Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session W2: Electronic Structure, Magnetism and Superconductivity of Sodium Cobaltate
Sponsoring Units: DCMPChair: Michelle Johannes, Naval Research Laboratory
Room: Morial Convention Center LaLouisiane C
Thursday, March 13, 2008 2:30PM - 3:06PM |
W2.00001: What we do and do not understand about electronic structure and superconductivity in sodium cobaltate? Invited Speaker: I will pose several questions and will attempt to answer at least some of them. (1) Why details of the electronic structure, including the Fermi surface topology, are particularly important for understanding superconductivity in this material? (2) What do standard band structure calculations get right, where they surely fail and where are they questionable? (3) How well do we understand the ARPES results? (4) What are possible effects of electron-electron correlations beyond LDA? Why LDA+U is worse than LDA for Na$_x$CoO$_2$ and to what extent can we believe DMFT? (5) What is the role of Na in the formation of the electronic structure and Fermi surface? To what extent the surface bands are the same as bulk? [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:42PM |
W2.00002: Pairing symmetry of the hydrated cobaltate superconductor Invited Speaker: We report NMR/NQR measurements on the hydrated cobaltate superconductor Na$_{x}$CoO$_{2}$*1.3H$_{2}$O at elevated pressures. The spin-lattice relaxation rate (1/$T_{1})$ decreases below $T_{c}$ with no coherence peak [1], and is in proportion to $T^{3 }$down to $T\sim T_{c}$/10, which provides compelling evidence for the existence of line nodes in the gap function [2,3]. The spin susceptibility obtained from the Knight shift measurement in a single crystal decreases below $T_{c}$ along all crystal-axis directions [4]. These results indicate anisotropic, spin-singlet pairing, and are most consistent with a $d$-wave gap. The electron correlations in the normal state are antiferromagnetic-like, which increases with decreasing Na-content [1,2]. The phase diagrams of $T_{c}$ and various physical properties as functions of Na-content [2], and pressure [3] will be presented, and the inter-relation between the superconductivity and the spin correlations will be discussed. \newline \newline References: \newline [1] T. Fujimoto, G. - q. Zheng, Y. Kitaoka, R.L. Meng, J. Cmaidalka, and C.W. Chu, Phys. Rev. Lett. \textbf{92}, 047004 (2004). \newline [2] G. - q. Zheng, K. Matano, R.L. Meng, J. Cmaidalka, and C.W. Chu, J. Phys.: Condens. Matter \textbf{18}, L63 (2006). \newline [3] E. Kusano, S. Kawasaki, K. Matano, G. - q. Zheng, R.L. Meng, J. Cmaidalka, and C.W. Chu, Phys. Rev. B \textbf{76}, 100506 (R) (2007). \newline [4] G. - q. Zheng, K. Matano, D.P. Chen and C.T. Lin, Phys. Rev. \textbf{B73}, 180503 (R) (2006). [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 4:18PM |
W2.00003: NMR studies of Na$_{x}$CoO$_{2}$ Invited Speaker: Sodium cobaltate Na$_{x}$CoO$_{2}$ is the parent phase of triangular-lattice superconductor Na$_{1/3}$CoO$_{2}$-[H$_{2}$O]$_{4/3}$. Co ions take a mixed-valence state of +4-x in this system. Since Co$^{4+}$ and Co$^{3+}$ possess spin S=1/2 and S=0, respectively, one may view the CoO$_{2}$ layers as charge doped S=1/2 triangular-lattice. CoO$_{2}$ layers exhibit a rich variety of strongly correlated electron behavior as a function of sodium concentration x, ranging from itinerant antiferromagnet (x$\sim $0.84 and 0.5), ``Curie-Weiss metal'' (x$\sim $0.71) to Fermi liquid (x$\sim $1/3). In this talk, I will discuss our NMR studies of Na$_{x}$CoO$_{2}$ for various Na concentration x, with particular emphasis on $^{59}$Co evidence for charge ordering in the insulating ground state of Na$_{0.5}$CoO$_{2}$ [1-4]. \newline \newline [1] F.L. Ning et al., arXiv : 0711.4023 \newline [2] F.C. Chou et al., arXiv : 0709.0085 \newline [3] F.L. Ning et al., PRL 94, 227004 (2005) \newline [4] F.L. Ning et al., PRL 93, 237201 (2004) [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:54PM |
W2.00004: Novel electronic states in Na$_x$CoO$_2$: Role of strong correlation and Na dopant order Invited Speaker: We argue that the strong Co intra-atomic Coulomb repulsion renormalizes the crystal field splitting and the bandwidths of the $t_{2g}$ complex in Na$_x$CoO$_2$, resulting in a single band crossing the Fermi level at all doping levels $x$ explored by ARPES experiments [1]. On this basis, we study the electronic states using a minimal electron-doped, one-band Hubbard model with large $U$ on the triangular lattice. The important role played by the off-plane Na dopants is taken into account by including the ionic electrostatic potential. We find a class of charge and spin density ordered states where the system alleviates antiferromagnetic (AF) frustration via charge inhomogeneity [2]. We show that the $\sqrt{3}\times2$ Na order at $x=0.5$ causes weak $\sqrt{3} \times1$ charge order in the Co layer and the emergence of AF order with small electron and hole Fermi surface pockets [2]. This theory of the ``0.5 phase'' is consistent with neutron scattering, NMR, Shubnikov-de Haas oscillations, and transport experiments. In the sodium rich phases, the high density of off-plane Na dopants (or dilute Na vacancies), in their ordered or disordered form, increases the tendency toward carrier localization in the Co plane [3], which competes with in-plane ferromagnetic (FM) correlations described by a renormalized Stoner theory [4]. We argue that the newly discovered electronic phases associated with Na vacancy order [5,6] can be described by a useful notion of ``super-Mottness'', where strong correlation effects on the superlattice structure give rise to the competition and possible coexistence of localized magnetic moments and itinerant FM carriers. \newline \newline [1] S. Zhou, M. Gao, H. Ding, P.A. Lee, and Z. Wang, Phys. Rev. Lett. 94, 206401 (2005). \newline [2] S. Zhou and Z. Wang, Phys. Rev. Lett. 98, 076401 (2007). \newline [3] C.A. Marianetti and G. Kotliar, Phys. Rev. Lett. 98, 176405 (2007). \newline [4] M. Gao, S. Zhou, and Z. Wang, Phys. Rev. B 76, 180402 (2007). \newline [5] M. Roger, et al., Nautre, 445, 631 (2007). \newline [6] F.C. Chou, et al., arXiv:0709.0085. [Preview Abstract] |
Session W3: Dislocation Patterns and Avalanches
Sponsoring Units: DCMPChair: Charles Reichardt, Los Alamos National Laboratory
Room: Morial Convention Center RO2 - RO3
Thursday, March 13, 2008 2:30PM - 3:06PM |
W3.00001: Scale-Free Intermittent Flow in Crystal Plasticity Invited Speaker: Under stress, crystals irreversibly deform through complex dislocation processes -- processes that intermittently change the microscopic material shape through isolated slip events. Using both model computer simulations and ultra-precise nano-scale measurements on nickel micro-crystals we directly determined the size of discrete slip events. The sizes range over nearly three orders of magnitude, and exhibit a shock and aftershock earthquake-like behavior over time. Analysis of the events reveals power-law scaling between the number of events and their magnitude, or scale-free flow. We show that dislocated crystals are a model system for studying scale-free behavior that is observed for many macroscopic systems. By analogy to plate tectonics, smooth macroscopic-scale crystalline glide arises from the spatial and time averages of disruptive earthquake-like events at the nano-scale. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:42PM |
W3.00002: Pattern Coarsening in a Two Dimensional Hexagonal System Invited Speaker: We have been studying the ordering, annealing, coarsening and alignment of two dimensional periodically ordered structures in thin films of diblock copolymers*. Coarsening by dislocation and disclination annihilation is clearly observed in AFM studies of monolayer films of cylindrical patterns with a time dependence given by t$^{\alpha }$, with $\alpha $ about 1/4. However in hexagonal structures the mechanism is less well defined and appears to involve the collapse of small grains entrained in the grain boundaries of larger domains. Remarkably the exponent of $\alpha $ about 1/4 remains. We also report on shear aligned samples and samples quenched in a gradient after alignment. * Harrison C, Angelescu DE, Trawick M, Cheng ZD, Huse DA, Chaikin PM, Vega DA, Sebastian JM, Register RA, Adamson DH, EUROPHYSICS LETTERS \textbf{67} 800-806 (2004) [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 4:18PM |
W3.00003: Domain Coarsening and Aging in Dislocation Glasses Invited Speaker: Dislocation systems were analyzed numerically with 1 and 3 glide axes, at T=0 and T$>$0, with and without climb. [1] Dislocation free domains formed even without shear, defined by dislocation rich domain walls. The domain formation was most pronounced in the presence of climb, somewhat counter-intuitively. The stability of domains was analyzed. The microscopic processes suppressing the climb-induced decay of domain walls were identified. The dislocation dynamics at low temperatures was markedly glassy. \underline {Aging:} Dislocations with glide only support minimal domain formation. The autocorrelation function showed aging, scaling with the waiting time as: C(t,t$_{w})$ = C$_{eq}$(t) C(t/t$_{w}^{\mu })$ and C$_{eq}$(t) $\sim $ t$^{-\beta }$, with $\mu $=0.65 and $\beta $=0.54. \underline {Freezing:} The effective diffusion constant decayed to zero as: D(t)$_{eff} \quad \sim $ t$^{-\gamma }$ , with $\gamma $=0.8. \underline {Coarsening:} Dislocations with glide and climb exhibited profound domain formation, the domains coarsening as L(t): L(t) $\sim $ t$^{1/z}$, with 1/z=0.17. The formation of domains without shear has been recently observed in GaAs by Rudolph and in dusty plasmas by Quinn and Joree. The domain coarsening was quantitatively captured in di-block copolymers [2], with 1/z=0.19, in good agreement with our results. \newline \newline [1] B. Bako, G. Groma, G. Gyorgyi and G.T. Zimanyi, Phys. Rev. Lett. \textbf{98}, 075701 (2007). \newline [2] P. Chaikin's talk, same session. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:54PM |
W3.00004: Dislocation Avalanches, Mean Free Path and Patterning Invited Speaker: Dislocations are the defects that carry plastic flow in crystalline materials. At the defect scale, plastic deformation occurs by dislocation avalanches, which are characterized by scale-free behavior within a bounded domain of amplitudes or energies. The probability distribution functions of these avalanches, their processes of initiation and termination and their contribution to dislocation storage during plastic flow are investigated using dislocation dynamics (DD) simulations. The model material is a copper single crystal strained along three high symmetry orientations. The distributions of avalanche amplitudes exhibit, for all orientations, a scaling exponent of 1.6, similar to what is reported in the literature. However, the average value of strain burst amplitudes, taken in the recorded domain of amplitudes, is found to be orientation-dependent. In parallel, a continuum model based on the notion of dislocation mean free path, which predicts the mechanical response of f.c.c. single crystals, was established with the help of DD simulations. It appears that both intermittent and continuum behavior exhibit the same orientation dependencies. Furthermore, for the three tested orientations, the ratio of the continuum mean free path to the average characteristic length traveled by dislocations during avalanches is a constant. This constant ratio simply results from an implicit coarse-graining procedure that is performed upon measuring different types of quantities in the simulated results. As a consequence, it appears that avalanche behavior can effectively be incorporated into continuum models for crystal plasticity. The present results are discussed with respect to the available experimental literature on the deformation of f.c.c. crystals. Finally, the formation of dislocation patterns, that is the self-organization properties of dislocations under strain, is tentatively discussed in terms of the properties of dislocation avalanches. [Preview Abstract] |
Session W4: Dynamics of Polymers
Sponsoring Units: DPOLYChair: Ralph Colby, Pennsylvania State University
Room: Morial Convention Center 206
Thursday, March 13, 2008 2:30PM - 3:06PM |
W4.00001: Dielectric and Viscoelastic Investigation of Entanglement Relaxation Invited Speaker: For a chain (probe) entangled with surrounding chains (matrix), the entanglement is released on large-scale motion of the matrix chains. This constraint release (CR) mechanism plays a central role in the current tube model for entangled chains. Since the distance for the lateral motion of the probe (= tube diameter) increases on CR, the tube model often utilizes the molecular picture of dynamic tube dilation (DTD) to represent the CR effect on the probe dynamics. In the simplest case of full-DTD, the relaxed portion behaves as a solvent giving no constraint and the tube diameter increases to the diameter in the corresponding solution. This talk utilizes cis-polyisoprene (PI) having the type-A dipole to test the validity of the DTD picture with the following strategy. The type-A dipole allows us to dielectrically evaluate the survival fraction f(t) of the dilated tube at time t. The full-DTD picture can be unequivocally tested by comparing the normalized viscoelastic modulus deduced from this picture (2.0-2.3th power of f(t)) with the viscoelastic data. The comparison indicated that the full-DTD picture works satisfactorily for monodisperse linear PI but not for blends of linear PI as well as for monodisperse star PI. The failure of the full-DTD picture for the last two systems is related to the fast relaxation modes in these systems. These modes leads to a significant increase of the fully dilated tube diameter over which the CR motion of the probe cannot occur in time, which naturally results in the failure of the full-DTD picture. Even for this case, the tube diameter increases to a level accepted by the CR motion. The partial-DTD picture considering this consistency with the CR motion was found to be valid for blends and star chains. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:42PM |
W4.00002: How does cohesive breakdown occur in entangled polymeric liquids? Invited Speaker: Entangled polymers are strongly viscoelastic materials with a characteristic relaxation rate that is a sensitive function of molecular weight and its distribution. At high Weissenberg number, i.e., when the rate of external deformation is far greater than the relaxation rate of entangled chains, a well entangled polymeric liquid yields like a solid before being forced to flow plastically. Recent particle tracking velocimetric observations (PTV, which are available for downloading at http://www3.uakron.edu/rheology/) show that the elastic yielding and subsequent flow take place inhomogeneously in both shear and extension. Most remarkably, an entangled polymer would suffer ``delayed'' cohesive structural breakdown after a step deformation. The transient cohesion provided by chain entanglement due to inter-molecular interactions is found to be higher at a higher applied rate. This talk will enumerate various PTV studies of step shear, startup shear and large-amplitude oscillatory shear to show how these crucial experiments have produced a phenomenological level understanding (\textit{J. Chem. Phys. }\textbf{2007}, $127$, 064903) of various flow phenomena in entangled polymers. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 4:18PM |
W4.00003: Dynamics of Polymer-Nanoparticle Mixtures Invited Speaker: Mixtures of polymers and particles occur in a variety of applications. Traditional applications of polymers in such systems include their role as colloidal stabilizers, and in rheological modifiers. Many of these applications are characterized by the feature that the polymer size is much smaller than the size of the particle. However, more recent developments in nano- and biotechnology applications have moved the polymer-particle mixtures from the ``colloid limit'' to the ``nanoparticle limit'' where the polymer size is comparable to or larger than the size of the particle. At the equilibrium level, the curvature of the particle now plays an important role in determining the interactions and phase behavior. At a dynamical level, conventional ``continuum'' wisdom no longer applies, and counterintuitive property relationships have been observed. This talk will focus on recent work in our group to develop and apply novel computer simulations to address the issue, ``how does the equilibrium, dynamical and property aspects of nanoparticle-polymer mixtures differ from their colloidal counterparts?'' Applications of our findings to the context and experiments of polymer nanocomposites will also be presented. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:54PM |
W4.00004: Shear Alignment and Realignment of Block Copolymer Microdomains in Thin Films Invited Speaker: Bulk block copolymers, like all liquid crystalline structures, are well-known to align under flow. In the past few years, we have shown that analogous flow alignment can be achieved in substrate-supported thin films ($<$100 nm thick) containing only one or a few layers of spherical or cylindrical nanodomains. Alignment can easily be imparted either by pulling a soft rubber pad in contact with the top surface of the film, or by flowing a nonsolvent fluid across the film. The latter geometry opens the possibility to ``write'' relatively complex patterns on the millimeter or submillimeter scale, where the nanodomain director follows the fluid streamline. Alignment can be achieved via either unidirectional or oscillatory shear, and is conveniently executed in a parallel-plate rheometer, where the substrate-supported film forms one ``plate'' and the ``gap'' is filled with the nonsolvent fluid. A threshold stress is required to achieve alignment of the microdomains, a stress which decreases steadily as the temperature is raised towards the polymer's order-disorder transition temperature. A simple melting-recrystallization model appears to capture the dynamics of overall alignment. Though no grain boundaries remain in well-aligned films, isolated dislocations persist. For sphere-formers, where two or more layers are required for alignment, the isolated dislocations are preferentially oriented in such a way as to facilitate sliding of the two layers of spheres past each other. Once a macroscopic orientation has been imparted to the film (over square-cm area), the microdomains can be reoriented by applying shear in a different direction, but a higher threshold stress is required than was needed for the initial alignment from the polygrain state. Recently, we have observed a sphere-to-cylinder transition in one particular block copolymer under shear, opening another possible mechanism for shear-induced alignment of the spheres which form when these cylinders relax. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:30PM |
W4.00005: Nanoparticle Ionic Fluids Invited Speaker: Nanoparticle ionic materials (NIMS) are a new class of organic-inorganic hybrid materials comprised of a nanoparticle core functionalized with a covalently-attached organic corona. These materials manifest a remarkable transition to a ``solvent-free'' colloidal liquid state near room temperature. Physical properties of these nanoparticle ionic fluids can be manipulated over an unusually wide range by varying geometric and chemical characteristics of the inorganic core and organic corona. On one end of the spectrum are materials with a high core particle contents, which display properties similar to fragile glasses, stiff waxes, and gels. At the opposite extreme are systems that spontaneously form particle-based ionic fluids characterized by transport properties remarkably similar to simple molecular liquids, but with high dielectric constants, conductivities, and refractive index. This talk will introduce nanoparticle ionic fluids based on charged and uncharged corona species, explore their applications, and will discuss physical and mathematical models for understanding their interactions, complex relaxation dynamics, and rheology. [Preview Abstract] |
Session W5: Sensing Science and Sensors for Industrial Applications
Sponsoring Units: FIAPChair: Mohsen Yaganeh, Exxon Mobil
Room: Morial Convention Center RO1
Thursday, March 13, 2008 2:30PM - 3:06PM |
W5.00001: Next-Generation Mid-Infrared Chemical Sensors – Challenges and Opportunities Invited Speaker: |
Thursday, March 13, 2008 3:06PM - 3:42PM |
W5.00002: Laser Interferometry for Harsh Environment MEMS Sensors Invited Speaker: Silicon-based MEMS technology has enabled the fabrication of a broad range of sensor and actuator systems that are having a great impact in areas that benefit from miniaturization and increased functionality. The main advantage of Si-based MEMS technologies is their possibility of integration with microelectronics thus allowing the economical production of smart microsystems. In the automotive industry for example, there is a need for inexpensive smart MEMS sensors for engine control applications. For instance, smart MEMS sensors capable of operating ``in cylinder'', where temperatures are around 400\r{ }C, could continuously monitor the combustion quality of the cylinders of automotive engines thus leading to reduced emissions and improved fuel economy. However, when the environment temperature is too high ($>$180\r{ }C), conventional Si-based microelectronics suffer from severe performance degradation, thus making smart Si-based MEMS impractical. Hence, further development, in terms of new MEMS materials and/or new technologies, is needed especially where high temperature capability is crucial to realizing improved electronic control. Remote sensing through optical signal detection has major advantages for safe signal transmission in harsh environments. It is highly resistant to electromagnetic interference (EMI) and radio frequency interference (RFI) and at the same time, it eliminates the necessity of on-board electronics, which has been one of the main obstacles in the development of smart MEMS sensors for high temperature applications. An economical way to deal with higher temperatures and other aggressive environmental conditions is to build MEMS sensors out of robust materials (e.g. Silicon nitride, SiC) and integrate them with optical signal detection techniques to form MOEMS. In this paper, we review recent trends for the use of laser interferometry for MEMS sensors in the context of using them for high temperature applications. Technological challenges faced in the development of these sensors, including sensitivity to measurement errors, packaging and cost reduction are also outlined. Finally, an overview of Fabry-Perot like MEMS sensors for high temperature applications is presented and issues facing their future progress and economical implementation are discussed. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 4:18PM |
W5.00003: Chemical and Physical Sensing in the Petroleum Industry Invited Speaker: World-scale oil, gas and petrochemical production relies on a myriad of advanced technologies for discovering, producing, transporting, processing and distributing hydrocarbons. Sensing systems provide rapid and targeted information that can be used for expanding resources, improving product quality, and assuring environmentally sound operations. For example, equipment such as reactors and pipelines can be operated with high efficiency and safety with improved chemical and physical sensors for corrosion and hydrocarbon detection. At the interface between chemical engineering and multiphase flow physics, ``multi-scale'' phenomena such as catalysis and heat flow benefit from new approaches to sensing and data modeling. We are combining chemically selective micro-cantilevers, fiber optic sensing, and acoustic monitoring with statistical data fusion approaches to maximize control information. Miniaturized analyzers represent a special opportunity, including the nanotech-based quantum cascade laser systems for mid-infrared spectroscopy. Specific examples for use of these new micro-systems include rapid monocyclic aromatic molecule identification and measurement under ambient conditions at weight ppb levels. We see promise from emerging materials and devices based on nanotechnology, which can one day be available at modest cost for impact in existing operations. Controlled surface energies and emerging chemical probes hold the promise for reduction in greenhouse gas emissions for current fuels and future transportation and energy technologies. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:54PM |
W5.00004: Silicon Carbide Micro/Nano Systems for Demanding and Harsh Environment Applications Invited Speaker: Micro/nano systems enable the development of smart products and systems by augmenting the computational ability of microelectronics with the perception and control capabilities of sensors and actuators. Micro/nano systems are also known as micro- and nanoelectromechanical systems (MEMS and NEMS), and have been commercialized in a wide range of applications including crash sensing, blood pressure measurement, optical projection, and fluid flow control to name a few. Silicon, in single- and polycrystalline forms, has been the platform semiconductor material underpinning the fabrication of the mechanical and electronic elements of micro/nano systems. However, the materials properties of silicon impose limitations on its use in harsh environment and demanding applications--for example, those involving operation in the presence of high temperatures, corrosive media, high shock loads, erosive flows, and/or high radiation, or involving performance requirements for the mechanical elements that are beyond silicon's capabilities. Silicon carbide (SiC) is an alternative platform semiconductor material that enables such applications because of its wider bandgap and higher melting/sublimation temperature, elastic modulus, fracture toughness, hardness, chemical inertness, and thermal conductivity. This talk will highlight our most recent SiC material, process, and device advances to enable sensing and actuation in applications such as propulsion instrumentation/control, power generation, resource exploration, nuclear reactor instrumentation, deep space exploration, and communications. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:30PM |
W5.00005: SiC Sensors in Extreme Environments: Real-time Hydrogen Monitoring for Energy Plant Applications Invited Speaker: Clean, efficient energy production, such as the gasification of coal (syngas), requires physical and chemical sensors for exhaust gas monitoring as well as real-time control of the combustion process. Wide-bandgap semiconducting materials systems can meet the sensing demands in these extreme environments consisting of chemically corrosive gases at high temperature and pressure. We have developed a SiC based micro-sensor for detection of hydrogen containing species with millisecond response at 600 \r{ }C. The sensor is a Pt-SiO$_{2}$-SiC device with a dense Pt catalytic sensing film, capable of withstanding months of continuous high temperature operation. The device was characterized in robust sensing module that is compatible with an industrial reactor. We report on the performance of the SiC sensor in a simulated syngas ambient at 370 \r{ }C containing the common interferants CO$_{2}$, CH$_{4}$ and CO [1]. In addition we demonstrate that hours of exposure to $\ge $1000 ppm H$_{2}$S and 15{\%} water vapor does not degrade the sensor performance. To elucidate the mechanisms responsible for the hydrogen response of the sensor we have modeled the hydrogen adsorptions kinetics at the internal Pt-SiO$_{2}$ interface, using both the Tempkin and Langmuir isotherms. Under the conditions appropriate for energy plant applications, the response of our sensor is significantly larger than that obtained from ultra-high vacuum electrochemical sensor measurements at high temperatures. We will discuss the role of morphology, at the nano to micro scale, on the enhanced catalytic activity observed for our Pt sensing films in response to a heated hydrogen gas stream at atmospheric pressure. \newline \newline [1] R. Loloee, B. Chorpening, S. Beers {\&} R. Ghosh, Hydrogen monitoring for power plant applications using SiC sensors, Sens. Actuators B:Chem. (2007), doi:10.1016/j.snb.2007.07.118 [Preview Abstract] |
Session W6: Strong Interacting Fermi Gases with Spin Asymmetry
Sponsoring Units: DAMOPChair: Roberto Diener, Ohio State University
Room: Morial Convention Center RO4
Thursday, March 13, 2008 2:30PM - 3:06PM |
W6.00001: Experiments in spin-polarized Fermi gases-- pairing without superfluidity? Invited Speaker: Fermionic superfluidity requires pairing of fermions. The nature of fermionic pairing in the strongly interacting regime both in the superfluid and possibly in the normal phase is of interest to condensed matter, nuclear and high energy physics. The experimental realization of high temperature superfluidity in ultracold Fermi gases opens a new approach to explore strongly interacting fermions both in the superfluid and normal phases. One question of relevance for example to superfluidity of quarks in cold baryonic matter as well as superconductivity has been the stability of the superfluid against an imbalance between the two strongly interacting fermionic components. An imbalance can be caused by different masses of the fermions or an externally applied magnetic field to a superconductor. In our experiments a density imbalance between two fermionic spin components is introduced. We will present the phase diagram of a spin-polarized Fermi gas of 6Li atoms at unitarity, mapping out the superfluid phase versus temperature and density imbalance. The nature of the phase transition changes from first-order to second-order at a tricritical point. At zero temperature, there is a quantum phase transition from a fully-paired superfluid to a partially-polarized normal gas at a critical spin polarization, known the Chandrasekhar-Clogston limit of superfluidity. These observations together with the implementation of an in situ ideal gas thermometer provide quantitative tests of theoretical calculations on the stability of resonant superfluidity. Pairing correlations in the superfluid and normal phases were explored in radio-frequency spectroscopy experiments. We studied how pairing correlations evolve across the superfluid to normal phase transition both as a function of temperature and spin imbalance. Even at spin imbalances above the Chandrasekhar-Clogston limit a gap in the single-particle excitation spectrum is observed. This indicates that the system is in a correlated state and the minority component is paired. The influence of final state interactions on the rf spectra will be discussed. Using a new superfluid 6Li spin mixture we demonstrate that pair dissociation spectra in the BEC-BCS crossover resemble asymmetric molecular dissociation spectra. Work done in collaboration with Y. Shin, A. Schirotzek and W. Ketterle, Department of Physics, MIT-Harvard Center for Ultracold Atoms, and Research Laboratory of Electronics, MIT, Cambridge, MA 02139. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:42PM |
W6.00002: Theory of RF Spectroscopy in Strongly Interacting Fermi Gases Invited Speaker: Radio frequency (RF) spectroscopy is an extremely powerful probe of the many-body state of a gas of cold atoms. For example spectra of cold Fermi gases have been used as evidence of superfluidity, and of pairing fluctuations in the normal state. Despite the large amount of information they contain, spectra of trapped gasses are not completely trivial to analyze. I will discuss the theory of RF spectroscopy, showing that the link between pairing and the observed spectra is very indirect, and that many of the ``pairing'' features occur even in a gas with no pairing whatsoever. I will also describe the important role played by final state interactions. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 4:18PM |
W6.00003: Phase separation in a spin polarized Fermi gas at the BEC-BCS crossover Invited Speaker: A strongly interacting ultra-cold gas of fermionic $^6$Li with unequal numbers of two spin components exhibits two distinct low temperature paired states.\footnote{G. B. Partridge, Wenhui Li, Y. A. Liao, R. G. Hulet, M. Haque and H. T. C. Stoof, \emph{Phys. Rev. Lett.} \textbf{97}, 190407 (2006) } Phase separation, where a uniformly paired core is maintained in the center of the trap by the expulsion of excess unpaired atoms, is observed at the lowest temperatures up to large number imbalance. Sharp boundaries, consistent with a first-order phase transition, are observed between the core and the unpaired atoms. Moreover, the superfluid core deforms markedly, becoming less elongated due to surface tension at the superfluid/normal boundary. At higher temperature, the core remains unpolarized up to a critical polarization, but does not deform. This temperature dependence is consistent with a tri-critical point in the phase diagram. Additionally, we are exploring the possibility that the large critical imbalance for loss of phase separation is a result of relatively small particle number ($10^5$) and high aspect ratio (30) elongated confinement. \vspace{1ex} \newline To date, no evidence for the long sought Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state has been observed in ultracold atomic gases. It is predicted that in 3D the FFLO phase occupies a very small region of the phase diagram,\footnote{D. E. Sheehy and L. Radzihovsky, \emph{Ann. Phys.} \textbf{322}, 1790 (2007)} whereas in 1D, the stability of the FFLO state is believed to be enhanced. Though the current optical potential is elongated, it is still in the 3D regime, and so a 2D optical lattice potential has been constructed to provide an array of 1D tubes. We will present results of our studies of the polarized Fermi gas in this 1D geometry. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:54PM |
W6.00004: Few-body physics of trapped unequal mass fermions Invited Speaker: The behavior of a two-component dilute Fermi gas exhibits an interesting dependence on the mass ratio between the two species. Our study tackles this system with 3-20 particles, using two independent techniques. First, an essentially exact diagonalization for 3-6 particles determines both the ground state and also the pattern of excited state energies, and our analysis permits an extraction of the dimer-dimer scattering length and effective range. Secondly, the nature of the system ground state is studied as a function of the mass ratio and the number of particles, up to N=20, using fixed-node diffusion Monte Carlo (DMC) techniques. By using two different solution techniques in their overlapping range of applicability from N=3-6, we are able to assess the accuracy of the nodal surface employed in the fixed-node DMC calculation. Physical properties such as the excitation gap will be analyzed over this range of particle number, and the intriguing unitarity limit is also considered. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:30PM |
W6.00005: Exploring an ultracold Fermi-Fermi mixture: interspecies Feshbach resonances of $^6$Li-$^{40}$K Invited Speaker: We report on the observation of interspecies Feshbach resonances in an ultracold mixture of two fermionic species, $^6$Li and $^{40}$K. Interpretation of the data unambiguously assigns molecular bound states to the various resonances and fully characterizes the ground-state scattering properties in any combination of spin states. Using this knowledge we hope to be able to produce $^6$Li-$^{40}$K molecules, cool them to quantum degeneracy, and study their BEC-BCS crossover. \newline \newline In collaboration with: F. Schreck, Institut fuer Quantenoptik und Quanteninformation, Oesterreichische Akademie der Wissenschaften, 6020 Innsbruck, Austria; E. Wille, Institut fuer Quantenoptik und Quanteninformation, Oesterreichische Akademie der Wissenschaften, 6020 Innsbruck, Austria and Institut fuer Experimentalphysik und Forschungszentrum fuer Quantenphysik, Universitaet Innsbruck, 6020 Innsbruck, Austria; F.M. Spiegelhalder, G. Kerner, D. Naik, A. Trenkwalder, G. Hendl, Institut fuer Quantenoptik und Quanteninformation, Oesterreichische Akademie der Wissenschaften, 6020 Innsbruck, Austria; R. Grimm, Institut fuer Quantenoptik und Quanteninformation, Oesterreichische Akademie der Wissenschaften, 6020 Innsbruck, Austria and Institut fuer Experimentalphysik und Forschungszentrum fuer Quantenphysik, Universitaet Innsbruck, 6020 Innsbruck, Austria; T.G. Tiecke, J.T.M. Walraven,Van der Waals-Zeeman Institute of the University of Amsterdam, 1018 XE, The Netherlands; S.J.J.M.F. Kokkelmans, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands; E. Tiesinga, P.S. Julienne, Joint Quantum Institute, National Institute of Standards and Technology and University of Maryland, Gaithersburg, Maryland 20899-8423, USA [Preview Abstract] |
Session W7: Nonlinear Dynamics of Neural Systems: A Statistical Mechanics of the Brain
Sponsoring Units: DBP GSNPChair: John Beggs, Indiana University
Room: Morial Convention Center RO5
Thursday, March 13, 2008 2:30PM - 3:06PM |
W7.00001: Representing Information with Correlated Neural Populations Invited Speaker: Using a multi-electrode array, we can record simultaneously from up to 50 ganglion cells, the output neurons of the retina, while stimulating with varied visual images generated on a computer monitor. We find that nearby ganglion cells, the output neurons of the retina, have spatial receptive fields that overlap significantly, leading to correlated firing and redundancy in the visual information that cells encode. Although the strength of correlations among pairs of cells is weak ($\sim$10\%), the effect in larger populations is dramatic: patterns of spiking and silence in groups of just 10 cells can occur with a probability $\sim$100,000-fold different from that predicted from statistical independence. We show that these strong network correlations can be explained by a model that includes all pairwise correlations, but no higher-order statistics. This model is identical to the Ising model, and predicts that larger populations may exhibit a form of freezing transition that allows for robust error correction. We have begun to explore these error-correcting properties in simple visual discrimination tasks using large populations of ganglion cells. We find that the correlations among neurons can dramatically reduce the discrimination error. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:42PM |
W7.00002: Exploring the network structure of concerted activity in the primate retina using maximum entropy methods Invited Speaker: All visual signals in the brain originate in the electrical activity of retinal ganglion cells (RGCs). Standard models implicitly assume that RGCs signal information independently of one another. However, several studies have demonstrated that significant concerted activity in pairs of RGCs may fundamentally alter visual signals. We recorded the electrical activity of several hundred RGCs in peripheral monkey retina. The regular mosaic organization of RGCs indicated that we recorded from nearly every cell in a region of the visual field. In the presence of constant illumination, pairs of RGCs fired synchronously several-fold more often than expected by chance, indicating significant network interactions. Synchrony was localized and universal amongst cells of the same type indicating that it arises from local and highly stereotyped circuitry. To test whether concerted firing can be explained by known pairwise interactions, we used a maximum entropy approach borrowed from statistical mechanics to predict concerted activity. The model accurately reproduced the data. This suggests that network interactions in the primate retina are well approximated by a nearest neighbor Ising model and concerted activity can be understood based on local interactions within a neural population. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 4:18PM |
W7.00003: A continuous phase transition in neocortical slice networks Invited Speaker: Recent experiments demonstrate that activity in neocortical circuits can propagate in the form of avalanches whose sizes follow a power law distribution, suggesting that these circuits operate near a continuous phase transition point. Computational models indicate that this critical point may be optimal for information processing. However, the existence of a power law is not sufficient to establish critical behavior because stochastic processes that do not undergo phase transitions can also produce power laws. We recorded power law distributions of neuronal avalanches from neocortical slices and then pharmacologically perturbed network activity. We then measured deviations from power law behavior. Here we show that these deviations covaried systematically with a control parameter, as would be expected for a continuous phase transition. A critical branching model captures this transition, while stochastic models do not. Our findings imply that the physical theory underlying continuous phase transitions can be fruitfully applied to neocortical circuits. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:54PM |
W7.00004: Patterns of Neural Activity in Networks with Complex Connectivity Invited Speaker: An understanding of emergent dynamics on complex networks requires investigating the interplay between the intrinsic dynamics of the node elements and the connectivity of the network in which they are embedded. In order to address some of these questions in a specific scenario of relevance to the dynamical states of neural ensembles, we have studied the collective behavior of excitable model neurons in a network with small-world topology. The small-world network has local lattice order, but includes a number of randomly placed connections that may provide connectivity shortcuts. This topology bears a schematic resemblance to the connectivity of the cerebral cortex, in which neurons are most strongly coupled to nearby cells within fifty to a hundred micrometers, but also make projections to cells millimeters away. We find that the dynamics of this small-world network of excitable neurons depend mostly on both the density of shortcuts and the delay associated with neuronal projections. In the regime of low shortcut density, the system exhibits persistent activity in the form of propagating waves, which annihilate upon collision and are spawned anew via the re-injection of activity through shortcut connections. As the density of shortcuts reaches a critical value, the system undergoes a transition to failure. The critical shortcut density results from matching the time associated with a recurrent path through the network to an intrinsic recovery time of the individual neurons. Furthermore, if the delay associated with neuronal interactions is sufficiently long, activity reemerges above the critical density of shortcuts. The activity in this regime exhibits long, chaotic transients composed of noisy, large-amplitude population bursts. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:30PM |
W7.00005: Optimal processing and the statistics of visual input signals Invited Speaker: Sensory information processing can be seen as a statistical estimation problem, where relevant features are extracted from a raw stream of sensory input containing an imperfect representation of those features. Broadly speaking, the optimal solution to the feature extraction problem depends on the statistical structure of those input signals. Here we study the statistics of natural visual input signals, and the optimal solution to the problem of visual motion detection. Motion detection is a biologically important feature estimation problem, as many animals use vision to estimate their motion through space. Many years ago, Reichardt and Poggio drew attention to two important aspects of this problem: First, computing motion from an array of photosensors is an irreducibly nonlinear operation, and second, biological versions of this operation seem mathematically tractable. To paraphrase, the problem is interesting but not hopelessly complicated. In this spirit I will discuss motion estimation in the visual system of the blowfly, with an emphasis on performance under natural conditions. As noted above, the array of photoreceptors in the retina implicitly contains data on self motion, but this relation is noisy, indirect and ambiguous due to photon shot noise and optical blurring, and also as a result of the structure of the natural environment. Further, natural variations in the visual signal to noise ratio are enormous, and nonlinear operations are especially susceptible to noise. One can therefore reasonably hope that animals have evolved interesting optimization strategies to deal with large variations in signal quality. I will present experimental data, both from sampling natural probability distributions, and from motion sensitive neurons in the fly brain, that illustrate some of these solutions and that suggest that the fly indeed approaches optimality. The implications of these findings and their possible generalizations will be discussed. [Preview Abstract] |
Session W8: Liquid Crystals II: Dynamics and Topology
Sponsoring Units: DFDChair: Robin Selinger, Kent State University
Room: Morial Convention Center RO6
Thursday, March 13, 2008 2:30PM - 2:42PM |
W8.00001: Finite element studies of the soft elastic response in liquid crystal elastomers Badel Mbanga, Jonathan Selinger, Robin Selinger When a liquid crystal elastomer film is stretched in a direction transverse to the nematic director, the resulting stress-strain curve typically displays a plateau region, showing a characteristic soft elastic response. Using 3-d nonlinear finite element simulation, we model the formation of orientational domains controlling this mechanical behavior. We investigate the force-displacement response as a function of strain rate, and explore geometric frustration arising from boundary conditions imposed by clamps. We also model mechanical response of polydomain films aligned under external strain. Our 3-d finite element algorithm is based on a Hamiltonian with terms representing elastic potential energy, kinetic energy, and coupling between elastic strain and nematic order. We assume that orientational order of the material relaxes quickly and remains in local quasistatic equilibrium with the instantaneous local strain. Internal dissipation is also included. We intend through this model to further our understanding of the basic physics governing the dynamic mechanical response of nematic elastomers and also provide a useful computational tool for design and testing of potential engineering device applications. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W8.00002: Fluctuating hydrodynamics of nematics for models of liquid-crystal based biosensors via lattice Boltzmann simulations Orlando Guzman, Jose Antonio Velez, David Casta\~neda Experimental biosensors based on liquid crystals (LC) use nematics to detect the presence of specific analytes, via the optical textures exhibited by the LC at long times. Efforts to model the time evolution of these textures have relied on relaxational models, ignoring transport phenomena. In this work we include hydrodynamics into a model for these LC biosensors, using lattice Boltzmann (LB) methods and assess the effect on the lifetime of multidomain structures, characteristic of high concentrations of analyte. We apply Yeoman's et al. LB algorithm, which reproduces the hydrodynamic equations developed by Beris and Edwards for LCs. We also take into account thermal fluctuations, by adding random perturbations to the hydrodynamic modes. Following Adhikari et al., their amplitude is determined by the Fluctuation-Dissipation theorem and we excite both hydrodynamic and the sub-hydrodynamic modes (also called ghost modes). As a result, we analyze the influence of the fluctuations and hydrodynamics on the movement of topological defects. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W8.00003: Viscous properties of bent core nematic liquid crystals measured using a nanoliter viscometer Christopher Bailey, John Harden, Antal Jakli Since the development of bent core liquid crystal mesogens showing the nematic phase, much work has been done to study the physical properties of these materials in the form of dielectric spectroscopy, dynamic light scattering, and magnetic field induced phase transitions. Some results of these studies showed interesting behaviors such as slow biaxial fluctuations and the possibility of long range tetrahedratic ordering above the nematic phase. Here we report rheological and optical studies on several bent core mesogens in their isotropic and nematic phases. For the rheological studies, we built a viscometer capable of measuring viscoelastic properties with two centipoise resolution with only using 10nL of fluid. Results show abnormal viscosity and optical behavior near the isotropic to nematic phase transition. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W8.00004: Electrically induced twist in smectic liquid-crystalline elastomers Jonathan Selinger, Robin Selinger, Christopher Spillmann, Jawad Naciri, B. R. Ratna Liquid-crystalline elastomers are cross-linked polymer networks covalently bonded to liquid-crystalline units, so that any change in the orientational order influences the shape of the polymer network. As a new approach to developing electrically controllable actuators, we prepare elastomers of chiral smectic-A liquid crystals, which have an electroclinic effect, i.e. a molecular tilt induced by an applied electric field. For thin films in a bookshelf orientation, one would expect the electroclinic effect to cause an in-plane shear of the elastomer, leading to a distortion from a rectangle to a parallelogram. Surprisingly, our experiments find instead that an electric field causes a twisting of the film out of the plane, leading to a helically curved shape. The twist is rapid and reversible, with a helical sense that depends on the sign of the applied field. To explain this electrically induced twist, we develop a continuum elastic theory based on the assumption that the film has an asymmetry between front and back, which can be attributed to the preparation conditions. We further present finite-element simulations of the twist, which show the dynamic shape change. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W8.00005: Topography induced homeotropic alignment of nematic liquid crystals Youngwoo Yi, Noel Clark, Joseph Maclennan, Vaibhav Khire, Christopher Bowman We probe the orientation of a nematic liquid crystal on a planar aligning polymer film, where the film is topographically modified with sub micron scale checkerboard patterns. Alignment is studied in hybrid LC cells with the patterned polymer film on one plate and a self-assembled monolayer (homeotropic) on the other plate. A transition to homeotropic alignment on the pattern is observed as the pattern scale is reduced. An analysis shows that as the size of a square well becomes smaller the elastic energy of LCs decreases more slowly than the surface anchoring energy of the well, so that when the size of the well becomes small enough the elastic energy in planar configuration becomes comparable or larger than the polar anchoring energy of the surface. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W8.00006: Flexoelectric effect in a bent-core liquid crystal measured by Dynamic Light Scattering Madhabi Majumdar, K. Neupane, James. T. Gleeson, Antal Jakli, Samuel Sprunt Flexoelectricity is a linear coupling between electric polarization and elastic flexure in liquid crystals [1]. Although typically quite weak in calamitic LCs, the flexoelectric effect has recently been shown, by direct (electromechanical) measurement of the flexure-induced polarization, to be enhanced by several orders of magnitude in certain bent-core nematic (BCN) liquid crystals [2]. We report here an application of dynamic light scattering to measure the flexoelectric coefficient ($e1 + e3$) of BCNs through coupling of polarization to elastic fluctuation modes of the optic axis. Our results agree in order of magnitude with the values obtained by the electromechanical method.\\~[1]~R.B. Meyer, \emph{Phys. Rev. Lett}. 22, 918 (1969).\\~[2]~J. Harden, B. Mbanga, N. Eber, K. Fodor-Csorba, S. Sprunt, J. T. Gleeson, A. Jakli, \emph{Phys. Rev. Lett}. 97, 157802 (2006). [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W8.00007: Measurement of the Converse Flexoelectric Effect of a Bent-Core Nematic Liquid Crystal John Harden, Richelle Teeling, Samuel Sprunt, James Gleeson, Antal Jakli Flexoelectricity is a linear coupling between bend or splay distortions and electric polarization$^{1}$. It is a unique property of orientationally ordered materials of which liquid crystals are the best known example. It has been shown that the bend flexoelectric coefficient in ``banana'' bent-core liquid crystals is three orders of magnitude higher than the effect found in calamitic liquid crystals$^{2}$. Using a Mirau interferometer attached to the objective port of a microscope, we were able to measure the converse effect. This polarity dependent flexing of a thin cell yielded displacements of 100nm when 100V DC was applied to a 1cm x 2cm x 25$\mu $m cell filled with the bent-core nematic liquid crystal 4-chloro-1,3-phenylene bis 4-[4'-(9-decenyloxy) benzoyloxy] benzoate (ClPbis10BB). The substrates were 100$\mu $m thick Mylar with ITO as a conducting layer. These preliminary experiments show the promise of new types of soft actuators or beam steering devices. References: $^{1}$Meyer R.B. (1969). \underline {Physical Review Letters} \textbf{22}(18): 918-921. $^{2}$Harden, J., B. Mbanga, et al. (2006). \underline {Physical Review Letters} \textbf{97}(15). Acknowledgement: NSF DMR-0606160 and NSF REU-0649017 [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W8.00008: Envelope Dynamics of an Experimental Electroconvection Pattern G. Acharya, G. Dangelmayr, I. Oprea, J.T. Gleeson A video displaying electroconvection of the nematic I52 is analyzed. Spatial Fourier transforms of the frames reveal that the dynamics is driven by four groups of oblique (relative to the director) modes corresponding to counterpropagating pairs of traveling waves. This is consistent with a stability analysis of the electrohydrodynamic equations, which predict for I52 a Hopf bifurcation with four oblique critical wave numbers. Beyond the linear stability analysis, a weakly nonlinear analysis tells that the evolution of the pattern is governed by four slowly varying (in space and time) envelopes. These four envelopes are extracted from the pattern using Fourier analysis, and analyzed using several diagnostic tools such as statistical analysis, Karhunen Loeve decomposition, and the computation of correlation lengths and times as well as Lyapunov exponents. The results of this analysis indicate that the pattern shows extensive spatiotemporal chaos. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W8.00009: Orientational order and topological defects on curved surfaces Robin Selinger, Jonathan Selinger, Alex Travesset Recent studies show a close relationship between geometry of surfaces and orientational order within the surfaces. Positive topological defects are attracted to regions of positive Gaussian curvature, and vice versa. To investigate this relationship, we develop a new computational approach to simulate orientational order on surfaces of arbitrary shape. We place xy spins on the surface in a disordered mesh constructed via random sequential absorption. We apply this approach to a sphere, a torus, and an ``egg-crate" surface, $z = A sin(kx) sin (ky)$. For the sphere, we find a total topological charge of +2 as required by the Gauss-Bonnet theorem. For the torus, defects form in pairs, +1 defects on the outer edge (with positive Gaussian curvature) and -1 defects on the inner edge (with negative Gaussian curvature). For the egg-crate surface, with a coarse mesh, a +1 defect forms at each max/min and a -1 defect at every saddle point; while for a finer mesh, defects anneal away. We analyze these simulation results in terms of a continuum elastic model. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W8.00010: Local photo-reorientation of a liquid crystal using a laser focused on an azo dye-based monolayer Yue Shi, Noel Clark The orientation adopted by molecules in an azobenzene-based self-assembled monolayer (azo-SAM) is perpendicular to the polarization of incident green light due to isomerization, aligning liquid crystal (LC) correspondingly. To study the local photo reorientation of the LC, the exciting laser is focused into a small spot on the azo-SAM, of a hybrid cell made with nematic LC sandwiched between the azo-SAM and a homeotropic surface. Under irradiation with changing polarization, a variety of interesting phenomena are observed, including winding of rings of reorientation and orientational slipping. Results vs. exciting light intensities and different rotating frequencies will be reported. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W8.00011: Topology and Geometry of 1D Translational Order on Curved Surface Xiangjun Xing It is shown that one dimensional translational order on two dimensional curved substrate is naturally described by differential forms. A new type of global dislocation defects is identified and its relation with the topological properties of the embedding (compact) manifold is explored using algebraic topological methods. The associated topological charge classifies all ground states with no local defects. The energetics of smectic order on curved substrate is also discussed. Coupling between nematic director field and extrinsic curvature is shown to be important. As a simple application, the phase diagram of smectic order on a torus is analyzed. Two phases are identified: a small/thin phase where the nematic director is locked by curvature and a large/fat phase where the director varies continuously with system parameters. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W8.00012: Coarsening of two-dimensional islands in freely suspended smectic A films Duong Nguyen, Cheol Park, Joseph Maclennan, Matthew Glaser, Noel Clark We have observed coalescence driven coarsening of islands (edge dilocation loops) in freely suspended Smectic A liquid crystal films. This is a good realization of a two-dimensional system, with films as thin as one molecular length (3-4 nm). The film is drawn by spreading material across a circular hole of about 4 mm in diameter, after which the film is suspended by the meniscus in contact with the edge of the hole. Islands are generated by blowing air parallel to the film surface, which produces a shearing force that breaks a thick region of the film into circular islands. Depending on the Smectic A material, we observe either strong or weak short-range repulsion between dislocation loops, leading to slow or fast coalescence, repectively. Over time, the average size of islands increases as the number of islands drops. The observed coasening dynamics is compared with theoretical predictions. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W8.00013: The Ground States of Nematic Order on a Sphere and Topological Defects Homin Shin, Mark Bowick, Xiangjun Xing We study the ground states of a spherical nematic order and the resulting configuration of topological defects. To emulate the ground state, we use hard rods confined on the surface of a sphere and very gradually compress the system upto the maximum packing density with Monte Carlo simulations. The nematic phases with four +1/2 disclination defects are clearly observed. Although the tetrahedral structure of four +1/2 defects is expected, we find all the defects most likely sitting on a great circle. The theoretical reasoning is provided with the calculation of defect energies in terms of the elastic anisotropy. Finally, we present that the allowance of some softness to the rods gives arise to qualitative changes in the director field surrounding the defect core. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W8.00014: Thermal-Cycle Memory Functions and Ising Dynamics Brad Johnson, David Patrick The Ising model provides a rich system for the study of a variety of correlated systems. In this talk, we present the results of numerical studies of 2- and 3-dimensional Ising spin systems subjected to thermal cycling from an ordered state to states with a fixed order parameter ($<$1), but with differing overall morphologies, and back to a quenched state. We find that for systems with initial states generated by thermal disordering above T$_{c}$, the initial state of a given order parameter has larger `islands' of like-spin (than the case for random disorder with the same overall order parameter) and consequent quenches of the state to T$<$T$_{c}$ result in a strong correlation to a particular final average order parameter. The function we find is given by $\left\langle S \right\rangle \approx \tanh (B\cdot S_{init} )$, where S$_{init}$ is the order parameter of the initial state, $<$S$>$ is the average quenched order parameter, and B is a constant that depends upon the morphology of the initial state. The reason for the strong correlation stems from the energies associated with spins at the borders of large clusters. This `memory effect' does not occur in 3D (due to the larger number of near-neighbors). Finally, we discuss the `memory function' in the context of interfacial states of liquid crystals. [Preview Abstract] |
Session W9: Theoretical Methods and Applications
Sponsoring Units: DCMPChair: Alexander Tchernatinsky, University of Minnesota
Room: Morial Convention Center RO7
Thursday, March 13, 2008 2:30PM - 2:42PM |
