Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B1: Strongly Correlated Electrons in One Dimension
Sponsoring Units: DCMPChair: Gil Refael, California Institute of Technology
Room: Morial Convention Center LaLouisiane AB
Monday, March 10, 2008 11:15AM - 11:51AM |
B1.00001: Narrow-gap Luttinger liquid in carbon nanotubes Invited Speaker: Single-walled carbon nanotubes are the thinnest and the cleanest among the currently available nanoscale quantum wires. Transport properties of nanotubes depend on the presence of a gap in electron spectrum, defining two main nanotube types, metallic and semiconducting. Semiconducting tubes attract interest in particular because of the sensitivity of their properties to external fields and doping. Among semiconducting tubes there is an interesting class of narrow-gap tubes, or so-called chiral metallic tubes, which exhibit a narrow semiconducting gap arising due to curvature [1]. The Luttinger liquid effects, which are strong in all nanotubes, are particularly interesting in the narrow-gap tubes. Interaction strongly affects the energy gap, reinforcing it and making it sensitive to the long-wavelength charge mode dynamics [2]. We discuss new types of charge carriers possible in the gapped states and their relation to recent experimental work [3]. \newline [1] C.L. Kane and E.J. Mele, Phys. Rev. Lett. 78, 1932 (1997) \newline [2] L. S. Levitov, A. M. Tsvelik, Phys. Rev. Lett. 90, 016401 (2003) \newline [3] V. V. Deshpande, M. Bockrath, arXiv:0710.0683 [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:27PM |
B1.00002: Observation of spin-charge separation and localization in one-dimensional quantum wires Invited Speaker: We have been able to measure hallmark properties of electrons confined to one-dimensional (1D) wires. Profoundly affected by interactions, the 1D electron liquid is a Luttinger-liquid. Single particle elementary excitations, which survive in spite of interactions in higher dimensions, completely lose their integrity in a Luttinger-liquid. Instead, the elementary excitations of the 1D electron liquid are all collective, with long range correlations and are spin-charge separated. In spite of the drastic influence of electron-electron interactions on the many-body states, the observation of these effects in experiment has been elusive. Our wires were fabricated from a GaAs/AlGaAs heterostructure using cleaved edge overgrowth. The sample I shall discuss contained two parallel wires, 20nm and 30nm thick, which were separated by a 6nm insulating AlGaAs barrier. A series of top gates allowed us to contact each wire separately, and thus allowed us to control both the energy and the momentum of the electrons tunneling between the wires. The resulting tunneling conductance was a direct measure of the spectral function in each of the wires, and thus enabled us to map the dispersions of the 1D many-body elementary excitations. Pushing the wires to low density allowed us to probe the regime where interactions dominate over kinetic energy. In this regime we clearly observed two spin modes and one charge mode of the coupled wires. Mapping the dispersion velocities as a function of decreasing density, we found good agreement between the data and theoretical calculations of the velocity of the antisymmetric charge mode of the coupled wires. The theory also predicted an additional symmetric charge mode, that was not observed. The spin velocities were found, within experimental precision, to be smaller than theoretically predicted. Reducing the density of electrons even further, we found an abrupt transition in the extent of the 1D states along the wires: At high densities they were extended and had well defined momenta, while at low densities they localized as a result of interactions and exhibited Coulomb blockade physics. A simultaneous measurement of the two-terminal conductance, which displayed the typical stepwise drop with decreasing density, showed that a localization transition was concurrent with each conductance drop. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 1:03PM |
B1.00003: The One-Dimensional Wigner Crystal in Carbon Nanotubes Invited Speaker: Electron-electron interactions strongly affect the behavior of low-dimensional systems. In one dimension (1D), arbitrarily weak interactions qualitatively alter the ground state producing a Luttinger liquid (LL) which has now been observed in a number of experimental systems. Interactions are even more important at low carrier density, and in the limit when the long-ranged Coulomb potential is the dominant energy scale, the electron liquid is expected to become a periodically ordered solid known as the Wigner crystal. In 1D, the Wigner crystal has been predicted to exhibit novel spin and magnetic properties not present in an ordinary LL. However, despite recent progress in coupled quantum wires, unambiguous experimental demonstration of this state has not been possible due to the role of disorder. We demonstrate using low-temperature single-electron transport spectroscopy that a hole gas in low-disorder carbon nanotubes with a band gap is a realization of the 1D Wigner crystal [1]. We observe for the first time three distinct regimes as a function of carrier density and axial magnetic field: (I) a completely spin and isospin polarized state, (II) an isospin polarized, spin antiferromagnetic state, and (III) an unpolarized state with a four-fold addition energy period. The transitions among these regimes can be quantitatively and intuitively explained using a Wigner crystal picture based on a gapped LL model [2] with the carriers represented by spatially localized solitons. Our observation provides a clean platform for testing theories of interacting electrons in 1D and also indicates the possibility of using this many-body state for solid-state quantum information processing. [1] V. V. Deshpande and M. Bockrath, arXiv:0710.0683v1 [cond-mat.str-el] [2] L. S. Levitov and A. M Tsvelik, Phys. Rev. Lett. 90, 016401 (2003) [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:39PM |
B1.00004: Fermi-Edge Singularity in a Spin-Incoherent Luttinger Liquid Invited Speaker: In recent years the spin-incoherent Luttinger liquid, obtained in the energy window $E_{\rm spin}\ll k_B T \ll E_{\rm charge}$, has attracted much attention because of its qualitatively distinct properties relative to the more familiar Luttinger liquid [1]. Some of the most remarkable effects appear in correlations in which the number of particles is abruptly changed, such as a single particle Green's function [2] or in the Fermi-edge singularity when a particle-hole pair is photo-excited [3]. In this talk, I draw on the methods developed in Ref.[2] to study the Fermi-edge singularity in the spin-incoherent Luttinger liquid [3]. Both cases of finite and infinite core hole mass are explored, as well as the effect of a static external magnetic field of arbitrary strength. For a finite mass core hole the absorption edge behaves as $(\omega-\omega_{\rm th})^\alpha/\sqrt{|\ln(\omega-\omega_{\rm th})|}$ for frequencies $\omega$ just above the threshold frequency $\omega_{\rm th}$. The exponent $\alpha$ depends on the interaction parameter $K_c$ of the interacting one dimensional system, the electron-hole coupling, and is independent of the magnetic field strength, the momentum, and the mass of the excited core hole (in contrast to the spin-coherent case). In the infinite mass limit, the spin-incoherent problem can be mapped onto an equivalent problem in a spinless Luttinger liquid for which the logarithmic factor is absent, and backscattering from the core hole leads to a universal contribution to the exponent $\alpha$.\\ \\ $[1]$ G. A. Fiete, Rev. Mod. Phys. {\bf 79}, 801 (2007).\\ $[2]$ G. A. Fiete and L. Balents, Phys. Rev. Lett. {\bf 93}, 226401 (2004).\\ $[3]$ G. A. Fiete, Phys. Rev. Lett. {\bf 97}, 256403 (2006).\\ [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 2:15PM |
B1.00005: Transition from a one-dimensional to a quasi-one-dimensional state in interacting quantum wires Invited Speaker: At low density, all electrons in a quantum wire occupy the lowest state of transverse quantization, and it is natural to view the system as one-dimensional. As the density is increased, the electrons start to populate the second subband, resulting in a transition to a quasi-one-dimensional state. I will discuss this transition in the presence of electron-electron interactions in a model that neglects electron spins. Clearly, in the non-interacting case the transition is accompanied by the emergence of a second gapless excitation mode. On the other hand, at very strong interactions, the one-dimensional electrons form a Wigner crystal, and the transition corresponds to it splitting into two chains. Unlike the non-interacting electrons, this two-row (zigzag) crystal still has only one acoustic excitation branch. This raises the question of how the nature of the transition to a quasi-one-dimensional state changes with interaction strength. We can show that in the vicinity of the transition already arbitrarily weak interactions open a gap in the second excitation mode. We then argue that only one gapless mode exists near the transition at any interaction strength. [Preview Abstract] |
Session B2: Magnetism in Semiconductors: New Frontiers
Sponsoring Units: GMAGChair: Scott Crooker, Los Alamos National Laboratory
Room: Morial Convention Center LaLouisiane C
Monday, March 10, 2008 11:15AM - 11:51AM |
B2.00001: Optical Properties of III-Mn-V Ferromagnetic Semiconductors Invited Speaker: We discuss the important role optical studies have played in our understanding of the electronic structure of III-Mn-V ferromagnetic semiconductors. These extensive studies have established the electronic structure is strongly affected by the strength of the exchange between the Mn local moments and the holes they introduce. Particular focus is given to Ga$_{1-x}$Mn$_{x}$As, where spectroscopic studies suggest the metallic state is unconventional. Finally, we will detail our recent experiments into the ultrafast manipulation of magnetism on the nanoscale. This work is in collaboration with D.B. Shrekenhamer, E.J. Singley, D.N. Basov (University of California, San Diego) J. Stephens, S. Mack, R.K. Kawakami, D.D. Awschalom(University of California, Santa Barbara), B.L. Sheu, N. Samarth (Pennsylvania State University), F. Chen, A. Azad, J. O'Hara, A.M. Dattelbaum, G. Montano, S. Crooker, and A.J. Taylor (Los Alamos National Laboratory). [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:27PM |
B2.00002: Spin Transport in Ferromagnet-Semiconductor Heterostructures Invited Speaker: Over the last two years, there has been significant progress in the integration of metallic ferromagnets with semiconductors, resulting in devices in which spin-polarized carriers are injected and detected electronically. I will discuss experiments on epitaxial Fe/GaAs Schottky tunnel barrier heterostructures patterned into lateral devices in which the ferromagnetic injection and detection contacts are separated by several microns.[1] The Schottky barrier consists of a highly-doped $n^+$ region ($n^+ \sim 5 \times 10^{18}$~cm$^{-3}$), and the channel of the device is $n$-doped GaAs ($n\sim 2 \times 10^{16 } - 1 \times 10^{17}$~cm$^{-3}$). A non-equilibrium spin polarization generated by electrical injection is detected potentiometrically using the non-local transport technique applied originally to metallic systems. An important aspect of this approach is the observation of spin precession and dephasing in the semiconductor channel (the Hanle effect), allowing for electrical measurements of the spin lifetime and diffusion length. We find a strong non-linear dependence of the spin polarization on the injection bias voltage, which we have investigated by preparing samples with different thicknesses of the $n^+$ region, thus varying the tunnel barrier profile. We find a systematic change in the spin accumulation observed under forward and reverse bias currents as the thickness of the $n^+$ region increases. Other aspects of these devices have also been explored. For GaAs channels that are doped near the metal-insulator transition, the non-equilibrium electron spin polarization leads to dynamic nuclear polarization, which has a profound impact on the electron spin dynamics at low temperatures. Finally, I will discuss some important considerations for applications in which a bias current flows in the detector. [1] X. Lou {\it et al.}, Nature Physics {\bf 3}, 197 (2007) [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 1:03PM |
B2.00003: Ultrafast Photoinduced Non-thermal Phenomena in (III, Mn)V Ferromagnetic Semiconductors Invited Speaker: Magnetic materials displaying carrier-mediated exchange interaction are ideal for non-thermal, potentially fast spin manipulation and detection. Prominent examples of such materials are Mn doped III-V semiconductors such as GaMnAs, in which the strong interaction of carriers (holes) and Mn ions results in high transition temperature ferromagnetism. The steady-state magnetooptical/transport measurements reveal rich magnetic memory effects and strong enhancement of ferromagnetism via external stimuli (i.e., light, electrical field or current). However, no timeresolved experiments in (III,Mn)V semiconductors have shown these collective magnetic phenomena, and hence their time scales are completely unknown. In this talk, I will present our recent observations in GaMnAs of: (1) ultrafast enhancement of ferromagnetism via photoexcited transient holes on a 100 ps time scale and (2) femtosecond detection of magnetic memory states. Our measurements reveal new fundamental collective magnetic processes at ultrafast time scales, and identify the critical roles of the Mn-hole correlation in these photo-induced cooperative behaviors. These results constitute the first evidence for \textit{ultrafast}, \textit{non-thermal} manipulation of the spin order in (III,Mn)Vs, which may represent as-yet-undiscovered universal features in all carrier-mediated ferromagnetic materials. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:39PM |
B2.00004: Ferromagnetism and localization in Ga$_{1-x}$Mn$_{x}$As, Ga$_{1-x}$Mn$_{x}$P, and in between Invited Speaker: Because of their potential as both injectors and filters of spin-polarized carriers, ferromagnetic semiconductors may play an important role in spin-based electronics, or \textit{spintronics}. Ferromagnetic semiconductors are formed by the substitution of a relatively small fraction of host atoms with a magnetic species. Ga$_{1-x}$Mn$_{x}$As has been the most thoroughly studied material among these, and ferromagnetism in it arises from hole-mediated inter-Mn exchange. The Curie temperature T$_{C}$ in Ga$_{1-x}$Mn$_{x}$As has been shown to increase with increasing concentration of substitutional Mn acceptors. However, room temperature ferromagnetism in this canonical system has been elusive due to challenges in materials synthesis---namely, raising $x$ while avoiding the formation of second phases or compensating defects. Increasing $p-d$ exchange by modifying the host semiconductor via anion substitution (e.g., replacing As with P) is a significantly less explored route by which T$_{C}$ may be raised. We are investigating the effect of anion substitution in ferromagnetic Ga$_{1-x}$Mn$_{x}$As$_{1-y}$P$_{y}$ formed by ion implantation followed by pulsed-laser melting. In the endpoint compound Ga$_{1-x}$Mn$_{x}$P T$_{C}$ is found to vary linearly with $x$, and non-metallic transport is observed for $x$ up to $\sim $4.2{\%}, corresponding to a T$_{C}$ of $\sim $62 K compared to $\sim $112 K for Ga$_{1-x}$Mn$_{x}$As with a similar $x$. Dilution of the endpoint compound Ga$_{1-x}$Mn$_{x}$As with P results in a precipitous decrease in T$_{C}$ to below 60 K for y=2.8{\%}. Remarkably, Ga$_{1-x}$Mn$_{x}$As$_{1-y}$P$_{y}$ films undergo a metal-insulator transition between $y$=1.5{\%} and 2.3{\%} even as $x$ is held approximately constant indicating that alloy disorder in the anion sublattice induces hole localization, which in turn may be responsible for a strong suppression of T$_{C}$. Thus, while anion substitution may enhance $p-d$ exchange, localization effects must be considered when developing a suitable picture for ferromagnetism in these materials. [Preview Abstract] |
Session B3: Silicon Photonics
Sponsoring Units: FIAPChair: Alex Bratkovsky, Hewlett-Packard Laboratories
Room: Morial Convention Center RO2 - RO3
Monday, March 10, 2008 11:15AM - 11:51AM |
B3.00001: Silicon lasers, integration, interconnect, etc. at Intel Invited Speaker: In this talk, we discuss integrated silicon photonic technologies that enable Tbit/s optical link for future VLSI interconnect applications. We also review the recent advances in various fundamental building blocks, including high-speed silicon optical modulator, SiGe detector, and hybrid silicon laser. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:27PM |
B3.00002: Ultrafast Silicon Photonics Modulators Invited Speaker: |
Monday, March 10, 2008 12:27PM - 1:03PM |
B3.00003: Silicon integrated nanophotonics for on-chip interconnects Invited Speaker: Current trend in microelectronics industry is to increase the parallelism in computation by multi-threading, by building large scale multi-chip systems and, more recently, by increasing the number of cores on a single chip. With such increase of parallelization the interconnect bandwidth between the racks, chips or different cores is becoming a limiting factor for the design of high performance computer systems. The on-chip ultrahigh-bandwidth silicon-based photonic network might provide an attractive solution to this bandwidth bottleneck. We will review recent results on silicon nanophotonic circuits based on photonic wires and photonic crystals. Strong light confinement at the diffraction limit enables dramatic scaling of the device area and allows unprecedented control over optical signals. Silicon nanophotonic devices have immense capacity for low-loss, high-bandwidth data processing that might enable the design of ultra-compact on-chip optical networks. In particular we will show recent results on design and characterization of various ultra-compact ($<$0.03mm2) silicon nanophotonic circuits as optical delay lines, electro-optic modulators, broadband optical switches, wavelength filters, etc. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:39PM |
B3.00004: Silicon Detectors, Sensors and Ultrafast Interconnect. Invited Speaker: |
Monday, March 10, 2008 1:39PM - 2:15PM |
B3.00005: Monolithic Integration of Photonic and Electronic Circuits in a CMOS Process Invited Speaker: We present our approach to a low-cost, highly scalable opto-electronic integration platform in silicon. We have developed a process in a commercial CMOS foundry that enables tightly integrated photonic devices and electronic circuits. The device library includes optical and opto-electronic components as well as electronic circuits. In this talk, we detail the performance of the building blocks and highlight the performance trade-offs encountered in integrating different functions on the same chip. We describe an opto-electronic circuit design toolkit, which is modeled after the standard electronic design flow and builds on commercial circuit design tools. The design flow includes automated design rule checking and layout-versus-schematic checks covering all types of circuit elements. As an example, we detail the design of a multi-channel transceiver chip with 10 Gbps/channel optical data transmission speed and report on its performance. [Preview Abstract] |
Session B4: Self-assembled Macromolecular Structures
Sponsoring Units: DPOLYChair: Spiros Anastasiadis, Foundation for Research and Technology, Hellas, Keraklion, Greece
Room: Morial Convention Center 206
Monday, March 10, 2008 11:15AM - 11:51AM |
B4.00001: Stimuli Responsive Vesicles, Micelles and Rods from Polypeptide-based Block Copolymers Invited Speaker: The self-assembly of amphiphilic block copolymers is dictated primarily by the balance between the hydrophobic core volume and the hydrophilic corona. In these studies, amphiphilic block copolymers containing poly(lysine) (P(Lys)) blocks were synthesized and their solution properties studied using dynamic light scattering, circular dichroism spectroscopy and transmission electron microscopy. The hydrophobic block used was either poly(butadiene) or a statistical copolymer of propylene oxide and ethylene oxide. The latter polymer exhibits a tunable critical point, below which the block copolymer is in the ``double hydrophilic'' limit. In these multiply-responsive materials, we exploit secondary structure changes that occur within the P(Lys) chain to observe changes in solution morphology as a function of solution conditions. At high pH, the P(Lys) chain assumes either an $\alpha$-helical or $\beta$-sheet conformation depending on temperature, while at lower pH the side chains become protonated, resulting in an expanded coil configuration. The effect of morphology changes due to these structural transitions will be discussed in the context of the interfacial curvature changes with pH and temperature. These dynamic materials have potential applications as viscosity modifiers, liquid crystals and gels. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:27PM |
B4.00002: Templated Self Assembly of Block Copolymer Thin Films Invited Speaker: A primary limitation of block copolymer films as templates for next generation electronic or data storage devices is the prohibitively long times required for thermally driven self-assembly and defect annihilation and long range order development. We demonstrate a high-throughput approach involving temporal zone (cold-hot-cold) annealing of block copolymer films well below their order-disorder transition temperature (T: HOT $<<$ T: ODT) that produces low defect concentrations, large grain size and a preferential alignment of the block microphase relatively rapidly. Promising results have been obtained by combining zone annealing with directed assembly on topographically patterned substrates. This combination results in the rapid development of long-range order which persists over the entire patterned area. The evolution of order in these templates is quantified using neutron reflection in conjunction with tomographic small angle scattering, and compared to scattering from model simulations to obtain a 3-D description of ordering within channel templates. The ability to rapidly achieve quantifiable long-range order in block copolymers (with inaccessible order-disorder transition temperatures) using non-destructive methods within templates suggests zone annealing as a robust nanomanufacturing method for guided self-assembly. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 1:03PM |
B4.00003: Single Molecules and Surface Induced Nanopattern in Ultrathin Blockcopolymer Films - Scanning Force Microscopy Invited Speaker: Supramolecular self-organization is a powerful tool for producing nanostructured patterns as an alternative to lithographic techniques. Block copolymers from incompatible fragments are well known for their ability to self-assemble into well-ordered periodic structures at a length scale of 10 – 100 nm. Besides length, composition, and the type and number of the constituent blocks, branching and chemical transformation provides a further parameter for controlling the molecular conformation and structure. In this work we describe a new approach to prepare polymeric supramolecular complexes using wedge-shaped amphiphilic molecules, which are able to self-assemble into cylindrical supramolecular structures. Scanning force microscopy on single macromolecules is demonstrated to provide a powerful tool to characterize the supramolecular complexes and structure formation in ultrathin films. The self-assembly properties of their complexes with homopolymers as well as block copolymers will be discussed. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:39PM |
B4.00004: Using block copolymer assembly to tailor surface properties. Invited Speaker: A challenge in the design of surfaces is to be able to control molecular function with nm-scale precision. This presentation discusses the use of block copolymers to deliver chemical functions to the near-surface region with precise control of surface functionality. Block copolymers prepared using either anionic or ATRP polymerization were used followed by subsequent functional group modification. By using block copolymers alone and in combination, it is possible to tailor not only surface properties, but thei mechanical behavior of the polymer surface region. The effect of surface composition and patterning on the biological response to self-organized surfaces will be discussed in the context of well characterized surfaces. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 2:15PM |
B4.00005: Integration of block copolymers into lithographic processes Invited Speaker: In the photolithographic process, information in the form of exposed and unexposed material is transferred to a sacrificial photoresist film in the exposure tool. Through a series of processing steps patterned features are ultimately created from that information that can then define components of devices or other functional materials. As feature dimensions shrink below 30 nm, however, the fidelity of the transferred information using current resists may not meet manufacturing requirements, particularly with respect to control over the precise size and shape of the patterned features, and the cost of manufacturing at ever higher resolution may become prohibitively expensive. We are investigating techniques to integrate self-assembling block copolymers into the lithographic process for improved information transfer, process control, and sub 15 nm resolution. We demonstrate strategies to reach these objectives and at the same time retain essential lithographic process attributes such as pattern perfection, registration, the ability to pattern non-regular device-oriented structures, and high volume manufacturing. [Preview Abstract] |
Session B5: Geometry and Elasticity
Sponsoring Units: DCMPChair: Mark Bowick, Syracuse University
Room: Morial Convention Center RO1
Monday, March 10, 2008 11:15AM - 11:51AM |
B5.00001: Generalized Crumpling: induced singularities in gently deformed elastic sheets Invited Speaker: If a thin disk of elastic material is confined in a shrinking sphere, the deformation of the disk is not smooth but nearly singular when its radius becomes larger than that of the sphere: the curvature at one point diverges as the thickness goes to zero. This talk considers {\it induced} singularities that arise from the interaction of these ``vertex" singularities with their environment. For example, if two vertices are present, the curvature on the line joining them also diverges, forming the familar ridge singularity [1]. Other induced singularities are coming to light. Here we consider two such singularities. The first is the induced vertex at the boundary [2] of a disk that has been compressed until it contains two interior vertices. Asymptotically, the triangular region bounded by the three vertices becomes arbitrarily flat as the sheet thickness goes to zero, while the curvature outside approaches a nonzero limit. The second singularity appears when a vertex is formed by forcing a flat sheet into a circular ring so that the sheet buckles. Then the ring force induces a singular radial curvature in the sheet. Remarkably this curvature just sufficient to make the mean curvature vanish where the the ring contacts the sheet [3]. We explore the generality of this curvature cancellation phenomenon. \newline [1] T. A. Witten {\sl Rev. Mod. Phys} {\bf 79} 643 (2007) \newline [2] E. Cerda, S. Chaieb, F. Melo, and L. Mahadevan, {\sl Nature} London {\bf 401} 46 (1999) \newline [3] T. Liang, T. A. Witten, {\sl Phys. Rev. E} {\bf 73} 046604 (2006) [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:27PM |
B5.00002: Columnar and crystalline monolayers on curved substrates Invited Speaker: We study thin self-assembled columns constrained to lie on a curved, rigid substrate. The curvature presents no {\sl local} obstruction to equally spaced columns in contrast to curved crystals for which the crystalline bonds are frustrated. Instead, the vanishing compressional strain of the columns implies that their normals lie on geodesics which converge (diverge) in regions of positive (negative) Gaussian curvature, in analogy to the focussing of light rays by a lens. The bending of the layers generates a pre-stress of geometric frustration in the ground state that exists prior to the inclusion of defects. This simple observation is the basis for a versatile analytical approach to calculate the geometrical forces between dislocations and Gaussian curvature in columnar as well as in crystalline monolayers. The resulting forces play an important role in stress relaxation dynamics, elastic instabilities, and melting. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 1:03PM |
B5.00003: The shape, stability and dynamics of elastic surfaces Invited Speaker: Bending a thin sheet is easier than stretching it, an observation which has its roots in geometry. We will use this fact to explain some unusual problems in biology, physics and geology. At the everyday scale, I will discuss the morphology of avascular algal blades, the dynamics of defects in an elastic ribbon, and the dynamics of prey capture by certain carnivorous plants. At the geological scale, I will try to explain the shape of island arcs on our planet. Finally, time permitting, I will discuss how we might extend these ideas to the macromolecular scale, to derive a mechanical model for the dynamic instability of a growing microtubule. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:39PM |
B5.00004: Elasticity and capillarity: wet hairs and origami Invited Speaker: Capillary forces are responsible for a large range of everyday observations : the shape of rain droplets, the imbibition of a sponge, the clumping of wet hair into bundles. Although they are often negligible on macroscopic structures, surface capillary forces may overcome volume forces at small scales and deform compliant micro-structures. Capillary-induced sticking can indeed prevent the actuation of mobile elements in micro-electro-mechanical systems (MEMS), or even cause their collapse. Capillary forces also have important consequences in biology such as the buckling of the airway lumen induced by surface tension, which can eventually cause the lethal closure of lung airways (known as neonatal respiratory distress syndrome). We will review a few experimental situations where capillary forces are able to deform two types of objects: rods, and thin sheets. For instance, the nanotubes of a ``carbon nanotube carpet'' self-assemble into conical ``teepee'' structures after the evaporation of a solvent and can produce intriguing cellular patterns. Similarly, macroscopic wet hairs tend to assemble into bundles through a cascade of successive pairings. Comparing attracting capillary forces to bending elasticity, leads to a characteristic ``elasto-capillary'' length. The case of thin sheets is challenging because of geometrical constrains, which generally leads to singularities. Can a thin sheet spontaneously wrap around droplet? We will describe in detail this ``capillary origami'' experiment. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 2:15PM |
B5.00005: Folding and swirling instabilities of viscous fluid threads in microchannels Invited Speaker: We study the behavior of viscous fluid threads formed by hydrodynamic focusing as they are swept along by the flow of a different outer fluid in hard microfluidic channels. By examining pairs of miscible liquids for which interfacial tension is essentially absent, such as silicone oils having different molecular weights, we reveal a rich variety of fluid instabilities that occur at low Reynolds numbers. When a single thread that propagates stably in the center of a straight channel encounters a divergence in the channel's width, the thread simply dilates if its viscosity is similar to that of the outer fluid. However, due to the extensional flow and deceleration in the diverging channel, a thread that is sufficiently viscous becomes unstable and reduces energy dissipation by performing sinuous bending oscillations, or `folding', rather than dilating. By tuning the flow rates, we reveal a novel period-doubling route to chaotic folding. The folding and stretching of a thread in a diverging channel provides a simple means of mixing viscous liquids and creating controlled viscosity gradients. Moreover, using a sequence of two cross-channels, we make a pair of viscous threads that become unstable when swept along near the walls of a straight channel as a result of the viscous torque induced by the velocity gradient. The amplification of lateral undulations ultimately causes the threads to break up and form an array of viscous swirls, the miscible counterparts of droplets. This swirling instability provides a means for producing discrete and uniform ephemeral swirls, the miscible counterpart of droplets. By injecting three different miscible liquids into a dual cross-channel geometry, we examine the complex patterns that form when several fluid instabilities interact and compete. Overall, we anticipate that these measurements will provide important insight into the behavior of flowing threads in which interfacial tension plays a more substantial role. [Preview Abstract] |
Session B6: Quantum Simulation and Quantum Information Theory in Cold Atoms
Sponsoring Units: GQI DAMOPChair: Lincoln Carr, Colorado School of Mines
Room: Morial Convention Center RO4
Monday, March 10, 2008 11:15AM - 11:51AM |
B6.00001: Cold atoms in 1D Invited Speaker: Ultracold atoms in optical lattices present a wealth of phenomena in one-dimensional geometries. In this talk, I will focus on specifically one-dimensional phenomena such as spin-charge separation in fermionic systems and the bosonic counterpart, as well as models of collective magnetism realized in ultracold atom systems. Special emphasis will be given to the observation of coherent quantum dynamics far from equilibrium which we can now simulate using the time-dependent density-matrix renormalization group method, in order to use cold atoms to address questions of relaxation to steady states in interacting quantum systems. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:27PM |
B6.00002: Probing Phase Transitions in Cold Atoms Invited Speaker: In this talk I will describe various interferometric probes, which can be used to study correlation functions of low-dimensional cold atom systems. These probes allow one to analyze both properties of phases with long or quasi-long range order and phase transitions. I will suggest the way of detecting fermionic superfluidity and the symmetry of the pairing gap. I will also discuss connections between interferometry in 1D bosonic systems with partition functions of some condensed matter models and with extreme value statistics. Finally I will describe the shot noise and argue how one can suppress it both for bosons and fermions using optical lattices. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 1:03PM |
B6.00003: Quantum Control with Ultracold Atoms Invited Speaker: Ultracold atoms have long been considered as a platform for quantum information processing. Of critical importance is the ability to coherently control both internal degrees of freedom such as spin and also interactions between atoms that depend on ``external'' motional degrees of freedom. In this talk I will review a variety of advances, including the ability to perform qudit operations such as state preparation and unitary gates, quantum-state reconstruction via continuous measurement, and cooling of atomic motion without decohering spin qubits. Two different platforms will be presented -- alkali atoms transported in a lattice with microwave-induced spin-flips, and alkaline-earth atoms in which quantum information is stored in nuclear spins. Microwave-induced spin flips provide a robust mechanism for inducing cold collisions between atoms that can form the basis of a quantum logic gate. As an alternative, cold alkaline-earth atoms are attractive since the ground state is a closed shell, with zero electron angular momentum. The nuclear spin is thus decoupled from the system and can acts as robust decoherence-free qubit. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:39PM |
B6.00004: Imaging single atoms in a three dimensional array Invited Speaker: We have demonstrated trapping and imaging of 250 single atoms in a 3D optical lattice. The 5 micron lattice spacing is large enough that individual atoms can be addressed using lasers and microwaves in a way that does not affect the quantum coherence of other atoms. Our goal is to use these trapped atoms as qubits. So far, we fill a random half of the lattice sites, but a combination of site-selective state changes and state-selective lattice translations should allow us to verifiably fill all vacancies. We will describe our experiments to date and our plans for entangling atoms and implementing a neutral atom quantum computer. Our lattice can readily be scaled to include thousands of trapped atoms. This work was performed in collaboration with Karl Nelson and Xiao Li. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 2:15PM |
B6.00005: Atomtronics Invited Speaker: We explore the analogy between ultracold atoms in optical lattices and electrons in crystal lattices. Of particular interest is atomtronics, where the analogy is extended to include electrical circuits and doped semiconductor materials. Lattice ``defects'' achieve behaviors similar to P-type and N-type semiconductor materials. Naturally the interest is to adjoin P-type and N-type atoms lattices to produce an atom diode, and then an NPN or PNP lattice ``sandwich" to achieve bipolar transistor-like behavior for ultracold atoms. [Preview Abstract] |
Session B7: Gene Regulation
Sponsoring Units: DBP GSNPChair: Sima Setayeshgar, Indiana University
Room: Morial Convention Center RO5
Monday, March 10, 2008 11:15AM - 11:51AM |
B7.00001: Max Delbruck Biological Physics Prize Talk: The Biophysics of Gene Regulation, Studied One Molecule at a Time Invited Speaker: Advances have led to a new field, dubbed single molecule biophysics. Prominent among the new technologies is the optical trap, or `optical tweezers.' Sensitive systems for measuring force and displacement in optical traps permit the nanomechanical properties of individual macromolecules to be explored with unprecedented precision, revealing behaviors heretofore obscured by ensemble-based approaches. This talk will focus on some of our current work with single-molecule systems, including transcription by RNA polymerase and structural transitions in nucleic acids. We developed high-resolution instrumentation that has broken the nanometer barrier and is thereby able to detect displacements down to the atomic level, in aqueous buffer at room temperature. Consequently, we can monitor the motions of RNA polymerase molecules in real time as these step from base to base along DNA. On the practical side, base-pair resolution makes it possible to sequence DNA in a new way, based on enzyme motions, and points to new directions in nanoscience. The improved stability afforded by the current generation of optical trapping apparatus has allowed us to reconstruct the complete energy landscapes for folding transitions in nucleic-acid hairpins. Recently, we have turned our attention to the problem of co-transcriptional folding, aptamers, and riboswitches formed in nascent mRNAs, and to the DNA or RNA sequence elements that regulate expression. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:27PM |
B7.00002: Using DNA mechanics to predict intrinsic and extrinsic nucleosome positioning signals Invited Speaker: In eukaryotic genomes, nucleosomes function to compact DNA and to regulate access to it both by simple physical occlusion and by providing the substrate for numerous covalent epigenetic tags. While nucleosome positions in vitro are determined by sequence alone, in vivo competition with other DNA-binding factors and action of chromatin remodeling enzymes play a role that needs to be quantified. We developed a biophysical, DNA mechanics-based model for the sequence dependence of DNA bending energies, and validated it against a collection of in vitro free energies of nucleosome formation and a nucleosome crystal structure; we also successfully designed both strong and poor histone binding sequences ab initio. For in vivo data from S.cerevisiae, the strongest positioning signal came from the competition with other factors rather than intrinsic nucleosome sequence preferences. Based on sequence alone, our model predicts that functional transcription factor binding sites tend to be covered by nucleosomes, yet are uncovered in vivo because functional sites cluster within a single nucleosome footprint and thus make transcription factors bind cooperatively. Similarly a weak enhancement of nucleosome binding in the TATA region becomes a strong depletion when the TATA-binding protein is included, in quantitative agreement with experiment. Our model distinguishes multiple ways in which genomic sequence influences nucleosome positions, and thus provides alternative explanations for several genome-wide experimental findings. In the future our approach will be used to rationally alter gene expression levels in model systems through redesign of nucleosome occupancy profiles. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 1:03PM |
B7.00003: Towards a Quantitative Understanding of Single-Gene Transcription Invited Speaker: The transcription of the genetic information in DNA into RNA is the first step in protein synthesis. This process is highly regulated and is carried out by RNA polymerase (RNAP), a complex molecular motor. Here we discuss some of the consequences of a Brownian ratchet model of transcription, which incorporates internal structural degrees of freedom of RNAP and kinetic barriers to backtracking of RNAP resulting from steric clashes with co-transcriptionally folded RNA. This approach was previously used (a) to successfully predict sequence dependent positions of pauses during the elongation process [1,2]; (b) to study the behavior of a number of mutants of RNAP, with different elongation behaviors, believed to involve different internal motions of the enzyme [3]; and (c) to gain insight into the interpretation of single-molecule transcription elongation experiments [2]. The same model can be used to characterize the stability of the elongation complex at specific termination sequences, places along DNA where, with high probability, RNAP releases the RNA transcript and disengages from the template. Recent experimental results on termination reinforce a picture of the elongation complex as a flexible structure, not a rigid body [4]. In more general terms, some of the modeling to be presented raises fundamental issues related to ``model comparison'' and ``model selection,'' the problem of identifying and characterizing quantitative models on the basis of limited sets of experimental data [5]. \newline \newline [1] Tadigotla V. R., \'O Maoil\'eidigh D., Sengupta A. M., Epshtein V., Ebright R. H., Nudler E., Ruckenstein A. E., Thermodynamic and Kinetic Modeling of Transcriptional Pausing. \it{Proc Natl Acad Sci U S A}\rm,103:4439-4444 (2006). \newline [2] D. \'O Maoil\'eidigh, Ph.D. Thesis, Rutgers University, 2006 \newline [3] Bar-Nahum, G., Epshtein, V., Ruckenstein, A. E., Rafikov, R., Mustaev, A. and Nudler E., A Ratchet Mechanism of Transcription Elongation and its Control. \it{Cell,} \rm120:183-193 (2005). \newline [4] Epshtein, V., Cardinale, C.J., Ruckenstein, A.E., Borukhov, S., and Nudler, E., An Allosteric Path to Transcription Termination. \it{Mol. Cell,}\rm28; 991-1001 (2007). \newline [5] Vasisht R. Tadigotla, Ph.D. Thesis, Rutgers University, 2006 [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:39PM |
B7.00004: Information flow and optimization in transcriptional regulation Invited Speaker: In the simplest view of transcriptional regulation, the expression of a gene is turned on or off by the changes in the concentration of a transcription factor (TF). Here we analyze transcriptional regulatory elements with the tools of information theory. Recent data on noise levels in gene expression are used to show that it should be possible to transmit much more than just one regulatory bit. Realizing this optimal information capacity would require that the dynamic range of TF concentrations used by the cell, the input/output relation of the regulatory module, and the noise levels of binding and transcription satisfy certain matching relations. This parameter-free prediction is in good agreement with recent experiments on the Bicoid/Hunchback system in the early Drosophila embryo, and this system achieves around 90\% of its theoretical maximum information transmission. The dependence of information capacity on parameters that govern gene expression for simple, single-input / single-output, genetic regulatory elements is systematically examined and the extensions of the work to genetic circuits consisting of several interacting elements are presented. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 2:15PM |
B7.00005: The Genomic Code for Nucleosome Positioning Invited Speaker: Eukaryotic genomes encode an additional layer of genetic information, superimposed on top of the regulatory and coding information, that controls the organization of the genomic DNA into arrays of nucleosomes. We have developed a partial ability to read this nucleosome positioning code and predict the in vivo locations of nucleosomes. Our results suggest that genomes utilize the nucleosome positioning code to facilitate specific chromosome functions including to delineate functional versus nonfunctional binding sites for key gene regulatory proteins, and to define the next higher level of chromosome structure itself. [Preview Abstract] |
Session B8: Colloidal Self-Assembly II
Sponsoring Units: DFDChair: Eric Weeks, Emory University
Room: Morial Convention Center RO6
Monday, March 10, 2008 11:15AM - 11:27AM |
B8.00001: Orthogonal Tracking Microscopy for Nanofabrication Research Matthew McMahon, Andrew Berglund, Peter Carmichael, Jabez McClelland, J. Alexander Liddle Constructing 2D lateral particle trajectories from digital video sequences of nanoparticle motion in a liquid is straightforward and fairly common, requiring only the use of centroid-finding algorithms. On the other hand, extracting particle trajectories in the third (out-of-plane) dimension has been more difficult, requiring detailed calibration of the radius of the defocused diffracted rings which result from vertical fluctuations of particle position. We introduce a new technique, termed orthogonal tracking microscopy or orthogonal projection microscopy, in which integrated micromirrors produce one or more reflected images of a particle within the same field of view as the direct image. The reflected images project vertical motion into lateral motion. Thus, we are able to construct a fully 3D particle trajectory from 2D digital video using only centroid-finding algorithms. We use this technique to study particle-surface interactions relevant to directed assembly of nanoparticles. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B8.00002: Self-Assembly of Colloidal Membranes Edward Barry, Zvonimir Dogic Symmetric monolayer membranes are observed to self-assemble in a colloidal suspension of hard rods with soft attractions. This attractive component to the interaction is enough to drive the self-assembly of stable two dimensional fluid-like surfaces of rods. Simultaneous measurements are made at both the molecular, via direct imaging of individual fluorescently labeled particles, and the continuum length scales. At the continuum scale, the elastic Hamiltonian for a two dimensional fluid-like surface is verified for a symmetric monolayer, and measured material constants such as the bending modulus and the area compression modulus are demonstrated to obey a simple elastic relationship. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B8.00003: Self-Assembled 3D Ordered Macroporous Structures for Tissue Engineering Scaffolds Wen-tau Juan, Kuo-yuan Chung, Narayan Mishra, Keng-hui Lin A simple, inexpensive and fast microfluidic method to fabricate three-dimensional ordered macroporous gel is demonstrated using alginate as the scaffold material. The microfluidic device consists of two concentric micropipettes where one is nested inside the other. Nitrogen gas and aqueous alginate solution with Pluronic F127 are pumped through the inner and the outer channel respectively. Under appropriate conditions, bubbles of a uniform size are generated within the device at few thousand Hz. We show the control over bubble size by the gas pressure and quantitatively predict the size dependence from the geometry of fluidic device. Monodisperse bubbles are collected and self-assemble into crystal structures as wet foam. The alginate solution between bubbles is crosslinked by divalent calcium ions and turns into 3D ordered macroporous gel where the pores are highly interconnected. The pore size can be directly controlled by the bubble size which ranges from few tens microns to few millimeters. This technique promises a versatile and robust way to make 3D ordered tissue engineering scaffolds. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B8.00004: Five-fold attractor in two-dimensional diffusion processes. Guillermo Ramirez-Santiago, Carlos I. Mendoza We introduce an algorithm to generate two-dimensional diffusion-limited star-branched polymers (DLSP) attaching monomers successively to a central colloidal particle with any desired number of reactive sites. The proposed algorithm produces star-shaped aggregates whose final structure at relatively large distances from the central colloid has five-fold symmetry independently of the initial number of reactive sites. Therefore, the final morphology can be considered as a universal attracting distribution for this irreversible diffusion-limited aggregation process. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B8.00005: Template-Guided Langmuir-Blodgett Deposition of Colloidal Particles Jaehyun Hur, You-Yeon Won We present a new method of fabricating highly-ordered two-dimensional (2D) colloid crystals with non-closed-packed symmetries. In this method, using the Langmuir-Blodgett (LB) monolayer deposition technique, we transfer a Langmuir monolayer of colloidal particles constructed at the air-water interface onto a substrate which contains micro-fabricated topological patterns. We demonstrate that by using this template-guided LB deposition method, a perfect single 2D colloid crystal structure that is homogeneous throughout the entire area of the patterned substrate can be economically fabricated under appropriate LB processing conditions. We investigate the effects of various control parameters (such as the initial particle density at the air-water interface, the substrate lifting speed, and the humidity condition during the LB monolayer deposition) on the structural properties of the resultant LB colloid monolayer. As the compression area or the lifting speed is increased, the average density of the deposited particles in the resultant LB colloid monolayer becomes reduced. The evaporation of water causes an undulation in the deposited particle density profile along the substrate lifting direction. We present a theoretical model which can quantitatively explain all these experimental observations. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B8.00006: Clustering in Hard Core/Soft Shoulder Lattice Gas Models Paul D. Beale, Charles A. Sievers, Matthew A. Glaser Isotropic hard core/soft shoulder interacting particle models have been shown to display a wide variety of thermodynamic phases: structured liquids, micellar solids, layered and columnar liquid crystals, and a variety of modulated solid phases. We have explored the phase diagram of a class of lattice gas models that are designed to approximate continuum models. We use generalizations of Baxter's hard hexagon model on a two-dimensional hexagonal lattice to model the hard core repulsions. The longer-ranged repulsive soft shoulder is included to induce a Klein/Likos clustering instability. The clustering instability creates softly interacting fluidic micelles, as well as several type of modulated solid phases. The lattice gas model allows for efficient Monte Carlo simulation in order to quickly explore the phase diagram. Two dimensional lattice gas models typically only display liquid phases with short-range order and solids with long-range order that is commensurate with the underlying lattice. Preliminary results indicate the model exhibits soft solid phases composed of fluidic micelles that form a quasi-long ranged solid phase characteristic of continuum solid phases in two dimensions. We will also present a mean field theory analysis of the initial clustering instability. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B8.00007: Elastic Theory of Defects in Toroidal Crystals Luca Giomi, Mark Bowick Crystalline assemblages of identical sub-units packed together and elastically bent in the form of a torus have been found in the past ten years in a variety of systems of surprisingly different nature, such as viral capsids, self-assembled monolayers and carbon nanomaterials. We investigate the structural properties of toroidal crystals and we provide a unified description based on the elastic theory of defects in curved geometries. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B8.00008: Two-dimensional hopping of aqueous colloidal clusters on commensurate surface wells. Minsu Kim, Stephen Anthony, Steve Granick Hopping of colloidal clusters in various shapes and sizes that are mainly confined within commensurate surface wells except for diffusing between them by Brownian motion is studied. The mobility of clusters decreases nonmonotonically with increasing cluster size. The mobility proceeds, depending on cluster shape, by different jumping mechanisms such as zigzagging or translation without rotation; this produces nonmonotonic changes of mobility when, at fixed cluster size, cluster shape varies. Unlike atomic clusters that change configuration and dissociate easily, these colloidal clusters are very stable and each type of jump can be identified separately. Hopping rate, diffusion and different jumping mechanisms that are associated with them will be discussed for various sizes and shapes of clusters. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B8.00009: Complexity from Specificity: Light Scattering and Colloidal Studies of Dscam Self-Association Jesse Collins, Natalie Arkus, Guangnan Meng, Michael Brenner, Dietmar Schmucker, Vinothan Manoharan The self-assembly of complex structures from nanometer-sized building blocks is of great technological importance(i.e. for the development of tissue scaffolds and photonic crystals) and is of significant basic scientific interest. Here I present light scattering and colloidal aggregation studies of Dscam, a protein with over 18,000 splice variants which all (or almost all) exhibit exclusively homophilic binding, and which is necessary for the generation of structural complexity in the brain of insects. Static and dynamic light scattering data reveal the statistical mechanical properties of Dscam self-association, including the free energy, second virial coefficient, and oligomer molecular weight. Finally, I demonstrate how to exploit Dscam's unprecedented level of molecular diversity and specificity for the self-assembly of custom nano- and micro-structures out of Dscam-conjugated colloids. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B8.00010: Non-spherical Depletants in Colloidal Suspensions Stephen Barr, Erik Luijten We investigate the effective interactions between spherical colloids induced by rigid rod-like depletants. The size disparity between the colloids and the rods makes conventional simulation methods inefficient. We overcome this by extending the generalized geometric cluster algorithm for colloidal suspensions [J. Liu and E. Luijten, Phys.\ Rev.\ Lett. \textbf{92}, 035504 (2004)] to systems of non-spherical particles. We investigate both uncharged and charged colloids and rods, where the electrostatic potential is modeled through a screened interaction. The dependence of the induced depletion potential on both the strength and the range of the electrostatic interactions is quantified. In case of a rod-sphere repulsion, the depletion attraction between the colloids is enhanced as the screening length becomes larger, owing to the increased effective size of the rods. Systems with a rod-sphere attractions are also explored. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B8.00011: Magnetically assembled ``ring-shaped'' colloidal particle structures Hui Son, Randall Erb, Bappaditya Samanta, Vincent Rotello, Benjamin Yellen We demonstrate a convenient method for assembling ring-shaped colloidal structures by applying uniform magnetic field to a mixture of 2.7-$\mu$m paramagnetic beads, 1-$\mu$m non-magnetic polystyrene beads, and a fluid dispersion of 10-nm iron oxide nanoparticles (i.e., ferrofluid). The ferrofluid serves as a magnetic contrast medium and induces dipole moments in both the paramagnetic and non-magnetic beads when an external magnetic field is applied. We discovered that for certain volume fractions of ferrofluid, the attractive forces generated between the smaller non-magnetic beads and the larger magnetic beads induce the non-magnetic particles to form a ring structure around the circumference of the paramagnetic beads. This method differs from similar self-assembly techniques in that the ring structures form solely through magnetic force, rather than depending on random motion and patterned bonding. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B8.00012: Effects of surface biotin density on lipid monolayer-assisted 2D crystallization of streptavidin at the aqueous solution-vapor interface Masafumi Fukuto, Matthew Lohr, Suntao Wang, Sumit Kewalramani, Lin Yang Adsorption and two-dimensional (2D) crystallization of soluble protein streptavidin on a biotinylated lipid monolayer at an aqueous solution-vapor interface have been studied extensively since the 1990s. These previous studies, largely based on fluorescence microscopy and \textit{ex-situ} electron microscopy measurements, revealed the effects of protein modifications and aqueous buffer conditions, such as pH and ionic strength. We have examined the dependence of 2D streptavidin crystallization on the areal biotin density in the lipid monolayer template, using Brewster-angle microscopy (BAM) and \textit{in-situ} x-ray reflectivity and grazing-incidence x-ray diffraction (GID). The lipid monolayer consisted of a binary mixture of DMPC and DPPE-x-biotin, and the biotin density was controlled by varying the lipid composition while keeping the area per lipid fixed. Both BAM and GID results demonstrate that in order for 2D crystallization of streptavidin to occur, the surface biotin density must exceed a threshold, corresponding to approximately two biotins per protein. The results highlight the importance of well-defined molecular orientations to the 2D crystallization of proteins. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B8.00013: Self-Assembly of 2D TMV Arrays on Substrate-Supported and Langmuir Lipid Monolayers Suntao Wang, Antonio Checco, Zhongwei Niu, Qian Wang, Masafumi Fukuto, Lin Yang Bionanoparticles (large proteins, viruses) are ideal building blocks for creating ordered two-dimensional (2D) arrays. These 2D protein crystals or ordered arrays are of great scientific and technological interest. Here, we demonstrate the use of in-situ x-ray scattering and Brewster angle microscopy (BAM) to monitor the formation of self-assembled, 2D ordered arrays by tobacco mosaic viruses (TMVs) on a lipid layer that was either supported by a solid substrate or formed at the liquid-vapor interface. The lipid monolayer not only confined the viral particles within a plane, but also provided the lateral mobility that is crucial for developing structural order. In-situ X-ray scattering was used to provide real time information on the structure of the virus array and guide optimizations of the surrounding chemical environment to improve in-plane structural order. The presence of Ca$^{2+}$ ions is also essential to the formation of well ordered, closely packed 2D arrays of TMV. Atomic Force Microscopy was also used to directly image the final structure to provide real space confirmation of developed structural order. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B8.00014: Non-equilibrium dynamics of virus capsid assembly Artem Levandovsky The process of self-assembly of nano-structures under non-equilibrium conditions has recently received a lot of attention in various fields. A viral shell (capsid) is, for sure, one of the most interesting biological structures that can spontaneously form (from statistical mechanics point of view) at the right pH and ionic strength. While the viral capsids are by far less complex than most other biological objects, the process of virus assembly remains poorly understood. Viruses are found to adopt many different shapes. The mechanisms involved in the self-assembly of capsids into a particular shape as well as the transitions from spherical to non-spherical shells are the subject of this presentation. We show that the kinetic formation of the protein building blocks into the intermediate states (dimmers, trimmers, pentamers and hexamers) can lead to the construction of shells with different morphologies. [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B8.00015: Thermal transport in colloidal silica system: effect of particle size and aggregation Gang Chen, Fan Jiang, Wenhua Yu, Jules Routbort Knowledge of the size and distribution of nanoparticles in solution is critical to understanding the observed enhancements of thermal conductivity in colloidal systems. We have applied small-angle x-ray scattering (SAXS) to study particle size and distribution of monodispersed and aggregated silica colloids. A hot-wire method has been used to measure thermal conductivity of the colloidal system. The results indicate that the thermal conductivity depends not only on the particle concentration, but also on the particle size and distribution. The experimental data contradict thermal transport models based on fluid interfacial layers or Brownian motion but shed light on the detrimental role of liquid-particle interface on the thermal transport properties. [Preview Abstract] |
Session B9: Pattern Formation and Nonlinear Dynamics
Sponsoring Units: GSNPChair: Vladimir Privman, Clarkson University
Room: Morial Convention Center RO7
Monday, March 10, 2008 11:15AM - 11:27AM |
