bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetism and cluster glass dynamics in geometrically frustrated LuFe2O4
Authors: Phan M. H. , Frey N. A. , Srikanth H. , Angst M. , Sales B. C. , Mandrus D.
Published: February 2009

We report on the magnetic properties of high quality LuFe2O4 single crystals grown by the floating zone method. dc and ac susceptibility measurements and analysis reveal a ferrimagnetic transition at ∼240 K followed by a re-entrant cluster glass transition below 225 K, with an additional magnetic transition around 170 K. Strong frequency dependence of the real (χ′) and imaginary (χ″) parts of the ac susceptibility observed at both these temperatures indicate glassy behavior and we quantitatively fit the data to a cluster glass model, τ=τo(Tf/Tg−1)−zv. Our studies show that these multiple transitions are consistent with the picture of ferrimagnetic clusters in the iron oxide planes with triangular lattice configuration favoring spin frustration and glass dynamics.
Superconducting and magnetic properties of Ni/Pb multilayered nanowires
Authors: Lin L. K. , Chi Y. S. , Chen T. M. , Shyu S. G. , Huang J. H. , Lee S. F.
Published: February 2009

Nickel/lead multilayered nanowires were prepared using standard three-electrode electrodeposition into anodic aluminum oxide porous templates. The diameters of the nanowires ranged from 50 to 200 nm. By means of electric and magnetic measurements, we found that the superconducting transition temperature, Tc, of the Pb layer persisted down to the smallest diameter without any noticeable dimensional effect. Tc was affected by the relative orientation of magnetizations in neighboring Ni layers. When the superconducting layers were driven normal under a large magnetic field, unusual resistance increase was observed with decreasing temperature, which was consistent with the Kondo effect.
A versatile and reliably reusable ultrahigh vacuum viewport
Authors: Weatherill K. J. , Pritchard J. D. , Griffin P. F. , Dammalapati U. , Adams C. S. , Riis E.
Published: February 2009

We present a viewport for use in ultrahigh vacuum (UHV) based upon the preflattened solder seal design presented in earlier work [Cox et al., Rev. Sci. Instrum. 74, 3185 (2003)]. The design features significant modifications to improve long term performance. The windows have been leak tested to less than 10−10 atm cm3/s. From atom number measurements in an optical dipole trap loaded from a vapor cell magneto-optical trap inside a vacuum chamber accommodating these viewports, we measure a trap lifetime of 9.5 s suggesting a pressure of around 10−10 Torr limited by background rubidium vapor pressure. We also present a simplified design where the UHV seal is made directly to a vacuum pipe.
Influence of doping (Ti, V, Zr, W) and annealing on the sp2 carbon structure of amorphous carbon films
Authors: Adelhelm C. , Balden M. , Rinke M. , Stueber M.
Published: February 2009

The influence of the transition metal (Ti, V, Zr, W) doping on the carbon matrix nanostructuring during the thin film growth and subsequent annealing is investigated. Pure and metal-doped amorphous carbon films (a-C, a-C:Me) were deposited at room temperature by nonreactive magnetron sputtering. The carbon structure of as-deposited and postannealed (up to 1300 K) samples was analyzed by x-ray diffraction (XRD) and Raman spectroscopy. The existence of graphenelike regions in a-C is concluded from a (10) diffraction peak. A comparison of the XRD and Raman results suggests that XRD probes only the small amount of 2–3 nm large graphenelike regions, whereas the majority of the sp2 phase is present in smaller distorted aromatic clusters which are probed only by Raman spectroscopy. Annealing leads to an increase in the graphene size and the aromatic cluster size. During the carbon film growth the addition of metals enhances ordering of sp2 carbon in sixfold aromatic clusters compared to a-C; Ti, and Zr showing the strongest effect, W the lowest. This order qualitatively corresponds with the catalytic activity of the respective carbides found during graphitization of carbide-doped graphites published in the literature. With annealing, carbide crystallite formation and growth occurs in a-C:Me films, which destroys the initial carbon structure, reduces the size of the initially formed aromatic clusters and the differences in carbon structure introduced by different dopants. For high annealing temperatures the carbon structure of a-C:Me films is similar to that of a-C, and is determined only by the annealing temperature.
Collisionless trapped electron and ion temperature gradient modes in an advanced tokamak equilibrium
Authors: Mahmood M. Ansar , Rafiq T. , Persson M. , Weiland J.
Published: February 2009

The linear stability of coupled collisionless trapped electron (TE) and ion temperature gradient (ITG) modes is investigated in an International Thermonuclear Experimental Reactor [C. Gormezano et al., Nucl. Fusion 47, S285 (2007)]-like magnetic field configuration. An advanced fluid model in the electrostatic limit and the ballooning mode formalism are used to derive an eigenvalue equation. The growth rates and real frequencies of the most unstable modes and their eigenfunctions are calculated. The coexistence of unstable ITG and TE modes and their relative strengths with respect to the fraction of trapped electrons, wave vector, and local plasma parameters, such as density gradient, electron/ion temperature gradient, and ion to electron temperature ratio are analyzed. The influence of geometrical factors, such as local/global magnetic shear and magnetic field curvature on both ITG and TE modes is also discussed.
A new approach to evaluate the thermodiffusion factor for associating mixtures
Authors: Abbasi Alireza , Saghir M. Ziad , Kawaji Masahiro
Published: February 2009