W9.00001: Polarization model revisited Michael Galperin, Abraham Nitzan, Mark A. Ratner We revisit a polaron model proposed by us as a possible mechanism for nonlinear conductance, and discuss difference in polaron formation within isolated system vs. molecular junction situation. Within one-level model we present approximate expression for electronic Green function corresponding to inelastic transport case, which in appropriate limits reduces to expressions presented previously for isolated molecule and for molecular junction coupled to slow vibration (static limit). Relevance of the isolated molecule-type consideration to describe properties of molecular junctions is discussed. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W9.00002: A theoretical investigation of the porphyrin-gold junction: applications to molecular wires. Matt McKenzie, Zorabel LeJeune, Jayne Garno, Bin Chen An important step in the miniaturization of electronic devices involves molecular wires and junctions at the nanoscale level. Porphyrins are a promising material for such objects because of their unique electronic, chemical, and optical properties. The model porphyrin used in this study is a free based tetra-substituted with two phenyl rings and two pyridyl rings as peripheral groups which could provide a mechanism for enhanced electron transfer. The goal of this study is to elucidate the electron transfer paths between the model porphyrins and Au(111). The orbital structures and properties are determined using Car-Parrinello molecular dynamics. The geometry of the porphyrin on the gold surface is explored; from a complete reorientation of the molecule with respect to the surface to different orientations of the pyridyl groups. The calculated electronic conductivity, using the Kubo-Greenwood formula, will be compared to experimental findings using conductive probe Atomic Force Microscopy. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W9.00003: Ion-Ion Interactions in Simple Metallic Systems: Beyond Linear Response James Porter, Neil Ashcroft, Geoffrey Chester We extend the formalism of electronic response theory to second order in perturbing pseudopotentials and examine the physical consequences on effective ionic pair potentials for certain simple metals, under standard conditions. The pseudopotentials, assumed to be transferable, are of the Ashcroft empty-core form. Our results show that inclusion of second-order response terms in the pair potentials leads to the deepest potential minima having locations that are within 8\% of the experimental nearest-neighbor distances for crystalline sodium, magnesium, aluminum, and metallic silicon, all in their standard one-atmosphere structures. Second-order response effects are found to become increasingly important as the valence increases. We briefly discuss two natural extensions of this research, namely to three-body potentials and to changes expected in pair potentials at higher densities. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W9.00004: Algorithm for extraction of quantum oscillation orbits from band structure data Patrick Rourke, Stephen Julian In determining the Fermi surface of a material, quantum oscillation measurements are often compared to band structure calculations. Each oscillation frequency corresponds to an electron (or hole) orbit on the Fermi surface, perpendicular to the applied magnetic field; only orbits enclosing areas that are locally extremal are detected. To facilitate comparisons between theory and experiment, we have developed an algorithm, ``SKEAF,'' which finds extremal orbits in band structure calculations and determines quantum oscillation frequencies, effective masses and band specific heat contributions. Our code uses a k-space supercell approach, and can successfully locate geometrically-complicated orbits. Example results will be presented for the heavy fermion material UPt$_{3}$. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W9.00005: Development of the relativistic tight-binding model for Platinum Alexander Tchernatinsky, J. Woods Halley As a first step in a program to understand the mechanism of oxygen reduction on a platinum surface in an aqueous environment, we developed a relativistic self consistent tight- binding model for platinum. We applied a scheme that we successfully used previously for the description of titanium (S.~Erdin,~et~al.,~PRB, \textbf{72}, 035405 (2005)) in which the electronic structure problem is described by an energy functional containing onsite terms depending self consistently on the local charge and interatomic terms. Due to the high atomic number of platinum, relativistic effects are known to be significant in the electronic structure. We include relativistic effects in the onsite functions of the tight binding model by making them self consistently dependent on the local Mulliken charge (as before) and also on the expectation values of the total atomic angular momentum number J of the atom and the occupation numbers of the 5d$_{5/2}$ and 5d$_{3/2}$ atomic orbitals in the tight binding basis. We find that this set of variables can uniquely describe the low energy states of the isolated platinum atom including relativistic effects. Their values are calculated self consistently in the tight binding model for the metal. The model was used to calculate the electronic structure of relaxed, low index platinum surfaces. Results will be compared with DFT results and with experiment. This work was supported in part by Minnesota Supercomputing Institute and U.S.DOE. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W9.00006: Equivalence of dipole correction and Coulomb cutoff techniques in supercell calculations Liping Yu, V. Ranjan, W. Lu, J. Bernholc, M. Buongiorno Nardelli In ab initio calculations for surfaces or non-periodic systems one often relies on the supercell approximation, where periodic replicas are separated by enough empty space to avoid spurious interactions between successive images. However, a vacuum separation is not sufficient to screen dipolar interactions that appear in asymmetrically charged or polar systems. Two solutions have been proposed in the literature: (i) the dipole correction, and (ii) Coulomb cutoff formalism that eliminates interactions between periodic replicas. We compare these methods under the same conditions in the framework of plane wave DFT calculations. It is found that the two methods produce equivalent results for the total energy, force, charge density and self-consistent potential. In band structure calculations, the results coincide for occupied states but differ for delocalized unoccupied ones in small supercells. This discrepancy can be used as a criterion to identify supercell sizes that are sufficiently large to obtain converged results. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W9.00007: Issues related to Convergence Properties of First Principles Full Potential Multiple Scattering Electronic Structure Calculations Aurelian Rusanu, G. Malcolm Stocks, Markus Eisenbach, Don M. Nicholson, Yang Wang Despite some clear advantages for specific problems, the implementation of full potential electronic structure methods based on the use of multiple scattering theory (MST) (KKR derived approaches) has received much less investment than alternate electronic structure methods. Here we describe some new techniques that facilitate an easy and accurate implementation of first principles full potential MST methods. The method consists of solving: (1) the full-potential single site scattering problem, where we avoid the usage of the shape function by surface integrals methods to determine the scattering matrices, and (2) the Poisson problem, where the site centered full-potential is constructed from a sphere bounded non-overlapping charge density and a smooth space-filling charge density. Specifically, we discuss issues related to accuracy and convergence properties of these techniques within the context of the order-N Locally Self-consistent Multiple Scattering (LSMS) method. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W9.00008: First-principles investigation of electronic band gap in multiwalled carbon nanotube: Role of mechanical deformations Pavan K. Valavala, Gregory M. Odegard, Ranjit Pati The carbon nanotube (CNT) structures have been the subject of intense research in recent years. Some studies have shown that the electronic band gap in single walled CNT can be modulated through mechanical deformations such as flattening. It has been shown that single walled CNTs undergo a semiconducting-metallic and semiconducting-metallic-semiconducting transition when subjected to deformations. However, the modulation of electronic band gap of multiwalled CNT under mechanical deformations has not been studied. We have used first- principles gradient density functional approach to explore the role of flattening on the electronic properties of MWCNT structures. The influence and the effect of flattening on the electronic properties of the constituent single walled CNTs are also explored. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W9.00009: Atomic-scale Analysis of the Interactions Between Atomic Hydrogen and Multi-walled Carbon Nanotubes Andre R. Muniz, Tejinder Singh, Dimitrios Maroudas We present a detailed atomic-scale analysis of the interactions of atomic hydrogen with the internal layers of multi-walled carbon nanotubes (MWCNTs). The analysis is based on a synergistic combination of classical molecular dynamics (MD) with first-principles density functional theory (DFT). The Adaptive Interatomic Reactive Empirical Bond Order (AIREBO) potential is employed in the MD simulations of H-MWCNT interactions and the resulting structural relaxations. Parameters that have been varied in our analysis include nanotube diameters, number of nanotube walls, inter-wall spacing, and temperature. The MD simulations reveal that, under certain conditions, H chemisorption onto internal MWCNT walls and H diffusion in the space between walls can induce the formation of inter-shell sp$^{3}$ C-C bonds. The MD mechanisms are in good agreement with our DFT calculations of optimal pathways of C-C bond formation and provide interpretations for the formation of nanocrystalline carbon, which has been observed experimentally upon H$_{2}$ plasma exposure of MWCNTs. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W9.00010: Electronic Property Control of Single-Walled Carbon Nanotubes by Functionalization Chiayun Wu, Young-Kyun Kwon Single-walled carbon nanotubes exhibit remarkable electronic properties. It is well known that there are two types of single-walled carbon nanotubes: metallic and semiconducting. However, separating semiconducting carbon nanotubes from metallic ones is a ``holy grail'' problem in nanoelectronics fields. Using ab initio density functional theory, we will present the effects of various functional groups, such as 4-bromobenzene diazonium tetrafluoroborate, on SWNTs. Modifications in electronic and transport properties due to such functionalization will be discussed. Possible mechanism converting metallic tubes to semiconducting ones will be addressed. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W9.00011: Design of Janus Nanoparticles with Atomic Precision Qiang Sun, Qian Wang, Puru Jena, Yoshi Kawazoe Janus nanoparticles, characterized by their anisotropic structure and interactions have added a new dimension to nanoscience because of their potential applications in biomedicine, sensors, catalysis and assembled materials. The technological applications of these nanoparticles, however, have been limited as the current chemical, physical, and biosynthetic methods lack sufficient size and shape selectivity. We report a technique where gold clusters doped with tungsten can serve as a seed that facilitates the natural growth of anisotropic nanostructures whose size and shape can be controlled with atomic precision. Using ab initio simulated annealing and molecular dynamics calculations on Au$_{n}$W (n$>$12) clusters, we discovered that the W@Au$_{12}$ cage cluster forms a very stable core with the remaining Au atoms forming patchy structures on its surface. The anisotropic geometry gives rise to anisotropies in vibrational spectra, charge distributions, electronic structures, and reactivity, thus making it useful to have dual functionalities. In particular, the core-patch structure is shown to possess a hydrophilic head and a hydrophobic tail. The W@Au$_{12}$ clusters can also be used as building blocks of a nano-ring with novel properties. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W9.00012: Uniaxial compression of group-IV nanoparticles from ab-initio molecular dynamics simulations Prasanjit Samal, Matteo Cococcioni Uniaxial compressions of isolated systems from ab-initio molecular dynamics are made possible through the extension of the electronic enthalphy method [Phys. Rev. Lett. 94, 145501 (2005)] previously introduced for finite systems under hydrostatic pressure. Through this novel approach experimental settings with nanoparticles indented between parallel plates can be reproduced and simulated more realistically, thus allowing for more reliable comparisons between experimental data and simulation results. Molecular dynamics simulations for some group-IV nanoparticles under uniaxial compression have been performed using this extended scheme. Comparison with the deformation of the same systems under hydrostatic loads will elucidate the differences and similarities in the nucleation events of structural transformations and in their kinetic pathways. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W9.00013: Explaining the electroluminescence of single molecules John Buker, George Kirczenow Experimental studies of electroluminescence from molecules on complex substrates have yielded intriguing relationships between the current-voltage characteristics and optical emission from such systems.\footnote{X. H. Qiu, G. V. Nazin, W. Ho, Science {\bf 299}, 542 (2003), S. W. Wu, N. Ogawa, W. Ho, Science {\bf 312}, 1362 (2006).} In this talk we propose a theoretical model that is able to account for many observed properties of these systems. We obtain distinct photon emission spectra and corresponding I-V curves for different couplings of the electrodes to the molecule, that are consistent with experimental data. We find emission to be highly dependent on the details of the tip/molecule and molecule/substrate coupling, and make further photon emission predictions for systems not yet achieved experimentally. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W9.00014: Toward quantitative acoustic molecular gas sensing Andi Petculescu Laser spectroscopy techniques provide unrivaled tools for detailed molecular sensing studies. Critical applications such as gas monitoring in life-support systems often require sensors that are not only fast and sufficiently accurate but also rugged and not needing extensive maintenance and calibration. Acoustic sensors are widely used to this end, based on measuring sound speed changes. These, however, can only constrain the overall molecular mass. For reasons unknown, researchers have habitually disregarded acoustic attenuation. Beside the classical sound loss mechanisms due to viscosity, heat conduction, and diffusion, the non-classical contribution to attenuation arises from the inability of internal molecular degrees of freedom to follow the acoustic temperature fluctuations. This connects acoustic attenuation to the molecular relaxation times. In the laboratory, acoustic studies of molecular relaxation in fluids require that the ambient pressure be varied over a wide range at a given frequency in order to cover the relaxation processes. In a fast-sensing device, this would be highly impractical. A novel algorithm is presented here relying on measuring sound speed and attenuation at one pressure and two frequencies to reconstruct the full frequency dependence of the effective specific heat for the main relaxation processes. This would enable a smart sensor to infer the concentration and nature of contaminant molecules in a base gas. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W9.00015: A delta-function model for three-dimensional axisymmetric crystals Ping Du, Harris Wong A surface energy polar plot contains two possible singularities: the cusps that give facets on an equilibrium crystal, and the circular arcs connecting the cusps that can lead to missing orientations. The common approach of specifying the surface energy usually cannot handle both singularities simultaneously. We model the surface stiffness to avoid missing orientations. Furthermore, a facet is represented by the Dirac delta function with the weight of the delta function equal to the width of the facet. Thus, both singularities are treated precisely. This approach has been shown to work for two-dimensional symmetric [1] and axially symmetric [2] crystals. Here, we apply the delta function to model three-dimensional axisymmetric crystals and obtain analytic solutions to the nonlinear differential equation governing the crystal shape. We find that at every point on the crystal surface the chemical potential is equipartitioned between the axial and the azimuthal component. \newline [1] Xin, T. and H. Wong, \underline {Surface Science} \textbf{487}, L529 (2001). \newline [2] Du, P. and H. Wong, \underline {Scripta Materialia} \textbf{55}, 1171 (2006). [Preview Abstract] |
Session W10: Focus Session: Hybrid Magnetic-Superconducting Systems III
Sponsoring Units: DMPChair: Ilya Vekhter, Louisiana State University
Room: Morial Convention Center RO8
Thursday, March 13, 2008 2:30PM - 3:06PM |
W10.00001: Scanning Tunneling Spectroscopy of Reentrant Superconductivity in the Ferromagnetic Superconductor ErRh$_{4}$B$_{4}$ Invited Speaker: ErRh$_{4}$B$_{4}$ is a reentrant superconductor. As temperature is lowered it transits to the superconducting state at 8.5 K, there is coexistence with magnetic ordering between 1.2K and 0.7K, and then it is a normal ferromagnetic material. We have used a scanning tunneling microscope, STM, to investigate the superconducting and magnetic properties of a ErRh$_{4}$B$_{4}$ single crystal from 300 mK up to 8.5K. We used a superconducting tip as the probe of the STM in order to enhance the different features present in the tunneling spectra corresponding to the ErRh$_{4}$B$_{4}$ density of states. The use of the superconducting probe allowed a detailed study of the destruction of superconductivity as ferromagnetic order develops. The perturbation of the ErRh$_{4}$B$_{4}$ superconducting density of states in the coexistence region is followed as function of temperature and external magnetic field. Spatial variations of the electronic density of states were detected by scanning tunneling spectroscopy maps at different temperatures and fields, and its correlation with the topographic features of the sample is investigated. These STM measurements allowed also the determination of a magnetic field vs. temperature phase diagram of the studied single crystal. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W10.00002: Vortex manipulation in S/F hybrid nanosystems Victor V. Moshchalkov, Alejandro Silhanek, Werner Gillijns, Alexei Aladyshkin, Niels Verellen Tunable field-induced superconductivity has been observed in superconducting films with a periodic array of magnetic dots with magnetization which is varied by using different hysteresis cycles to magnetize the dots. As a result, the shift of the Tc(H) curve to higher fields of a certain polarity (``magnetic bias'') can be controlled by tuning the strength of the field emanating from the dots and compensating the applied field. Static vortex patterns and dynamic effects, such as guided vortex motion and vortex ratchet effects, were studied in the superconductor/ferromagnet hybrids consisting from the superconducting film covered by magnetic dots, bars, loops with the in-plane magnetization. In-plane magnetic dipoles create asymmetric pinning sites responsible for the appearance of the magnetic dipole vortex ratchets theoretically predicted by Carneiro and now found experimentally. Switchable flux pinning landscape has been created by tuning the magnetic states of the in-plane magnetized loops (Phys.Rev.Lett. \textbf{98}, 117005 (2007), Appl. Phys. Lett. \textbf{90}, 182501 (2007), Phys. Rev. B (R) \textbf{76}, 60503 (2007)) [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W10.00003: Switchable pinning landscapes for flux quanta using arrays of ``magnetic vortices'' Javier E. Villegas, Kevin D. Smith, Lei Huang, Yimei Zhu, Rafael Morales, Ivan K. Schuller We constructed a superconducting/ferromagnetic hybrid system in which the \textit{ordering} of the pinning potential landscape for flux quanta can be manipulated. Flux pinning is induced by an array of magnetic nanodots in the ``magnetic vortex'' state, and controlled by the magnetic history. This allows switching \textit{on} and \textit{off} the collective pinning of the flux-lattice. In addition, we observed field-induced superconductivity that originates from the annihilation of flux quanta induced by the stray fields from the ``magnetic vortices.'' [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W10.00004: Vortex lattice matching effects in superconducting Mo$_{3}$Si films with magnetic pinning centers. David Perez de Lara, Alejandro Alija, Alejandro Junquera, Jose M. Colino, Jose I. Martin, Elena Navarro, Maria Velez, Jose V. Anguita, Jose L. Vicent Electron Beam Lithography and sputtering techniques have been used to fabricate arrays of Ni nanodots on Si (100) substrate. Nb films and amorphous Mo$_{3}$Si films were grown on top of the magnetic array by magnetron sputtering. The same arrays were used in both superconducting systems; therefore the same periodic pinning potential is induced in both systems. Magnetotransport and (I, V) curves were measured close to the films critical temperatures. Matching effects are observed between the periodic array of pinning potentials and the driving vortex lattice. The different vortex dynamics will be discussed in both systems. We want to thank Spanish Ministerio Educacion y Ciencia grants NAN2004-09087, FIS2005-07392. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W10.00005: Confined vortex phase in superconductor-ferromagnet nanocomposites Milorad Milosevic, Mauro Doria, Francois Peeters In the fifties, Abrikosov found the coexistence of superconductivity with an external magnetic field in the form of vortices, quantized filaments that cross the material. The puzzling properties of the recently discovered ferromagnetic superconductors are indicative of vortices, but stemming from the \textit{internal} magnetic field. Here we analyze this latter phenomenon and propose a new superconducting phase, made of \textit{confined vortex lines}, prior to the onset of experimentally visible spontaneous vortex phase. Similarly to exotic superconductors, internal complexes of vortex loops may also arise around embedded nanomagnets in artificial superconducting hybrids, where high density of magnetic particles can lead to a disordered vortex phase resembling a \textit{vortex glass}. Our simulations in the Ginzburg-Landau framework also show the remarkable, three-dimensional dynamic effects of the Lorentz force on vortex loops in applied dc current. This provides unique method for the experimental detection of the confined phase through transport measurements, both for bulk and mesoscopic samples. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W10.00006: Interplay of Superconducting with Magnetic Vortices A. Hoffmann, J.E. Pearson, G. Mihajlovi\'c, V. Metlushko, L. Fumagalli, J.C. Sautner, N. Jahedi Periodic arrays of magnetic structures are well known to give rise to commensurate pinning of superconducting vortices in adjacent superconducting films. We compared the pinning effects due to magnetic dots with either single domain or vortex magnetization configuration. There is a clear correlation between the magnetoresistance in the superconductor and the magnetization configuration of the magnetic dots, indicating that the pinning of the superconducting vortices is strongly enhanced for the magnetic vortex state. The origin of this enhanced pinning is due to the locally larger magnetic stray fields produced by the magnetic vortex cores. The absence of an asymmetry for parallel and anti-parallel orientation between the superconducting vortex flux and the magnetic vortex cores suggests that the enhanced pinning is not due to magnetostatic interactions but is rather due to the local suppression of superconductivity by highly localized, large perpendicular stray magnetic fields generated by the magnetic vortex cores. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W10.00007: Enhancement of superconductivity by parallel magnetic field in ultrathin $a$-Pb films. Ashwani Kumar, H. Jeffrey Gardner, Peng Xiong A modified dilution refrigerator equipped with \textit{in situ} film growth and a rotating sample stage is used to study the effect of magnetic field on ultrathin homogeneous Pb films. An insulating layer of Sb ($\sim $ 1 nm) is first deposited to ensure electrical and possibly structural uniformity down to a single atomic layer for the subsequently deposited Pb. Through incremental deposition of Pb, a film with increasing thickness (thus decreasing R and increasing T$_{C})$ is obtained and transport measurements in perpendicular and parallel magnetic field are performed \textit{in situ} at different film thicknesses. Any \textit{perpendicular} field is found to suppress superconductivity, and at sufficient strength induces an electrically inhomogeneous insulating state.$^{1}$ In contrast, the same film is at least two orders of magnitude less sensitive to a \textit{parallel} field. More strikingly, a moderate parallel magnetic field actually \textit{increases} the T$_{C}$ of the film, resulting in large negative magnetoresistance in the transition region in parallel fields as large as 3T. The dependence of this effect on the film thickness, impurities and temperature will be presented and discussed. $^{1}$J.S. Parker et al., Europhys. Lett. 75, 950 (2006). [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W10.00008: An Effective Action approach to inhomogeneous superconductors: Application to F/S/F spin valves Adnan Rebei The interplay between magnetism and superconductivity can be studied in simple systems such as ferromagnetic-superconductor-ferromagnetic tri-layers. Many recent experiments [1] measured a critical temperature dependence of the superconductor as a function of the relative orientations of the surrounding ferromagnetic layers. This layered system gives rise to an in-homogenous gap equation for the superconductor due to the polarization at the ferromagnetic-superconductor interfaces irrespective of the orientation of the magnetization in-plane or out-of-plane. Real space formulations are therefore better suited to study this kind of systems. We show that an Effective Action formulation similar to the one proposed by Weinberg [2] is very advantageous in this respect as was shown in normal-ferromagnetic-normal systems in [3]. \newline \newline [1] Yu. Rusanov, S. Habraken and J. Aarts, Phys. Rev. B 73, 060505(R) (2006); A. Singh, C. Sugers and H. v. Lohneysen, Phys. Rev. B 75, 024513 (2007); T. Ambrose (unpublished). \newline [2] S. Weinberg, Nucl. Phys. B 413, 567 (1994). \newline [3] A. Rebei and M. Simionato, Phys. Rev. B 71, 174415 (2005). [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W10.00009: Multilayered ferromagnet/superconductor nanostructures: proximity effect, decoupled superconductivity, and hierarchy of critical temperatures. Y. N. Proshin, N. G. Fazleev, M. G. Khusainov The four-layered F'/S'/F''/S'' nanostructure consisting of rather dirty superconducting (S) and ferromagnetic (F) metals is studied within the theory of the proximity effect taking detailed account of the boundary conditions. The new pi phase superconducting states are obtained for the F'/S'/F''/S'' nanostructure in addition to the known ``superlattice'' states. The dependence of the critical temperatures versus the F layers thicknesses is investigated. It is shown that the F'/S'/F''/S'' nanostructure can experience decoupled superconductivity. The latter manifests itself through a hierarchy of the critical temperature Tc, which can be different for different S' and S'' layers. An optimal set of parameters is determined, for which the difference between the critical temperatures for different S' and S'' layers becomes significant. The corresponding phase diagrams are constructed and discussed. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W10.00010: Paramagnetic Intrinsic Meissner Effect in Layered Superconductors Andrei Lebed Free energy of a quasi-two-dimensional superconductor with a coherence length perpendicular to the conducting layers being less than an inter-layer distance is calculated. The free energy is shown to differ from that in the textbook Lawerence-Doniach model at high fields, where the Meissner currents are found to create an unexpected positive magnetic moment due to shrinking of the Cooper pairs ``sizes'' by a magnetic field. This unique phenomenon - paramagnetic intrinsic Meissner effect (PIME) in a bulk [1] - is suggested to detect by measuring in-plane magnetization and torque in layered organic and high-Tc superconductors as well as in superconducting superlattices. \newline \newline [1] A.G. Lebed, Physical Review Letters, submitted. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W10.00011: Evidence of spontaneous vortex phase in ferromagnet-superconductor nanocomposites Tatiana Rappoport, Yutao Xing, Hans Micklitz, Milorad Milosevic, Ivan Solorzano-Naranjo, Elisa Baggio-Saitovitch The interplay between superconductivity and magnetism gives rise to many intriguing phenomena. We report a novel manifestation of this interplay: the appearance of a spontaneous vortex phase in superconducting films with embedded magnetic nanoparticles. These systems can be seen as artificial analogues of ferromagnetic superconductors. Unlike traditional vortices in superconductors, this vortex phase appears without any applied magnetic field. The vortices nucleate due exclusively to the stray field of the magnetic nanoparticles, which serve the dual role of providing the internal field while working as pinning centers. Transport measurements reveal that this vortices have a phase transition from a liquid to a disordered solid resembling a vortex glass. The transition is characterized by means of a scaling analysis. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W10.00012: Synchronization by internal cavity mode and resonant electromagnetic emission from intrinsic Josephson junction stacks Alexei Koshelev Intrinsic Josephson-junction stacks realized in mesas fabricated out of high-T$_c$ superconductors may be used as sources of coherent electromagnetic radiation. The major challenge is to synchronize Josephson oscillations in all junctions to get significant radiation. A simple way to solve this problem is to excite the in-phase Fiske mode when the Josephson frequency matches the Fiske-resonance frequency set by the stack lateral size[1]. A finite direct coupling to such mode exists in mesas with lateral modulation of the Josephson critical current identical in all junctions [2]. The powerful almost standing electromagnetic wave is excited inside the crystal in the resonance promoting full synchronization. We evaluate behavior of the I-V characteristics and radiated power near the resonance. We will discuss several relevant issues including (i) stability of the coherent state, (ii) mechanism of damping including external radiation and leaking of radiation into the bulk crystal, and (iii) angular dependence of external radiation. [1]L. Ozyuzer, \emph{et al.} Science \textbf{318}, 1291 (2007) [2]A. E. Koshelev and L. N. Bulaevskii, cond-mat 0708.3269 [Preview Abstract] |
Session W11: Theory of Superconductivity in Cuprates II
Sponsoring Units: DCMPChair: Mohit Randeria, Ohio State University
Room: Morial Convention Center RO9
Thursday, March 13, 2008 2:30PM - 2:42PM |
W11.00001: Theory of Strongly Correlated Superconductivity William Putikka Superfluid behavior is relatively common in strongly correlated fermion systems. This suggests there is a common reason for this behavior rooted in the strong correlations. I propose such a mechanism, developed in the context of the 2D $t$-$J$ model, where $d_{x^2-y^2}$ superconducting fluctuations have recently been observed$^1$. The $d_{x^2-y^2}$ fluctuations are {\it not} due to antiferromagnetic spin fluctuations; the AF fluctuations compete with superconducting fluctuations. Pairing fluctuations have their own origin based on strong correlations. The key to understanding how this comes about is the high temperature entropy. Strong correlations not only create order at low temperatures, they also reduce the entropy at high temperatures. The response of the system is to rearrange the degrees of freedom into separate spin and charge excitations. These excitations have very different energy scales, allowing the charges to develop pairing correlations before the spin degrees of freedom become coherent. At lower temperatures where the spins become coherent they determine the symmetry of the electronic pair wavefunction in a manner to best {\it avoid} the magnetic fluctuations in the system.\\ 1. WO Putikka and MU Luchini, PRL{\bf 96}, 247001 (2006). [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W11.00002: Magnetism and superconductivity in the t-t'-J model Federico Becca, Leonardo Spanu, Massimo Lugas, Sandro Sorella We present a systematic study of the phase diagram of the $t{-}t^\prime{-}J$ model by using the Green's function Monte Carlo (GFMC) technique, implemented within the fixed-node (FN) approximation and a wave function that contains both antiferromagnetism and d-wave pairing. This enables us to study the interplay between these two kinds of order and compare the GFMC results with the ones obtained by the simple variational approach. By using a generalization of the forward-walking technique, we are able to calculate true FN ground-state expectation values of the pair-pair correlation functions. In the case of $t^\prime=0$, there is a large region with a coexistence of superconductivity and antiferromagnetism, that survives up to $\delta_c \sim 0.10$ for $J/t=0.2$ and $\delta_c \sim 0.13$ for $J/t=0.4$. The presence of a finite $t^\prime/t<0$ induces a strong suppression of both magnetic (with $\delta_c \alt 0.03$, for $J/t=0.2$ and $t^\prime/t=-0.2$) and pairing correlations. In particular, the latter ones are depressed both in the low-doping regime and around $\delta \sim 0.25$, where strong size effects are present. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W11.00003: Phase Fluctuations in high-Tc Superconductors Yucel Yildirim, Weiguo Yin, Wei Ku Thermal fluctuation induced destruction of phase coherence of superconductivity in High-Tc superconductors is investigated via numerical quantum Monte Carlo solution of a new theoretical model at finite-temperature. Our simple effective boson Hamiltonian, derived from the pairing sector of the t'-t''-J model, can be considered as a natural extension of the hard-core boson with additional information about the internal structure of the local fermion pairs. The local solution is found to consist of d-wave pairing, hybridizing with neighboring p-wave pairs. The possible connection with the pseudo-gap phase will also be addressed. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W11.00004: Suppression of Superfluid Density in Underdoped Cuprates Wei-Cheng Lee, Jairo Sinova, Anton A. Burkov, Yogesh Joglekar, Allan H. MacDonald A key challenge for theories of high-T$_{c}$ cuprates is to explain why the superfluid density vanishes as the antiferromagnetic insulator state is approached. Viewing a cuprate as a doped Mott insulator gives a natural explanation of this property, but one that is not obviously compatible with the fact that superconductivity always vanishes at finite doping. We propose another possible explanation, starting from weak-coupling conventional d-wave BCS theory and calculating the correlation contribution to the superfluid density. We show that triplet fluctuations of the d-wave superconducting order parameter are canonically conjugate to antiferromagnetic fluctuations, and that this causes the correlation energy to increase in magnitude when superconductivity is weakened by a phase gradient. In our theory the inelastic neutron scattering resonance has the character of a magnetic plasmon rather than the character of an exciton. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W11.00005: Stability of inhomogeneous superconducting states in the doped t-J model Manuela Capello, Didier Poilblanc By using Variational quantum Monte Carlo techniques, we have investigated the stability of inhomogeneous RVB superconducting states in the t-J model at doping 1/8. We show that introducing half-filled charge domain walls involves very small energy costs, especially in the presence of tetragonal-lattice distortions [1]. This suggests that spontaneous unidirectional modulations of the hole density and the superconducting order parameter could easily be stabilized in real materials, in the presence of small perturbations. We argue that such a scenario is at play in the recently observed patterns of unidirectional domains in high-T$_c$ superconductors [1]. \newline [1] M. Capello et al. (in preparation). \newline [2] Y. Kohsaka et al. Science 315, 1380 (2007). [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W11.00006: ``Glue" approximation for the pairing interaction in the Hubbard model with next nearest neighbor hopping Ehsan Khatami, Alexandru Macridin, Mark Jarrell Recently, several authors have employed the ``glue" approximation for the Cuprates in which the full pairing vertex is approximated by the spin susceptibility. We study this approximation using Quantum Monte Carlo Dynamical Cluster Approximation methods on a 2D Hubbard model. By considering a reasonable finite value for the next nearest neighbor hopping, we find that this ``glue" approximation, in the current form, does not capture the correct pairing symmetry. Here, d-wave is not the leading pairing symmetry while it is the dominant symmetry using the ``exact" QMC results. We argue that the sensitivity of this approximation to the band structure changes leads to this inconsistency and that this form of interaction may not be the appropriate description of the pairing mechanism in Cuprates. We suggest improvements to this approximation which help to capture the the essential features of the QMC data. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W11.00007: Pairing glue in the Hubbard and t-J models: Insights from a dynamic cluster approximation study Thomas Maier, Douglas Scalapino, Mark Jarrell We will discuss dynamic cluster non-crossing approximation studies of the superconducting gap function $\Phi(k,\omega)$ in the 2D Hubbard and t-J models. The momentum and frequency dependence of the gap reflect the spatial structure and dynamics of the pairing interaction responsible for d-wave pairing. For both models we find that two mechanisms are simultaneously active. An instantaneous contribution to the gap arises from states whose energies reflect the Mott scale and which give rise to the exchange coupling $J$. The dominant contribution, however, is retarded and comes from energies that correspond to the spin fluctuation spectrum. It is therefore reasonable to speak of a dynamic ``pairing glue'' which gives rise to d-wave superconductivity in the Hubbard and t-J models. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W11.00008: Finite-Temperature Phase Diagram of the $d=3$ $tJ$ Model with Quenched Disorder A. Nihat Berker, Michael Hinczewski We study a quenched disordered $d=3$ $tJ$ Hamiltonian with static vacancies as a model of nonmagnetic impurities in high-$T_c$ materials.[1,2] Using a position-space renormalization-group approach, we calculate the evolution of the finite-temperature phase diagram with impurity concentration $p$, and find several features with close experimental parallels: away from half-filling we see the rapid destruction of a spin-singlet liquid phase (analogous to the superconducting phase in cuprates) which is eliminated for $p \ge 0.05$; in the same region for these dilute impurity concentrations we observe an enhancement of antiferromagnetism. The antiferromagnetic phase near half-filling is robust against impurity addition, and disappears only for $p \ge 0.40$. \newline \noindent [1] M. Hinczewski and A.N. Berker, Eur. Phys. J. B {\bf 51}, 461 (2006). \newline \noindent [2] M. Hinczewski and A.N. Berker, arXiv:cond-mat/0607171v1 [cond-mat.str-el]. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W11.00009: Dynamical Layer Decoupling in a Stripe-Ordered High-$T_c$ Superconductor Eduardo Fradkin, Erez Berg, Eun-Ah Kim, Steve Kivelson, Vadim Oganesyan, John Tranquada, Shoucheng Zhang In the stripe-ordered state of a strongly correlated two-dimensional electronic system, under a set of special circumstances, the superconducting condensate, like the magnetic order, can occur at a nonzero wave vector corresponding to a spatial period double that of the charge order. In this case, the Josephson coupling between near neighbor planes, especially in a crystal with the special structure of La$_{2-x}$Ba$_x$CuO$_4$, vanishes identically. We propose that this is the underlying cause of the dynamical decoupling of the layers recently observed in transport measurements at $x =1/8$. \newline [1] E. Berg et al, PRL {\bf 99}, 127003 (2007) [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W11.00010: Exact diagonalization study of electronic nematic and $d$-density wave states Hyeonjin Doh, Thomas Grzesiak, Hae-Young Kee It was proposed that the $d$-density wave (ddw) and the electronic nematic states are relevant phases in the phase diagram of high $T_C$ cuprates. The two phases break different symmetries, and their order parameters have been used to describe the characteristic broken symmetries. Here we show that the two order parameters transform into each other under a local gauge transformation, which implies that the order parameters cannot represent different broken symmetric states, if a Hamiltonian is invariant under such a transformation. The two order parameters describe distinctly different states, when the nearest neighbor hopping integral is finite, but the states are nearly degenerate at a strong coupling limit. We also present a phase diagram of a Hamiltonian with correlated hopping, and nearest neighbor repulsive interactions using Lanczos exact diagonalization where we find an interesting interplay between the ddw and nematic states. We compare our results with the previous mean field calculation, and discuss a possible relevance to a $d$-wave superconducting state. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W11.00011: Competition between superconductivity and antiferromagnetism as a boson-fermion mixture problem Gerardo Ortiz, Leonid Isaev, Cristian D. Batista It is widely believed that the key to understanding the mechanism of superconductivity in cuprate materials and heavy fermion compounds lays in the competition between two quantum orders: superconductivity and antiferromagnetism, which complicates the choice of relevant effective degrees of freedom. Here we present a new isomorphic mapping, preserving the dimension of the on-site Hilbert space, where holons are represented by fermionic and spinons -- by bosonic degrees of freedom, which, we claim, are the relevant ones in a certain regime (low density). Starting from t-J -- like models we show that the original Hamiltonian can be exactly mapped onto a mixture of interacting fermion and boson gases. It is then demonstrated that this system may be considered as dilute under certain experimentally reasonable conditions. In this limit we show that the fermion gas exhibits a pairing instability, caused by a two boson exchange, which implies superconductivity of holons in the original repulsive model. Although the long-range Coulomb interaction is, in principle, capable of suppressing this state, we argue about a physical mechanism for its compensation, which is relevant for real materials. Our results also lead to a mechanism for superfluidity in ultracold boson -- fermion gas mixtures. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W11.00012: Stability of nodal quasi-particles in superconductors with coexisting orders Erez Berg, Cheng-Chien Chen, Steven A. Kivelson The possible existence of nodal quasi-particles is one of the most distict properties of unconventional superconductors. Nodal quasi-particles have many unique experimental fingerprints, such as a linear temperature dependence of the superfluid density. It is therefore interesting to ask under what conditions can they exist generically. Here, we establish a condition for the perturbative stability of zero energy nodal points in the quasi-particle spectrum of superconductors in the presence of a general coexisting \textit{commensurate} order. The nodes are found to be stable if the Hamiltonian is invariant under time reversal followed by a lattice translation. The principle is demonstrated with a few examples. Some experimental implications of various types of assumed order are discussed in the context of the cuprate superconductors. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W11.00013: S-wave superconductivity from predominantly d-wave pairing Paul Oreto, Boris Spivak, Steven Kivelson We have studied the zero temperature properties of a system consisting of dilute superconducting droplets embedded in a weakly disordered Fermi liquid metal. Even under the assumption that the pairing interaction occurs exclusively in the d-wave channel, irregularities in the shape of the droplets, or impurities within the droplet generically induce local mixing of s-wave and d-wave components of the superconducting order. Moreover, we find that the Josephson coupling between the s-wave components of the order parameter on two droplets separated by a distance r falls off as a slower power law than does the d-wave component. Thus, for dilute enough droplets, even if the local gap structure within each droplet is dominantly d-wave-like, the globally phase coherent portion of the superconducting order must have s-wave symmetry. We speculate that this effect could lead to a region of globally s-wave superconductivity in highly overdoped cuprates. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W11.00014: Absence of superconductivity in the $\frac{1}{2}$-filled band Hubbard model on a triangular lattice Hongtao Li, R. Torsten Clay, S. Mazumdar The superconducting $\kappa$-(BEDT-TTF)$_2$X salts are strongly dimerized, with one hole per dimer unit cell. The occurrence of antiferromagnetism under ambient pressure, and the appearance of superconductivity under pressure has led several investigators to suggest that the superconductivity can be explained within an anisotropic triangular lattice $\frac{1}{2}$-filled band Hubbard Hamiltonian. Within this picture, pressure takes the system closer to the isotropic limit, when antiferromagnetism gives way to d-wave superconductivity. We have performed exact diagonalizations on a 16-site periodic anisotropic triangular lattice as a function of the Coulomb interaction (Hubbard $U$) and the anisotropy to investigate this claim. We calculate bond orders, double occupancies, spin-spin correlations, spin structure factors and $d_{x^2-y^2}$ superconducting pair-pair correlations. We are able to confirm the Mott metal-insulator transition and antiferromagnetism, but we do not find any hint of long range superconducting correlations. Neither is there any region of the Hubbard $U$ where these correlations are enhanced by the interaction strength. \footnote{Supported by DE-FG02-06ER46315}. [Preview Abstract] |
Session W12: Metal Insulator Transition II: Theory
Sponsoring Units: DCMPChair: Jim Freericks, Georgetown University
Room: Morial Convention Center 203
Thursday, March 13, 2008 2:30PM - 2:42PM |
W12.00001: Effect of frustration on charge dynamics for a doped two-dimensional triangular Hubbard lattice Takami Tohyama We examine the optical conductivity $\sigma(\omega)$ and the chemical potential $\mu$, together with the spin correlation, in the strong-coupling limit of a hole-doped two-dimensional triangular Hubbard model near half filling by using an exact diagonalization technique [1]. In contrast to the case of a square lattice without frustration, the doping dependences of $\mu$ and the Drude weight indicate that the charge degree of freedom is weakly coupled to the spin degree of freedom. However, we find that $\sigma(\omega)$ shows strong incoherent excitations extended to a higher energy region. This implies that geometrical frustration in strongly correlated electron systems influences incoherent charge dynamics. Momentum- dependent charge dynamics is also compared with that of the square lattice. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W12.00002: Exact diagonalization analysis of the Anderson-Hubbard model and comparison to real-space self-consistent Hartree-Fock solutions Xi Chen, Pakwo Leung, Robert Gooding We have obtained the exact ground state wave functions of the Anderson-Hubbard model for different electron fillings on a 4x4 lattice with periodic boundary conditions. When compared to the uncorrelated ground states (Hubbard interaction set to zero) we have found evidence of very effective screening, producing smaller charge inhomogeneities due to the Hubbard interaction, particularly at 1/2 filling, and have successfully modelled these local charge densities with non-interacting electrons that experience a static screening of the impurity potentials. Further, we have compared such wave functions to self-consistent real-space unrestricted Hartree-Fock solutions and have found that these approximate ground state wave functions are very successful at reproducing the local charge densities, and may indicate the role of dipolar backflow in producing a novel metallic state in two dimensions. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W12.00003: Electronic Griffiths phase near a disordered Mott transition in $D=2$ Eric Andrade, Eduardo Miranda, Vlad Dobrosavljevic We investigate the effects of weak and moderate disorder on the $T=0$ Mott Metal-Insulator Transition (MIT) in two dimensions, by solving the disordered Hubbard Model within the so-called Statistical Dynamical Mean Field Theory (statDMFT) {[}1]. In the weak disorder regime, we recover the results of the standard DMFT limit {[}2], including strong disorder screened close to the MIT. For moderate disorder, the screening of the disorder remains strong, but is reduced by fluctuation effects absent in high dimensions. For disorder strength $W$ smaller then the on-site interaction $U$, the transition retains the Mott character, where the local quasiparticles weights $Z_{i}$ vanish on all sites at the transition, indicating the transmutation of all electron into local magnetic moments. In contrast to the behavior in high dimensions {[}2], the corresponding distribution function $P\left(Z\right)$ is found to assume a singular form as the transition is approached, indicating the emergence of an electronic Griffiths phase {[}3] preceding the MIT. \\ {[}1] V. Dobrosavljevic and G. Kotliar, Phys. Rev. Lett. 78, 3943 (1997). \\ {[}2] D. Tanaskovic et al, Phys. Rev. Lett. 91, 066603 (2003). \\ {[}3] E. Miranda and V. Dobrosavljevic, Phys. Rev. Lett. 86, 264 (2001). [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W12.00004: An Exactly Solvable Model with a Tunable Mott Gap without Broken Symmetry Daniel Hansen, B. Sriram Shastry The 1d Hubbard model at half filling provides the only known example of a Mott Hubbard insulating state, with a Mott charge gap without any concomittant broken symmetry. Such a state has inspired much current work in correlated matter in low dimensions. We present a model, where the Mott gap can be manipulated and infact made to vanish with some parameter. Using the higher conserved currents found by Shastry in 1986 for the 1-d Hubbard model, we construct a new model {\em which does show a tunable Mott gap}. The model is given by the hamiltonian $$H = H_{Hubbard}(U) + \lambda \; I_3(U),$$ where $H_{Hubbard}$ is the Hubbard hamiltonian and $I_3$ is its third conserved current. The new model has exactly the same space time symmetries as the Hubbard model, but possesses {\em two parameters}, $U ,\; \lambda$. The phase diagram in $\lambda-U$ is explored using numerical methods and the Bethe Ansatz. It displays several interesting features including a ``superconducting'' type state. A significant role is played by a band transition at $U=0$ (similar to the Lifshitz transition), wherein the two fermi points of the Hubbard model break up into 6 Fermi points. We also find a variety of second order transitions. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W12.00005: Insulator-Insulator Transitions in an Ionic Hubbard Model Ara Go, Gun Sang Jeon We study the ionic Hubbard model in one and two dimensions at zero temperature. As the Hubbard interaction is increased, the system is known to evolve from a band insulator to a Mott insulator. The former phase is induced by the alternating on-site potential energy while the strong local Hubbard interaction drives the system towards correlated Mott insulator. In order to examine the transition nature, we perform the cellular dynamical mean-field calculation with an exact diagonalization technique employed as an impurity solver. From the computed local density of states we estimates the spectral gap as the interaction strength is varied. We also calculate the momentum-dependent density of states which exhibits characteristic features for different phases. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W12.00006: Metal-Insulator Transitions in the Periodic Anderson Model Giovanni Sordi, Adriano Amaricci, Marcelo Rozenberg We investigate the doping driven metal-insulator transition in the periodic Anderson model in the Mott-Hubbard regime, using dynamical mean-field theory. Upon electron doping of the Mott-insulator, a metal-insulator transition occurs, which shares the same qualitative features of the first order transition found in the single band Hubbard model. Surprisingly, upon hole doping, the metal-insulator transition is not first order. Thus our study demonstrate that the transition scenario of the single band Hubbard model is not generic for the periodic Anderson model, even in the Mott-Hubbard regime. {\sl Phys. Rev. Lett.} {\bf 99}, 196403 (2007) [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W12.00007: The Anderson-Mott transition for a correlated 2D system Maria Elisabetta Pezzoli, Federico Becca, Giuseppe Santoro, Michele Fabrizio The interplay of disorder and electron-electron interaction can lead a bidimensional system to different phase transitions. We show that the Gutzwiller wave function generalized for an inhomogeneous system and with a long-term Jastrow factor provides a proper variational description of the Mott insulating phase and of the compressible disordered phase. Moreover, we identify an order parameter for the disordered Mott transition both in the paramagnetic and in the magnetic case. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W12.00008: The 2D Metal-Insulator Transition: A percolative Monte Carlo study Donald Priour Jr., Sankar Das Sarma We examine the metal-insulator transition (MIT) in two dimensional electronic systems (e.g. n doped GaAs heterostructures) in the presence of a long-range Coulombic random potential set up by a nearby layer of charged impurities. An iterative scheme taking into account nonlinear screening is used to obtain the inhomogeneous ``landscape'' of electron-rich and electron-depleted regions. The percolation (or not) of electron rich areas (in the regime of electronic densities $n$ where linear screening breaks down) is determined by a variant of the Hoshen Kopelman algorithm. Identifying the percolation transition as the metal-insulator transition, we calculate the critical electron density $n_{c}$ as a function of the concentration of charged impurities, the separation $d$ of the impurity layer from the electronic layer, and the thickness of the impurity rich region; the effect of correlated impurity positions is also examined. Using finite size scaling analysis, we calculate the critical exponent $\delta$ for the asymptotic scaling $\sigma \propto (n - n_{c})^{\delta}$ in the vicinity of the MIT. We acknowledge support from US-ONR and NRI-NSF. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W12.00009: Quantum and thermal fluctuation effects on solid-light systems Martin Hohenadler, Markus Aichhorn, Charles Tahan, Peter Littlewood Several theoretical proposals of strongly correlated polariton systems which exhibit a quantum phase transition from a Mott insulator to a superfluid phase have recently been made. Here we study Mott phases of polaritons in a model of optical microcavities in a photonic crystal with nearest-neighbor photon hopping. The variational cluster approach takes into account quantum fluctuations exactly on the lengthscale of finite clusters, and yields phase diagrams and single-particle spectra at zero and finite temperature in one and two dimensions. The relation of the model to the well-known Bose-Hubbard model is explored, and implications of our findings concerning the stability of the Mott state at finite temperatures for technological applications are discussed. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W12.00010: Effects of electron correlations on delocalization and percolation of electronic states Kendall Mallory A simulation of the effects of electron correlations on the formation of delocalized states and percolating clusters is presented. The model includes disorder in site locations and energy and uses a semi-classical approach to finding matrix elements and diagonalizing the Hamiltonian. We are also looking for scale invariant properties in the system. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W12.00011: Dynamic mean-field theory of the ionic Hubbard model Ji-Woo Lee, Gun Sang Jeon We study the ionic Hubbard model in the infinite dimensions in the framework of dynamical mean-field theory. Exact diagonalization is used to obtain the impurity Green's function to satisfy the self-consistant equation. We obtain a phase diagram in the parameter space of local ionic potential strength, $\Delta$ and local repulsive energy, $U$ exhibiting three phases: Mott insulator, metal, band insulator. Analyzing the spectral density, we compare our results with those of iterative perturbation theory and quantum Monte Carlo study in two dimensions. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W12.00012: Evolution of the Mott-Hubbard insulator transition in bulk nonequilibrium; dynamical mean field theory Ryan Heary, Jong Han The equilibrium dynamical mean-field theory (DMFT) is extended to the steady-state nonequilirium in which the Hubbard lattice is populated by left and right movers. The nonequilibrium boundary condition is imposed in the non-interacting limit by applying a chemical potential shift between the left and right movers of the homogeneous bulk system. The success of this theory lies in the fact that the local Green function can be calculated nonperturbatively using the imaginary-time formulation of the steady-state nonequilibrium [1]. We study the evolution of metallic quasi-particle excitations near the Mott-Hubbard insulator transition as a function of the chemical potential bias. The nonequilibrium DMFT algorithm will be presented along with the nonlinear nonequilibirum destruction of the Kondo resonance. \newline [1] J. E. Han, R. J. Heary, Imaginary-time formulation of steady-state nonequilibrium: application to strongly correlated transport, {\it accepted to Phys. Rev. Lett.} arXiv:0704.3198. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W12.00013: Non-local Coulomb correlations in metals close to a charge order insulator transition Jaime Merino Recent extensions of dynamical mean-field theory (DMFT) to clusters either in its real space (CDMFT) or momentum space versions (DCA) have become important tools for the description of electronic properties of low dimensional strongly correlated systems. In contrast to single site DMFT, short range correlation effects on electronic properties of systems close to the Mott transition can be analyzed. We have investigated the charge ordering transition induced by the nearest-neighbor Coulomb repulsion V in the 1/4-filled extended Hubbard model using CDMFT. We find a transition to a strongly renormalized charge ordered Fermi liquid at V$_{\mathrm{CO}}$ and a metal-to- insulator transition at V$_{\mathrm{MI}}>$V$_{\mathrm{CO}}$. Short range antiferromagnetism occurs concomitantly with the CO transition. Approaching the charge ordered insulator, V$<$V$_ {\mathrm{MI}}$, the Fermi surface deforms and the scattering rate of electrons develops momentum dependence on the Fermi surface. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W12.00014: Exact steady state non-equilibrium DOS of the Hubbard model Alexander Joura, Jim Freericks, Thomas Pruschke Using a non-equilibrium Kadanoff-Baym-Keldysh formalism, we derive exact equations relating the retarded Green's function to the retarded self-energy for lattice electrons in presence of a constant and uniform electric field $E$. Such an approach allows us to go beyond linear response theory and study the behavior of systems for arbitrarily large electric fields. We find that the conventional dynamical mean-field theory (DMFT) algorithm is the same as in equilibrium except for a significantly modified lattice Dyson equation which couples together frequencies separated by the Bloch frequency. We apply the method to the Hubbard model and solve the model within the DMFT framework. As an impurity solver, we employ the numerical renormalization group (NRG). We discuss how the density of states (DOS) evolves as the electric field strength $E$ and interaction strength $U$ change. In particular, when both $E\ll1$ and $U\ll1$ the DOS is a set of equally spaced peaks (the so called Wannier-Stark ladder). Increasing $U$ leads to a broadening of the peaks, which finally merge and then evolve into a DOS that is quite similar to the equilibrium DOS. Increasing $E$ while keeping $U\ll1$ splits the peaks, resulting in novel behavior for the DOS, which is reminiscent of a metal-insulator transition (but the system carries current). [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W12.00015: Mott transition and Universality at finite temperatures Stefanos Papanikolaou, Rafael M. Fernandes, Eduardo Fradkin, Philip W. Phillips, Joerg Schmalian, Rastko Sknepnek We consider the finite temperature Mott critical point which has been the subject of recent experimental investigation. We demonstrate that this critical point is in the Ising universality class, consistent with all available experimental data. We show that, even though the thermodynamic behavior of the system near such a critical point is described by an Ising order parameter, the global conductivity depends on other singular observables and, in particular, the energy density, leading to the emergence of multiple crossover regimes. Finally, we show that in the presence of weak disorder the dimensionality of the system has crucial effects on the size of the critical region that is probed experimentally. ArXiv:0710.1627 and in press at Physical Review Letters. [Preview Abstract] |
Session W13: Quantum Monte Carlo
Sponsoring Units: DCOMPChair: Matthias Troyer, ETH Zurich
Room: Morial Convention Center 204
Thursday, March 13, 2008 2:30PM - 2:42PM |
