Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session D1: Fundamental Developments in Density Functional Theory
Sponsoring Units: DCOMPChair: Neepa Maitra, Hunter College of the City University of New York
Room: Morial Convention Center LaLouisiane AB
Monday, March 10, 2008 2:30PM - 3:06PM |
D1.00001: Nearsightedness in Density Functional Theory Invited Speaker: |
Monday, March 10, 2008 3:06PM - 3:42PM |
D1.00002: Density-functional theory of superconductivity Invited Speaker: A prominent challenge of modern condensed-matter theory is to predict reliably material-specific properties of superconductors, such as the critical temperature. The traditional model of Bardeen, Cooper and Schrieffer (BCS) properly describes the universal features that all conventional superconductors have in common, but it is not able to make accurate predictions of material-specific properties. To tackle this problem, a density-functional formalism has been developed [1] which describes superconductors in thermal equilibrium in terms of three quantities: the ordinary density, the superconducting order parameter, and the nuclear N-body density. These three ``densities'' are determined self-consistently through a set of Kohn-Sham equations. Approximations of the universal exchange-correlation functional are derived on the basis of many-body perturbation theory. In this way, a true ab-initio description is achieved which does not contain any adjustable parameters such as the $\mu $* of Eliashberg theory. Numerical results for the critical temperature, the isotope effect, the gap function and the jump of the specific heat will be presented for simple metals, for MgB$_{2 }$[2] and CaBeSi, and for calcium intercalated graphite (CaC$_{6})$ [3]. Furthermore, results for Li, Al, K, and H under pressure will be discussed. The calculations explain why Li and Al behave very differently, leading to a strong enhancement of superconductivity for Li and to a clear suppression for Al with increasing pressure [4]. For K we predict a behavior similar to Li, i.e. a strong increase of T$_{c}$ with increasing pressure. Finally, hydrogen is found to be a three-gap superconductor whose critical temperature increases with increasing pressure until about 100K (at 500 GPa). \\ \noindent [1] M. L\"{u}ders, M.A.L. Marques, N.N. Lathiotakis, A. Floris,G. Profeta, L. Fast, A.Continenza, S. Massidda, E.K.U. Gross, PRB \underline {\textbf{72}}, 024545 (2005). \\ \noindent [2] A. Floris, G. Profeta, N.N. Lathiotakis, M. L\"{u}ders, M.A.L. Marques, C. Franchini, E.K.U. Gross, A. Continenza, S. Massidda, PRL \underline {\textbf{94}}, 037004 (2005). \\ \noindent [3] A. Sanna, G. Profeta, A. Floris, A. Marini, E.K.U. Gross, S. Massidda, PRB (Rapid Comm.) \underline {\textbf{75}}, 020511 (2007). \\ \noindent [4] G. Profeta, C. Franchini, N.N. Lathiotakis, A. Floris, A. Sanna, M.A.L. Marques, M. L\"{u}ders, S. Massidda, E.K.U. Gross, A. Continenza, PRL \underline {\textbf{96}}, 047003 (2006). [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 4:18PM |
D1.00003: Remarks on Molecular Density Functional Theory Invited Speaker: The dft of finite molecular systems possesses unique special characteristics that produce challenges not yet met and promises not yet realized. I describe several of the problems in this subject with which we have been struggling. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:54PM |
D1.00004: The Partition Problem; Insights from Density Functional Theory Invited Speaker: How to partition a system into its components, the atoms in molecules problem and its multi-atomic generalizations, arises ubiquitously in physics, chemistry, and materials science. It is central to population analysis, chemical reactivity theory, issues of transferability, and relevant to computational methods for very large systems such as QM-MM and O(N) schemes. At issue is the decomposition of the total electron density into contributions from each part, whence the relevance of density functional theory. My collaborators and I have developed a new, exact scheme, partition theory, for that decomposition. It is based on the Perdew, Parr, Levy, and Balduz ensemble formulation of density functional theory. In this talk, the elements of partition theory will be described, including its formal structure, a dynamical version for efficient computation, and quantitative illustrations of its central features via the partition of very simple systems. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:30PM |
D1.00005: Restoring the Density-Gradient Expansion for Exchange in a GGA for Solid and Surfaces Invited Speaker: Density functionals for the exchange-correlation energy of a many-electron system are widely used in condensed-matter physics. Successful modern generalized gradient approximations (GGA's), developed largely for quantum chemistry, are biased toward free-atom energies. Recent ``GGA's for solids'' include PBEsol [1], a revised Perdew-Burke-Ernzerhof (PBE) GGA that improves equilibrium properties of densely-packed solids and their surfaces by recovering the first-principles density-gradient expansion for the exchange energy [2]. Results will be reported for the lattice constants of 20 solids and for the surface energy of jellium in the local spin density approximation and in the PBE and PBEsol GGA's. Other possible applications of PBEsol will be discussed. It will be argued (as in Ref. [3]) that the second-order gradient expansion is nearly converged for exchange, but not for correlation, in valence regions of typical solids (while atoms require a larger gradient coefficient for exchange). \newline [1] J.P. Perdew, A. Ruzsinszky, G.I. Csonka, O.A. Vydrov, G.E. Scuseria, L.A. Constantin, X. Zhou, K. Burke, http://arxiv.org/abs/0711.0156 \newline [2] P.R. Antoniewicz and L. Kleinman, Phys. Rev. B \textbf{31}, 6779 (1985). \newline [3] J.P. Perdew, L.A. Constantin, E. Sagvolden, and K. Burke, Phys. Rev. Lett. \textbf{97}, 223002 (2006). [Preview Abstract] |
Session D2: New Developments in HTSC II
Sponsoring Units: DCMPChair: Erica Carlson, Purdue University
Room: Morial Convention Center LaLouisiane C
Monday, March 10, 2008 2:30PM - 3:06PM |
D2.00001: Anomalous Electron-Phonon Coupling in Cuprates and its Doping Dependence Invited Speaker: It is well known that conventional superconductivity is mediated by phonons. Phonon renormalization at specific wavevectors (Kohn anomalies) appears in phonon dispersions in many of these compounds with high T$_{c}$s in agreement with LDA calculations. In the case of the cuprates, LDA calculations predict neither any significant Kohn anomalies nor electron-phonon coupling strong enough to account for high T$_{c}$ superconductivity. However, inelastic neutron and x-ray scattering experiments found huge softening and broadening of the bond-stretching phonons indicating that electron-phonon coupling in the cuprates is much stronger than expected from LDA. In the LaSrCuO family phonon renormalization has been observed in the vicinity of the reduced in-plane wavevector q$_{in}$=(0.25, 0) (in units of (2$\pi $/a,2$\pi $/a where a is the near-neighbor Cu-Cu distance). The effect is strongest at low temperatures and in compositions that exhibit the so-called stripe order where it occurs at the wavevector that corresponds to the charge order. Detailed \textbf{q}-dependent studies revealed that the underlying electronic instability is 2D in nature in the 214 compounds, i.e. for q$_{in}$=(0.25, K), it is peaked at K=0 with the full width at half maximum of about 0.15 r.l.u. The strength of this phonon renormalization tracks T$_{c}$ disappearing at the nonsuperconducting extremes of doping. In YBCO the similar phonon anomaly is quasi-1D with bond-stretching phonon renormalization occurring around q$_{in}$=(H, 0.25) for all investigated H (in units of (2$\pi $/a,2$\pi $/b where a/b is the near-neighbor Cu-Cu distance in the direction paralle/perpendicular to the Cu-O chains). Relationship between these effects and band structure will be explored. Experimental results will also be compared with expectations of LDA-based calculations as well as with predictions of models based on dynamic stripes. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:42PM |
D2.00002: Doping Dependent Anisotropic Electronic Scattering rate in LSCO Invited Speaker: An angle-resolved photoemission study of the scattering rate in the normal and superconducting states of the high-temperature superconductor La(2-x)Sr(x)CuO(4) as a function of binding energy and momentum will be presented. We report that, close to optimal doping, the scattering rate scales linearly with binding energy up to a high-energy scale E1. The scattering rate is found to be strongly anisotropic, with a minimum along the nodal direction of the superconducting gap. SInce both the degree of anisotropy and the energy dependence of the scattering rate appear to be strongly doping dependent, possible connections to a quantum-critical point will be discussed. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 4:18PM |
D2.00003: Fermi surface of underdoped cuprate revealed by quantum oscillations and Hall effect Invited Speaker: Despite twenty years of research, the phase diagram of high temperature superconductors remains enigmatic. A central issue is the origin of the differences in the physical properties of these copper oxides doped to opposite sides of the superconducting region. In the overdoped regime, the material behaves as a reasonably conventional metal, with a large Fermi surface [1]. The underdoped regime, however, is highly anomalous and appears to have no coherent Fermi surface, but only disconnected `Fermi arcs' [2]. We have reported the observation of quantum oscillations in the electrical resistance of the oxygen-ordered copper oxides YBa$_{2}$Cu$_{3}$O$_{6.5}$ [3] and YBa$_{2}$Cu$_{4}$O$_{8}$ [4], establishing the existence of a coherent closed Fermi surface at low temperature in the underdoped side of the phase diagram of cuprates, once superconductivity is suppressed by a large magnetic field. The low oscillation frequency reveals a Fermi surface made of small pockets, in contrast to the large cylinder characteristic of the overdoped regime. Moreover, the negative sign of the Hall effect at low temperature reveals that these pockets are electron-like rather than hole-like. We propose that the Fermi surface of these Y-based cuprates consists of both electron and hole pockets, probably arising from a reconstruction of the FS [5]. Work in collaboration with N Doiron-Leyraud, D. LeBoeuf and L. Taillefer from the University of Sherbrooke, J. Levallois and B. Vignolle from the LNCMP, A. Bangura and N. Hussey from the University of Bristol and R. Liang, D. Bonn, W. Hardy from the University of British Columbia. \newline [1] N Hussey et al, \textit{Nature} \textbf{425}, 814 (2003) \newline [2] M. Norman et al, \textit{Nature} \textbf{392}, 157 (1998) \newline [3] N. Doiron-Leyraud et al, \textit{Nature} \textbf{447}, 565 (2007) \newline [4] A. Bangura et al, submitted to \textit{Phys. Rev. Lett} (arXiv: 0707.4461) \newline [5] D. LeBoeuf et al, \textit{Nature} \textbf{450}, 533 (2007) [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:54PM |
D2.00004: Fermi orbits versus Fermi arcs Invited Speaker: We consider the effect of a short antiferromagnetic correlation length $\xi$ on the electronic bandstructure of the underdoped cuprates. Starting with a Fermi-surface topology thought to be consistent with that detected in magnetic-quantum-oscillation experiments, we show that a reduced $\xi$ gives an asymmetric broadening of the quasiparticle dispersion, resulting in simulated ARPES data very similar to those observed in experiment. Predicted features include the presence of `Fermi arcs' close to $a{\bf k}=(\pi/2,\pi/2)$, where $a$ is the in-plane lattice parameter, without the need to invoke a $d$-wave pseudogap order parameter. The statistical variation in the $k$-space areas of the reconstructed Fermi-surface pockets causes the quantum oscillations to be strongly damped, even in very strong magnetic fields, in agreement with experiment. (I would like to extend special thanks to the coauthors John Singleton and Ross McDonald and to E. Yelland and L. Taillefer for useful discussions). [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:30PM |
D2.00005: Fermi surface and anisotropic scattering in overdoped cuprates Invited Speaker: In the light of recent results detailing the emergence of small Fermi pockets and anomalous Hall coefficients in underdoped cuprates, we review here our measurements on the Fermiology and low temperature transport of cuprates that reside on the other side of the superconducting dome. Analysis of angle-dependent magnetoresistance and Hall coefficient data in Tl$_{2}$Ba$_{2}$CuO$_{6+\delta }$ and La$_{2-x}$Sr$_{x}$CuO$_{4}$ has uncovered a wealth of structure, not only in the (un-reconstructed) Fermi surface in overdoped cuprates, but also in the basal-plane transport scattering rate, in both the elastic and the inelastic channels. A striking correlation between the superconducting transition temperature and the strength of the anisotropic scattering is also revealed suggesting an intimate link between anisotropic scattering and the pairing mechanism itself. Finally, we discuss possible origins of these various anisotropic terms and how they might impact on our understanding of the evolution of the resistivity and the Hall coefficient across the entire cuprate phase diagram. [Preview Abstract] |
Session D3: Materials Physics in the Fast Lane
Sponsoring Units: DMPChair: Brian Schwartz, City University of New York
Room: Morial Convention Center RO2 - RO3
Monday, March 10, 2008 2:30PM - 3:06PM |
D3.00001: The Art and Materials Physics of the Motorcycle Invited Speaker: In 1871 Louis Guillaume Perreaux installed a compact steam engine in a commercial bicycle, and thus produced the world's first motorcycle. A steam engine was a logical choice, having steadily developed from the work of Savery and Newcomen in the 17th century to the point where Perreaux was able to make one small enough to use for this purpose. Unfortunately, it was a technological dead-end the moment it was created, since nine years earlier Alphonse Beau de Rochas had published the description of the four-cycle internal-combustion process. Significantly, the Michaux-Perreaux engine produced 1-2 hp in an overall machine that weighed 88 kg, whereas modern motorcycles produce 100 times more horsepower while weighing only twice as much. Examples I will show illustrate that developments in materials science over the past century are almost entirely responsible for making this possible. After a period of extraordinarily-rapid technological advance, by 1903 essentially all the components of a modern motorcycle were in place, and changes since then have been largely the result of evolutionary refinement in step with advances in materials science, rather than further revolutionary invention. Also, like many other objects of industrial design, motorcycles have played a variety of roles in society over the 137 years since the Michaux-Perreaux. I will discuss the interrelationship of the relevant technological, cultural, and aesthetic factors over the past century that have, amongst other things, resulted in standard production motorcycles -- incorporating such materials as carbon-fiber composites, maraging steels, and ``exotic'' alloys of magnesium, titanium and aluminum -- that can exceed 190 mph straight from the show room floor. For more information see http://www.optics.arizona.edu/ssd/aotm.html. Acknowledgment: I am grateful for the contributions of Ultan Guilfoyle to our joint work on the Solomon R. Guggenheim's ``The Art of the Motorcycle.'' [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:42PM |
D3.00002: Materials at 200 mph: Making NASCAR Faster and Safer Invited Speaker: You cannot win a NASCAR race without understanding science.\footnote{Diandra Leslie-Pelecky, \textit{The Physics of NASCAR} (Dutton, New York City, 2008).} Materials play important roles in improving performance, as well as ensuring safety. On the performance side, NASCAR limits the materials race car scientists and engineers can use to limit ownership costs. `Exotic metals' are not allowed, so controlling microstructure and nanostructure are important tools. Compacted Graphite Iron, a cast iron in which magnesium additions produce interlocking microscale graphite reinforcements, makes engine blocks stronger and lighter. NASCAR's new car design employs a composite called Tegris$^{TM}$ that has 70 percent of the strength of carbon fiber composites at about 10 percent of the cost. The most important role of materials in racing is safety. Drivers wear firesuits made of polymers that carbonize (providing thermal protection) and expand (reducing oxygen access) when heated. Catalytic materials originally developed for space-based CO$_{2}$ lasers filter air for drivers during races. Although materials help cars go fast, they also help cars slow down safely---important because the kinetic energy of a race car going 180 mph is nine times greater than that of a passenger car going 60 mph. Energy-absorbing foams in the cars and on the tracks control energy dissipation during accidents. To say that most NASCAR fans (and there are estimated to be 75 million of them) are passionate about their sport is an understatement. NASCAR fans understand that science and engineering are integral to keeping their drivers safe and helping their teams win. Their passion for racing gives us a great opportunity to share our passion for science with them. NASCAR$^{\mbox{{\textregistered}}}$ is a registered trademark of the National Association for Stock Car Auto Racing, Inc. Tegris$^{TM}$ is a trademark of Milliken {\&} Company. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 4:18PM |
D3.00003: Sox and Drugs: Baseball, Steroids and Physics Invited Speaker: The sports world is in an uproar over performance-enhancing drugs. In the United States steroids in baseball have received the most attention, in part because the purported effects are much more dramatic than in any other sport. From 1995-2003 a few players hit home runs at rates 20-50{\%} higher than the best sluggers of the preceding century. Could steroids really increase home-run performance that much? I will describe a model that combines estimates of the physiological effects of steroids, known baseball physics, and reasonable models of batting effectiveness for highly skilled hitters. A 10{\%} increase in muscle mass, which can reasonably be expected from steroid use, increases the speed of a batted ball by 3{\%}. Because home runs are relatively rare events on the tail of a batter's range distribution, even this modest change in ball speed can increase the proportion of batted balls that result in home runs by 30 -- 70{\%}, enough to account for the record-shattering performances of the recent past. I will also describe some of the attention -- both welcome and not -- that comes to the unsuspecting physicist who wades into such emotionally troubled waters. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:54PM |
D3.00004: Zero CTE Glass in the Hubble Space Telescope Invited Speaker: Orbiting high above the turbulence of the earth's atmosphere, the Hubble Space Telescope (HST) has provided breathtaking views of astronomical objects never before seen in such detail. The steady diffraction-limited images allow this medium-size telescope to reach faint galaxies fainter than 30th stellar magnitude. Some of these galaxies are seen as early as 2 billion years after the Big Bang in a 13.7 billion year old universe. Up until recently, astronomers assumed that all of the laws of physics and astronomy applied back then as they do today. Now, using the discovery that certain supernovae are ``standard candles,'' astronomers have found that the universe is expanding faster today than it was back then: the universe is accelerating in its expansion. The Hubble Space Telescope is a two-mirror Ritchey-Chr\'etien telescope of 2.4m aperture in low earth orbit. The mirrors are made of Ultra Low Expansion (ULE) glass by Corning Glass Works. This material allows rapid figuring and outstanding performance in space astronomy applications. The paper describes how the primary mirror was mis-figured in manufacturing and later corrected in orbit. Outstanding astronomical images taken over the last 17 years show how the application of this new technology has advanced our knowledge of the universe. Not only has the acceleration of the expansion been discovered, the excellent imaging capability of HST has allowed gravitational lensing to become a tool to study the distribution of dark matter and dark energy in distant clusters of galaxies. The HST has touched practically every field of astronomy enabling astronomers to solve many long-standing puzzles. It will be a long time until the end of the universe when the density is near zero and all of the stars have long since evaporated. It is remarkable that humankind has found the technology and developed the ability to interpret the measurements in order to understand this dramatic age we live in. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:30PM |
D3.00005: The Materials Science of Superheroes Invited Speaker: While materials scientists don't typically consult comic books when selecting research topics, innovations first introduced in superhero adventures as fiction can sometimes find their way off the comic book page and into reality. As amazing as the Fantastic Four's powers is the fact that their costumes are undamaged when the Human Torch flames on or Mr. Fantastic stretches his elastic body. In shape memory materials, an external force or torque induces a structural change that is reversed upon warming. Smart fabrics used in hiking clothing expand at low temperatures, while other materials increase their porosity at higher temperatures, allowing body heat and water vapor to escape. Some polymers can be stretched to over twice their normal dimensions and return to their original state when annealed, a feature appreciated by Mr. Fantastic. In order to keep track of the Invisible Woman, the Fantastic Four's arch nemesis Dr. Doom employed sensors in the eye-slits of his armored face-plate, using the same physics underlying night vision goggles. Certain forms of blindness may be treated using an artificial retina consisting of silicon microelectrode arrays, surgically attached to the back of the eye, that transmit a voltage to the optic nerve proportional to the incident visible light intensity (one of the few positive applications of Dr. Doom's scheming). Spider-Man's wall crawling ability has been ascribed to the same van der Waals attractive force that gecko lizards employ through the millions of microscopic hairs on their toes. Scientists have recently developed ``gecko tape,'' consisting of arrays of fibers that provide a strong enough attraction to support a modest weight. Before this tape is able to support a person, however, major materials constraints must be overcome (if this product ever becomes commercially available, I for one will never wait for the elevator again!) All this, and the chemical composition of Captain America's shield, will be discussed. [Preview Abstract] |
Session D4: Probing Spin and Charge States in Semiconductor Quantum Dots and “Molecules”
Sponsoring Units: DCMPChair: David Awschalom, University of California, Santa Barbara
Room: Morial Convention Center 206
Monday, March 10, 2008 2:30PM - 3:06PM |
D4.00001: Spin tunneling in optically excited quantum dot molecules: Controlling g-factors with electric field Invited Speaker: We have recently demonstrated coherent tunneling of electron and hole spin between two quantum dots using optical spectroscopy [1,2]. In the case of a hole spin, a very large and resonant enhancement or reduction of g-factor is controlled with an applied electric field [3]. This effect arises because of the corresponding enhancement or suppression of the hole wavefunction in the tunnel barrier for the bonding (symmetric) and anti-bonding (anti-symmetric) states, respectively. This effect was discovered for single holes, but also occurs for two-particle states (two holes or 1 hole and 1 electron). Using this effect to identify the symmetry of the wavefunction, we have now found that the energetic order of the bonding and anti-bonding molecular states goes through a reversal as a function of tunnel barrier thickness. That is, the bonding state is the low energy state for a 2nm barrier thickness (as expected in the simple particle-in-a-box model, or the one-band effective mass theory). But for thicknesses larger than 3nm, a transition occurs such that the anti-bonding state becomes the low energy state. This dramatic and non-intuitive effect arises from the spin-orbit interaction. \newline \newline [1] ``Optical Signatures of Coupled Quantum Dots,'' E. A. Stinaff \textit{et al}, \textit{Science }\textbf{311}, 636 (2006). \newline [2] ``Spin Exchange in Optically Excited Quantum Dot Molecules,'' M. Scheibner, M. F. Doty, I. V. Ponomarev, \textit{et al}., \textit{PRB} \textbf{75}, 245318 (2007). \newline [3] ``Electrically Tuneable g Factors in Quantum dot Molecular Spin States'' M.F. Doty \textit{et al., Phys. Rev. Lett.} \textbf{97}, 197202 (2006). [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:42PM |
D4.00002: Universal Quantum Gates for Two- and Three-Spin Qubits in Coupled Quantum Dots Invited Speaker: The ability to control the exchange coupling between coupled quantum dots allows for quantum gate operations on quantum dot spin qubits. Supplemented with single-spin rotations, the exchange coupling is universal for quantum computation on qubits that are formed by the spin 1/2 of single electrons. If qubits are formed by two spins, the requirement for single-spin rotations is reduced to the presence of a fixed inhomogeneous magnetic field, while for three spins, the exchange coupling is universal on its own. In this talk, we discuss the implementation of universal gate operations for two- and three-spin qubits in coupled quantum dots. In the case of the two-spin singlet-triplet qubit on a double quantum dot, we propose a set of universal gates that can be generated by controlling the electrostatic potential between the two dots without time-dependent control of the tunnel coupling between the dots [1]. This simplification should facilitate the implementation of quantum gates in the systems that are presently studied experimentally. We present explicit gate sequences for single-qubit rotations about two orthogonal axes, and a CNOT gate sequence, completing the universal gate set. Finally, the trade-off between leakage errors and simple operations will be briefly discussed. \newline [1] R. Hanson and G. Burkard, Phys.\ Rev.\ Lett.\ {\bf 98}, 050502 (2007). [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 4:18PM |
D4.00003: Nondestructive optical probe of coherent single spin dynamics in a quantum dot Invited Speaker: Understanding the coherent dynamics of a single electron spin in a quantum dot (QD) is important for potential applications in solid-state, spin-based quantum information processing. Here, results will be presented focusing on optical detection of a single spin and observation of the temporal evolution of the spin state. First, we demonstrate the detection of a single electron spin in a QD using a continuously averaged magneto-optical Kerr rotation (KR) measurement \footnote{J. Berezovsky, M. H. Mikkelsen, {\it et al.}, {\it Science} {\bf 314}, 1916 (2006).}. In contrast to many other single spin detection schemes, the KR measurement minimally disturbs the system, making it potentially useful for exploring quantum measurementphenomena or spin-photon entanglement. This continuous single QD KR technique is then extended into the time domain using pulsed pump and probe lasers, allowing the observation of the coherent evolution of an electron spin state with nanosecond temporal resolution \footnote{M. H. Mikkelsen, J. Berezovsky, {\it et al.}, {\it Nature Physics} {\bf 3}, 770 (2007).}. This provides a direct measurement of the electron g-factor and spin lifetime, and additionally serves as a sensitive probe of the local nuclear spin environment. Finally, we perform ultrafast coherent optical manipulation of the electron spin state in the QD using the optical Stark effect \footnote{J. Berezovsky, M. H. Mikkelsen, {\it et al.}, {\it submitted} (2007).}, where an off-resonant optical pulse induces rotations of the spin state through angles up to $\pi$ radians on picosecond timescales. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:54PM |
D4.00004: Spin Decoherence and Maxwell Angels Invited Speaker: The advantages of a quantum machine are rooted in the coherent superposition of its states. In the paradigmatic quantum system of a single-electron in the environment of a quantum dot of interacting nuclear spins, how does its spin coherence decay? Is the coherence doomed to dissipate (the H-theorem)? I shall present a theory of decoherence with a simple quantum explanation with no a priori stochastic assumption, based on two solutions [1,3] of the many-body dynamics of the single electron spin and a mesoscopic number of nuclear spins. The theory is followed by a description of the principle of how the coherence lost can be restored by controlling only the electron spin [2,4]. Work done in collaboration with Wang Yao, Renbao Liu, and Semion Saikin. \newline \newline [1] Wang Yao, Ren-Bao Liu, and L. J. Sham, Phys. Rev. B \textbf{74}, 195301 (2006). \newline [2] Wang Yao, Ren-Bao Liu, and L. J. Sham, Phys. Rev. Lett. \textbf{98}, 077602 (2007). \newline [3] S. K. Saikin, Wang Yao, and L. J. Sham, Phys. Rev. B \textbf{75}, 125314 (2007). \newline [4] Ren-Bao Liu, Wang Yao, and L. J. Sham, New J. Phys. \textbf{9}, 226 (2007). [Preview Abstract] |
Session D5: Circuit QED: Superconducting Qubits Coupled to Cavities
Sponsoring Units: DCMP GQIChair: Raymond Simmonds, National Institute of Standards and Technology, Boulder
Room: Morial Convention Center RO1
Monday, March 10, 2008 2:30PM - 3:06PM |
D5.00001: Superconducting qubits coupled to resonant cavities Invited Speaker: Coupling of superconducting qubits to resonant cavities and mechanical oscillators has opened new possibilities for quantum information processing and for the realization of quantum optics in solid-state devices. Together with steady improvements in superconducting qubits [1], this is due to the qubit-resonator coupling which can readily be made very large with respect to all dissipation rates. As a result, these solid-state systems can reach new parameter regimes currently unexplored in atomic based quantum optics [2]. A resonant cavity can also be used as quantum bus allowing entanglement to be generated controllably between qubits coupled to the same cavity, and regardless of the distance separating the qubits [3]. Because of the relatively large size of these cavities, this allows to couple and entangle multiple qubits, opening new avenues for scalable solid-state quantum computation. In this talk, I will review some of the key properties of superconducting qubits and how they can be strongly coupled to various types of resonant cavities. Focusing on superconducting charge qubits coupled to transmission line resonators, I will explain how the quantum state of the qubits can be coherently manipulated and probed by microwave irradiation of the resonator [4]. I will also present some of the recent theoretical proposals for the generation of entanglement in this system [5]. \newline [1] J. Koch, et al. Phys. Rev. A 76, 042319 (2007). \newline [2] D. I. Schuster, et al. Nature 445, 515 (2007). \newline [3] J. Majer, et al. Nature 449, 443 (2007). \newline [4] J. Gambetta, et al. arXiv:0709.4264 (2007). \newline [5] A. Blais, et al. Phys. Rev. A 75, 032329 (2007). [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:42PM |
D5.00002: Coherent manipulation of quantum information using two Josephson phase qubits coupled to a resonant cavity Invited Speaker: We have taken the first step towards the implementation of circuit quantum-electro dynamics (QED) quantum information processing with Josephson phase qubits. We have observed for the first time a coherent interaction between two phase qubits and an LC cavity formed by a ~7 mm long coplanar waveguide resonant at ~9 GHz. When either qubit is resonant with the cavity, we observe the vacuum Rabi splitting of the qubit's spectral line. In a time-domain measurement, we observe coherent vacuum Rabi oscillations between either qubit and the oscillator. Using controllable shift pulses, we have shown coherent transfer of a arbitrary quantum state. We first prepare the first qubit in a superposition state, then this state is transferred to the resonant cavity and then after a short time, we transfer this state into the final qubit. These experiments show that quantum information can be coherently stored and transferred between superconducting quantum bits using a resonant cavity. This opens up new possibilities for performing circuit QED and studying quantum information science. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 4:18PM |
D5.00003: Circuit QED: Coupling Superconducting Qubits via a Cavity Bus on a Chip Invited Speaker: Circuit quantum electrodynamics is a system, which allows us to do new experiments in quantum optics with a superconducting integrated circuit on a chip. In circuit QED, microwave photons are guided and confined by superconducting transmission lines and cavities, and can then be coherently coupled to a transmon qubit. This system leads to much stronger coupling of the ``light'' and ``matter'' than is possible with traditional atomic systems. Making use of that strong coupling it is possible to couple two qubits via the cavity[1]. I will show how one can use the cavity as a coupling bus which provides non-local and non-nearest neighbor coupling. The interaction is mediated by the exchange of virtual rather than real photons, avoiding cavity-induced loss. The same cavity is also used to perform multiplexed control and read-out of the two qubits. The coupling is effectively switchable which allows for time domain transfer of the quantum states between the qubits. [1] Coupling superconducting qubits via a cavity bus, J. Majer, J. M. Chow, J. M. Gambetta, Jens Koch, B. R. Johnson, J. A. Schreier, L. Frunzio, D. I. Schuster, A. A. Houck, A. Wallraff, A. Blais, M. H. Devoret, S. M. Girvin and R. J. Schoelkopf. Nature 449 443 (2007) [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:54PM |
D5.00004: Single artificial-atom maser Invited Speaker: Masers and lasers usually involve ensemble of atoms to be excited and stimulated for emission. As those atoms are only weakly coupled to the cavity mode, a large number of atoms and strong pumping are needed for lasing in order to overcome the cavity loss and the relaxation of atoms due to spontaneous emission into other modes. However, when the coupling becomes strong even a single atom is enough for lasing, as have been demonstrated with atoms in microwave/optical cavities. We have realized an analogous single artificial-atom maser in a superconducting circuit [1]. Josephson-junction charge qubit is used as an artificial atom with a large dipole. The qubit is coupled to a superconducting Nb coplanar-waveguide resonator at around 10~GHz and with a quality factor of 7600. The coupling strength between the qubit and the resonator is 80 MHz. Population inversion is generated by current injection: A current is injected through a voltage-biased electrode attached to the charge qubit via a highly resistive tunnel junction. In the so-called Josephson-quasiparticle process, the qubit is pumped incoherently to the upper state and emits photon into the cavity. This work is in collaboration with O. Astafiev, K. Inomata, A.O. Niskanen, T. Yamamoto, Yu.\ A. Pashkin, and J.S. Tsai. This work has been supported by RIKEN Frontier Research System and CREST-JST. \par Reference: [1] O. Astafiev {\it et al.}, Nature {\bf 449}, 588 (2007). [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:30PM |
D5.00005: Process Tomography of Quantum Memory in a Josephson Phase Qubit Invited Speaker: Quantum memory that can protect qubit states against decoherence is an important piece of a scalable quantum computing architecture. Coupling a qubit to a high-Q harmonic oscillator memory element is one example, but many other quantum systems could serve this role. We have used an atomic two-level state (TLS) in the amorphous AlO$_x$ tunnel barrier of a Josephson phase qubit as a prototype quantum memory element. The frequency-tunability of the phase qubit allows us to switch on and off the qubit-TLS coupling and thereby transfer arbitrary qubit states into the TLS, store them for some time and recall them later. We performed quantum process tomography to completely characterize the memory operation and the errors that occur during the state transfer and recall. The overall process fidelity is 78\%. The dominant operator-sum errors are dephasing-like ($\sim$12\%) and relaxation-like ($\sim$9\%), consistent with the measured T$_1$ and T$_2$ of the TLS. [Preview Abstract] |
Session D6: Long-Distance Charge Transfer in Biological Systems
Sponsoring Units: FIAP DPOLYChair: Nikolai Sergueev, University of Texas at Austin
Room: Morial Convention Center RO4
Monday, March 10, 2008 2:30PM - 3:06PM |
D6.00001: Theory of Electron Transfer and Transport Pathways in Biomolecules Invited Speaker: Electron transfer in proteins and nucleic acids occurs over large distances by a combination of short and long range tunneling mechanisms. Electron tunneling is facilitated by virtual oxidized and reduced states of the bridging macromolecule, and theoretical analysis reveals how a macromolecule's fold, energetics, and fluctuations influence the electron-transfer kinetics. Recent studies of protein electron transfer indicate when and why electron tunneling kinetics is sensitive to the structure of the protein's tunneling pathways. Electron transfer across protein-protein interfaces involves thin structured water layers that play a key role in tunneling mediation as well. Tunneling analysis that takes the dynamical fluctuations of the macromolecules into explicit account provides a unified view that links structure and function in protein electron transfer. In the case of DNA electron transport, a critical role is found for structural fluctuations and transport mediated by carrier injection to intervening bases, even at very short distances. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:42PM |
D6.00002: Long-Range Electron Transfer through Proteins and Solvents Invited Speaker: Reactions in which electrons tunnel long distances from donors (D) to acceptors (A) pervade solid-state physics, chemistry and biology. Theory suggests that the barriers to these tunneling processes depend strikingly on the composition and structure of the intervening medium. Poor coupling across nonbonded interfaces produces a strong bias in favor of covalent and hydrogen-bonded pathways between redox sites in proteins. The coupling disparity between bonded and nonbonded interfaces accounts in large part for the finding that protein electron-transfer rates do not exhibit a uniform dependence on distance, but instead depend critically on the composition of the medium between redox sites. Rates at a single D-A separation can differ by three orders of magnitude and D-A distances that differ by as much as 0.5 nm can produce identical rates. Our investigations of electron tunneling through proteins and solvents are aimed at elucidating the factors that determine long-range D-A couplings. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 4:18PM |
D6.00003: Correlated electron and proton transport in cytochrome c oxidase: Coulomb proton pump with kinetic gating Invited Speaker: I will discuss correlated transport of electrons and protons in cytochrome c oxidase, the terminal enzyme in the respiratory electron transport chain of aerobic organisms. This enzyme catalyzes the reduction of atmospheric oxygen to water in our cells, and utilizes the free energy of oxygen reduction for the creation the membrane proton gradient by pumping protons across the membrane. The proton gradient subsequently drives the synthesis of ATP. The details of the mechanism of this redox-driven proton pump are unknown. Computer simulations and theoretical modeling point to a possible mechanism of this biological molecular machine in which electron transport is coupled to proton translocation. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:54PM |
D6.00004: Theoretical/Computational Probes of Homogeneous and Interfacial Electron Transfer: Electronic Structure and Energetics Invited Speaker: Theoretical and computational techniques are used to elucidate the physical and chemical factors that control the kinetics of homogeneous and interfacial electron-transfer (ET) reactions. These latter include systems for which standard rate constants ($k^{0}(l))$ have been measured electrochemically for ET between substrate Au electrodes and redox couples attached to the electrode surfaces by variable lengths ($l)$ of oligomethylene (OM), oligophenylenevinylene (OPV) and oligophenyleneethynylene (OPE) bridges. These oligomers, spanning a range of $\sim $ 1-4 nm, are components of mixed self-assembled monolayers (SAMs), coupled to the substrate via S atom linkers. The mechanistic analysis of the kinetic behavior, including polaron-based activation and electronic tunneling, is supported by calculations of electronic structure and molecular and medium energetics. Band structure calculations for neat phenylthiolate SAMs on Au and Cu susbstrates were used to probe the properties of the interface, including surface dipole layer and work function, the electronic nature of the `thiolate' linker atoms, and the competition between direct and substrate-mediated coupling. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:30PM |
D6.00005: A nonadiabatic and nonlinear theory for electron transfer Invited Speaker: We propose a general theory both non adiabatic and nonlinear which extends those used for the standard theory of electron transfer (ET) in chemistry but also becomes equivalent to it far from the inversion point. In the vicinity of the inversion point, the model parameters may be finely tuned such that large amplitude electronic oscillations between the donor and an extrasite, associated with large amplitude and collective phonon oscillations at the same frequency, are spontaneously generated (coherent electron–phonon oscillator or CEPO). This extrasite is not a true acceptor but could play the role of a catalyst because by the CEPO it may trigger irreversible and ultrafast ET at low temperature toward a third site which is a real acceptor (while in the absence of catalyst, ET cannot occur). Such a trimer system may be regulated by small perturbations and behaves as a molecular transistor. We illustrate this idea by explicit numerical simulations on trimer models of the type donor-catalyst-acceptor. We discuss the relevance of our approach for understanding the ultrafast electron transfer experimentally observed in biosystems such as the photosynthetic reaction center. [Preview Abstract] |
Session D7: Locomotion in Complex Fluids
Sponsoring Units: GSNP DBPChair: Arshad Kudrolli, Clark University
Room: Morial Convention Center RO5
Monday, March 10, 2008 2:30PM - 3:06PM |
D7.00001: Propulsion in viscoelastic fluids: waving, flapping Invited Speaker: In this talk, we present recent results on low-Reynolds number locomotion in non-Newtonian fluids. We first consider waving motion, the prototypical biological situation arising e.g. in ciliary transport of mucus, or spermatozoa swimming in complex fluids. We use asymptotic methods to estimate the effect of viscoelastic stresses on the kinematics and energetics of locomotion and transport in complex fluids. In our second problem, we consider simple flapping motion. Because of Purcell's scallop theorem, reciprocal motion such as flapping is known to be ineffective in a Newtonian fluid. We show here instead that a fluid with normal stress differences - such as Oldroyd B - can be used to rectify flapping motion and generate non-zero average forces and flows. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:42PM |
D7.00002: Theory of swimming filaments in viscoelastic media Invited Speaker: Microorganisms often encounter and must move through complex media. What aspects of propulsion are altered when swimming in viscoelastic gels and fluids? Motivated by the swimming of sperm through the mucus of the female mammalian reproductive tract, we examine the swimming of filaments in nonlinearly viscoelastic fluids. We obtain the swimming velocity and hydrodynamic force exerted on an infinitely long cylinder with prescribed beating pattern. We apply these results to study the swimming of a simplified sliding-filament model for a sperm flagellum. Viscoelasticity tends to decrease swimming speed. The viscoelastic response of the fluid can change the shapes of beating patterns, and changes in the beating patterns can even lead to reversal of the swimming direction. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 4:18PM |
D7.00003: Undulatory swimming in a viscoelastic fluid Invited Speaker: Mammalian spermatozoa encounter complex, non-Newtonian fluid environments as they make their way through the female reproductive tract. The beat form realized by the flagellum varies tremendously along this journey. We will present recent progress on the development of computational models that couple the internal force generation of undulating flagella with the external dynamics of a complex fluid. An immersed boundary framework is used, with the complex fluid represented either by a continuum Oldroyd-B model, or a Newtonian fluid overlaid with discrete viscoelastic elements. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:54PM |
D7.00004: Large and limbless: the locomotion of snakes Invited Speaker: In efforts to understand snake locomotion, we consider one of their various gaits. By contracting and extending their bodies unidirectionally like a slinky, large snakes propel themselves in a straight line. In a combined experimental and theoretical investigation, we here report on the dynamics of a boa constrictor alongside the analysis of an n-linked extensible crawler model. Constraints on their locomotion are quantified and discussed, such as the elasticity, frictional anisotropy and abrasive wear of their skin. Also presented are certain snake behaviors that culminate in their tying themselves into knots. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:30PM |
D7.00005: Biological and robotic movement through granular media Invited Speaker: We discuss laboratory experiments and numerical simulations of locomotion of biological organisms and robots on and within a granular medium. Terrestrial locomotion on granular media (like desert and beach sand) is unlike locomotion on rigid ground because during a step the material begins as a solid, becomes a fluid and then re-solidifies. Subsurface locomotion within granular media is unlike swimming in water for similar reasons. The fluidization and solidification depend on the packing properties of the material and can affect limb penetration depth and propulsive force. Unlike aerial and aquatic locomotion in which the Navier-Stokes equations can be used to model environment interaction, models for limb interaction with granular media do not yet exist. To study how the fluidizing properties affect speed in rapidly running and swimming lizards and crabs, we use a trackway composed of a fluidized bed of of 250 $\mu m$ glass spheres. Pulses of air to the bed set the solid volume fraction $0.59<\phi<0.63$; a constant flow rate $Q$ below the onset of fluidization (at $Q=Q_f$) linearly reduces the material strength (resistance force per depth) at fixed $\phi$ for increasing $Q$. Systematic studies of four species of lizard and a species of crab (masses $\approx 20$ grams) reveal that as $Q$ increases, the average running speed of an animal decreases proportionally to $\sqrt{M/A-const}(1-Q/Q_f)$ where $M$ is the mass of the animal and $A$ is a characteristic foot area. While the crabs decrease speed by nearly $75 \%$ as the material weakens to a fluid, the zebra tailed lizard uses long toes and a plantigrade foot posture at foot impact to maintain high speed ($\approx 1.5$ m/sec). We compare our biological results to systematic studies of a physical model of an organism, a 2 kg hexapedal robot SandBot. We find that the robot speed sensitively depends on $\phi$ and the details of the limb trajectory. We simulate the robot locomotion by computing ground reaction forces on a numerical model of the robot using a soft-sphere Molecular Dynamics code. [Preview Abstract] |
Session D8: Focus Session: Granular Flows: Vibrated
Sponsoring Units: DFD GSNPChair: Mark Shattuck, City University of New York
Room: Morial Convention Center RO6
Monday, March 10, 2008 2:30PM - 2:42PM |
D8.00001: Particle kinematics in a 3-dimensional vibration-fluidized granular medium Hong-Qiang Wang, Narayanan Menon We report a study by high speed video imaging of particle motions in the bulk of a three dimensional granular gas. We fluidise with intense vertical vibration, delrin spheres of diameter, d=1.6 mm confined in a 3-dimensional volume (32d)$^3$. We isolate particles moving in a thin slice of this volume by illuminating with a laser sheet. We have developed a new algorithm to track with sub-pixel precision particles that are only partially illuminated or eclipsed by other particles. We will present data in the low-volume fraction regime for spatial profiles of the the kinetic temperature and number density, as well as for the velocity distribution. These results will be compared to predictions from hydrodynamic models. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D8.00002: Heating mechanism affects equipartition in a binary granular system Narayanan Menon, Hongqiang Wang Two species of particles in a binary granular system typically do not have the same mean kinetic energy, in contrast to the equipartition of energy required in equilibrium. We investigate the role of the heating mechanism in determining the extent of this non-equipartition of kinetic energy. In most experiments, different species of particle are unequally heated at the boundaries. We show by event-driven simulations that this differential heating at the boundary influences the level of non-equipartition even in the bulk of the system. This conclusion is fortified by studying a numerical model and a solvable stochastic model without spatial degrees of freedom. In both cases, even in the limit where heating events are rare compared to collisions, the effect of the heating mechanism persists. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D8.00003: Energy fluctuation, diffusivity and mobility in a 2D vibrated granular packing Eric Clement, Rim Harich, Nicolas Vandewalle, Geoffroy Lumay We present an experimental realization of a 2D vibrated granular packing. The new agitation method allows a spatially non synchronized influx of energy and the study of the vibrated packing at steady state. By image analysis of fast-camera movies, we obtain the velocity fluctuation spectra at different vertical levels and then, we separate the agitation velocities from the velocity fluctuations corresponding to the ``thermalized'' degrees of freedom. By measuring the corresponding particle diffusivities, we show that, in spite a large heterogeneity and anisotropy of the vibration, a relation between diffusivity and ``thermalized'' kinetic energy can be identified. We relate this type of fluctuation-dissipation relation to the mobility of macroscopic intruders of different sizes and weight moving in the vibrated granular packing. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:42PM |
D8.00004: ``Free Energy" in Vibrated Granular Non-Equilibrium Steady-States. Invited Speaker: Equilibrium statistical mechanics is generally not applicable to systems with energy input and dissipation present, and identifying relevant tools for understanding these far-from- equilibrium systems poses a serious challenge. Excited granular materials or granular fluids have become a canonical system to explore such ideas since they are inherently dissipative due to inter-particle frictional contacts and inelastic collisions. Granular materials also have far reaching practical importance in a number of industries, but accumulated ad-hoc knowledge is often the only design tool. An important feature of granular fluids is that the driving and dissipation mechanisms can be made to balance such that a Non-Equilibrium Steady-State (NESS) is achieved. We present strong experimental evidence for a NESS first-order phase transition in a vibrated two-dimensional granular fluid. The phase transition between a gas and a crystal is characterized by a discontinuous change in both density and temperature and exhibits rate dependent hysteresis. We measure a ``free energy''-like function for the system and compare and contrast this type of transition with an equilibrium first-order phase transition and a hysteretic backward bifurcation in a nonlinear pattern forming system. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D8.00005: Singular Energy Distributions in Granular Media Eli Ben-Naim, Annette Zippelius We study the kinetic theory of driven and undriven granular gases, taking into account both translational and rotational degrees of freedom. We obtain the high-energy tail of the stationary bivariate energy distribution, depending on the total energy $E$ and the ratio $x=\sqrt{E_w/E}$ of rotational energy $E_w$ to total energy. Extremely energetic particles have a unique and well-defined distribution $f(x)$ which has several remarkable features: $x$ is not uniformly distributed as in molecular gases; $f(x)$ is not smooth but has multiple singularities. The latter behavior is sensitive to material properties such as the collision parameters, the moment of inertia and the collision rate. Interestingly, there are preferred ratios of rotational-to-total energy. In general, $f(x)$ is strongly correlated with energy and the deviations from a uniform distribution grow with energy. We also solve for the energy distribution of freely cooling Maxwell Molecules and find qualitatively similar behavior. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D8.00006: Breathing Phenomena in Driven, Confined, Granular Chains Robert Simion, Adam Sokolow, Surajit Sen We consider a tapered granular alignment where the spherical grains progressively shrink in radius by a factor $q$. The system has a hard wall at one end and a piston at the other. We assume that the piston can be used to impart a force $F$ (time- dependent or otherwise) to an edge grain in the system. Extensive particle dynamics simulations and theoretical analysis reveal that such a system could revert back and forth between an oversqueezed state and a dilated state - i.e., ``breathe." The breathing is strongly dependent on the driving. When driven with a constant force, we show that $TF^ {1/6}$ is a constant for fixed $q$. More complex dynamics including nonlinear-resonance is observed when $F=F(t)$. The talk shall discuss the observed dynamical responses of the system. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D8.00007: Numerical Study of Particle Damping Mechanism in Piston Vibration System via Particle Dynamics Simulation Xian-Ming Bai, Binoy Shah, Leon Keer, Jane Wang, Randall Snurr Mechanical damping systems with granular particles as the damping media have promising applications in extreme temperature conditions. In particle-based damping systems, the mechanical energy is dissipated through the inelastic collision and friction of particles. In the past, many experiments have been performed to investigate the particle damping problems. However, the detailed energy dissipation mechanism is still unclear due to the complex collision and flow behavior of dense particles. In this work, we use 3-D particle dynamics simulation to investigate the damping mechanism of an oscillating cylinder piston immerged in millimeter-size steel particles. The time evolution of the energy dissipation through the friction and inelastic collision is accurately monitored during the damping process. The contribution from the particle-particle interaction and particle-wall interaction is also separated for investigation. The effects of moisture, surface roughness, and density of particles are carefully investigated in the simulation. The comparison between the numerical simulation and experiment is also performed. The simulation results can help us understand the particle damping mechanism and design the new generation of particle damping devices. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D8.00008: Breaking of granular jams with mechanical shocks Ke Chen, Andrew Harris, John Draskovic, Peter Schiffer We studied the brief granular flows initiated by breaking the jamming in a hopper using mechanical shocks. Jamming near the orifice of a hopper prevents granular materials from flowing spontaneously under gravity. Controlled mechanical shocks were applied from the bottom of the hopper to break the jamming and to initiate brief flows. The magnitude and the duration of the flows were measured. Preliminary results show that the probability of initiating a flow increases with the intensity of the shock, and reaches almost 100{\%} at the highest shock intensities. We also investigated the flow probability as a function of the ratio between the diameters of the orifice and the bead. Statistical characteristics of the flow magnitude and duration evolve with shock intensity as well as the ratio between the diameters of the orifice and the bead. This research was supported by the NASA through grant NAG3-2384 and the NSF REU program through grant DMR 0305238. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D8.00009: Spreading of a granular droplet Eric Clement, Ivan Sanchez, Franck Raynaud, Jose Lanuza, Bruno Andreotti, Igor Aranson The influence of controlled vibrations on the granular rheology is investigated in a specifically designed experiment in which a granular film spreads under the action of horizontal vibrations. A nonlinear diffusion equation is derived theoretically that describes the evolution of the deposit shape. A self-similar parabolic shape (the``granular droplet'') and a spreading dynamics are predicted that both agree quantitatively with the experimental results. The theoretical analysis is used to extract effective friction coefficients between the base and the granular layer under sustained and controlled vibrations. A shear thickening regime characteristic of dense granular flows is evidenced at low vibration energy, both for glass beads and natural sand. Conversely, shear thinning is observed at high agitation. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D8.00010: 2D granular avalanches with imposed vibrations Brian Utter, Dan Amon We present work on a 2D free surface granular flow experiment under vertical vibration. The experiment consists of photoelastic grains in a 2D circular drum which is rotated at a constant rate (f $<$ 1 mHz). We measure time series of the slope, particle trajectories, and image the bulk force network. Avalanche and build-up distributions exhibit a power-law dependence as previously observed. We then vibrate the drum vertically to determine the effect of external vibrations on this ``unjamming'' transition. While larger vibrations destabilize the pile and decrease the maximum angle of repose, small vibrations lead to a strengthening of the pile and tend to increase the critical angle of failure. In the absence of vibration, when the drum is rotated opposite the direction of steady rotation, the critical angle of the first failure decreases slightly from the steady-state value due to the lack of an established steady-state force network. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D8.00011: Bouncing trimer Stephane Dorbolo, Nicolas Vandewalle Trimers are composed of three stainless steel beads (1 cm of diameter) forming a solid equilateral triangle (2.5 cm of side). They are placed on a plate of an electromagnetic shaker. The system is shaken vertically. According to the acceleration, the trimer may spin, jump once every two periods or even every threee periods. Between these stable regimes, the system is chaotic. By measuring the time delay between two successive shocks (bead-plate), a mapping of the different regimes has been constructed. The spinning, 2-period and 3-period orbits occurs for the same acceleration whatever the frequency. However, the spin speed has been measured with respect of the frequency. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D8.00012: Dynamics of a single particle on a 2D driven granular lattice Jeffrey Olafsen, Kristin Combs, G. William Baxter Previous measurements have demonstrated interesting behavior in a novel bi-layer granular gas experiment of mechanically shaken particles. The results are of importance because the two layers are in ``thermal contact'' and yet have very different dynamical behaviors. The lower layer of particles demonstrates velocity statistics that are strongly correlated and non-Gaussian, while the upper layer of particles concurrently demonstrates a lack of correlations and Gaussian velocity statistics. Details of the collisions within each layer (intralayer) and between the layers (interlayer) are clearly of interest to understand the simultaneous behavior. Measurements are made for a single particle in the upper layer to examine the effects of interlayer collisions. In addition, velocity statistics in both layers are analyzed to determine effects of the sidewalls. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D8.00013: Structure and dynamics of a vibrated granular bead-chain Kevin Safford, Arshad Kudrolli, Yacov Kantor, Mehran Kardar We investigate the dynamics of a vibrated granular bead-chain with experiments and numerical simulations of random-walk models of polymers. Experiments are conducted with a chain composed of hollow 3~mm steel beads connected by flexible links confined to move on a 300~mm diameter rough circular bed. Observations made with digital imaging. We analyze the radius of gyration $R_g$, the structure factor of the chain configurations, and the diffusion of the center of mass. We find that $R_g$ and the structure factor scale with the exponent $\nu \sim 3/4$, consistent with the two dimensional self-avoiding random-walk model. Further, we observe confinement effects in the scaling of $R_g$ as the chain length increases relative to the size of the container. We perform simulations of non-self-avoiding walks confined to the same sized domain and find good agreement with experiment. The simulations show confinement effects dominate over self-avoided crossings in the experiments even when the length of chain is smaller than system size. We then experimentally examine the chain dynamics and find that the center of mass diffusion scales inversely as the length of the chain, consistent with the Rouse model of polymers. We observe an exponential decay in the the dynamical structure factor and compare this exponent with the measurement of the center of mass diffusion. [Preview Abstract] |
Session D9: Focus Session: Turbulence
Sponsoring Units: DFDChair: Haitao Xu, Gesellschaft fuer wissenschaftliche Datenverarbeitung mbH Goettingen
Room: Morial Convention Center RO7
Monday, March 10, 2008 2:30PM - 2:42PM |
D9.00001: Turbulent Viscosity Coefficient in 3-Dimesional Turbulence Hiroshi Shibata A new model for the large-eddy simulation (LES) is proposed. The LES has been accepted as the standard formalism for calculating observables concerning turbulence. In the application of the LES, several models are chosen. The purpose of the present paper is for us to propose one of the most physical models. The LES is usually written down as \begin{equation} \label{eq1} \frac{\partial U_i }{\partial t}+(\vec {U}\cdot \vec {\nabla })U_i =-\frac{1}{\rho }\frac{\partial P}{\partial x_i }+\nu _0 \Delta U_i -\frac{\partial Q_{ij} }{\partial x_j }. \end{equation} The above equation is rewritten as \begin{equation} \label{eq2} \frac{\partial U_i }{\partial t}+(\vec {U}\cdot \vec {\nabla })U_i =-\frac{1}{\rho }\frac{\partial P}{\partial x_i }+\nu \Delta U_i \end{equation} and $\nu $ is referred to as turbulent viscosity coefficient. The statistical mechanical method by Helfand[1] is extended replacing the relationship between the thermal velocity and the kinetic viscosity coefficient by the one between the turbulent velocity and the turbulent viscosity coefficient[2]. The major assumption here is the Gaussian statistics for the turbulent velocity. The concrete calculation using the lattice Boltzmann method is shown for 3-dimesional turbulence. [1] E. Helfand, Phys. Rev. 119,1(1960). [2] H. Shibata, J. Phys. Soc. Jpn. 76,024002(2007). [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D9.00002: Exploring the dynamics of the velocity gradient tensor Marco Martins Afonso, Charles Meneveau The dynamics of the velocity gradient tensor is investigated by means of analytical and numerical computations. Our starting point is the Lagrangian evolution equation of this tensor and a model for the pressure Hessian and viscous term proposed in Chevillard and Meneveau ({\it Phys.~Rev.~Lett.}~{\bf 97}, 174501, 2006). The model is based on the Recent Fluid Deformation (RFD) closure, which was introduced in order to overcome the unphysical finite-time blowup of the Restricted Euler model that neglects anisotropic pressure Hessian effects. Using matrix exponentials, the RFD closure takes into account both the geometry and the dynamics of the recent history of the deformation of a fluid particle, and requires the specification of a decorrelation time scale $\tau$. When this time scale is chosen too short (or, equivalently, the Reynolds number is too high), unphysical statistics are observed in the model. In order to understand this model in greater detail, the original, full matrix-exponential-based model is compared with its power- series expansion for small $\tau$. In particular, the time evolution in the so-called $R$-$Q$ plane is studied for the two approaches, and also, the effects of adding a Gaussian white noise are examined. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D9.00003: The Generalized Fractal Dimensions of a 2-D Compressible Turbulence Jason Larkin, Walter Goldburg, Mahesh Bandi Steady-state turbulence is generated in a tank of water 1m x 1 m x 0.3 m and the trajectories of particles floating on the surface are tracked in time. Initially the floaters are uniformly distributed. As time goes on they coagulate and form a fractal structure. The surface pattern reaches a steady state in approximately $t^*$ = 1 s. In the time interval $0 \alt t \alt 2t^*$, measurements are made of the generalized fractal dimensions $D_q(t)$ of the floating particles starting with the uniform distribution $D_q(0)$ =2. In the steady state, the pattern formed by the floaters continues to fluctuate at a time scale dictated by the underlying turbulent flow. This time scale is also of the order of 1 s. To understand the origin of the coagulation phenomenon, one must remember that the floaters form a compressible system, unlike the water molecules that drive them. The time evolution of the $D_q(t)$ are measured for a range of $q$ less than 10. The coagulated particles form into string-like structures having values of $D_q$ ranging down to approximately 1.5. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:42PM |
