Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session C1: Poster Session I |
Hide Abstracts |
Room: Colorado Convention Center Exhibit Hall F, 2:00pm - 5:00pm |
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C1.00001: POLYMERS I |
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C1.00002: Characteristic Time Scales of Pre-Glassy Dynamics Julieanne Heffernan, J. Budzien, Aaron T. Wilson, Robert J. Baca, Francisco Avila, John D. McCoy, Doug B. Adolf Continuing the scalar metric treatment of the glass transition, characteristic time scales based on the diffusion coefficient were investigated for simple model polymer systems. For all systems, regardless of the pressure-volume-temperature path, the resulting times form single-valued (system-dependent) functions of packing fraction. A ratio of time scales is shown to yield a smoother function upon collapse than do the time scales themselves, which suggests that extrapolations to the ideal glass transition are better performed in terms of this ratio of time scales. Finally, discontinuities in the characteristic times are explained in terms of caging behavior seen in the velocity autocorrelation functions. [Preview Abstract] |
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C1.00003: Theory of Relaxation, Physical Aging and Nonlinear Mechanical Properties of Polymer Glasses Kang Chen, Kenneth Schweizer The theory of Saltzman and Schweizer for collective dynamic barriers, segmental relaxation and the kinetic glass transition of deeply supercooled polymer melts has been generalized to the nonequilibrium glass. The critical idea is that the structural component of density fluctuations become frozen below T$_{g}$, which results in a crossover of the thermal dependence of the relaxation time to an Arrenhius form. Physical aging is modeled based on a first order kinetic equation for the time evolution of the dimensionless amplitude of density fluctuations. On intermediate time scales after a quench, the relaxation time grows with aging time as a power law with a temperature dependent exponent. The theoretical approach has been generalized to treat the consequences of applied stress which modifies the confining nonequilibrium free energy and accelerates relaxation. A constitutive equation has been developed and applied to compute stress-strain curves and yield points as a function of temperature and strain rate. The coupling of mechanical deformation and aging has also been studied to address the phenomenon of stress-induced ``rejuvenation''. It is possible to generalize the theory to treat the consequences of large amplitude chain deformation and strain hardening. Comparisons of the theoretical results with multiple experiments are encouraging. [Preview Abstract] |
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C1.00004: Changes in Segmental dynamics of PMMA Glass during Nonlinear Creep Deformation Hau-Nan Lee, Keewook Paeng, Stephen Swallen, Mark Ediger Reorientation of a dye molecule can be used to monitor the segmental dynamics of polymer melt. We utilize this technique to measure the mobility of lightly cross-linked polymethylmethacrylate (PMMA) film during tensile creep deformation. We have observed deformation-induced mobility from Tg-10 K to Tg -30 K. Generally we observe the largest changes in mobility if the sample is deformed at a lower temperature. At a given temperature, mobility enhancements correlate better with strain rate than with stress or strain. The largest change in mobility is about a factor of 200 (equivalent to a temperature increase of 12 K). After removing the stress, we see that the enhanced mobility disappears slowly. These are the first direct measurements of deformation-induced polymer mobility that allow a quantitative comparison to theory and simulation. [Preview Abstract] |
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C1.00005: Physical Aging in Confined Geometries: The Role of Stiff Backbone and Bulky Side Groups Connie B. Roth, Rodney D. Priestley, Manish K. Mundra, John M. Torkelson Although confinement effects on the glass transition have been well documented, little work has been done to probe the effects of confinement on low temperature structural relaxations. A decade ago the membrane community started reporting accelerated physical aging in free-standing films approximately 500 nm thick, relative to bulk films, in high free volume, stiff backbone polymers such as polysulfone (PSF) and poly(phenylene oxide) (PPO). Recently our group has reported suppressed physical aging in supported ultrathin films of poly(methyl methacrylate) (PMMA), a low free volume polymer. Here we compare the impact of confinement on physical aging in high free volume polymers with bulky side groups such as poly(tert-butyl methacrylate) (PtBMA) and poly(ethyl methacrylate) (PEMA). [Preview Abstract] |
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C1.00006: Polymer Nanocomposites Leading to the Suppression of Physical Aging: Effects of Attractive or Covalent Polymer-Nanofiller or Polymer-Substrate Interactions Perla Rittigstein, Rodney D. Priestley, John M. Torkelson Physical aging, or the change in properties accompanying the relaxation of a glass toward equilibrium, results in many deleterious effects ranging from embrittlement to reduction in permeability. Here we demonstrate how physical aging can be dramatically suppressed via the addition of well-dispersed nanofiller to a polymer matrix. Examples will be given associated with both real nanocomposites, in which silica or alumina nanoparticles or single-wall carbon nanotubes are dispersed in polymer, and model nanocomposites, with polymer of a known thickness confined between two silica slides. In both systems, we find a major reduction in physical aging rate, relative to neat polymer, when there are attractive polymer-substrate interactions, e.g., hydrogen bonds, or covalent bonds between the polymer and nanofiller. A molecular-level explanation of this effect will be provided which is related to a reduction in beta-relaxation dynamics caused by the attractive interactions/covalent bonding. In contrast, in cases where there is no attractive polymer-nanofiller interaction, e.g. polystyrene-silica nanocomposites with wetted interfaces, there is little or no effect of the nanofiller on aging rate. [Preview Abstract] |
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C1.00007: Functionalized Carbon Nanotubes in Modified Plant Oil Composites. Ian M. McAninch, Richard P. Wool Carbon nanotubes (CNTs) with their impressive mechanical properties are ideal reinforcement material. Acrylated epoxidized soy oil (AESO) has been previously shown to have favorable interactions with carbon nanotubes; however a mixture of aggregates and dispersed tubes were found even at low CNT concentrations. In order to prevent re-aggregation, the CNTs were functionalized with a 10 carbon long aliphatic chain. These aliphatic chains are similar to the fatty acids that make up soy oil. Functionalization was verified using XPS and IR spectroscopy. These functionalized CNTs were dispersed by mechanical shear mixing into AESO both with and without styrene as a comonomer. No large aggregates were observed in the liquid, uncured, samples or in the final cured composites. Dispersion in the solid composites was verified using optical and electron microscopy. Better dispersion also resulted in improved mechanical properties. [Preview Abstract] |
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C1.00008: The effect of soxhlet extraction on morphology and mechanical properties of Poly(DVB)polyHIPE Pornsri Pakeyangkoon, Manit Nithitanakul PolyHIPE (Polymerized High Internal Phase Emulsion) is a novel porous polymer, which is microporous materials and produce by polymerization of the continuous phase of system. Poly(DVB)PolyHIPE using two different system of three-component surfactant (S20M and S80M) with various porogenic solvents including T, CB, CB/T and PE has been successfully prepared. The phase morphology, mechanical properties and surface area measurement were investigated. After polymerization of continuous phase, the porous materials with interconnected were obtained. The cell size and surface area were found to improve by using various solvent, this is due to the ability of porogenic solvent and mixture of surfactant which prevent the Ostwald ripening (coalescence) of system. Moreover, the surface area and mechanical properties of the resulting materials were found to be depended on the soxhlet time. It can be concluded that the suitable soxhlet time for extraction was 6-12 hours and at this condition, high surface area with highest mechanical properties was obtained as compare to others conditions. [Preview Abstract] |
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C1.00009: A Mean Field Theoretic Study of Friction between Polyelectrolyte Polymer Brushes Jeffrey Sokoloff It is proposed that the fluctuations from the mean field theoretic parabolic monomer density profile for polymer brushes will result in a type of static friction between two polymer brush coated solid surfaces, which results from polymers that fluctuate out of the parabolic density profile belonging to one brush and get entangled with polymers belonging to the second brush. This occurs when the brushes are pushed together with a sufficiently large normal force so that the monomer density in the interface region separating the two polymer brushes is in the semidilute regime. The friction is not the usual static friction, in that when a force below this ``maximum force of static friction'' is applied, there is a ``creep velocity'' which is as large as a few millimeters per hour. At sufficiently light load so that the monomer density is in the dilute regime, the ``static friction'' goes away and there only exists a viscous kinetic friction (i.e., kinetic friction proportional to the sliding velocity) between the brushes. When the polymers are electrically charged, the counter ions produce additional osmotic pressure to support the load. Calculations of this additional load carrying mechanism using a Debye-Huckel theory treatment due to Miklavic and Marcelja, predict that the counterions do not provide a significant additional contribution to load carrying ability of polymer brushes. [Preview Abstract] |
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C1.00010: Collapse and Aggregation of Polymers in Solvents with Varing Molecular Sizes below $\theta$-Temperatures Gi Xue, Fangfang Tao, Xiaoliang Wang, Dongshan Zhou The collapse and interpenetration of polymer in various solvents were monitored by fluorescence below $\theta$-temperatures. Polystyrene was labeled with a donor and an acceptor groups respectively. Dilute and semidilute solutions in cyclohexane and DOP (dioctylphthalate) containing a mixture of the two polymers were monitored by nonradiative energy transfer. We found that PS collapsed to dis-entangled globules in DOP, while it collapsed and aggregated to interpenetrated particles in cyclohexane as the solutions were cooled from their $\theta$-temperatures. This can be attributed to the effect of viscoelasticity; namely, the diffusion coefficient of chain segments is very small in solvent with larger molar size due to the high viscosity, each chain becomes a tiny glassy ball before collision and association with other chains. The dis-entanglement of the glassy polymer was confirmed by a recently developed 1H solid state NMR. [Preview Abstract] |
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C1.00011: Lyotropic Phase Behavior of Concentrated Solutions of Diblock Copolymers in an Ionic Liquid Peter M. Simone, Timothy P. Lodge Concentrated solutions of three different poly((1,2-butadiene)-$b$-ethylene oxide) (PB-PEO) diblock copolymers in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF$_{6}$] were prepared, and the lyotropic phase behavior of the ordered copolymer nanostructures was investigated via small angle x-ray scattering (SAXS). The three copolymers contained PB blocks of the same length (9 kDa), and different length PEO blocks (3, 4, and 6 kDa). At moderately high concentrations (50-80 wt{\%} polymer) all three of the block copolymers adopted a lamellar morphology, while at very high concentrations (80-95 wt{\%} polymer) the two block copolymers with lower PEO weight fractions formed hexagonally packed cylinders. For lower concentrations ($<$40-50 wt{\%} polymer), a broadening of the SAXS peaks indicated a loss of long-range order in the copolymer nanostructure. [Preview Abstract] |
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C1.00012: Correlation length of a near-critical, eight-arm star polystyrene in methylcyclohexane Henry Timmers, Sarah Suddendorf, Angie Triplett, Nithya Venkataraman, D.T. Jacobs The turbidity of monodisperse, eight-arm star polystyrene in methylcyclohexane has been measured near each system's critical point and used to determine the correlation length amplitude $\xi _{0 }$at various molecular weights. The turbidity was determined from the measured ratio of the transmitted to incident light intensities as a function of temperature. Various systematic errors were explored and eliminated in obtaining a value of $\xi _{0}$ for each molecular weight of the branched polystyrene in methylcyclohexane. Ornstein-Zernike theory is used to fit the turbidity data. The values of $\xi _{0}$ and their dependence on molecular weight are comparable to that reported in the literature for a linear polystyrene of the same molecular weight in methylcyclohexane. We acknowledge the support from NSF-REU grant DMR 0243811 and to the donors of the American Chemical Society Petroleum Research Fund. [Preview Abstract] |
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C1.00013: The Glass Transition and Kinetics in Stacked Polystyrene Ultrathin Films Yung P. Koh, Sindee L. Simon The effect of nanoconfinement on glass transition phenomena is still not fully understood in spite of extensive work by the scientific community. Results depend on measurement method and sample preparation method, as well as on the type of confinement and on the glass-forming material.~ In this study, polystyrene of high molecular weight is constrained in stacked ultrathin film samples.~ Differential scanning calorimetry is used to examine the cooling rate dependence of Tg, the timescale for diffusion, and the kinetics of aging (structural recovery) as a function of film thickness and sample preparation method.~ Results are compared to those in the literature. [Preview Abstract] |
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C1.00014: Adhesion of Polymer Composite Melt to PTFE at Elevated Temperature David Pan, Thomas Debies, Dan McVeigh A novel technique for measuring the adhesive force between a thin molten polyester composite film and a PTFE surface is presented in the paper. The molten film was prepared by first depositing powdery composite particles on a substrate and then heating the powdery film on the substrate to the test temperature through a heated pressure nip between two conformable rollers comprising a PTFE overcoat at a speed up to 400 mm/s. The adhesive force is measured by wedging a bendable metal knife into the interface between the molten film and PTFE near the exit of the heating nip. Strain gauges were mounted on both sides of the metal knife. The differential strain gauge reading resulting from the degree of the knife bending is calibrated against standard weights. The technique was used to investigate the effects of the substrate, substrate roughness, molten film thickness, temperature and type of wax additive on the adhesion of the molten composite film to PTFE. On the effects of temperature and type of wax, we found that the adhesive force remains relatively constant at low temperature, and then drops significantly, in some instances to zero, as the temperature further increases. A phenomenological model is proposed to explain the effects of main temperature and the wax additive. The model is evidenced by the surface segregation of wax from the polyester composite film as determined by XPS. [Preview Abstract] |
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C1.00015: Nanomechanical Measurements on Ultra-thin Polymer Films Shanhong Xu, Paul OConnell, Greg McKenna A novel experimental technique for measuring the viscoelastic properties of ultrathin polymer films is described. The method is a scaled down version of the classic bubble inflation technique for measuring the biaxial creep compliance of films and is capable of measuring the creep compliance response of films at least as thin as 13 nm. Atomic force microscopy is used to image the nano-bubbles as a function of time, temperature and film thickness. The bubble shape (along with the known applied pressure and film thickness) gives the stress and strain in the film, from which the compliance may be calculated. Experiments on a range of polymers are presented and show a universal stiffening of the material in the rubbery regime as the thickness is reduced. This is in contrast to the degree to which the glass transition temperature is reduced which is material specific. [Preview Abstract] |
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C1.00016: Effects of Thickness, Molecular Weight and Temperature on Kinetics of Photo-induced Trans$\to $Cis Isomerization for Azobenzene Tagged to Polystyrene in Films Yohei Tateishi, Keiji Tanaka, Toshihiko Nagamura Photoisomerization of azobenzene (Az) tagged to polystyrene (PS) in films was studied as functions of thickness, PS molecular weight and temperature. The photoisomerization reaction in solid films was composed of fast and slow modes. The fractional of the fast mode ($I_{1})$ increased with decreasing thickness once the films became thinner than a few hundreds of nanometers. This can be explained in terms of a surface layer in which the photoisomerization proceeds faster, and the effect becomes remarkable with decreasing thickness due to a large surface to volume ratio. Since the thickness dependence of $I_{1}$ fraction was insensitive to the molecular weight of PS, it is likely that the kinetics was coupled with molecular motion with a relatively small scale. Comparing the temperature-$I_{1}$ relation with viscoelastic data of the PS, it was concluded that this was the case and the $\gamma $ and $\beta $ relaxation temperatures in the surface region were comparable to those in the bulk. Interestingly, the thickness of the surface layer, estimated through a layer model analysis, varied with changing temperature. [Preview Abstract] |
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C1.00017: Influence of Annealing and Solvent Treatment on Surface Structure of Diblock Copolymer Brushes Akira Otaka, Gokce Ugur, Bulent Akgun, Mark Foster A polymer brush is an assembly of polymer chains in which each chain is tethered to a surface or an interface. Diblock copolymer brushes have attracted enormous interest due to their stimuli-responsive behavior. Although diblock copolymer brush chemistry has been studied widely, the surface structure of the diblock copolymer brushes and how it is affected by annealing and contact with solvents have been studied little. We have studied how the surface structure of diblock copolymer brushes changes with annealing and treatment with different solvents. The brushes were synthesized on Si substrates using atom transfer radical polymerization (ATRP). The surface structure was investigated by atomic force microscopy (AFM) in tapping mode. Power spectral density (PSD) analysis of the AFM images provides information that is complementary to that from X-ray and neutron scattering studies. [Preview Abstract] |
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C1.00018: Surface Dynamics of Highly Branched Polystyrene in Melt Sewoo Yang, Jaesik Lee, Zhang Jiang, Sunil K. Sinha, Sanghoon Song, Hyunjung Kim, Suresh Narayanan, Mark D. Foster We have investigated the surface dynamics of highly branched polystyrene (PS) film using x-ray photon correlation spectroscopy (XPCS). Several highly branched PS chains of well-defined molecular architecture were synthesized anionically. Speckle patterns of films of the branched PS chains of thickness ($\sim $ 100 nm) were measured at temperatures above T$_{g}$ of the bulk chains and both the scattering of a single pattern analyzed and time correlation performed to obtain correlation functions. The temperature dependence for the q-dependent relaxation time was determined and estimates of the surface tensions of the different polymers derived from the static structure factors. The viscosities of the films determined by analyzing the XPCS data using capillary wave theory are compared with values obtained from bulk samples using rheological measurements. [Preview Abstract] |
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C1.00019: Interface Characterization of Photonic Films Created by Plasma Enhanced Chemical Vapor Deposition (PECVD) Someshwara Peri, Hyeonjae Kim, Mark Foster The structures of substrate/layer and layer/air interfaces in copolymer films made using PECVD have been probed for the first time using x-ray reflectivity (XR) and atomic force microscopy (AFM). Ellipsometry and XR measurements reveal that one can tune the visible light refractive index and XR refractive index by varying the monomer feed ratio in the copolymer films. Also, the structures of homopolymer octafluorocyclobutane (PP-OFCB) films made at different processing conditions were studied using XR and AFM. In addition to the narrow ``transition region'' next to the substrate, films deposited at power higher than 40 W exhibit structures adjacent to the air that differed from the structure in the ``center'' of the film. Uniform film structure can be achieved using any of three monomer feed locations. There are small differences between the structure deposited on 2'' diameter wafers and that deposited on 3'' diameter wafers, attributable to differences in flow dynamics. [Preview Abstract] |
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C1.00020: Spincoating Deposition of Thickness Gradient Polymer Films Monika Michalek, John Dutcher Thickness gradient polymer films can be prepared by dropping one drop of a polymer solution onto a spinning substrate at a position that is displaced from the axis of rotation. In these films, the thickness increases with increasing radial distance. To understand the deposition process for thickness gradient films in more detail, we have measured the intensity of laser light reflected from thickness gradient films during spincoating deposition. The results obtained at different radial distances will be discussed and compared with those obtained for polymer films of uniform thickness. [Preview Abstract] |
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C1.00021: Surface Dynamics of Molten Homopolymer Brushes Gokce Ugur, Bulent Akgun, William J. Brittain, Mark D. Foster, Suresh Narayanan, Heeju Lee, Sanghoon Song, Hyunjung Kim, Zhang Jiang, Sunil K. Sinha The surface dynamics of densely grafted polystyrene and poly($n$-butyl acrylate) homopolymer brushes of various thicknesses, synthesized using atom transfer radical polymerization were investigated by X-ray photon correlation spectroscopy for the first time. Brushes of thicknesses of 9, 26, 38, and 47 nm were considered. Within the range of time (0.2 -1100 s) and length scale (0.2-5 um) studied, no fluctuations of the brush surfaces were detectable for any thickness brush, presumably due to tethering of polymer chains to the substrate. Even 130 \r{ }C above the bulk glass transition temperature, no relaxation process was observed. The suppression of long-wavelength surface fluctuations on a molten homopolymer brush is consistent with the predictions of Fredrickson and co-workers. [Preview Abstract] |
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C1.00022: ABSTRACT WITHDRAWN |
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C1.00023: Capillary waves at polymer/polymer interface studied by resonant soft x-ray reflectivity (RSoXR) C. Wang, T. Araki, B. Watts, H. Ade Neutron reflectivity (NR) was previously used to study capillary waves at the PS/PMMA polymer/polymer interface. However, studies of thick films have been limited due to the small longitudinal coherence length of NR. Since RSoXR can study films up to micron thick and has much larger in-plane coherence length, it can be used to study capillary waves at polymer interfaces with relaxed thickness requirements. In addition, deuteration is not required for RSoXR and thus will eliminate the possible uncertainty induced by the isotope effect in the PS/PMMA system. The principles of RSoXR can also be extended to non-specular reflectivity, i.e. diffuse scattering, which probes the in-plane structure, offers a direct observation of the nature of the capillary wave spectra. The tunable sensitivity for selected interfaces using soft x-rays will allow the direct observation of a buried polymer/polymer interface. For the PS/PMMA bilayer, for example, there is strong reflection from the PS/PMMA interface at 281 eV, but almost no reflection from the PS/air interface. Hence, the diffuse scattering will be dominated by the structure function of PS/PMMA interface. In contrast, at 320 eV, the reflectivity is very high for the surface, and the diffuse scattering will be dominated by the contribution from the surface. Resonant diffuse scattering will thus lead to the direct observation of the capillary wave spectra at polymer/polymer interface. [Preview Abstract] |
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C1.00024: Synthesis of Polymeric Nanoparticles by Self-Assembly in Solution of Living Block Copolymers and Application of the Particles in Rubber Compounds Xiaorong Wang, James Hall, Sandra Warren, James Krom, Jeffery Magistrelli, Mindaugas Rackaitis, Georg Bohm Over the past number of years, we attempted to extend the self- assembly concepts of macromolecules in solutions to create nano- sized particles through industrially viable processes of significance for different applications. Depending on the type and macrostructure of the block-copolymers, the solvent, the concentration and other process parameters, a variety of core- shell nano-particles of different shapes (spheres, hollow spheres, ellipsoids, linear and branched strings etc.) and sizes have been reproducibly synthesized. Most of the nanoparticles were composed of a solid, highly crosslinked core and an elastomeric shell structure. The evolution and structure of the nano-particles during the different process steps involved were examined and characterized. The unique performance of spherical nanoparticles as performance enhancing additives and novel reinforcing agents was explored in rubber compounds. The mechanism of the reinforcement and the phase behavior of the particles in polymer matrices are also discussed. [Preview Abstract] |
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C1.00025: Interpretation of small angle neutron scattering measurements for LDOT-type styrenic block copolymers Junhan Cho, Inho Lee, Gangyoung Lee, Donghyun Lee, Doyeol Ryu, Jinkon Kim We investigate the scattering behavior of some LDOT-type styrenic block copolymers measured by small angle neutron scattering (SANS). The absolute scattering intensities and the resultant correlation lengths for the disordered samples are obtained as a function of temperature and pressure. The experimental scattering behavior is then analyzed by using random-phase approximation (RPA) analysis in connection with a molecular model for perturbed hard chains.~ With a proper choice of requisite molecular parameters, the scattering properties for the copolymers are predicted to be compared with those measured by SANS. Effective Flory-type interaction is obtained and discussed with regard to its role in the scattering behavior for the copolymers. [Preview Abstract] |
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C1.00026: Dynamic Structure Formation in Evaporating Bound Diblock Copolymer Solutions Suck Won Hong, Zhiqun Lin Drying of diblock copolymer solutions in the sphere-on-flat geometry is investigated. The synergy of controlled self- assemblies of diblock copolymers via irreversible solvent evaporation and their destabilization mediated by the interfacial interaction between the polymer and the substrate lead to the spontaneous formation of gradient concentric rings with fingering instabilities at the edge of the rings. Subsequent solvent annealing transforms them into well-ordered punch-hole-like structures. [Preview Abstract] |
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C1.00027: Relationship between mechanical properties and chain architectures of semicrystalline thermoplastic elastomeric triblock polyolefins Zhigang Wang, Glenn H. Fredrickson, Edward J. Kramer, Akio Tannna, Jeffrey Rose, Geoffrey W. Coates Semicrystalline thermoplastic elastomeric polyolefins, specifically triblock polypropylenes of syndiotactic polypropylene (sPP) as two end blocks and random, amorphous poly(ethylene-co-propylene) (PEP) copolymer block as the middle block were successfully synthesized by using stereoselective, living alkene polymerization catalysts in a sequential polymerization procedure. The fraction of ethylene (Fe) for PEP soft block is about 0.5 so that PEP block does not crystallize and microphase separation does not occur. A series of these sPP-b-PEP-b-sPP triblock copolymers with well-controlled chain architectures has been synthesized. Thermal analyses show that the length ratio of end block to middle block affects crystallization ability of the end blocks, indicating that sufficient length ratio is needed for crystallization. The semicrystalline sPP end blocks have crystallinities from 0 up to 13{\%} on the basis of the total triblock mass. The anchoring effects of the end block crystals on the mechanical properties of this series of triblock copolymers will be presented and discussed. [Preview Abstract] |
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C1.00028: Characterization of interactions in weakly interacting block copolymer by two-dimensional heterospectral analysis of WAXS and FTIR Hye Jeong Kim, Jin Kon Kim, Young Mee Jung, Seung Bin Kim We investigated, via 2D hetero-spectral correlation analysis of wide angle X-ray scattering (WAXS) and infrared (IR) spectroscopy, the specific chemical interactions existing in polystyrene-block-poly(n-pentyl methacrylate) copolymer (PS- PnPMA). The specific interaction between PS and PnPMA block is mainly arising from the dipole in the benzene ring of PS and the induced dipole in the PnPMA due to the cluster formation with a size of 1$\sim$2 nm. We found that the synchronous 2D WAXS-IR spectrum of the ordered state was completely different from that in the two disordered states. The CH group of the main chains of PS and PnPMA did not contribute to the cluster formation in the two disordered states, indicating that the main chains of PS and PnPMA blocks were uniformly distributed in the two disordered states. However, only the C=C group in the PS block contributed to the cluster at a disordered state below the LDOT, whereas both the C-C-O group in PnPMA and the entire phenyl ring as well as the C=C group in PS contributed to cluster formation at another disordered state above the UODT. [Preview Abstract] |
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C1.00029: Phase Behavior of poly(2-vinyl pyridine)-\textit{block}-poly(4-vinyl pridine) Copolymers Sung Hyun Han, Dong Hyun Lee, Jin Kon Kim Phase behavior of a block copolymer depends on the volume fraction ($f)$ of one block, total degree of polymerization (N), and the segmental interaction parameter ($\chi )$. Recently, we found that the order-to-disorder transition temperature (T$_{ODT})$ of polystyrene-\textit{block}-poly(2-vinylpridine) copolymer (PS-P2VP) was much lower than that of PS-\textit{block}-poly(4-vinylpridine) copolymer (PS-P4VP) at similar values of $f$ and N. The only difference between PS-P2VP and PS-P4VP is the different location of the nitrogen group in phenyl ring (2 versus 4-position). In this study, we studied, via small angle X-ray scattering (SAXS), rheometry, and birefringence, the phase behavior P2VP-\textit{block-}P4VP copolymers (P2VP-P4VP) with various $f$ and N. We determined the temperature dependence of $\chi $ between P2VP and P4VP from SAXS profiles in the disordered states with the aid of the random phase approximation. We found that the value of $\chi $ between P2VP and P4VP was relatively large. For instance, it is larger than $\chi $ between PS and P2VP. Thus, even when the molecular weight of symmetric P2VP-P4VP is ca. 12000, the T$_{ODT}$ is larger than 280 $^{o}$C. This work was supported by Creative Research Initiative Program supported by KOSEF [Preview Abstract] |
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C1.00030: Rheology On A Triblock Copolymer: Mechanical Hole Burning Spectroscopy Qian Qin, Gregory McKenna Mechanical hole burning spectroscopy was developed for probing the dynamic heterogeneity of polymeric materials. In this technique, a mechanical oscillatory perturbation is applied to the sample to excite the dynamic heterogeneities and this is followed by a small linear strain. Comparison between linear response after perturbation and that without any perturbation can result in vertical and horizontal modifications in the response that are referred to as ``holes.'' The holes are thought to be a signature of the dynamic heterogeneity of polymers. To quantitatively characterize the dynamic heterogeneity, a block copolymer was used and the study was performed in the vicinity of its order-disorder transition temperature. The results indicate that the mechanical holes can be successfully burned in the heterogeneous phase (T$<$TODT), while in the homogeneous or disordered phase, no apparent holes were observed. More importantly, the hole intensity decreases sharply in the vicinity of the ODT, which provides a correlation between hole intensity and the length scale of the heterogeneity. [Preview Abstract] |
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C1.00031: Symmetric Diblock Copolymers under Nano-Confinement Dong Meng, Qiang Wang We use both lattice Monte Carlo simulations and real-space self-consistent field calculations to study the self-assembled morphology of symmetric diblock copolymers under nano-confinement, either in thin films between two homogeneous surfaces or in nanopores. In the former case, we obtained 3D structures of mixed lamellae as an equilibrium morphology by 1D confinement (thin films) of 1D-structure (lamellae) forming copolymers. We also mapped out the system phase diagram as a function of surface preference and film thickness. In the latter case, we studied in detail the structures and phase transitions of various morphologies (concentric cylinders, the slab morphology and the mixed morphology) by systematically varying the pore diameter and surface preference, and compared with the thin-film case to understand their formation mechanisms. [Preview Abstract] |
