Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session X1: Magnetic Field Driven Quantum Phase Transitions
Sponsoring Units: DCMPChair: Joe Thompson, Los Alamos National Laboratory
Room: Colorado Convention Center Four Seasons 2-3
Friday, March 9, 2007 8:00AM - 8:36AM |
X1.00001: Field-tuned quantum critical points Invited Speaker: |
Friday, March 9, 2007 8:36AM - 9:12AM |
X1.00002: Dimensionality and Quantum Criticality: the case of the spin-dimer system BaCuSi$_2$O$_6$ Invited Speaker: Physical properties in the vicinity of a quantum critical point (QCP) are intimately related to system dimensionality. In heavy fermion systems, for instance, the origin of emergent superconductivity and non Fermi liquid behaviour at a pressure- tuned QCP has been suggested to be a consequence of reduced dimensionality. In this talk, I discuss the field-tuned Bose Einstein condensation (BEC) QCP in the spin-dimer system BaCuSi$_2$O$_6$ - also known as Han Purple. Long-range antiferromagnetic order in this system appears above a critical magnetic field $H_{c1}~\sim$~23.5 T due to the closure of the spin gap to triplon excitations. While the divergence of critical fluctuations tends to destabilise any reduction of dimensions at a fermionic QCP, contrary behaviour is demonstrated at the field-tuned bosonic QCP in BaCuSi$_2$O$_6$. The suppression of amplitude fluctuations at the BEC QCP in this system contributes to the effectiveness of geometrical frustration in reducing the effective dimensionality at the QCP. Experimental thermodynamic signatures are presented as evidence for a 2D BEC QCP in BaCuSi$_2$O$_6$. \newline \newline Work performed in collaboration with N. Harrison, C. D. Batista, L. Balicas, M. Jaime, P. A. Sharma, N. Kawashima, E. Palm, T. Murphy, and I. R. Fisher. \newline \newline [1] S. E. Sebastian et al., ``Dimensional reduction at a quantum critical point,'' Nature 441, 617-620 (2006) [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:48AM |
X1.00003: Fingerprints of Interacting Hardcore Bosons on a Lattice: Spin Dynamics in Dimer Spin Systems with Field-Tuned Quantum Criticality. Invited Speaker: Spin-dimer based magnetic insulators are model systems for the experimental and theoretical investigation of field-tuned quantum criticality and, in particular, the ground states of strongly interacting hardcore bosons (triplets), for which there are increasing parallels to ultra-cold atoms in optical lattices. We have investigated corresponding quantum phase transitions by inelastic neutron scattering (INS) in spin systems, which cover both the effect of dimensionality and the degree of quasi-particle mobility. These quantities characterize the triplet excitations and definite the magnon-`BEC' phases above the field-induced quantum critical point in these materials. Inorganic compounds like the 3D copper-halide family ACuCl$_{3}$ (A=K, Tl, NH$_{4})$, the strongly frustrated Shastry-Sutherland material SrCu$_{2}$(BO$_{3})_{2}$, and quasi-2D BaCuSi$_{2}$O$_{6}$ all show distinct spin dynamics associated with the boson system, which they represent. The fascinating quasi-1D limit is accessible in novel organic materials, which promote a characteristic quantum phase -- the Luttinger spin-liquid. Recent INS results, which explore this exciting quantum phase, will be compared to those obtained in higher dimensions and elaborate predictions by theory. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:24AM |
X1.00004: Field-induced magnetism and quantum criticality in superconducting CeRhIn$_{5}$ under pressure Invited Speaker: The antiferromagnet CeRhIn5 becomes superconducting under pressure, where superconducting state coexists with the helical magnetic state with Q=(0.5, 0.5, 0.293). Similarly to other heavy fermion superconductors, however, magnetism disappears when the antiferromagnetic transition temperature becomes equal to superconducting temperature, hiding a magnetic quantum critical point. Applying magnetic field reveals a low-temperature specific heat anomaly in the unconventional superconducting state, which defines a quantum phase transition from a solely superconducting state to a phase with coexisting magnetic and superconducting orders [1]. The field-pressure phase boundary at zero temperature is anticipated theoretically [2] and is strikingly similar to that in high-Tc cuprates [3], delineating a correlation between quantum criticality and unconventional superconductivity. \newline \newline [1] T. Park et al., Nature 440, 65 (2006) \newline [2] Q. Si et al., Nature 413, 804 (2001); E. Demler et al., Phys Rev. Lett. 87 067202 (2001) \newline [3] B. Lake et al., Nature 415, 299 (2002); H. J. Kang et al., Nature 423, 522 (2003) [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 11:00AM |
X1.00005: Magnetic-field quantum phase driven transitions in CeBiPt and CeCu$_{6-x}$Au$_{x}$ Invited Speaker: The half-Heussler compounds CeBiPt and LaBiPt are semimetals with very low charge-carrier concentrations as evidenced by Shubnikov-de Haas (SdH) and Hall-effect measurements. Elastic neutron-scattering results reveal a simple antiferromagnetic structure in CeBiPt below $T_{N}$ = 1.15 K. The band structure of CeBiPt sensitively depends on temperature, magnetic field, and stoichiometry. Above a certain, sample-dependent, threshold field ($B \quad >$ 25 T) the SdH signal disappears and the Hall coefficient reduces significantly. These effects are absent in the non-4$f$ compound LaBiPt. Electronic-band-structure calculations can well explain the observed behavior by a 4$f$-polarization-induced Fermi-surface modification. CeCu$_{6-x}$Au$_{x}$ orders for $x \quad >$ 0.1 with an incommensurate antiferromagnetic structure. Here we compare the magnetic fluctuation spectrum obtained from inelastic neutron scattering for a field-driven quantum phase transition at $x$ = 0.2 with that for zero-field transition at the critical concentration $x_{c}$ = 0.1. \newline \newline Work performed in collaboration with J. Wosnitza, G. Goll, A. D. Bianchi, B. Bergk, N. Kozlova, I. Ophale, S. Elgazzar, M. Richter, O. Stockert, T. Yoshino, T. Takabatake and M. Enderle. [Preview Abstract] |
Session X2: Understanding DNA and RNA machines
Sponsoring Units: DBPChair: Meredith Betterton, University of Colorado
Room: Colorado Convention Center Four Seasons 4
Friday, March 9, 2007 8:00AM - 8:36AM |
X2.00001: Invited Speaker: |
Friday, March 9, 2007 8:36AM - 9:12AM |
X2.00002: Invited Speaker: |
Friday, March 9, 2007 9:12AM - 9:48AM |
X2.00003: Hexameric DNA-based motor proteins Invited Speaker: Hexameric, ring-shaped motor proteins play important roles in a wide variety of cellular processes. They typically encircle a nucleic acid or protein substrate in order to perform a mechanical activity. Motivated by a desired to understand the advantages and peculiarities of this strikingly symmetric design, we have performed single-molecule measurements of several different DNA-based hexameric motors. I will discuss how, for the bacterial protein FtsK, the hexameric structure causes an anomalous coupling between rotation and linear motion. I will also present recent results in which we exploit the ability of certain hexameric helicases to encircle either single or double-stranded DNA in order to probe their activity in several different ways. Where relevant, I will introduce the novel magnetic-tweezer based measurement techniques we have devised for these experiments. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:24AM |
X2.00004: Stretching, Twisting, and Unzipping DNA Invited Speaker: DNA mechanics governs many essential cellular processes. During DNA replication, repair, recombination, and transcription, often a DNA double helix is unwound, its two strands are separated to expose the base sequence, and compensatory supercoils are generated in the remaining DNA. Our lab develops new single molecule optical trapping techniques to probe the mechanics of DNA as it is stretched, twisted, and unzipped. These approaches reveal interesting physical properties as well as permit direct investigation of the mechanisms of enzymes involved in these processes. I will discuss our recent work in these directions. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 11:00AM |
X2.00005: Invited Speaker: |
Session X3: DNA Organization within Chromosomes
Sponsoring Units: DCMPChair: Ralf Bundschuh, The Ohio State University
Room: Colorado Convention Center Korbel 2A-3A
Friday, March 9, 2007 8:00AM - 8:36AM |
X3.00001: Structure and Dynamics of the Kinetochore Invited Speaker: |
Friday, March 9, 2007 8:36AM - 9:12AM |
X3.00002: Structural Organization and Properties of DNA in the Cell Invited Speaker: The dynamical architecture of the cell nucleus can be regarded as one of the ``grand challenges'' of modern molecular and structural biophysics. The genomic DNA and the histone proteins compacting it into chromatin account for the major part of the contents of the nucleus. In my talk I will discuss the structural properties of the DNA from the 30nm fiber up the entire chromosome. For the 30nm range I present a model for the compactifiction and discuss the resulting phase diagram. I am going to reveal the fine-structure of the excluded-volume borderline. Furthermore, the effect of the Coulomb repulsion of the DNA linkers will be presented. Moving up further in the range (300-800nm), we will look at the entanglements of the polymer chain. On the level of the entire chromosome I will present a model for the chain that can successfully describe experimentally measured distance distributions on chromosome 1 in human cells using the notion of ``ridge'' regions (cluster of strongly expressed genes) and the ``antiridge'' regions. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:48AM |
X3.00003: Dynamics of Single Molecule DNA Invited Speaker: |
Friday, March 9, 2007 9:48AM - 10:24AM |
X3.00004: The Torsonal Rigidity of Single Chromatin Fibers Invited Speaker: |
Friday, March 9, 2007 10:24AM - 11:00AM |
X3.00005: Dynamics of Nucleosome Arrays Invited Speaker: DNA sites wrapped into chromatin are sterically occluded from proteins that must bind for processes such as RNA transcription and DNA repair. However, the role of chromatin compaction in biological function is poorly understood. To understand the biological functions of chromatin compaction, we constructed nucleosome arrays that are built with a tandem repeat of high affinity nucleosome positioning sequences, which contain probes for DNA accessibility and chromatin structure. I will describe our results that use restriction enzyme digestion and fluorescence resonance energy transfer to determine the probability for DNA site exposure within compacted nucleosome arrays and the time scale for changes in chromatin compaction. I will then discuss how these results help explain how proteins gain access to DNA sites buried within chromatin. [Preview Abstract] |
Session X4: Quantum Order in Chiral Magnets
Sponsoring Units: DCMPChair: Chandra Varma, University of California, Riverside
Room: Colorado Convention Center Korbel 2B-3B
Friday, March 9, 2007 8:00AM - 8:36AM |
X4.00001: Quantum order in chiral magnets: 3D Non-Fermi Liquid Phase and Blue Quantum Fog in MnSi Invited Speaker: The discovery of a distinct change from Fermi liquid to non-Fermi liquid resistivity and the observation of partial magnetic order in MnSi under high pressure [1,2] has generated great scientific interest in the properties of itinerant-electron systems with weak chiral spin-orbit interactions. Recent theoretical predictions include the spontaneous formation of a skyrmion phase at the boundary of conventional helical order [3] and the existence of a new type of Goldstone-like excitation, so called helimagnons [4]. New experimental work using sophisticated neutron scattering techniques and bulk properties exploring the question of skyrmion textures and helimagnon excitations, as well as studies of the thermal expansion under pressure using a newly developed ultra-high resolution neutron spin-resonance technique (Larmor diffraction) will be reviewed. \newline \newline [1] C. Pfleiderer, S. R. Julian, G. G. Lonzarich, Nature {\bf 414}, 427 (2001). \newline [2] C. Pfleiderer, et al., Nature {\bf 427}, 227 (2004). \newline [3] U. R{\"o}{\ss}ler, A. B. Bogdanov, C. Pfleiderer, Nature {\bf 442}, 797 (2006). \newline [4] D. Belitz, T. R. Kirkpatrick, A. Rosch, Phys. Rev. B {\bf 73}, 054431 (2006). [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 9:12AM |
X4.00002: Real Space Observation of Helical Spin Order Invited Speaker: When a symmetry gets spontaneously broken in a phase transition, topological defects are routinely formed. There are numerous examples of topological defects in condensed matter systems, such as, vortices in superconductors, vortices in superfluid helium, monopoles and strings in liquid crystals, etc. A similar picture would emerge in helimagnets. It is therefore interesting to deepen our understanding of how, what kind of, and why magnetic defects form and how they evolve after formation in helimagnets. In recent years, there have been significant advances in the experiment [1] and in the theories [2] of phases and textures in helimagnets. This will have a significant impact on our understanding of not only the puzzling behavior of the helimagnet MnSi with non-Fermi-liquid transport properties [3], but also phase transitions and phase diagrams in different condensed matter systems. \newpage In this paper, we describe the current status of our experiments. To see the helical spin order and magnetic defects in metal silicides such as (Fe, Co)Si and FeGe in real space, we used Lorentz electron microscopy, combined with the transport of intensity equation (TIE) analysis or holographic interference microscopy. This method has allowed us to find the topological defect similar to atomic dislocations in the crystal lattice. Furthermore, by applying magnetic fields, we directly observed the deformation processes of the helical spin order, accompanied by nucleation, movement, and annihilation of the magnetic defects. \newline \newline [1] M. Uchida \textit{et al.}, Science \textbf{311}, 359 (2006). \newline [2] U. K. R\"{o}{\ss}ler, A. N. Bogdanov, and C. Pfleiderer, Nature \textbf{442}, 797 (2006); B. Binz, A. Vishwanath, and V. Aji, Phys. Rev. Lett$.,$ \textbf{96}, 207202 (2006); S. Tewari, D. Belitz, and T. R. Kirkpatric, Phys. Rev. Lett., \textbf{96}, 47207 (2006). \newline [3] C. Pfleiderer \textit{et al}., Nature \textbf{427}, 227 (2004). [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:48AM |
X4.00003: Theory of the helical spin crystals Invited Speaker: Recent experiments in the ``partial order'' regime at high pressure in MnSi quite intriguingly suggest diffuse spin correlations and slow dynamics in a pure crystalline metal. As a starting point for a theoretical description of this phase, we are investigating the nature of its dominant spin correlations. Particularly, the observed location of maximal neutron scattering intensity around $\langle 110\rangle$ is difficult to explain in terms of fluctuating helical spin-density waves alone. We therefore investigate helical spin crystals. These are magnetic structures obtained by superimposing distinct spin spirals, via a process reminiscent of crystallization. Based on a phenomenological Landau description, we identify which spin crystal structures may be energetically stabilized and study their properties. One of these states, a bcc spin crystal, is compatible with existing data on MnSi from neutron scattering and magnetic field studies. It also shows new and interesting phenomena, such as symmetry stabilized topological textures, missing higher order Bragg reflections and an octupolar order parameter. Possible routes towards ``partial order,'' which requires the destruction of long-range order by some mechanism, will be briefly discussed. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:24AM |
X4.00004: Investigation of the Metallic State in Cubic FeGe beyond its Quantum Phase Transition Invited Speaker: FeGe and MnSi are prominent examples where the Dzyaloshinskii-Moriya interaction causes a modulation of the ferromagnetic structure as a consequence of the lack of inversion symmetry in the $B20$ structure (space group $P2_13 $). In FeGe, helimagnetism sets in through a first order phase transition at $T_{\rm C}=280\,$K with a saturated moment of $m=1 \mu_B$ per Fe atom. The helical modulation has a period of about $700\,$\AA\ and propagates along the spiral propagation vector ${\bf k} \parallel [1 0 0]$. It alters its direction to ${\bf k}\parallel [1 1 1]$ at $T_2\approx 211-245\,$K without a change in the period. In MnSi, however, the helical order occurs below $T_{\rm C}=29\,$K. The modulation has a wavelength of $175\,$\AA\ and the ordered moments of about $m=0.4\,\mu_B$ per Mn atom are perpendicular to ${\bf k}\parallel [1 1 1]$. It is well established that the second order phase transition is driven first order for a sufficiently weak magnetic interaction close to the critical pressure, $p_{\rm c}=1.46\,$GPa. In light of these structural and magnetic similarities between FeGe and MnSi, a volume compression in FeGe could tune its $T_C$ to zero temperature with the chance to reveal peculiar electronic ground state properties at the verge of the magnetic order. Indeed, the electrical resistivity measurements, $\rho(T) $, show a suppression of the helical order at $p_c\approx 19 \,$GPa. The strong deviations from a Fermi-liquid behavior in a wide pressure range above $p_c$ suggest that the suppression of $T_C$ disagrees with the standard notion of a quantum critical phase transition. Our band-structure calculations suggest that disorder due to zero-point motion is strong enough to close the narrow gap expected for compressed FeGe, stabilizing a new magnetic ground state above $p_c$. An anomaly observed at $T_X$ in the $\rho(T)$ curves recorded above $p_c$ might be related to this magnetic phase. The isothermal structural data at low temperature revealed a discontinuous change in the pressure dependence of the shortest Fe-Ge interatomic distance close to the $T_C(p)$ phase line. The $(T,V)$ phase diagram will be discussed and the connection with MnSi and the semiconducting properties of FeSi will be addressed. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 11:00AM |
X4.00005: Quantum PhaseTransitions and Exotic Phases in Metallic Helimagnets Invited Speaker: I will review some of the current theoretical understanding of the exotic properties of chiral magnets, in particular the metallic helimagnet MnSi. In the ordered phase, a helical Goldstone mode leads to corrections to Fermi-liquid behavior, and to a non-Fermi liquid single-particle relaxation rate [1]. On the phase boundary, a tricritical point pushes the quantum critical point to a nonzero external magnetic field, where the quantum critical behavior has been determined exactly [2]. In the disordered phase, an analogy with chiral liquid crystals suggests a first-order transition from a chiral liquid to a chiral gas as an explanation for neutron scattering data [3]. The observed non-Fermi-liquid transport behavior in the disordered phase [4] remains an open problem. \medskip\par\noindent [1] D. Belitz, T.R. Kirkpatrick, and A. Rosch, Phys. Rev. B {\bf 73}, 054431 (2006); Phys. Rev. B {\bf 74}, 024409 (2006). \smallskip\par\noindent [2] D. Belitz, T.R. Kirkpatrick, and J. Rollb{\"u}hler, Phys. Rev. Lett. {\bf 94}, 027205 (2005). \smallskip\par\noindent [3] S. Tewari, D. Belitz, and T.R. Kirkpatrick, Phys. Rev. Lett. {\bf 96}, 047207 (2006). \smallskip\par\noindent [4] C. Pfleiderer, S.R. Julian, and G.G. Lonzarich, Nature {\bf 414}, 427 (2004). [Preview Abstract] |
Session X5: Elasticity of Biological Membranes: From Physical Principles to Biological Processes
Sponsoring Units: DBPChair: Ranjan Mukhopadhyay, Clark University
Room: Colorado Convention Center Korbel 1A-1B
Friday, March 9, 2007 8:00AM - 8:36AM |
X5.00001: Biophysical studies of biological membranes Invited Speaker: |
Friday, March 9, 2007 8:36AM - 9:12AM |
X5.00002: The Beginning of the Ends: A Curvature-Mediated Mechanism for Localization of Lipids to Bacterial Poles Invited Speaker: In the past decade, intracellular fluorescence microscopy has fashioned a new appreciation for the diversity of ways in which proteins organize and segregate on bacterial membranes. Though some targeting anchors are known, cellular symmetry breaking ultimately requires molecular components that self-organize. We propose a novel equilibrium mechanism, based on the two-dimensional curvature of the membrane, for spontaneous lipid targeting to the poles and division site of rod-shaped bacterial cells. If one of the membrane components has a large intrinsic curvature, the geometrical constraint of the plasma membrane by the more rigid bacterial cell wall counteracts the attractive interaction between like lipids and leads to microphase separation. We find that the resulting clusters of high-curvature lipids are large enough to spontaneously and stably localize to the two cell poles and septal regions, and could have similar utility to lipid rafts as a stage for targeting proteins involved in a wide variety of biological processes. Recent evidence of localization of the phospholipid cardiolipin to the poles of bacterial cells suggests that protein targeting may depend on the membrane's heterogeneous lipid content. More generally, aggregates of lipids, proteins, and lipid-protein complexes may localize in response to features of cell geometry incapable of localizing individual molecules. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:48AM |
X5.00003: Fundamental physical mechanisms of membrane phase separation Invited Speaker: |
Friday, March 9, 2007 9:48AM - 10:24AM |
X5.00004: Membrane mechanics of nuclear division in yeast Invited Speaker: |
Friday, March 9, 2007 10:24AM - 11:00AM |
X5.00005: Fusion, Fission, and Membrane Microdomains Invited Speaker: In biology, curvature of intracellular membranes plays a key role in defining compartments to organize the interior of a cell and in creating the optimal shapes of organelles for function. We have studied how membrane curvature plays a crucial role in determining the energetics of membrane fusion in a number of systems. How the proteins that catalyze membrane fusion in cellular secretion and in viral fusion will be discussed in detail. Newer work on the role of proteins in the budding of viruses during assembly will be presented. The assembly of viruses also requires a concentration of viral protein components in the membrane. Recent experiments on cells expressing the influenza hemagglutinin show a clustering of proteins at many length scales. The dependence on cell cholesterol of this clustering (microdomain formation) will also be discussed. [Preview Abstract] |
Session X6: Networks in Genetic Regulation
Sponsoring Units: GSNP DBPChair: Erzsebet Ravasz, Los Alamos National Laboratory
Room: Colorado Convention Center 207
Friday, March 9, 2007 8:00AM - 8:36AM |
X6.00001: Check Point as Fixed point: Analysis of a Yeast Cell-Cycle Model Invited Speaker: The cell cycle regulation in the budding yeast \textit{Saccharomyces cerevisiae} is one of the best studied biological systems. Many major players and their interactions have been identified by decades of work in genetics and biochemistry as well as by the more recent effort in high throughput genomics and proteomics. On the other hand, current information about the network is mostly qualitative---while there is a circuit diagram (although it may not be complete) of who regulates whom, there is little quantitative information (e.g. the kinetic constants) about the regulation. Here we construct a model of yeast cell-cycle regulation from the known circuit diagram using ordinary differential equations and focus our attention on the global dynamic property and structural stability of the system. We found that certain qualitative conclusions about the system's behavior are very robust to parameter choices. In particular, each checkpoint can be a global attractor---when a checkpoint is on \textit{all} cell states evolve to the stationary state corresponding to the checkpoint arrest. Furthermore, there is a unique globally attracting trajectory for this dynamic system, which corresponds to the biological pathway of the cell cycle regulation. Substantial changes of certain parameters, especially when several parameters are changed simultaneously, can result in qualitative changes in the system's behavior. Typically, these not-so-robust parameters are associated with transitions between different cell-cycle phases and the corresponding abnormal behavior is often related to the arrest or bypass of a checkpoint. Our results reveal a robust picture of the yeast cell cycle regulation and the mechanisms under which the robustness can be compromised. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 9:12AM |
X6.00002: Boolean modeling of cellular regulatory networks Invited Speaker: Interaction between gene products forms the basis of essential processes like signal transduction, cell metabolism or embryonic development. Recent experimental advances helped uncover the structure of many cellular networks, creating a surge of interest in the dynamical description of gene regulation. Traditionally genetic and protein interactions are modeled by differential equations based on reaction kinetics, but these studies are greatly hampered by the sparsity of known kinetic detail. As an alternative, qualitative models assuming a small set of discrete states for gene products, or combinations of discrete and continuous dynamics, are gaining acceptance. Many results also suggest that the interaction topology plays a determining role in the dynamics of regulatory networks and there is significant robustness to changes in kinetic parameters. This presentation will focus on a Boolean model of the signal transduction network regulating drought response in plants. We integrate qualitative and indirect relationships into the simplest network consistent with all experimental observations, and express the regulation of network nodes as logical functions. Our model captures the regulation of more than forty identified network components, and accords well with previous experimental results at both the pathway and whole cell physiological level. We identify the dynamical repertoire of the network by varying process durations and initial conditions and by simulating gene disruptions, and find a remarkable robustness against a significant fraction of possible perturbations. Although qualitative, the model provides a ranking of disruptions and perturbations in the order of their severity. We experimentally test, and validate, the most surprising prediction. The success of this model illuminates the emergent (network-level) functional robustness of cellular regulatory networks. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:48AM |
X6.00003: Symmetry and the Self-Organized Evolution of Canalization in Boolean Networks Invited Speaker: Canalization of genetic regulatory networks have been argued to be favored by evolutionary processes due to the stability that it can confer to phenotype expression. Using an N-K Boolean network model of a genetic regulatory network, we explore whether a significant amount of canalization can arise in purely random networks in the absence of evolutionary pressures. We use a mapping of the Boolean functions in the Kauffman N-K model for genetic regulatory networks onto a k-dimensional Ising hypercube to show that the functions can be divided into different classes strictly due to geometrical constraints. The classes can be counted and their properties determined using results from group theory and isomer chemistry. We demonstrate that partially canalized functions completely dominate all possible Boolean functions, particularly for higher K. This indicates that partial canalization is extremely common, even in randomly chosen networks, and has implications for how much information can be obtained in experiments on native state genetic regulatory networks. Furthermore, we demonstrate that a highly canalized state evolves spontaneously from a competition between the nodes. Network finite-size effects are found to be important to that evolutionary process. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:24AM |
X6.00004: Quantitative aspects of gene regulation by small RNAs Invited Speaker: Small, non-coding RNAs (sRNAs) play an important role as genetic regulators in both prokaryotes and eukaryotes. Many sRNAs act through base-pairing interaction with target messenger RNAs (mRNAs) to regulate transcription, translation, and mRNA stability. sRNAs represent a novel form of genetic regulation distinct from more thoroughly studied protein regulators. This talk addresses quantitative aspectsof sRNA-mediated genetic regulation, focusing on noise, tunability, and feedback. In particular, we compare and contrast sRNA and protein regulators in an attempt to understand the compartive advantages of each form of regulation. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 11:00AM |
X6.00005: Gene regulatory networks: what is still missing? Invited Speaker: Gene regulatory networks have evolved to respond to a changing environment, serving the survival of the biological population. The topology of these networks has been investigated with the hope of gaining insight into their function or identifying the factors shaping their evolution. Recent studies have shown that gene regulatory networks have different in-degree and out-degree distribution, contain network motifs and are organized in a hierarchical set of layers. However, important pieces of information are still needed before the topological features of these networks can be correctly determined and their response to environmental changes can be modeled at increasingly large scale. [Preview Abstract] |
Session X7: Computational Nonequilibrium Many-body Physics: From Classical to Quantum Simulation
Sponsoring Units: DCOMPChair: David Langreth, Rutgers University
Room: Colorado Convention Center Korbel 4A-4B
Friday, March 9, 2007 8:00AM - 8:36AM |
X7.00001: Short-time dynamics of correlated quantum Coulomb systems Invited Speaker: Strong correlations in dense Coulomb systems are attracting increasing interest in many fields ranging from dense astrophysical plasmas, dusty plasmas and semiconductors to metal clusters and ultracold trapped ions [1]. Examples are bound states in dense plasmas (atoms, molecules, clusters) and semiconductors (excitons, trions, biexcitons) and many-particle correlations such as Coulomb and Yukawa liquids and crystals. Of particular current interest is the response of these systems to short excitations generated e.g. by femtosecond laser pulses and giving rise to ultrafast relaxation processes and build up of binary correlations. The proper theoretical tool are non-Markovian quantum kinetic equations [1,2] which can be derived from Nonequilibrium Green's Functions (NEGF) and are now successfully solved numerically for dense plasmas and semiconductors [3], correlated electrons [4] and other many-body systems with moderate correlations [5]. This method is well suited to compute the nonlinear response to strong fields selfconsistently including many-body effects [6]. Finally, we discuss recent extensions of the NEGF-computations to the dynamics of strongly correlated Coulomb systems, such as single atoms and molecules [7] and electron and exciton Wigner crystals in quantum dots [8,9]. \newline \newline [1] H. Haug and A.-P. Jauho, {\em Quantum Kinetics in Transport and Optics of Semiconductors}, Springer 1996; M. Bonitz {\em Quantum Kinetic Theory}, Teubner, Stuttgart/Leipzig 1998; \newline [2] {\em Progress in Nonequilibrium Green's Functions III}, M. Bonitz and A. Filinov (Eds.), J. Phys. Conf. Ser. vol. 35 (2006); \newline [3] M. Bonitz et al. Journal of Physics: Condensed Matter {\bf 8}, 6057 (1996); R. Binder, H.S. K\"ohler, and M. Bonitz, Phys. Rev. B 55, 5110 (1997); \newline [4] N.H. Kwong, and M.~Bonitz, Phys. Rev. Lett. {\bf 84}, 1768 (2000); \newline [5] {\em Introduction to Computational Methods for Many-Body Systems}, M. Bonitz and D. Semkat (eds.), Rinton Press, Princeton (2006); \newline [6] H. Haberland, M. Bonitz, and D. Kremp, Phys. Rev. E {\bf 64}, 026405 (2001); \newline [7] N.E. Dahlen, A. Stan and R. van Leeuwen, p. 324 in Ref. 2.; \newline [8] A. Filinov, M. Bonitz, and Yu. Lozovik, Phys. Rev. Lett. {\bf 86}, 3851 (2001); \newline [9] K. Balzer, N.E. Dahlen, R. van Leeuwen, and M. Bonitz, to be published [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 9:12AM |
X7.00002: Simulation of the Interaction of Intense Laser Pulses with Dense Plasma Invited Speaker: For some time now the interaction of intense laser beams with dense plasma has generated interest not only because of its relevance for the Fast Ignition concept in the field of Inertial Confinement Fusion (ICF) [1], but also due to the many fundamental physics problems related to it like laser energy deposition in plasma, the transport of the deposited energy via fast electrons or the propagation of ultra-high electric currents through plasma and many more. Of great interest at present are the details of the deposition and the transport of the energy of intense laser pulses in plasma. Of great importance in this context are collisions and collective effects. The proper equations are a set of classical relativistic Maxwell-Vlasov-Boltzmann equations. They are solved numerically with a Monte-Carlo Particle-In-Cell (MCPIC) [2] approach in three spatial dimensions. This quasi-particle method is capable of calculating effects as diverse as the degree of laser absorption in plasma, the generation of fast electrons, the relaxation of laser-generated non-Maxwellian electron and ion distribution functions due to collective effects and binary collisions, the propagation of electron driven heat waves into the plasma, or the generation of vast quasi-steady electric and magnetic fields. Details of the MCPIC-method applicable to systems of intense laser radiation interacting with plasma are presented. The application of the method to the acceleration of protons with intense lasers featuring collisional transport of fast electrons through solid density plasma and the excitation of teravolt electric fields is demonstrated [3]. \newline \newline [1] M. Tabak, J. Hammer, M. E. Glinski, W. L. Kruer, S. C. Wilks, and R. J. Mason, ``Ignition and high gain with ultrapowerful lasers,'' Phys. PLasmas \textbf{1}, 1626 (1994). \newline [2] ``Introduction to Computational Methods for Many-Body Systems,'' M. Bonitz and D. Semkat (eds.), Rinton Press, Princeton (2006) \newline [3] B. M. Hegelich, B. J. Albright, J. Cobble, K. Flippo, S. Letzring, M. Paffett, H. Ruhl, J. Schreiber, R. K. Schulze and J. C. Fern\'andez, ``Laser acceleration of quasi-monoenergetic MeV ion beams,'' Nature \textbf{439}, 441 (2006). [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:48AM |
X7.00003: Nonequilibrium dynamical mean-field theory Invited Speaker: Dynamical mean-field theory (DMFT) is establishing itself as one of the most powerful approaches to the quantum many-body problem in strongly correlated electron materials. Recently, the formalism has been generalized to study nonequilibrium problems [1,2], such as the evolution of Bloch oscillations in a material that changes from a diffusive metal to a Mott insulator [2,3]. Using a real-time formalism on the Kadanoff-Baym-Keldysh contour, the DMFT algorithm can be generalized to the case of systems that are not time-translation invariant. The computational algorithm has a parallel implementation with essentially a linear scale up when running on thousands of processors. Results on the decay of Bloch oscillations, their change of character within the Mott insulator, and movies on how electrons redistribute themselves due to their response to an external electrical field will be presented. In addition to solid-state applications, this work also applies to the behavior of mixtures of light and heavy cold atoms in optical lattices. \newline \newline [1] V. M. Turkowski and J. K. Freericks, Spectral moment sum rules for strongly correlated electrons in time-dependent electric fields, Phys. Rev. B {\bf } 075108 (2006); Erratum, Phys. Rev. B {\bf 73}, 209902(E) (2006). \newline [2] J. K. Freericks, V. M. Turkowski , and V. Zlati\'c, Nonlinear response of strongly correlated materials to large electric fields, in {\it Proceedings of the HPCMP Users Group Conference 2006, Denver, CO, June 26--29, 2006} edited by D. E. Post (IEEE Computer Society, Los Alamitos, CA, 2006), to appear. \newline [3] J. K. Freericks, V. M. Turkowski, and V. Zlati\'c, Nonequilibrium dynamical mean-field theory, {\it submitted to Phys. Rev. Lett.} cond-mat//0607053. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:24AM |
X7.00004: Signal transport and finite bias conductance in and through correlated nanostructures Invited Speaker: The problem of calculating the finite bias conductance through an interacting system has been formally solved by Meir and Wingreen using non-equilibrium Green function techniques [1]. In practice, the evaluation of these formulas for interacting systems is generally based on approximative schemes. Time dependent density matrix renormalization group methods (t-DMRG) [2] allow for an exact solution of the time-dependent evolution of many-body wavefunctions. We apply this technique to the problem of calculating the differential conductance of a strongly correlated nanostructure attached to one-dimensional noninteracting leads. By carefully monitoring the finite-size effects and the time-dependent dynamics, the differential conductance can be extracted from the t-DMRG results [3]. This talk will give an introduction to t-DMRG and its application to the calculation of transport properties. We present examples of signal propagation through interacting systems and how the linear and differential conductance varies for interacting systems tuned from weak to strong coupling. Refs: [1] Y. Meir and N. S. Wingreen, Phys. Rev. Lett. 68, 2512 (1992). [2] K. A. Hallberg, Adv. Phys. 55,, 477 (2006) and references therein. [3] G. Schneider and P. Schmitteckert, Conductance in strongly correlated 1D systems: Real-Time Dynamics in DMRG, condmat/0601389. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 11:00AM |
X7.00005: Calculations of molecular electronics with the DFT/NEGF method Invited Speaker: |
Session X8: Focus Session: Novel Superconductors VII: Triplet Pairing and Time Reversal Symmetry Breaking
Sponsoring Units: DMPChair: Igor Mazin, Naval Research Laboratory
Room: Colorado Convention Center Korbel 1C
Friday, March 9, 2007 8:00AM - 8:36AM |
X8.00001: High Resolution Polar Kerr Effect Measurements of Sr$_2$RuO$_{4}$: Evidence for Broken Time Reversal Symmetry in the Superconducting State Invited Speaker: Strontium ruthenate ({Sr$_2$RuO$_{4}$}) is an odd-parity superconductor, which has odd orbital angular momentum and symmetric spin-triplet (p-wave) pairing. Some of the possible p- wave states can further break time-reversal symmetry (TRS), since the condensate has an overall magnetic moment because of either the spin or orbital (or both) parts of the pair wave function. However, this TRS-breaking moment will be screened by the Meissner effect. Previously, tests for broken time- reversal symmetry in {Sr$_2$RuO$_{4}$} relied on surfaces and defects where the Meissner screening is not perfect. However, for an unambigous determination of this effect, a bulk measurement, such as measuring magneto-optic like effects, on high quality crystals, is needed. To this end we developed a new technique of measuring Polar Kerr Effect (PKE) at temperatures much below the transition temperature of {Sr$_2 $RuO$_{4}$} of 1.5 K. The technique is based on a fiber Sagnac interferometer with a zero-area Sagnac loop. This new technique allowed us to measure PKE with an accuracy of 10 nano-radian at 400 mK, while rejecting other artifacts like linear birefringence of the sample. The incident optical power was set to be below 2 micro-Watts in order not to heat up the sample locally at such low temperatures. We have observed non-zero Kerr rotations as big as 65 nanorad appearing below $T_c$ in domains comparable in size with the 25-micron-diameter optical beam. Our results imply a broken time reversal symmetry state in the superconducting state of {Sr$_2$RuO$_{4}$}, similar to $^3$He-A. More recent results on other oxide superconductors will also be described. This work was supported by Center for Probing the Nanoscale, NSF NSEC Grant 0425897 and by the Department of Energy grant DEFG03-01ER45925. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X8.00002: Theory of the high-frequency chiral optical response in a $p_x+ip_y$ superconductor Victor Yakovenko The optical Hall conductivity and the polar Kerr angle are calculated as functions of temperature for a two-dimensional chiral $p_x+ip_y$ superconductor, where the time-reversal symmetry is spontaneously broken. The theoretical estimate for the polar Kerr angle agrees by the order of magnitude with the recent experimental measurement in $\rm Sr_2RuO_4$ by Xia et al.,\ Phys.\ Rev.\ Lett.\ {\bf 97}, 167002 (2006). The theory predicts that the Kerr angle is proportional to the square of the superconducting energy gap and is inversely proportional to the cube of frequency, which can be verified experimentally.\\ Reference: cond-mat/0608148. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X8.00003: Instability of Singlet Superconductivity with Respect to the Appearance of a Triplet Component in a Vortex Phase. Andrei Lebed, Omjyoti Dutta We show [1] that a vortex phase in a singlet d(s)-wave superconductor is absolutely unstable with respect to a generation of a triplet component of a superconducting order parameter. The triplet component, which appears for both attractive and repulsive interactions in a triplet channel, is shown to break three important symmetries of an internal superconducting order parameter: spin-rotational, parity [1], and time-reversal [2] ones. As a result, Cooper pairs are characterized by non-zero angular momenta [2] and non-zero spins [1], polarized in a plane, perpendicular to the external magnetic field. The above mentioned effects are expected to be of the order of unity in almost all modern superconductors such as MgB$_{2}$, high-Tc, organic, and some others. [1] A.G. Lebed, Phys. Rev. Lett. \textbf{96}, 037002 (2006). [2] Omjyoti Dutta and A.G. Lebed, Nature, submitted (2006). [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X8.00004: Broken time-reversal symmetry in a vortex phase of a superconductor under perpendicular magnetic field. Omjyoti Dutta, Andrei Lebed A vortex phase in a singlet superconductor is absolutely unstable with respect to a generation of a triplet component of a superconducting order parameter. The triplet component, which appears for both attractive and repulsive interactions in a triplet channel, breaks spin-rotational and parity symmetries[1]. Here we show[2] that in a perpendicular magnetic field, in addition to the above mentioned symmetries, the order parameter also breaks time reversal symmetry. As a result, Cooper pairs carry non-zero angular momenta. The above mentioned effects are expected to be of the order of unity in almost all modern superconductors such as high-Tc and organic ones. We suggest experimental studies to discover triplet-singlet mixing phenomenon which characterizes this novel type-IV superconductivity. [1] A. G. Lebed, Phys. Rev. Lett. 96, 037002. [2]O. Dutta and A. G. Lebed, Nature, submitted (2006) [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X8.00005: Aligning chiral order parameter domains in Sr2RuO4: Josephson interferometry measurements Francoise Kidwingira, J.D. Strand, D.J. Van Harlingen, Y. Maeno There is compelling evidence that the ruthenate superconductor Sr2RuO4 forms chiral order parameters of the form px+ipy and px-ipy. In zero magnetic field, these states are degenerate and result in the formation of a dynamical domain structure that has been detected by Josephson interferometry experiments [1]. However, the degeneracy between the order parameters can be lifted by applying a magnetic field while cooling the system through the superconducting transition. We present Josephson interferometry measurements on field-cooled Josephson junctions that show evidence for domain alignment, manifested by the enhancement of the critical current and qualitative changes of the critical current modulation pattern in applied magnetic field. We also report evidence for memory effects in the domain chirality. [1] Francoise Kidwingira \textit{et al}., \textit{Science}, October 26 2006 (10.1126/science.1133239). [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X8.00006: Probing Bound States in p-wave Superconductors using Shot Noise C.J. Bolech, Eugene Demler The zero-energy bound states at the edges or vortex cores of chiral p-wave superconductors are expected to behave like Majorana fermions. We introduce a model Hamiltonian that describes the tunnelling process when electrons are injected into such states. Using a non-equilibrium green function formalism, we find exact analytic expressions for the tunnelling current and noise and identify experimental signatures of the Majorana nature of the bound states to be found in the shot noise. We discuss the results in the context of different candidate materials that are believed to support triplet superconductivity. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X8.00007: Skyrmion Flux Lattices in $p\,$-wave Superconductors Qi Li, John Toner, Dietrich Belitz In $p\,$-wave superconductors, topological excitations known as skyrmions are allowed, in addition to the usual vortices. In strongly type-II materials in an external magnetic field, a skyrmion flux lattice is expected to be energetically favored compared to a vortex flux lattice [1]. We analytically calculate the energy, magnetization curves ($B(H)$), and elasticity of skyrmion flux lattices in $p\,$-wave superconductors near the lower critical field $H_{c1}$, and use these results with the Lindemann criterion to predict their melting curve [2]. In striking contrast to vortex flux lattices, which {\it always} melt at an external field $H > H_{c1}$, skyrmion flux lattices {\it never} melt near $H_{c1}$. This provides a simple and unambiguous test for the presence of skyrmions. In addition, the internal magnetic field distributions (which are measurable by muon spin rotation techniques [3]) of skyrmion and vortex lattices are very different. \medskip\par\noindent [1] A. Knigavko, B. Rosenstein, and Y.F. Chen, Phys. Rev. B {\bf 60}, 550 (1999). \par\noindent [2] Qi Li, John Toner, and D. Belitz, cond-mat/0607391 \par\noindent [3] J.E. Sonier, J. Phys. Cond. Matt. {\bf 16}, S4499 (2004) [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X8.00008: Probing the superconducting order parameter of UPt3 by Josephson Interferometry J.D. Strand, F. Kidwingira, D.J. Van Harlingen, J.P. Davis, W.P. Halperin The unconventional superconductor UPt3 exhibits two superconducting transitions which are believed to correspond to two distinct superconducting phases. The symmetry of the order parameter in these phases has yet to be determined and the origin of the double transition remains an open question. We have fabricated Josephson junctions by evaporating copper and lead films onto UPt3 single crystals. Using a SQUID potentiometer, we observe critical currents that onset at the upper transition temperature of the crystal ($\sim $0.55K) and increase as the temperature is lowered. We are studying the magnetic field dependence of the critical current in edge and corner junctions to obtain direct information about the phase anisotropy of the order parameter and hence its pairing symmetry. We are particularly interested in measurements near the lower transition temperature ($\sim $0.50K) at which the order parameter is predicted to become complex and hence break time-reversal symmetry. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X8.00009: High Resolution Polar Kerr Effect Measurements of High-Temperature Superconductors: Evidence for Broken Time Reversal Symmetry Below the Pseudogap temperature Elizabeth Schemm, Jing Xia, Wolter Siemons, Gertjan Koster, Martin M. Fejer, Aharon Kapitulnik High resolution Polar Kerr Effect (PKE) measurements were performed on YBa$_2$Cu$_3$O$_{7-\delta}$ as a function of temperature for variety of doping levels. In order be able to measure effects beyond our old search for anyon superconductivity, we devised a new technique based on a fiber Sagnac interferometer with a zero-area Sagnac loop. With this technique we show a shotnoise-limited sensitivity of 100 nanorad/$\sqrt{Hz}$ with incident photon power of $\sim$ 10 $\mu$-Watt, in a wide temperature range from 0.3 K to room temperature. Our results indicate that a Time Reversal Symmetry Breaking (TRSB) signal appears in all underdoped YBa$_2$Cu$_3$O$_{7-\delta}$~ samples below the doping dependent pseudogap temperature. The effect increases with decreasing temperature and seems to saturate at a lower temperature close to $T_c$. The saturated size of the effect increases with increasing $\delta$. We will discuss possible origins of this effect and its presence in other high-Tc superconductors. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X8.00010: Scanning magnetic imaging of strontium ruthenate (Sr$_{2}$RuO$_{4})$ Clifford Hicks, Yoshiteru Maeno, Kathryn Moler Strontium ruthenate is a spin-triplet superconductor with, very likely, a time-reversal symmetry breaking $p_{x}\pm $\textit{ip}$_{y}$ orbital order parameter. This is suggested by several experiments, including recent observation of a Kerr effect that develops when Sr2RuO4 becomes superconducting. Such an order parameter should result in spontaneous edge and domain wall currents which would generate a real-space magnetic signal, but this field yet to be observed. Currently it is estimated that, within the sample and near an edge (in the bulk it is Meissner screened), the field should peak at about 6G. We believe the spontaneous edge field may actually be significantly smaller. We discuss its observability through scanning magnetic probe microscopy and describe current experimental efforts to image and measure this field. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X8.00011: Photoemission study of doping in the strontium ruthenate family Tim Kidd, Tonica Valla, John Rameau, Peter Johnson How dopants are incorporated into low dimensional correlated electron systems and their effects on such materials remain open questions despite an intense research effort over the past few decades. We have used photoemission spectroscopy to investigate the effects of dopants in these systems to measure their influence on electronic properties and phase transitions. In systems such as the high-Tc superconductors, disorder in the spatial distribution of dopants leads directly to nanoscale electronic disorder in the system. Despite this seeming randomness, photoemission studies have shown the electronic states become more well-defined with increasing concentration of dopants. In these systems, however, the dopants are usually incorporated outside the copper oxygen planes important for conduction and superconductivity. Here, we present photoemission data from the strontium ruthenate family that incorporate dopants directly into the conducting planes. Our results indicate that even relatively high dopant concentrations do not necessarily cause a large degree of disorder based broadening in the spectra. We have also detailed the influence of dopants like titanium on the associated magnetic and superconducting phase transitions in these materials. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X8.00012: Mixed pairing state in Ru microdomains embedded in bulk single-crystal Sr$_2$RuO$_4$ Zhenyi Long, Kelly McCarthy, Chrysafis Andreou, David Caven, Zhiqiang Mao, Hiroshi Yaguchi, Yoshiteru Maeno, Ying Liu We performed detailed tunneling measurements on Ru microdomains embedded in Sr$_2$RuO$_4$ using In-Ru/Sr$_2$RuO$_4$ junctions prepared by pressing freshly cut pure In wire onto a cleaved $ab$ face of a Ru-containing Sr$_2$RuO$_4$ single crystal. We observed a superconducting transition temperature of 0.45 K and a $T$ = 0 superconducting energy gap of 0.10 meV that are associated with the Ru microdomains. The systematic behavior observed in the tunneling spectra, including the presence of a Josephson coupling between In and Ru, a zero bias conductance peak (ZBCP), an unusually large magnitude and the temperature and magnetic field dependences of the superconducting energy gap, suggests that both $s$- and $p$-wave pairings are present simultaneously in the interior of a Ru microdomain. We propose the existence of a novel superconducting state featuring the mixing of the $s$- and the $p$-wave pairings in this unique superconducting system and discuss the properties of this mixed pairing state. [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X8.00013: Influence of Hydrostatic Pressure on Magnetoacoustic Properties of Sr$_{2}$RuO$_{4}$. O. Svitelskiy, S. Headley, S. Tozer, E. Palm, T. Murphy, A. Suslov, D. Shulyatev The interest in the nanolayered Sr$_{2}$RuO$_{4}$ crystal is due to its unconventional spin-triplet superconductivity. We used ultrasonic pulse-echo technique to obtain further insight into its electronic structure. We investigated longitudinal 30-300 MHz sound wave propagating in [100] direction of the single crystal Sr$_{2}$RuO$_{4}$, grown by floating zone technique, at temperatures down to 300 mK, in magnetic fields up to 18 Tesla, and under hydrostatic pressures up to 7.3 kbar. The decent quality of the material was confirmed by the T$_{c}$ value of 1.25 K. Under the hydrostatic pressure, T$_{c}$ and H$_{c2}$ decreased, the superconducting transition width increased, and the sound speed increased. Data analysis allowed us to separate electron and phonon contributions in the sound attenuation. [Preview Abstract] |