D9.00004: Particle Dynamics in Turbulence Invited Speaker: The interaction between particles and turbulence features in many environmental and engineering problems, e.g., the formation of rain, the dispersion of particulate pollutants, and sedimentation in rivers and oceans. In addition, tracer particles are routinely used in scientific research to study the flow itself. Understanding the behavior of particles in turbulent flows is not only an important practical problem, but also an intriguing scientific challenge. Our group has developed a three-dimensional Lagrangian Particle Tracking (LPT) system. Using high speed CMOS cameras, the system is capable of following simultaneously hundreds of particles in a turbulent flow with Taylor microscale Reynolds number $R_{\lambda}$ up to $10^3$. The LPT measurements provide both single- and multi-particle statistics following Lagrangian trajectories, at temporal resolutions better than the Kolmogorov time scales of the turbulence. Using the LPT system, we investigated the Lagrangian properties of turbulence by tracking tracer particles seeded in the flow. In the study of turbulent relative dispersion, our measurement of the separation of pairs of fluid elements in turbulence demonstrated that only when the separation between a time scale related to the initial separation between the pair and the turbulence integral time scale is large enough, or equivalently, at very large Reynolds numbers, the long-believed Richardson's $t^3$ law may be observed. Furthermore, measurements of multiple particles in the flow showed the evolution of geometric structures in turbulence. Due to its ability to follow individual particles, the LPT system is an ideal tool to study the behavior of non-tracer particles in turbulence. The inertial particles have density different from the fluid, but size smaller than the Kolmogorov length scale of turbulence. On the other hand, neutrally buoyant particles with size larger than the Kolmogorov scale behave very differently from inertial particles. We will present results from both cases. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D9.00005: Craig's XY-distribution and the statistics of Lagrangian power in two-dimensional turbulence. Colm Connaughton, Mahesh Bandi We study the probabilility distribution function (PDF) of injected power in numerical simulations of stationary 2D turbulence in the Lagrangian frame. The simulation mimics an electromagnetically driven fluid layer, a well-documented system for generating 2D turbulence in the laboratory. The forcing and velocity fields in the numerics are close to Gaussian, but the injected power PDF is sharply peaked at zero (suggesting a singularity) with asymmetric exponential tails. Large positive fluctuations are more probable than large negative ones leading to a net positive mean energy input. The main features of the power distribution are well described by Craig's XY distribution for the PDF of the product of two correlated normal variables. We show that the power distribution should exhibit a logarithmic singularity at zero and decay exponentially for large absolute values of the power. We calculate the asymptotic behaviour and express the asymmetry of the tails in terms of the correlation coefficient of the force and velocity and compare the measured PDFs with theoretical calculations. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D9.00006: Multiscale Sample Entropy of 2D Decaying Turbulence Ildoo Kim, Matthew Shtrahman, Xiao-Lun Wu Kolmogorov-Sinai entropy has been used to quantify degrees of complexity of spatiotemporally chaotic systems. However, it is not always convenient to implement in real experiments. Recently a Multiscale Sample Entropy (MSE) measure has been proposed, which allows easier analyses of time series. In this study, we have generated decaying turbulence in a two-dimensional soap film and have measured velocity fluctuations as functions of time and downstream distance using a laser Doppler velocimeter. We performed MSE analysis and found there is a time scale $\tau _0 $ at which the MSE is maximized. The value of $\tau _0 $, which correlates well with the large-eddy turn-over time, gets larger as turbulence decays. Other aspects of 2D turbulence are also analyzed using the velocity time series. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D9.00007: Conformal invariance in two-dimensional turbulence Guido Boffetta, Denis Bernard, Antonio Celani, Gregory Falkovich We show that some features of two-dimensional turbulence display conformal invariance. In particular, the statistics of vorticity clusters in the inverse cascade is equivalent to that of critical percolation, one of the simplest universality classes of critical phenomena. Vorticity isolines are therefore described by Stochastic Loewner Equation curves $SLE_{6}$. This result is generalized to a class of 2d turbulent systems, including Surface Quasi-Geostrophic turbulence (which corresponds to $SLE_{4}$) and Charney-Hasegawa-Mima turbulence. The picture emerging from our results is that conformal invariance may be expected for inverse cascades in two-dimensions therefore opening new perspectives in our understanding of 2d turbulent flows. References:\newline D. Bernard, G. Boffetta, A. Celani, and G. Falkovich, Nature Physics {\bf 2} 124 (2006) \newline D. Bernard, G. Boffetta, A. Celani, and G. Falkovich, Phys. Rev. Lett. {\bf 98} 024501 (2007) [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D9.00008: Mixing and entrainment of oceanic overflows: Implications for global climate evolution Robert Ecke, Jun Chen, Philippe Odier, Michael Rivera Oceanic overflows are important elements of the Earth's global thermohaline circulation but the mixing and entrainment that occur for such overflows is poorly understood. In particular, as overflow water moves down an inclined slope its stability is governed by the competition between stratification, which stabilizes the flow, and vertical shear, which tends to destabilize the flow. The properties of our laboratory experiment are designed to mimic oceanic overflows to the extent achievable on laboratory-accessible length scales. The flow exits a nozzle and flows along an inclined plane such that there is gravitational forcing of the flowing gravity current. Velocity and density fields are measured simultaneous using particle image velocimetry and planar laser induced fluorescence. The flow structure and dynamics of mixing at different downstream locations are investigated for a different levels of stratification and shear. The role of turbulence is examined by comparing cases of turbulent and laminar gravity currents. The implication of these results for ocean simulations and for understanding global climate are discussed. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D9.00009: Experimental Investigation of Homogeneity, Isotropy, and Circulation of the Velocity Field in Buoyancy-Driven Turbulence Quan Zhou, Chao Sun, Keqing Xia We present a direct multipoint velocity measurements of the 2D velocity field in the central region of turbulent Rayleigh- B\'{e}nard convection. The local homogeneity and isotropy of the velocity field are tested using a number of criteria and are found to hold to an excellent degree. The distribution of $\Gamma_r$ is found to depend on the scale $r$, reflecting strong intermittency. Besides, the slight asymmetry of the distribution tails reflects the fact that the velocity circulation structure functions (CSFs) are able to capture anisotropic coherent structures, such as thermal plumes, more effectively than longitudinal structure functions (LSFs) and transversal structure functions (TSFs). It is further found that velocity circulation has the same anomalous scaling exponents as LSFs and TSFs for low-order moments ($p<=5$). Whereas, for high-order moments ($p>5$), the anomalous scaling exponents for circulation are found to be systematically smaller than those of LSFs and TSFs. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D9.00010: Flow mode transitions in turbulent thermal convection Heng-Dong Xi, Ke-Qing Xia We report an experimental study of structures and dynamics of the large-scale mean flow in Rayleigh-B\'{e}nard convection cells with aspect ratio ($\Gamma$) 1, 1/2 and 1/3. It is found that both a single circulating roll flow structure and two vertically stacked counter-rotating rolls exist in the three aspect ratio cells. The average percentage of time that the large-scale mean flow spends in the single-roll mode (SRM) and the double-roll mode (DRM) are 87.1\% and $0.8\%$ for $\Gamma = 1$, 69.5\% and 7.9\% for $\Gamma = 1/2$, and 26.7\% and 34.1\% for $\Gamma = 1/3$. Several routes of transitions among the different flow modes are identified. In addition, different structures for the DRM are found and their relative weights are determined. We also show direct evidence that the SRM is more efficient for heat transfer than the DRM. Although the difference is very small, it shows how changes in internal flow state can manifest in the global transport properties of the system. It is also found that the time interval between successive flow mode transitions has an exponential distribution, suggesting a Poisson process for the underlying dynamics. The duration of the flow mode transition is found to be log-normally distributed. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D9.00011: The properties of elastic turbulence in semi-dilute polymer solutions Yonggun Jun, Victor Steinberg We studied elastic turbulence in Karman swirling flow of semi-dilute polymer solution. The concentrations of polymer solution used in the experiment were 100, 300, 500, 700, and 900 ppm, and the velocity fields to calculate the rms of the gradients of the tangential velocity, $\omega_{rms}$, were obtained using PIV. First we checked the saturation of $\omega_{rms}$ in the bulk, which represents the saturation of elastic stress. We found that $Wi_{bulk}=\omega_{rms}\tau$ saturates and approaches to unitary value as the polymer concentration increases. Here $\tau$ is the longest polymer relaxation time. Also we studied existence of the velocity boundary layer which is related to boundary layer of elastic stresses of elastic turbulence. The thickness of the boundary layer is the decreasing function of polymer concentration near the rotating upper plate but independent of concentrations near the wall. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D9.00012: Geometry of plane Couette flow transitional turbulence Predrag Cvitanovic, John Gibson, Jonathan Halcrow We propose to use a hierarchy of exact unstable invariant solutions of the Navier-Stokes equations -- corresponding to the recurrent coherent structures observed in experiments -- to construct a description of the spatio-temporally chaotic dynamics of turbulent fluid flows as a walk through the space of such structures. This description should allow us to obtain quantitative predictions of transport properties of fluid flows such as bulk flow rate and mean wall drag. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D9.00013: The heat transfer of water-based Al$_2$O$_3$ nanofluid in turbulent Rayleigh-B\'{e}nard convection Sheng-Qi Zhou, Rui Ni, Ke-Qing Xia We report experimental measurements of the convective heat transfer in water-based Al$_2$O$_3$ nanofluid in a cylindrical convection cell, which has 19 cm in both height and diameter. The nanofluid has been supplied by Nanophase Technologyies Inc. with an initial volume fraction ($\phi$) 22{\%}. It has been diluted into deionized water to obtain nanofluid of low volume fraction. The nominal diameter of Al$_2$O$_3$ particle is 45 nm. At the fixed heating power, $Q =500 W$, it has been found that the convective heat transfer coefficient ($h=Q/{\Delta T} $, $\Delta T$ is the temperature difference across the cell.) decreases to 2{\%} when $\phi$ varies from 0.03{\%} to 1.1{\%}. At $\phi =1.1{\%}$, we have measured the Nusselt number ($Nu$) as a function of Rayleigh number ($Ra$). It has been found that $Nu$ of nanofluid collapses on the $Nu \sim Ra$ scaling curve of pure water at higher $Ra$ ($4\times10^{9}$ to $1\times10^{10} $). While the deterioration of convective heat transfer has been observed at lower $Ra$ ($8\times10^{8}$ to $4\times10^{9} $), and the deterioration becomes more pronounced with decreasing $Ra$. Additional measurement on the thermal and flow structures is in progress to understand the convective heat transport in nanofluid. [Preview Abstract] |
Session D10: Focus Session: Hybrid Magnetic-Superconducting Systems I
Sponsoring Units: DMPChair: Lance Delong, University of Kentucky
Room: Morial Convention Center RO8
Monday, March 10, 2008 2:30PM - 3:06PM |
D10.00001: Odd-frequency pairing state in superconducting junctions Invited Speaker: We have theoretically studied the induced odd-frequency pairing states in ballistic normal metal/superconductor (N/S) junctions where a superconductor has even-frequency symmetry in the bulk. We demonstrate that the pair amplitude in the junction has an admixture of an odd-frequency component due to the breakdown of translational invariance near the N/S interface [1]. We have also studied about the proximity effect in junctions between diffusive normal metals (DN) and superconductors. It is revealed for spin-triplet superconductor that the resulting symmetry in DN is always odd-frequency spin-triplet [2]. The resulting quasiparticle density of state in DN has a zero energy peak [3]. This unusual proximity effect due to the generation of odd-frequency state is also expected in ferromagnet / superconductor junctions [4]. \newline [1] Y. Tanaka, et al, Phys. Rev. Lett. 99, 037005 (2007), M. Eschirig, et al, J. Low Temp. Phys. \textbf{147} 457 (2007). \newline [2] Y. Tanaka and A.A. Golubov, Phys. Rev. Lett. 98 037003 (2007). \newline [3] Y. Tanaka and S. Kashiwaya, Phys. Rev. B, 70, 012507 (2004). \newline [4] Y. Asano, Y. Tanaka and A. A. Golubov, Phys. Rev. Lett., 98, 107002 (2007). [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D10.00002: Superfluid density in the ferromagnetic layers of superconductor-ferromagnet hybrid structures Thomas Lemberger, Michael Hinton, Adam Hauser, Julian Hetel, Fengyuan Yang, Julia Meyer We have measured the areal superfluid density of superconductor-ferromagnet bilayers and trilayers. Samples are made by sputtering Nb and Ni films sequentially in an ultrahigh vacuum chamber with base pressure $<$10$^{-9}$ torr. Interfaces are cleaner when Ni is sputtered onto Nb for reasons related to disorder at the initial growth of Nb films. Superfluid density is measured using a low-frequency (50 kHz) two-coil technique with coils on opposite sides of the sample. These measurements provide the true T$_{C}$, i.e., the temperature below which superfluid exists. We find a nonmonotonic dependence of T$_{C}$ on ferromagnetic layer thickness, in agreement with resistive measurements of T$_{C}$. The superfluid density is also nonmonotonic. Even at large ferromagnetic layer thicknesses where T$_{C}$ is essentially constant, the areal superfluid density continues to increase, indicating that superfluid extends deeply into the ferromagnet layers. We will discuss these measurements in the context of theory of the superconductor-ferromagnet proximity effect. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D10.00003: Study of Inverse Proximity Effect in Ferromagnet/ Superconductor Bilayers Using a Sagnac Interferometer Jing Xia, A. Palevski, A. Kapitulnik It was recently proposed theroretically that ferromagnetic order can be induced in the superconductor in a ferromagnet/superconductor ($S/F$) bilayer structure through a so called inverse proximity effect. The proposal predicts a sizable magnetic moment in the ``S" layer that couples antiferromagnetically to the moment in the ``F" layer due to Cooper pairs near the interface formed with one electron in the F layer and one in the S layer. The induced magnetic moment is expected to penetrate the superconductor over a size of the Cooper pairs, i.e. $\xi_s$. In order to directly test this interesting scenario, we fabricated Ni/Pb and Ni/Al bilayer samples and probed the possible induced magnetic moments in the ``S" layer through high resolution Surface Magneto Optical Polar Kerr Effect (PKE) measurements on the ``S" layer side through the bilayer's $T_C$ using a Sagnac interferometer. The thickness of the ``S" layer was fabricated to be larger than the optical skin depth in order to make sure that our experiment doesn't pickup up any magnetic moment from the ``F" layer. $\xi_s$ dependence of the effect is studied by comparing the results in Ni/Pb and Ni/Al samples. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D10.00004: Investigation of the role of Al/CoFe interface in proximity effect of Nb/Al/CoFe K. Char, Junhyung Kwon, Wenjian Lu When a few nm-thick Al layer is inserted between Nb and ferromagnetic (F) layers such as CoFe, Ni, or CuNi, the superconducting critical temperature of the trilayers increase rapidly almost to a level of Nb/Al bilayers. In order to understand the role of Al/CoFe interface in the proximity effect of Nb/Al/CoFe, we have added Mg and Au scattering centers and found different behavior. The Mg scattering center did not change the critical temperature behavior of Nb/Al/CoFe, while the Au scattering centers reduced the critical temperature. The results point toward the importance of spin-orbit scattering. In addition, tunneling spectroscopy data on Nb/Al/F vs. on Nb/F will be presented in order to further characterize the role of Al/F interfaces. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D10.00005: Magnetic property change by superconductivity in Py-Nb Hybrid system David Murakami, Michael Hetman, Jiyeong Gu Recently ferromagnet/superconductor systems have attracted a great attention due to their scientific interest and potential for the technological application. So far, most of the work focused on the superconducting property change by magnetism in the hybrid system, and only few researches focused on the magnetic property change by superconductivity. In this presentation we will focus on the magnetic property change of the system by superconductivity when the system goes through the superconducting transition. We have investigated different types of hybrid structures including Py/Nb bilayer, multilayer, and composite systems. To separate out the signal from superconductor, we also measured the single Nb film. We measured the magnetization as a function of temperature and compared it between in a normal state and a superconducting state for the different types of structures. It showed strong magnetic history dependence. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D10.00006: Proximity effect in MgB$_{2}$-Permalloy hybrid system April O'Brien, Jiyeong Gu Recently ferromagnet/superconductor systems have attracted a great attention due to their scientific interest and potential for the technological application. So far, most of the work has been done for Nb with ferromagnetic metals or high temperature oxide superconductors with magnetic oxides. In this presentation we will discuss the proximity effect in magnesium diboride (MgB$_{2})$/Py thin films, where the superconducting property changes due to the adjacent Py layer. Even though the good electronic properties such as large critical current density and large coherence length of MgB$_{2}$ make it a good candidate for superconducting electronic applications, depositing a good-quality \textit{in-situ} MgB$_{2}$ thin film is still challenging. Especially for the multilayer deposition, it is ideal to deposit all the layers \textit{in-situ} without \textit{ex-situ} treatment. We used a sputtering to deposit the MgB$_{2}$ layer \textit{in-situ} and made a hybrid system with Py. We found that the substrate temperature during the MgB$_{2}$ deposition is the most important parameter to determine the superconducting transition temperature of the sample. In addition to the proximity effect we will also discuss the problem with a sputtering method for MgB$_{2}$ deposition. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D10.00007: Spin-switch effect in Permalloy-Niobium hybrid system Jiyeong Gu, Michael Hetman Proximity effect in ferromagnet ($F)$/superconductor ($S)$ systems has become a center of attention recently. It has been well known that the superconducting property is modified when the magnetization of the adjacent $F$ layer changes. Especially when the superconducting transition temperature changes as a function of the magnetization of the $F$ layers, the system can work as a switching device. In this presentation we compare three different Py-Nb hybrid structures; Py/Nb bilayer, Py/Nb/Py/FeMn spin valve, and Nb/Py/Nb/Py/FeMn ($S-F$-\textit{F$'$}) structure. We discuss the similarity and the difference in magnetic and transport properties of these three structures. When the Nb layer is thick in bilayer or $S-F-F'$ structure, the device showed a change between zero resistance (superconducting state) and finite resistance (normal state) by applying a small external magnetic field, where the shift in transition temperature is greater than the transition width, and this gave a huge magnetoresistance effect ([$R$(0)-$R(H)$]/$R$(0)=$\infty )$. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D10.00008: Anomalous transport property in the single-crystal Co nanowire with superconducting electrodes Jian Wang, Nitesh Kumar, Mingliang Tian, Qi Zhang, Jainendra Jain, Thomas Mallouk, Moses H.W. Chan Transport measurements were made on individual single-crystal Co nanowire with four focused ion beam (FIB) deposited tungsten (W) electrodes, which are superconducting below 5 K. It was found that the 2 microns long Co nanowire shows a sharp and large resistance peak near the onset transition temperature (T$_{C})$ of W and a rapid resistance drop below T$_{C}$. The large, 50{\%} resistance drop at low temperature suggests the proximity effect from superconducting W electrodes extends to a long fraction of the ferromagnetic Co nanowire. The resistance peak is not seen in the Au nanowire with same superconducting W electrodes. Measurements on a Co nanowire contacted with FIB deposited non-superconducting Pt electrodes show no change in resistance. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D10.00009: Transport properties of hybrid superconductor/ferromagnet nanowires fabricated by electrodeposition. Nitesh Kumar, Jian Wang, Qi Zhang, Mingliang Tian, Moses H.W. Chan We have fabricated multilayer nanowires with alternating superconducting and ferromagnetic segments using template-based electrodeposition. Nanowires are fabricated with different diameters and length, with individual segments on the length scale of few hundreds of nm to few microns, using both porous polycarbonate and anodized alumina membranes. We have used Pb as the superconducting and Co or Ni as the ferromagnetic components. Structural characterizations done with X-ray diffraction and Transmission Electron Microscope demonstrated that Pb and Co segments are good single crystal whereas Ni segments are polycrystalline. We have done electrical transport measurements on arrays of multilayer nanowires (embedded inside the template) showing interesting magnetoresistance behaviors below the superconducting transition temperature of Pb. Four terminal electrical measurements on a single multilayer nanowire are in progress. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D10.00010: Ferromagnetic Josephson Resonance Ivana Petkovic, Marco Aprili Ferromagnetic Josephson junctions with negative ($\pi -)$ coupling behave as phase sources, a potentially very useful component of quantum electronics. In order to elaborate sophisticated circuits, it is crucial to understand the interplay between spin- and superconducting phase dynamics. For that purpose, we fabricated strongly underdamped sub-micron Josephson junctions. We measured the critical current at zero voltage as a function of the applied magnetic field. The finite magnetization in the junction induces a shift in the Fraunhofer pattern which is invariant under time reversal. For a voltage such that the Josephson frequency matches the ferromagnetic resonance, we observe a reduction of the critical current due to the absorption of the Josephson radiation by the ferromagnetic layer. We have investigated the effect of an external microwave radiation and of the magnetic field. The resonances appear as satellites at every Shapiro step and they are shifted in energy by the magnetic field as expected. The high sensitivity of the ac Josephson effect to a small amount of spins opens up new routes for ESR in nano-magnetism. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D10.00011: Theory of spin wave excitation by Josephson current in a superconductor/ferromagnet/superconductor junction Shin-ichi Hikino, Michiyasu Mori, Saburo Takahashi, Sadamichi Maekawa The Josephson effect in a superconductor/ferromagnet/superconductor junction has been of considerable interest in recent years. Current-voltage (I-V) characteristics of superconducting weak links are studied by the resistively shunted junction (RSJ) model, which describes phase dynamics of superconductors (SC). The ferromagnet (F) has spin waves (SW). Therefore, in an S/F/S junction, it is important to treat the spin- and phase dynamics in an equal footing. However, the spin dynamics has not received much attention in the study of an S/F/S junction. ~We study the effect of the spin dynamics on the phase dynamics in an S/F/S junction. The RSJ model is extended to include the spin dynamics using gauge invariant phase difference between superconducting leads. We find that the I-V characteristics show step structures. The voltage at the steps is proportional to the SW energy in F. The origin of step structures will be discussed. [Preview Abstract] |
Session D11: Inhomogeneous Superconductors and Transport
Sponsoring Units: DCMPChair: Don Gubser, Naval Research Laboratory
Room: Morial Convention Center RO9
Monday, March 10, 2008 2:30PM - 2:42PM |
D11.00001: Evidence of Spatially Inhomogeneous Pairing on the Insulating Side of a Disorder-Tuned Superconductor-Insulator Transition K. H. Sarwa B. Tan, Kevin A. Parendo, Yen-Hsiang Lin, Allen M. Goldman The effect of a perpendicular magnetic field on disordered, amorphous and insulating indium oxide thin films has been investigated. The temperature dependence of the resistance in zero magnetic field, the magnetoresistance as a function of temperature, and the nonlinear current-voltage characteristics have been interpreted as evidence of the presence of superconducting islands on the insulating side of a chemical and/or structural disorder-tuned superconductor-insulator transition. The behavior is very similar to that observed in granular films which are composed of macroscopic grains of superconductor embedded in an insulator. However, characterization studies indicate that the films are fully connected, structurally homogeneous, and not granular. These results support theoretical models in which the destruction of superconductivity by disorder produces spatially inhomogenous pairing with a spectral gap. This work was supported in part by the National Science Foundation under grant NSF/DMR-0455121. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D11.00002: Disorder, Metal-Insulator crossover and Phase diagram in high-T$_{c}$ cuprates Florence Rullier-Albenque, Henri Alloul, Fedor Balakirev, Cyril Proust We have studied the influence of disorder induced by electron irradiation on the normal state resistivities $\rho $(T) of optimally and underdoped YBa$_{2}$Cu$_{3}$O$_{x}$ single crystals, using pulsed magnetic fields up to 60T to completely restore the normal state. We evidence that point defect disorder induces low $T$ upturns of $\rho $(T) which saturate in some cases at low $T$ in large applied fields as would be expected for a Kondo-like magnetic response. Moreover the magnitude of the upturns is related to the residual resistivity, that is to the concentration of defects and/or their nanoscale morphology. These upturns are found quantitatively identical to those reported in lower $T_{c}$ cuprates, which establishes the importance of disorder in these supposedly pure compounds. We therefore propose a realistic phase diagram of the cuprates, including disorder, in which the superconducting state might reach the antiferromagnetic phase in the clean limit. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D11.00003: Drag resistance in bilayer disordered superconducting thin films Yue Zou, Gil Refael, Jongsoo Yoon Highly disordered superconducting thin films exhibit a variety of novel phenomena, such as a possible metallic phase intervening the superconducting and the insulating state, and a huge peak in the magnetoresistance curve. Different theories have been proposed, including the quantum-vortex theory, the percolation picture of superconducting islands embedded in a normal metal, and the Bose metal theory. We propose that a drag resistance measurement in a bilayer setup would easily be able to determine which of the models applies. In such an experiment, two thin film superconductors are fabricated parallel to each other, separated by a thin insulator. A current bias is applied in one layer, and a voltage appears in the other due to the interaction between vortices (as in a Giaever transformer), or charge carriers (e.g., Coulomb drag), in different layers. Our calculation of the drag resistance in the various pictures will be discussed. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D11.00004: Granularity-induced field-hysteresis of transport critical current in patterned coated conductors A. A. Gapud, A. Khan, D. K. Christen, F. A. List III, R. Feenstra In superconducting coated conductors such as RABiTS and IBAD films, intergranular misorientations have been effectively minimized, but a small number of local, higher-angle misorientations remain. One important effect of such weak links is the hysteresis of the critical current density $J_{c}$ with respect to applied field $H$, brought about when large circulating currents trapped within adjacent grains produce a focused field within the grain \textit{boundaries }(GB's) which can partially cancel out $H$ when applied field is \textit{decreasing}, thus shifting the maximum $J_{c}$ from zero $H $to a finite field where the local field at the GB is at minimum. This effect has been seen recently in measurements of magnetization (\textit{induced}) currents, but has not been documented using transport (\textit{applied}) current. However, in samples that are \textit{patterned} into conduits 200 $\mu $m wide or less, the hysteretic effect on transport $J_{c}$ is clearly seen. This discrepancy between `magnetization $J_{c}$' and `transport $J_{c}$' may be due to differences in voltage criterion between the two types of measurement, as will be discussed. Systematic measurements and analyses will be presented, along with ramifications for applications. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D11.00005: Disorder induced resitivity upturns in metallic cuprates Brian M. Andersen, Wei Chen, Peter J. Hirschfeld We propose that experimentally observed resistivity upturn of cuprates at low temperature may be explained by properly accounting for the effects of disorder in a strongly correlated metallic host. Calculating DC transport using real space exact diagonization of a Hubbard model treated in an inhomogeneous Hartree-Fock approximation, we find that correlations induce magnetization around impurities with screening length which increases strongly as temperature decreases, giving rise to additional magnetic scattering which causes the resistivity upturn. This paramagnetic response together with the electronic band structure effect is capable of explaining the magnetoresistance as observed in disordered optimally doped YBCO. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D11.00006: Interrelations between superconducting properties and defect evolution in neutron irradiated YBa$_{2}$Cu$_{3}$O$_{7}$. Viorel Sandu, Gheorghe Aldica, Elena Sandu, Petru Nita We investigated the effect of neutron irradiation on the electric and magnetic properties, including the enhancement of the critical current density, of ceramic YBa$_{2}$Cu$_{3}$O$_{7}$(LiF)$_{x}$ samples. The superconducting properties of the virgin samples reach the optimal values for $x$ = 0.04. Up to the same $x$ value, the neutron irradiation up to 5$\times $10$^{17}$ neutrons/cm$^{2}$ enhances the superconducting response (critical temperature, transition width, etc) and double up the magnetic irreversibility. For $x \ge $ 0.08, all the superconducting properties are depressed whereas the increase of the irreversible magnetization is still present but less spectacular. The analysis of this behavior suggests a self-organization of the defects at low LiF content while for $x \ge $ 0.08 they are uniformly distributed within sample. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D11.00007: Negative Nernst effect in simulations of granular superconductors Andreas Andersson, Jack Lidmar The Nernst effect has recently become an important probe of superconducting fluctuations in high-Tc superconductors. The sign of the Nernst coefficient $\nu=E_y/(-B\partial_x T)$ is positive for ordinary vortex motion down an applied temperature gradient. Here we consider simulations of granular superconducting thin films in the vortex liquid regime. We find that the Nernst coefficient can become negative for certain magnetic fields. We attribute this observation to the motion of vortex vacancies in an otherwise pinned vortex solid. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D11.00008: Large oxygen-isotope shift above the quantum critical point of Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ John Mann, Pieder Beeli, Guo-meng Zhao We have studied the oxygen-isotope effect on the superconducting transition temperature $T_{c}$ in overdoped Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ with $x$ = 0.10, 0.20, and 0.25. We find the oxygen-isotope exponent $\alpha_{O}$ to be small ($\sim$0.02) for $x$ = 0.10 but substantial ($\sim$0.1) for $x$ = 0.20 and 0.25. The doping level above which $\alpha_{O}$ increases sharply coincides with a quantum critical point where the normal-state pseudogap starts to diminish. The present isotope-effect experiments provide direct and quantitative constraints on the pairing mechanism of high-temperature superconductivity in cuprates. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D11.00009: Isotope Effect in High-Tc Superconductors Dale Harshman, John Dow, Anthony Fiory For high-Tc superconductors in which transition temperatures, Tc, are reduced by doping, the oxygen isotope effect (OIE) coefficient in Tc is shown to increase systematically with the pair-breaking rate and with the valence difference between the substituted and native ions. Moreover, the OIE tends to zero as one approaches optimum (or ideal) stoichiometry at which the quality of the superconducting condensate is maximized. In materials with isovalent substitutions, e.g., Sr for Ba or Zn for Cu in YBCO, the small OIE of the parent compound is magnified, owing to pair-breaking disorder. In materials with heterovalent substitutions, e.g., La or Pr for Ba, where carrier densities are necessarily changed, pair breaking induces a much larger OIE. A seminal case is Pr-doped YBCO, where the decrease in Tc observed with Pr doping arises from pair-breaking caused by Pr-on-Ba-site defects. Without the defects, Tc is invariant, providing strong evidence against phononic mechanisms. The fact that Tc drops when Pr substitutes for Ba, but not for Y, indicates that the superconducting hole condensate resides in the BaO layers, where pair-breaking degrades Tc and dramatically increases the OIE. Superconductive pairing modeled on Coulomb coupling between the hole and the electron layers is shown to resolve the shortcomings in electron-phonon interactions. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D11.00010: Rich Behavior of the Critical Currents of Magnetically-Doped Superconducting Films in Applied Magnetic Fields Jeffrey Wasserman, Nina Markovic Films of MoGe were grown with varying dopant levels of cobalt contamination. Critical currents were measured in the presence of magnetic fields applied parallel to the film plane and perpendicular to current flow. Critical current curves reveal significantly different behavior depending on the polarity of the current with respect to the applied field. At sufficiently-high dopant concentrations, the critical current increases with increasing magnetic field intensity for one polarity of current. We will discuss these results in terms of pair-breaking effects of magnetic fields and magnetic impurities. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D11.00011: Analytic description of the transport $J_{c}(B)$ dependence of HTS thin films in small magnetic fields Jens H\"anisch, Francesco Grilli, Sebastian Engel, Bernhard Holzapfel Often, e.g. for deconvolution processes and field distribution calculations, an analytical function for the $J_{c}(B)$ dependence of high-$T_{c}$ thin films is needed. The parameters of these functions should still have a physical meaning regarding the intrinsic and extrinsic sample properties. Starting with the modified Kim model, described by Xu \textit{et al}., we found an excellent function by introducing a sharpness parameter $\beta $. This parameter describes the shape of $J_{c}(B)$ between the low-field plateau (single vortex pinning regime) and the power-law dependence at higher fields. The temperature dependence of all fitting parameters will be discussed. Furthermore, the importance of the field dependence of the $n$ value for distinguishing different pinning regimes will be illustrated. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D11.00012: Investigation of Vortex Pinning Anisotropy in the High Temperature Superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ Andra Petrean-Troncalli, Lisa Paulius, Heather Quantz, Valentina Tobos, Wai -K Kwok Columnar defects have proven to be highly effective at pinning vortices, but most studies have been performed with the defects oriented either \textit{perpendicular} or at large angles relative to the superconducting Cu-O planes. These studies have shown that the intrinsic pinning anisotropy of the crystal can actually be reversed by sufficiently strong columnar defects oriented \textit{perpendicular} to the Cu-O planes. We have preliminary data that indicate that the pinning anisotropy is actually enhanced for columnar defects introduced \textit{parallel} to the superconducting Cu-O planes. A single crystal of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ was polished down to a narrow width of 27 $\mu $m, allowing heavy ions to penetrate the crystal along the ab-plane. The crystal was irradiated with 1.4 GeV $^{208}$Pb$^{56+}$ ions to a dose matching field of 1T. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D11.00013: Conductance of d-wave superconductor/normal metal/d-wave superconductor junctions Dmytro Pesin, Anton Andreev, Boris Spivak We develop a theory of the low-temperature conductance of superconductor/normal metal/superconductor junctions in which the superconductors have d-wave pairing symmetry. We show that at low temperatures the conductance of the junction is determined by the inelastic relaxation time of quasiparticles in the bulk of d-wave superconductors, $G_{DND}\propto \sqrt{\tau^ {(d)}_{\epsilon}}$. Thus it greatly exceeds the conductance of the normal metal part of the junction, which is controlled by the elastic mean free path. This dependence of $G_{DND}$ on the inelastic relaxation time should be contrasted with that of the low-temperature conductance of the junction in the case of the s- wave superconductor leads, $G_{SNS}$. In the latter case the conductance is proportional to the first power of the inelastic electron relaxation time in the normal metal part of the junction, $G_{SNS}\propto \tau_{\epsilon}^{(n)}$ [1]. \newline [1] S. V. Lempitskii, Sov. Phys. JETP {\bf{58}}, 624 (1983); U. Gunsenheimer and A. D. Zaikin, Phys. Rev. B{\bf{50}}, 6317 (1994); F. Zhou and B. Spivak, JETP Lett. {\bf{65}}, 369 (1997). [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D11.00014: Interface superconductivity in bi-layers of insulating and overdoped metallic La$_{2-x}$Sr$_{x}$CuO$_{4 + \delta}$ Adrian Gozar, Gennady Logvenov, Anthony Bollinger, Ivan Bozovic We report on properties of thin superconducting (SC) sheets obtained in La$_{2-x}$Sr$_{x}$CuO$_{4 + \delta}$ bi-layers of overdoped, non-superconducting $(x = 0.45)$ and insulating $(x = 0)$ films grown by molecular beam epitaxy. Superconductivity is confined to a thickness of $\approx$ 2 nm from the interface. The observed transition temperatures have values $T_{c} \approx 15$~K, 30 K and 50 K depending on the layering sequence and oxidation state of the insulating material. Transport measurements are used to determine the screening properties of the quasi-2D SC sheets and the data in bi-layers are compared to results from single-phase films and bulk crystals. [Preview Abstract] |
Session D12: Theory of Ferroelectric, Multiferroic, and Other Structural Transitions
Sponsoring Units: DCMPChair: Craig Fennie, Argonne National Laboratory
Room: Morial Convention Center 203
Monday, March 10, 2008 2:30PM - 2:42PM |
D12.00001: Dynamics of domains switching in epitaxial BaTiO$_3$/SrTiO$_3$ superlattices from first principles Sergey Lisenkov, Inna Ponomareva, Laurent Bellaiche Superlattices (SL) consisting of alternating layers of perovskite oxides can possess properties that are dramatically different from those of bulk ferroelectrics. [BaTiO$_3$]$_n$/[SrTiO$_3$]$_n$ (BT/ST) SL with relatively large periods exhibit novel nanostripe domains for some specific epitaxial strains and within a particular temperature window [1]. Here, an effective Hamiltonian approach is used within molecular dynamics method to predict the evolution of these nanostripe domains in BT/ST SL under an {\it ac} electric field applied along the SL growth direction. For any investigated frequency, four different regions occur, depending on the magnitude of the electric field: Region I that consists of nanostripe domains in both BT and ST layers; Region II that exhibits nanostripe domains in BT layers while possessing monodomains in ST layers; Region III where bubble domains in BT layers coexist with monodomains in ST layers; and Region IV where monodomains form in both BT and ST layers. The dependency of the domain velocities, activation and critical fields on the field frequency is revealed. [1] Lisenkov et al., {\em Phys. Rev. B}, {\bf 76}, 020102(R) (2007). [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D12.00002: Ferromagnetic-like closure domains in ferroelectric ultrathin films: First principles simulation Javier Junquera, Pablo Aguado-Puente We simulate from first-principles the energetic, structural, and electronic properties of ferroelectric domains in ultrathin capacitors made of a few unit cells of BaTiO$_3$ between two metallic SrRuO$_3$ electrodes in short circuit. The domains are stabilized down to two unit cells, adopting the form of a domain of closure, common in ferromagnets but only recently detected experimentally in ferroelectric thin films. The domains are closed by the in-plane relaxation of the atoms in the first SrO layer of the electrode, that behaves more like SrO in highly polarizable SrTiO$_3$ than in metallic SrRuO$_3$. Even if small, these lateral displacements are essential to stabilize the domains, and might provide some hints to explain why some systems break into domains while others remain in a monodomain configuration. An analysis of the electrostatic potential reveals preferential points of pinning for charged defects at the ferroelectric-electrode interface, possibly playing a major role in films fatigue. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D12.00003: The theory of domain patterns in ferroelastics Allan Jacobs The theory of ferroelastic domain patterns is well developed and it explains qualitatively the domain patterns observed experimentally in many materials. It is known that much of the complexity of these patterns, which differ remarkably from those in conventional order-parameter systems, results from the differential rotation associated with domain walls. But there has been no direct confrontation between theory and experiment at a more quantitative level. I propose here such a confrontation. Specifically, I shall present predictions (obtained from numerical work) of the atomic displacements at the collision of orthogonal domain walls in tetragonal-orthorhombic ferroelastics and suggest that they be compared with HREM images of these materials. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D12.00004: Peculiar ordering in flat ferroelectric nanoparticles driven by lattice misfit Ivan Naumov, Alex Bratkovsky Flat \emph{free-standing} ferroelectric (FE)\ nanopartilcles tend to have a vortex-like polarization ordering with the in-plane polarization that curls around an out-of-plane vortex core axis. The question is, if such a structure is still a ground state in presence of noticeable misfit strains induced by a substrate, and whether the 180$^{\circ}$ stripe domains may form, similarly to the case of ultra thin FE films? Here we perform an \textit{ab initio} based study of disk- shaped Pb(Zr$_{1-x} $Ti$_{x}$)O$_{3}$ and BaTiO$_{3}$ nanoparticles that have the vortex ground state when no stress is applied. Our study leads to the following findings for the disks having circular and square footprint: (i) under strong enough compressive strains the curling state is no longer stable and yields to a multi-domain structure with an out-of-plane polarization, and different possible in-plane domains:\ triangle, stripe-like, or ``bubble'' in shape, (ii) each separate domain, regardless of its shape, runs through the entire thickness of a disk, and (iii) the 180$^{\circ}$ stripes occur only under special conditions depending on the shape and chemical composition of the nanostructures. Further, we discovered that starting with the vortex state and then increasing the compressive strains may lead to a metastable bi- or multi-domain phase different from the ground state obtained by gradual cooling at a fixed strain. This leads to a novel hysteretic behavior as a function of the misfit strain. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D12.00005: Static and Dynamic Properties of Ferroelectric Nanostructures and Multiferroic Bulk Systems: A Multiscale Approach T. Michael, S. Trimper, J.M. Wesselinowa Ferroelectric nanostructures and multiferroic bulk systems are studied in a multiscale approach. The excitation energy, associated damping of ferroelectric modes and polarization are presented as a function of temperature, defect concentration, size and shape. The softening of the mode is strongly influenced by the kind of doping ions, the surface configuration and the defect composition. The analysis is based on a modified Ising model in a transverse field. A Green's function technique in real space provides the static and dynamic properties, which differ significantly from the bulk behavior. Additionally, a mesoscopic approach is carried out similar to the Landau-Lifshitz equation with Gilbert damping for ferromagnets. The temperature dependence of the damping parameters is discussed. The analysis is extended to multiferroic bulk systems, where the magnetic moments interact via the Heisenberg model and the multiferroic coupling term differs for hexagonal and orthorhombic materials. We present the dielectric function and the dynamic properties of the coupled model by applying previous methods. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D12.00006: A new order parameter in complex dipolar structures Sergey Prosandeev, Laurent Bellaiche Microscopic models have been used to reveal the existence of a new order parameter that is associated with many complex dipolar structures in magnets and ferroelectrics. This overlooked order parameter involves a double cross product of the local dipoles with their positions. It provides a measure of subtle microscopic features, such as the helicity of the two domains inherent to onion states, curvature of the dipolar pattern in flower states or characteristics of set of vortices with opposite chirality (e.g., distance between vortices' centers and/or magnitude of their local dipoles). This work is mostly supported by DOE grant DE-FG02-05ER46188. We also acknowledge support from ONR grant N00014-04-1-0413 and NSF grants DMR-0701558, DMR-0404335 and DMR-0080054 (C-SPIN). Some computations were made possible thanks to the MRI Grants 0421099 and 0722625 from NSF. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D12.00007: Microscopic origin of Magnetic Ferroelectrics in Non-collinear Multiferroics Chen Fang, Jiangping Hu We propose a microscopic mechanism to understand the origin of magnetoelectric coupling in the best known multiferroic family $R$MnO$_3$ ($R$=Tb, Dy...). The mechanism lies in the vanishing of electric current in an insulator. A spontaneous electric polarization is thus necessary, as it causes an electric current through a spin-orbit coupling to cancel another nonzero local electric current induced by a non-collinear modulated magnetic structure. Within this counter-balance mechanism, the magnitude of the ferroelectric order is determined by the magnetic order parameter and the spin-orbit coupling strength. Based on the theory, we predict a general physical limit for the value of ferroelectricity. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D12.00008: A theory for the multiferroic compound LiCu$_{2}$O$_{2}$ Trinanjan Datta, Chen Fang, Jiangping Hu We investigate the possible coupling between ferroelectricity and magnetic structure in the zig-zag spin chain compound LiCu$_{2}$O$_{2}$. Based on a group theory analysis, we construct a multi-order parameter phenomenological model and show that a coupling involving the inter-chain magnetic structures and ferroelectricity is necessary in order to understand the experimental results of Park \emph{et. al}. The model is able to account for the electric polarization flip through $\pi /2$ and explain the occurrence of an electric polarization parallel to an applied external magnetic field. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D12.00009: Phase-field model of strain-induced grain-boundary premelting Nan Wang, Robert Spatschek, Alain Karma Grain-boundary premelting depends in a complex way on the relative magnitude of the solid-liquid interfacial free-energy and grain boundary energy as well as temperature and strain. We study this dependence in a bicrystal geometry using a phenomenological three-order parameter phase-field model. This model describes the short scale attractive or repulsive interaction between crystal-melt interfaces and macroscopic linear elasticity including the important effect of the density contrast between solid and liquid. The model exhibits a rich behavior characterized by single or multiple premelting transitions between dry or wet grain boundaries with different liquid layer thicknesses as a function of applied tensile stress. The results have important implications for the phenomenon of liquid metal embrittlement associated with the stress-driven penetration of nanometric liquid films along grain boundaries. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D12.00010: Incorporation of plasticity into the Landau-Ginzburg theory of martensitic phase transformations Roman Gr\"oger, Turab Lookman The Landau-Ginzburg theory of martensitic phase transformations has been utilized to reproduce the evolution of elastic texture in defect-free materials undergoing structural phase transformations. Generalizations of this theory to phase transformations that are accompanied by significant plastic distortions (as in U$_6$Nb) have been little studied. We propose a simple model that demonstrates how to incorporate plasticity into the Landau-Ginzburg theory. In the presence of topological defects such as dislocations, the usual Saint Venant compatibility constraint becomes an incompatibility constraint and this is represented by a tensor field $\eta_{ij}$. In our case, the components of $\eta_{ij}$ are expressed as gradients of the components of the Nye tensor that represent the dislocation density. The presence of dislocations induces large internal stresses in certain regions of the material, and these act as initiation sites for plastic deformation. When the external loading is applied, dislocations moving from these regions cause strain hardening that is detectable in experimental uniaxial measurements. This model serves as a starting point for further development of the framework of three-dimensional rate-independent theory of plasticity within the Landau-Ginzburg formalism. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D12.00011: Phonon Driven BCC to Orthorhombic Transformation in U-Nb Alloys Avadh Saxena, Turab Lookman The martensitic transformation in uranium alloys is of great strategic importance. We study the crystallography and model the well characterized BCC to orthorhombic phase tranformation in the shape memory alloy U-Nb for low Nb concentrations. Our predictions are consistent with the experimentally observed orientation relationship between the BCC and orthorhombic phases. We find that this temperature induced transformation is driven by a specific zone boundary phonon that couples to a particular shear mode. We also obtain a Landau free energy for this transformation. In addition, we compare our results with a similar shuffle based mechanism in a related martensitic alloy AuZn. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D12.00012: First-Principles Study of the Jahn-Teller Distortion in the Ti$_{1-X}$V$_{X}$H$_{2}$ and Zr$_{1-X}$Nb$_{x}$H$_{2}$ Alloys Ramiro Quijano, Romeo de Coss, David Singh The transition metal dihydrides TiH$_{2}$ and ZrH$_{2}$ present the fluorite structure (CaF$_{2})$ at high temperature but undergoes a tetragonal distortion with c/a$<$1 at low temperature. Electronic band structure calculations have shown that TiH$_{2}$ and ZrH$_{2}$ in the cubic phase display a very flat band at the Fermi level. Thus the low temperature tetragonal distortion has been associated to a Jahn-Teller effect. In order to understand the role of band filling in controlling the structural instability of the transition metal dihydrides, we have performed a first-principles total energy study of the Ti$_{1-X}$V$_{x}$H$_{2}$ and Zr$_{1-x}$Nb$_{x}$H$_{2}$ alloys. The calculations were performed using FP-LAPW method within the (DFT) and we use the GGA for exchange correlation functional energy. The critical concentration for which the Jahn-Teller effect is suppressed, was determined from the evolution of the tetragonal-cubic energy barrier. We discuss the electronic mechanism of the structural-instability, in terms of the band filling. From the obtained results we conclude that the tetragonal distortion in TiH$_{2}$ and ZrH$_{2}$ is not produced only by a Jahn-Teller Effect. This research was supported by Consejo Nacional de Ciencia y Tecnolog\'{\i}a (Conacyt) under Grant No. 43830-F. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D12.00013: Density functional analysis of long range order T.R.S. Prasanna A density functional analysis of order-disorder transitions in alloys shows that ordering energy is stored in superlattice wavevectors. Thermal vibrations play a key role and lower the transition temperature, T$_{c}$, to the experimental value (741 K) from the mean-field value (933 K) in the Bragg-Williams model for beta brass, $\beta $-CuZn. An isotope effect with 4 K difference in T$_{c}$ is predicted for $^{63}$Cu$^{64}$Zn and $^{65}$Cu$^{68}$Zn in a Modified Bragg-Williams model. The above conclusions are shown to be applicable in magnetic transitions as well. Theoretical analysis shows that thermal vibrations alter the exchange and total magnetic ordering energy. Every microscopic theory of magnetic and alloy phase transitions must satisfy the twin criteria that ordering energy is a) stored in superlattice wavevectors and b) a function of temperature due to thermal vibrations. An isotope effect is predicted to be a universal feature of alloy and magnetic phase transitions. The nuclear-nuclear energy term, $E_{n-n}$, converges without artificial parameters if zero point vibrations are included unlike the Ewald sum technique. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D12.00014: Ab-initio study of the structural and magnetic properties for the fcc Fe-Co alloy. Filiberto Ortiz-Chi, Aar\'on Aguayo, Romeo de Coss We have studied the structural and magnetic properties of the fcc Fe-Co alloy by means of first-principles calculations. For modeling the alloy we have used the ab-initio self-consistent Virtual Crystal Approximation. The ground state properties was calculated with the Fixed Spin Moment methodology and the Full-Potential LAPW method. For the exchange-correlation potential we have used the Generalized Gradient Approximation. For ferromagnetic fcc-Fe we find an anti-invar behavior (ELS$<$EHS) with the co-existence of two ferromagnetic states (metamagnetism). For the fcc-FeCo alloy we find a progressive evolution of the metamagnetism with the Co concentration. Using the calculated total-energy vs the lattice parameter and the Boltzman distribution function, we have obtained the lattice parameter as function of the temperature, in order to determine the thermal expansion coefficient $\alpha$ as function of the Co-concentration. We find that Fe65Co35 show an invar behavior. [Preview Abstract] |
Session D13: Computational Methods: Strongly Correlated and Many Body Systems
Sponsoring Units: DCOMPChair: Mark Jarrell, University of Cincinnatti
Room: Morial Convention Center 204
Monday, March 10, 2008 2:30PM - 2:42PM |
D13.00001: Maximally-localized Wannier functions for GW quasiparticles D.R. Hamann, David Vanderbilt Recent efforts carrying the GW many-body approximation to self-consistency have given improved electronic-structure results.$^{1}$ However, one is left with self-energy operators only on the grid of \textbf{k} points used for Brillouin-zone integration, unlike the case of DFT where the local self-consistent potential allows calculation of the band structure on arbitrary \textbf{k} points (e.g., along symmetry lines). As maximally-localized Wannier functions$^{2}$ (MLWF) provide a basis for a highly accurate approach to band interpolation, we have combined the \textsc{wannier90} code for MLWF with the self-consistent GW capabilities of the \textsc{abinit} code to efficiently extend the GW grid calculation to a full band structure. MLWF also provide an intuitive picture of the orbital character and bonding of groups of bands, as well as a quantitatively accurate measure of electric polarization.$^{2}$ Differences between quasiparticle$^{3}$ MLWF and their LDA counterparts examined to date (Si and perovskite SrZrS$_{3}$) have proven small, but the visualization of significant many-body effects through MLWF remains an intriguing possibility. 1. F. Bruneval \textit{et al}., Phys. Rev. B \textbf{74}, 045102 (2006). 2. N. Marzari and D. Vanderbilt, Phys. Rev. B \textbf{56}, 12 847 (1997). 3. M. van Schilfgaarde \textit{et al}., Phys. Rev. Lett. \textbf{96}, 226402 (2006). [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D13.00002: Ab initio calculation of the excited state properties of spiropyran Emmanouil Kioupakis, Steven G. Louie The photochromism of spiropyran/merocyanine molecules has been the subject of many experimental and theoretical studies. However, several questions remain open, in particular the excited state dynamics and the role of the triplet state. In this work, we use ab initio techniques based on Density Functional Theory and Green's functions methods based on the GW approximation to the electron self energy and Bethe-Salpeter equations to study the ground and excited states of spiropyran/merocyanine for various geometries. Our results are compared with previous work. This work was supported by National Science Foundation Grant No. DMR07-05941, the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Computational resources have been provided by NERSC and TeraGrid resources provided by SDSC and Indiana University. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D13.00003: All-electron GW calculation of vanadium dioxide Rei Sakuma, Takashi Miyake, Ferdi Aryasetiawan We present the results of the GW calculation of metallic and insulating vanadium dioxide using a full-potential LMTO basis set. Our calculations show that it is crucial to take into account both the frequency dependence and the off-diagonal elements of the self-energy. We find that the usual 1-shot GW scheme, where the frequency expansion of the self-energy is truncated in the first-order, yields a large error ($>0.1$eV) in quasiparticle energies due to the unsmoothness of the self- energy. In both phases, the dynamical correlation effect within RPA leads to a plasmon satellite above the Fermi level, but not below. This difference can be attributed to matrix element effects. Our 1-shot GW calculation does not reproduce insulating VO$_{2}$ due to the mixing of conduction and valence bands in the LDA calculation, and we reconstruct G and W by updating the quasiparticle wavefunctions and energies to obtain the band gap. The result indicates the importance of self-consistency in GW calculations. Our results are more in line with the Peierls picture of gap opening due to the lattice distortion, rather than the Motto-Hubbard picture of strong correlations. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D13.00004: GW-based \textit{ab initio} downfolding aiming at strongly correlated electron system Kazuma Nakamura, Taichi Kosugi, Yoshihide Yoshimoto, Ryotaro Arita, Masatoshi Imada Aiming at \textit{ab initio} description of real complex systems under effects of strong electron correlations, we develop a GW-based downfolding scheme formulated in the plane-wave basis set. Our method is successfully applied to organic conductors, the family of (BEDT-TTF)$_{2}$X. At the heart of our downfolding scheme lies utilizing the energy hierarchy of the system [1]: The low-energy hierarchy near the Fermi level (\textit{$\varepsilon $}$_{ f}\pm $2$\sim $3 eV) determines physics while is affected by the remaining high-energy part of hierarchy. We renormalize the high-energy part into low energy, based on the GW scheme. The renormalization generates a low-energy model characterized by renormalized transfers and effective screened Coulomb/exchange interactions, having frequency dependence arising from retarded screening by eliminated high-energy electrons. Thus, the low-energy frequency-dependent effective model is mapped out from the whole high- plus low-energy system in an \textit{ab initio} procedure. [1] F. Aryasetiawan \textit{et al}., Phys. Rev. B \textbf{70}, 19514 (2004); I. V. Solovyev and M. Imada, \textit{ibid}. \textbf{71}, 045103 (2005). [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D13.00005: GW Study of Actinides: $\alpha$-Uranium and $\delta$-Plutonium R.C. Albers, A.N. Chantis, M. van Schilfgaarde, T. Kotani We have applied the recently developed Quasiparticle Self-Consistent GW (QSGW) method to $\alpha$-U and $\delta$-Pu. This is the first time that the f-orbital electron-electron interactions in actinides have been treated by a first-principles method that goes beyond the level of the generalized gradient approximation. We show that the QSGW approximation for U predicts a significant f-band narrowing when compared to GGA band-structure results. However, because of the low f-electron occupation number in U, ground-state properties are not significantly affected. This provides the first formal justification for the success of the LDA and GGA calculations in describing the gound-state properties of this material. For Pu we find that QSGW, like conventional band-structure calculations, predicts a static magnetic ground-state in contradiction with experiment. A non-magnetic solution is also presented. For $\delta$-Pu we show that the QSGW approximation predicts even stronger band narrowing than for U. Because of this and a larger f-occupation, the ground state properties are affected much more significantly than for U. Overall, because of its better treatment of correlation, we suggest that the QSGW solution rather than GGA should be a better starting point for future Dynamical Mean Field Theory (DMFT) and other correlation methods. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D13.00006: The formation of stripes and the pairing of charge carriers in anisotropic materials K.J.E. Vos, C. Povey, J.M. Tipper We have examined the formation of stripes and pairing in the anisotropic t - J model. We have used exact diagonalization methods on several different cluster sizes to examine the underdoped region. Evidence of unidirectional stripe formation in the charge and spin correlations was found. We have determined that the formation of stripes parallel to the Cu-O-Cu bonds enhances pairing and in the bulk limit there is a finite range of doping concentration where hole pairs will form. As the material becomes more anisotropic there is a phase transition that destroys the stripe. These results are consistent with the experimental data. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D13.00007: Theory of the Normal State of the Copper-Oxide Superconductors Ting Pong Choy, Robert G. Leigh, Philip Phillips We show here that many of the normal state properties of the cuprates are consequences of the new charge 2e boson which we have recently (Phys. Rev. Lett. {\bf 99}, 46404 (2007) and arXiv:0707.1554) shown to exist in the exact low-energy theory of a doped Mott insulator. In particular, the 1) mid-infrared band, 2) the $T^2$ contribution to the thermal conductiivty, 3) the pseudogap, 4) the bifurcation of the electron spectrum below the chemical potential as recently seen in angle-resolved photoemission, 5) insulating behaviour away from half-filling, 6) the high and low-energy kinks in the electron dispersion and 7) T-linear conductivity all derive from the charge 2e boson. We also calculate the inverse dielectric function and show that it possesses two dispersing particle-hole branches as a function of momentum in the lightly doped regime. The second of the two branches is mediated by a new charge e composite excitation formed from the charge 2e boson and represents a distinctly new prediction of this theory. We propose that electron energy loss spectroscopy at finite momentum and frequency can be used to probe the existence of the second particle-hole branch. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D13.00008: Imaginary-time formulation for strongly correlated transport in steady-state nonequilibrium Jong E. Han, Ryan J. Heary We formulate steady-state nonequilibrium from the scattering state theory and show that an imaginary-time formalism exists for strongly correlated transport calculations in quantum dot devices\footnote{J. E. Han, R. J. Heary, Imaginary-time formulation of steady-state nonequilibrium: application to strongly correlated transport {\it accepted to Phys. Rev. Lett.} arXiv:0704.3198v2.}. Equilibrium imaginary-time method is extended to the steady-state nonequilibrium by mapping the chemical potential difference in the quantum got device into complex Matsubara voltage. Due to this formal similarity between equilibrium/nonequilibrium theories, we readily apply an equilibrium numerical technique to calculate the strong correlation effects in nonequilibrium. We use quantum Monte Carlo method to calculate Green functions at each Matsubara voltage and, after a numerical analytic continuation, we obtain nonperturbative Green function far from equilibrium. We show numerical results for the evolution of Kondo anomaly as a function of finite bias. We also discuss the splitting of Kondo anomaly at finite magnetic fields. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D13.00009: Electron fractionalization and statistics of holons in doped quantum dimer models Didier Poilblanc I introduce a doped two-dimensional quantum dimer model describing a doped Mott insulator and retaining the original Fermi statistics [1]. This model shows a rich phase diagram including a d-wave hole-pair superconductor breaking translation symmetry (supersolid) at small doping, a bosonic superfluid at large doping and an exotic intermediate phase in-between. The hole kinetic energy is shown to favor binding of topological defects to the fermionic holons turning them into bosons, in agreement with arguments based on RVB wave-functions. Results are discussed in the context of cuprates superconductors and compared with those of a related bosonic doped quantum dimer model [2]. [1] D. Poilblanc, arXiv:0711.2229. [2] A. Ralko, F. Mila, and D. Poilblanc, Phys. Rev. Lett. 99, 127202 (2007). [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D13.00010: Improvement on the STLS Approach and its Application to the Spin Fully-Polarized Low-Density Electron Gas Yasutami Takada, Kanako Yoshizawa Four decades ago, Singwi, Tosi, Land, and Sj\"olander (STLS) proposed a theory that treats the local-field factor $G(q)$ and the static structure factor $S(q)$ in a self-consistent fashion. Because of its simplicity in practical calculations and reasonably good results for the correlation energy, this STLS framework has been recognized as a powerful theoretical tool to study short-range correlation beyond the RPA. At the same time, however, it has been realized that the STLS scheme has several shortcomings; among others, it does not satisfy the Pauli principle as exemplified by the fact that the on-top parallel-spin pair distribution function $g_{\uparrow\uparrow} (0)$ becomes negative. In view of this situation, we propose an improved STLS approach in which a special procedure is added to the original STLS framework in order to impose the Pauli principle. This new scheme is successfully applied to the spin-fully polarized electron gas with the electron density parameter $r_s$ ranging from 1 to 100 to find that the correlation energy obtained by quantum Monte Carlo simulations is reproduced very accurately, along with satisfying $g_{\uparrow\uparrow}(0) = 0$ and the non- negativity condition $g_{\uparrow\uparrow}(r) \geq 0$. Implications of our results will be given in the context of the contribution to the density functional theory as well as to the spin structure of the Wigner-lattice state. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D13.00011: Role of phonons and of finite temperature on the spectral function of a single hole in a quantum antiferromagnet Satyaki Kar, Efstratios Manousakis Thermal broadening as well as the role of optical phonons are studied for a single-hole in a quantum antiferromagnet within the t-J (and the t-t'-t"-J) model. The non-crossing approximation (NCA) as well as the effect of vertex corrections (up to second order) are used to describe the coupling to spin waves and phonons up to intermediate range of coupling. Phonons at finite temperature are seen to broaden the quasiparticle peak and the string excitations are found to survive up to an intermediate phonon-coupling regime beyond which the NCA is expected to break down. The qualitative features of our results compare reasonable well with the recent high resolution angular resolved photoelectron spectroscopy. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D13.00012: The Green's Function of the 1D Breathing-Mode Polaron Glen Goodvin, Mona Berciu We apply the Momentum Average approximations MA(0) and MA(1) to study the properties of the one-dimensional breathing-mode polaron. The results are analytical, numerically trivial to evaluate, exact for both zero bandwidth and for zero electron-phonon coupling, and are accurate everywhere in parameter space. Comparison with recent numerical data confirms this accuracy. We also show that by applying MA as a variational method with a suitably chosen enlarged subspace, we can obtain extremely high accuracy for both ground state and higher energy state properties. With only a slight increase in computational effort this allows us to obtain ground state and momentum-dependent results well within 0.1\% error of the exact numerical data currently available. Although this work specifically looks at the breathing-mode model, we demonstrate that MA is applicable to all momentum dependent electron-phonon coupling models, and its accuracy can always be improved by systematically improving the approximation itself or by working in an enlarged variational subspace. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D13.00013: Competition between pseudogap phase and d-wave superconductivity in the limit of large number of coupled 1D chains Enrico Perfetto, Jose Gonzalez We study the electronic instabilities in a quasi-one-dimensional Hubbard model formed by a large array of Hubbard chains. We go beyond the renormalization group solution of the model, by incorporating the nonperturbative bosonization method for the analysis of the quasiparticle properties. We show that when the Fermi level is at the Van Hove singularity the anomalous electron dimension diverges at the Fermi points closer to the saddle points of the 2D dispersion. Such divergence competes with the d-wave superconducting instability, but the suppression of the quasiparticle weight around the hot spots at (pi,0) and (0,pi) takes place first. This behaviour survives when the number of coupled chains becomes large, suggesting that the development of the pseudogap should survive in the 2D limit. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D13.00014: Extensions of the Momentum Average approximation Mona Berciu, Lucian Covaci We consider a well studied problem, the formation of polarons. Even for the simplest electron-phonon interaction (the Holstein model), an exact solution is only known in the asymptotic limits of zero coupling or zero free-electron bandwidth. A simple analytical approximation that turns out to be accurate for all coupling strengths (the Momentum Average approximation) has only been found recently. We discuss the extension of this method to various other situations in which polaron physics might be important. We show how the Momentum Average approximation can be used in answering questions regarding coupling of electrons to multiple phonon branches, formation of polarons in the presence of magnetic fields and the existence of multiple electron bands. [Preview Abstract] |