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C1.00032: Depth Profiling using Nexafs Spectroscopy Karen Sohn, Sitaraman Krishnan, Marvin Paik, Christopher Ober, Ed Kramer, Daniel Fischer Bilayer systems consisting of a thin poly(methyl methacrylate) (PMMA) film on top of a poly(4-vinylpyridine) (P4VP) film were studied using NEXAFS spectroscopy, angle resolved XPS, and X-ray reflection. Depth profiling using NEXAFS spectroscopy was performed using two different methods, by varying the entrance grid bias (EGB) on the 3 grid high pass partial electron yield detector or by varying the electron emission angle. As the EGB becomes more negative or the emission angle becomes more glancing, the surface sensitivity increases. PMMA, which is unoriented, was used because it has a strong C 1s $\rightarrow \pi*$ signal at 288.8 eV from the carbonyl, whereas the unoriented P4VP has a strong signal from the C 1s $\rightarrow \pi*$ transition at 285.5 eV. By following these peaks as a function of emission angle or EGB in films of different PMMA thicknesses, the electron escape depth for NEXAFS spectroscopy can be calculated. [Preview Abstract] |
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C1.00033: Composition Dependence on the Closed-loop Phase Block Copolymer by Interaction Chromatography S.W. Hwang, S. Na, D.Y. Ryu, D.H. Lee, J.K. Kim, S. Park, T. Chang In this work our attention was directed towards the closed-loop transition behavior of polystyrene-poly(n-pentyl methacrylate), which have a narrower distribution in molecular weight as well as in chemical composition by interaction chromatography (IC). To find how the fractionation would affect the phase behavior of PS-b-nPPMA block copolymers, the mother block copolymer (homopolymer-free) was fractionated into five fractions. The mother copolymer and five samples with different lengths of chain in PPMA blocks are obtained, namely f1, f2, f3, f4, f5 depending on elution times. The microphase transitions of copolymers were measured by rheology, X-ray scattering, birefringence. f1 and f2 showed ordered structure within experimental temperature region, (110 $\sim$ 250), whereas f4 and f5 did disordered entirely. Although the mother sample fm gave the entire disordered state, the f3 which had similar to the molecular weight and composition of fm showed two transitions: the lower disorder-to-order transition (LDOT) and upper order- to-disorder transition (UODT) occurring at lower and higher temperature, respectively. [Preview Abstract] |
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C1.00034: Surface Characterization of Aliphatic Polyester \textit{-g-} Phosphorylcholine Copolymers Xiongfei Zhang, Todd Emrick, Shaw L. Hsu In order to control biodegradation behavior of a class of polyesters, hydrophilic functional groups were grafted onto the main chains. Phosphorylcholine (PC) molecules with azide attached at the end were synthesized. Due to their excellent biocompatibility and hydrophilicity, they have been covalently coupled to biodegradable aliphatic polyesters via a ``click'' cycloaddition reaction to produce amphiphilic graft copolymers. A series of copolymers were prepared by varying the molar incorporation of PC groups. Surface properties of the copolymers were examined to further explore their applications in drug delivery systems. Grazing angle reflection infrared spectroscopy was employed to determine segmental orientation at the film surface. XPS was used to verify surface composition. A water adsorption experiment was carried out to determine the water permeation rate. The improvement in hydrophilicity was confirmed by a water contact experiment. Results indicate that the graft copolymers were promising in drug delivery systems. [Preview Abstract] |
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C1.00035: Isolated nanomagnetic clusters formed by diblock copolymer phase separation inside nanopores of aluminum oxide membrane Priyanka Dobriyal, Thomas P. Russell, David Rider, Ian Manners Nanoporous alumina membranes are well known for generating 1-D polymeric nanostructures by wetting them with various polymer melts. In this work, cylindrical diblock copolymers of polystyrene-$b$-poly(ferrocenylsilane) (PS-$b$-PFS) was introduced into the nanopores of aluminum oxide template by capillary force, producing nanorods. Upon removal of the template, the resulting nanorods were microtomed and examined by TEM. The pore diameter was varied to obtain hexagonal PFS domains in PS matrix. Iron in PFS domains was oxidized to iron oxide by degrading the backbone of PFS with UV which cross links PS domain and ozone which oxidizes iron into iron oxide (Fe$_{2}$O$_{3})$. Hence, hexagonal clusters of iron oxide in the matrix of the other block were formed which have potential application in the field of magnetic data storage devices. [Preview Abstract] |
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C1.00036: Fabrication of Highly Distensible, Nanostructured Elastic Hydrogels from Block Copolymer Based Self-Assembly Chih-Yu Teng, Travis Bailey This work concerns the synthesis of a new class of highly distensible hydrogel membranes, based on a nanostructured solid phase framework that can be tunably expanded and contracted through the application of simple external stimuli. Our initial approach is based on blending sphere-forming poly(styrene-b-ethylene oxide) (PS-PEO) diblock copolymer amphiphiles with small amounts of the corresponding triblock copolymer (PS-PEO-PS). This triblock copolymer additive serves to create mechanical connectedness among the solid phase spherical PS domains, with its concentration and mid-block molecular weight dictating both the equilibrium dimensions and swelling dynamics of the gel. Precise control over molecular weight and composition of the copolymers was afforded through a combination of anionic and RAFT polymerization techniques, permitting the synthesis of a range of hydrogels with tunable swelling properties. Characterization of both the copolymer precursors and resultant nanostructured hydrogels through $^{1}$H NMR, SEC, TEM, dynamic melt-state rheology, and SAXS will be presented. [Preview Abstract] |
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C1.00037: Phase structures of a series of bent-core mesogen jacketed liquid crystalline block polymers Kishore Tenneti, Xiaofang Chen, Christopher Li, Lixia Rong, Benjamin Hsiao We report the phase structures of a series of mesogen-jacketed liquid crystalline (LC) block copolymers (PS-b-PVC) with bent- core mesogens. Samples whose LC blocks contained shorter ``tails'' (PVC10) exhibited morphologies with curved interfaces. However, no significant LC ordering was observed in these samples. Samples containing LC blocks with longer tails (PVC14) exhibited lamellar morphologies with columnar hexagonal (FH) LC ordering at symmetric volume fractions. As the fPS decreased, the LC ordering changed from FH to columnar Nematic. We propose that at asymmetric volume fractions, curved interface formation disrupts the LC FH formation because of the incompatibility between the curved IMDS and the macromolecular columns of the LC block. [Preview Abstract] |
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C1.00038: Phase Behavior of Rod-Coil Block Copolymer Blends Y. Tao, B.D. Olsen, Venkat Ganesan, R.A. Segalman Rod-coil block copolymers have unique self-assembly behaviors due to liquid crystalline interactions and have been shown to form only lamellar, hexagonal, nematic, and isotropic phases at equilibrium. The block copolymer domain size in the lamellar phase can be further controlled via blending of block copolymer with homopolymers. Coil homopolymers swell the coil-containing lamellae as expected. Liquid crystallinity, however, plays a significant role when rod homopolymers are added. Rod homopolymer interdigitates with the rod-blocks and the coils must rearrange to accommodate the increase in interfacial area. This results in an overall decrease in domain spacing with increasing rod content. Further, the addition of homopolymers is observed to alter the grain size and lamellar orientation in the thin film state which may have significant technological implications. These effects will be compared with self-consistent mean field models. [Preview Abstract] |
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C1.00039: pH-Responsive Nanostructures Assembled from Amphiphilic Block Copolymers Chen Xu, Bradford Wayland, Michael Fryd, Karen Winey, Russell Composto We present a novel route to assemble thin films containing pH-responsive nanostructures of hydrophilic cylindrical domains oriented perpendicular to a silicon substrate. The amphiphilic block copolymer, poly(styrene-$b$-acrylic acid) (PS-$b$-PAA) is prepared from the precursor, poly(styrene-$b$-\textit{tert}-butyl acrylate) (PS-$b$-P$t$BA), by an auto-catalytic reaction involving surface hydroxyl groups. The surface morphology and evolution of the nanostructures in aqueous solutions over a pH range of 2.6 to 9.1 are captured by in situ atomic force microscopy (AFM). The ordered PS-$b$-PAA films exhibit unique surface morphologies across three pH regimes. The equilibrium film thickness increases as pH increases, and is reversibly recovered upon decreasing pH. The water contact angle decreases by 30\r{ } as pH increases from 2.6 to 9.1, demonstrating that wettability can be tuned by varying the pH of the surrounding medium. Because of their pH-responsive behavior and small feature size, nanostructured devices designed from amphiphilic block copolymers have potential applications including sensors and membranes. [Preview Abstract] |
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C1.00040: Characterization of Nanostructures with Internal Phase Separation from Triblock Copolymers of PAA-b-PMA-b-PS Kelly Hales, Honggang Cui, Darrin Pochan, Zhiyun Chen, Karen Wooley The characterization and understanding of nano- through microstructures formed by charged, amphiphilic triblock copolymers of poly(acrylic acid)-b-poly(methyl acrylate)-b- polystyrene in water/tetrahydrofuran (THF) solvent mixtures is strongly dependent on block composition, solvent composition, and the presence of divalent, organic counterions. A variety of structures can be produced including polymer nanoparticles with bulk-like nanophase separation in the interior; bulk phase separation into a polymer-rich lamellar phase; spherical, cylindrical, and disk-like micelles; as well as toroidal assemblies. This poster will focus on the structure of polymer nanoparticles and networks formed in low water content systems. The size of the nanoparticles and whether separated nanoparticles vs. an interconnected network was formed can be controlled via solvent composition. Importantly, both the nanoparticles and network phases contain their own inherent nanostructure due to local phase separation of the block copolymers. Cryo-transmission electron microscopy (TEM), traditional TEM, and neutron scattering were used to examine these samples. [Preview Abstract] |
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C1.00041: Dual responsive PPO-P(Lys) block copolymer assemblies Gopal Venkatachalam, Sandeep S. Naik, Daniel A. Savin Dual responsive block copolymers consisting of poly(propylene oxide) and poly(L-lysine) (PPO-PLys) were synthesized and their solution self assembly investigated using dynamic light scattering and circular dichroism spectroscopy. PPO-PLys block copolymers were found to self assemble into spherical micelles in solution with a hydrodynamic radius dependent on both the pH, due to the helix-coil transition of the P(Lys) side chains in the corona, and the solution temperature. The basis for the pH response in these polypeptide-based hybrid block copolymers comes from the protonation or deprotonation in the side chains of the P(Lys) block, and the temperature response arises from the LCST behavior of the PPO block. The changes that occur in the solution morphology of these block copolymers were studied as a function of solution conditions. [Preview Abstract] |
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C1.00042: Block Copolymer Templates for Structured Nanocomposites Rafal A. Mickiewicz, Apostolos Avgeropoulos, Edwin L. Thomas Polymer systems containing nanoscale fillers have recently been the focus of much research activity due to the promise of unique physical properties. Increasingly, the ability to precisely control the distribution and orientation of the inclusions has been emphasized. The self assembly of block copolymers into periodic ordered domains provides a means to precisely control the microstructure of the resulting material. The goal is to design tailored functional materials by using the block copolymer as a template for the nanoparticles and the overall composite microstructure. The detailed behavior and morphology of the composite depends on many factors such as block copolymer chemistry and architecture, nanoparticle size and surface chemistry, as well as processing parameters used during co-assembly. In this study we aim to present ways to tune the microstructure of the block copolymer matrix using blends of triblock and diblock copolymers. In addition we explore the effect of the size of the nanoparticles on the morphology of the resulting composite. The motivation is to acquire a basic understanding of, and ability to control the morphological behavior of the nanocomposite in order to facilitate the rational design of tailored functional materials. Applications range from magnetic storage media, to photovoltaics, sensors, and thin film optical and electronic materials. [Preview Abstract] |
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C1.00043: Phase Transitions and Honeycomb Morphology in an Incompatible Blend of Enantiomeric Polylactide Block Copolymers Lu Sun, Jorge Ginorio, Lei Zhu, Lixia Rong, Igor Sics, Benjamin Hsiao Enantiomeric PLAs, poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), are known to form stereocomplexes. In this work, by using controlled ring-opening polymerization of L- and D-lactides from monohydroxyl-terminated hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(ethylene-\textit{co}-1-butene) (PEB) oligomers, respectively, well-defined PEO-$b$-PLLA (2k-5.4k) and PEB-$b$-PDLA (4.2-5.4k) block copolymers were synthesized. Quantitative stereocomplex formation was achieved by casting an equimolar mixture of incompatible PEO-$b$-PLLA and PEB-$b$-PDLA from chloroform at room temperature. Depending on different thermal histories, either lamellar or inverted cylindrical morphology was observed in the molten state. Intriguingly, novel honeycomb morphology with the minor PEB component forming the matrix was observed in the inverted cylindrical phase. [Preview Abstract] |
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C1.00044: Spin-on Di-block Copolymer Films for Covalent DNA Attachment Hernan Rengifo, Cristian Grigoras, Jingyue Ju, Jeffrey Koberstein We demonstrate that alkyne end-functional block copolymers can be used to immobilize fluorescently labeled dyes, RNA or DNA onto glass substrates by 1,3-dipolar cycloaddition reactions or so-called ``click chemistry''. Spin cast films of $\omega $-alkyne-terminated poly(methylmethacrylate-$b$-tert butyl acrylate) spontaneous form bilayer structures with thicknesses ranging from 5 to 30nm on silicon wafers or glass. The areal density of functional groups is controlled by adjusting the thickness of the monolayer through spin coating speed or solution concentration. The charge at the surface can be also controlled by de-protection of the \textit{tert}-butyl ester groups by exposure to UV radiation in the presence of photoacid generator to form carboxylic acid groups. The surface is characterized by contact angle, ellipsometry, fluorescent imaging and angle-dependent X-ray photoelectron spectroscopy (ADXPS). The technique is used to create DNA microarrays and the performance of these arrays is evaluated for DNA hybridization and DNA Sequencing by Synthesis. [Preview Abstract] |
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C1.00045: Zone Annealed Thin Block Copolymer Films on Chemical Micropatterns Sangcheol Kim, Brian C. Berry, Ronald L. Jones, Alamgir Karim, Robert M. Briber, Ho-Cheol Kim While zone refining techniques have long been utilized to produce very pure metals and semiconductors, a similar technique known as directional solidification has been used for organic materials such as organic alloys and liquid crystals. We extend the technique to thermally processed thin block copolymer films to understand the role of substrate defects on block copolymer ordering. To produce defects on the substrate, chemical patterns were created using a self-assembled monolayer of adsorbed octyldimethylchlorosilane with exposure to ultraviolet light. Zone annealing was performed on thin block copolymer films cast on hydrophilic/hydrophobic chemical patterns and the development of hole or island structures was observed. The variation of surface topology across the chemical pattern was investigated by optical microscope and scanning force microscope. When thin films were pulled at a constant speed across the boundary of the chemical patterns, the surface topology governed by chemical block wetting and the inversion transition depended not only on the preferential block affinity to the substrate, but was also the affected by the motion stage (defines thermal processing profile) speed$.$ [Preview Abstract] |
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C1.00046: Block copolymer lithography for growth of wide band gap nanostructures: Process control and optimization Kasiraman Krishnan, Azar Alizadeh, Oliver Boomhover, Kenneth Conway, Lauraine Denault, David Hays, Christopher Keimel, Rosalyn Neander, Seth Taylor, Andreas Stintz, Jay Brown, Sanjay Krishna, Edit Braunstein, Colin Jones Block copolymer lithography offers a promising route for fabricating wide band gap semiconductor quantum dots. A PS-PMMA block copolymer is self-assembled over a random copolymer brush to allow for perpendicular orientation of PMMA cylinders. The substrate consists of GaAs, SiO$_{2}$ and the brush layer. The block copolymer pattern is transferred to the SiO$_{2}$ by reactive ion etching. In the next step, growth of InAs quantum dots in the holes of the template is achieved by MBE. The quantum dots are further characterized by TEM and photoluminescence. The copolymer molecular weight is a critical parameter in determining density and size of the dots. Although use of lower molecular weight polymers can result in higher density of dots, it also leads to more processing challenges. We demonstrate the process optimizations involved in templating with lower molecular weight polymers, and in particular address the challenges in RIE. [Preview Abstract] |
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C1.00047: Alignment of spherical block copolymer microdomains with substrate features: effects of step edge height and film thickness Nathaniel T. Lawrence, Matthew L. Trawick, John M. Yarbrough, Gary M. Atkinson, Michael J. Fasolka, Douglas H. Adamson, Richard A. Register Diblock copolymers can be used as templates for nanolithography, but some applications would require at least local alignment and registration of copolymer microdomains with other features on a particular device. We present here the results of a systematic study of the alignment of spherical microdomains with step edges on a substrate as a function of both step edge height and polymer film thickness. The investigation used a combinatorial approach: we prepared a wafer with a series of step edges of a continuous range of heights along one direction, and applied a polymer film with a thickness gradient along the orthogonal direction. At polymer film thicknesses that are incommensurate with a single layer of microdomains, for which we expect the spontaneous formation of ``islands'' or ``holes'' of one commensurate thickness surrounded by another, the step edge acts as a nucleation site for boundaries between such regions, with the film on the high side of the step edge having fewer layers of spheres than the low side. We find that at some step heights, such a discontinuity in film morphology across the step edge is closely associated with the alignment of microdomains. [Preview Abstract] |
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C1.00048: Holographic patterning of block copolymers Michael Birnkrant, Christopher Li, Lalgudi Natarajan, Vincent Tondiglia, Pamela Lloyd, Richard Sutherland, Timothy Bunning The combination of a top down (holographic patterning (HP)) and a bottom up (Block Copolymer (BCP) self assembly) fabrication techniques resulted in a series of hierarchical nanostructures. This produced a long range uniform layered structure with a periodicity of $\sim $200 nm from HP and a finer structure on $\sim $10 nm scale due to BCP self assembly. The difference in refractive indexes of the BCP and crosslinked network results in a transmission spectrum with $\sim $100{\%} background transmission and a modest reflection notch in the visible region. Furthermore, upon heating, the BCP HP produces a red shift in the reflected wavelength of the material, which reverses upon cooling. Thus, combining HP and BCP offers a unique means to fabricate novel multifunctional optical components. [Preview Abstract] |
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C1.00049: Mechanism of Thermal Crystallization in Silk Fibroin Xiao Hu, David Kaplan, Peggy Cebe We investigated the formation of beta pleated-sheets in \textit{B. mori} silk fibroin films cast from water solution. Differential scanning calorimetry (DSC), its temperature-modulated variant (TMDSC), and the time-resolved techniques of Fourier Transform infrared spectroscopy and X-ray scattering, were used to monitor the detailed structural changes of silk fibroin during heating and isothermal crystallization above the glass transition temperature, Tg. Results show that bound water molecules inside the film play a very important role for the transformation from non-crystalline fibroin to crystalline beta sheet containing fibroin. Silk fibroin forms a new tighter structure with loss of intermolecular bound water during heating, which promotes the formation of the beta-sheets crystals above Tg. In addition, quick heating of the silk fibroin causes a water-induced glass transition, which results in a temporary water-silk structure where the bound water acts as a plasticizer for this polymeric system. This study will lead to a deeper understanding of the formation of beta pleated-sheets during the crystallization process in silk fibroin, with implications for the crystallization of the naturally occurring silk. [Preview Abstract] |
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C1.00050: Rigid Amorphous Fraction and Lamellar Structure in Nylon-6 Huipeng Chen, Peggy Cebe A three-phase model, comprising crystalline, mobile amorphous, and rigid amorphous fractions (RAF) has been applied in the study of semicrystalline Nylon-6. The samples were Nylon-6 alpha phase prepared by subsequent annealing of a parent sample. The samples were annealed at 110$^{\circ}$C, then briefly heated to 136$^{\circ}$C, then re-annealed at 110$^{\circ}$C. Temperature-modulated differential scanning calorimetry measurements allow the devitrification of the rigid amorphous fraction to be examined. We observe a lower endotherm, termed the ``annealing peak'' in the non-reversing heat flow after annealing. By brief heating above this endotherm and immediately quenching, the amount of RAF decrease substantially and the annealing peak disappears. The annealing peak corresponds to the point at which partial de-vitrification of the RAF occurs. None of these treatments affected the measured degree of crystallinity. Using a combined approach of thermal analysis with small angle X-ray scattering, we determine that the Homogeneous Stack Model is the correct one for Nylon-6. [Preview Abstract] |
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C1.00051: Tethered Polymer Chains on Single Crystal Surfaces Ryan Van Horn, Joseph X. Zheng, Huiming Xiong, Roderic P. Quirk, Bernard Lotz, Edwin L. Thomas, An-Chang Shi, Stephen Z. D. Cheng For years, tethered polymer chains have been studied for various applications, including responsive surfaces and protein adsorption. The tethering density of such systems is a highly important parameter in determining their conformations and subsequent properties, as shown by various theoretical and experimental works. Our group uses single crystals of crystalline-amorphous diblock copolymers to study tethered polymer chains. This system provides better control of tethering density and molecular weight as compared to previous methods. Our work focuses specifically on determining the transitions into each interaction regime. Zheng and coworkers found that the reduced tethering density is 3.7 for the onset of interchain interaction and 14.3 for the highly-stretched brush in PS-b-PEO and PS-b-PLLA. This work provides, for the first time, the values of these two onsets with high certainty; however, the systems studied do not provide general results. Work on expanding our knowledge of these transitions in other systems, PMMA-b-PLLA and PB-b-PEO, is currently underway. [Preview Abstract] |
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C1.00052: The effect of SAM interlayer on the crystalline orientation of PVDF-TrFE thin film in ferroelectric polymer capacitor Youn Jung Park, Seok Ju Kang, Cheolmin Park Poly vinylidene fluroride-\textit{co}- trifluoroethylene (P(VDF- TrFE)), one of the representative ferroelectric polymers, was used for a fabrication of metal-ferroelectrics-metal capacitor. As one way to control the crystalline structure of P(VDF-TrFE) in thin films, we formed organic buffer layer of self assembled monolayers(SAMs) between polymer films and bottom metal electrodes. The preferred orientation of P(VDF-TrFE) crystal with its $b$-axis perpendicular to the substrate without the insertion of an interlayer was changed into the orientation with $a$-axis of the crystal normal to the surface after the treatment of CH$_{3}$-terminated alkanethiolate SAMs on bottom electrodes. Other kinds of alkanethiolates with their terminal groups of --OH and --COOH did not induce such an orientation shift in crystalline polymer thin films. The evolution of ferroelectric crystalline microdomains of P(VDF-TrFE) in the presence of various alkanethiolate SAMs interlayer was characterized by atomic force microscope (AFM), transmission electron microscope (TEM), grazing x-ray diffraction(GIXD) and displacement-electric field hysteresis loop measurement. [Preview Abstract] |
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C1.00053: Influence of Confinement on Crystallization of Isotactic Polypropylene Xiaofeng Chen, Rahmi Ozisik, Sanat K. Kumar Crystallization of polymer chains in the presence of nanoparticles is an area of interest because there has been considerable progress in the synthesis of various types of nanoparticles and their use as fillers in polymeric matrices. Interactions between polymer chains and particles, effect of confinement due to nanoparticles, and shape and size of nanoparticles all play important roles in the crystallization behavior of polymers. In order to investigate these factors, computer simulations were performed on isotactic polypropylene nanocomposites. The results indicate that polymer-particle interaactions play an important role. When polymer particle interaction is strong, a high density layer near the particle surface is formed but helix formation does not take place due to the strong interaction. When interactions were not considered, particle acted as a nucleating agent for helix formation. Confinement also played an important role. [Preview Abstract] |
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C1.00054: A Comparative Study of Self-Seeding and Isothermal Crystallization in Polyethylene Solutions Howard Wang, Narayan Ch Das, Kaikun Yang, Boualem Hammouda Self-seeding and isothermal crystallization in polyethylene solutions have been comparatively studied using time-resolved small angle neutron scattering. Kinetics and morphology at various seeding conditions, crystallization temperatures, and molecular compositions are discussed. [Preview Abstract] |
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C1.00055: Semicrystalline/carbon nanotube nanohybrid shish-kebabs Christopher Li, Lingyu Li, Bing Li, Kishore Tenneti We recently reported periodically decorated carbon nanotubes (CNT) using solution crystallization, in which polymer lamellar crystals were solution grown on CNTs, resulting in nano hybrid shish-kebab (NHSK) structures. Herein we report using thin film method to obtain similar structure. Semicrystalline polymer (including polyethylene, PE, and isotactic polypropylene, iPP) and CNTs were spin coated on a carbon coated surface and crystallized at different crystallization temperatures. Dendrites of PE were first observed after spincoating. Upon melting/recrystallization, edge-on PE crystals were observed to periodically grow on CNTs. Giant defects were clearly observed. iPP single crystals were also observed to grow from CNT surface. Detailed structure analysis will be presented. [Preview Abstract] |
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C1.00056: Spectroscopic Analysis of Amorphous Fluorinated Polymers Yuning Yang, Shaw L. Hsu Polarized Raman spectra have been obtained for poly (vinylidene fluoride) (PVDF). The results encouraged us to revisit the Raman band assignment, especially within the conformational sensitive region (400 cm$^{-1} \quad \sim $ 1100 cm$^{-1})$. The potential energy distribution (PED) for Raman bands within this region is recalculated. The results are supported by the experimental polarized Raman study. Based on the calculated PED, spectroscopic features (band ratio between 648 cm$^{-1}$ and 550 cm$^{-1})$ are associated with the distribution of rotational isomeric states. Furthermore, a spectroscopic simulation of the amorphous phase of PVDF is carried out and proven successful by comparison to experimental amorphous spectra. Based on the simulation, the amorphous Raman bands are assigned to different conformational sequences (\textit{tttt}, \textit{tttg}, \textit{tgtg'}). This makes the quantitative analysis of the amorphous phase plausible. Amorphous PVDF chains in different solvents are analyzed by this spectroscopic method and may be used to explain solvent-induced polymorphism of PVDF. [Preview Abstract] |
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C1.00057: Phonons and Heat Capacity of Biodegradable Poly(lactic Acid) R. Stagraczynski, M. Pyda Phonon dispersion curves and density-of-states of biodegradable poly(lactic acid) PLA have been calculated based on an infinite chain using force field and the normal modes frequencies. The heat capacity was obtained from the dispersion curves via density-of-states. Results are compared to the heat capacity obtained using ATHAS-scheme where the vibrational heat capacity is separated into group and skeletal vibrations. The group vibrational heat capacity is approximated based on infra read and Raman spectroscopy. The calculated heat capacity$ C_{p}$ was compared with the experimental $C_{p}$ of biodegradable polymer PLA. [Preview Abstract] |
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C1.00058: Polymer Surface Modification by Adsorbing Functional Block Copolymer from a Supercritical Fluid Yong Chen, Jeiran Jahani, Jeffrey koberstein We demonstrate that a polymer surface may be functionalized by adsorption of a monolayer of poly(styrene-b-tert butyl acrylate) (PS-b-tBA) copolymer containing protected carboxylic acid functional groups. The deposited P(S-b-tBA) films spontaneously form bilayer structures on polystyrene substrates from a supercritical fluid mixture of scCO2 and hexane. The modified polystyrene substrates are characterized by AFM, contact angle, and angle-dependent X-ray photoelectron spectroscopy. These measurements show that the surface of the modified polystyrene substrate is flat with the PS part of the P(S-b-tBA) entangled with polystyrene substrate and the PtBA block presented at the air interface. Monolayer to submonolayer coverage is produced as the PtBA layer thickness increases from ca. 0.22 nm at 8 MPa to a maximum of ca. 2 nm for pressures of 18 MPa and higher. The tert-butyl ester groups on the polyacrylate blocks can be deprotected to form surface carboxylic acid groups for further functionalization by electroless deposition of nickel and bioconjugation with YRGDS. [Preview Abstract] |
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C1.00059: Self-Organization and Chain-Folding in Hybrid Coil-Coil-Cube Triblock Oligomers of Polyethylene-$b$-Poly(ethylene oxide)-$b$-Polyhedral Oligomeric Silsesquioxane (POSS) Jianjun Miao, Li Cui, Lei Zhu In this work, the crystallization and self-assembly behaviors of well-defined triblock oligomers polyethylene-$b$-poly(ethylene oxide)-$b$-polyhedral oligomeric silsesquioxane (POSS) (PE-$b$-PEO-$b$-POSS) were studied. The samples were characterized by differential scanning calorimetry, synchrotron small angle X-ray scattering (SAXS), and wide angle X-ray diffraction (WAXD). The orientations of PE and POSS crystals in a shear-oriented sample were determined by 2D WAXD and SAXS. The results also suggest that POSS molecules form an ABCA-stacked four-layer lamellar (trigonal) crystal sandwiched by two PE-PEO layers. The solution-cast sample shows a long period of 13.37 nm, corresponding to an extended chain conformation in the PE crystals. When grown from the melt, the $d$-spacing decreases to 10.13 nm, indicative of once-folded chain conformation for the PE block. [Preview Abstract] |
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C1.00060: METALS |
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C1.00061: Density-Functional Calculation for the Surface of Metal Wire Chin-Sheng Wu We apply the density-functional theory to calculate the surface electron structure of metal wire. We obtain the self-consistent solution of the charge distribution around the surface. The differential surface capacitance is obtained by differentiating the areal charge density with respect to the shift of Fermi levels. The electronic structure, potential, differential surface capacitance for the cylindrical metal wires are reported. Our calculations for the metal wire with the conduction electron densities r=2, 6, and 12 a.u. are performed. [Preview Abstract] |
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C1.00062: Exchange and Crystal Field Effects in the ESR spectra of Eu$^{2+}$ in LaB$_{6}$ Carlos Rettori, Jose Duque, Ricardo Urbano, Pablo Venegas, Pascoal Pagliuso, Zachary Fisk, Saul Oseroff The Electron Spin Resonance (ESR) spectra of Eu$^{2+}$ (4$f ^{7}$, $S $= 7/2) in LaB$_{6}$ single crystal show a single Dysonian resonance for the localized Eu$^{2+}$ magnetic moments. It is shown that the Eu$^{2+}$ ions are covalent exchange coupled to the (B) 2\textit{p - like }host conduction electrons. The anisotropy of the ESR spectra is interpreted in terms of an averaged positive fourth order cubic crystal field parameter. [Preview Abstract] |
