Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session J1: Poster Session II |
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Room: Baltimore Convention Center Exhibit Hall, 2:00pm - 5:00pm |
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J1.00001: METALS POSTER SESSION |
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J1.00002: Mechanical Spectral Hole Burning -- A Technique to Characterize Dynamic Heterogeneity Xiangfu Shi, Qian Qin, Gregory McKenna The Mechanical Spectral Hole Burning (MSHB) technique was developed and initially used to qualitatively characterize dynamic heterogeneity of polymer melts and solutions. Here we attempt to develop a quantitative relationship between the heterogeneity and the length scale, by developing a ``calibration'' for the dynamic heterogeneity, i.e. the frequency and magnitude of the burned hole and the size or length scale. Here we consider the local heterogeneity and well characterized structures of macroscopically homogeneous systems, such as miscible polymer blends and block copolymers in the disordered state, but approaching the phase boundary. We interrogate these materials using the MSHB method at different temperatures going through the phase boundary. Presently and for a styrene-isoprene-styrene triblock copolymer, we find that the hole amplitudes increase as the phase boundary is approached from the one-phase region, i.e., as the order-disorder transition is approached. Further work is ongoing to use miscible blends and other block copolymer systems to determine if the MSHB response can be related quantitatively to the known heterogeneities of the systems. [Preview Abstract] |
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J1.00003: Ferromagnetic single-electron transistors fabricated by atomic force microscopy Ruisheng Liu, Hakan Pettersson, Lukasz Michalak, Carlo Canali, Lars Samuelson We report on the fabrication and magneto-transport measurements of Ni/Au/Ni ferromagnetic single-electron transistors (F-SETs), fabricated by atomic force microscopy. By positioning a single Au disc (30 nm in diameter) into the gap between the Ni drain and source electrodes (of width 220 nm and 80 nm, respectively) step-by-step with Angstrom precision, and using plasma-processed NiO$_{x}$ as tunneling barriers, we can successfully fabricate F-SETs of high quality and substantial stability. The characteristic time interval of the device between two successive tunneling events is $\sim $10ps. The absence of any clear features in the transport related to the applied external magnetic field indicates that no spin-accumulation is maintained in the central Au disc. This interesting result indicates that the spin-relaxation time inside the central island should be shorter than 10ps. Based on these findings, we will discuss possible mechanisms of spin-relaxation in metal nano-structures triggered by spin-orbit interaction. [Preview Abstract] |
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J1.00004: Molecular dynamics simulations of equilibrium and undercooled liquid Ni Alvaro Posada-Amarillas, Efra\'{i}n Urrutia-Ba\~{n}uelos, Roberto N\'{u}nez-Gonzalez, I.L. Garz\'{o}n Molecular dynamics computer simulations were performed for equilibrium and undercooled liquid Ni. The Gupta potential was used to mimic the interatomic metallic interaction for different thermodynamic states. The inherent structures formalism is used to explore the potential energy landscape, showing a competition between periodic crystalline order (fcc, rhcp and bcc) and regular and distorted icosahedral order. For undercooled liquid Ni, a correlation between the entropy and the change in the crystalline (fcc) and icosahedral order is shown. This is the first simulation study which shows an interplay between the microstructural ordering in undercooled liquid metals and the entropy of the system. [Preview Abstract] |
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J1.00005: Elastic properties and phase stability of AgCl under external pressure Alejandro Bautista-Hern\'{a}ndez, Pedro H. Hern\'{a}ndez, Gregorio Hern\'{a}ndez-Cocoletzi, J. Francisco Rivas-Silva Ab initio total energy calculations have been performed to study AgCl phase stabilities. Our calculations are done using the density functional theory within the local density approximation, for the exchange and correlation energies. The ion-electron interactions are treated with the Troullier-Martins pseudopotentials. We determine the equation of state and free energy in the B1 (NaCl) and B2 (CsCl) phases. To obtain the elastic constants, we take the second derivative of the total energy respect to the applied deformations in both B1 and B2 phases. Our calculated structural parameters of the phases B1 and B2 of AgCl are compared with those available experimental data and theoretical calculations based on the local density approximation. The stability analysis based on elastic coefficients yields a critical pressure of 6 GPa at which the ideal lattice of B1 structure becomes unstable against homogeneous tetragonal shear deformation. The B2 structure is unstable in the interval of pressures 0-25 GPa. Therefore, a pressure induced phase transition between the structures B1 and B2 is not allowed elastically. We also explore the atomic structure of AgCl in the KOH phase. [Preview Abstract] |
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J1.00006: Single-Wall Gold Nanotubes R. Tugrul Senger, Sefa Dag, Salim Ciraci In recent ultra-high-vacuum transmission-electron-microscopy experiments evidence is found for the formation of suspended single-wall nanotubes (SWNTs) of gold. Similar to carbon nanotubes, the (n,m) notation defines the structure of the gold SWNTs. Experimentally, only the (5,3) tube has been observed to form among several other possible alternatives. Using first-principles calculations we demonstrate that gold atoms can form both freestanding and tip-suspended, chiral, single-wall nanotubes. Although freestanding, infinite (5,5) tube is found to be energetically the most favorable, the experimentally observed suspended (5,3) tube corresponds to a local minimum in the variation of wire-tension with the radius of the structure, which explains the experimental finding. Similarly, we predict the (4,3) tube as a favorable structure yet to be observed experimentally. Analysis of band structure, charge density, and quantum ballistic conductance suggests that the current on these nanotubes is less chiral than expected, and there is no direct correlation between the numbers of conduction channels and helical strands. \newline [1] R.T. Senger, S. Dag, and S. Ciraci, Phys. Rev. Lett. \textbf{93}, 196807 (2004). [Preview Abstract] |
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J1.00007: Role of relativity in electron impact ionization Bidhan C. Saha, Arun K. Basak, M. Alfaz Uddin The recently modified MBELL parameters[1] are generalized in terms of nl quantum numbers and applied to few selective systems. We intend to describe the procedure and to report the excellent predictive role of this simple procedure[2]. \newline \newline [1] A. K. F. Haque, M. A. Uddin, A.K. Basak, K. R. Karim and B. C. Saha, Phys. Rev. A 76 (in press) \newline [2] A. K. F. Haque, M. A. Uddin, A.K. Basak, K. R. Karim, B. C. Saha, and F. B. Malik, Phys. Rev. Lett (under consideration) [Preview Abstract] |
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J1.00008: Density-Functional Calculation for the Junction between Metal and Semimetal Chin-Sheng Wu . We apply the density-functional theory to calculate the junction characteristics between the metal and semimetal. We obtain the self-consistent solution of the charge distribution both when Fermi levels of the metal and semimetal are set equal and when they are set unequal. The differential junction capacitance is obtained by differentiating the areal charge density with respect to the shift of Fermi levels. The electronic structure, potential, differential junction capacitance for the interface are reported. Our calculations for the metal with the conduction electron densities r=2, 6, and 12 a.u., and the semimetal with r=24 a.u. [Preview Abstract] |
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J1.00009: Asymmetric confinement of an electron in a double-well potential and its relation with molecular lasers Ivan Marin-Enriquez, Jose Luis Marin-Flores, German Campoy-Guere\~{n}a, Raul Riera-Aroche The confinement of an electron in an asymmetric double-well is proposed as an alternative system in which transitions and tunneling between bound states can have a similar behavior as in double-square well structures, regarding lasing properties. We show that this system would be more reliable than actual devices, since the relevant factor related to its efficiency is sharper and can be shifted in wavelength in a wide range as compared with, namely, its closest partner, the square double-well potential. Since nowadays technology allows the deposition of atom by atom (or molecule by molecule) layers of active materials, we think that such a device is feasible of construction, and represents a very attractive subject for research at both experimental and theoretical physics. [Preview Abstract] |
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J1.00010: Wannier-Like Functions of Cesium Under High Pressure Eva Zurek, Ove Jepsen, Ole Krogh Andersen At ambient pressures cesium metal is found in the body centered cubic structure. At higher pressures, it undergoes a series of phase transitions which are accompanied by an electronic transition of the valence $s$ electron to $d$-like states. We first of all propose a procedure which may be used to obtain Wannier-like functions for metallic systems using the newly developed NMTO (Muffin Tin Orbitals of order $N$) method. Wannier-like orbitals for a subset of the experimentally determined crystal structures of cesium are calculated. The electron density obtained from these orbitals is compared with that obtained from standard LMTO calculations. These orbitals give clear evidence of the electronic transition: they are primarily $s$-like at ambient volumes and display an increasing amount of $d$-character for structures found at higher pressures. [Preview Abstract] |
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J1.00011: The effect of M (M=Ti, Cr, V, Nb) on transport and elastic properties of nanolayered ternary carbides M$_{2}$AlC Jeff Hettinger, Peter Finkel, Sam Lofland, Michel Barsoum, Adrish Gupta We report on a systematic investigation of the electronic, magneto-transport, thermal and elastic properties of the family of materials M$_{2}$AlC where M is Ti, V, Cr or Nb in the temperature range 4 to 300K. The elastic constants were measured for all compounds ultrasonic technique. The bulk moduli and anisotropic Young's moduli found to be varied in these compounds for various transition metal M. The Debye temperatures were high in the 640-710 K range and quite insensitive to composition. The Seebeck coefficient was a non-monotonic function of a temperature: at the lowest temperatures is small but increases with increasing temperature and saturates at 60-80 K and goes through zero again manifesting change in the dominating charge carrier type. The electrical conductivity, Hall coefficient and magnetoresistances are analyzed within a two-band framework assuming a temperature-independent charge carrier concentration. We concluded that there is little correlation between the Seebeck voltage and Hall number. As with other MAX-phase materials, all these materials are nearly compensated. Comparisons of these results will be presented. Results will be discussed in relation to theoretical work and recent measurements on related systems. [Preview Abstract] |
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J1.00012: Vacancy Scanning Tunneling Spectroscopy of Ag (111) Danda P. Acharya, Kai-F. Braun, Saw-Wai Hla We investigate the vacancy electronic structures on a Ag(111) surface at 4.6 K by using scanning-tunneling-microscopy (STM) manipulation and spectroscopy in an ultra-high-vacuum environment. The vacancies with single atom to over a hundred-atom sizes are created on atomically cleaned Ag(111) surface by controlled tip-sample contact and single atom manipulation. Conductance tunneling spectroscopy data are then acquired by using a lock-in amplifier attached to the STM. The shift of the on-set of Ag(111) surface state is observed in few-atom vacancies. For the larger vacancies, we are able to probe the bulk silver unoccupied s-p band using tunneling spectroscopy. This experiment demonstrates a unique capability of a combined STM manipulation and spectroscopy to probe bulk and surface properties of materials through engineered atom and nanoscale cavities. This work is financially supported by a US-DOE grant, DE-FG02-02ER46012, and a NSF-NIRT grant, DMR-0304314. [Preview Abstract] |
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J1.00013: Analytic treatment of metallic multilayer strength at all length scales Lawrence Friedman, Lei Fang Metallic multilayers can be used as ultra-high strength coatings. They exhibit a very pronounced size-effect where the mechanical strength depends on the layer thickness. Traditionally, the Hall-Petch Relation is used to describe the size effect. The Hall-Petch Relation is based on dislocation pileup theory, which states that the macroscopically observable strength is determined by dislocation obstacles and stress multiplication from pileups. However, more rigorous application of dislocation pileup theory as applied to multilayers predicts significant deviation from the Hall-Petch Relation due to elastic inhomogeneity, discreteness of dislocations and dislocation source operation. The necessary modifications to the Hall-Petch Relation are presented. An analytic formula accounting for these effects can only be obtained in a piecewise fashion. The variation of strength with layer thickness must be broken down into four length-scale regimes, and a simple analytic formula is obtained for each regime. This formulation allows one to bridge the length scales and predict multilayer strength from microscopic parameters (interface strength and dislocation source characteristics) and fundamental material parameters (elastic moduli, layer thickness and crystal structure). Finally, the theory is applied to Cu/Ni multilayers and theory prediction is compared with experimental data. [Preview Abstract] |
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J1.00014: Low-temperature Superplasticity of Dual-phase Ti$_{3}$Al Based Alloy Kai-Lin Yang, Chih-Ching Huang To date, the Ti$_{3}$Al based alloys have been widely reported their admirable superplasticity within the temperature ranges of 900-1000$^{o}$C. However, the superplastic behavior of temperature lower than 900$^{o}$C was seldom reported. The current paper will be aimed on the superplastic behavior at lower temperatures (700-900$^{o}$C). The phase transformation of $\beta \to \alpha _{2}$ in the two-phase ($\alpha _{2}+\beta )$ region during static annealing and superplastic straining at 700-1000$^{o}$C was occurred in this ($\alpha _{2}+\beta )$ dual-phase alloy. In addition, the misorientation angle distributions and texture characteristics of the $\beta $ and $\alpha _{2}$ phases were examined by the electron backscattered diffraction (EBSD) to complete the deformation behavior. Under the optimum LTSP condition at 850$^{o}$C and the strain rate of 5x10$^{-4}$ s$^{-1}$, texture evolution is also investigated. From the texture characteristics associated with microstructure, the transformation behavior related to the deformation behavior could be demonstrated more clearly. In this current study, the texture characteristics, phase transformation phenomena, and the deformation mechanism at lower temperatures (below 900$^{o}$C) will be related and discussed. [Preview Abstract] |
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J1.00015: ABSTRACT WITHDRAWN |
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J1.00016: Designing Laser Pulses to Control Molecular Motion Qinghua Ren, Gabriel Balint-kurti, Frederick Manby, Maxim Artamonov, Tak-San Ho, Herschel Rabitz A new method for incorporating polarization effects of the strong electric field into optimal control calculations is presented. An Electric-Nuclear Born-Oppenheimer approximation is introduced in which both the nuclear motion and the fluctuations of the external electric field are assumed to be slow compared with the speed at which the electronic wavefunction responds to these changes, which permits the generation of a potential energy surface that depends not only on the geometry of the nuclei, but also on the electric field strength and on the orientation of the molecule with respect to the electric field. As an illustration, it is firstly applied to the optimal control of the vibrational excitation of a hydrogen molecule aligned along with the field direction.$^{ [1]}$ And then it is extended to full three dimensions by allowing the H$_{2}$ molecules to rotate as well as vibrate.$^{ [2]}$ All calculation results show that it is possible to design shaped laser pulses that can excite H$_{2}$ either vibrationally, rotationally or both with a probability higher than 90{\%}. $^{[1]}$ G. G. Balint-kurti, F. R. Manby, Q. Ren, M. Artamonov, T. Ho, and H. Rabitz, J. Chem. Phys. 122, 084110 (2005). $^{[2]}$ Q. Ren, G. G. Balint-kurti, F. R. Manby, M. Artamonov, T. Ho, and H. Rabitz, J. Chem. Phys. (in press). [Preview Abstract] |
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J1.00017: Auger Parameter of Aluminum in aluminum compounds A. Chourasia, R. Miller, G. Nixon X-ray photoelectron spectroscopy has been employed to study 1s core level of aluminum in elemental aluminum, and in various aluminum compounds. The XPS spectra in the various regions have been recorded in the high resolution mode. The zirconium anode (energy = 2042 eV) has been used to access the 1s core level of aluminum. The energy difference between the 1s core level and the x-ray excited Auger line of aluminum in the XPS spectrum has been used to calculate the Auger parameter. Wagner plot has been constructed from these data. In this plot the insulating materials appear lower on the grid as compared to those showing metallic nature. The points do not lie on the same line with slope +1, distinctly indicating that aluminum exists in different chemical states in these materials. The estimation of bonding characteristics from this plot will be discussed. [Preview Abstract] |
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J1.00018: Electronic Structure of Sr$_{2}$RhO$_{4}$ investigated by ARPES Seunghyeok Lee, Bum-joon Kim, Jaejun Yu, Se-Jung Oh, Changyung Kim, I. Nagai, S.I. Ikeda We investigated the electronic structure of 4d-transition metal oxides Sr$_{2}$RhO$_{4 }$by high-resolution ARPES(angle-resolved photoemission spectroscopy) and compared the results with density-functional electronic structure calculation. Sr$_{2}$RhO$_{4 }$has the same crystal structure with Sr$_{2}$RuO$_{4 }$and just one more electron than Sr$_{2}$RuO$_{4}$ in 4d orbitals. But ARPES data reveal very different Fermi surface from that expected in the simple rigid-band picture, and especially the electronic structure related to the d$_{xy}$ band-the full occupation and missing at Fermi surface of the d$_{xy}$ band- is qualitatively different from Sr$_{2}$RuO$_{4}$. This turns out to be due to the fact that RhO$_{6}$ octahedra are rotated about the c-axis. We will discuss how the rotation of octahedra change the electronic structure by comparing the ARPES experimental results with density-functional electronic structure calculation in real crystal structure of Sr$_{2}$RhO$_{4}$. [Preview Abstract] |
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J1.00019: A Generalized Model for Dielectric Relaxation and Resonance James Baker-Jarvis, Michael Janezic, Pavel Kabos In this paper we develop a very general model from our previously developed statistical mechanical theory that describes relaxation, resonance, with temperature dependence. We also analyze the model in terms of entropy and a generalized Lyddane-Sachs-Teller relationship. The model is based on a correlation function approach and complex relaxation time from our previously developed theory. The commonly used Cole-Cole, Cole-Davidson and other models are related to this model as a special case. Using a fluctuation-dissipation theorem, we develop an algorithm for extraction of the loss properties of nanowires. [Preview Abstract] |
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J1.00020: SEMICONDUCTORS POSTER SESSION |
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J1.00021: HgSe Semiconductor Nanoclusters in Zeolite A.M.M. Abeykoon, E.A. Anokhina, Miguel Castro-Colin, W. Donner, A.J. Jacobson, Simon C. Moss Our primary challenge has been to produce HgSe semiconductor nanoclusters with a precise, controllable and narrow size distribution. Since a zeolite framework constrains the size and shape of species encapsulated within a pore, we use zeolite pores to contain our nanoclusters. Two different zeolite frameworks, Linde type L (tubular pore) and Faujesite (nearly spherical pore) are used in our study. Elemental Hg and Se are embedded into dehydrated zeolite cavities via vapor phase deposition. The AXS (Anomalous X-ray Scattering) technique is used in combination with the PDF (atomic Pair Distribution Function) technique to solve for the structure of the nanocluster. Optical absorption measurements are used to determine the band gap of the cluster. We intend to perform Raman Spectroscopy to complement our determination of local structure and to probe the electronic properties of the semiconductor nanoclusters. AXS and PDF data sets were collected at X7A beam line at the NSLS. Optical measurements were carried out in the UH Chemistry Department. Currently we are analysing our x-ray data sets to yield a successful model for the nanoclusters. [Preview Abstract] |
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J1.00022: Crystallographic analysis on growth process of GaAs nanocrystals deposited onto Si(100) surface Hiroyuki Usui, Kensuke Yamada, Hidehiro Yasuda, Hirotaro Mori Crystallographic analysis on growth process was carried out for GaAs nanocrystals deposited onto Si(100) by molecular beam epitaxy. Equilibrium crystal shapes of the GaAs nanocrystals at each growth stage were observed by cross-sectional transmission electron microscopy. At the initial growth stage, Stranski-Krastanov wetting layer and facetted nanocrystals with lower aspect ratio were formed. The GaAs nanocrystals at the middle growth stage formed regular pyramids consisting of {\{}111{\}} planes. At the later stage, the shape of GaAs nanocrystals was changed to dome structure consisting of {\{}111{\}} and {\{}311{\}} planes which has lower aspect ratio. This indicates that the GaAs nanocrystals introduced {\{}311{\}} facets to minimize the total surface free energy at the later stage because the surface free energy of {\{}311{\}} facets is smaller than that of {\{}111{\}} facets. [Preview Abstract] |
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J1.00023: Study of Interfacial Water Layer during Pentacene Monolayer Deposition Songtao Wo, Binran Wang, Yiping Wang, Randall Headrick, Alexander Kazimirov This study includes the formation of pentacene monolayer on SiO2 substrate. \textit{In situ} synchrotron x-ray scattering was used to probe the early stages of pentacene growth in real time and under conditions relevant to the fabrication of Organic Thin Film Transistors(OTFTs). Reflectivity measurements reveal that there is interfacial water layer between the pentacene monolayer and SiO2, which initially present on the substrate, and is trapped at the interface when the pentacene film is deposited. Anti-Bragg oscillations, observed for the first time during organic film growth, reveal that the first monolayer of pentacene completes fully before the second one nucleates. [Preview Abstract] |
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J1.00024: 3D Simulation of Phonon Modes in Semiconductor Nanocrystals Nicholas Jurasek, Shang-Fen Ren, Wei Cheng The goal of this current research is the visualization of different phonon modes of NCs with 3D simulation to help in the understanding of their behavior. In order to realize this goal, the numeric data output from the previous atomistic model calculations is used that provides a list of positions of atoms in NCs, the vibration frequencies and the displacement vectors of atoms in each phonon mode. These phonon modes are organized in different symmetries. Methods of visualizing the movement of these atoms are searched, and VMD from the University of Illinois is chosen as the best method to carry out it. VMD can create a 3-D image based on the positions and displacements of atoms, and it allows the user to rotate the structure around and to also ``fly'' through the structure. To use VMD, a program to convert the output of the simulation program into something that VMD can read is created. In this presentation, how VMD is used to carry out the 3D simulation of phonons in NCs will be discussed, and some interesting simulation results will be presented. These simulations can be used to understand more about the phonons and their related properties of NCs. [Preview Abstract] |
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J1.00025: Determination of electron penetration depth and stopping power of amorphous AlN using luminescence of Tm and Ho ions Muhammad Maqbool, Martin Kordesch Electron penetration depth and stopping power of amorphous AlN is determined using luminescence of Tm$^{+3}$ and Ho$^{+3}$ ions. Thin film bilayers of AlN:Ho and AlN:Tm are deposited on flat Silicon substrates by RF Magnetron sputtering at liquid nitrogen temperatures. In making a bilayer, 15.3 nm thick AlN:Ho film is first deposited on a flat Si(111) substrate of 2$\times $2 cm$^{2}$ surface area. On the top of this AlN:Ho film 37.8 nm thick AlN:Tm film is deposited to make it a bilayer. Electron beam of different energies, obtained from electron gun of the CL apparatus, is allowed to penetrate in the AlN:Tm/AlN:Ho bilayer film. Blue emission from Tm$^{+3}$ as a result from $^{1}$D$_{2} \quad \to \quad ^{3}$F$_{4}$ transition and green emission from Ho$^{+3}$ as a result from $^{5}$S$_{2} \quad \to \quad ^{5}$I$_{8}$ transition were used to track the electron beam penetrating in the bilayer. Energy of the beam just crossing 37.8 nm AlN:Tm film is recorded to obtain the stopping power experimentally. Experimental results are compared to the theoretical value using the established mathematical equations for stopping power. A percent deviation of 6.6{\%} is found in the experimental and theoretical results. [Preview Abstract] |
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J1.00026: Diffusion doping and Infrared Optical Properties of Transition Metal Ions in ZnSe windows and Cd$_{0.55}$Mn$_{0.45}$Te crystals Ei Nyein, Uwe Hommerich, Ivy Jones, Monique Calhoun, Erica Haynes, Sudhir Trivedi Transition metal (TM) doped II-VI semiconductors are of current interest for tunable solid state lasers in the mid-infrared (IR) spectral region as well as passive optical Q-switches. Laser action has been demonstrated at $\sim $2.3 $\mu $m from Cr: ZnSe, Cr: ZnS, Cr: CdSe, Cr: CdTe, Cr: Cr: CdMnTe, and at $\sim $4 $\mu $m from Fe: ZnSe. In this work, we report on the material preparation and IR optical properties of diffusion-doped TM ions (Cr$^{2+}$, Fe$^{2+}$, Co$^{2+})$ in polycrystalline ZnSe windows and Cd$_{0.55}$Mn$_{0.45}$Te crystals. Different dopant sources were employed in thermal diffusion experiments including TM chloride compounds, TM selenides, and elemental TM powders. The characteristic IR absorption bands from Cr$^{2+}$, Co$^{2+}$, and Fe$^{2+}$ doped II-VI hosts were observed in all samples, independent of the dopant source. Details of the absorption and luminescence properties of these TM: ZnSe and TM: CdMnTe samples will be presented at the conference. [Preview Abstract] |
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J1.00027: On the transfer matrix method and WKB approximation for systems with spatial-dependent effective mass Chun-feng Huang, S.D. Chao, D.R. Hang, Y.C. Lee A set of coupled differential equations is derived by considering the continuous limit of the transfer matrix method, which is a numerical approach for the one-dimensional structures such as the semiconductor heterostructures. By decoupling such a set of equations, an extension to the Wentzel-Kramers-Brillouin (WKB) method is obtained to incorporate effects due to the spatial-dependent effective mass. For a traveling wave, the decoupling is to ignore the reflection resulting from the variations of both the potential and effective mass. By considering a solvable fully-quantized system, it is shown that the extended WKB method provides good approximation for the states with the high eigenenergies. [Preview Abstract] |
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J1.00028: Unusual excitation spectra of magnesium-related donor impurities in silicon L.T. Ho It is well known that the group-II magnesium, when diffused into silicon, enters the silicon lattice interstitially and behaves as a helium-like double donor. Magnesium has also been found to be able to pair with dispersed oxygen in silicon to form magnesium-oxygen complex impurity, which is an interstitial double donor as well. Previous studies on the excitation spectra of these helium-like impurities indicate that the excited states of neutral magnesium donor or neutral magnesium-oxygen complex donor are all similar to those of hydrogen-like group-V donor impurities in silicon. Recently, however, from further study of the excited spectra of these magnesium-related donor impurities, we have observed some very unusual result totally unexpected and clearly different from a typical donor spectrum in silicon. [Preview Abstract] |
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J1.00029: LDA density functional study of lithium intercalated compounds in grephite Juan Salvador Arellano, Pablo de la Mora, M. J. Lopez, L. M. Molina, J. A. Alonso Local Density Functional (LDA) calculations have been done for different lithium intercalated graphite compounds. We have explored the most common ones namely LiC$_{6}$, and LiC$_{2}$. Also the LiC$_{3}$ has been studied. To explain some details about the structural and electronic properties of the last one, we have used two computer codes, FHI98MD and WIEN2k. This was motivated by a recent work for the LiC$_{3 }$compound, where it is said that lithium atoms positions are not in the middle plane between the graphene layers formed by the carbon atoms. Discussion of the three mentioned lithium intercalated compounds and somewhat about the superconductivity will be given during the presentation. [Preview Abstract] |
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J1.00030: Electronic structure of V-doped TiO2 Yushan Wang First-principles calculations using the full-potential linearized augmented plane-wave method have been performed to investigate the electronic structure of V-doped TiO2 in the anatase modification. In calculations with local density approximation (LDA) plus U ( Hubbard coefficient) approach, V 3d states were found to be completely spin- polarized with net magnetic moment generated. [Preview Abstract] |
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J1.00031: Structural deformation and instability of strong covalent solids Changfeng Chen, Hong Sun We employ first principles total-energy calculations to study the structural deformation modes at large strains and the lattice instabilities beyond the elastic limit for several strong covalent solids. We examine the atomistic bonding changes to elucidate the microscopic mechanism for the stress response and the orientational dependence of the peak stresses along different crystallographic directions and the bond breaking modes. Implications for the mechanical properties will be discussed. [Preview Abstract] |
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J1.00032: The 4x4 spin 1/2 representation in electron polarization in a magnetic field Keshav Shrivastava We introduce a 4x4 spin representation for spin 1/2. There are two spin orientations for a given value of g$_+$ and two for g$_-$ which arise for two signs of spin. This theory produces factors which affect the product m*g* so that if these factors are not taken into account, much larger values of the electron mass than those of the electron mass in a band arise. This predicted phenomenon is in accord with the experimental data. Similarly we find that factors arise in cyclotron frequency which affect the magnetoresistance and hence the g values. Accordingly, large g values are measured in the experimental data. When our factors are taken into account, the g values compare well with band values. The g*=g$_{\pm}-\delta$ describes the shift due to many-body interactions. [Preview Abstract] |
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J1.00033: An alternative model of the integer quantum Hall effect Tobias Kramer Contrary to common belief, the current emitted by a contact embedded in a two-dimensional electron gas (2DEG) is quantized in the presence of electric and magnetic fields. This observation suggests a simple, clearly defined model for the quantum current through a Hall device that does not invoke disorder or interactions as the cause of the integer quantum Hall effect (QHE), but is based on a proper quantization of the classical electron drift motion. The theory yields a quantitative description of the breakdown of the QHE at high current densities that is in agreement with experimental data. Furthermore, several of its key points are in line with recent findings of experiments that address the dependency of the QHE on the 2DEG bias voltage, results that are not easily explained within the framework of conventional QHE models. For additional information, see also http://arxiv.org/abs/cond-mat/0509451 (accepted by IJMPB) and the article references on http://people.deas.harvard.edu/$\sim$tkramer [Preview Abstract] |
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J1.00034: Determination of the structure of ZnS:ErF$_{3}$ TFEL phosphors using XAFS Stanislav Stoupin, Carlo Segre, Mark Davidson, Paul Holloway Many phosphor systems that provide useful levels of IR emission have a poor match between dopant atom and the host structure. An example of such a system is ZnS doped with rare-earth elements incorporated into the host as trivalent ions. For example, the size of Er is quite large compared to Zn and it does not easily substitute into the same crystallographic site. We have prepared a number of ErF$_{3}$ 1 mol\% doped ZnS thin film electroluminescent (TFEL) phosphors using RF magnetron sputtering followed by annealing at various temperatures. The intensity of the IR emission is found to be very sensitive to the post-deposition annealing. As the local chemical and structural environment of the dopant atom is very important to the performance of the phosphor we have used x-ray absorption fine structure spectroscopy (XAFS) to provide detailed information on the local environment surrounding the Er atom. The XAFS spectra are consistent with Er-F complex substitution into the Zn site accompanied by creation of S vacancies. The ZnS host lattice reveals significant disorder effects in the first Zn-S coordination shell. These structural features change with annealing temperature and are likely responsible for the observed changes in intensity of the various Er emission lines. [Preview Abstract] |
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J1.00035: Surface modification of InP wafers by pulsed UV laser radiation below ablation threshold Omar Musaev, O.-S. Kwon, D.-M. Zhu, D. Wieliczka, J. Wrobel Under multi-pulse laser irradiation below the threshold of ablation modification of surface layer of semiconductor can happen. The creation of point defects may lead to their subsequent aggregation and cause macroscopic damage of the irradiated surface if the number of pulses is large enough despite that the fluency is lower than ablation threshold. Low energy pulsed laser irradiation may also cause different structural changes in the surface layer of a compound semiconductor which manifests itself in changes of physical and chemical parameters of the surface. The surface of InP wafer irradiated by different numbers of pulses with different fluences below ablation threshold was analyzed with AFM and XPS. Mechanisms of defect creation and aggregation in InP surface under low intensity laser irradiation are discussed. [Preview Abstract] |
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J1.00036: Capacitance Dispersion in GaN Grown by Molecular Beam Epitaxy Mo Ahoujja, Rex Berney, Said Elhamri, Yung Kee Yeo GaN semiconductors are of great interest for applications in high-temperature and high-frequency electronic devices as well as blue to UV light emitting and detecting devices. However, an understanding of the role of dopants and defects in these semiconductors is essential for the realization of high performance devices. Si doped GaN samples grown by radio-frequency plasma activated molecular beam epitaxy on sapphire substrates were characterized by capacitance-voltage (C-V) measurements. The C-V measurements were performed with an LCR meter in series mode with an AC modulation level of 50 mV in the frequency range of 500 Hz to 1MHz. The junction capacitance of the Schottky diodes varied with both temperature and frequency. The variation of capacitance with frequency is known as capacitance dispersion. This dispersion is generally attributed to the slow response of deep levels to the high frequency AC signals. Consequently, it is suggested that native defects in GaN play a role in the observed capacitance dispersion. [Preview Abstract] |
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J1.00037: Intervalley Transitions in GaN Single Crystals S. Wu, D. Wang, R. Sobolewski, P. Geiser, J. Jun, J. Karpinski We present the direct observation of the intervalley transition of electrons between the conduction band $\Gamma $ valley and the L valley in GaN single crystals, using the two-color (ultraviolet-near infrared), femtosecond, pump-probe spectroscopy technique. We have found that scattering of electrons from the $\Gamma $ to L valley is faster than the optical phonon emission time, while the return process of electrons from L to $\Gamma $ is measurably slower. The developed model based on three-state rate equations fitted very well our experimental results. The fitting parameters showed that the $\Gamma $-- L scattering time is $\sim $250 fs, while the L--$\Gamma $ return time is $\sim $1 ps. The optical phonon emission time was found to be equal to $\sim $500 fs. The characteristic scattering times obtained in our experiments demonstrate that the dynamics of the intervalley transition process in GaN is substantially slower than the similar transitions earlier observed for GaAs. [Preview Abstract] |
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J1.00038: Positron annihilation induced Auger electron emission from silicon carbide surfaces S. Mukherjee, M. Nadesalingam, G. Brauer, T. Nozawa, A. Kohyama, A.H. Weiss Silicon carbide (SiC) in monocrystalline, hexagonal polytype form is a very interesting material for a wide class of novel applications in electronics. SiC fiber-reinforced SiC matrix composite materials (SiC/SiC) are considered to be the attractive candidates as materials for advanced energy systems, such as high performance combustion systems, fuel-flexible gasification systems, fuel cell / turbine hybrid systems, nuclear fusion reactors, and high temperature gas-cooled fission reactors. Positron Annihilation induced Auger Electron Spectroscopy (PAES) is an established tool to characterize the top most atomic surface layer of solids. Here, PAES has been used to study the surface of 6H-SiC after annealing under different thermal and ambient conditions. In addition, results of investigating the surface of a composite consisting of sintered SiC Nanopowder and fibres of pyrolytic carbon are presented and discussed. This research supported by the Welch Foundation under Y-1100. ~ [Preview Abstract] |