Session X9: Superconductivity: Electronic Transport Measurements of Copper Oxide Superconductors
Sponsoring Units: DMPChair: Mike Osofsky, Naval Research Laboratory
Room: Colorado Convention Center Korbel 1D
Friday, March 9, 2007 8:00AM - 8:12AM |
X9.00001: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 8:12AM - 8:24AM |
X9.00002: Quantum superconductor-to-insulator transition in 2D $Y_{1-x}Ca_xBa_2Cu_3O_{7-\delta}$ films Iulian Hetel, Thomas Lemberger This study investigates the relationship between the critical temperature, $T_C$, and the zero temperature superfluid density, $n_s(0)$, close to the quantum superconductor-to- insulator transition in 2D cuprate films. We use a two-coil technique, at frequencies up to 50 kHz, to measure the temperature dependence of superfluid density in severely underdoped $Y_{1-x}Ca_xBa_2Cu_3O_{7-\delta}$ films, as thin as 1-2 unit cells and with transition temperatures as low as 3 K. Superconducting films are grown by pulsed laser deposition on $SrTiO_3$ substrates, with thin insulating $PrBa_2Cu_3O_{7- \delta}$ protecting the film above and below. The zero temperature superfluid density in these films is comparable to values measured in thicker $Y_{1-x}Ca_xBa_2Cu_3O_{7-\delta}$ films, which suggests continuous superconducting layers. In 1-2 unit cell thick films, we find $T_C \propto n_s(0)$, which is expected in 2D. This result is different from the approximate $T_c \propto n_s(0)^{1/2}$ dependence previously reported in $YBa_2Cu_3O_{7-\delta}$ films and crystals and confirmed by our measurements on thicker $Y_{1-x}Ca_xBa_2Cu_3O_ {7-\delta}$ films. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X9.00003: Exploring the critical dynamic exponent using YBCO films and untwinned crystal in zero and non-zero magnetic field Su Li, Hua Xu, Steven M. Anlage, C. J. Lobb The phase transition in high Tc superconductors in zero and non-zero magnetic field has been intensely studied. However, there are debates on the critical dynamic exponent z from both simulations and experiments. We will report our result on z from our dc transport measurement in both YBCO films and untwinned crystals. Our results show that the finite size effects, which can cause misinterpretation in the film data, are absent in the crystal data, and the data on both the films and crystals give a very similar result for the critical dynamic exponent. (This work was supported by NSF grant number DMR-0302596) [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X9.00004: Upper critical field in electron-doped Pr$_{2-x}$Ce$_{x}$CuO$_{4-\delta }$ in parallel magnetic fields Pengcheng Li, F.F. Balakirev, R.L. Greene We report a comprehensive study of the resistive superconducting transition in the electron-doped Pr$_{2-x}$Ce$_{x}$CuO$_{4-\delta }$ films down to 1.5K for magnetic field up to 58T applied parallel to the conducting ab-planes. We find that the parallel critical field (H$_{c2//ab})$ exceeds 58T for underdoped and optimally doped films. For the overdoped films, 58T is sufficient to suppress the superconductivity. An H$_{c2//ab}$ -T phase diagram is established. A comparison between our experimental results and theories for orbital and spin pairbreaking effects will be presented. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X9.00005: $T_c$ suppression and resistivity in cuprates with out-of-plane defects Siegfried Graser, Thomas Dahm, Peter J. Hirschfeld, Lingyin Zhu The suppression of the critical temperature due to isotropic impurity scattering in a $d$-wave superconductor is expected to be described by the Abrikosov-Gor'kov formula. However recent experiments on cuprate superconductors with out-of-plane defects show a nearly linear decrease of the critical temperature as a function of the residual resistivity in contradiction to the Abrikosov-Gor'kov result. Other experiments suggest further that the $T_c$ suppression is stronger for out-of-plane than for in-plane impurities. Both results can be explained assuming that elastic forward scattering is the dominant scattering process for out-of-plane disorder. We present a simple model of forward scattering allowing an analytical solution that already includes the key features of $T_c$ suppression for out-of-plane defects. We also verify the intuitive result of this simple model by comparing it to numerical calculations assuming randomly distributed out- of-plane defects and a realistic band structure appropriate for YBCO. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X9.00006: c-axis magnetotransport and noise in underdoped La$_{2-x}$Sr$_x$CuO$_4$ I. Rai\v{c}evi\'{c}, J. Jaroszy\'{n}ski, D. Popovi\'{c}, G. Jelbert, C. Panagopoulos, T. Sasagawa We report a study of c-axis magnetotransport and noise on high quality single crystals of La$_{2-x}$Sr$_x$CuO$_4$ ($x=0.03$). The measurements were performed at temperatures 0.110$\leq T$(K) $\leq 50$ and fields $0\leq B$(T)$\leq 18$ parallel and perpendicular to the c axis. Our experiments have revealed for the first time a number of glassy features in the charge response at very low $T$, such as memory effects and history dependence. In the same $T$ range, we have observed positive magnetoresistance (MR), which exhibits hysteretic behavior. The hysteretic effects decrease with increasing $T$ and vanish at $\sim1.5$~K. The crossover from positive to negative MR takes place at higher $T$ and $B$. We have also observed switching fluctuations in the time-dependent resistance at the lowest $T$, with switching times varying from several minutes to several hours. The possible origins of the observed glassiness will be discussed. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X9.00007: Pulsed laser deposition growth and transport studies of superconducting La$_{1.85}$Y$_{0.15}$CuO$_{4}$ thin films Weiqiang Yu, B. Liang, P. Li, R. L. Greene, S. Fujino, T. Murakami, I. Takeuchi The recent MBE growth of the ``nominally undoped" cuprate superconductor (La,RE)$_{2}$CuO$_{4}$ has led to the speculation that this system is a band metal and not a ``doped Mott insulator" as found in all other superconducting cuprates $^1$. Here we report the first pulsed laser deposition (PLD) growth of insulating and superconducting La$_{1.85}$Y$_{0.15} $CuO$_{4}$ thin films, which are prepared under different oxygen conditions. We also report resistivity, Hall, Nernst, and magnetoresistance measurements, which show that La$_{1.85}$Y$_{0.15}$CuO$_{4}$ is an electron-doped, Mott-Hubbard system, where the carriers originate from oxygen nonstoichiometry produced by the oxygen reduction. This work is supported by NSF (DMR 0352735).\\ $^1$A. Tsukada et al., Sol. Stat. Commu. 133, 427 (2005). [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X9.00008: Superconducting Phase Diagram and Vortex-glass Scaling of the Electron-Doped Superconductor Sm$_{2-x}$Ce$_x$CuO$_{4-y}$ D.J. Scanderbeg, B.J. Taylor, R.E. Baumbach, K.T. Chan, M.B. Maple We report the growth and characterization of thin films of the electron-doped superconductor Sm$_{2-x}$Ce$_x$CuO$_{4-y}$ over a wide doping range 0.13 $\leq$ x $\leq$ 0.19. The shape of the superconducting dome is similar to other electron-doped compounds, such as Nd$_{2-x}$Ce$_x$CuO$_{4-y}$ and Pr$_{2-x}$Ce$_x$CuO$_{4-y}$, and shows a peak in the superconducting transition temperature T$_c$ at a doping level x $\approx$ 0.15. Magnetoresistance data $\rho$(H, T) in fields up to 17 T reveal a metal to insulator transition in the underdoped region. Analysis of Hall effect measurements from the underdoped to overdoped regime is presented along with vortex-glass (VG) scaling analysis of the transport measurements. VG scaling shows no change in the dynamical vortex behavior from the underdoped to overdoped regions. This research was sponsored by the DOE under Grant No. DE-FG02-04ER46105 and the CULAR program no. 9985-001. A portion of this work was performed at the National High Magnetic Field Laboratory (NHMFL), which is supported by NSF Cooperative Agreement No. DMR-0084173, by the State of Florida, and by the DOE. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X9.00009: Metal-insulator transition in YBa$_2$Cu$_3$O$_{\delta}$ Louis Taillefer, Cyril Proust, Nicolas Doiron-Leyraud, Mike Sutherland, D. Le Boeuf, J. Levallois, M. Nardone, H. Zhang, N. Hussey, S. Adachi, Ruixing Liang, D.A. Bonn, W.N. Hardy The non-superconducting ground state of the underdoped cuprates YBa$_2$Cu$_2$O$_{\delta}$ and YBa$_2$Cu$_4$O$_8$ was examined by measuring the electrical resistivity of high-quality single crystals in magnetic fields up to 60 T. A metal-insulator-like crossover is observed near a critical doping $p$~=~0.1, i.e., far in the underdoped region. This shows that the pseudogap phase present below $T^{\star}$ is a metal, characterized by a resistivity of the Fermi-liquid form, $\rho_0 + AT^2$, where $\rho_0$ is small, just as in the strongly overdoped regime. In the metallic phase, the transverse magneto-resistance undergoes a qualitative change between $p$~=~0.14, where it saturates at high field, and $p$~=~0.11, where it does not saturate. This would suggest that a change in the nature of the ground state occurs in the vicinity of $p$~=~1/8. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X9.00010: The Effect Of hBN addition on properties of Bi-2223 superconductors. Mustafa Akdogan, Erhan Budak, Ozgur Ozturk, Cabir Terzioglu, Ibrahim Belenli The effect of the addition of hBN (x = 0.005, 0.015, 0.025, 0.035 and 0.05 g.) in the \textit{Bi-2223} superconducting samples have been investigated in terms of magnetoresistivity, T$_{c}$, $J_{c}^{trans}$, and ac susceptibility. The samples were prepared by the conventional solid-state reaction method. We estimated the transition temperature, the activation energy values of the samples from the resistivity vs. temperature measurements in dc magnetic fields up to 0.7 T. We observed that transition temperature, $T_{c}$, and transport critical current density, $J_{c}^{trans}$, depend on the hBN addition. They both decrease with increasing x up to 0.025 g. With further increasing x up to 0.035 g. Activation energy, $U_{0},$ is calculated from resistivity versus temperature at various magnetic fields. It is observed that $U_{0}$ depend on both the hBN content of samples and the applied magnetic field. The ac susceptibility measurements were carried out at different values of the ac magnetic field amplitudes up to 555 A/m. The imaginary part of ac susceptibility measurements is used to calculate J$_{c}$ using the Bean's Model. The intergranular critical current density is also seen to vary with increasing hBN content. We extracted the peak temperature, $T_{p}$, and the pinning force density from our previous ac susceptibility measurements. The pinning force density varied with increasing the hBN content of samples. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X9.00011: Investigation of some physical properties of gold diffusion-doped Bi$_{1.8}$Pb$_{0.35}$Sr$_{1.9}$Ca$_{2.1}$Cu$_{3}$O$_{y}$ superconductors Ozgur Ozturk, Mustafa Yilmazlar, Tayfur Kucukomeroglu, Osman Gorur, Cabir Terzioglu We have investigated the effect of the gold-diffusion doped Bi-2223 superconducting samples on the crystal structure and superconducting properties using XRD, SEM, T$_{c}$, J$_{c}$, and room temperature resistivity measurements. Doping of Bi-2223 was carried out by means of gold diffusion from an evaporated onto pellets at the sintering. XRD patterns and SEM micrographs are used to obtain information about Bi-2223 phase ratio, lattice parameters calculations and grain size, respectively. The gold diffusion in the Bi$_{1.8}$Pb$_{0.35}$Sr$_{1.9}$Ca$_{2.1}$Cu$_{3}$O$_{y}$ has been also studied over the temperature range 500-800 $^{^{\circ}}$C$_{ }$using the technique of successive removal of thin layers and measurement of the sample resistivity at room temperature. The temperature dependence of the Au diffusion coefficient in the range 500-800 $^{^{\circ}}$C was described by the relation D=4x10$^{-4}$exp(- 1.08 eV/k$_{B}$T). For the Au-doped sample, the critical transition temperature and J$_{c}$ were increased from 100$\pm $1 K to 104$\pm $1 K and from 40 and 125 A cm$^{-2}$, respectively, in comparison with those of undoped Bi-2223. The possible reasons for the observed increases in critical temperature and critical current density due to Au diffusion were discussed. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X9.00012: The effect of \textit{Sm} substitution on properties of Bi$_{1.6}$Pb$_{0.4}$Sr$_{2}$Ca$_{2-x}$Sm$_{x}$Cu$_{3}$O$_{y}$ superconductors. Huseyin Aydin, Mustafa Yilmazlar, Ozgur Ozturk, Dincer Yegen, Cabir Terzioglu The effect of the partial substitution of \textit{Ca} by \textit{Sm} in the \textit{Bi-2223} superconducting samples have been investigated in terms of XRD, magnetoresistivity, critical temperature, transport critical current density, and ac susceptibility. The samples were prepared by the conventional solid-state reaction method. XRD patterns are used to calculate lattice parameters and obtain about Bi-2223 phase ratio. The volume fraction was determined from the intensities of Bi-2223 and Bi-2212 peaks. The room temperature XRD patterns of the samples showed the presence of Bi-2223 phase decreases with increasing the \textit{Sm} content. We estimated the transition temperature, the activation energy values of the samples from the resistivity vs. temperature measurements in dc magnetic fields up to 0.6 T. We observed that transition temperature, $T_{c}$, and transport critical current density, $J_{c}$\textit{trans}, depend on the Sm substitution. They both decrease with increasing the \textit{Sm} substitution. Activation energy, $U_{0},$ is calculated from resistivity versus temperature at various magnetic fields in both high temperature region and low temperature region. It is observed that $U_{0}$ depend on both the \textit{Sm} content of samples and the applied magnetic field. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X9.00013: Orientation and Grain Boundaries in Bulk Samples of Bi-2223 Phase. Murat Erdem, Ozgur Ozturk, Cabir Terzioglu, Ibrahim Belenli In polycrystalline high-T$_{c}$ compounds the critical current densities are mostly limited by the grain boundaries. Their characterization is of significant importance to understand and improve the superconducting properties. In our work, superconducting Bi$_{1.8}$Pb$_{0.35}$Sr$_{1.9}$Ca$_{2.1}$Cu$_{3}$O$_{y}$ samples were prepared by the standard solid-state reaction method. 20 samples were pressed in pellets and optimum annealing temperatures were found as 835$^{ o}$C for the first annealing and 830$^{ o}$C for the second annealing respectively. The optimum Between the first and second annealing procedure, the pellets are pressed piled up. After the second annealing, six bar samples were cut, three of them were perpendicular and the others parallel to the pressing surface. The transport T$_{c}$ and J$_{c}$ were measured using standard four-probe method. Micro structural examination of the samples was done using SEM. Degrading effect of the magnetic field on J$_{c}$ is much more significant for the perpendicular sample. This is in agreement with literature and ensures that our samples had preferred orientation. We have proved that preferred orientation in bulk pellet samples of (Bi, Pb)-2223 phase can be achieved by the method described in this work. Orientation of the magnetic field with respect to grain boundaries may be an important issue for transport critical current values in applied magnetic fields. [Preview Abstract] |
Session X12: Focus Session: Anomalous Hall Effect and Spin Orbit Coupling
Sponsoring Units: GMAG DMP FIAPChair: Partho Mitra, Pennsylvania State University
Room: Colorado Convention Center Korbel 3C
Friday, March 9, 2007 8:00AM - 8:36AM |
X12.00001: A Tunable Anomalous Hall effect in a Non-Ferromagnetic System Invited Speaker: The anomalous Hall effect (AHE) is a ubiquitous signature of ferromagnetism that has been known for almost as long as the Hall effect itself. Despite this, its theoretical origins nevertheless remain a subject of debate. In recent years, the physics behind the AHE has been employed to control spin transport in non-magnetic conductors via its sister phenomenon, the spin Hall effect (SHE). In this talk, I will present measurements of a magnetically-doped semiconductor quantum well that reveal a robust and tunable AHE, despite the absence of ferromagnetism. I will show that the AHE can be tuned in-situ by the application of a voltage to a nearby gate electrode and that this helps to uncover the origins of the effect. Most surprising is the fact that the parent material, ZnSe, is known to have only weak spin-orbit coupling, a property usually believed to be required for a strong AHE or SHE. This suggests that controllable semiconductor spin-transport might be realized in a larger class of materials than previously thought. Collaborators: L. S. Moore, H. T. Chou, K. C. Ku, G. Xiang, S. A. Crooker, N. Samarth, and D. Goldhaber-Gordon. See PRL 96, 196404. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X12.00002: Anomalous Hall Effect and Anomalous Nernst Effect in Ga1-xMnxAs Yong Pu, Jing Shi, Daichi Chiba, Fumihiro Matsukura, Hideo Ohno We have carried out systematic electrical and thermoelectric transport coefficient measurements on~a series of Ga1-xMnxAs samples (x from 0.01 to 0.07) with perpendicular magnetic anisotropy. 50 nm- thick Ga1-xMnxAs films were grown by molecular beam epitaxy on an InGaAs buffer layer with a tensile strain to induce the perpendicular anisotropy. Below the Curie temperature, we have observed a~non-zero transverse thermopower Sxy that accompanies the Hall resistance Rxy. Both Sxy and Rxy show abrupt jumps as the magnetization switches by an external magnetic field. The square hysteresis loops in Sxy and Rxy resemble those of the magnetization. Just as the anomalous Hall effect (AHE), the hysteresis loop in Sxy, i.e. the anomalous Nernst effect (ANE), is a consequence of the spin-orbit coupling in the ferromagnetic materials. We have measured both AHE and ANE over a wide range of temperatures in all samples, and found that the Hall resistance Rxy scales with the square of the longitudinal resistance Rxx. In contrast, the transverse thermopower Sxy is independent of the longitudinal thermopower Sxx over the same temperature range. These observations suggest that both AHE and ANE are of intrinsic or dissipationaless origin. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X12.00003: Sign change of anomalous Hall coefficient with temperature in Ga$_{1-x}$Mn$_{x}$Sb random alloys. M. Eginligil, G. B. Kim, H. Luo, B. D. McCombe We have observed sign changes as a function of temperature (T) in the anomalous Hall (AH) coefficient of ferromagnetic (FM) Ga$_{1-x}$Mn$_{x}$Sb films showing weakly localized behavior in the electrical transport. Low magnetic field measurements vs. T (below the Curie temperature, T$_{c}$, which is between 13K and 24K) show changes in the sign of the slope of the AH resistance vs. field. We attribute this unusual behavior to the movement of the chemical potential ($\mu )$ through the density of states (DOS) extrema in the spin dependent impurity band(s) as recently predicted theoretically [1]. We have developed a model based on the prediction that the AH coefficient depends on the local slope of the DOS in the hopping conduction regime. Our model uses the experimentally determined hole and Mn$_{Ga}$ concentrations to find the position of the $\mu $ vs. T. The two spin dependent impurity bands in the FM state are assumed to be gaussian. Below T$_{c}$ with increasing T, the spin-up and spin-down impurity bands move into the energy gap and converge. As T increases $\mu $ moves from its initial position on the positive slope of the low energy band (EB) through the minimum before continuing across the maximum of the higher EB. This analysis is in qualitative agreement with our experimental results. [1] Burkov and Balents, PRL, 91 (2003) Supported by NSF ESC 0224206 and University at Buffalo, SUNY [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X12.00004: Theory of Intrinsic Anomalous Hall Effect and Spin Hall Effect in Transition Metals Takuro Tanaka, Hiroshi Kontani, Jun-ichiro Inoue, Kosaku Yamada To elucidate the origin of anomalous Hall effect (AHE) in ferromagnetic transition metals, we study the intrinsic AHE based on a multi-orbital tight-binding model. A large anomalous velocity comes from the atomic $d$-orbital degrees of freedom. We derive a general expression for the intrinsic anomalous Hall conductivity (AHC) which is valid for any damping rate $\hbar/2\tau$. This expression enables us to calculate the AHC in metals with a wide range of resistivity $\rho$. The obtained AHC is almost constant with a value of $10^2\sim10^3\Omega^{-1}{\rm cm}^{-1}$ when $\rho$ is small, as found by Karplus and Luttinger. However, this relation does not hold any more in bad metals; we show that AHC is proportional to $\rho^{-2}$ when $\hbar/2\tau$ is larger than the minimum band-splitting measured from the Fermi level, $\Delta$. This crossover behavior of the intrinsic AHE, which was first derived by H. Kontani and K. Yamada [J. Phys. Soc. Jpn. {\bf 63} (1994) 2627], is recently observed in various ferromagnetic metals universally by A. Asamitsu et al. We also present the mechanism of spin hall effect in transition metal oxides. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:48AM |
X12.00005: Semi-classical theory of the Anomalous Hall Effect Invited Speaker: The new semi-classical theory of the anomalous Hall effect will be presented. Its goal is to derive all contributions to the Hall conductivity via a strongly simplified approach, based only on the semi-classical Boltzmann equation and wave packet equations of motion. This approach operates only with gauge invariant quantities and thus all expressions acquire a clear physical interpretation. In the diffusive regime the semi-classical approach leads to the same quantitative predictions as the formally exact approach based on the Kubo-formula. I will also discuss the 1-1 correspondence among different techniques applied to the problem of the anomalous Hall effect. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X12.00006: Anomalous Hall Effect in Superparamagnetic Co-(La,Sr)TiO$_{3}$ Thin Films Shixiong Zhang, Weiqiang Yu, Satish B. Ogale, Sanjay R. Shinde, Darshan C. Kundaliya, Joshua S. Higgins, Ranjan Sahu, Richard L. Greene, Thirumalai Venkatesan, Wangkong Tse, Shengyu Young, Lourdes G. Salamanca-Riba We performed a systematic study of the magnetic properties and the Hall effect on 5{\%} cobalt doped (La, Sr)TiO$_{3 }$thin films grown by pulsed laser deposition. The superparamagnetic nature of the system is established by several protocols of magnetic measurements. Nevertheless, the anomalous Hall effect (AHE) is observed in the system, the Hall resistivity vs magnetic field loops being found to be identical to the magnetic hysterisis loops. This once again (Phys. Rev. Lett. 92, 166601 (2004)) highlights the limitations of AHE as a tool to test the intrinsic nature of ferromagnetism in a diluted magnetic system. Possible reasons for the origin of the AHE in our system are discussed. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X12.00007: ABSTRACT HAS BEEN MOVED TO H12.00012 |
Friday, March 9, 2007 10:12AM - 10:24AM |
X12.00008: Mobility of Charge Carriers and Magnetoresistance of Dilute Magnetic Semiconduc. Michael Foygel, James Niggemann, Andre Petukhov We studied electrical transport in dilute magnetic semiconductors, which is determined by scattering of free carriers by localized magnetic moments. In our calculations of the scattering time and the mobility of the majority and minority-spin carriers we took into account both the effects of thermal spin fluctuations and of built-in spatial disorder of the magnetic atoms. These effects are responsible for the magnetic-field dependence of the mobility of the charge carriers. The application of the external magnetic field suppresses the thermodynamic spin fluctuations thus increasing the mobility and promoting negative magnetoresistance. Depending on the type of the carriers and on parameters of the impurity potential, scattering by built-in spatial fluctuations of the atomic spins increases or decreases with the magnetic field. The latter effect is due to the change in the magnitude of the random local Zeeman splitting with the magnetic field. Under certain circumstances it may promote positive magnetoresistance. We discuss the role of the above effects on mobility and magnetoresistance of semiconductors where magnetic impurities are electrically active or neutral. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X12.00009: Interplay of disorder and spin-orbit coupling in diluted magnetic semiconductors Xavier Cartoix\`a, Byounghak Lee, Nandini Trivedi, Richard M. Martin We address two open questions: (i) upon doping GaAs with Mn, where do the holes go? (ii) how does the disorder arising from the random Mn locations affect the carrier polarization? Using a realistic multi-band tight-binding model with disorder effects included exactly, we find upon examining the nature of the eigenstates that the system is composed of two components: for a given doping of Mn atoms, a fraction of holes are localized in the midgap region, whereas the rest hybridizes with the host valence band generating extended states. These fractions change in a systematic way with doping. We further find that while spin-orbit coupling reduces the spin polarization by mixing different spin states in the valence bands, disorder from the random locations of the Mn ions enhances the spin polarization. This is primarily because the additional field from the ferromagnetically aligned impurities polarizes the itinerant carriers. We also show that the ferromagnetic coupling between localized Mn moments exhibits a strong real space anisotropy. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X12.00010: Boundary conditions for spin diffusion Victor Galitski, Anton Burkov, Sankar Das Sarma We introduce a general method of deriving boundary conditions for spin-charge coupled transport in disordered systems with spin-orbit interactions and derive boundary conditions for spin diffusion in the Rashba model for various types of boundaries. Due to the surface spin precession, the boundary conditions are generally non-trivial and may contain terms, which couple different components of the spin density. We argue that boundary conditions and the corresponding electric-field-induced spin accumulation depend on the nature of the boundary and therefore the spin Hall effect in a spin-orbit coupled system can be viewed as a non-universal edge phenomenon. [Preview Abstract] |
Session X13: Focus Session: Layered Manganites
Sponsoring Units: DMP GMAGChair: John Mitchell, Argonne National Laboratory
Room: Colorado Convention Center Korbel 4C
Friday, March 9, 2007 8:00AM - 8:12AM |
X13.00001: Nonthermal Photoresponse in Epitaxial Thin Films of (La,Pr).67Ca.33MnO4: Correlation with Non-ohmic Electrical Transport and Magnetoresistance Anthony Davidson III, Rajeswari Kolagani, Grace Yong, Vera Smolyaninova, Mason Overby We have recently observed a non-thermal component of light induced resistance change in the vicinity of the insulator-metal transition, in epitaxial thin films of the CMR manganite material (La,Pr).67Ca.33MnO4 (LPCMO). LPCMO is known to have the co-existence of insulating and metallic regions. On cooling, the metallic regions grow at the expense of the insulating regions, giving rise to a percolative insulator-metal transition. Our results indicate that light may cause electronic changes in the insulating regions thus decreasing the electrical resistance. We will present our studies of the photoresponse and the correlation of the observed non-thermal photoresponse with magnetoresistance as well as current-voltage (I-V) characteristics. I-V measurements show that there is a current induced change in resistance which is not due to the joule heating effects. This effect is only seen in the metal-insulator transition range of the samples, similar to the nonthermal photoresponse, suggesting a common origin for these two phenomena. The effects of magnetic field however are seen to be distinct. A large magnetoresistance is seen at lower temperatures where the light and current induced effects are absent, thus suggesting a very different physical origin for the magnetoresistance. [Preview Abstract] |
Friday, March 9, 2007 8:12AM - 8:24AM |
X13.00002: Evolution of the CO-OO and AF ordering in the single-layer manganite $\rm Pr_{1-x}Ca_{1+x}MnO_4$ near half doping Feng Ye, J. A. Fernandez-Baca, Songxue Chi, Pengcheng Dai, J. W. Lynn, R. Mathieu, Y. Kaneko, Y. Tokura Manganese oxides have attracted considerable attention due to the CMR effect observed in the perovskite manganite $\rm A_{1-x}A'_{x}MnO_3$ near $x=0.3$. A peculiar charge/orbital (CO-OO) accompanied by antiferromagnetic (AF) order occurs when the carrier concentration is close to half doping (x=0.5). To understand the interplay between the charge, lattice and spin degrees of freedom in such insulating state, we used elastic neutron scattering to study the evolution of the CO-OO as well as the AF correlations in the single-layer manganite $\rm Pr_{1-x}Ca_{1+x}MnO_4$ ($x=0.40$, 0.45 and 0.50). Upon cooling, all three samples exhibit long-range CO-OO near 300~K. However, only the $x=0.50$ system displays long-range AF order at low temperatures. The CE-type AF correlations are quickly suppressed and become short-ranged as more $e_g$ electrons are introduced to the $\rm MnO_2$ plane. More interestingly, the CO-OO and AF order associated with Mn$^{3+}$ ions appears at incommensurate positions while the AF order associated with Mn$^{4+}$ ions remains commensurate. Our observations indicate that the orbital physics play an important role in the understanding of the electronic and magnetic properties of doped manganites. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X13.00003: Dimensional Crossover of Antiferromagnetism in Half-Doped La$_{0.5}$Sr$_{1.5}$MnO$_{4}$ Kuo-Sheng Chao, J. Okamoto, D. J. Huang, C. Y. Mou, H. -J. Lin, C. -H. Hsu, Y. Kaneko, R. Mathieu, Y. Tokura, C. T. Chen Like cuprates which exhibits high-temperature superconductivity, half-doped single-layered manganites such as La$_{0.5}$Sr$_{1.5}$MnO$_{4}$ have distinct features of the MnO$_{2}$ plane in the perovskite structure. In addition to charge-orbital order, of particular interest is the antiferromagnetism in half-doped manganites with the so called CE-type antiferromagnetic (AF) structure, which is essentially composed of ferromagnetic zigzag chains antiferromagnetically coupled to one-another. In this talk, we will report critical behavior and dimensional crossover of AF order in La$_{0.5}$Sr$_ {1.5}$MnO$_{4}$, based on measurements of resonant soft-x-ray magnetic scattering. A 2D incommensurate AF order exists at temperatures above the Neel temperature $T_N$. As the temperature cools across $T_N$, the interlayer exchange coupling prevails and the 2D incommensurability collapses to stabilize the 3D AF order. The measurements unravel spin correlations in the classical renormalized region for a non- standard (CE-type) antiferromagnet. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X13.00004: Role of oxygen in the orbital ordered state of La$_{0.5}$Sr$_{1.5}$MnO$_4$ John W. Freeland, Michel van Veenendaal, Ken Gray, Qing'an Li, Hong Zheng, John F. Mitchell In the single layer manganite, La$_{0.5}$Sr$_{1.5}$MnO$_4$ , experimental evidence points clearly to formation of orbital ordering but leaves the question open as to the exact nature of the state. Using oxygen K edge spectroscopy, we find that oxygen holes related to the Mn-O hybridization between O($2p$) and Mn($e_g$) states play an important role in the formation of the ordered groundstate. The large change in the number of $e_g$ related oxygen holes with the formation of the charge/orbital ordered states demonstrates it is not due to a locking in of orbitals which are fluctuating in orientation, but that the disordered state possesses a different orbital occupancy. The change in the number of $e_g$ holes occurs mainly within the ab plane and seems to be related to the crossover from ferromagnetic to anti-ferromagnetic correlations with the onset of the charge/orbital ordered state. This idea is supported by Mn$_4$O$_8$ cluster calculations. Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X13.00005: Orbital Ordering in Magnanites Probed with Soft X-Ray Scattering Stuart Wilkins, Natasa Stojic, Thomas Beale, Nadia Bingeli, Peter Hatton, D. Prabhakaran, Andrew Boothroyd, Massimo Altarelli Orbital ordering is important in the understanding of transition metal oxides as the magnetic and transport properties are strongly related to the orbital and charge degrees of freedom. In the case of La$_{0.5}$Sr$_{1.5}$MnO$_{4}$ we will present results using diffraction at the L-edges of Mn, for the orbital order superlattice relfection, and show that while there indeed does exist long range order of the orbitals the dominant process is cooperative Jahn-Teller distortions. By comparing these measurements with theoretical calculations we find a good general agreement showing a dominant orbital ordering of the $d_{x^2 - z^2} / d_{y^2 - z^2}$ type, in contrast to the $d_{3x^{2}-r^{2}}$ / $d_{3y^{2} - r^{2}}$ as previously proposed. In LaSr$_{2}$Mn$_{2}$O$_7$, which is found crystallographically to have virtually no Jahn-Teller distortion of the oxygen octahedra. We will show that even in the absence of Jahn-Teller distortions, this compound is strongly orbitally ordered. Fitting of the Mn $L$-edge resonance spectra demonstrates the presence of orbital ordering of the Mn$^{3+}$ ions within an almost cubic crystal field. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X13.00006: Rethinking the Orbital Ordering Transition: Using Coherent Soft X-ray Scattering to Study Dynamics in the CMR Manganites Joshua Turner, Jessica Thomas, John Hill, Mark Pfeifer, Karine Chesnel, Y. Tokura, Y. Tomioka, Steve Kevan The colossal magnetoresistance (CMR) phenomenon has baffled physicists since its discovery over a decade ago. Central to the puzzle is the short-range orbital ordering that arises in certain hole concentrations of the manganese oxides, even for low temperatures. We have used a Coherent Soft X-ray Scattering (CSXS) technique to resonantly enhance the orbital ordering contrast and measure speckle patterns from the domain structure of the self-assembling Mn d-orbitals. Our dynamics measurements suggest that the orbital domains remain static through the orbital ordering transition temperature -- challenging the previous belief of a mediation through slow, glass-like characteristics. Our experiments force us to rethink the role and nature of the orbitally-ordered state in the manganites, intrinsic to CMR. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X13.00007: Resonant soft x-ray scattering study on antiferromagnetic ordering of LaSr$_2$Mn$_2$O$_7$ J.-S. Lee, J. Koo, H. Jang, K.-T. Ko, H. J. Lee, Y. H. Jeong, K.-B. Lee, J.-H. Park, J.-Y. Kim, Y. Bang, T. Kimura, Y. Tokura Resonant soft x-ray scattering experiments at the Mn $L_{2,3}$- edge and O $K$-edge have been performed to probe the magnetic structure of LaSr$_2$Mn$_2$O$_7$ which is well known as the $A$-type antiferromagnetic (AFM) phase. At the low temperature, strongly resonant intensity of (001) AFM reflection was found. The temperature dependences of AFM resonance at both the Mn $L_{2,3} $- and O $K$-edge were relatively different, in which the order parameter at Mn $L_{2,3}$-edge showed an anomalous transition above N$\acute{\rm e}$el temperature (170 K). This feature could be regarded as the mixed valence state (Mn$^{3+}$ and Mn$^{4+}$) phenomenon, and besides, it could be supported by theoretical calculation and bulk measurement on magnetism. Detailed description will be discussed in presentation. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X13.00008: Reentrant orbital order and the true ground state of LaSr$_{2}$Mn$_{2}$O$_{7}$. S. Nyborg Ancona, S. Rosenkranz, R. Osborn, K. E. Gray, H. Zheng, Qing'An Li, J. F. Mitchell, Y. Chen, J. Lynn Strongly correlated electron systems, and colossal magnetoresistive materials in particular, exhibit a strong competition among orbital, charge and spin order. The phase diagram of the bilayer manganites, La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$, display interesting features near half doping, $x\approx0.5$, and it has been commonly accepted that CE order at $x=0.5$ is reentrant. Here, we present x-ray and neutron diffraction data of our purified La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ crystals contrasting the conventional wisdom. Our crystals exhibit CE-type orbital and charge order as the low-temperature ground state for $x=0.5$. For small deviations from $x=0.5$, the high temperature CE phase is replaced at low temperatures by an A-type antiferromagnetic phase {\sl without coexistence}. Larger deviations from $x=0.5$ result in a lack of CE-order at any temperature. Thus small compositional variations could explain why others commonly see this reentrance with coexistence. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X13.00009: Exotic Bilayer Manganite Phase Diagram near x=0.6 Hole Doping. Kenneth Gray, Hong Zheng, Qing'An Li, John F. Mitchell, Stine Nyborg Ancona, Stephan Rosenkranz, Ray Osborn Important modifications to the phase diagram of the bilayered manganites have been discovered through combined measurements of conductivity, magnetization and neutron and x-ray diffraction. The previously reported phase diagram for the CE-type orbital and charge order in LaSr$_{2}$Mn$_{2}$O$_{7}$ crystals (near hole doping of x=0.5) is qualitatively reproduced and graphically demonstrated for the bi-stripe (BIS) orbital and charge order in La$_{0.8}$Sr$_{2.2}$Mn$_{2}$O$_{7}$ crystals near x=0.6. Individual crystals, taken along the compositional gradient of a single boule made by the floating-zone technique with nominal composition x=0.61, display a large fraction of the phase diagram. As in the case of x=0.5, we find (1) crystals that enter the BIS phase at 240-300 K and reenter into an in-plane metal, A-type antiferromagnetic phase, and (2) crystals that retain BIS order down to at least 5 K. Surprisingly, the BIS state seems somewhat more stable than the CE state. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X13.00010: Polarized transmission EXAFS study of single crystal La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7 }$ as a function of temperature Jesse Guzman, G. Kurczveil, L. Downward, F. Bridges, J. Mitchell, H. Zheng The temperature dependence (3-300K) of the bilayer Colossal Magneto Resistive (CMR) manganites La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$, x=0.34 and 0.36, was studied using polarized, transmission EXAFS through a thin single crystal, with the x-rays polarization parallel to the c-axis or in the ab-plane. To eliminate Bragg diffraction effects from the single crystal, a 2D sample oscillator setup was used; the results of using this oscillator will be discussed. The data analysis shows a sharp increase in the width $\sigma $ of the pair distribution function (PDF) for the Mn-O peak near the ferromagnetic transition temperature T$_{c}$. This sharp increase in $\sigma ^{2}$ corresponds to a Jahn-Teller-like distortion as the temperature goes through T$_{c}$. Furthermore, a plot of the reduction in $\sigma ^{2}$, $\Delta (\sigma ^{2})$, vs. sample magnetization M shows a linear dependence with a large change in slope at M/M$_{o}\sim $2x, which is consistent with the recently proposed dimeron model proposed by Downward, et al. with regards to the LCMO system. Furthermore, there is some evidence for an increase in disorder below 75K which has not been discussed previously. Finally, another weaker step in $\sigma ^{2}$ at T$^{\ast } \quad \sim $250K, well above T$_{c}$ can be seen, which is comparable to a T$^{\ast }$in recent neutron scattering experiments, proposed to be a spin-liquid/spin-glass phase transition. Support: NSF DMR0301971. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X13.00011: Magnetic momentum density, Fermi surface and directional magnetic Compton profiles in LaSr$_{2}$Mn$_{2}$O$_{7}$ and La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ P. E. Mijnarends, S. Kaprzyk, B. Barbiellini, A. Bansil, Yinwan Li, J.F. Mitchell, P.A. Montano We have carried out first principles, all-electron computations of the magnetic momentum density $\rho_{mag}({\mathbf p})$ and magnetic Compton profiles (MCPs) for momentum transfer along the [100], [001], and [110] directions in LaSr$_2$Mn$_2$O$_7$ and La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ [1]. Parallel measurements of these three MCPs from a single crystal of La$_{1.2}$Sr$_{1.8} $Mn$_2$O$_7$ at 5 K in a magnetic field of 7 T are also reported. Here, we discuss details of the FS-related signatures in the first and higher BZs in the MCPs and show that high resolution magnetic Compton scattering experiments with a momentum resolution of 0.1 a.u. FWHM (full-width-at-half- maximum) or better will be necessary to observe this fine structure. We comment also on the feasibility of using positron annihilation spectroscopy in this connection. Work supported by the USDOE.\\ \mbox{[1]} Yinwan Li, P. A. Montano, J.F. Mitchell, B. Barbiellini, P. E. Mijnarends, S. Kaprzyk, and A. Bansil, Phys. Rev. Lett. 93 (2004) 207206. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X13.00012: Optic phonon anomaly as a precursor to polaron formation in a layered CMR manganite. Dmitry Reznik, Frank Weber, Nadir Aliouane, Dimitri Argyriou, Marcus Braden, Winfried Reichardt We found evidence of a precursor effect to polaron formation in the bond stretching phonons in the bilayer CMR manganite La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ at 10K. Inelastic neutron scattering measurements of these phonons in the XX0 direction show that both the transverse and longitudinal branches broaden abruptly from X=0.15, where they are resolution limited, to X=0.25 where FWHM=13meV. They then narrow again on approach to the zone boundary (X=0.5). The shell model predicts a downward dispersion following the cosine function for the transverse branch and upward dispersion for the longitudinal branch, which is exactly what is observed in undoped cuprates. But in La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ both branches show a steep downward dispersion above x=0.15. The anomalous dispersion and broadening can be understood as a precursor effect to the CMR transition at 125K because they appear at the same wavevectors as the polaron peaks in the same compound observed above 125K near q=(0.27,0.27,0). This behavior is very unusual, because such precursor effects have previously been associated with phase transitions where charge order appears on cooling, not on heating as in CMR manganites. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X13.00013: A local metallic state in globally insulating La$_{1.24}$Sr$_{1.76}$Mn$_{2}$O$_{7}$ well above the metal-insulator transition Zhe Sun, J. Fraser Douglas, Alexei V. Fedorov, Yi-De Chuang, Hong Zheng, John F. Mitchell, D. S. Dessau La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ is a typical colossal magnetoresistive oxide, and it shows a drastic transition from a low-temperature metal to a high-temperature insulator at 120K -160K. The famous CMR ( colossal magnetoresistive) effect usually accompanies the metal-insulator transition. Using angle-resolved photoemission spectroscopy (ARPES) we studied the electronic structure of a bi-layer manganite compound La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ (x=0.38). We found that in the insulating state there remain local metallic regions up to a very high temperature. In these metallic regions, the electronic behavior has minimal change with temperature. Our data indicate that the metal-insulator transition is a new type and an ``emergent'' phenomenon driven by the phase separation and percolation effect. The CMR effect can also be understood in the framework of the phase separation and percolation effect. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X13.00014: Angle resolved photoemission studies of colossal magnetoresistive bilayer manganites Norman Mannella, Wanli Yang, Kiyohisa Tanaka, Xingjiang Zhou, Jennifer Zheng, John Mitchell, Jan Zaanen, Tom Devereaux, Naoto Nagaosa, Zahid Hussain, Zhi-Xun Shen In this talk, we will discuss the results of some recent angle-resolved photoemission spectroscopy (ARPES) investigations in the colossal magnetoresistive (CMR) bilayer compound La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ (LSMO, x = 0.4) [1]. The temperature dependent evolution of the quasiparticle excitations in LSMO has been found to track remarkably well the DC conductivity, thus accounting for the macroscopic transport properties and the metal to insulator transition.~ Our results indicate that the microscopic mechanism leading to the MIT and the CMR effect in manganites is intrinsically a quantum effect linked to a crossover via the nodal QP collapse from a coherent polaronic conductor in the FM state below T$_{C}$ to a hopping regime with thermally activated single polarons in the paramagnetic state above T$_{C}$.~ The role of the exchange interaction is crucial in controlling the competition between coherence and localization effects. [1] N. Mannella et al., Nature \underline {438}, 474 (2005) [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X13.00015: Energy Range of State Conservation for Coulomb Interaction Effects in $La_{1.28}Sr_{1.72}Mn_2O_7$ Daniel Mazur, K. E. Gray, J. F. Zasadzinski, L. Ozyuzer, I. Beloborodov, H. Zheng, J. F. Mitchell Tunneling data on $La_{1.28}Sr_{1.72}Mn_2O_7$ crystals confirm Coulomb interaction effects through the $\sqrt{E}$ dependence of the density of states (DOS). Importantly, the data and analysis at high energy, $E$, show conservation of states: those removed from near $E_F$ are found between $\sim$30 and 70 meV from $E_F$. This quantum correction to the DOS agrees in magnitude with the $\sqrt{T}$ dependence of the bulk conductivity. Combining our results, with published theory and quantum interference data, we find a scattering time and Fermi velocity that agree reasonably well with recent ARPES results. [Preview Abstract] |
Session X14: Focus Session: Spin-Polarized Transport
Sponsoring Units: GMAG DMP FIAPChair: Dragomir Davidovic, Georgia Institute of Technology
Room: Colorado Convention Center Korbel 4D
Friday, March 9, 2007 8:00AM - 8:36AM |