B9.00001: Intermediate State in Type-I Superconducting Strip with Current Jacob Hoberg, Ruslan Prozorov The dynamic structure of the intermediate state was studied in pinning-free Pb strips using real-time magneto-optical visualization. It is found that topological hysteresis can be lifted by applying sufficiently large current. Namely, laminar structure that appears on flux exit in a static case is turned into tubular when the current is present. Temperature, magnetic field and current phase diagram is discussed. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B9.00002: Generation of dynamic self-propelled structures by symmetry breaking mechanism in driven magnetic layers on the surface of liquid Alexey Snezhko, Maxim Belkin, Igor Aranson Magnetic particles suspended over the surface of a liquid and energized by a vertical alternating magnetic field give rise to remarkable dynamic multi-segment magnetic structures (``snakes''). These structures (dynamic by nature) are directly related to surface waves in the liquid generated by the collective response of magnetic microparticles to the alternating magnetic field. The self-assembly process and existence of the magnetic snakes is accompanied by a generation of strong surface flows in the liquid. Properties of the snake and corresponding surface flows could be tuned by the parameters of the external magnetic driving. We demonstrate that above some critical frequency threshold magnetic snakes lose their stability and start to swim in the container. The effect is attributed to the development of symmetry breaking instability of the structure with respect to self-generated surface flows in the liquid. Parameters of the driving magnetic field are effectively used to control rich behavior of the dynamic magnetic swimmers. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B9.00003: Collective dynamics and pattern formation in 2D regular arrays of spherical particles in Stokes flow between two parallel walls Jerzy Blawzdziewicz, Eligiusz Wajnryb, Matthew Baron, Nidhi Khurana We present results of our numerical and theoretical investigations of collective dynamics of linear trains and regular square arrays of spherical particles suspended in a fluid bounded by two parallel walls. The simulations reveal propagation of particle-displacement waves, deformation and rearrangements of a particle lattice, propagation of dislocation-like defects in ordered arrays, and transitions between ordered and disordered regions that can coexist for a long time. We argue that ordered motion of the arrays is associated with the dipolar form of the quasi-2D asymptotic far-field flow produced by the particles. We also show that the overall deformation of the arrays can be described using a macroscopic theory where the array is treated as a 2D effective medium. The theory predicts a fingering instability near the array corners, and this instability is confirmed by our microscopic simulations. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B9.00004: Theory of slope-dependent disjoining pressure with application to Lennard-Jones liquid films Taeil Yi, Harris Wong A liquid film of thickness h $<$ 100 nm is subject to additional intermolecular forces, which are collectively called disjoining pressure P. Since dominates at small film thicknesses, it determines the stability and wettability of thin films. Current theory derived for uniform films gives P=P(h). This solution has been applied recently to non-uniform films and becomes unbounded near a contact line as h-$>$0. Consequently, many different effects have been considered to eliminate or circumvent this singularity. We present a mean-field theory of that depends on the slope as well as the height h of the film.[1] When this theory is implemented for Lennard-Jones liquid films, the new P=P(h,hx) is bounded near a contact line as h-$>$0. Thus, the singularity in P(h) is artificial because it results from extending a theory beyond its range of validity. We also show that the new can capture all three regimes of drop behavior (complete wetting, partial wetting, and pseudo partial wetting) without altering the signs of the long and short-range interactions. We find that a drop with an unbounded precursor film is linearly stable.1] Wu {\&} Wong, J. Fluid Mech. \underline {506}, 157 (2004)2] Yi {\&} Wong, J. Colloid Inter. Sci. \underline {313}, 579 (2007) [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B9.00005: The effects of chaotic mixing on patterns and fronts in an advection-reaction-diffusion system Tom Solomon, Jeffrey Boehmer We present experimental studies of the effects of fluid flows on reaction fronts and spatial patterns in the excitable Belousov- Zhabotinsky reaction. The flow is a square array of vortices, generated using magnetohydrodynamic techniques. Time-dependent forcing of the flow is achieved by displacing the fluid periodically in a circular manner relative to the vortex flow. Mixing of passive impurities in this flow is chaotic, with long- range transport that is typically diffusive (enhanced), although superdiffusion with L\'evy flights is also possible. Reaction fronts in this flow show small-scale patterns that reflect the stable and unstable manifolds that characterize chaotic mixing in this flow. Even on a larger scale, front structure reflects the underlying anisotropy of the vortex lattice. In many cases, the front mode-locks to the external forcing, lining up with the vortex array in a self-correcting manner. Self-generating trigger waves are also found in this system, producing both spiral and target patterns similar to those found in reaction- diffusion systems. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B9.00006: Spatial Forcing in Thermal Convection Gabriel Seiden, Stephan Weiss, Eberhard Bodenschatz An intrinsic characteristic of thermal convection is the preference of a particular wavenumber at onset. This fact renders experimental investigations of different important aspects, such as the exploration of phase space, difficult. The main tool employed to overcome this difficulty is spatial forcing, whereby, using external means, one forces a desired pattern and observes the evolution of the system thereafter. We present results on topologically and optically induced modulations of large aspect ratio isotropic (horizontal) and anisotropic (inclined layer) Rayleigh-B\'{e}nard convection. These include detailed bifurcation curves and phase diagrams for different forcing scenarios. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B9.00007: Renormalization group method for predicting frequency clusters in a chain of nearest-neighbor Kuramoto oscillators. Oleg Kogan, Gil Refael, Michael Cross, Jeffrey Rogers We develop a renormalization group (RG) method to predict frequency clusters and their statistical properties in a 1-dimensional chain of nearest-neighbor coupled Kuramoto oscillators. The intrinsic frequencies and couplings are random numbers chosen from a distribution. The method is designed to work in the regime of strong randomness, where the distribution of intrinsic frequencies and couplings has long tails. Two types of decimation steps are possible: elimination of oscillators with exceptionally large frequency and renormalization of two oscillators bonded by a very large coupling into a single one. Based on these steps, we perform a numerical RG calculation. The oscillators in the renormalized chain correspond to frequency clusters. We compare the RG results with those obtained directly from the numerical solution of the chain's equations of motion. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B9.00008: Pattern morphology and dynamical scaling in the Cahn Hilliard model Timothy Sullivan, Pushkar Dahal, Peter Palffy-Muhoray Numerical simulations were carried out in two-dimensions of the dimensionless Cahn-Hilliard equation. Simulations were run for a factor of ten in time beyond previously reported results. The simulations also covered a broad range of values of the mean composition,$\left\langle \psi \right\rangle _0 $. To determine the dynamical scaling exponent, $b,$ an equation of the form $R_G (t)=at^b+c$ was fit to a measure of average domain size. In contrast to previous results, we found that $b$ varied substantially with$\left\langle \psi \right\rangle _0 $. The largest deviation from the Lifshitz-Slyozov value of 1/3 occurred at $\left\langle \psi \right\rangle _0 =0.15$, where $b=0.221\pm 0.04$. We used a measure of the non-circularity of minority domains to show that for $\left\langle \psi \right\rangle _0 \mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle\sim}\vphantom{_x}}$}} 0.20$ the domain shapes are not scale invariant for times exceeding our simulation times. We also point out the possible existence of a phase boundary $\left\langle \psi \right\rangle _{0,c} $that separates a phase with circular domains of minority component from a phase with non-circular minority component domains. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B9.00009: Experimental Evidence for Mixed Reality States Vadas Gintautas, Alfred Hubler We present experimental data on the limiting behavior of an inter-reality system: a virtual pendulum with a bi-directional instantaneous coupling to its real-world counterpart [Gintautas \& Hubler, Phys.Rev.E 75, 057201 (2007)]. The data show that if the physical parameters of the simplified virtual system are close to the parameters of the real system, there is a phase transition from an uncorrelated dual reality state to a mixed reality state in which the motion of the two pendulums is highly correlated. As virtual systems better approximate real ones, even weak couplings in inter-reality systems may induce sudden transitions to mixed reality states. This phenomenon may be typical for systems with instantaneous coupling and was recently featured on the tip sheet of the American Physical Society [http://www.aps.org/about/tipsheets/tip68.cfm ]. We show that mixed reality states in physical systems are related to out-of- body experiences of humans in 3D-video feedback systems [H. H. Ehrsson, The Experimental Induction of Out-of-Body Experiences. Science 317, 1048 (2007)]. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B9.00010: Classical dynamics simulations on pattern formation by metallic spheres with induced electrostatic interactions Peter Fleck, Alfred Hubler We study classical dynamics simulations of metallic spheres immersed in a highly viscous, but weakly conducting medium while exposed to the electrostatic field of external electrodes of various geometries. We represent the system's charge dynamics by the spheres' multipole moments as induced by the electrodes. We simulate the sphere dynamics for various particle numbers retaining force contributions up to dipole-dipole order. Besides the electrodes' location and the spheres' initial positions, we find the system's dynamics to be governed by the ratio of a spheres motion time scale and a charge dynamics time scale alone. We find the spheres to form line arrangements between opposing electrodes for an important region of parameter space. We determine the phase boundaries of this line formation behavior in the sphere dynamics simulations. We find the phase diagram to be in good agreement with analytical predictions. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B9.00011: Hopping Conduction and Bacteria: Transport Properties of Disordered Reaction-Diffusion Systems Andrew Missel, Karin Dahmen Reaction-diffusion (RD) systems are used to model everything from the formation of animal coat patterns to the spread of genes in a population to the seasonal variation of plankton density in the ocean. In all of these problems, disorder plays a large role, but determining its effects on transport properties in RD systems has been a challenge. We present here both analytical and numerical studies of a particular disordered RD system consisting of particles which are allowed to diffuse and compete for resources ($2A\to A$) with spatially homogeneous rates, reproduce ($A\to2A$) in certain areas (``oases''), and die ($A\to0$) everywhere else (the ``desert''). In the low oasis density regime, transport is mediated through rare ``hopping events'' in which a small number of particles diffuse through the desert from one oasis to another; the situation is mathematically analogous to hopping conduction in doped semiconductors, and this analogy, along with some ideas from first passage percolation theory, allows us to make some quantitative predictions about the transport properties of the system on a large scale. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B9.00012: Efficient Desalination with Fractal Absorbers Martin Singleton, Gregor Heiss, Alfred Hubler A class of Ramified graphs (RG) is introduced as Iterated Function Systems (IFS) to optimally design networks for efficient reverse osmosis desalination in deep seawater. Different forms of the IFS are presented, along with a corresponding contractivity factor $s_c,$ in order to identify the attractors of the systems and their fractal dimension. Using the analogy to electrostatics, the diffusion equation is solved for the desalination systems under three different boundary conditions, i) all nodes having the same pressure difference across the absorbers, ii) all nodes producing permeate at identical rates, and iii) each node having the same salt node strength. Optimal branching angles and branch length ratios are found by phase-space and discrete simulated annealing search techniques for each boundary condition, which either maximize production of permeate or minimize expenditure of energy for different fixed numbers of absorbers. Dependence of desalination recovery ratios on the geometry and fractal dimension of the RG is also explored. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B9.00013: Static and Dynamical Properties of Polar Fluids Girija S. Dubey Molecular dynamics simulations are reported for a system whose pair potential can be separated as the sum of a Lennard-Jones term and a dipole-dipole interaction. The simulations were done for a chosen value of electric dipole moment and for a set of temperatures. Chain formation is observed when the potential is dipolar as well as when both terms are included. However. our simulation shows some differences in the pattern formation of the chains. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B9.00014: Statistical Light in Nonlinear Media as Photonic Plasma Dmitry Dylov, Jason Fleischer We consider the nonlinear propagation of partially-spatially-incoherent light as a photonic plasma. Using wave-kinetic theory and Wigner formalism, we interpret the speckles of statistical light as quasi-particles which can interact via the nonlinearity. We analytically derive a Bohm-Gross dispersion relation for these speckles and their Langmuir-type interaction waves, and identify an effective plasma frequency, effective Debye length, etc. Experimentally, we demonstrate this mapping by studying the nonlinear propagation of diffused light in a self-focusing photorefractive crystal. Observed phenomena include modulation instability, two-stream and bump-on-tail instabilities, wave collapse and optical turbulence. By recording a hologram of the internal dynamics, we observe speckle-wave and wave-wave interactions, in both position (x) space and momentum (k) space. The results generalize ideas from plasma physics, lead to new wave dynamics in nonlinear statistical optics, and allow the experimental study of phenomena that are difficult, if not impossible, to observe in material plasma. [Preview Abstract] |
Session B10: Gaps, Pseudogaps and Fermi Surfaces in Cuprates
Sponsoring Units: DCMPChair: Laura Greene, University of Illinois at Urbana Champaign
Room: Morial Convention Center RO8
Monday, March 10, 2008 11:15AM - 11:27AM |
B10.00001: Revealing pseudogap physics using lanthanide substituted Bi$_{2}$Sr$_{1.6}$Ln$_{0.4}$CuO$_{6+\delta}$ Daniel Garcia, Jeff Graf, Chris Jozwiak, Shuyun Zhou, Hiroshi Eisaki, Alessandra Lanzara Towards understanding the physics of the high-temperature superconducting cuprates, there has been growing interest in the role lattice strain plays between the copper oxide planes. We have examined Bi$_{2}$Sr$_{1.6}$Ln$_{0.4}$CuO$_{6+\delta}$ (Ln=La, Nd, Eu, Bi) near optimal doping using angle resolved photoemission spectroscopy. The increasing radius mismatch of the substituted lanthanide, which monotonically decreases the superconducting Tc, appears to also affect the electronic properties of these system. The effect of strain on Fermi arcs, superconducting gap and pseudogap physics will be discussed. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B10.00002: A Laser Based ARPES Study of the Nodal Region of the Cuprates J.D. Rameau, H.-B. Yang, G.D. Gu, P.D. Johnson A new laser based facility has been constructed at the NSLS for studies of strongly correlated electron systems. While recent studies of the nodal and near-nodal spectrum of the cuprates with low energy lasers appear to show good agreement with higher energy, synchrotron based ARPES, several adverse effects of performing ARPES with lasers remain to be explored. Using the new facility we show that the combination of high emission angles and low electron kinetic energies has many implications for the accurate measurement of any linewidth or band dispersion. We show that this is particularly so for the nodal and near-nodal single particle spectrum of the cuprates. The results and implications of our studies for future laser based ARPES are analyzed and discussed. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B10.00003: Evidence for pairing above Tc from the electronic dispersion in the pseudogap phase of cuprates A. Kanigel, U. Chatterjee, M. Randeria, M.R. Norman, J.C. Campuzano In the BCS paradigm for the superconducting state, electrons close to the Fermi level $E_{F}$ form Cooper pairs which condense into a zero center of mass momentum state. This results in a gap in the electronic excitation spectrum which is symmetrically centered about $E_{F}$. Above $T_{c}$ where the condensate is lost, the pairs dissociate, the energy gap collapses, and the the normal state Fermi surface appears. On the other hand, in the underdoped high temperature superconductors, instead of a complete Fermi surface above $T_{c}$, only disconnected Fermi arcs appear, separated by regions that still exhibit an energy (pseudo)gap. We show that in this pseudogap phase, the energy-momentum relation of electronic excitations near $E_{F}$ behaves like the dispersion of a normal metal on the Fermi arcs, but like that of a superconductor in the gapped regions. We argue that this dichotomy in the dispersion is hard to reconcile with a competing order parameter, but is consistent with pairing without condensation. Below $T_{c}$ the pairs condense and the electronic excitations, that were short-lived above $T_{c}$, become long-lived and exhibit a d-wave energy gap. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B10.00004: Possible Explanation of the Fermi Arcs in Cuprates, based on a clustered superconducting state above Tc Gonzalo Alvarez, Elbio Dagotto A previously introduced Landau-Ginzburg model [1] to describe the competition between antiferromagnetism and d-wave superconductivity in the cuprates is here further investigated. The state above the critical temperature Tc is made of superconducting (SC) clusters, with a nonzero amplitude of the SC order parameter but random phase factors, coexisting with antiferromagnetic(AF) regions. This state disappears above a higher temperature scale T*. The LDOS of this state is in good agreement with recent STM experiments [2]. Our main result is that the angle-resolved photoemission spectrum of this SC-AF clustered state contains Fermi surface arcs in the region Tc$<$T$<$T*, very similar to those observed experimentally [3]. Low energy states created at the interface between clusters are responsible for the arcs. [1] G. Alvarez et al., Phys. Rev. B 71, 014514 (2005). [2] K. K. Gomes et al., Nature 447, 569 (2007). [3] A. Kanigel et al., cond-mat/0708.4099 (2007). [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B10.00005: Understanding the Protected Nodes and the Fermi Arcs in the Cuprate Superconductors Qijin Chen, K. Levin We address a recent analysis of photoemission data which elucidates the superconducting phase of the underdoped cuprates. We first present a simple phenomenological approach to the spectral function which shows how the $d$-wave order parameter symmetry results in protected nodes, which, above $T_c$ broaden into Fermi arcs; this ``protection'' is associated with superconducting coherence rather than reduced thermal broadening. A microscopic theory, consistent with this phenomenology, is presented. It reconciles the observations that the excitation gap below $T_c$ is temperature independent while the superfluid density necessarily vanishes at $T_c$. \\ Reference: Q.J. Chen, K. Levin, and I. Kosztin, Phys. Rev. B \textbf{63}, 184519 (2001). [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B10.00006: Pseudogap correlations inside the superconducting dome Elena Bascones, Belen Valenzuela Recent experiments[1,2] have uncovered, two energy scales, an antinodal energy scale and a nodal one in the superconducting phase of underdoped hole doped cuprates. This finding reminds the nodal-antinodal dichotomy seen in the pseudogap state. The competing Yang-Rice-Zhang[3] scenario for the pseudogap has shown to be useful to understand these experiments[4]. Here we explore the effect of the competition between pseudogap and superconductivity on the condensation energy and superfluid density which show deviations from the standard BCS behavior. We compare the theoretical results with experiments. \newline [1] M. Le Tacon et al., Natur. Phys. 2, 537 (2006). [2] J. Mesot et al., Phys. Rev. Lett. 83, 840 (1999) K. Tanaka et al. Science 314, 1910 (2006); T. Kondo et al. Phys. Rev. Lett. {\bf 98}, 297004 (2007). [3] K-Y Yang, T.M. Rice and F-C Zhang, Phys. Rev. B 73, 174501 (2006). [4] B. Valenzuela and E. Bascones, Phys. Rev. Lett {\bf 98}, 227002 (2007). [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B10.00007: Pseudogap and Superconducting Gap - Same or Different? W.S. Lee, I.M. Vishik, K. Tanaka, R. Moore, D.H. Lu, T. Sasagawa, N. Nagaosa, T.P. Devereaux, Z. Hussain, Z.X. Shen The pseudogap state in underdoped cuprates has been one of the central questions in high-$T_{C}$ research. Recently, whether pseudogap and supercondicting gap are same energy gap or two different energy scales is strongly debated in data interpretation of single-particle spectrum, such as ARPES and STM. To gain further insight into this issue, detailed doping dependence and temperature dependence of the gap were studied using ARPES. In contrast to the behavior of the well-known pseudogap in the antinodal region, we found that the behavior of the gap is qualitatively different near the nodal region, a momentum space region overlooked in the previous measurements. This gap seems closely related to the superconductivity; it opens up at $T_{C}$ and reduces with decreasing doping in the deeply underdoped region following the trend of the superconducting dome in the heavily underdoped region of the phase diagram. The emerging two-gap phenomenon points to a picture of richer quantum configurations in high-$T_{C}$ superconductors. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B10.00008: Different temperature evolution of electronic states in superconducting state and normal state in underdoped Bi2212 high-Tc superconductor Kiyoshisa Tanaka, W.S. Lee, D.H. Lu, R. Moore, T. Sasagawa, Z. Hussain, Z.-X. Shen One of the most mysterious issues in high-Tc superconductor is an energy gap called ``pseudogap'' well above Tc, which exists over a wide region of compositions and temperatures. The origin of this pseudogap and its relation to the superconducting gap are believed to hold the key for understanding the mechanism of high-Tc superconductivity. Recent angle-resolved photoemission spectroscopy (ARPES) revealed the coexistence of two distinct energy gaps in heavily underdoped samples which have opposite doping dependence [1]. One gap can be assigned as pseudogap and the other gap as superconducting gap because of the positive correlation between the gap magnitude and Tc. This result suggests that pseudogap arises from another mechanism and gives profound implications on the mechanism of high-Tc superconductivity. More recently, this two gap feature has been observed in the temperature dependence even in near optimally doped samples [2]. Detailed temperature dependence of ARPES spectra will be shown and the special character of pseudogap state will be discussed. [1] K. Tanaka et al., Science, 314, 1910 (2006). [2] W.S. Lee et al., Nature, 450, 81 (2007). [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B10.00009: Temperature and doping dependent ARPES study of the gaps in Bi2212 H.-B. Yang, J.D. Rameau, P.D. Johnson, T. Valla, G.D. Gu, A.T. Tsvelik High-resolution angle-resolved photoemission (ARPES) is used to probe the development of the gaps around the Fermi surface in Bi2212. A new method of data analysis is presented to remove the complications associated with the experimental resolution. Normalizing by the Fermi function then allows the observation of both the occupied and unoccupied states. The results on the temperature and doping dependence of the gap show that the underdoped system in the normal state behaves differently from all region of the phase diagram in the superconducting state, and point to potentially different origins for the pseudogap. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B10.00010: Fermi arcs and phase diagram of the high-T$_{c}$ cuprates: Insights from Raman and angle resolved photoemission spectroscopies. James Storey, Jeffery Tallon, Grant Williams We calculate the B$_{1g}$ and B$_{2g}$ Raman responses of Bi-2212 from an ARPES-derived energy momentum dispersion and a model for the normal-state pseudogap. In light of these calculations, the Raman data demonstrates that the Fermi arc length remains finite in the pseudogap ground state. A re-examination of recent ARPES results is found to confirm this result. In addition, the presence of a van Hove singularity in the overdoped regime, as revealed by ARPES, allows us to propose a universal pairing potential that reproduces both the doping dependence of T$_{c}$, as well as the variation in T$_{c,max}$ between different species of cuprate superconductors. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B10.00011: Hall Effect Signature of Fermi Surface Reconstruction in High-$T_{c}$ Superconductors F.F. Balakirev, J.B. Betts, A. Migliori, I. Tsukada, Yoichi Ando, G.S. Boebinger The doping dependence of the Hall number in the normal state of two different HTS systems, La$_{2-x}$Sr$_{x}$CuO$_{4}$ and Bi$_{2}$Sr$_{2-x}$La$_{x}$CuO$_{6+\delta }$, exhibits an anomalous peak at optimum doping that emerges only at low temperatures. With increasing hole doping, as the pseudogap energy scale decreases, the peak onset is ascribed to the emergence of electron-like Fermi pockets in the Brillouin zone. The destruction of the peak beyond optimum doping suggests the destruction of the electron pockets and emergence of a large hole pocket, two phenomena that would result simultaneously with the loss of the Brillouin zone folding associated with the pseudogap state. The low temperature Hall resistance thus elucidates the Fermi surface evolution in the HTS cuprates, characterized by a zero temperature phase transition upon collapse of the pseudogap near optimum doping. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B10.00012: Energy gaps in failed superconductor La$_{1.875}$Ba$_{0.125}$CuO$_{4}$ R.-H. He, K. Tanaka, S.-K. Mo, T. Sasagawa, M. Fujita, N. Mannella, K. Yamada, Z. Hussain, Z.-X. Shen By angle-resolved photoemission spectroscopy with improved energy and momentum resolution, we find in the normal state of La$_{2-x}$Ba$_{x}$CuO$_{4}$ x = 1/8 a strong existence of the nodal quasi-particle together with a $d$-wave energy gap along the underlying Fermi surface extending over a significant range in the momentum space before an abrupt take-off of the gap close to the antinodal region. This suggests the presence of a novel nodal metal state, which is different from the one proposed that assumes a single $d$-wave extension of the pseudogap from the antinode toward the node along the whole underlying Fermi surface. This state is compatible with the static stripe ordering but only involves a precursor pairing of the electrons away from the antinodal region. We argue that the traditional pseudogap defined exclusively for the antinodal states has a distinct origin than its new nodal counterpart, i.e., a $d$-wave gap above T$_{c}$. Moreover, this normal state gap function is found to be quantitatively very similar with those of La$_{2-x}$Sr$_{x}$CuO$_{4}$ x$\sim $1/8 (T$_{c}<<$4K) in the superconducting state, pointing to a universal doping dependence of the pairing strength for La-based cuprates, which also highlights the inherent lack of a global phase coherence in La$_{2-x}$Ba$_{x}$CuO$_{4}$ x = 1/8 that makes it a failed superconductor. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B10.00013: Quantum Oscillations in the mixed state of d-wave superconductors Ashot Melikyan, Oskar Vafek We show that the low-energy density of quasiparticle states in the mixed state of ultra-clean $d$-wave superconductors is characterized by pronounced quantum oscillations in the regime where the cyclotron frequency $\hbar\omega_c\ll \Delta_0$, the $d-$wave pairing gap. Such oscillations as a function of magnetic field $B$ are argued to be due to the internodal scattering of the $d$-wave quasiparticles near wavevectors $(\pm k_D,\pm k_D)$ by the vortex lattice as well as their Zeeman coupling. The periodicity of the oscillations is set by the condition $k_D [hc/(eB)]^{1/2} \equiv k_D' [hc/(eB')]^{1/2}\pmod {2\pi}$. We find that there is additional structure within each period which grows in complexity as the Dirac node anisotropy increases. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B10.00014: Quantum Oscillations in the Underdoped Cuprate YBa$_{2}$Cu$_{4}$O$_{8}$ Edward Yelland, John Singleton, Chuck Mielke, Neil Harrison, Fedor Balakirev, Bogdan Dabrowski, Marcin Matusiak, John Cooper The quantum oscillations (QOs) seen in the underdoped cuprate superconductor YBa$_{2}$Cu$_{4}$O$_{8}$ (Y124) in magnetic fields up to $B$=85T [1] are strong evidence for a well-defined Fermi surface (FS) in Y124 at low temperature $T$ and high $B$. The QO frequency $F$=660$\pm $15T, implies a FS pocket with 2.4{\%} of the full Brillouin zone (BZ) area. Taken with earlier work [2], our data suggest FS pockets are generic to underdoped CuO$_{2}$ planes and give the first hint of doping dependence of the FS. We discuss the carrier concentration implied by the QOs within various models. Comparison of the T-linear specific heat $\gamma $ (from QO quasiparticle mass m*) to $\gamma $ estimated from zero-field specific heat measurements constrains the number of FS pockets present in the BZ and supports a reduced BZ due e.g. to a charge/spin density wave or ordered orbital currents. [1] E. A. Yelland et al, arXiv:0707.0057. [2] N. Doiron-Leyraud et al, Nature 446, 565 (2007) [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B10.00015: Fermi surface pockets in the underdoped cuprate YBa$_{2}$Cu$_{4}$O$_{8}$, - are they present in low magnetic fields? J. R. Cooper, M. Matusiak, J. W. Loram, E. A. Yelland, B. Dabrowski The observations of quantum oscillations in the underdoped cuprate superconductors, ortho-II YBa$_{2}$Cu$_{3}$O$_{6.5}$ [1] and YBa$_{2}$Cu$_{4}$O$_{8}$ (Y124) [2] at very high magnetic fields and low temperatures could lead to improved understanding of cuprate superconductivity. This will be especially true if the small Fermi surface (FS) pockets are still present at higher temperatures and lower magnetic fields. As pointed out in ref. [2] the pockets appear to have low Fermi energies $\sim $ 300 K, and could therefore give rise to T-dependent magnetic anisotropy in the normal state associated with Landau-Peierls diamagnetism. We report susceptibility anisotropy data for Y124 crystals up to 300 K, and discuss whether these data and zero field heat capacity data, are consistent with the properties of the FS pockets obtained from high field measurements. [1] N. Doiron-Leyraud, et al., Nature 447,565 (2007) [2] E.A. Yelland et al. arXiv:cond-mat/07070057. [Preview Abstract] |
Session B11: Magnetic Properties of Cuprate Superconductors
Sponsoring Units: DCMPChair: Malcolm Beasly, Stanford University
Room: Morial Convention Center RO9
Monday, March 10, 2008 11:15AM - 11:27AM |
B11.00001: Disappearance of antiferromagnetic spin excitations in overdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ John Tranquada, S. Wakimoto, K. Yamada, C.D. Frost, R.J. Birgeneau, H. Zhang We have used time-of-flight neutron spectroscopy to study magnetic excitations, for energies up to $\sim $100 meV, in overdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x$ = 0.25 and 0.30 [1]. Comparison of spectra integrated over the width of an antiferromagnetic Brillouin zone demonstrates that the magnetic scattering at intermediate energies, 20$<\omega <$100 meV, progressively decreases with overdoping. Previous work has shown that the low-energy magnetic excitations also disappear with overdopoing [2]. This strongly suggests that the magnetism is a vestige of the parent antiferromagnet; spatial segregation of the doped holes, as in the stripe picture, provides a natural way for this to occur. Both the magnetism and superconductivity disappear as the system becomes a homogeneous metal. [1] S. Wakimoto \textit{et al}., Phys. Rev. Lett. \textbf{98}, 247003 (2007). [2] S. Wakimoto \textit{et al}., Phys. Rev. Lett. \textbf{92}, 217004 (2007). [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B11.00002: Spin Susceptibility of Underdoped Cuprates: Insight from Stripe-Ordered $\bf La_{1.875}Ba_{0.125}CuO_4$ Markus H\"ucker, Gen D. Gu, John M. Tranquada The low-temperature decrease of the bulk magnetic susceptibility in underdoped high-temperature superconductors has commonly been cited as evidence for a pseudogap; however, the interpretations range from a Fermi-liquid perspective, with the susceptibility being proportional to the density of free carriers, to strong coupling pictures, with the susceptibility resulting from antiferromagnetic correlations among local moments. Analysis of the susceptibility of a particular cuprate, the stripe ordered $\rm La_{1.875}Ba_{0.125}CuO_4$, sheds new light on this remarkable system and puts tight constraints on possible interpretations. The recently discovered magnetic transition in high magnetic fields will also be discussed.\newline [1] M. H\"ucker, G. D. Gu, J. M. Tranquada, cond- mat/0503417v2.\newline [2] Q. Li, M. H\"ucker, G. D. Gu, A. M. Tsvelik, J. M. Tranquada, Phys. Rev. Lett. 99, 067001 (2007). [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B11.00003: Magnetic correlations on full chains of ortho-II YBCO6.5 Wei Chen, Peter J. Hirschfeld We propose that the $^{63}$Cu NMR line shape of chain Cu in YBCO6.5 Ortho-II samples may be explained by assuming induced magnetization due to oxygen vacancies and strong correlations in the chain. The model consists of CuO chains coupled to CuO2 plane with Hubbard correlations treated in unrestricted Hartree-Fock approxmiation, which induce magnetic moments around oxygen chain vacancies. The broadening and shift of NMR main line is consistent with increased chain magnetization at low temperature. Tunneling between chains and the plane induces magnetization on the planar coppers as well, which gives rise to quasi-1D like spin correlations in the plane. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B11.00004: Normal state magnetic susceptibility measurements in underdoped YBa$_2$Cu$_3$O$_{7-\delta}$ Brigitte Leridon, Philippe Monod, Doroth\'ee Colson Motivated by the observation of staggered magnetic moments by Fauqu\'e\textit{ et al.} [Phys. Rev. Letters 96, 197001 (2006)] using neutron diffraction in underdoped YBa$_2$Cu$_3$O$_{7-\delta}$, we have measured the magnetization of fourteen underdoped YBa$_2$Cu$_3$O$_{7-\delta}$ samples under 1 T from $T_C$ to 400 K. The oxygen contents are ranging from 6.43 to 7.00, and the critical temperatures $T_C$ from about 30 K to 91 K. We present here high resolution susceptibility data corresponding to a sensitivity of $10^{-8}$ $\mu_B$ per Cu atom. We separate the different contributions to the magnetization. We find: i) a small ferromagnetic contribution, visible at all temperatures within our range of measurement and consistent with a few ppm of $Fe_30_4$, ii) A ``1/T'' paramagnetic contribution attributed to a few percent of free Cu ions, iii) The contribution of the ``pseudogap'' which gives a susceptibility increasing with temperature excepted in the optimally doped samples where the susceptibility is flat as expected for Pauli paramagnetism. We discuss the physical implications of this contribution in light of experiments from Fauqu\'e \textit{et al}. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B11.00005: The Persistence of High-Frequency Spin Fluctuations in Overdoped Superconducting La$_{2-x}$Sr$_{x}$CuO$_{4}$ ($x$=0.22) O.J. Lipscombe, B. Vignolle, S.M. Hayden, D.F. McMorrow, T.G. Perring The wavevector and energy dependence of the spin fluctuations in overdoped superconducting La$_{1.78}$Sr$_{0.22}$CuO$_4$ ($T_c$=26~K) were investigated up to high energy (160~meV) by inelastic neutron scattering. Overdoping was found to suppress the strong magnetic response $\chi^{\prime\prime}(\mathbf{q},\omega)$ present in optimally doped La$_{2-x}$Sr$_{x}$CuO$_4$ which is peaked near 50~meV. The magnetic excitations were less dispersive than in the optimal compound and incommensurate at all energies investigated. Overdoping did not appear to weaken the high-frequency $E\ge 100$~meV response, suggesting that significant antiferromagnetic exchange couplings persist well into the overdoped part of the cuprate phase diagram. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B11.00006: Conditions for magnetically induced singlet d-wave superconductivity on the square lattice S.R. Hassan, B. Davoudi, B. Kyung, A.-M.S. Tremblay It is expected that at weak to intermediate coupling, d-wave superconductivity can be induced by antiferromagnetic fluctuations. However, one needs to clarify the role of Fermi surface topology, density of states, pseudogap, and wave vector of the magnetic fluctuations on the nature and strength of the induced d-wave state. To this end, we study the generalized phase diagram of the two-dimensional half-filled Hubbard model as a function of interaction strength $U/t$, frustration induced by second-order hopping $t^{\prime }/t$, and temperature $T/t$. In experiment, $U/t$ and $t^{\prime }/t$ can be controlled by pressure. We use the two-particle self-consistent approach (TPSC), valid from weak to intermediate coupling. D-wave superconductivity can be induced by magnetic fluctuations but only if they are near commensurate wave vectors and not too close to perfect nesting conditions where the pseudogap becomes detrimental to superconductivity. For given $U/t$ there is thus an optimal value of frustration $t^{\prime}/t$ where the superconducting $T_c$ is maximum. The symmetry d$_{x^{2}-y^{2}}$ vs d$_{xy}$ of the superconducting order parameter depends on the wave vector of the underlying magnetic fluctuations in a way that can be understood qualitatively from simple arguments. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B11.00007: Finite size effects in NMR $T_2$ relaxation by vortex vibrations Rachel Wortis, Eric Brown, Sinan Bulut Nuclear magnetic resonance measurements are a powerful probe of electronic behavior in superconductors, but a precise understanding of all relaxation mechanisms is required to draw accurate conclusions. A previous calculation of the rate of transverse relaxation in the cuprate superconductor YBa$_2$Cu$_3$O$_{7-\delta}$ based on an overdamped elastic model of vortex motions predicted relaxation rates orders of magnitude slower than those observed in experiments, despite strong experimental evidence pointing to vortex vibrations as the dominant relaxation mechanism. Here the finite size of the powder grains is explicitly included in the calculation, and we find that both the relaxation rate and the time dependence of the recovery seen in the experiments can be accounted for. The dependence of the relaxation rate on the size and shape of the samples is explored. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B11.00008: Evolution of the spin dynamics in electron-doped high-transition temperature superconductor Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta}$( Tc=24K, 27K) Jun Zhao, Pengcheng Dai, Shiliang Li, Hye Jung Kang, Louis-Pierre Regnault, Paul Steppens, Arno Hiess, Stephen Wilson We use both polarized and unpolarized neutron scattering to study spin excitations in electron doped cuprates Pr$_{0.88} $LaCe$_{0.12}$CuO$_{4-\delta}$(Tc=24K, 27K). We determine the dynamic susceptibility and its temperature dependence for energes between 0.5meV to 30meV in these samples. Our results show that the spectral weight of optimal doped PLCCO (27K) around resonance energy region ($\sim$10meV) is much stronger than that of the slightly underdoped 24K PLCCO. We also demonstrate that using polarized neutrons are necessary to isolate magnetic scattering from nonmagnetic processes. A determination of the evolution of spin excitations in PLCCO as the system is tuned from nonsuperconducting to superconducting states is important to our understanding of the role of magnetism in high-Tc superconductivity. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B11.00009: Incompatibility of Checkerboard Patterns with the Resonance Peak in Cuprate Superconductors Erica Carlson, Daoxin Yao Checkerboard patterns have been proposed to explain STM experiments on the cuprates BSCCO and Na-CCOC. However, simple checkerboard patterns are inconsistent with neutron scattering data, since they have low energy incommsensurate spin peaks rotated $45^o$ from the direction of the charge peaks.[1] Recently, more complicated checkerboard patterns have been proposed in order to reconcile this conflict. In this work, we have studied modulated checkerboards by using spin wave theory and analyzed noncollinear checkerboards as well. We find that the high energy response of the modulated checkerboards is inconsistent with neutron scattering results, since they fail to exhibit a resonance peak at ($\pi,\pi$), which has recently been shown to be a universal feature of cuprate superconductors. Furthermore, noncollinear checkerboards also lack a resonance peak. [2] However, stripe patterns naturally exhibit a resonance peak consistent with neutron scattering data. References: [1] D. X. Yao, E. W. Carlson and D. K. Campbell, Phys. Rev. B 73, 224525 (2006) [2] D. X. Yao and E. W. Carlson, arXiv:0708.1966 [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B11.00010: High field, low temperature 17O Knight shift in an underdoped High-Tc cuprate: La(2-x)Sr(x)CuO(4) for x =0.115, 0.15 Greg Boebinger, Robert Smith, Philip Kuhns, Arneil Reyes, Takashi Imai We use high magnetic fields ($>$30T) to suppress the superconducting Tc in order to investigate the normal state NMR properties of La(2-x)Sr(x)CuO(4) (LSCO) at low temperatures. Recent studies have shown glassy behavior and incommensurate spin-waves in LSCO, which has been discussed as evidencing a stripe ordered picture at low temperatures in the under-doped regime. We use 17O NMR as a local probe of the electron density on the planar oxygens. 17O Knight shift and linewidth were obtained over a wide temperature range in the normal state for under-doped (x=0.115) and optimally doped (x=0.15) LSCO. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B11.00011: Static Magnetic Order in Underdoped HgBa$_{2}$CuO$_{4+\delta }$ Yuan Li, Victor Baledent, Neven Barisic, Philippe Bourges, Yongchan Cho, Benoit Fauque, Yvan Sidis, Guichuan Yu, Xudong Zhao, Martin Greven It is believed by many that understanding the pseudogap phase is essential to understanding the mechanism of high-transition-temperature superconductivity. Here we present the first experimental identification, by polarized neutron diffraction, of an exotic magnetic order in the HgBa$_{2}$CuO$_{4+\delta }$ (Hg1201), which is considered the model high-Tc material with (i) simple tetragonal structure, (ii) large spacing between the CuO$_{2}$ planes, and (iii) the highest Tc among all single-layer compounds [1]. The order parameter develops below a characteristic temperature T$_{mag}$ that nicely agrees with the pseudogap temperature T* determined by DC transport. Our result is highly consistent with the previous work by B. Fauque \textit{et al. }on the bi-layer compound YBCO [2], and can be interpreted within an orbital-current picture that breaks time-reversal symmetry but preserves discrete translational symmetry. The magnitude and the orientation of the magnetic moments of the current loops are experimentally investigated. 1. X. Zhao \textit{et al.}, Adv. Mat. \textbf{18}, 3243 (2006). 2. B. Fauque \textit{et al.}, PRL \textbf{96}, 197001 (2006). [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B11.00012: Quantum Spin Excitations through the metal-to-insulator crossover in YBa$_2$Cu$_3$O$_{6+y}$ Shiliang Li, Zahra Yamani, Hye-Jung Kang, Kouji Segawa, Yoichi Ando, Xin Yao, H.A. Mook, Pengcheng Dai We use inelastic neutron scattering to study the temperature dependence of the spin excitations of a detwinned superconducting YBa$_2$Cu$_3$O$_{6.45}$ ($T_c=48$ K). In contrast to earlier work on YBa$_2$Cu$_3$O$_{6.5}$ ($T_c=58$ K), where the prominent features in the magnetic spectra consist of a sharp collective magnetic excitation termed ``resonance'' and a large ($\hbar\omega\approx 15$ meV) superconducting spin gap, we find that the spin excitations in YBa$_2$Cu$_3$O$_{6.45}$ are gapless and have a much broader resonance. Our detailed mapping of the spin excitations along the $a^\ast$- axis direction reveals a dispersion consistent with the ``hourglass'' like dispersion near the resonance, but the spin excitations are isotropic at lower energies. Since a fundamental change in the low-temperature normal state of YBa$_2$Cu$_3$O$_{6+y}$ when superconductivity is suppressed takes place at $y\sim0.5$ with a metal-to-insulator crossover (MIC), where the ground state transforms from a metallic to an insulating-like phase, our results suggest a clear connection between the large change in spin excitations and the MIC. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B11.00013: Magnetic excitations in the high-temperature superconductors HgBa$_{2}$CuO$_{4+\delta }$ and Nd$_{1.845}$Ce$_{0.155}$CuO$_{4}$ Guichuan Yu, Yuan Li, Eugene Motoyama, Philippe Bourges, Klaudia Hradil, Richard Mole, Martin Greven We report inelastic neutron scattering results for the magnetic excitations in hole-doped HgBa$_{2}$CuO$_{4+\delta }$ (Hg1201) and electron-doped Nd$_{2-x}$Ce$_{x}$CuO$_{4+\delta }$ (NCCO). The magnetic resonance mode has been observed previously in the superconducting state of several hole-doped systems. Recently, this mode has also been claimed to be present in the electron-doped compounds. We found in underdoped Hg1201 (T$_{c}\sim $85 K) the resonance-like feature appearing at rather high energy of 57(2)meV. Surprisingly, the dynamic susceptibility enhancement appears below the pseudogap temperature T$^{\ast }$ and shows no anomaly at T$_{c}$. Unlike recent reports for optimally-doped NCCO and (Pr,La,Ce)$_{2}$CuO$_{4+\delta }$, we found no evidence for a resonance mode in NCCO (x=0.155) in the 7-12 meV range. Instead, we identify two lower-energy features. One is associated with spectral weight redistribution below T$_{c}$ due to the electronic gap 2$\Delta $, the other already present in normal state is likely associated with the significant spin correlations in the electron-doped cuprates. [Preview Abstract] |
Session B12: Organic Conductors and Strongly Correlated 2D Systems
Sponsoring Units: DCMPChair: Victor Yakovenko, University of Maryland
Room: Morial Convention Center 203
Monday, March 10, 2008 11:15AM - 11:27AM |
B12.00001: The metal insulator transition in correlated quasi-one-dimensional organic conductors Claude Bourbonnais, Abdelouahab Sedeki We use a two-loop functional renormalization group approach to calculate the quasi- particle weight along the Fermi surface in the framework of the quasi-one- dimensional electron gas model which includes weak Umklapp scattering at half- filling. The location and evolution of cold and hot spots for electron-electron scattering is described and the Fermi surface reconstruction is clarified as a function of the amplitude of Umklapp scattering. The results are applied to the Mott transition in the Fabre salts series (TMTTF)$_2$X where the emergence of a Fermi surface is found as a consequence of electronic deconfinement under the application of hydrostatic pressure. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B12.00002: Numerical Study of Finite-Temperature Phase Transitions in Quasi-One-Dimensional Molecular Conductors Yuichi Otsuka, Hitoshi Seo, Yukitoshi Motome, Takeo Kato We have theoretically investigated the charge ordering, the dimer Mott, and the spin-Peierls phase transitions in quarter-filled quasi-one-dimensional organic conductors, such as DCNQI$_{2}X$ and TMTTF$_{2}X$, by considering the extended Hubbard model with electron-lattice couplings and inter-chain Coulomb interaction. We apply the stochastic-series-expansion quantum Monte Carlo method to the effective one-dimensional model obtained by the adiabatic and inter-chain mean-field approximation. Temperature dependences of the order parameters and the susceptibilities are calculated for the charge ordering, the dimer Mott, and the spin-Peierls transitions. The results show a competition between the charge-ordered and dimer Mott insulating states, consistent with our previous work [1], and both of them undergo the spin-Peierls transition at low temperatures. There, two types of spin-Peierls phases with spin gap appear in competition with showing different orderings of period four in lattice distortion and charge disproportionation. [1] H. Seo, Y. Motome, and T. Kato, J. Phys. Soc. Jpn. {\bf 76}, 013707 (2007). [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B12.00003: High field ESR on tau phase conductor --from GHz to THz-. Takahisa Tokumoto, J. van Tol, L.-C. Brunel, D. Graf, J.S. Brooks, Y. Oshima, G. Papavassiliou Organic conductors have exotic electronic and magnetic properties ranging from the possibility of unconventional, anisotropic superconductivity, to the observation of a variety of ground states such as charge-density waves, spin-density waves, field-induced spin density waves, and the observation of quantum Hall effect, or a spin-Peierls state. One example is tau - [P-(S, S) -- DMEDT-TTF]2(AuBr2)1+y ($\sim $0.75). The crystal structure of this quasi two dimensional organic compound is tetragonal with unit cell dimensions: a = 7.3546 A and c = 67.977 A. Even though there are no magnetic ions in the system, several measurements indicate magnetic ordering at low temperature and under magnetic fields. Furthermore, this compound exhibits field induced hysteretic MI transition above 38T with an inkling of weak ferromagnetism. We investigate the origin of the magnetic behavior by conducting a CW ESR study from 90GHz to 1.2 THz. Results and analysis will be presented. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B12.00004: First principles study of the charge transfer salt $\kappa$-(BEDT-TTF)$_2$Cu(CN)$_3$ Harald O. Jeschke, Hem C. Kandpal, Roser Valenti The charge transfer salt $\kappa$-(BEDT-TTF)$_2$Cu(CN)$_3$ has attracted a lot of attention due to experimental evidence that it is a realization of a spin liquid: no magnetic ordering was found down to 32 mK. For a good description of this behavior, it is crucial to consider the appropriate effective model for this system. Here, we present electronic structure calculations in the frame of density functional theory (DFT) and derive an effective model with the NMTO (N-th order muffin tin orbital) downfolding method and discuss its features. Since from X-ray diffraction, the structure of $\kappa$-(BEDT-TTF)$_2$Cu(CN)$_3$ has been determined without hydrogen positions, we first prepare a very similar structure including hydrogen atoms and carefully relax it using the projector augmented wave method. This structure is then used for the DFT and NMTO analysis. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B12.00005: Charge frustration and novel electron-lattice coupled phase transition in organic conductor DI-DCNQI$_2$Ag Hitoshi Seo, Yukitoshi Motome We have theoretically investigated the phase transition accompanying charge ordering in 1/4-filled quasi-one-dimensional organic conductor DI-DCNQI$_2$Ag. The nature of this phase transition at 220 K has been under debate since the first direct observation of charge ordering among organic conductors by Hiraki and Kanoda[1]. In this study, motivated by a recent synchrotron radiation x-ray study by Kakiuchi {\it et al.}[2], we investigate a three-dimensional interacting spinless fermion model coupled to the lattice degree of freedom. We have found that the peculiar ``spiral frustration'' existing in the interchain Coulomb interaction destabilizes simple Wigner crystal-type charge order and gives rise to a novel state where different chains show different orders and align periodically: charge order, lattice dimerization, and a co-existence of them. The co-existence of these two orders was in fact predicted in our previous study for the non-frustrated case[3], which was stabilized as a result of competing interactions. In contrast, the co-existence in the present study is obtained by compromising the frustration in charge sector. [1] K. Hiraki and K. Kanoda, Phys. Rev. Lett. {\bf 80} (1998) 4737. [2] T. Kakiuchi {\it et al.}, Phys. Rev. Lett. {\bf 98} (2007) 066402. [3] H. Seo, Y. Motome, and T. Kato, J. Phys. Soc. Jpn. {\bf 76} (2007) 013707. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B12.00006: Finite size effects in micro- and nanocrystals of (TMTSF)$_{2}$ClO$_{4}$ P. Dhakal, J.I. Oh, M.J. Naughton We report transport measurements in micro- and nanosized crystals of the molecular organic (super)conductor (TMTSF)$_{2}$ClO$_{4}$. Synthesized using the standard electrocrystallization process, various techniques such as focus ion beam-induced metallization, photo lithography, and atomic layer deposition were then employed to successfully make micro- and nano-electrodes contacting these samples. In addition, FIB sculpting was used to reshape crystals to reduce their sizes. We have successfully measured crystals with dimensions as small as 300 nm. We have observed that electrical transport behaviors of these finite sized crystals are very different from those of their parent bulk crystals. We will discuss our observations in terms of various origins such as finite size effects, fabrication-induced defects, and combinations thereof. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B12.00007: Pulsed-field study of the interference commensurate effect in quasi-one-dimensional organic conductors J. Roy, J.I. Oh, H. Yoshino, P. Dhakal, M.J. Naughton We report angle-dependent magnetoresistance oscillations for fields up to 43 T oriented mainly in the most conducting \textbf{\textit{x}}\textbf{-}\textbf{\textit{y}} plane, with small field component along the least conducting \textbf{\textit{z}} axis, in the q1d compounds (TMTSF)$_{2}$ClO$_{4}$ and (DMET)$_{2}$I$_{3}$ at 1.5 K. A hybrid plastic-metal cryoprobe system with pseudo dual-axis rotation has been built for these pulsed-field measurements. Due to the interference commensurate effect, (aka Lee-Naughton oscillations) [1-3], we have observed rich magnetoresistance oscillations, resulting from an interference effect of commensurate electron trajectories in the extended Brillouin zone. Also, we have found that, as theoretically expected [2], field-dependent magnetoresistance shows 1D and 2D transport behavior at local resistance maxima and$^{ }$minima (versus field angle), respectively. \newline [1] I.J. Lee and M.J. Naughton, Phys. Rev. B \textbf{57}, 7423 (1998).\newline [2] A.G. Lebed, \textit{et al}., Phys. Rev. Lett. \textbf{91}, 187003 (2003).\newline [3] H.I. Ha, \textit{et al}., Phys. Rev. B \textbf{73}, 033107 (2006). [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B12.00008: Acoustic properties of quasi-one-dimensional organic conductor (TMTSF)$_{2}$ClO$_{4}$ in the relaxed state: Superconductivity and FISDW anomalies. Alexandre Langlois, Mario Poirier, Claude Bourbonnais, Klaus Bechgaard Through competing electronic instabilities, the anion sublattice plays an important role in the rich phase diagram of the Bechgaard salts. In the quasi-one-dimensional organic conductor (TMTSF)$_{2}$ClO$_{4}$, anion ordering at 24 K affects the nesting properties of the Fermi surface and controls the stability of the superconducting phase below 1.2 K at ambient pressure. Moreover, the field induced spin density wave phases FISDW, one of the several features induced by a magnetic field in this compound, are also sensitive to the symmetry of the anions. In order to address the coupling issue between the lattice and these electronic instabilities, we have performed the first ultrasonic measurements on (TMTSF)$_{2}$ClO$_{4}$ in the relaxed state below 4 K using longitudinal and transverses waves (30-500 MHz). If low-frequency vibrating reed experiments have revealed magneto-elastic anomalies in the FISDW phases [1], the superconducting one was never investigated by similar techniques. We report anomalies in the ultrasonic velocity and attenuation for the superconducting and the FISDW phases. The coupling of these phases to the lattice is discussed in relation with the known T-B phase diagram. [1] X.D. Shi \textit{et al}., Phys. Rev. B. 50, 1984 (1994). [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B12.00009: Spin dynamics of the field-induced spin density wave phases in (TMTSF)$_2$ClO$_4$ in tilted magnetic fields L.L. Lumata, J.S. Brooks, A.P. Reyes, P.L. Kuhns, S.E. Brown, H.B. Cui, J.-I. Yamada The spin dynamics of the different subphases of the field-induced spin density wave (FISDW) ground state in (TMTSF)$_2$ClO$_4$ was investigated using simultaneous $^{77}$Se nuclear magnetic resonance (NMR) and electrical transport. The metallic and FISDW phases were accessed by rotating the sample along its most conducting axis in a constant magnetic field H and constant temperature. At a fixed field of 30 T and constant temperature 1.47K, we report observation of divergence in the spin-lattice relaxation rate 1/T$_1$ at angles corresponding to H$_{\perp}$=H$\cos(\theta)$ $\sim$ 6.4T, 15.9T, and 26T FISDW phase transitions. There is a gradual increase in 1/T$_1$ in the 2nd-order/FISDW cascade region and a nearly metallic-like behavior deep in the final$\backslash$reentrant SDW phase is observed. RF enhancements in the metallic and FISDW phases were also measured. The details will be discussed. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B12.00010: Interlayer Cooperon correction to angular-dependent magnetoresistance in layered metals Malcolm Kennett, Ross McKenzie Studies of angle-dependent magnetoresistance oscillations (AMRO) in the interlayer conductivity of layered metals have generally considered semi-classical electron transport. We consider a quantum correction to the semi-classical conductivity that arises from what can be described as an interlayer Cooperon. This depends on both the disorder potential within a layer and the correlations of the disorder potential between layers. We compare our results with existing experimental data on organic charge transfer salts that is not explained within the standard semi-classical transport picture. In particular, our results may be relevant for weak localization-like effects that have been seen when the applied magnetic field is close to parallel to the conducting layers. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B12.00011: Coulomb drag at zero temperature Alex Levchenko, Alex Kamenev We show that the Coulomb drag effect exhibits saturation at small temperatures, when calculated to the third order in the interlayer interactions. The zero-temperature transresistance is inversely proportional to the third power of the dimensionless sheet conductance. The effect is therefore the strongest in low mobility samples. This behavior should be contrasted with the conventional (second order) prediction that the transresistance scales as a certain power of temperature and is almost mobility-independent. The result demonstrates that the zero-temperature drag is not an unambiguous signature of a strongly-coupled state in double-layer systems. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B12.00012: Collective modes in quantum electron glasses and electron-assisted hopping Markus Mueller, Lev Ioffe We study electronic transport in Anderson insulators with strong Coulomb interactions in dimensions $d\geq 2$. Close to the metal insulator transition where the single particle localization length is much larger than interparticle-distance, the interactions lead to a strongly correlated quantum glass phase. Even though single particle excitations are localized and the system is insulating, there are collective electronic modes which remain delocalized down to parametrically small energies. These collective excitations serve as a continuous bath which can provide the activation energy for variable range hopping transport. This circumvents the energy conservation problem arising when only discrete particle-hole excitations are present. In contrast to the weak and material-dependent phonon-assisted hopping mechanism, the activation by an electronic bath leads to a nearly universal prefactor $e^2/h$ of the Efros-Shklovskii conductance, as is observed in many recent experiments. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B12.00013: Hall effect on the triangular lattice Gladys Leon, Christophe Berthod, Thierry Giamarchi, Andrew Millis We investigate the Hall effect on the two-dimensional triangular lattice. We calculate the high frequency Hall constant $R_H$ and its temperature dependence for three regimes of the Hubbard interaction $U$. In the non-interacting case $U=0$ we find that $R_H$ behaves at temperature $T=0$ like the classical dc Hall constant, $R_H\sim1/ne$. At high $T$ we find a positive $R_H$ increasing linearly with temperature, with a slope depending on the electron density. For small to moderate values of $U$, we study the effect of interactions on $R_H$ within second-order perturbation theory, and we find these effects to be small. The perturbation theory also shows that the electron self-energy is almost local ($k$-independent), suggesting the use of a local approximation as the Dynamical Mean Field Theory (DMFT) method to treat higher values of $U$. We therefore evaluate $R_H$ at large $U$ using both DMFT and the atomic limit of the self-energy, and we compare the results with those obtained at small $U$. Finally, we discuss the relevance of our calculations for the interpretation of recent Hall measurements in cobaltates. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B12.00014: Quantum Phase Transition in a Cold Atomic Spin-Boson Mixture Peter P. Orth, Ivan Stanic, Karyn Le Hur We theoretically implement a spin array in a tunable bosonic environment using cold bosonic atoms with two (hyperfine) ground states, trapped by different potentials [1]. The first specie lies in a deep optical lattice with tightly confining wells and forms a spin array; spin-up/down corresponds to occupation by one/no atom at each site. The second specie forms a superfluid reservoir. Different species are coupled coherently via laser transitions and collisions. Whereas the laser coupling mimics a transverse field for the spins, the coupling to the reservoir phonons (sound modes) induces a ferromagnetic (Ising) coupling as well as dissipation. This results in a peculiar ferro-paramagnetic quantum phase transition where the effect of dissipation can be studied in a controllable manner. \newline \newline [1] Peter P. Orth, Ivan Stanic, and Karyn Le Hur, arXiv:0711.2309 [cond-mat.other]. [Preview Abstract] |