W13.00001: The Continuous Time Quantum Monte Carlo method as a cluster solver in the Dynamical Cluster Approximation Karlis Mikelsons, Alexandru Macridin, Mark Jarrell, Emanuel Gull, Matthias Troyer, Sebastian Fuchs, Thomas Pruschke We have investigated the application of the Continuous Time Quantum Monte Carlo (CTQMC) method, based on interaction expansion, to solve the Hubbard model within the Dynamical Cluster Approximation (DCA). We show that CTQMC reproduces results obtained with the well known Hirsch - Fye method (HFQMC), including non-perturbative phenomena. We discuss the advantages and limitations of CTQMC as a cluster solver in the DCA. Since any QMC method suffers from fermion sign problem at low temperatures and large system sizes, we present the results of a detailed study of the sign problem within CTQMC, and compare it to HFQMC. We also discuss potential extensions of (CTQMC+DCA) for treating the phases with broken symmetry. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W13.00002: Continuous-time method for quantum impurity models Emanuel Gull, Philipp Werner, Olivier Parcollet, Matthias Troyer We present a new continuous-time quantum Monte Carlo algorithm for quantum impurity problems. The method allows an efficient simulation of large cluster impurity models with density density coupling. We compare the computational effort and average sign to alternative quantum Monte Carlo approaches, such as the discrete-time Hirsch-Fye algorithm [ J. E. Hirsch and R. M. Fye, Phys. Rev. Lett. 56, 2521 (1986)] and the weak coupling solver by Rubtsov, Savkin and Lichtenstein [Phys. Rev. B 72 035122 (2005)] [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W13.00003: Progress in the study of Molecular Hydrogen$-$Benzene binding$^\dagger$ Todd D. Beaudet, Michele Casula, Jeongnim Kim, Richard M. Martin In this work we present a quantum Monte Carlo study of the hydrogen-benzene system where binding is very weak. We demonstrate that the binding is well described at both the VMC and DMC levels by a single determinant correlated geminal wave function$^1$ with an optimized compact basis set that includes diffuse functions. Agreement between VMC and fixed node DMC binding energies is found to be within 0.18 mHa, suggesting the calculations are well-converged with respect to the basis. This relative insensitivity to basis set and superposition error is an advantage of the QMC methods we employ. Comparison is made with a Slater-Jastrow wave function at the DMC level using a trial function comprised of PBE single-body orbitals, empirical models and previous work$^2$. The physical underpinnings of the interaction will be discussed including the role of diffuse basis functions in this system. Progress on systems where binding is expected to be more favorable for practical hydrogen storage will also be presented.\vspace{0.2 cm} \\$^1$ M. Casula, C. Attaccalite, and S. Sorella, J. Chem. Phys. 121, 7110 (2004). \\$^2$ S. Hamel and M. C\^ot\'e, J. Chem. Phys. 121, 12618 (2004).\vspace{0.2 cm} \\$^\dagger$ Supported by NSF DMR03-25939 and A6062 ArmyUMC00005071-3 [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W13.00004: Geometry optimization with a noisy potential energy surface Jeffrey Grossman, Lucas Wagner Molecular and solid systems in the excited state and in the weak-binding regime (for example) are often not described well by current density functional(DFT) methods, often leading to inaccurate minimum energy structures. Quantum Monte Carlo(QMC) is a tempting method to improve on these deficiencies, since it offers a highly accurate fully correlated first principles description. However, QMC suffers from two major deficiencies: 1) forces are not easily calculated and 2) the energy is obtained with stochastic uncertainty, which makes optimization a challenging task. We examine several ways of compensating for this uncertainty while only using the reliable total energies obtained in QMC. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W13.00005: Quantum Monte Carlo calculations of NiO Ryo Maezono, Mike D. Towler, Richard. J. Needs We describe variational and diffusion quantum Monte Carlo (VMC and DMC) calculations [1] of NiO using a 1024-electron simulation cell. We have used a smooth, norm-conserving, Dirac-Fock pseudopotential [2] in our work. Our trial wave functions were of Slater-Jastrow form, containing orbitals generated in Gaussian-basis UHF periodic calculations. Jastrow factor is optimized using variance minimization with optimized cutoff lengths using the same scheme as our previous work. [4] We apply the lattice regulated scheme [5] to evaluate non-local pseudopotentials in DMC and find the scheme improves the smoothness of the energy-volume curve. \newline \newline [1] CASINO ver.2.1 User Manual, University of Cambridge (2007). \newline [2] J.R. Trail {\it et.al.}, J. Chem. Phys. {\bf 122}, 014112 (2005). \newline [3] CRYSTAL98 User's Manual, University of Torino (1998). \newline [4] Ryo Maezono {\it et.al.}, Phys. Rev. Lett., {\bf 98}, 025701 (2007). \newline [5] Michele Casula, Phys. Rev. B {\bf 74}, 161102R (2006). [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W13.00006: Efficient orbital storage and evaluation for quantum Monte Carlo simulations of solids Kenneth Esler Researchers have applied continuum quantum Monte Carlo methods to solids with great success, but thus far applications have been largely limited to crystals with simple geometry. In these simulations, three-dimensional cubic B-splines have proven to be a fast and accurate means of storing and evaluating electron orbitals. While B-splines require less memory than other spline interpolation schemes, modern cluster nodes often have insufficient memory to store the orbitals for more complex systems. We introduce three techniques, appropriate in different circumstances, to dramatically reduce the memory required for orbital storage, while retaining high accuracy: the generalized tiling of primitive-cell orbitals into a supercell of arbitrary shape, the use of nonuniform grids for localized orbitals, and the periodic replication of localized orbitals. We give examples for cubic boron nitride and w\"{u}stite (FeO), and show that these methods can reduce the memory used for orbital storage by more than two orders of magnitude. Finally, we introduce an open-source B-spline library to facilitate the incorporation of these methods into QMC simulation codes. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W13.00007: Triplet pairings and fermion wave functions nodal topologies Lubos Mitas, Michal Bajdich, Shuming Hu Fixed-node quantum Monte Carlo methods rely on accurate fermion nodes of trial wave functions. Recently, we have shown that BCS wave functions possess for generic singlet ground states possess the correct minimal number of two nodal cells. This contrasts with the Hartree-Fock wave functions which exhibit higher counts of four or more nodal domains resulting in incorrect nodal topologies. We prove that for fully spin-polarized systems one can show the same effect. As a simple example, we consinder the HF wave function for the lowest quartet of S symmetry and even parity for three electrons in a Coulomb potential. The wave function of this state $^4S(1s2s3s)$ has six nodal cells corresponding to 3! reordering of the radii. We show that pfaffian with triplet pairings is the simplest wave function which has the correct topology with two nodal cells. We further expand the study to some exactly solvable models to study the exact nodal structures dependence on potentials. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W13.00008: Compact and accurate quantum Monte Carlo wave functions for first-row atoms F. R. Petruzielo, Julien Toulouse, W. A. Al-Saidi, C. J. Umrigar, R. G. Hennig Many-body wave functions for the first row atoms (Li to Ne) are represented as expansions in eigenstates of $\hat{L}^2,$ $\hat{L}_z,$ $\hat{S}^2,$ $\hat{S}_z,$ multiplied by a Jastrow factor. This configuration state function (CSF) expansion provides a systematic means for improving a wave function by including CSFs corresponding to higher excitations. Optimization of all wave function parameters including Jastrow, CSF and orbital coefficients as well as basis exponents, starting from a simple initial guess, results in compact and accurate wave functions (low energy and variance of local energy). Further improvements by use of backflow transformations are explored. This work aims to develop insight into selecting the relevant CSFs particularly for large systems, where it is difficult to include all CSFs to a given order. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W13.00009: A DQMC study of cohesion energy of small Li clusters based on an RVB nodal structure Daniel Nissenbaum, Leonardo Spanu, Claudio Attaccalite, Bernardo Barbiellini, Arun Bansil We have carried out a diffusion Quantum Monte Carlo study (DQMC) of the cohesion energy of small (2, 4, and 8 atom) Li clusters based on Resonating Valence Bond (RVB) wavefunctions, and compared the results to the corresponding values obtained via wavefunctions utilizing a typical Hartree-Fock (HF) nodal structure (Jastrow-Slater wavefunctions). The RVB wavefunction allows more flexibility in the nodal structure than the HF wavefunction, and yields some improvement in the cohesion energy of Li$_2$, with comparable gain for the larger clusters. Interestingly, the variance of the local energy for the variationally optimized (VQMC) RVB wavefunction is found to be significantly smaller than for the VQMC-optimized Jastrow-Slater wavefunction, resulting in faster convergence of the DQMC calculations. This would make the RVB wavefunction a promising candidate for investigating larger and more complicated clusters. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W13.00010: An Improved Pressure Estimator for quantum Monte Carlo Jeremy McMinis, Jeongnim Kim, David Ceperley, Carlo Pierleoni Assaraf and Caffarel have developed a systematic method for deriving reduced variance estimators for observables for quantum Monte Carlo calculations and have applied to forces[1], the one body density and the spherical and system averaged pair density. It has yet to be applied to a thermodynamic observable. In this work we derive an expression for an improved pressure estimator for use in variational Monte Carlo and diffusion Monte Carlo calculations. We show that because the dependence of the trial wave function on the density is known, for the homogeneous electron gas this new estimator is accurate and efficient to implement. We discuss its application to many-body Hydrogen at high pressure. [1] Roland Assaraf and Michel Caffarel, J. Chem. Phys. 113, 4028 (2000) [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W13.00011: The Model Periodic Coulomb interaction in k-space: modelling the spherically averaged structure factor Rene Gaudoin, Idoia Gurtubay, Txema Pitarke Within Quantum Monte Carlo (QMC) calculations the Model Periodic Coulomb (MPC) interaction is a well know method to reduce finite size effects related to the long range nature of the Coulomb interaction. Recently we presented a method based on modelling the continuous-$k$ spherically averaged structure factor (SF) to understand and reduce Coulomb finite size effects. Here we show that our SF based method can be viewed as $k$-resolved MPC. This allows us to analyse the implicit assumptions that underlie MPC and what to do when these assumptions are not justified, i.e. in non-interaction Hartree-Fock systems or even surfaces. While we present data for the homogeneous electron gas the method itself is general. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W13.00012: Backflow transformation improves QMC calculations of silicon self-interstitial defects William D. Parker, Kevin P. Driver, John W. Wilkins, Richard G. Hennig Recent advances in quantum Monte Carlo (QMC) reduce error introduced by approximations. Direct improvement of the trial wave function through backflow transformation of the electron coordinates[1] produces a wave function closer to the ground state by moving electrons out of the way of a given electron. Adding plane waves of particle position to the Jastrow factor[2] augments the accounting for interparticle correlation in QMC calculations by capturing the ``corners'' of the simulation cell neglected when the Jastrow is only a function of pair separation. Hybrid density functionals have produced better starting trial wave functions for molecules by incorporating some exact exchange to more accurately describe electron-electron interactions. We apply backflow transformation, plane-wave-expanded Jastrow factors and hybrid functional trial wave functions to QMC calculations of silicon self-interstitial defects. [1] L\'opez-R\'ios {\it et al.}, Phys. Rev. E {\bf 74}, 066701 (2006). [2] Drummond {\it et al.}, Phys. Rev. B {\bf 70}, 235119 (2004). [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W13.00013: Density Dependence of Fixed-Node Errors Kevin Rasch, Lubos Mitas With both variational and diffusion Monte Carlo (VMC and DMC) methods, we calculate the ground state energy of isoelectronic free ions in the first row of the periodic table for both Hartree-Fock and Configuration Interaction based trial wave functions. As it is well-known, the fixed-node DMC is exact in the limit that the fermion nodes of the trial wave function are also exact. This study is focused on understanding of the density dependence of the fixed-node error since one expects that with increasing density of electrons the errors would be more pronounced due to higher frequency of sampling of the nodal regions and/or areas with low potential energy. For this purpose we construct Hartree-Fock and multi-reference wave functions and quantify the fixed-node biases. We compare strongly bonded highly localized cations, neutral atoms and weakly bonded anions. We compare the absolute and relative sizes of fixed-node errors and their relationships to multi-reference wave functions. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W13.00014: Slater determinant and pfaffian expansions for wave functions in electronic structure quantum Monte Carlo Michal Bajdich, Lubos Mitas We investigate several types of expansions in Slater determinants and pfaffians for trial wave functions in fixed-node quantum Monte Carlo. The long expansions in determinants are analyzed in order to identify the terms with the largest contributions towards decreasing the fixed-node errors. We further investigate the efficient mapping of these terms onto pfaffian expansions. We apply this technique to test the cases of molecular and atomic systems and we discuss the amounts of recovered correlation energy relative to the expansion size. Finally, following upon our previous study [1], we explore the use of multiple determinants and pfaffians for the accurate description of the wave functions of simple solids. [1] M. Bajdich et al. Phys. Rev. Lett. 96, 130201 (2006). [Preview Abstract] |
Session W14: Quantum Optics
Sponsoring Units: DAMOPChair: Mark Edwards, Georgia Southern University
Room: Morial Convention Center 205
Thursday, March 13, 2008 2:30PM - 2:42PM |
W14.00001: Quantum analysis of nonlinear beam splitter with second order nonlinearity. Hari Prakash, Devendra Kumar Mishra A linear beam splitter mixes two input modes having annihilation operators $\hat {a}$ and $\hat {b}$ and generate two output modes having annihilation operators $\hat {c}$and $\hat {d}$, which are linear in $\hat {a}$ and $\hat {b}$and may be written as $\hat {c}=t\hat {a}+ir\hat {b}$ and $\hat {d}=t\hat {b}+ir\hat {a}$, where t and r are the real coefficients of transmission and reflection, respectively, with r$^{2}$ + t$^{2}$ =1. We include the second order nonlinearity and as a result we find generation of second- harmonic non-classical light. If two coherent beams are mixed at input, squeezing and sub-Poissonian photon statistics are seen to be exhibited by the second-harmonic output beam. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W14.00002: Quantum control of EIT dispersion via atomic tunneling in a double-well Bose-Einstein condensate James Weatherall, Christopher Search, Markku Jaaskelainen We consider electromagnetically induced transparency (EIT) in an atomic Bose-Einstein Condensate (BEC) trapped in a double well potential. A weak probe propagates through one of the wells and interacts with atoms in a three-level $\Lambda$ configuration. The well through which the probe propagates is dressed by a strong control beam with Rabi frequency $\Omega_{\mu}$, as in standard EIT systems. Tunneling between the wells at the frequency $g$ provides a coherent coupling between identical electronic states in the two wells that leads to the formation of inter-well dressed states. The tunnel coupling results in the formation of two ultra-narrow absorption resonances for the probe field that are inside of the ordinary EIT transparency window, which can be interpreted in terms of the inter-well dressed states and the formation of a novel type of dark state involving the coupling laser and the inter-well tunneling. To either side of these ultra-narrow resonances there is normal dispersion with ultra-large slope controlled by $g$. For realistic values of $g$, the large slope of this dispersion yields group velocities for the probe field that are two orders of magnitude slower than standard EIT systems. Additionally, we discuss the effects of the inter-well coupling on the nonlinear susceptibility, $\chi^{(3)}$. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W14.00003: Electromagnetically-induced transparency (EIT) in a four-level atom system Chris Nelson, Cristian Bahrim We prove that the EIT technique can be used for slowing down \textit{simultaneously} two circularly--left and --right polarized laser pulses (probe fields) in the presence of a linearly polarized laser pulse (coupling field) using a four-level atom system prepared with a weak B-field. The simultaneous quantum interference between the coupling field and each of the two probe fields leads to the formation of a new type of EIT system: a \textbf{W-system}. By turning off one of the two probe fields, a standard EIT behavior can be recognized in two independent V-systems. Our novel EIT technique is applied to ultra-cold Mg atoms in low-lying states ($^{1}$S$_{0 }$-- ground state and $^{1}$P$_{1}$ -- first excited state). The density matrix formalism is employed for finding coherences between Zeeman levels of the upper $^{1}$P$_{1}$ state in our W-system. Solving the master equation for a population-trapped four-level atom system and assuming a zero population density on the Zeeman levels of the $^{1}$P$_{1}$ state give atomic susceptibilities for the dark states associated to the two probe fields. The analysis of the EIT behavior in our W-system is done by varying (1) the Rabi frequencies for the triplet $^{1}$P$_{1 }$state and (2) the detuning of the two probe lasers. The dependence of the EIT features with the B-field is also discussed. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W14.00004: Coherent optical spectroscopy of a strongly coupled semiconductor microcavity quantum-dot system Kartik Srinivasan, Oskar Painter Chip-based systems involving a semiconductor microcavity coupled to an embedded quantum dot (QD) offer a scalable, stable platform for optical cavity quantum electrodynamics. To harness this potential in a manner consistent with many protocols for quantum information processing, the system must be coherently probed and manipulated. However, experiments in these systems have largely relied on incoherent excitation through photoluminescence (PL). Here, we describe recent experiments [1] in which a fiber taper waveguide is used to perform steady-state coherent linear and nonlinear optical spectroscopy of a strongly coupled microcavity-QD system, probing the system on its photonic channel (rather than its matter channel, as in PL). Under weak driving, vacuum Rabi splitting is observed, while increasing the drive strength reveals saturation for an average intracavity photon number of less than one. [1] K. Srinivasan and O. Painter (2007), to appear in Nature, Dec. 6, 2007 (preprint: physics/0707.3311). [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W14.00005: Monitoring electron spin decoherence in a quantum dot by weak measurement Shu Hong Fung, Renbao Liu Based on the fluctuation-dissipation theorem, information about the dynamics of a system could be derived by noise spectra with passive probe, in lieu of active pump-control-probe procedures. For a quantum system, the passive probe still needs to be weak lest the system is disturbed by state collapse. We consider the weak measurement by Faraday rotation (FR) of a single spin in a quantum dot under an external magnetic field in the Voigt configuration. The quantum dot is repeatedly probed by linearly polarized laser pulses. The FR caused by a single spin is extremely small (about a few millionth rad), so the photon states with different rotation angles, and hence the electrons spin states, are only weakly distinguished. The polarized light beam is filtered by a polarized beam splitter and the reflected photons are counted. The second order correlation of the photon count shot noise oscillates with a rapid damping caused by the inhomogeneous broadening (which exists even for a single spin under repeated measurements). In the third order correlation, the single spin decoherence is singled out from the rapid inhomogeneous dephasing, which would otherwise require spin echo. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W14.00006: Spontaneous emission from a tunneling atom Daniel Braun, John Martin We study the tunneling of a two-level atom in a double well potential in while the atom couples either to a single electromagnetic field mode of a cavity to the full continuum of electromagnetic modes in three dimensions. Both studies are within the Lamb-Dicke regime concerning transitions to higher vibrational states, but beyond the Lamb-Dicke regime concerning the tunneling split groundstate. The first case leads to an extended Jaynes-Cummings model which can be solved exactly. We show that the coupling between internal and external degrees of freedom of the atom induced by the cavity mode can dramatically change the tunneling behavior. In general the tunneling process becomes quasiperiodic. If the cavity is fed with a coherent state, a collapse and revival of the tunneling can occur. Accessing the internal degrees of freedom of the atom with a laser allows to coherently manipulate the atom position, and in particular to prepare the atom in one of the two wells. In the second case, the tunneling process may decohere, depending on the wavelength corresponding to the internal transition and on the spontaneous emission rate. Interference fringes appear in the emitted light from a tunneling atom, or an atom in a stationary coherent superposition of its center--of--mass motion, if the wavelength is comparable to the well separation and if the external state of the atom is post-selected.\newline [1] J.~Martin and D.~Braun, arXiv:0704.0763 and arXiv:0707.4158. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W14.00007: Single photon nonlinearities using arrays of cold polar molecules T. Bragdon, R. M. Rajapakse, A. M. Rey, S. F. Yelin We model single photon nonlinearity via dipole-dipole interaction in cold polar molecules using the protected Dicke-like symmetric manifold for potential optical quantum computation processing. We report on potential decoherences described by phonon dispersion, spontaneous, and stimulated decays. We compare to individually addressed molecules from previous work, and discuss briefly the feasibility in optical quantum computation processing as an element of a controlled-Z gate. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W14.00008: Fermionization of strongly interacting photons in one-dimensional nonlinear medium Darrick Chang, Vladimir Gritsev, Giovanna Morigi, Vladan Vuletic, Mikhail Lukin, Eugene Demler Understanding strongly correlated quantum systems is a central problem in many areas of physics. The collective behavior of interacting particles gives rise to diverse fundamental phenomena such as confinement in quantum chromodyanmics, spontaneous symmetry breaking and phase transitions, and electron fractionalization in one dimensional systems and in the quantum Hall regime. While such systems typically involve strongly interacting massive particles, optical photons can also interact with each other in a nonlinear medium. In practice, however, such interactions are typically very weak. We describe a novel technique that allows the creation of a strongly correlated quantum gas of photons, which is made possible by the tight field confinement that can be achieved in a number of novel, one-dimensional optical systems. This confinement enables the generation of large optical nonlinearities via the interaction of photons with a nearby cold atomic gas, which can be further amplified by an optical Bragg grating that traps these photons within the medium. In its extreme, we show that a quantum light field can undergo {\it fermionization} in such one-dimensional media, which can be probed via standard photon correlation measurements. Realization of such systems can open a route for quantum simulators of {\it matter} Hamiltonians using light fields and novel applications in metrology and quantum information. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W14.00009: Entangled-photon absorption in semiconductor nanostructures Felipe Vallejo, Luis Quiroga We present a comparative study of two-photon absorption by semiconductor nanostructures for two kind of light: (i) Light with wave-like classical properties (laser light) and (ii) A new type of quantum entangled light. First, we report on results concerning entangled-photon absorption processes due to $s$ and $d$ final states in core-shell quantum dots. Second, within the framework of the effective-mass approximation both the classical as well as the entangled two-photon absorption in quantum wells (QWs) and quantum wires (QWRs) have been addressed. Results for non-entangled light with polarization both parallel and perpendicular to the directions of confinement are in perfect agreement with the ones already known. We proceed to extend those results to the less explored case of entangled light absorption in semiconductor nanostructures. The absorption spectra for entangled light is richer in structure and complexity as compared with the classical light case. We find that the absorption rate (cross-section) for entangled light depends additionally of a new important parameter, the entanglement time $T_e$, which gives rise to quantum interference effects. As a result, entangled photons produce entangled-induced transparencies. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W14.00010: Photon Tunneling Through Dielectric Bandgaps and Evanescent Gaps Natalia Rutter, Sergey Polyakov, Paul Lett, Alan Migdall We implement an optical tunneling testbed using the precise simultaneity of creation of twin photons produced by parametric down conversion and a Hong-Ou-Mandel interferometer. With this setup, we can measure photon traversal times of a sample with fs precision. We use this setup to compare the time for a photon to traverse dielectric stacks of odd versus even numbers of layers of alternating indexes of refraction. Preliminary data shows that subtle changes in the stack structure result in dramatic variations in photon traversal times ($\sim $10 fs) that can range from sub- to super-luminal. Our ultimate goal is to use this setup to investigate photon tunneling times in regions of true evanescent propagation and compare them to the traversal times in our dielectric stack bandgap samples where the propagation is oscillatory. This allows us to test the suitability of certain optical models of tunneling and highlight the pitfalls that occur when relying on conditional measurements. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W14.00011: Nonlinear wave scattering by small barrier potentials Wenjie Wan, Jason W. Fleischer Scattering by a barrier potential is a fundamental problem in wave physics, involving issues of boundary conditions, resonances, radiation, etc. While scattering in the linear case is well-known, the nonlinear case has received far less attention. In the nonlinear regime, self-interaction affects tunneling and re-radiation dynamics, often leading to new topological structures (e.g. dark solitons and vortices). Examples include many-body quantum systems, plasmas, and nonlinear optics. Here, we focus on the optical case by considering plane-wave scattering from an optically-induced barrier potential (step index) inside a photorefractive crystal. We experimentally demonstrate shock wave formation (dark soliton trains) in 1D and vortex generation in 2D, as a function of barrier height and input wave angle. We show numerically that these results arise from a combination of tunneling, scattering, and optical superflow around the boundary. Applications both within and beyond optics will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W14.00012: Photon localization and Dicke superradiance in atomic gases: crossover to a ``small world'' network Eric Akkermans, Aharon Gero, Robin Kaiser We study photon propagation in a gas of $N$ atoms, using an effective Hamiltonian that accounts for photon mediated atomic dipolar interactions. The configuration average density $P(\Gamma)$ of photon escape rates is obtained from the spectrum of the $N \times N$ random matrix $\Gamma_{ij} = \sin (x_{ij}) / x_{ij}$, where $x_{ij}$ is the dimensionless random distance between any two atoms expressed in units of the photon wavelength. A scaling function is defined to study photons escape rates as a function of disorder and system size. We show that for a strong enough disorder, photons do not escape the gas. This localization is described using a mapping of this problem onto statistical properties of random networks. We show that there is no localization phase transition as expected in disordered systems without correlation, but rather a cross-over between localized and delocalized photons. The mean field solution of this problem displays a ``small world'' behavior. In the Dicke limit, we recover localization associated to cooperative effects. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W14.00013: Fermi two atom problem in extended Fredrichs-Lee Model Kavan Modi, James Zabel, George Sudarshan In 1932 Fermi calculated the time required for excitation transition between two atoms. He found the minimum time to be the distance between the atoms divided by the speed of light. Recently, Hegerfeldt, using a very basic argument of analyticity of the wavefunction, showed that the excitation amplitude of the second atom must be finite for all times or zero for all times. We are studying this problem in detail using a modified Fredrichs-Lee model where two discrete states are connected by a continuum. We can solve for the transition amplitude exactly in our model, without assuming that a photon is the mediator between the two discrete modes. Our model should shed some light on the conceptual difficulties that have bothered the community for long. [Preview Abstract] |
Session W15: Quantum Entanglement II
Sponsoring Units: GQIChair: Ian Durham, Saint Anselm College
Room: Morial Convention Center 207
Thursday, March 13, 2008 2:30PM - 2:42PM |
W15.00001: Experimental demonstration of anyonic statistics with multiphoton entanglement Harald Weinfurter, Witlef Wieczorek, Christian Schmid, Nikolai Kiesel, Reinhold Pohlner, Jiannis Pachos Particles in nature are usually distinguished according to their statistics in two categories: bosons and fermions. However, if one considers only two spatial dimensions statistical behaviour ranging from bosonic to fermionic is found. Particles exhibiting such a behaviour are called anyons. Our experimental demonstration of anyonic statistics is based on a particular two-dimensional model: the toric code proposed by Kitaev [1]. There, anyons arise as excitations that are generated by local operations. We show that for this model anyonic behaviour is revealed for as little as four qubits [2]. This enabled us to experimentally demonstrate anyonic statistics in a quantum simulation with four-photon entanglement. \newline [1] A.~Yu.~Kitaev, Ann. Phys. {\bf 303}, 2 (2003). \newline [2] J.~K.~Pachos {\emph{et al.}}, arXiv:0710.0895v2 [quant-ph] (2007). [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W15.00002: Entanglement entropy and other observables of topological phases with finite correlation length Stefanos Papanikolaou, Kumar S. Raman, Eduardo Fradkin We elucidate the topological features of the entanglement entropy of a region in two dimensional quantum systems with a finite correlation length. Firstly, we suggest that simpler reduced quantities, related to the von Neumann entropy, could be defined to compute the topological entropy. We use our methods to compute the entanglement entropy for the ground state wave function of a quantum eight-vertex model in its topological phase, and show that a finite correlation length adds corrections of the same order as the topological entropy which come from sharp features of the boundary of the region under study. We also calculate the topological entropy for the ground state of the quantum dimer model on a triangular lattice by using a mapping to a loop model. The topological entropy of the state is determined by loop configurations with a non-trivial winding number around the region under study. Finally, we consider extensions of the Kitaev wave function, which incorporate the effects of electric and magnetic charge fluctuations, and use it to investigate the stability of the topological phase by calculating the topological entropy. arxiV: 0709.0729 [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W15.00003: Valence-Bond Monte Carlo for Chains of Non-Abelian Quasiparticles Huan Tran, Nick Bonesteel In non-Abelian FQH states, quasiparticles carry quantum numbers (topological charge) which characterize a degenerate Hilbert space. When these quasiparticles are close enough together, the degeneracy of this Hilbert space is lifted and the quasiparticles are said to interact.\footnote{A. Feiguin \textit {et al.}, PRL \textbf{98}, 160409 (2007).} Here we show that the valence-bond Monte Carlo method introduced by Sandvik\footnote{A. W. Sandvik, PRL \textbf{95}, 207203 (2005).} for spin-1/2 systems can be generalized to simulate 1D chains of such interacting non-Abelian quasiparticles. For uniform chains, our Monte Carlo results for the ground state energy agree with known exact values.$^1$ For random chains we confirm numerically that, as expected,\footnote{ N. E. Bonesteel and K. Yang, PRL \textbf{99}, 140405 (2007).} the ground state freezes into a random singlet phase. By suitably generalizing the notion of valence-bond entanglement entropy\footnote{F. Alet, \textit{et al.}, PRL \textbf{99}, 117204 (2007); R. W. Chhajlany \textit{et al.}, PRL \textbf{99}, 167204 (2007).} to the non-Abelian case we also confirm the predicted result$^3$ that in this phase the entropy of a block of length $L$ scales as $S^{\rm VB}_L \simeq \frac{\ln d}{3} \log_2 L$, where $d$ is the quantum dimension of the quasiparticles. Work supported by US DOE. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W15.00004: Geometry of metal-insulator transitions in one-dimension Noah Bray-Ali, Lorenzo Campos Venuti, Marco Cozzini, Paolo Zanardi We use the geometric approach to quantum critical points to study the metal-insulator transitions driven by chemical potential, $\mu$, or repulsion, $U$, in the one-dimensional Hubbard model. The transition to the band-insulator, as $\mu\rightarrow \mu_c,$ exhibits conventional scaling of the ground-state fidelity metric tensor $G_{\mu,\nu}\equiv{\rm Re}\left [\left \langle \partial_\mu\psi | \partial_\nu\psi\right \rangle - \left \langle \partial_\mu\psi |\psi\right\rangle\left\langle\psi| \partial_\nu\psi\right \rangle \right ]$. For example, the metric diverges as $G_{U,U}\sim 1/n$, where, $n\sim\sqrt{\mu-\mu_c}$, is the band filling. At the Mott transition, the metric behavior depends on the path of approach to the critical point. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W15.00005: Topological order at finite temperature: protected or not protected? Claudio Castelnovo, Claudio Chamon We investigate the behavior of the entanglement and topological entropy in the two- and three-dimensional toric code at finite temperature. From our results, we infer that quantum topological order is fragile with respect to thermal fluctuations in spite of the presence of a finite energy gap at zero temperature. In two dimensions, all topological order evaporates at any non-vanishing temperature in the thermodynamic limit. On the contrary, in three dimensions not all topological information is lost, although the topologically protected quantum information (qubit) stored in the ground state of the system is immediately degraded to topologically protected classical probabilistic information (pbit) at any infinitesimal temperature, in the thermodynamic limit. All information is eventually lost beyond a finite temperature phase transition. We comment on the implications of our results with respect to braiding operations and topological quantum computing. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W15.00006: Long distance entanglement mediated by gapped spin chains Aires Ferreira, Joao Lopes dos Santos This talk will describe an analytical approach for the computation of Long Distance Entanglement (LDE) mediated through one-dimensional quantum spin chains recently found in numerical studies \footnote{L. Campos Venuti, C. Degli Esposti Boschi and M. Roncaglia, Phys. Rev. Lett. \textbf{96} 247206 (2006).}. I review the formalism \footnote{A. Ferreira and J. M. B. Lopes dos Santos, \emph{submitted for publication in APS} pre-print: arXiv:0708.0320 (2007).} that allows the computation of LDE for weakly interacting probes with gapped many-body systems and show that, at zero temperature, a DC response function determines the ability of the physical system to develop genuine quantum correlations between the probes. In the second part of the talk, I show that the biquadratic Heisenberg spin-1 chain is able to produce LDE in the thermodynamical limit and that the finite antiferromagnetic Heisenberg chain maximally entangles two spin-1/2 probes very far apart. This is of crucial importance since feasible mechanisms of entanglement extraction from real solid state systems and their ability to transfer entanglement between distant parties are essential ingredients for the implementation of Quantum Information protocols, such as teleportation or superdense coding. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W15.00007: Entanglement of Impurities in Spin Chains Erik Sorensen, Nicolas Laflorencie, Ming-Shyang Chang, Ian Affleck Entanglement in $J_1-J_2$, $S=1/2$ quantum spin chains with an impurity is studied using analytic methods as well as large scale numerical density matrix renormalization group methods. The impurity contribution to the uniform part of the entanglement entropy, $S_{imp}$, is defined and analyzed in detail in both the gapless, $J_2\leq J_2^c $, as well as the dimerized phase, $J_2>J_2^c$, of the model. This quantum impurity model is in the universality class of the single channel Kondo model and we show that in a quite universal way the presence of the impurity in the gapless phase, $J_2\leq J_2^c$, gives rise to a large length scale, $\xi_K$, associated with the screening of the impurity, the size of the Kondo screening cloud. The universality of Kondo physics then implies scaling of the form $S_{imp}(r/\xi_K,r/R)$ for a system of size $R$. At the critical point, $J_2^c$, an analytic approach based on a Fermi liquid picture, valid at distances $r\gg\xi_K$ and energy scales $T\ll T_K$, is developed and analytic results at $T=0$ and $T\neq 0$ are obtained. In the dimerized phase an appealing picture of the entanglement is developed in terms of a {\it thin soliton} (TS) ansatz permitting variational calculations and the notions of impurity valence bonds (IVB) and single particle entanglement (SPE) are introduced. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W15.00008: Entanglement Entropy of States with Long-Range Magnetic Order Wenxin Ding, Nicholas Bonesteel, Kun Yang We study the bipartite entanglement entropy of spin models whose ground states have perfect ferromagnetic (FM) or antiferromagnetic (AFM) long-range order. For the FM case the entanglement entropy is taken to be one-half the quantum mutual information so as to properly take into account the ground state degeneracy. The calculation of the entropy for this case is then straightforward and agrees with previous work using a different approach. For the AFM case the problem is reduced to that of four coupled spins. This simplification allows us to determine the asymptotic behavior of the entropy analytically with results which agree well with exact numerical calculations. In both the FM and AFM cases we find the entropy grows logarithmically with block size, $N_1$. For example, if we take $N_1 = N/2$, where $N$ is the total number of spins, then in the FM case the entropy, $E$, scales as $E \simeq \frac{1}{2} \ln N_1$, and in the AFM case, $E \simeq \ln N_1$. In both cases the area law is clearly violated. Implications of these results for more general states with long range order are also discussed. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W15.00009: Entanglement Entropy in the Two-Dimensional Random Transverse Field Ising Model Stephan Haas, Rong Yu, Hubert Saleur We have applied the numerical strong disordered renormalization group method to the two-dimensional random transverse field Ising model, and studied the scaling behavior of the entanglement entropy. The leading term of the entanglement entropy scales linearly with the block size, following the so called \emph{area law}. However, besides this \emph{area law} contribution, a subleading logarithmic correction at the quantum critical point is resolved. This correction is understood from the point of view of an underlying percolation transition, both at finite and at zero temperature. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W15.00010: Entanglement Entropy and Complexity in Random Systems Rodriguez-Laguna Javier Entanglement is considered to be the hallmark of quantum physics, and entanglement entropy (EE) is one of its most natural measurements. Its utility as a marker for quantum criticality for random systems is well established. Recently, it has been shown that the scaling of the running-time in some quantum annealing methods is also related to it. In this work we show how the behaviour of this magnitude in some random systems can provide insight about the complexity of the structure of their quantum critical points. Moreover, we provide some hints that point towards a relation between the behaviour of the EE and the complexity class of classical problems. References: J. Rodriguez-Laguna, J. Phys. A: Math. Theor. 40, 12043 (2007), and JSTAT P05008 (2007). [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W15.00011: Entanglement Entropy Scaling in the Disordered Golden Chain Lukasz Fidkowski, Gil Refael, Nick Bonesteel, Kun Yang, Joel Moore For pure critical spin chains, the scaling of the entanglement entropy of a region of size $L$ with its complement is proportional to $\log L$ with the constant of proportionality being the central charge of the associated conformal field theory. Certain strongly disordered spin chains exhibit critical points with similar $\log L$ scaling. Here we study the disordered golden chain (modeled by fibonacci anyons), and show that the usual random singlet critical point achieved with random antiferromagnetic (AFM) couplings is unstable to ferromagnetic (FM) perturbations. We identify the new mixed FM-AFM fixed point and compute its entanglement entropy scaling. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W15.00012: Quantum Ergodicity and the Dynamical Generation of Entanglement in Kicked Coupled Tops Collin Trail, Vaibhav Madhok, Ivan Deutsch, Shohini Ghose, Leigh Norris, Arjendu Pattanayak We explain how the long-time average dynamically generated entanglement in a Hamiltonian bipartite system is related to the corresponding classical dynamics in the semiclassical limit. Where classical dynamics is chaotic, ergodic mixing leads to the generation of ``random quantum states.'' These states possess the typical entanglement of a state randomly sampled from the appropriate Hilbert space under the unitarily invariant Haar measure. We exemplify these results using a system of coupled kicked-tops in which entanglement and chaos arise from the same physical effect in contrast to previous studies. We present quantitive predictions of the dynamically generated entanglement, which is influenced by the time symmetries of the system and the structure of the Hilbert space, under a variety of different conditions, and show a close fit to numerical simulations. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W15.00013: Observation of two-particle Aharonov-Bohm interference Izhar Neder, Nissim Ofek, Yunchul Chung, Moty Heiblum, Diana Mahalu, Vladimir Umansky \textit{Nature} \textbf{448}, 333-337 (19 July 2007) We report the first observation of quantum interference between two independent {\&} non-interacting electrons in a unique interferometer proposed by Yurke et. al. [1] {\&} Samuelsson et. al. [2]. The interference fringes were observed only in the joint probability of electrons arrival at two different drains; hence being the quantum analogue to the Hanbury Brown {\&} Twiss (HBT) experiment with classical waves [3]. This, sought after, counter intuitive effect, is a direct result of the quantum exchange statistics of identical quantum particles. Our observation is a signature of orbital entanglement between two independent electrons, even tough they never interacted with each other. [1] B. Yurke {\&} D. Stoler, Phys. Rev. A46, 2229-2234 (1992) [2] P. Samuelsson, E. V. Sukhorukov {\&} M. Buttiker, Phys. Rev. Lett. 92, 02685 (2004). [3] R. Hanbury Brown {\&} R. Q. Twiss, Phil. Mag. 45, 663-682 (1954). [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W15.00014: Few-electron anisotropic quantum dots in low magnetic fields: exact-diagonalization results for excitations, spin configurations, and entanglement Constantine Yannouleas, Uzi Landman Following earlier studies\footnote{Y. Li, C. Yannouleas, and U. Landman, arXiv:0710.4325v1 [Phys. Rev. B (2007), in press]; C. Yannouleas and U. Landman, Rep. Prog. Phys. {\bf 70}, 2067 (2007)} for $N=2-3$ electrons, exact-diagonalization calculations for $N=4-6$ electrons in anisotropic quantum dots, covering a broad range of confinement anisotropies and strength of inter-electron repulsion, will be presented for zero and low magnetic fields. The excitation spectra are analyzed as a function of the magnetic field and of quantum-dot anisotropy. Analysis of the many-body wave functions through spin-resolved two-point correlations reveals that the electrons tend to localize forming Wigner molecules (WMs). For strong anisotropy, the WMs acquire a linear geometry, and the wave functions with a total spin projection $S_z=(N-2)/2$ are similar to the strongly entangled $W$ states. For intermediate anisotropy, the WMs exhibit a more complex structure. The degree of entanglement can be quantified through the use of the von Neumann entropy. [Preview Abstract] |
Session W16: Focus Session: Novel Biomedical Techniques
Sponsoring Units: DBPChair: Paul Gueye, Hampton University
Room: Morial Convention Center 208
Thursday, March 13, 2008 2:30PM - 3:06PM |
W16.00001: Novel Radiation Sources Based on Ultra-High-Power Lasers: New Capabilities for Radiology and Radiotherapy Invited Speaker: As the maximum power level of compact lasers steadily increases, new opportunities are enabled for their use in bio-medicine and medicine. For instance, the Diocles laser at the University of Nebraska, Lincoln, now produces a peak power of 150-terawatts (1.5x10$^{14}$ W) from a table-top-size system. When light at this power level is focused, it can accelerate electrons, and produce quasi-monoenergetic beams of x-rays, similar to those produced by much larger synchrotron light sources. Such MeV-energy beams create new opportunities in biomedicine, radiology and radiography. Examples to be discussed include structural analysis of bio-molecules, diffraction-enhanced imaging for computed tomography, and radio-sensitization-enhanced radiotherapy. This talk will describe the current status of laser-based x-ray technology, as well as the potential advantages and prospects for their use in medicine. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:42PM |
W16.00002: New techniques required to understand the by-stander effect in situ. Invited Speaker: The by-stander effect has been known for nearly a century under various names, of which the abscopal effect is probably the most well known. More recently the by-stander effect has received a lot of attention, and various models have been developed to assess the relative importance of the bystander effect in radiation treatment. It is clear that irradiated cells release factors that lead to alterations in the physiology of adjacent irradiated cells, both via inter-cellular junctions and through systemic factors. Most studies that have sought to identify the systemic factors and the cellular mechanisms that are responsible for the bystander effect have by necessity used in vitro systems. The purpose of this presentation is to alert the audience to the various techniques that are available to study the proteomic changes related to the bystander effect in situ. We shall pay attention to the use of MALDI-imaging to track spatial proteomic changes in tissue that have been exposed to microbeams. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W16.00003: The Physics of Up-Conversion Nanophosphors for Cancer Treatment Shuang-fang Lim, William Ryu, Robert Riehn, Robert Austin Upconversion phosphors are rare-earth containing crystalline materials which can convert infrared light to visible light (hence the name up-conversion), via excitation of real levels of rare earths. They overcome limitations of current organic fluorophors and quantum dots and have unique properties that enable advanced imaging, drug delivery applications and light-based chemotherapy. The process by which near -IR photons (980 nm) are summed in a rare-earth matrix to emit visible light is a fascinating problem in atomic physics combined with solid-state physics. We will present experimental results which probe this non-linear phenomena. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W16.00004: Disposal of Energy by UV-B Sunscreens Thomas Nordlund, Rajagopal Krishnan Ideal sunscreens absorb dangerous UV light and dispose of the energy safely. ``Safe disposal'' usually means conversion to heat. However, efficient absorption entails a high radiative rate, which implies high energy-transfer and other rates, unless some process intervenes to ``defuse'' the excited state. We studied the excited-state kinetics of three UV-B (290-320 nm) sunscreens by absorption, steady-state and time-resolved fluorescence. Excited-state rate analysis suggests that some sunscreens have low radiative-rate ``dark'' states, in addition to normal excited states.* We deduce dark states when sunscreens of high extinction coefficient do not show lifetimes and total emission consistent with such high radiative rates. A high radiative rate, accompanied by efficient fluorescence emission and/or transfer, may be unfavorable for a sunscreen. In spite of its dark excited state, padimate O shows significant re-emission of light in the UV-A (320-400 nm) and energy transfer to a natural component of excised skin, probably collagen. * Krishnan, R. and T.M. Nordlund (2007) J. Fluoresc. DOI 10.1007/s10895-007-0264-3. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:42PM |
W16.00005: Microdosimetry of ondividual cells with microbeam facilities Invited Speaker: This abstract has not been submitted. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W16.00006: Dynamical Systems, Cytokine Storms, and Blood Filtration Glenn Foster, Alfred Hubler Various infections and non-infectious diseases can trigger immune cells and the proteins (cytokines) the cells use to communicate with each other to be caught in a positive feedback loop; this ``cytokine storm'' is frequently fatal. By examining the network of cytokine-immune cell interactions we will illustrate why anti-mediator drugs have been generally ineffective in stopping this feedback. A more effective approach may be to try and reduce interactions by dampening many signals at once by filtering the cytokines out of the blood directly (think dialysis). We will argue that feedback on an out of control nonlinear dynamical system is easier to understand than its normal healthy state and apply filtration to a toy model of immune response. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W16.00007: Differentiation of benign epithelia, prostatic intraepithelial neoplasia and, stromal region of prostate biopsies using Raman spectroscopy Houbei Dai, Jagdish Thakur, Ratna Naik, Fazlul Sarkar, Wael Sakr, Greg Auner, Alex Cao, Abhilash Pandya, Vaman Naik A pilot study was performed to differentiate the benign epithelia (BE), prostatic intraepithelial neoplasia (PIN) and, stromal regions from deparaffinized prostate needle biopsies using Raman spectroscopy. Raman spectra were collected from six deparaffinized prostate tissues and all the three regions showed different Raman spectral features that may represent unique Raman signatures of these regions. One of the unique features observed in these spectra is that the ratio of the two peak heights at 1449 cm$^{-1}$ and 1338 cm$^{-1}$ is constant with different values for BE and PIN. While the peak at 1338 cm$^{-1}$ is quite weak in the stromal regions. Multivariate statistical methods clearly separated the spectra from three different regions of the tissues into three distinct groups. This study demonstrates the feasibility of using deparaffinized tissue biopsy samples for the diagnostic purpose. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W16.00008: Medical Applications of the Geant4 Simulation Toolkit Joseph Perl Geant4 is a toolkit for the simulation of the passage of particles through matter. While Geant4 was originally developed for High Energy Physics (HEP), applications now include Nuclear, Space and Medical Physics. Medical applications of Geant4 in North America and throughout the world have been increasing rapidly due to the overall growth of Monte Carlo use in Medical Physics and the unique qualities of Geant4 as an all-particle code able to handle complex geometry, motion and fields with the flexibility of modern programming and an open and free source code. Work has included characterizing beams and brachytherapy sources, treatment planning, retrospective studies, imaging and validation. This talk will provide an overview of these applications, with a focus on therapy, and will discuss how Geant4 has responded to the specific challenges of moving from HEP to Medical applications. [Preview Abstract] |
Session W17: General Biological Physics
Sponsoring Units: DBPChair: Dean Astumian, University of Maine
Room: Morial Convention Center 209
Thursday, March 13, 2008 2:30PM - 2:42PM |
W17.00001: Actin-mediated bacterial propagation as dissipative dynamics of F-actin concentration: onset of motion, comet profile, velocity fluctuations Vincenzo Benza Bacterial motion under the action of an actin gel network is described in terms of the F-actin concentration dynamics driven by polymerization, elasticity, and coupling with the bacterium. An explicit formula for the velocity clarifies the role of the different factors contributing to propagation. As regards the onset of motion, we find that smaller ratios of the branching/nucleation rates give rise to an increasingly long buildup time before startup. In the cruise regime the linear growth of the comet length versus velocity is analytically shown and numerically verified; as the length increases the concentration maximum decreases. Both features have been observed in kinematics experiments [1]. By expanding our previous work [2], we show that a larger elasticity modulus makes a larger velocity but with a smaller contribution from the interface polymerization. At steady state we find two regimes: constant velocity when the branching rate dominates over the nucleation rate, intermittent velocity when the two rates are comparable. In this case the concentration profile does not display macroscopic fluctuations, but the distance of its maximum from the bacterium surface oscillates: this behavior has its counterpart in the intermittency of the cruise velocity. [1] F.S.Soo, J.A.Theriot, Biophysical Journal 89 :703-723 (2005). [2] V.G.Benza, arXiv:q-bio/0702061 (February 2007) [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W17.00002: The contraction of Vorticella in different Ca concentration solutions Deependra Kantha, David Van Winkle The contraction of the stalk of \textit{Vorticella Convallaria} was studied in media with different concentrations of calcium ion solution. Seven solutions were prepared by adding different amounts of CaCl$_{2}$ in the range of 0.001M to 0.004M in 0.005M EGTA, 0.1M KCl and 0.02M MOPS. The pH values of the solutions were maintained between 6.7 and 6.9. The contractions were recorded as cines (image sequences) by a Phantom V5 camera (Vision Research) on a bright field microscope with 20X objective, with the image resolution of 256 $\times $ 128 pixels at 7000 pictures per second. The change in length of stalk as a function of time was analyzed to compute velocity, acceleration, force and force coefficient. The apparent force coefficient increases linearly with time until the whole stalk is contracting. Considering time dependence of force coefficient, the contracting length is modeled as: \[ L(t)=\frac{L_0 }{2}\left[ {\left( {1+\frac{C}{A}} \right)\exp \left( {\frac{-C+A}{2m}t} \right)+\left( {1-\frac{C}{A}} \right)\exp \left( {\frac{-C-A}{2m}t} \right)} \right] \] Where $L_0 $ is initial contractile length, $C=6\pi \eta r$, $A=\sqrt {C^2-4mK(t)} $, $\eta $ is coefficient of viscosity, $m$ is mass and$ r$ is radius of zooid. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W17.00003: Kinetics of Multidentate Ligand Binding Igor Goncharenko, Mike Colvin, Ajay Gopinathan The binding of multivalent ligands to cell surface receptors is an inherent feature of many biological processes and is technologically important in designing drug delivery systems. We analyze the binding and unbinding kinetics of a multi-armed ligand to normal and cancerous cells in terms of the residence time. By mapping the problem to a first passage time solution of the dynamics described by a multi-dimensional Fokker-Plank equation we are able to derive the residence times of the ligands on the cell surface as a function of the number of arms, binding affinity, polymer statistics of the linker arms as well as density, distribution and types of receptor binding sites. Our results point towards ways of optimizing these parameters so as to selectively target diseased cells with specially designed ligands that are capable of drug delivery. Our results also shed light on the recognition and response kinetics of a variety of cell types with specific functions that are triggered by the binding of surface receptors to exogenous ligands. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W17.00004: Single cell visualization of DNA repair \textit{in vivo}. Azadeh Samadani, Amy Rowat, Jennifer Makridakis, James Haber The creation of a DNA double-strand-break constitutes the most dangerous type of DNA damage. Inefficient response to DNA damage may lead to hypersensitivity to cellular stressors, susceptibility to genomic defects and resistance to apoptosis, which can lead to cancer. Current research on DNA repair has enabled numerous breakthroughs in our understanding of the DNA repair mechanisms at the population level. However, similar understanding at the level of single cells has been lacking mainly because of two reasons: 1) population level measurements do not visualize the repair process and therefore the exact mechanism by which the donor and recipient sequences are brought together is not well understood. 2) they are only sensitive to the mean of a distribution and usually hide the cell-to-cell variability of the repair processes. In my lab we utilize a multidisciplinary approach to address specific aspects of the DNA repair at the single cell level. By tagging several locations on DNA, its dynamic is visualized. furthermore the exact timing of the repair process is measured. In our experiments, individual cells are followed over long periods of time and many cellular generations in a microfluidic device, in which a precise control of the microenvironment of the cells is possible [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W17.00005: Current reversal in collective rocking ratchets induces by ground state instability Jose L. Vicent, Luis Dinis, Elvira M. Gonzalez, Jose V. Anguita, Juan M.R. Parrondo A collective mechanism for current reversal in rocking ratchets is proposed. The mechanism is based on a two-dimensional instability of the ground state of the system. We illustrate our results with numerical simulations and experiments using the dynamics properties of superconducting vortex lattice in Nb superconducting films fabricated on top of Si substrates with array of asymmetric nanodefects. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W17.00006: A Mathematical Exploration of MAP Kinase Behavior Rhys Adams, Gabor Balazsi Mitogen-Activated Protein (MAP) kinase pathways are highly conserved from yeast to humans and are implicated in cell survival and cell death. Signaling through these pathways starts with the phosphorylation of the most upstream component (MAP kinase kinase kinase, MAPKKK), continues with phosphorylation of a MAP kinase kinase (MAPKK), and ends with phosphorylation of the target MAP kinase (MAPK). Theoretical studies over the past few decades have generated important insights into the dynamical behavior and signal processing capability of these pathways, including bistability, oscillations, signal amplification, etc. Prompted by the possibility of complex behavior in simpler signaling units than a full MAP kinase pathway, we investigate the possibility of In-Band Detection (IBD) within a single step of the cascade. We show that a basal rate of target phosphorylation can lead to IBD in a simpler system than the one described before, and define a precise relationship between the various reaction rates that is necessary to obtain IBD. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W17.00007: Electric field control of the cell orientation Christopher Westman, Renat Sabirianov Many physiological processes depend on the response of biological cells to external forces. The natural electric field at a wound controls the orientation of the cell and its division.[1] We model the cell as an elongated elliptical particle with given Young's modulus with surface charge distribution in the external electric field. Using this simple theoretical model that includes the forces due to electrostatics and the elasticity of cells, we calculated analytically the response of the cell orientation and its dynamics in the presence of time varying electric field. The calculations reflect many experimentally observed features. Our model predicts the response of the cellular orientation to a sinusoidally varying applied electric field as a function of frequency similar to recent stress-induced effects.[2] \begin{enumerate} \item Bing Song, Min Zhao, John V. Forrester, and Colin D. McCaig, ``Electrical cues regulate the orientation and frequency of cell division and the rate of wound healing \textit{in} \textit{vivo}'', PNAS 2002, vol. 99 , 13577-13582. \item R. De, A. Zemel, and S.A. Safran, ``Dynamics of cell orientation'', Nature Physics 2007, vol.3, 655. \end{enumerate} [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W17.00008: X-ray studies of crystal transformation in dehydrating trehalose Duncan Kilburn, Paul Sokol The disaccharide trehalose is known to assist in stabilizing dehydrated biological cellular structure. It is present in relatively large quantities in certain organisms whose bodies remain viable for significant periods of time under conditions of extreme drought. Whilst trehalose may not be unique among the sugars in this function, there have been several studies investigating the influence of water on trehalose structure in the hope of determining the mechanism responsible for the properties noted above. We report real-time wide angle X-ray diffraction studies as the commonly occurring crystalline dihydrate form of trehalose is dehydrated at a range of temperatures (in the range 40-70 C) and forms the `alpha' crystalline form of anhydrous trehalose. We find that there is evidence of a two-step process: the dehydration, followed by a crystalline-crystalline transition. The speed of the latter transition is surprising because the dehydrated amorphous form of trehalose has a glass transition temperature of roughly 120 C. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W17.00009: Understanding a Period-Doubling Bifurcation in Cardiac Cells Carolyn Berger, Xiaopeng Zhao, David Schaeffer, Salim Idriss, Daniel Gauthier Bifurcations in the electrical response of cardiac tissue can destabilize spatio-temporal waves of electrochemical activity in the heart, leading to tachycardia or even fibrillation. Therefore, it is important to classify these bifurcations so that we can understand the mechanisms that cause instabilities in cardiac tissue. We have determined that the period-doubling bifurcation in paced myocardium is of the unfolded border-collision type. To understand how this new type of bifurcation manifest itself in cardiac tissue, we have also studied the role of calcium in inducing the bifurcation. We will discuss the nature of the unfolded border-collision bifurcation and present our results of dual voltage and calcium measurements in a frog ventricle preparation. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W17.00010: Line-defect spiral pattern formation during unstable spiral wave propagation in cardiac tissue Juan Restrepo, Alain Karma Spiral waves of voltage signaling in cardiac tissue are widely recognized to play an important role in the genesis of lethal heart rhythm disorders. Previous modeling studies have shown that the breakup of such waves, which has been proposed as a mechanism for heart fibrillation, can be mediated by a generic period doubling bifurcation. This bifurcation leads to beat-to-beat changes of action potential duration, and hence cellular refractoriness, known as alternans. Here we study the spatial pattern of the period two dynamics before spiral breakup. We find numerically that the line defects, the locus of all points where the dynamics has period one, can form either as a one- or a three-arm spiral pattern where each arm corresponds to a line defect emanating from the spiral core. Three-arm spirals form even when the spiral tip is meandering and lead to a greater dispersion of cellular refractoriness that is proarrhythmic. Analytical results are presented that shed light on the conditions for the formation of one- and three-arm line-defect spirals in the absence of meander. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W17.00011: Multifrequency EPR study of Vanadyl and Copper complexes in the characterization of electron paramagnetic tensor parameters and dynamic parameters Indra Sahu, Laxman Mainali, Keith Earle Vanadyl acetylacetonate, Vanadyl meso-tetraphenylporphine, Cupric acetylacetonate, and Cupric meso-tetraphenylporphine have been studied at S-, X-, K- and Q-band in the rigid limit and in the motional narrowing regime. Data have been analyzed using the Nested Sampling Algorithm developed by J. Skilling based on methods of Bayesian inference. The EasySpin software package is used to simulate the spectral fitting function used in the parameter estimation process. Two different sets of model parameters(A$_{\vert }$, A$_{-}$ , g$_{\vert }$, g$_{-}$, D$_{xy}$ , lw and A$_{iso}$, $\Delta $A, g$_{iso}$, $\Delta $g, D$_{xy}$, lw) have been used for the data analysis at the various frequencies, both independently and in a simultaneous multifrequency fit. After comparing the results at all the frequencies, it is seen that the magnetic tensor parameters defined from the individual frequency fits fluctuate more among each other in the motional narrowing regime but dynamic parameters do not. The model parameters are better fitted in the case of porphin complexes than that in acac complexes. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W17.00012: AC electrokinetics of dense inhomogeneous biological cells suspensions under nonuniform applied fields K.L. Chan, J.P. Huang, K.W. Yu When a biological cell is placed in a nonuniform AC (or DC) electric field, force would be induced because of the interaction between the induced electric dipole moment of the particle and the external electric field. This phenomenon is called dielectrophoresis (DEP). [1] In this study a new method is proposed to handle biological cells with arbitrary permittivity and conductivity profiles, and determine the importance of multipole effect as compared with the approximate point dipole calculation [2], which is valid if the external field is homogeneous. In real situations, cells often possess arbitrary graded profiles and the study of higher multipole effects can lead to a better understanding. We also extend the calculation to dense cells suspensions by the effective medium theories [3]. The study reveals significant effects on the DEP due to higher concentration. \newline \newline [1] \textbf{T. B. Jones,} \textit{Electromechanics of particles, Cambridge University Press, 1995} \newline [2] \textbf{C. Z. Fan, J. P. Huang, K. W. Yu,} \textit{J. Phys. Chem. B, 110, 25665 (2006).} \newline [3] \textbf{J. P}\textbf{. Huang}\textbf{, K. W. Yu}, \textit{Physics Reports 431, 87 (2006).} [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W17.00013: Folding of Pollen Grains Eleni Katifori, Silas Alben, Enrique Cerda, David Nelson, Jacques Dumais At dehiscence, which occurs when the anther reaches maturity and opens, pollen grains dehydrate and their volume is reduced. The pollen wall deforms to accommodate the volume loss, and the deformation pathway depends on the initial turgid pollen grain geometry and the mechanical properties of the pollen wall. We demonstrate, using both experimental and theoretical approaches, that the design of the apertures (areas on the pollen wall where the stretching and the bending modulus are reduced) is critical for controlling the folding pattern, and ensures the pollen grain viability. An excellent fit to the experiments is obtained using a discretized version of the theory of thin elastic shells. [Preview Abstract] |
Session W18: Focus Session: Dynamics of Nucleic Acid-Protein Interactions
Sponsoring Units: DPOLY DBPChair: Ferenc Horkay, National Institutes of Health
Room: Morial Convention Center 210
Thursday, March 13, 2008 2:30PM - 3:06PM |