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C1.00063: Evolution of small Ti clusters and the dissociative chemisorption of H$_{2}$ T.J. Dhilip Kumar, P.F. Weck, B. Naduvalath The addition of small Ti clusters in certain complex metal hydrides has been found to improve significantly the kinetics of H$_{2}$ adsorption and desorption processes. The catalytic activity of doped Ti in these hydrogen storage materials is not fully understood. Here we report a systematic study of the sequential growth of small Ti clusters from $n$ = 2 - 15 atoms and the dissociative chemisorption of H$_{2}$ on the minimum energy clusters using density functional theory under the generalized gradient approximation. It has been found that the low energy clusters follow a pentagonal growth pattern. The clusters Ti$_{7 }$and Ti$_{13}$ show higher stability with a configuration of pentagonal bipyramid and icosahedron structures, respectively. The second difference of binding energy plot indicates that these two clusters are highly stable which agrees with the experimental collision-induced dissociation studies. Subsequently, a systematic study of the chemical reactivity of small Ti$_{n}$ clusters with $n$ = 2 - 15 towards dissociative chemisorption of H$_{2}$ has been performed. It is found that the chemisorption occurs preferentially at the two adjacent edges of any Ti atom. The chemisorption energy as a function of cluster size shows considerable structural changes in the Ti$_{n}$ clusters due to H$_{2}$ adsorption and dissociation. [Preview Abstract] |
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C1.00064: Conductance of oblate or prolate nanometric metallic particles Nicolas Grigorchuk, Petro Tomchuk In the frame of kinetic method the conductivity of metallic nanoparticle having the shape of oblate or prolate ellipsoid of revolution in frequencies range, both exceeding, and smaller than the frequency of free run of an electron between walls of a particle is studied. The diffuse reflection of an electron from an intrinsic surface of a particle was chosen as the boundary conditions. For the case, when the electromagnetic wave length is great comparing to the typical sizes of a particle, the analytical expressions for the conductivity of an particle due to the degree of its oblateness or oblongneess and orientation with respect to the direction of an incident radiation are obtained. The dependence of the longitudinal and transverse conductance components on the particle size and electromagnetic radiation frequency is investigated as well. In the case of large eccentricities of particle the agreement with the knowing results for the thin films and thin wires are reached for those components. [Preview Abstract] |
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C1.00065: Metallic Nanoporous Films Fabricated by Etching W.J. Yeh, Shilpa Chava, Julia Tilles Nanoporous metal films are of interest in a wide range of applications such as catalysis, sensing, microfluidic control and filtration. In this presentation we will report our preliminary experimental results of metallic porous materials obtained from dealloying. Two different systems have been investigated. One is gold nanoporous films obtained from etching out the silver component from the silver-gold alloy. Another is lead porous material obtained by etching out tin from tin-lead alloy. It is found that after etching out the silver component, the continuously and homogenously silver-gold films were transferred to rigid nanoporous gold films with gold forming continuous network. The size of the opening part of the films can be tuned by changing the etching time. We are able to change the porous size from about 10 nm up to a few hundreds of nanometers. When the tin was etched off from tin-lead sheets, different microstructures were found for etching at different times. When it was etched for a short time, needle shaped structures with length size about a few hundreds of nanometer were found. As the etching time increased, sponge nanoporous structures were formed. [Preview Abstract] |
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C1.00066: Electronic and magnetic transport properties in YBCO, LCMO and related multilayers Xiaoshan Wu, Ju Gao We summarized the recently studies on the structures and properties in YBCO and LCMO films and related heterostructures. The single layer of LCMO may be separated into 3-sublayers with different lattice strain containing in the film. The conduction ``death'' layer, which related to the substrate and thickness, may relate to the interaction between the substrate and the film, or due to the larger lattice strain. Good surface morphology in YBCO film can be obtained with LCMO working as buffer. In heterostructures, both of superconductivity and magnetism may coexist with varying the thickness of each sublayer. In LCMO/YBCO/LCMO trilayers, the superconductivity disappears when the thickness of YBCO is less than that of LCMO, we argue that the Adreev effect play main roles on the transport properties. A negative thermal expansion behavior is also observed in LCMO and related heterostructures at low temperature. At high temperature, the lattice parameters increase with increasing temperature up to 600 $^{o}$C. [Preview Abstract] |
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C1.00067: Influence of chemical potential on mutual diffusion coefficient of binary alloys Gboyega Adebayo, Bede C. Anusionwu, Bamidele I. Adetunji We conduct a series of investigations using the Complex Formation Model (CFM) and relying on statistical theory to analyse the mixing behaviour of binary alloys when complexes are formed. A general equation for the mutual diffusion coefficients in binary alloys was derived. The chemical potential dependence of the transport coefficients of binary alloys has been interpreted by asumming the complex formed is strongly dependent on interatomic interaction of the constituent spieces. [Preview Abstract] |
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C1.00068: Optical Study of Lanthanide Metals at High Pressure R.G. Kraus, E.D. Emmons, J.S. Thompson, A.M. Covington We are currently investigating the pressure dependence of the photoluminescence spectra of lanthanides, including Gadolinium metal. This investigation is conducted at hydrostatic pressures up to 10 gigapascals. Static pressure is obtained using a four post diamond anvil cell. The photoluminescence is excited with a low power (5 mW) continuous wave Argon ion gas laser, and the spectrum is collected using a confocal Raman microscope. Through this investigation, we hope to obtain spectral signatures of phase transitions, thus creating a diagnostic for detection of phase transitions in shock wave experiments. Preliminary photoluminescence data will be presented. [Preview Abstract] |
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C1.00069: Light Pulse Excitation of Radiative Polaritons in a 2D Excitonic Layer Norman J.M. Horing, T. Yu Bagaeva, V.V. Popov, Jay Mancini The excitation of a polariton mode in a two-dimensional excitonic layer by a light-wave extinction front is analyzed here using the dyadic Green's function formalism. The electromagnetic response of the 2D excitonic layer includes inhomogeneous radiative exciton-polariton modes corresponding to the complex poles of the Green's function. [Preview Abstract] |
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C1.00070: Interlayer effect with Poly(N-vinylcarbazole) (PVK) as a carrier-blocking layer on performance of a hybrid OLED Joowon Lee, Jungsoo Park, Jaikyeong Kim, Jin Jang, Youngsoo Yoon We demonstrate high efficiency organic light-emitting devices (OLEDs) by incorporating of polymeric interfacial layer for carrier blocking effect. To prove the effect of interposition layer, we used an poly($N$-vinylcarbazole) (PVK) between a hole-conducting layer (PEDOT: PSS, poly(styrenesulfonate)) and an emitting layer (Alq$_{3})$. OLEDs with simple structure were controlled by a small molecule layers as a function of thickness and a polymer layer as a function of weight percent. The characteristics of hybrid structure with interfacial layer showed a peak efficiency of 8.56 cd/A, 2.30 lm/W in performance compared the device without interfacial layer of 0.31cd/A, 0.11 lm/W. The inserted PVK can efficiently enhance the hole-electron balance for achieve high recombination efficiency. Also, it showed the possibility of hybrid type to OLEDs production process. [Preview Abstract] |
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C1.00071: Green's function theory for defects in graphene sheets Raimundo Costa Filho, Joao Pereira Junior, Francois Peeters The presence of defects or impurities in graphene sheets is an important factor in its electronic and magnetic properties. In order to study such effects, we develop a Green's function formalism to calculate the electronic localized sates originated by defects or impurities on a graphene sheet. The equations of motion for the Green's functions are determined in the framework of the tight-binding model. The defects are represented by the absence of carbon atoms on the sheet, while impurities are introduced substitutionaly on the sheet. [Preview Abstract] |
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C1.00072: Evaluation of NiTi and NiPdTi Shape Memory Torque Actuators Alan Thompson, Jeff Sharp This work presents the evaluation of some thermal-mechanical transformation properties of NiTi and NiPdTi shape memory alloy (SMA) torque tubes processed under differing conditions. NiTi is the dominant alloy used in SMA applications, but its transformation temperatures are too low for certain applications. The substitution of Pd for Ni raises the transformation temperatures. One of the goals of our work is to determine if the NiPdTi alloy can be processed to achieve mechanical properties comparable to those of NiTi. Experiments employing a custom thermal-mechanical torque tube tester were conducted on Ni50Ti50. The transformation temperatures, recoverable and creep strains, and positive work done during the martensite-to-austenite transformation were obtained as a function of the number of cycles and loads. NiPdTi torque tubes are currently being fabricated for comparison. Graphical presentations of the percent strain versus temperature will be presented to demonstrate the transformation and mechanical behavior for the NiTi and NiPdTi SMA torque tubes at 10, 300, 500, and 600 cycles. Work per cycle versus load and versus number of cycles will be presented. [Preview Abstract] |
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C1.00073: Nonlinear acoustic effects from dislocation-based hysteretic kinking solids under stresses Peter Finkel, Aiguo Zhou, Michel Barsoum We argue that proposed recently mechanism explaining inelastic hysteresis in non-linear elastic systems indeed can be explained by means of formation of dislocation-based incipient kink bands (IKB). Using acoustic waves we investigated possible dislocation related mechanisms responsible for nonlinear dynamic response of IKB solids. In this work, for the first time we observed and reversibility of IKB formation directly under stress using ultrasonic waves and acoustic emission signatures during compression test of nanolaminated layered ternary carbide (MAX phases) samples. We confirm here that the dynamic behavior of these non-linear elastic systems is due to the interaction of dislocations with the stress waves. [Preview Abstract] |
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C1.00074: Tight-binding study of impurity problems in BCC transition metals Daniel Finkenstadt, Dimitrios Papaconstantopoulos, Mike Mehl Tight-binding provides a useful theoretical framework for studying defects and impurity problems, that would otherwise be very computationally demanding using density-functional and all-electron first-principles methods. Here we extend our tight-binding formalism, the NRL tight-binding method, to include alloy cases, specifically for BCC transition metals and their impurity formation energies. We have used a virtual-crystal approximation (VCA) of the impurity, onsite and hopping TB parameters, and we find that in many cases the VCA gives reasonable results for lattice and elastic constants, as well as for predicting correct ground-state structures. The VCA is then applied to calculate substitutional impurity defect energies and also interstitial defect energies, that we have examined previously with in tight-binding [PRB, in press, 2006]. [Preview Abstract] |
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C1.00075: The Refractive Relativity Theory (RRT) Eckart Colsman It is suggested that in the General Relativity Theory in the equation for the 4th dimension l=ct, the velocity of light c should replace the time t as a variable. This results in simplified algorithms and more transparency for the GRT. Replacing the time warp with a variable c results in a theory, which is a true complementary concept in the sense of Niels Bohr. It does not contradict any tests, which supports the GRT. The Refractive Relativity Theory (RRT): (A) calculates the bending of light by a body of mass with the theory of refraction per the Snell's law, (B) interprets the cosmological redshift as a universal change of c over time instead with the Doppler effect, (C) describes the gravity field as a dimensional gradient of c, (D) recognizes that the expansion rate of the universe is given by the constant of gravity G, (E) calculates the Hubble constant H to 68.65 km/sec/Mpc, (F) gives the Higgs field a clear definition and addresses the weight conundrum of the W and Z boson particles. Using c as a variable in the universe, the RRT shows the way to a theory unifying the General Relativity with the electromagnetic theory! [Preview Abstract] |
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C1.00076: LOW TEMPERATURE PHYSICS |
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C1.00077: Vibration Isolation for a Pulse-Tube Research Cryostat S.T.P. Boyd Commercial pulse-tube refrigerators (PTRs) now provide base temperatures $<$ 3K, low vibration, and long life. However, vibration levels are still often too large for LT and ULT measurements. One highly successful approach to vibration isolation in very small cryostats has been the use of 1-atm He exchange gas, in an envelope with a flexible element, interposed between the cold head and the cryostat. A design study to scale up this technique for a PTR research cryostat has previously been presented. However, some questions remained, given the violation of ``adiabaticity'' of the ``pulse tubes'' in the PTR and the potential for convective flow and Taconis oscillations of the exchange gas in the open geometry. We present experimental results obtained on the cryostat with a rigid exchange-gas volume, which permitted the variation of exchange-gas pressure. The news is all good so far: the heat exchangers perform well and in reasonable agreement with calculations, no evidence is seen of deleterious effects due to convection or Taconis oscillations or gas permeation, and the 2.8K PTR base temperature is only raised by 0.1K or less. Work to implement the fully-vibration-isolated cryostat is now underway. [Preview Abstract] |
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C1.00078: Fast Spin Relaxation in Solid $^{3}$He Matthias Kath, Erwin Schuberth To determine the spin structure of the nuclear magnetic ordered phases of solid $^{3}$He, neutron scattering would be the method of choice. For this experiment it is crucial to grow a single crystal within the sinter needed for cooling the solid to temperatures of the order of 0.5 mK and to aborb the major part ($>$ 90 {\%}) of the heat generated by the neutron capture. In this respect we studied the growth of crystals in Ag sinters of different pore sizes to find an optimal way to obtain preferably single crystalline samples. The ordered phases were indicated by a drop of the NMR intensity for the U2D2 phase and by an increase (30 {\%}) for the high field phase. Just below T$_{Neel }$(0.9 mK) we found a line splitting as expected for the U2D2 phase a) of about 20 kHz to the high frequency side of the Larmor line at 1980 kHz and b) 4 kHz on its low frequency side at 240 kHz. For unknown reasons the Larmor line itself was absent in the U2D2 phase. In the paramagnetic phase T$_{2}$ turned out to be about 4.6 ms. T$_{1}$ measurements were hampered by an unexpected fast initial recovery process prior to the longitudinal spin relaxation T$_{1}$ 250 ms at 10 mK). The efficient recovery of M$_{z}$ is possibly due to a sizeable demagnetizing field which is so strong that just above T$_{Neel}$ T$_{1}$ could not be determined. [Preview Abstract] |
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C1.00079: STATISTICAL AND NONLINEAR PHYSICS |
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C1.00080: Experimental study of the chaotic waterwheel George Rutherford, Amy Erxleben, Epaminondas Rosa, Jr. The chaotic waterwheel is often given as an example of a mechanical system that can exhibit chaotic behavior. Its early demonstration by Malkus and the realization that it can be modeled by the Lorenz equations has secured it a prominent place in almost every general presentation of chaos. It seems quite surprising, then, that no experimental investigations of this textbook system have ever been published. To fill this historic gap, and to initiate an experimental study of this incredibly rich dynamic system, our lab has constructed a research-grade waterwheel consisting of a vacuum-formed polycarbonate frame in which 36 cylindrical cells are mounted on an 18 inch diameter. The wheel and its axis can be tilted, and water is fed into the top of the wheel and drains out through thin tubes at the bottom of each cell. An aluminum skirt at the wheel's periphery passes through a variable gap magnet to provide magnetic braking. Angular time series data are collected with an absolute rotary encoder. The data are smoothed and angular velocity and acceleration are calculated via fast fourier transforms. The data show quasi-uniform rotation as well as periodic and chaotic motion and agree fairly well with computer simulations of the idealized wheel equations. We will discuss differences between the experimental data and the simulation predictions as well as plans for future studies. [Preview Abstract] |
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C1.00081: Growth law of the Phase Field Crystal model Mowei Cheng, James Warren We study the growth law of the characteristic length (averaged grain size) of the newly developed Phase Field Crystal model. We focus our study on the late time scaling regime after a quench from the high temperature isotropic liquid phase to the low temperature crystalline phase. [Preview Abstract] |
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C1.00082: Driven hard-core fluid on a ladder: shear-induced particle redistribution Ronald Dickman, Ronaldo Vidigal We study driven particle systems with excluded volume interactions on a two-level ladder with periodic boundaries, using Monte Carlo simulation, cluster mean-field theory, and exact (numerical) solution of the master equation. Particles on one level are subject to a drive that forbids motion along one direction, while in the other level the motion is unbiased; particles may jump between levels. Despite the symmetry of the rates for transitions between layers, the associated particle densities are unequal: at low densities there is an excess of particles in the non-driven layer, while at higher densities the tendency is reversed. Similar results are found for an off-lattice model. We quantify the reduction in the stationary entropy caused by the drive. [Preview Abstract] |
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C1.00083: Phase diagram and critical exponents for the pair annihilation model in one and two dimensions Adriana Dickman, Ronald Dickman We study the one and two-dimensional pair annihilation model (PAM) with diffusion using the pair approximation (PA) and Monte Carlo simulation. The model is defined on a $d$-dimensional hypercubic lattice where sites can be either occupied by a particle or vacant. The particles are allowed to diffuse with probability $D$; pairs of particles are annihilated with rate $1 $, and particles are created with rate $n\lambda/(2d)$, where $n$ is the number of occupied neighbors. The active and absorbing phases are separated by a continuous phase transition at $\lambda_c(D)$. We obtain the phase diagram and critical exponents for the model, which confirm that PAM belongs to the directed percolation (DP) universality class. The PA for the one and two-dimensional PAM indicates that for a diffusion rate greater than a certain value, $\lambda_c=0$. On the other hand, Katori and Konno [Katori, K., and Konno, N. (1992). Physica A {\bf 186}, 578] showed rigorously that $\lambda_c > 0$ for any diffusion rate, in one dimension. Our simulation results are consistent with this theorem, but suggest that the PA prediction is correct in two dimensions. [Preview Abstract] |
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C1.00084: Irreversible adsorption of nanoparticles with size dispersion N. A. M. Araujo, J. F. Marques, A. B. Lima, A. Cadilhe The effect of size dispersion on the irreversible adsorption of nanoparticles was studied with Monte Carlo simulation. To this end, we performed a tailored series of Monte Carlo simulations of single monolayer colloidal films with a Gaussian size dispersion around the mean value ranging from 3\% (monodisperse) to 20\% (polidisperse). To study possible ways of size selection, we used clean and patterned substrates to make a comparative study. In the latter case, the pattern consists of squares of size $\alpha$, a distance $\beta$ apart from each other. Adsorption can only take place inside the squares with excluded volume interaction. We follow the kinetics of adsorption and measure their resulting film coverage, mean particle size, dispersion, and the radial distribution of the distances. [Preview Abstract] |
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C1.00085: Response to disturbance and abundance of final state in logistic map Dan Shen, Wen-Xiu Wang, Yu-Mei Jiang, Ying-Mei Wang, Da-Ren He We propose a new definition of complexity. It is our hope that this definition can describe a clear boundary between simple systems and complex systems by showing that all the simple systems have zero complexity, and all the complex systems have positive complexity. The idea is that when a system evolves to a final state via a transient state, its complexity depends on the abundance of both the final state and the transient state. The abundance of the transient state may be described by the diversity of the response to disturbance. Complexity values of some example simple systems are analytically discussed. All the results show zero values. As examples for complex systems, numerically we discussed the complexity values of logistic map. The results show complexity values beyond zero in chaotic states. All the maximal values locate at the edges of chaos. [Preview Abstract] |
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C1.00086: Dephasing representation of quantum fidelity for general pure and mixed states Jiri Vanicek General semiclassical expression for quantum fidelity (Loschmidt echo) of arbitrary pure and mixed states is derived. It expresses fidelity as an interference sum of dephasing trajectories weighed by the Wigner function of the initial state, and does not require that the initial state be localized in position or momentum. This general \emph{dephasing representation} is special in that, counterintuitively, all of fidelity decay is due to dephasing and none due to the decay of classical overlaps. Surprising accuracy of the approximation is justified by invoking the shadowing theorem: \emph{twice}--both for physical perturbations and for numerical errors. It is shown how the general expression reduces to the special forms for position and momentum states and for wave packets localized in position or momentum. The superiority of the general over the specialized forms is explained and supported by numerical tests for wave packets, non-local pure states, and for simple and random mixed states. The tests are done in non-universal regimes in mixed phase space where detailed features of fidelity are important. Although semiclassically motivated, present approach is valid for abstract systems with a finite Hilbert basis provided that the discrete Wigner transform is used. This makes the method applicable, via a phase space approach, e. g., to problems of quantum computation. [Preview Abstract] |
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C1.00087: Precession and chaos in the Newtonian two-body problem in a spherical universe Martha I. Roseberry, John F. Lindner We generalize the Newtonian two-body problem from flat space to spherical space and realize much of the complexity of the three-body problem with only two bodies. We show analytically, by perturbation theory, that small, nearly circular orbits precess at rates proportional to the square root of their initial separations and inversely proportional to the square of the universe's radius. We show computationally, by creating initial velocity space plots of millions of orbits, that large orbits can exhibit extreme sensitivity to initial conditions, the signature of chaos. We acknowledge support from NSF-REU grant DMR 0243811. [Preview Abstract] |
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C1.00088: Mechanism of discrete breather excitation in driven micro-mechanical cantilever arrays Panagiotis Maniadis, Sergej Flach We study the pathway of generating discrete breathers (also known as intrinsic localized modes ``ILMs'') in damped and driven micro-mechanical cantilever arrays. Using the concept of the nonlinear response manifold we provide a systematic way to find the optimal parameter regime in damped and driven lattices where discrete breathers exist. Our results show that discrete breathers appear via a new instability of the manifold, different from the anticipated modulational instability known for conservative systems. We present several ways of exciting breathers, and compare also to experimental studies in anti-ferromagnetic layered systems. [Preview Abstract] |
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C1.00089: The ``Escape Set" elucidates chaos in the restricted three-body problem Evan Heidtmann, John F. Lindner We analyze solar escape as a special case of the restricted three-body problem. By numerically integrating the trajectories of millions of particles shot from Earth with different initial conditions, we determine their displacements after a fixed time. On a polar plot of (pseudo) energy radially and initial direction angularly, we color code these distances. The resulting ``Escape Set'' elucidates chaos in the three-body problem in the same way that the Mandelbrot set illustrates complexity in nonlinear maps: at low energies, the projectile is trapped; at high energies, it easily escapes; at intermediate energies, it executes intricate chaotic orbits with unbounded excursions and repeated returns. We acknowledge support from NSF-REU grant DMR 0243811. [Preview Abstract] |
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C1.00090: The Parallel Computational Complexity of the Percolation Model D.W. Blair, Jon Machta The parallel computational complexity of identifying cluster in the two-dimensional site percolation model is investigated. For a square lattice with sides of length $L$ and site occupation probability $p$, the running time of the parallel percolation algorithm we study scales as $log(D_f(L,p))$, where $D_f$ is the average value of the largest, finite cluster diameter in the lattice. We find that $D_f$ exhibits a continuous phase transition as $p$ approaches the critical percolation probability $p_c$ -- indicating that the parallel percolation simulation has a ``complexity critical point,'' corresponding to the structural percolation critical point. Our simulations also suggest that $D_f (L,p_c) \sim L^{d_{min}}$, and thus that the parallel percolation simulation runs in $O(log(L))$ time at $p_c$. [Preview Abstract] |
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C1.00091: Volatilities, Traded Volumes, and Price Increments in Derivative Securities Kyungsik Kim, Gyuchang Lim, Soo Yong Kim, Enrico Scalas We apply the detrended fluctuation analysis (DFA) to the statistics of the Korean treasury bond (KTB) futures from which the logarithmic increments, volatilities, and traded volumes are estimated over a specific time lag. For our case, the logarithmic increment of futures prices has no long-memory property, while the volatility and the traded volume exhibit the existence of long-memory property. To analyze and calculate whether the volatility clustering is due to the inherent higher-order correlation not detected by applying directly the DFA to logarithmic increments of the KTB futures, it is of importance to shuffle the original tick data of futures prices and to generate the geometric Brownian random walk with the same mean and standard deviation. It is really shown from comparing the three tick data that the higher-order correlation inherent in logarithmic increments makes the volatility clustering. Particularly, the result of the DFA on volatilities and traded volumes may be supported the hypothesis of price changes. [Preview Abstract] |
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C1.00092: Scaling Exponents in Financial Markets Kyungsik Kim, Cheol-Hyun Kim , Soo Yong Kim We study the dynamical behavior of four exchange rates in foreign exchange markets. A detrended fluctuation analysis (DFA) is applied to detect the long-range correlation embedded in the non-stationary time series. It is for our case found that there exists a persistent long-range correlation in volatilities, which implies the deviation from the efficient market hypothesis. Particularly, the crossover is shown to exist in the scaling behaviors of the volatilities. [Preview Abstract] |
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C1.00093: From Brownian motor to Brownian refrigerator Ryoichi Kawai, Christian Van den Broeck Onsager symmetry implies that a Brownian motor, driven by a temperature gradient, will also perform a refrigerator function upon loading. We analytically calculate the corresponding heat flux for an exactly solvable microscopic model and compare it with molecular dynamics simulations. [Preview Abstract] |
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C1.00094: ABSTRACT HAS BEEN MOVED TO R1 |
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C1.00095: Fluid flow through reconstituted marine quartz sediments - an interacting lattice gas simulation Allen Reed, Edward Braithwaite, Joe Gettrust, Ras Pandey A porous sediment sample (cylinder of 5.9 mm diameter) is reconstituted in the laboratory using marine quartz sands from the Northern Gulf of Mexico. Digitized computed tomography images of sub-sample (cylinder of 6.5 mm diameter), removed from different regions of the laboratory sample, provide the porous matrix for an interacting lattice gas simulation. A cubic lattice of size 100$^{3}$ is used to represent the sediment matrix of the order of 64 mm$^{3}$. Different regions of the reconstituted sample are represented by corresponding porous matrices, each with a unique pore distribution. Mobile particles, the constituents of an interacting lattice gas, are used to model the fluid, which flows through the porous media from a source at the bottom to a sink at the top. Fluid particles are driven by their concentration gradient and an external pressure bias against gravity. Variations of the root mean square displacement of each particle (tracer) and that of their center of mass with the time steps, mass transfer and flux are examined as a function of the external pressure bias. Transport properties, including the response of the fluid flux to pressure bias, will be presented. [Preview Abstract] |
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C1.00096: Self-organizing flow through a porous medium with slit: an interacting lattice gas simulation Joe Gettrust, Ras Pandey Flow of driven, interacting particles through a porous medium is complex and highly restricted especially at low porosity. How does a slit of higher porosity within the medium affect the flow and patterns in such a matrix? Computer simulations are performed to address this question using an interacting lattice gas model. A mixture of dissimilar particles (A,B) is driven by a hydrostatic pressure bias (H) and their concentration gradient upward against gravity from a source at the bottom. Density and mobility profiles are examined as a function of bias (H). Increasing the bias (H) seems to introduce long-range correlations among the constituents; this has considerable effect on their density profiles around the slit. [Preview Abstract] |
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C1.00097: Effects of molecular weight on eruptive flow of a multi-component system by interacting lattice gas Ras Pandey, Joe Gettrust Mobile particles (A,B), characterized by their molecular weights and interactions in an effective (interacting) medium, are driven by a hydrostatic pressure bias (H) and their concentration gradient upward against gravity from a source at the bottom. Initially, particles are randomly placed on a discrete lattice of size L$_{x}$ x L$_{y}$ x L$_{z}$ with equal proportion. Particles execute their stochastic movements with the Metropolis algorithm. Periodic (open) boundary conditions are used along the transverse (longitudinal) directions. Particles continue to enter the lattice from the source according to their concentration in the lattice and leave from the open ends when they attempt to hop out of the lattice (top and bottom). The net flow of particles reaches steady state at that point a self-organized structure emerges at each bias. Response of the flux density of constituents to pressure bias is examined as a function of their molecular weight. [Preview Abstract] |
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C1.00098: Role of Social Motility in Facilitating Collective Motion of Myxobacteria Yilin Wu, Nan Chen, Matthew Rissler, Yi Jiang, Dale Kaiser, Mark Alber Social motility is a unique form of behavior exhibited by a wide range of bacteria, including most pathogens that cause plant and animal disease. It is operated by type IV pili that attach to other cells' surfaces and pull; the retracting force pulls the cell forward. Experiments have demonstrated that social motility is important for the collective motion of bacteria colonies, and it facilitates the colonization of pathogens in hosts. We use a cell-based model to study the role of social motility in swarming of bacteria colonies. Mycococcus xathus, a species of myxobacteria, is our model bacteria because it exhibits typical social motility and has been well studied. Our simulation results suggest that social motility has an effect on alignment of neighboring cells, resulting in a highly ordered collective motion. We also show that social motility can significantly improve the swarming efficiency of bacteria. We track GFP labeled Mycococcus xathus cell cultures and derive model parameters from the cell motion data. Our work may shed light on the infection process of many diseases. [Preview Abstract] |