Session D14: Quantum Information Science in AMO
Sponsoring Units: DAMOP GQIChair: J. Y. Vaishnav, National Institute of Standards and Technology
Room: Morial Convention Center 205
Monday, March 10, 2008 2:30PM - 2:42PM |
D14.00001: Quantum Information Aspects of Cold Fermi Systems Razvan Teodorescu In the limit of fast switching of Feshbach resonance in cold fermionic systems, the dynamics is dominated by non-linear, coherent, multi-frequency quantum oscillations of the order parameter. This theoretical model is very rich and has known connections to several quantum field theories. In this talk, I will analyze the problem from the point of view of quantum information theory and indicate possible practical applications of the fast-switching regime. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D14.00002: Coherent Control of Trapped Bosons Analabha Roy, Linda Reichl We investigate the quantum behavior of a mesoscopic two-boson system produced by number-squeezing ultracold gases of alkali metal atoms. The quantum Poincare maps of the wavefunctions are affected by chaos in those regions of the phase space where the classical dynamics produces features that are comparable to $\hbar$. We also investigate the possibility for quantum control in the dynamics of excitations in these systems. Controlled excitations are mediated by pulsed signals that cause Stimulated Raman Adiabatic passage (STIRAP) from the ground state to a state of higher energy. The dynamics of this transition is affected by chaos caused by the pulses in certain regions of the phase space. A transition to chaos can thus provide a method of controlling STIRAP. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D14.00003: Phase transitions, entanglement and quantum noise interferometry in cold atoms Florian Mintert, Indubala Satija, Ana Maria Rey, Charles Clark Quantum entanglement represents one of the most fascinating features of quantum theory and has emerged as an important resource in quantum information science. Recent studies have suggested that the long range correlations that are established close to a quantum phase transition manifest themselves in a pronounced increase of entanglement. However, to show that is not an easy task given the fact that currently there is not consensus about the best method to define an entanglement measure for multi-particle systems. Using an entanglement measure that includes up to four point correlation functions we study the scaling properties of multi-particle entanglement in a one dimensional Ising chain around and at the critical point. Our study reveals that multiparticle entanglement indeed peaks at the phase transition, whereas pure biparticle entanglement measures often fail to reveal this feature. We discuss the connection between multiparticle entanglement measurements with noise correlations and the possibility of using these experimentally accessible quantities as a probe of entanglement in cold atomic systems. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D14.00004: ABSTRACT HAS BEEN MOVED TO SESSION W15 |
Monday, March 10, 2008 3:18PM - 3:30PM |
D14.00005: Topological Entropy of Quantum Hall states in rotating Bose gases Alexis Morris, David Feder Using exact numerical simulations of a small number of harmonically trapped ultracold alkali atoms at high rotation, we calculate the von Neumann entropy of the bosonic variant of the Laughlin and Pfaffian quantum Hall states. It has recently been shown that this entropy has a linear scaling with the boundary size. The y-intercept of this scaling relation corresponds to a universal quantity known as the topological entropy that is related to the quantum Hall filling factor. Through finite size scaling, we have extracted this quantity and compare the outcome to expected results. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D14.00006: Mapping photonic entanglement into and out of a quantum memory Hui Deng, Kyung S. Choi, Julien Laurat, H. Jeff Kimble Recent developments of quantum information science critically rely on entanglement. In particular, scalable quantum networks require capabilities to create, store, and distribute entanglement among distant nodes via photon channels. Atomic ensembles can serve as such nodes. In the photon counting regime, heralded entanglement between atomic ensembles has been demonstrated via probabilistic protocols. However, an inherent drawback of such protocols is the compromise between the fidelity of entanglement and its preparation probability, which hinders the schemes' scalability. Here we present a protocol where entanglement between atomic ensembles is created by coherent mapping of photonic entanglement. By splitting a single-photon and subsequent state transfer, we separate the generation of entanglement and its storage, enabling efficient scaling for high-fidelity quantum communication. After a programmable delay, chosen at 1us, the stored entanglement is mapped back into photon modes with an overall efficiency of 17{\%}. With improved retrieval efficiency and memory time, along with the development of on-demand single photon sources, our protocol enables the deterministic generation, storage, and distribution of entanglement among remote quantum memories for scalable quantum networks. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D14.00007: Broadband Multi-Spot Optical Beam Steering with Independent 2D Addressability for Quantum Information Processing Caleb Knoernschild, Changsoon Kim, Felix Lu, Jungsang Kim While quantum computation utilizing trapped ions or neutral atoms has seen significant advances in recent years, the necessary scalability of such implementations is limited in part by the distribution of laser resources. The capability to address multiple qubit locations with a single laser is an essential element in moving these experiments beyond individual quantum gate demonstrations. An optical system utilizing micro-electromechanical system (MEMS) technology can provide a scalable solution to address a qubit array with multiple independent beams concurrently. Broadband coatings can accommodate a large range of wavelengths, while fabrication techniques allow expansion to multiple parallel laser beams over a large number of trap locations. We demonstrate a two-spot beam steering system using MEMS mirrors that can simultaneously and independently illuminate any of 25 different locations within a 5x5 array with 2 laser beams of different wavelengths. Mirrors with settling times of $<$ 5$\mu $s have been fabricated allowing fast access times between qubits. Such systems can be used to implement two-qubit gates in a 1D or 2D array of qubits. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D14.00008: Phonon-Mediated Detection of Trapped Atomic Ions David Hume, Till Rosenband, David Wineland Both quantum information processing and quantum-limited metrology require sensitive detection of quantum states. Using trapped atomic ions, we investigate quantum non-demolition measurements in a two-species ion chain composed of Al$^{+}$ and Be$^{+}$. By mapping information from Al$^{+}$ to a shared phonon-mode then to Be$^{+}$ and detecting repetitively we have experimentally demonstrated a fidelity for state initialization and detection of 0.9994. We have also shown an increase in measurement efficiency through an adaptive procedure. Here we apply these ideas to the detection of states of multiple Al$^{+}$ using a single Be$^{+}$ ion, and describe the preparation of entangled states through measurement. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D14.00009: Towards a wire-mediated coupling of trapped ions Robert Clark, Tony Lee, Nikos Daniilidis, Sankaranarayanan S., Hartmut H\"{a}ffner Most schemes for ion trap quantum computation rely upon the exchange of information between ion-qubits in the same trap region, mediated by their shared vibrational mode. An alternative way to achieve this coupling is via the image charges induced in a conducting wire that connects different traps. This was shown to be theoretically possible by Heinzen and Wineland in 1990, but some important practical questions have remained unaddressed. Among these are how the presence of such a wire modifies the motional frequencies and heating rates of trapped ions. We thus have realized this system as a 1 mm-scale planar segmented rf ion trap combined with an electrically floating gold wire of 25 microns diameter and length 1 cm. This wire is placed close to trapped ions using a set of piezoelectric nanopositioners. We present here experimental measurements of the motional frequencies and heating rates of a single trapped calcium ion as the wire is moved from 3.0 mm to 0.2 mm away from the ion. We discuss the implications of these results for achieving wire-mediated coupling in the present apparatus, as well as in future improved setups. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D14.00010: Trapping and Detecting Polar Molecular Ions in a Closed-Cycle 4 K Ion Trap Gleb Akselrod, David Schuster, Paul Antohi, Ziliang Lin, Rob Schoelkopf, Isaac Chuang The rich internal structure of polar molecular ions make them attractive for interfacing with solid state systems in quantum information processing, yet it is this structure that makes trapping and detecting molecules difficult. We present an approach to this challenge which allows a superconducting cavity to be used as an integral part of a surface electrode ion trap, based on a closed-cycle cryostat operated at 4 K, and addressing the problem of loading molecules and detecting their presence. A mixture of Sr and SrCl ions is loaded into the trap by laser ablation. Subsequent laser cooling of the Sr ions sympathetically cools the molecular ions, producing a two-component Wigner crystal. ~This allows detection of the molecules using mass spectroscopy, observed indirectly through imaging of the Sr ions, or through coupling to microwave lines in the trap. Using a closed-cycle cryostat enables rapid testing and evaluation of ablation targets and trap geometries, at the cost of some trap vibration, measured to be below 160 nm in amplitude. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D14.00011: Microwave Cavity Quantum Electrodynamics in a Molecular Ion Trap David Schuster, Paul Antohi, Gleb Akselrod, Ziliang Lin, Isaac Chuang, David Demille, Robert Schoelkopf It has been proposed to use the rotational states of trapped neutral molecules inside of an on-chip superconducting microwave resonator for cavity quantum electrodynamics and quantum information processing. We investigate the potential of molecular ions, which have several properties that might be advantageous over neutral molecules. Ions can be loaded into deep RF traps which do not require ultra-stable lasers and trap independent of their rotational state. They can be cooled by both cryogenic buffer gas and sympathetic laser cooling with atomic ions. Further, the ion charge screens electric fields felt by the rotational dipole reducing dephasing due to trapping or spurious fields. In addition, this property should allow one to create stable ensembles of ions which could be used as a quantum memory. As with neutral proposals such a system could be both interrogated and manipulated with microwave fields. Finally, the system represents a high precision spectroscopy tool for studying microwave transitions of single molecules. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D14.00012: Cavity cooling of $^{88}Sr^+$ David Leibrandt, Yat Shan Au, Isaac Chuang Cavity cooling is a method of laser cooling which uses coherent scattering to cool atoms [V. Vuleti\'{c} and S. Chu, PRL \textbf{84}, 3787 (2000)]. The closed atomic transition used in Doppler cooling is replaced by a cavity resonance, so cavity cooling can be used to cool to sub-Doppler temperatures and is in principle applicable to complicated atoms or molecules without closed transitions. We describe an experiment to study three-dimensional cavity cooling of a single $^{88}Sr^+$ ion confined in a linear RF Paul trap. Large cooling rates can be attained by operating near the 422 nm $S_{1/2} \leftrightarrow P_{1/2}$ optical dipole transition and using a 5 cm long near-confocal Fabry-P\'{e}rot cavity with commercially available mirrors of finesse $10^4$. Given a cavity alignment error $\le 10$ $\mu$m and a trap frequency of 1 MHz, the resolved sideband cavity cooling limit is $\le$ 5 motional quanta. We present details of the experimental proposal and its implementation. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D14.00013: Microfabrication of Surface Electrode Ion Traps for Quantum Information Experiments Yufei Ge, Jaroslaw Labaziewicz, Paul Antohi, Isaac Chuang Surface electrode ion traps, while promising for large-scale quantum computation, have long been challenged by ion heating rates which increase rapidly as trap length scales are reduced. Through a series of measurements on over fifteen traps, we show that ion heating rates are surprisingly sensitive to electrode material and morphology, and in particular, to details of the fabrication procedure. For example, one $75$ $\mu$m size trap, made of chemically etched silver on a single crystal quartz substrate, showed a minimum heating rate of $\sim 40$ quanta/second, when prepared by annealing at $760^\circ$C in vacuum for one hour. This annealing smooths sharp edges, and significantly reduces breakdown voltage. However, if the annealing temperature is lowered to $720^\circ$C, leaving the breakdown voltage still robustly high, the heating rate jumps to $\sim1000$ quanta/second. With electroplated gold, on a silver seed layer, a record low heating rate of $\sim 2$ quanta/second is obtained. We present details of the fabrication procedures, evaluate alternative electrode materials such as niobium nitride, and explain how these measurements were obtained with an ion trap operated at $6$ Kelvin, containing a single strontium ion, sideband cooled to its quantum ground state of motion. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D14.00014: Temperature dependence of decoherence in ion traps Jaroslaw Labaziewicz, Yufei Ge, Shannon X. Wang, Ruth Shewmon, Isaac L. Chuang Dense arrays of trapped ions provide one way of scaling up ion trap quantum information processing. However, miniaturization of ion traps is currently limited by sharply increasing motional state decoherence at sub-100~$\mu$m ion-electrode distances. This decoherence has been demonstrated to be thermally driven, providing a plausible route to reduce it. In our experiment, we measure the heating rate out of the motional ground state of a single $Sr^+$ ion in a cryogenic surface electrode ion trap. We present our results on the temperature dependence of the heating rates as a function of electrode temperature in 10-100~K range. Heating rates at 6~K are observed to be as low as two quanta per second, but increase rapidly with temperature. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D14.00015: Individual ion addressing using a magnetic field gradient in a surface-electrode ion trap Shannon Wang, Jaroslaw Labaziewicz, Yufei Ge, Isaac Chuang The ability to address individual ions is an important issue in using multiple trapped ions to perform quantum operations. Previous efforts have included using precisely focused laser beams aimed at only one ion at a time, which poses a significant technical challenge. An alternative is to use field-dependent transitions and a magnetic field gradient to shift the transition frequencies of ions as a function of position. This requires good stability of the local field in order to achieve desired fidelity of quantum operations. In a cryogenic $Sr^+$ ion trap we use the $5S_{1/2} \rightarrow 4D_{5/2}$ transition as an optical qubit, which can be Zeeman shifted using a bias field generated by external coils. We present a scheme to create a local field gradient by integrating current sources onto a microfabricated surface-electrode trap. Taking advantage of the cryogenic environment, we stabilize the field at the trap site using superconducting rings as flux shields. The rings can be integrated with the trap, simplifying implementation and improving alignment to the ions. [Preview Abstract] |
Session D15: Focus Session: Foundations of Quantum Theory I
Sponsoring Units: GQIChair: Howard Barnum, Los Alamos National Laboratory and GSCCM Chair
Room: Morial Convention Center 207
Monday, March 10, 2008 2:30PM - 3:06PM |
D15.00001: LeRoy Apker Award Talk: Testing Hidden-Variable Theorems with Single-Photon Entangled States Invited Speaker: An ensemble of single photons created in a hyperentangled Bell state were used to test a broad class of Hidden-Variable Theorems (HVTs). Specifically, the class of HVTs based on the joint assumption of Realism and Non-Contextuality (NC) -- the premise that values associated with one observable are independent of which commuting observables may be measured simultaneously -- known as NCHVTs, and first examined by Bell as well as Kochen and Specker, were addressed using these single-photon states entangled in polarization and direction of momentum. A Clauser-Horne-Shimony-Holt (CHSH) Inequality was applied, with the factorization condition that is usually satisfied by a Non-Locality assumption being instead satisfied by the assumption of NC, due to the inherently local nature of detection events for single particles. The basis rotations and projections necessary for testing the CHSH Inequality were accomplished using interferometers and standard polarization optical elements. A violation of the CHSH Inequality was observed, ruling out either Realism or Non-Contextuality -- or possibly both. The tenability of Contextual HVTs remains, trivially, as the predictions of such a theory can map one-to-one to the predictions of Quantum Mechanics. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:42PM |
D15.00002: Weak values and the Leggett-Garg inequality in solid-state qubits Invited Speaker: The seminal paper of Aharonov, Albert, and Vaidman introduces the concept of a weak value as a statistical average over realizations of a weak measurement, where the system is both pre- and post-selected. By taking restricted averages, weak values can exceed the range of eigenvalues associated with the observable in question. We discuss how to implement a weak values measurement with solid-state qubits. In parallel activity, Leggett and Garg have devised a test of quantum mechanics for a single system using different ensembles of (projective) measurements at different times and correlation functions of those outcomes. The original motivation was to test if there was a size scale where quantum mechanics would break down. Introduced as a ``Bell-inequality in time'', the assumptions of macrorealism that could be verified by a non-invasive detector imply that their correlation function obeys a Leggett-Garg inequality that quantum mechanics would violate, formally similar to the inequality of Bell. We demonstrate that the proper notion of a classical weak value also demands these assumptions, and that furthermore a weak value can be non-classical if and only if a Leggett-Garg inequality can also be violated. We will discuss generalized weak values, where post-selection occurs on a range of weak measurement results. Our analysis is presented in terms of kicked quantum nondemolition measurements on a quantum double-dot charge qubit. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D15.00003: Negative probabilities and measurement disturbance Lars M. Johansen Feynman once said that ``the only difference between a probabilistic classical world and the equations of the quantum world is that somehow or other it appears as if the probabilities would have to go negative''. But what is the essential reason for probabilities going negative? Here it is demonstrated that negative probabilities are a direct consequence of measurement disturbance. The Margenau-Hill distribution, a quasiprobability taking negative values, is expressed in terms of the joint probability obtained in the successive measurement of two projectors. The quasiprobability takes negative values only if the measurement of the first projector disturbs the measurement of the second projector. The uniqueness of this quasiprobability follows by imposing a symmetry principle on measurement disturbance. The quasiprobability is made informationally complete by a complex extension. Conditions for informational completeness are derived. This quasiprobability also can be observed directly as a statistical average of pointer displacements in weak measurements. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D15.00004: The meaning of negative weak values James Troupe, Jeff Tollaksen A number of approaches to quantum mechanics incorporate negative values for quantities that were classically positive, such as the Wigner-Moyal density approach or the Feynman negative probability approach, etc. In the re-formulation of quantum mechanics using weak values and weak measurements, we encounter a new situation where weak values of projection operators turn out to be negative. We emphasize the differences between these negative weak values and the negative values encountered in the other formalisms: in the previous formalisms, the mathematical entity whose average yielded the negative values are not density operators. While they do yield the correct average of a function, they also have non-physical aspects, i.e. mathematical artifacts, when the densities become negative. The reason is that if we attempt to actually measure such ``negative" properties, then the result does not correspond to a physical observable in Hilbert Space. E.g. if we did attempt to cheat quantum mechanics by projecting onto p and x as densities simultaneously in Wigner-Moyal, then we obtain the parity operator, the most non-local result. On the other hand, in the case of weak values, we obtain a new situation: when we use a bonafide measuring device to measure these properties ideally, then the very same measuring device will yield the predicted negative weak values when the measurement interaction is simply weakened. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D15.00005: Frame representations of quantum mechanics and the necessity of negativity in quasi-probability representations Christopher Ferrie, Joseph Emerson Several finite dimensional quasi-probability representations of quantum states have been proposed to study various problems in quantum information theory and quantum foundations. These representations are often defined only on restricted dimensions and their physical significance in contexts such as drawing quantum-classical comparisons is limited by the non-uniqueness of the particular representation. Here we show how the mathematical theory of frames provides a unified formalism which accommodates all known quasi-probability representations of finite dimensional quantum systems. Moreover, we show that any quasi-probability representation satisfying two reasonable properties is equivalent to a frame representation and then prove that any such representation of quantum mechanics must exhibit either negativity or a deformed probability calculus. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D15.00006: Towards Testing Quantum Mechanics with Micro-Optomechanical Systems Dustin Kleckner, Susanna Thon, Evan Jeffrey, Dirk Bouwmeester We review our work in micro-optomechanical systems. Motivation for work on these systems is based in proposals to test quantum mechanics in new regimes. Although extremely challenging, creating a quantum superposition of a mirco-mechanical oscillator coupled to an optical cavity seems experimentally feasible with current technology. Additionally, the optomechanical systems used for this type of research have other applications, such as optical cooling, as recently demonstrated by several independent groups. Finally we will briefly discuss the direction of our research in the near future, including the use of conventional cryogenics to cool the resonator and the prospects for several related types of devices. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D15.00007: Decoherence and the Uncertainty Principle David Craig The uncertainty principle is normally understood as representing a limit on the fundamental accuracy of simultaneous measurements of incompatible observables. In the context of consistent/decoherent histories formulations of quantum theory, we show that it may also be understood as quantifying the degree of course-graining necessary in order for histories of a quantum system to decohere. This follows as a consequence of a new inequality bounding the interference between histories in a consistent histories formulation of quantum theory. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D15.00008: Noncontextuality and the Kochen-Specker Theorem Brian La Cour The question of noncontextuality in a simple, two-qubit system is considered. It is shown that quantum theory is consistent with a noncontextual hidden variable interpretation, contrary to the conclusions of the Kochen-Specker theorem. The key to the proof is the recognition of a subtle but fundamentally important assumption regarding the dependence of the hidden variable probability distribution on the particular set of mutually commensurate observables chosen for measurement. Recent experiments to test noncontextuality will be discussed and their results reconciled with a hidden variable interpretation. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D15.00009: Path detection and interference tradeoff in the double-slit experiment Julio Gea-Banacloche, Masanao Ozawa We study how the acquisition of ``which-path'' information leads to a loss of contrast in the double-slit interference setup. We show that neither the conventional uncertainty principle nor the recently-derived universal uncertainty principle place any restriction on the minimum root-mean-square momentum disturbance, once it is recognized that the which-path determination does not require one to measure $x$, but only an appropriate two-valued function of $x$. We then develop a description of the problem in terms of only two-valued variables, and consider a completely general measurement model, which allows us to distinguish between the measurement error and what we call the ``preparational error.'' We show that error-free which-path measurements are possible in this model that do not destroy the fringe visibility. On the other hand, we also show that there is a general tradeoff relation between preparational error and fringe visibility, which, for measurements obeying the ``non- destruction'' condition, reduces to Englert's inequality. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D15.00010: Exact mappings between quantum relativistic and quantum optical models A. Bermudez, M. A. Martin-Delgado, E. Solano We develop a novel quantum optical perspective into a couple of quantum relativistic systems: i) First we show how the two-dimensional extension of the harmonic oscillator, known as the Dirac oscillator, can be exactly mapped onto a chiral Anti-Jaynes-Cummings model of quantum optics. This equivalence allows us to predict a series of novel relativistic phenomena, such as spin-orbit {\it Zitterbewegung}. Furthermore, we also make a realistic experimental proposal, at reach with current technology, for studying the equivalence of both models using a single trapped ion [1]. ii) Second, we show that a relativistic version of Schr\"{o}dinger cat states, here called {\it Dirac cat states}, can be built in relativistic Landau levels when an external magnetic field couples to a relativistic spin $1/2$ charged particle. Under suitable initial conditions, the associated Dirac equation produces unitarily Dirac cat states involving the orbital quanta of the particle in a well defined mesoscopic regime. These states have a purely relativistic origin and cease to exist in the non-relativistic limit [2]. \newline [1] A. Bermudez et. al, Phys. Rev. A.\textbf{76}, 041801(R) (2007). \newline [2] A. Bermudez et. al, Phys. Rev. Lett.\textbf{99}, 123602 (2007). [Preview Abstract] |
Session D16: Biological Networks
Sponsoring Units: DBPChair: Eivind Almaas
Room: Morial Convention Center 208
Monday, March 10, 2008 2:30PM - 2:42PM |
D16.00001: Cooperation of multiple copies of noisy genes Aleksandra Walczak, Peter Wolynes The regulation of gene expression is influenced by the small numbers of protein and gene copies present in the cell. Noise properties arising from single copies of the gene are well known. In this talk we consider the case when a few copies of the same gene are present and actively transcribed in the cell. We use mathematical models which treat both the DNA and protein degrees of freedom stochastically. We study how the switching of one gene influences the switching behaviour of another gene, to which it is coupled by a mutual protein environment. We show that the genes loose properties defined by their individual parameters and take on the characteristics of a group to reach a new steady state. We show that system with multiple gene copies can be used to reduce noise or to modify the cooperativity of the regulatory characteristics of the circuit. These results can be useful for interpreting and designing bioengineering experiments in which there can be multiple copies of a gene. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D16.00002: Chaotic Gene Regulatory Networks Can Be Robust Against Mutations and Noise Volkan Sevim, Per Arne Rikvold Robustness to mutations and noise has been shown to evolve through stabilizing selection for optimal phenotypes in model gene regulatory networks. The ability to evolve robust mutants is known to depend on the network architecture. How do the state-space structures of networks with high and low robustness differ? Here we present large-scale computer simulations of a Random Threshold Network model of gene regulatory networks undergoing biological evolution. We show using damage propagation analysis and an extensive statistical analysis of state spaces of these model gene networks that the change in their dynamical properties due to stabilizing selection is very small. Therefore, conventional measures of stability do not provide much information about robustness in model gene regulatory networks. Interestingly, the networks that are most robust to both mutations and noise are highly chaotic. Chaotic networks are able to produce large attractor basins, which can be useful for maintaining a stable gene-expression pattern.\\ {[1] V.~Sevim and P.~A. Rikvold (2007), e-print arXiv:0708.2244.}\\ {[2] V.~Sevim and P.~A. Rikvold (2007), e-print arXiv:0711.1522.} [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D16.00003: A quantitative model of DNA replication in \textit{Xenopus} embryos: reliable replication despite stochasticity Scott Cheng-Hsin Yang, John Bechhoefer DNA synthesis in \textit{Xenopus} frog embryos initiates stochastically in time at many sites (origins) along the chromosome. Stochastic initiation implies fluctuations in the replication time and may lead to cell death if replication takes longer than the cell cycle time ($\sim $ 25 min.). Surprisingly, although the typical replication time is about 20 min., \textit{in vivo} experiments show that replication fails to complete only about 1 in 250 times. How is replication timing accurately controlled despite the stochasticity? Biologists have proposed two mechanisms: the first uses a regular spatial distribution of origins, while the second uses randomly located origins but increases their probability of initiation as the cell cycle proceeds. Here, we show that both mechanisms yield similar end-time distributions, implying that regular origin spacing is not needed for control of replication time. Moreover, we show that the experimentally inferred time-dependent initiation rate satisfies the observed low failure probability and nearly optimizes the use of replicative proteins. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D16.00004: Exponential sensitivity of noise-driven switching in genetic networks Pankaj Mehta, Ranjan Mukhopadhyay, Ned Wingreen Cells are known to utilize biochemical noise to probabilistically switch between distinct gene expression states. We demonstrate that such noise-driven switching is dominated by tails of probability distributions and is therefore exponentially sensitive to changes in physiological parameters such as transcription and translation rates. However, provided mRNA lifetimes are short, switching can still be accurately simulated using protein-only models of gene expression. Exponential sensitivity limits the robustness of noise-driven switching, suggesting cells may use other mechanisms in order to switch reliably. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D16.00005: Effects of coarse-graining on fluctuations in gene expression Juan Pedraza, Johan Paulsson Many cellular components are present in such low numbers per cell that random births and deaths of individual molecules can cause significant `noise' in concentrations. But biochemical events do not necessarily occur in steps of individual molecules. Some processes are greatly randomized when synthesis or degradation occurs in large bursts of many molecules in a short time interval. Conversely, each birth or death of a macromolecule could involve several small steps, creating a memory between individual events. Here we present generalized theory for stochastic gene expression, formulating the variance in protein abundance in terms of the randomness of the individual events, and discuss the effective coarse-graining of the molecular hardware. We show that common molecular mechanisms produce gestation and senescence periods that can reduce noise without changing average abundances, lifetimes, or any concentration-dependent control loops. We also show that single-cell experimental methods that are now commonplace in cell biology do not discriminate between qualitatively different stochastic principles, but that this in turn makes them better suited for identifying which components introduce fluctuations. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D16.00006: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 3:42PM - 3:54PM |
D16.00007: Regulatory control and the costs and benefits of biochemical noise Sorin Tanase-Nicola, Pieter Rein ten Wolde Experiments in recent years have vividly demonstrated that gene expression can be highly stochastic. We present a mathematical model that makes it possible to quantify the effect of protein concentration fluctuations on the growth rate of a population of genetically identical cells. The model predicts that the population's growth rate depends on how the growth rate of a single cell varies with protein concentration, the variance of the protein concentration fluctuations, and the correlation time of these fluctuations. The model shows that when the average concentration of a protein is close to the value that maximizes the growth rate, fluctuations in its concentration always reduce the growth rate. However, when the average protein concentration deviates sufficiently from the optimal level, fluctuations can enhance the growth rate of the population, even when the growth rate of a cell depends linearly on the protein concentration. We also apply our model to perform a cost-benefit analysis of gene regulatory control. Our analysis predicts that the optimal expression level of a gene regulatory protein is determined by the trade-off between the cost of synthesizing the regulatory protein and the benefit of minimizing the fluctuations in the expression of its target gene. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D16.00008: Effects of time-delayed negative feedback loops on noise-induced oscillations on the NF-KappaB signaling network Jaewook Joo, Jean-Loup Faulon NF-kappaB is a stimulus-responsive pleiotropic regulator of gene control. Our work was motivated by Nelson et al. [Science 306:704 (2004)], which showed noisy quasi-periodic oscillations of NF-kappaB translocation between cytoplasm and nucleus in single cells. Using both stochastic simulations and analytical approaches, we investigated the dynamic patterns of NF-kappaB translocation with a stochastic two-compartmental model, especially taking into account the interplay between intrinsic noise and delayed negative feedback loops of the NF-kappaB signaling system. We will present noise-induced oscillations of the NF-kappaB shuttling and the effects of time-delayed negative feedback loops on them. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D16.00009: How is the fitness landscaped upon which life evolves selected? Michael Deem We investigate the selective forces that promote the emergence of modularity in nature. We demonstrate the spontaneous emergence of modularity in a population of individuals that evolve in a changing environment. We show that the level of modularity correlates with the rapidity and severity of environmental change. The modularity arises as a synergistic response to the noise in the environment in the presence of horizontal gene transfer. We suggest that the hierarchical structure observed in the natural world may be a broken symmetry state, which generically results from evolution in a changing environment. The existence of such structure, therefore, need not necessarily rest on intelligent design or the anthropic principle. 1) J. Sun and M. W. Deem, Phys. Rev. Lett., to appear., arXiv:0710.3436 [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D16.00010: Early life was a generalist: protein modularity increase as evolution proceeds Jiankui He, Jun Sun, Michael Deem We study the evolution of modularity in protein-protein interaction network and protein domain-domain interaction networks. By introducing compositional age, we construct the interaction networks at different points in evolutionary time. We use the average-linkage hierarchical clustering method to reorganize the network matrix to identify the modules. With several different definitions of modularity, we compare the observed modularity at different evolutionary times in both E. coli and S.cerevisiae. We conclude that the modularity of protein-protein interaction network and domain-domain interaction network grows in evolution, validating recent theoretical predictions of spontaneous modularity in evolution [1]. \newline \newline [1] J. Sun and M. W. Deem, Phys. Rev. Lett, to appear, arXiv:0710.3436. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D16.00011: Genetic recombination models in molecular evolution Enrique Munoz, Jeong-Man Park, Michael Deem We introduce generalizations of two classical models of molecular evolution: the parallel or Crow-Kimura model, and the Eigen model. These generalizations include, in addition to point mutations and selection as driving forces for biological evolution, the presence of different forms of horizontal gene transfer and genetic recombination events between individuals in the population. We will present analytical solutions for these models, and compare our results with numerical solutions of the corresponding system of differential equations. We will also present stochastic simulation results for the single peak fitness case. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D16.00012: A minimal stochastic model of cell death signaling Subhadip Raychaudhuri Cell death (apoptosis) is mediated by a complex intracellular signaling network that involves a large number of components. We propose a minimal model of signaling network that can sense the strength of any extracellular stimuli such as the concentration of ligands and adapt to a fluctuating environment. Based on stochastic simulations we show that a three step slow, fast, slow pathway is enough to generate large cell to cell fluctuations under the conditions of weak stimulus. Such cell to cell stochastic fluctuations persist even in the presence of large number of molecules and cannot be captured by deterministic differential equation based models. We develop a probability distribution based approach that can characterize the stochastic fluctuations in such inherently stochastic signaling network. Interestingly, our results match with those obtained from kinetic Monte Carlo simulation of the full scale apoptotic network. Hence, our minimal signaling network can serve as a cell type independent general model of apoptosis signaling. We also discuss implications of our probability distribution based approach for diseases such as cancer that can result from disrupted apoptotic balance. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D16.00013: Temperature compensation model for the circadian clock of \textit{Neurospora crassa} Xiaojia Tang, Heinz-Bernd Sch\"uttler, Jonathan Arnold In the lowly bread mould, \textit{Neurospora crassa}, biomolecular reactions involving the \textit{white-collar-1} (\textit{wc-1}), \textit{white-colloar-2} (\textit{wc2}), and \textit{frequency} (\textit{frq}) genes and their products constitute building blocks of the biological clock that would response to temperature as well as light. The period of the biological clock remains stable in response to variation in ambient temperature, which is called a compensation phenomenon. Recent experimental results show evidences that the temperature compensation could be explained by the temperature sensitive translational control of production of two isoforms of the main oscillator protein FRQ: a long form FRQ$^{1-989}$ which is more abundantly produced at higher temperature; and a short from FRQ$^{100-989}$, more abundantly produced at lower temperature. With our recently developed method of genetic network identification, we are now simulating the network's temperature response based on published experimental data. These will serve as the starting point for a simulation-prediction-experiment-simulation workflow cycle. In this cycle, the maximally informative next experiment (MINE) technology will be employed to select the best experimental control parameters specifying the temperature response to be used in the next step of the workflow cycle. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D16.00014: Understanding the Role of Housekeeping and Stress-Related Genes in Transcription-Regulatory Networks Allison Heath, Lydia Kavraki, G\'abor Bal\'azsi Despite the increasing number of completely sequenced genomes, much remains to be learned about how living cells process environmental information and respond to changes in their surroundings. Accumulating evidence indicates that eukaryotic and prokaryotic genes can be classified in two distinct categories that we will call class I and class II. Class I genes are housekeeping genes, often characterized by stable, noise resistant expression levels. In contrast, class II genes are stress-related genes and often have noisy, unstable expression levels. In this work we analyze the large scale transcription-regulatory networks (TRN) of \textit{E. coli} and \textit{S. cerevisiae} and preliminary data on \textit{H. sapien}. We find that stable, housekeeping genes (class I) are preferentially utilized as transcriptional inputs while stress related, unstable genes (class II) are utilized as transcriptional integrators. This might be the result of convergent evolution that placed the appropriate genes in the appropriate locations within transcriptional networks according to some fundamental principles that govern cellular information processing. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D16.00015: The re-design of a theophylline riboswitch for DNT sensing Yaroslav Chushak, Nancy Kelley-Loughnane, Svetlana Harbaugh, Morley Stone Riboswitches are noncoding elements of mRNA that recognize and bind to small molecules and regulate the translation process of downstream genes. As an initial study, we used a theophylline riboswitch that regulates the expression of the Tobacco etch virus (TEV) protease placed downstream of the switch as a controlling element. Upon expression of TEV protease, an optical reporter is cleaved producing change in fluorescence resonance energy transfer (FRET) between BFP and eGFP. We altered the sensing domain of the original construct to create a synthetic riboswitch that responds to the presence of 2,4-dinitrotoluene (DNT) molecules. Computational analysis using Autodock4 and AMBER9 software packages showed that U24A mutant has a significantly higher binding affinity for DNT molecule compared to the original theophylline. Cells expressing the re-designed riboswitch showed a marked optical difference in fluorescence emission in the presence of DNT molecules, leading to the potential of using this construct in biosentinel applications of highly nitrated compounds. [Preview Abstract] |
Session D17: Focus Session: Time-Resolved Structural Investigations on Protein Folding and Function
Sponsoring Units: DBPChair: Aihua Xie, Oklahoma State University
Room: Morial Convention Center 209
Monday, March 10, 2008 2:30PM - 3:06PM |
D17.00001: Folding dynamics of a family of beta-sheet proteins Invited Speaker: Fatty acid binding proteins (FABP) consist of ten anti-parallel beta strands and two small alpha helices. The beta strands are arranged into two nearly orthogonal five-strand beta sheets that surround the interior cavity, which binds unsaturated long-chain fatty acids. In the brain isoform (BFABP), these are very important for the development of the central nervous system and neuron differentiation. Furthermore, BFABP is implicated in the pathogenesis of a variety of human diseases including cancer and neuronal degenerative disorders. In this work, site-directed spin labeling combined with EPR techniques have been used to study the folding mechanism of BFABP. In the first series of studies, we labeled the two Cys residues at position 5 and 80 in the wild type protein with an EPR spin marker; in addition, two singly labeled mutants at positions 5 and 80 in the C80A and C5A mutants, respectively, were also produced and used as controls. The changes in the distances between the two residues were examined by a pulsed EPR method, DEER (Double Electron Electron Resonance), as a function of guanidinium hydrochloride concentration. The results were compared with those from CW EPR, circular dichroism and fluorescence measurements, which provide the information regarding sidechain mobility, secondary structure and tertiary structure, respectively. The results will be discussed in the context of the folding mechanism of the family of fatty acid binding proteins. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:42PM |
D17.00002: Ultrafast studies of flavins and flavoproteins Invited Speaker: Flavin molecule plays an essential role as a cofactor in flavoproteins with its rich redox states. We have systematically studied the excited-state dynamics of three different redox states (oxidized, semiquinone, and fully reduced) in bulk solvent and in some important proteins. A series of new results were obtained and a correlation of structure-redox-dynamics-function in proteins was observed. We use two important proteins, photolyase and flavodoxin, to show these significant findings. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D17.00003: Coupled folding and binding kinetics in the intrinsically disordered peptide IA$_{3}$ Ranjani Narayanan, Omjoy Ganesh, Arthur Edison, Stephen Hagen IA$_{3}$ is an intrinsically disordered 68 residue peptide and is an endogenous inhibitor of yeast proteinase A (YPrA). X-ray crystallography of the IA$_{3}\cdot $YPrA complex [Li et al, Nat. Struct. Biol. (7), 113-117 (2000)] indicates that the N-terminus of IA$_{3}$ adopts an alpha-helical fold when it is bound to the YPrA active site. We have used equilibrium circular dichroism and multi-wavelength, nanosecond time-resolved laser temperature-jump spectroscopy to study the coupled folding and binding interaction of IA$_{3}$ with YPrA. Our initial measurements of the rate of helix formation in free IA$_{3}$ indicate mono-exponential folding kinetics that extrapolate to k$_{F} \sim $ 10$^{5}$/s at room temperature in aqueous solutions. By comparing this rate to the kinetics we observe for IA$_{3}$ interacting with YPrA, we can assess possible mechanisms for the coupled folding and binding of IA$_{3}$. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D17.00004: Impact of Salt and Water on Protein Structural Dynamics Anu Thubagere, Lorand Kelemen, Beining Nie, Sandip Kaledhonkar, Aihua Xie Water is known as the lubricant of life. Without water, most proteins would lose their biological functions. Extensive studies have been carried out on how high concentration salts (dissolved in water) alter the stability and solubility of proteins. Such effects are thought to be mediated via salt-water interactions and water-protein interactions. This classic research field is known as the Hofmeister Series. We report the effects of Hofmeister Salts on the structural dynamics of proteins. Photoactive yellow protein (PYP), a bacterial blue light photoreceptor protein, is employed as a model system in this study. Time-resolved FTIR spectroscopic techniques allow us to probe the structural changes in proteins. Our data reveal that high concentration salt solutions alter the proton transfer pathway and suppress conformational changes in PYP upon photo-excitation. This study opens up a new dimension in the field of Hofmeister series. Further theoretical and experimental studies are needed in order to understand the dynamic properties of salt-water interactions and water-protein interactions. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D17.00005: Direct protein photoinduced conformational changes using porphyrins. Lorenzo Brancaleon, Ivan Silva, Nicholas Fernandez, Eric Johnson, Samuel Sansone Most proteins functions depend on the interaction with other ligands. These interactions depend on uniquely structured binding sites formed by the folding of the proteins. Ligands can often prompt intended as well as ``accidental'' protein structural changes. One can foresee that the ability to prompt and control post-translational protein folding could be a powerful tool to investigate protein folding mechanisms but also to inhibit certain proteins or induce new properties to proteins. One possible way to produce such structural disruption is the combination of light and photoactive ligands. This option has been investigated in recent years by exploiting photoisomerization and other properties of non-physiological dyes. We used an alternative approach which uses porphyrins as the ``triggers'' of structural changes. The advantage of porphyrins is that they can be found naturally in living cells. The photophysical properties of porphyrins can induce local as well as long range effects on the structure of the bound protein. Porphyrins are known to produce structural changes in porphyrin-specific proteins, however the novelty of our results is that we demonstrated that these dyes can also produce structural changes in non-porphyrin-specific globular proteins. We will present an overview of our research to-date in this field and its potential applications. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D17.00006: Computer simulations of the folding mechanism of the GCN4 Leucine zipper Yanxin Liu, Prem Chapagain, Jose Parra, Bernard Gerstman A modified three dimensional lattice model incorporating a Monte Carlo Metropolis Algorithm is used to investigate the dimerization of the GCN4 Leucine zipper. The model is validated with heat capacity calculations that are seen to match well with experiment measurements. The free energy landscape is investigated as a function of temperature. Evidence of multiple meta-stable states is found during the simulation. The possible folding and dimerization mechanism of the Leucine zipper will be discussed. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D17.00007: Describing protein folding through the evolution of spatial density Xianghong Qi, John J. Portman The capillarity-like structure of folding nuclei is directly characterized for a wide range of two state folding proteins within a variational model that includes ``neutral'' cooperativity. We find that on average the volume of the folded core depends on the number of monomers as $V_{\rm{f}} \sim N_{\rm{f}}^{0.3}$. The precise relation agrees with the packing of rigid objects that are typically twice the size of a monomer in the native state. Focusing on the growth of the folded core and the interface region, we identify three different growth modes: core and interface consolidation, core dominated consolidation, and balanced growth. We also show in detail how the density of the core and interface of critical nuclei determine the common qualitative characterization as either diffuse or polarized. Such analysis will aid interpretation of $\phi$-value distributions in terms of the spatial density or compactness of the critical nucleus which is much more difficult to probe experimentally than the degree of similarity to the native state. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D17.00008: The trigger sequence in the protein folding and dimerization of the Leucine zipper coiled-coil motif Prem Chapagain, Yanxin Liu, Bernard Gerstman The existence of a trigger sequence in the protein folding and dimerization of the Leucine zipper coiled-coil structure is attracting increased interest. Also of interest is the presence of multiple meta-stable states in the folding and unfolding process. Using a computer lattice model, we investigate the effect of the trigger sequence by changing the strength of the propensity of the amino acids in the trigger sequence to form alpha-helix secondary structure. The results show that the trigger sequence is necessary for folding and dimerization. The trigger sequence also creates a folding and dimerization process that includes multiple meta-stable states. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D17.00009: Correlation functions of flexible macromolecules Donald Jacobs, Dennis Livesay, Oleg Vorov We present an ab initio calculation of conformational entropy, radii of gyration, X-ray and neutron scattering form-factors, correlation functions, and other observables describing proteins, polypeptides, nucleic acids, related macromolecules and artificial polymers [1]. The analytic form of the method minimizes computational costs and reveals relations between observables. We apply these methods to study thermodynamics of protein unfolding. The presented results agree with the results from experiment and simulation [2]. \newline [1] O.K.Vorov, A.Y.Istomin, D.R.Livesay, D.J.Jacobs, subm. to Phys.Rev.Lett., 2007. [2] O.K.Vorov, D.R.Livesay, D.J.Jacobs, to be subm. to Proc. Natl. Acad. Sci., 2007, in preparation. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D17.00010: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 5:18PM - 5:30PM |
D17.00011: Photocycle of a single photoactive yellow protein molecule studied by surface-enhanced Raman scattering Kaan Kalkan, Kushagra Singhal, Wouter Hoff, Aihua Xie We have demonstrated the detection of single molecules of photoactive yellow protein (PYP), by employing our novel surface-enhanced Raman scattering (SERS) active substrates. The Raman spectra reveal both ``receptor'' (G) and ``signaling'' (B) states of PYP at the single molecule level (at 514 nm excitation). The single molecule spectra are observed in terms of sudden appearance of discernable Raman peaks, each indicative of a PYP molecule finding a hot spot. The SERS spectra also exhibit various peaks, which are not normally Raman-active. Although the PYP has a long-lived signaling state (i.e., $\sim$0.3 s), the Raman peaks identifying this state are found to be dramatically narrow at the single molecule level for signal integration times of 0.25-0.5 s. In several instances, we observed subsequent change of the spectrum from B to G state. Although, the PYP is not chemisorbed on the metal nanoparticles, its short-term pysisorption is anticipated to allow for the capture of its photocycle at the single molecule level. In addition to narrower and better resolved peaks, the single molecule spectra also show variation in relative peak intensities. In particular, the C-C stretching and C-H bending modes of the aromatic ring of the chromophore inversely correlate at the single molecule level, while their intensities are comparable in the ensemble-average spectrum. [Preview Abstract] |
Session D18: Polymers at Surfaces
Sponsoring Units: DPOLYChair: Alfred Crosby, University of Massachusetts - Amherst
Room: Morial Convention Center 210
Monday, March 10, 2008 2:30PM - 2:42PM |
