Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session B1: 20th Anniversary of High Tc Superconductivity 'Woodstock' Session
Sponsoring Units: FHPChair: M. Brian Maple, University of California, San Diego
Room: Colorado Convention Center Four Seasons 2-3
Monday, March 5, 2007 11:15AM - 11:27AM |
B1.00001: Introduction and Overview of the 1987 `Woodstock' Session Invited Speaker: |
Monday, March 5, 2007 11:27AM - 11:39AM |
B1.00002: The Discovery of High-Tc Superconductivity and the Countdown to the Rally Invited Speaker: The guiding ideas on our road towards high-Tc superconductivity and the early work at the IBM Zurich Research Laboratory are briefly addressed. I will shed some light onto the environment and the decisive circumstances that in January 1986 led to the breakthrough with the discovery of superconductivity in the cuprates. The pre-``Woodstock'' period, which lasted less than a year, covers the time in which the Zurich team tested different La$_2$CuO$_4$-based compounds, confirmed the Meissner effect, and studied flux trapping in these new materials. It was also the time in which the news of the discovery started to spread and in which we experienced mixed reactions ranging from silent skepticism to polite (cautious) congratulations. This changed dramatically into excitement with the confirmation of the Zurich results by the Tokyo (S. Tanaka) and the Houston ( C.W. Chu) group, and cumulated in the take-off of the new field at the famous ``Woodstock Meeting of Physics'' after the discovery of the 90 K superconductor. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B1.00003: High $T_{c}$: The Discovery of RBCO Invited Speaker: It was said by Emerson that ``there is no history; there is only biography.'' This is especially true when the events are recounted by a person who, himself, has been heavily involved and the line between history and autobiography can become blurred. However, it is reasonable to say that discovery itself is not a series of accidents but an inevitable product of each development stage of scientific knowledge as was also pointed out by Holden et al. (1) The discovery of RBCO (2,3) with R = Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu is no exception. In this presentation, I will briefly recount several events that were crucial to the discovery of RBCO: those before 1986 (4) that sowed the seeds in our group important to our later high temperature superconductivity effort; those in 1986 (5) that were critical to our discovery of the 93 K RBCO soon after the discovery of the 35 K high temperature superconductor by M\"{u}ller and Bednorz (6); and those in 1987 when the barrier of the liquid nitrogen boiling temperature of 77 K was finally conquered. \newline \newline 1. G. J. Holton et al., American Scientist 84, 364 (1996). \newline 2. M. K. Wu et al., Phys. Rev. Lett. 58, 908 (1987). \newline 3. P. H. Hor et al., Phys. Rev. Lett. 58, 1891 (1987). \newline 4. C. W. Chu et al., S. S. Comm. 18, 977 (1976); C. W. Chu and V. Diatchenko, Phys. Rev. Lett. 41, 572 (1978); T. H. Lin et al., Phys. Rev. B(RC) 29, 1493 (1984); J. H. Lin et al., J. Low Temp. Phys. 58, 363 (1985). \newline 5. C. W. Chu et al., Phys. Rev. Lett. 58, 405 (1987); C. W. Chu et al., Science 235, 567 (1987). \newline 6. J. G. Bednorz and K. A. M\"{u}ller, Z. Phys. B64, 189 (1986). [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B1.00004: Some Prehistory to Woodstock Invited Speaker: I want to briefly describe the background surrounding two talks that provided a preview of the excitement that was to spill over at the '87 March APS meeting. The first was an unscheduled talk on LaBaCuO by Professor K. Kitazawa on Dec 5, 1986 at an MRS symposium on Superconducting Materials held in Boston. The second was a quasi- public disclosure by Professor Paul Chu regarding his work on superconductivity above liquid nitrogen temperatures on Feb 28, 1987 at UCSB. These talks form part of the prehistory to the what became known as the Wookstock of physics. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B1.00005: The 1987 High Tc `Woodstock' Session and High Tc at IBM Invited Speaker: |
Monday, March 5, 2007 12:15PM - 12:27PM |
B1.00006: Bell Labs and High Tc Invited Speaker: |
Monday, March 5, 2007 12:27PM - 12:39PM |
B1.00007: High Tc and Condensed Matter Theory in 1987 Invited Speaker: |
Monday, March 5, 2007 12:39PM - 12:51PM |
B1.00008: Early High Tc Activity in Japan: The Franco Rasetti Lecture Invited Speaker: From 1960 to 1980, R\&D of superconductivity in Japan was carried out mainly to improve A15 superconducting wires and magnets. Improvement of wires were made mainly in the National Institute for Metals, and improvements of superconducting magnets were made in the Japan Atomic Energy Research Institute for future nuclear fusion reactors, the National Railway Laboratory for future maglev trains and also in the Electo-Technical Laboratory for MHD generators. I began the research of BPBO in 1975 and at that time the research of oxide superconductors was limited only to my laboratory in the University of Tokyo. During the study of this new superconductor, we learned quite a lot on how to make ceramic samples, how to measure electrical conductivity and magnetic susceptibility at low temperatures. In 1982, Prof. S. Nakajima organized a rather small group for investigating ``New Superconducting Phenomena,'' and I became a member of the group. In 1985, Nakajima expanded the research group to include more than 5 experimentalists and 5 theoreticians. The title of the research was ``New Superconducting Materials'' and the funds came from the Ministry of Education of Japan. In late October, 1986, we followed the first paper of Bednorz and Muller, and immediately found the material includes high temperature superconductor and reported it to the group meeting held in early November. In early December, we confirmed La$_{2-x}$Ba$_x$CuO$_4$ is the real high temperature superconductor, the critical temperature is 28K. I sent a copy of our paper to Prof. Beasley of California and asked to inform this fact to his colleagues. Asahi Shimbun, the biggest newspaper in Japan announced this in its science section, and then many people knew the high temperature superconductor had been discovered. Then many physicists and chemists rushed to this field very quickly and many kinds of materials were synthesized. In the Government, the Ministry of Education, the Ministry of International Trade and Industry (MITI), and the Agency for Science and Technology began to make new development plans of their own. Superconductivity fever then started in Japan. ISTEC was established in early 1988 under the support of MITI and industries. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B1.00009: High Tc at BellCore Invited Speaker: |
Monday, March 5, 2007 1:03PM - 1:15PM |
B1.00010: High Tc at Stanford Invited Speaker: |
Monday, March 5, 2007 1:15PM - 1:27PM |
B1.00011: High Tc Superconductivity --1987 Invited Speaker: The discovery of superconductivity in the cuprate class of conducting oxides brought a flash of sunlight on one of the fields condensed matter physics that many of us had thought was rather mature and fairly well understood. Alas, it was not so. In addition to opening a whole new class of materials to the study of correlated motion of charge carriers, it opened a new mind-set that materials with complex chemical bonding can lead to totally new phenomena. The tasks of materials preparation escalated, and with it came the development of totally new spectral probes of the electron gas and the electronic structure in metals. The task is to use complexity so that the interplay of adjacent correlated motion can be used to generate new phenomena that will in turn perform novel functions. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 2:15PM |
B1.00012: Panel on Discovery of HTc |
Session B2: Spin Transfer-Driven Magnetic Excitations
Sponsoring Units: GMAGChair: Dan Ralph, Cornell University
Room: Colorado Convention Center Four Seasons 4
Monday, March 5, 2007 11:15AM - 11:51AM |
B2.00001: Spin Transfer Torque in Non-Uniform Magnetizations Invited Speaker: A spin polarized current exerts a torque when it passes through a region of non-uniform magnetization. In magnetic multilayers, this torque can reverse the magnetization of the layers or cause it to precess. In magnetic wires, it can move domain walls. These torques and their consequences can be largely understood from a series of simple models. However, experiments have become sophisticated enough to show that these simple models are not complete. In this talk, I will motivate the interest in these systems, describe the simple models that capture most of the physics, and highlight some the open questions that will be addressed in the later talks in this session. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:27PM |
B2.00002: Spin-Torque Diode Effect in Magnetic Tunnel Junctions Invited Speaker: Spin-injection magnetization switching (SIMS) technique [1] made it possible to control magnetization by a direct current. A discovery of spontaneous rf oscillation from CPP-GMR nano-pillars and a real time observation of the switching process have revealed essential amplification function of a precession in the magnetic nano-pillars under a direct current [2]. Beside of those progresses, developments of giant tunneling magneto-resistive (GTMR) effect using an MgO barrier [3] made it possible to utilize a very large resistance change according to the magnetization switching. In this talk, several attempts to utilize interplay between spin-torque and giant-TMR effect will be presented referring to a ``spin-torque diode effect'' [4] and other properties such like rf noise control and possible signal amplification using magnetic tunnel junctions (MTJs). \newline \newline [1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996) , L. Berger, Phys. Rev. B 54, 9353 (1996), and E. B. Myers, et al., Science 285, 867 (1999). \newline [2] S. I. Kiselev et al., Nature 425, 380 (2003), I. N. Krivorotov et al., Science, 307, 228 (2005). \newline [3] W. Wulfhekel, et al. Appl. Phys. Lett. 78, 509--511 (2001), M. Bowen, et al. Appl. Phys. Lett. 79, 1655--1657 (2001), J. Faure-Vincent, et al. Appl. Phys. Lett. 82, 4507--4509 (2003), S. Yuasa, et al., Jpn. J. Appl. Phys. Part 2, 43, L588 (2004), S. Yuasa, et al., Nature Mat. 3, 868 (2004), S. S. P. Parkin et al., Nature Mat. 3, 862 (2004), and D. D. Djayaprawira et al., Appl. Phys. Lett. 86, 092502 (2005). \newline [4] A. A. Tulapurkar, et al., Nature, 438, 339 (2005). [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 1:03PM |
B2.00003: Ferromagnetic resonance of individual nanomagnets driven by spin-polarized currents. Invited Speaker: Spin-polarized electrons passing through a nanoscale magnet can transfer their spin angular momentum to the magnetization, applying a torque far more efficiently than can be achieved by employing magnetic fields. We discuss the characterization of microwave-frequency magnetic excitations driven by these spin-transfer torques in metallic spin-valve devices and magnetic tunnel junctions using high-bandwidth electrical techniques. We first describe spontaneous magnetic oscillations that can be excited by DC currents, and then focus on a new form of ferromagnetic resonance (FMR) that uses high-frequency spin-polarized currents to excite resonance, instead of high-frequency magnetic fields. This technique allows measurements of individual magnetic samples orders of magnitude smaller than can be probed by traditional FMR, and should scale to much smaller devices as well. Using spin-transfer-driven FMR, we have been able to measure the spectra of normal modes for individual nanomagnets, including both the fundamental mode and higher-order more spatially-nonuniform modes. From the lineshapes, we can distinguish two different resonant regimes: simple FMR and phase-locking to a pre-existing DC-driven mode. The linewidths also enable efficient measurements of the magnetic damping parameter in single nanomagnets. We find that the FMR lineshapes differ between metallic spin valves and magnetic tunnel junctions when a DC current is applied, pointing to important differences in the fundamental mechanisms of the spin transfer torque in these two systems. This work was done in collaboration with P. M. Braganca, A. G. F. Garcia, I. N. Krivorotov, J. Z. Sun, J. C. Slonczewski, R. A. Buhrman, and D.C. Ralph. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:39PM |
B2.00004: X-Ray Imaging of Spin Transfer Induced Magnetization Reversal Invited Speaker: Time resolved x-ray microscopy allows one for the first time to image the magnetization switching process in a spin transfer structure. Instead of the coherent magnetization reversal, we observe switching by lateral motion of a magnetic vortex across a nanoscale element. The results of the first experiment demonstrates that the spin-switching can proceed in a new surprising mode that is quite different from uniform rotation. The switching can occur by lateral motion of a magnetic vortex across the magnetic film. The vortex structure is favored by the Oerstedt- field produced by the charge current while the spin current induces the lateral motion of the vortex leading to switching of M as soon as the center of the vortex moves out of the film and the resulting C-state relaxes into the uniform state. Our experiment also clearly shows that we have observed the effect of the torque induced by the Oerstedt field superimposed onto the torque produced by the spin current. The ratio of the torques from the Oersted field to spin transfer effects can be influenced by the size and thickness of the free layer, leading to different switching mechanisms. To obtain the spin torques without such bias, future experiments must eliminate or compensate the Oerstedt field. This appears to be possible for instance in lateral spin valve structures. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 2:15PM |
B2.00005: Temperature dependence of the spin torque effect in current-induced domain wall motion Invited Speaker: Rather than using conventional field-induced reversal, a promising approach for switching magnetic nanostructures is current-induced domain wall motion (CIDM), where due to a spin torque effect, electrons transfer angular momentum and thereby push a domain wall [1-4]. Since this interaction is strongly dependent on the wall spin structure, we have imaged domain walls in NiFe and Cobalt nanostructures and correlate the above mentioned effects with the imaged spin structure [1-4]. We find that both domain walls types can be moved due to the spin torque effect in the direction of the electron flow [2]. In addition to wall movement, changes in the wall spin structure have been predicted [2], and we have recently observed such wall type transformations using PEEM [3] and found that the velocity depends strongly on the wall type and the transformations occurring [3]. \newline Temperature dependent measurements of field- and current-induced wall motion have shown that the critical fields for field-induced wall motion decrease with increasing temperature, which can be attributed to thermal excitations. The critical current densities for current-induced motion though have been found to increase with increasing temperature, which is opposite to the behaviour due to thermal excitations [4], and might be due to the influence of thermally activated spin waves [4]. Using constrictions, we have been able to probe the interplay between current-induced motion and the attractive potential wells that the constrictions generate at variable temperature. We find that we can not only move domain walls with currents even into areas, where no current is flowing but the temperature dependence is also a sensitive probe separating the influence of thermal excitation vs. the intrinsic temperature dependence of the spin transfer torque. \newline \begin{enumerate} \item M. Kl\"{a}ui et al., PRL \textbf{94}, 106601 (2005); A. Yamaguchi et al., PRL \textbf{92}, 77205 (2004). \item A. Thiaville et al., EPL \textbf{69}, 990 (2005). \item M. Kl\"{a}ui et al., APL \textbf{88}, 232507 (2006). \item M. Laufenberg et al., PRL \textbf{97}, 46602 (2006). \end{enumerate} [Preview Abstract] |
Session B3: Phonons and Magnetic Frustration in Pyrochlores
Sponsoring Units: DCMPChair: Leon Balents, University of California, Santa Barbara
Room: Colorado Convention Center Korbel 2A-3A
Monday, March 5, 2007 11:15AM - 11:51AM |
B3.00001: Quenching ground state degeneracy in pyrochlore antiferromagnets Invited Speaker: The classical pyrochlore antiferromagnet (AFM) is considered the ``most'' geometrically frustrated system. Classically, this leads to the absence of any ordering transition at non-zero temperature, even in an applied magnetic field. We describe several mechanisms by which the ground state degeneracy can be split by fluctuations or other effects. We first consider quantum fluctuations, which may lead to the formation of a novel spin-liquid state or complex magnetic ordering. By deriving an effective Hamiltonian, we determine the quantum ground states for different values of spin $s$, resolving some of the ambiguities in existing large-$s$ spin-wave treatments. A quantitative application to recent experiments on the spinel chromites, ACr$_2$O$_4$ (A=Cd,Hg) shows that for the relevant $s=3/2$ the quantum effects are too weak to explain the observed ordering and the existance of a very robust magnetization plateau in a field. We then turn to a model of spin-lattice coupling that explains both the plateau formation and the observed ordering on the plateau. The predictions are confirmed by recent neutron scattering and x-ray scattering experiments (S. H. Lee et al.). The same model applied to zero magnetic field predicts a reduced but still large ground state degeneracy, including the states observed in both the Cd and Hg materials. This is consistent with the dominance of spin-lattice interactions, with weak additional effects determining the low field magnetic ordering. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:27PM |
B3.00002: Heisenberg antiferromagnet on the pyrochlore lattice: order from distortion Invited Speaker: The Heisenberg antiferromagnet on the pyrochlore lattice is an
example
of a highly frustrated system with a large degeneracy of the ground
state. The classical model with nearest-neighbor interactions shows
no signs of magnetic order down to very low temperatures. The
quantum
analog, with short enough spins, was considered a prime candidate for
a quantum-disordered ground state, such as a valence-bond liquid or
solid. At the same time, the large degeneracy makes this magnet
susceptible to a variety of nominally small perturbations. A
spin-lattice coupling leads to a spin-Peierls-like distortion of the
lattice. In contrast to spin chains, the spin-Peierls distortion
in a pyrochlore antiferromagnet occurs for any spin length $S$
remaining robust even in the classical
limit.
A recent experimental characterization of the $S=3/2$ prototype
CdCr$_2$O$_4$ [1] provided a test for the theoretical
model. This antiferromagnetic spinel exhibits a tetragonal lattice
distortion with an elongated unit cell $a=b |
Monday, March 5, 2007 12:27PM - 1:03PM |
B3.00003: Nonlinear optical signatures of the tensor order in Cd$_2$Re$_2$O$_7$ Invited Speaker: The pyrochlore oxide Cd$_2$Re$_2$O$_7$ undergoes a structural phase transition at $T=200$~K with an unusual tensor character.\footnote{J. C. Petersen \textit{et al.}, Nature Phys. \textbf{2}, 605 (2006); C. A. Kendziora \textit{et al.}, PRL \textbf{95}, 125503 (2005); I. A. Sergienko \textit{et al.}, PRL \textbf{92}, 065501 (2004).} The order parameter for this state is two-dimensional, and associated with a phonon mode with $E_u$ symmetry. In magnetically frustrated pyrochlores, magnetoelastic coupling to similar modes can induce fascinating magnetically ordered states;\footnote{Y. Yamashita and K. Ueda, PRL \textbf{85}, 4960 (2000); O. Tchernyshyov \textit{et al.}, PRL \textbf{88}, 067203 (2002) and PRB \textbf{66}, 064403 (2002).} Cd$_2$Re$_2$O$_7$ is nonmagnetic, so it is possible to study the structural instabilities of the pyrochlore lattice in isolation. We have used optical second harmonic generation with polarization sensitivity to resolve an ambiguity in the low-temperature crystal structure, and verify an auxiliary condition on the structure that is implied by the order parameter symmetry. We also show that the temperature-dependence of the order parameter is consistent with thermal occupation of a Goldstone mode that results from the $E_u$ order parameter symmetry. The methodology that we have developed may be applied more widely in characterizing ordered states in matter. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:39PM |
B3.00004: Frustrated and Satisfied Ground States in Pyrochlore Antiferromagnet Tb$_2$Ti$_2$O$_7$ Invited Speaker: The Rare-Earth Titanates have been a playground for the physics of geometrical frustration, as magnetic rare earth sites can be arranged on the pyrochlore lattice, a network of corner-sharing tetrahedra, and different rare earth elements display differing exchange interactions and anisotropies. Tb$_2$Ti$_2$O$_7$ is a particularly enigmatic pyrochlore antiferromagnet. It is reasonably well understood in terms of local Ising-like anisotropy, such that the Tb$^3+$ magnetic moments should point either directly into or out of the tetrahedra. The moments are antiferromagnetically coupled, and theory expects a phase transition to an ordered state at $\sim$ 1 K. However, in the abnsence of an external perturbation, it remains in a spin liquid state to very low temperatures, $<$ 0.05 K. We report new neutron scattering measurements on Tb$_2$Ti$_2$O$_7$ using new time-of-flight techniques at NIST, which show the spin liquid state can be brought to order in the presence of relatively weak [110] magnetic fields. We'll also discuss high resolution x-ray scattering results which show that the lattice is not a passive observe to the physics of frustration, but that it displays fluctuation effects typical of Jahn Teller physics, albeit at very low temperatures. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 2:15PM |
B3.00005: Neutron scattering studies of magnetic pyrochlores Invited Speaker: The pyrochlore antiferromagnets in which spins interact in a network of corner-sharing tetrahedra have macroscopic ground state degeneracy, that leads to exotic low temperature properties. Spinels AB$_2$O$_4$ realize the pyrochlore lattice if the B ions couple antiferromagnetically. This talk will start with a quick review of several novel properties found in spinels, such as the spin liquid state in ZnCr$_2$O$_4$, the 3D spin-Peierls transition in ZnCr$_2$O$_4$, the spin-orbital coupling in ZnV$_2$O $_4$, and the heavy fermionic behaviors in LiV$_2$O$_4$. A discussion will follow on our recent neutron and X-ray scattering works on ACr$_2$O$_4$ (A=Cd, Hg). We will show that the 3D spin-Peierls transition in CdCr$_2$O$_4$ is different from that observed in ZnCr$_2$O$_4$, and that the magnetic field-induced half-magnetization plateau state in HgCr$_2$O$_4$ has the P4$_3$32 symmetry. Our results provide direct tests of theoretical models proposed to understand the complex behaviors of the Heisenberg pyrochlore antiferromagnets. A quantum spin pyrochlore system will also be discussed. [Preview Abstract] |
Session B4: Population Imbalanced Superfluid Fermi Gases
Sponsoring Units: DAMOPChair: Allan Griffin, University of Toronto
Room: Colorado Convention Center Korbel 2B-3B
Monday, March 5, 2007 11:15AM - 11:51AM |
B4.00001: Phase-Separation in a Polarized Fermi Gas Invited Speaker: In fermionic systems, the formation of pairs between two constitute components is the essential ingredient of superfluidity and superconductivity. While many advances have been made in understanding pairing between components with equal chemical potentials, the possible pairing mechanisms and corresponding phases in systems with mismatched chemical potentials are topics of active debate. In contrast to the difficulties in generating magnetized superconductors, mismatched Fermi surfaces can be readily realized by creating an imbalance between the populations of two spin components in a gas of ultracold fermionic atoms. Exotic new states of matter are predicted for the unbalanced systems that, if realized, may have important implications for our understanding of nuclei, compact stars, and quantum chromodynamics. We investigate a strongly interacting Fermi gas of $^6$Li atoms with unbalanced populations by {\it in-situ} imaging of real-space density distributions \footnote{G.B. Partridge {\it et al.}, {\it Science} {\bf 311}, 503 (2006).}, \footnote{G.B. Partridge {\it et al.}, {\it Phys. Rev. Lett.} {\bf 97}, 190407 (2006).}. We observe two low-temperature regimes, both with an evenly paired core. At the lowest temperatures, an unpolarized core separates from the excess unpaired atoms by a sharp boundary, which is consistent with a phase separation driven by a first-order phase transition. Moreover, the unpolarized core deforms with increasing polarization, presumably due to surface tension at the superfluid/normal boundary. At higher but still degenerate temperatures, an unpolarized central core remains up to a critical polarization, but does not deform. In this case, the boundaries are not sharp, indicating a partially-polarized shell between the core and the unpaired atoms, consistent with a second-order phase boundary. The observed temperature dependence supports a phase diagram with a tricritical point. The phase- separated phase is only possible for temperatures below the tricritical point, while the higher temperature phase is a polarized superfluid. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:27PM |
B4.00002: Superfluidity in a Strongly Interacting Polarized Fermi Gas Invited Speaker: Cooper pairing is the underlying mechanism for the Bardeen-Cooper-Schrieffer superfluid state of equal mixture of two fermionic components. An interesting situation arises when the symmetry between the two components is broken, such as mass, density, or chemical potential. Is the pairing mechanism robust enough to overcome an asymmetric stress and keep driving superfluidity? Does a new form of superfluidity emerge out of two mismatched Fermi seas? We experimentally study these questions in an unequal mixture of strongly interacting ultracold fermionic atoms trapped in a three dimensional harmonic potential. We observe that due to strong interaction, the system maintains superfluidity up to a critical population imbalance, showing Pauli limit of superfluidity [1]. By correlating condensation fraction and in-situ density distribution, we identify that a superfluid has equal densities of two components and spatially separates from a normal gas of unequal densities [2]. Recent experimental results will be discussed. [1] M.W. Zwierlein et al., Science 311, 492 (2006). [2] Y. Shin et al., Phys. Rev. Lett 97, 030401 (2006). [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 1:03PM |
B4.00003: The Low-Temperature Phases of Polarized Fermionic Superfluids Invited Speaker: A number of recent experiments have achieved paired superfluidity of trapped fermionic atomic gases. Such pairing, occurring between two atomic hyperfine-state species (forming a pseudo-spin $1/2$ system), is possible due to the strong attractive interactions provided by a magnetic field tuned Feshbach resonance (FR). At equal populations, the superfluidity of resonantly interacting Fermi gases undergoes the well-studied crossover between Bardeen-Cooper-Schrieffer (BCS) pairing and Bose-Einstein condensation (BEC) as a function of FR detuning (or interaction strength). I will discuss recent work [1] aimed at understanding the case of {\it unequal\/} populations (i.e., imposed spin polarization), an easily controllable experimental knob that is predicted to interrupt the continuous equal-population BCS-BEC crossover, yielding a variety of distinct phenomena including regions of singlet paired superfluid, unpaired polarized normal Fermi liquid, polarized Fulde-Ferrell-Larkin-Ovchinnikov superfluid, polarized magnetic superfluid, and phase-separated mixtures of these uniform states. I will describe the low-temperature phase diagram of such polarized fermionic superfluids, focusing particularly on experimental signatures of the various phases in the inhomogeneous environment of the trap. [1] D.E. Sheehy and L. Radzihovsky, Phys. Rev. Lett. {\bf 96}, 060401 (2006); cond-mat/0607803 (Annals of Physics, in press). [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:39PM |
B4.00004: Surface tension and collective modes in population imbalanced Fermi gases in the BCS-BEC crossover regime Invited Speaker: Motivated by the striking experiments from MIT and Rice University, we study population imbalanced Fermi gases in the BCS-BEC crossover regime. We have calculated the surface tension in the boundary separating superfluid and polarized normal regions in a trap. We show how this surface tension can explain apparent inconsistencies between the two groups. Using several candidate equations of state, we calculate frequencies of the breathing mode, finding that collective mode measurements can distinguish between the various possibilities. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 2:15PM |
B4.00005: The important role of temperature in BCS--Bose-Einstein condensation crossover phenomena with population imbalance Invited Speaker: Any comparison between theory and experiment in the cold Fermi gases requires that one include the effects of non-zero temperature $T$. In this talk we show how to include finite $T$ in a way which is compatible with the generalized BCS-like ground state, assumed in essentially all $T=0$ calculations of gases with population imbalance. We use a pairing fluctuation theory of BCS--Bose-Einstein condensation (BEC) based on a $T$-matrix formalism. Distinguishing this theory from strict mean-field theories is our self-consistent treatment of incoherent, finite-momentum pairs along with single fermions. This leads to a pseudogap in the fermion excitation spectrum at finite $T$ which is necessary in order to arrive at physically meaningful transition temperatures $T_c(p)$, where $p$ is the polarization. We present phase diagrams in the $p$-$T$ plane with variable scattering length, $1/k_Fa$, and identify the regions where bulk superfluidity, normal phases and phase separation appears. For the trapped Fermi gases, we present particle density profiles for general $1/k_Fa$ as well as a detailed comparison with recent measurements at both MIT and Rice University. We find reasonably good agreement with these experimental data. \\ 1. C.-C. Chien, Q.J. Chen, Y. He, and K. Levin, \textit{Intermediate temperature superfluidity in an atomic Fermi gas with population imbalance}, Phys. Rev. Lett. 97, 090402 (2006). \\ 2. Q.J. Chen, Y. He, C.-C. Chien, and K. Levin, \textit{Stability conditions and phase diagrams for two component Fermi gases with population imbalance}, cond-mat/0608454; Phys. Rev. A 74, 06xxxx (2006). \\ 3. C.-C. Chien, Q.J. Chen, Y. He, and K. Levin, \textit{Finite temperature effects in trapped Fermi gases with population imbalance}, Phys. Rev. A 74, 021602(R) 2006. [Preview Abstract] |
Session B5: Adaptation in Biological Systems
Sponsoring Units: DBPChair: Ned Wingreen, Princeton University
Room: Colorado Convention Center Korbel 1A-1B
Monday, March 5, 2007 11:15AM - 11:51AM |
B5.00001: Precise Adaptation in Bacterial Chemotaxis through ``Assistance Neighborhoods'' Invited Speaker: The chemotaxis network in {\it Escherichia coli} is remarkable for its sensitivity to small relative changes in the concentrations of multiple chemical signals over a broad range of ambient concentrations. Key to this sensitivity is an adaptation system that relies on methylation and demethylation (or deamidation) of specific modification sites of the chemoreceptors by the enzymes CheR and CheB, respectively. It was recently discovered that these enzymes can access five to seven receptors when tethered to a particular receptor. We show that these ``assistance neighborhoods'' (ANs) are necessary for precise and robust adaptation in a model for signaling by clusters of chemoreceptors: (1) ANs suppress fluctuations of the receptor methylation level; (2) ANs lead to robustness with respect to biochemical parameters. We predict two limits of precise adaptation at large attractant concentrations: either receptors reach full methylation and turn off, or receptors become saturated and cease to respond to attractant but retain their adapted activity. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:27PM |
B5.00002: Adaptation in neural processing Invited Speaker: |
Monday, March 5, 2007 12:27PM - 1:03PM |
B5.00003: Adaptation by Plasticity of Genetic Regulatory Networks Invited Speaker: Genetic regulatory networks have an essential role in adaptation and evolution of cell populations. This role is strongly related to their dynamic properties over intermediate-to-long time scales. We have used the budding yeast as a model Eukaryote to study the long-term dynamics of the genetic regulatory system and its significance in evolution. A continuous cell growth technique (chemostat) allows us to monitor these systems over long times under controlled condition, enabling a quantitative characterization of dynamics: steady states and their stability, transients and relaxation. First, we have demonstrated adaptive dynamics in the \textit{GAL} system, a classic model for a Eukaryotic genetic switch, induced and repressed by different carbon sources in the environment. We found that both induction and repression are only transient responses; over several generations, the system converges to a single robust steady state, independent of external conditions. Second, we explored the functional significance of such plasticity of the genetic regulatory network in evolution. We used genetic engineering to mimic the natural process of gene recruitment, placing the gene \textit{HIS3} under the regulation of the \textit{GAL} system. Such genetic rewiring events are important in the evolution of gene regulation, but little is known about the physiological processes supporting them and the dynamics of their assimilation in a cell population. We have shown that cells carrying the rewired genome adapted to a demanding change of environment and stabilized a population, maintaining the adaptive state for hundreds of generations. Using genome-wide expression arrays we showed that underlying the observed adaptation is a global transcriptional programming that allowed tuning expression of the recruited gene to demands. Our results suggest that non-specific properties reflecting the natural plasticity of the regulatory network support adaptation of cells to novel challenges and enhance their evolvability. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:39PM |
B5.00004: How Large Asexual Populations Adapt Invited Speaker: We often think of beneficial mutations as being rare, and of adaptation as a sequence of selected substitutions: a beneficial mutation occurs, spreads through a population in a selective sweep, then later another beneficial mutation occurs, and so on. This simple picture is the basis for much of our intuition about adaptive evolution, and underlies a number of practical techniques for analyzing sequence data. Yet many large and mostly asexual populations -- including a wide variety of unicellular organisms and viruses -- live in a very different world. In these populations, beneficial mutations are common, and frequently interfere or cooperate with one another as they all attempt to sweep simultaneously. This radically changes the way these populations adapt: rather than an orderly sequence of selective sweeps, evolution is a constant swarm of competing and interfering mutations. I will describe some aspects of these dynamics, including why large asexual populations cannot evolve very quickly and the character of the diversity they maintain. I will explain how this changes our expectations of sequence data, how sex can help a population adapt, and the potential role of ``mutator'' phenotypes with abnormally high mutation rates. Finally, I will discuss comparisons of these predictions with evolution experiments in laboratory yeast populations. [Preview Abstract] |
Session B6: Control and Architecture in Directed Macromolecular Self-Assembly
Sponsoring Units: GSNP DPOLYChair: Alex Travesset, Iowa State University and Ames National Laboratory
Room: Colorado Convention Center 207
Monday, March 5, 2007 11:15AM - 11:51AM |
B6.00001: Geometry and universality in self-assembly Invited Speaker: I will discuss the use of ideas from geometry and topology in combination with the statistical mechanics of self-assembly to classify the possible types of mesoatoms that can constitute a library of raw materials for making mesomolecules and bulk materials. I will concentrate on mesoatoms made up of an ordered two-dimensional monolayer of particles on the surface of a liquid droplet. Both the shape and the topology of the two-dimensional surface can be varied as well as the architecture of the ordering particles. The topology of the surface and the symmetries of the two-dimensional order severely restrict the possible defect structure of the mesoatom, which in turn fixes its valency. Defective regions are natural places for biological activity, chemical linking, unusual elastic response and aggregation of disorder. Specific examples include crystalline and hexatic order of point particles on the sphere, paraboloid, torus and Gaussian bump, nematic order of nematogens on the sphere and torus, and vector order of polar units on the sphere. The Gaussian curvature of the underlying surface may also lead to new features in the ground state, such as extended defect arrays of various kinds and curvature-driven defect unbinding, all of which may be exploited via engineered or spontaneous self-assembly. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:27PM |
B6.00002: Colloidal atoms and molecules Invited Speaker: We describe two new types of colloidal particles that greatly expand the kinds of colloids and nanoparticles that are available for self-assembly. The first type, called colloidal molecules, consists of clusters of microspheres and/or nanospheres that take on well-defined geometries such as dumbbells, triangles, tetrahedra, octahedral, \textit{etc}. With these, we explore new two and three-dimensional phases, including dumbbells at a planar interface, and three-dimensional assemblies of tetrahedra. The second type, called colloidal atoms, consists of nearly spherical particles with a specific number of symmetrically-placed patches on their surface. These patches have symmetries similar to the colloidal molecules described above. The patches can be functionalized with single-stranded DNA that interacts specifically with complementary strands on other particles forming a network or crystal of colloidal particles. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 1:03PM |
B6.00003: Colloidal Armor Invited Speaker: Assembly of colloidal particles on fluid-fluid interfaces is a promising technique for synthesizing two-dimensional structured materials. We describe a microfluidic method that allows direct visualization and understanding of the dynamics of the growth of colloidal crystals on a curved interface. We show how this approach allows control over composition and size of the colloidal armor, including making janus shells. The two-dimensional granular shells have mechanical properties similar to other elastic-plastic materials. These features will be described and the influence of surfactants on the shells will be presented. Finally, gas bubbles covered with colloidal particles show unusual stability against gas dissolution, and we will explore this stability using experiments and numerical simulations. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:39PM |
B6.00004: Recent Advances in Solution-state Assembly of Synthetic Polymers into Well-defined Nanostructures. Invited Speaker: The solution-state assembly of synthetic amphiphilic block copolymers has emerged as a powerful tool to conveniently and rapidly afford discrete, well-defined nanoscale materials for study and application to advance nanoscience and nanotechnology. One of the key challenges has been the identification of appropriate polymer components and conditions to control the assembly mechanisms and produce complex materials of uniform size, narrow size distribution and having interesting morphologies. In addition, developing systems that are capable of undergoing assembly directly from aqueous solution and also those that contain complex internal phase segregated domains ($i.e.$ multicompartment micelles) are significant interests. This presentation will provide an update on our work to control the micelle morphologies and will describe recent di- and tri-block copolymer designs that allow for pH-triggered self assembly into amphiphilic core-shell micelles, without the use of organic solvents. Moreover, the aqueous solution-state assembly of novel amphiphilic hyperbranched copolymers will be discussed. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 2:15PM |
B6.00005: Non-ionic micelles and encapsulation. Invited Speaker: The development of self-assembly as a useful approach to the synthesis and manufacturing of complex systems and materials is a central theme in our research. Amphiphilic block copolymers of the poly(ethylene oxide)-poly(propylene oxide) (PEO-PPO) family (commercially available as Poloxamers) are well-known for self-assembling into (core-shell spherical) micelles and (cubic, hexagonal, and lamellar) lyotropic liquid crystals in water. We are interested on how the aqueous phase behavior and structure of these non-ionic polymeric amphiphiles can be modulated by the addition of organic solvents or solutes. Our studies (i) probe the amphiphile organization in both micellar solutions and lyotropic liquid crystals, (ii) combine macroscopic observations (e.g., concentration-temperature micellization phase boundaries, ternary isothermal amphiphile-water-cosolvent phase diagrams) with microscopic measurements (from small-angle neutron and X-ray scattering), and (iii) aim to relate the type of structure formed and its properties to the relative swelling of the polymer blocks and to the location of the solvent/solute in the amphiphile assembly. These studies address the following practical questions: What are the ``right'' components and conditions for self-assembly? What if the conditions are no longer ``right''? How can we ``help'' self-assembly? Modulation of structure-property relationships in amphiphile-containing media is central to formulation of pharmaceutics and personal care products. [Preview Abstract] |
Session B7: Emergent Patterns in Geophysical Processes
Sponsoring Units: DCMPChair: Raymond Goldstein, University of Cambridge
Room: Colorado Convention Center Korbel 4A-4B
Monday, March 5, 2007 11:15AM - 11:51AM |
B7.00001: Dynamics of precipitation pattern formation at geothermal hot springs Invited Speaker: The spectacular terraced landscape at geothermal hot springs is a world-wide phenomenon, shown here to arise from the nonlinear interplay between turbulent fluid transport and precipitation dynamics. The system is modeled successfully using a discrete space-time model, justified both from renormalization group considerations and our experience modeling phase transition kinetics in condensed matter systems. A variety of scaling laws are predicted and compared with field observations. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:27PM |
B7.00002: Scaling in Columnar Joints Invited Speaker: Columnar jointing is a fracture pattern common in igneous rocks in which cracks self-organize into a roughly hexagonal arrangement, leaving behind an ordered colonnade. We report observations of columnar jointing in a laboratory analog system, desiccated corn starch slurries. Using measurements of moisture density, evaporation rates, and fracture advance rates, we suggest an advective-diffusive system is responsible for the rough scaling behavior of columnar joints. This theory explains the order of magnitude difference in scales between jointing in lavas and in starches. We investigated the scaling of average columnar cross-sectional areas in experiments where the evaporation rate was fixed using feedback methods. Our results suggest that the column area at a particular depth is related to both the current conditions, and hysteretically to the geometry of the pattern at previous depths. We argue that there exists a range of stable column scales allowed for any particular evaporation rate. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 1:03PM |
B7.00003: How the Icicle Got Its Shape Invited Speaker: The growth of icicles is considered as a free-boundary problem. A synthesis of atmospheric heat transfer, geometrical considerations, and thin-film fluid dynamics leads to a nonlinear ordinary differential equation for the shape of a uniformly advancing icicle, the solution to which defines a parameter-free shape which compares very favorably with that of natural icicles. Away from the tip, the solution has a power-law form identical to that recently found for the growth of stalactites by precipitation of calcium carbonate. This analysis thereby explains why stalactites and icicles are so similar in form despite the vastly different physics and chemistry of their formation. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:39PM |
B7.00004: Snow spikes: formation of laboratory penitentes Invited Speaker: Spike-shaped structures are produced by light-driven ablation in very different contexts. Penitentes 1–-4 m high are common on Andean glaciers, where their formation changes glacier dynamics and hydrology. Laser ablation can produce cones 10–-100 microns high with a variety of proposed applications in materials science. We report the first laboratory generation of centimeter-scale snow and ice penitentes. Systematically varying conditions allows identification of the parameters controlling the formation of ablation structures. We demonstrate that penitente initiation and coarsening require cold temperatures, so that ablation leads to sublimation. Once penitentes have formed, further growth of height can occur by melting. The penitentes initially appear as small structures (3 mm high) and grow by coarsening to 1-5 cm high. Our results are an important step towards understanding ablation morphologies. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 2:15PM |
B7.00005: Triboelectrification and Razorbacks: Geophysical Patterns Produced in Dry Grains Invited Speaker: |
Session B8: Focus Session: Novel Superconductors II: CeCoIn5 and NaxCoO2
Sponsoring Units: DMPChair: Rongyin Jin, Oak Ridge National Laboratory
Room: Colorado Convention Center Korbel 1C
Monday, March 5, 2007 11:15AM - 11:51AM |
B8.00001: Influence of Al doping on the superconducting properties of magnesium diboride single crystals Invited Speaker: We present a systematic study of the evolution of the lower ($H_{c1}$) and upper ($H_{c2}$) critical fields, gap values and Sommerfeld coefficient of Mg$_{1-x}$Al$_{x}$B$_2$ single crystals (for $x = 0$, $0.1$ and $\geq 0.2$). We show that $H_{c1}$ and $H_{c2}$ deduced from Hall probe magnetization and specific heat measurements, respectively are both decreasing with increasing doping content. The corresponding anisotropy parameter $\Gamma_{H_{c2}}(0) = H^{ab}_{c2}(0)/H^c_{c2}(0)$ value also decreases from $\sim 5$ in pure MgB$_2$ samples down to $\sim 1.5$ for $x \geq 0.2$ whereas $\Gamma_{H_{c1}}(0)=H^c_{c1}(0)/H^{ab}_{c1}(0)$ remains on the order of 1 in all samples. The magnetic field dependence of the anisotropy parameter $\Gamma_{H_{c1}} \leq \Gamma(H) \leq \Gamma_{H_{c2}}$ has then been deduced from a detailled analysis of the angular dependence of the Sommerfeld coefficient for different values of the applied fields. The small and large gap values have been obtained both by fitting the temperature dependence of the zero field electronic contribution to the specific heat to the two gap model and by point contact spectroscopy measurements. Both measurements led to very similar values and the evolution of those gaps with Al concentration suggests that both band filling and interband scattering effects are present.\\ This work has been performed in collaboration with : L.Lyard, C.Marcenat, J.Marcus, Z.Holanova, P.Szabo, P.Samuely, B.W.Kang, H-J.Kim, H-S.Lee, H-K.Lee,S-I.Lee, S.Tajima and S.Lee. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B8.00002: De Haas-van Alphen Effect across the CeRh$_{1-x}$Co$_x$In$_5$ Phase Diagram Swee K. Goh, Johnpierre Paglione, M. Sutherland, C. Bergemann, T. A. Sayles, M. B. Maple We present de Haas-van Alphen (dHvA) data across the phase diagram of CeRh$_{1-x}$Co$_x$In$_5$ down to ultra-low temperatures. The chemical substitution of Co for Rh, which changes the electronic structure of CeRhIn$_5$ from a localized 4\textit{f} electron configuration at $x=0$ to itinerant behaviour at $x=1$, is analogous to the application of external pressure to the antiferromagnet CeRhIn$_5$, which was shown to exhibit a discontinuous evolution of its Fermi surfaces near 2.35GPa [Shishido \textit{et al.} J Phys. Soc. Jpn. \textbf{74}, 1103 (2005)]. Exploiting the rotational degree of freedom afforded by ambient pressure measurements of single-crystal specimens, we analyze both the Fermi surface geometry and quasiparticle effective masses of CeRh$_{1-x}$Co$_x$In$_5$ as a function of chemical substitution in order to investigate the evolution of electronic structure through the antiferromagnetic quantum critical point of this system. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B8.00003: Synthesis and Characterization of High Quality Single Crystal CeCoIn$_{5}$ Tesfaye Gebre, Abebe Kebede, T.D. Dail, T. Haywood, D. Kumar, J. Abiade, D. Worku, D. Seifu, K.A. Storr, F. Drymitis Novel electronic materials play an ever-increasing role in technological applications. They cover a rather broad spectrum of materials that exhibit exotic transport and magnetic properties. These include superconductors, narrow band gap semiconductors, electronically active polymers, colossal magneto resistive systems, nanoscale electronic materials, and Fullerenes, to name a few. Understanding the interplay between strong electronic correlations and structural properties of these materials is important. In this communication, we report the~growth and preliminary characterization of single crystal~Cerium based~intermetallic compounds that are known to show superconductivity at Tc = 2.3K. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B8.00004: NMR study of the Cd-doped CeCoIn$_{5}$ heavy-fermion superconductor R.R. Urbano, N.J. Curro, J.D. Thompson, Ben-Li Young, Long D. Pham, Z. Fisk We have investigated the local environment of In and Co sites of the heavy fermion compound CeCo(In$_{1-x}$Cd$_{x}$)$_{5}$ ($x$ = 0.0, 0.10 and 0.15) using Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) measurements. Recently, it was found that Cd-doping acts as an electronic tuning agent in CeCoIn$_{5}$ and that superconductivity (SC) and antiferromagnetism (AFM) coexist at ambient-pressure for $0.05 < x < 0.15$. The NMR/NQR spectra of In and Co indicate the presence of electronic inhomogeneity, and the spin-lattice relaxation rate $1/T_{1}$ shows dramatic change in the spin dynamics only below $T \simeq 2T_{c}$. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B8.00005: Impurities induced configuration space phase transition in the Fulde-Ferrell-Larkin-Ovchinnikov state Qian Wang, Chia-Ren Hu, Chin-Sen Ting We study the effects of impurities on the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state at low temperature in d-wave superconductors. At low impurity concentration, the order parameter remains two-dimensional lattice like. When the impurity concentration reaches certain level, nodal lines of the order parameter no-longer cross each other and the variation of the order parameter becomes essentially one dimensional. We will also discuss the effects of impurities on the quasi-particle density of states. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B8.00006: Fermi-liquid effects in the Fulde-Ferrell-Larkin-Ovchinnikov state of two-dimensional d-wave superconductors Matthias J. Graf, Anton B. Vorontsov We study the effects of Fermi-liquid interactions on quasi-two-dimensional d-wave superconductors in a magnetic field. The phase diagram of the superconducting state, including the periodic Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in high magnetic fields, is discussed for different strengths of quasiparticle many-body interactions within Landau's theory of Fermi liquids. Decreasing the Fermi-liquid parameter $F_0^a$ causes the magnetic spin susceptibility to increase, which in turn leads to a reduction of the FFLO phase. It is shown that a negative $F_0^a$ results in a first-order phase transition from the normal to the uniform superconducting state in a finite temperature interval. Finally, we discuss the thermodynamic implications of a first-order phase transition for the heavy-fermion superconductor CeCoIn$_5$. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B8.00007: Evolution of FFLO state of CeCo(In$_{1-x}$Cd$_{x})_{5}$ Yoshifumi Tokiwa, Filip Ronning, Joe Thompson, Roman Movshovich, Long Pham, Zachary Fisk Unconventional superconductor CeCoIn$_{5}$ at high magnetic field displays first order superconducting transition and an additional high field-low temperature superconducting phase (previously proposed to be an inhomogeneous superconducting FFLO state). Both phenomena were attributed to strong Pauli limiting effects. Our specific heat measurements on low Cd-doping (for In) samples, at fields close to the superconducting critical field $H_{c2}$, show that superconducting transition remains first order for samples with $H_{c2}$ up to 5.7 T (from 4.95 T in a pure compound), for field out of plane orientation ($H \quad \vert \vert $ c). We discuss systematic evolution of the proposed FFLO state with Cd content. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B8.00008: Specific Heat of Na$_{0.35}$CoO$_{2}\cdot $1.3H$_{2}$O: Effects of Sample Age; Two Energy Gaps; Non-Magnetic Pair Breaking N.E. Phillips, R.A. Fisher, N. Oeschler, R.J. Cava, M.-L. Foo, J.E. Gordon Specific-heat measurements on three samples of Na$_{0.35}$CoO$_{2}\cdot $1.3H$_{2}$O show an evolution of the superconductivity and its eventual disappearance with increasing sample age. The results, in combination with other recent work [1], provide a basis for understanding the extreme ``sample dependence'' of the properties of this material. Samples of different age are in effect samples of slightly different materials: A non-magnetic pair-breaking action produces a residual density of electron states that increases with sample age. It occurs preferentially in the electron band (one of two with different energy gaps) with the smaller gap, producing a change in the nature of the superconducting condensation. It also weakens the overall electron pairing of the superconducting state until it gives way to a competing ordering, possibly a CDW. The same combinations of features in the specific heat have been seen in measurements on other individual samples, showing that they are ``intrinsic''. The changes in the specific heat are evidently related to structural and electronic changes that occur on a similar time scale [1], and include an increasing concentration of O vacancies, which could be the pair-breaking scattering centers. \newline [1] P. W. Barnes et al., Phys. Rev. B \textbf{72}, 134515 (2005). [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B8.00009: Systematic ARPES study on high Na-level Na$_{x}$CoO$_{2}$ (x$>$0.75) Y.M. Xu, Z.H. Pan, P. Richard, R. Jin, B.C. Sales, D. Mandrus, F.C. Chou, Z. Wang, H. Ding The phase diagram of the cobaltae Na$_{x}$CoO$_{2}$, with varying Na concentration x, is very rich and complicated. At the high-doping regime (x$>$0.75), the system was found to be more correlated, with a spin-density-wave state emerging at low temperatures. A previous ARPES study has observed a usually large Fermi surface which apparently violates the Luttinger theorem at x = 0.8. We will report our recent ARPES results of the high Na-level Na$_{x}$CoO$_{2}$ (x$>$0.75) samples. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B8.00010: Complete d-Band Dispersion Relation in Sodium Cobaltates Dong Qian, L. Wray, D. Hsieh, L. Viciu, R.J. Cava, J.L. Luo, D. Wu, N.L. Wang, M.Z. Hasan We utilize fine-tuned polarization selection coupled with excitation-energy variation of photoelectron signal to image the complete d-band dispersion relation in sodium cobaltates. A hybridization gap anticrossing is observed along the Brillouin zone corner and the full quasiparticle band is found to emerge as a many-body entity lacking a pure orbital polarization. At low dopings, the quasiparticle bandwidth (Fermion scale, many- body EF $\sim$ 0.25 eV) is found to be smaller than most known oxide metals. The low-lying density of states is found to be in agreement with bulk-sensitive thermodynamic measurements for nonmagnetic dopings where the 2D Luttinger theorem is also observed to be satisfied. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B8.00011: Angle-resolved photoemission study of superconducting cobalt oxide Na$_{x}$CoO$_{2}$ yH$_{2}$O Takahiro Shimojima, Kyoko Ishizaka, Syuntaro Watanabe, Shik Shin, Takayuki Kiss, Tadashi Togashi, Takayoshi Yokoya, Petre Badica, Kazuyoshi Yamada, Kazumasa Togano Superconducting cobalt oxide Na$_{x}$CoO$_{2}$ yH$_{2}$O is studied by angle-resolved photoemission spectroscopy. We report the Fermi surface topology and electronic structure near the Fermi level in the normal state of Na$_{x}$CoO$_{2}$ yH$_{2}$O. Our result indicates the presence of the hexagonal Fermi surfce centered at $\Gamma $ point, while the small pocket Fermi surfaces along $\Gamma $-K direction are absent, similar to Na$_{x}$CoO$_{2}$. The top of the e$_{g}$' band, which is expected in band calculations to form the small pocket FSs, extends to within $\sim $30 meV below Fermi level, more closer to Fermi level than in Na$_{x}$CoO$_{2}$. Its possible role in superconductivity will be discussed, comparing with other experimental and theoretical results. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B8.00012: Phase Diagrams, Thermodynamic Quantities and Possible Two Different Superconducting States of Multiorbital Superconductor NaxCoO2.yH2O Masahito Mochizuki, Masao Ogata Motivated by recently reported experimental phase diagrams, we theoretically study the effects of CoO$_6$ distortion on the electronic structure in Na$_x$CoO$_2 \cdot y$H$_2$O by constructing the multiorbital model. By analyzing the model, we show the deformation of band dispersions and Fermi-surface (FS) topology caused by the variation of CoO$_2$-layer thickness. We propose that two different pairing states, an extended $s$-wave and a $p$-wave pairings, are possible depending on the layer thickness or the FS topology. Furthermore, microscopic calculations of thermodynamic quantities show that two different specific-heat data and two distinct superfluid- density data can be explained fairly well for these two pairing states. We also discuss that inconsistent and scattered experimental results on the magnetic properties, which have been reported from NMR/NQR, $\mu$SR and neutron-scattering measurements, can also be clarified well if we consider the strong layer-thickness dependence. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B8.00013: Unified origin for superconductivity and 3D magnetism in Na$_x$CoO$_2$ Kazuhiko Kuroki, Shuhei Ohkubo, Hidetomo Usui, Takumi Nojima, Ryotaro Arita, Seiichiro Onari, Yukio Tanaka The pairing mechanism of a hydrated cobaltate superconductor Na$_x$CoO$_2\cdot y$H$_2$O has been of great interest recently. Some experiments point toward unconventional pairing, while others suggest s-wave-like pairing. Recently, based on a fluctuation exchange study for an extended Hubbard model, we have proposed a possibility of unconventional s-wave pairing, where the nesting between the outer and the inner Fermi surfaces that arise due to the local minimum structure of the band at the $\Gamma$ point plays an important role. The superconducting gap changes sign between the inner and outer Fermi surfaces due to the repulsive interaction originating from the spin fluctuations at the nesting vector, while the gap does not change sign within each Fermi surface. We have further found that this nesting becomes three dimensional when a small hopping integral in the z-direction is considered, which gives rise to an in-plane ferromagnetic, out-of-plane antiferromagnetic spin correlation, consistent with the experiments for the non-hydrated Na-rich systems. The calculated magnetic ordering temperature and the spin wave dispersion explain well the experimental results. [Preview Abstract] |
Session B9: Superconductivity: Currents and Vortex Dynamics
Sponsoring Units: DCMPChair: Paul Barnes, Wright Patterson Air Force Base
Room: Colorado Convention Center Korbel 1D
Monday, March 5, 2007 11:15AM - 11:27AM |