W18.00001: Single-Molecule Dynamics of a DNA Aptamer Targeting VEGF Protein Invited Speaker: Single-molecule fluorescence resonance energy transfer (SMFRET) and SMFRET autocorrelation analysis were used to examine structural fluctuations in a DNA aptamer that binds and exhibits inhibition activity towards the vascular endothelial growth factor (VEGF) protein, a protein that is involved in macular degeneration. The aptamer's most stable conformation contains 4 Watson-Crick base pairs, resulting in only a shallow negative potential relative to the unfolded state. The weakly stable folded state and the manifold of unfolded structures quickly interconvert. In contrast, in the presence of the VEGF target, the aptamer folding rate constant decreases and the fluctuations in both the unfolded and folded states decrease in frequency, but are not eliminated. A possible relationship between activity and aptamer flexibility is discussed [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W18.00002: Thermal Disorder Effect on the DNA Electronic Structure Alexander Balaeff, Elizabeth Hatcher, Shahar Keinan, Ravindra Venkatramani, David Beratan We address the effect of the thermal dynamics of DNA structure on the energy and localization of the DNA electronic orbitals. Structural ensembles are generated for several DNA sequences by molecular dynamics simulations employing CHARMM and AMBER force fields. In the shortest sequences studied (CATG, GAAG, GATG, GAG), the highest occupied molecular orbitals (HOMOs) expectedly reside on the guanines (Gs), yet in a significant number of the structures the orbitals are observed to be delocalized between the Gs and the bridging adenines (As). Adding more Gs to the ends of the sequence expectedly shifts the orbitals toward the G clusters, yet the amount of orbital delocalization to the bridge is still significant. These observations suggest that a) contrary to the predominant view in the field, G-to-A thermal hopping may contribute significantly to the charge transfer in DNA even in the short-distance range, and b) the contribution of DNA structural fluctuations to triggering the charge transfer is as significant as that resulting from the ion gating mechanism. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W18.00003: Thermodynamic Restriction on Evolutionary Optimization of Transcription Factor Proteins Alexander Grosberg, Longhua Hu, Robijn Bruinsma Conformational fluctuations are believed to play an important role in the process by which transcription factor proteins locate and bind their target site on the genome of a bacterium. Using a simple model, we show that the binding time can be minimized, under selective pressure, by adjusting the spectrum of conformational states so that the fraction of time spent in more mobile conformations is matched with the target recognition rate. The associated optimal binding time is then within an order of magnitude of the limiting binding time imposed by thermodynamics, corresponding to an idealized protein with instant target recognition. Thus, we claim that it is possible for the overall binding rate of a transcription factor to approach the theoretical limiting value but only by a suitable choice of energy spectrum of conformational sub-states, and only if the dimensionless binding rate is of the order of one, or larger than one, where dimensionless binding rate is determined as the product of binding rate and the average time spent by the protein on one DNA base pair in one tour of 1D sliding along DNA. Numerical estimates suggest that typical bacteria operate in this regime of optimized conformational fluctuations. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W18.00004: Electron affinities of nucleobases, glycine and their complexes Ed S. Chen, Edward C. Chen The electron affinities of adnenine, guanine, and the amino acids except for glycine have not been measured in the gas phase. New valence state electron affinities of the subject molecules are reported from reduction potentials and literature anion photoelectron spectra. These are supported by quantum mechanical calculations. Multiple negative ion potential energy curves are calculated to consolidate reduction potentials, electron impact spectra in helium nanodroplets, negative ion mass spectra, electron transmission spectra, electron spin resonance data for DNA, A method of measuring negative valence state electron affinities using ESR data is proposed. The adiabatic electron affinities are: in eV~ Adenine, 1.08(5), Guanine, 1.65(10), Cytosine, 1.04(5), Thymine, 0.98(5), Uracil, 0.99(5), Glycine, 0.50(5) Adenine: thymine 1.40(5) eV. Excited dipole bound and valence state electron affinities are also identified. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W18.00005: Chemical physics of DNA packaging in a nucleosome core particle Andrew Spakowitz, Bariz Sudhanshu The fundamental unit of packaged DNA, the nucleosome core particle, contains 146 base pairs of DNA wrapped 1.7 times around a cationic protein complex called the histone octamer. A string of nucleosomes is organized into higher-order structures at several hierarchical levels to form chromatin, a remarkable complex that is compact yet maintains accessibility for gene expression. We develop a theoretical model of the nucleosome core particle in order to extract detailed quantitative information from single-molecule measurements of a single nucleosome under tension. We employ the wormlike chain model to describe the DNA strand as a thermally fluctuating polymer chain. The chain adsorbs on a spool that represents the histone octamer. This model is directly compared to single-molecule experiments conducted in Carlos Bustamante's lab; we find good agreement between our theory and the experimental data. Our model reveals the mechanism that underlies structural transitions that are apparent in the experimental measurements and predicts the conditions where these transitions occur. We proceed to construct a free energy surface to predict the dynamic response in a single-molecule experiment with a time-dependent rate of unwinding the nucleosome. The combination of single-molecule experiments and our theoretical modeling gives detailed information about the specific interactions between DNA and histone proteins. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W18.00006: DNA analysis in polymer nanofluidic devices Lasse Thamdrup, Anna Klukowska, Anders Kristensen Inexpensive polymer biochips with nanofluidic channels, for investigating confined DNA, are presented. The biochips are fabricated by thermal imprint in polymethyl methacrylate (PMMA) using a 4 inch diameter two-level hybrid stamp. The fluidic structures were sealed using thermal polymer fusion bonding. The stamp has nanometer- and micrometer-sized protrusions defined in a thermally grown SiO$_{2}$ layer and the sol-gel process derived duromeric polymer Ormocomp respectively. A durable chlorosilane based antistiction coating was applied by molecular vapour deposition. The polymer biochips were benchmarked against conventional fused silica based devices, by extending T4 GT7 bacteriophage DNA inside the nanochannels. The measured average extension length amounts to 20{\%} of the full contour length with a standard deviation of 4{\%}. These results are in good agreement with results obtained in fused silica devices. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W18.00007: Separation of long DNA molecules through cleavage of hydrogen bonds under a stretching force Lizeng Gao, Jiamin Wu, Jianzhong Wu, Di Gao We report that long DNA molecules of different lengths can be separated under a stretching force by cleaving hydrogen bonds that tether one end of the DNA to a substrate. This separation method can be implemented with a simple direct current electric field, does not require separation matrices, and in principle has no upper limit on the length of the DNA that can be efficiently separated. We here demonstrate efficient separation of lambda DNA (48,502 base pairs) from human genomic DNA ($>$ 100,000 base pairs) using this method. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W18.00008: Self-organized DNA/F-actin gels: entangled networks of nematic domains with tunable density John Butler, Olena Zribi, Ivan Smalyukh, Ghee Hwee Lai, Ramin Golestanian, Thomas Angelini, Gerard Wong We examine mixtures of DNA and F-actin as a model system of like-charged rigid rods and flexible chains. Confocal microscopy reveals the formation of elongated nematic F-actin domains reticulated via defect-free vertices into a network, all embedded in a mesh of random DNA. Synchrotron x-ray scattering results indicate that the DNA mesh squeezes the F-actin domains into a nematic state via the osmotic pressure of uncondensed counterions, so that the inter-actin spacing within the domains decreases with increasing DNA concentration. These observations are consistent with arguments based on electrostatics and nematic elasticity. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W18.00009: Histone code or not? Combinatorial pattern analyses of histone modifications Chongzhi Zang, Weiqun Peng, Zhibin Wang, Dustin E. Schones, Artem Barski, Suresh Cuddapah, Kairong Cui, Tae-Young Roh, Keji Zhao, Jeffrey Rosenfeld, Michael Zhang Eukaryotic genomes are organized into chromatin, the structure of which plays critical role in the program of gene expression. Chromatin structure and function is regulated by a myriad of posttranslational modifications on histone tails of the nucleosomes, the fundamental unit of chromatin. It remains unclear how different modifications interact. Based on high- resolution genomic maps of close to 40 histone methylations and acetylations in human T-cells obtained experimentally by ChIP- Seq technique, we investigated the combinatorial patterns of histone modifications at gene promoter regions. We found that a very limited number of patterns dominate. Modifications within a pattern are strongly correlated and each pattern is associated with a distinct gene expression distribution. Our results suggest that it is the patterns rather than the individual modifications that affect the downstream readout. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W18.00010: Changes of histone modification landscape in cell differentiation Weiqun Peng, Chongzhi Zang, Kairong Cui, Tae-Young Roh, Dustin Schones, Keji Zhao During eukaryotic cell differentiation chromatin structure undergoes important changes, as manifested by extensive alterations in histone modifications. It is hypothesized that the profile of these epigenetic markers serves as a signature of the cell identity. To test this, we analyzed high-resolution genomic maps of histone methylations during differentiation of pluripotent hematopoietic stem cells into erythrocyte precursor cells. Our results indicate significant changes in both the dominant patterns of histone modifications and the genes inside the patterns after differentiation. Our results suggest that certain modifications prepare the chromatin for future activation in stem cells and their erasure results in a permanent inactivation of associated genes in differentiation-committed cells. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W18.00011: Biophysical modeling of transcription initiation by bacterial RNA polymerase Marko Djordjevic RNA polymerase (RNAP) is a central enzyme in cell, which is responsible for gene transcription. As a first step of transcription initiation, RNAP binds to double stranded DNA and opens the two strands of DNA, which is referred to as the open complex formation. We will present the first quantitative model of the open complex formation by bacterial RNAP. The model is based on statistical physics and establishes an explicit relationship between the rate of transcription initiation and physical properties of promoter sequence and promoter-RNAP interactions [1]. The model leads to a very good agreement with the experiments, with no free parameters used in model testing. This agreement strongly supports both the quantitative model that we present and a qualitative mechanism on which the model is based. Bioinformatics applications of the presented work will also be discussed [2]. [1] M Djordjevic and R Bundschuh, under revision in \textit{Biophys. J.}, 2007 [2] M Djordjevic, to be submitted, 2007. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W18.00012: Nucleosome Positioning and Epigenetics David Schwab, Robijn Bruinsma The role of chromatin structure in gene regulation has recently taken center stage in the field of epigenetics, phenomena that change the phenotype without changing the DNA sequence. Recent work has also shown that nucleosomes, a complex of DNA wrapped around a histone octamer, experience a sequence dependent energy landscape due to the variation in DNA bend stiffness with sequence composition. In this talk, we consider the role nucleosome positioning might play in the formation of heterochromatin, a compact form of DNA generically responsible for gene silencing. In particular, we discuss how different patterns of nucleosome positions, periodic or random, could either facilitate or suppress heterochromatin stability and formation. [Preview Abstract] |
Session W19: Dopants and Defects in Semiconductors IV
Sponsoring Units: DMPChair: Matthew McCluskey, Washington State University
Room: Morial Convention Center 211
Thursday, March 13, 2008 2:30PM - 2:42PM |
W19.00001: The role of nitrogen vacancies and hydrogen in conductivity of InN Anderson Janotti, Chris G. Van de Walle Using first-principles methods we investigate the electronic properties and stability of the nitrogen vacancy and monatomic hydrogen in InN. We find that nitrogen vacancies act as shallow donors, but they have high formation energies in $n$-type InN. Therefore, N vacancies are unlikely to cause the observed unintentional $n$-type conductivity in as-grown InN. Hydrogen can occupy interstitial as well as substitutional sites in InN. Interstitial hydrogen has low formation energy, is stable in the bond-center configuration, and acts exclusively as a shallow donor (H$_i^+$). The calculated frequency of the H-N stretching mode is 3050 cm$^{-1}$. Hydrogen can also substitute for nitrogen in InN, bonding equally to the four In nearest neighbors in a multicenter-bond configuration [1]. Substitutional hydrogen also has low formation energy and, counterintuitively, forms a double-donor center. Our results suggest that monatomic hydrogen is a plausible cause of the unintentional $n$-type conductivity that is almost universally observed in as-grown InN.\newline [1] A. Janotti and C. G. Van de Walle, Nature Mater. {\bf 6}, 44 (2007). [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W19.00002: Thermopower measurements of n- and p-type InN J. W. Ager III, N. R. Miller, R. E. Jones, W. J. Schaff, W. Walukiewicz InN has the largest electron affinity, 5.8 eV, of any known semiconductor. At its surface, the Fermi level is pinned ca. 0.9 eV above the conduction band edge, leading to an electron accumulation layer in n-type material and an inversion layer in p-type material. Recently, we have used capacitance-voltage measurements with an electrolyte contact to deplete the surface inversion layer in Mg-doped InN and observe space charge due to ionized acceptors {[}1{]}. However, these measurements do not give information about the proportion of acceptors that are ionized, or about hole transport. Here, thermopower measurements are used to deduce the majority carrier type under the surface inversion layer in Mg-doped InN. Observation of a positive Seebeck coefficient provides direct and definitive evidence of mobile holes in InN:Mg. Temperature-dependent measurements from 200-300 K are consistent with degenerate conduction in the highly doped films. Modeling of the experimental data to determine the carrier effective masses and scattering mechanisms will be presented. {[}1{]} R. E. Jones \emph{et al.}, \emph{Phys. Rev. Lett.} \textbf{96}, 125505 (2006). [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W19.00003: Site Selective CEES and Nearfield Optical Spectroscopy on Nd:GaN N. Jha, P. Capek, V. Dierolf, E. Readinger, G. Metcalfe, H. Shen, M. Wraback We perform spatially resolved CEES on Nd ions that are in-situ-doped into GaN epitaxial films on c-plane sapphire substrate grown by plasma assisted molecular beam epitaxy. For a wide range of concentration (up to 8{\%}) we find in the emission a dominant incorporation site, which can be identified with good certainty as a substitutional `Ga' site. While resonant excitation yields strong emission signals even at high temperatures, indicating good intrinsic quantum efficiency, above bandgap excitation of the dominant incorporation site is rather inefficient. The conclusion is further supported by the observation of additional weak peaks in the spectra attributed to above band-gap excitation of minority sites (with presumably better excitation efficiencies). For the majority site, confocal and NSOM imaging under selective excitation show changes in emission intensity, excitation and emission wavelength on a submicron length scale. These observations are consistent with an interpretation that the changes are due to fluctuations in Nd-concentration that create fluctuation in the local strain fields that are caused by the substitution of the small Ga ion by a larger Nd ion (0.62A vs 0.99A). [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W19.00004: Excitation mechanisms of rare-earth ions in GaN Z. Fleischman, S. Penn, L. Maurer, Z. Dong, V. Dierolf Understanding the excitation mechanisms of rare earth ions in GaN is key to achieving more efficient EL emission from devices based on this material system. For that purpose, we performed site-selective photo- and cathodo-luminesence spectroscopy on Eu-doped GaN layers. We identified 8 different Eu environments, which are excited after the creation of electron-hole pairs through excitation channels with drastically different transfer efficiencies. In particular, we find one majority site and one minority site that is strongly coupled to an intrinsic GaN defect. For these most pronounced cases, resonant excitation shows that the majority site is 6 times more abundant than the defect-related site, while saturated CL data show only a factor of 3. This indicates that not all of the majority site ions are able to be excited electrically. We explain this and other observations with a model for the Eu excitation mechanism that involves two different intrinsic GaN defects acting as intermediate traps. We determined that the most efficiently excitable Eu are those near such a defect trap. Overall, this work shows that a higher population of defect traps in GaN is desirable for the most efficient rare earth luminescence. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W19.00005: First principles calculations for Gd doped GaN Chandrima Mitra, Walter Lambrecht Gd doped GaN has been reported by Dhar et al. to have magnetic moments of order a few 1000 $\mu_B$ per Gd in the very dilute limit of $10^{15}$ Gd/cm$^3$ and to show above room temperature ferromagnetism. Here we present first principle electronic structure calculations to study the spin splitting of the conduction band with varying concentration of Gd in GaN. Our calculations show that the spin splitting varies linearly with the concentration of Gd which suggests an almost zero splitting if one were to extrapolate to the 1 ppm dilute concentration of Gd. Thus the large magnetic moments cannot be explained simply by assuming donor electrons (for example from oxygen) will fill the spin-split conduction band. The spin polarization of the Ga and N atoms around Gd atom in a supercell of 1.5\% Gd were found to be small and to become negligible beyond second nearest neighbors. In these, studies, we either added oxygen or Si as co-dopants or a background charge to fill the spin-split conduction band. This indicates that the proposed model of Dhar of polarization of the host is not supported by our calculations. The magnetic exchange interaction parameter, for nearest neighbour Gd atoms have also been calculated by mapping the energy differences between the ferromagnetic and antiferromagnetic arrangement onto the Heisenberg's model. Effects of strain, supercell size and shape, and other dopants on the exchange interactions were investigated. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W19.00006: Inclined Dislocation Pairs in Green GaInN/GaN Light Emitting Diodes Grown on Bulk GaN Substrate Mingwei Zhu, Theeradetch Detchprohm, Shi You, Yong Xia, Wei Zhao, Yufeng Li, Jayantha Senawiratne, Christian Wetzel, Lianghong Liu, Edward Preble, Drew Hanser Inclined dislocation pairs (IDPs) were observed for the first time in the active region of ten-quantum-well green GaInN/GaN light emitting diodes grown on c-plane bulk GaN substrate. The two inclined dislocations (IDs) in one IDP usually start closely in the first three quantum wells and propagate to different sides of [0001] growth direction. By analyzing transmission electron microscopy images of IDs at different projection directions and reconstructing them in the three dimensional crystal, all IDs were found to tilt from the [0001] direction towards the $<$1-100$>$ directions by 18 - 23$^{\circ}$. The two IDs in one IDP tilt to equivalent $<$1-100$>$ directions separated by either 120$^{\circ}$ or 180$^{\circ}$. All of the IDs are edge-type with a Burger vector 1/3$<$11-20$>$. The creation of IDPs might help to release the strain accumulated in the active region. In spite of the existence of these IDPs in the active region, this green LED on GaN shows a 7-fold stronger photoluminescence than those on sapphire. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W19.00007: Electrical activation studies of Al$_{0.4}$Ga$_{0.6}$N and Al$_{0.5}$Ga$_{0.5}$N implanted with silicon for n-type doping. Elizabeth Moore, Yung Kee Yeo, Mee-Yi Ryu, Robert Hengehold A systematic electrical activation study of Si-implanted Al$_{x}$Ga$_{1-x}$N with Al concentrations of 40 and 50{\%} grown on sapphire substrates by MEMOCVD has been made as a function of ion dose and anneal temperature. The silicon ions were implanted at 200 keV with doses from 1x10$^{14}$ to 1x10$^{15}$ cm$^{-2}$ at room temperature. The samples were proximity cap annealed from 1150 to 1350 $^{o}$C for 20 minutes in a nitrogen environment. Hall-effect measurements were made from 10 to 700 K and cathodoluminescence measurements were taken at 7 K. Electrical activations of nearly 100{\%} were obtained for the Al$_{0.4}$Ga$_{0.6}$N:Si after annealing at 1350 \r{ }C for 20 minutes for doses of 1x10$^{14}$ and 5x10$^{14}$ cm$^{-2}$ and after annealing at 1200 \r{ }C for 20 minutes for the dose of 1x10$^{15}$ cm$^{-2}$. The Al$_{0.5}$Ga$_{0.5}$N:Si also had high activations of nearly 100{\%} for the two lower doses after annealing at 1300 \r{ }C for 20 minutes, while for a dose of 1x10$^{15}$ cm$^{-2}$, an activation of 66{\%} was obtained after the same annealing treatment. The highest room temperature mobility for the Al$_{0.4}$Ga$_{0.6}$N and Al$_{0.5}$Ga$_{0.5}$N samples are 61 and 55 cm$^{2}$/V$\cdot $s, respectively for the samples annealed at 1350 \r{ }C for 20 minutes. CL spectra support the electrical results in determining the optimal annealing conditions. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W19.00008: Effects of Dopants and Annealing on the Structure and Electronic properties of GaAsN Yu Jin, Matthew Reason, Hailing Chen, Cagliyan Kurdak, Rachel Goldman In this work, we investigate the effects of different n-type dopants and rapid thermal annealing (RTA) on the structure and electronic properties of GaAsN bulk-like films grown by molecular beam epitaxy. For as-grown GaAsN:Si and GaAsN:Te films, similar free carrier densities (n) and electron mobilities ($\mu )$ are observed. However, after post-growth RTA, a substantial increase in both n and $\mu $ is observed in the GaAsN:Te films, with negligible change in those of the GaAsN:Si films. Apparently, RTA reduces the concentration of N-related trapping and scattering centers in GaAsN:Te. On the other hand, the annealing process enhances the diffusion of Si, presumably leading to the formation of additional N$_{As}$-Si$_{Ga }$defect complexes. For both GaAsN:Te and GaAsN:Si films, x-ray rocking curves reveal reduced lattice parameters following annealing, suggesting a decrease in the interstitial [N], which leads to the improvement in electronic properties for the GaAsN:Te films. In the case of GaAsN:Si, the reduction in interstitial [N] is presumably balanced by an increase in the density of N$_{As}$--Si$_{Ga}$ defect complexes. The effect of dopants and annealing on the structure and electronic properties of InGaAsN will also be discussed. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W19.00009: Bismuth related changes in the electronic properties of high quality dilute GaAs$_{1-x}$Bi$_{x}$. Lekhnath Bhusal, Denis Karaiskaj, Ryan France, Aaron Ptak, Angelo Mascarenhas, Tom Tiedje In this work we will present the electronic and optical properties of dilute GaAs$_{1-x}$Bi$_{x}$ epitaxial layers for the range of samples with concentration up to $\sim $3{\%}. Variation of fundamental band gap (E$_{0})$ and the transition from the spin-orbit split off valance band (E$_{0}+\Delta )$ using the contactless modulated electroreflectance will be presented as a function of temperature (77-300K) and Bi concentrations. We also will discuss the isoelectronic codoping of Bi and nitrogen, as the excellent quality of GaAsBi samples presented in the work opens the path for the codoping of Bi with N to improve the electronic properties of dilute nitride III-V alloys. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W19.00010: A defect relaxation model for the carbon vacancy in SiC Jamiyanaa Dashdorj, Mary Ellen Zvanut Deep level intrinsic defects in high purity semi-insulating SiC play in important role in electrical compensation necessary to achieve high resistivity. We use a constant light intensity approach to steady-state photo-electron paramagnetic resonance (EPR) and time-dependent photo-EPR to study the ionization cross sections and relaxation energies of the well-studied defect, the positively charged carbon vacancy, V$_{c}^{+}$. As-grown 4H-SiC samples with room temperature resistivity of 10$^{10}$ Ohm-cm were studied at 4 K and 80 K. The shape of the absorption curve for V$_{c}^{+}$, as revealed by the steady state measurements, has a peak at 2.3 eV and photo-threshold at 1.8 eV. We interpret the difference between these two values as a structural relaxation upon release of an electron from the defect to conduction band. Consistently, preliminary time-dependent measurements reveal a Franck-Condon transition at 2.3 eV for release of an electron from the defect to the conduction band, and a second Franck-Condon transition at an energy greater than 1.3 eV for capture of electron at the defect from the valence band. The difference of no more than 1 eV may be interpreted as a relaxation energy, consistent with the interpretation of steady-state data. The details of the experimental approach and the formulation of the relaxation model will be presented. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W19.00011: Direct Imaging of Point Defect Configurations for Au Atoms Inside Si Nanowires K. van Benthem, S.H. Oh, A.Y. Borisevich, W. Luo, P. Werner, N.D. Zakharov, S.T. Pantelides, S.J. Pennycook Aberration-corrected scanning transmission electron microscopy (STEM) was used to directly image individual Au atom configurations inside Si nanowires grown by Au-catalyzed vapor-liquid-solid molecular beam epitaxy. A three-dimensional analysis of the nanowire microstructure revealed Au atom concentrations five orders of magnitude higher than equilibrium values in bulk silicon. Three distinct interstitial Au atom configurations were identified in addition to the substitutional configuration. The stability of the observed point defect configurations was further investigated by density-functional theory. The observed configurations are in excellent agreement with theory. However, a comparison of the number densities of the various configurations with the calculated formation energies indicates an effective temperature of approximately 1000 degrees centigrade suggesting significant beam-induced atomic diffusion. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W19.00012: Ni doping of semiconducting boron carbide S. Adenwalla, Jing Liu, Nina Hong Semiconducting boron carbide (BC) is intrinsically p-type. The addition of Ni has been shown to dope it n-type. I-V measurements on Ni doped BC on both p-and n-type Si (1*10$^{15}$ cm$^{-3}$ and 4.5*10$^{13}$ cm$^{-3})$ indicate changes in the doping level with increasing Ni concentration. At the highest dopant level, Ni doped BC on n-type Si showed n-n+ diode characteristics. The change of doping concentration was confirmed by the built-in potential increase from 0.1 V in the low Ni doped p-n diode to 1.2 V in the high Ni-doped p-n diode as well as by measurements of the reverse saturation current. The addition of Ni does not lead to significant structural changes in the BC as measured by x-ray diffraction. X-ray fluorescence data indicate an upper bound of 2 ppm for the Ni concentration. Using these results homojunction p-n diodes were fabricated from layers of undoped p-type BC and Ni doped n-type BC and characterized by I-V and capacitance-voltage (C-V) measurement. Homojunction devices are shown to be especially promising for thermal neutron detection. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W19.00013: First principles investigations into alkali intercalation of hexagonal boron nitride Bahadir Altintas, Cihan Parlak, Resul Eryigit, Cetin Bozkurt Although hexagonal boron nitride(hBN) is quite similar to graphite structurally, it has been very difficult to obtain any intercalation compound of hBN while there are hundreds of graphite intercalation compounds. We have investigated the possible intercalation of hBN by alkali atoms in the density functional theory framework by using pseudopotentials and plane-wave basis. The structural, electronic and lattice dynamical properties of hypothetical hBN analogues of LiC$_{3}$, LiC$_{6}$, KC$_{8}$, CsC$_{8}$, CaC$_{6}$ are calculated.We have found that although the electronic structure, band structure and fermi surface of alkali intercalated hBN is very similar to that of alkali intercalated graphite, the lattice dynamics show a set of negative frequency modes which indicate that alkali-intercalated hBN is not stable. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W19.00014: Effects of point defects on the electrical properties of aluminum antimonide: a first principles investigation Vincenzo Lordi, Daniel {\AA}berg, Paul Erhart A first principles study is conducted of the effects of point defects on the electrical properties of bulk AlSb, a material of interest for room temperature gamma radiation detection. Detailed calculations were performed for all native defects, including vacancies, antisites, interstitials, and split interstitials, and also for a variety of impurities (H, C, Si, Ge, Sn, P, O, S, Se, Te). Formation energies of each defect in different charge states were calculated to determine the equilibrium defect density and net carrier density. Carrier scattering rates for each defect were calculated using perturbation theory to determine the effects on electron and hole transport. The most detrimental, as well as innocuous, defects were identified. Relative solubilities of the impurities were examined along with their scattering rates to find efficient dopants that minimize mobility degradation. Finally, carrier trapping cross sections and energy levels were calculated to study the role of each defect in deep level trapping. [Preview Abstract] |
Session W20: Oxide Surfaces, Interfaces and Thin Films
Sponsoring Units: DCMPChair: Arthur Smith, Ohio University
Room: Morial Convention Center 212
Thursday, March 13, 2008 2:30PM - 2:42PM |
W20.00001: Electronic structure of epitaxial CrO$_{2}$(100) and CrO$_{2}$(110) films C. A. Ventrice Jr, H. Geisler, D. R. Borst, G. X. Miao, A. Gupta Half-metallic ferromagnets are conducting solids whose conduction electrons undergo magnetic ordering with a spin polarization of 100{\%} at 0 K. Although CrO$_{2}$ is predicted to be a half-metallic ferromagnet, previous attempts to make devices using CrO$_{2}$ have resulted in a degradation of performance instead of an enhancement. Using ultra-violet photoelectron spectroscopy at the CAMD synchrotron, we have measured the electronic properties of epitaxial CrO$_{2}$(110)/TiO$_{2}$(110) and CrO$_{2}$(100)/TiO$_{2}$(100) surfaces grown using a CrO$_{3}$ precursor. Clean, stoichiometric CrO$_{2}$ surfaces have been prepared either by exposure to white light, which desorbs OH groups from the surface, or by sputtering and annealing in O$_{2}$. The measurements of the CrO$_{2}$ surfaces show no emission at E$_{F}$ after sputtering and annealing the surfaces in oxygen. However, the white light prepared surfaces show a small density of states at E$_{F}$. Photon energy dependent photoemission experiments show no increase in the density of states at E$_{F}$ as the photon energy is lowered from 50 eV to 15 eV, which increases the bulk sensitivity of these measurements. These results indicate that CrO$_{2}$ behaves more like a semi-metal than a half-metal and that surface disorder can induce a semi-metal to semiconductor transition at its surfaces. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W20.00002: Variation of Growth Mode with Orientation for Epitaxial CrO$_{2}$ Films Krishna Chetry, Hunter Sims, William Butler, Arun Gupta Thin films of CrO$_{2}$ have been grown epitaxially on (110) and (100) oriented TiO$_{2}$ substrates. CrO$_{2 }$(100) films grow in a layer by layer mode, while an island growth mode is observed for CrO$_{2}$(110) films as confirmed from atomic force microscopy (AFM) studies. To better understand the differences in the growth mode, we have performed first principles-based calculations using density functional theory implemented within the VASP code to study the surface and interface energies of CrO$_{2}$ (100), (110), TiO$_{2}$ (100) and (110) systems. For these calculations a periodic repeating slab geometry is used with a sufficient vacuum width and thickness to converge the surface energy within 0.01J/m$^{-2}$ From our calculations we find that in case of (110) orientation $\sigma _{TiO_2} >\sigma _{CrO_2 } +\gamma $, where $\sigma $ is the surface energy and $\gamma $ is the interface energy between CrO$_{2}$ and TiO$_{2}$ system. This result is consistent with the island growth mode observed experimentally for (110) orientation. For the case of (110) orientation also we find that $\sigma _{TiO_2} <\sigma _{CrO_2 } +\gamma $ , which does not match with our experimental results. We speculate that formation of some oxygen deficient phase of chromium oxide in the very first monolayer, which then gets converted to CrO$_{2}$ by accepting oxygen from the second layer, favors the layer-by-layer growth mode in CrO$_{2}$ (100). [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W20.00003: Self-Assembled Epitaxial Multiferroic Nanocomposite Films Prepared by Polymer-Assisted Deposition Hongmei Luo, Hao Yang, Eve Bauer, T. Mark McCleskey, Anthony K. Burrell, Quanxi Jia Multiferroic materials, which show simultaneous electric and magnetic ordering, have attracted considerable interest recently due to their unusual physical properties and potential device applications. Here we demonstrate that a cost-effective chemical solution approach of polymer-assisted deposition (PAD) is a very promising technique to grow self-assembled epitaxial multiferroic nanocomposite thin films: such as BaTiO$_{3}$-NiFe$_{2}$O$_{4}$ films grown on (001)-oriented LaAlO$_{3}$ substrate. X-ray diffraction (XRD), atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM) analyses show clear epitaxial relationship between the two phases and the substrate. The ferroelectric BaTiO$_{3}$ grains are embedded in the ferrimagnetic spinel NiFe$_{2}$O$_{4}$ matrix. The composite films exhibit both ferroelectric and ferrimagnetic properties. The structure and properties will be discussed and compared with the nanocomposite films prepared by pulsed-laser deposition (PLD) method. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W20.00004: Structural and electrical properties of self-assembled (BiFeO$_{3})_{0.5}$:(Sm$_{2}$O$_{3})_{0.5}$ nanocomposite films Hao Yang, H. Wang, J.L. MacManus-Driscoll, Q.X. Jia Self-assembled (BiFeO$_{3})_{0.5}$:(Sm$_{2}$O$_{3})_{0.5}$ nanocomposite films were deposited on (001) SrTiO$_{3}$ and Nb-doped SrTiO$_{3}$ substrates by pulsed laser deposition using a single uniformly mixed target. Analysis from both high-resolution X-ray diffraction and transmission electron microscopy revealed self-assembled epitaxial two-phase BiFeO$_{3}$ (BFO) and Sm$_{2}$O$_{3}$ (SmO) composites in nanoscale. The BFO and SmO domains have grown alternately and vertically aligned with average column size of 10 nm. The dielectric properties of BFO:SmO nanocomposite films were investigated and compared with those of pure BFO and SmO thin films. The dielectric constant of the nanocomposites can be well described by a parallel connection of two individual dielectrics of BFO and SmO. On the other hand, the dielectric loss of nanocomposite films is lower than the theoretical value from such a parallel connection model. This might originate from the reduction of leakage current density of BFO phase in nanocomposite film due to the much larger interfacial area and the strong out-of-plane strain of each BFO column. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W20.00005: Checkerboard pattern formation in spinel oxide films Toshihiro Asada, Yoichi Horibe, Soonyong Park, Nara Lee, Makoto Tanimura, Sang-Wook Cheong A few-nanometer-size columnar superlattice with a checkerboard (CB) structure has been fabricated by harnessing Jahn-Teller structural distortions$^{1,2}$. In this talk, we will discuss the growth mechanism of the CB structures in thin films, obtained from the results of our transmission electron microscopy. Ionic diffusion for the phase separation occurs along the direction parallel to the twin boundaries in the high-temperature tetragonal phase. This anisotropic phase separation suggests the importance of the strain associated with the twin structure in the high-temperature tetragonal phase. 1. S. Yeo \textit{et al.}, Appl. Phys. Lett. \textbf{89}, 233120 (2006). 2. C. L. Zhang \textit{et al}., Appl. Phys. Lett. \textbf{90}, 133123 (2007). [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W20.00006: Thermal Stability of Epitaxial SrTiO$_{3}$ Thin Films on Si (001) G. Yong, Rajeswari Kolagani, S. Adhikari, W. Vanderlinde, L. Salamanca-Riba, Y. Liang, S. Friedrich Epitaxial SrTiO$_{3}$ on Si(001) is important for application as a high $\kappa$ (Kappa) gate oxide and is also suitable as a buffer layer for the subsequent growth of other perovskite oxide thin films to enable integration of perovskite oxide functionality with Si. For the latter application, the thermal stability of the interface structure in epitaxial SrTiO$_{3}$ thin films grown by molecular-beam epitaxy on Si (001) becomes a key issue since most of the perovskite layers are grown at relatively high temperatures (750-850\r{ }C). SrTiO$_{3}$ and Si are intrinsically thermodynamically unstable in proximity to each other, with some reduction of the rTiO$_{3}$ and oxidation of the Si to be expected. We have used x-ray diffraction and micro-analytical techniques including optical microscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to study the thermal stability of epitaxial SrTiO$_{3}$ thin films grown by molecular beam epitaxy (MBE) on Si (001). Elemental composition of the amorphous interface layer (AIL) and of the epitaxial SrTiO$_{3}$ film will change during various thermal treatments and gas environments encountered in the subsequent growth of other oxide layers. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W20.00007: Design and characterization of Pb(Zr,Ti)O$_{3}$ /CoFe$_{2}$O$_{4}$ Multilayer epitaxial thin films Nora Ortega, Ashok Kumar, Ram Katiyar Mutiferroics are a novel class of next generation multifunctional materials. Intensive research is being pursued towards the development of materials with high magnetoelectric (ME) coupling. We have fabricated epitaxial Pb(Zr,Ti)O$_{3}$--CoFe$_{2}$O$_{4}$ (PZT-CFO) multilayer (ML) thin films using pulsed laser deposition on (001) oriented lattice matched SrRuO$_{3}$/SrTiO$_{3}$ (SRO/STO) substrates. X-ray diffraction and Raman analysis revealed that PZT and CFO were in the perovskite and spinel phases respectively, in the multilayer thin films having high crystalline quality. The TEM and STEM line scan of the multilayer thin films showed that the layered structure was maintained. Magnetic hysteresis loop showed the ferromagnetic behavior of ML structure, which is independent of the ML configuration. Ferroelectric properties and temperature dependence magnetization will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W20.00008: Epitaxial oxide heterostructures on silicon Agham Posadas, J.W. Reiner, F.J. Walker, C.H. Ahn Silicon-based MOSFETs will soon be limited by the large off- state leakage current due to tunneling through the ~1 nm thick silicon oxynitride gate dielectric layer. One solution is to replace the silicon oxynitride with a high dielectric constant material, such as LaAlO$_3$, which has a relatively large dielectric constant of \~24 and band gap of 5.6 eV. We have recently grown LaAlO$_3$ epitaxially onto silicon via a transition layer consisting of SrTiO$_3$. The thickness of this SrTiO$_3$ layer is kept between 2 and 5 unit cells because of considerations of epitaxial strain and the atomic-scale interactions between the perovskite structure and the silicon substrate. The oxide heterostructures show atomically abrupt interfaces and dielectric constants close to the bulk value of LaAlO$_3$. Frequency and voltage dependent measurements of the complex impedance of the as-grown oxide heterostructures show a pinned Fermi level and a high density of interface states. Annealing at low temperatures in wet oxygen shows that the Fermi level can be unpinned, with a greatly reduced density of interface states. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W20.00009: Pulsed Laser Deposition of SrTiO$_{3}$ Thin Films: Time-Resolved X-ray Studies Joel Brock, Gokhan Arikan, John Ferguson, Arthur Woll X-ray scattering and reflectivity have been used to measure the static atomic structure of surfaces and interfaces for several decades. Modern synchrotron x-ray facilities now deliver sufficient flux to make time-resolved measurements on a wide variety of evolving surface and interface systems feasible. Here, we use time-resolved x-ray diffuse scattering studies of Pulsed Laser Deposition (PLD) of SrTiO$_{3}$ films on the (001) surface of SrTiO$_{3}$ to obtain detailed insight into the fundamental, atomic-scale growth mechanism. The data demonstrate that during layer-by-layer growth, islands are nucleated during the 1$^{st}$ pulse. The size of the pulse determines the nucleation density. During and in between subsequent pulses, the islands coarsen and coalesce. Line shape analysis of the diffuse scattering reveals that adatom capture occurs at the same time as coarsening. The time-resolved x-ray data obtained as a function of miscut further demonstrate that kinetic growth models based on adatom diffusion on singular surfaces are missing essential physical mechanisms which are step-edge density dependent. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W20.00010: Nucleation of MB$_2$ thin films on Si by low temperature CVD: island statistics and growth kinetics A. Yanguas-Gil, N. Kumar, J.R. Abelson HfB$_2$ thin films are deposited onto H-terminated Si by CVD using the single source precursor Hf(BH$_4$)$_4$, which affords extremely conformal and smooth film growth. HfB$_2$ films are technologically interesting as hard coatings and as impurity diffusion barriers. We analyze the nucleation process by in-situ spectroscopic ellipsometry (SE) and ex-situ AFM. The objective is to derive mechanistic information on the growth kinetics from a statistical analysis of the early stages of growth. The experimental results reveal that HfB$_2$ nucleates on Si(001) forming 3D islands and that there is an agreement between the SE data and the AFM measurements. Quantitative information on island statistics is extracted from AFM data taken after various growth times. The island height distribution functions are consistent with the presence of a physisorbed state of the precursor molecule. This situation is similar to that of the capture zone 2D island growth models in the literature and is in agreement with previous results on the steady state growth of HfB$_2$. The island spatial distribution and the correlation between island height and capture zone area indicate that nucleation is random and keeps on taking place until coalescence is reached. We show that the nucleation rate can be enhanced by surface activation, leading to a reduction in the grain size and lateral correlation length of the films. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W20.00011: STM study of MOCVD-grown a-plane ZnO thin films: film thickness, growth temperature, and substrate miscut effects O. Dulub, U. Diebold, G. Saraf, Y. Lu ZnO films with a-plane orientation were grown on r-plane (011bar2) sapphire substrates using metal-organic chemical vapor deposition (MOCVD). The surface morphology of ZnO films with various thicknesses (20 -- 2000 nm), growth temperatures (300 - 580$^{o}$C), and substrate miscut orientations and angles (0.2 - 2$^{o})$ was characterized by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Our results show strong dependence of surface morphology on growth parameters and the substrate miscut. STM images reveal uniform surfaces with small, rectangular terraces during the initial growth stage (20 nm-thick film). Films with thicknesses between 100 and 450 nm have a characteristic wave-like surface morphology with needle-shaped domains running predominantly along the crystallographic c-direction. Films with a thickness of 2000 nm exhibit more flat surfaces, but with c-oriented facets. We observed an increasing surface quality of the films with increasing growth temperature. Films grown at 580$^{o}$C exhibit the flattest surface morphology. Film morphologies show strong dependency on the substrate miscut angle as well as on the miscut direction at all growth conditions. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W20.00012: Avrami kinetics of oxide film decomposition on Si(100) Ikuya Kinefuchi, Hiroki Yamaguchi, Yukinori Sakiyama, Shu Takagi, Yoichiro Matsumoto The thermal decomposition of ultrathin oxide layers on silicon surface was investigated with TPD technique. Oxide layers were formed on Si(100) at 400$^{\circ}$C by exposure to O$_{2}$ molecular beam. The desorption spectrum of SiO for the initial coverages between 1.7 and 2.6 monolayers exhibits a relatively low peak at the lower temperature and a high peak at the higher temperature. The former peak corresponds to the unstable binding state, where O atoms are presumably trapped at the dangling bonds. The latter corresponds to the more stable binding states of O atoms at the dimer bridge sites and the dimer backbond sites. The most of O atoms are at the stable binding states, from which the desorption rate is well described by Avrami kinetics. This result is consistent with the reaction model which takes account of the void formation and growth as observed in STM studies. The rate-determining step is the reaction at void perimeter even if the overlap between voids becomes quite large. The Avrami exponents determined from our experiment suggest that the increase in the initial coverage makes the oxide adlayer more stable and suppresses the rate of void nucleation at the potential nucleation sites. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W20.00013: Effect of Surface Roughness on Oxidation: Changes in Scale Thickness, Composition, and Residual Stress Serif Uran, Boyd Veal, Marcos Grimsditch, John Pearson, Andreas Berger The effect of surface roughness on the properties of the oxide scale formed on Fe--Cr--Al alloys during oxidation in air at high temperatures has been investigated. Large and systematic differences in scale thickness, in the composition of the oxides forming the scale, and in the residual stress levels are found. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W20.00014: UHV Studies of Oxygen's Role in Hydrogen Sensing with a Platinum-Gate Silicon Carbide Field-Effect Device Roger Tobin, Yung Ho Kahng, Reza Loloee, Ruby Ghosh Silicon carbide-based field-effect devices with catalytic metal gates are promising as robust high-temperature gas sensors in harsh environments. We report ultrahigh vacuum studies of the gate surface chemistry of prototype Pt-SiO$_{2}$-SiC sensors. Oxygen plays a crucial role in the device's hydrogen-sensing behavior. Adsorbed oxygen can remove hydrogen by reacting to form water, which rapidly desorbs. In an oxygen-rich environment this reaction competes with the diffusion of adsorbed hydrogen to the interface bonding sites that give rise to the sensor signal. This competition reduces the sensor signal by decreasing the occupation of interface sites. The same reaction, however, is essential to the reversibility of the sensor, as oxygen is needed to fully deplete the sensor of hydrogen. Exposure to H$_{2}$S suppresses the sensor's response to alternating hydrogen and oxygen pulses, apparently by interfering with oxygen adsorption. Continued oxygen exposure, however, restores functionality by effectively removing the sulfur. [Preview Abstract] |
Session W21: Surfaces and Interfaces II
Sponsoring Units: DCMPChair: Will Castleman, Pennsylvania State University
Room: Morial Convention Center 213
Thursday, March 13, 2008 2:30PM - 2:42PM |
W21.00001: Nano-confined water on surfaces of metal oxide nanoparticles Andrey Levchenko, Juliana Boerio-Goates, Brian Woodfield, Alexander Kolesnikov, Nancy Ross, David Wesolowski, David Cole, Alexandra Navrotsky Nanolayers of water interacting with metal oxide surfaces demonstrate physical properties that are significantly different from those of bulk water and ice. Our recent water adsorption experiments suggest that the entropy of surface water is lower than those of bulk water and ice implying restricted motion of the water on the surface. We have studied dynamics of water on nanoparticles of two oxides, TiO$_{2}$ and SnO$_{2}$ by inelastic neutron scattering. Vibrational density of states for surface phonons of water confined by oxide surfaces has been calculated. Heat capacity of confined water has been measured by adiabatic calorimetry and compared with values derived from the vibrational density of states. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W21.00002: X-ray Reflectivity Study of Thermal Capillary Waves and the Interfacial Profiles of Water-Alcohol Mixtures Yoonnam Jeon, Jaeho Sung, Doseok Kim, Wei Bu, David Vaknin The liquid/vapor interfaces of water-alcohol (methanol, ethanol, and propanol) mixtures were investigated by X-ray reflectivity. Analysis of X-ray reflectivity data shows that the interfacial widths (surface roughnesses) of all mixtures at a fixed temperature depend solely on the surface tension of the mixture, and the intrinsic surface roughness is on the order of inter-atomic distances, and within error, independent of solution constituents. The implications of our results in the regard to the origin of the intrinsic roughness and the capillary wave-vector cutoffs applicable to X-ray scattering will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W21.00003: Viscoelacticity of Water in Sub-nanometer Gaps Tai-De Li, Elisa Riedo Direct and simultaneous measurements of the normal and lateral forces encountered by a nanosize spherical silicon tip approaching a solid surface in purified water are reported. For tip-surface distances, 0$\pm $0.03nm $<$ $d <$ 2nm, experiments and grand canonical molecular-dynamics simulations find oscillatory solvation forces for hydrophilic surfaces, mica and glass, and less pronounced oscillations for a hydrophobic surface, graphite. The simulations reveal layering of the confined water density and the development of hexagonal order in layers proximal to a quartz surface. For subnanometer hydrophilic confinement, the lateral force measurements show orders of magnitude increase of the viscosity with respect to bulk water, agreeing with a simulated sharp decrease in the diffusion constant. No significant viscosity increase is observed for hydrophobic surfaces. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W21.00004: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W21.00005: Study of Hydrophobic Water Interfaces with Phase-sensitive Sum-frequency Vibrational Spectroscopy Chuanshan Tian, Y. Ron Shen Self-assembled monolayer of octadecyltrichlorosilane (OTS) on fused silica has been used extensively as a representative hydrophobic surface in study of water/hydrophobic interfaces. However, the interfacial water structure and how it changes upon solvated ions are not clear. We have carried out a study on OTS/water interface using the newly developed phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS). It allows measurement of both real and imaginary parts of the surface spectral response with the latter playing a role equivalent to absorption and emission coefficients and provides information on net polar-orientations of various interfacial water species contributing to the different parts of the spectrum. The result shows that at low pH (pH$\sim $2), water molecules in both ice-like and liquid-like region have weak net polar-orientations with H pointing towards the liquid. At high pH (pH$\sim $11), they are well aligned with H pointing to the solid substrate. It is due to that OTS/silica is negatively charged at high pH because of the adsorption of OH$^{-}$ ions on methyl groups of OTS, so that the surface field will reorient part of the interfacial water molecules. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W21.00006: Electronic Structure and adsorption of Pentacene on Cu and Ag (110). Abdelkader Kara The adsorption of pentacene (C$_{22}$H$_{14})$ at coverages of one and 0.8 monolayer on Ag(110) and Cu(110) is studied using density functional theory. The unit cells for these systems are 6x2 and 7x2 for Ag(110) and Cu(110), respectively. The pentacene molecule adsorbs nearly flat (with a structural corrugation of about 0.6 ) at a position 2.5 above the surface. On Ag(110), the adsorbed pentacene is even flatter (0.45 corrugation) and sits higher (about 2.8 ) then the case of Cu. On Cu(110), most of the carbon atoms adsorb on top of Cu atoms, which is not the case on Ag(110). The resulting changes in the electronic states and the nature of the bonding will be discussed and comparison between the two systems will be presented. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W21.00007: Plasmonic Coupling of a Gold Colloid and a Gold Film Albert Chang, Fei Le, Felicia Tam, Naomi Halas, Peter Nordlander, Kevin Kelly Engineering of plasmon resonances is important for a variety of applications, including but not limited to surface enhanced Raman spectroscopy (SERS) and near field scanning optical microscopy (NSOM). Of special interest are systems where the plasmons of a nanoparticle and a thin gold film are coupled.~ This coupling allows for a greater degree of control of the plasmon resonance of the system, as well as a strong, localized, enhancement of the incident electric field.~ We demonstrate that this coupling, and the resulting enhancement, can be used for SERS applications, and explore its impact on potential NSOM applications. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W21.00008: Dipole interactions between dielectric spheres in AC electric fields Manish Mittal, Eric Furst Rheological properties of Electrorheological (ER) suspensions change dramatically on application of electric field. One of the key issues in the study of ER fluids is the nature and strength of the forces between suspended particles. Micron-sized dielectric spheres aggregate to form linear chains on the application of an AC electric field. The dipole-dipole attraction is the dominant force in this process. The dipole moment has contributions from Maxwell-Wagner charge distribution and the double layer polarization. Using optical tweezers the force between a pair of polystyrene spheres has been measured by observing the displacement of particle held in a static optical trap, of known trap stiffness, from its equilibrium position. At a fixed salt concentration, frequency and electric field strength the radial and tangential force have been measured as a function of the center-to-center separation (r) and angle ($\theta )$ with the electric field to create a 2-dimensional force map. Such a complete 2-dimensional interaction profile of micron-sized particles has never been measured before. Subsequently the effect of field, frequency and background salt concentration was studied. It was found that adding salt and increasing field frequency suppressed dipolar interactions. This effect can be explained qualitatively by the double layer polarization theory. Finally the effect of particle geometry was studied by measuring interactions between particles of different size and shape. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W21.00009: Gap-mode enhanced Raman scattering of organic monolayers on flat Au(111) surface Katsuyoshi Ikeda, Norihiro Fujimoto, Kohei Uosaki A metal-molecular junction is recognized as a fundamental issue of molecular-based nano-devices. Charge transport through organic layers is strongly affected by chemical and physical properties of metal-molecular interfaces. In order to investigate molecular structures adsorbed on metal surfaces, surface-enhanced Raman scattering (SERS) is widely utilized as a powerful spectroscopic tool. Because of the electromagnetic origin of SERS, however, conventional SERS spectroscopy is applicable only for ``rough'' metal surfaces. Since various adsorption sites are exposed on rough metal surfaces, it is difficult to obtain intrinsic information of metal-molecular junctions. Here, we provide a simple method of enhanced Raman spectroscopy for organic monolayers on ``flat'' metal surfaces based on gap-mode plasmon excitation. The gap-mode enhanced Raman spectra measured on ``flat'' Au(111) single crystal facets were compared with conventional SERS spectra, and adsorption site dependence of molecular structures was discussed. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W21.00010: Cooperative plasmon-mediated fluorescence of molecules near a metal nanoparticle V. N. Pustovit, T. V. Shahbazyan We study radiative and nonradiative decays of an ensemble of molecules attached to a metal nanoparticle. We show that when the system size is smaller than the radiation wavelength, the excited molecular dipoles are hybridized with each other via the nanoparticle surface plasmon, leading to cooperative plasmon-mediated emission similar to Dicke superradiance. In particular, an ensemble of N molecules located at random positions but at the same distance from nanoparticle surface has only 3 bright (superradiant) eigenstates each characterized by the single-molecule plasmon-enhanced radiative decay rate multiplied by approximately N/3, while the remaining N-3 states are optically dark (subradiant). The fluorescence quenching by the nanoparticle exhibits a similar behavior, with bright states having single-molecule nonradiative decay rate multiplied by the same factor and dark states having much longer but still finite non-radiative lifetime due to contribution of higher angular momenta. As a result, the radiation power of an ensemble is thrice that of a single molecule near a nanoparticle irrespective to total number of molecules. Calculations were performed for both perpendicular and parallel dipole orientations with respect to the nanoparticle surface. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W21.00011: Anisotropic mass enhancement factors of $\bar {\Gamma }$ state on Be(0001) surface TeYu Chien, Emile Rienks, Maria Jensen, Asier Eiguren, Eugene Chulkov, Philip Hofmann, Ward Plummer It is controversial on the values of mass enhancement factors, $\lambda $, of Be(0001) $\bar {\Gamma }$ state. Three possible explanations are: (1) $\lambda $ is anisotropic along the Fermi circle; (2) method-based difference for extracting $\lambda $; (3) failure of theoretical model for capturing EPC features. We demonstrate a systematically survey of $\lambda $ along the Fermi circle of Be(0001) $\bar {\Gamma }$ state. By adopting different methods to extract $\lambda $, the possibility of method-based inconsistency was ruled out. The trend of the anisotropic $\lambda $ is clear and is confirmed by theoretical calculations, though the values are inconsistent with experiment. With model data simulation, we are confident about the extracted $\lambda $ while we have noisy data. The possible explanation is that the DFT-LDA calculation can not catch the features of EPC on Be(0001) $\bar {\Gamma }$ state correctly. This work is financially supported by National Science Foundation (grants NSF-DMR-0451163). [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W21.00012: Electron-phonon induced complex quasiparticles in the 1x1 H/W(110) surface. Asier Eiguren, Claudia Ambrosch-Draxl We show that the solution of the complex Dyson equation for the electron-phonon problem induces several quasiparticle states for a given wave vector. The Dyson equation is considered in the full complex plane and it is solved without considering the imaginary part of the self-energy as an small parameter. By a first principle application of the formalism to the 1x1 H covered W(110) surface, we show that some aspects of the surface band splitting [Rotenberg et. al., Phys. Rev. Lett. \textbf{84}, 2925 (2000)] can be traced back to electron-phonon coupling, where we present the energy and lifetimes of each quasiparticle. Despite this breakdown of the single quasi-particle picture, it is remarkable that the spectral functions are very well Represented by the predicted multiple quasi-particles. From these results, we can deduce that some of the features that previously where prescribed in ARPES spectra as \textit{incoherent structure} could eventually be re-interpreted as contributions from additional quasi-particle states. Our method could also help to understand similar phenomena observed in high T$_{c}$ cuprates and various other surfaces. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W21.00013: Light-enhanced diffusion at the solid-liquid interface Sung Chul Bae, Janet Wong, Steve Granick Positively-charged rhodamine 6G molecules were allowed to adsorb onto quartz and mica surfaces and their translational and rotational diffusion was studied simultaneously by combined fluorescence correlation spectroscopy and time-correlated single photon counting. Surprisingly, the surface translational diffusion coefficient increased in direct proportion to the laser power used to excite these dye molecules. To elucidate the diffusion mechanism, we have investigated the excitation wavelength dependence, the spatial position dependence of diffusion coefficients, and the correlations between rotation and translation motion. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W21.00014: Epitaxial Growth of Quinacridone Derivative on Ag(110) Xiaobo He, Jinming Cai, Dondxia Shi, Werner A. Hofer, E. Ward Pluumer, Hongjun Gao The growth behavior of quinacridone derivative (QA16C) molecules on Ag(110) surface is studied using low-temperature scanning tunneling microscopy and low energy electron diffraction. At low coverage two distinct molecular orientations can be observed on silver terraces. At higher coverage up to 1 monolayer, depending on the growth temperature we observe two different structures on Ag(110) surface. At 100K the molecules organize in a complete monolayer on the surface, with a network-like structure. At 300K, the molecular orientation on the surface gives rise to row-like ordering, with a substantially higher molecular density. A theoretical analysis reveals that the structure of the molecular layer is controlled by the competition between molecular deposition rates and molecular diffusion along the surface. [Preview Abstract] |
Session W22: Focus Session: Organic Photovoltaics and LEDs
Sponsoring Units: DMP DPOLYChair: Chang Ryu, Rensselaer Polytechnic Institute
Room: Morial Convention Center 214
Thursday, March 13, 2008 2:30PM - 2:42PM |