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J1.00039: Anisotropy in the Amplitude of Shubnikov-de Haas Oscillations in AlGaN/GaN Heterostructures S. Elhamri, R. Berney, M. Ahoujja, G. Landis, W. Mitchel, W. Mitchell Variable temperature resistivity and Hall effect measurements were performed on several AlGaN/GaN heterostructures. The temperature dependence of the mobility and carrier density extracted from these measurements indicated the presence of a good quality two dimensional electron gas. At liquid helium temperatures, the carrier density and mobility in these samples ranged from about 2 x 10$^{12}$ to roughly 1 x 10$^{13}$ cm$^{-2}$ and from about 4000 to more than 50000 cm$^{2}$/Vs respectively. Variable field magnetoresistance studied were also conducted on several of these van der Pauw shaped samples at 1.2 K in fields up to 8 T. These measurements revealed the presence of Shubnikov-de Haas oscillations in the longitudinal magnetoresistance. However, the amplitude of the Shubnikov-de Haas oscillations originating from perpendicular van der Pauw positions were not isotropic. This anisotropy varied with the carrier density which was changed by illumination. The results of this study suggest that the anisotropy is either a manifestation of spatial carrier density inhomogeneities and/or an indication of a nonuniform scattering mechanism arising from an anisotropic interface roughness. [Preview Abstract] |
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J1.00040: Spin-Orbit Interaction for Cylindrical Nanotubes Godfrey Gumbs, Yonatan Abranyos When a gate voltage is applied perpendicular to the axis of nanotubes, this can lead to spin-orbit interaction (SOI) effects. This is of the same nature as the Rashba-Bychkov SOI at the interface of an asymmetric semiconductor heterojunction. We use a continuum model to obtain analytical expressions for the spin-split energy eigenstates for electrons on the surface of nanotubes in the presence of SOI. Each energy level could then be used to accommodate the two types of spin. For qubit functionality, one can use either one nanotube or a pair of coupled nanotubes. We investigate this scheme by calculating the exchange interaction energy and subsequently the entanglement of electron spins as they occupy allowed subbands. We also calculate the effect which SOI has on the plasma excitations since the exchange interaction can be mediated by long-range collective excitations. [Preview Abstract] |
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J1.00041: Conductivity for a Spin-Split 2D Electron System Godfrey Gumbs Recent research on spintronic systems has examined a wide variety of their transport and optical properties from both a theoretical and experimental perspective. By including spin-orbit interaction (SOI) in a model Hamiltonian, we calculate the low-frequency local conductivity, starting with Kubo's formula. For long mean-free paths, we obtain a closed-form analytical result which is a generalization of the Drude conductivity. By adding vertex corrections using maximally crossed Feynman diagrams to our result, we also obtain, to lowest order in the scattering parameter, the non-local conductivity is given in terms of function which shows that there is an ``interference" effect due to the SOI. This means that the electrons do not diffuse through the sample when the SOI is neglected, as it demonstrated several years ago. [Preview Abstract] |
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J1.00042: Effect of SOI on the Acoustoelectric Current Yonatan Abranyos, Godfrey Gumbs With a need to increase the accuracy of the measured acoustoelectric current for metrological applications, we investigate the role played by the Rashba spin-orbit (SO) coupling. With a model Hamiltonian which takes into account the confinement of electrons within a narrow channel between split metal gates, we demonstrate that the SO interaction increases the confinement of a captured electron in a moving quantum dot and may consequently improve the quenching of the quantized acoustoelectric current. [Preview Abstract] |
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J1.00043: A kinetic equation approach to the spin Hall effect in $p$-type bulk semiconductors S. Y. Liu, Norman J. M. Horing, X. L. Lei, V. Fessatidis A two-band kinetic equation method is employed here to investigate the spin-Hall effect (SHE) in a $p$-type Luttinger semiconductor. We find that the previously predicted solely intrinsic SHE arises from a dc-field induced polarization directly, or in other words, a stationary Rabi process, associated with all hole states below the Fermi surface. The total SHE, encompassing a mix of both extrinsic and intrinsic features, results essentially from a related polarization process involving electric field excitations between heavy- and light-hole bands, both directly and indirectly through scattering. In particular, we examine effects of long-range disorder on spin-Hall current (SHC) within the self-consistent Born approximation. In contrast to the vanishing SHC contribution due to short-range disorder, we show that long-range impurity scattering produces a nonvanishing SHC, independent of impurity density, having its sign opposite to that of the intrinsic SHC contribution and leading to a significant reduction of the total SHC. This disorder-generated SHE arises from a disorder-mediated interband polarization, related only to hole states near the Fermi surface. We analyze the hole density dependencies of SHC and spin mobility numerically: with increasing hole density, the SHC first increases and then decreases, while the spin mobility decreases monotonically. [Preview Abstract] |
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J1.00044: Kane model for the spin-splitting of holes in semiconductor heterostructures Arturo Wong, Francisco Mireles We present a theoretical study of the Rashba-like spin-orbit coupling in two dimensional hole systems formed in zincblende 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, light and split-off holes [1].~ 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 III-V semiconductor quantum wells. Our preliminary results indicate that the magnitude of the spin-splitting energies for heavy holes is generically larger than their counterparts for electrons. This results could be of relevance for the design and implementation of novel (hole) transport spintronic devices. [1] A. Wong and F. Mireles (in preparation) [Preview Abstract] |
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J1.00045: Structural, Magnetic and Transport Properties of Cobalt doped ZnO Epitaxial Thin Films Hyojin Lee, Sungho Lee, Fikret Yildiz, Yoon Hee Jeong Semiconductors with ferromagnetism at room temperature has been actively searched for in recent years; a prospect of spintronic devices using both charge and spin continuously gives impetus to the activities. Co substituted ZnO thin films, for example, were reported to show ferromagnetic properties at room temperature. However, various subsequent studies including ours do not seem to converge on a definite picture and controversy continues. What is needed is well synthesized and thoroughly characterized samples. A series of Zn$_{1-x}$Co$_x$O (0$=x=$0.2) thin films on sapphire (0001) substrates were epitaxially grown by using Laser MBE deposition technique, controlling laser fluence, substrate temperatures and oxygen partial pressures. In-situ RHEED pattern showed that the films grew in the layer by layer growth mode. We performed high resolution X-ray diffraction (XRD), SEM, TEM, and AFM measurements to confirm the structure and morphology of the films. XRD figures showed that the films were of single phase with the pure ZnO wurtzite structure. AFM measurements showed that the RMS values of roughness of the films were about 2$\AA$. Magnetization, resistivity, and Hall coefficient measurements were also carried out systematically as a function of $x$. These measurements revealed that Co doping brings about self-curing of oxygen vacancies. These results will be presented in detail. [Preview Abstract] |
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J1.00046: Magnetic and transport properties of cobalt doped ZnO Epitaxial thin films. Hyo-Jin Lee, Fikret Yildiz, S.-H Lee, Yoon-Hee Jeong Transition metal doped ZnO has become an important topic of scientific interest in view of diluted magnetic semiconductor (DMS) at room temperature. A series of Zn$_{1-x}$Co$_{x}$O (0$\le $x$\le $0.2) thin films on sapphire (0001) substrates were epitaxially grown by using Laser MBE deposition technique, controlling laser fluence, substrate temperatures and oxygen partial pressures. RHEED, X-ray diffraction (XRD), Atomic force microscope (AFM) scanning, magnetization and transport measurements were performed to investigate their structural, morphological, transport and magnetic properties. During the film growth, layer by layer growth mode were seen on RHEED pattern. XRD measurements showed that the films have the single phase of the pure ZnO wurtzite structure, there was not any secondary phase. The RMS values of roughness of the films were determined about 2$\AA$ range from AFM images. The electric and magnetic properties will be presented. [Preview Abstract] |
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J1.00047: Investigations of the magnetocrystalline anisotropy by the Planar Hall Effect in (Ga,Mn)As epilayers grown on vicinal GaAs substrates W. L. Lim, X. Liu, K. Dziatkowski, Z. Ge, S. Shen, J. K. Furdyna, M. Dobrowolska An asymmetric shift of the Planar Hall resistance is observed in the ferromagnetic (Ga,Mn)As epilayers grown on vicinal GaAs substrates, due to the superposition of both the in-plane Planar Hall Effect (PHE) and the out-of-plane Anormalous Hall Effect (AHE). The asymmetric shift reveals the direct manifestation of the strong magnetocrystalline anisotropy that confined the magnetization to the crystalline plane, providing a non-zero normal component of the magnetization with respect to the sample plane at low fields. This shift allows one to measure both components of AHE and PHE simultaneously in these (Ga,Mn)As thin films. We found that the ratio of the AHE resistance to the average PHE resistance change is independent of the vicinal angles. The asymmetry shift is found to be reduced with the increase of the temperature. At the temperature around 30K, the anisotropic magnetoresistance obtained by the PHE measurements shows a significant different profile near the switching fields during the magnetization reversal processes, indicating a complex competition between the cubic magnetocrystalline anisotropiy and the uniaxial anisotropy in these samples grown on vicinal GaAs substrates. [Preview Abstract] |
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J1.00048: Hydrostatic pressure enhancement of the Curie temperature and anomalous Hall effect in Sb$_{2-x}$Cr$_{x}$Te$_{3}$ Jeffrey S. Dyck, Andrew J. Luciana, Cestmir Drasar, Petr Lostak The narrow band gap tetradymite-type semiconductors with the form A$_{2}^{V}$B$_{3}^{VI}$ (A = Sb, Bi and B = Se, Te) are normally associated with thermoelectric cooling devices. However, Sb$_{2}$Te$_{3}$ doped with V or Cr and Bi$_{2}$Te$_{3}$ doped with Fe display a ferromagnetic transition at low temperatures. We have investigated electrical transport properties of bulk single crystals of the ferromagnetic diluted magnetic semiconductor Sb$_{2-x}$Cr$_{x}$Te$_{3}$ under varying pressure (0 GPa to 1.5 GPa), temperature (2 K to 300 K), and magnetic fields (0 T to 6 T). High pressure measurements afford a reversible way to tune both the electronic structure and magnetic interactions of these materials. The behavior of the Curie temperature T$_{C}$ can be monitored via the position of the peak in the resistance data associated with the ferromagnetic transition. We observe that T$_{C}$ increases with increasing pressure in this compound. The anomalous Hall effect with clear hysteresis is also observed below T$_{C}$, and trends with pressure will be discussed. [Preview Abstract] |
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J1.00049: INSULATORS AND DIELECTRICS POSTER SESSION |
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J1.00050: Polarizing and phase-shifting properties of multilayered X-ray mirrors with fluctuating parameters Arkady Satanin, Maria Barysheva, Nikolay Chkhalo It is well known that the multilayered periodical structures (MS) may be used for different X-ray optics applications: dispersion elements, mirrors, polarimeters etc. At the present the polarizing and phase-shifting properties of the MS attract much attention. It was found an intriguing behavior of the MS when the structures having almost the same reflection coefficients demonstrate the strong variations of the polarizing and phase-shifting characteristics. To understand this phenomenon we have investigated the distribution functions of polarization and phase-shift fluctuations for an array of identical MS depending on layer thickness fluctuations. It was shown both analytically and numerically that the relative small fluctuations of layer thickness and the deterministic (systematic) local period deviations may change dramatically the polarization and phase-shift of the MS. In our work we have shown that the polarization and phase-shift of the MS may be found from the reflection measurements. [Preview Abstract] |
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J1.00051: Crystal Structure, Magnetism, and Infrared Reflectivity of Double Perovskites Sr$_{2}$B$^{´}$UO$_{6}$ and Sr$_{2}$B$^{´}$UO$_{6-d}$, B$^{´}$=Mn, Fe, Co, Ni, Zn R.M. Pinacca, M. Del C. Viola, J.C. Pedregosa, R.E. Carbonio, M.J. Martinez-Lope, J.A. Alonso, F.P. De la Cruz, N.E. Massa Double perovskites Sr$_{2}$B$^{´}$UO$_{6}$ where B'=Mn, Fe, Co, Ni, Zn have been prepared as a polycrystalline powder by solid- state reaction. and studied by X-ray diffraction and magnetic measurements. At 300K, they present the same monoclinic distorted crystal structure, space group P2$_{1}$ /n. The perovskite lattice consists of a completely ordered array of B$^ {´}$O$_{6}$ and UO$_{6}$ octahedra exhibiting a slight tilting of the type a$^{-}$b$^{+}$a$^{-}$. Magnetic measurements show antiferromagnetism for the phases with B$^{´}$=Co, Ni and Mn at low temperature. The effective magnetic moment at T $\geq$ T$_ {Neel}$ (5.22l B/f.u. for Co phase and 3.26 B/f.u. for Ni phase) suggests an unquenched orbital contribution. The value for the Mn phase (5.74 B/f.u.) is consistent with that expected for high-spin Mn$^{2+}$ (5.91 B/f.u.). The topotactic reduction of the stoichiometric sample leads to oxygen deficient disordered perovskites, SrB$^{´}$$_{0.5}$U$_{0.5}$O$_{3-d}$ having an orthorhombic distorted structure, space group Pbnm, at 300K. We will also comment on the temperature dependent infrared reflectivity of Sr$_{2}$CoUO$_{6}$ and isomorphous compounds. [Preview Abstract] |
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J1.00052: Epitaxial Thin Films of the Oxynitride BaTaO$_2$N Grown by Pulsed-laser Deposition Weidong Si, Sangmoon Park, Eli Sutter, Thomas Vogt, Young-il Kim, Patrick Woodward Pulsed-laser deposition has been used to grow epitaxial thin films of the oxynitride BaTaO$_2$N on MgO substrates and SrTiO$_3 $ substrates with and without SrRuO$_3$ buffer layers. The thin films have a relatively high dielectric constant in the order of 200\,-\,250 from room temperature down to 4.2K with no significant frequency dependence. This special property, which has not been found before in other materials, potentially makes it a good candidate for many dielectric applications. We argue that a new mechanism is required to understand the dielectric properties of BaTaO$_2$N. [Preview Abstract] |
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J1.00053: Time-dependent space-charge-limited conduction Ho-Kei Chan, Yan Zhou, Chi-Hang Lam, F.G. Shin The concept of time-dependent space-charge-limited conduction (TDSCLC) was introduced in 2004 to account for the well-known observation of polarization offsets in compositionally graded ferroelectric films [1]. It is a generalization of Mott's steady-state SCLC model (J $\sim $ V$^{2})$ [2] to include two carrier types ($p$ and $n)$ and time dependence. Subsequently it was found to have a wider applicability in explaining the imprint phenomenon commonly observed in homogeneous ferroelectric films [3]. Here we would like to review the derivation of the TDSCLC formula and its relation to the steady-state SCLC model, and discuss its applicability to other electrical insulators. \newline \newline [1] H. K. Chan, C. H. Lam, and F. G. Shin, J. Appl. Phys. 95, 2665 (2004) \newline [2] R. Coelho, Physics of Dielectrics for the Engineer (Elsevier Scientific, New York, 1979), pp. 123-125 \newline [3] Y. Zhou, H. K. Chan, C. H. Lam, and F. G. Shin, J. Appl. Phys. 98, 024111 (2005) [Preview Abstract] |
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J1.00054: Mossbauer studies of mixed Fe-Mo oxides H.H. Hamdeh, H. Al-Ghanem, J.C. Ho, R.J. Willey, E. Marchese, W. Jablonski Mossbauer spectroscopy was carried out in conjunction with a catalytic study on mixed Fe-Mo oxides, among which Fe$_{2}$(MoO$_{4})_{3}$ has been widely used in selective oxidation of certain hydrocarbons. With the specific activity being sensitive to the catalyst particle size and morphology, an aerogel process was employed to prepare the samples covering Fe/Mo ratios from 1/2 to 2/1. Each set of Mossbauer data were well fitted to reflect one Fe3+ and two or three non-equivalent Fe2+ sites. Their isomer shift, quadruple splitting and molar fraction are analyzed in terms of temperature and Fe/Mo-ratio dependence. The Fe-rich samples exhibit magnetic ordering at low temperatures. [Preview Abstract] |
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J1.00055: Mid-infrared spectroscopy of Praseodymium doped KPb$_{2}$Br$_{5}$ and KPb$_{2}$Cl$_{5}$ Peter Amedzake, Ei Nyein, Uwe Hommerich, Sudhir Trivedi, John Zavada Rare-earth doped low phonon energy hosts are promising materials for mid-infrared (MIR) solid-state laser applications. Room temperature laser activity has been demonstrated at 7.2 $\mu $m, 5.2 $\mu $m and 1.6 $\mu $m from Pr: LaCl$_{3}$. In this work, we report on the material preparation and optical properties of Pr doped KPb$_{2}$Br$_{5}$ (KPB) and KPb$_{2}$Cl$_{5}$ (KPC). KPB and KPC are both non-hygroscopic and have low maximum phonon energies of 140cm$^{-1}$ and 200cm$^{-1}$, respectively. The small phonon energies lead to reduced non-radiative decay rates due to multiphonon relaxation. The preparation of Pr: KPB and Pr: KPC was based on a careful purification of the host materials followed by self-seeded Bridgman crystal growth. The characteristic absorption bands were obtained in the visible and IR spectral region. Under 1907 nm and 1550 nm excitations, both crystals exhibited broad MIR emission spectra centered $\sim $4.7 $\mu $m with a room-temperature lifetime of 4.5 ms. Results of temperature dependent and time-resolved emission spectroscopy of Pr: KPB and Pr: KPC will be presented at the conference. [Preview Abstract] |
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J1.00056: Planar faults in metastably retained hexagonal BaTiO$_{3}$. Yu-Chuan Wu, Hong-Yang Lu Hot-pressed BaTiO$_{3}$ ceramic samples contain predominantly h-BaTiO$_{3}$ as determined by XRD, and with a trace amount of t-BaTiO$_{3}$ as revealed by TEM. Extended planar stacking faults (EPSF's) found ubiquitously in hot-pressed samples having the eligible fault vectors \textbf{\textit{R}}$_{F(I)}$ = (1/2)\textbf{c} + \textbf{p}$_{(I)}$ and \textbf{\textit{R}}$_{F(II)}$ = (n/6)\textbf{c} + \textbf{p}'$_{(II)}$, containing both \textbf{p}$_{(I)}$ = 1/3$\langle $2\underline {11}0$\rangle $, \textbf{p}'$_{(II)}$ = 1/3$\langle $10\underline {1}0$\rangle $, n = 1, 2, 4, and 5, and \textbf{c} = [0001], have been determined by contrast analysis using the 2$\pi $\textbf{\textit{g}}$^{.}$\textbf{\textit{R}}$_{F}$ = 0 or 2n$\pi $ criteria crystallographically, and the number of layers along \textbf{\textit{c}} confirmed by high-resolution imaging uniquely. Two general types of EPSF's are found; one group with only three vectors of the six degeneracies of the \textbf{p}-component $\langle $10\underline {1}0$\rangle $ eligible and the other with all six degeneracies of the \textbf{p}-component $\langle $2\underline {11}0$\rangle $ eligible for the fault vector. The latter is analogously a $\pi $-rotation fault (as in DO$_{19}$ intermetallics) that a rotation about [0001] by 180$^{o}$ (or 60$^{o})$ arrives at another eligible fault vector. The metastable retention of the high-temperature h-BaTiO$_{3}$ phase is discussed in favour of oxygen deficiency. [Preview Abstract] |
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J1.00057: PIMC simulation of Ps in a dielectric solid Zachary Wolfson, Amy Bug Positronium (Ps) is commonly used to probe the pore space in insulating materials like polymers and low-k dielectrics. Such studies require an accurate model relating the lifetime to the size of the pore. The standard Tao-Eldrup approach models Ps as one quantum particle in the ground state of a spherical box. We go beyond Tao-Eldrup by modeling Ps exactly as two quantum particles at arbitrary temperature, using Path Integral Monte Carlo (PIMC) to simulate the electron and positron each as a classical polymer. As a further correction, we also include the dielectric response of the surrounding material. The total annihilation rate, which is the inverse of the lifetime, may be calculated from the pickoff and self annihilation rates via \[ \Gamma = \Gamma_{p.o.} + \kappa \Gamma_{self} \] We find that the material's polarization causes the Ps to be more attracted to the wall, decreasing its pickoff lifetime. The internal contact density, $\kappa$, decreases with increasing dielectric constant. The combination of using the two particle model as well as polarizable walls yields a lifetime close to the Tao-Eldrup result. [Preview Abstract] |
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J1.00058: Scaling laws In PZT/Si(001) Thin Films Juan Ram\'{i}rez, Alexander Cortes, Wilson Lopera, Maria Elena G\'{o}mez, Pedro Prieto Self-affine scaling behavior of ferroelectric Pb(Zr$_{1-x}$Ti$_{x})$O$_{3}$ (PZT) thin films grown on Si(001) substrates has been investigated by AFM \underline {A}tomic \underline {F}orce \underline {M}icroscope. PZT thin films were grown via Rf-sputtering technique at high oxygen pressures and at substrate temperatures 600 $^{o}$C varying the deposition time and keeping all parameters. growth constant. The $\alpha $-global rough-exponent was founded close to 0.7 indicating a correlated growth. Anomalous scaling behavior was founded in all PZT/Si surfaces. A value of 0.5.for the $\alpha $-local value was founded. The local value of the roughness exponent is associated to the diffusional process on the first stage growth. Saturation roughness showed oscillation dependence with the PZT-thickness. This dependence can be explained by the mismatch between Si substrate and PZT thin film. These results are corroborated calculating the grain size for all samples. This work was supported by COLCIENCIAS under the Excellence Center for Novel Materials Contract no 0043-2005. [Preview Abstract] |
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J1.00059: Crystal Structure and Relaxor-Type Transition In Praseodymium Doped SBT Jorge Mata*, Alejandro Duran*, Raul Escamilla, Eduardo Martinez*, Jesus Heiras*, Jesus Siqueiros* The effects of Pr substitution and ferroelectric response for the Sr$_{1-x}$Pr$_{x}$Bi$_{2}$Ta$_{2}$O$_{9}$ ceramic system were studied. Rietveld refined X-ray diffraction indicates that Pr ion progressively replaces Sr in the $A2_{1}$\textit{am }space group structure. The solubility of Pr in the solid solution is around 15 {\%}. The replacement induces change in the crystal structure and as consequence dielectric properties are affected. The displacement of TiO$_{6}$ octahedra with respect to Bi$_{2}$O$_{2}$ along the polarization axis decreases as Pr is increased. A notable decrease in the transition temperature (Tm) is observed. With Pr substitution the Tm shifts gradually from $\sim $305 $^{o}$C to $\sim $120 $^{o}$C. Apparently, the rotation at the ab-plane ($\beta )$ as well the octahedral distortion observed are strongly related to the coupling between Tm and x. However, the tilt angle ($\alpha )$ associated to the c-axis don't play an important role as it remains essentially constant. Relaxor type transition is observed as Pr increases due to polar microregions above the nominal ferroelectric transition. Local disorder induced by Pr ion is confirmed by the continuous increase in the diffuseness coefficient according to Isupov's model. Thanks to DGAPA-UNAM and CONACYT for funds through Proj. No. 40604-F, 47714-F, IN116703, IN100903 and to P. Casillas, J. Peralta, M. Sainz. [Preview Abstract] |
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J1.00060: Thermal stability and interfacial properties of ZrAl$_{x}$Si$_{y}$O$_{z}$ films prepared by pulse-laser deposition in high vacuum XiaoYan Qiu, HongWei Liu, JunMing Liu The thermal stability and interfacial properties of ZrAl$_{x}$Si$_{y}$O$_{z}$ film prepared by pulse-laser deposition (PLD) in high vacuum have been investigated. X-ray diffraction (XRD) and differential scanning calorimeter (DSC) investigation show that the crystallization temperature of ZrAl$_{x}$Si$_{y}$O$_{z}$ film is above 900\r{ }C. A dielectric constant of 12.9(at 1MHz) is obtained by measuring a Pt/ZrAl$_{x}$Si$_{y}$O$_{z}$/Pt structure. High-resolution transmission electron microscopy (HRTEM) and X-ray photon spectroscopy (XPS) analyses reveal that an amorphous Zr-silicide interfacial layer is formed at 700\r{ }C, but Pt/ ZrAl$_{x}$Si$_{y}$O$_{z}$ /Zr-silicide/Si capacitors still have good electrical properties, such as small equivalent oxide thickness of 0.5nm, flat band voltage of 0.43V and low leakage current density of 2.76mA/cm2 at 1V gate voltage. [Preview Abstract] |
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J1.00061: Fatigue endurance property of PZT thin films enhanced by Sr and Ba doping. Yang Wang, ZhenYu Li, QiYue Shao, JunMing Liu PZT thin films with Sr and Ba doping at A site, deposited by sol-gel and PLD method respectively, were investigated in our work. The enhancement of fatigue resistance property with increasing doping content were both observed in two kinds of samples. The experiments of dc conductivity vs temperature reveal that this behavior can be attributed to the reduction of oxygen vacancies, due to the stronger chemical bonding of Sr-O and Ba-O bonds compared to that of Pb-O bond suppressing the volatility of Pb ions and intensifying the stability of the metal-oxygen octahedron, since the defect chemistry is believed to play a key role in the fatigue of perovskite ferroelectrics. [Preview Abstract] |
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J1.00062: Trends in behavior for tunable ferroelectric material Daniel Potrepka Measurements in an environmental chamber pose problems in effectively coupling to rf- and microwave-tunable, ferroelectric devices. Capacitance can be measured but true device loss is shrouded by losses due to calibration error, impedance matching error from cables and packaging, and vibration interference. In this study, the true device losses of pulsed-laser deposited Barium Strontium Titanate films, at rf frequencies under DC bias are sought using a probe station, calibrated for matching. Breakdown of the device at the higher end of applied voltage and thin film material structure are characterized. Resulting losses and tunability versus temperature are commented upon. [Preview Abstract] |
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J1.00063: Thermal analysis and evidence of structural transition induced by praseodymium in SrTiO$_{3}$ *Alejandro Duran, **Francisco Morales, ***Luis Fuentes, +Javier Castro, *Jesus Siqueiros To explore the nature of ferroelectric behavior induced by Pr ion in the SrTiO$_{3}$ ceramic, we brought together a combination of Synchrotron x-ray powder diffraction and thermal analysis. Rietveld analysis shows a clear peak splitting strongly suggesting a cubic-tetragonal structural transition when 15 {\%} Pr substitutes the Sr site. To determine the temperature of the structural instability differential thermal analysis at high temperatures (30-900 $^{\circ}$C) and specific heat at low temperatures (2-300 K) were performed. The difference in the heat capacity for Pr doped and undoped samples is very small. A plateau in Cp/T-T at 110 K is a clear evidence of the competing effect of two kinds of order parameters, one represented by the polarization and the other by the rotation of the oxygen octahedral in the perovskite structure. Upon heating, DTA analysis shows an unexpected single endothermal broad anomaly at about 118 $^{\circ}$C. The occurrence of this thermal anomaly could be related to displacive structural contributions providing unambiguous evidence of diffuse phase transition and of the ferroelectric behavior observed at room temperature. Thanks are due to CONACYT P. 47714-F, DGAPA P. IN100903 and to P. Casillas for technical help. [Preview Abstract] |
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J1.00064: Structural and Dielectric Characterization of SrBi$_{2}$Ta$_{2}$O$_{9}$ Doped with Gadolinium Jorge Mata, Alejandro Duran, Raul Escamilla*, Eduardo Martinez, Jesus Siqueiros The results of the study of polycrystalline samples of SrBi$_{2}$Ta$_{2}$O$_{9}$ (SBT) doped with Gd, Sr-vacancies and a Bi-excess are presented. XRD structural characterization using Rietveld refinement revealed that the Gd-doped SBT with Sr-vacancies and Bi-excess (Sr$_{0.70}$?$_{0.15}$Gd$_{0.15}$Bi$_{2}$Ta$_{2}$O$_{9})$ is single phase. Furthermore, cationic disorder on the Bi$^{3+}$ and Sr$^{2+}$ crystallographic sites is promoted by the addition of the Gd$^{3+}$ ions. Replacement of Gd ions for Sr and vacancies in the SBT structure produces a shift toward lower values of the Curie Temperature (Tc) from 400 to 200 $^{o}$C in contrast with previous reports in the literature where a notable increase in Tc and the dielectric permittivity are found when the Sr- site is replaced with vacancies. Bi and Gd substitution and the cation vacancies at the Sr site in the Sr$_{0.70}$?$_{0.15}$Gd$_{0.15}$Bi$_{2}$Ta$_{2}$O$_{9}$ compound, enhance the structural distortion in the TaO$_{6}$ octahedra and leads to a larger ferroelectric spontaneous polarization. Acknowledgments: This work was partially supported by DGAPA-UNAM Project N$^{o}$ IN100903 and CONACYT No. 47714-F and 40604-F. The authors thank Pedro Casillas, M. Sainz, J. Peralta for their technical assistance. [Preview Abstract] |
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J1.00065: Thermo-Electromechanical Coupling Effects on Ferroelectric Perovskites Xiaowei Zeng, James Lee, Youping Chen We are investigating the thermo-electromechanical coupling effects on ferroelectric perovskite materials. This work is based on a shell model potential with parameters obtained from first-principles calculation. Molecular dynamics simulations are performed to investigate the dynamic response of the ferroelectric perovskites under thermal, mechanical and electric loadings. The phenomena of finite temperature phase transitions and polarization reorientation are observed. The temperature-pressure phase diagram is also obtained. [Preview Abstract] |
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J1.00066: SUPERCONDUCTIVITY POSTER SESSION |
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J1.00067: Effects of triplet pairing amplitudes in hybrid junctions of superconductor, ferromagnet, and normal metal. Na Young Lee, Han-Yong Choi, Hyeonjin Doh We calculate the critical temperature Tc and pairing amplitude of the hybrid junctions of superconductor (S), ferromagnet (F), and normal metal (N) by solving the Usadel equation in the dirty limit. S is a conventional singlet s-wave supercondcutor like Nb. The interface between S and N is modeled in terms of the interface resistance without the spin flips, while the interface between F and S or F and N is modeled by both the interface resistance and spin flip scatterings, parameterized by, respectively, $\gamma_b$ and $\gamma_m$. The spin flip scatterings induce the triplet pairing components from the singlet component. The Tc of the junction is determined by the critical order parameter, ie., the singlet pairing component. The $\gamma_b$ or $\gamma_m$ changes the Tc of the junctions indirectly by altering the singlet component by modifying the boundary conditions at the interfaces. We calculate the Tc and pairing amplitudes of S/N/F and F/S/F trilayers including $\gamma_b$ and $\gamma_m$ and investigate the effects of the triplet pairing components on the Tc and pairing amplitudes of the trilayers. [Preview Abstract] |
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J1.00068: Domain patterns in type-I superconducting films. Andrejs Cebers, Catherine Gourdon, Vincent Jeudy, Takanori Okada Like many other systems (magnetic fluids, Langmuir polarized layers{\ldots}) type-I superconducting (SC) films exhibit a phase modulation known as the intermediate state (IS). It consists of coexisting domains of the normal state (NS) and SC phases. The striking similarity of the domain patterns for various systems has stimulated the development of general models [1] based on the competition between the interface energy and the long-range interaction between domains. However, for superconductors, the long-range magnetic interaction between domains has to be modified to take properly into account screening currents [2]. This leads to substantial improvement of the description of the IS as it will be illustrated through 3 examples: (i) the impeded growth of NS circular domains, (ii) the stability limit of the circular shape, (iii) the nucleation and collapse of SC domains in the NS phase. [1] A.T. Dorsey and R.E. Goldstein, Phys. Rev. B 57, 3058 (1998) [2] A. Cebers et al, Phys. Rev. B 72, 014513 (2005) [Preview Abstract] |
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J1.00069: Specific heat of bosons among periodical layers Miguel A. Sol\'is, Marcela Grether It is well known that the specific heat of a 3D ideal boson gas shows continuity as a function of the temperature. However, interactionless bosons among periodic plane layers with variable penetrability, show a specific heat jump at the critical temperature, which increases as a function of the layer impenetrability. The jump resembles that of a conventional superconductor instead of that of a laminar cuprate. We expect that inclusion of inter-boson interaction leads to a more realistic specific heat. [Preview Abstract] |
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J1.00070: Thermodynamic properties of Ca$_{1.82}$Na$_{0.18}$CuO$_{2}$Cl$_{2}$ single crystals Kyung-Hee Kim, Heon-Jung Kim, Jung-Dea Kim, H.-G Lee, Sung-Ik Lee We report the experimental results on the reversible magnetization of high quality Ca$_{1.82}$Na$_{0.18}$CuO$_{2}$Cl$_{2}$ single crystals in the high-pressure condition of about 5.2 GPa. The superconducting transition temperature \emph{T}$_{c}$ ($\sim $ 27 K) of this single crystal is the highest reported in this family. The magnetizations are analyzed using the Hao-Clem model and the high-field scaling law. From these analysis, we have obtained various thermodynamic parameters such as the penetration depth and the critical fields, and clarified the dimensional nature of the superconductors. Even though the doping content between Ca$_{1.82}$Na$_{0.18}$CuO$_{2}$Cl$_{2}$ and La$_{1.82}$Sr$_{0.18}$CuO$_4$ is same, the physical properties of these materials are quite different. For example, $\lambda$$_{ab}$(0) is estimated to be 440 nm, which is larger than that of La$_{1.82}$Sr$_{0.18}$CuO$_4$. These changes are totally unexpected in this iso-structural superconductor. [Preview Abstract] |
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J1.00071: Linear Temperature Dependence of the Resistivity in the Pseudogap State A.A. Abrikosov A concept is proposed, explaining the characteristic features of the in-plane resistivity of high-$T_c $ layered cuprates above$T_c $: quasi-linear temperature dependence and absence of the residual resistance in the extrapolation of the curve $\rho \left( T \right)$ from $T>T_c $ to $T=0$. This concept is based on the idea of the pseudogap state structure described in previous works by the present author. This work was supported by the Department of Energy under the contracts {\#} W-31-109- ENG-38. [Preview Abstract] |
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J1.00072: Changes in the superconducting temperature by pressure in Nb$_{3}$Sn and its influence in the martensitic transition. Richart Falconi, Francisco Morales, Roberto Escudero, R. Webb High pressure experiments performed in a Nb$_{3}$Sn single crystal shows changes on the superconducting temperature and in the martensitic transition. Specific heat measurements performed in this crystal shows an anomaly at about 50 K associated to the martensitic transition. At ambient pressure, the electrical resistivity as a function of temperature shows a T$_{C}$ of 18.7 K with a transition width $\Delta $T$_{C}$ (10 to 90 {\%}) of 0.7 K. At low temperatures the R vs T curve show a T$^{2}$ behavior, specifically in the temperature range from 52 down to 23 K. The superconducting transition temperature decreases in linear form with a rate dTc/dP = - 0.79 K/GPa. The results are discussed in terms of the pressure effects on the martensitic transition via changes in the density of electronic states. [Preview Abstract] |
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J1.00073: Generating spin current by AC magnetic field Minhui ShangGuan, Qingfeng Sun, Jian Wang, Hong Guo We report a theoretical investigation on the possibility of generating pure spin current without an accompanying charge current, by applying a train of magnetic field pulses on the leads of a tunnel junction. When the duration of on-state of the pulse is shorter than the spin relaxation time while the off-state is longer than it, a spin current is driven through the two-probe device coherently. The time averaged spin current is found to be almost a periodic function of the pulse strength. A negative spin current can also be produced for a positive spin chemical potential, due to quantum interference among the Fourier modes of the photon assisted tunneling. [Preview Abstract] |
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J1.00074: Effect of Electron-Electron Scattering on the Conductance of a Single Quantum Wire Danhong Huang, Ken Lyo Electron-electron scattering conserves momentum and does not dissipate momentum in a low-density system where the Umklapp process becomes forbidden. However, it can still affect the conductance through the energy relaxation of carriers. We show that this effect can be studied with an arbitrary accuracy in a multi-level single quantum wire system with impurity and phonon scattering using a formally exact solution of the Boltzman transport equation. We study the effect of electron-electron scattering on the temperature and density dependence of the conductance at low temperatures. Some unique features in the calculated conductance as functions of the temperature and the density are predicted and physically explained. [Preview Abstract] |
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J1.00075: Density-matrix renormalization group study of pairing in Holstein-Hubbard model: effect of electronic band structure Masaki Tezuka, Ryotaro Arita, Hideo Aoki The density-matrix renormalization group, with a modification [1], is used to directly obtain correlation functions[2] when both electron-electron and electron-phonon interactions are strong with the Holstein-Hubbard model in a region intermediate between the adiabatic (Migdal's) and antiadiabatic limits. We found that the pairing correlation for a one-dimensional system at half-filling does not dominate over the charge density- wave(CDW) correlation, even when the phonon-induced attraction is comparable with the electron-electron repulsion. However, the pairing is shown to become dominant in a trestle lattice where the electron-hole symmetry is absent, which indicates that superconductivity can appear in a manner dependent on the lattice (hence the electronic band) structure even for the Holstein-Hubbard model with on-site interactions. Band- filling dependence is also discussed. [1] M. Tezuka, R. Arita, and H. Aoki, Physica 359B-361B, 708 (2005). [2] M. Tezuka, R. Arita, and H. Aoki, Phys. Rev. Lett. 95, 226401 (2005). [Preview Abstract] |
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J1.00076: Dynamics of magnetic flux jumps in highly dense MgB$_{2}$ samples C. Romero, O.A. Hernandez, F. Morales, A. Duran, D.H. Galvan, R. Escudero Flux jumps in magnetization curves were investigated experimental and theoretically in a polycrystalline MgB$_{2}$ dense sample. We performed magnetization cycles of M-H at different temperatures. Theoretical calculation were performed using an adiabatic critical-state model, that considers the heating effects due to the motion of flux lines, the instability process that originates flux jumps, and the flux redistribution towards a new metastable critical state. The flux jump and temperature dependence in the final state were successfully reproduced from experimental M-H curves. The number of the flux jumps and the jump size increase as the temperature is decreased corroborating the typical flux jump in hard type-II superconductor. [Preview Abstract] |
