrahim The dielectric function of the indirect gap semiconductor bulk Si has been measured experimentally in the temperature range 300 K to 1270 K, and has been modeled theoretically in the range 0 K to 1300 K. The observed low temperature optical peak at 1.1 eV is below the direct optical gap of the ideal lattice (about 3.0 eV). We attribute this peak to zero-point lattice vibrations which, by reducing the translational symmetry, allow direct optical transitions below the direct gap of the ideal lattice even at 0 K. A lattice dynamical calculation in which zero-point vibrations are included gives a good account of the temperature dependencies of the energies and the widths of the peaks in the observed dielectric function of bulk Si over the whole temperature range. We conclude that the dielectric function of bulk Si is very sensitive to the breaking of translational symmetry by the thermal motions of the atoms, and that zero-point vibrations play an essential role in a quantitative description of the dielectric function of bulk Si. The research was supported by Sfb 296 and NSERC Discovery Grants. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D40.00003: \textit{Ab initio} Optical Absorption by A Simple and Efficient Method: Single Excitation Configuration Interaction After Downfolding Kazuma Nakamura, Yoshihide Yoshimoto, Ryotaro Arita, Masatoshi Imada, Shinji Tsuneyuki We present a simple and efficient \textit{ab initio} method for calculating electronic excited states and optical absorption spectra of solids. The method is based on a single-excitation configuration-interaction calculation after downfolding to model Hamiltonians represented by maximally-localized Wannier functions. Single-excitation configurations are crucially important in evaluating a linear absorption, because they can describe a so-called excitonic effect; interactions in electron-hole pairs generated by excitations. A test was performed for a semiconductor GaAs, and detailed analyses for the resulting spectra and single-excitation many-body wavefunctions are presented. This work is supported by a Grant-in-Aid for Scientific Research in Priority Areas, ``Development of New Quantum Simulators and Quantum Design'' (No. 17064004) of the Ministry of Education, Culture, Sports, Science and Technology, Japan. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D40.00004: Band-offsets of semiconductor heterostructures using hybrid density functionals Amita Wadehra, John W. Wilkins, Richard G. Hennig, Gustavo E. Scuseria The performance of novel devices, e.g., HEMTs based on semiconductor heterostructures, depend strongly on their conduction and valence band-offsets. However, conventional density functional theory based on LDA and GGA fails for narrow-gap semiconductors such as InAs, predicting it to be a metal. An accurate treatment of such systems requires self-consistent DFT calculations with hybrid functionals such as B3LYP and HSE. B3LYP success for a wide variety of atoms and molecules is computationally challenging to translate to solids. We compare band gaps and band offsets of strained and unstrained InAs/InP and InAs/AlAs heterostructures calculated with these hybrid functionals. Our preliminary results agree well with experimental and other theoretical investigations. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D40.00005: B3LYP Works for Semiconductors Heterostructures Jeremy Nicklas, John Wilkins The B3LYP functional with Gaussian basis functions gives reliable valence band offsets (VBO) of heterostructures involving, for example, GaN, AlN, and InN. The density of states of the inner bulk layers in the heterostructure estimates the valence band offset while confirming the bulk bandgaps. The VBO is calculated for the cubic and hexagonal structures with a range of lattice constants. For example, cubic-AlN and cubic-GaN heterostructure with 8+8 (001) layers show decreasing VBO with increasing (001) planar lattice: 0.98 eV for 4.38 \AA, 0.97 eV for 4.45 \AA, and 0.85 eV for 4.52 \AA. These are consistent with previous GWA calculations [1]. [1] D. Cociorva, W. G. Aulbur and J. W. Wilkins, Solid State Communications 124, 63-66 (2002). [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D40.00006: Analysis of core-valence linearization in $G_{\mathrm{0}}W_{\mathrm{0}}$ calculations Ricardo Gomez-Abal, X. Li, M. Scheffler, C. Ambrosch-Draxl In recent years the $GW$ approach, typically applied as the first order correction to the Kohn-Sham (KS) eigenenergies ($G_{\mathrm{0}}W_{\mathrm{0}}$ approximation), has substantially improved the description of single-particle excitations in weakly correlated semiconductors and insulators. Most of the existing codes are based on the pseudopotential (PP) method. It is well known from ground-state calculations that the linear treatment of the core-valence exchange-correlation interaction is not always valid. However, in the $G_{\mathrm{0}}W_{\mathrm{0}}$-PP scheme, the self-energy is computed from the (pseudo-)valence states only, keeping the core-valence interaction at the KS level. There is no justification for such a ``core-valence linearization'' of the dynamical self-energy, a highly non-linear functional of the total density. Nevertheless, $G_{\mathrm{0}}W_{\mathrm{0}}$-PP results usually agree better with experiments than the all-electron ones. In this talk we analyze the reasons for this disturbing discrepancy and the validity of the ``core-valence linearization'' in the $G_{\mathrm{0}}W_{\mathrm{0}}$-PP scheme. Calculations are performed using our own all-electron $G_{\mathrm{0}}W_{\mathrm{0}}$ code, based on the Wien2k implementation of the FP-(L)APW+lo method. We compare our all-electron results with those obtained by computing the self-energy from the valence states only as well as with $G_{\mathrm{0}}W_{\mathrm{0}}$-PP calculations for selected materials (e.g. Si, NaCl,...). [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D40.00007: Band gap bowing in large size-mismatched II-VI alloys Su-Huai Wei, Chang-Youn Moon, Y.Z. Zhu, G.D. Chen Band gap bowing coefficients in large size-mismatched II-VI alloys M$^{II}$X$^{VI}_{1-x}$O$_x$ with M$^{II}$=Zn and Cd, and X$^{VI}$=S, Se, and Te in the zinc-blend structure are calculated using first-principles methods. We show that in these systems, the bowing coefficients are large and composition-dependent. The bowing coefficients increase as the size and chemical mismatch between the constituents increase. The bowing coefficients for the Zn alloys are larger than the corresponding Cd alloys, but smaller than the corresponding III-V alloys. We show that these results can be explained by the size and atomic eigenvalue differences between the constituents and the resulting band offsets and isovalent defect levels in these systems. Our results are compared with recent experimental data. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D40.00008: Large band gap bowing in CuAgGaS2 chalcopyrite semiconductors and its effect on optical parameters Chandrima Mitra, Walter Lambrecht CuxAg$_{1-x}$GaS$_2$ chalcopyrite semiconductors have been found to exhibit a large band gap bowing. Here we use full-potential linearized muffin-tin orbital calculations in the local density approximation of density functional theory to study the electronic band structure of these materials. The randomness in the alloy is treated by the special quasirandom structures. Some layered ordered compounds are studied as well. We find the band gap to depend strongly on the c/a ratio which varies nonlinearly with concentration, in agreement with experimental data by Matsushita et al. The bowing coefficient is found to be 0.74. We also calculate the indices of refraction and their dependence on concentration. Band gap corrections are adjusted using direct shifts to the conduction bands and adjusted for the end compounds. We find that the nonlinear behaviour of the band gap also leads to a non-linear behaviour of the index of refraction with x. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D40.00009: Optical properties of semiconductor alloys and superlattices HyeJung Kim, Yia-Chung Chang, Y.D. Kim Optical properties of semiconductor alloys and superlattices are studied using a full-potential linear augmented-Slater-type orbital (LASTO) method. The LASTO method is highly efficient for dealing with large supercells to model alloys and superlattices in reasonable time frame. Bulk properties such as band structures, equilibrium lattice constants and bulk moduli are in good agreement with existing data. We calculate optical bowing parameters and dielectric functions of zincblende semiconductors and our calculations are compared with ellipsometric measurements for InAs$_{x}$P$_{1-x}$, Zn$_{x}$Cd$_{1-x}$Se, Al$_{x}$Ga$_{1-x}$Sb alloys and (GaSb)$_{m}$(AlSb)$_{n}$ superlattices. To model alloys A$_{x}$B$_{1-x}$C, we consider 8-atom supercells of configurations B$_{4}$C$_{4}$, A$_{1}$B$_{3}$C$_{4}$, A$_{2}$B$_{2}$C$_{4}$, A$_{3}$B$_{1}$C$_{4}$, A$_{4}$C$_{4}$ (i.e., ratio of A to B atom equals 0, 0.25, 0.5, 0.75, 1). Atoms are allowed to relax to reach equilibrium positions. Complex dielectric functions are obtained after adding empirical GW corrections and semi-empirical excitonic effects. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D40.00010: First principles exploration of the possibility of high mobility phases and alloys of group IV semiconductors. Jay Sau, Marvin Cohen At present it is known that in some cases the mobility of Si can be increased through alloying of Si with Ge and straining Si epitaxially. Here we examine the possibility of higher mobility group IV semiconductors. Currently it appears that Ge based devices might become popular in the near future due to the higher mobility of Ge. Using Density Functional Theory and GW quasiparticle corrections together with k.p theory and EPM, we examine how strain and alloying Sn can be used to increase the mobility of Ge related semiconductors. In this study we account for alloy scattering due to Sn impurities in Ge using first principles calculations. We find that the effect of alloy scattering is not prohibitively large. Recently CVD based methods [Kouvetakis et al. APL, 81, 2992(2002)] have been developed to fabricate these alloys making such studies of increased technological relevance. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D40.00011: Band Edge Energies and Band Gaps of Quaternary GaInAsSb Alloys Rita Magri, Alex Zunger, Herbert Kroemer Quaternary alloys without a common atom such as (Ga,In)-(As,Sb) pose a difficult combinatorially design problem in that there are many different atomic configurations even when the system is constrained to be lattice-matched on a substrate. Using an atomistic pseudopotential approach\footnote{R. Magri, A. Zunger, H. Kroemer, JAP 98, 043701 (2005)} we have calculated the band edge energies of this quaternary random alloys as a function of Ga/In (x) and As/Sb (y) compositions assuming lattice-matching to either GaSb or InAs. The alloy is represented by a large supercell with random atomic occupations and atomic positions relaxed via the atomistic VFF functional. We find upwards bowing for both the conduction and valence band edge energies. On GaSb, the transition from staggered to broken-gap lineup is found to occur at x = 0.81 and y = 0.92, while on InAs it occurs at x = 0.59 and y = 0.62. We show that at the usual growth temperatures this quaternary alloy is not random but tends to exhibit an increased number of Ga-Sb and In-As bonds and a reduced number of In-Sb and Ga-As bonds. This effect brings the calculated band gaps in better agreement with experimental data. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D40.00012: Inverse Band Structure determination of optical properties of (In,Ga)(As,Sb) alloys Paulo Piquini, Alex Zunger InAs/GaSb superlattices and quantum wells present interesting band structure properties due to the overlap between the conduction band minimum of bulk InAs and the valence band maximum of bulk GaSb, which allow electrons to be transfered from the GaSb region to the InAs region. In long period (GaSb)$_n$/(InAs)$_n$ superlattices (SL's) one thus has a negative band gap that becomes positive as the period n decreases, due to the quantum-confinement. Using a supercell approach and calculating the electronic structure via the plane-wave empirical pseudopotential method we search for superlattice period and orientation (lattice matched to either GaSb and InAs) that gives a target band gaps in the mid infra-red region, e.g, 300 meV. This is performed by using the Inverse Band Structure approach, where a target value is first stated and a genetic algorithm search of the alloy configuration space is conducted. We study different compositions of In$_x$Ga$_{(1-x)}$As$_y$Sb$_{(1-y)}$ alloys and compare the results to those obtained by simpler $\vec k \cdot \vec p$ approaches. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D40.00013: First-principles investigation of the band gap evolution in (Pb,Sn)Te Xing Gao, Murray Daw The electronic structure of Sn-doped PbTe is interesting because of the so-called band inversion in its two end members, PbTe and SnTe [1,2]. Although, the electronic structure of these two compounds have been extensively studied by first-principles calculations, to our best knowledge, there are no direct first-principles calculations of the band gap evolution through the full range of alloying. We report a study of the electronic structure of this material through the full range of alloy content, combining SQS [3] and LDA. Our results show that disorder plays an important role in the electronic structure of this alloy. \newline [1] Dimmock, Melngailis, and Strauss, Phys. Rev. Lett. 16, 1193 (1966). \newline [2] Tung, and Cohen, Phys. Rev. 180, 823 (1969). \newline [3] Wei and Zunger, Phys. Rev. B 55, 13605 (1997). [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D40.00014: Structure, Electronic Structure and Phonons of ZnSnN2 Tula R. Paudel , Walter R. L. Lambrecht The structure and electronic structure of ZnSnN$_2$ was calculated using density functional theory in the local density density approximation (LDA) and the linearized muffin-tin orbital method (LMTO). The wurtzite lattice constant is found to be slightly greater then that of GaN. The system is found to have small direct band gap of 0.07eV at $\Gamma$ in LDA. The phonon frequencies at the $\Gamma$ were calculated by linear response theory and were labeled according to the symmetry. To the best of our knowledge this compound is studied for the first time both experimentally and theoretically. The stability of the compound will be discussed and the structural electronic and vibrational properties will be compared with other members of the Zn-IV-N$_2$ (IV=Si,Ge) compounds. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D40.00015: Electronic structure of SmN, DyN, and GdN B. Ruck, S. Granville, A.R.H. Preston, D.H. Housden, H.J. Trodahl, A. Bittar, J.E. Downes, K.E. Smith, P. Larson, W.R.L. Lambrecht The rare-earth nitrides lie on the boundary between metals and insulators, and as such present an exciting challenge to both experiment and theory. Incorporating the localized and correlated 4f electrons into band structure calculations is difficult, while the propensity of the rare-earth nitrides to oxidize in atmosphere impedes experimental studies. Here we present nitrogen K-edge x-ray absorption and emission measurements from SmN and DyN films grown in-situ at the synchrotron beamline, supported by resistivity and magnetization results. The x-ray data show a clear gap between the occupied and unoccupied states, implying that the materials are semiconducting. The density of states are in excellent agreement with band structure calculations performed in the LSDA+U scheme, as long as a rigid 1 eV upward shift is applied to the conduction band. [Preview Abstract] |
Session D41: Magnetism in 2 Dimensions
Sponsoring Units: DCMPChair: Jason Crain, National Institute of Standards and Technology
Room: Colorado Convention Center 504
Monday, March 5, 2007 2:30PM - 2:42PM |
D41.00001: Spin-polarized scanning tunneling spectroscopy of individual magnetic adatoms Yosef Yayon, Victor Brar, Michael Crommie, Lax Senapati, Steve Erwin An important goal in condensed matter physics is the ability to measure the spin-polarization state of a single magnetic atom or impurity. We have used spin-polarized scanning tunneling spectroscopy (SP-STS) to probe the local spin-dependent electronic structure of isolated Fe and Cr adatoms deposited onto magnetic Co islands on a copper surface. Individual Fe and Cr atoms prepared in this way show strong spin-polarized contrast in their dI/dV spectra. The spectra of Fe and Cr adatoms differ in that Fe atoms couple ferromagnetically to the Co islands while Cr adatoms couple antiferromagnetically to the islands. dI/dV spatial mapping reveals spin-dependent contrast in the spatial features of individual Fe and Cr atoms. Density functional theory calculations support our interpretation of the experimental results. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D41.00002: Spin-polarized metastable-atom deexcitation spectroscopy study of Xenon-adsorbed iron surfaces Yasushi Yamauchi, Mitsunori Kurahashi, Taku Suzuki, Xia Sun, Zhongping Wang The electron spin polarization at the interface between nonmagnetic and ferromagnetic medias is one of the essential factors that may alter the spin transport phenomena. To investigate fundamental aspects of induced spin polarization we have examined the adsorbate-covered magnetic surfaces by means of spin polarized metastable-atom deexcitation spectroscopy (SPMDS). Use of spin-polarized metastable helium atoms in triplet states moving at thermal energies gives rise to the ultimate surface sensitivity. Although Xenon can adsorb on surfaces at low temperatures by the van der Waals force, no electron exchange with surfaces, especially no spin interaction, is expected because of its closed shell structure. SPMDS spectra measured for Xenon-adsorbed iron surfaces show three prominent peaks that are the same as those previously reported for other surfaces by D. M. Oro, et al. [Phys. Rev. A 49 (1994) 4703]. Two peaks ($^{2}P_{1/2}$, $^{2}P_{3/2})$ at higher kinetic energies exhibit clear spin asymmetries while the other low energy peak has no appreciable spin asymmetry. The spin asymmetries will be discussed on the basis of spin polarization and deexcitation processes of metastable atoms. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D41.00003: Homochiral magnetism in low-dimensional magnets Stefan Blugel, Marcus Heide, Gustav Bihlmayer Spin structures observed in nanomagnets are commonly explained on the basis of the Heisenberg exchange and the magnetic anisotropy. Electrons propagating in the vicinity of inversion-asymmetric environments such as of surfaces, interfaces or in ultrathin films can give rise to the Dzyaloshinskii-Moriya (DM) interaction, typically unimportant for metals. Surprisingly, there is no hard number known from theory about its strength, as this requires supercomputing at the cutting edge. One deals with long-ranged complex magnetic structures in low-dimensions. Since the DM interaction arises from spin-orbit coupling, each atom of the long range structure has a different electronic environment and previous strategies, e.g.