Session B13: Focus Session: Simulations of Matter at Extreme Conditions II: Beryllium, Carbon, and Metals
Sponsoring Units: DCOMP GSCCMChair: Eric Schwegler, Lawrence Livermore National Laboratory
Room: Morial Convention Center 204
Monday, March 10, 2008 11:15AM - 11:27AM |
B13.00001: The Role of Anharmonicity in the Beryllium Equation of State Lorin X. Benedict, Andrea Trave, Christine Wu, Tadashi Ogitsu, Phil Sterne, Eric Schwegler We discuss the construction of a multiphase equation of state for Be from first principles, aimed at understanding the material's properties at extreme conditions. In addition to the usual computation of cold, quasiharmonic ion-thermal, and (negligible here) electron-thermal contributions, we consider the effects of strong anharmonicity in the bcc phase, and argue that the inclusion of such effects may greatly perturb the picture (in particular, the phase diagram) derived from assuming quasi-harmonic lattice dynamics. Our analysis involves studying the mean displacement from equilibrium of Be atoms in the lattice by DFT-molecular dynamics methods and comparing the results to those of the quasiharmonic theory. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B13.00002: Quantum molecular dynamics simulations of beryllium at high pressures Michael Desjarlais, Marcus Knudson The phase boundaries and high pressure melt properties of beryllium have been the subject of several recent experimental and theoretical studies. The interest is motivated in part by the use of beryllium as an ablator material in inertial confinement fusion capsule designs. In this work, the high pressure melt curve, Hugoniot crossings, sound speeds, and phase boundaries of beryllium are explored with DFT based quantum molecular dynamics calculations. The entropy differences between the various phases of beryllium are extracted in the vicinity of the melt curve and agree favorably with earlier theoretical work on normal melting. High velocity flyer plate experiments with beryllium targets on Sandia's Z machine have generated high quality data for the Hugoniot, bulk sound speeds, and longitudinal sound speeds. This data provides a tight constraint on the pressure for the onset of shock melting of beryllium and intriguing information on the solid phase prior to melt. The results of the QMD calculations and the experimental results will be compared, and implications for the HCP and BCC phase boundaries of beryllium will be presented. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B13.00003: Equation of state of beryllium from first-principles calculations Andrea Trave, Lorin Benedict, Tadashi Ogitsu, Christine J. Wu, Philip A. Sterne, Eric Schwegler The design of experiments of materials at extreme conditions of pressure and temperature is often based on hydrodynamic simulations, which make use of equation of state (EOS) models for the description of the systems under study. The validity of these models is extremely critical, and first-principles calculations can provide consistent and accurate parameters for the determination of the EOS in a wide range of thermodynamic conditions. Extensive density functional theory calculations at zero temperature have been performed for beryllium in various solid structures, in order to obtain accurate predictions for their compression curves, and phonon and electronic densities of states. Finite-temperature simulations have been used to further improve the models to include anharmonic effects. The melting line of beryllium has been obtained with first-principles two-phase simulations, which enables the construction of a multi-phase EOS for both liquid and solid beryllium. The results of these simulations provide useful indications on the relative stability of the various solid and liquid phases of beryllium in a region of the phase diagram lacking any experimental study so far. Prepared by LLNL in part under Contract W-7405-Eng-48 and in part under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B13.00004: Construction of a multi-phase equation of state for carbon at extreme pressures Alfredo A. Correa, Lorin X. Benedict, Stanimir A. Bonev, David A. Young, Eric Schwegler We describe the construction of a multi-phase equation of state for carbon at extreme pressures that is based on the results of first principles electronic structure calculations. Two solid phases (diamond, BC8) and the liquid are considered. Solid-phase free energies are built from a knowledge of cold curve and phonon calculations, together with first principles molecular dynamics calculations of the equation of state itself to extract anharmonic terms. The liquid free energy is constructed from a combination of molecular dynamics calculations and constraints determined from previously calculated melt curves, assuming a simple solid-like free energy model. The resulting equation of state is extended to more extreme densities and temperatures with a plasma-based free energy model. Comparisons to available experimental results are discussed. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B13.00005: Clustering in dense molten lithium Isaac Tamblyn, Jean-Yves Raty, Stanimir A. Bonev Molten lithium is investigated from zero to over nine-fold compression using first principles theory. Over this pressure range, we observe several electronic and structural transitions. The changes that lithium undergoes with increasing pressure are initially analogous to those predicted for liquid sodium [1]. However, upon further compression, effects due to increased core overlap lead to a new liquid phase composed of weakly bound lithium clusters. The properties of the proposed new liquid phases, the melting curve of lithium, and the implications of our findings for the stability of low-symmetry lithium solids will be discussed. [1] J.-Y. Raty, E.R. Schwegler, S.A. Bonev, Nature, 449, 448-451 (2007) [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B13.00006: Lithium at ultra-high pressures Andre Kietzmann, Ronald Redmer, Michael P. Desjarlais, Thomas R. Mattsson Lithium is a prototypical simple metal at standard conditions. However, by changing the density towards expanded or compressed states, the electrical conductivity shows strong variations. We have performed quantum molecular dynamics simulations for fluid lithium covering a wide range of densities and temperatures in order to derive the equation of state, the electrical conductivity, and information about structural and electronic changes along the expansion or compression. The electrical conductivity changes from the nonmetallic expanded fluid via the fluid metal region up to the degenerate electron liquid at high densities. We find a largely ordered ion structure at ultra-high densities reflecting a multi-center bonding situation in the liquid as predicted earlier for solid lithium. Supported by the DFG within SFB 652. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States DOE's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B13.00007: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 12:39PM - 12:51PM |
B13.00008: Optical Properties of LiH From Mixing Rules Daniel Horner, Joel Kress, Lee Collins We investigate the use of pressure and density matching mixing rules for predicting the optical properties and equation-of-state (EOS) of lithium hydride for densities from half to twice solid [0.78 g/cc] and temperatures from 0.5 to 3.0 eV. The mixing rules allow us to perform simulations of lithium and hydrogen separately and, from them, calculate properties of the mixture. Using the VASP code, we performed constant $(N, V, T)$ quantum molecular dynamics simulations for H, Li, and LiH with the results for the mixture (LiH) as a benchmark of the mixing procedures. A finite-temperature density functional theory formulation produces the electronic wave function at each time step within the generalized gradient approximation with projector augmented wave pseudopotentials. Optical properties were determined using the electronic wave function in a Kubo-Greenwood formula. We compare the frequency-dependent absorption coefficient, Rosseland Mean Opacity, and EOS computed via the mixing rules and those from a full LiH simulation. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B13.00009: First principles investigation of the dielectric function of gold under ultrafast laser excitation Tadashi Ogitsu, David Prendergast, Eric Schwegler, Yuan Ping, Andrew Ng Recently, a quasi-steady state in ultrathin, $\sim$30 nm gold foils exposed to an ultrafast laser pulse has been observed, which includes an enhanced interband transition peak at 2.6 eV in the imaginary part of the dielectric function [1]. Simulations of this system assuming a two-temperature model for the electronic and ionic degrees of freedom do not provide this enhancement in optical absorption, possibly indicating that both of these degrees of freedom are not in equilibrium. Our approach is to treat this as an inverse problem: to reproduce experiment by sampling various states of electrons and ions. We employ an efficient first principles technique to quickly estimate the dielectric function of this fcc metal for various finite temperature and non-equilibrium model distributions. Converged Brillouin zone sampling is achieved using a compact k-dependent Hamiltonian derived from first principles calculations [2]. [1] Y. Ping et al., Phys Rev Lett {\bf 96}, 25503 (2006). [2] E. L. Shirley, Phys Rev B {\bf 54}, 16464 (1996). [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B13.00010: High pressure lattice dynamics and elasticity of transition metals Daniel Orlikowski, Lorin Benedict, John Klepeis For continuum-level description of transition metals using equation of state and strength models, a large concerted calculation effort is required. We present here a subset of that work to provide Debye temperatures and elastic moduli for the equation of state (EOS) and strength models. DFT calculations for the phonons are performed to obtain the Debye temperature over the pressure range required by the EOS model. For the strength model, we have combined several sets of quantum-based, atomistic calculations with density functional theory (DFT) to develop elastic moduli over a wide range of temperatures (12,000 K) and pressures (4 Mbar). Our focus is upon vanadium but other transition metals will be presented tantalum and molybdenum. Our results are comparable to available experimental data. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B13.00011: A new wide-range equation of state for tungsten John H. Carpenter, Michael P. Desjarlais, Ann E. Mattsson, Kyle R. Cochrane A new wide-range equation of state for tungsten is described. Quantum molecular dynamics calculations in the warm dense matter region are combined with other experimental and theoretical calculations, providing a set of information on which to tune a model of the free energy landscape. The resulting model, describing the liquid, gas, and bcc solid phases, provides a good description of the liquid-vapor critical point, melt curve, static compression data, isobaric expansion data, and the Hugoniot. Finally, improvements in table generation greatly improve the resolution of phase boundaries. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B13.00012: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 1:39PM - 1:51PM |
B13.00013: Pressure Induced Solidification of Ta and Cu: A Comparison David Richards, James Glosli, Frederick Streitz Using powerful computers such as Blue Gene/L it is now possible to use classical molecular dynamics to simulate pressure induced solidification at size scales that are free of finite size effects. We present a comparison of the nucleation, growth, and coalescence of clusters during pressure induced solidification in large scale MD simulations of liquid Ta and Cu. We extract growth and nucleation rates from our simulations, as well as cluster size distributions that can be compared against the predictions of simple models. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B13.00014: New Phase Diagram of Ta: Bridging Laser Heated Diamond-Anvil Cell and Shock Melting Christine J. Wu, Per A. S\"oderlind, James N. Glosli, John E. Klepeis Determination of the melt line of materials under high pressures is essential for establishing its phase diagrams and has important implications for geophysics, material science, and high-pressure physics. So far, melting temperatures at high pressure are primarily measured by \textit{in situ} laser-heated diamond-anvil cell (DAC) or shock wave experiments. Often, these two methods yield significantly different results, particularly for non close-packed metals, such as bcc metals. For instance, anomalously flat melting slopes were reported for numerous bcc metals by laser-heated DAC. The flatness of the melting slope is in sharp contrast to the classical Lindemann behavior which shock-melting temperatures follow closely. In this presentation, we will report a novel phase diagram of Ta obtained from \textit{ab initio} methods, and molecular dynamics (MD) simulations, which resolves the long-standing controversy, and has significant impact on our understanding of phase diagrams of bcc metals. [Preview Abstract] |
Session B14: Casimir Forces, Collisions and Atomic Structure
Sponsoring Units: DAMOPChair: Ho Bun Chan, University of Florida
Room: Morial Convention Center 205
Monday, March 10, 2008 11:15AM - 11:27AM |
B14.00001: Measurement of near-field radiative heat transfer and implications for Casimir force measurements Arvind Narayanaswamy, Sheng Shen, Gang Chen Near--field force and energy exchange between two objects due to quantum electrodynamic fluctuations give rise to interesting phenomena such as Casimir and van der Waals forces, and thermal radiative transfer exceeding Planck's theory of blackbody radiation. Although significant progress has been made in the past on the precise measurement of Casimir force related to zero-point energy, experimental demonstration of near-field enhancement of radiative heat transfer is difficult. In this work, we present a sensitive technique of measuring near--field radiative transfer between a microsphere and a substrate using a bi--material atomic force microscope (AFM) cantilever, resulting in ``heat transfer-distance'' curves. Measurements of radiative transfer between a sphere and a flat substrate show the presence of strong near--field effects resulting in enhancement of heat transfer over the predictions of the Planck blackbody radiation theory. The implications of measurement of near-field radiative heat transfer for determining of the magnitude of the thermal component of the Casimir force will be discussed. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B14.00002: Casimir interactions between cold atoms and corrugated surfaces Diego A.R. Dalvit, Paulo A. Maia Neto, Astrid Lambrecht, Serge Reynaud The lateral Casimir-Polder force between an atom and a corrugated surface should allow the experimental study of non trivial geometrical effects in quantum vacuum. We apply the scattering approach to compute this force for an atom (or a BEC) above a corrugated surface, and compare our exact results with two commonly used approximations, the proximity approximation and the pairwise summation technique. We show that large corrections to these approximations could be measured using present-day technology with a BEC used as a vacuum field sensor. For details, see D.A.R. Dalvit et al, arXiv:0710.5249, 0709:2095. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B14.00003: Orientation dependence of Casimir force between uniaxial crystals Mark Romanowsky, Federico Capasso We present calculations showing that the Casimir force between uniaxial crystals depends on the orientation of the optical axes. For strongly anisotropic crystals, the Casimir force can be substantially different when the optical axes are perpendicular to the crystal surfaces versus the case in which they are parallel to the surfaces. We compute the force numerically for cases of experimental interest, and discuss the prospects for observing this effect. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B14.00004: Measurements of the Casimir interaction between a sphere and a rectangular corrugated plate Yiliang Bao, H. B. Chan We present measurements of the Casimir force gradient between a gold-coated sphere and a highly doped silicon plate with an array of nanoscale, high-aspect-ratio trenches. The Casimir force arises from quantum fluctuation of electromagnetic fields in vacuum and is strongly dependent on the boundary condition. While the majority of the precise measurements have been performed on the simple arrangement of plate-sphere or two parallel plates, few experiments have been done in geometries with interactions that deviate significantly form the pair-wise summation of two-body potentials. We choose one of the interacting surfaces to be an array of trenches with widths ranging from 200 nm to 500 nm and depth of 1um. Theoretical analysis predict that for perfectly conducting surfaces with such geometry, the Casimir force differs by up to 70 {\%} from pairwise summation at separation of 0.4 um. In our experiment, the force gradient is measured by the frequency shifts caused by Casimir force in the response of a periodically driven micromachined torsional oscillator, yielding a highly sensitive measurement of the force gradient for separations between 80nm and 500nm. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B14.00005: Numerical Evaluation of the Casimir Force in Concave Piston Geometries Martin Schaden Using a modified wordline approach[1] for a massless scalar field satisfying Dirichlet boundary conditions, I numerically calculate the Casimir force on a piston in a \textit{closed} \textit{concave} cavity of various shapes. There are no contributions from arbitrarily short paths and the Casimir \textit{force} on the piston is finite for all systems considered. I relate the Casimir interaction energy of concave cavities to a probability measure on the convex hulls of Brownian bridges. The resulting algorithm for Casimir forces in concave geometries is numerically stable, fast and very accurate. The results depend only on the number of hulls and points used and extrapolate readily to the continuum limit. I compare some of these numerical results to semiclassical estimates of the force[2] in similar piston geometries. [1] H. Gies, K. Langfeld and L. Moyaerts, JHEP 0306, 018 (2003); H. Gies and K. Klingmuller, Phys. Rev. D74, 045002 (2006). [2] L Mateescu and M. Schaden, [quant-ph/0705.3435]. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B14.00006: Measurements of the Casimir-Lifshitz force between a metal and a dielectric in fluid Jeremy Munday, Federico Capasso The Casimir force arises from the confinement of quantum fluctuations of the electromagnetic fields. The boundary conditions on these fields are dependent upon the electromagnetic properties of the interacting materials and affect both the magnitude and sign of the resulting force. We will discuss our experiments for measuring the Casimir-Lifshitz force between a metal and a dielectric immersed within a fluid and will describe situations which can give rise to a repulsive interaction. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B14.00007: On the repulsive Casimir force using metamaterials. Felipe Da Rosa, Diego Dalvit, Peter Milonni It is known for quite some time that Casimir repulsion between a dielectric and a magnetodielectric plate is possible, and the development of metamaterials brought this phenomenon closer to experimental possibilities. The purpose of this work is to discuss as realistically as possible the role that metamaterials play in the Casimir force and bring to the surface some aspects of this issue that were previously rarely mentioned, such as the typical anisotropy of metamaterials and the presence of a Drude background in its electric permittivity. We also study the Casimir and Polder force between an atom and a metamaterial, since this setup may be more suitable to the detection of repulsion forces. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B14.00008: Spin depolarisation of N$_{2}^{+}$ ($^{2}\Sigma ^{+})$ in collisions with $^{3}$He and $^{4}$He in a magnetic field Thierry Stoecklin, Gr\'egoire Guillon, Anatoli Voronin The possibility of tuning the interactions between atoms and molecules $^{[}$\footnote{ R. V. Krems, A. Dalgarno, J. Chem. Phys. 120, 2296 (2004)}$^{]}$ using a magnetic field has open new perspectives of controlling collisional energy transfer at very low temperature. In a first study dedicated to He-N$_{2}^{+}$ inelastic collisions we found that spin free collisions of N$_{2}^{+}$ with $^{3}$He and $^{4}$He exhibit a strong isotope effect in the ultra cold regime $^{[}$\footnote{ T.Stoecklin and A. Voronin, Phys Rev A. (2005), 72 : 042714.}$^{,}$\footnote{ G. Guillon, T. Stoecklin and A. Voronin, Phys. Rev. A. \textbf{75,} 052722 (2007).}$^{]}$. and recently found a similar effect for another ionic system: the He-CH$^{+} \quad ^{[}$\footnote{ T. Stoecklin and A. Voronin, Eur. Phys. J. D. (2007) DOI: 10.1140/epjd/e2007-00293-3}$^{] }$collision. In the present work, we compare first in the absence of an applied magnetic field the cross sections for the transitions changing the projection of the total angular momentum of N$_{2}^{+}(^{2}\Sigma )$ in collisions with $^{3}$He and $^{4}$He at very low collision energy$^{[}$\footnote{ G.guillon, T.Stoecklin and A. Voronin, Eur. Phys. J. D. (in press)}$^{]}$.In the second part of this contribution we investigate the effect of an applied magnetic field and compare the results obtained for the fundamental states of the two nuclear spin isomers of N$_{2}^{+}$. As a result of the different mechanisms of action of the spin rotation interaction for these two rotational levels we find a great difference of sensitivity to the applied magnetic field. Whereas even moderate values of the applied magnetic field (10 Gauss) completely modify the very low collision energy behaviour of the spin depolarisation cross section of the fundamental ortho level, we find that one has to apply magnetic fields two orders of magnitude larger to obtain similar effects for the fundamental para level. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B14.00009: Depolarization of Ne*(2p$_{i}$ [J=1]) atoms induced by collisions with He(1s$^{2}$) at 10 K $<$ T $<$ 3,000 K Cristian Bahrim, Vaibhav Khadilkar Depolarization mechanisms of Ne$^{\ast }$(2p$^{5}$ 3p; 2p$_{i}$ [J=1]) atoms induced by collisions with helium in a gas mixture at thermal equilibrium are analyzed using a close-coupling quantum method within a model potential approach [1]. Our goal is to explain measurements of alignment destruction (which is a mechamism that includes the disalignment and the depopulation of atoms) [2], disorientation [3], and disalignment [4] of Ne$^{\ast }$(2p$_{i }$[J = 1]) atoms induced by He-Ne collisions at 10 K $<$ T $<$ 3,000 K using the density matrix formalism. Such analysis offers accurate information about anisotropic atom-atom potentials and improves our model potential for the HeNe$^{\ast }$(2p$^{5 }$3p) system [1] by adding the polarizability of Ne$^{\ast }$ atoms on various 2p$_{i}$ states to the long-range potentials. Our results for neon excited on the 2p$_{2}$, 2p$_{5}$, 2p$_{7}$ and 2p$_{10}$ states are reported, and excellent agreement with available measurements [2, 3, 4] is found. Also the atomic polarizabilities for Ne$^{\ast }$on 2p$_{2}$, 2p$_{5}$, 2p$_{7}$ and 2p$_{10}$ states are reported. [1] Bahrim C, Kucal H and Masnou-Seeuws F 1997 \textit{Phys. Rev. A} \textbf{56} 1305. [2] Carrington C G and Corney A 1971 \textit{J. Phys. B }\textbf{4} 869. [3] Seo M, Shimamura T, Furatani T, Hasuo M, Bahrim C and Fujimoto T 2003 \textit{J. Phys. B }\textbf{36} 1885. [4] Nimura M, Hasuo M and Fujimoto T 2004\textit{ J. Phys. B. }\textbf{37} 4647. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B14.00010: Observation of the n($^3$He,t)p Reaction by Detection of Far-Ultraviolet Radiation Charles W. Clark, Alan K. Thompson, Michael A. Coplan, John W. Cooper, Patrick Hughes, Robert E. Vest We have detected Lyman alpha radiation as a product of the n($^3 $He,t)p nuclear reaction, induced in a $^3$He gas cell irradiated by a cold neutron beam at the NIST Center for Neutron Research. The predominant source of this radiation appears to be decay of the 2p state of tritium produced by charge transfer and excitation collisions with the background $^3$He gas. For atmospheric pressure and room temperature in the $^3$He cell, we find yields of tens of Lyman alpha photons for every neutron reaction. These results suggest a method of cold neutron detection that is complementary to existing technologies that use proportional counters. In particular, this approach may provide single neutron sensitivity with wide dynamic range capability, and a class of neutron detectors that are compact and operate at relatively low voltages. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B14.00011: Photo-Fragmentation of Lithium Atoms in FEL Radiation Fields Matthew Foster, J. Colgan, M.S. Pindzola, Alexander Dorn Multi-electron ejection from lithium induced by absorption of a single photon is a fundamental few-body reaction that tests the correlated interaction dynamics between atomic constituents. Experiments have been proposed using intense FEL radiation at FLASH in combination with reaction microscopes to measure four-body dynamics. These proposed experiments will first measure the double ionization dynamics from both the even parity Li ground state and the laser excited odd parity Li(2p $^2P^o$) state. We will present theoretical calculations using the time-dependent close-coupling method (TDCC) to assist in the experimental search for interesting correlation effects for both double and triple ionization of lithium. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B14.00012: Long range anions:diatomic, triatomic and polyatomic molecules Edward C. Chen, Edward S. Chen The observation of both long range and valence anions of O$_{2}$, NO, CS$_{2}$, N$_{2}$O, O$_{3}$, SF$_{6}$ , C$_{6}$F$_{6}$ CH$_{3}$NO$_{2}$, tetracene, anthracene, acridine, perylene, pyrene, naphthalene and the nucleobases is reported in negative ion mass spectrometry, photoionization, electron impact in nanodroplets, electron swarm experiments at low temperature and alkali metal beam studies.. New polarization or quadrupole bound electron affinities less than 0.1 eV are reported for the aromatic hydrocarbons, O$_{2}$, SF$_{6}$, CS$_{2}$ , C$_{6}$F$_{6}$ while dipole bound values less than 0.15 eV are reported for the nucleobases, N$_{2}$O and NO. The long range states act as gateways to valence states. This relationship is illustrated by Morse potential energy curves in single bond dissociations and in reaction coordinates analogous to Marcus parabolas. New adiabatic electron affinities are reported for some of these molecules, including (in eV) guanine, 1.64(2) adenine, 1.09(2); C$_{6}$F$_{6}$, 1.26(5); acridine, 1.09(2); perylene ,1.09(2); tetracene, 1.10(2), naphthalene, 0.19(2) eV. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B14.00013: The Effect of Birth Location in an Intense Laser Focus on the Non-Sequential Double Ionization Yield Jay Paquette, Jan Chaloupka Atoms in an intense laser field can become doubly ionized through a direct process known as rescattering, where a single electron is liberated through tunnel ionization and is driven back to the ion core by the laser field, leading to impact ionization and release of a second electron. In this quasi-classical description, the trajectory of the first electron will have a strong influence on the probability of release of a second electron, as evidenced by the reduction in yield with elliptical laser polarization. Even with linear polarization, the first electron can avoid a reencounter with the ion due to the v$\times $B term or the longitudinal electric field component (E$_{z})$ in the laser focus. Since the E$_{z}$ term is given from the requirement of zero divergence of the electric field, its magnitude will vary as a function of position within the laser focus. Using completely classical 3-D simulations, we demonstrate how longitudinal electric field variations affect electron trajectories, how the ion yields from various regions within the focus are affected, and the likelihood of observing this effect experimentally. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B14.00014: Modeling Plasma Induced Dispersion in a Modified Nonlinear Schr\"{o}dinger Equation Jeremy Gulley, William Dennis Ultrafast laser pulse propagation in dielectrics is often modeled by a modified nonlinear Schr\"{o}dinger equation (NLSE). At laser intensities sufficient to cause ionization, a plasma term can be included in the modified NLSE to account for free-carrier optical effects. This term is linear in the field and may be calculated from the classical Drude theory. We explore the consequences of a newly developed method for including dispersion relations of the plasma term as predicted by the Drude theory into the framework of the modified NLSE. The plasma induced dispersion terms can be shown to strongly effect ultrashort pulse evolution through regions of high plasma density after propagation distances as short as the skin depth. Our results suggest that, in regions high plasma density, plasma induced dispersion may be more significant than other linear or nonlinear dispersion corrections to the NLSE. [Preview Abstract] |
Session B15: Quantum Entanglement I
Sponsoring Units: GQIChair: Dagmar Bruss, Heinrich-Heine-Universitae Duesseldorf
Room: Morial Convention Center 207
Monday, March 10, 2008 11:15AM - 11:27AM |
B15.00001: Hyperentanglement-assisted dense coding: Beating the channel-capacity ``limit'' Julio T. Barreiro, Tzu-Chieh Wei, Paul G. Kwiat Dense coding was the very first quantum information protocol to be proposed and experimentally realized more than a decade ago. Today, however, the achieved channel capacity (CC) remains fundamentally limited as conceived for photons using linear optics: Alice can only decode three of four potential messages sent by Bob, due to the impossibility to deterministically resolve all four Bell states using conventional techniques; the maximum CC is thus limited to $\log_2 3 \approx 1.585$ bits. However, when the particles encoding the Bell pair are entangled in an additional degree of freedom, the complete and deterministic discrimination of all Bell states is possible. Via the process of spontanteous parametric down conversion, we produce photon pairs simultaneously entangled in polarization and orbital angular momentum. Using the auxiliary entanglement and a robust intermode coupling scheme, our experiment achieves CC=1.624(7) bits, the first to surpass the CC ``limit'' for linear photonic superdense coding. Our encoding is suited for quantum communication without alignment and satellite to satellite communication. Additionally, our scheme also enables the remote preparation of single-photon highly-entangled states. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B15.00002: Testing Quantum Devices: Practical Entanglement Verification in Bipartite Optical Systems Hauke Haseler, Tobias Moroder, Norbert Lutkenhaus We present a method to test quantum behavior of quantum information processing devices, such as quantum memories, teleportation devices, channels and quantum key distribution protocols. The test of quantum behavior can be phrased as the verification of effective entanglement. Necessary separability criteria are formulated in terms of a matrix of expectation values in conjunction with the partial transposition map. Our method is designed to reduce the resources for entanglement verification. A particular protocol based on coherent states and homodyne detection is used to illustrate the method. A possible test for the quantum nature of memories using two non- orthogonal signal states arises naturally. Furthermore, closer inspection of the measurement process in terms of the Stokes operators reveals a security threat for quantum key distribution involving phase reference beams. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B15.00003: Transmission of photonic Bell states over a 2x32dB, 144km free-space link Alessandro Fedrizzi, R. Ursin, T. Herbst, M. Nespoli, R. Prevedel, T. Scheidl, F. Tiefenbacher, T. Jennewein, Anton Zeilinger We successfully transmitted both photons of various Bell states over a 144 km free-space link between the islands of Tenerife and La Palma. Creating and transmitting more than $6\times10^6$ highly entangled photon pairs/s over the $2\times32$ dB channel we received $0.07$ pairs/s at the receiver. We were able to distinguish between $\left|\psi^-\right\rangle$ and $\left|\psi^+\right\rangle$ states and verified the presence of entanglement by violating a CHSH Bell inequality to $S=2.61\pm0.11$, 5 standard deviations above the classical limit of 2. Using a small and compact photon source, we effectively emulate quantum communication in a loss regime comparable to a two-link satellite communication scenario. Furthermore, we convincingly demonstrate the feasibility of 2-photon quantum communication protocols like dense coding, teleportation or quantum cryptography without reference frame over long distance. Finally, with a flight time of 0.5 ms the transmitted Bell states are the longest lived photonic Bell states ever reported. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B15.00004: Correlated photon generation via four-wave mixing in a birefringent semiconductor waveguide Daniel Rogers, Julius Goldhar, Christopher Richardson, Alessandro Restelli, Joshua Bienfang, Charles Clark The next generation of quantum cryptography will benefit from a fast and practical source of entangled photon pairs. Current methods of generating entanglement, whether in bulk nonlinear crystals or microstructure optical fibers, pose significant challenges to integration into fieldable quantum communications systems. In order to meet the demands of speed and practicality, we investigate third-order nonlinearity in a semiconductor waveguide as a source of correlated and ultimately entangled photon pairs. This device offers the advantages of a fast nonlinear response based on the optical Kerr effect and the relative ease of coupling to standard optical fibers. It is potentially useful for free-space and fiber-optic quantum key distribution as well as a host of other applications such as correlated photon metrology and two-photon interferometry. We show the feasibility of using phase-matched four-wave mixing in a birefringent AlGaAs waveguide to generate correlated photon pairs at wavelengths compatible with silicon detectors. We demonstrate the operation of such a device, consider the effects of loss and two-photon absorption, and evaluate the implications of birefringent phase matching. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B15.00005: Macroscopic quantum memories get entangled from far away Zhen-Sheng Yuan, Yu-Ao Chen, Shuai Chen, J\"{o}rg Schmiedmayer, Jian-Wei Pan We report an experimental implementation of a fundamental unit for long-distance quantum communication. By means of entanglement swapping, entanglement is generated between two remote atomic ensembles connected with either 6 m or 300 m fibre-based optical channel, where the flying qubits--two emitted photons from the atomic ensembles are sent to an intermediate station for a joint Bell-state measurement. Afterwards, the measurement induced entanglement between the atomic ensembles are verified by the violation of Bell's inequality or by an entanglement witness. The striking features, phase-error insensitiveness and scalable flexibility, promise the present setup as a fundamental unit for future quantum information processing. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B15.00006: Long-distance atom-photon entanglement and its coherence properties H. Weinfurter, M. Weber, J. Volz, W. Rosenfeld, M. Krug, F. Hocke, F. Henkel The distribution of entanglement between quantum memories at remote locations is one major challenge for the first demonstration of a quantum repeater. Entanglement between matter and light [1] is crucial for achieving this task. Here we report the observation of entanglement between a single trapped atom and a single photon, separated 300 m via an optical fiber. The entanglement is verified by appropriate correlation measurements of the atom-photon pair after communicating the photon through the fiber. In addition we measured the temporal evolution of the atomic density matrix after projecting the atom-photon pair via a state measurement of the photon onto a well defined atomic spin state. We find that the atomic Zeeman qubit decoheres after 100 $\mu$s. Our results represent important steps towards the realization of entanglement between single neutral atoms at distances of several 100 m. \newline [1] J. Volz, M. Weber, D. Schlenk et al., Phys. Rev. Lett. {\bf 96}, 030404 (2006). [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B15.00007: Entanglement dynamics in a qubit-nanomechanical resonator system Dian Wahyu Utami, Aashish Clerk Over the recent years, finding signatures of entanglement in macroscopic systems has been a central goal in many aspects of mesoscopic physics. Here we present our study of non-equilibrium entanglement dynamics between a qubit and a nanomechanical resonator that is coupled to a phonon bath. Unlike previous studies, we specifically look at dispersively coupled qubit-oscillator system which has been realized in many different experiments. Using master equation expressed in gaussian wigner functions, we obtained an elegant and intuitive expression for the entanglement. Two ways of generating entanglement were found; entanglement of the qubit to the amplitude of the driven oscillator and to the phase of the oscillator. A full analytical result within the zero temperature limit of the two cases were derived. We also investigate the effect of finite temperature to the entanglement dynamics and found the phase entanglement to be more robust against temperature. The result indicates that an indirect method to measure the presence of entanglement in the system is possible. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B15.00008: Generation of Entanglement Outside of the Light Cone James Franson The probability amplitude to emit a photon at one location and then annihilate it at another location is proportional to the Feynman propagator, which has nonzero values outside of the forward light cone. This does not allow messages to be transmitted faster than light, but it does allow correlations, entanglement, and mutual information to be generated outside the light cone. These effects are illustrated by considering two distant atoms, one of which is initially in its excited state and the other in its ground state. The probability amplitude for the two atoms to exchange a photon and make a transition to the other state is calculated using perturbation theory and commutator techniques, which gives a result proportional to the Feynman propagator. These effects can be interpreted as being due to the propagation of virtual photons outside of the light cone or as a transfer of pre-existing entanglement from the quantum vacuum. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B15.00009: Towards single time-bin entangled photons from a quantum dot. Christophe Couteau, Gregor Weihs We present our results on photon statistics of quantum dots. We show evidence for photon antibunching and cross-correlation measurements within the biexciton-exciton cascade. We also discuss directions towards a demonstration of time-bin entangled photons from a dot. Recently, it has been showed theoretically that a quantum dot could provide pairs of time-bin entangled photons. The temporal entanglement is due to the fact that we know the twin photons will come in pairs but we don't know when: either both the photons are really or both are late. Under the right conditions and using a double excitation of the dot corresponding to the two time bins, Simon and Poizat showed that one could get deterministic emission of pairs for time-bin entanglement using a single quantum dot. We will present our latest results on photon statistics and our demonstration of photon antibunching as well as cross-correlations within the biexciton-exciton cascade using an original new set-up. Based on these measurements we are confident that we will be able to demonstrate time-bin entangled photons in the near future. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B15.00010: Teleportation and Broadcasting of continuous variable entanglement Archan S. Majumdar, Satyabrata Adhikari, N. Nayak We present the first example for broadcasting of the entanglement of a two-mode squeezed state of the electromagnetic field shared by two distant parties into two nonlocal bipartite entangled states. Using the technique of covariance matrices we demonstrate the entanglement between the nonlocal output modes and the separability of the local output modes. We find the range of values for the squeezing parameter and the amplifier phase for which broadcasting of continuous variable entanglement can be implemented for physical states. We next present a scheme for teleporting two-mode entangled states of continuous variables from Alice to Bob. Our protocol is operationalized through the creation of a four-mode entangled state shared by Alice and Bob using linear amplifiers and beam splitters. Teleportation of the entangled state proceeds with local operations and the classical communication of four bits. We compute the fidelity of teleportation and find that it exhibits a trade-off with the magnitude of entanglement of the resultant teleported state. [arXiv:0708.1869; arXiv:0710.2777] [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B15.00011: Entanglement structure of two-mode squeezed states in absorbing and amplifying environment Phoenix S.Y. Poon, C.K. Law We examine the structure of entanglement for two-mode squeezed states interacting with symmetric linear baths [1]. In Fock space, $\rho^{T_A}$ is observed to be maintaining a block diagonal form as the system evolves. We explicitly obtain the eigenvalues and eigenvectors of $\rho^{T_A}$ (the partial transposition of density matrix $\rho$) as a function of time. The decoherence induced by the baths are shown to destroy the degeneracy of $\rho^{T_A}$, leading to a set of eigenvectors for the construction of entanglement witness operators. We prove that the eigenvectors are time-independent, which is an indicator for the robustness of entanglement of two-mode squeezed states in the presence of noise. \newline \noindent [1] Phoenix S. Y. Poon and C. K. Law, Phys. Rev. A \textbf{76}, 012333 (2007). [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B15.00012: Quantum Second Law with Entanglement Ladislav Andrey It is proved rigorously that every quantum operation generating quantum entanglement states in the multipartite quantum system of qubits leads to the reduction of quantum entropy of whole system. By other words the quantum entanglement can be the source of quantum information. As the consequence the novel formulation of quantum second law is presented with possible applications to progressively developing quantum information science and quantum communications. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B15.00013: Entanglement and Standard Thermodynamic Relations Inti Sodemann, Alonso Botero We re-examine thermal properties of the standard (Boltzmann) canonical ensemble from the point of view of canonical typicality. In this approach, the thermodynamic system is described by the reduced density matrix of a random pure state from an energy-constrained total Hilbert space for the system and environment, with the entanglement entropy playing the role of thermodynamic entropy. We examine the average and variance of the entanglement entropy over all pure states of the restricted total Hilbert space, and show correspondence with the expected results of standard statistical mechanics in the limit of a large environment. We study the correlation between energy and entropy fluctuations, and show that the temperature can also be defined from a variational principle minimizing deviations from a microscopic version of the first law of thermodynamics, involving energy and entropy fluctuations. [Preview Abstract] |
Session B16: Nonlinear Dynamics of Neuronal and Cardiac Systems
Sponsoring Units: DBPChair: John Beggs, Indiana University
Room: Morial Convention Center 208
Monday, March 10, 2008 11:15AM - 11:27AM |
B16.00001: Quantitative universality and non-local interactions in neural pattern formation Matthias Kaschube, Michael Schnabel, Siegrid Loewel, David Coppola, Leonard White, Fred Wolf The occurrence of universal quantitative laws in a strongly interacting multi-component system indicates that its behavior can be elucidated through the identification of general mathematical principles rather than by the detailed characterization of its individual components. Here we demonstrate that universal quantitative laws govern the spatial layout of orientation selective neurons in the visual cortex in three mammalian species separated in evolution by more than 50 million years. Most suggestive of a mathematical structure underlying this universality, the average number of pinwheel centers per orientation hyper-column in all three species is statistically indistinguishable from the constant $\pi$. Mathematical models of neural pattern formation can reproduce all observed universal quantitative laws if non-local interactions are dominant, indicating that non-local interactions are constitutive in visual cortical development. The spatial layout adheres to these laws even if visual cortical organization exhibits marked overall inhomogeneities and when neuronal response properties are experimentally altered. These results demonstrate that mathematical principles can shape the organization of the brain as powerfully as an organism's genetic make-up. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B16.00002: Spike-timing dependent plasticity in integrate-and-fire networks Chun-Chung Chen, David Jasnow We study plastic integrate-and-fire networks with spike-timing dependent plasticity. Following recent experiments, the long-term potentiation (depression) for causal (anti-causal) spike pairs is assumed to be additive (multiplicative) with reference to the existing synaptic strength. Assuming realistic physiological parameters, for time scales of minutes, the synaptic strength can be assumed fixed while neural activities equilibrate. A mean-field analysis in this regime predicts a first order phase transition for the neural activity. As the constant synaptic strength is increased, the network goes from a quiescent phase with only noise triggered activities, to a phase of persistent activity. The number of synapses per neuron controls the transition point in the synaptic strength. However, the activity level of the network just above the transition point is insensitive to the synapse number and represents a neural firing rate of about 20 to 30 Hz for the set of physiological parameters we considered. Simulations on random networks with fixed connectivities agree well with the mean-field predictions for a per-neuron synapse number of 10 or larger. Applying the plasticity rules and performing simulations covering physical times of days, at fixed depression factor for anti-causal spike pairs, the networks develop a unimodal distribution of synaptic strengths at small potentiation values for causal pairs, while run-away synaptic strengths are observed at large values. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B16.00003: A Maximum Entropy Model Applied to Temporal Correlations in Cortical Networks Wei Chen, Aonan Tang, Jon Hobbs, Jodi L. Smith, Hema Patel, Anita Prieto, John Beggs, David Jackson, Dumitru Petrusca, Matthew I. Grivich, Alexander Sher, Alan M. Litke Multi-neuron firing states are often observed, yet are predicted to be rare by models that assume independent firing. To predict these states, two groups recently applied a second-order maximum entropy model that used only observed firing rates and pairwise interactions as parameters (Schneidman et al., 2006; Shlens et al., 2006). Interestingly, these models predicted 90-99{\%} of network correlations. If generally applicable, this approach could vastly simplify analyses of complex networks. However, this work did not address the temporal evolution of correlated states. We applied the model to multielectrode data from cortical slices and cultures. In 8/13 preparations the observed sequences of correlated states were significantly longer than predicted by concatenating states from the model. We found a significant relationship between strong pairwise temporal correlations and observed sequence length, suggesting that pairwise temporal correlations may allow the model to be extended into the temporal domain. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B16.00004: Effective connectivity in a network of spiking cortical neurons Aonan Tang, Jon Hobbs, Wei Chen, Dumitru Petrusca, Matthew Grivich, Alexander Sher, Alan Litke, John Beggs The average cortical neuron makes and receives about 1,000 synaptic contacts. This anatomical information suggests that local cortical networks are connected in a fairly democratic manner, with all nodes having about the same degree. But the physical connections found in the brain do not necessarily reveal how information flows through the network. We used transfer entropy (Schreiber, 2000) to assess effective connectivity in cortical slice cultures placed on a 512 electrode array system (in collaboration with Alan Litke of UC Santa Cruz). These cultures (n = 6) were active for periods exceeding 1 hr, allowing us to collect long data sets for entropy statistics. Data were binned at ~1 ms to match the width of a single neural spike. Analysis revealed wide differences in node degrees, but did not clearly point to a small-world or a scale-free structure. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B16.00005: Inhibition is Needed to Learn Precise Multimodal Integration J. Leo van Hemmen Multimodal neuronal maps, combining input from two or more sensory systems, e.g., vision and audition, play a key role in processing and transforming sensory to motor information. For such maps to be of any use, the input from all participating modalities must be calibrated so that a stimulus at a specific spatial location is represented at an unambiguous position in the multimodal map. Here we discuss a method based on supervised spike-timing-dependent plasticity (STDP) to gauge input from different sensory modalities so as to ensure proper map alignment. We therefore analyze \emph{excitation-} and \emph{inhibition-mediated learning} in conjunction with the problem of how perfect a teacher should be. Analytical calculations and numerical simulations show on the one hand that inhibitory teacher input is essential if high-quality multimodal integration must be learnt rapidly. On the other hand, the quality of the resulting map is not limited by the quality of the teacher signal alone but rather by the accuracy of the input from other sensory modalities. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B16.00006: Distinguishing Similar Odor Stimuli in Nonlinear Recurrent Networks Stuart Wick, Martin Wiechert, Rainer Friedrich, Hermann Riecke The olfactory bulb (OB) is the first processing stage for olfactory information and receives input in the form of activity patterns across an array of discrete input channels (glomeruli). Experiments show that the OB decorrelates similar olfactory inputs: the output patterns are more distinct than the input patterns, which is likely to be important for downstream computations. The high dimensionality of odor space implies a fractured representation of odors on the two-dimensional array of glomeruli. The neural circuits achieving the decorrelation must therefore be non-trivial; their connectivity is, however, poorly known. We investigate what connectivities are optimally suited for this task. For neural networks with linear dynamics the connectivity can be given explicitly. Experiments indicate, however, that the bulbar dynamics are strongly nonlinear and must be minimally modeled by a piece-wise linear rectifier. We investigate the impact of the rectifier on two types of connectivities which are optimal for linear networks, but only one of which accomodates the rectifier. We test the performance of both types of networks by adapting them to an ensemble of odors and assessing their ability to decorrelate these and related odors at the same and other concentrations. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B16.00007: A low dimensional description of globally coupled heterogeneous neural networks of excitatory and inhibitory neurons Roxana A. Stefanescu, Viktor K. Jirsa Neural networks consisting of globally coupled excitatory and inhibitory non-identical neurons may exhibit a complex dynamic behavior including synchronization, multi-clustered solutions in phase space and oscillator death. We investigate the conditions under which these behaviors occur in a multidimensional parametric space defined by the connectivity strengths and dispersion of the neuronal membrane excitability. Using mode decomposition techniques, we further derive analytically a low dimensional description of the neural population dynamics and show that the dynamics of the entire network can be very well reproduced by this reduced system. Examples of networks of FitzHugh-Nagumo and Hindmarsh-Rose neurons are discussed in detail. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B16.00008: A Neuron-Based Model of Sleep-Wake Cycles Svetlana Postnova, Achim Peters, Hans Braun In recent years it was discovered that a neuropeptide orexin/hypocretin plays a main role in sleep processes. This peptide is produced by the neurons in the lateral hypothalamus, which project to almost all brain areas. We present a computational model of sleep-wake cycles, which is based on the Hodgkin-Huxley type neurons and considers reciprocal glutaminergic projections between the lateral hypothalamus and the prefrontal cortex. Orexin is released as a neuromodulator and is required to keep the neurons firing, which corresponds to the wake state. When orexin is depleted the neurons are getting silent as observed in the sleep state. They can be reactivated by the circadian signal from the suprachiasmatic nucleus and/or external stimuli (alarm clock). Orexin projections to the thalamocortical neurons also can account for their transition from tonic firing activity during wakefulness to synchronized burst discharges during sleep. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B16.00009: Interdependencies of Neural Impulse Pattern and Synchronization Hans Braun, Svetlana Postnova, Horst Schneider Neuronal synchronization plays a crucial role in many physiological functions such as information binding and wake-sleep transitions as well as in pathophysiological processes like Parkinson's disease and epileptic seizures. The occurrence of synchronized activity is often associated with significant alterations of the neuronal impulse pattern, mostly with a transition from tonic firing to burst discharges. We have used Hodgkin-Huxley type simulations to study how alterations of individual neurons' dynamics influence the synchronization in electrotonic coupled networks. The individual neurons have been tuned from tonic firing to bursting with chaotic dynamics in between. Our results demonstrate that these transitions have significant impact on the neurons' synchronization. Vice versa, the synchronization state can essentially modify the impulse pattern. The most remarkably effects appear when the individual neurons operate in a periodically tonic firing regime close to the transition to chaos. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B16.00010: Mosaics of retinal cells that transmit maximal information Tatyana Sharpee In the nervous system, visual signals are encoded by retinal ganglion cells into sequences of discrete electrical pulses termed spikes. Response regions of different ganglion cells tile the visual field and are arranged on approximately hexagonal lattice. Here we consider the optimal arrangement of response regions that would collectively allow for maximal information transmitted about the location of a point light source. We find that maximal information can be transmitted when at most three neighboring regions overlap and the average radius of response field is $\sim $0.67 of the distance between response field centers. This finding was obtained with no adjustable parameters and agrees with experimental measurements of retinal mosaics [1, 2]. \newline [1] D.M. Dacey and S. Brace, Visual Neuroscience 9:279-90 (1992). \newline [2] S.H. Devries and D.A. Baylor, J Neurophysiol. 78:2048-60 (1997). [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B16.00011: Phase synchronization analysis of voltage-sensitive dye imaging during drug-induced epileptic seizures. Daisuke Takeshita, Vassiliy Tsytsarev, Sonya Bahar Epileptic seizures are generally held to result from excess and synchronized neural activity. However, recent studies have suggested that this is not necessarily the case. We investigate how the spatiotemporal pattern of synchronization changes during drug-induced in vivo neocortical seizures in rats. Epileptic seizures are caused by the potassium channel blocker 4-aminopyridine, which is often used in experiments to induce epileptic seizures. In our experiments, the neocortex is stained with the voltage-sensitive dye RH-1691. The intensity changes in dye fluorescence are measured by a CCD camera and are consistent with the signal from local field potential recording. We apply phase synchronization analysis to the voltage-sensitive dye signals from pairs of pixels in order to investigate the degree to which synchronization occurs, and how spatial patterns of synchrony may change, during the course of the seizure. Our preliminary results show that two distant pixels are well synchronized during a seizure event. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B16.00012: Information transfer in ampullary electroreceptors Alexander Neiman, Tatiana Engel Many neurons in central nervous system and in sensory periphery are characterized by significant correlations between consequent interspike intervals of their stochastic spontaneous activity. Such non-renewal stochastic dynamics can result from internal properties of a neuron, such as spike-frequency adaptation, as well as from external perturbations or both. We consider one example of such system, peripheral ampullary electroreceptors in paddlefish. Spontaneous dynamics of electroreceptors is characterized by extended serial correlations of interspike intervals resulting from nonlinear interaction of two stochastic oscillators embedded into the system. Using computational modeling and approaches from information theory we show that these correlations significantly improve information transfer of weak external stimuli. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B16.00013: Spike-time-variability in stochastic Hodgkin-Huxley type neural models Peter Rowat When the classical Hodgkin-Huxley equations are simulated with Na- and K-channel noise and fixed applied current, the distribution of inter-spike intervals is bi-modal: one part is an exponential tail, as often assumed, while the other is a narrow gaussian peak centered at a short ISI value. The gaussian arises from bursts of spikes in the gamma-frequency range, the tail from the inter-burst-intervals, giving overall a very highcoefficient of variation: upto 2.5 for 180,000 Na-channels. Since neurons with a bimodal inter-spike interval distribution are common, it may be a useful model for any neuron with class 2 firing. The underlying mechanism is due to a sub-critical Hopf bifurcation together with a switching region in phase-space where a fixed point is very close to a system limit cycle. This mechanism may contribute to highly irregular spike times in cortex. Other mechanisms underlying neural variability will also be presented. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B16.00014: Ca$^{2+}$ Dynamics and Propagating Waves in Neural Networks with Excitatory and Inhibitory Neurons. Vladimir E. Bondarenko Dynamics of neural spikes, intracellular Ca$^{2+}$, and Ca$^{2+}$ in intracellular stores was investigated both in isolated Chay's neurons and in the neurons coupled in networks. Three types of neural networks were studied: a purely excitatory neural network, with only excitatory (AMPA) synapses; a purely inhibitory neural network with only inhibitory (GABA) synapses; and a hybrid neural network, with both AMPA and GABA synapses. In the hybrid neural network, the ratio of excitatory to inhibitory neurons was 4:1. For each case, we considered two types of connections, ``all-with-all" and 20 connections per neuron. Each neural network contained 100 neurons with randomly distributed connection strengths. In the neural networks with ``all-with-all" connections and AMPA/GABA synapses an increase in average synaptic strength yielded bursting activity with increased/decreased number of spikes per burst. The neural bursts and Ca$^{2+}$ transients were synchronous at relatively large connection strengths despite random connection strengths. Simulations of the neural networks with 20 connections per neuron and with only AMPA synapses showed synchronous oscillations, while the neural networks with GABA or hybrid synapses generated propagating waves of membrane potential and Ca$^{2+}$ transients. [Preview Abstract] |
Session B17: Lipid Bilayers: Structure and Function
Sponsoring Units: DBPChair: Linda Hirst, Florida State University
Room: Morial Convention Center 209
Monday, March 10, 2008 11:15AM - 11:27AM |