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C1.00099: Network description on competition and cooperation systems Yue-Ping Zhou, Hui Chang, Da-Ren He We propose a kind of network description on competition and cooperation systems. In the network the vertices are defined as the elements, which make competition and cooperation in some acts. They are competing because each of them tries to occupy a larger piece of ``cake'' that represents a kind of competition harvest. They are cooperating because they will not destroy each other. They coexist and each one gets a piece of cake. Two nodes are connected by an edge when they appear in a common act. The total weight of the cake a node gets is defined as the node weight. The statistical properties without involving the weight counting can show the situation how the elements are competing and cooperating in the acts. While the properties involving the weight counting can show the competition results and the relationship between the competition results and the network topological structure. We have done such network descriptions for some practical systems. The results show that the description may be helpful. [Preview Abstract] |
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C1.00100: Community and hierarchy in generalized collaboration networks Hui Chang, Bei-Bei Su, Da-Ren He We propose a generalization of social collaboration network, which we term generalized collaboration network. In this kind of network, nodes are defined as actors, two nodes are connected by an edge when they take part and collaborate in a common act. In an act each pair of actors has collaboration relationship therefore an act is described by a clique. Based on the common topological property, we proposed quantitative definitions of network community and hierarchy and their corresponding search algorithms. In a generalized collaboration network, two cliques can share a number of nodes. We define the connecting cliques as `1-hierarchy community' when they share at least one common node between each other; accordingly, we define a `2-hierarchy community' as a set of cliques which are mutually connected through at least two common nodes; in general, a `$h$-hierarchy community' is defined as the set of cliques which are mutually connected through at least $h$ nodes. We also propose a novel statistical parameter, i.e. degree of interweavement, to measure the connectivity of the whole network. [Preview Abstract] |
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C1.00101: Random field Ising model on networks with inhomogeneous connections Sang Hoon Lee, Hawoong Jeong, Jae Dong Noh We study a zero-temperature phase transition in the random field Ising model on scale-free networks with the degree exponent $\gamma$. Using an analytic mean-field theory, we find that the spins are always in the ordered phase for $\gamma<3$. On the other hand, the spins undergo a phase transition from an ordered phase to a disordered phase as the dispersion of the random fields increases for $\gamma > 3$. The phase transition may be either continuous or discontinuous depending on the shape of the random field distribution. We derive the condition for the nature of the phase transition. Numerical simulations are performed to confirm the results. [Preview Abstract] |
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C1.00102: Stability of large systems Harold Hastings We address a long-standing dilemma concerning stability of large systems. MacArthur (1955) and Hutchinson (1959) argued that more ``complex'' natural systems tended to be more stable than less complex systems based upon energy flow. May (1972) argued the opposite, using random matrix models; see Cohen and Newman (1984, 1985), Bai and Yin (1986). We show that in some sense both are right: under reasonable scaling assumptions on interaction strength, Lyapunov stability increases but structural stability decreases as complexity is increased (c.f. Harrison, 1979; Hastings, 1984). We apply this result to a variety of network systems. References: Bai, Z.D. \& Yin, Y.Q. 1986. Probab. Th. Rel. Fields 73, 555. Cohen, J.E., \& Newman, C.M. 1984. Annals Probab. 12, 283; 1985. Theoret. Biol. 113, 153. Harrison, G.W. 1979. Amer. Natur. 113, 659. Hastings, H.M. 1984. BioSystems 17, 171. Hastings, H.M., Juhasz, F., \& Schreiber, M. 1992. .Proc. Royal Soc., Ser. B. 249, 223. Hutchinson, G.E. 1959. Amer. Natur. 93, 145, MacArthur, R. H. 1955. Ecology 35, 533, May, R.M. 1972. Nature 238, 413. [Preview Abstract] |
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C1.00103: Empirical study on competition in audience rating of China Central Television programs Lingling Wu, Yue-Ping Zhou, Hui Chang, Da-Ren He In recent years, the competition in audience rating of China Central Television (CCTV) Programs becomes increasingly more vehement. Each program is trying its best to figure the new view to the masses. We have performed an empirical investigation on the competition situation by using a complex network description. In the network nodes are defined as television programs. Two nodes are connected by an edge when they are positioned in a common time period. The edge represents the competition and cooperation relationship between the programs. They are competing because each of them tries to get more audience rating; they are cooperating since each of them obtains some audience rating, or say, gets ``a piece of cake.'' Some empirical statistical rules have been obtained, such as an exponential degree distribution, the average path length 1.52, the $C(k) \sim k^{-1}$ scaling rule between the clustering coefficient and the degree, and the approximate zero assortativity. These observations may be helpful for the arrangement of the television programs. [Preview Abstract] |
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C1.00104: Empirical study on tourism network in China Yu Wang, Dan Shen, Hui Chang, Da-Ren He We have made an empirical investigation on the tourism in China with the complex network description. In the network the nodes are defined as scenic spots, two nodes are connected by an edge when they appear in a common tourist route. The tourist routes are defined as acts. The node weight is defined as the tourist number visiting the scenic spot. Some statistical properties have been obtained. The results certify that the degree distribution, the act degree distribution, and the dependence of the clustering coefficient on both of them obey exponential decays. The assortativity shows an extremely large number r = - 2.71. The node weight distribution also follows an exponential decay. [Preview Abstract] |
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C1.00105: Empirical study on Yangzhou University library borrowing network Jian-Ru Tao, Hui Chang, Da-Ren He We have done an empirical investigation on the Yangzhou University library borrowing data of the physics books by using complex network description. In the network the nodes are defined as books. Two nodes are connected by an edge when they relate to a common scientific subject. A scientific subject is defined as an act. The node weight is defined as the number of students who borrowed this book. Some empirical statistical results have been obtained. Both the node degree distribution and the act degree distribution and the dependence of the clustering coefficient on both of the degrees show exponential decays. The assortativity shows a positive number, r= 0.73. The distribution of the node weights shows a SPL function, which indicates an interpolation between a power law and an exponential decay. Based on these results we try to give some suggestions and advices on the management of physical books in the libraries. [Preview Abstract] |
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C1.00106: Empirical study on human acupuncture point network Jian Li, Dan Shen, Hui Chang, Da-Ren He Chinese medical theory is ancient and profound, however is confined by qualitative and faint understanding. The effect of Chinese acupuncture in clinical practice is unique and effective, and the human acupuncture points play a mysterious and special role, however there is no modern scientific understanding on human acupuncture points until today. For this reason, we attend to use complex network theory, one of the frontiers in the statistical physics, for describing the human acupuncture points and their connections. In the network nodes are defined as the acupuncture points, two nodes are connected by an edge when they are used for a medical treatment of a common disease. A disease is defined as an act. Some statistical properties have been obtained. The results certify that the degree distribution, act degree distribution, and the dependence of the clustering coefficient on both of them obey SPL distribution function, which show a function interpolating between a power law and an exponential decay. The results may be helpful for understanding Chinese medical theory. [Preview Abstract] |
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C1.00107: Empirical study on course selection network of Yangzhou University Chun-Hua Fu, Hui Chang, Da-Ren He We empirically investigate the course selection network of Yangzhou University. In the network the vertices are defined as courses, two vertices are connected by an edge when they are selected by the students in a common college. A college is defined as an act. The student number who selects the course is defined as the vertex weight. Some statistical properties have been obtained. The degree distribution shows an exponential decay. The clustering coefficient shows an exponential decay dependence on both the vertex degree and the vertex act degree. The assortativity shows an approximate zero number. The cumulative vertex weight distribution shows a power law. These results may be helpful for understanding the relationship between different scientific subjects in universities. [Preview Abstract] |
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C1.00108: Empirical study on salesmanship network of the top electronic companies in China Ling Chen, Yue-Ping Zhou, Da-Ren He We empirically investigate the top electronic company salesmanship network. In the network the vertices are defined as companies, two vertices are connected by an edge when they sale a common product. A product is defined as an act. The total sale income is defined as the vertex weight. Some statistical properties have been obtained. Among the properties, weight properties are more interesting. The cumulative vertex weight distribution shows a power law, the relationship between the averaged weight of the vertices, which have the same degree, and the vertex degree shows an exponential arising, and the relationship between the averaged weight of the vertices, which have the same act degree, and the vertex act degree shows a power law. These results indicate that the salesmanship competition is vehement in China. [Preview Abstract] |
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C1.00109: Empirical investigation on the transport network of the Yangtze River delta metropolis circle Si-Jun Pan, Chang-Gui Gu, Yu-Mei Jiang, Da-Ren He We empirically studied the combined transport network, which includes inter-urban bus network, railway network, river and sea ship network and airplane network of the Yangtze River delta metropolis circle from the viewpoint of complex network. The cities are defined as the nodes. Two nodes are connected by an edge when they are served by a bus, a train, a ship or an airplane directly. The total passenger number on the edge is defined as the edge weight. We propose a new statistical property addressed as ``incompleteness,'' which is defined as the edge difference of the networks from the complete graph. The results indicate that the combined one is very near to complete. Another new property is addressed as ``convenience.'' The convenience of an edge is defined as its weight. The total convenience of a road between two cities can be calculated by a rule of edge convenience similar to the rule of resistances in series and parallel connection. The total convenience of a network is defined as the average of all the edge convenience. The results show that the combined network has the largest convenience. [Preview Abstract] |
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C1.00110: Empirical study on a publisher questionnaire network Lei Yuan, Hui Chang, Da-Ren He The China High Education Publisher distributed ten thousands of questionnaires to university students for knowing their satisfaction degree on the published textbooks. We empirically investigate the statistical properties by using a network description. In the network the vertices are defined as textbooks, two vertices are connected by an edge when the students in a common specialty used them. A student specialty is defined as an act. The average satisfaction degree mark given by students on each textbook is defined as the vertex weight. Some statistical properties have been obtained. The degree distribution shows an exponential decay, while the act degree distribution shows a SPL function that indicates an interpolation between a power law and an exponential decay. Both the dependence of the clustering coefficient on the vertex degree and the cumulative vertex weight distribution show SPL functions. The assortativity shows a positive number, 0.14. [Preview Abstract] |
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C1.00111: Decay Processes of Three-Species on Networks Kyungsik Kim, Ki-Ho Chang We study the novel reaction-diffusion process of three-species on scale-free networks, which is significantly different from the chemical reaction systems manipulated from the deterministic rate equation. For our three-species process, the particle density scales as the power-law behavior existing the crossover, similar to the two-species reaction-diffusion system. From four kinds of the scaling exponent $\gamma$ in the degree distribution of networks, the fraction and the ratio are shown to increase linearly as time advances. The result that we analyzed and calculated is compared with other findings in the decay process. [Preview Abstract] |
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C1.00112: Mean-field solution of equilibrium ensembles of undirected networks with 3-edge interactions Peter Fleck, Noshir Contractor We study the equilibrium statistical mechanics of ensembles of undirected networks with triangle- and 3-star-type interaction among bi-valued edges. We present analytical expressions for the statistics' averages in mean-field approximation. We find this model's phase diagram to be identical after parameter substitution to that of a reduced model with triangle interactions only. For sufficiently large networks, both triangle- and 3-star-type interactions affect the network's topology very similarly. We find the mean-field solution to agree excellently with Markov Chain Monte Carlo simulations in an important part of parameter space. Implications for the analysis of network topologies are being discussed. [Preview Abstract] |
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C1.00113: A model of the sediment transport on a river network Xu-Ming Wang, Rui Hao, Jin-Feng Zhang, Jie Huo A dynamical model is proposed to mimic the sediment transport on a river network. A river can be divided into some segments. For the ith segment the schlepping sediment ability of the flow may be scouring or depositing, which is influenced by that of the (i- 1)th segment. In order to compare our model simulation results with the empirical data obtained in Yellow River, the model is equipped with an experiential relation between the flow rate and the depositing rate of the Yellow River. After this, the simulation results show an excellent agreement with the empirical conclusions obtained with the upper and middle parts of Yellow River when it is in the low-water periods (for instance, in Dec., Jan. and Feb.). This indicates that our model may successfully describe the scouring-depositing of river networks. [Preview Abstract] |
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C1.00114: Important relationships in the sediment-transporting network Jin-Feng Zhnag, Rui Hao, Jie Huo, Xu-Ming Wang Recently we have suggested a model for understanding the process of the sediment transportation and the sediment scouring- depositing behaviors. Some interesting and important properties are revealed. The so-called schlepping-sediment coefficient $k$ of a segment of the river is modulated by the just undergone state of scouring or depositing of its upriver segment and itself. The numerical result suggests a power relation between the schlepping-sediment coefficient $k(i)$ and the segments number $i$, which indicates that the schlepping-sediment ability of the segments reduce rapidly from the upriver to the downriver. Different from that, the relationship between the property and the quantity of the sediment scouring or depositing is exponential. If the property of scouring or deposition keeps the same along all segments the quantity of scouring or deposition increases, while if the property of scouring or deposition changes, the quantity decreases. These two important relationships can quantitatively interpret the deposition that usually occurs in the downriver. [Preview Abstract] |
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C1.00115: Predicting the state of scouring or deposition by a model of the sediment transport on a river network Rui Hao, Jin-Feng Zhang, Jie Huo, Xu-Ming Wang We constructed a model to describe the sediment transportation on the river network, which can indicate what state, scouring or deposition, will appear when the system, under certain conditions, evolves after a long time period and finally becomes stable. In the model a river segment, say the ith segment, can be classified into three types. The first one is actively- modulation type where the so-called impact factor of ith segment is larger than that of (i-1)th. The second one is passively- modulation type where the impact factor of ith segment is smaller. The third one is freely-modulation type where the two impact factors are equivalent. For the first type, the states, scouring or depositing, of the segments of the upriver are qualitatively the same as that the river source, while the states of the downriver change and distribute disorderly. For the second type, the states along a lone part of the river can qualitatively keep the same state as that of the source. A simpler case will appear in the third type: the state of the scouring or depositing on each segment equals, and are same as that of the source. [Preview Abstract] |
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C1.00116: MAGNETISM |
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C1.00117: Cluster Heat Bath Monte Carlo study of a quasi one-dimensional Ising antiferromagnet on a stacked triangular lattice E. Meloche, M. L. Plumer The results of extensive Monte Carlo simulations of the critical behavior of an Ising model on a stacked triangular lattice with anisotropic exchange coupling are discussed. We consider a system where the inter-plane coupling is ten times stronger than the nearest-neighbor antiferromagnetic in-plane interaction and also discuss the effects of a weaker ferromagnetic next-nearest-neighbor in-plane interaction. This work is motivated by recent experimental results on CsCoBr$_3$ which showed that the critical exponents associated with the upper magnetic phase transition are consistent with tricritical mean-field behavior.$^1$ The Histogram Monte Carlo simulations are carried out using the Cluster Heat Bath method$^2$ which improves the slow relaxation in these systems and finite-size scaling analysis is used to extract estimates for the critical exponents. Our results for the critical exponents are in agreement with previous Monte Carlo work on a similar model with isotropic exchange couplings and are consistent with XY universality. [1] Ming Mao, B. D. Gaulin, R. B. Rogge, and Z. Tun, Phys. Rev. B \textbf{66}, 184432 (2002). [2] F. Matsubara, A. Sato, O. Koseki, and T. Shirakura, Phys. Rev. Lett. \textbf{78}, 3237 (1997). [Preview Abstract] |
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C1.00118: Universal Scaling of the N\'eel Temperature of Near-Quantum-Critical Quasi-Two-Dimensional Heisenberg Antiferromagnets Daoxin Yao, Anders W. Sandvik In many strongly correlated materials, layered structures with weak interlayer couplings can lead to phase transitions, dimensional cross-overs, and related phenomena. We use a quantum Monte Carlo method to calculate the N\'eel temperature ($T_N$) of weakly coupled S=1/2 Heisenberg antiferromagnetic layers consisting of coupled ladders. This system can be tuned to three different two-dimensional scaling regimes for $T>T_N$: renormalized classical, quantum critical and quantum disordered regimes. From our calculations, the N\'eel temperature shows completely different behavior (which is determined by the single-layer phase) when interlayer layer coupling J' is extremely weak. The single-layer mean-field theory of the two-dimensional staggered susceptibility and interlayer coupling J' applies not only in the renormalized classical regime, but also in the quantum critical regime and part of the quantum disordered regime with a coordination number renormalization $k_2 \sim 0.65-0.70$. The product of J' and $\chi(\pi,\pi,0)$ is found to be a constant ($0.23$) in all the regimes. These constants could be very useful to extract the interlayer couplings experimentally. $J'S(\pi,\pi,0)/T_N$ distinguishes in the three regimes when J' is small. This study can be related to the high Tc cuprates with striped phase. (D. X. Yao and A. W. Sandvik, cond-mat/0606341) [Preview Abstract] |
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C1.00119: ABSTRACT HAS BEEN MOVED TO H10.00013 |
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C1.00120: Monte Carlo Simulation of Spin-Crossover Compounds Charles J. Pye, Carsten Timm A Monte Carlo simulation employing cluster updates is used to study a classical model of three-dimensional spin-crossover compounds. These compounds are characterized by magnetic ions that can be in a high-spin or low-spin state. We here consider the case of diamagnetic (S = 0) low-spin state appropriate for Fe(II) compounds. The values of the magnetization average, Binder cumulant, and high-spin/low-spin (HS/LS) fraction are studied over a wide range of values for the system size, temperature, magnetic field, HS/LS energy difference, nearest neighbor HS/LS coupling, and exchange interaction. We also address the phase diagram of the spin-crossover model. [Preview Abstract] |
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C1.00121: The Crystalline Electric Field Effect of Doped Quaternary Intermetallic Compounds W.C. Lee The temperature dependent magnetization data have been measured for the (R$_{x}$,R'$_{1-x})$Ni$_{2}$B$_{2}$C system with the two different applied magnetic fields (H$\parallel $and $\bot $ c-axis) where R and R' stand for the rare earth elements. Especially from the high temperature region of data, the dominant Stevens parameter B$_{2}^{0}$ of crystalline electric field effect were analyzed for the R = Ho and R' = Dy system. The broad minimum determined from the magnetization anisotropy in Stevens parameters was observed around x $\sim $ 0.3 system where T$_{c}$(superconducting critical temperature) and T$_{N}$ (N\"eel temperature) are similar and the deviation from de-Gennes scaling in T$_{c}$ versus de-Gennes factor of rare earth elements was observed. The minimum and change of B$_{2}^{0}$ will be compared and discussed with the crystalline electric field effects and pair breaking effect by some magnetic excitations. [Preview Abstract] |
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C1.00122: Magnetic properties of R$_{2}$TIn$_{3}$ (R= Nd,Ho,Er; T= Cu,Ag) compounds by $\mu$ SR Roger Waeppling, Ola Hartmann, Michael Kalvius, David Noakes, Noelia Marcano, Wolfgang Schaefer, Jiko Yakonthos The ternary R$_{2}$TIn$_{3}$ alloys crystallize in binary type crystal structures where the R ions occupy one site, while T and In ions shares the other site in random occupation. This disorder together with competing antiferromagnetic nearest neighbour and ferromagnetic next-nearest neighbour exchange leads to magnetic frustration. From susceptibility antiferromagnetic order with T$_{N }$in the 10K range was proposed. Subsequent neutron diffraction did not detect magnetic Bragg peaks. The present $\mu $SR data show that below the susceptibility peak temperatures the spin systems freeze into an only short-range ordered structure, long-range order being definitely absent. Persistent spin fluctuations were present in the limit T$\diamondsuit $0, a feature often observed by $\mu $SR in frustrated magnetic materials. [Preview Abstract] |
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C1.00123: Magnetism of frustrated Tb$_{2}$CuIn$_{3 }$by $\mu $SR Michael Kalvius, Roger Waeppling, David Noakes, Ola Hartmann, Noelia Marcano, Wolfgang Schaefer, Jiko Yakinthos The magnetic properties of Tb$_{2}$CuIn$_{3}$ differ markedly form those of other R$_{2}$CuIn$_{3}$ alloys. Neutron scattering data were interpreted in terms of the formation of a spin-glass-like state at 45K which coexists below T$_{N}$= 33K with long-range antiferromagnetic order. The origin of this unusual magnetic behaviour is thought to arise from frustration due to competing antiferromagnetic nearest neighbour and ferromagnetic next-nearest neighbour exchange together with structural disorder. The present $\mu $SR data definitely confirm spin glass magnetism below 45K. At 33K a second $\mu $SR signal appears which is compatible with long-range magnetic order. It shows, however, that the antiferromagnetic state is characterized by strong short range (over a few lattice distances) spin disorder. [Preview Abstract] |
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C1.00124: Magnetic excitation spectrum of the spin ladder system Cu$_{2}$Cl$_{4}$\textbullet D$_{8}$C$_{4}$SO$_{2}$. Vasile Ovidiu Garlea, Andrey Zheludev, Louis-Pierre Regnault, Jae-Ho Chung, Martin Boehm The new triangular spin ladder system, Cu$_{2}$Cl$_{4}$\textbullet H$_{8}$C$_{4}$SO$_{2}$, is remarkable is remarkable by its small ratio of spin gap energy and excitation bandwidth. It consists of infinite double chains of the edge-sharing octahedra CuCl$_{5}$O separated by large H$_{8}$C$_{4}$SO$_{2}$ molecules which ensure a weak enough inter-chain interaction. We report inelastic neutron scattering measurements carried out on deuterated single-crystal samples. Neutron scattering data collected in zero magnetic field revealed the presence of a gap excitation at approximately 0.58 meV with the global energy minimum located at q = (1/2, 0, 1/2). Our measurements show a considerable dispersion of the gap excitation along the chain direction and negligible dispersion perpendicular to it. A model for the ground state is proposed. [Preview Abstract] |
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C1.00125: Spin-Phonon coupling in frustrated magnet CdCr$_2$O$_4$ Rolando Valdes Aguilar, A.B. Sushkov, H.D. Drew, Y-J. Choi, S-W. Cheong Strongly frustrated magnetic materials resist magnetic ordering down to very low temperatures. The existence of order in some of these systems involves the occurrence of a spin-driven lattice distortion. The Cr spinels, with non-Janh-Teller active ions, are excellent examples of such frustrated systems where magneto-elastic coupling causes the ordering of the spins. The distinct distortions in these Cr spinels couple to different spin orders. This is the case in ZnCr$_2$O$_4$ and CdCr$_2$O$_4$, where the unit cell contracts and elongates along the \textit{c} axis, respectively. The contraction in ZnCr$_2$O$_4$ splits a triply degenerate infrared (IR) active phonon\footnote{A.B. Sushkov, et al. PRL \textbf{94} 137202 (2005)}. The magnitude of the splitting and the redistribution of spectral weight gives a sensitive measure of the spin correlations in this system. We report the temperature dependence of the IR active phonons in CdCr$_2$O$_4$. The phonon splitting is similar to ZnCr$_2$O$_4$, but with opposite spectral weight distribution. We discuss these effects in light of recent theoretical models\footnote{G-W.Chern, et al. PRB \textbf{74} 060405 (2006)}. [Preview Abstract] |
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C1.00126: NMR probe of spin gap characteristics in Cu$_{2}$Sc$_{2}$Ge$_{4}$O$_{13}$ C.N. Kuo, T.H. Su, C.S. Lue, G.J. Redhammer We report the results of a $^{45}$Sc nuclear magnetic resonance (NMR) study on the quasi-one-dimensional compound Cu$_{2}$Sc$_{2}$Ge$_{4}$O$_{13}$ at temperatures between 4 K and 300 K. This material has been a subject of current interest due to indications of spin gap behavior. The temperature-dependent NMR shift exhibits a character of low-dimensional magnetism with a negative broad maximum at $T_{max} \quad \sim $ 170 K. Below $T_{max}$, the NMR shifts and spin lattice relaxation rates clearly show activated responses, confirming the existence of spin gap in Cu$_{2}$Sc$_{2}$Ge$_{4}$O$_{13}$. The observed data can be well fit to the spin dimer model, yielding a spin gap value of about 275 K. A detailed analysis indicates that the nearly isolated dimer picture is proper for the understanding of gap feature in this material. [Preview Abstract] |
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C1.00127: Magnetic properties of vanadium oxide nanorods, nanotubes and nanourchins. Natasha Chernova, Chris Jacobs, Megan Roppolo, Kristin Butterworth, Chunmei Ban, M. Stanley Whittingham Vanadium oxide nanotubes (VONTs) were prepared by hydrothermal method and ion exchanged with BuLi. Vanadium oxide nanorods were synthesized by hydrothermal treatment of electrospun precursors. The compounds were characterized by x-ray diffraction, TGA, TEM, FTIR and magnetic susceptibility techniques. Magnetic properties of VONTs are found to be similar to that reported earlier [1]. Upon ion exchange with BuLi the amount of isolated V$^{4+}$ is preserved, but the spin-gap behavior becomes less pronounced. No hysteresis of magnetization is found as opposed to earlier report [1]. Vanadium oxide nanourchins have similar structure with VONTs; however, their magnetic properties are different. Vanadium oxide nanorods with the structure related to $\delta $-V$_{4}$O$_{10}$ show a large spin gap of 560 K. The V$^{4+}$ content is around 0.125 in this compound as follows from the analysis of the spin-gap behavior with the dimer model. [1] L. Krusin-Elbaum, D. M. Newns, H. Zeng, V. Derycke, J. Z. Sun and R. Sandstrom, Nature 431, 627 (2004). [Preview Abstract] |
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C1.00128: Magnetism driven by resonating valence bonds in low dimensional conjugated systems Sefa Dag, Vincent Meunier, William A. Shelton We report the theoretical study of the electronic and magnetic properties of two dimensional metal-$\pi \quad {\rm t}$conjugated complex (XPc) constituted of a transition metal ion and a benzene bridge as $\pi -{\rm t}$conjugation. Our investigations have been carried out by means of electronic structure calculations based on density functional theory within the local density approximation and led to find the many important ground state properties. In particular they result in an accurate description of the two-dimensional electronic structure, the insulating behavior, and the magnetic order in this system. Furthermore we reach the conclusion that band theory is well suited to describe the magnetic structure investigated here, since it possesses localized moments. We discuss the implications of our results on the magnetism of XPc structures from the view point of charge and spin density distributions, shape of the state orbitals, and their occupation numbers. [Preview Abstract] |
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C1.00129: Spin Redistribution by Entanglement in an Organic Magnet Andrey Zheludev, Ovidue Garlea, Sadofumi Nishihara, Yuko Hosokoshi, Arsen Gukasov, Alain Cousson, Katsuya Inoue Polarized neutron measurements of the spin density distribution in the organic S=1/2-tetramer system 2-[2$^\prime$,6$^\prime$-difluoro-4$^\prime$-($N$-\emph{tert}-butyl-$N$-oxyamino)phenyl]-4,4,5,5 -tetramethyl-4,5-dihydro-$1H$-imidazol-1-oxyl reveal a redistribution of spin populations within the magnetic molecular unit. The effect is driven by an exceptionally strong quantum entanglement of four spin wavefunctions. Experimental data are in quantitative agreement with quantum theoretical calculations and provide insight on the actual microscopic origin of the relevant entangling interactions. [Preview Abstract] |
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C1.00130: Oscillatory Exchange Coupling in Magnetic Molecules Haldun Sevincli, R. Tugrul Senger, Engin Durgun, Salim Ciraci First-principles calculations based on the spin-dependent density functional theory (DFT) have revealed that the magnetic ground state of a finite linear carbon chain capped by two transition metal (TM) atoms alternates between ferromagnetic and antiferromagnetic configurations depending on the number of carbon atoms. The character of indirect exchange coupling in this nanoscale, quasi-zero dimensional systems is different from those analogous extended structures consisting of magnetic layers separated by a non-magnetic spacer (or magnetic impurities in a non-magnetic host material) and hence cannot be described as a Fermi surface property of the extended spacer (or the host material). We present a tight-binding model which provides a theoretical framework to the underlying mechanism of the exchange coupling in molecular structures. The model calculations are capable of reproducing the essential features of the DFT results for the indirect exchange coupling and the atomic magnetic moments in the TM-C$_n$-TM structures as functions of the number of carbon atoms. In nanostructures consisting of a few atoms the concepts of extended wavefunctions and the band-theory loose their validity, and hence the oscillatory exchange coupling turns out to be a consequence of quantum interference effects due to the spin-dependent onsite and hopping energies. [Preview Abstract] |
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C1.00131: Inelastic Neutron Scattering Predictions for V$^{4+}$ Molecular Magnets J.T. Haraldsen, T. Barnes The two vanadium compounds (CN$_3$H$_6$)$_4$Na$_2$[H$_4$V$_6$O$_8$(PO$_4$)$_4$((OCH$_2$)$_3$CCH$_2 $OH)$_2$]$\cdot 14$H$_2$O and Na$_6$[H$_4$V$_6$O$_8$(PO$_4$)$_4$((OCH$_2$)$_3$CCH$_2$OH)$_2$]$\cdot 18$H$_2$O are thought to be magnetically well described as trimers of S=1/2 V$^{4+}$ ions, with a Heisenberg magnetic interaction between pairs. They are respectively isosceles (two interactions equal) and general (all interactions different) spin trimers. Inelastic neutron scattering can be used to test the assumed interaction pathways in these materials through comparisons of the measured and predicted INS structure factors. In this theoretical contribution we present results for these INS structure factors, which should be useful for the interpretation of INS experiments planned on these materials. With sufficient energy resolution one may also be able to observe interactions between the trimers, which have several possible superexchange pathways. We also discuss theoretical bulk magnetic properties, and compare them with experimental results in the literature. [Preview Abstract] |