Diffusion behaviors in associating mixtures present a larger degree of complexity than nonassociating mixtures. The direction of flow in associating mixtures may change with the variation of composition and temperature. In this paper a new viscous energy model is proposed for predicting the ratio of evaporation energy to viscous energy. The new model was implemented for prediction of thermodiffusion for acetone-water, ethanol-water, and isopropanol-water mixtures. In particular, this approach is implemented to predict the sign changes in the thermodiffusion factor for associating mixtures, which has been a major step forward in thermodiffusion studies for associating mixtures.
Optical properties and carrier dynamics of polarity controlled ZnO films grown on (0001) Al2O3 by Cr-compound intermediate layers
Authors: Kwon Bong-Joon , Sun Yuanping , Chung Jean Soo , Cho Yong-Hoon , Park J. S. , Yao T.
Published: February 2009

We report on optical properties and carrier dynamics of polarity controlled ZnO films grown on (0001) Al2O3 by Cr-compound intermediate layers using photoluminescence (PL), PL excitation, temperature dependent PL, and time-resolved PL techniques. At low temperature, a dominant donor-bound exciton peak at 3.36 eV has been observed in the O-polar ZnO grown on Cr2O3 intermediate layer, while a dominant acceptor-bound exciton peak at 3.32 eV has been observed in the Zn-polar ZnO grown on CrN intermediate layer. Native defects and/or impurities introduced by the Cr-compound intermediate layers play an important role in the optical properties of the polar ZnO samples.
Formation of the charge selective contact in solar cells with extremely thin absorber based on ZnO-nanorod/In2S3/CuSCN
Authors: Dittrich T. , Kieven D. , Belaidi A. , Rusu M. , Tornow J. , Schwarzburg K. , Lux-Steiner M. Ch.
Published: February 2009

Annealed and not-annealed solar cells with extremely thin absorber based on ZnO-nanorod /In2S3/CuSCN structures have been compared. Significantly higher external quantum efficiencies have been recorded on annealed devices. The temperature dependent current-voltage characteristics in the dark and under illumination were analyzed in detail. The short-circuit current density increased with the temperature and depended on the light intensity by a power law with a power coefficient of 0.85 that was independent of the annealing or measurement temperature. The temperature dependence of the ideality factor dominated the temperature dependencies of the diode saturation current density and of the open circuit voltage. The activation energies increased strongly after annealing. We propose that the limiting charge selective contact is driven away from the highly defective In2S3/CuSCN interface into the In2S3 layer due to stimulated by the annealing Cu diffusion.
High-frequency compact analytical noise model for double-gate metal-oxide-semiconductor field-effect transistor
Authors: Lázaro A. , Cerdeira A. , Nae B. , Estrada M. , Iñíguez B.
Published: February 2009

Silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistors (MOSFETs) are excellent candidates to become an alternative to conventional bulk technologies. The most promising SOI devices for the nanoscale range are based on multiple gate structures such as double-gate (DG) MOSFETs. These devices could be used for high-frequency applications due to the significant increase in the transition frequency fT for these devices. For low noise radiofrequency and microwave applications, high-frequency noise models are required. In this work, we present compact expressions to model the drain and gate current noise spectrum densities and their correlation for DG MOSFETs. These expressions depend on the mobile charge densities that are obtained using analytical expressions obtained from modeling the surface potential and the difference of potentials at the surface and at the center of the Si doped layer without the need to solve any transcendental equations. Using this model, the DG MOSFET noise performances are studied. The current and noise models can be easily introduced in circuit simulators.
A molecular dynamics study on heat transfer characteristics at the interfaces of alkanethiolate self-assembled monolayer and organic solvent
Authors: Kikugawa Gota , Ohara Taku , Kawaguchi Toru , Torigoe Eiichi , Hagiwara Yasumasa , Matsumoto Yoichiro
Published: February 2009

In this paper, we present molecular dynamics (MD) simulations of interfaces composed of self-assembled monolayers (SAMs) and solvents in order to investigate the heat transfer characteristics at the interface. Two typical normal alkylthiolate SAMs with different chain lengths, i.e., 1-propanethiol C3H7SH and 1-dodecanethiol (C12H25SH) chemically adsorbed on Au(111) substrate surfaces, were used, and toluene was adopted as the organic solvent. In addition to the SAM systems, an interface composed of the bare solid substrate and solvent (without SAMs) was analyzed for comparison. Nonequilibrium MD simulations, in which a temperature gradient perpendicular to the interface was imposed, were performed and the difference in thermal boundary resistance in the interface systems was discussed. We observed that the SAM interfaces have smaller thermal resistance when compared with that of the bare solid interface. In order to understand the mechanisms of the small resistance at the SAM-solvent interfaces, the vibrational character of molecules in each phase, which contacted each other at the interface was analyzed and a detailed adsorbed structure of solvent molecule in the interface region was extracted. As a result, a clear difference in these characters was found between the SAM interfaces and bare solid interface.
An ab initio Rice-Ramsperger-Kassel-Marcus/master equation investigation of SiH4 decomposition kinetics using a kinetic Monte Carlo approach
Authors: Barbato Alessandro , Seghi Carlo , Cavallotti Carlo
Published: February 2009