W22.00001: Probing Photoconductivity in Phthalocyanines by Terahertz Spectroscopy Chen Xia, Brian Kubera, Volodimyr Duzhko, Hefei Shi, Kenneth Singer, Jie Shan Liquid crystals (LCs) are a relatively new class of photoconductors. Surprisingly high carrier mobilities have lately been reported in LCs. Although the high molecular order in these systems has been recognized to play an essential role in the high carrier mobilities, the mechanism of charge transport and carrier photogeneration are still not well understood. In this work, we investigate phthalocyanine (Pc) derivatives belonging to a family of discotic LCs as a model system. Optical pump/terahertz probe spectroscopy was employed to measure frequency dependence of the photoconductivity in Pc from 0.2 to 2.5THz. Photoconductivity appears within $\sim $1ps (limited by time resolution of setup) after photoexcitation. It is followed by a fast decay of a few ps and a slow decay of 10's ps. Distinct frequency dependences were observed in the polycrystalline and liquid crystalline phases. The mechanism of charge transport and free carrier generation and recombination in the material will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W22.00002: Solution Processed Carbon Nanotube /PMMA Nano Composite Infrared Photodetectors Yi Liu, Liwei Liu, Paul Stokes, Qun Huo, Saiful I. Khondaker Solution processable nanostructured materials are of great interest for electronic and optical devices because of their enhance functionality, easy processibility, flexibility, and low cost of fabrication. We tested multi walled carbon nanotube networks dispersed in poly (methyl methacrylate) (PMMA) matrix for use as the infrared (IR) photodetectors at room temperature in ambient condition. Our study reveals both negative and positive infrared response depending upon the dark conductivity of the composite. The temperature dependence of resistance, photo intensities and bias voltages dependence of IR response will be presented. This work shows promising novel route for the fabrication of infrared bolometric photo detector based on solution-processed carbon nanotube/PMMA composites. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W22.00003: Triplet excitons in a ladder-type conjugated polymer: application in organic optoelectronics K. Yang, M. Arif, S. Guha Trace concentrations of metallic impurities present at the ppm level in conjugated polymers allow photophysical studies of triplet states at room temperature. We present temperature- dependent photoinduced absorption (PIA) and photoluminescence studies of diaryl (diphenyl)-substituted ladder-type poly (paraphenylene) (PhLPPP) containing a trace concentration of covalently bound Pd atoms. The T$_1$-T$_N$ peak ($\sim$1.3 eV) observed in the PIA blue shifts with increasing temperatures at a rate higher compared to the blue shift of singlet excitons in the sample with increasing temperatures. The temperature shift of the PIA signal arises both due to the temperature dependence of the triplet mobility as well as its mean free path. Our observations suggest that with increasing temperatures the triplet excitons remain localized on smaller chain segments. A triplet lifetime of $\sim$12 ms was determined at room temperature. Furthermore, we will discuss the application of triplet state enhanced polymers in photovoltaic applications. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W22.00004: Photovoltaic Effect in a Composite involving the Nonconjugated Conductive Polymer, Poly($\beta $-pinene) and C$_{60}$ Aditya Kumar Palthi, Ananthakrishnan Narayanan, Mrinal Thakur Photovoltaic effect in a composite involving a nonconjugated conductive polymer, poly($\beta $-pinene) and C$_{60}$ will be reported. The photovoltaic cell was fabricated using indium tin oxide coated glass as one electrode and aluminum as the other, with a poly($\beta $-pinene)-C$_{60}$ composite film sandwiched between the electrodes. Nitrogen laser (emission at 325 nm) and illuminant white light bulbs (emission at 300-700nm) were used as the light sources and the photo-voltage produced was recorded for different light intensities. The photo-voltage produced had a linear dependence on the light intensity. About 100 mV was generated for an intensity of $\sim $ 4mW/cm$^{2}$. Pristine poly($\beta $-pinene) has a photoluminescence peak at 360 nm for excitation at 280 nm. As we have observed, this photoluminescence is quenched when C$_{60}$ is added to poly($\beta $-pinene) to form the composite. Therefore, the observed photovoltaic effect appears to be a result of excited-state electron transfer from poly($\beta $-pinene) to C$_{60}$. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W22.00005: Femtosecond and CW transient studies of photoinduced charge transfer in donor/acceptor blends for organic solar cells Josh Holt, Sanjeev Singh, Tomer Drori, Alexandre Ndobe, Z. Valy Vardeny Current developments in organic solar cells ($\sim $6{\%} power conversion efficiency) require understanding and control of photoinduced charge transfer of donor-acceptor pairs. In this work we provide and compare evidence that poly(2-methoxy-5(2'-ethyl)hexoxy-phenylenevinylene) (MEH-PPV, donor) blended with 2,4,7-trinitrofluorenone (TNF, strong acceptor) form a below-gap charge transfer complex (CTC) state that extends absorption into the near infrared (NIR). Transient PA measurements show direct photoexcitation into the CTC state where significant charge species are initially photogenerated, the majority of which geminately recombine within $\sim $10 ps, but the few that escape geminate recombination are subsequently captured in long-lived traps. Polarons could also be photogenerated with high efficiency \textit{at NIR excitation}, with similar fate. This shows that a CTC state exists below the MEH-PPV optical gap, but with low dissociation efficiency, which leads to poor photovoltaic effect. We compare our results to those in blends of MEH-PPV/C$_{60}$, where apparently the obtained CTC state has a much higher dissociation efficiency. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W22.00006: Quantum efficiency in organic phototransistors William Hammond, Jiangeng Xue Organic optoelectronic devices hold a prominent role in current applied physics research. Although these devices inherently suffer from lower carrier mobility than inorganic devices, their mechanically flexible nature and low material costs enable new and interesting applications. Organic phototransistors, for example, may enable simplified circuits for large area and flexible sensors. Here we report the spectral dependence of the external quantum efficiency of organic phototransistors (OFETs) based on pentacene and C$_{60}$. Furthermore, we explore the gain mechanism in these devices and the effect of transistor structure on internal photomultiplication. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W22.00007: Ultrafast dynamics in blends of $\pi$-conjugated polymers/fullerenes Sanjeev Singh, Minghong Tong, ChuanXiang Sheng, Zeev Vardeny We have studied the ultrafast dynamics of photogenerated charges and excitons in a variety of $\pi$-conjugated polymer/fullerene blends using the transient pump-probe photomodulation (PM) spectroscopy with $\sim $ 100 fs resolution. These composites serve as active layers in organic photovoltaic devices with high power conversion quantum yield, due to the existence of a photoinduced charge transfer (PCT) reaction between the polymer and the fullerene molecules. Our transient PM spectrum spans a broad energy range from 0.1-2.4 eV, and this allows us to monitor the transient behavior of the various photoinduced absorption (PA) bands of polarons and excitons in the PM spectrum; as well as the transient exciton stimulated emission, and photobleaching (PB) of the ground state. The PB dynamics reflect the ground state recovery; hence, it can be used to determine the long-lived polaron photogeneration quantum efficiency in these systems. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W22.00008: Spin Response in Organic Light Emitting Diodes Fujian Wang, Cungeng Yang, Tomer Drori, Z. Valy Vardeny To understand the origin of the magnetic field effect in OLEDs, we studied the large magnetoresistance (MR) and magnto-electroluminescence (MEL) of OLEDs based on pristine MEH-PPV polymer, as well as MEH-PPV doped with various concentrations of radical impurities and C$_{60}$ molecules. In contrast to OLED based on pristine MEH-PPV that show MR and MEL up to 12{\%} at room temperature, we found in MEH-PPV:C$_{60}$ based OLED the MR and MEL decrease substantially with increasing C$_{60}$ molecule concentration. For MEH-PPV:C$_{60}$ devices with C$_{60}$ concentration of 50{\%}, the MR effect is less than 0.3{\%} at room temperature. In MEH-PPV: radical devices the MR and MEL effects again differ substantially from those found in pristine and C$_{60}$ doped MEH-PPV devices. At 50{\%} radical concentration the MR and MEL effects are about 1{\%} and 3.5{\%}, respectively. The results are discussed with existing models for the magnetic field effects in OLEDs. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:42PM |
W22.00009: Organic Semiconductors: devices, growth and ordered assembly Invited Speaker: Organic semiconductors are employed in devices such as field-effect transistors (FETs), light-emitting diodes and photovoltaic cells. Besides their technological interest, these devices are model systems to study physical processes in organic semiconductors [1]. Light-emitting field effect transistors (OLEFETs) based on organic semiconducting films are a novel class of devices integrating the transistor function with the light emission [2]. I will discuss LEFETs based on oligothiophenes [3] and oligoacenes [4] [5], in particular their optoelectronic properties and the films growth physics. A unique property of organic semiconductors is the ability to form ordered assemblies at surfaces that can be studied by scanning tunneling microscopy (STM). I will discuss the adsorption and self-assembly on different facets of copper of two organic semiconductors: the linear and planar quinquethiophene [6] and the branched non-planar rubrene [7]. These studies show the ability of organic semiconductors to form fascinating self-assembled motifs and are of paramount importance to understand the early stages of growth of organic films. Organic electrochemical transistors (OECTs) are expected to play a key role in future organic electronics. OECTs are ideal candidates for biosensing applications thanks to their low driving voltage and their ability to operate in aqueous environment. A great deal of work is needed to understand the device physics of OECTs and optimize their performance. I will discuss advances in the field drawing examples from studies on devices based on the conducting polymer PEDOT:PSS. \newline [1] G. G. Malliaras, R. H. Friend, Phys. Today 58, 53, 2005; \newline [2] F. Cicoira, C. Santato Adv. Funct. Mater. 17, 3421, 2007; \newline [3] F. Cicoira et al. Adv. Mater. 18, 169, 2006; \newline [4] F. Cicoira et al. Adv. Funct. Mater. 15, 375, 2005; \newline [5] F. Cicoira et al. J. Mater. Chem. in press; \newline [6] F. Cicoira et al. Small 2, 1366, 2006; \newline [7] F. Cicoira et al. J. Phys. Chem. A in press. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W22.00010: Spin injection effects on exciton distributions in conjugated organic semiconductors Mohammad Yunus, P. Paul Ruden, Darryl Smith Conjugated organic semiconductors are under rapid development as the active material in organic light emitting diodes (OLEDs). Electrons and holes injected into the organic semiconductor form bound singlet or triplet excitons. Singlet excitons may recombine radiatively giving rise to light emission whereas triplet excitons do not recombine radiatively. Thus the quantum efficiency of OLEDs is limited by the fraction of singlet excitons, \textit{$\chi $}$_{S}$. In this work, we explore theoretically an approach to control \textit{$\chi $}$_{S}$ through spin-polarized electron and hole injection from ferromagnetic contacts. Conventional ferromagnetic transition metals and half-metallic materials, such as LSMO, are considered as candidate electrode materials. Electron and hole transport in the organic semiconductor is treated through the conventional device equations with the formation of excitons described by a Langevin process. Once formed, the excitons may recombine or diffuse. Triplet excitons have a lower recombination probability and hence a longer diffusion length. The model calculations yield steady state spatial profiles for singlet and triplet exciton densities in the organic semiconductor. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W22.00011: Electrically detected coherent spin manipulation of polaron pairs in an MEH-PPV OLED Heather Seipel, Dane McCamey, Seoyoung Paik, Manfred Walter, Nick Borys, John Lupton, Christoph Boehme Understanding of spin relaxation in organic light emitting diodes (OLEDs) is important for determining the maximum device efficiencies, due to the spin dependence of electronic transitions in organic materials. Here, we present an experiment demonstrating that coherent spin motion of polaron spin pairs can influence the current through an OLED fabricated using the prototypical conjugated polymer poly[2-methoxy-5-(2$^{\prime }$-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV). We observe a change in the zero-bias photocurrent transient through an OLED device following a short, coherent microwave pulse resonant with polaron pair spin transitions. The shape of the current transient provides information about the recombination rates of polaron pairs in both the singlet and triplet configurations. By varying the length of this pulse, coherent Rabi oscillations are detected, which reveal that the polaron pairs responsible for the signal remain coherent for $>$0.5$\mu $s. Due to these extremely long coherence times, we conclude that spin mixing processes are unable to significantly influence the spin state of the polaron pairs. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W22.00012: Tunable and White Light Emitting Diodes of Single Component Fluorinated Benzoxazole Graft Copolymers Shih Jung Bai, Chien-Chang Wu Coil-like graft copolymers of poly(A$_{m}$-\textit{co}-B$_{(1-m)})$ containing identical heterocyclic aromatic benzoxazole with trifluoromethyl-ethyl as the backbone and pendants of \textit{mono}-hydroxl (A$_{m})$ and/or \textit{bi}-decyloxyl (B $_{(1-m)})$ on their phenylene ring were studied for luminescence properties. The copolymers were synthesized with molar fraction $m$ ranging from 0, 0.25, 0.5, 0.75 to 1, and then dissolved and spun onto Spectrosil$^{\mbox{{\textregistered}}}$ quartz slide or indium-tin-oxide (ITO) substrate. The fluorescence properties of copolymers were investigated by ultraviolet-visible absorption covering 185 nm to 800 nm and photoluminescence (PL) emission excited at 363 nm. The PL results exhibited an excellent chromatic tuning range from green to white emission as $m$ decreased. Aluminum electron injectors were evaporated onto the copolymer/ITO unit making it into \textit{mono-}layer light emitting diodes for current-voltage and electroluminescence (EL) responses. An emission threshold voltage of 6 V was achieved for all the \textit{mono}-layer copolymer devices. The Commission Internationale de l'Eclairage chromaticities of the EL emission were from (0.25, 0.53) to (0.24, 0.31) covering a wide visible range including white light emission. [Preview Abstract] |
Session W23: Focus Session: Manganites II
Sponsoring Units: DMP GMAGChair: Ichiro Takeuchi, University of Maryland
Room: Morial Convention Center 215
Thursday, March 13, 2008 2:30PM - 2:42PM |
W23.00001: Parameters controlling magnetic transitions in manganites B. Dabrowski, S. Kolesnik, O. Chmaissem, J. Mais Using description of structural and physical properties of perovskites in terms of the tolerance factor t(x,T,d), which is dependent on composition, temperature, and oxygen-content, we have established synthetic methods and studied composition-structure-properties for new manganites La$_{1-x-y}$Sr$_{x}$Ba$_{y}$MnO$_{3-d}$ far beyond solubility limits normally achieved during the synthesis in air. Parameters controlling magnetic transitions have been identified through examples of single-valent compounds of RMnO$_{3}$ (the Mn-O-Mn bond angles that can be equivalently described in terms of the tolerance factor) and Sr$_{1-x}$Ca$_{x}$MnO$_{3}$ (the tolerance factor and the variance of A-site ion sizes), and the mixed-valent La$_{0.5}$Sr$_{0.5-y}$Ba$_{y}$MnO$_{3}$ (the tolerance factor, variance of sizes, and the local strains described in terms of the elongated Mn-O bonds). By using an example of kinetically stable, atomically-ordered layered-perovskites RBaMn$_{2}$O$_{6}$ we show that the increase of Curie temperature T$_{c}$ and enhancement of colossal magneto-resistive effect at room temperature, can be achieved through reduction of variance of sizes and local strains. Work at NIU was supported by the NSF-DMR-0302617 and at ANL by the U.S. DOE under contract No. DE-AC02-06CH11357. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W23.00002: Phase separation in (001) and (110) La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ epitaxial films J. Fontcuberta, I.C. Infante, F. S\'anchez, M. Wojcik, E. Jedryka, S. Estrad\'e, J. Arbiol, F. Peir\'o Recent reports on the formation of a highly conducting layer at the interface between two insulators have driven a strong interest to interface physics. To explore strain and polarity effects on electronic phase separation (PS), we have grown epitaxial films of La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ (LCMO) on (001) and (110) SrTiO$_{3}$ (STO) substrates. (001) LCMO/STO and (110) LCMO/STO interfaces differ on the polarity sequence. Measurements of the LCMO layers show that the (110) LCMO films display always ``better'' magnetic properties that their (001) LCMO counter-partners and do not show any traces of PS. Moreover, we have analyzed the properties of (001) and (110) thin LCMO films capped with a thin STO barrier and compared with those of bare LCMO films. It turns out that STO capping induces PS in (001)LCMO films but not in (110) films. Reasons for this asymmetric behaviour will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W23.00003: Crystal Structure and Physical Properties of Oxygen Composition Controlled La$_{1-x}$Sr$_{x}$MnO$_{3+\delta }$ Single Crystals. Yuui Yokota, Jun-ichi Shimoyama, Tetsuro Ogata, Atsushi Nakamura, Hiraku Ogino, Shigeru Horii, Kohji Kishio The La$_{1-x}$Sr$_{x}$MnO$_{3+\delta }$ is known to have various magnetic and crystal structures as functions of $x$. Although this system has relatively large oxygen nonstoichiometry, the effects of excess oxygen on the crystal structure and physical properties have not been well understood. In the present study, the crystal structure and physical properties of excess oxygen controlled La$_{1-x}$Sr$_{x}$MnO$_{3+\delta }$ single crystals were systematically studied. Single crystals with nominal compositions of La$_{1-x}$Sr$_{x}$MnO$_{3+\delta }$ (0.05 $\le \quad x \quad \le $ 0.2) were grown by the floating zone method. Thin plate-like crystals were obtained from the grown boules and controlled the oxygen content by post-annealing in various atmospheres. With increasing \textit{$\delta $}, the orthorhombic of the as-grown La$_{0.95}$Sr$_{0.05}$MnO$_{3+\delta }$crystal changed to the rhombohedral through pseudo-cubic orthorhombic. In addition, ferromagnetic behaviors appeared accompanying the CMR effect and $T_{C}$ increased as a function of \textit{$\delta $}. These are attributable to an increase of valence of Mn by excess oxygen. Relationships among the Sr substitution level, excess oxygen content, crystal structure and magnetic behaviors of La$_{1-x}$Sr$_{x}$MnO$_{3+\delta }$ will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W23.00004: Improved CMR properties of RE-doped (La,Sr)MnO$_{3}$ single crystals Jun-ichi Shimoyama, Tetsuro Ogata, Yuui Yokota, Hiraku Ogino, Shigeru Horii, Kohji Kishio The relationships among crystal structure, $T_{C}$ and CMR effect have been eagerly studied for (La,Sr)MnO$_{3}$ system mainly as a functions of the Sr composition, $x$ thus far. In the present study, we have attempted to improve the CMR properties near room temperature of the present system by optimizations of $T_{C}$ and phase transition temperature between orthorhombic and rhombohedral through RE mixing and elimination of excess oxygen, $i.e.$ cation vacancies, for (La$_{1-x}$Sr$_{x})$MnO$_{3}$ single crystals with $x$ = 0.2 and 0.25, which have higher$ T_{C}$ than room temperature and essentially high electronic conductivity. Crystal boules with nominal compositions of La$_{0.8-z}$RE$_{z}$Sr$_{0.2}$MnO$_{y}$ and La$_{0.75-z}$RE$_{z}$Sr$_{0.25}$MnO$_{y}$ (RE = Pr, Nd, Sm : $z$ = 0 $\sim $ 0.3) were grown by the floating zone method. Crystal structure of La$_{0.75-z}$Pr$_{z}$Sr$_{0.25}$MnO$_{3}$ at $\sim $300 K changed from rhombohedral ($z$ = 0, 0.15, 0.25) to orthorhombic ($z$ = 0.3) due to a decrease in mean ionic radius of A site. In addition, Pr-doping systematically decreased $T_{C}$. Similar tendencies were confirmed for Nd- or Sm-doped samples. The RE-doped samples exhibited large CMR ratio at $\sim $300 K comparable to that of La$_{0.825}$Sr$_{0.175}$MnO$_{3}$ and much higher conductivity reflecting high Sr concentration when phase transition temperature and $T_{C}$ were optimized. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W23.00005: Unconventional spin-dynamics in a phase separate, weakly disordered perovskite manganite Feng Ye, Jaime Fernandez-Baca, Pengcheng Dai, Hye-Jung Kang, Jeffrey Lynn, Chenglin Zhang, S.-W. Cheong The intense investigation of perovskite manganites has revealed a variety of fascinating properties. The phenomena known as phase separation, the coexistence at different length scales of ferromagnetic, charge/orbital order has been recently recognized as an intrinsic feature of several strongly correlated electron systems. Using inelastic neutron scattering, we have studied the spin dynamics of the archetypical material (La,Pr)$_{7/8}$Ca$_{3/8}$MnO$_{3}$, where competing ground states coexist at low temperature. The low-$T$ spin wave excitations at H = 0 and 2T are drastically different. We discuss this difference in terms of magnetic excitations from ferromagnetic clusters of different length scales. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W23.00006: Phase separation in Pr$_{0.55}$Ca$_{1.45}$MnO$_{4}$ evidenced by magnetic excitations Songxue Chi, Pengcheng Dai, Feng Ye, Jaime Fernandez-Baca, Hye Jung Kang, Jeffrey W. Lynn, Ying Chen, Yoshio Kaneko, Yoshinori Tokura At doping levels x$<$0.5, a coexistence of commensurate (CM) and incommensurate (ICM) magnetic peaks are observed in single-layered manganites Pr$_{1-x}$Ca$_{1+x}$MnO$_{4}$ with elastic neutron scattering. Temperature dependence measurements of the magnetic intensities with different energy resolutions indicate a glassy nature of the magnetic moments. The magnetic excitation measurements using inelastic neutron scattering on the x=0.45 system reveal both symmetric and asymmetric behaviors about the CM peak positions. This strongly suggests two types of magnetic excitations originated from separated phases: the CE-type magnetic phase and an additional electronic phase caused by extra electrons introduced into the CE template. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W23.00007: Signature of Magnetic Phase Separation in Pr$_{1-x}$Ca$_{x}$MnO$_{3}$. Dalgis Mesa, Hao Sha, Jiandi Zhang, F. Ye, P.C. Dai, J. A. Fernandez-Baca, J. W. Lynn, Y. Tomioka, Y. Tokura Elastic neutron scattering has been used to study the evolution of the long-/short-range charge-orbital (CO-OO), ferromagnetic (FM), and antiferromagnetic (AF) correlations in the single crystals Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ (x = 0.3, 0.35 and 0.4). In contrast with Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (PCMO30), both the long-range CO-OO and AF ordering show precipitous decrease in intensity below 30K, where the short-range FM clusters are formed in the Pr$_{0.65}$Ca$_{0.35}$MnO$_{3}$ (PCMO35). Those results provide clear evidence of magnetic phase separation. The doping dependence results of the short-range magnetic correlations indicate that there is a critical doping $x_{cr}$ (close to $x$ = 0.35) for the existence of phase separation in the CO-OO ground state. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W23.00008: Anomalous Short Range Charge Ordering in La$_{1-x}$Ca$_{x}$MnO$_{3}$ Jing Tao, S.J. Pennycook, Y. Zhu Long range charge ordered (CO) phases have been observed at low temperatures in manganites in certain doping ranges. Structural characterization of the long-range CO phase showed that the CO superlattice is always along the Pnma a axis in bulk samples [1]. Here we report the observations of a short range CO phase in La$_{1-x}$Ca$_{x}$MnO$_{3}$ samples as a function of cation concentration $x$, temperature and magnetic field using in-situ electron microscopy. We find short range CO nanoclusters with the CO superlattice in both perpendicular directions (a and c axes) in single crystal domains. The a and c axis nanoclusters have different densities, and show different dependences on temperature and magnetic field. Time evolution of the CO nanoclusters with the CO superstructure along the anomalous direction (c axis) is also recorded, which implies that the energy barrier between the two types of the CO structure is very small [2]. [1]. P. G. Radaelli \textit{et al}., PRB 59, 14440 (1999) [2]. Research sponsored by the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering and by appointment to the ORNL Postdoctoral Research Program administered jointly by ORNL and ORISE. Work at BNL was supported by the U.S. DOE/BES under Contract No. DE-AC02-98CH10886. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W23.00009: Spatially-Resolved X-Ray Microstructural Studies of Bulk Phase Separation in Manganites J.D. Budai, D.D. Sarma, W. Liu, J.Z. Tischler, B.C. Larson, G. Shenoy, D. Topwal, S-W. Cheong We have used spatially-resolved, 3D x-ray microdiffraction to study phase separation in two directionally-solidified, transition-metal oxide systems: (Y,Eu)MnO and (Lu,LaSr)MnO. Both systems exhibit micron-scale coexistence of separate single-crystal lamellar domains (hex/ortho and hex/rhomb respectively) when grown by a floating zone method. Micron-resolution 3D x-ray microscopy reveals the domain morphologies, lattice orientations, and local strain fields within the phase-separated eutectic systems. The orientations of the lamellae are consistent with energetic predictions and the formation of low-energy, semi-coherent interfaces. In addition, we observe a bias for larger strain fluctuations in one phase. More generally, the microstructural features observed experimentally in these well-defined, micron-scale eutectic domains provide clues to the domain interactions believed to exist in similar, more homogeneous, nanoscale manganite systems. Structural studies at the nanoscale will be enabled in the future by advances in x-ray focusing optics. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W23.00010: Strain and Curent Induced Multiphase Coexistence in (La$_{0.5}$Pr$_{0.5}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ Probed by Magnetic Force Microscopy Frank Ruzicka, Changbae Hyun, Junwei Huang, Alfred Lee, Alex de Lozanne, Tara Dhakal, Jacob Tosado, Amlan Biswas The ferromagnetic (FM) domain structure of a thin film (La$_{0.5}$Pr$_{0.5})_{0.67}$Ca$_{0.33}$MnO$_{3}$ sample grown on a (110) NdGaO$_{3}$ (NGO) substrate was investigated using low-temperature magnetic force microscopy with temperature control, an external magnetic field and an electric field across the sample. We observed that the FM domains form stable patterns with in-plane magnetization at 78K. At higher temperatures the system enters a fluid phase separated (FPS) state. The FM domains change as the voltage across the sample increases. We believe that competition between a charge-ordered insulating (COI) phase and a ferromagnetic metallic (FMM) phase exists in this FPS state and the COI phase may be driven to an FMM phase by an electric field. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W23.00011: Domain Mapping of a Ca-doped Manganite Joshua Turner, K. Jessica Thomas, Mary Upton, John Hill, Jean Jordan-Sweet, Yoshinori Tokura, Yasuhide Tomioka, Stephen Kevan In the last few years, disorder has emerged as a key without which the colossal magnetoresistance (CMR) effect would not exist. Single crystals represent the simplest arenas to observe and study the peculiarities central to the manganites. This is in contrast to polycrystalline compositions for instance, where rampant strain fields can veil fundamental physics. By micro-focusing x-rays through a glass capillary, we have performed a microtopography technique to map the crystallographic domain structure of the Ca-doped manganite crystal, PCMO. This technique serves as a domain-mapping alternative tool to TEM that does not require challenging sample preparation procedures. Surprisingly, we find micron size domain structure. We suggest that the separation of crystallographic domains by twin boundaries is more relevant than previously believed. These boundaries could affect the large-scale, sub-micron size disorder that has been observed recently, and also may play a role in the heterogeneous nature of the CMR effect. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W23.00012: Temperature-Dependent Electronic Structure of the Colossal Magnetoresistive Manganite La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ via Hard X-Ray Photoemission Francesco Offi, Norman Mannella, Giancarlo Panaccione, Tommaso Pardini, Andrea Fondacaro, Piero Torelli, Simo Huotari, Mark West, John Mitchell, Charles Fadley We have studied single-crystal La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ with hard x-ray photoemission (HXPS) at an excitation energy of 7.7 keV. These more bulk-sensitive measurements reveal low-binding-energy satellites in the Mn 2p$_{3/2}$, 3s, and 3p core spectra that are consistent with previously observed satellites in Mn 2p$_{3/2}$ for other strongly-correlated materials, and which have been interpreted in terms of non-localized screening effects. The Mn 3s spectrum is agreement with recent soft x-ray measurements (Mannella et al., P.R.L. \textbf{92}, 166401 (2004)) in showing an increased multiplet splitting at temperatures 100 K or more above T$_{C}$, although the effect is here reduced. Core-normalized valence-band spectra exhibit enhancement of intensity at high temperature that is evidence of localization of Mn 3d-derived charge, in agreement with prior soft x-ray work. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W23.00013: Unusual temperature dependence of the oxygen-isotope effect on the exchange-energy of La$_{1-x}$Ca$_{x}$MnO$_{3}$ Guo-meng Zhao, John Mann We report magnetic susceptibility $\chi(T)$ measurements on oxygen-isotope exchanged La$_{1-x}$Ca$_{x}$MnO$_{3+y}$ up to 700 K. The $1/\chi(T)$ data show that the ferromagnetic exchange-energy $J$ depends strongly on the oxygen-isotope mass. The isotope effect on $J$ decreases with temperature up to 400 K and then increases again with temperature above 400 K. This unusual temperature dependence cannot be explained by existing theories of the colossal magnetoresistance effect for doped manganites. We suggest that a correct model for description of the physics of manganites should be based on double-exchange and the formation of localized bipolarons in the paramagnetic state. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W23.00014: Origin of the Non-Linear Pressure Effects in Perovskite Manganites Zhiqiang Chen, Trevor Tyson, Ken Ahn, Zhong Zhong, Jinzhu Hu High-pressure resistivity and x-ray diffraction measurements were conducted on La$_{0.85}$MnO$_{3-\delta }$ to $\sim $6 GPa and $\sim $7 GPa, respectively. At low pressures the metal-insulator transition temperature (T$_{MI})$ increases linearly up to a critical pressure, P* $\sim $ 3.4 GPa, followed by reduction of T$_{MI}$ at higher pressure. Analysis of the bond distances and bond angles reveal that a bandwidth increase drives the increase of T$_{MI}$ below P*. The reduction of T$_{MI}$ at higher pressures is found to result from Jahn-Teller distortions of the MnO$_{6 }$octahedra. The role of anharmonic interatomic potentials is discussed. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W23.00015: Phonon-induced magnetic deflagration and detection of very fast CMR in manganites Ferran Macia, Joan Manel Hernandez, Guillem Abril, Alberto Hernandez-Minguez, Francisco Parisi, Paulo V. Santos, Javier Tejada In this work we describe experiments in which we have used surface acoustic waves to induce controlled magnetic avalanches in (La, Pr)-based manganites. The avalanches propagate inside the sample following the law of the magnetic deflagration, and occur at well determined values of the temperature and the applied magnetic field, that depend on the phase separation fraction. Another important point is that the magnetic avalanche is accompanied by colossal variation of the electrical resistance in about 0.1 ms. F. Macia \textit{et al.}, Phys. Rev. B \textbf{76}, 174424 (2007). F. Macia \textit{et al}., Phys. Rev. B, submitted [Preview Abstract] |
Session W24: Glassy and Amorphous Systems
Sponsoring Units: DCMPChair: Bruno Uchoa, Boston University
Room: Morial Convention Center 216
Thursday, March 13, 2008 2:30PM - 2:42PM |
W24.00001: Dynamics of a polymer nanocomposite under stress Robert Riggleman, Juan de Pablo The response of polymer glasses to deformation has received much attention in recent years. Previous simulation studies have shown that strain applied to a polymer glass can destroy energy barriers that impede relaxation on the potential energy landscape. However, less attention has been placed on the response of a polymer glass to an applied stress, as in a creep experiment. We have performed molecular simulations of the non-linear creep of a coarse-grained polymer glass and a polymer nanocomposite in shear, compression, and extension. We study the effects of the applied stress on the dynamics of our system and find dynamic enhancement up to a factor of 100 compared to the quiescent polymer glass below its glass transition temperature, Tg. We find that the free volume changes induced by the stress are not indicative of the changes in dynamics; however, the strainrate is strongly correlated with the dynamics. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W24.00002: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W24.00003: Out-of-Equilibrium to In-Equilibrium Dynamics of SiO$_2$ Katharina Vollmayr-Lee, Jake Roman, Juergen Horbach We study the aging dynamics of SiO$_2$ (modeled by the BKS model) via molecular dynamics simulations. The system is well equilibrated at temperature $T_{\rm high}$, then quenched to $T_{\rm low}$ and observed after a waiting time $t_{\rm wait}$. We present results for the structure factor, for the mean squared displacement, and for the intermediate scattering function. The resulting relaxation times show for the largest investigated $T_{\rm low}$ that during the simulation run the system is first out-of equilibrium and then reaches equilibrium with relaxation times independent of $T_{\rm high}$ and $t_{\rm wait}$. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W24.00004: Double Percolation in the Intermediate Phase of Network Glasses J.C. Phillips, Gerry Lucovsky Intermediate Phases (IP) in network glasses, discovered by Boolchand and coworkers, exhibit two striking properties: a nearly reversible glass transition, and nearly zero internal network stress. Double percolation, a well-established phenomenon in polymer blend-carbon black composites and numerical simulations, explains quantitatively the ranges of the intermediate phase observed in ternary chalcogenide alloys. The mechanism underlying IP double percolation in network glasses is spinodal enthalpy-entropy balance. Triple percolation also explains many aspects of the phase diagrams of cuprate high-temperature superconductors. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W24.00005: Effects of relaxation on the energy landscape of amorphous silicon Houssem Kallel, Normand Mousseau, Francois Schiettekatte Amorphous silicon is used in many devices around us, included as a thin-film transistor in most flat screens, it also serves as the reference for the study of disordered network systems. Recently, differential scanning calorimetry and nanocalorimetry measurements (DSC) $^1$ have shown that the heat released as the temperature of the sample is raised following implantation, is temperature independent. To understand this behaviour, we characterize the energy landscape of model \emph{a}-Si. Using the activation-relaxation technique (ART nouveau) with the modified Stillinger-Weber potential, we generate models at four levels of relaxation and identify the relaxation mechanisms by analysing 100 000 events for each model. We find that while the distribution of the activation barriers shifts to higher energy as the system is relaxed, the distribution of the relaxation energies is almost unchanged. The relation between these two phenomena is consistent with the DSC measurements. This work is supported, in part, by NSERC, FQRNT and the CRC Foundation. HK is grateful for a scholarship from the Tunisian Ministry of Higher Education, Scientific Research and Technology. \newline $^1$ R. Karmouch \emph{et al.}, Phys. Rev. B \textbf{75}, 075304 (2007) [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W24.00006: Models of Ge$_x$Se$_{1-x}$ Marc-Andr\'e Malouin, Normand Mousseau We present numerical models of chalcogenide glasses constructed using the effective two and three body interaction potential developed by Mauro and Varshneya [1] combined with the activation-relaxation technique (ART nouveau) [2]. Structures are prepared starting from a random distribution, avoiding biases and crystalline remnants. Structural properties are studied mainly via characteristic system measurements including partial and total radial distribution functions, bond angle distributions, mean coordinations and bonds population. Results are shown for $Ge_{x}Se_{1-x}$ for various \emph{x} concentrations and compared to both experimental measurements and \emph{ab initio} simulation results. \newline [1] J.C. Mauro and A.K. Varshneya, J. Am. Ceram. Soc., 89 [7] 2323-6 (2006). \newline [2] R. Malek and N. Mousseau, Phys. Rev. E 62, 7723 (2000). [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W24.00007: Growth and properties of amorphous Ge$_{x}$Se$_{1-x}$ films prepared by pulsed laser deposition W.C. Liu, W. Zhou, G. Hoffman, R. Reano, R. Sooryakumar, P. Boolchand Chalcogenide glasses such as Ge$_{x}$Se$_{1-x}$ are interesting materials for their light induced effects. These phenomena include photdarkening, photofluidity, anisotropic optomechanical response and giant reversible photosoftening. Some of these effects have stimulated device applications that have led to mature technologies. In order to further investigate these systems, we report on the growth of thin films of Ge$_{x}$Se$_{1-x}$ (x $\sim $0.2) chalcogenide glass by pulsed laser deposition. The films were prepared using rotating targets of chalcogenide glasses that were prepared by conventional direct synthesis from elements in evacuated silica ampoules followed by melt quenching. Deposition was carried out in a vacuum chamber at room temperature on thermally oxidized silicon substrates. We will present results on the homogeneity, composition, and loss properties of these films, as well as their response to electron beams. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W24.00008: Particle Jumps in SiO$_2$: A Computer Simulation Robin Bjorkquist, Katharina Vollmayr-Lee, Juergen Horbach We use molecular dynamics simulations to investigate the aging dynamics of SiO$_2$ modeled by the BKS potential. The system is well equilibrated at temperature $T_{\rm high}$ and then quenched to $T_{\rm low}$. We characterize the dynamics by means of jumps in single-particle trajectories and measure the number of jump events, the number of jumping particles, and jump size. All measured quantities increase with increasing $T_{\rm low}$ and decrease with increasing waiting time. For the largest investigated temperatures $T_{\rm low}$ and for long enough waiting times the system reaches equilibrium and all measured quantities become independent of $T_{\rm high}$ and waiting time. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W24.00009: Flexible-Rigid elastic transition in sodium germanate glasses: A Brillouin light scattering study. W. Zhou, W.C. Liu, R. Sooryakumar, D.I. Novita, P. Chen, P. Boolchand Global structures in network glasses are characterized by their connectedness or mean co-ordination number, $r$, and have been classified as being flexible ($r\sim $2), intermediate($r\sim $2.4) or stressed rigid ($r\sim $2.7) . Recently these ideas have been extended from covalent networks to ionically bonded ones, as evidence of a reversibility window in (Na$_{2}$O)$_{x}$(GeO$_{2})_{1-x }$ glasses has emerged$^{2}$ in the 14{\%} $<$ x $<$ 19{\%} range. Glasses at x $<$ 14{\%} are viewed as stressed-rigid and those at x $>$ 19{\%} to be flexible. We have performed Brillouin light scattering measurements on this glass system, and will present results on the dependence of the longitudinal (LA) and transverse (TA) acoustic phonon velocities across a broad range of compositions, 0 $<$ x $<$ 30{\%}. Variations in C$_{11}$(x) and C$_{44}$(x) elastic constants across the three elastic phases will be discussed. $^{2}$ K.Rompicharla et al. (unpublished) [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W24.00010: Nature of electrical conductivity threshold in bulk (Ag$_{2}$Se)$_{x}$(GeSe$_{4}$)$_{1-x}$ glasses C. Holbrook, P. Boolchand, P. Chen, A. Pradel, A. Piarristeguy Bulk glasses were synthesized over the 0 $<$ x $<$ 25{\%} range, and examined in FT-Raman, m-DSC, Electric Force Microscopy (EFM) and complex impedance experiments. Ag$_{2}$Se as an additive to GeSe$_{4}$ base glass leads to macroscopic phase separation as revealed by bimodal T$_{g}$s ( base glass-T$_{g}$ =168\r{ }C, additive glass T$_{g}$ = 230\r{ }C at low x $<$ 15{\%} . In addition, at higher x ($>$ 16{\%}), a third T$_{g}$ near 211\r{ }C is observed, and its strength increases with increasing x. EFM confirms the heterogeneous character of the glasses displaying a conducting phase that is segregated at low x ($<$12{\%}), and which percolates at higher x ($>$16{\%}). Electrical conductivity results show a step-like jump of nearly 2 orders of magnitude in the 16{\%} $<$ x $<$ 20{\%} range. Frequency of the Raman active corner-sharing mode of GeSe$_{4 }$units (200 cm$^{-1})$ steadily decreases with increasing x, suggesting that the third phase (T$_{g}$=211 C) most likely is a conducting GeSe$_{4}$-Ag$_{2}$Se phase. These data are consistent with a volume percolation$^{2}$ of solid electrolyte phases near x$\sim $ 16{\%} contributing to the step-like jump in conductivity of glasses. $^{1}$ V.Balan et al. J.Optoelectronics Adv. Mater. 8, 2112(2006). $^{2 }$H. Scher and R.Zallen, J. Chem. Phys.53,3759(1970). * Supported by NSF grant DMR 04-56472 [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W24.00011: Synthesis of dry AgPO$_{3}$ glass and characterization by Raman, IR and m-DSC D. Novita, P. Boolchand Glass transition temperature of titled glass apparently vary over a wide range, 163\r{ }C$<$ T$_{g}<$ 254\r{ }C depending on the humidity of the ambient environment in which precursors are handled$^{1}$. We have examined a set of 4 samples (1(181\r{ }C),2(203\r{ }C), 3(242.7\r{ }C), 4(254\r{ }C) with different T$_{g}$s indicated in parenthesis, in m-DSC, Raman and IR experiments to elucidate the role of bonded water. Our results show that the glass transition endotherm of sample 4 (dry) is characteristic of a stressed-rigid glass, while that of sample 1 (wet) of a flexible glass. Although Raman scattering of samples 1 and 4 look superficially similar, they are different in details; the Boson peak in sample 4 has a stronger intensity than in sample 1. IR reflectance signal strength in the mid-IR range is weaker for sample 4 than for sample 1, and furthermore differs in details. These results show that presence of water disrupts the P-O-P chain network by replacing bridging O with terminal OH$^{-}$ ends. And once water is bonded, it is difficult to remove it completely. These results will be compared to previous reports in the field. $^{1}$ D.I.Novita and P.Boolchand Phys. Rev. B (in press) * Supported by NSF grant DMR 04-56472 [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W24.00012: Evidence for Quasi Tetrahedral S=As(S$_{1/2}$)$_{3}$ local structures in As$_{x}$S$_{1-x}$ glasses Ping Chen, P. Boolchand, C. Holbrook, K. Jackson, D. Georgiev, M. Micoulaut Raman scattering and modulated DSC on bulk As$_{x}$S$_{1-x}$ glasses in the 0.08 $<$ x $<$ 0.41 range is studied$^{1}$. Vibrational spectroscopy results supported by first principles cluster calculations reveal that modes near 335, 380 and 490 cm$^{-1}$ represent respectively symmetric stretch, asymmetric stretch and As=S stretch of Quasi- Tetrahedral (QT) S=As(S$_{1/2})_{3}$ local structures. Mode at 365 cm$^{-1}$ is attributed to both symmetric and asymmetric stretch of pyramidal (PYR) As(S$_{1/2})_{3}$ units. In addition, modes of S$_{n}$ chains near 460 cm$^{-1}$ and those of S$_{8 }$ rings near 475, 430, 217,and 150 cm$^{-1}$ are observed at x $<$ 0.25. Scattering strengths of modes of QT units are found to display a broad global maximum in the 0.20 $<$ x $<$ 0.30 composition range, while modes of PYR units increase monotonically with x, and modes of S$_{n}$ and S$_{8}$ rings monotonically decrease with x in the range examined. These findings are in harmony with the existence of a reversibility window (RW) in the 0.20 $<$ x $<$ 0.30 range. $^{1}$ D.G. Georgiev Ph.D. Thesis, Univ. of Cincinnati (unpublished) 2003. $^{2}$ D.G.Georgiev and P.Boolchand Phil. Mag.\underline {83}, 2941(2003). * Supported by NSF grant DMR 04-56472 [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W24.00013: Evidence of Intermediate Phase in (Na$_{2}$O)$_{x}$(GeO$_{2}$)$_{1-x}$ glasses V. Rompicharla, Ping Chen, D. Novita, P. Boolchand, M. Micoulaut, W. Huff Intermediate phases have been observed in covalent glasses, but ionically bonded network systems have received much less attention in this respect. We have now examined titled glasses in m-DSC, Raman scattering, IR reflectance and Birefringence experiments over wide range of soda concentration, 3 $<$ x $<$ 30{\%}. Thermal experiments reveal a sharp reversibility window (RW) in the 14{\%} $<$ x $<$ 19{\%} soda range, which correlates well with a broad global maximum in molar densities (germanate anomaly) . Raman and IR reflectance TO and LO mode frequencies exhibit anomalies between x$_{c}$(1) = 14{\%} (\textit{stress} transition) and x$_{c}$(2) = 19{\%} (\textit{rigidity} transition), with optical elasticity power-laws confirming the nature of the transitions. Birefringence measurements dramatize the macroscopically stress-free nature of the Intermediate Phase (IP) in the RW. These data also suggest that the germanate anomaly can be understood as a direct consequence of the multiscale structural self-organization of glasses in the IP. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W24.00014: Competing effects in the photo-crystallization of a-Se HARP films R.E. Tallman, B.A. Weinstein, A. Reznik, J.A. Rowlands, M. Kubota, K. Tanioka Photo-crystallization of amorphous (a-) Se is investigated as a function of temperature (T = 250--340 K) and exposure time by Raman scattering in photo-active films used in high-gain avalanche rushing photoconductor (HARP) cameras. We observe different temperature regimes that reflect the competition of viscosity-driven and strain-driven processes. No photo-crystallization is seen below 260K, and above 318K the crystallization rate is thermally enhanced. For 260--302 K the onset time for photo-crystallization decreases with increasing temperature, but the crystallization rate is T-independent. \textit{Surprisingly}, photo-crystallization is strongly suppressed in a 15 degree range around the glass transition at T$_{g} \quad \sim $ 310K. Our results are in qualitative accord with a theory by Stephens[1] that treats the effects of local strain on the growth of crystalline-Se domains within an a-Se matrix -- the strain arising from the density difference between the crystalline and amorphous phases. [1] R.B. Stephens, J. Appl. Phys. \underline {\textbf{51}}, 6197 (1981). [Preview Abstract] |
Session W25: Focus Session: Biopolymers: Molecules, Solutions and Networks II
Sponsoring Units: DPOLY DBPChair: Ting Xu, University of California, Berkeley
Room: Morial Convention Center 217
Thursday, March 13, 2008 2:30PM - 3:06PM |
W25.00001: Design of Responsive Peptide-based Hydrogels as Therapeutics Invited Speaker: Hydrogels composed of self-assembled peptides have been designed to allow minimally invasive delivery of cells in-vivo. These peptides undergo sol-gel phase transitions in response to biological media enabling the three-dimensional encapsulation of cells. Peptides are designed such that when dissolved in aqueous solution, exist in an ensemble of random coil conformations rendering them fully soluble. The addition of an exogenous stimulus results in peptide folding into beta-hairpin conformation. This folded structure undergoes rapid self-assembly into a highly crosslinked hydrogel network whose nanostructure is defined and controllable. This mechanism, which links intramolecular peptide folding to self-assembly, allows temporally resolved material formation. In general, peptides can be designed to fold and assemble affording hydrogel in response to changes in pH or ionic strength, the addition of heat or even light. In addition to these stimuli, DMEM cell culture media is able to initiate folding and consequent self-assembly. DMEM-induced gels are cytocompatible towards NIH 3T3 murine fibroblasts, mesenchymal stem cells, hepatocytes, osteoblasts and chondrocytes. As an added bonus, many of these hydrogels possess broad spectrum antibacterial activity suggesting that adventitious bacterial infections that may occur during surgical manipulations and after implantation can be greatly reduced. Lastly, when hydrogelation is triggered in the presence of cells, gels become impregnated and can serve as a delivery vehicle. A unique characteristic of these gels is that when an appropriate shear stress is applied, the gel will shear-thin, becoming an injectable low viscosity gel. However, after the application of shear has stopped, the material quickly self-heals producing a gel with mechanical rigidity nearly identical to the original hydrogel. This attribute allows cell-impregnated gels to be delivered to target tissues via syringe where they quickly recover complementing the shape of the tissue defect. This shear-thin delivery method is a convenient way to introduce cells to wound sites. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W25.00002: Synchrotron x-ray diffraction study on the size distribution and mechanical stability of microtubules by microtubule-associated-protein (MAP) tau M.C. Choi, U. Raviv, H. Miller, M. Gaylord, E. Kiris, D. Ventimiglia, L. Wilson, M.W. Kim, S. Feinstein, C.R. Safinya In neurons, microtubules (MTs), 25nm protein nanotubes, are used extensively as tracks for transporting nutrients and cellular components between the cell body and axons. MAP tau regulates microtubule assembly and, in a poorly understood manner, inter-MT interactions. Altered tau-MT interactions leads to MT depolymerization and tau tangles, which is implicated in a large number of neurodegenerative diseases. We will show that the size distribution and the enhanced mechanical stability of MTs by tau bindings are dependent on tau isoforms. Supported by DOE DE-FG02-06ER46314, NSF DMR-0503347, and NIH GM-59288, NIHI RO1-NS35010. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W25.00003: Effect of Mg Ions on Microrheological Properties of F-actin Solution across Isotropic-Nematic Phase Transition Jun He, Michael Mak, Yifeng Liu, Jay Tang We studied microrheological properties of F-actin across the isotropic-nematic phase transition region by video particle tracking and laser deflection particle tracking methods. The two methods give consistent results. As the nematic order parameter increases with the actin concentration, G$'_{x}$ (along the alignment) and G$'_{y}$ grow apart, with G$'_{y}$ larger than G$'_{x}$. The moduli scale with actin concentration as $G'_{x}\propto c^{0.54 \pm0.13}$ and $G'_{y}\propto c^{1.38\pm0.15}$. Furthermore, G$'$ and G$''$ dependence on [Mg$^{2+}$] were measured and compared for 1 mg/ml isotropic and 4 mg/ml nematic F-actin solutions. For isotropic phase, G$'$ increase with [Mg$^{2+}$] up to 6 mM and then plateaus; for nematic phase, G$'_{y}$ is larger and both G$'_{x}$ and G$'_{y}$ increase with [Mg$^{2+}$] monotonically all the way up to 16 mM, above which F-actin bundle formation occurs. In both isotropic and nematic phases, G$''$ only weakly depends on [Mg$^{2+}$]. In conclusion, particle tracking microrheology reveals rich rheological features of F-actin affected by I-N phase transition and by tuning weak electrostatic interactions among the filaments. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W25.00004: The stability of cellulose Tongye Shen, S. Gnanakaran Stabilities and dynamics of known forms of crystalline cellulose (an ordered assembly of beta-glucose polysaccharide chains) are dominated by hydrogen bonding (HB) interactions. A detailed understanding on how HB interactions contribute to overall thermostability of crystalline cellulose is essential for efficient enzymatic degradation of cellulosic structures and the eventual conversion to ethanol. We construct a statistical mechanical model of cellulose at the resolution of explicit HB networks. This model takes into account the essential physical interaction in terms of both intramolecular bonding between neighboring glucose units within a chain and intermolecular bonding between different cellulose chains. These calculations reveal stabilities of HB networks under various conditions, and the microscopic details (at the resolution of individual bonds) of breaking of HB network that leads to instability. By combining these observations with all-atom replica exchange dynamics simulations of short cellulose chains, we are able to capture the local disorder and amorphous nature of crystalline cellulose where the directionality of HB interactions play a critical role. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W25.00005: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W25.00006: Transient Binding and Viscous Dissipation in Semi-flexible Polymer Networks Oliver Lieleg, Mireille Claessens, Andreas Bausch Nature specifically chooses from a myriad of actin binding proteins (ABPs) to tailor the cytoskeletal microstructure. Herein, cells rely on the dynamics of the cytoskeleton as its structural and mechanical adaptability is crucial to allow for dynamic processes. A molecular understanding of such biological complexity calls for an in vitro system with well-defined structural rearrangements and cross-linker dynamics to elucidate the physical origin of the unique viscoelastic properties of cells. As we present here, the frequency-dependent viscoelastic response of cross-linked in vitro actin networks is determined by the binding kinetics of cross-linking molecules. Independent from the particular network structure, the viscous dissipation (loss modulus) exhibits a pronounced minimum in an intermediate frequency which is dominated by elasticity. We show that in this frequency regime the molecular origin of the viscoelastic response is given by the non-static nature of actin/ABP bonds as they are subjugated to chemical on/off kinetics. The time scale of the resulting stress release is set by the lifetime distribution of the cross-linking molecule and therefore can be tuned independently from other relaxation mechanisms. We speculate that unbinding of distinct cross-links might be the molecular mechanism employed by cells for mechanosensing. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W25.00007: : Microtubule Self- Assembly YongSeok Jho, M.C. Choi, O. Farago, MahnWon Kim, P.A. Pincus Microtubules are important structural elements for neurons. Microtubles are cylindrical pipes that are self-assembled from tubulin dimers, These structures are intimately related to the neuron transport system. Abnormal microtubule disintegration contributes to neuro-disease. For several decades, experimentalists investigated the structure of the microtubules using TEM and Cryo-EM. However, the detailed structure at a molecular level remain incompletely understood. . In this presentation, we report numerically studies of the self-assembly process using a toy model for tubulin dimers. We investigate the nature of the interactions which are essential to stabilize such the cylindrical assembly of protofilaments. We use Monte Carlo simulations to suggest the pathways for assembly and disassembly of the microtubules. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W25.00008: Direct Observation of Early-Time Hydrogelation in $\beta $-Hairpin Peptide Self-Assembly Tuna Yucel, Joel Schneider, Darrin Pochan Triggered hydrogelation of MAX 1 peptide (NH$_{2}$-(VK)$_{4}$-V$^{D}$PPT-(KV)$_{4}$-CONH$_{2})$ proceeds through peptide intramolecular folding into $\beta $-hairpins and immediate self-assembly into branched clusters of well defined (uniform, 3 nm cross section), semi-flexible, $\beta $-sheet-rich nanofibrils. Cryogenic transmission electron microscopy indicates that dangling fibrils extend from one growing cluster to another and lead to early, intercluster communication in solution. At the apparent percolation threshold, the dynamic shear modulus measured by oscillatory rheology ($G'(\omega ),G"(\omega )\propto \omega ^n)$ and the field-intensity autocorrelation function measured by dynamic light scattering ($g_1 (\tau )\propto \tau ^{-\beta '})$ show power-law behavior with comparable critical dynamic exponents ($n \quad \approx $ 0.47 and \textit{$\beta \prime $} $\approx $ 0.45). Finite interpenetration of percolating cluster with smaller clusters, along with permanent intercluster entanglements, increase the network rigidity. The self-assembly of MAX 1 peptide was compared and contrasted with the assembly of other biopolymeric networks in literature. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W25.00009: Loop Closure Dynamics of Flexible and Semi-flexible Polymer Jen-Fang Chang, Yeng-Long Chen It is widely believed that DNA looping due to multi-site DNA-binding proteins is important for DNA transcription, replication, and recombination. The chain closure problem has been studied in several Monte Carlo simulations to determine the chain closure probabilities and the chain conformation to infer dynamic properties. In this work, we investigate how the loop closure dynamics of flexible and semi-flexible polymers depend on the polymer length and the reactive site position using Brownian dynamics simulation, accounting for hydrodynamic interactions. Our study examines the probability of closing for two reactive sites along the chain and the shape of the loop formed by closing. In addition, intrachain hydrodynamic interactions are found to affect the diffusivity of circular chains compared to linear chains, in accord with experimental observations. We also consider the dynamics of chain closing under strong slit-like confinement as a function of slit height. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W25.00010: Equilibrium Size Distribution of Twisted Biopolymer Bundles Gregory Grason, Robijn Bruinsma Using a continuum elastic model of hexagonal filament packing, we demonstrate that molecular-scale chirality strongly affects the equilibrium properties of aggregates, or bundles, of biopolymers, such as DNA and F-actin. We show that biopolymers tend to form bundles with long-range, chiral stress patterns, twisting or braiding helically around the central bundle axis. Due to the build-up of elastic stress on the outer surface, the cohesive energy of chiral filament bundles has a non-monotonic dependence on filament number. As a consequence, we demonstrate for two cases--bundles with 1) columnar-hexagonal order and 2) columnar-solid order--that a stable phase of dispersed bundles is thermodynamically stable below the point of bulk condensation and below a critical surface energy for the bundle exterior. This work suggests that the large characteristic radius biopolymer bundles observed in {\it in vitro} studies of is not a product of some mysterious long-range force or kinetic limitations of bundle growth, but rather in-plane elastic stresses which result from the local preference for the chiral packing of filaments. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W25.00011: The Dependence of Actin Filament Assembly on Linking Agent Concentration Lam Nguyen, Qi Wang, Wei Yang, Linda Hirst This experimental and computational study focused on the properties of actin filament bundles and networks induced by the actin filament cross-linker, $\alpha $-actinin. Structural properties of actin filament networks linked by the $\alpha $-actinin protein were studied experimentally by using confocal microscopy. We varied the concentrations of actin and linking agents to study their effects on the properties of the bundles and networks. The study showed that the molar ratio of $\alpha $-actinin to actin played an important role in the properties of the network structure, determining the branching frequency of the bundles. An experimentally guided simulation based on the $\alpha $-actinin/actin filament system was carried out using CHARMM to attempt to replicate the features of the real system and therefore to study the physics behind the actin filament assembly process in different regimes. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W25.00012: Dimensional percolation of sheared nano-rod dispersions and consequences for highly anisotropic property tensors M. Gregory Forest, Xiaoyu Zheng, Ruhai Zhou, Richard Vaia The Doi-Hess theory for flowing nano-rod dispersion yields orientational probability distribution functions for the nano-particle phase in flow-processed thin films. These numerical databases for variable shear rate, particle aspect ratio and volume fraction are then combined with a Monte Carlo algorithm to populate sheared dispersions. Nano-rod cluster statistics are then computed to determine percolation thresholds, which yield transitions from zero to one, two and three dimensional percolating paths. Finally, effective property enhancements are computed which include standard volume-averaged homogenization and percolation cluster statistics. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W25.00013: Polymer Crystallization-Driven Gelation of an Ionic Liquid David Hoagland, John Harner Polyethylene glycol dissolves in the room temperature ionic liquid 1-ethyl-3-methylimidiazolium ethyl sulfate [EMIM][EtSO4] when heated above about 60C, the neat polymer's melting temperature. At typical polymer molecular weight and concentration, the homogeneous, slightly viscous solution solidifies during subsequent cooling, forming a semitransparent gel. For example, a 5 wt. percent solution of 6000 MW polymer produces a gel with modulus exceeding 1 KPa at 45C; cooled further to room temperature, the gel's modulus rises to a temperature-insensitive plateau of over 100 KPa. By DSC, rheology, and optical microscopy, gelation of the liquid is traced to kinetically frustrated polymer crystallization, a phenomenon previously reported for many pairings of crystallizable polymer and traditional solvent. Polarized optical microscopy reveals nucleation and growth of fibrillar polymer crystals during cooling, and these crystals, here with largest dimenions of tens to hundreds of microns, act as junction points. Melting is at a temperature higher than for gelation. Surprisingly, gelation can occur even when the starting polymer concentration is an order of magnitude below coil overlap. [EMIM][EtSO4] is hygroscopic, and with water uptake, the modulus drops. [Preview Abstract] |
Thursday, March 13, 2008 5:30PM - 5:42PM |
W25.00014: Avalanches, hardening and softening in dense cross-linked actin networks Jan Astrom, Sunil Kumar, Ilpo Vattulainen, Mikko Karttunen Actin filament networks enable the cytoskeleton to adjust to internal and external forcing. These active networks can adapt to changes by dynamically adjusting their crosslinks. Here, we study actin filaments as elastic fibers having finite dimensions. We employ a full three-dimensional model to study the elastic properties of actin networks by computer simulations. We model a dense actin network with the crosslinks being approximately 1$\mu$m apart. The results show that dense actin networks, without any pre-straining, are characterized by (a) strain hardening without entropic elasticity, (b) 'viscotic' hysteresis in the case of strong crosslinks, (c) avalanches of crosslink slippage leading to strain softening in the case of breakable crosslinks, and (d) spontaneous formation of stress fibers in the case of active crosslink formation and destruction. We will discuss the relation to recent experimental observations. [Preview Abstract] |
Thursday, March 13, 2008 5:42PM - 5:54PM |
W25.00015: Crystal aggregation in kidney stones; a polymer aggregation problem? J. Wesson, A. Beshensky, P. Viswanathan, W. Zachowicz, J. Kleinman Kidney stones most frequently form as aggregates of calcium oxalate monohydrate (COM) crystals with organic layers between them, and the organic layers contain principally proteins. The pathway leading to the formation of these crystal aggregates in affected people has not been identified, but stone forming patients are thought to have a defect in the structure or distribution of urinary proteins, which normally protect against stone formation. We have developed two polyelectrolyte models that will induce COM crystal aggregation \textit{in vitro}, and both are consistent with possible urinary protein compositions. The first model was based on mixing polyanionic and polycationic proteins, in portions such that the combined protein charge is near zero. The second model was based on reducing the charge density on partially charged polyanionic proteins, specifically Tamm-Horsfall protein, the second most abundant protein in urine. Both models demonstrated polymer phase separation at solution conditions where COM crystal aggregation was observed. Correlation with data from other bulk crystallization measurements suggest that the anionic side chains form critical binding interactions with COM surfaces that are necessary along with the phase separation process to induce COM crystal aggregation. [Preview Abstract] |
Session W26: Focus Session: Advances in Atmospheric Aerosol Science IV
Sponsoring Units: DCPChair: Laura Voss, Bowdoin College
Room: Morial Convention Center 218
Thursday, March 13, 2008 2:30PM - 3:06PM |
W26.00001: "The optics of atmospheric aerosol particles" Invited Speaker: |
Thursday, March 13, 2008 3:06PM - 3:42PM |