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C1.00132: First-principles study of multiferroic properties of Fe$_3$O$_4$/SrTiO$_3$ and Fe$_3$O$_4$/BaTiO$_3$ [001] superlattices Min Sik Park, A.J. Freeman Multiferroic superlattices are good candidates for the study of the interplay between magnetism and ferroelectricity, and important for multifunctional device applications operating at room temperature. Recently, it was found that the magnetoresistance arising at the Fe$_3$O$_4$/BaTiO$_3$ barrier is strongly bias dependent. \footnote{M. Ziese, A. Bollero, I. Panagiotopoulos, and N. Moutis, Appl. Phys. Lett., {\bf 88}, 212502 (2006).} We calculated the physical properties for multiferroic superlattices of spinel Fe$_3$O$_4$ (as a ferrimagnet) and perovskite SrTiO$_3$, and BaTiO$_3$ (as ferroelectric materials) by using first-principles density functional calculations with the highly precise full-potential linearized augmented plane wave (FLAPW) method. \footnote{Wimmer, Krakauer, Weinert, Freeman, Phys.Rev.B, {\bf 24}, 864 (1981).} At the interface, the half-metallicity of bulk Fe$_3$O$_4$ is destroyed, and magnetic moments that are different from bulk Fe are obtained. The ferroelectric instability of BaTiO$_3$ near the interface is also discussed. [Preview Abstract] |
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C1.00133: Structural and magnetic characterization of Fe-Mo Oxide nanoparticles J.C. Ho, H.H. Hamdeh, H. Al-Ghanem, T.J. Folkerts*, Y.Y. Chen, S.H. Wu, C.B. Tsai, T.H. Lin, S. Neeleshwar** Fe-Mo oxides with different Fe:Mo ratio x (0.5 to 1.5) were synthesized by aerogel process. As microns-diameter spherical shells comprising of nanoscaled needle-like grains, XRD patterns correspond to a beta-FeMoO$_{4}$ structure independent of x. SQUID measurements on two selected samples with x = 1.0 and 1.5, respectively, reveal a broad transition to antiferromagnetism at low temperatures. In addition, a slowly decreasing magnetic susceptibility with decreasing temperature below approximately 100 K for x = 1.0, but not for x = 1.5, presumably reflects a beta- to alpha-FeMoO$_{4}$ phase transition. After being annealed at 500$^{\circ}$C for 2 hr, both samples transform to a Fe$_{2}$(MoO$_{4})_{3}$ structure. The antiferromagnetic ordering becomes well defined at T$_{N}$ = 13 K. A secondary anomaly near 5 K likely arises from spin reorientation in the magnetic frustrated systems. These observations are corroborated by calorimetric and M\"{o}ssbauer measurements. Specific heat data also yield lattice softening and the associated entropy change. \newline *On leave from Barton County Community College, Great Bend, Kansas. \newline **Present address: Indraprastha University, Delhi, India [Preview Abstract] |
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C1.00134: Purely Magnetic Nature of Spin-Reorientation Transitions in Orthoferrites. Ya. B. Bazaliy, L. T. Tsymbal, V. N. Derkachenko, V. I. Kamenev, G. N. Kakazei, F. J. Palomares, P. E. Wigen Magnetic and structural characteristics of ErFeO$_3$, TmFeO$_3$ and YbFeO$_3$ single crystals were studied in the vicinity of the spin-reorientation transitions. In all crystals the behavior of the absolute value of the magnetization $M(T)$ and the rotation angle $\theta(T)$ are incompatible with the conventional Landau mean field theory but can be described with no fitting parameters by the proposed modified mean field theory. X-ray measurements found no symmetry-lowering lattice distortions and suggest a purely magnetic nature of the transition. Successful fitting of experimental data in several materials with a wide range of magnetic parameters demonstrates the generality of the proposed description of $\Gamma_4(G_x, F_z) \to \Gamma_{24}(G_{xz}, F_{xz}) \to \Gamma_2(G_z, F_x)$ orientation phase transitions in orthoferrites. [Preview Abstract] |
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C1.00135: Observation of Strain and Temperature Induced Changes in the Band Structure of Thin La$_{0.8}$MnO$_{3-\delta }$ films T. A. Tyson, Q. Qian, M. DeLeon, C. Dubourdieu , L. Fratila, Y. Cai, K. Ahn Mn K-edge resonant inelastic x-ray scattering measurements were performed on ultrathin and thick films of La$_{0.8}$MnO$_{3-\delta }$,. The measurements reveal that strain causes large shifts of the Mn 3d and Mn 4p/4s bands above the Fermi level. While the thick films track the behavior of bulk samples, the thinnest film is found to exhibit a switch from a localized 3d band at high temperature to a delocalized metal band at low temperature. The strain induced switching behavior opens the possibility of tuning the transition to higher temperatures for device applications in this class of manganite materials. [Preview Abstract] |
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C1.00136: Magnetic phase separation in electron-doped Bi1-xCaxMnO3 systems Yuhai Qin, Trevor Tyson The manganite system Bi1-xCaxMnO3 possesses intriguing properties in the low bismuth doping region. In this electron doped region (0.6$<$x$<$1), a large ferromagnetic (FM) moment of $\sim $1.2 Bohr magnetons per Mn site is found for x$\sim $0.875. The magnetic moment per Mn site maintains a value $\sim $1/3 the theoretical limit even in fields a high as 60 T. The physical origin of this high moment region is not well understood. Various models including canted ferromagnetism and ferromagnetic clusters hosted by an antiferromagnetic background have been proposed. In our previous work, we have conducted small-angle neutron scattering (SANS) on Bi0.125Ca0.875MnO3 polycrystalline samples which revealed the existence of FM clusters embedded in an AFM background. 55Mn NMR results give more evidence supporting of this heterogeneous phase model: resonance signals from both AFM and FM phases were identified. More recent progress from multiple-temperature Bi-L3 edge XAFS and TEM/EDS Bi distribution measurements will be presented as well. This work is supported by NSF DMR-0512196. [Preview Abstract] |
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C1.00137: Electric-field effect on La$_{0.9}$Ba$_{0.1}$MnO$_{3}$ strained films. Cheng-Pang Lin, Y.H. Ho, Hsiung Chou Electric field effect on superconducting films can modulate the carrier concentration of the films and the superconducting transition temperatures. Although the change of the carrier concentration is relatively small at a field of around 10$^{8}$ V/cm, T$_{C}$ can be altered in a range as wide as few degrees. In this study, a La$_{0.9}$Ba$_{0.1}$MnO$_{3}$ strained films with a thickness of 20nm was constructed into similar geometry as earlier reports, various gate voltages, equivalent to 0$\sim $10$^{6}$ V/cm, were applied while measuring resistances as a function of temperature around the metal-insulator transition temperatures. It is found that the T$_{C}$ and the resistance at constant temperature changes sharply at low gate voltage and saturated to a certain value at large gate voltage. [Preview Abstract] |
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C1.00138: Ferromagnetic and reetrant spin-glass state in La$_{0.7}$Ca$_{0.3}$Mn$_{1-y}$V$_{y}$O$_{3}$ J. S. Park, Y. P. Lee, Y. S. Lee, J-H. Kang We have investigated the influence of vanadium (V)-doping on the electronic and the magnetic properties of La$_{0.7}$Ca$_{0.3}$Mn$_{1-y}$V$_ {y}$O$_{3}$ prepared by the conventional solid state reaction. The resistivity upturn below the Curie temperature was observed in the V-doped manganites with 0.1 $\leq $ y $\leq $ 0.2 due to the orbital ordering (OO). The V doping into the Mn sites results in an increase of the ferromagnetic (FM) superexchange interaction favoring the ferromagnetic insulating (FMI) phase. The magnetic relaxation of the explored compounds is found in both reentrant spin glass and high-temperarure FM phases. The heavy V -doping in the La$_{0.7} $Ca$_{0.3}$MnO$_{3}$ compound induces the reentrant behavior due to formation of OO FMI regions. [Preview Abstract] |
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C1.00139: Crystal Growth of Perovskite Mn-Oxides CaMnO$_{3}$ and La$_{0.7}$Ca$_{0.3}$MnO$_3$ Using the Floating-Zone Method Benjamin White, J.A. Souza, T. Huynh, C.A.M. dos Santos, K.J. McClellan, J.J. Neumeier High quality, centimeter-sized single crystals of CaMnO$_{3}$ and La$_{0.7}$Ca$_{0.3}$MnO$_3$ have proven difficult to grow. Crystals of this size could be used to conduct neutron diffraction and other measurements. The goal of this study was to investigate the growth process using the optical floating- zone method in an NEC model SC1-MDH optical furnace. Growth parameters which were varied include the growth rate, rotation rate, and starting composition of the feed and seed rods. Crystal quality was determined through x-ray analysis, optical microscopy, SEM, iodometric titration, and magnetic measurements. These measurements and other general observations will be presented. [Preview Abstract] |
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C1.00140: High Resolution Neutron Diffraction Study of $\rm La_{1-x}Sr_{x}CoO_{3}$ [$0.2 N. Sundaram, I. E. Anderson, D. P. Belanger, K. Kamazawa, D. Louca, Th. Proffen The crystal structure and properties of $\rm La_{1-x}Sr-{x}CoO_3$ ($\rm LaSrCoO$) have been extensively studied in bulk crystals. We have extended the studies to nanoparticles of $\rm LaSrCoO$ in the size range of 10-50 nm, synthesized using a hetero-nuclear complex precursor. Particle sizes were determined from TEM measurements. High-resolution neutron diffraction experiments were carried out on the NPDF diffractometer at the Los Alamos National Laboratory to analyze the local structure above and below $T_c$. Data were collected at $T=12, 100$ and $300$ K. We examined the implication of the particle size on the local and average structures using both routine Rietveld and PDF techniques. The `$r$' range dependent PDF analyses indicate distortions from the average crystal structure. The Jahn-Teller distortions, observed earlier in bulk compounds, are not observed in these nanoparticle systems. Preliminary magnetic measurements show that, although $T_c$ is similar to that of the bulk, the spin-glass-like behavior persists even above the critical composition $x=0.18$, unlike the bulk. $M$ vs $T$ plots indicate inhomogeneity. Based on these initial results, the local structure-property correlations will be discussed for all the compositions studied. [Preview Abstract] |
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C1.00141: Magnetic properties of Na$_{1-x}$Ca$_{x}$Mn$_{2}$O$_{5}$ (x=0, 0.05, 0.1) multiferroic materials Hsiung Chou, S. Y. Huang, C. C. Yu, C. J. Yeh, C. W. Lee Multiferroics in which the ferromagnetism and ferroelectrism coexist has been extensively studied for their potential application in spintronics. Na$_{1-x}$Ca$_{x}$Mn$_{2}$O$_{5}$ (x=0, 0.05, 0.1) is formed by standard sol gel technique and annealed at around 1000$^{o}$C. X-ray diffraction indicates the compound is composed of Na$_{1-x}$Ca$_{x}$Mn$_{2}$O$_{5}$ (115) as the main phase and of Na$_{1-y}$Ca$_{y}$Mn$_{1}$O$_{3}$ (113) as the minor phase with a percentage around 3{\%} to 20{\%}. The FC and ZFC curves in a magnetization measurement split below 75K indicating a very complex domain dynamics in which two FM transitions and one AFM transition are observed. The high temperature FM transition occurs at 82K for the parent compound and is enhanced with the Ca doping; while the low temperature FM transition at 40K is independent of the Ca doping. The AFM transition temperature is suppressed from 18.2K to around 5K with the Ca content. The holes that introduced by Ca doping are localized and partially drive the valence of Mn to 4+ which may enhance the double exchange coupling for forming FM phase, therefore, the AFM transition temperature is suppressed and the FM transition temperature is enhanced. As a result of this, the effective moment decreases with the Ca doping. [Preview Abstract] |
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C1.00142: A Charge Density Wave Transition in Non Superconducting Na$_{0.35}$CoO$_{2}\cdot $1.3H$_{2}$O R.A. Fisher, H. Fu, D.-H. Lee, N. Oeschler, N.E. Phillips, R.J. Cava, M.-L. Foo For most superconducting samples of Na$_{0.35}$CoO$_{2}\cdot $1.3H$_{2}$O the transition to the superconducting state occurs near 4.5 K. For some non-superconducting samples a different transition, near 7 K, shows the existence of another ordering that competes with the electron pairing of the superconducting state. Specific-heat measurements in zero field and 9 T show that the 7-K transition is essentially independent of field, which is suggestive of a CDW transition. The specific-heat anomaly is consistent with a CDW on 1/4 of the Fermi surface and an order parameter with a temperature dependence similar to that of the BCS transition. A theoretical study using a band-structure fit to ARPES data for Na$_{0.3}$CoO$_{2}$ supports the presence of CDW order. Under renormalization group flow an onsite plus nearest-neighbor Hubbard interaction leads to an effective low-energy electron-electron interaction containing scattering processes that favor a CDW with waves of period 3$^{\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} }$a, where a is the lattice constant. A mean-field analysis confirms that this effective low-energy interaction can lead to real-space density modulations with period 3$^{\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} }$a. [Preview Abstract] |
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C1.00143: Orbitons versus Vibronic Excitations Kai P. Schmidt, Markus Grueninger, Goetz S. Uhrig A large number of transition-metal compounds show orbital ordering at low temperatures. But the experimental observation of the corresponding elementary excitations of an orbitally ordered state, the so-called orbital waves (orbitons), turned out to be rather difficult, and a watertight experimental proof for the existence of orbitons is still lacking. It is therefore natural to ask whether other degrees of freedom are involved in transition-metal compounds which complicates the search for orbital waves. Here the coupling of orbital and lattice degrees of freedom is investigated. We calculated the kinetic and the spectral properties of the orbital waves for a one-dimensional toy-model. [Preview Abstract] |
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C1.00144: Ferromagnetic Resonance Studies of Magnetic Anisotropy in Sr$_{2}$FeMoO$_{6}$ Tetiana Nosach, Mark Ebrahim, Steve Greenbaum, Yuhang Ren, Adyam Venimadhav, Qi Li We investigated magnetic anisotropic parameters of a 400-nm ferromagnetic half-metallic Sr$_{2}$FeMoO$_{6}$ thin film by ferromagnetic resonance (FMR). The resonance field and line width were recorded as a function of relative angle between applied magnetic field and crystallographic axes of the sample. The resonance field varies sinusoidally and considerable line width broadening occurs when the field aligned axis is rotated orthogonal to the spectrometer field. This is consistent with the presence of higher order components of anisotropy field. Further, we obtain the values of the cubic and the uniaxial magnetic anisotropy fields from the analysis of the FMR data. [Preview Abstract] |
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C1.00145: Energy of domain walls in ferrite films M.E. Gomez, P. Prieto, A. Mendoza, O. Guzman MnZn Ferrite films were deposited by RF sputtering on (001) single crystal MgO substrates. AFM images show an increment in grain size with the film thickness. Grains with diameter between $\Phi$ $\sim $ 70 and 700 nm have been observed. The coercive field H$_{c}$ as a function of the grain size reaches a maximum value of about 80 Oe for $\Phi_{c} \sim $ 300 nm. The existence of a multidomain structure associated with a critical grain size was identified by Magneto-optical Kerr effect technique (MOKE). The transition of the one-domain regime to the two-domain regime was observed at a critical grain size of $D_{c} \sim $ 530 nm. This value agree with values predicted previously. The Jiles-Atherton model (JAM) was used to discuss the experimental hysteresis loops. The $k$ pinning parameter obtained from JAM shows a maximum value of $k$/$\mu _{o }$= 67 Am$^{2}$ for grains with $L_{c} \sim $ 529 nm. The total energy per unit area $E$ was correlated with $k$ and $D$. We found a simple phenomenological relationship given by $E \quad \alpha $ \textit{kD; }where $D$ is the magnetic domain width. [Preview Abstract] |
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C1.00146: Possibilities of Vector Magnetometer in Studies of Magnetic Macroscopic Quantum Tunneling ( MQT ) Alexander Bazhan Magnetization and thermoremanent magnetization processes of samples of randomly oriented magnetic nano-particles with superparamagnetic properties and thermally activated relaxation in high temperatures are of interest in studies of magnetic MQT at low temperatures in nanoscale technologies. Thermoremanent magnetization processes are of interest as deviations from thermal relaxation regime are easily detected by studies of magnetic viscosity. Variation of magnetization with time at magnetic field cutting off after samples cooling are studied in experiments. Vector magnetometer presents possibility to study the decay of magnetic field dependencies and the decay of time dependencies in magnetic fields of perpendicular to magnetic field sample magnetic moment components after sample cooling and ninety-degrees rotation with respect to magnetic field. Comparison of time dependencies of magnetic moments components of samples of randomly oriented magnetic present information about the same physical parameters, including MQT, which determine relaxation processes of magnetic nano-particles. [Preview Abstract] |
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C1.00147: Dichroism Soft X-ray Absorption Spectromicroscopy and Antiferromagnetic Surfaces and Interfaces Hendrik Ohldag, Joachim St{\"o}hr, Andreas Scholl Synchrotron based dichroism x-ray absorption spectro-microscopy (dichroism XAS) is an excellent tool for the investigation of magnetic heterostructures, because of its ability to address antiferromagnetic (AF), ferromagnetic (FM), chemical and structural order of different elements in an unknown sample. Even more important, dichroism XAS can be used as a contrast mechanism in a photoemission electron microscope (PEEM) to characterize surfaces, buried interfaces and nanostructures with high spatial resolution ($\sim$50nm) and temporal resolution (100ps). In this paper we present results of our research focusing on AF/FM exchange coupling In this area dichroism XAS has helped to improve our insight tremendously over recent years because conventional magnetic imaging techniques are not able to address the AF order which is a key ingredient in these systems. For the first time we were able to correlate the AF, FM, crystallographic and interfacial domains. Furthermore we could identify and analyze thestructure of lateral AF domain walls and extract quantitative information about the relevant magnetic anisotropy energies in NiO. [Preview Abstract] |
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C1.00148: Exchange Interaction Contribution to Magnetoelectric Effect in Ferrite-Piezoelectric Bilayer at Magnetoacoustic Resonance M.I. Bichurin, O.V. Rjabkov, S.V. Averkin, V.M. Petrov, G. Srinivasan The report is on the effects of exchange interactions on magnetoelectric coupling at the coincidence of electromechanical resonance and ferromagnetic resonance in a bilayer of ferrite and piezoelectric thin film. A tangentially magnetized ferrite film is considered. Giant magnetoelectric voltage coefficient on the order of 75 -- 100~V/cm Oe is predicted for yttrium-iron garnet -- lead zirconate-titanate bilayers at 5~GHz. The influence of exchange interactions is stronger with frequency and manifests as increase in the peak value of magnetoelectric coefficient. The phenomenon is of importance for the synthesis of novel bilayers for multifunctional microwave devices (sensors, transducers) based on magnetoelectric effect. The research was supported by grants from the NSF (DMR-0606153; NIRT-0609377; ECCS-0621907). [Preview Abstract] |
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C1.00149: Magnetoelectric Interactions in Ferromagnetic-Piezoelectric Layered Nanostructures V.M. Petrov, G. Srinivasan, M.I. Bichurin, D. Viehland, Ce-Wen Nan A theoretical model is presented for low frequency magnetoelectric (ME) coupling in ferrite-piezoelectric nanobilayers of ferrite and piezoelectrics. The nanocomposite is considered as a homogeneous medium with piezoelectric and magnetostrictive subsystems and we take into account clamping effect of the substrate in determining the ME voltage coefficient. The model could be used to estimate the ME couplings from known material parameters (piezoelectric modules, magnetostriction, stiffness, geometrics). Expressions for ME coefficient are obtained using the solution of the elastostatic and electrostatic equations. The effect of material parameters on the ME susceptibility is studied. Increasing the substrate thickness results in a decrease in the ME interaction due to the clamping caused by the substrate. With increasing substrate thickness the theory also predicts a shift in the volume fraction of PZT that corresponds to peak ME coefficient. The research was supported by grants from the NSF (DMR-0606153; NIRT-0609377; ECCS-0621907). [Preview Abstract] |
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C1.00150: Magnetic Properties of Fe-Co Catalysts for Carbon Nanofiber Synthesis O. Malkina, C. Finkel, K. D. Sorge, T. Leventouri, J. D. Fowlkes, P. D. Rack, K. L. Klein, A. V. Melechko, M. L. Simpson The magnetic properties of Fe-Co alloys used as catalysts for vertically-aligned carbon nanofiber (VACNF) growth are presented. 10 nm thick layers of Fe$_x$Co$_{100-x}$ of varying composition ($10 \le x \le 75$) are deposited on Si wafers by a co-sputtering technique. VACNFs are then grown by Plasma-Enhanced Chemical Vapor Deposition (PECVD) in an atmosphere of NH$_3$ and C$_2$H$_2$ at a temperature of $\sim 570^{\circ}$C. The catalyst particles on the tips of the VACNFs are 30--80 nm in size after growth. The magnetic properties are investigated at various stages of the fabrication process by SQUID magnetometry in a field range of $|H|\le 10$ kOe and temperatures of $T = 5$--300 K. The particles are ferromagnetic with moderate coercivity and remanence. The magnetization, however, is lower than expected at each stage. [Preview Abstract] |
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C1.00151: Ferromagnetic Resonance Studies in Cobalt Nanodot Arrays Mark Ebrahim, Tetiana Nosach, Steve Greenbaum, Yuhang Ren, Chae-Hyun Kim, Hao Zeng We used ferromagnetic resonance technique to investigate magnetic properties of Cobalt nanodot arrays. The resonance field and the line width were measured with varying the angle between the applied field and film plane. The general behavior of the applied field vs. resonance frequency is described using an effective out-of-plane magnetic anisotropy, possibly originating from the surface anisotropy. The anisotropy field inferred from our experiments is H$_{A}\sim $ 1 KOe. Considerable line width broadening occurs when the field aligned axis is rotated orthogonal to the spectrometer field. This corroborates the presence of higher order components of anisotropy field. Furthermore, we obtain gyromagnetic factor g = 2.1, which is consistent with the value determined for the fcc Co grains. [Preview Abstract] |
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C1.00152: Properties of hexagonally ordered CoFe -- alloy nanowire arrays Petru Fodor, Georgy Tsoi, Lowell Wenger The structural and magnetic properties of an array of micron-long Co$_{0.45}$Fe$_{0.55}$ alloy nanowires with diameters ranging from 12 to 52 nm electrodeposited in porous anodic alumina templates have been studied using X-ray diffractometry, scanning electron microscopy, and magnetization measurements. The nanowires are found to crystallize in a body-centered-cubic (bcc) structure along the (110) axis for all diameters studied. The magnetization curves indicate a highly anisotropic behavior with the easy axis along the nanowire axis and a coercivity of 3500 Oe at room temperature. Results from the magnetization hysteresis and magnetic relaxation measurements suggest that the magnetization reversal takes place through localized nucleation in volumes smaller than the physical volume of the nanowires. The localization of the nucleation combined with the cooperative effects arising from magnetostatic interactions between the nanowires, limits the coercivity of the nanowire arrays. [Preview Abstract] |
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C1.00153: Orientational Distributions and Nematic Order of Rodlike Iron Nanoparticles in Magnetic Inks Vemuru Krishamurthy, Gary Mankey, Bin He, Maihua Piao, John Wiest, Dave Nikles, Lionel Porcar, Lee Robertson The suitability of nanoparticles for magnetic recording applications depends on the control of the orientational order in an external force field, such as a shear flow or a magnetic field. Using small angle neutron scattering (SANS), we have investigated the orientational order of iron nanoparticles dispersed in cyclohexanone. The particles are rod shaped and polydispersed with an average length of 200 nm, an average diameter of 25 nm. SANS shows an anisotropy, which is a measure of orientational order, for 3.2 vol. $\%$ and 3.9 vol. $\%$ iron particle dispersions at shear rates of 0-4000 s$^{-1}$ and/or in a magnetic field of 0-180 Oe. The scattering anisotropy could be fitted by a model considering Onsager distribution of the orientation of the particles. The orientational distribution parameter $\alpha$ and orientational order parameter {\it S} indicate that the particles start to orient either in a shear flow of 100 s$^{-1}$ or in a magnetic field of 20 Oe. The orientational structure of the dispersion is reversible in shear flow, but irreversible in the magnetic field. [Preview Abstract] |
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C1.00154: Dumbbell-like Composite Nanoparticles: Chemical Synthesis and Catalytic Applications Chao Wang, Shouheng Sun Dumbbell-like NM-Fe$_3$O$_4$ (NM=Au, Ag, Pt and Pd) nanoparticles are synthesized by epitaxial growth of Fe$_3$O$_4$ on noble metal nanoparticle seeds in organic solution. The size of the noble metal and Fe$_3$O$_4$ particles in these dumbbell structures can be well controlled (from 2 nm to 20 nm). These dumbbell nanoparticles are superparamagnetic at room temperature, and also show plasmonic absorption (from Au or Ag). Compared to the single-component noble metal nanoparticles, the dumbbell particles show enhanced catalytic properties, like Pt-Fe$_3$O$_4$ for oxygen reduction and methanol oxidation in the fuel cell, as well as Au-Fe$_3$O$_4$ for the growth of silicon nanowires. [Preview Abstract] |
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C1.00155: Relaxation and Hyperthermia Investigation on Magnetic Nanoparticle suspensions V. Bubulac, P.P. Vaishnava, R. Tackett, C. Sudakar, R. Naik, G. Lawes We have examined the N\'{e}el and Brownian relaxation mechanisms and hyperthermia characteristics of aqueous suspension of $\gamma $- Fe$_2$O$_3$ and Fe$_3$O$_4$:Co$_{x}$ ( 0 $\le $ x $\le $ 0.15) nanoparticles by ac and dc magnetization measurements and specific absorption ratio (SAR) values. The structural properties were investigated by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). The ac susceptibility measurement showed dissipation peaks associated with N\'{e}el relaxation in $\gamma $- Fe$_2$O$_{3}$ and Fe$_3$O$_4$ samples but only the Fe$_3$O$_{4}$ sample showed a significant Brownian relaxation peak near the melting temperature of the carrier fluid. The specific absorption rate (SAR) value for the Fe$_{3}$O$_{4}$ sample is five times larger than that of the $\gamma $- Fe$_{2}$O$_{3}$ sample, which we attribute to reduced steric hindrance to rotation. Changes in the structural, magnetocrystalline, and SAR values on incorporating Co ions in Fe$_{3}$O$_{4}$ will be presented. [Preview Abstract] |
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C1.00156: Synthesis and characterization of Fe/Au nanoparticles. Shahid Ahmad, Saleem Rao, Donny Magana, Geoffrey Strouse, Shahid Shaheen Magnetic nanoparticles are the materials of interest for their fundamental properties, such as single domain state, quantization of spin waves, and coercivity enhancement, and for their potential applications in magnetic recording, sensing, and biological diagnosis and treatment. The core/shell structured nanoparticles, with magnetic materials as core and gold as shell, are of special interest, because the gold shell protects the oxidization of magnetic core and is biocompatible and easy to functionalize. Here we report the synthesis and characterization of Fe/Au nanoparticles with different atomic concentrations of Fe and Au. The nanoparticles were annealed under the flow of nitrogen gas and their magnetic behavior was characterized as a function of time and annealing temperature. A large change in blocking temperature (18K to 210K) was observed on gradual annealing up to 400$^{0}$C. The XPS data shows the presence of Au at the surface and no indication of iron or iron oxide. Also TEM images do not show any agglomeration of particles and the particle size remains unchanged on annealing. [Preview Abstract] |
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C1.00157: Spin structures and interactions of geometrically - confined head-to-head domain walls M. Klaui, M. Laufenberg, D. Bedau, L. Heyne, D. Backes, F. Junginger, H. Ehrke, S. Cherifi, Andrea Locatelli, T. Kasama, R. Dunin-Borkowski, F. Nolting, L. Heyderman, U. Rudiger Using photoemission electron microscopy (XMCD-PEEM) and electron holography we have obtained high-resolution images of the spin structure of the domain walls, which allows us to determine the wall type and the wall width for different Co [1] and NiFe [2] wire and ring geometries. We determine the phase transition between the different domain wall types as a function of the geometrical parameters (width, thickness) [1,2]. Comparison with theoretical calculations [3] and micromagnetic simulations reveals the importance of local energy minima. The geometry-dependent height of the energy barriers separating the two wall types is derived. The energy barrier height is then directly measured by high-temperature (up to 600 K) imaging of thermally activated transitions from transverse to vortex walls [2]. By varying the spacing between domain walls, we determine the coupling strength that leads to a shift in the phase boundary [4]. \newline [1] M. Klaui et al., APL 85, 5637 (2004); [2] M. Laufenberg et al., APL 88, 52507 (2006); [3] R.D. McMichael and M. J. Donahue, IEEE Trans. Magn. 33, 4167 (1997); [4] M. Laufenberg et al., APL 88, 212510 (2006). \newline [Preview Abstract] |
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C1.00158: Enhance in the magneto-optical properties of magnetic grating JinBae Kim, Geon Joon Lee, YoungPak Lee, Joo Yull Rhee, Chong Seung Yoon The magneto-optical properties of one-dimensional magnetic grating structure of Co$_{2}$MnSi film were investigated. By using the interference pattern of two femtosecond-laser beams, a selective-area annealing of the as-deposited Co$_{2}$MnSi film was performed and one-dimensional magnetic grating structures were fabricated. The atomic-force-microscopy image showed regularly-spaced alternating lines with a periodicity of 2\textit{ $\mu $}m. The corresponding magnetic-force-microscopy image also revealed periodic patterns of two different magnetic states in the sample. The longitudinal Kerr rotations of the zeroth- and the first-order diffracted beams were measured. The longitudinal Kerr rotation of the first-order diffracted beam is nearly 18 times larger than that of the zeroth-order beam. [Preview Abstract] |
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C1.00159: Electric Conductivity of Isolated Hollow Sphere Toshifumi Terui, Takashi Nagase, Hiroyuki Hasegawa, Rieko Ueda, Shinro Mashiko, Yasuharu Koduka, Quan-Lin Ye, Hirofumi Yoshikawa, Mototaka Onishi, Kunio Awaga In magnetic materials of nano-meter size, peculiar magnetic structures and characteristics are expected. The hollow sphere is an interesting material in such nano magnetic materials because of the unique structure. The diameter and the thickness of shell of the hollow sphere can be accurately controlled between 100-500nm and 40nm respectively. It is necessary to examine the physical properties about isolated hollow sphere of ferromagnet to apply this material as nano-spin material. Therefore, we combined the top down and the bottom up technique to isolate the hollow sphere on the surface. For example, the nano-gap electrodes were fabricated by EB lithography and FIB. Manipulation of the hollow sphere were also performed by SEM with probe. [Preview Abstract] |
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C1.00160: Exchange and Magnetic Anisotropic Interactions Alexander Bazhan Studies of exchange and magnetic anisotropic interactions, which determine antiferromagnetic (AF) orderings of magnetic ions particularly in used in HTS tetragonal Mott insulators, by studies of magnetic field behaviour of separate components of sample magnetic moments are discussed. Antiferromagnetic orderings are described by phenomenological theory of magnetic symmetry with representations of interacting magnetic moments by antiferromagnetic and magnetic vectors. Magnetic field behaviour of AF orderings in such representations are discussed with introduction of model which connect transformations of absolute values of AF vectors on magnetic field with transformations of magnetic susceptibilities along AF vectors, expressions of which are presented in equations of magnetic states. Advantages of vector magnetometer are discussed in studies of interactions, which determine AF orderings and weak ferromagnetism in AF materials and in studies of AF orderings of Cu moments in Mott insulators, which are of interest in studies of transformations of AF orderings, when correlated electron-hole carriers are introduced. [Preview Abstract] |
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C1.00161: Selection rules in magnetic superlattices and efficient spin inversion devices Jose Luis Cardoso, Pedro Pereyra Analysing spin transport of quasi-2D electrons gas moving through a semiconductor wave guide subject to a sectionally homogeneous tilted magnetic field, we found well-defined selection rules for resonant and antiresonant transmission. Based on this selection rules and the band shift induced by the magnetic field strength and tilting angles, we propose an efficient spin inversion device. For polarised incoming electron beams, we can determine from our theoretical approach, physical conditions (field strength and tilting angle) for spin inversion efficiency up to $80 \%$. We visualise this mechanism in terms of conductance and the wave function amplitude and phase bahavior along the superlattice. We discuss several examples with different geometrical parameters and magnetic strength values. [Preview Abstract] |
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C1.00162: Thin-Film Magnetic Moment Measurements Using Anisotropic Magnetoresistance Fabio da Silva, Sean Halloran, Anthony Kos, Willard Uhlig, John Unguris, David Pappas The Anisotropic Magnetoresistance (AMR) effect is used to estimate the magnetic moment of thin-film Permalloy samples. The method uses the angular dependence of the AMR to measure the shape anisotropy field of high aspect ratio rectangular bars. The measurement is directly proportional to the saturation magnetization and the thickness of the sample via the demagnetizing factor. Relative uncertainties of 0.1 \% of magnetic moments less than 10$^{11}$ A$\cdot$m$^{2}$ can be obtained. These uncertainties are mainly due to the sample geometry and its influence on the domain structure, and the magnetic field uncertaities. AMR results are supported by scanning electron microscopy with polarization analysis, conventional magnetometry measurements, and micromagnetic simulations. [Preview Abstract] |