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J1.00077: Flux jumps in irradiated MgB$_{2}$ dense samples E. Verdin, C. Romero, F. Morales, E. Adem, J. Rickards, A. Duran, D.H. Galvan, M.B. Maple, R. Escudero This work shows magnetic flux jumps and changes in specific heat measurements studied in MgB$_{2}$ dense bulk samples irradiated: with $^{60}$Co (500 mrad), electrons (500 mrad), and protons (1x10$^{6}$ cm$^{2})$. Magnetic susceptibility measurements $\chi $(T) show that the Tc ($\sim $38.5 K) is independent of irradiation doses. M vs H data display flux jumps strongly dependent of the temperature. Specific heat measurements show an increase in the magnitude of Cp/T vs T curves just in the transition temperature for the irradiated protons, suggesting enhanced local disorder. The behavior of specific heat data at low temperature is analyzed using a two band model in order to clarify the effect of local disorder with the gap superconducting structure. [Preview Abstract] |
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J1.00078: Vortex nucleation in a Layered Superconductor Patricia Salas, Miguel A. Solis We propose that the paired fermions (electrons) in a layered superconductor, such as cuprates, be treated as interacting bosons in the Bose-Einstein regime. We solve the time dependent Gross-Pitaevskii equation for these bosons inside a rotating layered superconductor which is simulated by an external periodical potential of the Kronig-Penney type perpendicular to the planes while bosons are allowed to be free in the other two directions [1]. Among the several numerical methods that exist to solve the Gross-Pitaevskii Equation, we use a time iteration procedure, based on the discretization of time and space [2]. We annalize the vortex nucleation and its influence in the thermodynamic properties of the superconductor. [1] See M.A. Solis and M. Grether, ``Specific heat of bosons among periodical layers," in this proceedings. [2] S. K. Adhikari and P. Muruganandam, J. Phys. B {\bf 35}, 2831 (2002). [Preview Abstract] |
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J1.00079: Gap Distributions in Cuprate Superconductors Ashot Melikyan, Brian M. Andersen, Tamara S. Nunner, P. J. Hirschfeld Recent Scanning Tunneling Microscopy (STM) data on $\rm Bi_2Sr_2CaCu_2O_{8+x}$ impose stringent restrictions on the spatial distribution of the superconducting order parameter and scalar potential due to impurities. In Ref. [1] it was shown that these distributions are inconsistent with a conventional mean-field approach where the inhomogeneities in the LDOS are driven by a scalar impurity potential. It was further demonstrated that the salient experimental features of the LDOS spatial modulations can be obtained if the pairing coefficient itself is assumed to be enhanced by the dopant atoms. Here, we report additional features of the LDOS that support the conclusions of Ref. [1]. [1] T. S. Nunner, B. M. Andersen, A. Melikyan, and P. J. Hirschfeld, Phys. Rev. Lett. 95, 177003 (2005) [Preview Abstract] |
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J1.00080: Holonomic quantum computation using rf superconducting quantum interference devices coupled through a microwave cavity Jiada Sun, Peng Zhang, Zidan Wang, Changpu Sun We propose a different scheme to realize holonomic quantum computation with rf superconducting quantum interference device (SQUID) qubits in a microwave cavity. In this scheme associated with the non-Abelian holonomies, the single-qubit gates and a two-qubit controlled-PHASE gate as well as a controlled-NOT gate can be easily constructed by tuning adiabatically the Rabi frequencies of classical microwave pulses coupled to the SQUIDs. The fidelity of these gates is estimated to be possibly higher than 90{\%} with the current technology. [Preview Abstract] |
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J1.00081: An improved system for SQUID-detected MRI at microtesla fields S. Barriga, P. Koo, A. Hunt, S. Busch, D. Kinion, M. Hatridge, W. Myers, M. M\"o{\ss}le, A. Pines, John Clarke, M. M\"uck We perform magnetic resonance imaging (MRI) by detecting protons precessing at 5.6 kHz in a 132-$\mu $T field with a superconducting gradiometer coupled to a superconducting quantum interference device (SQUID). We have designed, built, and tested an improved system with an increased signal to noise ratio, intended to reduce data acquisition time and/or increase spatial resolution. By using a SQUID with a lower noise and an input coil inductance optimally matched to that of the gradiometer we have reduced the intrinsic magnetic field noise referred to one pickup loop by a factor of four to 0.4 fT Hz$^{-1/2}$. To take advantage of this reduced noise we have enclosed the entire system in a 6-mm thick aluminum shield that attenuates 5.6-kHz magnetic noise by a factor of about 60. Our new coils provide more homogeneous imaging fields and field gradients and are more compact than those employed in our prototype. Finally, we align the precession field perpendicular to the fields generated by the building elevator to minimize shifts in the precession frequency. We present images obtained in our new system. This work was supported by the USDOE. [Preview Abstract] |
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J1.00082: Pulsed Tera-hertz Radition from Femto-second Laser Excited Superconductive YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ Antenna Shyh-Shii Pai, Cheng-Chung Chi We have observed the ultrashort electromagnetic pulse radiation from a current-biased bow-tie structure of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ thin film dipole antenna on MgO using 100 fs, 750 nm laser pulses. With the electro-optic detection, we obtained the THz pulses with 1.0 ps full width at half maximum, containing frequency components up to 1.0 THz. The THz peak amplitude dependence shows the saturation and a nonlinear behavior with a higher excitation pumping power and with the applied bias currents. The saturation on the dependence with the excitation powers exhibits the bolometric heating in nature. However, the nonlinear characterization of the THz radiation from the superconductive thin film antenna revealed that the inadequacy of pure supercarrier approximation on a two-fluid model. The ultrashort transient response and the deviation from the classical theory are discussed in relation to the quasiparticle dynamics of the nonequilibrium mechanism. [Preview Abstract] |
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J1.00083: Vortex avalanche in gold-coated and finite-sized MgB$_{2}$ thin films. Eun-Mi Choi, Hyun-Sook Lee, Sung-Ik Lee, \AA. A. F. Olsen, D.V. Shantsev, T.H. Johansen The vortex avalanches in carbon-free MgB$_{2}$ thin films were studied using magneto-optical imaging. The main focus of this study was the effect of a gold coating and the sample size on the occurrence of the dendritic flux avalanches. For this purpose, we prepared two different sets of MgB$_{2}$ films. In the first, the films had gold coatings of various thicknesses, and in the second set bare MgB$_{2}$ films were patterned into long strips of different widths. The dendritic flux avalanches essentially disappeared for gold coatings thicker than 2.5 $\mu $m. Also for strip widths less than 0.3¥ª3 mm$^{2}$ the formation of flux dendrites are suppressed. The results are consistent with a thermomagnetic origin of the dendritic instability. [Preview Abstract] |
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J1.00084: Effect of disorder in neutron irradiated MgB$_{2}$ thin films investigated by transport measurements in high magnetic field Valeria Ferrando, Ilaria Pallecchi, Chiara Tarantini, Marina Putti, Xiaoxing Xi, Carlo Ferdeghini We analyse a series of MgB$_{2}$ thin films grown by Hybrid Physical Chemical Vapor Deposition (HPCVD), where disorder is introduced by neutron irradiation. Increasing progressively the neutron fluence, T$_{c}$ monotonously decreases down to 2K and $\rho _{0}$ increases by one order of magnitude. A complete characterization of this series of samples is presented. In particular, we propose high field magnetoresistivity and critical field as a method to study the effect of disorder in the two bands. Differently from polycrystalline samples, H$_{c2}$, after a first increase at the lowest neutrons fluences, only weakly depends on the irradiation level. This suggests that in thin films H$_{c2}$ and resistivity are affected by different mechanisms induced by irradiation. From a quantitative analysis of the magnetoresistivity curves as a function of the angle between the field and the crystalline axes, we extract the scattering times in $\pi $- and $\sigma $-bands. We find that the unirradiated films have less mobile $\pi -$bands than $\sigma -$ones; upon irradiation, the scattering rates of both bands progressively increase, allowing to estimate resistivity values consistent with the measured ones. [Preview Abstract] |
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J1.00085: Tuning inter-band scattering mechanism in MgB$_{2}$ through ion irradiation S. Patnaik, S.D. Kaushik The effect of light and heavy ion irradiation on Magnesium Diboride thin films is studied. MgB$_{2}$ is a two-band superconductor and its electromagnetic response is determined by the relative strength of scattering between the isotropic $\pi $ band vis \'{a} vis the planar $\sigma $ band. Our results indicate that by suitably choosing the type of ion, fluence, and energy we can control the inter and intra band scattering and thereby alter the physical properties of this intermetallic superconductor. For example, we find that while the anisotropy in upper critical field can be increased with point defects created by Si$^{+8}$ ions, it can only be decreased by extended defects formed by heavy ion Au$^{+15}$ at same fluence. Similarly while the upper critical field in the direction parallel to c-axis of the film shows enhancement post irradiation with heavy ions, it exhibits some decrease with light ion irradiation. The curvature in H-T phase diagram is also dependent on the defect type. We understand these phenomena within the framework of disorder in multiband superconductivity. [Preview Abstract] |
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J1.00086: Study of MgB$_{2}$ Films on Niobium Substrate Chenggang Zhuang, Dan Yao, Fen Li, Kaicheng Zhang, Qingrong Feng, Zizhao Gan We have successfully fabricated polycrystalline MgB$_{2}$ films on metal niobium by using the hybrid physical-chemical vapor deposition technique. T$_{C}$ (onset) of these samples ranged from 38.5 K to 39.4 K, with $\Delta $T, 0.1 K $\sim $ 0.3 K. The observed T$_{C}$ was the highest among all the MgB$_{2}$ films over metal substrates reported to date. Thicknesses of the films were about 1.0 \textit{$\mu $}m. XRD indicated that lattice constants approached the values of the bulk. The film surface was visible with hexagonal plate-shaped MgB$_{2}$ crystallites but not dense enough, shown by SEM observation. A line scanning spectra of EDX on the cutting cross section exhibited that there was an oxygen-rich region at the interface. Also, the diffusion of the Mg atoms deeply into the film has resulted in the tenacity and adherence of the film to the substrate. TEM investigation proved the existence of this buffer layer, $\sim $100 nm. Estimated using magnetic hysteresis loops and Bean model, J$_{C}$ was above 2.30 $\times$ 10$^{4}$ A/cm$^{2}$ at 10 K in zero field. The synthesis of MgB$_{2}$/Nb films with thickness above one micron, showing certain tenacity, is an important and significant step towards the application of the 2$^{nd}$ generation MgB$_{2}$ superconductor wires or tapes. [Preview Abstract] |
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J1.00087: Nonequilibrium Photoresponse of Current-Biased, Epitaxial MgB$_{2}$ Microbridges M. Khafizov, D. Wang, X. Li, R. Sobolewski, Y. Cui, X. X. Xi We report nonequilibrium, subnanosecond-wide voltage transients generated by current-biased MgB$_{2}$ microbridges excited by 100-fs optical pulses. Our epitaxial MgB$_{2}$ films were grown by the HPCVD technique and the microbridge structures, embedded into coplanar waveguides, were patterned by a conventional photolithography and ion milling. The electrical photoresponse transients, measured in the superconducting state, could be decomposed into two elements: the fast, $\sim $100-ps-wide, dominating pulse attributed to the kinetic inductive response, and the slow, several-ns-long signal representing the thermal resistive response. The characteristics of the fast response follow the Rothwarf-Taylor model, although in order to explain the peculiar features of this nonequilibrium signal one needs to incorporate into the model the two distinct superconducting gaps. The nonequilibrium photoresponse of MgB$_{2}$ microbridges makes them attractive as fast photodetectors. [Preview Abstract] |
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J1.00088: The loss of the anisotropy in the electrical conductivity in MgB$_{2}$ under pressure Ulises Estevez, Pablo de la Mora MgB$_{2}$ is a multiple band superconductor, with two $\sigma $-bands and two $\pi $-bands. The $\sigma $-bands that are highly anisotropic are the responsible of the superconductivity in this compound. It has been shown that with Sc, C and Al doping the $\sigma $-bands reduce their anisotropy, but for the case of Al and C doping the bands fill up and as consequence the number of $\sigma $-carriers reduce and disappear which leads to $T_{c}$ reduction. In this work it is shown that pressure reducing the MgB$_{2}$-cell parameters which leads to an increase of the interplane $\sigma $-orbitals overlap. This leads to an increase of the $\sigma $-bands electrical conductivity in the c-direction, in other words, a reduction of the anisotropy of the $\sigma $-bands, on the other hand there is no band filling therefore no reduction of $\sigma $-carriers. This reduction as function of pressure follows a similar trend as $T_{c}$, thus showing that the anisotropy in the $\sigma $-bands could be an important factor of the high $T_{c}$ in MgB$_{2}$. [Preview Abstract] |
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J1.00089: MAGNETISM POSTER SESSION |
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J1.00090: Research on Dynamic Model Building of Active Magnetic Bearing Jian Shi, Guo-zheng Yan, Kun-dong Wang As for AMB(active magnetic bearing), many researchers and engineers use theoretical method presented by G. Schweitzer to get the dynamic model. But the model built in this way can not suspend the magnetic system effectively sometimes. Furthermore if any original physical parameter is changed or lost, the theoretical method will be out of work at all. This paper presents a simple and convenient experimental method to build the primary dynamic model of AMB. There is a special advantage that this process has no relation to any original parameter except the mass of rotor which can be obtained easily however. Base on the dynamic model, feedback control system is designed which can make AMB system suspended stably. Through online identifying with frequency method we get the so-called precise model. Finally three models from experiment, theoretical method and online identification respectively, are compared. Result shows that the model from experiment is much closer than that from theoretical method to the precise model. [Preview Abstract] |
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J1.00091: Spin liquids of a frustrated SU(3) quantum magnets on a triangular lattice Peng Li, Shun-Qing Shen The spin disorder state of the bilinear-biquadratic model near the edge of antiferromagnetic region on a triangular lattice is studied in a point of view of frustrated SU(3) quantum magnets. A mean field theory in the quasi-particle representation is established. From the quasi-particle spectra, we show that the density-of-states rise linearly from zero in the region where the SU(3) symmetry is broken. As a consequence, the specific heat at low-temperature in this region exhibits the power law, $C \propto T^{2}$, which may give an explanation for a recent experiment on NiGa2S4. A finite susceptibility is also obtained in the zero temperature limit, which is also in agreement with the experiment. [Preview Abstract] |
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J1.00092: Linear and nonlinear magneto-optical properties of Fe/Au multilayered films and Fe-Au alloy films Y. H. Hyun, Y. P. Lee, K. W. Kim, R. Yang, Q. Y. Jin In this study, the magnetic and the magneto-optical properties of Fe/Au multilayered films (MLF) and Fe-Au alloy films were comparatively studied to elucidate the peculiar magneto-optical properties of Fe/Au MLF. (3.0 nm Fe / t$_{Au}$)$_{20}$ MLF (where t$_{Au}$ = 1.0, 2.0, 2.5 and 3.0 nm) and Au$_{1-x}$Fe$_ {x}$ (0 $<$ x $<$ 1) alloy films of about 100 - 150 nm in thickness were prepared by rf-sputtering onto glass substrates at room temperature. The structures of these films were studied by x-ray diffraction. The magnetic properties were investigated by using a vibrating sample magnetometer. The magneto-optical properties of Fe-Au alloys and Fe/Au MLF were measured by using magneto-optical Kerr effect (MOKE) and magnetization-induced second-harmonic generation (MSHG). The MOKE was obtained in the equatorial mode, and the MSHG measurements were performed in the longitudinal mode. The MSHG results were analyzed in connection with the MOKE and the VSM results. [Preview Abstract] |
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J1.00093: Magnetic Properties and Electronic Structure in Transition Metal Doped La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ K.K. Yu, S.J. Jun, J.S. Park, J.Y. Kim, Y.P. Lee, K.H. Han, Y.S. Lee, J.-H. Kang The magnetic properties of La$_{0.5}$Ca$_{0.5}$Mn$_{0.98}$TM$_ {0.02}$O$_{3}$ (TM = Cr, Ti) perovskites were studied by using the temperature dependences of magnetization and coercive field. The transition-metal doping like La$_{0.5}$Ca$_{0.5}$Mn$_{1-y} $TM$_{y}$O$_{3}$ (TM = transition metal) can alter the Mn$^{3+} $/Mn$^{4+}$ratio, and lead to significant modifications in the magnetic properties. The observed reductions of Curie temperature and magnetization are interpreted with the calculated ratio of Mn$^{3+}$/Mn$^{4+}$according to the Curie- Wiess law. The value is determined to be 0.48 for the TM-doped samples. The Mn L- and O K-edge were also measured by x-ray absorption fine-structure spectroscopy. The domain- wall pinning was investigated, as well, with the temperature dependence on coercivity at a constant field. [Preview Abstract] |
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J1.00094: Magnetic behaviour of Sr$_{2-x}$La$_{x}$IrO$_{4}$ Carlos Cosio, Gustavo Tavizon, Pablo de la Mora, Roberto Escudero Magnetic properties of Sr$_{2}$IrO$_{4}$ are an interesting topic because the anisotropic behaviour showed in the single crystal case, as well to the variable valence states of iridium. By solid state reaction we have obtained polycrystalline samples of the solid solution Sr$_{2-x}$La$_{x}$IrO$_{4}$ and performed structural refinements based on the X-ray powder pattern. We have observed that, as a consequence of distortions in the [IrO6] octahedra, the symmetry of the compounds in the solid solution Sr$_{2-x}$La$_{x}$IrO$_{4}$ goes from the K$_{2}$NiF$_{4}$ (SG 139) type to I4$_{1}$/acd (SG 142). This explanation also is supported by internal parameters optimization with electronic structure calculations using the Wien2K code. On the other hand, magnetic and electrical behaviour characterization of samples in the range of 2-300 K shows that this system can be thought as a weak ferromagnet in all the solubility range. For the high temperature regime the observed effective magnetic moment of iridium ion diverges from the free ion case. [Preview Abstract] |
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J1.00095: Field dependence of RF susceptibility in the vicinity of a ferromagnetic transition in single crystals CeAgSb$_2$ R. Prozorov, M.D. Vannette, S.A. Law, S.L. Bud'ko, P.C. Canfield A 10 MHz resonant technique was used to study ferromagnetic transition in single crystals CeAgSb$_2$. Detailed measurements of field and temperature dependencies of the dynamic susceptibility in the vicinity of the ordering temperature were performed. It was found that a sharp peak in zero-field response rapidly smears out in a weak ($\sim$100 Oe) magnetic field. Obtained results are compared to DC magnetic susceptibility and resistivity, - both showing no similar effect. The possible relation our measurements to study dynamics of a ferromagnetic transition is discussed. [Preview Abstract] |
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J1.00096: Determination of complex magnetism in a homologous series of compounds. R.W. McCallum, Y. Janssen, TA. Lograsso, K.A. Gschneidner, Jr., V.K. Pecharsky, B.N. Harmon Pr$_{(n+1)(n+2)}$Ni$_{n(n-1)+2}$Si$_{n(n+1)}$, where n = 2, 3, and 4, forms a homologous series of hexagonal compounds whose basic structural unit is a trigonal prism of Pr atoms with its axis parallel to the c-axis. Between 100 and 400 K, their dc susceptibility, $\chi {\rm s}$ measured with H$\vert \vert $c and H$\bot $c on a single crystal follows a Curie-Weiss law. $_{ }$In all compounds, the component of M$\vert \vert $c orders ferromagnetically with T$_{c}$ increases with n. For H$\bot $c, a peak is observed in the low-field M vs T plots below T$_{c}$ suggesting antiferromagnetic order. For H$\bot $c at 5 K, all three compounds exhibit a metamagnetic transition between 2 T and 3 T. Based on the systematics of the properties of the members of the series, a model for site specific interactions has been developed for comparison with first principles calculations. [Preview Abstract] |
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J1.00097: The spin structure of maghemite investigated by ${}^{57}$Fe NMR Seong-Joo Lee, Soonchil Lee Maghemite($\gamma$-Fe${}_{2}$O${}_{3}$) is a ferrimagnetic material known for a long time but its detailed spin structure is not fully understood yet. Interpretation of the hyperfine field measured by the M\"ossbauer study, which has been the main tool to study local spin structure of this material, has been controversial. Maghemite has an inverse spinel structure where magnetic Fe ions can occupy either the octahedral or the tetrahedral sites. The NMR spectrum shows two separate peaks in zero external field. We split these peaks clearly by applying external magnetic field and compared the peak intensities. The result shows that the hyperfine field at the nuclear spins in the octahedral site is larger than that in the tetrahedral site. The field dependence of the resonance frequency shows that the spins at the octahedral site are antiparallel with external field while those at the tetrahedral site are canted and parallel to external field. [Preview Abstract] |
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J1.00098: Spin structure of NiFe/FeMn/NiFe trilayers A.M. Alsmadi, S.G.E. te Velthuis , G.P. Felcher , H.G. Yoon, C.G. Kim Using polarized neutron reflectometry we studied the layer-by-layer spin structure of NiFe(t)/FeMn(15nm)/NiFe(5nm) trilayers with, t = 3.8, and 12 nm in. For both samples the hysteresis curves show two clearly separated loops, each corresponding to the magnetization reversal of one of the two NiFe layers. For the first sample of t = 3.8 nm the neutrons were reflected without undergoing a spin inversion, indicating that at all fields the magnetization of both NiFe layers are either parallel or antiparallel to the field. However for the second sample of t = 12 nm, at the field where the magnetization of the top NiFe layer starts being hysteretical, the presence of non-zero neutron spin-flip reflectivities signals that some component of the magnetization is perpendicular to the applied field at some depth in the film. The data are consistent with a magnetic configuration where the NiFe layer magnetizations are canted with respect to each other. The polarized neutron results are discussed in terms of the detail FM/AF/FM interactions. [Preview Abstract] |
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J1.00099: Superlattice effects on spin precession in lateral ferromagnetic heterostructures Nikolay Polushkin, Steven Michalski, Roger Kirby Wang and Tilley have developed a model to describe spin wave behavior in systems of magnetic stripes separated by non-magnetic spacers.$^{1}$ A key parameter defining spin-wave excitation features in periodic systems is the QD product, where Q is the wavenumber of excited magnons and D the superlattice period. In the long-wavelength limit when QD$<<$1, spin-wave frequencies can be calculated analytically. We have extended the Wang and Tilley model to describe spin-wave behavior in striped media with stripes of two different magnetizations. The analysis reveals the existence of a basic difference in the spectra for different geometries of the applied magnetic field. For instance, two dominating modes occurring under parallel field orientation reduce to a single mode if the field is oriented in the transverse direction. This crossover reflects the different arrangement of the effective medium permeability tensor, which depends on the geometry of the field. This analysis is applied to our pump-probe studies of patterned heterostructures of submicron-width Fe-V stripes. $^{1}$X.Z. Wang and D.R. Tilley, Phys. Rev. B 50, 13472 (1994). [Preview Abstract] |
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J1.00100: Spin-state Transition Associated with the Magnetization Jump on Y$_{1-x}$Sr$_{x}$CoO$_{3-\delta}$ Compounds (x=0.67, 0.75) M. Izumi, Y.F. Zhang, S. Sasaki, O. Yanagisawa Perovskite-type compounds Y$_{1-x}$Sr$_{x}$CoO$_{3}$ (x=0.67, 0.75) were prepared by the conventional solid state method and sintered under air and O$_{2}$ flow. There is a DC magnetization jump during the field cooling at 0.01 T and then it comes normally back to high temperature in the specimens, which is called as magnetic memory effect. The Co$^{3+}$ ions spin state transition from low to intermediate state results in the magnetization jump. The jump temperature ($T_{J})$ and magnetization increase with the x increases. With increasing the magnetic field, the magnetization jump disappears. Air-processed samples exhibit higher $T_{J}$ and larger magnetization than O$_{2}$-processed samples. The main reason is the different content of Co$^{3+}$ ions due to the oxygen deficiency under the different prepared conditions. Samples with x=0.75 show the higher Curie temperature ($T_{C})$ and at almost the same temperature the cusp appears in the zero-field curve. At the same time, there is another small cusp in the zero-field curve in x = 0.75 samples. [Preview Abstract] |
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J1.00101: Scaling of hysteresis in phenomenological models of thin ferromagnetic films Eshel Faraggi Explicit solutions are derived for several phenomenological models of magnetization reversal in thin ferromagnetic films driven by a saw-tooth magnetic field. For a domain wall velocity that is linear in the magnetic field it is found that the dynamic coercive field, and hence the scaling of hysteresis, follows a square-root power-law in the slope of the magnetic field, shifted by the depinning field. For a more general domain wall velocity different power-law exponents are found, yet the overall form for the scaling of the area of the hysteresis loop remains a power-law shifted by the depinning field. This shifted power-law could be interpreted to be a crossover between adiabatic and dynamic regimes. [Preview Abstract] |
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J1.00102: Magnetic Properties and Magnetization Dynamics in Amorphous Ferromagnetic Microwires Jo\~ao Paulo Sinnecker, Rafael Novak The dynamics of the domain walls during magnetization reversal processes in amorphous ferromagnetic microwires is studied. The studies have been based in measurements made in bmagnetic bistable Fe$_{77.5}$Si$_{7.5}$B$_{15}$ microwires . In bistable microwires the magnetization reversal process generally happens through the propagation of a single domain wall along the axial direction, i.e., a large Barkhausen jump. This reversal process is initiated at the wire ends, where a complex domain structure, characterized by the presence of closure domains, is found. Samples with diameters between 20 and 40 $\mu$m, and lengths between 7 and 12 centimeters have been studied. The dynamics has been investigated through the analysis of the signal induced in search coils by the domain walls movement. The influence of externally applied magnetic fields in the dynamics of domain wall propagation has been investigated. Through these studies it was possible to distinguish the different magnetization processes taking place at the extremities and the central part of the microwires. [Preview Abstract] |
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J1.00103: Dissipative dynamics of vortices in a spiral state and transport in the spin-glass phase of La$_{2-x}$Sr$_x$CuO$_4$ V. Juricic, L. Benfatto, A.O. Caldeira, C. Morais Smith Doping of an antiferromagnet with holes may lead to a spiral rearrangement of the spins, as it has been argued by Shraiman and Siggia. The formation of a spiral is consistent with the incommensurate magnetic order observed by the neutron scattering experiments in the spin-glass phase of La$_{2-x}$Sr$_x$CuO$_4$. The spiral state has a chiral degeneracy that leads to the formation of topologically nontrivial vortex-like defects. We propose that the dissipative dynamics of these defects is responsible for the transport properties in the spin-glass phase of cuprates [1]. Using the collective-coordinate method, we show that the defects are coupled to a bath of magnons. The resulting effective action, after the magnons have been integrated out, indicates that the motion of the defects is damped due to the scattering by the magnons. Assuming that the holes are attached to the vortices, we have calculated the corresponding in-plane resistivity, which exhibits an anisotropy and linear temperature dependence in agreement with experimental data. References: [1] V. Juricic, L. Benfatto, A. O. Caldeira, and C. Morais Smith, Phys. Rev. B {\bf 71}, 064421 (2005). [Preview Abstract] |
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J1.00104: Spin fluctuations and quantum criticality in a CePt ferromagnetic Kondo lattice Julio Larrea J., M.B. Fontes, E. Baggio-Saitovitch, A. Eicher, M. Continentino We report on an study of the quantum critical behavior of a ferromagnetic (FM) CePt Kondo lattice using ac susceptibility ($\chi_{ac}$) and electrical resistance ($R(T)$) measurements under high pressures ($P \leq 15$ GPa). Our results shows that the FM ordering disappears at the critical pressure $P_{C} \sim$ 12.1 GPa, which is seen as a vanishing of the Curie temperature ($T_{C}$) and the anisotropic FM magnons ($\Delta$). This $P_ {C} $ is taken as a quantum critical point (QCP) and separates the FM ordering from paramagnetic (PM) state at zero temperature. In the vicinity of $P_{C}$, a non Fermi liquid behavior (NFL) is observed in the $R(T)$ data as a temperature dependence $T^{1.3}$, which is ascribed to the FM-QCP transition. Beyond the QCP, the system recovers the truly Fermi liquid (FL) behavior. For our knowledge, CePt is the only Ce Kondo lattice that shows a direct FM-QCP transition [1]. Our analysis of the $R(T)$ data, using scaling relations and a spin wave scenario, suggest that the two dimensional FM spin fluctuations is the mechanism to accounts for this ferromagnetic instability. [1] J. Larrea J. et al, Phys. Rev. B 72, 035129 (2005). [Preview Abstract] |
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J1.00105: Magnetism and hybridization effects in UCuSn Sami El-Khatib, Anna Llobet, Agus Purwanto, A. Alsmadi, Heinz Nakotte UCuSn crystallizes in an orthombically-distorted structure that is closely related to the hexagonal GaGeLi structure. Bulk studies reveal that UCuSn undergoes two magnetic transitions at about 60 and 25 K. Here, we present more detailed neutron-diffraction studies that performed on the High-Intensity Powder Diffractometer (HIPD) at the Manuel Lujan, Jr. Neutron Scattering Center at Los Alamos. Neutron-diffraction studies provide clear evidence for a non-collinear configuration of the magnetic moments in both magnetic phases. We propose a simultaneous existence of two magnetic allowed-symmetry structures as one possible explanation to fit the observed magnetic intensities below 25 K. We will discuss the relationship between magnetic and structural properties in UCuSn. We observe regular thermal contraction with decreasing temperature down to 60 K, below which we observe discontinuities for the interatomic spacings, d$_{U-U}$, d$_{U-Cu}$ and the cell parameters, which provides a measure of the hybridization effects in UCuSn. We were able to correlate the structural changes with the magnetism in UCuSn. [Preview Abstract] |
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J1.00106: Hartree-Fock calculations of the frustrated 2D Hubbard model Matthew Enjalran Motivated by recent work on the materials Na$_x$CoO$_2$$\cdot$ y$H$_2$O and a class of organic conductors ($\kappa$-(BEDT-TTF)$_2$X), where strongly correlated electrons are confined to a geometrically frustrated triangular lattice, we investigate the frustrated Hubbard model via mean-field theory. We report preliminary results from Hartree-Fock calculations of the charge and magnetic properties of our model on the triangular and square lattices. We also discuss the potential application of the constrained path quantum Monte Carlo (CPQMC) method to the study of frustrated 2D Fermi systems. [Preview Abstract] |
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J1.00107: Colossal resistivity change besides magnetoresistance: an extended theoretical framework for electronic transport of manganites Shuai Dong, KeFeng Wang, Han Zhu, XiaoYan Yao, JunMing Liu Current theoretical approaches to manganites mainly stem from magnetic framework, in which the electronic transport is thought to be spin-dependent. However, quite a number of experimental observations can yet not be reasonably explained. An extended framework for electronic transport of manganites has been proposed, in which the total resistivity has been partitioned into two parts: $\rho _{s}$ and $\rho _{c}$ in terms of two different mechanisms: spin-dependent and charge-dependent. Correspondingly, the colossal magnetoresistance (CMR) classification inherited from Aliaga et al has been extended as: CMR2 and XR, where CMR2 is the classical spin-dependent process while XR is spin-independent. We emphasize the important role of XR which helps to understand the true mechanism of CMR. [Preview Abstract] |
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J1.00108: The enhanced magnetoresistant effects and cluster-glass state induced by A-site cational size disorder KeFeng Wang, LiFeng Wang, JunMing Liu In pervoskite-structure oxides, not only the A-site cational mean
radii, but
also the variance of the A-site cational radii, controls the
physical
properties of the sample. The disorder effects induced by A-site
cational
size mismatch in large band-width manganite
La$_{0.55}$Sr$_{0.45}$MnO$_{3}$
have been carefully studied by preparing a series of samples
which have the
same $ |
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J1.00109: Resistor-network study of tunneling magnetoresistance in ordered arrays of magnetic nanoparticles Dimitris Kechrakos, Kalliopi Trohidou The tunneling magnetoresistance (TMR) of a hexagonal array of dipolar interacting magnetic nanoparticles is studied using a resistor network model and micromagnetic configurations obtained from Monte Carlo simulations. Analysis of the field-dependent TMR and the corresponding magnetization curve shows that dipolar interparticle interactions suppress the maximum TMR effect, increase or decrease the field-sensitivity depending on the direction of applied field and produce strong dependence of TMR on the direction of applied magnetic field. The peak in TMR for a unipolar sweep of the magnetic field is associated with the critical field for irreversible rotation of the magnetization, rather than the coercive field as commonly interpreted. This behavior is evident in strongly interacting arrays with the magnetic field at a nearly right angle to the array. Under these conditions the difference between the coercive and critical field is maximized. The relation of our simulations to recent TMR measurements in self-assembled Co nanoparticle arrays is discussed. [Preview Abstract] |
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J1.00110: Epitaxial half metallic/oxide semiconductor schottky contacts J. Jhaveri, A. Venimadhav, Qi Li Half metallic/semiconductor junctions are useful for spintronic devices. With such an interface, one can potentially inject a large percentage of spin polarized electrons into the semiconductor in a active spintronic device. We fabricated half-metallic Sr$_{2}$FeMoO$_{6}$ (SFMO) and 0.1{\%} Nb doped SrTiO$_{3 }$(Nb: STO) schottky contacts using Pulsed Laser Deposition (PLD). Sr$_{2}$FeMoO$_{6 }$films were grown epitaxially on Nb-SrTiO$_{3}$ with Ar and small amounts of O$_{2}$ (0.02{\%}) while the substrate was kept at 800$^{o}$ C. The current versus voltage characteristics of the junction were measured on 1 mm$^{2}$ contact area. The I-V curves were measured at several temperatures and in each case, the junction exhibited a clear rectifying behavior. A strong rectifying behavior was observed at room temperature with a small activation voltage of 0.2 V. Temperature dependence of I-V characteristics shows a clear schottky nature of the contacts. The barrier height and ideality factor of the junction were calculated using the thermionic model given by $I =I_{s}$(e$^{V/nkT}$ -1). Here, $I_{s }=A A$* $T^{2}e^{q\phi /kT}$, where $A $is the active area, $A$* is the effective Richardson constant, \textit{$\phi $} is the Schottky barrier height, and $n $is the ideality factor. We found that the ideality factor was approximately constant as the temperature was varied with small deviations, and the barrier height decreased with temperature and more abruptly below 100K. [Preview Abstract] |
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J1.00111: Spin-polarized sources for Si-based spin injection-detection devices Sebastiaan van Dijken, C. Boothman, M. Thiebault, C. Murray, G. Feng, J.M.D. Coey We have explored magnetite/Si and Fe/MgO/Si contacts as possible sources of highly spin-polarized currents. Reactive sputtering of magnetite from a pure Fe target results in the growth of single crystal films with a (111) texture on both Si(001) and Si(111) substrates. The evolution of the film magnetization with temperature exhibits a sudden decrease around 120 K, which is characteristic for the Verwey transition in magnetite and indicates high quality film growth. Electrical transport across the magnetite/Si interface is dominated by tunneling through (at low temperatures) and thermionic emission across (at elevated temperatures) a Schottky barrier. The Schottky barrier height is 0.52 eV and 0.65 eV for magnetite films on Si(111) and Si(001), respectively. Direct rf magnetron deposition of MgO on Si(001) results in tunnel barriers with a weak (001) film texture. The injector current (at fixed bias voltages) varies non-monotonically with tunnel barrier thickness. This dependence is qualitatively explained by a competition between a reduction in current due to a thicker tunnel barrier and an increase in current due to a reduction of the Schottky barrier height and Si depletion width. [Preview Abstract] |