B17.00001: Zoology of giant unilamellar vesicles Yan Yu, Stephen M. Anthony, Sung Chul Bae, Steve Granick Lipid vesicles, especially giant unilamellar vesicles (GUVs) are often used as simplified models for biological membranes, but their polymorphous panoply of shapes and shape changes is notorious to those who work with them. This affords opportunities to study why phospholipid membranes so often fail to minimize their surface area to adopt spherical shapes. For example, tube-like membranes are formed when flow is introduced during hydration and when certain types of lipids or polymers are inserted into the membrane. This talk will describe the evolution of GUVs from spherical to pearl-like and to tube-like shapes, and back again reversibly. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B17.00002: Adhesion induces and localizes phase separation in lipid membranes Markus Deserno, Vernita Gordon, Caroline Andrew, Stefan Egelhaaf, Wilson Poon We study, using confocal microscopy, model membranes in the form of giant unilamellar vesicles (GUVs) consisting of a mixture of two lipids. We demonstrate that, near a demixing transition, adhesion can favor phase separation, and thus induce the formation of well-defined, localized heterogeneities in a variety of lipid systems. We outline a theoretical framework in which this may be understood as the result of suppressing thermal fluctuations in the adhering areas and consequently favoring demixing. Our findings have important implications for the mechanisms by which biomembranes may create and stabilize functional heterogeneities, such as rafts and focal adhesion sites. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B17.00003: Cholesterol Effect on Phase Behavior in Ternary Lipid Membrane---X-ray Diffraction and AFM. Jing Yuan, Alex Kiss, Yohanes Pramudya, Lam Nguyen, Linda Hirst There is growing evidence that lipid membranes are not uniform, but contain lipid microdomains or ``rafts'', which are enriched in cholesterol, saturated long-chained lipids, and particular proteins. The effects of cholesterol on lipid ordering and phase separation in lipid-rafts-contained model membrane systems have been investigated by Synchrotron X-ray Diffraction and Atomic Force Microscope (AFM). We have measured bilayer d-spacings in two ternary lipid mixtures: DOPC/eSM/Cholesterol and DOPC/DPPC/Cholesterol, as cholesterol content is varied. Mixtures containing intermediate amounts of cholesterol exhibited two phases, and for DOPC/eSM/Cholesterol with 10{\%} and 12{\%} cholesterol, three d-spacings were observed, indicating the possible coexistence of three different phases: liquid disordered (ld) phase, liquid ordered (Lo) phase, and gel phase. AFM images of supported lipid bilayers on mica substrates contained clearly visible raft-like micro-domains in the similar cholesterol amount range. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B17.00004: Thermodynamic properties of planar membranes: applications to stripped phases Francisco J. Solis, Chloe Funkhouser, Katsuyo Thornton Multicomponent membranes can have shapes that are planar at large length scales while retaining complex morphologies at smaller scales. We explore the properties of these membranes that arise from the planarity condition. We show that planarity requires that the average stress tensor of the membrane be parallel to the planar directions. We apply this description of planar membranes to the case of striped (lamellar) morphologies. In this case we determine the possible shapes of membranes, their stability and the thermodynamic equations of state satisfied by their intensive variables. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B17.00005: Phospholipid Membranes Restructure Locally Where Nanoparticles Bind Bo Wang, Yan Yu, Stephen Anthony, Sung Chul Bae, Steve Granick In the field of surface science, it is known that metal and semiconductor surfaces may respond to their environment by restructuring. Similar issues are more significant in nanoscience, since large populations of the atoms/molecules reside on the surfaces. It is natural to inquire whether analogous restructuring might also be characteristic of phospholipid membranes, bearing mind that no bulk exists at all in this case. We show here that the two categories of reconstructions, phase state and local curvature, of unicomponent lipid bilayers can occur through non-specific interactions when charged nanoparticles adsorb. This coupling not only modulates the short-range molecular orientation and packing, but also is believed to be responsible for long-range interaction and transportation on fluctuating membranes. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B17.00006: Multiscale Modeling of supported lipid bilayers Roland Faller, Chenyue Xing, Matthew Hoopes The study of lipid structure and phase behavior at the nano scale length is of importance due to implications in understanding the role of the lipids in biochemical membrane processes. We performed a variety of simulations in homogeneous and heterogeneous membrane systems to elucidate such behaviors. Our simulations demonstrate that various coarse grained simulation models can predict different aspects of lipid phase separation and describe the change of the system under the influence of a support. The simulations are performed using models at different length scales ranging from the all atom scale to a scale where lipids are modeled by only three interaction sites. We are able to follow transformations, such as lipids phase transitions. These phase transitions are determined by analyzing parameters like area per lipid head group, the deuterium order parameter and dynamic properties. Additionally, we characterize individual lipid molecules using rotational correlation functions to classify different dynamic populations and we study the stability of artificially designed patterns. We discuss the changes of the system phase behavior as well as differences between the two leaflets as induced by the support. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B17.00007: Synergy of Membrane Curvature-Stabilization and Electrostatic Interaction leads to Formation of Block Liposomes by Colossal Charged Lipids Alexandra Zidovska, Kai K. Ewert, Cyrus R. Safinya, Joel Quispe, Bridget Carragher, Clinton S. Potter Recently, we have reported block liposomes (BLs), a new vesicle phase formed in mixtures of MVLBG2, DOPC and water (A. Zidovska et al., \textit{Submitted}, 2007), where MVLBG2 is a newly synthesized highly charged (16+) lipid (K. Ewert et al., \textit{JACS, }2006) with giant dendrimer-like headgroup. BLs are liposomes consisting of distinctly shaped nanoscale spheres, pears, tubes, or rods connected into blocks. In this work we investigate the contribution of spontaneous curvature and membrane charge density to the formation of BLs. By comparing with a system of matching membrane charge density but zero spontaneous curvature and by screening the charge of MVLBG2 but keeping the curvature constant, we were able to identify both, spontaneous curvature and membrane charge, as critical parameters for BLs-formation. The effect of salt and pH on the shape evolution of the BLs was also carefully studied. Funding provided by DOE DE-FG-02-06ER46314, NIH GM-59288, NSF DMR-0503347. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B17.00008: Photo-induced Phase Separation Phenomena in Lipid Tubules Linda Hirst, Jing Yuan The self-assembly of biological amphiphiles has proved a fascinating topic in recent years, the hollow cylindrical lipid tubule morphology being of particular interest due to its potential relevance to intercellular transporting channels and applicability to controlled-release systems, chemical micro-reactors and nano-conduits. Co-existence of the liquid-ordered and liquid-disordered phases in the lipid bilayer has recently been observed in biologically-relevant three-component giant unilamellar vesicles. We have generated stable, photo-induced micron-scale phase separation in lipid tubules formed from ternary lipid mixtures, inducing a new bilayer disc structure. This investigation not only aids in our understanding of lipid sorting phenomena in cell membranes (suggesting a mechanism for bilayer disc formation in retinal rod-cells), but is also a fascinating route to the generation of new, functional structures. This work is supported by the MARTECH and the Institute of Molecular Biophysics, both at Florida State University. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B17.00009: Dynamics of Multi-Component Model Membranes Studied via Light and X-Ray Scattering Kevin Johnson, Maikel Rheinst\"adter We study the dynamics of multicomponent biological model membranes (phospholipid, ethanol, cholesterol systems) via X-ray and light scattering to probe the dynamics of such membranes in solid supported and freestanding configurations. Collective molecular motions may play a significant role in different biological functions such as transmembrane transport and pore opening processes. Our main research objective is to quantify collective molecular motions in membranes and establish relationships to key physiological and biological functions of the bilayers. The phase diagram of this system with varying cholesterol and ethanol concentrations at set temperatures is determined using X-ray diffraction techniques and the mesoscopic membrane dynamics is then measured using time correlation light scattering techniques. The results can be compared to molecular dynamics simulations in a coarse grained membrane model. The dynamics shows propagating and relaxating processes, which allow to determine, e.g. the elasticity parameters of the bilayers. By understanding the mesoscopic properties of membranes with selected composition, membranes with specific properties can be designed. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B17.00010: The Impact of Collective Molecular Dynamics on Physiological and Biological Functionalities of Artificial and Biological Membranes Maikel Rheinstadter We use neutron, X-ray and light scattering techniques to determine dynamical and structural properties of artificial and biological membranes. The combination of various techniques enlarges the window to length scales from the nearest-neighbor distances of lipid molecules to more than 10$^{-6}$m, covering time scales from about 0.1 ps to 1 s. The main research objective is to quantify collective molecular fluctuations in these systems and to establish relationships to physiological and biological functions of the bilayers, such as transmembrane transport. The motivation for this project is twofold: 1) By understanding fundamental properties of bilayers at the microscopic and mesoscopic level, we aim to tailor membranes with specific properties such as permeability and elasticity. 2) By relating dynamical fluctuations to physiological and biological functions, we can gain a deeper understanding of the bilayers on a molecular scale that may help optimizing the transmembrane transport of certain drugs. We show how bilayer permeability, elasticity and inter protein excitations can be determined from the experiments. M.C. Rheinst\"{a}dter et al., Phys. Rev. Lett. 93, 108107 (2004); Phys. Rev. Lett. 97, 048103 (2006); Phys. Rev. E 75, 011907 (2007);J. Vac. Soc. Technol. A 24, 1191 (2006). [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B17.00011: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 1:27PM - 1:39PM |
B17.00012: Effect of anesthetics on bending elasticity of lipid membranes Zheng Yi, Nagao Michihiro, Dobrin Bossev Change in physical and chemical properties of bio-membranes is of great interest for understanding the mechanism of anesthetic action on membranes. Hypothetically the anesthetic alters the lipid membrane structure (promoting pore formation across membranes or at least switching transmembrane channels) and therefore the biophysical properties of the membrane. We have used neutron spin echo (NSE) spectroscopy to study the effect of anesthetic molecule, lidocaine, on the bending elasticity (BE) of lipid membranes. BE of lipid bilayers made of (1,2-Dimyristoyl-\textit{sn}-Glycero-3-Phosphocholine) DMPC and 1,2-Dipalmitoyl-\textit{sn}-Glycero-3-Phosphocholine (DPPC) have been measured at different temperatures and different in the fluid (L$_{\alpha })$ phase. Using Zilman-Granek theory the BE were obtained from the decay of the NSE intermediate scattering function. We have found that in the presence of lidocaine the BE of DMPC and DPPC bilayers increases. The results were correlated with those from differential scanning calorimetry. Increase in the lidocaine concentration leads to decrease in the liquid/crystalline transition temperature. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B17.00013: Interactions between non-steroidal anti-inflammatory drugs and lipid membranes Mohan Boggara, Ramanan Krishnamoorti Chronic usage of Non-steroidal anti-inflammatory drugs(NSAIDs) leads to gastrointestinal toxicity and clinical evidences point the cause to direct interactions between NSAIDs and phospholipid membranes. Also, NSAIDs pre-associated with phospholipid vesicles are shown to be safer and therapeutically more effective than unmodified ones. Our initial experiments and simulations on the partitioning of Aspirin and Ibuprofen clearly indicate role played by the drug structure in drug-membrane interactions. Those results motivated systematic molecular dynamics simulations of membranes with NSAIDs of different size, structure and pKa values. Our results suggest high partition coefficients for these NSAIDs in the membrane compared to water and thinning effect on the bilayer. Our small angle neutron scattering and reflectivity studies on DMPC-Ibuprofen systems indicate that the drug affects both $\sim $5 nm thick bilayer and overall $\sim $100 nm diameter vesicle, indicating that NSAIDs affect vesicles on various length scales. We will discuss the structural perturbations to membranes due to NSAIDs at clinically relevant molar ratios and their implications on the use of vesicles as delivery vehicles for NSAIDs. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B17.00014: Calorimetric and Optical Studies of Cholesterol-Rich Filamentous, Helical Ribbon, and Crystal Microstructures Klaida Kashuri, Germano S. Iannacchione, Yekaterina A. Miroshnikova, Yevgeniya V. Zastavker Calorimetry (differential-scanning and modulation) and optical phase contrast microscopy studies have been performed on the filamentous, helical ribbon, and crystal microstructures formed in Chemically Defined Lipid Concentrate (CDLC). CDLC is a quaternary sterol system consisting of a cholesterol, bilayer-forming amphiphiles, micelle-forming amphiphiles, and water. Phase contrast microscopy confirms the presence of the three microstructure types in all samples studied. Sample size and temperature scan rate were varied on samples ranging from 1 to 20 mg and rates from 0$:$017 to 1 degree C/min, respectively. Thermal profiles are strongly dependent on sample size, scan rate, and thermal history. These scans also reveal numerous ``transition'' features, likely due to melting of various microstructures in CDLC, that generally shift to higher temperatures with increasing sample size. These results indicate that the filamentous, helical ribbon, and crystal microstructures formed in CDLC may be coexisting in a meta-stable chemical equilibrium with each other and the solvent environment from which they grow. [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B17.00015: Structural and dynamical properties of water at the bilayer DPPC membrane interface Jianping Gao, Charles Cleveland, Uzi Landman The properties of interfacial water near the DPPC lipid bilayer surface are probed by a nano-size quartz tip through large scale atomistic molecular dynamics simulations. The water films confined between the bilayer membrane surface and a crystalline wetting quartz surface are kept in contact with a water reservoir at 293K. The distance between the solid tip and the gel phase membrane is varied between 0.5 to 2 nanometers. Layering of the confined water film is found near the solid tip while the water at the membrane surface remains unlayered. Some water is trapped in the cavities between the head groups of the lipid molecules. The recorded solvation force does not show oscillations due to the rough nature of the membrane surface. [Preview Abstract] |
Session B18: Focus Session: Mechanical Properties of Polymers: Fracture and Adhesion
Sponsoring Units: DPOLY FIAPChair: Theresa Hermel-Davidock, Dow Chemical Company
Room: Morial Convention Center 210
Monday, March 10, 2008 11:15AM - 11:51AM |
B18.00001: High Strain Deformation and Fracture of Self-Assembled Polymer Gels Invited Speaker: Triblock copolymers with poly(methyl methacrylate (PMMA) endblocks and a poly(n-butyl acrylate) (PnBA) midblock form thermoreversible gels in a variety of alcohols. The mechanical response of the gels is determined by the nature of the PMMA aggregates that are bridged by PnBA midblocks. The close proximity of the ordering temperature to the glass transition of the PMMA aggregates gives rise to a remarkably strong temperature dependence of the relaxation time for the polymer gels. At low temperatures, where the relaxation times are very large, the gels can be deformed to very large strains prior to solid-like fracture. At intermediate temperatures the materials flow, but strain localization leads to a melt fracture phenomenon, and at higher temperatures the materials behave as viscous polymer solutions. We have used these gels as a model material for studying rate effects in the high strain deformation and fracture of soft solids. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B18.00002: Reinforcement of Epoxies Using Single Walled Carbon Nanotubes Ramanan Krishnamoorti, Jitendra Sharma, Tirtha Chatterjee The reinforcement of bisphenol-A and bisphenol-F epoxies using single walled carbon nanotubes has been approached experimentally by understanding the nature of interactions between the matrices and nanotubes. Unassisted dispersions of single walled carbon nanotubes in epoxies were studied by a combination of radiation scattering (elastic small angle scattering and inelastic scattering), DSC based glass transition determination, melt rheology and solid-state mechanical testing in order to understand and correlate changes in local and global dynamics to the tailoring of composite mechanical properties. Significant changes in the glass transition temperature of the matrix can successfully account for changes in the viscoelastic properties of the epoxy dispersions for concentrations below the percolation threshold, while above the percolation threshold the network superstructure formed by the nanotubes controls the viscoelastic properties. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B18.00003: Mechanical and Electrical Properties of Organogels with Multiwall Carbon Nanotubes Mohammad Moniruzzaman, Karen Winey Organogels are fascinating thermally reversible viscoelastic materials that are comprised of an organic liquid and low concentrations (typically $<$2 wt {\%}) of low molecular mass organic gelators. We have fabricated the first organogel/carbon nanotube composites using 12-hydroxystearic acid (HSA) as the gelator molecule and pristine and carboxylated multi-wall carbon nanotubes as the nanofillers and 1,2-dichlorobenzene as the organic solvent. We have achieved significant improvements in the mechanical and electrical properties of organogels by incorporating these carbon nanotubes. For example, the linear viscoelastic regime of the HSA organogel, an indicator of the strength of the gel, extends by a factor of 4 with the incorporation of 0.2 wt{\%} of the carboxylated nanotubes. Also, the carbon nanotubes (specially the pristine tubes) improve the electrical conductivity of the organogels, e.g. six orders of magnitude enhancement in electrical conductivity with 0.2 wt{\%} of pristine tubes. Differential scanning calorimetry experiments indicate that the nanotubes do not affect the thermoreversibility of the organogels. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B18.00004: Identification of key deformation mechanisms of polyethylene materials via in-situ x-ray scattering Theresa Hermel-Davidock, Brian Landes, Mehmet Demirors Changes in the microstructure of ethylene based copolymers can be used to modify and enhance their mechanical performance. In this study, the effects of comonomer content, molecular weight, and molecular weight distribution on the mechanical performance of select polyethylene polymers were examined. Two key performance parameters for commercial polymer materials, especially in the area of blown film applications, are tear resistance and puncture resistance. However, polyethylene films which exhibit very similar morphology often exhibit very different Dart and Elmendorf tear values which cannot be differentiated by standard tensile test methods. Alternative methods to link morphology and mechanical response need to be found. Wide-angle and small-angle x-ray scattering was collected during in-situ tensile testing to understand how compositional and structural differences affect the mechanical response of semicrystalline polyethylene polymers. Microstructural changes observed during the in-situ deformation process are correlated to Elmendorf tear performance for both intrinsic and blown film samples. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 1:03PM |
B18.00005: Brittle-tough transitions during crack growth in toughened adhesives Invited Speaker: The use of structural adhesives in automotive applications relies on an effective understanding of their performance under crash conditions. In particular, there is considerable potential for mechanics-based modeling of the interaction between an adhesive layer and the adherends, to replace current empirical approaches to design. Since energy dissipation during a crash, mediated by plastic deformation of the structure, is a primary consideration for automotive applications, traditional approaches of fracture mechanics are not appropriate. Cohesive-zone models that use two fracture parameters - cohesive strength and toughness - have been shown to provide a method for quantitative mechanics analysis. Combined numerical and experimental techniques have been developed to deduce the toughness and strength parameters of adhesive layers, allowing qualitative modeling of the performance of adhesive joints. These techniques have been used to study the failure of joints, formed from a toughened adhesive and sheet metal, over a wide range of loading rates. Two fracture modes are observed: quasi-static crack growth and dynamic crack growth. The quasi-static crack growth is associated with a toughened mode of failure; the dynamic crack growth is associated with a more brittle mode of failure. The results of the experiments and analyses indicate that the fracture parameters for quasi-static crack growth in this toughened system are essentially rate independent, and that quasi-static crack growth can occur even at the highest crack velocities. Effects of rate appear to be limited to the ease with which a transition to dynamic fracture could be triggered. This transition appears to be stochastic in nature, and it does not appear to be associated with the attainment of any critical value for crack velocity or loading rate. Fracture-mechanics models exist in the literature for brittle-ductile transitions in rate-dependent polymers, which rely on rate dependent values of toughness with unstable branches. The present observations do not appear to follow the type of behavior that would be expected from these models, but are consistent with prior observations of fracture instabilities in bulk model rubber-toughened epoxies. Some alternative models for the transitions in fracture mode will be discussed. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B18.00006: Controlling polymer adhesion with surface wrinkles Edwin Chan, Erica Smith, Ryan Hayward, Alfred Crosby One of nature's solutions in controlling adhesion is through the intelligent design of patterned interfaces. For example, the feet of geckos and some insects are decorated with fibrillar structures designed specifically for locomotion, i.e. enhanced control of adhesion and release. Recently, there have been significant efforts in mimicking these materials to develop synthetic analogs to tailor polymer adhesion. However, challenges such as reusability and fabrication scalability limit the succesful applications of these materials. In this work, we present an alternative approach to the design of a patterned adhesive that utilizes surface wrinkles to contol the adhesion of a poly($n$-butyl acrylate) (P$n$BA) elastomer. Using a probe-type contact adhesion test, we experimentally show that surface wrinkles enhance adhesion of the P$n$BA elastomer based on a mechanism termed contact line splitting. We demonstrate that the efficiency of this contact line splitting mechanism is coupled with the wavelength of the wrinkles. Furthermore, the geometry of the surface wrinkles facilitates a repeatable interfacial response necessary to function as a reuseable adhesive. Our approach provides a simple and scaleable strategy to the design of patterned adhesives that is amenable to a variety of polymeric materials while facilitating enhanced control of interfacial response. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B18.00007: Soft-soft nanocomposite adhesives made from colloidal particles Costantino Creton, Fanny Deplace, Michael Rabjohns, Andrew Foster, Peter Lovell, Chunghong Lei, Joseph Keddie, Keltoum Ouzineb, Jeanne Marchal Nanocomposites are often made with a dispersed hard phase in a soft matrix. In some cases however it might be interesting to obtain the stiffness from a continuous honeycomb structure and the toughness from the inside of the cells. Colloidal latex particles provide some unique opportunities to design nanocomposites. We have synthesized core-shell particles of acrylic copolymers where the outside shell contains a small amount of crosslinkable groups. These groups can then be used to introduce crosslinking points in and between the shells during the drying of the latex dispersion into a homogeneous film. The resulting films have very different large strain properties while the small and intermediate strain properties remain controlled by the viscoelastic properties of the inside of each particle. We show that films made in that way can have a very pronounced strain softening at intermediate strains (characteristic of viscoelastic liquids) while retaining a pronounced hardening at large strains (characteristic of viscoelastic solids'). This particular balance of properties results in a strong adherence to low energy surfaces while retaining a good resistance to creep. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B18.00008: Dangling chain effect on the modulus of polyurethane networks Bruno Fayolle, Julie Diani, Pierre Gilormini While the theory of elasticity has been well verified for the non ideal structures accounting for contribution of entanglements, some effort is needed to investigate the contribution of dangling chains. In order to establish a quantitative contribution of dangling chains, networks with a controlled architecture, i.e. where architecture is determined by synthesis of well-characterized reactants, are required. In this work, polyurethane networks based on poly(propylether) and poly(tetramethyl adipate) crosslinked by triisocyanate were prepared and studied. Different polyether molar masses are chosen from 430 g/mol up to 4000 g/mol. By varying the stoechiometric ratio r = [NCO]/[OH] between 0.4 and 1, dangling chains are introduced, provided that the reaction between NCO groups is negligible. After synthesis, the Young's modulus (E) of the networks has been measured from tensile tests according to the neo-Hookean law. The molar mass of elastically active network chains (ENAC) is determined from E. Since this molar mass is close to the molar mass of each diol used for synthesis at r=1 (``ideal'' network), a correction taking into account the number of dangling chains (b) calculated from r is proposed. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B18.00009: Morphological Determinants of Yield Stress for Semicrystalline Ethylene / Methacrylic Acid Copolymers Robert Scogna, Richard Register Reducing the crystal thickness of ethylene/$\alpha$-olefin copolymers typically results in a decrease in the measured yield stress. However, statistical incorporation of methacrylic acid, also a noncrystallizable comonomer, actually increases the yield stress at room temperature. The yield stress for ethylene/methacrylic acid (E/MAA) copolymers as a function of temperature and test rate is described using a model which accounts for polyethylene crystal plasticity through thermal nucleation of screw dislocations in addition to the effects of incomplete relaxation of the amorphous fraction at the strain rate employed. This is possible using a small number of physically reasonable best-fit parameters. Yield stress master curves can be constructed for any material that obeys the model; such curves have been constructed for a low-density polyethylene and five copolymers of varying MAA content from data taken at various strain rates and temperatures. The master curves clearly show that this unusual behavior of the yield stress is caused by the increase in $\beta$ relaxation temperature with increasing MAA content, as seen via dynamic mechanical testing. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B18.00010: On the statistics of Gaussian two and three-dimensional networks: Fluctuations of junctions and collapse driven by structure. Michael Lang, Sergey Panyukov, Michael Rubinstein, Jens-Uwe Sommer We investigate ideal Gaussian networks both analytically and with computer simulations using the Bond Fluctuation model with and without excluded volume interactions. The focus of this study is on fluctuations and the collapse of networks with different connectivity and dimensionality. We show that the size of a perfect square 2D network made from $n^2$ Gaussian chains with $N$ monomers each is $R_{g} \sim N^{1/2}log(n)$. Thus fluctuations in two-dimensional networks diverge logarithmically with the size of these films while fluctuations of three-dimensional networks $\sim N^{1/2}$ and do not increase with their size. We study the cross-over between two and three-dimensional networks by following the dependence of junction fluctuations on the thickness of films. The results of model systems are compared with more realistic networks in order to understand the effect of disorder on the properties of the network and fluctuations of network junctions. [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B18.00011: Physical understanding of the bulk modulus of polyisoprene by molecular dynamics simulations Julie Diani, Bruno Fayolle, Pierre Gilormini The temperature dependence of the Young's modulus of amorphous polymers has been explained well in the literature, however, the moderate drop of the bulk modulus of a factor 2 or 3 is still not fully understood and the question of a possible entropic contribution to the bulk deformation is still open. In this work, we propose to address the question of the physical source of the bulk modulus of the polyisoprene cis1-4, using molecular dynamics simulations. Using Material Studio MD simulations software, we built several periodic virtual polyisoprene. The materials were submitted to volume contractions at various temperatures. During the simulations, we monitored among other parameters, the volume, the pressure, the temperature, the bonded, nonbonded and kinetic energies of the cell. These data were used in a classical thermodynamics analysis to assess the physical source of the bulk modulus below and above the glass transition temperature. The results showed that the Van der Waals interactions control the bulk modulus below the glass transition. Above the glass transition the internal energy contribution to the bulk modulus is driven by the Van der Waals interaction but a non negligible entropic contribution appeared which at a first order may be considered as independent on the temperature. [Preview Abstract] |
Session B19: Novel Surface Instrumentation and Techniques
Sponsoring Units: DCMPChair: John Wendelken, Oak Ridge National Laboratory
Room: Morial Convention Center 211
Monday, March 10, 2008 11:15AM - 11:27AM |
B19.00001: Spectromicroscopic characterization of Ag surfaces by energy-filtered LEEM Y. Fujikawa, T. Sakurai, R.M. Tromp Low energy electrons have been applied successfully for both structural and spectroscopic studies since the early days of modern surface science. While the reciprocal space analysis has been combined with both microscopic and spectroscopic studies, there is still very limited availability of spectromicroscopic analysis because it is not easy to achieve both high spatial and spectroscopic resolution at low energies. We present spectromicroscopic characterization of Ag islands on Si substrates using a newly developed energy-filtered Low Energy Electron / Photo-Electron Emission Microscope capable of 5D (2D spatial + 2D reciprocal + energy) surface analysis. The electron energy loss signal from Ag surface plasmons (SP) was imaged in real space with a resolution of 35 nm or less, while the SP energy dispersion was obtained from a 6 $\mu $m diameter area on Ag(111) within $\sim $1 second. HeI photoemission spectra were obtained from a complex Ag island, selecting Ag(111) and Ag(100) areas with a $\phi $4 $\mu $m aperture during PEEM observation. Full dispersion data covering the full reciprocal-energy space were obtained from both surfaces, reflecting their respective symmetries. The ability to perform detailed spectromicroscopic experiments in a standard lab environment is of key importance for nanoscale analysis of novel structures and materials. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B19.00002: Enhanced depth-resolution in multilayer nanostructures from standing-wave excited photoemission Sven Doering, Mihaela Gorgoi, Ruslan Ovsyannikov, See-Hun Yang, Mark Huijben, Franz Schaefers, Daniel Buergler, Claus Schneider, Charles S. Fadley, Walter Braun, Carsten Westphal The depth resolution for studying buried layers and interfaces in multilayer structures with photoemission can be enhanced by exciting with a standing-wave field created by reflection from a multilayer mirror substrate. Combining experiment with x-ray optical simulations can provide information on depth profiles of different chemical species and their magnetic properties. We have applied this method, using both rocking curves and sample scans over a wedge profile, to several types of samples: bare multilayers of Si/Mo and epitaxial SrTiO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_{3}$, and epitaxial MgO on a wedge of Fe on top of a GaAs/AlAs multilayer. Both soft x-ray excitation at the TU Dortmund DELTA facility and hard x-ray excitation at BESSY II in Berlin have been utilized, with the hard x-rays for the first time permitting the study of deeper layers and interfaces. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B19.00003: Interface characterization using atomic core-level shifts Erik Holmstrom, Weine Olovsson, Igor Abrikosov, Anders Niklasson, M. Gorgoi, Olof Karis, Svante Svensson, F. Schafers, W. Braun, G. Ohrwall, G. Andersson, B. Johansson, M. Marcellini, W. Eberhardt We propose a nondestructive technique based on atomic core-level shifts to characterize the interface quality of thin film nanomaterials. Our method uses the inherent sensitivity of the atomic core-level binding energies to their local surroundings in order to probe the layer-resolved binary alloy composition profiles at deeply embedded interfaces. From an analysis based upon high energy x-ray photoemission spectroscopy and density functional theory of a Ni/Cu fcc (100) model system, we demonstrate that this technique is a sensitive tool to characterize the sharpness of a buried interface. We performed controlled interface tuning by gradually approaching the diffusion temperature of the multilayer, which lead to intermixing. We show that core-level spectroscopy directly reflects the changes in the electronic structure of the buried interfaces, which ultimately determines the functionality of the nanosized material. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B19.00004: Manifestation of Work Function Difference in High Order Gundlach Oscillation Chun-Liang Lin, Shin-Ming Lu, Wei-Bin Su, Hwa-Te Shih, Bi-Fen Wu, Yeong-Der Yao, Chia-Seng Chang, Tien-Tzou Tsong Gundlach oscillation (or standing-wave state) is a general phenomenon manifesting in the tunneling spectrum acquired from a metal surface using scanning tunneling spectroscopy. Previous studies relate the energy shift between peaks of the lowest-order Gundlach oscillation observed on the thin film and the metal substrate to the difference in their work functions. By observing Gundlach oscillations on Ag/Au(111), Ag/Cu(111) and Co/Cu(111) systems, we demonstrate that the work function difference is not the energy shift of the lowest order but the ones of higher order where a constant energy shift exhibits. Higher order Gundlach oscillations can thus be applied to determine the work function of thin metal films precisely. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B19.00005: Application of Gundlach Oscillation in Scanning Tunneling Spectroscopy on Nano-Scale Imaging Shin-Ming Lu Gundlach oscillation observed with scanning tunneling spectroscopy (STS) is a phenomenon of field-emission resonance through standing-wave states in the tip-sample gap. No matter if the sample is the thin film or the bulk, peak characteristic of Gundlach oscillation can always appear in the tunneling spectrum. We use STS to study Gundlach oscillation on Ag/Si(111)7$\times $7 and Au(111) surfaces. It is shown that the spectral intensity of the Gundlach oscillation peaks can vary with observed locations. The spatial mappings of spectral intensities at peaks can reveal the contrasts of the interface structure on Ag/Si(111)7$\times $7 and herringbone reconstruction on Au(111) surfaces. The contrast can be attributed to the local variation of the electron transmissivity which affects the transmission background superposing with Gundlach oscillation in the tunneling spectrum. In our observation, the spatial resolution of the mapping is 1 nm and can be preserved even the tip is away from the sample by 60 angstrom. Gundlach oscillation may be useful for the nano-scale imaging of the soft material. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B19.00006: Standing Friedel waves Jun-Qiang Lu, X.-G. Zhang, SokratesT. Pantelides The electron density around defects in a metal is known to exhibit Friedel oscillations. Here, we report simulations that demonstrate a dynamic analogue of the static Friedel oscillation in nanoscale devices. We use a spot gate capacitively coupled to a nanowire or a two-dimensional electron gas, a setup that can be implemented with a sharp STM tip. The application of an AC voltage generates a dynamic standing Friedel wave (SFW), near the spot gate and edges and boundaries. The SFW wave length is controlled by the AC frequency and the device's Fermi velocity, whereby the latter can be measured. The SFW amplitude exhibits resonant behavior at AC frequencies that are related to eigenenergy spacings in the device, allowing their direct measurement. Spin-polarized SFW may be generated in a graphene nanoribbon. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B19.00007: Molecular Etching of Pure and Mn Intercalated TiSe$_{2}$ using an STM Tim Kidd, Brett Gamb, Polina Skirtachenko, Laura Strauss STM is normally a non-destructive technique. However, some materials, such as the transition metal dichalcogenides (TMDCs), are so weakly bonded that STM measurements performed in air can change the surface topology. We performed STM measurements on single crystals of TiSe$_{2}$ and Mn$_{0.05}$TiSe$_{2}$ in air. Under normal scanning conditions, the surface of both TiSe$_{2}$ and Mn$_{0.05}$TiSe$_{2}$ would spontaneously etch, removing single and double molecular layers. In pure TiSe$_{2}$, step edges were unusually rounded and the etching predominately occurred along the scanning direction. In Mn$_{0.05}$TiSe$_{2}$, the etching occurred even more readily. However, step edges in these samples were much straighter, reflecting the crystal symmetry. The symmetry was also seen in the etching of these samples, as triangular pits were often formed along with the usual etching along the scanning direction. The differences seen in the etching of Mn$_{0.05}$TiSe$_{2}$ samples indicate intercalated ions can affect both intra- and inter-layer bond strengths. The etched material disappeared completely from the sample, suggesting that TiSe2 molecules are energized sufficiently to sublimate from the sample. This research indicates that the etching process can be controlled to induce complex nanostructures in the surface of TMDCs. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B19.00008: Effect of Surface Stress on the Stiffness of Cantilever Plates John Sader, Michael Lachut Measurements over the past 30 years have indicated that surface stress can significantly affect the stiffness of microcantilever plates. Several one-dimensional models based on beam theory have been proposed to explain this phenomenon, but are found to be in violation of Newton's third law, in spite of their good agreement with measurements. In this talk, we shall review this work and rigorously examine the effect of surface stress on the stiffness of cantilever plates using a full three-dimensional model. This study establishes the relationship between surface stress and cantilever stiffness, and in so doing elucidates its scaling behavior with cantilever dimensions. Use of short nanoscale cantilevers thus presents the most promising avenue for future investigations. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B19.00009: Direct determination of transient heating in a nanoconfined environment by ultrafast electron diffraction Ryan A. Murdick, Ramani K. Raman, Yoshie Murooka, Richard J. Worhatch, Chong-Yu Ruan Temperature is generally ill-defined at the statistical limit, such as on the ultrashort time scale and in mesoscopic systems. Understanding the thermal energy relaxation and transport at such limits is key to nanoelectronics and energy research. We report on the development of \textit{local} temperature determination on the atomic scale using the technique of ultrafast electron diffraction. Lattice heating characteristics are elucidated by determining the dynamical Debye-Waller factor and local bond stretches, allowing the time scale of electron-phonon coupling and local heating at different length scales to be detailed. We compare these measured temperatures with predictions from two-temperature model (2TM) and determine that strongly coupled `hot' phonons dominate lattice heating at short times (10 ps), and cannot explained by 2TMs. Models beyond the 2TM are developed to explain our data. We also discuss other novel heating scenarios at the statistical limits. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B19.00010: Surface Femtochemistry with Hyperthermal Energy Ion Beams M.P. Ray, R.E. Lake, C.E. Sosolik We are investigating the interactions of hyperthermal energy ions with ultrathin film Schottky diode devices. Specifically, we apply a bias voltage across the device in order to alter the charge transfer dynamics between an incident ion and the metal surface of the Schottky diode. This is an extension of previous work where thermal energy atoms were used to excite electrons-hole pairs and ballistically transport electrons through an ultrathin metal film [1]. In our experiment, we modify the surface electron energy distribution by ballistically transporting electrons to the surface of the thin film. This allows us to tailor the energy level crossings between the incident ion and the metal film and to change the neutralization probability of the scattered beam. Varying the bias voltage will open the possibility for tunable chemical reactions. Preliminary results are presented and discussed in the context of basic ion-surface interactions. [1] H. Nienhaus, H.S. Bergh, B. Gergen, A. Majumdar, W.H. Weinburg and E.W. McFarland, Physical Review Letters \textbf{82}, 446 (1999). [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B19.00011: Hyperthermal Energy Ion Scattering as a Time Resolved Probe of Pico- to Femtosecond Surface Excitations R.E. Lake, M.P. Ray, C.E. Sosolik Trajectories of hyperthermal energy alkali ions scattered from single crystal metal surfaces are well described by binary classical collisions with a strong dependence on the atom-surface mass ratio. Such simple events allow hyperthermal ion scattering to be used as a time resolved probe for studying novel surface effects such as femtosecond scale electron transport and ballistic electron excitations. In this talk, three ion scattering cases from the 1-1000 eV energy regime are discussed. K$^{+}$ scattered from Ag(100) is compared to Na$^{+}$ scattered from Cu(100) in terms of interaction potential, image charge effects and neutralization probability. Secondly the heavy atom Cs$^{+}$-Ag(100) system will be presented including a discussion of an anomalous high energy peak possibly attributable to a collective surface response. Finally a method for probing hot electron excitation by an alkali beam scattered from a biased atomically ordered ultrathin film device will be discussed including charge transfer predictions based on rate equations and quantum mechanical 1/N and dynamic matrix renormalization group codes. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B19.00012: An Investigation of Nitride Formation on InP Surface After Nitrogen Ion implantation Mohammad Reza Hantehzadeh The effect of nitrogen ion implantation and change in physical characteristics of InP [100] surface after nitrogen ion implantation is investigated. The energetic 30 KeV nitrogen ions with different doses were implanted into [100] InP surface at about 500-700 k. The formation of different phases of indium nitride and change in surface morphology after the implantation is studied. The annealing effect on change in nitride phase in a nitrogen environment at temperatures above 900 k is observed. The morphology and phases of the surface after ion implantation is characterized using AFM and XRD. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B19.00013: Sputter Deposition System for High Throughput Fabrication of Composition Spread Thin Films John Gregoire, Frank DiSalvo, Hector Abruna, Robert Bruce van Dover We describe a custom built sputtering system that can deposit composition spreads in an effectively UHV environment but which does not require the high-throughput paradigm to be compromised by a long pumpdown each time a target is changed. The system employs four magnetron sputter guns in a cryoshroud (getter sputtering) which allows elements such as Ti and Zr to be deposited with minimal contamination by oxygen or other reactive background gasses. Other features of the deposition system will be presented, most notably the ability to quickly measure deposition profiles from individual deposition sources. We discuss the possibility of calculating codeposited film composition from these profiles, noting that codeposition affords resputtering phenomena which are absent in single-source deposition. To demonstrate the efficacy of this system, we describe our study of combinatorial libraries of electrocatalyst materials for fuel cell applications. This study includes a high-throughput parallel screening of composition spreads using a fluorescence indicator. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B19.00014: Discharging Optics in Vacuum Mark Girard, Dennis Ugolini We have studied using UV illumination to remove excess surface charge from fused silica optics. We commissioned and calibrated a commercial Kelvin probe to measure the surface potential of charged optics in vacuum. Using a Xenon light source and a monochromator, we directed UV light at the sample and were able to remove the excess charge. We determined that the discharging rate scaled linearly with the intensity of the light and the charge density on the surface. By varying the wavelength of the light, we saw a peak discharge rate at 215nm in both uncoated and coated optics. The Kelvin probe also allows us to study the sign of the charge carriers and other techniques for charge removal. [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B19.00015: Ordering process in metallic thin films investigated with angle-resolved photoelectron spectroscopy Dah-An Luh, Cheng-Maw Cheng, Chi-Ting Tsai, Ku-Ding Tsuei, Jian-Ming Tang We report the observation of the ordering process in real time of a Ag thin film on a Au(111) surface by measuring the in-plane dispersion of quantum well states using temperature-dependent angle-resolved photoelectron spectroscopy. Low-temperature deposited Ag films on a Au(111) substrate were annealed to yield atomically flat films, and the in-plane dispersion of quantum well states was measured in real time during annealing. Our results revealed that isolated ordered patches, fully crystallized along the surface normal, are formed as an intermediate step in the process of film crystallization. We observed the transition from localized states in a partially ordered film to free-electron-like states in a fully ordered film. This process may be general in many other systems of metal thin films. [Preview Abstract] |
Session B20: Focus Session: Growth, Kinetics and Quantum Effects in Metal Thin Films
Sponsoring Units: DMPChair: Robert Bartynski, Rutgers University
Room: Morial Convention Center 212
Monday, March 10, 2008 11:15AM - 11:27AM |
B20.00001: Growth instabilities and adsorbed impurities: Nanostructuring of vicinal surfaces controlled by adsorbates Ajmi BHadj Hamouda, T.L. Einstein, P.E. Hoggan, A. Pimpinelli A kinetic Monte Carlo study of the meandering instability of a vicinal surface growing by step flow is performed. Impurities are co-deposited during growth, and they are shown to be responsible for quantitative and qualitative modifications of the surface morphology. In particular, impurities make adatom diffusion less dependent on the deposition rate, affecting thus the wavelength of the meandering. Impurities also act as nucleation centres, causing small stepped pyramids to appear on the surface. Comparison with step- flow experiments on vicinal Cu(100) make plausible the hypothesis that many previously unexplained features of the meandering instability in this system are due to impurities. The density of nano-pyramids can be tuned by varying the impurity concentration. Our results show also that the step bunching instability is strongly affected by adsorbated impurities having lower diffusion rate than adatoms. Such impurities slow the adatoms diffusion and weakens the instability, even removing it at large impurities concentration. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B20.00002: Effects of deposition angle in metal(100) epitaxial growth Jacques G. Amar, Valery Borovikov, Yunsic Shim The effects of oblique incidence on the surface roughness and morphology in Cu/Cu(100) epitaxial growth are investigated via multiscale kinetic Monte Carlo simulations in which the effects of shadowing and short-range (SR) and long-range (LR) attraction during deposition are taken into account via molecular dynamics simulations. Somewhat surprisingly, while the effects of deposition angle are found to be relatively weak at $200$ K, at a slightly lower temperature ($160$ K) both the surface roughness and the growth exponent $\beta$ depend strongly on deposition angle even for moderate deposition angles. These results resolve a long-standing puzzle regarding the growth behavior of Cu/Cu(100) over this temperature range. We also find that while the effects of shadowing and SR attraction play important roles, for moderate deposition angle the effects of LR attraction are relatively weak. Our results also demonstrate that, in general, the effects of deposition angle must be considered in low-temperature growth even for moderate deposition angles. Results for the surface morphology and roughness at higher temperatures and for very large deposition angles are also presented and compared with experiments. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B20.00003: Shape fluctuations of small Pb(111) islands at low temperatures. M. Hupalo, M.C. Tringides With STM we have measured shape fluctuations of monolayer Pb(111) islands grown on top of Pb mesas of controllable height to determine their step energy. Islands as small as 10nm in diameter were used to test the limit of the thermodynamic analysis. It found that the mean square deviations of the fluctuations obey a linear relation on RT where R is the island radius and T the temperature in agreement with the standard analysis.The extracted step energy 140eV/atom is in good agreement with the one obtained in the literature on islands of much larger sizes[1]. Fluctuation magnitude of bilayer island grown on top of 5-layer isladn(which corresponds to a magic 7-layer height) are much lower and the corresponding step free energy is three times higher than for monatomic steps. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B20.00004: Connector Model for Many-Body Interactions at Surfaces from First Principles Yogesh Tiwary, Kristen Fichthorn First-principles calculations based on DFT indicate that pair, trio, four- and five-body atomic interactions are significant in Al clusters on Al(110). These many-body interactions are a signature of ``elastic screening'' of long-ranged, substrate-mediated, elastic interactions between atoms in dilute adlayers by direct bonding and short-ranged substrate relaxation in dense clusters. As a consequence of this screening, we developed the Connector Model to effectively describe the energies of compact clusters. Adsorbate structures and interactions are described in terms of single-atom, many-body connector units, which link to one another to form the structures that can occur in thin-film and crystal growth. The additive connector energies can be effectively incorporated into a lattice-based Hamiltonian to study thermodynamics and kinetics at surfaces. The Connector Model is considerably more efficient than lattice-gas Hamiltonian approaches, which would require a large number of terms to accurately capture many-body effects. Details of the connector model and its application to predicting the shapes of compact clusters on Al(110) including the chain-to-island transition will be discussed. This model can be extended to other crystalline surfaces. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B20.00005: Double-layer island decay on Ag(111): A molecular dynamic simulation Berk Onat, Sondan Durukanoglu We have performed molecular dynamic simulations to investigate double-layer island decays on Ag(111) using the interaction potentials based on the embedded atom method with a specific aim to observe the effect of varying island size and temperature on interlayer mass transport between two layers of the adatom islands. Our preliminary results indicate that decay rates of adatom islands show different characteristics with varying adatom island size. From an analysis of MD simulations, we further examine how the activation barriers for several diffusion processes taking place during adatom island decay change with respect to varying tempterature and island size. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B20.00006: Kinetics of Facile Bilayer Island Formation for Ag on NiAl(110) J.W. Evans, Yong Han, D.-J. Liu, Baris Unal, F. Qin, D. Jin, C.J. Jenks, P.A. Thiel STM studies reveal that deposition of Ag on NiAl(110) at 127 K and above leads to bilayer-by-bilayer growth of a nearly-strain-free film with Ag(110) structure [1]. This growth mode is attributed to Quantum Size Effects (QSE) associated with electron confinement in the Ag film. Our focus here is on analysis the initial nucleation and growth of bilayer Ag(110) islands on NiAl(110) which is facile even at 127K despite requiring uphill transport of Ag. DFT analysis for supported Ag films determines adatom adsorption energies (which display QSE), interaction energies, and various relevant diffusion barriers. Kinetic Monte Carlo simulation of an atomistic lattice-gas model incorporating these energies highlights the role of strongly anisotropic interactions in facilitating bilayer island formation. \newline [1] B. Unal et al., Phys. Rev. B 76 (2007) 195410. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B20.00007: Self-organization and pattern selection under nanosecond pulsed laser-induced melting of ultrathin metal films Ramki Kalyanaraman, Christopher Favazza, Justin Trice, Radhakrishna Sureshkumar When an ultrathin metal film is rapidly melted by nanosecond (ns) laser pulses, ensuing hydrodynamic instabilities and/or fluid flow due to surface tension gradients lead to self-organizing patterns of ordered nanostructures. The extremely fast heating O(100 K/ns) and resolidification (O(10 K/ns) in such ns melting experiments permits quenching in of the morphology. Thus, multiple pulses of such fast melting/resolidification cycles in the film allow different stages of the patterning process to be identified and studied. We show that pattern formation via a thin film hydrodynamic dewetting instability and thermocapillary flow can compete and the dominating mechanism is one which has shorter time scale. We have explored this behavior for a large variety of metals, including Ti, V, Mn, Fe, Co, Ni, Cu, and Ag. This ns melting approach permits the robust self-organization of a wide variety of nanoscale structures, including nanoholes, nanostars, nanoparticles and nanowires. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B20.00008: Quantum engineering of apparent tunneling height in ultra thin Pb films Jungdae Kim, Shengyong Qin, Chih-Kang Ken Shih The thickness dependence of tunneling decay constant ($\kappa )$ for ultra thin Pb films is studied with various sample biases by using low temperature STM. It is found that quantum well states (QWS) have a strong influence on the tunneling decay constant $\kappa $. While the decay constant versus layer thickness ($\kappa $ vs. L) clearly shows bilayer oscillations, we found that the apparent contrast in $\kappa $ vs. L also show strong bias dependence. Depending on the bias voltage, contrast reversal in the apparent oscillation of $\kappa $ vs. L can be precisely tuned when the tunneling into the sample empty states. This result also shows that $\kappa $-oscillation does not necessarily imply the work function oscillation. We further show that in this case, the parallel component of crystal momentum plays a critical role in tunneling process and is largely responsible for the observed phenomena. On the other hand, at negative sample bias, we show that the measured decay constants well reflect the variation of surface workfunction. In this case, the layer-dependent surface work functions indeed show bi-layer oscillations and the signature of phase slip due to non-perfect phase matching between Fermi wavelength and the vertical lattice constant. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B20.00009: Why Is The Size Dependence Of the Scanning Tunneling Microscopy Workfunction Order Of Magnitude Larger Than That Of Photoemission ? Wei-Xue Li, Xin Liu, S.B. Zhang Quantum size effect (QSE) has been studied extensively, as a primary driving force for nano technology. Recent scanning tunneling microscopy workfunction of ultrathin Pb(111) film found however order of magnitude larger of QSE than that of photoemission. By first-principles calculations, we show that the QSE is not merely a size effect but symmetry driven: being maximal at the $\overline{\Gamma}$ point (i.e., the center of the surface Brillouin zone) derived from interlayer coupling of $p_z$ orbital, but could be vanishingly small at other symmetry points from in-plane $p_{x,y}$ orbital. The $\overline{\Gamma}$ valley states have the slowest decay. Thus they are the ones being picked up by near-field techniques such as the scanning tunneling spectroscopy. While whole surface Brillouin zone contributes equally to photoemission, and only marginal QSE effect occurs. For this reason, symmetry could be essential for all near-field physics and chemistry. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B20.00010: Tuning Surface Energy Landscapes in Metallic Quantum Films using Alkali Adsorbates Alexander Khajetoorians, Shengyong Qin, Wenguang Zhu, Holger Eisele, Zhenyu Zhang, Chih-Kang Shih Quantum confinement shows a strong interplay with growth and kinetics in thin metal systems where the Fermi wavelength has a special relationship to the surface normal lattice constant. In the case of Pb/Si(111) systems, this relationship reveals an interesting thickness-dependent bilayer oscillation in the density of states and surface energy up to a phase. In this paper, we report on a novel effect: tuning of the energy landscape of a flat-top quantum Pb mesa using Cs adsorbates. Using STM/STS, we show that depositing Cs adsorbates on a thin Pb mesa promotes quantum stable Pb nanoislands on preferentially unstable thicknesses. Thickness-dependent nanoisland densities show a strong bilayer oscillation correlating with quantum stability. By modifying the Cs coverage on the mesa surface, we can tune the lateral size distribution of the nanoislands and the overall amplitude of the island density oscillation. Nanoisland formation is linked to a step decoration of Cs adatoms along the step edge of the nanoisland. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B20.00011: Super-oscillations in the Interlayer Lattice Relaxation of Quantum Pb Films. Yu Jia, Biao Wu, T.L. Einstein, H.H. Weitering, Zhenyu Zhang Using first-principles total energy calculations, we study the interlayer relaxation in lead films and observe oscillations of the interlayer lattice relaxation with layer number (distance from vacuum) and with film thickness. By analyzing the charge distribution along the direction perpendicular to the film, we show that the former oscillations of lattice relaxation are induced by Friedel oscillations in charge density that decays slowly, following a 1/r law rather than a 1/r$^{2}$ law found in most metal films. The superoscillations are attributable to interference of the Friedel oscillations from the two boundaries of the film. Our results suggest that Friedel oscillations may be responsible for the quantum size effect observed in lead films. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B20.00012: Modifying the Adsorption of Molecules at Metal Surfaces by Quantum Confinement of Electrons Levan Tskipuri, Robert Bartynski We have studied the bonding of CO on several ultrathin Cu and Co films that exhibit metallic quantum well (MQW) states, whose energies change as a function of overlayer thickness, using inverse photoemission (IPE), reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). For Co system, which has unoccupied MQW states that do not cross the Fermi level, a CO 2$\pi $-induced feature is observed in IPE at 3.8 eV above E$_{F}$. CO desorbs at 375 K ( 30 K lower than for hcp Co surfaces) and a second TPD feature at 230 K appears upon low temperature ($\sim $ 100K) dosing. These TPD peak temperatues change as a function of film thickness and are correlated with two different C-O stretch vibrational frequencies observed in the IR spectra. The intensity of the C-O stretch feature in IRAS spectra, and the peak CO desorption temperature in TPD from CO on Cu MQWs both show modulations that are correlated with MQW states crossing the EF. We have also studied the influence of MQW states on the adsorption properties of the dimethyl disulfide (thiol) molecule (CH$_{3}$S)$_{2}$, which forms a self-assembled monolayer when adsorbed on the Cu(100) surface. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B20.00013: Crystal structure and height selection for Indium growth on Si(111) interfaces J. Chen, M. Hupalo, M. Ji, C.Z. Wang, K.M. Ho, M.C. Tringides The growth of In/Si(111) has been studied with SPA-LEED and STM to identify whether QSE-driven height selection with mono-disperse distribution is possible. The motivation is to discover other metals besides Pb/Si(111) with high degree of self-organization. Indium growth only on one substrate (Si(111)-Pb-$\alpha -\surd $3x$\surd $3) has resulted in uniform height In(111) 4-layer flat top islands (with the fcc(111) orientation different from the bct In bulk structure). This allotropic transition is observed at low temperatures T$<$250K and coverages $\theta <$6ML. Otherwise bct(110) oriented Indium islands are observed with continuously increasing height/size aspect ratios. These results suggest two stabilizing energetic effect for the fcc In(111) height selection i.e. Quantum Size Effects (QSE) stabilize uniform height and orientation dependent surface energy favours the different crystallography. The contribution of each effect was studied with first principles calculations and both the height selection and the allotropic transition can be quantitatively explained. -/a [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B20.00014: Non-classical scaling in the Pb/Si(111) coarsening at low temperature Myron Hupalo, R. Feng, E.H. Conrad, C. A. Jeffrey, P. F. Miceli, S. Hayden, M. Gramlich, P. J. Ryan, C. Kim, M.C. Tringides Recent coarsening experiments monitoring the evolution of a mixture of stable and unstable islands in Pb/Si(111) towards a mono-disperse 7-layer height distribution have revealed novel features that extend the classical curvature driven growth. Two complementary techniques are used, X-ray scattering and STM. In particular the coverage $\theta $ , temperature T and flux F dependence are the opposite of what is expected from the classical analysis. The coarsening time $\tau $ increases with increasing temperature T, coverage $\theta $ and decreases with increasing flux rate F according to the scaling relation $\tau $F=constant. These paradoxical results can be understood from the island stability dependence on lateral size L in addition to the QSE-driven well--analyzed height dependence. The decay constant of an unstable island is an increasing function of its lateral size and for sizes larger than Lc$\sim $50nm the unstable islands do not decay but grow in the next stable height. Since the lateral size increases with T, $\theta $ and decreases with F this can account for the novel coarsening results. [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B20.00015: Surface, quantum well, and bulk states in Ag films Nathan Speer, Ching Wei, Tom Miller, Tai Chiang Atomically uniform Ag films grown on Si(111) substrates show, in addition to the usual Shockley Surface state, multiple surface states in pockets within the d-band manifold as observed by angle-resolved photoemission spectroscopy. At low coverages, quantum well states are resolved. As the film thickness increases, quantum well states evolve into the bulk band continuum plus separate surface states. The results are compared to a density functional theory calculation. [Preview Abstract] |