D18.00001: Surface Segregation in Blends of Chains with Two Architectures Mark Foster, Sewoo Yang, Nam-Heui Lee, David Wu Blends of chains of two architectures, e.g. linear chains with pom-pom branched polymers, present opportunities for tailoring bulk rheology and surface character of the blends. Pom-pom chains contain a central linear portion between two junction points from which multiple arms extend. We have studied the effect of varying the relative length of the central linear portion of the pom-pom chain on the size of a single chain and on the bulk and surface thermodynamics of blends of the pom-pom chains with linear analogs. The strength of segregation to the surface and substrate interface of a blend film is seen with neutron reflectivity and surface enhanced Raman spectroscopy to depend on the relative length of the central linear portion of the pom-pom. Comparison will be made to a linear response theory that considers the number and type of chain ends and junction points. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D18.00002: Single chain mobility at an interface of a liquid polymer Jingfa Yang, Jiang Zhao Interfacial diffusion of single chains of polystyrene-b-polyisoprene (PS-b-PI) at the interface between polyisoprene and its non-solvent, DMF, was studied by fluorescence correlation spectroscopy. The diffusion coefficient of PS-b-PI probe was found to be two orders of magnitude high than that in the bulk PI, indicating a lower interfacial viscosity. The experimental data also exhibit a very weak dependence of the interfacial diffusion coefficient on the molecular weight of the liquid polymer. The possible mechanism was discussed. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D18.00003: Molecular origin of oil resistance of polyacrylonitrile: CN interactions at the surface Veronique Lachat, Ali Dhinojwala, Dennis Peiffer, Mohsen Yeganeh Nitrile rubber (NR) is a random copolymer of acrylonitrile and butadiene and is one of the best oil resistance polymers. The superior oil resistance property of nitrile rubber is thought to be directly related to the amount of acrylonitrile used in NR. Here, we report for the first in-situ sum frequency generation spectroscopy characterization of polyacrylonitrile/oil interactions. We demonstrate that CN-CN interaction is the chief reason for superior oil resistance property of NR. At room temperature, the interaction between the polymer chains is much stronger than the interaction between the polymer and solvent molecules including water and heptane. However, at high temperatures, the interaction between the nitrile groups of the polymer weakens making the interaction between the nitrile groups and the surface hydroxyls of the substrate and water possible. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D18.00004: Effect of Hydrogen Bonding on Colloidal Nanocrystal Growth: The Case for PbS Lixin Zhang, Shengbai Zhang The adsorption of methylamine (CH$_{3}$NH$_{2})$ on rocksalt PbS(111)-S surface is studied by first-principles total energy calculation. It was found that nitrogen lone pairs on the CH$_{3}$NH$_{2 }$ can form dative bonds with surface sulfur atoms. Such an interaction is unique in two ways: first it is non-local. Charge transfer takes place from CH$_{3}$NH$_{2}$ not only to the closest S but also to surface S further away. Second, the interaction is strongly affected by hydrogen bonds formed between CH$_{3}$NH$_{2}$ and solution molecules such as H$_{2}$O. Only by the latter effect can the PbS nanocrystals assume the (111) facets in a colloidal growth in agreement with experiment. In the past, studies of nanostructures have taken the assumption that, in the presence of a solution, the relative stability among the various facets will not change, at least not dramatically. Our study shows that such an assumption is not necessarily true. The significance of the study therefore goes beyond just the PbS or PbTe systems. Its basic principles should apply to most colloidal growth of solid-state nanostructures with broad implications. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D18.00005: Conformational behavior of polymers adsorbed on nanotubes Simcha Srebnik, Inna Gurevitz We study the interaction of a dilute solution of semiflexible polymers with a weakly attractive infinitely long nanotube using Monte Carlo simulation. Apart for bending stiffness of the polymer chains, the only interactions considered in our model are weakly attractive short-ranged Lennard-Jones interactions between the monomers and with the surface. These nonspecific interactions are found to result in stable helical and multi-helical adsorbed conformations for semiflexible chains. Adsorption of these chains is found to occur in a sequential manner through tight wrapping of the polymer around the nanotube. Adsorption occurs quickly and is characterized by a sharp peak in the heat capacity. A second transition follows whereby opening and reorganization of the adsorbed chains into nearly perfect helices and multiple helices. Extension of the model to block and triblock copolymers reveals rich conformational behavior. These results are discussed on physical grounds and implications towards polymer-carbon nanotubes composites are offered. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D18.00006: Origin of glass transition temperature behavior in polymer nanocomposites Jamie Kropka, Venkat Ganesan, Peter Green Local composition variations inherent in multi-component materials, even when the material constituents are miscible, generally lead to heterogeneous behavior in the properties of mixtures relative to their single component counterparts. In contrast, experiments have suggested that some polymer nanocomposite materials exhibit changes in their bulk T$_{g}$ without displaying excess heterogeneity in their dynamics, as measured mechanically, relative to the neat polymer. Incoherent neutron scattering measurements on materials that fit this description, C$_{60}$-PMMA mixtures, suggest that modifications of the polymer melt dynamics are limited to the vicinity of the particles. A model by which the localized modifications of polymer dynamics can account for the apparent homogeneous change in T$_{g}$ is proposed to explain the experimental findings. Computations based on percolation theory support the model. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D18.00007: Directed Self-Assembly of Gradient Concentric Carbon Nanotube Rings Suck Won Hong, Wonje Jeong, Hyunhyub Ko, Vladimir Tsukruk, Michael Kessler, Zhiqun Lin Hundreds of gradient concentric rings of linear conjugated polymer, (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylenevinylene], i.e., MEH-PPV) with remarkable regularity over large areas were produced by controlled, repetitive ``stick- slip'' motions of the contact line in a confined geometry consisting of a sphere on a flat substrate (i.e., sphere-on-flat geometry). Subsequently, MEH-PPV rings exploited as template to direct the formation of gradient concentric rings of multiwalled carbon nanotubes (MWNTs) with controlled density. This method is simple, cost effective, and robust, combining two consecutive self-assembly processes, namely, evaporation-induced self- assembly of polymers in a sphere-on-flat geometry, followed by subsequent directed self-assembly of MWNTs on the polymer- templated surfaces. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D18.00008: Use of Fluorescence Correlation Spectroscopy for Studying Polyelectrolyte-Nanoparticle Interaction in Aqueous Solution Nadia Edwin, Denis Pristinski, Chengqing Wang, Vivek Prabhu Formation of polyelectrolyte complexes is very facile via layer-by-layer sequential adsorption of oppositely charged species method. This method has been used to fabricate versatile materials with tailored properties. However, the fundamental assembly mechanisms of these advanced engineering materials and the adsorption kinetics of these systems is not completely understood. We use fluorescence correlation spectroscopy (FCS), which analyzes fluctuations in the fluorescence emission of molecular ensembles and provides the concentration, mobility, and dynamics of fluorescently labeled molecules, to study the interaction of polyelectrolyte-nanoparticle assembled structures to establish what's driving the adsorption of these systems, the dependence of molecular weight and the effects of variations in the solution environment with pH and ionic strength. Layer-by-layer assembly was performed on fluorescent dye-labeled spherical nanoparticles and amine and carboxyl functionalized polyelectrolytes in aqueous solution. Changes in the dynamics of the polyelectrolyte-nanoparticle system in response to various salt and pH conditions will be presented. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D18.00009: Directed Nanoparticle Assembly onto Random Copolymer Templates: Kinetics and Surface Considerations Marla McConnell, Shu Yang, Russell Composto Recent efforts have focused on the development of nanoparticle arrays with controlled spacing. In this study, poly(styrene-ran-acrylic acid) films were prepared by spin-casting poly(styrene-ran-t-butyl acrylate), followed by thermal deprotection. Silica nanoparticles (10-15 nm in diameter) coated with self-assembled monolayers (SAMs) of (3-aminopropyl)triethoxysilane were covalently attached to the PS-ran-PAA films with an EDC/NHS coupling reaction. To measure the kinetics of nanoparticle attachment, films of either 25 or 50 weight percent acrylic acid were reacted with nanoparticle suspensions from 0.005 to 0.1 weight percent for varying lengths of time. SEM imaging of the nanoparticle surfaces showed that the particles were well dispersed, and that particle coverage increased with increasing AA and nanoparticle concentration, and time. SAMs containing an acrylic acid moiety were used as a non-swelling control surface, and particle attachment to these surfaces follow different kinetics than those observed for the polymeric substrates. The swelling of the polymeric substrates under the reaction conditions was found to influence the observed coverage kinetics, so film swelling was monitored with environmental AFM. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D18.00010: Studies of the Dynamics of Alkane Nanoparticles S.-K. Wang, M. Bai, H. Taub, M. Rheinstadter, J. R. D. Copley, V. Garcia Sakai, G. Gasparovic, U. G. Volkmann, F. Y. Hansen Our AFM and synchrotron x-ray scattering measurements on dotriacontane (C$_{32}$H$_{66}$ or C32) deposited on SiO$_{2}$-coated Si(100) substrates reveal mesa-shaped nanoparticles that have an orthorhombic structure in which the C32 molecules are aligned perpendicular to the SiO$_{2}$ surface.$^{2}$ To investigate their dynamical properties, we have used both the backscattering and disk chopper spectrometers at NIST covering a wide range of time scales (1 ps -- 4 ns). Elastic scans obtained on both spectrometers show step-like changes in intensity as a function of temperature indicating the existence of phase transitions below the bulk C32 melting point. One of these steps occurs at the crystalline-to-rotator phase transition of bulk C32, but there is a second step at still lower temperature of unknown origin. The similarity of the elastic scans on the two spectrometers suggests that the crystalline-to-rotator phase transition involves diffusive motion spanning a wide range of time scales. $^{2}$M. Bai\textit{ et al.}, Europhys. Lett. \textbf{79}, 26003 (2007). [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D18.00011: Hierarchically Ordered Plasmonic Mask for Photo-lithography Woo Soo Kim, Edwin L. Thomas A new high density nanolithography method for the fabrication of a hierarchically ordered plasmonic mask employs silver (Ag) nano-particles (NPs) attached to the surface of an amine- functionalized two-dimensional (2D) pattern fabricated by laser interference lithography (IL). The bi-functional sol-gel hybrid material (BFHM) is a negative-tone resist and can be directly patterned by IL. Since the BFHM has both an amine-function and a methacryl function on each molecule, photopolymerization provides network formation and a set of binding sites for the Ag NPs. The Ag NPs were then attached onto the BFHM pillars by immersing the patterned sample in a solution. Hierarchically ordered arrays of Ag NPs could be made by a block copolymer comprised of 40nm diameter spherical P2VP domains having a spacing of 88nm, forming a hexagonal pattern covering the hexagonally arrayed BFHM pillars. Lithography experiments using 430 nm wavelength light demonstrate transfer of both a 350 nm periodic pattern and a 88nm patten to a positive-tone photoresist via plasmonic field enhancement arising from the collective and individual excitation of the closely spaced interacting Ag NPs on the hierarchically patterned BFHM. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D18.00012: Dynamics of an Adsorbed Polymer Chain Joshua Kalb, Sanat Kumar Because of the current precision in fluorescent labeling, it is possible to label single polymers such as DNA or PEG and track their dynamical and equilibrium properties in the bulk as well as near attractive surfaces[Maier et. al., Macro 2000][Sukhishvili et. al., Macro 2002]. Recent evidence from these experiments and related simulations has shown that the dynamics of a single polymer near an attractive surface appear diffusive, however further evidence coming from the 'diffusion coefficient' implies a different process other than diffusion is at work such as reptation, `hovercrafting', or `hopping' [Sukhishvili et. al., Macro 2002]. In general, these possible dynamical behaviors are determined by the length of the polymer itself as well as the microscopic details of the attractive surface which include the density, strength, and distribution of attractive surface sites[Desai et. al., PRL 2007][Qian et. al., PRL 2007]. In this presentation, we investigate the effects of microscopic surface effects on single polymer dynamics through Monte-Carlo and molecular dynamics simulations. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D18.00013: Chasing drops: Following escaper and pursuer drop couple systems Aisha Leh, Rafael Tadmor, Preeti Yadav, Prashant Bahadur, Kumud Chaurasia, Lan Dang We study the Marangoni flow induced by two chemically different drops resting on a solid surface in air. We show that in all the systems studied, the Marangoni flow is induced at the solid-liquid interface as opposed to the air-liquid interface. This is true even for the case of water drop and alcohol drop on a glass surface (corresponding to the classical ``tears of wine'' case). Thus we explain the drop motion as a result of a surface tension gradient which takes place primarily at the air-surface region (and less at the drop-substrate or drop-air interfaces). The discontinuous motion of the drops, characterized by stops and jumps as in a ``stick slip'' mechanism is explained by an increase in the Laplace pressure that creates a higher anchoring pinning effect at the front edge of the moving drop. We discuss this in connection to the ``tears of wine'' case. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D18.00014: The Measurement of Surface Rheological and Surface Adhesive Properties using Nanosphere Embedment Stephen Hutcheson, Gregory McKenna In previous work, we determined the actual rheological behavior at the surface of a polystyrene film with nanometer scale resolution by applying a viscoelastic contact mechanics model to experimental data in the literature. The goal of our current research is to build upon this analysis and use nanosphere embedment experiments to probe the nanorheological behavior of polymer surfaces near the glass transition, in the melt state and in the solid rubbery state. An atomic force microscope (AFM) is used to measure the embedment depth as nanoparticles are pulled into the surface by the thermodynamic work of adhesion. The results show that, with properly designed experiments, both the surface adhesion properties and the surface rheological properties can be extracted from nanosphere embedment rates. We include work on a phase separated copolymer and a commercially available polydimethylsiloxane (PDMS) rubber. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D18.00015: Slip behavior of the confined polymer melt near periodically roughened surface: comparison between molecular dynamics and continuum simulations Anoosheh Niavarani, Nikolai Priezjev Molecular dynamics (MD) simulations are used to investigate the behavior of the slip length in the Couette flow of a polymer melt. For atomically smooth surfaces and weak wall-fluid interactions, the shear rate dependence of the slip length is a non-monotonic function, with a distinct local minimum. For corrugation wavelengths larger than the radius of gyration of polymers, the decay of the slip length with corrugation amplitude obtained from MD simulations agrees well with the continuum predictions for the following cases: (1) Stokes solution with constant local slip length, (2) Stokes solution with local shear-rate-dependent slip length, and (3) Navier- Stokes solution with local rate-dependent slip length. If the corrugation wavelength is less than or on the order of the radius of gyration, the continuum predictions (the Stokes solution) overestimate the values of the slip length extracted from MD simulations. The analysis of the conformational properties of the polymer melt indicates that polymer chains tend to stretch in the direction of shear at the peaks of the sinusoidal wave and align themselves along the bottom of the grooves. [Preview Abstract] |
Session D19: Focus Session: Dopants and Defects in Semiconductors I
Sponsoring Units: DMPChair: Anderson Janotti, University of California, Santa Barbara
Room: Morial Convention Center 211
Monday, March 10, 2008 2:30PM - 3:06PM |
D19.00001: Stability and Dynamics of Frenkel Pairs in Silicon Invited Speaker: Extensive EPR experiments on electron irradiated \emph{p}-Si observe only signals identified as arising from isolated vacancies and interstitial impurities. Subsequent experiments and calculations demonstrated that isolated interstitials in \emph{p}-Si diffuse athermally according to a charge-carrier- mediated mechanism. The overall conclusion has been that Si FPs either rapidly recombine or dissociate, even at cryogenic temperatures. More recent X-ray scattering experiments, however, suggest that Si FPs persist at temperatures up to 150 K. We report first-principles calculations of Si FP properties and resolve the apparent conflict between experiments. We find that the vacancy and interstitial components of a proximal FP interact electronically, suppressing the previously identified athermal interstitial diffusion. Such proximal FPs are bound only by the presence of barriers to either recombination or dissociation. Further, metastable FPs may lower their energy by transferring electrons from the interstitial to the vacancy component. We show that EPR studies of FPs are likely unable to distinguish between FPs and isolated vacancies. In addition, calculated diffusion barriers for FP components indicate that FPs should anneal at temperatures similar to those for isolated vacancies: $\sim$150 K. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D19.00002: Defect Formation Energies without the Band-Gap Problem: Combining DFT and \textit{GW} for the Silicon Self-Interstitial P. Rinke$^1$, A. Janotti, C.G. Van de Walle, M. Scheffler For the self-interstitial in silicon, a defect of high technological relevance, density functional theory (DFT) in the widely applied local-density approximation (LDA) underestimates the formation energies of different configurations in the neutral charge state by $\sim$1.5 eV compared to diffusion Monte Carlo calculations [1,2]. We attriubte this to artificial self-interaction and the absence of the derivative discontinuity in the LDA exchange-correlation potential that give rise to the band-gap problem. We present a new formalism that combines LDA with quasiparticle energy calculations in the $G_0W_0$ approximation to overcome these deficiencies. The formation of the neutral defect is expressed as successive charging of its 2+ charge state, for which the defect level is unoccupied, permitting a decomposition into a lattice (LDA) and an electron addition part ($G_0W_0$) [3]. The $G_0W_0$ corrections increase the LDA formation energy by $\sim$1.1~eV. Moreover, the $G_0W_0$-corrected charge transition levels agree well with recent measurements [4]. [1] Batista {\it et al.} PRB {\bf 74}, 121102(R) (2006), [2] Leung {\it et al.} PRL {\bf 83}, 2351 (1999), [3] Hedstr\"om {\it et al.} PRL {\bf 97}, 226401 (2006), [4] Bracht {\it et al.} PRB {\bf 75}, 035211 (2007) [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D19.00003: Density functional study of charged self-interstitials in silicon Amita Wadehra, John W. Wilkins, Richard G. Hennig The self-interstitials in silicon created by ion-beam processing determine migration rate of impurities, time evolution of dislocations and dopant-enhanced diffusion. The large mobility of interstitials makes their experimental observation difficult. Electron-assisted transport mechanism suggests that charged states with different migration barriers and minimum energy positions are responsible for this rapid migration. Therefore, it becomes necessary to identify the stable ground state charges for each geometry and electron chemical potential. Recent studies of energetics and migration of these defects have largely concentrated on neutral defects and a few on charged single interstitials. We present a density functional study of electronic structure and energetics of charged single-, di- and tri-interstitials in silicon. An analysis of stability of five different charged states in various geometries is provided through formation energies. The diffusion pathways of these charged interstitials are also discussed. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D19.00004: Large scale ab initio calculations for shallow acceptor levels in bulk Si Lin-Wang Wang Accurate calculation of shallow donor levels in conventional semiconductors is a long standing challenge due to the large supercell needed for such calculation. We have used the charge patching method and local density approximation (LDA) to study the acceptor levels in bulk Si, including B, Al, Ga, In and Tl acceptors. The atomic positions are relaxed under LDA using 512 atom cells, and 64,000 atom supercells are used to calculate the acceptor energy levels to achieve the converged results. The calculated impurity binding energies reproduce the experimental trend from B to Tl. However, there is still a significant difference between the calculated binding energies and the experimental results, especially for Tl. This raises the question of whether the LDA can be used to calculate the shallow impurity level accurately. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D19.00005: Relative stability of extended interstitial defects in silicon: large-scale classical MD and first-principles DFT. Hyoungki Park, John Wilkins Extensive simulations for defected Si reveal the relative thermal and temporal stability of extended interstitial defects: {311} and {111} rod-like defects, and Frank dislocation loops. Classical molecular dynamics simulations provide the atomic configurations of those defects, and show that the energetically favored structures change from {311} rod-like defects to Frank loops as the number of interstitals increases, which is consistents with the experimentally-observed transition from rod-like defects to Frank loops after long, high-temperature annealing processes [1,2]. This relative stability is validated with massively parallelized density-functional calculations of ~1500-atoms 2D supercells. Relaxation of experimentally-observed-size defect cluster demonstrates the energetic hierarchy is dependent on the number of interstitials in the defect clusters. [1] L. S. Robertson et al., J. Appl. Phys. 87, 2910 (2000). [2] G. Z. Pan et al., J. Non-Crystalline Solids 352, 2506 (2006). [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D19.00006: Ortho-para transition of interstitial H$_{2}$ in Si Michael Stavola, Chao Peng, Megan Lockwood Interstitial H$_{2}$ in Si is a nearly-free rotator and has ortho and para species with the nuclear spins of the two protons aligned either parallel or antiparallel [1]. If one waits a sufficiently long time at low temperature, H$_{2}$ will relax to its lower energy para state. The ortho-para (o-p) transition for H$_{2}$ in Si has been observed in recent Raman studies [2]. We have performed IR absorption experiments to investigate issues that have proved difficult to study by Raman. When a Si sample containing H$_{2}$ is stored for a month or more at 77K, the 3618.4 cm$^{-1}$ IR line assigned to o-H$_{2}$ [1] is reduced in intensity because, when the o-p transition occurs, p-H$_{2}$ is not seen by IR. When this sample was annealed at room temperature, the ortho population characteristic of room temperature was recovered with a time constant of $\approx $6 hrs. Our IR studies of the kinetics of the o-p transition complement recent Raman results and suggest that the cause of the o-p transition is not yet understood. \newline [1] M. Stavola \textit{et al.}, Physica B \textbf{340-342}, 58 (2003). \newline [2] M. Hiller \textit{et al.}, Phys. Rev. Lett. \textbf{98}, 055504 (2007); \textbf{99}, 209901 (2007). [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D19.00007: Carbon Impurity Effects on Structural and Magnetic Properties of Manganese Doped Silicon Joshua LaRose, Roger Pink, Tara P. Das, Mengbing Huang, Jian-Qing Wang The recent finding of room temperature ferromagnetism in Mn-doped Si may open up a promising route toward Si-based spintronics. In this work, we investigate effects of co-doped carbon on the structural and magnetic properties of Si:Mn, in a hope to identify possible microstructures responsible for ferromagnetism. Carbon atoms of 0.25 at. {\%} are uniformly doped within the 250-nm surface layer of Si(100) using ion implantation. The C-rich Si and the Si control are subsequently implanted at 300 \r{ }C with Mn ions, yielding a concentration profile of Mn ([Mn] $\sim $ 0.25 at. {\%}) within the depth of 160 nm. Post-implantation annealing is conducted in the range of 800-1000 \r{ }C. Ion channeling measurements suggest that Mn could occupy several lattice sites in Si including the tetrahedral interstitial and substitional site, with respective occupancy affected by carbon impurity and thermal annealing. The Hartree-Fock Cluster Method is used to calculate the binding energies of Mn for different lattice sites in Si. These structural information are compared with the results of superconducting quantum interference device (SQUID) experiments. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D19.00008: Thermal Stability and Laser Annealing of Si$_{1-y}$C$_{y}$ alloys Stefan Zollner, P. Grudowski, V. Dhandapani, G. Spencer, A. Thean Dilute alloys of silicon and carbon are metastable, but can be produced (up to 3{\%} C) using nonequilibrium growth techniques, such as chemical vapor deposition. In such alloys, carbon atoms are located at lattice sites (preferred for device applications) or at interstitial sites. Other impurities (such as H) can be introduced during the growth process. Since Si:C alloys are metastable, they usually do not survive typical source-drain dopant activation anneals (on the order of 1000C for 5s). Also, Si:C alloys implanted with NMOS dopants do not recrystallize during conventional source-drain dopant activation anneals. We report here that the vertical lattice constant of as-grown Si:C alloys (measured by XRD) decreases after laser annealing ($\sim $1 ms near the melting point), leading to an increase in the measured substitutional carbon content. This indicates a conversion of interstitial carbon defects into substitutional carbon or an evolution of hydrogen. We describe our results using a multiscale model applicable to thermal processing over a broad range of temperature and anneal times. Our model describes both solid-state regrowth and the loss or gain of substitutional carbon after annealing. We also present NMOS transistor results, where embedded Si:C alloy source-drain stressors lead to a reduction in the channel resistance. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D19.00009: Chalcogen dopants for infrared optoelectronic Si Jeffrey Warrender, Brion Bob, Michael Aziz, Supakit Charnvanichborikarn, James Williams, Malek Tabbal, Atsushi Kohno Doping Si with a chalcogen in excess of the solubility limit has been shown to result in subbandgap optical absorption and sensitive photodetection, suggesting potential for chalcogen-doped Si as an infrared optoelectronic material. We investigated optical absorption and photovoltaic energy conversion using S, Se, and Te as dopants. We achieved supersaturation of the chalcogen dopant by ion implantation followed by pulsed laser melting and rapid solidification. We observed broadband subbandgap absorption for all dopants over a wavelength range from 1 to 2.3 microns. The subbandgap absorption and photovoltaic response depended sensitively on the chalcogen dose, laser processing, and thermal annealing conditions. We correlate these observations with the corresponding influence of the processing conditions on the material's crystalline quality, chalcogen dopant depth profile, carrier concentration profile, and dopant activation. We found good agreement between the chalcogen depth profiles obtained from experiments and a 1-dimensional model for plane-front melting, solidification, liquid-phase diffusion, and kinetic solute trapping. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D19.00010: H-shuttling within a Hf-defect complex in Si/SiO2/HfO2 structures A.G. Marinopoulos , I. Batyrev, X. Zhou, R. Schrimpf, D. Fleetwood, S.T. Pantelides It was recently shown that, following irradiation of Si-SiO2/HfO2 structures by X-rays or constant-voltage stress, both oxide- and interface-trap densities exhibit oscillations with switch-bias annealing that are much larger than those previously observed in Si/SiO$_{2}$ devices. Here we describe a particular defect complex that can account for the observations. The complex comprises a suboxide Hf-Si bond and an interfacial dangling bond (P$_{b}$ center). With the aid of first-principles calculations we show that this defect possesses a symmetric double-well minimum and can provide trapping sites for H atoms near the interface. In the first site, the H atom passivates the dangling bond; in the second site the H atom resides near the center of the Hf-Si bond. A moderate intervening barrier (1.2 eV) suggests a relatively easy hopping of H atoms between these two energy minima, aided by the applied field and temperature. This shuttling mechanism can explain the observed oscillations in the interface trap densities during switched-bias conditions. This work was supported in part by the AFOSR and the DOE. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D19.00011: Transition from high to low $1/f$ noise regimes in Field Oxide Field Effect Transistors (FOXFETs) Xing Zhou, Daniel Fleetwood, Ronald Schrimpf, Laura Gonella, Federico Faccio The excess low frequency ($1/f)$ noise of parasitic field oxide FETs from a 130 nm technology has been found to vary by more than $\sim $6 orders of magnitude with gate voltage, above the nominally measured device threshold. We find that this variation is due to a transition from noisy subthreshold conduction to full conduction in strong inversion at a point that is more than 5 V above the standard extrapolated threshold voltage. This field oxide structure has a length of $\sim $ 1 micron and a width of $\sim $ 200 micron. We attribute the conduction below to a noisy, subthreshold (perhaps even percolative) path at lower voltages, with a significant contribution from the high density of defects at the Si/SiO$_{2}$ interface in this parasitic FOXFET structure. The noise above the ?true? threshold (as determined with assistance from the noise measurements) follows a standard number fluctuation model, when the subthreshold conduction regime is separated out in the analysis. This work was supported in part by the US Navy. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D19.00012: Effects of Aging and Humidity on Low-Frequency Noise of Metal-Oxide-Semiconductor (MOS) Transistors Aritra Dasgupta, S.A. Francis, D.M. Fleetwood Low frequency noise measurements can provide a non-destructive method of measuring radiation hardness and/or reliability of MOS transistors. We have been studying the effects of moisture exposure at elevated temperatures on MOS low frequency noise. The devices under test were manufactured in the 1980s and came from two different process lots. The results show that the normalized 1/$f$ noise magnitudes $K$ of the pMOS transistors increased significantly with exposure to humidity at elevated temperatures, while the changes in the 1/$f$ noise magnitudes of the nMOS transistors were mostly much less. To estimate the energy dependence of the defects responsible for the noise, we evaluated the gate voltage dependence (Vg-Vt) of the noise, where Vt is the threshold voltage. We find, for the pMOS transistors, Svd$\sim $(Vg-Vt)$^{-1}$, whereas, for the humidity exposed nMOS transistors, Svd$\sim $(Vg-Vt)$^{ -1.25}$ These deviations from an inverse square gate voltage dependence indicate a strong energy dependence of the defect distribution due to humidity exposure. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D19.00013: Disorder--recrystallization effects following low-energy beam--solid interactions Matthew J. Beck, D. M. Fleetwood, R. D. Schrimpf, S. T. Pantelides Classical MD simulations have shown that thermal-spike-related disorder, including local melting, should be widely expected following high energy ($>$1 keV) recoils resulting from beam-- solid interactions during ion-beam processing. In contrast, the formation of isolated point defects by direct atomic displacement is expected for low energy ($<$1 keV) recoils. Using state-of-the-art dynamical DFT calculations of \emph{c}- Si systems we show that recoils of much less than 1 keV result in highly disordered regions which persist for 100s of fs. Therefore, the production of beam-induced defects, as well as the post-implant yield of active dopants, following low-energy beam--solid interactions is controlled by dynamic recrystallization processes. This work was supported in part by the AFOSR through a MURI grant. [Preview Abstract] |
Session D20: Focus Session: Growth and Properties of Novel Semiconductor and Related Nanostructures
Sponsoring Units: DMPChair: Ray Phaneuf, University of Maryland
Room: Morial Convention Center 212
Monday, March 10, 2008 2:30PM - 3:06PM |
D20.00001: Silicon Nanomembranes Invited Speaker: Silicon nanomembranes (SiNMs) are extremely flexible, strain-engineered, defect-free, thin \underline {single-crystal} sheets, with thicknesses from several 100 nm to less than 10 nm. Their novelty is several-fold: they are flexible, they are readily transferable to other hosts and conform and bond easily, they are stackable, and they can take on a large range of shapes (tubes, spirals, ribbons, wires) by engineering the strain and patterning the geometry. One can thus think of SiNMs as having inexact and tunable dimensionality, from 3-D to 0-D (when growth of quantum dots is included). Many properties of bulk Si are modified by thinness, strain, shape, and size, including band structure and quantum properties, electronic transport, phonon distributions, and mechanical properties. Because they are so close, the two surfaces of the membrane can influence each other's behavior, and the surface also becomes a significant influence on overall SiNM properties. After a review of SiNM fabrication, strain engineering, and transfer, we overview some of the unexpected physical and electronic properties of SiNMs. These include surface transfer doping via surface structures or adsorbed layers, through-membrane elastic interactions to create periodic strain lattices, energy level splitting and shifting with strain and quantum size effects, and orientation-dependent mobility enhancement with strain. SiNMs provide the potential for new or enhanced application of Si in fast flexible electronics; quantum electronics, new nanophotonic, optoelectronic, and thermoelectric devices; and chemical and biological sensors. These applications will be briefly outlined. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D20.00002: Controlled displacement of nanoscale structures using an electron wind force C.G. Tao, W.G. Cullen, E.D. Williams Electromigration is widely used to drive mass transfer in the fabrication of nanogaps, and will be a crucial issue for the structural stability, reliability and performance of nanoscale electronic devices. Using a combination of scanning tunneling microscopy and scanning electron microscopy, we directly observe the biased motion of monatomic islands driven by the electron wind force on patterned single-crystal Ag(111) thin films. The island motion can be steered by changing the direction of the applied electric current. For monatomic adatom islands, the biased motion is opposite to the current direction and along the wind force direction, while vacancy islands move in the opposite direction. The measured dependence of the drift velocity on the island size, yields the product of the diffusion constant and the magnitude of the wind force, giving the diffusion constant $D$ = 1.56 $\times $ 10$^{6}$/$z*$ (nm$^{2}$/s) for an effective charge of $z*$ =360 [1]. The wind force acts even more strongly on C$_{60}$-decorated structures, as observed by directionally-oriented bending of step edges. The wind force needed to cause the observed structure distortions is $F$ = 0.13 meV/nm, about 3 times the corresponding wind force acting on the bare steps. [1] A. Bondarchuk, et al. PRL 99, 206801 (2007). [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D20.00003: Lateral alloy segregation in thin heteroepitaxial films Christian Ratsch, Jason Reich, Xiaobin Niu, YoungJu Lee, Russel Caflisch We have studied the segregation and alloy formation of thin heteroepitaxial films. We use an atomistic strain model that has a cubic geometry and includes nearest neighbor bonds, next nearest neighbor bonds, and bond bending terms. Our motivation is the well established fact that for many heteroepitaxial systems growth proceeds in the Stranski-Krastanov growth mode, where islands form after the formation of a wetting layer. Recent results indicate that intermixing and thus vertical variations of the alloy concentration are a crucial factor in controlling the formation and thickness of the wetting layer. Our results suggest that in addition to vertical segregation there is also lateral segregation. Thermodynamically, the system prefers to have one big feature of the epilayer material that is embedded in the substrate but is near the surface. In practise, there will be a typical separation distance of these features because of kinetic limitations. We postulate that this lateral segregation and the separation of these features is ultimately responsible for the lateral placement of islands on the surface. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D20.00004: Two Species Diffusion Model of Self-Organized Evolution on Patterned GaAs(001) Surfaces* Hung-Chih Kan, Erin Flanagan, Tabassom Tadayyon-Eslami, Subramaniam Kanakaraju, Chris Richardson, Ray Phaneuf We report on numerical simulations of the self-organized evolution on GaAs(001) surface, pre-patterned with square arrays of pillars, during homo-epitaxial growth. Our experiments showed that lithographically fabricated, flat-topped cylindrical pillars evolved into a universal, downward paraboloidal shape, for initial diameters of the pillar ranging from 0.7um to several microns. In modeling this behavior, we construct a two-species diffusion model to simulate the growth. We consider the diffusion of both the Ga atom and As$_{2}$ dimers deposited on terraces between the concentric loop steps which make up the sidewall of the pillar. The stoichiometry for incorporating the diffusing Ga atoms and As$_{2}$ dimers into solid GaAs at the step edges produces boundary conditions that couple the flux of both diffusising species. We compare the results of our numerical simulation to the observed self-organization of the topography. *supported by the Lab for Physical Sciences and by~NSF{\#} DMR-0705447.~~ [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D20.00005: Directed Matrix Seeding of Nitride Semiconductor Nanocrystals A.W. Wood, B.L. Cardozo, W. Ye, X. Weng, R.S. Goldman, Y.Q. Wang The controlled formation of semiconductor nanocomposites offers a unique opportunity to tailor functional materials with a variety of novel properties. A promising approach to nanocomposite synthesis is matrix-seeded growth, which involves ion-beam-amorphization of a semiconductor film, followed by nanoscale re-crystallization via annealing. In this work, we are studying the formation and evolution of N ion-implanted InAs and GaAs (InAs:N, GaAs:N). The InAs:N and GaAs:N nanocomposites are synthesized using 100keV ion-implantation with a dose of 5x10$^{17}$cm$^{-2}$, at 77K and 300C, respectively. In all cases, the as-implanted structures are primarily amorphous, and after appropriate rapid thermal annealing (RTA) sequences, zincblende (ZB) InN and GaN [1] nanocrystals are formed. We are also developing a novel approach to \textit{direct} the seeding of nanostructure arrays, using a combination of focused-ion-beam (FIB) implantation in combination with conventional ion implantation. To date, we have demonstrated the selective positioning of wurtzite (WZ) and ZB GaN nanocrystals using 75keV and 100keV N implantation, followed by FIB patterning and 800C RTA. The growth mechanisms and structural evolution of nitride crystallites will also be discussed. [1] X. Weng, et al, \textit{J. Appl. Phys}., \textbf{92} 4012 (2002) [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D20.00006: Nanofabrication of carbon materials Dinko Chakarov, Hans Fredriksson, Bengt Kasemo We demonstrate a process for fabrication of nanostructures on the surfaces of highly oriented pyrolytic graphite (HOPG) and glassy carbon (GC) samples. Using hole-mask colloidal lithography (HCL), nanosized etch masks with three different feature diameters were prepared by identical processes on each of the two surface types. Oxygen reactive ion etching (RIE) was then used to transfer the mask pattern onto the surfaces. The structures were characterized using atomic force- (AFM), scanning electron microscopy (SEM) and optical spectrophotometry. The identical preparation schemes applied to the two materials yield structures with remarkably different shape and sizes. For example the process that yields 361 nm high and 37 nm diameter structures on glassy carbon yields 120 nm high and 119 nm diameter structures on HOPG. In general, the diameters of the fabricated GC nano-features are always at least 80 nm smaller than those of the corresponding HOPG structures, and the GC structure heights are more than three times that of the HOPG structures. These differences are attributed to different (an)isotropic etching behavior of the two materials. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D20.00007: Fabrication of Metallic Nanoporous Films by Selective Chemical Etching Shilpa Chava, Wei Jiang Yeh The objective of this study is to synthesize and characterize different nanoporous structures by chemical etching. The experiments were conducted on three different materials: We treated 6 carat white gold (Au/Ag alloy, 1:3 ratio by weight) with 70{\%} HNO$_{3}$ to grow Au nanoporous, the 50/50 solder wire (Pb/Sn alloy) with 93{\%} H$_{2}$SO$_{4}$ to create Pb porous and Imitation Italian gold leaf (Cu/Zn alloy, 82/18 by wt. {\%}) with NaOH solution (5 gm NaOH per 100 ml distilled H$_{2}$O) for Cu porous. The free-standing porous films have been analyzed by scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), high quality x-ray mapping (XRM). We observed the composition of the porous materials at every stage of chemical dealloying and conducted tests with different process parameters to optimize the size of self-ordered porous structures. Our experiments resulted in sponge like Au nanoporous of 10-200 nm, Pb pores of 10-300 nm and Cu pores of 10-150 nm. The results showed a technically improved fabrication of different nanoporous materials with high surface area and well defined pore morphology. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D20.00008: Formation of Periodic 2D Metallic Nanostructures by Template-Assisted Electrodeposition Mu Wang, Bo Zhang, Yuyan Weng, Ruwen Peng, Naiben Ming Two-dimensional ordered metallic nanostructures on solid surface with specific patterns may have potential applications in photonics and optoelectronics. Yet it remains a challenge to produce regular nanostructures over a large area with low cost and with a simple method. Here we report a novel method to fabricate well-aligned copper nanowire array on silicon surface by template-assisted electrodeposition. The template is introduced onto silicon surface by nanoimprinting. With our previously reported unique electrodeposition system [1-2], we find that the array of straight copper wires with their width varying from 400 nm to 20 nm can be fabricated. The wire width can be tuned by the control parameters in electrodeposition. It is shown that this method is not limited to straight wires only. It can be used to form more complicated patterns. The physical properties of the metallic nanostructures are also discussed. [1] M. Zhang, S. Lenhert, M. Wang, L. Chi, et al., Adv. Mater. 16, 409 (2004) [2] M. Wang, S. Zhong, X. Yin, J. Zhu, et al., Phys. Rev. Lett., 86, 3827 (2001) [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D20.00009: Phase transition induced surface electronic states on Pb/Si(111) surface Hsing-Yi Chou, Wei-Bin Su, Chi-Lun Jiang, Ming-Chi Yang, Chun-Liang Lin, Chia-seng Chang, Tien-Tzou Tsong It is known that the 1$\times $1 phase of a monolayer Pb on Si(111) surface at room temperature may undergo a phase transition into a $\surd $7x$\surd $3 phase at a low temperature below 250K. We use scanning tunneling spectroscopy to study electronic structures on both 1$\times $1 and $\surd $7x$\surd $3 phases. Our observation reveals that the electronic structures of Pb overlayer are significantly affected because of phase transition. In tunneling spectra there appears two distinct peaks on $\surd $7x$\surd $3 phase but they disappear on 1$\times $1 phase, indicating that the phase transition can induce the formation of the surface electronic states on $\surd $7x$\surd $3 phase. Moreover, the peak intensity is location-dependent and the relative strength at the low-energy peak can be reversed at the high-energy peak. These phenomena can be qualitatively explained by Kronig-Penney model. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D20.00010: Fabrication of High-Aspect-Ratio Nanogaps Alexandra Fursina, Sungbae Lee, Douglas Natelson For nanoscale electrical characterization and device fabrication it is often desirable to fabricate planar metal electrodes with separations well below 100 nm running parallel over a macroscopic width. In this work we demonstrate a self-aligned process to accomplish this goal using a thin Cr film as a sacrificial etch layer. The resulting gaps can be as small as 10 nm and have aspect ratios exceeding 1000, with excellent interelectrode isolation. Two separate lithographic patterning steps are used to define first and second electrodes while the interelectrode separation is controlled by the oxidation of a Cr layer deposited upon the first electrode. Advantageously, only a $\mu $m-alignment of first and second electrodes is required and the described method effectively does not have limitations on the gap width while the length of the gap is controlled by the Cr layer thickness. In addition to fabrication of Ti/Au electrodes on Si substrates, our technique was also demonstrated to work for other electrode metals (Pt, Fe, etc.) even on such relatively reactive substrates as magnetite, F$_{3}$O$_{4}$, films, thus demonstrating the flexibility and utility of this method. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D20.00011: Subtle role played by H in Si thin-film growth from radicals: key atomic-scale mechanisms revealed by DFT calculations F. Montalenti, S. Cereda, Leo Miglio, F. Zipoli, M. Bernasconi, M. Ceriotti Breaking silane molecules and creating reactive radicals in the gas phase is an efficient strategy for growing Si films at high growth rates and/or moderate temperatures. In a seminal experimental paper [1], the possibility of obtaining crystalline growth down to T$\sim$200$^\circ$C, was clearly demonstrated under high dilution of radicals in H. Several interpretations, in some cases controversial, have been given for explaining this evidence. Here we shall show that a clear understanding can be reached by relying on DFT calculations. Starting by a fully hydrogenated Si(001)(1$\times$2) surface, typical of low-temperature growth, we first illustrate the role played by SiH$_3$ in removing adsorbed H, therefore creating empty sites for further SiH$_3$ adsorption [2]. The adsorbed sylil, however, is frozen in its initial, non-epitaxial configuration, so that crystalline growth cannot take place. We demonstrate that further incoming hydrogen can easily transform silyl into SiH$_2$ which, in turn, incorporates into epitaxial sites crossing a barrier of only $\sim$1 eV [3], compatible with Ref. [1] conditions. [1] C.C. Tsai et al., J. Non-Cryst. Solids 114, 151 (1989). [2] S. Cereda et al., Phys. Rev. B 75, 235311 (2007), Phys. Rev. Lett. (in press). [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D20.00012: Kinetic Monte Carlo Simulation of Plasma Deposition of Silicon Thin Films Sumeet Pandey, Dimitrios Maroudas We report results from kinetic Monte Carlo simulations of plasma deposition of silicon thin films under conditions that render the SiH$_{3}$ radical the dominant deposition precursor. The transition probabilities for the various kinetic events accounted for in the simulations are based on first-principles density functional theory (DFT) calculations of the corresponding optimal pathways on the H-terminated Si(001)-(2$\times $1) surface and on molecular-dynamics simulations on hydrogenated amorphous silicon film surfaces. The relevant surface transport and reaction processes include SiH$_{3}$ diffusion, SiH$_{3}$ chemisorption and insertion into Si-Si bonds, surface H abstraction reactions, surface hydride dissociation reactions, as well as SiH$_{4}$ and Si$_{2}$H$_{6}$ desorption into the gas phase. Surface etching is predominantly observed over the 373-640 K temperature range. The surface compositions obtained are in good agreement with experimental measurements on films deposited under similar growth conditions. At 500 K, surface SiH$_{2}$ formed by surface trihydride dissociation reactions is the dominant surface hydride species. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D20.00013: Isotropic interfaces in a structurally anisotropic organic thin film C. G. Tao, Q. Liu, B. Riddick, W. G. Cullen, D. Evans, J. Reutt-Robey, J. D. Weeks, E. D. Williams We investigate the interfacial boundary fluctuations of Acridine-9-Carboxylic Acid (ACA) deposited on Ag(111) using UHV STM. The ACA molecule is anisotropic in shape and intermolecular interactions, and has been shown to exhibit a disordered 2D gas phase on Ag(111) at low coverage\footnote{B. Xu et al., J. Phys. Chem. B 110, 1271 (2006)}. At higher coverage, the molecules arrange in domains of ordered chain-like structures which coexist with the disordered phase. We measure the real-time fluctuations at the phase boundaries, and show that these fluctuations are governed by molecular exchange between the two phases. Due to structural anisotropy, there are two types of domain boundaries with significantly different molecular interactions. Surprisingly, the fluctuation magnitudes, mobilities, and free energies are nearly equal for the two boundary types. A lattice-gas statistical model is presented which includes the influence of molecular conformations on substrate interactions, and reproduces the essential features observed experimentally: domain shapes, boundary fluctuations, and phase densities. [Preview Abstract] |
Session D21: Focus Session: Clusters, Cluster Assemblies, Nanoscale Materials II
Sponsoring Units: DCPChair: Ulises Reveles, Virginia Commonwealth University
Room: Morial Convention Center 213
Monday, March 10, 2008 2:30PM - 3:06PM |
D21.00001: From Superatoms to Cluster Assembled Materials Invited Speaker: A collaborative effort with the theoretical group of S.N. Khanna at VCU has led to the concept of superatoms comprised of clusters which mimic elements of the periodic table. The latest advances will be presented which support the contention that there should be no limitation in developing a 3-D periodic table based on this idea. As the behavior of clusters can be controlled by size and composition, the superatoms offer the potential to create unique compounds with tailored properties. One of the prime objectives of current research is to lay the foundation for forming new nanoscale materials utilizing these ``elements'' as the building blocks. This is viewed as one of the most promising frontiers in materials research. The current status of success in this endeavor will be discussed. AWC, Jr. gratefully acknowledges the United States Air Force Office of Scientific Research, Grant {\#}FA9550-07-1-0151, the U. S. Department of Energy, Grant No. DE-FG02-02ER46009, and the U.S. Department of the Army through a MURI Grant {\#}W911NF-06-1-0280, for financial support of the experimental work reported herein. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D21.00002: From Designer Clusters to Synthetic Crystalline Nano-Assemblies Meichun Qian, S. Khanna, A. Reber, A. Castleman, A. Sen, A. Ugrinov, K. Davis, S. Peppernick, M. Merritt Clusters have the potential to serve as building blocks of materials, enabling the tailoring of materials with novel properties. We have recently proposed a new protocol that combines gas phase investigations to examine feasible units, theoretical investigations of energy landscapes to identify potential motifs, and synthetic chemical approaches to synthesize cluster assemblies. We had earlier applied the protocol to As$_{7}^{3-}$ based cluster assemblies. In this work, we extend our investigations to cluster assembled materials based on As$_{11}^{3-}$ units as building blocks. By varying the alkali cation and introducing crypts, it is possible to form materials with arsenic clusters arranged to form one dimensional chain, two dimensional layers or three dimensional lattices and X-ray studies provide information on bond lengths etc. Theoretical studies have been carried out to examine their microscopic structure and electronic properties. It will be shown that these new compounds have the tunable electronic and optical properties. The theoretical predictions on the As$_{11}^{3-}$Crypt(K)$_{3}$ and [As$_{11}$Cs$_{2}$]$^{1-}$Crypt(K) are in good agreement with the experimental observations. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D21.00003: Geometries and stabilities of Ag-doped Si$_{n}$ (n =1 - 13) clusters: a first-principles study Feng-chuan Chuang, Yun-Yi Hsieh, Chih-Chiang Hsu, Marvin Albao The structures of AgSi$_{n}$ (n = 1 - 13) clusters are investigated using first-principles calculations. Our studies suggest that AgSi$_{n}$ clusters with n = 7, and 10 are relatively stable isomers and that these clusters prefer to be exohedral rather than endohedral. Moreover, doping leaves the inner core structure of the clusters largely intact. Additionally, the plot of fragmentation energies as a function of silicon atoms shows that the AgSi$_{n} $ are favored to dissociate into one Ag atom and Si$_{n}$ clusters. Alternative pathways exist for n $>$ 7 (except n = 11) in which the Ag-Si cluster dissociate into a stable Si$_{7}$ and a smaller fragment AgSi$_{n-7}$. The AgSi$_{11}$ cluster dissociates into a stable Si$_{10}$ and a small fragment AgSi. Lastly, our analysis indicate that doping of Ag atom significantly decreases the gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital for n $>$ 7. Reference: J. Chem. Phys. 127, 144313 (2007). [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D21.00004: Transition Metal Doped and Encapsulated ZnO Cages Marcela R. Beltran, Arthur Reber, Shiv N. Khanna ZnO is a wide band gap semiconductor with potentail for applications. We had earlier shown that Zn$_{12}$O$_{12}$, Zn$_{15}$O$_{15}$, Zn$_{18}$O$_{18}$, and Zn$_{21}$O$_{21}$ clusters are particularly stable and exhibit cage structures. In this work, we examine the possibility of making magnetic materials by either substituting Zn atoms by transition metal atoms or by encapsulating transition metal clusters inside the cages. Our studies are carried out within a density functional framework employing gradient corrected functionals. The studies cover all the 3d elements and examine the strength of the magnetic moment as well as the nature of the coupling between the local moments. We also present results on the coupling between the endohedral transition metal cluster and the transition metal atoms substituting for Zn atoms within the cage. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 4:18PM |
D21.00005: DFT study of a carotenoid-porphyrin-C$_{60}$ light-harvesting molecular triad Invited Speaker: The abundance of solar energy reaching the earth presents an attractive alternative energy source. Nearly 75\% of the solar energy striking the upper atmosphere reaches the earth in the form of photons of energies typically higher than 1 eV. Biological light-harvesting systems are highly efficient in utilizing the solar radiation. Bio-mimetic molecules are investigated to mimic the photosynthesis process efficiently in laboratory. We present a computational study of the process in a bio-mimetic carotenoid-porphyrin-C$_{60}$ molecular triad which is about 5 nm long. The description of the photo-induced charge separation process requires accurate excited state energies and coupling between electrons and the phonons of the system. Since charge-transfer excitations create large changes in a molecular dipole moment, changes in excited-state energies due to coupling between a ~5 nm molecular photovoltaic and the surroundings (solvent and spectator molecules) also has to be taken into account. A density functional theory based method including all these effects to describe the photo-induced charge separation process will be presented. ~ [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:54PM |
D21.00006: Spin Accommodation and Reactivity of Superatoms. Invited Speaker: We have recently discovered novel effects that may allow tuning of the reactivity of small clusters by controlling their spin excitation, electronic structure, and local geometry. These findings offer the prospect of designing novel catalysts through cluster assemblies where chosen clusters, called superatoms, serve as elemental building blocks. Taking aluminum as an example, I will present our recent findings that illustrate how reactive clusters can be made non-reactive while inert species can be made reactive by adding hydrogen atoms. These findings offer a microscopic understanding of the recent experimental reactivity studies on aluminum and aluminum-hydrogen clusters that show variable reactivity in even electron systems and rapid etching in odd electron systems. It is shown that the reactivity of even electron clusters is governed by a spin transfer, from the triplet oxygen to the cluster, that fills the spin down antibonding orbitals on oxygen. Theoretical investigations show that when the spin transfer cannot occur, the species is unreactive, and when spin accommodation is possible, more subtle effects appear. Secondly, I will examine the reactivity of aluminum clusters with simple nucleophiles such as water. The reactivity and nature of the ensuing products is wildly variable with the size and shape of the cluster. Again, the electronic structure and local coordination of the active sites allow for an understanding of changing barrier heights and resulting reactivity. This work provides a framework with which new catalysts may be designed. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D21.00007: Peierls distortion of endohedral atoms in clathrate I Hidekazu Tomono, Kazuo Tsumuraya The guest atom displacements in type II clathrates have been reported on experimental and theoretical points of view. The displacements are reported to be 0.6 {\AA} from the cage center of the Si$_{28}$ cage to the hexagonal in the hydrogen terminated double caged Si$_{28}$ cluster [1]. The distortion can be expected to occur in the type I clathrate which forms with bamboo structures in the x, y, and z directions. The guest atoms show Peiels distortion when we calculate the equilibrium distances between the two Na atoms which locate at the neighboring Si$_{24}$ cages in the bamboo structure using periodic density functional calculation. The binding energy between the guest atoms is $-0.10$ eV/Na$_{2}$. We also confirm the tendency of the Peierls distotion from the force directions of guest atoms in the double unit cells that contain four Na atoms in one dimension; We will propose the cohesion mechanism of the clathrates that the clathrates are precipitated states of the connecting endohedral atoms in the 14 group atoms. So are the hydroclathrates in which the guest molecules bind each other with chains. [1] H. Takenaka and K. Tsumuraya, Mater. Trans. 47, 63 (2006). [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D21.00008: [Te$_{2}$As$_{2}$]$^{2-}$: A Planar Motif with Potential for Ferromagnetism Shiv Khanna, Arthur Reber, Meichun Qian, Angel Ugrinov, Ayusman Sen Here we report the synthesis and crystal structure of [K(18-crown-6)]$_{2}$[Te$_{2}$As$_{2}$], the first four-membered ring Zintl anion of elements from groups XV and XVI, isolated from an ethylenediamine solution of As, K, and As$_{2}$Te$_{3}$ at room temperature. X-ray analysis indicates that the [Te$_{2}$As$_{2}$]$^{2- }$anion has an unexpected planar rhombic structure with alternating bonds. First-principles electronic structure investigations within the density functional framework, indicate that the Te$_{2}$As$_{2}$K$_{2}$ motif possesses a triplet ground state where the spin configuration leads to a distortion of the square geometry into rhombus structure marked by two Te-As shorter bond length pairs joined by longer bond lengths. A NICS analysis reveals that the triplet motif has a net aromatic character. Supercell calculations on the periodic solid show that the spin moments on the individual motifs order ferromagnetically thus offering the potential of an aromatic ferromagnet made of traditionally non-magnetic elements. [Preview Abstract] |
Session D22: Focus Session: Organic Electronics: Synthesis and Materials
Sponsoring Units: DMP DPOLYChair: Lynn Loo, Princeton University
Room: Morial Convention Center 214
Monday, March 10, 2008 2:30PM - 3:06PM |