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J1.00112: High coercive cobalt ferrite Co$_{x}Fe$_(3-x)$O$_{4}$ nano-composite thin films prepared by spin-on process P. Talagala, R. Naik, G. M. Tsoi, L. E. Wenger, R. Suryanarayanan, V. M. Naik Spin coating technique have been employed to synthesize Co$_{x}$Fe$_{3-x}$O$_{4}$ (0.5 $\le $ x $\le $ 1.9) nano-composite thin films of $\sim $10 nm crystalline size on sapphire, glass, and aluminum substrates. The films were annealed in various environments such as air, H$_{2}$/Ar, and vacuum. The structural properties were characterized by XRD and Raman Spectroscopy. The magnetic hysteresis measurements of the films exhibit a large coercivity H$_{C }$(15 - 17 kOe) at 5 K. Low temperature processed films demonstrate supermagnetic characteristics with H$_{C}$ = $\sim $150 Oe and reduced remanence M$_{r}$/M$_{s}=\sim $ 0.05 at 300K. These films show a spin-glass behavior with the blocking temperature T$_{B}$ $\propto $ H$^{2 }$for low fields and T$_{B} \quad \propto $ H$^{2/3}$ for high fields. High temperature processed films with various annealing conditions demonstrate H$_{C}$ as high as 1.8 kOe at 300K. Estimated magnetic anisotropy value of the films is in the range of 0.8 -- 2.4 x 10$^{6}$ erg/cm$^{3}$. Optical absorption spectra exhibit band gaps in the visible range of 1.4 -1.7 eV and in the IR range (1.0 eV and 0.7 eV). Annealing in hydrogen shows a reduction of resistivity. Further details of the structural, electrical and magnetic properties of the films will be presented. [Preview Abstract] |
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J1.00113: Structural and Magnetic Ordering of the Interface of Fe/Gd Multilayers Ataur Chowdhury, Andrea Freitag Multilayers of Fe/Gd were fabricated with magnetron sputtering to study the magnetic and structural ordering of the interfaces of these multilayers. Two groups of samples, one with fixed Gd layer thickness and one with fixed Fe layer thickness, were prepared to study the effect of layer thickness on the structural properties of these multilayers. The samples were investigated using x-ray diffraction and Mossbauer spectroscopy. Two major structures were identified with x-ray diffraction, and two more minor structures were identified from Mossbauer measuremnt. The composition and the thickness of the interface was found to change with both Fe and Gd layer thicknesses. Results of Mossbauer measurement also show that an amorphous Gd-Fe component at the interface with an average composition close to that of the intermetallic compound GdFe[sub2] is likely to be the origin of perpendicular magnetic anisotropy (PMA) of Fe/Gd multilayers. Experimental results clearly suggest that PMA in Fe/Gd multilayers and amorphous Gd-Fe films may have the same origin. [Preview Abstract] |
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J1.00114: Properties of magnetoelectric susceptibility Junyi Zhai, Jiefang Li, Dwight Viehland, M.I. Bichurin The magnetoelectric (ME) susceptibility is the principle property of ME materials, determining the connection between polarization (or electric induction) and an external magnetic field. Since measurement of the ME susceptibility over a wide frequency range [1] and the design of new ME devices require more information about the ME susceptibility, the present work has focused on this property in detail. First, we consider the ME susceptibility as a complex parameter with both real and the imaginary parts, advancing a methodology for measurement of these values. Second, we have analyzed the ME susceptibility, for example a trilayer laminate composite of Terfenol-D/PZT, Terfenol-D, and found a maximum value of $\sim $3.5x10-7s/m in the electromechanical resonance range. In addition, we have studied the internal structure of the ME susceptibility: i.e., its dependencies on phase volume fractions, layer thickness, and choice of materials couple. Our results for the ME susceptibility will allow it to be more correctly used both as a fundamental materials property and also in potential ME device applications. \newline [1] M.I. Bichurin, V.M. Petrov,Yu.V. Kiliba, and G. Srinivasan. Phys. Rev. B 66, 134404 (2002). [Preview Abstract] |
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J1.00115: Domain wall phase diagram and wall widths M. Klaui, M. Laufenberg, D. Backes, U. Rudiger, C. A. F. Vaz, J. A. C. Bland, L. J. Heyderman, F. Nolting The details of the spin structure of domain walls have recently become the focus of intense research due to the fundamental physical questions associated with domain walls (wall width, magnetoresistance, current-induced domain wall propagation). Using photoemission electron microscopy (XMCDPEEM) 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 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]. Comparison with theoretical calculations [2] and micromagnetic simulations reveals the importance of local energy minima and the geometry-dependent height of the energy barriers separating the two wall types can be 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. By varying the spacing between domain walls, we determine the coupling strength that leads to a shift in the phase boundary. [1] M. Klaui et al., APL \textbf{85}, 5637 (2004); [2] R.D. McMichael and M.J. Donahue, IEEE Trans. Magn. \textbf{33}, 4167 (1997); [Preview Abstract] |
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J1.00116: Electric Dipole Transitions at Magnetoacoustic Resonance M.I. Bichurin, V.M. Petrov, O.V. Ryabkov, A.V. Filippov, A.A. Ivanov, G. Srinivasan Ferromagnetic-ferroelectric composites show giant magnetoelectric (ME) effects that are facilitated by the sample response to electric, magnetic, and elastic forces. Composites consisting of magnetostrictive ferrites and piezoelectric lead zirconate titanate (PZT) or lead magnesium niobate-lead titanate (PMN-PT) are found to show strong ME coupling. Such materials also provide us with unique opportunities for theoretical and experimental studies on ME coupling when the magnetic and/or electric subsystems show resonance behavior. Two types of resonances are of importance: electromechanical resonance (EMR) for the piezoelectric component and ferromagnetic resonance (FMR) for the magnetic component. At the coincidence of EMR and FMR, i.e., at the magneto-acoustic resonance (MAR) ME interaction becomes stronger [1]. This work focuses on electric dipole transitions in multilayer ferromagnetic-ferroelectric composites, such as yttrium iron garnet (YIG) and PZT, at MAR. Expressions have been obtained for ME susceptibility and the ME coefficient. The results indicate the potential for novel microwave devices based on ME interactions at MAR. Supported by grants from the ARO, ONR and NSF.\newline [1] M.I. Bichurin, V.M. Petrov, O.V. Ryabkov, S.V. Averkin and G. Srinivasan, \textit{Phys. Rev. B}. \textbf{72}, 060408(R) (2005). [Preview Abstract] |
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J1.00117: Enhancement in Magnetoelectric Effects at Thickness Modes of Layered Ferromagnets and Ferroelectrics D.A. Filippov, M.I. Bichurin, V.M. Petrov, V.M. Laletsin, G. Srinivasan, C.W. Nan Magnetoelectric (ME) effects in magnetic - piezoelectric heterostructures are caused by mechanical coupling between magnetic and piezoelectric layers. We reported earlier on the theory and observation of a resonant enhancement in the ME effects when the electrical subsystem is driven to resonance, i.e., electromechanical resonance (EMR) associated with radial acoustic modes [1]. Here we discuss the theory and data for ME effects associated with thickness EMR modes. Profiles of ME voltage coefficients versus frequency were estimated for trilayers based lead zirconate titanate and the following ferromagnetic phases: cobalt ferrite, nickel ferrite and lithium ferrite and Fe, Co and Ni. The results are compared with data on samples 10 mm in diameter and 2 mm in thickness. An enhacement in the ME voltage due to radial modes is observed at 350 kHz. A similar behavior due to the thickness mode is observed at 1.5-2 MHz, in agreement with the theory. Calculated ME voltage coefficients versus frequency profiles are in excellent agreement with data. - supported by an NSF grant. \newline [1] D. A. Filippov, M. I. Bichurin, V. M. Petrov, V. M. Laletin, G. Srinivasan, Phys. Solid State \textbf{46}, 1674, (2004). [Preview Abstract] |
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J1.00118: Nonextensivity in Magnetic Nanocluster Ensembles Christian Binek, Srinivas Polisetty, Xi He, Tathagata Mukherjee, Rajesh Rajasekeran, Jody Redepenning We study the scaling behavior of dipolar interacting nanoparticles in 3D samples of various sizes but constant particle density. Ferromagnetic $\gamma $-Fe$_{2}$O$_{3}$ clusters embedded in a polystyrene matrix are fabricated by thermal decomposition of metal carbonyls. Transmission electron microscopy reveals a narrow size distribution of 12 nm clusters. They are randomly dispersed in the matrix with an average separation of 80 nm. Magnetization isotherms of these single domain particle ensembles are measured by SQUID magnetometry above the blocking temperature T$_{B}$ =115K where non-equilibrium effects are avoided. After demagnetization corrections which convert the applied magnetic fields into internal fields, H, a data collapse is achieved when scaling the magnetic moment, m, and H by appropriate factors. The latter are theoretically predicted functions of the number of particles and determined here numerically. Scaling of H takes into account the nonextensive (NE) behavior of dipolar interacting particles. In the case of long range interactions a scaling schema has been proposed by Tsallis and confirmed by simulations. The controversial field of NE thermodynamics requires however experimental evidence provided here. [Preview Abstract] |
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J1.00119: Magnetic and magneto-optical properties of Co$_{2}$MnSi magnetic grating J.B. Kim, G.J. Lee, Y.P. Lee, C.S. Yoon, J.Y. Rhee, K.W. Kim The magnetic and the magneto-optical properties of periodic magnetic structure, produced by the femtosecond-laser induced crystallization of amorphous Co$_{2}$MnSi films, were elucidated. The amorphous Co$_{2}$MnSi films have been prepared by rf-magnetron sputtering on a glass substrate at room temperature. In order to form a grating structure, two-beam interference of femtosecond laser pulses was employed. The atomic-force-microscopy and the transmission-electron-microscopy results show regularly-spaced alternating lines with a periodicity of 2\textit{ $\mu $}m. The magnetic-force-microscopy results reveal the periodic patterns of magnetic domains. The Kerr hysteresis loops of sample were measured by using a magneto-optical microscope. The enhanced magneto-optical properties were observed in the first-order diffracted beam. It was also found that the films with a spatially periodic magnetic structure can be relatively easily fabricated by selectively crystallization of a paramagnetic Co$_{2}$MnSi amorphous film, which crystallizes into a ferromagnetic phase upon illumination with sufficiently intense laser light. [Preview Abstract] |
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J1.00120: Non-lithographic Fabrication of Magnetic Nanodot Arrays Bi-Ching Shih, Min Ni, Hao Zeng Highly-ordered magnetic nanodot arrays with tunable size and interdot spacing have been fabricated by electron-beam evaporation through ultra thin porous anodic alumina (PAA) templates. The PAA templates were fabricated by a 2-step anodization process in a 0.3 M oxalic acid electrolyte at room temperature, with an anodization voltage ranging from 20 to 60 V. The aspect ratio were controlled by the second anodization time. In order to get porous arrays with through holes, the aluminum layer and the barrier layer were removed by wet chemical etching. The template was then placed on a solid substrate as a shadow mask for subsequent deposition of magnetic nanodots. The magnetic properties of Ni nanodot arrays with 50 nm diameter and 80 nm spacing were studied in detail. The dots are superparamagnetic at room temperature. They become ferromagnetic at approximately 170 K. The coercivity at low temperatures is found to be much higher than that of continuous Ni films, due to a transition of magnetization reversal mechanisms from domain wall motion to spin rotation. A significant reduction in the Curie temperature is observed, which is attributed to the finite size effect. [Preview Abstract] |
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J1.00121: Magnetic Thermal Hysteresis in (Co, Fe)/Tb Multilayers Maria Hossu, Ali Koymen In plane and out of plane magnetic thermal hysteresis have been measured to investigate the ferrimagnetic behavior of (Co, Fe)/Tb multilayers. Due to antiferromagnetic coupling between the (Co, Fe) and Tb, the multilayers behave like artificial ferrimagnets. For [Co (30{\AA})/Tb (45 {\AA})] $_{8}$ the measurement of magnetic moment (M) as a function of temperature shows that magnetic phase transition occurs at different temperatures during the heating (superheating) and cooling (supercooling) cycles resulting in a characteristic \textbf{bow-tie} shaped thermal hysteresis curve for \textbf{M (}T\textbf{)}.The width of the thermal hysteresis was measured to be to be around 90 K in an external magnetic field of 2000 Oe. Increasing the field to 4000 Oe reduces the thermal hysteresis width to about 40 K and above 6000 Oe the thermal hysteresis disappears. Co/Tb multilayers with thicker layers show the same trend, however, the width of the thermal hysteresis is generally smaller at a given magnetic field. [Preview Abstract] |
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J1.00122: Magnetic properties of double-layer vanadium oxides and vanadium oxide nanotubes. Natasha A. Chernova, Samuel T. Lutta, Chris Jacobs, M. Stanley Whittingham, Peter Y. Zavalij Double-layer V$_{8}$O$_{20}$ compounds\footnote{ P. Y. Zavalij and M. S. Whittingham, Acta Cryst. B55, 627 (1999).} intercalated with various cations and the vanadium oxide nanotubes (VONTs) VO$_{2.4}$[C$_{12}$H$_{28}$N]$_{0.31}\cdot $0.56H$_{2}$O were prepared using hydrothermal method and characterized by x-ray diffraction, TGA, TEM, FTIR. Magnetic properties were studied using SQUID magnetometer. Depending on the intercalated cation type, the magnetic susceptibility of V$_{8}$O$_{20}$ compounds reveal presence of small clusters or infinite linear chains with antiferromagnetic (AF) exchange. VONTs show a spin gap, which is well described by the simple model of magnetic dimers with AF exchange. Various ion exchange and red-ox reactions are performed on VONTs. Structure and magnetic properties of the resulting compounds are discussed. No ferromagnetic response from the VONTs reacted with BuLi is found\footnote{ L. Krusin-Elbaum, D. M. Newns, H. Zeng, V. Derycke, J. Z. Sun and R. Sandstrom, Nature 431, 627 (2004).}. The work is supported by the National Science Foundation through grant DMR 0313963. [Preview Abstract] |
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J1.00123: Thermal and Quantum Peierls Transitions in Organic Charge-Transfer Salts Sharon Bewick, Zoltan Soos The choice of donors (D) and acceptors (A) governs the charge-transfer $\rho $ in organic CT salts with mixed one-dimensional DADA stacks. Strong D and A yield $\rho \quad \sim $ 0.9 stacks of radical ions with thermally accessible spin and charge degrees of freedom whose Peierls transition can be described by a Hubbard model with site energies. The same microscopic model describes CT salts with smaller and variable $\rho \quad \sim $ 0.5 in which neutral-ionic and/or Peierls transitions occur in the ground electronic state. Quantum transitions are driven by volume changes, with negligible thermal population of excite states. CT salts with thermal or quantum Peierls transitions are identified. Conflicting magnetic, vibrational and structural data in several CT salts are resolved in terms of mobile spin solitons, a dimerized ground state and a Peierls transition beyond the crystal's thermal stability. [Preview Abstract] |
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J1.00124: Avalanches in Mn12-Acetate: ``Magnetic Burning" Sean McHugh, Y. Suzuki, D. Graybill, M.P. Sarachik, N. Avraham, Y. Myasoedov, H. Shtrikman, E. Zeldov, R. Bagai, N.E. Chakov, G. Christou From local time-resolved measurements of fast reversal of the magnetization in single crystals of the molecular magnet Mn$_{12}$-acetate, we have shown[1] that the magnetization avalanche spreads as a narrow interface that propagates through the crystal at a constant velocity roughly two orders of magnitude smaller than the speed of sound. This phenomenon is closely analogous to the propagation of a flame front (deflagration) through a flammable chemical substance. The propagation speed of the avalanche depends on the energy stored in each molecule, which can be controlled and tuned using an external magnetic field. We report studies of propagation speed with different external fields in Mn$_{12}$-acetate. \newline \newline [1] Yoko Suzuki, M.P. Sarachik, E.M. Chudnovsky, S. McHugh, R. Gonzalez-Rubio, N. Avraham, Y. Myasoedov, H. Shtrikman, E. Zeldov, N.E. Chakov and G. Christou, Phys. Rev. Lett. 95, 147201 (2005). [Preview Abstract] |
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J1.00125: Time Evolution of the Magnetization of Mn-12 Acetate above 1.8 K. Reem Jaafar, Y. Suzuki, D. Graybill, M.P. Sarachik, R. Bagai, G. Christou It is well known that the decay of the magnetization of the molecular magnet Mn12-acetate is not a simple exponential. From measurements using a commercial Quantum Design MPMS magnetometer between 3 K and 1.8 K, we present the results of a detailed study of the time evolution of the magnetization as it increases from zero (following zero-field cooling) in response to longitudinal magnetic fields up to 400 Oe. Fits to the data will be shown for distributions of characteristic times (Gaussian, Lorentzian and others) of different widths in response to magnetic fields near and away from a tunneling resonance. The effect of temperature and transverse field will be discussed. [Preview Abstract] |
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J1.00126: Edge excitations in quantum spin chains Changfeng Chen We explore the nature of the topological edge excitations in quantum spin chains using both analytical valence-bond-solid analysis and numerical simulations. The results reveal new excitation modes that are associated with a topological order in the spin chains. Experimental implications will be discussed. [Preview Abstract] |
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J1.00127: Record Two-Halide Exchange in Cu(2,5-dmpz)Br$_{2}$: Theory and Experiment R. Butcher, C.P. Landee, M.M. Turnbull, J. Novoa, J. Ribas Cu(2,5-dmpz)Br$_{2}$ (2,5-dmpz = 2,5-dimethylpyrazine) is an antiferromagnetic rectangular lattice, with the S=1/2 copper ions bridged along one axis through the pyrazine molecules and through Cu-Br. . . Br-Cu contacts in a transverse direction. Large orbital overlap is obtained by the very short Br-Br distances plus the linear arrangement of the copper and bromine atoms. The magnetic susceptibility corresponds to an exchange strength of J/k$_{B} \quad \approx $ 160 K, a record for exchange for two bromide contacts. Numerical techniques have been developed for calculating two-halide contact magnetic exchange interactions from first principles [1,2]. They predict the same exchange strength for Cu(2,5-dmpz)Br$_{2}$ based only on the structural parameters of the compound. We present the magneto-structural correlations in Cu(2,5-dimethylpyrazine)Br$_{2}$ and compare this to two other known pyrazine complexes; Cu(pyrazine)CuBr$_{2}$ and Cu(2,3-dimethylpyrazine)Br$_{2}$. 1. M. Deumal \textit{et al}, \textit{Polyhedron }\textbf{22 } 2235-2239 ( 2003). 2. M. Deumal \textit{et al}, \textit{Euro. J. Inorg. Chem.} \textbf{2005}, 4697-4706. [Preview Abstract] |
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J1.00128: Magnetic properties of Co-Rh and Ni-Rh nanostructures Javier Guevara, Tristana Sondon, Andres Saul We study the evolution of the magnetic properties of Co-Rh and Ni-Rh nanostructures (free-standing monolayers and wires) on Rh content ($x$). It is known that dimensionality affects the magnetic properties of the materials. Rh is non magnetic in bulk, but shows magnetic order as free-standing monolayer or nano- wire, being 1.03$\mu_B$ and 0.26$\mu_B$ the corresponding magnetic moments, calculated by using the $ab initio$ Wien-2k code. In the case of Co$_{1-x}$Rh$_x$ and Ni$_{1-x}$Rh$_x$ wires, the evolution of the Rh magnetic moment is similar for both cases, being enhanced with respect to pure Rh wire value and reaching the largest values at x=.5 (1.36$\mu_B$ and 1.31$\mu_B$ respectively). For $x$ $>$ 0.5 all the magnetic moments fall. For $x$ $<$ .5 the Co and Ni magnetic moments reach values larger than the corresponding pure wire and than their saturation magnetic moments. In the case of mixed monolayers the Rh and Ni magnetic moments are not enhanced as Co or Ni content increases, however Co $\mu$ is enhanced without crossing its saturation value. By alloying with Co or Ni, Rh wires enhanced their magnetic moments, while mixed monolayers' Rh magnetic moments are not enhanced. [Preview Abstract] |
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J1.00129: $^{51}$V-NMR investigation of the spin-frustrated magnet Ni$_{3}$V$_{2}$O$_{8}$. W.G. Clark, P. Ranin, Guoqing Wu, G. Gaidos, G. Lawes, A.P. Ramirez, R.J. Cava, M. Horvatic, C. Berthier The Ni$^{2+}$ ions in the Kagom\'{e}-staircase lattice compound Ni$_{3}$V$_{2}$O$_{8}$ form an electron spin $S$~=~1 system with a geometrically frustrated magnetization and a rich variety of phases below 10 K. Here, we report a study of the local magnetic field and its fluctuations using $^{51}$V NMR measurements of the spectrum, the spin-phase memory time ($T_{2})$, and the spin-lattice relaxation rate (1/$T_{1})$ over a broad range of magnetic field and temperature. Above 10 K, the local field from the Ni$^{2+}$ ions follows a Curie-Weiss law. In the ordered phases below 10 K, the NMR spectrum has a very broad structure that changes according to the particular phase. Also, below 10 K, $1/T_1 \propto T^2$. These features suggest that 1/$T_{1}$ is dominated by Ni$^{2+}$ electron moment fluctuations whose dynamics are driven by two-dimensional antiferromagnetic coupling. [Preview Abstract] |
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J1.00130: Variation of critical temperature with dilution in a 3D-XY insulating ferromagnet G.C. DeFotis, R.A. Huddleston, B.C. Rothermel, J.H. Boyle, E.S. Vos, Y. Matsuyama, A.T. Hopkinson A very rare 3D-XY insulating ferromagnet is the molecular material Fe(III) bis(diselenocarbamate) chloride, ordering near 3.4 K. The XY anisotropy arises because of a large zero-field splitting of the quartet crystal field ground term, with a positive axial splitting parameter making the (+1/2,-1/2) Kramers doublet low-lying. Intermolecular exchange interactions operate via Fe-Se...Se-Fe contacts. We have succeeded in diluting this material to significant degrees with a diamagnetic near structural isomorph, Zn(II) bis (dithiocarbamate). Analysis of dc susceptibility and magnetization data leads to estimates of the ferrromagnetic ordering temperature in undiluted, 0.137, and 0.202 Zn-mole fraction diluted material. Compositions are determined by atomic absorption spectrophotometry. The ordering temperature decreases moderately with increasing dilution, at a rate typical of three-dimensional systems and much slower than in two-dimensional materials. It is more difficult to distinguish among spin interaction models based on the data. [Preview Abstract] |
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J1.00131: A Yttrium Iron Garnet-Lead Zirconate Titanate Phase Shifter G. Srinivasan, A.S. Tatarenko, M.I. Bichurin Tunable microwave phase shifters are of interest for miniature oscillators and phased array antenna systems. Traditional ferrite phase shifters use magnetic tuning systems that are slow, demand high power, and are not miniature in size. Here we discuss the design, fabrication and characterization of a novel electric field tunable phase shifter based on a yttrium iron garnet (YIG) -- lead zirconate titanate (PZT) bilayer. The electrical control of the phase shift is realized through magnetoelectric (ME) interaction. The phase shifter consisted of a microstrip transmission line with stubs of $\lambda $/8 and 3$\lambda $/8 lengths for generating circularly polarized microwave magnetic field in the YIG-PZT resonator. The ME resonator was made from 124 micron thick (100) YIG film on GGG and 0.5 mm thickness PZT with electrodes. The operating frequency of the phase shifter was set by applying appropriate bias magnetic field. The phase shift vs. electric field E characteristics was linear or quadratic in E, depending on the operating frequency. The maximum phase shift was 180 deg. and showed an insertion loss of 1.5-2.0 dB at 5 GHz and 3-4 dB at the frequency 10 GHz. The ME phase shifter is capable of rapid tuning, miniature in size and dissipates practically zero power. -- The work was supported by grants from ONR, ARO and NSF. [Preview Abstract] |
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J1.00132: A Magnetic Field Tunable Lens of Left-Handed Material A.S. Petrov, R.V. Petrov, M.I. Bichurin, G. Srinivasan, D. Viehland The design, fabrication and characterization of lenses of left handed materials (LHM) based on ferrite spheres are presented. A unit-cell constructed from cross-wire resonators with 2-mm diameter polycrystalline yttrium iron garnet spheres at the center was used. The ferrite sphere facilitated LHM elements with magnetic-field dependent permeability. The focal length of the LHM lens, therefore, could be tuned with a magnetic field. The studies show that the YIG-cross-wire resonators are ideal tunable LHM lenses. The lens characteristics were measured in the frequency range from 1 to 18 GHz. For example, the focal point of the lens could be tuned over a length of 2 cm by varying the magnetic field from 4100 to 4125Oe at 11.5 GHz. - supported by a grant from the NSF. [Preview Abstract] |
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J1.00133: A Negative-Index Metamaterial Unit Cell R.V. Petrov, M.I. Bichurin, D. Viehland, G. Srinivasan Metamaterials with a negative index of refraction can be realized in an array of straight wires and split-ring resonators (SRRs). Such structures will have a negative permittivity and permeability at a specific frequency. Here we provide the details on the design and characterization of a modified negative-index material (NIM) that uses cross-wire resonators (CWR). The resonators were made of copper wires having a diameter of 0.2 mm. Comparison of properties at 11.5 GHz for NIMs constructed with SRR and CWR is presented. For the NIM lens with CWR, precise control of the focal point is possible; the edges in the profile for intensity vs. distance are well defined, and the maximum achievable focal length is higher than for the split-ring NIMs. - supported by a grant from the NSF. [Preview Abstract] |
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J1.00134: Study of Complete Elliptic Integrals with a goal of Improving Computation of Axisymmetric Fields Joseph W. Rudmin In recent years I have been investigating the Complete Elliptic Integral of the First Kind, K, with a goal of finding a rapidly convergent power series R such that K can be written in terms R with the non-analyticities subtracted off or divided out. The singularities in K are logarithmic in nature, so that power series expressions of K converge slowly. Progress in this study will be presented. [Preview Abstract] |
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J1.00135: Magnetization dynamics of two interacting spins in an external magnetic field William Coffey, Hamid Kachkachi, Yuri Kalmykov, Sergey Titov The longitudinal relaxation time of the magnetization of a system of two exchange coupled spins subjected to a strong magnetic field is calculated exactly by averaging the stochastic Gilbert-Landau-Lifshitz equation for the magnetization, i.e., the Langevin equation of the process, over its realizations so reducing the problem to a system of linear differential-recurrence relations for the statistical moments (averaged spherical harmonics). The system is solved in the frequency domain by matrix continued fractions yielding the complete solution of the two spin problem in external fields for all values of the damping and barrier height parameters. The magnetization relaxation time extracted from the exact solution is compared with the inverse relaxation rate from Langer's theory of the decay of metastable states [J. S. Langer, Ann. Phys. (N.Y.) \textbf{54}, 258 (1969)], which yields in the high barrier and intermediate-to-high damping limits the asymptotic behavior of the greatest relaxation time. [Preview Abstract] |
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J1.00136: A Comparison of Various Update Schemes Used in Monte Carlo Simulations of Stoner-Wohlfarth Particles Christopher Russell, Karl Unruh The hysteresis loops for non-interacting Stoner-Wohlfarth particles have been simulated by the Monte Carlo (MC) method as a function of the temperature, the magnitude of the angular aperture used to update the magnetization direction, and the total number of MC steps. Comparing the coercivity obtained from these hysteresis loops with those obtained from N\'{e}el's relaxation model has allowed two different sets of consistent MC parameters to be determined (in the sense that the simulated hysteresis loops will reproduce the temperature dependence of the N\'{e}el coercivity). The first set allows the use of a physical temperature but requires the magnitude of the update aperture to be temperature dependent (at a fixed number of MC steps). The second set allows the use of a constant update aperture but requires the introduction of an effective temperature (again at a fixed number of MC steps). [Preview Abstract] |
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J1.00137: Toy model of an electron interacting with two classical spins William Schwalm, Juana Moreno We consider a single electron governed by a tight-binding Hamiltonian, $H\,=\,H^{(o)}+V$, where $H^{(o)}$ stands for the band structure of the crystal and $V$ is a double-exchange type interaction between the electron and two localized classical spins, $\vec{S}_1 \cdot \vec{J} + \vec{S}_2 \cdot \vec{J}$, where $\vec{S}_1$ and $\vec{S}_2$ are classical spins and $\vec{J}$ is electron angular momentum. We study the dynamics of the system as a function of the distance and the angles between the two spins. By summing over electron states we learn about the effective interaction between the two spins. We discuss the connection of this model with ferromagnetism in dilute magnetic semiconductors. [Preview Abstract] |
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J1.00138: Relaxation of the magnetization in biaxial superparamagnetic particles in the presence of a strong uniform magnetic field Y. Kalmykov, W. Coffey, B. Ouari, S. Titov The longitudinal relaxation time of the magnetization and the spectrum of the complex magnetic susceptibility are evaluated for biaxial single domain ferromagnetic particles in the presence of a strong uniform magnetic field. The relaxation time is estimated for all dissipation regimes, i.e., very low damping, intermediate-to-high damping, and turnover, using the method of Coffey \textit{et al.} [Adv. Chem. Phys. \textbf{117}, 483 (2001)]. It is shown that the simple asymptotic formulae for the greatest relaxation time so obtained are in complete agreement with the relaxation time calculated from the infinite hierarchy of linear differential-recurrence equations for the statistical moments. This hierarchy, which governs the relaxation of the magnetization of an individual particle, is derived by averaging the governing stochastic Gilbert equation over its realizations. The exact solution of the system of moment equations is obtained by matrix continued fractions. Simple analytic equations, which allow one to accurately predict the spectrum of the longitudinal complex susceptibility for wide ranges of the barrier height and dissipation parameters, are also proposed. [Preview Abstract] |
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J1.00139: A Novel Class of Helical Nanostructures: Paradigms for their Design and Synthesis Using Imprint Lithography and Carbon Nanotubes David Smalling This research focuses on the use of solenoid-like configurations which are both microscopic and macroscopic, to make energy conversions between electrical energy and other forms. Numerous conceptual models are to be evaluated for the construction of an environment in which a charged particle could be allowed travel along a helical path of extremely small pitch and comparatively large large radius. The two main types of solenoid designs discussed, are lithographically synthesized solenoids and coiled carbon nano tube solenoids. For future identification the family of structures described will be referred to as Zeta ($\zeta$) Structures. In the case of a $\zeta$ solenoid, the objective would be to create a solenoid whose general structure is macroscopic but comprises a conductive trace which is on the nano-scale, In the case of such a solenoid the current flowing \emph{I} and the magnetic flux \emph{B} would be related approximately by $B \approx 1.256\times10^{3}I$. Such a situation gives rise to a very fascinating relationship between B and I. This means that if a current of say 1 ampere was made to flow through the solenoid, a magnetic flux of 1250 Tesla would be generated. The defining characteristic of $\zeta$ structures is their overall macroscopic dimensions which comprise high level nano-scale repetition. This research discusses theoretical propositions for the development of a class sub micron structures defined by a unique helical foundation, to be used for the generation of magnetic fields. [Preview Abstract] |
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J1.00140: Comparative Study of the Magnetic Properties of Ba-Ferrite Thin Films Deposited on Different Substrates W. J. Yeh, Joel Krehbiel, A. R. Abuzir The Barium ferrite (Ba$Fe_{12}$$O_{19}$) film is an attractive candidate for the self-biased microwave circulator application. The requirement for this application is that the Barium ferrite films have excellent c-axis orientation with moderate $H_{c}$ value and good squareness, so that Barium ferrite films can be self-biased to eliminate the requirement of the external DC magnetic field. We have used the magnetron sputtering to deposit Ba-ferrite films onto silicon, MgO and sapphire substrates. All of the films were deposited in the same Ar + $O_{2}$ atmosphere at elevated substrate temperatures to optimize the magnetic properties in terms of c-axis orientation. The films were studied by VSM, XRD, SEM and MOKE. It was found that Barium ferrite thin films on silicon substrates had coercivity value of 3000 Oe with squareness of 0.6 with moderate c-axis orientation. Films deposited on MgO showed better c-axis orientation. However, the values of $H_{c} $ and squareness were low. Films deposited on sapphire showed the best c-axis orientation. The coercivity value of 3000 Oe and the squareness of 0.93 in perpendicular direction were achieved. Based on our results sapphire is the promising substrate choice for growing Barium ferrite films for the self- biased circulator application. [Preview Abstract] |
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J1.00141: Control of magnetic and ferroelectric phase in multiferroic (Tb, Bi)MnO$_{3}$ system Tae-Hwan Jang, Muralidharan Rajaram, Chan-Ho Yang, Tae Yeong Koo, Yoon Hee Jeong Various magnetic ferroelectrics have been discovered and extensively studied because of the possibility of coupling between electric and magnetic degrees of freedom and their potential technological applications. However, the two transitions from electric and magnetic origins have a big difference in their temperature range of occurrence and there is a rare possibility to get two critical points in the same temperature region. We report highly polarizable cation modification effects of multiferroic TbMnO$_{3}$ polycrystalline system on magnetic and ferroelectric phase transition. Random replacement of Tb$^{3+}$ with Bi$^{3+}$ induces low frequency relaxation in magnetic susceptibility reminiscent of relaxor behavior. The special role of Bi$^{3+}$ ions is emphasized because the anomalous behavior has not been observed in other non-magnetic cation substituted TbMnO$_{3}$ systems. Various accompanying phenomena such as magnetic and ferroelectric relaxational behavior, structural anisotropy, and tuning of electromagnetic coupling are also discussed. Controllability of both magnetic and ferroelectric phase by the combination of Bi-substitution and driving frequency suggests a new possibility for getting two transitions having different origin at the same temperature point. [Preview Abstract] |
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J1.00142: Role of oxygen vacancy in ferromagnetic Mn-doped ZnO films S.Y. Park, Y.J. Yoo, P.J. Kim, Y.P. Lee, T.H. Kim, J.-H. Kang We report that Zn$_{1-X}$Mn$_{X}$O (x = 0.4) films grown by reactive magnetron co-sputtering have ferromagnetism at temperatures above 300 K. The ferromagnetic behavior is sensitive to not only the carrier concentration but also the film growth parameters, such as the substrate, substrate temperature, deposition rate and oxygen partial pressure (P$_{O2})$ during deposition. In this study, we focused on the role of oxygen vacancy for the ferromagnetism in Zn$_{1-x}$Mn$_{x}$O films with x $\le $ 0.05. In order to fabricate high-quality samples, the preparation was performed in an ultrahigh vacuum, and the accurate P$_{O2}$ was monitored using a residual gas analyzer during deposition. The magnetic and the structural properties of films were characterized by SQUID and XRD, respectively. The normal-mode and oxygen-resonance-mode RBS were carried out to confirm the mount of Mn, Zn and O contents. The film, prepared at an oxygen partial pressure of 2.2x10$^{-7}$ Torr, exhibits a strong ferromagnetism with $T$c above 300 K, while the films at an partial pressure of higher than 1.2x10$^{-6}$ Torr show the nonmagnetic behaviors. Our results can be elucidated with a theoretical model by Coey[1] that the oxygen vacancy could results in the magnetic ordering of Mn-doped ZnO film by the enhancement of overlap between $s$-band and imputity $d$-band. [1] J. M. D. Coey et al., Nat. Mater. \textbf{4}, 173 (2005). [Preview Abstract] |
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J1.00143: Magnetoelectric effect in HoMnO$_{3}$ N. Hur, A. Saxena, M.F. Hundley, S.B. Kim, S.-W. Cheong Multiferroic rare earth manganites have attracted a renewed interest because of recent observations of the giant coupling between ferroelectricity and magnetism in these materials. In particular, hexagonal HoMnO$_{3}$ has been one of the most actively studied systems among them. For incidence, novel dielectric phase transitions and electric field induced ferromagnetism have been observed recently. However, any direct observation of magnetoelectric effect has not been made. In this presentation, I will discuss the effect of magnetic structure change on the electric polarization in HoMnO$_{3}$ studied by measuring the polarization change as a function of the temperature and magnetic fields. [Preview Abstract] |
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J1.00144: Negligible Magnetism in transition metal (Cr, Mn, Fe, Co) doped Nb:SrTiO3 Shixiong Zhang, Satish Ogale, Wegdan Ramadan, Sankar Dhar, Darshan Kundaliya, Thirumalai Venkatesan, Lianfeng Fu, Nigel Browning In order to realize the possibility of carrier induced ferromagnetism in transition metal (TM) doped SrTiO$_{3}$, we have performed magnetic, electronic transport, and micro-structural measurements on 2{\%} TM doped SrTi$_{0.98-x}$Nb$_{x}$O$_{3-\delta }$ (x=0.000, 0.005,0.010) thin films. Unfortunately, no unambiguous signature of ferromagnetism is observed in this system, even though its conductivity is seen to be greatly improved by the enhancement of carrier density due to Niobium doping. High-resolution Z-contrast Scanning Transmission Electron Microscopy shows no evidence of clustering, while Electron Energy Loss Spectroscopy indicates that the TM is enriched on the surface of the film whereas Niobium is mostly distributed in the bulk. It is believed that the absence of strong ferromagnetism in this system may be attributed to a spatial separation of the spins contributed by the TM and the carriers from Nb in the SrTiO$_{3}$ matrix. [Preview Abstract] |
