Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Z1: Solid-State Spin Qubits: Coherence Control and Protection
Sponsoring Units: DCMPChair: Sophia Economou, Naval Research Laboratory
Room: Ballroom A1
Friday, March 25, 2011 11:15AM - 11:51AM |
Z1.00001: Control of single-spin decoherence by dynamical decoupling and spin bath manipulation Invited Speaker: Controlling the interaction of a single quantum system with its environment is a fundamental challenge in quantum science and technology. We dramatically suppress the coupling of a single spin in diamond with the surrounding spin bath by using high-fidelity double-axis dynamical decoupling [1]. The coherence is preserved for arbitrary quantum states, as verified by quantum process tomography. The resulting coherence time enhancement is found to follow a general scaling with the number of decoupling pulses. No limit is observed for the decoupling action up to 136 pulses, for which the coherence time is enhanced more than 25 times compared to spin echo. Furthermore, we have exploited multi-pulse sequences to enhance the sensitivity of single-spin magnetometry and to measure properties of the decoupling sequences themselves [2]. In this talk, I will present an overview of this work combined with our latest results on coherent manipulation of the spin bath environment. \\[4pt] [1] Universal dynamical decoupling of a single solid-state spin from a spin bath, G. de Lange, Z.H. Wang, D. Rist\`{e}, V.V. Dobrovitski, and R. Hanson, Science 330, 60 (2010). \\[0pt] [2] Single-spin magnetometry with multi-pulse sequences, G. de Lange, D. Rist\`{e}, V. V. Dobrovitski, R. Hanson, arXiv:1008.4395 (2010). [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:27PM |
Z1.00002: Control of electron spin decoherence in nuclear spin baths Invited Speaker: Nuclear spin baths are a main mechanism of decoherence of spin qubits in solid-state systems, such as quantum dots and nitrogen-vacancy (NV) centers of diamond. The decoherence results from entanglement between the electron and nuclear spins, established by quantum evolution of the bath conditioned on the electron spin state. When the electron spin is flipped, the conditional bath evolution is manipulated. Such manipulation of bath through control of the electron spin not only leads to preservation of the center spin coherence but also demonstrates quantum nature of the bath. In an NV center system, the electron spin effectively interacts with hundreds of $^{13}$C nuclear spins. Under repeated flip control (dynamical decoupling), the electron spin coherence can be preserved for a long time ($>$1~ms). Therefore some characteristic oscillations, due to coupling to a bonded $^{13}$C nuclear spin pair (a dimer), are imprinted on the electron spin coherence profile, which are very sensitive to the position and orientation of the dimer. With such finger-print oscillations, a dimer can be uniquely identified. Thus, we propose magnetometry with single-nucleus sensitivity and atomic resolution, using NV center spin coherence to identify single molecules. Through the center spin coherence, we could also explore the many-body physics in an interacting spin bath. The information of elementary excitations and many-body correlations can be extracted from the center spin coherence under many-pulse dynamical decoupling control. Another application of the preserved spin coherence is identifying quantumness of a spin bath through the back-action of the electron spin to the bath. We show that the multiple transition of an NV center in a nuclear spin bath can have longer coherence time than the single transition does, when the classical noises due to inhomogeneous broadening is removed by spin echo. This counter-intuitive result unambiguously demonstrates the quantumness of the nuclear spin bath. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 1:03PM |
Z1.00003: Single spin qubits in self-assembled quantum dots Invited Speaker: The search for a highly coherent electronic spin in the solid state has led most spectacularly to the NV colour centre in diamond. Have self-assembled quantum dots, InGaAs in GaAs, been left behind? The advantages of self-assembled quantum dots are considerable - there is a strong optical transition, advanced heterostructure technology and post-growth processing techniques - but so far the spin coherence has been at best modest. This talk will present some possible ways to out fox the decoherence processes in a semiconductor with the goal of creating a highly coherent spin. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:39PM |
Z1.00004: Preserving electron spin coherence in solids by optimal dynamical decoupling Invited Speaker: To exploit the quantum coherence of electron spins in solids in future technologies such as quantum computing, it is first vital to overcome the problem of spin decoherence due to their coupling to the noisy environment. Dynamical decoupling, which uses stroboscopic spin flips to give an average coupling to the environment that is effectively zero, is a particularly promising strategy for combating decoherence because it can be naturally integrated with other desired functionalities, such as quantum gates. Errors are inevitably introduced in each spin flip, so it is desirable to minimize the number of control pulses used to realize dynamical decoupling having a given level of precision. Such optimal dynamical decoupling sequences have recently been explored. The experimental realization of optimal dynamical decoupling in solid-state systems, however, remains elusive. Here we use pulsed electron paramagnetic resonance to demonstrate experimentally optimal dynamical decoupling for preserving electron spin coherence in irradiated malonic acid crystals at temperatures from 50K to room temperature [1]. Using a seven-pulse optimal dynamical decoupling sequence, we prolonged the spin coherence time to about 30 ms; it would otherwise be about 0.04 ms without control or 6.2 ms under one-pulse control. By comparing experiments with microscopic theories, we have identified the relevant electron spin decoherence mechanisms in the solid. Recently, we demonstrate experimentally that dynamical decoupling can preserve bipartite pseudo-entanglement in phosphorous donors in a silicon system [2]. In particular, the lifetime of pseudo entangled states is extended from 0.4 us in the absence of decoherence control to 30 us in the presence of a two-flip dynamical decoupling sequence. \\[4pt] [1]. Jiangfeng Du, Xing Rong, Nan Zhao, Ya Wang, Jiahui Yang~and R. B. Liu, Preserving electron spin coherence in solids by optimal dynamical decoupling, Nature~461, 1265-1268 (2009). \\[0pt] [2] Ya Wang, Xing Rong, Pengbo Feng, Wanjie Xu, Bo Chong, Ji-Hu Su, Jiangbin Gong, and Jiangfeng Du, Preservation of bipartite pseudo-entanglement in solids using dynamical decoupling, submitted to Phys. Rev. Lett. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 2:15PM |
Z1.00005: Theory of qubit dephasing in a large nuclear spin bath Invited Speaker: This abstract not available. [Preview Abstract] |
Session Z2: Pseudogap in High Tc Cuprates
Sponsoring Units: DCMPChair: Martin Greven, University of Minnesota
Room: Ballroom A2
Friday, March 25, 2011 11:15AM - 11:51AM |
Z2.00001: Loop-Current Order in Several Families of Cuprates Invited Speaker: In high temperature copper oxides superconductors, a novel long range 3D magnetic order associated with the pseudogap phase has been identified in two different cuprate families - ${\rm YBa_{2}Cu_3CuO_{6+x}}$ (YBCO),\footnote{B. Fauqu\'e, Y. Sidis, V. Hinkov, S. Pailh\`es, C.T. Lin, X. Chaud, and P. Bourges, {\it Phys. Rev. Lett.} {\bf 96}, 197001 (2006).} ${\rm HgBa_2CuO_4}$ (Hg1201)\footnote{Y. Li, V. Bal\'edent, N. Barisi\'c, Y. Cho, B. Fauqu\'e, Y., Sidis, G. Yu,, X. Zhao, P. Bourges, and M. Greven, {\it Nature} {\bf 455}, 372 (2008).} - over a wide region of temperature and doping. That magnetic order, evidended using polarized neutron diffraction, respects the translation symmetry of the lattice and can be described as a Q=0 antiferromagnetism with active role of in-plane oxygens atoms. Such a magnetic order can be associated with orbital moments in the circulating currents phase proposed by C. Varma. Similar magnetic ordering is observed in the archetypal cuprate ${\rm La_{2-x}Sr_xCuO_4}$ (LSCO) system below 120 K for x=0.085.\footnote{V. Bal\'edent, B. Fauqu\'e, Y. Sidis, N. B. Christensen, S. Pailh\`es, K. Conder, E. Pomjakushina, J. Mesot, and P. Bourges {\it Phys. Rev. Lett.} {\bf 105}, 027004 (2010).} In contrast to the previous reports, the magnetic ordering in LSCO is {\it\bf only} short range with an in-plane correlation length of $\sim$ 10 \AA\ and is bidimensional (2D). Such a less pronounced order suggests an interaction with other electronic instabilities. In particular, LSCO also exhibits a strong tendency towards stripes ordering at the expense of the superconducting state. Additional polarized neutron diffraction measurements have been performed in YBCO.\footnote{V. Bal\'edent, D. Haug, Y. Sidis, V. Hinkov, C.T. Lin and P. Bourges, preprint} At lower doping (8.5 \%), the magnetic order is observed at lower temperature ($\sim$ 150 K) than the generally assumed value for the pseudogap. It tends to vanish for dopings where the nematic electronic liquid crystal phase sets up. Recently, two others cuprates families have been studied: ${\rm Bi_2Ca_2SrCu_2O_{8+\delta}}$ (Bi2212) and electron doped ${\rm Nd_{2-x}Ce_xCuO_4}$ (NCCO). In both families, a magnetic order related to the pseudogap phase has been also observed. The recent results will be discussed during the talk. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:27PM |
Z2.00002: Novel magnetic excitations in a model cuprate high-$T_{c}$ superconductor Invited Speaker: Magnetic fluctuations might be essential to the mechanism of high-temperature superconductivity in the cuprates. For a long time, such fluctuations have been theoretically regarded as arising from the antiferromagnetic correlations within the copper-oxygen layers, and experimental studies of magnetic excitation spectrum have mainly been carried out near the corresponding wave vector (1/2,~1/2). Following neutron diffraction experiments which demonstrated the universal existence of a ``$q$~=~0 antiferromagnetic order'' in the pseudogap phase of three different cuprates [1-3], our recent inelastic neutron scattering experiments on the model compound HgBa$_{2}$CuO$_{4+\delta }$ (Hg1201) revealed the existence of unusual magnetic excitations that weakly disperse throughout the entire Brillouin zone [4,5]. Like the $q$~=~0 antiferromagnetic order, the new excitations are observed in the pseudogap phase and therefore appear to be associated with the order. The excitations possess very large spectral weights at well-defined characteristic energies that are comparable to the resonance energy [6] and to those of electron-boson-coupling features observed in a wide range of cuprates, highlighting their possible influence on the electronic structure. These findings demonstrate that the pseudogap state is a distinct phase of matter rather than a mere crossover. They furthermore cast doubt on the presumed predominant importance of the wave vector (1/2,~1/2) in the magnetic excitation spectrum, and have the profound implication that a single-band description of the cuprates is insufficient. \\[4pt] [1] B. Fauque \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{96}, 197001 (2006). \\[0pt] [2] Yuan Li \textit{et al.}, \textit{Nature} \textbf{455}, 372 (2008). \\[0pt] [3] V. Baledent \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{105}, 027004 (2010). \\[0pt] [4] Yuan Li \textit{et al.}, \textit{Nature} \textbf{468}, 283 (2010). \\[0pt] [5] Yuan Li \textit{et al.}, unpublished. \\[0pt] [6] G. Yu \textit{et al.}, \textit{Phys. Rev. B} \textbf{81}, 064518 (2010). [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 1:03PM |
Z2.00003: Collective Modes in Cuprates and their coupling to Fermions Invited Speaker: The quantum-critical fluctuations of the loop current order observed universally in underdoped Cuprates have been derived and shown to be local in space and power law in time. The coupling of these fluctuations to fermions are shown to promote d-wave pairing as well as to give the Marginal Fermi liquid single particle spectra in the normal state [1]. Three collective fluctuations modes in the loop order modes are derived [2]. They are massive weakly dispersive magnetic modes. Two of these branches have been discovered. Experiments are suggested to discover the third branch. \\[4pt] [1] V. Aji, A. Shekhter and C.M. Varma, Phys. Rev. B. 81, 06451 (2010).\\[0pt] [2] Yan He and C.M. Varma, arXiv:1008.3182. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:39PM |
Z2.00004: Disentangling Cooper-pair formation above {$T_{c}$} from the pseudogap state in the cuprates Invited Speaker: The discovery of the pseudogap in the cuprates created significant excitement amongst physicists as it was believed to be a signature of pairing, in some cases well above room temperature. This was supported by a number of experiments detecting phase-fluctuating superconductivity above {$T_{c}$}. However, several recent experiments reported that the pseudogap and superconducting state are characterized by different energy scales, and likely compete with each other, leaving open the question of whether the pseudogap is caused by pair formation. To address this issue, we investigate the spectral weights, which are easier to quantify and in many cases interpret than the spectral feature, which is traditionally used. A key such measure is the density of states at the Fermi energy $D(E_{F})$. In conventional, clean superconductors this weight is zero below $T_{c}$, but can be finite if there are strong impurity scattering effects. In such cases $D(E_{F})$ reflects the pair breaking states. A separate scenario is a generic ``density wave state" in the absence of pairing, which leads to a decrease of the $D(E_{F})$ due to the opening of the density wave gap. In addition there is also the possibility of the coexistence of superconductivity and the density wave state - inhomogeneous superconductors such as the cuprates, where superconducting and non-superconducting patches coexist in the sample. One can then expect that the temperature dependence of $D(E_{F})$ can be used to distinguish between these scenarios and disentangle the electronic ground states of the cuprates. Since the spectral gap in the cuprates displays significant momentum dependence, in our study we use the intensity of the spectral function at $E_{F}$, $I(E_{F}, k)$, which when integrated over all momenta equals $D(E_{F})$. This allows us to isolate the behavior at a specific $k$-point and avoid smearing due to averaging. In this talk, we report the discovery of a spectroscopic signature of pair formation and demonstrate that in a region commonly referred to as the ``pseudogap", two distinct states coexist: one that persists to an intermediate temperature {$T_{pair}$} and a second that extends up to { $T^{*}$}. The first state is characterized by doping independent scaling behavior and is due to pairing above {$T_{c}$}, but significantly below {$T^{*}$}. The second state is the ``proper" pseudogap - characterized by the loss of low energy spectral weight, anomalies in transport properties and the absence of pair formation. {$T_{pair}$} has a universal value around 120-150K even for materials with very different {$T_{c}$} and it likely sets limit on the highest attainable {$T_{c}$} in the cuprates. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 2:15PM |
Z2.00005: Phenomenology of electronic nematic and smectic states in STM studies of high T$_c$ cuprates Invited Speaker: Electronic liquid crystals are phases in which electronic structure of a material breaks the spatial symmetries of its crystal lattice: electronic nematic only breaks the point group symmetry, while smectic (stripe) additionally breaks the translational symmetry. Here I define two independent order parameter fields for nematic and smectic that can be constructed from STM data. Using these order parameters we find long range intra-unit cell nematicity in the pseudogap states [1]. In contrast, we observe many topological defects that disorder the smectic fields. However, these defects reveal a remarkable coupling between smectic tendency and fluctuations in the nematic order. From these observations, we propose a Ginzburg-Landau free energy describing the quantum nematic/smectic coupling and demonstrate how it can explain the coexistence of these states and correctly predict their interplay [2]. In principle, this understanding may enable us to disentangle the complexities of the system specific cuprate phase diagrams.\\[4pt] [1] M. J. Lawler, K. Fujita, Jhinhwan Lee, A. R. Schmidt, Y. Kohsaka, Chung Koo Kim, H. Eisaki, S. Uchida, J. C. Davis, J. P. Sethna, Eun-Ah Kim, ``Intra-unit-cell electronic nematicity of the high Tc copper-oxide pseudogap states'', Nature {\bf 466}, 347 (2010).\\[0pt] [2] A. Mesaros, K. Fujita H. Eisaki, S. Uchida, J.C. Davis, S. Sachdev, J. Zaanen, M.J. Lawler, and Eun-Ah Kim, ``How topological defects couple the smectic and nematic electronic structure of the cuprate pseudogap states'', submitted (2010). [Preview Abstract] |
Session Z9: Complex Fluids, Polymers, Gels
Sponsoring Units: DFDChair: Corey O'Hern, Yale University
Room: D220
Friday, March 25, 2011 11:15AM - 11:27AM |
Z9.00001: An anisotropic continuum model for flow, aggregation and microstructure evolution in magnetorheological fluids Murat Ocalan, Gareth McKinley The complexities associated with the transport of magnetorheological (MR) fluids under non-uniform magnetic and flow fields pose unresolved problems for generating accurate computational models. The evolutions of the electromagnetic and rheological properties of MR fluids are strong functions of the suspension microstructure; however, the geometrical features that lead to the field non-uniformities are often of a much larger length scale. To address these commonly occurring flow problems, we develop an anisotropic continuum model for MR fluids in which the electromagnetic stress is incorporated into the constitutive model for the viscoplastic stress generated in the bulk fluid by considering the generation and distortion of suspension microstructure under flow. The new model is incorporated into both a single-phase and a two-phase continuum description of the suspension. The aggregation dynamics and the evolution of MR fluid microstructure are observed in unique ferromagnetic microfluidic channels that replicate flow conditions of practical interest. The predictions of the newly developed models are verified with the experimental observations of microstructure evolution and macroscopic measurements of fluid rheology. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z9.00002: Electrorheological response of dense strontium titanyl oxalate suspensions Carlos Orellana, Jinbo He, Heinrich Jaeger Strontium Titanyl Oxalate (STO) particles were synthesized using a new method of precipitating the STO out of a water solution by adding alcohol. When dispersed in silicon oil, dense STO suspensions exhibit a high static yield stress in the presence of an electric field (200kPa at 5kV/mm), high shear stress at high shear rates and low current densities. We also find that the yield stress increases roughly linearly with applied field. This behavior is a key characteristic of a polar molecule dominated electrorheological effect. We also observed stress stiffening with time under low shear, stress oscillations, and stress reduction with strain. These effects can be accounted for by the interaction of permanent dipoles with the particles, the creation of shear bands of a few particles in width and the lack of self-diffusion in the samples. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z9.00003: Molecular Simulations of Particle Nanorheology Mir Karim, Rajesh Khare Over the past few years, experimental and theoretical developments in the field of microrheology have enabled determination of the local mechanical properties of complex materials. In this presentation, we will extend this approach to determine the local viscoelastic properties of polymeric materials using molecular dynamics (MD) simulations. Molecular simulations provide the unique ability to explicitly account for the intermolecular interactions in the system. Thus an approach based on molecular simulations allows for the determination of the viscoelastic properties at the nanoscale. The specific system that is studied in this work consists of a polymeric melt in which the polymers are modeled as bead-spring chains. We will present a comparison of the results obtained from the passive and the active nanorheology approaches. A discussion of the parameter (e.g. amplitude and frequency) ranges that allow usage of these techniques will also be presented. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z9.00004: Microscopic Approach for the Friction on a Spherical Particle in Dense Liquids: Hydrodynamics and Beyond Umi Yamamoto, Kenneth Schweizer We propose a new microscopic, non-mode-coupling, statistical dynamical approach to deriving the Stokes-Einstein (SE) friction coefficient of a large spherical particle dissolved in a dense fluid. The real space method is based on including as a slow variable the force exerted on a particle by the surrounding fluid. By exploiting the appropriate separation of time and length scales, and the Kirkwood superposition approximation for multi-point correlations, the SE result is obtained including the slip and stick limits plus the crossover function. This advance provides the foundation for developing a unified theory of friction for nanoparticles that includes both hydrodynamics and the non-hydrodynamic contribution associated with material-specific particle-fluid and particle-particle forces. Applications to nanoparticles in unentangled and entangled polymer solutions and melts, under various interfacial polymer-particle structure conditions, will be reported. Questions of particular interest include how the non-hydrodynamic friction contribution scales with particle radius, the role of length-scale-dependent viscosity in polymer liquids, and the conditions required for crossover to the hydrodynamics-dominated regime. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z9.00005: Rheology of bacterial flagella suspensions Sevim Yardimci, Thomas Gibaud, Daniel Chen, Edward Barry, Zvonimir Dogic The mechanical behavior of a suspension of rigid and semiflexible filaments has been studied in great detail. In comparison the effect of the filament geometry has been relatively unexplored. We present experimental results on the rheological behavior of suspensions of curly and straight flagella with an identical average contour length. We find that both suspensions are trapped in a glassy state and exhibit a solid-like behavior. We observe that the scaling of viscoelastic moduli is highly dependent on filament geometry. Taken together, this highlights the role of filament geometry in suspension mechanics. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z9.00006: Brownian Dynamics simulations of dilute graphene solutions under flow Yueyi Xu, Micah Green Many graphene-based materials (such as thin conductive films and nanocomposites) are processed in the liquid phase and require the conformation and alignment of graphene in solution to be precisely controlled. However, prior studies of conformation dynamics of sheetlike macromolecules such as graphene have been limited to equilibrium behavior, and there have been no studies of the dynamics of sheetlike macromolecules on flow processing timescales. Here we develop Brownian Dynamics (BD) algorithms in order to quantify the effects of flow processing on graphene conformation. The method is conceptually similar to those used for linear polymers; we coarse-grain the sheet using a bead-rod lattice of arbitrary 2-D connectivity and develop a novel theoretical framework for bending and metric forces. Using this technique, we simulate the conformation dynamics of dilute sheetlike macromolecule solutions in shear flow and compute the corresponding solution properties as a function of flow strength, sheet size, and solvent quality. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z9.00007: Orientation Dependent Gelation of Platelet Suspensions Ya-Wen Chang, Andres Mejia, Zhengdong Cheng Gelling behavior of colloidal suspensions of disk-shaped particles has long been used as an ideal system for studying the formation of arrested state of matter. High aspect ratio synthetic $\alpha $-Zirconium phosphate ($\alpha $--ZrP) monolayer platelets have recently received our attention as a new type of liquid crystal building blocks. We report the phase diagram of charged $\alpha $-Zirconium phosphate platelet suspensions across the isotropic (I) --nematic (N) region versus salt concentrations. Typical electrostatic screening induced flocculation and gelation of platelet suspensions were observed. The morphological and rheological characteristics of liquid crystalline and colloidal gel phases were studied with polarized optical imaging and rotational/oscillatory rheometer. At high ionic strengths ($>$10mM), a re-entrance of fluidic liquid crystal phase occurs when particle volume fractions are above the arrested gel phase. We contribute this behavior to the competition between the driving forces for isotropic/nematic and sol/gel transitions of attractive colloidal platelets. Strong particle alignment hinders gelation, which usually demonstrates the ``house of card'' configuration in platelet suspensions; Isotropic suspensions flocculate and gel easily, as we confirmed experimentally. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z9.00008: Dynamic separation of macromolecules under temperature gradient Yusuke Maeda, Axel Buguin, Albert Libchaber Thermophoresis is a motion of suspensions in a fluid that are subjected to a temperature gradient. Although its effect is widely studied in case of single solute in water, little is known about how the mixture of different solutes is affected. We heated water with an infrared laser by $\Delta T_{\max } $=5C and $\nabla T$=0.25C/um to induce thermophoresis of polyethylene glycol (PEG) and DNA. PEG is depleted from the hot region and results in a stationary gradient of its high volume fraction $\phi $. Under this high concentration of PEG, DNA of small concentration is submitted to thermophoresis and osmotic pressure difference. The DNA shows regime of depletion, ring-like localization and accumulation as the volume fraction of PEG increases. As the osmotic force depends on the size of trapped solutes, DNA of different size accumulates at different regions. Depending whether the DNA size is below or above 5kbp a different scaling of position versus DNA size is observed. Thermal separation is a general phenomenon. It applies also to RNA and microbeads. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z9.00009: Non-affine deformations in flexible and semi-flexible polymer gels Anindita Basu, Qi Wen, Xiaoming Mao, Tom Lubensky, Paul Janmey, Arjun Yodh We test the validity of affine deformation assumption in flexible and semi-flexible polymer networks by embedding different-sized fluorescent tracer beads within model polymer networks and quantifying their displacements under shear. A conventional rheometer is used with a confocal microscope for this purpose. Non-affinity is quantified as a function of applied strain, polymer chain density, cross-link concentration, network morphology, reaction kinetics and size of probe particles used. ~Non-affinity measurements in flexible polymer gels are in qualitative agreement with current theories in rubber elasticity. ~For semi-flexible bio-polymer gels, measurements indicate that non-affine deformations are small for networks of thinner, relatively flexible filaments and get smaller as strain increases into non-linear elastic regime. These small measures are consistent with the entropic model for non-linear elasticity of semi-flexible gels. However, as filament stiffness and mesh size increase, the deformations become more non-affine, as predicted by the enthalpic bending and stretching models of non-linear elasticity. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z9.00010: Drying of polymer films: study of demixing phenomena Julie Fichot, Rodolphe Heyd, Marie-Louise Saboungi, Christophe Josserend, Emilie Combard, Jean Francois Tranchant Understanding the mechanisms that control the stability of polymeric films is important in beauty care. We have prepared films starting from a water-soluble organic polymer, a preservative and water. We study the drying of these films as a function of several physicochemical parameters that control their interfaces such as temperature, humidity and the nature of the support. The viscoelastic properties of the solutions before spreading out are analyzed with a rheometer in order to adjust the temperature. The topography of the films is observed by optical microscopy and the evolution of the drying is determined with a precision gravimetric balance. The behavior of the films on a nanometric scale is followed by AFM. During the drying process, droplets appear on the surface of the film, made up of water surrounded by a shell of preservative. As the films dries, the water evaporates from the droplets and the preservative spreads on the surface of the film, leading to the formation of craters on the surface of the dried film. The dimensions and numbers of the craters depend strongly on the type and concentration of the preservative employed. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z9.00011: Multiple Particle Collision Dynamics Simulations of the Effect of Catenation on the Structural and Dynamic Properties of Ring Polymers in Solution Govind Hegde, Rajesh Khare Multiple particle collision dynamics (MPCD) is a particle based mesoscale simulation technique that coarse-grains the solvent while preserving the hydrodynamics, thus enabling simulations over longer length and time scales as compared to molecular dynamics (MD) simulations. In this work, MPCD is used to study the effect of topology on the structural and dynamic behavior of complex fluids. The systems of interest in this work are the dilute solutions of ring and catenated ring polymers. MPCD simulation results are compared with those obtained from MD simulations in which the hydrodynamic interactions are governed by the explicit intermolecular interactions. Different chain topologies are considered such as catenated as well as multi-catenated rings. Results will be presented for the effect of chain length on the radius of gyration and chain diffusion coefficient for the various topologies studied. Our results will also be compared with previous theoretical and experimental work reported in literature. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z9.00012: Strongly anisotropic polymer networks Stephan Ulrich, Annette Zippelius, Panayotis Benetatos We investigate a network of worm-like chains, which are strongly oriented along a preferred direction due to an external field, boundary conditions, or a nematic environment. We discuss the effects of random permanent cross-links, whose density may follow an arbitrary distribution along the alignment direction. We show that the tilt modulus is unaffected by cross-links. As the cross-link density is increased beyond the gel point, the network develops a stiffness to in-plane shear deformations. Results for the shear elasticity and fluctuations of the polymer chains are presented. The case of cross-linking the chains on one end only is highlighted, it constitutes a simple model for polymer brushes. Moreover force-extension curves are presented for a toy model that consists of two cross-linked chains. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z9.00013: Structure and Dynamics of Water Absorbed in Polyamide Marco Laurati, Paul Sotta, Didier Long, Ludovic Odoni, Veronique Bossenec, Thierry Badel, Arantxa Arbe, Angel Alegria, Juan Colmenero We present results of elastic and inelastic neutron scattering, dielectric spectroscopy and MD simulations concerning the structural organization and the dynamics of water absorbed in an amorphous polyamide material. We find that, different from predictions of available models of water absorption in polyamide, only a small fraction of water binds to the amide groups while most of it organizes into aggregates. Such structural model is supported by results on the microscopic dynamics of water, which can be described as diffusive motions with a relaxation time following a VFT dependence on temperature, similarly to bulk water. Measured average diffusion coefficients of water absorbed in Polyamide are approximately two orders of magnitude smaller than in bulk water, revealing the confinement effect of the polymer matrix. [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z9.00014: Order parameter defining liquid-liquid transition in water J. Raul Grigera, Osvaldo Chara, Andres McCarthy Water presents both open tetrahedral and compact hexagonal structures. Although several order parameters have been proposed to quantify this, all of them are only applicable to data produced by simulation. We present an order parameter ($P_{r})$ that is calculated from the radial distribution function $g(r)$, also available from experiment. We hereby extract the tetrahedral and hexagonal components from the $g(r)$, each one reconstructed as the sum of a Freundlich distribution for the first peak, two subsequent Gaussian distributions, and a sigmoidal to account for the rest. The order parameter can be calculated from the relative contribution of tetrahedral over hexagonal contribution. We obtained the $P_{r}$ for SPC/E water model from molecular dynamics simulations of water at different pressures and temperatures. At 300K, the pressure in which both, tetrahedral and hexagonal contributions become equal ($P_{r }$= 0), a structural crossover is found in the vicinity of 2kbar, close to the pressure at which the `'anomalous'' behavior manifests. Having computed $P_{r}$ for this wide range of pressure and temperature we then calculate the HDL spinodal, the coexistence line, the second critical point, and the Widom line. [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z9.00015: Particle and fluid diffusivity of non-colloidal suspensions Emmanouela Filippidi, Alexandre Franceschini, Chui-Lai Cheung, Jacob Tutmaher, Sean Paradiso, Tarun Jain, David Pine Suspensions of non-colloidal spheres at moderate volume fractions (0.2-0.4) under slow periodic strain undergo a phase transition from an absorbing to an active fluctating state. Particle trajectories change from reversible below the critical strain to irreversible above. We measure the fluid diffusivity of the fluorescently labelled fluid and compare it with the particle diffusivity in order to obtain a measure of the coupling between the two. Of particular interest is how the fluid diffusivity changes near the onset of irreversibility of the particle trajectories. [Preview Abstract] |
Session Z10: Novel Instrumentation and Techniques in Surface Science
Sponsoring Units: DCMPChair: Ray Phaneuf, University of Maryland
Room: D221
Friday, March 25, 2011 11:15AM - 11:27AM |