B9.00001: Critical current density distribution in YBCO coated conductors measured with a ``magnetic knife'' Jens H\"{a}nisch, Stephen P. Ashworth, Fred M. Mueller, Yates Coulter, Vladimir Matias We present a simple and straightforward tool for investigating the spatial $J_{c}$ distribution in YBCO coated conductors. The samples were prepared by co-evaporation and PLD on IBAD and RABiTS templates. The tool we have used is a ``magnetic knife'' [1] consisting of several Nd$_{2}$Fe$_{14}$B permanent magnets in close proximity to the superconducting tape. A 200 $\mu $m wide zone of low magnetic field $B$ is embedded in a background field of around 600~mT. This region is scanned across the sample, in the direction perpendicular to the direction of tape current flow. The critical current for each position is measured with a four-point technique at 75 K (liquid N$_{2}$ at ambient pressure). The raw data are deconvoluted with a Fourier inversion method. Several techniques for obtaining the magnetic field distribution in the magnetic knife (simulations and measurements) will be compared and discussed. Finally, differences in the current distributions for different coated conductor samples will be discussed. [1] ten Haken et al., Physica C 334, 163 (2000) [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B9.00002: Testing the limits for critical currents in YBa$_{2}$Cu$_{3}$O$_{7}$ films Leonardo Civale, Boris Maiorov, Scott Baily, Honghui Zhou, Frank Hunte, Igor Usov, Stephen Foltyn, Terry Holesinger, Quanxi Jia, Judith MacManus-Driscoll, Haiyan Wang Vortex pinning in YBa$_{2}$Cu$_{3}$O$_{7}$ films can be very strong. At low temperatures and in the absence of applied magnetic field (\textbf{H}), critical current densities J$_{c}$ of about 20{\%} of the depairing limit have been obtained. This is as high as the best achieved in commercial Nb-based superconducting wires after decades of optimization. Remarkably, similar J$_{c}$s are attained in YBa$_{2}$Cu$_{3}$O$_{7}$ films grown by various methods that produce vastly different nanostructures, suggesting that perhaps we are close to an effective J$_{c}$ limit regardless of the details of the pinning mechanisms. In contrast, the different types of pinning centers (either naturally occurring or artificially introduced by material nanoengineering) produce distinctively different J$_{c}$ behavior as a function of \textbf{H} strength and orientation. I will present a comparison of pinning mechanisms in YBa$_{2}$Cu$_{3}$O$_{7}$ films and will analyze the possibilities of further improvements. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B9.00003: Irreversibility line of YBa$_{2}$Cu$_{3}$O$_{7}$ films as a function of angle and field up to 50 Tesla S.A. Baily, B. Maiorov, F. Hunte, H. Zhou, S.R. Foltyn, Q.X. Jia, L. Civale, F.F. Balakirev, M. Jaime Studying the irreversibility line (resistivity=0) in high T$_{c}$ superconductors is scientifically and technologically relevant because the critical current drops to zero at this vortex solid-liquid transition. We have used low current transport measurements to study the irreversibility line of YBa$_{2}$Cu$_{3}$O$_{7}$ films in fields up to 50 T. Electronic mass anisotropy can describe most of the angular dependence, but fails to account for deviations along the crystalline axes. Correlated pinning causes a large increase in the irreversibility field along the $a-b$ planes, and a small $c$-axis peak. Inclusion of BaZrO$_{3}$ not only adds $c$-axis correlated defects, but increases the overall irreversibility field and alters the shape of the resistivity vs. magnetic field curve in the liquid state. We will discuss the results in terms of vortex pinning, the corresponding types of phase transitions, micro-structural analysis, and information obtained from critical current measurements. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B9.00004: AC current driven vortex dynamics in YBCO thin films and coated conductors Andrea Lucarelli, Ran Yang, Gunter Luepke, Francesco Grilli, Timothy Haugan, George Levin, Paul Barnes The effect of an AC current and a static magnetic field on the vortex dynamics in YBa$_{2}$Cu$_{3}$O$_{7-x}$ (YBCO) thin films and coated conductors is studied by time-resolved magneto-optical imaging. Our measurements show that the AC current enables the vortex lattice in the YBCO thin film to reorganize into two coexisting states with different characteristics: a quasi-static state in the sample interior and a dynamic state near the edges. Vortices and anti-vortices, induced by the current during the cycle, penetrate from the edges into the sample and interact with the pinned vortices altering the flux lattice. We compare the AC current driven vortex dynamics in YBCO thin films and coated conductors with particular focus on the AC loss characteristics of multifilamentary samples. Finite-element method (FEM) calculations adopting a recently developed method, are used to compute current density, field profiles and AC losses during the cycle. The model assumes a thermal activation of the magnetic flux that leads to a nonlinear dependence of the electric field and current density. The FEM calculations show a very good agreement with the measured data. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B9.00005: Influence of disorder on the vortex pinning and cutting of YBa$_2$Cu$_3$O$_7$ films B. Maiorov, L. Civale, Q.X. Jia, H. Zhou, S.R. Foltyn, T.G. Holesinger, S. Baily, H. Wang, J.L. MacManus-Driscoll, T.N. Haugan, P.N. Barnes Flux cutting and recombination has been used to explain high critical current densities ($J_c$) observed in experiments done in Force-Free (FF) and Variable Lorentz Force(VLF) configurations \textit{i.e.}, when the current $I$ is totally or partially aligned to the applied magnetic field $H$. However, the effect of different pinning centers has not been studied. We present angular and field $J_c$ studies in FF and VLF configurations and study the effects of random, correlated and extended defects on the $J_c$ of YBa$_2$Cu$_3$O$_7$ films. Results are analyzed in terms of vortex pinning at different defects and vortex cutting mechanism. We show that pinning greatly influences $J_c$ in FF and VLF, up to the point of obtaining $J_c$ higher than $J_c(H=0) $ up to magnetic fields as high as 3T [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B9.00006: Evolution of the vortex-solid to vortex-liquid melting line inY$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{6.96}$ and YBa$_{2}$Cu$_{3}$O$_{6.5}$ to 45 tesla B.J. Taylor, M.B. Maple By extending magneto-transport measurements to fields up to 45 tesla, we have been able to examine the vortex glass melting line of Y$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{6.96}$ (x = 0 - 0.4) thin film samples and that of an oxygen deficient YBa$_{2}$Cu$_{3}$O$_{6.5}$ single crystal over an extended temperature range, 0.03~$T_{c}~\leq~T~\leq~T_{c}$, larger than heretofore reported. The melting lines are analyzed in the context of the model of Blatter \& Ivlev (BI) [PRL $\bf{70}$, 2621 (1993)] with temperature dependent parameters, $\xi$, $\lambda$, etc. The temperature dependence of the relaxation time of a single vortex flux line, displaced by quantum/thermal fluctuations, is deduced such that the \textit{entire} melting line of each sample can be fit to smoothly by the modified expression of BI, implying that the physical mechanism responsible for the manner and conditions of the melting of the vortex solid can be described smoothly over the entire temperature $-$ field range. This research was sponsored by the DOE under Research Grant No. DE-FG02-04ER46105. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by NSF Cooperative Agreement No. DMR-0084173, by the State of Florida, and by the DOE. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B9.00007: Vortex pinning in single crystal CaC$_{6}$ Ulrich Welp, Daniel Rosenmann, Ruobing Xie, David Hinks, Helmut Claus, Goran Karapetrov, John Schlueter, Wai-Kwong Kwok, Lisa Paulius Crystals of the new graphite intercalation superconductor CaC$_{6}$ were synthesized in a liquid transport process in which graphite single crystals are exposed to an eutectic Ca-Li melt at 350 $^{\circ}$C. The resulting samples displayed a sharp superconducting transition at 11.6 K. X-ray diffraction reveals the rhombohedral CaC$_{6}$ structure with no indication of graphite second phases. The phase diagram and the vortex pinning properties were determined using magnetization and Hall magnetometry measurements. The irreversibility line for fields applied along the c-axis lies close to the upper critical field and displays down to temperatures of 2 K a linear temperature dependence with a slope of about -230 G/K. An analysis based on the Bean critical state model yields a critical current density of 10$^{4}$ A/cm$^{2}$ at 4.5 K and zero field. The effect of particle irradiation on the flux pinning properties of CaC$_{6}$ will be presented. This work was supported by the US Department of Energy, BES-Materials Sciences, under Contract DE-AC02-06CH11357. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B9.00008: Magnetic relaxation and critical currents of NdBCO coated conductors A.O. Ijaduola, S.H. Wee, A. Goyal, J. Li, P.M. Martin, J.R. Thompson, D.K. Christen A magnetic study of the critical current density $J_c$ and magnetic relaxation (`creep') effects in thin NdBa$_2$Cu$_3$O$_{7-\delta}$ (NdBCO) superconducting films of thicknesses 0.7 and 2.1 $\mu$m was conducted. These films, doped with BaZrO$_3$, were deposited by a PLD process on `IBAD' substrates. The $J_c$ values display broad peaks near the c-axis. This is associated with densely spaced columnar defects distributed about this axis. We analyzed the magnetic relaxation data using the Maley expression for the activation energy $U$ as a function of current density $J$. The data are described fairly well by the collective creep interpolation formula $U=U_0\left[\left(J_{c0}/J\right)^\mu-1\right]$ with $U_0$, $J_{c0}$, and $\mu$ treated as fitting parameters. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B9.00009: Vortex Phases in Mesoscopic Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ Single Crystals in Magnetic Fields Near \textit{ab}-plane Jovan Mirkovic, Yuimaru Kubo, Masahi Kohri, Kohei Kawamata, Takashi Yamamoto, Itsuhiro Kakeya, Kazuo Kadowaki In order to study the vortex matter in the layered superconductors in magnetic fields parallel to the \textit{ab}-plane, we performed the in-plane resistivity and the $c$-axis resistivity measurements on the bulk and mesoscopic Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ single crystals. In the bulk samples, a boundary between the strong pinning phase and the weak pinning vortex phase was found at about 3$^{o}$ away from the \textit{ab}-plane, indicating possible crossover from the vortex chain + lattice phase into tilted (vortex chain) phase. The vortex phase in the parallel magnetic fields, exhibited a strongly non-Ohmic behavior, indicating the possible two-stage melting phase transition. It was found that the vortex lock-in transition in mesocopic crystals, becomes considerably broad in high magnetic fields, while exhibiting the sharp features in low magnetic field region. The first penetration field of vortex pancakes demonstrates a nontrivial field dependence. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B9.00010: Peak Effect in Polycrystalline Vortex Matter Ivo Dimitrov, Nikos Daniilidis, Charles Elbaum, Jeff Lynn, Xinsheng Ling The peak effect (PE) in $J_{c}$ in weakly-disordered type-II superconductors is believed to mark the transition between a disordered vortex state and a quasi-ordered Bragg glass regime. Some strongly-disordered type-II superconductors, such as the binary alloy V-21at.\%Ti, also exhibit ``peak effect'' at temperatures close to $H_{c2}$, despite lack of atomic long-range order and presence of sample composition inhomogeneities. SANS field-cooled measurements on a V-21at.\%Ti sample show that both deep in the mixed state and close to the PE transition, there exist no long-range orientationally-ordered vortex lattices (VL's). The neutron scattering data analysis shows that the diffraction radial widths do not change significantly as a function of field, suggesting that VL states ordered on the scale of $\mu m$ exist. We conjecture that the ``peak effect'' in V-21at.\%Ti corresponds to the disordering of ordered VL Larkin domains. The V-21at.\%Ti peak effect phase diagram is mapped via ac susceptometry. This measurement reveals that the peak effect disappears below a certain field, as has been reported in other superconductors. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B9.00011: Magnetocaloric Studies of the Peak Effect in Nb Nikos Daniilidis, Ivo Dimitrov, Vesna Mitrovi{\'c}, Charles Elbaum, Xinsheng Ling We report a magnetocaloric study of the peak effect and Bragg glass transition in a Nb single crystal. The thermomagnetic effects due to vortex flow into and out of the sample are measured. The magnetocaloric signature of the peak effect anomaly is identified. It is found that the peak effect disappears in magnetocaloric measurements at fields significantly higher than those reported in previous ac-susceptometry measurements. Investigation of the superconducting to normal transition reveals that the disappearance of the bulk peak effect is related to inhomogeneity broadening of the superconducting transition. The emerging picture also explains the concurrent disappearance of the peak effect and surface superconductivity, which was reported previously in the sample under investigation. Based on our findings we discuss the possibilities of multicriticality associated with the disappearance of the peak effect. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B9.00012: Collapse of the critical state in superconducting niobium Ruslan Prozorov, Daniel V. Shantsev, Roman G. Mints Giant abrupt changes in the magnetic flux distribution in niobium foils were studied by using magneto-optical visualization, thermal and magnetic measurements. Uniform flux jumps and sometimes almost total catastrophic collapse of the critical state are reported. Results are discussed in terms of thermomagnetic instability mechanism with different heat removal channels. Video figures are available at: http://www.cmpgroup.ameslab.gov/supermaglab/video/Nb.html [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B9.00013: Analysis of collective pinning and depinning of the flux line lattice in pristine 2H-NbSe$_{2}$ Jonghee Lee, Hui Wang, Michael Dreyer, Barry I. Barker Larkin and Ovchinnikov predicted collective pinning of the flux line lattice (FLL) in type II superconductors several decades ago. The collective pinning results from the interplay between strong vortex-vortex interaction and randomly distributed weak pinning centers in a media. The evidence of collective pinning was previously observed at a magnetic field, H, close to H$_{c2}$ in current-driven transport experiments on the macroscopic scale. But there still exists a lack of understanding of collective pinning on the microscopic level. In this talk, we show collective pinning and depinning of the FLL in pristine 2H-NbSe$_{2}$ in a long time series (15 days), measured by a low temperature scanning tunneling microscope. We observed the motion of the FLL within an area of 400 nm $\times $ 400 nm, with an initial magnetic field of 0.5 T. The motion was caused by the very slow decay of magnetic field ($\sim $ 5 nT/s) in a defective superconducting magnet. The average speed of FLL was $\sim $ 2.5 pm/s, lower than previously reported. Using highly time resolved data, we will further discuss the average direction of motion, the strength of pinning centers in pristine 2H-NbSe$_{2}$, flux line mass, and the difference between current-driven and field-driven FLL motions. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B9.00014: Paramagnetic effect in nano-opal-lead structure Fulin Zuo, Hengsheng Zhang, Di Wu, Jing Shi We report magnetic studies of the paramagnetic effect observed in the superconducting nano-structured opal-lead system. Positive magnetization is clearly observed when the sample is cooled in field. The paramagnetic effect is strongly dependent on the applied field, cooling rate and the background magnetization. The results suggest that the paramagnetic moment is due to flux trapping and the competition between the positive and negative moments due to the temperature dependence of penetration depth. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B9.00015: A concept of the upper threshold field for dendritic vortex avalanches in superconductors. Vitaliy Yurchenko, D.V. Shantsev, I. Maasilta, K. Senapati, T.H. Johansen Dendritic vortex avalanches in superconductors (SC), associated with thermo-magnetic instabilities, are known to destroy a metastable critical state and severely detriment performance of SC applications. Several thermo-magnetic models have been proposed to determine conditions under which a superconductor is to undergo a dendritic magnetic avalanche. However, there remains an omnipresent experimental fact that has not been given a theoretical explanation: instabilities always disappear above some upper threshold field. Our recently developed model [Phys. Rev. Lett. 97, 077002 (2006)] predicts divergence of the threshold field at low values of the critical current j$_{c}$. In its turn, at increasing magnetic field j$_{c}$ becomes strongly suppressed by the field. We explain disappearance of the dendritic avalanches in terms of the divergence of the upper threshold field due to a strong dependence of the critical current on magnetic field j$_{c}$(B) and support it by the results of our recent magneto-optical investigations of dendritic flux avalanches in NbN thin films. We then verify the model in a range of controllably varied j$_{c}$ values. [Preview Abstract] |
Session B10: Non-Fermi Liquids
Sponsoring Units: DMPChair: Anton Burkov, Harvard University
Room: Colorado Convention Center Korbel 1E
Monday, March 5, 2007 11:15AM - 11:27AM |
B10.00001: Nonanalytic Magnetic Response of Fermi- and non-Fermi Liquids Andrey Chubukov, Dmitrii Maslov, Ronojoy Saha We revisit the issue of the non-analytic dependence of the static spin susceptibility of a 2D Fermi liquid on temperature and a magnetic field, $\chi_s (T, H) = \chi_0 + A T f_\chi (\mu_B |H|/T)$. We show that in a generic Fermi liquid the prefactor $A$ is expressed via complex combinations of the Landau parameters, and does not reduce to the backscattering amplitude, contrary to the case of the specific heat $C(T, H)$. We show that this distinction with the specific heat is mostly relevant near a ferromagnetic QCP -- the non-analytic terms in $\chi_s (T,H) $ are less singular near QCP than those in $C(T, H)$. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B10.00002: Derivation of the Marginal Fermi Liquid for the Cuprates Vivek Aji, Chandra Varma The statistical mechanics of the time-reversal and inversion symmetry breaking order parameter, possibly observed in the pseudogap region of the phase diagram of the Cuprates,can be represented by the Ashkin-Teller model. We add kinetic energy and dissipation to the model for a quantum generalization and show that the correlations are determined by two sets of charges, one interacting locally in time and logarithmically in space and the other locally in space and logarithmically in time. The quantum critical fluctuations are derived and shown to be of the form postulated in 1989 to give the marginal fermi-liquid properties. The model solved and the methods devised are likely to be of interest also to other quantum phase transitions. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B10.00003: Pomeranchuk instabilities of Fermi fluids in the spin channel Kai Sun, Congjun Wu, Eduardo Fradkin, Shou-Cheng Zhang We study the Pomeranchuk instabilities of the Fermi surface in the spin channel. It is shown that the instabilities will lead to two classes of the ordered phases, the $\alpha$ and $\beta$-phases, named by analogy to the superfluid $^3$He-A and B-phases. The Fermi surfaces in the $\alpha$-phases exhibit spontaneous anisotropic distortions, while those in the $\beta$-phases remain circular with non-trivial spin configurations in momentum space. The low energy excitations of the ordered phases are studied by RPA approximation. In the $\alpha$-phases, the density excitations exhibit anisotropic overdamping and the spin density excitations are nearly isotropic and underdamped at small propagating wavevectors. The $\beta$-phases shows a Lifshitz- like instability in the $p$-wave channel, and will stabilize a chiral ground state inhomogeneity. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B10.00004: Theory of non-fermi liquid in a diagonal electronic nematic state Hae Young Kee, Ying-Jer Kao We study the fluctuation effects of the diagonal electronic nematic order on a two dimensional square lattice. It has been shown that there exists a quantum critical point between the diagonal nematic and isotropic phases.[1] We study the correlations near the critical point, where the singular forward scattering leads to a non-Fermi liquid behavior over the whole Fermi surface except along the (0,$\pi$) and ($\pi$,0) directions. We will also discuss the decay rate of the single-particle excitations as functions of temperature and chemical potential. \newline \newline [1] Hyeonjin Doh, Nir Friedman, Hae-Young Kee, Phys. Rev. B 73, 125117 (2006) [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B10.00005: Diamagnetism of nodal fermions in semimetals: graphene and significant others Amit Ghosal, Pallab Goswami, Sudip Chakravarty Nodal fermionic excitations are interesting examples of the simplest fermionic quantum criticality in which the dynamic critical exponent $z=1$, and the quasiparticles are well defined. They arise in a number of physical contexts. We derive the scaling form of the diamagnetic susceptibility, $\chi$, at finite temperatures, $T$, and finite chemical potential, $\mu$. From measurements in graphene, or in $Bi_{1-x}Sb_{x}$ ($x=0.4$), one may be able to infer the striking quantum critical Landau diamagnetic susceptibility of the system at $T=0$ and $\mu=0$, $\chi\propto - H^{-1/2 }, \; H\to 0$, where $H$ is the magnetic field. Although the quasiparticles in the mean field description of the proposed $d$-density wave (DDW) condensate in a high temperature superconductors is another example of nodal quasiparticles, the crossover from the high temperature behavior, $\chi\propto - T^ {-1} $, and the quantum critical behavior takes place at a far lower temperature due to the reduction of the velocity scale from the fermi velocity $v_{F}$ in graphene to $\sqrt{v_{F}v_{DDW}}$, where $v_{DDW}$ is the velocity in the direction orthogonal to the nodal direction at the Fermi point of the spectra of the DDW condensate. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B10.00006: Self-energy corrections to anisotropic Fermi surfaces Rafael Roldan, M. Pilar Lopez-Sancho, Francisco Guinea, Shan-Wen Tsai The electron-electron interactions affect the low-energy excitations of an electronic system and induce deformations of the Fermi surface. These effects are especially important in anisotropic materials with strong correlations, such as copper oxides superconductors or ruthenates. In this talk I will analyze the deformations produced by electronic correlations in the Fermi surface of anisotropic two-dimensional systems, treating the regular and singular regions of the Fermi surface on the same footing. Simple analytical expressions are obtained for the corrections, based on local features of the Fermi surface, as the Fermi velocity and curvature. It will be shown that, even for weak local interactions, the behavior of the self-energy is non trivial, showing a momentum dependence and a self-consistent interplay with the Fermi surface topology. Applications of the method to cuprates- and Sr$_{2}$RuO$_{4}$-like Fermi surfaces will be shown. R. Roldan, M.P. Lopez-Sancho, F. Guinea and S.-W. Tsai; cond-mat/0603673 [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B10.00007: Gradient Expansion approach to interacting Fermi Liquids Ryuichi Shindou, Leon Balents Starting from the Keldysh equation for a general multiple band Fermi liquid (FL), we project out fully occupied / empty bands and derive the SU(2) reduced Keldysh equation (RKE), only to discuss the low-energy property of those quasi-particles which are constrained within a single Fermi surface. The RKE thus derived characterizes quasi-particle dynamics in terms of Berry's curvatures defined in the (d+1) dual space. Namely, in addition to the well-studied ``k-space magnetic field'', our Fermi liquid formulations naturally introduce ``k-space electric field'', as the Berry's curvature in frequency and momentum space. When solving the derived RKE in favor of spectral functions, we observed that these artificial electromagnetic fields enter into the linear response of the spectral weight against real applied electromagnetic fields. This theoretical observation naturally lets us raise several photoemission experiments as the candidate experimental tool to measure both U(1) and SU(2) artificial fields in a momentum resolved way, which is widely demanded from the (spin) galvanomagnetic community. Restricting ourselves to the U(1) FLs, we further derive the U(1) Boltzmann equation out of this RKE, to find that not only the artificial magnetic field but also the electric fields enters into the effective EOM for quasi-particles as the Lorentz force in k-space. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B10.00008: Effect of Nonmagnetic Impurity in Nearly Antiferromagnetic Fermi Liquid: Magnetic Correlations and Transport Phenomena Hiroshi Kontani, Masanori Ohno In nearly AF metals such as high-Tc superconductors (HTSCs), heavy fermion systems and organic superconductors, a single nonmagnetic impurity frequently causes nontrivial widespread change of the electronic states. To elucidate this long-standing issue, we study a Hubbard model with a strong onsite impurity potential based on an improved fluctuation-exchange (FLEX) approximation, which we call the $GV^I$-FLEX method. We find that (i) both local and staggered susceptibilities are strongly enhanced around the impurity. By this reason, (ii) the quasiparticle lifetime as well as the local DOS are strongly suppressed in a wide area around the impurity (like a Swiss cheese hole), which causes the ``huge residual resistivity'' beyond the s-wave unitary scattering limit. We stress that the excess quasiparticle damping rate caused by impurities has strong momentum-dependence due to non-s-wave scatterings induced by many-body effects, so the structure of the ``hot spot/cold spot'' in the host system persists against impurity doping. This result could be examined by the ARPES measurements. In addition, (iii) only a few percent of impurities can causes a ``Kondo-like'' upturn of resistivity ($d\rho/dT<0$) at low temperatures when the system is very close to the AF quantum critical point (QCP). We also discuss the impurity effect in the superconducting state. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B10.00009: Kondo physics in a dissipative environment K. Ingersent, M. T. Glossop, N. Khoshkhou In recent years impurity models with quantum critical points have attracted much interest. Well-studied examples include the pseudogap and Bose-Fermi Kondo models. In the former model, the depletion of the host density of states at the Fermi level can destroy the Kondo effect; in the latter case, Kondo screening competes with coupling to a dissipative bosonic bath representing, e.g., collective spin fluctuations of the host. The physics of both models is dominated by an interacting quantum critical point. Here, we focus on the more general case of a magnetic impurity interacting with a pseudogap fermionic density of states $\rho(\epsilon)\propto |\epsilon|^r$ and with a bosonic bath having a spectral function $B(\omega)\propto \omega^s$. Perturbative renormalization-group (RG) studies of the resulting model, discussed in relation to Kondo temperature suppression in underdoped cuprates [1], have established a rich phase diagram with three stable and two critical fixed points. We report nonperturbative results for this model, obtained using a Bose-Fermi numerical RG approach [2]. We discuss the phase diagram for the Ising-anisotropic case, together with quantum critical properties probed via response to a local magnetic field. [1] M.\ Vojta and M.\ Kir\'{c}an, PRL {\bf 90}, 157203 (2003). [2] M.\ T.\ Glossop and K.\ Ingersent, PRL {\bf 95}, 067202 (2005); PRB (2006). [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B10.00010: Nonequilibrium steady-state density of states for a strongly correlated electron system in the presence of a large electric field. Alexander Joura, Jim Freericks The electronic density of states (DOS) of the Falicov-Kimball model in a constant uniform electric field $E$ is calculated using a Kadanoff-Baym-Keldysh nonequilibrium Green's function technique and dynamical mean-field theory. When the electron-electron interaction $U$ vanishes, the DOS is the Wannier-Stark ladder of delta functions spaced by the Bloch frequency. If $U$ is increased, the delta function peaks initially broaden due to the scattering, but ultimately evolve into a continuous structure for large $U$'s. As $E$ is increased from small values, where linear response theory can be used and we see broadened Wannier-Stark peaks, the DOS develops a shape with large peaks at miniband edges, separated in energy by $U$. We verify the accuracy of our calculations by checking the DOS against frequency-moment sum rules, and an independent transient-response calculation of the Green's functions at long times. While our formalism has been applied to the Falicov-Kimball model, it can also be directly extended to other models like the Hubbard or periodic Anderson model, by using more complicated impurity problem solvers. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B10.00011: Experimental Evidence of Spin-Incoherent Luttinger Liquid State in Semiconductor Quantum wires Mustafa Muhammad, Steven Herbert, Richard Newrock, Philippe Debray We have measured the Coulomb drag between two spatially separated parallel quantum wires in the absence of tunneling to experimentally probe the recently proposed spin-incoherent Luttinger liquid (SILL) state. This new state is considered to exist in one-dimensional electron systems when the electron density is sufficiently low and the electron-electron interaction is strong, leading to \textit{J$<<$T$<<$E}$_{F}$, where $J $is the exchange coupling of spins and $E_{F}$ the Fermi energy. The measured drag resistance in the strictly one-dimensional (1D) transport regime is found to follow a power-law temperature dependence with a negative exponent (-0.65) in the temperature range 70mK -- 1.2K, in excellent agreement with the prediction of SILL theory for 4k$_{F}$ backscattering. The drag resistance is also found to decay exponentially with wire separation confirming the backscattering nature of the momentum transfer process. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B10.00012: Hall effect in strongly correlated low dimensional systems Gladys Leon, Christophe Berthod, Thierry Giamarchi We investigate the Hall effect in a quasi one-dimensional system made of weakly coupled Luttinger liquids at half filling. A memory function approach is used to compute the Hall resistivity ($R_H$) in the presence of umklapp scattering along the chains. In this approximation, the Hall resistivity decomposes into two terms linear in the magnetic field: an infinite frequency limit term and a memory function term. We investigate the case of zero umklapp scattering, where the memory function vanishes and the Hall resistivity is given by a simple formula corresponding to non-interacting fermions, in agreement with former results made on weakly coupled Luttinger Liquids in the absence of dissipation along the chains. With umklapp scattering present, we find a negative power-law correction to the free-fermion value (band value), with an exponent depending on the Luttinger parameter $K_{\rho}$. We also calculate $R_H$ for the case of noninteracting fermions with umklapp scattering present using Feymnan diagrams to compare with the limit $K_{\rho} \to 1$ of the power-law result. At high enough temperature or frequency, the Hall coefficient approaches the band value $R_H^0$. cond-mat/0608427 [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B10.00013: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 1:51PM - 2:03PM |
B10.00014: ABSTRACT WITHDRAWN |
Session B11: Exotic Quantum Phases and Transitions
Sponsoring Units: DMPChair: Shailesh Chandrasekharan, Duke University
Room: Colorado Convention Center Korbel 1F
Monday, March 5, 2007 11:15AM - 11:27AM |
B11.00001: Spinon Deconfinement at the Quantum Critical Point of $2+1$ D Antiferromagnets Zaira Nazario, David I. Santiago The natural spin 1 excitations of $2+1$ D antiferromagnets are made of constituent confined quarks of spin 1/2, spinons. The quantum paramagnetic phase possesses quantum tunneling events or instantons, which confine the spinons. There have been recent suggestions of new critical points where spinons are deconfined. Instanton events which cause the spinon confinement disappear at the deconfined critical point because the massless spinons screen them effectively and because instanton tunneling becomes infinitely costly. We point out that this happens irrespective of the intrinsic spin of the antiferromagnet. Hence spinons are deconfined irrespective of microscopic spin. Berry phase terms relevant to the paramagnetic phase make the confinement length scale diverge more strongly for half-integer spins, next strongest for odd integer spins, and weakest for even integer spins. There is an emergent photon at the deconfined critical point, but the ``semimetallic'' nature of critical spinons screens such photon making it irrelevant to long distance physics and the deconfined spinons behave as strictly free particles. A unique prediction critical free spinons at the quantum critical point is an anomalous exponent $\eta$ for the susceptibility exactly equal to one. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B11.00002: Experimental Consequences of O(3) Deconfined Criticality in 2+1 D Antiferromagnets David I. Santiago, Zaira Nazario The paramagnetic phase of 2+1 D antiferromagnets can be described in terms of electrodynamics of charged, massive bosonic spinons interacting through an emergent compact U(1) gauge field. Spinons in the paramagnet are confined due to the presence of nontrivial tunneling effects, instantons which provide a long range interaction between the gauge fields and the charges that gaps the gauge fields and provides a linear potential for the charges. The instantons responsible for spinon confinement in the paramagnetic phase vanish at the quantum critical point. Therefore, spinons are deconfined at criticality. We have recently obtained the effective theory that describes the universal physics of these deconfined critical points. From the deconfined critical theory, we calculate the critical Neel field propagator and find a critical exponent eta=1. We also obtain measurable effects and quantities that follow from the prediction eta=1 and serve as characterization of O(3) deconfined criticality. Those are the inelastic and elastic neutron scattering response, Nuclear Magnetic Resonance (NMR) response, magnetic field response and the specific heat. All of these response functions serve to define the O(3) deconfined universality class. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B11.00003: Emergent supersymmetry at a critical point of a lattice model Sung-Sik Lee We present a two dimensional lattice model which exhibits an emergent space-time supersymmetry at a critical point. The lattice model consists of spinless fermion on the honeycomb lattice and boson on the triangular lattice which is dual to the honeycomb lattice. It will be shown that there is only one relevant perturbation at the supersymmetric critical point and the critical theory becomes the 2+1 dimensional N=2 Wess-Zumino theory with two copies of chiral multiplets. Exact values of scaling dimensions can be obtained due to the emergent superconformal symmetry although the critical theory is the interacting theory. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B11.00004: Critical exponents in a transition between an AFM and a valence bond crystal Samuel Moukouri, Kenneth Graham We use the two-step density-matrix renormalization group method to extract the critical exponents $\beta$ and $\nu$ in the transition from a N\'eel $Q=(\pi,\pi)$ phase to a magnetically disordered phase with a spin gap. We find that the exponent $\beta$ computed from the magnetic side of the transition is consistent with that of the classical Heisenberg model, but not the exponent $z\nu$ computed from the disordered side. We also show the contrast between integer and half-integer spin cases. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B11.00005: Self-optimized resonating-valence-bond trial wavefunctions Kevin Beach, Anders Sandvik The spin singlet ground state of a quantum antiferromagnet can be expanded in the overcomplete basis of valence bond states.~[1] To first approximation, the weight associated with each configuration is factorizable into a product of individual bond amplitudes. For nonfrustrated antiferromagnets with local interactions, mean field calculations indicate that the amplitudes are generically powerlaw in the bond length with exponent d+1, where d is the dimension of the lattice. Such states can be employed as the initial trial state for a valence bond projector calculation of the exact ground state.~[2] Moreover, the amplitudes can be determined self- consistently by measuring the statistics of the bonds appear in the projected state and feeding this information back into the trial state. It is also possible to build some of the neglected bond-bond correlations into the trial state itself. The next level of approximation is to factorize the weights in terms of amplitudes that depend on the lengths and orientations of two valence bonds. Again, these amplitudes can be self-optimized in a simulation by matching them to the bond-- bond correlations of the projected state. \newline [1] K.\ S.\ D.\ Beach and A.\ W.\ Sandvik, Nucl.\ Phys.\ B \textbf{750}, 142 (2006).\newline [2] A.\ W.\ Sandvik, Phys.\ Rev.\ Lett.\ \textbf{95}, 207203 (2005). [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B11.00006: Simulating finite-momentum states of quantum spin systems in the valence bond basis Anders Sandvik, Kevin Beach Quantum spin systems such as the Heisenberg model can be simulated numerically in the valence bond basis, as an alternative to the standard basis of eigenstates of the $S^z_i$ operators [1]. One advantage of this approach is that also the triplet sector can be studied based on the configurations generated in the singlet sector [1,2]. This way an improved estimator for the singlet-triplet gap can be constructed. Here we show that also finite-momentum triplet states can be studied [in practice for $q$ close to $0$ or $\mathbf{\pi}$ due to a phase problem], thus allowing us to calculate the triplet dispersion $E(q)$. Matrix elements $\langle T(q)|S^z_q|0\rangle$ are also accessible. These matrix elements give directly the magnon weight in the dynamic structure factor $S(q,\omega)$. We also discuss how deconfined spinon excitations can be detected in this approach. \hskip10.5cm [1] A. W. Sandvik, Phys. Rev. Lett. \textbf{95}, 207203 (2005).\hfill\break [2] K. S. D. Beach and A. W. Sandvik, Nucl. Phys. B \textbf{750}, 142 (2006). [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B11.00007: Spin-liquid phase in a spin-1/2 quantum magnet on the kagome lattice Sergei Isakov, Yong Baek Kim, Arun Paramekanti We study a model of hard-core bosons with short-range repulsive interactions at half filling on the kagome lattice. This model is equivalent to an easy-axis spin-$1/2$ quantum model with no special conservation laws. Using quantum Monte Carlo numerics, we find that this model exhibits a continuous superfluid-insulator quantum phase transition, with exponents $z=1$ and $\nu=0.67(5)$. We show unambiguously that the insulator is a Z$_2$ fractionalized spin liquid phase with short-ranged density and bond correlations, topological order, and exponentially decaying spatial vison correlations. In addition, we map out the finite temperature phase diagram. A Kosterlitz-Thouless finite temperature superfluid-insulator transition becomes strongly first order as the strength of the repulsive interactions increases. This is consistent with the zero temperature transition to the fractionalized phase. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B11.00008: First-order phase transition in a gauge theory of $S=1/2$ quantum antiferromagnets in the deep easy-plane limit Asle Sudbo, Steinar Kragset, Eivind Smorgrav, Joakim Hove, Flavio Nogueira We perform large-scale Monte Carlo simulations on an effective gauge theory for a deep easy-plane antiferromagnet, including a Berry phase term that projects out the $S=1/2$ sector. Without a Berry phase term, the model exhibits a phase transition in the $3DXY$ universality class associated with proliferation of gauge-charge neutral $U(1)$ vortices. The instantons that eliminate the phase transition in the gauge-charged sector are suppressed by the Berry phases. The result is a {\it first-order} phase transition. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B11.00009: Non-Abelian Anyon Superconductivity Waheb Bishara, Chetan Nayak Non-Abelian Anyons are proposed to exist in certain spin models and in Quantuam Hall systems at certain filling fractions. In this work we studied the ground state of dynamical $SU(2)$ level $\kappa$ Chern Simons non-abelian anyons at finite density and no external magnetic field. We find that in the large $\kappa$ limit the topological interaction induces a pairing instability and the ground state is a superconductor with $\it{d+id}$ gap symmetry. We also develop a picture of pairing for the special value $\kappa=2$ and argue that the ground state is a superfluid of pairs for all values of $\kappa$. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B11.00010: Strongly correlated fermions on frustrated lattices Frank Pollmann, Kirill Shtengel, Joseph Betouras, Erich Runge, Peter Fulde Systems with frustrated interactions are generally characterized by a high density of low--lying excitations which leads to a high susceptibility and thus interesting physical effects. We study a novel class of strongly correlated fermions on frustrated lattices which allows for excitations which carry fractional charges [1]. For a systematic study, we firstly consider a model of spinless fermions on a geometrically frustrated planar pyrochlore (checkerboard) lattice. An effective Hamiltonian is derived for the strongly correlated limit which describes the low--lying excitations. We solve the fermionic sign problem for the latter Hamiltonian and thus make it possible to apply quantum Monte Carlo methods [3]. The ground state is shown to be charged ordered and fractional charges are linearly confined. Secondly, we consider a model of spinful fermions on the kagome lattice and study the interplay between charge -- and spin -- degrees of freedom. [1] P.~Fulde, K.~Penc, and N.~Shannon, Annalen der Physik {\bf 11}, 892 (2002) [2] E.~Runge and P.~Fulde, Phys. Rev. B {\bf 70}, 245113 (2004) [3] F.~Pollmann, J.~J.~Betouras, K.~Shtengel, and P.~Fulde, Phys. Rev. Lett. \textbf{97}, 170407 (2006) [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B11.00011: Spinless Fermions on a Checkerboard Lattice Kirill Shtengel, Frank Pollmann, Joseph Betouras, Peter Fulde We present a study of the low-energy physics of a spinless fermionic model on a checkerboard lattice at half-filling. The bosonic version of this model has been recently studied and found to have several unusual features. Fermionic models tend to be more interesting: the inherent sign problem resulting from the fermionic statistics makes them notoriously difficult to handle. The low-energy physics of the model can be described by a fermionic quantum loop model on the square lattice. We found a non-local transformation that can, in certain cases, cure the sign problem. We also identified a large class of fluctuationless states specific to the fermionic models -- a result hinting at a possible explanation of the extended ground-state entropy recently found in a few other fermionic models. Finally, we looked at the so-called Rokhsar-Kivelson quantum critical point, where we found the exact ground state(s) as well as studied the low-lying excitations. This allowed us to make several educated guesses about the phase diagram for the model in question. [1] F.~Pollmann, J.~J.~Betouras, K.~Shtengel, and P.~Fulde, Phys. Rev. Lett. \textbf{97}, 170407 (2006) [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B11.00012: Spinless charges on the triangular lattices in the strong repulsion limit: possibility of a new charge ordered liquid Nobuo Furukawa, Chisa Hotta We propose a new type of charge ordered liquid state in the spinless fermion system on a triangular lattice under strong inter-site Coulomb interactions, $V$. In the absence of fermion hoppings, the ground state is disordered due to geometrical frustration. Introduction of hopping terms lifts the degeneracy and drives the system to a metalic state with possible partial charge orders, which we call a ``pinball liquid''. There, a gapless charge liquid component moves around a possible long range ordered Wigner crystal solid component. This liquid state is dominant over wide range of filling, $n=1/3 \sim 2/3$. When an anisotropy in $V$ exceeds its critical value at half-filling $n=1/2$, an metal-insulator transition accompanied by another charge order with a different periodicity is induced. Relevance to the organic conductors $\theta$-ET$_2$X which show novel nonlinear transport properties is discussed.\\ REFERENCES:\\ \qquad cond-mat/0605045, cond-mat/0607181, cond-mat/0607717. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B11.00013: Local density of states in electronic nematic phase Hyeonjin Doh, Hae-Young Kee We study spatial patterns of local density of states in electronic nematic phase in the presence of a non-magnetic impurity. Since the Fourier transform of the spatial pattern represents the symmetry of an electronic structure of a system, the local density of state can be a direct probe for the isotropic-nematic phase transition. In this work, we show local density of states near the nematic-isotropic phase transition tuned by a magnetic field, and discuss its application to the bilayer ruthenate, Sr$_3$Ru$_2$O$_7$. [Preview Abstract] |
Session B12: Focus Session: Spin Injection
Sponsoring Units: GMAG DMP FIAPChair: Paul Crowell, University of Minnesota
Room: Colorado Convention Center Korbel 3C
Monday, March 5, 2007 11:15AM - 11:51AM |
B12.00001: Highly Efficient Room Temperature Spin Injection Using Spin Filtering in MgO Invited Speaker: Efficient electrical spin injection into GaAs/AlGaAs quantum well structures was demonstrated using CoFe/MgO tunnel spin injectors at room temperature. The spin polarization of the injected electron current was inferred from the circular polarization of electroluminescence from the quantum well. Polarization values as high as 57{\%} at 100 K and 47{\%} at 290 K were obtained in a perpendicular magnetic field of 5 Tesla. The interface between the tunnel spin injector and the GaAs interface remained stable even after thermal annealing at 400 $^{o}$C. The temperature dependence of the electron-hole recombination time and the electron spin relaxation time in the quantum well was measured using time-resolved optical techniques. By taking into account of these properties of the quantum well, the intrinsic spin injection efficiency can be deduced. We conclude that the efficiency of spin injection from a CoFe/MgO spin injector is nearly independent of temperature and, moreover, is highly efficient with an efficiency of $\sim $ 70{\%} for the temperature range studied (10 K to room temperature). Tunnel spin injectors are thus highly promising components of future semiconductor spintronic devices. \newline \newline Collaborators: Roger Wang$^{1, 3}$, Gian Salis$^{2}$, Robert Shelby$^{1}$, Roger Macfarlane$^{1}$, Seth Bank$^{3}$, Glenn Solomon$^{3}$, James Harris$^{3}$, Stuart S. P. Parkin$^{1}$ \newline $^{1 }$\textit{IBM Almaden Research Center, San Jose, CA 95120} \newline $^{2}$\textit{ IBM Zurich Research Laboratory, S\"{a}umerstrasse 4, 8803 R\"{u}schlikon, Switzerland } \newline $^{3}$\textit{ Solid States and Photonics Laboratory, Stanford University, Stanford, CA 94305} [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B12.00002: Electrical spin injection from Fe into Al$_{x}$Ga$_{1-x}$As quantum well spin-LEDs Imran Khan, Manuel Diaz-Avila, Mesut Yasar, Athos Petrou, Aubrey T. Hanbicki, George Kioseoglou, Berend T. Jonker We have studied Fe spin LEDs in which electron-hole recombination takes place either in GaAs or in Al$_{x}$Ga$_{1-x}$As quantum wells (QW). The dependence of the electroluminescence circular polarization P on temperature T in these two types of devices at fixed magnetic field is compared. The polarization in the Al$_{x}$Ga$_{1-x}$As QW LEDs decreases much more slowly with temperature compared with the GaAs QW LEDs; the polarization of the former persists up to room temperature. The improved high temperature performance of the Al$_{x}$Ga$_{1-x}$As spin LEDs is tentatively attributed to the localization of the recombining electron-hole pairs by potential fluctuations in the QW. These sites have zero-dimensional character suppressing the Dyakonov-Perel spin scattering mechanism. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B12.00003: Spin injection from Fe into GaAs quantum wells populated by electrons or holes: A comparison A. Petrou, M. Yasar, I. Khan, M. Diaz-Avila, G. Kioseoglou, A.T. Hanbicki, B.T. Jonker We have studied the circular polarization of band-edge electroluminescence (EL) from three types of AlGaAs(n)/GaAs(i)/AlGaAs(p) light emitting diodes (LEDs) in which the electrons are injected from ferromagnetic Fe contacts. In the first (second) group the GaAs quantum well is populated by electrons (holes) due to excess n-type (p-type) doping in the n-AlGaAs (p-AlGaAs) barrier. In the third device type the GaAs quantum wells are empty and these LEDs are used as reference samples. We have compared the magneto-optical characteristics (dependence of the EL circular polarization P as function of magnetic field, current, and photon energy) of these three groups. Significant differences have been identified which must be taken into account in order to determine accurately the spin injection efficiency of these devices. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B12.00004: Generation and detection of spin current in GaAs with MgO tunnel barriers Replace Y.J. Park, M. van Veenhuizen, J.S. Moodera, C.H. Perry, D. Heiman The MgO tunnel barrier has been proven as one of best candidates for the spintronic memory and switching devices. When one injects and detects spin polarized carriers efficiently into (and out of) semiconductors, the use of tunnel barrier (TB) is expected to avoid the conductivity mismatch and provide a high feasibility for the fabrication of a spin transistor. To reach this goal evaluation of the TB on a semiconductor is an important issue. In this work, we report the combination of spin dependent photocurrent generation and electrical detection as an efficient technique for understanding the role of the MgO TB grown on GaAs. We used (100)GaAs/MgO/Fe structures prepared in an MBE chamber. Our results show that spin filtering effects are largely influenced by the quality of MgO TBs. The estimated photocurrent polarization reaches up to approximately 80{\%} at RT in a certain forward bias region which is associated with transport processes. The efficient room temperature spin filtering for GaAs/MgO/Fe structures observed here has not been reported yet for either Fe/GaAs or Fe/Al$_{2}$O$_{3}$/GaAs structure. The possible origin will be discussed in detail. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B12.00005: Electrical spin injection from Fe$_{1-x}$Ga$_{x}$ (001) films into AlGaAs/GaAs(001) LEDs G. Kioseoglou, A.T. Hanbicki, O.M.J. van 't Erve, C.H. Li, M. Osofsky, S.-F. Cheng, B.T. Jonker Electron spin polarizations of 40-70{\%} have been obtained in GaAs due to electrical injection from Fe or FeCo contacts using surface-emitting spin-LEDs. In such LEDs, since Fe has its magnetization easy axis in the substrate plane, a large magnetic field ($>$2.2 tesla) along the surface normal is required to saturate the magnetization out-of-plane. We have grown epitaxial films of Fe$_{1-x}$Ga$_{x}$ (0 $<$ x $<$ 0.75), a material noted for its high magnetostriction, on AlGaAs/GaAs (001) heterostructures, and summarize the structure, magnetization, spin polarization, and results for electrical spin injection into AlGaAs/GaAs. The out-of-plane saturation field and magnetization decrease rapidly with Ga content, but the point contact spin polarization remains near that of Fe for x $\le $ 0.5. Electrical spin injection from an Fe$_{0.5}$Ga$_{0.5}$ contact produces an electron spin polarization of 30{\%} in the GaAs at 20 K, similar to that obtained from Fe contacts, but with out-of-plane saturation fields as low as 0.4 T. Post-annealing at low temperature increases the electron spin polarization up to 40{\%} and it will be discussed at the meeting. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B12.00006: Electron distribution among the $\Gamma $, L, and X GaAs conduction band valleys in an Fe/GaAs(n) Schottky barrier. Stuart Holmes, J. Laloe, I. Farrer, Imran Khan, Mesut Yasar, Manuel Diaz-Avila, Athos Petrou The electroluminescence (EL) spectra from Fe/GaAs(n)/InGaAs/GaAs(p) spin-LEDs have a complicated composition. In addition to the $e_1 \ell_1 $light and $e_1 h_1 $ heavy hole excitonic features the band-edge EL contains the following phonon replicas: $e_1 h_1 -TA$, $e_1 \ell_1 -LO$, $e_1 h_1 -LO$, $e_1 h_1 -LO-TA$, and $e_1 h_1 -LO-LA$. The replicas are interpreted as due to recombination processes that involve electrons occupying the L and X valleys of the GaAs(n) conduction band. The high momentum electrons are promoted to the higher energy L and X valleys by the strong electric field at the Fe/semiconductor interface [1]. \newline [1] S. Saikin et al, J. Phys: Condens.Matter \textbf{18}, 1535, (2006) [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B12.00007: Electrical spin injection into the ground and excited states of uniform InAs quantum dots M. Yasar, I. Khan, M. Diaz-Avila, A. Petrou, G. Kioseoglou, C.H. Li, B.T. Jonker Spin-polarized electrons from Fe contacts were injected into zero-dimensional InAs quantum dots (QDs) where they recombine with unpolarized holes. Using standard MBE growth techniques, the QD density was reduced resulting in uniform dot-size distribution. In these new samples the broad electroluminescence (EL) observed previously in the high QD density LEDs is replaced by distinct features associated with the atomic-like s-, p-, d-, and f-shells of the QDs. As the diode bias voltage was increased, higher energy shells became populated. The circular polarization of these features was studied as function of applied magnetic field, bias, and temperature. Significant differences were observed in the behavior of the various EL features. The polarization dependence on magnetic field confirms spin injection from Fe. Furthermore, higher energy shells exhibit correspondingly higher polarization values consistent with optical pumping studies. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B12.00008: Measurement of the spin detection efficiency of the s, p,d, and f shells in InAs QDs using optical pumping M. Diaz-Avila, M. Yasar, I. Khan, A. Petrou, C.H. Li, G. Kioseoglou, B.T. Jonker We have carried out an optical pumping study of n-i-p AlGaAs(n)/GaAs(i)/AlGaAs(p) quantum wells (QW) structures that incorporate a single layer of InAs quantum dots (QDs) in the GaAs QW. The electron-hole pairs were photo-excited in the GaAs QWs and were subsequently captured by the InAs QDs. The resulting photoluminescence (PL) spectra contain features associated with recombination processes that involve electrons occupying the s- , p-, d-, and f-shells. The circular polarization of these features varies from 10 {\%} to 30 {\%} and increases monotonically from the s- to the f-shell. From these circular polarization measurements the spin detection efficiency for each shell was determined as function of temperature and longitudinal magnetic field applied along a direction perpendicular to the QD plane (z-axis). The optical pumping results, in combination with magneto-EL studies of Fe spin LEDs that have the same parameters and were grown under identical conditions were used to determine the injected electrons spin polarization in these devices. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B12.00009: Model for the Bias Dependence of the Sign of Spin Injection in Ferromagnetic Metal/Semiconductor Schottky Tunnel Contacts P.P. Ruden, D.L. Smith We examine theoretically the spin-polarized electron injection across a ferromagnetic metal/semiconductor Schottky tunnel barrier. The example structures we model consist of thin, heavily doped regions near an Fe/GaAs interface in which the band bending associated with the Schottky barrier is accommodated, and lightly doped bulk regions.$^{1,2}$ Under reverse bias electrons tunnel from the Fe through the space charge layer into the lightly doped region; under forward bias electrons tunnel from the GaAs into the Fe. Because Fe is ferromagnetic, the transmission is spin-dependent. The shape of the tunnel barrier depends strongly on bias. By considering the asymptotic forms of the wave functions, we show that the bias dependence of the spin-dependent transmission coefficients generally can induce a change in the sign of the spin-polarized current. Model calculations reveal that spin-polarized transport is sensitive to the tunnel barrier properties. The results are in good agreement with recent experimental data. 1) A.T. Hanbicki et al., Appl. Phys. Lett. 80, 1240 (2002). 2) S.A. Crooker et al., Science 309, 2191 (2005). [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B12.00010: Hot electron injection, vertical transport, and electrical spin detection in Silicon Ian Appelbaum, Biqin Huang, Igor Altfeder, Douwe Monsma In our devices, spin-dependent hot electron transport through metallic ferromagnetic thin films is used to polarize a charge current injected into the conduction band of Si, and then to analyze the remaining polarization after vertical drift. Our measurements of a clear spin-valve signature indicate substantial electron spin polarization after transport through several microns of Si. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B12.00011: All Epitaxial Heterostructure for Spin Injection from a Half Metal into Silicon Maitri Warusawithana, Darrell Schlom, James Eckstein Using reactive molecular-beam epitaxy, epitaxial La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ / SrTiO$_{3}$ / Si heterostructures have been grown. The SrTiO$_{3}$ layer, just a few unit cells thick, serves simultaneously as a tunnel barrier and as a means to reduce reaction between the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ and the underlying Si. The growth of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ at MBE-compatible pressures requires ozone, which readily oxidizes bare Si and would destroy the chances for epitaxial growth. In contrast, epitaxial SrTiO$_{3}$ can be grown on (001) Si using molecular oxygen via a complex, but established process. Once the SrTiO$_{3}$ film is complete, ozone is turned on for the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ growth. The thin SrTiO$_{3}$ layer acts as a diffusion barrier for oxygen limiting the formation of SiO$_{2}$ at the SrTiO$_{3}$/Si interface. X-ray diffraction measurements show that the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ layer has good crystalline quality with rocking curve full width at half maximum values of the 200 peak of less than 0.5\r{ }. Furthermore, electrical transport measurements indicate that the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ layer is ferromagnetic and metallic below $\sim $370 K with a resistivity $<$100 $\mu \Omega $--cm at 4.2 K. Possible devices for tunneling spins into Si and for detecting spin carrier density inside a Si channel will be discussed. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B12.00012: Non-equilibrium Fe-Si thin films as potential spin injection materials Jian Zhou, Erik Helgren, Li Zeng, Frances Hellman Fe-Si thin films are potential spintronics materials for its tunable structural, magnetic, and electric properties [1]. Our goal is to inject spin polarized electrons from iron-silicides into Si through a Schottky barrier, which is formed by choosing the proper doping level for Si, and a suitable Iron-silicide composition. We prepared Fe$_{1-x}$Si$_{x}$ (x = 0.25 - 0.5) films by electron beam co-evaporation from Fe and Si sources onto Si substrates under ultra-high vacuum conditions. Growth at 300 \r{ }C leads to a homogeneous Fe$_{1-x}$Si$_{x}$ magnetic alloy with both Tc and room temperature magnetization monotonically decrease with an increasing x. X-ray diffraction patterns show that a thin seed layer of FeSi at interface reduces the lattice mismatch between Si substrate and the bcc Fe$_{1-x}$Si$_{x}$ film, so that epitaxial growth can be realized. The seed layer also plays the role of reducing interdiffusion. A clear interface at Iron-silicide and silicon is obtained, resulting in a good Schottky barrier with height around 0.7 eV. By adjusting the Fe$_{1-x}$Si$_{x}$ composition, the resistivity of iron-silicide can be tailored. Ferromagnetic Fe$_{55}$Si$_{45}$ shows resistivity of 10$^{-3}$ ohm-cm, and magnetization 100 emu/cc at 300 K. By increasing the iron-silicide resistivity, one of the main obstacles for spin-injection from metal to semiconductor -- the resistivity mismatch -- can be overcome. Experiments based on spin-valve-type magneto-resistance for spin injection detection will be discussed. [1]. Ionescu et al. Physical Review B 71, 94401 (2005). [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B12.00013: Spin Injection and Spin Dynamics at CuPc/GaAs (100) Interface Huanjun Ding, Yongli Gao, Marina Sanchez-Albaneda, Mirko Cinchetti, Jan-Peter W\"ustenberg, Oleksiy Andreyev, Michael Bauer, Martin Aeschlimann Spin injection from GaAs (100) to organic semiconductor copper phthalocyanine (CuPc) has been investigated experimentally with spin-resolved two-photon photoemission (SR-2PPE) spectroscopy. The spin-polarized electrons are originally generated in GaAs through optical pumping with femtosecond time resolution and injected into CuPc film. We observed an enhancement in spin polarization at the interface after initial CuPc deposition. This demonstrates that interface spin scattering is insignificant, which is similar to our previous results of spin injection at CuPc/Co interface. The spin polarization dropped when the CuPc film became thick, an effect attributed to bulk attenuation in CuPc. The lifetime of the unoccupied orbits in CuPc was also studied with red-blue excitation of photon energy of 1.6 eV and 3.2 eV, respectively. There was a strong asymmetry in the time-resolved spectra, and an unexpected long lifetime for the low intermediate state was observed. A simple explanation of this phenomenon will be discussed. [Preview Abstract] |
Session B13: Focus Session: Cobaltates and Manganites
Sponsoring Units: DMP GMAGChair: Weida Wu, Rutgers University
Room: Colorado Convention Center Korbel 4C
Monday, March 5, 2007 11:15AM - 11:27AM |
B13.00001: Tuning Physical Properties via Isovalent Doping in Ba$_{2-x}$Sr$_{x}$CoO$_{4}$ Hao Sha, Jiandi Zhang, Q. Huang, V.O. Garlea, B.C. Sales, D. Mandrus, R. Jin It is known that monoclinic Ba$_{2}$CoO$_{4}$ is an antiferromagnetic (AFM) insulator with Neel temperature $T_{N}$=25K. We found that isovalent Sr substitution for Ba drastically changes the structural and magnetic properties of Ba$_{2-x}$Sr$_{x}$CoO$_{4}$ system. With increasing $x$, $T_{N}$ initially increases then decreases after reaching the maximum at $x$=0.5. Correspondingly, its crystal structure changes from monoclinic ($x<$0.5) to orthorhombic ($x\ge $0.5) at room temperature. The correlation between structure and physical properties will be discussed. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B13.00002: Spin-state transitions and magnetic polaron in lightly doped La$_{1-x}$Sr$_{x}$CoO$_{3}$. A. Podlesnyak, M.W. Haverkort, K. Conder, E. Pomyakushina, Daniel Khomskii Using the inelastic neutron scattering (INS) technique, we identified the energy levels of the thermally excited states of Co$^{3+}$ ions in both LaCoO$_{3}$ and La$_{0.998}$Sr$_{0.002}$CoO$_{3}$. In LaCoO$_{3}$ an excitation at \textit{$\sim $}0$.$6 meV appears at T$>$30K, whose intensity follows the bulk magnetization. Within a model including crystal field interaction and spin-orbit coupling we interpret this excitation as originating from a transition between thermally excited states located about 120 K above the ground state. Since the $g$-factor obtained from the field dependence of the INS is \textit{g$\sim $ }3, we interpret this state as a high-spin state of Co$^{3+ }$. The lightly doped material \textit{x$\sim $}0$:$002 exhibits paramagnetic properties at low temperatures. An INS peak at energy transfer \textit{$\sim $}0$.$75 meV was observed in it already at $T $= 1$:$5 K. We propose that the holes introduced in the LS state of LaCoO$_{3}$ by doping are extended over the neighboring Co sites, forming thus magnetic polaron and transforming all the involved Co ions (e.g. 6 of them) to the high-spin state. Similarly to LaCoO$_{3}$, we interpret the INS transition at 0.75 meV as that on these high-spin Co$^{3+}$ ions. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B13.00003: Incommensurate Spin Correlations in La$_{1-x}$Sr$_{x}$CoO$_{3}$ D. Phelan, D. Louca, K. Kamazawa, S.-H. Lee, S. Rosenkranz, J.F. Mitchell, S.N. Ancona, M.F. Hundley, Y. Motome, Y. Moritomo Non-magnetic, insulating LaCoO$_3$ is transformed into a metallic, spin cluster ferromagnet when holes are added to the system by replacing La$^{3+}$ with Sr$^{2+}$. Previous work has shown that this transition results from the percolation of isotropic, ferromagnetic clusters. Here, we present elastic neutron scattering data which shows that a short-ranged, anisotropic incommensurate magnetic phase also appears as holes are added. We have studied this incommensurate phase in detail for a number of concentrations, spanning the phase diagram above and below the percolative phase transition, and the incommensurability increases with the hole concentration. From the evolution of the incommensurate signal strength with x, we concluded that the the incommensurate phase is competing with the FM clustering. The spin incommensurability may originate from a local ordering of magneto-polarons. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B13.00004: A non-Griffiths-like clustered phase above the Curie temperature of the doped perovskite cobaltite La$_{1-x}$Sr$_{x}$CoO$_{3}$ Chunyong He, Maria Torija, Jing Wu, Jeff Lynn, John Mitchell, Chris Leighton The existence of preformed clusters above the \textit{Tc} of the doped perovskite manganites is well established and, in many cases, conforms to the expectations for a Griffiths phase. We show here that the phase-separated perovskite cobaltite (La$_{1-x}$Sr$_{x}$CoO$_{3})$ also exhibits a clustered state above the \textit{Tc} in the ferromagnetic phase. The formation of magnetic clusters at a well-defined temperature ($T$*) is revealed in the small-angle neutron scattering, d.c. susceptibility, and resistivity. Remarkably, this clustered state has none of the characteristics of a Griffiths phase; the deviation from Curie-Weiss behavior is opposite to expectations and is not field dependent, and $T$* does not correspond to the undiluted \textit{Tc}. These results demonstrate that although the Griffiths phase may occur in many systems with quenched disorder, it is not universally applicable to the randomly doped transition metal oxides. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:39PM |