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J1.00145: Dependence of Ferromagnetic Properties on Carrier Transfer in (Ga$_{1-x}$Mn$_{x})$N H. C. Jeon, S. J. Lee, T. W. Kang, T. W. Kim, Joongoo Kang, K. J. Chang Among many types of DMS materials, (Ga$_{1-x}$Mn$_{x})$As DMS have been mostly studied. However, since the highest T$_{c}$ obtained from the (Ga$_{1-x}$Mn$_{x})$As has been 172 K, which is too low for practical applications. As alternative DMS materials with the high T$_{c}$, (Ga$_{1-x}$Mn$_{x})$N DMSs are of current interest because their T$_{c}$ values can be as high as room temperature. Theory predicts that the ferromagnetic properties observed in GaMnN material system depends on the occupancy of the Mn energy band in GaMnN and the position of the Fermi level relative to this band. Carriers (holes) in the Mn energy band are needed to mediate ferromagnetic interaction: the depletion and enhancement of carrier concentration in the band will change the ferromagnetic properties of GaMnN. Even though many studies concerning the growth and characterization of (Ga$_{1-x}$Mn$_{x})$N have been carried out, systematic studies are required to understand the carrier-mediated ferromagnetism which is assumed to play an important role in enhancing the Tc by increasing hole carrier concentrations. This work will demonstrate this concept by studying the carriers to mediate ferromagnetism in various GaMnN structures, hence affecting the ferromagnetic properties of the GaMnN. [Preview Abstract] |
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J1.00146: Magnetism of Discrete, L1$_{0}$ Ordered FePt Nanoparticles Hao Zeng, Michael DeMarco, Daren Li, Ping Liu Discrete, L1$_{0}$ ordered FePt nanoparticles have been made by annealing the as-synthesized nanoparticles in the presence of NaCl powder.$^{1}$ These particles exhibit high degree of chemical ordering and very large coercivity at room temperature. The temperature dependent magnetic properties of these non-aggregated nanoparticles have been studied systematically as a function of particle size. The magnetization reversal behavior of 4 nm particles can be well explained by Stoner-Wohlfarth coherent rotation model, while that of 8 nm and 15 nm particles is more complicated. Mossbauer spectroscopy measurements indicate the existence of a paramagnetic phase in these highly ordered nanoparticles. The amount of the second phase decreases with temperature but persists even in liquid He temperature. This is attributed to canted spins at the particle surfaces. $^{1}$K. Elkins, D. Li, N. Poudyal, V. Nandwana, Z. Jin, K. Chen and J.P. Liu, J. Phys. D: Appl. Phys. 38, 2306 (2005). [Preview Abstract] |
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J1.00147: Silicon and III-V compound nanotubes: Structural and electronic properties Engin Durgun, Sefaattin Tongay, Salim Ciraci Unusual physical properties of single-wall carbon nanotubes have started a search for similar tubular structures of other elements. We present a theoretical analysis of single-wall nanotubes of silicon and group-III-V compounds. Starting from precursor graphenelike structures we investigated the stability, energetics, and electronic structure of zigzag and armchair tubes using the first-principles pseudopotential plane wave method and finite temperature ab initio molecular dynamics calculations. We showed that (n,0) zigzag and (n,n) armchair nanotubes of silicon having $n \ge 6$ are stable but those with $n < 6$ can be stabilized by internal or external adsorption of transition metal elements. Some of these tubes have a magnetic ground state leading to spintronic properties. We also examined the stability of nanotubes under radial and axial deformation. Owing to the weakness of radial restoring force, stable Si nanotubes are radially soft. Undeformed zigzag nanotubes are found to be metallic for $6 \le n \le 11$ due to the curvature effect; but a gap starts to open for $n \ge 12$. Furthermore, we identified stable tubular structures formed by the stacking of Si polygons. We found AlP, GaAs, and GaN (8,0) single-wall nanotubes stable and semiconducting. Our results are compared with those of single-wall carbon nanotubes.[1] [1] E. Durgun, S. Tongay, and S. Ciraci Phys. Rev. B 72, 075420 (2005) [Preview Abstract] |
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J1.00148: COMPLEX STRUCTURED MATERIALS POSTER SESSION |
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J1.00149: Self-Assembly of Nanoporous Silica: Synthesis, Morphogenesis, Functionalization, and Applications Yaroslav Kievsky, Igor Sokolov We study the process of self-assembly and growth of nano(meso)porous silica particles and film via surfactant templating. Process of formation of the mesoporous silica includes growth of the liquid crystalline template and solidification of this template via polymerization of silica precursor. Material obtained as a result of such synthesis (e.g., MCM-41) features highly uniform porosity, a large variety of shapes and their sizes. To control the assembly of the desired shapes, we study their morphogenesis. New conditions of self-assembly are found to form monoshaped nanoporous fibers. Recently suggested Origami-type mechanism for synthesizing a rich family of nanoporous silica shapes (cones, hollow tubes, and helixes) is examined. Shape details and their evolution are analyzed by means of XRD, SEM, TEM, AFM, and optical microscopy techniques. The shapes can possibly serve as templates for various electronic and optical applications. Nanoporous shapes are the prospective hosts for lasing dyes (sealing laser dye molecules inside the silica pores saves them from oxidation and prevents their dimerization). Color coded silica beads are interesting for tagging in security applications and labeling in biology. Diffusion from the nanoporous shapes can be used for a control drug release. Another application of mesoporous silica is the coating of optical fibers by uniform low refractive index film with a good adhesion -- a possible host for laser dyes or quantum dots. [Preview Abstract] |
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J1.00150: Synthesis of Hollow Carbon Nanofibers and Filled Carbon Nanotubes using Chemical Vapor Deposition Technique. Kevin Shay, Evgeniya Moiseeva, Gamini Sumanasekera Hollow carbon nanofibers (CNFs) and filled carbon nanotubes were created using chemical vapor deposition (CVD). Hollow carbon nanofibers were synthesized in porous anodic aluminum oxide templates (AAO) by CVD of methane (CH$_{4})$ with ferrocene (C$_{10}$H$_{10}$Fe), indium tin oxide (ITO) and iron nitride (Fe(NO$_{3})_{3})$ as catalyst at 910\r{ }C. Carbon nanotubes were filled with iron using CVD of methane. The source of iron in this process was ferrocene. Both the nanostructures were viewed under SEM and TEM. The CNFs grown within the pores of the aluminum oxide membranes are uniform with lengths of about 50 mm and outer diameters of about 200 nm. A diameter of 20-30 nm was measured for the iron filled sample. The magnetic transport properties of the samples will be presented. [Preview Abstract] |
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J1.00151: Raman studies of crystallization and photo-darkening in HARP a-Se targets R.E. Tallman, B.A. Weinstein, A. Reznik, J.A. Rowlands, M. Kubota, K. Miyakawa, Y. Ohkawa, K. Tanioka, T. Kawai The build-up of light-induced defects in the a-Se targets of High-gain Avalanche Rushing Photoconductor (HARP) cameras is important for the robustness of these detectors. Recent experiments show that the photo-induced dark spots in HARP targets exhibit irreversible and reversible components, i.e., spot transparency is only partly restored by heating to 35C, $\sim $1C below the a-Se glass transition.[1] We report Raman studies of laser(2mW HeNe)-induced crystallization in a-Se camera targets. The rate of increase in the trigonal c-Se Raman intensity (233 cm$^{-1}$ peak) is measured \textit{vs.} exposure time for local temperatures in the range 25 -- 65 C, as found from Stokes-to-anti-Stokes ratios. Lateral- and depth- profiling by optical microscopy easily correlate the Raman-detected crystallization regions with visual damage. Further studies \textit{vs.} temperature and HARP-target usage are in progress to unravel the damage mechanisms. [1] A. Reznik \textit{et. al.}, submitted to J. Non-Cryst. Solids [Preview Abstract] |
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J1.00152: Equilibrium Atomic Configurations Studies and Electron Paramagnetic Resonance of Binuclear Niobium Molecules in Li-Nb Phosphate Glass Dielectrics. Sabrina Arrington-Peet, Rakhim Rakhimov, Vladimir Gavrilenko Equilibrium atomic configurations of binuclear niobium complexes Nb-O-Nb both free-standing and that embedded in lithium niobium phosphate glass (LNPG) are studied by total energy minimization method using \textit{ab initio} pseudopotentials within density functional theory. Measurements of electron paramagnetic resonance revealed nonequivalent distribution of the charge and electron spin density between two Nb atoms. Measurements and theoretical analysis indicate that niobium atoms could exist in either of the two forms, Nb$^{4+}$-Nb$^{4+}$ or Nb$^{3+}$-Nb$^{5+}$. The Nb atoms in freestanding molecule are equivalent with oxygen atom having only one local equilibrium position in the center of the molecule, indicating centro-symmetric structure. When the Nb complex is embedded into the glass the effect of external potential in LNPG resulted in an appearance of two non-equivalent configurations for the central oxygen atom in the molecule. Predicted nonequivalent geometries of the Nb-O-Nb complex embedded into LNPG is confirmed experimentally. Mechanism of the distortion is discussed. [Preview Abstract] |
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J1.00153: X-ray absorption spectroscopic studies of the coordination environment of rare earth ions in novel rare earth phosphate glasses Hasitha Ganegoda, Kanishka Marasinghe, Nathaniel Wyckoff, Richard Brow, Carlo Segre, Mali Balasubramanian Rare earth (RE) phosphate glasses (RE$_{2}$O$_{3}$-P$_{2}$O$_{5})$ have exciting magnetic and optical properties with numerous potential applications. These properties depend heavily on the atomic structure, especially the rare earth coordination environment. A series of rare earth (namely neodymium, praseodymium, gadolinium, and erbium) phosphate glasses having low rare earth ion concentrations have been studied with rare earth L$_{III}$ edge X-ray absorption spectroscopy (XAS). Rare earth coordination environment appears to undergo an abrupt change when the RE$_{2}$O$_{3}$ concentration exceeds approximately 15 mol{\%}. Implications of this structural change will be discussed and results from XAS technique will be compared with those obtained with X-ray and neutron diffraction techniques. [Preview Abstract] |
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J1.00154: Electron density, geometric structure and vibration spectra of endohedral M$_{1-3}$\@C$_{60}$ (M = Mn, Co, Fe, Gd) metallofullerenes G. P. Li, R. Sabirianov, W. N. Mei, P. A. Dowben, X. C. Zeng Endohedrally doped buckminster fullerene C60 with metallic clusters of M$_{1-3}$, (M = Mn, Co, Fe and Gd) are studied by using density functional theory (DFT), projector augmented wave (PAW) and pseudo-potential (PP) methods. Optimized molecular geometries are obtained from varying the positions of these two/three metal atoms together with the C$_{60}$ cage and finding the energy minimum while considering different multiplicities and magnetic configurations. Subsequently the vibrational frequencies of the cluster, electric and magnetic dipole moments are calculated and compared with experiments. Upon the molecular properties are thoroughly studied, we construct different crystal models utilizing these metallofullerenes and employ the local density approximation (LDA)+U techniques to account for many-body effects. [Preview Abstract] |
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J1.00155: Energy Radiation from Plasmons on Nanotubes Antonios Balassis, Godfrey Gumbs A formalism has been developed for calculating the rate of transfer of energy from a current of charged particles to multi-walled and a linear array of nanotubes. Numerical calculations are presented for tubules of various radii and impact parameters of the moving charged particles. The dispersion relation of the coupled tubules interacting through the Coulomb interaction has several branches corresponding to excitation of electrons within a subband as well as between energy subbands. The electric field of the current excites these modes but one of them is unstable and radiates energy. This is demonstrated in the spectrum of energy transfer with a ``dip'' unlike the other plasmon modes which have a ``peak'' for arbitrary impact parameter. The single-particle excitation spectrum does not have an instability for any charged particle velocity or impact parameter. [Preview Abstract] |
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J1.00156: Hysteresis in single-walled carbon nanotube field--effect transistors: Experiments, a model, and implications S. Kar, S. Talapatra, A. Vijayaraghavan, C. Soldano, R. Vajtai, S.K. Nayak, O. Nalamasu, P.M. Ajayan Hysteresis in carbon nanotube FETs is understood to be due to charge injection from the nanotube to its surrounding dielectric. We present a simple yet effective model to understand and analyze this phenomenon, wherein the charge injection and its subsequent redistribution has been modeled as a series RC circuit. A set of experiments validates this model, and also fits previously published data by other groups. Our work provides an in-depth picture of the parameters, which play a crucial role in modifying the transfer characteristics in nanotube FETs under different experimental conditions. [Preview Abstract] |
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J1.00157: Networking Carbon Nanotubes for Integrated Electronics. J.M. Romo-Herrera, M. Terrones, H. Terrones, V. Meunier The unique electronic and mechanical properties of individual Carbon Nanotubes (CNTs) have attracted much interest as candidates for molecular electronic devices and reinforced materials. However, their integration in organized architectures remains a major challenge. Recent breakthroughs reported on the Self-Assembly of 1D Nanostructures[1], and on the coalescence mechanism for interconnecting CNTs[2], point to the possibility of designing and obtaining Ordered Networks based on CNTs (ON- CNTs). We propose a set with different complex architectures of ON- CNTs based on --but not limited to-- armchair and zigzag nanotubes. In addition to the study of the energetics of the structures, we have systematically investigated their electronic transport properties in the framework of the Landauer-Buttiker formalism and equilibrium Green functions. To take curvature into account, we employed a semi-empirical Hamiltonian based on 4 orbitals (s,px,py,pz) per carbon atom. Further insight is obtained analyzing the electron pathways from a scattering point of view, which allows a real-space analysis of the wave function from the transmitted electrons across the structure. [1]Whang D \textit{etal.} \underline {\textbf {Nanoletters}},\textbf{3} $(2003)$. Tao A \textit{etal.} \underline {\textbf{Nanoletters}}\textbf{,3 }$(2003)$. [2]Terrones M \textit{etal.} \underline {\textbf{PRL}},\textbf {89} $(2002)$. Endo M \textit{etal.} \underline {\textbf{Nanoletters}},\textbf{5 }($2005)$. [Preview Abstract] |
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J1.00158: Band structure and absorption spectrum of double-walled zigzag carbon nanotubes in an electric field Yen-Hung Ho, G. W. Ho, M. F. Lin, C. P. Chang The electronic structure of the (9,0)-(18,0) double-walled zigzag carbon nanotubes in the presence of a uniform transverse electric field is studied by the tight-binding model. The electric field could induce the semiconductor-metal transition, change the direct gap into the indirect gap, alter the subband curvatures, destroy the double degeneracy, produce the new band- edge states, make more subbands group around the Fermi level, and widen the $\pi$-band width. Such effects are directly reflected in density of states and optical excitation spectra. The absorption spectra exhibit a lot of prominent peaks, mainly owing to the rich one-dimensional energy subbands. The intensity, the number, and the frequency of absorption peaks are strongly modulated by the electric field. The modulation of electronic and optical properties is amplified by the parallel magnetic field. The predicted electronic and optical properties can be, respectively, verified by the conductance measurements and the optical spectroscopy. [Preview Abstract] |
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J1.00159: Electronic and optical properties of finite carbon nanotubes in an electric field R.B. Chen, C.H. Lee, M.F. Lin, C. P. Chang The effects, caused by the geometric structure and a transverse electric field ($\bf E$), on the electronic and optical properties of quasi-zero-dimensional finite carbon nanotubes are explored by employing the tight-binding model coupled with a curvature effects. It is found that the electronic properties (discrete state energies, symmetry of electronic states, energy spacing, and state degeneracy) are significantly affected by $\bf E$ and the geometric structure (radius, length, and chirality). The electric field, by lowering the symmetry of finite carbon nanotubes, modifies the electronic properties. Thus, the optical excitation spectra, excited by electric polarization parallel to the nanotube axis, exhibit rich delta- function-like peaks, which reveal the characteristics of the electronic properties. Therefore it follows that geometric structure and $\bf E$ influence the low-energy absorption spectra, i.e., the change of frequency of the first peak, the alternation of the peak height, and the production of the new peaks. Moreover, the very complicated optical absorption spectra are characteristic for individual the chiral carbon nanotube due to its specific geometric structure. Above all, the predicted absorption spectra and the associated electronic properties could be verified by optical measurements. Supported by NSC 94-2112-M-022-001; NSC 94-2112-M-165-001; NSC 94-2112-M-006-002. [Preview Abstract] |
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J1.00160: Electronic excitations and deexcitations in narrow-gap carbon nanotubes C. W. Chiu, Y. H. Ho, M. F. Lin, F. L. Shyu Temperature could induce some free carriers in narrow-gap carbon nanotubes. Such carriers exhibit rich intraband single- particle excitations and collective excitations. Only the low- frequency e-h excitations are the effective deexcitation channels in the carrier decay. The Coulomb decay rates are dominated by the screened loss function and the carrier distribution function. They are very sensitive to the changes in nanotube radius and temperature, but not wave vector. Narrow- gap carbon nanotubes quite differ from moderate-gap and metallic carbon nanotubes in electronic excitations and carrier relaxation. [Preview Abstract] |
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J1.00161: Efficacy of the Thermalized Quantum Potential Approach for Modeling Nanoscale Semiconductor Devices. Shaikh Ahmed, Dragica Vasileska, Gerhard Klimeck, Christian Ringhofer We propose a novel parameter-free effective potential scheme for use in conjunction with particle-based simulations of alternative semiconductor device technologies. The method is based on perturbation theory around thermodynamic equilibrium and leads to a quantum potential which depends on the energy and wavevector of each individual electron. The computation of the quantum potential involves only the evaluation of pseudo-differential operators and can be effectively facilitated using Fast Fourier Transform (FFT) algorithms. Our investigations suggest that for low doping densities, as it is usually the case in alternative device structures, such as dual-gate, FinFET and nanowire devices, the Hartree correction term (computationally expensive) can be neglected and the quantum correction for the Barrier potential/field term (one-dimensional and time-independent adding no additional computational cost) needs be included in the model only. Excellent agreement has been obtained with our Effective Potential Monte Carlo device simulations data and the results that utilize the NEGF formalism. [Preview Abstract] |
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J1.00162: Optical Trapping and Torquing of C60 Nanorods Douglas Bonessi, Keith Bonin, Thad Walker We have optically trapped and studied C60 polymer nanorods with diameters of 300-500 nm and lengths of 1-3 microns.~ The primary goal is to measure opto-electronic properties of the C60 polymer rods.~ This is accomplished by observing the rotational motion of the rods in a water sample when a light torque is applied. We also hope to report on opto-electronic properties of even smaller rod-shaped objects (of diameters less than 100 nm).~ [Preview Abstract] |
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J1.00163: Heavy Ion Interactions with Nanotubes and Nanowires B.W. Jacobs, V.M. Ayres, R.M. Ronningen, A.F. Zeller, E.H. Carey, M.A. Crimp, M.P. Petkov, S.L. Rutledge, J.B. Benavides, H.C. Shaw, D. Liu, J.B. Halpern, M.Q. He, G.L. Harris We will present investigations of the response of single and multi wall carbon nanotubes, electrospun carbon nanofibers and gallium nitride nanowires to high-Z heavy ion radiation that simulates space radiation environments. The heavy ion radiation experiments were performed at the National Superconducting Cyclotron Laboratory at Michigan State University, whose available beams and beam energies well match the energy spectra of abundant charged particles in galactic cosmic rays. In these experiments, the nanotubes and nanowires were irradiated in two separate series of experiments using Krypton primary beams with mass numbers 86 and 78 respectively. Explorations of the fundamental mechanisms of radiation interactions and damage propagation in reduced dimensionality systems, and in nanoscale systems with high surface to bulk ratios and small overall volumes, will be presented. [Preview Abstract] |
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J1.00164: A Novel Vibrational Mode in Carbon Systems Morinobu Endo, Mauricio Terrones, Y.A. Kim, Takuya Hayashi, Hiroyuki Muramatsu, Riichiro Saito, Federico Villalpando-Paez, Shin Grace Chou, Mildred S. Dresselhaus We report on the identification of a novel resonant Raman mode located at \textit{ca.} 1850 cm$^{-1}$, which is related to vibrations of linear carbon chains. This mode, termed ?the coalescence inducing mode? (CIM), was observed during the merging of highly purified double-walled carbon nanotubes (DWNTs) induced by thermal annealing and boron doping. In our case, the CIM mode arises from the generation of short 1D carbon chains established covalently between adjacent tubes. These chains trigger nanotube coalescence via a zipper model, and as the tubes merge the CIM vibration disappears. Theoretical calculations demonstrate that this vibrational frequency corresponds to linear chains with a few carbon atoms. The CIM mode could now be used to identify sp hybridized carbon in various systems such as irradiated graphite, polymerized C$_{60}$ molecules, functionalized fullerenes and nanotubes, carbynes, polymers, nanographites, etc. [Preview Abstract] |
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J1.00165: Growth and characterization of aligned ZnO nanorods T. Williams, K. Zhang, D. Hunter, K. Lord, A.K. Pradhan We report the growth of aligned ZnO nanostructures on pulsed-laser deposited ZnO epitaxial film-substrates synthesized by vapor transport of the evaporation of pure Zn metal powders followed by subsequent condensation. Remarkable arrangements of uniform ZnO nanorods were found on ZnO film by varying the synthesis conditions. The nanorods grow preferentially from a hexagonal ZnO base. High-density aligned nanorods of 60-100 nm diameter and 10-20 $\mu $m in length grew at the center of the substrate over a large area. Structural and spectroscopic properties clearly indicate that the nanorods are of superior and defect-free in quality. The ZnO nanorods have also been grown on Si and sapphire substrates. Details of the results will be presented. [Preview Abstract] |
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J1.00166: Intensity dependent Raman spectra of pristine and functionalized carbon nanotubes. Charles Blackledge, Deokjin Yu, Maxim Tchoul, Xiaoming Jiang, Warren Ford, James Wicksted Raman spectra of polymer functionalized and pristine single wall carbon nanotubes (SWNTs) are presented. The intensity ratio of the disorder (D band) to the tangential (G band) increases after polymer functionalization. Generally Raman spectra of ensembles of SWNTs indicate sample inhomogeneity such as the presence of many different nanotube symmetries and diameters. Laser induced changes in SWNT Raman spectra were also investigated. We observe temporally the laser induced loss of the intensities of the RBMs from small SWNTs and the metallic G band. This is attributed to selective oxidation of smaller carbon nanotubes that primarily consist of metallic SWNTs in our samples. Under identical conditions, the spectral changes of pristine samples are greater than those of polymer functionalized SWNTs. These results confirm that pristine small diameter SWNTs are more reactive than large ones. [Preview Abstract] |
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J1.00167: Percolation Model of Magnetoelectric Effects in Ferrite/Piezoelectric Bulk Composite V.M. Petrov, M.I. Bichurin, D.S. Tuskov, G. Srinivasan A bulk composite consisting of ferrite-ferroelectric phases shows magnetoelectric (ME) effects that are product properties of the magnetostrictive deformation and piezoelectric charge generation$. $Bulk composites with strong ME effects are desirable over layered samples due to superior mechanical strength. Here we discuss a percolation approach for modeling the ME effect in bulk composite [1]. We calculated percolation threshold corresponding to structural phase transition of bulk composites from the state with ME interactions to a state with no ME interactions.Expressions for longitudinal and transverse low-frequency ME voltage coefficients have been obtained. Volume fractions corresponding to peak ME voltage coefficients are given. We obtained a piezoelectric volume fraction of 0.45 for maximum ME effect in bulk samples of lead zirconate titanate and nickel-cobalt ferrite. These estimates are in excellent agreement with data. supported by a grant from the NSF. \newline \newline [1] V. M. Petrov, M. I. Bichurin, V. M. Laletin, N. Paddubnaya, and G. Srinivasan, \textit{Magnetoelectric Interaction Phenomena in Crystals-NATO Science Series II. Vol. 164}, Eds. M. Fiebig, V. V. Eremenko, and I. E. Chupis (Kluwer Academic Publishers, London, 2004), p.65-70. [Preview Abstract] |
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J1.00168: Analysis of the Behaviour of Nanocomposites by Multi-scale Simulation Woong Lee, Woo-Sung Jung, Jae-Min Myoung Hierarchical multi-scale simulation scheme was introduced for the analysis of the mechanical behaviour of nanocomposites. Aluminium matrix composites reinforced with uniformly dispersed carbon nanotube (CNT) was chosen as a model system. Ab initio simulation was first carried out to estimate inter-atomic potential between a CNT and an Al atom and to determine type of bonding between CNT and Al. This fundamental information was then passed onto the molecular dynamic (MD) simulation in which the interface properties between the matrix and the reinforcement was characterised in terms of load-displacement behaviour by simulating a CNT pullout test. Continuum analysis wad carried out with this interface properties to finally extract representative properties of CNT-reinforce Al matrix nanocomposites using representative volume element approach. Fracture behaviour was also estimated by analysing possible crack paths within the nanocomposites. [Preview Abstract] |
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J1.00169: Microscopic Mechanism of Reinforcement and Conductivity in Polystyrene/Carbon Nanotube Composites T.-E. Chang, S. Rhodes, A. Kisliuk, W. J. Brittain, A. P. Sokolov Carbon nanotube (CNT) is one of the most studied materials for the polymer reinforcement due to their extraordinary mechanical properties, superior thermal and electronic properties, and high aspect ratio. However, to unlock the potential of CNTs for applications, CNTs must be well dispersed in polymer matrix and the microscopic mechanism of polymer reinforcement by CNTs must be understood. The main goal of the presented research is to analyze structure and conductivity and enhance mechanical properties of polystyrene/carbon nanotube composites. Polystyrene matrix composites reinforced with single-wall carbon nanotubes (SWNTs) were prepared with different nanotubes concentrations. Microscopic structure and conductivity of composites is analyzed using Raman scattering and dielectric measurement. Studies on the tangential mode in the Raman spectra indicated well-dispersed SWNTs in polymer matrix. We show that conductivity appears in composites already at very low concentrations, hinting at the formation of a `percolative' network even below 0.5{\%} of the SWNT. [Preview Abstract] |
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J1.00170: Adsorption of Supercritical Carbon Dioxide in Aerogels as Studied by SANS and Neutron Transmission Techniques George Wignall, Yuri Melnichenko, David Cole, Henrich Frielinghaus Small-angle neutron scattering (SANS) has been used to study the adsorption behavior of supercritical carbon dioxide (CO2) in porous Vycor glass and silica aerogels. Measurements were performed at T = 35C and 80C as a function of pressure up to P = 25 MPa. The neutron transmission data were used to monitor the ``excess'' adsorption of CO2 at different pressures. The adsorption of CO2 is significantly higher in aerogels than in activated carbons and silica gels. SANS data have revealed the existence of highly compressed adsorbed phase with the density close to the density corresponding to van der Waals volume of carbon dioxide. The results demonstrate the utility of SANS combined with transmission measurements to study the adsorption of supercritical fluids in porous materials. [Preview Abstract] |
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J1.00171: First-principles phonon calculation for Al-(Re,Mn)-Si 1/1-1/1-1/1 approximants N. Nagasako, R. Asahi, T. Takeuchi A very low thermal conductivity ($\sim $1 W/Km) and a relatively large Seebeck coefficient in Al-based Mackay-type icosahedral quasicrystal have attracted much interest aiming for thermoelectric applications. We performed first-principles phonon calculations for the Al-(Re,Mn)-Si 1/1-1/1-1/1 approximants [1] to elucidate mechanism of their low thermal conductivity. The total energy and atomic force were calculated for a modeled Al$_{17}$(Re,Mn)$_{4}$Si$_{2}$ unit cell containing 138 atoms using the projector augmented wave method implemented in VASP code [2], and were then utilized for obtaining a phonon dispersion by the direct method [3]. The calculated lattice specific heat showed very good agreement with experiment, confirming validity of the present calculations. We found localized optical modes with a relatively low frequency ($\sim $2 THz) in the phonon dispersion. We discuss the low thermal conductivity in comparison between the Al-Re-Si and Al-Mn-Si systems via averaged group velocity and umklapp phonon scattering related to these optical modes. [1] Takeuchi et al., Phys. Rev. B70, 144202 (2004). [2] G. Kresse, J. Furthmuller, Phys. Rev. B54, 11169 (1996). [3] MedeA-Phonon, Materials Design, Inc. (2003) based on K. Parlinski, Phonon 3.11 (2002). [Preview Abstract] |
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J1.00172: Interaction of Surface Modified Carbon Nanotubes with Supercritical Carbon Dioxide Nihat Baysal, Banu Unsal, Rahmi Ozisik The properties of carbon nanotube (CNT)-polymer nanocomposites are far below than those calculated, mainly due to poor dispersion or interface quality. This is particularly difficult for single walled carbon nanotubes (SWNTs) as they tend to form bundles or ropes that are difficult to exfoliate. Supercritical fluid (SCF) assisted processing is one of the methods that can be used to exfoliate/disperse CNTs along with modifiying the interface of the CNTs. Molecular dynamics simulations were performed to understand how the surface modifiers behave near SWNT surface with and without the presence of SCF molecules. It is also important to understand the diffusivity of SCF molecules between SWNT bundles and the effect of surface modifiers on diffusion. Octane and \textit{n}-perflourooctane molecules were used as surface modifiers with varying tethering density and carbon dioxide (CO$_{2}$) was chosen as the SCF. Results showed that the system with highest number of \textit{n}-perfluorooctanes presented the highest degree of success in separating the SWNTs in the presence of CO$_{2}$. [Preview Abstract] |
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J1.00173: The influence of electric field on optical properties of the few-layer graphene with AB-stacking Chilang Lu, Y.C. Huang, M.F. Lin, C.P. Chang, R.B. Chen The effect of perpendicular electric field {\bf E} on optical properties of the AB-stacked few-layer graphene, made up of two, three, or four graphite sheets, is explored through the gradient approximation. In contrast to the featureless optical spectra of graphene, the low-energy absorption spectra of few-layer graphene with AB-stacking exhibit many jumping structures, which result from the band edge states caused by the stacking effect, in the absence of electric field. Remarkably, {\bf E} causes the subband (anti)crossing, changes the subband spacing, produces the scillating bands, and increases the edge states. It, therefore, follows that the field-modulating-spectra with sharp peaks are generated. Moreover, the frequency of peak, which is strongly dependent on the layer number and the field strength, is predicated. Above all, the predicted absorption spectra and the associated electronic properties could be verified by the optical measurements. [Preview Abstract] |
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J1.00174: Cluster Formation and Corralling Effect in Two Dimensional Binary Mixtures Dongsheng Zhang, Marcelo Carignano, Igal Szleifer We study the formation of ordered clusters of nanoparticles in binary mixtures in two dimensions using Monte Carlo simulations. The necessary conditions for the formation of highly ordered clusters are: the size mismatch between the two components and a long-range soft repulsions between the large particles. The degree of order of the clusters depends on the concentration of large particles. We show the conditions upon which compression of the layer induces squeezing out of the large particles from the interface. Our findings explain recent experimental observations on poloxamer-lipid mixtures and provide guidelines for how to form ordered clusters of nanoparticles in two dimensions. [Preview Abstract] |
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J1.00175: A molecular-dynamics study of defects and failure mechanisms in strained heteroepitaxial interfaces Jarrod E. Schiffbauer, Martina E. Bachlechner The formation and growth of pits and pit-like structures through a delamination-driven mechanism have been observed in the analysis of comprehensive molecular dynamics simulations of a laterally strained Si(111)/Si$_{3}$N$_{4}$(0001) interfacial system. These phenomena contribute to the ultimate mechanical failure of the simulated samples. Several factors play a key role the nucleation and growth of the pits including temperature and strain rate. Here we present an overview of the dependence of pit nucleation and morphology on both global and local conditions for a representative range of temperatures and applied strain rates. [Preview Abstract] |
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J1.00176: The hierarchy of 1D-, 2D- and 3D-dimensional LPS in Cu-Pd and Ag-Pd: A first-principles study S. B\"arthlein, E. Winning, S. M\"uller, G.L.W. Hart, A. Zunger Throughout many decades the marvelous variety of so-called long-periodic superlattices (LPS) - being constructed from stacks of $L1_2$ with a certain period $M$ after which an anti-phase boundary is introduced - has been a matter of interest for binary compounds consisting of Pd and noble metals such like Cu, Ag and Au. Whereas $Au_{0.75}Pd_{0.75}$ puts forth the $D0_{23}$ (i.e. $M=2$) structure as ground state [1], $Cu_{0.75}Pd_{0.75}$ and $Ag_{0.75}Pd_{0.75}$ exhibit LPS3 (i.e. $M=3$) as $T=0$K ground state. Moreover, the formation enthalpies for a whole class of superlattices are almost degenerated. In order to control the energetical hierarchies of {\em all} LPS in question the usual 1D-LPS configuration space must be overcome. We therefore demonstrate how for each system a set of suitable interactions can be constructed by first principles means. Questions concerning a complete description and exhaustive predictive power will be asked and answered by a mixed-space cluster expansion that allows us to handle 1D-LPS with average $M$, 2D- and 3D-LPS with arbitrary integer $M$ and to predict their energies in order to investigate their abilities to serve as ground state candidates. [1] S. Barabash et al., submitted to Phys. Rev. B [Preview Abstract] |
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J1.00177: Statistical studies of the propagation of a signal through a disordered system Joshua Dachman-Soled, Gabriel Cwilich We have considered the propagation of a signal through a certain layered nanostructure by modeling it as a collection of random one- dimensional interfaces, through which a coherent wave can be transmitted or reflected while being scattered at each interface. We have previously used a simple model where a persistent random walk ( the ``t-r'' model in 1- D) is used as a representation of the propagation of the signal in such a medium $^{(1)}$ We will present here some extensions of those results to analyze the total transmission through a chain when a random distribution of the transmittance of the scatterers is included. Some of the statistical properties of the system will be discussed, and some results of possible extensions to more than one dimension in the context of this model will be presented. (1)\textit{ Nanotechnology}, v13, 274 , (2002). [Preview Abstract] |
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J1.00178: Density Functional Study of a Typical Thiol on Perfect Au(111) Surfaces Guan Wang, William Sandberg, Steven Kenny We investigated systematically the static, dynamical and mechanical properties of a typical thiol-Au surface system at the density functional level. The computational approach uses optimized atomic-like orbits for efficiency and parallel computation. Ab initio results show the equilibrium configuration of the thiol on gold surfaces is in agreement with other computational methods. We found, from ab initio molecular dynamics simulations, that this tethering system keeps stable at 300, 350, and 370 degrees K. We studied the energy profiles that reflect the behaviors of the tethered thiol under normal and parallel stretches. The obtained rupture force is comparable with AFM measurements. We found the rupture force of thiols on gold surfaces is directional. We also show the process by which multiple successive ruptures of the tether occur with the surface. These results should be useful in understanding failure dynamics of tailored biosurfaces in microfluidic devices where fluidic sheer and normal forces are of concern. [Preview Abstract] |
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J1.00179: Magnetic, magneto-optical and dielectric properties of ferromagnetic semiconductors CdCr$_{2}$S(Se)$_{4}$ from first-principles calculations Guang-Yu Guo, Hai-Qing Lin Recently, CdCr$_{2}$S$_{4}$ has been found to exhibit fascinating relaxor ferroelectricity and colossal magnetocapacitive effect.To understand this and other interesting properties, we have calculated the electronic structure, exchange coupling, optical dielectric constant and magneto-optical Kerr rotation of CdCr$_{2}$S$_{4}$ and CdCr$_{2}$Se$_{4}$. The calculations are based on density functional theory with generalized gradient approximation plus on-site Coulomb correlation (GGA+U). The all-electron full potential linearized augmented plane wave (FLAPW), frozen core full potential projector augmented wave (PAW), and also all-electron tight-binding linear muffin-tin orbital methods (TB-LMTO) are used. In this contribution, we will present the obtained exchange coupling parameters, magneto-optical Kerr rotation spectra and dielectric constants. In particular, we will discuss the possible origins of the obtained ferroelectricity in these materials. [Preview Abstract] |