D22.00001: The role of symmetry and charge delocalization in two-dimensional molecules conjugated molecules for optoelectronic applications Invited Speaker: Our group is investigating whether star molecules offer any advantage over linear polymers when used as the active layer in light-emitting diodes (LEDs), organic transistors (OFETs) or in photovoltaics (PVs). Specifically, we are investigating the role of architecture, synthesizing some novel molecules that contain a central tetra substituted phenyl ring. These molecules have a tendency to pi-pi stack, further delocalizing the carriers. The synthetic strategy used to prepare these molecules is versatile so that the four arms in the molecules do not have to be identical. The placement of differing arms also affords the opportunity to study the effect of symmetry on the properties of these molecules. The HOMO and LUMO levels can be changed via the type and placement of arms. Recent results obtained with these materials and their applications in photovoltaics and light-emitting diodes will be described. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D22.00002: Ultra-pure organic semiconductors with improved charge carrier transport properties Clara Santato, Fabio Cicoira, Francesca Di Maria, Manuela Melucci, Giovanna Barbarella The electrical performance of devices based on organic semiconducting films depend critically on the synthesis and processing of the materials. The quality of organic semiconductors depends on synthetic conditions and purification procedures, which have not yet been optimized. Ultra-pure semiconductors permit to establish sound correlations between molecular structure, functional properties of the films and performance of devices based thereon, e.g. field-effect transistors (FETs). Understanding structure-property relationships in films is the crucial condition for predicting chemical structures with superior properties. We report on FETs based on oligothiophene that exhibit dramatically improved charge transport, due to the ultra-purity of the organic semiconductors. The latter was achieved using a synthetic methodology that takes advantage of (i) heterogeneous catalysts, (ii) microwave activation to shorten reaction times and suppress side reactions. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D22.00003: Side Chain Effects on the Structure and Dynamics of PPEs in different Complex Fluids Yunfei Jiang, Dvora Perahia, Yiqing Wang, Uwe H. F. Bunz Poly (para phenyleneethynylenes) (PPEs) are electro-optically active macromolecules with immense potential in organic electro-optical devices. The polymer backbones are substituted by side chains that affect their conformation, association modes and dynamics. Consequently, they modify the optical response of the polymer. The present work introduces a small angle neutron scattering (SANS) and neutron spin echo (NSE) measurement of PPE substituted with a bulky triisopropylsiloxyl (TIPS) side chain in toluene. The results are compared with those previously obtained results of alkyl substituted PPEs. While alkyl substituted PPEs interacts via pi-pi stacking and forms a fragile gel, TIPS-PPE associates predominantly via the side chains and forms a stable gel over a large temperature range. The dynamics of PPEs in gel includes both fast and slow components and a cooperative dynamics between the solvent and PPE molecules. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D22.00004: Thermal, structural, and electrical characterization of two high performance semiconducting polymers L.J. Richter, A.J. Moad, D.M. Delongchamp, R.J. Kline, D.J. Gundlach, D.A. Fischer, I. McCulloch, M. Heeney Polymer semiconductors are inexpensive solution processable alternatives to amorphous silicon for applications in flexible large area electronics. Recently, thin films of spun-cast poly(2,5-bis(3-alkylthiophen-2yl)thieno[3,2-b]thiophene) (pBTTT) have been demonstrated to exhibit exceptional hole mobilities in thin film transistors (TFTs) after heating into a mesophase. Poly(dialkylthieno[3,2-b]thiophene-2,5-bithiophene) (pTTBT) is a newly synthesized isomer of pBTTT with side chains attached to the thienothiophene rather than the bithiophene unit. This subtle structural change results in a very different response to heating. FTIR, spectroscopic ellipsometry (SE), AFM, X-ray diffraction, and NEXAFS were utilized to determine the root of the different thermal behavior. The structural transitions of the isomers are generally similar; however, the side chain melting transition Tm occurs about 50\r{ }C lower in pTTBT than in pBTTT. The significant drop in Tm appears to correlate with a subtle decrease in main chain packing interactions. Both materials exhibit high hole mobility, even in their respective mesophases. The slight overall higher order in pBTTT is reflected in the device performance. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D22.00005: Synthesis and characterization of conducting polymer inserted carbon nanotubes A. Jeong Choi, Young Woo Nam, Yung Woo Park The carbon nanotubes filled with the photo-conducting polymer poly($N$-vinyl carbazole) and the conducting polymer polypyrrole were prepared by polymerizing the monomers inside the nanotubes using the supercritical carbon dioxide. The endohedral nanotubes were characterized by HRTEM and $^{1}$H NMR, which confirmed that the inserted material was indeed the conducting polymer [1]. I-V characteristics of the polymer inserted carbon nanotubes are presented. \newline [1] Johannes Steinmetz, Soyoung Kwon, Hyun-Jung Lee, Edy Abou-Hamad, Robert Almairac, Christophe Goze-Bac, Hwayong Kim, Yung-Woo Park,, Chem. Phys. Lett., \textbf{431}, 139 (2006) [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D22.00006: X-ray scattering Study of Ordering in Liquid Crystalline Semiconducting Polymers Michael Chabinyc, Michael Toney, Iain McCulloch, Martin Heeney The electrical performance of thin-film transistors, TFTs, formed with semiconducting polymers is approaching that of amorphous silicon. The highest performance TFTs are obtained from semiconducting polymers with liquid crystalline (LC) mesophases. Thermal annealing of these materials after deposition can increase the field effect mobility by as much as an order of magnitude. We will report the results of detailed x-ray scattering studies of the impact of thermal annealing on the crystalline ordering in thin films of poly(2,5-bis(3-n-alkyl-2yl)thieno[3,2-b]thiophene), PBTTT. The packing structure of this polymer has a lamellar stacking direction due to the alkyl sidechains and a $\pi $-stacking direction due to the planar backbones. Measurements of the lamellar d-spacing show that the lamellar packing in the as-cast state is distinct from the packing in annealed films. The change in ordering occurs after the film is heated into the LC mesophase. The LC mesophase is highly ordered with maintenance of both the lamellar and $\pi $-stacking of the polymer chains. The largest differences in these phases are found in the ordering of the sidechains with smaller changes in the $\pi $-stacking. The influence of these phases on electrical transport will be discussed. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D22.00007: Improving the Electrical Conductivity of Polyaniline Through Molecular and Structural Control Joung Eun Yoo, Kwang Seok Lee, Yueh-Lin Loo We have investigated the electrical conductivity of polyaniline (PANI) that is template synthesized with a polymer acid of poly(2-acrylamino-2-methyl-1-propanesulfonic acid), PAAMPSA, as a function of the polymer acid molecular characteristics, including its molecular weight and molecular weight distribution. The electrical conductivity of PANI-PAAMPSA increases with decreasing PAAMPSA molecular weight. Additionally, PANI that is doped with narrow molecular weight distribution PAAMPSA is twice as conductive as PANI that is doped with PAAMPSA of comparable molecular weight having a broader molecular weight distribution. We can further increase the electrical conductivity of PANI-PAAMPSA post-synthesis by exposing the polymer film to dichloroacetic acid (DCA). In this case, the conductivity of PANI-PAAMPSA improves by more than two orders of magnitude. UV-vis-NIR spectroscopy and X-ray photoelectron spectroscopy suggest that DCA moderates the ionic interactions between PANI and PAAMPSA, allowing the polymer blend to adopt a structurally more favorable extended chain conformation. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D22.00008: Surface photoisomerization activity vs. functionalization of azobenzene derivatives Luis Berbil-Bautista, Jongweon Cho, Niv Levy, Matthew J. Comstock, Dan Poulsen, Jean M.J. Frechet, Michael F. Crommie Azobenzene and its derivatives can be reversibly photoisomerized between their \textit{cis} and \textit{trans} conformations in solution. The photoisomerization process is wavelength selective and results in a large length change. Hence, it is ideally suited to actuating molecular nanomachines on surfaces. However, it has recently been shown [1] that to recover photoisomerization activity on a metallic surface molecules must be functionalized with bulky spacing groups to decouple the optically active part of the molecule from the surface. This results in various trade-offs between molecular optical activity and overall flexibility/functionality. We have explored the photoisomerization activity of different azobenzene derivatives on metallic surfaces using a scanning tunneling microscope (STM) with optical access to the sample. The effects on molecular photo-activity and self-assembly for different substituent groups has been studied. [1] Matthew J. Comstock, Niv Levy, Armen Kirakosian, Jongweon Cho, Frank Lauterwasser, Jessica H. Harvey, David A. Strubbe, Jean M. J. Fr\'echet, Dirk Trauner, Steven G. Louie, and Michael F. Crommie Phys. Rev. Lett. \textbf{99}, 038301 (2007) [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D22.00009: Electrical and Optical Properties of a Novel Nonconjugated Conductive Polymer, Polynorbornene Ananthakrishnan Narayanan, Aditya Kumar Palthi, Mrinal Thakur We report electrical and optical properties of a novel nonconjugated conductive polymer, polynorbornene which has an isolated double bond in the repeat. Electrical conductivity of this polymer increases by more than ten orders of magnitude to about 0.01 S/cm upon doping with iodine. Optical absorption measurements of the polymer film have been made at different dopant concentrations. For a lightly doped polymer, two absorption peaks: one corresponding to cation radicals and the other corresponding to charge transfer between the double bond and the dopant were observed at 4.20 eV (295nm) and 3.13 eV (396nm) respectively. FTIR spectroscopic measurements have shown a reduction in the intensity of the C=C stretching and =C-H bending vibration bands upon doping indicating formation of radical cations. Photoluminescence studies have shown an emission band with a peak at $\sim $ 425nm when excited at 300nm. Nonlinear optical studies of this novel nonconjugated conductive polymer are in progress. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D22.00010: Quadratic Electro-optic Measurements in the Nonconjugated Conductive Polymer, Poly($\beta $-pinene) at 800 nm and 1550 nm Jitto Titus, Ananthakrishnan Narayanan, Mrinal Thakur Electro-optic effect in the nonconjugated conductive polymer, iodine-doped poly($\beta $-pinene) measured at 633 nm has been recently reported. In this presentation, results of quadratic electro-optic measurements at longer wavelengths will be reported. The electro-optic measurement has been made using the field-induced birefringence technique in the cross-polarized geometry with lock-in detection. Films with a medium doping level of iodine have been used in the measurements. Modulation depths of about 1.1{\%} at 800 nm and 0.06{\%} at 1550 nm were observed for an applied ac field of about 1 Volt/$\mu $m and for a film thickness of about 1 $\mu $m. More detailed measurements are in progress. The results are highly promising for applications of these materials in electro-optic modulators in the channel waveguide configuration. This exceptionally large quadratic electro-optic effect has been attributed to the confinement of this electronic system within a sub-nanometer dimension and the special electronic structure of this doped system. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D22.00011: Enantiotropic Polymorphism in Di-Indenoperylene Theo Siegrist, Michael Heinrich, Jens Pflaum, Ashutosh Tripathi, Wolfgang Frey, Michael Steigerwald The enantiotropic polymorphic phase transformation of di-indenoperylene (DIP), an organic semiconductor material, with transition temperature of 403 K, has been structurally characterized using single crystal X-ray diffraction. Both the low temperature $\alpha $- and the high temperature $\beta $-phase have a herringbone-type structure, with the $\alpha $-phase being triclinic with doubled unit cell volume compared to the monoclinic $\beta $-phase. In the latter, the molecules have a more upright orientation in the herringbone plane. The epitactic transformation from the $\beta $- to the $\alpha $-phase involves strong shearing displacements as well as bending and torsional deformations of the DIP molecules. The $\beta $-phase of DIP is equivalent to the thin film phase. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D22.00012: Direct Nanoscale Characterization of Submolecular Mobility in Complex Organic Non-linear Optical Systems Daniel Knorr, Tomoko Gray, Tae-Dong Kim, Jingdong Luo, Alex Jen, Rene Overney For organic non-linear optical (NLO) materials composed of intricate molecular building blocks, the challenge is to deduce meaningful molecular scale mobility information to understand complex relaxation and phase behavior. This is crucial, as the process of achieving a robust acentric alignment strongly depends on the availability of inter- and intra-molecular mobilities outside the temperature range of the device operation window. Here, we introduce a nanoscale methodology based on scanning probe microscopy that provides direct insight into structural relaxations and shows great potential to direct material design of sophisticated macromolecules. It also offers a means by which mesoscale dynamics and cooperativity involved in relaxation processes can be quantified in terms of dynamic entropy and enthalpy. This study demonstrates this methodology to describe the mesocale dynamics of two systems (1) organic networking dendronized NLO molecular glasses that self-assemble into physically linked polymers due to quadrupolar phenyl-perfluorophenyl interactions and (2) dendronized side-chain electro-optic (EO) polymers. For the self assembling glasses, the degree of intermolecular cooperativity can be deduced using this methodology, while for the dendronized side-chain polymers, specific side chain mobilities are exploited to improve EO properties. [Preview Abstract] |
Session D23: Focus Session: Triangular Lattice and Spinels
Sponsoring Units: DMP GMAGChair: Art Ramirez, Lucent
Room: Morial Convention Center 215
Monday, March 10, 2008 2:30PM - 2:42PM |
D23.00001: Magnetic ordering and structural transition in layered Li$_{2}$RuO$_{3}$ Devina Pillay, Michelle Johannes Li$_{2}$RuO$_{3}$ is a layered, triangular-lattice metal oxide system much like Na$_{x}$CoO$_{2}$, NaNiO$_{2}$ and LiNiO$_{2}$ with the exception that one of every three transition metal ions (Ru) is replaced by a Li ion. This results in a honeycomb arrangement of spin-carrying ions and eliminates the magnetic frustration intrinsic to the triangular lattice. Here we investigate the electronic structure of Li$_{2}$RuO$_{3}$, especially in relation to its magnetic ordering both in-plane and between adjacent planes. We find that the dimerization of Ru atoms within the metal-oxide planes acts in conjunction with magnetic ordering to establish a gapped, magnetic ground state. The change in the energy level spectrum brought on by the formation of spin-polarized Ru-Ru molecular orbitals replaces the expected Jahn-Teller mechanism as a way of relieving a degeneracy at the Fermi energy. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D23.00002: Magnetic interactions in geometrically frustrated triangular lattice antiferromagnet CuFeO2 Jaime Fernandez-Baca, Feng Ye, Randy Fishman , H. J. Kang, J. W. Lynn, Tsuyoshi Kimura Geometrically frustrated magnetic systems have received considerable attention due to their extraordinary magnetic properties. The delafossite CuFeO2 is of particular interest because it exhibits multiferroic behavior with either the application of a magnetic field or introduction of nonmagnetic impurities. Our recent [1] on CuFeO2 shows that spin waves in this material can be explained by antiferromagnetic interactions up to third nearest neighbors within the hexagonal plane as well as out-of-plane coupling, indicating that the quasi-like Ising nature of this material results from the delicate balance between competing interactions. Two energy dips in the spin wave dispersion occur at the incommensurate wavevectors associated with multiferroic phase, and are dynamic precursors to the magnetoelectric behavior. In this talk we will present preliminary measurements of the field magnetic field dependence of these excitations as CuFeO2 approaches the multiferroic phase [1] Ye et al. \textit{ Phys. Rev. Lett}. \textbf{99}, 157201 (2007) [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D23.00003: Neutron scattering investigation of Lithium based spinels. Wouter Montfrooij, Marcus Petrovic, Michael Kraus, Alexander Schmets Lithium based spinels Li$_{x}$M$_{2}$O$_{4}$, (with M a transition metal like Mn, V, Ti) offer a rich variety of ground states, depending on the transition metal in question. What makes these materials particularly attractive for both fundamental research as well as for applications is that the Li atoms can easily be extracted from the crystal without affecting the overall spinel structure. The oxidation state of the mixed valent transition metal M will change as a function of Li removal, with the result that the system can go from a disordered state to a long range ordered state. We present new neutron scattering results on a variety of Lithium spinels [Li$_{x}$Mn$_{2}$O$_{4}$, LiCoVO$_{4}$, LiCo$_{0.94}$Fe$_{0.06}$VO$_{4}$, LiNiVO$_{4}$, and LiNi$_{0.94}$Fe$_{0.06}$VO$_{4}$]. We discuss the changes in ground states as a function of Lithium content, and we review how the magnetic properties of the transition metal ions influence the electronic properties of the system. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D23.00004: Elastic properties of the vanadate spinel MnV$_{2}$O$_{4}$ V. Keppens, Y. Luan, V.O. Garlea, R. Jin, D. Mandrus Spinel vanadates AV$_{2}$O$_{4}$ are known to undergo a cubic-to-tetragonal structural phase transition (SPT) at temperature T$_{S}$ and order magnetically at lower temperature T$_{N}$. ZnV$_{2}$O$_{4}$ is characteristic of the entire series and has received extensive theoretical attention. When Mn occupies the A site there is an additional superexchange interaction between Mn and V. This superexchange interaction leads to ferrimagnetic order at about 56 K, involving a ferromagnetic configuration of the V spins. The current work focuses on the elastic properties of MnV$_{2}$O$_{4}$. Resonant Ultrasound Spectroscopy (RUS) has been used to measure the elastic response of the sample, as a function of temperature (5-300K) and magnetic field (0-7 Tesla). The temperature dependence of the frequencies is found to be quite unusual, displaying a softening over a large temperature range. Measurements in magnetic field reveal an additional feature near 50 K, which could represent a striking manifestation of direct spin-orbital coupling. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D23.00005: Neutron Scattering Study of Magnetic and Orbital Order in MnV$_{2}$O$_{4}$ Stephen Nagler, Ovidiu Garlea, Ronying Jin, David Mandrus, Doug Abernathy, Bertrand Roessli, Martha Miller, Arthur Schultz, Qingzhen Huang, Chris Frost Neutron diffraction and inelastic scattering has been used to study the spinel system MnV$_{2}$O$_{4}$. The results confirm the existence of two phase transitions. The first, near 56 K, is from a paramagnetic state to a collinear ferrimagnetic phase. There is a second simultaneous structural and magnetic transition at 53 K to a tetragonal structure with a non-collinear ferrimagnetic state. The low T magnetic structure has been definitively resolved, and is seen to be associated with a gap in the magnetic excitation spectrum. The magnetic structure is compatible with a staggered orbital ordering.Inelastic scattering has been carried out using both triple axis and time of flight techniques, and several branches of magnetic excitations are visible. These observations put tight constraints on theoretical models for MnV$_{2}$O$_{4}$. An initial account of some of the results of this work appears in arXiv:0711.1844v1. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D23.00006: Magnetic and magnetoelastic properties of Ge-substituted cobalt ferrite Naresh Ranvah, Ikenna Nlebedim, Yevgen Melikhov, John Snyder, David Jiles, Anthony Moses, Paul Williams Research in past has shown chemical substitution of Fe$^{3+}$ by trivalent ions (Cr$^{3+}$, Mn$^{3+}$ and Ga$^{3+})$ to alter the properties of these materials, notably reducing Curie temperature and magnetic anisotropy, which lead to increase in permeability. The change in properties has been understood in terms of site preference of the dopant cation. The current study investigated the temperature dependence of magnetic and magnetoelastic properties of germanium substituted cobalt ferrite, Co$_{1-x}$Ge$_{x}$Fe$_{2-2x}$O$_{4}$, over a temperature range of 10 -- 400 K. Both magnetic hysteresis loops and magnetostriction loops have been measured for several selected compositions with 0$\le $x$\le $0.8. At room temperature, saturation magnetization, magnetic anisotropy and magnetostriction were seen to decrease with increasing Ge-content. This new class of materials is therefore highly suitable for magnetic sensor and actuator applications in which the magnetic properties can be tailored to meet specific needs. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 4:18PM |
D23.00007: Charge order and frustrated magnetism in the orbitally-degenerate triangular metallic antiferromagnet AgNiO2 Invited Speaker: We explore the electronic ground state in the orbitally degenerate triangular metallic antiferromagnet AgNiO$_2$. In high-resolution neutron diffraction we observe a structural transition below 365 K to a tripled unit cell in the triangular layers with a periodic arrangement of expanded and contracted NiO$_6$ octahedra, naturally explained by a three-sublattice ($\sqrt{3} \times \sqrt{3}$) charge order pattern on the triangular lattice of Ni sites. Band-structure calculations suggest that charge order occurs in order to lift the orbital degeneracy and is favoured by the weak electron delocalization over local Jahn Teller distortions found in more insulating systems. An unusual magnetic order is observed at low temperatures with only one third of sites (the electron-rich Ni sites) carrying a magnetic moment arranged in an unexpected collinear stripe order pattern on an antiferromagnetic triangular lattice. Possible mechanisms stabilizing the observed ground state will be discussed. E. Wawrzy\'{n}ska, R. Coldea, E.M. Wheeler, I.I. Mazin, M.D. Johannes, T. S\"{o}rgel, M. Jansen, R.M. Ibberson, P.G. Radaelli, Phys. Rev. Lett. 99, 157204 (2007). We acknowledge support from EPSRC UK. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D23.00008: Electrical, Thermal, and Magnetic Properties of Single Crystal CaMn$_2$O$_4$ Marokite B.D. White, J.J. Neumeier, J.A. Souza, C. Chiorescu, J.L. Cohn CaMn$_2$O$_4$ was first described [1] in 1963 as a natural mineral called Marokite. Since its discovery, it has been studied as a minor structural impurity phase in CMR- related CaMnO$_3$ and for its structural similarities to high-pressure phases of spinel-oxide compounds. However, little attention has previously been paid to physical properties beyond its temperature-dependent magnetization. We will present a detailed physical properties study of CaMn$_2$O$_4$ single crystals grown by the optical floating zone method. [2] These measurements, several of which display anisotropy as a result of an orthorhombic crystal structure, include electrical transport, thermal transport, thermal expansion, heat capacity, and magnetization. \newline [1] C. Gaudefroy, G. Jouravsky, F. Permingeat, Bull. Soc. Fran\c c. Min\'er. Crist. {\bf86} (1963) 359. [2] B. D. White, C. A. M. dos Santos, J. A. Souza, K. J. McClellan, J. J. Neumeier submitted to J. Cryst. Growth. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D23.00009: X-ray diffraction and reciprocal space mapping in ZnMnGaO$_{4}$ films with checkerboard nanostructures A.A. Sirenko, S.M. O'Malley, P.L. Bonanno, A. Kazimirov, S. Park, S.-W. Cheong Reciprocal space maps (RSM) in ZnMnGaO$_{4}$ films with checkerboard nanostructures were measured with the energy of the x-ray beam of 10.53 keV at the A2 beamline at Cornell High Energy Synchrotron Source (CHESS) using a four-circle diffractometer. Structural properties of the checkerboards, such as elastic strain, relaxation effects, twists, and tilts of the nanodomains, were analyzed using H-K, H-L, and K-L cross sections of the RSM's measured around various symmetric and asymmetric reflections (022), (004), (044), (226), (222) of the spinel structure. Work at Rutgers was supported by the DE-FG02-07ER46382 and the NSF-DMR- 0706326. Work at NJIT was supported by the NSF-DMR-0546985. The Cornell High Energy Synchrotron Source is supported by the NSF and the NIH/NIGMS under Award No. DMR-0225180. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D23.00010: Magnetic coupling in CoCr$_2$O$_4$ and MnCr$_2$O$_4$: an LSDA+$U$ study Claude Ederer, Matej Komelj Competing magnetic interactions can lead to very interesting magnetic structures and pronounced effects due to spin-lattice coupling. Chromium spinels with additional magnetic $A$-site cations are prime examples for such systems, and have recently regained attention due to the discovery of a small electric polarization in CoCr$_2$O$_4$ [1]. The multiferroic character of CoCr$_2$O$_4$ is supposedly a result of its inversion symmetry-breaking magnetic structure, which has been classified as a ``ferrimagnetic spiral'' [2]. To achieve a better understanding of the complicated magnetic structure in CoCr$_2$O$_4$ and similar systems, an {\it ab initio} determination of the magnetic coupling constants is very desirable. Here, we present results of LSDA+$U$ calculations of the magnetic coupling constants in both CoCr$_2$O$_4$ and MnCr$_2$O$_4$ [3]. We carefully assess the predictive power of such calculations, and then give quantitative estimates for the strengths of the most prominent magnetic interactions. Our results highlight the possible importance of $AA$ interactions in spinel systems with magnetic ions on both $A$ and $B$ sites, and provide an important link between previous theory and experimental observations. \par \noindent [1] Y. Yamasaki {\it et al.}, Phys. Rev. Lett. 96, 207204 (2006). [2] D. H. Lyons {\it et al.}, Phys. Rev. 126, 540 (1962). [3] C. Ederer and M. Komelj, Phys. Rev. B 76, 064409 (2007). [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D23.00011: Crystal and Magnetic Structure of FeV$_{2}$O$_{4}$ with e$_{g}$ and t$_{2g}$ orbital degeneracies Sungdae Ji, J.-H. Kim, S.-H. Lee, Q. Huang, T. Katsufuji We have performed neutron diffraction measurements on a powder and a single crystal of a spinel FeV$_{2}$O$_{4}$ that has e$_ {g}$ as well as t$_{2g}$ orbital degeneracy. Our data show that upon cooling this system undergoes three successive phase transitions: a cubic-to-tetragonal structural transition at 140 K, a tetragonal-to-orthorhombic transition at 110 K accompanied by a ferri-magnetic order and an orthorhombic-to-orthorhombic phase transition with another magnetic order at 80 K. The magnetic structures of the two magnetic phases were refined by the group theoretical analysis of our powder diffraction data. We will also discuss implication of the magnetic structures regarding to the orbital states of FeV$_{2}$O$_{4}$. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D23.00012: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 5:18PM - 5:30PM |
D23.00013: The Dzyaloshinskii-Moriya Anisotropy in Hyper-Kagome Lattice for $\mbox{Na}_4 \mbox{Ir}_3 \mbox{O}_8 $ Gang Chen, Leon Balents The effect of quantum correction on the classical spin order in the iridium antiferromagnet $\mbox{Na}_4 \mbox{Ir}_3 \mbox{O}_8 $ are considered. The spin-wave spectrum is calculated to the linear order. The nearest-neighbor exchange interactions between the $\mbox{Ir}$spins as well as the Dzyaloshinskii-Moriya interactions are taken into account. Numerical simulation by exact diagonalization and experimental results are used to compare with the theoretical prediction. [Preview Abstract] |
Session D24: Superlattices and Nanostructures: Electronic Properties I
Sponsoring Units: DCMPChair: Alexander Govorov, Ohio University
Room: Morial Convention Center 216
Monday, March 10, 2008 2:30PM - 2:42PM |
D24.00001: Recent Progress on Modeling H Passivated CdS Nanocrystals using ab initio techniques Chad Junkermeier, Jinling Zhou, James P. Lewis Spherical CdS nanocrystals with shells of H atoms are studied via an \textit{ab initio} tight-binding analysis. Starting from the bulk zinc blende structure of CdS, these nanocrystals undergo relaxation as the geometries optimize to configurations that minimize internal forces. H atoms are then attached, to passivate the surface, and then whole structure is relaxed. We will present our latest results. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D24.00002: Measurements of Charge Transport in Arrays of Lead Selenide Nanocrystals Kenneth MacLean, Tamar Mentzel, Scott Geyer, Venda Porter, Moungi Bawendi, Marc Kastner We report electrical transport measurements of self-assembled arrays of PbSe nanocrystals (NC). NCs $\sim $6.2 nm in diameter are colloidally synthesized and drop cast onto an inverted field effect structure. The NCs self assemble into hexagonal close-packed arrays with $\sim $2 nm inter-particle spacing. The current is immeasurable in as deposited arrays. After annealing at 400K for $\sim $30 minutes, the arrays become less ordered and the inter-particle spacing decreases to $\sim $1 nm as evinced from TEM images and glancing incidence small angle x-ray scattering experiments. As a result of these changes, the conductance increases by more than 6 orders of magnitude. We measure the current in these devices as a function of source-drain voltage, gate voltage and temperature. We find that the temperature dependence of the conduction is strong at zero-bias and grows weaker with application of a source-drain bias. This implies that the conductance is thermally activated and the field serves to reduce the activation energy. We also find that the gate modulates the activation energy to conduction. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D24.00003: Structural and Electronic Properties of IV-VI Semiconductor Nanodots Roman Leitsmann, Friedhelm Bechstedt The characterization of nanostructure properties versus dimension and surface passivation is of increasing importance for the nanotechnology. Especially the stoichiometry, geometry, and the electronic states of IV-VI semiconductor nanodots are of special interest [1,2]. We use ab initio methods to calculate structural and electronic properties of colloidal IV-VI semiconductor nanodots as a function of the dot diameter. A method to passivate the non-directional dangling bonds at the nanodot surfaces is derived and used to study the confinement effect on the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) states. In addition we take the influence of relativistic (spin-orbit coupling -- SOC ) and excitonic effects into account. While the SOC leads to a considerable decrease of the HOMO-LUMO gap, excitonic effects play a minor role. [1] JACS 128, 10337 (2006) [2] JACS 129, 11354 (2007) [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D24.00004: Density functional theory studies of core-shell semiconductor nanoparticle quantum dots Brent Walker, Shaun Hendy, Richard Tilley In going from the macroscale to the nanoscale, quantum-mechanical effects become increasingly important and may mean that nanostructures of a material exhibit very different properties from the corresponding bulk. This is especially noticeable in the case of the optical properties of semiconductor nanoparticles (or quantum dots), which display a number of remarkable features (including very distinct peaks, and tunability across a broad range of wavelengths), due to quantum confinement. Our work involves modeling Si-Ge core-shell nanoparticles using large-scale computer simulations based on the density functional and time-dependent density functional theories. These simulations in particular provide us with predictions of the geometric structures and optical absorption spectra of nanoparticles in an accurate and computationally efficient way, and allow us to study the systematic trends in these properties as the composition and size of the nanoparticle change. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D24.00005: Conduction Mechanism in Arrays of Lead Selenide Nanocrystals Tamar Mentzel, Kenneth MacLean, Scott Geyer, Venda Porter, Moungi Bawendi, Marc Kastner We perform transport measurements of a PbSe nanocrystal solid which serves as the channel of a field-effect transistor. We find that a simple model of hopping between intrinsic localized states describes the conduction mechanism. From the field effect, we see that the majority carriers are holes, which are thermally released from acceptor states. At low source-drain voltages, the activation energy for the conductivity is given by the energy required to generate holes plus the activation over barriers resulting from site disorder. At high source-drain voltages the activation energy is given by the former only. The thermal activation energy of the zero-bias conductance indicates that the Fermi energy is close to the highest-occupied valence level, the 1S$_{h}$ state, and this is confirmed by field-effect measurements, which give a density of states of approximately 8 per nanocrystal as expected from the degeneracy of the 1S$_{h}$ state. Using the Thomas-Fermi screening length in the NC solid, we find that the gate serves to modulate the charge density in the monolayer closest to the gate, while successive monolayers are screened from the field. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D24.00006: Quasi 1-D electronic structure of silver nanowires A. Sekharan, W. Zhao, F. Womack, F. Wang, O. Kizilkaya, R. Kurtz, P. Sprunger Epitaxial Ag nanowires have been found to self-assemble on Cu(110) at coverages exceeding 1.2 ML. The low energy electronic structure of these nanowires has been characterized by ARPES. Previous STM, LEED, and LEEM data reveal that the Ag nanowires grown on Cu(110) are approximately 2 nm ($\sim $12 nm) in height (width). The nanowires orient with the long axis parallel to the [\={ }110] substrate direction The ARPES results reveal that the valence bands within the Ag nanowire are strongly anisotropic with clear band dispersion in the along-wire direction, but no dispersion in the across-wire direction. ARPES identified two low-energy electronic bands, with strong dispersion close to the Fermi energy. The first band, which crosses the Fermi energy, suggests the metallic nature of nanowires. However, there is an avoided crossing of the second band, perhaps due to many-body effects. While discussing the quasi 1-D electronic structure, we will emphasize its connection to many-body effects and the one dimensional nature of nanowires. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D24.00007: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 3:54PM - 4:06PM |
D24.00008: Measurements of the conduction band energy surrounding individual InGaAs quantum dots by Cross-Sectional Ballistic Electron Emission Microscopy (XBEEM) C. Marginean, J.P. Pelz, S.Y. Lehman, J.G. Cederberg Cross-sectional ballistic electron emission microscopy (XBEEM) at room temperature was used to measure the conduction band (CB) energy in the ``wetting layer'' around and \textit{behind} cleaved InGaAs quantum dots (QDs). Samples with a $\sim $2 nm thick In$_{0.4}$Ga$_{0.6}$As layer embedded in n-doped (5 x 10$^{16}$ cm$^{-3})$ GaAs were grown by organometallic vapor phase epitaxy, then cleaved \textit{ex situ} and 7nm-thick Au Schottky barrier (SB) contacts deposited on the cleaved edge using a shadow mask [1]. With reverse bias $V_{rev}$ = 0V, Schottky barrier heights (SBHs) over different QDs were measured to range from $\sim $0.78 eV - 0.82 eV, compared to $\sim $ 0.84 eV over the wetting layer next to the QDs and $\sim $0.913 eV SBH over the adjacent GaAs. With $V_{rev}$ = 1V, the SBH over the QDs were reduced by $\sim $30 -- 50 meV compared to a much smaller ($<$ 5 meV) measured decrease over the GaAs due to image force lowering, indicating that SBH over the QDs was due to the CB of the wetting layer at an estimated depth of 6 -- 9 nm \textit{behind} the QDs. The XBEEM transmission over the QDs was also strongly enhanced by the applied reverse bias, for reasons that are not yet clear. Work supported by NSF Grant No. DMR-0505165. [1] C. Tivarus \textit{et al}., PRL \textbf{94}, 206803 (2005). [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D24.00009: Shifting the Reference: Co on Cu(775) M.B. Yilmaz, S. Wang, K.R. Knox, N. Zaki, J.I. Dadap, R.M. Osgood, T. Valla, P.D. Johnson Bimetallic metal stepped surfaces enable easy and controlled variation of surface electronic structure. Use of vicinal substrates varies the electronic structure compared to that of flat surfaces directly as a result of the ordered step array and indirectly by affecting the growth mode of the adlayer. We have used ARUPS measurement to probe the coverage-dependent electronic structure of the Co/Cu(775). The regular step structure on our substrate gives rise to Umklapp features and shift in the surface state binding energy ($\Delta $E=110 meV). It also displays a shift in the reference frame, which is in agreement with earlier vicinal-angle guidelines. However, in the presence of even 0.03 ML Co, a dramatic shift in the reference plane occurs, which is in accord with a change in the surface atomic structure. At 0.06 ML, new electronic features are observed, namely a quantum well state that results from the hybridization of Co with the Cu(775) and a nondispersive d-band due to island growth of Co. This latter band smoothly evolves into a Co d-band similar to that previously observed on Co/Cu(111). [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D24.00010: Electronic nanoholography Christopher R. Moon, Laila S. Mattos, Brian K. Foster, Gabriel Zeltzer, Hari C. Manoharan We have developed a quantum holographic method to advance information density beyond the areal limits set by the discreteness of matter. We present experiments on information encoding using nanoscale writing with degenerate two-dimensional electrons. We show ``pages'' (letters encoded at specific energies) materialized by precisely engineering electron scattering environments with the tip of a scanning tunneling microscope. We then demonstrate that multiple pages can be encoded into the same region of space, using energy as a third holographic dimension. This form of holography produces non-volatile subnanometer features, tens of times smaller than the most precise optical or scanned-probe lithography, and information densities exceeding 5 bits per square nanometer. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D24.00011: Exploiting Resonances in Laser Photoemission J.I. Dadap, M.B. Yilmaz, K. Knox, N. Zaki, R.M. Osgood, P.D. Johnson Laser photoemission is attracting new interest due to its ability to increase photoemission probe depth and to gain insight into surface-electron dynamics. We present new data on the use of energy levels that are resonant with the excitation-photon energy. Our surface system is the regular array of nanometer-scale steps on Cu(775), which yield information on surface electron confinement. We probe this nanostructured system with 2-photon photoemission. Our tunable, fs optical parametric amplifier (OPA) source allows resonance mapping with photon energies $\sim $ 4.2-4.6 eV to obtain a comprehensive map of the unoccupied state manifold. We observe, in addition to the surface state and image states, bandfolding from Umklapp due to the periodic steps, and, for the first time, the existence of a weak unoccupied state. The OPA allows observation of even this relatively weak unoccupied state as well as the rapid interrogation of electronic structure. The origin of this intermediate state is discussed. In addition, we present the intensity dependence of the measured linewidth and the position of the resonances. [Preview Abstract] |
Session D25: Theory and Simulations I
Sponsoring Units: DPOLYChair: Arthi Jayaraman, University of Illinois at Urbana-Champaign
Room: Morial Convention Center 217
Monday, March 10, 2008 2:30PM - 2:42PM |
D25.00001: Thermodynamically Consistent Nonrandom Mixing on a Bethe Lattice Scott Milner For over 40 years, engineering calculations of nonrandom mixing effects in lattice-based calculations of free energies of mixing in both small-molecule and polymeric solutions (e.g., the non-random two-liquid model, or NRTL) have been based on a strange approximation. By ``strange'', I mean that the approximation violates some commonsense sum rules in how the lattice is filled; namely, that ``something is next to everything'', and ``everything is next to something''. The resulting theories are thermodynamically inconsistent, and explicitly depend on combinations of interaction energies of which the exact mixing free energy is demonstrably independent. To remedy this, I have extended the exact solution of the Ising model on a Bethe lattice to an $n$-component mixture with arbitrary pairwise interactions. Explicit and practical expressions are obtained for the entropy and average energy per site, which incorporate nonrandom mixing in a thermodynamically consistent way. Although it still has mean-field exponents for the binary mixture critical point, the shape of the coexistence curve lies much closer to the exact results for the Ising model in $d=3$ dimensions than do previous engineering-level theories. In addition, the model may be generalized easily to deal with mixtures of species occupying different numbers of sites on the lattice. Thus the model can be used to compute phase behavior for mixtures of molecules of different sizes, including polymeric solutions. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D25.00002: Mixture Properties of Flexible Chains: Comparisons between Experiment, Simulation and Theory; Contrasts between Lattice and Continuum Ronald White, Jane Lipson We present new theoretical results for a series of binary chain-molecule mixtures using both the hard-sphere, and the square-well potentials. We compare these results to simulation data, and contrast them to those obtained using the analogous lattice version of the theory. We discuss all of our findings in the context of experimental data for hydrocarbon chain mixtures. In the course of these studies we consider mixtures of components with varied chain lengths and energetics, and examine the effects of changing composition, temperature, and density. In addition, by calculating the free energy over a wide range of the P,V,T and composition space, we are able to characterize coexistence, both liquid-vapor as well as liquid-liquid partial miscibility. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D25.00003: A Multichain Self-Consistent Field Theory for Correlations in Polymers: Chain Swelling in Polymer Blends David Wu The self-consistent mean field theory of polymers has been highly successful as a tractable computational framework for capturing the thermodynamics and structure of polymer systems. One notable limitation has been the neglect of fluctuations and correlations, which can be important in a variety of physical circumstances. One such circumstance involves the non-Gaussian conformations (swelling) of branched polymers. We present a method for calculating these correlations with an extension of the SCF theory when applied to multiple chains. As an example of the methodology, we show how the crossover from swollen to screened conformations occurs in a blends of star and linear polymers. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D25.00004: Numerical Renormalization Group for Coarse Graining Field-Theoretic Fluid Models Michael Villet, Glenn Fredrickson Statistical field theory models have proven to be valuable tools for studying the equilibrium behavior of polymeric fluids, but direct simulation of these field theories without use of the mean field approximation is computationally demanding. Computational resources can be extended to simulate larger systems by discretizing the field variables with a coarsely spaced lattice, but indelicate coarse graining risks truncation of important short-wavelength physics. We investigate numerical renormalization group transformations in tandem with complex Langevin simulations as a systematic approach to coarse graining field-theoretic fluid models, using a simple repulsive Yukawa fluid as a test system. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D25.00005: Continuous translocations in connected chambers under pseudo-hydrodynamic force Erica Saltzman, Murugappan Muthukumar Experimental separation of polydisperse synthetic and biopolymers is frequently conducted via combination of electrophoretic or hydrodynamic flow with a series of obstacles or traps. In order to understand the interaction of entropic escape and biased diffusion processes, we conduct simulations on a generic model system. Brownian dynamics simulations are performed on linear chains confined in a series of chambers connected by narrow pores. A uniaxial force designed to mimic the effect of solvent flow acts on each bead of the chain, leading to translocation between chambers. Translocation events are separated by periods of trapping, which shorten with increasing chain length; for long chains individual translocations become indistinguishable. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D25.00006: A two-scale-two-mode dynamic self-consistent theory of entangled interfaces in polymer fluids under flow. Yitzhak Shnidman, Ismael Yacoubou-Djima Tracking conformation statistics on the Kuhn scale is essential for modeling interfacial phenomena in polymer fluids under flow. Successive entanglements partition entangled chains into strands that are in one of two modes: entangled or dangling. Strands follow different differential evolution equations for the second moment of their end-to-end distance, depending on their mode. Dangling strands are governed by the FENE-P equation. For entangled strands, a different evolution equation was proposed by G. Marrucci and G. Ianniruberto, \textit{Phil. Trans. R. Soc. Lond. A} \textbf{361}, 677 (2003). On the Kuhn scale, strand's conformation statistics is sampled by random walks in an effective potential, which are regulated by the evolving second moment of its end-to-end distance. Conformation statistics of a dangling strand is adequately modeled by Wiener (uncorrelated) random walks, but stretching of entangled strands under flow induce correlations between successive steps requiring a persistent random walk model (I. Yacoubou-Djima and Y. Shnidman, http://arxiv.org/abs/0708.2679v1). A two-scale-two-mode subchain propagation scheme, starting from free segments evolved by a probabilistic transport equation, allows a self-consistent calculation of evolving interfacial structure and rheology. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D25.00007: Twinkling Fractal Theory of the Glass Transition. Richard Wool A new approach to the glass transition temperature T$_{g}$ considers the interaction of particles with an anharmonic potential U(x), and Boltzmann population $\phi $(x) $\sim $ exp --U(x)/kT. As T$_{g}$ is approached from above, solid clusters of atoms form and percolate at T$_{g}$. However, the solid percolation cluster is in dynamic equilibrium with its surrounding liquid and ``\textit{twinkles}'' as solid and liquid atoms interchange. The \textit{twinkling} frequency F($\omega )$ is related to the vibrational density of states G($\omega )$ $\sim \quad \omega ^{df}$ and the energy difference $\Delta $U $\sim $ (T$^{2}$-T$_{g}^{2})$ via F($\omega ) \quad \sim $ G($\omega )$ exp -$\Delta $U/kT, where d$_{f}$ = 4/3 is the fracton dimension. F($\omega )$ controls the rate dependence of T$_{g}$, physical aging, yield stress, heat capacity C$_{p}$, T$_{g}$ of thin films, etc. When T $<$ T$_{g}$, the non-equilibrium volume development $\Delta $V, is determined by the fractal structure at T$_{g}$.$_{ }$The thermal expansion coefficients in the liquid and glass are related via $\alpha _{g}$ = p$_{c}\alpha _{L}$. For a Morse potential U(x) = D$_{o}$[1-exp $a$x]$^{2}$, we predict that T$_{g}$ = 2D$_{o}$/9k, and $\alpha _{L}$ = 3k/[4D$_{o}$aR$_{o}$]. For atoms with R$_{o}\approx $ 3 {\AA}, bond energy D$_{o} \quad \approx $ 2-10 kcal/mol and anharmonicity factor $a\approx $2/{\AA}, we obtain $\alpha _{L}$ T$_{g} \quad \approx $ 0.03, and modulus E $\sim $ 1/$\alpha _{L}$, which were found for a broad range of polymers. The yield stress $\sigma _{y}$ is determined by the onset of the twinkling fractal state as $\sigma _{y}$ = {\{}0.16 E [p$_{s}$-p$_{c}$] D$_{o}$/V$_{m}${\}}$^{1/2}$ where V$_{m}$ is the molar volume. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D25.00008: Nascent Polymerized Chain Crystallization on Surface Simulated by the Growing Chain Molecular Dynamics Xiaozhen Yang To understand nascent structure of polymerized chain on a catalyst surface, we have developed a code of growing chain molecular dynamics (GCMD), which describes aggregation behavior of growing chain with increase of repeat units during polymerization. This simulation shows that on the surface the growing chain has a nucleation process before certain chain length and an ordered structure growth process. Meantime, chain folding behavior was surprisingly observed. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D25.00009: Modeling Vapor Deposition Polymerization: Kinetic Monte Carlo Approach Sairam Tangirala, Yiping Zhao, David P. Landau A Kinetic Monte Carlo method is employed to model vapor deposition of growing, linear-polymer thin films which have applications ranging from microelectronic interconnects to biotechnology. Our 1+1 dimensional lattice model [1] implements various dynamical processes that occur during the film-growth, including random-angle deposition, monomer adsorption, free-monomer diffusion, and polymer-end flips. The temperature ($T$) is parametrized using the diffusion coefficient $(D=\exp(-\Delta E_a/k_BT))$, where $\Delta E_a$ is the activation energy for surface diffusion. The diffusion coefficient ($D$) and the deposition rate ($F$) play an important role in the growth process through the ratio $G$ ($=D/F$). We study the polymer chain length distribution, average polymer-chain length, film density, film height, surface-width, and radius of gyration as a function of $G$, system size ($L$), and time. Since polymers have much more complicated structures and interactions than those of organic materials, we find novel behaviors that are different from inorganic thin film growth. [1] W. Bowie and Y.-P. Zhao, \emph{Surf.\ Sci.\ Lett.} \textbf{563}, L245 (2004). [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D25.00010: Amphiphilic Systems under shear flow Hongxia Guo Phase behavior and the related physical and rheological properties of the amphiphilic systems including liquid crystals, diblock copolymers and surfactants are of wide-spread interest, e.g. in industrial processing of layered materials or biological applications of lipid membranes. For example, submitted to an applied shear flow, these lamellae show an interesting coupling of the layer orientation and the flow field. Despite an extensive literature dealing with the shear-induced transition, the underlying causes and mechanisms of the transition remain largely speculative. The experimental similarities between systems of different molecular constituents indicate, that the theoretical description of these reorientations can be constructed, from a common generic basis. Hence one can develop an efficient computer model which is able to reproduce the properties pertinent to real amphiphilic systems, and allows for a large-scale simulation. Here, I employed a simplified continuum amphiphilic computer model to investigate the shear--induced disorder-order, order-order and alignment flipping by large-scale parallelized (none) equilibrium molecular dynamics simulation [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D25.00011: Formation and structure of amorphous carbon char from polymer materials John Lawson, Deepak Srivastava Amorphous carbonaceous char produced from burning polymer solids has insulating properties that makes it valuable for aerospace thermal protection systems as well as for fire retardants. A pyrolytic molecular dynamics simulation method is devised to study the transformation of the local microstructure from virgin polymer to a dense, disordered char. Release of polymer hydrogen is found to be critical to allow the system to collapse into a highly coordinated structure. Mechanisms of the char formation process and the morphology of the resulting structure are elucidated. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D25.00012: Fluctuations in Confined Homopolymers Studied by Fast Off-Lattice Monte Carlo Simulations Yuhua Yin, Qiang Wang The conventional molecular simulations of many-chain systems are hindered by explicit excluded-volume interactions and expensive pair-potential calculations. The former greatly reduces the chain relaxation towards equilibrium configurations and the efficiency of sampling the configurational space, while the latter becomes computationally very expensive for concentrated polymeric systems. Fast off-lattice Monte Carlo (FOMC) simulations overcome these limitations, where individual polymer segments are modeled as volumeless points with the excluded-volume interactions modeled by either solvent quality, Helfand compressibility, or incompressibility constraint commonly used in polymer field theories. By dividing the simulation box into cells and assigning polymer segments to a cell, the short-range interactions can be readily evaluated without expensive pair-potential calculations. To demonstrate the great advantages of FOMC simulations, we have studied homopolymers confined between two parallel surfaces, and compared the results with self-consistent field calculations and field-theoretic simulations (FTS). Since FOMC simulation is particle-based, it avoids the unsolved ?sign problem? encountered in FTS. For the systems we have studied, FOMC simulations can sample the whole spectrum of fluctuations and are several orders of magnitude faster (more efficient) than FTS. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D25.00013: Computational Modeling of Aging Effects of Epoxy Polymers. Thomas Clancy, Sarah-Jane Frankland, Thomas Gates Due to the increased usage of non-metallic materials in aircraft, there is interest in the effects of aging on the performance of these materials. In order to gain insight into the molecular mechanisms of failure or reduced performance of these materials, computational modeling has been performed. Crosslinked epoxy systems were studied at the atomistic level. Atomistic models of crosslinked epoxy polymers were built by performing molecular dynamics simulations of unreacted epoxy and crosslinker molecules, followed by the formation of chemical crosslinks. Further molecular dynamics simulations were employed to equilibrate the models. These atomistic crosslinked epoxy models were also built with a range of moisture content. In addition, the crosslinking density was varied. The mechanical properties of these atomistic models were calculated in order to assess the effect of hygrothermal aging on the epoxy models. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D25.00014: Conformation and collapse of a polymer chain in explicit solvent: A solvation potential approach Mark P. Taylor The conformation of a polymer chain in solution is intrinsically coupled to the chain's local solvent environment. In much of the theoretical work on polymers in solution the effects of solvent are treated implicitly and explicit chain-solvent coupling is ignored. Although a formally exact treatment of chain-solvent coupling can be constructed, the required many-body solvation potential is not practical to compute. We have recently shown that for short hard-sphere and square-well chain-in-solvent systems this many-body solvation potential can be made tractable via an ``exact'' decomposition into a \textit{set} of two-site potentials [1]. Here we use these exact short chain results, combined with the pure solvent potential of mean force, to construct approximate two-site solvation potentials for long chains under a range of solvent conditions [2]. Monte Carlo simulations for full chain-in-solvent systems verify the accuracy of our solvation potential mapping. We use this approach to study the role of solvent in both driving and inhibiting chain collapse in square-well systems and discuss the possibility of solvent driven chain collapse in the symmetric hard-sphere chain-in-solvent system. [1] M. P. Taylor and G. M. Petersen, J. Chem. Phys. \textbf{127}, 184901 (2007). [2] M. P. Taylor and S. Ichida, J. Polym. Sci., Part B: Polym. Phys. \textbf{45}, 3319 (2007). [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D25.00015: Statistical Mechanical Theory of Phase Separation and Structure in Dense Polymer-Particle Mixtures Lisa Hall, Ken Schweizer Microscopic liquid state theory has been applied to investigate phase separation and structure of dense mixtures of hard spherical particles and flexible polymer chains in the presence of interfacial attractive interactions. The entire range of filler loading, from the dilute particle regime to the colloid science relevant case of ultra-high particle volume fraction with dilute polymer additives, has been studied for the first time. Many body effects can result in large quantitative, or even qualitative, changes of spinodal demixing boundaries compared to a low order virial treatment. In the temperature-particle volume fraction representation both upper and lower critical temperatures are present, separated by a miscibility window. Entropic effects dominate for weak interfacial attractions (high temperature) resulting in depletion phase separation with a critical point at roughly 10{\%} filler loading. At relatively high interfacial cohesion (low temperature) a network bridging transition occurs characterized by a highly asymmetric spinodal boundary which depends sensitively on attraction spatial range. Deep contact or bridging minima in the particle potential of mean force can occur, which raises the possibility that kinetic gelation or aggregation pre-empts equilibrium phase separation. The evolution of the real space correlations and scattering structure factors as phase separation is approached has been studied in detail. [Preview Abstract] |
Session D26: Focus Session: Photophysics of Cold Molecules III
Sponsoring Units: DCPChair: Wolfgang Jaeger, University of Alberta
Room: Morial Convention Center 218
Monday, March 10, 2008 2:30PM - 3:06PM |
D26.00001: Spectroscopy of large hydrogen clusters in He droplets and H$_{2}$ droplets. Invited Speaker: Clusters of molecular hydrogen (H$_{2}$) at low temperatures have been attracted much attention because of the possible superfluid phase of molecular hydrogen. Parahydrogen has been predicted to undergo Bose-Einstein condensate (BEC) and to exhibit a superfluid phase below 6 K. However, since the freezing point of H$_{2}$ (14 K) is much higher than the predicted superfluid transltion temperature, the supercooling of bulk H$_{2}$ system has not been achieved despite many attempts. Clusters are known to exhibit lower freezing and melting temperatures than their bulk system due to the size effect. In addition, the melting temperature may become significantly lower than the freezing temperature in such clusters, and coexistence of liquid and solid phases between the melting and freezing temperatures has been predicted theoretically. Thus, clusters of molecular hydrogen are very appealing system for the observation of possible superfluid phase of molecular hydrogen. Since superfluid is a macroscopic property, we have studied properties of hydrogen clusters with fairly large size ($N=100 - 10^{6}$) by using He droplet spectroscopy. Some advantages of using droplet spectroscopy for this study include (1) cluster size can be precisely controlled by its pickup process, and (2) the temperature of clusters is well defined. Laser induced fluorescence of several molecules doped in H$_{2}$ clusters showed clear evidence of non-rigidity of hydrogen clusters at 0.4 K or 4 K. We have also observed a clear difference in the LIF spectra between {\it parahydrogen} and {\it orthohydrogen} clusters. We will discuss the properties of large parahydrogen clusters from the dependence on the cluster size and concentration of orthohydrogen. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D26.00002: Hydrogen clusters that remained fluid Kirill Kuyanov-Prozument, Andrey Vilesov \textit{Para}-H$_{2}$ may constitute the only other superfluid besides helium. The superfluid transition temperature is predicted to be around 2 K, well below freezing of H$_{2}$ at 13.8 K. Numerous attempts to supercool macroscopic H$_{2}$ samples proved to be unsuccessful. Our approach includes formation of H$_{2}$ clusters in a pulsed cryogenic nozzle beam expansion of a neat $p$H$_{2}$ gas as well as \textbf{\textit{X}}\textbf{{\%}} of $p$H$_{2}$ diluted in He and interrogation via Coherent Anti-Stokes Raman Scattering. At \textbf{\textit{X}}\textbf{ = 2 -- 100 {\%}} the frequency of the vibrational Q$_{1}$(0) line in clusters remains constant at about $\nu $ = 4149.7 cm$^{-1}$ very similar to 4149.6 cm$^{-1}$ as in solid $p$H$_{2}$ and lower than in liquid $p$H$_{2 }$at 18 K (4151.9 cm$^{-1})$. The rotational S$_{0}$(0) transition show some characteristic crystal field splitting having magnitude of about 6 cm$^{-1}$. The splitting pattern is different from that in the \textit{hcp} solid, suggesting different structure in solid $p$H$_{2}$ clusters. At \textbf{\textit{X}}\textbf{ $\le $ 2 {\%}}, the frequency of the Q$_{1}$(0) line increases to about 4150.5 cm$^{-1}$, which is consistent with that expected in the supercooled liquid. The S$_{0}$(0) transition in these clusters, consisting of about 5 x 10$^{4}$ molecules, appears as a single line at the same frequency as in liquid $p$H$_{2}$. The temperature of these supercooled clusters is estimated to be less than about 1 K. Possible superfluidity of the clusters is discussed. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D26.00003: Three-body interactions in liquid and solid hydrogen: Evidence from vibrational spectroscopy Robert Hinde In the cryogenic low-density liquid and solid phases of H$_2$ and D$_2$, the H$_2$ and D$_2$ molecules retain good rotational and vibrational quantum numbers that characterize their internal degrees of freedom. High-resolution infrared and Raman spectroscopic experiments provide extremely sensitive probes of these degrees of freedom. We present here fully-first-principles calculations of the infrared and Raman spectra of liquid and solid H$_2$ and D$_2$, calculations that employ a high-quality six-dimensional coupled-cluster H$_2$-H$_2$ potential energy surface and quantum Monte Carlo treatments of the single-molecule translational degrees of freedom. The computed spectra agree very well with experimental results once we include three-body interactions among the molecules, interactions which we also compute using coupled-cluster quantum chemical methods. We predict the vibrational spectra of liquid and solid H$_2$ at several temperatures and densities to provide a framework for interpreting recent experiments designed to search for superfluid behavior in small H$_2$ droplets. We also present preliminary calculations of the spectra of mixed H$_2$/D$_2$ solids that show how positional disorder affects the spectral line shapes in these systems. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D26.00004: Rotational spectrum of small, doped $^{3}$He clusters Tatjana Skrbic, Saverio Moroni, Stefano Baroni In recent years, symmetry-adapted imaginary-time correlation functions have been extensively used to study the rotational spectrum of doped $^{4}$He clusters within the frame of the reptation quantum Monte Carlo method. The success of this approach relies on the choice of suitable correlation functions, whose spectral resolution is dominated by few, well separated eigenvalues of the Hamiltonian. Under these conditions, reliable excitation energies can be extracted by inverse Laplace transform. This method has been tailored for bosons, due to the positivity of the ground-state wave-function and to the distinctive scarcity of low-lying states. For sufficiently small systems, however, the states of the discrete spectrum can be calculated in the same manner also with Fermi statistics, using appropriate generalizations of the correlation functions. We present rotational spectra for small $^{3}$He clusters doped with molecules --such as CO2 and OCS-- whose effective moments of inertia, in $^{4}$He clusters, feature a non-trivial dependence on the system size, with a pronounced turnaround for less than 10 atoms. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 4:18PM |
D26.00005: Quantum melting and superfluidity of molecular hydrogen clusters Invited Speaker: Clusters of parahydroge comprising between 10 and 50 molecules have been extensively studied by computer simulations based on the continuous-space Worm Algorithm, which allows one to go down to temperatures as low as a few hundredths of a K. These clusters display an intriguing interplay of liquid- and solid-like behavior as a function of both temperature and cluster size. In this sense, their physics is far richer than that of helium clusters. An intriguing phenomenon predicted by our simulations is {\it quantum melting}, whereby clusters in some size range (roughly between 22 and 30 molecules) are observed to go from rigid, solid-like, to essentially structureless and liquid-like as the temperature is lowered, due to the onset of quantum exchange cycles involving all the molecules in the cluster. At low temperature these clusters turn superfluid; their local superfluid response has been analyzed, and found to be essentially uniform throughout the system in the $T\to 0$ limit, even in clusters with a pronounced shell structure. In particular, exchanges involving molecules in the inner and outer shells are shown to be underlying the superfluid response. This system can also allow one to gain insight into the relationship of the superfluid properties with Bose condensation, and aspect that has been thoroughly investigated. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D26.00006: Alkaline Earth Metal Atom Complexes with HCN Trapped On/In Helium Droplets: Vibrational Excitation Induced Solvation and Desolvation Gary Douberly Infrared laser spectroscopy is used to probe the rotational dynamics of the binary HCN-M (M=Ca, Sr) complexes, either solvated within or bound to helium droplets. The ``surface bound'' spectral signatures reported previously for the HCN-alkali atom complexes are observed for both species, while a second band is observed for HCN-Ca that corresponds to a solvated species. IR-IR double resonance spectroscopy is used to probe the interconversion of the two distinct HCN-Ca populations. Above a threshold droplet size, vibrational excitation results in the solvation of the surface bound HCN-Sr complex. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D26.00007: Imaging Photoelectron Dynamics in Doped Helium Droplets Chia Wang, Oleg Kornilov, Darcy Peterka, Jeong Kim, Oliver Gessner, Daniel Neumark Photoionization of He droplets doped with Xe and Kr atoms have been investigated by photoelectron imaging utilizing VUV synchrotron radiation. Photoelectron images were recorded over a wide range of He droplet sizes, photon energies, and dopant pick-up conditions. Significant ionization of dopants was observed at 21.6 eV, the absorption maximum of 2${ }^1P$electronic excited state of He droplets, suggesting an indirect ionization via excitation transfer. Photoelectron images and spectra indicate multiple pathways for photoelectrons generated by this process to escape the droplet. Special attention is paid to the excitation transfer dynamics and the electron relaxation in He droplets. It is found that excitation transfer from 2${ }^1P$state to dopants competes with relaxation to the lower 2${ }^1S$ state. The excitation is likely a localized exciton that transfers the energy to the dopant via a dipole-dipole hopping mechanism. The conduction band of He droplets as a function of droplet size is also observed. The conduction band edge reaches the bulk limit for the largest He droplets. The electron under the conduction band becomes trapped and forms an electron bubble that escapes the droplet by transcending a barrier near the liquid/vapor interface. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D26.00008: Interchange-Tunneling Splitting in HCl Dimer in Helium Nanodroplets Dmitry Skvortsov, Russell Sliter, Myong Yong Choi, Andrey F. Vilesov Infrared spectra of HCl dimers have been obtained in helium nanodroplets. The splitting in the vibrationally excited state of the bonded H-Cl stretching band ($v_{2})$ in (H$^{35}$Cl - H$^{37}$Cl) dimers was obtained to be 2.7 cm$^{-1}$ as compared to 3.7 cm$^{-1}$ in free dimer. From the splitting, the strength of the interchange-tunneling interaction in liquid helium was obtained to be 0.85 cm$^{-1}$, which is about a factor of two smaller than in the free dimer. The results are compared with the previous spectroscopic study of (HF)$_{2}$ in He droplets as well as to the theoretical study of (HF)$_{2}$ and (HCl)$_{2}$ dimers in small He clusters. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D26.00009: Path integral studies of methane rotations in $^4$He clusters Nikolay Markovskiy, Chi Mak Path integral simulations have been carried out to study the rotations of a methane inside a single shell of $^4$He atoms at 0.3~K to address the question of whether dopant molecule rotations can be used to probe the quantum statistics and superfluidity of the shell. We examined the effects of the probe molecule on the $^4$He exchanges and their counter effects on the renormalized rotation constant of the probe systematically by varying the intrinsic moment of inertia of the methane. The observed effects show strong dependence on the intrinsic moment of inertia of the rotating probe, with a heavy probe favoring stronger templating of the $^4$He density and a corresponding suppression of exchanges in the shell, as well as a large renormalization in the probe's effective rotation constant, while a light probe shows almost no effect on the shell density or the effective rotation constant. These results can be rationalized in terms of a rotational smearing effect and suggest that there is no clearly quantifiable relationship between the superfluid fraction of the shell and the renormalized rotation constant of the probe for cases where the probe molecule has weak anisotropic interactions with the $^4$He atoms. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D26.00010: Pump-probe spectroscopy of Rg-Br$_2$ linear isomers Jordan Pio, Craig Bieler, Wytze van der Veer, Kenneth Janda We have recorded and analyzed the \mbox{\textit{X}$\rightarrow$\textit{B}} spectra for three Rg--Br-Br linear isomers [Rg = He, Ne, Ar] using pump-probe spectroscopy. This work is an interesting test case for the transition from quantum to quasi-classical dynamics, and how the dynamics are interconnected with changes in the potential energy surface. Helium is not only much lighter than argon, but the He-Br$_2$ potential well is much shallower than that of Ar-Br$_2$. Excitation spectra to individual Rg-Br$_2$ (\textit{B}, $\nu$') intermolecular potentials were recorded by probing the Br$_2$ (\textit{B}, $\nu$') asymptotic limit of the potential while scanning the pump laser. The continuum spectra of the three species are very different, with the He-Br$_2$ spectrum peaking at threshold while the Ar-Br$_2$ spectrum is negligible at threshold and strongly blue shifted. The linear Ne-Br$_2$ bond energy was measured to be \mbox{71 $\pm$ 3 cm$^{-1}$} by the threshold energy for the onset of the continuum. Since excitation tends to move electron density to the $\sigma^*$ orbital of the Br-Br bond near the rare gas atom, the intramolecular stretching vibration (Br-Br) and the intermolecular stretching vibration (Rg-Br) are strongly coupled. The experiments will be compared to a two dimensional model using the best available potential energy functions. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D26.00011: Time-resolved photoionization of He droplets using high-harmonic Oleg Kornilov, Oliver Gessner, Mathew Leonard, Stephen Leone, Daniel Neumark, Chun-Te Peng, Chia Wang Helium droplets are widely used as nanocontainers for matrix-isolated rotational, vibrational and electronic spectroscopy. Their superfluid nature and low temperatures (0.37K) provide gentle environment for embedded atoms, molecules and complexes. However, most of the traditional spectroscopic techniques are not efficient for pure droplets, because of the very high energies of electronic transitions. One of the recent studies [1] conducted using synchrotron light demonstrated very interesting phenomena in photoionization of pure He droplets. It has been shown that below the threshold for He atom photoionization essentially zero kinetic energy electrons are emitted independent of the wavelength of the photoionizing radiation. In this contribution a new experiment will be presented utilizing a novel source of VUV radiation based on the high-harmonic generation. In this process femtosecond pulses of radiation are created, which will be used in a VUV-pump/IR-probe scheme to study dynamics of photoionization of He droplets. First results towards the time-dependent photoelectron spectra will be presented. [1] D. Peterka \textit{et al}, Phys. Rev. Lett. \textbf{91}, 043401 (2003) [Preview Abstract] |