X14.00001: Tuning the Exchange Bias in Spin Valves by an Electric Current Invited Speaker: An electrical current can transfer spin angular momentum to a ferromagnet. This novel physical phenomenon, called spin transfer, offers unprecedented spatial and temporal control over the magnetic state of a ferromagnet and has tremendous potential in a broad range of technologies, including magnetic memory and recording. Recently, it has been predicted [1] that spin transfer is not limited to ferromagnets, but can also occur in antiferromagnetic materials and even be stronger under some conditions. This talk will discuss our recent experiments [2] that demonstrate the transfer of spin angular momentum across an interface between ferromagnetic and antiferromagnetic metals. The spin transfer is mediated by an electrical current of high density ($\sim $1012 A/m2) and revealed by variation in the exchange bias at the ferromagnet/antiferromagnet interface. We find that, depending on the polarity of the electrical current flowing across the interface, the strength of the exchange bias can either increase or decrease. This finding is explained by the theoretical prediction that a spin polarized current generates a torque on magnetic moments in the antiferromagnet. Current-mediated variation of exchange bias could be used to control the magnetic state of spin-valve devices, e.g., in magnetic memory applications. [1] A. S. Nunez, R. A. Duine, P. Haney, and A. H. MacDonald, Phys. Rev. B 73, 214426 (2006). [2] Z. Wei et al., cond-mat/0606462. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X14.00002: Spin transfer in exchange biased magnetic nanopillars Nickolas Anthony, Sergei Urazhdin We present a study of the effect of current on the magnetic state of nanopatterned ferromagnetic/antiferromagnetic bilayers. We show that the magenetic state of the antiferromagnet can be affected by a high current density. First, the exchange bias can be altered by applying a pulse of current. The change is accompanied by an increase of coercivity. Additionally, the magnetic anisotropy depends on the value and the direction of the applied current. Our findings cannot be explained by the Joule heating, and indicate that a spin transfer effect similar to that previously demonstrated for ferromagnets is also responsible for the current-induced effects in nanopatterned antiferromagnets. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X14.00003: Specific Resistance and Scattering Asymmetry of Py/Pd, Fe/V, Fe/Nb, and Co/Pt Interfaces Amit Sharma, Tony Romero, Nikoleta Theodoropoulou, Reza Loloee, William Pratt Jr., Jack Bass The properties of interfaces between normal (N) and ferromagnetic (F) metals, described by specific resistance, AR* (A = area, R = resistance), and scattering asymmetry, $\gamma $, are of interest to optimize current-perpendicular-to-plane (CPP) magnetoresistance (MR) and current-induced magnetization-switching (CIMS) in nanopillars. Sputtered standard Py/Cu, Co/Cu, and Fe/Cr interfaces have 2AR* $\sim $ 1 f$\Omega $m$^{2}$ and $\gamma \quad \sim $ 0.7. Recently, sputtered F/Al interfaces with F = Py, Co, Fe, and Co(91)Fe(9) were found to have very large 2AR$^{\ast }$ $\sim $ 9 f$\Omega $m$^{2}$, but small $\gamma \quad \le $ 0.1 [1]. In hopes of finding interfaces with both large 2AR* and larger $\gamma $ than for F/Al, we have determined 2AR* and $\gamma $ at 4.2K for sputtered Py/Pd, Fe/V, Fe/Nb, and Co/Pt pairs, where we've matched crystal structures of the F and N metals. We will present our data and our derived values of 2AR* and $\gamma $. [1] N. Theodoropoulou et al., J. Appl. Phys. \textbf{99}, 08G502 (2006); \textit{ibid}., IEEE Trans. on Magn. (Submitted). [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X14.00004: Current-perpendicular-to-plane magnetoresistance of multilayered Co/Cu nanocolumns by scanning tunneling microscope P. Morrow, A. Kar, X. Tang, T. Parker, G. -C. Wang, T. -M. Lu, J. Y. Dai In this work we present a method to measure the current-perpendicular-to-plane magnetoresistance (CPP-MR) of a small number of multilayered nanocolumns using a nonmagnetic STM. Samples were grown on Au-coated Si substrates by oblique angle thermal deposition from separate Co and Cu sources. We set the layer thicknesses and column lengths at 5-15 nm and 200-700 nm, respectively. SEM images show column diameters of about 25-100 nm, and the multilayer structure is confirmed by EELS and HR-TEM. VSM analysis gives coercivities of a few tens to several hundred G. In the MR measurement, mechanical contact was established between the STM tip and a small number of as-deposited nanocolumns, and dynamic hysteresis loops of resistance vs. magnetic field (up to 2.5 kG) were then collected in air at room temperature. The observed MR ratio for most samples was on the order of 1{\%}, which is posited to be due to the same physical mechanism as the GMR effect, but with significantly less efficacy. Cautions in the experiment and factors that may facilitate higher MR are also discussed. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X14.00005: Large Magnetoresistance in Co/Ni/Co Ferromagnetic Single Electron Transistors Ruisheng Liu, Hakan Pettersson, Lukasz Michalak, Carlo Canali, Dmitry Suyatin, Lars Samuelson We report on magnetotransport investigations of Co/Ni/Co ferromagnetic single electron transistors, fabricated using a high-precision alignment procedure invoked during e-beam writing. As a result of reduced size, the devices exhibit single-electron transistor characteristics at 4.2K. Magnetotransport measurements carried out at 1.8K reveal TMR traces with negative coercive fields, which we interpret in terms of a switching mechanism driven by the shape anisotropy of the central wire-like Ni island. A large TMR of about 18{\%} is observed within a small source-drain bias regime. The TMR decreases rapidly with increasing bias, which we primarily attribute to the excitation of magnons in the central island. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X14.00006: Ballistic Anisotropic Magnetoresistance in Electrodeposited Co Nanocontacts Andrei Sokolov, Chunjuan Zhang, Evgeny Y. Tsymbal, Jody Redepenning, Evgeny Kiriranov, Bernard Doudin As dimensions of a metallic conductor is reduced, spin-dependent conductance quantization in units of $e^{2}$/$h_{ }$leads to unusual magnetoresistive phenomena. One of them is ballistic anisotropic magnetoresistance (BAMR), a quantized change in the ballistic conductance according to the direction of magnetization. Here we present a first observation of BAMR in Co electrodeposited nanocontacts by \textit{in-situ }investigation of their spin-dependent transport properties. We compare the results from electrochemically synthesized and break junction contacts. By measuring the conductance as a function of the applied magnetic field direction at saturation, we find the step-wise variation of the ballistic conductance, signature of the BAMR effect. Our results show that BAMR can be positive and negative, and have symmetric and asymmetric angular dependence. This behavior is explained using a simple tight-binding model in terms of the effect of the spin-orbit interaction on the electronic band structure of nanocontacts. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X14.00007: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 9:48AM - 10:00AM |
X14.00008: First principles calculations of anisotropic magnetoresistance in ferromagnetic nanocontacts D. Jacob, J. Fernandez-Rossier, J.J. Palacios Here we present {\it ab initio} transport calculations of ferromagnetic nanocontacts [1] including the spin-orbit (SO) coupling for the very first time. Due to the SO coupling the conductance of the nanocontact changes with the direction of the magnetization giving rise to the so-called anisotropic magnetoresistance (AMR). We investigate the magnitude of the AMR effect while going from the atomic contact regime (BAMR) [2] to the tunneling regime (TAMR) [3]. Our work is motivated by recent experiments on ferromagnetic nanocontacts [4,5] which report much larger AMR values than those usually obtained for bulk materials in agreement with recent electronic structure calculations of ideal monatomic Ni chains [2].\\ {\bf References:}\\ $[1]$ D. Jacob, J. Fern\'andez-Rossier, and J. J. Palacios, Phys. Rev. B {\bf 71}, 220403(R) (2005).\\ $[2]$ J. Velev et al., Phys. Rev. Lett. {\bf 94}, 127203 (2005).\\ $[3]$ L. Brey, C. Tejedor, and J. Fern\'andez-Rossier, Appl. Phys. Lett. {\bf 85}, 1996 (2004).\\ $[4]$ M. Viret et al., Eur. Phys. J. B {\bf 51}, 1 (2006).\\ $[5]$ K. I. Bolotin and F. Kuemmeth, and D. C. Ralph, Phys. Rev. Lett. {\bf 97}, 127202 (2006). [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X14.00009: Superconductivity suppression by ferromagnetism in bi- and tri-layers of La$_{0.7}$Ca$_{0.3}$MnO$_3$\ ferromagnets and high-T$_c$\ YBa$_2$Cu$_3$O$_{7-\delta}$ Norbert M. Nemes, Flavio Y. Bruno, Mar Garcia-Hernandez, Axel Hoffmann, Suzanne G. E. te Velthuis, Cristina Visani, Carlos Leon, Zouhair Sefrioui, Jacobo Santamaria Large magnetoresistance occurs in F/S/F trilayers of highly spin-polarised La$_{0.7}$Ca$_{0.3}$MnO$_3$\ ferromagnet and high-T$_c$\ superconductor YBa$_2$Cu$_3$O$_{7-\delta}$ for antiferromagnetic (AF) alignment of the manganite layers. We discuss the relative importance of spin diffusion across the superconductor, proximity effect at the F/S interface and stray fields due to domain walls of the ferromagnet based on magnetoresistance and magnetisation measurements of bilayers and trilayers of various thickness of superconductor and ferromagnet. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X14.00010: Spin injection and imbalence in ferromagnet/ superconductor/ ferromagnet double tunnel junctions. Hyunsoo Yang, See-Hun Yang, Stuat Parkin The interplay between magnetism and superconductivity can be explored in double tunnel junctions (DTJs) with a superconducting (SC) middle electrode, CoFe/ MgO/ Al/ MgO/ CoFe, prepared using metal shadow masks and magnetron sputter deposition. Due to the strong competition between magnetism and superconductivity induced by the accumulation of spin polarized electrons in the SC, the superconducting gap is reduced with increasing bias voltage for anti-parallel alignment of the two ferromagnetic electrodes, as predicted theoretically [1]. We find that a large inverse (or negative) TMR is observed around gap energy and almost negligible TMR at zero bias. [1] S. Takahashi, H. Imamura, and S. Maekawa, Phys. Rev. Lett. \textbf{82}, 3911--3914 (1999). [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X14.00011: Cooper pair phase oscillation in thin Al superconductor induced by effective Zeeman splitting from spin injection Guo-xing Miao, Jochem Nietsch, Jagadeesh Moodera By placing a superconductor (S) and a ferromagnet (F) in close contact, the superconductivity proximity effect induces transient Cooper pairs in F leading to FFLO [1,2] state, while the ferromagnetic proximity effect will populate the S region with non-equilibrium parallel spins. In our experiment, the spins are induced from both sides of the superconductor symmetrically through thin Al$_2$O$_3$ tunnel barriers. By toggling the two F layers between parallel and anti-parallel, we can effectively turn on and off the spin imbalance in the Al layer creating $>$ 1000\% MR. The Tc of Al layer is shifted between parallel and antiparallel states as a net result of the non-equilibrium spin population. Such Tc shift is observed to oscillate with Al layer thickness, which is a clear evidence that the effective Zeeman splitting caused by parallel spin population can also induce FFLO states in superconductors. The CPP conductance in such a structure also show dramatic difference between the two states. 1. P. Fulde and R.A. Ferrel, Phys. Rev. 135, A550 (1964) 2. A. I. Larkin and Y. N. Ovchinnikov, Sov. Phys. JEPT 20, 762 (1965) [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X14.00012: ABSTRACT WITHDRAWN |
Session X17: Structure and Dynamics of Polymer Films
Sponsoring Units: DPOLYChair: Joona Bang, Korea University
Room: Colorado Convention Center 102
Friday, March 9, 2007 8:00AM - 8:12AM |
X17.00001: In-situ Nanoparticles Direct Self-Assembly of Block Copolymer Thin Films. Ranjan Deshmukh, Gavin Buxton, Nigel Clarke, Russell Composto Nanocomposite films have been prepared by thermally decomposing the organometallic precursor, (1,1,1,5,5,5-hexafluoroacetylacetonato)silver(I), AgHFA, in block copolymer poly(styrene-b-methyl methacryalate), PS-b-PMMA films. The decomposition of AgHFA at 185 \r{ }C results in the in-situ formation of silver nanoparticles (Ag NP) distributed throughout the PS-b-PMMA films. RBS, AFM and TEM were used to study the depth profile of silver, evolution of the surface and internal block copolymer morphology for films containing 0, 2, 3.5 and 5-wt {\%} Ag. The in-situ formed Ag NP segregate to the surface of the PS-b-PMMA films and influence the self-assembly process of the block copolymer and vice versa. The Ag NP selectively partition into the PMMA block of the perpendicular lamellae at the surface forming high-density 1D arrays at the surface. The Ag NP further retard the kinetics of flipping from the perpendicular to parallel lamellae morphology at the surface. As the Ag content is increased from 0 to 5-wt {\%} the perpendicular lamellae morphology is completely pinned (i.e. metastable). We have also used cell dynamic simulation (CDS) method to capture the evolution of block copolymer morphology by varying the substrate-polymer interactions. [Preview Abstract] |
Friday, March 9, 2007 8:12AM - 8:24AM |
X17.00002: Real-Time Guided Wave Depolarized Light Scattering of Block Copolymer Thin Films during in Situ Annealing Jeffrey Wilbur, Nitash Balsara, Zhuangxi Fang, Maurice Newstein, Bruce Garetz The guided wave depolarized light scattering technique for measuring grain structure in block copolymer thin films has been further developed to enable us to measure scattering in films heated above the glass transition temperature. We previously published work in which we used a prism to couple a plane-polarized beam into and out of the transverse magnetic (TM) mode of a glassy polymer film, measured the extent of depolarized scattering into the transverse electric (TE) mode within the film, and correlated the intensity of the TE signal to grain structure. Through the design and incorporation of a grating coupler system in which the polymer film is deposited onto the coupler itself, we have supplanted the prism coupler and removed the requirement that the measured film be glassy, enabling measurements of grain structure during annealing. Coupled with our recently developed theoretical solution for GWDLS, we are able to analyze the grain structure averaged over a 1 cm cross section of a film as it changes in real time. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X17.00003: Structural Characterization of Asymmetric Block Copolymer Thin Films using Resonant Soft X-Ray Scattering J. M. Virgili, J. B. Kortright, N. P. Balsara, R. A. Segalman Resonant soft X-ray scattering (RSOXS) is a powerful tool for structural characterization of block copolymer thin films over very large areas ($\sim $9000 $\mu $m$^{2})$. We study a single layer of cylinders formed from an asymmetric poly(styrene-$b$-isoprene) (PS-PI) block copolymer thin film using X-rays tuned to the carbon $\pi $* resonance. These results are compared to bulk structural data obtained by conventional small angle X-ray scattering (SAXS). We demonstrate that the cylinder-to-cylinder spacing is conserved between the bulk and thin film form. In sphere-forming PS-PI block copolymers, we observe a sphere-to-sphere spacing in thin films that is between the bulk nearest neighbor and bulk lattice spacing. RSOXS' capability in probing complex multi-block copolymers by tuning to different energy edges and also of probing a variety of film thickness effects will also be discussed. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X17.00004: Decay length of phase coherent block copolymer films: neutron reflectivity, analytical theoretical and simulation studies Junhan Cho, Kwanwoo Shin, Kwangsoo Cho, Wonyoung Jung, Sangbo Na Phase coherent thin films of symmetric diblock copolymers are investigated to study relationship between processing condition and thermodynamic responses for the manufacture of nanopatterned copolymer films. Temperature effects on phase coherent profiles and their decay lengths for some styrenic block copolymer films are firstly probed by using neutron reflectivity measurements. Pressure effects on those properties are secondly probed by using a recent field-theoretic simulation method for compressible systems. An analytical Landau theory in connection with a molecular model for perturbed hard chains is then introduced to give a molecular prediction of the reflectivity and simulation results for the copolymers. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X17.00005: Silicon Wire Grid Polarizer for Deep UV Fabricated by Diblock Copolymer Lithography Koji Asakawa, Young-Rae Hong, Vincent Pelletier, Douglas Adamson, Richard Register, Paul Chaikin Thin-shaped polarizers able to continuously polarize deep ultraviolet (UV) light below 300 nm were demonstrated using silicon wire grids with a 16.5 nm half-pitch stripe pattern. Conventional wire grid polarizers are capable of polarizing infrared and even some visible light, but are not for wavelengths below 300 nm due to their large pitch. To realize a deep UV polarizer, both a highly reflective material and an ultra-fine stripe pattern are required. While our group recently demonstrated aluminum wire polarizers [1], silicon shows greater reflectivity in the deep UV than aluminum. Block copolymer lithography was used to fabricate this ultra fine stripe pattern. A thin film of cylinder-forming polystyrene-polyhexylmethacrylate diblock copolymer (PS-PHMA, 21-64 kg/mol) was shear-aligned [2] and used as a mask for reactive-ion etching (RIE) to generate the 16.5 nm half-pitch stripe pattern of silicon nanowires on a transparent substrate. The finished polarizer showed polarization ability starting from the visible region and extending below 300 nm, including good polarization efficiency for 200 nm UV light. 1) Pelletier, et. al. Appl Phys Lett, 88, 211114 (2006), 2) Angelescu et al. Adv. Mater. 17, 1878 (2005) [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X17.00006: Why Does the Effect of the Free Surface on the Tg-Confinement Effect Depend So Strongly on Polymer Species? John M. Torkelson, Manish K. Mundra The effect of confinement on the glass transition temperature in thin polymer films has been heavily examined in the case of polystyrene and poly(methyl methacrylate). However, related effects have seen very limited study in other polymer systems. Here we demonstrate that simple adjustments to the polymer repeat unit structure leading to a greater requirement for cooperativity of the segmental mobility associated with Tg result in larger effects of the Tg-confinement effect. This is seen both with regard to the magnitude of the reduction in Tg at a given film thickness and with regard to the film thickness at which reductions in Tg from the bulk value are observed. Sets of data in which an increase in the size of a rigid side group (polystyrene vs. poly(4-methyl styrene) vs. poly(t-butyl styrene)) or an increase in the polymer backbone rigidity (polystyrene vs. polycarbonate vs. polysulfone) lead to an increase in the Tg-confinement effect will be described. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X17.00007: Dynamics of water on self-assembled monolayers J. Matthew D. Lane, Michael Chandross, Mark J. Stevens, Gary S. Grest, Christian D. Lorenz Nanoscale surface interactions of water at polymeric interfaces are important for a number of applications including biophysics, nanofluidics and MEMS devices. Using molecular dynamics simulations we investigate the fundamental properties of water at interfaces for a variety of water models and surfaces. We report results for the contact angle and wetting dynamics of TIP3P and SPC/E water on self-assembled monolayers (SAMs) of alkylsilanes on amorphous silica and alkanethiols on gold with various hydrophilic and hydrophobic head groups. For comparison we also investigate water interactions on bare amorphous silica substrates. Simulations of the dynamics of water confined between two surfaces coated with SAMs will also be presented. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X17.00008: Phase Transitions of Hexadecanethiol Self-Assembled Monolayers on Polycrystalline Silver Studied by NanoDSC Liang Hu, Leslie Allen Hexadecanethiol self-assembled monolayers (SAMs) grown on polycrystalline Ag planar surfaces (2D) and nanoparticles (3D) are studied by measuring heat capacity with NanoDSC. Two different melting transitions with peak temperature TM1=115$^{\circ}$C and TM2=127$^{\circ}$C are observed, which indicates two ordered phases exist. SAMs with TM2 are thermodynamically preferred, and the melting characteristics are comparable to that of Ag-Alkanethiolate layered materials precipitated from solution as reported in the literature. Three different states of the system can be obtained by specific heating and cooling schedule: (1) upon fully annealing, a single phase with TM2 is observed; (2) partially annealing results in the coexistence of both phases; (3) upon rapid quenching from high temperature, a single phase with TM1 can be obtained. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X17.00009: Kinetics of growth and assembly of ordered array of non-coalescing water droplets over evaporating polymer solutions Vivek Sharma, Mohan Srinivasarao Breath figures form over cold solid or liquid substrates on contact with humid air. Typically, the water drops exhibit a range of sizes, self-similar growth marked by coalescence in late stages, and final surface coverage is limited to 0.55. Breath figures formed on evaporating polymer solutions in contact with the blast of humid air, have drops that can grow without coalescence, self-assembling into close packed arrays of nearly monodisperse drops with surface coverage approaching 0.90. In this study, we elucidate the mechanism of drop growth, by considering the growth kinetics of a droplet population under the mass and heat transport of water vapor that are intimately coupled with the corresponding fluxes of the evaporating solvent. We examine the role of solvent and polymer in controlling the kinetics of growth and assembly of droplets, which eventually evaporate away producing a polymer film with ordered array of air bubbles. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X17.00010: Molecular Dynamics in Self-Assembled Monolayers Jason Bochinski, Derrick Stevens, Mary Scott, Laura Guy, Casey deDeugd, Laura Clarke Silane self-assembled monolayers (SAMs) are an important tool for both scientific research and technological applications. Despite their widespread use, few experimental investigations have addressed molecular motion within these films, which offer a unique and useful physical system for fundamental scientific studies, such as observing dipolar and other glass transitions in two-dimensions. In addition, relaxations such as ``rotator'' phases where molecular groups rotate in a plane parallel to the surface have been correlated with film conductivity, adhesive, and wetting properties. We utilize surface-sensitive, dielectric relaxation spectroscopy to probe molecular motion as a function of temperature within silane chemistry-based monolayers formed upon interdigitated electrodes. Our latest results exploring a previously published motion as well as comparisons to linear polymer films will be discussed. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X17.00011: Thickness study of Langmuir-Blodgett Films of Copolymers of Vinylidene Fluoride with Trifluoroethylene using X-ray Reflectivity Jihee Kim, Stephen Ducharme, Shireen Adenwalla Nanometer thickness scale control is one of the advantages of ferroelectric polymer films made by Langmuir-Blodgett (LB) deposition technique, compared to conventional techniques, such as solvent spin coating and casting, but polymers do not always form true monomolecular layer. Therefore, film thickness must be calibrated independently. We report measurements of sample thickness using x-ray reflectivity (XRR), a powerful tool to measure the thickness of ultrathin films, which is also suitable for polymer LB films. The XRR data from LB films of copolymers of vinylidene fluoride with trifluoroethylene deposited on thick silicon wafers exhibit up to six interference oscillations. Fitting by the Kiessig fringe method results thickness measurements averaging 2.8 $\pm $ 0.2 nm, 2.6 $\pm $ 0.2 nm, and 2.3 $\pm $ 0.2 nm per LB transfer, for copolymers consisting of 80{\%}, 70{\%} and 50{\%}, respectively, of vinylidene fluoride, which means that 1 LB transfer consists of approximately 6 molecular layers. The results are consistent with ellipsometric measurements made on similar films [M. Bai \textit{et al.}, \textit{J. Appl. Phys. }\textbf{95}, 3372 (2004)]. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X17.00012: Electro-Optic Polymer Films for Reconfigurable Photomask Applications Adam Fontecchio, Anna Fox Holographically formed polymer dispersed liquid crystal (H-PDLC) films have unique electro-optic properties including the ability to selectively reflect or transmit a particular wavelength as a function of bias applied to the film. The proposed application formed in this medium is a real-time dynamically reconfigurable mask for the UV exposure step in the photolithographic process. Current photolithography technology requires a static mask to prevent UV exposure of selected areas on the patterning surface in order to form structures in photoresist. The exposure process must be repeated several times with different masks to fabricate 3D structures. A real-time reconfigurable mask allows simple generation of 3D structures including peaks, valleys, and grades in the resist substrate. The H-PDLC photomask consists of a film array with independent control over each pixel. Bias is applied across the selected pixels to allow UV exposure in that region. Change in applied bias allows transmission, reflection, and grayscale during a single UV exposure. Proof of concept is shown for a 9-pixel array with passive-matrix style control over each pixel. Samples of cured photoresist in several configurations using the 9-pixel mask are evaluated. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X17.00013: Interfacial Characteristics of a Potentially Anti-befouling Highly Rigid Ionomer Christopher J. Cornelius, Cy H. Fujimoto, Lilin He, Dvora Perahia Ionic polymers with a potential to form a water purification membrane inhibit biofouling activity. The onset of biofouling involves release of polysaccharides by bacteria that adhere to the membrane surface, serve as bedding and nutrients and allow further bacterial growth. This stage is controlled by the interfacial energy and morphology of the membranes. It is often followed by irreversible structural changes, as fouling propagates. The surface energy as a function of time of contact with water of sulfonated substituted \textit{para}phenylenes was studied as a first step in understanding the mechanism of adhesion of bacteria to the membrane. Surface energies increase with increasing exposure times of the membranes with water, indicative of rearrangements in which more sulfonated groups migrate to the surface. Ongoing studies are currently on the way to correlate adhesion of alginate, a model polysaccharide and the interfacial characteristics of the membranes. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X17.00014: Interfacial Effects of Nanometer Fluorinated Segments on Energy Controlled Responsive Polymeric Films Dvora Perahia, Alma Gonzales, Dennis W. Smith Jr. Responsive polymeric thin films with controlled surface energy, dielectric constant and structure are critical for a variety of immerging technologies such as microfluidics, nano-electro-optical devices and nano-biotechnology. Introducing nanometer scale fluorinated segments into polymers offers a means to control surface energies and dielectric constants while enhancing significantly the chemical and thermal stability of materials. Fluorinated and protonated segments are highly incompatible and differ in their interfacial affinities. Introducing nanometer scale fluorinated segments offers a mechanism for the polymer to rearrange and respond to changes in the environment while retaining the integrity of the layers. The interfacial energies couples with structure were studies as a function of temperature for PFCB \textit{(perfluoro cynclobutadiene)} based co-polymers. The structure varies from amorphous to liquid crystalline to crystalline with very small variations in comparison to protonated co-polymers. The polymer-air interface is found to be fluorine rich, however interfacial energy is found to be a complex function of the amount of fluorine and the conformation of the fluorinated segments at the interface. [Preview Abstract] |
Session X18: Biopolymers II: Simulations
Sponsoring Units: DPOLY DCOMPChair: Daniel Savin, University of Vermont
Room: Colorado Convention Center 103
Friday, March 9, 2007 8:12AM - 8:24AM |
X18.00002: Statistical Mechanics of Membrane Proteins. Karim Wahba, Robijn Bruinsma The statistical mechanics of polymers is applied to the self-assembly of membrane proteins. We find a two stage process as a function of the attractive interactions between the membrane bound portions of the polymer. First individual trans-membrane helices develop connected by Gaussian loops. Secondly the alpha helices interact with each other, both entropically through the loops and enthalpically through the attractive interaction. In the phase diagram the first stage is found to be a continuous phase transition where as the aggregation of the helices is seen to be first order. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X18.00003: Mechanical unfolding of proteins: reduction to a single-reaction coordinate unfolding potential, and an application of the Jarzynski Relation Peter Olmsted, Daniel West, Emanuele Paci Single molecule force spectroscopy (AFM, optical tweezers, etc) has revolutionized the study of many biopolymers, including DNA, RNA, and proteins. In this talk I will discuss recent work on modelling of mechanical unfolding of proteins, as often probed by AFM. I will address two issues in obtaining a coarse-grained description of protein unfolding: how to project the entire energy landscape onto an effective one dimensional unfolding potential, and how to apply the Jarzynski Relation to extract equilibrium free energies from nonequilibrium unfolding experiments. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X18.00004: Boundary Element Microhydrodynamics: Stagnation of flow in protein cavities Sergio Aragon, David Hahn A very precise boundary element solution of the exact Stokes flow surface integral equation has been implemented in our Fortan 90 program BEST. In our previous work (Aragon {\&} Hahn, Biophys. J. 2006, 91: 1591-1603; J. Chem. Theory and Comput. 2006, 2: 1416-1428) we obtained very precise values of the tensorial transport properties (translation, rotation, and intrinsic viscosity) for a large set of proteins with a uniform water hydration thickness of 0.11 nm. In this work, we utilize the surface stress distribution thus obtained to evaluate the flow field as a function of distance away from the hydrodynamic surface for a variety of surface features in a dimpled sphere (test case) and for the proteins myoglobin, lysozyme, and human serum albumin. We demonstrate that solvent in small to large pockets on the hydrodynamic surface moves with the protein with distances up to 2 nm for deep pockets regardless of the direction of motion of the protein. On the other hand, the fluid flow pattern on protruding portions of the hydrodynamic surface decays much more rapidly with distance from the surface. The implications of these results with respect to the amount of water of associated with the surface and the rate of transport to active enzymatic sites in stirred solutions is discussed. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X18.00005: Electrostatic theory of viral self-assembly: Structure and Kinetics Tao Hu, Boris Shklovskii Viruses self-assemble from identic capsid proteins and their genome consisting, for example, of a long single stranded (ss) RNA. For a big class of T = 3 viruses capsid proteins have long positive flexible N-terminal tails. We explore the role played by the Coulomb interaction between the N-terminal tails and negative ss RNA molecule in the kinetics of virus self-assembly. Capsid proteins stick to unassembled chain of ss RNA (antenna) and slide on it towards the self-assembly site. We show that due to such one-dimensional diffusion the virus self-assembly is more than ten times faster than the case involving only three-dimensional diffusion. In the assembled virus, the ss RNA strongly interacts with the brush of tails rooted at the inner surface of the capsid. We show that viruses are most stable when the total length of ss RNA is close to the total length of the tails. For such a structure the absolute value of total (negative) charge of ss RNA is approximately twice larger than the charge of the capsid. This conclusion agrees with available structural data. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X18.00006: Electrophoresis of DNA on a disordered two-dimensional substrate Cynthia J. Olson Reichhardt, Charles Reichhardt We propose a method for electrophoretic separation of DNA in which adsorbed polymers are driven over a disordered two-dimensional substrate which contains attractive sites for the polymers. Using simulations of a model for long polymer chains, we show that the mobility increases with polymer length, in contrast to gel electrophoresis techniques, and that separation can be achieved for a range of length scales. We demonstrate that the separation relies on steric interactions between polymer segments, which prevent substrate disorder sites from trapping more than one DNA segment each. Since thermal activation does not play a significant role in determining the polymer mobility, band broadening due to diffusion can be avoided in our separation method. [1] Phys. Rev. E 74, 051908 (2006). [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X18.00007: Shape of DNA in a box Ya Liu, Bulbul Chakraborty, Jane' Kondev The statistical and mechanical properties of DNA are known to change dramatically when the persistence length of DNA is comparable to the linear dimensions of the viral capsid and DNA is strongly bent. Based on modeling semiflexible DNA by worm-like chain model, we make use of the Bond Fluctuation Algorithm to study the behavior of DNA in confinement. The effective persistence length is extracted from tangent-tangent correlation function. We will present results for (a) the dependence of effective persistence length on the bare persistence length and (b) a shape transition accompanying increasing confinement. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X18.00008: Polymer dynamics in a tight squeeze Jeremy Schmit, Ercan Kamber, Joshua Kalb, Bulbul Chakraborty, Jane' Kondev Chromosomal DNA is confined to a space roughly an order of magnitude smaller than its natural radius of gyration due to the nuclear or cellular membrane. In addition to altering the observed static distributions of the chromosome, this confinement has the potential produce dynamics that differ from the Rouse/Zimm dynamics of the free chain. We propose a model for studying the dynamics of polymers under confinement that can be extended more generally to particle diffusion in a crowded environment. We find that as the size of the confining box is reduced, initially the relaxation times of the Rouse modes decrease due to the reduced phase space accessible to the polymer. However, in the strongly confined regime the relaxation times increase with decreasing box size due to jamming. We demonstrate this non-monotonic behavior using a lattice toy model as well as Monte Carlo simulations. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X18.00009: $\lambda$-DNA thermal migration in a microchannel Jennifer Kreft, Yeng-Long Chen The Ludwig-Soret effect, the migration of a species due to a temperature gradient, has been extensively studied without a complete picture of its cause. Many particle and polymer solutions have been used to study the phenomena; here we investigate the dynamics of DNA confined to a microchannel and subjected to a thermal gradient using a combination of Brownian dyanmics and the lattice Boltzmann method. We observe that the DNA molecules will migrate to colder regions of the channel, an observation also made in the experiments of Duhr, et al (Eur. Phys. J. E, \textbf{15}, (2004)). In fact, the thermal diffusion coefficient found for a range of temperature gradients and DNA molecular weights agrees nearly quantitatively with the experimental value. Furthermore, we use the simulation to understand how the interaction of the DNA with the solvent leads to thermal migration via the thermal fluctuations-fluid momentum flux coupling. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X18.00010: Force-extension relation of DNA-histone complexes A.J. Levine, Mark L. Henle, Tom Chou In eukaryotic cells, DNA is packaged inside the nucleus in the form of chromatin, a structure whose basic repeat unit, known as the nucleosome, consists of DNA wrapped around a cylindrical complex of histone proteins. In order for the cell to function properly, these nucleosome complexes must be stable at equilibrium. At the same time, the cell must be able to gain access to the genomic information contained within the DNA, which it can achieve by exerting forces on the nucleosomes that cause the DNA to unwrap from the histones. Single molecule mechanical manipulation techniques, in which DNA/histone complexes are disrupted by an external force, can provide information not only about the equilibrium structure of these complexes, but also about the forces and displacements required to access the DNA in the nucleosome. In this talk, we derive the force-extension relation for these complexes. We allow for the DNA to unwrap from the histones in both a continuous and discontinuous fashion; that is, we allow the histones to ``pop'' off of the DNA, releasing a large amount of DNA in the process. We also include the conformational fluctuations of the unwrapped portions of the DNA. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X18.00011: Monitoring the Bending Stiffness of DNA Chongli Yuan, Xiongwen Lou, Elizabeth Rhoades, Huimin Chen, Lynden Archer In eukaryotic cells, the accessibility of genomic sequences provides an inherent regulation mechanism for gene expression through variations in bending stiffness encoded by the nucleic acid sequence. Cyclization of dsDNA is the prevailing method for determining DNA bending stiffness. Recent cyclization data for short dsDNA raises several fundamental questions about the soundness of the cyclization method, particularly in cases where the probability of highly bent DNA conformations is low. We herein evaluate the role of T4 DNA ligase in the cyclization reaction by inserting an environmental sensitive base analogue, 2-amino purine, to the DNA molecule. By monitoring the 2-AP fluorescence under standard cyclization conditions, it is found that in addition to trapping highly-bent cyclic DNA conformations, T4 DNA ligase enhances the apparent base pair flip out rate, thus exaggerating the measured flexibility. This result is further confirmed using fluorescence anisotropy experiments. We show that fluorescence resonance energy transfer (FRET) measurements on suitably labeled dsDNA provides an alternative approach for quantifying the bending stiffness of short fragments. DNA bending stiffness results obtained using FRET are compared with literature values. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X18.00012: Validity of the bead-spring model for describing the linear viscoelastic properties of single-strand DNA under strongly denaturing conditions Semant Jain, Ronald Larson Using a normal mode analysis, we predict the infinite dilution linear viscoelastic properties of single strand-DNA molecules and compare the results to the linear viscoelastic data of Shusterman et al. (Shusterman, Alon, Gavrinyov, and Krichevsky, 2004) obtained by monitoring the diffusion of a fluorescently labeled terminus of the molecule. To compute the overall best global fit, we constrain the hydrodynamic interaction parameter, $h*$, equilibrium rms spring extension, $b$, and the number of Kuhn steps per spring, $N_{K,S}$, to be equal for the strands compared. The fits using the bead-spring model for all but 23100 base ss-DNA strands match the experimental data at long times with significant deviations at intermediate and short times. However, parameters fitted separately to all individual strand lengths predict results well. The best-fits to data for 2400 and 6700 base pairs yield $N_{K,S} \quad \sim $ 12 and $h*$ = 0.12. These values are similar to those found for conventional polymers such as polystyrene which have been successfully modeled with $N_{K,S} \quad \sim $ 7 and $h*$ = 0.15, indicating ss-DNA and polystyrene exhibit analogous hydrodynamic behavior. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X18.00013: Dynamics of particles with key-lock interactions Nicholas Licata, Alexei Tkachenko We present a theoretical discussion of particles which interact through the reversible formation of multiple key-lock bridges. Two potential experimental realizations include DNA- grafted particles which interact with a two-dimensional DNA substrate, and particles grafted with antibodies interacting with a protein substrate. We argue there is a percolation transition characterized by the average number of bridges realized between a particle and the substrate. The transition separates a regime in which particles are localized from a diffusive regime where they explore the substrate surface through mutiple breaking and reconnecting of bridges. This diffusion behavior is dispersive, characterized by $\langle r^ {2}(t) \rangle \sim t^{\alpha}$ with $\alpha<1$. The distribution of departure times in a multi-particle system is calculated in two different models which account for the particle dynamics above and below the percolation threshold. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X18.00014: Molecular Dynamics Simulation of semi-flexible filament assembly Lam T. Nguyen, Qi Wang, Ziyad Muslimani, Linda S. Hirst An MD simulation has been developed to study semi-flexible filament bundling and network formation by cross-linkers in solution. We aim to model and understand the ordered mesoscale structures observed experimentally by F-actin, in which different network configurations occur for different concentrations of cross-linking protein. [1]. We use a bead-rod model for the semi-flexible filament and linkers, which can be easily adapted to different cross-linking proteins. Electrostatic interactions were shown to be the main mechanism for the aggregation process, Coulombic forces between excess charges on the proteins dominate at the long range inducing the assembly, while at short range the Van der Waals interaction and specific binding potentials of the proteins have been taken into account. We discuss the effects of screening the Coulomb interaction not only for the linker conc. at which a phase transition occurs but also on bundling and network configuration above the transition point. These results are in good agreement with experimental observations of actin filament bundling and assembly. [1] \textbf{L.S. Hirst} et al J. CHEM. PHYS. 123, 104902~(2005) [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X18.00015: Ion condensation near patterned surfaces Yury Velichko, Francisco Solis, Sharon Loverde, Monica Olvera de la Cruz Using the exactly solvable model we have studied ion condensation near patterned surfaces. Competition between electrostatic energy of interaction with the surface and the energy of ionic crystal of correlated condensed ions result in reach phase behavior. Our results show that the structure of ionic crystal of condensed ions essentially depends on the surface charged density and the size of the pattern. Our results can contribute to understanding of different closed phenomena, like surface pattern recognition, separation and synthesis of molecules near patterned surfaces. [Preview Abstract] |
Session X19: Surfaces, Interfaces, and Colloids II
Sponsoring Units: DCPChair: Steve George, University of Colorado
Room: Colorado Convention Center 104
Friday, March 9, 2007 8:00AM - 8:12AM |
X19.00001: Low Energy Inelastic Helium Atom Scattering from a Monolayer Solid. L.W. Bruch, F.Y. Hansen A time-dependent wave packet calculation for inelastic low energy atomic scattering by a physisorbed monolayer\footnote{L. W. Bruch and F. Y. Hansen, J. Chem. Phys. {\bf 122}, 114714 (2005)} has demonstrated that $^4$He atoms incident on an incommensurate monolayer solid of Xe/Pt(111) readily excite shear horizontal monolayer phonons. Adding an absorbing potential has enabled the calculation to be extended to such long times that transient trapping has decayed and the scattering event is nearly complete. Extending the spatial grids has enabled the use of more nearly monochromatic, spatially broad, wave packets and the inelastic intensities are found to be sensitive to variations of scattering energy by 0.3 meV at incident energies of 4 to 9 meV. Trends for the inelastic intensities at low incident energies, near thresholds, are determined and correlations between the inelastic scattering results and the time development of the elastic scattering are discussed. These calculations were stimulated by the systematic body of experimental data available for $^4$He scattering by a Xe/Pt(111) monolayer\footnote{L. W. Bruch, A. P. Graham, and J. P. Toennies, J. Chem. Phys. {\bf 112},314 (2000)}. [Preview Abstract] |
Friday, March 9, 2007 8:12AM - 8:24AM |