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C1.00163: A theoretical study of volume-dependent exchange interactions in MnAs Biplab Sanyal, Camille Aron, Olle Eriksson We have studied magnetic interactions in zinc-blende MnAs as a function of volume by {\it ab-initio} density functional calculations. The calculated Heisenberg pair-exchange parameters show a strong volume dependence. The calculated Curie temperatures, obtained from by Monte-Carlo simulations using {\it ab-initio} parameters and a classical Heisenberg Hamiltonian, show a decreasing trend upon compression. Analysis of cross-sections of Fermi surfaces indicate a nesting feature for compressed volumes, which can give rise to competing exchange interactions. Calculations of self-consistent noncollinear spin configurations indicate that spins deviate strongly from collinear ordering for low volumes whereas they align in a collinear ferromagnetic fashion for high volumes. Also, for expanded volumes, the calculated Curie temperature is around 600 K signifying the possibility of using MnAs for future room-temperature spintronic applications. [Preview Abstract] |
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C1.00164: Efficient spin injection from a ferromagent into a semicondutor V. Karpan, M. Zwierzycki, P.J. Kelly It has been proposed that the spin-dependence of the interface resistance between (001) bcc Fe and zinc blende semiconductors such as GaAs, InAs or ZnSe can be so large that direct spin injection should be possible. This would not contradict the conductivity mismatch analysis in which spin-dependent interface resistance is neglected. In PRB {\bf{67}}, 092401 (2003) however, it was pointed out that even a modest amount of interface disorder would be sufficient to destroy the spin-dependence. We circumvent the problem entirely by inserting a buffer layer (BL) between Fe and InAs to prevent Fe minority-spin states coupling to the semiconductor directly while still preserving the transmission spin-polarization. We identify a very promising candidate BL and demonstrate by explicit calculation that it preserves the transmission polarization of the ideal epitaxial structure remarkably well. Disorder at the Fe|BL or at the BL|InAs interface are shown to have small effect on the large transmission polarization. The same BL is expected to work equally well for Fe|GaAs and Fe|MgO interfaces. [Preview Abstract] |
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C1.00165: Theory of ultrafast demagnetization in transition metals - the role of single-particle and collective excitations Lukasz Cywinski, L.J. Sham An optical excitation with a short pulse causes the magnetization of an itinerant ferromagnet (e.g. Fe,Ni,Co) to decrease by at least a couple percent during one picosecond [1]. A consistent microscopic theory of such an ultrafast demagnetization process is still missing. We introduce a non- equilibrium generalization of the Stoner model, within which we calculate the demagnetization due to Eliot-Yafet spin relaxation of single-particle excitations (recently proposed as a mechanism of demagnetization in [2]). We show that in order to account for the observed magnitude of demagnetization, the scattering of optically excited electrons with collective excitations (spin waves) has to be included. 1. E. Beaurepaire et. al., Phys. Rev. Lett. {\bf 76}, 4250 (1996) 2. B. Koopmans et. al., Phys. Rev. Lett. {\bf 95}, 267207 (2005) [Preview Abstract] |
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C1.00166: Understanding laser-induced ultrafast demagnetization in ferromagnets: First-principles and two-level model investigation G.P. Zhang, T.F. George, T. Anderson, W. H\"ubner Ultrafast demagnetization in ferromagnets attracts lots of attention both experimentally [1] and theoretically [2]. However, up to now, there is no clear understanding whether the observed signal represents a magnetization, while the experimental results are very controversial. In this study, a two-level model is used to simulate the demagnetization process. All the transition matrix elements are computed using the Wien2K code and from bulk nickel. The pump and probe signals and the magnetization for this two level system are computed directly. This should provide insight into the laser-induced ultrafast demagnetization process. [1] E. Beaurepaire {\it et al}, PRL {\bf 76}, 4250 (1996); B. Koopmans {\it et al}, PRL {\bf 85}, 844 (2000); L. H. Andrade {\it et al}, PRL. {\bf 97}, 127401 (2006). [2] G. P. Zhang and H\"ubner, PRL {\bf 85}, 3025 (2000); R. Gomez-Abal, O. Ney, K. Satitkovitchai, and W. H\"ubner, PRL {\bf 92}, 227402 (2004). [Preview Abstract] |
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C1.00167: Electrically controllable spin filtering and switching in multiferroic tunnel junctions Sheng Ju, Tian-Yi Cai, Guang-Yu Guo, Zhen-Ya Li We propose an electrically controllable spin filter based on multiferroic tunnel junction. This novel spin filter combines the exchange splitting of ferromagnets and asymmetry in energy potential due to the screening of ferroelectric polarization charges at electrodes. Transfer matrix calculations show an enhanced spin filtering efficiency, depending on the magnitude and orientation of ferroelectric polarization. A transition from a positive tunneling magnetoresistance to a negative one is also found. Furthermore, an electric controllable switching between multiple resistive states via magnetoelectric coupling is also described, which will open a new logic programming in the future spintronics. [Preview Abstract] |
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C1.00168: ABSTRACT HAS BEEN MOVED TO W12.00015 |
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C1.00169: In-situ Structure and Transport Correlations in Magnetic Tunnel Junctions Ann Chiaramonti, Amanda Petford-Long, Bernd Kabius, Jon Hiller, William Egelhoff, Mark Scheeff The final transport properties of magnetic tunnel junctions are dictated by their underlying magnetic domain and microstructure. As such, atomic-level characterizations combined with local, site-specific transport measurements are essential to truly understand their fundamental behavior. In this poster, we present local, nanoscale, site-specific transport measurements of magnetic tunnel junctions with MgO and Al$_2$O$_3$ tunnel barriers and FeCo-based ferromagnetic layers made using a novel $\emph{in-situ}$ TEM nanobiasing holder. When combined with the simultaneous nanoscale microstructure and interfacial characterization available in the TEM, tunneling measurements from a fully described nanoscale region of the specimen can be obtained. The resulting I-V curves can be fit to the Simmons model [J.G. Simmons, J. App. Phys. $\bf{34}$, 6 (1963)], allowing structure-transport relationships to be directly studied for a variety of tunnel junction materials and compositions. [Preview Abstract] |
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C1.00170: Persistent spin current in mesoscopic ferrimagnetic spin ring Ming-Che Chang, Jing-Nuo Wu, Min-Fong Yang Using a semiclassical approach, we study the persistent magnetization current of a mesoscopic ferrimagnetic ring in a nonuniform magnetic field. At zero temperature, there exists persistent spin current because of the quantum fluctuation of magnons, similar to the case of an antiferromagnetic spin ring. At low temperature, the current shows activation behavior because of the field-induced gap. At higher temperature, the magnitude of the spin current is proportional to temperature $T$, similar to the reported result of a ferromagnetic spin ring. [Preview Abstract] |
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C1.00171: Spin polarization in Co-Pt alloys J. Pulikkotil, V. Antropov, M. Faiz, R. Panguluri, B. Nadgorny, C. Kaiser, S. Parkin The degree of spin polarization in the system of disordered Co-Pt alloys has been studied using density functional approach. The electronic structure of several ordered intermetallics have been analyzed in details. Our analysis is focussed on the difference between magnetization and the degree of spin polarization as a function of Pt concentration, measured by spin tunneling spectroscopy{[}1{]} and Andreev reflection spectroscopy{[}2{]}. Several factors influencing the deviation of these quantities from a linear behavior have been identified. We attempt to explain the dependence of spin polarization on magnetization observed experimentally by both techniques. We also discuss the effect of different tunnel barriers observed in Ref.{[}1{]}. In general, experimental tendencies have been confirmed using $ab$-$intio$ methods, and we consider the possible origin of spin polarization in these alloys. \par \noindent {[}1{]} C. Kaiser, S. van Dijken, S.-H. Yang, H. Yang, and S. S. P. Parkin, Phys. Rev. Lett. 94, 247203 (2005) \par \noindent {[}2{]} R. P. Panguluri $et$ $al$, unpublished [Preview Abstract] |
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C1.00172: Carrier properties of hydrogen-implanted ZnO and Mn-doped ZnO films S.Y. Park, P.J. Kim, H.W. Lee, Y.P. Lee, J.-H. Kang, T.H. Kim Recently, many research groups have been interested in the carrier properties of transition metal (TM)-doped ZnO as well as undoped ZnO due to the application for spintronics and optoelectronics. Especially, the magnetic properties of TM-doped ZnO (TM: Mn, Fe, and Co) are theoretically expected to be connected with the characteristics of charge carrier characteristics which are formed by hydrogen contamination. In this study, we present the effects of hydrogen implantation on the magnetic properties for Mn-doped ZnO and undoped ZnO film. All of the films were grown by using UHV magnetron co-sputtering. These films were implanted with 30 keV H- ions at a dose of 6.9x10$^{16}$ cm$^{-2}$ at room-temperature. The lattice parameters of the implanted samples were increased owing to the interstitial H and the ion-induced crystalline damages. The time-of-flight secondary-ion mass-spectroscopy results show that H was implanted uniformly with keeping the original crystalline structures. Their magnetic properties were determined by using a SQUID. The $M-H$ curves reveal that the H-implanted film with post-annealing as well as the as-grown film exhibit the ferromagnetic behavior at room temperature with a $T$c above 350 K, while the H-implanted film prepared at a higher oxygen pressure shows the nonmagnetic behaviors. The detailed analysis of the magnetic properties for implanted Mn-doped ZnO film will be discussed in comparison with the as-grown samples. [Preview Abstract] |
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C1.00173: The Chiral Anomalous Hall Effect in re-entrant \underline {Au}Fe alloys. Frederik Fabris, Vivien Zapf, Alex Lacerda, Paulo Pureur, Jacob Schaf, Valdemar Vieira, Ian Campbell The Hall Effect has been studied in a series of \underline {Au}Fe samples in the re-entrant concentration range, as well as in part of the spin glass range. The Hall coefficient has been measured in function of temperature and applied magnetic field. The present experimental data demonstrate that is indeed a negative contribution in addition to the canonical Karplus-Luttinger term. An anomalous Hall contribution linked to the tilting of the local spins can be identified, confirming theoretical predictions of a novel topological Hall term induced when chirality is present. This effect can be understood in terms of Aharonov-Bohm-like intrinsic current loops arising from successive scatterings by canted local spins. The experimental measurements indicate that the chiral signal persists, meaning scattering within the nanoscopic loops remains coherent, up to temperatures of the order of 150 K. [Preview Abstract] |
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C1.00174: Magnetostriction and effect of stress on hysteresis and anhysteretic magnetization of multilayered FeNi-Fe heterostructures Peter Finkel, Ed Garrity We report on the low-field magnetic properties of thin film FeNi-Fe multilayered samples under tensile stress. Anhysterretic magnetization as a function of stresses was measured using a conventional vibrating sample magnetometer combined with a specially designed loading fixture providing controlled uniaxial stresses. Stresses are deduced from the characteristic resonant frequency of the sample in the fixture. Anhysteretic permeability was extracted from the anhysteretic $B-H$ curves constructed by degaussing the sample at given longitudinal dc field. The magnetostriction contribution to dc magnetization under elastic stress and the effect of the plastic strain on the hysteresis loops were measured. The large positive magnetostriction of FeNi layer is found to compensate negative magnetostiction of the Fe layer. This leads to higher susceptibility and lower coercivity for low tensile stress. The drop in coercivity was particularly sensitive to film stress/strain. Larger stresses result in plastic strain of the sample which induces an increase in dislocation density and subsequently domain wall pinning. This causes an increase in coercivity and decrease in anhysteretic permeability at the highest stresses. The paper summarizes these effects. [Preview Abstract] |
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C1.00175: Atomistic modeling of surface alloy formation for Fe deposited onto flat and stepped Pt substrates Cary Pint, Guillermo Bozzolo, Jorge Garces Fe-Pt alloys are of significant importance toward applications of high-density magnetic recording media. In this work, we apply the BFS method for alloys to study the energetic pathway for subsurface Fe-Pt alloy formation upon deposition of Fe atoms on (flat) Pt(100), Pt(111), and (stepped) vicinal Pt(997) substrates. We observe the preference for Fe atoms to decorate Pt step edges upon Fe deposition on Pt(997), allowing the formation of an ordered Fe-Pt surface alloy at the edges of the monatomic surface steps with increasing thermal energy. Compared to the formation of a surface alloy from deposition on a low-index Pt surface, we observe that step decoration of the Fe atoms on Pt(997) reduces the strain energy required for Fe surface penetration, resulting in a lower energy barrier associated with the formation of an Fe-Pt subsurface alloy. The results are found to be in agreement with experiment, and the formation of the Fe-Pt subsurface alloy in each case is explained by a simple analysis emerging from the competition between BFS strain and chemical energy contributions. [Preview Abstract] |
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C1.00176: Breaking Of Ergodicity In A Canonical Spin System Bruno Goncalves, Fausto Borgonovi, G. Luca Celardo Breaking of ergodicity occurs when the phase space of an anisotropic spin system is disconnected. The related 11phase'' separation produces a power-law divergence of the recurrence time within one disconnected portion (demagnetization time) at some critical energy. Such average time can be also evaluated for a system in contact with a thermal bath and found to diverge at low temperature even for few particle systems. [Preview Abstract] |
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C1.00177: ABSTRACT WITHDRAWN |
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C1.00178: Torque Magnetometry and Thermomagnetic Capacity Studies on a 2-d Cr$^{4+}$ Antiferromagnet Narpinder Kaur, Saritha Nellutla, Youn-jung Jo, Luis Balicas, Johan van Tol, Naresh Dalal We report torque magnetometry and magnetic heat capacity measurements on a rare complex, Cr$^{IV}$-Diethylenetriamine diperoxo. The motivation here was to search for a simple spin-gap system that could exhibit a Bose-Einstein type condensation (BEC) of magnons. Our earlier reported magnetization and specific heat ($C_{p})$ measurements had indicated that this compound is a 2-d antiferromagnet, with a $T_{N}$ of 2.55 K in zero-field [1]. These magnetization and $C_{p}$ data have now been augmented by use of additional magnetic fields, and the newly found B-$T$ phase diagram is seen to be clearly parabolic. Torque magnetometry confirmed the $C_{p}$ data and has enabled measurements close to the $T \quad \to $ 0 K, B $\sim $ 12.5 T region. Measurements in the dilution fridge are planned to extract the critical exponent ($\alpha )$ from the relation $k_{b}T_{c} \quad \sim $ (B$_{c}$-B)$^{\alpha }$ . We surmise that this system will constitute a simple new model for examining the BEC of magnons in detail. [1] C.M. Ramsey, B. Cage, P. Nguyen, K.A. Abboud, N.S. Dalal, Chem. Mater. \textbf{15}, 92 (2003). [Preview Abstract] |
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C1.00179: Structural and Magnetic Properties of Gd$_{3}$N@C$_{80}$ Jing Lu, Renat Sabirianov, W.N. Mei, Yi Gao, X.C. Zeng, Chungang Duan Using relativistic and on-site correlation corrected density functional theory, we have investigated the structural and magnetic properties of recently synthesized Gd$_{3}$N@C$_{80}$. The most stable structure of Gd$_{3}$N@C$_{80}$ has the three magnetic Gd ions pointing to the centers of hexagons in C$_{80}$. The magnetic ground state of this structure has the three coplanar spins ($S$ = 7/2) offset by 120$^{o}$ angles. At the same time, the state with the highest multiplicity, where all the spins are parallel aligned, is found only about 4.5 meV higher in energy. Therefore, at room temperature, we expect Gd$_{3}$N@C$_{80}$ to be paramagnetic with spin fluctuating between different multiplicities. [Preview Abstract] |
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C1.00180: SUPERCONDUCTIVITY |
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C1.00181: Using Inhomogeneity to Raise Superconducting Critical Temperatures Yen Lee Loh, Erica W. Carlson There has been a surge of interest in the physics of inhomogeneous superconductors due to recent experiments that have detected strong local inhomogeneity in the high-temperature cuprate superconductors. An important question is whether local inhomogeneity helps or harms superconductivity, or if it is irrelevant. We present a study of inhomogeneous 2D XY models, which describe superconducting films with low superfluid density. Using Monte Carlo simulation and finite-size scaling, we show that \emph{certain} types of inhomogeneity (``frameworks'') can significantly increase the transition temperature while preserving the zero-temperature long-wavelength properties. We support our conclusions with corresponding analytic results for Ising models. [Preview Abstract] |
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C1.00182: Structural and electrical properties of $Bi_{2-x}M_xIr_2O_7$ ($M$= $Na^+$, $Ca^{2+}$ and $Ce^{4+}$ ) pyrochlores Carlos Cosio-Casta\~neda, Oliver Mart\'{\i}nez-Anaya, Gustavo Taviz\'on, Pablo de la Mora In $\alpha$-pyrochlores, $A_2B_2O_7$, it is possible to appreciate different magnetic couplings depending on A and B spin-arrangement. In order to investigate how these magnetic and electrical properties are related with the iridium oxidation-state, we investigate the $Bi_{2-x}M_xIr_2O_7$ system ($M$ = $Na^+$, $Ca^{2+}$ and $Ce^{4+}$). Polycrystalline samples have been synthesized by the conventional solid-state reaction method. Single-phase polycrystalline samples were confirmed by $X$-ray diffraction patterns. Structural characterizations were performed by $X$- ray Rietveld refinement of powders and the temperature dependence of the electrical resistivity was studied in the 10- 300 K range. The samples showed a metallic-type behavior that depended on the Iridium oxidation-state. This behavior could be explained on the basis of the hole/electron doping when we introduce some different $Bi^{3+}$-substituent metals in order to change the iridium electronic state. In this way, $Ce^{4+}$- doping will produce a change from $Ir^{4+}$ ($t_{2g}^5e_g^0$, electronic state in octahedral coordination) to $Ir^{3+}$ ($t_ {2g}^6e_g^0$) [Preview Abstract] |
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C1.00183: Structural and superconducting flux-pinning effects of self-assembled CeO2 nanodots on laser ablated YBa2Cu3O7 single layer thin films Talisha Haywood, Dhananjay Kumar, Abebe Kebede, Jeremiah Abiade, Tesfaye Gebre For electrical power applications, high temperature superconductors such as YBa2Cu3O7 (YBCO) need to possess a high critical current density (Jc) under high magnetic fields. Therefore it's critical to improve their physical properties for enhanced flux pinning. In this communication we present the magnetic field behavior of Jc as function of CeO2 nanodot inclusions in the matrix of laser ablated superconducting thin films. [Preview Abstract] |
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C1.00184: ABSTRACT WITHDRAWN |
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C1.00185: Interaction effects in the transport of two-dimensional holes in GaAs. Jian Huang, D. S. Novikov, D. C. Tsui, L. N. Pfeiffer, K.W. West The power-law increase of the conductivity with temperature in the nominally insulating regime, recently reported for the dilute two-dimensional holes [cond-mat/0603053], is found to systematically vary with the carrier density. Based on the results from four different GaAs heterojunction-insulated-gate field-effect-transistor samples, it is shown that the power law exponent depends on a single dimensionless parameter, the ratio between the mean carrier separation and the distance to the metallic gate that screens the Coulomb interaction. This dependence suggests that the carriers form correlated state in which the interaction effects play a significant role in the transport properties. [Preview Abstract] |
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C1.00186: Superconducting transport through a vibrating molecule Alex Zazunov, Reinhold Egger, Christophe Mora, Thierry Martin Nonequilibrium electronic transport through a molecular level weakly coupled to a single coherent phonon/vibration mode has been studied for superconducting leads. The Keldysh Green function formalism is used to compute the current for the entire bias voltage range. In the subgap regime, multiple Andreev reflection (MAR) processes accompanied by phonon emission cause rich structure near the onset of MAR channels, including an even-odd parity effect that can be interpreted in terms of an inelastic MAR ladder picture. Thereby we establish a connection between the Keldysh formalism and the Landauer scattering approach for inelastic MAR. [Phys. Rev. B 73,214501 (2006)] [Preview Abstract] |
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C1.00187: Numerical Calculations of The Total AC Losses of YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ Coated Conductors with The Ferromagnetic Substrate and Stabilizer Doan Nguyen, Pamidi Sastry, Justin Schwartz A numerical model is developed to calculate total AC loss of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ coated conductors with a ferromagnetic substrate and Cu stabilizer when carrying a AC transport current in a AC applied background magnetic field. The time evolutions of current and magnetic field distribution along the width of the conductor are calculated by solving Poisson's equation for the vector potential. Apart from the AC loss dissipated in the superconducting layer, the ferromagnetic loss generated by the substrate and eddy current loss dissipated in the Cu stabilizer were also modeled. In the calculations, the superconducting property is assumed to follow power-law characteristics. In general, other practical properties of conductors, including field-dependence of critical current density $J_{c}$, n-value and non-uniform distribution of $J_{c}$ can also be accounted for in the numerical calculations. Comparisons between numerical, analytical and experimental results are presented. [Preview Abstract] |
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C1.00188: Vortex Noise in the Superconducting Transition of Tin Film Hengsheng Zhang, Qian Chen, Fulin Zuo We report preliminary noise studies in the superconducting transition of thin Tin (Sn) films. Voltage noises are measured as a function of temperature and current. The noise spectral power S$_{v}$ depends strongly on the resistance and current. For small R, S$_{v}^{1/2}$ is linear with R and is replaced by logarithmic R dependence for large R. The peak in S$_{v}^{1/2}$ follows closely but always shifted down from dR/dT. I-V characteristics and voltage noises are measured simultaneously to reveal the nature of the excess noises. The I-V curves are indicative of the presence of Kosterlitz-Thouless transition (KT). The noise dependence on voltage suggests strongly the nature of vortex shot noise with a characteristic V$^{1/2}$ dependence for small V and reduction of S$_{v}$ due to strong correlation of vortices at large V. [Preview Abstract] |
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C1.00189: On the character of the coherent and incoherent excitations of electron-doped SrTiO$_{3}$ Yukiaki Ishida, Ritsuko Eguchi, Masaharu Matsunami, Koji Horiba, Munetaka Taguchi, Ashishi Chainani, Yasunori Senba, Haruhiko Ohashi, Hiromichi Ohta, Shik Shin Lightly-electron-doped SrTiO$_{3}$ shows little sign of electron correlation effects in the transport and thermodynamic properties, but photoemission spectra show not only coherent excitations from the Fermi level but also broad incoherent excitations positioned $\sim $1.5 eV below the Fermi level [1]. We have investigated the near-$E_{F}$ electronic structures of Nb-doped SrTiO$_{3}$ thin film [2] by performing resonant photoemission at the Ti 2$p$ and O 1$s$ absorption edges. Both the coherent and incoherent excitations showed giant resonances at the Ti 2$p$ edge, while at the O 1$s$ edge, resonance occurred mainly in the incoherent excitations. This indicates that the coherent excitations have mainly Ti 3$d$ character, while the incoherent excitations have mixed character of Ti 3$d$ and O 2$p$ states. We attribute the incoherent and coherent excitations to locally and non-locally screened final states, respectively, similar to that argued in Ca$_{1-x}$Sr$_{x}$VO$_{3}$ [3]. [1] A. Fujimori \textit{et al.}, Phys. Rev. B \textbf{46,} 9841 (1992). [2] S. Ohta, H. Ohta \textit{et al.}, Appl. Phys. Lett. \textbf{87}, 092108 (2006). [3] R.J.O. Mossanek, M. Abbate, and A. Fujimori, cond-mat/0606253. [Preview Abstract] |
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C1.00190: Precise determination of the superconducting gap along the diagonal direction of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+y}$: Evidence for extended $s$-wave gap symmetry Guo-meng Zhao We have analyzed the data of electron self-energy along the diagonal ($\Gamma-Y$) direction for a nonsuperconducting La$_{1.97}$Sr$_{0.03}$CuO$_{4}$ (LSCO) crystal and a superconducting Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+ y}$ (BSCCO) crystal with $T_{c}$ = 91 K. The high-resolution spectra of the second derivative of electron self-energy show clear peak features associated with the phonon modes strongly coupled to electrons. By precisely matching the phonon structures of the superconducting BSCCO with those of nonsuperconducting LSCO, we accurately determine the diagonal superconducting gap to be 9$\pm$1 meV for the BSCCO. The substantial non-zero diagonal gap observed in the optimally doped cuprate definitively rules out seemingly well accepted $d$-wave gap symmetry and strongly supports an extended $s$-wave gap symmetry with eight line nodes. [Preview Abstract] |
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C1.00191: Interplay between strain and the chain charge density wave in Sr14Cu24O41 Andrivo Rusydi, P. Abbamonte, H. Eisaki, Y. Fujimaki, N. Motoyama, S. Uchida, Y.-J. Kim, M. R\"ubhausen, G.A. Sawatzky Using resonant soft x-ray scattering (RSXS) and x-ray absorption (XAS), we have studied interplay between strain and chain charge density wave (CDW) in Sr14Cu24O41 (SCO). We have found a direct evidence of holes modulation in the chain. The hole modulation is pinned in the strain wave of the chain-ladder buckling and amplifies its modulation through the 4kF -CDW instability. Further, we use the coherent interference between the Thomson and resonant scattering to show that the holes are localized in the strain minima and the ratio of the the charge and strain amplitude, W, is estimated to be 278/A. This all explains both the insulating behavior of the chains and the apparent CDW in inelastic neutron scattering. [Preview Abstract] |
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C1.00192: Scanning Hall probe microscopy of vortex structures in mesoscopic BSCCO disks Milorad Milo\v{s}evic, Malcolm Connolly, Simon Bending It is now well established that vortex behavior in mesoscopic superconductors is strongly influenced by the sample geometry [L.F Chibotaru et al. Nature (2000)]. In particular, the topological confinement directly reflects on the symmetry of multi-vortex structures, especially in highly symmetric samples such as disks, squares, and equilateral triangles [B.J. Baelus et al. PRB (2002), I.V. Grigorieva et al. PRL (2006)]. Advances in nanofabrication techniques have stimulated the use of transport and magnetic imaging measurements to deduce the arrangement of vortices indirectly. Recent breakthroughs in scanning Hall probe microscopy (SHPM) at the University of Bath have made it possible to image vortex configurations \textit{directly}. Using photolithographic patterning and Ar+ ion beam milling, we have machined arrays of isolated micro-disks suitable for investigation by SHPM. Disk diameters are of the order of the SHPM scanning range, making it possible to image entire samples and hence assess the interaction between the sample geometry, the Meissner screening currents, and the multi-vortex configurations. The results are fully corroborated by Ginzburg-Landau theory, where finite demagnetization effects were taken into account and the stray magnetic field has been simulated in 3D. [Preview Abstract] |
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C1.00193: Vortex-cavity pinning force in a superconducting film Golibjon Berdiyorov, Milorad Milosevic, Francois Peeters The interaction of a vortex with a circular perforation in a superconducting film is calculated within the Ginzburg-Landau formalism. Due to the local compression of vortex currents and their adhesion to the edge of the perforation, a local repulsive component is added to the generally attractive pinning force. The dependence of the repulsive part on the size of the cavity is examined. The associated deformation of the vortex core and its dynamics is obtained. [Preview Abstract] |
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C1.00194: ME-$\mu $SR study in YBCO vortex states. T.H. Le, C. Boekema We are analyzing $\mu $SR vortex data of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ (T$_{c}$ = 91 K). The average superconducting grain size is $\sim $20 $\mu $m. The $\mu $SR data are recorded in a transverse 1 kOe field and at temperatures below 10 K. The $\mu $SR technique is used to probe the magnetic fields in the cuprate vortex state. The $\mu $SR signals show an oscillatory time dependence. To determine the frequency-dependent signals, we use the Maximum Entropy (ME) transform technique. [2] The ME-Burg algorithm removes noise, and does not suffer from Fourier-like truncation effects. The frequency signals are better fit with Lorentzians than static Gaussians. This Lorentzian behavior indicates the existence of dynamic magnetism in and around the vortex cores. This is consistent with earlier YBCO vortex ME-$\mu $SR results [3] and the SO(5) modeling [4] of cuprate superconductivity, predicting the existence of antiferromagnetism in the vortex states. Research is supported by NSF-REU and WiSE at SJSU. [1] C. Boekema \textit{et al}, Physica C282-287 (1997) 2069. [2] J Lee \textit{et al,} J Appl Phys 95 (2004) 6906 and ref therein; AIP www: Virtual J Appl of Superconductivity 2004 V6 Iss11. [3] C. Boekema \textit{et al}, 8th Int M2S-HTSC Conf, Physica C in press. [4] H-D Chen \textit{et al,} Phys Rev B70 (2004) 024516; SC Zhang, Science 275 (1997) 1089. [Preview Abstract] |
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C1.00195: STM differential conductance in disordered Lu$_{2}$Ni$_{2}$B$_{2}$C superconductor at low temperatures and high magnetic fields Sasha Dukan, Joseph Porembski, Karl Tata The tunneling conductance between a surface of a disordered Lu$_{2}$Ni$_{2}$B$_{2}$C superconductor and a Scanning Tunneling Microscope (STM) tip in a high magnetic field and at zero temperatures is calculated. In the clean system we find that when the STM tip is placed at the position of a vortex, the differential conductance $\sigma $(V,B) has an algebraic dependence on a bias voltage V reflecting the presence of gapless points in the quasiparticle excitation spectrum of a superconductor in high magnetic fields. When non-magnetic impurities are introduced in the system, the differential conductance at zero bias voltage becomes finite indicating the broadening of the gapless or near gapless regions in the quasiparticle excitation spectrum. We plot the differential conductance $\sigma $(V,B) as a function of disorder parameters for wide range of magnetic field strengths B in the mixed state. [Preview Abstract] |
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C1.00196: Superconductivity of YB$_{6 }$. Energy gap and electron-phonon interaction studies by point-contact spectroscopy Pavol Szabo, Jozef Kacmarcik, Peter Samuely, Jan Girovsky, Slavomir Gabani, Karol Flachbart, Takao Mori Ytrium hexaboride has the second highest critical temperature, $T_{c} \quad \sim $ 8 K, among all borides. The paper deals with the point-contact-spectroscopy study of its superconducting energy gap. The temperature dependence of the energy gap and the strength of the superconducting coupling are determined as well. By the same method also the electron-phonon interaction in the system is addressed. [Preview Abstract] |
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C1.00197: Long range proximity effect in High Tc Josephson NanoJunctions : a quantitative study. Jerome Lesueur, Nicolas Bergeal, Marco Aprili, Takis Kontos, Martin Sirena, Giancarlo Faini, Jean Pierre Contour Proximity effect (PE) between a superconductor (S) and a normal metal (N) has been a powerful tool to study conventional superconductors. In High Tc (HTc) compounds, low quality interfaces and poor Fermi wave-vector match with most of common metals usually considerably reduce the proximity effect, and make really difficult its study. We have designed Josephson NanoJunctions in which two S reservoirs are connected through an N layer at a nanoscale, where N is a lightly disordered HTcS, whose Tc has been reduced by ion irradiation. In these SNS junctions, Cooper pairs propagate through the N layer by PE. In this situation with no metallurgical interfaces within the same material, we have shown that a long range PE takes place, which can be quantitatively described by the quasi-classical approach of the diffusive Usadel equations. The Josephson coupling temperature can be computed. The role of the order parameter symmetry will be also discussed. [Preview Abstract] |