Session D27: Focus Session: Magnetic Nanowires and Nanodots I
Sponsoring Units: GMAG DMPChair: Paul Crowell, University of Minnesota
Room: Morial Convention Center 219
Monday, March 10, 2008 2:30PM - 2:42PM |
D27.00001: The anomalous exchange bias effect in core-shell Co/CoO nanoparticles Mikhail Feygenson, Yiu Yuen, KiSub Kim, Meigan Aronson We study the anomalous exchange bias effect in Co/CoO nanoparticles by means of neutron and x-ray scattering and magnetic experiments. The Co nanoparticles were prepared in oleic acid by thermal decomposition of Co$_{2}$(CO)$_{8}$ and were subsequently oxidized. Co core- CoO shell nanoparticles with differing core and shell dimensions were obtained. The magnetic measurements indicated that there is an optimal ratio of the core and shell dimensions which maximizes the exchange bias field. Anomalous small angle x-ray scattering experiments using core-shell contrast and energy analysis provide high accuracy measurements of the core and shell, and their respective size distributions. Neutron diffraction measurements find that oxidation introduces a new modulation wave vector for the magnetization, leading to the increasing magnetic decompensation of the core-shell interface. It is our proposal that this interface moment enhances the exchange coupling of the core and shell, and leads to the extraordinarily large exchange bias effect. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D27.00002: Artificial Nanomagnet with Lateral Confinement Lifeng Yin, Noppi Widjaja, Jian Shen For artificially low dimensional nanodots, the exchange interaction and dipole-dipole magnetostatic interaction can no longer stabilize the long range magnetic order at finite temperature. However, the electron-mediated indirect exchange interaction could be enhanced at surfaces due to the symmetry broken. A collective ferromagnetic behavior in two-dimensional Fe dot assemblies grown on a single crystal Cu(111) surface has been reported$^{[1]}$. These Fe nanodots were grown using a novel method called buffer-layer assisted growth. The ferromagnetic ordering temperature appears to depend not only sensitively on the average spacing between the dots, but also strongly associated with the presence of surfaces states. The vicinal surfaces have been found a rich variety of novel behavior that results from broken translational symmetry by surface atomic steps. The presence of a free-electron-like Shockley surface state on the corresponding flat Cu(111) surface will be interrupted on vicinal surface. More interestingly, a switch between two qualitatively different regimes at a miscut of 7$^{o}$ takes place$^{[2]}$. In this work, a curve-polished Cu(111) (0$\sim $8$^{o}$ miscut) substrate is used to tune the surface electronic states, and in turn influences the electron-mediated indirect exchange interaction of Fe nanodots. [1] J. P. Pierce et al., Phys. Rev. Lett. 92, 237201 (2004). [2] J.E. Ortega et al., Phys. Rev. Lett. 84, 6110 (2000). [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D27.00003: Magnetic Correlations In A Magnetite Nanoparticle Assembly Investigated Using Polarized SANS Kathryn Krycka, Charles Hogg, Yumi Ijiri, Ryan Booth, Julie Borchers, Wangchun Chen, Mark Laver, Thomas Gentile, Brian Maranville, Benjamin Breslauer, Sara Majetich Using small angle neutron scattering (SANS) with polarization analysis, we have studied ferromagnetic magnetite monodisperse nanospheres in order to determine the field (0 and 1.3 Tesla) and temperature (50, 100, and 200 K) dependence of the magnetic interparticle correlations. These particles were 7 nm in diameter with an average edge-to-edge separation of 2.5 nm. Preparation techniques are described elsewhere [1]. An FeSi supermirror polarized the incident neutrons, and a polarized 3He cell was used as a spin analyzer. While a typical magnetic SANS experiment observes the convolution of the nuclear and magnetic terms, we have implemented and further developed an algorithm to separate the four spin dependent cross sections. This provides an unambiguous separation and measurement of magnetic and nuclear contributions. At low temperatures, magnetic correlation lengths have been found to be significantly larger than at high temperatures.\newline [1] J. Am. Chem. Soc. 2002, 124, 8204-8205. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D27.00004: Magnetic Nanocheckerboards with Tunable Sizes in the Mn-Doped CoFe$_{2}$O$_{4}$ Spinel Chenglin Zhang, C.M. Tseng, C.H. Chen, S. Yeo, Y.J. Choi, S.-W. Cheong In the Mn-doped CoFe$_{2}$O$_{4}$ spinel, a highly ordered array of two types of rectangular nanorods, $\sim $300 nm in length and a few nanometer in size, is achieved through chemical phase separation mediated by cooperative Jahn-Teller distortions. At room temperature, the magnetic nanorods with composition close to CoFe$_{2}$O$_{4}$ interlace with the paramagnetic counterparts and form a highly organized checkerboard pattern in the cross section. The checkerboard size, varying in the range of $\sim $3 nm and $\sim $80 nm, is tunable with composition as well as with the isothermal annealing time This may be of potential significance to the next generation magnetic storage. The magnetic nanocheckerboards exhibit a nearly ideal configuration for perpendicular recording media. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D27.00005: Magnetic interactions among Co nanowires Hafsa Khurshid, Michael Bonder, Gearoge Hadjipanayis Magnetic nanowires have recently gained much attention because of their potential applications in magnetic recording, sensors and other electronic devices. The magnetic properties of nanowires are determined by the competition between magnetocrystalline and shape anisotropy resulting from the reduced dimensionality of the system. In this study we present results on the magnetic interactions among arrays of electrodeposited Co nanowires as a function of inter-wire spacing and nanowire diameter. X-Ray diffraction and electron microscopy reveal that the nanowires exhibit the hexagonal closed packed polycrystalline structure. The direction of the magnetic easy axis is controllable as a function of wire diameter. By increasing the diameter of nanowires from 30 to 188 nm, the magnetic easy axis switches from perpendicular to parallel to the nanowire's major axis. This is further evidenced by a decrease in coercivity from 1200 to 100 Oe and a reduced loop squarness. The magnetic interactions were probed using delta M plots. All samples exhibit a negative delta M curve indicative of magnetostatic interactions. As the inter-wire spacing increases there is a broadening of the dipolar component of delta M plots indicating an increase in the switching field distribution. This work was supported by NSF Grant{\#} DMR-0302544. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D27.00006: Memory Effects and Inter-particle Interactions in Co Nanoparticles Embedded in Carbon Matrix Peng Liu, Michael Bonder, George Hadjipanayis In this work we have studied the magnetic properties of face-centered-cubic (fcc) Co nanoparticles made by the cluster gun. The zero field-cooled (ZFC) and field-cooled (FC) $M(T)$ curves at different fields show that the blocking temperature is shifted to lower temperature when the applied magnetic field is increased. This behavior could be due to a decreased energy barrier at increased filed or to inter-particle dipole-dipole interactions. M vs H/T data above the blocking temperature show that the latter might be responsible for this behavior. The dynamics of the FC magnetization were also studied. The $M(T)$ curves on FC samples obtained with the magnetic field on and off at different temperatures, show that the sample remembered its thermal history and demonstrated a memory effect at temperatures lower than the blocking temperature. However, this memory effects were not observed in the ZFC samples. The magnetic relaxation with a change at low temperature also shows a memory effect at temperature below the blocking temperature. The $M(T)$ curves at different fields and memory effects indicate that the dynamics of nanoparticles are due to the distribution of particle sizes and inter-particle interactions. Work Supported by NSF GRANT {\#} DMR-0302544. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D27.00007: Probing the effect of interparticle interactions in ferrite nanoparticles using the reversible transverse susceptibility method M.B. Morales, P. Poddar, N.A. Frey, H. Srikanth, S.A. Morrison, E.E. Carpenter Spin dynamics in magnetic nanoparticles is an issue of current interest. It is important to understand how interparticle interactions in a 3-dimensional arrangement of nanoparticles as well as their surface functionalization would affect the global magnetic response, in particular, the magnetic anisotropy. We report the influence of surface functionalization and systematic dipolar interactions strength variation on the magnetic properties of surfactant-coated monodispersed manganese zinc ferrite (Mn$_{0.68}$Zn$_{0.25}$Fe$_{2.07}$O$_{3})$ nanoparticles of 15 nm mean particle size using temperature and field-dependent reversible transverse susceptibility measurements at a 12 MHz resonant frequency. Our experiments reveal that the characteristic features in the transverse susceptibility --viz. the position, height and symmetry of the peaks at anisotropy fields -are extremely sensitive to interparticle interactions and surface chemistry of the nanoparticles. In contrast to earlier theoretical suggestions, our experimental results suggest that the transverse susceptibility technique can be used effectively even for strongly interacting magnetic nanoparticle systems. Work at USF supported by NSF through a GOALI grant from DMII. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D27.00008: Effects of magnetic interactions in Ni nanowire arrangements Ovidiu Trusca, Dorin Cimpoesu, Leonard Spinu, John B. Wiley, Jin Hee Lim Systems of magnetic nanowires are considered strong candidates for many technological applications. The main parameter controlling the frequency response of magnetic nanowires assemblies is their aspect ratio (length to diameter ratio) [1], that can be tuned by changing the dimensions of wires. We modified the nanowires aspect ratio by keeping constant the length and changing the diameter. This required designing templates of different diameters with the same average distance between the pores. Two sets of Ni nanowires samples with diameters of 40, 60, 80 nm and constant length of 500 and 1000 nm respectively, obtained by electrodeposition, were studied using X-band ferromagnetic resonance measurements at room temperature. The two series of samples are ideally candidates for verifying the models recently proposed to describe the interactions in such systems [2]. As the nanowire's diameter increases, the peak observed in the angular dependence of the FMR resonant field diminishes. [1] A. Fert, L. Piraux, J. Magn. Magn. Mater, vol. 200, pp. 338-358, 1999. [2] I. Dumitru, et al., IEEE Trans.Mag 42(10), 3225, 2006. Work supported by Louisiana Board of Regents Contract {\#}LEQSF(2007-12)-ENH-PKSFI-PRS-04. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D27.00009: Fabrication of GaMnAs Magnetic Semiconductor Nanodot Arrays S. Bennett, L. Menon, D. Heiman Ordered arrays of GaMnAs ferromagnetic semiconductor nanodots were fabricated using anodic porous alumina templates as etch masks. In this study we used nanochannel porous alumina membranes as masks for thermal evaporation coupled with reactive ion etching for the fabrication of organized hexagonal arrays of both manganese doped semiconductor and metal alloy comprised nanodots. The GaMnAs nanodots have diameters $\sim $40nm and dot periodicity of $\sim $80 nm. Field-cooled and zero-field-cooled magnetization measurements demonstrate that the dots are superparamagnetic at room temperature with a blocking temperature of T=30 K, below which they are ferromagnetic. This illustrates that arrays of uniform and highly-ordered nanodots can be fabricated inexpensively, rapidly and over large length scales for semiconductors which cannot be formed by techniques of self-assembly. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D27.00010: Synthesis of surface functionalized magnetic nanoparticles and their polymer nanocomposites M.J. Miner, M.B. Morales, P. Poddar, H. Srikanth, S.M. Skidmore, T.M. Weller Magnetic nanoparticles embedded in polymer matrices are good examples of functional nanostructures with excellent potential in applications such as tunable microwave devices, EMI shielding, flexible electronics etc. Control over the dispersion of the nanoparticle phase embedded in a polymer matrix is critical and often challenging. To achieve excellent dispersion, competition between polymer-polymer and polymer-particle interactions must be balanced to avoid clustering of particles in polymer nanocomposites. In earlier work, we had demonstrated the successful synthesis of 2$\mu $m thick spin coated nanocomposite PMMA films with Fe$_{3}$O$_{4}$ (mean size 15nm) nanoparticle inclusions exhibiting superparamagnetic behavior. In this work, we will present our attempts to achieve thicker films more suitable for microwave applications and a study of the role of surface functionalization of ferrite nanoparticles synthesized using co-precipitation and hydrothermal routes. Cross-sectional SEM and TEM studies as well as magnetic characterization using a Physical Property Measurement System will be presented and discussed. We will also report on the microwave response of these films using a coplanar waveguide fixture. Work at USF supported by NSF through a GOALI grant from NSF-DMII. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D27.00011: Magnetic behavior of PdNi nanowires and extended thinfilms as a function of the film thickness Juan-Carlos Gonzalez-Ponz, John Henderson, Enrique Del Barco, Barbaros \"Ozyilmaz Recently Pd$_{1-x}$ Ni$_{x}$ alloy has attracted considerable attention as ferromagnetic electrodes in carbon based lateral spin valves. Its wetting properties on carbon nanotubes (CNT) leads to transparent contacts, while its room temperature ferromagnetic behavior provides a means for spin injection. Surprisingly, in the case of CNTs a tunneling barrier between the PdNi and the CNT is unnecessary for spin injection, making PdNi ideal for electrodes in carbon-based electronic devices. Here we report studies of both the anisotropic magneto-resistance (AMR) of PdNi nanowires with varying widths and the ferromagnetic resonance (FMR) behavior of PdNi thin films with varying thickness. The AMR revealed strong angular field dependence with respect to the nanowire, indicating magnetization tilted out of the plane of the wire. The tilt angle decreases with increasing the nanowire width. Room temperature broad-band (5-50GHz) FMR measurements of extended films show in-plane magnetization and out-of-plane uniaxial anisotropy ($K)$, which is not large enough to overcome the demagnetization energy. We speculate that the constriction of a dimension in the film plane modifies the demagnetization factors allowing the out-of-plane anisotropy to push the magnetization out of the plane for small nanowire width. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D27.00012: Synthesis and Characterization of Iron-Nickel (Fe$_{x}$Ni$_{1-x})$ Nanowires Rakesh Shah, Igor Dubenco, Amelia Church, Xianfeng Zhang, Shane Stadler, Saikat Talapatra, Naushad Ali Electrochemical deposition method was used to synthesize ordered arrays of Fe$_{x}$Ni$_{1-x}$ (25 $<$ x $<$ 85) nanowires into porous anodic alumina template. These nanowires were structurally characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). An estimate regarding the elemental composition of the nanowires was obtained by Energy dispersive spectroscopy (EDS). X-ray diffraction studies revealed that the nanowires exhibit a phase transition from face-centered-cubic (FCC) to base-centered-cubic (BCC) crystal structure with increasing iron concentration. The effect of the variation of the compositional ratio of iron and nickel on the magnetic properties of the nanowires will also be presented. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D27.00013: Growth of Co nanoclusters on rutile TiO2 (110) surface En Cai, Xuewen Wang, Jiandi Zhang Fabrication of magnetic nanodots with uniform size and density is one of the key issues of studying the structure and property of a nanomagnet system. We report here our studies on the growth of Co nanoclusters on rutile TiO$_{2}$ (110) surface. Well ordered TiO2 (110) surface is prepared in the UHV chamber via Ar$^{+}$ sputtering and annealing. Co deposition is carried out in situ by molecular beam epitaxy and characterized with STM. Growth parameters are tuned to optimize the uniformity of dot size and density. Co dot coverage, size and density are investigated as functions of deposition rate and time as well as post-annealing temperature. Our results show that uniformity of the dots mainly depends on deposition rate, and the density of the dots primarily depends on the coverage of the dots, while the size of the dots depends mainly on the deposition rate and post-annealing temperature instead of coverage. Growth mechanism will be discussed. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D27.00014: Tuning of the magnetocrystalline anisotropy in Co$_x$Fe$_{3-x}$O$_4$ nanoparticles through cobalt doping Ronald Tackett, Sudakar Chandran, Ratna Naik, Gavin Lawes, Corneliu Rablau, Prem Vaishnava We report on the effect of cobalt doping on the magnetocrystlline anisotropy of Co$_x$Fe$_{3-x}$O$_4$ nanopaticles. The Co$_x$Fe$_{3-x}$O$_4$ ($0 \leq x \leq 0.15$) nanoparticles were synthesized through the coprecipitation of ammounium hydroxide in an environment of Fe$^{2+}$, Fe$^{3+}$, and varying concentrations of Co$^{2+}$. The size and crystallinity were confirmed using transmission electron microscopy, with a mean size of 17 $\pm$ 4 nm which was found to be constant across the different cobalt dopings. The magnetic properties were investigated through the use of dc and ac magnetic susceptibility, with the effective magnetocystalline anistropies being extracted from these data. The effective magnetocrystalline anisotropy found from each method were, within acceptable experimental error, found to agree, as well as increase linearly with cobalt doping. The effective anisotropy values were found to increase in magnitude by 100$\%$ as the cobalt fraction was increased from $x = 0$ to $x = 0.1$. This trend allows for the tuning of the magnetic isotropy of iron oxide nanoparticles through cobalt doping. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D27.00015: Preparation of cobalt-ferrite nanoparticles within a biopolymer template Marco Garza, Virgilio Gonz\'alez, Alejandro Torres-Castro, Mois\'es Hinojosa, Ubaldo Ort\'iz Using an in-situ co-precipitation reaction from solid dissolutions of stoichiometric amounts of Fe (III) and Co (II) inorganic salts, it was prepared highly loaded nanocomposites (as high as 75{\%} w/w) of cobalt-ferrite nanoparticles within a chitosan matrix, with particle size of about 7 nm, narrow particle size distribution and superparamagnetic character. Nanocomposite samples were characterized by high resolution transmission electron microscopy (HRTEM), UV-vis spectrometry and magnetic measurements by SQUID, using magnetization-field dependent, M(H), and magnetization-temperature dependent, M(T), studies. [Preview Abstract] |
Session D28: Focus Session: Transport in Nanostructures III: Single Molecules
Sponsoring Units: DMPChair: Ward Plummer, University of Tennessee
Room: Morial Convention Center 220
Monday, March 10, 2008 2:30PM - 3:06PM |
D28.00001: Understanding the Conductance of Single-Molecule Junctions from First Principles Invited Speaker: Discovering the anatomy of single-molecule junctions, in order to exploit their transport behavior, poses fundamental challenges to nanoscience. First-principles calculations based on density-functional theory (DFT) can, together with experiment, provide detailed atomic-scale insights into the transport properties, and their relation to junction structure and electronic properties. Here, a DFT scattering state approach [1] is used to explore the single-molecule conductance of two prototypical junctions as a function of junction geometry, in the context of recent experiments. First, the computed conductance of 15 distinct benzene-diamine-Au junctions is compared to a large robust experimental data set [2]. The amine-gold bonding is shown to be highly selective, but flexible, resulting in a conductance that is insensitive to other details of the junction structure. The range of computed conductance corresponds well to the narrow distribution in experiment, although the average calculated conductance is approximately 7 times larger. This discrepancy is attributed to the absence of many-electron corrections in the DFT molecular orbital energies; a simple physically-motivated estimate for the self-energy corrections results in a conductance that is much closer to experiment [3]. Second, similar first-principles techniques are applied to a range of bipyridine-Au junctions. The extent to which Au-pyridine link bonding is affected by the constraints of forming bipyridine-Au junctions is investigated. In some contrast to the amine case, the computed conductance shows a strong sensitivity to the tilt of the bipyridine rings relative to the Au surfaces. Experiments probing the conductance of bipyridine-Au junctions are discussed in the context of these findings. [1] H. J. Choi et al, Phys Rev B, 76, 155420 (2007) [2] L. Venkataraman et al, Nano Lett 6, 458 (2006) [3] S. Y. Quek et al, Nano Lett. 7, 3477 (2007) [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D28.00002: Single Molecule Conductance and Contact Chemistry Latha Venkataraman, Young Park, Adam Whalley, Masha Kamenetska, Michael Steigerwald, Colin Nuckolls, Mark Hybertsen Our previous experiments probing the conductance of single molecule circuits with amine-gold linkages have demonstrated the relationship between the electrical characteristics and the intrinsic molecular properties such as their length, conformation, gap between the highest occupied and lowest unoccupied molecular orbitals and the alignment of these levels to the metal Fermi level. Here we study different chemical linker groups expected to form donor-acceptor bonds to gold. We measure transport through single molecule junctions by repeatedly forming and breaking Au point contacts with a modified STM in a solution of the molecules terminated by Amine, Dimethyl Phosphine and Methyl Sulfide linker groups. The clear molecular signatures allow us to demonstrate a systematic dependence the link group. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D28.00003: Conductance Trends in Single Molecule Junctions Formed Using Donor-Acceptor Links: Theoretical Analysis Max B. Koentopp, Latha Venkataraman, Michael L. Steigerwald, Mark S. Hybertsen The conductance of single molecule junctions using amine-gold links has been understood based on formation of a donor- acceptor bond involving the N lone pair and the s-orbital on an undercoordinated Au site on the electrode. Experiments probing junctions formed with alkanes terminated by dimethyl phosphines and methyl sulfides also show an unambiguous conductance signature. The structure and bonding in these junctions is analyzed using density functional theory based calculations. Like the amine link, the dimethyl phosphine and methyl sulfide bond to an under-coordinated Au site through a donor-acceptor motif. While the bond energy for the amine and methyl sulfide links are similar (0.6 eV), the dimethyl phosphine is significantly stronger (1.2 eV). Trends in measured junction conductance (amine $<$ sulfide $<$ phosphine) are analyzed in terms of available electronic channels. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D28.00004: Molecular Conductance of oligophenylene-vinylene in Metallic Break Junctions Patrick Wheeler, Meng Lu, David Corley, James Tour, Doug Natelson Break junctions between a metallic tip and a metallic substrate have proven to be extremely useful tools for characterizing single-molecule electrical conductance. Conductance measurements while repeatedly breaking and reforming junctions are conducive to rapid statistical characterization. We will present preliminary results of room temperature break junction conductance measurements on amine-terminated oligophenylene-vinylene (OPV) oligomers. Recent low temperature measurements of OPV oligomers in the electromigrated gap configuration imply a large renormalization downward of the HOMO-LUMO gap. Since the HOMO-LUMO gap is correlated with the conductance and the tunneling coefficient, beta, break junction measurements should provide clarification about the HOMO-LUMO gap in these molecules. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D28.00005: Probing mechanisms of electrical conduction in single organic molecules Lyudmyla Adamska, Ivan Oleynik, Mortko Kozhushner The electrical conduction of relatively long (1-2 nm) single organic molecules occurs via resonant tunneling of charge carriers, electrons and/or holes, through the energy levels of negative molecular ion (electrons) and/or positive molecular ion (holes). The position of these resonant energy levels with respect to the Fermi levels of the anode and cathode determines the relative contributions of electron and hole conduction to the resonant current. These resonant levels depend on the applied bias, and are also influenced by several physical factors such as the polarization of the molecule, image potential and metal/molecule interfaces that are difficult to control under conditions of real experiment. In this presentation we suggest a method of \textit{unambiguous experimental} determination of specific type of the conduction mechanism (electron or hole conduction) which is based on the idea of utilizing experimental techniques of nanocalorimetry. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D28.00006: Single Molecule Junctions: Conductance, Formation and Persistence Statistics. Maria Kamenetska, Michael Frei, Mark Hybertsen, Latha Venkataraman We measure the conductance of single molecules attached to gold electrodes by repeatedly forming and breaking Au point contacts with a modified STM in a solution of molecules. Conductance traces measured while pulling the point-contacts reveal steps due to the formation of single molecule junctions which can be elongated without a significant change in junction conductance. To better understand the mechanical stability of these single molecule junctions, we analyze data sets of 20000 or more individual conductance traces for a series of diamine molecules, measuring the distance over which junctions can persist. We find that the distance that a junction can be pulled is affected by the metal-molecule binding energy. In addition, we see an unambiguous relationship between geometry and stability, where both the length of the molecule as well as the atomic configuration of the contact electrode affect the distance over which a junction can persist. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D28.00007: Tunneling Transport through Long Molecular Chains Emil Prodan, Roberto Car The Riemann structure of the bands and other properties of the evanescent Bloch functions have been used to derived an asymptotic expression for the tunneling conductance through long molecular chains. Our results give the contact conductance in terms of an overlap integral of three well defined and physically relevant quantities. In particular, this formula shows how the conducting states of the leads couple to the evanescent Bloch functions of the insulating chain. The theory is applied to amine-linked alkyl and aromatic chains and the results are compared with the experiment. Using these applications, we discuss the key aspects and advantages of the theory. Extensions to spin dependent transport will be also discussed. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D28.00008: Transport properties of molecular wires from ab initio calculations Kenji Hirose, Nobuhiko Kobayashi Understanding of electron transport through nanostructures becomes important with the advancement of fabrication process to construct atomic-scale devices. Due to the drastic change of transport properties by contact conditions to electrodes in local electric fields, first-principles calculation approaches are indispensable to understand and characterize the transport properties of nanometer-scale molecular devices. Here we focus on the transport properties of molecular wires bridged between metallic electrodes, especially on the effects of contacts to electrodes and on the dependence of the length of molecular wires on transport properties. We use an ab initio calculation method based on the scattering waves, which are obtained by the recursion-transfer-matrix (RTM) method, combined with non-equilibrium Green's function (NEGF) method. We find that conductance shows exponential behaviors as a function of the length of molecular wires due to tunneling process determined by the HOMO-LUMO energy gap. From the voltage drop behaviors inside the molecular wires, we show that the contact resistances are well separated for the long molecular wires. We will present detailed data of electronic states at contacts to metallic electrodes under strong electronic fields and will discuss the polarization, screening effect, and potential barrier formation at contacts on the transport properties of molecular wires, comparing them with those of metallic atomic wires. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D28.00009: The molecular electronics of protein fragments David Cardamone, George Kirczenow Small fragments of polypeptide chains provide a uniquely scalable, customizable basis for nanoelectronic devices. Using a combination of \emph{ab initio} and semi-empirical techniques, we arrive at a quantitative understanding of the charge transport properties of these molecules. This allows us to investigate their chemical and physical properties, such as lead-molecule bonding geometry, lead-induced distortion of molecular structure (e.g., molecular stretching), and device properties. We explain the observed current rectification in these molecules and further predict negative differential resistance, opening the way to protein-based nanoelectronic devices. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D28.00010: Ab initio studies of electronic transport through amine-Au-linked junctions of photoactive molecules David A. Strubbe, Su Ying Quek, Latha Venkataraman, Hyoung Joon Choi, J.B. Neaton, Steven G. Louie Molecules linked to Au electrodes via amine groups have been shown to result in reproducible molecular conductance values for a wide range of single-molecule junctions [1,2]. Recent calculations have shown that these linkages result in a junction conductance relatively insensitive to atomic structure [3]. Here we exploit these well-defined linkages to study the effect of isomerization on conductance for the photoactive molecule 4,4'-diaminoazobenzene. We use a first-principles scattering-state method based on density-functional theory to explore structure and transport properties of the cis and trans isomers of the molecule, and we discuss implications for experiment. [1] L Venkataraman et al., Nature 442, 904-907 (2006); [2] L Venkataraman et al., Nano Lett. 6, 458-462 (2006); [3] SY Quek et al., Nano Lett. 7, 3477-3482 (2007). [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D28.00011: Electronic spectrum and orbital filling in a single-molecule junction Edgar A. Osorio, Kevin O'Neill, Maarten Wegewijs, Nicolai Stuhr-Hansen, Jens Paaske, Thomas Bjornholm, Herre van der Zant We study single-electron tunneling in three-terminal devices in which a single molecule bridges the gap between source and drain electrode. The molecular devices are made by electromigration and at low temperatures excitations appear in the stability diagram. For the OPV-5 molecule more than fifteen different excitations are visible, of which twelve match RAMAN spectra and the remaining ones are due to vibratrions of the molecule attached to gold electrodes at energies below 10 meV. Similar to carbon nanotubes, the observation of a singlet-triplet transition allow us to determine the orbital filling and spin configuration of the molecule. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D28.00012: Gate effects on electronic transport in alkanedithiol single molecular junctions. Hyunwook Song, Takhee Lee, Youngsang Kim, Heejun Jeong We investigate the gate effects on the electronic transport properties in alkanedithiol single molecular junctions. Using electromigration-induced break junction technique, we fabricated an array type of electrode pairs with nanometer-sized separation on top of naturally oxidized aluminum gate electrodes. The alkanedithiol molecules were bridged between the nanometer-sized gap that was achieved by breaking gold nanowires fabricated using electron-beam lithography with a controlled passage of current. The electric potential applied to the aluminum gate electrode shifts the molecular energy levels relative to the Fermi energy in the metallic contacts. We will discuss the gate-bias dependent current-voltage characteristics and other observed transport properties of alkanedithiol single molecular junctions in the off-resonant tunneling transport regime. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D28.00013: Modeling of N@C$_{60}$ single-molecule transistors Carsten Timm, Jacob E. Grose, Wolfgang Harneit, Daniel C. Ralph We report on recent experimental and theoretical results for single-molecule transistors involving endohedral N@C$_{60}$ fullerene molecules. In this talk, we will focus on the theoretical modeling. The observed differential conductance shows strong evidence for the exchange interaction between electrons in the fullerene LUMO and the nitrogen p-electrons, favoring an antiferromagnetic interaction. In addition, soft vibrational modes are seen, which are attributed to oscillations of the molecule as a whole. We discuss a model Hamiltonian that reproduces the main features of the experimental conductance. [Preview Abstract] |
Session D29: Focus Session: Carbon Nanotubes and Related Materials IV: Graphene
Sponsoring Units: DMPChair: Igor Herbut, Simon Fraser University
Room: Morial Convention Center 221
Monday, March 10, 2008 2:30PM - 3:06PM |
D29.00001: Electronic properties of Dirac fermions in epitaxial graphene Invited Speaker: Graphene, atomically thin layers of graphite, has attracted a lot of research interest because of its intriguing physics as well as its technological potential for next generation electronic devices. I will first present a detailed characterization of the growth of atomically thin films of epitaxial graphene on SiC, by using low energy electron microscopy (LEEM). The electronic properties of the films are hence studied by angle resolved photoemission spectroscopy (ARPES). Data as a function of doping, temperature and sample thickness are presented and the role of disorder and many body interactions will be discussed. Finally, the presence of a bandgap in the spectra of Dirac fermions will be presented and its potential for bandgap engineering will be discussed. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D29.00002: Electronic Confinement in Epitaxial Graphene As Seen by ARPES David Siegel, Shuyun Zhou, Alexei Fedorov, Andreas Schmid, Farid El Gabaly, Alessandra Lanzara The epitaxial growth of graphene on 6-H SiC and its electronic structure have been studied with low energy electron microscopy (LEEM) and angle-resolved photoemission spectroscopy (ARPES) respectively. Some of the critical growth parameters that determine sample homogeneity and domain properties have been identified. The resulting electronic structure presents features that generally agree with the conical dispersion of Dirac quasiparticles, however deviations are observed near the Dirac point energy. The dependence of these deviations on real-space electronic confinement is discussed. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D29.00003: Tuning the phonon self-energy of a graphene bilayer Jun Yan, Erik Henriksen, Philip Kim, Aron Pinczuk We use low temperature Raman spectroscopy and the electric field effect to investigate the coupling of long wavelength optical phonons (the G-band) with charge carriers in bilayer graphene. The charge tunable phonon spectra exhibit a remarkable symmetry which reflects the underlying particle-hole symmery of the electron band structrue. The change of phonon line-width is interpreted as a Landau damping of the phonon into resonant electron-hole pair transitions. The phonon energy exhibits an intriguing non-monotonic evolution with charge density. We found that the electron-hole pair excitation stiffens (softens) the lattice vibration when its energy is smaller (larger) than the phonon energy, in agreement with theoretical predictions for deformation electron-phonon coupling. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D29.00004: Graphene on a graphite surface: effect of interlayer coupling. Adina Luican, Guohong Li, Eva Y. Andrei We present low temperature high magnetic field scanning tunneling microscopy and spectroscopy on a sheet of graphene suspended above a graphite substrate. The sheet consists of two regions that couple to the substrate with different strengths resulting in two distinct sequences of Landau Levels (LL). One region exhibits a sequence that is typical of single layer graphene (square root dependence on field and level index) with a reduced Fermi velocity that is renormalized by electron-phonon interactions. The sequence in the other region is anomalous and, according to recent theoretical work [1], it can be attributed to a bilayer with interlayer coupling that is $\sim $ 10 times weaker than that of normal bilayers. We find that the difference between the two regions is also reflected in the values of the Fermi velocity, suggesting that electron-phonon renormalization is suppressed by interlayer coupling. \newline [1] \textit{M. Pereira, F.M. Peeters and P. Vasilopoulos, Phys. Rev. B 76, 115419 (2007)} [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D29.00005: Scanning tunneling microscopy/spectroscopy studies of local electronic structure in Epitaxial Graphene Nikhil Sharma, Michael Sprinkle, Claire Berger, Walter DeHeer, Phillip First Epitaxial growth of graphene on hexagonal SiC by thermal desorption of Si has produced high quality films, providing a potential route to wafer-scale graphene electronics. However, many aspects of this new electronic material system remain to be understood. Using scanning tunneling microscopy and spectroscopy (STM/STS), we investigate the layer-dependent effect of atomic defects and deposited metal islands on the local electronic structure of epitaxial graphene. Metal islands locally dope the graphene due to the work function difference between materials, and atomic defects can have a similar effect. The lateral gradient in carrier density and the possible transition from hole- to electron- doping (PN junction) is investigated experimentally for these cases. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D29.00006: Transport in multilayered epitaxial graphene Claire Berger, Xiaosong Wu, Mike Sprinkle, Xuebin Li, Fan Ming, Walt de Heer We present recent results of electronic transport in multilayered epitaxial graphene (EG) grown by thermal decomposition of SiC wafers. Because of the rotational stacking of the layers, it was recently shown theoretically that the system should retain essentially the same band structure as single layer graphene. The system consists of a charged layer at the SiC/EG interface, as revealed by the period of the Shubnikov-de Haas oscillations (a few $10^{12}/cm^2$), and quasi-neutral layers on top. We discuss possible effects of the multilayering in the transport properties, such as the large positive increase in field of the magnetoresistance $\rho_{xx}$, the weak amplitude of the Shubnikov-de Haas and the overall features of the Hall effect, in particular the anomaly of the Hall resistance $\rho_{xy}$ observed at low field. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D29.00007: Electronic compressibility of a graphene bilayer Silvia Viola Kusminskiy, Johan Nilsson, David Campbell, Antonio Castro Neto We calculate the electronic compressibility arising from electron-electron interactions for a graphene bilayer within the Hartree-Fock approximation. We show that, due to the chiral nature of the particles in this system, the compressibility is rather different from those of either the two-dimensional electron gas or ordinary semiconductors. We find that an inherent competition between the contributions coming from intra-band exchange interactions (dominant at low densities) and inter-band interactions (dominant at moderate densities) leads to a non-monotonic behavior of the compressibility as a function of carrier density. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D29.00008: Charge inhomogeneity in a single and bilayer graphene Hari Dahal, Tim Wehling, Kevin Bedell, Jian-Xin Zhu, Alexander Balatsky We study the possibility of charge ordered state in both single and bilayer graphene using a real space tight binding model. We find that the single layer graphene always remains in a liquid phase; the reason being the higher kinetic energy compared to the potential energy. The bilayer graphene on the other hand can have an inhomogeneous distribution of the charge, namely the charge density wave (CDW) state. The CDW state is commensurate with the lattice. The charge ordered state is stabilized by the Coulomb interaction of the carriers of two layers. We also predicted a kinetic energy driven (KID) inhomogeneous phase. This phase can be stabilized by the inter layer hopping energy. The KID phase and the CDW phase compete with each other below the half filling whereas they cooperate above half filling. For the physical parameter of bilayer graphene CDW phase always wins over the KID phase. Hari P. Dahal, Tim O. Wehling, Kevin S. Bedell, Jian-Xin Zhu, Alexander V. Balatsky [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D29.00009: Interplay of Coulomb interactions and rippling of monolayer graphene: RG approach Oskar Vafek, Vladimir Juricic, Igor Herbut The effects of electron-electron Coulomb interactions and rippling disorder of a mono-layer graphene are studied at half-filling using renormalization group. It is found that the system flows to an infra-red stable line of fixed points which is accessible perturbatively and along which the zero temperature minimal metallic conductivity is non-universal and enhanced relative to the clean non-interacting fixed point. An estimate of the typical random vector potential representing ripples in graphene brings the theoretical value of the minimal conductivity into the vicinity of $4e^2/h$. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D29.00010: Graphene in a magnetic field and a superlattice potential J.M.B. Lopes dos Santos, N.M.R. Peres, A.H. Castro Neto The problem of electrons in a periodic potential in the presence of a magnetic field is revisited here in the context of graphene, by considering a superlattice periodic perturbation on the Dirac-Weyl equation for massless fermions. We solve the problem of a periodic potential for massless Dirac Fermions in a magnetic field. The relevance for graphene physics arises from the possibility of a superlattice modulation, both in single-layer graphene, due to the substrate, and in few layer graphene due to rotational stacking faults, which give rise to long wavelength moire patterns [1,2]. \par\noindent [1] J. Hass. \emph{et. al} arXiv:0706.2134v1 [cond-mat.mtrl-sci]\par\noindent [2] JMB Lopes dos Santos, NMR Peres and AH Castro Neto,arXiv:0704.2128v1 [cond-mat.mtrl-sci] [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D29.00011: Quenching of quantum Hall effect and the role of undoped planes in epitaxial graphene Pierre Darancet, Nicolas Wipf, Didier Mayou We propose a mechanism for the quenching of the Shubnikov de Haas oscillations and the quantum Hall effect observed in epitaxial graphene. This involves a coupling between the uncharged rotationally stacked layers and the charged graphene layer at the interface. In a magnetic field, the extraordinary graphene $n=0$ Landau level of the uncharged layers produces a high density of states at the Fermi level. Consequently we find that the scattering time of the conduction electron in the charged plane is magnetic field dependent and reduced to the order of the cyclotron orbit period. This scenario also explains quantitatively the recent observation of a linear magnetoresistance in epitaxial graphene. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D29.00012: Anomalous Thermoelectric Effect in Graphene Tomas Lofwander, Mikael Fogelstrom We present calculations of the thermal and electric linear response in graphene, including disorder in the self-consistent t-matrix approximation [1]. For strong impurity scattering, near the unitary limit, the formation of a band of impurity states near the Fermi level leads to that Mott's relation holds at low temperature. For higher temperatures, there are strong deviations due to the linear density of states. The low-temperature thermopower is proportional to the inverse of the impurity potential and the inverse of the impurity density. Information about impurity scattering in graphene can be extracted from the thermopower, either measured directly, or extracted via Mott's relation from the electron-density dependence of the electric conductivity.\newline \newline [1] T. L\"ofwander and M. Fogelstr\"om, Phys. Rev. B {\bf 76}, 193401 (2007). [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D29.00013: Charge Carrier Interaction with a Purely Electronic Collective Mode: ``Plasmarons" and the Infrared Response of Semi-metal Bismuth N. Peter Armitage, Riccardo Tediosi, Enrico Giannini, Laszlo Forro, Dirk van der Marel We present a detailed pressure and temperature- dependent optical study of single-crystal bismuth using infrared reflectivity and ellipsometry. In the ambient pressure optical conductivity, an anomalous temperature dependent mid-infrared absorption feature is observed. An extended Drude model analysis reveals that it can be connected to a sharp upturn in the scattering rate, the frequency of which exactly tracks the strongly temperature dependent plasmon frequency. We interpret this absorption and increased scattering as direct optical evidence for a charge carrier interaction with a collective mode of purely electronic origin, here electron-plasmon scattering. The observation of a ``plasmaron'' as such is made possible by the exceptional properties of semi-metal bismuth, but it is also likely relevant to the low energy transport and thermodynamic properties of other semi-metals, like graphite and graphene. As a function of pressure, we observe massive changes in bismuth's optical and infrared conductivity as the material approaches a Lifshitz-like metal/insulator transition. [Preview Abstract] |
Session D30: Focus Session: Carbon Nanotubes and Related Materials V: Nanotube Transport
Sponsoring Units: DMPChair: Oleg Prezhdo, University of Washington at Seattle
Room: Morial Convention Center 222
Monday, March 10, 2008 2:30PM - 3:06PM |
D30.00001: Electrical, Mechanical, and Optical Studies of Carbon Nanotubes of Known Chiral Index. Invited Speaker: Because small changes in the crystal structure (chirality) of carbon nanotubes can produce large changes in their electrical properties, it is important to understand the relationship between structure and transport properties, both for basic science and for applications. We have developed a unique set of tools for characterizing and manipulating nanotubes that allow for detailed studies of the properties of known-chirality nanotubes. Completed and ongoing studies include: structure-correlated optical properties; tube-tube interactions; variable electron-phonon coupling; electromechanical properties; structure-correlated electrical transport; nanotube intermolecular heterojunctions; and mechanical stiffness and strength. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D30.00002: Electron field emission from Carbon Nanotube Composites: Transport and Local Electric Fields David Carey, Thomas Connolly, Richard Smith, Jonathan Coleman Electron field emission characterisation of carbon nanotube polymer composites has been performed where emission at low nanotube mass fractions ($<$10{\%}) has been observed. Nanotubes have been embedded in two different types of polymer: PmPV, a conjugated polymer and PVA, polyvinyl alcohol. It shown that for nanotubes embedded in PmPV, the field emission is strongly influenced by charge transport through the film. For nanotubes in PVA a transition from bulk transport to a Fowler-Nordheim emission mechanism is seen as the mass fraction exceeds about 1-2{\%}. Estimates of the local field as a function of mass fraction are also shown. The potential role of nanotube -- polymer composites produced by solution processing to large area cathodes with a controllable mass fraction will be discussed. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D30.00003: Metal-Semiconductor-Metal (MSM) Photodetectors with Single-walled Carbon Nanotube Film Schottky Electrodes on GaAs Ashkan Behnam, Jason Johnson, Yongho Choi, Leila Noriega, G\"unhan Ertosun, Zhuangchun Wu, Andrew Rinzler, Pawan Kapur, Krishna Saraswat, Ant Ural We fabricate and experimentally characterize the dark and photocurrent in metal-semiconductor-metal (MSM) photodetectors with transparent and conductive single-walled carbon nanotube (CNT) film electrodes on GaAs. The dark current measurements of MSM structures reveal that the CNT film forms a Schottky contact on GaAs substrates. The Schottky barrier height and the CNT film workfunction are extracted to be approximately 0.55 and 4.6 eV, respectively, based on dark current measurements as a function of temperature. We also study the effect of device geometry on the dark current of the CNT film-GaAs MSM devices. Furthermore, we find that CNT film MSM devices exhibit a significantly lower dark current and higher normalized photo-to-dark current ratio compared to metal control samples. We explain these observations by comparing the interfaces in these structures. This work opens up the possibility of integrating CNT films as Schottky electrodes in conventional semiconductor electronic and optoelectronic devices. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D30.00004: Anomalous Coulomb oscillation in crossed carbon nanotubes Seung Jae Baek, Dongsu Lee, Seung Joo Park, Yung Woo Park, Johannes Svensson, Mats Jonson, Eleanor E. B. Campbell Single-walled carbon nanotube (SWCNT) crossed junctions separated by an insulating layer were fabricated to investigate the double quantum dot modulated by a single gate (DQD-sG). Anomalous Coulomb oscillations were observed on the lower CNT at low temperature, where the behavior was interpreted by the concept of a double quantum dot (DQD) system http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal\&id =APPLAB000089000023233107000001\&idtype=cvips\&gifs=yes [1]. To understand it more clearly, we have intentionally fabricated crossed CNTs without oxide layer in between. The observed anomalous Coulomb oscillations indicate that the contact resistance between the two tubes becomes a potential barrier splitting the initial single QD into the DQD, and the back-gate modulates the energy levels of the DQD. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D30.00005: Strain Tuning of the Photocurrent Spectrum in Single wall Carbon Nanotubes Prasanth Gopinath, A. Mohite, H. Shah, J. Lin, B. Nagabhirava, T. Bansal, B. Alphenaar The electronic structure of a single-wall nanotube (SWNT) can be substantially modified by the application of uniaxial strain. We use displacement photocurrent spectroscopy to study the effect of uniaxial strain on the optical transitions of a SWNT in the energy range 0.5eV-3eV. This broad energy range allows us to compare the strain dependence of the lowest (E11) and higher order (E22 and E33) optical transitions of semiconducting SWNT's. As predicted by a simple non-interacting model, we observe an energy shift of each transition with increasing strain. By fitting the model to the magnitude and slope of the energy shift for the lowest energy transition (E11) the nanotube chirality can be identified uniquely. For the higher energy transitions, the data deviates significantly from the non-interacting model, presumably because of the influence of the excitonic binding energy. Finally, we observe a large reversible increase in the magnitude of the photocurrent around the ground state (E11) energy regime with an applied strain of 0.01{\%}. We attribute this to reversible strain induced defect states opening up within the bandgap of the SWNT. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D30.00006: Negative magneto resistance in carbon nanotubes: A first principle study Li Chen, Saroj Nayak We have studied spin transport through single wall carbon nanotube (SWNT) with nickel contacts using \textit{ab initio} density functional theory and green's function based Landauer B\"{u}ttiker formalism. Our results show enhanced current for anti-parallel alignment compared to that obtained for parallel alignment. This non Julli\'{e}re's model results due to finite size effect and this talk will present a detailed atomic level mechanism of such phenomena. Our results will be discussed in the light of recent experimental studies. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D30.00007: Observation of Spin-Orbit Coupling in Clean Carbon Nanotubes Ferdinand Kuemmeth, Shahal Ilani, Paul McEuen, Dan Ralph The electronic states in defect-free carbon nanotubes (NTs) are widely believed to be four-fold degenerate, due to independent spin and orbital symmetries, and also to possess electron-hole symmetry. We report measurements demonstrating that in clean NTs the spin and orbital motion of electrons are coupled, thereby breaking all of these symmetries. This spin-orbit coupling is directly observed as a splitting of the four-fold degeneracy of a single electron in an ultra-clean quantum dot. Application of a parallel magnetic field reveals that the coupling favours parallel alignment of the orbital and spin magnetic moments for electrons and anti-parallel alignment for holes. We further show that SO coupling determines the filling order in the many-electron ground states, in a way different than that expected from electron-electron interactions. At low magnetic fields we find that the two-electron ground state is neither a spin-triplet nor a spin-singlet, but a Slater determinant in which the spin and orbital wavefunction are entangled. Our findings have important implications for spintronic applications in NTs and provide a mechanism for all-electrical control of spins. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D30.00008: Infrared Photoconductivity of Individual Carbon Nanotubes Using Fourier-Transform Spectroscopy Sami Rosenblatt, Matthew Sfeir, Yang Wu, Hugen Yan, Christophe Voisin, Bhupesh Chandra, Robert Caldwell, Yuyao Shan, Tony F. Heinz, James Hone, G. Larry Carr, James A. Misewich We have developed a novel infrared spectroscopy technique for studying the photocurrent response of individual semiconducting carbon nanotubes. We use a synchrotron-based, broadband infrared light source coupled to a Fourier-transform spectrometer and microscope to induce currents in electrically biased nanotubes. This approach enables the rapid acquisition of high-resolution photocurrent spectra near the bandgap of the larger diameter ($>$ 1.7 nm) nanotubes commonly produced by synthesis using chemical vapor deposition. We have recorded optical transitions with energies as low as 0.4 eV for individual nanotubes. The structures used in these measurements consisted of isolated nanotubes with well-separated metal contacts on a Si back gate. In addition to describing the experimental approach and results, we will discuss the sensitive dependence of the measured photocurrent on the electrical biasing conditions. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D30.00009: Conductance enhancement of carbon nanotubes through metallization Caterina Soldano, Li Chen, Swastik Kar, Saikat Talapatra, Robert Vajtai, Saroj Nayak, Pulickel Ajayan A novel method for building high-conductance device using carbon nanotubes is presented. The process involves a systematic, repeatable and controllable enhancement of the conductance using a rapid high-voltage cycling conducted in vacuum; this process leads to orders of magnitude drop in the two-terminal resistance. Electron microscopy analysis indicates that the high-bias cycling of nanotubes causes sufficient Joule heating for the platinum to migrate from the contact regions and decorate the outer surface of the nanotubes, giving rise to enhanced metallization. Pre- and post-metallization characterization is presented. It is believed that the conductance enhancement is due to a combination of a decrease in disorder density in the tube and an increase in the number of available channels for conductance. Those outcomes are investigated in the light of recently predicted theoretical models. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D30.00010: Carbon nanotubes as tunable Luttinger liquids Wade DeGottardi, Tzu-Chieh Wei, Smitha Vishveshwara We investigate the properties of single-walled carbon nanotubes in transverse electric and magnetic fields. We find via band structure calculations that these fields can break particle- hole symmetry as well as that of the two Dirac points. Additionally, the speed of the left and right movers is generally different in the presence of both electric and magnetic fields. We consider the effect of these fields on Coulomb interactions within the tube and show that they can be used to tune the interaction parameter K associated with the Luttinger liquid properties of the tube. Finally, we discuss finite size effects and Coulomb blockade physics in this context. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D30.00011: Massively Parallel Imaging and Electrical Characterization of Carbon Nanotube Transistors with Scanning Thermocurrent Microscopy Adam Tsen, Huseyin Kurt, Luke Donev, LiHong Herman, Jiwoong Park Electrical characterization of individual carbon nanotubes (CNTs) is a painstaking and time-consuming process, requiring a serial study of individual CNT devices. We present a novel method utilizing a laser-based setup$^{1}$ to simultaneously image and electrically characterize a multitude of CNTs in a parallel transistor array geometry, foregoing the need to create individual CNT devices in the study of their electronic transport. The diffraction-limited laser spot induces local heating of the CNT and affects overall conductance under applied bias, presenting a spatially-resolved visual image of the CNT in our measurement scheme. Furthermore, this change in conductance displays gate dependence similar to that of overall conductance of the CNT, allowing us to probe the local electronic properties of the CNT simultaneously. Using this technique we are then able to both image with diffraction-limited resolution and electrically characterize up to hundreds of CNTs rapidly in the simplest of device geometries. 1. Y. H. Ahn, A. W. Tsen, B. Kim, Y. W. Park, and J. Park, ``Photocurrent Imaging of p-n Junctions in Amibipolar Carbon Nanotube Transistors,'' \textit{Nano Letters}, vol. 7, no. 11, pp. 3320-3323, 2007. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D30.00012: Modeling Nanotube Networks For Semiconductor Channels and Sensors Meg Noah, Young-Kyun Kwon We present a model to characterize ensembles of NT networks using properties of individual from measurement and from ab initio computations, and including changes in the presence of gases like NH$_{3}$, pressure, or external fields. From these, we simulate networks for user-specified channel shape, size and inhomogeneous NT mixtures. For example, the ensemble \textit{IVg }characteristics of 2 by 2 micron network can readily be compared to ensembles of 20 by 40 micron networks or annular networks with 1 micron and 3 micron radii for a mixure of nanotubes characterized by independent length distributions for each chirality and then compared subjected to different environmental conditions. Validation with experimental data resulting from inhomogeneous NT mixtures is presented. Our goal is to optimize nanomanufacturing parameters like channel size for a user-defined application be it gas sensor, pressure actuator, or semiconductor answering questions like: ``What is the statistical conductivity enhancement in the presence of NO$_{2}$?'' ``What radii yield the most semiconducting for 1 micron SWNT?'' and ``How does conductivity change as a function of gas density?'' We focus on the fundamental understanding of nanocomposites. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D30.00013: The Control of Electron Transport Related Defects in \textit{In situ} Fabricated Single Wall Carbon Nanotube Devices Zhixian Zhou, Alaska Subedi, Gyula Eres, Rongying Jin, David Mandrus Metallic single wall carbon nanotube (SWNT) devices were characterized using low temperature transport measurements to study how the growth conditions affect defect formation in carbon nanotubes. Suspended carbon nanotube devices were grown \textit{in situ} by a molecular beam growth method on a pair of catalyst islands located on opposing Au electrodes fabricated by electron beam lithography. We present experimental evidence that transport related defect formation in carbon nanotubes, in addition to the well known growth temperature dependence, is also affected by the nature and the composition of the carbon growth gases. [Zhou \textit{et al}., Appl. Phys. Lett. 89, 133124 (2006)] We have also investigated how the transport related defects affect the performance of SWNT field-effect-transistors, revealing significantly different impacts of the defects on semiconducting SWNT devices with Schottky and Ohmic contacts. [Zhou \textit{et al}., unpublished] [Preview Abstract] |
Session D31: Focus Session: Computational Nanoscience II: Nanowires and Transport
Sponsoring Units: DMP DCOMPChair: Serdar Ogut, University of Illinois at Chicago
Room: Morial Convention Center 223
Monday, March 10, 2008 2:30PM - 3:06PM |