The unimolecular reaction of decomposition of SiH4 to SiH2 and H2 and the bimolecular reaction between SiH3 and H were investigated by solving the master equation using a stochastic kinetic Monte Carlo (KMC) approach. Rice-Ramsperger-Kassel-Marcus (RRKM) microcanonical kinetic constants were determined using classic transition state theory for the reaction of decomposition to SiH2 and H2 and microcanonical J-resolved variational transition state theory for decomposition to SiH3 and H. Structures of reactants and transition states were determined at the B3LYP/aug-cc-pVTZ level, while energies were calculated at the CCSD(T) level and extended to the complete basis set limit. Unimolecular kinetic constants were directly computed from the results of KMC simulations using a new algorithm while bimolecular rate constants were calculated from stochastic reaction probabilities. The simulation results are in good agreement with experimental data for the unimolecular decomposition of SiH4, which is in the falloff regime in the temperature (1100–1700 K) and pressure (10−3–101 bar) range investigated. The calculated high and low pressure limit kinetic constants for SiH4 decomposition to SiH2 and H2 are k∞=1.2×1013T0.477 exp(−28 988/T) and k0=1.4×1042T−7.245 exp(−33 153/T). The calculated Troe falloff parameter is Fcent=0.979 exp(−T/1427)+0.021 exp(T/1489). The rate of the bimolecular reaction between SiH3 and H to give SiH2 and H2 is pressure independent between 10−3 and 100 bar and slightly temperature dependent between 300 and 2000 K. The kinetic constant interpolated in this temperature and pressure range is 6.9×1011T0.736 exp(134.8/T(K)) cm3 mol−1 s−1, which is among the highest values proposed in the literature for this process.
Molecular dynamics simulations on crystallization of polyethylene/fullerene nanocomposites
Authors: Yang Hua , Zhao Xiao Jun , Li Ze Sheng , Yan Feng Dong
Published: February 2009

Molecular dynamics simulations of polyethylene/fullerene (PE/C60) nanocomposites with different fullerene contents were performed. During the isothermal relaxation process of the PE/C60 nanocomposites, subglobules first appear around C60 molecules and the subglobules coalesce into single globule, then the globule evolves as an ordered structure with the C60 molecules being located at the free surface of the cluster. The simulation results show that the attractive van der Waals interactions control the relaxation process, and as the C60 content increases, the crystallinity of the ultimate structures of PE/C60 nanocomposites decreases.
Exact dynamics of driven Brownian oscillators
Authors: Xu Rui-Xue , Tian Bao-Ling , Xu Jian , Yan YiJing
Published: February 2009

Exact quantum master equation for a driven Brownian oscillator system is constructed via a Wigner phase-space Gaussian wave packet approach. The interplay between external field and dissipation leads to this system an effective field correction that enhances the polarization. This cooperative property is resulted from an effective bath response to the external field applied on the system. It is important in the low-frequency driving and intermediate bath memory region. We demonstrate this non-Markovian effect on the linear response and nonlinear dynamics and analyze the results together with analytical asymptotic expressions.
Physical analysis of the radiation shielding for the medical accelerators
Authors: Li Q. F. , Xing Q. Z. , Kong C. C.
Published: February 2009

Radiation safety standards today require comprehensive shielding protection schemes for all particle accelerators. The original shielding system of BJ-20 (BeiJing-20 MeV), the high-energy medical electron linac, was designed only for the 18 MeV level. And the dose caused by the lost electrons in the 270° bending magnet system was neglected. In this paper, the leakage dose of BJ-20 is carefully analyzed. The radiation leakage dose distribution of the photons coming from the accelerator head is obtained for energy levels of 6, 12, 14, and 18 MeV. The dose of the photoneutrons is especially analyzed for the 18 MeV level. The result gives that even neglecting the dose from the 270° bending magnet system, the shielding system is still not enough for the energy levels lower than 18 MeV. The radiation leakage produced by electrons that are lost in the 270° bending magnet system has been particularly studied. Using beam transport theory and Monte Carlo sampling methods, which have been combined in calculations, we have obtained the distribution of the energy, position, and direction of the lost electrons. These data were then further processed by the Monte Carlo N-particle (MCNP) code as input data. The results show that when the electron loss rate in the 270° bending magnet system is 13.5%, the radiation leakage dose of the photons generated by the lost electrons is 0.1% higher than that at the isocenter, and the corresponding relative leakage dose of the photoneutrons reaches 0.045% around an angle of 170° at 18 MeV level. Both of these parameters exceed radioprotection safety standards for medical accelerators. The original shielding design is therefore not suitable and is also incomplete since the radiation produced by the electrons being lost in the 270° bending magnet system was neglected and the leakage dose for the low-energy levels was not considered in the original design. Our calculations provide a very useful tool for further optimization and design improvement that will enable this radiation shielding to conform to present day safety standards.
Validating optical emission spectroscopy as a diagnostic of microwave activated CH4/Ar/H2 plasmas used for diamond chemical vapor deposition
Authors: Ma Jie , Ashfold Michael N. R. , Mankelevich Yuri A.
Published: February 2009

Spatially resolved optical emission spectroscopy (OES) has been used to investigate the gas phase chemistry and composition in a microwave activated CH4/Ar/H2 plasma operating at moderate power densities (∼30 W cm−3) and pressures (≤175 Torr) during chemical vapor deposition of polycrystalline diamond. Several tracer species are monitored in order to gain information about the plasma. Relative concentrations of ground state H (n=1) atoms have been determined by actinometry, and the validity of this method have been demonstrated for the present experimental conditions. Electronically excited H (n=3 and 4) atoms, Ar (4p) atoms, and C2 and CH radicals have been studied also, by monitoring their emissions as functions of process parameters (Ar and CH4 flow rates, input power, and pressure) and of distance above the substrate. These various species exhibit distinctive behaviors, reflecting their different formation mechanisms. Relative trends identified by OES are found to be in very good agreement with those revealed by complementary absolute absorption measurements (using cavity ring down spectroscopy) and with the results of complementary two-dimensional modeling of the plasma chemistry prevailing within this reactor.
Temperature dependent leakage current behavior of pulsed laser ablated SrBi2Ta2O9 thin films
Authors: Roy A. , Maity S. , Dhar A. , Bhattacharya D. , Ray S. K.
Published: February 2009