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C1.00198: Observation Of Shapiro Steps In The IV Characteristics Of Thin Nb Films Deposited On Square Arrays Of Magnetic Quantum Dots? David Mast, Yong Song We have studied the IV characteristics of a superconducting thin film of niobium that has been deposited over a 2D array of 200nm diameter Fe quantum dots. A set of dips appear in the dynamical resistance vs current curves of this Nb film below its superconducting transition temperature when the film is biased with both dc and ac currents. These local minima in the dynamical resistance display a well defined power and frequency dependence like that observed for Shapiro Steps in 2D Josephson Junction Arrays (JJAs). The results of the frequency and the power dependence of these minima will be compared to previous experiments investigating integer Shapiro steps in site-disordered niobium-gold SNS JJAs. The influence of temperature and applied magnetic fields on these minima is also reported. [Preview Abstract] |
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C1.00199: Solvent effects on chiroptical properties of carbonyl functional group molecules Watheq Al-Basheer, Richard Pagni, Robert Compton Solvent effects on Optical Rotatory Dispersion (ORD) and Circular Dichroism CD of carvone enantiomers and $R$-(+)-3-methylcyclopentanone ($R$3MCP) are studied for 35 common solvents. Solvent effects are significantly attributed to the solute-solvent electrostatic and Van der waals interactions. Hartree-Fock and Density Function theoretical calculations of $R$3MCP CD and ORD in solvation are also employed to support the experimental findings and observed to have good agreement with experimental results. Enantiomers ($R$,$S)$ of chiral molecules are known to exhibit optical activity effects which are equal in magnitude and opposite in sign. For some carbonyl molecules (possessing C=O) the equatorial and axial conformers also exhibit CD and ORD of opposite sign but not necessarily the same absolute magnitude for the $n\to \pi $* ($n\to $3s) molecular transition. Temperature dependent variations of CD and Raman spectra are shown to be a useful technique to study the conformer's populations and energy difference of $R$3MCP. Thermodynamic constants of $R$3MCP in 35 solvents, will be presented and related to solvent polarity parameters. [Preview Abstract] |
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C1.00200: Estimation of $\epsilon_H$ in cuprates Jungseek Hwang, Thomas Timusk The quantity $\epsilon_{H}$ is the contribution to the dielectric constant from all the high frequency spectral weight but excluding the low frequency free carrier or intraband contribution. Recently, the accuracy of the various methods of finding $\epsilon_H$ has becomes an important issue because the optical scattering rate is very sensitive to the value of $\epsilon_H$, especially at high frequency. In strongly correlated systems including cuprates it is often difficult to determine where the dividing line between the free carriers and the interband absorption lies. Here we explain a reliable method to estimate $\epsilon_H$ in Bi-based cuprates. We will show $\epsilon_H$ of various Bi-2212 systems extracted using the method and discuss some important issues related to $\epsilon_H$. [Preview Abstract] |
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C1.00201: A new concept of concomitant field free, low-field SQUID MRI. Byeong Ho Eom, Konstantin Penanen, Inseob Hahn We report a new MRI concept using a magnetic field configuration that eliminates the concomitant field effect, especially for low-field MRI. In low field MRI, the image acquisition time and/or the image resolution are constrained by small image encoding magnetic field gradients. Image encoding gradients always entail the undesirable concomitant fields according to the Maxwell equations. These fields are necessarily proportional to the encoding gradients, and result in various image artifacts. To reduce or correct the artifacts, the gradients should be small so that the magnetic field variation in field of view is a small fraction of the static homogeneous field. We obtained a field configuration such that the magnitude of the field changes monotonically along one direction by removing the constraint of uniaxial spin precession and linearity of field gradients. This field configuration is used to encode projections and the image is obtained from back projection reconstruction. A current configuration was obtained to produce this field configuration. An implementation design is proposed based on expanding the field and current configuration in terms of harmonics. [Preview Abstract] |
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C1.00202: NMR study of Nb-deficient effect in NbB$_{2}$ T.H. Su, C.S. Lue Observations of the Nb-deficiency enhanced superconducting temperature ($T_{C})$ in NbB$_{2}$ have been reported. In order to provide experimental information for this effect, we have carried out the nuclear magnetic resonance (NMR) study of Nb$_{1-x}$B$_{2}$ (x = 0, 0.13, 0.20, and 0.26) in the normal state. From both $^{11}$B and $^{93}$Nb NMR spin-lattice relaxation rates, we can deduce the B-2$p$ and Nb-3$d$ partial Fermi-level density of states (DOS) for each individual composition. The result indicates that the B-2$p$ and Nb-3$d$ partial Fermi-level DOS increase with Nb deficient level. Such a tendency is consistent with the trend of the superconducting temperature, revealing that the observed $T_{C}$ enhancement is strongly correlated to the increase of the Fermi-level DOS induced by the Nb deficiency. [Preview Abstract] |
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C1.00203: Phase Diagram Of UGe$_{2}$: The Magnetic Transition within the Ferromagnetic Phase and the Superconducting Transition; the Effect of Magnetic Field on the Ambient-Pressure Ferromagnetic Phase N.E. Phillips, F. Bouquet, R.A. Fisher, F. Hardy, N. Oeschler, J.C. Lashley, J. Flouquet, A. Huxley Superconductivity in UGe$_{2}$ occurs near 1.2 GPa at the 0-K termination of the phase boundary (T$_{x}$, P$_{x})$ of a magnetic transition that occurs within the ferromagnetic phase. Ambient-pressure specific-heat measurements show a hysteretic transition at T$_{x}$(0) $\sim $ 22 K, reminiscent of the CDW/SDW transition in $\alpha $-U, and consistent with the suggestion that the transition in UGe$_{2}$ is also a CDW/SDW transition. The magnetic field dependence of the specific heat, at ambient pressure, demonstrates the presence of structure in the electron density of states and an unusual nature of the ferromagnetic ordering at the Curie temperature. Specific-heat measurements to 1.8 GPa give an estimate of the latent heat of the transition and determine the phase boundary for 1 $\le $ T $\le $ 11 K. Contrary to expectations, the onset temperature of the superconducting transition is independent of pressure in the region in which it was observed, 1.08 $\le $ P $\le $ 1.35 GPa. [Preview Abstract] |
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C1.00204: Evidence of Ferromagnetic Spin Fluctuations in the single crystalline MgCNi$_{3}$ superconductor Dong-Jin Jang, Hyun-Sook Lee, Hye-Gyong Lee, Sung-Ik Lee Despite strong expectation of the ferromagnetic ground state or the strong ferromagnetic fluctuation in MgCNi$_{3}$, the real ground state shows only superconductivity without any evidence of magnetism. At least the ferromagnetic spin fluctuations should be appeared in this strong ferromagnetic Ni based compound, but no clear observation of them is absent. Lack of the single crystalline and use of the poly-crystalline MgCNi$_{3}$ had hindered to obtain the intrinsic signal from the spin fluctuations due to the strong scattering of the defects or grain boundaries. To clarify this issue, we prepared single crystalline MgCNi$_{3}$ and measured the resistivity as a function of temperature and field. Important role of the spin fluctuations as the pair breaker in the MgCNi$_{3}$ single crystals was confirmed in these measurements while comparing the prediction from the modified Eliashberg equation including the spin fluctuations by O. V. Dolgov \textit{et. al.} The unusual normal-state resistivity and upward curvature of $H_{c2}(T)$ near $T_{c}$ also supported the existence of the additional scattering due to the spin fluctuation scattering in addition to the typical electron-phonon scattering in BCS theory. [Preview Abstract] |
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C1.00205: Observation of a vortex lattice spinodal and a Bragg glass in MgB$_{2}$ Hyun-Sook Lee, S.K. Gupta, Byeongwon Kang, Sung-Ik Lee We observed a fully disordered (Spinodal) and a fully ordered (Bragg glass) vortex lattice in MgB$_{2}$ single crystals by using fast current measurements to measure $I_{c}$ for variously prepared vortex states. The obtained $I_{c}$ showed a very different behavior depending on the vortex states, such as Zero Field Cooled (ZFC), ZFCW (warmed), FC, and FCW. We noticed that the values of $I_{c}(T)$ for the FC and the FCW states for MgB$_{2}$ showed clear history dependence with two reversible points. The one point above the $T_{peak}$ was exactly the same as the spinodal point obtained by using the same method as reported in PRL 92, 227004 (2004). The other point below the $T_{peak,}$ was consistent with the Bragg glass point measured by using the time-resolved transport measurements reported in PRL 96, 017009 (2006). We obtained $H-T$ diagram including several vortex states. The theoretical spinodal line predicted by Li \textit{et al. }based on one-gap superconductivity described the case for NbSe$_{2 }$well, but could not explain the spinodal line for MgB$_{2}$. [Preview Abstract] |
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C1.00206: Microwave measurements of CeCoIn$_5$ using a micro-stripe line I. P. Nevirkovets, O. Chernyashevskyy, J. B. Ketterson, C. Petrovic We have studied microwave signal transmission through a structure consisting of a meander-type micro-strip line and a ferromagnetic or superconducting sample placed adjacent to it. The setup, which currently operates from 0 to 9 Tesla and from room temperature to 1.7 K, is suitable for studying both ferromagnetic resonance, and temperature/field dependent superconducting properties. Using this setup we have observed the superconducting transition of a single crystal of CeCoIn$_5$ in the frequency range from 5 to 19 GHz. The technique should be applicable to the study of wavelength-dependent collective modes. [Preview Abstract] |
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C1.00207: A general computational approach for tailoring superconducting properties in BCS systems Jin-Cheng Zheng, Yimei Zhu We present a general computational approach for tailoring superconducting properties, such as by varying transition temperature, energy gap, critical field, in the framework of BCS theory. The relative changes in superconducting properties due to chemical doping, hydrostatic pressure, and strain fields are studied in terms of electronic and phonon contributions in a unique way that these components can be easily computed by first principles calculations based on density functional theory. Several rules for tailoring superconducting properties are derived. Some typical examples including niobium compounds and magnesium diborides and related materials will be given to illustrate the applications of these rules. [Preview Abstract] |
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C1.00208: SOCIETY OF PHYSICS STUDENTS |
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C1.00209: Surface Magnetic Anisotropy of Vertical Ni Nano-Columns as Revealed by Magneto-Optical Kerr Effect. Judah Henry, Fu Tang, Gwo-Ching Wang The surface magnetic properties of vertical Ni nano-columns grown on a native oxide Si(110) substrate by oblique angle deposition with uniform substrate rotation were studied using the longitudinal Magneto-Optical Kerr Effect (MOKE). The vertical nature of the $\sim $250 \textit{nm} high columns was verified using Scanning Electron Microscopy (SEM) imaging. X-ray diffraction showed that the bulk nano-columns exhibit a vertical (220) texture. Contrary to the magnetic isotropy observed from the vibrating sample magnetometer (VSM) measurements, the MOKE hysteresis loops display azimuthal magnetic anisotropy. Using the method proposed by Tang, \textit{et al.}$^{1}$, the coupled hysteresis loops obtained in the MOKE experiment were decomposed into polar and longitudinal components by addition and subtraction of loops 180\r{ } apart. The polar part has an obvious azimuthal anisotropy while the longitudinal component shows an approximate isotropy. It is proposed that the oblique angle deposition results in the surface microstructure differing from the bulk microstructure, yielding magneto-crystalline or shape anisotropy on the surface. This work was supported by the NSF 0453231. $^{1}$. F. Tang, et al. J. Appl. Phys. 93 (7), 4194 (2003). [Preview Abstract] |
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C1.00210: Investigation of Deformations regarding THUNDER Actuators. Paul Harris A new type of piezoelectric composite actuator called THUNDER, which was originally developed by NASA, has potential applications in micro robotics, aeronautics, and hydraulics. The manufactured process produces internal stresses with accompanying structural deformation. It is the goal of this research to characterize these deformations. Detailed measurements were taken with a laser micrometer on several different types of actuators. Several functional forms were used in an attempt to fit the data. The data was best fit by a circular segment function. We also used a transcendental equation to be able to compare to other single point published values. We found the range of curvature to be between 4.97x10$^{3}$mm$^{1}$ and 1.29x10$^{2}$ mm$^{1}$. Preliminary experimental results of voltage induced deformations will be presented. [Preview Abstract] |
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C1.00211: A Fluorescence Spectroscopic Analysis of the Binding of Pyrene to Cytochromes P450 1A2 and 3A4. Judah Henry, F. Peter Guengerich, Glenn Marsch Fluorescence spectroscopy was used to study cytochromes P450 1A2 and 3A4. Spectra of P450s were acquired in the presence and absence of acrylamide quencher. In both P450s, quenching revealed three distinguishable species of amino acid fluorescence, with maxima at 297, 323, and 345 nm. The 345 nm tryptophan fluorescence was quenched by low levels of acrylamide; the 297 nm tyrosine fluorescence was resistant to quenching. The 323 nm fluorescence was observed at intermediate concentrations of quencher. Stern-Volmer plots of P450 quenching were non-linear, but were well-fitted to a superposition of linear plots for each fluorophore species. The effect of the P450's binding on pyrene fluorescence was also examined. Upon binding to P450 1A2, the intensity of the 383 nm pyrene vibronic band was decreased relative to the intensities of the 372 and 393 nm bands. Both P450's showed binding of the pyrene, but 1A2 demonstrated significantly more excimer emission than did the 3A4, which suggests that more than one pyrene molecule binds to 1A2's active site. The results of these analyses will be used in further characterization of these enzymes. [Preview Abstract] |
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C1.00212: Numerical simulations for parabolic pulse shaping in non-linear media R.C. Nora, C.G. Durfee, L.D. Carr Pulses with parabolic temporal profiles have the property that they can propagate through non-linear media in a self similar manner. Parabolic pulses have been generated experimentally in fiber amplifiers. Input pulses develop into parabolic pulses by the combined action of group velocity dispersion, non-linear refractive index, and gain. In this work, we are exploring the feasibility of generating ultrafast parabolic pulses in laser resonators. We have successfully numerically simulated the generation of parabolic pulses in fiber amplifiers using two different algorithms, the Cayley method, and fourth order Runge-Kutta, to solve the Nonlinear Schrodinger equation with gain and periodic boundary conditions. In contrast to fiber amplifiers, pulses in laser resonators must maintain a stable pulse shape on each round trip through the optical cavity. We are exploring the prediction that a time dependent saturable gain will stabilize the pulse in the oscillator and yield parabolic pulses. [Preview Abstract] |
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C1.00213: SEMICONDUCTORS |
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C1.00214: Edgemagnetoplasmons in a partially screened two-dimensional electron gas A.J. Dahm, M.I. Goksu, Mokyang R. Kim, K.A. Mantey We report a study of edgemagnetoplasmons in a partially-screened system of electrons on a helium surface. We compare experimental results with theories of Fetter and of Mikhailov and Volkov on the frequency, damping, and penetration-depth dependence on magnetic field, temperature-dependent damping, and the dependence of the frequency on screening. We show explicitly the dependence of frequency on edge density profile. [Preview Abstract] |
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C1.00215: Magnetotransport in a Modulated 2DES Godfrey Gumbs We calculate the quantum magnetotransport coefficients of a two- dimensional electron gas (2DEG) in the presence of one-dimensional (1D) electrostatic modulation in the x-direction. The calculations are performed when a low-intensity, low-frequency external electric field is applied. The system is also subjected to a perpendicular magnetic field. The Kubo formula for the conductivity is employed. The eigenstates, which depend on the strengths of the magnetic field and modulation potential, are calculated and then used to determine the conductivity. We study the longitudinal resistivity and the transverse resistivity as functions of the the modulation potential and the magnetic field. We demonstrate that the effect of finite frequency is to reduce both the longitudinal and transverse resistivities. A low concentration of impurities is also included in these calculations. [Preview Abstract] |
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C1.00216: On different mobilities in the low-field insulator-quantum Hall transition Z.-H. Sun, J.-Y. Lin, J.-H. Chen, C.-T. Liang, Y.F. Chen, Gil-Ho Kim, D.H. Youn, C.M. Jeon, J.M. Baik, J.-L. Lee, C.F. Huang We report that the quantum mobility (determined from the Shubnikov-de Haas oscillations) can be much smaller than the Drude mobility (determined from the crossing point) when there exists an approximately temperature ($T$)-independent point in $\rho_{xx}$ for which the corresponding Landau level filling factor $\nu \geq 3$ [1]. Moreover, recently it has been pointed out that the mobility can be renormalized and increases with increasing $T$ [2]. Our experimental results, together with the seminal paper [2] strongly suggest that different mobilities should be considered in the low-field insulator-quantum Hall (I- QH) transition. Most importantly, corrections to the argument raised by Huckestein must be made to obtain a correct insight into the direct low-field I-QH transition [1]. \newline \newline [1] B Huckestein, PRL 81, 3141 (2000) and references therein. \newline [2] G M Minkov et al., PRB 74, 045314 (2006). [Preview Abstract] |
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C1.00217: Rashba spin-splitting in wurtzite based two dimensional holes systems Arturo Wong-Lopez, Francisco Mireles We present a theoretical study of the Rashba-like spin-orbit coupling in two dimensional hole systems formed in wurtizite semiconductor heterostructures.~ Using an 8X8 band Kane model within the envelope function approximation we derive exact analytical expressions for the Rashba-like Hamiltonians of the heavy and light holes.~ The linear and cubic dependence in the wave vector of such Hamiltonians will be discussed. Simple analytical expressions for the spin-orbit coupling parameters can be also extracted from our model. A variational approach has been used in order to estimate the hole spin-splitting energies in typical group III nitrides semiconductor quantum wells. These results could be of relevance for the design and implementation of novel (hole) transport spintronic devices. [Preview Abstract] |
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C1.00218: Interplay of the Rashba and Dresselhaus spin-orbit coupling in the optical spin susceptibility of 2D electron systems Catalina Lopez-Bastidas, Jesus A. Maytorena, Francisco Mireles Electrical manipulation of electron and hole spins without use of ferromagnetic materials and/or external magnetic fields is currently one of the important goals in the field of spintronics. The presence of a sizeable spin-orbit interaction (SOI) in low-dimensional semiconductor structures and its modulation possibility make it a good mechanism for the access and manipulation of the carriers spin states. The spin susceptibility gives the average spin polarization induced via electric- or magnetic- dipole interactions and can be used to obtain a magnetic susceptibility or the electric-field-induced spin orientation factor. Charge or spin Hall conductivities can be related to this susceptibility making it an interesting quantity in the study of spin dynamics of a 2D electron gas (2DEG). In this work we present calculations of the frequency-dependent spin susceptibility tensor of a 2DEG with competing Rashba and Dresselhaus SOI. We show that the interplay between both types of spin-orbit coupling gives rise to a rich anisotropic spectral behavior of the spin density response function. Strong resonances are thus developed in the susceptibility as a consequence of the angular anisotropy of the energy spin-splitting. This modulable optical response may be useful to experimentally probe spin accumulation and spin density currents in such systems. [Preview Abstract] |
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C1.00219: Spin Quantum Kinetics in Relaxation and Transport of Semiconductors Han-Chieh Lee, Chung-Yu Mou, Stephen A. Lyon Generalized Kadanoff-Baym Equation (GKBE) with spin degree of freedom is firstly presented and its theoretical framework of applications, which aims to semiconductor quantum kinetics in femtosecond and nanometer scales, demonstrated. The GKBE was constructed by Green functions thermally averaging Pauli equation of motion with using Langreth theorem. As applied for relaxation, Kadanoff-Baym ansatz was made and carrier-carrier scattering (CCS) with random-phase approximation considered. The derivation can simulate an evolution of excited carriers spreading via CCS, buildup of magnetic field by Rashba effect and formation of spin relaxation, where energy non-conserving event and memory effect are figured out. For transport, retarded Green functions were retrieved from spin Dyson equation as an input for GKBE with the presence of electron-phonon (impurity) interaction. The part is useful for spin Hall effect in precisely estimating spin current and accumulation in nanostructures or ballistic regime. [Preview Abstract] |
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C1.00220: Growth of Single Crystalline ZnGeN$_{2}$ from Zn/Ge Melts Timothy J. Peshek, Kathleen Kash, John C. Angus We present the first evidence for the growth of ZnGeN$_{2}$ from a nitrogen saturated Zn/Ge melt. ZnGeN$_{2}$ is the II-IV-V$_{2}$ compound semiconductor analogue of GaN. Results from photoluminescence spectroscopy and X-ray diffraction suggest that it is nearly identical to GaN in band gap and lattice constants, making it potentially suitable as a GaN substrate or replacement. To date, reported methods of synthesis have been limited to vapor phase deposition on highly lattice-mismatched substrates. The present technique, which does not require the use of substrate, yields hexagonal prisms of ZnGeN$_{2}$, 40-50 $\mu$m in length along the [0001] direction, and 3-5 $\mu$m wide in cross-section, capped by a round, polycrystalline dome of stoichiometric ZnGeN$_{2}$. This morphology is highly suggestive of a VLS-like growth mechanism and is evidence that ZnGeN$_{2}$ may melt congruently at the pressures and temperatures employed. [Preview Abstract] |
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C1.00221: Synthesis and characterization of ZnS doped with metallic impurities. Estela Gomez, Enrique Sanchez-Mora, Rutilo Silva, Leticia Perez-Hernandez, Cesar Lopez-Garcia, Victor Lozada-Dircio Zinc sulfide (ZnS) is a wide band gap and direct transition semiconductor. It is an important material for detection emission and modulation of visible and ultraviolet light, and for electroluminescent devices among other applications. The object of this work was to deposit by the sol-gel method/deep coating, ZnS, ZnS:Mn and ZnS:Sm films (5 coatings) on glass substrate. The samples were characterized to study the surface morphology, composition and some optical properties. SEM micrographs show a porous surface morphology with agglomerate type defects. FTIR spectra show the presence of surface O-H and S-O groups. By AES it was determined the composition of the films, and UV-Vis spectra confirmed the ZnS compound formation. This work has been partially supported by VIEP-BUAP, Project No. 11/EXC/06/G. [Preview Abstract] |
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C1.00222: Optical and Structural Properties of Luminescent MgZnO Nanoalloys Leah Bergman Mg(x)Zn(1-x)O alloys are promising wide-bandgap semiconductors for UV applications, and also of considerable interest from a fundamental viewpoint. These optical alloys may enable the tuning of the bandgap and the luminescence at the range of $\sim $ 3.0 for ZnO of the wurtzite structure up to $\sim $ 7 eV for the MgO of the rocksalt structure. We will present studies on the photoluminescence and Raman properties of Mg(x)Zn(1-x)O nanocrystallites. For the studied composition range of 0-26{\%} Mg, the room temperature UV-PL was found to be tuned by $\sim $ 0.3 eV towards the UV-spectral range. For that composition range the first-order LO Raman mode was found to exhibit a significant blueshift of $\sim $ 33 cm$^{-1}$ indicating that a good solid solution was achieved at the nanoscale. At higher composition ranges a PL blue shift of at least 1.3 eV was achieved. Issues such as excitonic emissions, alloy spectral broadening, phonon symmetry, as well as temperature and pressure response of the nanoalloys will be presented. [Preview Abstract] |
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C1.00223: Current voltage measurements on magnetic ZnO /Y$_{1}Ba_{2}Cu_{3}O_{7-\delta}$ thin film heterostructures N. Ghosh, H. Schmidt, H. Hochmuth, M. Lorenz , Q. Xu , M. Grundmann, G. Wagner We have grown ca. 200 nm magnetically doped ZnO films on Y$_{1}Ba_{2}Cu_{3}O_{7-\delta}$ on r-plane sapphire by pulsed laser deposition for future Andreev Reflection(AR) measurements to determine spin polarization. The critical current density, 5 x 10$^{6}$ A/cm$^{2}$, of the YBCO films at zero field and 77K [1]remains in same range after growth of granular doped ZnO and critical temperature is still above 77K. Four-point transport measurements across the ZnO/YBCO interfaces with junction area ( A $\geq$ 1 mm$^{2}$) resulted in nonlinear I-V characteristics. The observed conductivity fluctuations at zero bias in the low temperature range are masked by heavy noise due to the large A lying in the Maxwell range ($\it{d} $$\gg$$\it{l}$ , $\it{d}$ = contact radius, $\it{l}$ = mean free path). Fitting the conductivity data according to the Blonder-Tinkham-Klapwijk 0K theory shows that the superconducting gap ($\Delta$) of YBCO and ZnO/YBCO barrier strength (Z) amount to $\leq$ 20.4 ($\pm$1) meV and $\geq$0.74($\pm$ 0.23), respectively. Smaller junction area with reduced scattering AR measurements are under work. [1]M.Lorenz etal. IEEE Transactions on Applied Superconductivity 11(2001)3209. [Preview Abstract] |
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C1.00224: Surface morphology evolution of GaN thin films directly grown on c-plane sapphire substrate by hydride vapor phase epitaxy Sang-Hwa Lee, Hyeokmin Choe, Taegeon Oh, Boa Shin, Jaewon Jeon, Chinkyo Kim Epitaxial GaN thin films were grown with no {\it intentional} buffer layers at 1050~$^{\rm \circ}$ on c-plane sapphire substrates by hydride vapor phase epitaxy. Scanning electron microscopy (SEM), synchrotron x-ray scattering, and transmission electron microscopy (TEM) were utilized to investigate the nucleation mechanism and the evolution of surface morphology. It turned out that unintentional buffer layers accomodating lattice mismatch between GaN and sapphire substrate were spontaneously formed. The surface morhologies of subsequently grown layers showed very different characteristics which were not observed in typical two step growth of GaN films. [Preview Abstract] |
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C1.00225: Effect of delayed coalescence of buffer layers on the properties of subsequently grown GaN thin films Chinkyo Kim, Hyeokmin Choe, Youngboo Moon, Shi-Jong Leem The coalescence time of low-temperature-grown GaN buffer layers was controlled by varying NH$_3$ flow rates and the effect of delayed coalescence on the characteristics of subsequently grown GaN thin films was investigated by utilizing x-ray diffraction (XRD), atomic force microscopy (AFM) and Hall measurement. The buffer layers were grown by metal-organic vapor phase deposition (MOCVD) at 560~$^{\rm \circ}$C and the growth time for 90$\%$ coalescence of buffer layers was controlled by varying NH$_3$ from 2 slm to 11 slm. 2$\mu$m- thick GaN layers were deposited on the low-temperature-grown buffer layers. The carrier mobility, full width at half maximum (FWHM) of (002) Bragg peaks, and pit density measured for these samples showed that increased coalesced time of buffer layers improved the subsequently grown GaN film quality. This dependence on the coalescence time was attributed to increased grain size due to delayed coalescence during buffer layer growth. [Preview Abstract] |
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C1.00226: Growth of M-plane gallium nitride on $\gamma $-LiAlO$_{2}$ (100) substrate by radio frequency plasma-assisted molecular beam epitaxy Chia Ho Hsieh , M.H. Gao, Y.L. Chen, W.Y. Pang, Y.I. Chang, Ikai Lo, M.C. Chou We present a study of the growth of GaN (1\underline {1}00) thin films on$\gamma $-LiAlO$_{2}$ (100) by radio frequency plasma-assisted molecular beam epitaxy. The films were monitored in-situ by reflection high-energy electron diffraction. After growth, the samples are characterized by double crystal X-ray diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence measurements. X-ray diffraction measurements show the pure M-plane GaN of the samples. The cross-sectional TEM micrographs show anisotropy in defect structure. The morphological images show that the lattice lines are either parallel or perpendicular to the GaN $c $axis. [Preview Abstract] |
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C1.00227: Improvement of GaN Epilayer by Multi-Step Method in Molecular-Beam Epitaxy Yen-Liang Chen, Ming-Hong Gau, Chia-Ho Hsieh, Wen-Yuen Pang, Yu-Chi Hsu, Ikai Lo, Wan-Tsang Wang, Jih-Chen Chiang The quality of GaN template layer plays a very important role in high electron mobility transistors. We propose a special method in the growth of molecular beam epitaxy to enhance the quality of structure and the morphology of GaN. In our study, we used a nitrogen-rich GaN growth condition to deposit the initial varied layer. Then, we changed the N/Ga ratio stepwise to the growth condition of gallium-rich GaN and grew the epitaxy layer right away. In X-ray diffraction analysis, the full width at half-maximum (FWHM) value of rocking curves of GaN(002) is 59.43 arcsec. In atomic force microscopy (AFM) analysis, the surface is rather flat with a rms roughness of 1.622 nm over a 5 $\mu $m$^{2}$ area. [Preview Abstract] |
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C1.00228: Structural Stability, Phase Transitions and Piezoelectric Anomalies in ordered Sc$_{0.5}$Ga$_{0.5}$N Alloys. Ahmad Alsaad Local-density approximation (LDA) within density functional theory (DFT) and Berry phase approach within modern theory of polarization are performed to predict the structural and piezoelectric properties of ordered Sc$_{0.5}$Ga$_{0.5}$N alloys under tensile and compressive in-plane strain. This alloy is found to exhibit a tremendous piezoelectric response, associated with a phase transition from nonpolar $p$6$_{3}$/\textit{mcc}(D$_{6h})$ space group to a polar $p$6$_{3}$\textit{mc}(C$_{6v})$ structure at fixed Ga and Sc compositions when continuously changing the experimental accessible parameter (i.e., the tensile and compressive strain). The mechanism of the effects behind such anomalies behavior are revealed and explained. [Preview Abstract] |
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C1.00229: Study of two-subband population in Fe-doped AlxGa1-xN/GaN heterostructures by persistent photoconductivity effect Jenn-Kai Tsai, Ikai Lo, M. H. Gau, Y. L. Chen, J. Z. Chang, W. T. Wang, J. C. Chiang The electronic properties of Fe-doped Al$_{0.31}$Ga$_{0.69}$N/GaN heterostructures have been studied by Shubnikov-de Haas measurement. The lowest two subbands of the two-dimensional electron gas in the heterointerface were populated. After the low temperature illumination, the electron density increases from 11.99x10$^{12}$ cm$^{-2}$ to 13.40x10$^{12}$ cm$^{-2}$ for the first subband and from 0.66x10$^{12}$ cm$^{-2}$ to 0.94x10$^{12}$ cm$^{-2}$ for the second subband. The persistent photoconductivity effect ($\sim $13{\%} increase) is mostly attributed to the Fe-related deep-donor level in GaN layer. The second subband starts to populate when the first subband is filled at a density of 9.40x10$^{12}$ cm$^{-2}$. We obtained the energy separation between the first and second subbands to be 105 meV. [Preview Abstract] |
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C1.00230: Influence of threading dislocation scattering on the electron mobility in AlGaN/GaN heterostuctures grown by PAMBE Ming-Hong Gau, Yan-Liang Chen, Chia-Ho Hsieh, Hsu-Yu Chi, Pang-Wen Yuen, Iaki Lo, Wan-Tsang Wang, Jih-Chen Chiang In order to understand the influence of threading dislocation on the transport properties in two dimensional electron gas (2DEG) system, series of modulation doped Al$_{0.25}$Ga$_{0.75}$N were grown on GaN with different threading dislocation density by plasma-assisted molecular beam epitaxy (PAMBE). We have observed the distinct characterization in F.W.H.M of (102) rocking curve in GaN $^{1}$, which is proportional to total threading dislocation density, when the AlN buffer layer and the following GaN epilayer were grown under III-stable or V-stable condition.$^{ }$The dependence of mobility on carrier concentration was found to follow a bell-shaped curve, and the peak of curve was shifted from 1x10$^{13}$ cm$^{-2}$ to 5x10$^{12}$ cm$^{-2}$ as F.W.H.M of (102) rocking curve reduced from 1200 sec to 800 sec. We believed that the shift was resulted from the interaction between free electron and charged dislocation when dislocation scattering dominates the transport characteristic. 1. M.H. Gau \textit{et al.} March Meeting of APS (2005) [Preview Abstract] |