X19.00002: Study of Oxidation of Silicon by X-ray Photoelectron Spectroscopy A.R. Chourasia, William Johnston, R.L. Miller, Ryan Jacob The oxidation of silicon has been investigated using the technique of x-ray photoelectron spectroscopy. The silicon substrates have been exposed to oxygen at a partial pressure of 5 $\times $ 10$^{-6}$Torr. The exposure was done at substrate temperatures of 100\r{ }C, 200\r{ }C, 300\r{ }C, 400\r{ }C, 500\r{ }C, and 600\r{ }C, The substrates have been analyzed by angle resolved XPS. The magnesium anode (energy = 1253.6 eV) have been used for this purpose. The silicon 2p and oxygen 1s core level regions have been investigated. The spectral data have been recorded at 10\r{ }, 20\r{ }, 40\r{ }, 60\r{ },80\r{ }, and 90\r{ } take-off angles. The QUASES software has been used to determine the thickness of the oxide layer formed on the substrates. The experimental data on the thickness of the oxide layer has been correlated with that obtained from the QUASES model. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X19.00003: Ab Initio Calculations for Br and Cl adsorption on the Ag(100) surface Tjipto Juwono, Ibrahim Abou Hamad, Per Arne Rikvold Ab-initio density-functional methods have been used to find the ground-state configurations of Br and Cl adsorbates on Ag(100) surfaces with coverages of $1/9$, $2/9$, $1/4$, $1/3$, and $1/2$ monolayers. The supercell slab method was used to calculate the electron-density distributions for each configuration. The charge-transfer function, surface dipole moments, adsorbate resident charge, and adsorption energies were calculated and compared with results from electrochemical adsorption exeperiments and Monte Carlo simulations. The lateral adsorbate-adsorbate interactions and the binding energies were extracted from the adsorption energies using a lattice-gas model. The calculated quantities are weakly dependent on the coverage, and the overall shape of the charge-transfer function is nearly coverage independent. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X19.00004: Cracks, Meltdowns and Crossover Sizes: An abrupt change in sublimation kinetics associated with the thermally-activated introduction of disclination charge in crystallites. Alex J. Levine, Moumita Das, Don Blair Recent experiments and numerical studies of the sublimation kinetics of 2d colloidal crystals show an abrupt increase in the sublimation rate at a particular crystallite size [J. R. Savage {\it et. al.} Science {\bf 314}, 795(2006)]. Motivated by this observation, we propose that the abrupt change in the sublimation kinetics is due to the thermally activated introduction of a disclination charge leading to large internal stresses. These stresses are then relaxed by a fission event precipitating the break-up of the remaining crystallite. We use our numerical simulations to show that the average disclination charge indeed increases to one at the `crossover size' corresponding to the observed change in sublimation rate. Using the Griffith criterion for the spontaneous propagation of microscopic cracks, we see that the effect should depend sensitively upon the range of the attractive interparticle potential. We test this prediction using numerical simulations of the sublimating system. Where that potential is short-ranged, the crystal is brittle allowing for the proposed mechanism. For longer-ranged potentials, however, the material is more ductile preventing this abrupt fission event. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X19.00005: The structure of a C$_{60}$ monolayer on Ag(111) Renee Diehl, Hsin Li, Kelly Hanna, Wolfgang Moritz The structure of a monolayer of C$_{60}$ on Ag(111) was studied using dynamical LEED, the first such study for C$_{60}$ molecules. The C$_{60}$ monolayer adopts a commensurate hexagonal (2$\surd $3x2$\surd $3)R30\r{ } structure with a nearest-neighbor separation of 10.0 {\AA}. LEED intensities were measured at a sample temperature of 32 K using incident beam energies up to 600 eV. The LEED analysis was performed using conventional methods adapted for large molecular adsorbates, with up to 15 phase shifts to describe the scattering potential. The structure of the monolayer consists of C$_{60}$ molecules occupying the top sites with their hexagonal faces down, at a distance of 2.27 {\AA} above the Ag(111) surface. There is an accompanying deformation of the Ag surface that involves the downward displacement of the Ag atoms nearest to the C$_{60}$ molecules, consistent with a charge transfer between the surface and the molecule. The C$_{60}$ molecules have a significant librational motion about their vertical axes, even at 32 K. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X19.00006: Accelerated Molecular Dynamics Simulation of Thermal Desorption Kelly Becker, Kristen Fichthorn Thermal desorption has been the focus of much surface science research recently. Alkane desorption experiments on graphite [1] show a prefactor that is constant with chain length, while experiments on magnesium oxide [2] show a prefactor that increases with chain length. We utilize an all-atom model to study alkane desorption from graphite. Transition state theory is used to obtain rate constants from the simulation. Accelerated molecular dynamics techniques are used to extend the simulations to experimentally relevant temperatures. Our results provide an explanation [3] for this seemingly contradictory functionality of the prefactor. We also examine the effect that film structure has on the rate of desorption and the shape of the desorption profile through varying coverage. [1] K.R. Paserba and A.J. Gellman, \textit{J. Chem. Phys. }\textbf{115}, 6737 (2001). [2] S.L. Tait et al., \textit{J. Chem. Phys. }\textbf{122}, 164707 (2005). [3] K.E. Becker and K.A. Fichthorn, \textit{J. Chem. Phys.} \textbf{125, }184706 (2006). [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X19.00007: Irreversible nanoparticle adsorption on patterned substrates A. Cadilhe, N. A. M. Araujo, V. Privman The adsorption of nanoparticles on a surface has interest in fields like heterogeneous catalysts and quantum dots. We simulate the monolayer adsorption of nanoparticles on patterned substrate. We adopted a pattern consisting of equal squares of size $\alpha$ and a distance $\beta$ apart from each other, and characterize the system, by reckoning the mean value and variance of the distance between the nanoparticles and the radial distribution function of their distances. Proper control of $\alpha$ and $\beta$ parameters leads to morphologies range from lattice to homogeneous, with interesting non-trivial behaviors in between. Our study shows the relevance of geometrical constraints to obtain different morphologies of colloidal monolayer films with potential for practical applications. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X19.00008: Subsurface oxygen stabilization by a third species: Carbonates on Ag(210) Pushpa Raghani, Letizia Savio, Andrea Gerbi, Luca Vattuone, Mario Rocca, Nicola Bonini, Stefano de Gironcoli Subsurface species have often been invoked to explain the activation of catalytic surfaces for specific reactions. In particular, subsurface oxygen is thought to be important for the chemistry of Ag catalysts. Here we show by high resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS) combined with density functional theory (DFT) and density functional perturbation theory (DFPT) that on Ag(210) surface, the subsurface oxygen is stabilised more efficiently by carbonates than by oxygen adatoms or when there is no supersurface oxygen present. Experimentally a maximum of six subsurface oxygens are found to be stabilised by each carbonate. These results could have an importance in catalytic reactions where subsurface oxygen is known to play a crucial role. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X19.00009: Computer simulations of the Adsorption of Xenon onto a C$_{60}$ monolayer on Ag (111) Silvina Gatica, Milton Cole, Renee Diehl We performed Grand Canonical Monte Carlo simulations to study the adsorption of Xenon on a substrate composed of C$_{60 }$molecules placed on top of a Ag(111) surface. The C$_{60}$ molecules form a commensurate structure at a distance of 0.227 nm above the Ag surface. The interaction potential between the Xe atoms and the substrate has two contributions: from the C$_{60}$ molecules and from the Ag atoms. In the simulations, the interaction with the Ag surface was computed using an \textit{ab initio} van der Waals potential, varying as 1/d$^{3}$. The interaction between the Xe atoms and each C$_{60}$ molecule was computed using a potential previously developed by Hernandez et.al. (E. S. Hernandez, M. W. Cole and M. Boninsegni, ``\textit{Wetting of spherical surfaces by quantum fluids}'', J. Low Temp. Phys. 134, 309-314 (2004)), who integrated the Lennard Jones interaction over the surface of a spherical buckyball. The total potential has especially attractive 3-fold sites, positioned~ 0.4 nm above the point between each three buckyballs. ~The low coverage uptake populates those sites, and then continues forming a monolayer. The adsorption isotherms show several steps, typical of substrates that have distinct adsorption sites. We compare the results with the experimental data. ~ [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X19.00010: First-principles study of polar molecule adsorption on hydrogenated diamond (001) Yong-Hyun Kim, S. B. Zhang, Yang Yu, L. F. Xu, C. Z. Gu Density functional theory calculations reveal that adsorption of small polar molecules such as H$_{2}$O, HF, and HCl on hydrogenated diamond (001) surfaces can result in unexpected dihydrogen bonding. This implies that in a C-H bond, H may be more electronegative than C, thus deviating from the widely-accepted Pauling's electronegativity scale, C(2.55) \textit{vs} H (2.20). Detailed analysis of the self-consistent charge densities confirms that electrons are indeed accumulated more at the H site than at the C site with respect to the free atoms. It further explains why dihydrogen bond can form for H$_{2}$O and HF on the surfaces, but not for NH$_{3}$. The true physical origin for the well-known reduction of the work function due to surface hydrogenation is now attributed to the replacement of loosely bonded surface C $\pi $ electrons by more tightly bonded C-H $\sigma $ electrons. We also propose that the favorable formation of the dihydrogen bonds may contribute to the observed p-type conductivity of diamond surfaces in acidic conditions. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X19.00011: STM/S study of polycyclic aromatic hydrocarbons on Co (0001) Daejin Eom, Michael Lefenfeld, Kwang Taeg Rim, Li Liu, Shengxiong Xiao, Colin Nuckolls, Tony Heinz, George Flynn The fascinating physical properties of carbon nanotubes (CNT) have attracted attention for more than a decade. Both practical and scientific uses of CNTs have, however, been hindered by the incomplete synthetic control of nanotube structure (diameter and chiral angle). Understanding of growth at the microscopic level may advance our ability to control nanotube chirality. We have consequently explored the interaction of a cobalt substrate, a common catalyst for the growth of CNTs, with polycyclic aromatic hydrocarbon molecules, such as hexabenzocoronene (HBC). Using ultra-high vacuum, low-temperature scanning tunneling microscopy (STM), we have examined the changes in HBC topographic features and vibrational spectra that are induced by thermal annealing of the adsorbed molecules. The potential of hydrocarbon molecules like HBC as end caps for seeded growth of CNT of specific chirality will be discussed. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X19.00012: Electrocatalytic property of PtBi and PtPb line compounds via DFT Lin-Lin Wang, D. D. Johnson A major obstacle to practical, mass market fuel cell (e.g. hydrogen and direct methanol) technology is CO poisoning of Pt anode. Pt alloys, such as disordered Pt$_{x}$Ru$_{(1-x)}$, are known to have an increased CO-tolerance. There has been significant effort to understand the mechanism for increased CO- tolerance and to design better catalyst via alloyed nanoparticles and surface alloys. Alternatively, Pt intermetallic compounds, such as with Bi and Pb, have been observed to improve dramatically the CO-tolerance. [E. Casado- Rivera et al. ChemPhysChem 4, 193 (2003) and J. Am. Chem. Soc. 126, 4043 (2004)] Here we use density functional theory to study the adsorptions of CO, H and OH on these materials. We find that (100) and (110) surfaces of PtBi and PtPb line compounds have lower cleavage energy than (001) surface. Adsorption energies and electronic structure are examined to explain the increased CO-tolerance. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X19.00013: Surface adsorbates and mechanical dissipation in micromechanical resonators Thomas Metcalf, Bradford Pate, Jeffrey Baldwin, Brian Houston, Maxim Zalalutdinov Temperature dependence measurements of the mechanical quality factor of a silicon micromechanical resonator found a pronounced dissipation peak in the neighborhood of 160 K, the magnitude of which reduced dramatically upon in-situ annealing\footnote{Haucke, \textit{et al.}, Appl. Phys. Lett. \textbf{86}, 181903 (2005)}. Present in all of the resonator's normal modes, the peak is sufficiently broad so that the mechanical dissipation was observed to decrease with increasing temperature near room temperature, indicating that this loss mechanism contributes significantly to the room-temperature dissipation value. The leading candidates for the origin of the dissipation are surface adsorbates (e.g. water). We report an investigation of the dissipation of micromechanical resonators (between $\sim100$ K and room temperature) with carefully prepared and characterized surfaces as a function of adsorbate and of adsorbate coverage. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X19.00014: An STM Study of the Interaction of hexabenzocorone on a Ru(0001) Surface Kwang Taeg Rim, Daejin Eom, Li Liu, Shengxiong Xiao, Michael Steigerwald, Mark Hybertsen, Colin Nuckolls, George Flynn The interaction of hexabenzocorone (hbc) with Ruthenium surface has been investigated using Scanning Tunneling Microscopy in ultrahigh vacuum. The images obtained at 77K and 4.5K, after hbc molecules were vacuum deposited at 325$^{o}$C onto a pristine Ru(0001) surface, exhibit surface bound molecules with off-centered bonding sites. After annealing the hbc-Ru complex at 600$^{o}$C for 15min, hbc molecules appear to be dehydrogenated to form the bowl-shaped C$_{48}$. The nature of the interaction and the dehydrogenation of the hbc on Ru(0001) surface will be discussed with DFT calculation, the supplemental IR absorption, and Temperature Programmed Desorption studies. The possibility of growing carbon nanotubes on C$_{48}$ end-cap upon dosing with C$_{2}$H$_{2}$ will also be discussed. [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X19.00015: Commensurate ground states of monolayers on surfaces Alexandre Tkatchenko We present a procedure for generation of \textit{all} commensurate monolayer-surface structures of a given symmetry, up to a certain number of adsorbate particles (atoms or molecules), $N_{ads}$, in the unit cell. It is shown that each cell is related to a well-defined sequence of Fourier terms of the single particle-surface potential. Most importantly, the knowledge of Fourier amplitudes alone is sufficient to exactly predict the ground states of commensurate structures in multi-adsorbate unit cells. The impact of the developed theory for theoretical (i.e. DFT simulations) and experimental (LEED studies) determination of commensurate monolayer ground states is briefly discussed. Furthermore, the experimental results for epitaxy of alkali atoms on the Ag(111) surface and iodine on the Pt(111) surface can be described by this approach, in contrast to previous epitaxy theories. [Preview Abstract] |
Session X21: General Theory
Sponsoring Units: DCOMPChair: David Tanner, University of Florida
Room: Colorado Convention Center 106
Friday, March 9, 2007 8:00AM - 8:12AM |
X21.00001: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 8:12AM - 8:24AM |
X21.00002: Topological Characterization of Time Reversal Invariant Systems Rahul Roy We study $Z_2$ invariants for time reversal invariant systems in two and dimensions and discuss in particular the novel fourth $Z_2$ invariant in three dimensions. We present heuristic as well as rigorous arguments justifying the invariance of the $Z_2$ topological numbers and discuss the consequences of the formulation in terms of the Chern numbers to the surface state spectrum. We also present and discuss models in which the various 3d topological phases can be seen. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X21.00003: Renormalization of QED at Finite Temperatures. Samina Masood Renormalization of QED at different temperatures is studied in the background of real particles. It is explicitly shown that the second order thermal modifications to electromagnetic properties of a hot medium are nonzero at temperatures below the electron mass. However the second order contributions are smaller than the first order contribution at these temperatures which ensures the renormalization of QED in this situation. However, the situation changes at high temperatures. The overlap of hot infrared singularities in Lorentz invariance breaking formalism with usual cold ultraviolet divergences of vacuum has to be handled in a special order to avoid un-removable singularities. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X21.00004: A Black Body Radiation Law in a Gravity Free Vacuum Clarence A. Gall The interpretation of black body radiation in terms of Planck's quantum hypothesis in 1900 largely defined the science of the twentieth century. Einstein was one of the vocal few in opposition to this approach because it led to the rejection of strict causality in science. He sought without success a solution of the radiation problem without light quanta, a problem he viewed as incredibly important as well as difficult. His approach appears to have involved `the energy principle.' This paper proposes a solution to this problem that is considered to meet Einstein's criteria. It uses an apportioning function developed on the basis of Einstein's energy-mass principle $(E=mc^{2})$ that distributes the total thermodynamic Stefan-Boltzmann energy over the entire wavelength range. This black body distribution function $\left( I_{\lambda }d\lambda =\frac{\sigma T^{6}}{w^{2}}\lambda e^{-\frac{T}{w}\lambda }d\lambda \right) $ also results in a new temperature scale with units of reciprocal wavelength, which unifies the thermodynamic and colour scales. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X21.00005: Einstein's Obscure 1918 Special Relativity Paper Tom Morton Few people are aware of the existence of Einstein's 1918 paper ``Dialogue about Objections to the Theory of Relativity.'' In 1918 he attempted to resolve some perceived asymmetry issues in Special Relativity but did not reach any sufficiently satisfactory conclusion. This presentation reviews Einstein's 1918 efforts. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X21.00006: What are the Sands of Titan Telling Us About Gravity? Jerry Jensen The mass distributions of Mars and Venus are determined by carefully measuring the perturbations of our orbiting probes. All volcanic peaks on the planet Mars exhibit positive Bouguer gravity anomalies. Mars chasma exhibit negative Bouguer gravity anomalies. The exact opposite is true of Venus: Our orbiting probes indicate Venus mountains exhibit negative Bouguer gravity anomalies while all Venus chasma exhibit positive gravity anomalies. These incongruencies can be identified as artifacts, if the Newtonian equivalence principle incorrectly predicts the masses of orbiting bodies. The curious spectra and surface features of Titan and other moons are better explained if much greater masses can be assigned to the outer planetary systems. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X21.00007: Unifying the Geometry of General Relativity with the Virtual Particle Nature of Quantum Theory John Laubenstein General Relativity (GR) and Quantum Electro-Dynamics (QED) utilize different underlying assumptions regarding the nature of vacuum and space-time. GR requires the actual geometry of space-time to change in the presence of mass resulting in gravitation. QED operates within flat space-time and propagates forces through the exchange of virtual photons. Efforts to unify these theories are -- despite their mathematical elegance -- complex, cumbersome and incomplete. The inability to achieve unification may suggest a need to re-think basic conceptual models. The IWPD Research Center has found evidence suggesting that time -- as a unique degree of freedom -- may be illusionary. Our research suggests that time may be ``embedded'' within a spatial dimension through a geometric manipulation of the light cone in Minkowski space-time. This interpretation of space-time provides predictions that are experimentally verifiable and suggests a conceptual path for the unification of GR and QED. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X21.00008: Rotational and Vibrational Kinetic Energies Should be Included in Mass-Energy Calculations Stewart Brekke Conversion of mass to energy involves more than rest mass. At the subatomic, nuclear, atomic and molecular levels there are rotational, spin and vibratory kinetic energies although small may account for some discrepancies between theory and experiment. In pair annhilation spin and vibratory kinetic energies should be included in calculations. Thus, $[ E_0 = 2m_0c^2 + .5I \omega^2_r_p + .5\omega^2_r_e + (n + 1/2) \hbar \omega_v_p + (n + 1/2) \hbar \omega_v_e + 1/2mv^2_p + 1/2mv^2_e ]$ if the positron and electron are going slowly. The $\omega_r$ and the $\omega_v$ are the rotational angular velocity iand vibrational angular frequency. The mass energy equation for a nucleus and a molecule is therefore $[E_0 = m_0c^2 +1/2I\omega_r^2 + (n +1/2)\hbar \omega_v + 1/2mv^2]$. On a universal scale planets, stars, galaxies and galactic groups will have rotational Orbital and vibrational factors which should be included in any mass-energy conversions. Some of the energy attributed to binding energy on a nuclear level may actually be vibrational and rotational kinetic energy. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X21.00009: Electricity from Gravity Roy Masters Einstein's cosmological constant as gravity, will unify quantum mechanics to general relativity and link gravity to electromagnetism. Then, an electromagnetic vacuum engine driven by the force that spins, moves, and sustains mass at the subatomic level, will do free, what generators cannot. Flowing outward-bound sinusoidally from its source, this gravity force assumes a three-dimensional spherical universe. Lines of force intersect, spinning into gyroscopic particles and passes as time-present, with a \textit{compression} gravity of space-time curvature continuum unifying all mass. The spaces between approaching masses suffer a decrease of right-angled \textit{vacuum} \textit{energy}, increasing external pressures, \textit{pushing }them together. Ubiquitous gravity now interacts electromagnetically with mass. Gravity's ``heat energy'' operates below absolute zero and squeezes mass into thermonuclear ignition of stars. Creation needs a gravity field for the propagation of light that will make sense of its wave/particle behavior. Creation from a white hole recycles down through a black one, into new beginnings of galaxies. ``Vacuum energy'' will light cities and factories; faster than light spacecraft will raise silently from the ground utilizing the very gravity it defies, propelling us to the stars. [Preview Abstract] |
Session X22: Focus Session: Deformation and Fracture
Sponsoring Units: GSNP DMPChair: Ian Robertson, University of Illinois at Urbana-Champaign
Room: Colorado Convention Center 108
Friday, March 9, 2007 8:00AM - 8:36AM |
X22.00001: Crystal strength by direct computation Invited Speaker: The art of making materials stronger goes back to medieval and even ancient times. Swords forged from Damascus steels more than 10 centuries ago possessed a unique combination of hardness and flexibility, two qualities that are difficult to attain simultaneously. The skills of metalworking were based on empirical knowledge and were passed from the master smith to his pupils. The science of physical metallurgy came about only in the XX century bringing with it new methods for finding out why some materials are strong while others are not. Soon it was realized that, when it comes to metal strength, it is all about crystal defects -- impurities, dislocations, grain boundaries, etc. - and how they are organized into crystal microstructure. This understanding has since resulted in new effective methods of material processing aiming to modify crystal microstructure in order to affect material's properties, e.g. strength and/or hardness. Remarkably and disappointingly, general understanding that microstructure defines material's response to external loads has not yet resulted in a workable physical theory of metal strength accounting for the realistic complexity of material microstructure. In this presentation I would like to discuss a few tidbits from computational and experimental research in our group at LLNL on crystal defects and their contributions to material strength. My selection of the examples aims to illustrate the major premise of our work that the mechanisms by which the microstructure affects crystal strength are multiple and complex but that there is hope to bring some order to this complexity. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X22.00002: Modeling of Self-Healing in Materials Reinforced with Nanoporous Fibers. Vladimir Privman We report on our group's progress towards continuum rate equation modeling, as well as numerical simulations, of self-healing of fatigue in composites reinforced with glue carrying nanoporous fibers. We conclude that with the proper choice of the material parameters, effects of fatigue can be partially overcome: fracture and degradation of mechanical properties can be delayed. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X22.00003: Effects of grain boundary constraints on properties of polycrystalline materials Kimberly McGarrity, Erin McGarrity, Phillip Duxbury, Bryan Reed, Elizabeth Holm Grain boundary networks are engineered by increasing the fraction of boundaries which have favorable properties. Favorable boundaries have either low grain boundary misorientation or they are special boundaries, such as coincident site lattice boundaries. Significant improvement in properties such as corrosion resistance, critical current in superconductors and mechanical strength and toughness occur, provided percolating grain or grain boundary structures can be engineered. We demonstrate that grain boundary constraints shift percolation thresholds from their uncorrelated values and that the behavior near threshold is also modified. The origin of these behaviors is an enhanced clustering of weak boundaries induced by grain boundary constraints. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X22.00004: Modeling the Evolution of Subsurface Microstructures During Wear of Metal Single Crystals Corbett Battaile, Somuri Prasad, Joseph Michael Friction can lead to complex mechanical and microstructural evolution near the worn surface, and these changes can impact the properties of the material. Recent results from tribological experiments on nickel single crystals reveal the formation of microstructural features ranging from nanometers (very near the surface) to microns in size. The formation and mechanical response of these zones is sensitive to crystallography, and can dramatically alter the frictional properties of the material itself. We have modeled these phenomena using a combination of dislocation plasticity, microstructure formation, and grain boundary sliding. The loading conditions are adopted from an analysis of static frictional contact. A phenomenological treatment of wear debris and asperity-mediated contact is included to appropriately describe the mechanical mixing that occurs very near the contact interface. We will provide an overview of the experimental evidence, discuss the wear model in detail, and present results for kilocycle wear on nickel single crystals in different crystallographic orientations. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X22.00005: Spontaneous shear localization in a model brittle solid Craig E. Maloney, Mark O. Robbins A better understanding of the failure of brittle materials is practically important in situations ranging in scale from nano-indentation to earthquake physics. Recent discrete models of this failure focused on geometries such as uniaxial tension or anti-plane strain where creation of free surfaces dominates. They are not appropriate for modeling the formation of shearing systems where frictional sliding of material in intimate contact and plastic deformation are important. We present results on a novel approach which introduces damage directly into particle based simulations. When loaded, the model exhibits a period of bursts of spatially correlated damage accumulation followed by a period of catastrophic weakening during which a geometrically complex through-going fault network forms, strikingly reminiscent of both laboratory experiments and geophysical observations at the field scale. We will discuss: spatial correlations in damage, evolution of the geometry of the fault system, and the dependece on confining pressures. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X22.00006: Simulations of Nanoscale Mechanical Contacts with Intervening Adsorbates Shengfeng Cheng, Mark Robbins Molecular simulations are used to investigate the role of intervening adsorbed molecules in nanometer scale mechanical contacts between nominally spherical tips and flat elastic substrates. Previous studies show that atomic scale deviations from the sphere that are present on any tip constructed from discrete atoms can have profound effects on contact areas, adhesive energies, and lateral stiffness. We find that including adsorbed molecules in contacts reduces the variation with tip geometry, but introduces new effects. One is that tip geometry affects the number of atoms that are pushed out of the contact and the resulting pressure distribution. The pressure at the center of the contact may be smaller than at the edge. We also find that the presence of adsorbates influences frictional behavior of contacts. For some cases the frictional force is proportional to area for bare tips and proportional to load when adsorbed molecules are present. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X22.00007: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 9:48AM - 10:00AM |
X22.00008: Rubber Band Recoil Romain Vermorel, Nicolas Vandenberghe, Emmanuel Villermaux When an initially stretched rubber band is suddenly released at one end, an axialstress front propagating at the celerity of sound separates a free and a stretched domain of the elastic material. As soon as it reaches the clamped end, the front rebounds and a compression front propagates backward. When the length of the compressed area exceeds Euler critical length, a dynamic buckling instability develops. The rebound is analysed using Saint-Venant's theory of impacts and we use a dynamical extension of the Euler-Bernoulli beam equation to obtain a relation between the buckled wavelength, the initial stretching and the rubber band thickness. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X22.00009: String breaking and the Petersburg Paradox Jake Fontana, Peter Palffy-Muhoray The Petersburg Paradox(1) provides a simple paradigm for systems that show critical sensitivity to rare events. The breaking strength of filaments, yarns and strings is determined by the presence of defects. In a given sample, the largest defect determines the stress at which failure occurs, and since the defect distribution is a function of sample size, the breaking strength of strings depends on their length. Analogy with the Petersburg paradox suggests that the breaking strength should vary approximately linearly with the logarithm of the length. We have carried out experiments to measure the breaking strength of samples of polyester sewing thread and of monofilament fishing line ranging in length from \textit{1mm} to \textit{1km}. We describe our experiments, present the results, and, compare fits of our data to Weibull and mean field failure statistics and the predictions from analogy with the Petersburg Paradox. 1. I. Todhunter, \textit{A History of Mathematical Theory of Probability}, (Chelsea, New York, 1949) [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X22.00010: Simulations of aging and plastic deformation in polymer glasses Mya Warren, Joerg Rottler Experiments on a broad class of amorphous glassy materials show that their mechanical behavior strongly depends on the time since vitrification. The slow relaxation of configurational degrees of freedom, or aging, generally increases the material's resistance to applied stress. In this study, we investigate the interplay between aging and plastic deformation in a simple model for polymer glasses by means of molecular dynamics simulations. We determine the macroscopic creep compliance for different loading conditions and aging times and find excellent qualitative agreement with experiments: compliance curves can be shifted to form a universal master curve, and the applied stress can reduce the effective age of the glass (mechanical rejuvenation). We then measure microscopic, local relaxation times and show that they correlate well with the aging characteristics of the macroscopic creep response. In addition, we explore the evolution of several measures of local order during aging and discuss their role in the mechanical behavior. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X22.00011: Deformation mechanism of silver nanowires Marcel Lucas, Austin Leach, Matt McDowell, Ken Gall, Simona Hunyadi, Catherine Murphy, Elisa Riedo Silver is the metal which exhibits the highest electrical and thermal conductivity, and has potential applications in electronics, photonics and catalysis. Silver nanowires could serve as interconnects between electronic circuits, catalysts in chemical reactions, or substrates for surface-enhanced Raman spectroscopy. Understanding how their mechanical properties are affected by their structure (size, cross-section geometry) is essential for their integration in nanodevices. Recently, silver nanowires have been synthesized in aqueous solution without surfactant or catalyst. These nanowires were characterized by Atomic Force Microscopy (AFM) and have a diameter ranging from 20 to 40 nm. Their deformation mechanism was studied by AFM nanoindentation and the results were correlated with atomistic simulations of silver nanowires with a pentagonal cross section. [Preview Abstract] |
Session X24: Conducting Polymers and Devices
Sponsoring Units: DPOLY DMPChair: Pawan Kahol, Missouri State University
Room: Colorado Convention Center 201
Friday, March 9, 2007 8:00AM - 8:12AM |
X24.00001: Dissipative effects in the electron transport through conducting polymers Natalya Zimbovskaya, Grigory Zimbovskiy Here, we study the effects of stochastic nuclear motions on the electron transport in doped polymer fibers assuming the conducting state of the material. We treat conducting polymers as granular metals and apply the quantum theory of conduction in mesoscopic systems to describe the electron transport between the metalliclike granules. To analyze the effects of nuclear motions we mimic them by the phonon bath, and we include the electron-phonon interactions in consideration. Our results show that the phonon bath plays a crucial part in the intergrain electron transport at moderately low and room temperatures suppressing the original intermediate state for the resonance electron tunneling, and producing new states which support the electron transport. [Preview Abstract] |
Friday, March 9, 2007 8:12AM - 8:24AM |
X24.00002: Tuning the Electrical Conductivity of Polyaniline by Controlling the Molecular Characteristics of the Polymer Acid Template Joung Eun Yoo, Tracy Bucholz, Yueh-Lin Loo We have investigated the electrical conductivity of polyaniline (PANI) that is template synthesized with a polymer acid as a function of the polymer acid molecular characteristics. The polymer acid of choice is poly(2-acrylamino-2-methyl-1-propanesulfonic acid) and it was synthesized by both conventional free-radical polymerization (PAAMPSA) and atom transfer radical polymerization (ATRP); aPAAMPSA. The synthesis of aPAAMPSA is kinetically controlled and thus aPAAMPSA has a narrower molecular weight distribution compared to PAAMPSA. In general, PANI-aPAAMPSA is about twice as conductive as PANI-PAAMPSA of comparable molecular weights. The difference in conductivity is correlated with different crystalline structures observed between PANI-PAAMPSA and PANI-aPAAMPSA. Specifically, the structure of PANI-aPAAMPSA resembles that of PANI doped with AAMPSA, the monomer of PAAMPSA. These results are further corroborated by UV-vis-NIR experiments where the polaron peak becomes broader and is red-shifted with PANI-aPAAMPSA. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X24.00003: Electrical Conductivity Measurements of Nanofibers Electrospun from Polyaniline/Polyethylene Oxide Blends Saima Khan, Aurangzeb Khan, Martin Kordesch Electrically conducting fibers of polyaniline doped with Camphorsulfonic acid PAn.HCSA in the Polyethylene Oxide (PEO) matrix were prepared using the non-mechanical electrospinning technique. The morphology of the fibers was studied using the scanning electron microscope (SEM) and Transmission electron microscope (TEM), showing a uniform thickness along the fiber length. The fibers had a diameter ranging from 800nm to 2$\mu $m. The electrical conductivity of the non-woven fibrous mat and the cast film was measured using the four-point probe method, for different concentrations of Pan.HCSA in the blend. Some possible factors affecting the electrical conductivity of the fibers/films were discussed. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X24.00004: Exciton localization and delocalization in phenyl-cored thiophene dendrimers$^{\ast}$ Muhammet Erkan Kose, Kwiseon Kim, Will J. Mitchell, Nikos Kopidakis, Garry Rumbles, Sean E. Shaheen A group of $\pi $-conjugated dendrimers, which are soluble organic molecules consisting of a core group to which branched arms (dendrons) are attached, were synthesized and characterized$^{1}$. These dendrimers have a phenyl core with 3 or 4 arms, i.e., 3 or 4 thiophene dendrons. It has been shown that these dendrimers present a viable alternative to polymers in organic photovoltaic devices with PCBM as the acceptor$^{2}$. We studied electronic structure of these dendrimers by density functional theory and restricted CI singles methods. Quantum mechanical calculations aimed at predicting the optical properties as well as the spatial location of excitons upon photoexcitation were performed. In particular, correlated electron-hole probability diagrams and transition density plots, to be presented here, reveal the nature of excitonic behavior in the dendrimers. [1] Mitchell, W. J.; Kopidakis, N.; Rumbles, G.; Ginley, D. S.; Shaheen, S. E., \textit{J. Mater. Chem. }\textbf{15}, 4518 (2005). [2] Kopidakis, N.; Mitchell, W. J.; van de Lagemaat, J.; Ginley, D. S.; Rumbles, G.; Shaheen, S. E.; Rance, W. L., \textit{Appl. Phys. Lett. }\textbf{89}, 103524 (2006). *This work was supported by the NREL LDRD program and the Xcel Energy Renewable Development Fund, and done in collaboration with the NREL Chemical and Biosciences Center and NCPV. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X24.00005: Semiconducting molecular crystals: Bulk in-gap states modified by structural and chemical defects S. Haas, C. Krellner, C. Goldmann, K. P. Pernstich, D. J. Gundlach, B. Batlogg Charge transport in organic molecular crystals is strongly influenced by the density of localized in-gap states (traps). Thus, a profound knowledge of the defect states' origin is essential. Temperature-dependent space-charge limited current (TD-SCLC) spectroscopy was used as a powerful tool to quantitatively study the density of states (DOS) in high-quality rubrene and pentacene single crystals. In particular, changes of the DOS due to intentionally induced chemical and structural defects were monitored. For instance, the controlled exposure of pentacene and rubrene to x-ray radiation results in a broad over-all increase of the DOS. Namely, the ionizing radiation induces a variety of both chemical and structural defects. On the other hand, exposure of rubrene to UV-excited oxygen is reflected in a sharp peak in the DOS, whereas in a similar experiment with pentacene oxygen acts as a dopant, and possible defects are metastable on the time-scale of the measurement, thus leaving the extracted DOS virtually unchanged. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X24.00006: Optical Characterization of a Single Cavity in Random Laser Polymer Film Z.Valy Vardeny, Abdullah Tulek Random lasing action with coherent feedback mechanism was previously attributed to inter-particle scattering of light forming close loops. A competing explanation is the formation of natural random cavities in the gain medium. We demonstrate the existence of such random cavities in thin polymer films by optically characterizing some of the lasing key properties such as size, threshold, and azimuthal emission intensity distribution. The optical properties of random cavities are compared with those of a fabricated microdisk having similar size, and the similarities and differences are emphasized. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X24.00007: Unidirectional Emission from Asymmetric Polymer Microcavities Abdullah Tulek, Z.Valy Vardeny Symmetric polymer microcavities that support whispering gallery modes such as microrings and microdisks have been extensively studied due to their ease of fabrication and high quality factors. However an important drawback is their isotropic emission. Two different microcavity configurations; namely a spiral, and a microdisk that contain a linear defect positioned in a specific orientation, have been proposed to overcome this problem. We measured the laser properties of such microcavities with DOO-PPV polymer as gain medium. Using the spiral microcavity we found unidirectional emission with a contrast ratio of about 8 in an angular emission window of $\sim $10$^{0}$. For the `defected' microdisk the observed contrast ratio was $\sim $ 8, and emission window was $\sim $25$^{0}$. Different orientations and the sizes of the defect have been studied thoroughly. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X24.00008: Molecular Spectroscopy Using Slow Surface Plasmon Polaritons Michael Preiner, Ken Shimizu, Nazanin Davani, Jason Fabbri, Nicholas Melosh Surface plasmon polaritons have recently attracted interest for both fundamental science and device applications. In addition to the normal surface plasmon mode, in metal-insulator-metal junctions a slow mode exists that is of particular interest for studying molecular-scale junctions. Accessing the slow mode is of great utility for studying the properties of these junctions, as the field intensity of the slow mode can be thousands of times larger than that of the normal surface mode inside the junction. We have demonstrated the ability to optically couple into the slow modes of molecular junctions using short-period diffraction gratings, and will discuss applications of this technique. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X24.00009: Mechanism for interaction between gases and Phthalocyanine films Amos Sharoni, Corneliu Colesniuc, Jeongwon Park, Forest I. Bohrer, Andrew C. Kummel, William C. Trogler, Ivan K. Schuller Phthalocyanines (Pcs) have been identified as promising candidates for electro-chemical gas sensors. They show potential for chemical selectivity via manipulation of the metal center and substitution of functional groups on the organic ring. However, for a given analyte and Pc, it is not clear which properties define the strength of response. To address this issue, we measured time-dependent current responses, of cobalt and metal free Pc, at constant voltage during exposure to various analyte vapor doses. The current in the device reduces with dosing; the response is linear with concentration and is found to follow first order kinetics. We show that the response is dominated by analyte interactions with the central Pc cavity; coordination strength governs CoPc responses, and hydrogen bonding ability governs H$_{2}$Pc responses. A model for the phthalocyanine electrical response and binding with analytes will be discussed. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X24.00010: The Electronic structure at the copper phthalocyanine to P(VDF-TrFE) copolymer thin films interface Jie Xiao, Carolina Ilie, Peter Dowben Copper phthalocyanine (CuPc) is an organic semiconducting material which has a potential application in organic thin film transistors. We explore the band offsets of CuPc deposited on crystalline polyvinylidene-trifluroethylene P(VDF-TrFE) copolymers through combined photoemission and inverse photoemission studies. Other work indicates that dipole orientation affects the band alignments of adsorbate molecules, so the possibility exists that the band offsets of the adsorbate molecule may be affected by ferroelectric phase transition in the P(VDF-TrFE) copolymer thin film substrate. Alternatively such a change, in the band alignment of CuPc, may be affected by a reversal of dipole orientation in the adjacent P(VDF-TrFE). [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X24.00011: Transport properties and non-volatile memory application of self assembled nanoparticle array by microtubules Mei Xue, K.L. Wang, Jing Zhou, Bruce Dunn A method of self-assembly of gold nanoparticles with the diameter of around 1nm is developed by the use of bio species (microtubule) and transport properties of nanoparticle arrays are investigated. Via self-assembly, the attachment sites of gold nanoparticles to the microtubule can be controlled. The density of the gold nanoparticles in our experiments achieved is on the order of 10$^{9 }$cm$^{-2 }$and can be extended to as high as 10$^{13}$ cm$^{-2}$. A transport bi-stability is observed in a sandwich structure of Au/ MT + gold nanoparticle array / P$^{+}$ Si substrate. On the basis of detailed analysis of the temperature and electrical field dependences, a band model incorporating electron-tunneling is suggested to explain the observed bi-stability and other transport characteristics. The retention time is also measured to be larger than 10$^{5}$s. The operation and endurance of this memory device are confirmed. With its simple structure and the compatible fabrication process with conventional MOS, this MT/Au nanoparticle array holds a great potential for memory applications. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X24.00012: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 10:24AM - 10:36AM |
X24.00013: Nondestructive Memory Elements Based on Polymeric Langmuir-Blodgett Thin Films T.J. Reece, S. Ducharme Ferroelectric field effect transistors (FeFETs) have attracted much attention recently because of their low power consumption and fast nondestructive readout. Among the ferroelectric thin films used in FET devices; the ferroelectric copolymer of polyvinylidene fluoride, PVDF (C$_{2}$H$_{2}$F$_{2})$, with trifluoroethylene, TrFE (C$_{2}$HF$_{3})$, has distinct advantages, including low dielectric constant, low processing temperature, low cost and compatibility with organic semiconductors. By employing the Langmuir-Blodgett technique, we are able to deposit films as thin as 1.8 nm. We discuss the characterization, modeling and fabrication of metal-ferroelectric-insulator-semiconductor (MFIS) structures incorporating these films. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X24.00014: Salt-induced phase transitions in charged polymerized membranes Angelo Cacciuto, Erik Luijten We study the behavior of charged, fully polymerized membranes in the presence of multivalent salt by means of molecular dynamics simulations. At moderate salt concentrations the interplay between the electrostatic interactions and the in-plane elasticity of the membrane gives rise to a novel multi-step folding transition pathway that is qualitatively different from the folding induced by generic attractive interactions at low temperatures. Furthermore, the number of folds in the membrane is greatly reduced when the salt concentration exceeds a critical value, indicating a reentrant transition. Both observations can be viewed as the two-dimensional counterpart of the behavior displayed by flexible linear polyelectrolytes in multivalent salt solutions [1]. \newline \newline [1] P.-Y. Hsiao and E. Luijten, Phys. Rev. Lett. 97, 148301 (2006). [Preview Abstract] |
Session X25: Liquid Crystalline And Amorphous Polymers
Sponsoring Units: DPOLYChair: Andrey Dobrynin, University of Connecticut
Room: Colorado Convention Center 203
Friday, March 9, 2007 8:00AM - 8:12AM |
X25.00001: Swelling and Shrinking Dynamics of Monodomain Nematic Elastomers Kenji Urayama, Ryo Mashita, Yuko Arai, Toshikazu Takigawa We demonstrate that the swelling and shrinking of monodomain nematic elastomers in solvents exhibit unusual dynamics because of the presence of shape and volume variation modes with markedly different rates. A variation in the degree of orientational order induced by temperature (T) jumps causes a spontaneous deformation along the director as well as a change in the chemical potential of the solvent inside the gel. The former effect results in an almost instantaneous shape change, whereas the latter drives a slow volume change governed by the diffusion of polymer networks. The markedly different rates of these two modes cause unique dynamics: (i) a pronounced over- or undershoot of the specimen dimensions occurs in the direction where the shape and volume variations act to change the dimensions in the opposite manner, and (ii) a large dimensional change (more than 50 percent of the total change) takes place with almost no delay after the T-jumps in the direction where these two effects on the dimension synchronize. [Preview Abstract] |
Friday, March 9, 2007 8:12AM - 8:24AM |