Polycrystalline SrBi2Ta2O9 (SBT) thin films were grown on Pt/Ti/SiO2/Si substrates by pulsed laser ablation technique. Phase analyses of the deposited films were studied by grazing incidence x-ray diffraction. Microstructural and interfaces of the SBT film were investigated using a field emission scanning electron microscope. The dc leakage current behavior was studied at different temperatures, and the current transport mechanism was investigated. The calculated activation energies from the Arrhenius plot were attributed to the shallow traps existing near the conduction band of the SBT thin films. The current-voltage plot could be clearly separated into three regions, i.e., Ohm’s law, trap-filled limited, and Child’s law. At a low electric field, the current density–voltage characteristics showed the Ohmic behavior. Lampert’s theory of space charge limited conduction was found to be suitable to explain the current conduction through SBT films. The trap-filled limited voltage increases with increasing temperature up to 100 °C and then decreases with temperature.
Superparamagnetic relaxation time of a single-domain particle with a nonaxially symmetric double-well potential
Authors: Usov N. A. , Grebenshchikov Yu. B.
Published: February 2009

The analytical estimates for the relaxation time of a single-domain particle with a nonaxially symmetric double-well potential are obtained in the limits of low and high dissipation regimes. The expressions derived turn out to be valid also for an important case of spheroidal nanoparticle whose easy anisotropy axis is inclined at an arbitrary angle to a shape anisotropy axis. The analytical estimates are compared with the numerical simulation results obtained for the same particle by means of solution of the stochastic Landau–Lifshitz equation at various values of the dimensionless damping parameter κ. It is shown that in the case of moderate reduced energy barrier the high damping limit occurs at κ≥1.0, whereas the low damping limit corresponds to very small values of the damping parameter, κ≤0.003. For intermediate values of κ, a heuristic interpolation formula for the particle relaxation time is suggested. It describes the numerical simulation results with accuracy of several percent.
An ultrahigh-vacuum apparatus for resonant diffraction experiments using soft x rays (hν=300–2000 eV)
Authors: Takeuchi T. , Chainani A. , Takata Y. , Tanaka Y. , Oura M. , Tsubota M. , Senba Y. , Ohashi H. , Mochiku T. , Hirata K. , Shin S.
Published: February 2009

We have developed an ultrahigh-vacuum instrument for resonant diffraction experiments using polarized soft x rays in the energy range of hν=300–2000 eV at beamline BL17SU of SPring-8. The diffractometer consists of modified differentially pumped rotary feedthroughs for θ-2θ stages, a sample manipulator with motor-controlled x-y-z-, tilt (χ)-, and azimuth (ϕ)-axes, and a liquid helium flow-type cryostat for temperature dependent measurements between 30 and 300 K. Test results indicate that the diffractometer exhibits high reproducibility (better than 0.001°) for a Bragg reflection of α-quartz 100 at a photon energy of hν=1950 eV. Typical off- and on-resonance Bragg reflections in the energy range of 530–1950 eV could be measured using the apparatus. The results show that x-ray diffraction experiments with energy-, azimuth-, and incident photon polarization-dependence can be reliably measured using soft x rays in the energy range of ∼300–2000 eV. The facility can be used for resonant diffraction experiments across the L-edge of transition metals, M-edge of lanthanides, and up to the Si K-edge of materials.
Optically active centers in Eu implanted, Eu in situ doped GaN, and Eu doped GaN quantum dots
Authors: Bodiou L. , Braud A. , Doualan J.-L. , Moncorgé R. , Park J. H. , Munasinghe C. , Steckl A. J. , Lorenz K. , Alves E. , Daudin B.
Published: February 2009

A comparison is presented between Eu implanted and Eu in situ doped GaN thin films showing that two predominant Eu sites are optically active around 620 nm in both types of samples with below and above bandgap excitation. One of these sites, identified as a Ga substitutional site, is common to both types of Eu doped GaN samples despite the difference in the GaN film growth method and in the doping technique. High-resolution photoluminescence (PL) spectra under resonant excitation reveal that in all samples these two host-sensitized sites are in small amount compared to the majority of Eu ions which occupy isolated Ga substitutional sites and thus cannot be excited through the GaN host. The relative concentrations of the two predominant host-sensitized Eu sites are strongly affected by the annealing temperature for Eu implanted samples and by the group III element time opening in the molecular beam epitaxy growth. Red luminescence decay characteristics for the two Eu sites reveal different excitation paths. PL dynamics under above bandgap excitation indicate that Eu ions occupying a Ga substitutional site are either excited directly into the D50 level or into higher excited levels such as D51, while Eu ions sitting in the other site are only directly excited into the D50 level. These differences are discussed in terms of the spectral overlap between the emission band of a nearby bound exciton and the absorption bands of Eu ions. The study of Eu doped GaN quantum dots reveals the existence of only one type of Eu site under above bandgap excitation, with Eu PL dynamics features similar to Eu ions in Ga substitutional sites.
Physical limits for high ion charge states in pulsed discharges in vacuum
Authors: Yushkov Georgy Yu. , Anders André
Published: February 2009