Z10.00001: Development of a device-oriented UHV scanning probe microscope based on quartz sensors Jacob Tosado, William G. Cullen, Michael S. Fuhrer Scanning tunneling microscopy (STM) provides atomic-scale spatial resolution and performs local electronic spectroscopy of conducting materials. The recent emergence of graphene has highlighted the ability to tune carrier density by applying a gate voltage. However, preparation of samples as field-effect-transistors necessitates a dielectric substrate below the device, which is problematic for STM. Driven by the need to carry out high resolution imaging in ultrahigh vacuum, we are now developing an instrument which combines STM with atomic force microscopy (AFM) using a quartz sensor. This combination allows AFM approach and navigation, with uncompromised STM performance due to the very high stiffness of the quartz sensor. Primary features of the microscope design include in-situ exchange of probes and samples, with flexibility in probe and sample geometries and multiple contacts to both probe and sample. The microscope is housed in a UHV chamber with complete surface preparation and analysis capability. This talk will cover unique design features as well as testing of the microscope concept. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z10.00002: Low-temperature STM/STS study on superconducting FeSe films Xucun Ma, Canli Song, Yilin Wang, Peng Cheng, Lili Wang, Ke He, Xi Chen, Qikun Xue By using molecular beam epitaxy (MBE) technique, we have prepared single crystalline and atomically flat FeSe thin films on graphene-terminated SiC substrate. Low temperature scanning tunneling microscopy/spectroscopy (STM/STS) measurements reveal that the local superconducting gap in the quasiparticle density of states remains robust down to two triple layers ($\sim $1.1 nm), and that the FeSe films show a novel thickness-dependent superconductivity transition behavior. We show that the superconductivity of the FeSe films can be manipulated by concise control of their surface structures. \textit{dI/dV} mapping of the vortex lattice confirms that FeSe is a typical type II superconductor. Understanding these properties may help us to unravel the mechanism of the recently discovered Fe-based superconductors and even the long-term studied cuprates. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z10.00003: In situ coherent x-ray scattering and STM studies of hexagonally reconstructed Au(001) in Electrolytes Michael S. Pierce, Vladimir Komanicky, Andi Barbour, Daniel Hennessy, Jun-Dar Su, Alec Sandy, Hoydoo You We have studied the dynamics of Au(001) and Au(111) surfaces in situ in 0.1 M HClO4 electrolyte solution using coherent x-ray scattering experiments and STM microscopy. Our coherent x-ray scattering experiments measure a correlation time for the surface as a function of applied potentials. Coherent x-ray scattering differs from the ordinary x-ray diffraction in sensitivity to the structural and temporal details. The correlation times were obtained from measurements conducted while the surface is in equilibrium and the ordinary surface scattering intensity is constant. The correlation time changes from high $10^3$ seconds to low $10^2$ seconds. The correlation times of reconstructed surfaces at low potential are at least an order of magnitude smaller than those measured at the reconstructed surfaces in vacuum. The correlation times also change dramatically in response to the applied potential. These experiments also represent the first successful application of coherent x-ray scattering to the study of electrochemical interfaces in situ. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z10.00004: Local transport measurements at mesoscopic length scales on epitaxial graphene using scanning tunneling potentiometry Weigang Wang, Ko Munakata, Michael Rozler, Francoise Kidwingira, Malcolm Beasley By contrast to quantum transport measurements across nanostructures (single molecular, carbon nanotube, or lithographically manufactured), local transport measurements on macroscopic samples at mesoscopic length scales are relatively uncharted territory. Scanning tunneling potentiometry (STP) is the natural tool to perform such measurements. Due to its characteristic materials parameters, thin epitaxial graphene on silicon carbide is an attractive model system for search of quantum mechanical effects in local transport. We report results of STP measurements on epitaxial graphene at room temperature. In addition to the expected residual resistivity dipoles, we have observed features in the measured potential that are counter intuitive to classical diffusive considerations. Based on these results, we conclude that a more complete theoretical description of STP measurement is necessary. Work supported by AFOSR. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z10.00005: Possible microscopic origin of large broadening parameter in point Andreev reflection spectroscopy Jian Wei, Goutam Sheet, Venkat Chandrasekhar We report on the low frequency noise in ballistic point-contacts between a silver tip and a niobium foil. The ballistic nature is confirmed by point-contact Andreev reflection (PCAR) spectroscopy. As has been found by others, a broadening parameter $\Gamma$ needs to be introduced to fit the PCAR spectra. For contacts with higher resistance, a larger $\Gamma$ is required to fit the PCAR spectra, and we find that random two level fluctuations dominate the noise spectra. This finding suggests that two level fluctuations contribute to the broadening of the gap features. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z10.00006: Scanning SQUID measurements of the superconducting state of $\delta$-doped SrTiO$_3$ heterostructures Julie A. Bert, Minu Kim, Chris Bell, Harold Y. Hwang, Kathryn A. Moler The discovery of interface superconductivity in complex oxide heterostructures has generated significant excitement. We used scanning SQUID microscopy to investigate the magnetic properties of one such heterostructure, $\delta$-doped structures in SrTiO$_3$ thin films. We have observed a diamagnetic response and imaged vortices providing further evidence of a two-dimensional superconducting state. Finally we measured the magnetic susceptibility from which we observe spatial inhomogeneities in the superconducting response and can estimate the temperature dependence of the magnetic penetration depth. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z10.00007: Probing Transient Structures during interfacial charge transfer mimicking solar cells and heterogeneous catalysis Lin Chen, Xiaoyi Zhang, Grigory Smolentsev, Jianchang Guo, Klaus Attenkofer, Andrew B. Stickrath, Di-Jia Liu, Nosheen Gothard Photoinduced charge transfer at interfaces is a key process in photocatalysis and dye sensitized solar cells (DSSCs). Using X-ray transient absorption (XTA) spectroscopy, we extracted metal center surrounding transient structural information in a DSSC mimic, namely the RuN3 dye on the TiO$_{2}$ nanoparticle surfaces. Structural evolution of the adsorbed dye sensitizer and the rearrangement of the nanocrystal surface associated with the electron density shift during and after the interfacial charge injection were investigated. The other interfacial charge transfer system is a suspension of Pt nanoparticles on TiO$_{2}$ where the photoexcitation induces redox reactions and generate hydrogen fuel. The preliminary XTA results demonstrate the feasibility of the method in probing heterogeneous catalytic systems. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z10.00008: Composition of CuAu alloy and changes upon corrosion studied by Hard X-ray Photoelectron Spectroscopy Parasmani Rajput, Blanka Detlefs, Ajay Gupta, Dieter Kolb, Jorg Zegenhagen Metals and their alloys are highly susceptible to corrosion in wet environment. Dealloying is a particular type of corrosion, attacking practically all metals in industrial use: When an alloy is coming into contact with an electrolyte, the less noble metal may go into solution, typically causing crack formation and subsequent material failure upon stress. We used bulk sensitive Hard X-ray photoelectron spectroscopy (HAXPES) with an excitation energy of 6 keV as a new powerful tool to investigate the chemical composition of alloys and changes upon dealloying, studying Cu$_{x}$Au (with x = 4.1) alloy films of 9 to 50 nm thickness. Morphology, structure and composition were further characterized by atomic force microscopy, X-ray reflectivity and quantitative X-ray fluorescence. The HAXPES analysis revealed that chemical shifts of metal core-levels, i.e. Au4$f$ and Cu3$s$, can be used as a benchmark for the alloy composition. HAXPES as a function of electron emission angle allowed depth sensitive determination of the chemical composition before and after dealloying in sulphuric acid. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z10.00009: X-Ray Characterization of Electrodeposited Alloy Thin Films Nicholas Wozniak, Daniel McNeel, Alyssa Frey, Jennifer Hampton Particle-Induced X-ray Emission (PIXE) was used to measure the composition of electrodeposited NiFe and NiFeCu thin films. The thin films were deposited on gold-plated silicon wafers, and PIXE spectra were analyzed to obtain the Ni, Fe, Cu, and Au content in each sample. By comparing the Ni, Fe, and Cu content in a sample to the Au content in the same sample, the relative amounts of deposited material between samples could be measured. The effect of the deposition solution, deposition parameters and duration of deposition was explored. The results were compared to those measured with Energy Dispersive Spectroscopy. The results show that PIXE can measure the total deposited metal in a sample over at least four orders of magnitude with similar fractional uncertainties. The technique is also sensitive enough to observe the variations in alloy composition due to sample non-uniformity or variations in deposition parameters. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z10.00010: Thermal conductance of interfaces with molecular layers - low temperature transient absorption study on gold nanorods supported on self assembled monolayers Wei Wang, Jingyu Huang, Catherine Murphy, David Cahill While heat transfer via phonons across solid-solid boundary has been a core field in condense matter physics for many years, vibrational energy transport across molecular layers has been less well elucidated. We heat rectangular-shaped gold nanocrystals (nanorods) with Ti-sapphire femtosecond pulsed laser at their longitudinal surface plasmon absorption wavelength to watch how their temperature evolves in picoseconds transient. We observed single exponential decay behavior, which suggests that the heat dissipation is only governed by a single interfacial conductance value. The ``RC'' time constant was 300ps, corresponding to a conductance value of 95MW/m$^{2}$K. This interfacial conductance value is also a function of ambient temperature since at temperatures as low as 80K, which are below the Debye temperature of organic layers, several phonon modes were quenched, which shut down the dominating channels that conduct heat at room temperature. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z10.00011: Contact voltage-induced softening transition of gold-on-gold contacts at cryogenic temperatures Diana Berman, Matthew Walker, Jacqueline Krim A series of experiments were performed in vacuum environments to investigate the impact of contact voltage on the surface softening transition for gold-on-gold contacts at cryogenic temperatures [1]. The purpose of this work was twofold: (1) To examine whether asperity heating models already validated for high temperature contacts were also applicable at cryogenic temperatures, and (2) to explore the implications and validity of prior suggestions that contact temperatures between 338 and 373 K are high enough to dissociate adsorbed film and/or push them aside, but low enough to prevent asperities from becoming soft and adherent. Measurements on two distinct RF MEMS switch types were performed in the temperature range 79 - 293K and for contact voltages ranging from 0.01 to 0.13 V. Contact resistance values at all temperatures were observed to be lower for higher contact voltages associated with increased heating and softening effects. In-situ removal of adsorbed species by oxygen plasma cleaning resulted in switch adhesive failure. Switches that had not been cleaned meanwhile exhibited distinct reductions in resistance at contact temperatures close to 338 K, consistent with suggestions that films begin to desorb, disassociate, and/or be pushed aside at that temperature. Funding agencies: AFOSR, DARPA, and NSF DMR. [1] D. Berman, M. Walker, J. Krim, J. Appl. Phys., 108, 044307 (2010). [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z10.00012: Reduction of a polar oxide surface in a strong DC-field Wolfram Steurer, Svetlozar Surnev, Giovanni Barcaro, Alessandro Fortunelli, Falko P. Netzer Polar oxide surfaces are of fundamental scientific interest because of their inherent instability in bulk samples on electrostatic grounds. Here we report first experimental evidence of field-induced reduction of a polar oxide surface by applying homogeneous external DC-fields. Ultrathin Ni-oxide nanostructures immersed into an Ag(100) substrate have been grown by reactive evaporation and have subsequently been exposed to electric fields in the range of 0.5-1.6 V/nm. We achieve such high fields in a setup resembling a plate capacitor where the Ag(100) substrate (with the deposited NiO film) acts as the cathode with a counter electrode placed 800nm apart. For fields exceeding the threshold of 0.9 V/nm, oxygen atoms are torn away from the surface, thus, efficiently reducing the initially highly-ordered Ni-oxide film. The remaining Ni atoms on the surface are highly mobile and cluster together. No oxide reduction occurs if the field polarity is inverted. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z10.00013: Interference between Surface and Bulk Scattering in Nanoscale Conductors Swarbhanu Chatterjee, Alexander Meyerovich We analyze the quasiclassical and quantum interference between the bulk and boundary scattering channels in thin high quality films. The effective transport time is calculated beyond the Matthiessen's approximation as an expansion in inverse bulk mean free path. The interference corrections to resistivity exhibit a crossover between two regimes that are characterized by distinct dependences on temperature and/or impurity concentration. In our quasiclassical approximation the results reduce to a transparent analytical expression. We discuss differences between quantum and quasiclassical approaches and experimental implications of our results. [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z10.00014: Quantitative Transmission Electron Microscopy of Nanoparticles and Thin-Film Formation in Electroless Metallization of Polymeric Surfaces Aniruddha Dutta, Helge Heinrich, Stephen Kuebler, Chris Grabill, Aniket Bhattacharya Gold nanoparticles(Au-NPs) act as nucleation sites for electroless deposition of silver on functionalized SU8 polymeric surfaces. Here we report the nanoscale morphology of Au and Ag nanoparticles as studied by Transmission Electron Microscopy (TEM). Scanning TEM with a high-angle annular dark-field detector is used to obtain atomic number contrast. From the intensity-calibrated plan-view scanning TEM images we determine the mean thickness and the volume distribution of the Au-NPs on the surface of the functionalized polymer. We also report the height and the radius distribution of the gold nanoparticles obtained from STEM images taking into consideration the experimental errors. The cross sectional TEM images yield the density and the average distance of the Au and Ag nanoparticles on the surface of the polymer. [Preview Abstract] |
Session Z13: Granular Materials II
Sponsoring Units: GSNP DFDChair: Arshad Kudrolli, Clark University
Room: D225/226
Friday, March 25, 2011 11:15AM - 11:27AM |
Z13.00001: Perfect fluid flow from the impact of a dense granular jet Wendy W. Zhang, Jake Ellowitz, Nicholas Guttenberg, Herve Turlier, Sidney R. Nagel Axisymmetric collision of a cylindrical water jet with a circular target generates a thin conical sheet, also known as a water bell [Cheng et al. Phys. Rev. Lett. 99, 2007]. Intriguingly, recent experiments on granular jet impact in the regime of dense inertial flow reveal similar behavior: the angles by which the collimated sheets of particles are ejected from the target agree closely with the angles measured in the water-bell experiments [Clanet, C. J. Fluid Mech. 430, 2001] . This quantitative correspondence suggests that the collective granular motion during impact can be modeled as an incompressible, continuum fluid. Since viscous effects are weak in water-jet impact and the granular jet is comprised of non-cohesive particles (hence possessing zero surface tension), the simplest scenario is that the continuum motion corresponds to the flow of a perfect fluid. We show an exact solution of 2D perfect fluid impact agrees quantitatively with 2D discrete-particle simulation results. Therefore, the emergence of a highly collimated outgoing sheet does not necessarily signal the creation of a thermodynamic liquid phase. Such a coherent outcome results generically when the motion is nearly incompressible and dominated by inertia. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z13.00002: Jet-Induced Granular 2-D Crater Formation with Horizontal Symmetry Breaking Abe Clark, Robert Behringer We investigate the formation of a crater in a 2-D bed of granular material by a jet of impinging gas, motivated by the problem of a retrograde rocket landing on a planetary surface. As the strength and height of the jet are varied, the crater is characterized in terms of depth and shape as it evolves, as well as by the horizontal position of the bottom of the crater. The crater tends to grow logarithmically in time, a result which is common in related experiments. We also observe an unexpected horizontal symmetry breaking at certain well-defined conditions. We present data on the evolution of these asymmetric states and attempt to give insights into the mechanism behind the symmetry-breaking bifurcation. This horizontal symmetry breaking is highly suggestive of a pitchfork bifurcation, and we give evidence to classify it as forward or backward in different regimes of operation. As we will demonstrate, the formation of an asymmetric crater could be of considerable practical concern for lunar or planetary landers, particularly in the case of a backward pitchfork bifurcation, which is characterized by hysteresis and very rapid transitions. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z13.00003: Simulations of granular jet impact deadzone formation Nicholas Guttenberg, Jake Ellowitz, Wendy Zhang, Herve Turlier, Sidney Nagel Motivated by granular experiments showing the emergence of continuum-like dynamics when a dense jet hits a target, we simulate the impact of 2D and 3D granular jets of frictional, cohesion-less grains upon a fixed target. This is a inertial, dense jet regime where the motion is essentially incompressible. Impact deflects the material in the jet into a hollow conical sheet. The cone angles measured in simulation are consistent with previous experimental studies of the 3D granular jet impact. In addition, experiments have revealed the formation of a ``dead zone,'' a region where the grain motion is negligibly small. The simulation shows that this dead zone can only form when a no-slip boundary condition is enforced at the target. The presence or absence of the dead zone leads to a change in cone angle consistent with the experimentally observed differences in cone angle between the 3D granular flow and the corresponding water bell flow. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z13.00004: Endless penetration in impact cratering J. Carlos Ruiz-Suarez, Felipe Pacheco-Vazquez, J. Manuel Solano-Altamirano, Gabriel Caballero-Robledo The phenomena of impact cratering have been in the minds of physicists at least for two decades; the reason being the interest for elucidating the intriguing rheological response produced by granular systems when they are penetrated. With the great amount of work done in this regard, one could think that the problem is reasonably well understood. However, we study here a fascinating phenomenon never observed before in granular penetration experiments: depending on the mass of a projectile colliding onto a granular bed, it either stops at a given depth like normally expected, or keeps sinking with a terminal velocity as if the medium were a newtonian fluid. Understanding this intriguing behaviour could help us to know the subtleties of intrusion phenomena in granular media. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z13.00005: Avalanches of Singing Sand in the Laboratory Simon Dagois-Bohy, Sylvain Courrech Du Pont, St\'ephane Douady The song of dunes is a natural phenomenon that has arisen travellers' curiosity for a long time, from Marco Polo to R.A. Bagnold. Scientific observations in the XXth century have shown that the sound is emitted during a shear flow of these particular grains, the free surface of the flow having coherent vibrations like a loud speaker. The sound emission is also submitted to a threshold effect with many parameters like humidity, flow speed, surface of the grains. The sound has been reproduced in laboratory avalanche experiments close to the natural phenomenon on field, but set in a channel with a hard bottom and a few centimeters of sand flowing, which contradicts explanations of the sound that involve a sand dune under the avalanche flow. Flow rates measurements also show the presence of a plug region in the flow above the sheared band, with the same characteristic length as the coherence zones of the sound. Finally we show experimentally that the Froude number, once modified to take into account the height of this plug band, is the parameter that sets the amplitude of the sound, and produces a threshold that depends on the grain type. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z13.00006: Shear strength of vibrated granular/granular-fluid mixtures Brian Utter, Ralph Herman, Ben Foltz The behavior of dense granular materials can be characterized by the continuous forming and breaking of a strong force network resisting flow. This jamming/unjamming behavior is typical of a variety of systems and is influenced by factors such as grain packing fraction, applied shear stress, and the random kinetic energy of the particles. We present experiments on shear strength of granular and granular-water mixtures under the influence of external vibrations, one parameter that leads to unjamming. We use low vibration ($<$ 1g) and slow shear and measure avalanching statistics in a rotating drum and the torque required to move a stirrer through a sand/water mixture. We find that external vibration (i) increases granular strength at small vibrations in the dry system, (ii) removes history dependence (memory), and (iii) decreases shear strength at all accessible saturation levels in the sand-fluid system. Additionally, shear strength is found to be smallest for both dry and completely saturated mixtures. Additional ongoing experiments probe beyond a dimensionless acceleration of 1 and explore jamming and surface chemistry effects in the avalanching flow of granular/fluid mixtures. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z13.00007: Collisions between solitary waves in granular alignments Surajit Sen, Diankang Sun Solitary waves arise naturally when an unloaded alignment of elastic spheres, that is held between fixed end walls, is perturbed at one end. Unlike most known classes of solitary waves, those in granular materials are special and tend to break down and reform during any collision. Here we present what happens when two solitary waves of unequal magnitude suffer head-on and overtaking types of collisions. We will show that these collisions provide ways for solitary waves to not only become smaller but also become larger (within bounds) and that they are the underlying reason behind the emergence of the quasi-equilibrium phase. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z13.00008: Oil in Water Emulsion Flow in a 2D Hopper Xia Hong, Dandan Chen, Kenneth Desmond, Eric Weeks Granular flows are still somewhat poorly understood. One such case is the flow of 2D disks through a hopper. In a~prior experiment~by To~(K. To, et al. PRL 86(1) 2001), they found that as 2D disks flow through a hopper they may jam due to arch formations~at the hopper~exit, and that the jamming probability can be increased by enhancing the static friction between the disks.~In our study we remove the effects of static friction by using quasi-2D oil in water emulsion droplets flowing through a hopper to understand the role of friction in jamming. The droplets feel a viscous friction, but no static friction. Similar to the granular experiment, our oil droplets flow due to gravity. We have observed the transition between jammed and unjammed flows in our setup, and we are currently investigating its nature as the hopper size changes. In our experiments, jamming seems to occur only for very small hopper openings, and arches are always unstable. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z13.00009: Water Retention of Mixed Hydrogel Particles and Sandy Soil Yuli Wei, Douglas Durian We study the water-holding capacity of mixed hydrogel particles and a model sandy soil. To probe static behavior, we develop a custom pressure plate method that measures the expelled water per unit pressure increment per unit cross-sectional area; results are analyzed in terms of the water-accessible pore areas in the granular packing. To probe dynamic behavior, we build a raindrop impingement set-up that measures the retained water inside a dry granular packing during steady rain at a fixed rate. The percentage saturation of the granular packing is deduced. In both studies, we first determine the influence of the packing height and then of the gel concentration and size. Results from pressure plate method show that the swollen hydrogel particles partially clog the pores in the sandy soil, so that less water could be expelled for a given pressure increment. The total water-accessible area determined from the expelled water curve decreases exponentially as the gel concentration increases. Large hydrogel particles are less efficient in clogging the pores when no extra confinement is applied on the packing. Results from the raindrop impingement measurements also show that the water-holding capacity of sandy soil is improved by addition of hydrogel particles. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z13.00010: Granular ``electrophoresis'': in situ measurement of charge and size of freely-falling grains Scott Waitukaitis, Gustavo Castillo, Sebastian Gonzalez, Heinrich Jaeger We present measurements of tribocharged, chemically identical grains falling from a hopper. Tribocharging is the transfer of electrical charge between contacting surfaces. Granular interactions are governed by contacts, and not-surprisingly tribocharging can have important effects on bulk granular behavior. What is surprising is that this occurs even in grains of the same material. Typically same chemistry tribocharging (SCT) correlates with the particle size distribution: larger particles charge positively and smaller particles negatively. However, the detailed mechanism of SCT remains elusive. We have developed an experimental technique to make \emph{in situ} measurements of the particle size and charge on small ($\sim$100-500 $\mu$m) grains. With high speed videography of freely-falling grains we resolve particle sizes down to a few microns, charges as small as a few thousand electrons, and forces as small as a few picoNewtons. Our results confirm the qualitative charge segregation observed in previous SCT experiments and provide quantitative measurement for theoretical comparison. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z13.00011: Strain-stiffening in random packings of granular chains Heinrich Jaeger, Alice Nasto, Dylan Murphy, Eric Brown We report on triaxial compression experiments performed to characterize the mechanical response of random packings of granular particles. For a wide variety of particle shapes, the packings yield when the shear stress exceeds a value on the order of the confining pressure. In contrast, granular chains consisting of flexibly connected beads exhibit strain stiffening (i.e., the effective modulus increases with strain), sustain stresses far beyond the confining pressure, and do not yield until the chains break. The critical chain length required for significant strain-stiffening to occur corresponds to the minimum circumference of closed loops the chains are able to form during the formation of the packings. This strain-stiffening behavior is similar to that found in polymer materials, and chain packings therefore may serve as a model system to quantify the contribution of pure entanglement effects to the strength of polymer materials in the absence of Brownian motion. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z13.00012: Anisotropic diffusion of vibrated semi-flexible granular rods Vikrant Yadav, Arshad Kudrolli We discuss the diffusive dynamics of semi-flexible granular rods as a function of their concentration in a vertically vibrated container. These rods are composed of short beaded chains and are tracked with a camera, and their trajectories used to analyze the rotational and translational displacement as a function of area fraction $\phi$. We observe that the diffusion in the parallel and perpendicular direction in the body frame of reference deviate from those calculated for thermally excited elastic polymer rods. In particular we find that the diffusion perpendicular to the major axis in dilute regime is observed to be greater than that in the parallel direction due to rotation about the major axis of the rod. The motion is observed to become sub-linear above $\phi = 0.48$ and $0.54$ in the perpendicular and parallel directions, respectively, both lower than for spherical particles. Rotational diffusion is also investigated and found to deviate systematically from exponential decays with increase in $\phi$. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z13.00013: Vibrofluidized melting of geometrically cohesive granular media Nick Gravish, Geoffrey Russell, Scott V. Franklin, David Hu, Daniel I. Goldman Dry granular media composed of particles of special shapes (e.g. long rods or c-shaped particles) can display cohesive effects through particle geometry alone. We study the solid to gas transition in piles of c-shaped particles under vertical vibration as we vary acceleration and frequency. A cylindrical solid of particles is formed with wall angles near $90^\circ$ and is placed on a solid surface. For fixed frequency as acceleration increases, the pile undergoes two transitions. The first is from the solid-like state to a liquid-like state in which the wall angles relax but the mobile particles remain spatially localized. The second is from the liquid-like state to the gaseous state in which particles become separated (not entangled). Using video and accelerometer measurements, we record the temporal evolution of the spatial density and pile-plate collisional impulse. A critical energy scale, set by the particle geometry and gravitational potential energy, governs the liquid-gas transition. [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z13.00014: Switchable capillary bridges in sphere packings Christoph G\"ogelein, Martin Brinkmann, Matthias Schr\"oter, Stephan Herminghaus If one adds a small amount of water to a heap of sand, it becomes paste-like since the grains get interconnected by capillary bridges. Due to this effect, we can easily sculpture wet sand (e.g., building a sand castle), whereas a heap of dry grains ripples away and cannot sustain any shape. In the present work, we use a non-Brownian suspension of glass spheres immersed in a binary liquid mixture. The suspending water-lutidine mixture exhibits a well studied lower critical point slightly above ambient temperature. Hence, the mixture starts to phase separate upon heating. Since the water-rich phase wets the hydrophilic glass spheres, capillary bridges are formed between adjacent particles. If the system is cooled below the demixing temperature, the bridges disappear within a few seconds by intermolecular diffusion. Thus, this systems offers the opportunity to switch the capillary bridges on and off by altering the temperature. In this presentation, we will show the temperature-induced formation of capillary bridges using confocal and bright light microscopy [1]. Furthermore, we will discuss the effect of capillary bridges on random sphere packings using a fluidized bed setup. \\[4pt] [1] C. G\"ogelein, M. Brinkmann, M. Schr\"oter, and S. Herminghaus, Langmuir 26 (2010) 22, 17184. [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z13.00015: Stable Solitary Waves in Granular Alignments Yoichi Takato, Surajit Sen We study the propagation of an impulse in a loaded chain of elastic spheres where the spheres are held between fixed walls. We show that for a certain critical loading, propagating impulses develop into solitary waves and these solitary waves are not measurably affected by wall collisions, the latter being typically the case with granular solitary waves. The properties of these special solitary waves and of possible connections between this problem and the Fermi-Pasta-Ulam problem will be addressed. [Preview Abstract] |
Session Z14: Focus Session: Statistical Mechanics of Complex Networks III
Sponsoring Units: GSNPChair: Adilson Motter, Northwestern University
Room: D227
Friday, March 25, 2011 11:15AM - 11:27AM |
Z14.00001: Frequency of Relevant Nodes with Different Function Classes in Critical Boolean Networks Shabnam Hossein, Matthew Reichl, Kevin E. Bassler Boolean networks have two phases of dynamical behavior, fixed and chaotic, depending on the update functions of the nodes. Boolean functions can be categorized by their symmetry properties, which are related to their canalization properties. Canalization is a type of network robustness, which was first introduced to explain the stability of phenotype expression of biological systems. For networks with 3 inputs per node, the 256 possible Boolean functions can be divided into 14 classes that correspond to the group orbits of rotation plus parity. For critical networks at the boundary of the fixed and chaotic phases, we analytically derive the frequency of the different types of Boolean functions among the relevant nodes that control the dynamics. By setting up a set of differential equations that determines the relevant nodes through a pruning process, we can find the average number of nodes in each of the classes. Then, considering the effects of fluctuations, the probability distribution of the number of relevant nodes is accurately derived. We find that in critical networks the frequency of relevant nodes is inversely correlated with canalization. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z14.00002: Two-Component Diffusion Anna Stephenson, Jayanta Rudra When two diffusing components propagate through the same material, the space that is occupied by one component may not be occupied by the other. This interaction, which is purely geometrical and non-chemical in nature, plays an important role in the dynamics of two-component flow. It redefines the diffusing space for each of them and each component sees its surrounding space being constrained by the other. We use a novel approach to describe the diffusion of two components through a discrete lattice of a narrow channel. Specially, we look at the influence of a fast component on a slow one and vice versa. We express the time evolution of the joint probability distribution of two diffusing components in terms of a modified Master equation such that both of these may not occupy the same lattice site at the same time. From this restricted time evolution of the joint probability distribution we then calculate relative flow rate of the component and infer whether such a channel could be used as a molecular sieve to separate a slow component from a fast one. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z14.00003: Backbones and borders from shortest-path trees Daniel Grady, Christian Thiemann, Dirk Brockmann One of the most important tasks in complex network research is to distinguish between vertices and edges that are topologically essential and those that are not. To this end, a variety of vertex and edge centrality measures have been introduced, ranging from measuring local properties (degree, strength) to quantities that depend on the global structure of the graph (betweenness). Here we introduce a novel technique based on the family of shortest-path trees, which is applicable to strongly heterogeneous networks. This approach can identify significant edges in the network, distinct from conventional edge betweenness, and these edges make up a network backbone relevant to dynamical processes that evolve on such networks. We will show that important network structures can be extracted by investigating the similarity and differences of shortest-path trees and show that tree dissimilarity in combination with hierarchical clustering can identify communities in heterogeneous networks more successfully than ordinary reciprocal-weight distance measures. We demonstrate the success of this technique on complex multi-scale mobility networks. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z14.00004: Interactive Network Exploration using Shortest-Path-Tree Tomography Christian Thiemann, Dirk Brockmann The shortest-path tree of a node contains information on the whole network as observed from a specific node, thus combining local and global information in a two- dimensionally embeddable sub-network. We developed new visualization software that reduces a complex network to its nodes' shortest-path trees and allows for interactive exploration of this network in a structured way. In this talk I will present various example networks and also briefly talk about off-spring projects that have been sparked by looking at networks in this way, including a simplified view on disease spreading on networks. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z14.00005: Searching for the extreme values of the mutual information between two interacting subsystems Ilya Grigorenko, Vincent Crespi We have considered two interacting subsystems represented by classical spins with a long-range interaction immersed in a thermal reservoir. We searched for maxima and minima of the mutual information between the subsystems by tuning the interaction parameters within only one subsystem: the parameters of the second subsystem - which can be thought of as an environment for the first one - and the interaction parameters between the first subsystem and the environment remain unchanged. We have identified the conditions leading to maximisation and minimisation of the mutual information between the subsystems, and their relation to a degeneracy of the energy spectrum that is spontaneously engineered by the optimisation procedure. We interpret the spatially inhomogeneous structure of the optimised subsystem in terms of information heterogeneity. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z14.00006: Synchronization Dynamics of Coupled Anharmonic Plasma Oscillators John Laoye, Uchechukwu Vincent, Taiwo Roy-Layinde The synchronization of two identical mutually driven coupled plasma oscillators modeled by anharmonic oscillators was investigated. Each plasma oscillator was described by a nonlinear differential equation of the form: $\ddot{x} + \epsilon (1 + x^2)\dot{x} + x + \kappa x^2 + \delta x^3 = F \cdot \cos(\omega t)$. The model employed the spring-type coupling. Numerical simulations, including Poincar\'{e} sections, time series analysis, and bifurcation diagram were performed using the fourth-order Runge-Kutta scheme. The numerical value of the threshold coupling K$_{th}$ was determined to be approximately 0.15. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 1:03PM |