W26.00002: The Molecular Picture Behind Resonance Phenomena in Aerosol Spectra Invited Speaker: The study of icy aerosol particles with sizes in the submicron range is a highly interdisciplinary subject at the junction of nanosciences, atmospheric physics, and astrophysics. The microphysics of aerosol clouds in the atmospheres of planets and their moons, such as ammonia clouds on Jupiter and Saturn or methane aerosols on Titan, is currently very much in the focus of the scientific community. Particularly broad interest has been sparked by the recent Cassini-Huygens mission to Saturn's moon Titan, which has illuminated the importance of methane clouds for Titan's weather and their analogy to the role of water ice clouds in Earth's atmosphere. The present contribution focuses on the influence of intrinsic particle properties, such as shape, size or architecture, on infrared optical properties of icy aerosol particles. Intrinsic particle properties manifest themselves in mid-infrared extinction spectra by modifying the structure of vibrational bands. We ultimately aim at unravelling the microscopic origin of the characteristic patterns found in the spectra of these weakly bound molecular aggregates. To this end we compare our experimental results with different model calculations combining molecular dynamics simulations with vibrational quantum dynamics. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W26.00003: Investigations of Hygroscopic Growth and Phase Transitions of Atmospheric Particles by Noncontact Atomic Force Microscopy (AFM). Benjamin Ocko, Susan Oatis, Matthew Strasberg, Stephen Schwartz, Antonio Checco Aerosol particles (nanometer to micrometer sized particles suspended in air) affect atmospheric radiation and cloud microphysics. A correct description of their behavior in the atmosphere is essential to accurate climate modeling. The processes by which initially hydrophobic particles become hygroscopic, accrete water from the vapor, undergo phase transition from solid particles to solution droplets are important but not well understood at a fundamental level. We have carried out AFM studies to measure changes in particle size and morphology as a function of the relative humidity for particles of sodium chloride (a substance whose bulk hygroscopic properties are well characterized) deposited on substrates with differing surface energies (Silicon Oxide, Carboxy- and Methyl-terminated organic thin-films). For particles with height $>$ 50 nm, deliquescence was observed with a relative humidity near 75{\%} ($\pm $2{\%}), is consistent with measurements of suspended aerosols. These preliminary results demonstrate that environmental AFM is a viable probe for studying the hygroscopic behavior of salt nanoparticles on solid supports. Supported by the U.S. Department of Energy, DE-AC02-98CH. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W26.00004: Extinction by Single and Multiple Particles Matthew Berg, Christopher Sorensen, Amit Chakrabarti The combined effect of scattering and absorption is referred to as extinction and is responsible for the redistribution of radiant energy by a particle. This presentation will show that extinction is due to wave interference. Simulations of the energy flow caused by the interference graphically demonstrate how extinction redistributes the energy of incident light. Both single and multi-particle systems are considered. A conceptual, phase-based explanation is given that builds on previous work and illustrates the physical meaning of the optical theorem. Implications regarding the measurement of extinction are discussed. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W26.00005: Scattering Patterns for Spherical and Non-spherical Particles Christopher Sorensen Aerosols affect our climate directly by scattering and absorbing light. These optical properties depend on the size, shape, and composition. We have recently described patterns that appear in the phase function for spherical particles, Mie scattering, when the scattered intensity is plotted versus the scattering wave vector q = 2ksin(theta/2) [1, 2]. These patterns involve three different power law regimes and a quasi- universality on the phase-shift parameter rho= 2kR(m-1), where R is the radius and m the refractive index. Similar patterns appear for non-spherical particles. These patterns give us an empirical description of scattering by particles of arbitrary shape and refractive index. In other work we have explored the consequences of symmetry or its lack on polarization and backscattering. These results can be useful for predicting the scattering of atmospheric aerosol particles. [1] C.M. Sorensen and D.F. Fischbach, Opt. Commun. 173,145 (2000). [2] M.J. Berg, C.M. Sorensen, and A. Chakrabarti, Applied Optics 44, 7487-7493 (2005). I acknowledge very useful collaborations with M. Berg and A. Chakrabarti. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:54PM |
W26.00006: Understanding aerosol-cloud interactions Invited Speaker: The effects of aerosols on clouds (known as the ``aerosol indirect climatic effect'') are thought to have a net climatic cooling effect which partially offsets greenhouse gas warming. Regional impacts of aerosols on precipitation and cloudiness can be even stronger. Despite its importance, the complex and multi-scale nature of aerosol-cloud interactions makes quantitative assessments of the indirect effect one of the most uncertain components of anthropogenic climate change. This talk will present the approaches used to observationally study them and represent them in models. We will provide an assessment of what has been learned and point out key research challenges for the future. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W26.00007: Alkyl Polyoxyethylene Surfactant Residue Distribution after Sessile Droplet Evaporation. Kwaichow Chan, Scott Pierce, Yunji Mi, Heping Zhu Post-evaporation residues of an Alkyl Polyoxyethylene surfactant on a slightly hydrophilic surface are studied. ~An edge-detecting contrast recognition algorithm is used to measure the areas of small (7-30$\mu $m diameter) surfactant ``islands'' which form during the evaporation process as a result of flows within the droplet and surface tension gathering. ``Island'' distributions broaden with increased concentration, accompanied by higher mean interior island size. Fitting the histogram of island sizes beginning with the principle peak reveals a general form: $y=(6\times 10^8)Cx^{-\alpha}$ where y is frequency, $C$ the concentration and \textit{$\alpha $ }is a constant having value between 2.55 and 3.00. Log-log plots evidence a linear behaviour over two orders of magnitude. Total area covered does not increase in a linear fashion with concentration, as one may expect. Rather, a ``critical'' concentration is achieved at approximately 0.15{\%}, above which area increase is less pronounced. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W26.00008: ABSTRACT WITHDRAWN |
Session W27: Focus Session: Spin Glasses and SrCu2(BO3)2
Sponsoring Units: GMAGChair: Matt Stone, Oak Ridge National Laboratory
Room: Morial Convention Center 219
Thursday, March 13, 2008 2:30PM - 2:42PM |
W27.00001: Unconstrained Chern-Simons theory for the geometrically frustrated spin compound SrCu$_2$(BO$_3$)$_2$ Cristian Batista, Pinaki Sengupta, Suchitra Sebastian, Neil Harrison We show that an unconstrained Chern-Simons theory -- where the local densities are determined in a self-consistent manner -- correctly reproduces the sequence of magnetization plateaus recently observed in the geometrically frustrated spin compound SrCu$_2$(BO$_3$)$_2$ in an external magnetic field. The theory predicts that at the plateaus, the triplets are arranged in stripe patterns which is consistent with NMR experiments at and close to the 1/8 plateau. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W27.00002: Fractalisation drives crystalline states in the frustrated spin system SrCu$_2$(BO$_3$)$_2$ Suchitra Sebastian, N. Harrison, P. Sengupta, C.D. Batista, S. Francoual, E. Palm, T. Murphy, H.A. Dabkowska, B.D. Gaulin Geometrical frustration in the spin dimer material
SrCu$_{2}$(BO$_{3})_{2}$ leads to a singlet Shastry-Sutherland
groundstate at low magnetic fields, but complex spin
superstructures at higher fields. Our magnetisation measurements
reveal a fine substructure of quantum Hall-like plateaux at all
$1/q$ ratios $2 |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W27.00003: Doped Valence Bond Solid and Superconductivity on the Shastry-Sutherland Lattice Bohm-Jung Yang, Yong Baek Kim, Jaejun Yu, Kwon Park Motivated by recent experiments on SrCu$_2$(BO$_3$)$_2$, we present a theoretical framework for understanding the ground states of the doped Mott insulator on the Shastry-Sutherland lattice. To provide a unified theoretical framework for both the valence bond solid state found in undoped SrCu$_2$(BO$_3$)$_2$ and the doped counterpart in on-going experimental pursuit, we analyzed the $t$-$J$-$V$ model via the bond operator formalism. The interplay between strong dimerization and the nearest-neighbor repulsive interaction leads to different behaviors of the doped holes determining the overall phase diagram. Specifically, if the nearest-neighbor repulsive interaction, $V$, is smaller than a critical value, $V_c$, the hole-pairing within dimers is preferred, resulting in the $S$- wave superconductivity at any non-zero $x$. On the other hand, if $V$ is larger than $V_c$, the density of paired-holes within the same dimers vanishes at finite $x$ and the plaquette $D$-wave superconductivity with a peculiar spatial pattern emerges. Implications to future experiments are discussed. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W27.00004: Critical Properties of the Unconventional Spin-Peierls System TiOBr J.P. Clancy, B.D. Gaulin, F.C. Chou TiOBr is one of only three inorganic compounds which have been shown to undergo a spin-Peierls (SP) transition, developing a dimerized singlet ground state at low temperatures. However, unlike conventional SP systems, TiOBr exhibits not one, but two successive phase transitions: a continuous transition to an incommensurate SP state at T$_{c2}$$\sim$48K, followed by a discontinuous transition to a commensurate SP state at T$_{c1}$$\sim$27K. We have performed x-ray diffraction measurements on single crystal TiOBr, and carried out a detailed analysis of the critical exponent $\beta$ near T$_{c2}$. Our results yield a transition temperature of T$_{c2}$=47.85(5)K and an exponent of $\beta$=0.30(3). This value is consistent with conventional 3D Ising-like behavior ($\beta$=0.326), and closely compares to the exponent reported for CuGeO$_{3}$ ($\beta$=0.36(3)) [1], the canonical inorganic SP system. In contrast with measurements performed on isostructural TiOCl [2], we observe no evidence of commensurate dimerization fluctuations in either the incommensurate SP phase (T$_{c1}$$<$T$<$T$_{c2}$) or the so-called pseudogap phase (T$_{c2}$$<$T$<$T*$\sim$100-150K). Furthermore, the incommensurate scattering between T$_{c1}$ and T$_{c2}$ appears to be shifted in Q-space relative to the commensurate scattering below T$_{c1}$. [1] M.D. Lumsden et al., PRL {\bf 76}, 4919 (1996). [2] J.P. Clancy et al., PRB {\bf 75}, 100401(R) (2007). [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W27.00005: Thermal properties and Ehrenfest relation in an ``ideal" spin glass. S.L. Bud'ko, P.C. Canfield, G.M. Schmiedeshoff In order to test whether the Ehrenfest relation is applicable to the spin glass ``transition,'' we report temperature-dependent heat capacity and thermal expansion measurements on an ``ideal'' spin glass material: single grain icosahedral Gd-Mg-Zn quasicrystal and its non-magnetic counterpart, Y-Mg-Zn. Spin-glass state related signatures are clearly seen in the Gd-Mg-Zn data sets. Directly measured pressure dependence of the freezing temperature in Gd-Mg-Zn is compared to its evaluation through the Ehrenfest relation. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W27.00006: Anomalous field-sweep rate dependence of magnetization process of a spin-glass system Fe$_{x}$Mn$_{1-x}$TiO$_{3}$. N. Miura, H. Aruga Katori, A. Ito, M. Schoenhart, R. Groessinger, N. V. Kozlova, K. Doerr A mixed compound Fe$_{0.50}$Mn$_{0.50}$TiO$_{3}$ is a typical ising spin-glass. When a magnetic field is applied to the spin-glass state with a rather fast sweep rate 0.2 $<$ \textit{dB/dt} $<$ 1 T/s by a steady field magnet, the sample shows step-like jumps in the magnetization, whereas in a pulse magnet with an even faster sweep rate above 700 T/s, it shows a smooth metamagnetic transition. The field of the step-like jump is decreased as the sweep rate is increased, which is contrary to any relaxation phenomena with a slow response time. In the case of smooth metamagnetic transition at faster sweep rates, the transition field is increased as the sweep rate is increased. In this study, we investigated the magnetization process in the intermediate range of field sweep rate between 1.0 and 124 T/s using an ultra-long pulse magnet. It was found that as the sweep rate is increased, the jump behavior changes over to the metamagnetic behavior at around 2.5 T/s, where the transition field takes the minimum. The observed results are suggestive of some locally grown non-equilibrium spin alignment. The jump behavior is discussed in connection with similar phenomena in perovskite manganites. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W27.00007: The Free Energy Landscape of a Two-Dimensional Spin Glass from a Hard-Spin Mean-Field Treatment Michael Mihalco, Susan McKay The two-dimensional Ising antiferromagnet on a triangular lattice shows no ordered phase at finite temperatures until frustration is relieved, often accomplished through dilution (Grest and Gabl PRL 43, Netz and Berker PRL 66) or by the addition of randomly placed ferromagnetic bonds. Using the second approach, we have created a two-dimensional spin-glass phase with tunable frustration and explored its free energy landscape using a hard-spin mean-field approach (Netz and Berker PRL 66). The Monte Carlo implementation of this method quickly provides a self-consistent solution of site magnetizations, and we have used the overlaps and internal energies of various solutions to explore the energy landscape as a function of temperature and the number of frustrated plaquettes. Results indicate a multiplicity of solutions and a rich structure of overlaps, which are sensitive to the temperature and level of frustration. This method shows clearly the distinctions between the spin-glass phase and the bordering ferromagnetic ordering that occurs once the ferromagnetic interactions dominate. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W27.00008: Field-Driven Hysteresis of the d=3 Ising Spin Glass: Hard-Spin Mean-Field Theory Burcu Y\"ucesoy, A. Nihat Berker Hysteresis loops are obtained in the Ising spin-glass phase in $d=3$, using frustration-conserving hard-spin mean-field theory.[1] The system is driven by a time-dependent random magnetic field $H_Q$ that is conjugate to the spin-glass order $Q$, yielding a field-driven first-order phase transition through the spin-glass phase. The hysteresis loop area $A$ of the $Q-H_Q$ curve scales with respect to the sweep rate $h$ of magnetic field as $A-A_{0}$ $\sim $ $h^{b}$. In the spin-glass and random-bond ferromagnetic phases, the sweep-rate scaling exponent $b$ changes with temperature $T$, but appears not to change with antiferromagnetic bond concentration $p$. By contrast, in the pure ferromagnetic phase, $b$ does not depend on $T$ and has a sharply different value than in the two other phases. \newline \noindent [1] B. Y\"ucesoy and A.N. Berker, Phys. Rev. B {\bf 76}, 014417 (2007). [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W27.00009: Quantum Induced Asymmetric Phase Diagrams of Spin-Glass Systems C. Nadir Kaplan, A. Nihat Berker The spin-1/2 quantum Heisenberg spin-glass system is studied in all spatial dimensions $d$ by renormalization-group theory.[1] Strongly asymmetric phase diagrams in temperature and antiferromagnetic bond probability $p$ are obtained in dimensions $d\geq3$. The asymmetry at high temperatures approaching the pure ferromagnetic and antiferromagnetic systems disappears as $d$ is increased. However, the asymmetry at low but finite temperatures remains in all dimensions, with the antiferromagnetic phase receding to the ferromagnetic phase. A finite-temperature second-order phase boundary directly between the ferromagnetic and antiferromagnetic phases occurs in $d\geq6$, resulting in a new multicritical point at its meeting with the boundaries to the paramagnetic phase. In $d=3,4,5$, a paramagnetic phase reaching zero temperature intervenes asymmetrically between the ferromagnetic and reentrant antiferromagnetic phases. There is no spin-glass phase in any dimension. \newline \noindent [1] C.N. Kaplan and A.N. Berker, arXiv:0709.3589v1 [cond-mat.dis-nn], Phys. Rev. Lett., in press. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W27.00010: Spin phonon induced colinear order and magnetization plateaus in triangular and kagome antiferromagnets. Applications to CuFeO$_2$ Fa Wang, Ashvin Vishwanath Coupling between spin and lattice degrees of freedom are important in geometrically frustrated magnets where they can lead to degeneracy lifting and novel orders. We show that moderate spin-lattice couplings in triangular and Kagome antiferromagnets can induce complex colinear magnetic orders. When classical Heisenberg spins on the triangular lattice are coupled to Einstein phonons, a rich variety of phases emerge, including the experimentally observed four sublattice state and the five sublattice 1/5th magnetization plateau state seen in the magneto-electric material CuFeO$_2$. In addition we predict magnetization plateaus at 1/3, 3/7, 1/2, 3/5 and 5/7 at these couplings. Strong spin-lattice couplings induce a striped colinear state, seen in $\alpha$-NaFeO$_2$ and MnBr$_2$. On the Kagome lattice, moderate spin-lattice couplings induce colinear order, but an extensive degeneracy remains. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W27.00011: Jahn-Teller coupling and magnetic ground state in vanadium spinels$^1$ Gia-Wei Chern, Oleg Tchernyshyov The interplay of orbital, lattice, and spin degrees of freedom in vanadium spinels has attracted much interest among researchers. The V$^{3+}$ ion has two electrons occupying three degenerate $t_{2g}$ orbitals and is thus Jahn-Teller active. It also has a total spin $S=1$ in accordance with Hund's rules. Moreover, the V$^{3+}$ ions sitting on the $B$-site of spinel form a pyrochlore lattice, the interactions between these localized spin and orbital degrees of freedom are thus geometrically frustrated [1]. Here we present a theoretical model for the ground states of vanadium spinels. We view all of the vanadates (Cd, Zn, Mg on the one hand and Mn on the other) within the same model in which the influence of Mn is simulated by a magnetic field. In the case of MnV$_2$O$_4$, our calculation yields a ground state with antiferro-orbital ordering accompanied by a tetragonal structural distortion with lattice constants $a=b>c$. In addition, the V spins develop an orthogonal antiferromagnetic order in the $ab$ plane on top of a ferromagnetic moment along the $c$ axis. The results are consistent with a recent experimental characterization of MnV$_2$O$_4$ [2]. In particular, we will discuss the important role played by cooperative Jahn-Teller interaction and spin-orbital coupling in stabilizing the orthogonal spin configuration. [1] O. Tchernyshyov, Phys. Rev. Lett., {\bf 93}, 157206 (2004). [2] V. O. Garlea {\em et al.}, cond-mat/0711.1844. $\,^1$NSF Grant No. DMR-0348679 [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W27.00012: AC Susceptibility and Heat Capacity Studies of Geometrically Frustrated Pyrochlores Daniel Antonio, Andrew Cornelius, Jason Gardner Materials with a geometrically frustrated magnetic pyrochlore lattice have been of interest due to their unusual ground states. Tb$_{2}$Ti$_{2}$O$_{7}$ is known for going to a spin liquid ground state and does not transition to a long-range ordered state down to at least 50 mK [1]. Ho$_{2}$Ti$_{2}$O$_{7}$ has a macroscopically degenerate spin ice ground state which resembles that of the proton ordering in water ice [2]. Heat Capacity measurements of Tb$_{2}$Ti$_{2-x}$Sn$_{x}$O$_{7}$ were done from 300 K to 0.36 K and AC magnetic susceptibility measurements of Ho$_{2-x}$Y$_{x}$Ti$_{2}$O$_{7}$ and $_{2-x}$La$_{x}$Ti$_{2}$O$_{7}$were done for frequencies from 10 Hz to 10 kHz down to 1.8 K, both in magnetic fields up to 9 T. These experiments were performed to further understand the factors leading to their unusual behavior and the effects of introducing disorder through doping with nonmagnetic elements. Determination of the effect of an external field on the hyperfine crystal field at the Tb sites was done. In addition, unusual behavior in the ac susceptibility of the Ho samples at lower temperatures was observed. [1] J.S. Gardner \emph{et al.}, Phys. Rev. Lett. \textbf{82}, 1012 (1999) [2] S.T. Bramwell and M.J.P. Gingras, Science \textbf{294}, 1495 (2001) [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W27.00013: Local studies of AC demagnetization in a model frustrated magnet Jie Li, Xianglin Ke, Cristiano Nisoli, Paul Lammert, Vincent Crespi, Peter Schiffer We have studied the process of ac demagnetization using a model system consisting of single-domain ferromagnetic islands arranged on perpendicular square lattices such that the interaction between the islands are frustrated by the geometry of the arrays. The sample is first subjected to an oscillating magnetic field whose magnitude is stepped down by different step sizes and the sample is rotating at the same time. We find that the net moment of the arrays can be readily brought to near zero with the magnetic field step size below a certain threshold value. However the interaction energy of the demagnetized array state continues to decrease linearly with decreasing step size even as the step size approaches zero. We characterize the result in terms of developing local correlations between the island moments, with strong analogies to other athermal magnetic systems. This research has been supported by the Army Research Office. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W27.00014: Dimensional Crossover in ZnMn$_{2}$O$_{4}$ William Ratcliff, Ying Chen, Goran Gasparovic, Yiming Qiu, Qing Huang, Jeffrey Lynn, Sunmog Yeo, Sang Cheong, Paula Piccoli, Arthur Schultz ZnMn$_{2}$O$_{4}$ crystallizes at high temperatures as a cubic spinel. At lower temperatures, it undergoes a Jahn Teller distortion which lowers its symmetry to tetragonal. At lower temperatures (TN\~{ }60 K), the system orders magnetically. Fits to the order parameter, line shape of powder diffraction peaks, and the direct observation of rods of scattering in single crystal diffraction experiments show the system to be two dimensional. This is likely due to the interplay between orbital ordering and frustration. In this talk I discuss the magnetic structure and spin waves of this system. I will also discuss an interesting crossover in the dimensionality of the magnetism in this compound. [Preview Abstract] |
Session W28: Superlattices and Nanostructures: Electronic Properties II
Sponsoring Units: DCMPChair: Jong Han, State University of New York at Buffalo
Room: Morial Convention Center 220
Thursday, March 13, 2008 2:30PM - 2:42PM |
W28.00001: Tunnel spectroscopy in an ac-driven triple dot quantum shuttle J. Villavicencio, I. Maldonado, R. S\'anchez, E. Cota, G. Platero Within the framework of a fully quantum mechanical approach we use the density matrix master equation formalism to study the electronic transport in a triple dot quantum shuttle in the presence of an ac-field. We show that the ac-field induces photo-assisted tunneling which manifests itself as sidebands in the electronic current. We also show that these new tunneling sidebands can be explained in terms of simple sum rules involving the number of absorbed and emitted photons and the oscillator states participating in the process. Finally, we demonstrate that the tunneling channels can be controlled by manipulating the frequency and intensity of the ac-field, giving rise in particular to coherent destruction of tunneling (CDT). [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W28.00002: Fabrication and Characterization of a CMOS-Based Quantum Dot Device Ming Xiao, E. Yablonovitch, H.W. Jiang Silicon-based single electron devices are particularly attractive for implementing quantum information processing due to the extremely long electron spin lifetimes. We report here the demonstration of a stack-gated CMOS structure that can define a quantum dot in the few-electron regime and can be integrated with a sensitive, high bandwidth field effect transistor. Multiple lower layer side gates, as small as 50nm, on an ion-implanted Si/SiO2 wafer electrostatically define the quantum dot. A top gate that controls the electron population in the quantum dot is then fabricated on top of an isolating Al2O3 layer made by atomic layer deposition (ALD). The low-temperature ALD process provides excellent device stability while preserving the integrity of the side gates. We found that the devices can be operated effectively both in the accumulation mode and in the depletion mode. Transport through the quantum dots in the few-electron regime for currents less than 100fA can be reliably studied with a good reproducibility. We will detail our fabrication and characterization processes in this presentation. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W28.00003: Transport through a triple quantum-dot ring in high-spin state Yunori Nisikawa, Takahide Numata, Akira Oguri Using the numerical renormalization group (NRG), we study transport through a triple quantum-dot ring in high-spin state, connected to two noninteracting leads symmetrically. The transport is determined by two phase shifts for quasi-particles with even and odd parities. An isolated triple quantum-dot ring has a high-spin ground state of $S = 1$ caused by a Nagaoka ferromagnetism, when it has one extra electron introduced into a half-filling. The results show that the conduction electrons screen the local moment via two separate stages with different energy scales. The half of the $S = 1$ is screened first by one of the channel degrees, and then at very low temperature the remaining half is fully screened to form a Kondo singlet. A two-terminal conductance in the series configuration is suppressed similar or equal to 0, while plateau of a four-terminal parallel conductance reaches a unitary limit value similar or equal to $4e2/h$ of two conducting modes. We also present the relation between electronic states and thermodynamic quantities calculated using NRG eigen energies. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W28.00004: Valley Splitting in Different Landau Levels of a Si/SiGe Quantum Point Contact Lisa McGuire, K.A. Slinker, Mark Friesen, Srijit Goswami, J.O. Chu, Robert Joynt, S.N. Coppersmith, Mark A. Eriksson Si/SiGe is an interesting platform for spin physics and quantum information due to the weak spin-orbit coupling in Si and the presence of nuclear spin zero isotopes. However, silicon has a near degeneracy of orbital states in the conduction band arising from multiple valley minima, which could enhance decoherence rates and complicate qubit operation. Recent measurements in a quantum point contact have shown that the valley splitting is large, of order 0.5 - 2 meV$^{1}$. Here, we investigate fundamental mechanisms of valley splitting by taking into account the valley couplings between Landau levels. We also account for the dependence of valley splitting on materials parameters such as miscut angle and device orientation. From our data, we are able to extract distinct valley splittings from the lowest two Landau levels, which vary similarly as a function of gate voltage (i.e., channel width). We are further able to place bounds on local variations of the tilt angle of the quantum well interface. Work supported by ARO, NSA, and NSF. 1 S. Goswami et al., Nature Physics 3 (1): 41-45 JAN 2007. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W28.00005: Counting statistics and conditional evolution in a quantum electromechanical system Steven Bennett, Aashish Clerk We present a theoretical study of full counting statistics (FCS) and conditional evolution of a quantum point contact (QPC) coupled to a mechanical oscillator. Such a system has recently been studied in several experiments. Starting from a microscopic model, we derive a master equation for the reduced density matrix that contains several important differences from the usual equation used to describe conditional position evolution ({\it i.e.} the dynamics of the oscillator inferred from a particular measurement outcome of current through the QPC). The master equation may then be solved analytically to obtain the FCS and conditional evolution. We find that the oscillator can significantly affect the FCS, leading to a highly non-Gaussian distribution characterized by an enhanced third moment even for very weak coupling. In the conditional evolution we find clear evidence that the back-action of the QPC on the oscillator cannot simply be described as coupling to an effective thermal bath. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W28.00006: Thermo-electric effects in nanoscale systems out of equilibrium Yonatan Dubi, Roberto D'agosta, Massimiliano Di Ventra As technology advances into the nanoscale regime, probing the electronic properties of nanoscale circuits has become a major challenge. Specifically, it has been suggested that thermo-electric effects may serve as a tool to study electronic properties of nanoscale systems, and experiments on thermo-power in quantum point contacts (QPCs) and molecular circuits have been performed. On the theoretical side, however, linear-response theory is inadequate to determine the dynamical formation of the thermo-electric effect. Here, we propose a novel scheme to calculate dynamical thermo-electric effects in nanostructures arbitrarily far from equilibrium using a local generalization of the Lindblad master equation. We demonstrate the method by calculating the charge imbalance of a QPC in the presence of Coulomb interactions and a temperature gradients, and obtain the long-time energy distribution in the QPC out of equilibrium. Our suggested scheme can be implemented into stochastic time-dependent current-density functional theory [PRL, 98, 226403 (2007)], thus providing a valuable tool in studying the interplay of charge and energy currents for arbitrary many-body systems. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W28.00007: Universality and the thermal dependence of the conductance of nanodevices Luiz N. Oliveira, Antonio C. Seridonio, Makoto Yoshida The conductance of a quantum wire side-coupled to a quantum dot will be discussed. In this device, plots of the conductance $G$ vs.\ the gate voltage $V_g$ applied to the dot display Fano antiresonances due to the interference between the current traversing the~wire and the flux of electrons that hop to the dot to bypass the adjacent section of the wire; at fixed $V_g$'s, the interference accounts for a variety of thermal dependences $G(T)$. Analytical renormalization-group arguments will be presented that map $G(T)$ to the universal curve $g(T/T_K)$ for the conductance of the spin-degenerate Anderson impurity Hamiltonian, with temperatures normalized by the Kondo temperature $T_K$. This linear, universal mapping will be shown to (i) generate curves in excellent agreement with the measurements of Sato~et al.~[Phys.\ Rev.\ Lett.\ {\bf 95}, 066801 (2005)] and justify those authors' phenomenological description of their data; (ii) fit novel numerical renormalization-group data for the conductance of the side-coupled device; and (iii) link $G(T)$ to the conductance of the single-electron transistor. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W28.00008: Electronic Properties of Quantum Point Contacts in a Quantum Ring A. Devrim Guclu, Cyrus J. Umrigar, Harold U. Baranger We investigate the electronic properties of a narrow constriction (quantum point contact) in a quantum ring using variational and diffusion Monte Carlo methods. Quantum point contacts are basic building blocks of nanoscale devices. The experimental control over their width allowed the observation of conductance quantization in integer steps of $G_0 =2e^2/h$. However, a puzzling additional structure is also observed around $0.7G_0$ in some devices. One possible explanation is the formation of a local quasi-bound state. Here, we present a first quantum Monte Carlo calculation showing that electrons can be strongly localized in the constriction, with a well quantized electron number $N$ that varies abruptly as the width of the point contact decreases. We also study the low-lying excited states and investigate the possibility of a spin texture as a function of applied gate voltage. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W28.00009: Piezoelectric models for semiconductor quantum dots Morten Willatzen, Benny Lassen, Danielle Barettin, Lok Lew Yan Voon The importance of fully-coupled and semi-coupled piezoelectric models for quantum dots will be presented and compared to corresponding results for one-dimensional heterostructures. Electron energies differences of up to around 30 meV were found possible for GaN/AlN quantum dots. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W28.00010: High sensitivity cantilevers for measuring persistent currents in normal metal rings Ania Bleszynski Jayich, Will Shanks, Rob Ilic, Jack Harris We propose a new approach to measuring persistent currents in normal metal rings. By integrating micron-scale metal rings into sensitive micromechanical cantilevers and using the cantilevers as torque magnetometers, it should be possible to measure the rings' persistent currents with greater sensitivity than the SQUID-based and microwave resonator-based detectors used in the past. In addition, cantilever-based detectors may allow for measurements in a cleaner electromagnetic environment. We have fabricated ultra sensitive cantilevers with integrated rings and measured their mechanical properties. We present an estimate of the persistent current sensitivity of these cantilever-based detectors, focusing on the limits set by the cantilever's Brownian motion and the shot noise in the laser interferometer that monitors the cantilever. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W28.00011: Electron Pair Resonance in the Coulomb Blockade Mikhail Raikh, Eran Sela, Heung-Sun Sim, Yuval Oreg, Felix von Oppen Transport through a nanostructure in the regime of Coulomb blockade is dominated by {\em elastic} single-electron cotunneling. We study many-body corrections to the cotunneling current via a localized state with energy $\epsilon_d$ at large bias voltages $V$. We show that the transfer of electron pairs, enabled by the Coulomb repulsion in the localized level, results in ionization resonance peaks in the third derivative of the current with respect to $V$, centered at $eV = \pm 2\epsilon_d/3$. Our results predict the existence of previously unnoticed structure within Coulomb-blockade diamonds. Remarkably, this new structure emerges within the standard Anderson Hamiltonian conventionally used for description of transport through nanostructures. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W28.00012: Quantum Dot Array for Simulating the Hubbard Model Ching-Tzu Chen, Dennis M. Newns, Chang C. Tsuei, Jeffrey J. Urban As the simplest theory that incorporates strong onsite Coulomb repulsion, the Hubbard model is fundamental to the understanding of phenomena in strongly correlated systems, e.g. the metal-insulator transition in transition-metal oxides. It is further widely believed that the 2D square-lattice Hubbard model captures the puzzling physics of high-temperature superconductivity. Since an analytical solution of the model is not viable, we propose to study its properties through a ``quantum simulator'' made from quantum dot arrays (QDAs). In this talk, we first demonstrate the correspondence between the Hubbard Hamiltonian and the simplified model for the QDA. We then address the scaling of the crucial parameters for designing a practical simulator. Two systems for implementing the QDA are proposed -- the self-assembled nanocrystal arrays, and the lithographically defined 2D-electron-gas QDA. Preliminary experimental results derived from these systems will be presented. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W28.00013: Correlation effects with low electron density along a potential barrier in quantum point contacts Karan Aryanpour, Jong Han We study correlated electrons effect in the conductance through Quantum Point Contacts (QPC). Conductance of a QPC is quantized in steps of $G_{0}=2e^{2}/h$ (e the charge of an electron and h Planck's constant). Experiments also reveal an additional shoulder near $0.7G_{0}$ referred to as the {\it 0.7 Anomaly}. Evidence supporting spin $1/2$ magnetic moment formation in the conductance channel has motivated scenarios such as the Kondo effect. We intend to address whether or not the $0.7$ anomaly is a many-body effect associated with the formation of a spin $1/2$ magnetic moment in the conductance channel. We employ the Quantum Monte Carlo (QMC) technique for electrons on a 1-D QPC lattice with an adiabatic potential barrier. The QPC lattice includes the Hubbard lattice in the QPC region and two leads modeled by semi-infinite chains and we calculate the conductance along the chain using the Kubo formula. The physics is determined by the competition between the many-body interaction and the small kinetic energy at the top of the potential barrier. Due to the singular nature of the density of states at low electron density along with the spatial inhomogeneity, the many-body effects are expected to differ from conventional wide-band limit physics. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W28.00014: Persistent currents in normal metal rings Hendrik Bluhm, Nicholas C. Koshnick, Julie A. Bert, Martin E. Huber, Kathryn A. Moler We have measured the magnetic response of more than 20 individual mesoscopic gold rings at low temperatures. The rings were characterized one by one using a scanning SQUID microscope, which also enabled in situ measurements of the sensor background. All measured rings show a paramagnetic linear susceptibility and a poorly understood anomaly around zero field, both of which we attribute to unpaired defect spins. The response of some sufficiently small rings also has a component that is periodic in the flux through the ring. Its period is close to $h/e$, and its sign and amplitude vary from ring to ring. Including rings without a detectable periodic response, the amplitude distribution is consistent with predictions for the typical $h/e$ persistent current in diffusive metal rings. The temperature dependence of the response, measured for two rings, is also consistent with theory. [Preview Abstract] |
Session W29: Focus Session: Carbon Nanotubes and Related Materials XIV: Theory and Sensing
Sponsoring Units: DMPChair: Sankar Das Sarma, University of MarylandBob Haddon, University of California, Riverside
Room: Morial Convention Center 221
Thursday, March 13, 2008 2:30PM - 3:06PM |
W29.00001: Electron-phonon interaction and excited states relaxation in carbon nanotubes Invited Speaker: We will discuss the role of electron-phonon interaction on excited states relaxation and phonon spectra in carbon nanotubes (CNTs). The electron-phonon interaction leads to the polaronic effects of the charge carriers, but it also renormalizes the energy and the lifetime of phonons. We present a theoretical model that predicts the changes induced in the phonon modes of CNTs as a function of the charge carrier doping, i.e. position of the Fermi level. In agreement with the predictions, our experiments show sharpening and blue shifts of the G-phonons of metallic CNTs, but only blue shifts for semiconducting CNTs, making the Raman scattering a useful probe of local doping of CNTs [1]. The non-equilibrium dynamics of charge carriers under external electric field is determined by the electron-phonon scattering. The hot carriers under unipolar transport conditions can be produced, leading to the strong impact excitation and light emission, which intensity is determined by electric field, phonon scattering, and impact excitation cross section [2, 3]. In the reverse process of photoconductivity, light is absorbed creating excited states. We will discuss electronic relaxation of high energy excited states leading to the free carriers, contributing to the photoconductivity, and phonon relaxation, leading to the bound excitons [4]. The later can contribute to the photocurrent only after ionization by the external field [5]. Finally, we will discuss the role of phonons in the long puzzling question regarding the nature of the dominant decay channel of the low energy excited states and the potential of optoelectronic applications of CNTs. [1] J.C. Tsang, M. Freitag, V. Perebeinos, J. Liu, and Ph. Avouris, Nature Nanotechnology 2, 725 (2007); [2] J. Chen, V. Perebeinos, M. Freitag, J. Tsang, Q. Fu, J. Liu, Ph. Avouris, Science 310, 1171 (2005); [3] V. Perebeinos and Ph. Avouris, Phys. Rev. B. 74, 121410(R), (2006); [4] T. Hertel, V. Perebeinos, J. Crochet, K. Arnold, M. Kappes, Ph. Avouris, arXiv:0710.3154; [5] V. Perebeinos and Ph. Avouris, Nano Lett. 7, 609 (2007) [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W29.00002: Study of Butane adsorption on Purified HiPco Carbon Nanotubes Toyo Furuhashi, Dinesh Rawat, Aldo Migone We investigated the adsorption characteristics of butane on purified HiPco single-walled carbon nanotubes for coverages in the first layer. We measured 9 full isotherms between 180 and 260K. The results for butane are compared with those obtained in a previous study of ethane adsorption on the same substrate. Comparable values for the specific surface area of the substrate were found when this quantity was measured using either ethane or butane. This strongly suggests that both of these species have access to essentially the same adsorption sites. We also determined that the strength of binding of the hydrocarbon chains to the SWNT bundles correlates, roughly linearly, with molecular length. An increase in chain length provides a greater number of contacts between sites on the substrate and the adsorbate species, thus, resulting in an increase in the values of the binding energies. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W29.00003: Adsorption Kinetics of CH$_{4}$ on Purified HiPco Single-Walled Carbon Nanotubes Murat Bulut, Dinesh S. Rawat, Aldo D. Migone We present the results of an adsorption kinetics study of CH$_{4}$ on two sets of purified HiPco SWNTs. We monitor the time evolution of the pressure from the instant at which gas is added to the cell containing the nanotubes to the moment at which equilibrium is reached. The two sets of samples were baked, under vacuum, at different temperatures (300 and 400$^{\circ}$C). The difference in baking temperatures resulted in a difference in the specific surface areas; the sample treated at 400$^{\circ}$C has a 15{\%} larger surface area than the sample treated at 300$^{\circ}$C. It also caused a difference in the kinetic behavior of the samples; the equilibration times for the two samples differ by a factor of 3. Moreover, the sample heated at 400$^{\circ}$C exhibits two distinct equilibration times, while the one heated at 300$^{\circ}$C exhibits only one. These changes are probably the result of partially opening the purified SWNTs by baking them at 400$^{\circ}$C, which does not occur when we heat them only to 300$^{\circ}$C. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W29.00004: Adsorption kinetics of diatomic molecules on carbon nanotube bundles Jared Burde, Mercedes Calbi A Kinetic Monte Carlo algorithm is used to explore the kinetics of adsorption of diatomic adsorbates on one-dimensional chains of sites. In particular, we monitor the evolution of the orientational configuration of the adsorbate as equilibrium is being reached at different values of temperature and chemical potential. We also analyze the dependence of the orientational evolution of the phases on the interactions between the molecules and on the presence of adsorption sites with different energies. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W29.00005: Equilibration times of adsorption on external surfaces of carbon nanotube bundles Nayeli Zuniga, Jared Burde, Mercedes Calbi We investigate the adsorption kinetics of gases on the exterior of a carbon nanotube bundle by monitoring the uptake and exchange of particles in regions of the surface characterized by different binding energies. By using a Kinetic Monte Carlo scheme, we follow the time evolution of the gas uptake for different values of external pressure and temperature. The presence of adsorption sites with different energies gives rise to distinctive features on the equilibration time as function of the coverage. We show that preliminary experimental results for CF$_{4}$, Ar, and CH$_{4 }$on nanotube bundles with closed ends are consistent with our results. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W29.00006: Adsorption kinetics of binary mixtures on carbon nanotube bundles Seyoum Tsige, Jared Burde, Mercedes Calbi We examine kinetic selectivity effects that take place during the adsorption of a binary mixture on one-dimensional chains. After reaching equilibrium at the same chemical potential, the species with the higher binding energy will enjoy the greatest coverage. However, the weaker-binding species has faster adsorption kinetics and is able to reach a coverage higher than its equilibrium value before the stronger species can adsorb significantly. The result of this process is an `overshoot' in the fractional coverage of the weaker species that is reached at a time long before the system equilibrates. We analyze the appearance of this overshoot as a function of the temperature, chemical potentials, and energy parameters of the system. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W29.00007: Hydrogen sensing properties of palladium-decorated carbon nanotube circuits Vaikunth Khalap, Tatyana Sheps, Alexander Kane, Philip Collins Sensitive hydrogen gas sensors can be fabricated from carbon nanotube circuits decorated with palladium metal, and we have investigated the responsible physical mechanisms using isolated, single-walled carbon nanotubes (SWCNTs). Hydrogen sensitivity arises from two active mechanisms, neither of which is the mere adsorption of Pd onto pristine SWCNTs. The first mechanism relies on the chemical sensitivity of Schottky barriers present when semiconducting SWCNTs are contacted by metals. Pd decoration of the barrier region, or the use of pure Pd as the contact metal, produces a modest H$_{2}$ sensitivity, if any. A more sensitive mechanism involves Pd-decorated defect sites, which in both metallic and semiconducting SWNTs results in reversible conductance swings of 100{\%}. This presentation will review the temporal dynamics and pressure dependence of both mechanisms. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W29.00008: Electrical Measurement of Single Molecule Catalysis using Carbon Nanotubes Brett R. Goldsmith, Alexander A. Kane, Vaikunth Khalap, John Coroneus, Gregory A. Weiss, Philip G. Collins We demonstrate single molecule chemical sensors based on single-walled carbon nanotubes (SWNT). The architecture uses a SWNT conductor having a single, reactive species covalently bonded to the sidewall [1]. Dynamics of the molecule are electrically transduced as it interacts with its surrounding environment. As a test case, we investigate the catalytic modification of EDC by a carboxylate. After creating a carboxylate terminus on the SWNT, the circuit is monitored for several hours and through hundreds of individual EDC reactions. Statistical analysis determines the lifetime of the carboxyl-EDC complex, as well as the catalytic turnover rate, from discrete events. Because the carboxylate site can be readily derivatized with proteins, peptides, or other functional molecules, the technique shows promise as a tool for single molecule research independent of optics and scanning probe microscopy. 1. B.R. Goldsmith, et al. \textit{Science} \textbf{315}, 77 (2007). [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W29.00009: Artificial introduction of defects in carbon nanotubes through Argon and Hydrogen ion irradaition, and application to chemical sensors Prabhakar Bandaru, Jeffrey Nichols, Mark Hoefer The goal of this study is to quantify the effect of defect density on the electrochemical properties of multi-walled CNTs. Consequently, ion irradiation, with argon (Ar) and hydrogen (H), individually, has been performed to systematically incorporate defects into vertically aligned MWCNTs. Raman spectroscopy was used to characterize the amount of disorder within the nanotube samples. The electrochemical behavior of the irradiated MWCNT samples was then characterized through cyclic voltammetry (CV) measurements. Raman spectroscopy revealed an increase in the disorder in MWCNTs with the argon and hydrogen irradiation, as evidenced by an increase in the I$_{D}$/I$_{G}$ peak intensity ratio. However, Ar is intercalated into the CNTs, and charges the nanotubes (forming dangling bonds), while H treatment terminates residual CNT dangling bonds. In CV, we have seen that only the Ar treated samples exhibit \textit{perfect} reversible Nernstian behavior characteristic of ideal electrodes. Hydrogen treated CNT electrode ensembles seem to exhibit quicker response, as glucose sensors, with exquisite ($\sim $ 1 $\mu $M) sensitivity [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W29.00010: Structure and thermodynamics of a Neon monolayer adsorbed on carbon nanotube bundles Oscar Vilches, Michel Bienfait, Mark Johnson, Stephane Rols, Subramanian Ramachandran We report results from neutron diffraction measurements of five submonolayer coverages of Ne adsorbed on single-wall, closed-end carbon nanotube bundles (SWCNB). Our recent thermodynamic study of Ne adsorbed on SWCNB, Phys. Rev. B, 76, 075404 (2007), showed one-dimensional (1d) solid behavior below 4K at doses less than 0.08 monolayer, which crossed over to 3d-like behavior above 16K without signature of a phase transition. Above 0.18 monolayer there is a 2d-like solid behavior below 8K, with Debye temperatures in the 45 to 53K range, similar to those found for 2d Ne/graphite, but there is no melting transition at 13.5K as seen in 2d. Our structure measurements on Ne adsorbed on SWCNB similar to those used for the thermodynamic study were performed at 2K using beam line D20 at ILL. Results show the shift from a 1d to a 2d solid structure with increasing coverage. Lattice parameters, the relationship between the thermodynamic and structural measurements, and theoretical expectations will be shown and discussed. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W29.00011: Structure of a DNA-carbon nanotube hybrid using replica exchange molecular dynamics Robert Johnson, A.T. Charlie Johnson, Michael Klein DNA-carbon nanotube hybrids (DNA-CN) are novel nanoscale materials that consist of single-wall carbon nanotubes coated with a self-assembled monolayer of single stranded DNA (ssDNA). Many recent experiments have demonstrated that this nanomaterial is an ideal candidate for a variety of nanotechnological applications. Despite the importance of this material, a complete understanding of its structural and physical properties is lacking. Recent molecular dynamics (MD) simulations of this nanomaterial have provided information about the self-assembly mechanisms and possible ssDNA conformations that characterize DNA-CN. However, MD simulations of biopolymers at low temperatures (T $\sim $ 300 K) result in kinetic trapping and limits sampling of ssDNA configurational space. Here, we present the results of large scale replica exchange molecular dynamics simulations that provide robust sampling of the multitude of ssDNA conformations about SWCN. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W29.00012: Photon sensing with carbon nanotubes: low-temperature photothermal effect on carbon nanotube transistors LiHong Herman, Adam Tsen, Jiwoong Park While thermal effects on the electrical properties of carbon nanotubes (CNTs) are an area of great interest, the effect of single photons on the low temperature conductance of CNTs has not been carefully studied. We recently developed a low-temperature fiber-based laser confocal microscope with simultaneous electrical measurement capability and diffraction-limited laser illumination and detection. In our experiment, CNT devices functionalized with gold nanoparticles were illuminated with a focused laser beam while their conductance was measured at temperatures as low as 300 mK. Photon absorption by either CNTs or gold nanoparticles results in local heating and provides spatially-resolved information about the thermal effects on transport in the CNT as the laser is scanned across the device. Our technique can be easily extended to the study of single-photon effects in other nanostructures including nanowires and graphene in the near future. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W29.00013: Transport and Charge Sensing in $^{12}$C and $^{13}$C Carbon Nanotube Double Quantum Dots Hugh Churchill, David Marcos, Andrew Bestwick, Jennifer Harlow, Carolyn Stwertka, Susan Watson, Charles Marcus We report measurements of gate-defined carbon nanotube double quantum dot devices with a charge sensor fabricated on the same nanotube. The methane used during growth controls the $^{13}$C content of the nanotubes. $^{12}$C nuclei have zero nuclear spin, and $^{13}$C nuclei have spin 1/2. We compare samples with natural abundance (1\%) and enriched (99\%) $^{13}$C content. A strong isotope effect is observed in the magnetic field dependence of transport at finite bias. Fast control of these devices is demonstrated using a pulsed-gate technique. [Preview Abstract] |
Session W30: Nanotubes and Nanowires III: Other Properties II
Sponsoring Units: DMPChair: Michael Fuhrer, University of Maryland
Room: Morial Convention Center 222
Thursday, March 13, 2008 2:30PM - 2:42PM |
W30.00001: Nanotube Radio Kenneth Jensen, Jeff Weldon, Henry Garcia, Alex Zettl We have constructed a fully functional, fully integrated radio receiver from a single carbon nanotube. The nanotube serves simultaneously as all essential components of a radio: antenna, tunable band-pass filter, amplifier, and demodulator. A direct current voltage source, as supplied by a battery, powers the radio. Using carrier waves in the commercially relevant 40-400 MHz range and both frequency and amplitude modulation techniques, we demonstrate successful music and voice reception. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W30.00002: Template-Based Electroless deposition and TEM Analysis of TiO$_{2}$ Nanotubes Isabel Schultz, Haidong Liu, Zuxin Ye, Wenhao Wu We report on the fabrication of TiO$_{2}$ nanotubes using a template-based electroless deposition method. We used anodic aluminum oxide membranes of pore diameter $\sim $ 200 nm as the templates. The TiO$_{2}$ nanotubes were fabricated by first dipping the membranes into a titanium tetrafluoride solution at 60 \r{ }C for 30 minutes, and then annealing at 225 \r{ }C for 2 hours. The composition of the nanotubes was verified using the energy dispersive spectroscopy. We used scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to study the surface morphology of the nanotubes extracted after the membranes were dissolved. The nanotubes were found to be uniform along the length of the nanotube. For TEM analysis, an ion-milling technique was also used to produce a thin sample region so that the cross-section of the nanotubes remaining in their original pore channels could be directly imaged. The thickness of the TiO$_{2}$ nanotubes is about 50 nanometers, and could be controlled by varying the deposition time. Potential applications of these semiconducting TiO$_{2}$ nanotubes for forming nanostructured semiconducting interfaces will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W30.00003: Electron Microscopy Analyses of Nanowires Electrochemically Deposited into Porous Membranes Haidong Liu, Zhiping Luo, Zuxin Ye, Wenhao Wu We fabricated single crystal nanowires of Zn, Sn and Pb by electrochemically depositing materials into the pores of porous anodic aluminum oxide membranes and polycarbonate membranes. We applied an \textit{in situ} self-contacting technique to electrically contact single nanowires with macroscopic electrodes of Au, Sn, and Pb pre-fabricated on the membrane surfaces. We observed an anomalous long-range proximity effect in this nanowire/electrode system. In this talk, we describe electron microscopy methods we used to analyze the structure and the composition of the nanowires/electrode system. These included analyses of extracted nanowires using the scanning electron microscope (SEM) and the transmission electron microscope (TEM). Nanowires remaining in their original pore channels were also analyzed with TEM using samples prepared by ion-milling and ultramicrotomy. These analyses revealed that the nanowires were single crystalline. Furthermore, the interface between the nanowires and the electrodes were directly imaged. The chemical compositions of the nanowires were also confirmed by the energy dispersive spectroscopy (EDS) analyses and mappings. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W30.00004: Characterization of individual SnO$_{2}$ nanowires with surface science techniques Khabibulakh Katsiev, Ulrike Diebold, Andrei Kolmakov Tin oxide is widely used as a solid-state gas sensor for detection of combustible and toxic gases. Recently, the use of nanobelts and nanoribbons has been suggested as novel materials for gas sensing applications. Large surface-to-volume ratio of the semiconducting metal oxide nanobelts and the congruence of the carrier screening length with their lateral dimensions make them highly sensitive and efficient transducers of surface chemical processes into electrical signal. The surface morphology of an individual nanobelts (NB) was studied with STM. Atomically resolved STM images of NBs reveal an 1X1 (101) SnO$_{2}$ structure on the top surface of the NB. The thermal stability of the NBs was studied with SEM. The critical temperatures were determined, where structural changes occur in UHV, O$_{2}$, and air. XPS was used to characterize chemical composition and monitor the cleanness of the NB material. Ca and C contamination was detected on as-grown SnO$_{2}$ nanobelts. O plasma, ozone treatment, and annealing in oxygen were used to remove the contaminants. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W30.00005: Structure and Stability of Metal Oxide Nanowires De Nyago Tafen, James Lewis We present a comprehensive theoretical study --- within the framework of {\it ab initio} density functional theory method --- of the structural and stability properties of metal oxide nanowires. We consider nanowires with $\langle$100$\rangle$ growth direction with several diameters and surface facet configurations. A stability analysis of the results obtained for theses nanowires is used to determine the most stable geometries. We show that the perimeter of the nanowires is a meaningful dimensional parameter, and that the surface facets play a central role on the energetics of the nanowires. The results are compared to available experimental data. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W30.00006: First principles investigation of InN non-polar surfaces and nanowires Aleksandrs Terentjevs, Alessandra Catellani, Giancarlo Cicero In the last years InN nanostructures have been proposed for application in solar cells, because of the outstanding electronic properties of this nitride compound. An increase of solar energy conversion in these kinds of cells requires a deep knowledge of surfaces properties, and on the effect of confinement on the electronic properties of the material. Here we present an investigation of the structural and electronic properties of InN nanowires as obtained by means of \textit{ab initio} Density Functional calculations. First we discuss the results for the clean (1-100) and (11-20) faces, which are usually exposed in nanostructures, then we show how InN electronic properties change in nanowires due to confinement effects. We will finally present a possible InN functionalization pathway based on the use of molecules containing thiol groups. Our results show that thiols may attach to the surface following an exothermic process (dissociation energy is about 2.5 eV/mol for the (11-20) surface), thus they represent an effective anchoring group for the realization of hybrid InN based devices. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W30.00007: Van Der Waals Interaction between Two Parallel Radially Deformed Single Wall Carbon Nanotubes Adrian Popescu, Lilia Woods, Igor Bondarev The van der Waals potential energy is calculated between two parallel infinitely long radially deformed single walled carbon nanotubes within the pairwise Lennard-Jones approximation for extended systems. The nanotubes will undergo different geometrical radial shape transitions if an external hydrostatic pressure with an increasing strength is applied. We describe these shapes with analytically in order to facilitate the calculations. The most preferred mutual orientations are determined in all considered cases in terms of their potential well depths, equilibrium distances, and geometrical parameters. We find that the interaction evolves in such a way as to keep the distance between the interacting surfaces comparable to the graphene-graphene distance in graphite. In addition, the universal graphitic potential concept is extended to radially deformed carbon nanotubes. These results can be used as a guide for future experiments to investigate interactions between deformed carbon nanotubes. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W30.00008: Atomic Structure of the Si(111)-4x1-In System Barry Haycock, J.D. O'Mahony The indium-induced 4 x 1 reconstruction on silicon (111) has been extensively studied due to its unique physical structure and electronic properties, which indicate so-called ``Quantum Wire Behaviour''. The likely crystallographic structure of this system has been hotly debated since 1999. The structure was mathematically modeled by Tsay using a planewave calculation method in 2005, which yielded results of atomic positions that matched closely to the Bunk model. In this study we model the this system using the molecular dynamics (MD) package Fireball, taking as a starting point the atomic positions of the Bunk model. This ab-initio tight-binding MD method has the advantage of being able to operate with a very large number of atoms per cell in a single calculation, thus allowing for a very large superslab in the calculation model. This allows for greater surface area and thus is expected to produce a more accurate surface characteristic calculation result. The results of this calculation are compared to the recent results of Tsay. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W30.00009: Vapor-solid-solid growth mechanism driven by epitaxial match between solid AuZn alloy catalyst particle and ZnO nanowire at low temperature Rodrigo Lacerda, Leonardo Campos, Matteo Tonezzer, Andre Ferlauto, Rogerio Paniago, Sergio Oliveira, Luiz Orlando Ladeira, Vincenzo Grillo The present paper provides a comprehensive picture of the precise mechanism of ZnO vapor-solid-solid nanowire growth at low temperatures and gives the fundamental reasons responsible [1]. We demonstrate by using a combination of synchrotron XRD and high resolution TEM that the growth dynamics at low temperatures is not governed by the well-known VLS mechanisms. Based on the Au-Zn phase diagram, temperature measurement and temperature size effects, we show that growth occurs via VSS. The precise composition of the Au-Zn catalyst nanoparticle has been determined to be $\gamma $-AuZn. Furthermore, we experimentally observe that there is an indication of a epitaxial relationship between the ZnO nanowires and the $\gamma $-AuZn seed particle. A critical new insight on the driving factor of VSS growth is proposed in which the VSS process occurs by a solid diffusion mechanism that is driven by a preferential oxidation process of the Zn inside the alloy catalyst induced by an epitaxial match between the ZnO(10-10) plane and the $\gamma $-AuZn(222) plane. [1] L. C. Campos et al, Adv. Materials (accepted for publication). [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W30.00010: Temperature effects in the stability of pure and doped gold nanowires Antonio J.R. da Silva, Edwin Hobi Jr., Adalberto Fazzio Gold nanowires have attracted a great deal of attention, one of the reasons being the ability to form linear gold chains that are one atom wide and that have just a few atoms in length. One of the amazing characteristics of these wires is their stability, even at ambient temperatures. Therefore, it is very important to understand why they are stable and why they break. Moreover, inserted impurities, such as H and C, can dramatically affect their stability. In the present work we use state-of-the-art ab initio molecular dynamics to perform simulations of pure and doped Au nanowires. We propose a general mechanism that helps to explain the stability of these wires and the rupture process. We show that triplets of Au atoms in the neck of atomically thin nanowires have an instability towards rupture at a length around 6.0-6.1 angstroms. At this length, the potential energy surface (PES) describing the motion of the central atom changes from a single minimum at the middle of the triplet, to a double minimum profile. This provides a consistent picture of the mechanism of rupture of metallic nanowires. Impurities such as C and H also have this behavior. However, as they have stiffer bonds, these PES instabilities always happen in pure Au bonds. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W30.00011: Surface Effects on the Electronic, Magnetic and Structural Properties of Free-standing InP nanowires Tome Schmidt Nanowires due to their large surface-to-volume ratio of particular interest is the understanding of the surface structure and the electronic effects due to surface states. In this work we investigate the structural and electronic properties of InP nanowires aligned along the [111] direction, in which the surface have been studied by passivating with hydrogens or OH radicals and also oxidized. The magnetic properties of Mn doped InP nanowires with different surface termination have been also investigated. Our {\it ab initio} density functional calculations show that hydrogen passivation removes the surface states, opening up the band gap. Our results for oxygen adsorbed on the hydrogen passivated InP nanowires, show that there are many configurations where the oxygens are chemisorbed processes. For Mn doped InP nanowires our results reveal that the surface of the nanocrystals play a fundamental role on the impurity stability and on the magnetic properties of InP nanowires. The formation energy of pairs of Mn impurities in unpassivated nanowires are lower than that of the bulk InP. Most of the Mn pair configurations present FM coupling and their prefer to be inside the nanowire and not on the surface. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W30.00012: Shift- and injection-current optical response of BN nanotubes Norberto Arzate, Fred Nastos, Raul A. Vazquez-Nava, Miguel Gonzalez, Bernardo Mendoza, John Sipe We present the frequency dependence of the electrical shift and injection currents optically generated on BN nanotubes. The shift and injection currents are a second order effect that is associated to a divergence of the nonlinear susceptibility at zero frequency. The shift current is generated with linearly polarized light and the injection current can be generated with circularly polarized light. We make use of density functional theory and pseudopotentials to calculate the nanotube structures and their electronic states. The current-coefficient calculations are done in optical rectification, within the independent particle approximation and within a full band structure scheme. We also obtain the frequency dependence for the shift distance that the center of electron charge moves in the shift-current process and for the swarm velocity or maximum velocity acquired by the electrons in the injection process. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W30.00013: An ab initio study of the interaction of transition metal atoms with single-wall armchair SiC nanotubes Kazi Alam, Asok Ray A systematic study of Fe atom encapsulated and adsorbed in armchair SiC nanotubes has been performed using hybrid density functional theory calculations within finite cluster approximation. A detailed comparison of the binding energies, equilibrium positions, Mulliken charges and spin magnetic moments of Fe atoms has been performed for three types of nanotubes. The electronic states, HOMO-LUMO gaps, and changes in gaps with respect to the bare nanotube gaps have been investigated as well. Binding energies of the encapsulated and adsorbed Fe atoms indicate that these structures are stable and show site dependence. For both cases significant band gap decrease is observed for type 1 nanotubes enabling band gap tailoring. This decrease is not observed for the other two types in both cases of interactions. All the structures are found to have magnetic ground states with very high magnetic moments indicating the possibility of them being used as nanomagnets. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W30.00014: Energetics of Cu Nanowires Mine Konuk, Berk Onat, Sondan Durukanoglu We have calculated activation energies for several single atom and vacancy diffusion mechanisms on the $<$100$>$ and $<$110$>$ axially oriented, rectangular Cu nanowires with a particular interest in determining the effect of varying cross-sectional area on the activation barriers for the investigated processes. The calculations are performed using the nudged elastic band technique based on the interaction potential obtained from the embedded atom method. Our results on activation barriers for adatom diffusion mechanisms indicate a clear dependence on the cross-sectional area of the nanowires. We, furthermore, find that the energy barrier for single vacancy diffusion is decreasing drastically near the outer wall compared to barriers for single vacancy diffusions taking place in the interior region of the nanowire. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W30.00015: {\it Ab Initio} Computational Studies of the Electronic and Optical Properties of SiC Nanotubes Guang-Yu Guo, I-Jen Wu Since the discovery of carbon nanotubes (CNTs) in 1991 by Iijima, carbon and other nanotubes have attracted considerable interest worldwide because of their unusual properties and also great potentials for technological applications. Though CNTs continue to attract great interest, other nanotubes such as BN nanotubes (BN-NTs) may offer different opportunities that CNTs cannot provide. We have carried out systematic computational studies of various physical properties of SiC nanotubes. In this talk, we will present ab initio calculations of electronic, linear and nonlinear optical properties of SiC nanotubes [1]. \newline [1] I.J. Wu and G.Y. Guo, Phys. Rev. B {\bf 76}, 035343 (2007). [Preview Abstract] |