B13.00005: Competing ferromagnetic and incommensurate order in perovskite cobaltites La$_{1-x}$Sr$_x$CoO$_3$ Invited Speaker: Many phenomena observed in complex oxides, and in particular their enhanced response to external fields, are intimately linked to the existence of short-range order such as formation of stripes, ladders, checkerboards or phase separation. This nanoscale disorder results from the delicate balance of spin, orbital, charge, and strain degrees of freedoms that leads to competing groundstates with incompatible order. Inhomogeneity in the form of phase separation is also believed to play a key role in the magnetoresistance of doped cobalt perovskites La$_{1-x}$Sr$_x$CoO$_3$. In this system, doping holes into the non- magnetic, insulating parent compound leads to spin-glass behavior at low doping, with nanoscale ferromagnetic clusters forming within a non-fermomagnetic matrix. Percolation of these isolated clusters then leads to the ferromagnetic and coincident metal- insulator transition at x=0.18. Utilizing elastic and inelastic neutron scattering, we have studied in detail the evolution of the static and dynamic spin correlations in these systems. Within the cluster-glass phase, we observe the formation of static ferromagnetic droplets below the spin- glass freezing temperature. Aided by the double exchange, the ferromagnetic correlations grow rapidly with doping. At the onset of metallicity, the correlation length increases abruptly, but remains finite.\footnote{D. Phelan, D. Louca, S. Rosenkranz,S.-H. Lee, Y. Qiu, P.J. Chupas, R. Osborn, H.Zheng, J.F. Mitchell, J.R.D. Copley, J.L. Sarrao, Y. Moritomo, Phys. Rev. Lett. {\bf 96}, 027201 (2006)} Concurrent, and in competition with the ferromagnetic correlations, we also observe the formation of a short- range, incommesurate (IC) spin structure below the spin-freezing or ferromagnetic transition temperature. The incommensurate wavevector increases continuosly with doping, with the intensity of the IC correlations increasing in the insulating phase with doping, but dropping in the metallic state. This IC order could result from correlations between local Co$^{3+}$-Co$^{4+}$ clusters in the form of short-range stripes. \newline \newline Work supported by US DOE BES-DMS DE-AC02-06CH11357 and NSF DMR-0454672. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B13.00006: Thickness dependence of the structural, magnetic, and electronic, properties of epitaxial La$_{0.5}$Sr$_{0.5}$CoO$_{3}$ films on SrTiO$_{3}$(001) substrates Manish Sharma, Maria Torijja, Chris Leighton Bulk La$_{1-x}$Sr$_{x}$CoO$_{3}$ (LSCO) materials have received considerable attention due to the existence of spin-state transitions, magnetoelectronic phase separation, and giant anomalous Hall effect. In our prior work we have established optimized conditions for the deposition of high quality epitaxial LSCO thin films. In this work, we provide a comprehensive study of the variation of structural, morphological, magnetic, and electronic properties as a function of film thickness. This investigation has been carried out as a controlled function of oxygen stoichiometry using the total sputtering pressure (30, 70, and 140 mTorr, at an O$_{2}$ / Ar ratio of 0.4) as the control parameter. High resolution WAXRD, x-ray rocking curves, x-ray reflectivity, AFM, d.c. magnetometry, and electronic transport measurements have all been employed. Our results indicate that the thickness dependence of the electronic and magnetic properties is dominated by the sensitive interplay between oxygen stoichiometry and strain relaxation. The behavior at very low thickness is discussed in terms of the known phenomenology of the magnetoelectronic phase separation in this material. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B13.00007: Fabrication, characterization, and magnetic and electronic properties of epitaxial La$_{0.5}$Sr$_{0.5}$CoO$_{3}$ films M.A. Torija, M. Sharma, J. Wu, C. Leighton Bulk La$_{1-x}$Sr$_{x}$CoO$_{3}$ (LSCO) has received considerable attention with regard to magnetic phase separation. Fabrication of epitaxial films would provide a means to study this phase separation under dimensional confinement. We have investigated the properties of epitaxial films of x = 0.5 LSCO on SrTiO$_{3}$ (001) by reactive d.c. sputtering from compound targets. Structure and properties were studied as a function of sputtering pressure, O$_{2}$/Ar ratio, and post-deposition O$_{2}$ treatment. Optimized conditions result in single phase, stoichiometric, substantially relaxed, epitaxial films. At a fixed O$_{2}$ / Ar ratio, two distinct regimes of total sputtering pressure ($P)$ occur. Films grown at $P>$ 50 mTorr have properties close to bulk; they are ferromagnetic ($T_{C} \quad \approx $ 230 K, $M_{S} \quad \approx $ 2 $\mu _{B}$ / Co), have a metallic-like $\rho (T)$ at all $T$, and exhibit 10 {\%} magnetoresistance at $T_{C}$. For $P \quad <$ 50 mTorr, we obtain lower surface roughness and narrower rocking curves but with low moment, insulating $\rho (T)$, and increased out-of-plane lattice parameter due to O deficiency. Similarly, a dramatic enhancement in physical properties, and elimination of a CoO minority phase, is obtained when cooling in 500 Torr of O$_{2}$. The results demonstrate good control over the oxygen stoichiometry, and therefore physical properties. Work supported by NSF DMR. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B13.00008: Doping Dependence of Polaron Hopping Energies in La$_{1-x}$Ca$_x$MnO$_3$ ($0\leq x\leq 0.15$) Krishna Neupane, Joshua Cohn, John Neumeier Measurements of the low-frequency ($f\leq 100$~kHz) permittivity at $T\leq 160$~K and dc resistivity ($T\leq 430$~K) are reported for La$_{1-x}$Ca$_x$MnO$_3$ ($0\leq x\leq 0.15$). Static dielectric constants are determined from the low-$T$ limiting behavior of the permittivity. The estimated polarizability for bound holes $\sim 10^{-22}$~cm$^{-3}$ implies a radius comparable to the interatomic spacing, consistent with the small polaron picture established from prior transport studies near room temperature and above on nearby compositions. Relaxation peaks in the dielectric loss associated with charge-carrier hopping yield activation energies in good agreement with low-$T$ hopping energies determined from variable-range hopping fits of the dc resistivity. The doping dependence of these energies suggests that the orthorhombic, canted antiferromagnetic ground state tends toward an insulator-metal transition that is not realized due to the formation of the ferromagnetic insulating state near Mn$^{4+}$ concentration $\approx 0.13$. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B13.00009: Magnetic susceptibility of La$_{0.7}$Ca$_{0.3}$MnO$_3$ at very low magnetic fields in the vicinity of the ferromagnetic transition John J. Neumeier, Jose A. Souza, Yi-Kuo Yu Magnetic susceptibility ($\chi$) measured at magnetic fields $H$ as low as 0.2 Oe is reported for La$_{0.7}$Ca$_{0.3}$MnO$_ {3}$. A pronounced enhancement in $\chi$ is observed in the region above the critical temperature $T_c$ at very low $H$. As the magnetic field is increased, this feature is shifted toward $T_c$, eventually vanishing near $H$ = 400 Oe. Electrical resistivity measurements show a positive magnetoresistance effect between 0 and 500 Oe in a temperature range slightly above $T_c$. The results are discussed in a scenario of frustrated magnetism and the possibility of a Griffiths singularity is addressed.$^1$ $^1$ Chan, Goldenfield, and Salamon, Phys. Rev. Lett. 97 (2006) 137201. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B13.00010: Critical Exponents and Pressure Dependence of $T_c$ of La(Ca)MnO$_3$ Jose A. Souza, B. D. White, J. J. Neumeier, Y.-K. Yu, C. A. M. dos Santos Measurements of heat capacity and thermal expansion for La$_{1- x}$Ca$_{x}$MnO$_{3}$ with $x$ = 0, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, and 1 are reported. Using a model proposed previously (Souza et al. Phys. Rev. Lett. 94, 207209 (2005)), which utilizes both heat capacity ($C_{P}$) and thermal expansion coefficient ($\mu$) data, the pressure dependencies of $T_c$, d$T_{c}$/d$P$, are obtained for all samples. d$T_{c} $/d$P$ decreases as the Ca doping increases. Critical behavior using both $C_{P}$ and $\mu$ is evaluated for the samples. The critical exponent $\alpha$ increases from 0.13, for LaMnO$_{3} $ to 0.97 for $x$ = 0.30. As Ca content is increased further, $\alpha$ drops reaching 0.11, for CaMnO3. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B13.00011: High Pressure Effects on Structural and Transport of self-doped Manganite La$_{0.9}$MnO$_{3}$ Zhiqiang Chen, Trevor Tyson, Zhong Zhong The effects of hydrostatic pressure up to 6 GPa and 11 GPa, respectively on the electrical resistivity and structural properties are systematically investigated on the self-doped Maganite La$_{0.9}$MnO$_{3}$. We find a maximum shift of the peak resistivity with pressure occurs at $\sim $ 3.4 GPa in the La deficient system similar to the chemically doped manganite systems previously studied by our group. The unusual pressure dependence of resistivity can be related with the competition between ferromagnetic Double Exchange interaction and antiferromagnetic superexchange mechanism. The x-ray diffraction reveals a single-phase crystallographic phase of monoclinic space group up to 11 GPa. Electronic structure simulations of the pressure dependence on the stability of the magnetic phases are being conducted. This work was supported by NSF DMR-0512196. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B13.00012: Effects of internal structural parameters on the properties of Ba-substituted La$_{0.5}$Sr$_{0.5}$MnO$_{3}$ Omar Chmaissem, Bogdan Dabrowski, Stanislaw Kolesnik, Leopoldo Suescun, James Mais, Timothy Maxwell, James D. Jorgensen Barium substituted La$_{0.5}$Sr$_{0.5-x}$Ba$_{x}$MnO$_{3}$ materials have been synthesized and investigated using neutron powder diffraction. We show that Ba substitution suppresses the low temperature orbital-ordering previously observed in La$_{0.5}$Sr$_{0.5}$MnO$_{3}$, and demonstrate the evolution of the magnetic and nuclear structures as a function of increasing Ba content. All samples exhibit paramagnetic and ferromagnetic properties near room temperature. The effects of A-site ionic size, size variance, and strains in the lattice on the ferromagnetic ordering temperature, T$_{C}$, are discussed and compared with other members of the general La$_{0.5}$(Ca,Sr,Ba)$_{0.5}$MnO$_{3}$ series. Depending on the substitution path, the relationship between T$_{C}$ and $<$r$_{A}>$ is either nearly constant or looks like an inverted parabola. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B13.00013: Structural and magnetic properties of SrMn$_{1-x}$Ru$_{x}$O$_{3}$ perovskites B. Dabrowski, S. Kolesnik, O. Chmaissem, T. Maxwell Ferromagnetism of SrRuO$_{3}$ is unique among 4d transition metal based perovskite oxides. On substitution of Mn its T$_{C}$ decreases from 163 K to 0 for x$\sim $0.5-0.6 followed by a formation of an antiferromagnetic insulating state at a quantum critical point. The other end member of the SrMn$_{1-x}$Ru$_{x}$O$_{3}$ family, a cubic perovskite SrMnO$_{3}$ is a G-type antiferromagnet with T$_{N}$=233 K. We have synthesized the complete SrMn$_{1-x}$Ru$_{x}$O$_{3}$ solid solution. The polycrystalline samples were characterized by neutron difraction, magnetic, and transport experiments. The incorporation of Ru in the SrMnO$_{3}$ matrix (0.1$\le $x$\le $0.4) results in a phase transition to a C-type antiferromagnetic state accompanied by a cubic-tetragonal transition. The intermediate substitution level induces a spin-glass behavior, due to competing ferro- and antiferromagnetic interactions. Mixed valence Mn$^{3+}$/Mn$^{4+}$ and Ru$^{4+}$/Ru$^{5+}$ pairs introduce additional frustration to the magnetic states. The glassy behavior can be observed for x up to 0.7 in the tetragonal structure. Supported by NSF (DMR-0302617) and the U.S. Department of Education [Preview Abstract] |
Session B14: Focus Session: Magnetic Nanowires, Nanodots, Multilayers
Sponsoring Units: GMAG DMPChair: Justin Shaw, National Institute of Standards and Technology, Boulder
Room: Colorado Convention Center Korbel 4D
Monday, March 5, 2007 11:15AM - 11:51AM |
B14.00001: Enhancement of AMR in Permalloy Point Contacts due to Quantum Interference. Invited Speaker: We measure the low-temperature resistance of mechanically-stable permalloy break junctions as a function of contact size and the magnetic field angle in applied fields large enough to saturate the magnetization. We show that the size of the anisotropic magnetoresistance (AMR) signal at low temperature can increase dramatically as the contact cross section is narrowed to the nanometer-scale regime. For metallic devices with $R$ larger than $\sim 1$\ k$\Omega$ we observe AMR effects larger than in bulk devices, with an angular variation that can deviate from the sinusoidal bulk dependence, and which are associated with fluctuations in $dV/dI$ of similar magnitude as a function of $V$. Even more strikingly, we find that point contacts which are completely broken, so as to enter the tunneling regime, also exhibit a tunneling anisotropic magnetoresistance effect (TAMR) as large as 25\% when the magnetic-moment directions in the two contacts are rotated together while remaining parallel. We propose that these large AMR and TAMR effects are the result of mesoscopic quantum interference which depends on the orientation of the magnetization, leading to fluctuations of conductance and the spin-dependent local density of states. These fluctuations should affect a broad variety of nanoscale devices that contain magnetic components, producing strong perturbations in measurements of low-temperature spin-dependent transport. This work was done in collaboration with F. Kuemmeth and D. C. Ralph. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B14.00002: Temperature dependence of anisotropic magnetoresistance fluctuations and observations of atomic motion in ferromagnetic metal break junctions Sufei Shi, Kirill Bolotin, Ferdinand Kuemmeth, D.C. Ralph Recent studies of the resistance of nanometer-scale magnetic junctions as a function of the angle of an applied magnetic field have found very large anisotropic magnetoresistance (AMR) signals, compared to bulk samples. One proposed mechanism is that coupling between the ferromagnetic moment and electron orbits may lead to conductance fluctuations due to quantum interference when the moment is rotated. Here we report that the large AMR signals are suppressed with increasing temperature ($T)$, consistent with a quantum-interference mechanism. We also note that as $T$ is increased to 32 K, most of our samples exhibit an increasing amount of time-dependent two-level resistance noise due to atomic rearrangements. We conclude that higher-$T$ measurements of magnetic nanocontacts generally involve averages over many atomic configurations. In some samples, the atomic switching rates depend strongly on the magnetic field angle, so that even at low $T$ the resistance can sometimes switch abruptly between repeatable values as the field angle is rotated. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B14.00003: Magnetic Stripe Domains in Thermally Evaporated Ni Strips Soo Hyung Lee, Frank Zhu, Chia-Ling Chien, Nina Markovic We have studied thermally evaporated thin Ni strips with varying widths and geometry. Magnetic force microscope images showed the presence of magnetic stripe domains. Wide Ni strips in their as-prepared-state exhibited stripe domains oriented perpendicular to the edge of the samples. In contrast, narrow Ni strips showed stripe domains that were parallel to the edge. Changes to the geometry of the strips caused competition of different stripe orientations. After we demagnetized the strips with an in-plane field, stripe domains followed the field's direction, which was at an arbitrary angle to the edge of the sample. We will discuss these results in terms of existing theoretical models. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B14.00004: Oscillatory Curie Temperature of Fe/ Cu-wedge/ Fe/ Cu(111) nanodots. Noppi Widjaja, W.C. Lin, K. Fuchigami, M.A. Torija, M.-T. Lin, E.W. Plummer, J. Shen The interactions between two layers of Fe nanodots were studied as a function of the thickness of a Cu spacer layer. The bottom Fe-dot layer was grown on a Cu(111) substrate cleaned \textit{in situ} by cycles of Ne-ion sputtering and annealing, employing a inert gas buffer layer assisted growth (BLAG) technique. Subsequently, a wedge-shaped Cu spacer layer was evaporated to cap the Fe-dots, followed by the growth of the top Fe-dot layer using the same BLAG method. Wedge-shaped samples are crucial for this study because the effects we are looking for are often subtle and would have been obscured by fluctuations in preparation conditions if we had to prepare a separate sample for each thickness. In-situ magneto-optical Kerr effect (MOKE) measurements were utilized to determine the local Curie temperature (T$_{C})$ at various positions on the wedged samples, and regular oscillations in T$_{C }$ as a function of the spacer layer thickness were observed. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B14.00005: Temperature dependence of the uncompensated magnetization in Fe$_{x}$Ni$_{1-x}$F$_2$/Co bilayers David Lederman, Miyeon Cheon, Zhongyuan Liu A giant uncompensated magnetization in Fe$_{x}$Ni$_{1-x}$F$_2
$/Co was observed in the hysteresis loops at low temperatures
($T |
Monday, March 5, 2007 12:39PM - 12:51PM |
B14.00006: Magnetic properties of one dimensional Ni/ Cu (Al) /Ni nanowires: Role of non-magnetic spacer Partha Pratim Pal, Ranjit Pati One dimensional (1-D) magnetic multilayered nanowires with alternating ferromagnetic and non-magnetic structures arranged in sequence have been the subject of intense research in recent years for their potential applications in magneto-electronics or spintronics. We have used first-principles periodic density functional theory to study the stability, electronic, and magnetic properties of Ni/Cu/Ni and Ni/Al/Ni nanowires. The thickness of the non-magnetic spacer layer is systematically changed to explore the role of non-magnetic spacer in controlling the interlayer magnetic coupling and hence the magnetic properties of these 1-D nanowires. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B14.00007: Dipole-Exchange modes in ferromagnetic nano-wires of arbitrary cross sections Rodrigo Arias, Douglas Mills We present a method that allows to calculate the eigen- frequencies of dipole-exchange modes in ferromagnetic nano- wires of arbitrary cross sections. The method is setup for calculating modes of long wavelength along the main direction of the nano-wire, but it could be extended to short wavelengths. The basis of the theoretical approach is the extinction theorem, under a form appropriate for the inclusion of the exchange interaction. Appropriate integral equations in the form of contour integrals around the periphery of the wire are obtained for the magnetostatic potentials and magnetization of the modes. We perform a numerical analysis of the eigen- frequencies of geometries of interest, like rectangular and elliptical cross sections, recovering the appropriate limiting values of the magnetostatic or exchange dominated regimes [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B14.00008: High Magnetization FeCo/Pd multilayers Michael Walock, Frank Klose, Mairbek Chshiev, Gary Mankey, William Butler A high saturation magnetization is advantageous in magnetic recording. ~Currently, the peak of the Slater-Pauling curve is the BCC FeCo system with a saturation magnetization of 2.45 T.~ Recently, a magnetization of 2.57 T in the FeCo layers of a [40 nm Fe$_{30}$Co$_{70}$ /1.7 nm Pd]x25 superlattice has been reported [1, 2]. ~This behavior may be attributed to an enhanced Fe moment in the expanded FCC matrix, and an accompanying induced moment in the Pd. Our theoretical calculations show an atomic moment enhancement, but this is not great enough to overcome the overall magnetization density reduction caused by the incorporation of Pd in the matrix. The overall effect is a reduced magnetization. Through variation of the FeCo composition and Pd layer thickness, and the combinatorial methods of structural and magnetic characterization, we will gain insight into the magnetic structure of this tertiary thin film system. [1] K. Noma, M. Matsuoka, H. Kanai, Y. Uehara, K. Nomura, and N. Awaji. IEEE Trans. Magn. \textbf{42}, 140 (2006). [2] \textit{ibid}. \textbf{41}, 2920 (2005). [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B14.00009: Modulating magnetic properties of the one dimensional (1-D) Fe/Pt /Fe multilayered Nanowires: A first principles study Puspamitra Panigrahi, Ranjit Pati Using first-principles density functional theory within the Local Spin Density Approximation (LSDA), we have predicted the stability, electronic and magnetic properties of 1-D ferromagnetic Fe/Pt/Fe multilayered nanowires. The thicknesses of the magnetic and non-magnetic spacer layers are systematically varied to study the evolution of magnetic properties with the spacer size. The stability of the nanowire is found to increase with the increase of the thickness of the Pt spacer. Furthermore, by increasing the thickness of the Pt layer in the nanowire, we found that the average magnetic moment per Fe atom in the ferromagnetic configuration can be enhanced significantly. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B14.00010: Imprinting magnetic vortices into heterostructure multilayer rings Volker Rose, Vitali Metlushko, Bojan Ilic, John W. Freeland While thin (5 nm) microscale single-layer ferromagnetic rings usually only exhibit a one-step switching between opposite bidomain states, also referred to as onion states, we show that in a trilayer (NiFe/Cu/Co) ring structure the interlayer dipolar interactions can lead to the stabilization of flux-closure vortex states. Using X-ray resonant magnetic scattering we have studied magnetic interactions in a series of single- (5 nm NiFe and Co) and multilayer (NiFe (5 nm)/Cu (3 nm)/Co (5 nm)) continuous films and patterned ring arrays. Each ring has a width of 0.75 $\mu $m and an outer diameter of 2 $\mu $m. In the NiFe and Co single-layer rings the spin switching occurs from an initial onion state to a final reverse onion state. By contrast, in a NiFe/Cu/Co multilayer ring the magnetostatic coupling strongly affects the reversal and gives rise to the nucleation of a well-defined vortex state in the NiFe layer. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B14.00011: Size dependence and metastability of the vortex state in magnetic nanodots Igor V. Roshchin, Chang-Peng Li, X. Batlle, J. Mejia-Lopez, D. Altbir, A. H. Romero, Ivan K. Schuller Magnetization reversal in structures with sizes comparable to or smaller than ferromagnetic domains has received much attention recently. We report on the magnetization switching as a function of size in sub-100 nm magnetic nanodots fabricated using anodized nanoporous alumina masks.[1] The hysteresis loops for the samples where a vortex is observed exhibit a non-zero remanence in accordance with Monte Carlo simulations.[2] Even if the vortex state is the ground state the dot may get stuck in a metastable single domain state. The size range for this metastability is determined from the energy diagrams for various vortex and non-vortex states obtained from micromagnetic simulation. Effects of commensurability and other parameters affecting stability of the vortex state will be discussed. Work is supported by AFOSR, US DOE, Spanish MECD, Catalan DURSI, FONDECYT, and CONACYT. \\ $[1]$ C.-P. Li et al., J. Appl. Phys. \textbf{100}, 074318 (2006). \\ $[2]$ J. Mejia-Lopez, J. Appl. Phys. \textbf{100}, in press (2006). [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B14.00012: Optical and magneto-optical properties of composite noble-metal-ferromagnetic thin films Michelle Sestak, Jonathan Skuza, R. Alejandra Lukaszew We will report on the optical and magneto-optical properties of thin films composed of a magneto-optically active part and a noble metal that acts as a plasmonic counterpart: magnetoplasmonic materials. We will show that such combination leads to structures with novel properties that can be used to achieve more efficient and sensitive nanophotonic devices based on surface plasmon resonance. We will compare a material consisting of a noble/ferromagnetic/noble metal (ie. Au/Co/Au) trilayer, in which the optical and magneto-optical response can be tailored by varying the thickness of the different layers, with a nanocomposite material made with magnetic nanoclusters embedded on a noble metal matrix. The clusters are ion-implanted onto the noble metal matrix and their size and penetration depth can be tailored by choosing the ion-implantation conditions. We will show how sub-nanometer modifications can enhance the magneto-optical response of the system. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B14.00013: The importance of the dipolar interaction strength in magnetization hysteresis curves of two-dimensional nanomagnet arrays Richard Klemm, Marisol Alcantara Ortigoza, Talat Rahman Recently, Takagaki and Ploog [Phys. Rev. B {\bf 71}, 184439 (2005)] used a fourth-order Runge-Kutta technique to integrate the Landau-Lifschitz-Gilbert equations for square lattices of $N\times N$ magnetic nanodots with dipolar interdot interactions. Some of their results appeared to differ qualitatively from the second-order Runge-Kutta results obtained for the same systems by Kayali and Saslow [Phys. Rev. B {\bf 70}, 174404 (2004)], both in the hysteresis area $A_N$ and in the number of steps of the magnetization hysteresis loops. We [Phys. Rev. B {\bf 74} (22), xxxxxx (2006), in press] show that these differences are not due to inaccuracies in either calculation or to the potentially different magnetic induction sweep rates used, but can be attributed entirely to different choices of the dipolar interaction strength $h_{\rm dip}\propto a^{-3}$, where $a$ is the two-dimensional lattice constant. [Preview Abstract] |
Session B16: Focus Session: Spin Transport and Orbital Polarization Effects
Sponsoring Units: GMAG DCOMP DMPChair: Adnan Rebei, Seagate Research
Room: Colorado Convention Center Korbel 4F
Monday, March 5, 2007 11:15AM - 11:27AM |
B16.00001: Spin-dependent electronic transport through magnetic molecules Carsten Timm, Florian Elste Electronic transport through magnetic molecules has recently received considerable attention. This is partly motivated by the idea to integrate spintronics with molecular electronics. This talk highlights a number of interesting effects we predict for tunneling through single magnetic molecules and molecular monolayers weakly coupled to metallic leads. The results are obtained in a rate-equation approach which treats the intra-molecular interactions exactly and works also for situations far from equilibrium (large bias voltage). Effects to be discussed include fingerprints of magnetic excitations seen in inelastic tunneling beyond the sequential-tunneling approximation, very slow spin relaxation, giant spin amplification, and negative differential conductance at high temperatures. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B16.00002: Spin-polarized transport through the endohedral fullerene Gd@C$_{82}$ Laxmidhar Senapati, Steven C. Erwin We investigate theoretically the spin-polarized electron transport through an endohedral metallofullerene Gd@C$_{82}$ sandwiched between magnetic Fe contacts. Using density-functional theory and the Landauer-Buttiker formalism, we demonstrate that the total current depends on the relative orientation of the Gd spin and the two Fe-contact spins. Specifically, the current varies by 15-20{\%} as the orientation of the Gd spin moment is changed with respect to that of Fe contacts. We also studied the effect of changing the strength of the coupling between the Fe contacts and the fullerene molecule. We find that strong coupling leads to metallic current-voltage characteristics, while weak coupling leads to Coulomb blockade. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B16.00003: Magnetic phenomena, spin orbit effects, and electron transport in Pt nanowire contacts Alexander Smogunov, Andrea Dal Corso, Erio Tosatti We present a first-principles DFT study of the electronic, magnetic, and transport properties of short monatomic Pt nanowire contacts. For an infinite tipless Pt wire a fully relativistic calculation, including spin-orbit effects, yields a ferromagnetic ground state already for the unstressed wire [1]. We found that short 3-atom and 5-atom stressed wires in contact with nonmagnetic Pt leads remain locally magnetic, with the magnetization parallel to the wire axis, owing to orbital magnetism. Ballistic conductance of these nanocontacts is calculated using the scattering-based method [2]. Preliminary results indicate a ballistic conductance for a stressed 5-atom wire of about 2.0 $G_0$ ($G_0 = 2e^2/h$ is the conductance quantum) for parallel magnetization, 2.3 $G_0$ for perpendicular magnetization, and 2.4 $G_0$ in the nonmagnetic case. The former is in closest agreement with experimental values reported in break junctions [3].\newline [1] A. Delin and E. Tosatti, Phys. Rev. B {\bf 68}, 144434 (2003); R. Weht, A. Smogunov, A. Delin, A. Dal Corso, and E. Tosatti, in preparation. \newline [2] A. Smogunov, A. Dal Corso, and E. Tosatti, Phys. Rev. B {\bf 70}, 045417 (2004); A. Dal Corso, A. Smogunov, and E. Tosatti, Phys. Rev. B {\bf 74}, 045429 (2006).\newline [3] R.H.M. Smit {\it et al.}, Phys. Rev. Lett. {\bf 87}, 266102 (2001). [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:27PM |
B16.00004: Progress in ab initio methods for spin transport Invited Speaker: Numerical simulations have an important role in spintronics, here one envisages the use of the spin as well as the electron charge for electronic applications. In this talk I will present our code {\it Smeagol} [1] which combines the non-equilibrium Green function formalism with density functional theory and it has been specifically designed for magnetic devices. With {\it Smeagol} I will first investigate the possibility of large ballistic magnetoresistance in nickel point contacts, addressing the effects of local exchange and correlation functionals as well as the possible presence of oxygen impurities. Then I will describe an attempt to integrate the fields of spin- and molecular-electronics by constructing spin-valves using organic molecules. I will demonstrate that it is possible to obtain different transport behaviour, large magnetoresistance [1] as well as current rectification and spin-diode effects by simply selecting the molecule and the anchoring groups. Finally I will show how {\it Smeagol} is a valuable tool for simulating spin-polarised STM images. \begin{thebibliography}{99} \bibitem{smeagol} A. R. Rocha, {\it et al.}, Phys. Rev. B {\bf 70}, 085414 (2006), Nature Materials, {\bf 4}, 335 (2005). \end{thebibliography} [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B16.00005: Calculations of spin-disorder resistivity from first principles Aleksander Wysocki, Kirill Belashchenko, Julian Velev, Mark van Schilfgaarde Spin-disorder resistivity of Fe and Ni is studied using the noncollinear density functional theory. The Landauer conductance is averaged over random disorder configurations and fitted to Ohm's law. The distribution function is approximated by the mean-field theory. The dependence of spin-disorder resistivity on magnetization in Fe is found to be in excellent agreement with the results for the isotropic s-d model. In the fully disordered state, spin-disorder resistivity for Fe is close to experiment, while for fcc Ni it exceeds the experimental value by a factor of 2.3. This result indicates strong magnetic short-range order in Ni at the Curie temperature. We suggest that the analysis of calculated and measured spin-disorder resistivity provides a powerful method to extract information on the temperature dependence of the magnetic short-range order parameter in ferromagnetic metals. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B16.00006: Size-dependent alternation of magnetoresistive properties in atomic chains R. Tugrul Senger, Engin Durgun, Haldun Sevincli, Hatem Mehrez, Salim Ciraci Spin-polarized electronic and transport properties of carbon atomic chains are investigated when they are capped with transition-metal (TM) atoms like Cr or Co, using density functional theory. The magnetic ground state of the TM-C$_n$-TM chains alternates between the ferromagnetic (F) and antiferromagnetic (AF) spin configurations as a function of $n$. The desirable AF state is obtained for only even-$n$ chains with Cr; conversely only odd-$n$ chains with Co have AF ground states. We present a simple model that can successfully simulate these variations, and the induced magnetic moments on the carbon atoms. Depending on the relative strengths of the spin-dependent couplings and the on-site energies of the TM atoms there induces long-range spin polarizations on the carbon atoms which mediate the exchange interaction. When connected to appropriate electrodes these atomic chains display a strong spin-valve effect. Analysis of electronic and magnetic properties of these atomic chains, and the indirect exchange coupling of the TM atoms through carbon chain will be presented. $^1$E. Durgun \textit{et al.}, J. Chem. Phys. \textbf{125}, 121102 (2006). $^2$E. Durgun \textit{et al.}, Phys. Rev. B \textbf{74} (\textit{in press}). [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B16.00007: T-dependent matrix elements in x-ray magnetic circular dichroism Yongbin Lee, Bruce Harmon, Alan Goldman, Jonathan Lang Dramatic changes in the Er L$_{2}$ and L$_{3}$ XMCD spectra in Er$_{2}$Fe$_{17}$ as a function of temperature have been investigated with detailed experiments and first principles calculations. This study seeks to understand the fundamental mechanisms governing the spectral shape and magnitude of the L$_{2}$ and L$_{3}$ XMCD spectra as a step toward developing XMCD as a quantitative probe for rare earths similar to its effectiveness for transition metals via the use of sum rules (which do not work for rare earths). The calculations simulate the key thermal physics by evaluating the spin polarized band structures obtained with the 4f moment on the Er atom constrained to values of 0, 1, 2, and 3 Bohr magnetons. Both the theory and our experiments, performed at the Advanced Photon Source, show the XMCD L$_{2}$-edge spectrum changes sign as the temperature is lowered, and the L$_{3}$-edge spectrum also shows systematic and significant changes. We will discuss the effects of dipole matrix elements, spin-orbit coupling, hybridization between 5d-3d orbital, and magnetic anisotropy on the XMCD spectra. Quadrupole transitions and core hole effects will be also discussed. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B16.00008: Orbital Polarization in Itinerant Magnets Igor Solovyev The correct description of the orbital magnetism is one of the longstanding problems in the density functional theory (DFT). One possible solution is to extend DFT by considering explicit dependence of the exchange-correlation energy on the orbital degrees of freedom. Since the angular momentum operator does not commute with electrostatic potential, it is not an observable except a small atomic region where this potential is nearly spherical. Hence, the orbital magnetism is an atomic property, and we inevitably have to deal with the problem of on-site Coulomb interactions and screening of these interactions in solids.$^1$ For itinerant systems, this screening can be evaluated in the random-phase approximation (RPA), by considering the strong-coupling limit. Then, the orbital polarization can be computed as the self-energy correction in the static version of the GW method, without any adjustable parameters.$^2$ This opens a formal way for combining the spin itineracy in the local-spin-density approximation (LSDA) with the atomic orbital magnetism. RPA can be further improved by restoring the spin polarization of LSDA through the local- field corrections. Numerical applications reveal a remarkable improvement for the orbital magnetization and magnetocrystalline anisotropy energies of transition metals and actinide compounds. $^1$ I.~V.~Solovyev~\textit{et al.}, Phys.~Rev.~Lett.~\textbf{80}, 5758 (1998). $^2$ I.~V.~Solovyev, Phys.~Rev.~Lett.~\textbf{95}, 267205 (2005). [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B16.00009: Magnetic circular dichroism and the orbital magnetization of ferromagnets Ivo Souza, David Vanderbilt The spontaneous magnetization of ferromagnets has both spin and orbital contributions, ${\bf M}={\bf M}_{\rm spin}+{\bf M}_{\rm orb}$, which can be separated out via gyromagnetic measurements. Recently\footnote{D. Ceresoli, T. Tonhauser, D. Vanderbilt, and R. Resta, {\it Phys. Rev. B} {\bf 74}, 024408 (2006).} it was found that, when expressed as a bulk property of the Bloch electrons, the orbital magnetization itself consists of two terms, ${\bf M}_{\rm orb}= \widetilde{\bf M}_{\rm LC}+\widetilde{\bf M}_{\rm IC}$, which can be loosely interpreted as the localized and itinerant contributions, respectively. Interestingly, $\widetilde{\bf M}_{\rm LC}$ and $\widetilde{\bf M}_{\rm IC}$ are separately gauge-invariant, which raises the possibility that they may be independently measurable. We show that indeed they are related to the magnetic circular dichroism (MCD) spectrum by a subtle sum rule. MCD, the difference in absorption between left- and right-circularly-polarized light, is given by $\sigma_{{\rm A},\alpha\beta}^{(2)}(\omega)$, the absorptive part of the antisymmetric conductivity. We derive the following sum rule for the interband contribution: $\int_0^\infty \vec\sigma_{\rm A}^{(2)}(\omega)d\omega= (2\pi e c/\hbar)\big( \widetilde{\bf M}_{\rm LC}-\widetilde{\bf M}_{\rm IC}\big)$, where $\vec\sigma_{\rm A}^{(2)}(\omega)$ is a pseudo-vector. Hence, by combining the results of gyromagnetic and magneto-optical experiments, $\widetilde{\bf M}_{\rm LC}$ and $\widetilde{\bf M}_{\rm IC}$ can in principle be measured independently. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B16.00010: Alternative approach to ab-initio NMR spectra for periodic systems Timo Thonhauser, Arash Mostofi, Nicola Marzari, David Vanderbilt, Raffaele Resta We propose a novel finite-differences approach for computing the NMR response in periodic solids that is based on the theory of orbital magnetization\footnote{T.~Thonhauser, D.~Ceresoli, D.~Vanderbilt, and R.~Resta, Phys. Rev. Lett. {\bf 95}, 137205 (2005).} recently introduced by some of us. Instead of obtaining the shielding tensor from the response to an external magnetic field, we derive it directly from the orbital magnetization appearing in response to a microscopic magnetic dipole. This procedure has an established parallel in the case of electric fields, where Born effective charges are often obtained from the polarization induced by a sublattice displacement instead of the force induced by an electric field. Among the advantages of the present approach are its simplicity and its applicability to situations in which linear-response theory would be cumbersome. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B16.00011: Orbital magnetization in a supercell framework: Single {\bf k}-point formula Davide Ceresoli, Raffaele Resta The position operator ${\bf r}$ is ill-defined within periodic boundary conditions: owing to this, both the macroscopic (electric) polarization and the macroscopic orbital magnetization are nontrivial quantities. While the former has been successfully tamed since the early 1990s, the latter remained a long-standing unsolved problem. A successful formula within DFT for crystalline systems has been recently found.\footnote{ D. Ceresoli, T. Thonhauser, D. Vanderbilt, R. Resta, Phys. Rev. B {\bf 74}, 024408 (2006).} The formula is based on a Brillouin-zone integration, which is discretized on a reciprocal-space mesh for numerical implementation. We find here the single ${\bf k}$-point limit, useful for large enough supercells, and particularly in the framework of Car-Parrinello simulations for noncrystalline systems. We validate our formula on the test case of a crystalline system, where the supercell is chosen as a large multiple of the elementary cell. Rather counterintuitively, even the Chern number (in 2d) can be evaluated using a single ${\bf k}$-point diagonalization. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B16.00012: Orbital magnetism of mesoscopic integrable system Ming Lou, Slava Serota In the mesoscopic regime, magnetic properties (such as orbital magnetism) are sensitive to whether the corresponding classical dynamics is chaotic or integrable. Non-interacting electron gas in a rectangular box is proposed as a ``generic'' model to study orbital magnetism of integrable system. We derived the exact energy level correlation function for this system, including the perturbation by magnetic field. Combining the exact correlation function and Imry's formalism, we calculated the orbital magnetic susceptibility and discussed the field dependence at$T\to 0$ and temperature dependence at $B\to 0$. As a result, the susceptibility $\chi \sim \left| {\chi _L } \right|\sqrt {k_F L} \left\{ {\begin{array}{l} \log \left( {{\phi _0 } \mathord{\left/ {\vphantom {{\phi _0 } \phi }} \right. \kern-\nulldelimiterspace} \phi } \right)\mbox{ }(T\to 0,\phi <<\phi _0 ) \\ \log \left( {{\sqrt {\Delta E_F } } \mathord{\left/ {\vphantom {{\sqrt {\Delta E_F } } T}} \right. \kern-\nulldelimiterspace} T} \right)\mbox{ }(B\to 0,T<<\sqrt {\Delta E_F } ) \\ \end{array}} \right.$, where \textit{$\chi $}$_{L}$ is the Landau susceptibility, $k_{F}$ the is Fermi vector, $L$ is the rectangle's side, \textit{$\phi $}$_{0}$ is the magnetic flux quantum, $\Delta $ is the mean level spacing, and $E_{F}$ is the Fermi energy. For high temperature and large field, the mesoscopic part of susceptibility exponentially vanishes and only the bulk Landau diamagnetism is left. The logarithmic divergence at zero field and zero temperature is consistent with previous numerical calculations and is a manifestation of pronounced non-self-averaging properties of integrable systems. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B16.00013: Orbital Ordering in Cs$_{2}$AgF$_{4}$ - an electronic structure study Deepa Kasinathan, Klaus Koepernik, Ulrike Nitzsche, Helge Rosner The argentate Cs$_{2}$AgF$_{4}$, first synthesized in 1974 has many similarities to the high-T$_{c}$ cuprates, featuring AgF$_{2}$ sheets in place of CuO$_{2}$ sheets. While the undoped cuprates are antiferromagnetic, this argentate is ferromagnetic with a T$_{Curie}$ of about 15 K. Density functional calculations in the proposed tetragonal structure produce an itenerant half-metallic ferromagnet. Recent inelastic neutron scattering experiments have suggested an orthorhombic structure that allows an orbitally ordered ferromagnetic ground state. We performed electronic structure calculations using the LDA+U method on this system and were able to obtain an orbitally ordered ground state, not only for the newly proposed orthorhombic lattice but also for the orginal tetragonal lattice by constraining the bravais lattice and allowing the change of the atomic basis. In specific, very small changes in the position of the in-plane fluorine atoms already trigger an orbital ordering. Our calculated energy scale shows that this orbitally ordered state should be stable for all temperatures, consistent with the experiment. [Preview Abstract] |
Session B17: Focus Session: Pentacene and Field Effect Transistors
Sponsoring Units: DPOLY DMPChair: Chang Y. Ryu, Rensselaer Polytechnic Institute
Room: Colorado Convention Center 102
Monday, March 5, 2007 11:15AM - 11:51AM |
B17.00001: Organic Field Effect Transistors Invited Speaker: This talk will present an introduction to organic field effect transistors (OFETs) including results for pentacene thin film transistors, which have become a benchmark for organic electronics. We will also discuss the use of high capacitance gate dielectrics in OFETs to achieve large two dimensional carrier densities and metallic conductivity in OFETs. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B17.00002: Growth of pentacene on inorganic and organic dielectrics and sub-micron channel oTFT fabrication G. Leising, B. Stadlober, U. Haas, A. Haase, H. Gold We have fabricated pentacene thin films on different organic and inorganic dielectrics at four substrate temperatures with different film thicknesses ranging from the submonolayer over the multilayer to the ``thick'' film regime. These films were characterized by AFM and analyzed by means of scaling and rate equation theory in order to deduce the molecular growth dynamics. We found that on all substrates and in a certain substrate temperature range the growth can be well described as diffusion-limited aggregation. A critical island size was deduced from the scaling of the distribution density of the grain areas and the power-law dependence of the saturated nucleation density on the deposition rate. OTFTs with a channel length down to 300 nanometers have been fabricated by nanoimprint-lithography, using stamps made by e-beam-lithography and reactive ion etching. Due to a combination of different effects these transistors show high quality electrical characteristics. In conclusion, we observed no principal limitation for the downscaling of pentacene-based oTFTs due to short channel effects concerning all relevant parameters such as threshhold voltage, mobility, on-current and on-off ratio as long as the morphology is characterized by large and well-ordered crystallites. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B17.00003: The improvement of out of plane crystalline size of pentacene thin films on plastic substrates by transfer printing S.A. Solin, Y. Shao, D.R. Hines, E.D. Williams Pentacene thin films on plastic substrates were fabricated using the transfer printing method. [1] The out of plane structural order, structure disorder factor and crystalline size were studied by X-ray diffraction.[2] The effects of transfer printing control parameters, such as temperature and pressure, on the crystalline size and structural disorder were quantified using paracrystal theory. The calculated average number of reflecting net planes in the crystalline domains N and the disorder factor g$_{II}$ agree with the $\alpha^{\star}$ law. The 12$\sim$16\% improvement of out of plane crystalline size was observed for pentacene printed at 100 -120 $ ^{o}C$. Higher printing pressure 600 PSI improved the crystallinity above that obtained with low pressure 100 PSI. Pentacene printed at 120 $ ^{o}$C and 600 PSI showed both optimal growth axis crystalline size of 283 $\AA$ and mobility 0.237 cm$^{2}$/Vs, respectively. The optimized crystalline size shows a direct correlation with the improvement of the carrier mobility of pentacene thin film transistors. [1] D. R. Hines $\textit {et al}.,$ Appl. Phys. Lett. 86, 163101 (2005). [2] Y. Shao $\textit {et al}.,$ J. Appl. Phys. 100, 44512 (2006). [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B17.00004: Enantiotropic Polymorphs in Pentacene Theo Siegrist, Celine Besnard, Simon Haas, Mark Schiltz, Philip Pattison, Dmitry Chernyshov, Bertram Batlogg, Christian Kloc The high temperature structural phase transformation in bulk pentacene has been characterized by X-ray single crystal and powder diffraction. A well-defined transition temperature of 463K was observed in single crystals. In contrast, pentacene powders heated above the phase transformation temperature do not always fully convert, and upon cooling, coexistence of the two polymorphs in varying concentrations is observed down to room temperature. The 1$^{st}$ order phase transformation is isostructural, whereby the close packed herringbone-type layers shift against each other, keeping the same symmetry. The present results demonstrate that the structure of pentacene first reported in 1961 is actually stable only at high temperatures, and thus metastable at room temperature. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B17.00005: Effect of impurities on pentacene thin film growth for field-effect transistors Elba Gomar-Nadal, Brad R. Conrad, Ellen D. Williams The presence of impurities in organic semiconductors is one of the factors that limit device performance. Among all organic semiconductors, pentacene has shown the highest mobility reported to date. The effect of a controlled introduction in pentacene thin films of a well characterized impurity, 6,13-pentacenequinone (PnQ), was investigated. Since the majority of charge carriers in organic field effect transistors (OTFT) are located at the semiconductor-dielectric interface, this work focuses on the correlation between pentacene ultrathin film morphology and the overall OTFT device performance. The introduction of large amounts of PnQ revealed the presence of crystalline domains characteristic of pure PnQ submonolayer growth. The change of crystalline structure of the initial submonolayer for smaller amounts of PnQ is being investigated. The transistor mobility is dramatically reduced by increasing the degree of PnQ in the source material. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B17.00006: Aggregation of pentacene molecules on SiO2 substrates and its influence on the FET characteristics. Genki Yoshikawa, J. T. Sadowski, A. Al-Mahboob, Y. Fujikawa, T. Sakurai, Y. Tsuruma, S. Ikeda, K. Saiki Pentacene is one of the most promising materials for organic field effect transistors (OFETs). In order to improve the FET performance, dielectric layers, such as SiO2, are commonly modified by the self-assembled monolayers (SAMs), such as hexamethyldisilazane (HMDS). Owing to utilization of these SAMs, the performance of the pentacene FET has exceeded that of amorphous Si FET. However, we have found that pentacene molecules deposited on HMDS-treated SiO2 substrates aggregate with time even under ultra-high-vacuum (UHV) and ambient temperature conditions. We constructed an in situ atomic force microscopy (AFM)-FET measurement system and found that the FET mobility significantly decreased with the aggregation. Thus, this aggregation should be one of the major origins of the instability and irreproducibility of pentacene-based devices. In order to reveal the mechanism of the aggregation, we carried out an in situ and real time observation of growth and the aggregation of pentacene molecules on the several substrates, such as clean SiO2 and HMDS, under UHV conditions with low energy electron microscope (LEEM). We have found that pentacene tends to aggregate on the substrate with lower surface energy. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B17.00007: Orientation of Pentacene Molecules on SiO$_{2}$: From a Monolayer to the Bulk Fan Zheng, Byoung-Nam Park, Soonjoo Seo, Paul G. Evans, Franz J. Himpsel The orientation of pentacene films on SiO$_{2}$ is studied for the thickness range from a monolayer to 150 nm by polarization-dependent NEXAFS spectroscopy (Near Edge X-ray Absorption Fine Structure). All films exhibit a strong polarization dependence of the $\pi ^{\ast }$ orbitals, which indicates that the pentacene molecules are highly oriented. However, the degree of orientation varies with the rate at which pentacene molecules are deposited. This difference can be explained by a previously-proposed mixture of the bulk phase and a metastable thin film phase. Faster rates favor the thin film phase and slower rates the bulk phase. Our NEXAFS results extend previous structural observations down to the first layer at the oxide interface, which is critical for the performance of devices. Including a finite distribution of the molecular tilt angles in the data analysis accounts for residual disorder. Damage to the molecules by hot electrons from soft x-ray irradiation eliminates the splitting between nonequivalent $\pi ^{\ast }$ orbitals, indicating a breakup of the pentacene molecule. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B17.00008: Structure of a pentacene monolayer deposited on SiO$_{2}$: Role of trapped interfacial water Songtao Wo, Binran Wang, Hua Zhou, Yiping Wang, Jonathan Bessette, Randall L. Headricka, Alex C. Mayer, George G. Malliaras, Alexander Kazimirov \textit{In situ }synchrotron x-ray reflectivity is used to probe the early stages of pentacene growth in real time, under conditions relevant to the fabrication of organic thin film transistors. The results reveal that there is an interfacial water layer initially present on the SiO2 substrate and that this water layer is still present at the interface after the deposition of a pentacene thin film. The thickness of the trapped interfacial water layer does not significantly change subsequent to film deposition, even after exposure to atmospheric pressure or during vacuum annealing at 70 \r{ }C. However, a water layer is observed to form on the free surface of pentacene after sufficient exposure to water vapor, and the thickness of this layer can be reduced by subsequent vacuum annealing. These observations are correlated with organic thin film transistor mobilities measured at atmospheric pressure versus under vacuum. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B17.00009: Molecular Scale Structure of Pentacene Interfaces Soonjoo Seo, Paul Evans The adsorption of organic molecules on metals, insulators, and semiconductors has been an important issue in the organic semiconductor research. The morphology and crystal structure of the first few molecular layers at organic-inorganic interfaces, in particular, affects the electrical properties of organic thin films. The first upright layer of pentacene on Si (111) forms on top of a disordered layer on which strongly bonded pentacene molecules are formed. The microstructures of interfaces between organic molecules and insulators lack understanding in molecular orientation, packing and degree of disorder. Low temperature scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements were performed to probe molecular-scale structures of an upright layer of pentacene molecules. Our approach uses the disordered layer as a conducting analog of an insulating surface to enable a high-resolution structural study of the relevant crystalline phase of pentacene. STM and STS can be used to gain further understanding of structural defects such as vacancies, dislocations and grain boundaries within and between islands. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B17.00010: Pentacene Molecules on Inert Surfaces Yina Mo, Paul Maragakis, Efthimios Kaxiras We study the energetics and dynamics of pentacene molecules in vacuum and saturated diamond (111) surface and silica surfaces. Force field molecular dynamics simulations are applied to capture the van de Waals type interactions among the pentacene molecules and the substrates. The herringbone arrangement of the molecules is found to be optimal both in vacuum and on various inert surfaces. A 90 degree rotation of the entire structure relative to that experimentally reported is identified on the silica surfaces. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B17.00011: Charge-Transport Parameters in Molecular Organic Semiconductors. Veaceslav Coropceanu, Pavel Paramonov, Roel S. S\'anchez-Carrera, Demetrio A. da Silva Filho , Jean-Luc Bredas In this contribution we will discuss the present state-of-the-art in the derivation of electronic and electron-phonon coupling constants in organic semiconductors from quantum-chemical calculations. We will reveal some of the shortcomings of the current models used to depict organic semiconductors and also the paths to be followed to achieve significant improvements. The contributions of both intra-molecular and inter-molecular vibrations to the electron-phonon interaction will be discussed in detail. Our results show that for an adequate description of the charge transport in organic semiconductors both local and non-local electron-phonon mechanisms should be taken into account. In the case of oligoacene crystals several phonon modes that contribute most strongly to the modulation of the transfer integrals were found to display large nonlinear electron-phonon couplings. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B17.00012: Scanning Tunneling Microscopy and Spectroscopy of Pentacene films Deposited on SiC Sandeep Gaan, Roberto Duca, Randall Feenstra Among various organic semiconductors, pentacene (Pn) has attracted much attention because of its ability of form ordered structures and its relatively high electron and hole mobilities. We have used SiC surfaces etched at 1600\r{ }C in 1 atm of hydrogen to form atomically flat substrates for Pn deposition. Oxidizing these substrates prior to Pn deposition electronically decouples the molecular films from the substrate. Scanning tunneling microscopy (STM) and spectroscopy (STS) was performed at room temperature on in-situ deposited Pn films. STM reveals a dendritic morphology of the films, consistent with prior reports [1]. We find a step height of 1.43$\pm $0.10 nm indicating that the Pn molecules are standing up, confirming the relatively weak interaction between the substrate and the film. STS reveals a band gap of about 2.0 eV, which is attributed to the edges of HOMO and LUMO bands of the molecules. Measurements over a wide range of tunnel currents are in progress, in an effort to deduce any transport limitations in the films. Supported by NSF. [1] F.-J. Meyer zu Heringdorf et al., Nature \textbf{412}, 517 (2001) [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B17.00013: Time Resolved Microscopy of Charge Trapping in Polycrystalline Pentacene Michael Jaquith, Erik Muller, John Marohn The microscopic mechanisms by which charges trap in organic electronic materials are poorly understood. Muller and Marohn recently showed that electric force microscopy (EFM) can be used to image trapped charge in working pentacene thin-film transistors [E. M. Muller \textit{et al.}, \textit{Adv. Mater.} \textbf{17} 1410 (2005)]. We have made a new discovery by imaging trapped charge in pentacene films with much larger grains. In contrast to the previous study in which charge was found to trap inhomogeneously throughout the transistor gap, we find microscopic evidence for a new trapping mechanism in which charges trap predominantly at the pentacene/metal interface in large-grained devices. We conclude that at least two charge trapping mechanisms are at play in polycrystalline pentacene. We have made localized measurements of the trap growth over time by performing pulsed-gate EFM experiments. Trap formation is not instantaneous, taking up to a second to complete. Furthermore, the charge-trapping rate depends strongly on gate voltage (or hole concentration). This kinetics data is consistent with the hypothesis that traps form by chemical reaction. [Preview Abstract] |
Session B18: Structure and Dynamics in Polymer Nanocomposites
Sponsoring Units: DPOLYChair: Mitch Anthamatten, University of Rochester
Room: Colorado Convention Center 103
Monday, March 5, 2007 11:15AM - 11:27AM |