Z14.00007: Visual analytics for discovering node groups in complex networks Invited Speaker: Given the abundance of relational data from a variety of sources, it is becoming increasingly more important to be able to discover hidden structures in the topology of real-world complex networks. In this talk, I will extend the usual definition of groups as densely connected sets of nodes and show that many real networks have groups distinguished by a diverse combinations of node properties, but not by the density of links alone. To overcome the virtually unlimited ways to potentially distinguish groups, we have developed an \textbf{exploratory} analysis tool that exploit human visual ability. In this visual analytical approach, the user input from \textbf{visual interaction} is integrated into the analysis to discover unknown group structures, rather than simply detecting a known type of structure. I will also address the problem of determining an appropriate number of groups, when it is not known \textit{a priori}. I will demonstrate that our method can effectively find and characterize a variety of group structures in model and real-world networks, including community and $k$-partite structures defined by link density, as well as groups distinguished by combinations of other node properties. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z14.00008: A Network Approach to Rare Disease Modeling Susan Ghiassian, Sabrina Rabello, Amitabh Sharma, Olaf Wiest, Albert-Laszlo Barabasi Network approaches have been widely used to better understand different areas of natural and social sciences. Network Science had a particularly great impact on the study of biological systems. In this project, using biological networks, candidate drugs as a potential treatment of rare diseases were identified. Developing new drugs for more than 2000 rare diseases (as defined by ORPHANET) is too expensive and beyond expectation. Disease proteins do not function in isolation but in cooperation with other interacting proteins. Research on FDA approved drugs have shown that most of the drugs do not target the disease protein but a protein which is 2 or 3 steps away from the disease protein in the Protein-Protein Interaction (PPI) network. We identified the already known drug targets in the disease gene's PPI subnetwork (up to the 3rd neighborhood) and among them those in the same sub cellular compartment and higher coexpression coefficient with the disease gene are expected to be stronger candidates. Out of 2177 rare diseases, 1092 were found not to have any drug target. Using the above method, we have found the strongest candidates among the rest in order to further experimental validations. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z14.00009: Flavor network and the principles of food pairing Yong-Yeol Ahn, Sebastian Ahnert, James Bagrow, Albert-Laszlo Barabasi We construct and investigate a flavor network capturing the chemical similarity between the culinary ingredients. We found that Western cuisines have a statistically significant tendency to use ingredient pairs that share many flavor compounds, in line with the food pairing hypothesis used by some chefs and molecular gastronmists. By contrast, East Asian cuisine tend to avoid compound sharing ingredients. We identify key ingredients in each cuisine that help us to explore the differences and similarities between regional cuisines. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z14.00010: Relationship between structural and functional networks in complex systems with delay Toni Perez, Victor Eguiluz, Javier Borge-Holthoefer, Alex Arenas Functional networks of complex systems are usually obtained from the analysis of the temporal activity of their components, and are often used to infer their unknown underlying connectivity. Here we investigate on this challenge from a fundamental physical perspective, analyzing the functional network resulting from the simplest dynamical system with delay presenting a synchronous dynamics on a given topology. We have found the conditions for the emergence of locked dynamical states and the equations relating topology and function in a system of diffusively delay-coupled elements in complex networks. We solve exactly the resulting equations in motifs (directed structures of three nodes), and in directed networks. The mean-field solution for directed uncorrelated networks shows that the clusterization of the activity is dominated by the in-degree of the nodes, and that the locking frequency decreases with increasing average degree. We find that the exponent of a power law degree distribution of the structural topology, $\gamma$, is related to the exponent of the associated functional network as $\alpha =(2-\gamma)^{-1}$, for $\gamma < 2$. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z14.00011: Why hubs may not be the most efficient spreaders Lazaros Gallos, Maksim Kitsak, Shlomo Havlin, Fredrik Liljeros, Lev Muchnik, H.E. Stanley, Hernan Makse The origin of a spreading process in a complex network can drastically influence the extent of the area that spreading can reach. In principle, the network hubs should be the most efficient spreaders. Here, we find that, in contrast to common belief, there are plausible circumstances where the best spreaders do not correspond to the best connected nodes or to the most central nodes (high betweenness centrality). Using the SIR model we find that: {\it (i)} The most efficient spreaders are those located within the core of the network as identified by the $k$-shell decomposition analysis. {\it (ii)} When multiple spreaders are considered simultaneously, the distance between them becomes the crucial parameter that determines the extent of the spreading. Similarly, we find that, in the SIS model, infections persist in the high $k$-shells of the network. Our analysis provides a plausible route for an optimal design of efficient dissemination strategies. [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z14.00012: Finite size scaling theory for discontinuous percolation transitions B. Kahng, Y.S. Cho, S.W. Kim, J.D. Noh, D. Kim Finite-size scaling (FSS) theory has been useful for characterizing phase transitions. When the phase transition is continuous, the critical behavior of a system in the thermodynamic limit can be extracted from the size-dependent behaviors of thermodynamic quantities. However, FSS approach for discontinuous transitions arising in disordered systems has not been studied yet. Here, we develop a FSS theory for the discontinuous PT in the modified Erd\"os-R\'enyi model under the Achlioptas process. A scaling function is derived based on the observed fact that the derivative of the curve of the order parameter at the critical point $t_c $ diverges with system size in a power-law manner, which is different from that for continuous percolation transitions. Numerical simulation data for different system sizes are well collapsed onto a scaling function. [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z14.00013: Monte Carlo Simulations of Metastable Decay in the Ising Model on the Hyperbolic Plane Howard L. Richards, Mallory A. Price, Julie E. Lang Consider a regular network of Ising spins with short-ranged, ferromagnetic interactions and a weak, negative magnetic field. The system evolves under single-spin-flip Metropolis dynamics from an initial state of all spins ``up'' ($s_i = +1$, $\forall i$). For Euclidean networks in less than 6 dimensions, decay from the ``metastable'' state occurs in a finite time (measured in Monte Carlo steps per spin) through the nucleation and growth of one or more finite critical droplets. For networks on the hyperbolic plane, however, we show that the size of a critical droplet diverges at a nonzero magnetic field -- the spinodal field. We then use Monte Carlo simulations on the $\{5, 4\}$ grid to demonstrate the divergence of the lifetime of the metastable state at nonzero spinodal fields. [Preview Abstract] |
Session Z22: Properties of Semiconducting Nanosystems
Sponsoring Units: DCMPChair: Stefan Badescu, Air Force Research Laboratory, Dayton, Ohio
Room: D163
Friday, March 25, 2011 11:15AM - 11:27AM |
Z22.00001: ABSTRACT WITHDRAWN |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z22.00002: Conductivity of Thin Film Structures Fabricated by E-Beam Lithography from Gold Nanoparticle Resists Stefan Dickert, Myoung-Hwan Park, Colin Jermain, Vincent M. Rotello, Mark T. Tuominen Drop- and spin-coated solutions of ligand coated nanoparticles act as a novel ``direct write'' e-beam resist, which can be prepared with metallic, magnetic and semiconducting nanoparticles (Y.Ofir. et. al, Adv. Mater. 20, 2561-2566 (2008)). We prepared thin films from gold nanoparticles in which we varied the ligand length, ligand type and the film thickness. Small angle X-Ray scattering experiments as well as SEM imaging of the samples were performed to determine structural properties of the nanoparticle films at various stages of the fabrication process, after drop coating, ebeam exposure and annealing. We further performed resistance measurements in the 2-350K temperature range and report different conductivity mechanisms based on the ligand type and film thickness, ranging from insulating to Mott hopping conduction to metallic. We observed different results for Thioalkylated trimethyl ammonium (TMA) and thioalkyl tetra (ethylene glycol)ated trimethyl ammonium (TTMA) ligands. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z22.00003: Terahertz Ionization of Highly Charged InGaAs Quantum Posts Christopher Morris, Dominik Stehr, Tuan-Anh Truong, Hyochul Kim, Pierre Petroff, Mark Sherwin, Craig Pryor Quantum posts (QPs) are quantum dot based nanostructures grown by MBE. They form short In$_{.4}$Ga$_{.6}$As cylinders embedded in an In$_{.1}$Ga$_{.9}$As quantum well (QW). Terahertz absorption measurements are performed on 30 nm high QPs with $\sim $6 electrons per post and a charge density of $\sim $2.4x10$^{11}$/cm$^{2}$ in the surrounding well. Comparison of spectra from QP and reference QW samples shows an absorption feature due to the QPs absent in the QW sample. Temperature dependent measurements show this absorption is due to electrons in the QPs. 8 band k.p calculations of post and well energies are performed as a function of the number of electrons in the posts, and the absorption is determined to be due to an ``ionizing'' transition from the posts to the well. The highest filled QP state absorbs a terahertz photon, transitioning to a weakly bound unfilled post state $\sim $20 meV higher. From there, the electron quickly scatters into the two dimensional electron gas in the quantum well matrix. Coulomb repulsion from QP electrons locally depopulates the quantum well states, leaving open states for the QP electrons. QPs represent a promising structure for investigation of Coulomb blockade physics and ionizing transitions in artificial atom systems. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z22.00004: The dipole moments of ZnO nanorods Sefa Dag, Shuzhi Wang, Lin-Wang Wang A self-consistent linear scaling three dimensional fragment (LS3DF) method is used to study the dielectric properties of large ZnO nanorods. Our ab initio calculations show that the ZnO nanorod with unpassivated (10-10) side surface has a large dipole moment which is caused by both surface and interior bulk dipoles. An systematic analysis is carried out, and we found that the biggest contribution to the total dipole moment is from the (10-10) surface. Dielectric screening model is used to illustrate how the dipole moment changes with nanostructure size and geometry. We also show the effect of the dipole moment on the interior electronic structure of the nanorod. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z22.00005: Natural interface states in coherent and isovalent III-V heterostructures Voicu Popescu, Alex Zunger Interface states occur in semiconductor heterojunctions whenever a significant perturbation is present, caused by interface defects, lattice mismatch, discontinuities in the effective mass or sharp variations in the potential across the interface. We discuss the natural interface states appearing in perfectly coherent and isovalent III-V heterojunctions when a $\Gamma$-well and an $X$-anti-well coexist in the conduction band. We use empirical pseudopotential calculation to illustrate this type of states for a few III-V heterostructures. For InP/GaP the interface localised states lie energetically in the band-gap and possess, because of their mixed $\Gamma-X$ character, a strong optical signature. This allows us to provide a different interpretation of the photoemission data existent in the literature for InP/GaP quantum wells and dots. We further discuss the presence of the interface localised states in other III-V heterojunctions, investigating the conditions under which they might be experimentally observed. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z22.00006: Electronic Phase Diagram of Single-Element Silicon ``Strain'' Superlattices Zheng Liu, Wenhui Duan, Jian Wu, Max Lagally, Feng Liu The evidence that the band gap of Si changes significantly with strain suggests that by alternating regions of strained and unstrained Si one creates a single-element electronic heterojunction superlattice (SL), with the carrier confinement defined by strain rather than by the chemical differences in conventional compositional SLs. Using first-principles calculations, we map out the electronic phase diagram of a one- dimensional pure-silicon SL. It exhibits a high level of phase tunability, e.g., tuning from type I to type II. Our theory rationalizes a recent observation of a strain SL in a Si nanowire and provides general guidance for the fabrication of single-element strain SLs. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z22.00007: Quantum Boundary Effect in Nanomaterials: Undo the Quantum Size Effect by Surface Passivation of Silicon Nanofilms Yiyang Sun, Xin Liu, Shengbai Zhang It is well known that when the size of a semiconductor is reduced, its band gap will increase due to the increased kinetic energy of the electrons and holes. However, first-principles calculations reveal that there should also be a quantum boundary effect (QBE), which can drastically change the band gap to the extent that the quantum size effect (QSE) is completely erased. It is found that, for Si(001) nanofilms, surface passivations could show such a strong QBE. While the films are passivated by hydrogen, they show a clear QSE with significant increase in band gap. When some of the hydrogen atoms are replaced by =NH ligands, however, the band gap recovers to that of bulk silicon even for film size as small as two nanometers. The concept of zero confinement state for semiconductors will be introduced. It elucidates why it is possible to remove the seemingly universal QSE. The finding here could be highly desirable for certain applications of nanostructured semiconductors where gap increasing due to QSE is detrimental. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z22.00008: Excitation energy dependence of the exciton inner ring Yuliya Kuznetsova, Jason Leonard, Leonid Butov, Joe Wilkes, Alex Ivanov, Arthur Gossard We report on the excitation energy dependence of the inner ring in the emission pattern of indirect excitons. The contrast of the inner ring is found to increase with excitation energy until it reaches the direct exciton plus LO phonon energy and saturate at higher excitation energies. The data show that excitation by low-energy laser light tuned to the direct exciton resonance can effectively suppress the laser-induced heating of the exciton gas. The observed dependence is explained in terms of exciton transport and cooling. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z22.00009: Dynamic coherent exciton condensates in a semiconductor planar microcavity Chih-Wei Lai, Kyaw Zin Latt, Yi-Shan Li, Sheng-Di Lin We observed a coherent exciton state with a long decay time $\sim $1000 ps in a semiconductor planar microcavity structure. The lifetime of cavity polariton condensates previously reported has been limited to $\sim $10 ps. The sample consists of InGaAs quantum wells positioned near anti-nodes of the photon field in a GaAs $\lambda $-cavity sandwiched by GaAs/AlAs-based Bragg mirrors. Under a pulsed excitation above the stop-band of the Bragg mirrors (excess energy $>$150 meV), spatially coherent exciton emissions were observed to last for $\sim $1 ns. Conventional dynamic exciton-polariton condensates with a $\sim $10 ps lifetime were observed under a near-resonant (excess energy $\sim $ 6 meV) ps pulsed excitation. Dynamics of spatial coherence, energy relaxation, and spin polarization were characterized by time-resolved spectroscopies, including double-slit and polarimetry experiments. The fluctuation of excitonic emissions was characterized by a photon-correlation measurement. The existence of such a long-lived coherent exciton state is attributed to formation of dark excitons under an excitation with significant excessive energy. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z22.00010: Amplitude oscillations in a non-equilibrium polariton condensate Richard Brierley, Peter Littlewood, Paul Eastham Like cold atomic gases, semiconductor nanostructures provide new opportunities for exploring non-equilibrium quantum dynamics. In semiconductor microcavities the strong coupling between trapped photons and excitons produces new quasiparticles, polaritons, which can undergo Bose-Einstein condensation. Quantum quenches can be realised by rapidly creating cold exciton populations with a laser [Eastham and Phillips, PRB 79 165303 (2009)]. The mean field theory of non-equilibrium polariton condensates predicts oscillations in the condensate amplitude due to the excitation of a Higgs mode. These oscillations are the analogs of those predicted in quenched cold atomic gases and may occur in the polariton system after performing a quench or by direct excitation of the amplitude mode. We have studied the stability of these oscillations beyond mean field theory. We show that homogeneous amplitude oscillations are unstable to decay into lower energy phase modes at finite wavevectors, suggesting the onset of chaotic behaviour. The resulting hierarchy of decay processes can be understood by analogy to optical parametric oscillators in microcavities. Polariton systems thus provide an interesting opportunity to study the dynamics of Higgs-like modes in a solid state system. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z22.00011: Coherence dynamics of a long-lived excitonic condensate in an optical microcavity Kyaw Zin Latt, Yi-Shan Li, Sheng-Di Lin, Chih-Wei Lai We report dynamics of long-range spatial coherence of an excitonic condensate with a $\sim $1000 ps life-time in a planar Fabry-Perot microcavity. The sample consists of three sets of three InGaAs(8nm)/GaAs (14nm) quantum wells positioned near anti-nodes of the photon field in a GaAs $\lambda $-cavity sandwiched by two GaAs/AlAs-based Bragg mirrors. Conventional dynamic exciton-polariton condensates with a $\sim $10 ps lifetime were observed under a near-resonant (excess energy $\sim $ 6 meV) ps pulsed excitation at a 50 degree incident angle. Under an excitation above the stop-band of the Bragg mirrors (excess energy $\sim $170 meV), an excitonic state with 100-$\mu $eV luminescence linewidth was observed to last for $\sim $1 ns. Coherence dynamics were characterized by time-resolved double-slit experiment in a confocal geometry with a ps streak camera system as a function of excitation intensity (fluence) and temperature. The visibility of interference fringes reached above 0.3 within 40 ps and remained above 0.1 up to $\sim $1ns for a double-slit-distance of 12 $\mu $m. [Preview Abstract] |
Session Z23: Superconductivity: Thermodynamics, etc.
Sponsoring Units: DCMPChair: S.L. Budko, Ames Laboratory
Room: D165
Friday, March 25, 2011 11:15AM - 11:27AM |
Z23.00001: Evolution of ground state and upper critical field in R$_{1-x}$Gd$_x$Ni$_2$B$_2$C (R = Lu, Y): Coexistence of superconductivity and spin-glass state S.L. Bud'ko, V.G. Kogan, H. Hodovanets, S. Ran, S.A. Moser, M.J. Lampe, P.C. Canfield We report effects of local magnetic moment, Gd$^{3+}$, doping ($x \leq 0.3$) on superconducting and magnetic properties of the closely related Lu$_{1-x}$Gd$_x$Ni$_2$B$_2$C and Y$_{1- x}$Gd$_x$Ni$_2$B$_2$C series. The superconducting transition temperature decreases and the heat capacity jump associated with it drops rapidly with Gd-doping; qualitative changes with doping are also observed in the temperature-dependent upper critical field behavior, and a region of coexistence of superconductivity and spin-glass state is delineated on the $x - T$ phase diagram. The evolution of superconducting properties can be understood within the Abrikosov-Gor'kov theory of magnetic impurities in superconductors taking into account the paramagnetic effect on upper critical field and the details of the $x - T$ phase diagrams. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z23.00002: Micro-Calorimeter for Heat Capacity Studies of Sub-Microgram Superconducting Crystals Carlos Chaparro, U. Welp, L. Fang, W.K. Kwok, M. Eskildsen A steady-state ac-temperature micro-calorimeter for heat capacity measurement of sub-microgram superconducting samples is presented. It utilizes thermocouple thermometers nano-patterned onto 150 nm thick Si$_{3}$N$_{4}$ membranes. Theoretical models, mathematical relations describing the operation of the calorimeter and calibration procedures are discussed. The system achieves a resolution of 10$^{-4}$; and allows for measurements from 5 K to room temperature. The calorimeter can be continuously rotated in a split-coil magnet generating up to 8 T. To demonstrate the performance of our device we present measurements of the specific heat of single crystals of SmFeAsO$_{0.85}$F$_{0.15}$ (120 $\mu $m wide and 10 $\mu $m thick, crystal w1) and of BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ (150 $\mu $m wide and 30 $\mu $m thick). [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z23.00003: Search for quantum critical behavior in the specific heat of HTS cuprates Jeffery Tallon, Felix Barber, John Cooper, John Loram Much evidence has been accumulated suggesting the presence of a quantum critical point in the lightly overdoped regime for high-$T_{c}$ superconductors, around $p$=0.19 holes per Cu. It is however not decisive. In the neighborhood of a quantum critical point the normal-state electronic specific heat should contain a $T$ln($T)$ term. While this would be concealed by the onset of superconductivity it is in principle recoverable using the entropy balance that exists in a second-order phase transition. This paper reviews the data for Y$_{0.8}$Ca$_{0.2}$Ba$_{2}$Cu$_{3}$O$_{7-d}$ and Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+d}$ and places limits on the magnitude and doping evolution of a $T$ln($T)$ term in the specific heat across the phase diagram. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z23.00004: Thermodynamic effects in superconducting hybrid devices Laetitia Pascal, Sukumar Rajauria, Hung Nguyen, Bernard Pannetier, Frank Hekking, Herve Courtois Investigating thermal transport in hybrid superconducting nanostructures can yield a better understanding of such devices and give access to new and useful phenomena. Sub-gap biased superconducting-insulator-normal metal (SIN) junctions exhibit electron cooling, which is useful for achieving electronic temperatures below the cryostat bath temperature. We have designed an experiment to allow independent monitoring of the electron and phonon populations temperatures. An electronic cooler was studied under out-of-equilibrium conditions, in both cooling and heating regimes. The results are interpreted using a thermal model, which takes into account the electron, phonon and photon heat transfer. The Kapitza and electron-phonon couplings, along with a generalized circuit theory approach for the photon heat transfer, will be discussed. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z23.00005: Caloric determination of the anisotropic phase diagram of BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ crystals Wai-Kwong Kwok, Ulrich Welp, Carlos Chaparro, Lei Fang, Alexei Koshelev We report specific heat measurements on a series of BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ single crystals with phosphorous contents ranging from optimal doping (x$\sim $0.3, T$_{c}$ = 29.5 K) to highly overdoped (x$\sim $0.6, T$_{c}$ = 11K). We find a sharp superconducting transition at T$_{c}$ for all doping levels, a suppression of the $\Delta $C-step at T$_{c }$with increasing doping and enhanced magnetic field dependence at higher doping. The phase diagrams determined from specific heat data show a decrease of dH$_{c2}$/dT with increasing doping and a nearly constant superconducting anisotropy of $\Gamma \sim $2.5. Our results will be compared with the proposed ``universal'' scaling of $\Delta $C$_{p}$/T$_{c}$ and dH$_{c2}$/dT due to quantum criticality and non Fermi liquid behavior [1] and due to strong pair-breaking and non-magnetic interband scattering [2], respectively. \\[4pt] [1] J. Zaanen, Phys. Rev. B 80, 212502 (2009) \\[0pt] [2] V. G. Kogan, Phys. Rev. B 80, 214532 (2009) [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z23.00006: Growth of thin superconducting films and heterostructures by atomic layer deposition Jeffrey Klug, Thomas Proslier, Nicholas Becker, Jeffrey Elam, James Norem, John Zasadzinski, Michael Pellin We report the use of atomic layer deposition (ALD) to synthesize thin superconducting films and superconductor-insulator (S-I) heterostructures. ALD uses sequential self-saturating surface chemical reactions to produce uniform coatings with atomic scale control on substrates with arbitrary shape. The ALD process therefore offers the possibility of conformally coating complex shapes with precise, layered structures with tightly constrained morphology and chemical properties. Among other applications, such coatings may enable the production of superconducting radio frequency (SRF) structures with significantly better performance and yield than those obtained from bulk niobium. Furthermore, the atomic-scale thickness control afforded by ALD enables the study of superconductivity and associated phenomena in homogeneous layers in the ultra-thin film limit. In this respect, we will present results of ALD-grown Nb$_{1-x}$Ti$_x$N-based films and S-I heterostructures. Our program looks both at the metallurgy and superconducting properties of these coatings, and also their performance in working SRF structures. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z23.00007: Laser Processing on the Surface of Niobium Superconducting Radio-Frequency Accelerator Cavities Senthilraja Singaravelu, Michael Klopf, Geoffrey Krafft, Michael Kelley Superconducting Radio frequency (SRF) niobium cavities are at the heart of an increasing number of particle accelerators.~ Their performance is dominated by a several nm thick layer at the interior surface. ~Maximizing its smoothness is found to be critical and aggressive chemical treatments are employed to this end.~ We describe laser-induced surface melting as an alternative ``greener'' approach.~ Modeling guided selection of parameters for irradiation with a Q-switched Nd:YAG laser.~ The resulting topography was examined by SEM, AFM and Stylus Profilometry. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z23.00008: Charge order phase diagrams of hole-doped cuprates Robert Markiewicz, Jose Lorenzana, Goetz Seibold, Arun Bansil ``Stripe''-like phases in the cuprates can be dominated by either spin or charge fluctuations. We calculate the phase diagram of charge-order phases stabilized by a lattice distortion using Gutzwiller approximation (GA) + RPA, and compare it to the magnetic phase diagrams [1]. The stripe periodicity is determined by Fermi surface [double] nesting, and hence is very similar for charge or magnetic stripes. A detailed analysis of the susceptibility reveals that the leading charge instability in Bi2201 and Bi2212 is an electronically driven phonon soft mode associated with a ``Pomeranchuk wave.'' This instability has the pseudogap doping dependence and shares many properties with the phase seen in scanning tunneling microscopy (STM) Work supported in part by the USDOE and by a Marie Curie Grant. \\[4pt] [1] R.S. Markiewicz {\it et al.}, Phys. Rev. B{\bf 81}, 014509 (2010). [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z23.00009: Using Disorder to Detect Locally Ordered Electron Nematics via Hysteresis Erica Carlson, Karin Dahmen The interplay between charge, orbital, and lattice degrees of freedom in correlated electron systems has resulted in many proposals for new electronic phases of matter. An electron nematic breaks the point group symmetry of the host crystal, often from $C_6$ or $C_4$ rotational symmetry to $C_2$. Electron nematics have been reported in several condensed matter systems including cuprate and iron arsenic based high temperature superconductors, and they have been proposed to exist in many other materials. However, the combination of reduced dimensionality and material disorder typically limits the spatial range over which electron nematic order persists, rendering its experimental detection extremely difficult. Despite the tantalizing possible connection between the phase and high temperature superconductivity, there is surprisingly little guidance in the literature about how to detect the remaining disordered electron nematic. We propose a general method for detecting disordered electron nematics in bulk condensed matter systems using nonequilibrium methods. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z23.00010: Superconducting-to-Normal State Switching Experiments using Graphene-based Josephson Junctions Joseph Lambert, Steven Carabello, Roberto Ramos We report results of ongoing superconductor-to-normal state switching experiments using graphene-based Josephson junctions. These devices consist of a single-layer graphene flake contacted by two superconducting parallel leads separated by a few hundred nanometers. Through the proximity effect, the superconducting state is induced in the graphene region below the leads and the Josephson supercurrent is mediated through the normal graphene region by multiple Andreev reflections. The Josephson effect has been firmly demonstrated in these devices, where supercurrents in the hysteretic current-voltage characteristic, Shapiro steps, the Fraunhofer-like diffraction pattern in the critical current versus external magnetic field, and the current-phase relationship have been observed. We report on work in progress, in measuring I-V characteristics, thermal activation and microwave resonant activation in graphene-based junctions, at various temperatures below 1 Kelvin. We modulate the density of charge carriers using a back-gate voltage, which tunes the critical current. This provides another knob for studying these state switching properties. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z23.00011: Noise spectroscopy: a sensitive probe to explore hot electron effect in highly correlated systems Sudeshna Samanta, Arup K. Raychaudhuri Non-linear electrical conductance in ferromagnetic insulating (FMI) state of manganites can give rise to reversible colossal electro-resistance and current induced resistance change due to heating of the electrons in the system. In FMI state ($<$120K), the temperature of the lattices or phonon ($T_{ph})$ and electrons ($T_{e})$ in the sample can decouple by high input power density giving rise to heating of the electronic bath. We investigated whether white noise like Nyquist noise can be used to measure $T_{e}$ (which is expected to be larger than $T_{ph})$ when the two baths get decoupled. The use of the Nyquist noise to measure $T_{e}$ assumes that the electron bath forms a proper temperature bath in equilibrium. A dc stressing current was used to heat the electron bath while a small ac signal was used to measure the noise. With enhanced power input to the electron system, the white noise enhances and there is a large deviation from the simple estimate of Nyquist relation (4$k_{B}T_{e}R_{sample})$ indicating that the electron system is not in thermal equilibrium and is a non-ergodic system where Fluctuation Dissipation Theorem has broken down. [Preview Abstract] |
Session Z25: Novel Superconductors II
Sponsoring Units: DCMPChair: Chih-Kang Ken shih, University of Texas at Austin
Room: D166
Friday, March 25, 2011 11:15AM - 11:27AM |