Short-pulse high-current discharges in vacuum were investigated with the goal to maximize the ion charge state number. In a direct extension of previous work [G. Y. Yushkov and A. Anders, Appl. Phys. Lett. 92, 041502 (2008)], the role of pulse length, rate of current rise, and current amplitude was studied. For all experimental conditions, the usable (extractable) mean ion charge state could not be pushed beyond 7+. Instead, a maximum of the mean ion charge state (about 6+ to 7+ for most cathode materials) was found for a power of 2–3 MW dissipated in the discharge gap. The maximum is the result of two opposing processes that occur when the power is increased: (i) the formation of higher ion charge states and (ii) a greater production of neutrals (both metal and nonmetal), which reduces the charge state via charge exchange collisions.
Producing metastable nanophase with sharp interface by means of focused ion beam irradiation
Authors: Barna Árpád , Kotis László , Lábár János , Osváth Zoltán , Tóth Attila L. , Menyhárd Miklós , Zalar Anton , Panjan Peter
Published: February 2009

Amorphous carbon/nickel double layers were irradiated by 30 keV Ga+ ions via focused ion beam. The effect of irradiation on the concentration distribution of all constituents was studied by Auger electron spectroscopy depth profiling and cross sectional transmission electron microscopy, while the morphology change of the sample was determined by atomic force microscopy. The Ga+ ion irradiation results in the formation of metastable Ni3C layer with a uniform thickness. The C/Ni3C and Ni3C/Ni interfaces were found to be sharp up to a fluence of 200 Ga+ ions/nm2.
Artificial bacterial flagella: Fabrication and magnetic control
Authors: Zhang Li , Abbott Jake J. , Dong Lixin , Kratochvil Bradley E. , Bell Dominik , Nelson Bradley J.
Published: February 2009

Inspired by the natural design of bacterial flagella, we report artificial bacterial flagella (ABF) that have a comparable shape and size to their organic counterparts and can swim in a controllable fashion using weak applied magnetic fields. The helical swimmer consists of a helical tail resembling the dimensions of a natural flagellum and a thin soft-magnetic “head” on one end. The swimming locomotion of ABF is precisely controlled by three orthogonal electromagnetic coil pairs. Microsphere manipulation is performed, and the thrust force generated by an ABF is analyzed. ABF swimmers represent the first demonstration of microscopic artificial swimmers that use helical propulsion. Self-propelled devices such as these are of interest in fundamental research and for biomedical applications.
A picosecond time-resolved electron energy spectrometer based on Čerenkov radiation
Authors: Elberson Lee N. , Ping Yuan , Shepherd Ronnie L. , Patel Pravesh K. , Mackinnon Andrew J. , Hill Wendell T.
Published: February 2009

The energy spectrum of relativistic electrons is an important characterization of high intensity laser-matter interactions. We present a technique that utilizes Čerenkov radiation to measure the time-resolved energy distribution of electrons. Electrons escaping from targets irradiated by high-intensity laser pulses were measured, demonstrating the feasibility of such a novel diagnostic. Limitations on the time resolution of this diagnostic are also discussed.
Development of atomic force microscope with wide-band magnetic excitation for study of soft matter dynamics
Authors: Kageshima Masami , Chikamoto Takuma , Ogawa Tatsuya , Hirata Yoshiki , Inoue Takahito , Naitoh Yoshitaka , Li Yan Jun , Sugawara Yasuhiro
Published: February 2009

In order to probe dynamical properties of mesoscopic soft matter systems such as polymers, structured liquid, etc., a new atomic force microscopy apparatus with a wide-band magnetic cantilever excitation system was developed. Constant-current driving of an electromagnet up to 1 MHz was implemented with a closed-loop driver circuit. Transfer function of a commercial cantilever attached with a magnetic particle was measured in a frequency range of 1–1000 kHz in distilled water. Effects of the laser spot position, distribution of the force exerted on the cantilever, and difference in the detection scheme on the obtained transfer function are discussed in comparison with theoretical predictions by other research groups. A preliminary result of viscoelasticity spectrum measurement of a single dextran chain is shown and is compared with a recent theoretical calculation.
Optical anisotropy of electronic excitations in elliptical quantum dots
Authors: Singha Achintya , Pellegrini Vittorio , Kalliakos Sokratis , Karmakar Biswajit , Pinczuk Aron , Pfeiffer Loren N. , West Ken W.
Published: February 2009

The authors report that anisotropic confining potentials in laterally coupled semiconductor quantum dots (QDs) have large impacts in optical transitions and energies of intershell collective electronic excitations. The observed anisotropies are revealed by inelastic light scattering as a function of the in-plane direction of light polarization and can be finely controlled by modifying the geometrical shape of the QDs. These experiments show that the tuning of the QD confinement potential offers a powerful method to manipulate electronic states and far-infrared intershell optical transitions in QDs.
70 nm resolution in subsurface optical imaging of silicon integrated-circuits using pupil-function engineering
Authors: Serrels K. A. , Ramsay E. , Reid D. T.
Published: February 2009