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J1.00180: Ferromagnetic properties of epitaxial manganite films on SrTiO$_{3}$/Si heterostructures D. Hunter, B. Lasley, K. Lord, T.M. Williams, R.R. Rakhimov, A.K. Pradhan We report on the growth and magnetic properties of epitaxial LSMO films on Si (100) and Si (111) substrates using SrTiO$_{3}$ template layer, which demonstrate magnetic and electrical properties at and above room temperature. The growth conditions were optimized by varying substrate temperature and oxygen partial pressure. The grain of 20 nm in size coalesced with increasing substrate temperature, revealing very smooth film surface. The magnetization data show magnetic transition and magnetic hysteresis at and above room room-temperature. The ferromagnetic resonance studies show anisotropic effects related to ferromagnetic properties of films. The details of the structural and magnetic properties will be presented. [Preview Abstract] |
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J1.00181: Polarizability of carbon nanotube peapods from first-principles Kiseok Chang, Young-Gui Yoon We calculate polarizability of carbon nanotube peapod from first-principles. The calculated result gives information about electronic screening effects of carbon nanotubes. Recently, the structure has been suggested as bucky shuttle memory devices and three-terminal switching devices. The polarizability and the screening effects may play an important role for the control of that type of devices. We discuss our results in the context of nanoelectronics application. [Preview Abstract] |
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J1.00182: Structural and electronic properties of a bucky-ferrocene molecule from first-principles Jinwoo Jeong, Young-Gui Yoon Bucky-ferrocene molecules have been synthesized from fullerene and ferrocene molecules. The molecules have features of both components. We performed first-principles electronic structure calculations of a bucky-ferrocene based on C$_{60}$ fullerene and ferrocene molecules in a supercell geometry. We got local minimum structures with geometry optimization from different initial configurations. Orientation of methyl groups and pentacene leads to states in small energy range. Possible device applications of the molecule are discussed. [Preview Abstract] |
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J1.00183: Supramolecular Lysophospholipid-Carbon Nanotube Complexes Enable Cellular Studies Apparao Rao, Yonnie Wu, Jessica Moore, Pu-Chun Ke Single walled carbon nanotubes (SWNTs) have found tremendous applications in electronics, nanophotonics, chemical and biosensing, and very recently, in nanomedicine. The bottleneck for many of these applications is the inherent insolubility of SWNTs due to their mutual interactions. Here we report that lysophospholipids, or single-chained phospholipids offer unprecedented solubility for SWNTs. The biocompatibility of lysophospholipids is unsurpassed since they occur naturally in the cell membrane. Using transmission electron microscopy we show lysophospholipids wrap SWNTs as striations whose size and regularity are affected by the polarity of the lysophospholipids. These findings shed light on the debate over the binding mechanism of amphiphilic polymers and cylindrical nanostructures and has implications on the design of novel supramolecular complexes and nanodevices. Since the head groups of lysophospholipids can be functionalized with tags such as quantum dots, antioxidants, and monoclonal antibodies, our method opens the door for utilizing nanomaterials for \textit{in vivo }imaging, gene and drug therapy, and novel nanomedicine. [Preview Abstract] |
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J1.00184: Time-resolved photoluminescence of single wall carbon nanotubes Christophe Voisin, S\'{e}bastien Berger, Guillaume Cassabois, Claude Delalande, Philippe Roussignol, Xavier Marie Time-resolved photoluminescence measurements are performed on ensembles of isolated (6,5) single wall carbon nanotubes after a selective picosecond excitation of the second transition of the semiconducting nanotubes. These measurements give insight into the carrier dynamics in the bright nanotubes. The carrier dynamics in bright nanotubes is non exponential reflecting the inhomogeneous distribution of life times. However two contributions to the relaxation are distinguishable within the experimental resolution : a fast component of the order of 10~ps and a smaller long living component of about 100 ps. When cooling the sample down to 10 K, the relative weight of the slow component increases with a characteristic temperature of 150 K. Only a small increase of about 30\% of the quantum yield is observed at low temperature. No clear relation between emission energy and life time is observed. In light of recent publications these results suggest that the monoexponential relaxation process in one single nanotube might be either fast or slow and that the proportion of both kind of processes is modified by the temperature with a typical energy of 15~meV. [Preview Abstract] |
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J1.00185: Effects of the Modification of Single- and Double-Wall Carbon Nanotubes: A Theoretical Study of Field Emission Properties Brahim Akdim, Tapas Kar, Xiaofeng Duan, Ruth Pachter In this work, we report first principles calculations of modified single-wall carbon nanotubes (SWCNTs) by O$_{3}$ and --COOH functionalization, as well as of double-wall carbon nanotubes (DWCNTs) by Cs surface adsorption. Properties related to field emission, including the effects of an applied electric field to mimic the emission environment, as well as structural parameters, are discussed. The results, which suggest a suppression of the emission upon ozone adsorption and carboxy-functionalization in SWCNTs, but an enhancement with Cs surface adsorption, are explained in detail. Finally, the effects of treating DWCNTs with Cs adsorption, as compared to our previous results for SWCNTs, are summarized. [Preview Abstract] |
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J1.00186: Dissociation of Water on Ti-Decorated C nanostructures Yang Lei, Wenguang Zhu, Sheng Meng, Zhenyu Zhang, Zhengxiao Guo Direct thermal splitting of water produces pure hydrogen but requires exceptionally high temperatures, making practical implementation challenging. Here we use first-principles simulations to show that water can dissociate on Ti decorated carbon nanostructures, such as graphenes and nanotubes. The behavior of water molecules approaching a Ti adatom is investigated. A single water molecule dissociates freely on a Ti-decorated graphene layer. A small energy barrier has to be overcome to dissociate a second water molecule, with the released H atoms readily combining to form H$_{2}$. As the number of water molecules further increases, the Ti adatom will dissociate from the graphene layer. We find similar results for when a Ti atom is adsorbed on a carbon nanotube. When more Ti atoms are adsorbed on the outside of the carbon nanotube, the energy barrier against dissociating water molecules is greatly decreased. We also explore the effects of replacing Ti by other potential catalytic elements. [Preview Abstract] |
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J1.00187: Field Emission of Electrons From Transparent Carbon Nanotube Sheets. Alexander Kuznetsov, Alexander Zakhidov, Mei Zhang, Sergey Lee, Ray Baughman, Anvar Zakhidov The well-ordered aligned arrays of multiwalled carbon nanotubes were used to make strong and transparent carbon nanotube sheets, prepared by dry spinning from oriented MWCNT forests [1]. Study of electron field emission (FE) shows phenomenally low threshold fields $<$ 0.5 V/mm with very high current densities and steep I-V curves. Also some~unusual features observed in IV curves including step-like current increase and hysteresis behavior. Some possible explanations are proposed. Such CNT transparent sheets can be used as high current, high stability field emission sources which are at the same time transparent and can be used as an optical polarizer. The prototype of high luminosity fluorescent lamp with~CNT sheets as a cathode was created. \newline \newline [1] M.Zhang, S.Fang, A.Zakhidov, S.B.Lee, A.Aliev et.al., Science, 309,(2005) 1215 [Preview Abstract] |
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J1.00188: Microwave plasma enhanced chemical vapor deposition growth of small-diameter CNTs using the polymer based catalysts Peng Wang, Jennifer Lu, Otto Zhou Microwave plasma enhanced chemical vapor deposition (MPECVD) has emerged to be a promising candidate method to realize the growth of carbon nanotubes (CNTs) at a temperature of several hundred degrees Celsius. Among various catalysts, metal-containing polymers have the advantage of easy particle size and density controlling. Here we report to produce carbon nanotubes by using metal-containing polymers at 604 $^{o}$C. There are two to six wall layers. The diameter is consistently in the range of 4 to 7 nm. The control of the density and the length of the as grown nanotubes are discussed. The field emission property is characterized. [Preview Abstract] |
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J1.00189: H\"uckel theory of large-scale planar organic structures William Schwalm, Mizuho Schwalm H\"uckel theory applies, if at all, to the $\pi$ orbitals of planar organic molecules, treating only one $p_z$-like basis function for each atomic site. The only advantage of this is that one has closed formulas for properties of rather large, complex structures. We develop $\pi$-orbital Green function calculations for $\vec{k}_{\|}$ resolved density of states of ordered layers of organic adsorbates, such as might pertain to angle-resolved photoemission. Simple-minded though it may be, the theory leads to closed formulas for a wide variety of molecular architectures. [Preview Abstract] |
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J1.00190: Quantum transport through a carbon nanotube under a time-dependent electric field. Monica Pacheco, Pedro Orellana We investigate quantum transport through single-wall carbon nanotubes (CNT) connected to leads in the presence of an externally imposed time-dependent transverse electric field. A time-oscillating potential uniform in space induces the apparition of side-bands in the spectrum and therefore the tunneling current is drastically modified. To obtain the conductance and the density of states of the CNT, we adopt the equation of motion approach and we calculate the retarded and advanced Green's function defined in terms of the creation and annihilation operator of the electron in the CNT. We analyze the conductance spectra as a function of the frequency and amplitude of the external time-varying potential. We found that above a critical value of the electric field intensity, an enhancement of the conductance, or suppressed resistance, as a function of the electric field strength occurs. This effect can be explained in terms of photon-assisted transport. The conductance increases displaying oscillations which amplitude shows a strong dependence on the frequency of the electric field. For low frequencies, in the microwave range, the oscillations evolve to a structure of well defined steps. The steps in the normalized resistance as a function of the field strength appear for energies corresponding to the Van Hove singularities of the LDOS of the CNT. [Preview Abstract] |
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J1.00191: Carbon Nanotubes as Counter Electrodes for Gratzel Solar Cells Hasan Shodive, Ali Aliev, Mei Zhang, Sergey Lee, Ray Baughman, Anvar Zakhidov The role of interfaces is very critical for solar cell devices which use nanostructured materials. Dye Sensitized Solar Cells (DSSC) are devices which parts are interfacial in character and physico --chemical processes occur at the interface of two distinct media. DSSC are of great interest due to combination of their high efficiency and relatively low cost. An effective counterelectrode with high electrochemical activity is an important component of DSSC to enhance its practical utility. Presently used Pt coated ITO counterelectrode can not be applied in flexible DSSC architectures, while there is a growing need for flexible anodes which are transparent and have desired interface characteristics. In this work in order to search for such materials for counter electrode in dye sensitized solar cells, newly developed strong and transparent and modified carbon nanotube sheets [1] are used in interfacial counter electrode. To increase the electrochemical activity of the anode the CNT sheets are coated with highly conductive SWCNT and compared with pure multiwall CNT sheets. We show that the transparent sheets of SWCNT/MWCNT perform as a flexible anode and as electrochemical catalyst and also can be used in tandems of dye sensitized solar cells as transparent charge recombination or interconnect layers. [1] M. Zhang, S.Fang, A.Zakhidov, S.B.Lee, A.Aliev et.al., Science, 309,(2005) 1215 [Preview Abstract] |
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J1.00192: Is a DNA-Wrapped Metallic Nanotube Still Metallic? S.E. Snyder, S.V. Rotkin Modeling suggests that essential changes in electronic band structure can result from the helical wrapping of single-stranded DNA around a single-walled carbon nanotube (SWNT). The metal-to-semiconductor transition of a metallic nanotube is one of many possible changes. As SWNT/DNA hybrids become more popular, it is of great practical importance to determine how the charged backbone of single-stranded DNA may affect SWNT properties. As a first approximation, the change in band structure is modeled by considering the Coulomb interaction with a charged helical wrap. A range of models is developed to address a variety of wrappings of both metallic and semiconducting tubes. Parameters of the model include width, charge profile, and axial and angular displacement of the helix, periodicity along the helix, and, indirectly, chiral angle of the wrap. Band gap changes are shown to be fairly robust for axial and angular shifts of the helix. Trends of the band gap and strength of the potential perturbation with tube diameter and wrapping angle are explored. DNA is just one of several polymers which are presumed to helically wrap SWNTs and thereby disperse SWNTs in solution. Therefore, our study is relevant for electronics applications which require individual nanotubes dispersed via polymer wrapping. [Preview Abstract] |
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J1.00193: Parameter identification procedure for the Density-Gradient model Petru Andrei The existing methods for the computation of parameters of the Density-Gradient model [M. G. Ancona et al, \textit{Phys. Rev. B} \textbf{39}, 9536 (1989)] are based on the comparison of the model with one-dimensional Poisson-Schr\"{o}dinger computations. While this approach gives good results for devices in which the carriers are confined in only one direction, it is not appropriate for devices in which the carriers are confined in more than one direction. Such devices include short-channel MOSFET and SOI devices, FinFETs, etc. In this presentation we propose a new identification technique based on the effective-mass Schr\"{o}dinger equation. Our identification technique can be easily applied to 2- and 3-dimensional semiconductor systems. The model parameters are found by using the electron and hole concentrations computed in the framework of the effective-mass Schr\"{o}dinger equation. Special attention is paid to the computation of the electron effective mass entering in the Density-Gradient equations since these equations are widely used in n-channel nanoscale transistor simulations. [Preview Abstract] |
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J1.00194: The Lowest Energy Structures of Si-24 Cluster J. Rufinus We present the results of computational calculations of the lowest energy structures of Silicon 24 cluster. The calculations were done in the framework of DFT with PBE exchange functional. The initial structures were calculated using a Genetic Algorithm. Approximately 1000 structures were recalculated using Gaussian 03 (with the basis set 6-31G) to determine the lowest energy structures. [Preview Abstract] |
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J1.00195: Density Functional Study of Strain Effects on the Energy Gap in Silicon Nanoclusters Xihong Peng, Azar Alizadeh, Nitin Bhate, Surya Ganti, Pradeep Sharma, Saroj Nayak, Sanat Kumar It is well known that nanoscale Si clusters are photo luminescent. In parallel, industry routinely exploits the dramatic strain dependence of the band gap of bulk Si for electronics applications. We have discovered, using density functional calculations that the combined role of finite size and strain leads to previously unanticipated effects on the energy gap in Si clusters. Both hydrostatic and non-hydrostatic strains were applied to the clusters with sizes up to 2nm. For the hydrostatic strain, which maintains the tetrahedral bonding network in Si, the effects of strain on the energy gap exhibits size dependence. The behavior can be classified in three distinct regimes that will be discussed in detail. For non-hydrostatic strains, which result in distortion of the tetrahedral bonds, the coupling between the energy gap and strain is largely independent of the cluster size. The mechanisms responsible for the coupling between the gap and strain are proposed. The relative shifts of the energy gap for hydrostatic and non-hydrostatic strains are compared. [Preview Abstract] |
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J1.00196: FLUIDS AND SOFT MATTER POSTER SESSION |
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J1.00197: Anomaly in the heat capacity of triethylamine and water Ruth Shewmon, Ryan Hartschuh, D.T. Jacobs The heat capacity C$_{p}$ of the liquid-liquid mixture triethylamine-water has been precisely measured using our own computer-based data acquisition and control, adiabatic calorimeter. A step process of adding heat and then waiting several minutes for a stable temperature assures equilibrium values for the heat capacity from the known heat added and the resulting change in temperature. For a sample with a much larger concentration of triethylamine than the critical concentration, we observe strikingly different behavior in the heat capacity than that observed near-critical. The unusual behavior is reproducible and may be due to a structure forming in the sample. We acknowledge support from NSF-REU grant DMR 0243811. [Preview Abstract] |
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J1.00198: Extrinsic homogeneity of non-cubic periodic minimal surface families Gerd E. Schroeder, Andrew Fogden, Stephen T. Hyde Infinite periodic minimal surfaces (IPMS) form the structural basis of cubic phases in copolymer, lipid or surfactant self-assemblies. Although few non-cubic equilibrium phases have been reported to date, there are compelling reasons to study non-cubic IPMS: first, they offer possible transition structures between bicontinuous cubic phases. Second, the reason why soft systems (that favour homogeneity but not explicitly symmetry) exhibit phases of cubic symmetry is still uncertain. We use the concept of {\em medial surfaces} to quantify {\em packing homogeneity}, i.e.~variations of the channel radius within the structure. This analysis is carried out on IPMS families that are degradations of the cubic P, D and G surfaces (the rPD, rG, tG, tD tP, and H). We show that the cubic G and D are locally maximally homogeneous, whereas the cubic P has at least one relative that is more homogeneous. We discuss the implications for molecular self-assembly (chain stretching), and compare this result to analyses of curvature fluctuations.\\ G.E.~Sch\"oder, S.J.~Ramsden, A.~Fogden, S.T.~Hyde, Physica A {\bf 339}, 137--144 (2004) and G.E.~Schr\"oder, S.J.~Ramsden, A.G.~Christy, S.T.~Hyde, Eur.~Phys.~J.~B {\bf 35}, 551--564 (2003) [Preview Abstract] |
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J1.00199: Synchrotron studies of nanocrystal thin film self-assembly Diego Pontoni, Kyle Alvine, Antonio Checco, Oleg Gang, Ben Ocko, Peter Pershan, Francesco Stellacci This work is aimed at exploring the mechanisms of formation and dissolution of nanocrystal 2D and 3D assemblies in the presence of nano-thin wetting films. In particular monolayers of bimodal/polydisperse thiol-stabilized gold nanocrystals are studied. In-situ synchrotron X-ray grazing incidence diffraction reveals that upon adsorption of a good solvent for the nanocrystals, the monolayer first expands and then undergoes a transition to a bilayered/disordered state. A detailed physical model for the corresponding X-ray reflectivity data suggests that the transition starts when the substrate is completely covered by the monolayer as a result of its expansion into 2D voids. Further liquid adsorption forces the expulsion of nanocrystals from the monolayer by vertical displacement. The first nanocrystals to be expelled are the large ones. The whole process is reversible upon slow decrease of the adsorbed liquid. [Preview Abstract] |
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J1.00200: Self-Assembly of Porphyrin J-Aggregates Valentinas Snitka, Mindaugas Rackaitis, Gintare Navickaite The porphyrin nanotubes were built by ionic self-assembly of two oppositely charged porphyrins in aqueous solution. The porphyrins in the acid aqueous solution self-assemble into J-aggregates, wheels or other structures. The electrostatic forces between these porphyrin blocks contribute to the formation of porphyrin aggregates in the form of nanotubes, enhance the structural stability of these nanostructures. The nanotubes were composed mixing aqueous solutions of the two porphyrins - anionic Meso-tetra(4- sulfonatophrnyl)porhine dihydrochloride (TPPS$_{4})$ and cationic Meso-tetra(4-pyridyl)porphine (T$_{4}$MPyP). The porphyrin nanotubes obtained are hollow structures with the length of 300 nm and diameter 50 nm. Photocatalytic porphyrins are used to reduce metal complexes from aqueous solution and to control the deposition of Au from AuHCl$_{4}$ and Au nanoparticles colloid solutions onto porphyrin nanotubes. Porphyrin nanotubes are shown to reduce metal complexes and deposit the metal selectively onto the inner or outer surface of the tubes, leading to nanotube-metal composite structures. [Preview Abstract] |
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J1.00201: Longitudinal complex dielectric polarizability and relaxation times of polar molecules in a biaxial liquid crystal phase Yuri Kalmykov, William Coffey, Bachir Ouari, Sergey Titov The longitudinal relaxation time and the complex dielectric polarizability of rod-like molecules with dipole moment parallel to the long axis in a biaxial nematic liquid crystal are calculated using as model the rotational Brownian motion in a mean field potential so reducing the problem to a set of linear differential-recurrence relations for the appropriate equilibrium orientational correlation functions. The solution of this set is obtained by matrix continued fractions. Simple analytic equations (based on the exponential separation of the time scales of the intrawell and overbarrier (interwell) relaxation processes), allowing one to understand the qualitative behavior of the system and accurately predicting the longitudinal complex polarizability for a wide range of the barrier height and anisotropy parameters, are proposed. [Preview Abstract] |
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J1.00202: Critical behavior at the isotropic to nematic phase transition in a bent-core liquid crystal David Wiant, Strahinja Stojadinovic, Krishna Neupane, Sunil Sharma, Katalin Fodor-Csorba, Antal Jakli, James Gleeson, Samuel Sprunt Magnetic birefringence and dynamic light scattering measurements of orientational order parameter fluctuations at the isotropic- nematic phase transition of a bent-core liquid crystal reveal a pretransitional temperature dependence consistent with the standard Landau-deGennes mean field theory. However, the transition in the bent-core compound is more weakly first-order ($T_{NI}-T^\ast \approx 0.4^\circ$C), the leading Landau coefficient is $\sim 10$ times lower, and the viscosity associated with nematic order fluctuations is $\sim 50$ times higher, than typically observed in calamitic (rod- shaped) liquid crystals. These anomalies can be explained by an unconventional optically isotropic phase composed of complexes of bent-core molecules. Also, we will present preliminary magnetic birefringence and density measurements at temperatures above the nematic-isotropic transistion which support the existence of an optically isotropic state. [Preview Abstract] |
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J1.00203: Dielectric Properties of Binary Mixtures of MBBA and 7CB John Gamble, Shila Garg The dielectric properties of mixtures of 4-methoxyybenzylidene-4-butylaniline (MBBA) and 4-n-heptyl-4'-cyanobiphenyl (7CB) were investigated at different molar concentrations. The isotropic-nematic phase boundary was determined for various mixtures in order to calculate the nematic range. For some concentrations, there was a second transition to a higher order smectic phase, most likely a Smectic G. Our previous work on MBBA and 5CB indicated that there was strong interaction between the two mesogens at certain concentrations. We find similar strong interactions for mixtures around the 1:1 molar concentration. Planar and homeotropic cells were constructed in order to measure the two different dielectric permittivities, $\varepsilon _{\vert \vert } $ and $\varepsilon _\bot $. The cells were maintained at a fixed reduced temperature $\tau $ during the measurements, since the nematic range and transition temperatures were different for each mixture. From the $\varepsilon _{\vert \vert } $ and $\varepsilon _\bot $ data, we calculated the dielectric anisotropy $\Delta \varepsilon =\varepsilon _\parallel -\varepsilon _\bot $ as a function of molar concentrations of the mixtures. [Preview Abstract] |
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J1.00204: Thermal Conductivity of Silica Aerogels from Computer Simulations Brian Good Aerogels are of current interest to the aerospace community; their light weight and low thermal conductivity make them suitable for use in lightweight thermal insulation. The gels typically exhibit a complex structure; the smallest observed feature is a 2-5nm ``primary'' particle of amorphous silica. The primary particles aggregate to form ``secondary'' particles an order of magnitude larger, and these, in turn, form pearl-necklace structures whose details depend on the density. The gels appear to exhibit fractal dimensionality, at least over a small range of length scales. In this work, we investigate the relationship between the structure of the gels, their dimensionality and density, and their thermal conductivity. We model the secondary-particle aggregate structure using a modified diffusion limited cluster aggregation (DLCA) scheme, which produces qualitatively different structures at low and high densities that are consistent with experimental observation. At lower densities, we find evidence for a transition from fractal behavior at small length scales to compact mass scaling at larger lengths. We model the thermal conductivity by assigning a size-dependent thermal conductance to each secondary particle, and an effective thermal conductivity to the gas assumed to be present in the pores between the particles. The conductivity of the gel-gas network is obtained using standard numerical techniques. The scaling of the thermal conductivity with density, fractal dimension, and the ratio of silica to gas conductivities is discussed. [Preview Abstract] |
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J1.00205: Dynamics of multiple glass states Nora Graneto, Andrzej Latka, Piotr Habdas We study concentrated colloidal suspensions, a model system which exhibits a glass transition. Using microscopy techniques we directly observe the microscopic behavior of the colloidal particles as the glass transition is approached. Moreover, by adding polymer to the colloidal suspension we induce ``sticking'' between the colloidal particles. When such attractive force is introduced to a hard-sphere colloidal glass, the system enters a liquid phase. Increasing the polymer concentration even further causes the system to enter another glass state -- an attractive glass. We investigate the particle dynamics as system enters multiple glass states. Particularly, we focus on single particle jumps and effect of caging on the particles' dynamics. [Preview Abstract] |
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J1.00206: Statistical Mechanics of Hydrodynamically Coupled Colloidal Spheres in Optical Vortices Yael Roichman, Dov Levine, David G. Grier Optical vortices are ring-like optical traps created by strongly focusing a helical mode of laser light. Colloidal particles trapped in an optical vortex are driven around its circumference by the vortex's orbital angular momentum flux. A set of such circulating spheres may be viewed as a system of perpetually sedimenting particles. Like sedimenting spheres, colloid in a vortex experience a uniform body force. They also interact with each other hydrodynamically. Unlike sedimenting spheres, those in an optical vortex remain in the field of view indefinitely and can be studied in great detail. In addition, the circulating colloid can be subjected to a nonuniform static potential energy landscape created by varying the intensity of light around an optical vortex's circumference. This elegant model system allows us to study the effect of hydrodynamic coupling on the mutual sedimentation of several particle with varying sizes. We find that sedimentation of a bidispered mixture of colloids forms stable couples of particles as opposed to the less stable structure of monodipersed particles sedimenting. We also demonstrate that fluctuations in the particles' trajectories can be used to gauge their system's departure from equilibrium. [Preview Abstract] |
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J1.00207: Measuring the transverse magnetization of rotating ferrofluids Manfred L\"{u}cke, J. Embs, S. May, C. Wagner, A. Kityk, A. Leschhorn We report on measurements of the transverse magnetization of a ferrofluid rotating as a rigid body in a constant magnetic field, ${\bf{H}}_0$, applied perpendicular to the axis of rotation. The rotation of the fluid leads to a non-equilibrium situation, where the ferrofluid magnetization, ${\bf{M}}$, and the magnetic field within the sample, ${\bf{H}}$, are no longer parallel to each other. The off-axis magnetization perpendicular to ${\bf{H}}_0$ is measured as a function of both the applied magnetic field, $H_0 $, and the angular frequency $\Omega$. The latter ranges from a few Hz to frequencies well above a characteristic inverse Brownian relaxation time. Our experimental results strongly indicate that the transverse magnetization is caused only by a small fraction of the collodial ferromagnetic particles. The effect of the polydispersity of the ferrofluid is discussed. Experimental results are compared to predictions based on several theoretical models. A single-time relaxation approach for the so-called effective field and a field dependent Debye relaxation of ${\bf{M}}$ yield reasonably good shapes of the curves of transverse magnetization versus $\Omega$. However, like the other models they overestimate their magnitudes. [Preview Abstract] |
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J1.00208: Effect of Associative Polymers on the Foaming Properties of Surfactant Solutions Alfredo Cervantes, Emmanuel Robles, Heriberto Acu\~{n}a, Rogelio Gamez, Amir Maldonado Aqueous foams are materials which have many industrial applications. Their stability is affected by three mechanisms: bubble coalescence (film rupture), coarsening (gas diffusion) and drainage (gravity-driven liquid flow). The aim of this work is to obtain some insight into the effect of associative polymers on the foamability, foam stability and drainage of surfactant solutions. The foams were produced by air bubbling and by the turbulent mixing method. The surfactant is SDS and the associative polymers studied are HEUR and POE-Stearate. We studied the effect of polymer concentration for each macromolecule. The results show that two opposite effects are present when the polymer concentration is increased: for low polymer concentrations, foamability and foam stability is higher than for high concentrations. Results are discussed in terms of the properties of the solution: surface tension, electrical conductivity, bulk viscosity, etc. [Preview Abstract] |
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J1.00209: Elasticity of the FCC Hard Sphere Crystal from two point microrheology Matthew Sullivan, Kun Zhao, Andrew Hollingsworth, P.M. Chaikin, William B. Russel We present a measurement of the full elastic tensor of a hard sphere colloidal crystal. Our system consists of concentrated suspensions of monodisperse PMMA-PHSA particles suspended in a density-matching solvent mixture of decalin and tetrachloroethylene. Single crystals are heterogeneously nucleated from a surface template to produce large face centered cubic (FCC) single crystals. The two-point displacement correlations are measured using confocal microscopy and video analysis. These show a $1/r$ decay with distance and an angular dependence that can be used to determine the three elastic constants of the FCC crystal. [Preview Abstract] |
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J1.00210: Computer simulations of SDS/dodecanol (hexadecanol) monolayers at water/air interface Hector Dominguez, Margarita Rivera Simulations of monolayer mixtures of amphiphlic molecules were investigated to study the effects of the polar head groups in the structure of the monolayer at the water/air interface. The binary mixture was composed of sodium dodecyl sulfate (SDS) and dodecanol (hexadecanol) molecules For the low dodecanol concentration molecules two monolayer mixtures were prepared. In the first monolayer all the dodecanol molecules were placed together in the center of the simulation box whereas in the second monolayer those molecules were uniformly distributed in the surface area in such a way that they were a part from each other. Simulations of both systems indicate that the dodecanol tails in the first monolayer are more straight and more ordered than those in the second monolayer. When simulations of SDS/hexadecanol are performed it is observed that the tilt angle and the order of the tails are different respect to the SDS/dodecanol mixtures. [Preview Abstract] |
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J1.00211: Role of membrane composition in the formation and stability of buds Kurt A. Smith, Anna C. Balazs Using dissipative particle dynamics we study the behavior of buds in amphiphilic bilayer membranes. These buds occur during phase separation when line tension between the two phases comes to dominate the bending rigidity of the membrane. We show that the inclusion of twin-tail amphiphiles, which segragate to the interface between the two phases, is a means to controlling the onset of budding. In addition they stabilize the neck when budding does occur, by acting essentially as stitches, thereby increasing the energy needed to detach the bud from the membrane. [Preview Abstract] |
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J1.00212: Competing interactions in two dimensional Coulomb systems: Surface charge heterogeneities in co-assembled cationic-anionic incompatible mixtures Sharon Loverde, Yuri Velichko, Monica Olvera de la Cruz A binary mixture of oppositely charged components confined to a plane such as cationic and anionic lipid bilayers may exhibit local segregation. The relative strength of the net short range interactions, which favors macroscopic segregation, and the long range electrostatic interactions, which favors mixing, determines the length scale of the finite size or microphase segregation. The free energy of the system can be examined analytically in two separate regimes, when considering small density fluctuations at high temperatures, and when considering the periodic ordering of the system at low temperatures (F. J. Solis, S. I. Stupp and M. Olvera de la Cruz \textit{J. Chem. Phys}. 122 (5), 054905 (2005)). A simple Molecular Dynamics simulation of oppositely charged monomers, interacting with a short range Lennard Jones potential and confined to a two dimensional plane, is examined at different strengths of short and long range interactions. The system exhibits well-defined domains that can be characterized by their periodic length-scale as well as the orientational ordering of their interfaces. By adding salt, the ordering of the domains disappears and the mixture macroscopically phase segregates in agreement with analytical predictions. [Preview Abstract] |
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J1.00213: Circular Couette cell for two-dimensional turbulence experiments in sheared flow: initial results. John V. Ulmen, Paul W. Fontana, Martin Kearney-Fischer An experiment to study turbulence in quasi-two-dimensional flows with a controlled mean flow shear has been built. Experiments are underway to investigate the suppression of turbulent transport by sheared flow as seen in geostrophic flows and laboratory fusion plasmas. The apparatus, a circular Couette cell, uses a liquid film of dilute soap solution suspended freely in an annular channel with a rotating outer boundary. The channel is $7~cm$ wide with an average radius of $46.5~cm$, and can be rotated at angular speeds exceeding $10~rad/s$. Mean flow profiles will be presented showing the effect of air resistance on the flow; damping lengths on the order of $1~mm$ are observed. Turbulence is driven independently via electromagnetic forcing. The rate of turbulence injection can be varied continuously, and its spatial scale corresponds to the spatial frequency of an array of NdFeB magnets. Diagnostics include particle imaging velocimetry, two-point laser Doppler velocimetry, and thickness measurements via reflection interferometry. Initial results and plans for upcoming measurements will be presented. [Preview Abstract] |
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J1.00214: Droplet pinch off of diluted polymer solutions Christian Wagner, Rainer Sattler, Jens Eggers Tiny amounts of polymers can alter the flow behaviour of simple liquids dramatically. An aesthetic and instructive example is the detachment process of a droplet of a polymer solution, e.g. a diluted DNA solution. It is characterized by the suppression of the pinch off finite time singularity and the formation of a cylindrical filament between the droplet and the nozzle. On later stages of the experiments beads on a string a formed. Their generation can be well explained with scaling arguments of the intrinsic time scales of the experiment. The dramatic increase of resistance against the flow is macroscopically described by the elongational viscosity. It is a crucial parameter in many different industrial processes where contraction flows are generic. A pure elongational flow stretches the macromolecules at maximum and we use different experimental techniques simultaneously to relate macroscopic flow profiles with microscopic polymer configurations. [Preview Abstract] |
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J1.00215: Motility in multicomponent fluids Robert Magerle, Stephan Weiss, Nicolaus Rehse During annealing in air thin films of polystyrene-block-polypropylene and similar block copolymers decompose and form a phase separating multi-component polymeric fluid where domains coarsen similar as in a binary mixture. At a certain threshold of domain size, two domains form a droplet which starts to move spontaneously across the surface. We show that a chemical reaction with oxygen is required for this motion for which we have determined the effective activation energy. We discus a model for droplet motion and speculate about the relevance of the underlying physics for the motility of biological cells and their ultra-cellular compartments. [Preview Abstract] |
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J1.00216: Characterization of diffusion in supersaturated solutions using T-sensors Kristen Bloschock, Russel Ross, Edward Van Keuren Measurement of the molecular diffusivity has provided much of the evidence for cluster formation in supersaturated solutions. The recent development of the T-sensor, a microfluidics laminar flow device, enables rapid determination of molecular diffusion coefficients. In this device, fluids in two channels meet and undergo laminar flow, with diffusion occurring perpendicular to the direction of flow in the channel. We have used fluorescence imaging of molecules in a T-sensor to study diffusion in supersaturated solutions of several dyes. When they self-assemble, these molecules exhibit characteristic changes in fluorescence emission. Therefore, using imaging we can determine both the state of aggregation and the diffusion coefficient. We will present results of experiments with one channel containing a molecular solution and the second a miscible non-solvent; the diffusional mixing induces nucleation and growth of nanocrystals. [Preview Abstract] |