\ applying the generalized Bloch theorem, fail. But if DM is important, the so-far anticipated collinear magnetism become unstable, and homochiral spin structures occur. We developed a perturbative strategy implemented into the FLAPW code {\tt FLEUR} which can cope with this challenge. We show by first-principles calculations based on the vector-spin density formulation of the DFT that the DM interaction is indeed sufficiently strong to compete with the interactions that favor collinear spin alignment. We predict new magnetic phases in thin films which had been overlooked during the past 20 years. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D41.00004: Temperature-induced Domain Shrinking in Dipolar Frustrated Ising Ferromagnet Danilo Pescia, Alessandro Vindigni, Oliver Portmann, Paolo Politi Motivated by recent experimental observations on ultrathin Fe/Cu(001) films, we performed a theoretical analysis of magnetic domain pattern evolution in 2D Dipolar Frustrated Ising Ferromagnet. Due to the competition between long-ranged dipolar interaction and nearest neighbor ferromagnetic exchange interaction, the ground state is given by a succession of saturated domains of positive and negative magnetization, which alternate in a sharp striped pattern of characteristic domain width $L_{gs}$. Close to the Curie temperature $T_C$, the Mean Field theory predicts the occurrence of a cosine modulation with a much smaller spatial period ($L(T_C$)). We found that these two limits are connected continuously in the temperature range $0\le T \le T_C$. But, as translational invariance does not hold, the interplay between thermal fluctuations and the two competing interactions gives rise to a non-trivial magnetization profile at intermediate temperatures. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D41.00005: Controlling Spin-Density Wave Periodicity in Thin Cr$_{1-x }$V$_{x}$ Films Oleg Krupin, Eli Rotenberg, S. D. Kevan Chromium is an itinerant antiferromagnet with a spin-density wave (SDW) ground state driven by a nesting of Fermi surface sheets around the Gamma and H points of the Brillouin zone. Periodicity of the SDW plays an important role in mediating magnetic interactions in magnetic multilayer structures providing a giant magnetoresistance effect and potentially interesting for application in spintronic devices. Therefore control of SDW in thin chromium films is of the high importance. It requires a detailed understanding of phenomena related to stabilization of SDW. We used angle-resolved photoemission to characterize spin-density wave and Fermi surface topology in thin Cr$_{1-x }$V$_{x }$films as a function of the film thickness, temperature, composition and hydrogen surface coverage. A key feature of our results is the ability to control the magnetic structure of thin films of Cr with an external perturbation: balancing the surface energetic interactions favored commensurate state of SDW vs. the energy associated with Fermi surface topology stabilizing SDW incommensurate phase in the bulk. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D41.00006: Manipulating spins using spin-valves of self-assembled molecular wires Vladimir Burtman, Valy Z. Vardeny We studied spin transport using spin-valves of self-assembled monolayer (SAM) devices sandwiched between two ferromagnetic electrodes, namely La0.33Sr0.66MnO3 (LSMO) and Co having different coercive fields. The SAM film contained isolated molecular wires that bond with both electrodes, in an otherwise insulating molecular matrix that bond only with one electrode. The relative resistance change, or magnetoresistance (MR), DR/R between the device resistance with the electrodes magnetizations parallel and anti-parallel to each other serves as a figure of merit and show spin injection through the isolated molecular wires. We found a giant MR of up to 80{\%} at 10K. The MR response was measured at various temperatures and biasing voltages to obtain the complete magneto-transport characteristic properties of the organic spin-valve devices. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D41.00007: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 3:54PM - 4:06PM |
D41.00008: Structural and magnetic properties of ferromagnetic metal-oxide films grown by polymer assisted deposition M. Jain, P. Shukla, M. F. Hundley, A. K. Burrell, T. M. McCleskey, Q. X. Jia Ferromagnetic metal-oxide films such as doped lanthanum manganites have been extensively investigated over the past decade due to their potential applications in different fields. To grow these oxide films, the most widely used approaches are physical vapor deposition, chemical vapor deposition, and chemical solution deposition techniques. One of the challenges in solution-based processes of such oxide films has been to produce high quality multilayer films and at the same time to control the stoichiometry. We describe a solution route called polymer-assisted deposition (PAD) to grow such oxide films. High quality epitaxial single layer and multilayer coated films of La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) and La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) have been grown by PAD. Multilayer is used to effectively take the advantages of both LSMO and LCMO with an aim to achieve large values of magnetoresistance (MR) near room temperature. An MR value as high as -66{\%} at 5 T has been obtained at 295K for the multilayer-coated films with LSMO/LCMO volume ratio of 60/40. The successful growth of epitaxial doped lanthanum manganites with desired properties by PAD shows that PAD is a feasible alternative approach to the growth of high quality metal-oxide films. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D41.00009: Growth, structure and magnetic properties study of CVD cobalt layers Nirmalendu Deo, Harold S. Gamble Chemical vapour deposition (CVD) of cobalt was performed on oxidised silicon wafers at the temperature ranges 300-450 $^{o}$C, in hydrogen ambient, from cobalt tricarbonyl nitrosyl. For deposition of cobalt at 300 $^{o}$C the layer was found to be highly resistive ($\sim $250$\mu \Omega $cm). As the deposition temperature increases the layer resistivity decreases, and at 450 $^{o}$C the layer resistivity was reduced to $\sim $30$\mu \Omega $cm. Thus reduced resistivity is taken as evidence that the cobalt layer is purer. X-ray diffraction of the cobalt layers reveal both hcp and fcc peaks. The AES analysis shows that cobalt layer deposited at 300 $^{o}$C contains 26atom {\%}O, 10atom {\%}N. At higher deposition temperature of 400$^{o}$C and above the impurities was 1{\%} or less as documented by AES. At 300 $^{o}$C deposited cobalt layer the surface looks agglomerated as seen by SEM. At 350 $^{o}$C the grain structure is elongated and at 400 $^{o}$C and above the grain structure changes to hexagonal structure. At this temperature the cobalt phase-change occurs from hcp to fcc. The roughness of cobalt layer is higher in lower deposition temperature but this is only due to higher layer thickness measured by AFM. VSM shows, the saturation of magnetisation (Ms) for layers deposited at 400 $^{o}$C and 450 $^{o}$C is consistent with the bulk value of 1422 emu/cm$^{3}$. As the cobalt deposition temperature increases, the layer coercivity decreases from 705 to 400-Oe. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D41.00010: Combinatorial Exploration of Magnetostriction of Fe$_{1-x-y}$Ga$_{x}$M$_{y}$ Ternary Alloys Jason Hattrick-Simpers, Kyu Sung Jang, Samuel E. Lofland, Noble Woo, Bruce Van Dover, Manfred Wuttig, Ichiro Takeuchi Fe$_{80}$Ga$_{20}$ is a well known magnetostrictive material, which owes its large magnetostriction to a tetragonal distortion of the Fe lattice from a local ordering of Ga clusters. Here we will report on the synthesis and characterization of thin film combinatorial Fe-Ga-Pd and Fe-Ga-Al ternary spread samples. The composition spread samples were synthesized in an ultra high vacuum (10$^{-9}$ Torr) co-sputtering chamber. Magnetic properties were mapped through the use of a room temperature scanning SQUID and a high throughput magneto optical kerr effect (MOKE) system. Magnetostrictive measurements were performed on micromachined cantilever libraries at room temperature. The correlation between magnetic and magnetostrictive properties across the composition phase diagram of the two systems will be discussed. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D41.00011: Memory interference in stage-2 CoCl$_{2}$ graphite intercalation Masatsugu Suzuki, Itsuko Suzuki, Motohiro Matsuura Memory interference effects of aging behavior in stage-2 CoCl$_ {2}$ GIC ($T_{cu}$ = 8.9 K and $T_{cl}$ = 6.9 K)$^{1,2}$ have been studied by low frequency ($f$ = 0.1 Hz) AC magnetic susceptibility and genuine thermoremant magnetization experiments. When the system is aged at multiple stop temperatures ($T_{s}$) for wait times (typically $t_{w} = 3.0 \times 10^{4}$ sec) during a zero-field cooling (ZFC) protocol, the AC magnetic susceptibility exhibits multiple aging holes (dips) at the stop temperatures ($T_{s} < T_{cu}$) on reheating. The depth of the aging hole at $T_{s}$ = 6.0 K is logarithmically proportional to the wait time. The depth of thr aging hole (for the same $t_{w}$) exhibits a local maximum at 6.5 K just below $T_{cl}$. It drastically decreases with increasing temperature and reduces to zero above $T_{cu}$. The genuine thermoremnant magnetization (TRM) measurement also indicates that the memory of the specific spin configurations imprinted at multiple stop temperatures between $T_{cl}$ and $T_ {cu}$ for a wait time during the field-cooled (FC) protocol can be retrieved on reheating. \\ 1. M. Suzuki, I.S. Suzuki, and M. Matsuura, Phys. Rev. B\textbf {73}, 184414 (2006).