D31.00001: First Principles Studies of Tapered Silicon Nanowires: Fundamental Insights and Practical Applications Invited Speaker: Nanowires (NWs) are often observed experimentally to be tapered rather than straight-edged, with diameters (d) shrinking by as much as 1 nm per 10 nm of vertical growth. Previous theoretical studies have examined the electronic properties of straight-edged nanowires (SNWs), although the effects of tapering on quantum confinement may be of both fundamental and practical importance. We have employed ab initio calculations to study the structural and electronic properties of tapered Si NWs. As one may expect, tapered nanowires (TNWs) possess axially-dependent electronic properties; their local energy gaps vary along the wire axis, with the largest gap occurring at the narrowest point of the wire. In contrast to SNWs, where confinement tends to shift valence bands more than conduction bands away from the bulk gap, the unoccupied states in TNWs are much more sensitive to d than the occupied states. In addition, tapering causes the band-edge states to be spatially separated along the wire axis, a consequence of the interplay between a strong variation in quantum confinement strength with diameter and the tapering-induced charge transfer. This property may be exploited in electronic and optical applications, for example, in photovoltaic devices where the separation of the valence and conduction band states could be used to transport excited charges during the thermalization process. In order to gain insight into TNW photovoltaic properties, we have also carried out calculations of the dipole matrix elements near the band edges as well as the role of metal contacts on TNW electronic properties. Finally, a combination of ab initio total energy calculations and classical molecular dynamics (MD) simulations are employed to suggest a new technique for bringing nanoscale objects together to form ordered, ultra high-aspect ratio nanowires. This work was supported in part by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D31.00002: Structure and electronic properties of silicon nanowires grown along the [110] direction: role of surface reconstruction T. Akiyama, K. Nakamura, T. Ito Silicon single-crystal nanowires (SiNWs) are attracting great interest for future nanoscale devises in recent years. So far, the [110] grown SiNWs with diameters below 4 nm have been successfully fabricated by various methods\footnote{Ma {\it et al.}, Science {\bf 299}, 1874(2003); Wu {\it et al.}, Nano Lett. {\bf 4}, 433(2004).}. Although the size dependence in electronic and optical properties for the [110] grown SiNWs terminated by H-atoms have been intensively studied\footnote{Zhao {\it et al.}, Phys. Rev. Lett. {\bf 92}, 236805(2004).}, effects of surface reconstructions on the electronic structure have been rarely examined. Here, we investigate the atomic and electronic structures of SiNWs along the [110] direction with \{001\} and \{111\} facets using first-principles pseudopotential method. The calculations for SiNWs whose diameters are $\sim$4 nm demonstrate that the reconstructions on these facets are strongly dependent on H-chemical potential $\mu_{\rm H}$: The SiNW consisting of monohydride \{001\} and H-terminated \{111\} facets is stabilized for high $\mu_{\rm H}$($\geq$-0.75 eV) while the pristine SiNW stabilized for low $\mu_{\rm H}$($\leq$-0.82 eV). The reconstructions with partially hydrogenated facets appear for -0.82$<$$\mu_{\rm H}$$<$-0.75 eV. Peculiar features in the electronic structure are also found in partially hydrogenated SiNWs. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D31.00003: Modeling of Polycrystalline and Wurtzite Si Nanowires with Symmetry-Adapted Objective Molecular Dynamics Traian Dumitrica, Dong-Bo Zhang, Ming Hua The stability and properties of the most promising ground state candidate Si nanowires with less than $10$~nm in diameter is comparatively studied with molecular dynamics coupled with non-orthogonal tight-binding and classical potential models. The computationally-expensive tight-binding treatment becomes tractable due to the substantial simplification in the number of atoms introduced by the presented symmetry-adapted objective molecular dynamics scheme. It indicates that the achiral polycrystalline of five-fold symmetry and the wurtzite wires of three-fold symmetry are the most favorable quasi one-dimensional Si arrangements. Quantitative differences with the classical model description are noted over the whole diameter range. Using a Wulff energy decomposition approach it is revealed that these differences are caused by the inability of the classical potential to accurately describe the interaction of Si atoms on surfaces and strained morphologies. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D31.00004: \textit{Ab initio} calculations for the electronic properties of zinc-doped indium phosphide nanowires Manuel Alemany, Xiangyang Huang, Murilo L. Tiago, L.J. Gallego, James R. Chelikowsky $p$-type indium phosphide nanowires are known to function as working devices when assembled with $n$-type nanowires, and thus are seen as very promising building blocks for highly integrated electronic devices within the semiconductor industry. In this work, we have characterized the impurity state responsible for current flow in zinc-doped indium phosphide nanowires through first-principles calculations based on a real-space implementation of density-functional theory and pseudopotentials. The binding energy of the acceptor state is predicted to range from the value of the acceptor state in the bulk to up to values of approximately 0.2 eV in the thinner nanowires as a result of the two-dimensional quantum confinement. Our results show that, in thin nanowires, quantum confinement can move the defect level deep into the energy gap. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D31.00005: Molecular-Dynamics Simulations of Nanowire Growth Tomorr Haxhimali, Dorel Buta, Mark Asta, Jeffrey Hoyt This talk will present results of molecular dynamics simulations investigating the mechanisms of nanowire growth from a liquid. We investigate the model system of elemental Si, modeled with the classical Stillinger-Weber potential. The work aims to investigate the effect of nanowire size on the intrinsic growth mechanisms and the relations between solid-liquid interface velocity, growth direction and driving force. Results will be presented for nanowires with diameters ranging from 5-10 nm, and will be compared with simulations for bulk Si modeled with the same potential. The consequences of these findings for the mechanisms of nanowire growth from liquid catalysts by the vapor-liquid-solid mechanism will be discussed. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D31.00006: First-principles study of the electronic and magnetic properties of Fe-Co nanowires Dangxin Wu, Ping Liu, Qiming Zhang, Ruqian Wu Fe-Co nanowires provide a potential way to produce high-performance nanocomposite permanent magnets due to their high Curie temperature, large magnetization and appreciable anisotropy. In this talk we present our recent results of first-principles investigation of this matter. The calculations use both PAW method and FLAPW method, based on density functional theory. The structures of Fe-Co nanowires were optimized by PAW method and then the electronic structure and magnetic properties such as saturation magnetization and anisotropy energies are studied by FLAPW method. The effects of size and composition of the nanowires on the magnetic properties are also studied and compared with those of bulk Fe-Co materials. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D31.00007: Magnetic Co impurity in Gold Nanowires Edison da Silva, Renato Pontes, Antonio J.R. da Silva, Adalberto Fazzio Nanoscale electric contacts using suspended gold nanowires (NWs) have recently been made and were imaged by electron microscopy. Using tools derived from Density Functional Theory (DFT) we study the role of magnetic impurities in these NWs with the possibility of spintronic applications. Here we study structural and transport properties of a gold nanowire with one Co impurity as function of tension applied to the NW. Co added new features to the physics of this system. We present studies of structure and also electronic transport using the same DFT formalism [1] that show the effect of the spin anisotropy introduced by Co. In particular, we present results of two geometries, one where the Co atom is connected to two Au atoms of the lead and another where it is in the middle of the suspended neck, in a linear configuration. In the former case we observe an interference between the s and d channels, leading to a Fano-like structure in the transmittance, whereas in the latter configuration due to the local symmetry there is a decoupling between these two channels and the transmittance has a simple peak around the Co d-states, leading to a large spin polarized transport. [1] F. D. Novaes, A.J.R. da Silva, and A. Fazzio, Braz. Jou. Phys. \textbf{36 }(3A): 799-807 (2006) [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D31.00008: Transport in Carbon Nanotube Junctions K.H. Khoo, James R. Chelikowsky There is growing interest in the use of carbon nanotube thin films as transparent electrical conductors and thin-film transistors owing to their high optical transmittance, low sheet resistivity, and ease of fabrication. [1,2] A major contribution to the sheet resistivity originates at nanotube junctions, as electrical contact is typically poor between adjacent nanotubes. It is thus important to characterize carbon nanotube junctions in order to understand the conduction properties of nanotube thin films. To this end, we have performed \textit{ab initio} density functional theory calculations to investigate the structural, electronic and transport properties of carbon nanotube junctions as a function of nanotube chirality and contact geometry \newline [1] Z. Wu \textit{et al.}, Science \textbf{305}, 1273 (2004) \newline [2] E. S. Snow, J. P. Novak, P. M. Campbell, and D. Park, Appl. Phys. Lett. \textbf{82}, 2145 (2003). [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D31.00009: Schottky Barrier Heights in CNT-Metal Junctions from First-principles Nicholas Singh-Miller, Nicola Marzari Fundamental understanding of the electronic properties at the junction between a carbon nanotube (CNT) and a substrate is important for the practical application of CNT-based devices. Here, we use density functional theory (DFT) to probe the properties of the CNT-metal interface, paying particular attention to the Schottky barrier heights (SBH). We focus on the junction between a semiconducting (8,0)CNT and aluminum or palladium, chosen as paradigmatic examples of a simple metal and a transition metal, respectively. We obtain SBHs from the potential lineup, examining the effects of geometry at the interface and the functionalization of the CNT on the SBH. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D31.00010: Transport properties of transition-metal-encapsulated Si cages Lingzhu Kong, James R. Chelikowsky We performed density functional pseudopotential calculations of the spin dependent transport through transition-metal-atom-encapsulated Si cages Si$_{12}X$($X$=Mn, Fe and Co). The effect of the metal atom on conductance is studied. Mn and Fe doped systems show highly spin polarized transmission whereas the magnetization in Co doped system is quenched. It is found that electrons are transferred from Si atoms into the minority $d$ orbitals of the metal atoms. The conductance decreases as these electrons become localized around the encapsulated atoms. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D31.00011: Transport Properties through Nanomaterials by First-principles Calculations Hiroshi Mizuseki, Rodion Belosludov, Sang Uck Lee, Yoshiyuki Kawazoe Nanoscale molecular devices are potential candidates for this next step, and they would make it possible to realize the most advantageous devices. Our group has covered a wide range of nanoscale materials[1] such as self-assembled nanowires on Si(001) [2, 3], quantum length dependence of conductance in oligomers [4] and single-molecule rotation switch [5] and so on. In this presentation, we will present our recent study on the transport properties of these nanoscale materials using the nonequilibrium Green's function formalism for quantum transport and the density functional theory (DFT) of electronic structures using local orbital basis sets. References 1. http://www-lab.imr.edu/$\sim $mizuseki/nanowire.html 2. J.-T. Wang, C. Chen, E. G. Wang, D.-S. Wang, H. Mizuseki, and Y. Kawazoe, Phys. Rev. Lett., 97 (2006) 046103. 3. R. V. Belosludov, A. A. Farajian, H. Mizuseki, K. Miki, and Y. Kawazoe, Phys. Rev. B, 75 (2007) 113411. 4. Y. X. Zhou, F. Jiang, H. Chen, R. Note, H. Mizuseki, and Y. Kawazoe, Phys. Rev. B, 75 (2007) 245407. 5. Y. Y. Liang, F. Jiang, Y. X. Zhou, H. Chen, R. Note, H. Mizuseki, and Y. Kawazoe, J. Chem. Phys. 127 (2007) 084107. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D31.00012: Ab initio non-equilibrium Green's function study on the growth of metallic bridge in mixed conductor atomic switch Tomofumi Tada, Zhongchang Wang, Tingkun Gu, Satoshi Watanabe A novel atomic switch [1] composed of a mixed conductor, Ag$_{2}$S or Cu$_{2}$S, has attracted much attention. To investigate the electronic properties of the atomic switch, we have examined interface structure and electron transport of Ag/Ag$_{2}$S/Ag and Cu/Cu$_{2}$S/Cu using ab initio non-equilibrium Green's function method. In Ag/Ag$_{2}$S/Ag, we found a spontaneous growth of a metallic bridge composed of a Ag atomic chain when a unidirectional stress is applied to Ag$_{2}$S [2]. On the contrary, a metallic bride does not appear in Ag$_{2}$S and Cu$_{2}$S without stress. We also examined the influence of Ag/Cu addition on the structural and transport properties of Ag$_{2}$S and Cu$_{2}$S atomic switches, and found that the Ag/Cu addition leads to the metallization in the both systems. However, clear growth of the atomic bridge is confirmed only in Ag/Ag$_{2+\delta }$S/Ag. The metallic nature in Cu/Cu$_{2+\delta }$S/Cu is related to the growth of electron charge network at the Fermi level. 1) K. Terabe, et al., Nature 433, 47 (2005). 2) Z. Wang, T. Kadohira, T. Tada, S. Watanabe, Nano Letters 7, 2688 (2007). [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D31.00013: Ab initio transport properties of platinum chains calculated by taking into account spin orbit effects Victor Garcia-Suarez, David Zsolt Manrique, Colin Lambert, Jaime Ferrer The transport properties of infinite and finite platinum chains are calculated by using a combination of Density Functional Theory and Non-Equilibrium Green's Functions Formalism, as implemented in the Smeagol Code. We show that spin orbit effects, which are included fully self-consistently in our calculations, are of paramount importance to determine accurately the electronic and transport characteristics of these systems. For infinite chains we find that under special circumstances which depend on the type of chain (linear or zigzag), length and spin orientation relative to the chain, the conductance can be totally suppressed, giving rise to large magnetoresistive ratios. In the case of finite chains between bulk electrodes the spin-orbit effect plays also a crucial role and gives results which agree better with experiments. [Preview Abstract] |
Session D32: Focus Session: Spin Transfer Torque I
Sponsoring Units: GMAG DMP FIAPChair: Bill Rippard, National Institute of Standards and Technology, Boulder
Room: Morial Convention Center 225
Monday, March 10, 2008 2:30PM - 2:42PM |
D32.00001: Switching behavior of a Stoner-Wohlfarth particle subjected to spin-torque effect Huy Pham, Dorin Cimpoesu, Alexandru Stancu, Leonard Spinu The concept of the ``spin-transfer torque'' proposed by Slonczewski and Berger offers a new way of controlling the magnetization reversal in ferromagnetic multilayer systems, which replaces the conventional method utilizing magnetic field. The novel technology is expected to reduce the switching time of magnetization as well as to increase the recording density of the magnetoresistive random access memories. In this paper the switching properties of a Stoner-Wohlfarth magnetic particles, subject to a continuous or short magnetic field pulses, and to a short current pulse are presented. The theoretical investigation of precessional motion is described by using phenomenological modified Landau-Lifschitz-Gilbert equation with a spin-transfer torque term included. The switching under the influence of spin transfer torque is discussed as a function of the applied field strength and direction, and also as a function of the length of the current pulse. The main goal is to determine the parameters of field pulse for that the fast and stable switching can be achieved. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D32.00002: Dynamical Coupling of Nanomagnets due to Spin Transfer Sergei Urazhdin, Weng-Lee Lim, Nicholas Anthony, Andrew Higgins Spin transfer devices typically incorporate a thick magnet polarizing the electric current, and a thin layer driven by spin torque. However, spin torque acting on both layers is significant in devices with comparable thickness of magnetic layers. Moreover, dynamics of one of the magnetic layers results in oscillations of the polarization of the current flowing through the other layer, which can lead to dynamical coupling between them. We discuss results of simulations and measurements, demonstrating several consequences of such dynamical coupling. First, the dynamics of both layers are always simultaneously excited by the current. Second, the critical current for the onset of magnetic dynamics is scaled by the ratio of the thicknesses of the magnetic layers, diverging when the two are the same. This behavior is caused by the coupled precession of two magnetic layers reducing the efficiency of spin transfer. Below the critical current, a hysteretic regime is found in which a dynamical state and a static parallel configurations are possible. This regime may explain the 1/f noise and broad precession peaks that are often observed in the spectra of current-induced excitations. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D32.00003: Magnetic propeller driven by spin transfer Weng Lee Lim, Nicholas Anthony, Andrew Higgins, Sergei Urazhdin Spin-transfer devices usually contain two magnetic layers in which the thicker layer polarizes the electron current and the thinner layer experiences dynamics due to spin transfer. However, both magnetic layers can polarize current and experience simultaneous dynamics when the thickness of magnetic layers is similar (symmetric nanopillars). We investigated current-driven magnetization switching in \textit{symmetric} nanopillars with structure Ni$_{80}$Fe$_{20}$= Py(4nm)/Cu(3.5nm)/Py(4nm). Time-resolved measurements of resistance for both directions of current and magnetic field showed reversible switching of magnetization between parallel (P) and anti-parallel (AP) states with unusual dependence on the current. We observed that the dwell times displayed two different dependences on the current $I$ for different values of applied field $H$. At large $H$, the dwell time in the P state $t_{P}$ decreases with increasing $I$ while the dwell time in the AP state $t_{AP}$ increases, similarly to asymmetric devices. However, at small $H$, both $t_{P}$ and $t_{AP}$ decrease with increasing $I.$ We explain this unusual behavior by a thermal activation model involving four-cycle sequential reversal of two magnetic layers. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:42PM |
D32.00004: Coherence of spin-torque microwave oscillators Invited Speaker: Recently discovered effect of microwave generation in current-driven magnetic nano-structures caused by the spin-transfer torque opens a possibility for the development of a new class of tunable microwave auto-oscillators. The spin-torque oscillators (STO) are strongly nonlinear as their frequency $\omega (P)$ and total (positive plus negative) damping $\Gamma (P)$ are dependent on the oscillation power $P$ . We developed a theory of the generation linewidth of a \textit{nonlinear} auto-oscillator, and showed that the nonlinear frequency shift (characterized by the coefficient$N=d\omega /dP)$ leads to an effective increase of the phase noise. In a strongly supercritical regime, when the oscillation energy $E(P)$ is much larger than the thermal energy $k_{B}T$, the generation linewidth of a STO can be written as $\Delta \omega =\Delta \omega _0 [1+(N/\Gamma _{eff} )^2]$, where $\Delta \omega _0 =\Gamma (0)[k_B T/E(P)]$ is the oscillator linewidth without account for the nonlinear frequency shift and $\Gamma _{eff} =d\Gamma /dP$ is the effective nonlinear damping of the oscillator. Our theory explains the following features of the STO linewidth observed in experiment: (i) general linewidth narrowing with the increases in the bias current $I$ and the oscillation energy $E(P)$; (ii) presence of a minimum in the linewidth dependence on the orientation of the external bias magnetic field; (iii) linear dependence of the linewidth on the absolute temperature. Our theory also demonstrates that in the array of $n$ phase-locked STO the generation linewidth decreases linearly with the increase on the number of oscillators $n$, while the generated power $P $ increases as $n^2$. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D32.00005: Magnetization excitations in magnetic nanopillars induced by a d.c. spin polarized current Nikoleta Theodoropoulou, Amit Sharma, William Pratt Jr., Jack Bass We have measured spin-transfer-torque driven magnetization dynamics at 293K in Py(24nm)/Cu(10)/Py(6) magnetic nanopillars, with the top Py(6) magnetic layer and part of the Cu layer shaped into a 140x70 nm$^{2}$ ellipse, and the rest left extended. Among the more interesting results are sharp peaks at zero applied field when a large enough negative d.c. current, I, is applied. The oscillations extend from 0.6 to 4 GHz. They disappear when a magnetic field, $\mu _{0}$H, larger than 10 mT is applied in the plane of the layers, but persist up to 0.2 T when $\mu _{0}$H is applied perpendicular to this plane. The peaks persist up to 5-9 times the switching current and appear to be current-hysteretic. At $\mu _{0}$H =0, the frequency of the oscillations increases with I ($\sim $40MHz/mA). Except for the direction of I, these observations seem to be consistent with the vortex dynamics reported by the NIST and Cornell groups. If time permits, we will present results on magnetic nanopillars where the Py(24) layer has been replaced by NiCr, which inverts the spin asymmetry. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D32.00006: Resonant Spin-Transfer-Driven Switching of Magnetic Spin Valves Assisted by Microwave Current Pulses Yong-Tao Cui, Jack C. Sankey, Chen Wang, Kiran V. Thadani, Zhi-Pan Li, Robert A. Buhrman, Daniel C. Ralph Spin transfer torque from an electrical current can reverse the magnetization in a nanomagnet. We show experimentally that applying a microwave-frequency current pulse in addition to a DC pulse can improve switching characteristics at low temperature by exciting a nanomagnet resonantly at its precession frequency. We study spin valve nanopillars with the structure IrMn(8 nm)/permalloy(4 nm)/Cu(8 nm)/permalloy(4 nm) where exchange bias causes an initial offset angle of $\sim $45 degrees between the permalloy magnetizations. We apply nanosecond-scale microwave-frequency current pulses prior to completing the switching with a DC current pulse. We find that the probability of successful switching has a resonant dependence on frequency, and it also depends on the phase of the microwaves at the moment when the DC pulse is applied. With a microwave pulse, the DC pulse length required for switching is shorter and has a narrower distribution compared to switching driven by a DC pulse alone. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D32.00007: Phenomenological model for size-dependent switching behavior in spin transfer torque devices Venkatesh Chembrolu, Yves Acremann, John Paul Strachan, Xiaowei Yu, Ashwin Tulapurkar, Jordan Katine, Mathew Carey, Tolek Tyliszczak, Joachim Stohr Recent results based on time resolved x-ray imaging of magnetization dynamics in nano-magnetic devices have shown size dependent trends in the switching behavior. Samples with a lateral dimension of 100x180nm show a vortex-driven switching mechanism, whereas smaller samples with a lateral dimension of 110x150nm do not switch by a vortex. Further studies have shown that when a non-zero angle in introduced between the fixed and the free layers, vortex-driven switching becomes manifest in samples with smaller dimensions also. Here, we would like to present a phenomenological model based on linearlized LLG equations to explain the various regimes of observed switching behaviors. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D32.00008: Linewidths of Spin-Torque-Driven Nanomagnetic Oscillators as a Function of Field Angle and Temperature Kiran V. Thadani, Z.-P. Li, O. Ozatay, J.C. Sankey, I.N. Krivorotov, Y.-T. Cui, R.A. Buhrman, D.C. Ralph, G. Finocchio In a magnetic multilayer device, spin-transfer torque from a direct current can excite steady-state magnetic oscillations. We observe that the linewidths of the oscillations decrease dramatically as an applied magnetic field is rotated away from the magnetic easy axis towards the in-plane hard axis. Micromagnetic simulations show that the spatial coherence of the oscillations improves greatly as the field is rotated, and their amplitude increases, making them less susceptible to thermal fluctuations. We report the temperature dependence of the linewidths for the field directions giving the minimum linewidths. It has been suggested previously that the linewidths are dominated by fluctuations of the precession angle of the nonlinear oscillator [1,2]. We analyze the mechanisms governing our linewidths by comparing them to micromagnetic modeling. [1] J. C. Sankey et al., Phys. Rev. B 72, 224427 (2005). [2] J.-V. Kim et al., cond-mat/0703317. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D32.00009: Enhancement in spin-torque efficiency by nonuniform spin current generated within a tapered nanopillar spin valve P.M. Braganca, O. Ozatay, A.G.F. Garcia, O.J. Lee, D.C. Ralph, R.A. Buhrman When modeling spin torque related phenomena, it is generally assumed that the polarization of the incident current is spatially uniform and invariant in time across the surface of the free layer nanomagnet. This is not necessarily the case for a relatively thick, low saturation magnetization reference layer that is patterned as part of a nanopillar structure, where the role of the reference layer on the spin torque dynamics of the system is considerably more complex. Here, we discuss the results of spin-torque micromagnetic simulations, confirmed by both dc and short-pulse switching measurements of nanopillar spin valve structures, which reveal that the use of this type of reference layer can result in non-uniform polarization of the current that impinges onto the free layer. This effect can enhance magnetic reversal in the nanosecond-switching regime over the case of a fixed and uniformly magnetized reference layer, substantially reducing the current amplitude required for magnetic reversal with a given ns pulse-width. We will discuss these results, which differ substantially from descriptions provided using macrospin approximations, and describe a nanopillar spin-torque device configuration that simulations and experiments indicate could be quite effective in reducing the spin torque switching current for MRAM applications. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D32.00010: Theory of Ferromagnetic Resonance in Perpendicularly Magnetized Nanodiscs; Excitation by Injected AC Current Rodrigo Arias, Douglas Mills Recent experiments explore the ferromagnetic resonance (FR) response of nanodiscs incorporated into nanopillars, where a DC spin torque current has a small AC component superimposed. For such a circular perpendicularly magnetized disc, we develop the theory of the FR response via AC current. Earlier we discussed the vortex state induced by the DC Oersted field in such a sample, and the nature of the spin waves in the presence of the vortex$^{2}$. The present study explores the linear response of the disc, when a small AC current is superimposed on the DC current. A Green's function approach allows us to describe the linear response of the system. We argue that the AC component of the Oersted field is responsible for spin wave excitation; the modes excited thus differ from those observed in ferromagnetic resonance studies via microwaves. We shall present calculations which explore the spectrum and eigenvectors of modes excited by modulation of the DC current, their width as a function of DC current, and their intensity. $^{2}$R. E. Arias and D. L. Mills, Phys. Rev. B\textbf{75}, 214404 (2007). [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D32.00011: Quantum transport in Spin Torque Transfer Devices Sayeef Salahuddin, Deepanjan Datta, Prabhakar Srivastava, Supriyo Datta We present a simulation of tunneling based Spin Torque Transfer (STT) devices using the Non Equilibrium Greens Function (NEGF) formalism in the ballistic regime. Our method is based on effective mass treatment of the magnetic contacts and tunneling oxide, including the effect of transverse modes in the transverse direction. We show that it is possible to achieve a quantitative agreement with experiments for both the tunneling magneto resistance (TMR) and the amplitude of the switching current with the same set of device parameters [1]. We shall talk about some implications of these results in the context of improving the device performance. We shall also briefly discuss how the nature of the torque may change if there is spin flip scattering. [1] S.Salahuddin, Deepanjan Datta, Prabhakar Srivastava and Supriyo Datta, proceedings of International Electron Devices Meeting (IEDM), 2007. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D32.00012: Spin Pumping of Current in Non-Uniform Conducting Magnets Wayne Saslow Using irreversible thermodynamics we show that current-induced spin transfer torque within a magnetic domain implies spin pumping of current within that domain. This has experimental implications for samples both with conducting leads and that are electrically isolated. These results are obtained by deriving the dynamical equations for two models of non-uniform conducting magnets: (1) a generic conducting magnet, with net conduction electron density $n$ and net magnetization $\vec{M}$; and (2) a two-band magnet, with up and down spins each providing conduction and magnetism. For both models, in regions where the equilibrium magnetization is non-uniform, voltage gradients can drive adiabatic and non-adiabatic bulk spin torques. Onsager relations then ensure that magnetic torques likewise drive adiabatic and non-adiabatic currents -- what we call bulk spin pumping. For a given amount of adiabatic and non-adiabatic spin torque, the two models yield similar but distinct results for the bulk spin pumping, thus distinguishing the two models. As for recent spin-Berry phase work, we find that within a domain wall the ratio of the effective emf to the magnetic field is approximately given by $P(2\mu_{B}/e)$, where $P$ is the spin polarization. The adiabatic spin torque and spin pumping terms are shown to be dissipative. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D32.00013: First principles calculation of the spin transfer torques Ke Xia, Yuan Xu, Shuai Wang A first principles method was developed to calculate the spin transfer torques in the noncollinear magnetized system. We found that the behavior of spin torques in a spin valve depends on the materials(Co, Ni and Ni$_{80}$Fe$_{20}$ were taken into account). The formulism is also applied to the anti- ferromagnetic domain wall. It is found that the spin torques could be exerted over a long range in the anti-ferromagnetic materials and move the domain wall away, which can detected by measureing the resistance of an antiferromagnetic point contact. [Preview Abstract] |
Session D33: Focus Session: Spin Dependent Phenomena in Semiconductors: III
Sponsoring Units: DMP GMAG FIAPChair: Chris Palmstrom, University of Minesota
Room: Morial Convention Center 224
Monday, March 10, 2008 2:30PM - 3:06PM |
D33.00001: Theory of Current-Induced Domain Wall Creep in (Ga,Mn)As. Invited Speaker: A magnetic domain wall (DW) can interact with electrical current and as a result its displacement is induced by the application of the current, showing the possible electrical control of the magnetization direction. Thus electrical current in this non-uniform spin texture is now drawing much attention from the technological point of view, in addition to the longstanding interests from the fundamental physics points of view. Recently, systematic experimental data of the dependence of DW velocity spanning five decades on current density have been obtained in a microstructure made from a ferromagnetic semiconductor (Ga,Mn)As, providing deep insight to outstanding physics of DW dynamics. The quantitative analyses showed that the current-driven motion in (Ga,Mn)As can be explained by spin-transfer mechanism under currents beyond a threshold value. The linear mobility tempts ones to expect there is always an equivalent magnetic field which has the same effect upon DWs as the current does. Here we make a detailed comparison between these two sources of drive but in the subthreshold, ``creep,'' regime, where the velocity obeys an Arrhenius scaling law. The observed scaling law for the two drives is incompatibly different from each other, i.e., the effect of a driving current and field are not equivalent. We offer theory which explains the important features of experiment. When described by an Arrhenius law it is found that the barriers diverge as the drive approaches zero, manifesting the system is in a ``glassy" state. While the field driven case is compatible with the random field universality class, the case of current induced creep is not to fit any known such class. The work reported is the result of collaborations with M. Yamanouchi, F. Matsukura, S. E. Barnes, S. Maekawa, and H. Ohno. [Ref. Science 317, 1726-1729 (2007).] [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D33.00002: Analyzing the influence of magnetic domain walls on longitudinal and transverse magnetoresistance in tensile strained (Ga,Mn)As Gang Xiang, Nitin Samarth We present a theoretical analysis of magnetoresistance in (Ga,Mn)As epilayers with perpendicular magnetic anisotropy [Phys. Rev. B {\bf 76}, 054440 (2007)]. The model reproduces the field-antisymmetric anomalies observed in experimental measurements [Phys. Rev. B {\bf 71}, 241307(R) (2005)] of the longitudinal magnetoresistance in the planar geometry (magnetic field in the epilayer plane and parallel to the current density), as well as the unusual shape of the accompanying transverse magnetoresistance. As in the case of metallic ferromagnets with perpendicular anisotropy [Phys. Rev. Lett. {\bf 94}, 017203 (2005), the magnetoresistance characteristics are attributed to circulating currents created by the presence of magnetic domain walls. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D33.00003: Manipulation of Magnetic Domain Walls in Patterned (Ga,Mn)As Devices Andrew Balk, Meng Zhu, Nitin Samarth, David Awschalom Ferromagnetic semiconductors such as (Ga,Mn)As provide new opportunities for the electrical manipulation of magnetic domain walls in a different regime of parameter space compared with ferromagnetic metals [Chiba et al, PRL 96, 096602 (2006)]. Here, we discuss different approaches to pinning and controlling magnetic domain walls in laterally patterned (Ga,Mn)As microdevices with perpendicular magnetic anisotropy. The pinning/depinning of domain walls is monitored using measurements of the magnetoresistance, the anomalous Hall effect and high speed Kerr microscopy. The domain wall pinning potential is engineered using a variety of schemes, including lateral shape engineering and lithographically integrated micromagnets. We find that even simple schemes (such as lateral notches) can significantly enhance domain wall pinning in relatively large (micron scale) devices. Supported by ONR MURI. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D33.00004: Spin valve effect in self-exchange biased ferromagnetic metal/semiconductor heterostructures Meng Zhu, Mark Wilson, Ben-Li Sheu, Partha Mitra, Peter Schiffer, Nitin Samarth The systematic engineering of exchange biased ferromagnetic semiconductor spin valve devices is important for developing proof-of-concept semiconductor spintronics devices (such as spin torque oscillators). Here, we report magnetization and current-perpendicular-to-the-plane (CPP) magnetoresistance measurements in hybrid ferromagnetic metal/semiconductor heterostructures built from MnAs and (Ga,Mn)As [APL 91, 192503 (2007)]. We observe an exchange biased CPP spin valve effect in MnAs/(Ga,Mn)As bilayers, and discuss the dependence of the exchange field and the spin valve effect on (Ga,Mn)As layer thickness. We also demonstrate the CPP spin valve effect and exchange biasing in MnAs/ p-GaAs/ (Ga,Mn)As trilayers, and discuss the dependence of both phenomena on the doping and thickness of the non-magnetic spacer layer. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D33.00005: The Influence of the Doping Profile on Spin Transport in Fe/GaAs Schottky Tunnel Barrier Heterostructures Q. Hu, E.S. Garlid, K.S.M. Reddy, J. Zhang, T. Kondo, P.A. Crowell, C.J. Palmstr$\O$m A strong non-monotonic dependence of the spin polarization on the bias across the injector has been observed in recent studies of spin transport in Fe/GaAs heterostructures. We have conducted a study of spin transport in non-local Fe/GaAs spin valves in which the doping profile of the Schottky barrier has been systematically modified. The samples were $\mbox{Fe}/n^ {+}/n\mbox{-GaAs}$ heterostructures in which the thickness $d$ of the $n^{+}$ layer ($n^{+}$ fixed at $5\times10^{18}$ cm$^{-3}$) was varied from 5 nm to 50 nm while $n \approx 5\times10^{16}$ cm$^{-3}$ in the 2.5 $\mu$m thick channel. We performed non- local spin valve measurements at 15 K for unannealed samples and after annealing at 200$^{\circ}$C and 250$^{\circ}$C. For $d$ less than 10 nm, no spin accumulation is observed under either forward or reverse bias. For $d \approx $ 15 nm, spin accumulation is observed under forward bias only. Spin accumulation is observed for both bias polarities at larger thicknesses, with an optimal $d \approx $ 20-25 nm. Although this overall trend with $d$ is observed in both unannealed and annealed samples, the sign and magnitude of the non-local signal can change upon annealing. These results suggest that spin accumulation is sensitive to both the tunnel barrier profile and interfacial conditions. This work was supported by ONR and the NSF MRSEC, IGERT, and NNIN programs. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D33.00006: Single domain switching by spin-polarized current in GaMnAs nanodevice A. Chernyshov, M. Overby, L.P. Rokhinson, J.K. Furdyna, X. Liu Dilute magnetic semiconductors (DMS) have a potential to bring electrostatic control into magnetic domain and bridge the gap in control efficiency between conventional ferromagnetic materials and semiconductors. A significant progress has been demonstrated in current-induced magnetization reversal, where DMS materials show a few orders of magnitude current reduction compared to the conventional ferromagnets. In this work we demonstrate and investigate in-plane single domain magnetization rotation and reversal in GaMnAs nanodevices by spin-polarized dc electric current. Single domain is defined lithographically, which eliminates unambiguity associated with previously investigated multi-domain switching. The magnetization orientation can be controllably switched between two [100] and [010] easy axes or reversed. Current alone is not sufficient to switch the magnetization and have been aided by small in-plane magnetic field ($\sim $10mT). We observe linear dependence of critical currents with respect to magnetic field and analyze it in terms of current induced torque on the domain walls. The critical current densities are of the same order as for out-of-plane magnetization switching reported by Ohno, et. al. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D33.00007: Magnetoresistance Enhancement through a Resonant Tunneling Diode based in the GaMnAs/AlGaAs Materials System Edward Likovich, Kasey Russell, Wei Yi, Venkatesh Narayanamurti, Keh-Chiang Ku, Meng Zhu, Nitin Samarth A resonant tunneling diode was fabricated with ferromagnetic GaMnAs emitter and quantum well regions and a nonmagnetic p- GaAs collector. Negative differential resistance (NDR) associated with resonant tunneling of holes was observed at 4K, which is below the Curie temperature for GaMnAs. If the device bias is held constant and the magnetic field is swept, our device exhibits either positive or negative tunneling magetoresistance (TMR) up to 30\%, depending on device bias. Current-voltage sweeps reveal the source of the magnetoresistance as a shift in the NDR features to higher bias when the magnetizations of the GaMnAs films become antiparallel. We attribute this bias shift to an increase in tunneling conductivity from the emitter to quantum well for antiparallel GaMnAs magnetization alignment. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D33.00008: Dissipationless anomalous transport properties and Mott relation in Ga$_{1-x}$Mn$_{x}$As Yong Pu, Jing Shi, Daichi Chiba, Fumihiro Matsukura, Hideo Ohno We have found an anomalously large Nernst effect (ANE) accompanying the anomalous Hall effect (AHE) in~a series of Ga$_{1-x}$Mn$_{x}$As (x=0.04-0.07) ferromagnetic semiconductor samples with perpendicular anisotropy. Without applying a magnetic field, none-zero ANE and AHE are observed, and both effects are very well scaled with the sample magnetization. We have developed a method, which dose not depend on the accuracy of magnetization measurement, to study the anomalous transport effects. By measuring AHE and ANE under zero magnetic field and over a wide range of temperatures, we have demonstrated the dissipationless origin of the anomalous electrical and thermoelectric transport properties in these samples.~ Furthermore, we have successfully verified the Mott relation for the off-diagonal transport coefficients in the regime of dissipationless transport that may not depend on scattering. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D33.00009: Localization effects in ferromagnetic Ga$_{1-x}$Mn$_{x}$Sb random alloys M. Eginligil, G. B. Kim, H. Luo, B. D. McCombe, Y. S. Yoon, J. P. Bird We have investigated temperature and magnetic field dependence of resistance (R$_{xx})$ in MBE-grown ferromagnetic Ga$_{1-x}$Mn$_{x}$Sb films in which the ferromagnetism is mediated by holes [1]. Samples with higher carrier densities (6.7 x 10$^{19 }$cm$^{-3}$ and 1.3 x 10$^{20 }$cm$^{-3}$, with Curie temperatures, T$_{c}$, of 13 K and 24 K respectively) show metal-like behavior in the temperature dependence of R$_{xx}$. These samples exhibit small positive magnetoresistance (MR) up to 0.5 T between 1.6 K and 20 K, followed by negative MR up to 10 and 11 T (at 2.4 K), respectively. Samples with lower carrier densities (2.9 x 10$^{19 }$cm$^{-3}$ and 3.9 x 10$^{19 }$cm$^{-3}$, with T$_{c}$'s of 13 K and 24 K, respectively) show ``weak'' thermally activated behavior and negative MR between 1.6 K and 50K (but no positive MR at low fields). The latter samples exhibit low field positive MR between 35 mK and $\sim $ 400 mK, followed by negative MR up to 8 T. R$_{xx}$ increases with decreasing temperature for both samples at zero field, and the magnitude of the change increases with applied magnetic field. These results will be discussed in terms of localization behavior in this system. [1] Eginligil et al.,\textit{ in press} PhysicaE (2007) [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D33.00010: Huge tunneling magnetoresistance ($>$18300{\%}) in semiconductor based magnetic tunnel junctions with zinc-blende MnAs nanoparticles Pham Nam Hai, ByungHo Yu, Shinobu Ohya, Masaaki Tanaka Zinc-blende (ZB) MnAs nanocrystallite is a new prospective material for semiconductor spintronics, since it is expected to be haft-metallic. However, there is no report on the magneto-transport characteristics of ZB MnAs nanoparticle system. In this paper, we report on the huge tunneling magnetoresistance (TMR) effect in MBE-grown magnetic tunnel junctions (MTJs), whose structure is (from the top to the bottom) hexagonal MnAs film (20 nm) / GaAs (1 nm) / AlAs (2.1 nm) / GaAs:MnAs (10 nm), revealing the haft-metallicity of ZB MnAs nanocrystallite. Here, the GaAs:MnAs layer contains ZB MnAs nanoparticles embedded in a GaAs matrix. The tunnel resistance decreases sharply with increasing the magnetic field, resulting in a huge TMR ratio = ($R_{max}-R_{min})$/$R_{min }>$ 18300{\%}. The TMR ratio decreases quickly with increasing the bias voltage and temperature, but survives up to 100 K. Such a huge TMR effect can be explained by an unique combination of Coulomb Blockade effect and large Zeeman splitting in half-metallic ZB MnAs nanoparticles. A magnetic-field dependent electromotive force emerged from those MTJs was also observed. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:30PM |
D33.00011: Ultrafast Photoinduced Demagnetization in (III,Mn)V Ferromagnetic Semiconductors Invited Speaker: Ultrafast light-induced demagnetization, in which photoexcitation leads to a decay of magnetization in less than a picosecond, has been recently observed in (III,Mn)V materials [1]. To explain these measurements, we have proposed a theory of ultrafast magnetization dynamics within the sp-d model [2]. We have calculated the spin-flip scattering between the localized spins and the carriers strongly excited by the laser pulse. In this process the energy is pumped into the localized spin system, while the angular momentum is transferred to the carriers, leading to their dynamical spin polarization. For significant ultrafast demagnetization, this polarization has to be efficiently relaxed by spin-orbit assisted scattering of carriers - otherwise a ``spin bottleneck'' can occur, in which the carriers' spin polarization quickly becomes large enough to suppress further spin-flip scattering. Because of that, and also due to their larger exchange coupling to Mn spins, the holes (having a very short spin relaxation time) are much more important than photoelectrons for demagnetization. Since the spin-flip transition rate is proportional to the carrier temperature, the time-scale for this two-step process of demagnetization is given by the energy relaxation time of very hot holes. I will discuss in detail the application of this theory to (III,Mn)V semiconductors taking into account their valence band structure, and the fact that their optical properties are strongly affected by disorder inherent to these materials. \newline 1. J. Wang, C. Sun, J. Kono, A. Oiwa, H. Munekata, L. Cywinski, and L.J. Sham, Phys. Rev. Lett. {\bf 95}, 167401 (2005) 2. L. Cywinski and L.J. Sham, Phys. Rev. B, {\bf 76}, 045205 (2007) [Preview Abstract] |
Session D35: Focus Session: Negative Index Materials II
Sponsoring Units: FIAPChair: Anatoly Zayats, Queen's University of Belfast
Room: Morial Convention Center 227
Monday, March 10, 2008 2:30PM - 2:42PM |
D35.00001: Negative Index Metamaterials for Superlenses Based on Metal-Dielectric Nanocomposites Latika Menon, Wentao Lu, Adam Friedman, Steven Bennett, Donald Heiman, Srinivas Sridhar Negative index metamaterials are demonstrated based on metal-dielectric nanocomposites. The nanocomposites are prepared using a versatile bottom-up nanofabrication approach involving the incorporation of vertically aligned metal nanowires such as Au and Ag inside dielectric aluminum oxide nanotemplates. Aluminum oxide nanotemplates with specific pore dimensions are fabricated by means of electrochemical anodization. Following this, Au/Ag nanowires with specific wire dimensions are electrodeposited inside the pores. Optical absorbance measurements show resonance peaks corresponding to transverse and longitudinal surface plasmon modes. Peak position and intensity are found to be strongly dependent on nanocomposite dimensions, filling factor (ratio of the volume of metal versus the volume of dielectric) and angle of incidence with respect to the wire axis. A quantitative model based on effective medium theory is in excellent agreement with experimental data, and points to specific composite configurations and wavelength regimes where such structures can have applications as negative refraction media for superlens imaging. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D35.00002: Anisotropic nanostructured metamaterials for broadband all-angle negative refractionand flat lens imaging Wentao Lu, Srinivas Sridhar We show that a metamaterial consisting of aligned metallic nanowires in a dielectric matrix has strongly anisotropic optical properties. For filling ratio $f<$1/2, the composite medium shows two surface plasmon resonances (SPRs): the transverse and longitudinal SPR with wavelengths $\lambda _{t}<\lambda _{l}$. For $\lambda _{t}>\lambda _{l}$, the longitudinal SPR, the material exhibits Re $\varepsilon _{//}<$ 0, Re $\varepsilon _{\bot }>$ 0, relative to the nanowires axis, enabling the achievement of broadband all-angle negative refraction and flat lens (superlens) imaging systems. High performance systems made with Au, Ag or Al nanowires in nanoporous templates are designed and predicted to work from the infrared up to ultraviolet frequencies. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D35.00003: Achieving sub-diffraction imaging through bound surface states in negative-refracting photonic crystals at the near-infrared Rohit Chatterjee, Nicolae Panoiu, Kai liu, Zachary Dios, Ming Bin Yu, My The Doan, Laura Kaufman, Richard Osgood, Chee Wei Wong We report the observation of imaging beyond the diffraction limit due to bound surface states in negative refraction photonic crystals. We achieve an effective negative index figure-of-merit [-Re($n)$/Im($n)$] of at least 380, $\sim $125$\times $ improvement over recent efforts in the near-infrared, with a 0.4 THz bandwidth. Supported by numerical and theoretical analyses, the observed near-field resolution is 0.47$\lambda $, clearly smaller than the diffraction limit of 0.61$\lambda $. Importantly, we show this sub-diffraction imaging is due to resonant excitation of surface slab modes, allowing evanescent wave amplification. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:42PM |
D35.00004: Optical Super Lens: from near-field to far field Invited Speaker: Recent theory predicted a new class of meta structures made of engineered sub wavelength entities - meta ``atoms'' and ``molecules'' which enable the unprecedented electromagnetic properties that do not exist in the nature. For example, artificial plasma and artificial magnetism, and super lens that focuses far below the diffraction limit. The metamaterials may have profound impact in wide range of applications such as nano-scale imaging, nanolithography, and integrated nano photonics. I'll discuss a few recent experiments that demonstrated these intriguing phenomena. We showed, for the first time, the high frequency magnetic activity at THz generated by artificially structured ``meta molecule resonance'', as well as the artificial plasma. Our experiment also confirmed the key proposition of super lens theory by using surface plasmon. We indeed observed optical superlensing which breaks down so called diffraction limit. I'll also discuss nano plasmonics for imaging and bio-sensing. The surface plasmon indeed promises an exciting engineering paradigm of ``x-ray wavelength at optical frequency.'' [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D35.00005: Surface plasmon polariton bound state and negative index imaging at the dielectric edge Igor Smolyaninov, Yu-Ju Hung, Ehren Hwang, Christopher Davis Negative refraction of surface plasmon polaritons at the dielectric edge has been studied using near-field and far-field optical microscopy techniques. Edge plasmon polariton state has been observed. Magnified negative index imaging has been demonstrated using a far-field optical microscope. Good agreement between theoretically calculated and experimentally measured images has been demonstrated. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D35.00006: Linear and nonlinear optical devices based on plasmonic negative index metamaterials. Igor Smolyaninov, Idan Mandelbaum, Louise Sengupta, Yu-Ju Hung, Ehren Hwang, Christopher Davis Fabrication of three-dimensional photonic metamaterials faces numerous technological challenges. On the other hand, many new concepts and ideas in the optics of metamaterials may be tested much easier in two spatial dimensions using planar optics of surface plasmon polaritons. In this talk we will describe applications of plasmonic negative index metamaterials in various novel microscopy, waveguiding and switching devices. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D35.00007: Metaplasmonics and Epsilon-Near-Zero Metamaterials for Optical Nanocircuits, Wave-Bending Tunneling Elements, and Nanoantennas Nader Engheta, Andrea Alu, Mario Silveirinha, Brian Edwards, Michael Young, Jingjing Li Negative-permittivity plasmonic media, engineered epsilon-near-zero (ENZ) metamaterials, and zero-index materials may be exploited as building blocks for synthesis of more complex metamaterials and structures. We have been exploring fundamental concepts and various potential applications of plasmonic materials and ENZ metamaterials, and have studied several metaplasmonic-based structures, devices, and nanocircuits. Among these: (1) we will discuss some of the processing features of meta-nanocircuits, in which the arrangement of plasmonic and nonplasmonic nanostructures may provide the functionalities of optical circuits capable of tailoring electric fields within subwavelength regions and loading and tuning optical nanoantennas; (2) we will present theoretical and experimental results on ENZ-based supercoupling and wave-bending tunneling phenomena in waveguides with ultranarrow subwavelength channels and bends; and (3) we will mention our designs and analyses of optical nanoantennas and arrays inspired from microwave antennas. Future directions in these areas will also be forecasted. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D35.00008: Quantum Josephson Junction Metamaterials Laura Adams, Hua Xu, Steven Anlage Recent breakthroughs in the field of artificial electromagnetic materials, known as metamaterials, have opened the door to creating structures which exhibit extraordinary properties not generally found in nature such as negative index of refraction. By engineering structures that are small compared to the wavelength of operation, material parameters such as (electric) permittivity and (magnetic) permeability can be designed. However there are significant limitations in these structures due their sensitivity to losses which increase with decreasing size. One straightforward way of getting around this issue is to use superconductors which also have the advantage of tunability. Even more sophisticated are arrays of Josephson Junctions (two superconducting islands separated by a thin insulating barrier) which have the distinct advantage of behaving as quantum metamaterials due to their nonlinear microwave inductance tunable by external DC and AC fields. We will discuss the tunability of these arrays as a function of temperature and magnetic field and the possibilities for negative permeability over a wide range of microwave frequencies. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D35.00009: Experimental realization of a generalized superlens using negative refraction at infrared wavelengths Ravinder Banyal, B.D.F. Casse, W.T. Lu, Y.J. Huang, S. Selvarasah, M. Dokmeci, C.H. Perry, S. Sridhar We demonstrate experimentally using a near-field scanning optical microscope the imaging of a point source by a generalized superlens fabricated in InGaAsP/InP heterostructure at wavelengths around $\lambda $= 1.5 $\mu $m. The theory of superlens imaging with lens equation u + v = $\sigma $d gives excellent explanation of wave refraction and imaging formation of our superlens with an effective lens property $\varepsilon _{eff}$= 0.43. This can be used as the basis for design optical elements made of photonic crystals. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D35.00010: An Experimental Near-Field Focusing Plate A. Grbic, L. Jiang, R. Merlin Using a patterned grating-like surface, which we refer to as a near-field plate [1], we experimentally demonstrate focusing of 1.027 GHz radiation well beyond the diffraction limit. A near-field plate is a finely structured surface that acts as a modulated surface reactance [2]. Its ability to provide subwavelength resolution relies on the strong near-field coupling that exists between the reactive surface elements of the plate. This coupling sets up a highly oscillatory electromagnetic field at the plate surface which can focus in the near-field. The plate consists of an array of interdigitated capacitors printed on an electrically thin dielectric substrate. It focuses microwaves emanating from an S-polarized cylindrical source (vertical line current) to a focus with a null-to-null beamwidth equal to one-tenth of the wavelength. Passive surfaces that can focus electromagnetic energy to extreme subwavelength dimensions offer an advantage with respect to slabs in that they obviate the need for the 3D fabrication techniques. Applications in antennas, beam-shaping devices, wireless non-radiative power transfer systems, microscopy and lithography will be discussed. [1] R. Merlin, Science \textbf{317}, 927 (2007). [2] A. Grbic and R. Merlin, arXiv:0708.0049. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D35.00011: Radiationless Electromagnetic Interference: Perfect Focusing with Evanescent-Field Plates R. Merlin Planar subwavelength structures are described, which rely on a hitherto unrecognized property of Maxwell's equations to provide superlensing, i.~e., electromagnetic focusing well beyond the diffraction limit, at arbitrary frequencies [1]. The resulting fields bear a striking resemblance to those of negative-index slabs. The structures' design is related to that of the Fresnel plates in that diffraction forces the input field to converge to a spot on the focal plane. Unlike the diffraction-limited zone plates, for which focusing results from standard interference of traveling waves, the subwavelength plates control the near field and, as such, their superlensing properties originate from a new, static form of interference. ~~~~~~~ ~~~~~~~~~~[1] R. Merlin, \textit{Science }\textbf{317}, 927 (2007). [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D35.00012: Contribution of electric quadrupole resonance in optical metamaterials David Cho, Feng Wang, Xiang Zhang, Y. Ron Shen Optical metamaterials are artificial structures composed of nanoscale units with unit dimension smaller than optical wavelength. They can exhibit negative index of refraction when both effective permittivity and permeability are negative. Although, negative permittivity is straightforward to obtain, negative permeability is nonexistent in nature. Only recently has it been achieved using strong magnetic resonances in suitably designed metal plasmonic nanostructures. However, similar to the magnetic resonance, electric quadrupole resonance can also be greatly enhanced by plasmon resonances. The contribution of the electric quadrupole resonance to the effective properties of metamaterials has not been well understood and often neglected. We show by simulation that, for many metamaterial structures, electric quadrupole radiation is comparable to that of magnetic dipole and we propose an experimental scheme to determine individual contribution of the electric dipole, magnetic dipole and electric quadrupole. We also show that the electric quadrupole radiation can greatly affect effective permeability, and therefore is of central importance for designing metamaterials with negative permeability. [Preview Abstract] |
Session D36: Focus Session: Hydrogen Storage I: Chemical Hydride and Complex Metal Hydride Materials I
Sponsoring Units: DMP FIAPChair: Chris G. Van de Walle, University of California, Santa Barbara
Room: Morial Convention Center 228
Monday, March 10, 2008 2:30PM - 3:06PM |
D36.00001: Nanoscale Hydrides in Porous Carbon Scaffolds Invited Speaker: Light element and complex anion hydrides (such as LiH, MgH$_{2}$, NaAlH$_{4}$, and LiBH$_{4})$ are being studied intensely as reversible hydrogen storage materials for fuel cell powered vehicles because they have high gravimetric and volumetric capacities. However, the \textit{rates} of dehydrogenation and rehydrogenation of these hydrides are typically much too slow to be compatible with proton exchange membrane fuel cell temperatures. The slow rates originate, at least in part, from the high activation energies for diffusion associated with the ionic and covalent bonds found in these materials. In contrast to metallic systems, ionic and covalent bonds are directional. Thus, the transition states for atomic rearrangement occur in particularly unfavorable bonding configurations. This increases the activation energies for diffusion and results in slow phase transformation and hydrogen sorption kinetics. Overcoming these kinetic limitations has become a critical element in the development of light-metal complex hydrides for practical storage applications. Small quantities of catalytic additives have been shown to greatly improve the rates of hydrogen exchange in MgH$_{2}$ and NaAlH$_{4}$. Another approach involves formation of nanoscale hydrides by incorporation into nanoporous scaffolds. The small pores of the scaffold limit the sizes of the hydrogenated and dehydrogenated phases and thus the diffusion lengths to nanoscale dimensions. These relatively short diffusion lengths reduce diffusion times and therefore, increase overall rates of hydrogen sorption. The size limitations also result in increased interfacial area between reacting phases, which improves hydrogen capacity retention during cycling. This talk will focus on the preparation, characterization, and hydrogen sorption behavior of LiBH$_{4}$, MgH$_{2}$ and NaAlH$_{4}$ incorporated into nanoporous carbon aerogels. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D36.00002: A Study of the Structural and Dynamical Properties of Lithium Borohydride Confined within Nanoporous Framework Structures using Neutron Scattering Investigations Ceris Hamilton, Michael R. Hartman, Hui Wu, Terrence J. Udovic, John. J. Rush, Adam F. Gross, John J. Vajo, Theodore F. Baumann Lithium borohydride, LiBH$_{4}$, is a complex metal hydride that shows great promise as a hydrogen storage medium with a volumetric hydrogen density of 122 kg H/m$^{3}$ and a gravimetric hydrogen density of 18.5 wt. {\%}. We have previously reported on the structural and dynamical properties of neat $^{7}$Li$^{11}$BH$_{4}$ as determined by neutron powder diffraction, neutron vibrational spectroscopy, and quasielastic neutron scattering. Here we report on recent measurements undertaken to investigate the changes in the structural and dynamical properties that are observed when this material is confined within nanoporous structures with pore sizes ranging from 4 nm to 25 nm. These materials exhibit a reduction in the structural transition and melting temperatures, which we associate with a marked decrease in the activation energy for reorientational motions of the [BH$_{4}$]$^{-}$ tetrahedra. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D36.00003: Density Functional Screening of Metal Hydride Reactions Karl Johnson, Sudhakar Alapati, Bing Dai, Ki-Chul Kim, David Sholl The on-board storage of hydrogen is one of the most vexing problems associated with the development of viable fuel cell vehicles. Hydrides of period 2 or 3 metals can store hydrogen at high gravimetric and volumetric densities. However, existing hydrides either have unacceptable thermodynamics or kinetics. New materials for hydrogen storage are therefore needed. We demonstrate how first principles density functional theory (DFT) can be used to screen potential candidate materials for hydrogen storage. We have used DFT calculations in conjunction with a free energy analysis to screen over a million reactions involving 212 known compounds. This approach has identified several interesting reaction schemes that have not yet been explored experimentally. We have computed the phonon density of states and used this information to predict the van't Hoff plots for some of the most promising candidate reactions identified though our modeling. We have also examined the thermodynamics of thin films and nanoparticles for selected metal hydrides by accounting for the surface energies of the films or nanoparticles. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D36.00004: Reversible Hydrogen Storage in the Lithium Borohydride -- Calcium Hydride Coupled System Frederick Pinkerton, Martin Meyer We report large reversible hydrogen storage in a new coupled system, LiBH$_{4}$/CaH$_{2}$, via the reaction 6 LiBH$_{4}$ + CaH$_{2}$ $\leftrightarrow $ 6 LiH + CaB$_{6}$ + 10 H$_{2}$ having a theoretical hydrogen capacity of 11.7 wt{\%} and an estimated reaction enthalpy of $\Delta $H = 59 kJ/mole H$_{2}$. Samples that include 0.25 mole (18.2 wt{\%}) TiCl$_{3}$ reproducibly store 9.1 wt{\%} hydrogen, corresponding to 95{\%} of the available hydrogen. H$_{2}$ is the only evolved gas detected by mass spectrometry. X-ray diffraction confirms that the sample cycles between LiBH$_{4}$ and CaH$_{2}$ in the hydrogenated state and LiH and CaB$_{6}$ in the dehydrogenated state. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D36.00005: Structures and Bonding of Li-B-N-H Quaternary Hydrides Hui Wu, Wei Zhou, Terrence Udovic, John Rush, Taner Yildirim Complex metal hydrides are of great interest for hydrogen-storage applications due to their potential high hydrogen capacity. Intense efforts has been made on the Li-B-N-H system, with the discovery of several novel quaternary phases. There have been prior studies investigating the structure of one of these new phases (Li$_{4}$BN$_{3}$H$_{10})$. However, all these studies were undertaken on hydrides without any isotope enrichment, thus resulted in diffraction data of limited quality and yielded structures with questionable bond lengths and uncharacteristically deformed anion groups. So far, no studies have been reported on the isotopically labeled samples, which are necessary to determine correct structures for these hydrides. We report for the first time the crystal structures of Li$_{2}$BNH$_{6}$ and Li$_{4}$BN$_{3}$H$_{10}$ derived from high-resolution neutron diffraction data on samples labeled with $^{7}$Li, $^{11}$B, and D. Our refined structures clarify the prevailing structural discrepancies. We also report corresponding neutron vibrational spectra combined with first-principles calculations to gain more insight between structure and bonding. The configurations of both BH$_{4}^{-}$ and NH$_{2}^{-}$ anions and the structural variations upon compositional changes will be discussed. Our study provides implications to the mechanisms of hydrogen absorption/desorption in these complex hydrides. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D36.00006: Thermodynamic guidelines for the prediction of hydrogen storage reactions and their application to destabilized hydride mixtures Donald Siegel, C. Wolverton, V. Ozolins We propose a set of thermodynamic guidelines aimed at facilitating more robust screening of hydrogen storage reactions. The utility of the guidelines is illustrated by reassessing the validity of reactions recently proposed in the literature and through vetting a list of more than 20 candidate reactions based on destabilized LiBH$_4$ and Ca(BH$_4$)$_2$ borohydrides. Our analysis reveals several reactions having both favorable thermodynamics and relatively high hydrogen densities (ranging from 5--9 wt \% H$_2$ and 85--100 g H$_2 $/l), and demonstrates that chemical intuition alone is not sufficient to identify valid reaction pathways. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D36.00007: Crystal Chemistry of the Perovskite-type Hydride NaMgH$_{3}$ Terrence Udovic, Hui Wu, Wei Zhou, John Rush, Taner Yildirim The crystal structure, lattice dynamics, and local metal-H bonding configuration of the perovskite hydride NaMgH$_{3}$ were investigated using combined neutron powder diffraction, neutron vibrational spectroscopy, and DFT calculations. NaMgH$_{3}$ crystallizes in the orthorhombic GdFeO$_{3}$-type perovskite structure (\textit{Pnma}) with $a^{-}b^{+}a^{-}$ octahedral tilting in the temperature range of 4-370~K. In contrast with previous structure studies, the refined Mg-H lengths and H-Mg-H angles indicate that the MgH$_{6}$ octahedra maintain a near ideal configuration, which is corroborated by bond valence methods and our DFT calculations, and is consistent with perovskite oxides with similar tolerance factor values. The temperature dependences of the lattice distortion, octahedral tilting angle, and atomic displacement of H are consistent with the recently observed high H mobility at elevated temperature. The stability and dynamics of NaMgH$_{3}$ are discussed and rationalized in terms of lattice distortion, cation octahedra tilting, and local bonding configurations in the observed perovskite structure. Further experiments reveal that its perovskite crystal structure can be used to improve the slow hydrogenation kinetics of some strongly bound light-metal-hydride systems such as MgH$_{2}$ and to design new alloy hydrides with desirable hydrogen-storage properties. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D36.00008: Bonding changes in compressed NaBH$_{4}$ probed by inelastic X-ray scattering Andrew Cornelius, Ravhi Kumar, Malcolm Nicol, Michael Hu, Paul Chow Hydrogen storage for commercial applications is an ongoing challenge in materials science research in recent years. Complex borohydrides are technologically promising materials due to their light weight and high gravimetric and volumetric hydrogen density. So far knowledge of the structural and bonding changes in these systems is elusive due to low z elements and lack of in-situ experimental probes. Here we present the first experimental results of boron K-edge inelastic X-ray scattering performed on NaBH$_{4}$ revealing the nature of bonding changes during compression up to 12 GPa. NaBH$_{4}$ undergoes structural phase transition from cubic (Fm-3m) to tetragonal (P421/c) above 6 GPa and to orthorhombic (Pnma) above 8.3 GPa. The high pressure tetragonal and orthorhombic phases show weakening of B-H bonding during phase transition. Further, NaBH$_{4}$ may be considered as a representative example for isostructural systems since a similar structural sequence is also observed in KBH$_{4}$ on compression. The experimental details and the inelastic x-ray scattering results will be presented. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D36.00009: On the Formation of LaFe$_{5}$H$_{n}$ Jan Herbst, Louis Hector, Jr. Formation of a LaFe$_{5}$H$_{n}$ hydride is explored by means of density functional theory. Enthalpies of formation $\Delta$H with respect to the elemental metals and H$_{2}$ are calculated for various hydrogen configurations in four prototype crystal structures. We find $\Delta$H $<$ 0 in many cases, suggesting the existence of LaFe$_{5}$H$_{n}$, as does Miedema's semi-empirical model. $\Delta$H is a minimum for the LaFe$_{5}$H$_{n}$ stoichiometry with hydrogen occupying the 4e, 8g, and 16m sites in the orthorhombic Cccm structure. Phonon dispersion relations and elastic constants computed for that structure exhibit no anomalies, demonstrating vibrational stability. Similar results for LaFe$_{5}$ indicate that compound may form under pressure. [Preview Abstract] |