Session B21: Focus Session: Clusters, Cluster Assemblies, Nanoscale Materials I
Sponsoring Units: DCPChair: Shiv Khanna, Virginia Commonwealth University
Room: Morial Convention Center 213
Monday, March 10, 2008 11:15AM - 11:51AM |
B21.00001: Physical and chemical properties of supported, suspended and trapped clusters Invited Speaker: Investigations of physical systems of small sizes and reduced dimensionalities, exhibiting discrete quantized energy level spectra and specific structures and morphologies, open avenues for systematic explorations of the emerging unique behavior of materials at the nanoscale, as well as of the transition from the atomic and molecular domain to the condensed phase regime. We will discuss computer simulations and accompanying experiments exhibiting emergent physical and chemical behavior at the nanoscale, focusing on several confinement configurations -- free, supported and trapped clusters. These include investigations of metallic, semiconducting, superconducting [1] and mixed metal/molecular [2] suspended nanowires, recent advances in understanding the structure and catalytic activity of supported gold nanoclusters and methods for controlling their dimensionality and properties [3], strongly correlated states and formation of electron (Wigner) clusters in 2D quantum dots, and crystallization of trapped finite boson clusters [4]. \newline \newline [1] A. Marchenkov, Z. Dai, C. Zhang, R. N. Barnett, U. Landman, Phys. Rev. Lett. 98, 046802 (2007); A. Marchenkov, Z. Dai, B. Donehoo, R. N. Barnett, U. Landman, Nature Nanotech. 2, 481 (2007). \newline [2] C. Zhang, R.N. Barnett, U. Landman, Phys. Rev. Lett., in press (2007). \newline [3] U. Landman, B. Yoon, C. Zhang, U. Heiz, M. Arenz, Topics in Catal. 44, 145 (2007). \newline [4] C. Yannouleas and U. Landman, Reports on Progress in Physics 70, 2067 (2007). [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B21.00002: Electric Dipole Moments of Neutral Sodium Clusters John Bowlan, Anthony Liang, Walt de Heer Recent ab initio studies have predicted large electric dipole moments in neutral Na clusters. We performed an electric deflection experiment on these clusters at 20 K and have found upper bounds on their dipole moments that are orders of magnitude smaller than predicted, and consistent with zero. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B21.00003: Enhanced magnetic moments of Sc, Ti and V doped Na$_{n }$(n =4, 5, 6) clusters J. Ulises Reveles, Kalpataru Pradhan, Prasenjit Sen, Shiv N. Khanna Theoretical studies on the geometry, electronic structure and spin multiplicity of Sc, Ti and V doped Na$_{n}$ (n = 4, 5, 6) clusters have been carried out within a gradient corrected density functional approach. Two complementary approaches including all-electron calculations on free clusters, and supercell calculations using planewave pseudopotential and projector augmented wave formalisms have been carried out. It is shown that spin magnetic moments of the transition metal atoms, the magnitude of host polarization, and the sign of the host polarization all change with the number of alkali atoms. In particular the transition metal atoms are shown to attain spin moments that are higher than their atomic values. The role of hybridization between the transition atom $d$-states and the alkali \textit{sp}-states is highlighted to account for the evolutions in the spin moments and host polarization. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B21.00004: Structure, Growth and Optical Properties of Si$_{n}$O$_{m}$ Nanoparticles: From SiO molecules to Silicates in Circumstellar Space Selvarengan Paranthaman, Arthur Reber, Penee Clayborne, Shiv Khanna, A. Welford Castleman Jr. Synergistic effort combining formation of clusters in molecular beams and first principles electronic structure studies within a gradient corrected density functional scheme are employed to examine the geometries, stability, vibrational frequencies and optical properties of Si$_{n}$O$_{m}$ clusters. It is proposed that the oxygen enrichment needed to form silicates in interstellar space, starting from SiO molecules can occur via two processes. (1) Chemically driven compositional separation in (SiO)$_{n}$ motifs resulting in oxygen rich and oxygen poor regions, and (2) reaction between Si$_{n}$O$_{m}$ clusters leading to oxygen richer and poorer fragments. Theoretically calculated optical and infrared spectra of Si$_{n}$O$_{m}$ clusters exhibit features observed in the extended red emissions and blue luminescence from interstellar medium indicating that the Si$_{n}$O$_{m}$ fragments could be contributing to these spectra. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 1:03PM |
B21.00005: Cage Clusters of Gold and Tin: Golden Buckyballs and Stannaspherene Invited Speaker: Photoelectron spectroscopy (PES) yields direct electronic structure information for size-selected clusters. Combining PES with theoretical calculations has become an effective approach to obtain structural information for small and medium-sized clusters. We present recent discoveries of two classes of cage clusters in gold and tin. Negatively charged gold clusters (Au$_{n}^{-})$ have been shown to exhibit a remarkable structural diversity from 2D structures for $n$ = 4-12 and the pyramidal structure for $n$ = 20. Using PES and DFT calculations, we have found that gold clusters with $n$ = 16-18 possess unprecedented hollow cage structures. We have been able to successfully dope a variety of transition-metal atoms into the empty spaces in the golden cages, confirming their structural robustness, as well as demonstrating chemical tuning of their electronic, magnetic, and catalytic properties. Unlike carbon, the heavier congeners of the group 14 elements are not known to form hollow cage structures similar to the fullerenes. In PES studies of tin clusters, we noted that the spectrum of Sn$_{12}^{-}$ is distinctly different from that of its neighbors or its Si/Ge counterpart. This observation led to our discovery of a highly symmetric and stable icosahedral Sn$_{12}^{2-}$ cage, for which we coined a name ``stannaspherene'' to describe its high symmetry and spherical pi bonding. We have also shown that all transition metals including the f-block elements can be doped inside Sn$_{12}^{2-}$ to form a whole class of endohedral stannaspherenes, which may be used as potential building blocks for new cluster-assembled materials. In a preliminary experiment to synthesize stannaspherene in the bulk, a new cluster, Pd$_{2}$@Sn$_{18}^{4-}$, was crystallized and characterized, suggesting all stannaspherene and endohedral stannasphernes may be fabricated in the bulk under suitable conditions. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:39PM |
B21.00006: Cooperative effect between electronic and geometric structures in binary clusters of superatoms Invited Speaker: The fabrication of cluster-assembled materials is dependent upon finding a suitable building block for a cluster that is chemically stable and that interacts weakly with other clusters of the same material. For designing the characteristics of clusters, binary systems are very important to create functionality of materials, and application of hetero-atom doping has been undertaken in the present study to two prototypes: metallic aluminum (Al) clusters and covalent silicon (Si) clusters. In particular, efforts to examine the superatom behavior involved in electronic and geometric shell closings have focused on substituting the central atom in Al$_{12}$X and MSi$_{16}$. Binary aluminum and silicon clusters of Al$_{12}$X and MSi$_{16}$ were generated by a double laser vaporization method, and the electronic and geometric stabilities for the resulting hetero-atom encapsulated clusters were examined experimentally. For aluminum-based binary superatoms of Al$_{12}$X, both experimental and theoretical results show that Al$_{12}$Si has high ionization energy and low electron affinity, and Al$_{12}$P has low ionization energy, both with the icosahedral structure having a central Si or P atom, revealing that Al$_{12}$Si and Al$_{12}$P exhibit rare-gas-like and alkali superatoms, respectively. Experiments confirmed the possibility that the change in the total number of valence electrons on substitution could produce ionically bound binary superatom complexes; the binary cluster salts (Al$_{12}$P)$^{+}$F$^{-}$ and (Al$_{12}$B)$^{-}$Cs$^{+}$. For silicon-based binary superatoms of MSi$_{16}$, on the other hand, results obtained by mass spectrometry, anion photoelectron spectroscopy, and adsorption reactivity towards H$_{2}$O show that the neutral cluster doped with a group-4 atom features an electronic and a geometric closing at n=16. The MSi$_{16}$ cluster with a group-4 atom undergoes an electronic change in (i) the number of valence electrons when the metal atom is substituted by the neighboring metals with a group-3 or -5 atom, and in (ii) atomic radii with the substitution of the same group elements of Zr and Hf. The reactivity of a halogen atom with the MSi$_{16}$ clusters reveals that VSi$_{16}$F forms a superatom complex with ionic bonding. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B21.00007: Stability and Magnetic Coupling between Mn doped Stannaspherenes Anil Kandalam, Gang Chen, Puru Jena The discovery of carbon fullerenes has stimulated considerable interest in the search of cage clusters involving not only group IV elements but also metallic systems. The recent discovery of stannaspherene, a highly stable Sn$_{12}^{2-}$ hollow cage cluster with a reported diameter of 6.1 {\AA} has triggered a renewed interest in the search for stable endohedral cage complexes with 3$d$ transition metal atoms as dopants. It is anticipated that these complexes may carry large magnetic moments and can be used as building blocks for novel magnetic materials. We report the first density functional theory based study of the interaction between two Mn doped stannaspherenes (Mn@Sn$_{12})$ and show that Mn@Sn$_{12}$ is not only highly stable and carry a magnetic moment of 5 $\mu _{B}$, but the clusters retain their structural identity even when they interact with each other. Equally important, the magnetic coupling depends strongly on the orientation of the clusters. We believe that these new results would encourage the scientific community to explore the possibility of synthesizing novel magnetic materials with magnetic element doped Sn$_{12}$ clusters as building blocks. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B21.00008: Structural and electronic properties of chiral gold nanoclusters Ignacio Garzon, Itzel Santizo, Luis Perez Chiral structures had been found as the most stable isomers of bare and thiolate-passivated gold nanoparticles of several sizes, from density functional calculations (DFT). These results provided theoretical support for the existence of chirality in metal clusters, suggested by the intense optical activity measured from the metal-based electronic transitions of size-separated glutathione-passivated gold nanoclusters, and more recently, of penicillamine-passivated gold clusters with metal core mean diameters of 0.57, 1.1, and 1.75 nm. A further structural analysis using the Hausdorff chirality measure, as well as, a semiclassical calculation of the circular dichroism spectrum, has confirmed the existence of chirality in Au nanoparticles. In this work, the structural and electronic properties of chiral Au nanoclusters are studied by using global optimization methods combined with semiempirical many body potentials and first principles density functional calculations. In particular, we study the Au$_{34}$ cluster. For this system there exists experimental evidence on the energetic stability of a chiral structure with $C_{3}$ symmetry. Our calculations theoretically confirm these results, providing further evidence on the existence of chiral gold nanoclusters. [Preview Abstract] |
Session B22: Focus Session: Organic Electronics: FETs I
Sponsoring Units: DMP DPOLYChair: Ting Xu, University of California, Berkeley
Room: Morial Convention Center 214
Monday, March 10, 2008 11:15AM - 11:27AM |
B22.00001: In-crystal carriers in organic single crystal transistors Jun Takeya, Y. Tominari, M. Yamagishi, Y. Iwasaki The intrinsic semiconductor character of organic semiconductors, consisting of one molecular species, causes peculiar features different from those of doped inorganic semiconductors, when they are built in field-effect transistors (FETs). The effect can be most drastic with purified organic single crystal devices because of their minimized impurity (defect) concentrations of $\sim $ 10$^{14}$ cm$^{-3}$, resulting in relatively long length scale (more than 1 $\mu $m) of band bending and carrier distribution in the direction of the crystalline thickness. In this presentation, we report effects of the in-crystal carriers in two FET structures of clean rubrene single-crystals with the thickness comparable to the length scale of the distribution. With a double-gate device incorporating two transistor structures on the both sides of the crystal, we found that each transistor cross-talks with each other, inducing high-mobility (higher than 30 cm$^{2}$/Vs) carriers inside the crystal. The similar cross-talking events are observed also for a device with an acceptor layer of F$_{4}$-TCNQ on one side of the rubrene crystal. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B22.00002: Defect healing at room temperature in pentacene thin films and improved transistor performance. Wolfgang Kalb, Fabian Meier, Kurt Mattenberger, Bertram Batlogg We observed a healing of defects at room temperature in the prototypical organic semiconductor pentacene. Pentacene thin-film transistors were fabricated and characterized by in situ gated four-terminal measurements. Under high vacuum conditions (base pressure of order 10E-8 mbar), the device performance is found to improve with time. The effective field-effect mobility increases by as much as a factor of two and the contact resistance decreases by more than an order of magnitude. Oxygen/nitrogen exposure and annealing experiments show the improvement of the electronic parameters to be driven by a thermally promoted process and not by chemical doping. The spectral density of trap states was extracted from the transistor characteristics with a powerful scheme which allows for a calculation of the trap densities with high accuracy in a straightforward fashion. We show the performance improvement to be due to a reduction in the density of shallow traps $<$0.15 eV from the mobility edge, while the energetically deeper traps are essentially unaffected. This peculiar effect is a direct consequence of the weak intermolecular interaction which is characteristic of organic semiconductors. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B22.00003: Impurities and carrier trap formation in rubrene Leonidas Tsetseris, Sokrates Pantelides Rubrene has emerged recently as a very promising system for applications in organic electronics. In particular, measured carrier mobilities of rubrene films have been among the highest values reported for organic semiconductors. Here we present the results of first-principles calculations on impurities in rubrene crystals. We find that the most stable atomic H defect induces deep traps in the energy band gap of this prototype organic semiconductor, but we further show that the formation of a stable pair of vicinal H impurities eliminates these traps. We thus establish a dual role of hydrogen in carrier trap formation in rubrene devices. We also find that the incorporation of oxygen and water-related impurities is energetically favorable, and we address their possible role in the appearance of energy levels in the band gap of rubrene. Finally, we discuss the relevance of our findings for the development and operation of rubrene-based devices. This work was supported in part by DOE Grant DEFG0203ER46096. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:27PM |
B22.00004: Organic Field Effect Transistors Based on Micro and Nano-sized Single Crystalline Semiconductors. Invited Speaker: Organic single crystals are of interest for fundamental charge transport studies. Recently, we have shown that organic semiconductor single crystals can be patterned over a large area using vapor phase growth method. This opens up new possibilities of using organic single crystals for high performance devices. In this talk, I will present our recent work in solution deposition of single crystals by various methods and their device performance. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B22.00005: Field-effect modulated Seebeck coefficient in pentacene and rubrene K.P. Pernstich, B. Roessner, B. Batlogg We report on the first study of the charge carrier concentration and the temperature dependence of the Seebeck coefficient $S$ for two prototypical organic semiconductors measured in a field-effect transistor (FET) structure. As a basic transport property of solids, the Seebeck coefficient provides deep insights into the nature and dynamics of charge carriers. Using a FET structure enables the variation of the Fermi-level position in the active semiconductor region while measuring $S$, which is essential for determining individual contributions to the thermopower. The sign of the measured Seebeck coefficient is consistent with hole transport, and $S$ is in the range of 0.3-1 mV/K, it is independent of temperature between 295 K and 200 K, and interestingly it decreases for both semiconductors as $S \propto $\textit{$\vert $V}$_{g}$\textit{$\vert $}. The measured $S$ is quantitatively described by $S=(k/e)(E(V_g )/kT+A)$. The Fermi-level position $E(V_{g})$ is obtained from analyzing the transistor's characteristic which then allows to calculate the parameter $A$. For both semiconductors we find that $A$ is in the range of 1.7-3.6, just as in conventional semiconductors. The results are well described by solely considering a realistic density of in-gap trap states and band-like transport of quasiparticles that are subjected to scattering. There is no need to invoke self-trapping of massive charge carriers. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B22.00006: Contact-correlated bias stress instability in pentacene thin film transistors K. Tsukagoshi, S.D. Wang, T. Minari, T. Miyadera, Y. Aoyagi The bias stress effect and large contact resistance are the remaining problems for practical applications of organic thin film transistors (OTFTs). The bias stress effect in top-contact pentacene TFTs was observed to be correlated with the metal/organic contact. The drain current decay under the bias stress condition results from the combination of the contact resistance change and the channel resistance change arising from the threshold voltage shift in the channel. The transistors with the different contacts (gold and copper) show the very similar channel properties. On the other hand, the bias-stress-induced contact resistance change in the gold transistor is much larger than the corresponding channel resistance change, so that the bias stress effect can be even dominated by the gold contact. In contrast, the copper contact is more stable, showing the small contact resistance change. The temperature variance measurements on contact resistance indicate that, the gold contact has a wider trap distribution compared with the copper contact, that is, there are much more deep trap states at the gold contact. Therefore, the time-dependent charge trapping in the deep traps is proposed to be the mechanism of the bias stress effect in OTFTs. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B22.00007: Negative Magnetoresistnace of Organic Field Effect Transistors Masaya Nishioka, Yeonbae Lee, Allen Goldman, Yu Xia, Daniel Frisbie The magnetoresistance (MR) of organic field effect transistors has been studied. Both pentacene film, tetracene single crystal and rubrene single crystal devices exhibit negative MRs of up to 1.2 {\%} at 9 T. This has been demonstrated to not be contact related. The effect has been found to increase with decreasing temperature between 150 and 300 K. On the other hand, this effect is not strongly affected by the magnetic field direction. The phenomenon may result from the action of the magnetic field on the hopping transport of carriers. However, the possibility of a magnetocapacitance effect that would increase the number of carriers cannot be ruled out. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B22.00008: Pentacene Thin-Film Transistors With Organophosphonate Self-Assembled Monolayer Modified Gate Dielectrics Ian Hill, Matthew McDowell, Joseph McDermott, Jeffrey Schwartz, Steven Bernasek, Jaehyung Hwang, Antoine Kahn Organophosphonate-based self-assembled monolayers synthesized from aliphatic, heterocyclic aromatic, and aromatic moieties were used to modify silicon dioxide gate dielectric in pentacene organic thin-film transistors. Striking improvements in the subthreshold performance of these devices is noted, with a dependence on the molecular species used in fabricating the monolayer. All modified gate dielectrics outperform the untreated surface. As determined by parameters such as the subthreshold slope, threshold voltage magnitude and uniformity, and the on/off ratio, the aromatic species outperform the heterocyclic species, which outperform the aliphatic species. Origins of the improvement, based on energetic alignment of the molecular levels of the SAM and the pentacene, and charge trap densities will be discussed. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B22.00009: Percolative Effects on Noise in Pentacene Transistors Brad Conrad, William Cullen, Winston Yan, Ellen Williams Noise in pentacene thin film transistors has been measured as a function of device thickness from well above the effective conduction channel thickness to only two conducting layers. Over the entire thickness range, the spectral noise form is 1/f, and the noise parameter varies inversely with gate voltage, confirming that the noise is due to mobility fluctuations, even in the thinnest films. Hooge's parameter varies as an inverse power-law with conductivity for all film thicknesses. The magnitude and transport characteristics of the spectral noise are well explained in terms of percolative effects arising from the grain boundary structure. http://arxiv.org/abs/0710.2700v2 [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B22.00010: Surface-Treatment Effects on the Pentacene-Based Organic Field-Effect Transistors with Anodized Gate Dielectrics Yeon Taek Jeong, Christopher Lombardo, Davianne Duarte, Ananth Dodabalapur The realization of low operating voltage organic field-effect transistors (OFETs) is technologically important with many methods having been proposed to achieve this goal. The use of anodized high-$k$ dielectrics is very promising in that the approach is applicable to obtaining gate dielectrics at low temperature. We report on the device characteristics and proper surface-treatment effects on low voltage OFETs with anodized Ta$_{2}$O$_{5}$ and SiO$_{2}$ gate dielectrics. Pentacene-based OFETs with anodized Ta$_{2}$O$_{5}$ gate dielectric obtained from an e-beam-evaporated and a sputtered Ta thin film layer exhibited the saturation mobility of 0.52 cm$^{2}$/Vs and 0.45 cm$^{2}$/Vs at $V_{ds}$ = -10V, respectively. Moreover, a hexamethyldisilazane (HMDS) and a mono-dodecyl phosphate surface treatment resulted in enhanced mobility and significantly reduced gate leakage current. In the case of anodized SiO$_{2}$ devices, an octadecyltrichlorosilane (OTS) treatment increased the saturation mobility from 0.38 cm$^{2}$/Vs to 0.88 cm$^{2}$/Vs at $V_{ds}$ = -10V. The OTS treatment also proved to reduce gate leakage current by more than 90{\%}. In related work, we will discuss the fabrication of all-organic dual-channel devices. These devices are promising because of their probable applications to organic sensing and CMOS transmission gates. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B22.00011: Drift mobility and frequency response of diode connected organic field effect transistors Brian Cobb, Ananth Dodabalapur Common methods of estimating the mobility of carriers in organic field effect transistors (OFETs) include extraction from output or transfer characteristics. Mobilities extracted from transfer curves are realistic only in certain steady-state regimes. Optical time-of-flight methods measure charge mobility in directions perpendicular to the semiconductor-dielectric interface, perpendicular to the direction of transport in a functioning OFET. We have developed a method that allows the extraction of the drift mobility of charges through an active channel in an OFET by measuring the frequency response of a diode connected device. An AC signal of varying frequency was applied to both the Gate and Drain nodes, while the rectified output voltage was measured at the Source node using an oscilloscope. This rectified voltage remains relatively steady at lower frequencies, then displays a marked fall. This pole was found to correspond to the transit time of the carriers through the channel. The fabrication of the OFETs, calculation of drift mobility from the frequency of the pole, and the effects of varying channel length will be reported. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B22.00012: Field-induced polymorphous disorder and bias-stress instability of pentacene organic thin-film transistors Masahiko Ando, Claudia Duffy, Jessica Winfield, Takashi Minakata, Henning Sirringhaus We propose a field-induced polymorphous disorder model to explain bias-stress instability in pentacene organic thin-film transistors Field-effect mobility at 0.7 cm$^2$/Vs and threshold voltage, Vth, at 0 V were obtained by using highly crystalline zone-casted pentacene semiconductor on benzocyclobutene insulator. Vth shifted up to +25V with positive gate bias-stress at +40 V for 15 hours and recovered after gate bias removal. Vth recovery was drastically accelerated by direct photo-excitation of pentacene and it indicated electrons were trapped in pentacene and not in BCB. After annealing at 130 C in N2, the initial electrical performance were recovered. Micro-Raman spectroscopy of pentacene at the channel revealed that shape of the C-H vibrational peaks at around 1160 cm$^{-2}$ changed reversibly in accordance with the positive shift and recovery of Vth. Our pentacene films with average d-spacing at 14.3 A were considered to be composed of a mixture (mosaic) of two kind of polymorphs with d-spacing at 14.1 A and 14.5 A. The polymorphous mixture should be disordered by electric field to create electron traps and induce Vth shift. [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B22.00013: Prediction of the absolute charge mobility of molecular crystals Alessandro Troisi I propose a computational protocol to predict the absolute mobility of molecular semiconductors without adjustable parameters. The system dependent parameters are computed using a combination of classical molecular dynamics simulations and quantum chemical methods. The model used to connect the computable quantities with the observable temperature dependent mobility takes into account the effects of molecular reorganization energy and the fluctuation of the transfer integral due to thermal motions. The absolute value of the hole mobility, computed for the case of rubrene, is in excellent agreement with the experiments. The possibility of using computational chemistry methods to improve the theoretical models of charge transfer will be discussed in some detail. The predictive capabilities of the model presented in this work will be further validated considering the recent THz spectroscopy measurements performed by R. van Laarhoven in Eindhoven and the results in literature on temperature dependent band structure and density of state tails. [Preview Abstract] |
Session B23: Focus Session: Perovskite Cobaltites
Sponsoring Units: DMP GMAGChair: Stephan Rosenkranz, Argonne National Laboratory
Room: Morial Convention Center 215
Monday, March 10, 2008 11:15AM - 11:27AM |
B23.00001: Direct Electron Spectroscopy Investigation of the Spin State of LaCoO$_3$ J. Stanley, J. Hinton, N. Sundaram, B. S. Mun, A. Bostwick, E. Rotenberg, D. P. Belanger, G.-H. Gweon The spin state of LaCoO$_3$ and related properties have been a topic of high interest lately. Here we investigate the electronic structure of LaCoO$_3$ using core level and valence band photo-electron spectroscopy. We compare the competing spin models in the literature by using our data obtained as a function of incident photon energy and temperature. We are able to make an interesting contribution to the discussion of the spin state of this material. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B23.00002: Field Effect on Spin Correlations in La$_{1-x}$Sr$_x$CoO$_3$ Daniel Phelan, Despina Louca, Seunghun Lee, Stine Ancona, Stephan Rosenkranz, John Mitchell La$_{1-x}$Sr$_x$CoO$_3$ evolves from a non-magnetic, insulating ground state in the parent compound (x=0) into a metallic, ferromagnetic cluster-glass (x$>$0.18). For intermediate concentrations, the ground state is characterized by a competition between ferromagnetic and incommensurate spin correlations, which leads to short-range clustering. The field effect on these competing spin correlations has been investigated by neutron diffraction from a single crystal of La$_{.85}$Sr$_{.15}$CoO$_3$ under an externally applied vertical magnetic field ranging from 0 to 7 T. The intensity of the incommensurate reflections is signficantly reduced under 7 T. On the other hand, the ferromagnetic signal is enhanced by more than an order of magnitude under 7 T, while simultaneously narrowing in reciprocal space, implying an increase in the correlation length, which cannot be understood simply by a reorientation of ferromagnetic clusters along the direction of the applied field. Instead, the data is interpreted in a phase-separated scenario in which the applied field polarizes regions which are initially not ferromagnetic in zero field. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B23.00003: Ionic Size Effects and Magnetic Incommensurability in Cobaltite Juan Yu, Despina Louca, Daniel Phelan, K. Yamada LaCoO$_{3}$ in which Co$^{3+}$ has nearly degenerate spin states shows unusual magnetic behavior attributed to the fact that the different states can co-exist. The cobalt ions interact with each other via complex magnetic coupling, that is further complicated by the addition of Co4+ ions with hole doping. In the case of hole doped La$_{1-x}$Sr$_{x}$CoO$_{3}$ (LSCO), we found that the system exhibits a rich phase diagram, dominated by two competing magnetic phases, one that is ferromagnetic and metallic and another that is incommensurate with the lattice and insulating. Our recent elastic neutron scattering measurements on single crystals of La$_{1-x}$Ba$_{x}$CoO$_{3}$ (LBCO, with x = 0.03, 0.06, 0.10 and 0.15) also identified the coexistence of these two phases. Compared to LSCO, the LBCO series shows different incommensurability, but has a similar trend in the progression of intensity with increasing x. However, the LBCO series shows a much longer correlation length particularly in the direction perpendicular to (111), and a significantly stronger incommensurate peak than in LSCO. The incommensurate and ferromagnetic-like intensities exhibit identical temperature dependence in LBCO unlike in LSCO, in which ferromagnetic scattering had a higher onset temperature than the incommensurate phase at identical compositions. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B23.00004: Electronic structure of La$_{1-x}$Sr$_{x}$CoO$_{3}$ investigated from doping evolution of its soft x-ray spectra Yinwan Li, David Ederer, Thomas CallCott, J. W. Freeland Perovskite cobalt oxide LaCoO$_{3}$ attracted a lot of attention because of its spin state transition around 90K, the interpretation of which is still under debate. We performed soft X-ray absorption and emission experiments on La$_{1-x}$Sr$_{x}$CoO$_{3}$ at O-K edge and Co-L edge with various doping from x = 0 to x = 30. The doping evolution of the spectra can be understood as a result of introduction of holes at the top of valence band accompanied with a lowering of the Fermi level. The significant change of oxygen K edge spectra with doping indicates that the top of the valence band is largely of oxygen character, indicating strong Co 3d -- O 2p hybridization. The bottom of unoccupied band has a double peak feature indicating a splitting e$_{g}$ band. This split is the largest in the undoped sample and becomes smaller with doping. A gap of less than 1eV is observed from the excitation feature in the emission spectra and its change with doping again indicates a lower Fermi level in doped samples. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B23.00005: Thickness dependent magnetotransport properties of epitaxial La$_{0.5}$Sr$_{0.5}$CoO$_{3}$(001) films M. Sharma, M. Torija, C. Leighton, M. Varela Thin films of the doped perovskite cobaltite La$_{1-x}$Sr$_{x}$CoO$_{3}$ offer an ideal system to study the effect of dimensional confinement on spin state transitions and magnetoelectronic phase separation, and have application possibilities as electrodes in ferroelectric memory and solid oxide fuel cells. In this work we present the magnetotransport properties of epitaxial La$_{0.5}$Sr$_{0.5}$CoO$_{3}$ (001) thin films deposited on SrTiO$_{3}$ (001) by high pressure reactive sputtering. The films were structurally characterized by high-resolution x-ray diffraction, scanning probe microscopy, and STEM. Films with thickness $>$ 100 {\AA} exhibit bulk-like ferromagnetic metallic characteristics with the conventional negative MR in the vicinity of T$_{C}$ and a large AMR at low T. In stark contrast, films with thickness below 60 {\AA} exhibit reduced magnetization and a crossover to an insulating-like temperature dependence of the resistivity. This crossover is accompanied by a large negative MR at low T which bears a striking resemblance to that seen in bulk at x $<$ 0.17, which is known to be due to an intercluster ``GMR'' effect. In essence, x = 0.5 films on STO, when sufficiently thin, behave much like x $<$ 0.17 bulk samples, i.e. phase separation is evidenced at the LSCO/STO interface. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B23.00006: Heat capacity investigation of phase separation and spin-state transitions in La$_{1-x}$Sr$_{x}$CoO$_{3}$ Chunyong He, Heng Zheng, John Mitchell, C. Leighton We present a heat capacity study (to 0.3 K) on La$_{1-x}$Sr$_{x}$CoO$_{3}$ single crystals (0.00 $<$ x $<$ 0.30). In doped samples we observe three contributions at low $T$; a lattice term ($\propto \quad T^{3})$, an electronic term ($\propto \quad T)$, and a third term proportional to $T^{2}$. Remarkably, the x dependence of the electronic and $T^{2}$ contributions reflects very clearly the known magnetic phase separation, indicating that the $T^{2}$ term is a signature of the non-F matrix. Possible origins related to AF fluctuations will be discussed. At the lowest $T$ the nuclear hyperfine contribution provides a further probe of magnetic order. The electronic contribution also gives the density of states at the Fermi level which, in combination with the hole density from Hall effect, suggests a large effective mass indicative of strong correlations. Finally, the end-member LaCoO$_{3}$ shows a striking Schottky anomaly providing new information on the controversial spin-state transition. In particular, we find further evidence of the around 0.5 meV excitation recently observed by inelastic neutron scattering. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B23.00007: Phase separation and inhomogeneity in La$_{1-x}$Sr$_{x}$CoO$_{3}$ single crystals W.G. Moulton, Robert Smith, Michael Hoch, Phillip Kuhns, Arneil Reyes, Gregory Bobinger, John Mitchell, Chris Leighton Nanoscale inhomogeneity in La$_{1-x}$ Sr$_{x}$CoO$_{3}$ has been studied in single crystal samples for 0.05$\le $ x $\le $0.3 using $^{139}$La NMR as a probe of the internal magnetic field distribution. The results show that phase separation occurs for x near the metal-insulator critical concentration x$_{C }$=0.17. The phase separation is confined to a much smaller range of x than that previously found in polycrystalline samples even though the bulk magnetic properties are similar. In single crystals with increasing x large, spin polarized (FM) regions merge into networks while smaller regions, identified as spin polarons, coexist with the larger regions. Insight into phase separation has been obtained by simulating the spectra with statistical model A phase diagram summarizing the evolution of the inhomogeneous internal field distribution with x and T will be presented. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B23.00008: Strongly Inhomogeneous Conduction in Cobaltite Films. Vladimir Orlyanchik, Michael Weissman, Maria Torija, Manish Sharma, Chris Leighton The prototypical doped perovskite cobaltite, La0.5Sr0.5CoO3, has received considerable attention in bulk form. It was shown to be a ferromagnetic material with metallic conductivity. Here we show that the films, like bulk samples of the same composition, show resistance increasing with temperature when measured by Van der Pauw technique. However, when the resistance is measured in strip-shaped samples it reveals negative temperature dependence. Noise measurements performed in relatively large (e.g. 10-10 cm3) samples, revealed strongly non-Gaussian conductance fluctuations in the form of random telegraph noise. Field and temperature dependencies of Boltzmann factors of individual switchers show them to represent magnetic rotations rather than fluctuations between different thermodynamic phases (metallic and insulating). We argue that the inhomogeneous magnetic orientation dependent conductivity arises from strain-driven domain texture. The highly inhomogeneous conductivity may account for the unusual geometry dependent conductance. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B23.00009: The spin state issue in the $R$BaCo$_2$O$_{5.5}$ cobaltates Hua Wu, Z. Hu, T. Burnus, D. I. Khomskii, L. H. Tjeng The double perovskites $R$BaCo$_2$O$_{5+\delta}$ ($R$=rare earth, 0$\le\delta\le$1) display intriguing phenomena such as charge and orbital ordering, as well as antiferromagnetic to ferromagnetic transition, depending on the oxygen concentration. In particular, the $\delta$=0.5 system shows a giant magnetoresistance effect, and its metal-insulator transition has been often interpreted in terms of a spin-state transition [1,2], which, however, is fiercely debated [3,4]. To address the spin-state issue, we performed density-functional theory calculations which include a mean-field correction for the correlation effects caused by the Co $3d$ electrons. We have investigated various scenarios with different combinations of the low-, intermediate- and high-spin (LS, IS, and HS) states. Our results show that the pyramidally coordinated Co$^{3+}$ ions are exclusively in the HS state since [3], in disagreement with [1,2]. The octahedrally coordinated Co$^{3+}$ can be stabilized into a LS-HS ordered state if we take into account the superstructure recently reported [4]. Our results put limits as to how much spin-state transition could accompany the metal-insulator transition. [1] C. Frontera $et$ $al$., Phys. Rev. B {\bf 65}, 180405(R) (2002). [2] A. A. Taskin $et$ $al$., Phys. Rev. Lett. {\bf 90}, 227201 (2003). [3] Z. Hu $et$ $al$., Phys. Rev. Lett. {\bf 92}, 207402 (2004). [4] D. D. Khalyavin $et$ $al$., Phys. Rev. B {\bf 75}, 134407 (2007) [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B23.00010: Magnetic anisotropy and switching in Pr$_{0.5}$Sr$_{0.5}$CoO$_{3}$ using RF transverse susceptibility N.A. Frey, H. Srikanth, D.D. Stauffer, C. Leighton Pr$_{0.5}$Sr$_{0.5}$CoO$_{3}$ has been a system of current interest that appears very different from manganites or even other cobaltites. It has a ferromagnetic transition at 230 K and an anomalous magnetic transition at 120 K which is manifested in steps in temperature dependent magnetization and coercivity. The origin of this transition is still under investigation and recent experiments have suggested a change in magnetic anisotropy associated with this anomalous transition. In this work, we report systematic investigations of the magnetic anisotropy and switching fields across the 120 K transition in polycrystalline Pr$_{0.5}$Sr$_{0.5}$CoO$_{3}$ using a very sensitive RF transverse susceptibility method based on a tunnel diode oscillator resonant at 12 MHz. Our experiments reveal evidence for a sharp change in the anisotropy at 120 K with the anisotropy field (H$_{k})$ dropping from around 1.8 kOe to 750 Oe on cooling. By tracing the evolution of the anisotropy and switching peaks, we are able to discern behavior associated with separate ferromagnetic phases with 120 K signaling the transition between the two. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B23.00011: Influence of cation and oxygen vacancy ordering on magnetic properties of RE$_{1-x}$Sr$_{x}$CoO$_{3-\delta }$ (RE=Ho, La) S. Kolesnik, B. Dabrowski, O. Chmaissem, J. Mais, A. Baszczuk The attractive physical properties of RE$_{1-x}$Sr$_{x}$CoO$_{y}$ (RE - rare earth and Y) are strongly dependent upon overall oxygen content y and the ordering of oxygen and RE/Sr, as well as charge and spin ordering on the Co$^{3+}$/Co$^{4+}$- sites. Structural, magnetic and thermoelectric properties of Ho$_{1/3}$Sr$_{2/3}$CoO$_{y}$ have been studied within the oxygen content range 2.67$<$y$<$2.80. The studied samples demonstrate antiferromagnetic order (G-type) and semiconductive behavior. No ferromagnetic ordering has been observed and compared with A. Maignan et al. on a closely related Sr$_{2/3}$Y$_{1/3}$CoO$_{y}$ with similar y. We have observed that a the same high degree of oxygen ordering and antiferromagnetic structure exists for both cation disordered (La) and ordered (Y,Ho) materials. We conclude that the high degree of oxygen vacancy ordering determines the observed properties via strong localization of Co$^{3+ }$electrons. Work at NIU was supported by the NSF-DMR-0302617 and at ANL by the U.S. DOE under contract No. DE-AC02-06CH11357. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B23.00012: Orbital Ordering in Room Temperature Ferromagnet Sr$_3$YCo$_4$O$_{10.5}$ Studied by a Resonant X-ray Scattering Hironori Nakao, Tetsuya Murata, Daisuke Bizen, Youich Murakami, Shintaro Ishiwata, Wataru Kobayashi, Ichiro Terasaki Sr$_{1-x} R_x$Co$_4$O$_{10.5}$ ($R=$Y and lanthanide,
$0.2 |
Session B24: Focus Session: Transport in Nanostructures II: Strong Correlations
Sponsoring Units: DMPChair: Sergio Ulloa, Ohio University
Room: Morial Convention Center 216
Monday, March 10, 2008 11:15AM - 11:51AM |
B24.00001: Symmetries and interaction effects in carbon nanotube quantum dots Invited Speaker: By controlling the contact transparency within the same nanotube quantum dot, we observe the conductance evolving from the well- developed Coulomb blockade through the Kondo regime to the mixed valence regime. We work with high quality nanotubes, where energy subbands are doubly-degenerate, resulting in the SU(4) Kondo effect for one, two, and three electrons filling two degenerate orbitals. As the contacts are made more transparent, the sample enters the mixed valence regime, where different charge states within a pair of orbitals are hybridized. The hallmark of this regime in nanotube conductance is washing out of single-electron features at low temperature. In our measurement the level broadening is close to the charging energy and level spacing (both $\approx 10$ meV). Nevertheless, the low temperature regime is established only at temperatures of the order or less than 0.5 meV. The same low energy scale is also apparent from the width of the zero-bias peak in the tunneling density of states. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B24.00002: SU(4) Kondo effect in coupled quantum dots in parallel: Evidence of marginal fixed point Mikio Eto We theoretically study the Kondo effect in coupled quantum dots in parallel, using the scaling and NRG methods. The double quantum dots are capacitively coupled to each other, whereas they are attached to separate leads.\footnote{A.\ Huebel, J.\ Weis and K.von Klitzing, 17th International Conference on the Electronic Properties of Two-Dimensional Systems (EP2DS, 2007).} The SU(4) Kondo effect is realized when the energy levels are matched between the quantum dots. We show that (i) the Kondo temperature $T_K$ decreases with increasing $|\Delta|$, where $\Delta$ is the level separation between the dots, obeying a power law [crossover from SU(4) to SU(2) Kondo effect]. (ii) The exponent of the power law is not a universal value in general.\footnote{M.\ Eto, J.\ Phys.\ Soc.\ Jpn.{\bf 74}, 95 (2005).} This is an evidence of the marginal fixed point of SU(4) Kondo effect.\footnote{L.\ Borda {\it et al}., Phys.\ Rev.\ Lett.\ {\bf 90}, 026602 (2003)} (iii) The conductance through one of the quantum dots may show a non-monotonic behavior as a function of temperature $T$ although the total conductance is a universal function of $T/T_K$. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B24.00003: Exact-diagonalization treatment of the non-universal transport regime in few-electron quantum dots Leslie O. Baksmaty, Constantine Yannouleas, Uzi Landman Recently, experimental studies\footnote{M. Avinun-Kalish {\it et al.\/}, Nature {\bf 436}, 529 (2005)} have revealed a distinct second transport regime in the behavior of transmission phases obtained via Aharonov-Bohm interferometry using small quantum dots (QDs); namely, a non-universal regime for QDs with $N < 10$ electrons, in addition to the earlier known universal one for larger QDs with $N > 14$. Sophisticated (beyond-the-mean-field) many-body methods are needed for describing this non-universal regime. Here, we study the transport properties of small QDs using exact-diagonalization (EXD) calculations in conjunction with Bardeen's theory of quasiparticle mediated conductance.\footnote{For an adaptation of the formalism to QDs, see S.A. Gurvitz, arXiv:0704.1260v1} We will present EXD calculations\footnote{C. Yannouleas and U. Landman, Rep. Prog. Phys. {\bf 70}, 2067 (2007)} for anisotropic QDs, and for a wide range of anisotropies and strengths of inter-electron repulsion. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B24.00004: Quantum dot in a Aharonov-Bohm interferometer: magnetic flux-dependent pseudogap in the Kondo regime. Pascal Simon, Luis Dias da Silva, Nancy Sandler, Sergio Ulloa We study a quantum dot embedded in one of the arms of a Aharonov-Bohm interferometer threaded by a magnetic flux $\Phi$. In the regime where a single resonant mode propagates in the interferometer's ``free arm", the system can be described by an effective one-channel Anderson impurity model coupled to a non-constant, flux-dependent density of states (DoS). We present numerical renormalization-group results for the Kondo temperature $T_K$, phase shift and finite-temperature linear conductance. For $\Phi\neq 0$, the ground state of the system is Kondo-like, with a renormalized $T_K$. For $\Phi=0$, the effective DoS \textit{vanishes} at the Fermi energy and the system maps into the pseudogap Anderson model, which displays a quantum critical transition between Kondo and non-Kondo phases [1]. Signatures of these effects appear in the conductance and transmission phase-shifts across the system. This setup constitutes an experimental realization of a tunable pseudogap Anderson Hamiltonian, allowing for an experimental probe into the non-trivial properties of such a model. \newline [1] L.G.G.V. Dias da Silva et al, PRL {\bf 97} 096603 (2006). \newline Supported by NSF-IMC/NIRT. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 1:03PM |
B24.00005: Two-particle processes in quantum dots Invited Speaker: Inelastic two-electron processes in transport through quantum dots can lead to unexpected effects. At the low temperature Kondo regime, transport is described by an effective low-temperature theory in terms of weakly interacting quasiparticles. Despite the weakness of the interaction, we find that the backscattering current and hence the shot noise are dominated by two-quasiparticle scattering. We show that the simultaneous presence of one- and two- quasiparticle scattering results in a universal average charge 5/3e as measured by shot-noise experiments. As will be presented, recent experimental data measured in the vicinity of the Kondo limit supports our findings. Furthermore, this experiment suggests that many-body effects are not restricted to the low temperature regime only. Extending our study to the high temperature regime, we found under general conditions in Coulomb blockaded quantum dots, signatures of transfer of electron pairs. Those results on many- body corrections to the cotunneling current will be discussed in a related talk by M.E. Raikh. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B24.00006: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 1:15PM - 1:27PM |
B24.00007: Finite-temperature conductance signatures of a quantum-critical transition in a double quantum-dot device Nancy Sandler, Luis Dias da Silva, Kevin Ingersent, Sergio Ulloa We present conductance results for a quantum-dot system containing one dot in the Kondo regime coupled to two leads in parallel with a noninteracting resonant level. The system can be mapped onto a single-impurity Anderson model with a pseudogapped effective density of states [1]. The finite-temperature linear conductance $G(T)$ of this double-dot device is obtained via numerical renormalization-group calculations. The position of the single-particle levels can be controlled with gate voltages so that the effective density of states vanishes in power-law fashion at the Fermi energy of the leads; within this regime, further tuning can drive the system through a quantum critical point separating Kondo and unscreened phases. Signatures of both effects appear in $G(T)$, with a prominent feature at the scale $T^*$ marking the crossover from the high-temperature quantum-critical regime to a low-temperature Fermi liquid. These results open the way for experimental verification of the effect, and in principle allow access to a quantum critical point in a unique tunable system. [1] L.G.G.V. Dias da Silva et al., PRL {\bf 97}, 096603 (2006). [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B24.00008: Conductance signatures of quantum phase transitions in asymmetric double quantum dots W. Brian Lane, Kevin Ingersent, Luis Dias da Silva, Nancy Sandler, Sergio Ulloa Double quantum dots (DQDs) are currently of great theoretical and experimental interest. A DQD device in which dot 1 is in the Kondo regime and dot 2 acts as a noninteracting resonant level can be tuned to access a pair of quantum phase transitions separating Kondo-screened and local-moment phases [1]. This talk focuses on the effects of introducing a nonzero Coulomb interaction $U_2$ on the second dot. For small $U_2$, the system continues to exhibit two quantum phase transitions, although their nature is markedly different than for $U_2=0$. However, stronger interactions $U_2 > U_{2,c}$ suppress the local-moment phase and destroy the quantum phase transitions. We use numerical renormalization-group techniques to identify signatures of these behaviors in the linear conductance of the DQD device. [1] L. G. G. V. Dias da Silva, N. P. Sandler, K. Ingersent, and S. E. Ulloa, Phys. Rev. Lett. {\bf 97}, 096603 (2006). [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B24.00009: Symmetries and conductance in Kondo quantum dots Edson Vernek, Carlos B\"usser, Enrique V. Anda, Nancy Sandler, Sergio E. Ulloa The role of symmetries on nanoscale structures is essential for the different physical behavior exhibited in these systems while its understanding offers deeper insights into the observed properties. The ability to fabricate structures such as quantum dot arrays with tailored symmetries provides further motivation to understand the interplay of geometrical and orbital symmetries in interacting systems at low temperatures, where quantum coherence and Kondo correlations determine the electronic properties. In this work we study the transport properties of three interconnected quantum dots coupled to different leads in a triangular geometry. Conductance calculations carried out in a finite-U slave-boson mean field approximation show excellent agreement with results from the embedded cluster approximation (ECA), highlighting the rich features of the various physical regimes. We focus on and compare two important geometries with: {\em equilateral} (all couplings and leads identical) and {\em isosceles} (one lead and respective couplings different from the others) symmetries. In the first case, we show that only two degenerate orbitals contribute to the Kondo state conductance. Further, we show that the presence of an open third lead in all cases introduces dephasing which affects differently the various features of the conductance. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B24.00010: Kondo correlations and transport in single and triple quantum dots with damped lead hoppings: a tDMRG study. Luis Dias da Silva, Fabian Heidrich-Meisner, Adrian Feiguin, Elbio Dagotto We study the transport properties of one and three quantum dot systems with the time-dependent Density Matrix Renormalization-Group method (tDMRG). As previously noted [1,2], finite-size effects make the tDMRG description of the strongly interacting Kondo regime a numerically demanding task. We address this issue by introducing an exponential decay in the hopping terms in the leads ($t_n \propto \Lambda^{-n/2}$), recently introduced in cluster embedding methods [3]. For a given system size, results for $\Lambda>1$ show several improvements over the undamped ($\Lambda=1$)[1,2] case: the Kondo plateau in the differential conductance is correctly obtained deeper in the strongly interacting regime; steady-state current plateaus remain well defined for longer time scales. These results show that, with the proposed modification, the characterization of Kondo correlations in the transport properties can be substantially improved, at less computational cost. [1] K. A. Al-Hassanieh et al. PRB, 73, 195304 (2006). [2] F. Heidrich-Meisner et al. arxiv:0705.1801 (2007). [3] E. Anda et al., pre-print (Nov. 2007). [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B24.00011: Comparative Study of Quantum Monte Carlo and GW methods of treating electronic correlations in nanoscale junctions Catalin D. Spataru, Xin Wang, Mark S. Hybertsen, Andrew J. Millis We analyze the impact of electronic correlation in nanoscale junctions, focusing on the on-molecule interactions. To discuss the essential physics, we restrict attention to a single resonance coupled to metallic leads and including the local Coulomb interaction, the single-impurity Anderson model. Self-consistent GW results for the linear response conductance, orbital filling and spectral function are compared to numerically exact Quantum Monte Carlo calculations. Our analysis suggests that while the GW approximation may be useful for molecular conductors in the non-resonant tunneling regime, it is not accurate for nanoscale junctions in intermediate to strongly correlated and Kondo regimes. [Preview Abstract] |
Session B25: Polymer Blends
Sponsoring Units: DPOLYChair: Rick Register, Princeton University
Room: Morial Convention Center 217
Monday, March 10, 2008 11:15AM - 11:27AM |