We present experimental evidence for the resolution-enhancing effect of an annular pupil-plane aperture when performing nonlinear imaging in the vectorial-focusing regime through manipulation of the focal spot geometry. By acquiring two-photon optical beam-induced current images of a silicon integrated-circuit using solid-immersion-lens microscopy at 1550 nm we achieved 70 nm resolution. This result demonstrates a reduction in the minimum effective focal spot diameter of 36%. In addition, the annular-aperture-induced extension of the depth-of-focus causes an observable decrease in the depth contrast of the resulting image and we explain the origins of this using a simulation of the imaging process.
Gradient index circuit by waveguided metamaterials
Authors: Liu Ruopeng , Yang Xin Mi , Gollub Jonah G. , Mock Jack J. , Cui Tie Jun , Smith David R.
Published: February 2009

Metamaterials are artificially structured materials that provide considerable flexibility for control of electromagnetic waves. The metamaterial concept can also be applied to the design of planar waveguiding structures. Here, we illustrate this design approach with the development of two-dimensional (2D) planar gradient index (GRIN) circuits. To form the structure, we make use of a 2D complementary split ring resonator, which exhibits an electric response to guided transverse-electric waves. To confirm the properties of the planar GRIN structure predicted from numerical simulations, we present experimental results for a beam-steering and a focusing GRIN circuit. These examples illustrate the versatility of the metamaterial approach in the design of complex waveguiding structures.
Small anisotropy, weak thermal fluctuations, and high field superconductivity in Co-doped iron pnictide Ba(Fe1−xCox)2As2
Authors: Yamamoto A. , Jaroszynski J. , Tarantini C. , Balicas L. , Jiang J. , Gurevich A. , Larbalestier D. C. , Jin R. , Sefat A. S. , McGuire M. A. , Sales B. C. , Christen D. K. , Mandrus D.
Published: February 2009

We performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe1−xCox)2As2. Unlike the high-temperature superconductor cuprates and 1111-phase oxypnictides, Ba(Fe1−xCox)2As2 showed practically no broadening of the resistive transitions under magnetic fields up to 45 T. We report the temperature dependencies of the upper critical field Hc2 both parallel and perpendicular to the c-axis, the irreversibility field Hirrc(T), and a rather unusual symmetric volume pinning force curve Fp(H) suggestive of a strong pinning nanostructure. The anisotropy parameter γ=Hc2ab/Hc2c deduced from the slopes of dHc2ab/dT=4.9 T/K and dHc2c/dT=2.5 T/K decreases from ∼2 near Tc, to ∼1.5 at lower temperatures, much smaller than γ for 1111pnictides and high-Tc cuprates.
Proton exchange membrane fuel cell modeling based on adaptive focusing particle swarm optimization
Authors: Qi Li , Weirong Chen , Junbo Jia , Thean Cham Yew , Ming Han
Published: February 2009

According to the modeling principle of proton exchange membrane fuel cell (PEMFC), adaptive focusing particle swarm optimization (AFPSO), which had preferable ability of global search and search rate was proposed to research a set of optimized parameters in the mechanism model. The comparison between experimental results and simulation results demonstrates that AFPSO can make the simulation results fit the experiment data with higher precision and has manifest superiority and validity for estimating the model parameters. Hence AFPSO achieves significant effect for optimizing the model parameters of PEMFC.
Dislocation onset and nearly axial glide in carbon nanotubes under torsion
Authors: Zhang D.-B. , James R. D. , Dumitrică T.
Published: February 2009

The torsional plastic response of single-walled carbon nanotubes is studied with tight-binding objective molecular dynamics. In contrast with plasticity under elongation and bending, a torsionally deformed carbon nanotube can slip along a nearly axial helical path, which introduces a distinct (+1,−1) change in wrapping indexes. The low energy realization occurs without loss in mass via nucleation of a 5-7-7-5 dislocation dipole, followed by glide of 5-7 kinks. The possibility of nearly axial glide is supported by the obtained dependence of the plasticity onset on chirality and handedness and by the presented calculations showing the energetic advantage of the slip path and of the initial glide steps.
Optical transitions and multiphonon Raman scattering of Cu doped ZnO and MgZnO ceramics
Authors: Huso Jesse , Morrison John L. , Mitchell James , Casey Erin , Hoeck Heather , Walker Chris , Bergman Leah , Hlaing Oo W. M. , McCluskey M. D.
Published: February 2009

Cu doped ZnO and MgZnO ceramics were created via a process of cold pressing and annealing, and their optical properties and phonon dynamics were studied. It was found that the ceramics exhibit infrared absorption peak energies at 5783 and 5822 cm−1, indicative of intraband transitions in a substitutional Cu ion of oxidation state +2. The UV photoluminescence (PL) intensity of the ceramics was found to weaken significantly relative to an undoped sample. The low PL intensity is discussed in terms of the CuxZn1−xO alloy system and the indirect bandgap of the CuO end member, as well as in terms of the nonradiative Cu centers. Due to the weak PL, up to ten LO multiphonons were observed in the Raman spectra, pointing to a strong polaron coupling. The resonance behavior of the highest intensity mode was found to exhibit outgoing resonance characteristics.
Fourier synthesis predicting onset of the initial instability during directional solidification
Authors: Wang Zhijun , Wang Jincheng , Yang Gencang
Published: February 2009

With the combination of the Fourier synthesis method and the time-dependent linear stability analysis, a simple model was developed to describe the dynamic evolution of the initial interface instability. By using this simple model, the early instability stage of a planar interface during directional solidification of binary alloys is predicted and how the interface microstructure generates from the background infinitesimal fluctuations is also revealed. The results from this model are in good agreement with those from quantitative phase field simulations and previous experiments.
Enhanced flux pinning by BaZrO3 and (Gd,Y)2O3 nanostructures in metal organic chemical vapor deposited GdYBCO high temperature superconductor tapes
Authors: Chen Yimin , Selvamanickam Venkat , Zhang Yifei , Zuev Yuri , Cantoni Claudia , Specht Eliot , Paranthaman M. Parans , Aytug Tolga , Goyal Amit , Lee Dominic
Published: February 2009