B18.00001: Externally Activated, Thermodynamically Governed Dispersion Behavior of Silica Nanoparticles in PMMA Frederick Beyer, Philip Costanzo Dispersion of nanoparticles is a topic of significant interest to the industrial, government, and academic communities as a route for creating new materials that combine the properties of the matrix polymer with functional properties of the dispersed particulate phase. We have previously demonstrated that the inclusion of a Diels-Alder linkage in the ligands use to compatibilize nanoparticles allows external control over the miscibility of those particles in the polymer matrix. Above a known temperature range, the ligands separate at the Diels-Alder linkage site, changing the miscibility of the particle. Here we are investigating the dispersion of silica particles approximately 40 nm in diameter in a PMMA matrix, using SAXS, TEM, and optical microscopy to characterize the dispersion of the particles. Annealing the samples causes in a change in the morphology of the materials, with the formation of larger aggregates and depletion zones. The optical clarity of the PMMA is substantially degraded by this process. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B18.00002: Single Chain Mean Field Theory (SCMF) in Polymer Nanocomposites Sudeepto Sen, Sanat Kumar, Igal Szleifer Recent experimental work in our group has shown that grafting the surface of inorganic nanoparticles with polymer brushes improves the dispersion of the particles in a polymer matrix. Since improved dispersion is critical to the enhancement in material properties of the resultant nanocomposite, we are motivated to study the thermodynamics of the interface structure in the polymer particle system since the interface itself is a major determinant of nanocomposite properties. We will present the results from recent and ongoing SCMF studies on a model polymer nanocomposite system with grafted particles and explore the thermodynamics of the particle/chain miscibility region along with explicit knowledge of the polymer chain structures (both matrix and grafted). Dependence on various parameters such as the molecular weights of the grafted and matrix chains respectively, the density of the grafted chains and the particle size will also be explored. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B18.00003: Improved Mechanical Properties by Grafting Nylon 6, 10 to Single Wall Carbon Nanotubes M. Moniruzzaman, Jayanta Chattopadhay, William E. Billups, Karen I. Winey We have prepared nylon 6, 10 nanocomposites using functionalized single wall carbon nanotubes (SWNT) using our interfacial \textit{in situ }polycondensation method. To improve mechanical properties, SWNT were functionalized with specific groups --(CH$_{2})_{n}$COCl with n of 4 or 9 to facilitate covalent bonding between the SWNT and the nylon matrix via alkyl segments. After synthesis, the SWNT/nylon 6,10 nanocomposites were melt spun into fibers. Relative to nylon 6,10 prepared in the same manner, the 1wt{\%} SWNT/nylon 6,10 composite shows significant increases in tensile modulus (160{\%}), strength (160{\%}) and toughness (140{\%}) with only modest reduction in the strain to break. The alkyl segments at the SWNT/nylon 6,10 interface appear to be critical to observed improvements. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B18.00004: Mechanical properties of nanocomposite systems George Papakonstantopoulos, Manolis Doxastakis, Mihail Vladkov, Jean-Louis Barrat, Juan de Pablo We employ molecular simulations to investigate the alteration of the mechanical properties upon addition of nanoparticles to a polymer matrix in the melt and the glass regime. In the glass regime, the formation of a stiffer glassy layer is apparent in the vicinity of the nanoparticles. Between particles an increase of the population of local high moduli domains suggests that a second mechanism of property improvement is the formation of a glassy network percolating throughout the material. The distribution of local moduli and the correlation of the non-affine displacements provides interesting insights into the inhomogeneity and the fragility of the nanocomposite and these systems. In the melt regime, relaxation times and viscosity are found to be higher for the nanocomposite systems than the pure polymer. Calculation of chain bridges between the particles is compared to the chain length to examine the bridge formation hypothesis. A primitive path analysis is performed to investigate the effect of inclusions on the entanglement length; results suggest that the entanglement length decreases upon addition of nanoparticles. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B18.00005: Morphological dependence on the conductance of polymer/MWNT electrospun mats Derrick Stevens, Satyajeet Ojha, Wesley Roberts, Seth McCullen, Russell Gorga, Laura Clarke Porous, electrically conducting structures are an invaluable resource for improving and developing technologies such as electrostatic dissipating filters and tissue scaffolds. Fibrous mats of electrospun polymers doped with multi-walled carbon nanotubes (MWNT) are an ideal candidate for such materials. A characterization of the morphological effect on conductance for these mats is presented here. Changes in the percolating behavior were investigated under the influence of different processing parameters. Alterations of fiber size, alignment, and construction (single component vs core-sheath bicomponent) were performed. For each set of processing parameters, electrospun mats were produced with varying carbon nanotube doping levels above and below the anticipated percolating region. Multiple parameters describing the percolation were calculated and compared for each of the processing regimes. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B18.00006: Effect of MWNT and Carbon Nanofiber Orientation in Polymer Nanocomposites on Electrical Conductivity Karen I. Winey, Lai-Ching Chou, Minfang Mu Poly(methyl methacrylate) (PMMA) composites were prepared by a coagulation method containing multi-wall carbon nanotubes (MWNT) or carbon nanofibers (CNF). The critical concentrations for electrical conductivity are 0.25wt{\%} and 4.0wt{\%} for MWNT and CNF composites, respectively. These isotropic composites were melt spun into fibers to align the fillers and the extent of alignment was quantified using x-ray scattering methods to measure the azimuthal full width at half-maximum (FWHM). The electrical conductivity shows a strong dependence on filler alignment and the presence of a critical alignment (FWHM$_{c})$ indicates a percolation threshold with respect to alignment. The critical alignment shifts to lower FWHM (more aligned) as the MWNT loading increases. We are also exploring the filler alignment and electrical conductivity of nanocomposites with both MWNT and CNF. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B18.00007: Controlling the Average and Local Glass Transition Temperatures of PMMA-SWCNT Nanocomposites Perla Rittigstein, T. Ramanathan, L. Catherine Brinson, John M. Torkelson The presence of nanofillers in a polymer matrix can drastically alter the chain segmental mobility and limit the polymer conformations, changing the properties of the system. Here, fluorescence is used to measure local and average Tg's in poly(methyl methacrylate) (PMMA)/single-walled carbon nanotubes (SWCNT) nanocomposites. SWCNT functionalized with amide groups via chemical modification of carboxyl groups introduced on the carbon nanotube surface, allow for covalent bonding with PMMA. In this study, three side groups, (CH$_2$)$_{12}$, a phenyl ring and (CH$_2$)$_2$, were used in the amide functionalized SWCNT. The PMMA-grafted SWCNT was blended with bulk PMMA at different compositions to form polymer nanocomposites. The results show that, relative to the bulk Tg of PMMA, the increases in local and average Tg of PMMA-SWCNT nanocomposites, ranging from 3 K to 32 K, correlate with the length and flexibility of the side group on the amide functionalized SWCNT. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B18.00008: The Aging Effect on Segmental Dynamics of PMMA brushes Studied by Incoherent Neutron Scattering Pinar Akcora, Victoria Garcia-Sakai, Linda Schadler, Sanat Kumar We have performed quasi-elastic neutron scattering experiments on nanocomposites comprised of a PMMA homopolymer mixed with silica particles on which a PMMA brush is attached. The PMMA brushes were 55k in molecular weight with a grafting density of 0.3 chains/nm$^2$, while we considered two matrices, 20k and 200k PMMA homopolymers, respectively. (The particle weight fraction was varied in the range 0-0.25 in a series of samples.) In all cases there are very strong aging effects, emphasizing the long equilibration times that these hybrid materials require. After a week of equilibration at 150$^{\circ}$C, we have found that in the case of wet brushes (i.e., the 20k matrix) the mean-square displacement only changes weakly with decreasing particle concentration. In contrast, the MSD decreased strongly with increasing particle concentration in the case of non-wetting matrices. These results stress that confinement effects (which increase with increasing particle content) play a secondary role to the effect of wetting behavior in determining the thermomechanical properties of these nanocomposites. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B18.00009: Polymer Diffusion in Single Wall Nanotube / Polystyrene Nanocomposites Minfang Mu, Russell J. Composto, Karen I. Winey In addition to providing exception properties, single wall carbon nanotubes (SWNT) enable fundamental studies of polymer diffusion. SWNT and small SWNT bundles have high aspect ratios ($>$ 30:1) and diameters on the order of nanometers, such that nanocomposites containing $\sim $ 1wt{\%} SWNT provide a fixed set of nanoscale obstacles. Here we will report our first results of polymer diffusion in these materials. A SWNT / polystyrene nanocomposite is prepared by a coagulation method and compression molding. Trace diffusion experiments are performed using deuterated polystyrene (dPS) where the concentration profile is characterized by forward recoil spectrometry (FRES) as a function of diffusion time and temperature. The SWNT filler, particularly with the creation of a SWNT network, inhibits the diffusion of dPS and the diffusion coefficient decreases by one order of magnitude in the nanocomposite. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B18.00010: Origin of dynamical properties in PMMA-C$_{60}$ nanocomposites Jamie Kropka, Peter Green To develop further insight into the mechanism(s) by which nanoscale fillers influence the properties of polymer nanocomposites (PNCs), we evaluate the thermal and viscoelastic behavior of a model PNC, narrow molecular weight distribution PMMA into which C$_{60}$ fullerene particles are incorporated. Differential scanning calorimetry and dynamic mechanical analysis measurements indicate systematic increases of the PNC T$_{g}$ over the C$_{60}$ composition range. Oscillatory shear rheological measurements reveal an increase in the longest relaxation time of the polymer chains accompanies this change in T$_{g}$. An assessment of particle dispersion within the polymer host suggests that the changes in the material properties are due to polymer-particle interfacial interactions, as confinement of polymer molecules between filler particles is unlikely. The interfacial interactions lead to an increase in the local friction of the system, which ultimately suppresses polymer dynamics. The suppression of local polymer dynamics is evaluated using incoherent neutron scattering. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B18.00011: Instabilities in Nanoporous Media Jiun-Tai Chen, Mingfu Zhang, Thomas Russell We studied Rayleigh instabilities in thin polymer films confined within nanoporous alumina templates. Thin films of PMMA were generated by filling cylindrical nanopores of an anodic aluminum oxide (AAO) membrane with a PMMA solution in chloroform followed by solvent evaporation. Undulations in the film thickness were observed that were induced by a Rayleigh instability when the PMMA nanotubes were annealed above the glass transition temperature (Tg). The amplitude of the undulations increased with time and eventually bridged across the cylindrical nanopore in the AAO membrane, resulting in the formation of polymer nanorods with periodically encapsulated holes. A similar behavior was observed when PMMA films were confined within carbon nanotubes (CNT). The Rayleigh instabilities in these confined geometries offer a novel means of controlling and fabricating the polymer nanostructures. These compartmentalized nanorods may have potential applications as delivery devices. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B18.00012: Dynamic Consequences of the Fractal Network of Nanotube - Poly(ethylene oxide) Nanocomposites Tirtha Chatterjee, Ramanan Krishnamoorti In this work, SWNTs are successfully dispersed in a PEO matrix with the aid of an anionic surfactant. A geometric percolation at $\sim $ 0.1 vol {\%} of SWNTs ($\phi _{c})$ is observed and indicates a good state of dispersion of tubes with an effective aspect ratio of $\sim $ 650. At compositions ($\phi )$ well above the percolation threshold, the melt state rheological properties of the nanocomposites are dominated by the self-similar fractal network of the nanotubes (verified by scattering measurements) and demonstrate `time-temperature-concentration' superposition. The scaling of the network elastic strength, G $\left( {\propto \left( {\phi -\phi _c } \right)^{3.4}} \right)$, and critical strain for the onset of shear-thinning, $\gamma _{c} \quad \left( {\propto \left( {\phi -\phi _c } \right)^{-1.9}} \right)$, reveal a bond-bending mechanism to bear stress as expected from the strong short-range interactions between nanotubes. The onset of non-linearity and the damping behavior of the network show concentration invariance when represented against the local strain experienced by the network elements, with the onset occurring at a local strain value of 0.1. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B18.00013: Gelation of Freely Associating Single-Wall Carbon Nanotube Networks Daniel Chen, Larry Hough, Mohammad Islam, Arjun Yodh We report on the rheological evolution of a model filamentous network comprised of a semidilute dispersion of Single-Wall Carbon Nanotubes (SWNT). Using microrheology, we follow the gelation of a surfactant stabilized SWNT suspension from an initial sol of contacting but unbonded SWNT, endpointing in a gel network with a finite, zero-frequency elastic modulus. The SWNT network exhibits all the hallmarks of a critical sol-gel transition, including divergence of the viscosity as the gel point is approached from below, emergence of a finite elastic shear modulus above the gel point, and power law scaling of the viscosity and shear modulus below and above the gel point, respectively. The viscoelastic moduli exhibit a remarkable collapse under time-cure superposition, a footprint of self-similarity in inter-tube bond connectivity. Additionally, we present a scheme to spatially map rheological inhomogeneities in the network during gelation. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B18.00014: Preparation and rheology of supercritical CO$_{2}$-based dispersed polymer-clay nanocomposites Rangaramanujam Kannan, Steven Horsch, Esin Gulari Effective dispersion of the fillers in a polymer matrix has been a key challenge in the field of nanocomposites. Supercritical carbon dioxide (scCO$_{2})$ appears , PS/clay, The nanocomposites are characterized using WAXD, SEM, TEM, Rheology and DSC. The high degree of dispersion achieved through sc-CO$_{2}$ appears to result in an order of magnitude increase in the rheological properties of PS, associated with an increase in the T$_{g}$ of around 13\r{ }C, at 10{\%} clay loading. These moduli improvements are significant better than those obtained with conventional, chemically-modified intercalated clay nanocomposites. The degree of enhancement in the properties appears to be strongly dependant on the polymer-clay interactions, and how it is promoted by the supercritical fluid. In the case of PDMS nanocomposites, where the clay-polymer interactions were weak, the modulus increase at low frequencies (for sc-CO$_{2}$ processed system) was only a factor of 2. In the case of PVME- I30P clay nanocomposites, the modulus increase was substantial even at moderate loadings and dispersions, perhaps to be hydrogen-bonding interactions. Our results indicate that scCO2 can not only disperse nanoclays in polymers, it can also significantly enhance clay-polymer interactions. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B18.00015: ABSTRACT WITHDRAWN |
Session B19: Focus Session: New Frontiers in Imaging II
Sponsoring Units: DCPChair: Galina Pavlovskaya, Colorado State University
Room: Colorado Convention Center 104
Monday, March 5, 2007 11:15AM - 11:51AM |
B19.00001: NMR mapping of ionic currents and electro-osmotic flow in microsystem channel networks Invited Speaker: Magnetic resonance tomography is known to provide images the contrasts of which are determined by a combination of several parameters. On the one hand these can be system parameters like spin density, relaxation times and diffusion coefficients. On the other hand, the contrasts will be affected by experimental parameters like echo time, repetition time, and by the type of the radio frequency pulse sequence used to generate the signals. In contrast to this, we are interested in ``maps'' where a well defined system parameter is quantitatively encoded in gray shades or colors. A frequently employed technique of this sort is mapping of pressure induced flow. Apart from this, the objective of the present study is to examine and compare maps of the ionic current density and electro-osmotic flow in channel networks on a microscopic length scale. As a paradigm for complex pore spaces, model objects of random and correlated site percolation clusters were fabricated and filled with electrolyte solutions. The experimental maps were compared with computational fluid dynamics simulations based on finite element techniques. The patterns observed in maps of the current density, pressure induced and electro-osmotic flow velocity strongly deviate from each other. This is due to the different transport resistance characteristics and the different nature of the driving forces. The patterns of the spatial distribution of the electric current density measured in the pore space of plastic objects (no electro-osmotic flow superimposed), for example, is totally different from those found in ceramic objects (electro-osmotic flow superimposed). Vortices and recirculation patterns have been observed for all transport quantities, but at different sites. The findings can be explained and elucidated on the basis of the computational fluid dynamics simulations and experiments with test objects especially designed for this purpose. \newline \newline (1) B. Buhai and R. Kimmich, Phys. Rev. Letters \textbf{96}, 174501 (2006). \newline (2) B. Buhai, T. Binser, and R. Kimmich, Appl. Magn. Reson., in press. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B19.00002: Insights into the Distribution of Water in Operating Proton-Exchange Membrane Fuel Cells Using H-1 NMR Imaging. Roderick Wasylishen, Kirk Feindel, Steven Bergens The operation of proton-exchange membrane fuel cells (PEMFCs) depends critically on the amount and distribution of water throughout the FC (K.W. Feindel, S.H. Bergens, and R.E. Wasylishen \textit{ChemPhysChem}, \textbf{2006}, $7$, 67-75). To study in-situ the distribution of water in an operating PEMFC using H-1 NMR imaging we constructed PEMFCs to operate within a 10 or 30 mm coil. Recent results provide the first images of the in-plane distribution of water within the PEM. The influence of gas flow configuration in a self-humidifying PEMFC was investigated, and the results are qualitatively in agreement with predictions from several theoretical models. The maximum power output occurs when water begins to accumulate in the cathode flow field, and the integral of the image intensity from the PEM correlates with the power output. The use of H-D exchange has been found effective to introduce contrast in H-1 NMR images. Our results demonstrate that H-1 NMR imaging is capable of providing information on the in-situ operation of PEMFCs that is difficult or impossible to obtain with other PEMFC diagnostic techniques. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B19.00003: Concentration and Velocity Measurements of Both Phases in Liquid-Solid Slurries Stephen Altobelli, Kimberly Hill, Arvind Caprihan Natural and industrial slurry flows abound. They are difficult to calculate and to measure. We demonstrate a simple technique for studying steady slurries. We previously used time-of-flight techniques to study pressure driven slurry flow in pipes. Only the continuous phase velocity and concentration fields were measured. The discrete phase concentration was inferred. In slurries composed of spherical, oil-filled pills and poly-methyl-siloxane oils, we were able to use inversion nulling to measure the concentration and velocity fields of both phases. Pills are available in 1-5mm diameter and silicone oils are available in a wide range of viscosities, so a range of flows can be studied. We demonstrated the technique in horizontal, rotating cylinder flows. We combined two tried and true methods to do these experiments. The first used the difference in T1 to select between phases. The second used gradient waveforms with controlled first moments to produce velocity dependent phase shifts. One novel processing method was developed that allows us to use static continuous phase measurements to reference both the continuous and discrete phase velocity images. ? [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B19.00004: NMR velocity imaging of single liquid drops A. Amar, S. Stapf, B. Bluemich Liquid-liquid extraction processes are often found in industrial applications when a bulk phase needs to be purified from dissolved components. The extraction strategy consists of dissolving the impurities into a second, carrier phase, with optimal performance being guaranteed by maximizing both contact interface area and mass transfer rate, in the shape of a swarm of dispersed droplets. Their buoyancy-driven flow within the continuous medium induces internal fluid motion driven by momentum transfer at the drop surface. This convective transport enhances mass transfer and the efficiency of an extraction column. However, understanding mass transfer depends on a proper description of the flow field inside and outside the drops. For that purpose, a cell was built that enables the levitation of a single drop within a counterstream of water. NMR velocity imaging was then applied to drops of different fluids to monitor the internal dynamics as a function of drop size, age, and interface tension. Vortex-type patterns in at least part of the drop were observed where their size and velocity magnitude depended on the system impurity concentration. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B19.00005: Velocity, correlation time and diffusivity measurements in highly turbulent gas flow by an MRI method Zhi Yang, Ben Newling We present non-invasive, quantitative MRI wind-tunnel measurements in flowing gas (velocity $>$ 10 m/s) at high Reynolds numbers (Re $>$ 10$^{5})$. Our measurement method is three-dimensional and has the potential for saving time over traditional pointwise techniques. The method is suitable for liquids and for gases. We demonstrate the use of the technique on different test sections (bluff obstruction, clark Y-wing and cylinder). The mean velocity of gas flowing past those sections has been measured. We also investigate methods to measure flow correlation times by changing the acquisition interval between excitation of the sample and detection of the signal. This may be accomplished by making separate measurements or by using a multiple-point acquisition method. A measurement of correlation time allows us to map turbulent diffusivity. The MRI data are compared with computational fluid dynamics. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 1:15PM |
B19.00006: Recent progress in NMR/MRI in petroleum applications Invited Speaker: NMR has become an important technique for characterization of porous materials. In particular, its importance in petroleum exploration has been enhanced by the recent progress in NMR well-logging techniques and instruments. Such advanced techniques are increasing being accepted as a valuable service especially in deep-sea exploration. This paper will outline the recent progress of MR techniques at Schlumberger-Doll Research. {\bf Well-logging} - The second generation NMR well-logging tool and the 2D NMR methods (D-T2, etc) enable measurements at several depths from the well bore allowing a one-dimensional profiling of the fluid. Such data have allowed quantification of fluid invasion during drilling, obtaining the properties of native fluids and identifying oil/gas zones. {\bf MRI}- Rocks from oil reservoirs are heterogeneous (e.g. large range of pore sizes and porosity variation) due to the complex geological and geochemical histories. The spatial pattern of the heterogeneity has not been well studied. We have developed several NMR techniques to quantify pore length scale previously. In order to predict flow over a large length scale, it is necessary to determine spatial heterogeneity and pore connectivity over the relevant size. We have performed MRI on a series of carbonate rocks and found interesting patterns of the heterogeneity characteristics. {\bf Mathematics} - It is well known that the Laplace inversion is non-unique and the resulting spectrum can be strongly dependent on the prior constraints, specific algorithm and noise. However, the different spectra can all be solutions consistent with data. It would be useful to have a robust criterion -- independent of algorithms -- to determine the properties of the resulting spectrum. Several methods will be described to examine the statistics of the solutions, uncertainty of the spectrum and its integrals and resolution. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B19.00007: MRI measurements of heterogeneity in carbonate rock cores Andrew Pomerantz, Eric Sigmund, Yi-Qiao Song Magnetic resonance imaging (MRI) provides spatially resolved measurements of the spin-spin relaxation time ($T_{2})$ of brine that saturates the pores of carbonate rocks.~ Images with millimeter resolution reveal relaxation that can be well described by a double exponential in each voxel. From these images, it is possible to describe the length scales and extent of spatial heterogeneity both qualitatively and quantitatively. Qualitatively, the fitted values from each voxel can be combined into a histogram to make a $T_{2}$ distribution, and histograms produced at different resolutions can be compared to each other and to the Laplace inversion of CPMG data for the whole core. Quantitatively, experimental semi-variograms can be constructed and analyzed using geostatistical techniques. In general, heterogeneity both above and below the 1~mm imaging resolution is observed, although the extent of heterogeneity is found to vary greatly between rock cores. In many cases, the qualitative features of the $T_{2}$ distribution for the entire core are manifest in almost every individual voxel of microliter volume, indicating significant heterogeneity at short length scales. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B19.00008: Visualization of the secondary flow formation during contraction flow of a viscoelastic fluid by NMR methods Galina Pavlovskaya Viscoelastic fluids are known to exhibit a variety of flow transitions at low Re numbers. These fluids are also known to form secondary flows in the corners of abrupt contractions during the entry flow. We have applied NMR methods to determine the areas where the vortices are formed during the entry flow of a viscoelastic fluid in a tubular abrupt contraction. We have measured volume-averaged dispersion coefficients at different axial locations above the contraction entry plane. We also have measured velocity fields in the same spatial locations. In both types of experiments the flow encoding time was varied. We found no time dependence in the velocity fields while dispersion coefficient profiles were strongly dependent on the flow encoding time in the areas where vortices were formed. Based upon these results some information about the dynamics of the vortex formation in this type of flow could be deduced. [Preview Abstract] |
Session B20: Focus Session: Properties of Ferroelectrics and Relaxors
Sponsoring Units: DMPChair: Hans Christen, Oak Ridge National Laboratory
Room: Colorado Convention Center 105
Monday, March 5, 2007 11:15AM - 11:27AM |
B20.00001: Multiple-Scattering of Millimeter Waves in Random Dielectrics Dustin McIntosh, John A. Scales, L. D. Carr, Valentin Freilikher, Yu. P. Bliokh We investigate millimeter wave localization in random binary-layered dielectrics composed of sub-wavelength scatterers. We measure the broad-band phase-dependent reflection and transmission response of the system. The random dielectrics exhibit band gaps and transmission resonances, the hallmark of localization. The band gaps correspond to forbidden mode propagation; the resonances to effective cavities in the system. These cavities are associated with enhanced attenuation and slow light which we observe in both experiment and theory. These effects are a result of weak multiple-scattering by the layer boundaries due to the disorder in the dielectric stack. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B20.00002: Effect of Stress on Dielectric Loss Behavior in Ba0.7Sr0.3TiO3 Thin-Films Ricardo Zednik, Paul McIntyre, John Baniecki, Masatoshi Ishii, Kazuaki Kurihara, Kazunori Yamanaka We present the results of a systematic dielectric study of barium strontium titanate thin-film planar capacitors measured over a wide temperature range of 20 K to 575 K for frequencies between 1 kHz and 1 MHz. Sputter deposition on substrates with different thermal expansion coefficients were used to produce barium strontium titanate thin-films that differ only in their stress states. Over a narrow temperature range, near room temperature, the dielectric dispersion can be understood in terms of the phenomenological Curie-von Schweidler relationship (Universal Relaxation Law), with the complex permittivity following a power law dependence on frequency. However, outside this range, dielectric-loss peaks are observed in the temperature and frequency domains that can be closely fit to the Vogel-Fulcher expression used in describing relaxors. Additionally, the loss peaks shift with the stress state of the film. These observations suggest that the relaxor-like dielectric behavior of our barium strontium titanate thin-films may be affected by a possible phase transition. The dependence of the dielectric loss behavior on film stress in the Curie-von Schweidler and Vogel-Fulcher regimes will be contrasted and mechanisms for the observed stress dependent dielectric loss behavior discussed. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B20.00003: Ferroelectric Phase Transition Study of Coupling KTN Perovskites Oxide by Scanning Microwave Microscope. Shuogang Huang, Mark Reeves, Jennifer Sigman, David Norton, Hans Christen We used a scanning near field microwave microscope to determine the thin film dielectric properties of KTN near transition temperature. For solid solution K(NbxTa1-x)O3 thin film deposited on MgAl2O3 substrate a 1st order phase transition was observed and for KTN 1x1 super lattice two 2nd order phase transitions were observed. Then a finite element method simulation was applied to numerically calculate the dielectric constant of the samples in difference phases. The results show a strong consistent with the previous x-ray and capacitance measurements. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:27PM |
B20.00004: Interaction of Terahertz Radiation with Ferroelectrics Invited Speaker: Ferroelectric crystals have long been used as acoustic transducers and receivers. An extensive toolset has been developed for MHz-frequency acoustic wave generation, control, guidance, and readout. In recent years, an analogous toolset has been developed for terahertz wave transduction and detection. Femtosecond optical pulses irradiate ferroelectric crystals to generate responses in the 0.1-5 THz frequency range that are admixtures of electromagnetic and polar lattice vibrational excitations called phonon-polaritons. Spatiotemporal femtosecond pulse shaping may be used to generate additional optical pulses that arrive at specified times and sample locations for control and manipulation of the THz waves. Femtosecond laser machining may be used for fabrication of waveguides, resonators, and other structures that are integrated into the ferroelectric host crystal. Finally, real-space imaging of the THz fields can be executed with variably delayed femtosecond probe pulses, permitting direct visualization of THz wave spatial and temporal evolution. This ``polaritonics'' toolset enables multiplexed generation of arbitrary THz waveforms and use of the waveforms within the ferroelectric host crystal or after projection into free space or an adjacent medium. The polaritonics platform will be reviewed and several new developments and applications will be presented. These include spectroscopy of relaxor ferroelectrics, whose temperature-dependent dielectric responses in the GHz-THz regime reveal complex polarization dynamics on well separated fast and slow time scales; direct measurement of phonon-polariton lattice vibrational displacements through femtosecond time-resolved x-ray diffraction; generation of high polariton field amplitudes and pulse energies; use of large-amplitude polariton waves to drive nonlinear lattice vibrational responses; and enhancement of optical-to-THz conversion efficiency through a pseudo-phase-matching approach that circumvents the very large disparity between refractive index values at optical and THz frequencies. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B20.00005: Elastic constants and sound velocities in single crystal transition metal scandates Matthew G. Hilt, K.A. Pestka II, Jin H. So, J.D. Maynard An important effect on thin films deposited on substrates is the strain induced by lattice mismatch. Different perovskite structured transition metal scandates have similar a-axis lattice parameters but slightly different c-axis lattice parameters. By adjusting the transition metal content, the c-axis lattice parameter may be controlled, so that if these materials are used as substrates, lattice mismatch may be greatly reduced. To further match lattices dynamically, it is necessary to know the elastic constants of the scandate materials. However, only small single crystals of GdScO3, DyScO3, SmScO3, and NdScO3 have been fabricated. By using the small sample version of resonant ultrasound spectroscopy, we have determined the elastic constants and sound velocities for several transition metal scandates. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B20.00006: Additional mode of PbZr$_{x}$Ti$_{1-x}$O$_{3}$ films Chi Yat Yau, Relva Buchanan In addition to the phonon modes predicted by the selection rule, a phonon mode at 45 cm$^{-1}$ (lower than soft mode frequency) was observed in the first order Raman spectrum of ferroelectric PbZr$_{x}$Ti$_{1-x}$O$_{3}$ films with $x$ = 0 - 1. Thus, this additional mode is not a zone-center mode. Its existence in the first order spectrum is due to the zone folding in structures with strain, defect and disorder. This additional mode, like soft mode, also significantly downshifts as approaching the phase boundaries, e.g. at $x$ = 0.5 and 0.95, or with the grain size change. The similar change of the additional mode and the soft mode implies a strong interaction between the soft mode and the additional mode. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B20.00007: Phonon anomalies induced by polar nano-regions in a relaxor ferroelectric Guangyong Xu, Jinsheng Wen, Chris Stock, Peter Gehring Inelastic neutron scattering was used to measure both acoustic and optic phonons polarized along (110) (T2 mode) in the relaxor ferroelectric compound PZN-4.5PT. In the low temperature rhombohedral phase, a single domain state was achieved by cooling the single crystal sample under an external electric field of 2 kV/cm along the [111] direction. Phonon measurements were performed near the (2,2,0) and (2,-2,0) Bragg peaks. We have found that the TA2 phonon couples closely to the diffuse scattering, which arises from polar nano-regions in the system. With the redistribution of diffuse scattering under the external field (see Ref. 1), a clear hardening of TA2 mode was observed near the (2,2,0) Bragg peak, while the TA2 mode near (2,-2,0) Bragg peak softens significantly and becomes over-damped. Our results indicate local inhomogeneities such as the PNR can have direct and significant effects on the lattice dynamics and stability of the whole system. Ref. 1: ``Electric-field-induced redistribution of polar nano-regions in a relaxor ferroelectric'', Guangyong~Xu, Z.~Zhong, Y.~Bing, Z.-G.~Ye, and G.~Shirane, Nature Materials 5, 134, (2006). [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B20.00008: Cubic ground state of field-cooled PbMg$_{1/3}$Nb$_{2/3}$O$_3$ Peter Gehring, Chris Stock, Guangyong Xu, Haosu Luo, Hu Cao, Jiefang Li, Dwight Viehland, Gen Shirane Neutron and x-ray diffraction techniques have been used to study the competing long and short-range polar order in the relaxor PbMg$_{1/3}$Nb$_{2/3}$O$_3$ (PMN) under the influence of an external [111]-oriented electric field. While the bulk unit cell remains cubic for electric fields up to 8\,kV/cm, a suppression of the diffuse scattering and a concomitant enhancement of the Bragg peak intensity is observed below $T_c=210$\,K, indicating a more ordered structure with increasing electric field yet an absence of a long-range ferroelectric ground state. The electric field strength has little effect on the diffuse scattering above $T_c$. The absence of hysteresis suggests that the ground state of PMN may not be a frozen glassy phase, but may be better understood in terms of random fields introduced through the presence of structural disorder. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B20.00009: Dielectric Spectroscopy and conductivity relaxation of PSN-PST relaxor thin films Margarita Correa, Natasan Baskaran, Ram Katiyar -abstract- Relaxor ferroelectric materials exhibit singular dielectric relaxation. They have large dielectric constant, high piezo and electrostrictive coefficients that make them useful for sensors, actuators and ferroelectric related devices. We have prepared PSN-PST relaxor thin films by pulsed laser deposition technique. Studies of dielectric properties, as a function of temperature (100-650K) and frequency (40 Hz --1 MHz) have shown that the materials have diffuse phase transition along with the frequency dispersion. The magnitude of dielectric constant and the temperature of maximum permitivitty (T$_{m})$ differ from those in the bulk form. However, its conductivity behavior follows similar trend in both bulk and thin film forms. The ac conductivity exhibits frequency independent plateau at lower frequencies followed by a dispersion region at higher frequencies. The observed dispersion of conductivity with frequency can be described by a power law $\sigma \left( \omega \right)=\sigma _{dc} +A\omega ^n$ with n$>$1. The unusual high n value is due to glassy like nature of the relaxor system and it is explained in terms of polaron assisted ionic mechanism. Detailed results will be presented. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B20.00010: Quantitative Huang-scattering analysis of local structure in the relaxor-based piezoelectric PZN-4.5{\%}PT Branton J. Campbell, Vayee Vue, Daniel Robertson, Stephan Rosenkranz, Peter Lee, Stine N. Anacona, Ray Osborn Polar nano-regions (PNR) in ferroelectric relaxor materials like Pb(Zn$_{1/3}$Nb$_{2/3})$O$_{3}$ and Pb(Mg$_{1/3}$Nb$_{2/3})$O$_{3}$ are of pressing applied interest due to their influence on the remarkable piezoelectric properties of their solid solutions with PbTiO$_{3}$. In the recent literature, x-ray and neutron single-crystal diffuse scattering techniques have been shown to provide qualitative insight into the local structure of these materials. Here, we present a quantitative three-dimensional Huang-scattering analysis of 80 keV x-ray single-crystal diffuse scattering data from PZN-4.5{\%}PT. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B20.00011: NMR Observation of Impurity-Pair Ordering in Weakly Disordered Solid Solutions David Ailion, Bostjan Zalar, Andrija Lebar Breaking of the average cubic symmetry in Li-doped potassium tantalate (K$_{1-x}$Li$_{x}$TaO$_{3})$ was observed with quadrupole-perturbed $^{7}$LiNMR at temperatures (150-400 K) far above the nominal glass transition temperature (80 K). The observed spectrum consists of contributions from both isolated Li ions (i.e., with no nearest neighbor Li) and from Li pairs. The isolated Li ions move among six equivalent off-center sites in a potential having cubic symmetry. These have zero average electric-field gradient and, hence, exhibit no quadrupole splitting. In addition, very low intensity, but well resolved, quadrupole satellites having a temperature-dependent splitting were observed. This splitting indicates that the various Li pair configurations are not all equally probable. These are the first observations of biased Li-ion ordering that persists in the paraelectric phase at temperatures high above the glass phase. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B20.00012: Local Basis Set Supercell Studies of (K,Na)NbO$_3$ Solid Solutions Ricardo Kagimura, Malliga Suewattana, David J. Singh We report density functional supercell calculations for (K,Na)NbO$_3$ perovskite solid solutions using the local basis SIESTA code. We did detailed comparisons of results for ferroelectric structures and vibrational frequencies obtained with SIESTA with those obtained using all-electron full potential LAPW calculations, and used these comparisons to establish compact but accurate choices of basis set and pseudopotentials for the SIESTA calculations. Supercell calculations using SIESTA are used to investigate the dependence of ferroelectric polarization and local structure on the K/Na ordering. This work was supported by the DOE ORNL LDRD program and the Office of Naval Research. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B20.00013: Effect of elastic strain and Sc dopant concentration-dependent cell volume on the electrical properties of Epitaxial (Ba,Sr)TiO$_{3}$ thin films WooYoung Park, CheolSeong Hwang, John D. Baniecki, Masatoshi Ishii, Kazuaki Kurihara, Kazunori Yamanaka We present the results of a systematic study of the correlation between dopant concentration-dependent elastic strain and dielectric properties in (Ba,Sr)TiO$_{3 }$films. For this work, undoped and Sc-doped (Ba,Sr)TiO$_{3}$ thin film capacitors epitaxially grown on SrTiO$_{3}$ substrate were prepared by a sputtering deposition method. Sc-doped BST capacitors exhibit significantly higher permittivity and lower leakage current density, but little effect on the loss tangent, as compared to nominally undoped BST capacitors. The Ti/(Ba+Sr) ratio of the films and Sc dopant concentration-dependent unit cell volume, as determined by x-ray sin$^{2}\psi $ analysis, are consistent with the preferential B-site occupancy of the Sc dopant. Furthermore, this work suggests that dopant concentration-dependent elastic strain as well as the 2D clamping effect of thin films on a thick substrate$^{1}$ must be considered to fully understand the dielectric behavior of perovskite titanate thin films. 1. N. A. Pertsev, A. G. Zembilgotov, and A. K. Tagantsev, Phys. Rev. Lett. 80, 1988 (1998) [Preview Abstract] |
Session B21: Society of Physics Students
Sponsoring Units: SPSChair: Gary White, Society of Physics Students/AIP-Education
Room: Colorado Convention Center 106
Monday, March 5, 2007 11:15AM - 11:27AM |
B21.00001: Electronic Structure Calculations of Si, SiC, Si$_{3}$N$_{4}$ and SiO$_{2}$ Ryan Jacob, A.R. Chourasia The band structure of Si, SiC, Si$_{3}$N$_{4}$ and SiO$_{2}$ have been studied using a DFT computational approach implemented in CRYSTAL98. The Becke exchange has been employed. Two types of correlations have been explored: Perdew, Wang, and Lee, Yang, Parr. The atomic basis sets with a polarization function have been optimized for each configuration in these materials. The density of states in the valence and conduction bands has been computed in each case. The projected density of states of the constituents has also been computed. The band gap and the dielectric constant have been calculated for these materials. These values have been compared with the available experimental data. The correlation between the electronic polarizability and the Auger parameter determined previously from x-ray photoelectron spectroscopy will be presented. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B21.00002: Electronic energy transfer in the layered single crystal Sm[Ag$_{.5}$Au$_{.5}$(CN)$_{2}$]$_{3}$ Stefan Oehrlein, Christie Larochelle The aim of this study is to examine the energy transfer between [Ag$_{.5}$Au$_{.5}$(CN)$_{2}$]$_{3}$ donors and the Sm$^{3+}$ acceptors with which they have been doped. The single crystals of [Ag$_{.5}$Au$_{.5}$(CN)$_{2}$]$_{3}$ have been doped with varying concentrations of Samarium. Energy transfer was characterized using steady-state excitation and emission spectra, lifetime measurements and time-resolved spectra. Data was obtained as a function of temperature between 77 K and room temperature. Strong luminescence is seen at all temperatures in the mixed-metal system, a contrast from the luminescence displayed in the pure gold and silver dicyanide donors. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B21.00003: Anomalous elastic response of diamond single crystals to shock compression Keith McLaughlin, Mikhail Ladanov, Ivan Oleynik, Sergey Zybin, Mark Elert, C.T. White We have performed large-scale molecular-dynamics simulations of shock-wave propagation in single-crystal diamond and observed an anomalous elastic response of the material in the intermediate range of shock-wave intensities between the elastic-plastic split shock-wave regime and the graphitization regime of shock compression. The anomalous elastic response is characterized by the absence of plastic deformations in highly uniaxially compressed material. The unusual materials response in shock-compressed diamond is attributed to unique and complex constitutive relationships: both shear and longitudinal stresses are non-monotonic functions of compression. This example clearly demonstrates the necessity of generalization of the notion of the Hugoniot elastic limit (HEL) to include critical shear stresses in a criterion of materials yielding upon shock compression. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B21.00004: Magnetic Properties of Manganese(III) in Cluster-based Coordination Polymers. Kevin J. Little, D.M. Pajerowski, M.W. Meisel, D.R. Talham, H. Zhou, A. Lachgar Magnetization studies, using a SQUID magnetometer operating down to 2~K and up to 7 T, were performed on one-, two-, and three-dimensional coordination polymers built with [Mn($L$)]$^+$ complexes and [Nb$_6$Cl$_{12}$(CN)$_6$]$^{4-}$ clusters. All three structures exhibited paramagnetic properties and no signs of long-range ordering down to 2 K. The magnetic properties are dominated by the Mn(III) ions, $S = 2$ and $g \approx 2$, per formula unit, and deviations between the samples at low temperatures are associated with the different anisotropic Mn(III) environments of each material. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:27PM |
B21.00005: Thickness dependent properties of CMR Manganite thin films on lattice mismatched substrates: Distinguishing Strain and Interface Effects Anthony Davidson III, Rajeswari Kolagani, Ellisaveta Bacharova, Grace Yong, Vera Smolyaninova, David Schaefer, Rajeh Mundle Epitaxial thin films of CMR manganite materials have been known to show thickness dependent electrical and magnetic properties on lattice mismatched substrates. Below a critical thickness, insulator-metal transition is suppressed. These effects have been largely attributed to the role of bi-axial lattice mismatch strain. Our recent results of epitaxial thin films of La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) on two substrates with varying degrees of compressive lattice mismatch indicate that, in addition to the effect of lattice mismatch strain, the thickness dependence of the properties are influenced by other factors possibly related to the nature of the film substrate interface and defects such as twin boundaries. We have compared the properties of LCMO films on (100) oriented LaAlO$_{3}$ and (001) oriented NdCaAlO$_{4 }$both of which induce compressive bi-axial strain. Interestingly, the suppression of the insulator-metal transition is less in films on NCAO which has a larger lattice mismatch. We will present results correlating the electrical and magneto transport properties with the structure and morphology of the films. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:51PM |
B21.00006: Aspects of melting in thin molecular films adsorbed onto a graphite substrate using molecular dynamics simulations Cary Pint This work is conducted in order to study the behavior of very thin (nearly 2-D) films of molecules adsorbed onto a graphite substrate when the film is exposed to temperatures near the temperature-induced melting point. Simulations of both the linear normal alkanes (C$_{n}$H$_{2n+2})$ and perflourinated alkanes (C$_{n}$F$_{2n+2})$ of different chain lengths indicate that the nature of the melting transition in thin films composed of these molecules are dependent upon features of the molecules themselves, such as their chain length and flexibility. This is due to thermal fluctuations that dominate the behavior of the film prior to the melting point which seem to depend on these molecular features, as well as on aspects of the film- such as the adsorbed solid phase. A picture of melting in (quasi) 2-D films based on ``allowed'' thermal fluctuations is presented, and is expected to be generalizable to more complex films of molecules on a variety of different solid substrates. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B21.00007: First-principles molecular reactive dynamics of energetic materials Aaron Landerville, Ivan Oleynik, Carter White The understanding of initiation chemistry of shock-compressed energetic materials on the atomic scale is of fundamental importance for developing a predictive theory of initiation of detonation. We have performed first-principles density-functional modeling of reactive molecular collisions of PETN and RDX molecules aimed at elucidating the first chemical events that trigger the chemistry behind the shock wave front. We will discuss fundamental mechanisms responsible for the transformation of mechanical energy from the shock wave into molecular degrees of freedom that result in excitation of a reaction mode and eventual bond breaking. We will also discuss the stereochemistry of initial reaction events, unimolecular, bi-molecular or multi-molecular nature of initial reactive events and the relationship of the simulated reactive collisions with a non-equilibrium shock wave environment. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B21.00008: Determining the Time Evolution of Bose-Einstein Condensate Merideth Frey Bose-Einstein condensates (BEC) offer a macroscopic way to analyze the quantum mechanical world. In order to measure most properties of these condensates, the cooled atomic gas must be released from its potential trap and allowed to expand. Oftentimes an analytic solution for the time-evolution of the BEC wavefunction after release from the trap cannot be found and a numerical solution is needed. By applying a recent numerical method for solving the Gross-Pitaevskii equation, the time-evolution for BEC after release from potential traps of various geometries can be found. For this project, spherically and cylindrically symmetric traps are analyzed. When applicable, the numerical results are compared with analytical solutions to evaluate the error in the method. Numerical solutions will also be found for potential trap geometries that produce interesting interference effects due to the quantum behavior of Bose-Einstein condensates. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B21.00009: Regulating the Tumor Cell Population Dynamics by Controlling the Proliferation Rate Sarah Hirschbeck, Mitra Shojania Feizabadi The two-compartment model of cancer cell population dynamics introduces two subpopulations for a tumor (proliferating and quiescent). In precious theoretical models, the interaction of tumor cells with chemotherapeutic drugs is expressed as an additional term which reduces the size of subpopulations because of the killing effect of the drug with different killing rates. We develop a simple mathematical model for a more realistic interaction of anti-cancer drugs with tumor cells. The key assumption used in developing this model is that the anti-cancer drug not only kills the subpopulations but also decreases the proliferating rate of the proliferating subpopulation during the course of therapy. Finally, we present the numerical result for the evolution of the subpopulations based on this model. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B21.00010: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 1:39PM - 1:51PM |
B21.00011: Coherent Population Exchange between BCS Atom Pairs and Ground Molecular BEC Atom Pairs Andrew Robertson, Hong Ling Two fermionic atoms of opposite spins (represented by the hyperfine Zeeman sublevels)~can be combined either into a molecule highly localized in real space or into a Bardeen-Cooper-Schrieffer (BCS) pair highly correlated in momentum space.~ In an attractive fermionic system with atoms of opposite spins, we show that the simultaneous presence of a Feshbach resonance and an optical laser coupling can lead to a coherent superposition between the ground molecular state and the~BEC state.~By optically perturbing the system, we demonstrate numerically stable~coherent atom-molecule oscillations. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B21.00012: Cr$^{3+}$ optical probes of structural transformation in porous alumina M.L. Lombardi, J.K. Krebs High surface area aluminum oxide is useful as a catalytic support for gas conversion devices. Upon heat treatment at temperatures in the range of 700 to 1200$^{ o}$C, the high surface area structures undergo a series of transformations terminating in a dense ceramic composed of particles with the corundum structure. These structural transformations occur as the porosity of the material decreases, thus reducing the catalytic surface area. Among the various porous aluminum oxide structures, theta alumina is the highest temperature form that maintains porosity. Lanthanum impurities have been shown to increase the conversion temperature for the theta to corundum transition by preventing boundary creep. We report on an optical technique to observe the structural transition by monitoring the spectrum of trivalent chromium impurities. Since the energies of the d electrons of the chromium impurities shift with the local environment of the ion, optical spectra of the impurities provide sensitive probes for the local structure of the host alumina. We monitor the local structure as a function of heat treatment for various lanthanide concentrations. The temperature of the theta to corundum transition is unaffected by lanthanum addition at concentrations below 0.2 molar percent and increases significantly at higher concentrations. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B21.00013: Raman Spectroscopy of Carbon Nanotubes Richard Bergstrom Jr., Ernst Knoesel Raman spectroscopy is an optical technique used to analyze vibrational modes in materials. Raman is used in carbon nanotubes for the study of phonon modes which produce the vibrations. There are two principle bands of phonons in carbon nanotubes, the G-mode and the Breathing mode. Through the examination of the Breathing mode phonons we can determine the nanotube type and size. By studying the relationship between the two phonon modes we would like to gain insight into the conductivity and the heat transfer between nanotubes. [Preview Abstract] |
Session B22: Systems Far from Equilibrium
Sponsoring Units: GSNPChair: Beate Schmittman, Virginia Polytechnic Institute and State University
Room: Colorado Convention Center 108
Monday, March 5, 2007 11:15AM - 11:27AM |
B22.00001: Aging in disordered magnets and local scale-invariance Michel Pleimling, Malte Henkel The aging of the bond-disordered two-dimensional Ising model quenched to below its critical point is studied through the two-time autocorrelator and thermoremanent magnetization (TRM). The corresponding aging exponents are determined. The form of the scaling function of the TRM is well described by the theory of local scale-invariance.\\[0.5cm] M. Henkel and M. Pleimling, Europhys. Lett. {\bf 76}, 561 (2006). [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B22.00002: Minimal Modeling of Driven Dissipative Systems Yair Shokef, Dov Levine By simple modeling of dissipative interactions we resolve fundamental questions related to systems far from thermal equilibrium, such as granular materials, foams and colloidal suspensions. We solve the non-Boltzmann energy distribution, demonstrate the violation of time-dependent fluctuation- dissipation relations, show that different measures of effective temperatures generally differ, and address further issues such as ergodicity breaking and detailed balance violation. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B22.00003: Work Distributions far from Equilibrium in Quantum Spin Chaines Sven Dororsz We are investigating the non equilibrium steady state for different quantum spin chaines by an exact numerical calculation. The systems, initially in a canonical state, are driven out of equilibrium by a periodic external force which couples to each site. Motivated by the Jarzynski relation and the fluctuation theorem for quantum systems we analyze in detail the probability distribution P(W). Both finite and infinite temperatures are discussed. General properties independent of the nature of the interaction are identified and we observe two dynamic regimes in the limit of small and large frequencies. The intermediate regime is caracterized by resonance peaks in the distribution. The moments of the distribution can be exploited to discuss the possible definitions of a Jarzynski-operator for quantum systems. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B22.00004: Conjugate Field and Fluctuation-Dissipation Relation for the Dynamic Phase Transition in the Two-dimensional Kinetic Ising Model D.T. Robb, P.A. Rikvold, A. Berger, M.A. Novotny The two-dimensional kinetic Ising model, when exposed to an oscillating magnetic field with zero time-average, has been shown to exhibit a nonequilibrium, second-order dynamic phase transition (DPT), whose order parameter $Q$ is the period-averaged magnetization. It has been established that this DPT falls in the same universality class as the equilibrium phase transition in the two-dimensional Ising model in zero applied field. Here we apply a square-wave field with (for the first time) a non-zero period-averaged magnetic field, $H_b$, and study the scaling of the dynamic order parameter with respect to $H_b$. We find evidence that the field scaling exponent, $\delta_{\mathrm{d}}$, at the critical period of the DPT is equal to the exponent for the critical isotherm, $\delta _{\mathrm{e}}$, in the equilibrium Ising model. A finite-size scaling analysis of the dynamic order parameter in the critical region provides further support for this result. We also demonstrate numerically that a fluctuation-dissipation relation (FDR), with an effective temperature $T_{\mathrm{eff}}\left(T, P, H_0\right)$ depending on the period, and possibly the temperature and field amplitude, holds for the dynamic order parameter and its conjugate field. This FDR justifies our use of the scaled variance of $Q$ as a proxy for the nonequilibrium susceptibility, $\partial \langle Q \rangle / \partial H_b$. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B22.00005: A possible classification of nonequilibrium steady states. Royce K.P. Zia, Beate Schmittmann We propose a general classification of nonequilibrium steady states in terms of their stationary probability distribution and the associated probability currents. The stationary probabilities can be represented graph-theoretically as directed Cayley trees; closing a single loop in such a graph leads to a representation of probability currents. This classification allows us to identify all choices of transition rates, based on a master equation, which generate the same nonequilibrium steady state. We explore the implications of this freedom, e.g., for entropy production, and provide a number of examples. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B22.00006: Noise induced chaos in optically driven colloidal rings. Yael Roichman, George Zaslavsky, David G. Grier Given a constant flux of energy, many driven dissipative systems rapidly organize themselves into configurations that support steady state motion. Examples include swarming of bacterial colonies, convection in shaken sandpiles, and synchronization in flowing traffic. How simple objects interacting in simple ways self-organize generally is not understood, mainly because so few of the available experimental systems afford the necessary access to their microscopic degrees of freedom. This talk introduces a new class of model driven dissipative systems typified by three colloidal spheres circulating around a ring-like optical trap known as an optical vortex. By controlling the interplay between hydrodynamic interactions and fixed disorder we are able to drive a transition from a previously predicted periodic steady state to fully developed chaos. In addition, by tracking both microscopic trajectories and macroscopic collective fluctuations the relation between the onset of microscopic weak chaos and the evolution of space-time self-similarity in macroscopic transport properties is revealed. In a broader scope, several optical vortices can be coupled to create a large dissipative system where each building block has internal degrees of freedom. In such systems the little understood dynamics of processes like frustration and jamming, fluctuation-dissipation relations and the propagation of collective motion can be tracked microscopically. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B22.00007: Fluctuation-dissipation relations in driven dissipative systems Guy Bunin, Yair Shokef, Dov Levine Exact theoretical results for the violation of time dependent fluctuation-dissipation relations in driven dissipative systems are presented. The ratio of correlation to delayed response in the stochastic model introduced in [Phys. Rev. Lett. 93, 240601 (2004)] is shown to depend on measurement time. The fluctuation temperature defined by this ratio differs both from the temperature of the environment performing the driving, and from other effective temperatures of the system, such as the average energy (or ``granular temperature''). General explanations are given for the time independence of fluctuation temperature for simple measurements or long measurement times. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B22.00008: Time Correlations on the Ziff-Gulari-Barshad Model with Random Defects C.S. Dias, A. Cadilhe We studied a generalized version of the Ziff, Gulari, and Barshad model (1986), i.e., of the $A + B_2$ reaction, in order to accomodate the presence of a given fraction of inert sites present on the substrate. Specifically, we show their impact on the kinetics of the catalysis, particularly on the time correlation of the reactants distribution on the surface, to monitorize the evolution, in time, of a list of reactants of both species. We also characterize the reactant cluster structure in the presence of impurities by resorting to the Hoshen-Kopelmann algorithm. We have found two diferent regimes of the time correlation, namely, an initial exponential decay at short times, and a second regime given by a streched exponential decay at late times of the number of surviving particles. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B22.00009: Renormalization Group Treatment of the Trapping Reaction Jack Hanson, Scott McIsaac, Benjamin Vollmayr-Lee We consider the trapping reaction $A+B\to A$, with diffusing traps ($A$) and particles ($B$), where the traps additionally undergo either an annihilation ($A+A\to\emptyset$) or coalescence ($A+A\to A$) reaction. This two-species reaction-diffusion system exhibits asymptotic power law decays in both the trap and particle densities, and simple scaling in the trap-trap ($AA$) and particle-trap ($AB$) correlation functions. However, simulations indicate the induced particle-particle correlations scale as $C_{BB}(x,t) = t^{\phi} f(x/t^{1/2})$ with an anomalous dimension $\phi$ [B.P. V-L and R.C. Rhoades]. We perform a one-loop renormalization group calculation of this exponent for $d<2$ --- which involves 59 diagrams --- and demonstrate that the anomalous dimension is universal and is due to a renormalization of the initial particle density. Our results are compared to the simulation data. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B22.00010: Comparing Extremal and Hysteretic Optimization in Spin Glasses Bruno Goncalves, Stefan Boettcher We compare the capabilities of the $HO$\footnote{PRL 89, 150201} and the $EO$\footnote{PRL 86, 5211} heuristic in finding spin glass ground states. Using a one-parameter model recently discussed by Katzgraber and Young \footnote{PRB 67, 134410} that interpolates between the mean-field, infinte-dimensional SK spin glass and the finite-dimensional EA lattice spin glass, we evaluate the heuristics as a function of that parameter. Our results show interesting variations in algorithmic behavior that elucidates their properties. It may also indicate a transition in the physics between high and low-dimensional spin glasses. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B22.00011: Shock-induced crystalline instabilities Ramon Ravelo, Brad Lee Holian, Timothy C. Germann Uniaxial deformations of single crystals such as those produced under planar shock loading can produce structural instabilities which compete with defect nucleation mechanisms. In fcc single crystals under (110) shock loading, the resulting body-centered orthorhombic crystal structure develops a long-wavelength dynamical instability associated with tetragonal shear distortions, which occurs at lower strains (pressures) than those predicted by the vanishing of the elastic constants at finite pressure (stiffness coefficients). \footnote[2]{J. Wang, S. Yip, S.R. Phillpot, D. Wolf, Phys. Rev. Lett. {\bf 71}, 4182 (1993)} The criterion for these instabilities is derived and verified by equilibrium and non-equilibrium molecular dynamics simulations [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B22.00012: Hydrodynamic solutions of spatially-varying 1D exclusion processes Greg Lakatos, Tom Chou We analyze the open boundary partially asymmetric exclusion process with smoothly varying internal hopping rates in the infinite-size, mean field limit. The mean field equations for particle densities are written in terms of Ricatti equations with the steady-state current $J$ as a parameter. These equations are solved both analytically and numerically. Upon imposing the boundary conditions set by the injection and extraction rates, the currents $J$ are found self-consistently. We find a number of cases where analytic solutions can be found exactly or approximated. Results for $J$ from asymptotic analyses for slowly varying hopping rates agree extremely well with those from extensive Monte Carlo simulations, suggesting that mean field currents are exact as long as the hopping rates vary slowly over the lattice. If the forward hopping rate is greater than or less than the backward hopping rate throughout the entire chain, the three standard steady-state phases are preserved. Our analysis reveals the sensitivity of the current to the relative phase between the forward and backward hopping rate functions. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B22.00013: Off-Lattice 3D Eden Cluster Growth Model Eric Kuennen The Eden model for 2D clusters is understood to be in a large universality class of models and phenomena which have 1D surfaces with growth dynamics as predicted by the KPZ equation. However, the growth behavior of 3D Eden clusters, and that of the KPZ equation for 2D surfaces, is less well understood and a matter of some controversy. Determining which growth phenomena belong to the KPZ universality class in 3D is an important unsolved problem in statistical physics. Previous studies of the Eden model in 3D have all used an underlying lattice and grew clusters vertically from a flat substrate. Since Eden clusters grown on a lattice exhibit significant anisotropies, and in many natural phenomena growth occurs radially from a seed, in this paper, I propose a 3D Eden model for off-lattice clusters grown radially from a seed. With large-scale computer simulations, I investigate the kinetic roughening of the surface by estimating the surface-width growth exponent, in order to determine whether 3D Eden growth indeed belongs to the KPZ universality class. Noise-reduction techniques are used, and for validation the model is applied to a flat substrate geometry, which makes it possible to estimate the roughness exponent as well. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B22.00014: Nucleation and growth of islands during submonolayer deposition on Ag/Ag(100) Nuno Araujo, Antonio Cadilhe The growth of multilayer thin films is strongly influenced by the formation of the first layer. We introduce a kinetic Monte Carlo model to study the nucleation and growth of the first layer in the regime of high values of the incoming flux. We simulate the deposition of Ag adatoms on the Ag(100) substrate at a temperature of 200 K for values of the incident flux of particles ranging between 0.01 ML/s and 1.00 ML/s. To characterize the nucleation process we studied the dependence of the mean island density during growth and the island size distribution on the incident flux of particles. Varying the flux of incident adatoms allows us to tune the relevance of the different elemental processes taking place during the deposition stage. In the limit of high fluxes, we show that scaling functions do not match and the island size distribution function does not have a maximum value coincidental with the mean island size. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B22.00015: Surface Growth Modeling of Load Balancing in Parallel Discrete Event Simulations (PDES) Poonam Verma, Mark Novotny We study a non-equilibrium surface growth model of load balancing for conservative Parallel Discrete Event Simulations (PDES) [Korniss et al., Science 299, {\bf 677} (2003); Guclu et al., Phys. Rev. E {\bf 73}, 066115 (2006)]. Load balancing improves the performance of the parallel simulations by distributing the work load over all processors evenly. These models for static load balancing are in the Kardar-Parisi-Zhang (KPZ) universality class, with the KPZ process often mixed with a Random Deposition (RD) process [Kolakowska et al., Phys. Rev. E {\bf 73}, 011603 (2006)]. We study how the utilization and the desynchronization behave when the load changes randomly during the simulation. We compare the static and dynamic load balancing results for the models of PDES. The underlying framework proposed in [L. N. Shchur and M. A. Novotny Phys. Rev. E {\bf 70}, 026703 (2004)], is that the Local Simulated time (LST) is associated with the nodes and not with the processing elements. [Preview Abstract] |