B25.00001: Effective Coordination Number and Interaction Parameter In Simple Models of Polymer Blends David Morse One challenge faced when trying to quantity corrections to the RPA in either simulations or experimental measurements of correlations in polymer blends is the need to estimate the temperature or parameter dependence of a self-consistent field interaction parameter $\chi(T)$. In simulations of simple models, a useful independent definition of $\chi(T)$ may be obtained from a thermodynamic perturbation theory in which the difference between AB and AA interactions is treated as a perturbation of a one-component melt. For lattice models, this yields a value that, in the limit of long chains, is related to the Flory-Huggins value by replacing the coordination number $z$ by an effective coordination number given by the average number of inter-molecular contacts per monomer, as proposed previously by M. Mueller. Generalizations of the idea are provided for continuum models, and for shorter chains. An analytic theory is presented that quantitatively predicts the N-dependence of the effective coordination number found in lattice simulations. The approach provides a clean to way to separate RPA from non-RPA effects in the analysis of simulations. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B25.00002: Nucleation in Polymer Blends Edward Feng, Nitash Balsara Balsara and co-workers used small angle neutron scattering experiments on binary homopolymers blends to determine the size of the critical nucleus during phase separation. This suggests measuring the size of a single critical nucleus through a measurement of total density fluctuations. We carefully analyze this idea through kinetic Monte Carlo simulations of the Ising model, performing simulations of phase separation that conserve and do not conserve the magnetization. Calculations of the structure factor and spin-spin correlation function reveal clear differences in the nucleation mechanism for these two dynamics. Simulations that conserve the magnetization qualitatively agree with the experimental results on binary blends. Moreover, we calculate the cluster distribution during nucleation to determine the critical nucleus size of the Ising model. Comparing this result with the value determined from the structure factor shows qualitative agreement with increasing supersaturation. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B25.00003: Molecular dynamics simulations of constraint release effects in entangled binary blends of linear polymers Zuowei Wang, Ronald G. Larson We present extensive molecular dynamics simulations of the dynamics of entangled binary blends consisting of long test chains diluted in shorter chain matrix. The ratio between the long and short chain lengths is varied by a factor of ten covering the crossover from the chain reptation regime to the tube Rouse relaxation regime. Consistent with Neutron Spin Echo experiments, the dynamic structure factor of the long chains is found to decay faster in the matrix with shorter chain lengths, owing to the stronger constraint release effect. Correspondingly the monomers and centers of mass of the long chains show a faster time-dependent diffusivity than that expected from pure reptation. The simulation results for the diffusion properties agree qualitatively with the predictions based on constraint release Rouse motion model at long time scales, but show deviations from the theoretical predictions in the intermediate time regime. Our preliminary analysis of diffusion of the matrix chains in the tube-region of the long chains indicates that this discrepancy results from neglect of the broad distribution of the lifetimes of constraint release events in the theoretical treatment. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B25.00004: Flory Theorem for Structurally Asymmetric Mixtures Andrey Dobrynin, Frank Sun, David Shirvanyants, Gregory Rubinstein, Michael Rubinstein, Sergei Sheiko, Hyung-Il Lee, Krzysztof Matyjaszewski The generalization of the Flory theorem for structurally asymmetric mixtures was derived and tested by direct visualization of conformational transformations of brushlike macromolecules embedded in a melt of linear chains. Swelling of a brush molecule was shown to be controlled not only by the degree of polymerization of the surrounding linear chains, \textit{NB}, but also by the degree of polymerization of the brush's side chains, $N$, which determines the structural asymmetry of the mixed species. The boundaries of the swelling region were established by scaling analysis as $N^2$ $<$\textit{NB} $<$\textit{NA/N}, where \textit{NA} is the degree of polymerization of the brush backbone. Experiment and theory demonstrated good agreement. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B25.00005: The Molecular Weight and Composition Dependence of Measured Flory-Huggins Interaction Parameters for Blends of Model Polyolefins Alisyn Nedoma, Megan Robertson, Nisita Wanakule, Nitash Balsara The thermodynamics of binary blends of model polyolefins (deuterated polybutadiene and polyisobutylene) was studied using small angle neutron scattering (SANS) and small angle light scattering (SALS). SANS experiments on single phase blends reveal that the Flory-Huggins interaction parameter, chi, for this system depends on both blend composition and component molecular weights. This is in contrast to numerous previous studies on thermodynamics of polymer blends where chi is reported to be independent of molecular weight. The phase separation temperatures of our blends, determined by SALS and SANS, will be compared to predictions based on the measured chi parameters. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B25.00006: Porod SAXS Studies of Shear-Induced Droplet Deformation in a Concentrated Immiscible Polymer Blend Wesley Burghardt, Kristin Brinker Droplet deformation, break-up and coalescence of immiscible polymer blends under flow is fundamental to understanding the effect of processing on ultimate blend properties, as well as the excess rheological properties associated with deformation of the multiphase structure. Rheo-optical methods have frequently been employed to gain insight into these processes, but in most cases, optical methods are restricted to quite dilute concentrations, owing to concerns of high turbidity and multiple scattering. Here we explore used of time-resolved synchrotron small-angle x-ray scattering as an alternative method to study multiphase materials under shear. Typical blend droplet sizes of $\sim $ 1 micron are large relative to length scales typically probed by SAXS; however, the wide-angle limit of small-angle scattering (i.e. the Porod regime) is directly sensitive to interfacial structure of multiphase materials and, when rendered anisotropic by shear flow, provides direct insights into the deformation and orientation of interface. We report scattering studies in the flow-gradient plane of a polystyrene/poly(methyl methacrylate) blend, which is approximately viscosity matched, to step-strain deformations. Postulating that each droplet is deformed to a geometrically similar shape, data are analyzed in the context of a model of Porod scattering from ellipsoids. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B25.00007: Measurements of the Onsager coefficient in a phase-separating polymer blend Amish Patel, Nitash Balsara Phase separation in a polymer blend of critical concentration was studied using time-resolved Small Angle Neutron Scattering (SANS). To start off, the blend was homogenized by taking it well into the 1-phase region of the phase diagram. It was then quenched into the 2-phase region by using a rapid change in pressure. Since, the sample enters the two-phase region at the critical point, it is expected to phase separate by spinodal decomposition. Hence, the time dependent SANS intensity was fit to the Cahn-Hilliard-Cook equation for systems undergoing spinodal decomposition. The fit parameters were then used to calculate the non-local Onsager coefficient at several different pressures. The functional form of the Onsager coefficient, as a function of the wave-vector, was compared to that predicted by theory. Finally, the molecular parameters that go into the theory were extracted. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B25.00008: Structure And Dynamics Of Semi-crystalline Polyethylene Oxide / Polyvinyl Acetate Blends James Runt, Daniel Fragiadakis The structure and dynamics of semi-crystalline, melt-miscible polyethylene oxide / polyvinyl acetate (PEO/PVAc) blends were investigated using small-angle X-ray scattering and dielectric relaxation spectroscopy. PEO/PVAc blends with selected compositions were crystallized at various temperatures. Small-angle X-ray scattering was used to quantitatively determine the semi-crystalline microstructure, including the location(s) of the non-crystallizable PVAc in the structure. Segmental and local dynamics were studied using broadband dielectric relaxation spectroscopy. We attempt to clarify the origin of an additional relaxation, located at intermediate frequencies between the segmental and local processes, which has been proposed to be related to initial stages of crystallization even in blends lacking macroscopic crystallinity. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B25.00009: The Structure and Thermodynamics of Cellulose Acetates. Mark Dadmun, Rujul Mehta, Gary Lynn Cellulose acetate (CA) polymers having different degrees of substitution (DS) vary widely in their properties, such as glass transition, miscibility, chemical resistance, and gas permeation. These variations hint at differences in their molecular structure. We have systematically studied the variation in the molecular structure of CA with DS, utilizing small angle neutron scattering (SANS). SANS measurements were carried out in 1{\%} (w/v) solutions in dDMSO (which is sufficiently dilute to avoid intermolecular scattering). The polymer chains display rigidity along their main chain and have persistence lengths in the range of 12 to 14 nm. The effect of DS on the extent of miscibility is explained on the basis of available intra-molecular hydrogen bonding sites along the polymer chain. SANS measurements were also carried out on 50-50(w/w) miscible blends of two sets of CA, where the difference in the DS between the polymers varied from 0.05 to 0.25. Random phase approximation analysis provides an estimate of the Flory-Huggins interaction parameter between the two polymers in the blend. The influence of both the amount of substitution and hydrogen bonding on the polymer miscibility will be discussed. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B25.00010: Effect of solvent evaporation and coagulation on morphology development of asymmetric membranes Neelakandan Chandrasekaran, Thein Kyu Miscibility behavior of blends of amorphous polyamide (PA) and polyvinylpyrrolidone (PVP) was studied in relation to membrane formation. Dimethylsulfoxide (DMSO) and water were used as solvent and non-solvent, respectively. Differential scanning calorimetry and cloud point measurements revealed that the binary PA/PVP blends as well as the ternary PA/PVP/DMSO system were completely miscible at all compositions. However, the addition of non-solvent (water) to this ternary system has led to phase separation. Visual turbidity study was used to establish a ternary liquid-liquid phase diagram of the PA-PVP/DMSO/water system. Scanning Electron Microscopy (SEM) showed the development of finger-like and sponge-like cross sectional morphologies during coagulation. Effects of polymer concentration, PA/PVP blend ratio, solvent/non-solvent quality, and evaporation time on the resulting membrane morphology will be discussed. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B25.00011: The Glass Transition and Dynamics in Athermal Poly(a-Methyl Styrene)/Oligomer Blends Wei Zheng, Sindee Simon The glass transition and dynamics in athermal blends of poly(a-methyl styrene) (PaMS) and its short chain oligomers are investigated using differential scanning calorimetry (DSC). A methodology is described to partition the calorimetric transition in order to obtain effective Tgs for each component of the blend. The dependences of these effective Tgs on overall blend composition are described by the Lodge-McLeish model although the self-concentration effect is less than expected based on the Kuhn length. The kinetics associated with the glass temperature, Tg, are examined by studying the cooling rate dependence of Tg for the pure components and the blends, as well as by examining the enthalpy overshoots in the heating DSC scans. Extension of Colmenero's model to describe the dynamics in these materials will be discussed. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B25.00012: Effect of compositional heterogeneity on the phase structure and crystallization behavior of polypropylene in-reactor alloys. Dujin Wang, Haijin Zhu, Benjamin Monrabal, Charles C. Han Although the compositional heterogeneity and chain structure of PP/EPR in-situ blends have been extensively investigated, little is known about the conclusive relationship among the molecular/phase structure and the ultimate mechanical properties due to its complex compositions in such system. A systematic study was conducted on the compositional heterogeneity, phase structure, the crystallization and subsequent melting behavior of two in-reactor alloys EB-P and EP-P. The composition of the alloys and the chain structure of each component were characterized by preparative TREF and 13C-NMR technique. The results showed that the excellent balance between toughness and rigidity of EB-P primarily benefits from the polyethylene homopolymer (HPE) phase and the ethylene-$\alpha $-olefin copolymer (EC) component, which is enriched at the interface between the dispersed phase (HPP) and the matrix (HPE). As for EP-P, the amorphous EC and the interpenetrating phase are mainly responsible for the outstanding low temperature impact toughness. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B25.00013: Mesoscopic drop dynamics and rheological modeling for polymer blends Yuanze Xu, Wei Yu, Jianmao Yang Fundamental challenge to rheology for polyblends originates from the dynamic coupling between interfacial morphology and viscoelastic flow. This work will outline our approaches. The framework of irreversible thermodynamics was employed to model immiscible viscoelastic blends considering the chain conformation, the drop deformation, drop break-up and collapse and the hydrodynamic interaction of drops in concentrated blends. The theoretical predictions were proved by measuring the drop dynamics in a four-roll mill rheometer with success. Great challenge to the model exists in systems where the drop dynamics severely deviating from emulsions of Newtonian droplets. A new type of drop merging by a string pulling two beads together (BSB) was observed and analyzed. The relevant material and process conditions causing BSB or beads-on-string are discussed elucidating its origin of non-linear viscoelasticity of polymers. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B25.00014: Self Similar Growth of Polyolefin Blends On a Micro-Nano Granule Reactor Charles Han, Jiang Du, Kun Meng, Xia Dong, Jin-yong Dong, Dujin Wang A Ziegler-Natta/metallocene hybrid catalyst was used in this MRGT (Multi-catalyst Reactor Granule Technology) synthesis. Isotactic polypropylene/polyethylene-co-octene (iPP/PEOc) polymer blends were prepared on the micro-nano granule reactors. A self-similar growth mechanism has been observed and deciphered. The self-similar structure is extended and observed at least for 5 decades in a combined real and reciprocal spatial range. With thermal treatment, structure growth and crystallization kinetics has been studied on these single reactor granules. [Preview Abstract] |
Session B26: Focus Session: Photophysics of Cold Molecules II
Sponsoring Units: DCPChair: Roland Wester, University of Freiburg
Room: Morial Convention Center 218
Monday, March 10, 2008 11:15AM - 11:51AM |
B26.00001: Molecular collision studies with Stark-decelerated beams Invited Speaker: Molecular scattering behaviour has generally proven difficult to study at low collision energies. We formed a molecular beam of OH radicals with a narrow velocity distribution and a tunable velocity by passing the beam through a Stark decelerator [1]. The transition probabilities for inelastic scattering of the OH radicals with Xe atoms were measured as a function of the collision energy in the range of 50 to 400 wavenumbers. The behaviour of the cross-sections for inelastic scattering near the energetic thresholds was accurately measured, and excellent agreement was obtained with cross-sections derived from coupled- channel calculations on ab initio computed potential energy surfaces [2]. For collision studies at lower energies, the decelerated beams of molecules can be loaded into a variety of traps. In these traps, electric fields are used to keep the molecules confined in a region of space where they can be studied in complete isolation from the (hot) environment. Typically, 10$^5$ state- selected molecules can be trapped for times up to several seconds at a density of 10$^7$ mol/cm$^3$ and at a temperature of several tens of mK [3]. The long interaction time afforded by the trap has been exploited to measure the infrared radiative lifetime of vibrationally excited OH radicals, for instance, as well as to study the far-infrared optical pumping of these polar molecules due to blackbody radiation [4]. As an alternative to these traps, we have demonstrated an electrostatic storage ring for neutral molecules. In its simplest form, a storage ring is a trap in which the molecules - rather than having a minimum potential energy at a single location in space - have a minimum potential energy on a circle. To fully exploit the possibilities offered by a ring structure, it is imperative that the molecules remain in a bunch as they revolve around the ring. This ensures a high density of stored molecules, moreover, this makes it possible to inject multiple - either co-linear or counter propagating - packets into the ring without affecting the packet(s) already stored. We have recently demonstrated a prototype molecular synchrotron, which will be used as a low-energy collider for neutral molecules in the future [5].\newline [1] H.L. Bethlem, G. Berden, and G. Meijer, Phys. Rev. Lett. 83, (1999) 1558-1561.\newline [2] J.J. Gilijamse, S. Hoekstra, S.Y.T. van de Meerakker, G.C. Groenenboom, and G. Meijer, Science 313, (2006) 1617-1620.\newline [3] S.Y.T. van de Meerakker, P.H.M. Smeets, N. Vanhaecke, R.T. Jongma, and G. Meijer, Phys. Rev. Lett. 94, (2005) Artn. 023004.\newline [4] S. Hoekstra, J.J. Gilijamse, B. Sartakov, N. Vanhaecke, L. Scharfenberg, S.Y.T. van de Meerakker, and G. Meijer, Phys. Rev. Lett. 98, (2007) Artn. 133001.\newline [5] C.E. Heiner, D. Carty, G. Meijer, and H.L. Bethlem, Nature Physics 3, (2007) 115-118. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B26.00002: Magnetoelectrostatic trapping of neutral OH molecules Brian Sawyer, Benjamin Stuhl, Benjamin Lev, Mark Yeo, Dajun Wang, Jun Ye Advances in cold molecule production promise to profoundly impact research on precision measurement, quantum information, and controlled chemistry. To this end, we employ a Stark decelerator to remove 99.5{\%} of the center-of-mass kinetic energy of a supersonic beam of ground-state OH molecules. We subsequently trap a 70 mK sample of the decelerated molecules at a density of $>$10$^{5}$ cm$^{-3}$ within a magnetic quadrupole whose center lies $\sim $1cm from the decelerator exit. Our magnetoelectrostatic trap (MET) design allows for the addition of an electric field of variable magnitude to the trapped sample to facilitate polar-molecule collision studies. We report progress toward observation of cold collisions between samples of polar molecules. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B26.00003: Photodissociation of SO$_2$ as a way to cold atoms and molecules Lisdat Christian, Oleg Bucicov, Marcin Nowak, Sebastian Jung, Eberhard Tiemann We discuss the possibility to use the photodissociation of cold SO$_2$ molecules to produce internally and translationally cold photofragments SO and O. It is expected from our measurements of the molecular Stark effect~[1] that the dissociation pathways and excess energies of the fragments are tunable by electric fields~[2]. Cold SO$_2$ molecules are produced by Stark deceleration. We have realized a Stark decelerator that is able to slow down packages SO$_2$ in weak-field seeking levels to a few 10~m/s center of mass velocity. A Stark decelerator with 326~stages is required for this purpose, since the ratio of Stark shift to initial kinetic energy is small for SO$_2$. The photofragments SO and O have triplet ground states, while the ground state of SO$_2$ is diamagnetic. In combination with the photodissociation at the threshold we want to employ this constellation to accumulate fragments in a magnetic trap by dissociating SO$_2$ as it is stopped by electric fields in the center of the trap. \newline [1] J. Phys. B \textbf{39}, S1085 (2006). \newline [2] Phys. Rev.~A \textbf{74}, 040701(R) (2006). [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B26.00004: Alternating gradient focusing and deceleration of large molecules Kirstin Wohlfart, Fabian Gr\"atz, Frank Filsinger, Gerard Meijer, Jochen K\"upper During the last decade, fascinating progress has been made in the spectroscopy of the ``molecular building blocks of life''. Meanwhile, our group has been developing methods to decelerate neutral, polar molecules using time varying inhomogeneous electric fields. Extending these techniques to bio-molecules would allow, for instance, to increase observation times for precision spectroscopy or to separate different conformers. However, for such large molecules all states are practically high-field seeking. Therefore, alternating gradient focusing has to be applied. Here, we demonstrate the focusing and deceleration of benzonitrile (C$_7$H$_5$N) from a molecular beam. Benzonitrile is prototypical for large asymmetric top molecules that exhibits rich rotational structure and a high density of states. It is decelerated in its absolute ground state from 320~m/s to 289~m/s, and similar velocity changes are obtained for excited rotational states. We are setting up a longer alternating gradient decelerator, which will enable us to decelerate benzonitrile or larger molecules to much lower velocities and to thereby completely separate the decelerated packet from the rest of the beam pulse. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 1:03PM |
B26.00005: Production and Trapping of Ultracold Polar RbCs Molecules Invited Speaker: Our group has recently demonstrated the ability to assemble ultracold, polar molecules from laser-cooled atoms. We use photoassociation followed by stimulated emission pumping to produce RbCs molecules in their absolute ground state, at temperatures $T\sim 100\mu \mbox{K}$. In recent work, we have moved towards the goal of accumulated large, high-density samples of ultracold RbCs. Here we present new results on the trapping and collisional properties of RbCs in levels of high vibrational excitation. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B26.00006: Multistage Zeeman deceleration of hydrogen S.D. Hogan, A. Wiederkehr, M. Andrist, H. Schmutz, B. Lambillotte, F. Merkt With the goals of: (i) performing ultra-high resolution spectroscopy with long interaction times between a cloud of cold atoms or molecules and a narrow bandwidth radiation field, and (ii) studying cold reactive collisions in which the kinetic energies and quantum states of the colliding particles may be controlled to a high degree, a multi-stage Zeeman decelerator for neutral radicals has recently been developed in our laboratory. This instrument relies on the same concept of phase stability employed in charged particle accelerators. It opens up the possibility to manipulate the translational motion of a wider range of species than has been demonstrated using other quantum-state-selective techniques such as multi-stage Stark deceleration, and applies to a very different class of species than those to which Rydberg Stark deceleration is appropriate. The results of a recent series of experiments in which we have decelerated ground state hydrogen will be presented along with progress toward three-dimensional magnetic trapping of the decelerated radicals. In these experiments magnetic fields of 1-2~T are pulsed in each of the coils which make up the decelerator for tens of microseconds, with rise and fall times shorter than 5~$\mu$s. We have characterized the decelerated part of the gas pulse and studied the effect of zero field time windows, in which electron spin flips can occur, on the deceleration process. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B26.00007: Kinetics of Cold Molecule Production in ``Kinematic'' Cooling Jeffrey Kay, Kevin Strecker, David Chandler ``Kinematic'' cooling is a general technique by which a vast array of molecules can be translationally cooled using crossed atomic and molecular beams. The success of the technique relies primarily on the existence of an approximate mass degeneracy between the molecule to be cooled and its atomic (or molecular) collision partner. Here, we discuss factors that affect the efficiency of cold molecule production by this method, as well as schemes that may allow tunability of the velocity and temperature of the cold molecules on a fine scale. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B26.00008: A new source for quantum optics with biomolecules and biomolecular clusters Markus Marksteiner, Philipp Haslinger, Hendrik Ulbricht, Markus Arndt We present recent progress towards matter wave experiments with amino acids, polypeptides and large biomolecular clusters. All successful experiments on macromolecule interferometry so far, with fullerenes, fullerene derivates and large perfluoroalkyl-functionalized azobenzenes used effusive beam sources. The combination of Stark deflectometry with quantum interferometry also allowed us to create a new device for precisely measuring electric susceptibilities of large molecules in the gas phase. In order to apply quantum interference to molecules of biological interest, we have now implemented a pulsed laser desorption source. The combination of UV laser desorption into an intense noble gas jet and single-photon ionization by a VUV excimer laser (157nm) allows us to observe intense neutral jets of amino acids (e.g. Tryptophan), nucleotides (e.g. Guanin) and polypeptides ranging from tri-peptides to Gramicidin. Remarkably, we also found a new method for producing large neutral amino acid clusters, such as for instance Trp$_{30}$, with masses exceeding 6000 amu: the addition of alkaline Earth salts in the desorption process leads to the inclusion of at least one metal atom per complex and is sufficient to catalyze the cluster formation process. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 2:15PM |
B26.00009: Collisions of ultracold molecules Invited Speaker: In our experiments we routinely produce ultracold trapped samples of dimer molecules out of a Cs atomic gas by exploiting the atom-dimer coupling near Feshbach resonances. We explore the rich molecular structure for the Cs dimers near the atomic threshold by consecutive state transfer after initial dimer production and produce atom-dimer mixtures for which we measure the atom-dimer collisional rate as a function of magnetic field at temperatures down to 40 nK. We find resonant enhancement of this rate for sufficiently small dimer binding energies for which coupling to an Efimov trimer state is possible. We also produce pure dimer samples for which we measure the collisional loss rate. For a weakly bound molecular s-state this rate depends strongly on temperature and on the applied magnetic field. We will also discuss first results from our experiment on producing ultracold ro-vibrational ground state molecules for the case of Cs dimers and RbCs starting from weakly bound molecules which initially are produced on a Feshbach resonance. [Preview Abstract] |
Session B27: Focus Session: Molecular Magnets I
Sponsoring Units: GMAGChair: Stephen Hill, University of Florida
Room: Morial Convention Center 219
Monday, March 10, 2008 11:15AM - 11:51AM |
B27.00001: Self-Assembled Growth and Magnetism of Ordered Cluster Arrays Invited Speaker: It is generally recognized that the fabrication of magnetic storage media with bit densities of Gigabytes per square inch and more is out of reach of currently available thin film technologies. Patterned media may therefore set off to challenge thin film media as they allow in principle for bit densities several orders of magnitude larger than what is currently feasible. In this talk I will show how nanoclusters can be fabricated on substrates directly by self-assembled growth, and how their magnetism and their lateral arrangement on the substrate can be controlled. Buffer layer assisted growth is used to form clusters of controlled density and size, in the range between a few atoms to several nanometers diameter. The clusters are randomly distributed over the bare substrate surface. The cluster nucleation on the buffer layer and their growth after making contact with the substrate was studied with variable temperature scanning tunneling microscopy, and will be discussed in the talk. The investigation of the cluster magnetism with X-ray magnetic circular dichroism revealed size and strain effects as well as mutual dipolar and cluster-substrate interactions. We found a pronounced dependence of the magnetic anisotropy on the substrate material. On Pt, for instance, the preferential magnetization direction is out-of-plane, while it is in-plane on Ag. The application of self-assembled clusters as individually addressable magnetic units requires their controlled arrangement into well-defined ordered arrays. We are therefore guiding the clusters with energetic sinks provided by periodic network structures prefabricated on the substrate. We use mechanically extremely stable, electronically insulating boron nitride nanomesh monolayers as template surfaces. Repeated cluster deposition cycles increase the cluster density on the nanomesh, eventually resulting in an densely packed, ordered cluster array with a cluster-cluster distance corresponding to the BN nanomesh periodicity of 3.2 nm. These cluster layers offer densities of magnetic elements as high as 80 x 10$^{12}$ clusters per square inch. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B27.00002: Transition linewidth of Mn12-Acetate Beth Parks, Kurt Andresen, Christopher Beedle, David Hendrickson The single-molecule magnet Mn$_{12}$-acetate forms macroscopic crystals in which quantum tunneling of the magnetic moment can be observed. This quantum tunneling occurs due to localized defects in the crystal structure. These same defects are thought to be responsible for broadening the linewidth of transitions between adjacent levels, such as m$_{s} = 10$ to m$_{s} = 9$. It is possible to test this understanding by observing the linewidth of the transition during the tunneling process. If the same defects are responsible for both effects, then as tunneling progresses, the linewidth should change. Results of this measurement of the linewidth obtained using terahertz time-domain spectroscopy will be presented. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B27.00003: Low temperature relaxation crossover in one dimensional chain-like molecular magnet [Fe$^{II}(\Delta)$Fe$^{II}(\Lambda)$(ox)$_{2}$(phen)$_{2}$]$_{n}$ J.L. Her, C.P. Sun, S. Taran, C.C. Chou, C.L. Chan, C.C. Lin, L.L. Li, K.J. Lin, H.D. Yang The frequency-dependent ac susceptibility, thermoremanent magnetization relaxation (TRM) and magnetic field dependent magnetization have performed on a hand-aligned partially orientated molecular magnet compound [Fe$^{II}(\Delta )$Fe$^{II}(\Lambda )$(ox)$_{2}$(phen)$_{2}$]$_{n}$,[1] which exhibits one dimension chain like structure. The ac susceptibility shows spin glass-like relaxation at temperatures between 7.8 and 8.2 K. In addition, the TRM results show various relaxation behaviors below T$_{m}$ $\sim $ 8.6 K, indicating that there is a remnant instability at low temperature. It might be caused by the complex interaction within and/or between the chains and the stacked layers. With slowly sweeping the magnetic field, a step-like behavior in the magnetic hysteresis loop was observed below T$_{m}$. The possible origins for these properties are discussed. \newline [1] L. L. Li, K. J. Lin, C. J. Ho, C. P. Sun, and H. D. Yang, Chem. Commun. \textbf{12}, 1286 (2006). [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B27.00004: A novel experiment using rotating magnetic fields to study the pumping spin states in molecular magnets Alberto Hernandez-Minguez, Ferran Macia, Joan Manel Hernandez, Carla Carbonell, Roger Amig\'o, Javier Tejada We report here a new experimental technique to monitor spin population dynamics in molecular magnets. This deals with a huge rotating magnetic field initially applied along the easy magnetization direction, $z$--axis, that rotates with components parallel and perpendicular to the $z$ axis. This technique allows us to probe spin relaxation on reasonably fast time scales detecting the inversion of the whole spin states. The population of spin levels depends on the frequency of the rotating magnetic field. This very new technique could help to carry out new experiments in a number of different fields, broadening substantially the scope of their use until now. A Hern\'{a}ndez-M\'{\i}nguez \textit{et al.}, Appl. Phys. Lett. \textbf{91}, 202502 (2007) [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B27.00005: Anomalous magnetic relaxation in 2D layered organic-based magnet [Fe(TCNE)(NCMe)$_2$][FeCl$_4$] Jung-Woo Yoo, V. N. Prigodin, K. I. Pokhodnya, Joel S. Miller, A. J. Epstein The magnetic relaxation of the 2D organic-based magnet [Fe(TCNE) (NCMe)$_2$][FeCl$_4$] was explored using both static and dynamic measurements. Static $M$($H$,$T$) studies showed that the ferrimagnetic order between the spins in Fe$^{2+}$ ($S$ = 2) and the spin in (TCNE)$^-$ ($S$ = 1/2) occurs principally within the plane of [Fe(TCNE)(NCMe)$_2$]$^+$ layers, with no magnetic coupling to the $S$ = 5/2 of the [FeCl$_4$]$^-$, which is located between layers [1]. The DC magnetic relaxation in ZFC states shows the memory effects similar to that observed in superparamagnetic systems. This reflects the weak magnetic coupling between the layers enabling the bistable nature between FC and ZFC states [1]. The memory effects disappear when the system is cooled in field supporting bistable nature of interlayer coupling. The dynamic susceptibility near the critical $T$ shows two relaxation processes the possible origins of which will be discussed. \newline [1] K. I. Pokhodnya, ${\it et}$ ${\it al}$. J. Am. Chem. Soc., ${\bf 118} $, 12844 (2006). [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B27.00006: Ferromagnetic resonance (FMR) of a room temperature organic based mixed valent Vanadium Hexacyanochromate magnet K$_{1.54}$V$^{II}_{0.77}$V$^{III}_{0.08}$[Cr$^{III}$(CN)$_{6}$](SO$_{4})_{0.16}$3.1H$_{2}$O. N.P. Raju, J.W. Yoo, Amber C. McConnell, William W. Shum, Kendric J. Nelson, Joel S. Miller, A.J. Epstein Recently, organic materials both magnetic and non-magnetic have been receiving significant attention for their potential applicability in spintronics devices such as spin-valves, memory devices, spin-transistors etc. Here we report magnetic and X-band FMR studies of an organic based mixed valent Vanadium Hexacyanochromate magnet K$_{1.54}$V$^{II}_{0.77}$V$^{III}_{0.08}$[Cr$^{III}$(CN)$_{6}$](SO$_{4})_{0.16}$3.1H$_{2}$O with an ordering temperature well above room temperature ($>$340K). Temperature dependencies of FMR parameters including intensity, linewidth, resonance field will be discussed in terms of the coexistence of long-range magnetic ordering and spin-glass-like behavior in this material. [1] {\O}yvind Hatlevik et. al. Adv. Mater. \textbf{11}, 914 (1999). [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B27.00007: Origin of edge magnetism in zig-zag graphene nanoribbons Jeil Jung, Allan MacDonald We explore the physical origins of edge magnetism in zig-zag terminated graphene nanoribbons addressing the role of exchange effects and the band structure in the ferromagnetic alignment of the spins along the edge, the preference of anti-ferromagnetic inter-edge ground state to the ferromagnetic one, and the microscopic physics of the spin stiffness along an edge. Our analysis of the qualitative physics will rest largely on unrestricted Hartree-Fock theory calculations for $\pi$-band model Hamiltonians with long-range Coulomb interactions. Unlike Hubbard models, or ab initio LDA calculations, this type of theory can consistently account for non-local exchange effects. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B27.00008: Magnetic Properties and Inelastic Neutron Scattering for a Spin Hexamer: Application to the V$_{6}$ Molecular Magnets J. T. Haraldsen, T. Barnes, J. Sinclair, J. Thompson, R. Sacci, J. Turner We present the study of the magnetic susceptibility and inelastic neutron scattering energies and intensities for a spin hexamer formed by two interacting spin S and S' trimers. Using an isotropic Heisenberg Hamiltonian, we conclude that, regardless of spin, the structure factors for the magnetic excitations will have a specific function form which is dependent on the symmetric parts of the hexamer being excited. This work is then compared to previous work performed on two vanadium compounds (CN$_3$H$_6$)$_4$Na$_2$[H$_4$V$_6$O$_8$(PO$_4$)$_4$((OCH$_2$)$_3$CCH$_2 $OH)$_2$]$\cdot 14$H$_2$O and Na$_6$[H$_4$V$_6$O$_8$(PO$_4$)$_4$((OCH$_2$)$_3$CCH$_2$OH)$_2$]$\cdot 18$H$_2$O, which are thought to be magnetically well described as trimers of S=1/2 V$^{4+}$ ions. We show that it is possible to that a very weak inter-trimer interaction exists and we predict the energy and momentum transfer dependence for these compounds that may be observable with inelastic neutron scattering. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B27.00009: Can large magnetic anisotropy and high spin really coexist? Claudia Loose, Eliseo Ruiz, Jordi Cirera, Joan Cano, Santiago Alvarez, Jens Kortus This theoretical study discusses the interplay of the magnetic anisotropy and magnetic exchange interaction of two Mn$_6$ complexes. Our results for two polynuclear Mn$_6$ complexes show a very strong dependence of the D value on the spin of the ground state while the energy barriers are practically constant. Thus, complex 2 with a large spin (S = 12) favoured by ferromagnetic interactions has a small D value, while the lower spin complex 1 (S = 4) has a large D value. Therefor we suggest, that a large magnetic anisotropy is not favoured by a high spin state of the ground state.\\ E.Ruiz et al. Chem. Comm. 2008, DOI: 10.1039/b714715e [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B27.00010: Time Evolution of Electric Properties of Mn$_{12}$-Acetate Film Measured with Self-assembled Tunneling Junction Lianxi Ma, Chi Chen, Glenn Agnolet we report the results of tunneling measurement of Mn$_{12}$-Ac thin film ($\sim $2 monolayer) at the time right after the film is made and after 6 months. We found that for the fresh film the differential conductance can change suddenly at bias voltage about -0.1 V and staircase form of $I-V$ curves are observed. At about 0 V bias voltage, we see the conductance changes from minimum to maximum as the initial resistance decreases. For the film stored for 6 months, however, all of these properties have been lost and high frequency oscillation on conductance is frequently observed. Also for the stored sample, clean $I-V$ curves are rarely seen. All of these results indicate that deterioration of the molecules after 6 months. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B27.00011: Bottom-up creation and adsorption of hybrid organic-inorganic magnetic molecules on metal substrates Daniel Wegner, Ryan Yamachika, Yayu Wang, Michael F. Crommie, Mark Pederson, Bart M. Bartlett, Jeffrey R. Long Charge-transfer compounds of the type M[TCNE]$_x$ (M: transition-metal, TCNE: tetracyanoethylene) form an important group of molecule-based ferromagnets with potential applications due to their high Curie temperature. Despite extensive studies the origin of magnetic coupling is not well understood due to a lack of sufficient structural characterization, largely attributed to disordered growth. Using a bottom-up approach, we have used a scanning tunneling microscope to arrange single V atoms and TCNE molecules to form charge transfer complexes of different size and geometry. By tunneling spectroscopy we prove chemical bonding as well as the emergence of magnetic properties. Special attention is given to the influence of the metallic substrate on the properties of TCNE and the implications for future organic-inorganic nanoscopic devices. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B27.00012: Temperature- and field dependent Raman spectra of the single-molecule magnet Mn$_{12}$-acetate. Robert Furstenberg, Christopher A. Kendziora, Javier Macossay Mn$_{12}$-acetate and related single-molecule magnets are promising candidates for high-density magnetic storage devices, spintronics components and even quantum computing applications. However, certain aspects of their fundamental properties are still not completely understood. For example, the importance of spin-vibron interactions in the calculation of the magnetic anisotropy of Mn$_{12}${\_}acetate has been proposed a few years ago [1]. Some experimental evidence has already been provided [2]. We present further evidence in support of this theory. A detailed Raman scattering study was performed by measuring polarization-dependent spectra in the 35-1500cm$^{-1}$ frequency region, temperature range of 6-300K and in magnetic fields of up to 9T. A symmetry analysis of the vibrational modes revealed the presence of acetic acid disorder-related isomers with C$_{1}$ symmetry. This confirms an earlier low-temperature X-ray study [3]. The observed temperature dependence of vibrational modes was interpreted in terms of spin-vibron interactions. The influence of external magnetic field on Raman spectra will also be discussed. [1] M. Pederson et al., \textit{Phys. Rev. Lett}. 89, 097202 (2002); [2] A.B. Shushkov et al., \textit{Phys. Rev.} B 66, 144430 (2002); [3] Cornia et al., \textit{Acta Crystallogr., Sect. C: Cryst. Struct. Commun.,} 58, 371 (2002) [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B27.00013: Order from disorder in the molecular-based spin ladder Cu(Qnx)Br$_{2}$ Christopher Landee, Chris Sataline, Brian Keith, Igor Partola, Mark Turnbull Copper quinoxaline dibromide is a molecular-based antiferromagnetic spin ladder in which Cu$_{2}$Br$_{4}$ dimers are linked into ladders by the quinoxaline molecules. The rung exchange occurs through the bridging bromide ions while the rail exchange occurs through the organic molecules [1]. It is possible to introduce randomness into this ladder by replacing bromide ions by chlorides, by replacing the quinoxaline molecules by the structurally equivalent 2,3-dimethylpyrazine, or by substituting diamagnetic cadmium ions into the copper sites. In all cases, order occurs at low temperatures ( 4 to 6 K) as evidenced by FC/ZFC studies, hysteresis and remnant magnetizations, as well as sharp anomalies in the susceptibility. [1] C. P. Landee \textit{et al}, \textit{Polyhedron }\textbf{22}, 2325-2329 (2003). [Preview Abstract] |
Session B28: Focus Session: Optical Properties of Nanostructures II: Graphene, Graphite and Related Materials
Sponsoring Units: DMPChair: Shigeo Maruyama, University of Tokyo
Room: Morial Convention Center 220
Monday, March 10, 2008 11:15AM - 11:27AM |
B28.00001: Optical studies of multilayer graphene in magnetic fields Hsiang-Lin Liu, G. L. Carr, K. A. Worsley, M. E. Itkis, E. Bekyarova, R. C. Haddon, A. N. Caruso We report the optical properties of multilayer graphene thin films grown on silicon substrate. The room-temperature reflectance and transmittance of the samples were measured over the energy range from the far-infrared to near-infrared. To extract the optical constants of the films, we analyzed all of the layers of this thin-film structure using a Drude-Lorentz model. From the parameters obtained, we compute the optical constants. With decreasing temperature, the far-infrared transmittance of the samples is increasing up to 4 {\%} down to 2 K. Interestingly, in an applied magnetic field of up to 10 Tesla, the giant positive magneto-optical effects over 20 {\%} are observed in the far-infrared region from 2 K to 300 K. Possible origin of these will be discussed. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B28.00002: Edge states and optical transition energies in carbon nanoribbons Jie Jiang, Wenchang Lu, Jerry Bernholc The local density of states (LDOS) of edge states and optical transition energies in carbon nanoribbons are investigated with density-functional calculations. The LDOS in either magnetic or non magnetic phases show peaks both below and above the Fermi level. The peaks in the two phases are localized in different energy ranges. Moreover, the LDOS in the two phases have the same decay shapes. The defects at zigzag edges are found to destroy spin-polarization in edge states. They also tend to increase the decay length in edge states by mixing defect and edge states. Thus, the LDOS measured by scanning tunneling spectroscopy shows different features depending on the edge quality. We also find that the optical transition energies $E_{ii}$ are not affected by the spin-polarization. However, edge effects tend to increase $E_{ii}$ values by 1.25/W, where W is the ribbon width. Therefore, the ratios of $E_{ii}$ in nanoribbons for different i are changed from those observed in single-wall carbon nanotubes. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B28.00003: Raman Studies of Electron-Phonon Coupling in Graphene and Graphite Intercalated Compounds J. Camacho, M. Y. Sfeir, A. T. Bollinger, A. C. Walters, C. A. Howard, M. Ellerby, J. A. Misewich, T. Valla Effects of the electron-phonon interaction in carbon-based materials can be seen in many physical properties, ranging from relatively high-Tc superconductivity in doped fullerenes and graphite intercalated compounds to being a limiting factor on the mobility of carriers in carbon nano-tubes. Despite the intensive research, these effects are not completely understood. Here we present Raman scattering studies of several long wavelength optical phonons in different graphene-based materials. We find that the frequency and line-width of some modes are very sensitive to electric field doping and to chemical doping of graphene sheets, reflecting the changes in interactions of these modes with charge carriers in the system. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B28.00004: Optical Selection Rule from Inversion Symmetry Breaking and Valley Optoelectronics in Graphene Wang Yao, Di Xiao, Qian Niu Inversion symmetry breaking allows contrasted circular dichroism in different $k$-space regions, which takes the extreme form of optical selection rules for interband transitions at high symmetry points. In graphene systems with broken inversion symmetry, this enables valley dependent interplay of electrons with light of different circular polarizations, in analogy to spin dependent optical activities in semiconductors. We discuss graphene based valley optoelectronics applications where light polarization information can be interconverted with electronic information. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B28.00005: Gate-Variable Optical Transitions in Graphene Feng Wang, Yuanbo Zhang, Chuanshan Tian, Caglar Girit, Alex Zettl, Michael Crommie, Y. Ron Shen Two-dimensional graphene monolayers and bilayers exhibit fascinating electrical transport behaviors. Using infrared spectroscopy we find that they also have strong interband transitions and, remarkably, their optical transitions can be significantly modified through electrical gating. This unique gate-dependence of interband transitions adds a valuable dimension for optically probing graphene bandstructure. For a graphene monolayer, it yields directly the linear band dispersion of Dirac fermions, while in a bilayer it reveals a dominating van-Hove singularity arising from interlayer coupling. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B28.00006: Filling-Factor-Dependent Magnetophonon Resonance in Graphene Kostyantyn Kechedzhi, Mark Goerbig, Jean-Noel Fuchs, Vladimir Falko We describe a peculiar fine structure acquired by the in-plane optical phonon at the $\Gamma$-point in graphene when it is brought into resonance with one of the inter-Landau-level transitions in the material. The effect is most pronounced when this lattice mode is in resonance with inter-Landau-level transitions $-,1\Rightarrow 0 $ and $0\Rightarrow+,1$ at a magnetic field $B_0 \approx 30$ T. The predicted mode splitting may be used to measure directly the strength of the electron-phonon coupling, and also to distinguish between circularly (left- and right-hand) polarized lattice modes. A similar effect is predicted in bilayer graphene in lower magnetic fields. \newline \newline [1] M.O. Goerbig, J.-N. Fuchs, K. Kechedzhi, V. I. Fal'ko, PRL 98, 087402 (2007). [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B28.00007: Raman Spectrum of Strained Single Layer Graphene Minsheng Wang, Song Han, Kang L. Wang Two-dimensional single layer graphene (SLG) films are very attractive due to their unique electrical and optical properties. In this work, silicon nitride, silicon oxide and polyimide are used to induce strain in SLG sheets. Micro-Raman spectra reveal the evolution of graphene feature peaks under various stress conditions. Different peak position, width and shape are observed due to different stress type, level and direction. Changes of the second order D peaks show that strain affects not only the phonon spectrum but also the electron band through a double resonant Raman process. Relative intensity changes between D and G bands indicate that defects are introduced into graphene during some of the deposition processes, which usually are not observed in spin-coated SLG samples. Strain effects also can be utilized to further modify the electron band structure of graphene. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B28.00008: Infrared probe of charge dynamics in graphene transistors Zhiqiang Li, Erik Henriksen, Zhigang Jiang, Zhao Hao, Michael Martin, Philip Kim, Horst Stormer, Dimitri Basov We report on infrared (IR) spectroscopy of charge dynamics in monolayer graphene. IR reflectance and transmission measurements were performed on graphene transistors as a function of gate voltage. From these data, we obtained the optical conductivity of graphene at various carrier densities. The dominant feature of the optical conductivity is an interband transition with the onset at twice the Fermi energy, which evolves systematically with gate voltage. Similar behavior was observed with the Fermi level on either side of the Dirac point. We will compare these results with theoretical predictions and discuss several new aspects of the charge dynamics in graphene uncovered by this work. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B28.00009: The G-band phonon frequency in single layer graphene Hyungbin Son, Alfonso Reina, Mildred Dresselhaus, Jing Kong Recent experimental studies on the high-frequency phonon modes of as-deposited n-graphene layer (nGL) films report that the frequency of the G-band in single graphene layer is generally higher than that in nGL films (n>2) and highly ordered pyrolytic graphite (HOPG). However, our results show that the frequency of the G-band in single graphene layer approaches that in HOPG when the single graphene layer transfered to different substrate, sonicated, or exposed to NaOH. These results suggest that the difference in the frequency of the G-band in single graphene layer and HOPG is due to the strain generated in the deposition process. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B28.00010: Spatially resolved and polarized Raman spectroscopy of graphene Duhee Yoon, Hyerim Moon, Hyeonsik Cheong Graphene samples were prepared by micromechanical cleavage of graphite flakes on silicon wafer, which was covered with a 300-nm silicon oxide layer. Raman spectra of a single-layer graphene were clearly differentiated from those of a few layers of graphene sheets. We compared the spatially resolved micro-Raman spectra with atomic force microscopy to determine the number of layers for each sample, and variations of Raman spectra, which intensity and shift of Raman peak, were observed through the Raman image. The Raman G peak, corresponding to Raman active mode E$_{2g}$, was observed at $\sim $1590 cm$^{-1}$, and G' peak due to double resonance Raman scattering was observed at $\sim $2700 cm$^{-1}$. We performed polarized Raman spectroscopy of a single-layer graphene. The intensity of G peak was independent of polarization, in agreement with Raman tensor calculation. The variation of Raman intensity of G' peak was measured as a function of the analyzer angle. The intensity was maximum for parallel polarization and was minimum for perpendicular polarization. The depolarization ratio was 1/3. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B28.00011: Probing Edge defects in \textit{n} (\textit{n}=1,2..) Graphene Layer system via Raman Scattering Humberto Gutierrez, Awnish Gupta, Peter Eklund Results of a microRaman study (spot size $\sim $0.7 microns; 514.5 nm excitation) of an edge (or boundary) of n- layer graphene films is presented. Graphene (n=1 layer) exhibits a very narrow Lorentzian D-band at $\sim $1344 cm$^{-1}$ with FWHM $\sim $15 cm$^{-1}$. For 2$<$n$<$5, this narrow peak is found to split into four bands. Interestingly, the D band intensity of the edge is quite strong (1/4 of the G-band) If te defects are truly localized on the edge, this implies a better resonance than found for defects at the interior, or, on the other hand, the range of the defects may extend a long distance into the interior of the films and involve many sites. Polarized Raman studies on this D band were made with the incident field at an angle $\theta $ with the respect to the average direction of the edge. The scattered light was collected either parallel (H) or perpendicular (V) to the edge. The polar intensity plots I$_{V}(\theta )$ and I$_{H}(\theta )$ were found to exhibit a quadrupolar and dipolar pattern, respectively. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B28.00012: X-ray absorption spectroscopy of graphite oxides Hae-Kyung Jeong, Leyla Colakerol, Han-Jin Noh, Yun Pyo Lee, Mei Hua Jin, Per-Anders Glans, Jae-Young Kim, Kevin E. Smith, Chong Yun Park, Young Hee Lee We have investigated electronic structures of graphite oxide using x-ray absorption spectroscopy at carbon and oxygen K-edges. Unoccupied states such as $\pi ^{\ast }$ and $\sigma ^{\ast }$ states originating from sp$^{2}$ hybridization in graphite are also visible in the graphite oxide, even though the graphite oxide experiences a severe oxidation. Additional electronic states of the graphite oxide compared to that of pure graphite are ascribed to the functional groups such as epoxide, carboxyl, and hydroxyl groups that are present in the graphite oxide. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B28.00013: Symmetry breaking in epitaxial graphene probed by ARPES Aaron Bostwick, Taisuke Ohta, Jessica McChesney, K. V. Emtsev, Th. Seyller, Karsten Horn, Eli Rotenberg The energy bands of $n$-doped graphene on SiC(0001) have been observed to deviate significantly from the expected conical shape near the Dirac crossing[Bostwick et al 2007, Zhou et al 2007]. Two scenarios have been proposed to explain these deviations, either as originated from the real part of the electronic self-energy due to many body interactions[Bostwick et al], or from ``A-B'' symmetry-breaking due to chemical interactions between the graphene and the underlying layer[Zhou et al]. In this talk we present a number of arguments against the A-B symmetry breaking scenario, among which is the observed \textbf{k}-dependent intensity of the Fermi surface as measured by ARPES. We show that the observed intensity distribution is directly sensitive to the degree of A-B symmetry breaking and show that the upper limit for an energy gap in graphene on SiC is much smaller than the observed deviations in the band structure. [Preview Abstract] |
Session B29: Focus Session: Carbon Nanotubes and Related Materials II: Graphene Transport
Sponsoring Units: DMPChair: Shan-Wen Tsai, University of California, Riverside
Room: Morial Convention Center 221
Monday, March 10, 2008 11:15AM - 11:51AM |
B29.00001: Intrinsic and Extrinsic Limits of Mobility in Graphene Invited Speaker: Graphene is an exciting new condensed matter system, both for the opportunity to observe the physics associated with massless Dirac Fermions in the laboratory, and because of materials parameters which make it attractive for technological applications. However, in the few years since the experimental realization of graphene, progress toward cleaner (higher mobility) samples has largely stalled. I will discuss experiments performed on atomically-clean graphene on SiO$_{2}$[1] in ultra-high vacuum to determine the intrinsic and extrinsic limits of mobility in graphene[2,3], which point out both the promise of the material as well as the technological challenges that lie ahead in realizing better graphene samples. Intrinsic scattering by the acoustic phonons of graphene[3] limits the room-temperature mobility to 2 x 10$^{5}$ cm$^{2}$/Vs at a carrier density of 10$^{12}$ cm$^{-2}$, higher than any known material. However, extrinsic scattering due to charges in the substrate[2] and substrate polar optical phonons[3] currently impose much more severe limits on the mobility, pointing out the importance of substrate choice for graphene devices[4]. \newline [1] M. Ishigami, et al., \textit{Nano Letters} \textbf{7}, 1643 (2007). \newline [2] J. H. Chen, et al., arXiv:0708.2408. \newline [3] J. H. Chen, et al., arXiv:0711.3646. \newline [4] J.-H. Chen, et al., \textit{Advanced Materials} \textbf{1}9, 3623 (2007). [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B29.00002: Intrinsic and Extrinsic performance limits of graphene device on SiO$_{2}$ Jianhao Chen, Chaun Jang, Shudong Xiao, Masa Ishigami, Michael Fuhrer We have measured the temperature-dependent resistivity of clean graphene devices on SiO$_{2}$ from 16K to 485K in ultra high vacuum[1]. Longitudinal acoustic phonons, intrinsic to graphene, give rise to the measured resistivity linearly dependent on temperature from 16 to $\sim $200K. Above 200 K, a sharp upturn in resistivity is observed due to remote interfacial phonon (RIP) scattering by the polar optical phonons of the SiO$_{2}$ substrate. Combining the contributions from intrinsic and extrinsic phonons, we are able to explain the complete temperature and carrier density dependence of the graphene resistivity on SiO$_{2}$. For a technologically relevant carrier density of n = 10$^{12}$ cm$^{-2}$ at room temperature, the intrinsic phonon scattering will only limit the mobility to $\sim $2x10$^{5}$ cm$^{2}$/Vs, while the extrinsic RIP scattering from SiO$_{2}$ will limit the mobility to $\sim $4x10$^{4}$cm$^{2}$/Vs. [1] J. H. Chen, et al., http://arxiv.org/abs/0711.3646 [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B29.00003: Single-particle relaxation time versus scattering time in 2D graphene layers Euyheon Hwang, Sankar Das Sarma We calculate the transport scattering time ($\tau_t$) and the single particle relaxation time ($\tau_s$) for disordered graphene in the lowest order of the electron-impurity interaction (Born approximation). We find that the ratio of $\tau_t$ to $\tau_s$ is always greater (less) than two for charged Coulomb (short-ranged neutral) scatterers. Thus, the calculated scattering time ratio can be a good criterion of directly selecting the dominant scattering mechanism in graphene. As a direct consequence of scattering times we calculate graphene mobility, damping rate, and density of states of single particle state. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B29.00004: The Coulomb Impurity Problem in Graphene Vitor Pereira, Johan Nilsson, Antonio Casto Neto We address the problem of an unscreened Coulomb charge in graphene and calculate the local density of states and displaced charge as a function of energy and distance from the impurity. This is done nonperturbatively in two different ways: (1) solving the problem exactly by studying numerically the tight-binding model on the lattice and (2) using the continuum description in terms of the 2D Dirac equation. We show that the Dirac equation, when properly regularized, provides a qualitative and quantitative low energy description of the problem. The lattice solution shows extra features that cannot be described by the Dirac equation: namely, bound state formation and strong renormalization of the van Hove singularities. \newline [Reference: Phys. Rev. Lett. 99, 166802 (2007)]. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B29.00005: Coulomb Impurity Screening in Graphene Valeri Kotov I will discuss the vacuum polarization charge density around a Coulomb impurity with charge $Z|e|$. Perturbation theory in powers of $Z\alpha$ (where $\alpha = e^{2}/v_{F}$ is the effective coupling constant in graphene), shows that the polarization charge is localized at the impurity site. An exact calculation, based on the Green's function in a Coulomb field, leads to a non-perturbative result, valid to all orders in $Z\alpha$ [1]. Taking into account also electron-electron interactions in the Hartree approximation, we solve the problem self-consistently in the subcritical regime, where the impurity has an effective charge $Z_{\mbox{eff}}$, determined by the localized induced charge. We find that an impurity with bare charge $Z=1$ remains subcritical, $Z_{\mbox{eff}} \alpha < 1/2$, for any $\alpha$, while impurities with $Z=2,3$ and higher can become supercritical at certain values of $\alpha$. \newline [1] I.S. Terekhov, A.I. Milstein, V.N. Kotov, and O.P. Sushkov, arXiv:0708.4263. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B29.00006: MagnetoResistance of Graphene-based spin valves. Luis Brey, Herbert Fertig In this work we present a detailed study of the conduction properties of wide graphene strips, with two different models for the source and drain leads. We reconfirmed that for undoped graphene, the system can be described by a {\it conductivity} in the $L \rightarrow \infty$ limit even when defects are absent from the system, and examined this behavior with respect to a broad range of lead parameters. Our results indicate that the conductance is relatively insensitive to the electronic structure of the leads. We then compute the conductivity of a simple three stripe spin-valve device with graphene acting as the non-magnetic material between the ferromagnetic leads. Two types of ferromagnetic lead systems were considered: one with a single ($s$) orbital for each spin state, with band centers separated in energy to induce spin polarization, and another with a narrow $d$ band which was taken to be spin-polarized. We find that the conductivity depends only weakly on the relative spin orientations of the leads, and therefore the magnetoresistance is rather small for most circumstances, largely due to the insensitivity of the conductivity with respect to conditions in the leads. Our results indicate that, although graphene has properties that make it attractive for spintronic devices, the performance of a graphene-based spin-valve is likely to be poor. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B29.00007: Spin transport in graphene strongly coupled to ferromagnetic leads Jelena Trbovic, Hagen Aurich, Gunnar Gunnarsson, Christian Schoenenberger We study low temperature spin transport in graphene layers by using NiPd alloy as ferromagnetic contacts. This type of contacts has been successfully used in realizing carbon nanotube-based spin devices. The measurements are done in the temperature range between 240 mK and 1.6 K with average electrode separation of 0.7 $\mu $m. We find a clear two-terminal spin-valve signal while sweeping the magnetic field in plane of the device, with about 3{\%} effect. However, the signal rapidly decays with increasing temperature and vanishes above 1.6 K. We believe that the observed rapid dephasing is due to the strong coupling of PdNi contacts to the graphene layer. In addition, three-terminal measurements (quasi non-local) have been done in the same temperature range in order to study the influence of a single NiPd electrode on the observed spin transport. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B29.00008: Interference of Electron Waves in a Ballistic Graphene Transistor. Sungjae Cho, Michael Fuhrer We have prepared single- and few-layer graphene samples by mechanical exfoliation of Kish graphite on SiO$_{2}$/Si substrates. We have fabricated graphene field-effect transistors by electron beam lithography followed by thermal evaporation of Cr/Au or Permalloy source and drain electrodes; the conducting silicon underneath 300 nm silicon dioxide serves as a back gate electrode. We find that at low temperatures that the two-dimensional plot of conductance as a function of gate voltage and drain voltage shows an interference pattern of maxima and minima which occur along diagonal lines. We analyze the pattern in terms of interference of electron waves reflected between source and drain electrodes. The slope of the lines measures the compressibility of the two-dimensional electron system, and has strikingly different dependence on carrier density (gate voltage) for single- and few-layer graphene samples, as expected theoretically. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B29.00009: Local Gating of Graphene Devices via Contactless Top Gates Jairo Velasco Jr., Gang Liu, Chun Ning Lau Graphene devices with local electrostatic gates are promising candidates for investigation of novel phenomena such as Klein tunneling and the veselago lensing effect. However, it is experimentally challenging to fabricate local gates without inadvertent introduction of dopants or defects. We have developed a novel lithography process that enables fabrication of contactless, suspended top gates above single and bi-layer graphene devices. Using this technique, we have demonstrated graphene p-n junctions. We will discuss latest progress towards electrical transport of such devices in the zero-magnetic field regime, as well as in the quantum Hall regime. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B29.00010: Current-Voltage Characteristics of Electrolyte-Gated Graphene Field-Effect Transistors Inanc Meric, Sebastian Sorgenfrei, Melinda Han, Barbaros Oezyilmaz, Philip Kim, Kenneth Shepard We investigate the current-voltage characteristics of graphene field-effect transistors (FET) with ionic-solution gating. Single-layer graphene FETs are fabricated with different device dimensions and electrolytically gated with a potentiostat in which a Pt counter electrode and an Ag/AgCl reference electrode in a feedback configuration hold the solution at a desired potential. This setup enables the gating of graphene with high efficiency due to the short Debye length and high dielectric constant in ionic solutions, leading to enhanced measured transconductances. Electrolytic gating has direct applicability to field-effect sensor applications of graphene devices. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B29.00011: Fabrication of gated suspended graphene devices Kirill Bolotin, Martin Klima, Kenneth Sikes, Geoff Fudenberg, James Hone, Philip Kim, Horst Stormer We find that graphene acts as a catalyst for the vapor-phase etch of silicon dioxide: silicon dioxide under graphene is etched much faster compared to the bare surface. This is consistent with the presence of a trapped water layer between graphene and the silicon dioxide substrate which accelerates etching of the substrate. This unusual property allows us to fabricate devices where a large-area graphene flake is suspended over a micron-sized trench with the unetched silicon substrate serving as a gate electrode. Electronic transport in the resulting devices suggests enhanced sample mobilities near the Dirac point. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B29.00012: High resolution, temperature dependent Raman spectroscopy of graphene Sebastian R\'emi, Constanze Metzger, Billy Hubbard, Claire Thomas, Bennett B. Goldberg, Anna Swan Single and bi-layer graphene are studied with high resolution, temperature dependent Raman scattering. The electron-phonon coupling in graphene depends sensitively on both the concentration of charge carriers and the temperature. Raman spectroscopy directly probes electron-phonon coupling, and has been used to examine the stiffening of the G-band, phonon damping [1] and spatial inhomogeneities in the carrier density [2]. Our measurements are performed between room temperature and 4K in a confocal scanning Raman system. The samples are back-gated, allowing us to tune the carrier density and spectroscopically map the Raman response. We will discuss our recent measurements. [1] J. Yan, Y. Zhang, P. Kim, and A. Pinczuk, \textit{Phys. Rev. Lett, }\textbf{98}, 166802 (2007) [2] C. Stampfer, et al. Arxiv, cond-mat 0709.4156v1 [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B29.00013: Studies of limitations on the mobility and mean free paths in graphene devices. Xu Du, Ivan Skachko, Eva Y. Andrei The Dirac Fermion nature of the quasiparticles in graphene has led to many predictions for novel phenomena such as specular Andreev reflections at graphene-superconductor interfaces and a negative index of refraction for transmission of charge across graphene p-n junctions. These predictions presuppose ballistic transport, which requires long mean free paths compared to the distance between leads. However, within current fabrication techniques, the mean free paths of charge carriers in graphene devices are often too short for ballistic transport. The reduced mean free path is primarily due to excess scattering introduced by extrinsic factors such as material imperfections, substrate contamination, e-beam resist residue, chemical doping, contact potential and contact geometry. We will discuss the results of systematic studies of extrinsic factors, highlighting the case of graphene SNS weak links, and will propose strategies to increase the mean free path. [Preview Abstract] |