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C1.00231: Magnetic Properties of (Cr,Ga)N Diluted Magnetic Semiconductor J. Rufinus We study the magnetic properties of Cr-doped GaN Diluted Magnetic Semiconductor (DMS) using first principles Density Functional Theory. The effect of Cr clustering on the magnetic properties of this DMS material was investigated. We found that the Cr atoms tend to bind more strongly to N atoms than to Ga atoms, resulting in larger overall magnetic moments and lower total energy. This may explain the role of the anions in this DMS material. [Preview Abstract] |
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C1.00232: High pressure magnetotransport properties of single crystals and thin films of the diluted magnetic semiconductor Sb$_{2-x}$V$_{x}$Te$_{3}$ P. C. Quayle, J. S. Dyck, Y.-J. Chien, Z. Zhou, C. Uher, P. Lostak Bulk, single crystals of the narrow band gap semiconductor Sb$_{2-x}$V$_{x}$Te$_{3}$ doped with x = 0.03 display a ferromagnetic transition at 22 K. Both carrier concentration and Curie temperature have been shown to be strongly affected by pressure in these crystals, thus high pressure experiments allow a way to study the magnetic interactions in these materials. Recently, by employing molecular beam epitaxy (MBE) thin film growth, the vanadium concentration in Sb$_{2-x}$V$_{x}$Te$_{3}$ was increased by an order of magnitude, resulting in Curie temperatures of 104K and 177K for x = 0.15 and 0.35, respectively. Here, we present the temperature dependent magnetotransport properties of both single crystal and MBE thin film samples for pressures up to 1.5 GPa. Trends with pressure will be discussed in light of RKKY-type models for the ferromagnetic interaction. [Preview Abstract] |
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C1.00233: Synthesis, Characterization and Gas Sensing Properties of SnO$_{2}$-\textit{x}Fe$_{2}$O$_{3}$(\textit{x} = 0 to 1) Thin Films G. Setzler, C. Sudakar, M.B. Sahana, P.P. Vaishnava, Ron Baird, G.W. Auner, G. Lawes, R. Naik, V.M. Naik Due to special properties and enhanced performance, nanocrystalline composites are being explored for use in potential applications in gas sensors, solar energy conversion and photocatalysis. We report the properties of SnO$_{2}$ -- $x$Fe$_{2}$O$_{3}$ ($x$ = 0 to 1) nanocrystalline composite thin films grown by the spin-on metal-organic decomposition method using Fe and Sn ethyl hexanoate on sapphire substrates. The films, on annealing at 600$^{o}$C, are comprised of SnO$_{2}$ and Fe$_{2}$O$_{3}$ nanoparticles, as evidenced by X.ray diffraction and Raman spectra. Ultraviolet-visible spectra indicate a decreasing band gap of this composite from $\sim $3.9 to 2.25eV with increasing Fe$_{2}$O$_{3}$ concentration. These samples were also tested for response to CO gas in a nitrogen environment. Parameters such as annealing temperature, operating temperature and CO concentration were varied, showing an increase in sensitivity with operating temperature, and a decreasing sensitivity with increasing Fe$_{2}$O$_{3 }$concentration. [Preview Abstract] |
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C1.00234: Photocatalytic properties of TiO$_{2}$-Fe$_{2}$O$_{3}$ and ZnO-Fe$_{2}$O$_{3}$ films. Estela Gomez, Enrique Sanchez-Mora, Rutilo Silva-Gonzalez Morphological, optical and photocatalytic properties of TiO$_{2}$-Fe$_{2}$O$_{3}$ and ZnO-Fe$_{2}$O$_{3}$ samples were studied. The layers were deposited on glass substrate by the sol-gel method. The photocatalytic activity of the samples was performed by means of the photodecomposition of methylene blue (MB) under visible light illumination. The FTIR results indicate that the samples present surface OH that are bound to the Ti and Fe and Zn atoms. The absorption edges are displaced toward to low energies as the concentration of Fe$_{2}$O$_{3}$ increases in TiO$_{2}$ and ZnO respectively. Here two mechanisms are observed during the photodecomposition of the methylene blue. [Preview Abstract] |
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C1.00235: Influence of stoichiometry of V$_{2}$O$_{5}$ thin films on the electrochemical properties M.B. Sahana, C. Sudakar, C. Thapa, G. Lawes, R. Baird, G.W. Auner, K.R. Padmanabhan, R. Naik, V.M. Naik V$_{2}$O$_{5}$ has been widely used in a variety of technological applications such as solid state battery cathodes, solar cell windows, and electrochromic devices, as it allows easy intercalation/deintercalation of different ions due its open layered structure. However, the electrochemical properties of V$_{2}$O$_{5}$ critically depend on the details of its composition, and preparation methods, thus requiring optimization of several growth parameters. We present experimental results on the effect of V$_{2}$O$_{5}$ stoichiometry on electrochemical characteristics, including the intercalation capacity and Li$^{+}$ diffusion coefficient. We have prepared V$_{2}$O$_{5}$ thin film samples with different stoichiometry by spin coating with three precursors of different carbon to vanadium ratio: (i) a sol gel inorganic,(ii) a sol-gel organic , and (iii) a metalorganic precursor. The stoichiometry of V$_{2}$O$_{5 }$is found to be very sensitive to carbon to vanadium ratio in the precursor. Films prepared from the metalorganic precursor, with a high carbon to vanadium ratio, are highly non-stoichiometric, as determined using Raman and UV-visible spectroscopy and high resolution electron microscopy and possess a higher intercalation capacity and a larger Li$^{+}$ diffusion coefficient. [Preview Abstract] |
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C1.00236: The adsorption of water molecules on the Si(001)-2$\times$1 surface Sang-Yong Yu, Hanchul Kim, Ja-Yong Koo The water molecules exist even in the ultrahigh vacuum. On the one hand, they contaminate the surface of Si(001) wafer and deteriorate the films grown on the surface. On the other hand, they can be used to grow good oxides on the Si(001) surface at high temperature. We investigated the adsorption of water molecules on the Si(001)-2$\times$1 surface by scanning tunneling microscopy. We could find two types of adsorption configuration. In one configuration the water molecule sits on top of one Si dimer, in another configuration the water molecule adsorbs on two Si atoms of neighboring Si dimers in the same dimer row. The ratio between the two features at high temperatures show interesting properties. [Preview Abstract] |
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C1.00237: One-beam coherent control effect at Si(111) surfaces: linear optical formalism and layer-by-layer analysis A.I. Shkrebtii, J.E.N. Arzate, J.L. Cabellos, B. Mendoza, F. Nastos, J.E. Sipe We demonstrated how the so-called one-beam coherent control effect might be used as a complementary probe in surface physics to extract information about surface structure, electronic states, bonding, \textit{etc}. In [1] this effect has been developed in terms of a \textit{nonlinear} optical formalism. In contrast to [1], we presented a complementary ballistic photocurrent formalism that is \textit{linear}. We demonstrated that the non-diagonal elements of the linear optical tensor contribute to this coherent control effect, which is due to the quantum interference between the two orthogonally polarized beams. Such ballistic photocurrent injection depends on the relative phase between these beams \textit{($\phi $}$_{b}$\textit{ - $\phi $}$_{c})$ as \textit{dJ }$^{a }$\textit{/dt = $\eta $ }$^{abc }$\textit{$\vert $E}$^{b}$\textit{$\vert \quad \vert $E}$^{c}$\textit{$\vert $ sin($\phi $}$_{b}$\textit{ - $\phi $}$_{c}).$ As the second step, we developed a microscopic formalism to \textit{separate} the optical response produced by each atomic layer of a semi-infinite crystal. Finally, we applied this formalism to the clean Si(111) (2$\times $1), Si(111):H (1$\times $1) and Si(111):As(1$\times $1) surfaces. The research was supported by NSERC Discovery Grant. [1] J.E. Sipe and A.I. Shkrebtii, Phys. Rev. B, \textbf{61}, 5337 (2000) [Preview Abstract] |
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C1.00238: Low-energy Landau levels and magneto-optical properties of the zigzag graphene ribbons Y.C. Huang, C.P. Chang, M.F. Lin The Peierl's coupling tight-binding model is employed to investigate low-energy Landau levels and magneto-absorption spectra of the zigzag graphene ribbons. The Landau-level features vary with the ribbon width: the greater the ribbon width, the more the Landau levels. What should be noticed is that the positions of Landau levels change little in spite of the increasing of the ribbon width. The Landau level energy satisfy the relation $E_n\propto\sqrt{|n|B}$ , where n is subband index and B is field strength. The associated absorption spectra exhibits richer discrete Landau peaks. The transition channels related to Landau peaks are identified. The peak positions are independent of the ribbon width. Above all, a formula is presented to simulate the magneto-absorption spectra of a graphene in a very simple form. [Preview Abstract] |
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C1.00239: Magnetoelectronic properties of a single-layer graphite Y.H. Lai, J.H. Ho, M.F. Lin The magnetoelectronic structure of a single-layer graphite is mainly determined by the strength, the period, and the direction of the modulated magnetic field. Such field could induce the destruction of state degeneracy, the drastic change of energy dispersion, the increment of band-edge states, and the alternation of band width. Most of energy bands become nondegenerate, and the flat bands are replaced by the parabolic bands. Density of states exhibits the linear energy dependence, the square-root divergences, the logarithmic divergences, the discontinuous structures, and the delta-function-like divergences. These special structures directly reflect rich energy spectra. [Preview Abstract] |
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C1.00240: Theoretical study of adsortion of Sb on GaAs(110) surface Norberto Arzate, Raul Vazquez, Bernardo Mendoza We present a model for studying the adsorption of Sb atoms on a clean GaAs(110) surface and analyse the consequences of the adsorption on the reflectance anisotropy spectroscopy (RAS). A $1 \times 3$ unit cell is taken as a basis of the Sb-covered GaAs(110) structure. The relaxed coordinates of each configuration have been obtained thanks the use of the ABINIT code [1,2], that is based on pseudopotentials and planewaves. We allow the Sb-covered surface to be disordered by letting every surface atom move freely around its equilibrium position. In order to obtain a representative RAS spectrum of the surface we generate an ensemble with $\cal N$ different structural realizations of the surface and the ensemble RAS average is performed. \newline [1] X. Gonze, et al., Computational Materials Science {\bf 25}, 478 (2002). \newline [2] X. Gonze, et al., Zeit. Kristallogr. {\bf 220}, 558 (2005). \newline [3] Bernardo S. Mendoza, N. Arzate and R.A. V\'azquez-Nava, phys. stat. soli. (c) {\bf 2}, 4031 (2005). [Preview Abstract] |
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C1.00241: Examination of the Properties of the Ab-Initio Calculations for the Electronic Structure of GaAs and InAs G.L. Zhao, H. Jin, D. Bagayoko At present, most of the ab-initio density functional calculations for the studies of the electronic structure of solid state materials utilized non-orthogonal basis sets. The mathematical and physical properties of these methods have not been carefully studied and can be different from those utilizing orthogonal basis sets in quantum mechanics theory. We present some of the mathematical properties of the methods, based on the variational theory of Ritz-process. The utilizations of non-orthogonal basis sets in the ab-initio calculations could lead to a non-uniformity in approaching their solutions. We also studied the properties of the ab-initio calculations for the electronic structure of GaAs and InAs. Acknowledgments: this work was funded in part by NSF (Award No. 0508245) and ONR (Grant No: N00014-05-1-0009). [Preview Abstract] |
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C1.00242: Reflectance anisotropy spectroscopy of CdTe(001) surfaces Raul Vazquez, Norberto Arzate, Bernardo Mendoza The spectroscopical reflectance anisotropy (RA) response of (001) surfaces of CdTe, which exhibits a $(2 \times 1)$ and $(2 \times 1)$ surface reconstructions, is studied using two theoretical aproaches: an $ab$ $initio$ pseudopotential calculation in the framework of the density functional theory and within the local density approximation (DFT-LDA) is used to obtain the relaxed atomic positions, and a microscopic formulation based on a semi-empirical tight binding (SETB) approach which includes spin-orbit (SO) interactions[1] is used to obtain the RA spectra. We show RA spectrum of each surface reconstruction and compare our theoretical results with experimental results[2]. We find a good agreement between experimental and theoretical spectra. We show how the RA changes when SO is taken into account and compare our theoretical results with experimental measurements of Ref. 2. [1] R.A. V\'azquez-Nava, B.S. Mendoza and C. Castillo, Phys. Rev. B {\bf 70}, 165306 (2004). [2] R. E. Balderas-Navarro, K. Hingerl, W. Hilber, D. Stifter, A. Bonanni and H. Sitter, J. Vac. Sci. Technol. B {\bf 18}, 2224 (2000). [Preview Abstract] |
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C1.00243: Effects of spin filter on singlet and triplet exciton fractions in organic light-emitting devices Lianbin Niu, Xiaohong Zhang, Fujia Zhang In recent years spin and magnetic field effects are investigated in organic light-emitting devices (OLEDs). We studied the electroluminescence current efficiency at room temperature under parallel and antiparallel magnetic field in OLEDs with spin filter. The magnetic field applied parallel to the electrode allows the magnetization of the electrode to be switched independently. The current efficiency for the antiparallel magnetic field is found to be enhanced as compared to the parallel one. We show that this increase is an evidence of spin injection and a consequence of the magnetic-field dependence of the singlet-to-triplet exciton ratio in OLEDs, suggesting that the spin-polarized electrons were injected into the organic molecules from the spin filter under electrical excitation. Therefore, the use of spin filter demonstrates a pathway to tune the optoelectronic properties that are related to singlet and triplet states in OLEDs. [Preview Abstract] |
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C1.00244: INSULATORS AND DIELECTRICS |
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C1.00245: An optical criterion to obtain crystalline miscible mixtures in alkali halides Ricardo Rodriguez-Mijangos, Gustavo Vazquez-Polo, Raul Perez-Salas This work gives a novel criteria for obtain solid solutions in alkali halide solids and discuss some results obtained in the development of ternary and quaternary miscible crystalline dielectric mixtures of alkali halides. The mixtures are miscible in any proportion of their concentration. The obtaining of these mixed crystals is quite related to the F center through to the F center through the behavior observed in the optical absorption F band as a function of the lattice constant of the alkali halide where this was formed ( F center). Visually, in an energy curve of the F band versus lattice constant, an agglomeration of points is observed , which sets the standard for considering that if the materials that corresponds to these points were to be mixed, a solid solution could be obtained, meaning, a single phase crystal , which resulted in ternary and quaternary mixed crystals. Thus, the optical absorption F band allows obtaining a numerical criteria, based on the percentage of the F band energy, in order to obtain solid solutions. [Preview Abstract] |
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C1.00246: Synthesis, characterization and dielectric properties of new BLnZT nanostructured ferroelectric thin films P. Prieto, M.E. Gomez, E. Delgado, C. Ostos In this work we have investigated the synthesis and ferroelectric properties of Ba$_{1-y}$ La$_{2y/3}$Ti$_{1-x}$Zr$_{x}$O$_{3}$ (BLZT) compounds maintaining the stoichiometry and perovskite structure. In order to obtain the BLZT, conventional ceramic method and a novel oxalate-peroxide process were compared. By using powder X-ray diffraction the ranges of solid solution for the two methods are built-up and compared. The morphology of pure BLZT is studied by SEM. The composition is determined by ICP. Samples showed dielectric constants significantly higher than PZT compounds. BLZT films were deposited by RF-magnetron sputtering under high-oxygen pressure on different substrates at 873 K. The films reveal [00l] epitaxial reflections corresponding to perovskite single-phase compounds, showing stoichiometries corresponding to BLnZT (Ln=Nd, La). Ferroelectric measurements through hysteresis P-E curves were obtained in Ba$_{0.90}$La$_{0.067}$Ti$_{0.91}$Zr$_{0.09}$O$_{3}$, capacitor structures showing clear ferroelectric behavior with P$_{r}$, P$_{s}$ and E$_{c}$ of 11.9 $\mu $C/cm$^{2}$, 36.8 $\mu $C/cm$^{2 }$and 38.6 kV/cm, respectively [Preview Abstract] |
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C1.00247: Structural and spectroscopic characterization of double perovskites La$_{2}$Co$_{4/3}$M$_{2/3}$O$_{6}$ (M= V, Nb or Ta) and La$_{4/3}$Bi$_{2/3}$Co$_{4/3}$Ta$_{2/3}$O$_{6 }$ V.C. Fuertes, M.C. Blanco, D.G. Franco, J.M. De Paoli, F.P. de la Cruz, N.E. Massa, R.E. Carbonio We report the synthesis of the title compounds and the refinement of their structures using Rietveld analysis of powder XRD data. All belong to the monoclinic space group P2$_{1}$/n. While the crystallographic formula can be written as La$_{2}$(Co)$_{2a}$(Co$_{1/3}$M$_{2/3})_{2b}$O$_{6}$ for M= Nb or Ta, indicating the highest possible order for the cations, we find La$_{2}$(Co$_{2/3}$V$_{1/3})_{2a}$(Co$_{2/3}$V$_{1/3})_{2b}$O$_{6 }$for V replacement indicating the highest disorder configuration. The cell volume for La$_{2}$Co$_{4/3}$M$_{2/3}$O$_{6}$ decreases as the ionic radius of the M$^{5+}$ cation decreases. These compounds were previously shown to be ferrimagnetic with Curie temperatures in the range 60-70 K. Since the incorporation of cations with the sp hybridized lone pair can potentially induce ferroelectric properties, we also synthesized La$_{4/3}$Bi$_{2/3}$Co$_{4/3}$Ta$_{2/3}$O$_{6}$ where we found a partial cations disorder. Crystal and magnetic structures refined with powder neutrons diffraction measurements as well as the infrared spectroscopic characterization, now in progress, will be presented. [Preview Abstract] |
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C1.00248: Synthesis of CaCu$_{3}$Ti$_{4}$O$_{12}$ by wet-chemistry methods Wai-Ning Mei, Jianjun Liu, Yucheng Sui, Robert W. Smith CaCu$_{3}$Ti$_{4}$O$_{12}$ was reported to have a giant dielectric constant up to 10$^{4}$ at room temperature, which has tremendous potential for technological application. Solid-state synthesis of CaCu$_{3}$Ti$_{4}$O$_{12 }$from component oxides requires repetitive grinding and firing at high temperature and long time (1000 $^{o}$C, 20 hours). However, the resulting sample may still contain secondary phase or unreacted components. Whereas, the wet-chemistry method starts with a homogeneous liquid solution of cation ingredients with metal ions already mixed at the atomic scale. Therefore, the pure sample in nanometer scale can be obtained at low temperature with short time. In this work, we synthesized CaCu$_{3}$Ti$_{4}$O$_{12}$ by using two solution combustion methods at 700 $^{o}$C and 5 hours. Two CaCu$_{3}$Ti$_{4}$O$_{12}$ precursors were obtained by combusting citrate-nitrate gel and acetate-nitrate solution, respectively. The phases of the precursors after treated at different temperature were characterized by using the X-ray diffraction. [Preview Abstract] |
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C1.00249: Protective SiAlON Coating with Embedded High Temperature Corrosion Sensor S.B. Rivers, J.I. Krassikoff, G. Bernhardt, C.J. Bocchino, R.J. Lad We have fabricated oxidation resistant and wear resistant SiAlON films by RF magnetron co-sputtering of Al and Si targets in Ar/O$_{2}$/N$_{2}$ mixtures. The SiAlON films are amorphous, extremely smooth, and have very low oxygen permeability making them extremely attractive as passivation layers on a variety of sensors and other microelectronic devices used in high-temperature oxidizing environments. To analyze the performance of these films as passivation coatings, we incorporated a corrosion sensor at the SiAlON-substrate interface to monitor oxygen penetration through the coating. This corrosion sensor is a serpentine metal pattern. The conductivity changes as a function of time are monitored giving an \textit{in situ} quantitative measure of the oxidation rate of the metal sensor due to oxygen diffusion through the SiAlON film. [Preview Abstract] |
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C1.00250: Ceramic Polymer Composites Jayakumari Isac, Sabu Thomas, Soosy Kuryan Nanocrystaline Powders of PZT ceramic material were prepared by conventional ceramic preparation technique. Studied its particle size using Xrd and SEM. Prepared composites out of it by mixing it with Polystyrene by melt mixing method. Studied its mechanical, Electrical and surface tensional properties. Addition of fillers enhances the mechanical and electrical properties of polystyrene. All the results are in close agreement with its uses in electronic industry. [Preview Abstract] |
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C1.00251: Major Improvements in Photorefractive Two-Beam Coupling Using Synergistically Doped Potassium Niobate Dean Evans, Gary Cook, Mohammad Saleh Significant improvements have been made in photorefractive potassium niobate by substitutionally replacing potassium ions in the host lattice with ions of appropriate size, charge, and coordination number. The modified lattice accounts for the enhanced optical and electrical properties, as well as a substantial increase in trap density ($>$30X's). These material modifications lead to a considerable improvement in photorefractive two-beam coupling efficiency (i.e. photorefractive gain). Two-beam coupling via photorefractive gratings has been demonstrated in both transmission and self-pumped reflection geometries. [Preview Abstract] |
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C1.00252: Photoluminescence of Pb$_{1-X}$Cd$_{X}$J$_{2 }$ alloys --new radiation detector materials Yuriy Gnatenko, Igor Beynik, Pavlo Skubenko It is well known that lead iodide (PbJ$_{2})$ crystals are promising materials for radiation detectors operating at room temperature. In this paper we are reporting the first study of photoluminescence (PL) spectra of 2H-Pb$_{1-X}$Cd$_{x}$J$_{2}$ (X=0.02 -- 0.40) alloys. It was shown that for 2H-PbJ$_{2}$ PL line of free excitons corresponds to E=2.497 eV. Other exciton line at E=2.492 eV is assigned to bound excitons. The most intensive PL wide structural band is near energies 2.38-243 eV which corresponds to the recombination of donor-acceptor pairs. For Pb$_{1-X}$Cd$_{x}$J$_{2}$ crystals the exciton lines are shifted to the short-wavelength region ( for X=0.10 and X=0.30 the position of bound exciton line corresponds to 2.567 eV and 2.654 eV, respectively. It indicates about the formation semiconductor alloys. Analysis of form and energy position of PL lines shown that for X$\le $0.20 these alloys are homogeneous. At larger value X the formation of CdJ$_{2}$ clusters in PbJ$_{2}$ crystals take place. For X$\ge $0.40 the crystals are strongly inhomogeneous. [Preview Abstract] |
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C1.00253: Optical properties of epitaxial Ba$_{1-x}$Sr$_{x}$TiO$_{3}$ films on MgO(100) substrates S.G. Choi, Q.X. Jia, A.M. Dattelbaum, S.T. Picraux We present optical properties of Ba$_{x}$Sr$_{1-x}$TiO$_{3}$ (0.0 $\le \quad x$ $\le $ 1.0) films grown epitaxially by pulsed laser deposition (PLD) on MgO (100) substrates. Nominal thickness of the grown layers was 5000{\AA}. Room-temperature pseudodielectric function spectra $<\varepsilon >$ = $<\varepsilon _{1}>+i<\varepsilon _{2}>$ of the alloys were measured by spectroscopic ellipsometry from 1.5 to 5.0 eV. The spectra obtained exhibit two critical-point structures at $\sim $ 4.2 and $\sim $ 4.8 eV as well as the fundamental absorption edge (the $E_{0})$ below 4 eV. Analysis of second energy derivatives calculated numerically from the measured date yielded critical-point energies of interband transitions. Dependence of critical-point energies on alloy composition is discussed. [Preview Abstract] |
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C1.00254: Magneto-optic Properties of Co$_{1+x}$Ge$_{x}$Fe$_{2-2x}$O$_{4}$ Seong-Jae Lee, Sang-Hoon Song, David Jiles Recently, cobalt ferrite composites and their modifications based on chemical substitution have been actively investigated as part of the development of materials for highly sensitive non-contact stress and torque sensors. Research into the magneto-optic properties of cobalt ferrites and their modifications has also been undertaken because of their potential application to magneto-optic recording media. In the present study, the polar Kerr rotations for a series of Ge-substituted cobalt ferrite Co$_{1+x}$Ge$_{x}$Fe$_{2-2x}$O$_{4}$ samples have been investigated. The samples were prepared by standard powder ceramic techniques and the photon energy range of the magneto-optic polar Kerr spectra was measured between 1.5 and 3.0 eV with increments of 0.05 eV. Comparison the polar Kerr rotation spectra for Co$_{1+x}$Ge$_{x}$Fe$_{2-2x}$O$_{4 }$with those of recently reported CoFe$_{2-x}$Ga$_{x}$O$_{4}$ samples and the analyses of the origin of the peaks in the polar Kerr spectra and the observed shift of peak position resulting from Ge-substitution using the intervalence charge-transfer (IVCT) transition, in which an electron from a metallic cation is transferred to a neighboring cation through an optical excitation, will be presented. [Preview Abstract] |
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C1.00255: Field-induced rectification in rutile single crystals. John R. Jameson, Yoshiaki Fukuzumi, Koji Tsunoda, Zheng Wang, Peter B. Griffin, Yoshio Nishi A previously unknown resistive memory effect is reported in rutile titanium dioxide. Two Pt electrodes were placed on the surface of a rutile crystal, and a large voltage was applied between them. Afterwards, the device allowed current to pass in the direction of the voltage, but not in the other direction. The orientation of this rectification could then be switched by applying a large voltage of opposite sign. The effect was observed with electrodes on the (100) or (110) surfaces, but not the (001) surface. A plausible explanation is the field-induced motion of oxygen vacancies, which the large voltage might cause to pile up under the negative electrode, eliminating a Schottky barrier at that interface, but leaving a Schottky at the positive electrode intact. Parallels are drawn to other memory effects in titanium dioxide. [Preview Abstract] |
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C1.00256: Dielectric properties of correlated oxides: a GGA+U electric-enthalpy approach Oswaldo Dieguez, Paolo Umari, Matteo Cococcioni, Nicola Marzari The dielectric response of correlated-electron oxides is often overestimated in common density-functional theory approaches, such as the local-density or the generalized-gradient approximations, in a failure that closely mirrors the underestimation of the Kohn- Sham band gap. A Hubbard $U$ correction can greatly improve the description of the electronic ground-state for these difficult systems; in addition, $U$ can be derived fully from first-principles using a linear-response approach,\footnote{M.\ Cococcioni and S.\ de Gironcoli, Phys.\ Rev.\ B {\bf 71}, 035105 (2005)} making the approach parameter-free. Here, we show that an electric-enthalpy formulation allows to straightforwardly calculate the dielectric properties in the presence of a Hubbard $U$ term, leading to much closer agreement with experiments for the paradigmatic case of $3d$ transition-metal oxides. [Preview Abstract] |
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C1.00257: Dielectric Properties of Barium Titanate/ Polybenzoxazine Composites for Electrical Applications Gasidit Panomsuwan, Hatsuo Ishida, Hathaikarn Manuspiya Polymer-ceramic composites are candidate materials for electrical applications, especially embedded capacitors. In this study, barium titanate (BaTiO$_{3})$ powders prepared from sol-gel process were incorporated into polybenzoxazine matrix to form composites with 0-3 connectivity. The dielectric properties of these composites, including dielectric constant and loss tangent, were studied as a function of BaTiO$_{3}$ contents in the frequency range of 1 kHz-10 MHz at room temperature. The benzoxazine coating on BaTiO$_{3}$ powders have been used to prevent the agglomeration of ceramic fillers. The distribution of BaTiO$_{3}$ powders in the matrix was investigated by using scanning electron microscope (SEM). Moreover, the dielectric constant of these composites will be fitted with theoretical models for 0-3 connectivity (Yamada's model). [Preview Abstract] |
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C1.00258: Ferroelectric properties of lanthanum and praseodymium doped bismuth titanate thin films Menka Jain, BoSoo Kang, Quanxi Jia Ferroelectric thin films have received considerable attention recently due to their potential applications in the nonvolatile random access memory. Bi-layered ferroelectrics materials such as Bi$_{3.25}$La$_{0.75}$Ti$_{3}$O$_{12}$ (BLT) and SrBi$_{2}$Ta$_{2}$O$_{9}$ are particularly attractive for such applications due to their lower operating voltage and improved fatigue behavior compared to lead zirconium titanate. We have used a chemical solution deposition technique to grow thin films of BLT and Bi$_{4-x}$Pr$_{x}$Ti$_{3}$O$_{12}$ (BPT) on Pt/Si substrates with and without conductive LaNiO$_{3}$ layer. The remanent polarization (2$P_{r})$ of BLT film on Pt/Si, under an applied field of 396 kV/cm, was found to increase from 26 \textit{$\mu $}C/cm$^{2}$ to 43.14 \textit{$\mu $}C/cm$^{2}$ if the BLT film was deposited on LaNiO$_{3}$/Pt/Si substrate. Importantly, BLT films on LaNiO$_{3}$/Pt/Si showed no degradation in the switchable polarization ($P_{sw}-P_{ns})$ after 10$^{10}$ switching cycles. Ferroelectric and leakage current characteristics of BLT and BPT will be discussed as well in this presentation. [Preview Abstract] |
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C1.00259: Biaxial Stress-Induced Domain Wall Motion at Room Temperature in Polycrystalline Lead Zirconium Titanate Thin Films Ricardo Zednik, Paul McIntyre Wafer curvature methods can be used to impose pure biaxial tensile and compressive stresses on thin-films. This makes it possible to study the isolated effects of biaxial stress on the ferroelastic domains in ultra-fine grained PZT. Electrical measurements, such as capacitance-voltage and polarization-field hysteresis, were conducted as a function of applied stress and complemented with in-situ high resolution synchrotron X-ray diffraction measurements performed at the Stanford Synchrotron Radiation Laboratory. Systematic correlation of synchrotron scattering data with the electrical properties of the films shows that applied biaxial stress results in a marked change in the film's ferroelastic domain populations at room temperature. The large magnitude changes in ferroelectric and dielectric properties of thin film capacitors are consistent with the observed changes in relative volume fractions of the in-plane (a-axis) and out-of-plane (c-axis) oriented tetragonal PZT domains. This fully-reversible effect is symmetric in both tensile and compressive stress states. Our results, obtained from columnar-structure, fiber-textured PZT thin films, will be compared to reported data for ferroelastic domain wall motion in bulk and epitaxial specimens to assess the influence of PZT crystallite size and sample geometry on this phenomenon. [Preview Abstract] |
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C1.00260: Cation-, dipole-, and spin-order in perovskite oxides. Xiaoli Tan, R. William McCallum, Vladimir Antropov Single phase compounds based on the Pb(Fe$_{2/3}$W$_{1/3})$O$_{3}$ perovskite oxide were designed and synthesized to develop long range cation order on the B-site in the ABO$_{3}$ structure. Pure Pb(Fe$_{2/3}$W$_{1/3})$O$_{3}$ oxide displays 1:1 cation order and electric dipole order both at only the nanometer scale. As a consequence of the nanoscale polar order, it shows a relaxor ferroelectric behavior with zero remanent polarization. Chemical doping schemes were designed to develop long range cation order. Presumably a long range polar order will accompany the long range cation order. Therefore, a normal ferroelectric behavior with some remanent polarization is expected. Since perovskite oxide structure favors the G-type antiferromagnetic order, an uncompensated antiferromagnetic order is also expected since the different occupancy of the Fe$^{3+}$ on the two B-site sublattices. As a consequence, this will lead to an oxide with net magnetization. In this experimental study, the Pb(Fe$_{2/3}$W$_{1/3})$O$_{3}$ perovskite oxide was chemically modified with Zn-, Sc-, and In-dopants. Long rang B-site cation order was successfully developed. Ferroelectric and magnetic properties measurements have shown that these double-perovskite compounds are promising magnetoelectric multiferroic materials. [Preview Abstract] |
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C1.00261: Effect of solvent on the conformation and dynamics of Aspartic Acid Protease by a coarse-grained bond-fluctuating Monte Carlo simulation Ras Pandey, Barry Farmer In a coarse-grained description of a protein chain, all of the 20 amino acid residues can be broadly divided into three groups, hydrophobic (H), polar (P), and electrostatic (E). A protein can be described by tethered nodes in a chain with a node representing the amino acid group. Aspartic acid protease consists of 99 residues in a well-defined sequence. The specific sequence of H, P and E nodes tethered together by fluctuating bonds is placed on a cubic lattice where empty lattice sites constitute an effective solvent medium. The amino groups (nodes) interact with the solvent (S) sites with appropriate attractive (HS) and repulsive (PS) interactions with the solvent. Each node executes its stochastic movement with the Metropolis algorithm. Variations of the root mean square displacements of the center of mass and that of its center node of the protease chain, and its gyration radius with the time steps are examined for different solvent strength. The structure of the protease swells on increasing the solvent interaction strength which tends to enhance the relaxation time to reach diffusive behavior of the chain. [Preview Abstract] |
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