Session B23: Focus Session: High Pressure II - Earth and Planetary Materials
Sponsoring Units: DMP DCOMPChair: Alexander Goncharov, Carnegie Institution of Washington
Room: Colorado Convention Center 110
Monday, March 5, 2007 11:15AM - 11:51AM |
B23.00001: High-pressure investigations of Earth's interior Invited Speaker: In the first half of the talk, the electronic structure of iron in ferromagnesium silicate perovskite will be discussed. Knowledge of iron valences and spin states in silicate perovskite is relevant to our understanding of the physical and chemical properties of Earth's lower mantle such as transport properties, mechanical behavior, and element partitioning. In this study, we have measured the electronic structure of the iron component of an aluminous Fe-bearing silicate perovskite sample, (Mg$_{0.88}$Fe$_{0.09})$(Si$_{0.94}$Al$_{0.10})$O$_{3}$, close to a pyrolite composition, using synchrotron M\"{o}ssbauer spectroscopy (SMS) and laser heated diamond anvil cells at high-pressure and temperatures at beamline 3-ID of the Advanced Photon Source. Evaluation of the spectra provided the isomer shift and the quadrupole splitting of the iron component in silicate perovskite, which gives information on valence and spin states under lower mantle conditions. In the second half of the talk, experiments on the melting curve of iron at high-pressures will be presented. Seismological observations indicate that Earth's iron-dominated core consists of a solid inner region surrounded by a liquid outer core. Previously, melting studies of iron metal at high-pressures and temperatures were performed by shock-compression, resistive- and laser-heating in diamond anvil cells using visual observations or synchrotron x-ray diffraction and theoretical methods. However, the melting curve of iron is still controversial. Here, we will present a new method of detecting the solid-liquid phase boundary of iron at high-pressure using $^{57}$Fe SMS. The characteristic SMS time signature is observed by fast detectors and vanishes suddenly when melting occurs. This process is described by the Lamb-M\"{o}ssbauer factor $f$ = exp(-$k^{2}<$x$^{2}>)$, where $k$ is the wave number of the resonant x-rays and $<$x$^{2}>$ is the mean-square displacement of the iron atoms. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B23.00002: Infrared Reflectance of Magnesiowustite(Mg$_{1-x}$Fe$_{x}$O): Experiment and Theory Tao Sun, Philip B. Allen, David G. Stahnke, Steven D. Jacobsen, Christopher C. Homes We measured the optical reflectance spectra(0$\sim $32300 cm$^{-1})$ of magnesiowustite(Mg$_{1-x}$Fe$_{x}$O, x=0.06, x=0.27) at 6K and 295K, using a Bruker IFS 113v spectrometer. Kramers-Kronig relations are used to extract the corresponding dielectric functions. The Infrared parts of the spectra resemble those of pure MgO, while showing much smaller temperature dependence. There are two factors determining the structure of dielectric functions: a) anharmonic phonon-phonon interactions, b) disorder scattering. A breathing-shell model is used to evaluate factor a) in pure MgO, and a supercell is built to estimate the influence of factor b) in Fe doped MgO. Our results will be useful for computing the heat conductivity of magnesiowustite in the earth's lower mantle. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B23.00003: Dynamical mean-field theory study of the high pressure behavior of FeO A.C. Kollias, R.E. Cohen First-principles calculations have an important part in the development of our understanding of Earth's interior, including geophysical and geochemical phenomena. Proper treatment of iron bearing minerals is fundamental in this respect. Unfortunately standard density functional theory (DFT) approaches such as the local density (LDA) or the generalized gradient approximations (GGA) fail in describing qualitative features of simple iron containing minerals; for example the insulating nature and magnetic structure of many metal oxides such as FeO. The LDA+U approximation, self-interaction correction (SIC), and dynamical mean-field theory (DMFT) have demonstrated significant improvement in the physical description of transition metal and rare earth compounds. Presented results will focus on theoretical predictions obtained with the DMFT method. The high pressure behavior and high-spin-low-spin phase transition for iron oxide in the distorted rocksalt (B1) structure. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B23.00004: Elastic Signature of the High-Spin to Low-Spin Transition in Magnesiow\"{u}stite Cesar da Silva, Renata Wentzcovitch, Taku Tsuchiya It has been reported that the high to low-spin spin transition in ferrous iron in magnesiow\"{u}stite (Mw) under pressure is accompanied by considerable volume reduction and changes in elastic properties. Using an LDA+U method with consistently calculated U, we investigate the elastic signature of this transition. We confirm that there is large contrast in elasticity across this transition. However, this contrast is temperature sensitive. We address the geophysical signature of this phenomenon. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B23.00005: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 12:39PM - 12:51PM |
B23.00006: \textit{Ab initio} Study of the Composition Dependence of the Pressure-Induced Spin Transition in Perovskite (Mg,Fe)SiO$_{3}$ Amelia Bengtson, Kristin Persson, Dane Morgan We present \textit{ab initio} calculations of the zero-temperature compositional dependent spin transition in (Mg,Fe)SiO$_{3}$ perovskite at pressures relevant to Earth's lower mantle. Equations of state are fit for a range of compositions and used to predict the Fe$^{2+}$ high- to low-spin transition pressure and associated changes in volume and bulk modulus. We predict a relatively constant transition pressure for x $<$ 0.25, but a significant decrease for higher Fe concentrations, contrary to the trend for rocksalt (Mg,Fe)O, suggesting the mechanism for spin transition is highly dependent on the structural environment of Fe. The spin transition is dominated by both the spin flip energy and the change in volume from high- to low-spin. Furthermore, volume trends show that high-spin Fe$^{2+}$ is larger than Mg$^{2+}$ even under pressure, but low-spin Fe$^{2+}$ is smaller at ambient conditions and approximately the same size as Mg$^{2+}$ under pressure, which has implications for correctly calculating Fe partitioning coefficients in the lower mantle. We also find the spin transition pressure differs between Fe$^{2+}$ and Fe$^{3+}$; therefore the coupling behavior of these two species must be examined closely. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B23.00007: First-principles investigation of the spin state of ferrous iron in MgSiO$_3$ under pressure Yonggang Yu, Ryan Requist, Koichiro Umemoto, Renata Wentzcovitch We present a density functional study of the pressure induced spin transition in ferrous iron in MgSiO$_3$ perovskite and post-perovskite. We address the influence of iron concentration and configuration (structural and magnetic), as well as technical issues such as the nature of the exchange correlation (XC) functional (LDA versus PBE-GGA) on the spin transition pressure. Supercells containing up to 160 atoms were adopted to tackle these issues. We show that there are preferred configurations for high-spin and low-spin iron and that the spin transition pressure depends strongly on iron concentration and XC functionals. We also address the possibility of a structural change accompanying the spin transition. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B23.00008: Structure and freezing of MgSiO$_3$ liquid in Earth's interior Lars Stixrude, Bijaya Karki Silicate liquids are primary agents of mass and heat transport, yet little is known of their physical properties or structure over most of the mantle pressure regime. We have applied density functional theory within the local density approximation to the study of silicate liquids via Born-Oppenheimer first principles molecular dynamics. The simulations are performed in the NVT ensemble with a Nose thermostat. We find that over the pressure regime of Earth's mantle the mean Si-O coordination number increases nearly linearly with compression from four-fold to six-fold. The Gr\"uneisen parameter of the liquid increases markedly on compression, in contrast to the behavior of mantle crystalline phases, and in accord with expectations based on the pressure-induced change in structure of the liquid. The density contrast between liquid and crystal decreases nearly five-fold over the mantle pressure regime and is 4 \% at the core-mantle boundary. The melting curve, obtained via integration of the Claussius-Clapeyron equation yields a melting temperature of $5400 \pm 600$ K at the core mantle boundary. Our results support the notion of buoyantly stable silicate melts at the core-mantle boundary. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B23.00009: Consequences of the Quasiharmonic Approximation: Tests and Predictions Pierre Carrier, Jun Tsuchiya, Renata M. Wentzcovitch The quasiharmonic approximation (QHA) is extremely useful since it allows the computation of thermodynamic properties if one knows the volume dependence of the vibrational density of states. It has an important consequence: the structure and vibrational properties of the solid depend on volume alone. The temperature dependence occurs via extrinsic volumetric effects. We present here a criterion to determine the pressure- temperature range of validity of the QHA, apply it to and test it in MgSiO3-perovskite, and inspect the possibility of a simple volumetric depended of other properties such as acoustic velocities, i.e., ``Birch's Law.'' [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B23.00010: Quantum Monte Carlo Benchmarks Functionals for Silica Polymorphs K.P. Driver, J.W. Wilkins, R.G. Hennig, C.J. Umrigar, G. Scuseria, B. Militzer, R.E. Cohen For many silica polytypes, the local density approximation (LDA) does a better job than the generalized gradient approximation (GGA) in predicting structural properties and bulk moduli. However, gradient corrections to the charge density are necessary for accurate phase energy differences \footnote[1]{Th. Demuth et al., J. Phys.: Cond. Matter 11, 3833 (1999).}. Functionals that go beyond GGA may improve the accuracy of both structures and energies. For example, a meta-GGA functional, TPSS, and hybrid functionals B3LYP and HSE \footnote[2]{J. Heyd et al., J. Chem. Phys. 121, 1187 (2004).} have shown improvement in other systems \footnote[3]{E. R. Batista et al., Phys. Rev. B 74, 121102(R) (2006).}. We compare results from these functionals for structural properties, energy differences, and bulk moduli for a few high pressure phases of silica, and benchmark the results with Quantum Monte Carlo (QMC). Preliminary QMC results indicate that careful wavefunction optimization and finite size effects are of particular importance in obtaining accurate silica phase properties. Supported by DOE(DE-FG02-99ER45795), NSF (EAR-0530301, DMR-0205328), and Sandia National Laboratory. Computation at OSC and NERSC. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B23.00011: Quantum Monte Carlo Simulations of the Quartz to Stishovite Transition in SiO$_2$ R.E. Cohen, Mike Towler, Pablo Lopez Rios, Neil Drummond, Richard Needs The quartz-stishovite transition has been a long standing problem for density functional theory (DFT). Although conventional DFT computations within the local density approximation (LDA) give reasonably good properties of silica phases individually, they do not give the energy difference between quartz and stishovite accurately. The LDA gives stishovite as a lower energy structure than quartz at zero pressure, which is incorrect. The generalized gradient approximation (GGA) has been shown to give the correct energy difference between quartz and stishovite (about 0.5 eV/formula unit) (Hamann, PRL 76, 660, 1996; Zupan et al., PRB 58, 11266, 1998), and it was generally thought that the GGA was simply a better approximation than the LDA. However, closer inspection shows that other properties are not better for the GGA than the LDA, so there is room for improvement. A new density functional that is an improvement for most materials unfortunately does not improve the quartz-stishovite transition (Wu and Cohen, PRB 73, 235116, 2006). We are performing QMC computations using the CASINO code to obtain the accurate energy difference between quartz and stishovite to obtain more accurate high pressure properties, and to better understand the errors on DFT and how DFT can be improved. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B23.00012: First-principles calculations of thermodynamic properties and phase transitions in Al$_{2}$O$_{3}$ and Ga$_{2}$O$_{3}$ at high temperature and high pressure Bin Xu, Jianjun Dong Using \textit{ab initio} density functional theory and statistical quasi-harmonic approximation theory, we have calculated thermodynamic potentials of mineral Al$_{2}$O$_{3}$ materials and the related Ga$_{2}$O$_{3}$ materials over a wide range of temperature and pressure (T-P) conditions. The equilibrium T-P phase diagrams are predicted to understand the trend of pressure induced phase transitions in group IIIB oxides. Furthermore, we theoretically explored the possible new high-pressure structures of Ga$_{2}$O$_{3}$. Finally, we derived experimentally measurable thermal properties, such as lattice thermal expansion, heat capacity, and isothermal compressibility. Our calculated thermal properties are in excellent agreement with available experiments. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B23.00013: Proton behaviour, structure and elasticity of serpentine at high-pressure Mainak Mookherjee, Lars Stixrude Serpentine occurs in oceanic crust as the alteration product of ultramafic rocks and is a possible candidate for carrying water to the deep earth. The presence of sub-surface serpentine may be manifested by mud volcanoes, high electrical conductivities, and seismic anomalies. Using density functional theory, we predict a phase transition in serpentine near 22 GPa. The phase transition is caused by a re-orientation of the hydroxyl vector coupled with changes in the di-trigonal rings of SiO$_{4}$ tetrahedra. The symmetry of the crystal-structure remains unaffected. Evidence of pressure-induced hydrogen bonding is absent in serpentine, as evident from the reduction of O-H bond length upon compression. Results of compression for the low-pressure phase is well represented by a fourth order Birch-Murnaghan finite strain expression with $K_O $= 63 GPa, ${K}'_O $= 10.2 and $K_O {K}''_O $ = -120, where $K$ is the bulk modulus, prime indicates pressure derivatives, and O refers to zero pressure. At low pressures, the elastic constant tensor is highly anisotropic with $C_{11}^o \sim 2.4\times C_{33}^o $, and becomes more isotropic with compression. We find an elastic instability near 36 GPa that may be related to experimentally observed amorphization. [Preview Abstract] |
Session B24: Focus Session: Reversibly Associating Polymers: Theory & Experiments
Sponsoring Units: DPOLYChair: Sanat Kumar, Columbia University
Room: Colorado Convention Center 201
Monday, March 5, 2007 11:15AM - 11:51AM |
B24.00001: Reversible Associating Polymers as Biological Mimics Invited Speaker: |
Monday, March 5, 2007 11:51AM - 12:03PM |
B24.00002: Architectural effect on the self-assembly of supramolecular triblock copolymer melts Won Bo Lee, Richard Elliott, Kirill Katsov, Glenn H. Fredrickson Thermoreversible, supramolecular self-assembly in multi-block copolymer melts is studied within the framework of self-consistent field theory. This approach is adapted to study a system composed of two chemically distinct, but reactive homopolymer species: a linear A-homopolymer with a single reactive group at one of the ends, and a linear B-homopolymer with reactive groups at both ends. Reversible bonding occurs between the functional groups from different polymer species so that the reacting system can contain A, B, AB and ABA (co)polymer species whose overall volume fractions are controlled by the segmental incompatibility, bonding strength and homopolymer chain lengths. Architectural variations of these copolymers, arising from the differing lengths of A and B homopolymers, have a dramatic effect on not only the micro-phase separation but also on the extent of reversible bonding. Two characteristic phase diagrams are constructed to illustrate this behavior and possible technological applications are discussed. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B24.00003: Osmotic Properties of Acrylic Triblock Copolymer Gels Rafael E. Bras, Kenneth R. Shull Triblock copolymer solutions are of interest both practically and scientifically. This interest stems largely from their ability to self assemble into soft elastic solids. Acrylic triblock copolymers solvated in alcohols are of particular interest as they exhibit a rapid thermo-reversible transition from a viscous liquid at elevated temperatures (but still less than 100 C) to a strong elastic solid at room temperature. The rapid transition is a result of the increasing interaction parameter between the polymer endblock and solvent upon cooling, resulting in self assembly and the eventual stabilization of endblock aggregates below the endblock glass transition temperature. Several methods, including vapor pressure osmometry, small angle x-ray scattering, and rheometry, were used to examine the properties of semidilute PMMA-PnBA-PMMA acrylic triblock solutions in the temperature regime near the critical micelle temperature. The measured osmotic pressure is controlled by contacts between the well solvated midblocks, the solvent-endblock interaction energy, and the elasticity of the triblock network. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B24.00004: Computational study of gel transition and jamming in an ensemble of reversible associating polymers Arlette Baljon, Danny Flynn, David Krawzsenek The sol/gel transition is studied in model telechelic polymers. We find that, as in the case of glass-forming liquids, the transition range is very broad. Four characteristic temperatures for gel formation are calculated: (1) the temperature below which relaxation times no longer show Arrhenius dependence on temperature; (2) the Vogel-Fulcher temperature at which the structural relaxation time extrapolates to infinity; (3) the analogue of the crossover temperature defined by Mode Coupling Theory; (4) the ``jamming'' transition temperature at which the number of reversible bonds sharply increases and their distribution over micelles changes qualitatively. These temperatures are obtained from measurements of diffusivity, specific heat, and network topology. The effect of external shear will be discussed as well. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B24.00005: Phase Behavior of Semi-Flexible Polymer Gels Venkatram H. Padmanabhan, Sanat K. Kumar Histogram Reweighting Monte Carlo simulations were performed to obtain the sol-gel phase diagrams for a system of semi-flexible polymer chains in a cubic box with periodic boundary conditions. Our calculations stress the importance of patchiness -- in its absence the systems form standard liquid crystalline phases. In contrast, for strong patch interactions, with decreasing temperature, the chains experience a local ``arrest'' in positions due to the interchain interactions, thus slowing down its dynamics. The physical aggregation of these chains is similar to that of Flory Type -- III gels. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B24.00006: Modeling the Crystallization of Proteins Hongjun Liu, Sanat Kumar, Shekhar Garde We have used molecular dynamics and monte carlo simulations to understand the pathway to protein crystallization. We find that models which ignore the patchy nature of protein-protein interactions only crystallize inside the metastable gas-lqiuid coexistence region. In this regime they crystallize through the formation of a critical nucleus. In contrast, when patchiness is introduced we find that there is no need to be inside this metastable gas-liquid boundary. Rather, crystallization occurs through an intermediate which is composed of disordered aggregates. These are formed by patchy interactions. Further, there appears to be no need for the formation of a critical nucleus. Thus the pathways for crystallization are strongly controlled by the nature of protein-protein interactions, in good agreement with current experiments. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B24.00007: Physical Properties of Anionic Peptide Amphiphile Fibers Grown in the Presence of Polyion Salt Megan Greenfield, Yuri Velichko, Samuel Stupp, Monica Olvera de la Cruz We analyze the structure and mechanical properties of self-assembled gels formed by anionic peptide amphiphiles (PA) in the presence of cationic peptides and polyion salt. The PA molecules, which are composed of a hydrophobic alkyl tail, a beta-sheet forming region, and a hydrophilic epitope region, self-assemble into cylindrical micelles in water with multivalent salt. The fibers grow in one dimension by forming an internal beta sheet along the middle segment; the hydrophobic tail hides inside the fiber and the epitope region is exposed on the surface. Rheology and electron microscopy are used to investigate the physical properties of the resulting PA gels. The PA-fibers form a self-supporting gel at a concentration of one weight percent. Our experimental results show a strong dependence on the nature and valency of the polyions. We will present a theoretical model that incorporates both PA self-assembly and gelation of PA-fibers in the presence of polyion salt. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B24.00008: Reversible and Tunable Network Formation of Ca$^{2+}$-Sensitive Biomaterials Shana Topp, Vikram Prasad, Gianguido C. Cianci, Eric R. Weeks, Justin P. Gallivan A major goal of polymer science is to develop ``smart'' materials that sense chemical signals in complex environments and respond with predictable changes in their mechanical properties. We present a genetic toolbox of natural and engineered protein modules that can be rationally combined in manifold ways to create reversible self-assembling materials that vary in their composition, architecture, and mechanical properties. The protein calmodulin, which binds Ca$^{2+}$ with micromolar affinity but does not bind Mg$^{2+}$, was employed to develop materials that could discriminate between the two cations. Using calmodulin and other modules in the toolbox, we produced several materials that reversibly self-assemble in the presence of Ca$^{2+}$ and characterized these materials using particle-tracking microrheology. Since the properties of these materials could be predicted from the dilute solution behavior of their component modules, an expanded toolbox may be generally useful for creating new stimuli-sensitive materials. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B24.00009: Folding and Aggregation of Mucin Domains. Brigita Urbanc, Rama Bansil, Bradley Turner Mucin glycoproteins consist of tandem repeating glycosylated regions flanked by non-repetitive protein domains with little glycosylation. These non-repetitive domains are involved in polymerization of mucin via disulfide bonds and play an important role in the pH dependent gelation of gastric mucin, which is essential to protecting the stomach from autodigestion. We have examined the folding and aggregation of the non-repetitive sequence of von Willebrand factor vWF-C1 domain (67 amino acids) and PGM 2X (242 amino acids) using Discrete Molecular Dynamics (four-bead protein model with hydrogen bonding and amino acid-specific hydrophobic/hydrophilic and electrostatic interactions of side chains). Simulations of vWF C1 show 4-6 $\beta $-strands separated by turns/loops with more loops at lower pH. A simulation of several vWF C1 proteins at low pH shows aggregates still with a high content of $\beta $-strands and enhanced turn/loop regions. For the PGM 2X simulation the contact map shows several salt bridges enclosing hairpin turns. The implications of these simulations for describing the aggregation/gelation of PGM will be discussed. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B24.00010: Shape-Memory Network Polymers Containing Reversible H-Bonding Associating Groups Jiahui Li, Mitchell Anthamatten Thermally reversible secondary interactions such as H-bonding can be used to stabilize mechanically strained states at low temperatures. A new type of lightly crosslinked shape-memory polymer is reported which contains reversible H-bonding ureidopyrimidinone (UPy) side-groups. Butyl acrylate, UPy containing monomer, and crosslinkable monomers were copolymerized and cast into films. Experiments on those elastomers using thermal-mechanical analysis showed clear shape-memory effects. The shape recovery ratio of those elastomers is nearly 100{\%}, and the shape fixity ratio is typically 90{\%}. Interestingly, the materials' shape recovery rate exhibits Arrhenius-like temperature dependence. Activation energies were obtained by fitting the TMA creep experiment data, and they were compared to the dynamics of UPy H-bonding dissociation. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B24.00011: Miscibility studies on blends containing telechelic supramolecular polymers Michelle Wrue, Mitchell Anthamatten The incorporation of associating end groups influences the phase behavior of polymer blends. We are studying the effects of the incorporation of strong, site-selective, hydrogen-bonding groups in various polymer blend systems. We have synthesized several telechelic ureidopyrimidone (UPy) functionalized polymers. These self-complimentary functional groups have the ability to form four hydrogen bonds. When these groups are incorporated into a polymer blend, the miscibility is altered. Laser light scattering has been employed to study the phase behavior of these systems. Data from systems in which only one polymer was functionalized indicate a reduced miscibility when compared with the unfunctionalized parent blend. We are also investigating the effect of functionalizing both polymers of the blend with UPy end groups. We predict enhanced miscibility in such systems. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B24.00012: Design and characterization of well-defined supramolecular polymers Kathleen Schaefer, Matthew Kade, Craig Hawker, Edward Kramer Polymeric materials with well-defined and controllable temperature dependent properties are of interest both for technological applications and fundamental physical studies. Melt processing requires low viscosity, while resistance to fracture is desirable at material operating temperatures, and these two properties are often mutually exclusive. Through controlled radical polymerization (ATRP) we have synthesized tailor-made polymers with MHB groups specifically located at one or both chain ends or randomly along the backbone to provide thermal tunability, and by changing the nature of the MHB group (complementary or self-complementary) we can control the specificity and type of the polymer-polymer interaction. As a simple model system, we investigate the case of two end-functional MHB homopolymers that form a novel supramolecular diblock copolymer. Two energies are expected to be important in this system---$\chi $N, the Flory-Huggins interaction parameter times the degree of polymerization, which describes the polymer-polymer interaction, and $\varepsilon $, the binding energy of the MHB group. Using deuterium labeled polymers in various multilayer thin film structures, dynamic secondary ion mass spectrometry (dSIMS) allows each of these parameters to be measured independently and these values used to design technologically and physically interesting new materials. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B24.00013: Structure and stability of oligomer/$\alpha $-cyclodextrin inclusion complexes. Marcus Hunt, Silvia Villar, Marian Gomez, Alan Tonelli, Maury Balik Cyclomaltohexaose ($\alpha $-cyclodextrin, $\alpha $-CD) can form inclusion complexes (ICs) with polymer molecules in the columnar crystal in which $\alpha $-CD molecules stack to form a molecular tube. Complementary water vapor sorption and wide-angle X-ray diffractomery (WAXD) were performed on oligomer/$\alpha $-CD ICs to probe their structures and stabilities. To discern the effect of guest molecule hydrophobicity on water adsorption isotherms, polyethylene glycol (PEG, MW = 600 g/mol) and hexatriacontane (HTC) guests were used. Sorption isotherms for PEG/$\alpha $-CD IC are similar to those obtained for pure $\alpha $-CD and PEG, suggesting the presence of dethreaded PEG in the sample. WAXD collected before and after water vapor sorption of PEG/$\alpha $-CD IC indicated a partial conversion from columnar to cage crystal structure, the thermodynamically preferred structure for pure $\alpha $-CD, due to dethreading of PEG. This behavior does not occur for HTC/$\alpha $-CD IC. Sorption isotherms collected at 20, 30, 40 and 50\r{ } C allowed the calculation of differential heats of adsorption and integral entropies of adsorbed water, while solid-state $^{13}$C NMR suggested a dramatic increase in HTC and $\alpha $-CD mobilities upon complexation. [Preview Abstract] |
Session B25: Focus Session: Adhesion, Swelling, and Elastic Properties of Thin Polymer Films
Sponsoring Units: DPOLYChair: Theresa Hermel-Davidock, Dow Chemical
Room: Colorado Convention Center 203
Monday, March 5, 2007 11:15AM - 11:51AM |
B25.00001: Interaction and Viscoelastic Deformation of Polymeric Surfaces Measured with the Atomic Force Microscope Invited Speaker: Methods are described for the measurement and analysis of deformable surfaces with the atomic force microscope (AFM). It is shown how to obtain the zero of separation and how to calibrate the photo-diode for quantitative force measurement [1]. The properties of viscoelastic materials (relaxation times, Youngs moduli) may be extracted by modeling particular sorts of force measurements [2]. Results are shown for a biopolymer agar [3], and for a polyelectrolyte polydimethylsiloxane [4], both of which are viscoelastic, and for polystyrene, which is elastic [5]. The potential for using the AFM as a nanorheometer is discussed. \newline \newline [1] P. Attard, ``Friction, Adhesion, and Deformation: Dynamic Measurements with the Atomic Force Microscope'', J. Adhesion Sci. Technol. 16, 753--791 (2002). \newline [2] P. Attard, ``Interaction and Deformation of Viscoelastic Particles. Non-adhesive Particles'', Phys. Rev. E 63, 061604 (2001) \newline [3] J. W. G. Tyrrell and P. Attard, ``A Viscoelastic Study Using and Atomic Force Microscope Modified to Operate as a Nanorheometer'', Langmuir 19, 5254--5260 (2003) \newline [4] G. S. Gillies, C. A. Prestidge, and P. Attard, ``An AFM Study of the Deformation and Nano-rheology of Cross-Linked PDMS Droplets'', Langmuir 18, 1674--1679 (2002) \newline [5] M. W. Rutland, J. W. G. Tyrrell, and P. Attard, ``Analysis of Atomic Force Microscopy Data for Deformable Materials'', J. Adhesion Sci. Technol. 18, 1199--1216 (2004) [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B25.00002: Surface wrinkling of grafted polymer brushes and its effect on interfacial adhesion Christopher M. Stafford, Heqing Huang, Jun Young Chung Grafted polymer brushes present a simple and convenient route to generate a rich and diverse parametric space that expresses the chemical complexity found at materials surfaces and interfaces. For example, adhesion at the interface can be tuned by controlling the chemistry, length, and density of the grafted polymer brush. If constructed on a soft elastomer, these polymer brushes can undergo wrinkling at the interface, creating a powerful combination of surface chemistry and topography, both of which play a critical role in interfacial adhesion. Here, we present strategies to control the adhesion strength through changes in brush length and composition, solvent quality, and modulus of the elastomer. A phenomenological model for the development and growth of the wrinkled surface is developed to explain the observed results. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B25.00003: Experiments of compaction of an elastic sheet closely-packed in a rigid container Deboeuf Stephanie, Boue Laurent, Adda-Bedia Mokhtar, Boudaoud Arezki When crumbling a sheet of paper, it is known that deformation essentially appears in localized and lineic singularities, the so-called developable cones and ridges. But how relate the emergence of such patterns to the global shape of the sheet and to the compression force needed to compact it? We try to answer to such a question in the case of a model experiment of compaction. When a circular sheet is pulled through a rigid hole, first it exhibits a conical shape -developable cone-, then it develops folded patterns to achieve close-packed configurations. Our experimental set-up allows simultaneously for the observation and statistical characterization of folded configurations and for the measure of mechanical forces. Different patterns of folds are observed and systematically characterized as a function of the packing fraction of the sheet within the hole. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B25.00004: Dependence of the in-plane modulus of thin free-standing polymer films Adam N. Raegen, Kari Dalnoki-Veress We employ an axi-symetric deformation and modulus test to measure the in-plane elastic modulus of free-standing polymer films. This method measures the deformation of a thin spincast film when a flat circular punch is brought into contact with the sample. The use of a small, flat punch minimises uncertainties in the experiment, while the use of spincast films provides a very smooth surface. We will present a study of the dependence of the elastic modulus on the film thickness and the annealing history of the samples. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B25.00005: Measuring Correlation Functions and Elastic Constants of 2D Layers of Block Copolymers by Single Crystal Diffraction Gila Stein, Edward Kramer, Xuefa Li, Jin Wang Monolayers of spherical-domain block copolymer, exhibiting hexagonal symmetry with a periodicity of 29nm, are laterally confined in hexagonal wells 12$\mu$m wide and 26nm deep that span the entire area of a 2-inch diameter silicon wafer. At 210 $^{\circ}$C, films that are 41nm thick (monolayer plus brush) form oriented single crystals in each well, where the close- packed rows of the lattice are aligned parallel to the edges. The structure is characterized with grazing-incidence small- angle x-ray diffraction, and results are interpreted within the KTNHY framework for a 2D crystal. Translational order decays algebraically with a correlation function of the form g$_t$(r) $\sim$r$^{-0.25}$, and from the magnitude of the decay exponent, the 2D shear modulus of the crystal $\mu=1.7\times10^ {-4}$N/m can be extracted. Orientational order is long-range, with a full width at half maximum of 1.1$^{\circ}$. Decreasing the film thickness by 1nm produces hexatic ordering with a translational correlation length on the order of 0.5$\mu$m, and diminished orientational order with a full width at half maximum of 2.4$^{\circ}$. These results agree qualitatively with high resolution scanning force microscopy images of the lattice. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B25.00006: Polymer-Solid Interface Connectivity and Adhesion: Design of a Pressure Sensitive Adhesive Shana P. Bunker, Richard P. Wool Adhesion at polymer-solid interfaces was explored for a new bio-based PSA in terms of sticker groups $\phi _{X }$ on the polymer, receptor groups $\phi _{Y}$ on the solid and the strength of the X-Y acid-base interaction, $\chi $. The polymer-solid interface models of Gong, Lee and Wool were extended with new percolation models of entanglements and interface strength to determine the optimal sticker group concentration $\phi $*$_{X}$. For the general case where $\phi _{Y}$ and $\chi $ are constant, it is predicted that when $\phi _{X}<\phi $*$_{X}$, the peel strength behaves as G$_{1c}\sim \phi _{X}$/$\phi $*$_{X}$ and the locus of failure is adhesive between the polymer and the solid. However, when $\phi _{X}>\phi $*$_{X}$, failure occurs cohesively in a polymer-polymer interface adjacent to the solid and the strength decreases as G$_{1c}\sim \phi $*$_{X}$/$\phi _{X}$. The switch from adhesive to cohesive failure can be understood in terms of the changes in the chain conformations of the adhered chains and their decreasing interpenetration X with the bulk chains, via X$\sim $1/r, where r = $\chi \phi _{X}\phi _{Y}$. The optimal value of $\phi _{X}$ which maximizes the adhesion and determines the mode of failure is given by $\phi $*$_{X}\approx $0.07/C$_{\propto }$, and for typical values of the characteristic ratio C$_{\propto }$ in the range 7-10, $\phi $*$_{X}\approx $1{\%} mole fraction, corresponding to about 2 sticker groups per critical entanglement length M$_{c}$. Supported by USDA [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B25.00007: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 1:03PM - 1:15PM |
B25.00008: Welding Immiscible Polymer with Supercritical Fluid Xiaochu Wang, Isaac Sanchez Polymer adhesion between two immiscible polymers is usually poor because there is little interpenetration of one polymer into the other at the interface. Thus, increasing the width of the interfacial zone can enhance adhesion and mechanical properties. In principle, this can be accomplished by exposing the solid polymer materials to high pressure CO2. The CO2 acts as a common solvent and promotes interpenetration. It also increases the mobility at the interface which helps to promote the ?welding? of the two polymers. A combination of the gradient theory of inhomogeneous systems and the Sanchez-Lacombe Equation of State was used to investigate this phenomenon. We calculate the interfacial density profile, interfacial thickness and interfacial tension between the two polymers with and without CO2. We find that the interfacial tension is decreased and the interfacial thickness is increased with high pressure CO2, which means that the presence of CO2 does help polymer welding. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B25.00009: Off-Specular Neutron and X-ray Reflectometry for the Structural Characterization of Buried Interfaces Kristopher Lavery, Vivek Prabhu, Eric Lin, Wen-li Wu, Kwang-Woo Choi, Sushil Satija, Matthew Wormington For applications in semiconductor processing and nanotechnology, the lateral structure of interfaces can significantly affect the performance of a given device. For example, roughness on the edges of developed photoresist patterns can reduce the performance of the final devices. Off-specular reflectometry is a non-destructive scattering technique sensitive to lateral compositional variations at surfaces and interfaces. It is particularly well-suited as a means of measuring the form and amplitude of surface roughness, as well as separating contributions from physical roughness and gradients in material density. In this work, model rough surfaces were prepared on float glass substrates and the roughness and lateral correlation lengths were cross-correlated using neutron and x-ray off-specular reflectometry measurements. These techniques were extended to observe the lateral correlation length of the reaction-diffusion front in a model photoresist using a polymer-polymer bilayer designed to mimic an ideal lithographic line edge. These experiments highlight the advantages of the technique for the investigation of buried interfaces while illustrating how x-ray and neutron techniques work complementarily to measure interfacial roughness. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B25.00010: Confinement Effects on the Swelling Behavior of Thin Polymer Films Aleta Hagman, Kenneth R. Shull, Jin Wang, Martin Tolkiehn, Xuefa Li, Suresh Narayanan Marker motion can be used to measure polymer dynamics in polymer/metal nanocomposite thin films. In our current experiments, thermally evaporated gold particles (few nanometers in diameter) act as a marker layer between two polymer layers with different mobilities. To create the differing mobilities we are using a low molecular weight poly(2-vinyl pyridine) (PVP) on one side of the marker layer and a high molecular weight PVP on the other. The low molecular weight PVP will swell into the higher molecular weight polymer, when heated above the polymer glass transition temperature, creating a flux of the marker atoms in the opposite direction. We have used X-ray standing waves (XSWs), generated by total external reflection above a mirror surface, as a probe to monitor the time evolution of the gold nanoparticle distribution in the nanocomposite ultrathin films. Swelling kinetics are obtained from the measured time-dependence of the nanoparticle distribution. We will also discuss the effects the free surface and the mirror surface have on the amount of swelling observed. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B25.00011: Equilibrium and Kinetic Water Uptake in Ultrathin Chitosan Films Chris Murray, John Dutcher We present the results of ellipsometry and surface plasmon resonance experiments of equilibrium and kinetic water uptake in ultrathin films of chitosan that have been prepared by spincoating from dilute solution. Exposure of the films to increasing relative humidity results in increases in thickness and mass due to the absorption of water from the surrounding atmosphere. The equilibrium water content measured at different RH values can be interpreted in terms of either the average size of water clusters forming within the polymer matrix, or multilayers of water adsorbed onto specific sites within the matrix. Measurement of the time-dependence of water uptake allows the calculation of the diffusion coefficient of water in the films. We observed dramatic reductions in the water uptake of chitosan films that have been heated to temperatures 150 $^{o}$C $< \quad T \quad <$ 200 $^{o}$C: the equilibrium water content is reduced by more than a factor of four and the diffusion coefficient of water in the films decreases by more than a factor of two. We suggest that this irreversible reduction in the equilibrium water content and the diffusion coefficient for water in the films is caused by the formation of new inter- and intra-molecular bonds within the polymer matrix, and we present one possible mechanism that is consistent with all of our observations. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B25.00012: Grafting of Telechelic Polymers onto Functionalized Substrate in Polymeric Matrices Rujul Mehta, Zhenyu Huang, Haining Ji, Jimmy Mays, Mark D. Dadmun We have investigated the grafting of polymer chains on an inorganic surface by reaction of the functional end-groups on the polymer chains. Specifically, polystyrene (PS) terminated at one or both ends with carboxylic acid groups is grafted onto silica modified with epoxy groups by spin coating a thin film of a blend of reactive and non-reactive PS onto the silica wafer and promoting the grafting reaction. This generates a model system to study the impact of the volume fraction of reactive polymer and the chain length of both reactive and matrix polymers on the resultant grafted brush. The interfacial excess isotherms are correlated to a theoretical model proposed by Shull, which describes the end adsorption of polymers at polymer/substrate interfaces in brushes based on scaling arguments and self-consistent field theory. Comparison is made between the characteristics of brushes formed from PS chains that are reactive at one or both ends. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B25.00013: Kinetics of Grafting and Loop Formation of Telechelic Polymers on Solid Substrate Mark Dadmun, Zhenyu Huang, Haining Ji, Jimmy Mays A simple system was used to examine loop formation at polymer/substrate interfaces. Telechelic polystyrene terminated with carboxylic acid end groups was grafted from the melt onto a silicon wafer modified with epoxy-terminated silane layers. This study focuses on understanding how molecular weight, annealing temperature, and surface functionality affect the grafted amount and grafting kinetics. It was found that the practical limit of the grafted amount scales as $R_{g}$ leading to a -0.5 power law for the surface density as a function of molecular weight. The kinetic results suggest that the grafting process is most likely reaction controlled. By labeling the free carboxylic acid groups on singly bound chains with a fluorescence probe, the time dependence of the amount of the singly bound chains was monitored using fluorimetry. The obtained results provide a method to control the loop formation at interfaces and understand their structure and properties. [Preview Abstract] |
Session B26: Focus Session: Protein Folding: Theory and Simulations II
Sponsoring Units: DCP DBPChair: Jose Onuchic, University of California, San Diego
Room: Colorado Convention Center 205
Monday, March 5, 2007 11:15AM - 11:51AM |
B26.00001: Invited Speaker: |
Monday, March 5, 2007 11:51AM - 12:27PM |
B26.00002: Protein folding and dynamics from simulations of coarse protein models. Invited Speaker: The dynamics and folding transitions of proteins are studied by computer simulations of coarse-grained models. The simulations are related to experimental studies of the unfolding of proteins under mechanical force, and the effects of mutations on the folding rates using phi-value analysis. Coarse protein models have also been useful in studies of slow conformational transitions. Applications to the helix-to-sheet transition of an arc repressor mutant, and the open-to-closed transition of the calmodulin C-terminal domain indicate that local unfolding events can contribute significantly to the slow dynamics of these proteins. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 1:03PM |
B26.00003: Invited Speaker: |
Monday, March 5, 2007 1:03PM - 1:15PM |
B26.00004: The folding of an ``average'' beta trefoil protein. Shachi Gosavi, Pat Jennings, Jose Onuchic The beta-trefoil fold is characterized by twelve beta strands folded into three similar beta-beta-beta-loop-beta (trefoil) units. The overall fold has pseudo-threefold symmetry and consists of a six stranded-barrel, capped by a triangular hairpin triplet. The loops connecting the beta-strands vary in length and structure. It is these loops that give the fold its varied binding capability and the binding sites lie in different parts of the fold. The beta-trefoil proteins have little sequence similarity (sometimes less than 17{\%}) and bind a range of molecules, including other proteins, DNA, membranes and carbohydrates. Protein folding experiments have been performed on four of the beta trefoils, namely, interleukin-1 (IL1B), acidic and basic fibroblast growth factors (FGF-1 and FGF-2) and hisactophilin (HIS). These experiments indicate that the proteins fold by different routes. Folding simulations of the proteins identify the possible folding routes and also show that the shapes of the barriers are different for the different proteins. In this work, we design a model protein which contains only the core fold elements of the beta-trefoil fold. We compare the folding of this ``average'' protein to the folding of His, FGF and IL1B and make some connections with function. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B26.00005: Life in a Crowd: Macromolecular Crowding and Confinement Effects on Protein Interactions in Living Systems Margaret Cheung Biological polymers carry out their functions in living systems where the environment is very concentrated or crowded by macromolecules. Physically, the composition of a cell is more than ``a sack of water''; its consistency is closer to Jell-O. Experiments suggests that, because of this macromolecular crowding effect that confines polymeric dynamics, the kinetics and thermodynamics of protein folding and the association rate constants of protein-protein interactions in a cell (in vivo) are very different from that ina diluted test tube (in vitro). In order to quantitatively understand macromolecular crowding and confinement effects on protein dynamics, we used coarse-grained models that physically captured interactions between crowders and a protein. The folding rates of a model protein nonmonotonically increased with the volume fraction of the crowders. At lower volume fractions, depletion-induced attractions from crowders could be mapped according to the spherical confinement model. A result of spherical confinement was the destabilization of denatured states by disallowing extended configurations that were longer than the pore size. However, at higher volume fractions, conformational fluctuations of a protein were susceptible to the shape of the confining condition. Thus, an approximation of the spherical confinement to mimic crowding effects was no longer effective. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B26.00006: 2D IR Spectroscopy of Ubiquitin Unfolding Dynamics Ziad Ganim, Hoi Sung Chung, Andrei Tokmakoff The unfolding dynamics of ubiquitin have been studied using a combination of amide I 2D IR spectroscopy and spectral calculations drawing on structures from molecular dynamics simulations. Equilibrium temperature-dependent 2D IR spectra and transient 2D IR spectra following a nanosecond temperature jump are used to follow the unfolding of ubiquitin's $\beta$-sheet. The equilibrium 2D IR spectra show two features that arise from delocalized $\beta$-sheet vibrations of which differ by whether C=O oscillators vibrate parallel or perpendicular to its strands. Spectral changes in the transient difference spectrum start with an abrupt blue shift of the perpendicular diagonal region, which corresponds to the disruption of hydrogen bonds between water and solvent-exposed peptide groups. This change is followed over $\mu$s to ms time scales by a blue shift of the perpendicular region and disappearance of a cross peak, which reflect the gradual unfolding of the $\beta$-sheet of the protein. The experiments are compared with 2D IR spectra calculated from molecular dynamics trajectories of ubiquitin unfolding using a structure-based model for protein amide I spectroscopy. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B26.00007: Generalization of distance to higher dimensional objects, and applications to biopolymer folding Steven Plotkin The measurement of distance between two objects is generalized to the case where the objects are no longer points but are one-dimensional (strings) or many-dimensional (differential manifolds). Applications to biopolymer folding will be discussed. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B26.00008: Collapse transition for self-avoiding random walks with hydrophobic interaction on a 2 dimensional lattice Mathieu Gaudreault, Jorge Vinals We study the collapse transition of a protein model with an explicit coarse-grained model of solvent hydrophobicity using Monte Carlo simulation. The protein is modelled as self- avoiding random walk with nearest neighbour interaction on a two dimensional lattice by using the pivot algorithm. Without the solvent, universal quantities of the chain around the transition temperature are well known. Hydrophobicity is modelled through a lattice of solvent molecules in which each molecule can have q states, depending of an orientation variable. Only one state is energetically favoured, when two neighbouring solvent molecules are both in the same state of orientation. The monomers are placed in interstitial position of the solvent lattice, and are only allowed to occupy sites surrounded by solvent cells of the same orientation. The potential of mean force between two interstitial solute molecules is calculated, showing that the strength of attraction increases by increasing the free energy of H-bond formation while its range decreases. We also show that the temperature of the collapse transition is shifted in the presence of solvent, while the universal quantities of the protein transition are conserved. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B26.00009: ABSTRACT WITHDRAWN |
Session B27: Focus Session: Computational Nanoscience II-Methods and Applications
Sponsoring Units: DMP DCOMPChair: Sanjay Khare, University of Toledo
Room: Colorado Convention Center 301
Monday, March 5, 2007 11:15AM - 11:27AM |
B27.00001: Kinetic Monte Carlo simulations of Ag(111) island coarsening Giridhar Nandipati, Yunsic Shim, Jacques Amar, Altaf Karim, Abdelkader Kara, Talat Rahman The results of parallel kinetic Monte Carlo simulations of submonolayer island coarsening on the Ag(111) surface are presented. Our simulations are carried out using a large database of activation barriers which has been generated from previous self-learning kinetic Monte Carlo simulations of small and medium-size clusters. In this database, which includes both single-atom and multi-atom concerted moves, interactions between a central atom and all other adatoms within the first two nearest-neighbor rings are taken into account, while the symmetry of the (111) surface is also used. In order to reach extended time and length-scales we have implemented a novel parallel kinetic Monte Carlo scheme in which processor domains are dynamically assigned in order to minimize boundary events. Preliminary results using an open database corresponding to a true self-learning kinetic Monte Carlo simulation will also be presented. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B27.00002: Self-Learning Off-Lattice Kinetic Monte Carlo method as applied to growth on metal surfaces Oleg Trushin, Abdelkader Kara, Talat Rahman We propose a new development in the Self-Learning Kinetic Monte Carlo (SLKMC) method with the goal of improving the accuracy with which atomic mechanisms controlling diffusive processes on metal surfaces may be identified. This is important for diffusion of small clusters (2 - 20 atoms) in which atoms may occupy Off-Lattice positions. Such a procedure is also necessary for consideration of heteroepitaxial growth. The new technique combines an earlier version of SLKMC [1] with the inclusion of off-lattice occupancy. This allows us to include arbitrary positions of adatoms in the modeling and makes the simulations more realistic and reliable. We have tested this new approach for the case of the diffusion of small 2D Cu clusters diffusion on Cu(111) and found good performance and satisfactory agreement with results obtained from previous version of SLKMC. The new method also helped reveal a novel atomic mechanism contributing to cluster migration.~We have also applied this method to study the diffusion of Cu clusters on Ag(111), and find that Cu atoms generally prefer to occupy off-lattice sites. [1] O. Trushin, A. Kara, A. Karim, T.S. Rahman~ Phys. Rev B 2005 [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B27.00003: Diffusion Limited Processes Using Accelerated Molecular Dynamics Erdi Bleda, Xing Gao, Murray Daw We present a systematic microscopic approach to diffusion-limited processes for intermetallic alloys using Accelerated Molecular Dynamics. On-the-fly kinetic Monte Carlo is combined with the Dimer Method to find the saddlepoints exiting a valley, based on energetics from the Embedded Atom Method. With this technique, we compute the tracer diffusivities as a function of composition and temperature for strongly ordered (Cu$_{3}$Au), weakly ordered (Ag-Au) and weakly clustered (Cu-Ni) alloys. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B27.00004: Extension of Mean-Field Nucleation Theory with Long-Range Interactions John A. Venables, James Degraffenreid, Ramon Grima Mean-field nucleation theory is an important tool in understanding various adsorbate-substrate systems, particularly in the context of epitaxial growth. Conventional mean-field theory does not take into account nonlocal interactions, but these can be important in the nucleation and growth of various nanostructures. An approach due to Ovesson [1] is based on the assumption that the change of saddle-point energy in a potential field equals the average changes at the neighboring binding sites, but this assumption is not generally satisfied. We reformulate the theory in a more general sense, as an extension of the work of Grima and Newman [2] and Venables \textit{et al.} [3]. This leads to a continuum mean-field description in a general potential field, in which the transport coefficients are intrinsically connected with the interaction potential and with microscopic parameters. Computational examples are presented for Ge/Si(001) material parameters. \newline \newline [1] S. Ovesson, \textit{PRL} \textbf{88}, 116102 (2002);\newline [2] R. Grima and T.J. Newman, \textit{PRE} \textbf{70}, 036703 (2004);\newline [3] J.A. Venables \textit{et al.}, \textit{PRB} \textbf{74}, 075412 (2006) [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B27.00005: Capture-Zone Areas \& the Wigner Distribution: New Case of Universal Scaling of Spacings in Fluctuating Systems A. Pimpinelli, T.L. Einstein When investigating scaling of island sizes during growth in $d$ dimensions, one should consider the distribution of the areas of proximity cells around nucleation centers, i.e. capture zones (CZ). Using data from kinematic Monte Carlo studies,\footnote{ Mulheran et al., PRB {\bf 53} ('96) 10261, {\bf 54} ('96) 11681; EPL {\bf 49} ('00) 617, {\bf 65} (’04) 379. Amar, Family, et al., PRL {\bf 74} ('95) 2066; PRB {\bf 64} (’01) 205404. Evans, Bartelt, et al. PRB {\bf 66} (’02) 235410; SSR {\bf 61} ('06) 1.} we find that the CZ distributions in both $d$ = 1 and $d$ = 2 are well described by the generalized Wigner distribution (GWD) from random-matrix theory: $P_\varrho(s)=as^\varrho\exp(-bs^2)$. $P_\varrho(s)$ accounts for a broad range of fluctuation phenomena, inc.\ the terrace-width distribution (TWD) on vicinal surfaces. For CZ distributions, we find $\varrho = i + d/2$, where $i$ is the critical nucleus size. We present a phenomenological justification by constructing a Langevin equation similar to that used in accounting for the equilibration of TWDs.\footnote{A. Pimpinelli, H. Gebremariam, \& T.L. Einstein, PRL 95 ('05) 246101} We discuss implications for processing and analysis of experimental data. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B27.00006: Feature detection for large-scale molecular dynamics simulations Hyoungki Park, David Richie, Jeongnim Kim, Joseph Gorse, John Wilkins Advances in computer hardware and numerical methods compound the analysis of complex, large-scale evolutionary phenomena. Progress comes from just-in-time analysis and data compression. Real-time multiresolution analysis (RTMRA) on dynamical quantities (e.g., positions and local energies of atoms) -- based on simple Haar wavelets -- compresses data more than 100-fold while retaining 0.1 \AA r.m.s. resolution. Further, RTMRA techniques enable a sophisticated event detection scheme capable of identifying meta-stable structures and detecting infrequent events, e.g., structural transitions, in the presence of thermal noise. As an example, the dynamics over a broad temperature range of silicon defect systems yields visually clear diffusion mechanisms for small silicon interstitial clusters (single-, di-, and tri-interstitial), and initiating growth of extended defects such as the extended {311} defects. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B27.00007: Objective Molecular Dynamics Traian Dumitrica, Richard James We present a generalization of periodic molecular dynamics that we term {\it objective molecular dynamics}. It is a method of doing molecular dynamics for a restricted set of atoms, nonperiodically mapping the time-dependent displacements of this small set of atoms onto the full, typically infinite structure, such that the full structure satisfies exactly the full, unconstrained set of equations of molecular dynamics subject to certain group-invariant initial conditions. The method is applicable to a wide variety of interesting molecular structures including the tails, capsids and other parts of many viruses, carbon nanotubes, many of the common proteins, C$_{60}$ and many other nanostructures now being synthesized, especially via the process of self-assembly. Overall, the strength of the proposed symmetry-based approach is that (i) it heavily reduces the computational effort through a drastic reduction in the number of atoms to be accounted for, (ii) it is compatible with full quantum mechanics, and (iii) the implementation can be done in a general framework, allowing for simulations of a larger class of structures. In addition (iv) the scheme is ideal for obtaining nanomechanical responses since it allows for applying various mechanical deformations. The method is illustrated by simulations of carbon nanotubes. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B27.00008: Decorrelation of samples in Quantum Monte Carlo calculations and applications to metallic nanoclusters Daniel Nissenbaum, Bernardo Barbiellini, Arun Bansil We discuss decorrelation of samples in Quantum Monte Carlo (QMC) ground-state energy calculations for large lithium and water nanoclusters and show how accurate results can be obtained without the need for decorrelating samples. The scaling of the integrated autocorrelation time $\tau$ is analyzed as a function of nanocluster size. $\tau$ is found to scale quadratically in Li nanoclusters, which adds a quadratic factor to the scaling of the total computation time in this metallic case, a factor which does not appear in computations of non-metallic H$_{2}$O nanoclusters. We choose nanoclusters which are relatively large in the context of QMC to demonstrate the application of these techniques - lithium nanoclusters with up to 64 atoms and water nanoclusters with up to 20 molecules. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B27.00009: Linear Scaling NanoScience Simulations for Petascale Computing Zhengji Zhao, Lin-Wang Wang, Juan Meza There are many large-scale nanoscience problems that require \emph {ab initio} accuracy total energy calculations and atomic relaxations. Unfortunately, the traditional direct \emph {ab initio} method scales as $O(N^3)$, where $N$ is the number of atoms in the system, and most of the $O(N)$ methods studied in the last decade have various numerical convergence problems and computer parallelization issues. In this talk, we present an alternative $O(N)$ method which divides the whole system into small fragments. By combining the fragments in an ingenious pattern, the artificial boundary effects of the spatial division can be canceled out. As a reasult, the difference between this method and the direct \emph {ab initio} calculation is smaller than errors introduced by other numerical approximations, and the method scales almost linearly to the number of processors. We have used this method to calculate nanostructures with more than ten thousand atoms using thousands of processors under the conventional planewave pseudopotential approach. We will demonstrate that this approach provides a practical way for future petascale computation in materials/nanomaterials science. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B27.00010: Including spin-orbit coupling in materials-specific studies of spin transport. A.A. Starikov, P.J. Kelly Spin-orbit coupling (SOC) plays a crucial role in magnetoelectronics: it is the origin of anisotropic magneto-resistance (AMR), prevents half-metallic ferromagnets from having 100\% spin polarization, gives rise to spin-flip scattering which ultimately destroys the spin polarization of a current in non-magnetic materials - to mention but a few of its effects. Nevertheless, it has been virtually ignored in theoretical transport studies. To redress this neglect, we have developed a method based upon Linearized Muffin-Tin-Orbitals suitable for studying spin-dependent transport in nanostructures which includes SOC and provides a framework for modelling layered magnetic systems with non-collinear magnetizations. As a first application and test of the method, we study the AMR effect in ferromagnetic alloys. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B27.00011: Modeling the deformation of materials with stochastic fractal microstructure M.A. Soare, R.C. Picu Many materials with heterogeneous multiscale fractal structure are found in nature. Examples include biological tissues and bone, some rock such as sandstones, and aero-gels. In such materials the amount of geometrical detail observed in the microstructure increases from scale to scale in a self-similar manner, they lack characteristic length scales and the Hausdorff dimension is smaller than that of the embedding space. Furthermore, the microstructure is multiscale and stochastic, in the sense that the generating operators that map the geometry from one scale to the next are stochastic. In this work, we develop a method by which boundary value problems can be solved for these complex multiscale materials with minimal computational effort. Use is made of the scaling properties of the geometry and of stochastic finite elements in which the solution is approximated using chaos polynomials. The talk will review the formulation and a number of examples used for verification. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B27.00012: Binding energies of CO$_{2}$ with some ionic liquids William Eucker, John Bendler Room temperature ionic liquids (RTILs), a novel class of materials with negligible vapor pressures and potentiality as benign solvents, may be an ideal chemical for carbon dioxide (CO$_{2})$ gas sequestration. \textit{Ab initio} computational modeling was used to investigate the molecular interactions of simple RTIL anions hexafluorophosphate (PF$_{6}^{-})$ and tetrafluoroborate (BF$_{4}^{-})$ with CO$_{2}$. Electronic potential energy surface (PES) scans of a comprehensive sampling of 1:1 anion-CO$_{2}$ orientations were computed using Spartan '02 with Dunning's correlation consistent basis sets. Qualitatively, the PES scans yielded deeper, more numerous and radially closer active sites surrounding BF$_{4}^{-}$ anion as compared with the PF$_{6}^{-}$ anion. Quantitatively, the binding energies of 17.87 kJ/mol and 25.24 kJ/mol were extracted from the identified global energy minima for the PF$_{6}^{-}$ and BF$_{4}^{-}$ systems, respectively. The smaller BF$_{4}^{-}$ anion was concluded to bind more strongly to the CO$_{2}$. However, literature-reported experimental Henry's law constants for CO$_{2}$ dissolved in imidizolium based RTILs show greater gas solvation in the PF$_{6}^{-}$ system. The discrepancy between the energetics calculation results and the experimental solvation data will be discussed. [Preview Abstract] |