X25.00002: Effects of Long Wave-length Thermal Fluctuations on the Elasticity of Nematic Elastomers Xiangjun Xing, Aparna Baskaran We study the long wavelength fluctuations of the nematic director as well as the phonon fields in nematic elastomers. These fluctuations have important effects on the elasticity in the large deformation regime. We calculate the nonlinear stress-strain relations for several simple geometry of deformation. We also analize the correlation functions of the nematic director and phonon fields in the presence of large uniform strain deformations. All of these results can be directly measured by experiments. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X25.00003: Synthesis of Optimal and Imperfect Main Chain Smectic Elastomers Harshad Patil, Ronald Hedden Liquid crystalline polymers (LCPs) and elastomers (LCEs) are mesomorphic polymers that exhibit unique rheological characteristics. Smectic LCE, rubber-like materials which exhibit lamellar mesophases, exhibit high values of the mechanical loss factor (tan delta) over several decades of frequency. We are designing a model LCE system for study of the underlying molecular-level relaxation mechanisms responsible for this broad spectrum of relaxation times. We seek to distinguish to what extent the dynamic evolution of defects (e.g. focal conic defects and dislocations) contributes to the broad loss spectrum, as opposed to other ordinary processes such as relaxation of dangling and free chains. We are studying main chain, smectic LCPs and LCEs consisting of alternating flexible siloxane segments and mesogens. Elastomers are prepared by a three-monomer (A2 + B2 + A4) non-linear polycondensation. Synthetic methods are needed to prepare ``optimal'' and ``imperfect'' networks by introducing controlled amounts of dangling and free chains, and to control the phase behavior of the networks through mesogen chemical structure and crosslinking. Dynamic mechanical behavior of model networks, in conjunction with X-ray and neutron scattering studies, will distinguish the underlying physical processes responsible for the broad loss spectrum in smectic LCE. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X25.00004: Construction of Chiral Propeller Architectures from Achiral Molecules Kwang-Un Jeong, Deng-Ke Yang, Matthew J. Graham, Brian S. Knapp, Frank W. Harris, Stephen Z.D. Cheng Achiral BPCA-Cn-PmOHs construct chiral propeller structures in an N phase. The origin of chiral N phases in these achiral molecules comes from the twisted conformation of head-to-head dimers, indicating that neither molecular chirality, nor molecular bends, nor molecular tilting is necessary to form a chiral phase. The Frank-Pryce spherulitic N droplets and finger-print textures result from the single-twisting of chiral conformers, while the first-time observed propeller-patterned chiral N droplets are attributed to the double-twisting of chiral conformers in the N phase. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X25.00005: Small angle X-ray scattering studies of side chain liquid crystalline block copolymers. Eric Verploegen, Lu Tian, Paula Hammond A series of well defined smectic side chain liquid crystalline (LC) block copolymers with a low T$_{g}$ siloxane center block has been synthesized via anionic polymerization. The presence of a smectic liquid crystalline phase and the block copolymer mesophase are observed across various temperature ranges via Small Angle X-ray Scattering (SAXS) and Grazing Incidence Small Angle X-ray Scattering (GISAXS). The influence of various types of confinement and mechanical deformation upon the morphologies of the liquid crystalline and block copolymer mesophases was investigated. The interactions between the smectic LC and the block copolymer morphologies and their influence upon their respective orientations in response to the various confinement and mechanical deformations are detailed. Additionally, it was found that modifications to the liquid crystalline moiety were key in the clearing points for the smectic liquid crystalline phase, as well as significantly influencing the nanophase segregation of the block copolymer. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X25.00006: Solvent induced shape changes in liquid crystal elastomers Attilio Golemme, Tibor Toth-Katona, Jeremy Neal, Peter Palffy-Muhoray Liquid crystal elastomers are exceptionally responsive due to coupling between orientational order and mechanical strain.~ Changes in orientational order can give rise to mechanical deformations. Orientational order can be changed by a variety of excitations, including chemical concentration fields.~We have studied the dynamics of shape changes in LCE samples due to exposure to organic solvents and solvent vapors.~Unlike isotropic elastomers, which simply swell, LCEs show dramatic anisotropic shape changes when exposed to solvents.~We present results for the excitation and relaxation dynamics of shape changes for a variety of materials in response to the presence of different solvents.~ The absorption of solvents can cause a nematic-paranematic phase transition.~ We discuss possible applications [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X25.00007: Bloch wall defects in nematic thin films: experiments and simulations Mohan Srinivasarao, Jian Zhou, Jung O. Park, Gino De Luca, Alejandro D. Rey We study Bloch wall defects formed by quenching nematic thin films from planar anchoring to homeotropic anchoring through a temperature-driven anchoring transition. We show direct visualization of two types of Bloch walls, pure twist walls and diffuse walls, using fluorescence confocal polarizing microscopy (FCPM) technique. We describe the simulations of the evolution of the Bloch walls with varying anchoring strengths using Frank elasticity, which agree remarkably well with the experimental FCPM observation.A pure twist wall exists if the ratio of sample thickness to surface extrapolation length p is smaller than or close to 1; while a diffuse Bloch wall is obtained if p is much greater than 1. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X25.00008: Abnormal Slowdown of Longitudinal Diffusion of F-actin across Isotropic to Nematic Phase Transition Jun He, Jorge Viamontes, Jay Tang F-actin is a semi-flexible macromolecule. Above a few tenths of a percent in volume fraction, F-actin solution undergoes an isotropic (I) to nematic (N) phase transition. By tracking fluorescently labeled F-actin in a network of unlabeled filaments, we studied the diffusion behavior of F-actin across the I-N phase transition and found an abnormal slowdown of longitudinal diffusion after the system enters the transition region. In contrast, for an ordinary liquid crystalline I-N phase transition, there is an abrupt increase of longitudinal diffusion coefficient at the transition point. By comparing the diffusion behaviors of F-actin, microtubule and fd virus in F-actin solution and studying the apparent viscosity dependence on divalent counter-ion concentration, we attribute this counter-intuitive phenomenon to counter-ion condensation induced weak attraction between filaments in nematic phase. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X25.00009: Dynamic Fragility and the Glass Transition: Is there a relationship? Gregory McKenna, Qian Qin There have been multiple efforts over the years to correlate dynamic fragility, i.e., a Tg normalized temperature dependence of the dynamics, with various thermodynamic and dynamic parameters. Here we make a case that the dynamic fragility m=dln(viscosity)/d(Tg/T) evaluated at T=Tg is in fact strongly correlated to the glass transition Tg itself except for inorganic network glasses. We compile literature data for dynamic fragility m for six types of glass forming liquids: polymers, small molecule organics, hydrogen bonding organics, inorganics, ionic and metallic glass formers and find that different categories of glass forming liquids exhibit different behaviors in terms of the correlation between m and Tg, a correlation not previously examined. For hydrogen bonding organics, polymeric and metallic glass formers, there is a near linear increase in m with increasing Tg. For inorganic glass formers, m appears almost independent of Tg, remaining nearly constant over a wide range in Tg. We also investigated the apparent activation energy Eg at Tg and found that Eg increases with the square of Tg for hydrogen bonding organics, polymeric and metallic glass forming liquids, while Eg of the inorganics has a linear dependence on Tg. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X25.00010: Prediction of Creep Behavior in PMMA James Caruthers, Rebecca Martin, Grigori Medvedev Recently proposed thermoviscoelatic constitutive model (TVEM) of Caruthers et al. [1] has shown promise in being able to describe within a single set of material parameters a wide range of experiments including yield, stress and enthalpy relaxation, and nonlinear stress-strain behavior under complex loading histories. The TVEM program consists in performing a number of linear relaxation experiments (i.e. for small deviations from equilibrium in relaxing quantity) in order to determine the shear, bulk, and enthalpy relaxation spectra which serve as input to the TVEM constitutive equations. Once these memory functions have been set, TVEM must be able to predict results of a non-linear experiment under an arbitrary thermal and loading history without any further adjustment of model parameters. Following this program we carried out an extensive study of relaxation behavior of lightly cross-linked PMMA using TMA, DMA, and DSC techniques. The nonlinear experiments chosen for validation of TVEM were the creep experiments below Tg where we studied the dependencies on load, temperature, and aging time. We also performed multi-step loading-unloading experiments in both linear and non-linear regimes. In this report the predictive capabilities of TVEM are presented and critically analyzed. 1. J.M. Caruthers, D.B. Adolf, R.S. Chambers, P. Shrikhande - Polymer, 45, 4577 (2004) [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X25.00011: Strain Hardening and Plastic Deformation in Polymer Glasses Robert S. Hoy, Mark O. Robbins Although entropic network models are often used to fit stress-strain curves for polymer glasses, we show that these models do not correctly capture the physics of glassy strain hardening. We examine the relationship between strain hardening and plastic deformation in model polymer glasses over a wide range of entanglement densities and temperatures. While the total stress in densely entangled samples is well fit by the Langevin model, the dissipative component of the stress is always Neohookean. This indicates that the nonlinear corrections to the stress are associated with internal energy storage, contrary to entropic models. In the athermal limit, plastic dissipation is directly proportional to the rate of damage of van der Waals bonds. At large strains, both are proportional to entanglement density. The energy dissipated per damaged bond is independent of entanglement density and strain at moderate strains, but increases at high strains for the more densely entangled systems due to increased energy barriers. The partitioning of plastic events into strain-activated and thermally-activated events is discussed. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X25.00012: The influence of nonlinearity on the timescale of volume relaxation Prashanth Badrinarayanan, Sindee Simon The relationship between the timescales of volume and enthalpy relaxation has been studied extensively in the literature with differing results. Based on volume, enthalpy, and creep relaxation studies for polyetherimide, polystyrene, and selenium, a general picture was developed for the relationship between the relative timescales of different properties which was consistent with the data in the literature. According to the general picture, the timescales of different properties are similar at temperatures above the nominal value of T$_{g}$; however, the time scales diverge at temperatures below T$_{g}$ with volume and creep exhibiting longer relaxation timescales compared to enthalpy. However, when the timescales are re-analyzed using the cooling rate dependence of T$_{g}$ from capillary dilatometry and DSC, no divergence between the timescales of volume and enthalpy was observed, in contradiction with the general picture. It is hypothesized that the divergence in timescales observed in earlier work is due to the pronounced nonlinearity of volume relaxation compared to enthalpy relaxation. In this work, we use capillary dilatometry to test this hypothesis; in particular, we examine the effect of the magnitude of temperature down jumps on the volume relaxation timescale for polystyrene. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X25.00013: Photothermal studies of polymers using polarized light Marshall Thomsen, Daeha Joung, Don Snyder Visible light has been used as the pump beam in surface thermal lensing experiments involving nominally transparent polymers. A small portion of the pump beam is absorbed by the sample, producing local heating and a thermal bump. The nature of the bump depends on thermal, optical, and mechanical properties of the sample. The presence of the bump is detected by a weaker probe beam scattered off the surface. We have used a polarized probe beam and have observed the reflected beam as a function of polarization. The resulting time dependence is unlike anything observed in the absence of polarizers. These experiments suggest that photothermal techniques using polarized light can provide new insight into structural changes in polymers. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X25.00014: Gradient Copolymers Yield Uniquely Broad Glass Transition Temperatures in Comparison with Block Copolymers and Polymer Blends Jungki Kim, Michelle M. Mok, Christopher L.H. Wong, Robert W. Sandoval, John M. Torkelson Gradient copolymers, which can be made by controlled radical polymerization or ring-opening metathesis polymerization, are distinct from random and block copolymer because of the gradient in comonomer composition along the copolymer backbone. As a result of this gradient along the chain, in the ordered lamellar state gradient copolymers are believed to exhibit a sinusoidal composition profile that is distinct from the ``square-wave'' composition profile observed in ordered lamellar block copolymers. This difference in the manner in which the local composition varies in the ordered state leads to dramatic differences in the glass transition behavior of gradient copolymers and block copolymers and similarly between gradient copolymers and polymer blends. Five gradient copolymer systems have been examined allowing for study of the effects of the enthalpic incompatibility of the comonomer units, the Tg differences among the homopolymers, and hydrogen bonding effects leading to random copolymers exhibiting higher Tgs than those found in block copolymers. We show via differential scanning calorimetry and dynamic mechanical analysis that single, continuous Tg breadths as large as 70-100 K are possible in gradient copolymers. [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X25.00015: Utilizing Nanoparticle Surface Plasmons for Surface-Initiated Polymerization and Conformational Switching of Polymers Nelson Nunalee, Jack Mock, Ashutosh Chilkoti, Stefan Zauscher Spherical gold nanoparticles on the order of 50nm in diameter experience a localized surface plasmon resonance peak at an incident light wavelength of around 550nm. This resonance is a result of an extremely efficient coupling of the incoming oscillating electric field with the free electrons in the gold. Some of the light is absorbed, while some is scattered. The absorbed portion of light is lost to phonons in the gold, which results in localized heating. Our research seeks to capitalize on this heating to switch the conformational state of surface grafted stimulus-responsive poly(N-isopropylacrylamide) (pNIPAAm). Furthermore, we seek to harness the strong amplification of scattered light at the plasmon resonance to induce near-field surface-initiated polymerization of pNIPAAm. We will report on the progress of our research, which aims to utilize these plasmonic effects as the basis for nanofabrication and sensing devices. [Preview Abstract] |
Session X26: Focus Session: Charge Transport in Nanostructures IV
Sponsoring Units: DCPChair: Paul McEuen, Cornell University
Room: Colorado Convention Center 205
Friday, March 9, 2007 8:00AM - 8:36AM |
X26.00001: Molecular Junction Transport - Some Vibronic Effects Invited Speaker: The behavior of molecular transport junctions in the coherent tunneling (Landauer-Imry) regime is rapidly becoming understood. But vibronic effects characterize molecules, and understanding how they act in such junctions is a significant issue. This talk will deal with the role of both weak and strong vibronic interactions in molecular junctions. The weak mixing appears in IETS spectra, and can be handled by perturbation theory in the coherent tunneling limit. It provides some quantitative comparisons between calculation and experiment, And can clarify pathways for transport. But strong vibronic interaction requires a more elaborate analysis, and changes the mechanisms for transport. Hysteresis and switching behaviors will be discussed. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X26.00002: Vibrationally induced two-level systems in single molecule junctions Robert Thijssen, Darko Djukic, Sander Otte, Rolf Bremmer, Jan van Ruitenbeek It is found that differential conductance spectra of small single molecules contacted by metal electrodes display positive or negative peaks. The positions in energy of these peaks correspond with the energies of local vibration modes of the molecule in the junction. A model of vibrationally induced two-level systems is made in order to explain the physics responsible for the observed features. A molecule in an atomic junction can be contacted in two geometrically different configurations, each of which results into a different junction conductance. These two energy minima are separated by a large energy barrier. Only by vibrationally exciting the molecule above the barrier, a transition between the configurations is possible. This results in a sudden jump in conductance and a peak in differential conductance. The vibrationally induced two-level switching is expected to be quite general, since we have observed dI/dV peaks in many different single molecule-metal junctions. It acts as an intrinsic amplification mechanism for local vibration mode features, even when large conductance fluctuations are present. Therefore it could be exploited as a new spectroscopic tool for identifying local vibration mode energies. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X26.00003: Electronic and Vibronic Spectroscopy of Molecular Junctions James Kushmerick Transition voltage spectroscopy and inelastic electron tunneling spectroscopy are used to explore charge transport in molecular junctions. Our recent work has shown that a mechanistic transition occurs from direct tunneling to field emission in molecular junctions. The magnitude of the voltage required to enact this transition is molecule-specific, and thus constitutes a form of spectroscopy. We demonstrate that the transition voltage of a conjugated molecule depends directly on the manner in which the conjugation path is extended. Furthermore using inelastic electron tunneling spectroscopy to measure the vibronic structure of non-equilibrium molecular transport, aided by a quantitative interpretation scheme based on non-equilibrium Greens function/density functional theory methods, we are able to characterize the actual pathways that the electrons traverse when moving through a molecule in a molecular transport junction. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X26.00004: Charge Modification of Vibrational Features in Inelastic Electron Tunneling Spectroscopy Lam Yu, James Kushmerick Inelastic electron tunneling (IET) spectroscopy of Au-decandithiol-Ni atoms-benzenethiol-Au, Au-decandithiol-Au colloids-benzenethiol-Au and Au-decandithiol-benzenethiol-Au junctions are investigated by cross-wire tunnel junctions. Both the IET spectroscopic features' intensities and line shapes are observed to be significantly modified by the presence of a metal-sandwich layer. We attribute the vibronic features modification to the interaction between the metallic electronic levels and molecularly coupled phonons in the molecular junctions. Our results provide experimental insights into understanding the origin of some of the differences observed in two previous molecular IET spectroscopy experiments by Kushmerick et al. (Nano lett. 2004, 4, 639) and Wang et al. (Nano lett. 2004, 4, 643). [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X26.00005: Dynamic Molecular Nanostructures Assembled with Atomic Manipulation Brian K. Foster, Hari C. Manoharan Molecular nanostructures of CO were engineered on a Cu(111) surface by single molecule manipulation in a custom-built low-temperature scanning tunneling microscope (STM). The structures were designed to allow for well-defined motion of either a single molecule or linked sets of molecules on the surface, for the purpose of studying the system's dynamic translational behavior. Dynamics such as meta-stability, bi-stability and molecular vibration were observed; STM measurements were used to decipher dynamic behavior though coupling to electronic charge via elastic and inelastic tunneling. We also explore the coupling of vibrational modes of individual molecules (at THz frequencies) to the molecular translation rates (at kHz frequencies or below). The dynamic behavior of such structures offers the potential to control and transmit information across surfaces. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X26.00006: Classical Nuclear Motion in Quantum Transport Claudio Verdozzi, Gianluca Stefanucci, Carl-Olof Almbladh A quantum-classical scheme is presented to study nuclear motion in time-dependent quantum transport. The nuclei are treated in the Ehrenfest approximation. We illustrate the method in terms of model systems results. We show how electron-lattice interactions may induce dynamical Peierls distortions in short wires, and change their conducting behavior. We also show time-resolved results for current-induced molecular desorption and suggest that AC biases could provide a way to tailor electromigration. The results illustrate the importance of non-adiabatic effects for transient phenomena in nanodevices. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X26.00007: Inelastic effects in noise properties of molecular junctions Michael Galperin, Abraham Nitzan, Mark A. Ratner The effect of electron-phonon coupling on the current noise in a molecular junction is investigated within a simple model. The model comprises a one-level bridge representing a molecular level that connects between two free electron reservoirs and is coupled to a vibrational degree of freedom representing a molecular vibrational mode. The latter in turn is coupled to a phonon bath that represents the thermal environment. We focus on the zero frequency noise spectrum and study the changes in its behavior under weak and strong electron-phonon interactions. In the weak coupling regime we find that the noise amplitude can increase or decrease as a result of opening of an inelastic channel. In particular the relative Fano factor decreases with increasing off resonance distance and junction asymmetry. For resonant inelastic tunneling with strong electron-phonon coupling the differential noise spectrum can show phonon sidebands in addition to a central feature. A striking crossover of the central feature from double to single peak is found for increasing asymmetry in the molecule-leads coupling or increasing electron-phonon interaction. A possible use of noise data from scanning tunneling microscopy experiments for estimating the magnitude of the electron-phonon interaction on the bridge is proposed. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X26.00008: \textit{In-situ} Inelastic Electron Tunneling Spectroscopy of Oligoaniline Molecular Junctions Heayoung Yoon, Masato Maitani, Lintao Cai, David Allara, Theresa Mayer Recently, several studies have reported that self assembled monolayers of oligoaniline dimmers showed room temperature bistable switching behavior. In this talk, we will discuss the electrical and spectroscopic properties of thiol-substituted oligoaniline (OA) molecular junctions at the interface of lithographically-defined bottom metal nanowire contacts and metal nanowire top contacts. The junctions showed reproducible room temperature bistable switching with the threshold voltages of approximately $\pm $ 1.5 V and I-V(T) showed the dominant transport mechanism is coherent tunneling. Inelastic electron tunneling (IET) spectra in low and high current states were obtained at 5 K using a standard AC modulation technique to collect the second harmonic signal directly. The observed IET peaks in a plot of d2I/dV2 versus V were compared to infrared and Raman spectra for the OA self-assembled monolayers. The measurement confirms that the measured transport properties of molecular junctions are due to the intended molecule rather than process induced artifacts. In addition, the intensity change of vibrational modes of the benzene ring (185mV) and the quinon (197mV) of the OA in the low to the high current state suggest that the switching behavior is attributed to an inherent molecular feature of the OA molecules that form the junction. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X26.00009: Shot noise measurements on a single molecule Jan M. van Ruitenbeek, Oren Tal Fabrication of molecular junctions with diverse and controlled functionality requires a fundamental understanding of the relation between the structure and conductance properties of these junctions. We address this issue using simple organic molecules (e.g., hydrogen, carbon monoxide, and benzene) as a molecular bridge between two Pt electrodes formed by the mechanical break junction technique. Shot noise is used to reveal the number of conductance channels through the molecular junction, and their probabilities, while point contact spectroscopy yielded its characteristic vibration modes. This diverse information combined with theoretical calculations allows us to present a detailed picture of the relation between the conductance and the junction structure. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X26.00010: Quantum-state-resolved probing of molecular inner-sphere reorganization using a single-molecule transistor Nathalie de Leon, Wenjie Liang, Qian Gu, Hongkun Park The coupling of electron transport through a single molecule to various degrees of freedom, such as spin, charge, and vibrations, can be probed using a single molecule transistor. The addition or subtraction of electrons usually accompanies a change in molecular geometry, a phenomenon known as inner-sphere reorganization. We have studied the effects of inner-sphere reorganization on electron transport using two complexes, ferrocene and Fe(bpy)$_{3}^{n+}$ (n = 1, 2, 3), as model systems. The reported energies of vibrational excitations in Fe(bpy)$_{3}^{n+}${\-} are in agreement with existing IR and Raman spectroscopic data. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X26.00011: Quantum Channels and Conductance Oscillations in Metal/Molecule/Metal Switches Feng Miao, Douglas Ohlberg, R. Stanley Williams, C. N. Lau We investigate conductance switching in Pt/stearic acid monolayer/Ti devices by pressure-modulated conductance microscopy. For devices with conductance G$>>$G$_{Q}$ and G$<<$ G$_{Q}$, where G$_{Q}$ is the conductance quantum, localized pressure-induced conductance peaks are observed, indicating formation of nanoscale conductance pathways on the electrodes. For devices with G$\sim $ 1- 2 G$_{Q}$, in addition of conductance peaks, we also observed conductance dips and oscillations in response to localized pressure. These results suggest formation of quantum conductance channels in our devices, and can be satisfactorily modeled by considering interfering electron waves between two partially transmitting electrodes. Moreover, the force dependence of such conductance modulations is fully consistent with this model. Our findings underscore the possible use of these devices as atomic switches. [Preview Abstract] |
Session X27: Nanowires
Sponsoring Units: DCMPChair: Dan Finkenstadt, Naval Research Laboratory
Room: Colorado Convention Center 301
Friday, March 9, 2007 8:00AM - 8:12AM |
X27.00001: Superconductor-Insulator Transition in Epitaxial Niobium Nanowires Timothy McArdle, Kevin Inderhees, Paul Welander, James Eckstein As the dimensions of a superconducting nanowire are reduced, it undergoes a transition from a superconductor to an insulator. Near Tc, thermally activated phase slips cause this insulating state, and it is believed that in extremely narrow wires quantum phase slips appear and become dominant at low temperature. However, the exact nature of the S-I transition, specifically what parameters control it, is not clear. We report on recent studies of nanowires fabricated using electron beam lithography from single-crystal niobium films grown by ultra-high vacuum molecular beam epitaxy. Since the films are single crystal, the role of disorder is reduced. Our films are 100 angstroms thick, have transition temperatures near 7.2 K, and residual resistance ratios of around 5, typical for ultra-thin single-crystal niobium films. The wires are 10 $\mu $m long and range in width from 35 to 200 nm. \newline This work was supported by the DOE BES at the F. Seitz Materials Research Laboratory at the University of Illinois, Urbana. [Preview Abstract] |
Friday, March 9, 2007 8:12AM - 8:24AM |
X27.00002: Imaging of Few-electron InAs Quantum Dots in InAs/InP Nanowires Erin E. Boyd, Halvar J. Trodahl, Ania Bleszynski, Michael Stopa, R.M. Westervelt, Linus E. Froberg, Lars Samuelson InAs quantum dots are promising contenders for nanoelectronics, spintronics and quantum information processing.~ Their large g-factor makes manipulation of electron spins easier at higher temperatures.~ InAs dots, as small as 10 nm long holding only a few electrons, can be formed by InP barriers in InAs/InP nanowire heterostructures grown using chemical beam epitaxy.~ Coulomb blockade transport measurements done using metal contacts and a back gate show excellent results [1].~ Using a liquid-He cooled scanning probe microscope, we imaged an InAs quantum dot that holds only one-electron, with the conducting tip as a movable gate [2].~ Simulations of electron wavefunctions in the dot show the effect of the back gate and the moveable tip. [1] M. Bj\"{o}rk\textit{ et al.}, Nano Letters\textbf{ 4}, 1621 (2004) [2] A. Bleszynski\textit{ et al.}, 28th Int. Conf. Physics of Semiconductors, 2006 [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X27.00003: Imaging Few-Electron Double Quantum Dots in InAs/InP Nanowires Halvar J. Trodahl, Erin E. Boyd, Ania Bleszynski, R. M. Westervelt, Linus E. Froberg, Lars Samuelson InAs quantum dots formed in InAs/InP nanowire heterostructures are attractive candidates for nanoelectronics, spintronics and quantum information processing.~Tunnel-coupled double InAs dots defined by InP barriers can be grown using chemical beam epitaxy; each dot can be small enough to hold just a few electrons. It is difficult to lithographically define gates small enough to individually address each dot. With use of a liquid-He cooled scanning probe microscope (SPM), the Coulomb blockade conductance of a single InAs quantum dot in an InAs/InP nanowire has been imaged, using the SPM tip as a movable gate [1]. This approach can individually tune the charge on each InAs dot in an InAs/InP nanowire. We plan to use this technique to investigate tunnel-coupled InAs double dots. \newline [1] A. Bleszynski et al., 28th Int. Conf. on the Physics of Semiconductors, 2006. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X27.00004: Elastodynamics of GaN nanowires at microwave frequencies Colm Flannery, Kristine Bertness, Sudook Kim, Ward Johnson Single-crystal III-nitride nanowires have been the subject of extensive research because of their potential usefulness in innovative electronic, optoelectronics, and sensing applications. They also have been recognized as having several characteristics, including a relatively high surface-to-volume ratio, that are attractive for nanoelectromechanical systems (NEMS) applications, such as mass sensing. In this report, we present Brillouin-light-scattering (BLS) measurements and theoretical modeling of thermally excited vibrational modes in free-standing GaN nanowires with hexagonal cross sections ($\sim $70 nm to $\sim $200 nm in diameter) grown from a substrate by catalyst-free molecular beam epitaxy to a length of $\sim $8 $\mu $m. A series of modes with frequencies between 8 GHz and 50 GHz are seen in the BLS spectra with axial wavelengths between 270 nm and 1600 nm. Modeling of the normal modes was performed under the approximations of infinite length and circular cross section. The lowest-frequency BLS peak is identified as the lowest-order flexural mode. Stronger higher-frequency modes include ones with greater phase variation around the circumference. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X27.00005: Surround Gate Germanium Nanowire Field Effect Transistors Li Zhang, Ryan Tu, Hongjie Dai Surround gate (SG) GeNW field effect transistors (FETs) were successfully constructed using a novel self-aligned fabrication approach based on the core-shell GeNWs formed with a single-crystalline Ge core and concentric shells of nitride and silicon passivation layer by chemical vapor deposition (CVD), an Al$_{2}$O$_{3}$ gate dielectric layer by atomic layer deposition (ALD) and an Al metal SG shell by isotropic magnetron sputter deposition. Individual SG GeNW FETs show improved switching over GeNW FETs with planar gate stacks owing to improved electrostatics. FET devices comprised of multiple quasi-aligned SG GeNWs in parallel afford on-currents exceeding 0.1mA at low source-drain bias voltages. Capacitance-voltage characteristics of individual GeNW FETs with topgate or SG were directly measured for the first time using a novel method to reduce background capacitance to less than 50aF via device design, ground plane shielding, interprobe shielding, and use of a capacitance bridge circuit. In addition, the mobility of the GeNW FETs was extracted according to the direct electrical and capacitance measurements. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X27.00006: Low temperature electron transport studies of Bi nanowires Zuxin Ye, Hong Zhang, Haidong Liu, Wenhao Wu We report the electron transport studies of Bi nanowires at sub-Kelvin temperatures. Bi nanowiers with a nominal diameter $\sim $ 80nm and a length $\sim $ 10 $\mu $m were electrochemically deposited into the pores of ion track etched polycarbonate membranes. Electric contacts of single Bi nanowires were \textit{in-situ} formed on the on-membrane macroscopic electrodes during the electrochemical deposition. Electron transport properties were measured at temperatures from 60mK to 20K. The temperature dependence of resistance showed a quick drop when the samples were cooled below 0.6K, resembling a superconducting transition. The samples had a finite resistance at low temperature far below the transition, instead of having a zero resistance as regular superconductors. I-V curves showed a zero-bias resistance valley and multiple non-zero-bias peaks symmetrically distributed on both sides of the central valley at T $<$ 0.6K. The magnetic field dependence of resistance showed an unusual hysteresis loop with a butterfly shape when the magnetic field was swept along a close cycle between -0.6T and +0.6T. All these features gradually vanished when an applied magnetic field was increased to 0.6T. We will discuss the origin of this unusual low temperature behavior of Bi nanowires and its relation with the microscopic structures. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X27.00007: Nanoscale Thermal Imaging Kamal Baloch, Todd Brintlinger, Yi Qi, David Goldhaber-Gordon, John Cumings We present real time, in-situ, high resolution thermal imaging of metallic nanowires. The nanowires are grown on the front-side of silicon nitride membranes. Resistive heating along the wires produces thermal gradients which melt/freeze 20-200nm diameter indium islands deposited by thermal evaporation on the back-side of the membrane. These transitions can be imaged using a transmission electron microscope operating in dark-field mode such that contrast corresponds to the phase of an individual island. Global changes in temperature can be used to calibrate the melting point of individual islands and to account for the presence of the $\sim $100nm thick silicon nitride membrane. Thermal modeling confirms the imaged thermal behavior. This technique could be generally employed for thermal imaging of nanowires and nanotubes, wherein the nanoscale systems are imaged in-situ and under electrical bias. Results of local resistive heating in a carbon nanotube device will also be shown [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X27.00008: X-ray Radiation Damage Studies of Individual Nanotubes and Nanowires H.D. Mo, C. Nelson, C.-C. Kao, M. Sfeir, A. Bollinger, I. Bozovic, J. Misewich, A. Stein, W. Liu, P. Zschack, N. Bozovic The development of techniques for x-ray studies of individual nanomaterials is motivated by the spectroscopic, structural, and dynamic information that x-rays provide. In combination with other probes (e.g., STM), x-ray techniques promise the complete characterization of nanomaterial properties and functionality, which can be used as feedback for the synthesis of useful nanomaterials. The feasibility of x-ray studies of individual nanomaterials is approaching due to ongoing improvements in x-ray focusing optics and synchrotron radiation sources that together lead to increasing flux densities. However one possible barrier concerns the effects of high intensity x-ray beams on hard nanomaterials, about which little is currently known. Therefore here we report on x-ray damage studies of individual carbon nanotubes and SrRuO3 nanowires. Samples of the two systems were exposed to microfocused x-rays on APS beamline 34-ID for variable amounts of time. Pre-and post-exposure SEM imaging was used to qualitatively study the effects on carbon nanotubes, and real-time monitoring of sample integrity was provided by measuring a current passing through the SrRuO3 nanowires during the exposure. This research is supported by the DOE, under contracts DE-AC02-98CH10886 (BNL) and W-31-109-ENG-38 (ANL). [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X27.00009: In-situ and ex-situ analysis of terminal lengths and growth rates of vertically aligned carbon nanotubes grown by acetylene and alcohol catalytic chemical vapor deposition J. Jackson, K. Belay, A. Puretzky, D. Geohegan, H. Christen, H. Cui Controlled synthesis of carbon nanotubes (CNTs) at a fast growth rate which produced long, vertically aligned, and containing required number of walls, from MWNT to double- and single-walled nanotubes by varying processing parameters have been achieved in a thermal chemical vapor deposition (CVD) process. Both in-situ optical monitoring and ex-situ characterization methods have been used to determine the terminal lengths and growth rates of CNTs for acetylene, methanol and ethanol feedstocks grown on silicon substrates coated with an aluminum underlayer ($\sim $10 nm), molybdenum ($\sim $ 0.2 nm), and iron ($\sim $ 1nm). Maximum growth rates of CNTs were 3-8 times and 50-125 times slower for ethanol and methanol, respectively, compared when using an acetylene feedstock. The terminal lengths of CNT arrays were estimated as: from 200 mm to a few mm for acetylene, $\sim $ 20 mm for methanol, and $\sim $ 8 mm for methanol. The terminal lengths, growth rates, the number of walls, and quality of the carbon nanotubes for all three feedstocks show similar temperature dependences. This indicates the existence of a common mechanism responsible for the activation and deactivation of catalyst nanoparticles at different growth temperatures, probably related to catalytic activity and/or the oxidation state of the catalyst nanoparticles. The restart of growth was also observed while controlling subsequent methanol and hydrogen gas flows. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X27.00010: Electrical Properties of Individual Semiconducting Oxide Nanobelt and Their Applications* Yi Cheng, P. Xiong, L. Fields, J.P. Zheng, R. Yang, Z.L. Wang Field-effect transistors (FETs) with multi-terminal electrical contacts were fabricated on individual oxide (SnO$_{2}$ and ZnO) nanobelts. Simultaneous two-terminal and four-terminal measurements enable direct correlation of the FET characteristics with the nature of the contacts. Nanobelt FETs with Schottky contacts were found to exhibit n-channel, p-channel or ambipolar characteristics transistors depending on the properties of the contacts. In contrast, low-resistance ohmic contacts on the nanobelts lead to high-performance n-channel depletion mode FETs with well-defined linear and saturation regimes, ``on/off'' ratio as high as 10$^{7}$ at ambient conditions$^{[1]}$. The electron concentration and effective carrier mobility of the nanobelts in different gases at various temperatures were determined from FET measurements on the channel-limited devices. Sensitive electrical response of the SnO$_{2}$ nanobelt FETs to gas flow containing 0.2-2{\%} H$_{2}$ was observed at room temperature$^{[2]}$. The effort to utilize the channel-limited nanobelt FETs for protein detection will also be reported. *Supported by NSF NIRT grant ECS-0210332. $^{[1] }$Y. Cheng \textit{et al.}, Appl. Phys. Lett. \textbf{89}, 093114 (2006). $^{[2] }$L.L. Fields \textit{et al}., Appl. Phys. Lett. \textbf{88}, 263102 (2006). [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X27.00011: Nonequilibrium electromechanical noise in a nanomechanical resonator Patrick Truitt, Jared Hertzberg, Keith Schwab Current carrying electrons passing through a diffusive conductor can undergo elastic collisions with defects or surface boundaries and thus impart momentum to the lattice. At sufficiently low temperatures, where the electron-phonon scattering length is longer than the mean free path, this electromechanical noise can be driven out of equilibrium with the conductor's thermal noise (Joule heating). The resulting force from elastic collisions on a doubly-clamped beam was predicted by Shytov et al [1]. We will discuss our low temperature measurements of a gold-coated, radio-frequency nanomechanical resonator. We current bias the conducting layer and monitor the position of the resonator with an RF-SET. From mechanical noise thermometry, we then compare the observed electromechanical noise force with the theory.{\\}{\\}[1] A.V. Shytov, L.S. Levitov, and C.W.J. Beenakker, Phys. Rev. Lett. \textbf{88}, 228303 (2002) [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X27.00012: Electrical-transport properties of individual single-crystalline IrO$_2$ nanorods Y. H. Lin, T. C. Lee, Y. C. Sun, W. B. Jian, H. M. Chang, Y. S. Huang, J. J. Lin We have studied the electrical-transport properties of individual single-crystalline IrO$_2$ nanorods (NRs) prepared by MOCVD. With the help of e-beam lithography, individual NRs are contacted by Cr/Au submicron electrodes from above. Utilizing different probe configurations, not only the intrinsic properties of the NRs but also the temperature dependence of the contact resistance, $R_{\rm{contact}}$, has been determined down to liquid-helium temperatures. Our measured resistivity behavior of the NRs is in close agreement with the current theoretical understanding of this material. On the other hand, we found that the temperature behavior of the $R_{\rm{contact}}$ obeys the law log$R_{\rm{contact}}\propto T^{-1/2}$ over a wide temperature range from 100 K down to liquid-helium temperatures. This later conduction process is ascribed to the hopping of electrons through nanoscale metal granules accidentally formed at the contact region during the thermal evaporation of the submicron electrodes. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X27.00013: Silicon nanoporous pillar array: template for fabricating silicon-based nanocomposites with enhanced physical properties Xin Jian Li, Xiao Nan Fu, Hai Jun Xu, Wei Fen Jiang A triple hierarchical structure, silicon nanoporous pillar array (Si-NPA), was formed on silicon wafers by a hydrothermal method. The structure of Si-NPA is characterized by the regular array of micron-sized silicon pillars, quasi-identical nanopores densely distributing over each pillar, and silicon nanocrystalllites composing the walls of the nanopores. Utilizing the excellent structural regularity and high chemical reactivity of Si-NPA, patterned nanocomposites of CdS, carbon nanotubes (CNTs), Au, and Fe$_{3}$O$_{4}$/Si-NPA were fabricated. Their elemental compositions, morphologies and microstructures were characterized. Ideal physical properties of I-V curve in CdS/Si-NPA heterojunction, field emission in Si-NPA, CNTs/Si-NPA, Au/Si-NPA, and humidity/gas sensitivity in Fe$_{3}$O$_{4}$/Si-NPA were observed and the corresponding mechanisms were analyzed. These results indicate that Si-NPA could be employed as an ideal template to assembly silicon-based functional nanosystems, and might find multiple applications in fabricating novel electronic devices. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X27.00014: Growth and Characterization of ZnSe Nanowires on Au-catalyzed Ge substrates Tina Lin, Benjamin Cooley, Nitin Samarth Semiconductor nanowires derived from ZnSe are of interest for semiconductor spintronics because of the relatively long spin lifetimes in bulk n-ZnSe. The metal-catalyzed growth of ZnSe nanowires has already been demonstrated on a variety of substrates, including GaP and Si.[1,2]. Here we exploit the formation of a low melting point Au-Ge eutectic alloy to initiate the growth of ZnSe nanowires on Au-covered Ge substrates. ZnSe is deposited under ultrahigh vacuum conditions using solid source molecular beam epitaxy. Scanning electron microscopy reveals the formation of dense random arrays of ZnSe nanowires with typical lengths in the range $\sim 1 - 3 \mu$m and diameters of $\sim 10 - 30$ nm. We report a systematic study of the effects of growth temperature, thickness of the gold layer, and ZnSe deposition thickness on the characteristics of the resulting ZnSe nanowires. We also report characterization of these nanowires using high resolution scanned probe and electron microscopies, as well as low temperature optical spectroscopy.\\ 1. Y. F. Chan {\it et al.}, Appl. Phys. Lett. 83, 2665 (2003).\\ 2. A. Colli {\it et al.}, Nanotechnology 16, S139 (2005). [Preview Abstract] |
Session X31: Synthesis of Nanotubes and Nanowires
Sponsoring Units: DCMPChair: Paola Barbara, Georgetown University
Room: Colorado Convention Center 401
Friday, March 9, 2007 8:00AM - 8:12AM |
X31.00001: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 8:12AM - 8:24AM |
X31.00002: Bismuth Nanowires: Synthesis, Microscopy and Transport Properties Jason Reppert, Jian He, Malcolm Skove, Brad Edwards, Terry Tritt, Apparao Rao Thermoelectric materials approaching the atomic level possess unique quantum confinement properties that have generated much interest in recent history. Theoretical investigations have suggested that nanowires with diameters $<$10 nm will possess a ZT $>$2. Previously, bismuth nanowires have been successfully synthesized by means of electrochemical deposition, liquid-phase pressure injection, and vapor-phase deposition. Here, we report the synthesis of bismuth nanowires via the pulsed laser deposition method (PLD). Using this approach, we have been successful in producing nanowires ranging in diameters of 10 - 20 nm, with the majority ranging 10 -- 12 nm, and lengths 200 - 300 nm. The structure of the as-prepared nanowires was characterized using scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy, x-ray diffraction and electron diffraction. The HRTEM images of the bismuth nanowires show a crystalline Bi core that is wrapped in an amorphous oxide layer. The lattice spacing of planes parallel to the length of the Bi core was found to be 0.328 nm, corresponding to the (012) planes of Bi. Temperature dependent thermopower measurements obtained from our narrow diameter Bi nanowires will be presented. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X31.00003: Growth of Carbon Nanotubes on Metallic Superalloys Saikat Talapatra, Swastik Kar, Sunil Pal, Pethuraja Gopal, Lijie Ci, Robert Vajtai, Pulickel Ajayan There are several advantages of growing carbon nanotubes (CNT) directly on bulk metallic substrates, for example in the formation of robust CNT-metal contacts during growth. Recently, we have shown that multi-wall carbon nanotubes can be grown on Inconel 600, a super alloy, using vapor phase catalyst delivery. The single-step growth of high-quality aligned nanotubes (comparable to those grown on SiO$_{2}$ substrates) show encouraging electrical and mechanical properties. The in situ growth opens up a large number of possibilities for nanotube-based devices. Here, we present detailed investigations on the kinetics of the growth under various experimental conditions, and analyze the nanotube growth mechanism on the generic super alloy systems in the framework of our investigations. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X31.00004: VLS growth of $<$111$>$ oriented Silicon nanowires on Si (111) and Si (100); Growth rate dependence of growth defects Joonho Bae, Shawn Coffee, John Ekerdt, Chih Kang Shih Recently, models have been developed to explain the relation between the growth direction and the diameter of VLS grown silicon nanowires. In this study, we present experimental evidences showing growth rate dependence of growth defects such as bending and kink formation of silicon nanowires grown by SiCl$_{4}$ as a precursor and H$_{2}$ as a carrier gas. We find that the high growth rate tends to result in nanowires with less growth defects permitting well oriented nanowires. By applying this finding and controlling growth conditions, large area silicon nanowires along $<$111$>$ direction were successfully demonstrated on Si (111) and Si (100) substrates. On Si (111) substrates, we achieve large area vertically aligned [111] oriented nanowires. On Si (100) substates, nanowires with four different $<$111$>$ orientations form a large area of inter-lacing network pattern. The underlying growth mechanism and pattern formation are discussed. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X31.00005: Self-assembled Silicon Nanotubes: new 1D semiconductors Ming Xie, Jiesheng Wang, Yoke Khin Yap Silicon nanotubes (SiNTs) have recently attracted attention because of the peculiar properties. SiNTs is also compatible to the present Si microelectronic technology. Theoretically, many research groups have investigated the possible existence of SiNTs. Experimentally, amorphous SiNTs have been synthesized by using template methods. Nevertheless, these SiNTs cannot form good crystal structure due to disordered aggregation of silicon atoms in the inner wall of the templates. Recently, self- assembled silicon nanotubes were reported, which have good crystal structure under supercritically hydrothermal conditions. Here we report self-assembled SiNTs via dual RF-plasma treatments. This technique is compatible to the present integrated circuit technology without involving excessive synthesis pressures and temperatures. Furthermore, our SiNTs are vertically aligned on substrates, which can be easily extracted for devices fabrication. Tunneling spectroscopy was used to characterize the local density of states of these SiNTs. Results indicate that these SiNTs are p-type semiconductors, a new 1D semiconductor for future nanoelectronic devices. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X31.00006: Controlled In-Situ Microscopy Studies of the Effect of Heterogeneous Catalysis on the Growth, Structure and Composition of GaN Nanowires M. Taheri, T. Lagrange, B. Simpkins, N. Browning Controlled growth of GaN nanowires is vital to the efficient production of blue light emitting diodes and other nanoscale optoelectronic devices. The VLS (vapor-liquid-solid) method, which often uses foreign metal catalysts, is a mechanism by which nanowires grow using chemical vapor deposition. Defects, grain boundaries and impurity incorporation that arise during heterogeneous catalysis of nanowires can negatively impact a wire's electronic properties. These properties are directly related to the microstructure and composition of a wire. To understand the specific effects of these metal catalysts on a nanowire's electrical characteristics, a clear understanding of the structural evolution as a function of catalyst (Ni) must be obtained. This is achieved by using in-situ microscopy-based growth methods. In-situ TEM annealing is coupled with high resolution TEM/STEM and Energy Dispersive Spectroscopy (EDS) to study the structure and chemistry of the growing Ni-catalyzed GaN nanowires. Concurrently, laser controlled methods using a Dynamic Transmission Electron Microscope (DTEM) are used to monitor the various stages of the VLS method during microstructural evolution. Each step of wire nucleation and growth during VLS is triggered by a pulsed laser while being monitored in-situ in the TEM. Trends in impurity incorporation with morphology and defect concentration are compared for nanowires grown by both methods of in-situ TEM growth, and to those grown the conventional CVD-based VLS methods. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X31.00007: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 9:24AM - 9:36AM |