\\ 2. M. Suzuki, I.S. Suzuki, and M. Matsuura, J. Phys. Condensed Matter in press, Proceeding of HFM 2006, Osaka, Japan (August, 2006). [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D41.00012: Experimental Determination of the Heats of Formation for the Ordered Intermetallics in the Fe-Pt System David Berry, Katayun Barmak, Ysela Chiari Accurate heats of formation are necessary to examine phase stability and to aid in the modeling of phase transitions. However, most reported heats of formation, particularly for intermetallic compounds, are available only as the result of theoretical calculations with little or no experimental verification. For the Fe-Pt system, in which a phase transition from the disordered A1 phase to the ordered L1$_{0}$ phase is of great current interest for application in ultrahigh density magnetic recording media, only a few sets of calculated heats of formation are available, for which there is sizable disagreement. Using non-isothermal differential scanning calorimetry (DSC) of sputter-deposited multilayer thin films, the heats of formation of the ordered intermetallic phases, namely L1$_{2}$ Fe$_{3}$Pt and FePt$_{3}$, and L1$_{0}$ FePt, are measured. These values are then compared with the first principles calculated values available in the literature, where there is good qualitative agreement; however, all of the calculated values have underestimated the total heats of formation. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D41.00013: Resistivity, transverse magnetoresistance and Hall effect induced by electron-surface scattering on thin gold films deposited onto preheated mica substrates under high vacuum Raul C. Munoz, Juan P. Garcia, Ricardo Henriquez, German Kremer, Luis Moraga We report measurements of the resistivity $\rho $, transverse magnetoresistance $\Delta \rho $/$\rho $ and Hall effect carried out on 4 gold films (thickness of 69, 93, 150 and 185 nm) evaporated onto mica substrates under high vacuum, where the signal is primarily determined by electron-surface scattering. The experiments were performed at low temperatures T (4K $\le $ T $\le $ 50K) under high magnetic field strengths \textbf{B} (1.5 T $\le $ B $\le $ 9 T). $\rho $, $\Delta \rho $/$\rho $ and the Hall tangent tan($\theta )$ = E$_{H}$/E$_{x}$ (E$_{H}$ stands for the transverse Hall field, E$_{x}$ for the longitudinal field) depend on film thickness. Sondheimer's theory predicts $\rho $ and tan($\theta )$, but leads to $\Delta \rho $/$\rho $ \textit{one order of magnitude smaller than observed}. Calecki's model predicts $\rho $ and tan($\theta )$, but leads to $\Delta \rho $/$\rho $ \textit{several orders of magnitude smaller than observed}. The failure of current theories to predict all 3 transport coefficients is the first compelling evidence pointing to the need of a new, fresh theory to describe size effects arising from electron-surface scattering in metallic films in the presence of a magnetic field. Work funded by FONDECYT 1040723. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D41.00014: Kapitza Pendulum Effect in Weakly Disordered Continuously Degenerate Magnetic Materials Igor Fomin Limitation of general Larkin-Imry-Ma mechanism of disruption of a long-range order by a weak disorder is discussed. It is shown, that the long-range order can be preserved if not all components of the continuously degenerate order parameter are coupled to the relevant random field. The order parameter in that case has a freedom to adjust to disorder so that the disrupting effect of the disorder is excluded. For a weak disorder energy gain due to the adjustment is greater then the gain at the disruption. The mechanism of adjustment is analogous to that, known for a mechanical pendulum with the vibrating point of suspension (Kapitza pendulum). Physically relevant examples of 3D ferromagnet with a 2D random anisotropy and superfluid $^{3}$He in aerogel are considered. [Preview Abstract] |
Session D42: Self-Ordering Nanostructures
Sponsoring Units: DCMPChair: Karsten Pohl, University of New Hampshire
Room: Colorado Convention Center 505
Monday, March 5, 2007 2:30PM - 2:42PM |
D42.00001: Novel mechanisms for self-assembled pattern formation in nanoscopic metal films R. Kalyanaraman, J. Trice, C. Favazza, D. Thomas, R. Sureshkumar Classical hydrodynamic theory of dewetting of spinodally unstable thin films (\emph{Vrij, Disc. farad. Soc. 1966}) predicts a monotonic increase in patterning length scales with increasing film thickness. We verified this effect for nanoscopic Co metal films following melting by ns laser pulses for thickness regime $h\leq h_{c}\sim8\, nm$ (\emph{Favazza et al. Nanotechnology, 2006}). However, a dramatic change is observed beyond this thickness $h_{c}$, with length scales decreasing with increasing $h$. This novel behavior arises from strong thickness dependence of heating by ultrafast laser light resulting in thermocapillary effects, whose magnitude and sign are thickness dependent. We modified the classical theory, according to which the instability occurs when the stabilizing capillary force is overcome by destabilizing attractive long-range interactions, to include thermocapillary effects. The modified theory accurately predicts the experimentally observed trend. This result suggests that a variety of new length scales can be accessed by robust self-assembly via dewetting of metal films under ultrafast light. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D42.00002: Growth and ordering of gas films on a decagonal AlNiCo quasicrystal surface W. Setyawan, R.D. Diehl, N. Ferralis, M.W. Cole, S. Curtarolo The growth and ordering of Ne, Ar, Kr, and Xe films on a decagonal surface of Al$_{73}$Ni$_{10}$Co$_{13}$ quasicrystal are studied with Grand Canonical Monte Carlo using Lennard-Jones interactions. We observe interesting phenomena that can only be attributed to the quasicrystallinity and/or corrugation of the substrate, including structural evolution of the overlayer films from commensurate pentagonal to incommensurate triangular, substrate-induced alignment of the incommensurate films, and density increase in each layer with the largest one observed in the first layer and in the smallest gas. 2D quasicrystalline epitaxial structures of the overlayer form in all the systems only in the monolayer and at low pressure. The final structure of the films is a triangular lattice with a considerable amount of defects except in Xe/QC. Here a first-order transition occurs in the monolayer resulting in an almost perfect triangular lattice. The next layers of Xe/QC have hexagonal close-packed structures. By simulating fictitious gases, we find that the existence of the transition is correlated with the size mismatch between adsorbate and substrate's characteristic lengths. A simple rule is proposed to predict the phenomenon. We extend the theory to other gases/substrates of technological interest. Research is sponsored by NSF and ACS-PRF. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D42.00003: An Addressable Supramolecular Rotary Switch Featuring Distinguishable Positions Embedded In A Two-Dimensional Porphyrin-Based Porous Network Meike Stoehr, Nikolai Wintjes, Hannes Spillmann, Andreas Kiebele, Serpil Boz, Thomas Jung, Fuyong Cheng, Davide Bonifazi, Francois Diederich In recent years, the attempts to build artificial functional devices from single molecules by the ``bottom-up'' approach were strongly in the focus of surface nanoscience. First experiments with molecules manually arranged by the STM tip gave first hints on the powerful possibilities of such a device. Nevertheless, a simple way to produce supramolecular devices parallel in vast amounts has never been shown before. Herein, we report on a highly complex supramolecular device that reminds of a mechanical rotary switch. It is fabricated on a Cu(111) surface following the ``bottom-up'' approach. Self-assembly of a specially designed porphyrin molecule leads to the formation of porous networks featuring chiral cavities which serve as molecular stators for multi-state molecular rotors. By STM, this rotational behavior was analyzed in detail while the energy barrier for rotation was estimated to be 0.28~eV. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D42.00004: Creation of open networks from perylene derivatives Manfred Matena, Meike Stoehr, Markus Wahl, Tomas Samuely, Thomas A. Jung, Lutz H. Gade Self-assembly of molecules on surfaces directed by supramolecular interactions has been widely explored. The perylene derivative (DPDI) we analyzed is modified on the surface in order to achieve self-assemblies. This modification is temperature-induced, thus providing an additional feature to the control of self-assemblies in contrast to usual approaches that make use of molecular properties already inherent to the molecules [1]. Thin films of DPDI were prepared on Cu(111) and investigated with STM. Depending on the coverage before annealing, three different H-bond assemblies are generated, since in a thermally induced reaction the end groups of the molecule are modified and it can then act as both a H-bond donor and acceptor. For a similar perylene derivative (TAPP), an open quadratic assembly is found on Cu(111), which is not based on temperature-induced modification. If both molecules are present on the surface, no intermixing occurs. Instead a separation into two porous networks happens after annealing. [1] M. Stoehr et al., Angew. Chem. Int. Ed. 44 (2005) 7394 [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D42.00005: Statistical Mechanics of Pore Self-Ordering in Porous Alumina Arief Budiman, Daniel Lo Porous alumina has been used as natural templates in forming various ordered nanostructures and has also a potential as catalysts or molecular sieves. Optoelectronic applications typically require a perfect spatial pore ordering, while chemical applications demand less ordering. More recently, pore self-ordering has also been observed in other material systems. A general framework to understand the self- ordering is still lacking. We will focus on the filled hexagonal lattice of pores in porous alumina and describe their pore-ordering by their elastic interaction and their configurational entropy using cluster variation method. Phase diagrams describing the location of ordered and disordered phases as functions of interpore distance, surface tension, and temperature are obtained. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D42.00006: Characterization of molecular linkages of Zeolite microcrystal assembly Heeju Lee, Jin Seon Park, Kyung Byung Yoon, Hyunjung Kim We have measured x-ray reflectivity curves of silicalite-1 microcrystal (MC) monolayers on Si wafers using two different types of molecular linkages, namely, through chloropropyl (CP) groups and through CP/polyethylene imine/CP groups. While the scanning electron microscope images of the two MC monolayers are indistinguishable of molecular linkage between the monolayers and the substrate, their reflectivity curves are distinctively different, despite the fact that the thicknesses of the molecular linkage layers ($\sim $1-2 nm) are negligibly small compared to the thicknesses of MC monolayers ($\sim $320 nm). We demonstrated that x-ray reflectivity is a very useful tool for the characterization of very thin layers of molecular linkages existing between much thicker MC monolayers and the substrate. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D42.00007: Determination of the hyperpolarizability components of hemicyanine dyes by measuring the anisotropic fluorescence and second harmonic of the dyes uniformly aligned within zeolite channels Doseok Kim, Taekyu Shim, Myounghee Lee, Bumku Rhee, Hyeonsik Cheong, Hyunsung Kim, Kyungbyung Yoon Unidirectional ensemble of hemicyanine molecules was prepared by inserting the molecules into the vertical channels of a uniformly-oriented zeolite (silicalite-1) film grown on a glass substrate. Fluorescence from this sample excited with light polarized along the vertical channel was 50 times larger than that excited with light polarized orthogonal to the vertical channel direction. This vertically aligned hemicyanine dyes were used to determine the ratio of the molecular hyperpolarizability components $\beta_{zxx}$/$\beta_{zzz}$ of hemicyanine. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D42.00008: Orientational Order and Hyperpolarizability of Nonlinear Chromophore Molecules Supported in Amphiphilic 4-Helix Bundle Peptides Grazia Gonella, Andrey Tronin, Michael J. Therien, Hai-Lung Dai, J. Kent Blasie The designed nonlinear optical chromophore, (Polypyridyl)Ruthenium-(Porphinato)Zinc(II) (Ru-PZn), incorporated in a monolayer of amphiphilic 4-helix bundle peptides which is used to provide control of the chromophore orientational order on a silica substrate, has been examined by optical Second Harmonic Generation (SHG). The single monolayer of the H6H20 AP0 [1] peptide covalently attached to an alkylated silica surface with thiol end groups can be used to host and support Ru-PZn cofactor. It has been found that the cofactor's hyperpolarizability tensor is dominated by its component along the conjugation axis as suggested by Karki et al. for similar systems [2]. The tilt angle of the principal symmetry axis of the chromophore molecule from the surface normal has been determined as well as the absolute magnitude of the molecular nonlinear polarizability through comparison with a quartz crystal.\\ \\ 1. Strzalka, J. et al., Nano Lett. (2006) in press\\ 2. Karki, L. et al., J. Am. Chem. Soc. 120 (1998), 2606 [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D42.00009: \textit{Ab initio }simulations of alkyl-terminated Si(001) surfaces. Giancarlo Cicero Self assembled monolayers (SAMs) are ordered molecular assemblies formed by the adsorption of an active surfactant on a solid surface. The interest in the area of self-assembly, and specifically in SAMs, stems partially from their perceived relevance to science and technology. In contrast to ultrathin films made by, for example, chemical vapour deposition, SAMs are highly ordered and oriented and can incorporate a wide range of groups both in the molecular chain and at the chain termination. Therefore, a variety of surfaces with specific interactions can be produced with fine chemical control. In particular, SAM are used in cantilever based detection, as the first step towards the realization of surfaces with specific sensing properties. Understanding how the surface stress and the mechanical properties of a cantilever change upon functionalization is fundamental to achieve accurate quantitative analysis. Here we present \textit{ab initio }simulations of SAM formation on Si(001) surface to make contact with some recent experimental results [1], in which well packed and ordered alkyl-terminated silicon surfaces were obtained. We will show how the Si(001) surface stress and its mechanical properties (elastic constants) change when organic molecules are attached to it. In particular we will discuss the effect of increasing the surface coverage and the length of the alkyl chain used for the functionalization process. [1] Cerofolini G. F. \textit{Semicond. Sci. Technol. }\textbf{18}, 423-429 (2003). [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D42.00010: Structure and morphology of (111) textured Au/Co/Au trilayers grown on glass by MBE. Divine Kumah, J.R. Skuza, A. Cebollada, C. Clevaro, J.M. Garcia Martin, R.A. Lukaszew, Roy Clarke A complete structural and morphological study as a function of Co thickness is presented in a series of Au/Co/Au trilayers grown by MBE on glass substrates. A combined AFM, RHEED, SAXRR and XRD characterization allows determining the optimum deposition conditions that lead to the fabrication of highly textured, flat and continuous layered structures. Development of (111) texture upon annealing the Au layer grown on glass is followed in situ using RHEED. High crystalline quality is confirmed by XRD measurements. A simultaneous in-plane and out-of-plane Co lattice expansion is observed for the thinnest Co layers, converging to bulk values for thickest films. The roughness of the Co layer extracted from the SAXRR data is similar to that of the Au buffer layer, indicative of a conformal growth. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D42.00011: Silicon-on-insulator for symmetry-converted growth Y. Fujikawa, Y. Yamada-Takamura, G. Yoshikawa, T. Ono, M. Esashi, P. Zhang, M. G. Lagally, T. Sakurai Because of its well-established processability, good electronic-transport properties, and ability to form a stable insulating oxide, silicon will remain the essential semiconductor for fabrication of electronic devices. Most device fabrication uses Si(001) and hence most of fundamental research, including heteroepitaxy and integration, has focused on Si(001), a square lattice. Materials having 3- or 6-fold symmetries, a major and important class with key properties, have intrinsic difficulty in their growth on Si(001) because the symmetry mismatch induces polycrystallization at the interface and degrades the film quality. We present a general solution for this long-standing problem that allows maintaining the Si(001) bulk material for those aspects of device fabrication that require it while making possible the growth of 3-fold symmetric structures. We utilize silicon-on-insulator (SOI) in which Si(111) is bonded to Si (001). A 14 nm-thick Si(111) template layer is bonded to Si(001) via the buried oxide. Using the surface preparation method recently developed for the Si(001)-SOI surface, this SOI structure provides a uniform Si(111)-7x7 clean surface. Wurtzite GaN is grown directly on this SOI structure, forming of uniform N-polar film. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D42.00012: Nanotube Formation from Self-Bending Nanofilms Driven by Atomic-scale Surface Stress Imbalance Ji Zang, Minghuang Huang, Feng Liu We present a novel mechanism for fabricating nanotubes by self-bending of nanofilms under intrinsic surface stress imbalance due to surface reconstruction. A freestanding Si nanofilm may spontaneously bend itself into a nanotube without external stress load, and a bilayer SiGe nanofilm may bend into a nanotube with Ge as the inner layer, opposite of the normal bending configuration defined by misfit strain. Such rolled-up nanotubes can accommodate a high level of strain, even beyond the magnitude of lattice mismatch, greatly modifying the tube electronic and optoelectronic properties. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D42.00013: Formation of metallic periodic nanostructures by spontaneous oscillation in self-organized electrocrystallization Mu Wang, Ruwen Peng, Guobin Ma, Xiping Hao, Naiben Ming A unique electrodeposition system is designed to self-organize metallic periodic nanostructures on the surface of solid substrate, which consists of an ultrathin electrolyte layer about 300 nm in thickness. In this system the metal electrodeposits (copper, cobalt, zinc and silver) are formed robustly on the solid substrate (silicon wafer or glass plate), possess considerably low branching rate, and usually cover with periodic metal/metal oxide nano-nodules. Both potentiostatic and galvanostatic modes can be applied to generate such structures. Following issues are focused in this presentation: (1) The mechanism for the formation of the periodic nanostructures on the filaments; (2) The experimental conditions to control the periodicity of these periodic spatio-temporal structures; (3) The electric properties of this nano-nodules chains. \newline References: \newline [1] Yu-Yan Weng, et al. Phys. Rev. E 73, 051601 (2006) \newline [2] Yuan Wang, Yu Cao, Mu Wang, et al., Phys. Rev. E, 69, 021607 (2004) \newline [3] M. Zhang, et al., Adv. Mater. 16, 409 (2004) \newline [4] S. Zhong, Y. Wang, Mu Wanget al., Phys. Rev. E, 67, 061601 (2003) [Preview Abstract] |
Session D43: Synthesis and Growth of Nanostructured Materials