Session B28: Focus Session: Carbon Nanotube Optoelectronics
Sponsoring Units: DMPChair: Tony Heinz, Columbia University
Room: Colorado Convention Center 302
Monday, March 5, 2007 11:15AM - 11:51AM |
B28.00001: Excited states and electro-optics of carbon nanotubes Invited Speaker: We will discuss experimental and theoretical results on nanotube excited state production and luminescence through photoexcitation, electron-hole recombination and hot carrier impact excitation. The effects of an external electric field, as well as environmental effects on the absorption and emission spectra will be examined. Finally, nanotube photoconductivity and photovoltage and the role of the substrate and defects on these processes will be analyzed. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B28.00002: Electrically driven thermal light emission from individual single-walled carbon nanotubes Y.K. Kato, D. Mann, A. Kinkhabwala, E. Pop, J. Cao, X. Wang, L. Zhang, Q. Wang, H. Dai, J. Guo Light emission from carbon nanotubes offer unique opportunities in nano-optoelectronics, because of their chirality dependent electronic structure, availability of high quality electrical contact, and very high aspect ratio. We study electrically- driven light emission from individual single-walled carbon nanotubes, including both quasi-metallic and semiconducting species.\footnote{D. Mann et al., submitted for publication (2006).} Our field effect transistor structure utilizes a clean, as-grown nanotube suspended across a trench, allowing for low contact resistance and good isolation from the substrate. The spectra from quasi-metallic nanotubes reveal pronounced peaks in the visible and infrared corresponding to E$_{11}$ and E$_{22}$ transitions. The emission rates show strong correlation with electrical power dissipated in the devices, consistent with thermally excited emission due to resistive heating. We observe similar behavior for the semiconducting devices, although electroluminescence in these nanotubes has been explained by either carrier injection or impact excitation. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B28.00003: Electroluminescence Properties of Carbon Nanotube Network Transistors Benoit St-Antoine, Elyse Adam, Carla Aguirre, David Menard, Richard Martel Carbon nanotubes network transistors (CNNT) open a promising route for the integration of nanotubes in electronics for that they circumvent major issues related to their fabrication. [1] They also reduce device-to-device discrepancies because they combine the properties of an ensemble of nanotube species. Here, we investigated the optoelectronic properties of the CNNT fabricated from different nanotube sources and found bright electroluminescent (EL) emission. The EL is specific to the nanotube source and can be linked using absorption spectra to their diameter distribution. (1) E.S. Snow, P.M. Campbell, M.G. Ancona, Appl. Phys. Lett., 2005, 86, 033105. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B28.00004: Localized Photoresponse and Raman Spectra of Long Carbon Nanotube FET's James Tsang, Marcus Freitag, Phaedon Avouris The spatially resolved photoresponse, and Raman spectra of CVD grown carbon nanotube field effect transistors with channel lengths between 2 and 50$\lambda $m have been measured using conventional imaging techniques at photon energies between 1.4 and 2.7eV. A strong localized photoresponse including both the short circuit photocurrent and the high impedance photovoltage is observed even at zero bias with spatially resolvable contributions from the Schottky barriers, from observable inhomogeneities and from fluctuations all along the device. The magnitude of the photoresponse from defects such as tube crossings and fluctuations in the tube environment can be comparable to or stronger than that arising from the charge separation at the Schottky barriers. The Raman spectra show high quality CNTs with some correlation between the spatial positions of weak D lines, when observed, and changes in the photoresponse. Comparisons of the Raman spectra and the intensity of the photoresponse show the presence of significant potential fluctuations on the micron length scale along these devices [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 1:03PM |
B28.00005: Interplay Between Transport and Optical Properties in Carbon Nanotube $p-n$ Diodes Invited Speaker: The $p-n$ junction diode is the basis for nearly all-modern semiconductor electronics. It is the basis for transistors and optical devices. For any new material system, therefore, a proper characterization of the $p-n$ junction is crucial for their development into electronic devices. In this talk, I will demonstrate the formation of $p-n$ junction diodes along individual single-walled carbon nanotubes (SWNTs). The $p-n$ junction is formed using a novel electrostatic doping technique using a pair of split gate electrodes, and can exhibit \textit{ideal diode} behavior, the theoretical limit of performance for any diode. The low background leakage currents coupled with a built-in electric field region to transport the quasi-particles makes these diodes ideal for studying the optical response of SWNTs. I will show that the photocurrent spectroscopy of these diodes is able to provide a comprehensive probe of the excited states in SWNTs. A series of narrow excitonic resonant peaks is observed over a wide spectral range, including the first exciton peak ($E_{11})$, which defines the optical gap. At an intermediate energy the onset of continuum (electronic band gap) is observed and demonstrate large exciton binding energies. Because of the large exciton binding energies, the large photocurrent derived from $E_{11}$ excitons is not expected. Here, I will describe several characteristics related to these peaks such as the origin of the quasi-particle (electron and hole) currents, quantum efficiency, and the role of many-body effects in determining the dark (ideal diode) and excited (excitonic) states of SWNTs. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B28.00006: Impact Excitation by Hot Carriers in Carbon Nanotubes Vasili Perebeinos, Phaedon Avouris We find in Ref. 1 and 2, that the impact excitation processes in nanoscale devices are much more efficient than in conventional bulk semiconductors due to the enhanced Coulomb interaction in low dimensions. In semiconducting carbon nanotubes, we calculate the impact excitation rates to be 4-5 orders of magnitude larger than in bulk semiconductors [2]. The impact excitation rate is much higher in nanotubes than the impact ionization, which neglects electron-hole interaction of the produced electron-hole pair, while their difference is negligible in bulk materials. The angular momentum conservation law plays a crucial role in determining the threshold energy of the impact excitation. The spectra of the produced excitons depends strongly on the bias and not constrained by the dipole selection rule as in the photoluminescence. The triplet excitons have approximately equal probability to be produced, unlike 1/4 statistical fraction for the independently injected electrons and holes. [1] J. Chen, V. Perebeinos, M. Freitag, J. Tsang, Q. Fu, J. Liu, Ph. Avouris, Science 310, 1171, 2005. [2] V. Perebeinos and Ph. Avouris, Phys. Rev. B. 74, 121410(R), 2006. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B28.00007: Exciton Annihilation Processes in Individual Single-Wall Nanotubes Aditya Mohite, Prasanth Gopinath, Hemant Shah, Bhaskar Nagabhirava, Tanesh Bansal, Bruce Alphenaar Field enhanced photocurrent measurements of individual single-wall nanotubes show that bound exciton dissociation occurs through two distinct processes. At low fields, the barrier to field ionization is not surmounted but bound carriers can still dissociate by tunneling into the free carrier states. At high fields (approximated by the binding energy divided by the Bohr radius, or,$E_b /r)$ the bound excitonic state is destroyed. We measure the photocurrent of a SWNT capacitor, in which the nanotubes lie on a 100 nm oxide dielectric on doped silicon substrate. This allows us to apply extremely large electric fields across the nanotube. Excitons do not contribute to the photocurrent unless dissociation into free carrier states occurs. At fields below $1\times 10^8V/m$ the exciton peak increases according to Fowler-Nordheim field dependence. At a field of approximately $1.2\times 10^8V/m$the photocurrent rapidly increases by more than an order of magnitude suggesting a huge increase in the exciton dissociation rate. This corresponds to the predicted field required for exciton annihilation to occur. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B28.00008: The role of intrinsic regions in nanotube photodiodes Derek Stewart In this work, we consider the impact of intrinsic regions on transport in carbon nanotube diodes. Recently nanotube diodes have been fabricated in a split-gate geometry where a central intrinsic region separates two regions gated p-type and n-type, respectively. These devices show near ideal diode behavior and can also act as photodetectors. We use a self-consistent non-equilibrium Green's function approach to examine how the central intrinsic region affects the properties of a nanotube p-i-n photodiode. The charge and potential along the diode are determined self-consistently for systems with different intrinsic layer lengths. We find that the intrinsic region has little effect on the dark current in the device. However, as the size of the intrinsic region increases, the photocurrent grows as well. The presence of a central intrinsic region also leads to greater power conversion efficiency in nanotube photodiodes. These changes in the photoresponse can be related to charge redistribution caused by the introduction of the intrinsic layer. This leads to a reduction of the flat band regions near the leads, while unmasking the van Hove singularities in the central intrinsic region that enhance the photoresponse for higher photon energies. This effect is quite general and may be observed in similar p-n junctions (i.e. nanowires) where the density of states is quasi-one dimensional. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B28.00009: Dependence of Raman-active modes on the external voltage in single-wall carbon nanotube thin films Giovanni Fanchini, Goki Eda, Husnu Emrah Unalan, Manish Chhowalla We report on Raman measurements under the application of an external voltage in gap-cell devices made by transparent and conducting single-wall carbon nanotube (SWNT) thin films [1] Two different Raman excitation wavelengths (785 and 633 nm) were used. Application of voltage results in downshifts of the D and G modes and in reduction of their intensity. The intensities of the radial breathing modes increase with voltage in metallic SWNTs, while decreasing in semiconducting SWNTs. A model explaining the phenomenon in terms of both direct and indirect (Joule heating) effects of the field is proposed. Our work rules out the elimination of large amounts of metallic SWNTs in thin film transistors using high field pulses. Our results support the existence of Kohn anomalies in the Raman-active optical branches of metallic graphitic materials. Additional Raman measurements in SWNT thin film transistors at varying source-drain voltage and gate voltage will be presented as well. [1] G Fanchini, et al, submitted [2] S.Piscanec et al, PRL 93 (2004) 185503 [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B28.00010: Absolute Absorptivity of Single-walled Carbon Nanotubes Employing a Pyroelectric Detector Katherine Hurst, Anne Dillon, John Lehman Optical properties are important for determining fundamental characteristics of carbon single-walled nanotube (SWNT) samples including purity, chirality, and tube diameter. Previously, we have estimated the volume fraction of metallic versus semiconducting tubes for highly purified SWNT bucky-paper on a pyroelectric detector from spectral responsivity measurements and an effective medium approximation to determine the dielectric function (1). Pyroelectric detector-based measurements are based on the thermalization of photons within the SWNT coating and provide a robust technique for measuring absolute absorptivity at normal incidence. Alternatively, we perform transmissivity measurements of SWNTs by employing a gold-black coated pyroelectric detector. Spectral responsivity measurements are made by direct substitution against a NIST calibrated detector such that quantitative changes in the volume fraction and purity of SWNT samples are revealed. These results will be compared to specular transmissivity measurements made by UV-VIS spectrometry. Raman spectroscopy will also serve to verify nanotube properties. (1) K.E.H. Gilbert, J.H. Lehman, A.C. Dillon and J.L. Blackburn Appl. Phys. Lett. 88, 143122 (2006). [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B28.00011: Photoresponse of Suspended Carbon Nanotube Networks: Single-Walled Carbon Nanotube Infrared Bolometer Mikhail E. Itkis, Ferenc Borondics, Aiping Yu, Robert C. Haddon The photoresponse of a single-walled carbon nanotube (SWNT) film is dramatically enhanced when the nanotube film is suspended between electrical contacts in vacuum. We show that the change in electrical conductivity is bolometric (caused by heating of the SWNT network). Electron-phonon interactions lead to ultrafast relaxation of the photoexcited carriers and the energy of the incident infrared radiation is efficiently transferred to the crystal lattice. The photoinduced changes in resistance occur as result of temperature changes rather than by photoexcited holes and electrons and we consider the implications of this result for the band and exciton models in carbon nanotubes. We show that the infrared photoresponse of suspended SWNT films is sufficiently high that they may function as the sensitive element of an infrared bolometric detector. M.E.Itkis, F.Borondics, A.Yu, R.C.Haddon, \textit{Science} \textbf{312}, 413 (2006) [Preview Abstract] |
Session B29: Dense Granular Flows and Jamming
Sponsoring Units: DFDChair: Martin van Hecke, University of Leiden
Room: Colorado Convention Center 303
Monday, March 5, 2007 11:15AM - 11:27AM |
B29.00001: Affine and Non-Affine motion in a Granular Couette Experiment Brian Utter, Robert Behringer We characterize local motion of grains in a 2D granular Couette shear. In steady state, grains exhibit a shear band, where the grains are dilated near the shearing surface, $r = 0$. The mean velocity is in the tangential direction, and decays somwhat faster than exponentially. We characterize the local motion by tracking small clusters of particles. The overall motion of the cluster can be described in terms of a smooth affine part, and a non-affine part that is not captured by the smooth deformation. We determine the measure of non-affine motion, $D^2_{min}$ of Falk and Langer. This quantity shows characteristic distributions that initially grow roughly as power laws, but are then cut off exponentially. Distributions of non-affine displacements for individual particles are roughly guassians. The width of these distributions, the widths of the distributions for $D^2_{min}$ and previously measured diffusivities show essentially identical variation with local shear rate. We understand the formation of the shear band from an initially homogeneous packing in terms of outwardly directed diffusion next to the shearing surface. In the steady state, there is a balance between inward diffusion from a density gradient, and ourward diffusion driven by the shearing. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B29.00002: Nonlinear elastic stress response in granular materials Brian Tighe, Joshua Socolar We study the response of two-dimensional granular materials to a local boundary force, for which classical elasticity predicts identical stress states in the cases of isotropic and hexagonally anisotropic materials. We probe the differences in these two cases by including corrections from the full nonlinear elasticity theory. Additionally, we model the effect of discrete microstructure by taking the magnitude of multipole stress response terms, which are induced in the nonlinear system, as material parameters. By so incorporating both anisotropy and microstructure, reasonable fits are obtained for experimental stress response profiles in hexagonal packings of photoelastic grains, while either correction alone is insufficient. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B29.00003: Upward penetration through a granular medium D. Costantino, T.J. Sheidemantel, M.B. Stone, J. Cole, C. Conger, K. Klein, M. Lohr, W. McConville, Z. Modig, P. Schiffer We measure the force needed to push a flat plunger upwards through a granular medium. The plunger begins flush with the base of the grains' container, and we focus upon the force necessary to initiate motion.~The data show that this break-out force increases monotonically with plunger diameter and pile height as expected.~In contrast to previous measurements of the force needed for vertical penetration from above and of the horizontal drag force, this break-out force has a strong dependence on the diameter of beads making up the pile.~The nature of this bead size dependence can be altered by using different methods to form the grain pile.~Implications for the relevant force chain network will be discussed.~Research supported by NASA grant NAG3-2384 and the NSF REU program. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B29.00004: Sound and Force Propagation in Granular Materials Clifford E. Chafin, Karen E. Daniels A characteristic of granular materials under stress is a highly nonuniform distribution of forces. These localized force chains are prominent of 2-D packings of photoelastic particles, but their role in sound propagation is unclear. We mechanically excite 100 $\mu$s square wave pulses and periodic waveforms through such packings. We report optical measurements of changes in the force chain network using a high speed camera, and simultaneous acoustic measurements from biaxial accelerometers of similar size and mass to the photoelastic particles. These measurements provide amplitude and speed (time of flight and group velocities) of the response both on and off the force chain network. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B29.00005: Testing the equal-probability assumption of jammed particle packings Guo-Jie Gao, Jerzy Blawzdziewicz, Corey O'Hern The Edwards' entropy formalism provides a statistical mechanical framework for describing dense granular systems. In addition, experiments on vibrated granular columns and numerical simulations of quasi-static shear flow of dense granular systems have provided evidence that the Edwards' theory may accurately describe certain aspects of these systems. However, a fundamental assumption of the Edwards' description---that all mechanically stable (MS) granular packings at a given packing fraction are equally likely---has not been explicitly tested in dense granular systems. We investigate this assumption by generating all mechanically stable hard disk packings in small systems using a protocol in which we successively compress or decompress the system followed by energy minimization. We then apply quasi-static shear flow at fixed pressure to these MS packings to study the frequency with which MS packings occur during the shear flow. We find that the MS packings do not occur with equal probability during the shear flow, in fact, there is a significant reduction in the number of accessible MS packings at large shear strain. Thus, the Edwards' entropy formalism should be re-examined in light of our findings. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B29.00006: Experimental study of the compaction dynamics for 2D granular pile of spherical and cylindrical grains Geoffroy Lumay, Nicolas Vandewalle, Francois Ludewig We present an experimental study of the compaction dynamics for two-dimensional granular systems. The compaction of a pile of spherical grains and of a pile of cylindrical grains have been studied. Compaction dynamics is measured at three different scales : the macroscopic scale through the normalized packing fraction, the mesoscopic scale through the normalized fraction of ideally ordered domains in the system, and the microscopic scale through the grain mobility. Moreover, the ideally ordered domains are found to obey a growth process dominated by the displacement of domain boundaries. A global picture of compaction dynamics relevant at each scale is proposed. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B29.00007: Packing and segregation in thermally cycled granular materials Ke Chen, John Draskovic, Julia Cole, Andrew Harris, Casey Conger, Matthew Lohr, Kit Klein, Thomas Scheidemantel, Peter Schiffer We have studied the change of packing fraction of granular materials and the displacement of an intruder in a granular pile under thermal cycling. We find that the packing fraction of granular materials increases with thermal cycling, i.e., heating the sample and returning it to ambient temperature. This effect appears to be related to the difference in thermal expansion between the container and the grains, and it increases monotonically with increasing cycle temperature. The packing fraction further increases under multiple thermal cycles and the increasing packing fraction can be fit to a double exponential decay toward the random close packing. We also find that spherical intruders in granular piles can move downward with thermal cycling, and that this effect depends on the relative density of the grains and the intruder as well as the relative thermal expansion of the grains and the container. This research was supported by the NASA through grant NAG3-2384 and the NSF REU program through grant DMR 0305238. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B29.00008: Experimental Characterization of the Jamming Transition in a Granular Material Trushant Majmudar, Robert Behringer We describe experiments to test recent predictions for the jamming transition in disordered solids. Here, our system is a 2D granular material consisting of photoelastic disks. By observing these particles through crossed circular polarizers, it is possible to a) accurately determine particle contacts, b) via an appropriate computational procedure, calculate the vector contact forces between particles, and c) from the contact forces compute the Cauchy stress. Simulations (e.g. by O'Hern et al., Donev et al.) for frictionless particles predict a discontinuous increase in the contact number, $Z$ at the jamming transition, given by a critical packing fraction, $\phi_c$. Above jamming, $Z$ should then increase as a power law in $\phi -\phi_c$ with an exponent of 0.5 to 0.6. The pressure, $P$ is also predicted to grow as a power law. Additionally, Senkes and Chakraborty have predicted the behavior of $P$ and $Z$ using a meanfield entropy-based description. Our experiments support all of these predictions. There is a rapid increase in $Z$ at $\phi_c$, and power law increase of $Z$ and $P$ above the transition. There is also reasonable agreement between the data and the predictions of Senkens and Chakraborty. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B29.00009: Stress, strain rate, and free volume in dense granular flow Chris Rycroft, Ken Kamrin, Martin Bazant There have been many attempts to describe dense granular flow with continuum models, but a complete theory is still lacking. Often, these models make assumptions about microscopic quantities (such as shear stress, strain rate, and local density) and here we present Discrete Element Method (DEM) simulations to directly measure these in a variety of different non-homogeneous granular flows. Motivated by previous work, we compute these quantities on a mesoscopic length scale of several particle diameters, and examine both spatial distributions, and correlations between the variables. We investigate the validity of the commonly-used Mohr-Coulomb incipient yield hypothesis, which states that the ratio of shear stress to normal stress should be everywhere constant in a flowing granular material. Our results also show some striking correlations between strain rate and local density, which suggest a phase transition between static and flowing granular materials. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B29.00010: The force network in emulsions and the role of external stress Jing Zhou, Tim Prisk, Su Long, Habib Skaff, Qian Wang, Todd Emrick, Anthony D. Dinsmore Direct imaging of emulsion droplets labeled with fluorescent nanoparticles using confocal microscopy is a valuable experimental tool for studying granular materials in three dimensions. By measuring individual droplet-droplet contacts inside the frictionless emulsion piles, we visualize the force network and calculate the orientations, positions, and magnitudes of forces and their statistical distributions. We find that large forces are more likely to align parallel to each other, leading to long-range, chain-like correlations of magnitude and direction of contact force. Furthermore, we investigate how the force network evolves with time and how it changes under various external stresses. We also measure the contact force at the bottom of emulsion piles and compare to previous surface measurements and the measurement inside the bulk. This work may shed light on the aging and macroscopic viscoelasticity of granular systems. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B29.00011: The packing and compaction dynamics of granular polymers Ling-Nan Zou, Xiang Cheng, Heinrich Jaeger, Sidney Nagel While the packing of hard spheres has been the subject of intense research, the packing of objects with reduced symmetries is far less well-studied, both experimentally and theoretically. Here, we report an experimental study on the packing of a granular polymer analogue --- chains of hollow, spherical brass beads, 1.9 mm in diameter, ranging in length from a 1 to 42 700 beads per chain. In particular, we systematically measure the density $\rho$ of the bead-chain pack as a function of the number of beads per chain $M$ (\textit{i.e.} the molecular weight of the granular polymer). The density decreases from the random close packed density $\rho_{RCP} \approx 0.64$ for single beads to an asymptotic density $\rho_\infty \approx 0.39$ in the limit of very long chains; the form of the density fall-off is rather slow, the effect is noticeable even when $M$ is much larger than the chain persistence length. In terms of dynamics, the compaction of bead-chain packs appears to obey the same logarithmic relaxation form found in the compaction of single bead packs [1], but with a puzzling, $M$-dependent sensitivity to initial conditions. We shall discuss these results in the context of, and attempt to make connections to, the packing of single hard spheres on one hand and the physics of polymer melts on the other. [1] J. B. Knight \textit{et al.}, Phys. Rev. E \textbf{51}, 3957 - 3963 (1995). [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B29.00012: The Stochastic Flow Rule and Rate Sensitivity in Dense Granular Flows. Ken Kamrin, Chris H. Rycroft, Martin Z. Bazant The Stochastic Flow Rule (SFR) is a constitutive law which, when used with limit-state Mohr-Coulomb plasticity for stresses, gives predictions for the mean velocity field in quasi-2D dense granular flows. It is based on a simple microscopic flow mechanism, where ``spots'' of free volume perform random walks along slip-lines, biased by stress imbalances upon local fluidization. The SFR has recently been shown to predict dense granular flows in diverse geometries--- e.g. draining silos, annular Couette cells, and plate-dragging experiments--- without the use of fitting parameters. However, a significant rheological change occurs in certain geometries--- e.g. inclined plane flow and gravity-free horizontal shear flow--- where the packing fraction is nearly uniform and a distinct stress/strain-rate relationship arises. In this talk, we review the SFR and propose a simple explanation of when and why rate sensitivity occurs, depending on the slip-line geometry. We also postulate how rate-dependent terms may be combined with the SFR to create a more universal theory of dense flows. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B29.00013: The Dynamics of Sandpile Model and Its Application to Earthquakes Yunfan Gong Just from the simple yet widespread power laws, it seems unlikely to differentiate self-organized criticality (SOC) from other mechanisms proposed for power-law relationships. Here we report SOC phenomenon in a sandpile model driven by chaos. We characterize SOC by analyzing times series from the system. Surprisingly, we find that the microscopic dynamics of the complex sandpile system can be best approximated by a very simple one-order autoregressive (AR) model. Meanwhile, the AR model can well reproduce almost all power-law behaviors of the sandpile model, suggesting a similar dynamics between the complex sandpile system and the simple one-order AR model. Next, real earthquake time series including Harvard catalog and source time functions (STFs) are analyzed along the same lines. The one-order linear dynamics fitted from the STFs is in excellent agreement with that of the sandpile model, whereas the optimal two-order dynamics fitted from the STFs is a false mode and should be rejected. Our results support that earthquakes can be considered as a SOC process and suggest that they may be governed by sandpile models with high order ($\geq 2$) dynamics. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B29.00014: Smoothing a Rock by Chipping Sidney Redner, Paul Krapivsky We investigate an idealized model for the size reduction and smoothing of a polygonal rock due to repeated chipping at corners. Each chip is sufficiently small so that only a single corner and a fraction of its two adjacent sides are cut from the object in a single chipping event. After a large number of chipping events, the shape is not circular, with the distribution of facet lengths and corner angles broadly distributed. In the long-time limit, the shape of the object is not a unique, but rather is characterized by large sample-to-sample fluctuations. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B29.00015: Comparison of the influence of a strong current and of a spark on the distribution of the resistance of a contact between two grains Stephane Dorbolo, Alexandre Merlen, Eric Falcon, Matthieu Creyssels, Bernard Castaing, Nicolas Vandewalle The distribution of the electrical resistance of a contact between two stainless steel beads is a log normal. When a current is injected through a contact, the voltage is not univocally determined. The system exhibits hysteresis. A chain of beads have been used to make some statistic and to determine how a strong current or a electric spark modify the distribution of the resistance. A strong current changes the distribution of resistance into a nearly gaussian distribution. The contacts are soldered by the current. On the other hand, a spark only modifies the highest resistances. The value of the minimum resistance that is modified is determined by the distance between the spark and the bead chains. [Preview Abstract] |
Session B30: Instabilities and Turbulence
Sponsoring Units: DFDChair: Robert Ecke, Los Alamos National Laboratory
Room: Colorado Convention Center 304
Monday, March 5, 2007 11:15AM - 11:27AM |
B30.00001: Lagrangian statistics in two-dimensional turbulence Michael Rivera, Robert Ecke Using data obtained from a stably stratified shallow layer of fluid, we generate Lagrangian trajectories from which a number of statistical quantities can be calculated. Of particular interest are the Lagrangian structure functions of the velocity difference and the acceleration statistics. We find Kolmogorov like scaling in the Lagrangian structure functions (when plotted using ESS) in the direct enstrophy cascade range, and deviations from Kolmogorov in the inverse energy cascade range. This is somewhat surprising because there is a marked lack of intermittency in the inverse energy range. Intermittency is associated with deviations from Kolmogorov scaling in three-dimensional experiments. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B30.00002: Pressure Fluctuations in Two-dimensional Turbulence Yonggun Jun, X.L. Wu We investigate pressure fluctuations in two-dimensional (2D) turbulence driven electromagnetically in a freely suspended soap film. The reduced probability distribution function (PDF), $P(p/\sigma_{p})$, is found to be universal for different Reynolds numbers and consists of asymmetrical exponential wings, where $\sigma_{p}\equiv\langle p^{2}\rangle^ {1/2}$ is the standard deviation. The calculated pressure skewness $S_{p}=\langle p^{3}\rangle/\sigma_{p}^{3}\simeq-0.5$ is significantly smaller than predictions by simple 2D models (Holzer and Siggia, Phys. Fluids A5, 2525 (1993)) but surprisingly close to 3D calculations using a random velocity field with a Kolmogorov energy spectrum $E(k)\propto k^{-5/3}$. The pressure spectrum $E_{pp}(k)$ scales approximately as $E_ {pp}(k)\propto k^{-7/3}$ in the energy inverse-cascade subrange and $k^{-5}$ in the enstrophy cascade subrange. These observations suggest that pressure fluctuations is essentially a large-scale phenomenon and the presence of an enstrophy cascade has no effect on the tails of $P(p/\sigma_{p})$. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B30.00003: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 11:51AM - 12:03PM |
B30.00004: Three regularizations as turbulent subgrid models Jonathan Pietarila Graham, Darryl Holm, Pablo Mininni, Annick Pouquet Geophysical and astrophysical phenomena involve a huge range of scales. The number of degrees of freedom are inconceivable for numerical simulations to achieve, and truncation of the omitted scales removes important physics. Regularization subgrid models for this closure problem have recently emerged. Unlike many Large Eddy Simulations (LES), these models have guarantees on the computability of their solutions, conserve energy, and recover the physical equations as the filter width vanishes. Three regularizations can be viewed as LES with successively more complex subgrid-stress terms: the Clark, Leray, and alpha models. Comparing these, we establish the affects of each term. As each has different small-scale energy spectra this can shed light on the link between small-scale properties of the flows and their intermittent behavior. We find that Leray fails to recover large-scale anisotropy in our flow and the time scale for the development of turbulence. The Clark and alpha models both perform well in these regards but require extra dissipative for adequate computational gains. We also test the helicity of vortex tubes, Beltramization of the flow, and statistical properties for the subgrid models. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B30.00005: Simultaneous Velocity Discrimination Method of Two-Phase Flows Using Time Resolved Stereo PIV and PTV P.B. Vanderwerker, Y. Chen, M.M. Torregrosa, F.J. Diez, S. Photos, D. Troolin Multiphase jets laden with particles appear in many engineering and environmental processes. Typical examples are sprays containing liquid fuel drops in combustion processes, air jets laden with coal particles in a power plant, and the dispersion of harmful substances like soot and pollutants from steady exhaust flows, among others. Studies of particle-laden turbulent flows suggest that particle distribution is not uniform but preferential. In order to understand the mechanism of particle dispersion, time resolved simultaneous 3D velocity measurements of the disperse phase and of the fluid flow were made. Two-phase discrimination algorithms were developed based upon the filtering methodology proposed by Khalitov {\&} Longmire (2002), allowing for complete separation of the two-phases in stereo PIV images. The different filtering methods studied include separation of the two-phases using: (1) particle size discrimination, (2) particle intensity discrimination, (3) particle size and intensity discrimination, and (4) fluorescent particles for one of the two-phases. This methodology also enables time-resolved instantaneous 3D velocity fields using PTV and PIV on the disperse phase and fluid flow phase respectively. These allow visualization of 3D turbulent coherent structure evolution in the fluid as well as the evolution of the dispersed phase. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B30.00006: Measurement of entrainment and mixing in oceanic overflows Philippe Odier, Jun Chen, Michael Rivera, Robert Ecke The mixing and entrainment processes existing in oceanic overflows, e.g., Denmark Strait Overflow (DSO), affect the global thermohaline circulation. Owing to limited spatial resolution in global climate prediction simulations, the small-scale dynamics of oceanic mixing must be properly modeled. We have built a facility (Oceanic Overflow Facility) allowing the study of a gravity current along an inclined plate, flowing into a steady ambient medium. At small values of the Richardson number, the shear dominates the stabilizing effect of the stratification and the flow at the interface of the current becomes unstable, resulting in turbulent mixing. In addition, the level of turbulence is enhanced by an active grid device. Using PIV and PLIF to measure, respectively, the velocity and density fields, we characterize the statistical properties of the mixing. We also study the entrainment of the ambient fluid by the flow. An accurate parametrization of the mixing and entrainment can be a valuable input for ocean circulation models. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B30.00007: Finite Size Effects in the Quasi-geostrophic Inverse Cascade Colm Connaughton In the standard statistical theory of quasi-geostrophic turbulence forced at intermediate scales, two cascades are produced. Energy flows to large scales and potential enstrophy flows to small scales. The inverse cascade of energy is very similar to that which occurs in purely two-dimensional hydrodynamics. In that case, interesting phenomena occur if the friction between the fluid layer and the substrate is sufficientlyweak to allow the inverse cascade to reach the size of the system. Most striking among these is the spontaneous emergence of very intense coherent vortices which suppress turbulent fluctuations. A similar situation can arise in the quasi-geostrophic inverse cascade if the Ekman damping is weak enough, the scenario which I will describe in this talk. The situation is richer because large scale coherence can be obtained either through the formation of large vortices or through the formation of zonal jets. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B30.00008: Measured oscillations of the velocity and temperature fields in turbulent Rayleigh-B\'{e}nard convection in a rectangular cell Sheng-Qi Zhou, Chao Sun, Ke-Qing Xia We report experimental measurements of the velocity and temperature oscillations in a rectangular cell. The aspect ratios are $\Gamma_x=1$ and $\Gamma_y=1/4$ so that the large scale convective flow is confined in the plane of $\Gamma_x=1$. From particle image velocimetry (PIV) measurement it is found that the large-scale flow plane aligns along the diagonal plane of the cell. The large scale circulation is found to be oscillatory based on analysis of the autocorrelation functions of the velocity and temperature fields. It is well known that oscillations of velocity and temperature exist in cylindrical cells. The fact that they are now also found in a rectangular cell suggests that the oscillation phenomenon is an intrinsic character of the convective flow rather than the geometric character of convection cell. In the range of Ra from $3.5\times10^{10}$ to $9\times10^{11}$, it found that the oscillation frequency of temperature $f_{T}\sim Ra^{0.45}$ and that of the velocity $f_{V}\sim Ra^{0.51}$, which are close to results from previous measurements made in cylindrical cells. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B30.00009: A connexion between turbulence in Rayleigh Taylor flows and turbulence in other buoyant flows Olivier Poujade An increasing number of numerical simulations and experiments describing the turbulent spectrum of Rayleigh-Taylor (RT) mixing layers came to light over the past few years. Results reported in recent studies allow to rule out a turbulence {\it \`a la Kolmogorov} as the main mechanism acting on a self similar RT turbulent mixing layer. In this case, the injected power is due to buoyancy motion on a broad range of length scales. We have generalised Lin's spectral equation to buoyant flows and we have shown that this injected power tends to accumulate at large scales so that big whirls can get bigger as the mixing layer thickness increases. Only a small fraction of this power is transferred to small scales through a Kolmogorov cascade and dissipated. This balance between the accumulation of energy at large scales and the buoyancy production can also be applied to Rayleigh-B\'enard instabilities. It explains the Bolgiano-Obukov scaling predicted and experimentally observed for these flows. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B30.00010: Unsteady Kelvin-Helmholtz instability of an inmiscible interface with a large contrast in viscosity Harunori Yoshikawa, Jose-Eduardo Wesfreid We studied a stability problem of two-layer oscillatory flows, especially with an interest in the case of a large contrast in viscosity at the interface. Preceding experimental studies showed that static deformations of the interface, often referred to as ``frozen waves,'' happened beyond a threshold. Theoretically, we examined exhaustively the linear stability of the system for any viscosity contrast. What we found are: (i) destabilizing effect by the viscosity contrast and (ii) frequency dependence of the wave length selected by the linearly most instable mode. Instability is provoked by a smaller excitation in the case of large viscosity contrast within a certain band of frequency. The second point shows a deviation from the classical KHI. For high frequencies, the most instable mode has the capillary wave length, while for low frequencies, a longer one. Within an intermediate frequency range, the most instable mode can have a shorter wave length than the capillary one, depending on the viscosity contrast. We also realized a model experiment in a small-frequency range rarely investigated in the preceding studies. The two fluids were chosen so that the contrast in viscosity was very large ($10^4$ times difference in kinematics viscosity). Interface behavior was determined in detail. Results were in good agreement with our theoretical predictions. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B30.00011: Growth of Convective and Absolute Instabilities in Co-flowing Jets Andrew Utada, Alberto Fernandez-Nieves, David Weitz We have shown recently that the dripping-to-jetting transition in co-flowing liquids is controlled by two non-dimensional numbers: the capillary number (\textbf{\textit{Ca}}$_{out})$ of the outer liquid and the Weber number of the inner liquid (\textbf{\textit{We}}$_{in})$. When jetting is forced by \textbf{\textit{Ca}}$_{out}$, the diameter of the jet narrows in the downstream direction and the drop size scaling is well predicted assuming that the Rayleigh-Plateau instability is convected along the jet to cause its break up. However, when jetting is forced by \textbf{\textit{We}}$_{in}$, the diameter of the jet widens in the downstream direction and the resultant drop size can not be predicted assuming that the Rayleigh-Plateau instability causes the jet to break up. Instead, we believe these jets break due to absolute instabilities. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B30.00012: Lattice Boltzmann Model for Two-Dimensional Flow of Immiscible Fluids between Closely-Spaced Plates Alex Fore, Robert Sekerka, Michael Widom We formulate a Lattice Boltzmann (LB) model for simulation of two-dimensional flow of nearly-immiscibile fluids between closely spaced parallel plates. We treat displacement of a more viscous fluid by a less viscous fluid, as in a Hele-Shaw cell. The nearly two dimensional flow leads to the well-known Saffman-Taylor instability. We use a binary (A-B) LB model to simulate the problem. We account for the effects of the thin dimension between the plates via a drag force that we obtain by averaging the equations of motion over the thin dimension. We consider the A-B solution to be a regular solution with a strongly repulsive potential and use the effective potential method, consistent with equilibrium thermodynamics, to model non-ideal solutions. To control the viscosity of each phase we use a mixing rule for the relaxation time that depends linearly on mole fraction. We use a gradient energy on the mole fraction to attain control over the interface width and surface tension. We use this model to simulate viscous fluid displacement in a rectangular Hele-Shaw cell. Preliminary results display the Saffman-Taylor instability which we compare with a classical linear stability analysis. We have also observed time development of nonlinear fingering patterns. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B30.00013: A Numerical Study of Shock-Bubble Multiple Interaction Lingling Wu, Xiaolin Li This paper studies the numerical solution of shock-bubble multiple interactions through reflecting walls. Front tracking method is applied to track the dynamic motion of the interface and FFT method is used to analyze the enstrophy changes during the process. Our results suggest that the enstrophy is a monotonically increasing function of the Mach number and the bubble radius when the density ratio of the two fluids inside and outside the bubble is fixed. Moreover from light-in/heavy-out to heavy-in/light-out, enstrophy is a monotonic function of the Atwood number. The analysis of these different cases provides a quantitative understanding about the vorticity generation in the turbulent mixing as a result of the Richtmyer-Meshkov instability induced by shock-contact interaction. We have also compared numerical solutions with and without tracking of the contact surface. The comparison shows that the untracked solution suffered substantial loss of enstrophy due to numerical diffusion. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B30.00014: Non-Brownian microrheology of a fluid-gel interface Erik K. Hobbie, Sheng Lin-Gibson, Satish Kumar We use stroboscopic video microscopy to study the stability of a planar fluid-gel interface under simple steady shear flow. External mechanical noise plays a role analogous to temperature, with periodic fluctuations associated with the repeated build-up and release of stress. We relate the high frequency motion of the interface to the rheological properties of the underlying gel, pointing toward potential applications in the area of non-Brownian optical microrheology. At low frequency, the data suggest a breakdown of linear response, which we interpret as the emergence of an instability that is intrinsic to the driven interface. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B30.00015: Spin dewetting of wetting and partially wetting fluids Shomeek Mukhopadhyay, Robert Behringer One of the classical results of fluid dynamics is the free-surface flow of a viscous liquid in a vertically rotating cylinder, where the free surface becomes a paraboloid. This solution neglects both viscosity and surface tension, and makes the unphysical prediction that the fluid height can become negative beyond a certain critical angular velocity (for a given fluid height). We perform experiments with completely wetting PDMS oil on silcon wafer, where beyond the critical angular velocity, the central region never dewets, but goes to a nominally flat state over long times. The dynamics of the transition to this final state depends on the angular speed and the initial radius of the dewetting region. There is a marked difference in the spin-up and spin-down dynamics. When the completely wetting liquid is replaced by a partially wetting liquid a dry central spot opens up, occasionally leaving a droplet trail. In both cases the contact line does not develop any azimuthal instabilities. Collaboration with Tom Witelski and Mihaela Froehlich. [Preview Abstract] |
Session B31: Cold Fusion II
Sponsoring Units: DCMPChair: Edmund Storms
Room: Colorado Convention Center 401
Monday, March 5, 2007 11:15AM - 11:27AM |
B31.00001: Future Power Production by LENR with Thin-Film Electrodes George H. Miley, Heinz Hora, Andrei Lipson, Nie Luo, P. Joshi. Shrestha PdD cluster reaction theory was recently proposed to explain a wide range of Low energy Nuclear Reaction (LENR) experiments\footnote{G. H. Miley, H. Hora, et al., 233rd Amer Chem Soc Meeting, Chicago, IL, March 25-29, 2007.} If understood and optimized, cluster reactions could lead to a revolutionary new power source of nuclear energy. The route is two-fold. First, the excess heat must be obtained reproducibly and over extended run times. Second, the percentage of excess must be significantly (order of magnitude or more) higher than the 20-50\% typically today. The thin film methods described here have proven to be quite reproducible, e.g. providing excess heat of 20-30\% in nine consecutive runs of several weeks each. However, mechanical separation of the films occurs over long runs due to the severe mechanical stresses created.. Techniques to overcome these problems are possible using graded bonding techniques similar to that used in high temperature solid oxide fuel cells. Thus the remaining key issue is to increase the excess heat. The cluster model provides import insight into this. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B31.00002: Engineering of Condensed Matter Nuclear Physics: Heterodyne Behavior in Condensed Matter Nuclear Systems Mitchell R. Swartz Previously, we reported methods\footnote{M.R. Swartz, Bull. of the APS, 50, \#1, part 2, 1203 (2005).}$^,$\footnote{M.R.Swartz, Proc ICCF10, 29-44; 45-54, and 213-226 (2006).} to semi-quantitatively measure and control tardive thermal power (TTP) which develops long after the termination of electric input power in condensed matter high-deuteron-flux Phusor devices providing (Pt/D2O/Pd; ~0.5 cm$^3$) peak excess power ratios circa 2.30+/-.84 $^2$$^,$$^3$. Now we report one method to improve excess energy using heterodyned CMN systems using both normal and TTD operation - heterodyne operation (that is, ` $hetero$ ' for other, and ` $dyne$ ' for power). By augmenting the conventional excess energy produced by CMN active systems (normal operation) with the additional energy (``other power'') resulting from the time integral of TTP (``heat after death''), the net time-integrated excess energy (output energy beyond that applied as the input energy) is greater than we have previously reported$^2$ and may be maximized using TTD drive techniques$^1$. Initial experiments of heterodyned active samples, capable of excess heat operation at the optimal operating point, have yielded excess energy increases of up to four times beyond that obtained without heterodyned operation. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B31.00003: Maruhn-Greiner Maximum for Confirmation of Low Energy Nuclear Reactions (LENR) via a Compound Nucleus with Double Magic Numbers Heinrich Hora, George Miley One of the most convincing facts about LENR due to deuterons (ds) or protons of very high concentration in host metals of palladium is the measurement of the large scale minimum in the reaction probability with product elements centered around the nucleon number A = 153. The local maximum was measured\footnote{J. Maruhn et al, Phys. Rev. Letters 32, 548 (1974)} in this region is similar to fission of uranium at A = 119 where the local maximum follows the Maruhn-Greiner mechanism$^1$. We suggest this phenomenon can be explained by the strong screening of the Maxwellian ds on the degenerate rigid electron background within the swimming electrons at the metal surface or thin filem interfaces. The deuterons behave like neutrals at distances of above 2 picometers (pm) and form clusters due to soft attraction in the range of thermal energy; 10 pm diameter clusters can react over long time scales (10$^6$ s) with Pd leading to double magic number compound nuclei 306x126 \footnote{] H. Hora, G.H. Miley, CzechJ. Phys. 48, 1111 (1998)} decaying via fission to an A=153 element distribution. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B31.00004: The Science of Low Energy Nuclear Reactions Edmund Storms The large literature describing the anomalous behavior attributed to cold fusion or low energy nuclear reactions has been critically described in a recently published book\footnote{Edmund K Storms, The Science of Low Energy Nuclear Reactions, in press (2006). Also see: http://www.lenr-canr.org/StudentsGuide.htm }. Over 950 publications are evaluated allowing the phenomenon to be understood. A new class of nuclear reactions has been discovered that are able to generate practical energy without significant radiation or radioactivity. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B31.00005: Time Resolved, High Resolution Gamma Ray and Integrated Charged and Knock-on Particle Measurements of a Pd:D Co-deposition Cell Lawrence P.G. Forsley, Gary Phillips, Jay Khim, Pamela Mosier-Boss, Frank Gordon, Stanislaw Szpak Time resolved, with a 10 second interval, high resolution gamma ray measurements using a high efficiency cryogenically cooled gamma ray detector have been taken simultaneously with a CR-39 integrating charged particle detector on a series of experiments in conjunction with the Navy SPAWAR Pd:D co-deposition cell. These results include anomalous, coincident, gamma ray emissions from witness materials in the cell in conjunction with the CR-39 data. There is evidence of a variety of knock-on particles as well. The copious data, exceeding 10,000 tracks/mm2, offers a means to distinguish among various condensed matter nuclear science theories. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B31.00006: Two-level systems and a low-energy oscillator: Excitation transfer and energy exchange Peter Hagelstein, Irfan Chaudhary We first consider one set of matched two-level systems that are coupled to an oscillator with an energy much lower than that of the two-level systems. We show that energy can be exchanged between the two systems coherently, illustrating the effect both with the results of a direct numerical calculation, and also with an analytic result. We then show that excitation can be transferred between two sets of two-level systems that are coupled indirectly through a low-energy oscillator. We illustrate the effect with a direct numerical calculation, and also with an analytic result. Finally, both of these effects are significantly enhanced when energetic loss channels are open to the oscillator. This is illustrated with numerical and analytic calculations. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B31.00007: Material Science Developments Enhancing Excess of Power Reproducibility V. Violante, M. Bertolotti, E. Castagna$^1$, M. McKubre, F. Sarto, C. Sibilia$^1$, F. Tanzella$^2$, T. Zilov Material science research activities have been carried out in order to increase the reproducibility of the excess of power production during electrochemical loading of palladium with deuterium. In the past a wide work was developed to obtain a metallurgical structure of the palladium able to ensure a significant loading of deuterium above the threshold of 0.95 (D/Pd atomic fraction). It was observed that the high loading of the Pd cathode with deuterium was a necessary condition to have the occurrence of the excess of power production. The more recent work was mainly oriented to optimize the material properties in order to have a significant improvement of the reproducibility of the excess of power. During the last campaign of experiments more than 50\% of the experienced cathodes produced excess of heat ranging from 30\% up to more than 100\% of the input. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B31.00008: Quantization of Differences Between Atomic and Nuclear Rest Masses and Self-organization of Atoms and Nuclei F.A. Gareev, I.E. Zhidkova We come to the conclusion that all atomic models based on either the Newton equation and the Kepler laws, or the Maxwell equations, or the Schrodinger and Dirac equations are in reasonable agreement with experimental data. We can only suspect that these equations are grounded on the same fundamental principle(s) which is (are) not known or these equations can be transformed into each other. We proposed a new mechanism of LENR: cooperative processes in the whole system – nuclei + atoms + condensed matter - nuclear reactions in plasma - can occur at smaller threshold energies than the corresponding ones on free constituents. We were able to quantize\footnote{F.A. Gareev, I.E. Zhidkova, E-print arXiv Nucl-th/ 0610002 2006.} phenomenologically the first time the differences between atomic and nuclear rest masses by the formula: $\delta$$\delta$M =n$_1$/n$_2$ X 0.0076294 (in MeV/ c$^2$), n$_i$=1,2,3,.... Note that this quantization rule is justified for atoms and nuclei with different A, N and Z and the nuclei and atoms represent a coherent synchronized systems - a complex of coupled oscillators (resonators). The cooperative resonance synchronization mechanisms can explain how electron volt (atomic-) scale processes can induce and control nuclear MeV (nuclear-) scale processes and reactions., [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B31.00009: Anomalous Nuclear Phenomena Associated with Ultrafast Processes Xingliu Jiang, Xiaoping Zhou, Lijun Han , Liyin Wang Localized nuclear reactions on the tips of the surface of electrodes in electrolysis cells have been observed by using solid detectors CR-39 and autoradiography in our laboratory at the period of May, 1989. A physical model\footnote{Jiang Xingliu, Lei Jinzhi, Torsion field and tapping the zero-point energy in an electrochemical system, J. of New Energy, 4(2), 93(1999).} of transient vortex dynamics with torsion coherence with the zero point energy has been proposed by Xingliu Jiang based on the ultrafast processes of tripple phases area of tip effect on the electrode surface. Considering the large equivelent capacitance of electrochemical double layer, it is presumed that the double layer can exhibit nonlinear electrical response with spatial and temporal variations confined to micreoscopic areas by tip effect. Recent work\footnote{B. Naranjo, J.K. Gimzewski \& S. Putterman, Observation of nuclear fusion driven by a pyroelectric crystal, Nature, 434, 1115(2005).} reveals that nuclear reactions which usually occur at the field of high energy states, could be created in the systems of far from equilibrium with nonlinear beharvior at room tempurature.Our current understanging of science is like a puzzle with a large missing piece-zero point energy. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B31.00010: Search for Charged Particle Tracks Using CR-39 Detectors to Replicate the SPAWAR Pd/D External Field Co-Deposition Protocol Winthrop Williams A solution of 0.031 M PdCl2 and 0.30 M LiCl in D2O was electrolyzed between Pt anode and Ag cathode wires at currents ranging from 100 microamps to 100 milliamps in two similar series-connected plastic butyrate cells. Pd and D were co- deposited onto the Ag cathodes. CR-39 detectors adjacent to the Ag cathode wires were used to search for charged particle tracks in each cell. An external magnetic field was applied to one of the two cells.\\ \\ Throughout the experiment, ambient temperature, current through and voltage across each cell were monitored. Current was applied in a stepped fashion, starting at 0.1mA increasing by factors of 2 to 5 up to 100mA. [Preview Abstract] |