Z25.00001: Momentum-dependent multiple gaps of MgB$_{2}$ probed by electron tunneling spectroscopy on MgB$_{2}$/native oxide/Pb junctions Ke Chen, Wenqing Dai, Qi Li, X.X. Xi Distinct multi-band superconductivity is a unique feature that distinguishes MgB$_{2}$ from all other phonon-mediated Bardeen-Cooper-Schrieffer (BCS) superconductors. According to a first-principles calculation employing an anisotropic Eliashberg formalism [Choi \textit{et al.} Nature 418, 758 (2002)], there is a distribution of superconducting energy gap values on the Fermi surface of MgB$_{2}$ (two $\sigma $ bands and two $\pi $ bands). However, only two distinct gaps have been observed experimentally, leading to the suggestion that consideration of the fully anisotropic electron-phonon interaction may not be necessary for real MgB$_{2}$ samples. Here, we present an electron tunneling spectroscopy study on MgB$_{2}$/native oxide/Pb junctions that clearly shows the distribution of energy gaps. By deconvoluting the tunneling spectrum based on the density of state of Pb, we derive the momentum-dependent energy gaps of MgB$_{2}$, which are in good agreement with the anisotropic Eliashberg calculation. The result affirms the importance of the anisotropic electron-phonon interaction in MgB$_{2}$ as well as its important impact on device applications. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z25.00002: Non-Cuprate Superconductor with Cubic Structure and T$_{c}$ = 85 K J.M. Estrada, E. Chavira, I. Rosales, O. Novelo, E. Fregoso, E.E. Marinero, M. Nishioka, V. Garcia-Vazquez, M. Suchomel We have synthesized a new superconductor material, namely: \textbf{Ba(Yb}$_{0.38}$\textbf{ In}$_{0.10}$\textbf{Sn}$_{0.42}$\textbf{ Pb}$_{0.10}$\textbf{)O}$_{2.66}$, by solid-state reaction in air and ambient pressure. The new compound is determined to have a T$_{c}$ of 85 K, which is new record for a non-cuprate structure. Employing synchrotron XRD and Rietveld refinement, 5 structural phases are identified in the reaction products. The phases identified are: BaTb$_{0.5}$Sb$_{0.5}$ O$_{3}$ (41.3{\%}), Yb$_{2}$BaCuO$_{5}$ (26.1{\%}), CuO (22.6{\%}), Yb$_{2}$Cu$_{2}$O$_{5}$ (4.4{\%}) and Ba$_{1.99}$Y$_{1.01}$ Cu$_{3}$O$_{8}$ (5.6{\%}). The microstructure exhibits cubic morphology (SEM) and EDX analysis is utilized to determine the stoichiometry of the new superconducting material, \textbf{Ba(Yb}$_{0.38}$\textbf{ In}$_{0.10}$\textbf{Sn}$_{0.42}$\textbf{ Pb}$_{0.10}$\textbf{)O}$_{2.66}$, which is isostructural to the cubic BaTb$_{0.5}$Sb$_{0.5}$ O$_{3}$ phase. Magnetic and Resistance measurements vs T indicate a superconducting transition at T$_{c}$ at 85 K. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z25.00003: Suppression of time reversal symmetry breaking superconcutivity in Pr(Os,Ru)$_{4}$Sb$_{12}$ and (Pr,La)Os$_{4}$Sb$_{12}$ Lei Shu, W. Higemoto, Y. Aoki, A.D. Hillier, K. Ohishi, K. Ishida, R. Kadono, A. Koda, O.O. Bernal, D.E. MacLaughlin, Y. Tunashima, Y. Yonezawa, S. Sanada, D. Kikuchi, H. Sato, H. Sugawara, T.U. Ito, M.B. Maple Zero-field muon spin relaxation ($\mu$SR) experiments have been carried out in the Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ and Pr$_{1-y}$La$_y$Os$_4$Sb$_{12}$ alloy systems to investigate broken time-reversal symmetry (TRS) in the superconducting state, signaled by the onset of a spontaneous static local magnetic field $B_{\rm s}$. In both alloy series $B_{\rm s}$ initially decreases linearly with solute concentration. Ru doping is considerably more efficient than La doping ,with a $\sim$$50\%$ faster initial decrease. The data suggest that broken TRS is suppressed for Ru concentration larger than 0.6, but persists for essentially all La concentrations. Our data support the theory of TRS-breaking superconductivity via crystal-field excitonic Cooper pairing of Koga, Matsumoto, and Shiba. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z25.00004: Signatures of half-quantum vortices in magnetoresistance of perforated samples Victor Vakaryuk, Valerii Vinokur Recent cantilever magnetometry measurements of annular micron-size samples of $\rm Sr_2RuO_4$ [1] have revealed evidence for the existence of half-quantum vortices (HQVs) in this material. Here we suggest to look for HQVs at temperatures close to $T_c$ in magnetoresistance of ``punctured'' (perforated) $\rm Sr_2RuO_4$ samples which consist of an array of regularly spaced micron-size holes in an otherwise uniform superconducting matrix. Due to the dissipative nature of resistive measurements signatures of HQVs might be seen even if their thermodynamic stability is not expected. We analyze the dependence of magnetoresistance on the thermodynamic stability of HQVs and point out features which may help to identify them. \\[4pt] [1]~J.~Jang, D.G.~Ferguson, V.~Vakaryuk, R.~Budakian, S.B.~Chung, P.M.~Goldbart, Y.~Maeno (2010). [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z25.00005: Nonintegral flux penetration in superconductors with broken time-reversal symmetry via bent domain walls David Ferguson, Paul Goldbart $\textrm{Sr}_2\textrm{RuO}_4$ is a candidate material for realizing a superconducting state that spontaneously breaks time-reversal symmetry~[1]. In such a state, the spatial pattern of the superconductivity may be broken up into regions of differing chirality, separated by domain walls. Here, we show that, near to bends in such domain walls, nonintegral (and even nonquantized) multiples of the superconducting magnetic flux quantum would penetrate the system~[2]. We discuss the implications of this ``bend flux'' effect for various experimental probes that are sensitive to time-reversal symmetry breaking. For the example of scanned-probe magnetic imaging, the observation of localized-nonquantized flux penetrating a z-axis surface of the sample, could be interpreted in terms of the presence of bent walls between domains of opposing chirality, and hence would be suggestive of time-reversal symmetry-breaking superconductivity. Alternatively, if observations should reveal localized but only {\it quantized\/} flux, this would suggest either (i)~the absence of domain walls or (ii)~their presence, but as a parallel array of straight walls. \\[4pt] [1]~C.\ Kallin and A.\ J.\ Berlinsky, J.\ Phys.\ Cond.\ Mat.{\bf 21\/}, 164210 (2009). \hfil\break\noindent [2]~D.\ G.\ Ferguson and P.\ M.\ Goldbart, arXiv:1011.2765v1 (2010). [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z25.00006: Scenarios for half-integer fluxoid behavior of annular rings of Sr$_2$RuO$_4$ Raffi Budakian, David Ferguson, Paul M. Goldbart, Joonho Jang, Victor Vakaryuk Recently, cantilever torque magnetometry experiments on annular rings of superconducting Sr$_2$RuO$_4$ have revealed half- height steps in the magnetization~[1]. These features are suggestive of the existence, in these annular samples, of half- quantum fluxoid states (i.e., the coreless analogs of half- quantum vortices). We consider the existence and energetic stability (for various forms of triplet superconductivity) of half-quantum fluxoid states in annular samples. We also consider alternative scenarios that could give rise to magnetization steps. One particular scenario requires the presence, in the bulk of the sample, of thermodynamically stable ``wall vortices'' By analyzing the equilibrium state of the superconductor, as a function of the applied magnetic field, we conclude that any wall-vortex scenario consistent with the observations of Ref.~[1] would require a (to date, unexplained) fine tuning of various material parameters.\\[4pt] [1]~J.\ Jang, D.\ Ferguson, V.\ Vakaryuk, R.\ Budakian, S.\ Chung, P.Goldbart, and Y.\ Maeno, (2010 unpublished) [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z25.00007: The Atomic-Scale Onset of Charge Density Waves in NbSe$_{2}$ Chockalingam S P, L. Zhao, C. Arguello, E. Rosenthal, C. Gutierrez, J. Kang, A. Pasupathy, S. Jia, R. Cava Many modern complex materials exhibit spatially ordered electronic states such as charge and spin density waves, and scanning tunneling microscopy (STM) has recently been used successfully to visualize some of these phases. Unfortunately such phases often occur simultaneously in a single sample making the interpretation of experimental data difficult. In order to gain insight into the nature of spatial order in a simple material, we study the transition-metal dichalcogenide NbSe$_{2}$, which displays a 2D charge density wave (CDW) phase at low temperature. How does this CDW phase manifest itself in real space at the atomic scale? In order to answer this question, we have performed variable-temperature STM experiments on NbSe$_{2 }$single crystals at various temperatures around T$_{c}$ (33.5 K). We discover that static, short-range CDW order exists around crystal defects and impurities at temperatures up to 3T$_{c}$. We will describe the temperature evolution as well as the energy dependence of the short-range CDW using spectroscopic mapping. Our experiments provide a high-resolution measurement of the changes in electronic structure caused by the formation of CDW in the dichalcogenides. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z25.00008: Multiband Superconductivity in $2H$-NbSe$_{2}$ Probed by Cryomagnetic STM Spectroscopy Igor Fridman, J.Y.T. Wei, C. Kloc, V. Lukic, Rongwei Hu, C. Petrovic Using a novel magnetic field geometry, we study multiband pairing in single crystals of superconducting $2H$-NbSe$_{2}$ under finite superfluid momentum. Spectroscopy and conductance imaging were performed with a scanning tunneling microscope (STM) at 300 mK and in a field of up to 9 T, applied in the $ab$-plane. We observed multiple spectral features that evolve systematically with field, and a two-sloped zero-bias conductance that dips anomalously at 0.7 T. Our analysis yields distinct evidence for multiple gaps coming from the various Fermi-surface sheets, and has possible implications on the origin of the coexisting charge density wave order. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z25.00009: Spin density in UCoGe Jonathan Taylor, Jonathan Duffy, Matthew Butchers, Chris Stock, E. Bauer Below T = 0.8K, superconductivity and ferromagnetism (T$_{C}$ = 3K) coexist in UCoGe. The total magnetic moment is 0.03 $\mu_{B}$ at low temperatures. UCoGe is considered to be a weak itinerant ferromagnet. Recent theoretical studies Predict UCoGe indicate that UCoGe is ferromagnetic, but that the small total magnetic moment arises from the near cancellation of large U $5f$ spin and orbital moments. Theory also predicts there to be a Co spin moment. However, the total moment is predicted to be considerably larger than observed experimentally. Using magnetic Compton scattering experiments together with KKR electronic structure calculations we have measured the spin density of the ferromagnetic superconductor UCoGe and determined that at 1.3K the U$5f$ and Co$3d$ spin moments are antiparallel, in agreement with theory, but disagreeing with polarized neutron diffraction results. The measured total spin moment is determined to be -0.35$\mu_{B}FU^{-1}$, and the orbital moment hence determined to be 0.5$\mu_{B}FU^{-1}$. These moments are reduced with respect to the LDA calculations. The small ratio of U orbital to spin moments indicates that the 5f moment is itinerant. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z25.00010: Interplay of charge density modulations and superconductivity Jason W. Sadowski, K. Tanaka Although charge density waves (CDW) in transition metal dichalcogenides (TMDs) have been studied for over three decades, there is yet no consensus on the mechanism for CDW in two-dimensional TMDs. Moreover, the layered 2H-type TMDs which exhibit coexistence of CDW order and superconductivity (SC) present an intriguing opportunity for studying the interplay of SC and charge density modulations. In particular, 2H-NbSe$_2$ is most suitable for such investigation as its transition temperatures for CDW and SC are well separated, and as it is easy to cleave is ideal for surface-sensitive measurements. The relation between CDW and SC in this material is still under debate, with one experiment indicating an enhancement of SC by CDW [1] and another implying a competition of the two kinds of orders [2]. Motivated by these experiments, we study the effects of Fermi surface structure on CDW and its interplay with SC in terms of the Bogoliubov-de Gennes formalism. \\[4pt] [1] Kiss {\it et al.}, Nat. Phys. {\bf 3}, 720 (2007). \\[0pt] [2] Borisenko {\it et al.}, Phys. Rev. Lett. {\bf 102}, 166402 (2009). [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z25.00011: First-principles study of the noncentrosymmetric superconductors Li$_2$Pt$_3$B and Li$_2$Pd$_3$B Tatsuya Shishidou, Tamio Oguchi Effect of spin-orbit coupling (SOC) associated with lack of space-inversion symmetry has been a central issue in condensed-matter physics. Li$_2$Pt$_3$B and Li$_2$Pd$_3$B are superconducting below 2.7 K and 7 K, respectively, and have the same crystal structure (cubic $P4_3 32$, No.~212), which is noncentrosymmetric and is characterized by highly distorted corner-sharing BPt(Pd)$_6$ octahedra. Despite the same valency and structure, they show quite different superconducting behavior. NMR measurements [1] indicate that Li$_2$Pt$_3$B would be a spin-triplet superconductor with line nodes in the gap function while Li$_2$Pd$_3$B is a conventional spin-singlet $s$-wave superconductor. SOC would be a key to understand this difference. To clarify the electronic band structure and Fermi surface of these compounds, we performed density-functional (GGA PBE) calculations with FLAPW method. Relativistic effects were fully taken into account. The band structures calculated are in good accordance with previous work[2] and the spin splitting due to SOC is quite significant in Li$_2$Pt$_3$B. This work was supported by a MEXT KAKENHI on Innovative Areas ``Topological Quantum Phenomen''. [1] M.~Nishiyama, Y.~Inada, and G.-q.~Zheng, PRL {\bf 98}, 047002 (2007); PRB {\bf 71}, 220505(R) (2005). [2] K.-W.~Lee and W.~E.~Pickett, PRB {\bf 72}, 174505 (2005). [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z25.00012: The electron-boson spectral density function of underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and YBa$_2$Cu$_3$O$_{6.50}$ Jungseek Hwang We investigate the electron-boson spectral density function, $I^2\chi(\omega,T)$, of CuO$_2$ plane in underdoped Bi$_2$Sr$_2 $CaCu$_2$O$_{8+\delta}$ (Bi-2212) and underdoped YBa$_2$Cu$_3 $O$_{6.50}$ (Y-123) systems using the Eliashberg formalism. We apply a new (in-plane) pseudogap model to extract the electron- boson spectral function. For extracting the spectral function we assume that the spectral density function consists of two components: a sharp mode and the broad Millis-Monien-Pines (MMP) mode. We observe that both the resulting spectral density function and the intensity of the pseudogap show strong temperature dependences: the sharp mode takes most spectral weight of the function and the peak position of the sharp mode shifts to lower frequency and the depth of pseudogap, $1-\tilde {N}(0,T)$, is getting deeper as temperature decreases. We estimate fictitious (maximum) superconducting transition temperatures, $T_c(T)$, from the extracted spectral functions at various measured temperatures using a generalized McMillan formula. The estimated (maximum) $T_c$ also shows a strong temperature dependence; it is higher than the actual $T_c$ at all measured temperatures and decreases with temperature lowering. Since as lowering temperature the pseudogap is getting stronger and the maximum $T_c$ is getting lower we propose that the pseudogap may suppress the superconductivity in cuprates. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z25.00013: Modulation of Spin-Orbit interaction and superconductivity in two-dimensional electron gas at the Mott-Insulator - Band-Insulator interface: LaTiO3 - SrTiO3 Johan Biscaras, N. Bergeal, A. Kushwaha, T. Wolf, A. Rastogi, Ramesh Chandra Budhani, Jerome Lesueur It has been shown recently that a two-dimensional electron gas could form at the interface of two insulators such as LaAlO$_{3}$ and SrTiO$_{3}$ [1], or LaTiO$_{3}$ (a Mott insulator) and SrTiO$_{3}$ [2]. We present low temperature transport and magneto-transport measurements on LaTiO$_{3}$/SrTiO$_{3}$ hetero-structures, whose properties can be modulated by field effect using a metallic gate on the back of the substrate. The corresponding phase diagram has been investigated, and superconductivity evidenced for the first time in this system which involves a Mott insulator [3]. We will discuss the role of the confinement potential and the SrTiO$_{3}$ band structure on the phase diagram, and show the specific role of the spin-orbit coupling measured by localization corrections to the magnetoconductivity. Finally, the superconducting to insulator transition will be analyzed. [1] N. Reyren et al, Science 317, 1196 (2007) [2] A. Ohtomo et al, Nature 419, 378 (2002) [3] J. Biscaras et al, Nature Commun, DOI: 10.1038/ncomms1084 (2010) [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z25.00014: Upper Critial Field Calculations in p-Wave Tripet Ferromagnetic Superconductors Christopher L{\"o}rscher, Richard Klemm We report $H_{c2}(T)$ calculations for novel triplet ferromagnetic superconductors using a uniaxially anisotropic pairing interaction, obtained by means of the linearized Gor'kov gap equation. In addition to the intrinsic anisotropy of the p-wave states, we also include effective mass anisotropies in our calculations. We investigate the $H_{c2}(T)$ phase diagrams for several combinations of anisotropy, leading to novel $H_{c2}(T)$ properties, including upward curvature. We discuss the relevance of our results to experiments on UCoGe. [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z25.00015: Identifying d-vectors in spin-orbit coupled multi-orbital superconductors Christoph Puetter, Hae-Young Kee In multi-orbital systems, Hund's interaction has been recognized to play a significant role in spin-triplet pairing. On the other hand, spin-orbit coupling has been treated as a perturbation, which is not a good approximation in 4d or 5d transition metal compounds. We have treated both effects on an equal footing in t2g orbital systems and studied their combined effect on spin-triplet superconductivity. We also discuss the implications of our results for spin-triplet candidate materials. [Preview Abstract] |
Session Z26: Focus Session: Iron Based Superconductors -- Electronic Anisotropy
Sponsoring Units: DMP DCOMPChair: Rajiv Singh, University of California, Davis
Room: D162/164
Friday, March 25, 2011 11:15AM - 11:51AM |
Z26.00001: In-plane electronic anisotropy of underdoped iron arsenide superconductors Invited Speaker: Common to the high Tc cuprates, superconductivity in the Fe arsenides and related compounds is associated with suppression of an antiferromagnetic ground state. On the underdoped side of the phase diagram, in addition to the antiferromagnetic transition, the materials also suffer a phase transition that breaks the 4-fold rotational symmetry of the high-temperature crystal structure, this occurring at either the same or higher temperature than the Neel transition. Emerging evidence based on measurements of detwinned single crystals reveals a dramatic in-plane electronic anisotropy associated with this nematic transition.\footnote{In collaboration with: J.-H. Chu, H.-H Kuo, J. Analytis, M. Yi, D. H. Lu, Z. X. Shen, A. P. Sorini, A. F. Kemper, S.-K. Mo, T. P. Devereaux, R. G. Moore, M. Hashimoto, W. S. Lee, Z. Hussain, K. De Greve, P. L. McMahon, Y. Yamamoto (Stanford); Z. Islam (APS,ANL); A. Dusza, A. Lucarelli, F. Pfuner \& L. Degiorgi (ETHZ).} [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z26.00002: Superconducting properties and the interplay between magnetism and superconductivity in 1111 Fe arsenides as revealed by torque magnetometry Gang Li, Gael Grisonnanche, Benjamin Conner, Nikolai Zhigadlo, Sergiy Katrych, Zbigniew Bukowski, Janusz Karpinski, Luis Balicas We performed a study of the angular dependence of the magnetic torque in LaFeAsO$_{0.9}$F$_{0.1}$ single crystals. We developed a method to separate the magnetic and the superconducting components inherent to the FeAs layers and which are superimposed onto the reversible torque signal $\tau_{\rm{rev}} (\theta, H, T)$. We show that by exploring the amplitude of the superconducting component in $\tau_{\rm{rev}} (\theta)$ as a function of $H$, it is possible to extract the thermodynamic value of the superconducting upper critical field $H_{c2}$. This so obtained value can be used to extract the field and the temperature dependencies of respectively, the superconducting anisotropy and the superfluid density through the Kogan formalism. We observe a strong temperature and field dependence of the superconducting anisotropy as expectable within a multi-band superconducting scenario. The resulting $T$-dependence of the superfluid-density resembles the behavior previously reported for LaFePO and which was ascribed to nodal superconductivity. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z26.00003: Properties of de-twinned iron-arsenide SrFe$_2$As$_2$ Erick Blomberg, M.A. Tanatar, A. Kreyssig, N. Ni, A. Thaler, R.W. Hu, P.C. Canfield, S.L. Bud'ko, A.I. Goldman, R. Prozorov The iron-pnictides, AFe$_2$As$_2$ (A = alkali earth metal), undergo a tetragonal to orthorhombic structural transition below $T_s$, which is in the range between 130~K to 210~K depending on the compound. To release elastic deformation the orthorhombic phase is spontaneously divided into four degenerate, equally populated, structural domains. This makes any measurements of in-plane anisotropy extremely difficult. Unlike high$-T_c$ Y-Ba-Cu-O which is orthorhombic already at room temperature, detwinning of pnictides is more difficult, because of lower $T_s$. We developed a technique of mechanical de-twinning of these materials that allows transport, x-ray and direct optical measurements [1]. Here we report polarized light microscopy, synchrotron X-ray analysis and AC transport measurements on SrFe$_2$As$_2$, which represents a clean case of first-order magnetic/structural transition. \\[4pt] [1] M. A. Tanatar, et al. Phys. Rev. B \textbf{81} 184508 (2010). [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z26.00004: Magnetic Field Effect on the In-plane Electrical Resistivity of FeTe$_{1-x}$Se$_x$ Single Crystals Yimin Xiong, Amar Karki, Brian Sales, Rongying Jin The in-plane electrical resistivity ($\rho_{ab}$) of FeTe$_{1- x}$Se$_x$ single crystals is measured as a function of temperature (T), magnetic field (H), and the angle ($\theta$) between H and electric current (I). The results reveal that $\rho_{ab}$ strongly depends on both H and $\theta$, indicating the participation of spin scattering in the electrical transport. The underlying physics will be discussed. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z26.00005: Anisotropy of normal and superconducting states of FeSeTe M.A. Tanatar, E.C. Blomberg, J.-Ph. Reid, J. Hu, J. Quian, Z.Q. Mao, Louis Taillefer, R. Prozorov We report anisotropic electrical and thermal transport measurements in non-superconducting parent FeTe and superconduicting optimally doped FeTeSe. Intrinsic in- plane anisotropy of the electrical resistivity was measured in mechanically detwinned crystals of the parent compound [1]. In-plane and inter-plane heat transport was used to probe the symmetry of the superconducting gap in the material close to optimal doping (Tc=15~K). The results are compared to those of superconducting FeSe [2] and doping evolution of thermal conductivity in BaFe2As2 doped with cobalt [3,4]. [1] M.A. Tanatar, {\it et al.} Phys. Rev. B {\bf 81}, 184508 (2010). [2] J. K. Dong, {\it et al.} Phys. Rev. B {\bf 80}, 024518 (2009). [3] M. A. Tanatar, {\it et al.} Phys. Rev.Lett. {\bf 104}, 067002 (2010). [4] J.-Ph. Reid, {\it et al.} Phys. Rev. B {\bf 82}, 064501 (2010). [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z26.00006: Pulsed-field contactless mapping of the anisotropic upper critical field in LiFeAs superconducting crystals K. Cho, H. Kim, M.A. Tanatar, R. Prozorov, Y.J. Song, Y.S. Kwon, W.A. Coniglio, C.C. Agosta, A. Gurevich Angle - resolved measurements of the upper critical field were performed using a tunnel diode resonator in the stoichiometric iron arsenide superconductor LiFeAs ($T_c$=18~K) in pulsed magnetic fields up to 50~T at temperatures down to 0.6~K. Complete $H^{\parallel c}_{c2}(T)$ and $H^{\bot c}_{c2}(T)$ curves with $T \to 0$ extrapolated values of $H^{\parallel c}_{c2}(0)=17 \pm 1 $ T and $H^{\bot c}_{c2}(T)=26 \pm 1$ T were obtained. The anisotropy, ${\gamma _{H_{c2}} } \equiv H_{c2}^{ \bot c}/H_{c2}^{\parallel c} \approx 2$, close to $T_c$ has revealed the essentially three-dimensional electronic structure of the material. Both temperature - dependent $H_{c2}(T)$ can be well fit within a single set of band structure, magnetism and scattering parameters. In a configuration with $H \parallel c$, $H^{\parallel c}_{c2} (T)$ is limited by orbital effects with modest scattering. In the perpendicular orientation, $H^{\bot c}_{c2}(T)$ shows a notable low-temperature saturation and a strong departure from the orbital Werthamer-Helfand-Hohenberg model. Instead, fitting results suggest paramagnetic Pauli limiting and the development of a spatially - modulated superconducting state. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z26.00007: Symmetry breaking orbital anisotropy observed in detwinned Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ above the spin density wave transition Ming Yi, D.H. Lu, J.-H. Chu, J.G. Analytis, A.P. Sorini, A.F. Kemper, S.-K. Mo, R.G. Moore, M. Hashimoto, W.-S. Lee, Z. Hussain, T.P. Devereaux, I.R. Fisher, Z.-X. Shen Nematicity has recently been observed in the competing phases in proximity to the superconducting phase in the cuprates. Similarly, the iron pnictides exhibit symmetry breaking competing phases in the form of a tetragonal to orthorhombic structural transition and a collinear spin density wave (SDW) transition in the underdoped regime. Evidence for strong in-plane anisotropy in the SDW state has been reported by neutron scattering, scanning tunneling microscopy, and transport measurements, but the nature of this nematic behavior is still elusive. Here we present the results of an ARPES study of detwinned single crystals of underdoped Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$, resolving single domain electronic structure in the orthorhombic SDW state which exhibits strong in-plane anisotropy consistent with other probes. The anisotropy is evident in a large splitting of the dxz and dyz bands, which is seen to develop almost fully above the onset of the long range magnetic order. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z26.00008: Nematic spin fluid in the tetragonal phase of BaFe$_{2}$As$_{2}$ Leland Harriger, Huiqian Luo, Mengshu Liu, Toby Perring, Chris Frost, Jiangping Hu, Mike Norman, Pengcheng Dai We use inelastic neutron scattering to demonstrate the presence of a large spin anisotropy above $T_{N}$ in the unstressed tetragonal phase of BaFe$_{2}$As$_{2}$. In the low-temperature orthorhombic phase, we find highly anisotropic spin waves with a large damping along the AF $a$-axis direction. On warming the system to the paramagnetic tetragonal phase, the low-energy spin waves evolve into quasi-elastic excitations, while the anisotropic spin excitations near the zone boundary persist. These results strongly suggest that the spin nematicity we find in the tetragonal phase of BaFe$_{2}$As$_{2}$ is the source of the electronic and orbital anisotropy observed above $T_{N}$ by other probes, and has profound consequences for the physics of these materials. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z26.00009: Reconstructed electronic structure from orbital ordering and antiferromagnetism in the iron pnictides Weicheng Lv, Philip Phillips Recent experimental developments have unambiguously demonstrated the in-plane electronic and magnetic anisotropy of the iron-based superconductors. It has been argued that this nematic state can arise from orbital ordering physics. Including an energy splitting term that breaks the degeneracy of the Fe $d_{xz}$ and $d_{yz}$ orbitals, we solve the multi-orbital Hubbard model within a mean-field approximation. Despite sensitivity of the resulting state to the input parameters, we find that a weak orbital order that places the $d_{yz}$ orbital slightly higher in energy than the $d_{xz}$ orbital, along with the interactions $U$ and $J$ being of intermediate strength, is compatible with current experimental results. In this regime, the stripe antiferromagnetism is further stabilized and the existence of the Dirac cones is preserved. Furthermore, this anisotropic electronic state leads to the observed resistivity anisotropy and STM interference patterns. Finally the relation between orbital order and superconductivity is discussed. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z26.00010: Magnetic Torque Evidence for Broken Rotational Symmetry in the Tetragonal Phase of BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ Single Crystals Shigeru Kasahara, Hongjie Shi, Ryuji Okazaki, Kenichiro Hashimoto, Minoru Yamashita, Takasada Shibauchi, Takahito Terashima, Yuji Matsuda The emergence of broken four-fold symmetry is found in the tetragonal phase of BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ single crystals [1] by in-plane anisotropy measurements of magnetic susceptibility. Magnetic torque detects a spontaneous growth of two-fold oscillations under in-plane field rotations, whose amplitude is linked to an order parameter of an electronic ``nematic'' phase. Our findings reveal that the spontaneous rotational symmetry breaking sets in far above the tetragonal to orthorhombic structural transition, which might be also linked to the unconventional superconductivity of this system [2,3]. \\[4pt] [1] S. Kasahara, et al., Phys. Rev. B 81, 184519 (2010). \\[0pt] [2] K. Hashimoto, et al., Phys. Rev. B 81, 220501(R) (2010). \\[0pt] [3] Y. Nakai, et al., Phys. Rev. B 81, 020503(R) (2010). [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z26.00011: Interplay of orbital ordering and magnetism in the parent compounds of the iron pnictides Andriy Nevidomskyy The neutron scattering experiments on the parent compounds of the 122 family of the iron pnictide superconductors show a pronounced orhthorhombic anisotropy in the spin wave spectra [1], also observed in resistivity measurements on detwinned crystals [2,3]. Orbital ordering of the d$_{xz}$ and d$_{yz}$ orbitals may be a possible explanation for this behaviour [4]. In this work, we establish the effect of orbital ordering on the magnetism and study their interplay through a combination of the first-principles band theory calculations and phenomenological analysis in the framework of the Landau theory. This enables us to establish the coupling between the orbital and magnetic degrees of freedom in these materials. Consequences for the symmetry of the superconducting order parameter are discussed.\\[4pt] [1] J. Zhao et al., Nature Phys. {\bf 5}, 555 (2009) \\[0pt] [2] M.A. Tanatar at al., Phys. Rev. B {\bf 81}, 184508 (2010).\\[0pt] [3] J.-H. Chu et al., Science {\bf 329}, 824 (2010). \\[0pt] [4] C.-C. Chen et al., Phys. Rev. B {\bf 82}, 100504(R) (2010). [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z26.00012: Orbital-ordering and In-plane Anisotropy in Low-moment Ground-state of Parent Compounds of Iron-based Superconductors Masahiko Machida, Hiroki Nakamura Since the discovery of the iron-based superconductor, a large discrepancy between experimental observations and first-principles calculations in the magnetic moment of the antiferromagnetic state of the parent compounds has been intensively debated. The observed moment values are about 3 to 5 times smaller than those of the calculation although there is a variety of the difference depending on the materials. Very recently, an interesting calculation data fully reproducing the observed low moment has been suggested by F. Cricchio et al., (Phys. Rev. B 81 (2010) 140403) who performed first-principles calculations using a LDA+U scheme. In this study, we suggest that the new state is a possible candidate to well explain the other data, e.g., strong anisotropy in spatial patterns measured by STM and magnetic excitations found by neutron scattering. Furthermore, we compare the result with other theoretical works reproducing similar low moment in terms of orbital ordering. [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z26.00013: Conductivity Anisotropy in the Antiferromagnetic State of Iron Pnictides Belen Valenzuela, Elena Bascones, Maria J. Calderon Recent experiments on iron pnictides have uncovered a large in- plane resistivity anisotropy with a surprising result: the system conducts better in the antiferromagnetic $x$ direction than in the ferromagnetic $y$ direction [1]. We address this problem by calculating the ratio of the Drude weight along the $x$ and $y$ directions, $D_x/D_y$, for the mean-field $\bf{Q}= (\pi,0)$ magnetic phase diagram of a five-band model for the undoped pnictides [2,3]. We find [4] that $D_x/D_y$ ranges between $0.2 < D_x/D_y < 1.7$ for different interaction parameters. Large values of the orbital ordering favor an anisotropy opposite to the one found experimentally. On the other hand, $D_x/D_y$ is strongly dependent on the topology and morphology of the reconstructed Fermi surface. Our results point against orbital ordering as the origin of the observed conductivity anisotropy, which may be ascribed to the anisotropy of the Fermi velocity. [1] J.-H. Chu et al., Phys. Rev. B 81, 214502 (2010); J.-H. Chu et al., Science 329, 824 (2010); M. Tanatar et al., Phys. Rev. B 81, 184508 (2010). [2] M.J.Calderon, B. Valenzuela, E. Bascones, Phys. Rev. B 80, 094531 (2009). [3] E.Bascones, M.J. Calderon, B. Valenzuela, Phys. Rev. Lett. 104, 227201 (2010). [4] B.Valenzuela, E. Bascones, M.J. Calderon, Phys. Rev. Lett. 105, 207202 (2010). [Preview Abstract] |
Session Z27: Focus Session: Semiconductor Qubits - Theory and Experiment
Sponsoring Units: GQIChair: Xuedong Hu, SUNY Buffalo
Room: C155
Friday, March 25, 2011 11:15AM - 11:27AM |