Session W31: Focus Session: New Materials and Properties of Complex Oxides
Sponsoring Units: DMP GMAGChair: Amos Sharoni, University of California, San Diego
Room: Morial Convention Center 223
Thursday, March 13, 2008 2:30PM - 2:42PM |
W31.00001: Magnetic and Structural Properties of Sr-Doped Ba$_{2-x}$Sr$_{x}$CoO$_{4}$ Hao Sha, Jiandi Zhang, Q. Huang, V.O. Garlea, B.C. Sales, D. Mandrus, R. Jin We have studied the structural and magnetic properties of a newly synthesized compound Ba$_{2-x}$Sr$_{x}$CoO$_{4}$ with different doping ($x)$ levels. Monoclinic Ba$_{2}$CoO$_{4}$ is an antiferromagnetic (AFM) insulator with N\'eel temperature $T_{N}$ = 25 K and a two-dimensional character with spins aligned in the \textit{ac} plane. The isovalent Sr doping causes changes in both crystal structure and magnetic properties. With increasing $x$, $T_{N}$ initially increases then decreases after reaching the maximum at $x$=0.5. Correspondingly, its crystal structure changes from monoclinic ($x <$ 0.5) to orthorhombic ($x\ge $0.5) at room temperature. The correlation between crystal structure and physical properties will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W31.00002: Electronic structure changes in novel $J_{eff}$=1/2 system: Ruddlesden-Popper series Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ (n=1, 2, and $\infty )$ S.J. Moon, J.S. Lee, W.S. Choi, T.W. Noh, H. Jin, J. Yu, Y.S. Lee, V. Durairaj, G. Cao, A. Sumi, H. Funakubo We investigated the electronic structures of Ruddlesden-Popper series Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ (n=1, 2, and $\infty )$ compounds with optical spectroscopy and first-principles calculation. Among Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$, while SrIrO$_{3}$ is a metal, Sr$_{2}$IrO$_{4}$ and Sr$_{3}$Ir$_{2}$O$_{7}$ are insulators. In optical conductivity spectra \textit{$\sigma $}(\textit{$\omega $}), we found unique bandwidth-driven changes of the electronic structures which were quite different from those of 3$d $or 4$d \quad S$=1/2 systems. From the comparison between \textit{$\sigma $}(\textit{$\omega $}) and the results of first-principles calculation, we found that the intriguing changes of the electronic structures can be realized by the cooperative interaction between the SO coupling and the electron correlation. These results clearly demonstrate that Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ should be considered as a $J_{eff}$=1/2 single band system. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W31.00003: Electrical Resistance of Quasi-1D Li$_{0.9}$Mo$_{6}$O$_{17}$ at Very High Magnetic Field Carlos A.M. dos Santos, J. Moreno, B.D. White, J.J. Neumeier, L. Balicas Recently, photoemission experiments, band structure calculations, tunneling, and the description of the electrical resistivity by two power-law terms suggest that Li$_{0.9} $Mo$_{6}$O$_{17}$ is an excellent example of a metallic Luttinger-liquid (LL) [a,b]. The crossover from metallic to insulating-like behavior near $T_M$ = 28 K was addressed by thermal expansion experiments which suggest that a dimensional crossover sets the stage for superconductivity [b]. To obtain more information about the crossover at $T_M$, magnetoresistance measurements were performed under very high magnetic field (0 $< H <$ 23 tesla). The results show that the minimum at $T_M$ increases with increasing $H$. The power-law temperature dependence of the electrical resistance at $T_M (H)$ is also evaluated. [a] C. A. M. dos Santos, M. S. da Luz, Yi-Kuo Yu, J. J. Neumeier, J. Moreno, and B. D. White. Submitted to Phys. Rev. Let. (2007). [b] C. A. M. dos Santos, B. D. White, Yi-Kuo Yu, J. J. Neumeier, and J. A. Souza, Phys. Rev. Let. {\bf98}, 266405 (2007). [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:42PM |
W31.00004: Unconventional magneto-transport in novel layered cobalt oxides Invited Speaker: Among strongly correlated transition-metal oxides, cobalt oxides are known to have unique features arising from the spin-state degree of freedom tightly coupled with Co valence. The Co$^{4+}$ ion in the low spin-state is responsible for anomalous metallic states such as large thermopower in Na$_{x}$CoO$_{2}$ and unconventional superconductivity in hydrated Na$_{x}$CoO$_{2}$. The Co$^{2+}$ ion favors the high-spin state, which makes magnetic insulators. The Co$^{3+}$ ion is most interesting in the sense that the low-, intermediate- and high-spin states are nearly degenerate, where a spin-state crossover/transition occurs with temperature or pressure. Recently we have discovered two complex layered cobalt oxides, which exhibit unprecedented transport originated from interplay between charge, orbital and spin-states. The first one is SrCo$_{6}$O$_{11}$, in which the Co-O Kagome lattice and two-types of Co-O pillars are stacked along the c axis [1]. The conduction electrons in the Kagome lattice interact with Ising spins in the pillars, and shows two-step plateau in the magnetoresistance along the c axis. The second one is Sr$_{3}$YCo$_{4}$O$_{10.5}$, which exhibits a ferromagnetic insulating state below 340 K. Various substitutions of Sr, Y and Co sites dramatically suppress this ferromagnetic state, and concomitantly modify the magneto- and thermoelectric transport. We will discuss the structure-property relationship based on structure analyses. The main part of this work was done in collaboration with S. Ishiwata, W. Kobayashi, and M. Takano. \newline [1] S. Ishiwata et al., Chem. Mater. 17, 2789 (2005)~; Phys. Rev. Lett. 98, 217201 (2007) \newline [2] W. Kobayashi et al. Phys. Rev. B 72, 104408 (2005)~; S. Ishiwata et al. Phys. Rev. B75, 220406(R) (2002) [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W31.00005: Search for Half-Metallic Antiferromagnetism in Double Perovskites V. Pardo, W. E. Pickett The wide class of double perovskite oxides was proposed earlier (PRB 57, 10613 [1998]) as promising for producing a half-metallic antiferromagnet [HMAFM] (more correctly, a spin-compensated half metal). Here we present examples of the affects of structural distortions on the electronic and magnetic properties in selected members. For La$_2$CrNiO$_6$ the idealized cubic perovskite structure had led to a spin-antiparallel state with net moment of 0.6 $\mu_B$, but a ferromagnetic half-metallic state (4 $\mu_B$) was 150 meV per metal atom lower in energy (within local density approximation). Starting with experimental information on LaCrO$_3$ and LaNiO$_3$ and their alloys, we have relaxed the volume and the (seven) internal coordinates within the orthorhombic Pnma space group. The charge states can be characterized by Cr$^{4+}$ and Ni$^{2+}$. The ferromagnetic state is lower by 50 meV within the generalized gradient approximation. Using LDA+U (U=3 eV on each transition metal ion) opens a gap of 0.6 eV (FM insulator) and is favored by 120 meV over the antialigned state. Although no HMAFM state is obtained, these results show that structural relaxation must be taken into account, and that in some cases (as here) it may make the antialigned state more favorable. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W31.00006: Electrical transport and thermodynamic properties of SrNbO$_{3.41}$ Ariana de Campos, Ann Deml, B.D. White, C.A.M. dos Santos, M.S. da Luz, J.J. Neumeier In 1991, Lichtenberg et al.$^1$ reported the electric conductivity of SrNbO$_{3.41}$ revealing quasi-1D behavior. This system offers many possibilities to vary the compositional, structural, chemical, and physical properties.$^1$ Depending upon the temperature range and crystallographic direction, it exhibits metallic behavior or a metal-semiconductor transition. In this work, the properties of SrNbO$_{3.41}$ single crystals are revisited. The single crystals were grown by the floating zone method and characterized by x-ray diffraction. Electrical resistivity as a function of temperature was measured with four-probe and Montgomery methods. We will also report results of heat capacity and thermal expansion measurements. $^1$F. Lichtenberg et al., Z. Phys. B {\bf 84}, 369 (1991); F. Lichtenberg et. al., Prog. Solid State Chem. {\bf 29}, 1-70 (2001). [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W31.00007: Extreme electron-phonon coupling in magnetic rubidium sesquioxide Robert de Groot, Jisk Attema, S. Riyadi, Greame Blake, Gilles de Wijs, Thomas Palstra Rb$_2$O$_3$ is a black, opaque oxide. Early work suggests that the stability range of the sesquioxide phase in the rubidium-oxygen phase diagram is rather broad. Rb$_2$O$_3$ remains cubic down to the lowest temperature measured (5~K). The oxygens form dumbbells with interatomic distances in between those of peroxide and superoxide anions, and strong athermal motion persists down to low temperatures. [1] Electronic-structure calculations show that the dynamics at low temperature is caused by 6 phonon modes of zero frequency, which induce a very strong electron-phonon interaction. The softness of half of these modes is suppressed by the application of pressure. Calculated using the average oxygen positions, rubidium sesquioxide is a half-metallic ferromagnet. [2] \newline [1] CR CHIM (11-13): 591-594 NOV 1999\newline [2] JACS 127 (46): 16325-16328 NOV 23 2005 [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W31.00008: Gigantic optical magneto-electric effect in CuB$_{2}$O$_{4}$ Mitsuru Saito, Kouji Taniguchi, Takahisa Arima It has been recognized since 1960s that magneto-electric (ME) materials may also show an optical magneto-electric (OME) effect showing up as a change in optical absorption with reversal of the propagating direction of light. The OME effect is an interesting object of scientific research and provides possibilities for applications. However, the changes in absorption coefficient ever discovered were very small (less than 0.2 {\%}). We present a gigantic OME effect in a noncentrosymmetric weak ferromagnet CuB$_{2}$O$_{4}$, in which the absorption coefficient changes by a factor of three with reversal of a very weak magnetic field of 300 Oe. This magnitude of OME effect enables us to observe it by a CCD camera with linearly polarized near-infrared and visible light. Spectroscopic study and comparison of OME effect with magnetization indicate an important role of canted antiferromagnetic spin ordering and local symmetry of a square Cu$^{2+}$ site. The gigantic OME effect can be applicable to optical devices like magnetic switching of color in the future. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W31.00009: Resonant inelastic X-ray scattering study of quasi-zero-dimensional copper metaborate Jason Hancock, Guillaume Chabot-Couture, Martin Greven, Guerman Petrakovskii, Kenji Ishii, Jun'ichiro Mizuki CuB$_2$O$_4$ consists of many CuO$_4$ plaquettes separated by B ions. We report a study of the electronic excitation spectra of this system in order to explore the relationship between excitation symmetry and the resonant inelastic X-ray scattering (RIXS) technique. We find a small number of well separated features in the experimentally accessible range of 0.5-15 eV energy transfer, and weak dispersion is suggestive of the quasi-zero-dimensional nature of this system. Systematic trends in the data are suggestive of a composite nature to one of the observed features. Using a cluster model, we describe these unexpected trends and clarify how the choice of experimental geometry selectively influences the sensitivity to particular excitation symmetries in the RIXS experimental technique. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W31.00010: Powder neutron diffraction study of quasi-one-dimensional Li$_{0.9}$Mo$_{6}$O$_{17}$ Mario S. da Luz, C.A.M. dos Santos, B.D. White, J.J. Neumeier, Q. Huang, J.B. Leao, J.W. Lynn The crystallographic structure of quasi-one-dimensional Li$_{0.9}$Mo$_6$O$_{17}$ was investigated by Rietveld refinement of powder neutron diffraction data at temperatures in the range 5 K $< T <$ 295 K. Structural parameters, atomic positions, occupation numbers, and isotropic thermal parameter $B_{iso}$ will be reported. The occupancy was refined revealing a Li occupancy greater than 0.9. Bond valences sums will also be reported for various Li and Mo sites. At room temperature, the crystal was found to exhibit monoclinic symmetry with space group P21/m and lattice parameters $a$ =12.7506(1) \AA, $b$ = 5.5242(1) \AA, $c$ = 9.4913(2) \AA \, and $\beta$ = 90.593(1)$^o$. Good agreement between the temperature dependence of lattice parameters and high resolution thermal expansion results$^*$ was obtained. $^*$C. A. M. dos Santos, B. D. White, Yi-Kuo Yu, J. J. Neumeier, and J.A. Souza, Phys. Rev. Lett. {\bf98}, 266405 (2007). [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W31.00011: Unusual Physical Properties of Ca$_{3}$Co$_{4}$O$_{9}$ Rongying Jin, Larry Allard, Doug Blom, Sriparna Bhattacharya, Veerle Keppens, Brian Sales, David Mandrus We have investigated the structural and physical properties of Ca$_{3}$Co$_{4}$O$_{9}$ single crystals including electrical and thermal conductivity, thermopower, specific heat, magnetic susceptibility, and electron diffraction. The study reveals many interesting features that are unique to Ca$_{3}$Co$_{4}$O$_{9}$. In addition to high thermopower and low thermal conductivity, the low-temperature specific heat yields large electronic specific heat coefficient, suggesting strong electron-electron correlation. However, the electronic specific heat coefficient is dramatically decreased under magnetic field, implying the modification of electronic density of states by magnetic field. The magnetic susceptibility data indicate that there are several magnetic transitions above room temperature. The possible correlation between charge, spin, and lattice will be explored. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W31.00012: X-ray absorption and x-ray magnetic dichroism study on Ca$_3$CoRhO$_6$ and Ca$_3$FeRhO$_6$ Tobias Burnus, Zhiwei Hu, Julio C. Cezar, Seiji Niitaka, Hua Wu, Hidenori Takagi, Chun Fu Chang, Nicholas B. Brookes, Ling-Yun Jang, Keng S. Liang, L. Hao Tjeng The valence-state of the transition-metal ions in the chain-like compounds Ca$_3$CoRhO$_6$ and Ca$_3$FeRhO$_6$ is currently an issue under debate. Using numerical simulations and x-ray absorption spectroscopy at the Rh-$L_{2,3}$, the Co-$L_{2,3}$, and the Fe-$L_{2,3}$ edges we reveal a Co$^{2+}$/Rh$^{4+}$ configuration in Ca$_3$CoRhO$_6$ and Fe$^{3+}$/Rh$^{3+}$ in Ca$_3$FeRhO$_6$. X-ray magnetic circular dichroism at the Co-$L_{2,3}$ edge shows that the Co$^{2+}$ ions carry a giant orbital moment of about $1.7\mu_B$. We attribute this to a $d_1^0d_1^2$ ground state for the high-spin Co $3d^7$ configuration in trigonal prismatic coordination. The intrachain-ferromagnetic coupling of two neighboring Co ions is mediated by a low-spin Rh$^{4+}$ ion ($S = 1/2$) in between. The results agree with our recent ab-initio study [Hua Wu {\it et al.}, Phys. Rev. B {\bf 75}, 245118 (2007)]. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W31.00013: Anisotropy in magnetic properties of single crystal LiFePO$_{4}$ Gan Liang, Keeseong Park, John Markert, Jiying Li, David Vaknin We report the experimental and theoretical results on the anisotropies in the magnetic properties and x-ray absorption spectra of single crystal LiFePO$_{4}$. A mean-field theory is developed to explain the observed strong anisotropies in Lande g-factor, paramagnetic Curie temperature, and effective moment for LiFePO$_{4}$ single crystals. The values of the in-plane nearest- and next-nearest-neighbor spin-exchange ($J_{1}$ and $J_{2})$, inter-plane spin-exchange ($J_{\bot })$, and single-ion anisotropy ($D)$, obtained recently from neutron scattering measurements, are used for calculating the Curie temperatures with the formulas derived from the mean-field Hamiltonian. It is found that the calculated Curie temperatures match well with that obtained by fitting the magnetic susceptibility curves to the modified Curie-Weiss law. [Preview Abstract] |
Session W32: Focus Session: Domain Wall Motion and Itinerant Magnetism
Sponsoring Units: GMAG DMP FIAPChair: Rembert Duine, Utrecht University
Room: Morial Convention Center 225
Thursday, March 13, 2008 2:30PM - 2:42PM |
W32.00001: Domain wall motion under the non-uniform transverse magnetic field with rigid domain structure Chun-Yeol You Another method for the domain wall movement in a nanowire geometry with rigid domain structure will be proposed. We find that a transverse domain wall move to the energy minimum position under a non-uniform transverse magnetic field in order to minimize the Zeeman energy. By the collective coordinate approach, the domain wall dynamics under non-uniform transverse magnetic field in nanowire geometry is investigated. The validity of concept of the present method and the domain wall equation of motion are confirmed by micromagnetic simulations. It is found that the domain wall velocity of a few 100 m/s can be obtained for the moderate conditions based on the analytic and numerical studies. The direction of the domain wall movement depends only on the magnetization direction inside of the domain wall itself, not on the one of the domain. Therefore, it is possible to achieve field driven domain wall motion with rigid domain structures. The non-uniform transverse magnetic field driven domain wall motion has a superior nature of the rigidity of the domain structure during the domain walls movement in addition to the all advantages of conventional field driven domain wall movement. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W32.00002: Crossed Ratchet Effects for Magnetic Domain Wall Motion Jose I. Martin, A. Perez- Junquera, V.I. Marconi, A.B. Kolton, L.M. Alvarez- Prado, Y. Souche, A. Alija, M. Velez, J.V. Anguita, J.M. Alameda, J.M.R. Parrondo The driven motion of domain walls in extended amorphous magnetic films patterned with a periodic array of asymmetric holes has been studied experimentally and theoretically. We find two crossed ratchet effects of opposite sign that change the preferred sense for domain wall propagation, depending on whether a flat or a kinked wall is moving. These crossed effects have an interesting consequence with potential applications: the system keeps memory of the sign of the last saturating state even in a zero magnetization configuration. By solving numerically a simple $\phi^4$-model we show that the essential physical ingredients for this effect, the competition between drive, elasticiy and asymmetric pinning, are quite generic and could be realized in other experimental systems involving elastic interfaces moving in multidimensional ratchet potentials. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W32.00003: In Situ TEM Observation of Current-Induced Domain Wall Motion in Patterned Permalloy Wires Todd Brintlinger, John Cumings Using a transmission electron microscope (TEM) operating in Lorentz mode, we observe the movement of a domain-wall due to the flow of current in a permalloy wire. The wire is formed on electron-transparent silicon nitride membranes using standard electron beam lithography and thermal vacuum deposition. The resulting wire geometry is 30 nm thick, $\sim $100nm wide, and microns long. A custom-built electrical measurement stage and palladium leads deposited on top of the permalloy wires allow in situ measurements on the wire in the TEM. Lorentz mode imaging (Fresnel contrast) allows the determination of the domain wall location. We observe the domain wall to move in the direction of electron flow, with a current density of around 1 x 10$^{11}$ A/m$^{2}$ being required to move the wall. We will present the nanofabrication process, results, and interpretation of these experiments. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:42PM |
W32.00004: Stochastic Current-Driven Domain-Wall Motion Observed by X-Ray Microscopy Invited Speaker: Transmission x-ray microscopy can directly visualize the influence of a spin-polarized current on the magnetization of micro- and nanostructures. We investigate the stochastic motion of domain walls in curved wires [1] and the motion of vortices in squares [2]. To observe domain-wall motion pulses of nanosecond duration and high current density are send through permalloy wires and either move or deform the domain wall. The current pulses have nanosecond duration and a high current density of up to 1.0 10$^{12}$ A/m$^{2}$ and drive the wall either undisturbed, i.e. as a composite particle through the wire or causes structural changes of the magnetization. Repetitive pulse measurements reveal the stochastic nature of current induced domain-wall motion. From the experiments we estimate the ratio between the degree of nonadiabaticity and the Gilbert damping parameter indicating the importance of the nonadiabatic contribution to current driven domain-wall motion. To compare experimental results with theory the spin-torque transfer model of Zhang and Li [3] is implemented in the micromagnetic framework OOMMF [4]. The code is applied to determine the current-induced domain wall velocity using the material parameters of permalloy. The simulations support the interpretation of the experimental results. Sinusoidal high-density currents are applied to micrometer-sized permalloy squares containing ferromagnetic vortices. Spin-torque induced vortex gyration on the nanosecond timescale is observed. The phase of the gyration in structures with different chirality are compared to an analytical model and micromagnetic simulations, considering both alternating spin-polarized currents and the current's Oersted fields. This analysis reveals that spin-torque is the main source of motion. Supported by the DFG via SFB 668 and GK 1286 as well as by the U.S. DOE Contract No. DE-AC02-05-CH11231. References: \newline [1] G. Meier, M. Bolte, R. Eiselt, U. Merkt, B. Kr\"{u}ger, D. Pfannkuche, D.-H. Kim, and P. Fischer, Phys. Rev. Lett. 98, 187202 (2007) \newline [2] B. Kr\"{u}ger, A. Drews, M. Bolte, U. Merkt, D. Pfannkuche, and G. Meier, Phys. Rev. B 76, in press (2007). \newline [3] S. Zhang and Z. Li, Phys. Rev. Lett. 93, 127204 (2004). \newline [4] M. Donahue and D. Porter, Interagency Report NISTIR 6376, National Institute of Standards and Technology, Gaithersburg, MD (Sept. 1999). [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W32.00005: The dynamics of field and current-driven magnetic domain wall depinning Geoffrey Beach, Carl Knutson, Maxim Tsoi, James Erskine The depinning of a magnetic domain wall from a well-defined potential well was studied experimentally on timescales ranging from minutes down to tens of nanoseconds. At longer timescales, the behavior follows the classical Neel-Brown model of thermal activation, one of the few observations of this process for the ideal case of a single energy barrier. Below one microsecond, however, the depinning rate becomes independent of the activation volume and assumes a more universal behavior. This transition is due to a vanishing of the energy barrier at a critical field, beyond which the rate of depinning depends primarily on the torque supplied by the field and spin current. A dc spin-polarized current flowing across the domain wall has the effect of lowering the energy barrier by an amount that is predominantly quadratic in current, independent of its direction. This is seen to arise from a shift of the wall in the energy potential due to the adiabatic component of spin-transfer torque. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W32.00006: Dynamics of domain walls in nanostrips via collective coordinates D. Clarke, O. Tretiakov, G.-W. Chern, Ya. B. Bazaliy, O. Tchernyshyov The rich internal structure of domain walls in nanostrips [1-2] greatly affects the motion when an external magnetic field or electric current is applied, leading to reduced mobility when the driving force is strong. We generalize Thiele's equations [3] to describe arbitrary wall motion with any number of collective coordinates [4]. The formalism is sufficiently general as to allow the inclusion of spin current, and can be applied to films with in- or out-of-plane magnetic anisotropy. We examine a model wall [5] with two soft modes corresponding to the coordinates of a vortex core. As in a one-dimensional domain wall [6], the system has a steady-state regime below a critical field and an oscillatory regime above it. We calculate the drift velocity in both regions. The results are compared to numerical simulations and to available experimental data [7]. This work was supported in part by the NSF Grant DMR-0520491. [1] R. D. McMichael and M. J. Donahue, IEEE Trans. Magn. 33, 4167 (1997). [2] O. Tchernyshyov and G.-W. Chern, Phys. Rev. Lett. 95, 197204 (2005). [3] A. A. Thiele, Phys. Rev. Lett. 30, 230 (1973). [4] O. Tretiakov et. al, arXiv:0705.4463 [5] H. Youk et al., J. Appl. Phys. 99, 08B101 (2006). [6] N. L. Schryer and L. R. Walker, J. Appl. Phys. 45, 5406 (1974). [7] G. S. D. Beach et al., Nature Mater. 4, 741 (2005) [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W32.00007: Domain wall motion by subcritical harmonic current Yury Adamov, Artem Abanov, Jairo Sinova We consider the behavior of the domain wall in the bianisotropic magnetic wire. We show that while the domain wall cannot be moved by the constant current below the certain threshold value (critical current), if we add an alternating component to the current the domain wall starts to move with nonzero velocity. We obtain the analytic expression for this velocity and make suggestions of possible experiments. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W32.00008: Theory of Electromotive Force Induced by Domain Wall Motion Shengyuan Yang, Di Xiao, Qian Niu We formulate a theory on the dynamics of conduction electrons in the presence of moving magnetic textures in ferromagnetic materials. We show that the variation of local magnetization in both space and time gives rise to topological fields, which induce electromotive forces on the electrons. Universal results are obtained for the emf induced by both transverse and vortex domain walls traveling in a magnetic film strip, and their measurement may provide clear characterization on the motion of such walls. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W32.00009: Oscillatory domain wall motion in a single-crystal ultrathin Au/Co/Au system Keoki A. Seu, Sujoy Roy, Sungkyun Park, Charles M. Falco, Stephen D. Kevan We have used x-ray photon correlation spectroscopy together with resonance soft x-ray scattering to measure domain dynamics in a Au/Co/Au system that exhibits a spin reorientation phase transition (SRT) in the temperature range of 200-300 K. The incoming photon energy was tuned at the Co L$_{3}$ edge and the coherence is established with a $\sim$10 um pinhole. The resultant speckle pattern is measured with a CCD camera in time as a function of temperature 200 K to 300 K. The correlation coefficient, which is an indicator of domain wall dynamics, shows damped oscillatory behavior in time. The period of the oscillations is approximately 120 sec. The frequency and damping constant were found to depend on the length-scale and temperature changes through the phase transition. Our results show that the SRT dynamics on a mesoscopic length scale and slow time scale can be surprisingly complex. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W32.00010: Thermodynamics of Itinerant Magnets: A Simple Classical Model with Longitudinal Spin Fluctuations James Glasbrenner, Aleksander Wysocki, Kirill Belashchenko The effects of longitudinal spin fluctuations (LSF) on the thermodynamics of magnetic metals are studied using a model Hamiltonian with only one ``itinerancy parameter.'' We performed Monte Carlo simulations and compared the results with the mean-field theory. A non-trivial complication is the choice of phase space measure. We explored two options: the ``classical'' measure and the ``flat'' measure. Our central result is that magnetic short-range order is always weak, and the mean-field theory is in a very good agreement with Monte Carlo results. Additionally, the results are very sensitive to the choice of the phase space measure, which is a limitation of our model. Nevertheless LSF are essential for the correct description of magnetic thermodynamics and their absence in the adiabatic approximation leads to unphysical results for itinerant systems. Deviations from the Curie-Weiss law due to LSF are also observed and discussed. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W32.00011: High Pressure Study of Magnetic Order in an Itinerant Electron System: Investigating Weak vs. Strong Coupling Rafael Jaramillo, Y. Feng, J. C. Lang, Z. Islam, T. F. Rosenbaum We measure directly the spin- and charge-density-wave order parameters of the itinerant antiferromagnet Cr via x-ray diffraction as the system is driven towards its quantum critical point with pressure using diamond anvil cell techniques. The exponential decrease of the spin and charge diffraction intensities with pressure confirms the harmonic scaling of spin and charge in this incommensurate system, while the evolution of the incommensurate ordering vector provides important insight into the difference between tuning with pressure and chemical doping. Measurement of the charge density wave over several orders of magnitude of diffraction intensity provides the clearest demonstration to date of a weakly coupled BCS-like ground state. Evidence for coexistence of this weak coupling ground state with incipient magnetic fluctuations at high temperatures in chromium and other more strongly coupled systems raises intriguing questions about the meaning of weak vs. strong coupling and suggests a new category of quantum phase transitions. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W32.00012: Dynamic susceptibility of itinerant ferromagnets in the ordered state Matthew Vannette, Sergey Bud'ko, Paul Canfield, Ruslan Prozorov Measurements of radio-frequency dynamic susceptibility of ferromagnets exhibit striking differences between local moment and itinerant systems. Whereas local moment systems show a sharp peak at the Curie temperature ($T_c$) which evolves to higher temperatures and lower amplitudes with applied dc magnetic field, itinerant systems show a broad maximum at temperatures well below $T_c$. The itinerant system's maximum is suppressed in amplitude and shifts to lower temperatures with applied dc magnetic field. Existing Stoner or spin fluctuations theories derive strictly zero-field susceptibility and we propose a generalization of these models to incorporate the effect of applied dc field. A good agreement between our semi-phenomenological approach and experimental results obtained on several generally accepted itinerant materials with various $T_c$'s is presented. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W32.00013: Elementary excitations in antiferromagnetic Heisenberg spin segments. Marco Affronte, Alberto Ghirri, Marco Evangelisti, Andrea Candini, Stefano Carretta, Paolo Santini, Giuseppe Amoretti, Rachel Davies, Grigore Timco, Richard Winpenny We report on \textit{ac}-susceptibility, low temperature magnetization and specific heat measurements on molecular compounds, shortly named (Cr$_6$)$_2$, (Cr$_7$)$_2$, Cr$_8$Cd, (NiCr$_6$)$_2$Zn, that comprise different variants of spin arrays. These systems constitute real examples of collections of identical antiferromagnetic Heisenberg spin segments. We indeed show that this picture, with dominant exchange term in the spin Hamiltonian ($J/k_B$ ranging from 13 to 16 K in all compounds) and weak anisotropy term, fits well the measured physical properties. The character of energy spectra and the low lying magnetic excitations are discussed accordingly. The direct comparison of experimental results and of the energy spectra of these variants with those of similar cyclic spin systems evidences effects associated to: i) the breaking the cyclic boundary conditions and ii) odd and even nuclearity of spin segments. [Preview Abstract] |
Session W33: Focus Session: Spins in Narrow Gap Semiconductors
Sponsoring Units: GMAG FIAP DMPChair: Berry Jonker, Naval Research Laboratory
Room: Morial Convention Center 224
Thursday, March 13, 2008 2:30PM - 2:42PM |
W33.00001: Spin Injection, Manipulation, and Detection, in InAs Nanodevices G.M. Jones, B.T. Jonker, B.R. Bennett, J.R. Meyer, M.E. Twigg, T.L. Reinecke, D. Park, S.V. Pereverzev, C.S. Badescu, C.H. Li, A.T. Hanbicki, O. Van'terve, I. Vurgaftman In this talk the authors will discuss their progress using InAs heterostructures to produce spin-polarized injection and detection, as well as manipulation of coherent spin-polarized electrons for a spin-based FET (SpinFET). High-quality n-type InAs heterostructures demonstrate many favorable characteristics necessary to the study of spin dynamics, including 2DEG's with small effective mass (m* = 0.023) and large g-factor (g = -15). Previously, high-mobility InAs heterostructures have been demonstrated in which electrons pass ballistically over hundreds of nanometers up to room temperature. Our devices seek to exploit the strong Spin-Orbit effect present in InAs to manipulate coherent spin-polarized electrons during transport, by producing perpendicular electric field using isolated top-gates fabricated over the electron transport region. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W33.00002: Electrical Spin Injection into InAs Wetting Layer Connie H. Li, George Kioseoglou, Aubrey T. Hanbicki, Ramasis Goswami, Steve Hellberg, Berry T. Jonker, Mesut Yasar, Athos Petrou InAs is an attractive material for optoelectronic and high-speed transistor devices due to its small bandgap and high electron mobility. Owing to its large Rashba spin-orbit coupling, the 2DEG formed in InAs-based heterostructures has also been proposed for spin transport within a spin FET$^{1}$. Here we demonstrate efficient spin injection from Fe into a thin ($\sim $3ML) InAs wetting layer (WL) that forms on GaAs before the formation of QDs. Cross sectional TEM shows that the WL is continuous laterally over many microns, and transport measurements reveal 2DEG-like behavior. The WL electroluminescence is readily distinguished from that of the QDs, and dominates emission at higher biases over a wide temperature range up to RT. We measure an optical circular polarization of 26{\%} at 5K due to the injection of spin-polarized electrons from a reverse-biased Fe Schottky contact, which corresponds to an electron spin polarization $>$50{\%} after lifetime corrections, demonstrating that even this remarkably thin layer supports high spin polarization. This polarization stayed relatively constant up to 60K, and decreased to $\sim $6{\%} at RT, consistent with D'yakonov-Perel spin relaxation mechanism. Supported by ONR, NRL core funds, and NSF. $^{1}$ S. Datta and B. Das, Appl. Phys. Lett. \textbf{56}, 665 (1990). [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W33.00003: Spin polarized current in an InSb film Matthew Frazier, J.J. Heremans, Giti A. Khodaparast Recently, there has been much interest in developing and exploring spin based semiconductor devices and phenomena. One of the key challenges in developing spin based devices is to generate, control, and measure spin currents directly. In this talk, we report interband circular photogalvanic (CPG) effects using pulsed near-infrared radiations in an InSb film grown by the MOCVD technique. The film is n-type Te-doped with electron density of $\sim $ 6.0 x10$^{15}$ cm$^{-3}$ and mobility of 58,500 cm$^{2}$/Vs at 100 K. We observe a CPG current whose direction and magnitude depend on the helicity of the incident light, the angle of the incidence, and temperature. Our observation is important to understand zero-field spin splitting mechanisms in a system with strong-spin orbit interaction. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W33.00004: Spin coherence times in n-type InSb thin films R.L. Kallaher, J.J. Heremans In order to investigate the spin coherence times in the narrow gap semiconductor InSb, low temperature magnetoresistance measurements were performed on Te-doped thin films of InSb in weak perpendicularly applied magnetic fields. The measured changes in the resistance, as a function of applied field, in these n-type films show anti-localization phenomena that occur as a consequence of the strong spin-orbit interaction present in InSb. Hence, the magnitude of both the spin and the phase coherence times of the electrons in InSb can be determined by fitting the measured magnetoresistance curves to a localization theory that includes the effects of spin-orbit scattering. Such fits reveal that for the Te-doping levels investigated, the spin coherence times vary from approximately 20 ps to 200 ps at low temperatures, with very weak or no temperature dependence below 10 K. Furthermore, by analyzing the spin coherence times in films with different Te doping densities, it is shown that the Elliot-Yafet mechanism is responsible for the spin decoherence in doped InSb at low temperatures. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W33.00005: Control and probe of carrier and spin relaxations in InSb based quantum wells Kanokwan Nontapot, R.N. Kini, G.A. Khodaparast, N. Goel, T.D. Mishima, M.B. Santos The growing interest in spin-related phenomena and devices has prompted intense activity in the science and engineering of narrow gap semiconductors (NGS). NGS offer several scientifically unique features such as small effective masses, large g-factors, high intrinsic mobilities, and large spin- orbit coupling effects. In this work we report the dynamics of photo-excited carrier/spin in several InSb based quantum wells (QWs) using standard pump-probe spectroscopy and magneto- optical Kerr (MOKE) effect. Our InSb QWs are grown on GaAs (001) substrates with the Al$_{x}$In$_{1-x}$Sb barrier layers. We studied one undoped and five doped QWs with the electron concentrations in the wells ranging from ${\sim}1-4.4{\times}10^ {11}cm^{-2}$, where only the ground-state subband is occupied. The electron mobility in the samples are ranging from ${\sim} 70,000-100,000 cm^{2}/Vs$ at 4.2 K. We observed strong dependence of the dynamics to the density of photo-excited carriers and the pump photon energy and only weak variation with changing the samples' temperature. Our results are important to understand different relaxation mechanisms in NGS with strong-spin orbit interactions. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W33.00006: Temperature dependence of spin-resolved transverse magnetic focusing in InSb- and InAs heterostructures J.J. Heremans, R.L. Kallaher, R.B. Lillianfeld, Hong Chen, N. Goel, S.J. Chung, M.B. Santos, W. Van Roy, G. Borghs Spin-orbit interaction in InSb/InAlSb and InAs/AlGaSb heterostructures leads to spin-split orbital motion. The spatial separation of semiclassical cyclotron orbits is experimentally detected by measuring the magnetoresistance of lithographically patterned mesoscopic ballistic transverse magnetic focusing geometries. The 1st and 2nd focusing maxima show multiplet structure, consistent with spin-split ballistic orbits and spin-dependent reflection off the focusing barrier. In InSb, the transverse magnetic focusing effect is observable up to 150 K, while the multiplet structure in the maxima shows a more pronounced temperature dependence and fades by 80 K. In InAs, the transverse magnetic focusing effect persists to 60 K, and the multiplet structure is visible up to 20 K. The difference in temperature dependence between maxima and multiplet structure indicates that a length scale separate from the mean-free-path governs the observation of splitting, pointing to the use of transverse magnetic focusing for quantifying the temperature dependence of spin coherence. (NSF DMR-0618235, DMR-0080054, DMR-0209371) [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W33.00007: External electric field effects on AAS oscillations in narrow gap semiconductors R. B. Lillianfeld, R. L. Kallaher, D. E. Davis, J. J. Heremans, Hong Chen, N. Goel, S. J. Chung, M. B. Santos, W. Van Roy, G. Borghs We present experiments on quantum interference phenomena in semiconductors with strong spin-orbit interaction, using mesoscopic parallel ring arrays fabricated on InSb/InAlSb and InAs/AlGaSb heterostructures. A front gate modulates the spin-orbit interaction, which in turn affects the oscillatory interference phenomena. The experiments investigate the low temperature resistance of the ring arrays as a function of weak perpendicularly applied magnetic fields as well as applied gate voltage. The low field magnetoresistance in the arrays has the h/2e periodicity characteristic of Altshuler-Aronov-Spivak (AAS) oscillations. Despite reduced gate action typical of narrow-gap heterostructures (characterized by Hall measurements), we note an effect on the oscillatory magnetoresistance. The AAS oscillation magnitudes acquire a quasi-periodic modulation as function of gate voltage, and the localization background broadens at higher electron densities. The nature of these influences is examined. (NSF DMR-0618235, DMR-0080054, DMR-0209371) [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W33.00008: Measurements of spin polarization and spin relaxation in 2D electron gases S.M Frolov, A. Venkatesan, W. Yu, J.A. Folk, W. Wegscheider Pure spin currents are generated and detected using quantum point contacts in narrow channels of a GaAs/AlGaAs 2D electron gas. A spin relaxation length of 50 microns is achieved due to a cancellation of Rashba and Dresselhaus spin-orbit interactions along 110 crystal direction. Spin currents observed at zero magnetic field are correlated with the 0.7 conductance feature of quantum point contacts, suggesting static spontaneous spin polarization. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W33.00009: Separation of the Rashba and Dresselhaus terms using the Square and Rectangular Loop Arrays in InGaAs/InAlAs Quantum Wells Takaaki Koga$^{1}$, Minu Kim, Yoshiaki Sekine The spin interference effect [1,2] was investigated for square and rectangular loop arrays that were nanolithographically defined in InGaAs/InAlAs quantum wells both theoretically and experimentally. In theory, interference between the following spin wave functions were considered : $\Psi ^{CW}$=\textbf{R}$_{-x}$(\textit{$\theta $}$_{v})$\textbf{R}$_{y}$(\textit{$\theta $}$_{h})$\textbf{R}$_{x}$(\textit{$\theta $}$_{v})$\textbf{R}$_{-y}$(\textit{$\theta $}$_{h})\Psi _{i}$ (wave function after the clockwise path in a rectangular loop) and $\Psi ^{CCW}$=\textbf{R}$_{y}$(\textit{$\theta $}$_{h})$\textbf{R}$_{-x}$(\textit{$\theta $}$_{v})$\textbf{R}$_{-y}$(\textit{$\theta $}$_{h})$\textbf{R}$_{x}$(\textit{$\theta $}$_{v})\Psi _{i}$ (wave function after the counter-clockwise path), where the spin rotation operators \textbf{R}$_{\xi }$(\textit{$\theta $}$_{v,h})$ were obtained from solving the Poisson and Schrdinger equations self-consistently including the Rashba and Dresselhaus Hamiltonians. Then, the gate-dependence of the norm $\vert \Psi ^{CW}+\Psi ^{CCW}\vert ^{2}$, averaged over all directions for the initial spin ($\Psi _{i})$, were compared to the gate-dependence of the AAS oscillation amplitude in the experiment. We propose that the measurement of the spin interference effect is a reliable method for the simultaneous determination of the Rashba and Dresselhaus terms quantitatively. [1] Koga \textit{et al.}, PRB \textbf{70}, 161302(R) (2004);\textit{ ibid.} \textbf{74}, 041302(R) (2006). [2] Koga \textit{et al.}, phys. stat. sol. (C) \textbf{3}, 4220 (2006). [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W33.00010: Aharonov-Casher effect in hole ring with spin-orbit interaction Mario Borunda, Alexey Kovalev, Tomas Jungwirth, Laurens Molenkamp, Jairo Sinova We study the quantum interference effects induced by the Aharonov-Casher phase in a ring structure two-dimensional heavy-hole (HH) system with spin-orbit interaction. The influence of the spin-orbit interaction on the transport causes interference effects which are a signature of the topological Aharonov-Casher phase. We present numerical calculations of the magnetoconductance and spin dependent transport in realistic semiconductor ring structures. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W33.00011: Plasmons of a two-dimensional electron gas with Rashba and Dresselhaus spin-orbit coupling. Jesus A Maytorena, Catalina Lopez-Bastidas, Elmer Cruz We calculate the dielectric response of a two-dimensional electron gas with both Rashba and Dresselhaus spin-orbit (SO) coupling within the self-consistent-field approach. We obtain the dispersion relations of the collective modes and the regions of single-particle excitations, related with intra- and inter-spin-split transitions. The interplay of both types of SO couplings give rise to an angular anisotropy of the spin-splitting energy. As consequence, the plasmon spectrum and the Landau damping regions show a dependence on the direction of the wave vector transfer. This response is in contrast to that of vanishing Rashba or Dresselhaus case. We also discuss the dependence of this spectral characteristics on the electron density and SO coupling strengths, and derive expressions for the intra- and inter- SO plasmons in the long-wavelength limit. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W33.00012: Magnetoplasmon excitations in Rashba spintronic quantum wires: Maxons, rotons, and negative-B dispersion Manvir Kushwaha We report on the theoretical investigation of plasmon excitations in a quasi-two-dimensional electron gas in the presence of a harmonic potential, a perpendicular magnetic field, and the spin-orbit interaction (SOI) induced by the Rashba effect. The resultant system is a Q1D quantum wire with free propagation in the y direction and magnetoelectric quantization along the x. The problem involves three length scales: ${\it l}_0=\sqrt{\hbar/m^*\omega_0}$, ${\it l}_c=\sqrt{\hbar/m^*\omega_c}$, and ${\it l}_{\alpha}=\hbar^2/(2m^*\alpha)$, which characterize the relative strengths in the interplay of confinement, the magnetic field, and the Rashba SOI. The resulting Schr\"odinger-like equations are two coupled equations, which cannot be solved in an explicit form. However, invoking the limits of a strong magnetic field, ${\it l}_c \ll {\it l}_0$, and $k_y{\it l}_0\ll 1$ allow us to solve this set of coupled equations exactly. We then derive and discuss the dispersion relations for collective excitations within the framework of Bohm-Pines' RPA. The intrasubband and intersubband magnetoplasmons in a Q1DEG are characterized, respectively, by the negative-B dispersion and the magnetoroton excitation. Here we scrutinize the effect of the Rashba SOI on these characteristics in depth. We observe that the SOI modifies drastically the behavior of both magnetoplasmons in the long wavelength limit and may render them relatively more susceptible to the Landau damping in the short wavelength limit. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W33.00013: Resonant spin polarization and Hall conductances in a two-dimensional electron gas Degang Zhang, Yao-Ming Mu, Chin-Sen Ting We have studied transport properties in a two-dimensional electron gas with equal Rashba and Dresselhaus spin-orbit interactions under a perpendicular magnetic field. By employing the exact solution for this system, we found resonant charge and spin Hall conductances at a certain magnetic field, where all the nearest-neighboring Landau levels cross. Near the magnetic field, there exists a resonant spin polarization, which can also induce resonant charge and spin Hall effects. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W33.00014: Spin signal recovery in a two-dimensional electron gas with a quantum point contact Jae-Seung Jeong, Hyun-Woo Lee We study transport properties of the spin-polarized current in a two-dimensional electron gas(2DEG) including a quantum point contact(QPC) in the presence of Rashba spin-orbit(RSO) coupling. Spin-resolved conductance is investigated numerically using a recursive Green function method, with special attention to the quantum effects of spin-charge transport channels. It is found that when the conductance is examined as a function of the RSO coupling strength, the conductance modulation ratio, defined as the ratio between the maximum and minimum conductances, can be enhanced by the QPC in the ballistic and weakly diffusive regime as well. Decaying rate of the spin-polarization can be also reduced. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W33.00015: Semiclassical Equations of Motion for Bloch Electrons in External Fields, Including Spin-Orbit Interaction. W.C. Kerr This talk considers an electron moving in a periodic potential with spin-orbit interaction and perturbed by external slowly varying electric field and uniform magnetic field. Superposition of the time-independent Bloch spinor states of the unperturbed Hamiltonian gives a wave packet state with both wavevector space and spin-orientation amplitude factors. The time-dependent variational principle produces equations of motion for the centers of the wave packet in both configuration and wavevector space and for the spin-orientation factors. For spinless electrons this procedure yields the familiar semiclassical equations of motion augmented by an orbital magnetic moment contribution to the Bloch band energy and an ``anomalous velocity'' proportional to a Berry curvature.\footnote{M. C. Chang \& Q. Niu, Phys. Rev. B {\bf{53}}, 7010 (1996)} The inclusion of spin-orbit interaction gives additional contributions to the velocity involving different Berry curvatures. One is a spin-dependent contribution to the magnetic moment, and another is an electric-dipole-like contribution that is also proportional to the spin operator. [Preview Abstract] |
Session W35: Focus Session: Nanotechnology II
Sponsoring Units: FIAPChair: Curt Richter, National Institute of Standards and Technology
Room: Morial Convention Center 227
Thursday, March 13, 2008 2:30PM - 3:06PM |
W35.00001: TEBAL: Nanosculpting devices with electrons in a transmission electron microscope Invited Speaker: Manipulation of matter on the scale of atoms and molecules is an essential part of realizing the potential that nanotechnology has to offer. In this talk I will describe transmission electron beam ablation lithography (TEBAL), a method for fabricating nanostructures and fully integrated devices on silicon nitride membranes by nanosculpting evaporated metal films with electron beams. TEBAL works by controllably exposing materials to an intense and highly focused beam of 200 keV electrons inside the transmission electron microscope (TEM). The effect of electron irradiation can be used to controllably displace or ablate regions of the metal with resolution on the scale of tens of atoms per exposure. In situ TEM imaging of the ablation action with atomic resolution allows for real-time feedback control during fabrication. Specific examples presented here include the fabrication and characterization of nanogaps, nanorings, nanowires with tailored shapes and curvatures, and multi-terminal devices with nanoislands or nanopores between the terminals. These nanostructures are fabricated at precise locations on a chip and seamlessly integrated into large-scale circuitry. I will discuss how the combination of high resolution, geometrical control and yield make TEBAL attractive for many applications including nanoelectronics, superconductivity, nanofluidics and molecular (DNA) translocation studies through nanopore-based transistors. References: 1) M.D. Fischbein and M. Drndic, ``Sub-10 nm Device Fabrication in a Transmission Electron Microscope'', Nano Letters, 7 (5), 1329, 2007. 2) M. D. Fischbein and M. Drndic, ``Nanogaps by direct lithography for high-resolution imaging and electronic characterization of nanostructures'', Applied Physics Letters, 88 (6), 063116, 2006. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W35.00002: Atomic Precision in Nano-Electronics Douglas Strachan, Danvers Johnston, Beth Guiton, Ye Lu, Sujit Datta, Peter Davies, Dawn Bonnell, Charlie Johnson One of the greatest challenges in developing molecular-scale devices is to fabricate and monitor their formation with atomic precision. Recently, we have developed an electromigration technique that employs feedback for controllably electromigrating a nano-scale electrode with atomic precision at room temperature [1]. We will discuss our recent progress advancing this technique towards atomically precise nano-electronics. This will include in-situ transmission electron microscopy which shows evidence for highly crystalline electrode formation and the parallel fabrication of nanogaps for scaling-up to very large-scale integrated-circuits. Our results have implications on the development of a wide range of novel molecular-scale devices. Funding provided by: NSF-NSEC/NBIC DMR-0425780, NSF-NIRT Grant No. 0304531, and MRSEC award No. DMR05-20020. [1] D. R. Strachan, et al., Appl. Phys. Lett. 86, 043109 (2005). [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W35.00003: Quantifying the properties of nano-composites. Murray Daw, Bo Zhang, Jian He, Terry Tritt With the proliferation of nano-composites produced for possible thermoelectric application, we ask the question: To what extent is a given nano-composite like other composites? Or, in other words, when do we know that we have something new? To address this we apply the classical theory of composites to specific nano-composites grown and characterized at Clemson. The theory is very simple and assumes explicitly very simple properties of the materials, the most important being Fourier's Law/Ohm's Law. Given this assumption, the theory of composites can be applied to the nano-composites based on what is known of the microstructure. This ``classical'' result then forms the basis by which the properties can be compared to determine if non-classical effects are being observed. One simple theory is the application of rigorous bounds, such as the Hashin-Strikman Bounds which are based only on very simple microstructural descriptors. Another simple theory is the application of FEM, which can be constructed directly from SEM images of the samples using the NIST code ``OOF''. The FEM produces specific predictions for the composite properties. We find that the Hashin-Strikman Bounds are very useful for analyzing the thermal conductivities of composites, but are too loose to be useful for low-temperature electrical conductivity of composites composed of metals and insulators, where the FEM technique can be applied successfully. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W35.00004: `Focused Assembly' of V$_{2}$O$_{5}$ Nanowires for Fabrication of Metallic Nanowire Sensors Tae Hyun Kim, Sung Myung, Kwang Heo, Seunghun Hong We present a method named `focused assembly' for high-precision assembly of pristine V$_{2}$O$_{5}$ nanowires (NWs) on solid substrates. In this method, 'microscale' self-assembled monolayer patterns with 'gradient' surface molecular density `focused' the assembly of V$_{2}$O$_{5}$ NWs onto the `nanoscale' regions on a metallic thin film just like a lens focuses the light. The assembled NWs could be utilized as a shadow mask during the ion-milling process to generate metallic NW-based devices. As a proof of concepts, we successfully demonstrated the fabrication of metallic NW-based sensors to detect thiol molecules or hydrogen gas under ambient conditions. This focused assembly phenomenon gives us new insights about the directed assembly process of nanostructures. Furthermore, this approach provides us an easy, but efficient, means to mass-produce NW-based devices for various practical applications such as field effect transistors, chemical sensors and nanoscale interconnectors. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W35.00005: Nanomechanical Spectroscopy: A novel route to label-free chemical sensing Peter Greaney, Jeffrey Grossman We propose a novel spectroscopic technique in which the vibrational modes of an analyte molecule are probed directly using a nanoscale mechanical resonator. It is anticipated that such ``nanomechanical spectroscopy'' can provide a method for label free chemical sensing. We elucidate the concept of the nanomechnical spectroscope with the example of using an array of carbon nanotubes to detect a series of simple test molecules. In these examples, energy is transferred between the molecular vibrations of the analyte and specific phonon modes of the carbon nanotubes. Molecular dynamics simulations are used to explore the feasibility of this energy exchange for chemical sensing, and limits of both sensitivity and selectivity. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W35.00006: `Lens' Effect in Directed Assembly of Nanowires on Gradient Molecular Patterns Moon Gyu Sung, Sung Myung, Jiwoon Im, Seunghun Hong We report a new phenomenon, named here as the \textit{`lens'} effect, in the directed-assembly process of nanowires (NWs) on self-assembled monolayer (SAM) patterns. In this process, the adsorption of NWs is focused in the nanoscale regions at the center of microscale SAM patterns with gradient surface molecular density just like an optical lens focuses light. As a proof of concepts, we successfully demonstrated the massive assembly of V$_{2}$O$_{5}$ NWs and single-walled carbon nanotubes (swCNTs) with a nanoscale resolution using only microscale molecular patterning methods. This work provides us with important insights about the directed-assembly process, and from a practical point of view, it allows us to generate nanoscale patterns of NWs over a large area for mass fabrication of NW-based devices. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W35.00007: In$_{2}$O$_{3}$ Nanoparticles for Gas Sensors Zengxing Zhang, Daniela Caruntu, Charles J. O'Connor, Weilie Zhou In the last decades, sensors based on nanostructured materials have attracted much attention. Generally, nanosensors often demonstrate excellent sensitivities because of their high specific surface area and comparable size to the detected targets (chemicals or biomolecules). So far, lots of efforts have been put on the fabrication of nanowires based nanosensors. In this talk, we reported our recent work on employing wet-chemically synthesized indium oxide (In$_{2}$O$_{3})$ nanoparticles for gas sensing. The nanoparticles were self-assembled between the gold electrodes patterned on silicon substrates covered with thermal oxide film using e-beam nanolithography. Several gases, such as ammonia (NH$_{3})$, hydrogen sulfide (H$_{2}$S), etc., were used for the testing. The results exhibit that the sensitivity can be reached down to PPM order. In addition, the sensitivity in terms of nanoparticle size, temperature, etc, were also investigated. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W35.00008: Environment of TiO$_{2 }$ nanoparticles as an important factor to achieve highly efficiency on dye sensitized solar cells Tereza Paronyan, M.C. Lin Amorphous TiO2 nanoparticles were synthesized by sol gel technique. Environment of nanoparticles was neutralized by ammonia, and pH5.8 of TiO2 gel was achieved in result, which is close to the point of zero charge (PZC) of anatase TiO2. Highly interconnected, mesoporous, transparent films were fabricated from the TiO2 colloid with pH5.8. AFM, SEM XRD analyses were carried out for the investigation the size of nanoparticles, the surface morphology and the crystal structure of films. Volt-amperic characteristics showed an improvement in the cell efficiency along with the increasing of pH TiO2 colloid. The cells parameters (Voc, FF, Jsc, $\eta )$ were studied depending on the pH of TiO2 colloid. Increasing pH of the colloid from 2.1 to 5.8 enhanced the overall conversion efficiency of the dye-sensitized solar cells by approximately 30{\%} , and 9.2 {\%} of efficiency was achieved with N719 dye under illumination by simulated AM1.5 solar light (100 mW/cm-2). [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W35.00009: Electron Thermal Microscopy of Multiwalled Carbon Nanotubes Kamal Baloch, Todd Brintlinger, John Cumings A thorough electrical and thermal characterization of carbon nanotubes (CNTs) is essential for their application as electrical and thermal devices. We demonstrate high-resolution thermal imaging of multiwall CNT using electron thermal microscopy. This is achieved by observing the solid to liquid phase transition of low melting point indium islands in a transmission electron microscope. High resolution thermal maps thus obtained provide a qualitative analysis of transfer of heat along a CNT suggesting a trend of CNT acting as a heat spreader. Also, important parameters like thermal conductivity of the CNT can be extracted by finite element modeling. The temperatures involved are $\sim $200C and the samples can be operated over several voltage cycles. Experimental technique, high resolution maps, real time videos, and simulation results will be presented. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W35.00010: The Nuclear Environment for Electron Spins in a Double Quantum Dot David Reilly, Jacob Taylor, Jason Petta, Charles Marcus, Micah Hanson, Art Gossard We report measurements examining the nuclear spin environment for electrons in a GaAs double quantum dot. The hyperfine field, which drives transitions of a two-electron spin state, is detected via spin-to-charge transfer and rf-QPC readout. Fluctuations of the hyperfine field are measured to be broadband, with spectral content ranging from milliseconds to the decorrelation time of $\sim$ 10 seconds. In addition, we demonstrate dynamic nuclear polarization (DNP) using a cyclic gate-pulse sequence. Relaxation of the DNP is studied using time-resolved measurements and found to be sensitive to the spin-state of electrons. The presence of a small DNP is found to suppress hyperfine fluctuations by a factor of $\sim$ 100, leading to a time-ensemble dephasing time, $T_2^*\sim$ 1 microsecond for electron spins. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W35.00011: Nanoscale probing of material properties across antiparallel domain wall in ferroelectrics Vasudeva Rao Aravind, Lili Tian, Nozomi Odagawa, Samrat Choudhury, Pavel Capek, Volkmar Dierolf, Anna N. Morozovska, Eugene A. Eliseev, Long-qing Chen, Yasuo Cho, Sergei Kalinin, Venkatraman Gopalan Although the intrinsic width of an ideal antiparallel ferroelectric domain wall is expected to be step-like on a unit cell level ($\sim $0.5nm), we have recently shown that actual widths of these walls can extend to $\sim $20-100nm in lithium niobate and lithium tantalate (L.Tian \textit{et al}., Physical Review Letters (\textit{in review})). In this work we study the variation of material properties coercive field and switchable ferroelectric polarization as a function of the distance from the domain wall in lithium niobate. Until recently, the study of these material properties on the nanoscale were limited by the lack of theoretical modeling of the instrument resolution limits. Using experimental results and theoretical modeling we demonstrate the relation between intrinsic width of the domain wall and its effect on material properties. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W35.00012: Electrostatic and Nanotechnology – Multidisciplinary Approach - for Space Radiation Shielding Ram Tripathi, John Wilson, Robert Youngquist For the success of NASA's new vision for space exploration to Moon, Mars and beyond, exposures from the hazards of severe space radiation in deep space long duration missions is ``a must solve'' problem. The exploration beyond low Earth orbit to enable routine access of space will require protection from the hazards of the accumulated exposures of space radiation. There is a need to look to new horizons for newer technologies. The present multidisciplinary investigation explores the feasibility of using the active electrostatic shielding in concert with the state-of-the-art materials shielding and protection technologies. The full space radiation environment has been used, for the first time, to explore the feasibility of multidisciplinary shielding. The goal is to repel enough positive charge ions so that they miss the spacecraft without attracting thermal electrons and further attenuate the exposure using nano-materials. Conclusions are drawn for the future directions of space radiation protection. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W35.00013: Probing Local Structures in ZrO$_{2}$ Nanocrystals Using EXAFS Y.L. Soo, P.J. Chen, S.H. Huang, T.J. Shiu, T.Y. Tsai, Y.H. Chow, Y.C. Lin, S.C. Weng, S.L. Chang, J.F. Lee, C.L. Cheung, R.F. Sabirianov, F. Namavar, W.N. Mei Extended x-ray absorption fine structure (EXAFS) has been employed to investigate the local structures surrounding Zr in cubic zirconia thin films prepared by an ion beam assisted deposition technique. These materials have demonstrated promising mechanical properties such as improved hardness and lubricant wettability compared to yttria-stabilized zirconia. To verify the cubic structure of zirconia in films prepared under different growth conditions and to fully understand the mechanism leading to their unique physical properties, the structural information is a required prerequisite. Since zirconia is in the form of nanosized crystallets, conventional x-ray diffraction method is not useful for this purpose. Our x-ray results reveal cubic-like structure with O vacancies around Zr in several nanocrystal samples. Powders of cubic zirconia prepared using chemical methods were also measured for comparison. [Preview Abstract] |