Session B32: Strong Field and Ultrafast Physics
Sponsoring Units: DAMOPChair: Marc Vrakking, Institute for Atomic and Molecular Physics
Room: Colorado Convention Center 402
Monday, March 5, 2007 11:15AM - 11:27AM |
B32.00001: Spatial Dependence of High Harmonic Generation in Hydrogen Atom Seth Ross, G.P. Zhang We used the hydrogen atom as a model and computed the continuum mavefunction and the transition matrix elements. The total quantum number used is 200 and the number of plane waves is 100. We have done dynamical simulations to mimic the laser and electron interaction. Fianlly we compute the power spectrum by Fourier transformation of the dipole matrix. This gives us the opportunity to see the spatial dependence of high harmonic generations in the hydrogen atom. References: H. Niikura {\it et al}, Nature {\bf 417}, 917 (2002); {\bf 421}, 826 (2003); G. P. Zhang, Phys. Rev. Lett. {\bf 95}, 047401 (2005); G. P. Zhang and T. F. George, Phys. Rev. A {\bf 74}, 023811 (2006) [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B32.00002: Demonstration of a high brightness injection-seeded soft x-ray laser amplifier using a dense plasma Eduardo Granados, Yong Wang, Miguel A. Larotonda, Mark Berrill, Brad M. Luther, Dinesh Patel, Carmen S. Menoni, Jorge J. Rocca There is a great interest in the generation of high brightness beams of soft x-ray light. We have conducted a table-top experiment in which we have demonstrated the generation of an intense soft x-ray laser beam by saturated amplification of high harmonic seed pulses in a dense transient collisional soft x-ray laser plasma amplifier created by heating a solid titanium target. Amplification of the seed pulses in the 32.6 nm line of Ne-like Ti generates laser pulses of sub-picosecond duration that are measured to approach full spatial coherence. The peak spectral brightness is estimated to be $\sim $ 2$\times $10$^{26}$ photons/( s mm$^{2}$ mrad$^{2}$ 0.01{\%} bandwidth). The scheme is scalable to produce extremely bright lasers at very short wavelength with full temporal and spatial coherence for applications. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B32.00003: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 11:51AM - 12:03PM |
B32.00004: Quasi Phase Matching and Quantum Path Control of High Harmonic Generation using Counterpropagating Light Xiaoshi Zhang, Amy Lytle, Oren Cohen, Henry Kapteyn, Margaret Murnane We demonstrate the first use of a 3-pulse train of counterpropagating pulses to enhance the coherent upconversion of an intense ultrashort laser pulse into the extreme ultraviolet region of the spectrum. This all-optical quasi-phase-matching technique uses interfering beams to scramble the quantum phase of the generated high-order harmonics, to suppress emission from out-of-phase regions. A wavelength selective enhancement in the flux of up to $\approx $ 300x is observed at photon energies around 70 eV in Argon, that cannot otherwise be phase matched. We also show that further very large enhancements are possible, presenting a real prospect for orders-of-magnitude improvement in coherent upconversion of lasers into the soft x-ray region of the spectrum. Finally we show that by adjusting the intensity of the counterpropagating light, we can selectively enhance different electron quantum path trajectories, demonstrating attosecond time-scale coherent control of the radiating electron wavefunction. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B32.00005: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 12:15PM - 12:27PM |
B32.00006: Controlling rotational revivals in asymmetric tops Vinod Kumarappan, Lotte Holmegaard, Simon Viftrup, Christer Bisgaard, Henrik Stapelfeldt, Edward Hamilton, Tamar Seideman We use improved experimental and theoretical tools to demonstrate a novel method for controlling the revival structure of strong-field alignment of asymmetric top molecules. Experimentally, iodobenzene molecules (which is a near-prolate top) are cooled to 1 K using an Even-Lavie supersonic valve and non-adiabatically aligned using 800 nm pulses of durations ranging from 200 fs to 2 ps. The alignment is probed by velocity map imaging of I$^{+}$ fragments produced by Coulomb explosion of the molecules using a 25 fs pulse focused tightly to restrict the volume probed. We show that as the fluence of aligning pulse is increased, the revival structure is simplified to a nearly period pattern reminiscent of symmetric tops. Theoretically, non-perturbative solution of the Schrodinger equation demonstrates the generality of the effect, and emphasizes the importance of this new control scheme for the alignment and revival dynamics of asymmetric tops. Classically, the simplified motion at high fluence corresponds to stable rotations about the slowest principal axis (the C-axis) of the molecule. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B32.00007: Heterodyne control of attosecond pulse generation Thomas Pfeifer, Lukas Gallmann, Mark J. Abel, Phillip M. Nagel, Aurelie Jullien, Daniel M. Neumark, Stephen R. Leone Adding a weak laser field at a different color to the fundamental in high-order harmonic generation results in a new type of heterodyne mixing in the kinetic energy term of the active electron. Analytical calculations and quantum simulations show that the effect of the weak field is amplified by the strong fundamental laser field that acts as the local oscillator [1]. The photon energy of different attosecond pulses within the produced pulse trains can thus be significantly modified. Two important applications for this phenomenon are the generation of isolated attosecond pulses with multi-cycle driving fields and the shaping of attosecond pulse trains. \newline \newline Ref.: [1] T. Pfeifer et al., Phys. Rev. Lett. 97, 163901 (2006) [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B32.00008: Measurement of higher-order moments of a rotational wave packet dynamics and alignment Klaus Hartinger, Randy Bartels Field free molecular alignment, attributed to the revivals of a rotational wave packet, has been an area of very active research recently, with numerable potential applications$^1$. While there is very rich structure and temporal dynamics in the quantum rotational wave packet, so far, measurements have been restricted to just the first moment of the wave packet, i.e., a measurement of $\langle\langle\cos^2\theta\rangle\rangle$. This measure probes the transient alignment of the molecules, but does not reveal the complete dynamics of the quantum wave packet. A measurement of the rotational wave packet dynamics with a linear optical technique depends only on $\langle\langle\cos^2\theta\rangle\rangle$ and does not provide information on higher order moments of the alignment. Third-order nonlinear interactions provide information on the $\langle\langle\cos^4\theta\rangle\rangle$ moment and provide additional information about rotational wave packet dynamics. We present third harmonic generation experiments measuring the transient THG susceptibility, which includes the $\langle\langle\cos^4\theta\rangle\rangle$ dynamics.\\ $^1$T. Seideman, Adv. in At., Mol. and Opt. Phys., 52 (2006) [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B32.00009: In-situ probe of ionization and coherent buildup for high-order harmonic generation in hollow waveguides using counterpropagating light Amy Lytle, Xiaoshi Zhang, Margaret Murnane, Henry Kapteyn, Oren Cohen We use counterpropagating light to directly observe, in-situ, the coherent buildup of high harmonic generation in a hollow waveguide. We measure, for the first time, the phase mismatch, (i.e. coherence lengths) for high photon energies that cannot be phase matched using conventional approaches. We also probe the transition through phase matching, the ionization level at which different harmonic orders are generated, and the change in the coherence length as the intensity of the guided mode evolves along the fiber. These results demonstrate that the hollow waveguide geometry possesses exceptional coherence and a ``quasi-one-dimensional'' plane wave geometry, in analogy to conventional fiber optics in the visible. This in-situ information also directly prescribes the optimal structures or pulsetrains required for implementing quasi phase matching. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B32.00010: High harmonic generation from ions in a capillary discharge Tenio Popmintchev, David M. Gaudiosi, Oren Cohen, Margaret M. Murnane, Henry C. Kapteyn, Michael Grisham, Brendan Reagan, Mark Berrill, Jorge J. Rocca, Barry C. Walker We demonstrated a significant extension of the high harmonic spectra from noble gases by generating harmonics from ions in a capillary discharge plasma. The discharge plasma eliminates ionization-induced defocusing and ionization loss, allowing photon energies of 160 eV, 170 eV and 275 eV to be generated from xenon, krypton and argon ions, respectively. In addition to extending the spectra, harmonic generation in a capillary discharge results in an enhancement of the flux of up to two orders of magnitude near the harmonic cutoff observed in a hollow waveguide. The use of a capillary discharge plasma as a new medium for high harmonic generation shows great promise for extending efficient harmonic generation to shorter wavelengths. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B32.00011: Computational Study of Orientation-dependent Molecular High Harmonic Spectra Anthony Dutoi, Tamar Seideman Recently, there has been much interest in high harmonic generation (HHG) by aligned molecules [{\it Phys. Rev. A} {\bf 67} 023819, {\it Nature} {\bf 432} 867, {\it Nature} {\bf 435} 470]. During HHG, an electron is ionized and driven back to the cation by a strong, low-frequency field, and radiation is emitted at harmonics of this driving pulse. Because this process is sensitive to the orientation of a molecule, rotational dynamics can be probed on very short time scales. We are working to predict the time-dependent HHG spectra for aligned rotational wavepackets. In conjunction with experiment, these simulations should be valuable for studying the loss of rotational coherence in media such as dense gases. Within the presented formalism, Born-Oppenheimer rotational dynamics are handled exactly, while HHG at any given orientation is estimated by numerical time integration of a one-electron Schr\"{o}dinger equation. Propagation outside of the integration grid can be handled using an analytical Volkov propagator at the expense of ignoring the cation field at this distance. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B32.00012: Effect of nuclear motion on the absorption spectrum of dipicolinic acid Petra Sauer, Yuri Rostovtsev, Roland Allen A current scientific challenge is the rapid detection of chemical and biological substances, including bacterial spores. A significant component of spores is the molecule dipicolinic acid (DPA or 2,6-pyridinedicarboxylic acid) and its salts. A variety of spectroscopic detection schemes are being explored, including fluorescence spectroscopy, ultraviolet and visible resonant Raman spectroscopy, and FAST CARS. Using semiclassical electron-radiation-ion dynamics (SERID), we have examined the effect of nuclear motion, resulting from both finite temperature and the response to a radiation field, on the line broadening of the excitation profile of DPA. With nuclei fixed, we find a relatively small broadening associated with the finite time duration of an applied laser pulse. When the nuclei are allowed to move, the excitation spectrum exhibits a much larger broadening, and is also reduced in height and shifted toward lower frequencies. In both cases, the excitation is due to well-defined $\pi$ to $\pi*$ transitions. The further inclusion of thermal motion at room temperature broadens the linewidth considerably because of variations in the molecular geometry: Transitions that had zero or negligible transition probabilities in the ground state geometry are weakly excited at room temperature. [Preview Abstract] |
Session B33: Focus Session: Quantum Foundations I
Sponsoring Units: GQIChair: Carlton Caves, University of New Mexico
Room: Colorado Convention Center 403
Monday, March 5, 2007 11:15AM - 11:51AM |
B33.00001: Almost quantum theory: classical theories with a constraint on knowledge Invited Speaker: What kind of theory would be appropriate for an agent living in a world that is essentially classical but where there is a fundamental restriction on how much knowledge can be acquired about the physical state of any system? Formalizing such a restriction, one can define several toy theories that are found to have a rich structure similar to that of quantum theory, including a notion of coherent superposition and entanglement. These theories are also found to have analogues of a wide variety of quantum phenomena, such as complementarity, interference, teleportation, no-cloning, and many quantum cryptographic and communication protocols. The diversity and quality of these analogies provides compelling evidence for the view that quantum states are not states of reality -- as most interpretations suggest -- but rather states of knowledge that are incomplete (and cannot be completed). The question ``what is the nature of the reality to which this knowledge refers?" remains open in this research program but the phenomenon of contextuality (a consequence of the Bell-Kochen-Specker theorem) provides, I argue, our best clue for how to answer it. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B33.00002: Cloning, Broadcasting and the de Finetti theorem in Generalized Probablistic Theories Matthew Leifer, Howard Barnum, Jonathan Barrett, Alexander Wilce We give a lightning overview of a framework for generalized probablistic theories, proposed by Barrett, that includes classical probability and quantum theory as special cases. The framework also includes theories that support ``superquantum'' correlations, which violate Bell inequalities to a larger extent than quantum theory whilst still not allowing signalling. In recent years, many similarities between quantum entanglement/nonlocality and ``superquantum'' correlations have been found by researchers studying quantum information and foundations. These can be seen to emerge from the common structure of all theories in Barrett's framework. In particular, some results from quantum information that can be generalized to all theories in the framework are described, including versions of the no-cloning theorem, the no-broadcasting theorem and the de Finetti theorem. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B33.00003: Quantum Mechanics on Phase Space and Informational Completeness Franklin E. Schroeck, Jr. The formalism of quantum mechanics on phase space is reviewed and the informational completeness of it is discussed. Applications to quantum computing will be mentioned. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B33.00004: Quasi-Orthonormal Bases for the Space of Density Operators Christopher A. Fuchs, D. M. Appleby, Hoan B. Dang Recently there has been much interest in the quantum information community to prove (or find a counterexample to) the existence of so-called symmetric informationally complete measurements (SICs). In this talk we show that there should be even more interest. For, under a robust measure of orthonormality for operator bases (one that does not build in any symmetry at the outset), one can show that SICs, if they exist, come as close as possible to being orthonormal bases for the space of density operators. Moreover, in contrast to the usual expression of the superposition principle (where bases are taken to be orthogonal sets of state vectors), writing a superposition principle in terms of SICs leads to a more intrinsically-quantum representation for quantum states. This is because the basis states, rather than being the easiest to eavesdrop on, are actually the hardest. Moreover, they fulfill a few other extreme quantum properties which will be outlined. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B33.00005: Macrorealism Emerging from Quantum Physics Caslav Brukner, Johannes Kofler I will give a novel theoretical approach to macroscopic realism and classical physics within quantum theory. While conceptually different from the decoherence program, it is not at variance with it. It puts the stress on the required precision of our measurement apparatuses such that quantum effects can still be observed. In the first part of the talk I will show that for unrestricted measurement accuracy a violation of macrorealism (i.e., a violation of the Leggett-Garg inequalities) is possible for arbitrary large systems. In the second part, I will show that, given the restriction of coarse-grained measurement resolution, not only macrorealism becomes valid but even the classical Newtonian laws emerge out of the quantum laws. Thus, even if an object were sufficiently isolated from its environment to avoid decoherence and fully obeys the laws of quantum physics, it will appear to behave classically under coarse-grained measurements. In the final part of the talk I will argue that since larger and larger systems require better and better measurement precision to see quantum effects -- and infinite precision cannot be reached in a world with finite resources --, there could be a fundamental limit on the dimensionality of the object above which its quantum features cannot be observed. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B33.00006: Critical sets of rays in four dimensions proving the Bell-Kochen-Specker theorem P.K. Aravind In recent years several sets of rays have been discovered in four and more dimensions that provide illustrations of the Bell-Kochen-Specker (BKS) theorem. These demonstrations acquire additional significance through the fact that they provide proofs of Bell's nonlocality theorem as well if used in conjunction with the right kind of entanglement. This talk will provide a brief overview of this field and then concentrate on two 60-ray sets in four dimensions recently discovered by the author. Both sets embed a geometrical structure within them, known as Reye's configuration, that permits the identification of smaller subsets (``critical sets'') that provide noncoloring proofs of the BKS theorem. (Reye's configuration is a set of 12 points and 16 lines with the property that four lines pass through every point and three points lie on every line. A ``critical set'' of rays is one that provides a minimalist noncoloring proof of the BKS theorem in the sense that the deletion of even a single ray from it makes the proof fail). This talk will discuss the ``quantum geometry'' of the two 60-ray sets and their relationship to each other. The application of these results to quantum information processing will be briefly considered. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B33.00007: Probing Contextuality with Pre- and Post-selection Jeff Tollaksen By analyzing the concept of contextuality (Bell-Kochen-Specker) in terms of pre-and-post-selection (PPS), it is possible to assign definite values to observables in a new way. Physical reasons are presented for restrictions on these assignments. When measurements are performed which do not disturb the pre- and post-selection (i.e. weak measurements), then novel {\it experimental} aspects of contextuality can be demonstrated including a proof that every PPS-paradox with definite predictions implies contextuality. Certain results of these measurements (eccentric weak values with e.g. negative values outside the spectrum), however, cannot be explained by a ``classical-like'' hidden variable theory. Surprising theoretical implications are discussed. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B33.00008: On the logical structure of Bell theorems. Jonathan Walgate, Anne Broadbent, Hilary Carteret, Andre Methot Some specific predictions of quantum mechanics are inconsistent with local realism, a phenomenon known as nonlocality. Despite overwhelming evidence for quantum mechanics, the practical difficulties of detector efficiency and coordinating space-like separated measurements have provided loopholes for a classical worldview. New experiments have been proposed to meet these challenges, based around a new kind of nonlocality proof called an ``EPR Bell inequality''. Much investment is now being made to realize these proofs experimentally. We show all these proposals are fundamentally flawed. We focus on a series of designs that have appeared in \textit{PRL} and \textit{PRA} for loophole-free Bell experiments. These experiments use hyperentangled two-photon quantum states to generate experimental data supposedly at odds with local realism. We show how to produce identical results using a single coin, and explain where the logical flaw can be found. To understand our nonclassical world, we must understand precisely the experimental evidence for nonlocality. This tempting shortcut is a logical, theoretical and experimental dead end. http://www.arxiv.org/abs/quant-ph/0512201. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B33.00009: Tests of local realism and the coincidence-time problem Jan-{\AA}ke Larsson This paper analyzes effects of time-dependence in the Bell inequality, and other tests of local realism. Generalized tests are derived for the case when coincidence and non-coincidence [and hence whether or not a pair, or triplet, contributes to the actual data] is controlled by timing that depends on the detector settings. Needless to say, these tests are violated by quantum mechanics and could be violated by experimental data provided that the loss of measurement data through failure of coincidence is small enough, but the quantitative bound is more restrictive in this case than in the previously analyzed ``efficiency loophole.'' [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B33.00010: Entanglement and correlations in mixed-state quantum computation Animesh Datta, Steven Flammia, Carlton Caves, Guifre Vidal A very intriguing model of mixed-state quantum computation is the `power of one qubit' [E. Knill and R. Laflamme, Phys. Rev. Lett 81, 5672 (1998)], which has one pure qubit and n qubits in the completely mixed state. This model is known to evaluate the normalized trace of a unitary matrix with fixed accuracy efficiently, and offers an exponential speed-up over the best known classical algorithm. We show that this model involves entangled states. We also show that, on one hand, these states have no more than a constant amount of entanglement (as measured by the negativity), while on the other, they have an exponentially high operator Schmidt rank. Since quantum systems with limited Schmidt rank are known to be simulatable classically in an efficient manner, this suggests that the advantage of mixed-state quantum computation may stem not from the amount of entanglement but the degree of correlations (as quantified by the operator Schmidt rank) the system possesses. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B33.00011: Multiply constrained bounds on measures of entanglement Anil Shaji We place bounds on non-operational measures of entanglement using multiple operational measures as constraints. Non-operational measures like the entanglement of formation, tangle and concurrence are physically significant, but they do not admit efficient procedures for computing because computing them involves finding optimal pure state decompositions for mixed states. On the other hand, there are operational measures of entanglement that can be computed relatively easily for arbitrary states. Bounding non-operational measures using a single operational measure as constraint has previously been done. We generalize this method to more than one constraint. We work out examples in which bounds are obtained for the entanglement of formation, tangle and concurrence of a family of states using the operational entanglement measures constructed from two positive, but not completely positive maps as constraints. The two maps are the partial transpose map and the $\Phi$-map introduced by Breuer [H-P. Breuer, e-print, quant-ph/0605036]. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B33.00012: Graphical description of the action of Clifford operators on stabilizer states Matthew Elliott, Bryan Eastin, Carlton Caves We introduce a graphical representation of stabilizer states, which reduces to standard graphs for graph states. The effects of Clifford operators on stabilizer states are then translated into graph operations on the corresponding stabilizer state graphs, and we find that they are completely described in terms of loop complementation and local complementation. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B33.00013: Entanglement and the second law Ian Durham The second law of thermodynamics is, in reality, a strong argument about the nature of probabilities. In essence, the same can be said of Bell's inequalities, of which thermodynamic variations have been found. The tantalizingly close nature of these two arguments has potentially profound implications for quantum theory and, in particular, practical quantum computing. This presentation explores this relationship and its potential implications. [Preview Abstract] |
Session B38: Focus Session: Bioinstrumentation and Biophotonic Technologies
Sponsoring Units: GIMS DBPChair: Andreas Mandelis, University of Toronto
Room: Colorado Convention Center 501
Monday, March 5, 2007 11:15AM - 11:51AM |
B38.00001: Fourier-Domain Biophotoacoustic Sub-surface Depth Selective Amplitude and Phase Imaging of Turbid Phantoms and Biological Tissue Invited Speaker: A novel photothermoacoustic imaging modality utilizing a frequency-swept (chirped) intensity-modulated laser source and coherent frequency domain signal processing (``biophotoacoustics'') was introduced for non-invasive imaging of biological tissues. The developed frequency-domain imaging system takes advantage of linear frequency modulation waveforms to relate depth of tissue chromophores to the frequency spectrum of the detected acoustic response and of a narrow signal detection bandwidth to improve signal-to-noise ratio (SNR). Application of frequency-domain photothermoacoustic (FD-PTA) imaging was demonstrated using turbid phantoms and ex-vivo specimens of chicken breast with embedded absorbing inclusions simulating tumors. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B38.00002: An application of fast response Polarized Light Microscopy Deependra Kantha, David Van Winkle A fast response polarized light microscope was designed based on the algorithm by Shribak et. al (Applied Optics, vol. 42, 3009-3017). A pulsed laser beam was passed through two Pockels cells aligned at different angles with respect to optical axis. The retardance of the Pockels cell was controlled by external switches and power supplies. The electronics circuit in the system allows change of the retardance of the Pockels cell each millisecond for four milliseconds. In four milliseconds, four images of a birefringent sample, formed by different states of polarized light are recorded. The images are added appropriately to calculate retardence amplitude and phase by using codes written in imageJ software. The microscope was used to show the retardance and phase of a rabbit muscle fiber. Recordings were also taken of the contraction of Vorticella convallaria but the changes were too fast to yield retardance images. This type of microscope can be used to study different kinds of biological functions that change on a timescale slower than four milliseconds but faster than two seconds. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B38.00003: Evaluation of optical excitation conditions for ruthenium complex for biosensor optodes Sean Pieper, Zhong Zhong, Kevin L. Lear, Ken Reardon Development of a fiber optic biosensor incorporating genetically engineered enzymes which catalyze chlorinated ethenes in an oxygen-consuming reaction for in situ monitoring of groundwater contaminants motivates optimization of optode excitation conditions. These conditions affect the sensitivity, signal-to-noise, and optode service life impacting the quality of the overall biosensor. Optodes are generally comprised of a fluorophore conjugated with a polymer as a substrate cross linked at the distal end of a fiber optic. We investigate the excitation conditions of tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II) chloride (Ru(dpp)3) conjugated with poly(vinyl alcohol) (PVOH) as an optode. A reported advantage of Ru(dpp)3 is that it has no emission spectral shift occurring under varying chemical and environmental conditions. Photostability degradation due to photobleaching of Ru(dpp)3 with PVOH as a substrate is explored by varying the optical irradiance of the fluorophore containing optode. Other issues relating to practical implementation of Ru(dpp)3 as oxygen sensitive biosensors will be discussed. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B38.00004: Two-Photon Microscope with Spectral Resolution Russell Fung, Mike Melnichuk, Anurag Chaturvedi, Devin Gillman, Valerica Raicu Two-photon microscopy has many distinct advantages over other types of microscopy: it is faster, there is no out-of-plane photobleaching, and using near-infrared laser light (to produce visible fluorescence signal) allows deeper penetration into thick samples. We have built a two-photon microscope based on a novel design that uses a diffractive optic, a nondescanned detection scheme and an EM-CCD camera to produce spectrally resolved fluorescence images of samples after only one full scan of the sample and with relatively high speed. Our design is readily extended to incorporate control in the excitation channel through pulse shaping using spatial filtering in the frequency domain. This microscope, in conjunction with Fluorescence Resonance Energy Transfer (FRET) between fluorescent tags, has been used to detect interactions between proteins in various systems including yeast (Saccharomyces cerevisiae) cells. Also, its exquisite sensitivity makes it suitable to spectrally resolve signals from single quantum dots and single molecules. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B38.00005: Exploration of detection sensitivity of biomarker acetone in aqueous samples using cavity ringdown spectroscopy Armstrong Mbi, Chuji Wang Breath acetone is a biomarker for diabetes (Type 1). Currently, high sensitivity breath gas analysis is mainly performed by gas chromatography-mass spectrometry (GC-MC). We are developing a potable ringdown spectrometer for diabetes diagnostics using non-invasive breath gas analysis. The ringdown spectrometer consists of a compact Nd: YAG laser source operating at 266 nm, a atmospheric gas cell of 43 cm in length, a miniature detector, and a data processing section. In this work, the exploration of detection sensitivity of acetone in aqueous samples using cavity ringdown spectroscopy is presented. Pure acetone is diluted in distilled water in different concentrations ranging from 0.5 drop/liter to 8 drops/liter, or 730 ppbv - 12 ppmv in gas phase. The instrument performance using two sampling methods is evaluated. With the mirror reflectivity of 99.98{\%}, the spectrometer demonstrates a detection limit of acetone of 450 ppbv (based on 1-$\sigma )$, which is slightly lower than the threshold number of acetone concentration in normal human breath. Preliminary results from actual breath gases are also presented. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B38.00006: Optofluidic intracavity spectroscopy of single cells in a passive Fabry-Perot resonator Hua Shao, Weina Wang, Susan Lana, Kevin Lear Considerable effort has been devoted to analyzing complex biological systems such as living cells by combining photonic and microfluidic techniques. Cells in biocavity lasers developed by Gourley et al produced rich multimode spectra that multivariate analysis correlated with the cell type. Optofluidic intracavity spectroscopy (OFIS) reported here operates on a similar principles but does not require gain media. It measures transmission spectra of individual cells in a passive Fabry-Perot (FP) cavity. Non-normal incidence identified the relative order of the various transverse modes to verify the applicability of different simplified models of the cavity modes. Distinctive spectral features, including transverse mode spacing and the number of modes were used to differentiate red and white human blood cells, for example. OFIS measurements of canine lymphoma cells produced repeatable transmission spectra. Continuing investigations on the capability of OFIS to distinguish cancer cells will be reported. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B38.00007: Magnetically Directed Cell Co-Localization Edward Felton, Daniel Reich, Yoojin An, Christopher Chen The ability to control the movement and location of biological cells has led to novel approaches to several areas of interest, from tissue engineering to the study of cell-cell interactions. We have introduced ferromagnetic nanowires as a tool for applying forces to cells; their high remanent magnetization allows cells bound to nanowires to be manipulated in low-strength magnetic fields. Micropatterned magnetic structures generate magnetic fields that can precisely guide cells into predetermined positions on substrates in culture, and cells can be restricted to localized areas through chemical functionalization of the substrate. We have used these directed cell assembly techniques to organize cells into a variety of patterns with a single cell type, and have extended its utility to include two cell types. We have created regular arrays of cells in which heterotypic cells pairs are magnetically trapped at each array site. This method of producing large numbers of isolated heterotypic cell pairs is potentially useful in studies of cell-cell interactions between different cell types. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B38.00008: Detection of cancer protein using Spectroscopic Ellipsometry as Surface Plasmon Resonance Mode Yunbog Kim, Dongryul Jeon, Min-ah Woo, Myunghaing Cho Since the first application of surface plasmon resonance (SPR) for biosensing almost two decades ago, SPR has made great strides in terms of both the instrumentation and the application. We used spectroscopic ellipsometry as an SPR sensor to detect the reaction of HER2 protein of SKBR3 cancer cells with its antibody. Since the Psi value of ellipsometry is related to the reflectivity of P wave, the surface plasmon signal can be measured using spectroscopic ellipsometry. A glass plate coated with 50 nm-thick gold film was dipped in HER2 antibody solution for 1 hour. The substrate was then dipped in a soup containing broken SKBR3 cells to induce HER2 antibody-antigen reaction. The pure gold film exhibited a SPR peak at 2.04 eV. After the adsorption of HER2 antibody, the peak shifted to 1.99eV. After dipping in the soup of SKBR3 cells, the peak shifted to 1.96 eV. We believe this shift is due to the change in surface plasmon caused by binding of HER2 protein and antibody. The AFM images of the samples supported our conclusion. Our result adds an example to the possibility of using spectroscopic ellipsometry as an SPR mode for detecting cancer cells. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B38.00009: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 1:27PM - 1:39PM |
B38.00010: In-line Phase Contrast Imaging of Soft Tissue in the Mammalian Cochlea Lixin Fan, C. Rau, I. Robinson, C.-P. Richter Soft tissue has been visualized in a mammalian cochlea with hard X-rays in-line phase contrast imaging at the UNICAT beamline 34 ID-C, APS. The sensation of hearing results from a series of complex events that transform acoustic pressure waves into the perception of sound. During the normal hearing process, sound energy is converted to mechanical energy by the middle ear, which then is converted to motions in the structures of the cochlea. To date, many aspects of the sound induced vibrations are still unclear. Firstly, mechanics of the cochlea are likely to changes by the manipulations, and secondly, cochlear micromechanics are unexplored for the cochlear middle section. Therefore, our objective is to measure the motion patterns of cochlear tissues in a closed cochlea. Thick mammalian cochlear slices have been imaged and were compared with those obtained by light microscopy. Furthermore, intact cochleae have been imaged to identify the soft tissue structures involved in the hearing process. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B38.00011: Dental Photothermal Radiometry: Theoretical Analysis. Anna Matvienko, Raymond Jeon, Andreas Mandelis, Stephen Abrams Dental enamel demineralization in its early stages is very difficult to detect with conventional x-rays or visual examination. High-resolution techniques, such as scanning electron microscopy, usually require destruction of the tooth. Photothermal Radiomety (PTR) was recently applied as a safe, non-destructive, and highly sensitive tool for the detection of early dental demineralization, artificially created on the enamel surface. The experiments showed very high sensitivity of the measured signal to incipient changes in the surface structure, emphasizing the clinical capabilities of the method. In order to analyze the biothermophotonic phenomena in a tooth sample during the photothermal excitation, a theoretical model featuring coupled diffuse-photon-density-wave and thermal-wave fields was developed. Numerical simulations identified the effects on the PTR signal of changes in optical and thermal properties of enamel and dentin as a result of demineralization. The model predictions and experimental results will be compared and discussed. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B38.00012: ABSTRACT HAS BEEN MOVED TO J21.00010 |
Monday, March 5, 2007 2:03PM - 2:15PM |
B38.00013: ABSTRACT HAS BEEN MOVED TO J21.00009 |
Session B39: Focus Session: Materials and Applications for Solar Energy I
Sponsoring Units: FIAP DMPChair: Mike McGehee, Stanford University
Room: Colorado Convention Center 502
Monday, March 5, 2007 11:15AM - 11:51AM |
B39.00001: Multiple Exciton Generation for Highly Efficient Solar Cells Invited Speaker: In order to utilize solar power for the production of electricity and fuel on a massive scale, it will be necessary to develop solar photon conversion systems that have an appropriate combination of high efficiency and low capital cost ({\$}/m$^{2})$. One new potential approach to high solar cell efficiency is to utilize the unique properties of semiconductor quantum dot nanostructures to control the relaxation dynamics of photogenerated carriers to produce either enhanced photocurrent through efficient multiple exciton generation (MEG) or enhanced photopotential through hot electron transport and transfer processes. To achieve these desirable effects it is necessary to understand and control the dynamics of electron relaxation, cooling, multiple exciton generation , transport, and interfacial electron transfer of the photogenerated carriers with fs to ns time resolution. We have been studying these fundamental dynamics in bulk and nanoscale semiconductors (quantum dots, quantum wires, and quantum wells) using femtosecond transient absorption, photoluminescence, and THz spectroscopy. This work will be summarized and recent advances in creating multiple excitons from a single photon will be discussed, including a unique model to explain efficient MEG based on the coherent superposition of multiple excitonic states. Various possible configurations for quantum dot solar cells that could produce ultra-high conversion efficiencies for the production of electricity, as well as for producing solar fuels (for example, hydrogen from water splitting), will be discussed, along with associated thermodynamic calculations that show the increase in the maximum theoretical gain in solar photon conversion efficiency for both electricity and fuel production. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B39.00002: Optical properties of II-VI structures for solar energy utilization Joshua Schrier, Denis Demchenko, Lin-Wang Wang Although II-VI semiconductor materials are abundant, stable, and have direct band gaps, the band gaps are too large for optimal photovoltaic efficiency. However, staggered band alignments of pairs of these materials, and also the formation of intermediate impurity levels in the band gap (which has been demonstrated to increase the efficiency as compared to both single-junction devices), could be utilized to improve the suitability of these materials for solar energy utilization. Previous theoretical studies of these materials are limited, due to the well-known band gap underestimation by density-functional theory. To calculate the absorption spectra, we utilize a band-corrected planewave pseudopotential approach, which gives agreements of within 0.1 eV of the bulk optical gaps values. In this talk, I will present our work on predicting the optical properties of ZnO/ZnS and ZnO/ZnTe heterostructures, nanostructures, and alloys. This work was supported by U.S. Department of Energy under Contract No.DE-AC02-05CH11231 and used the resources of the National Energy Research Scientific Computing Center. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B39.00003: Local Structures Around S in CdS:O Thin Films Photovoltaic Materials Probed by S K-edge X-ray Absorption Fine Structures Y. L. Soo, W. H. Sun, S. C. Weng, Y. S. Lin, S. L. Chang, L. Y. Jang, X. Wu, Y. Yan Local Structures around S in thin films of CdS:O have been investigated using EXAFS and NEXAFS techniques at the S K absorption edge. Our S K-edge EXAFS results clearly indicate the presence of S-O bonds that coexist with S-Cd bonds in the oxygen-containing samples. The S K-edge NEXAFS data further identify SO$_{3}$ and SO$_{4}$ complexes in the samples. As indicated by our previous results on Cd K-edge EXAFS, Cd atoms are predominantly bonded with S. These x-ray results demonstrate that the oxygen atoms actually combine with S to form SO$_{3}$ and SO$_{4}$ complexes instead of being incorporated into the CdS host. In combination with the evidence of nanoparticles revealed by TEM, our results suggest that oxygen-free CdS nanocrystals are formed in the films due to the O content. The bandgap of the samples is therefore found to increase with O concentration as opposed to the bandgap decrease for O doping expected in the band anticrossing model. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B39.00004: P-type InGaN alloys D.M. Yamaguchi, R.E. Jones, N.R. Miller, E.E. Haller, J.W. Ager, K.M. Yu, W. Walukiewicz, H. Lu, W.J. Schaff We have demonstrated via electrolyte-based capacitance-voltage (CV) measurements that a set of Mg-doped In$_{1-x}$Ga$_{x}$N thin films (x=.05,.30,.33,and .80) exhibit bulk p-type activity. There is a change in the slope of the Mott-Shockley plots of In$_{1-x}$Ga$_{x}$N with x $\le $ .33 which is consistent with p-type bulk material underneath an n-type surface inversion layer. In contrast, CV analysis of Mg-doped In$_{0.2}$Ga$_{0.8}$N indicates p-type activity throughout the film. These results are consistent with surface Fermi level pinning at --4.9 eV with respect to the vacuum level. Based on the known valence band offsets between GaN and InN, a surface inversion layer is predicted for In$_{1-x}$Ga$_{x}$N with x $\le $ .66 and a surface Schottky barrier for x $>$ .66. These results provide the first evidence of p-type doping of InGaN alloys in the whole composition range. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 1:03PM |
B39.00005: Seven Excitons per Single Photon Using Semiconductor Nanocrystals Invited Speaker: The efficient conversion of photon energy into electrical charges is a central goal of much research in physics, chemistry, and biology. A usual assumption is that absorption of a single photon by a material produces a single electron-hole pair (exciton), while the photon energy in excess of the energy gap is dissipated as heat. In 2004, we reported for the first time that nanocrystals (NCs) of PbSe could respond to absorption of a single photon by producing two or more excitons with the unity probability (Phys. Rev. Lett. 92, 186601, 2004). Our more recent findings indicate that this carrier multiplication process can generate multiple charges with quantum efficiencies that correspond to the ultimate limit dictated by energy conservation. For example, for photon energy of 7.8 energy gaps, a maximal possible number of photogenerated excitons based on energy considerations is 7, which is exactly the number measured in our experiments (Nano Lett. 6, 424, 2006). Another unexpected feature of carrier multiplication is that it results in unusual distributions of carrier populations that cannot be described by Poisson statistics. Specifically, by selecting certain photon energies, we obtain photoexcited NC ensembles with nearly pure single multiplicities (i.e., all excited NCs contain the same number of excitons) that can be tuned in the controlled way from 1 to 7 (Phys. Rev. Lett. 96, 097402, 2006). While the exact mechanism for carrier multiplication in NCs is still under debate, one factor, which likely contributes to high efficiencies of this process, is a unique property of the NCs to produce significant carrier-carrier interactions as indicated, e.g., by our previous Auger recombination studies (Science 287, 1011, 2000). This confinement-enhanced Coulomb coupling can lead to the unusual mechanism for direct photogeneration of multiexcitons via virtual single-exciton states, which can explain our observations of very short, sub-200 femtosecond buildup times of multiexciton populations in the regime of carrier multiplication (Nature Phys. 1, 189, 2005). [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B39.00006: Theoretical investigation of vacuum thermionic energy conversion devices for efficient conversion of solar to electrical energy Joshua Smith, Robert Nemanich, Griff Bilbro A vacuum thermionic energy conversion device (TEC) would offer the potential of efficiently converting solar energy directly to electrical work. These devices consist of a heated emitter electrode and a collector electrode separated by an evacuated interelectrode space. Models for such conceptual devices are developed, and efficiency is calculated by considering electron transport across the device as well as Stefan Boltzmann radiation. A device operating with an emitter and collector temperature of $775K$ and $375K$, respectively is considered. The conceptual TEC features diamond materials having low emission barrier heights as electrodes. Hydrogen terminated diamond is known to have a negative electron affinity (NEA) and nitrogen or phosphorus doping introduces donor levels at $~1.7eV$ and $~0.6eV$, respectively, below the conduction band minimum. For the devices considered, the barrier heights are $1.1eV$ and $0.5eV$ for the emitter and collector, respectively. The Richardson constant is $10A/cm^{2}K^{2}$, consistant with experimental results. Assuming an emissivity of 0.5, the device has a Carnot efficiency of 0.52, and a calculated absolute efficiency of 0.17 at a maximum power of $0.25W/cm^{2}$. The theory is extended to include the negative space charge effect, and the NEA properties of the materials are shown to mitigate the space charge effect and increase output power. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B39.00007: Novel photophysics and tandem device designs for solar hydrogen production Justin Johnson, Matt Law, Nathan Neale, Arthur Frank, Josef Michl, Arthur Nozik Solar hydrogen production by water photolysis could provide a means for generating large quantities of clean, transportable fuel cheaply and efficiently for a wide variety of energy uses. Previous schemes of solar hydrogen production have not resulted in a suitable combination of high efficiency, low cost, and good long-term stability to meet requirements for their practical utilization in large-scale energy production. Revolutionary technologies and application of novel photophysical concepts represent a pathway toward overcoming current barriers and achieving an entirely practical method for producing solar fuels. One such concept is the utilization of tandem device designs, which allow for the incorporation of visible/near-IR absorbing materials into the device, thus increasing solar flux harvesting. Moreover, including molecules capable of charge multiplication (multiple electrons/holes per single photon) holds the prospect for additional gains in solar-to-hydrogen efficiencies. Current progress as well as future challenges for developing such devices will be discussed, including simulations, fundamental spectroscopic experiments, and device design and construction. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B39.00008: Optimization of Nanostructured ZnO / Conjugated Polymer Photovoltaic Devices Dana Olson, Yun-Ju Lee, Erik Spoerke, Darren Dunphy, James Voigt, Julia Hsu, Matthew White, Sean Shaheen, David Ginley Nanostructured oxide semiconductor / conjugated polymer composites are promising systems for low cost photovoltaic devices. The use of nanostructures increases the heterojunction areas, resulting in more effective capturing of photogenerated charges. We have fabricated arrays of ZnO nanorods by low-temperature solution growth on patterned ITO substrates. The dense ZnO nanorod arrays are subsequently infiltrated with poly(3-hexylthiophene) (P3HT), and the devices are completed by depositing Ag top electrodes. Depending on the seeding conditions, we can control the alignment of ZnO nanorods on ITO: ordered (aligned perpendicular to the substrate) versus disordered. We will study the effects of nanorod array morphology and growth chemistry, as well as processing conditions used to infiltrate P3HT into the ZnO nanorod arrays. We will also examine surface treatment and modification of ZnO prior to polymer infiltration to enhance electron transfer efficiency at the ZnO/P3HT heterojunctions. Finally, these results are correlated with the device data to observe the effects of ZnO nanorod ordering, interfacial treatment, and the infiltration process on the device performance. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B39.00009: Theoretical Insights on Interfacial Charge Transfer across the P3HT/Fullerene Photovoltaic Heterojunction from ab Initio Calculations Yosuke Kanai, Jeffrey C. Grossman Within the current effort to develop more efficient and less expensive solar cell devices, the polymer/fullerene photovoltaic (PV) structure is considered to be very promising. The crucial component of such a PV structure is the nano-scale heterojunction interface of the polymer and the fullerene. This interface must facilitate the dissociation of the exciton which is formed in the polymer, so that separated charges can be generated across the interface. Our current understanding of the charge separation mechanism at an atomistic level is rather limited, slowing the progress in the structural design of the heterojunction interface. We employ ab initio calculations to investigate and characterize the charge transfer state which is responsible for the charge separation process. Our results elucidate several important phenomena regarding this mechanism, which lies at the heart of higher power conversion efficiency in polymeric solar cell devices. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B39.00010: Hybrid Tandem Solar Cells: CIGS/DSC with Carbon Nanotube Interlayer anvar Zakhidov, William Shafarman, Mei Zhang, Shaoli Fang, Ray Baughman Multi-junction solar cells enable harvesting of wider regions of the solar radiation spectrum leading thereby to increased overall efficiencies. We present here a first study of a hybrid monolithic structure composed of\textit{ dye sensitized solar cells (}DSCs) with thin film inorganic CIGS$.$ We have created several architectures of monolithic multi-junction cells and address fundamental connectivity issues by using sheets of strong, transparent carbon nanotubes (T-CNTs) recently produced at UTD [1] as a uniform interlayer platform. Free-standing T-CNT networks can be laminated onto any surface and their advanteges as transparent interlayers in tandems is shown here for a tandem in which a un-finished CIGS ( top ITO is absent) is coated by T-CNTs. Such CIGS with T-CNT shows Voc=0.6 V and Isc $\sim $ 10 mA/cm2. It has been combined with DSC playing role of a photoactive counter-electrode, with iodine based electrolyte and Ru-dye on TiO2 mesoscopic electrode. The tandem demonstrated Voc= 0.82 V, which is higher than Voc of our sole DSC-CNT and Isc= 1mA/cm2, smaller than photocurrent of single DSC due to unbalanced current. The physics of processes of charge recombination in hybrid tandems is discussed . [1] M. Zhang, S. Fang, A. Zakhidov, S. B. Lee, A. Aliev, R.H. Baughman, \textbf{\textit{Science,}} 309,(2005) 1215 [Preview Abstract] |
Session B40: QHE in Graphene
Sponsoring Units: FIAPChair: Michael Fuhrer, University of Maryland
Room: Colorado Convention Center 503
Monday, March 5, 2007 11:15AM - 11:27AM |
B40.00001: The Nature of Quantum Hall States near the Charge Neutral Dirac Point in Graphene Zhigang Jiang, Yuanbo Zhang, Yanwen Tan, Horst Stormer, Philip Kim We investigate the quantum Hall (QH) states near the charge neutral Dirac point of a high mobility graphene sample in high magnetic fields ($B$). We find that the QH states at filling factors $\nu=\pm2$ show thermally activated behavior with an energy gap as large as $\sim890$ K at $B=45$ T. This large energy gap between the $n=0$ Landau level (LL) and the $n=1$ LL, enables us to observe a well-defined QH effect in graphene over a wide temperature range and even up to room temperature. In addition, the data reveal an activation energy gap at filling factor $\nu=1$, which is considerably larger than the previous studied spin states at $\nu=\pm4$ and shows a square root dependence on $B$, suggesting a many-body origin of this state. Such an origin is further supported by tilted field measurements, in which the $\nu=\pm1$ gaps are found to depend only on the normal component of the field with respect to the graphene plane. We therefore propose that the $\nu=\pm1$ states arise from the lifting of the sublattice degeneracy of the $n=0$ LL. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B40.00002: Room Temperature Quantum Hall Effect in Graphene Andre Geim, Philip Kim, Kostya Novoselov, Zhigang Jiang, Horst Stormer, Yuanbo Zhang, Sergey Morozov, Uli Zeitler The quantum Hall effect (QHE) is an example of those few quantum phenomena that occur on a truly macroscopic scale, and it has been attracting intense interest since its discovery in 1980. As many other quantum phenomena, the observation of the QHE requires temperatures $T$ typically below 4K. Efforts to extend the QHE temperature range by using semiconductors with small effective masses of charge carriers have so far failed to reach temperatures above 30K. We show that in graphene -- a single layer of carbon atoms densely packed in a honeycomb crystal lattice -- the QHE can be observed even at room temperature. This is due to the highly unusual nature of charge carriers in graphene, which behave as massless relativistic particles (Dirac fermions) and move with little scattering under ambient conditions. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B40.00003: Quantum transport of massless Dirac fermions in graphene Kentaro Nomura, Allan MacDonald Graphene is a two-dimensional carbon material with a honeycomb lattice and Dirac-like low-energy excitations. Motivated by recent graphene transport experiments, we have undertaken a numerical study of the conductivity of disordered two-dimensional massless Dirac fermions. It has been found that in the long-range Coulombic scattering case the conductivity depends linearly on the carrier density and that the minimum conductivity $\sigma_{\rm min}$ $\stackrel{\approx}{\_\_}$ e$^{2/h}$, consistent with experiments if inter-valley scattering is assumed to be negligible. We study the transport properties of graphene in the presence of a perpendicular magnetic field. In a strong field the conductivity reveals the massless Dirac fermion Landau level structure and quantized Hall conductivities. On the other hand, in the strongly disordered (weak field) regime in which Landau level maxing is important, the conductivity closes to e$^{2/h}$ for the long-range scattering case. K. Nomura, A. H. MacDonaold, cond-mat/0606589. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B40.00004: Dirac and Normal Fermions in Graphite and Graphene: Implications to the Quantum Hall Effect Igor Luk'yanchuk, Yakov Kopelevich Spectral analysis of Shubnikov de Haas (SdH) oscillations of magnetoresistance and of Quantum Hall Effect (QHE) measured in quasi-2D highly oriented pyrolytic graphite (HOPG) reveals two types of carriers: normal (massive) electrons with Berry phase $0$ and Dirac-like (massless) holes with Berry phase $\pi$. We demonstrate that recently reported integer- and semi-integer QHE for bi-layer and single-layer graphenes take place simultaneously in HOPG samples. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B40.00005: Delocalization of electrons in the lowest Landau level of disordered graphene Pallab Goswami, Xun Jia, Sudip Chakravarty We investigate analytically and numerically the effects of disorder on the density of states and localization of relativistic two dimensional fermions in the lowest Landau level. We have used the numerical technique pioneered by Huckestein, to establish the localization-delocalization transition and calculate the localization length exponents for different types of disorder. For some combinations of disorder the localization-delocalization transition is shown to belong to a different universality class compared to the localization-delocalization transition in the lowest Landau level of nonrelativistic fermions. Our results have direct relevance to the integer quantum hall effect observed in graphene. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B40.00006: Quantum Hall Stripes in Graphene Jianhui Wang, Andrew Iyengar, Herbert Fertig, Luis Brey We study unmodulated stripes in graphene with a high, half integer filling factor. The ground-state energy and stripe density are calculated in the Hartree-Fock approximation to determine the effect of the differing Landau indices on the two sublattices in the single-particle wavefunctions. For appropriate filling factors, the stripes should support a spontaneous polarization of the valley degree of freedom in the limit where electron-electron interactions may be treated as SU(2) symmetric. The implications of the groundstate structure for collective modes and stability of the stripe state will be considered. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B40.00007: Localization and resistance oscillations in n-layer graphene Neal Staley, Haohua Wang, Conor Puls, Jeremy Forster, Kelly McCarthy, Ben Clouser, Ying Liu Single and double-layer graphene have attracted much attention recently because of their unusual electronic band structures and novel physical properties. Theoretical calculations on n-layer graphene (nLG) revealed varying electronic properties, either semiconducting or semimetallic, depending on $n$ as well as the stacking pattern. We have prepared nLG devices, with $n$ ranging from 1 to 5, using a lithography-free, ``all-dry'' process, and measured the conductance of these devices as a function of the temperature, magnetic field, and the gate voltage. The conductance was found to exhibit quantum oscillations and magnetic field dependence that appear to have resulted from weak localization effects. Results of other measurements will also be presented. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B40.00008: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 12:51PM - 1:03PM |
B40.00009: Quantum Hall ferromagnetism in monolayer and bilayer graphene Kentaro Nomura, Allan MacDonald Graphene is a two-dimensional carbon material with a honeycomb lattice and Dirac-like low-energy excitations. Recent experiments and theoretical studies have clarified the unconventional quantum Hall effects that occur in both graphene systems because of their chiral band structures. In this contribution we address the influence of interaction on the quantum Hall effect in single-layer[1] and bilayer graphene, concentrating on the competition between disorder and interactions and on the variety of broken symmetry states that occur at integer filling factors. We also comment on the unusual fractional quantum Hall effect in bilayer graphene, which is strongest at filling factor $\nu$=2/5 rather than at filling factor $\nu$=1/3. \newline \newline [1] K. Nomura, A. H. MacDonald, Phys. Rev. Lett. 96, 256602 (2006). [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B40.00010: Magnetic field effects in bilayer graphene systems Daniel Arovas, Srinivas Raghu, Taylor Hughes, Andrei Bernevig Recently, it has been shown that bilayer graphene systems display interesting electronic properties: examples include the quantum Hall effect with additional plateaus at zero hall conductivity, and electronic gaps tunable by a bias voltage. Here, we study an experimentally relevant problem of the effect of an external applied magnetic flux density on the bulk band structure and edge dynamics of bilayer graphene. We combine exact diagonalization studies with analytical transfer matrix methods and systematically study the quantized Hall conductivity, edge state structure, and possible topological phases of the system over a range of externally applied fields. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B40.00011: Biased bilayer graphene: semiconductor with a gap tunable by electric field effect Eduardo V. Castro, J.M.B. Lopes dos Santos, N.M.R. Peres, K.S. Novoselov, S.V. Morozov, A.K. Geim, F. Guinea, Johan Nilsson, A.H. Castro Neto A graphene bilayer with an electrostatic potential difference between layers~--~biased bilayer~--~has been experimentally realized recently. Using a tight binding description we demonstrate that the externally applied gate bias effectively controls the electronic gap between the valence and the conduction bands of bilayer graphene. Applying the theory to the description of magneto-transport data (Shubnikov-de Haas measurements of the cyclotron mass) we extract the value of the gap as a function of the electronic density. We show that the gap can be tuned between zero and mid-infrared energies using fields still below the electric breakdown of SiO$_2$. The opening of a gap is clearly seen in the quantum Hall regime, where the zero-energy double step characteristic to the anomalous quantum Hall effect in unbiased bilayer graphene, splits into two, giving rise to an additional plateau at zero Hall conductivity, besides the standard quantum Hall sequence. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B40.00012: Quantum Hall effect in carbon nanotubes and curved graphene strips Jose Gonzalez, Enrico Perfetto We show the development of Landau levels for thick carbon nanotubes in a transverse magnetic field, making use of a long-wavelength description in terms of Dirac fermion fields. For values of the magnetic length smaller than the nanotube radius, quantized longitudinal currents are carried by states localized at the flanks of the nanotube. We find that the Hall conductivity is given by even multiples of $2 e^2 /h$, and clarify the transition to the typical odd-integer quantization of graphene as the nanotube is unrolled to form a curved strip. We also show that the absence of significant backscattering interactions opens the possibility to observe a robust chiral liquid at the flanks of the nanotube. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B40.00013: Long-range interactions and Pseudo-relativistic Phenomena in Disordered Graphene: The Zero-bias Anomaly William Shively, Dmitri Khveshchenko Two-dimensional graphene creates a window into new and unusual transport phenomena, which can be described in terms of the propagation of non-interacting Dirac quasiparticles (DQP). In such a system, Coulomb interactions also remain unscreened, and it is of interest how such long-ranged correlations might significantly affect DQP excitations. Using single-particle tunneling measurements, the DQP densities of states are computed analytically, in the presence of mild impurities and for energies ranging between the diffusive and the ballistic limits. Interesting interplay between the Coulomb interactions and the DQP is best revealed in the ballistic regime, whereas in the diffusive limit we recover what is essentially the conventional 2DEG. The evolution of the anomalous exponent characterizing the ``zero-bias'' anomaly in the ballistic regime is discussed. [Preview Abstract] |