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J1.00217: Self-organizing flow in a multi-component driven system - interacting lattice gas simulation Ras Pandey, Joe Gettrust We consider a mixture of mobile particles ($A,B$) described by their molecular weight, interaction, and miscibility gap on a discrete lattice of size $L_x \times L_y \times L_z$. The source of particles is at the bottom ($z=1$) of the lattice with open top ($z=L_z$). The Metropolis algorithm is used for stochastic moves of particles with a hydrostatic pressure bias (H). Periodic boundary conditions are used along the transverse directions with open longitudinal ends. Particles continue to enter the lattice from the source and are driven by bias, concentration gradient, and thermal energy against gravity. Net flow of particles occurs and a steady-state is reached with a self-organized morphology. Self-organizing structures and flow flux rates are examined as a function of bias for a range of molecular weight ratios for non-equilibrium steady-state system with non-conservative mass/volume. Response of the flux density shows linear and non-linear behavior depending on the range of the bias with an eruptive response at extreme values of the bias. [Preview Abstract] |
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J1.00218: Distributions and seeps of driven mixture around a slit in porous media - interacting lattice gas simulation Joe Gettrust, Ras Pandey We consider a porous medium with a slit on a discrete lattice of size $L_x \times L_y \times L_z$. The porous matrix is generated by a random distribution of immobile barriers on a fraction of the lattice sites. A longitudinal slit of width $L_s$ spans from bottom to top through the center of the lattice. The source of particles specified by their molecular weight, interaction, and miscibility gap is connected to the bottom ($z=1$) of the lattice with an open top ($z=L_z$). The Metropolis algorithm is used for stochastic moves of particles with a hydrostatic pressure bias ($H$). Periodic boundary conditions are used along the transverse directions with open longitudinal ends. Particles continue to enter the lattice from the source. Particles flow from bottom to top reaching a steady-state where we examine their seeps and distributions. We find that the steady-state distributions of constituents and their local mobility in the slit and the surrounding regions depend on bias and porosity with strong correlations at high bias. [Preview Abstract] |
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J1.00219: Self-Organized Criticality in a Bead Pile Megan Miller, Tuan Nguyen, Elizabeth Baker, D.T. Jacobs This experiment examined a conical bead pile and the distribution of avalanche sizes when using uniform 3mm stainless steel spheres (``beads''). A bead pile is built by pouring beads onto a circular base where the bottom layer of beads has been glued randomly. Beads are then individually dropped from a fixed height after which the pile is massed. This process is repeated for thousands of bead drops. By measuring the number of avalanches of a given size that occurred during the experiment, the resulting distribution could be compared to a power law description as predicted by self-organized criticality. We had found in an earlier experiment that glass beads dropped from a small height were consistent with a simple power-law, but if dropped from larger heights then a power-law times an exponential was needed. The stainless steel beads always had a distribution that deviated from a simple power-law with larger deviations as the beads were dropped from larger heights. In addition, we observed a different power-law exponent than that found for glass beads yet a similar dependence on drop height. We acknowledge support from NSF-REU grant DMR 0243811. [Preview Abstract] |
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J1.00220: Effect of Relative Material Stiffness of Binary Mixture Components to Compression Craig Bentham, Meenakshi Dutt, Bruno Hancock, James Elliott Typical powder blends will have constituent particles which will differ in material properties, such as stiffness, resulting in variation of coordination number and contact slipping state, at a local particle scale. At a macroscopic scale, the relative properties of the various components will influence the force and stress response of the blend to an external load or strain. Of particular curiosity is the distribution of load as a function of contact stiffness. We present numerical results from our studies on binary mixtures with components whose relative stiffness (Young's modulus) has been varied systematically. These mixtures settle under gravity followed by compression at a constant strain rate. In addition, we discuss the correlations between the contact slipping state, stiffness, and the load sustained. [Preview Abstract] |
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J1.00221: Granular templating: effects of boundary structure on particle packings under simultaneous shear and compression Bruno Hancock, Meenakshi Dutt, Craig Bentham, James Elliott We present our findings on the effect of various confining substrates, both crystalline and amorphous, on spherical granular particles, packed under gravity followed by the simultaneous application of shear and compressive strains. We show that a substrate templated with the BCC (100) plane, whose void and particle packing structures are identical, is most suitable for inducing crystallization mimicking the substrate structure. Our results for both templated substrates show the compressive strain dominating the shear strain. [Preview Abstract] |
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J1.00222: A Particle-Substrate Model and Its Applications to Cooling and Driven Granular Systems Robert Behringer, Meenakshi Dutt A complete understanding of the microscopic dynamics of a monolayer of identical spheres moving on a substrate must encompass the effects of collisions and the substrate on the particles. We begin from first principles by considering collections of spherical frictional particles that roll and slip on a flat static substrate. We present a numerical model which accounts for collisional and surface frictional dissipation and their influence on particle dynamics for a quasi 2-dimensional cooling granular material. We apply this model to a simulation of the granular collider experiment (Painter et al., Physica D (2003)), in which collections of particles collided as they moved radially inward on a substrate. We find an agreement between the experimental and numerical results. We extend this model further to study a horizontally vibrated particle-substrate system. We show that the ratio of the substrate acceleration to the particle-substrate static frictional force per unit mass (Kondic, Phys Rev. E (1999)) dominates the individual particle dynamics and the collision dynamics. We will present results from our numerical experiments which further higlight the critical role of static friction, relative to the driving acceleration. [Preview Abstract] |
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J1.00223: Blank |
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J1.00224: Quicksand Daniel Bonn Quicksand is the generic name for unstable soils reputed to trap anyone who treads on it. Popular wisdom has it that one should not move when trapped in quicksand, as motion makes one sink in even deeper and that once trapped, it is difficult to escape. We provide an explanation for these observations by studying the most commonly encountered form of natural quicksand. We show that a spectacular liquefaction of the material occurs when a stress is applied to the material: the liquefaction is the reason why one sinks away, and it is more pronounced for larger stresses. By constructing ’laboratory quicksand’, we demonstrate that the liquefaction is due to the structure: quicksand is a loose granular packing of sand particles stabilized by a clay matrix that forms a particulate gel. The stress liquefies the clay matrix, and the granular assembly collapses, expulsing water. This results in a densely packed system that practically impossible to dilate: it is for this reason that once trapped it is difficult to get out of quicksand. A sinking test demonstrates that, due to buoyancy, it impossible to drown in the quicksand. [Preview Abstract] |
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J1.00225: Properties of uniformly heated and sheared granular gases. A DSMC simulation. Francisco Vega Reyes We study a monodisperse hard sphere granular gas by means of the Direct Monte Carlo Simulation (DSMC) of the associated Enskog-Boltzmann equation. Due to the kinetic energy loss in the interparticle collisions, the granular temperature decays to the zero unless there is some kind of energy input. We consider two types of energy input. One, heating, process through which kinetic energy is added directly to the particles; and two, shearing of the granular gas. We focus particularly in the uniform steady states. Contrary to elastic hard sphere gases, in granular gases the steady uniform shear state is possible (i.e., a constant linear velocity profile with steady uniform density and temperature). This state is a reference point in the study of granular gases. We analyse the relevant magnitudes (like stress tensor) in the different energy input cases, as a function of the inelasticity. [Preview Abstract] |
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J1.00226: Networks of Liquid Bridges and Clusters in Wet Granular Systems Mario Scheel, Stephan Herminghaus, Ralf Seemann The macroscopic mechanical properties of a dry granulate change dramatically when small amounts of liquid are added. This is due to capillary bridges forming between mutually adjacent grains in the pile, which exert an attractive force by virtue of the surface tension of the liquid. If much more liquid is added, the liquid clusters, and the stability of the pile is reduced. Although the tensile strength of wet granulates can be roughly estimated from the capillary forces, a quantitative theory of the mechanical properties of granulate requires a detailed understanding of the topology of the complex network of capillary bridges and clusters. We have determined the macroscopic properties in model granulates with a vertical fluidization experiment, as well as the microscopic geometry of the distribution of liquid within the pile via x-ray microtomography. The transition from capillary bridges to clusters or the percolation can be clearly observed in both the fluidization experiments and the tomographic imaging. [Preview Abstract] |
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J1.00227: Flow of Polymer Solutions in Nansocale Slit Pores Jaime A. Millan, Mohamed Laradji, Yongmei Wang We present results from extensive and systematic molecular dynamics simulations of semi-dilute and cocentrated polymer solutions in nanoscale slit pores. We will particulary report on the effects of the polymer concentration, molecular weight and flow rate on the profiles of the velocity field, polymer concentration, and radii of gyration of the polymer chains across the slit. [Preview Abstract] |
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J1.00228: Kinematics of a sphere moving through a non-Newtonian liquid Hans Wheelersburg, Nora Graneto, Piotr Habdas When a sphere moves through a non-Newtonian liquid, it creates an envelope of sheared liquid around the sphere. By using different tube sizes and pulling the sphere at different velocities we examine the dimensions of the envelope and study the effect of the walls of the tube on the sphere in motion. Also we study how the drag force acting on the sphere varies with the tube size, sphere velocity, sphere size, and the properties of the non-Newtonian liquids (i.e. yield stress). In a Newtonian liquid, the drag force acting on the sphere is proportional to the velocity of the sphere and follows the Stokes Law. The drag force acting on a sphere moving through a non-Newtonian liquid does not follow this dependence and we are currently investigating the relationship of the drag force on velocity in non-Newtonian liquids. [Preview Abstract] |
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J1.00229: Spreading of Advected Tracers in a Creeping Flow in a Rectangular Channel Miron Kaufman We have developed an analytical solution [M.Kaufman, 2003 AIChE Annual Meeting Conference Proceedings] of the Navier-Stokes equation for the two-dimensional incompressible flow in a rectangular cavity in the limit of zero Reynolds number. An analytical solution for the fluid velocity along the axis of a parallelepiped, again in the Stokes limit, is also known. In this work we combine the two solutions to get analytically the three-dimensional creeping flow inside a channel in the shape of a parallelepiped. We integrate numerically the velocity vector to get trajectories of tracers advected by the fluid. We analyze the spreading in space of the tracers by calculating the time evolution of the entropy. [Preview Abstract] |
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J1.00230: Spinning Rods: Experiment \& Theory Jonathan Toledo, Richard M. McLaughlin, Roberto Camassa, Terry Jo Leiterman, Richard Superfine, Jing Hao, Leandra Vicci, Adam Brooks Mixing and transport properties of the fluid motion induced by a slender rod attached to a plane sweeping out a cone are examined in a table- top experiment. This experiment is used to validate an exact solution for the motion of a spheroid sweeping out a double cone in free space and an asymptotic solution for a slender body attached to a plane. Validation of the theory is used to examine mixing and transport of the same experiment on the microscale. [Preview Abstract] |
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J1.00231: Influence of Pressure on Fast Dynamics in Polymers B. Begen, A. Kisliuk, A.P. Sokolov, V.N. Novikov, K. Niss, A. Chauty-Cailliaux, C. Alba-Simionesco, B. Frick Recent experiments reveal a strong correlation between the fast dynamics and the fragility of glass forming liquids. It is known that both density and thermal energy influence significantly the structural relaxation and the glass transition in most of the glass forming systems. However, not much is known about the influence of volume and temperature on the fast dynamics. In this contribution, we present light and neutron scattering study of the influence of pressure on fast dynamics and elastic properties in polymers. Our results show that the boson peak frequency increases with pressure stronger than the sound velocity. We observed that the boson peak intensity decreases under pressure stronger in Raman scattering than in neutron scattering suggesting a decrease in the light-to-vibrations coupling coefficient $C(v)$. We argue that variations in $C(v)$ might be related to amplitude of structural fluctuations. We speculate that change in disorder and/or overall density under pressure is the main cause for the observed variations. [Preview Abstract] |
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J1.00232: Fragility, spatially heterogeneous dynamics, and enhanced translational self-diffusion in low molecular weight glassformers Marie K. Mapes, Stephen F. Swallen, M.D. Ediger Recent measurements of self-diffusion coefficients in prototypical small molecule glassformers reveal behaviors believed to be universal for low molecular weight fragile glasses. These behaviors include enhanced translational motion and a self-diffusion controlled crystallization regime. Similar enhancements in self-diffusion relative to the Stokes-Einstein relation have been found for two glassformers of similar fragility: $o$-terphenyl with around 100 times faster self-diffusion than expected at T$_{g}$ +3 K, and \textit{tris}-naphthylbenzene, with around 400 times faster diffusion at T$_{g}$. The temperature range of enhanced motion also overlaps a range of self-diffusion controlled crystal growth. The origin of enhanced diffusion is often attributed to spatially heterogeneous dynamics thought to exist in fragile glassformers. Relaxation time distributions in the literature for $o$-terphenyl and \textit{tris}-naphthylbenzene, and their relationships with enhancement of self-diffusion, fragility, and heterogeneous dynamics are discussed. [Preview Abstract] |
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J1.00233: Dynamics of magnetic colloids Artur Erbe, Michael Koeppl, Larysa Baraban, Florian Merkt, Paul Leiderer Colloidal suspensions are fascinating examples of soft matter, but also model systems for studying the behavior of atoms and molecules. Here we present the dynamic behavior of superparamagnetic particles and the fabrication and characterization of magnetically anisotropic particles. Transport of the superparamagnetic particles~(diameter of 4.7~$\rm \mu$m) is characterized through narrow constrictions on the scale of $60 - 100$~${\rm \mu}$m in an externally applied magnetic field. In this system we observe the formation of lanes depending on the interparticle interactions and the particle densities. In order to create a larger range of interparticle interactions we also produce anisotropic particles by evaporation of metallic caps on top of the colloidal particles~(for these experiments we use nonmagnetic silica colloids). These particles can have a well-defined magnetic moment, given by the material of the cap. We determine this moment using the magnetooptic technique. [Preview Abstract] |
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J1.00234: Informational Self Organization in Isotopically Random Soft Structures Alexander A. Berezin Stable isotopicity can potentially account for self organization and informational proliferation in soft matrices, gels and even fluids. This can be a key to explain seeming ``impossibility'' of such claims as memory effects in water. Within isotopicity paradigm (A.A.Berezin, Isotopic diversity in natural and engineering design, In: Design and Nature, WIT Press, 2004, 411- 419) pattern enhancement upon successive dilutions can be attributed to ``avalanche'' cascade formation of informationally rich and numerically redundant ``Arnold tongues'' akin to strange attractors. Furthermore, even biological and medical ``phenomenology'' which we currently put under umbrella term ``immune systems'' can be operationally interpreted through quantum computer informational processing in isotopic neural networks using diversity of nuclear spins of different isotopes. Under action of informational attractor (analogy with Aristotelian causa finalis) ``decay'' process of patterns can be slower than build up of informational redundancy (morphic resonance). The latter can be envisioned as a kind of informational ``Huygens principle'' in which formation of global (wave) front (here: informational front) results from superposition of virtual sources at previous stage of cascade process. Nonlocal aspects of isotopic quantum computing can potentially provide connector to infinititude (perhaps even Cantor uncountable) library of patterns and serve as yet another bridge between physics and biology. [Preview Abstract] |
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J1.00235: Quantifying spatiotemporal chaos in Rayleigh-Benard convection Magnus Einarsson, M.R. Paul, P.F. Fischer One of the biggest challenges of modern science is to gain further understanding of large spatially extended systems driven far-from-equilibrium. Such systems often exhibit spatiotemporal chaos yielding aperiodic behavior in both space and time that is difficult to characterize. Diagnostics based upon the Lyapunov exponents and eigenvectors are explored as a way to quantify spatiotemporal chaos in Rayleigh-Benard convection in an experimentally realistic cylindrical domain. This is done through large-scale parallel numerical simulations and the results are used to gain new physical insight into the behavior of spatiotemporal chaotic systems. [Preview Abstract] |
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J1.00236: Modeling microcapsules moving through microchannels of differing geometries Alexander Alexeev, Rolf Verberg, Anna Balazs We study the dynamic behavior of deformable microcapsules in microchannels, which are roughly comparable in size to the diameter of the capsules. The capsules model synthetic polymeric microcapsules or biological cells, such as leukocytes. The microcapsules are driven to move through the channels by an imposed pressure gradient or flow field. Each microcapsule consists of an elastic shell that is filled with a viscous fluid. To model this multi-component system, we combine the lattice Boltzmann model for fluid dynamics and the lattice spring model for the micromechanics of elastic solids. We determine the effects of the mechanical properties of the elastic capsule and the characteristics of the imposed flow on the dynamics of the capsules as they move inside microchannels with sinusoidal and undulating geometries. We also isolate conditions that lead to a trapping of the capsule inside the channel. The findings provide insight into the behavior of cells within biological vessels and provide guidelines for regulating the motion of polymeric microcapsules with microfluidic devices. [Preview Abstract] |
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J1.00237: Deviation of a Jet at a T junction at low Reynolds number Pascal Panizza, Wilfried Engl, Kouske Ohata, Annie Colin We study the hydrodynamic behaviour of a laminar jet flowing through a channel when it reaches a junction. We observe the existence of two possible flow regimes, namely the splitting and deviation of a jet in the most simple microfluidic configuration, namely a T junction. The transition between the two regimes is not monitored by the shape of the T junction nor by capillary effects, but can be easily anticipated in terms of the hydrodynamic properties of the flow. We present a simple hydrodynamic model which is in very good agreement with observed experimental jet behaviour. The transition between both regime acts as a flow or viscosity comparator. We show how this effect can be used for the design of digital and integrated microfluidic devices [Preview Abstract] |
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J1.00238: Fluids Moving Past Smooth Hydrophilic Surfaces -- Slip? Janet Wong, Sung-Chul Bae, Liang Hong, Steve Granick Recent experiments challenge the traditional view that fluids satisfy the ``no-slip'' boundary condition. While the amount of slip observed differs between research groups, it is now generally accepted that the no-slip boundary condition need not hold strictly -- if the surface is smooth and hydropobic. What happens if the surface is wetted by the moving fluid? This work describes hydrodynamic studies of fluid between two crossed cylinders in a surface forces apparatus, using a new method of mica cleaving that appears to produce different findings from those obtained from classical methods. Aqueous salt solutions with various ion concentrations were used with mica as the confining surface. The role of salt concentration was explored. Independently, the fluorescence lifetime of fluorescent dyes was used to measure the near-surface ionic conditions independent of a force measurement. [Preview Abstract] |
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J1.00239: Enhanced Fluid Mixing in Nanochannels: A Molecular Dynamics Study Eric C.J. Oliver, Gary W. Slater The efficient mixing of fluids is of paramount importance in several applications such as lab-on-a-chip and microfluidic devices. The main limitation to efficiency is that on small scales where the Reynolds number of the flow is low, mixing is dominated by diffusion. Purely diffusive motion is very slow and is an inefficient mixing mechanism unless the channel width is extremely small. Starting with the basic result for diffusive mixing of a binary fluid in a Poiseuille flow we explore methods to enhance the level of mixing between the two fluid species. We simulate the system using Molecular Dynamics and model the fluids as assemblies of Lennard-Jones beads. In order to increase the rate of mixing we have forced lateral motion in the fluid using configurations of mid-stream posts. Specifically, posts set in a prism-like structure have proven to be extremely well suited to reducing the channel length required to achieve complete mixing. In order to measure efficiency we have proposed a mathematical function that quantifies the level mixing associated with a fluid element. Furthermore, we have developed a basic theory for the position of the mixing front in a flow with spatially dependent velocity and diffusion coefficient. [Preview Abstract] |
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J1.00240: Effects of Membrane Material Properties on the Deformation of Elastic Capsules in a Shear Flow Kit Yan Chan, Charles Eggleton The deformation process of elastic capsules in a simple shear flow is studied numerically using the immersed boundary method to probe the influence of membrane material properties. Membrane models that are representative of linear elastic membrane (Hookean law), strain hardening membrane (Skalak), and strain softening membrane (Mooney-Rivlin, neo-Hookean) and the Evans-Skalak model developed from thermodynamic principles to represent the behavior of a lipid bilayer and a cytoskeletal network, are used to study the effects of membrane material properties on the response of the capsule at various shear rates. Simulation results indicate that both the time to reach steady state and the final steady shape of the capsule are sensitive to the choice of the membrane model and parameter values used. These experimentally measurable quantities may be compared with computations for determining suitable model of a particular capsule of interest, and the associated material properties. Local strain and energy distributions computed also provide additional information that is not easily accessible experimentally. [Preview Abstract] |
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J1.00241: Molecular dynamics modeling of nanoparticle dielectrophoresis Emppu Salonen, Emma Terama, Ilpo Vattulainen, Mikko Karttunen We have used molecular dynamics simulations to study nanoparticle dielectrophoresis (DEP), i.e., motion induced by a coupling of the particle polarization to a non-uniform electric field. In the first simulations we have considered the case of a single spherical colloid in a non-conductive solvent [1]. This approach has given qualitative information on the limitations of DEP transport due to the thermal motion of the manipulated particles at different DEP coupling strengths. In addition, we have studied changes in the DEP transport efficiency due to particle aggregation. The results of the modeling are compared to analytical studies [2,3] as well as experiments.\\ ~\\ ~[1] E. Salonen et al., Eur. Phys. J. E. 18 (2005) 133.\\ ~[2] J. P. Huang et al., Phys. Rev. E 67 (2003) 021403.\\ ~[3] J. P. Huang et al., Phys. Rev. E 69 (2004) 051402. [Preview Abstract] |
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J1.00242: An internal splash: Levitation and Long Transients of Falling Spheres in Stratified Fluids Richard Parker, Byron Huff, Richard McLaughlin, Roberto Camassa We present measurements regarding new phenomena arising with falling bodies in sharply stratified (two layer) fluids.\footnote{N. Abaid, D. Adalsteinsson, Akua Agyapong, and R. M. McLaughlin, ``An Internal Splash: Falling Spheres in Stratified Fluids,'' Physics of Fluids, 16, no. 5, 1567-1580, 2004.} Specifically, the newly observed phenomena in which a heavy falling sphere stops and reverses its motion before ultimate descent to the bottom is further explored through careful adjustment of the bottom layer fluid density . An experimental study is presented which documents the bounce amplitude and long layer residence times as function of the bottom layer density for several spheres of different densities. Finally, we present the behavior associated with beads of adjustable density and varying size. [Preview Abstract] |
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J1.00243: Flow through a laboratory sediment sample: hydraulic conductivity by computer simulation modeling Edward Braithwaite, Allen Reed, Ray Seyfarth, Ras Pandey, Joe Gettrust Digitized catscan image of a three dimensional laboratory sediment sample provides a host matrix of size $100^3$. The porosity of the sample is 0.282, i.e., below the site percolation threshold of a cubic lattice. Despite a high fraction of sediment barriers, visualizations reveal that the ramified channels of connected pores span across the sample. A coarse grained description is used to model fluid by interacting particles in spirit of classical lattice gas. As in experiments, the source of the fluid is connected at one end of the sample in our computer simulations. The fluid constituents execute their stochastic motion via Metropolis algorithm and flow through the sample with periodic boundary condition across transverse boundaries. From the net flow of fluid, we estimate the flux density and the hydraulic conductivity in steady-state. We also consider pressure bias (H) to investigate the response. Results of transport, flux rate, and distribution of fluid will be presented. [Preview Abstract] |
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J1.00244: Acoustic modulation of pharmaceutical sprays Sorin Mitran, Hugh Smyth, Anthony Hickey We consider the effect of acoustical radiation upon the particle size distribution in pharmaceutical sprays. A mixed particle hydrodynamic-Eulerian technique is used to model the propagation of acoustic waves through the spray and the effect of the acoustic radiation upon spray droplets. A model for droplet breakdown based upon normal mode oscillations is employed to capture acoustically induced modifications of the particle size distribution. Measurements are presented for metered dose inhalers and compared to theoretical predictions. [Preview Abstract] |
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J1.00245: Arranging Droplets for Microfluidics Craig Priest, Enkhtuul Surenjav, Magdalena Ulmeanu, Stephan Herminghaus, Ralf Seemann The online generation, organization and manipulation of monodisperse droplets in confinement present new possibilities for microfluidics. Droplets make excellent compartments for handling minute quantities of chemical, biological and particulate materials. When confined, a high dispersed phase volume fraction emulsion, i.e. gel emulsion, will self-organize into well-defined arrangements. The possible arrangements are dependent on the droplet size and the length-scale of the confining geometry. In this work, we have studied the organization of monodisperse gel emulsions in microchannels for application in microfluidic processing. The high degree of spatial order exhibited by gel emulsions facilitates the precise manipulation of a droplet, or group of droplets, using geometry alone. [Preview Abstract] |
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J1.00246: PHASE TRANSITIONS AND STRONGLY CORRELATED SYSTEMS POSTER SESSION |
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J1.00247: Critical Behavior Of Uniformly Uniaxial Magnets Demetris Nicolaides Real magnetic systems never obey rotational invariance because the lattice itself is not completely rotation-invariant. The simple case of phase transitions in a magnetic system with broken rotational symmetry, described by a single, m-component vector order parameter, with uniform uniaxial perturbation in only the first component, is a prototype example. Renormalization group theory found that this kind of magnets experience phase transition of the second order, into either the easy axis of magnetization, or that they have critical behavior analogous to an (m-1)-component spin system. The present work extends this study to cover the more complex case of two coupled, continuous, anisotropic, m-component vector order parameters, having frozen-in, uniform, uniaxial anisotropies. These anisotropies are due to nonmagnetic impurities, and affect the trial (mean-field) critical temperature of only the first component of each order parameter. The study is done with the help of a phenomenological model which considers fluctuations partially. It is shown that when both coupled vector order parameters are anisotropic, the order-disorder phase transition is always of the second order, into either the uniaxial or the (m-1)- isotropic phase. This is despite the fact that for the isotropic case of two coupled order parameters, the phase transition is a fluctuation-induced first order. [Preview Abstract] |
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J1.00248: Dynamical mean-field theory of the Hubbard-Holstein model at half-filling: electron-phonon interaction effects on the finite temperature Mott transition Jae Hyun Yun, Han-Yong Choi, Gun-Sang Jeon, Hyun C. Lee, Tae-Ho Park We study the Hubbard-Holstein (HH) model which indludes both the electron-electron and electron-phonon interactins characterized by $U$ and $g$, respectively, by empolying the dynamical mean-field theory in combination with Wilson's numerical renormalization group technique. We fix $g=0.1W$ ($W$=bandwidth of the conduction electrons) and calculate the finite temperature phase diagram of metal-insulator transition in the $U-T$ plane. We will discuss in particular how the critical $U$ values, $U_{c1}$ and $U_{c2}$ , and the critical temperature $T_{c}$ of the Mott transition are changed by the electron-phonon interaction. [Preview Abstract] |
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J1.00249: Magnetic and transport properties of V-doped La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ J.S. Park, K.K. Yu, Y.P. Lee, Y.S. Lee, J.-H. Kang The effects of substitution of V for Mn on the electronic and the magnetic properties of La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ have been investigated. The samples show a reentrant magnetic behavior as the V doping is increased. The V doping into the Mn sites leads to a mixed-phase ground state consisting of the ferromagnetic insulating and the ferromagnetic metallic phases at low temperatures. The magnetic relaxation was observed in both the ferromagnetic and the reentrant spin-glass phases, which means the existence of frustration and magnetic disorder in the samples. It was found that the V-doped manganites with aging effect bear the characteristics of a reentrant ferromagnet. [Preview Abstract] |
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J1.00250: Heat bath approach to Landau damping and Pomeranchuk quantum critical points Johan Nilsson, Antonio Castro Neto We study the problem of the damping of collective modes close to a Pomeranchuk quantum critical point in a Fermi liquid. In analogy with problems in dissipative open quantum systems, we derive the Landau damping of a Fermi liquid by integrating out a macroscopic number of degrees of freedom from a generating functional. Being a reformulation of the linearized Boltzmann equation this approach reproduces well-known results from the theory of Fermi liquids. We also study the Bethe-Salpeter equations within the Landau theory and discuss the implications of these results on quantum phase transitions of the Pomeranchuk type and its dynamical exponent, z. We apply our results to the electronic nematic instability and find z=3 in the collisionless limit. [Preview Abstract] |
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J1.00251: Magnetic Phases of $\lambda $-(BETS)$_{2}$FeCl$_{4}$ investigated by proton NMR spectroscopy. Guoqing Wu, W.G. Clark, P. Ranin, S.E. Brown, L. Balicas, L.K. Montgomery The organic conductor $\lambda $-(BETS)$_{2}$FeCl$_{4}$, is of interest because of its unusual phases, which include a paramagnetic metal (PM), an antiferromagnetic insulator (AFI), and a field-induced superconducting phase. Important drivers for these phases are the 3d Fe$^{3+}$ moments (spin $S_{d}$ = 5/2) from the FeCl$_{4}^{-}$ anions and the $\pi $ conduction electrons (spin $S_{\pi }$ = 1/2) in the BETS donor molecules, which generate a correlated $\pi $-d electron system. Here, we report a proton NMR spectroscopy study of these phases in a small ($\sim $3~$\mu $g) single crystal of $\lambda $-(BETS)$_{2}$FeCl$_{4}$ using an applied field of 9 T over the temperature ($T)$ range 2-180 K. The results show a complex spectrum that broadens and is shifted as $T$ is lowered in the PM phase, and additional changes associated with the PM-AFI transition. The main spectral features at all $T$ are attributed to the large dipolar field from the 3d Fe$^{3+}$ ions at the proton sites. A phenomenological model provides a reasonable fit to them. On lowering $T$ through the PM-AFI transition at 3.5 K, the spectrum smears and its second and first moments change discontinuously. These features indicate that the transition is first order and that the $\pi $-d interaction is important for its properties. The work at UCLA is supported by NSF Grants DMR-0334869 (WGC) and 0203806 (SEB). [Preview Abstract] |
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J1.00252: Improved thermoelectric characteristics of misfit-layered cobaltites Ca$_{3}$Co$_{4-x}$Fe$_{x}$O$_{9+\delta }$. Chia-Jyi Liu, Li-Chen Huang, Wen-Ching Hung, Jeng-Lung Chen, Ching-Lin Chang We have measured the electrical resistivity, Seebeck coefficients and thermal conductivity as a function temperature for a new series of oxides Ca$_{3}$Co$_{4-x}$Fe$_{x}$O$_{9}$ (x=0, 0.05, 0.1, 0.15, 0.2) prepared by the conventional solid state reaction. Structural parameters were refined with a superspace group of X2/m(0b0)s0 using powder X-ray diffraction data. With the substitution of Fe$^{+2 }$ for Co$^{+3}$, the resistivity ($\rho )$ decreases, while the thermoelectric power (S) also increases simultaneously. In the low temperature regime from 15 K to 60 K, the electrical conductivity follows the Mott's law of the form exp[T$_{0}$/T$^{-1/4}$], suggesting the variable-range-hopping transport. The thermoelectric power also shows the same transport mechanism in the same temperature regime. The x = 0.05 sample exhibits a higher power factor value (3.3 $\mu $W/K$^{2}$-cm) than that of undoped Ca$_{3}$Co$_{4}$O$_{9}$ (1.2 $\mu $W/K$^{2}$-cm) at 300K, indicating the improvement of the thermoelectric characteristics upon Fe substitution for Co. [Preview Abstract] |
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J1.00253: Computational Toolkit for First-Principles Multicomponent Alloy Thermodynamics Teck Tan, Nikolai Zarkevich, D.D. Johnson A ``toolkit'' for the simulation of alloy thermodynamics has been developed by integrating first-principles, electronic-structure calculations and the cluster expansion (CE) with Monte Carlo (MC) methods. This Thermal Toolkit (TTK) is aimed at producing reliable thermodynamics of alloys with limited input from the user. Given an alloy, TTK first generates a comprehensive set of structures, automatically submits an electronic-structure calculation to determine the structural energies, stores the structure and its energy in a database,{\footnote {The ``Structural Database'' http://data.mse.uiuc.edu}} then constructs the CE via the structural inversion method that conforms to a set of mathematical conditions to produce an optimal truncated cluster expansion.{\footnote {Nikolai Zarkevich and D.D. Johnson, Phys. Rev. Letts 92, 255702 (2004)}} Using this optimal CE, a MC code (included in TTK) can be used to calculate thermodynamic properties, such as structural phase diagram (T vs c). We present here example application and functionality of TTK on binary and ternary alloy. [Preview Abstract] |
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J1.00254: Resonant X-ray Scattering Study of Quadrupolar Ordering in DyB$_4$ Sungdae Ji, C. Song, J. Koo, K.-B. Lee, Y.J. Park, J.S. Rhyee, B.K. Cho Resonant x-ray scattering was performed on DyB$_4$ to observe a quadrupolar ordering. It is demonstrated that anisotropic tensor susceptibility (ATS) resonances due to ordering in Dy $4f$ states can be seperated from those due to atomic displacements, such as Jahn-Teller distortion, by tuning the incident x-ray energies. An ATS resonance at 7.792 keV, corresponding to antiferro-type quadrupolar (AFQ) ordering, was observed to have different azimuthal angular dependence and polarizations from those of magnetic resonance. The AFQ order parameter is developed concomitantly with monoclinic phase transition below 12.3 K, and its magnitude is proportional to quadratic of the monoclinic angle. [Preview Abstract] |