Session W36: Focus Session: Materials for Photovoltaics and Photocatalysis III
Sponsoring Units: DMPChair: David Ginley, National Renewable Energy Laboratory
Room: Morial Convention Center 228
Thursday, March 13, 2008 2:30PM - 3:06PM |
W36.00001: TBD Invited Speaker: |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W36.00002: Hole Mobility Studies on Thiophene-Based Conjugated Polymers Developed for Use in Organic Electronic Devices N.C. Heston, J. Mei, D.B. Tanner, J.R. Reynolds In optimizing organic electronic devices, such as solar cells and field effect transistors, the mobility plays a crucial role affecting many aspects of performance, including: charge separation efficiencies, carrier densities, and drain currents. By fabricating hole-dominated devices and fitting the measured current-voltage characteristics to the field-dependent space-charge-limited mobility model we were able to measure hole mobilities in a set of conjugated polymers including p-Pt-BTD-Th, p-Pt-BTD-EDOT, and both regio-regular and regio-randem P3HT. These materials have been shown to exhibit promise as active layers in organic solar cells, light-emitting diodes, and field effect transistors. We present the results of these measurements and the effects induced by thermal annealing. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W36.00003: Molecular and electronic structure at C$_{60}$:pentacene interfaces S.W. Robey, D.B. Dougherty, W. Jin, W.G. Cullen, G.J. Dutton, J.E. Reutt-Robey Successful utilization of organic donor-acceptor systems for photovoltaic applications requires understanding factors controlling molecular and electronic structure at interfaces. We have used STM, STS, and photoemission to study the donor- acceptor system C$_{60}$:pentacene. At low coverage, C$_{60}$ deposited on a well-ordered pentacene bilayer structure on Ag (111) adsorbs in between two adjacent pentacene rows. Isolated C$_{60}$ molecules are easily observed at room temperature indicating that the mobility of C$_{60}$ on pentacene is significantly smaller than on metal surfaces. Some images of C$_{60}$ reveal structure that may indicate a preferred C$_{60} $ orientation. Electrostatic contributions to intermolecular interactions are discussed to help explain C$_{60}$ adsorption between pentacene molecules. With increasing coverage, C$_{60} $ forms linear chains, still locked to underlying pentacene rows. A further increase in coverage results in domains of disordered C$_{60}$ that we propose result from competing C$_ {60}$- C$_{60}$ and C$_{60}$-pentacene interactions. Information on nanoscale transport gaps and band alignment was obtained using constant-current distance-voltage spectroscopy. A gap of 4.5 eV is found over the linear C$_{60}$ chains compared with a gap of 3.6 eV for the surounding pentacene bilayer. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W36.00004: Porous nanocrystalline TiO$_{2}$ thin films for dye-sensitized solar cells Xiaojuan Fan, Claudia Swanson, David Rogow, Akhilesh Tripathi, Scott Oliver We report a rapid and low cost method to fabricate porous TiO$_{2}$ thin films used as anode electrodes for solid state dye-sensitized solar cells. Polymethylmethacrylate (PMMA) gel was used as template to define a network co-structure with alkali titanium oxide, then spin cast on substrates. After thermally removing polymer, smooth and crack-free large area TiO$_{2}$ thin films with fine pores were generated. Thin film structures were detected by powder {\&} grazing incident X-ray diffraction. Film thickness can be controlled over a range of tens of nanometers to several microns by precursor viscosity, spin coating speed and coating times. The SEM image shows the highest quality porous TiO$_{2}$ film derived from a certain concentration of precursor. The above TiO$_{2}$ thin films were then used to fabricate solid state dye sensitized solar cells. Porphyrine dye and poly(ethylene glycol) electrolyte with I$^{-}$/I$_{3}^{-}$ redox couple were used in the cells. Current-voltage curves were recorded. The open circuit voltage boosts to more than 1.0 V. The reasons for the high open circuit voltage probably will be discussed. Overall photo-electricity conversion efficiency reaches 2.05{\%} under an illumination of one solar unit (AM1.5, 100 mW/cm$^{2})$. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W36.00005: TiO2 nanowire sensitized by natural dyes for solar cell applications Sheng Meng, Jun Ren, Efthimios Kaxiras We investigate the electronic coupling between a semiconductor TiO$_2$ nanowire and a natural dye sensitizer based on time-dependent first-principles calculations. The model dye molecule, cyanidin is found to dissociate into the quinonoidal form upon adsorption, rendering its highest occupied molecular orbitals (HOMO) located in the middle of TiO$_2$ bandgap and its lowest-unoccupied molecular orbital (LUMO) at the bottom of TiO$_2$ conduction band. The visible light absorption is greatly enhanced with two prominent peaks at 460 nm and 650 nm. The excited electrons are injected into the TiO$_2$ conduction within a ultrafast timescale of $<$50 fs, with negligible non-radiative energy dissipation and recombination. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W36.00006: Electronic structure of N3 DYE molecules on the TiO$_{2}$(110) surface and on anatase nanoparticle films Eric Bersch, Sylvie Rangan, Jean-Patrick Theisen, Robert A. Bartynski, Judith D. Sorge, Dunbar P. Birnie We have used direct and inverse photoemission to measure the occupied and unoccupied electronic states, and their alignment with the band edges of the substrate, of N3 dye adsorbed on the rutile TiO$_{2}$(110) surface and on anatase TiO$_{2}$ nanoparticle thin films. In dye-sensitized solar cell applications, the HOMO-LUMO gap determines the useful portion of the solar spectrum, and charge transfer of photoexcited electrons to the substrate depends on the alignment of the LUMO to the TiO$_{2}$ conduction band edge. Samples were prepared and passivated with a pivalate layer in UHV, then sensitized in air in a solution of N3 dye in acetonitrile. STM measurements show that the pivalic acid forms an ordered overlayer on the TiO$_{2}$(110) surface and that the N3 dye molecules can be imaged after sensitization. Our spectroscopic measurements show that contamination (presumably from water in the ambient) is significantly reduced and that the N3 HOMO occurs at 1.1 eV above the TiO$_{2}$ valence band edge, and the LUMO is found 0.3 eV above the conduction band edge. Comparison with experimental and theoretical values from the literature will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W36.00007: Electronic structure of N3 DYE molecules on the ZnO single crystal and epitaxial film surfaces Jean-Patrick Theisen, Eric Bersch, Sylvie Rangan, Yicheng Lu, Robert Bartynski Most dye-sensitized solar cells use TiO$_{2}$ nanoparticle films as the electrode, but ZnO offers an interesting alternative. We have used direct and inverse photoemission to measure the occupied and unoccupied electronic states, and their alignment with the band edges of the substrate, of N3 dye adsorbed on ZnO(0001), ZnO(11-2), epitaxial ZnO a-plane film surfaces, and ZnO nanopillars. As the unoccupied states of ZnO are of \textit{sp}-character and of relatively low cross section, the LUMO of the dye is easily observed. Samples were prepared and passivated with a pivalate layer in UHV, then sensitized in air in a solution of N3 dye in acetonitrile. As opposed to the case of the TiO$_{2}$(110) surface, STM measurements indicate that the pivalic acid does. From UPS, the N3 HOMO is found at $\sim $0.8 eV above the ZnO valence band edge, and the LUMO is found $\sim $1.5 eV above the conduction band edge for the epifilm. Differences in dye adsorption and orbital alignment for these different ZnO surfaces will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W36.00008: Towards visible light activity of wide band gap photocatalysts: Surface functionalization of ZnO with ZnS Jayeeta Lahiri, Matthias Batzill We show that at the ZnO/ZnS interface the band alignment is favorable for reducing the photo excitation threshold energy; signifying that the combination of two wide band gap photocatalysts can yield a material with visible light activity. Modification of ZnO with a sub monolayer ZnS is investigated by scanning tunneling microscopy (STM) and photoemission spectroscopy. STM studies indicate that the ZnS grows by nucleation and spreading of 2D clusters of monolayer height ($\sim $ 2.5 {\AA}). Photoemission spectroscopy is used to measure the band alignment between ZnO and ZnS, as well as measure the changes in the surface charge region and work function. An increase in work function by 1.1 eV is observed and a staggered band alignment is found with ZnS states effectively narrowing the band gap for photo excitation from 3.4 to 2.7 eV. We propose that the combination of these structural and electronic properties of the modified ZnO surface result in an improved, visible light active photocatalyst. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W36.00009: Energy bands and point defects in CuInSe$_{2}$ and CuGaSe$_{2}$ calculated by Quasiparticle Self-Consistent $GW$ Mark van Schilfgaarde, Takao Kotani CuIn$_{x}$Ga$_{1-x}$Se$_{2}$, or CIGS, is emerging as a leading candidate for second-generation solar cell applications. Here we present the bulk energy band properties and dielectric response of CuInSe$_{2}$ and CuGaSe$_{2}$, computed within the Quasiparticle Self-Consistent \emph{GW} (QS$GW$) approximation. QS$GW$ has been proven to be a very reliable, true \emph{ab initio} predictor of QP levels in a wide variety of materials systems; it is expected to be similarly reliable for chalcopyrite semiconductors. The fundamental gap agrees well with experiment. Also, the electron and hole effective masses are evaluated. Various kinds of point defects were considered using certain approximations to QS$GW$. Of particular interest are low-energy cation defects (antisites and vacancies). Rather unusual properties of these levels are found, owing to the unique role that shallow Cu $d$ states play in CIGS. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W36.00010: Synthesis of type II core/shell nanowires for photovoltaic application Weilie Zhou, Kai Wang, Jiajun Chen, Yong Zhang, John Pern, Yanfa Yan, Angelo Mascarenhas The core/shell semiconducting nanowires based on II-VI semiconductors, involving with type II band energy alignments, are predicted to be a new kind of nanostructured materials for efficient charge separation for stable and efficient photovoltaic devices. In this talk, we report a successful synthesis of II-VI semiconducting core/shell nanowires by a relatively simple and low-cost approach. The structures and optical properties were characterized by applying a set of comprehensive techniques. A sharp interface and the epitaxial relationship between the core and shell were observed. Two excitonic absorption peaks were clearly found at respective excitonic bandgaps, indicating a good crystallinity of both the core and shell. Compared to the single component nanowires, the PL spectrum of the core/shell nanowires shows a reduction in intensity and a slight blue shift at the band edge emission of the core nanowires, which may partially arise from spatially charge separation between the core and shell. The direct growth of core/shell nanowires represents a major step toward fabricating a low-cost, high efficiency and stable solar cell. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W36.00011: Nanocoax Solar Cells M.J. Naughton, K. Kempa, Z.F. Ren, J. Rybczynski, T. Paudel, Y. Gao, Y. Xu A novel architecture for high efficiency solar energy conversion, employing separated photo-- and --voltaic pathways and antenna-based light collection, is described. This \textit{material-independent} architecture solves the ``thick-and-thin'' conundrum of solar photovoltaics, wherein solar cells must be thick enough to absorb light yet thin enough to allow for charge extraction. Our solar cells are comprised of arrays of high aspect ratio, vertically-aligned, nanoscale, metallic coaxial wires (\textit{nanocoax}) which are indeed simultaneously thick (tall) and thin (narrow). Photons captured by nanoscale antennas are channeled axially as TEM-mode radiation in the nanocoax annulus, which is filled with a PV medium. This annulus is unprecedentedly thin radially ($\sim $100 nm), such that exciton lifetimes and subsequent electron and hole diffusion lengths of virtually any PV material are sufficiently long to enable highly efficient solar energy conversion. We discuss results with radial $p-i-n $junctions using $ a$-Si PV and carbon nanofiber coax center conductors, where nanocoax solar cell efficiencies exceed those of comparable planar junctions. Moreover, this nanoscale architecture can be considered a feasible portal to 3$^{rd}$ generation solar power. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W36.00012: Low temperature pulsed electrically detected magnetic resonance on a-Si:H p-i-n solar cells Thomas Herring, Heather Seipel, Dane McCamey, Christoph Boehme, Craig Taylor, Jian Hu, Feng Zhu, Arun Madan Hydrogenated amorphous silicon (a-Si:H) has become one of the most important semiconductor materials, with applications including solar cells and thin film transistors. In spite of this, and more than 30 years of intensive studies of this material, the microscopic nature of various recombination mechanisms in this material are still not well understood. Recently, pulsed electrically and optically detected magnetic resonance (p-EDMR, p-ODMR, respectively) spectroscopy has provided a method for directly and quantitatively observing some of these microscopic processes. Here, we present p-EDMR measurements on a-Si:H p-i-n solar cells at temperatures T $\le $ 40K, with a comparatively low light excitation density. After a short, coherent microwave excitation, we record transients for a range of externally applied magnetic fields. The results show the presence of a number of resonances, which we discuss with regard to previous continuous wave (cw-) ESR and cw-EDMR studies, as well as cw- and p-ODMR measurements. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W36.00013: Carrier Multiplication in Semiconductor Nanocrystals: Theoretical Screening of Candidate Materials Jun-Wei Luo, Alberto Franceschetti, Alex Zunger The process of Direct Carrier Multiplication (DCM) involves the creation of TWO electron-hole pairs as a results of exciting a nanostructure by ONE photon with energy two times larger than the band gap $E_g$. The ratio $R(E)=\rho_{XX}(E)/\rho_X(E)$ between the biexciton ($XX$) and monoexciton ($X$) density of states [Nano Lett. {\bf6}, 2191 (2006)] is a ``figure of merit'' of the DCM process. Using the ``Truncated Crystal'' approximation to the electronic structure of nanocrystals based on the atomistic, semi-empirical pseudopotential approach we calculated $R(E)$ for nanocrystals of GaSb, InAs, InP, GaSb, InSb, Ge, Si, and PbSe. We found that InSb, GaSb, Ge, and PbSe quantum dots have larger DCM figure of merit than the other quantum dots. Our calculations suggest that there are three requirements for high DCM efficiency: (1) Small nanocrystal band gap ($<$ 1.6 eV) to match the solar spectrum, (2) high degeneracy of the band-edge states, and (3) heavy electron and hole effective mass. We conclude that nanocrystal made of HgS, HgTe, HgSe, PbS, PbTe, and Sn are also good candidates for DCM-based solar cells. [Preview Abstract] |
Thursday, March 13, 2008 5:30PM - 5:42PM |
W36.00014: Exploring the Use of Self-Assembled InGaAs/GaAsP Quantum Dots as Intermediate Band Solar Cells (IBSC) Voicu Popescu, Gabriel Bester, Alex Zunger It has been recently proposed that the efficiency of photovoltaic solar cells based on wide-gap III-V absorbing materials can be enhanced if quantum dots are embedded in such matrices, leading to confined electron and hole states that can be excited to the band edges of the wider-gap matrix material, thereby capturing the lower energy (IR) solar photons. Such proposals, however, were not scrutinized so far by modern quantum-dot calculations. We apply our pseudopotential Linear-Combination-of Bloch-Bands (LCBB) approach to this problem. Lens-shaped dots of InGaAs were vertically stacked with varying dot-dot separation. The effects of spin-orbit, multi-band, and multi-valley coupling are included by a direct diagonalization of the atomistic problem. A matrix of GaAsP was chosen so as to strain-balance the system epitaxially on a GaAs(001) substrate. We will discuss the energies of the band edges of the matrix material, and those of the confined dot levels relative to the expected values for ideal IBSC operation, as well as their variation with respect to either the vertical dot-dot separation, or the band gap of the matrix material. [Preview Abstract] |
Session W37: Focus Session: Multiferrocity in BiFeO$_3$-based films
Sponsoring Units: DCMPChair: Craig Fennie, Argonne National Laboratory
Room: Morial Convention Center 229
Thursday, March 13, 2008 2:30PM - 2:42PM |
W37.00001: First-principle calculations of electronic structure of bismuth ferrate and manganate with the mullite structure Jen-Chang Chen, Ching-Ming Wei We applied ab initio total-energy calculation to study the electronic structure of Bi$_2$Fe$_4$O$_9$ and Bi$_2$Mn$_4$O$_{10}$ single crystals. Both crystals are orthorhombic with the centro-symmetric structure and belong to the mullite-type material. Most of bismuth-based oxides exhibit good ferroelectricity, pyroelectricity and piezoelectricity. However, little researches of electronic structure and properties have been done on the bismuth ferrate and bismuth manganate. In this study, the band structure, density of state, partial density of state and magnetic moment of both bismuth ferrate and bismuth manganate at zero Kevin were calculated. The effect of Fe and Mn atom within the structure were discussed. The phase stability of these two crystals was also examined. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W37.00002: The structural, magnetic, and electric properties of Sr and Ca-doped BiFeO$_{3}$ films. Chan-Ho Yang, M. Huijben, Y.-H. Chu, L.W. Martin, M. Holcomb, R. Ramesh, M. Chi, N. Browning Many perovskite oxides, as holes are doped, exhibit unusual physical phenomena such as superconductivity, colossal magnetoresisitance, metal-insulator transition, and charge ordering. We are expoloring the consequences of hole doping into a well known multiferroic, the antiferromagnetic ferroelectric, BiFeO$_{3}$. In this presentation, the systematic investigation on structural, electric, and magnetic properties will be presented for nominally hole-doped BiFeO$_{3}$ obtained by partially substituting the Bi ions by divalent ions (such as Ca and Sr). The parent BiFeO$_{3}$ is unique single-compound multiferroic material with ferroelectricity and magnetic ordering at room temperature, has polarization along [111] direction and is a G-type antiferromagnet. The divalent ion doping weakens the ferroelectric properties but improves magnetization owing to cluster-glass-like magnetism. We will discuss the magnetic properties based on superexchange mechanism. The structural characterization by X-ray diffraction, scanning microscopy and TEM will be presented. The SQUID and the dielectric constant measurements as a function of magnetic field and temperature will be also reported. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W37.00003: Phase diagram for Bi1-xCaxMnO3 (x $<$ 0.4) Yuhai Qin, Trevor Tyson, Sang-Wook Cheong, Xiao-nong Xu The multiferroic BiMnO3 system, in which ferroelectronic and ferromagnetic orders can coexist, has attracted much research work in the past years for its potential technological applications. For the more general system Bi1-xCaxMnO3, the phase diagram for the Ca rich region (x $>$ 0.4) has been established[1]. In order to understand the multiferroic behavior near the x=0 system, the hole-doped region (0$<$x$<$0.4) was investigated. We have completed the magnetic, transport, and structural phase diagram of Bi1-xCaxMnO3, by performing detailed structural (XRD and XAFS), magnetization (ZFC/FC) and electrical measurements on Bi1-xCaxMnO3 (0$<$x$<$0.4), showing the transition form the highly distorted monoclinic phase to the orthorhombic phase. This work is supported by NSF DMR-0512196 and DOE Grant DE-FG02-07ER46402. [1] H. Woo, et al, Phys. Rev. B: Condensed Matter and Materials Physics 63, 134412/1 (2001). [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W37.00004: Combinatorial discovery of morphotropic phase boundary in a lead-free high $T_{c}$ piezoelectric perovskite Bi$_{1-x}$(RE)$_{x}$FeO$_{3}$ S. Fujino, D. Kan, A. Varatharajan, C.J. Cheng, V. Nagarajan, M. Murakami, S.-H. Lim, D. Hunter, C.J. Fennie, L. Salamanca-Riba, M. Wuttig, I. Takeuchi We have recently discovered a morphotropic phase boundary (MPB) in Bi$_{1-x}$Sm$_{x}$FeO$_{3}$ which has a simple perovskite structure. We have systematically investigated compositionally varied Sm doped BiFeO$_{3}$ thin films using the combinatorial approach and found that ferroelectric properties and piezoelectric properties show pronounced enhancement at the MPB. The samples were fabricated by combinatorial pulsed laser deposition on SrTiO$_{3}$ (001) substrates with a SrRuO$_{3}$ buffer layer. The boundary is a rhombohedral to pseudo-orthorhombic structural transition which exhibits a ferroelectric to antiferroelectric transition at approximately Bi$_{0.86}$Sm$_{0.14}$FeO$_{3}$ with intrinsic d$_{33}$ comparable to those of PbZr$_{0.52}$Ti$_{0.48}$O$_{3}$ thin films. Transmission electron microscopy (TEM) reveals presence of nanodomains at the MPB. TEM also reveals onset and formation of antiferroelectric domains as the composition is swept near the MPB, where electron diffraction patterns show systematic structural tilt transitions of the system as a function of Sm doping. Finally, we report on structural transitions and ferroelectric properties in other Bi$_{1-x}$(RE)$_{x}$FeO$_{3}$ systematically studied by the composition spread technique. This work is supported by NSF MRSEC, ARO, and the W. M. Keck Foundation. Research at UNSW is supported by DEST Australia, Australian Research Council Grant DP 0666231 and ARNAM Travel Grant. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W37.00005: The effect of structural and chemical perturbations in multiferroic BiFeO$_{3}$ epitaxial films Dae Ho Kim, H.N. Lee, M. Varela, H.M. Christen, M.D. Biegalski, C.J. Callender, D.P. Norton The compatibility between the lone-pair driven ferroelectric distortion and antiferromagnetic order in BiFeO$_{3}$ attracts a lot of attention. A detailed understanding of ferroelectric properties in BiFeO$_{3}$ is gained by investigating the effect of structural/chemical perturbations in strained epitaxial films with chemical modifications. Our work shows that the ferroelectric polarization along [111] exhibits weak dependency on epitaxial strain on a (001) substrate. To examine the role of distortions induced by magnetic ions, we have grown BiFe$_{1-x}$Cr$_{x}$O$_ {3}$ epitaxial films and observed a ferroelectric to antiferroelectric transition with increasing the Cr content. Furthermore, epitaxial films of Bi$_{1-y}$Ba$_{y}$FeO$_{3}$ were grown to investigate the effect of structural variation in connection with change in the valence of Fe ion. The results reveal a high stability of the ferroelectric distortion in epitaxial BiFeO$_{3}$ films. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W37.00006: Structural, electrical and magnetic properties of Bi(Fe$_{x}$,Ti$_{1-x})$O$_{3}$ thin films N.M. Murari, R. Melgarejo, R. Thomas, R.S. Katiyar Multiferroic materials have recently seen a surge of research activities due to its applications in the memory, spintronics, sensors devices and micro mechanical systems (MEMS). BiFeO$_{3}$ is a naturally occurring multiferroics. However, the large leakage current is a concern for practical applications and substitution at Bi and Fe is commonly employed to circumvent this problem. Here, Fe substitution by Ti is considered for this purpose. Thin films on Pt/Ti/SiO$_{2}$ /Si were grown by chemical solution deposition (CSD) and characterized for structure and symmetry by XRD and Raman scattering, surface morphology by AFM, dielectric properties by impedance analyzer, and the leakage current by I-V measurements. Magnetic (M-H) and electric (P-V) hysterisis loops were also studied. Variation of dielectric constant ($\varepsilon _{r})$ and loss tangent (tan$\delta )$ with temperature, frequency and temperature were systematically studied. Further, the effect of Ti substitution on the electrical properties will be presented. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W37.00007: Piezoelectricity and structure of epitaxial ferroic thin films at high electric fields Alexei Grigoriev, Ribecca Sichel, Ho Nuyng Lee, Chang-Beom Eom, Zhonghou Cai, Eric C. Landahl, Bernhard Adams, Eric M. Dufresne, Paul G. Evans With mastering the techniques to grow nearly perfect epitaxial thin oxide films, there are emerging opportunities to control the structure and properties of oxide materials using extremely high electric fields. To unveil the piezoelectric and structural properties of Pb(Zr,Ti)O3 and BiFeO3 epitaxial thin films at electric fields which are a few times stronger than the low-frequency dielectric breakdown field, we employed time-resolved structural measurements synchronized with electric field pulses of a nanosecond duration. At these extreme fields we measured record-high piezoelectric strains and explored nonlinearities in piezoelectric responses predicted to occur due to the changes in interatomic interactions. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W37.00008: Giant Polarization Rotation in BiFeO3/SrTiO3 Thin Films. M.C. Langner, Y.H. Chu, L.M. Martin, M. Gajek, R. Ramesh, J. Orenstein We use optical second harmonic generation to probe dynamics of the ferroelectric polarization in (111) oriented BiFeO3 thin films grown on SrTiO3 substrates. The second harmonic response indicates 3m point group symmetry and is consistent with a spontaneous polarization normal to the surface of the film. We measure large changes in amplitude and lowering of symmetry, consistent with polarization rotation, when modest electric fields are applied in the plane of the film. At room temperature the rotation is an order of magnitude larger than expected from reported values of the dielectric constant and increases further (as 1/T) as temperature is lowered. We propose a substrate interaction model to explain these results. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W37.00009: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 4:18PM - 4:54PM |
W37.00010: Exchange biasing with multiferroic: electric field effects on magnetic and magnetotransport properties Invited Speaker: Room-temperature multiferroic materials are scarce and display a weak magnetoelectric coupling and thus huge difficulties exist for controlling the magnetic state by using an electric field or viceversa. A possible alternative to circumvent this limitation is to exploit the clamping of ferroelectric and antiferromagnetic domains in biferroic materials and use a suitable exchange-bias existing with ferromagnetic materials to tune the magnetic response of the ferromagnet. In this presentation we shall overview recent experiments on exchange-biasing using hexagonal YMnO$_{3}$ biferroics and Permalloy as a soft-ferromagnet. Exchange-bias on ferromagnetic materials is most commonly evidenced by their magnetic response, although magnetotransport measurements are also very adequate to monitor the exchange bias. We will present and discus first how exchange-bias is manifested and monitored. Next, we will describe the effects of an electric field, biasing the ferroelectric (and antiferromagnetic) epitaxial layer, on the exchange bias. We will show that under appropriate conditions, magnetization can be switched by application of a suitable electric field. We will discuss the significance of the results with particular attention to role of current leakages across the ferroelectric. \newline \newline In collaboration with X. Mart\'{\i}, Institut de Ci\`{e}ncia de Materials de Barcelona-CSIC, Spain; V. Laukhin, Institut de Ci\`{e}ncia de Materials de Barcelona-CSIC and Institut Catal\`{a} de Recerca i Estudis Avan\c{c}ats (ICREA), Barcelona, Catalonia, Spain; V. Skumryev, Institut Catal\`{a} de Recerca i Estudis Avan\c{c}ats (ICREA) and Departament de F\'{\i}sica, Universitat Aut\`{o}noma de Barcelona, Spain; D. Hrabovsky and F. S\'{a}nchez, Institut de Ci\`{e}ncia de Materials de Barcelona-CSIC, Spain; M. Varela, Departament de F\'{\i}sica Aplicada i \`{O}ptica, Universitat de Barcelona, Spain; U. L\"{u}ders and J.F. Bobo, LNMH ONERA-CNRS, France. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W37.00011: Investigation of the electrical switching of magnetization through exchange interactions with a magnetoelectric multiferroic Alfred Lee, Alex de Lozanne, Ying-Hao Chu, Lane W. Martin, Mikel Barry, Qian Zhan, Pei-Ling Yang, Kilho Lee, Z. Q. Qiu, R. Ramesh The coupling between antiferromagnetic (AF) and ferromagnetic (FM) ordering at an interface between the two types of materials has been well established and provides the basis for modern day hard drives.~ BiFeO$_{3 }$is a magnetoelectric multiferroic material which shows coupling between ferroelectric (FE) and AF phases.~ The Curie and Ne\'{e}l temperatures are $\sim $820 and 370\r{ }C, respectively, permitting room temperature operation.~ Manipulation of the FE ordering via external electric fields affects the AF ordering which in turn permits control over the magnetization of an adjacent FM material.~ We present the results of our investigation into the control of magnetic domains using electric fields.~ The object of investigation is a multilayer film of Co$_{0.9}$Fe$_{0.1}$/BiFeO$_{3}$/SrRuO$_{3}$/SrTiO$_{3}$(001) patterned into islands.~ A magnetic force microscope is used both to image the magnetic domains and to apply the potential across the film. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W37.00012: Anisotropic photo-control of spontaneous polarization in BiFeO$_{3}$ thin films: Indications of photo-assisted magnetoelectric effect D.S. Rana, I. Kawayama, H. Murakami, M. Tonouchi The terahertz (THz) emission from multiferroic BiFeO$_{3 }$(BFO) due to ultra-fast depolarization of electric order helps in evaluating its ferroelectric behavior [1]. In this work, we investigate THz-emission in BFO films on (LaAlO$_{3})_{0.3}$(Sr$_{2}$AlTaO$_{6})_{0.7}$ (LSAT) substrates along $(100)$, $(110)$ and $(111) $directions. THz emission amplitude (E$_{THz})$ and its electric field dependence are similar in BFO $(100)$ and BFO $(110)$; E$_{THz}$ as function of electric field translates to ferroelectric-like hysteresis loop, and sign and amplitude of THz-emission are commensurate with the applied electric field. On the contrary, in BFO $(111)$ the hysteretic loop is absent, E$_{THz}$ varies linearly with electric field and the polarity of THz-emission is opposite to that of the electric field. These results suggest that THz-emission is mainly a result of ultrafast depolarization but with a superimposed character of ultrafast reorientation or demagnetization of antiferromagnetic (AFM) order. Though AFM order in BFO makes the determination of magnetoelectric (ME) coupling rather difficult, the photo-assisted electric field control of AFM order shown in the present work is suggestive of photo-assisted ME effect. 1. K. Takahashi \textit{et al.}, Phys. Rev. Lett. 96, 117402 (2006). [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W37.00013: Measurements of Bandgap of Epitaxial BiFeO$_{3}$ Films by UV-VIS Absorption and Cathodoluminescence Spectroscopies A.J. Hauser, J. Zhang, L. Mier, R. Ricciardo, P.M. Woodward, T.L. Gustafson, L.J. Brillson, F.Y. Yang We report measurements of the bandgap of pure-phase epitaxial BiFeO$_{3}$ thin films on (001)-oriented SrTiO$_{3}$ substrates, via UV-VIS absorption and cathodoluminescence (CL) spectroscopies. 70 nm thick BiFeO$_{3}$ films were grown using ultrahigh vacuum RF magnetron sputtering at substrate temperatures between 500 \r{ }C and 600 \r{ }C. X-ray diffractometry shows that samples grown in this temperature range are epitaxial and pure-phase. UV-VIS absorption spectra show a consistent bandgap of 2.5 $\pm $ 0.03 eV for all growth temperatures. A small tail in the UV-VIS absorption spectra just below the band gap extends down to 2.2 eV, indicating some electronic states within the bandgap. The bandgap was confirmed via CL measurements, where a bandgap of 2.46 $\pm $ 0.01 eV was obtained for samples at growth temperatures of 550 \r{ }C and 600 \r{ }C. To our knowledge, this report is the first sdetailed measurement of electronic band and defect structure for epitaxial BiFeO$_{3}$ films and confirms theoretical predictions. [Preview Abstract] |
Session W39: Focus Session: Jamming II: Packing and Force Networks
Sponsoring Units: GSNPChair: Gregg Lois, Yale University
Room: Morial Convention Center 231
Thursday, March 13, 2008 2:30PM - 2:42PM |
W39.00001: A phase diagram for jammed matter reveals the nature of the random loose and random close packing of spheres Ping Wang, Chaoming Song, Hernan A. Makse We employ statistical mechanics of jammed matter to demonstrate the phase diagram of all available jammed configurations of frictional and frictionless granular packings. This provides a statistical definition of RLP and RCP, predicts their density values in close agreement with simulations, and establishes the concomitant equations of state relating observables such as the coordination number, $z$, entropy, $S$, and volume fraction, $\phi$. We show that the RCP state is not a unique point in the phase space but extends along a line of zero compactivity, a temperature-like variable, predicted to be at a constant $\Phi_{RCP} = 0.634$, but with different $z$. The lowest density of RLP appears as a line of infinite compactivity parameterized by $z$, ending at the minimum possible density theoretically predicted to be $\Phi_{RLP} = 0.543$. The nature of the disorder of the packings is statistically characterized by the entropy which is shown to be larger in the random loose case than in the random close case. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W39.00002: Random loose packing of rough spheres Greg Farrell, Michael Martini, Narayanan Menon We report experiments in which random loose packings of spheres are created by sequential deposition of monodisperse (3.18$\pm$0.05mm) PMMA beads of high sphericity in a fluid. The deposition speed is controlled by varying the relative densities of the spheres and the fluid, as well as by varying fluid viscosity. As in the work of Onoda and Liniger, we find that the volume fraction of the sediment asymptotically approaches a lower limit as we approach neutral buoyancy. However, we find that deposition in increasingly viscous fluids has the same effect. We also study the effect on the packing of controlled chemical roughening of the surface of the spheres. The volume fractions attained can be significantly lower than the often-quoted volume fraction of 0.555 for random loose packing. Thus there is no unique volume fraction for the random loose packing of spheres; the measured volume fraction, even in the limit of slow deposition dynamics is determined by particle surface properties. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W39.00003: Tunable Random Packings Geoffroy Lumay, Nicolas Vandewalle We present an experimental protocol that allows one to tune the packing fraction $\eta$ of a random pile of ferromagnetic spheres from a value close to the lower limit of random loose packing $\eta_{RLP} \simeq 0.56$ to the upper limit of random close packing $\eta_{RCP} \simeq 0.64$. This broad range of packing fraction values are obtained under normal gravity in air, by adjusting a magnetic cohesion between the grains during the formation of the pile. Attractive and repulsive magnetic interactions are found to deeply affect the internal structure and the stability of sphere packing. After the formation of the pile, the induced cohesion is decreased continuously along a linear decreasing ramp. The controlled collapse of the pile is found to generate various and reproducible values of the random packing fraction $\eta$. (see New Journal of Physics 9 406 (2007)). [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W39.00004: Analysis of Configurational Entropy in Jammed Granular Matter Christopher Briscoe, Ping Wang, Chaoming Song, Hernan Makse Energy fluctuations in jammed granular matter are negligible and cannot control the statistical mechanics. It is of interest to explore volume fluctuations in an effort to describe the statistical mechanics of jammed matter, originally proposed by Edwards. Current studies have introduced the concept of a theoretical phase diagram for jammed matter, providing plausible statistical explanations for the RCP and RLP states, along with intermediate jammed states, as a function of coordination number, z, friction coefficient $\mu$, and volume fraction, $\phi$. Entropy can be derived from this theoretical framework by means of a Hamiltonian, with energy and temperature replaced by volume, W, and compactivity, X, where X is an analogue of temperature. Our present efforts are to calculate the Shannon entropy of jammed granular packings along various paths of the phase diagram, resulting in an extensive entropy density, and X, as a function of $\phi$, providing a new equation of state for jammed granular matter. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W39.00005: Jamming of Frictional Spheres Abdullah Cakir, Leonardo Silbert Packings of monodisperse, frictional spheres are studied for a wide range in particle friction coefficient on approach to the jamming transition - the point where the packing loses mechanical stability. In particular, we focus on dynamical properties through the study of the vibrational normal modes. The dynamical matrix includes terms that take into account the rotational degrees of freedom due to non-central forces in the presence of friction. The influence of friction on the normal mode frequencies and particle polarization vectors are examined. Distinct from frictionless systems, for finite friction, the normal modes now allow for particle rotations. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W39.00006: Hard Discs on the Hyperbolic Plane Carl Modes, Randall Kamien We examine a simple hard disc fluid with no long range interactions on the two dimensional space of constant negative Gaussian curvature, the hyperbolic plane. This geometry provides a natural mechanism by which global crystalline order is frustrated, allowing us to construct a tractable model of disordered monodisperse hard discs. We extend free area theory and the virial expansion to this regime, deriving the equation of state for the system, and compare its predictions with simulation near an isostatic packing in the curved space. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W39.00007: Random subcubes as a toy model for constraint satisfaction problems Thierry Mora, Lenka Zdeborova abstract-Many hard combinatorial problems, such as Random Satisfiability, have been shown to reproduce some salient properties of glassy materials. In particular, it has been proved that the configurational landscapes of the hardest problems are made of many disconnected ergodic components, leading to rich phase diagrams. Here we present an exactly solvable random-subcube model inspired by the structure of hard constraint satisfaction and optimization problems. Our model reproduces the structure of the solution space of the random satisfiability and coloring problems, and undergoes the same phase transitions. Distance properties, and their relation to ergodicity, are studied. The model can also be generalized to define a continuous energy landscape useful for studying several aspects of glassy dynamics. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W39.00008: Random Packing of Platonic Solids Taber Hersum, Martin-D. Lacasse, Hubert King While a large amount of literature has been devoted to the packing of spheres, very little is known about the packing of regular polyhedra, such as platonic solids. This presentation will describe numerical work on the packing of model systems in which mono-disperse tetrahedra, cubes, dodecahedra, and icosahedra are randomly packed. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W39.00009: Experiments on Random Packings of Tetrahedrons Alexander Jaoshvili, Paul Chaikin We have performed experiments related to the random packing of tetrahedral. The main experiments are MRI scans of tetrahedral dice from which we determine their positions and orientations. We have done a direct analysis of the dice packing. The dice differ from mathematical tetrahedral in having slightly rounded vertices and edges. We have performed a best fit to each die to a perfect tetrahedral and then relaxed the packing to reduce the resulting overlaps. Analyzed data for the dice, the relaxed tetrahedra and simulations include the packing density, the orientational and spatial correlation functions, the average coordination number as well as the distribution of point to face, edge to edge and edge to face contacts. We also study the boundary effects from the walls and the distribution of constraints per particle. Our measurements indicate that random packed tetrahedral have a very small spatial or orientational correlation length. They are more random, that is with smaller and shorter range correlations than what has been found for spheres, or ellipsoids. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W39.00010: Local pressure distributions in the force network ensemble for granular media Brian Tighe We present an analytic calculation of the probability distribution of pressure on individual grains in a static granular packing. We maximize entropy within an ensemble of all possible force networks on a fixed contact network, which incorporates force balance on each grain. Similar to energy in the microcanonical ensemble, the average pressure in each configuration is fixed. Subject to this global constraint alone, entropy maximization would yield a pressure distribution with an exponential tail. We demonstrate that, as a direct consequence of local force balance, there exists an additional global conserved quantity. Maximizing entropy while also respecting this new conserved quantity, we find a pressure distribution that, in frictionless packings, grows as a power law for small pressures and decays with a Gaussian tail. The form of the distribution is confirmed by numerics. As we increase the coefficient of friction, the tail approaches an exponential. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W39.00011: Using MR Elastography to Image Force Chains in a Quasi-Static Granular Assembly L. Sanfratello, S.A. Altobelli, R.P. Behringer, E. Fukushima Questions about the internal structure of dense granular assemblies remain unanswered for lack of 3D experimental data. It is known from 2D observations and from the boundaries of 3D systems that non-uniform stresses are present on container boundaries as well as at the bottom of granular piles. These forces are seen in 2D to be distributed by force chains, where most of the stress is transmitted through a small number of chains with much of the assembly transmitting little or none of the force. However, force chains have yet to be fully visualized in 3D. We propose a variation of magnetic resonance elastography (MRE) to image 3D force chains within a densely packed granular assembly. MRE is an MRI technique whereby small periodic displacements within an elastic material can be measured. Multiple bipolar motion encoding gradients incorporated into a typical pulse sequence, and applied at the frequency of mechanical oscillations, are used to detect the displacements. We have verified our MRE technique using a gel (Perma-Gel). We now extend this method to image force chains within a 3D granular assembly of particles under stress, on top of which is superimposed a small-amplitude vibration. It is our hypothesis that significant coherent displacements will be found only along force chains while most particles will move randomly. Experimental results will be presented. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W39.00012: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W39.00013: Contact Percolation in Dense Granular Flow Fuping Zhou, Deniz Ertas Steady-state rheology of spheres are studied in the dense flow regime with three-dimensional molecular dynamics simulations in two different geometries: Simple shear flow and gravity-driven chute flow. The same set of constitutive equations, which are only a function of the local dimensionless strain rate, $I$, are found to characterize bulk macroscopic observables such as density, internal Coulomb coefficient and scaled velocity fluctuations in both cases. A transition has been identified at a finite (non-universal) value of $I=I_{c}$, corresponding to the percolation transition of the instantaneous contact network. For $I \quad < \quad I_{c}$, an infinite contact network spans the system. The flow dilates and the internal Coulomb coefficient increases with increasing $I$. For $I > \quad I_{c}$, the instantaneous contact network is broken into finite clusters. The system dilates further with increasing $I$ while the internal Coulomb coefficient becomes independent of $I$, resulting in a maximum tilt angle for steady chute flow. Scaled velocity fluctuations exhibit power-law dependence on $I$ on both sides of $I_{c}$, with a minimum at the transition. The transition is distinct from the ``jamming'' transition at $I$ = 0 associated with the \textit{rigidity} percolation of the contact network. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W39.00014: Dynamics of the Granular Jamming Transition. Mahesh Bandi, Andras Libal, Michael Rivera, Robert Ecke We experimentally study the force fluctuations felt by a probe disk as it is dragged through a two-dimensional bi-disperse system of randomly packed photo-elastic disks. The fluctuations are studied as a function of packing fraction where the system goes from an unjammed to a jammed state with increasing packing fraction. As the system approaches the Jamming Point, the fluctuations are expected to diverge and become increasingly intermittent. We will present preliminary results of the force fluctuations felt by the probe disk as measured by a force transducer and compare them with visual data as obtained from the force-chains formed by the photo-elastic disks. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W39.00015: Stick-Slip and Granular Force Networks Robert Behringer, Peidong Yu We describe friction/failure experiments for a granular system consisting of photoelastic particles. The goal of the experiments is to provide a microscopic understanding of stick-slip friction for an object that is pulled across a granular material. The granular material consists of a photoelastic disks (bidisperse distribution) that are confined to a vertically oriented channel. A slider that is rough at the grain scale is pulled across the upper surface of the material. The pulling is accomplished by a screw-driven platform that is connected to the slider by a spring. Photoelastic image data are acquired by a camera and light source that move with the platform. Non-periodic stick-slip occurs for the regime of parameters studied here. During a stick event, force builds up in a strong network of force chains in the granular material. When one or more of the chains break, a slip event occurs. Energy changes from these events are power-law distributed. Analysis of failure points and slip events yields the effective friction coefficients, which are broadly scattered. An alternative description involves modeling the force chain network as a collection of springs. Failure of one spring can lead to a cascade and hence the broad distribution of energy losses. [Preview Abstract] |
Session W40: Focus Session: Networks, Regulation, and Pathways in Cell Biology
Sponsoring Units: DBP GSNPChair: Rahul Kulkarni, Virginia Polytechnic Institute and State University
Room: Morial Convention Center 232
Thursday, March 13, 2008 2:30PM - 3:06PM |
W40.00001: Metabolism and evolution: A comparative study of reconstructed genome-level metabolic networks Invited Speaker: The availability of high-quality annotations of sequenced genomes has made it possible to generate organism-specific comprehensive maps of cellular metabolism. Currently, more than twenty such metabolic reconstructions are publicly available, with the majority focused on bacteria. A typical metabolic reconstruction for a bacterium results in a complex network containing hundreds of metabolites (nodes) and reactions (links), while some even contain more than a thousand. The constrain-based optimization approach of flux-balance analysis (FBA) is used to investigate the functional characteristics of such large-scale metabolic networks, making it possible to estimate an organism's growth behavior in a wide variety of nutrient environments, as well as its robustness to gene loss. We have recently completed the genome-level metabolic reconstruction of \textit{Yersinia pseudotuberculosis}, as well as the three \textit{Yersinia pestis} biovars \textit{Antiqua}, \textit{Mediaevalis}, and \textit{Orientalis}. While \textit{Y. pseudotuberculosis }typically only causes fever and abdominal pain that can mimic appendicitis, the evolutionary closely related \textit{Y. pestis} strains are the aetiological agents of the bubonic plague. In this presentation, I will discuss our results and conclusions from a comparative study on the evolution of metabolic function in the four \textit{Yersiniae} networks using FBA and related techniques, and I will give particular focus to the interplay between metabolic network topology and evolutionary flexibility. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W40.00002: Level architecture in genetic regulatory networks and the role of microRNAs J. M. Schwarz It is well known that genes that code for proteins regulate the expression of each other through protein-mediated interactions. With the discovery of microRNAs$^1$ (miRNAs), it has been conjectured that there are many such regulatory miRNAs in the cell that are never transcribed into proteins but are important for regulation and, hence, could explain the nature of the non-coding (or junk) DNA.$^2$ Furthermore, miRNAs are highly conserved molecules. So, just as genes that code for proteins form regulatory networks, we conjecture that miRNAs form a higher-level regulatory network amongst themselves as mediated by the genes-coding-for-proteins regulatory network to form a complex organism. We investigate this conjecture within the framework of random Boolean networks where the two-level architecture is modelled via two coupled random Boolean networks with one network taking precedence over the other for various input/output values. Aspects of the evolution of the lower-level network will also be addressed. $^1$ D. P. Bartel, Cell {\bf 116}, 281 (2004). $^2$ J. S. Mattick, Sci. Amer. {\bf 291}, 60 (2004). [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W40.00003: Tailoring the metabolism against mutations Natali Gulbahce, Adilson E. Motter, Eivind Almaas, Albert Laszlo Barabasi In the post-genomic era, organisms can be modelled at the whole-cell level in silico via steady state methods to describe their metabolic capabilities. We use two such methods, Flux Balance Analysis and Minimization of Metabolic Adjustment to explore the behavior of cells (of E. coli and S. cerevisiae) after severe mutations. We propose experimentally feasible ways of modifying the underlying biochemical reaction network of a mutant cell such that cell functionality, in particular growth rate, is significantly improved. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W40.00004: Form, Function, and Evolvability in Biological Networks Andrew Mugler, Etay Ziv, Ilya Nemenman, Chris H. Wiggins A driving problem in systems biology for several years has been exploring the extent to which the topology of a small biological network constrains or guides its function. The absence of such constraint would allow a given network to evolve without rewiring its underlying form. We introduce a quantitative measure of this evolvability that does not rely on pre-defining the preferred function of a given topology. We then study the stochastic description of the experimental setup of Guet [1], treating chemical inducers as functional inputs and the expression of a reporter gene as the functional output. We take an information-theoretic approach, allowing the system to set parameters that optimize signal processing ability, thus enumerating the highest-fidelity functions. We find that, while all networks studied are highly evolvable by our measure--meaning that the function has little dependence on location in parameter space--the evolvability is correlated with individual topological features. Certain topological attributes, then, are shown (with statistical significance) to convey evolvability to biological networks. [1] C. C. Guet et al., Science \textbf{296}, 1466 (2002). [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 4:18PM |
W40.00005: Noisy out of necessity: Probabilistic behavior during cellular differentiation Invited Speaker: Diverse organisms ranging from bacteria to mammalian stem cells undergo pluripotent differentiation where a single cell can commit to one out of several cell fates. How do underlying genetic circuits comprised of interactions between genes and proteins allow cells to ``choose'' a specific cell fate and execute the appropriate differentiation program? To address this question we investigate a simple bacterial differentiation system utilizing mathematical modeling and quantitative single cell measurements. In particular we are interested in elucidating the role of circuit dynamics and stochastic behavior in cellular differentiation. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W40.00006: Analysis of temperature-dependent changes in the metabolism of \textit{Yersinia pestis}. Ali Navid, Eivind Almaas The gram-negative bacterium \textit{Yersinia pestis} is the aetiological agent of bubonic plague, a zoonotic infection that occurs through the bite of a flea. It has long been known that \textit{Y. pestis} has different metabolic needs upon transition from the flea gut environment (26 \r{ }C) to that of a mammalian host (37 \r{ }C). To study this and other outstanding questions about metabolic function of \textit{Y. pestis}, we used the available genomic, biochemical and physiological data to develop a constraint-based flux balance model of metabolism in the avirulent 91001 strain (biovar Mediaevalis) of this organism. Utilizing two sets of whole-genome DNA microarray expression data, we examined the system level changes that occur when \textit{Y. pestis} acclimatizes to temperature shifts. Our results point to fundamental changes in its oxidative metabolism of sugars and use of amino acids, in particular that of arginine. This behavior is indicative of an inefficient metabolism that could be caused by adaptation to life in a nutrient rich environment. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W40.00007: Information processing in the {\it E. coli} Chemotaxis Network Lin Wang, Sima Setayeshgar Biochemical signal transduction, broadly defined as the conversion of the concentration of an input signal to an output response, is the most basic level of biological information processing. The chemosensory pathway in bacterial chemotaxis is the best-characterized signal transduction network, and as such it provides an ideal system for probing the physical principles governing complex cellular signaling and response. Using an experimentally realistic stochastic implementation of the {\it E. coli} chemotaxis network and motor response, we investigate optimality of the chemotactic response in terms of input/output information transmission. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W40.00008: Correlated Phenotypic Transitions to Competence in Bacterial Colonies Inbal Hecht, Eshel Ben-Jacob, Herbert Levine Genetic competence is a phenotypic state of a bacterial cell in which it is capable of importing DNA, presumably to hasten its exploration of alternate genes in its quest for survival under stress. Recently, it was proposed that this transition is uncorrelated among different cells in the colony. Motivated by several discovered signaling mechanisms which create colony- level responses, we present a model for the influence of quorum- sensing signals on a colony of \emph{B. Subtilis} cells during the transition to genetic competence. Coupling to the external signal creates an effective inhibitory mechanism, which results in anti-correlation between the cycles of adjacent cells. We show that this scenario is consistent with the specific experimental measurement, which fails to detect some underlying collective signaling mechanisms. Rather, we suggest other parameters that should be used to verify the role of a quorum-sensing signal. We also study the conditions under which phenotypic spatial patterns may emerge. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W40.00009: On the Selection of Bistability in Genetic Regulatory Circuits Cheol-Min Ghim, Eivind Almaas Bistability is a defining character of switching and memory devices. Many regulatory circuits observed in cellular reaction networks contain ``bistability motifs'' that endow a cell with efficient and reliable switching between different physiological modes of operation. One of the best characterized system, the \textit{lac} operon in \textit{E. coli}, has been shown to display a saddle-node bifurcation when induced by nonmetabolizable lactose analogue inducers, such as isopropylthio-$\beta$-D-galactoside (IPTG) and thio-methyl-galactoside (TMG). Motivated by the absence of bifurcation in the same system with its natural inducer, lactose, we studied the conditions for bistability and rationalized its fitness effects in the light of evolution. Stochastic simulations as well as mean-field approach confirm that history-dependent behavior as well as nongenetic inheritance, being realized by bistability motifs, may be beneficial in fluctuating environments. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W40.00010: Mutual information in random Boolean models of regulatory networks Joshua Socolar, Andre Ribeiro, Bj\"orn Samuelsson, Jason Lloyd-Price, Stuart Kauffman In a large, complex network of interacting elements, such as a genetic regulatory network within a cell, the average of the mutual information over all pairs of elements is a global measure of how well the system can coordinate its internal dynamics. We study the average pairwise mutual information $\cal{I}$ in random Boolean networks (RBNs) as a function of the distribution of Boolean rules implemented at each element, assuming that the links in the network are randomly placed. As the number $N$ of network nodes approaches infinity, $N\cal{I}$ exhibits a discontinuity at parameter values corresponding to critical RBNs. For finite systems, $N\cal{I}$ peaks near the critical value, but slightly in the disordered regime for typical parameter variations. The source of high values of $N\cal{I}$ is the indirect correlations between pairs of elements from different long chains with a common starting point. The contribution from pairs that are directly linked approaches zero for critical networks and peaks deep in the disordered regime. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W40.00011: Dynamical properties of structured Boolean networks Andrew Pomerance, Wolfgang Losert, Michelle Girvan, Edward Ott Boolean networks have been used since the 60s as a model for genetic control networks. In this model, each node takes on the value 0 or 1, modeling whether a gene is expressed or not, and updates at each time step according to a function of the value of its inputs. Random boolean networks (RBNs), where each node is randomly connected to other nodes and the function governing the dynamics is initially randomly generated, have been particularly well-studied. In particular, since these are deterministic, finite systems, the system must eventually settle into a periodic or fixed point attractor. A key question has been the scaling of the number of attractors with system size. In this talk we present results on how network structure effects the behavior of Boolean networks with randomly assigned dynamical rules. For example, we show that the number of attractors is dramatically increased by the addition of community structure to the network from the baseline RBN count with the same number of nodes. Furthermore, imposing bipartite structure on the network has little effect on the number of attractors. [Preview Abstract] |
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