Session B41: Nanotechnology
Sponsoring Units: FIAPChair: Eric Dufresne, Yale University
Room: Colorado Convention Center 504
Monday, March 5, 2007 11:15AM - 11:27AM |
B41.00001: Transport of Nanomaterials in Air and Aquatic Systems -- An Ontological Approach Ashok Vaseashta Advances made over the last few years provide new opportunities for scientific and technological developments in nanostructures and nanosystems with unique architectures, desired characteristics and improved functionality. Despite major developments in this field, there is a significant gap in our knowledge of the environmental, health, and ecological impacts associated with nanostructured materials. Since innovations in the field of nanotechnology occur faster than the policymakers can develop safe handling practices; a comprehensive and fundamental investigation is necessary based on dynamic transport of nanomaterials in the environment and its impact on human health and ecology. A matrix of parameters which govern transport of nanomaterials such as exposure routes, chemical composition, surface structure, solubility, size and shape effects, toxicity, absorption, distribution, metabolism, agglomeration, and excretion rate and mechanisms is proposed in this investigation. The complex nature of naturally occurring and engineered nanomaterials and transport either in the environment or via different exposure routes with human body necessitate an ontological modality. A theoretical basis for ontologies used for transport of nanomaterials in the environment such as air, water, and soil and human body will be presented. A comprehensive investigation will prove beneficial to risk assessment and ensuring safe practice in nanotechnologies. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B41.00002: Some Relaxed Equilibrium Configurations for a Few Hydrogen Atoms Inside a Unit Graphene Cell of Variable Volume Juan Salvador Arellano A set of hydrogen atoms (1 to 9) are considered to be inside a graphene cell with parameters a = 4.6117 and c = 5 or 10 a.u. The carbon coordinates are frozen, but hydrogen atoms are relaxed to find favorable configurations with local minimal energy. The fhi98md-LDA code has been used to do the calculations. For some sets of hydrogen atoms it has been obtained two possible configurations. The configurations are analyzed with the DOE (USA Department of Energy) criteria, respect to the characteristics that a material must meet to be considered as a good candidate to store hydrogen. Some of these criteria, for example the gravimetric density and the hydrogen volumetric concentration are discussed in particular. Higher values than 6.5 weight \% and 65 $kg/m^3$, of stored hydrogen required by the DOE are obtained for some of the studied graphene cells. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B41.00003: Gas sensors based on single-wall carbon nanotubes and polypyrrole-coated carbon nanotubes Young Wook Chang, Je Seung Oh, Seung Hwan Yoo, Ji Hun Kim, Hyang Hee Choi, Kyung-Hwa Yoo We have fabricated gas sensors based on single-wall carbon nanotubes and detected NH3 and NO2 gas. At the room temperature, the absorbed gas molecules are not easily detached from the CNT surface. So, we have testes the gas sensor at high temperatures and investigated the temperature dependences of electrical properties of CNTs above the room temperatures. Depending on the gas atmosphere and the temperature, large hysteresis has been observed. In addition, in order to improve the properties of gas sensor, we have electro-deposited polypyrrole onto CNTs and compared with SWNT without polypyrrole. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B41.00004: The Intensity and the Lifetime Variation of the Single Quantum Dot with its Mirror Image System Jui Wen Chou We propose to observe the oscillation effects on the spontaneous emission intensity and the lifetime under the condition of distance variation of the single quantum dot and its mirror image system. We start from the wave function and use the superposition principle to predict the self-interference phenomena of the system; and the lifetime of the system is theoretically predicted damping oscillated. The spontaneous emission intensity and the lifetime were observed as the function of the distance between the single quantum dot and its mirror image. We first demonstrate the method of observing self-interference and superradiant spontaneous emission at room temperature by the single quantum dot with its mirror image distance variation system. Because of the phenomena is observable at room temperature, it could be potentially applied as the basic quantum bit in the quantum information processing and computation. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B41.00005: `Sliding Kinetics' of Carbon Nanotubes on Self-Assembled Monolayer Patterns Jiwoon Im, Minbaek Lee, Sung Myung, Juwan Kang, Dong Joon Lee, Seunghun Hong Recently, self-assembled monolayer (SAM) patterns were utilized to guide the `assembly' and `alignment' of single wall carbon nanotubes (swCNT) on solid substrates for the large scale fabrication of swCNT-based devices (Nature 425, 36 (2003); Nature Nanotechnology 1, 66 (2006)). Herein, we present the experimental results and theoretical model describing the new adsorption kinetics of swCNTs onto SAM patterns including the `sliding motion' of swCNTs. The adsorption behavior of swCNTs on large-size SAM patterns is similar to Langmuir isotherm, while that on the nano-scale patterns shows a significant deviation which can be explained by the `sliding motion' of adsorbed nanotubes (J. of Chem. Phys. 124, 224707 (2006)). The `sliding chamber' experiment confirms that swCNTs can align along the SAM patterns by sliding motion right above the SAM surfaces. We performed extensive study regarding the adsorption behaviors of swCNT on various SAM patterns as well as the solvent effect on CNT adsorption. These results could provide important guidelines for large-scale directed assembly of swCNT-based devices in the near future. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B41.00006: Synthesis and nano-placement of nanoparticle using cage-shaped protein S. Kumagai, S. Yoshii, K. Yamada, K. Nishio, N. Matsukawa, K. Iwahori, I. Yamashita Nanoparticles (NPs) have been attracting considerable attention and the placement of a single NP at will is fundamental technique for nanodevices. We artificially synthesized a variety of uniform NPs ($\phi $6nm) within the cage-shaped protein, apoferritin and studied the placement of ferritin (apoferritin with NP core, $\phi $12nm). We numerically analyzed the interaction between negatively charged ferritin and positively charged nano-disk on the negatively charged SiO2 surface, which can be realized at neutral pH. The calculated free energy potential profile derived from electrostatic interaction, osmotic pressure and van der Waals force showed that a $\phi $15nm positively charged disk could attract a single ferritin molecule in the solution with the Debye length of 14nm. Using the conditions, a single ferritin molecule was placed successfully on the every disk arranged quadrilaterally with 100nm interval. Heat treatment under O2 gas removed protein shell selectively and left NP array. The electrostatic interaction with long range effect has been thought unsuitable for the nano-placement, but it was clearly demonstrated that the electrostatic interaction achieves handling of molecules with nanometric resolution. This study is partially supported by MEXT, Japan. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B41.00007: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 12:39PM - 12:51PM |
B41.00008: Nanophotonic Circuit Elements Aric Sanders, Norman Sanford, John Schlager, Eric Dufresne, Mark Reed In order to realize nanoscale photonic circuits, sub-diffractive light guides, switches, and other active elements must be first realized. We have demonstrated that silver nanowires can act as plasmonic fibers, redirecting and distributing light at the nanoscale. In addition, silver nanowires can act as sensitive switches based on the strong polarization dependence of the plasmon polariton modes carried on their surface. Finally, the role of gallium nitride nanowires as active optical elements is presented. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B41.00009: Applications of Nanotemplates and Nanoparticles in Nanomanufacturing and Nanomedicine E. Gultepe, D. Nagesha, C. Fantasia, S. Lloyd, S. Tai, S. Sridhar Nanotemplates and nanoparticles have potential for use in the area of nanomanufacturing and biomedical applications. Using controlled anodization, nanoporous surface can be achieved on metals such as aluminum and titanium. We are using highly ordered nanoporous alumina as a template for drug delivery and to assemble nanoelements such as latex beads and super coiled DNA by the means of electrophoresis. We have developed a variety of platforms incorporating superparamagnetic iron oxide nanoparticles (SPIONs) for targeted delivery, magnetic hyperthermia and as a contrast agent for magnetic resonance imaging. The results of cell studies on these platforms will be discussed. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B41.00010: Molecular switch modifications by single atom manipulation M. Mamatkulov, L. Stauffer, Ph. Sonnet, M. Martin, M. Lastapis, D. Riedel, G. Dujardin The operation of a single molecule as a molecular nanomachine requires controlling the interaction between the molecule and its surrounding with an atomic scale precision. Recently, it has been demonstrated that a single biphenyl molecule adsorbed on a Si(001) surface behaves as bistable molecule at low temperature. By means of ab-initio calculations, we study the underlying physics of this system. A variety of configurations for the adsorbed biphenyl molecule have been investigated [1]. We show that, during its adsorption on Si(001), one hydrogen atom dissociates from one phenyl and bonds to a neighbouring surface silicon atom. After desorbing this hydrogen with a STM tip, the dynamics of the adsorbed biphenyl molecule is strongly modified: it becomes a multistable molecule having four stable states. Local density of states calculations have been performed and compared to the experimental STM topographies. A good agreement has been observed, allowing a deeper understanding of this system [2]. Our study emphasizes that, by means of a single atom manipulation, one expect to be able to control the intrinsic performance of molecular device. [1] M. Mamatkulov et al., Phys. Rev. B 73 (2006) 035321 [2] M. Martin et al., Phys. Rev. Lett (accepted) [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B41.00011: Novel Parametric Actuation Scheme in Piezoelectric NEMS Rassul Karabalin, Sotiris Masmanidis, Ron Lifshitz, Michael Cross, Michael Roukes Resonant nanoelectromechanical systems (NEMS) are attracting interest in a broad variety of applications ranging from ultrasensitive mass and force detectors to quantum limited devices. However, an efficient, fully integrated scheme for actuation and detection remains a challenge. This talk reviews our recent progress in addressing this problem. First, we obtain excellent actuation efficiency using multilayered piezoelectric nanostructures. Second, we employ the piezoelectric properties of these structures for parametric amplification of mechanical motion. Finally, we excite arrays of coupled NEMS resonators and find that their response agrees with theoretical predictions. We use such parametrically-driven resonant devices to demonstrate a thousand-fold amplitude gain, as well as a significant quality factor enhancement, both in vacuum and in air, suggesting that this scheme may provide significant improvement to sensor performance. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B41.00012: Hysteretic melting and freezing of nanoscale indium islands using local thermal cycling for phase-change memory nodes Todd Brintlinger, Kamal Hussain Baloch, Yi Qi, William G. Cullen, David Goldhaber-Gordon, John Cumings Using a transmission electron microscope (TEM) operating in dark-field mode, the melting and freezing transition in nanoscale (approximately 20-200nm diameter) metal islands can be imaged at video rates (33ms/frame). The metal, typically indium, islands are thermally evaporated on one side of a 100nm thick SiN membrane. Local thermal gradients produced by Joule heating of lithographically defined electrodes on the opposite side of the membrane show a hysteretic effect in the melting/freezing of the metal islands. Read and write cycles are accomplished with 5-10 microW power, while a quiescent power of 80-100 microW is required to keep an island near its melting point. The hysteresis indicates a finite nucleation energy during freezing of individual islands. While TEM is not a practical readout mechanism, the behavior suggests a type of phase-change memory node on an inherently nanometer scale. Results for all the aforementioned will be shown, including micrographs, video, and related discussion. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B41.00013: Application of mesoscopic light transport theory to ultra-early detection of cancer in a single biological cell Prabhakar Pradhan, Hariharan Subramanian, Yang Liu, Young Kim, Hemant Roy, Vadim Backman We report application of mesoscopic light transport theory to ultra-early detection of carcinogenesis in a single biological cell. In early stages, the progression of carcinogenesis is accompanied by nanoscale morphological changes in the internal structure of a biological cell. Such changes result in nanoscale alterations of the refractive index distribution in cells, which modulate the nanoscale light transport properties of a cell. Understanding the mesoscopic/nanoscopic light transport properties of cells is therefore important for detecting the progression of carcinogenesis in a cell or tissue. We have developed a new technique -- partial wave spectroscopy (PWS) -- to measure changes in nanoscale light transport properties associated with the nanometer scale refractive index alterations in a cell. $^{ }$We quantify the cell's refractive index fluctuations using mesoscopic light transport theory. Our results using rat/human cells demonstrate that the PWS technique is able to detect ultra-early refractive index fluctuation changes at the nanoscale associated with the progression of carcinogenesis in these cells. This method can be used as a potential biomarker for ultra-early detection of cancer. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B41.00014: Multiple-layer SOI based on Single-Crystal Si Nanomembrane Transfer Weina Peng, Michelle Roberts, Eric Nordberg, Frank Flack, Paula Colavita, Robert Hamers, Donald Savage, Max Lagally, Mark Eriksson Silicon-on-insulator (SOI) has many advantages over bulk Si including the reduction of parasitic resistance and increased device speed. Multiple-layer SOI, having more device layers per unit area, enables 3D process integration as well as applications in optics. However, it is impossible to achieve such a system by growth techniques (one can grow only non-crystalline Si on SiO$_{2})$, and multiple Smart Cut transfers used to create single layer SOI may be prohibitively expensive. We present here a novel method to fabricate such a multiple SOI system using transferred Si nanomembranes$^{ }$and subsequent oxidation. The surface roughness and interface quality are examined respectively by AFM and cross-sectional SEM. Low surface roughness (0.176nm) and smooth interfaces are achieved. As an example optical application, we apply the multilayer system to fabricate a Si-based Bragg reflector. The specular reflectivity of one, two, and three-membrane mirrors is measured using FTIR. High specular reflectivity, above 99{\%}, is achieved for three stacked membranes. Comparison of the measured reflectivity with theoretical calculations shows good agreement. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B41.00015: Inelastic Tunneling Spectroscopy Study of nm-thick oxides in Metal-Oxide-Semiconductor Structure Jino Lee, Kookrin Char, Sangjin Hyun, Sangbom Kang, Siyoung Choi Using inelastic electron tunneling spectroscopy (IETS) technique which can probe phonon modes and defect states in the tunneling barrier, we investigated Si(n+) -- HfSiO(2.0nm)-- Si(p+) MOS capacitor samples. It is an efficient way to study the properties of MOS capacitors because there can be a measurable tunneling current, as the insulating barrier of the MOS devices approaches a few nm. We measured the phonon spectra between 10meV and 70meV and identified each peak using the reference data in previously reported papers by others. When comparing with the data obtained from the sample of Si(n+) -- SiO2(1.2nm) -- Si(p+) MOS capacitor, we observed the additional intensive phonon mode peak in the HfSiO gate oxide. We believe that this peak is enhanced by interface effects between the substrate and the insulating barrier. In the higher energy regime, features that are associated with the trap-related states were observed. The spatial location relative to the interfaces and the energy level of trap-related states can be estimated through simple modeling, when a same defect state can be identified simultaneously in the forward and backward biases.. [Preview Abstract] |
Session B42: Focus Session: Kinetics of Self-Assembly at Surfaces
Sponsoring Units: DMPChair: Ray Phaneuf, University of Maryland
Room: Colorado Convention Center 505
Monday, March 5, 2007 11:15AM - 11:51AM |
B42.00001: Theory of kinetics of surface evolution: simulations, Langevin, and Fokker-Planck approach Invited Speaker: The study of the the time evolution of morphological features at crystal surfaces has become a topic of great importance, motivated in part by the need of achieving controlled fabrication of nanostructures, and in part by the fundamental statistical mechanics questions that it raises. For the evolution of nanostructure, the control of step dynamics is crucial, since the steps are the fundamental building blocks of crystalline surfaces. Kinetic Monte Carlo (KMC) simulations, coupled to Langevin-type analysis of the scaling properties of step dynamics is a powerful tool for assessing the universal features of surface fluctuations close and far from equilibrium. We discuss the results of KMC simulations of unstable growth of vicinal surfaces, that exhibit anomalous scaling exponents as well as multiscaling. We also discuss a tentative interpretation of the numerics within the Langevin approach, as well as the usefulness of the results in interpreting experimental data. Other theoretical tools, such as the spacing distribution of the eigenvalues of random matrices have been employed, e.g. for investigating the evolution of the terrace width distribution (TWD) on stepped surfaces. We discuss KMC simulations of the relaxation of non equilibrium surface structures towards the equilibrium state. Our results show that relaxation far from equilibrium may be driven by microscopic processes, such as detachment of three-bonded atoms, that differ from those that drive step fluctuations close to equilibrium. Applications of the formalism to out-of-equilibrium states, such as step flow growth, are discussed. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B42.00002: Strain induced metastability in the shape evolution of self-assembled nanoislands on Si(111): real-time electron microscopy observations and numerical simulations Nikhil Medhekar, Vivek Shenoy, James Hannon We present real-time Low-Energy Electron Microscopy (LEEM) observations of the growth and equilibrium shapes of (7x7) reconstructed domains on (1x1) reconstructed Si(111) surface which show several intriguing features in its shape evolution due to strain mediated interactions. We find that the shapes of large domains are fundamentally different from the compact shapes of smaller domains. In contrast, large islands show more ramified shapes resembling branched pine-tree when grown at faster rate and connected-triangles morphology when growth is near equilibrium. Using a phase-field model, we show that the key to understanding this behavior is the strain induced metastability of domain shapes that are trapped in the local minima of the complex energy landscape. The consideration of growth shapes that show spontaneous formation of side branches is necessary to establish the presence of unstable orientations and thus, our work shows that in estimating the thermodynamic and kinetic parameters, the conclusions solely drawn based on the analysis of equilibrium shapes can be erroneous. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B42.00003: Atom-Scale Mechanisms for Unstable Growth on Patterned GaAs(001) Tabassom Tadayyon-Eslami, Hung-Chih Kan, Lynn Calhoun, Ray Phaneuf Molecular beam epitaxy on patterned GaAs(001) under standard conditions of temperature ($\sim $600\r{ }C), rate ($\sim $ 0.3 nm/s) and flux ratio (As$_{2}$/Ga$\sim $10:1) leads to a transient instability toward perturbation of the flat surface [1]. Lowering the temperature through approximately 540$^{o}$C, roughly coincident with the preroughening temperature changes the mode of this instability [2]; however, as we show in this talk, observations of the As$_{2}$ flux dependence rule out both preroughening and a reconstructive phase transition as driving the growth mode change. Instead, we find evidence that the change in unstable growth mode can be explained by a competition between decreased adatom collection rate on small terraces and a small anisotropic multi-step Ehrlich-Schwoebel barrier. We relate these effects to the up-down symmetry breaking term which commonly appears in continuum equations for growth. [1] H.-C. Kan, S. Shah, T. Tadayyon-Eslami and R.J. Phaneuf, Phys. Rev. Lett., \textbf{92}, 146101 (2004). [2] T. Tadayyon-Eslami, H.-C. Kan, L. C. Calhoun and R. J. Phaneuf, Phys. Rev. Lett., \textbf{97}, 126101 (2006). [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B42.00004: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 12:27PM - 12:39PM |
B42.00005: Multiscale simulations of self-assembly of CdTe nanoparticles into sheets Zhenli Zhang, Zhiyong Tang, Nicholas Kotov, Sharon Glotzer By controlling the organic stabilizers on the surfaces of CdTe nanodots, these particles are found in experiments to self-assemble into one-dimensional wires and two-dimensional sheets[1,2]. To explore the underlying mechanisms for the two processes we perform simulations on multiple scales ranging from quantum mechanics to mesoscale stochastic simulations[2,3]. The simulations and corresponding energy analysis demonstrate that a delicate balance of anisotropic forces between nanoparticles is responsible for the different nanostructures they form. In particular, we show how nanoparticle shape, directional hydrophobic attraction, and electrostatic interactions determine the anisotropy of the interaction and final self-assembled structures. \medskip [1] Tang ZY, Kotov NA, Giersig M, Science, 297, 237-240, 2002. [2] Tang ZY, Zhang ZL, Wang Y, Glotzer SC and Kotov NA, Science, 314, 274-278, 2006. [3] Zhang ZL, Tang ZY, Kotov NA and Glotzer SC, preprint. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B42.00006: Charging in CdSe nanocrystals and mechanistic elucidation of the electrophoretic deposition of nanocrystal films Shengguo Jia, Sarbajit Banerjee, Irving Herman The charge on nanocrystals is not only used to stabilize the colloidal systems but also to assemble these materials into novel films and superlattices. Here, we propose a model for charging in nanocrystals involving the dissociation of ligand molecules from specific surface sites. We also develop a mechanistic model to explain the electrophoretic deposition of nanocrystal films based on electrophoretic mobility measurements, photoluminescence from nanocrystal solutions and films, and observations from deposition experiments. Even though equally thick nanocrystal films are obtained on both negative and positive electrodes, the numbers of positive and negative nanocrystals are not equal in solution. After appropriate reprecipitation cycles, the nanocrystals are ``sticky'' enough to be deposited on the electrodes and nanocrystal films can be formed by electrophoresis. The limiting factor for the maximum thickness to which the films can be grown is the concentration of the minority charged crystals (negatively charged nanocrystals in this case). The charge on the nanocrystal surfaces can be adjusted by the addition of ligands. This work was supported primarily by the MRSEC Program of the NSF under Award No. DMR-0213574 and by NYSTAR. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B42.00007: Phase Field Crystal Modeling of Island Formation and Dislocation Nucleation During Strained Film Growth Zhi-Feng Huang, Ken Elder We study the process of nanostructure self assembly during epitaxial growth of strained solid films through the use of the phase field crystal model. The model is derived from density functional theory and incorporates anisotropy, elasticity and plastic deformations on atomic length and diffusive time scales. We particularly address the formation and evolution of islands/mounds in strained thin films following an initial morphological instability and the nucleation and climb of misfit dislocations. The relation between film structural properties and materials and growth conditions are also discussed. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B42.00008: An Investigation into InAs/GaAs Thin Film Growth Maria Mignogna, Kristen Fichthorn Quantum dots self-assemble due to Stranski-Krastinov growth in heteroepitaxial systems with a lattice mismatch above 2\%, for example in the deposition of InAs on GaAs (001). However, there are many questions left unanswered about quantum dot growth, such as the role of strain in the wetting layer. Simulation techniques such as molecular dynamics (MD) can provide insight at the atomic scale. An empirical potential to study this system has recently been developed [1]. Using NPT MD, we studied the thermal properties and melting of bulk GaAs, as well as the stability of the GaAs(001)$\beta$2(2x4) reconstruction against melting. To probe diffusion and the preferred Ga-atom binding sites, we calculated the minimum potential-energy surface for a gallium atom on the GaAs (001) $\beta$2(2x4) reconstruction. We also evaluated various diffusion pathways and energy barriers using the nudged elastic-band method. The potential captures the location and energy of the deepest binding minimum as compared to DFT values and also achieves good agreement for the diffusion barriers. We used accelerated MD simulations to obtain diffusion coefficients as a function of temperature and these compare favorably to previous results from experiment and DFT. [1] T. Hammerschmidt, PhD Thesis (2006) [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B42.00009: Nanocrystal Formation in Ion-Beam Synthesized GaAs:N and InAs:N A. Wood, W. Ye, X. Weng, P.T. Wang, R.S. Goldman, Y.Q. Wang Ion-implantation followed by thermal annealing offers a unique approach to custom tailoring of semiconductor nanocomposites. For N ion-implanted GaAs (GaAs:N), an amorphous layer with crystalline GaAs remnants is often observed. Subsequent furnace or rapid-thermal annealing (RTA) leads to the formation of zincblende (ZB) GaN nanocrystals [1], which transform to wurtzite (WZ) following extended furnace annealing [2]. For N ion-implanted InAs (InAs:N), nanocrystal formation and evolution has not been previously reported. We are studying the formation and evolution of GaAs:N and InAs:N nanocomposites, synthesized using 100keV ion-implantation with a dose of 5x10$^{17}$cm$^{-2}$, at 300C and 77K. In all cases, the as-implanted structures are primarily amorphous. For GaAs:N, RTA up to 625C leads to an amorphous layer with crystalline GaAs remnants, while RTA in the range 675-700C results in both ZB and WZ nanocrystallites. For InAs:N, 500C RTA leads to the formation of ZB InN-rich and InAs-rich nanocrystals, with amorphous matrices and domains. We will discuss the role of crystalline remnants in the nucleation and growth of ZB nanocrystals, and the mechanisms of the ZB-WZ transformation. [1] X. Weng, et al, \textit{J. Appl. Phys}., \textbf{92} 4012 (2002) [2] X. W. Lin, et al, \textit{Appl. Phys. Lett}. \textbf{67}, 2699 (1995) [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B42.00010: Dual-surfactant effect on enhancing Zn-Doping of GaP Junyi Zhu, Gerald Stringfellow, Feng Liu We report first-principles calculations demonstrating a dual-surfactant effect of Sb and H on enhancing Zn-doping in vapor phase epitaxially grown GaP thin films. The combined effects of Sb and H lower significantly the doping energy of Zn in GaP, while neither Sb nor H can work alone as effectively. The role of H is to provide the extra electron accommodating the p-type dopant incorporation to satisfy the electron counting rule. Our finding has an important general implication that p-type doping in III-V thin films can be achieved by chemical deposition with H, but difficult by physical deposition without H. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B42.00011: Design Principles Incorporating Surface Dynamics for Creating Ordered Organic Nanostructures on Si and SiC Dimerized Surfaces via Car-Parrinello Molecular Dynamics Robin Hayes, Mark Tuckerman Self-assembled organic nanostructures on Si-type semiconducting surfaces promise to impact nanoelectronics, sensors, and nanolithography. Experimentalists have long exploited cycloaddition reactions between small conjugated molecules and Si surface dimers, but with limited success in creating well-ordered structures. Cycloaddition of 1,3-cyclohexadiene (CHD) to the Si(100)-2x1 surface provides a rich test case to explore the role surface dynamics plays in adduct formation because of the variety of bonding motifs and discrepancy of the product distribution with thermodynamic predictions. Car-Parrinello molecular dynamic simulations reveal that the local surface environment, including dimer tilt angle and dimer flipping, matters. CHD often travels over several dimers before forming an adduct by a two step process. First the C=C reacts with a ``down'' Si. The intermediate can persist for over 4 ps and can cause nearby dimers to flip, allowing CHD to complete the reaction with any of the adjacent Si. Thereby, accounting for most of the experimental product distribution. Previously formed adducts protect Si within a 5.5 $\AA$ radius and direct the surface exploration of unbound CHD. These principles are tested on reactions between 1,3-CHD and the closely related 3C-SiC(001)-3x2 surface. [Preview Abstract] |
Session B43: Focus Session: Materials for Quantum Information Processing I
Sponsoring Units: DMPChair: Steve Lyon, Princeton University
Room: Colorado Convention Center 506
Monday, March 5, 2007 11:15AM - 11:27AM |
B43.00001: Sputtered Gold as an Effective Schottky Gate for Strained Si/SiGe Nanostructures Gavin Scott, Ming Xiao, Ed Croke, Eli Yablonovitch, HongWen Jiang Metallization of Schottky surface gates by sputtering Au on strained Si/SiGe heterojunctions enables the depletion of the two dimensional electron gas (2DEG) at a relatively small voltage while maintaining an extremely low level of leakage current. A fabrication process has been developed to enable the formation of sub-micron Au electrodes sputtered onto Si/SiGe without the need of a wetting layer. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B43.00002: Phosphorus Donors in Highly Strained Silicon M. S. Brandt, H. Huebl, A. R. Stegner, M. Stutzmann, G. Vogg, F. Bensch, E. Rauls, U. Gerstmann Donors in strained Si layers have been proposed for quantum computing applications. The lifting of the six-fold valley degeneracy, characteristic for unstrained Si, leads to a suppression of the Kohn-Luttinger oscillations in strained layers which would otherwise limit the exchange interaction of neighboring qubits. Via electrically detected magnetic resonance, we have determined the hyperfine interaction of phosphorus donors in fully strained Si thin films grown on virtual Si$_1$$_-$$_x$Ge$_x$ substrates with $x\leq0.3$, extending the regime investigated earlier by a factor of 20 to higher strains. For highly strained epilayers, hyperfine interactions as low as 0.8 mT are observed [1], significantly below the limit predicted by valley repopulation. Within a Green's function approach, density functional theory shows that the additional reduction is caused by the volume increase of the unit cell and a relaxation of the Si ligands of the donor. [1] H. Huebl et al., Phys. Rev. Lett. \textbf{97}, 166402 (2006). [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B43.00003: Process integration and electron spin coherence of donor atom implants in silicon T. Schenkel, A. Persaud, A. M. Tyryshkin, S. A. Lyon, J. Bokor, C. C. Lo, R. deSousa, I. Chakarov We implanted low doses (2 to 4 x $10^{11}_ cm^{-2})$ of P, Sb, and Bi ions into isotopically enriched silicon (28-Si) and characterized diffusion, electrical activation and electron spin coherence after rapid thermal annealing. Phosphorus and bismuth both exhibit enhanced segregation to an imperfect Si/SiO2 interface, while dopant movement is suppressed for antimony ions. Pulsed electron spin resonance shows that spin echo decay is sensitive to the dopant depths, and the interface quality. At 5.2 K, a spin de-coherence time, T2, of 0.3 ms is found for Sb profiles peaking 50 nm below a Si/SiO2 interface, increasing to 0.75 ms when the surface is passivated with hydrogen. These measurements provide benchmark data for the development of devices in which quantum information is encoded in donor electron spins [1]. [1] T. Schenkel, et al., Appl. Phys. Lett. 88, 112101 (2006). [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B43.00004: Double donors in Si quantum computer architecture Maria J. Calderon, Belita Koiller, Sankar Das Sarma We discuss the possibility of performing single spin measurements in Si-based quantum computers through electric field control of electrons bound to double donors near a barrier interface[1]. In particular, we investigate the feasibility of shuttling donor-bound electrons between the double donor impurity in the bulk and the Si/SiO$_2$ interface by tuning an external electric field. We find that both the required electric fields and the tunneling times involved are probably too large for practical implementations. We also investigate operations with double donors in their first excited state: In this case ionization fields are smaller and tunneling times are faster, as required in spin-to-charge conversion measurements. This work is supported by LPS and NSA. \newline \newline [1] M.J. Calderon, B. Koiller, and S. Das Sarma, cond- mat/0610089. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B43.00005: Electric-field driven donor-based charge qubits in semiconductors Belita Koiller, Xuedong Hu, Sankar Das Sarma We theoretically investigate donor-based charge qubit operation driven by external electric fields [1]. We consider initially a single electron bound to a shallow-donor pair in GaAs: This system allows the basic physics of the problem to be presented. In the case of Si, heteropolar configurations such as P-Sb$^+$ pairs are also considered. For both homopolar and heteropolar pairs, the multivalley conduction band structure of Si leads to short-period oscillations of the tunnel-coupling strength as a function of the relative position of the donors. However, for any fixed donor configuration, the response of the bound electron to a uniform electric field in Si is qualitatively very similar to the GaAs case, with no valley quantum interference-related effects, leading to the conclusion that valley interference does not prevent the coherent manipulation of donor-based charge qubits by external electric fields. We also discuss the effect of perturbations due to additional distant donors. [1] B. Koiller, X. Hu, and S. Das Sarma, Phys. Rev. B 73, 045319 (2006) [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B43.00006: Suppressing anisotropic hyperfine induced electron spin echo modulations in Si:P Wayne Witzel, Xuedong Hu, Sankar Das Sarma In previous work,\footnote{W.M. Witzel, {\it et al.}, Phys. Rev. B {\bf 72}, 161306(R) (2005).} our theory of spectral diffusion (SD) agrees well with electron spin echo decay measurements\footnote{A.M. Tyryshkin, {\it et al.}, cond-mat/0512705.} in Si:P. In addition to SD decay, these experiments show strong electron spin echo envelope modulations (ESEEM) that significantly reduce spin coherence at short echo times relative to SD decay. Strong demands imposed by fault tolerant quantum computing require suppression (or exploitation) of this effect in order to realize spin-based quantum computation in Si:P systems. It is known that these modulations, caused by anisotropic hyperfine interactions with $^{29}$Si nuclei, can be suppressed via isotopic purification, or by applying a strong, $\sim 10$~T,\footnote{S. Saikin and L. Fedichkin, Phys. Rev. B {\bf 67}, 161302(R) (2003).} magnetic field. Our insights lead to an alternative approach that eliminates predominant modulations at modest magnetic fields ($\sim 1$~T) with little need for isotopic purification. Our calculations are in remarkable agreement with experiment, showing good theoretical understanding of refocused electron spin coherence in Si:P systems. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 1:03PM |
B43.00007: Electron transport through STM-patterned dopants in silicon Invited Speaker: The recent adaptation of scanning probe systems for nanoscale device fabrication has opened the door to creating electronic devices in silicon with single atom precision. Using a combination of STM lithography and molecular beam epitaxy we show how we can pattern planar, highly doped P layers in silicon down to the atomic-scale and electrically contact them outside the microscope environment. Having developed this technology we demonstrate conduction through silicon nanowires with widths down to 8nm that still exhibit ohmic conduction with resistivities as low as 3x10$^{-6}\Omega $m. We present a study to determine what ultimately limits conduction in these systems as well as studies of tunnel gaps as charge detectors, ordered dopant arrays and transport through silicon dots. We will present an overview of devices that have been made with this technology and highlight some of the challenges to achieving truly atomically precise devices such as a silicon based quantum computer. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B43.00008: Electron Spin Resonance on Arrays of Etched Quantum Dots in $^{28}$Si/SiGe Jianhua He, A. M. Tyryshkin, S. A. Lyon, D. E. Savage, M. A. Eriksson Relaxation times of 2-dimensional electrons in Si quantum wells (QW) in Si/SiGe heterostructures are found to be shorter than the extremely long relaxation of 3-dimensionally confined donor- bound electrons in Si. Confining the electrons into quantum dots (QD) could suppress the Dyakonov-Perel spin relaxation due to fluctuating Rashba fields, and thus lead to long relaxation times for electrons in QDs. We have developed a reliable, low- defect density process to pattern large area 2D arrays (several cm$^2$) of nominally 100nm dots with a 200nm pitch in a CVD grown $^{28}$Si/SiGe quantum well. The process involves nanoimprinting, reactive ion etching and wet etching. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) imaging has been used to characterize the etch depth (various depths up to 60nm) and uniformity. After etching we find an electron spin resonance signal with a g-factor of 1.9998, which is considerably shifted from that of the unetched QW (2.0003). This line exhibits anisotropies of its g-factor and linewidth that are similar to those of 2D electrons, as might be expected for large, flat QDs. This new line is also broader (420 mG) than that from the unetched QW (90 mG) which could result from an inhomogeneity in dot sizes. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B43.00009: Electron Spin Resonance of Electrons in a Large-Area Silicon MOSFET Shyam Shankar, A. M. Tyryshkin, Sushobhan Avasthi, S. A. Lyon Spins of electrons in two-dimensional (2D) semiconductor heterostructures are considered as qubit candidates for quantum information processing. Electron spin resonance (ESR) of silicon MOSFETs can be useful in characterizing electrons in 2D structures, but previous attempts have been inconclusive. To have sufficient signal for ESR measurements, a large area n- channel silicon FET with a 100nm thick oxide was made using standard processing techniques. Two ESR signals were seen at temperatures below 20K with a gate bias above the threshold voltage of 0.9V. A weak signal with a linewidth of 1G, at g=1.9988(1) may be similar to one seen by Wallace and Silsbee (PRB 1991). A stronger signal is found at g=2.0000(1) with a linewidth of 400mG. This signal shows a noticeable increase in g- factor from 1.9999 at 1V to 2.0000 at 1.7V gate bias and a corresponding decrease in linewidth from 500mG to 400mG. A small g-factor and linewidth change is also seen when the FET is rotated with respect to the applied magnetic field. The signal intensity shows non-Curie temperature behavior below 10K. Such a signal, possibly from conduction electrons or electrons in shallow traps, has not been reported before and is being further investigated. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B43.00010: Electrically Detected Magnetic Resonance of Shallow Donors in Accumulation Layer MOSFETs Cheuk Chi Lo, J. Bokor, T. Schenkel, R. de Sousa, Jianhua He, G. Sabouret, S. Shankar, F. R. Bradbury, A. M. Tyryshkin, S. A. Lyon The ability to read out the spin state of a single donor-bound electron is an essential, but not yet demonstrated, capability for building a quantum computer processor using the spins of electrons bound to Si donors as the qubits. We present measurements of the spins of ensembles of shallow donors embedded in the channel of a MOSFET. Our approach is based on spin-dependent transport arising from the fact that the scattering cross-section of conduction electrons with donor electrons depends on whether the two electrons form a spin singlet or triplet. Our measurements are done on accumulation layer MOSFETs doped with phosphorus or antimony. In continuous wave EDMR experiments, with large area devices (up to 100x100 microns) the measured signals, $\Delta$R/R of the channel, is of the order 10$^{-6}$. The EDMR signal is a function of the field modulation frequency, with pronounced passage effects observed above 1kHz. This result implies that the spin relaxation time, T$_1$, of the donors in the FET channel may be quite long, in excess of 1ms at 5K. A long T$_1$, of this magnitude, suggests that it may be possible to scale the devices to submicron dimensions and read out the state of an individual donor electron spin. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B43.00011: Large Tunable Valley Splitting in a Si/SiGe Quantum Point Contact Lisa McGuire, K.A. Slinker, Mark Friesen, Srijit Goswami, O.J. Chu, Robert Joynt, S.N. Coppersmith, Mark A. Eriksson Quantum dots formed in Si/SiGe two-dimensional electron gases are of interest for use in semiconductor quantum computing due to their potentially long spin lifetimes. However, silicon has a near degeneracy in orbital states due to the presence of multiple valley minima. If the splitting between valley states is smaller than the spin splitting, decoherence rates will be enhanced, complicating qubit operation. Here we present measurements of the valley spitting in a quantum point contact (QPC). The valley splitting is shown to be large, of order 0.5 - 2 meV, over the entire measured range in gate voltage and magnetic field. These results are in contrast with numerous past measurements of valley splitting in laterally unconfined Si/SiGe systems, such as Hall bars. We discuss a physical mechanism, based on interference between states on neighboring atomic steps at the quantum well interface, that explains the discrepancy between the experiments on laterally confined and laterally unconfined systems. We find, in all cases, the magnitude of the valley splitting is suppressed due to atomic steps, but this suppression is substantially lifted in laterally confined nanostructures like QPCs. Work supported by ARO, NSA, and NSF. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B43.00012: Multiscale analytic calculation of valley splitting in silicon quantum wells. Sucismita Chutia, Susan Coppersmith, Mark Friesen Valley splitting in Si/SiGe quantum wells is a central issue for Si based quantum dot quantum computers. The valley coupling arises due to the mixing of states at a sharp quantum well interface. The effective mass theory provides an essential tool for studying the valley splitting in various geometries. However, the magnitude of the splitting must still be determined microscopically, e.g., from atomistic theories. Here, we develop a multiscale theory that bridges the effective mass and atomistic approaches. Since the valley coupling occurs within just a few atomic layers of the interface, we splice a tight binding treatment of the interface into an effective mass treatment of the extended wavefunction. This multiscale theory yields analytical solutions for the valley splitting with no adjustable parameters.~ [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B43.00013: Enhancement mode single electron transistor in pure silicon Binhui Hu, C.H. Yang, G.M. Jones, M.J. Yang Solid state implementations of lateral qubits offer the advantage of being scalable and can be easily integrated by existing main stream IC technologies. In addition, the two Zeeman states of an electron spin in a quantum dot (QD) provide a promising candidate for a qubit. Spins in lateral QDs in the GaAs/AlGaAs single electron transistors (SETs) have been intensively investigated. In contrast, Si provides a number of advantages, including long spin coherence time, large g-factor, and small spin-orbit coupling effect. We have demonstrated Si SET in the few electron regime.* In this talk, we will report the isolation of a single electron in a Si QD using a fabrication technique that incorporates the standard Al/SiO2/Si system with an enhancement mode SET structure. Our SET is built in highly resistive Si substrates with bilayer gates. The high purity Si minimizes the potential disorder from impurities. The top gate induces 2D electrons, and several side gates help define the tunneling barriers, fine tune the shape of the QD, and control the number of electrons in it. We will discuss the operating principle, computer simulation, and low temperature transport data. *APPLIED PHYSICS LETTERS 89, 073106 (2006) [Preview Abstract] |
Session B44: Focus Session: Nanoscale Transport - Molecules I
Sponsoring Units: DMPChair: J.B. Neaton, Lawrence Berkeley National Laboratory
Room: Colorado Convention Center 507
Monday, March 5, 2007 11:15AM - 11:51AM |
B44.00001: Conductance of Single Molecule Junctions: Dependence on Structure and Conformation Invited Speaker: We recently demonstrated that the conductance of single molecule junctions formed by breaking Au point contacts in an environment of molecules with amine linkages can be measured reliably and reproducibly$^{1}$. We have now studied junctions formed by approximately 30 different amine terminated molecules, allowing systematic study of the correlation between molecular properties and single molecule junction conductance. This talk will focus on the relation between molecular conductance and molecule conformation for the simple case of a biphenyl, two benzene rings linked together by a single C-C bond. Our results from a series of seven biphenyl derivatives show that the molecular junction conductance depends on the twist angle. Specifically, we find that the planar molecule has the highest conductance, and the conductance for the series decreases with increasing twist angle, consistent with a cosine squared relation predicted theoretically$^{2}$. 1. L. Venkataraman, J.E. Klare, I.W. Tam, C. Nuckolls, M.S Hybertsen and M. Steigerwald, \textit{Nano Letters}, vol. 5, pp. 458-462, 2006. 2. L. Venkataraman, J.E. Klare, C. Nuckolls, M.S Hybertsen and M. Steigerwald, \textit{Nature}, vol. 442, pp. 904-907, 2006. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B44.00002: DFT-based transport calculations for single molecules: Can Coulomb blockade effects be reproduced by local functionals? Max Koentopp, Kieron Burke In principle, time-dependent current density functional theory (TDCDFT) allows for exact calculations of the electronic transport properties of single molecules. In practice, one is forced to make approximations for the exchange-correlation functional employed, and the computationally less costly ground-state DFT in a local approximation (GGA) is used. This introduces errors that can lead to an overestimation of the calculated current by one to two orders of magnitude. The use of local approximations to the exchange-correlation functional also leads to the inability to reproduce Coulomb blockade effects. We will discuss the origin and scope of these errors. Then, model calculations for molecules where Coulomb blockade effects have been observed experimentally will be presented, and the mechanism for the failure to reproduce Coulomb blockade effects will be explained. \newline \newline [1] M.Koentopp, K.Burke, F.Evers, PRB Rapid Comm. {\bf 73}, 121403(R) (2006) [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B44.00003: First-principles Studies of Single-molecule Conductance in Amine Linked Junctions Su Ying Quek, Mark S. Hybertsen, Latha Venkataraman, Michael Steigerwald, Colin Nuckolls, Steven G. Louie, J.B. Neaton Recently, it was discovered that the conductance of single molecule junctions with amine linkages to Au electrodes can be reliably and reproducibly measured. We compute and examine the conductance of prototypical single molecule junctions formed with amine-Au links using a first-principles scattering state method based on density functional theory. In particular, we elucidate the nature of the scattering states that give rise to the computed conductance, and relate the transmission spectra of each junction to intrinsic molecular and amine link properties. We explore the sensitivity of our results to specific contact geometries. The results are discussed relative to the measured distribution of conductance for each molecule. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B44.00004: Electronic transport through alkane chains: the case of end group functionalization G. Kim, W. Lu, S. Wang, M. Buongiorno Nardelli, J. Bernholc Using first-principles calculations, we have investigated the mechanism of metal/molecule coupling and its influence on the electronic transport properties in the prototypical case of long hydrocarbon (alkane) chains sandwiched between gold contacts. In our study, 1-Pentanethiol [CH$_3$-(CH$_2$)$_4$- SH] , 1-Pentylamine [CH$_3$-(CH$_2$)$_4$-NH$_2$], octanediamine [C$_8$H$_{16}$(NH$_2$)$_2$] and octanedithiol [C$_8$H$_{16}$(SH) $_2$] are anchored to ideally terminated Au (111) surfaces in order to investigate the effects of the functionalization of the end groups on the conduction properties. The results indeed show that the end group functionalization plays a crucial role in controlling the electronic transport through the molecule: the effective contact resistance of the amine/Au system is much smaller than that of the thiol/Au one, giving rise to a large difference in the I-V characteristics. Our results are in good agreement with recent experimental measurements of the tunneling current through these functional groups [1]. \\ \\ $[1]$ C. Chu and G. Parsons, to be published (2006) [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B44.00005: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 12:39PM - 12:51PM |
B44.00006: Transport in Molecular Junctions with Different Metallic Contacts John Lawson, Charles Bauschlicher Ab initio calculations of phenyl dithiol connected to Au, Ag, Pd, and Pt electrodes are performed using non-equilibrium Green's functions and density functional theory. For each metal, the properties of the molecular junction are considered both in equilibrium and under bias. In particular, we consider in detail charge transfer, changes in the electrostatic potential, and their subsequent effects on the IV curves through the junctions. Gold is typically used in molecular junctions because it forms strong chemical bonds with sulfur. We find however that Pt and Pd make better electrical contacts than Au. The zero-bias conductance is found to be greatest for Pt, followed by Pd, Au, and then Ag. (Physical Review B, 74, (2006), p 125401) [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B44.00007: Effects of -NO$_{2}$ substitution on charge addition and reorganization energies in phenylene ethynylene oligomers Steven Robey, N. E. Gruhn, J. Cizek, J. M. Tour Reports of non-linear transport in molecular-scale junctions have stimulated suggestions for computing and switching applications based on molecular electronics. One of the most widely referenced results is reported negative differential resistance (NDR) behavior in -NO$_{2}$ substituted oligo- phenylene ethynylenes (OPE). Theoretical work has invoked the importance of charge addition effects on conformation and electronic structure and polaronic effects to provide potential explanations for this behavior. We have investigated charge addition effects for pristine versus -NO$_{2}$ substituted OPE self-assembled monolayers using photoelectron spectroscopy in combination with ``doping'' with K. Results are consistent with differences arising from filling of levels associated with the - NO$_{2}$ group. We have investigated polaronic effects using photoelectron spectroscopy and optical absorption to guide calculations of reorganization energies for pristine versus -NO$_ {2}$ substituted OPE's. We find theoretical evidence for increased reorganization energy with -NO$_{2}$ substitution for anionic species by about 33 percent, with experimental values of the reorganization energy ranging from about 0.2 eV to 0.4 eV and theoretical values about 0.2 eV. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B44.00008: Theoretical Analysis of the Trends in Single Molecule Junction Conductance Formed Using Amine-Gold Links Mark Hybertsen, Michael Steigerwald, Latha Venkataraman, Colin Nuckolls The conductance of single molecule junctions using amine-gold links has now been measured for several families of molecules. Systematic trends are revealed with length, conjugation, conformation and substituents. The amine link group binds to undercoordinated gold atoms in the junction through a donor- acceptor type bond. The frontier orbitals that result are consistent with flexible and reproducible electronic coupling between the electrodes and the molecules. A simple proxy for the Au link site in each electrode allows direct calculation of the tunnel coupling through the molecule. The calculated trends in tunnel coupling are in excellent agreement with experiment within each family studied. Surprisingly, the trends between different families are also reproduced, albeit with a modest offset between families. The comparison between families is proposed to also involve small changes in the alignment of the gateway states controlling tunnel coupling with the electrode Fermi Energy. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B44.00009: Electronic transport through single molecules: effects of strain and contacts Helio Chacham, Ronaldo Batista, Mario Mazzoni, Ignacio Garzon, Marcela Beltran, Pablo Ordejon, Emilio Artacho We will present theoretical investigations on single-molecule electron transport. We will focus on the following systems: a) Connected Au nanoparticles: we performed a first principles study [1] of the electronic properties of lattices of Au nanoparticles functionalized by the conjugated molecules BDMT and BDCT. Distinct behaviors of the electron hopping matrix elements between particles as a function of compression are predicted for functionalized lattices. b) Current rectification with symmetric molecules: In an interesting experiment, Reichet et al. [2] measured the current through symmetric organic molecules and obtained asymmetric IV curves when the Au contacts are pulled apart. We show, by means of first-principles calculations, that this effect can originate from the formation of small Au chains between the molecule and the Au surfaces in an asymmetric way. [1] R. J. C. Batista et al, Phys. Rev. Lett. 96, 116802 (2006). [2] J. Reichet et al., Phys. Rev. Lett. 88, 176804 (2002). [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B44.00010: Controlled Fabrication and High-Resolution Imaging of Molecular-Scale Three-Terminal Devices Douglas R. Strachan, Danvers E. Johnston, Beth S. Guiton, T.-H. Park, M.J. Therien, Peter K. Davies, Dawn A. Bonnell, A.T. Charlie Johnson One of the biggest challenges to developing molecular-scale three terminal devices is to precisely fabricate and monitor the formation of the nanometer-scale electrodes (nanogaps). Recently, electromigrated nanogaps have been developed which provide sufficient gate-coupling to produce such devices. We have developed a technique for forming electromigrated nanogaps in a transmission electron microscope (TEM) in order to monitor their formation with high-resolution imaging in real time. The technique relies on computer-controlled electromigration using feedback to produce the nanogaps at room temperature. This TEM imaging allows us to monitor the dynamics of the device evolution, where the gaps remain ordered and clear of residue during the process. Using this technique, we find clear visual evidence for the importance of joule heating in the formation of electromigrated nanogaps. These electrodes can also be directly used in the construction of three terminal nanometer-scale devices. Our results have implications on the development of a wide range of novel molecular-scale devices. This work was supported by the National Science Foundation (NIRT Grant No. 0304531 and MRSEC award DMR05-20020). [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B44.00011: Movies of Electromigration During the Formation of Break Junctions Thiti Taychatanapat, K. I. Bolotin, F. Kuemmeth, D. C. Ralph Breaking metal wires by electromigration is a useful technique for making contacts for single-molecule devices. However, some research groups have found that a high percentage of gaps formed during electromigration (10-30{\%}) can contain metal nanoparticles which produce artifacts in the device's electrical characteristics that might be mistaken for molecular signals. Other groups, using slightly different electromigration protocols, observe these artifacts at much lower rates. Here we investigate this issue by examining the electromigration process in real-time using a scanning electron microscope. We provide direct confirmation for arguments that the amount of series resistance in the electromigration circuit is a critical parameter in controlling whether nanoparticles are formed within the device. By observing devices to which metal nanoparticles have been attached using linker molecules, we are also able to estimate the effective temperature experienced by molecular adsorbates during electromigration. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B44.00012: Formation of a Metallic Contact: Jump to Contact Revisited. C. Untiedt, M.J. Caturla, M.R. Calvo, J.J. Palacios, R.C. Segers, J.M. van Ruitenbeek The process of adhesion between two metallic surfaces has been described so far as involving an instability leading to a jump from tunneling into contact[1,2]. In the last decade some experiments have shown that this is not always the case, and sometimes the transition from tunnelling to metallic contact goes smoothly[3-5]. We have observed that the configuration and material composition of the electrodes before contacts largely determines the presence or absence of a jump. Moreover, when jumps are found preferential values of conductance have been identified. Through combination of experiments, molecular dynamics, and first-principles transport calculations these conductance values are identified with atomic contacts of either monomers, dimmers or double-bond contacts. These results provide basic understanding of fundamental interactions between surfaces at the nanoscale. [1]N. Agrait, et al. Phys. Rep {\bf 377},81 (2003) [2]U. Landman et al. Science {\bf 248}, 454 (1990) [3]G. Cross et al. Phys. Rev. Lett. {\bf 80}, 4685 (1998) [4]A. Halbritter et al. Phys. Rev. B. {\bf 68}, 035417 (2003) [5]L. Limot et al. Phys. Rev. Lett. {\bf 94}, 126102 (2005). [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B44.00013: High Quality Nanogap Electrodes for Electronic Transport Measurements of Single Molecules Danvers E. Johnston, Douglas R. Strachan, Beth S. Guiton, Peter K. Davies, Tae Hong Park, Michael J. Therien, A. T. Charlie Johnson Electromigrated metal electrodes and resulting devices have shown great promise in moving towards the realization of single molecule-based electronic devices holding the potential for a wide range of electronic applications. At present, a major concern is that the electronic behavior of such devices may be greatly influenced by residual nanoscale metal particles. We have developed a computer controlled electromigration (CCE) process for creating nanogaps at room temperature which allows us to characterize a bare nanogap before putting a molecule into the nanogap.$^{1}$ This is very different from other approaches used in the field where nanogaps are formed at low temperature with molecules already attached to the nanowire by employing a simple ramp up in voltage. Among the bare nanogaps we produced using CCE, tunneling behavior is observed with no indication of transport signatures associated with metal particle formation. Details of molecular measurements utilizing these clean gaps will be discussed. This work was supported by the National Science Foundation (NIRT Grant No. 0304531 and MRSEC award DMR05-20020). $^{1}$D. R. Strachan, D. E. Smith, D. E. Johnston et al., Appl. Phys. Lett. \textbf{86} 043109 (2005). [Preview Abstract] |
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