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J1.00255: Smoothing and Bootstrapping in Numerical Studies of Lattice Statistical Models Voltaire Teodorescu We investigate the applicability and estimate efficiency for Monte Carlo type simulations in lattice models of magnetic materials. In particular, an improved algorithm based on smoothed bootstrapping of Markov chains is applied to the two-dimensional Ising model. We report the results based on several smoothing functions. [Preview Abstract] |
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J1.00256: Magnetic and transport properties of La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/BaTiO$_{3}$ and La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/BaTiO$_{3}$ bilayered films V.G. Prokhorov, Y.P. Lee, K.K. Yu, S.Y. Park, V.L. Svetchnikov The magnetic and the transport properties of La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/BaTiO$_{3}$ (LCMO/BTO) and La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/BaTiO$_{3}$ (LSMO/BTO) bilayered films, prepared by the rf-magnetron sputtering, have been investigated. For comparison, the same data for the LCMO and the LSMO films, deposited on the bare LaAlO$_{3}$ (LAO) substrates, are obtained as well. The microstructural analysis reveals that LCMO/BTO and LSMO/BTO have the biaxial tensile in-plane and compressive out-of-plane strains, while LCMO/LAO and LSMO/LAO are reversely in the compressive in-plane and tensile out-of-plane strains. The films with biaxial tensile in-plane lattice strains undergo the magnetic transition at a higher temperature than those with biaxial compressive ones. This suggests that the Mn-O-Mn bond angle, controlled by the lattice strain, plays more important role in the formation of spin ordering in the manganite film than the modification of Mn-O bond length. The observed enhancement of magnetoresistance at room temperature in LSMO/BTO provides an advance for the development of new hybrid ferromagnetic/ferroelectric devices. [Preview Abstract] |
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J1.00257: Kondo Transport at Low Temperature and Electron-electron Interaction in Dilute Mn-doping Na$_{0.7}$CoO$_{2}$ Systems Jincang Zhang, Zaiqing Zhang, Yan Xu, Chao Jing, Shixun Cao The layer transition-metal oxide Na$_{\gamma }$CoO$_{2}$ engendered much interest to physicists due to its promising thermoelectric properties. Its anomalous large thermoelectric power and low electrical resistivity in combination with low thermal conductivity is difficult to be understood in the framework of conventional band picture. In this paper, we present the results of low-temperature transport behaviour and its dependence on applied magnetic field for dilute Mn doping Na$_{0.7}$Co$_{1-x}$Mn$_{x}$O$_{2}$ systems with x=0, 0.03, 0.07, 0.1. The results show that the transport properties are very sensitive to Mn doping and a novel resistivity minimum was found with a characteristic of metallic-semiconductor transition at low temperature for all the doping samples. And the best fitting was made in the framework of Kondo scattering and the electron-electron (e-e) interaction in a wide temperature range of 2-100 K. The weak dependent on the external field prove that the present Na$_{0.7}$Co$_{1-x}$Mn$_{x}$ is a strong disorder system and there exists an enhancing e-e interaction, which reflects a typical characteristic of strong correlation systems. The results prove that the layer Na$_{\gamma }$CoO$_{2}$ is a kind of typical Kondo-like oxide as like the dilute convention alloys. [Preview Abstract] |
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J1.00258: Study of the one-band model for colossal magnetoresistive manganites using the Truncated Polynomial Expansion Monte Carlo Method Cengiz Sen, Elbio Dagotto The recently proposed Truncated Polynomial Expansion Method (TPEM) for spin-fermion systems is tested using the one-band double exchange model with finite Hund coupling $J_{\rm H}$ in the clean limit. Two dimensional lattices as large as 48$\times$48 are studied, far larger than those that can be handled with standard exact diagonalization (DIAG) techniques for the fermionic sector. Phase diagrams are obtained, showing first-order transitions separating ferromagnetic metallic from insulating states. A huge magnetoresistance is found at low temperatures by including small magnetic fields, in excellent agreement with experiments. By comparing results between the two methods, TPEM and DIAG, on small lattices, and by analyzing the systematic behavior with increasing cluster sizes, it is concluded that the TPEM is accurate to handle realistic manganite models on large systems. Our results pave the way to a frontal computational attack of the colossal magnetoresistance phenomenon using double-exchange like models, on large clusters, and including quenched disorder. [Preview Abstract] |
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J1.00259: The effect of substrate induced strain on the electronic soft matter in thin films of the manganite (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ ($y$ = 0.5, 0.6) Jacob Tosado, Tara Dhakal, Sunghee Yun, Amlan Biswas For most hole-doped manganites, the low temperature phase is either a ferromagnetic metal (FMM) or charge-ordered insulator (COI). At certain compositions the competition between the FMM and COI phases leads to a mixed phase state in which both these phases coexist. In this mixed phase state, changing the temperature, magnetic field or strain modifies the spatial arrangement and relative proportions of these two phases. This property of manganites is reminiscent of electronic soft matter. We have studied the effect of substrate-induced strain on this electronic soft matter by measuring the magneto-transport properties of (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LPCMO,$ y$ = 0.5, 0.6) thin films of different thicknesses. The temperature --magnetic field ($T-H)$ phase diagram of LPCMO shows four distinct phases \textit{viz}., the COI phase, the FMM phase, a static phase separated (SPS) state and a fluid phase separated (FPS) state. Our experimental results show that as the thickness of the thin film increases (i.e. the substrate induced strain decreases), the SPS state expands and covers more area in the phase diagram at the expense of the FPS state. [Preview Abstract] |
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J1.00260: SURFACES, INTERFACES, AND THIN FILMS POSTER SESSION |
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J1.00261: Structure of Photonic Multilayer Films Created by Plasma Enhanced Chemical Vapor Deposition (PECVD) Someshwara Peri, Brian Habersberger, Bulent Akgun, Mark Foster, Hao Jiang, Timothy Bunning, Charles Majkrzak The structures of copolymer films created by PECVD were studied for the first time using neutron reflectivity (NR) and x-ray reflectivity (XR). Both homopolymer films of benzene (PP-B) and octafluorocyclobutane (PP-OFCB) and copolymer films of HMDS and OFCB made using different feed ratios were considered. In HMDS-OFCB copolymer films, we observed a transient structure next to the substrate as seen previously for homopolymer OFCB films. PP-B films swelled approximately 200 {\%} in deuterated-THF vapor and PP-OFCB swelled approximately for 30 {\%} in deuterated-toluene vapor, indicating that for these processing conditions PP-B films are much less highly crosslinked than are PP-OFCB films. We have measured for the first time the variation in crosslink density with depth for homopolymer films created by PECVD. [Preview Abstract] |
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J1.00262: TOF MS studies of the BN and carbon laser ablated plasma (YAG-laser, \textit{$\lambda $}$_{exc}$ = 1064 nm). Vladimir Makarov, Dachun Huang, Artuto Hidalgo, Gerardo Morell, Brad Weiner In the present study, TOF MS spectra of the ablated ions from the BN-ceramic, amorfus carbon, graphite and fullerene-60 surfaces were recorded for different distances between the target surface and work (analytical) area of the TOF MS and for different radiation density of the laser radiation. The averaged energy and temperature of the ablated ions were estimated. The mechanism of the plasma formation in all cases studied was analyzed and discussed. [Preview Abstract] |
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J1.00263: Pair distribution functions of silicon/silicon nitride interfaces Deng Cao, Martina E. Bachlechner Using molecular dynamics simulations, we investigate different mechanical and structural properties of the silicon/silicon nitride interface. One way to characterize the structure as tensile strain is applied parallel to the interface is to calculate pair distribution functions for specific atom types. The pair distribution function gives the probability of finding a pair of atoms a distance $r $apart, relative to the probability expected for a completely random distribution at the same density. The pair distribution functions for bulk silicon nitride reflect the fracture of the silicon nitride film at about 8 {\%} and the fact that the centerpiece of the silicon nitride film returns to its original structure after fracture. The pair distribution functions for interface silicon atoms reveal the formation of bonds for originally unbound atom pairs, which is indicative of the interstitial-vacancy defect that causes failure in silicon. [Preview Abstract] |
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J1.00264: Interaction of H atom with Si(111)4x1-In surface Sang-Yong Yu, Geunseop Lee, Hanchul Kim, D. Lee, Ja-Yong Koo Using scanning tunneling microscopy (STM) measurements and \textit{ab initio} calculations, the interaction of an H atom with the Si(111)4x1-In surface at room temperature is investigated. It is found that the H atom preferentially adsorbs at the bridge site between the In chain and the Si Seiwatz chain and break an In-Si bond to form a Si-H bond. Experimentally, the adsorption of H is influenced by the registry of the Si substrate and preferentially occupies one of the two zigzag In chains to the other, while the little difference is found in the calculated adsorption energy. The adsorbed H atom induces not only a localized lattice distortion but also a distant electronic perturbation near the Fermi level, which appears as out-of-phase period-doubled (x2) charge ordering in the filled- and the empty-state STM images. These perturbations induced by the two nearby H atoms in the same row interfere to make the inbetween x2 modulation enhanced or suppressed depending on the H-H distance. The x2 perturbed region remains metallic, distinguishing it from the insulating low-temperature 4x2 (8x2) phase. The observed x2 perturbation away from the H-adsorbate is found to be the theoretical ground state of the In/Si(111) surface predicted by density-functional theory, which is stabilized by the presence of the surface defect. [Preview Abstract] |
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J1.00265: Tetracene thin film morphology on hydrogen-passivated Si(100) Jun Shi, Andrew Tersigni, Xiaorong Qin Initial stage of vacuum evaporated tetracene films on hydrogen-passivated Si(100) substrates has been investigated by scanning probe microscopy. Three-dimensional crystalline islands and dendrites have been obtained at low deposition rates, exhibiting contributions of lateral and vertical edge-diffusions in growth and local surface effects. The absence of the 3D structures has been achieved at a proper range of deposition rates and a layer-by-layer thin film growth mode has been obtained. Results suggest that the obtained thin film phase defines a kinetic path for the formation of uniform films. [Preview Abstract] |
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J1.00266: Oxidation of Hafnium studied by X-ray photoelectron spectroscopy. A Chourasia, W. Johnston The oxidation of hafnium has been studied by X-ray photoelectron spectroscopy. Two types of experiments were conducted. In both types, thick films of hafnium were deposited on a metal substrate using the electron beam deposition technique. In one set of experiments, the substrate temperature was in the range between 100C and 500C. The deposition chamber was filled with oxygen at a very low pressure while the film was getting deposited. In the second set of experiments, substrate was kept at room temperature during the hafnium film deposition. The film was then subjected to post-deposition annealing in an oxygen atmosphere. The hafnium XX core level and oxygen 1s core level were recorded in the high resolution mode. The thickness of the oxide films have been determined from the XPS spectra using the Quantitative Analysis by SES software. The study provides a comparison of the oxide formation as a function of substrate temperature in the two cases. [Preview Abstract] |
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J1.00267: Self Alignment of Wafer Stacks via Pattern Modification of Self Assembled Monolayer Surface Energies Ernest Walker, Hans Hallen Self assembled monolayers (SAMs) are deposited on oxide layer, and characterized by ellipsometry and contact angle. Vinyl-terminated SAMs are oxidized to carboxyl-termination, which changes the wetting characteristics. Measurements of sliding friction between combinations of these layers is measured and discussed. The relative surface energies can also be obtained. From these data, the relative importance of friction and forces resulting from surface energy gradients can be determined, and wafer self-alignment masks evaluated. [Preview Abstract] |
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J1.00268: Reciprocal Space Mapping of Thin La$_{x}$ Mn O$_{3}$ Films of Varying Thickness M.A. DeLeon, T. Tyson, C. Dubourdieu, J. Bai In order to understand the growth of ultrathin manganite films on substrates, we made reciprocal space maps in the 004, 404, and 044 pseudocubic directions of La$_{x}$MnO$_{3}$ films deposited on LaAlO$_{3}$ (LAO) via metal-organic chemical vapor deposition. The x-ray maps exhibit the strain effects of the lattice-substrate mismatch and the effects of a twinned interface on the growth of films. Reciprocal space mapping provides more accurate lattice parameters and strain effects from lattice-substrate interface mismatch. A characteristic feature of the LAO substrates is twinning, which results in multiple structural components of the films, similar lattice parameters but differing orientation. These characteristics reveal crucial aspects of deposition processes to be inspected in assuring high-quality films. This research is supported by NSF DMR-0209243 and DMR-0512196. [Preview Abstract] |
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J1.00269: H Adsorption on RH (110) Surface Shao-Ping Chen We have used the density functional theory to study the H adsorption phenomena on Rh (110) surface with H coverage from 0.33 to 2.00 mono-layers. We found H atom favors the three- fold coordinated site as observed experimentally. We confirmed the existence of 1x3-H, 1x2-H, 1x3-2H, 1x1-2H ordered structures. We also found that the proposed 1x2-2H structure for coverage of 1.0 is not the lowest energy configuration. We have proposed a new 1x2-2H structure which needs to be tested by future experiments. [Preview Abstract] |
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J1.00270: Influence of diffusion on submonolayer island growth during hyperthermal deposition onto Cu(100) and Cu(111) Mika Jahma, Marko Rusanen, Ismo Koponen, Tapio Ala-Nissila We consider the influence of realistic island diffusion rates to homoepitaxial growth on metallic surfaces using a recently developed rate equation model [1] which describes growth in the submonolayer regime with hyperthermal deposition. To this end, we incorporate realistic size and temperature-dependent island diffusion coefficients for the case of homoepitaxial growth on Cu(100) and Cu(111) surfaces [2]. We demonstrate that the generic features of growth remain unaffected by the details of island diffusion, thus validating the generic scenario of high density of small islands found experimentally and theoretically for large detachment rates. However, the details of the morphological transition and scaling of the mean island size are strongly influenced by the size dependence of island diffusion. This is reflected in the scaling exponent of the mean island size, which depends on both temperature and the surface geometry.\newline {[1]} I.T. Koponen et. al, Phys. Rev. Lett. \textbf{92}, 086103 (2004)\newline {[2]} M.O. Jahma et. al, Surface Science, in press [Preview Abstract] |
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J1.00271: Self-assembly of nanoparticle/copolymer mixtures Jinbo He, Ravisubhash Tangirala, Kevin Sill, Todd Emrick, Thomas Russell, Xuefa Li, Jin Wang, Yao Lin, Alexander Boker, Anna Balazs Mixtures of polystyrene-\textit{block}-poly (2-vinylpyridine) with tri-$n$-octylphosphine oxide-(TOPO)-covered CdSe nanoparticles were chosen to test the theoretical prediction of synergistic effects between two self-organization systems. Preliminary results confirmed that hierarchical structures were provided, with poly (2-vinylpyridine) cylindrical microdomains oriented normal to the surface and CdSe-TOPO nanoparticles self-assembly at the surface, that balanced the surface tensions of the P2VP with that of PS. Detailed structure evolution was revealed by \textit{in-situ} grazing incidence small angle x-ray scattering (GISAXS) during thermal annealing. Results from these studies indicated that the orientation of the mcrodomains began at the free surface and propagated in the film towards the substrate. This one-step self-orienting, self-assembly process, without the use of external fields, opens a simple route for fabrication of nanostructured materials having hierarchical order, with applications including chemical sensing, separation, catalysis, high-density data storage and photonic materials.. [Preview Abstract] |
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J1.00272: Equilibrium shape and dislocation nucleation in strained epitaxial nanoislands J. Jalkanen, O. Trushin, K. Elder, E. Granato, T. Ala-Nissil\"{a}, S.-C. Ying We study numerically the equilibrium shapes, shape transitions and dislocation nucleation of small strained epitaxial islands with a two-dimensional atomistic model, using simple pair potentials [1]. We first map out the phase diagram for equilibrium island shapes as a function of island size and lattice misfit with the substrate. When the interactomic potential minimum depth $\epsilon$ is the same for substrate and adsorbate, the latter either wets the surface or has one of three generic equilibrium island shapes. As $\epsilon$ favours substrate-adsorbate interface, Stranski-Krastanow growth mode emerges between complete wetting and island formation zones while in the opposite case the islanding zone is extended. A simulation based extrapolation scheme predicts an optimal island shape for attractive $\epsilon.$ The energy barriers and transition paths between different island shapes and dislocation nucleation in initially coherent islands are investigated with Nudged Elastic Band method. We also discuss the elastic behaviour of these systems in terms of the Phase Field Crystal model [2]. [1] J. Jalkanen, O. Trushin, E. Granato, S. C. Ying, and T. Ala-Nissila, Phys. Rev. B 72, 081403 (2005) [2] K. Elder and M. Grant, Phys. Rev. E {\bf 70}, 051605 (2004) [Preview Abstract] |
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J1.00273: The Structure of Adsorbed Methane on the MgO(100) Surface from Inelastic Neutron Scattering and First-Principles Calculations M.L. Drummond, B.G. Sumpter, W.A. Shelton, Jr., J.Z. Larese The adsorption of molecules onto a solid surface is a phenomenon with important ramifications in areas such as catalysis, corrosion, and electronics. We have used plane-wave, psuedopotential-based density functional calculations, in conjunction with inelastic neutron scattering (INS), to evaluate the structure of methane on the MgO(100) surface. The combination of high-resolution INS and high-quality calculations has proven extremely versatile in determining the structural arrangement, despite the existence of multiple energetically favorable arrangements. The structures for adsorbed methane mono-, bi-, and higher order adlayers are discussed. [Preview Abstract] |
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J1.00274: c(4x2) Reconstruction of Manganese Tetramers on Mn3N2 (001) Studied by Scanning Tunneling Microscopy Rong Yang, Haiqiang Yang, Arthur Smith We have investigated the growth of antiferromagnetic Mn3N2 on MgO(001) by molecular beam epitaxy. Two orientations [(010) and (001)] of this structure are grown controllably on MgO(001), depending on the growth conditions. The bulk structure is face- centered tetragonal with 2 layers of MnN followed by 1 layer of Mn . Here we present results for the (001) surface. STM images show smooth terraces and atomic steps. On some of the terraces a unique and new reconstruction is seen, resolved as square Mn tetramers in a c(4$\times$2) structural arrangement. Two domains of the tetramer reconstruction, rotated by 90 $^\circ$ to each other, occur. A model is presented for this square Mn tetramer reconstruction, in which the Mn atoms of the tetramer layer belong to the Mn layer at the surface in the MnN- MnN-Mn stacking sequence. The work is supported by NSF9983816 and 0304314. [Preview Abstract] |
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J1.00275: Intrinsic Surface Reconstructions of Zinc-Blende GaN(001) Studied by Scanning Tunneling Microscopy Arthur R. Smith, Muhammad Haider, Rong Yang, Costel Constantin, Hamad Al-Brithen, Erdong Lu, Nancy Sandler, Pablo Ordejon The intrinsic structures of semiconductor surfaces are important for epitaxial growth and future applications. Here we present a scanning tunneling microscopy study of the clean zinc-blende GaN(001) surface, free from the presence of arsenic. We find a sequence of reconstructions for the clean surface, beginning with 4x3 and including c(4x12), 4x7, c(4x16), 4x9, and c(4x20). Generally, all of these reconstructions are row-like. The 4x3 is a semiconducting surface structure with a bandgap, measured using tunneling spectroscopy, of 1.14 eV; the 4x3 is obtained under Ga-poor surface conditions and is compared with a model calculated using ab-initio techniques. The other reconstructions are metallic, obtained under Ga-rich surface conditions. They can be modeled using simple adatom schemes on top of a bulk-like Ga atom termination. [Preview Abstract] |
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J1.00276: First-principles study of adsorption and migration on the (001) surfaces of cubic BN Hiroaki Koga, Tsuyoshi Miyazaki, Satoshi Watanabe, Takahisa Ohno This first-principles study examines the behavior of boron and nitrogen adatoms on the dimer-reconstructed (001) surfaces of cubic boron nitride (cBN), to find low-barrier migration pathways that control the vapor-growth of cBN. Adatom dynamics on the surfaces of wide band gap semiconductors such as cBN (with $\sim $6 eV gap) and diamond is an uncharted field of surface science: Because these materials have very strong, directional bonds, the adatoms may choose unlikely adsorption sites and migration pathways, to minimize the induced strains. For example, we have found that a N adatom on a N dimer row of cBN(001) migrates using a site-exchange reaction, rather than migrating on the dimer row. The calculated energy barrier for the site-exchange pathway is mere 0.9 eV, in contrast with the large barrier (over 4 eV) of the dimer row pathway. At the meeting, we report such low-barrier pathways on cBN(001), and also discuss a new approach to cBN epitaxy that exploits them. [Preview Abstract] |
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J1.00277: Anchoring to the Nanoworld: Spectroscopic Investigation of Atomic Metal Strings Adsorbed on GaN Che-Chen Chang, Chien-Hua Lung It is now possible to produce structures that are only a few tens to a few nanometers in size using techniques which involve the self assembly or the chemical functionality-guided arrangement of atoms, molecules and nanospecies. Their devices, however, are useless unless they are able to communicate with the outside world and with one another. This study explored the possibility of fabricating metal contacts from the bottom up on semiconductors using a linear metal atom string complex, which was chelated by dipyridylamino ligands and terminated at both ends by one Cl atom each, for CVD. More than 75{\%} of the string complex exposed at 105 K to the GaN(0001) surface was chemisorbed dissociatively, with one of its terminal Cr-Cl bonds disrupted and the other pointing freely outwards. The chelate structure of the adsorbed complex was stable until reaching $\sim $340 K, at which temperature detachment of some of its ligands occurred. The ligand desorption followed the first-order kinetics, with a desorption energy of 21.5 kcal/mol. The detachment of the ligand from the complex caused the metal chain axis to lie down on the surface. Consequently, the Cr-Cl bond on the free end of the chain interacted with the surface, causing other ligands to desorb from the surface at the higher substrate temperature of $\sim $540 K. [Preview Abstract] |
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J1.00278: Negative differential resistance of TEMPO on Si(111) Ann-Sofie Hallback, Harold J W Zandvliet, Bene Poelsema Negative differential resistance (NDR) has been observed for individual 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) molecules, adsorbed on Si(111) at room temperature. Measurements were performed in ultra high vacuum (UHV) using scanning tunneling microscopy (STM) and - spectroscopy (STS). NDR effects were observed exclusively at negative bias voltage on the used n-type Si(111) sample. TEMPO was observed to adsorb preferentially at corner adatom sites of the Si(111)-7x7 structure. Although the Si(111)-7x7 reconstruction was conserved, local defects were observed in the vicinity of the TEMPO adsorbates. [Preview Abstract] |
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J1.00279: Reflectance anisotropy spectra of CdTe$(001)$ surfaces Raul Vazquez, Bernardo Mendoza, Norberto Narzate We calculate reflectance anisotropy (RA) spectra of a clean Cd-terminated CdTe$(001)$ surface which exhibits $c(2 \times 2)$ reconstruction, and clean Te-terminated CdTe surfaces with $(2 \times 1)$, $(1 \times 2)$ and $c(2 \times 2)$ surface reconstructions. Theoretical spectra are obtained from two aproaches: an $ab$ $initio$ pseudopotential calculation in the framework of the density functional theory and within the local density approximation (DFT-LDA), and a microscopic formulation based on a semi- empirical tight binding approach which includes spin-orbit (SO) interactions$^1$. We show how RA spectrum changes when SO coupling is taken into account and compare our theoretical results with experimental results$^2$. We find a good agreement between experimental and theoretical spectra. \newline [1] R. A. V\'azquez-Nava, B. S. Mendoza and N. Arzate, Phys. Stat. Sol. b {\bf 242} 3022 (2005); R. A. V\'azquez-Nava, B. S. Mendoza and C. Castillo Phys. Rev. B {\bf 70}, 165306 (2004) \newline [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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J1.00280: Local Valence Structure of Fe in Fe3Si/GaAs(001) Kuan-Li Yu, Hsueh-Hsing Hung, Ling-Yun Jang, Ming-Wei Hong, Ray Nien Kwo A high quality, eptiaxial magnetic films of Fe$_{3}$Si on GaAs(001) substrates were studied in our experiment. Fe$_{3}$Si is ferromagnet and can be regarded as a Heusler-like alloy with a composition of Fe$_{2}$FeSi. There is possibility that Fe$_{3}$Si is a half metal which is an ideal candidate for spin injection, although the calculated density of states for bulk Fe$_{3}$Si does not predict half-metallic behavior. By using diffraction anomalous fine structure measurement, there is chance to look at the anomalous effect on the different atom sites. Two diffraction of Fe$_{3}$Si were measured as (002) and (004) to understand the local electron density structure in different Fe sites and the spectra show significant different features. By carefully handling these DAFS results, there is chance to understand the structure of the local density of state of the Fe atoms in different crystal sites. [Preview Abstract] |
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J1.00281: Angle-resolved Ultra-violet Photoelectron Spectroscopy Study of Epitaxial CrO$_{2}$ films grown on TiO$_{2}$ Substrates Daniel Borst, Carl Ventrice, G. Miao, Arun Gupta Chromium dioxide is predicted to be a half-metallic oxide. Although there is experimental evidence that CrO$_{2}$ is half-metallic at low temperature, attempts to make devices based on CrO$_{2}$ have yielded very low efficiencies. To study the electronic properties of the surface region of CrO$_{2}$, we have performed ARUPS measurements on epitaxial CrO$_{2}$ films. The CrO$_{2}$ thin films have been deposited on (100) and (110)-oriented TiO2 substrates by chemical vapor deposition, using CrO$_{3}$ as a precursor. The effects of sputtering of the CrO$_{2}$ films to remove the outer layer of Cr$_{2}$O$_{3}$ and of annealing the films in oxygen to heal surface defects has been studied. Sputtering results in shifts in the onset of valence emission away from the Fermi edge by as much as 0.5 eV, which is opposite of what would be expected for a half-metallic system. Annealing of the films above 450 $^{o}$C results in spectra similar to Cr$_{2}$O$_{3}$. [Preview Abstract] |
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J1.00282: Ga-doping effect on superconductivity for Ba8Si46 clathrates Ruihong Zhang, Yang Liu, Xingqiao Ma, Ning Chen, Guohui Cao, Yang Li We present a joint experimental and theoretical study of the superconductivity and electronic structures in type-I Ga-doped silicon clathrates. The superconducting critical temperature in Ba8Si46-xGax is shown to decrease strongly with gallium content increasing. These results are corroborated by first-principles simulations calculated from the density-functional theory with plane waves and pseudopotentials. The simulations show that Ga doping results in a large decrease of electronic density of states in Fermi level, which can explain the superconducting critical temperature decrease with Ga-doping in the BCS theoretical frame. [Preview Abstract] |
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J1.00283: The Ferromagnetic Hafnium Dioxide Thin Films Prepared by Pulsed Laser Deposition Yung M. Huh, R. Vaselaar, J. Zhang, D.J. Sellmyer The ferromagnetic hafnium dioxide (HfO2) thin films were prepared using pulsed laser deposition (PLD) system. Ferromagnetic moment was completely suppressed and revived as HfO2 film was annealed in vacuum and air. The variation of ferromagnetic moment was investigated to study the origin of the ferromagnetism in HfO2 system. X-ray diffraction showed that the c-plane is perpendicular to the growth direction. Strong anisotropic moment was observed for applied magnetic fields along the parallel and perpendicular to the c-plane. [Preview Abstract] |
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J1.00284: Inelastic electron tunneling spectroscopy of decanethiol on Au(111) at elevated temperatures Ann-Sofie Hallback, Harold J W Zandvliet, Bene Poelsema We have studied decanethiol self-assembled monolayers (SAMs) on Au(111) in UHV by inelastic electron tunneling spectroscopy (IETS) performed with a scanning tunneling microscope (STM). This study reveals inelastic peaks, which can successfully be assigned to molecular vibration modes (Au-S or S-C stretch modes, and C-C stretch mode or CH$_{2}$ modes). It is remarkable that these modes can be detected already at 77 K, i.e. at a much higher temperature than commonly used in IETS measurements. [Preview Abstract] |
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J1.00285: Quantum size effect on the diffusion barriers and growth morphology of Pb/Si(111) Tzu-Liang Chan, Cai-Zhuang Wang, Myron Hupalo, Michael Tringides, Kai-Ming Ho Using first-principles total energy calculations, we have studied the diffusion barriers of Pb adatoms on a free-standing Pb(111) film as a function of film thickness. We found that diffusion of an adatom on the Pb film has very low barriers (less than 60 meV). A bi-layer oscillation in the diffusion barriers due to quantum size effect(QSE) is observed, with lower barrier on the odd-layered,relatively unstable Pb films. The diffusion barrier difference between the odd-and even-layered film is found to be as large as 40 meV. This big difference in the diffusion barriers due to QSE is the origin of the intriguing growth morphology of Pb islands on Si(111) surface observed in the STM experiments: the growth of a Pb layer on Pb islands with unstable heights starts from the periphery and moves towards the center, while the nucleation of the next layer on stable Pb islands starts away from the periphery. [Preview Abstract] |
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J1.00286: Substrate Dependence on the Allotropic Transformation and the Morphology of Ultrathin Bi Films on Si(111) S. Yaginuma, T. Nagao, K. Nagaoka, M. Maekawa, J.T. Sadowski, M. Saito, Y. Fujikawa, T. Sakurai, T. Kakayama Bismuth film grows via an intriguing thickness-driven allotropic transformation from a {\{}012{\}}-oriented phase to a bulk-like (001) phase. We compare the growth of Bi films on the Si(111)-7x7 surface with that on the Si(111)-$\beta -\surd $3x$\surd $3-Bi surface by use of electron diffraction and scanning tunneling microscopy. Small amount of Bi(001) nanocrystals can nucleate on the 7x7 substrate from the very beginning of the growth $\sim $ 1 monolayer (ML). They stabilize with 3 bilayer height in a perfect commensurate manner on the 7x7 substrate because of lattice contraction as predicted from our \textit{ab initio} calculations, and thus lead to the growth of the highly-crystalline (001) films. On the $\beta -\surd $3x$\surd $3-Bi surface, on the contrary, the (001) nuclei is not observed in the initial stage of the growth and the Bi{\{}012{\}} films survive, and thus the nucleation of the (001) phase is inhibited up to $\sim $ 15 ML. [Preview Abstract] |
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J1.00287: Comparative studies of Pb films on different substrates by first-principles calculations Yu Jia, Biao Bu, H.H. Weitering, Zhenyu Zhang Quantum growth of ultrathin Pb on Ge(111) and Cu(111) substrates up to more than 25 monolayers are studied using total energy calculations within density functional theory. Our studies show that the surface energy, work function, and lattice relaxation of these films all oscillate strongly with the film thickness. The oscillation pattern is always even-odd with interruptions by crossovers, in good agreement with experimental observations. However, the positions of the crossovers and the separation between the crossovers depend on which substrate is used. These results are rationalized based on the interplay between Friedel oscillations, quantum size effects, and charge transfer effects. [Preview Abstract] |
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J1.00288: Characterization of Noble Gas Ion Beam Fabricated Single Molecule Nanopore Detectors Ryan Rollings, Bradley Ledden, John Shultz, Daniel Fologea, Jiali Li, John Chervinsky, Jene Golovchenko Nanopores fabricated with low energy noble gas ion beams in a silicon nitride membrane can be employed as the fundamental element of single biomolecule detection and characterization devices [1,2].~With the help of X-ray Photoelectron Spectroscopy (XPS) and Rutherford Backscattering (RBS), we demonstrate that the electrical noise properties, and hence ultimate sensitivity of nanopore single molecule detectors depends on ion beam species and nanopore annealing conditions. .1. Li, J., D. Stein, C. McMullan, D. Branton, M.J. Aziz, and J.A. Golovchenko, \textit{Ion-beam sculpting at nanometre length scales.} Nature, 2001. \textbf{412}(12 July): p. 166-169. 2. Li, J., M. Gershow, D. Stein, E. Brandin, and J.A. Golovchenko, \textit{DNA Molecules and Configurations in a Solid-state Nanopore Microscope.} Nature Materials, 2003. \textbf{2}: p. 611-615. [Preview Abstract] |
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J1.00289: Simulation of self-organized evolution of patterned GaAs(001) surfaces during homo-expitaxial growth Erin Flanagan, Hung-Chih Kan, Tabassom Tadayyon-Eslami, Subramaniam Kanakaraju, Chris Richardson, Raymond Phaneuf We report on both physically based and phenomenological simulations for morphological evolution of patterned GaAs(001) surface during homo-epitaxial growth. We compare these simulations with the experimental observations of homo-epitaxial growth on substrates patterned with arrays of cylindrical pillars. Our atomic force microscope (AFM) characterization indicates that the pillars evolve in a self-organized manner, i.e. the shape evolution of the pillars seems to be insensitive to its initial diameter in the [-110] direction. [Preview Abstract] |
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J1.00290: Study of Nanopore Sculpting with Noble Gas Ion Beams at Various Energies Bradley Ledden, Eric Krueger, Jiali Li We report on experiments using noble gas beams: Helium, Neon, Argon, Krypton, and Xenon, at energies of 1keV, 3keV, and 5keV to controllably fabricate nanopores in freestanding silicon nitride membranes. Utilizing computer simulations (SRIM and TRIM), and a surface adatom diffusion model to describe the phenomenon of nanopore formation, we evaluate the conditions, specifically the ion energy, responsible for highly controllable fabrication of solid state nanopores. Additionally, we present methods to determine the thickness of the grown film as well as the thickness profile of the nanopore. [Preview Abstract] |
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J1.00291: Surface-enhanced Raman spectroscopy of the nitrided SiO$_{2}$/SiC interface S.H. Choi, M. Park, D. Wang, J.R. Williams, W. Lu, S. Dhar, L.C. Feldman Performance of silicon carbide (SiC) field effect devices is limited by poor channel mobility due to the high density of oxide/dielectric interface states, (D$_{it})$. In general D$_{it}$ is crystal-face dependent. Carbon clusters generated by the high temperature oxidation process, are assumed to be the major contributor to this high defect density. Nitric oxide (NO) annealing reduces the interface states and results in an enhanced mobility, although the physical/chemical mechanisms are not yet established. We report a surface enhanced Raman spectroscopy (SERS) analysis of the 4H-SiC/SiO$_{2}$ interface, clearly showing carbon cluster bands, on the C (carbon-terminated) face. The concentration of the carbon clusters on the C-terminated interface is much higher than that of the Si-face, consistent with the higher D$_{it}$ on the C-face. Furthermore, NO annealing results in a reduction of the cluster concentration. This result provides direct experimental evidence that carbon clusters exist at the as-grown interface and the effect of NO anneal at atmospheric pressure suggest removal of these interfacial carbon defects. [Preview Abstract] |
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J1.00292: Surface Demixing in a AuSn Liquid Alloy Venkat Balagurusamy, Reinhard Streitel, Oleg Shpyrko*, Peter Pershan, Ben Ocko, Moshe Deutsch We present results of X-ray reflectivity studies of the eutectic AuSn alloy liquid-vapor interface. The analysis shows that in common with the BiSn eutectic, there is surface demixing that extends to more than one monolayer. This is in contrast to a common presumption that the Gibbs adsorption predicts complete demixing only in the surface monolayer. The composition profiles can be explained by surface segregation theory for attractive interaction between Sn and Au atoms, similar to BiIn [1] and BiSn [2]. \newline [1] E. DiMasi, H. Tostmann, O. G. Shpyrko, P. Huber, B. M. Ocko, P. S. Pershan, M. Deutsch, and L. E. Berman, Phys. Rev. Lett. \textbf{86}, 1538 (2001) \newline [2] O. G. Shpyrko, A. Y. Grigoriev, R. Streitel, D. Pontoni, P. S. Pershan, M. Deutsch, and B. M. Ocko, Phys. Rev. Lett. \textbf{95}, 106103 (2005) \newline *Present address: Center for Nanoscale Materials, ANL [Preview Abstract] |
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