X31.00008: Optimum growth of vertically-aligned boron nitride nanotubes at low temperatures Jiesheng Wang, Ming Xie, Yoke Khin Yap Boron nitride nanotubes (BNNTs) are well recognized as the candidate that will complement the uses of carbon nanotubes in nanotechnology. However, high growth temperatures ($>$1100 ${^\circ}$C), low production yield, and impurities have hindered research and applications of BNNTs. We have recently reported the first success of growing pure BNNTs by RF-plasma enhanced pulsed-laser deposition at 600 ${^\circ}$C. These BNNTs can be grown vertically aligned into arrays of regular patterns, and can be used for applications without purification. In this work, we have compared the growth of these BNNTs by a series of catalysts. Electron microscopy images indicate the growth of pure BNNTs with high structural order. UV Raman Spectroscopy demonstrates a peak at 1372 cm$^{-1}$, which corresponds to a E2g mode of h-BN networks of these BNNTs. The effect of catalyst, growth temperatures, ambient gas pressures, substrate bias voltages and the growth mechanism will be described in detail in the meeting. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X31.00009: Controlling the dielectrophoretic assembly of single-walled carbon nanotubes Sarbajit Banerjee, Brian White, Blake Rego, Stephen O'Brien, Nicholas Turro, Irving Herman The AC dielectrophoretic assembly of single-walled carbon nanotubes (SWNTs) represents an attractive approach for the fabrication of SWNT devices. The dielectrophoresis approach relies on the deposition of water-soluble surfactant-wrapped individualized SWNTs in electrode gaps. We have tested a variety of different anionic, non-ionic, and cationic surfactants for their ability to dissolve SWNTs. The zeta potential of the dissolved nanotubes, which is a measure of their surface charge, can be adjusted by varying the surfactant, the pH, and the surfactant concentration. The resulting modulation in surface conductance has implications for the chiral selectivity of the dielectrophoretic process. The surfactant-wrapped SWNTs have been precisely positioned in device geometries by designing appropriate electrode structures based on electric-field simulations. The influence of the surfactant on the transport properties of these devices will also be discussed. This work is primarily supported by the Nanoscale Science and Engineering Center at Columbia University, which is supported under NSF Award Number CHE-0641523. It is also partially supported by the MRSEC program of the NSF under Award Number DMR-0213574 and by NYSTAR. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X31.00010: Au-Induced Assembly of Protective Carbon Shells on Ge Nanowires Eli Sutter, Peter Sutter Semiconductor nanowires (NWs) are promising materials for novel devices. Among them group IV semiconductors offer compatibility and facile integration with conventional electronic circuitry. Given the large surface-to-volume ratio of NWs, it is especially important that the NW surface be protected against oxidation to avoid uncontrolled property changes. For Ge NWs such oxidation protection is not provided by a stable native oxide (as is the case for Si). Hence, any Ge NW-based devices will require the development of passivation or encapsulation techniques. Here we discuss real-time observations by high-resolution transmission electron microscopy during annealing of individual carbon-supported Ge NWs [1]. At moderate temperatures ($\sim $300C) even thick oxide layers on the Ge NWs are reduced rapidly. This is followed by the assembly of crystalline carbon shells that depends critically on traces of Au on the NW surface originating from the Au/Ge catalyst nanoparticles used for the NW synthesis. We demonstrate that the C-shells provide efficient protection of the Ge NW surface against oxidation in ambient air. More generally, our results point at using metal surface decoration to trigger the encapsulation of a wide variety of NW materials in protective C-shells. \newline [1] E. Sutter, P. Sutter, Adv. Mater. 18, 2583 (2006). [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X31.00011: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 10:12AM - 10:24AM |
X31.00012: High Yield Fabrication of Single-Walled Carbon Nanotube Devices via Self-Assembly George S. Tulevski, James B. Hannon, Ali Afzali Single-walled carbon nanotubes (SWCNTs) are attractive materials for many technological applications. Success in the large-scale integration of SWCNTs will depend upon progress in processing to address challenges such as separation, chemical doping and selective placement. This work will highlight recent progress in the selective placement of SWCNTs into predefined positions on gate oxide surfaces, allowing for the fabrication of large arrays of SWCNT devices. SWCNTs are first functionalized with organic compounds that selectively bind to metal oxide surfaces. Electron beam lithography is then employed to pattern hafnium oxide trenches into which the functionalized SWCNTs selectively bind. The surface functionalization is shown to be fully reversible. Once the nanotubes are assembled into the trenches, the molecules are then removed leaving the unfunctionalized SWCNTs behind. This technique allows for hundreds of working devices to be fabricated with high yield. The electrical properties of the subsequent devices are excellent, showing no performance deterioration as a result of the placement process. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X31.00013: New High Aspect-Ratio Titania Nanotubes Eugen Panaitescu, Christiaan Richter, Latika Menon Titanium oxide nanotubes show great promise in photocatalytic, gas sensing, biological, and other applications. Techniques for the fabrication of titania nanotubes include electrodeposition in polymer molds starting from alumina templates, anodization of titanium in fluoride containing solutions, and hydrothermal treatment of nano- and micropowders. We have developed a new synthesis route for the production of new ultra-high aspect-ratio (over 1000:1) titania nanotubes by anodization in chloride containing acid solutions. The fabrication process occurs rapidly, in a fraction of the time when compared with other methods such as anodization in the highly toxic fluoride-containing electrolytes. We have demonstrated nanotubes with diameters as small as 25 nm, and lengths of up to 50 $\mu $m, and we have produced them with varying carbon content through the addition of organic acids in the electrolyte. This opens up new possibilities for many advanced applications of such nanotubes. Various synthesis conditions (pH, chloride content, electrolyte nature), and their influence on morphology, composition, and crystalline structure will be presented. Preliminary results on photocatalytic and transmission properties will also be discussed. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X31.00014: Template-Grown TiO2 Single-Nanowires for Gas Sensing Yaping Dan, Stephane Evoy, A. T. Charlie Johnson A number of contemporary research efforts are directed towards realization of an ``electronic nose'' system where a sensor array is coupled to signal-conditioning electronics and sensor responses fed to odor recognition algorithms to perform detection and classification of vapors. Metal oxides thin films and nanowires are candidates for use in such systems, with the latter having performance advantages associated with their small footprint and enhanced quasi one-dimensional carrier confinement. Here we report experiments exploring the use of template-grown TiO$_{2}$ single-nanowires for gas sensing. TiO$_{2}$ nanowires were prepared by electroplating Ti(OH)$_{x}$ sol-gel into anodic aluminum oxide membranes and then annealing at 450\r{ }C for 12 h. These nanowires are typically 10$\pm $1$\mu $m long and 100$\pm $20nm in diameter. When the temperature is elevated from 20\r{ }C to 200\r{ }C, the conductance of a single nanowire increases from 30pS to 330pS, from which an activation energy of 0.51 $\pm $ 0.02 eV is extracted. When exposed to 20{\%} O$_{2 }$at a working temperature of 200\r{ }C, the conductance of the wires increases by 100{\%} within a few seconds. We will report on sensing experiments for O$_{2}$, H$_{2}$ and CO with different concentrations as well as the effects of sample annealing and working temperature. This work was supported by the National Science Foundation NIRT Grant {\#}0303981. [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X31.00015: Self assembly of organic nanostructures and dielectrophoretic assembly of inorganic nanowires. Geetha Dholakia, Steven Kuo, E. L. Allen Self assembly techniques enable the organization of organic molecules into nanostructures. Currently engineering strategies for efficient assembly and routine integration of inorganic nanoscale objects into functional devices is very limited. AC Dielectrophoresis is an efficient technique to manipulate inorganic nanomaterials into higher dimensional structures. We used an alumina template based sol-gel synthesis method for the growth of various metal oxide nanowires with typical diameters of 100-150 nm, ranging in length from 3-10 $\mu $m. Here we report the dielectrophoretic assembly of TiO$_{2}$ nanowires, an important material for photocatalysis and photovoltaics, onto interdigitated devices. Self assembly in organic nanostructures and its dependence on structure and stereochemistry of the molecule and dielectrophoretic field dependence in the assembly of inorganic nanowires will be compared and contrasted. Tunneling spectroscopy and DOS of these nanoscale systems will also be discussed. [Preview Abstract] |
Session X32: Quasi-One-Dimensional Quantum Gases
Sponsoring Units: DAMOPChair: Ana Maria Rey, Harvard-Smithsonian Center for Astrophysics/ITAMP
Room: Colorado Convention Center 402
Friday, March 9, 2007 8:00AM - 8:12AM |
X32.00001: Decoherence dynamics in low-dimensional cold atoms condensates Anton Burkov, Mikhail Lukin, Eugene Demler We report on a theoretical study of the dynamics of decoherence of a matter-wave interferometer, consisting of a pair of low-dimensional cold atoms condensates. We identify two distinct regimes in the time dependence of the coherence factor of the interferometer: quantum and classical. Explicit analytical results are obtained in both regimes. In particular, in two-dimensional (2D) condensates in the classical (long time) regime, we find that the dynamics of decoherence is universal, exhibiting a power-law decay with an exponent proportional to the ratio of the temperature to the Kosterlitz-Thouless temperature of a single 2D condensate. In the one-dimensional (1D) case we find a nonanalytic time dependence of decoherence,which is a consequence of the nonhydrodynamic nature of damping in 1D liquids. [Preview Abstract] |
Friday, March 9, 2007 8:12AM - 8:24AM |
X32.00002: Crossover to a quasi-condensate in a weakly interacting trapped 1D Bose gas Karen Kheruntsyan, Isabelle Bouchoule, Gora Shlyapnikov One-dimensional (1D) Bose gases are remarkably rich physical systems exhibiting properties not encountered in 2D or 3D. Here we study the exactly solvable 1D model of bosons interacting via a repulsive delta-function potential. Specifically, we discuss the system in the context of a harmonically trapped, weakly interacting 1D Bose gas at ultra-low temperatures and analyze the transition from a fully decoherent regime to a coherent, quasi-condensate regime. By finding the characteristic critical temperature and atom number that depend explicitly on the interaction strength and the trap frequency, we specify the conditions for identifying this transition as an interaction-induced crossover. We contrast this to the finite-size Bose-Einstein condensation (BEC) phenomenon studied previously in the context of an ideal trapped 1D Bose gas. We predict that for sufficiently weak confinement one expects to observe the interaction-induced crossover scenario, rather than the finite-size BEC. The situation is reversed for strong confinement. We identify typical experimental parameters that enable the realization of either of these two competing scenarios. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X32.00003: Quantum Monte Carlo study of a 1D phase-fluctuating condensate Charlotte Gils, Lode Pollet, Alice Vernier, Frederic Hebert, George Batrouni, Matthias Troyer Starting from a microscopic description, we numerically investigate the low temperature behaviour of a trapped one dimensional Bose gas with repulsive interactions. For a sufficient number of particles and weak interactions, we identify a pronounced quasicondensate regime in temperature, where density fluctuations are negligible while phase fluctuations are considerable. In the weakly interacting limit, we find good agreement of our results with those obtained using a mean-field approximation. In addition, we study the system in parameter regimes which are beyond the accessibility of mean-field approaches. A phase-fluctuating condensate exists also in these cases, but phase-correlation properties are qualitatively different. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X32.00004: Spin dynamics in the two-component strongly repulsive 1D Bose gas Mikhail Zvonarev, Thierry Giamarchi, Vadim Cheianov We investigate spin diffusion in the two-component one- dimensional Bose gas in the limit of strong repulsion. While the spectrum of charge excitations can be linearized in such a system, it remains quadratic in the spin sector, and the Luttinger Liquid description is not applicable. However, we showed that dynamical Green's functions of the system can still be found by using a mapping onto an effective spinless model. In this way we get an exact analytic expression for the one-particle and spin-spin Green's functions and found an anomalously low spin-diffusion rate. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X32.00005: Dynamical correlation functions of the 1D Bose gas (Lieb Liniger model) Jean-Sebastien Caux, Pasquale Calabrese The momentum- and frequency-dependent correlation functions (one-body and density-density) of the one-dimensional interacting Bose gas (Lieb-Liniger model) are obtained for any value (repulsive or attractive) of the interaction parameter. In the repulsive regime, we use the Algebraic Bethe Ansatz and the ABACUS method to reconstruct the correlators to high accuracy for systems with finite but large numbers of particles. For attractive interactions, the correlations are computed analytically. Our results are discussed, with particular emphasis on their applications to quasi-one-dimensional atomic gases. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X32.00006: Full distribution functions of interference contrast in low-dimensional bose gases Adilet Imambekov, Vladimir Gritsev, Eugene Demler We consider interference experiments with two independent low dimensional bose condensates. Full distribution function of fringe visibilities is determined by higher order correlation functions within individual condensates and contains non trivial information about quantum and thermal fluctuations in the system. We develop a general method for calculating distribution functions of the interference amplitude and apply it to one and two dimensional condensates both at zero and finite temperatures. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X32.00007: Strongly correlated bosons on optical superlattices: Dynamics and relaxation in the superfluid and insulating regimes Marcos Rigol, Maxim Olshanii, Alejandro Muramatsu We study the nonequilibrium dynamics of hard-core bosons (HCB's) on one-dimensional lattices. The dynamics is analyzed after a sudden switch-on or switch-off of a superlattice potential, which can bring the system into insulating or superfluid phases, respectively. A collapse and revival of the zero-momentum peak can be seen in the first case. We study in detail the relaxation of these integrable systems towards equilibrium. We show that after relaxation time averages of physical observables, like the momentum distribution function, can be predicted by means of a generalization of the Gibbs distribution. [M. Rigol, A. Muramatsu, and M. Olshanii, Phys. Rev. A 74, 053616 (2006).] [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X32.00008: Spectroscopy and quantum quench dynamics of interacting one-dimensional Bose condensates Vladimir Gritsev, Anatoli Polkovnikov, Eugene Demler We discuss applications of the exact solution of the quantum sine Gordon model to study non equilibrium dynamics of two coupled interacting one dimensional Bose liquids. In particular, we consider a set up in which a sudden quench of the tunneling amplitude introduces oscillations in the relative phase of the two condensates. We demonstrate that the power spectrum of the interference amplitude oscillations should reveal the non trivial excitation spectrum of the quantum sine Gordon model. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X32.00009: Onset of Chaos and Thermalization in a One-Dimensional Bose-Hubbard Lattice in the Mean-Field Regime Douglas Mason, Amy Cassidy, Vanja Dunko, Maxim Olshanii The goal of this work is to identify a Chirikov threshold for the onset of chaos and, beyond the threshold, to study thermalization in a one-dimensional Bose-Hubbard Model. In the mean-field approximation the problem is conceptually close to the one of the beta-f Fermi-Pasta-Ulam model. In the regime of well developed chaos the atomic momentum distribution is shown numerically to converge to the predictions of the grand canonical ensemble, simulated in turn using the Monte Carlo method. We find good agreement between our analytical predictions and the results of our numerical calculations. We discuss the relevance of our results to the recent Newton's Cradle experiments on relaxation of an ensemble of bosonic atoms in a one-dimensional optical trap [T. Kinoshita, T. Wenger, D.S. Weiss, Nature (London) 440, 900 (2006)]. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X32.00010: Dynamics of Bose gases in Y-shaped potential and Andreev-type reflection Akiyuki Tokuno, Masaki Oshikawa, Eugene Demler Recently, guiding of atoms in low-dimensional magnetic traps has been actively studied. While the theory of one-dimensionally trapped atoms has been vigorously studied, much of the dynamical aspects remain unexplored. As a simple yet nontrivial example, we study the real-time dynamics of BEC in the Y-shaped potential. Collective nature of the transport is considered by treating each one-dimensional channel as a Tomonaga-Luttinger liquid. We analytically investigate the reflection and transmission at the center of the potential, for a high-density packet moving from one side to the other sides. Even though we study a system of bosons, we find that the reflection at the center of the potential exhibits an Andeev-type reflection reminiscent of that at a normal-superconductor interface in electron systems. This could be attributed to the fermionic nature of the repulsively interacting Bose systems in one dimension. In addition, we shall also discuss the dynamics in the ring type interferometer which consists of two symmetric Y-junction. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X32.00011: Ultracold bosons in one-dimensional incommensurate superlattices Tommaso Roscilde, Ignacio Cirac Motivated by recent experiments (L. Fallani et al., cond-mat/0603655), we numerically investigate the ground-state properties of strongly interacting ultracold bosons in a one-dimensional quasi-periodic superlattice, modeled by the Bose-Hubbard Hamiltonian in an incommensurate cosine potential. In the weakly interacting regime, the incommensurate potential (IP) is known to lead to Anderson localization when exceeding a given critical strength. We find that strong repulsion, giving rise to a Mott-insulating state for the system without the IP, introduces an extremely rich physical scenario. For repulsion values away from the Tonks limit an IP added to the Mott phase is effectively screened by a fraction of the particles, and drives the system to a superfluid phase for the remaining fraction. For larger IPs, a cascade of incompressible insulating states appears with incommensurate fractional fillings. The change of filling from a state to the next is usually accompanied by significant particle number fluctuations without superfluidity, namely by Bose-glass behavior in narrow parameter ranges. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X32.00012: Damping of condensate oscillations of a trapped Bose condensate in a 1D optical lattice at finite temperatures Emiko Arahata, Tetsuro Nikuni We study Landau damping of dipole oscillations of a Bose condensate in a 1D optical lattice at finite temperatures. Assuming that an additional trap potential is highly anisotropic, in which the radial confinement is much tighter than the axial confinement, we derive a quasi-1D model of the Gross-Pitaeavskii equation and the Bogoliubov equations that include the effect the excitations in the radial direction. We calculate the Landau damping rate and investigate its dependence on the lattice depth, compare our result with the experiental data on collective modes in an optical lattice [F. Ferlaino et. al., Phys. Rev. A 66, 011604(2002)]. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X32.00013: Glassy behavior of Bose-Bose mixtures in one-dimensional optical lattices Ignacio Cirac, Tommaso Roscilde We numerically investigate the properties of strongly repulsive two-boson mixtures in one-dimensional optical lattices, targeting their ground state either by slow cooling from high temperature, or by a slow change in the Hamiltonian parameters starting from the weakly interacting regime. The two bosonic species have very different effective masses, so that the slow bosons can act as an effective potential to the faster ones. When the interspecies repulsion is strong compared with the intraspecies one, a phase-separated ground state is masked by an exponentially large number of metastable \emph{quantum emulsion} states, in which the two species are fragmented into microscopic droplets. The quantum emulsion states can be regarded as the out-of-equilibrium realization of a localization phenomenon, in which each species acts as a random potential to the other one, effectively localizing it. Quantum Monte Carlo investigations reveal an extremely slow relaxation of the system towards equilibrium, typical of a glassy phase. Increasing the intraspecies repulsion for the fast bosons drives them through a quantum phase transition to the superfluid state. [Preview Abstract] |
Session X33: Focus Session: Quantum Information at the AMO/Condensed-Matter Interface
Sponsoring Units: GQI DAMOPChair: Barry Sanders, University of Calgary
Room: Colorado Convention Center 403
Friday, March 9, 2007 8:00AM - 8:36AM |
X33.00001: Quantum simulation of magnetism using optical lattices Invited Speaker: Physical simulation as a means for resolving outstanding quantum many-body problems was first proposed by Feynmann in 1981. Since then, physicists have dreamed of using physical quantum simulation as a quantitative tool. Ultra-cold atoms trapped in an optical lattice are now emerging as an ideal tool for quantum simulation of a wide range of many-body quantum models, including the Hubbard model and quantum magnetism. I will review the developing field of quantum simulation using ultra-cold atoms and highlight our progress on simulating quantum magnetism. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X33.00002: Quantum phase transitions of light Charles Tahan, Andrew Greentree, Jared Cole, Lloyd Hollenberg Recently, condensed matter and atomic experiments have reached a length-scale and temperature regime where new quantum collective phenomena emerge. Finding such physics in systems of photons, however, is problematic, as photons typically do not interact with each other and can be created or destroyed at will. Here, we introduce a physical system of photons that exhibits strongly correlated dynamics on a meso-scale. By adding photons to a two-dimensional array of coupled optical cavities each containing a single two-level atom in the photon-blockade regime, we form dressed states, or polaritons, that are both long-lived and strongly interacting. Our zero temperature results predict that this photonic system will undergo a characteristic Mott insulator (excitations localised on each site) to superfluid (excitations delocalised across the lattice) quantum phase transition. Each cavity's impressive photon out-coupling potential may lead to actual devices based on these quantum many-body effects, as well as observable, tunable quantum simulators. We explicitly show that such phenomena may be observable in micro-machined diamond containing nitrogen-vacancy colour centres and superconducting microwave strip-line resonators. (Nature Physics, December 2006) [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X33.00003: Universal and measurable entanglement in the spin-boson model Angela Kopp, Karyn Le Hur We study the entanglement between a qubit and its environment by calculating the von Neumann entropy of the spin in the delocalized phase of the spin-boson model. Using a well-known mapping between the spin-boson model with Ohmic dissipation and the anisotropic Kondo model, we obtain exact results for the entanglement entropy $E$ at arbitrary dissipation strength $\alpha$ and level asymmetry $h$. We show that the Kondo energy scale $T_K$ controls the entanglement between the qubit and the bosonic environment. For $h \ll T_K$, we find that $E=E(h=0)-\frac{2e^{b/(2-2\alpha)} \Gamma[1+1/(2-2\alpha)]}{\pi \ln 2 \Gamma[1+\alpha/(2-2\alpha)]} (\frac{h}{T_K})^2$, where $b=\alpha \ln \alpha + (1-\alpha) \ln (1-\alpha)$. The universal $(h/T_K)^2$ scaling reflects the Fermi liquid nature of the Kondo ground state. In the limit $h \gg T_K$, E vanishes as $(T_K/h)^{2-2\alpha}$, up to a logarithmic correction. We thoroughly explore the phase space $(\alpha, h)$; for a given $h$, the maximal entanglement occurs in the crossover regime $h \sim T_K$. We also emphasize the possibility of measuring this entanglement using charge qubits subject to electromagnetic noise. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X33.00004: Quantum Information Transport in Nuclear Spin Chains. Paola Cappellaro, David Cory In many solid-state proposals for quantum computers, the transport of information over relatively short distances inside the quantum processor itself is an essential task, and one for which relying on photons, and therefore on a frequent exchanging of information between solid-state and light qubits, could be too costly. Quantum wires based on spins could be a viable alternative leading to much theoretical work on quantum information transfer in linear spin chains. Experimental studies on nuclear spin systems in solid-state by NMR (the most natural implementation of such models) has been up to now prevented by the unavailability of the desired (Heisenberg) Hamiltonian, since the naturally occurring interaction assumes the dipolar form. We present here a similarity transformation between the Heisenberg Hamiltonian and an interaction which is achievable with the collective control provided by rf pulses in NMR. Not only this second Hamiltonian allows us to simulate the information transport in a spin chain, but it also provides a means to observe its signature experimentally. With this scheme it will be possible to study experimentally, in solid-state NMR systems, the transport of polarization beyond exactly solvable models. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X33.00005: Cavity QED in the mesoscopic regime Pascal Degiovanni, Valentin Bonzom, Hichem Bouzidi, Arnaud Le Diffon, Clement Ruef, Tristan Meunier, Jean-Michel Raimond We report on a recent study of the behavior of N atoms resonantly coupled to a single electromagnetic field mode sustained by a high-Q cavity, containing a mesoscopic coherent field. Using a simple effective Hamiltonian model, we show that the strong coupling between the cavity and N atoms/qubits produces an atom-field entangled state, involving N+1 nearly coherent components slowly rotating at different paces in the phase plane. The periodic overlap of these components results in a complex collapse and revival pattern for the Rabi oscillation. Decoherence induced by cavity relaxation, qubit relaxation and dephasing are taken into account. We propose a simple model based on the stochastic quantum trajectories approach. Its results are successfully compared to numerical simulations. Explicit predictions for Rydberg atoms and circuit QED experiments are obtained and suggest that these effects may be observable in the near future. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X33.00006: Diverging Length Scale, Scaling, and Universality of Entanglement Near a Quantum Phase Transition Han-Dong Chen In this work, we show that an important quantity to study about entanglement near a quantum phase transition is the two-body entanglement S(i,j), which measures the entanglement between two separated degrees of freedom (ij) and the rest of system. We establish its relation to correlation functions in the long range limit. Away from the critical point, S(n) saturates with a characteristic length scale $\xi_E$, as the distance n increases. The entanglement length $\xi_E$ diverges near the critical point with the same critical exponent as correlation length. At the critical point, S(n) follows a power law. The universality and finite size scaling of entanglement are demonstrated in a class of exactly solvable spin model. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X33.00007: Landau-Zener dynamics in qubits-oscillator settings Sigmund Kohler, Martijn Wubs, Peter H\"anggi, Keiji Saito, Yosuke Kayanuma In a Cooper-pair box realization of a qubit, the energy splitting of the logical states can be tuned upon variation of the penetrating magnetic flux. Then the coupling of the qubit to a circuit QED oscillator can induce Landau-Zener transitions between the qubit levels. By summing a perturbation series to all orders, we obtained an exact expression for the corresponding LZ transition probability. Moreover, we determined the parameters for which a non-adiabatic transition is accompannied by single-photon generation and showed that LZ transitions can create qubit-oscillator entanglement in a controlled manner [1]. Replacing the oscillator by a quantum heat bath, we encounter a nontrivial problem of dissipative quantum mechanics which can be solved in a similar way. As a main application, we discuss the determination of both the reorganization energy and the integrated spectral density of the bath [2]. Moreover, this provides a convenient test bench for numerical schemes for real- time dissipative quantum dynamics. \\{} [1] K. Saito \textit{et al.}, Europhys.\ Lett.\ \textbf{76}, 22 (2006). \\{} [2] M.\ Wubs \textit{et al.}, Phys.\ Rev.\ Lett.\ \textbf{97}, 200404 (2006). [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X33.00008: Coherence control via dynamical decoupling of an electron spin in a quantum dot Wenxian Zhang, V.V. Dobrovitski, Nikolaos Konstantinidis, Lea F. Santos, Lorenza Viola, B.N. Harmon An electron spin in a quantum dot is a promising system for applications in coherent spintronics and quantum computation, but the interaction with the nuclear spins leads to fast decoherence. Subjecting the electron spin to a suitable pulsed control field decouples it from the nuclear spin bath and suppresses decoherence. We study numerically and analytically several most promising decoupling protocols, taking into account the intra-bath coupling, using fully quantum mechanical treatment of the system plus bath dynamics. We show that some high-level protocols extend the coherence time by 3 orders of magnitude for an arbitrary initial spin state. Moreover, we present the protocols which preserve a known initial state with near-to-optimal fidelity for arbitrarily long times. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X33.00009: Entanglement entropy of bilinear fermionic systems Letian Ding, Noah Bray-Ali, Stephan Haas We work out a bound of the block entropy $S_L$ for systems of spinless fermions with generic, bilinear interactions. Experimentally relevant examples include p-wave superconductors and cold atom gases near a Feshbach resonance. We find that the block entropy does {\it not} obey an area law $S_L \sim cL^{d-1}$ law whenever the system has a $d-1$ dimensional surface of gapless excitations. For other systems, such as a p-wave superconductor with Fermi points, the block entropy does obey the area law, but with a coefficient that diverges as the gap closes. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X33.00010: Adiabatic Preparation of Topological Order Alioscia Hamma, Daniel Lidar Topological order characterizes those phases of matter that defy the standard description in terms of symmetry breaking and local order parameters. Topological order is found in nature in the fractional quantum Hall effect. Topologically ordered systems have ground state degeneracy that is robust against perturbations, which has given the root to topological quantum information processing. We discusss the second order quantum phase transition between a spin-polarized phase and a topologically ordered string-net condensed phase. Next we show how to prepare the topologically ordered phase through adiabatic evolution in a time that is upper bounded by $O(\sqrt{n})$. This provides a physically plausible method for constructing a topological quantum memory. We discuss applications to topological and adiabatic quantum computing. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X33.00011: Quantum phase transition from magnetic to topological order Alioscia Hamma, Wen Zhang, Stephan Haas, Daniel Lidar We present a numerical study of the quantum phase transition from the magnetically ordered phase to the topologically ordered phase of a $n$-spins $1/2$ system. We show that the derivative of von Neumann entropy of a plaquette diverges at the critical point, signaling a second order quantum phase transition. Moreover, we compute the finite-size scaling of the Topological Entropy, showing how this quantity detects the passage to the topologically ordered phase. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X33.00012: Replacing energy by von Neumann entropy in quantum phase transitions Xun Jia, Angela Kopp, Sudip Chakravarty In the thermodynamic limit two distinct states of matter cannot be analytic continuations of each other. Classical phase transitions are characterized by non-analyticities of the free energy. For quantum phase transitions the ground state energy often assumes the role of the free energy. But in a number of important cases this criterion fails, such as the three- dimensional metal-insulator transition of non-interacting electrons in a random potential. It is therefore essential that we find alternative criteria that can track fundamental changes in the internal correlations of the ground state wavefunction. Here we propose that QPTs are generally accompanied by non- analyticities of the von Neumann (entanglement) entropy. In particular, the entropy is non-analytic at the Anderson transition, where it exhibits unusual fractal scaling. We also examine integer quantum Hall effect from this perspective. [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X33.00013: Statistical quantum mechanics and entanglement in anisotropic Heisenberg model. You-Ling Chiang, Armen Kocharian, Chee Yang The single site quantum and thermal entanglement, concurrences, quantum phase transitions and corresponding quantum critical points are studied in small spin $s={1\over 2}$ and $1$ in ferromagnetic and antiferromagnetic Heisenberg dimers. The grand canonical ensemble of Heisenberg clusters is also used for exact calculations of thermal properties, quantum and thermal entanglements of the various spin and fermionic lattice models in the presence of magnetic field. We study the magnetic phase transitions and crossovers driven by external field and temperature. The comparison with the exact solution for Heisenberg model in thermodynamic limit for the limiting cases is also provided. The small Ising, Heisenberg and Hubbard clusters are also used for comparison with the exact Bethe ansatz solutions and predictions of traditional mean field theory and developed perturbation theory about generalized self- consistent solution. [Preview Abstract] |
Session X34: Shape Changes in Biological Membranes
Sponsoring Units: DBPChair: Peter Jung, Ohio University
Room: Colorado Convention Center 404
Friday, March 9, 2007 8:00AM - 8:12AM |
X34.00001: Polyunsaturated Fatty Acids in Lipid Bilayers and Tubules Linda S. Hirst, Jing Yuan, Yohannes Pramudya, Lam T. Nguyen Omega-3 polyunsaturated fatty acids (PUFAs) are found in a variety of biological membranes and have been implicated with lipid raft formation and possible function, typical molecules include DHA (Docosahexanoic Acid) and AA (Alphalinoleic Acid) which have been the focus of considerable attention in recent years. We are interested in the phase behavior of these molecules in the lipid bilayer. The addition of lipid molecules with polyunsaturated chains has a clear effect on the fluidity and curvature of the membrane and we investigate the effects the addition of polyunsaturated lipids on bilayer structure and tubule formation. Self-assembled cylindrical lipid tubules have attracted considerable attention because of their interesting structures and potential technological applications. Using x-ray diffraction techniques, Atomic Force Microscopy and confocal fluorescence imaging, both symmetric and mixed chain lipids were incorporated into model membranes and the effects on bilayer structure and tubule formation investigated. [Preview Abstract] |
Friday, March 9, 2007 8:12AM - 8:24AM |
X34.00002: Dynamics of Encapsulation and Budding in Lipid Membranes Kurt Smith The behavior of lipid membranes is important in cell biology, as well as in the development of synthetic vesicles for drug delivery and other applications. The fundamental role of the membrane is to control the passage of matter into and out of a cell or vesicle. We have examined two related processes - the encapsulation of a particle by an adhesive membrane (as in endocytosis) and the budding and vesiculation of a phase separated membrane domain. These processes require changes in membrane topology (i.e. pinch-off) which involve molecular-scale rearrangements. Thus they cannot be fully understood through a macroscopic free energy formulation. Using dissipative particle dynamics, we examine the pathway through which pinch-off occurs, and find that it depends upon the nucleation of a pore at the membrane neck. We use simulations to predict the range of conditions under which pinch-off is possible. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X34.00003: Lipid tubules Formed by Flow-Controlled Hydration Jing Yuan, Linda S. Hirst Self-assembled cylindrical tubules from lipid molecules have attracted considerable attention because of their interesting supramolecular structures and technological applications. Schnur et al. [1] reported the formation of tubular microstructures from a series of diacetylenic phospholipids after liposomes were cooled through their chain melting transition. After that, several methods have been developed to fabricate such unique microstructures mainly by means of deforming preformed Giant unilamellar vesicles. Here we present a simple strategy to construct lipid microtubules through a flow-controlled lipid hydration. Fluorescent microscopy and Confocal Laser Microscopy were used to visualize the formation and the structure of the lipid tubules. Tubules were found to develop following the direction of the dynamic flow with highly parallel alignment. At high flow speeds, partial cross-linking of the lipid tubules was observed. To demonstrate the generality of this method, different types of phospholipids, such as Phosphatidic Acid (PA), Phosphatidylserine (PS), Phosphatidylethanolamine (PE), and Phosphatidylglycerol (PG) were investigated. \newline \newline [1] J.M. Schnur et al, Science, 264, 945 (1994). [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X34.00004: Spontaneous Formation of Lipid Nanotubes and Lipid Nanofibers from Giant Charged Dendrimer Lipids Alexandra Zidovska, Kai K. Ewert, Cyrus R. Safinya, Joel Quispe, Bridgett Carragher, Clinton S. Potter Liposomes have attracted much scientific interest due to their applications in model cells studies and in drug encapsulation. We report on the discovery of new vesicle phases formed in mixtures of MVLBG2, DOPC and water. MVLBG2 is a newly synthesized highly charged (16+) lipid (K. Ewert et al., \textit{JACS, }2006) with giant dendrimer headgroup thus leading to a high spontaneous curvature of the molecule. In combination with zero-curvature DOPC, MVLBG2 exhibits a rich phase diagram showing novel vesicle morphologies such as bones, lipid nanotubes and nanofibers as revealed by differential contrast microscopy (DIC) and cryo-TEM. At the micron scale DIC reveals a new phase consisting of bone-like vesicles. This novel morphology persists down to the nanometer scale as shown by cryo-TEM. The nanotubes are of diameter 10-50 nm, length $>$ 1$\mu $m and consist of a single lipid bilayer. A surprising new morphology arises resulting from a spontaneous topological transition from tubes to lipid nanorods. Funded by DOE DE-FG-02-06ER46314, NIH GM-59288, NSF DMR-0503347. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X34.00005: Hydrodynamic extrusion of membrane nanotubes: the role of the cytoskeleton Karine Guevorkian, Nicolas Borghi, S\'{e}bastien Kremer, Axel Buguin, Fran\c{c}oise Brochard We have investigated membrane-cytoskeleton adhesion properties by extrusion of tubes from tethered vesicles and cells using hydrodynamic flows. Our experimental results show that impermeable membranes (giant vesicles) act as entropic springs, i.e. the extruded tubes reach a stationary length, whereas porous membranes (vesicles decorated with pores) lead to tubes, which extrude at constant velocity without reaching a stationary length. On the other hand, experiments on red blood cells (RBC) suggest that the dynamics of extruded tubes is dominated by the detachment of the membrane from the cytoskeleton and the flow of lipids through the binding membrane proteins. We have estimated the membrane-cytoskeleton binding energy and the viscosity of the membrane for RBC-s. Tube extrusion from other cell types (S180, MDCK, BON) show phenomena such as healing time for the membrane-cytoskeleton rebinding, and cell aging (breakage of the tube after a few consecutive extrusions). We will discuss how these phenomena depend on the properties of the cytoskeleton and on the presence of cell adhesion molecules. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X34.00006: Shape transformations of active tubular membranes Elnaz Alipour-Assiabi, Thomas Powers Motivated by the action of enzymes that flip lipid molecules from one monolayer to another in a lipid bilayer membrane, we study shape instabilities of a tubular membrane driven by lipid-flipping. We begin with the instability of a tube with a fixed lipid number density distance, determining the relative importance of solvent viscosity, membrane viscosity, and bilayer friction. Then we consider the case of a uniform density of enzymes acting at a fixed rate. Implications for experiments will be discussed. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X34.00007: Effective surface tension of red blood cell membranes induced by cytoskeleton meshworks Rui Zhang, Frank Brown The membrane of red blood cell (RBC) consists of a lipid bilayer and a two dimensional cytoskeleton meshwork underneath. Its elastic properties are therefore different from a simple lipid bilayer. We introduced a simple entropic spring model to study the meshwork. In this model, adjacent nodes of the meshwork interact with each other through the link of an entropic spring. We run Monte Carlo and Brownian dynamics simulations, and developed some simple analytical theories to understand the simulation results. For a complete meshwork, we found that the cytoskeleton meshwork produced an effective surface tension to the RBC membrane, as far as the height fluctuation of the membrane is considered. This surface tension depends on the wave length of the fluctuation, and shows a crossover at the wave length of the average mesh size. We also studied the case when a fraction of randomly chosen links are disconnected from the nodes, possibly with the help of ATP. In this case, the surface tension changes with the fraction of connected links. Most interestingly, we found a percolation phase transition of the surface tension at long wave length limit. We discussed the experimental results related to our theory. Our model may improve the understanding of certain functions the RBC membrane related to its elastic properties. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X34.00008: Synthetic antimicrobial oligomers induce composition-dependent topological transition in membranes Lihua Yang, Vernita Gordon, Abhijit Mishra, Kirstin Purdy, John Cronan, Abhigyan Som, Gregory Tew, Gerard C.L. Wong Antimicrobial peptides comprise a key component of innate immunity for a wide range of multicellular organisms. Recently, their synthetic analogs have demonstrated broad-spectrum antimicrobial activity via permeating bacterial membranes selectively, although the precise molecular mechanism underlying the activity is still unknown. We systematically investigate interactions and self-assembled structures formed by model bacterial membranes and a prototypical family of phenylene ethynylene-based small molecule antimicrobials with controllable activity and selectivity. Synchrotron small angle x-ray scattering (SAXS) results correlate antibacterial activity and the induced formation of an inverted hexagonal phase, and indicate that the organization of negative curvature lipids such as DOPE are crucially important. Preliminary killing assays of DOPE-deficient mutant bacteria agree with the x-ray results. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X34.00009: Pore formation by antimicrobial peptides: structural tendencies in bulk and quasi-2D membrane systems Vernita Gordon, Lihua Yang, Matthew Davis, A. Som, G. Tew, Gerard Wong Antimicrobial peptides are cationic, amphiphilic structures that are key components of innate immunity. A prototypical family of synthetic analogs are the phenylene ethynylene antimicrobial oligomers (AMOs), which have hydrophobic alkyl chains connected to cationic hydrophilic regions. Synchrotron small-angle x-ray scattering (SAXS) shows that when AMO is mixed with concentrated model membranes, initially in the form of Small Unilamellar Vesicles, the sample forms the inverted hexagonal phase. This is a 3-dimensional phase characterized by a regular array of size-defined water channels. We demonstrate how this structural tendency is expressed when AMOs interact with dilute model membranes in the form of Giant Unilamellar Vesicles (GUVs). Using confocal microscopy, we see that applying AMO to the GUVs causes small encapsulated molecules to be released while large molecules are retained, indicating that size-defined pores have been created. Examining the partial release of polydisperse intermediately-sized molecules allows a closer measurement of the pore size, and there are indications that this single-vesicle microscopy will allow elucidation of the kinetics of the pore-forming process. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X34.00010: Role of membrane bending in ASAP1 protein activity Beatriz E. Burrola Gabilondo, Ruibai Luo, Wolfgang Losert, Paul A. Randazzo ASAP1 is part of the protein machinery that alters membranes and the actin cytoskeleton in cellular structures, called invadopodia, that mediate invasion of mammary cell carcinoma and uveal melanoma. The molecular mechanism by which ASAP1 contributes to these structures is not well defined. ASAP1 induces the hydrolysis of GTP that is bound to the protein Arf. Another activity is to deform lipid bilayers into tubules. We have set out to test the hypothesis that the enzymatic GAP activity is related to the mechanical activity. We contrast several reaction schemes for GAP activity, including steps that would be sensitive to physical changes in the membrane. We compare the numerical model predictions to data obtained from kinetics experiments. We are also developing assays such as FRET and tools like laser tweezer forcing of vesicle deformations to be used to determine the effect of ASAP1 and mutants with defects in enzymatic activity on the physical state of lipid vesicles. The ramifications of the results to the role of ASAP1 in invadopodia formation will be discussed. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X34.00011: Shape transformations of human red blood cells under osmotic deflation-inflation Gerald Lim, Michael Wortis We systematically study the mechanics of osmotically driven shape transformations of human red blood cells, based on a computational model we developed earlier that successfully describes the stomatocyte-discocyte-echinocyte shape transformations, which are driven by the bilayer couple mechanism. We obtain a surprisingly complex energy landscape, the prominent feature of which is a tricritical point that gives rise to self-intersection of the main minimum-energy surface in a line and shape transformations exhibiting hysteresis, metastability, and re-entry. These occur in physically accessible regions of parameter space and, thus, can be tested experimentally. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X34.00012: Dissipative Particle Dynamics Simulations of Deformable Red Blood Cells in Small Blood Vessels Igor Pivkin, Peter Richardson, George Karniadakis Explicit simulations of the blood cellular components require computational methods capable of tracking time-varying fluid-solid interface. The Dissipative Particle Dynamics (DPD) is an inherently adaptive method and potentially very effective in simulating complex fluid systems. In DPD, the fluid and solid objects are represented as a collection of interacting points, each representing a group of atoms or molecules. The red blood cell model takes into account bending and in-plane energies as well as constraints of constant surface and volume. We will present results of simulations of the deformable red blood cells in a small blood vessel using DPD. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X34.00013: Nucleation of holin domains and holes optimizes lysis timing of \textit{E. coli} by phage $\lambda$ Gillian Ryan, Andrew Rutenberg Holin proteins regulate the precise scheduling of \textit{Escherichia coli} lysis during infection by bacteriophage $\lambda$. Inserted into the host bacterium's inner membrane during infection, holins aggregate to form rafts and then holes within those rafts. We present a two-stage nucleation model of holin action, with the nucleation of condensed holin domains followed by the nucleation of holes within these domains. Late nucleation of holin rafts leads to a weak dependence of lysis timing on host cell size, though both nucleation events contribute equally to timing errors. Our simulations recover the accurate scheduling observed experimentally, and also suggest that phage-$\lambda$ lysis of \textit{E.coli} is optimized. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X34.00014: Physiological role of stochastic calcium signaling in subcellar microdomains Yohannes Shiferaw Calcium (Ca) plays an important role in regulating various cellular processes. In a variety of cell types, Ca signaling occurs within microdomains where Ca channels deliver localized pulses of Ca which activate a nearby collection of Ca sensitive receptors. The small number of channels in these microdomains ensures that the signaling process is stochastic. The aggregate response of several thousand of these micro-domains yields a whole cell response which dictates the observable cell behavior. Here, we study analytically the statistical properties of a population of these micro-domains in response to a trigger signal. We apply these results to understand the relationship between Ca influx and Ca release in cardiac myocytes. In particular, we explain why the global response is graded with respect to total Ca influx, even though Ca response at the micro-domain level is all-or-none. [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X34.00015: A Systematic Study of Bilayer Failure on Engineered Surfaces Morgan Mager, Nicholas Melosh Ever since the invention of black lipid membranes, supported lipid bilayers have been an important tool for studying integral membrane proteins as well as fundamental bilayer behavior. In spite of this, these structures have a relatively short lifetime and little is known about their failure mechanisms. By systematically altering the geometry and surface chemistry of microfabricated pores, we are able to isolate the importance of several distinct failure mechanisms. These include pressure fluctuations, unsupported area, surface energy of the pore wall and surface roughness. We will also demonstrate that, even when not actively controlled, these parameters can inadvertently be altered depending in processing conditions. [Preview Abstract] |