Sponsoring Units: DCMPChair: Chris Palmstrom, Univeristy of Minnesota
Room: Colorado Convention Center 506
Monday, March 5, 2007 2:30PM - 2:42PM |
D43.00001: Detailed Structure Determination of Nanostructures by Low-Energy Electron Diffraction. Michel Van Hove, G.M. Gavaza, Z.X. Yu, L. Tsang, C.H. Chan, S.Y. Tong Detailed structural information of nanostructures (e.g. bond lengths and bond angles) is needed to allow the understanding and prediction of their physical and other properties. To that end we have extended the theoretical capabilities of surface structure determination by Low Energy Electron Diffraction (LEED) to nanostructures. Our new computational method has a compute time that scales as NlogN in terms of the number N of independent atoms, in contrast to N$^{2}$ or N$^{3}$ which typical of conventional LEED calculations of electron multiple scattering. We exhibit the resulting ability to solve detailed nanostructures of different kinds: buckyballs adsorbed on a Cu(111) surface; endohedral and exohedral buckyballs with additional Li or Cu atoms; adsorbed carbon nanotubes; and silicon nanowires. This demonstration of the capability to solve nanostructures should spur the development of experimental methods to measure electron diffraction from nanostructures. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D43.00002: Modeling the Role of Ligands in Controlling the Sizes, Shapes and Supramolecular Ordering of Quantum Dots Michael Tambasco, Sanat Kumar, Igal Szleifer The density of electronic states controls many physical properties of a quantum dot and can be tuned by altering the dot's size, shape, or composition. In colloidal methods, ligands are used to control quantum dot size, shape, and polydispersity; however, there exists no a-priori means of describing specific conditions that will optimize the synthesis procedure. We apply a mean field theory to study the role of ligands in quantum dot synthesis with particular emphasis on non-spherical shapes. We examine the effects of ligand type and concentration on thermodynamic and structural properties, and compare our results with available data. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D43.00003: Migration-influenced island size distribution in interfacial growth Da-Jun Shu, Mu Wang By considering island migration, island interaction and temperature, we demonstrate that distribution of both island size and spatial dispersion on surface can be tuned. It is shown that at lower temperature and stronger inter-island interaction, a narrow size distribution is accompanied by spatial uniformity when island migration is free; yet both size and spatial distribution become random when island migration is forbidden. At higher temperature and weaker inter-island interaction, however, entropy effect dominates, and a narrow size distribution is associated with a fluctuant spatial distribution when island migration is forbidden. These unexpected features are enlightening to fabricate quantum dots where uniformity in both size and spatial distributions are essential. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D43.00004: A level-set method for self-organized pattern formation during heteroepitaxial growth Christian Ratsch, Young-Ju Lee, Xiaobin Niu, Russel Caflisch We have developed an island dynamics model for heteroepitaxial growth that employs an island dynamics model with the level-set technique in combination with a fully self-consistent elastic model. At every timestep in the simulation, we solve the elastic equations for the entire system. This is possible within our approach because the numerical timestep can be chosen much larger than in an atomistic simulation. At every lattice site strain then changes the local bonding, and thus the potential energy surface for adatoms and the microscopic parameters of the simulation. In particular, strain changes the diffusivity of adatoms and enhances the rate of detachment from island edges. We will show how islands become smaller and more regular upon increasing strain. The reason is that bigger islands are typically more strained than smaller islands, and thus their growth is slowed down. We also present results that show that strain moves the system from layer-by-layer growth to the formation of coherent islands as a mechanism for strain relieve. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D43.00005: Phononic Band Gaps in Colloidal Crystals at Hypersonic Frequencies George Fytas, Wei Cheng, Eugenia Nunez, Ulrich Jonas, Nikolaos Stefanou The phononic properties of fabricated closed packed fcc colloidal crystals were investigated by high resolution Brillouin light scattering spectroscopy in the GHz frequency range. The dispersion relation has revealed two phononic band gaps: (i) a Bragg --gap occurring at the boundary of the first Brillouin zone and (ii) a hybridization-gap resulting from the interaction of particle eigenmodes with the acoustic mode of the effective medium. Crystallinity is a prerequisite for the appearance only of the Bragg-gap. Depending on the particle size and the speed of sound in the infiltrated fluid, the frequency and the width of the Bragg-gap can be tuned. Since hypersonic crystals can simultaneously exhibit phononic and photonic band gaps in the visible spectral region, the technological applications could range from tunable filters and heat management to acoustic-optical devices. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D43.00006: Local crystal structure of iron oxide nanoparticles synthesized from Ferritin Michael Krispin, Marcus Preisinger, Peter Pfalzer, Siegfried Horn, Daniel Strongin We have investigated the size dependence of the local crystal structure of nanosized iron oxide by extended x-ray absorption fine structure (EXAFS) at the iron K-edge. Hematite ($\alpha $-Fe$_{2}$O$_{3})$ nanoparticles of different diameters were produced by thermal treatment of horse spleen ferritin molecules and remineralized apo-ferritin molecules, respectively. The structure of these particles was compared to $\alpha $-Fe$_{2}$O$_{3}$ and $\gamma $-Fe$_{2}$O$_{3}$ nanopowder references. The Fourier transformed EXAFS spectra of the nanoparticles differ significantly from hematite and maghemite reference spectra and change systematically as a function of particle diameter, signalling a corresponding evolution of the structure. We show that the Fe--O bond length decreases with decreasing diameter of the particles and with decreasing particle density. This is explained by a core-shell model, in which the fraction of a $\gamma $-Fe$_{2}$O$_{3}$ like particle shell increases while the hematite core decreases with decreasing particle size. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D43.00007: Viscoplasticity and granularity in films of colloidal nanocrystals Dongyun Lee, Shengguo Jia, Sarbajit Banerjee, Joze Bevk, Irving Herman, Jeffrey Kysar Thin films composed of colloidal CdSe nanocrystals have been electrophoretically deposited onto Au/Si substrates with thicknesses ranging from 300 to 3200 nm. The mechanical properties of these films have been measured by nanoindentation. Indentation is carried out to 25{\%} of total thickness of the films, and the elastic modulus and hardness of the films are measured at 10{\%} of the total film thickness to minimize substrate effects. In addition, the force is held at peak load for up to 20 s to observe the creep behavior of the films. The elastic modulus and hardness of 3.2 nm nanocrystal films are $\sim $10 GPa and $\sim $450 MPa, respectively. Furthermore, after particle cross-linking and partial ligand removal, the films exhibit compaction of the cores. This mechanical response suggests these nanocrystal films have polymeric features that can be attributed to the organic ligands and granular characteristics due to the inorganic cores. Both characteristics have also been confirmed by investigating larger nanocrystals and by removing the capping ligands. This work was supported primarily by the MRSEC Program of the NSF under Award No. DMR-0213574 and by NYSTAR. Nanoindentation studies at the Oak Ridge National Laboratory SHaRE User Center were sponsored under DE-AC05-00OR22725. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D43.00008: Doping colloidal nanocrystals: the role of surfactacts Mao-Hua Du, Steven Erwin, Alexander Efros The intentional doping of nanocrystals (NCs) with impurity atoms will be critical to their functioning in a variety of technologies. For NCs grown by colloidal synthesis, doping efforts have been far less successful than for their bulk counterparts. We recently proposed a theoretical model that explains this difference [1]. The central idea is that because the temperatures used in colloidal growth are low (250-300 C), thermal equilibrium is never established. Instead, kinetic factors -- such as impurity adsorption on the NC surface -- play a dominant role in dopant incorporation. Here we consider another kinetic factor known to strongly affect doping in colloidal NCs: the surfactant molecules that are added to passivate the growing NC. We show that the binding strength between the surfactant and dopants in solution strongly affects the dopant sticking coefficient on the NC surfaces, and the dopant solubility in solution. We focus on Mn doping of CdSe NCs, and use first-principles calculations to shed light on the competition between dopant-surface, surfactant-surface, and dopant-surfactant interactions. Our findings are consistent with experimental results for Mn incorporation in CdSe NCs. [1] S.C. Erwin, L. Zu, M.I. Haftel, Al.L. Efros, T.A. Kennedy, and D.J. Norris. Doping semiconductor nanocrystals. Nature \textbf{436}, 91 (2005). [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D43.00009: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 4:18PM - 4:30PM |