We have formed BaZrO3 nanocolumns and (Gd,Y)2O3 nanoprecipitates in reel-to-reel metal organic chemical vapor deposition (MOCVD) processed (Gd,Y)Ba2Cu3O7−x coated conductors and increased the critical currents (Ic) of the conductors in applied magnetic fields to remarkable levels. A (Gd,Y)Ba2Cu3O7−x tape of 1 m in length with 6.5% Zr-additions and 30% composition rich in both Gd and Y showed Ic values of 813 A/cm width at (self-field, 77 K) and above 186 A/cm width at (1 T, 77 K). The strongly enhanced flux pinning over a wide range of magnetic field orientations can be attributed to the bidirectionally aligned defect structures of BaZrO3 and (Gd,Y)2O3 created by optimized MOCVD conditions.
High-pressure induced phase transitions of Y2O3 and Y2O3:Eu3+
Authors: Wang Lin , Pan Yuexiao , Ding Yang , Yang Wenge , Mao Wendy L. , Sinogeikin Stanislav V. , Meng Yue , Shen Guoyin , Mao Ho-kwang
Published: February 2009

We investigated high-pressure induced phase transitions in Y2O3 and Eu-doped Y2O3 (Y2O:Eu3+) using angular dispersive synchrotron x-ray diffraction, Raman spectroscopy, and photoluminescence (PL). With increasing pressure, we observed a series of phase transformations in Y2O3:Eu3+, which followed a structure sequence of cubic→monoclinic→hexagonal, while Y2O3 followed a sequence of cubic→hexagonal. During decompression, both hexagonal structured Y2O3 and Y2O3:Eu3+ transformed into monoclinic phases which were quenchable back to ambient pressure. Raman and PL measurements shed additional light on the different phase transition behavior in these two samples.
Difference in the nonlinear optical response of epitaxial Si on Ge(100) grown from SiH4 at 500 °C and from Si3H8 at 350 °C due to segregation of Ge
Authors: Valev V. K. , Leys F. E. , Caymax M. , Verbiest T.
Published: February 2009

The properties of epitaxial strained Si on Ge (001) grown from SiH4 at 500 °C and from Si3H8 at 350 °C have been investigated as a function of film thickness using second harmonic generation (SHG). A clear difference in the corresponding signal amplitude, for both the interface and the “bulk” contributions, is observed. After analysis of the nonlinear susceptibility tensor components, this difference is attributed to the segregation of Ge to the SiO2/Si interface. It is demonstrated that when employed in combination with more standard experimental techniques, SHG can be a valuable tool for probing and characterizing the SiO2/Si/Ge interfaces.
Room temperature GaAs exciton-polariton light emitting diode
Authors: Tsintzos S. I. , Savvidis P. G. , Deligeorgis G. , Hatzopoulos Z. , Pelekanos N. T.
Published: February 2009

Room temperature GaAs polariton emission is demonstrated under electrical injection. Temperature and angle-resolved electroluminescence measurements on a polariton light emitting diode clearly show the persistence of Rabi splitting and anticrossing behavior at temperatures as high as 315 K. We show that by increasing the number of quantum wells in the structure, the cutoff temperature for the strong coupling regime can be pushed beyond room temperature, in good agreement with theory. Our results suggest that optimally designed GaAs microcavities are perfectly suited for room temperature polaritronics.
Laser-generation of ultrasonic X-waves using axicon transducers
Authors: Passler K. , Nuster R. , Gratt S. , Burgholzer P. , Paltauf G.
Published: February 2009

Photoacoustic and classical ultrasound imaging are both based upon ultrasonic waves but use different contrast mechanisms. For the development of a scanning acoustic microscope that uses both contrasts, an axicon transducer generating nondiffracting ultrasound, also called X-waves, by illumination with short laser pulses is investigated. Such a transducer provides simultaneously high depth of field and high lateral resolution. In this work, the spatial and temporal characteristics of the laser-generated X-waves are investigated using experiments and theoretical simulations. The experimental results reveal a characteristic spatial pulse width of 50 μm and a focal depth that complies well with the theoretical predictions.
Experimental two-dimensional field mapping of total internal reflection lateral beam shift in a self-collimated photonic crystal
Authors: Garcia-Pomar J. L. , Gollub J. N. , Mock J. J. , Smith D. R. , Nieto-Vesperinas M.
Published: February 2009

A lateral beam shift is demonstrated both theoretically and in microwave experiments when total internal reflection takes place at the boundary of a self-collimating two-dimensional photonic crystal consisting of an array of high index dielectric cylinders. We further show the dependence of this shift on the cut of the last row of cylinders that defines the crystal interface.
Thin-film characterization by terahertz time-domain spectroscopy using grating-assisted excitation of guided modes
Authors: Laamiri Y. , Garet F. , Coutaz J.-L.
Published: February 2009