Session X35: Focus Session: Nucleic Acid Protein Interaction
Sponsoring Units: DBPChair: Dean Astumian, University of Maine
Room: Colorado Convention Center 405
Friday, March 9, 2007 8:00AM - 8:36AM |
X35.00001: Accuracy of Localization Methods for Individual Fluorescent Probes Invited Speaker: Recent technological developments have made light microscopy of single molecules possible. The limited number of photons available from a single fluorescent molecule makes image analysis a statistical analysis. Consequently, optimal data analysis is as important to experimental resolution as improved experimental conditions, such as photobleaching rates of fluorescent probes. The simple case of \textit{localization accuracy} provides a pertinent example. In theory, conventional lens-based light microscopy can determine the position of a point-like object with an accuracy that increases infinitely with the number of photons producing it. In practice, a finite signal-to-noise ratio limits localization accuracy and so may the choice of statistical estimator. Some estimators are easier to apply than others, but their relative virtues in regards to accuracy is unclear, or only known numerically for specific cases. We analyze three popular estimators under ideal conditions, find exact analytical results for their accuracy, and clear up a confusion in the literature. Next we test our results for accuracies against ideal real data, and find results that change our view of these estimators for practical use. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X35.00002: Phenotypic consequences of promoter-mediated transcriptional noise: Experiment and computational modeling Gabor Balazsi, William Blake, Michael Kohanski, Kevin Murphy, James Collins A more complete understanding of the causes and effects of gene expression noise is needed to elucidate whether the resulting phenotypes are disadvantageous or confer some adaptive advantage. We introduce mutations within the promoter region of an engineered, repressible \textit{Saccharomyces cerevisiae} GAL1 promoter to show that the level of gene expression noise is affected by the sequence of the TATA box. Through computer simulations, we identify transcription scaffold stability as a critical noise-mediating factor. We demonstrate that TATA box-dependent, increased gene expression noise can be beneficial after an acute change in environmental conditions. First, we illustrate computationally how a stable transcription scaffold can enable increased cell-cell variability at steady state. Second, we experimentally verify our computational prediction that the increased gene expression noise enabled by TATA-containing promoters confers a clear benefit in the face of an acute environmental stress. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X35.00003: Role of boundary constrains in DNA looping problem. Alexei Tkachenko We present a theoretical study of the effects of boundary constrains on DNA looping. The developed Effective Hamiltonian description enables one to calculate the looping probability density (so called J-factor), in a much simpler way than by traditional methods. Our approach is applicable to a variety of in-vitro and in-vivo problems, ranging from DNA cyclization, to protein-mediated DNA looping. In particular, it will be demonstrated that the existing controversy between various DNA cyclization experiments can be attributed to the variation in the boundary conditions. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X35.00004: Kinetic Accessibility of Buried DNA Sites in Nucleosomes Wolfram M\"obius, Richard A. Neher, Ulrich Gerland Motivated by recent experiments on nucleosome accessibility [1,2] we study the transient exposure of protein-binding DNA sites within nucleosomes using a theoretical model for spontaneous partial DNA unwrapping from histones. We focus on the functional dependence of the rates for site exposure and re-burial on the site position, which is pertinent to gene regulation. We find the dependence to be roughly described by a random walker model. Close inspection however reveals a surprising dependence of the re-burial rates on the length of unwrapped DNA. We show that this corresponds to a physical effect of flexibility-assisted barrier crossing, which we characterize within a toy model, the {\it semiflexible Brownian rotor}.\newline [1] G. Li, M. Levitus, C. Bustamante, and J. Widom, Nat. Struct. Biol. {\bf 12}, 46 (2005)\newline [2] M. Tomschik, H. Zheng, K. van Holde, J. Zlatanova, and S. Leuba, Proc. Natl. Acad. Sci. U.S.A. {\bf 102}, 3278 (2005)\newline [3] W. M\"obius, R.A. Neher, and U. Gerland, Phys. Rev. Lett. {\bf 97}, 208102 (2006) [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X35.00005: Coarse-Grained Modeling of Molecular Machines in AAA+ Family Kenji Yoshimoto, Charles L. Brooks III We present a new coarse-grained model of the large protein complexes which belong to AAA+ (ATPase associated with diverse cellular activities) family. The AAA+ proteins are highly efficient molecular machines driven by the ATP (adenosine triphosphate) binding and hydrolysis and are involved in various cellular events. While a number of groups are developing various coarse-grained models for different AAA+ proteins, the molecular details of ATP binding and hydrolysis are often neglected. In this study, we provide a robust approach to coarse-graining both the AAA+ protein and the ATP (or ADP) molecules. By imposing the distance restraints between the phosphates of the ATP and the neighboring $C_{\alpha}$ of the proteins, which are used to conserve a typical motif of ATP binding pocket, we are able to predict large conformational changes of the AAA+ proteins, such as replicative hexameric helicases. In the case of the hexameric LTag (large tumor antigen), the backbone RMSD between the predicted ATP-bound structure and the X-ray structure is 1.2 $\AA$, and the RMSD between the predicted ADP-bound structure and the X-ray structure is 1.5 $\AA$. Using the same approach, we also investigate conformational changes in the hexameric E1 protein, whose X-ray structure was recently solved with ssDNA, and give some insights into the molecular mechanisms of DNA translocation. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X35.00006: A Simple Model of Nucleosome Localization David Schwab, Robijn Bruinsma It has recently been shown that nucleosomes localize to preferred locations along DNA. This localization is a result of the sequence dependent bending stiffness of dsDNA, which must be wrapped around a histone protein to form a nucleosome. As a simple model of nucleosome localization, we study a one-dimensional hard-core gas in a random potential. We numerically solve for the density profile and other thermodynamic quantities using as input both randomly generated potential profiles and experimental energy landscapes. We compare with the annealed average, inspired by the Random Energy Model, and find that the quenched and annealed averages differ significantly above the localization temperature, implying sequence induced structural organization long before the system has frozen. Although information about the ground state is preserved at higher temperatures, there exist massive structural reorganizations at fixed temperature when the chemical potential is lowered. This offers another perspective on why different cells, with different chemical potentials, have different gene expression. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X35.00007: Dynamics of assembly of proteins along a stretched DNA Ranjith Padinhateeri, John Marko We study the dynamics of filling of a one-dimensional lattice by k-site-long hard particles. We show that a model with adsorption, desorption and diffusion of k-mer particles can mimic in vitro experiments involving assembly of proteins along a stretched DNA. We study the dependence of force on the protein assembly dynamics and final filling. We also show that in a regime when adsorption rate is much larger than desorption rate, and no diffusion, one gets a power-law-like filling dynamics soon after jamming. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X35.00008: Hidden Markov Analysis of Tethered Particle Motion Phil Nelson, John Beausang Tethered particle experiments use light microscopy to measure the position of a micrometer-sized bead tethered to a microscope slide via a ~micrometer length polymer, in order to learn about the behavior of the invisible polymer. Currently, this method is being used to measure rate constants of DNA loop formation and breakdown mediated by repressor protein that binds to the DNA. We report a new technique for measuring these rates using a modified hidden Markov analysis that directly incorporates the diffusive motion of the bead, which is an inherent complication of tethered particle motion because it occurs on a time scale between the sampling frequency and the looping time. We compare the looping lifetimes found with our method, which are consistent over a range of sampling frequencies, to the lifetimes obtained via the traditional threshold-crossing analysis, which vary depending on how the raw data are filtered. Our method does not involve filtering, and so is able to detect short-lived looping events and sudden changes in looping behavior. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X35.00009: Model for the simultaneous evolution of protein sequences and conformations Longhua Hu, Alexander Grosberg Protein molecule folds because its sequence is quenched while its conformation dynamically evolves governed by the quenched sequence. Sequence design procedures known in the literature usually operate by computationally annealing the sequence on the background of properly quenched conformation. There are suggestions in the literature to invigorate both the sequence design and the computational folding algorithms by considering the simultaneous evolution of both sequence and conformation, assuming that these two sets of degrees of freedom interact with thermostats of two different temperatures. To examine this procedure, we study the model of random walks on the graph in which each vertex represents the state of a protein, including both sequence and conformation. The graph has bonds of two sorts, some represent change of conformation (physical motion), while others represent change of sequence (`mutation'). We show that when sequence and conformation dynamics are governed by different temperatures, there cannot be any equilibrium, and we analyze the stationary currents in the system which are realized by never stopping cascade of sequence rearrangements followed by conformational moves followed by sequence moves and so on, ad infinum. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X35.00010: Unwinding of double-stranded DNA and branch migration of Holliday junctions by hexameric motor proteins Noah Ribeck, Omar A. Saleh Ring-shaped hexameric helicases are critical components of the DNA replication machinery in eukaryotes and bacteria. It has been shown that in vitro, certain hexameric helicases such as Mcm4,6,7 from eukaryotes, and DnaB from E. coli can translocate while encircling either single-stranded DNA (while opening a DNA fork in advance of the protein) or while encircling double-stranded DNA. Further, the latter translocation mode can drive branch migration of Holliday junctions. Using magnetic tweezers, we have performed single-molecule measurements of the activity of DnaB and the Mcm complex during both fork-opening and branch migration. We will report on progress of measurements of velocity of these motors in each mode, and relate the results to theoretical models of active and passive unwinding. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X35.00011: How cells decide between life and death: predictions from stochastic simulation Subhadip Raychaudhuri, Eric Willgohs, Thuc-Nghi Nguyen Recent experiments show that cells experiencing oxidative stress conditions trigger both apoptotic (programmed cell death) and survival pathways. Cross-talk between those two complex signal transduction networks, in turn, crucially decides between life and death of a cell. We have developed a Monte Carlo stochastic simulation method that can predict the outcomes of cellular decision-making (between life and death) under oxidative stress in a probabilistic manner. Even under identical cellular conditions our stochastic simulations can lead to differential cellular response as observed in recent in vitro experiments. Interestingly, our numerical experiments indicate that spatial heterogeneity and localization of signaling molecules, in addition to the structure of the signaling networks, are crucial to such a stochastic outcome of cell signaling. By performing sensitivity analyses under a variety of physiological conditions we are able to identify some of the critical regulators of apoptotic cell death signaling under oxidative stress. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X35.00012: Evolution of codes, crosstalk, and sequence niches in biomolecular signaling Christopher Myers Signaling and regulation in cellular networks is mediated through biomolecular interactions, which can be somewhat promiscuous, involving the molecular recognition of broad sets of binding targets. This leads to some basic questions concerning crosstalk among similar sets of biomolecules: does it occur, to what extent can it be avoided, how can phenotypic errors due to crosstalk be minimized, and when might crosstalk be advantageous? Beyond biology, questions of this sort have connections to phase transitions in constraint satisfaction problems, and to the theory of message coding in noisy channels. Expanding upon my previous work exploring the nature of the satisfiability (SAT-UNSAT) transition in a simple model of protein-protein interactions, this talk will investigate the role of sequence evolution in shaping high-dimensional sequence niches and biomolecular codes. [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X35.00013: Transcriptional Interference: A quantitative approach to in vivo dynamics of RNAP on DNA. Kim Sneppen We present a mathematical model for transcriptional interference by RNA polymerase traffic in Escherichia coli. The model deals with the interference between the two promoters pA and pS. The RNAPs are injected onto the DNA through binding and formation of sitting duck complexes at the respective promoters, followed by subsequent formation of elongating complexes. Finally we discuss a combination of modeling and in vivo-experiments can be used to infer the interference-recruitment game that govern the core of the genetic switch in the temperate bacteriophages 186.\\ K. Sneppen, I.B. Dodd, K.E. Shearwin, A.C. Palmer, R.A. Schubert, B.P. Callen, and J.B. Egan. J. Mol. Biol. 346:399 (2005) [Preview Abstract] |
Session X38: Focus Session: Negative Index Materials: Concepts to Applications III
Sponsoring Units: FIAPChair: Nader Engheta, University of Pennsylvania
Room: Colorado Convention Center 501
Friday, March 9, 2007 8:00AM - 8:36AM |
X38.00001: Negative Refraction in Metamaterials and Surface Plasmonics Invited Speaker: In this talk, we explore general requirements on negative-refraction metamaterials imposed by the fundamental principle of causality [1]. We show that the causality imposes lower limit on optical losses at and near the observation frequency. We also discuss specific, exactly solvable models of nanoplasmonic materials that possess the negative refraction [2] and compare them with the predictions of the exact theory based on the dispersion relations. From the requirements of causality, we derive the dispersion relation for squared refraction index $n^2(\omega )$ of a metamaterial (optically-linear refraction is considered). Using this relation, we derive a rigorous criterion of the negative refraction with a low optical loss at observation frequency $\omega $ as \[ \frac{2}{\pi }\int\limits_0^\infty {\frac{{\varepsilon }''(\omega ){\mu }'(\omega )+{\mu }''(\omega ){\varepsilon }'(\omega )}{\left( {\omega _1^2 -\omega ^2} \right)^2}} \omega _1^3 d\omega _1 \le -1, \] where ${\varepsilon }'(\omega )$ and ${\mu }'(\omega )$ are the real parts of the permittivity and permeability, and ${\varepsilon }''(\omega )$ and ${\mu }''(\omega )$ are their imaginary parts describing the electric and magnetic losses. This criterion imposes the lower limits on the electric and magnetic losses in the region of the negative refraction. If these losses are eliminated or significantly reduced by any means, including the compensation by active (gain) media, then this criterion is violated and the negative refraction will disappear. This criterion can be particularly useful in designing new left-handed materials: testing the expected polarizabilities of a medium against this criterion would check the compliance with the causality and verify the design feasibility. As examples of this general criterion, we consider negative refraction of surface plasmon polaritons in three exactly-solvable nanoplasmonic systems: metal nanolayer in dielectric, dielectric nanolayer in metal, and a dielectric nanolayer on metal surface. In all these three cases, the regions of negative refraction are also those of very strong absorption where extinction of the optical wave occurs on a distance on order of the wavelength. This is in accord with the obtained criterion. \textbf{References} 1. M. I. Stockman, \textit{Does Nature Allow Negative Refraction with Low Losses in Optical Region?}, cond-mat/0611350 (2006). 2. M. I. Stockman, \textit{Slow Propagation, Anomalous Absorption, and Total External Reflection of Surface Plasmon Polaritons in Nanolayer Systems}, Nano Lett. \textbf{6}, 2604-2608 (2006). [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X38.00002: A new mechanism for negative refraction and focusing using selective diffraction from surface corrugation W. T. Lu, Y. J. Huang, P. Vodo, R. K. Banyal, C. H. Perry, S. Sridhar Refraction at a smooth interface is accompanied by momentum transfer normal to the interface. We show that corrugating an initially smooth, totally reflecting, non-metallic interface provides a momentum kick parallel to the surface, which can be used to refract light negatively or positively. This new mechanism of negative refraction is demonstrated by visible light and microwave experiments on grisms (grating-prisms), and is used to create a new optical device, a grating lens. A plano-concave grating lens is demonstrated that focuses plane microwaves to a point image. Single-beam all-angle-negative-refraction is achieved by incorporating a surface grating on a flat multilayered material. These results show that customized surface engineering can be used to achieve negative refraction even though the bulk material has positive refractive index. The surface periodicity provides a tunable parameter to control beam propagation leading to novel optical and microwave devices. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X38.00003: Resonant enhancement of absorption in the superlens Leonid Alekseyev, Zubin Jacob, Evgenii Narimanov The Veselago lens (also known as the super lens) [1], which is a slab made of left handed material with $\varepsilon $=-1 and $\mu $=-1 has interesting properties like perfect lensing [2] and cloaking [3]. When a source is placed in front of it there are regions of high field intensity in such a lens , known as anomalously localized resonant regions [3]. For practical applications of the superlens taking advantage of these properties, the effect of finite loss on the device performance is critical [4] . We calculate the absorption loss of dipole radiation by an $\varepsilon <$0 and $\mu <$0 slab and find resonant enhancement of absorption in the superlensing regime. \newline [1] V. G. Veselago, ``The electrodynamics of substances with simultaneously negative values of permittivity and permeability,'' Sov. Phys. Usp. \textbf{10}, 509 (1968). \newline [2] J. B. Pendry, ``Negative refraction makes a perfect lens,'' Phys. Rev. Lett. \textbf{85}, 3966-3969 (2000). \newline [3] Graeme W. Milton and Nicolae-Alexandry P. Nicorovici ``On the cloaking effects associated with anomalous localized resonance,'' Proc. R. Soc. A (2006) 462, 3027-3059. \newline [4] V. A. Podolskiy and E. E. Narimanov, ``Near-sighted superlens,'' Opt. Lett. \textbf{30}, 75-77 (2005) [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X38.00004: Negative Index Metamaterials at Optical Frequencies: Theory and Experiment E. Ponizovskaya, A.M. Bratkovsky Pafomov and Veselago showed in 1950-60s that negative refraction should occur in homogeneous media with simultaneously negative dielectric permittivity and magnetic permeability, $\epsilon<0$, $\mu<0$. Pendry (2000) speculated that the ideal Veselago lens can produce sub- wavelength resolution. We find a strong effect of surfaces on resolution limit and nontrivial relation of subwavelength imaging to EM ``softness'' of the lens [1]. We have designed the metamaterial by means of FDTD modeling, which is a stack of metallic films with periodic hole arrays separated by dielectric layers (called ``fishnet'', FN) to work at IR wavelengths $\lambda=1.5- 1.6 \mu$m. The FN samples have been fabricated by nanoimprint lithography[2]. The transmission and reflectance characteristics of the samples have been measured by laser spectroscopic ellipsometry and showed unambiguously that the FN supports the ``backward'' waves and have overall negative index of refraction at IR frequencies. We also show that single layers of FN structure have positive index. We also address the questions of countering losses by using gain medium, limits sub-wavelength resolution, and focusing. 1. A.M. Bratkovsky, A.Cano, and A.P. Levanyuk, Appl. Phys. Lett. 87, 103507 (2005). 2. W. Wu et al., cond-mat/0610352. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X38.00005: Surface resonant states in acoustic and elastic metamaterials Muralidhar Ambati, Dongmin Wu, Xiang Zhang We discuss a new type of surface acoustic wave at an interface between two media, one of which is a metamaterial. In contrast to the familiar case where the surface acoustic waves exist at solid-fluid and solid-solid interfaces, these unique waves exist because of the negative material responses in metamaterials. We explore the existence of a surface wave a) at the interface of a fluid and an acoustic metamaterial modeled as a fluid and b) at the interface of a solid and an elastic metamaterial modeled as a solid. The latter case is for shear waves with horizontal polarization (SH). In each of these two cases, first, we discuss the necessary and sufficient conditions on the material properties of metamaterials for the existence of surface modes. Second, we offer the microscopic picture of these surface modes in terms of particle trajectories at the interfaces. Next, we examine the unique characteristics of these surface states; as a result, we propose and numerically demonstrate an acoustic superlens for sub-diffraction limited imaging. Finally, we provide a design for metamaterials that can lead to the surface states and make a significant impact in ultrasonic imaging. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X38.00006: Embeddable Metal Coil Microparticles for 3-D Metamaterial Applications C. K. Harnett, E. V. Moiseeva, Y. M. Senousy We will discuss our recent efforts in strain-based self-assembly of metal/insulator bilayers for production of freestanding metal coil microparticles in the sub-200 micron diameter range. This process has generated microparticles having metal coils in three orthogonal planes, demonstrating a possible route to polarization-insensitive metamaterials. Other designs such as ``Swiss roll'' shapes, coils in only one or two planes, or coils in non-orthogonal planes are also achievable by this method. Because the coils are assembled out-of-plane using strain mismatch between two thin films, particle diameter is governed mainly by film thickness, rather than by lithographic patterning as in planar split-ring resonators. These microparticles may be detached from the substrate by etching. Assortments of particles may function as taggants with an engineered spectral signature in the microwave to terahertz range. Surprisingly, such particles are robust enough to be embedded and detached into a flexible polymer cast over the substrate. Detaching entire arrays preserves particle orientation and any spatial gradients designed into the array. Stacking arrays of embedded resonant particles, or distributing freestanding particles uniformly throughout a dielectric matrix, will enable production of machinable, three-dimensional electromagnetic metamaterials. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X38.00007: Nanophotonic structures for Negative Refraction at NIR and VIS Ravinder Banyal, B.D.F. Casse, Plarenta Vodo, W.T. Lu, Srinivas Sridhar Nanophotonic structures were fabricated for negative index refraction at NIR and VIS frequencies. Structures include prisms and flat lenses fabricated in Si using nanolithography. Refraction and imaging experiments were carried out at NIR and VIS. Natural divergence of laser beam, collimation and guiding of the light within the device structures are also discussed. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X38.00008: Towards 3D and Multilayer Electromagnetic Metamaterials Structures in the THz Range B. D. F. Casse, H. O. Moser, J. W. Lee, S. Inglis, M. Bahou, L. K. Jian V. G. Veselago predicted that left-handed materials would exhibit a plethora of unusual effects such as a negative index of refraction as used in Snell's law, a reverse Doppler and \v {C}erenkov effect. This novel class of materials, following Pendry's recipes, can potentially restore evanescent waves to focus subwavelength details in an image. Micron-size electromagnetic metamaterials (EM$^3$) structures which exist so far were produced and characterized as single layer structures. Furthermore the structures were produced with a low yield. In the first part of the talk, we will present techniques for producing copious amount of EM$^3$ chips via the LIGA process using Synchrotron radiation and demonstrate assembly of the first multilayer THz EM$^3$ structures. The planar micro- or nanoEM$^3$ produced so far are also highly anisotropic. Recently, we proposed schemes to produce more isotropic structures, within the same matrix, via tilted X-ray exposures that were introduced in the LIGA process years ago. In the second part of the talk, we will show the results of microfabrication of nearly 3D EM$^3$ structures for the THz range. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X38.00009: Nonlinear Absorption in Nanoparticle Suspensions and Aerogels Yi Huang, Ashish Agarwal, Piotr Lesiak, Nick Kotov, David Carroll, Peter Palffy-Muhoray Nonlinear optics can be used to study materials structure by inferring material properties from characteristics of the mechanisms responsible for the nonlinear response. We have carried out ns and ps Z{\_}scan experiments to determine the nonlinear absorption and refraction in pure and dye containing aqueous solutions of Au nanorods and self-assembled polymer coated Ag nanowire glass gels. The enhancement of nonlinear absorption by the presence of nanoparticles can be related to the nanoparticle shape, linear response as well as concentration. We present our results for nonlinear refraction as well as nonlinear absorption, and discuss our results in terms of simple models. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X38.00010: Plasma-like negative capacitance in a nano-colloid Jason Shulman, Feng Chen, Stephen Tsui, Yuyi Xue, C. W. Chu A negative capacitance has been observed in an electrorheological fluid consisting of urea-coated Ba$_{0.8}$Rb$_{0.4}$TiO(C$_{2}$O$_{4})_{2}$ nanoparticles in silicone oil. The response is linear over a broad range of conditions. Previously, it was shown that this phenomenon originates at the surfaces of the nanoparticles. In this work, we demonstrate that the low frequency dispersions of both the resistance and capacitance are consistent with the free-carrier plasma model, while the transient behavior demonstrates an unusual energy storage mechanism. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X38.00011: ABSTRACT HAS BEEN MOVED TO S33.00014 |
Session X39: Focus Session: Physics & Technology of III-V Semiconductors in Infrared & THZ Imaging I
Sponsoring Units: FIAPChair: Gamani Karunasiri, Naval Postgraduate School
Room: Colorado Convention Center 502
Friday, March 9, 2007 8:00AM - 8:36AM |
X39.00001: High Performance Thermal Imaging Using Quantum Well Infrared Photodetector Arrays Invited Speaker: Quantum well infrared photodetector (QWIP) technology has opened up new opportunities to realize focal plane arrays (FPA) for high-performance thermal imaging [1]. High thermal and spatial resolution, low 1/f noise, low fixed-pattern noise, and high pixel operability makes QWIP FPAs appropriate for many applications. Due to their narrow absorption bands with relative spectral widths $\Delta \lambda $/$\lambda $ of the order of 10{\%}, QWIPs are particularly suitable for thermal imaging applications involving several atmospheric transmission bands or several colors within the same band. For dual-band/dual-color FPAs, QWIP technology has the unique property that the active region for the long-wavelength band is transparent for the short-wavelength band. In this talk, I will report on typical QWIP structures optimized for thermal imaging applications and on the performance of some state-of-the-art QWIP cameras which were jointly realized by the Fraunhofer-Institute for Applied Solid State Physics (Freiburg, Germany) and AIM Infrarot-Module GmbH (Heilbronn, Germany). Besides imagers for the 8 -- 12 $\mu $m long-wavelength infrared (LWIR) and 3 -- 5 $\mu $m mid-wavelength infrared (MWIR) regimes, these include a LWIR/MWIR dual-band QWIP FPA with 384x288 pixels which, at 6.8 ms integration time, exhibits a noise-equivalent temperature difference as low as 20.6~mK in the LWIR and 26.7~mK in the MWIR spectral bands. A specially designed diffraction grating is used for optical coupling of both spectral regimes. The array, which is based on a photoconductive QWIP for the MWIR and a photovoltaic ``low-noise'' QWIP for the LWIR, allows for synchronous and pixel-registered image acquisition in both bands. This functionality yields several advantages, including better distinction between target and background clutter, operation in a much wider range of ambient conditions, and the ability of remote absolute temperature measurement. \newline \newline [1] H. Schneider and H. C. Liu, \textit{Quantum Well Infrared Photodetectors: Physics and Applications}, ISBN 3540363238, Springer Series in Optical Sciences Vol. 126, 2006. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X39.00002: Detection of 3.4 THz Radiation from a Quantum Cascade Laser using Microbolometer Infrared Camera Barry Behnken, Gamani Karunasiri, Michele Lowe, Danielle Chamberlin, Peter Robrish, J. Faist Microbolometer infrared cameras are traditionally used for imaging objects in the 8-12 $\mu$m atmospheric window. Their use for imaging in the terahertz frequencies (0.1 -- 10 THz) is relatively unknown. In a recent experiment, a microbolometer camera with 160x120 pixels was used for real time detection of 3.4 THz radiation from a quantum cascade laser. The focal plane array of the camera consists of 50x50 $\mu$m$^{2}$ pixels made of a composite film of Si$_{3}$N$_{4}$ membrane and VO$_{x}$ thin resistive layer for sensing the temperature change. The laser was operated at 20 K with a peak power of 10 mW and duty cycle of 7{\%}, providing an average power of roughly 700 $\mu$W. Initial experiments were carried out without the focusing Ge lens of the camera, since the antireflection coating on it was found to absorb most of the THz laser power. Recently we have incorporated a picarin lens, capable of 80{\%} transmittance at THz frequencies, to enhance the light collection and improve detection capability. Video recordings of the laser beam interacting with various objects, with and without the use of focusing optics, will be presented. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X39.00003: Terahertz ellipsometry using electron-beam based sources T. Hofmann, U. Schade, M. Mross, T. Iowell, M. Schubert Spectroscopic ellipsometry is known as a viable and precise technique for the investigation of optical material properties in the far-infrared to the VUV spectral region. Generalized ellipsometry in the THz frequency domain allows in combination with strong magnetic fields at low temperatures investigation of low energy electron dynamics in semi- and superconducting materials. We report here on the first successful application of this technique to investigate condensed matter samples in the frequency range from 0.7 to 8 THz using a high-brilliance Terahertz synchrotron radiation source and a Smith-Purcell-effect Terahertz radiation source. We discuss and present THz range physical material properties due to bound and unbound charge resonances in low-dimensional semi- and superconducting materials. This research will provide understanding of optical properties for novel materials, inspire new designs, and accelerate development of optical Terahertz devices. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X39.00004: III-V semiconductor based infrared detectors and focal plane arrays David Z.-Y. Ting, S. V. Bandara, C. J. Hill, S. D. Gunapala, J. K. Liu, J. M. Mumolo, S. A. Keo, E. R. Blazejewski, Y.-C. Chang, S. B. Rafol We report recent results in III-V semiconductor based infrared detectors, including quantum well infrared photodetector (QWIP) based simultaneous dual-band infrared focal plane array, quantum dot infrared photodetector (QDIP) based large-format long wavelength infrared focal plane array, and development in far infrared photodetectors. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X39.00005: Development of GaAs blocked-impurity-band detectors for the far-infrared Lothar A. Reichertz, Jeffrey W. Beeman, Reinhard Katterloher, Nancy M. Haegel, Eugene E. Haller The far-infrared (or THz) region of the electromagnetic spectrum requires improved photon detectors, especially for large array formats for highly sensitive imaging in astronomy. For wavelengths greater than 120 $\mu$m stressed Ge photoconductors are currently being used with a cut off at 210 $\mu$m. GaAs is a promising material to overcome this limitation due to its lower donor binding energy. The problem of high dark currents in GaAs can be solved by using a multilayered blocked-impurity-band (BIB) detector concept. This allows for a more heavily doped infrared active layer which enables a thinner device and a further extension of the long wavelength cut off. Such a planar structure allows lithographic processes for much larger array formats than currently possible in stressed Ge technology. Although BIB technology is well established in Si, its transfer to other materials has proven difficult. Only recently is GaAs approaching the needed levels of purity and interface control. We have demonstrated spectral response in a multi layer GaAs BIB structure extending to 500 $\mu$m, using material grown through vapor phase epitaxial techniques. The test structures were grown on 4 inch wafers, demonstrating feasibility for a 32 x 32 (and ultimately larger) array. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X39.00006: The Study of Temperature Dependence and High Temperature Operation of Photoresponse in Superlattice Infrared Photodetectors Shih-Hung Lin, J.H. Lu, C.H. Kuan, J.Y. Feng, T.S. Lay For temperature dependence of photoresponse in superlattice with a single barrier, we have compared two samples' performance and concluded four factors are in effect including doping density in superlattice, externally applied bias, the single barrier's thickness and energy height. Doping density and temperature will change electron distribution in the first miniband of superlattice, and thereby the photoresponse. Scattering during the transport through barrier is increased with applied bias. Barrier thickness and height influence ballistic transport behavior and tunneling mechanism, respectively. Based on these factors, we design a structure of superlattice integrated with quantum wells to demonstrate photoresponse under high-temperature operation. By understanding those factors' effect, it is advantageous to design SLIPs for high-$T$ applications. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X39.00007: Effect of the Inclusion of Magnetically Active Imbedded Nanocomposites (MAINs) on the Absorption and Response of Infrared Detectors Gamini Ariyawansa, V.M. Apalkov, N. Dietz, S.G. Matsik, A.G.U. Perera The performance of heterojunction infrared detectors can be improved by MAINs in the emitter layer which use free carrier absorption. As the free carrier absorption requires a phonon or similar particle to conserve momentum, the additional disorder introduced by the MAINs increases absorption, leading to improved response. As the GaMnN MAINs show ferromagnetic behavior, the application of a magnetic field can lead to further response increases. Calculations will be presented on the effect of the size and density of the GaMnN MAINs on the expected absorption and response for Tunable Magnetically Active Imbedded Nanocomposite InfraRed (TMAINIR) sensors in the LWIR and THz ranges. The addition of 10 nm diameter GaMaN MAINs with a density of $10^{11}$ cm$^{-2}$ will lead to an increase in the calculated peak response from 15 to 75 mA while also shifting the peak wavelength from 8 to 6 $\mu$m for a design with a threshold of 15 $\mu$m. Applying an electric field can further increase the peak response up to $\sim 85$ $\mu$m. Results will be presented on the effect of the GaMnN MAINs on the expected absorption and response for detector designs in the LWIR and THz ranges. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X39.00008: A new mechanism for THz-frequency radiation generation: Nonlinear strain waves in piezoelectrics Evan Reed, Michael Armstrong Using molecular dynamics simulations and analytics, we show that extremely large strain amplitude THz frequency acoustic waves can spontaneously form in crystalline GaN at the front of a shock wave and generate THz frequency radiation at an interface with AlN or another piezoelectric material. This new mechanism for the generation of THz radiation can be realized using a table-top ultrafast laser and has fundamentally different limiting properties than existing nonlinear optical ultrafast techniques for THz generation. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X39.00009: Electronically Tunable Grating-Gate Terahertz Detectors E.A. Shaner, M.C. Wanke, Mark Lee, A.D. Grine, J.L. Reno, S.J. Allen Spectroscopy in the millimeter-wave to THz frequencies has received a great deal of recent interest for security applications and chemical identification. This talk will address detectors that utilize plasmons in high-mobility GaAs/AlGaAs quantum well structures to provide a frequency tunable detector response. In particular, recent advances on the grating-gate detector, including membrane and split-gate versions, will be presented. The discussion will include our understanding of the detection mechanisms involved as well as the noise equivalent powers that have been achieved in the various geometries. Currently, the grating-gate style of detector covers a frequency range from 150GHz to 1THz at temperatures ranging from 4K to 80K, however, the ultimate frequency and temperature limits of these detectors are not currently known. The ability to tune the detector response by simply changing a gate voltage leads to an attractive `spectrometer-on-a-chip' where no moving parts would be needed for THz spectral analysis. To date we have achieved spectral scans from 600GHz to 1THz in 12.5ms with a measured 15GHz minimum linewidth. *Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X39.00010: Brewster angle phenomenon in two-dimensional metallic photonic crystals and its application to polarization beam splitting Xinhua Hu, Kai-Ming Ho The authors show the Brewster angle phenomenon of p waves in two-dimensional metallic photonic crystals of rectangular lattices with effective medium theory and accurate multiple-scattering calculations. The Brewster angle can be tuned over a wide range by varying the structure parameters. Based on this phenomenon, the authors propose a polarization beam splitter which can completely separate the p and s waves at an incident angle of 45$^{\circ}$ $\pm$ 10$^{\circ}$ in a very wide wavelength range (1$\sim$500 micron). [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X39.00011: Dynamics of 980 nm VCSELs Characterized Using Temperature Dependent RIN Spectra Rashid Safaisini, Ahmad N. Al-Omari, John R. Joseph, Kevin L. Lear High speed optical interconnects are replacing copper connections in ever shorter link distance, low cost, high speed communication systems employing directly modulated laser diodes. In very short distance systems ($<$1 m), laser diode bandwidth is a major limitation. Laser intensity noise spectra contain valuable information on laser dynamics including resonant frequency and damping. In this work, vertical cavity surface emitting lasers (VCSELs) are fabricated from a metal-organic chemical vapor deposition (MOCVD) grown AlGaAs structure on an n-type substrate. The intrinsic resonance frequency and damping constant of 980 nm VCSELs are measured. Two methods are used: constant temperature and varying injection current and constant injection current with varying temperature. After fitting the spectra to the conventional damped oscillator model to extract resonance frequency and damping, K-factor analysis is performed both on the conventional constant temperature data as well as the constant current data. The results of the two approaches are compared. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X39.00012: Detection of Nanomechanical Device Motion by Spatiotemporal Stroboscopic Interferometry Joseph Losby, F. Giesen, J. Moroz, A. Fraser, M. Belov, G. McKinnon, Y. Ning, W. Hiebert, M.R. Freeman Actuation and detection of nanomechanical device motion in the ultrahigh frequency regime remains a considerable challenge. We have performed broadband characterization of the displacement of silicon NEMS cantilevers and doubly-clamped beams by stroboscopic optical interferometric detection synchronized to pulsed electrostatic actuation. Initial test structures have yielded time-domain measurements of the response of structures having fundamental resonant frequencies up to 580 MHz. Calibration of the vertical displacement sensitivity and imaging of out-of-plane flexural modes will also be discussed. This study aims to extend the potential of interferometry for research on nanomechanical systems. [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X39.00013: Terahertz imaging using the Jefferson Lab -- FEL high power broadband terahertz source J. Michael Klopf, Matthew Coppinger, Nathan Sustersic, James Kolodzey, Gwyn P. Williams Imaging using THz radiation is of considerable interest due to the non-ionizing nature of the radiation as well as the relative transmission, absorption, and reflection of various materials of interest. With a source of sufficient power and spectral characteristics, it is possible to realize imaging capabilities that were not previously possible. At the Jefferson Lab -- Free Electron Laser Facility, a high power broadband source has been commissioned, providing an ideal resource for the development of THz imaging technology. Even with this high power source now available, significant challenges remain in creating detector arrays with sufficient responsivity and creating optical systems to provide the illumination and resolution necessary to create usable images. In collaboration with a group from the University of Delaware, unprecedented imaging tests have been performed toward this goal and are presented here. [Preview Abstract] |
Session X40: QHE: Antidots and Bilayers Systems
Sponsoring Units: FIAPChair: Herb Fertig, Indiana University
Room: Colorado Convention Center 503
Friday, March 9, 2007 8:00AM - 8:12AM |
X40.00001: Realization of a primary-filling e/3 quasiparticle interferometer F.E. Camino, W. Zhou, V.J. Goldman We report experiments on a quasiparticle interferometer where the entire system is on the f=1/3 primary fractional quantum Hall plateau. Electron-beam lithography is used to define an electron island separated from the 2D bulk by two wide constrictions, much less depleted than in our prior work [1]. This results in the entire electron island being at filling f=1/3 under quantum-coherent tunneling conditions. For the first time in such devices we report interferometric Aharonov-Bohm-like conductance oscillations. The flux and charge periods of the interferometer device are calibrated with electrons in the integer regime. In the fractional regime, we observe magnetic flux and charge periods $h/e$ and $e/3$, respectively, corresponding to creation of one quasielectron in the island. These periods are the same as in quantum antidots, but the quasiparticle path encloses no electron vacuum in the interferometer. Quantum theory predicts a $3h/e$ flux period for charge $e/3$, integer statistics particles. Accordingly, the observed periods demonstrate anyonic statistics of Laughlin quasiparticles. \newline [1] F. E. Camino et al., PRL 95, 246802 (2005); PRB 72, 075342 (2005). [Preview Abstract] |
Friday, March 9, 2007 8:12AM - 8:24AM |