D43.00010: Sub-100 nm interferometric lithography realized with table top extreme ultraviolet lasers Mario Marconi, Przemyslaw Wachulak, Dineshchandra Patel, Maria Gabriela Capeluto, Carmen Menoni, Jorge Rocca We demonstrated patterning of arrays of nano-dots with feature sizes below 100 nm by interferometric lithography using a table top extreme ultraviolet 46.9 nm wavelength laser. The interferometric lithography setup was based on a Lloyd's mirror interferometer and multiple exposures. That allowed the patterning of arrays of nano-dots over areas of 500$\times $500 $\mu $m$^{2}$ on commercial photoresists with different motifs. This new technique demonstrates the printing capability of nano-scale patterns with a compact table-top set up at extreme ultraviolet wavelengths. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D43.00011: Chemical Nanomachining of Si Jeremy Robinson, Paul Evans, J. Alex Liddle, Oscan Dubon We demonstrate a simple process for chemical nanomachining of reproducible Si nanonstructures. Using a stencil mask containing windows of various geometries, we evaporate Au onto a Si surface rinsed in HF. The pattern formed by the spontaneous oxidation of Si at and around each patterned Au feature serves as a mask for the underlying Si and permits the use of simple wet chemistry to produce highly ordered nanostructures of diverse shapes including rings, pillars, wires, and nanopores. Pillars are formed by etching a Si sample patterned with an array of nominally 1 nm-thick Au squares having a side dimension of 200 nm. Remarkably, the Au capped core of these pillars can be removed by briefly rinsing the Au-patterned sample with HF prior to etching with KOH. When the Au-squares are sufficiently close together, the anodic oxide patterns surrounding the Au squares overlap to form a continuous surface oxide. Etching in this case with HF followed by KOH produces a continuous Si film with holes. Thus, this unique catalyzed patterning process opens the door for the rapid, parallel fabrication of a variety of nanostructures that are unfeasible or impractical to fabricate with traditional processing routes. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D43.00012: Sculpting of Nanopores in Silicon-Nitride Membranes Kristin Lucas, Jeffrey L. Wasserman, Soo Hyung Lee, Nina Markovic Fabrication of controllable-diameter nanopores in a suspended membrane is of great interest for dynamic stencil deposition techniques, as well as DNA sequencing and other applications. We have developed a method for the production of nanometer-scale pores in a silicon-nitride membrane. We punch holes in a membrane of low-stress silicon-nitride through focused ion beam or lithographic techniques. By exposing the holes to an electron beam we can shrink the pore diameter down to a few nanometers. We can also produce complex pore shapes through selective sculpting of the pore. We will discuss the details of the process and its applications. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D43.00013: Nanocrystalline Composite Media in the GeO$_{2}$-Ga$_{2}$O$_{3}$ Mullite-Type System: Synthesis and Conventional as Well as Synchrotron Based Characterization Kristina E. Lipinska-Kalita, Patricia E. Kalita, Oliver Hemmers, Cedric L. Gobin, Gino Mariotto, Thomas Hartmann, Longzhou Ma In the frame of our extensive project on nanocomposites based on dielectric matrices, we designed and synthesized a series of optically transparent glass-based composites, containing nanometer-sized crystals embedded in an isotropic host matrix. We determined the structure of the nanocrystals to be an orthorhombic GeO$_{2}$-Ga$_{2}$O$_{3}$ mullite-type phase, isostructural with SiO$_{2}$-Al$_{2}$O$_{3}$ mullite. In order to characterize the materials synchrotron X-ray diffraction, optical spectroscopy and electron microscopy were used. High-pressure compression and decompression studies up to 40 GPa were also performed to investigate the structural integrity of the nanocrystals. This work is the first report of a controlled, successful synthesis of Ge-Ga mullite type nanocrystalline composites. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D43.00014: GaAs$_{x}$P$_{1-x}$ Superlattice nanowires. Romaneh Jalilian, Sharvil Desai, Anton Sidorov, Zhiqiang Chen, B. K. Pradhan, Gamini Sumanasekera Semiconductor alloys provide a natural tool to tune the bandgap by managing the spatial spreading and distribution of alloys sequentially along a nanowire. A growth technique has been developed using laser ablation to synthesize ternary alloys of GaAs$_{x}$P$_{1-x}$ superlattice nanowires. These superlattice nanowires exhibit very interesting structures; alternating crystalline structures of GaAs$_{x}$P$_{1-x }$alloys grow in one direction. Continuous modulation of growth routine, specially the ablation time can control the composition of the segments in the superlattice nanowires. SEM, HRTEM, SAD, EDS, EELS, XRD, XPS have been used to study the morphology, crystalline structures and relative distribution of gallium, arsenic and phosphorus in ternary alloys superlattice. Optical absorption, luminescence and electrical properties have been explored. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D43.00015: Quantitative structural characterization of annealed InAs/GaSb superlattices Ge Liu, Bernd Fruhberger, Ivan K. Schuller, Heather J. Haugan, Gail J. Brown MBE grown InAs/GaSb superlattices without \textit{in situ} thermal processing were post-growth annealed in vacuum and analyzed by a combination of XRD and structural refinement. The refinement shows a FWHM of satellite peak broadening and a d-spacing contraction in InAs constituent layers, which are caused by interfacial roughness and inhomogeneous strain, when the annealing temperature is above 200 $^{o}$C. Furthermore, the annealing above 450 $^{o}$C destroys the superlattice structure and annihilates all satellite peaks. An additional series of satellite peaks showing InSb-like behavior was found in annealed superlattices which is reduced by further annealing at 200 $^{o}$C. The refinement reveals that these peaks arise from the ``superlattice'' composed of InSb-like structure. The surface morphology studied using AFM shows that the InAs/GaSb superlattice surfaces evolve from flat plane, porous structure, and pebble structure to island structure as the annealing temperature increases. [Preview Abstract] |
Session D44: Focus Session: Spin and Nonlinear Dynamics in Optical Nanostructures
Sponsoring Units: DMPChair: Gabriel Bester, National Renewable Energy Laboratory
Room: Colorado Convention Center 507
Monday, March 5, 2007 2:30PM - 2:42PM |
D44.00001: Optical control and determination of charge in self-assembled quantum dots M. Korkusinski, P. Hawrylak, A. Babinski, M. Potemski, S. Raymond, J. Lapointe, Z. Wasilewski We present a theory and experiment allowing for optical control of charge in a single InAs/GaAs quantum dot (QD) in magnetic fields up to 23 T [1]. The charge is controlled by excitation energy and power and is determined by comparing the experimental PL spectra of the QD to the ones calculated for N electrons and one hole using the parabolic confinement and the CI technique for many-carrier states. The number N is determined from the characteristic features in PL [2]. For N=4 electrons in low fields the degenerate p shell is half-filled and the system is in a triplet state. At larger fields the degeneracy is removed and a triplet-singlet transition occurs. This transition is seen as a discontinuity in the magnetic-field dependence of PL lines. In even higher fields, electrons increase their polarization through spin-flip transitions, which also leads to discontinuities of the PL spectra. Also, as the magnetic moment of electrons increases, the electron-hole exchange leads to the appearance of multiple PL lines. [1] A. Babinski et al, Physica E 26, 190 (2005) [2] A. Wojs and P. Hawrylak, Phys. Rev. B 55, 13066 (1997) [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D44.00002: Spin effects in coupled quantum dots under ac electric fields Lilia Meza-Montes, Arezky H. Hernandez, Sergio E. Ulloa Spin control has recently attracted attention for applications in spin-based devices. Different effects and applied fields have been suggested to accomplish the goal. We explore the time evolution of electronic spin in coupled quantum dots under harmonic electric fields. Using the Floquet formalism, we obtain the time dependent wave function in terms of the Floquet states and the quasi-energy spectrum for a single electron in double InSb dots. The spatial part of the wave function includes the SIA and BIA spin-orbit effects. The spectral force is analyzed at anti-crossings of the quasi-energy bands as a function of the field strength. The resulting dynamical symmetries and the way they reflect in the time evolution of the spin clouds will be discussed. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D44.00003: Qubit identification and entanglement in tunneling and F\"{o}rster coupled quantum dots Juan E. Rolon, Sergio E. Ulloa We investigate the possibility of qubit coherent manipulation using the multi-excitonic optical spectrum features of a quantum dot molecule (QDM), a system of two vertically coupled InAs/GaAs self-assembled quantum dots. The spectrum is modeled using a Hamiltonian that incorporates coupling dependence on several experimental parameters, such as gate voltage, optical excitation intensity and its detuning. We use realistic structure parameters to describe the important coupling constants, including electron and hole tunneling, and Coulomb correlations that depend on the QDM strain field, and interdot distance [1]. We also incorporate the role of the F\"{o}rster-Dexter resonant energy transfer processes, as well as, exciton oscillator strengths extracted from available PL spectroscopy data. The dynamics given by the time evolution of the density matrix and the qubit-qubit entangling interaction is monitored by calculations of the entanglement of formation [2] for the suitable excitonic molecular states. We discuss how to optimize Rabi flops and entanglement via gate-controlled adiabatic passage through a level anticrossing [3]. [1] G. Bester, A. Zunger, PRB 71 075325 (2005) [2] W.K. Wootters, PRL 80(10) 2245 (1998) [3] K. Bergmann, Rev. Mod. Phys. 70(3) (1998) [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D44.00004: Spin Interactions in Optically Excited Quantum Dot Molecules Invited Speaker: Recently we have demonstrated controlled interaction between QDs [1] -- a key requirement for the use of QDs as basic building blocks in novel information processing technologies, as e.g. in quantum computation or spintronics. Here we delineate for the first time the origin of the exchange coupling between spins [2] in optically excited QD molecules, and we trace its atomic to molecular evolution. We have performed photoluminescence spectroscopy on single InAs/GaAs QDMs. The QD molecules were formed by the subsequent growth of two closely spaced layers of self-assembled QDs. The two QD layers were embedded in a diode structure in order to controllably tune different excitonic charge states through molecular resonances. The resulting optical spectra of double QDs exhibit a rich variety of fascinating features. Distinct patterns of anticrossings formed by the various excitonic and biexcitonic charge states allow us to determine the rules governing the state energies and quantum mechanical coupling between two QDs. Prominent spin fine structure in the molecular spectra is understood in terms of the interplay between h-h, e-h, and e-e exchange interactions. This work sets the stage for using laser fields to execute two-qubit operations in quantum dots. \newline \newline [1] E. A. Stinaff, et al., Science \textbf{311}, 636 (2006) \newline [2] M. Scheibner, et al., cond-mat/0607241 [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D44.00005: Theory of Spin States in Coupled Quantum Dots. Ilya Ponomarev, Matt Doty, Michael Scheibner, Allan Bracker, Dan Gammon, Tom Reinecke The system of vertically coupled self-assembled quantum dots (CQDs) tuned by external electric field is a promising candidate as a basis for coherent optical spin manipulation in quantum information applications and spintronics [1]. We have developed a theoretical model that describes spin states of neutral and charged excitons in CQDs [2]. In this approach the electric field induced resonant tunneling of the electron and hole states occurs at different biases due to the inherent asymmetry of CQDs. The truncated many-body basis configurations for each molecule are constructed from antisymmetrized products of single-particle states. The interplay between tunneling, electron-electron, hole-hole and electron-hole exchange interactions splits the states with different spin-projections. The model explains a rich diversity of spectral line patterns in photoluminescence spectra observed in recent experiments. [1] E.A.Stinaff et al., Science 311, 636 (2006). [2] I.V. Ponomarev et al., Phys. Stat. Sol. (b), 243, 3869. (2006) [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D44.00006: Polarized stimulated emission from photonic molecule states in coupled microdisk lasers X. Li, B.J. Cooley, N. Samarth, F.M. Mendoza, R.C. Myers, D.D. Awschalom Recent studies have demonstrated the engineering of spin coherence via photon-spin interactions in microdisk lasers [S. Ghosh {\it et al.}, Nature (Materials) {\bf 5}, 261 (2006)], motivating the extension of such measurements to pairs of microdisks coupled through the evanescent electromagnetic field. Such coupled microdisks behave like ``photonic molecules'' (PMs) with bonding and antibonding states for the confined photon modes [A. Nakagawa {\it et al.}, Appl. Phys. Lett. {\bf 86}, 04112 (2005)]. We describe the fabrication and optical characterization of different PM geometries, consisting of laterally coupled GaAs/GaAlAs microdisks of both circular and elliptical shape. Steady state photoluminescence measurements reveal bonding and antibonding modes with distinct geometry-dependent polarization characteristics that are consistent with finite-difference time-domain simulations. We also discuss time-resolved optical measurements that probe both carrier and spin dynamics in these PMs. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D44.00007: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 4:18PM - 4:30PM |
D44.00008: Spin Multiphoton Antiresonance at Finite Temperatures Christian Hicke, Mark Dykman Weakly anisotropic $S>1$ spin systems display multiphoton antiresonance. It occurs when an Nth overtone of the radiation frequency coincides with the distance between the ground and the Nth excited energy level (divided by $\hbar$). The coherent response of the spin displays a sharp minimum or maximum as a function of frequency, depending on which state was initially occupied. We find the spectral shape of the response dips/peaks. We also study the stationary response for zero and finite temperatures. The response changes dramatically with increasing temperature, when excited states become occupied even in the absence of radiation. The change is due primarily to the increasing role of single-photon resonances between excited states, which occur at the same frequencies as multiphoton resonances. Single-photon resonances are broad, because the single-photon Rabi frequencies largely exceed the multi-photon ones. This allows us to separate different resonances and to study their spectral shape. We also study the change of the spectrum due to relaxational broadening of the peaks, with account taken of both decay and phase modulation. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D44.00009: Nonlinear interlevel optical phenomena in quantum dots Victor Bondarenko, Yang Zhao Nonlinear interlevel optical phenomena caused by the electron-electron interaction in quantum dots are investigated theoretically within the semiclassical density matrix formalism. A special attention is paid to the intrinsic optical bistability. Obtained analytical relations and results of numerical simulations reveal role of driving charasteristic parameters of quantum dot systems as well as of the incident radiation in the phenomena. Self-consistent treatment of the electron-electron interaction is shown to be of crucial importance. A proper microscopical treatment is shown to be needed for accurate description of the phenomena. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D44.00010: Origin of second-harmonic generation of Si nanoinclusions in glass E.J. Adles, D.E. Aspnes We applied our anisotropic bond model (ABM) to clarify the origin of the second-harmonic-generation (SHG) signals observed by Figliozzi et al.[1] for Si nanoinclusions in glass. The ABM describes nonlinear-optic (NLO) responses in terms of radiation from anisotropically and anharmonically bound bond charges, and differs from conventional force formulations by (1) incorporating anisotropy at the bond level and (2) describing observed NLO intensities as a coherent superposition of radiation from these charges accelerated by the driving field. It therefore provides specific information about the origins of NLO signals at the atomic level. Here, SHG signals from the glass and bulk of the Si inclusions are found to be essentially nonexistent, as expected,[2] in the former case as a result of cancellation of radiation fields of bonds oriented in random directions, and in the latter case due to dielectric screening. Our calculations show that SHG is dominated by gradient effects, specifically from the variation in field across the inclusion (spatial-dispersion and crossed-beam effects), consistent with experiment. The large interface field gradient contributes a weak signal from charge motion transverse to the bond direction. [1] P. Figliozzi et al. Phys Rev Lett 94 (2005). [2] V. L. Brudny et al. Phys Rev B 62 (2000). [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D44.00011: Rabi Coupling Between IR-active Phonon and Cavity-resonant Electromagnetic Modes Gavin K. Brennen, H.M. Lawler, Sanjiv Shresta, J.N. Byrd We predict an approximately 200 micron Rabi coupling between a cavity-resonant electromagnetic mode and the infrared-active phonon of an enclosed GaAs sample. This prediction follows from our quantized description of the electromagnetic field, the phonon field, and their interaction. We believe the prediction to be supported by recent observations of geometry-enhanced terahertz emission, and boundary-condition dependent phonon-polariton spectra in pump-probe optical studies. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D44.00012: Electron-Photon interaction associated Uncertaity Relation based Tunneling in a Parallel Double Quantum Dot System Kao-Chin Lin A new mechanism of electron-photon interaction in a parallel double quantum dot (DQD) system is studied. The electron is allowed to transit between dots due to the electron-photon interaction. When the electron in quantum dot m (QDm) transits to the adjoining QDm(m,m1,2 and mm), it is allowed to tunnel into leadm , which is connected to QDm , via energy-time uncertainty relation in a very short time interval. Like the Kondo resonant peak in Anderson model, the new mechanism of the electron-photon interaction exhibits peaks which depends logarithmically on temperature. The character temperature obtained is found to be higher than the Kondo temperature in some situations. Unlike the Kondo effect, the quantum mechanical tunneling associated the electron-photon interaction is not always on resonance. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700