Using a grating coupler, the guided modes of a dielectric slab can be efficiently excited by a terahertz beam delivered by a classical terahertz time-domain setup. By analyzing the variation in the transmission spectrum induced by a thin film deposited over the device, the refractive index as well as the absorption coefficient of the film material can be determined with a great accuracy because of the increased interaction length with the guided terahertz signal. The principle of such a technique is demonstrated in the case of a 7.1 μm thick film of Shipley 1828 photoresist.
An analytical solution for motion of an elliptical void under gradient stress field
Authors: Dong X. , Li Z.
Published: February 2009

We present a rigorous analytical solution for motion of an elliptical void under gradient stress field. An elliptical void will move to the region of high stress at a velocity inversely proportional to an equivalent void size, strongly affected by the shape parameter of the void, average stress, stress gradient, and stress states.
Nonlinear prism based on the natural ferroelectric domain structure in calcium barium niobate
Authors: Molina P. , Álvarez-García S. , Ramírez M. O. , García-Solé J. , Bausá L. E. , Zhang Huaijin , Gao Wenlan , Wang Jiyang , Jiang Minhua
Published: February 2009

We show the ability of calcium barium niobate as a nonlinear material to produce conical second harmonic generation in a broad frequency range. The nonlinear properties of this system are based on the broad sized microdomain distribution appearing naturally in the as-grown crystal. The large angular deflection obtained for conical second harmonic generation allows considering this system as a nonlinear prism with variations in the second harmonic generation angle as large as 35° in the wavelength range of 430–660 nm.
Band gap engineering of GaN nanowires by surface functionalization
Authors: Fang D. Q. , Rosa A. L. , Frauenheim Th. , Zhang R. Q.
Published: February 2009

We investigated [0001] bare and functionalized gallium nitride (GaN) nanowires by using the density-functional theory. Passivation of GaN nanowires with various functional groups (H, NH2, OH, and SH) show distinct electronic properties. We found that the band gap for the nanowires with partial surface coverage is dependent on the coverage ratio and adsorption sites. In view of the importance of surface states to the properties of nanowires, we suggest that the electronic and optical properties can be modulated by controlling the surface states of nanowires by functionalization.
Micellization Behaviour, DNA Binding, Antimicrobial, and Cytotoxicity Studies of Surfactant - Cobalt(III) Complexes Containing Di- and Tetramine Ligands
Authors: Kumar Rajendran Senthil , Arunachalam Sankaralingam , Periasamy Vaiyapuri S. , Preethy Christo P. , Riyasdeen Anvarbatcha , Akbarsha Mohammad A.
Published: February 2009

A new class of surfactant–cobalt(iii) complexes, cis-[Co(en)2(C11H23NH2)Cl](ClO4)2 (1), (en = ethylenediamine) and cis-[Co(trien)(C11H23NH2)Cl](ClO4)2 (2) (trien = triethylenetetramine) have been synthesized and characterized by UV/Vis, IR, 1H NMR, and 13C NMR spectroscopic methods and elemental and metal analysis. The critical micelle concentration (CMC) values of these surfactant–cobalt(iii) complexes in aqueous solution were obtained from conductance measurements. The specific conductivity data (at 298, 308, 318, and 328 K) served for the evaluation of the temperature-dependent CMC and the thermodynamics of micellization (ΔGm0, ΔHm0, and ΔSm0). Absorption spectroscopy, emission spectroscopy, and viscosity measurements have been used to investigate the binding of these complexes with calf thymus DNA (CT-DNA). The intrinsic binding constants (Kb) of complexes 1 and 2 were determined as 1.70 × 104 M–1 and 2.91 × 104 M–1, respectively, which suggests that complex 2 binds more strongly to CT-DNA than complex 1. These complexes were screened for their antimicrobial and cytotoxic activities against certain human pathogenic microorganisms and cervical cancer cells. The complexes showed moderate antibacterial and antifungal activities against certain selected microorganisms. The cytotoxic property of the complexes was tested on human cervical cancer cells, SiHa, adopting the 3-(4,5-di-methylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay and specific staining techniques. The complexes affected the viability of the cells significantly and the cell death was through apoptosis as seen in the changes in the nuclear morphology and cytoplasmic features.
Water-Mediated Synthesis of 2-Substituted Benzimidazoles by Boric Acid and Glycerol
Authors: Mukhopadhyay Chhanda , Tapaswi Pradip Kumar , Butcher Ray J.
Published: February 2009

A very simple, mild, and highly efficient green catalyst has been developed for the synthesis of 2-substituted benzimidazoles by treatment of substituted ortho-phenylenediamines and aldehydes in water at 80°C in the presence of boric acid (5 mol-%) and glycerol (0.05 mL). This methodology has been standardized on 55 substrates, and nine new compounds have been synthesized.
Vapour-Phase Polymerization of Pyrrole and 3,4-Ethylenedioxythiophene Using Iron(III) 2,4,6-Trimethylbenzenesulfonate
Authors: Subramanian Priya , Clark Noel , Winther-Jensen Bjorn , MacFarlane Douglas , Spiccia Leone
Published: February 2009

A new iron(iii) 2,4,6-trimethylbenzenesulfonate (MSA) with composition [Fe(OH2)5(MSA)3] has been prepared from the reaction of Fe(OH)3·xH2O and three molar equivalents of 2,4,6-trimethylbenzenesulfonic acid and used as oxidant in the preparation of highly conducting polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) films for the first time. PPy and PEDOT films grown on non-conducting overhead transparency (polyethylene terephthalate films) using a vapour phase polymerization technique exhibited very high conductivities; 200 ± 20 S cm–1 for PPy and 1000 ± 80 S cm–1 for PEDOT.