Session B30: Focus Session: Carbon Nanotubes and Related Materials III: Synthesis
Sponsoring Units: DMPChair: Chun Ning Lau, University of California, Riverside
Room: Morial Convention Center 222
Monday, March 10, 2008 11:15AM - 11:51AM |
B30.00001: Novel Functions in Double Walled Carbon Nanotubes Invited Speaker: Nano-sized carbon nanotubes with hollow core were observed when hydrocarbons were catalytically decomposed in the existence of nano-sized catalyst such as iron at higher temperature. Up to now, this catalytic chemical vapor deposition (CCVD) method has been utilized as the most powerful technique for the selective and large-scale production of carbon nanotubes. Since large amount of multi-walled carbon nanotubes (up to 250 ton/year) are available, much efforts has intensified on the development of their industrial usages. A recent hot topic has focused on the synthesis of double walled carbon nanotubes (DWNTs) because these tubes are more thermally and chemically stable when compared to single wall carbon nanotubes (SWNTs); they also exhibiting the 1D character of a quantum wire. In addition, DWNTs could also be used in the fabrication of electron field emitter and nano-composites. Very recently, we have successfully prepared highly pure and crystalline DWNTs by the combination of the CCVD and the subsequent oxidative purification process. In this talk, I will describe the preferential growth of DWNTs over SWNT or MWNTs, their structural characterizations using various analytic techniques and their possible applications. We found that these coaxial tubes consist of two relatively round, small and homogeneous-sized (below 2 nm in the outer shell) concentric tubules and are packed in a hexagonal array. Then, I will discuss a novel and stable structure consisting of flattened tubules containing two SWNTs via the coalescence of two adjacent tubes, chemical doping effect as a tunable way of electronic structure of DWNTs, and formation of atomic scale metal wires in the hollow core of DWNTs. Finally, I will report their transport properties as well as their performance in field effect transistors as compared with those of SWNTs. Therefore, in the near future, it may be possible that DWNTs replace SWNTs in specific carbon nanotube devices used today. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B30.00002: Electron Spin Resonance in Single-Walled Carbon Nanotubes W. D. Rice, J. Kono We have performed electron spin resonance (ESR) measurements on various types of single-walled carbon nanotube (SWNT) samples. As catalyst particles were chemically removed from the SWNTs, the linewidth of the conduction electron spin resonance (CESR) signal became smaller, reaching 43 G at 5 K for acid-purified HiPco SWNTs. For every type of SWNT tested, a ferromagnetic resonance (FMR) signal was observed; we show that this is attributed to catalyst particles. The $g$-factor for the CESR signal was slightly shifted from the free electron value; as purity increased, the resonance moved closer to $g$ = 2.003. The conduction electron signal increased as the temperature was decreased, indicating that we are observing both Pauli and Curie paramagnetism. No spin gap for either laser-oven acid-purified or HiPco acid-purified SWNTs was seen when the temperature was taken from 295 K to 5 K. In addition, the FMR signal decreased as the temperature was decreased. SWNTs suspended in aqueous surfactant solutions were also measured. As a function of purity, the FMR signal was substantially decreased. We show that as a function of metal catalyst content, the ESR lineshape of SWNTs changes significantly. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B30.00003: IPS verification of the integrity of CNTs walls after purification. Patricio Haberle, Samuel Hevia, Rodrigo Segura, Manish Chhowalla We report results from measurements by inverse photoemission spectroscopy (IPS) from single wall carbon nanotubes (SWCNTs). We have used this technique to verify the influence of strong purification procedures on the integrity of the tube walls. The purification included an HNO$_{3}$ immersion in a solution for 3 hours, before a 400 \r{ }C annealing. Even though the thin layers of CNTs present a high conductivity, the treated tubes trap the electronic charges from the IPS electron beam. A possible explanation for this apparent inconsistency is that the acid treatment induces the formation of charge traps in the outside tubes of the bundles. RAMAN spectroscopy from the same samples, with and with out the acid treatment, show almost no variations. The IPS signal is extremely sensitive to the conditions of the tube's external wall, while Raman spectroscopy can detect signals form inner tubes. Using IPS may then prove useful to determine electronic quality of CNTs arrays. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B30.00004: High surface area, porous nanotube film supercapacitors. Rajib K. Das, Ryan M. Walczak, John R. Reynolds, Andrew G. Rinzler Recently, I-H Kim \textit{et al.} [a] described high performance supercapacitors based on ruthenium oxide electrodeposited on multi-walled carbon nanotube mats. We recently developed a method for producing enhanced porosity single wall carbon nanotube (SWNT) films based on co-filtration of sacrificial nano-spheres and the SWNTs in the filtration based film fabrication. Here we follow Kim \textit{et al.} in electrodepositing ruthenium oxide onto the porous SWNT films. Performance of the devices will be discussed. a. I-H Kim, J-H Kim, Y-H Lee and K-B Kim J. Electrochem. Soc. 152, A2170 (2005) [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B30.00005: Controlled Fabrication of Single Electron Transistors from Single-Walled Carbon Nanotubes Paul Stokes, Saiful I. Khondaker Single-walled carbon nanotubes (SWNTs) are considered to be an ideal material for quantum electronic applications such as single electron transistors (SETs). However, fabrication of SET based devices is still in its infancy. Controlled fabrication of SWNT-SETs has been demonstrated by introducing kinks using AFM. However, AFM manipulation is time consuming and reproducibility can be extremely challenging. Here, we show a novel approach to fabricate controllable and reproducible SETs using SWNT. SWNTs were placed on 100 nm wide local Al/Al$_{2}$O$_{3}$ bottom gates and then contacted with Pd source and drain electrodes with 1 um spacing on Si/SiO$_{2}$ substrates. The Al gate serves two purposes (i) it defines tunnel barriers at the edges of the gate electrodes by introducing buckles, and (ii) it acts as a local gate to tune the number of carriers in the central island. Low temperature electronic transport measurements show coulomb oscillations up to 125 K. The stability diagram shows a charging energy of $\sim $ 13 meV and energy level spacing of $\sim $ 5 meV. These energies are consistent with a quantum dot size of $\sim $100 nm, thus verifying the dot is defined and controlled by the 100 nm wide aluminum oxide gate. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B30.00006: Nanotube enabled thin film transistors utilizing low mobility organic semiconductors. Bo Liu, Mitchell McCarthy, Youngki Yoon, Doyoung Kim, Zhuangchun Wu, Franky So, Paul H. Holloway, John R. Reynolds, Jing Guo, Andrew G. Rinzler We describe a novel organic thin film transistor architecture enabled by single walled carbon nanotubes. Initial devices exhibit 2 orders of magnitude current modulation at useful currents despite the use of low mobility organic semiconductors (that exhibit no detectable current in a conventional TFT architecture). Modeling shows that the present devices function principally via Shottky barrier modulation, however as the source drain distance is reduced bulk modulation should also occur, with corresponding improvements in performance. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B30.00007: Structural and Electronic Properties of Single-Walled Carbon Nanotube Heterojunctions Joydeep Bhattacharjee, Young Woo Son, Bhupesh Chandra, James Hone, Jeffrey B. Neaton Inspired by recent experiments[1], we present a systematic approach to construct structural models of mostly linear single walled carbon nanotube (SWCNT) heterojunctions. A minimum number of 5-7 defects is found to be required to join two SWCNTs of differing chiralities. Using nearest-neighbor tight-binding and first-principles density functional theory, we explore the sensitivity of the heterojunction electronic structure and transport properties to different arrangements of the interfacial 5-7 defects, and discuss their implications for future experiments and nanoelectronic applications. \begin{thebibliography}{1} \bibitem{bj} B. Chandra, J. Hone, {\it Unpublished}. \end{thebibliography} [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B30.00008: Synthesis and Structure of Carbon Nanotube Junctions and Co$_{9}$S$_{8}$ Nanowire-filled Carbon Nanotubes Wenzhi Li, Gaohui Du We describe the synthesis of carbon nanotube junctions and Co$_{9}$S$_{8}$ nanowire-filled carbon nanotubes by pyrolysis of thiophene on cobalt catalyst in chemical vapor decomposition. The formation of these nanostructures is strongly dependent on the thiophene vapor concentration during the material synthesis. The carbon nanotube junctions have hollow channels while the filled carbon nanotubes have solid Co$_{9}$S$_{8}$ cores. The encapsulated Co$_{9}$S$_{8}$ nanowires are single crystals, and their lengths are about 10 $\mu $m with their [110] direction parallel to the axis of the carbon nanotubes. It is postulated that the filling of the Co$_{9}$S$_{8}$ nanowires results from the volume increase of the catalyst induced by a phase transition from cobalt to cobalt sulfide and the spatial confinement of the carbon nanotubes as nano-molds. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B30.00009: Adsorption of Alcohols and Alkanes on Single-Walled Carbon Nanotubes Erik Alldredge, Stefan Badescu, Thomas Reinecke, Navdeep Bajwa, F. Keith Perkins, Eric Snow Recent experiments with arrays of carbon nanotubes (CNTs) reveal a strong electrical response during exposure to polar alkane derivatives such as linear alcohols C$_{n}$H$_{2n+1}$OH, which is in contrast with the weak response to linear alkanes C$_{n}$H$_{2n+2}$. To develop an understanding of the microscopic mechanisms involved, we perform detailed \textit{ab initio} calculations of adsorption geometries and charge configurations for the size parameter $n$ from 1 to 8 on pristine zig-zag and armchair CNTs. We use Density Functional Theory with localized orbitals in a cluster approach, along with the M05-2X functional appropriate for the weak interactions of physisorption for these systems. We find that adsorption energies are larger for alcohols than for alkanes and increase linearly in energy with length of the molecule $n$ for both alcohols and alkanes (at 35 meV and 40 meV per additional CH$_{2}$, respectively). This is found to be in good agreement with the binding energy per additional CH$_{2}$ estimated from the fast conductance response measurements for both alcohols and alkanes using a simple kinetic theory model. We estimate small charge transfers for all molecules, which suggest that the electric response is dominated by the scattering from the dipole moments of the adsorbates. [This work is supported by the Office of Naval Research.] [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B30.00010: Adsorption of neon and tetrafluoromethane on carbon nanohorn aggregates: differences in specific surface area values Vaiva Krungleviciute, Masako Yudasaka, Sumio Iijima, Aldo Migone We have measured adsorption isotherms for two different adsorbates, neon and tetrafluoromethane, on dahlia-like carbon nanohorn aggregates. The experiments were performed at similar relative temperatures for both gases. The measurements were conducted to explore the effect of adsorbate diameter on the behavior of the resulting adsorbed systems. We measured the effective specific surface area value of the nanohorn sample using both gases, and we found that this quantity was about 22{\%} smaller when we determined this quantity using tetrafluoromethane, the larger molecule. Isosteric heat and binding energy values were also determined from our measurements. We will compare our experimental results with those from a computer simulation study performed by Prof. M. Calbi. The simulations help us understand the source of the observed differences in the measured specific surface values, as well as the coverage dependence of the isosteric heat of adsorption for both gases. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B30.00011: Application of nanohorns to anti-cancer drug carriers Masako Yudasaka, Minfang Zhang, Kumiko Ajima, Jin Miyawaki, Tatsuya Murakami, Kunihioro Tsuchida, Sumio Iijima Potential applications of single-wall carbon nanohorns (SWNH) that have shown no acute toxicity in various tailored animal experiments, to the drug delivery system have been studied. We previously reported that the drugs were able to be incorporated inside SWNH at room temperature through liquid phase, and chemical modifications with hydrophilic molecules enhanced dispersion of SWNHs in aqueous solutions. The modifications with the tumor-targeting molecules were also possible. The contrast agent attachments enabled the in vivo visualization of SWNHs by magnetic resonance imaging. We show in this report how the effects of anti-cancer drugs were influenced by being incorporated inside SWNHs, and discuss its reasons. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B30.00012: Building Physical Carbon Nanoparticles from Small-World Networks: Density Functional Theory Calculations Jeremy A. Yancey, M.A. Novotny, Steven R. Gwaltney We have performed B3LYP/6-31G* Density Functional Theory calculations on carbon cluster nanoparticles built with (pseudo) small-world network topologies to determine whether they are stable and can exist in nature. Such particles may have novel material properties due to their (pseudo) small-world nature [1]. We have embedded a ring of carbons with one or more small-world connections made with and without additional carbon atoms. No carbon is allowed to make more than four bonds. The energy per atom of these (pseudo) small-world carbon systems is compared with benchmark carbon clusters including monocyclic rings, linear rods, graphene fragments, and various fullerenes from C$_{20}$ to C$_{60}$. The energy per atom and vibrational frequency calculation results for these materials indicate that there are pure-carbon small-world nanomaterials that are reasonable for real world synthesis. We present both NMR and IR spectra for these nanoparticles. [1] M.A. Novotny, \textit{et al}, J. Appl. Phys, \textbf{97}, 10B309 (2005). [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B30.00013: First-Principles Study of Carbon Nanoframeworks Tailored for Hydrogen Storage Eunja Kim, Philippe Weck, Balakrishnan Naduvalath, Hansong Cheng, Boris Yakobson Based on first-principles calculations, we propose a novel class of 3-D materials consisting of small diameter single-walled carbon nanotubes (SWCNTs) functionalized by organic ligands as potential hydrogen storage media. Specifically, we have carried out density functional theory calculations to determine the stable structures and properties of nanoframeworks consisting of (5,0) and (3,3) SWCNTs constrained by phenyl spacers. Valence and conduction properties, as well as normal modes, of pristine nanotubes are found to change significantly upon functionalization, in a way that can serve as experimental diagnostics of the successful synthesis of the proposed framework structures. Ab initio molecular dynamics simulations indicate that such systems are thermodynamically stable for on-board hydrogen storage. In order to increase the hydrogen uptake in the interstitial cavity of such nanoframeworks, we are currently investigating the possibility of Li deposition on these nanostructures. [Preview Abstract] |
Session B31: Focus Session: Computational Nanoscience I: Electronic and Optical Properties of Nanoclusters
Sponsoring Units: DMP DCOMPChair: James R. Chelikowsky, University of Texas at Austin
Room: Morial Convention Center 223
Monday, March 10, 2008 11:15AM - 11:51AM |
B31.00001: Optical Properties of Free and Embedded Small Nanoparticles Invited Speaker: It is well known that the absorption spectra, as well as the effective dielectric function, of nanoparticles in vacuum or surrounded by a dielectric medium can be obtained by classical Mie and Maxwell-Garnett theories. A limit as to how the particles can be for the theory to apply has not been established. Here I present theoretical results on the optical properties of small Ag, Au, and Si and Ge nanoparticles with tens of atoms in vacuum and in an embedded dielectric medium obtained from first-principles density-functional calculations. In particular, I will discuss the role that $d$-electron play on the optical properties of Ag and Au nanoparticles, and the cases when classical Mie and Maxwell-Garnett theories can be applied for nanoparticles of just few atoms in size and whose atoms are in bulk-like and not bulk-like positions. Comparison will be made for nanoparticles in vacuum and embedded in an alumina matrix. The quantum-mechanical results indicate that small nanoparticles in alumina can have an imprint on the effective dielectric function that is several times larger than would be predicted by Maxwell-Garnett theory for same-size particles. This work was supported by a GOALI NSF grant, DOE, the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, and Alcoa Inc. Collaborators: S. \"{O}g\"{u}t, K. Jackson, J. Jellinek, A. Halabica. R. F. Haglund, R. Magruder, S.J. Pennycook and S.T. Pantelides. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B31.00002: Ab initio calculation of temperature effects in the optical response of open-shell sodium clusters Marie Lopez del Puerto, Murilo Tiago, James Chelikowsky We incorporate the temperature effect on the optical absorption spectra of open-shell sodium clusters by combining pseudopotentials, Langevin molecular dynamics and time-dependent density functional theory. We have done calculations for several open-shell sodium clusters, for which experimental data is available for comparison. We find that the positions of the lower energy peaks of the calculated spectra correspond very well to the peaks in the experimental spectra. We fit the width of the peaks in the lower temperature calculations to the corresponding experimental result to obtain the instrumental line width. We then use this same width for the high temperature calculations and find very good agreement with experiment. Finally, we analyze the transitions that contribute to the observed peaks in the absorption spectra and we plot the effective valence charge density for specific transitions for each cluster. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B31.00003: Bethe-Salpeter and Quantum Monte Carlo Calculations of the Optical Properties of Carbon Fullerenes P.R.C. Kent, M.L. Tiago, F.A. Reboredo, Randolph Q. Hood We have calculated the low energy optical excitations of the carbon fullerenes C20, C24, C50, C60, C70, and C80. Properties are calculated via the GW-Bethe-Salpeter Equation (GW-BSE) and diffusion Quantum Monte Carlo (QMC) methods. We compare these approaches with time dependent density functional results and with experiment. GW-BSE and QMC have previously shown good agreement for small molecules, but this is the first study of these methods for these larger yet prototypical nanostructures. The first ionization potentials are consistently well reproduced and are similar for all the fullerenes and methods studied. However, electron affinities and first triplet exciton show substantial method and geometry dependence. GW-BSE yields triplet energies around 1eV below the QMC results. We discuss the possible reasons for these differences. Research at Oak Ridge National Laboratory performed at the Materials Science and Technology Division, sponsored by the Division of Materials Sciences, and at the Center for Nanophase Materials Sciences, sponsored by the Division of Scientific User Facilities, U.S. Department of Energy. Research at Lawrence Livermore National Laboratory was performed under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B31.00004: Efficient first principles quasiparticle states using optimal Brillouin zone sampling David Prendergast, Steven G. Louie We present a methodology for accurate evaluation of quasiparticle states within the GW approximation [1], exploiting optimal Brillouin zone sampling [2]. This approach permits fast, efficient sampling of the Brillouin zone using a compact k-dependent Hamiltonian. Applications to systems with complicated dispersion or large numbers of atoms are permitted with favorable computational scaling and straightforward exploitation of existing parallelized numerical libraries. As input, this method requires only standard density functional theory calculations of eigenstates and eigenenergies on a very coarse k-point grid. For systems with large numbers of atoms, a single k-point is often sufficient. K-point convergence of the dielectric matrix and self-energy is readily achieved leading to accurate GW quasiparticle states. [1] M. S. Hybertsen and S. G. Louie, Phys. Rev. B {\bf 34}, 5390 (1986). [2] E. L. Shirley, Phys. Rev. B {\bf 54}, 16464 (1996). [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B31.00005: First Principles Absorption Spectra of Intermediate Size Ag$_n$ ($n=10-20$) Clusters Kopinjol Baishya, Juan Carlos Idrobo, Serdar Ogut, Mingli Yang, Koblar A. Jackson, Julius Jellinek First principles optical absorption spectra, obtained within time-dependent density functional theory (TDDFT), for the ground state and low-energy isomers of Ag$_n$ ($n=10-20$) are presented. Overall, our theoretical results exhibit quite good agreement with spectra obtained for Ag clusters trapped in rare- gas matrices. We show that the classical predictions from Mie- Gans theory using the bulk dielectric function of Ag are in rather good agreement with experimental results and TDDFT spectra for isomers of various shapes. We analyze the orbital character of the optical excitations, and unexpectedly find that the $d$ electrons of Ag$_n$ clusters in this size range have a significant contribution to low-energy optical excitations, unlike the case for smaller Ag$_n$ ($n < 9$) clusters. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B31.00006: Trends in the Electronic Structure and Vibrational Dynamics of 34 atom Ag-Cu Nanoalloy (Ag$_{n}$-Cu$_{34-n}$) H. Yildirim, A. Kara, T.S. Rahman We report results of a systematic study of the electronic and geometric structures as well as vibrational and thermodynamical properties of Ag$_{n}$Cu$_{34-n}$, using density functional theory and model interaction potentials. A detailed analysis of the effect of coordination and atomic environment shows the limitation of coordination to \textit{solely} explain the changes in the structural and vibrational characteristics. On the other hand, a combination of coordination and environment, as expressed in the elemental characteristics of the neighbors, shows systematic trends in the bond length and vibrational free energy distribution. In addition, Cu atoms in the Cu-core/Ag-shell structure, with coordination 6 show a bi-modal feature with 2 distinct ranges of bond lengths. A global analysis shows that the fluctuations in the vibrational free energy depend on the elemental environment which is more pronounced in the case of copper atoms. Correlations between the d band characteristics with coordination/bond length and environment will also be discussed. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B31.00007: Static dipole polarizabilities of icosahedral fullerenes from C$_{60}$ to C$_{2160}$ by all-electron density-functional theory Rajendra Zope, Tunna Baruah, Mark Pederson, Brett Dunlap The electronic response of C$_{60}$, C$_{180}$, C$_{240}$, C$_{540}$, C$_{720}$, C$_{960}$, C$_{1500}$, and C$_{2160}$ fullerenes is characterized by determining their static dipole polarizabilities by all-electron density- functional theory. We first determine the dipole polarizabilities of C$_{60}$, C$_{180}$, C$_{240}$, and C$_{540}$ fullerenes by the finite-field method, using 35 basis functions per atom (NRLMOL basis set), and using the PBE-GGA. In the second set of calculations the sum-over-states (SOS) polarizabilities for all fullerenes from C$_{60}$ through C$_{2160}$ are determined by our fully analytic formulation of density functional theory(ADFT). The 6-311G(d,p) basis set is used in the ADFT calculation, which amounts to 38800 basis functions for the largest fullerene in this series, namely C$_ {2160}$. The SOS polarizabilities are roughly 4 times larger than the finite-field polarizabilities. When scaled by a correction factor obtained within linear response theory, the SOS polarizabilities are within 1-3\% of the finite-field polarizabilities. The polarizability per carbon atom increases from 1.34 \AA$^3$ in C$_{60}$ to 4 \AA$^3$ in C$_{2160}$ while the ratio of fullerene polarizability to its volume approaches unity pointing to quenching of quantum size effects by C$_{2160}$. The results show previous tight-binding calculations greatly exaggerate the electronic response of large fullerenes. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B31.00008: Heusler clusters Alexey Zayak, Scott Beckman, Murilo Tiago, Peter Entel, James Chelikowsky Real space pseudopotential calculations are used in order to investigate the properties of Heusler clusters. Bulk-like clusters with various Ni-Mn-Ga compositions have been examined in the size range from 15 up to 169 atoms. Among these compositions the closest to the stoichiometric Ni$_2$MnGa are the most stable. These clusters retain tendency for tetragonal distortion, which is inhabited from the bulk properties. Although, the surface effects dominate suppressing the tetragonal structure in the smaller clusters, the bigger clusters develop a bulk-like tetragonal distortion. We predict the existence of switchable Ni-Mn-Ga clusters, which might be of great interest for the nano-Magnetic-Shape-Memory technology. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B31.00009: Tangential Ligand-Induced Strain in Icosahedral Au13 Leeor Kronik, Olga Guliamov, Anatoly Frenkel, Laurent Menard, Ralph Nuzzo A quantitative comparison of first principles calculations with extended x-ray absorption fine structure and transmission electron microscopy measurements provides strong evidence that Au$_{13}$ nanocrystals are stabilized in a slightly distorted icosahedral structure by on-top phosphine ligands and a combination of on-top and bridging thiol ligands. Importantly, the ligands change the icosahedral strain (i.e. the radial- tangential bond length ratio) significantly, with the tangential bonds within the Au core exhibiting much more disorder than the radial ones. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B31.00010: Structural Properties and Phase Transitions in Small Gold Nanoclusters Yanting Wang, Sergey Rashkeev Small gold nanoclusters (below 5 nm in diameter) exhibit good catalytic activity. Molecular dynamics simulations combined with the parallel tempering method have been used to investigate the bulk and surface structural properties and phase behavior of small gold nanoclusters (10$^{1}$-10$^{4}$ atoms). For small clusters, the transition from solid to liquid does not occur at a definite temperature. Instead, one gets a temperature range in which the transition state from solid to liquid is observed, and this range become broader for nanoclusters with smaller number of atoms. In this work, we perform an analysis of structural and dynamic properties of gold nanoparticles of different sizes and show that the nature of the solid-liquid phase transition in very small nanoparticles (with tens of atoms) is radically different from that of the mid-sized ones (10$^{2}$-10$^{4}$ atoms). The surface characteristics of the particle (the presence of low-coordinated atoms at the surface and the dynamic fluxionality, i.e., an ability of the surface to reconstruct) that define its catalytic behavior are also investigated and analyzed. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B31.00011: The peculiar distribution pattern of Pd on the PdAu bimetallic nanoclusters Dingwang Yuan, Xingao Gong, Ruqian Wu Through systematic density functional calculations, we found that Pd atoms in PdAu nanoclusters may only take the (111) facets while leave the (001) facets with pure Au. This is promoted by the tendency that Pd prefers to form bonds with Au, rather than Pd. The segregation from the (001) facet to the (111) facet appears to occur easily. The local activity of Pd somewhat depends on the size of cluster and site of substitution. The peculiar distribution pattern of the active constituent should strongly alter the chemical properties of bimetallic nanoclusters toward catalyzing reactions. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B31.00012: Computational model for the production of monodisperse silver spheres in solution Daniel Robb, Ionel Halaciuga, Vladimir Privman, Dan Goia We report the results of computational modeling of the production of monodisperse, spherical silver particles through the rapid mixing of reducing agent and silver-amine complex solutions, in the absence of a protective colloid. We find that the process can be modeled effectively by a two-stage reaction mechanism used previously to model the production of gold particles [1]. Here, we treat both the equilibrium concentration of silver atoms and the surface tension of silver particles as free parameters in our simulation, finding that the reaction time scale is fit by a narrow region of this parameter space. As in the previous work on gold particles, a kinetic `dimer suppression factor' is required to limit the number of final particles produced. In addition, we consider an extension of the two-stage reaction model which incorporates the effect of the silver-amine complexation reaction on the availability of Ag monomers. [1] J. Park, V. Privman, and E. Matijevic, \textit{J. Phys. Chem. B} \textbf{105}, 11630 (2001). [Preview Abstract] |
Session B32: Focus Session: Nanocontacts and Inhomogeneous Magnetic States
Sponsoring Units: GMAG DMPChair: Andrei Slavin, Oakland University
Room: Morial Convention Center 225
Monday, March 10, 2008 11:15AM - 11:51AM |
B32.00001: Theory of magnetic interactions and transport in tunnel junctions and point contacts Invited Speaker: |
Monday, March 10, 2008 11:51AM - 12:03PM |
B32.00002: Fabrication of point contacts by FIB patterning B. O'Gorman, M. Tsoi Nanoscale electrical contacts currently receive an increased amount of attention due to their capability to produce extremely high current densities needed, e.g., in experiments on current-driven precession and reversal of magnetization. Here we describe a new technique for the fabrication of such point contacts using a focused ion beam (FIB) patterning. FIB-fabricated point contacts combine the robustness and size-control of other lithographical methods with the flexibility of mechanical techniques to produce contacts to samples of arbitrary shape and composition. After sample coverage with a thin insulating layer (SiO), an FIB is used to mill a 100-nm-diameter hole through the insulator. Electrical contact to the sample is then made in-situ by filling the hole with a metal (Pt) using the ion beam assisted chemical vapor deposition capability of our FIB system. We have demonstrated the use of two such contacts (as an emitter and collector) in a transverse electron focusing (TEF) experiment. The contacts were made to a single crystal of bismuth, ballistic electrons were injected into the crystal through the emitter, and then focused onto the collector by a magnetic field. We see the expected voltage peaks at the collector as a function of the applied magnetic field. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B32.00003: Electronic excitations in four structurally similar but magnetically different Nickel chain compounds V.C. Long, Y.H. Chou, I.A. Cross, A.C. Kozen, L.A. LaViolet, C.A. Miller-Shelley, J.R. Montague, E.P. Plumb, S.A. McGill, X. Wei, B.R. Landry, K.R. Maxcy-Pearson, M.M. Turnbull, C.P. Landee, R.D. Willett We report the low temperature zero-field and magnetic field- dependent optical spectra of four Ni$^{2+}$ chain compounds: NENP (Ni[$en$]$_2$NO$_2$ClO$_4$), NENB (Ni[en]$_2 $NO$_2$BF$_4$), NTNB (Ni[$tn$]$_2$NO$_2$BF$_4$), and NINO (Ni[tn] $_2$NO$_2$ClO$_4$). The four differ in the counterions isolating the chains and the rings coordinated to the Ni$^{2+}$ ions. Despite the structural similarities, only three of the compounds exhibit the Haldane gap typical of a spin-1 chain; the fourth, NTNB, behaves like spin glass, likely due to finite chain effects.$^1$ We focus on the near infrared spin-forbidden (SF) electronic $d-d$ transitions and the visible Ni$^{2+}$-to-NO$_2^- $ charge transfer (CT) band. The zero-field absorption spectra differ in the $en$ and $tn$ ring compounds but are nearly identical in compounds with identical rings. The SF and CT band absorption intensities depend on field in a way that reflects the magnetic ground state. In the Haldane compounds, the onset of intensity changes occurs above the crossover field, whereas in NTNB the field-dependent absorption intensities respond to any finite field.$^1$ R.D. Willett and C. J. Gomez-Garcia, EMRS2007, Strasbourg. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B32.00004: Theory and application for chain formation in break junctions Yuriy Mokrousov, Alexander Thiess, Stefan Bluegel, Stefan Heinze We introduce a generic model for chain formation in break junctions by formulating separate criteria for the stability and producibility of suspended monoatomic chains based on total energy arguments. Using first-principles calculations [1], we apply our model to break junctions of $4d$ and $5d$ transition-metals (TMs), as well as Ag and Au, including the effects of spin-polarization and spin-orbit coupling. Thereby, we can explain the physical origin of the experimentally observed trend of increasing probability for the creation of long suspended chains in break junctions for $5d$-TMs at the end of the series [2] and suppressed chain elongation for $4d$ elements. Moreover, we clearify why the probability of chain elongation is greatly enhanced by the presence of oxygen in experiments with Au and Ag. Our model also allows us to make predictions on the ballistic transport properties of suspended chains. [1] Y.Mokrousov {\sl et al.}, Phys.\ Rev.\ B\ {\bf 72}, 045402 (2005). [2] R.H.M.Smit {\sl et al.}, Phys.\ Rev.Lett.{\bf 87}, 266102 (2001). [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B32.00005: Anderson model of local magnetism at a break junction nanocontact Paola Gentile, Michele Fabrizio, Giuseppe E. Santoro, Erio Tosatti Atoms at break junction nanocontacts in nearly magnetic heavy transition metals such as Pt and Pd may develop a nonzero magnetization. Since here the nanocontact is strongly electronically tied to the two bulk leads, it is not automatically clear what the correct physical picture of the system should be, and in particular whether the nanocontact should or should not become analogous to a Kondo impurity as in quantum dot devices. To clarify that, we consider one (or more) impurity sites inserted into a linear chain (representing the nonmagnetic leads), every site endowed with orbitally degenerate orbitals, large spin orbit coupling, and Hund's rule exchange; neighboring sites connected by electron hopping and by intersite ferromagnetic exchange. The mean-field solution when the impurity site (where Hund's rule exchange is made stronger) is locally magnetic shows a ferromagnetic polarization around it, in agreement with realistic density functional calculations for nanocontacts consisting of monatomic chain segments. Our results suggest that this type of nanocontact, regarded as an Anderson impurity, is ferromagnetically coupled to the leads, and hence that Kondo screening does not occur in this case. The physical consequences for the conductance through the chain are discussed. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 1:15PM |
B32.00006: Dynamics of a pinned magnetic vortex Invited Speaker: Disks patterned from soft ferromagnetic films typically form a single magnetic vortex for diameters on the order of a few microns or less. The vortex dynamics include both ordinary spin waves and a gyrotropic mode, in which the vortex core undergoes circular motion about its equilibrium position [1, 2]. This mode has sub-GHz frequencies which ideally depend only on the aspect ratio (diameter over thickness) of the disk [2, 3]. We have used time-resolved Kerr microscopy to investigate the gyrotropic mode as a function of the equilibrium position of the core, which can be tuned by an applied field with a sensitivity of $\sim 1$~nm/Oe. In the limit of high excitation amplitude, the gyrotropic frequency $f_G$ is independent of the vortex core position, as previously predicted and observed [1, 3]. For small amplitudes, however, we observe unexpected fluctuations in $f_G$ as a function of the applied field. The average core displacement between consecutive frequency peaks, as well as the average frequency shift, is observed to be independent of disk diameter. These observations indicate that the fluctuations are due to a distribution of nanoscale defects that pin the vortex core by lowering its energy [4]. Furthermore, they are consistent with a model in which the frequency shift for a particular fluctuation is a direct measure of the interaction energy of the vortex core with one defect. By mapping $f_G$ as a function of orthogonal in-plane static fields, we image the 2D spatial distribution of defects with nanoscale resolution. \newline [1] K. Yu Guslienko \textit{et.al.}, JAP \textbf{91}, 8037 (2002). \newline [2] J. Park \textit{et.al.}, PRB \textbf{67}, 020403(R) (2003). \newline [3] V. Novosad \textit{et.al.}, PRB \textbf{72}, 024455 (2005). \newline [4] R. L. Compton and P. A. Crowell, PRL \textbf {97}, 137202 (2006). [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B32.00007: Angular Dependence of Vortex Annihilation Field in Asymmetric Co Nanodots$^{\ast }$ Randy Dumas, Kai Liu, Thomas Gredig, Chang-peng Li, Ivan K. Schuller Magnetization reversal via a vortex state is a common occurrence in sub-micron magnetic nanodots. We have investigated arrays of 500 nm polycrystalline Co dots prepared by e-beam lithography. The circular symmetry of the dots has been broken by introducing a flat edge to the dots. Magneto-optical Kerr effect (MOKE) measurements and micromagnetic simulations confirm the reversal mechanism of the dots is via the nucleation, propagation, and annihilation of a vortex core. The asymmetric dot shape favors vortex nucleation from the flat edge and therefore allows for control over the vortex chirality. Additionally, by modifying the applied field sweep, we can control which side of the dot the vortex annihilates from. We have studied the vortex annihilation field as a function of the angle between the applied field and the flat edge of the dot. At small angles, the annihilation field depends on the chirality of the vortex and annihilation is easier from the flat edge of the dot. The difference in annihilation fields for the two chiralities is strongly dependent on the angle of the applied magnetic field. This behavior is due to the complex motion of the vortex core across an asymmetric dot during reversal. *Work supported by ACS-PRF, AFOSR-MURI, and the Alfred P. Sloan Foundation. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B32.00008: Influence of excitation fields on vortex core dynamics in micron-sized magnetic disks Xuemei Cheng, David Keavney, Kristen Buchanan, Ralu Divan Magnetization vortices in micron-sized magnetic disks have been of great interest because of potential applications in memory devices. Theory predicts a rich spectrum of excitations including the fundamental or gyrotropic mode. Experimentally the gyrotropic mode is observed in some experiments while in others a linear or elliptical trajectory is seen. We have imaged free vortex core motion in permalloy disks of 6 $\mu $m diameter using time-resolved x-ray photoemission electron microscopy at beamline 4-ID-C of the Advanced Photon Source with 90 ps temporal resolution. We demonstrate that the vortex core motion trajectory depends on the magnitude of the excitation field. The vortex core exhibits a gyrotropic trajectory under low excitation fields, while under high excitation fields the core shows a more linear trajectory. We find that if the initial displacement of the core is greater than 20{\%} of the disk radius, transient magnetic domains appear in the first 1 ns after removal of field. These domain states then profoundly influence the subsequent motion. The core oscillation frequencies are consistent with theoretical predictions, regardless of the excitation amplitude. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B32.00009: Direct Observation of Magnetic Vortex Cores using Scanning Electron Microscopy with Polarization Analysis (SEMPA) Seok-Hwan Chung, Daniel Pierce, John Unguris Magnetic singularities associated with magnetic vortex cores are a common feature in patterned magnetic nanostructures. Their small size, on the order of 10 nm, makes them technologically interesting, but also difficult to measure or image directly. We used Scanning Electron Microscopy with Polarization Analysis (SEMPA) to image magnetic vortices in a wide variety of patterned nanostructures. Since SEMPA can measure both the in-plane and the out-of-plane component of the surface magnetization, SEMPA can potentially determine both the chirality and the polarity of the vortex core, simultaneously. Samples consisted of NiFe (25nm) / Ta (3nm), and other soft magnetic films, patterned by electron beam lithography and lift-off into disks with various diameters. The films were grown on 85nm thick SiN membranes to reduce image degradation from backscattered electrons. The experimental results were compared to micromagnetic simulations and the vortex core profile showed a good correspondence with theoretical predictions, which considers only the exchange and magnetostatic energy. This work has been supported in part by the NIST-CNST/UMD-NanoCenter Cooperative Agreement. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B32.00010: Transition states of magnetization reversal in ferromagnetic nanorings Gabriel Chaves-O'Flynn, Andrew Kent, Daniel Stein Thin ferromagnetic rings are of interest for fundamental studies of magnetization reversal, in part, because they are a rare example of a geometry for which an analytical solution for the rate of thermally induced switching has been determined [1]. The theoretical model predicts the transition state to be either a global magnetization rotation of constant azimuthal angle or a localized fluctuation, denoted the instanton saddle. Numerically we have confirmed that for a range of values of external magnetic field and ring size the instanton saddle is energetically favored [2]. The model takes the annular width to be small compared to the mean radius of the annulus; in which case the main contribution to the magnetization energy comes from the surface magnetostatic energy. We present numerical micromagnetic calculations of the activation energy for thermally induced magnetization reversal for the two different transition states for the case when the annular width is equal in magnitude to the mean radius of the ring. Results of the total and surface magnetostatic energies are compared for different ring sizes. [1] K. Martens, D.L. Stein, A.D. Kent, PRB 73, 054413 (2006) [2] G.D. Chaves-O'Flynn, K. Xiao, D.L. Stein, A. D. Kent, arXiv:0710.2546 (2007) [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B32.00011: Probing a SET nanomagnet with shot noise. L.D. Contreras-Pulido, J. Fernandez-Rossier, R. Aguado Although recent experiments show that single atomic spins [1] and molecular magnets [2] can be proved via transport measurements, their magnetic properties can hardly be tuned once they are fabricated. In a recent Letter [3], we have shown that a single-electron transistor (SET) based upon a II--VI semiconductor quantum dot and doped with a single-Mn ion behaves like a quantum nanomagnet with magnetic properties which can be controlled electrically. Conversely, the electrical properties of this SET depend on the quantum state of the Mn spin. Here, we extend these previous ideas and study the shot noise of this kind of nanomagnets. Our results reveal that shot noise contains much more information that the one contained in the average current. Interestingly, important quantities of the nanomagnet like the spin relaxation time and information about current-induced spin precession can be directly extracted from shot noise measurements. [1] Cyrus F. Hirjibehedin et al, Science, 317, 1199 (2007). [2] Moon-Ho Jo et al, Nanoletters, 6, 2014, (2006); H. B. Heersche et al., Phys. Rev. Lett. 96, 206801 (2006). [3] J. Fernandez-Rossier and R. Aguado, Phys. Rev. Lett. 98, 106805 (2007). [Preview Abstract] |
Session B33: Focus Session: Spin Dependent Phenomena in Semiconductors: II
Sponsoring Units: DMP GMAG FIAPChair: Mark van Schilfgaarde, Arizona State University
Room: Morial Convention Center 224
Monday, March 10, 2008 11:15AM - 11:27AM |
B33.00001: Imaging Drift and Diffusion of Accumulation from the Spin Hall Effect N.P. Stern, D.W. Steuerman, S. Mack, A.C. Gossard, D.D. Awschalom The spontaneous generation of spin polarization near sample edges by the spin Hall effect when electron currents flow in a metal or semiconductor with spin-orbit coupling has attracted recent attention due to the elegant and complex spin-orbit physics as well as the potential for all-electrical spin generation in spintronics devices. Optical techniques in semiconductors allow for spatial resolution of the electrically generated spin accumulation, a feature not present in all-electrical measurements. We use Kerr rotation microscopy to image the spatial and temporal evolution of spin accumulation produced by the extrinsic spin Hall effect in n-GaAs devices. Measurements in a variety of device geometries, including arms transverse to a channel, reveal the unambiguous contribution of longitudinal spin drift in accumulation profiles\footnote{N. P. Stern, D. W. Steuerman, S. Mack, A.C. Gossard, and D. D. Awschalom, \textit{Appl. Phys. Rev. Lett.} \textbf{91}, 062109 (2007)}. We develop one- and two- dimensional drift-diffusion modeling to explain the observed features, providing a more complete understanding of observations of spin accumulation and the spin Hall effect. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B33.00002: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 11:39AM - 11:51AM |
B33.00003: Fractional Charge and Topological Pumping in the Quantum Spin Hall Insulators Taylor Hughes, Xiao-Liang Qi, Shoucheng Zhang We study the physics at the edge of a 2d topological (quantum spin Hall) insulator. This system is known to be topologically non-trivial and a profound manifestation of topologically non-trivial states of matter is the occurrence of fractional charge. In this work, we show that a magnetic domain wall at the edge of the quantum spin Hall insulator carries one half of the unit of electron charge, and we propose an experiment to directly measure this fractional charge on an individual basis. As an additional consequence, a rotating magnetic field can induce a topologically pumped dc electric current, and vice versa. Finally, we discuss an interacting version of this model in which the fractional charge is carried by the fundamental excitations. These physical phenomena can be derived from a generic topological effective action for topological insulators and are directly related to the physics of the second Chern number as will be described in another talk. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B33.00004: Topological Field Theory of Time-Reversal Invariant Insulators Xiao-Liang Qi, Taylor Hughes, Shou-Cheng Zhang We show that the fundamental time reversal invariant (TRI) insulator exists in the $4+1$ dimension, where the effective field theory is described by the $4+1$ dimensional Chern-Simons theory, and the topological properties of the electronic structure is described by the second Chern number. These topological properties are the natural generalizations of the time reversal symmetry breaking (TRSB) quantum Hall insulator in $2+1$ dimension. The TRI quantum spin Hall insulator in $2+1 $ dimension and the topological insulator in $3+1$ dimension can be obtained as descendants from the fundamental TRI insulator in $4+1$ dimensions through dimensional reduction. The effective topological field theory, and the $Z_2$ topological number for the TRI insulators $2+1$ and $3+1$ dimensions are naturally obtained from this procedure of dimensional reduction. All physically measurable topological response functions of the TRI insulators are completely described by our effective topological field theory. As a consequence of the effective theory, we predict the existence of fractional charge induced by a magnetic domain wall on the edge of quantum spin Hall insulator, which will be presented as a separate talk. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B33.00005: A many-body generalization of the $Z_2$ topological invariant for the quantum spin Hall effect Sung-Sik Lee, Shinsei Ryu We propose a many-body generalization of the $Z_2$ topological invariant for the quantum spin Hall insulator, which does not rely on single-particle band structures. The invariant is derived as a topological obstruction that distinguishes topologically distinct many-body ground states on a torus. It is also expressed as a Wilson-loop of the SU(2) Berry gauge field, which is quantized due to the time-reversal symmetry. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B33.00006: Shot noise in the mesoscopic spin Hall effect Ralitsa Dragomirova, Liviu Z\^arbo, Branislav Nikoli\' c The spin Hall effect has recently attracted a lot of attention as a promising all-electrical scheme to generate and manipulate pure spin currents by utilizing spin-orbit (SO) coupling in semiconductor nanostructures. Injection of unpolarized charge current through the longitudinal leads of a four-terminal two-dimensional electron gas (2DEG) with the Rashba SO coupling and/or SO-dependent scattering off extrinsic impurities is responsible not only for the pure spin Hall current in the transverse electrodes, maximized when the ballistic sample size is comparable to the {\it mesoscale} defined by the spin precession length, but also for random time-dependent current fluctuations. We extend the Landauer-B\"uttiker scattering formalism to calculate spin-resolved shot noise in multiterminal nanostructures for arbitrary polarization of the injected current and analyze the shot noise of transverse pure spin Hall current and zero charge current or transverse spin current and non-zero charge Hall current, driven by unpolarized or spin-polarized longitudinal charge current respectively. Since any spin-flip event (instantaneous or due to precession) within the 2DEG acts as an additional source of noise, we demonstrate that these spin and charge shot noises offer a unique tool to differentiate between intrinsic and extrinsic microscopic SO mechanisms behind the spin Hall and charge Hall effects in paramagnetic systems. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B33.00007: Beyond the spin-Hall effect Dimitrie Culcer, Roland Winkler Considerable progress has been made in recent years in the electrical manipulation of spins in semiconductors. An extraordinary amount of experimental and theoretical work have culminated in the prediction and discovery of the spin-Hall effect and of spin generation by an electric field. In the past year alone the spin-Hall effect was observed at room temperature and several groups successfully measured spin currents directly. However, we will demonstrate that the symmetry of crystal lattices allows spin currents other than the spin-Hall current and the spin current response to an electric field can be very complex. We will discuss samples in which such currents are expected to arise. The presence of non- spin-Hall currents has important and interesting consequences with regard to the manipulation of spins by electric fields. We will moreover show that spin currents and bulk spin densities in an electric field arise from linearly independent contributions to the density matrix, and that the presence of a nonequilibrium spin density has a profound effect on the spin current [1, 2]. [1] D. Culcer and R. Winkler, arxiv:0708.4009, to appear in Phys. Rev. Lett. [2] D. Culcer and R. Winkler, arxiv:0710.5260, submitted to Phys. Rev. B. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B33.00008: Universal quantized spin-Hall conductance fluctuation in graphene Zhenhua Qiao, Jian Wang, Yadong Wei, Hong Guo We report a theoretical investigation of quantized spin-Hall conductance fluctuation of graphene devices in the diffusive regime. Two graphene models that exhibit quantized spin-Hall effect (QSHE) are analyzed. Model-I is with unitary symmetry under an external magnetic field $B\ne 0$ but with zero spin- orbit interaction, $t_{SO}=0$. Model-II is with symplectic symmetry where $B=0$ but $t_{SO} \ne 0$. Extensive numerical calculations indicate that the two models have exactly the same universal QSHE conductance fluctuation value $0.285 e/4\pi$ regardless of the symmetry. Qualitatively different from the conventional charge and spin universal conductance distributions, in the presence of edge states the spin-Hall conductance shows an one-sided log-normal distribution rather than a Gaussian distribution. Our results strongly suggest that the quantized spin-Hall conductance fluctuation belongs to a new universality class. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B33.00009: Spin-Hall edge spin polarization in a ballistic 2D electron system Eugene Mishchenko, Vladimir Zyuzin, Peter Silvestrov Universal properties of spin-Hall effect in ballistic 2D electron systems are addressed. The net spin polarization across the edge of the conductor is second order, $\sim\lambda^2$, in spin-orbit coupling constant independent of the form of the boundary potential, with the contributions of normal and evanescent modes each being $\sim\sqrt{\lambda}$ but of opposite signs. This general result is confirmed by the analytical solution for a hard-wall boundary, which also yields the detailed distribution of the local spin polarization. The latter shows fast (Friedel) oscillations with the spin-orbit coupling entering via the period of slow beatings only. Long-wavelength contributions of evanescent and normal modes exactly cancel each other in the spectral distribution of the local spin density. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B33.00010: Network Model for $Z_2$ Quantum Spin-Hall Effects with Disorder Hideaki Obuse, Akira Furusaki, Shinsei Ryu, Christopher Mudry We study the effects of static disorder on the $Z_2$ quantum spin-Hall effect for noninteracting electrons propagating in two dimensions. To this end, a network model realizing the $Z_2$ quantum spin-Hall effect is constructed to account for the effects of static disorder on the propagation of noninteracting electrons subjected to spin-orbit couplings with the time-reversal symmetry. This network model is different from past network models belonging to the symplectic symmetry class in that the propagating modes along the links of the network has a single Kramers doublet. By investigating this network model numerically, it is found that a two-dimensional metallic phase of finite extent is embedded in insulating phases. We also find that the Anderson localization-delocalization transition between the metallic and $Z_2$ insulating phases belong to the conventional symplectic universality class in two dimensions. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B33.00011: Intrinsic spin-Hall effect in the presence of an in-plane magnetic field Luyao Wang, ChonSaar Chu, Anatoly Malshukov The intrinsic spin-Hall effect (SHE) induced by a driving electric field E$_{x}$ in the presence of an in-plane magnetic field $\vec {B}$ in a 2D semiconductor strip is studied. In the diffusive regime, the spatial distribution of spin densities S$_{i}$ (\textit{i=x, y ,z}) is calculated from a spin diffusion equation derived from nonequilibrium Green's function. For the case of Rashba spin-orbit interaction (SOI), we find that the spin polarization S$_{z}$ normal to the 2D strip remains zero with or without the in-plane magnetic field. For the case of Dresselhaus SOI, where cubic term is included, the symmetry of S$_{z}$ with respect to the in-plane magnetic field depends on the orientation of the $\vec {B}$ field. With $\vec {B}$ along $\hat {x}$, S$_{z}$ exhibits symmetric dependence on B$\hat {x}$. However, with a transverse in-plane magnetic field, along $\hat {y}$, at the edge of the strip exhibits asymmetric dependence on B$\hat {y}$. These results lead to a possible diagnostic tool for the identification of the SOI in the system. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B33.00012: Phase transition between quantum spin Hall and ordinary insulating phases Shuichi Murakami, Satoshi Iso, Yshai Avishai, Masaru Onoda, Naoto Nagaosa We theoretically study the phase transition between the quantum
spin Hall (QSH) and insulator phases, which involves a change
of the $Z_2$ topological number. We deal with 2D and 3D systems
without impurity and interaction. We introduce a parameter $m$
controlling the phase transition, and we study whether the gap
closes or not by one-parameter tuning. In general, level
repulsion prevents the gap from closing.
In fact, the physics of the $Z_2$ topological number is encoded
in the problem whether the gap closes by tuning a single
parameter. In 2D [1], as well as in the 3D inversion-symmetric
systems [2], the gap closes at one point, $m=m_0$, whereas in
3D inversion-asymmetric systems [2], there appears a finite
regime for $m$ ($m_1 |
Monday, March 10, 2008 1:39PM - 1:51PM |
B33.00013: A voltage probe of the spin Hall effect Yuriy Pershyn, Massimiliano Di Ventra The spin Hall effect does not generally result in a transverse voltage. We predict that in systems with inhomogeneous electron density in the direction perpendicular to main current flow, the spin Hall effect is instead accompanied by a transverse voltage. We find that, unlike the ordinary Hall effect, this voltage is quadratic in the longitudinal electric field for a wide range of parameters accessible experimentally. We also predict spin accumulation in the bulk and sharp peaks of spin Hall induced charge accumulation near the edges. Our results can be readily tested experimentally, and would allow the electrical measurement of the spin Hall effect in non-magnetic systems and without injection of spin-polarized electrons. \newline [1] Yu. V. Pershin and M. Di Ventra , J. Phys.: Cond. Matt. (in press), arXiv:cond-mat/0703310. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B33.00014: Spin-charge Separated Excitations in a Topological Insulator Dung-Hai Lee, Ying Ran, Ashvin Vishwanath We construct a simple, controllable, two dimensional model based on a topological band insulator. It has many attractive properties. The main conclusions are: (1) The quasiparticles exhibit spin-charge separation. (2) It suggests an alternative way to classify $Z_2$ topological insulator without resorting to the sample boundary. (3) The quasiparticle condensation triggers a phase transition from a spin liquid to an insulating easy-plane ferromagnet. [Preview Abstract] |
Session B35: Focus Session: Emerging Materials and Devices I
Sponsoring Units: FIAP DMPChair: Stefan Zollner, Freescale Semiconductor, Inc.