Z27.00001: Quantum dot charge stability diagram from a generalized Hubbard model Xin Wang, Shuo Yang, Sankar Das Sarma We develop a theory for the charge stability diagram in solid state quantum dot spin qubits using a general form of the Hubbard model. We argue that the extended Hubbard model (with both on-site and inter-site Coulomb repulsion) is the minimal model to describe the system. The appropriate parameters of the Hubbard model can be read off by comparing our theoretically derived results with the experimental charge stability plots. We make predictions on how the charge stability diagram depends on various parameters of the Hubbard model, especially the spin-exchange and hopping energies. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z27.00002: Microscopic theory for the charge stability diagram of coupled quantum dot systems Shuo Yang, Xin Wang, Sankar Das Sarma We present a quantitative microscopic theory for the charge stability diagram of coupled quantum dot systems. Using the configuration interaction method we obtain a generalized Hubbard model, from which the charge stability diagram is calculated and compared with experiments. We establish an exact connection between experimental measurements and the microscopic theory, and predict some experimentally observable quantum effects. We also map the classical capacitance model to the extended Hubbard model, and argue that the effect of spin-exchange and various hopping terms cannot be expressed in the capacitance model. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z27.00003: Formation and electrical characterization of directed self-assembled Ge/Si quantum dot Dongyue Yang, Chris Petz, Jeremy Levy, Jerrold Floro Directed self-assembly of sub-10 nm Ge islands is a candidate for producing laterally coupled quantum dot molecules with geometrically-defined spin exchange couplings. The islands are created by the nucleation of Ge islands on nanoscale SiC templates defined by direct-write electron-beam lithography.\footnote{O. Guise, J. Ahner, J. John T. Yates, V. Vaithyanathan, D. G.Schlom, J. Levy, Appl. Phys. Lett. 87, 1902 (2005).} Ge islands are coupled through ohmic contacts to the Si capping layer, and geometries can be defined that are suitable for either vertical or lateral transport.We describe low-temperature magneto-transport measurements on individual and small arrays of Ge islands grown on semi-insulating silicon substrates. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z27.00004: The Kondo Effect in a Double Quantum Dot Sami Amasha, Ileana Rau, Andrew Keller, Jordan Katine, Hadas Shtrikman, David Goldhaber-Gordon A quantum dot consists of a confined droplet of electrons connected to electron reservoirs by tunnel barriers. When the dot has an odd number of electrons it has a net spin. The electrons in the reservoir can screen this spin via the Kondo effect, which corresponds to a many-body, highly correlated electron state. We study a lateral GaAs/AlGaAs double quantum dot, where one or both of the dots can be in the Kondo regime. The dots are also coupled to each other, and this inter-dot interaction can compete with the Kondo effect. We report transport measurements in this system at low electron temperatures and for a variety of inter-dot couplings. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z27.00005: Exhibition of tunnel coupling of negatively charged dangling bonds on Si Surface Using Scanning Tunneling Microscope M. Baseer Haider, L. Livadaru, J. Pitters, R. Wolkow We have performed Scanning tunneling microscopy study of hydrogen terminated Si (100). We will show that single Si atoms in a solid state environment can be served as quantum dots. These negatively charged quantum dots can be tunnel coupled to the nearby Si quantum dots. We will demonstrate that this tunnel coupling can be controlled by adjusting the separation between the two Si atomic quantum dots. Moreover electron occupation in the tunnel coupled Si quantum dots can be controlled. We have used this tunnel coupling effect of Si atomic quantum dots to fabricate Quantum Cellular Automata Cells. Quantum Cellular Automata are used to transmit binary information through electrostatic interaction between adjacent cells without the transfer of charge from one cell to the next. Devices based on Quantum Cellular Automata will consume much less power compared to the conventional transistor based devices. Moreover, since there is no transfer of charge so power dissipation during its operation is minimal compared to conventional semiconductor devices. This Si based Quantum Cellular Automat Cell works at room temperature. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z27.00006: The RKKY Interaction and the Nature of the Ground State of Double Dots in Parallel Manas Kulkarni, Robert Konik We argue through a combination of slave boson mean field theory and the Bethe ansatz that the ground state of closely spaced double quantum dots in parallel are Fermi liquids. We do so by studying the dots conductance, impurity entropy, and spin correlation. In particularly we find that the zero temperature conductance is characterized by the Friedel sum rule, a hallmark of Fermi liquid physics, and the impurity entropy vanishes in the limit of zero temperature, indicating the ground state is a singlet. This conclusion is in contradistinction to a number of numerical renormalization group studies. We suggest a possible reason for the discrepancy. Our findings are also consistent with a 1/N diagrammatic approach to the same setup. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z27.00007: Frequency-dependent Fano factor of multilevel systems with inelastic decay processes Farzad Qassemi, Bill Coish, Joakim Bergli, Frank K. Wilhelm We study the frequency-dependent noise of electrons passing through a multilevel quantum dot or molecule accounting for ``dark'' states through which current is prohibited and inelastic transitions between the levels. Our theory results in simple closed-form expressions directly relating the frequency-dependent noise to inelastic decay rates in the limit where the rates are widely separated. To demonstrate the method, we apply it to evaluate the shot noise for electrons passing through single and double quantum dots in the presence of multiple spin decay mechanisms. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z27.00008: Electron Localization in the Inhomogeneous Electron Gas: Quantum Point Contacts Abhijit C. Mehta, Cyrus J. Umrigar, A. Devrim Guclu, Harold U. Baranger We use Quantum Monte Carlo (QMC) techniques to investigate the behavior of electrons in an inhomogeneous quasi-one-dimensional wire as a model of quantum point contact geometries. Previous QMC work by Guclu et al. demonstrated that electrons can be strongly localized in quantum point contacts, and this result was reproduced by Welander et al. using LSDA calculations. We model a quantum point contact as a constriction in a quantum ring, and we use variational and diffusion Monte Carlo to investigate the effects of different point contact lengths and geometries on the electronic properties of the QPC. A key issue is how robust the previous results are to the length of the constriction, the depth and steepness of the confining potential, and to increasing the density of the electrons in the high-density lead region. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z27.00009: Electron pair tunneling resonance in a double-dot interferometry Jinhong Park, H.-S. Sim It is difficult to experimentally detect an electron pair tunneling resonance in a quantum dot with repulsive Coulomb interactions, since it is usually masked by lower-order single-electron tunneling processes. We propose to use an Aharonov-Bohm interferometry consisting of two quantum dots for the detection. We find that in the second harmonics of the interference current, pair tunneling processes give a leading non-monotonous contribution around the bias voltages at which pair tunneling resonances appear. The second-harmonics differential conductance shows the signal of a pair tunneling resonance as well as the destructive interference of two pair tunneling resonances. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z27.00010: Giant current fluctuations in an overheated single-electron transistor Matti Laakso, Tero Heikkil\"a, Yuli Nazarov Interplay of cotunneling and single-electron tunneling in a thermally isolated single-electron transistor leads to peculiar overheating effects. In particular, there is an interesting crossover interval where the competition between cotunneling and single-electron tunneling changes to the dominance of the latter. In this interval, the current exhibits anomalous sensitivity to the effective electron temperature of the transistor island and its fluctuations. We present a new theoretical method for the study of the temperature fluctuations and induced fluctuations of other quantities, e.g., current, based on the Fokker--Planck equation. We apply this method to the study of the current and temperature fluctuations in an overheated SET around the crossover interval. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z27.00011: Electron exchange between quantum dot and ring by jumping in magnetic field Igor Filikhin, Sergei Matinyan, James Nimmo, Branislav Vlahovic Semiconductor heterostructures as quantum dots (QD) or quantum rings (QR) demonstrate discreet atom-like energy level configuration. In the presented work we show that in the weak coupled Double Concentric Quantum Ring (DCQR) electron position jumping can exist due to the energy level crossing. We study DCQR composed of GaAs in an Al$_{0.70}$Ga$_{0.30}$As substrate under influence of magnetic field. In our model the DCQR is considered in three dimensional space within single sub-band effective mass approach [1]. Magnetic field is applied in z direction, perpendicular to the DCQR plane. The electron position in DCQR is defined by effective radius which is radius of most probable localization of a single electron. We study electron structure of QD located at the center of QR. The electron position jumping between QD and QR is considered. Discussed will be possibility of experimental implementations of the jumping effect for composite object of QD and QR. \\[4pt] [1] I. Filikhin, V. M. Suslov and B. Vlahovic, Phys. Rev. B 73, 205332 (2006). [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z27.00012: Quantum phase transition of light as a control of the entanglement between interacting quantum dots Angela Barragan, Carlos Vera-Ciro, Ian Mondragon-Shem We study coupled quantum dots arranged in a photonic crystal, interacting with light which undergoes a quantum phase transition. At the mean-field level for the infinite lattice, we compute the concurrence of the quantum dots as a measure of their entanglement. We find that this quantity smoothly changes in the vicinity of the phase transition, and in a step-like fashion in the Mott-insulator phase. This behavior can be externally monitored through the second-order correlation function for the light in each lattice site. For the finite case, we discuss boundary induced effects using a mean-field ansatz, as well as the impact of having finite temperatures on the entanglement of the quantum dots. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z27.00013: First-principles study of the energy and spin structure of excited states of NV$^{- }$center in diamond and its corresponding Hubbard model parameters Sangkook Choi, Manish Jain, Steven G. Louie A negatively charged nitrogen-vacancy pair defect (NV) in diamond is one of the promising candidates to embody a qubit for quantum computation in solid states. It is an individually addressable quantum system that may be initialized, manipulated, and measured with high fidelity at room temperature due to a long coherence time of the spin in the ground states and long-life time of the excited states. The knowledge of the electronic and spin structures of the NV center in the ground as well as excited state is crucial in understanding them. Here, we evaluate the energies and spin structures of its excited states employing the first-principles GW-BSE methods. We further obtain the Hubbard model parameters for this defect system by comparing the excited-state energies from our ab-initio GW-BSE calculation with those from the model Hamiltonian. This work was supported by NSF Grant No. DMR10-1006184, the U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by DOE at LBNL's NERSC facility. [Preview Abstract] |
Session Z29: Focus Session: Superconducting Qubits - Coherence and Materials III
Sponsoring Units: GQIChair: John Teufel, National Institute of Standards and Technology
Room: C148
Friday, March 25, 2011 11:15AM - 11:27AM |
Z29.00001: Improving the Quality Factor of Microwave Compact Resonators K. Geerlings, S. Shankar, E. Edwards, L. Frunzio, R.J. Schoelkopf, M.H. Devoret Superconducting microwave resonators are now widely used for coupling to superconducting qubit systems. Compact resonators [1] consisting of an interdigitated capacitance and a meander inductance take up much less space than a typical coplanar waveguide resonator. Since the design of compact resonators and qubits share common features, qubit decoherence mechanisms can be studied through the measurement of resonator loss. We measured of order 100 resonators and have achieved internal quality factors in excess of 300,000. Results indicate loss appears to be due to spurious two level systems. Loss increases when the participation of surfaces in the energy density is increased. Thus a large separation of electrodes is preferred, in agreement with the findings of other groups. Work in progress involves the combination of these resonators with transmon qubits. Work supported by IARPA, ARO and the NSF. \\[4pt] [1] M.S. Khalil, F.C. Wellstood, and K.D. Osborn, arXiv:1008.2929 [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z29.00002: Radiative Losses in Superconducting Coplanar Resonators James Wenner, R. Barends, R.C. Bialczak, Y. Chen, J. Kelly, M. Lenander, E. Lucero, M. Mariantoni, M. Neeley, A.D. O'Connell, P. O'Malley, D. Sank, A. Vainsencher, H. Wang, M. Weides, T. White, Y. Yin, J. Zhao, A.N. Cleland, John M. Martinis Radiation is a potential loss mechanism in superconducting qubits. Radiation loss was studied in superconducting coplanar resonators, which are important both in coupling superconducting qubits and because they provide a simple system to quantitatively measure the resulting effects. We fabricated 8 GHz resonators and measured the resulting reduction of the high-power Q. We found it was necessary to design the resonators carefully to reduce stray coupling between the resonators so that losses would be dominated by radiation. The radiation loss is measured to be 30 times greater than predicted by a simple theoretical model, but was predicted accurately by simulation data. We attribute this to the effects of the device mount and the finite substrate height on the radiation pattern. We conclude that radiation is an unlikely decoherence mechanism for the present generation of qubits and resonators. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z29.00003: Minimal resonator loss for circuit quantum electrodynamics Rami Barends, N. Vercruyssen, A. Endo, P.J. de Visser, T. Zijlstra, T.M. Klapwijk, P. Diener, S.J.C. Yates, J.J.A. Baselmans, H. Wang, M. Hofheinz, J. Wenner, M. Ansmann, R.C. Bialczak, M. Lenander, E. Lucero, M. Neeley, A.D. O'Connell, D. Sank, M. Weides, A.N. Cleland, J.M. Martinis In Josephson quantum information processing superconducting coplanar waveguides are used as memory elements and coupling buses. Quality factors of these resonators reach up to a million at high excitation powers, but decrease down to below 100x10$^3$ at the single photon level in the presently used materials, such as Al and Nb. We report quality factors of up to 500x10$^3$ by using NbTiN or Re and removing the dielectric from regions with high electric fields. Using a model-analysis and by a comparison with Ta, the crucial sources of intensity-dependent loss are dielectrics on the surface of the metal and substrate. Our approach shows that using non-oxidizing superconductors such as Re and NbTiN and removing dielectrics is a straightforward route to high quality factors in the single photon regime. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z29.00004: Low-loss superconducting microwave resonators with NbN films C. Song, B. Xiao, M. Ware, B.L.T. Plourde The native oxide that forms on the surface of most superconducting thin films contains a distribution of two-level system (TLS) defects that results in a significant microwave loss channel at low temperatures and powers. One of the key limitations in the quality factor of microwave devices in this regime for superconducting quantum information processing schemes is due to this surface loss mechanism. Thus, nitride superconducting materials are promising candidates due to their lack of a significant surface oxide. We have fabricated coplanar waveguide microwave resonators from reactively sputtered NbN films on sapphire and Si substrates. We characterize the resonators with measurements of the center frequency and quality factor as a function of temperature and power. In the low-temperature and low-power limit, we have observed quality factors for NbN resonators in excess of 200,000. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z29.00005: Low Loss Superconducting Titanium Nitride Coplanar Waveguide Resonators Michael Vissers, David Wisbey, Jiansong Gao, Jeffrey Kline, Martin Weides, David Pappas The introduction of new, low loss superconducting materials will be necessary for the improvement of superconducting qubits. To fulfill this aim, thin films of titanium nitride (TiN) were sputter-deposited onto intrinsic Si and c-plane sapphire wafers with and without SiN buffer layers. The films were then fabricated into RF coplanar waveguide resonators, and internal quality factor measurements were taken at millikelvin temperatures in both the high and low power limits, i.e. many and single photon regimes, respectively. At high power, internal quality factors (Qi's) higher than 10$^7$ were measured for multiple TiN films with a predominantly (200) orientation. Films that showed significant (111) texture invariably had much lower Qi's in this regime, on the order of 10$^5$. Our studies show that the (200) TiN is favored for growth at high temperature on either bare Si or substrates with SiN buffer layers. However, growth on bare sapphire or Si (100) at low temperature resulted in primarily a (111) orientation. Ellipsometry and Auger measurements indicate that the (200) TiN growth on the bare Si substrates is correlated with the formation of a thin, $\sim $2nm, layer of SiN during the pre-deposition procedure. We found that TiN grown on these surfaces also showed significant increases of Qi in the low power limit, while thicker SiN buffer layers resulted in reduced Qi's. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z29.00006: Dielectric loss measurements using an embedded transmission line resonator Bahman Sarabi, M.J.A. Stoutimore, Moe Khalil, Sergiy Gladchenko, Alexander Kozen, Gary Rubloff, F.C. Wellstood, J.C. Lobb, K.D. Osborn Lossy dielectrics are a major source of decoherence in superconducting qubits. Superconducting linear resonators have proven to be ideally suited for measuring loss in different dielectrics due to their versatility and relative simplicity in design, fabrication, and measurement. We will present data from samples where the low-loss coplanar resonators are fabricated on top of AlOx dielectric films grown using atomic layer deposition (ALD). Although the low-power loss can be extracted from this geometry, embedding the dielectric under study between metal films has advantages that we will discuss. In addition, ALD films can be grown conformally and without pinholes to small thicknesses in comparison to conventional PECVD films. This allows us to make lumped-element resonators with a relatively small footprint, which can easily be embedded within the transmission line. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z29.00007: Design and Fabrication of High Q Titanium Nitride Resonators David Wisbey, Jiansong Gao, Michael Vissers, Jeffrey Kline, Martin Weides, David Pappas Titanium nitride (TiN) is a new material that shows promise in quantum information circuits as a low loss material for resonators, and as a multiplexed kinetic inductance photon detector. We have measured lumped element LC resonators and coplanar waveguides resonators. For the lumped element resonator we report internal quality factor (Qi) of over 300,000 at low power, in the single photon regime, and 4 million at high power, and for a half wave coplanar waveguide we report low power Qi of 800,000 and high power Qi of 5 million. We found that overetch in single layer devices can shift the resonance frequency and affect the internal quality factor Qi, and that as the trench depth grew, both the resonance frequency and internal quality factor increased. When designing resonators it is important to know quantities such as the kinetic inductance, superconducting transition temperature (Tc), penetration depth, and amount of overetch so the resonator can be accurately simulated. [Preview Abstract] |
Session Z33: Quantum Fluids and Solids II
Sponsoring Units: DCMPChair: John Davis, University of Alberta
Room: C143/149
Friday, March 25, 2011 11:15AM - 11:27AM |
Z33.00001: Dissipation of compound torsional oscillator loaded with solid $^4$He containing $^3$He impurity level from 0.3 to 25 ppm Patryk Gumann, Michael Keiderling, David Ruffner, Harry Kojima High sensitivity of supersolid phenomenon in solid $^4$He to low levels of $^3$He impurity($x_3$) is a puzzle not yet understood. We have analyzed the data taken using our compound torsional oscillator on the variation of dissipation as $x_3$ was varied between 0.3 and 25 ppm. The compound oscillator allows studies of the dissipation at two oscillator mode frequencies(0.5 and 1.2 kHz). Arrhenius plots of temperatures, where peaks in dissipation occur, vs. frequency allow extracting the activation energy and the characteristic time. The data are consistent with distributions of activation energy whose widths increase with $x_3$ but the mean value of $\sim$ 430 mK independent of $x_3$. The characteristic time varies approximately as $\propto x_3^{2/3}$. Temperature dependence of the dissipation is consistent with Debye model but frequency dependence is not. We give an interpretation of the characteristic time in terms of diffusion of $^3$He along dislocation lines. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z33.00002: The rotation anomaly of high quality $^{4}$He single crystals Xavier Rojas, Ariel Haziot, Joshua T. West, Moses H.W. Chan, Humprey Maris, S\'ebastien Balibar We have built a transparent torsional oscillator in order to monitor the growth of $^{4}$He crystals near 20 mK. It allows us to measure the rotational inertia of high quality oriented single crystals and compare it with low quality crystals or polycrystals grown at constant volume. It is also possible to vary the $^{3}$He concentration from 0 to 0.3 ppm (natural purity). Since the change in TO period associated with the change in shear modulus of the He sample could be calculated, we could see if supersolidity is really due to superflow along dislocation lines. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z33.00003: Shear Modulus of Solid Helium Confined in Aerogel Arif Rabbani, John Beamish Torsional oscillator experiments on $^4$He show supersolid behavior which appears to be associated with disorder. However, confining helium in the pores of an aerogel does not enhance the supersolid decoupling, even though x-ray measurements confirm that the crystals are highly disordered. Solid helium's shear modulus also shows anomalous behavior below 150 mK, stiffening as mobile dislocations are pinned by $^3$He impurities at low temperatures. A highly porous material such as aerogel should also provide effective pinning sites for dislocations. We have made shear modulus measurements on solid $^4$He grown in a 95\% porosity aerogel. We see large modulus decreases as the samples are warmed but these occur at much higher temperatures and over a broader range than in bulk $^4$He. The frequency dependence of the modulus and dissipation are consistent with a thermally activated process. The activation energies are roughly 10 to 15 K and may be associated with vacancy motion. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z33.00004: Possible glass anomalies in the shear modulus and dielectric function of solid helium Jung-Jung Su, Matthias J. Graf, Alexander V. Balatsky The shear modulus of solid $^4$He exhibits an anomalous change at low temperature that is qualitatively similar to a frequency change in torsional oscillator experiments. We propose that in solid $^4$He the stiffening of the shear modulus with decreasing temperature can be described with a generalized susceptibility including a glassy backaction by assuming a distribution of temperature-dependent relaxation times $\tau(T)$. The glass susceptibility captures the freezing out of glassy degrees of freedom below a characteristic crossover temperature $T_X$, when the dynamic response of the solid satisfies $\omega \tau(T_X) \sim 1$, thus leading to a viscous response. We predict that the maximum change of the amplitude of the shear modulus and the height of the dissipation peak are independent of the applied frequency $\omega$. Recent measurements of the dielectric function $\epsilon(\omega)$ by the UFL group show a similar amplitude increase. We propose that changes in $\epsilon(\omega)$ are due to the glassy dynamics of low-lying excitations and are related to the shear modulus through acousto-optical coupling. We predict a dissipation peak in the imaginary part of the dielectric function, where the change in the real part is largest. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z33.00005: Stress induced roughening of superclimbing dislocation in solid $^4$He Darya Aleinikava, Anatoly Kuklov We investigate numerically superclimb [1] of dislocation in solid $^4$He biased by externally imposed chemical potential $\mu$. The effective action takes into account quantum phase slips in the core superfluid as well as the core displacement in Peierls potential within the Granato-L\"{u}cke string model. The bias produces stress on the core and this can result in dislocation roughening. Such roughening is characterized by hysteretic behavior at temperatures (T) below some threshold $T_{\rm hyst}$. At $T>T_{\rm hyst}$ strong resonant peaks develop in the dislocation differential response. These peaks exhibit periodic behavior vs $\mu$, with the period determined by Peierls potential and dislocation length. We explain these effects by thermally assisted tunneling of jog-antijog pairs across the barrier created by Peierls potential and the bias. Since superclimbing is controlled by core superflow, speed of sound along the superfluid core exhibits dip-like features at the peak positions. We propose that this effect is seen in the mass transport experiment [2].\\[4pt] [1] S. G. S\"{o}yler, et al, Phys. Rev. Lett. {\bf 103}, 175301 (2009).\\[0pt] [2] M. W. Ray and R. B. Hallock, Phys. Rev. Lett. {\bf 105}, 145301 (2010). [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z33.00006: Andreev-Lifshitz Theory Applied to Normal Solids under Pressure Matthew Sears, Wayne Saslow On letting the superfluid density go to zero, the Andreev-Lifshitz hydrodynamic theory of supersolids becomes applicable to an ordinary solid.\footnote{A. F. Andreev and I. M. Lifshitz, Sov. Phys. JETP 29, 1107 (1969).} Under applied pressure $P_{a}$, needed to produce solid He$^3$ and He$^4$ or to be of geophysical relevance, the system has both an elastic stress $\lambda_{ik}$ and an internal pressure $P$, with $P\delta_{ik}=P_{a}\delta_{ik}+\lambda_{ik}$ in equilibrium. $P$ may be thought of as being due to a vacancy fluid. For $P_a$ small compared to the bulk modulus, Maxwell relations give $P\sim P_{a}^{2}$. The dynamical equations lead to three sets of propagating elastic modes (longitudinal and transverse sound) and two diffusive modes (one largely of entropy density and one largely of vacancy density -- or, more generally, defect density), all of which we study for non-zero $P_{a}$.\footnote{M. R. Sears and W. M. Saslow, Phys. Rev. B 82, 134304 (2010).} The vacancy diffusion mode has diffusion constant $D_{L}\sim P_{a}^{2}$, and is diffusive because its associated internal pressure fluctuation $P'$ nearly cancels its lattice stress fluctuation $\lambda'_{ik}$. This mode permits the system to respond differently to transducers with different surface treatments. We specifically have in mind solid $^4$He, which requires $P_a \sim 25$ bars to solidify; however, the results should apply to any solid under pressure. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z33.00007: Quantum phases of grain boundaries in solid $^4$He Debajit Goswami, Kinjal Dasbiswas, Alan Dorsey First-principles Monte Carlo simulations show that the core of a dislocation in solid $^4$He is superfluid and has Luttinger-liquid like properties [Phys. Rev. Lett. {\bf 99}, 035301 (2008), Phys. Rev. Lett. {\bf 103}, 175301 (2009)]. Low angle grain boundaries in crystals can be thought of as a linear array of dislocations, suggesting that a grain boundary in solid $^4$He can be modeled as an array of coupled Luttinger liquids. By exploiting this analogy, in this work we study the quantum phases of the grain boundary, as well as analogies with dipolar condensates in one dimensional optical lattices. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z33.00008: Supersolid $^4$He monolayer Massimo Boninsegni The conditions for the existence of a low temperature supersolid $^4$He monolayer are investigated by Monte Carlo simulations. The crystalline film considered here is one that not registered with the underlying substrate crystal structure. Its superfluid response is underlain by large zero-point motion of atoms in the direction perpendicular to the substrate. It is proposed that the physics described here might be observable in a helium film adsorbed on a Ni substrate. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z33.00009: U(1) $\times$ U(1) Kosterlitz-Thouless transition of the Larkin-Ovchinnikov phase in an anisotropic two-dimensional system Chungwei Lin, Xiaopeng Li, W. Vincent Liu We study Kosterlitz-Thouless (KT) transitions of the Larkin-Ovchinnikov (LO) phase for a two-dimensional system composed of coupled one-dimensional tubes. The main character of LO phase is a stripe structure (periodic in only one direction) in the order parameter. The low energy excitations involve the deformation of the stripe configuration and the fluctuation of the phase which can be described by two anisotropic XY model. We compute from a microscopic model the coefficients of XY model from which the KT transition temperatures are determined. We found the $T^{KT} \propto t_{\perp}$ for small intertube tunneling $t_{\perp}$. As $t_{\perp}$ increases the system undergoes a first-order transition to normal phase at zero temperature. Our method can be used to determine the Goldstone excitations of any stripe order involving charge or spin degrees of freedom. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z33.00010: Small-angle Neutron Scattering Measurements of Liquid Helium Mixtures Confined in MCM-41 Helmut Kaiser, Timothy Prisk, Paul Sokol, Ian Steward, Claudia Pantalei Small-angle neutron scattering (SANS) was used to study the isotopic distribution of liquid helium mixtures confined in MCM- 41, a silica glass with a 2D hexagonal net of monodisperse cylindrical pores, as a function of filling and He$^{3}$ concentration. The ordered pore array of MCM-41 gives rise to Bragg reflections with intensities determined by both how the liquid fills the pores and how the isotopes are distributed within the pores. The modulation in peak intensity can be modeled by writing down a form factors for cylindrical objects with varying scattering length density. Comparison will be made with small-angle X-ray (SAXS) scattering measurements performed with synchrotron light on liquid helium mixtures confined in aerogel. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z33.00011: Deep Inelastic Neutron Scattering Study of Nanoconfined Liquid Helium Mixtures Paul Sokol, Timothy Prisk, Narayan Das The single-particle momentum distribution $n(p)$ plays a central role in the contemporary understanding of quantum many- body systems, especially the helium liquids. The superfluid behavior of liquid He$^{4}$ below the famous lambda-point temperature is associated with the Bose condensation of a macroscopic fraction of the He$^{4}$ atoms to the zero momentum state. This manifests itself in $n(p)$ as a $\delta$-function singularity at $p = 0$. Similarly, the Fermi liquid character of He$^{3}$ is associated with a sharp discontinuity in the Fermi surface at the Fermi momentum $p_{F}$. Using the Wide Angular Chopper Spectrometer at the Spallation Neutron Source, we recently carried out a deep inelastic neutrons scattering study of dilute He$^{3}$ + He$^{4}$ solutions confined in mesoporous MCM-41 in order to investigate the effects of confinement on the non-classical momentum distribution of an isotopic helium solution. The Bose condensate fraction, Fermi surface, average isotopic kinetic energies, and related work in the literature will be discussed. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z33.00012: A Luttinger liquid core inside helium-4 filled nanopores Adrian Del Maestro We study the effects of confining helium-4 at low temperatures inside long narrow pores with nanometer radii using worm algorithm path integral quantum Monte Carlo. The results show that even in the phase with a finite superfluid response, the interaction between helium atoms and the surface of the pore induces radial density oscillations which decay as the center of the pore is approached. For some special radii, an ``inner tube'' of helium atoms exists, and a careful scaling analysis confirms that it behaves like a one dimensional Luttinger liquid at low energies. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z33.00013: $^4$He Adsorption on a Single Graphene Sheet: Path-integral Monte Carlo Study Yongkyung Kwon, David Ceperley We have performed path-integral Monte Carlo calculations to study $^4$He adsorption on a single graphene sheet, where the $^4$He-substrate interaction is described by the sum of the helium-carbon pair potentials. Among those proposed to account for helium scattering data on the graphite surface, we employ three different types of the inter-atomic pair potentials; a spherical 6-12 potential, an anisotropic 6-12 potential, and an anisotropic Yukawa-6 potential. Regardless of the choice of the pair potential, a first $^4$He monolayer is found to show the C$_{1/3}$ commensurate structure at a surface density of 0.0636~\AA$^{-2}$ and to go through the domain wall phases for densities above the commensurate one before crystallizing into an incommensurate triangular solid. Below the commensurate density, however, the low-temperature phase of this helium adlayer varies depending on the choice of the $^4$He-substrate interaction. The calculation based on the spherical pair potentials suggests a superfluid liquid phase at lower densities while incorporation of anisotropy into the helium-carbon pair potential results in a low-density state of a solid with clustered vacancies. Finally we observe van der Waals correlation between the upper monolayer and the one below the graphene sheet. The effects of this interlayer correlation on a possible formation of stable vacancies will be discussed. [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z33.00014: Commensurate-Incommensurate Transition in $^{4}$He Monolayer Adsorbed on a C$_{60}$ Molecule Hyeondeok Shin, Yongkyung Kwon Path-integral Monte Carlo calculations have been performed to study adsorption of $^{4}$He on a single C$_{60}$ fullerene molecule. In order to account for helium corrugations on the molecular surface, the sum of all interatomic pair potentials between a carbon atom and a $^{4}$He atom is used for the $^{4}$He-C$_{60}$ interaction. The radial density distributions reveal a layer-by-layer growth of $^{4}$He with the first adlayer being located at a distance of $\sim$ 6.2 $\AA$ from the center of a C$_{60}$ molecule. This first layer is found to exhibit various quantum states as the number of adsorbed $^{4}$He atoms $N$ varies. For $N$=32 the helium layer shows a commensurate solid structure with twenty helium atoms being localized on the tops of the hexagon centers of the C$_ {60}$ surface and the other twelve atoms above the pentagon centers. As more $^{4}$He atoms are added, a commensurate-incommensurate transition is observed. After going through various domain wall states the first layer is crystallized into an incommensurate solid for $N \sim 52$. We find that solid states observed for $N$=32,44, and 48 do not show any superfluid response even below 0.2 K while domain-wall fluids formed with 45 to 47 $^4$He atoms show significant superfluid fractions below 0.6 K. Finally different quantum states oberseved in the first $^4$He layer around a C$_{60}$ are compared with phase diagrams determined for the helium monolayer on a graphite surface. [Preview Abstract] |
Session Z35: Insulators and Dielectrics II
Sponsoring Units: DCMPChair: Oleksandr Kutana, Naval Research Laboratory
Room: C140
Friday, March 25, 2011 11:15AM - 11:27AM |