X40.00002: $\nu =1$ quantum Hall state in a lateral periodic quantum anti-dot array W. Pan, J.L. Reno, J.A. Simmons, D. Li, S.R.J. Brueck The quantum Hall ferromagnetism (QHF) at the Landau level filling $\nu $=1 in 2DES has been extensively studied over the years. Due to strong Columbic interaction, at $\nu $=1, all the electron spins align with the external magnetic, giving rise to a ferromagnetic order. Consequently, the energy gap (Eg) of the $\nu $=1 state is much larger than that of bare Zeeman splitting (Ez). Previous experimental studies focused mostly on the clean limit of sample quality where the electron-electron interaction is strong. On the other hand, theories have shown that a phase transition from the QHF state to a quantum Hall spin glass state can occur as sample disorder increases. To study this phase transition, we used a lateral quantum anti-dot array, where the electronic potential modulation can be viewed as a special form of sample disorder. More importantly, this disorder can be continuously tuned by varying electron density. We observed that for small potential modulation Eg at $\nu $=1 is much larger than Ez, indicating a ferromagnetic order. As the modulation strength increases, Eg first decreases slowly and after a critical value of modulation, the decrease rate accelerates and Eg approaches the Ez limit, signaling a possible transition from a ferromagnetic state to a spin glass state. Tilting magnetic field results will also be discussed. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X40.00003: Coulomb blockade of anyons: coherent quasiparticle transport in multi-antidot systems Dmitri Averin, James Nesteroff We have developed a model for transport of anyonic quasiparticles of primary quantum Hall liquids through systems of multiple antidots weakly coupled to external edges. At energies smaller than the energy gap of the antidots, the quasiparticles behave as {\em hard-core} anyons, i.e., exhibit fractional exchange statistics, while the on-site Coulomb interaction suppresses the double occupancy of the antidots (thus simulating Fermi exclusion). The hard-core condition implies that the quasiparticle exchanges affect transport only in systems with closed loops. Coherent tunnel coupling of the antidots leads to the formation of stationary states and associated resonant conductance peaks for tunneling between the edges through these states. We have calculated the tunnel conductance for double- and triple-antidot systems and shown that in the case of three antidots connected in a loop, the anyonic exchange statistics manifests itself in both the relative positions and amplitudes of the peaks. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X40.00004: Tunable-Coupling Quantum Antidot Molecule Wei Zhou, F.E. Camino, V.J. Goldman We report experiments on two double-antidot devices. The molecule is formed by two equal-size lithographic antidots fabricated from a very low-disorder GaAs/AlGaAs heterostructure. The two antidots are close enough so that the states bound on each antidot become hybridized and form bonding and antibonding states, like in a diatomic molecule or a qubit [1]. We observe resonant tunneling peaks on the f=1 and 1/3 quantum Hall plateaus. The quantum-coherent coupling between the antidots can be tuned by a gate bias and by magnetic field $B$. The f=1 conductance peaks display three regimes as a function of $B$: (i) one peak per period $\Delta =h/2eS$, like in single antidot, but the total area $2S$ contributing. (ii) At higher $B$, the peak splits into two overlapping peaks; (iii) at yet higher $B$, nearly sinusoidal double-frequency (one oscillation per $\Delta /2$) conductance oscillations are observed. The fractional regime shows bonding-antibonding split peaks that display a charge $e/3$ back-gate coupling.\newline [1] Averin and Goldman, Solid State Commun. 121, 25 (2002). [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X40.00005: Shot Noise in Anyonic Mach-Zehnder Interferometer Kam Tuen Law, Dima Feldman, Yuval Gefen, Alexei Kitaev, Ady Stern Recently a new type of interferometer for quantum Hall systems, an electronic Mach-Zehnder interferometer, was designed. We demonstrate that shot noise in such an interferometer can be used to probe the charge and statistics of quantum Hall quasiparticles. The dependence of the noise on the magnetic flux through the interferometer allows for a simple way to distinguish Abelian and non-Abelian quasiparticle statistics. In the Abelian case, the Fano factor is always lower than an electron charge. In the non-Abelian case, the maximal Fano factor as a function of the magnetic flux exceeds the charge of an electron. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X40.00006: Fractional charge revealed in computer simulations of resonant tunneling in the fractional quantum Hall regime Evgueni Tsiper The concept of fractional charge is central to the theory of the fractional quantum Hall effect (FQHE). Quasiparticles of fractional charge have been first observed by Goldman and Su [1] in resonant tunneling through a quantum antidot. In [1] a periodic sequence of resonant tunneling events was observed as either the magnetic field H or the backgate voltage Vg were varied. The tunneling events are thought of in terms of a quasiparticle tunneling through the bulk of the fractional state between the outer edge of the sample and the inner edge formed around the antidot. The periodicities in H and in Vg were related to the quasiparticle charge e*. Here I use exact diagonalization as well as configuration space renormalization (CSR) to study finite clusters large enough to contain two independent edges. I analyze the conditions of resonant tunneling between the two edges. The ``computer experiment'' reveals a periodic sequence of resonant tunneling events consistent with the experimentally observed fractional quantization of electric charge in units of e/3 and e/5 [2]. \newline [1] V.J. Goldman and B. Su, Science, 267, 1010, 1995. \newline [2] E.V. Tsiper, Phys. Rev. Lett., 97, 76802, 2006. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X40.00007: Spin Polarization of Bilayer $\nu $=2 Quantum Hall States Probed by NMR Norio Kumada, Koji Muraki, Yoshiro Hirayama In a bilayer system at total Landau level filling factor $\nu $=2, the interplay between intralayer and interlayer interactions leads to quantum Hall (QH) states with various spin configurations: the ferromagnetic (F), canted antiferromagnetic (CAF) and spin-singlet (SS) states. Recently, by measuring the nuclear spin relaxation rate 1/$T_{1}$, which probes the in-plane spin fluctuations, we have demonstrated the existence of the Goldstone mode predicted for the CAF state [1]. In this study, we report the direct measurement of the spin polarizations of these states by using a current pump and resistively detected NMR technique. The double-quantum-well sample used has a tunneling energy gap of 8 K. Since electron spins add effective Zeeman field to nuclear spins, the electron spin polarization appears as a shift in the nuclear resonant frequency, i.e., the Knight shift. As the bias potential between the two layers is increased, the spin polarization changes from full to zero continuously, indicating the phase transitions from the F to SS states via the CAF state. The combination of the data for the Knight shift and 1/$T_{1}$ allows for more complete understanding of the spin ordering including both in-plane and out-of-plane components and fluctuations in these states. [1] N. Kumada, K. Muraki and Y. Hirayama, Science 313, 329 (2006). [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X40.00008: Collective excitations of a crystal of CP(3) skyrmions in a bilayer quantum Hall system Ren\'{e} C\^{o}t\'{e} , Dominique Boisvert, J\'{e}r\^ome Bourassa, Herbert A. Fertig Recent experiments [1,2] in a bilayer quantum Hall system suggest that the quasiparticles at filling factor $\nu=1$ could have both spin and pseudospin textures i.e. they could be CP3 skyrmions. At very low temperature, these skyrmions condense to form a Skyrme crystal. In a previous work [3], we have identified the regions of stability of this crystal in the parameter space of the bilayer system i.e. as a function of filling factor, interwell separation, potential bias, Zeeman and tunnel couplings. In this talk, we derive the spin and pseudospin response functions as well as the collective excitations of the CP(3) Skyrme crystal in the Generalized Random-Phase Approximation(GRPA). We study the behavior of the NMR relaxation time computed from the GRPA spin response function and discuss its relevance for the experiments of Refs. [1,2]. \\(1) I. B. Spielman et al., Phys. Rev. Lett. {\bf 94}, 76803 (2005).\\(2) N. Kumada et al., Phys. Rev. Lett. {\bf 94}, 96802 (2005). \\(3) J. Bourassa et al., Phys. Rev. B {\bf 74}, 195320 (2006). [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X40.00009: Interlayer tunneling studies of highly imbalanced bilayer 2D electron systems at \boldmath{$\nu_T = 1$} A.R. Champagne, J.P. Eisenstein, L.N. Pfeiffer, K.W. West When the separation between two parallel 2-dimensional electron systems (2DES) becomes comparable to the average distance between electrons within a single layer, the system can support a quantum Hall state with total filling factor $\nu_T=1$. This state can be described as a Bose condensate of excitons. Previous studies [1] have shown that close to the $\nu_T=1$ phase boundary, a small imbalance in the number of electrons in each layer can strengthen the condensate. We report on interlayer tunneling measurements of the effect of large imbalances as a function of the interlayer spacing. We explore the possibility of competing order between the excitonic state and the (1/3, 2/3) fractional states in the individual layers. This work was supported by the NSF and the DOE. \newline [1] I. B. Spielman, et al., Phys. Rev. B {\bf 70}, 081303 (2004). [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X40.00010: Zero Bias Tunneling Resonance at Filling Factor $\nu_T=1$ in GaAs Hole Bilayers Shashank Misra, Nathaniel C. Bishop, Emanuel Tutuc, Mansour Shayegan Previous tunneling and transport measurements on bilayer two dimensional carrier systems at total filling factor $\nu_T = 1$ provide strong evidence for an excitonic ground state with small but finite dissipation. We present, for the first time, tunneling conductance measurements of bilayer two dimensional {\it hole} systems in a strongly interacting regime ($1.1 < d/l_B < 1.3$, where $d$ is the interlayer distance and $l_B$ is the magnetic length). We find that the zero bias tunneling resonance at $d/l_B=1.2$ has a larger amplitude and a weaker temperature dependence than existing data from electron samples ($d/l_B=1.5-1.8$) for the range of temperatures where we have taken data ($300mK-600mK$). This work was supported by the DOE and NSF. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X40.00011: Activation Energies and Dissipation in Biased Quantum Hall Bilayer Systems at Total Filling Factor $\nu=1$. Bahman Roostaei, H.A. Fertig, K.J. Mullen, Steven Simon Electrons in a closely spaced bilayer semiconductor structure, such as a double quantum well, are thought to form an interlayer coherent state when a perpendicular magnetic field is applied such that the total Landau level filling factor $\nu$ is 1. When the Zeeman energy is sufficiently large to polarize electron spins, the low energy excitations are thought to be topological pseudospin meron-antimeron pairs[1]. These objects carry charge $\pm e/2$,vorticity, and electric dipole moments perpendicular to the layers. Disorder is likely to unbind merons from antimerons and allow them to diffuse through the system independently[2]. Due to their different dipole moments, the various types of merons and antimerons may then in principle be distinguished in transport activation energies by an interlayer bias potential. We report on estimates of these energy differences in various circumstances, and discuss the connection of our results with recent experiments[3].\newline [1]K.Moon,et. al., PRB {\bf 51},5138(1995). \newline [2]H.A.Fertig,G.Murthy, PRL {\bf 95},156802(2005). \newline [3]R.D.Wiersma,et.al., PRL {\bf 93},266805(2004). [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X40.00012: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 10:24AM - 10:36AM |
X40.00013: Increases in Electron Drag in Intermediate Magnetic Fields Sanghun An, Gokul Gopalakrishnan, Yuko Shiroyanagi, Dongkyun Ko, Tom Gramila, Loren Pfeiffer, Ken West \\It has been reported that measurements of electron drag in intermediate magnetic fields show anomalous dependences on temperature and magnetic field. Although extensive theoretical investigations of drag in magnetic fields have been done, the behavior currently lacks a theoretical explanation. The experimental findings show that for intermediate magnetic fields, where the Landau level spacing is comparable to temperature, electron drag is substantially increased, even while very little change is observed in the longitudinal resistivity, suggesting that the effect's origin does not lie in changes in the density of states. There is, furthermore, very little dependence on temperature, and the increase varies roughly as the cube of magnetic field. We report drag measurements designed to examine the potential role of spin in the effect and detailed measurements examining the specific temperature dependence, as approaches towards the identification of the source of this unusual behavior. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X40.00014: Electron Drag in High Filling Factors Gokul Gopalakrishnan, Sanghun An, Yuko Shiroyanagi, Dongkyun Ko, Thomas Gramila, Loren Pfeiffer, Kenneth West Electron drag measurements, which permit direct detection of electron-electron scattering rates, have revealed a number of surprising behaviors in bilayer electron systems in the presence of magnetic fields. Among these is an oscillation in the polarity of the drag voltage with the difference in filling factors of the two layers, measured at high filling factors. Recent theoretical examinations of this sign reversal have pointed to particle-hole asymmetry induced by Landau quantization[1], and disorder induced mixing of Landau levels[2] as the source. In addition to the sign reversal is an unusual temperature dependence. Experiments[3] have suggested an activated behavior in this regime, with the activation energy related to the Zeeman energy. This result, however, differs functionally from the theoretical calculation in [1]. We report on measurements that examine this potential contradiction in order to investigate the nature of Coulomb drag in this regime. \newline [1]I. V. Gornyi, et al, PRB 70, 245302 (2004) \newline [2]R. Bistritzer, A. Stern, PRL 96, 226801 (2006) \newline [3]K. Muraki, et al, PRL 92, 246801 (2004) [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X40.00015: Interedge coherent line junctions in Quantum Hall systems Emiliano Papa In this talk I will address the properties of quantum Hall line junctions (QHLJ) that occur near barriers separating electron gases on quantum Hall plateaus. In narrow barriers where electron tunneling can occur at any point, the low energy physics of the QHLJ is described by the massive quantum sine-Gordon model. We propose procedures to study a sort of properties of these systems in relation with recent experimental studies. The spectrum of the quantum sine-Gordon model consists of topological particles, solitons, antisolitons, and when forward interactions are strong enough also their bound states can form. In presence of a chemical potential however that couples with these charges the spectral gap can be suppressed. When this chemical potential exceeds the mass of the soliton ($\Delta/2$), a finite density of solitons appears in the ground state, distributed on a Fermi sea according to their statistics, embedded in their interactions (or their scattering matrix). The low-energy physics of this system then will be of particle-hole type formed around these Fermi points. The properties of this metallic state, namely the value of the Luttinger liquid (LL) parameter $K$ and the Fermi velocity can be accessed with the thermodynamic Bethe ansatz. Experimentally there are two quantities that offer the measurement of two combinations, the product and the ratio of the LL parameter $K$ and the Fermi velocity, namely the Drude weight and charge susceptibility, respectively. [Preview Abstract] |
Session X41: Semiconductors: Electronic and Optical Properties
Sponsoring Units: FIAPChair: Angelo Mascarenhas, National Renewable Energy Laboratory
Room: Colorado Convention Center 504
Friday, March 9, 2007 8:00AM - 8:12AM |
X41.00001: Wire-like characteristics induced by vertically electronic coupling in stacked InAs/GaAs quantum dots superlattices Tzung-Te Chen , Yang-Fang Chen, Jyh Shyang Wang, Ying-Sheng Huang , Ru Shang Hsiao, Jenn Fang Chen, Chih Ming Lai, Jim Young Chi The wire-like characteristics of stacked InAs/GaAs quantum dots induced by vertically electronic coupling were demonstrated by surface photovoltaic and photoluminescence measurements. It is found that the surface photovoltaic signal can be enhanced by up to one hundred times due to the wire-like behavior along the growth direction. The emission from the cleaved edge surface is strongly anisotropic, which can be turned from the in-plane to normal to the plane polarization by changing the spacer thickness. Additionally, the electroluminescence of stacked quantum dots near 1.3 um based on the wire-like characteristics has a much better performance than that of uncoupled quantum dots. [Preview Abstract] |
Friday, March 9, 2007 8:12AM - 8:24AM |
X41.00002: Giant Spin-Orbit Bowing in GaAs$_{1-x}$Bi$_{x}$ A. Mascarenhas, B. Fluegel, S. Francoeur, S. Tixier, E. C. Young, T. Tiedje We report a giant bowing of the spin-orbit splitting energy $\Delta _{0}$ in the dilute GaAs$_{1-x}$Bi$_{x}$ alloy for Bi concentrations ranging from 0 to 1.8 {\%}. This is the first observation of a large relativistic correction to the host electronic band structure induced by just a few percent of isoelectronic doping in a semiconductor material. It opens up the possibility of tailoring the spin-orbit splitting in semiconductors for spintronic applications. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X41.00003: Valence Band Anticrossing in GaBi$_{x}$As$_{1-x}$ Kirstin Alberi, O.D. Dubon, W. Walukiewicz, K.M. Yu, K. Bertulis, A. Krotkus Recently, significant attention has been devoted to exploring the large bandgap bowing and spin-orbit splitting in GaBi$_{x}$As$_{1-x}$. alloys. We attribute the origins of these effects to a restructuring of the alloy valence band induced by an anticrossing interaction between the delocalized GaAs $p$-like states and the resonant localized Bi $p$-like states. Hybridization of like-symmetry states leads to the splitting of the heavy hole, light hole and spin-orbit split-off bands into sets of $E_{+}$ and $E_{-}$ subbands. The splitting is confirmed experimentally by photomodulated reflectance spectroscopy in alloys with Bi concentrations up to $x$ = 0.084. The bandgap bowing is a direct consequence of the strong upward shift of the uppermost heavy and light hole $E_{+}$ bands with increasing Bi concentration, while the much slower ascent of the spin-orbit split-off $E_{+}$ band produces the large rise in the spin-orbit splitting energy. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X41.00004: Interactions of Ga$_n$As$_{n}$ Clusters with CGaAs Cages: Possible Nanostructures Ajit Hira, Matilda Fernandez Extending our work on fullerene-alkali complexes$^{1}$, we now examine the interactions of small Ga$_{n}$As$_{n}$ clusters (n = 1 thru 10) with mixed CGaAs cage clusters. First, we derive the physical and chemical properties of the GaAs clusters, including their binding energies, bondlengths, ionization potentials and charge distributions. The geometries of the small gallium arsenide clusters are based on full optimizations. Electron correlation effects are included for binding energies and optimal intermolecular bondlengths. Next we focus on the physical and chemical properties of 60-atom mixed CGaAs cages. The optimization of the cages is subject to symmetry constraints. The third phase of the investigation examines the interactions of the small GaAs clusters with the CGaAs cages. For these complexes various properties, including dissociation channels and dissociation energies, are tabulated. We also explore the implications of this research for the design of nanostructures. 1. Daniel Bulnes, Nichole Moya-Leyba, Erica Velarde and Ajit Hira, " Theoretical Study of Na$_{3}$C$_{60}$ and Na$_{4}$C$_{60 }$Clusters: Pathways to Nanoscale Contacts", \textit{Bull. Am. Phys. Soc}. 50, 1475 (March 2005\textbf{).} [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X41.00005: Persistent Currents in Higher Genus Materials and Structures David Schmeltzer, Avadh Saxena Persistent charge and spin currents have been usually studied in an isolated ring. However, recent semiclassical calculations and an experiment performed on sixteen connected GaAS/GaAlAs rings provide motivation to study persistent currents in higher genus structures. To this end, we introduce Dirac's second class constraint to explore high genus materials and ring structures. As a specific example we apply this method to compute the persistent charge current in a system of coupled rings. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X41.00006: Optical Transport Imaging for the Measurement of Electric Field and Minority Carrier Diffusion Length N.M. Haegel, P. Andrikopoulos, T.J. Mills An optical imaging technique is used to determine electric field and minority carrier diffusion length in planar heterostructures. We combine an optical microscope inside an SEM and image the spatial distribution of luminescence due to diffusion and drift from point source excitation. By comparing peak luminescence intensity with and without applied field, a quantitative measure of electric field value is obtained, while the transport image illustrates the field direction. Drift behavior has been imaged in a dimensionally confined region in a p type AlGaAs/GaAs double heterostructure where electron motion transitions from quasi 1D to 2D. Comparison to finite element modeling confirms the approach and demonstrates the sensitivity of the technique to local material and field variations. The technique can also be used for a 2 point measure of contact resistance. In the absence of applied field, the luminescence distribution provides a measure of minority carrier diffusion length, in a contact-free manner from a single image. This is demonstrated for a range of minority carrier diffusion lengths from $\sim $ 1.5 to 60 $\mu $m in III-V materials. The technique provides direct access to minority carrier mobility for studies of anisotropy, field dependence and domain behavior. We acknowledge support from NSF DMR-0203397 and DMR-0526330. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X41.00007: Electrical measurement of quantum interference population control in (111) GaAs Jared Wahlstrand, Ryan Smith, Jessica Pipis, Peter Roos, Steven Cundiff Two-color quantum interference control is the interference between absorption pathways, such as one- and two-photon absorption. Depending on the symmetry of the crystal, it can result in a ballistic current being injected or modulation of the carrier population. Previously, population control has been measured using an all-optical technique. We present results of an experiment in which we measured the carrier population modulation electrically using a Au/Ge electrode structure patterned on (111) GaAs. This is a more practical technique for potential applications, such as carrier-envelope phase detection.\footnote{T. M. Fortier, P. A. Roos, D. J. Jones, S. T. Cundiff, R. D. R. Bhat and J. E. Sipe, Phys. Rev. Lett. 92, 147403 (2004)} [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X41.00008: Exciton binding energy and electron effective-mass in strain compensated InGaAsN/GaAs single Quantum Well Lifang Xu, Dinesh Patel, Carmen Menoni, Jengya Yeh, Luke Mawst, Nelson Tansu A detailed lineshape analysis of the temperature dependent photoluminescence spectra of In$_{0.4}$Ga$_{0.6}$As$_{1-y}$N$_{y}$/GaAs quantum well (QW) (y=0; 0.005) is carried out. The analysis extracts the binding energy of the e$_{1}$-hh$_{1}$ ground-state exciton which equals 9.72$\pm $1.24 meV and 17.5$\pm $0.9 meV for InGaAs and InGaAsN (N=0.5{\%}) single QW sample, respectively. By using a fractional dimension exciton binding energy model, an electron effective mass of m$_{e}$*=(0.11$\pm $0.015)m$_{0 }$ is determined for the highly strained dilute nitride single QW. [Preview Abstract] |
Session X43: Electronic and Spin Properties of Quantum Dots and Quantum Point Contacts
Sponsoring Units: DCMPChair: Stefan Badescu, Naval Research Laboratory
Room: Colorado Convention Center 506
Friday, March 9, 2007 8:00AM - 8:12AM |
X43.00001: The Kondo box in the presence of exchange interaction Stefan Rotter, Hakan E. Tureci, Yoram Alhassid, A. Douglas Stone We study the problem of a quantum dot with finite level spacing which is coupled antiferromagnetically to a Kondo spin (``Kondo box''). In particular, we investigate the influence of a ferromagnetic exchange interaction among the dot electrons and the effect of an applied Zeeman field. This problem is addressed with the help of a numerical algorithm that allows for an exact diagonalization of the Hamiltonian in a good total spin basis [1] of the dot plus the Kondo spin. We discuss how the competition between the ferromagnetic exchange and the antiferromagnetic Kondo interaction affects the ground-state spin of the system. \newline [1] H. Tureci and Y. Alhassid, Phys. Rev. B 74, 165333 (2006). [Preview Abstract] |
Friday, March 9, 2007 8:12AM - 8:24AM |
X43.00002: Measurements of Universal Kondo Scaling Behavior in a Quantum Dot Michael Grobis, Ileana Rau, Ronald Potok, David Goldhaber-Gordon, Hadas Shtrikman At zero temperature, a many-body Kondo singlet forms between a spin-1/2 quantum dot and electrons in nearby, tunnel-coupled reservoirs. In transport measurements, the characteristic signature of the Kondo singlet is a narrow zero bias conduction enhancement. Finite temperature and bias destroy the Kondo singlet and suppress this enhancement. At low temperature and bias (eV, kT $<<$ kT$_{K})$, the evolution of the Kondo conduction is predicted to show universal scaling in eV/kT. However, such behavior has not been examined thoroughly in experiments. To address this issue, we have performed detailed transport measurements through a Kondo quantum dot at finite bias and temperature. We have measured the scaling power law and lowest order expansion coefficient of the universal scaling function, as well as the expected deviations from universality at higher energies. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X43.00003: Kondo Screening Cloud and Charge Quantization in Mesoscopic Devices Rodrigo G. Pereira, Nicolas Laflorencie, Ian Affleck We propose that the finite size of the Kondo screening cloud, $\xi_K$, can be probed by measuring the charge quantization in a one-dimensional system coupled to a small quantum dot. When the chemical potential in the system is varied at zero temperature, one should observe charge steps that are influenced by the Kondo effect when the system size is comparable to $\xi_K$. We show that the ratio of the width of the plateaus with an odd number of electrons to the width of the plateaus with an even number is a universal scaling function of $\xi_K/L$. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X43.00004: Observation of quantum charge pumping in a gate-confined open dot with symmetrically configured pumping gates Shih-ying Hsu, Kai-Ming Liu Open quantum dots formed out of a high mobility two dimensional electron gas from negatively biased sub-micron metal gates are fabricated. Two quantum point contacts (QPCs) with independently adjustable transmission mode numbers are deviced as the two entrances of the quantum dots. Two additional metal gates are introduced to function as the pumping gates. The overall gate configuration is symmetric. We observe a dc current with the two pumping gates ac biased in the quantum charge pumping mode where the ac bias voltages are at the same frequency, 0.1$\sim $10MHz, but maintain a given phase difference $\phi $. The system remains longitudinally symmetric in this quantum pumping mode. The current amplitude drops with the opening up of the quantum dot. Moreover, we find that the current amplitude decreases with increasing the total transmission mode number in an inverse relation. By switching over to a rectification mode of gate biasing, the dc current characteristic is drastically changed. This leads to strong evidence that we have observed the quantum charge pumping. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X43.00005: Transient current in a quantum dot asymmetrically coupled to metallic leads Ali Goker, Peter Nordlander We use the time-dependent non-crossing approximation to study the transient current for a single electron transistor attached asymmetrically to two metallic leads. We investigate the effects of the bandwidth of the leads, the effect of dot energy level position, the effect of asymmetry in the couplings, and the effects of temperature. In the short timescale, the current reaches a maximum before it starts decaying. In the long timescale, we observe sinusoidal modulations of the current. The frequency of these oscillations is linearly proportional to the bandwidth of the conduction electrons in the leads. The amplitude of these oscillations are found to increase as the temperature is reduced and saturate for temperatures below the Kondo temperature. We discuss the microscopic nature of these oscillations and comment on the possibilities for their experimental detection. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X43.00006: Positive correlation in multi-level transport through a tunable quantum dot Yiming Zhang, Leonardo DiCarlo, Douglas McClure, Michihisa Yamamoto, Seigo Tarucha, Charles Marcus, Micah Hanson, Art Gossard We report measurements of shot noise auto- and cross- correlation in a tunable quantum dot with two or three leads. As the Coulomb blockade is lifted at finite source-drain bias, the current noise evolves from super-Poissonian to sub- Poissonian in the two-lead case, and the cross-correlation evolves from positive to negative in the three-lead case. The observed super-Poissonian noise and positive cross-correlation are shown to be consistent with transport through excited states. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X43.00007: Electron Localization in Strongly Correlated Quantum Dots A. D. G\"u\c{c}l\"u, Amit Ghosal, C. J. Umrigar, Harold U. Baranger We investigate the electronic properties of quantum dots in the low density regime up to $r_s\sim60$ using variational and diffusion quantum Monte Carlo methods. Quantum dots are highly tunable systems that allow the study of the physics of strongly correlated electrons. With decreasing electronic density, interactions become stronger and electrons are expected to localize at their classical positions, as in Wigner crystallization in an infinite two-dimensional system. We have studied several multi-determinental wave functions each built from single-particle states of very different nature -- LDA, Hartree, or floating Gaussian orbitals -- all optimized using an energy minimization technique. We study the density, pair-density, power spectrum, and addition energy as a function of increasing interaction strength. The main physical picture that emerges is: The system (i) first experiences a competition between different possible classical configurations, namely either the magic angular momentum states of the shell structure or those of the quantum mechanical symmetry, and (ii) then finally reaches the strongly localized regime consistent with the classical ground state. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X43.00008: Magnetic-field evolution of collective excitations in AlGaAs/GaAs few-electron quantum dots in the mK regime. Sokratis Kalliakos, Cesar Pascual Garcia, Vittorio Pellegrini, Aron Pinczuk, Brian S. Dennis, Loren N. Pfeiffer, Ken W. West, Massimo Rontani, Guido Goldoni, Elisa Molinari Spin transitions and interactions in few-electron quantum dots (QDs) are investigated by resonant inelastic light scattering (ILS). Here we present the observation of inter-shell excitations in GaAs/AlGaAs QDs that are fabricated by combining e-beam nano-lithography with high quality reactive ion etching. The interpretation of the experimental results by numerical evaluations within a full configuration interaction approach highlights the importance of the exchange and correlation effects in these systems. We show that, under the impact of a perpendicular magnetic field, the evolution of electronic spin and charge inter-shell excitations at mK temperatures reveal that significant changes in the ground state occur even at moderate magnetic fields. These experiments demonstrate that ILS enables the study of few-electron effects in QDs under the extreme conditions of low temperatures and high magnetic fields. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X43.00009: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 9:48AM - 10:00AM |
X43.00010: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 10:00AM - 10:12AM |
X43.00011: Electronic Structure of PbSe/PbS Core-Shell Quantum Dots Adam Bartnik, Efrat Lifshitz, Frank Wise The electronic structure of PbSe/PbS core-shell Quantum Dots (QDs) is calculated within a 4-band envelope function theory and compared to experimentally observed absorption spectra [1]. Our theory extends the isotropic effective mass approximation used successfully in PbS and PbSe core QDs [2] to be valid across discontinuous barriers in material parameters. Even though the band gaps of PbSe and PbS present a Type-II heterostructure, the model predicts that at typical QD sizes, these Type-II effects will not be seen. In fact, the wavefunctions are predicted to extend evenly over both materials except in the largest of sizes. This unusual lack of confinement is explained, and is shown to agree well with recent experimental results. [1] E. Lifshitz, et. al. Air-stable PbSe/PbS and PbSe/PbSe$_{x}$S$_{1-x }$core-shell nanocrystal quantum dots and their applications. \textit{Journal of Physical Chemistry B}, 2006. [2] I. Kang and F. W. Wise. Electronic structure and optical properties of PbS and PbSe quantum dots. \textit{J. Opt. Soc. Am. B}, 14(7):1632, July 1997. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X43.00012: Investigation of Individual InGaAs Quantum Dots by Cross-Sectional Ballistic Electron Emission Microscopy (BEEM) S.Y. Lehman, J.P. Pelz, C. Marginean, J.G. Cederberg Quantum dots (QDs) in III-V semiconductors are of great technological interest, but electronic properties of individual QDs are difficult to measure. We are using cross-sectional BEEM for this purpose. Multiple layers of InAs and In$_{0.4}$Ga$_{0.6}$As QDs were grown by organometallic vapor phase epitaxy, with the thickness of deposited material ranging from subcritical to that generating 5 x 10$^{10}$ dots/cm$^{2}$. The QD layers were separated by n-doped (5 x 10$^{16}$ cm$^{-3})$ GaAs/Al$_{0.3}$Ga$_{0.7}$As layers of combined thickness ranging from 130 to 190 nm. The sample was cleaved \textit{ex situ} and 5nm-thick Au Schottky barrier (SB) contacts were deposited on the cleaved edge using a shadow mask [1]. UHV BEEM at 300 K was used to locate and image the QDs and adjacent layers. The local conduction band energy (as measured by the SB height) ranged from $\sim $0.7 eV at cleaved or near-surface QDs up to $\sim $1 eV over the AlGaAs layers. Surprisingly, the BEEM current amplitude over particular QD layers appears to vary non-monotonically with In content. On-going work will be discussed to use low temperature BEEM with an applied reverse bias to measure the energy depth and physical depth of the cleaved QDs. Work supported by NSF Grant No. DMR-0505165. [1] C. Tivarus \textit{et al}., PRL \textbf{94}, 206803 (2005). [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X43.00013: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 10:36AM - 10:48AM |
X43.00014: Composite fermion solid and liquid states in two component quantum dots Chuntai Shi, Gun Sang Jeon, Jainendra K. Jain We consider correlated states of a quantum dot, at high magnetic fields, assuming electrons with two components. This model has possible relevance to quantum dots in a bilayer system, a two valley system, in graphene, or for ordinary GaAs based quantum dots in the limit of small Zeeman energy. We show that both the liquid states and crystallites (the latter occurring at large angular momenta) are accurately described in terms of composite fermions. The residual interaction between composite fermions is important, however, and causes complex nearest and next-nearest neighbor spin correlations in the composite fermion crystallite. [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X43.00015: Dephasing of an electron Mach-Zehnder interferometer capacitively coupled to a quantum dot Seok-Chan Youn, Heung-Sun Sim, Hyun-Woo Lee We theoretically investigate an electron Mach-Zehnder interferometer capacitively coupled to a quantum dot. We derive the current and connect its interference behavior to the statistics of the charge fluctuation of the dot. The interference is analyzed in the two limiting cases of fast and slow charge fluctuations: For the case of fast fluctuations where the dwell time of the dot is much smaller than the flight time of the interferometer, the visibility is reduced due to the charge fluctuations, while in the opposite limit the behavior of the interference can be understood by screening effects. The connection to recent experimental and theoretical works will be discussed. [Preview Abstract] |
Session X44: Focus Session: Nanoscale Transport - Molecules III
Sponsoring Units: DMPChair: Mark Hybertsen, Brookhaven National Laboratory
Room: Colorado Convention Center 507
Friday, March 9, 2007 8:00AM - 8:12AM |
X44.00001: Landauer conductance and twisted boundary conditions for Dirac fermions Shinsei Ryu, Christopher Mudry, Akira Furusaki, Andreas Ludwig We apply the generating function technique developed by Nazarov to the computation of the density of transmission eigenvalues for a finite graphene sheet in which a two-dimensional freely propagating massless Dirac fermion is realized. By modeling ideal leads attached to the sample as a conformal invariant boundary condition, we relate the generating function for the density of transmission eigenvalues to the twisted chiral partition functions of fermionic (c=1) and bosonic (c=-1) conformal field theories. We also discuss the scaling behavior of the ac Kubo conductivity and compare its \textit{different} $dc$ limits with results obtained from the Landauer conductance. Finally, we show that the disorder averaged Einstein conductivity is an analytic function of the disorder strength, with vanishing first-order correction, for a tight-binding model on the honeycomb lattice with weak real-valued and nearest-neighbor random hopping. [Preview Abstract] |
Friday, March 9, 2007 8:12AM - 8:24AM |
X44.00002: Electron turbulence in nanoscale junctions Neil Bushong, John Gamble, Massimiliano Di Ventra Electron transport through a nanostructure can be characterized in part using concepts from classical fluid dynamics. [1] It is then natural to ask how far the analogy can be taken, and whether the electron liquid can exhibit nonlinear dynamical effects such as turbulence. Here we present a first-principles study using time-dependent current density functional theory of electron transport in nanojunctions which reveals that the electron liquid indeed exhibits behavior quite similar to that of a classical fluid. For example, a transition away from symmetric flow occurs at higher current densities, just as in the classical Navier-Stokes case. We will also discuss the behavior of the velocity correlation tensor in both laminar and turbulent regimes, as well as spontaneous symmetry breaking. Work supported in part by NSF and DOE. \newline \newline [1] R. D'Agosta and M. Di Ventra, J. Phys. Cond. Matt. in press. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X44.00003: Conductance switching and electronic states in polymer nanodevices Nikolai Zhitenev, Alexander Sidorenko, Donald Tennant, Raymond Cirelli Organic materials offer new electronic functionality not available in the inorganic devices. However, the integration of organics within nanoscale electronic circuitry poses new challenges for material physics and chemistry. To rationally control the conducting properties of small devices, the electronic states in organics have to be optimized relative to the Fermi level of metal contacts. We demonstrate a novel approach to create and chemically modify such electronic states in thin polyelectrolyte films. Nanoscale devices fabricated using integrated shadow masks and the polyelectolyte film grafted to electrodes display reversible switching between conducting and non-conducting states. The conductance is related to the creation and annihilation of the chain of the electronic levels in the polymer. The electronic properties and the switching dynamics are broadly tunable by the chemical composition of the polymers. The open design of our nanodevices allows us to perform the chemical conversion targeting primarily carboxyl groups inside the completed junctions. The conduction memory effect is observed in devices with lateral size down to 30 nm. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 9:12AM |
X44.00004: Pair Tunneling through Single Molecules Invited Speaker: Coupling to molecular vibrations induces a polaronic shift, and can lead to a negative charging energy, U. For negative U, the occupation of the ground state of the molecule is even. In this situation, virtual pair transitions between the molecule and the leads can dominate electron transport. At low temperature, T, these transitions give rise to the charge-Kondo effect [1]. We developed the electron transport theory through the negative-U molecule [2] at relatively high T, when the Kondo correlations are suppressed. Two physical ingredients distinguish our theory from the transport through a superconducting grain coupled to the normal leads [3]: (i) in parallel with sequential pair-tunneling processes, single-particle cotunneling processes take place; (ii) the electron pair on the molecule can be created (or annihilated) by two electrons tunneling in from (or out to) opposite leads. We found that, even within the rate-equation description, the behavior of differential conductance through the negative-U molecule as function of the gate voltage is quite peculiar: the height of the peak near the degeneracy point is independent of temperature, while its width is proportional to T. This is in contrast to the ordinary Coulomb-blockade conductance peak, whose integral strength is T-independent. At finite source-drain bias, V$>>$T, the width of the conductance peak is $\sim$V, whereas the conventional Coulomb-blockade peak at finite V splits into two sharp peaks at detunings V/2, and -V/2. Possible applications to the gate-controlled current rectification and switching will be discussed. \newline \newline [1] A. Taraphder and P. Coleman, Phys. Rev. Lett. 66, 2814 (1991). \newline [2] J. Koch, M. E. Raikh, and F. von Oppen, Phys. Rev. Lett. 96, 056803 (2006). \newline [3] F. W. J. Hekking, L. I. Glazman, K. A. Matveev, and R. I. Shekhter, Phys. Rev. Lett. 70, 4138 (1993). [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X44.00005: Tuning the Kondo effect with a mechanically controllable break junction J. J. Parks, A. R. Champagne, G. R. Hutchison, S. Flores-Torres, H. D. Abruna, D. C. Ralph We study electron transport through C$_{60}$ molecules in the Kondo regime using a mechanically controllable break junction. By varying the electrode spacing, we are able to change both the width and height of the Kondo resonance, indicating modification of the Kondo temperature and the relative strength of coupling to the two electrodes. The normalized linear conductance as a function of $T/T_\textrm{K}$ agrees with the scaling function expected for the spin-1/2 Kondo problem. The same devices can also exhibit finite-bias inelastic Kondo features at an energy that corresponds to the $H_g(1)$ intracage vibrational mode of C$_{60}$. Changes in electrode spacing can tune the energy and amplitude of these signals. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X44.00006: Bandgap enhancement of phonon occupation Lutfe Siddiqui, Avik Ghosh, Supriyo Datta We explore the effect of electron transport through a vibration-coupled quantum dot weakly coupled with semiconducting leads and the thermal environment by writing master equations in the electron-phonon fock space. We show that the presence of bandgap in the semiconducting leads results in an enhancement of phonon occupation compared to the case of metallic contacts under certain bias as different phonon absorption processes get shut off by the bandgap. We also show that the presence of bandgap can lead to more than one negative differential resistance (NDR) peaks in the conductance spectrum of the dot. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X44.00007: Interference Effects in Nanoscale Electron-Phonon Transport Andrei Sergeev, Michael Reizer, Vladimir Mititn Using the quantum transport equation we investigate interplay of the electron-phonon interaction and disorder in low-dimensional conductors. Interference of electron scattering mechanisms is beyond the Landau Fermi-liquid picture and result in nontrivial corrections, which violate the Mathiessen rule. Interfer-ence effects have been known for some time, however, the research in this field was mainly limited to disordered bulk materials and thin metallic films. In low dimensions, we found strong enhancement of the interference effects. As in the electron-phonon kinetics [1], this enhancement is due to a smaller electron momentum transferred in electron-phonon scattering processes in low dimensions. \newline [1] A. Sergeev et al., Phys. Rev. Lett. 94, 136602 (2005). [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X44.00008: Tuning interacting electron and phonon quantum states in molecular nanostructures via atomic manipulation Gabriel Zeltzer, Hari C. Manoharan We investigate atomically precise nanostructures assembled from CO molecules on Cu(111) using a custom-built low-temperature ultrahigh vacuum (UHV) scanning tunneling microscope (STM). The atomic manipulation capability of this instrument allows single molecule placement to desired locations, thus enabling the construction of nanostructures with \AA ngstrom level precision. We demonstrate the control of electronic and vibrational states within several quantum corral geometries, and investigate possible interactions between these two quantum degrees of freedom. Dependencies on corral shape and size are presented. $dI/dV$ spectroscopy reveals the ability to engineer the corral eigenstates with sub-meV accuracy. We also briefly discuss the design and performance specifications of our atomic manipulation STM which enable this class of experiments. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X44.00009: Coulomb gas scaling of the non-equilibrium spin-boson model Aditi Mitra, Andrew Millis The nonequilibrium ``spin-boson model,'' a localized electronic level coupled to a fluctuating two-state system and to two electronic reservoirs, is solved via an Anderson-Yuval-Hamann mapping onto a plasma of alternating positive and negative charges time-ordered along the two ``Keldysh'' contours needed to describe nonequilibrium physics. The interaction between charges depend on whether their time separation is small or large compared to a dephasing scale defined in terms of the chemical potential difference between the electronic reservoirs, and a decoherence scale defined in terms of the current flowing from one reservoir to another. A nonequilibrium scaling transformation is introduced. An important feature is the presence in the model of a new coupling, essentially the decoherence rate, which acquires an additive renormalization similar to that of the energy in equilibrium problems. The resulting flow equations are used to study the competition between the dephasing-induced formation of independent resonances tied to the two chemical potentials, and the decoherence which cuts off the scaling and leads to effectively classical long-time behavior. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X44.00010: Numerical Estimation of Keldysh-Countour Path Integrals for Nonequilibrium Problems Andrew Millis, Philipp Werner We propose an idea for simulating the dynamics of open (coupled to reservoirs) systems in a nonequilibrium steady state and present preliminary numerical results for the nonequilibrium spin boson and Anderson models. The method builds on the observation (Phys. Rev. Lett. ${\bf 94}$ 076404) that out of equilibrium the Keldysh time evolution operator exhibits an exponential time decay, and uses the stochastic hybridization expansion techniques of Phys. Rev. Lett. ${\bf 97}$, 07640. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X44.00011: Full Counting Statistics for a Single-Electron Transistor: Nonequilibrium Effects at Intermediate Conductance Yasuhiro Utsumi, Dimitri Golubev, Gerd Sch\"on We calculate the probability distribution of current for a single-electron transistor (SET) with intermediate strength conductance where quantum fluctuations of the charge play a dominant role. The calculations are based on the multichannel anisotropic Kondo model in the Majorana representation and the fermionic Keldysh generating functional. The effects of quantum fluctuations are taken into account by the summation of a certain subclass of diagrams, which corresponds to the leading logarithmic approximation in the sense that the result is consistent with the RG analysis. We have shown that in non-equilibrium situations quantum fluctuations of the charge induce lifetime broadening for the charge states of the central island. Consequences which can be detected in experiments include a suppression of the probability of currents larger than the average value. Y. Utsumi, D. Golubev, G. Schoen; PRL. 96, 086803 (2006) [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X44.00012: Vibronic coupling effect on the electron transport through molecules Masaru Tsukada, Kunihiro Mitsutake Electron transport through molecular bridges or molecular layers connected to nano-electrodes is determined by the combination of coherent and dissipative processes, controlled by the electron-vibron coupling, transfer integrals between the molecular orbitals, applied electric field and temperature. We propose a novel theoretical approach, which combines \textit{ab initio} molecular orbital method with analytical many-boson model. As a case study, the long chain model of the thiophene oligomer is solved by a variation approach. Mixed states of moderately extended molecular orbital states mediated and localised by dress of vibron cloud are found as eigen-states. All the excited states accompanied by multiple quanta of vibration can be solved, and the overall carrier transport properties including the conductance, mobility, dissipation spectra are analyzed by solving the master equation with the transition rates estimated by the golden rule. We clarify obtained in a uniform systematic way, how the transport mode changes from a dominantly coherent transport to the dissipative hopping transport. [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X44.00013: Quantum open systems approach to single-molecule devices Yongqiang Xue, William Kennerly Experimental advances in electrically and optically probing individual molecules have provided new insights into the behavior of single quantum objects and their interaction with the nanoenvironments without requiring ensemble average. Single-molecule devices are open quantum systems whose dynamics are intrinsically stochastic and are subject to dissipation and decoherence through system-environment correlation. New concepts and computational techniques may be needed to unravel the rich physics underlying single-molecule measurements. In this talk, we discuss our efforts in developing quantum open systems theory of single-molecule electronics and optics, building on concepts and techniques from quantum optics and quantum measurement. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700