Room: Morial Convention Center 227
Monday, March 10, 2008 11:15AM - 11:51AM |
B35.00001: The Nanoelectronics Research Initiative and Beyond CMOS Research Activities in the US Invited Speaker: The six leading Semiconductor Companies in the US have joined forces with Federal and State government to form the Nanoelectronics Research Initiative in 2005. The goal is to find new information processing paradigms, systems and devices which will extend Moore's Law functional scaling into the indefinite future. The research activities are guided by 5 central research vectors which define the scope and content of the program and are listed below. \begin{enumerate} \item Computational state variables other than electronic charge \item Non-equilibrium systems out of equilibrium with the thermal environment \item Novel information transport mechanisms \item Nanoscale thermal management \item Directed self assembly of complex heterostructures \end{enumerate} The current NRI research effort consists of 56 projects at 25 universities and 3 research centers in a coherent program where each project is aligned with one or more of the research vectors. During the past two years, significant progress has been made in a number of areas including spin wave, generation, detection and characterization, room temperature DMS materials, femptosecond magnetic domain switching characterization, improved MQCA structures, multiferroic and, magnetoelectric materials and devices, non-conformational metal insulator phase transitions in VO$_{2}$ and ferromagnetic ring nanodevices. A brief discussion and references will be provided. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B35.00002: Diamond Nanoelectronics Igor Altfeder, Jacqueline Krim, Andrey Voevodin Thin films of ultra-nanocrystalline diamond (UNCD) represent an extremely promising nanoelectronic material. The electronic devices based on UNCD can operate at temperatures exceeding by an order of magnitude the working temperature of silicon-based devices. This talk will describe the recent STM/AFM study of CVD-grown UNCD films. The most important advances and challenges of UNCD-electronics, which will be discussed, are (a) the possibility of controlled doping of these films, (b) the influence of doping on chemical structure of UNCD surfaces and interfaces, and (c) exploring extremely low surface adhesion/friction of UNCD for design of MEMS. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B35.00003: Low frequency Noise in Top-Gated Ambipolar Carbon Nanotube Field Effect Transistors Guangyu Xu, Fei Liu, Song Han, Koungmin Ryu, Alexander Badmaev, Chongwu Zhou, Kang L. Wang Low-frequency noise of top-gated ambipolar carbon nanotube field effect transistors (CNT-FET) with aligned CNT growth onto the quartz substrate is presented. The noise of top-gated CNT-FETs in air is lower than that of back-gated devices, and is comparable with that of back-gated devices in vacuum. This shows that molecules in air act as additional scattering sources, which contribute to the noise. Different noise amplitudes in the electron-conduction region and hole-conduction region are due to both the Schottky barriers (SB) with respect to the conduction band and valance band and the scattering in the channel. The SB contact determines the sample conductance, and thus the noise; the channel scattering also determines the noise. The impact of channel length to the noise amplitude is discussed. This device offers a potential low noise CNT-FET structure. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B35.00004: Nanopositioning of Individual Vertical Aligned Carbon Nanotubes on Interconnects Reginald C. Farrow, Amit Goyal, Sheng Liu, Zafar Iqbal, Gordon A. Thomas, Linus A. Fetter Electrophoresis has been used to deposit single wall carbon nanotubes in arrays of sub-100 nm windows in insulating thin films over metal interconnects. The number of nanotubes that are deposited depends on the electric field and the geometry of the windows and nanotubes. Surface charge on the insulator causes the windows to become nanoscopic electrostatic lenses. Under certain readily achievable conditions \textit{only one nanotube will be deposited} at the base of a window since each deposited nanotube modifies the electric field. This discovery enables the process integration of vertical aligned carbon-based electronics with more traditional technologies such as complementary metal oxide semiconductor (CMOS) using the \textit{current generation of lithography and process technology}. Devices such as vertical field effect transistors and interconnected nanoprobe arrays may now be fabricated in the metal levels of CMOS integrated circuits to facilitate three-dimensional polylithic circuit architectures. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B35.00005: Impact Ionization in Photocurrent Measurements of Carbon Nanotube p-n Junctions Nathaniel Gabor, Z. Zhong, K. Bosnick, J. Park, P.L. McEuen We investigate the photocurrent response at a nanotube gated p-n junction using a focused laser illumination source. Scanned photocurrent imaging demonstrates that photocurrent response occurs primarily in the p-n junction. Measurements in an optical cryostat down to 4K reveal large photoresponse and unusual step-like structure in the reverse bias photocurrent. We relate the intersection point of the forward bias photocurrent to the flat band condition in the device and infer the band gap, which is in excellent agreement with the band gap determined by thermal activation and diameter measurements. The striking photocurrent steps in reverse bias occur at intervals roughly equal to the band gap. We attribute these steps to impact ionization and carrier multiplication in the junction region of the device. By measuring the photon energy dependence of the impact ionization process, we determine that ionization occurs with high probability for carriers in the second and higher subbands of carbon nanotubes. These results show that nanotube p-n junctions provide an ideal system for probing carrier dynamics and interactions of electrons and holes in nanotubes. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B35.00006: High Performance Silicon Nanowire Field Effect Transistor Qiliang Li, Xiaoxiao Zhu, Yang Yang, Dimitris Ioannou, John Suehle, Curt Richter We report the fabrication and characterization of double-gated Si nanowire field effect transistors with excellent electrical characteristics and a small subthreshold slope: $\sim $ 85 mv/dec. The Si nanowires were grown by chemical vapor deposition at pre-defined location on a 50 nm thermal SiO$_{2}$ (bottom gate oxide). The source/drain electrodes (Al) were formed by using photolithographic alignment and metal lift-off processes. The nanowires were then covered with HfO$_{2}$ via atomic layer deposition. A thin layer of SiO$_{2}$ was deposited on the HfO$_{2}$ as a buffer layer before the top gate electrode formation (Al, using photolithographic and lift-off processes). This self-aligned process enables the integration of a large number of high-quality nanowire transistors for electronic circuitry. We have investigated the effect of device structure and annealing conditions on the final device performance, and developed theoretical models to assist the device optimization. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B35.00007: Effects of Non-Ideal Edges in Graphene Nanoribbons D. Basu, M.J. Gilbert, L.F. Register, S.K. Banerjee, A.H. MacDonald We report quantum mechanical transport simulations of the edge effects of nanoribbons of two-dimensional (2D) graphite sheets or graphene. Semiconducting graphene nanoribbons have the potential to augment Si technology because of their excellent electronic properties. In practice we find that scattering from the vacant sites in an otherwise perfect armchair edge of graphene reduces its transmission characteristics drastically. These effects decrease as the widths of the ribbons increase and as the number of steps along the edges decrease. However, band gap of these semiconducting graphene decreases as the width increases, leading to an increase in the band-to-band leakage current. We conclude that without atomic precision to define perfect edges, it may not be practical to use very narrow graphene layers as a semiconducting material for field effect transistors (FETs). Our tight-binding treatment of vacancies allows us to study not only FET-like devices, but also the effect of disorder that breaks symmetry in the graphene sheet for more exotic applications such as pseudospin-type devices. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B35.00008: Determination of Non-Accumulative Effects in PCMO Resistive Switches Stephen Tsui, Nilanjan Das, Y.Q. Wang, Y.Y. Xue, C.W. Chu In recent years, the observation of electric field induced resistive switching occurring at the interface between a Ag electrode and Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (PCMO) thin film has stirred a great deal of activity. The controllable switching, depending on the applied voltage polarity, associated with this and other perovskite oxide systems may very well be a means to develop new nonvolatile memory devices. However, a consensus has not yet been reached on the origins of the physical mechanism, be it lattice rearrangement, electromigration, charge trapping, or carrier doping. An important issue is whether the switching behaves in an accumulative fashion, e.g. driven by a change in oxygen stoichiometry through ion-migration. We explore the situation through transport properties, switching characters and the size dependence of the switching area. Our results indicate that a large scale accumulation driven mechanism is not likely for the switching and that a local structural rearrangement may be a more reasonable physical process. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B35.00009: A Physical Model for Resistive Switching In Metal-Oxide Interface Nilanjan Das, Stephen Tsui, Yaqi Wang, Yuyi Xue, Ching-Wu Chu Resistive switching in metal-oxide interface has been studied extensively and different models have been proposed. We have investigated the switch in metal-PCMO ($\Pr_{0.7} Ca_{0.3}MnO_3)$ sample. Interface R and C, both have been found to be frequency independent almost up to 10 MHz. Also the activation energy for both the states (High and Low) are almost the same with bulk as found in R(T) plot. The physical picture will be very shallow potential wells, which may not be enough for retention observed. A pure electronic process (trapping and de-trapping in defects) of carrier only, as suggested earlier, will not be correct answer. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B35.00010: Percolative model for resistance switching S. H. Chang, S. C. Chae, J. S. Lee, S. B. Lee, D.-W. Kim, B. Khang, T. W. Noh There have been research efforts on resistive switching in numerous insulating oxide films for the next nonvolatile memory device. Recently percolation has been considered as a key concept to explain unipolar memory switching [1]. The rupture process of conducting path is closely related to thermal heat budget induced by Joule-heating effect. In this process, the thermal heat dissipation during rupture could play important role in details of unipolar memory switching. In this study, we investigated correlation between the resistance switching behaviors of Pt/NiO/Pt capacitor structures and thermal heat dissipation as a function of the bottom electrode and temperature. Our modified percolative simulation and finite element analysis demonstrated these phenomena. [1] S. C. Chae \textit{et al}., Adv. Mat., to be published (2007). [Preview Abstract] |
Session B36: Focus Session: Advances in Scanned Probe Microscopy I: Low Temperatures
Sponsoring Units: GIMSChair: Joseph A. Stroscio, National Institute of Standards and Technology
Room: Morial Convention Center 228
Monday, March 10, 2008 11:15AM - 11:51AM |
B36.00001: Subkelvin spin polarized STM: measuring magnetization curves of individual adatoms Invited Speaker: Magnetic nanostructures consisting of a few atoms on non-magnetic substrates are explored as model systems for miniaturized data storage devices and for the implementation of novel spin-based computation techniques. Since these nanostructures are well defined and controllable on the atomic scale, they are ideally suited to study the fundamentals of magnetic interactions. We used spin polarized scanning tunneling spectroscopy at subkelvin temperatures to image the magnetization of individual adatoms as a function of an external magnetic field. This allows to directly measure their magnetic interactions at very low energy scale. We will present the design of the 300mK STM [1] and then focus on the results. Interestingly, Co atoms on Pt(111) behave paramagnetic even at very low temperatures, 300 times smaller than the previously reported giant barrier between up and down spin [2]. A peculiar variation in the saturation flux density, which is measured for each atom, is found. This is attributed to their mutual indirect exchange via the substrate electrons. Indeed, we observe an interaction between the adatom and a Co monolayer stripe oscillating with distance between ferromagnetic and antiferromagnetic coupling on the scale of the Fermi wavelength. \newline [1] J. Wiebe et al., Rev. Sci. Instrum. 75, 4871 (2004). \newline [2] P. Gambardella et al., Science 300, 1130 (2003). [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B36.00002: Investigation of Acceptor States and Landau Levels in (In,Mn)As by Scanning Tunneling Spectroscopy Young Jae Song, Nikolai Zhitenev, Joseph Stroscio, Gregory Rutter, Phillip First Increased interest in spin-based electronics as a replacement for charge-based electronics has led to significant scientific attention on dilute magnetic semiconductors (DMS). Magnetically doped III-V semiconductors are a strong research focus, with the aim of achieving higher Curie temperatures by understanding the microscopic nature of ferromagnetism in these DMS materials. In this presentation, we discuss our recent study of single Mn acceptor states in InAs(110). Mn impurities deposited at low temperature are substituted by using STM atom manipulation techniques to exchange a Mn atom with a surface In atom [1]. Voltage-dependent imaging and scanning tunneling spectroscopy (STS) reveal a number of electronic states associated with the Mn acceptor state and the Landau levels in the 2D subbands of an accumulation layer as a function of applied magnetic field. This work has been supported in part by the NIST-CNST/UMD-NanoCenter Cooperative Agreement, NSF grant ECS-0404084, and Dept. of Commerce/NIST grant 60NANB7D6166. [1] Dale Kitchen \textit{et al}, Nature 442, 436 (2006) [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B36.00003: Atomic manipulation and tunneling spectroscopy on vacancy of Ag(111) surface studied with LT-STM Danda P. Acharya, Kai F. Braun, Saw W. Hla The evolution of the surface state and the effect of vacancies on the Ag(111) surface are investigated at an atomic scale by combining scanning tunneling microscopy and spectroscopy and atom manipulation at 5 K. Various vacancy sizes, from one atom to hundreds of atoms, on Ag(111) are first created by tip-sample contact in a controlled manner. Using lateral manipulation, a vacancy is filled one at a time and the corresponding differential conductance spectra are recorded. Small energy shifts in the onset of surface state are observed. The shift is more pronounced for small size vacancy and becomes less and less pronounced for larger size vacancies. The observed dI/dV intensities at different size of vacancies clearly reveals that the surface state onset on Ag(111) disappears after reaching towards the 6$^{th}$ layers. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B36.00004: Direct detection of force gradient using atomic force microscopy with very small oscillation amplitude Toshu An, Atsushi Nomura, Takahiro Nishio, Toyoaki Eguchi, Kotone Akiyama, Yukio Hasegawa Using a quartz, which has a self-sensing capability, simple configuration of atomic force microscopy (AFM) is realized, and because of its high stiffness frequency-modulation (FM) operations with a small oscillation amplitude below 100 pm is possible. The small amplitude AFM operation enhances sensitivity of short range forces. Moreover, a force gradient can be directly detected from force vs. distance measurements. We carried out FM-AFM at low temperature (LT) in ultra-high vacuum using a quartz length-extension resonator with a tungsten tip (Spring constant and resonant frequency of the resonator are 540 000 N/m and 1 MHz, respectively) (An et al., APL \textbf{76}, 133114 '05). The system was simply made by attaching an extra electrode to the tip-holder of our LT-scanning tunneling microscope cooled by $^{3}$He (APL \textbf{88}, 113115 '06). Direct detection of the force gradient was performed on the Si(111) 7x7 surface using very small oscillation amplitude of 70 pm at 2.4 K. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B36.00005: Low-Temperature Nanotribology of Au and Pb using a Quartz Tunning-Fork Carlos Untiedt, Giovanni Saenz-Arce Quartz Tunning-Fork (TF) has been in recent years successfully implemented in force detection schemes for scanning probe microscopy (SPM) applications. Here we report its use as a nanotribometer for measuring friction in atomic size areas. The idea behind such a friction detector, is to take advantage of the large Q-factor of a TF ($Q_{air}\sim6000$, $Q_{vac}\sim20000$ at room temperature) which in our set-up depends of the SPM tip-sample dissipative forces. We have measured the Q-factor and the resonance frequency for various TFs. As a first step in the use of a TF as a nanotribometer, we studied the reactive forces that are associated with the combined local elastic properties of the sample and tip and calculated the damping rate associate with changes of the tip-sample distance. Finally, we show the variation of the measured damping rate and local spring constants with tip-sample distances for Au and Pb. For the measurement we have used our TF-nanotribometer at different temperatures ranging from 1.5K to room temperature in high vacuum. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B36.00006: A Nano-Scale Scanning SQUID Susceptometer for the Measurement of Isolated Magnetic Moments Nicholas C. Koshnick, Martin E. Huber, Julie Bert, Hendrik Bluhm, Jeffrey Large, Hal Edwards, Kathryn A. Moler Superconducting Quantum Interference Devices (SQUIDs) are well known as excellent magnetic field sensors. We present a scanning DC SQUID susceptometer that is designed to couple well to nanometer-sized objects. Its gradiometric design and local field coils allow for cancellation of the applied field so that dynamic range issues do not limit the SQUID's sensitivity. Integrated modulation coils linearize the signal and allow for optimal performance at all applied fields. Planar coaxial shielding, enabled by a multi-layer niobium process, results in a low inductance (100 pH) millimeter scale design where the pickup loops can be optimized independently from the junction and shunt resistor parameters. The sensor loop is on a terraced structure so that it can be scanned approximately 100 nm from the sample surface. Focused Ion Beam milling is used to fabricate pickup loops with inner diameters between 250 nm and 2 microns with line widths of approximately 200 nm. A white noise sensitivity of 0.8 $\mu\Phi_0/\sqrt{Hz}$ gives an estimated spin sensitivity of 80 $\mu_B/\sqrt{Hz}$ at 4 Kelvin. We will also report on on-going scanning susceptometry measurements, and on the spin sensitivity at low temperatures. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B36.00007: Absolute in-situ calibration piezoelectric quartz tuning fork force Sensors. Sanjay Bidasaria, Alexei Marchenkov A method has been developed for absolute calibration of piezoelectric quartz tuning forks for use as force sensors with nano-newton resolution. The performance of the forks in a cryogenic environment is investigated. The mechanical properties of the forks are extracted from the frequency dependent admittance and compared to the exact model of a vibrating cantilever in a helium atmosphere. The method is verified by simultaneous application of calibrated point loads to the cantilever in vacuum. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B36.00008: A UHV-LT-STM System for Optical Experiments David R. Daughton, Donghun Lee, Jay A. Gupta The combination of optical techniques and scanning tunneling microscopy (STM) provides insight into a diverse set of physical processes including surface chemistry, surface-photon interactions, and spin scattering in semiconductors. We present a novel cryogenic temperature, ultrahigh vacuum STM which incorporates a maneuverable, high numeric aperture lens, with sub-5 micron spot size, in proximity to the tunnel junction. Modifications to our microscope have been made to improve upon the 12.5 K base temperature and 10 pm tip stability. Our initial efforts are focused on studies of photo-chemical reactions and chemical identification by tip-enhanced Raman spectroscopy (TERS). UHV deposition techniques have been developed for a variety of molecules well suited for TERS studies including azulene, azobenzene, methylene blue, and C60. Electrochemically and chemically-etched Ag and Au tips are optimized for field enhancement with characterization by scanning electron microscopy and collection of the plasmon emission from the tip. Raman spectra have been collected from molecule-coated tips in vacuum to test the optical setup for TERS. http://www.physics.ohio-state.edu/$\sim $jgupta [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B36.00009: Photon-induced Molecular Motion Probed by STM Jun Zhang, Kevin Kelly To understand the mechanics of nanoscale motion and manipulation in true molecular machines, we have investigated fullerenes and fullerene-based derivatives with an eye towards the molecular rolling motion on surfaces. Assisted by scanning tunneling microscopy (STM), we have successfully observed the conformational changes through the chemomechincal interaction with the external photonic and tunneling electron excitations. In particular, we investigated a fullerene dimer structure which included an azo-based linkage. This system demonstrated a mechanical switching by incident irradiation, due to the azo transformation between the ``cis'' and ``trans'' states. This is particularly exciting because the hinge-like lateral motion is coupled to the surface through the axle and the rolling motion of the fullerene wheel. The results of these studies underscore the ability to control designed motion in molecular-sized nanostructures through specific macroscopic excitations. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B36.00010: Using a Geophone for Vibration Cancellation in a STM Alan Fang, Zhanybek Alpichshev, Aharon Kapitulnik We demonstrate a method for using a geophone (velocity-sensitive vibration sensor) for reducing the vibration-induced tunnel current noise in a Scanning Tunneling Microscope (STM). Some simple analog circuitry compensates for the transfer function of the geophone and STM head. This error signal is then fed back to the scan piezos. Although the geophone is placed one meter away from the STM head, (due to the low temperature and UHV requirements) we show a noise reduction performance of approximately 5x or better. Further improvements to the system are discussed. [Preview Abstract] |
Session B37: FQHE in Higher Landau Levels
Sponsoring Units: FIAPChair: Chetan Nayak, Microsoft Station Q
Room: Morial Convention Center 229
Monday, March 10, 2008 11:15AM - 11:27AM |
B37.00001: Evolution of the Fractional Quantum Hall States in the Second Landau Level H.C. Choi, W. Kang, S. Das Sarma, L.N. Pfeiffer, K.W. West Study of the energy gap of the fractional quantum Hall effect (FQHE) in the second Landau level will be presented. Two symmetrically doped GaAs/AlGaAs quantum well samples with densities $n = 3.2 \times10^{11}$cm$^{-2}$ and $n = 2.8\times10^{11}$cm$^{-2}$ with respective mobilities of $\mu = 28.3\times10^6 $cm$^2$/Vs and $\mu = 10.5\times 10^6$cm$^2$/Vs were studied. In the higher mobility sample, clear FQHE states are observed at filling factor $\nu = $ 5/2, 7/3, 8/3, 14/5, 11/5, 12/5, 16/7, and 19/7. Some of the higher order FQHE states disappear in the lower mobility sample, and clear FQHE states are observed at $\nu = $ 5/2, 7/3, 8/3, 14/5, and 11/5. The energy gaps of the FQHE states at $\nu = $ 5/2, 7/3 and 8/3 in the higher mobility sample are found to exceed 500mK. The energy gaps of the $\nu = $ 5/2, 7/3 and 8/3 states in the lower mobility sample are typically reduced by more than 50\% in comparison. Our measured gap for $\nu=$5/2 state, which is less than 1/5 of the theoretical gap, can be understood when the finite width correction and disorder broadening are factored in. Evolution of the energy gap with mobility shows that the even-denominator FQHE state at $ \nu = $ 5/2 is the most robust FQHE state in the second Landau level. In addition, the $\nu = $ 7/3 and 8/3 states are unlikely to be the second Landau level analog of the Laughlin states at $\nu = $ 1/3 and 2/3 in the lowest Landau level. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B37.00002: Fractional Quantum Hall Effect and Electron Correlations in Partially Filled First Excited Landau Level George Simion, John J. Quinn The possibility of using non-Abelian quasiparticle excitations in quantum computing has led to a revival of interest in the fractional quantum Hall (FQH) states of excited Landau levels.We present a quantitative study of most prominent incompressible quantum Hall states in the partially filled first excited Landau level (LL1) which have been recently studied experimentally by Choi et al.(cond-mat:0707.0236v2). The pseudopotential describing the electron- electron interaction in LL1 is harmonic at short range. It produces a series of incompressible states which is different from its LL0 counterpart. The numerical data indicate that the most prominent states $\nu=\frac{5}{2}$, $\frac{7}{3}$, and $\frac{8} {3}$ are not produced by Laughlin correlated electrons, but result from a tendency of electrons to form pairs or larger clusters which eventually become Laughlin correlated. States with smaller gaps at filling factors $\frac{14}{5}$, $\frac{16} {7}$, $\frac{11}{5}$, $\frac{19}{7}$ are Laughlin correlated electrons or holes and fit Jain's sequence of filled $\rm{CF}^4 $ levels. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B37.00003: Fractional quantum Hall effect in higher Landau levels Michael R. Peterson, S. Das Sarma The fractional quantum Hall effect in the second Landau level (LL), particularly at filling factor 5/2, has seen a resurgence of research activity since its possible use in fault tolerant topological quantum computation was pointed out[1]. We do not, however, have a complete understanding of the FQHE in the second LL(SLL) compared with the corresponding lowest LL situation. For instance, while the Moore-Read Pfaffian state is the leading candidate for the 5/2 FQHE, it has only a moderate overlap ($\sim$0.9) with the exact wavefunction for finite size systems of electrons interacting through the Coulomb interaction. In this work we consider the finite thickness of the electrically polarized quasi-2D quantum confinement in three models: Zhang-Das Sarma, infinite square-well, and Fang-Howard potentials, respectively. We calculate overlap between the Laughlin(fillings 1/3 and 1/5) or Pfaffian(filling 1/2) and the corresponding exact state, obtained by exact diagonalization, in the lowest, second, and third LLs as a function of the layer thickness. We find that the Pfaffian state becomes a nearly exact description of the physics at filling factor 1/2 in the SLL for a finite value of thickness. We also show the comparative trends in the ground state energy and the excitation gap as a function of layer thickness, comparing among the first, second, and the third LLs. We acknowledge support from Microsoft Q Project. [1] Das Sarma et al. PRL 94, 166802(2005) [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B37.00004: Spin Order in Paired Quantum Hall States Ivailo Dimov, Bertrand Halperin, Chetan Nayak We consider quantum Hall states at even-denominator filling fractions, especially $\nu=5/2$, in the limit of small Zeeman energy. Assuming that a paired quantum Hall state forms, we study spin ordering and its interplay with pairing. We give numerical evidence that at $\nu = 5/2$ an incompressible ground state will exhibit spontaneous ferromagnetism. The Ginzburg-Landau theory for the spin degrees of freedom of paired Hall states is a perturbed CP$^2$ model. We compute the coefficients in the Ginzburg-Landau theory by a BCS-Stoner mean field theory for coexisting order parameters, and show that even if repulsion is smaller than that required for a Stoner instability, ferromagnetic fluctuations can induce a partially or fully polarized superconducting state. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B37.00005: Confinement of fractional quantum Hall states in the first excited Landau level Michael Manfra, Robert Willett, Loren Pfeiffer, Kenneth West The quasiparticles of certain exotic quantum Hall states in the first excited Landau level including $\nu $=5/2 and $\nu $=12/5 are believed to obey non-Abelian statistics. Manipulation of such quasiparticles is crucial to recent proposals of topologically protected quantum computation. Most schemes to determine the statistics of the quantum Hall quasiparticles rely on the manipulation of the correlated state in confined geometries. As a preliminary step in this direction, we report on the magnetic field and temperature dependences of transport through quantum point contacts (qpc's) in the regime where the first excited Landau level is partially occupied in the confined region. Our high density (n$\sim $4x10$^{11}$cm$^{-2})$ and high mobility GaAs samples are of sufficient quality such that well-defined quantum Hall states are resolved at $\nu $=8/3, 5/2, and 7/3 in the bulk at low temperature. In particular, we have studied the impact of confining geometry design and the size of the qpc opening on the stability of higher order fractional states in the qpc. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B37.00006: Confinement of Fractional Quantum Hall States Robert Willett, Michael Manfra, Ken West, Loren Pfeiffer Confinement of small-gapped fractional quantum Hall states facilitates quasiparticle manipulation and is an important step towards quasiparticle interference measurements. Demonstrated here is conduction through top gate defined, narrow channels in high density, ultra-high mobility heterostructures. Transport evidence for the persistence of a correlated state at filling fraction 5/3 is shown in channels of 2$\mu $m length but gated to near 0.3$\mu $m in width. The methods employed to achieve this confinement hold promise for interference devices proposed for studying potential non-Abelian statistics at filling fraction 5/2. R.L. Willett, M.J. Manfra, L.N. Pfeiffer, K.W. West, Appl. Phys. Lett. \textbf{91}, 052105 (2007). [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B37.00007: Fractional Quantum Hall Hierarchy and the Second Landau Level Parsa Bonderson, J.K. Slingerland We generalize the Haldane-Halperin hierarchy picture to apply to non-Abelian fractional quantum Hall states, and propose trial wave functions to describe the observed Hall conductance plateaus in the second Landau level. These hierarchy states are constructed over the Moore-Read state, the expected description of the $\nu = 5/2$ plateau, and thus all have electron pairing in the ground state and an excitation spectrum that includes non-Abelian anyons of the Ising model $\sigma$-vortex type. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B37.00008: Edge States and Interferometers in the Pfaffian and anti-Pfaffian States Waheb Bishara, Chetan Nayak In this work we use two theoretical candidates for describing the $\nu=5/2$ Quantum Hall state, the Moore-Read Pfaffian and its particle-hole conjugate, to calculate the conductance of a two point contact interferometer in the weak tunneling regime. We invoke the appropriate edge theory and calculate the conductance as a function of temperature and voltage, and we establish the connection to the underlying bulk topological theory. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B37.00009: Spectrum of Quantum Entanglement in Fractional Quantum Hall States Hui Li, F.D.M. Haldane We present numerical studies of the bipartite entanglement in fractional quantum Hall (FQH) states. We partitioned the (spherical geometry) Landau-level orbitals into two hemispheres: the entanglement spectrum derives from the Schmidt decomposition $|\psi\rangle = {\sum}_{i}\exp(-{\beta_{i}}/{2}) |\psi_{A}^{i}\rangle\otimes|\psi_{B}^{i}\rangle$, where $|\psi_{A}^{i}\rangle$ (or $|\psi_{B}^{i}\rangle$) are orthonormal. The $\beta_{i}$ are ``energy levels'' of a system with thermodynamic entropy at ``temperature'' $k_{B}T =1$ equivalent to the entanglement entropy. The \textit{entanglement spectrum}, \textit{i.e.}, the relation between the $\beta_i$ and the quantum numbers that classify $|\psi_{A}^{i}\rangle$ (or $|\psi_{B}^{i}\rangle$), serves as a ``fingerprint'' of the topological phase of the FQH state, and reveals much more information than just the entanglement entropy, a single number. The spectrum of, \textit{e.g.}, the $1/3$ Laughlin state has far fewer levels than expected for a generic wavefunction, and its low-energy spectrum corresponds to that of a conformal field theory (CFT). We studied the wavefunctions that interpolate between the Laughlin state and the ground state of a realistic Coulomb interaction potential at $\nu = 1/3$: the generic number of levels is restored, but the low-lying CFT structure remains essentially unchanged. We also describe the interpolation between the Moore-Read state and the Coulomb interaction ground state at $\nu = 5/2$. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B37.00010: Model Wavefunctions For Non-Abelian Quasiparticles B. Andrei Bernevig, F.D.M. Haldane We present model wavefunctions for quasiparticle (as opposed to quasihole)excitations of the $Z_k$ parafermion sequence (Laughlin/Moore-Read/Read-Rezayi) of Fractional Quantum Hall states. These states satisfy two generalized clustering conditions: they vanish when either a cluster of $k+2$ electrons is put together, or when two clusters of $k+1$ electrons are formed at different positions. For Abelian Fractional Quantum Hall states ($k=1$), our construction reproduces the Jain quasielectron wavefunction, and elucidates the difference between the Jain and Laughlin quasiparticle constructions. For two (or more) quasiparticles, our states differ from those constructed using Jain's method. By adding our quasiparticles to the Laughlin state, we obtain a hierarchy scheme which gives rise to a non-abelian $\nu=\frac{2} {5}$ FQH state. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B37.00011: Jack Polynomials, Exclusion Statistics, and non-Abelian FQHE States at $\nu$ = $k/(km+r)$ F. D. M. Haldane, B. Andrei Bernevig We describe a general family of non-Abelian FQHE states at
$\nu$ = $k/(km+r)$ with polynomial wavefunctions
$\prod_{i |
Monday, March 10, 2008 1:27PM - 1:39PM |
B37.00012: Searching for anyons in a realistic model of fractional quantum Hall liquids Zi-Xiang Hu, Xin Wan, Peter Schmitteckert We study quasihole/particle excitations in a microscopic model of fractional quantum Hall liquids with long-range Coulomb interaction and an edge confining potential. We find with a local trapping potential quasihole/particle states can emerge from the Laughlin and the Moore-Read states. The presence of Abelian and non-Abelian quasiholes has a distinct effect on the corresponding edge spectra. The stability of quasiholes/particles depends on the detail of the confining potential and the trapping potential. We discuss the relevance of the calculation to the high-accuracy generation and control of individual anyons in potential experiments, in particular, in the context of topological quantum computing. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B37.00013: Probing Non-Abelian Statistics in $\nu=12/5$ Quantum Hall State Kam Tuen Law The tunneling current and shot noise between two Fractional Quantum Hall edges in $ \nu=12/5 $ state in electronic Mach- Zehnder Interferometer with two quantum point contacts (QPCs) is studied. We show that the tunneling current and shot noise can be used to probe the existence of non-Abelian statistics in the $ k=3 $ Read-Rezayi state. More specifically, the dependence of the current on the Aharonov-Bohm flux in the Read- Rezayi state is asymmetric under the change of the sign of the applied voltage. This property is absent in the Laughlin states. Moreover the Fano factor can exceed 12.7 electron charges in the $k=3 $ Read-Rezayi state. This number is much greater than the maximum possible Fano factor in all Laughlin states and the Moore-Read state which was shown previously to be $ e $ and $ 3.2 e $ respectively. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B37.00014: Effect of Landau Level Mixing on Braiding Statistics Steven H. Simon We examine the effect of Landau level mixing on the braiding statistics of quasiparticles of abelian and nonabelian quantum Hall states. While path dependent geometric phases can perturb the abelian part of the statistics, we find that the nonabelian properties remain unchanged to an accuracy that is exponentially small in the distance between quasiparticles. [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B37.00015: Studying topological order in quantum Hall states using entanglement entropy calculations. Masud Haque, Oleksandr Zozulya, Kareljan Schoutens, Ed Rezayi, Nicolas Regnault We present calculations of the entanglement entropy in fractional quantum Hall (FQH) states. Calculating the entanglement entropy between spatially separated regions allows us to probe the topological order in Laughlin and Moore-Read states. The entanglement entropy is also found to be a sensitive indicator of quantum phase transitions between FQH and non-FQH states. [Preview Abstract] |
Session B38: Structure, Electronic Properties, Spectroscopy, and Ionic Transport in Insulators
Sponsoring Units: DCMPChair: Sudakar Chandran, Wayne State University
Room: Morial Convention Center 230
Monday, March 10, 2008 11:15AM - 11:27AM |
B38.00001: Perturbed Angular Correlation Study of ZrSiO$_{4}$ and HfSiO$_{4}$ Herbert Jaeger, Sean McBride Time-differential perturbed angular correlation spectroscopy is a powerful technique to study short-range interactions between probe nuclei and host crystals. The technique requires a very low concentration of radioactive probe nuclei and relies on the probes substituting for atoms of the host crystal. We report on PAC measurements of synthetic zircon and hafnon using $^{181}$Ta-probes to study the EFG at Zr and Hf lattice-sites. The quadrupole coupling constants for both zircon and hafnon decrease linearly with increasing temperature. The slope of $\nu _{Q}$ vs. T increases above 800$^{\circ}$C (zircon) and 1100$^{\circ}$C (hafnon) as a consequence of a change in the Zr-O and Hf-O coordination. We also observe a systematic reduction of the anisotropy value $\vert$A$_{2}$$\vert$ that is correlated with the onset of the structural change. Changes in the anisotropy are usually related to dynamic interaction and can be due to nuclear decay after effects. In our case we believe that we see evidence of a trapped electronic defect, giving rise to a rapidly decaying high-frequency quadrupole interaction. At higher temperatures the defect detraps and no longer causes a reduction in the anisotropy. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B38.00002: Stability and lattice dynamics of SiO$_2$ cristobalite Sinisa Coh, David Vanderbilt Among the phases of SiO$_2$ are alpha and beta cristobalite. Despite early indications that the higher-temperature beta phase might be cubic (Fd$\overline{3}$m), it is now accepted that it is in fact tetragonal (I$\overline{4}$2d), and that the experiments suggesting a cubic structure were averaging spatially or dynamically over tetragonal domains. Recently, Zhang and Scott (J.\ Phys.\ Cond.Matt.\ {\bf 19}, 275201) suggested that the lower-temperature alpha phase, widely accepted to be tetragonal (P$4_12_12$), might be an artifact in a similar way. With this motivation we investigate the energy landscape in the vicinity of cristobalite phases using first-principles calculations. We use the ABINIT implementation of density-functional theory in a plane-wave pseudopotential framework. We find that both the P$4_12_12$ alpha and I$\overline{4}$2d beta phases are local minima, thus reinforcing that the identification of the alpha phase as belonging to the P$4_12_12$ structure. We compute the frequencies of phonon modes at high-symmetry k-points in both structures and compare with experiment. We also identify a minimum-energy path connecting the alpha and beta phases through an intermediate orthorhombic phase (P$2_12_12_1$), and find a surprisingly low barrier of $\sim$5\,meV per formula unit. We note that a simple rigid-unit mode picture gives a good rough description of these energetics, and we map out the minimum-energy path in the space of rigid unit rotations in a physically insightful way. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B38.00003: First Principles Modeling of BaCeO$_{3}$: Stabilization of O Vacancies Joseph Bennett, Andrew Rappe We use first-principles density functional theory (DFT) calculations to investigate the ground state structures of both BaCeO$_{3}$ (BC) and Pd-doped BC (BCP) perovskites of general formula ABO$_{3}$. The relaxed structures match closely with recent experimental scattering studies, and also provide a local picture of how the BC perovskite lattice accommodates Pd. Both stoichiometric and oxygen-deficient materials are considered, and structures with an O vacancy adjacent to each Pd are predicted to be favored. The oxidation state of Pd in each doped structure is investigated through a structural analysis, the results of which are supported by an orbital-resolved projected density of states. The local bonding environments around Pd as well as the electron fillings of atomic orbitals on Pd and its neighbors are examined. A particular focus is the character of the highest occupied molecular orbital (HOMO) and the few lowest unoccupied molecular orbitals (LUMO). These electronic states are compared with expectations based on crystal field splittings in the computed atomic geometries. The vacancy stabilization by Pd in BCP is explained through redox chemistry and lattice strain relief. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B38.00004: Dynamical structure factor of LiF for all wave vector transfers: New results and insights Q. Kou, M.C. Troparevsky, A.G. Eguiluz(1), B.C. Larson, J.Z. Tischler, P. Zschack We report a theoretical-experimental investigation of the dynamical structure factor of LiF. The calculations are done within the TDLDA of time-dependent density functional theory; the measurements correspond to non-resonant inelastic x-rays scattering (NIXS) cross sections, obtained in absolute units. The TDLDA spectra contain one adjustable parameter: a ``scissors-operator'' shift of the conduction bands. This parameter is determined in view of the NIXS line shape for $q$= 6{\AA}$^{-1}$ ($q //$(111)). The TDLDA spectra display a non-trivial semi-quantitative agreement with the NIXS data, for all wave vectors (directed along the three high-symmetry directions); indeed, the line shape changes drastically from the coherent-response small-$q$ regime, to the incoherent-response large-$q$ regime. The picture of the excitations which emerges offers an alternative view relative to a seminal investigation involving an approximate solution of the Bethe-Salpeter equation; W. A. Caliebe et al. Phys. Rev. Lett. 84, 3907 (2000). [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B38.00005: Simulation of Li ion diffusion near electrolyte-metal interface -- Li$_3$PO$_4$ and Li Yaojun Du, Xiao Xu, N.A.W. Holzwarth Motivated by recent technological interest in Li$_3$PO$_4$-based electrolytes developed at Oak Ridge National Laboratory\footnote{J. B. Bates, N. J. Dudney, and co-workers, {\em{Solid State Ionics}}, {\bf{53-56}}, 647-654 (1992).} for use in rechargeable solid-state batteries and other technologies, we have used first-principles modeling techniques to study Li ion diffusion near idealized interfaces between Li$_3$PO$_4$ and Li metal. Using the nudged elastic band method, migration energy barriers for Li ion diffusion across interfaces in different crystallographic directions are calculated for both vacancy and interstitialcy mechanisms. Preliminary results find interface migration barriers as low as 0.2 eV and 0.26 eV for the vacancy and interstitialcy mechanisms, respectively. This suggests that interface diffusion barriers are likely to be comparable or lower than the corresponding migration barriers within crystalline Li$_3$PO$_4$.\footnote{Y. A. Du and N. A. W. Holzwarth, {\em{Phys. Rev. B}}, {\bf{76}}, 174302 (2007).} [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B38.00006: QNS study on superprotonic conductor Yutaka Ikedo, Hiroshi Nozaki, Masahiko Harada, Jun Sugiyama, Taku Sato, Yasumitsu Matsuo, Yiming Qiu, John Copley Cesium hydrogen selenate, CsHSeO$_4$, and related materials $M$H$X$O$_4$, where $M$ = Cs, Rb, K and $X$ = S, Se, are considered to be a promising candidate as electrolyte materials for fuel cells. In order to clarify the mechanism of their high proton conductivity, quasielastic neutron scattering (QNS) measurements were carried out using single crystal samples of CsHSeO$_4$ on disk chopper spectrometer (DCS) in NIST Center for Neutron Research at temperatures mainly above T$_C$ (= 401 K), at which CsHSeO$_4$ undergoes a structural phase transition from a low-T orthorhombic phase (Phase II) to a high-T tetragonal phase (Phase I). High proton conductivity, i.e., super-protonic ionic conductivity is observed only in Phase I. The analysis of the QNS spectrum suggests an anisotropic proton diffusion in Phase I, in spite of the isotropic behavior reported by AC conductivity measurements. The present QNS result thus provides crucial information on the proton diffusion pass in these materials. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B38.00007: Proton Transport in Mixed Rare-Earth/Alkaline Earth Metaphosphate Glasses Gabriel Harley, Lutgard C. De Jonghe The transport properties of [La$_{(1-x)}$M$_{x}$]-P$_{3}$O$_{9}$ metaphosphate glasses, where M$_{x}$ is Ba, Sr, Ca, and 0$\le $x$\le $0.8 were investigated in the 300 -- 500\r{ }C range. Protons are found to be incorporated as charge compensation for the substituting divalent cation. A model for proton conduction in phosphate glasses is presented where aliovalent cations within the phosphate network act as trapping sites for protons. Protons are transported via trapping center to trapping center along phosphate tetrahedra. The diffusion of protons is found to be between $\sim $10$^{-8}$ cm$^{2}$/s and $\sim $10$^{-6}$ cm$^{2}$/s in the 300-500\r{ }C range. The conductivity increases two orders of magnitude from the unsubstituted to the 60{\%} substituted glass reaching a maximum conductivity of $\sim $10$^{-6}$ S/cm at 450\r{ }C. The average transport distance between proton centers predicted by the electrical analysis is on the order of tens of nanometers, which is the same magnitude of proton-proton distance calculated from the structural data. The activation energy is found to be independent of concentration though dependant on modifying substitutional cation, and increases from 0.92 eV for Ba to 1.02 for Sr. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B38.00008: Evidence of structure dependence of hyperpolarizability in octupolar molecules Claudia Cardoso, Paulo Abreu, Fernando Nogueira It was recently reported that second-order hyperpolarizability increases significantly upon introduction of positive charges at the pyridyl end groups in trispyridyl octopolar chromophores, when compared with the neutral species. We performed ab-initio and semi-empirical calculations for the geometries, electronic localization function and first hyperpolarizabilities of a series of 6 trispyridyl molecules in neutral and protonated forms. Ab-initio calculations correctly reproduce the large hyperpolarizability values of the protonated octopolar molecules. Semi-empirical calculations predict somehow smaller hyperpolarizability values but reproduce the trend of the experimental and ab-initio values. Linear response TDDFT calculations are in good agreement with the experimental absorption spectra, reproducing the red-shift of the peaks with protonation. A correlation between the molecular structures and the first hyperpolarizability was established. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B38.00009: UV Spectra of rutile and anatase phases of TiO$_2$ using ABINIT2NBSE H.M. Lawler, J.J. Rehr, S. Dalosto, Z.H. Levine, E.L. Shirley We have developed an interface between the electronic structure code ABINIT and the optical spectroscopy code NBSE, developed at NIST. NBSE calculates dielectric spectra at various levels of theory, including iterative solution of the Bethe-Salpeter equation (BSE) for electron-hole states. From a single input file the interface executes a complete calculation, from the self-consistent ground-state to the dielectric spectrum. The interace can also treat momentum-transfer dependence of the dielectric function. As an application, we present calculations of the birefringent spectra of rutile and anatase TiO$_2$. Not surprisingly, we find that excitonic interactions dominate low-energy features of the spectra. We also address the role of planar-bonding, oxygen $\pi$-orbitals near the Fermi level, and the unfilled titanium d-band in the birefingent properties of these systems. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B38.00010: Zeeman Effect in TiO$_{2}$:Cr$^{3+}$ Michael Crawford, Xing Wei, Stan Tozer We will describe the results of Zeeman effect measurements for single crystals of rutile TiO$_{2}$ doped with Cr$^{3+}$. These measurements, performed at the National High Magnetic Field Laboratory in magnetic fields with strengths up to 45 T, utilized the near-infrared luminescence of Cr$^{3+}$ at a temperature of T = 1.4 K. The Cr$^{3+}$ luminescence spectra show the evolution with field strength of the splitting of the Cr$^{3+}$ zero-phonon line at 12,684 cm$^{-1}$ in magnetic fields applied parallel or perpendicular to the crystallographic $c$-axis. In the former case the zero-phonon line splits into four Zeeman components, while for the latter case three components appear. These results will be discussed and compared to earlier measurements made in weaker magnetic fields. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B38.00011: Raman electronic paramagnetic resonance (Raman-EPR) of Cr$^{3+}$ in ruby X. Lu, S. Venugopalan, Hyunjung Kim, M. Grimsditch, S. Rodriguez, A.K. Ramdas We have observed the Raman-EPR of the Zeeman split $^{4}$A$_{2}$ ground state of the Cr$^{3+}$ ion in Al$_{2}$O$_{3}$:Cr, i.e., ruby, exploiting the resonance conditions associated with the R$_{1}$ line. Employing a tunable dye laser with a photon energy E$_{L}$ in the vicinity of the Zeeman components of the R$_{1}$ luminescence, we observe the Stokes and anti-Stokes Raman transitions with shifts corresponding to the intra $^{4}$A$_{2}$ ground state levels split by the external magnetic field (\textbf{B}). The proximity of the incident and the scattered radiation to the Zeeman components of R$_{1}$ leads to selective dramatic resonance enhancements of the intensities of EPR transitions brought about as a function of \textbf{B} and E$_{L. }$The microscopic mechanism for the resonance enhancement involves the `in resonance' and `out resonance' conditions fulfilled by the virtual transitions from the sublevels of $^{4}$A$_{2}$ ground state to the sublevels of $^{2}$E by the incident and the scattered radiation in a two step process. Raman-EPR of the Zeeman sublevels of $^{2}$E excited state of R$_{1}$ is also observed. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B38.00012: Trivalent chromium probes in mixed dicyanoargentate-dicyanoaurate single crystals C.L. Larochelle, J.K. Krebs Single crystals of the form R[M(CN)$_2$]$_3$ (R=trivalent ion, such as a rare earth; M=Ag, Au, or both) have a layered structure consisting of alternating layers of M(CN)$_2^-$ ions and R$^{3+}$ ions. Recent work on this type of crystal has focused on energy transfer from the metal dicyanide donor to the rare earth acceptors, specifically Tb$^{3+}$, Eu$^{3+}$, Sm$^{3+}$, and Ce$^{3+}$. Crystals of this type are particularly interesting because the luminescence energies are tunable, changing with changes in the temperature as well as Ag/Au ratio. Doping these single crystals with chromium can provide an opportunity to study the crystal field strength at the ion site because the energy levels in chromium are very sensitive to the ion's environment. We present steady-state excitation and emission results, along with lifetime measurements for a series of crystals of the type Cr$_{0.1}$La$_{0.9}$[Ag$_{x}$Au$_{1-x}$(CN)$_2$]$_3$, with $x$=1, 0.5, 0.75, 0.9. These measurements indicate that the chromium emission is of an unusually low energy in these crystals. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B38.00013: Ce/Cr:YAG Based Yellow/Red Phosphors Jinke Tang, Wendong Wang, Sheng Teng Hsu, Brian Sullivan Ce:YAG (yttrium aluminum garnet) is the current industrial material of choice for phosphor for white LEDs, but suffers from poor color rendering index. We have investigated Cr and Ce co-doped YAG. Our investigation suggests Cr addition can drastically enrich the emission spectrum of Ce:YAG in the red region, which is realized by a non-radiative energy transfer from the Ce3+ 2D3/2 level to Cr3+ 4T level. The emission spectrum excited at 467 nm blue wavelength shows that the addition of Cr induces strong peaks in the red region. Comparison with the emission spectrum of the Cr-only sample reveals that the intensity of red Cr emission is much higher in Ce and Cr co-doped samples, which suggests energy transfer from Ce to Cr. Excitation spectra collected at the Cr red emission show a large excitation peak at 458 nm and suggest energy transfer from the 2D3/2 level of Ce3+ to the 4T level of Cr3+, which is responsible for the enhanced red emission from Cr. At the same time, radiative transfer from Ce3+(2D3/2) to Cr3+(4T) through the absorption of the yellow emission by Cr seems limited. The orders of magnitude increase in the Cr red emission in Ce/Cr:YAG compared to Cr:YAG suggests that the former is an efficient red and yellow phosphor. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B38.00014: Spectral Intensities of Transitions Between Stark Levels of Er$^{3+}$(4f$^{11})$ in Single Crystal, Ceramic, and Nanocrystalline Y$_{2}$O$_{3}$ Kelly Nash, John Gruber, Dhiraj Sardar, Uygun Valiev, Nikolai Ter-Gabrielyan, Larry Merkle, Arockiasamy Michael Similarities and differences among the optical properties of Er$^{3+}$:Y$_{2}$O$_{3}$ in single crystal, polycrystalline (ceramic), and nanocrystalline forms are discussed based on spectra obtained between 400 nm and 1700 nm and temperatures between 8 K and 300K. The observed crystal-field splitting and the measured intensities of transitions between the $^{2S+1}$L$_{J}$ manifolds of Er$^{3+}$(4f$^{11})$ in both the C$_{2}$ and C$_{3i}$ sites are analyzed in terms of models that invoke the mixing of states of opposite parity through the odd terms in the crystal-field Hamiltonian. The inversion symmetry of C$_{3i}$ sites limits electronic transitions to magnetic dipole transitions between the $^{4}$I$_{13/2}$ and $^{4}$I$_{15/2}$ manifolds. For Er$^{3+}$ ions in C$_{2}$ sites, the forced electric-dipole transitions along with some magnetic dipole contribution in certain cases, are allowed between the J+1/2 Stark levels within all manifolds. Within the instrumental resolution, there are some important differences between intensities of transitions depending on particle size of the Er$^{3+:}$Y$_{2}$O$_{3}$. [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B38.00015: Judd-Ofelt analysis and crystal-field modeling of Er$^{3+}$ transitions in YAlO$_{3}$ Sree R. Chandrasekharan Optical absorption and emission intensities are investigated for trivalent Er$^{3+}$ ions in YAlO$_{3}$ crystal. The Judd-Ofelt model is applied to the room temperature absorption intensities of Er$^{3+}$(4f$^{11})$ transitions in YAlO$_{3}$ to obtain the intensity parameters which are then used to calculate the spontaneous emission probabilities, branching ratios, radiative decay rates, and radiative lifetimes of the Er$^{3+}$ transitions from the upper multiplet manifolds to the corresponding lower-lying multiplet manifolds in YAlO$_{3}$. The room-temperature fluorescence lifetimes and the emission cross sections of selected intermanifold transitions are determined. From the calculated radiative lifetimes and the measured fluorescence lifetimes, the quantum efficiency of the sample has been found. The 8K absorption spectrum has been examined as well. Selected manifolds have been analyzed in terms of crystal field splitting using current models and minimization methods to establish the parameters of Er$^{3+}$ in C$_{s}$ symmetry sites. The optical and spectroscopic characteristics of Er$^{3+}$:YAlO$_{3 }$show that this material has a potential for both 1.5$\mu $m and 544.96 nm stimulated emissions. [Preview Abstract] |
Session B39: Focus Session: Collective Dynamics of Self-Driven Particles
Sponsoring Units: GSNP DFDChair: Sriram Ramaswamy, Indian Institute of Science, Bangalore
Room: Morial Convention Center 231
Monday, March 10, 2008 11:15AM - 11:51AM |
B39.00001: Polar and apolar active matter Invited Speaker: Assemblies of interacting self-driven units form a new type of \textit{active} soft matter with collective behavior qualitatively different from that of its individual constituents, nonequilibrium phase transitions, and unusual mechanical and rheological properties. Examples include cytoskeletal filaments crosslinked by motor proteins, bacterial colonies, migrating cells, and vibrated layers of granular rods. In this talk I will review our work on using nonequilibrium statistical physics to derive a continuum description of these systems from specific models of single particle dynamics. This approach aims at understanding the interplay between physical mechanisms (such as formation or loss of physical connections, excluded volume effects, directional forces) and biochemical or other processes in regulating the large-scale organization and function of active matter. I will contrast the behavior of units with a head and a tail that can exhibit a macroscopic polar state, where all organisms move coherently in a preferred direction, with that of units with head-tail symmetry, that can order in a nematic state, with no net motion on macroscopic scale. Finally, I will use a simple model of active rods on a substrate to discuss the interplay between equilibrium steric effects and self-propulsion in controlling order and fluctuations in active fluids. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B39.00002: Swarming and swirling in self-propelled polar granular rods Arshad Kudrolli, Geoffroy Lumay, Dmitri Volfson, Lev Tsimring We discuss the dynamics of ``self-propelled" polar rods experimentally and numerically. In the experiment, the polar motion was achieved by vibrating rods with asymmetric mass distribution. In the numerics, we postulate a driving force acting along the axis of the rod. We observe aggregation of rods at the boundaries because of the inability of rods to turn around and escape for high enough density under low noise conditions. As vibration strength and thus noise is increased, the aggregation reduces and a uniformly distributed state displaying local orientation order and swirls are observed. We observe greater than $\sqrt{n}$ density fluctuations which are in a qualitative agreement with the Toner-Tu model, but this agreement should not be over-emphasized since the model is directly applicable to a nematic regime. Our findings elucidate an important and interesting interplay between the shape and the directed motion in {\em realistic} self-propelled rods which affects the phenomenology of their collective dynamics. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B39.00003: Rectification of Swimming Bacteria and Self Driven Particle Systems by Arrays of Asymmetric Barriers Charles Reichhardt, Mew Bing Wan, Cynthia Olson Reichhardt, Zohar Nussinov We show that the recent experimental observation of the rectification of swimming bacteria in a system with an array of asymmetric barriers occurs due to the ballistic component of the bacteria trajectories introduced by the bacterial ``motor.'' Each bacteria selects a random direction for motion and then moves in this direction for a fixed period of time before randomly changing its orientation and moving in a new direction. In the limit where the bacteria undergo only Brownian motion, rectification by the barriers does not occur. We also examine the effects of steric interactions between the bacteria and observe a clogging effect upon increasing the bacteria density. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B39.00004: Delay induced instabilities in self-propelling swarming particles Eric Forgoston, Ira Schwartz We consider a general model of self-propelling biological or artificial individuals interacting through a pairwise attractive force in a two-dimensional system in the presence of noise and communication time delay. Previous work has shown that a large enough noise intensity will cause a translating swarm of individuals to transition to a rotating swarm with a stationary center of mass. In this work, we use numerical simulations to show that with the addition of a time delay, the model possesses a transition that depends on the size of the coupling parameter. This transition is independent of the swarm state (traveling or rotating) and is characterized by the alignment of all of the individuals along with a swarm oscillation. By considering the mean field equations without noise, we show that the time delay induced transition is associated with a Hopf bifurcation. The analytical result yields good agreement with numerical computations of the value of the coupling parameter at the Hopf point. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B39.00005: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 12:39PM - 12:51PM |
B39.00006: From Cannibalism to Active Motion of Groups Pawel Romanczuk, Lutz Schimansky-Geier The detailed mechanisms leading to collective dynamics in groups of animals and insect are still poorly understood. A recent study by Simpson et. al. suggests cannibalism as a driving mechanism for coordinated migration of mormon crickets [1]. Based on this result we propose a simple generic model of brownian particles interacting by asymmetric, non-conservative collisions accounting for cannibalistic behavior and the corresponding avoidance strategy. We discuss our model in one and two dimensions and show that a certain type of collisions drives the system out of equilibrium and leads to coordinated active motion of groups.\newline [1] Stephen J. Simpson, Gregory A. Sword, Patrick D. Lorch and Iain D. Couzin: \emph{Cannibal crickets on a forced march for protein and salt}, PNAS, 103:4152-4156, 2006 [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B39.00007: Spatial instability and bioturbulence in highly concentrated bacterial suspensions Andrey Sokolov, Igor Aranson We present an experimental study of spatio-temporal organization and transition to complex collective swimming regimes in highly concentrated suspensions of Bacillus subtilis. Experiments are performed in a free-standing thin-film sample with controlled thickness. Novel non-invasive high-resolution optical coherence tomography technique is used to probe the density distributions in the film in real time. Increasing the film thickness beyond certain threshold triggered a transition from quasi-to-dimensional collective swimming to three-dimensional turbulent state which is attributed to Oxygentaxis. We have studied effect of the controlled oxygen concentration on the bacterial collective behavior and transition to turbulent bioconvection. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B39.00008: Non-Coalescent, Self-Assembling Water Drops: Phase transitions, flows and hydrodynamics Mohan Srinivasarao, Vivek Sharma We study the collective nucleation, growth and self-assembly of non-coalescent water drops. These form and organize over evaporating polymer solutions exposed to a draft of moist air. The creation and evolution of a population of drops towards a closed packed array occurs in response to heat and mass fluxes involved in droplet condensation and solvent evaporation. We elucidate the kinetics and dynamics of droplet growth and assembly, by accounting for various transport and thermodynamic processes. These water drops template hexagonally ordered arrays of holes in polymer films. We thus have a useful and economical method for manufacturing porous films requiring only a drop of polymer solution (dilute) and a whiff of breath! [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B39.00009: Active nematics: fluctuations and coarsening Sriram Ramaswamy, Shradha Mishra, Francesco Ginelli, Hugues Chate, Sanjay Puri Nonequilibrium steady states with spontaneous nematic order are known to arise in collections of amoeboid cells as well as granular-rod monolayers. Recent studies [EPL 62 (2003) 196-202; PRL 96, 180602 (2006); PRL 97 (2006) 090602; Science 317 (2007) 105] have established that these states differ radically from thermal equilibrium systems of the same spatial symmetry. This talk will present results from our studies of microscopic as well as coarse-grained models of active nematics, highlighting the unique, fluctuation-dominated character of coarsening in these systems. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B39.00010: Long-range correlations in simulations of suspensions of swimming microorganisms Patrick Underhill, Juan Hernandez-Ortiz, Michael Graham Simulations of large populations of hydrodynamically interacting swimming particles have been performed at low Reynolds number in periodic and confined geometries. Our simulations show that the interactions of the particles lead to long-range spatial correlations in the fluid at scales larger than the size of a single organism. These long-range correlations lead to a large enhancement in the fluid transport properties. The diffusivity of passive, non-Brownian tracer particles diverges in the periodic geometry with increasing the simulation box size. This collective motion depends on the method the organism uses for propulsion. Simple scaling arguments have also been developed that can capture much of the physics of both the swimmer and tracer motions. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B39.00011: Simulated Flocking Dynamics of 2D Self-propelled Hard Particles Donald Blair Following a recent demonstration of long-lived giant number fluctuations in a swarming, granular nematic (Narayan et. al, Science {\bf 317}, 105 (2007)), we perform 2D simulations of hard, self-propelled particles which communicate only through contact. We vary particle end-shape, polarity, and aspect ratio and explore the effects on order, on the development of density fluctuations, and on the evolution of the swarm boundary. Connections to various forms of active matter (swimming bacteria, crawling cells) will be discussed. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B39.00012: Cell swarming leads to vortex flow in early embryo formation Ariel Balter, James A. Glazier A forming embryo can be though of as a confined region of incompressible medium. Vortex flow is observed in early embryo formation from \em drosophila \em fruit flies to mammals. The Navier-Stokes equation for fluid flow in a cavity is known to have stable vortex solutions. A model for cell motion in which cells move independently of their neighbors corresponds to high Reynolds number (\em Re\em) incompressible flow. An alternative cell-swarming model in which cells do influence their neighbors motion (through a mechanism known as \em contact following\em) corresponds to a flow model that is similar to low \em Re \em incompressible flow. Both models can potentially lead to stable vortex formation in a confined cavity. We investigate the applicability of both models to real biological systems [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B39.00013: Active elastic dimers: self-propulsion and current reversal on a featureless track Vijay Kumar Krishna Murthy, Sriram Ramaswamy, Madan Rao Directed motion without an imposed external gradient is seen not only in living systems but also in agitated granular matter. The essential ingredients are an external energy input and an inherent asymmetry. Unlike traditional ``Brownian ratchet models'', the asymmetry of interest in the above systems is \emph{internal} to the motile objects, and does not lie in an external periodic potential. In this work, we present a Brownian inchworm model of a self-propelled elastic dimer in the absence of an external potential. Nonequilibrium noise together with a stretch-dependent damping form the propulsion mechanism. Our model connects three key nonequilibrium features -- position-velocity correlations, a nonzero mean internal force, and a drift velocity. Our analytical results, including striking current reversals, compare very well with numerical simulations. The model unifies the propulsion mechanisms of DNA helicases, polar rods on a vibrated surface, crawling keratocytes and Myosin VI. We suggest experimental realizations and tests of the model. [Preview Abstract] |
Session B40: SPS Undergraduate Research and Outreach I
Sponsoring Units: SPSChair: Gary White, Society of Physics Students/American Institute of Physics
Room: Morial Convention Center 232
Monday, March 10, 2008 11:15AM - 11:27AM |
B40.00001: Summer 2007: My SPS intern experience and working with ComPADRE Andrew Coughlin For the summer of 2007 I was enrolled in the Society of Physics Students Internship program hosted by the American Institute of Physics in College Park, Maryland. My time at was spend working for the American Physical Society outreach department. My primary task was to expand and maintain Physicstogo.com which is part of the ComPADRE group of outreach websites funded by the National Science Foundation. More specifically, I searched for materials to add to the website, cataloged citation information, and updated the biweekly magazine portion of the website. Work experience was not the only thing gained from this internship. As a group of 8 interns we attended an event on Capitol Hill, met with members of congress, and toured nearly every monument and museum in the area. The 2007 SPS internship was an amazing opportunity and a summer memory that will remain with me for the rest of my life. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B40.00002: Demonstrating Levitation and Suspension of a Superconductor on a Magnetic Track Charles P. Strehlow, M.C. Sullivan The suspension and levitation of superconductors by permanent magnets is one of the most fascinating consequences of superconductivity, and a wonderful instrument for generating interest in low temperature physics. We present a novel classroom demonstration of the levitation/suspension of a superconductor over a magnetic track that maximizes levitation/suspension time, separation distance between the magnetic track and superconductor as well as insulator aesthetics. A theoretical explanation of the levitation/suspension and a simple mathematical model of the lateral restoring forces are discussed. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B40.00003: Comprehensive study on deformation of metal samples based on measurements of temperature, in-plane displacement and stress-strain characteristics John Gaffney, Christopher Schneider, Sanichiro Yoshida We have studied the dynamics of deformation of metal samples. Our efforts are directed at finding some correlation between the stage of deformation (elastic, plastic, pre-fracturing stage, etc) and changes in properties of the sample such as the change in temperature across the surface, the stress strain characteristics, and the in-plain displacement on the sample surface. To study this, we simultaneously applied three independent data collection systems; a tensile machine to obtain the stress strain curve, an optical interferometer to study both the vertical and horizontal displacement of the surface, and two thermistors to obtain a continuous temperature reading as the sample deformed. With two thermistors, we were able to study how the temperature changed in different locations on the surface of the sample. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B40.00004: Low-temperature calorimeter for magnetocaloric-effect measurements in high magnetic fields Travis Miller, Yasumasa Takano The magnetocaloric effect, in which sweeping a magnetic field results in a temperature variation, is a powerful tool for detecting phase transitions in magnetic samples. The effect is particularly useful near the zero-temperature limit where a transition line becomes horizontal in the field-temperature phase diagram, a temperature region in which specific heat fails to exhibit sharp anomaly at the transition. At temperatures below 200 mK, however, eddy current heating produces a temperature background that becomes relative in magnitude to the temperature change of genuine features, seriously limiting sensitivity. This causes numerous problems in trying to extracting clear data. We describe a new calorimeter design which overcomes this problem in experiments using a dilution refrigerator in magnetic fields up to 20 T. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B40.00005: Driven Intrinsic Localized Modes in a Coupled Pendulum Array. Ritoban Basu Thakur, Lars English, Albert Sievers Intrinsic localized modes (ILMs), also called discrete breathers, are directly generated via modulational instability in an array of coupled pendulums. These ILMs can be stabilized over a range of driver frequencies and amplitudes. They are characterized by a $\pi $-phase difference between their center and wings. At higher driver frequencies, these ILMs are observed to disintegrate via a pulsating instability, and the mechanism of this breather instability is investigated. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B40.00006: Quartz tuning fork as a viscometer for Helium liquids J. Jhaveri, M. Gonzalez, P. Bhupathi, Y. Lee Oscillating beams serve as simple systems for measuring effects of energy dissipation as a result of interaction with their environment. Especially in miniature mechanical oscillators, the enhanced surface-to-volume ratio signifies the importance of damping caused by drag force. We have investigated the mechanical response of commercial miniature quartz tuning forks with a natural resonant frequency of 32.768 kHz. The changes in resonance frequency and damping have been measured at various Helium and Nitrogen gas pressures and various temperatures. Our results will be compared with theoretical predictions in order to extend its application to the sub-millikelvin temperature range as an effective thermometer in superfluid $^{3}$He. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B40.00007: Demonstrating the Principle of an rf Paul Ion Trap Andrew Johnson, James Rabchuk An rf ion trap uses a time-varying electric field to trap charged ions. This is useful in applications related to quantum computing and mass spectroscopy. There are several mechanical devices described in the literature which have attempted to provide illustrative demonstrations of the principle of rf ion traps, including a mechanically-rotating ``saddle trap'' and the vertically-driven, inverted pendulum$^{1,2}$. Neither demonstration, however, successfully demonstrates BOTH the sinusoidal variation in the electric potential of the rf trap AND the parametric stability of the ions in the trap described by Mathieu's equation. We have modified a design of a one-dimensional ponderomotive trap$^{3}$ so that it satisfies both criteria for demonstrating the principle of an rf Paul trap. Our studies indicate that trapping stability is highly sensitive to fluxuations in the driving frequency. Results from the demonstration apparatus constructed by the authors will be presented. $^{1}$ Rueckner, W., et al., ``Rotating saddle Paul trap,'' Am. J. Phys., 63 (2), February 1995. $^{2}$ Friedman, M.H., et al., ``The inverted pendulum: A mechanical analogue of a quadrupole mass filter,'' Am. J. Phys., 50 (10), October 1982. $^{3}$ Johnson, A.K. and Rabchuk, J.A., ``A One-Dimensional Ponderomotive Trap,'' ISAAPT 2007 spring meeting, WIU, March 30, 2007. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B40.00008: Experimental characterization of piezoelectric THUNDER actuator shape Paul Harris A new type of piezoelectric composite actuator called THUNDER, which was originally developed by NASA, has potential applications in micro robotics, aeronautics, acoustics and hydraulics. The manufacturing process produces internal stresses with accompanying structural deformation. It is the aim of this research to characterize these deformations. Detailed measurements were taken by a motion control LabView data acquisition system and measured with a laser micrometer on several different types of actuators. Several functional forms were used in an attempt to fit the data. The data was best fit by a circular segment function. We also used a transcendental equation to be able to compare to other single point published values. We found the range of dome heights to be between 10.15 mm and 1.45 mm. For one particular model, the manufacturing difference was found to be 16{\%} with an experimental error of 0.5{\%}. The robust experimental data is vital to the development of our finite elements models. Preliminary experimental results of voltage induced deformations will be presented. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B40.00009: A Relativistic Understanding of Rotating Reference Frames Kacey Meaker, Tom Michalik The purpose of this research is to obtain a greater understanding of relativistic acceleration and rotation. Particles in a rod experiencing constant acceleration have hyperbolic worldlines. A simple global rigid rotating frame cannot be physically realized, because the force needed to maintain an object in circular motion approaches infinity. This understanding will be discussed in this presentation. [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