Z35.00001: Opacification of dielectrics oxides investigated by infrared emittance spectroscopy Myriam Eckes, Domingos de Sousa Meneses, Mohammed Malki, Patrick Echegut With increasing temperature, some compounds that are transparent in the near infrared range at room temperature become progressively opaque towards the liquid phase. Such a behavior deeply impacts their thermal radiative properties which knowledge is of main importance in crystal growth processes for example. To understand this phenomena, infrared emittance spectra were acquired from room temperature up to the liquid state on crystalline LaAlO$_{3}$ and LiAlO$_{2}$. The samples are heated with a CO$_{2}$ laser that allows a direct measurement of emittance up to their melting point and in a wide spectral range, i.e. between 50 and 10000 cm$^{-1}$. In the transparency region, we observed an increase of emittance up to 1000 K for LaAlO$_{3}$ (more than 1000 K before the solid to liquid transition) and no significant increase of emittance for LiAlO$_{2}$ before the solid to liquid phase transition. The experimental data have been fitted with a dielectric function model including a Debye relaxation term. The absorption mechanism at the origin of the opacification is thermally activated and has a relaxation time compatible with a motion due to electrons. Electrical conductivity measurements of these compounds were also made up to 800K. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z35.00002: Non-equilibrium ballistic phonon transport in microstructures Jared Hertzberg, Obafemi Otelaja, Richard Robinson We demonstrate a method to locally excite and detect phonon modes in silicon microstructures. Decay of quasiparticles injected into an adjacent superconducting film excites phonons in a non-thermal spectral distribution [1]. Phonons of frequency of order 100 GHz are detected by the excitations they cause in a second superconducting film, after ballistically traversing microstructures of 10 to 50 micron dimension. Measurements are made at temperatures of 0.3 to 1.2 K. Such a device advances the goal of building a nanoscale phonon spectrometer to study acoustic confinement and surface scattering effects. This work is supported by KAUST (KUS-C1-018-02), NSF (DMR 0520404), and DOE (DE-SC0001086). \\[4pt] [1] W. Eisenmenger, A. H. Dayem, Phys. Rev. Lett. 18, 125 (1967). [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z35.00003: Dielectric measurements above 100 GHz using a high-Q open hemispherical resonator Rezwanur Rahman, John Scales High-Q cavities can be used to study materials (or perturbations to materials) whose effects are too small to be seen by other methods. We have developed a millimeter wave cavity operating above 100 GHz in order to measure the dielectric properties of thin films and ultra low loss materials. The cavity is a open hemispherical resonator. Millimeter waves are introduced and measured via 2 closely spaced sub-wavelength holes in the center of a 15cm spherical, copper mirror. Cavity perturbation techniques are applied to extract the complex permittivity of a sample. This is a paraxial system and axissymmetric modes are of primary interest but nonaxisymmetric modes area also generated and need to be dealt with. Applications to thin films and other materials will be shown. \noindent This was was partially supported by US Department of Energy under grant DE-FG02-09ER16018 [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z35.00004: ABSTRACT WITHDRAWN |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z35.00005: Comparison between Resonant Inelastic X-Ray Scattering and the Dynamical Structure Factor Chunjing Jia, Cheng-Chien Chen, Brian Moritz, Adam Sorini, Thomas Devereaux Momentum dependent resonant inelastic X-ray scattering (RIXS) is an effective probe of many-body excitations. Theoretical and experimental work has shown that under certain circumstances RIXS can be viewed as an approximate probe of the dynamical structure factor $S(q,\omega)$. We perform cluster diagonalization combined with the bi-conjugate gradient stabilized method to model the RIXS spectra and $S(q,\omega)$ for the single-band and multi-orbital Hubbard models. While these two cross sections share some similar features, there are significant quantitative differences, which highlight the qualitative distinction between these two probes. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z35.00006: Observations of ferroelastic switching by Raman spectroscopy in 18-percent ceria-stabilized zirconia Amy Bolon, Juan Munoz Saldana, Molly Gentleman Ferroelastic switching has been shown to be responsible for significant increases in the toughness of tetragonal zirconia ceramics. Observations of switching and measurements of coercive stress have generally been limited to TEM studies on large single crystals. In this study we show that it is possible to observe ferroelastic switching in 18 mole-percent ceria stabilized zirconia using polarized confocal Raman spectroscopy. Observations were made on bulk polycrystalline samples indented with a standard Vicker's indent and exhibited reorientation of crystal domains along the crack as well as near the crack tip. Coercive stress measurements were made by loading the samples uniaxially while making measurements of domain orientation. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z35.00007: Impact of Bond Coordination and Percolation on Mechanical Properties of a-SiC:H Thin Films Sean King, Jeff Bielefeld, Brian Daly Plasma Enhanced Chemically Vapor Deposited a-SiC:H thin films are compelling materials for both semiconductor nano-electronic and MEMS/NEMS technologies due to the extreme chemical inertness of this material and the ability to tune a variety of material properties across an extreme range of values. As one example of the latter, we demonstrate that using PECVD the Young's modulus of a-SiC:H thin films can be varied from $<$ 10 GPa to $>$ 200 GPa and the Hardness can be varied over an equally impressive range of $<$ 0.5 to $>$ 30 GPa. Utilizing Fourier Infrared-Transform Spectroscopy, we show that this remarkable range in materials properties is achieved primarily via the incorporation of terminal hydrogen groups which lowers the overall connectivity of the Si-C network bonding. We find that once the average network coordination number for Si and C falls below 2.6, the Si-C network becomes under constrained and there is a loss of rigidity percolating through the system. There results are compared and found to be in agreement with constraint theory for amorphous materials. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z35.00008: First Principles Investigation of Structure and Electronic Properties of $a-$ Si$_{3}$N$_{4}$ Ravi Pramod Vedula, Nathan L. Anderson, Alejandro Strachan We use a combination of molecular dynamics with empirical inter-atomic potentials and density functional theory (DFT) calculations to generate an ensemble of statistically independent, well relaxed $a$-Si$_{3}$N$_{4}$ structures. Variations in the annealing conditions used to generate the structures lead to zero-stress structures spanning a wide range of densities (2.6g/cm$^{3}$ to 3.1g/cm$^{3})$ but exhibiting very similar cohesive energies. The bulk modulus was found to be varying between 110-180 GPa depending on the density. The predicted density variation agrees well with the range in experimentally observed density, resulting from different fabrication conditions. The radial distribution functions and angle distributions for different densities are in good agreement with diffraction experiments; further validating our models. The slow annealing procedure used to generate the structures leads to well equilibrated structures with relatively small density of coordination defects and several defect free structures. We also compute the formation energy and charge transition levels for the defects found. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z35.00009: Thermal transport and surface sensitivity in suspended amorphous silicon nitride thin films R. Sultan, A.D. Avery, D. Bassett, B.L. Zink Thermal transport in disordered materials continues to provide surprising new results, which often have direct consequences for applications ranging from quantum computation to cutting-edge cosmology. For example, some of the most sensitive detectors of radiation currently in use are thermal detectors that use highly sensitive micromachined thermometers to register the temperature rise caused by absorption of incident light or particles. To achieve this sensitivity, the thermometer is commonly thermally isolated using free-standing amorphous silicon-nitride membranes. As a result, the heat flow through this material is often a critical design parameter. In this talk we present recent measurements of thermal conductivity of a large number of suspended silicon-nitride structures. The results show not only deviation from previously reported measurements, but also very clear but somewhat puzzling dependence on the surface preparation of the structures. Such surface-sensitivity is expected at very low temperatures, but is seen in our experiments even near $300$ Kelvin, where one normally expects heat flow to be dominated by carriers with very short mean-free-paths. We discuss possible interpretation of our results and compare to other recent surprises in the thermal properties of disordered materials. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z35.00010: Accessing short and intermediate range orders of silicate glasses by infrared spectroscopy Domingos de Sousa Meneses, Cristiane N. Santos, Myriam Eckes, Yann Vaills, Patrick Echegut The characterization of short and intermediate range orders in glasses is a very active field since this knowledge is of main importance for understanding how order impacts their properties. The chemical simplicity of binary silicate glasses makes them model systems that are suitable to show how their dielectric functions include such kind of information. We show that it is possible to extract from infrared reflectivity measurements no solely quantitative information on short range order, i.e. populations of Q$^{n}$ tetrahedral units (n : number of bridging oxygens) but also intermediate range information like the presence and evolution of 3D network silicate clusters and silicate sheet clusters. Examples will be given for alkaline silicates glasses and discussed in the light of predictions obtained from structural glass models and literature results. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z35.00011: In situ high-temperature infrared emissivity spectroscopy of silicate glasses and glass-ceramics Cristiane N. Santos, Domingos de Sousa Meneses, Valerie Montouillout, Patrick Echegut Glasses and glass-ceramics are materials of widespread application in industry, building, photonics, microelectronics and medicine. Glass-ceramics are obtained by controlled glass crystallization, and many efforts have been done in the last years to better understand the structural changes occurring in this process. Here we show that in situ infrared emissivity spectroscopy is also a suitable technique for this purpose and a wide spectral and temperature range could be accessed (25-16000 cm$^{-1}$ and 400-1700 K, respectively). We use a home-made instrument composed of two spectrometers, and a CO$_{2}$ laser for locally heat the glass samples up to the melt. A dielectric function model was applied to fit the experimental data and compute the materials optical properties. We show that using new decomposition procedure quantitative information on the distribution of the $Q^{n}$ tetrahedral units (n being the number of bridging oxygen) can be obtained. The results at room temperature are in good agreement with recent molecular dynamics simulations. The major changes occur during quartz crystallization, with a remarkable increase of $Q^{4}$ units. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z35.00012: CrN electronic structure and vibrational modes: an optical analysis Xunyuan Zhang, Daniel Gall The electronic structure of the paramagnetic insulating phase of CrN is investigated using optical spectra from epitaxial CrN(001) layers. The imaginary part of the dielectric function indicates direct interband transitions at \textit{h$\omega $} = 0.64, 1.5 and 2.9 eV, and suggests a depletion in the density of states at the Fermi level. This is attributed to local magnetic moments that cause splitting of the $t_{2g}$ bands and the formation of an indirect band gap of 0.19$\pm $0.46 eV, as estimated by comparing the optical transition energies with reported direct gap energies from calculations with different magnetic ordering and Coulomb interaction terms. The dielectric function shows a strong resonance at \textit{h$\omega $}$_{0}$ = 48.7$\pm $0.2 meV, and values of dielectric constants \textit{$\varepsilon $}$_{dc}$ = 53$\pm $5 and \textit{$\varepsilon $}$_{\infty }$ = 22$\pm $2, respectively, providing values for transverse and longitudinal optical phonon frequencies at the zone center of 11.7 and 18.2 THz, respectively. The vibrational frequencies are confirmed by Raman spectroscopy peaks at 800, 1170, and 1330 cm$^{-1}$ which are attributed to 2TO(X), 2LO(X), and 2LO(L) modes.\\[4pt] X.Y. Zhang and D. Gall, \textit{Phys. Rev. B} \textbf{82}, 045116 (2010). [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z35.00013: Surface states control and thermoelectric properties of CrN films Antia S. Botana, Alberto Pi\~neiro, Victor Pardo, Daniel Baldomir The manipulation of the electronic structure of a material by quantum confinement has attracted much attention recently, e.g. the appearance of conducting surface states or the novel phenomena at the interface between oxides being the most notorious. CrN is a degenerate semiconductor with large thermoelectric power in the bulk[1]. We have performed electronic structure calculations in thin CrN films and studied the evolution of the electronic structure and conduction properties (calculated using Boltzmann formalism) with thickness, focussing on the evolution of the Seebeck coefficient and electrical conductivity. We have utilized a density functional theory (LDA+U) formalism for our calculations with the U value (4 eV) that reproduces the experimental gap for the material as a bulk. When nanostructured, (almost conducting) surface states arise due to the dangling bonds at the surface that reduce the symmetry of the octahedral crystal field around Cr. When the film is relaxed, a gap opens and the material remains semiconducting. In addition, we have analyzed the role of these surface states in a possible improvement of the CrN thermoelectric properties by studying the figure of merit dependence with thickness. \newline [1] C. X. Quintela et al., Appl. Phys. Lett. 94, 152103 (2009). [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z35.00014: Temperature Dependence of Internal Deformation Field in Zeolites Wonsuk Cha, Sanghoon Song, Nak Cheon Jeong, Tung Pham, Ross Harder, Gang Xiong, Kyung Byung Yoon, Ian K. Robinson, Hyunjung Kim We studied temperature dependent internal deformation field distributions in zeolite microcrystals using coherent x-ray diffraction. We measured the coherent x-ray diffraction patterns around (200) and (020) Bragg peaks of the crystals. The three-dimensional real space images were obtained by phasing and inverting the oversampled diffraction patterns using the phase retrieval algorithm combined with error reduction and hybrid input-output method. The internal deformation fields show unusual temperature dependent behaviors which might be originated from the synthesis and calcination process. [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z35.00015: Tension in the Initial Growth Stages of Sputter Deposited WSi$_{2}$ on Si in Multilayers Albert Macrander, Kimberley MacArthur, Bing Shi, Ray Conley We concentrate on the initial stages of growth of sputtered amorphous multilayers with equal WSi$_{2}$ and Si layer thicknesses of 5.5 nm, and we report observations of tension in the first 1.1 nm of WSi$_{2}$ grown on Si layers. Measurements of wafer curvature were made \textit{in-situ} in the sputtering chamber. Stresses in the conjugate interface of Si on WSi$_{2}$ were observed to be significantly smaller. A clear asymmetry in the stress of these conjugate interfaces rules out an explanation based solely on lattice misfit. We find a value of 1.3 x 10$^{10}$ dynes/cm$^{2}$ for the biaxial film stress at the WSi$_{2}$ on Si interface grown at 2.3 mTorr of Ar pressure, a value comparable to values calculated for hybridization of Si(111) facets by various adatoms [1]. Our observations thereby support a model for chemically induced changes in packing density during sputtering of the interfaces. As an example of a particular application, WSi$_{2}$/Si multilayers consisting of many hundreds periods have been used to make lenses for nanofocusing of hard x-rays [2-3].\\[0pt] [1] D. Vanderbilt, PRL 59, 1456 (1987). [2] H.C. Kang et al. PRL 96, 127401 (2006); APL 92, 221114 (2008). [4] L. Zhou et al. PRB 82, 075408 (2010). [Preview Abstract] |
Session Z39: Surfaces, Interfaces and Colloids
Sponsoring Units: DCPChair: Phillip Geissler, University of California, Berkeley
Room: A124/127
Friday, March 25, 2011 11:15AM - 11:27AM |
Z39.00001: Two-dimensional soft solids: a rheological study Gabriel Espinosa, Duyang Zang, Dominique Langevin, Bernard Binks Many soft solids, such as concentrated suspensions, emulsions, foams, behave in a similar way under an applied shear: they exhibit a Maxwell-type relaxation with a characteristic relaxation time that varies inversely with the applied shear rate. When the storage and loss moduli are measured at different frequencies and constant shear rate, the curves obtained can be rescaled [1]. We will show here that the behavior in two dimensions can be strikingly similar. We will present data on monolayers of nanoparticles and on mixed layers made with DNA and surfactant. The physical origin of the relaxation time will be discussed. The nonlinear behavior will be also discussed. Depending on the compaction degree, the layers can behave as brittle or plastic solids. This has been confirmed by images of the layers after a shear deformation made using Brewster angle microscopy. \\[4pt] [1] H. M. Wyss, K. Miyazaki, J. Mattsson, Z. Hu, D. R. Reichman, and D. A. Weitz, Phys. Rev. Lett. \textbf{98}, 238303 (2007). [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z39.00002: ABSTRACT WITHDRAWN |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z39.00003: Direct measurement of short range colloidal interactions using digital holographic microscopy Rebecca W. Perry, Jerome Fung, David M. Kaz, Guangnan Meng, Vinothan N. Manoharan Using digital holographic microscopy, we record the 3D positions of micron-sized polystyrene particles as they approach and retreat from each other. Analysis of the holograms using an exact solution for the scattering from pairs of spheres allows us to separate vibrational, translational, and rotational motion. The vibrational mode supplies the information needed to characterize the inter-particle interactions down to separation distances on the order of 10 nm. In particular, we study an aqueous system of one micron diameter sulfate-coated polystyrene beads suspended in a solution of 95 nm hydrogel particles. The attractive depletion interaction we measure deviates from the Asakura-Oosawa model, likely because of electrostatic interactions at these short distances. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z39.00004: Fabrication of Monolayer of Polymer/Colloids Hybrid at a Water-Air Interface Chi-Chih Ho, Ting-Hui Chen, Po-Yuan Chen, Keng-Hui Lin, Wen-Tau Juan, Wei-Li Lee Polymer-assisted assembly of polystyrene (PS) colloids with diameter ranging from 100 nm to 1 $\mu$m at water-air interface is demonstrated. Initially, PS colloids were slowly spread onto water surface and crystallized into triangular lattice with finite separation between colloids due to the coulomb repulsive force. By adding merely 1-3 ppm water-soluble polyethylene oxide (PEO) in water, the colloids gradually moved closer. Eventually, the separation between colloids was equal to or less than 60 nm determined from its diffraction pattern by a laser beam. In addition, the Brownian motion of colloids was suppressed by the PEO adsorption effect which was demonstrated from the analysis of colloids trajectory recorded by ultra high speed camera. We showed that the resulting monolayer of colloids /PEO hybrid can be deposited on various substrates, including a plastics sheet, curved surface and even across 10 $\mu$m-diameter hole. Our method may further extend the scope of nanosphere lithography technique for large area nanostructure fabrication. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z39.00005: Heterogeneous crystallization of hard-sphere colloids near a wall Kyril Sandomirski, Elshad Allahyarov, Hartmut L{\"o}wen, Stefan Egelhaaf Confocal microscopy experiments and equilibrium Brownian Dynamics computer simulations were combined to investigate heterogeneous crystallization near a hard wall in a suspension of hard-sphere colloids. Particles near the wall initially rearrange, before an extended regime of steady-state crystal growth is observed. Finally, a depletion zone develops which slows down the progressing crystal-fluid interface. In good agreement between experiment and simulation, the steady-state growth velocity shows a maximum in its dependence on the bulk volume fraction. Beyond this, these techniques allow us to obtain local microscopic information on the level of individual particles, namely the temporal evolution of the density profiles in the fluid and crystal phase as well as the width of the interface. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z39.00006: $^{19}$F NMR Study of Molecular Aggregation of Lithium Perfluorooctylsulfonate in Water at Temperatures from 30 to 250 \r{ }C Dobrin Bossev, Mutsuo Matsumoto, Masaru Nakahara $^{19}$F NMR chemical shifts have been measured to determine the critical micelle concentration (cmc) and aggregation number ($m)$ of lithium perfluorooctylsulfonate (LiFOS) in water at temperatures ranging from 30 to 250 \r{ }C under the saturation pressure. The cmc slightly increases in the temperature range from 30 to 100 \r{ }C, whereas above 125 \r{ }C the increase is much steeper; cmc is 6.5, 13.2, and 161 mmol kg$^{-1}$ at 30, 100, and 250 \r{ }C, respectively. The aggregation number, estimated on the basis of a mass action model exhibits a rapid decrease in the temperature range of 30 - 125 \r{ }C and becomes almost constant at higher temperatures; $m$ is 36, 6.8, and 2.4 at 30, 100, and 250 \r{ }C, respectively. Thermodynamic parameters indicate similar values for the free energy at all temperatures, and constant values for the enthalpy and entropy of aggregation at high temperatures. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z39.00007: Dielectric dispersion of clustered living cells via the boundary integral equation method Sai Kit Yung, Kin Lok Chan, Kin Wah Yu We have developed a boundary integral equation (BIE) method for computing the dielectric response of clusters of biological particles like living cells. In the BIE, we formulate a surface integral equation for the scalar potential for an arbitrary number of particles of various shapes [1]. BIE method avoids matching the complicated boundary conditions on the surfaces of the particles. Numerical solutions of the eigenvalue equation yield a dielectric dispersion spectrum through the spectral representation [2]. While BIE method is valid for arbitrary surfaces, we confirm it for two approaching cylinders and a concentric cylinder. In many typical cases, the numerical results are in excellent agreement with the exact analytic results. Moreover, BIE method offers a convenient way to compute the alternating current responses, and hence the dielectric dispersion of clustered cell suspensions. \\[4pt] [1] Yu KW, Wan JTK, Computer Physics Communications 142, 368 (2001). \\[0pt] [2] Huang JP, Yu KW, Phys. Rep. 431, 87 (2006). [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z39.00008: Multi scale computer simulations of the self-assembly of block copolymeric beta-peptides Jagannath Mondal, Arun Yethiraj There is considerable interest in a class of molecules made from $\beta$-amino acids (which contain an additional backbone carbon atom when compared with natural amino acids). Block copolymers of $\beta$-peptides, where one block is hydrophobic and the other is hydrophilic, self-assemble into micelles. In this work we use computer simulations to provide insight into the self-assembly of these molecules. All-atom simulation results for the free energy of association of a pair of these block co polymeric $\beta$- peptides show that a {\it homochiral} hydrophobic block promotes self assembly compared to a {\it heterochiral} hydrophobic block, consistent with experiment. We have also developed a coarse-grained model for these block co-polymers and simulations using this model show that these molecules spontaneously forms micelles, and the morphology of these micelles is concentration dependent, with spherical micelles at low concentrations and worm-like micelles at high concentrations. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z39.00009: Ab initio Study of Structure and Hydrogen Bonding of Cellulose Crystals and Surfaces James Davenport, Yan Li We have studied the equilibrium structure and hydrogen bonding of cellulose crystals and surfaces using semi-empirical dispersion corrections to density functional theory (DFT+D)[1], which has been shown to be an efficient alternative to more advanced methods for weakly bound aromatic assemblies[2]. The predicted crystal structures for both I$_\alpha$ and I$_\beta$ phases agree well with experiments. The cohesive energy was decomposed into interchain and intersheet interactions and analyzed in terms of hydrogen bonding and van der Waals dispersion forces. Both interactions were found to be responsible for holding cellulose sheets together. In particular, the dispersion corrections to DFT proved to be indispensable in reproducing the equilibrium intersheet distance and binding strength. Adsorption energy and configuration of water molecules on cellulose surfaces were found to depend sensitively on the surface orientation, adsorption site and contribution from vdW interactions.\\[4pt] [1] S. Grimme, J. Comput. Chem. 27, 1787 (2006).\\[0pt] [2] Y. Li, D. Lu, H-V. Nguyen and G. Galli, J. Phys. Chem. A 114, 1944 (2010). [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z39.00010: Interfacial free energy and stiffness of the solid-melt interface of NaCl Tatyana Zykova-Timan, Erio Tosatti, Daan Frenkel The importance of the interfacial free energy for the equilibrium morphology of crystals is well understood. In contrast, much less is known about to the so-called ``interfacial stiffness'' that governs fluctuations of, e.g., solid-liquid interfaces. We carried out molecular dynamics simulations of capillary wave fluctuations on various faces of NaCl crystals in contact with its melt, that provides new information on the behaviour of this interface at the atomistic level. The capillary fluctuations connect directly with the interfacial stiffness, and indirectly also to the interface free energy. In our simulations we studied the (100)-liquid interface and adjacent vicinals. From the angular dependence of the surface stiffness, we deduce an estimate of NaCl(100)-melt interfacial free energy and discuss limitations of the fluctuation approach. Finally we compare this estimate of the surface free energy with values obtained through other methods [1,2,3] and discuss the differences [4]. \\[4pt] [1] T. Zykova-Timan, D. Ceresoli, U. Tartaglino, and E. Tosatti, Phys. Rev. Lett. 94, 176105 (2005). [2] C.Valeriani, E. Sanz and D. Frenkel, J. Phys. Chem. ,122, 194501 (2005). [3] T. Zykova-Timan, C. Valeriani, E. Sanz, E. Tosatti and D. Frenkel, Phys. Rev. Lett., 100, 036103 (2008). [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z39.00011: Utilization of Metal Oxides and Chalcogenides Stabilized in Organic Solvents Lester Lampert, Robby Flaig, Jorge Camacho, James Hamilton Metal oxides and metal chalcogenides are important materials for a variety of applications including photocatalysis for decomposition of water, conductive and optical coatings, catalysts, photovoltaics, pryoelectrics, self-cleaning surfaces, pigments, and high efficiency Li-insertion materials in batteries among many other applications. Fundamental discoveries of surprising solubility of insoluble materials such as single and multi-walled carbon nanotubes and graphene has lead us to discover that certain metal oxides and metal chalcogenides such as TiO2 are soluble in certain solvents. Due to the industrial importance of TiO2, discovering stable pure solvent systems demonstrates a possibility to avoid surface modification of TiO2 nanoparticles by use materials such as of (3-methacryloxypropyl)-trimethoxysilane and various other methods of artificial stabilization. We have created thin films of TiO2, transparent ultraviolet (UV) --absorptive polymers, and Li-ion battery anodes with graphene-TiO2 hybrid materials. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z39.00012: EPR, Endor and DFT Studies on X-Irradiated Single Crystals of L-Lysine HCl 2H$_{2}$O and L-Arginine HCl H$_{2}$O Yiying Zhou, William H. Nelson When proteins and DNA interact, arginine and lysine are the two amino acids most often in close contact with the DNA. In order to understand the radiation damage to DNA in vivo, which is always associated with protein, it is important to learn the radiation chemistry of arginine and lysine independently, and then complexed to DNA. This work studied X-irradiated single crystals of L-lysine$\cdot$HCl$\cdot$2H$_{2}$O and L-arginine$\cdot$HCl$\cdot$H$_{2}$O with EPR, ENDOR techniques and DFT calculations. In both crystal types irradiated at 66K, the carboxyl anion radical and the decarboxylation radical were identified. Specifically, the calculations performed on the cluster models for the carboxyl anion radicals reproduced the proton transfers to the carboxyl group from the neighboring molecules through the hydrogen bonds. Moreover, computations supported the identification of one radical type within irradiated arginine as the guanidyl radical anion with an electron trapped by the guanidyl group. Based on the radicals detected in the crystal irradiated at 66K and at 298K, and the annealing experiments from the irradiation at 66K, the mechanisms of the irradiation damage on lysine and arginine were proposed, and the possible effects of irradiated arginine and lysine to the DNA within chromatin were analyzed. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z39.00013: Detection of Nitro aromatics via fluorescence quenching of pegylated and siloxanated 4, 8-dimethylcoumarins Abhishek Kumar, Mukesh Pandey, Jayant Kumar There is considerable interest in developing chemical sensors for detection of trace explosives. Optical sensors, which rely on the change in optical properties of the material, proved to be very effective. Therefore, there is a need to develop materials for optical sensors which can interact specifically with analytes and detect them sensitively. Here, we report the synthesis of co-polymers of 4, 8-dimethylcoumarins with poly (ethylene glycol) (PEG) and polydimethylsiloxane (PDMS) using \textit{Candida Antarctica lipase} as a catalyst under solvent-less condition. The low T$_{g}$ of PEG and PDMS may facilitate porous structure in solid films which allows quencher molecules to easily diffuse in and out of these films. In addition, the co-polymers prevent aggregation and lend themselves easily for thin film fabrication which otherwise is difficult because of low molecular weight of coumarin. Fluorescence quenching of these co-polymer in presence of nitro aromatics, 2,4-dinitrotoulne and 2,4,6-trinitrotoluene, in solution and in vapor phase will be reported. [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z39.00014: Correlated electrolyte solutions and ion-induced attractions between nanoparticles Jos Zwanikken, Monica Olvera de la Cruz Information about the degree of association can be obtained from a nonlinear Debye-Hueckel theory [1], in agreement with simulation and experimental results [2], in strong contrast with the widely applied (linear) Debye-Hueckel limiting law. The radial distribution functions calculated within this nonlinear theory are indistinguishable from molecular dynamics simulations of the restricted primitive model for divalent salts up to 0.1 molar concentrations. We apply the method to study the cohesive effects of strong couplings between ions on the effective interactions between nanoparticles, and the screening cloud around functionalized nanoparticles.\\[4pt] [1] J. W. Zwanikken, and M. Olvera de la Cruz, Phys. Rev. E 82, 050401(R) (2010).\\[0pt] [2] C. Valeriani, P. Camp, J. Zwanikken, R. van Roij, and M. Dijkstra, J. Phys.: Condens. Matter 22, 104122 (2010); Soft Matter 6, 2793 - 2800 (2010). [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z39.00015: ABSTRACT WITHDRAWN |
Session Z40: Semi Crystalline Polymers: Morphology and Electronics
Sponsoring Units: DPOLYChair: Robert Hoy, Yale University
Room: A122/123
Friday, March 25, 2011 11:15AM - 11:27AM |