Session D37: Optical Properties of Semiconductors (mostly nitrides)
Sponsoring Units: FIAPChair: Mark Holtz, Texas Tech University
Room: Morial Convention Center 229
Monday, March 10, 2008 2:30PM - 2:42PM |
D37.00001: Magnetooptic Ellipsometry: Determination of free charge carrier properties in semiconductor device structures Mathias Schubert, Tino Hofmann The standard tool for the electrical characterization of free charge properties in semiconductor layer structures is the electrical Hall effect. However, besides the requirement for electrical contacts, the application of this technique to investigate complex heterostructures is very difficult and reliable deconvolution of the individual layer contributions to the measured Hall-Voltage virtually impossible. In our contribution we show that magnetooptic ellipsometry at long wavelengths when applied to conducting or semiconducting multilayer structures can yield equivalent and even much increased information. Our technique allows the independent measurement of free charge carrier density, type, mobility, and effective mass including anisotropy without any electrical contact in buried structures, and which may have been inaccessible to any true electrical evaluation thus far. We present results for multilayer AlGaP and GaInP samples with different doping concentrations. Furthermore, multilayer LED device structures were investigated and we demonstrate that magnetooptic ellipsometry allows deconvolution of the individual free charge carrier contributions of the p- and n-type regions of the device structure. We predict a realm of applications for magnetooptic ellipsometry in future materials research and engineering. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D37.00002: Cyclotron mass of two-dimensional holes in (100) oriented GaAs/AlGaAs heterostructures T.M. Lu, Z.F. Li, D.C. Tsui, M.J. Manfra, L.N. Pfeiffer, K.W. West We have measured the microwave cyclotron resonance of low-density two-dimensional hole gases in (100) oriented heterostructures, including single interface heterostructures (SIHs) and 20nm-wide quantum wells(QWs). It was found that in the case of the SIHs, the cyclotron mass is relatively insensitive to the hole density, ranging from $0.25\times10^{10}$/cm$^2$ to $13.1\times10^{10}$/cm$^2$, and has a value of approximately $0.5m_{e}$. On the other hand, for holes in the QWs, the cyclotron mass decreases from $0.48m_{e}$ at density $p=6.9\times10^{10}$/cm$^2$ to $0.29m_{e}$ at $p=0.8\times10^{10}$/cm$^2$. In this talk, the absorption spectra will be shown, and a qualitative explanation will be presented. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D37.00003: High-Gap Semiconductors and Their Absorption Mechanisms at Millimeter and Submillimeter Wavelengths J.M. Dutta, C.R. Jones, V. Parshin, B. Garin, V. Polyakov, A. Rukovishnikov Experimental data has been applied to extend the available theoretical models to elucidate the predominate mechanisms of absorption for mm and sub-mm electromagnetic waves in high-gap semiconductors, especially CVD diamond and SiC, which are among the most promising low-loss materials commercially available. Dielectric properties of CVD diamonds, as measured by several laboratories, over a wide range of temperature and frequency will be discussed. The experimental data gathered, both from dielectric and electrical measurements, has been applied to extend available theoretical models for CVD diamonds. Similar studies are undertaken on SiC to determine the primary loss mechanisms for SiC in the mm wavelength range. Preliminary experimental results will be presented. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D37.00004: Propagation of light in birefringent tilings Angelo Mascarenhas, Brian Fluegel, Lekhnath Bhusal Two-dimensional tilings of an optically birefringent material are proposed as an orientational superlattice for light. The light modes that result from loss-free refraction are analyzed. It is shown that the behavior for light propagation in such lattices leads to totally bound as well as propagating states. The behavior of light propagation in several such tilings is compared. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D37.00005: Properties of Bulk GaN Crystals grown from the Solution Madhu Murthy, Jaime A. Freitas Jr., Micheal A. Mastro, Rachel M. Frazier, Boris N. Feigelson We investigated the structural and optical properties of bulk GaN single crystal grown from solution at temperatures $<$ 900$^{\circ}$C and a pressure of $\sim $ 0.3Mpa [1] on HVPE substrates. The X-ray diffraction peaks measured on both Ga- and N-face of the epitaxial film are 34.561 and 34.612 \textit{arcsec}, respectively, which are two orders of magnitude lower than the substrate. The photoluminescence measurements performed on both faces of the sample, showed a dominant intense emission near 3.47 eV, which is assigned to recombination process involving shallow impurities and excitons [2]. Two additional bands observed at 3.25 eV and 2.25 eV are assigned to donor-acceptor recombination process and the yellow band, respectively. The relative decrease of these bands intensities suggests a reduction of the concentration of defects incorporated in the epitaxial film, as compared with the substrate. These variations are consistent with different incorporation rate of point defects at different crystal surface polarity. [1] B.N. Feigelson, et al., J. Cryst. Growth, 281 (2005) 5. [2] M. Murthy, et al., J. Cryst. Growth, 305 (2007) 393. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D37.00006: Optical properties of MOCVD selective area growth of GaN D.Y. Song, A. Chandolu, L. Tian, N. Stojanovic, S.A. Nikishin, M. Holtz Selective area growth (SAG) is useful for overcoming mismatch between non-native substrates and epitaxial materials. SAG of GaN is carried out using metallorganic chemical vapor deposition with silicon dioxide as the mask material with openings ranging from 500 nm to several microns. This talk addresses optical properties of completed GaN pyramidal islands grown using SAG. SEM-based cathodoluminescence (CL) is used to investigate the bandgap emission. The optical properties at different positions on the pyramids are related to overgrowth conditions which determine the luminescence properties. The apex region is almost fully relaxed, while the pyramid base exhibits a red-shifted CL spectrum. This shift is attributed to stress and impurity incorporation in the overgrown sidewall region. The red shift observed in CL spectra on the pyramid sidewall region gradually increases from apex to base, varying by $\sim $ 40 meV, when they exceed the size of the opening in the silicon dioxide mask. However, the pyramid has almost uniform luminescence properties when overgrowth does not occur. The CL line width is narrowest at the pyramid apex, suggesting a decrease in the dislocation density. The authors acknowledge support from the National Science Foundation (ECS--0609416 and ECS--0304224) and the J. F Maddox Foundation. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D37.00007: Insights into the Electronic Structure of Single-Crystal AlN with NRIXS Joseph Bradley, Gerald Seidler, Rafael Dalmau, Kenneth Nagle, Zlatko Sitar A fundamental description of the mechanisms underlying the diverse applications of w-AlN requires a complete understanding of its short-range and long-range electronic structure. Here, we report momentum-transfer (q) dependent nonresonant inelastic x-ray scattering (NRIXS) measurements of single crystal w-AlN, surveying the low-energy plasmon regime, as well as the complex behavior at the Al K and L edges. This complete determination of the dynamic structure factor S(q,w) over a wide range of q and w both within and perpendicular to the basal plane allows for a uniquely detailed perspective on chemical bonding and low-energy electronic response in w-AlN. Our results are compared and contrasted with two independent ab initio theoretical treatments. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D37.00008: Optical properties of AlN epilayers probed by polarization resolved photoluminescence A. Sedhain, N. Nepal, M.L. Nakarmi, J.Y. Lin, H.X. Jiang, J.H. Edgar Polar and non-polar AlN homo- and hetero-epitaxial layers were grown on sapphire and AlN bulk substrates by metal organic chemical vapor deposition. Polarization resolved deep ultraviolet photoluminescence (PL) spectroscopy was employed to investigate the optical properties of these samples. For the first time, B and C valence band related free exciton (FX) transitions with a dominant \textbf{E}$_{- }$\textbf{c} polarization were directly observed from PL. The emission energy peaks of B and C excitons were found to locate at 199 and 212 meV higher than that of the A-exciton transition possessing the \textbf{E}$\vert $\textbf{c} polarization. A more comprehensive picture of the valence band structure of AlN is thus directly observed from PL measurements. AlN homo-epilayers in all orientations (a-, c-, and m-plane) were found to be strain free and have a nearly identical band gap of 6.099 eV at 10 K. The band edge peak intensity ratios of a-, c-, and m-plane homo-epilayers were roughly 32:5:1 and line width was found to be the smallest in a-plane homo-epilayer. Our results also indicated that built-in electric fields are almost absent in all AlN homo-epilayers. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D37.00009: Excitons in AlN under pressure. Niels E. Christensen, Robert Laskowski First-principles calculations based on solution of the Bethe-Salpeter equation of excitons in the high-pressure AlN phase show that a delocalization-localization transition occurs as additional pressure is applied. The transition, which is associated with a sudden increase in exciton binding energy, is related to a pressure-induced rearrangement of the energy bands. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D37.00010: Dresselhaus Effect in Bulk Wurtzite Structures Wan-Tsang Wang, Yia-Chung Chang, Jih-Chen Chiang, Yu-Chi Hsu The wurtzite bulk inversion asymmetry (WBIA) terms in the$k\cdot p$Hamiltonian have been derived from the linear combination of atomic orbital method, and the analytic form of Dresselhaus effect is obtained via an eight-band$k\cdot p$Hamiltonian. It is found that WBIA terms lead to not only a spin-degenerate line (along the kz axis) but also a minimum-spin-splitting surface. Furthermore, they can induce large spin splitting energies in wide-gap wurtzite materials such as GaN. Consequently, the D'yakonov-Perel' (DP) spin-relaxation mechanism can be effectively suppressed for all spin components in the [001] wurtzite quantum wells (QWs) at a resonance condition through appropriate sample design or the application of a suitable gate bias. Therefore, wurtzite QWs (e.g., InN/GaN or GaN/AlN) are potential candidates for spintronic devices such as the resonance spin lifetime transistor. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D37.00011: Optical phonon decay in GaN and AlN D. Y. Song, P. Pandit, A. Chandolu, M. Basavaraj, S. A. Nikishin, M. Holtz The intrinsic phonon decay properties of high-quality crystalline III-nitrides are found to be critical to device self-heating. An excellent approach for examining self-heating, as well as for understanding the properties of phonons, is micro-Raman scattering. This talk reports micro-Raman studies of the A$_{1}$(TO), E$_{1}$(TO), E$_{2}$2, A$_{1}$(LO), and E$_{1}$(LO) symmetry phonons of GaN and the A$_{1}$(TO), E$_{1}$(TO), E$_{2}$2 and A$_{1}$(LO) symmetry phonons of AlN from 13 to 375 K. By applying anharmonic decay theory to the observed temperature dependences of the phonon energies and linewidths, the phonon decay mechanisms of these zone-center vibrations have been determined. Thermal expansion is taken into account using published temperature-dependent coefficients. Both GaN and AlN A$_{1}$(TO) and E$_{1}$(TO) vibrations are described by symmetric two-phonon decay. The GaN E$_{2}$2 decays via creation of three phonons, however, the AlN E$_{2}$2 decays symmetrically into two phonons. The GaN and AlN LO bands are interpreted by an asymmetric two-phonon decay. Phonon lifetimes are obtained based on the observed linewidths, and the dependence allows us to estimate the intrinsic phonon life time for each vibration. The authors acknowledge support for this work by the National Science Foundation (ECS--0609416 and ECS--0304224) and the J. F Maddox Foundation. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D37.00012: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 4:54PM - 5:06PM |
D37.00013: Large Optical Absorption Variation in Green GaInN/ GaN Multiply Quantum Well under High Charge Carrier Density. Wei Zhao, Mingwei Zhu, Yufeng Li, Yong Xia, Jayantha Senawiratne, Theeradetch Detchprohm, Christian Wetzel Blue GaInN/GaN multiple quantum well (MQW) light emitting diodes (LED) typically show a strong efficiency droop under high injection current density. The effect is even further pronounced for green LEDs. To identify the reason, 535nm emitting MQW epilayers were studied under a continuous wave high density photon excitation to simulate electrical injection conditions. Varying the excitation density over four orders of magnitude, the transmission and absorption were obtained. By comparing the signal of various sample structures, such as GaN/sapphire and full MQW/GaN/sapphire structures, the MQW effect could be isolated. At a wavelength of 514 nm and carrier injection density of 5 x 10$^{22}$ cm$^{-2}$ s$^{-1}$, the MQW showed an absorption enhancement as large as 20{\%}. Following a standard derivation procedure, a very large nonlinear absorption coefficient of 2.6 cm/W was derived. The enhancement of absorption may indicate a new recombination process that becomes effective under high carrier density. The effect could play a role in the observed efficiency droop of LEDs and play a major role in the limitations of GaInN/GaN laser diodes. This work was supported by DOE/NETL. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D37.00014: The influence of O$_{2}$ on the properties of InN thin films A. Dixit, C. Sudakar, J.S. Thakur, V.M. Naik, R. Naik, G. Lawes InN thin films often exhibit characteristics typical of degenerate semiconductors. This degenerate behavior is typically attributed to the inclusion of oxygen vacancies or the presence of structural defects. To help clarify this point, we studied the effect of O$_{2}$ in two different sputter deposited InN thin films. One film was prepared using pure In metal sputtered in an argon/nitrogen atmosphere and the other sample was prepared from an In$_{2}$O$_{3}$ target, which leads to incorporation of some oxygen in the resultant film. We studied the structural, optical and electronic properties of these films in detail. These measurements showed that even the small amount of O$_{2}$ from the oxide target, drastically affects the growth and structure of the InN film, resulting in polycrystalline samples. However, for both samples, the absorption spectra showed an optical band edge close to 1.84 eV with carrier concentration is $\sim $10$^{21}$ cm$^{-3}$ , showing the degenerate nature of both films. This study suggests that the degenerate nature of the InN films is unchanged on increasing the concentration of oxygen impurities or adding structural defects. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D37.00015: Effect of hydrogen on surface electron accumulation in InN films Rudra Bhatta, Brian Thoms, Mustafa Alevli, Nikolaus Dietz Effect of hydrogen on electron accumulation in InN films has been studied by high resolution electron energy loss spectroscopy. N-H loss features are observed in the HREEL spectra from the InN surface after atomic hydrogen cleaning. Heating for 30 s at 500$^{\circ}$C results in the disappearance of all adsorbate loss features indicating desorption of hydrogen and production of a bare InN surface. HREEL spectra taken from both bare and hydrogenated surfaces show a broad loss feature due to conduction band plasmon excitations. As the incident electron energy is decreased from 35 to 7 eV, which results in shorter penetration depths and increasing surface sensitivity, the energy of plasmon loss feature increases by 600 cm$^{-1}$ in spectra from both the bare and hydrogenated surfaces. The increase in plasmon loss energy with increasing surface sensitivity indicates higher carrier concentration on the surface, i.e. surface electron accumulation. This demonstrates that surface electron accumulation exists in the absence of surface indium overlayers or droplets and is unaffected by surface hydrogen adsorption. [Preview Abstract] |
Session D38: Metals: Defects and Elastic Properties
Sponsoring Units: DCMPChair: George M. Stocks, Oak Ridge National Laboratory
Room: Morial Convention Center 230
Monday, March 10, 2008 2:30PM - 2:42PM |
D38.00001: Atomic short-range order effects on magnetostriction in Fe-rich Fe-Ga Yevgeniy Puzyrev, G. E. Ice, G. M. Stocks, R. McQueeney, Yingzhou Du We have measured diffuse x-ray scattering from an Fe-rich Fe-Ga BCC single crystal. Measurements were made on beamline 33-ID at the Advanced Photon Source using a wavelength dispersive spectrometer to suppress Compton, Fluorescence and Resonant Raman backgrounds. Data was collected over a large volume in reciprocal space and measurements were made at two energies to maximize and minimize the x-ray scattering contrast between Fe and Ga. We recovered short-range order (SRO) parameters for the crystal. Using these SRO parameters we use KKR-CPA and locally self-consistent multiple scattering (LSMS) calculations to study the effects of local atomic environment on electronic and magnetic structure of the alloy Research sponsored by the Division of Materials Sciences and Engineering. Research in part performed on Beamline 33-ID at the Advanced Photon Source which is sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D38.00002: First-principles study of helium bubble formation at a palladium lattice vacancy Pei Lin, Yan Wang, Mei-Yin Chou Helium ($^{3}$He) generated from the tritium decay is one of the main reasons for macroscopic radiation damage in the structural components of nuclear devices such as fission reactors and tritium storage media. In contrast to the hydrogen isotopes, helium with its closed electron shell is inert inside metals and tends to aggregate into bubbles which can cause deterioration of materials and influence the lifetime of reactor components. To examine this behavior, we have performed \textit{ab initio} calculations of helium atoms inside palladium (Pd) using density functional theory (DFT) and the projector augmented wave (PAW) method within the generalized gradient approximation (GGA). We find that He diffuses easily in a defect-free Pd lattice. However, it is energetically favorable for multiple He atoms to be trapped at an isolated Pd vacancy site, forming a cluster of up to 8 atoms. The atomistic mechanisms of He-vacancy interaction in Pd are investigated by studying the corresponding electronic structural properties. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D38.00003: Embedded Atom Method Potential for Titanium-Vanadium Alloys Michael R. Fellinger, John W. Wilkins Titanium alloys are important materials for aerospace applications due to their large strength-to-weight ratio and their ability to resist corrosion. Vanadium is an important alloying element for titanium since it stabilizes the high temperature bcc phase of titanium at lower temperatures, and bcc-stabilized titanium alloys generally showed improved hardness and forgeability [1]. Titanium-vanadium is also a reasonable starting point in the study of more complex titanium alloys of commercial importance. The calculation of many alloy properties requires the use of large numbers of atoms simulated over long periods of time. These calculations are currently only feasible through the use of classical interatomic potentials. An embedded atom method (EAM) potential for titanium-vanadium is presented, and thermodynamic and mechanical properties of this alloy are calculated using the potential. The results are compared to density functional theory results and experimental results when available. \newline \newline [1] M. J. Donachie, Jr., \emph{Titanium: A Technical Guide}, 2nd ed. ASM International: Metals Park, OH (2000). [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D38.00004: Atomistic dislocation simulation of aluminum using a tight-binding method Chee Gan, Siu-Sin Quek Atomistic simulation of dislocation in aluminum has been performed using a tight-binding method where the parameters are based on the works of Mehl and Papaconstantapoulous at the Naval Research Laboratory. We study the dissociation of a perfect edge dislocation (the dislocation line is along the $[11{\overline 2}]$ direction) of Burgers vector of $\frac{a}{2}[1{\overline 1} 0]$ into two partials of $\frac{a}{6}[2 {\overline 1}{\overline 1}] $ and $\frac{a}{6}[1 {\overline 2}1] $ on the $(111)$ slip plane. By performing a large scale atomistic relaxation, we observe a separation of partials of about 14~\AA\ and a stacking fault region. We will comment on the estimate of partials separation predicted by the elasticity theory, which relates to certain quantities such as the stacking fault energy. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D38.00005: First-principles calculation of dislocation properties of ductile rare-earth intermetallic compounds Min Ji, Cai-zhuang Wang, Kai-Ming Ho, Qian Chen, Xiang-Yang Liu, Bulent Biner We have used first-principles calculations to study the mechanical properpies of rare-earth intermetallic B2 compounds which exhibit significant ductility. According to Peierls-Nabarro model and slip plane observed in tensile experimens, we have calculated and compared the {110} gamma surface energy for both brittle NiAl and ductile YCu. We also compared unstable stacking fault and twinning energy for a series of B2 compounds with different ductilily. Correlation between these energetics and the ductilily are discussed. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D38.00006: Effect of Chemistry on Dislocation Core Properties in $\alpha$-Fe: AN \emph{ab initio}-Based Approach Zhengzheng Chen, Nicholas Kioussis, Nasr Ghoniem, Tadashi Hasebe Screw dislocations in $\alpha$-Fe and its alloys play an important role on the low-temperature mechanical properties. The solute atom can cause a significant local reconstruction of the dislocation core and therefore affect the mobility. Since direct investigations of the solute-dislocation interaction by first principles calculation remains a difficult problem, we employ a hybrid coupling approach that includes atomistic dislocation modeling with ab initio parameterization of the inter-row interactions, proposed by Suzuki. Using this approach, we have investigated the change of core structure and the $a/2\langle111\rangle$ screw dislocation mobility induced by impurities of Cu and Cr. We find that Cu induces a change from a non-degenerate (P=0, where P is the core polarization) core structure in $\alpha$-Fe to a degenerate (P=1) one, while Cr impurity does not change the P at any concentration. We have also studied the behavior of these systems under stress, and found that Cr impurities lower the mobility of the screw dislocation, while Cu induces that the dislocation of Fe-Cu system under stress exhibits a peculiar \emph{stable} $\rightarrow$ \emph{metastable} $\rightarrow$ \emph{stable} transition, and strengthens $\alpha$-Fe. The above conclusions are supported by molecular dynamics calculation, which also show that Cu impurities, in addition to changing the core polarization, dramatically increase the edge components of screw dislocation in Fe. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D38.00007: Dependence of the Strain Rate Sensitivity of Crystalline Materials on the Distribution of Obstacles to Dislocation Motion Renge Li, Zhijie Xu, Catalin Picu The strength and strain rate sensitivity of metals is usually described in terms of the concentration of obstacles to dislocation motion, i.e. the mean of the obstacle spatial distribution function. In this study we investigate the role of higher moments of this distribution function on these parameters. It is shown that large local fluctuations of obstacle density influence to a large extent the strain rate sensitivity of the material, while the effect on the strength (critical resolved shear stress) is smaller. It is shown that a large reduction of the strain rate sensitivity is associated with a change in the dislocation motion mode from smooth to jerky. Populations composed from obstacles of same strength but different activation energy, as well as obstacles of same activation energy and different strength are also studied. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D38.00008: Elastic properties of $\gamma $-Pu by Resonant Ultrasound Spectroscopy Izabela Stroe, A. Trugman, J. Betts, A. Migliori, C. H. Mileke Despite of intense experimental and theoretical work on Pu, there is still little understanding of the strange properties of this metal. We used resonant ultrasound spectroscopy method to investigate the elastic properties of pure polycrystalline Pu at high temperatures. Shear and longitudinal elastic moduli of the $\gamma $-phase of Pu were determined simultaneously and the bulk modulus was computed from them. A smooth linear and large decrease of all elastic moduli with increasing temperature was observed. We calculated the Poisson ratio and found that it increases from 0.242 at 519K to 0.252 at 571K. . [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D38.00009: The 50 K anomaly in the shear modulus of $\beta $-PdH$_{0.71}$ Douglas Safarik, Ricardo Schwarz When palladium hydride, PdH$_{x}$, is rapidly cooled to liquid helium temperature and then slowly reheated, both the heat capacity and electrical resistivity show a peak in the range 50~$<$~$T$~$<$~80~K, depending on the composition $x$. This ``50~K anomaly'' has been previously explained in terms of formation of long-range ordered hydrogen superlattice structures. However, several aspects of the 50~K anomaly are inconsistent with an ordering phase transition, namely, the temperature of the anomaly depends on the rate of cooling, and the magnitude of the anomaly is larger for a fast cooling rate than for a slow cooling rate. We have studied the 50~K anomaly by measuring the elastic constants of single-crystal PdH$_{0.71}$ in the temperature range 1.4~$<$~$T$~$<$~300~K during both fast cooling and slow warming. During warming, we observed a peak in the shear modulus $C^{\prime }$~=~($C_{11}$~-~$C_{12}$/2) at 55~K, which we attribute to the 50~K anomaly. In contrast, we observed no peak in the temperature dependence of the shear modulus $C_{44}$ or of the bulk modulus $B$. We propose that the 50~K anomaly arises not from the formation of long-range ordered hydrogen superlattice structures, but from freezing of the hydrogen short-range order as the hydride is cooled. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D38.00010: Field theoretical approach to deformation dynamics. Sanichiro Yoshida Based on a recent gauge theory called physical mesomechanics, an attempt is made to formulate the deformation dynamics of solid-state materials comprehensively. In this formalism, deformation is described as a linear transformation of the position vector connecting two nearby points of the material; the transformation is global in the elastic regime and local in the plastic regime. Request of local invariance leads to a Maxwell type field equation, in which a symmetry charge analogous to the electric charge is defined. Dynamics in the plastic regime is characterized by transverse force proportional to rotational displacement, as opposed to translational displacement in the elastic regime, and longitudinal force proportional to velocity. The transverse force is a restoring force, which can be interpreted as the recoverability mechanism that the material regains in the plastic regime. The longitudinal force can be interpreted as a field force acting on the above-mentioned charge, which is basically an energy dissipating force causing the irreversibility of plastic deformation. Fracture is considered to be the situation where the material completely loses recoverability. Supporting experimental data will be presented. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D38.00011: Where is the Simple Hexagonal Structure in Tin? Michael Mehl, Daniel Finkenstadt The heavier elements of periodic table column IV exhibit number of structural phase transitions under pressure. Si and Ge transform from the ground-state diamond structure to, successively, the $\beta$-Sn structure, a body-centered orthorhombic structure, the simple hexagonal structure, etc., ending at a close-packed phase (fcc or hcp) near 200 GPa. Tin also transforms from diamond to $\beta$-Sn, but then to a body-centered tetragonal phase, ending with the body-centered cubic phase. The simple hexagonal phase is not seen, despite the fact that numerous tin-rich alloys exhibit a simple hexagonal structure. To understand this we performed DFT calculations on tin in various crystal structures, using both full-potential LAPW and VASP with a PAW potential. Surprisingly, we find that the simple hexagonal phase is degenerate with $\beta$-Sn over pressures at which the $\beta$-Sn phase is seen experimentally. This holds both for LAPW and VASP calculations, in both the LDA and the GGA. We explore reasons for the lack of a tin simple hexagonal phase, including zero point and spin-orbit effects. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D38.00012: {\em Ab initio} study of Fe-rich Fe-Cu alloys David Reith, Raimund Podloucky Cu precipitates are important for strengthening steel. Our {\em ab initio} study aims to model the thermodynamical stability of Cu precipitations in $\alpha$-Fe. As a first step, a density functional theory (DFT) supercell approach is applied to study $Fe_{1-x}Cu_x$ alloys at small concentrations $x$. From the DFT total energies a strongly nonbonding substitutional energy $E_{subs} \approx 0.7eV$ is derived, which is significantly larger than results of a previous DFT study [1]. Based on force constants derived by the same DFT approach the temperature dependent vibrational free energy is determined [2]. In particular at higher temperatures the vibrational entropy significantly reduces the formation energy. Finally, by using the entropy of mixing the dilute Fe-Cu alloy becomes stabilized. The derived phase diagram is in good agreement with experimental data [3]. According to our analysis, the vibrational free energy is very important for a correct modelling of the phase stability of Fe-rich Fe-Cu alloys. [1] C. Domain et al., PRB, 65, 024103 (2001) [2] D. Alf{\`e} et al, PRB, 65, 045123 (2001) [3] B. Predel, Landolt-B{\"o}rnstein - IV, Springer, Volume 5d (1994) [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D38.00013: Strain-induced interactions in size-mismatched alloys: A Kanzaki force approach Alejandro Diaz-Ortiz, Oleg Shchyglo, Vladimir Bugaev, Helmut Dosch, Alexander Udyansky, Harald Reichert, Ralf Drautz A perturbative approach to determining the strain-induced effective interactions in binary alloys with large atomic-size mismatch is presented. Using the chemical energy as the reference state, the strain-induced energy of the alloy is cast into a many-body (Kanzaki) force expansion that depends on both the configurational and displacive degrees of freedom. It is shown that the $k$-space energy expansion is valid for all wave-lengths. The theory is then applied to the Cu$_3$Au alloy where, due to the large difference between atomic sizes, considerable relaxations are observed from first-principles calculations. We found that the inhomogeneous contribution (\boldmath{$k$}$\neq$0) dominates the strain energy in Cu$_3$Au, whereas the homogeneous part ({\boldmath $k$}=0), notwithstanding its configurational dependence, contributes only a few percent. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D38.00014: New candidates for the Pt$_8$Ti structures in intermetallics Erin Gilmartin, Jacqueline Corbitt, Gus Hart The only known intermetallic structure with an 8:1 stoichiometry is that of Pt$_8$Ti. Because of its uniqueness, this structure has been studied in Pt, Pd, and Ni rich systems. However, these metals have only been paired with a handful of other elements. Are there more elements that when alloyed with Pt, Pd, or Ni order with the Pt$_8$Ti structure? We explored $\approx$40 different Pd- and Pt-based binary systems. We calculated their formation enthalpies for the Pt$_8$Ti structure, compared the value to the tie line between pure Pd/Pt and experimentally-observed ground states. We find that there are other (beyond those experimentally observed) possible alloys with this structure. These new Pt/Pd-rich alloys could fin application in the jewelry and catalysis industries. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D38.00015: Properties of the type I Ge-based clathrates Ba$_{8}$Al$_{13}$Ge$_{33}$ and Ba$_{8}$Al$_{16}$Ge$_{30}$ Emmanuel Nenghabi, Charles Myles The type I clathrate lattice is simple cubic with 46 atoms per unit cell. The cages in this lattice can host ``guests'' and the framework can have substituted atoms. Here, we focus on the ``alloy'' system Ba$_{8}$Al$_{x}$Ge$_{30-x}$ (x is an integer; 0$<$x$<$15). The Ba are guests and Al substitutes for some Ge framework atoms. Using the local density approximation (LDA), we have calculated some properties of the type I clathrates Ba$_{8}$Al$_{13}$Ge$_{33}$ and Ba$_{8}$Al$_{16}$Ge$_{30}$. Our calculations of the equilibrium structures predict that Ba$_{8}$Al$_{16}$Ge$_{30 }$and Ba$_{8}$Al$_{13}$Ge$_{33}$ have approximately the same lattice constant and that Ba$_{8}$Al$_{13}$Ge$_{33}$ is expected to be slightly more stable than Ba$_{8}$Al$_{16}$Ge$_{30}$. Our band structures and electronic density of states results predict that Ba$_{8}$Al$_{13}$Ge$_{33}$ is metallic and that Ba$_{8}$Al$_{16}$Ge$_{30}$ is a semiconductor with an indirect fundamental band gap of 0.3 eV. The vibrational spectrum predicts low frequency rattling modes caused by the Ba guests that are loosely bound in the Al-Ge framework cages. Such modes may scatter the heat-carrying acoustic vibrational framework modes, potentially reducing the thermal conductivity. [Preview Abstract] |
Session D39: Focus Session: Econophysics and Applications Outside of Physics
Sponsoring Units: GSNPChair: Victor Yakovenko, University of Maryland
Room: Morial Convention Center 231
Monday, March 10, 2008 2:30PM - 2:42PM |
D39.00001: Image Segmentation in Linear Time using the Potts Model Frank W. Bentrem A computational method is described which efficiently segments digital grayscale images using the Q-state Ising (or Potts) model. Since the Ising model was first proposed in 1925, physicists have studied lattice models to gain deep insights into ordered/disordered systems. Some researchers have realized that digital images may be modeled in much the same way as these physical systems (i.e., as a square lattice of numerical values). A major drawback in using this technique for image segmentation is that it processes in exponential time. Advances have been made via certain approximations to reduce the segmentation process to power-law time. However, real-time processing (such as for sonar imagery) requires much greater efficiency. We describe an energy minimization technique using four Potts (Q-Ising) models which processes in linear time. The technique is demonstrated on acoustic seafloor images as well as medical images. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D39.00002: Battles between an insurgent army and an advanced army - focus on strategy Surajit Sen, Linda Shanahan Detailed and aggregate analyses of the outcome of past battles focusing on rates of troop losses or on the ratios of forces on each side is at the heart of present knowledge about battles. Here we present non-equilibrium statistical mechanics based studies of possible outcomes of well matched strategic battles by a ``blue'' army against insurgency based attacks by well matched opponents in a ``red'' army in red territory. We assume that the red army attacks with randomly varying force levels to potentially confuse and drive the blue's strategies. The temporal evolution of the model battles incorporate randomness in the deployment of the reds and hence possess attendant history dependence. Our results reveal that while unpredictable events play a major role in battles, a balance between risk of exposure in a battlefield and the use of short range intelligence is needed in determining whether one side can decimate the other, and hence force a battle to end. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D39.00003: Understanding individual human mobility patterns Marta C. Gonz\'{a}lez, Cesar A. Hidalgo, Albert-L\'{a}szlo Barab\'{a}si Understanding human mobility patterns is of major importance for a number of areas, ranging from urban planning to traffic forecasting, transportation geography, and preventing the spread of biological and mobile viruses. Yet, in the absence of tools to monitor the time resolved location of a large number of individuals, our understanding of the basic laws governing human trajectories remains limited. Here we study the individual mobility pattern of mobile phone users whose position is tracked in a time resolved manner. We find that the displacement distribution of the whole population can be approximated with a truncated L\'{e}vy statistics, in agreement with earlier measurements. We show, however, that the main contribution to the observed distribution comes from the differences in the travel pattern of individuals. Furthermore, we find that the individual trajectories are bounded in space and are highly anisotropic, an effect that increases with the trajectory's radius of gyration. After we correct for differences in the radius of gyration and anisotropy all individuals are described by the same universal mobility pattern. These results open new avenues for modeling human motion, with important impact on agent based modeling, epidemic prevention, emergency response and urban planing. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D39.00004: Epidemics with Multistrain Interactions: Cross Immunity and Antibody-Dependent Enhancement Simone Bianco, Leah Shaw Dynamics of epidemic spread is a problem of global interest. In this work we investigate the dynamical properties of a multistrain disease in a population where the strains interact via antibody-dependent enhancement (ADE) and cross immunity. ADE is a property of some multistrain diseases, such as dengue fever and Ebola, in which the antibodies generated by a primary infection with a strain tend to increase the infectiousness of a secondary infection with a different strain. After a primary infection, cross immunity provides temporary reduced susceptibility to the other strains. The presence of chaotic outbreaks and desynchronization between strains has already been observed in a model with no cross immunity if the ADE is sufficiently strong. The addition of weak cross immunity provides a stabilizing effect, while strong cross immunity leads to large amplitude chaotic outbreaks. A stochastic version of the model is also considered. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D39.00005: Panic reactions and global disease dynamics Rafael Brune, Christian Thiemann, Bernd Blasius, Theo Geisel, Dirk Brockmann We analyze spatially extended disease dynamics in a system in which individuals change their dispersal characteristics in response to the local infection level. The key question is to what extent infectious wave front dynamics and the time course of the global infection change in response to host awareness and individuals trying to avoid infection by increased dispersal. We investigate two qualitatively different responses to the local degree of infection. In one system (panic reaction) the local diffusion coefficient increases with the concentration of infecteds, in the other system (directed reaction) individuals drift proportional to infection level gradients. For both systems we develop a mean field model. Although one expects that the individual rationale of avoiding an epidemic wave mitigates disease dynamics we find extended parameter regimes in which this rationale actually facilitates epidemic spread. Finally we investigate the dynamics of a fully stochastic system in which the effects prevail but which also show an increased extinction probability of the epidemic as a function of increasing dispersal response. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D39.00006: Fractal Hearts are Healthy Hearts---Are Fractal Companies Healthy Companies? Bronson Argyle, Gus Hart Fractal analyses of cardiac rhythms have implied that healthy individuals have complex cardiac behavior whereas aged or unhealthy individuals show either more random or more periodic behavior. Does this marker of ``complexity = health'' show up elsewhere? Can this technique be used in other fields as well? Specifically, does a Detrended Fluctuation Analysis of S{\&}P 500 bid prices reveal long range correlations in volatility, similar to those observed in cardiac interbeat intervals? If such correlations exist, measures of market complexity could be compared with individual securities to assess corporate strength and vitality. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 4:18PM |
D39.00007: Economic Fluctuations and Statistical Physics: Quantifying Extremely Rare and Much Less Rare Events Invited Speaker: Recent analysis of truly huge quantities of empirical data suggests that classic economic theories not only fail for a few outliers, but that there occur similar outliers of every possible size. In fact, if one analyzes only a small data set (say $10^4$ data points), then outliers appear to occur as ``rare events.'' However, when we analyze orders of magnitude more data ($10^8$ data points!), we find orders of magnitude more outliers---so ignoring them is not a responsible option, and studying their properties becomes a realistic goal. We find that the statistical properties of these ``outliers'' are identical to the statistical properties of everyday fluctuations. For example, a histogram giving the number of fluctuations of a given magnitude $ x$ for fluctuations ranging in magnitude from everyday fluctuations to extremely rare fluctuations that occur with a probability of only $10^{-8}$ is a perfect straight line in a double-log plot. Quantitative analogies between financial fluctuations and earthquakes will be discussed. Two unifying principles that underlie much of the finance analysis we will present are scale invariance and universality [R. N. Mantegna and H. E. Stanley, {\it Introduction to Econophysics: Correlations \& Complexity in Finance\/} (Cambridge U. Press, 2000)]. Scale invariance is a property not about algebraic equations but rather about functional equations, which have as their solutions not numbers but rather functional forms. The key idea of universality is that the identical set of laws hold across diverse markets, and over diverse time periods. This work was carried out in collaboration with a number of students and colleagues, chief among whom are X. Gabaix (MIT and Princeton) and V. Plerou (Boston University). [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D39.00008: Nonstationary increments and variable diffusion processes in financial markets Joseph L. McCauley, Kevin E. Bassler, Gemunu H. Gunaratne Fat tailed returns distributions and Hurst exponent scaling for financial markets have been reported for more than a decade. The sliding interval technique used in those analyses implicitly assumes that the increments are stationary, an assumption that generally contradicts the facts that the increments are uncorrelated. We show that the data exhibit nonstationary, uncorrelated increments, implying diffusive dynamics with a variable diffusion coefficient, but there is no evidence for either fat tails or Hurst exponent scaling in daily FX returns. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D39.00009: A Model for Nonstationary Market Dynamics with Nontrivial Dynamical Scaling Min Liu, Kevin E. Bassler In a recent empirical analysis of the Euro/Dollar exchange rate [Bassler, et al., PNAS 104, 17287 (2007)] it was found that during certain periods of the day the market returns scale with Hurst exponents H that are significantly different from 1/2. In some of these periods it is less than 1/2, while in others it is greater than 1/2. In this talk we will propose a possible origin for this behavior and other stylized market facts, including short time negative autocorrelations of returns, in terms of a nonstationary compound Poisson process with a time-dependent intensity rate function that results from a changing bid-ask spread in the microscopic market. The model correctly describes the dynamic scaling behavior of a simple reaction-diffusion model of a limit-order book. That model, like the Euro/Dollar exchange rate, has nonstationary return increments and a Hurst exponent H not equal to 1/2. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D39.00010: path integral approach to closed form pricing formulas in the Heston framework. Damiaan Lemmens, Michiel Wouters, Jacques Tempere, Sven Foulon We present a path integral approach for finding closed form formulas for option prices in the framework of the Heston model. The first model for determining option prices was the Black-Scholes model, which assumed that the logreturn followed a Wiener process with a given drift and constant volatility. To provide a realistic description of the market, the Black-Scholes results must be extended to include stochastic volatility. This is achieved by the Heston model, which assumes that the volatility follows a mean reverting square root process. Current applications of the Heston model are hampered by the unavailability of fast numerical methods, due to a lack of closed-form formulae. Therefore the search for closed form solutions is an essential step before the qualitatively better stochastic volatility models will be used in practice. To attain this goal we outline a simplified path integral approach yielding straightforward results for vanilla Heston options with correlation. Extensions to barrier options and other path-dependent option are discussed, and the new derivation is compared to existing results obtained from alternative path-integral approaches (Dragulescu, Kleinert). [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D39.00011: Modeling income distribution as a sum of additive and multiplicative stochastic processes Anand Banerjee, Victor Yakovenko We obtained an analytical stationary solution of the Fokker-Planck equation for a stochastic process that is a sum of the additive and multiplicative processes. The stationary probability distribution function smoothly interpolates between an exponential distribution at the low end and a power law at the high end. It may have different applications in physics. Here we apply it to income distribution in a society by modeling income as a stochastic process. We analyze the personal income distribution data in USA from the Internal Revenue Service. Using just three fitting parameters (the average income in the exponential part, the power-law exponent, and the crossover point between the exponential and the power laws), we obtain very good fits of the IRS data for a range of years. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D39.00012: Power-law behavior and condensation phenomena in disordered urn models: Analysis and its application to macro-economics Jun-ichi Inoue, Jun Ohkubo We investigate equilibrium statistical properties of a disordered urn model. New types of urn models are proposed, in which quenched disorder parameters play an important role in generating power-law behavior. By choosing an arbitrary energy function for each urn, one can construct a lot of urn models, and we assume that the energy function contains a disordered parameters. We evaluate the occupation probability $P(k)$ that an arbitrary urn has $k$ balls by using the concept of statistical physics of disordered systems. In our new disordered urn model, we find that above critical density $\rho_{\rm c}$ for a given temperature, condensation phenomenon occurs and most of the balls are condensed into an urn with the lowest energy level. As the result, the occupation probability changes its scaling behavior from an exponential-law to a heavy tailed power-law in large $k$ regime. We also discuss an application of our results for explaining of macro economy, in particular, emergence of wealth differentials. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D39.00013: The Macro Model of the Inequality Process and the Surging Relative Frequency of Large Wage Incomes John Angle Particles are randomly paired in the Inequality Process (IP), a particle system scattering a positive quantity, wealth. Each particle has a parameter, $\omega$, the fraction of wealth lost in a loss whose probability is 0.5. The stationary distribution of the wealth of particles with $\omega _{\psi}$ is approximated by a $\Gamma $ pdf, the IP's macro model, with shape and scale parameters expressed in terms of $\omega_{\psi }$. The model's dynamics are driven by the product, $\tilde {\omega}_t \mu _t $, where $\tilde {\omega }_t $ is the harmonic mean of the $\omega$'s in the population at time t and $\mu _t$, the population mean of wealth at time t. This $\Gamma $ pdf model fits the annual distribution of annual wage income in the U.S. 1961-2003. These data also confirm that the time-series of scalar statistics of wage income that labor economists think are produced by the U.S. distribution of wage income being ``hollowed out'' (bimodal), the increasing dispersion of wage income and the surging relative frequency of large wage incomes, are produced by the distribution being stretched over larger wage incomes, as implied by the IP's macro model when $\tilde {\omega }_t \mu _t $ increases. The IP's macro model includes wage income distribution dynamics into statistical mechanics. To appear in The Econophysics of Markets and Business Networks. [Preview Abstract] |
Session D40: SPS Undergraduate Research and Outreach II
Sponsoring Units: SPS DBPChair: Gary White, Society of Physics Students/ American Institute of Physics
Room: Morial Convention Center 232
Monday, March 10, 2008 2:30PM - 2:42PM |
D40.00001: Oxide Reliability of SiC MOSFETs Enrique Carrion, Moshe Gurfinkel, John Suehle SiC is one of the materials that presents the most promise for harsh environment electronics. Its ability to operate under high temperature and high power, as well as under radiation, made it the material of choice for this study. SiC MOSFETs constitute an important step towards the development of the next generation of resistant electronics. The eventual industrial manufacturing of this type of field effect transistor depends on the effectiveness to improve its performance. Currently, a sudden current degradation, and an unsatisfactory low mobility are observed during the operation of these devices. In this work, we studied both of these drawbacks as a function of temperature. The devices used were SiC nMOSFETs with a SiO$_{2}$ oxide. Two types of measurements (ultra fast and conventional) were performed during this experience in order to observe 8 decades of current degradation. From our experience, it was observed that as the temperature was lowered the threshold voltage (V$_{TH})$ increased, while the mobility and the drain current (I$_{D})$ decreased. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D40.00002: X-ray degradation studies of Nafion in a PEM fuel cell Rebecca Jenkins, Juan Fragoso The overall goal of this research is to test for degradation of the Polymer Electrolyte Membrane (PEM) fuel cells due to exposure to ionizing radiation. We have successfully developed a Membrane Electrode Assembly (MEA) that can be fully disassembled down to the bare Proton Exchange Membrane (PEM) and reassembled repeatedly. This is crucial for testing the degradation effects on the individual components of the MEA. It was also important to establish baseline repeatability of the polarization curves of the MEAs. Therefore, we systematically varied different parameters to test their effect as well as to establish consistent experimental procedures. Hydration of the fuel cell has been found to be crucial for repeatable results. These polarization curves showed voltages that ranged from 0.4V to 1.0V and current densities up to 11mA/cm2. The Nafion can then be exposed to an x-ray source and the respective polarization data can be studied. A working fuel cell has also been built that fits into the microwave cavity of an electron paramagnetic resonance spectrometer. This allows for study of in situ behavior of free radicals formed in a normal operational fuel cell as well as fuel cells with x-ray exposed membranes. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D40.00003: Polymer Nanocomposite Gyroids Chris Knorowski, Joshua Anderson, Alex Travesset Self-assembled polymer phases are increasingly being used in the development of nanocomposite materials. The polymer matrix provides a template for nanoparticles added to the system, transferring the structure of the polymer to the nanoparticles. We perform Molecular Dynamics simulations of these polymer nanocomposite materials and characterize their phase diagrams. Two striking results are found. First, a specific interaction of the polymer and the nanoparticles is required for a successful templating. Second, the presence of nanoparticles can change the pure polymer phase entirely. For instance, a small nanoparticle concentration turns a polymer system from a hexagonal phase into a gyroid phase, both for the polymers and the nanoparticles. In fact, the gyroid is the most prevalent phase over a wide range of interaction strengths and polymer composition. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D40.00004: Sol-gel synthesis and characterization of terbium doped tin-oxide Rebecca Sobel, Christie Larochelle Rare earth doped tin oxide nanocrystals emit visible light when excited in the ultra-violet. Using a sol-gel process, we embedded Tb$^{3+}$ doped SnO$_2$ nanocrystals in silica glass and characterized the samples using x-ray diffraction, photoexcitation and emission spectroscopy, and transmission electron microscopy. We synthesized four sets of samples, SnO$_2$-99SiO$_2$, 3SnO$_2$-97SiO$_2$, 5SnO$_2$-95SiO$_2$, 7SnO$_2$-93SiO$_2$ with constant weight ratios of Tb$^{3+}$ to measure the effects of varying the molar concentrations of Tin-Oxide on the photoluminescence properties of the nanocrystals. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D40.00005: Generation of 279nm Light for Single Photon Ionization of Laser Cooled Rubidium Lucas Willis, Michael Lim The ionization of rubidium for the formation of ultracold plasma is often done by a two photon process; a 479nm photon ionizes the rubidium from an excited state pumped by the 780nm trapping beams. We detail the generation and characterization of this 297nm light from a Nd:YAG pumped dye laser and a tracking doubling crystal. Supported by Rowan University College of Liberal Arts and Sciences, Research Corporation grant CC6180 and NSF grant PHY-0613659. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D40.00006: Bio-Photonic Detection of Various Cellular Cultures Patrick Hann, Maria Garzon, Erik Pfeiffer, Samuel Lofland, Ernst Knoesel Since it is non-invasive, there has been increased research in the field of bio-optics. Many biological systems display an unusual phenomenon, delayed luminescence, produced by what is known as bio-photons. We present an apparatus and procedure for the detection of these ultra-weak photonic emissions using a single photon detection device. The results of bread yeast, saccramyces, and algae will be presented and compared to other reports in the literature [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D40.00007: Epitaxial Thin Film Growth of CMR Manganites on Silicon: The Effect of Thermal Stress Sanjay Adhikari, Bao Ha, Grace Yong, David Schaefer, Rajeswari Kolagani Our research addresses some of the challenges associated with growing epitaxial thin films of the CMR manganite material, Nd$_{1-x}$Sr$_{x}$MnO$_{3}$ (NSMO) on Silicon for application as a bolometric x-ray sensor. Due to the chemical incompatibility between NSMO and Silicon, the formation of amorphous SiO$_{2}$ and crystal lattice mismatch issues, `buffer layers' and `template layers' of other suitable materials need to be interposed between NSMO and the Silicon substrate. Even with such schemes in place, there exists a mismatch between the thermal expansion coefficients of Silicon ($\alpha _{Si}$=2.618$\times $10$^{-6}$K$^{-1}$ at 300K) and NSMO ($\alpha _{NSMO}\sim $3$\times $10$^{-5}$K$^{-1})$. This large mismatch induces thermal stresses that deteriorate the film properties. Our research investigates how the thermal stress evolves as a function of the thickness of the multi-layers, and how the process parameters such as the film growth kinetics and thermal kinetics can be optimized to minimize the stress. We are using the Pulsed Laser Deposition technique for thin film growth and characterizing the properties of the sensor layer using X-ray diffraction, electrical resistance measurements, optical microscopy and atomic force microscopy. Acknowledgement: We acknowledge support for this research from Lawrence Livermore National Laboratory. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D40.00008: Synthesis and Characterization of CrAlC Thin Films Juan Roche, Jeffrey Hettinger, Samuel Lofland , Ted Scabarozi We have synthesized and characterized Cr2AlC thin films grown on substrates Al2O3, MgO and seed layers of VC, and TiC at room temperature up to 850$^{\circ}$C. Texture films were successfully grown above 550$^{\circ}$C while Raman spectroscopy shows vibrations down to 500$^{\circ}$C. Films below 500$^{\circ}$C down to room temperature show texturing upon annealing at 750$^{\circ}$C. The films were prepared using RF magnetron sputtering from elemental targets. Electrical transport shows metallic behavior of the films down to 10 K. EDS was used to verify chemistry from which the MA ratios were found that a slight deviation still allowed formation of the MAX phase. X-ray diffraction shows that when the chemistry is off it results in secondary phases of Cr26C6 and Cr2Al. Atomic Force Microscopy (AFM) shows smoother films at lower temperatures and rough at higher temperatures with a surface roughness $>$ 20 nm. Friction test results will be presented. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D40.00009: Spatially resolved quasiparticle tunneling spectra in the vortex state of optimally hole-doped YBa$_{2}$Cu$_{3}$O$_{x}$ (Y-123) M.S. Grinolds, A.D. Beyer, M.L. Teague, N.-C. Yeh We report cryogenic scanning tunneling spectroscopic (STS) studies of superconducting single crystalline Y-123 (T$_{c}$ = 93 K) as a function of magnetic field. We study and model the influence of competing orders (COs), which coexist with superconductivity (SC), on the quasiparticle (QP) excitation spectra. The spatial dependence of the QP tunneling spectra is probed via STS to quantify the presence and spatial extent of SC and CO. Zero-field spatial maps of the QP spectra (100$\times $100 nm$^{2})$ in Y-123 exhibit long-range spatial homogeneity of SC ($\Delta _{SC }$= 23$\pm $1 meV) associated with the spectral coherence peaks and the presence of CO (V$_{CO }$= 33$\pm $2 meV) that gives rise to the spectral satellite features at $\Delta _{eff}$ = [($\Delta _{SC})^{2}$+(V$_{CO})^{2}$]$^{1/2}$. Conductance maps of the Y-123 in finite fields demonstrate spatially varying spectra consistent with the periodicity $a_{0}$ of the vortex lattice, with pseudogap (PG) like features at $\sim $V$_{CO}$ inside the vortex core and SC gap features remaining at $\sim \Delta _{SC}$ outside the vortex core. Moreover, conductance histograms of the vortex state reveal that the ratio of the areas associated with $\Delta _{SC}$ and V$_{CO}$ is comparable to ($a_{0}$/\textit{$\xi $}$_{ab})^{2}$, (\textit{$\xi $}$_{ab}$: in-plane SC coherence length). These results therefore suggest the important role of COs in the cuprate QP excitations. This work is supported by NSF Grant DMR-0405088. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D40.00010: High Frequency Electrical Properties of Carbon Nanotubes Dobromir Kamburov, Beth Parks, Zhaohui Zhong, Paul McEuen We report on measurements of the high frequency electrical properties of single-walled carbon nanotubes. These measurements are accomplished by incorporating a single nanotube into a microwave stripline and using optical pulses from a femtosecond laser to create short electrical pulses on the stripline. By varying the time delay between the pulses, it is possible to determine the frequency dependence of the response of the nanotube. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D40.00011: Current State of Research of Alternate Fuel Sources for Passenger Vehicles Lee Massey The purpose of this project is to report on the current state of research in the field of alternate fuel sources for passenger vehicles. Because the number of alternate fuel options is very large, this study focuses on selected bio-fuels and briefly describes a couple of the most popular non-bio and non-renewable alternatives. The fuel and energy sources studied are compared using well-to-wheel and well-to-tank net energy balances. Data also includes relative production capabilities by volume in terms of current fossil fuels. Qualitative data includes production methods and transportability. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D40.00012: Universal Properties of Population Dynamics with Fluctuating Resources Sayak Mukherjee, Hans-Karl Janssen, Beate Schmittmann Starting from the well-known field theory for directed percolation, we describe an evolving population, near extinction, in an environment with its own nontrivial spatio-temporal dynamics. Here, we consider the special case where the environment follows a simple relaxational (Model A) dynamics. Two new operators emerge, with upper critical dimension of four, which couple the two theories in a nontrivial way. While the Wilson-Fisher fixed point remains completely unaffected, a mismatch of time scales destabilizes the usual DP fixed point, suggesting a crossover to a first order transition from the active (surviving) to the inactive (extinct) state. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D40.00013: First-principles density-functional theory investigation of FOX-7 Brian Vohaska, Michael Conroy, Ivan Oleynik, Carter White Due to the expense and difficulty of experimental investigation of the chemical and physical properties of energetic materials (EMs), computational methods provide a unique opportunity for accurate determination of the chemical and physical properties of EM molecular crystals based on underlying atomic structure. In this presentation, we discuss the results of first-principles density functional theory (DFT) calculations of hydrostatic and uniaxial compression of the important energetic material, FOX-7. The calculated equilibrium properties, such as lattice parameters, elastic constants, and the bulk modulus will be reported and compared with experiment, as well as the isothermal equation of state. Due to the anisotropic nature of energetic molecular crystals, physical properties such as cohesive energy, band gap, and stress-strain relationships are reported as functions of each uniaxial compression studied. In addition, the shear stress behavior upon uniaxial compression will be discussed, as well as its possible relation to anisotropic shock-sensitivity in FOX-7. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D40.00014: Variational Wavefunction Monte Carlo method applied to electrons in a two dimensional square lattice with zero doping Sunita Kannan, Courtney Lannert We present the theoretical results from the Variational Wavefunction Monte Carlo method applied to electrons in cuprates, of a two dimensional square lattice with zero doping. Since the true Hamiltonian of the cuprates is not definitively known, much study has gone into identifying the best possible Hamiltonian. To do this, we vary the terms in the Extended Heisenberg Hamiltonian - the neighbor spin coupling term J, the spin next-neighbor term J' and the spin ring exchange term Jring, where each variation represents a different electronic interaction. We then use the variational approach to find the best groundstate wavefunction for each model Hamiltonian. Once we find the best groundstate wavefunction for each Hamiltonian, we can deduce the magnetization predicted by that model. Hence, by comparing our results for the magnetization to known experimental results, we can identify the most suitable model. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D40.00015: Quantum Criticality and Neutron Scattering Solutions for a Spin-1/2 Ladder Model Justin Cohen, Jeremiah Barry, Mark Meisel Exact solutions for a two dimensional, $S = 1/2$ quantum spin ladder model are obtained through mapping the Hamiltonian and correlation functions onto those of a one dimensional Ising chain model [1]. These solutions include a three dimensional ground state phase diagram, establishing states of ladder rung singlets, triplets, and alternating singlets and triplets in terms of interaction parameters and applied magnetic field. Evidence of quantum criticality is uncovered for select regions of the phase diagram through explorations into ladder site correlations and correlation lengths. Neutron scattering solutions for scattering intensities provide insight into the energy spectra associated with various rung spin configurations. \\$[1]$ J. H. Barry and M. W. Meisel, \textit{Phys. Rev. B} \textbf{58}, 3129 (1998). [Preview Abstract] |
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