Z40.00001: Structure Evolution of Propylene-1-Butylene Random Copolymer under Uniaxial Stretching: from Unit Cells to Lamellae Yimin Mao, Christian Burger, Xiaowei Li, Benjamin Hsiao Crystallization changes of propylene-1-butylene (P-H) random copolymer with low butylene content (5.7 mol\%) under uniaxial tensile deformation at high temperature (100 $^{\circ}$C) was investigated using time-resolved wide- and small-angle X-ray scattering (WAXS/SAXS) techniques. Structure and preferred orientation at length scales of crystal unit cell and lamellae were investigated explicitly using 2D whole pattern analysis. $\gamma$-phase was found to be the dominant initial modification which was transformed into $\alpha$-phase during stretching, forming more stable parallel packed polymer chains in the unit cell. 2D WAXS analysis enabled us to identify three orientation modes from different crystal forms, i.e., $\gamma$-phase with tilted cross-$\beta$ configuration, $\alpha$-phase with parallel chain packing and \emph{a}-axis orientation of $\alpha$-form crystals in daughter lamellae. 2D SAXS analysis based on stacking model enabled us to understand the development of the four-point pattern under deformation. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z40.00002: An In-Situ X-ray Scattering Study during Uniaxial Stretching of Ionic liquid/Ultra-High Molecular Weight Polyethylene Blend Xiaowei Li, Yimin Mao, Hongyang Ma, Benjamin S. Hsiao The 1-docosanyl-3-methylimidazolium bromide ionic liquid (IL) was incorporated into ultra-high molecular weight polyethylene (UHMWPE) to form IL/UHWMPE blend by solution mixing. The structure evolution of this blend system during uniaxial stretching was followed by in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. During deformation at room temperature, the elongation-to-break ratio of the IL/UHMWPE blend increased by 2 - 3 times compared with pure UHMWPE sample, where the blend did not lose the tensile strength. Deformation-induced phase transformation from orthorhombic to monoclinic phase was observed in both blend and neat UHMWPE. During deformation at high temperature (120 $^{\circ}$C), no phase transformation was observed in both samples. However, the blend showed better toughness, higher crystal orientation, and tilted lamellar structure at high strains. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z40.00003: Microscopic kinetic model for polymer crystal growth Wenbing Hu Linear crystal growth rates characterize the net result of competition between growth and melting at the liquid-solid interfaces. The rate equation for polymer crystal growth can be derived with a barrier term for crystal growth and with a driving force term of excess lamellar thickness, provided that growth and melting share the same rate-determining steps at the growth front. Such an ansatz can be verified by the kinetic symmetry between growth and melting around the melting point of lamellar crystals, as made in our recent dynamic Monte Carlo simulations. The profile of the growth/melting front appears as wedge-shaped, with the free energy barrier for intramolecular secondary crystal nucleation at its top, and with the driving force gained via instant thickening at its bottom. Such a scenario explains unique phenomena on polymer crystal growth, such as chain folding, regime transitions, molecular segregation of polydisperse polymers, self-poisoning with integer-number chain-folding of short chains, and colligative growth rates of binary mixtures of two chain lengths. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z40.00004: Polyethylene crystallization in compatibilized polyethylene/polyamide 6 blends Simona Ceccia, Katerina Hynstova, Alexandra Fabre, Lise Trouillet-Fonti, Didier Long, Paul Sotta Blends of semicrystalline polymers can exhibit much better properties than each of the pure polymers regarding e.g. impact/modulus compromise. Controlling the crystallization mechanisms (nucleation, kinetics) is a key factor to obtain the desired morphologies which lead to these unique properties. We have studied the crystallization of polyethylene (PE) in blends of PE and polyamide 6 (PA) compatibilized by PE functionalized with maleic anhydride (PE-g-MA, 1 wt{\%} MA) obtained by reactive blending. Samples with different amounts of PA6 (0-60 {\%}vol) have been investigated by polarized optical microscopy and Differential Scanning Calorimetry. The samples were heated at a temperature above the melting temperature of PE and below the melting temperature of PA, and then cooled at the selected crystallization temperature. We describe how the crystallization kinetics is modified by the presence of PA and MA. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z40.00005: Kinetic Partitioning of 1-Butene Defect in Random Propylene 1-Butene Copolymers by Time-Resolved FTIR Carolina Ruiz-Orta, Rufina G. Alamo Two different types of regularity bands are identified in a time-resolved FTIR crystallization of a series of random isotactic propylene 1-butene copolymers with a concentration of 1-butene from 2 to 19 mol{\%}. The first type are bands associated with 31 helices of isotactic sequences of different n length (n, number of monomer units). The second are regularity bands at 830, 920, 1010 and 1240 cm-1 associated with continuous sequences of the chain that include the 1-butene comonomer. Conformational changes during the isothermal crystallization process were monitored with these regularity bands. The variation of the intensity of regularity band at 920 cm-1 with crystallinity content and a shift of the 841 cm-1 (n = 12) at lower frequencies correlate with the content of comonomer included in the crystal. Changes in frequency of the n = 12 band with varying crystallization temperature (Tc), and with crystallization time at a fixed Tc are associated with the kinetic partitioning of the comonomer units, that was quantified with data obtained from 13C NMR. The frequency shift is absent in copolymers with co-units that are excluded from the crystalline regions, such as the 1-octene comonomer. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z40.00006: Homogeneous bulk, surface, and edge nucleation in crystalline nanodroplets Jessica L. Carvalho, Kari Dalnoki-Veress We present a study on the homogeneous nucleation of dewetted poly(ethylene oxide) droplets on a substrate that is itself crystallisable. While the chemical properties of the substrate prepared in either the amorphous or crystalline state are identical, the surface landscape varies widely. We observe a large difference in the substrate's nucleating ability depending on how it is prepared. Furthermore, the scaling dependence of the nucleation rate on the size of the droplets depends on the substrate surface properties. The birth of the crystalline state can be directed to originate predominantly within the bulk, at the substrate surface or at the droplet's edge depending on how we tune the substrate. \\[4pt] J.L. Carvalho and K. Dalnoki-Veress, Phys. Rev. Lett in press, 2010. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z40.00007: Molecular simulation of homogeneous crystal nucleation of n-alkane melts Peng Yi, Gregory Rutledge One of the most important phenomena in molecular systems is homogeneous nucleation of the crystal phase from a melt. This phenomenon is particularly interesting for chain molecules due to their strong anisotropy and their conformational flexibility. In this work we report the results of molecular simulations of homogeneous crystal nucleation of n-eicosane (C20) from the melt. A realistic united atom force field was employed. The crystal phase and melting behavior were first determined by ramping temperature in a set of MD simulations. The nucleation trajectory was then sampled using MD simulations at about 20{\%} supercooling; and the nucleation free energy was sampled using Monte Carlo umbrella sampling method for three temperatures, ranging from 10{\%} to 20{\%} supercooling. A first-passage time technique was used to determine the critical nucleus and the nucleation rate. Detailed examination of the simulations reveals the critical nucleus to be a bundle of stretched segments about 8 CH2 groups long, organized into a cylindrical shape. The remaining CH2 groups form a disordered interfacial layer. By fitting the nucleation free energy curve to the cylindrical nucleus model, the crystal-melt interfacial free energies are calculated to be about 10 mJ/m2 for the side surface and 4 mJ/m2 for the end surface. We also discussed the effect of using different nucleus definitions [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z40.00008: Hierarchal Ordered Structures in Hybrid Functional Supramolecules and Macromolecules Stephen Cheng, Wen-Bin Zhang, Chien-Lung Wang, Xinfei Yu, Yiwen Li, Xuehui Dong, Ryan Van Horn To create new functional materials for advanced technologies, control over their hierarchical structure and order is vital for obtaining the desired properties. We utilized and functionalized fullerene (C$_{60})$ and polyhedral oligomeric silsesquioxane (POSS), and assembed both of these particles with polymers to form those hierarchical structures. In order to do so, we have developed a novel way of attaching C$_{60}$ and POSS onto other organic materials in a highly efficient and controlled manner via ``click'' chemistry. The structure of this assembly along with the resulting ordered structures were analyzed to determine their structure-property relationships. Examples are materials of POSS-[60]Fullerenoacetate conjugate crystals, giant surfactant of polystyrene-(carboxylic acid-functionalized polyhedral oligomeric silsesquioxane) amphiphile and porphyrin-C$_{60}$ hybrids for intra- and inter-columnar in supramolecular double cable structures. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z40.00009: Nanoconfined Ferroelectricity in Polymers Lei Zhu, Fangxiao Guan, Jing Wang, Qing Wang In this presentation, a low loss polystyrene (PS) was grafted as side chains onto the P(VDF-CTFE) main chain. After PVDF crystallization, dielectric PS side chains were segregated to the crystalline-amorphous interface, forming a finite confinement layer for ferroelectric PVDF crystals. We speculated that less space charge was induced during electric poling because of the nanoscale confinement effect. Consequently, a fast discharge speed, relatively high energy density, and low losses were achieved. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z40.00010: Quantifying the rapid dynamics of polyelectrochromic switching in polymer acid-doped polyaniline Jacob Tarver, Yueh-Lin Loo Films cast from particles comprising polyaniline template synthesized on poly(2-acrylamido-2-methyl-1-propanesulfonic acid) exhibit polyelectrochromism and possess superior switching times ($<$10 s) and enhanced stability relative to other polymer acid-doped polyaniline systems. Solvent annealing in dichloroacetic acid induces polymer chain relaxation and further improves the speed ($\sim $1 s) and stability of electrochromic cycling. Electrochromic responses before and after solvent annealing can be described by Avrami kinetics that capture the influence of the film's mesoscale structural development and reveal variations in the dimensionality of reaction fronts when switching between the insulating and conducting states. Transitions from the conductive state are best fit by an Avrami exponent of 1.5 and are consistent with a reaction homogenously initiated throughout the film. Transitions to the conductive state require fits with exponents $\ge $2, suggesting nucleation and auto-accelerated propagation of conductive pathways away from the film/electrode interface in a manner analogous to polymer crystallization. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z40.00011: Enhanced RF Heating of Poly(N-Isopropylacrylamide) Gels by Utilization of Multiferroic Nanoparticles Ezekiel Walker, Yukikuni Akishige, James Roberst, Tong Cai, Zhibing Hu, Arup Neogi Poly(N-Isoproylacrylamide) polymer hydrogels possess the special property of a discontinuous volumetric phase transition. This phase transition can be induced by external stimuli such as temperature, light, electric or magnetic fields, PH, and others. Of great interest is the functionalization of the gels to external stimuli for faster and more uniform spatial response. An efficient route to functionalization, specifically for termperature and light, is to utilize nanoparticles with EM resonances in the polymer network. The nanoparticles would be distributed throughout the gels, and an EM source resonant with the nanoparticle-gel structure would be used to induce a hysteresis-like effect to heat the gels, thereby, electromagnetically controlling the phase of the gel. KF-BaTiO$_{3}$ and BiFeO$_{3}$ are two sets of promising multiferroic nanoparticles that have exhibited resonances in the GHz region. Here, we present our findings for the functionalization and enhancement of the gels for radio-frequency light using KF-BaTiO$_{3}$ and BiFeO$_{3}$. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z40.00012: Manipulating functional properties at the interface of composite organic semiconductors Panagiotis Maniadis, Turab Lookman, Avadh Saxena, Daryl Smith The formation of interfaces between different conjugated polymers is very important for the function of organic solar cells and other organic semiconductor devices. We propose a mechanism to control the properties of these interfaces with the addition of specially designed macro-molecules, with functional units. We develop the framework, using self consistent field theory (SCF), to study the concentration and the correlation function related to these functional units. When the functional units include a dipole moment, the average dipole moment and the standard deviation is expressed as a function of the SCF propagators. For electrostatic dipoles we also calculate the electric field difference created by the dipoles, as well as the charge distribution. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z40.00013: The Role of Exciton Dynamics in Photorefractivity in Organic Semiconductors Mohammad Samiullah A theory of the photorefractive effect in organic photorefractive materials based on formation, diffusion and dissociation of excitons in organic polymer semiconductors will be presented. A comparison with experimental results shows that the exciton dynamics plays an important role in the creation of the space charge field, and should be considered when selecting candidates for photorefractivity in organic systems. [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z40.00014: Smart lens made of dielectric elastomer: simulation study Hong Tang Electroactive Polymers (EAPs) are polymers that exhibit a change in size or shape when stimulated by an electric field. The common applications of this type of material are in actuators and sensors. A typical characteristic property of an EAP is that they will undergo a large amount of deformation while sustaining large forces. It has been demonstrated that EAPs can exhibit a strain from 10{\%} to 300{\%}. A dielectric elastomer (DEA) is a compliant capacitor, where a passive elastomer film is sandwiched between two compliant electrodes. When a voltage is applied, the electrostatic pressure arising from the Coulomb forces acting between the electrodes, therefore the electrodes squeeze the elastomer film. Based on the finite element analysis, we simulated the deformation of a polymer lens made of transparent dielectric elastomer materials under an application of electric field, which is provided by the transparent thin metal layers coated on the upper and lower surfaces of the lens. The focus of the lens can be adjusted by the applied electric field strength. By designing the electrode configuration on the lens surfaces, one can achieve both the positive and negative adjustment for the focus length. [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z40.00015: ABSTRACT WITHDRAWN |
Session Z42: Biopolymers: Molecules, Solutions, Networks, and Gels
Sponsoring Units: DPOLYChair: Jan-Michael Carrillo, University of Connecticut
Room: A302/303
Friday, March 25, 2011 11:15AM - 11:51AM |
Z42.00001: How do polymers degrade? Invited Speaker: Materials derived from agricultural products such as cellulose, starch, polylactide, etc. are more sustainable and environmentally benign than those derived from petroleum. However, applications of these polymers are limited by their processing properties, chemical and thermal stabilities. For example, polyethylene terephthalate fabrics last for many years under normal use conditions, but polylactide fabrics cannot due to chemical degradation. There are two primary mechanisms through which these polymers degrade: via hydrolysis and via oxidation. Both of these two mechanisms are related to combined factors such as monomer chemistry, chain configuration, chain mobility, crystallinity, and permeation to water and oxygen, and product geometry. In this talk, we will discuss how these materials degrade and how the degradation depends on these factors under application conditions. Both experimental studies and mathematical modeling will be presented. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z42.00002: Stretching semiflexible filaments with quenched disorder Panayotis Benetatos, Eugene M. Terentjev Many biopolymers, such as DNA, are characterized by sequence heterogeneity. At large scales, this heterogeneity may behave as a quenched random variable. We consider a wormlike chain with uncorrelated quenched disorder in its arc-length dependent spontaneous curvature. In the weakly bending approximation, we obtain analytic results for the elastic response to a stretching force applied at its end-points. We show that the effect of quenched disorder does not always reduce to a simple renormalization of the bending stiffness of the pure system. We also discuss a formally similar disordered system where a stretched wormlike chain is subject to random uncorrelated transverse forces. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z42.00003: Chains Are More Flexible Under Tension Andrey Dobrynin, Jan-Michael Carrillo, Michael Rubinstein The mechanical response of networks, gels, and brush layers is a manifestation of the elastic properties of the individual macromolecules. The two main classes of models describing chain elasticity include the worm-like and freely jointed chain models. The selection between these two classes of models is based on the assumptions about chain flexibility. We are proposing a unified chain deformation model that describes the force deformation curve in terms of the chain bending constant, $K$, and bond length, $b$. This model demonstrates that the worm-like and freely jointed chain models correspond to two different regimes of polymer deformation, and the crossover between these two regimes depends on the chain bending rigidity and the magnitude of the applied force. Polymer chains with bending constant $K>1$ behave as a worm-like chain under tension in the interval of the applied forces $f\le KkT/b$ and as a freely jointed chain for $f\ge KkT/b$. ($k$ is the Boltzmann constant and $T$ is the absolute temperature.) The proposed crossover expression for chain deformation is in excellent agreement with the results of the molecular dynamics simulations of chain deformation and single molecule deformation experiments of biological and synthetic macromolecules. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z42.00004: Statistical Mechanics of Helical Wormlike Model Ya Liu, Toni Perez, Wei Li, James Gunton, Amanda Green The bending and torsional elasticities are crucial in determining the static and dynamic properties of ~biopolymers such as dsDNA and sickle hemoglobin. We investigate the statistical mechanics of stiff polymers ~described by the helical wormlike model. We provide a numerical method to solve the model using a transfer matrix formulation. The correlation functions have been calculated and display rich profiles which are sensitive to the combination of the temperature and the equilibrium torsion. The asymptotic behavior at low temperature has been investigated theoretically and the predictions fit the numerical results very well. Our analysis could be used to understand the statics of dsDNA and other chiral polymers. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z42.00005: DNA walks one step at a time in electrophoresis Juan Guan, Bo Wang, Steve Granick Testing the classical view that in DNA gel electrophoresis, long polymer chains navigate through their gel environment via reptation, we reach a different conclusion: this driven motion proceeds by stick-slip. Our single-molecule experiments visualize fluorescent-labeled lambda-DNA, whose intramolecular conformations are resolved with 30 ms resolution using home-written software. Combining hundreds to thousands of trajectories under amplitudes of electric field ranging from zero to large, we quantify the full statistical distribution of motion with unprecedented statistics. Pauses are seen between steps of driven motion, probably reflecting that the chain is trapped inside the gel matrix. The pausing time is exponentially distributed and decreases with increasing electric field strength, suggesting that the jerky behavior is an activated process, facilitated by electric field. We propose a stretch-assisted mechanism: that the energy barrier to move through the gel environment is first overcome by a leading segment, the ensuing intramolecular stress from stretching causing lagging segments to recoil and follow along. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z42.00006: The effects of end-interactions on semiflexible polymers looping Jaeoh Shin, Wokyung Sung Biopolymer looping is a ubiquitous dynamic process that occurs in cell, such as gene regulation and protein folding. We study the dynamics of looping for a variety of chain contour and persistence lengths via simulation and analytical theory. To speed up the looping time in simulation, which is very long for the short, rigid chains, we use the path integral hyperdynamics method. We analyze the effects of static and hydrodynamic interactions between the end beads on the looping time. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z42.00007: Modeling of biomimetic peptoid polymers Dina Mirijanian, Steve Whitelam Peptoids are sequence-specific, oligo-N-substituted glycine polymers that can mimic the structural motifs and functionalities of proteins. Recently, novel sheet-like nanostructured materials have been self-assembled from peptoids under physiological conditions. These structures are biocompatible and may be selectively functionalized. We have constructed atomistic models of peptoids using high level ab initio calculations to guide the parameterization of a classical force field based on the CHARMM22 peptide force field. Atomistic molecular dynamics simulations show the accessible configurations of peptoids in water to be markedly different from those of peptides. We have also used our parameterized force field to study the molecular structure of peptoid sheet-like nanostructures. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z42.00008: Periciliary Layer as a Protective Barrier of Human Airways Liheng Cai, Brian Button, Richard Boucher, Michael Rubinstein The human airway surface layer consists of an overlaying gel-like mucus layer and a lower periciliary layer (PCL) protecting epithelial surface from mucus and the pathogens it contains. We investigated the permeability of the PCL using polymers that can readily penetrate through mucus. We found that in dilute solutions dextran larger than $\sim $30 nm are excluded from the PCL, whereas dextran smaller than that can penetrate the PCL. The penetration depth increases (distance of dextran from epithelial surface decreases) as the dextran size decreases. We also found that the PCL can be compressed by semidilute solutions of dextran larger than 50 nm with concentration above a certain value, at which the solution correlation length (osmotic pressure) is about 30 nm (300 Pa). Above this concentration the height of the PCL decreases with the increasing concentration (osmotic pressure). The dependence of the PCL height on correlation length for semidilute solutions that compresses it is similar to the dependence of distance from cell surface on dextran size for the case of the PCL penetration by smaller polymers from dilute solutions. Our results suggest that the PCL protects the airways by limiting the penetration of inhaled infectious particles. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z42.00009: Cellulose aerogel from ionic liquid solution dried by silylation Dmitry Rein, Yachin Cohen Aerogels are a class of materials characterised by a highly porous structure with low solids content. There is much interest in cellulose aerogel (aerocellulose) as a biodegradable and sustainable material. Cellulose lyogel can be fabricated from its solution in ionic liquids (IL) by coagulation with a nonsolvent such as water. However, subsequent drying capillary forces in the gel pores that result in severe shrinkage and pore closure. The use of supercritical fluids for drying or freeze-drying entails high equipment and energy requirements. We describe the fabrication and structure of aerocellulose fabricated from IL solution with a simple novel drying process: Addition of a compatible reactive agent (trimethylchlorosilane) and its diffusion into the water-swollen cellulose hydrogel pores results in a reaction with water as well as the pore surface hydroxyl groups. The remaining hydrophobic compound (hexamethyldisiloxane-HMDS), which fills the initially hydrophilic cellulose hydrogel pores, has a low intrinsic surface tension in the pores allowing easy drying with minimal shrinkage. Furthermore it allows modification of the pore surface and even fabrication of cellulose-polysiloxane composites. Relations between aerocellolose processing conditions and the resulting structural features will be discussed. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z42.00010: Macroscopic structure and properties of aqueous methylcellulose gels Tirtha Chatterjee, Roland Adden, Meinolf Brackhagen, Alan I. Nakatani, David Redwine, Robert L. Sammler Cold semi-dilute aqueous methylcellulose (MC) solutions are known to undergo thermoreversible gelation when warmed. Here, studies on two MC materials, which contrast in thermal gelation performance (gel temperature, hot gel modulus etc.) even though they were prepared with similar methyl ether substitution levels and molecular weight distributions are presented. Small-angle neutron scattering (SANS)* measurements reveal differences in their gel structures which presumably are relevant to their thermal gelation performances. MC gel with higher gel temperature and lower hot gel modulus contains a single temperature invariant characteristic length ($\sim $ 1000 {\AA}). However, besides this length scale, an additional and distinct smaller structure is also observed for the material with the lower gel temperature and the higher hot gel modulus. Further, in this case, the characteristic length scale decreases as temperature rises where as, the other length scale (smaller in size) remain almost temperature-invariant. The smaller domain size of the gel structure leads to the higher hot gel modulus for these methylcellulose materials. *Performed at NG3 beamline, NCNR, NIST. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z42.00011: Responsive Gelation in Physical Double Network Hydrogels from Artificial Protein Polymers B.D. Olsen, M.J. Glassman Artificial protein polymers with responsively associating groups on two different length scales are engineered in order to form physical double networks with potential application as shear-thinning hydrogels that may be toughened after injection. Gel-forming molecules are prepared by conjugating poly(N-isopropylacrylamide) (PNIPAM) at both ends of an artificial protein polymer to form PNIPAM-protein-PNIPAM triblock copolymers. Aggregation of the polymer endblocks forms a longer length scale network, while associating coiled-coil groups within the protein midblock form a shorter length scale network. At low temperatures where the coiled-coil domains are physically crosslinked but the copolymer endblocks are soluble, the materials form soft shear-thinning hydrogels. Elevating the temperature results in self-assembly of the second network, as manifest by stiffening of the gels. The structure of the materials is characterized using light scattering, X-ray scattering, and microscopy. Kinetics of the second network formation are characterized by linear oscillatory shear rheology, and nonlinear rheology is used to characterize the effect of the second network on the yield stress in these gels. [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z42.00012: Universality in Nonlinear Elasticity of Biological and Polymeric Networks and Gels Jan-Michael Carrillo, Andrey Dobrynin Networks and gels are part of our everyday experience starting from automotive tires and rubber bands to biological tissues and cells. Biological and polymeric networks show remarkably high deformability at relatively small stresses and can sustain reversible deformations up to ten times of their initial size. A distinctive feature of these materials is highly nonlinear stress-strain curves leading to material hardening with increasing deformation. This differentiates networks and gels from conventional materials, such as metals and glasses, showing linear stress-strain relationship in the reversible deformation regime. Using theoretical analysis and molecular dynamics simulations we propose and test a model that describes nonlinear mechanical properties of a broad variety of biological and polymeric networks and gels by relating their macroscopic strain hardening behavior with molecular parameters of the network strands. This model provides a universal relationship between the strain-dependent network modulus and the network deformation and explains strain-hardening of natural rubber, synthetic polymeric networks, and biopolymer networks of actin, collagen, fibrin, vimentin and neurofilaments. [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z42.00013: Criticality and isostaticity in fiber networks Xiaoming Mao, Olaf Stenull, Tom C. Lubensky, Chase P. Broedersz, Fred C. Mackintosh We investigated the elastic response of model semiflexible networks based on diluted periodic lattices, using a new effective medium theory and numerical simulations. In this model, central forces link nearest neighbor sites and bending forces link second neighbor sites along fibers. We found that by turning on fiber bending rigidity, the central force rigidity critical point became unstable, and the lattices lose rigidity at a lower threshold that is independent of fiber bending rigidity. We calculated scaling relations and exponents at both critical points. In addition to the bending and stretching dominated regimes, we identified a novel bend-stretch coupled regime in the vicinity of the central force critical point, in which the shear modulus exhibits a fractional power-law dependence on both the fiber bending and stretching rigidities. [Preview Abstract] |
Session Z43: Liquid Crystalline Order in Polymer and Complex Fluids
Sponsoring Units: DPOLYChair: Chinedum Osuji, Yale University
Room: A306/307
Friday, March 25, 2011 11:15AM - 11:27AM |
Z43.00001: Nonlinear Dielectric Response of the Liquid Crystal 8CB near the phase transitions Hannah Buchanan The nonlinear dielectric response of the liquid crystal (LC) 8CB (4'-octyl-4-cyanobiphenyl) was measured near the smectic-nematic and nematic-isotropic phase transitions. The sample was filled in a commercially available LC capacitor cell of dimensions (1 cm $\times $ 1 cm $\times $ 9 $\mu $m). The cell was mounted in a temperature-controlled environment with a stability and resolution of 1 mK. The capacitance of the cell was measured at different temperatures in the range 25 -- 45\r{ }C covering both phase transitions, and over a range of frequencies up to 100 kHz, and a signal level in the range of 0 -- 5V, using a lock-in amplifier (SRS830) and LCR meter (Fluke PM6304). Nonlinear effects were observed in the capacitance even at a 200 mV signal level, and very large changes in the capacitance, both linear and nonlinear, were observed in the nematic phase, near each phase transition. *Undergraduate physics students **Recent physics graduates [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z43.00002: A Novel Liquid Crystal Elastomer with Large Spontaneous Length Changes Patricia Cladis, Simon Krause, Yusril Yusuf, Shohei Hashimoto, Leonid Fel, Shoichi Kai, Heino Finkelmann An order parameter describes an elastomer where a liquid crystal is cross linked at a temperature T$_{L}$ far from its clearing temperature. At T$_{L}$, there is a first order constant density phase transition where the order parameter vanishes and the elastomer has cubic symmetry. Below T$_{L}$ it is an icosahedral nematic and above a biaxial nematic. This theory quantitatively accounts for spontaneous shape change, nonlinear elasticity and gelation dynamics in a novel liquid crystal elastomer. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z43.00003: Magnetic field directed self-assembly of liquid crystalline block copolymers for membrane applications Manesh Gopinadhan, Pawel W. Majewski, Chinedum O. Osuji The use of magnetic fields is presented as a facile approach to the control of long range order and alignment of block copolymers. Using SAXS we demonstrate the alignment of lamellar and hexagonally packed cylinder morphologies of a poly(ethylene oxide)-based LC diblock copolymer by slow cooling in the presence of the field through the order-disorder transition. Non-degenerate alignment of the lamellar system is enabled by sample rotation and alignment in the system is shown to be driven by the diamagnetic anisotropy of the LC mesogen, and not anisotropy resulting from crystallization of the PEO block. We consider the effects of lithium doping and field strength on the order-disorder transition of the system, and the effect of lithium content on the critical field required for attaining well aligned films. The controlled alignment of PEO channels over large areas offers a route to selective ion transport in solid state batteries. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z43.00004: Temperature Dependent Self-Assembly of Side-Group Liquid Crystalline Block Copolymers in LC Solvent Paul Pirogovsky, Zuleikha Kurji, Rohan Hule, Julia Kornfield Side Group Liquid Crystalline Polymers (SGLCPs) take on anisotropic conformations in a small molecule LC solvent. This conformation is affected by both the temperature and the sense of attachment of the mesogenic side group (either parallel or perpendicular to the backbone). Covalently linking the SGLCP with a random coil (PS) block leads to a block copolymer that self assembles into anisotropic micelles. Transmission Electron Microscopy was used to view these micelles in real space and determine their size and structure. Small Angle Neutron Scattering (SANS) was performed on dilute (1 wt{\%}) solutions of a series of SGLCPs (homopolymers and block copolymers with a range of polystyrene block lengths) dissolved in deuterated 5CB. A rubbed alignment layer and a magnetic field were used to break symmetry and allow the ordered structure to be seen. PS-b-SGLCP block copolymers were seen to form self-assembled structures that changed qualitatively with the changing PS block length and the temperature. Most intriguingly, several polymers exhibit mutually orthogonal anisotropies at different length scales. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z43.00005: In-Situ X-Ray Diffraction Study of the Elongation Behavior of a Main Chain Liquid Crystal Elastomer Sonal Dey, D.M. Agra-Kooijman, Leela Joshi, Satyendra Kumar, Wanting Ren, Whitney M. Kline, Anselm C. Grrifin We studied the structural evolution of a main chain smectic elastomer under uniaxial stress and during strain recovery. At low strains, both the alkyl chains and the smectic layers are oriented on average parallel to the strain direction. At higher strains, the development of layer undulations is observed at $\sim $50{\%} which gradually evolve into chevron-like smectic structures as revealed by the four diffuse spots in small angle x-ray diffraction patterns. This is accompanied with an enhancement of the smectic order as inferred from higher intensity of small angle peaks and larger orientational order parameter, $S$ [for example, $S $(20{\%}) = 0.33; $S $(110{\%}) = 0.86]. During strain recovery, two relaxation rates of 6.5 min and 38.5 min are observed which are associated with relaxations of the mesogenic part and the elastomer network, respectively. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z43.00006: Evaporative Self-Assembly and Formation of the Lyotropic Liquid Crystalline Phase of Poly(3-hexyl thiophene) Min Sang Park, Avishek Aiyar, Jung Ok Park, Elsa Reichmanis, Mohan Srinivasarao In this study, we electrically and optically interrogated the evolution of the thin film structure in conjugated systems using poly(3-hexylthiphene) (P3HT) as a model semiconducting polymer. In an effort to understand the electrical properties of the conducting channel in terms of polymer chain orientation and relaxation in solution, we performed in-situ micro-Raman measurements using polarized incident light. We measured the extent of molecular chain alignment during the process of film formation and showed the existence of a lyotropic liquid crystal phase at the three-phase contact line. The variation of frequency dispersion and the shift of position for Raman active mode, combined with the structural anisotropy of P3HT films, suggest a phase transition to the lyotropic liquid crystalline phase. The orientational order of P3HT chains in the liquid crystalline phase was quantified as a function of evaporation time using solidified solvent, 1,3,5-triclorobenzene (1,3,5-TCB). [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z43.00007: Lyotropic nematic droplets of single-walled carbon nanotubes in super acids: equilibrium shape and dynamics Natnael Behabtu, Budhadipta Dan, Lee Taewoo, Ivan Smalyukh, Matteo Pasquali The balance between surface tension, elastic energy and surface anchoring creates nematic droplets with elongated shape, called tactoids. Here we report equilibrium shape and coalescence dynamics of single-walled carbon nanotube (SWNT) nematic droplets in superacids. Surprisingly, we observe bipolar tactoids with uniform aspect ratio. The uniform aspect ratio is not expected based on theoretical arguments that indicated that aspect ratio should drop with domain volume. Bipolar order contrasts recent findings of uniform order in liquid crystalline tactoid droplets of aqueous SWNTs stabilized with bile salts. The coalescence dynamics is highly influenced by the drop elasticity. Coalescence pathways that lead to defect creation are not observed, unlike spherical nematic droplets. Instead coalescence proceeds by tangentially matching the nematic director of two drops. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z43.00008: The unique behavior of chiral templated polymers swollen with liquid crystals Michael McConney, Jennifer Hurtubise, Vincent Tondiglia, Timothy White, Timothy Bunning Liquid crystals (LC) have the capability of impart a degree of order onto the molecular structure of polymers, essentially acting as ``smart solvents.'' Furthermore, structured materials have the ability to force liquid crystals into structures that are far from the bulk LC structure. Our studies are aimed at exploring the interplay between LC templated polymers and polymer templated LCs. Specifically we investigated a unique swelling-deswelling phase transition involving an ordered liquid crystal solvent and a structured polymer. The polymer is formed through anisotropic photopolymerization of LC monomers in a chiral LC fluid, is only attached from one surface of a single cell, and only spans a fraction of the cell thickness. The details of the transition were studied with various techniques including DSC, confocal microscopy, polarized microscopy, and visible/near-IR spectrometry. The unusual thermally induced structural changes result in large changes to the reflection wavelength of the cholesteric LC cells. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z43.00009: The Shape of Structured Polymer Vesicles Mark Bowick, Min-Hui Li, Homin Shin, Xiangjun Xing, Zhenwei Yao Polymer vesicles are stable robust vesicles made from block copolymer amphiphiles. Recent progress in the chemical design of block copolymers has lead to the creation of a variety of polymer vesicles with varying internal structure, functionality and shape. By choosing suitable liquid-crystalline polymers for one of the copolymer components one can create vesicles with internal nematic or smectic order. This talk will address the possible shapes of these \textit{structured }vesicles as well as their inherent topological defects. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z43.00010: Curvature-induced transitions in two-dimensional nematics Badel L. Mbanga, C.D. Santangelo, G.M. Grason Anisotropic particles absorbed to a fluid interface are known to significantly alter the structure and mechanics of these surfaces, possibly even stabilizing surfaces of complex, bicontinuous topology, as in ``bijels'' formed from arrested spinodally- decomposing fluid mixtures. We study the intricate interplay between the morphology of these interfaces and the alignment of anisotropic elongated particles with nematic order. Here we present results from computer simulation of nematic order on hyperbolic surfaces using a model that accounts for the contribution of both the intrinsic and extrinsic curvatures to the energetics of topological defects defects. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z43.00011: Effect of chain stiffness on structural and thermodynamic properties of polymers Kiran Khanal, Jutta Luettmer-Strathmann The stiffness of the chains affects many properties of polymers. We investigate structural and thermodynamic properties of a bond- fluctuation lattice model for semiflexible polymer chains. Monte Carlo simulations for polymer melts for a range of values of the bending penalty, density, and temperature show elongation of the polymer conformations with increasing chain stiffness but no transition to a nematic phase. Results for average bead-bead interaction energy and bending energy were studied separately and showed that the bending energy is almost independent of the filling fraction, suggesting that the stiffness of the chains can be treated as a single chain property. We calculated the bending energy from the partition function of a pair of neighboring bonds and find excellent agreement between theory and simulation. Equation of state properties of the lattice model were determined from insertion methods and thermodynamic integration. We develop a theoretical description of these properties and use it to determine model parameters for real polymer melts from a comparison with experimental data. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z43.00012: Symmetric reflection band broadening of weakly polymer stabilized cholesteric thin films using low DC electric fields Madeline Dunning, Christopher Bailey, Anastasia Voevodin, Vincent Tondiglia, Lalgudi Natarajan, Timothy White, Timothy Bunning We report on a new, low field electro-optical effect in weakly polymer stabilized cholesteric liquid crystals with negative dielectric anisotropy. By applying low DC electric fields ($<$3V/$\mu $m), a symmetric broadening of the cholesteric reflection band can be seen, resulting in band width increases by factors of two or more. An intensive study of the various experimental parameters combined with numerical calculations of the transmission spectra, indicate that the polymer interacts with the electric fields resulting in an approximately constant pitch gradient across the cell thickness. Our results show that the maximum pitch distortions reach values of approximately 15{\%} the zero voltage value for notches in the visible range (pitches of 300-400nm), but increase along with the pitch. Possible physical mechanisms will be explored and discussed that might explain this interesting electro-optical effect. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z43.00013: Structure of Rigid Hard-Ring Fluids Mariam Nouri, Marc Robert Structure of fluids of molecules consisting of rigid rings of hard spheres is studied in two, quasi two, and three dimensions, using Monte Carlo computer simulations in the canonical ensemble. For rings of various size and for a wide range of densities, results are reported for the pair distribution function of the ring centers and for the pair distribution of the ring orientations. For dense fluids in two dimensions, a shoulder, precursor of the freezing transition, is observed in the second peak of the pair distribution function of the ring centers, as previously seen in the simple hard-sphere fluid. In quasi two dimensions, where the centers of the rings are confined to a plane but the rings themselves can wobble out of plane, a liquid crystalline nematic phase is observed at sufficiently high densities. Results are also presented for three dimensions. [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z43.00014: Fast Off-Lattice Monte Carlo Simulations with a Novel Soft-Core Spherocylinder Model Jing Zong, Xinghua Zhang, Qiang (David) Wang Fast off-lattice Monte Carlo simulations with soft-core repulsive potentials that allow particle overlapping give orders of magnitude faster/better sampling of the configurational space than conventional molecular simulations with hard-core repulsions (such as in the Lennard-Jones potential).\footnote{\textit{Q. Wang and Y. Yin}, \textbf{J. Chem. Phys., 130}, 104903 (2009).} Here we present our fast off-lattice Monte Carlo simulations on the structures and phase transitions of liquid crystals and rod-coil diblock copolymers based on a novel and computationally efficient anisotropic soft-core potential that gives exact treatment of the excluded-volume interactions between two spherocylinders (thus the orientational interaction between them favoring their parallel alignment). Our model further takes into account the degree of overlap of two spherocylinders, thus superior to other soft-core models that depend only on their shortest distance. It has great potential applications in the study of liquid crystals, block copolymers containing rod blocks, and liquid crystalline polymers. [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z43.00015: Solvent involved self-crystallization of C$_{70}$ molecules into high definition cube microstructure Chibeom Park, Hee Cheul Choi C$_{70}$ molecules dissolved in mesitylene (good solvent) are self-crystallized into cube shape microstructure by the addition of isopropyl alcohol (poor solvent). Through control experiments attempted with different types of alcohols as well as in the replacement of mesitylene with other similar solvents, such as toluene, m-xylene, and m-dichlorobenzene, it is confirmed that mesitylene plays a critical role to guide C$_ {70}$ molecules to form cube microcrystal with high definition edges and surfaces. Thermal gravimetric and crystallographic analyses show that the crystal structure is simple cubic whose unit cell is composed of one C$_{70}$ and two mesitylene molecules. The photoluminescence intensity from C$_{70}$ cube crystals are enormously increased compared to C$_{70}$ powder. Such abnormal photoluminescence increase is mainly attributed to the high crystallinity of C$_{70}$ cubes as confirmed by time-resolved photoluminescence lifetime measurements. [Preview Abstract] |
Session Z44: Focus Session: Organic Electronics and Photonics -- New Materials and Applications
Sponsoring Units: DMP DPOLYChair: Rui He, Columbia University
Room: A309
Friday, March 25, 2011 11:15AM - 11:27AM |
Z44.00001: Ambipolar Electric Double Layer Transistors Using Organic Single Crystals Taishi Takenobu, Di Wen, Hidekazu Shimotani, Shimpei Ono, Yoshihiro Iwasa Among organic devices, ambipolar transistors are very unique device, in which both electrons and holes are equally mobile and we are able to observe light emission through the recombination of them. Progress in the applications of such light-emitting transistors (LETs) based on organic single crystals has provided possibilities in developing organic laser. However, in these LETs, the current density is still low for lasing, and, therefore, a different device structure is necessary to overcome this issue. Here we show the first demonstration of organic ambipolar electric double layer transistors (EDLTs), in which the gate dielectric is not a conventional insulator but an electrolyte. The peculiar merit of EDLT is extremely high conductivity due to the huge capacitance of the EDL formed at the organic/electrolyte interfaces. Consequently, we can increase current density. In this study, we used rubrene single crystal and ion-gel as the active material and electrolyte, respectively. These present results will provide a prospect for further development in LET operation. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z44.00002: Refractive Indices of Specific Nonconjugated Conductive Polymers: Organic Nanometallic Systems Prashant Dubey, Sapana Shrivastava, Mrinal Thakur Exceptionally large quadratic electro-optic effect and two-photon absorption coefficients have been recently reported for nonconjugated conductive polymers after doping. These polymers include: cis- and trans- polyisoprene, poly($\beta $-pinene) and others. The large optical nonlinearities observed in these materials have been attributed to the nanometallic state with subnanometer dimensions that is formed upon doping and charge-transfer. Measurement and calculation of linear refractive indices of these novel nonlinear optical systems before and after doping are important. Linear absorption coefficients (UV-Visible) of trans-1,4-polyisoprene have been measured for different doping levels of iodine. Refractive indices have been calculated using Kramers-Kronig transformation of absorption data for different doping levels. Numerical integration using MATLAB software was used for these calculations. Refractive indices at specific wavelengths have been determined by measuring reflectivity at normal incidence. The calculated and measured values of refractive indices have been compared. Results on calculations and measurements on these novel systems will be discussed. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z44.00003: ABSTRACT WITHDRAWN |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z44.00004: Sub-diffraction limited features in three-dimensional photopatterned, two-photon excimer-forming fluorescent dye-doped films Christopher Ryan, Brent Valle, Joseph Lott, Jack R. Johnson, Jie Shan, Kenneth D. Singer, Christoph Weder, David A. Schiraldi 3D Photopatterning is a key process in optical data storage, photolithography and other applications. Two photon active systems are a popular choice to pattern in 3D. The main challenges pertain to the contrast and density of the patterned features. By making use of a thermal threshold process which induces dye deaggregation, high contrast features are written with diameters smaller than the system's diffraction limit. A polymer film was doped with a two photon active dye that possesses two distinct fluorescence states in its monomer and excimer phases. The film's phases are stable at room temperature, and have a threshold response to heating. By selective exposure to a pulsed 675 nm source, the film is photopatterned in 3D as the focused pulses are absorbed and thus anneal the sample. Because the change is physical rather than chemical, the mechanism is shown to be a threshold process. As a result, sub diffraction limit photopatterns are demonstrated in the medium. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z44.00005: Low threshold conjugated polymer lasers by intrinsically directed resonator design Alexander Kuehne, David Weitz Creation of laser cavities requires external imposition of an optical feedback system onto the gain medium. By contrast, we use functional conjugated polymers that can be chemically or physically patterned from within to form diffractive laser resonators. One realization is based on a chemically-modified polyfluorene, which can be patterned into distributed feedback (DFB) resonators of any desired grating period. A different route is by physically pattering conjugated polymers into sub-micrometer colloids via microfluidics. These systems can be tuned with respect to particle size and their arrangement in the resulting laser structure. We show random lasing for a photonic-glass, self-assembled from monodisperse conjugated polymer particles. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z44.00006: Effects of polar analytes on the transport properties of organic semiconductor field-effect chemical sensors Davianne Duarte, Brian Cobb, Ananth Dodabalapur Chemical recognition or sensing in organic thin film transistors (TFTs) can be achieved by direct analyte interaction or the inclusion of specific receptor molecules added to the semiconducting surface. Overall, sensing is dependent on the interactions occurring between the molecule and the OTFT active region, which includes the semiconductor and semiconductor-insulator interface. The magnitude of the interaction will depend on the molecules polarizability and the partition function of the analyte vapor. We employ a range of analytes (cyclohexane, ethanol, and styrene) with different solvations parameters (polarizability/dipolarity levels) to gain more clarity on their effects of the charge transport properties in OTFTs. Receptors are used to understand in more detail the physical and chemical interactions, which contribute to the sensor response. The receptors themselves have diverse polarizability/dipolarity parameters, which produce varying sensing behaviors dependent on the solvation parameters of the analytes. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z44.00007: Chemical vapor sensors using a poly(triarylamine) semiconductor modified through incorporation of organic receptors Brian Cobb, Davianne Duarte, Ananth Dodabalapur A poly(triarylamine) (PTAA) semiconducting active layer has been employed in order to produce OFET chemical sensors displaying excellent stability in air, with minimal bias stress effects. We propose a ``chemical fingerprint'' sensing array comprised of a single base polymer (PTAA), with the selectivity of individual devices modified through the incorporation of a variety of small molecule receptors. This allows for consistent device operation and optimization of the array. The effect of various receptors will be discussed, and reported while sensing alcohol vapors. Different methodologies will be proposed for incorporation of receptors into the device, including incorporation into the PTAA film and separate deposition on top of the PTAA film. The relative merits of each approach will be discussed, including the effect on both threshold voltage and carrier mobility. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z44.00008: Nonadiabatic exciton dynamics in conjugated polymers Adam Willard, Peter Rossky The results of mixed quantum/classical simulations of the nonadiabatic excited state dynamics of sexithiophene and the C60-sexithiophene interface are presented. The model is capable of describing the photogeneration and subsequent time-evolution of excitons in conjugated polymers at model bulk heterojunction interfaces. The effect of chain length and electric field on exciton mobility is discussed for both single-chain and $\pi$-stacked sexithiophene oligomer. In addition the dynamics of exciton dissociation at the C60-sexithiophene interface are described. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z44.00009: ABSTRACT WITHDRAWN |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z44.00010: Solution Processable Organic Solar Cell Microarrays for Use in MEMS Jennifer Trinh, Jason Lewis, Patrick Toglia, Xiaomei Jiang We have developed an innovative way to fabricate organic solar arrays for application as DC power supplies in electrostatic MEMS devices. The generation 1 microarray consists of 20 small (1 mm\textsuperscript{2}) solar cells connected in series (total device area of 2.2 cm\textsuperscript{2}). The device uses an active layer of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC$_{61}$BM), which are mixed together (1:1 mass ratio) in appropriate solvent. We manipulated active layer nanomorphology by choice of solvents and annealing conditions. The optimized generation 1 device has an open-circuit voltage of 11.5V, short-circuit current density of 1 mA/cm\textsuperscript{2}, and a power conversion efficiency of 2\% under simulated solar AM1.5 illumination. The generation 2 microarray has a new design with reduced series resistance and improved cell occupancy. The generation 2 arrays have demonstrated improved device efficiency and power output density. Detailed analysis of device physics in both generation microarrays will be presented. The procedure described has potential for producing microarrays as small as 0.01 mm\textsuperscript{2}. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z44.00011: Synthesis and characterization of a new TPA-Thiophene based molecule for potential Organic PV applications Prashant Sarswat, Amarchand Sathyapalan, Michael Free Triphenylamine (TPA) containing molecules possess unique optical and photovoltaic properties. These molecules show very strong visible absorption due to unique electronic structure. Another class of molecules, thiophene derivatives and resulting polymers have higher hole mobility amongst other conjugate co-polymers. In view of these aspects, we have designed a new conjugated thiophene containing molecule with TPA. A solution processable technique is reported to synthesize this newly designed star shaped molecule with TPA derivative as its core and thiophene derivative as branches. The synthesized molecule is systematically characterized by studying Mass spectroscopy, Raman spectroscopy, UV-Vis spectroscopy, and cyclic voltammetry. This molecule shows solubility in various common organic solvents, broad absorption in spectral range of 300- 650 nm, and good thermal stability. An atomistic simulations based on density functional theory was carried out to validate HOMO and LUMO levels. The cyclic voltammetry analysis is consistent with atomistic simulation results. A 3D simulated orbital image reveals that the HOMO level corresponds to oxygen as well as carbon in a benzene ring. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z44.00012: Quadratic Electro-optic Effect in the Nonconjugated Conductive Polymer Trans-1,4-polyisoprene Measured at 1.55 $\mu $m Sapana Shrivastava, Mrinal Thakur Quadratic electro-optic effect in the nonconjugated conductive polymer, iodine-doped trans-1,4-polyisoprene has been measured at a longer wavelength (1.55 $\mu $m) and x-ray diffraction of the polymer film has been studied. Optical absorption spectra of trans-1,4-polyisoprene (TPI) show two peaks after doping, one at 4.2eV (295 nm) due to radical cation and the other at 3.2eV (388 nm) due to charge-transfer from double to dopant. X-ray diffractometer scans show that the trans-1,4-polyisoprene film has $\gamma $-phase crystal structure. Upon doping intensities of two peaks were observed increase without significant changes in overall peak positions. Field induced birefringence technique has been used to measure quadratic electro optic effect at 1.55 $\mu $m (which is away from resonance) and an exceptionally large Kerr coefficient (2.5x10$^{-10}$ m/V$^{2})$ has been obtained. This large nonlinearity has been attributed to the subnanometer-size metallic quantum dot structure produced upon doping of this polymer. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z44.00013: Photo-active Silicon Containing Polymer Films; An Approach Towards All-Solution Processable Devices Scott LeFevre, Xinxing Liu, Leonard Interrante, Chang Ryu The continuous drive for smaller, cheaper electronic devices elucidates the necessity for the design and incorporation of materials with improved properties along with a greater ease of processability. Polycarbosilanes (PCSs) are a class of organic-inorganic hybrid materials which exhibit increased chemical and thermal stability, appreciable hydrophobicity, and significant mechanical robustness. Recently, we have developed UV-crosslinkable cyclolinear PCSs by the incorporation of aryl substituted disilacyclobutane (DSCB) rings into the backbone. This novel material exhibits a low dielectric constant (k), making it an attractive new material in the development of interlayer dielectrics. Preliminary data shows that there is a maximum UV-crosslinking depth of approximately 30 microns. Additionally we have successfully demonstrated the resist capabilities of this material by patterning micro-scale (about 10 microns) features with a surface roughness variation of approx. 1 percent. Finally, new approaches towards taking advantage of the reactivity of polysilanes and the effect of molecular weight and polydispersity will be discussed. [Preview Abstract] |
Session Z45: Bose-Einstein Condensates, Matter Optics, and Atomic Interferometry
Sponsoring Units: DAMOPChair: Zhenhua Yu, Ohio State University
Room: A310
Friday, March 25, 2011 11:15AM - 11:27AM |
Z45.00001: New Facility to Probe Physics With Degenerate Bose and Fermi Gas Subhadeep De A new facility to produce dual species degenerate Bose and Fermi gas is under construction at JQI. This apparatus is designed to create degnerate mixtures of bosonic rubidium ($^{87}$Rb) and fermionic lithium ($^{6}$Li). A degenerate Bose-Fermi mixture supports many quantum phase transitions, giving an experimental platform to study many-body statics, dynamics, and perhaps precision measurements. High T$_c$ superconductivity could be probed, where fermions are bound into Cooper pairs by boson mediated interactions. Dual species heteronuclear molecules with large permanent electric dipole moment may lead to a system for implementing quantum bits. A spin-polarized, non-interacting, degenerate $^{6}$Li gas coupled to $^{87}$Rb atoms in an optical lattice will give rise to a long range, spin-dependent interactions to realize quantum magnetism and potentially supersolidity. Far red-detuned lattices are far weaker, in recoil units, for Li as compared to Rb. So, in the Mott phase of Rb - one atom per lattice site - the three body recombination of Li-Li-Rb is greatly suppressed. Thus the wide $^{87}$Rb-$^{6}$Li Fesbach resonance at 1.1 kG is expected to effectively control fermion mediated interactions. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z45.00002: Solitons and Breathers in Strongly Repulsive Bose-Einstein Condensates William Reinhardt, Indubala Satija, Bryce Robbins, Charles Clark Collisional dynamics of solitary matter waves of hard core bosons, consisting of dark and bright waves as well as supersonic periodic trains, reveals remarkable richness and coherence, with the phase of the condensate playing a key role. Depending upon the condensate density, we see two distinctive effects in the collision of these waves: intuitively expected repulsive collision due to hard core boson constraint and also collisions in which they ``pass through'' each other without distortion. In addition to confirming the soliton status of the solitary waves, our studies reveal a variety of multi-solitons including a family of breathers, that can be demonstrated in an optical lattice with appropriate phase imprinting. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z45.00003: Macroscopic two-state systems in trapped atomic condensates Dmitry Mozyrsky, Dmitry Solenov We consider a macroscopic two-sate system based on persistent current states of a Bose-Einstein condensate (BEC) of interacting neutral atoms confined in a ring with a weak Josephson link [1]. We demonstrate that macroscopic superpositions of different BEC flows are energetically favorable in this system. Moreover, a macroscopic two-state dynamics emerges in the low energy limit. We also investigate fundamental limitations due to the noise inherent to the interacting BEC of Josephson-ring geometry. We show that the coherent macroscopic dynamics is readily measurable for an experimentally accessible range of parameters. \\[4pt] [1] D. Solenov and D. Mozyrsky, arXiv:1009.1901 [cond-mat.quant-gas]. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z45.00004: Analysis of a free oscillation atom interferometer Rudra Kafle, Dana Anderson, Alex Zozulya We analyze a Bose-Einstein condensate (BEC) - based free oscillation atom Michelson interferometer in a weakly confining harmonic magnetic trap. A BEC at the center of the trap is split into two harmonics by a laser standing wave. The harmonics move in opposite directions with equal speeds and return under the influence of the trapping potential at their classical turning points. The harmonics are allowed to pass through each other and a recombination pulse is applied when they overlap at the end of a cycle after their return at the second time. We derive an expression for the contrast of the interferometric fringes and obtain the fundamental limit of performance of the interferometer in the parameter space. The results are also compared and contrasted with the results of our previous work on a single- and double reflection atom Michelson interferometers. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z45.00005: Atomic population distribution in the output ports of a waveguide interferometer with optical splitting and recombination Ebubechukwu Ilo-Okeke, Alex Zozulya Manipulation of atomic Bose-Einstein condensates (BECs) in an atom waveguide interferometer relies on the use of off-resonant laser pulses to split a cloud of BEC into two clouds that travel along different paths and are then recombined using the same laser pulses. During the evolution of the condensates, residual spatial-dependent phase is accumulated due to the confining potential and the inter-atomic interactions within the condensates. Additional phase is accumulated due to the mode-entangled state of the clouds after splitting that causes each atom in the condensate to evolve at a different rate. The recombination laser pulses are sensitive to these phases and the population of atoms in the atomic samples that emerge after recombination depends on them. We derive an expression for the probability density distribution of observing any number of atoms in the clouds after recombination and give a parameter space for optimum operation of the interferometer in the presence of phase due to mode-entangled state of BEC clouds and the residual spatial-dependent phase. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z45.00006: Momentum-space engineering of gaseous Bose-Einstein condensates Brandon Benton, Jeffrey Heward, Mark Edwards, Charles Clark We show how the momentum distribution of gaseous Bose--Einstein condensates can be shaped by applying a sequence of standing--wave laser pulses. We present a theory, whose validity was demonstrated in an earlier experiment,\footnote{L.\ Deng, et al., PRL {\bf 83}, 5407 (1999)} of the effect of a two--pulse sequence on the condensate wavefunction in momentum space. We generalize the previous result to the case of $N$ pulses having arbitrary pulse areas and separated by arbitrary time intervals and show how these parameters can be engineered to produce a desired final momentum distribution. We find that several momentum distributions, such as single--state distributions or a range of momentum states which are important in initial state selection in atom--interferometry applications, can be engineered with high fidelity with two or three pulses. We present several examples of such distributions and show how the fidelity improves as more pulses are added. We also give some ideas of how these momentum distributions can be applied to atom interferometry. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z45.00007: Enhanced atom interferometry through quantum information science Mark Edwards, Brandon Benton, Michael Krygier, Charles W. Clark New designs for atom interferometers can be inspired by quantum information science (QIS). QIS--inspired atom interferometer (AI) designs have the potential for producing AIs with enhanced sensitivity and robustness. We compare the sensitivity of a standard Mach--Zehnder (M--Z) Bragg AI with an AI whose design is based on the idea of decoherence--free subspaces (DFS).\footnote{D.A.\ Pushin, M.\ Arif, and D.G.\ Cory, Phys.Rev.\ A {\bf 79}, 053635 (2009)} We studied the performance of both atom interferometers using an enhanced version of a previously developed Bragg interferometer prototyping model.\footnote{S.E.\ Simsarian et al., Phys.\ Rev.Lett.\ {\bf 85}, 2040 (2000).} This model approximates the effect on the condensate of multiple Bragg pulses separated by time delays using two elements: the effect of a single pulse and condensate evolution between pulses. The overall effect is rapidly approximated by following the steps of the interferometric process. We describe this model and then present the comparison of the performance of the M--Z interferometer with that of the DFS--inspired interferometer. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z45.00008: Eigenvalues of the linearized collision operator for a Bose-condensed gas Erich Gust, L.E. Reichl Beginning from a Hamiltonian description, we have derived the kinetic equation for the elementary excitations of an interacting gas of bosons with and without a condensate. This kinetic equation is the quantum analog of the Boltzmann equation for a classical gas. Linearizing the kinetic equation about the equilibrium state allows us to treat it as a linear integral operator equation and find its eigenvalues. These eigenvalues give the rates at which different eigenmodes relax to equilibrium. They also provide a relationship between the microscopic properties of the interacting particles and the macroscopic properties of the gas. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z45.00009: Single-Particle Spectral Density of a Bose-Condensed Gas in the Two-Fluid Hydrodynamic Regime Emiko Arahata, Nikuni Tetsuro, Allan Griffin In Bose supefluids, the single-particle Green's function can be directly related to the superfluid velocity-velocity correlation function in the hydrodynamic regime. An explicit expression for the single-particle spectral density was originally written down by Hohenberg and Martin in 1965, starting from the two-fluid equations for a superfluid. We give a new simple derivation of their results. Using these results, we calculate the relative weights of first and second sound modes in the single-particle spectral density as a function of temperature in a uniform Bose gas. We show that the second sound mode makes a dominant contribution to the single-particle spectrum in relatively high temperature region. We also discuss the possibility of experimental observation of the second sound mode In a Bose gas by photoemission spectroscopy. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z45.00010: A new self-consistent perturbation expansion for Bose-Einstein condensates satisfying conservation laws and Goldstone's theorem Takafumi Kita Quantum-field-theoretic descriptions of Bose-Einstein condensates (BEC) have suffered from the lack of self-consistent approximation schemes satisfying Goldstone's theorem and dynamical conservation laws simultaneously. I will report on a new perturbation expansion of the kind developed recently and its predictions on the single-particle and two-particle excitations of BEC. It is shown that the single-particle Bogoliubov excitations are generally different from the two-particle density fluctuations in contradiction to the conclusion of previous studies. I will elucidate some properties of the two distinct modes in connection with Goldstone's theorem on spontaneously broken symmetries. References: T. Kita, Phys. Rev. B80, 214502 (2009); Phys. Rev. B81, 214513 (2010). [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z45.00011: Hamiltonian monodromy in the spectrum and dynamics of a spin-1 Bose condensate Austen Lamacraft A spin-1 Bose condensate in a magnetic field in the single mode approximation -- valid for sufficiently small condensates -- represents a rather simple dynamical system. In this talk I will show that there is nevertheless scope for some rather unusual behavior. In particular, this system displays the phenomenon of \emph{Hamiltonian monodromy}, a topological obstruction to the existence of global action-angle variables. I will discuss the signatures of this phenomenon in the classical dynamics and quantum spectrum [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z45.00012: Ground States of a Mixture of Two Species of Spinor Bose Gases with Interspecies Spin Exchange Yu Shi We consider a mixture of two species of spin-1 atoms with both interspecies and intraspecies spin exchanges, and find out the exact ground states in various parameter regimes. We describe the quantum phase diagrams in the space of parameters, including intraspecies and interspecies spin coupling strengths as well as the magnetic field. The boundaries where the ground states are either continuous or discontinuous are determined, with the latter identified as where quantum phase transitions take place. Interspecies spin exchange leads to features beyond the usual ones of a single species of spinor atoms. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z45.00013: Bound states of a localized magnetic impurity in a superfluid of paired ultracold fermions David Pekker, Eric Vernier, Martin Zwierlein, Eugene Demler The nature of states formed around localized impurities can be a valuable probe of the properties of a quantum many body system. We propose using a deep optical lattice to localize an impurity atoms. We show that such and impurity atom acts like a magnetic impurity and leads to the formation of a pair of Shiba bound states inside the superconducting gap as well as possible bound states below the Fermi. The properties of these localized bound states can be read out using RF spectroscopy. [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z45.00014: Spectroscopy for cold atom gases in periodically modulated optical lattice potential Akiyuki Tokuno, Thierry Giamarchi Cold atoms in optical lattices are vigorously studied experimentally and theoretically as one of the candidates for a quantum simulator. At the same time, further development of probes to microscopic structure of systems is needed. We propose a novel spectroscopy in cold atom experiments by use of periodic phase-modulation of optical lattice potentials. Corresponding to the statistics of atoms, we formulate the different observables: The energy absorption rate for bosonic atom gases, and the doublon production rate for fermionic atom gases. These observables are formulated within the linear response theory. Interestingly they are given by the imaginary part of the retarded current-current correlation function which is familiar as a quantity corresponding to an optical conductivity. As an example, we discuss one-dimensional Mott insulating state, and also compare our spectroscopy with another known spectroscopy by amplitude-modulation of an optical lattice. [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z45.00015: Subwavelength localization of a single atom via resonance fluorescence photon statistics M. Al-Amri, Shuai Yang, M. Suhail Zubairy A subwavelength localization scheme of a single atom is investigated. The localization is based on the interaction of the two-level atom with a standing wave laser field. The photon statistics of resonant fluorescence depends on the Rabi frequency of the driving laser field and thus the position of the atom inside the standing wave. We show that this dependence can be used to localize the atom to a subwavelength accuracy. [Preview Abstract] |
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