bluuebunny/crossref_metadata_2025 huggingface dataset:
Vascularized materials: Tree-shaped flow architectures matched canopy to canopy
Authors: Kim Sunwoo , Lorente Sylvie , Bejan Adrian
Published: October 2006

In this paper we develop flow architectures for “vascularizing” smart materials that have self-healing capabilities. The flow architectures are configured as two trees matched canopy to canopy. A single stream flows through both trees and bathes every subvolume (crack site) of the material. Several types of tree-tree configurations are optimized. Trees that have only one level of branching and bathe a rectangular domain have optimal external shapes that are nearly square. They also have optimal ratios of channel sizes before and after branching. Trees optimized on square domains perform nearly as well as trees on freely morphing rectangular domains. The minimized global flow resistance decreases slowly as the number of subvolumes increases. It is more beneficial to bathe the entire volume with a single (optimized) one-stream architecture than to bathe it with several streams that serve small clusters of volume elements. These conclusions are reinforced by an analytical optimization of the same class of architectures in the limit of a large number of assembled subvolumes. We also show that the freedom to morph the design and to increase its performance can be enhanced by using tree-tree architectures with more than one level of branching.
Correlation length and negative phase velocity in isotropic dielectric-magnetic materials
Authors: Mackay Tom G. , Lakhtakia Akhlesh
Published: October 2006

A composite material comprising randomly distributed spherical particles of two different isotropic dielectric-magnetic materials is homogenized using the second-order strong-property-fluctuation theory in the long-wavelength approximation. Whereas neither of the two constituent materials by itself supports plane wave propagation with negative phase velocity (NPV), the homogenized composite material (HCM) can. The propensity of the HCM to support NPV propagation is sensitive to the distributional statistics of the constituent material particles, as characterized by a two-point covariance function and its associated correlation length. The scope for NPV propagation diminishes as the correlation length increases.
Ruddlesden-Popper phases as thermoelectric oxides: Nb-doped SrO(SrTiO3)n (n=1,2)
Authors: Lee Kyu Hyoung , Kim Sung Wng , Ohta Hiromichi , Koumoto Kunihito
Published: October 2006

A class of materials known as superlattices has shown substantial promise for potential thermoelectric (TE) applications because of its low thermal conductivity. We have investigated natural superlattice Ruddlesden-Popper (RP) phases [S. N. Ruddlesden and P. Popper, Acta Crystallogr. 10, 538 (1957)] to elucidate their potential as TE materials. The TE properties of Nb-doped SrO(SrTiO3)n (n=1,2) with a RP structure were measured, and the origin of the TE properties is discussed from the viewpoint of the structure of the TiO6 octahedron. Compared with the cubic perovskite-type Nb-doped SrTiO3, the lattice thermal conductivity decreased by more than 50% (4.4–5Wm−1K−1) at room temperature and by 30% (1.9–2.2Wm−1K−1) at 1000K. There was a decrease in electrical conductivity owing to the randomly distributed insulating SrO layers in polycrystalline RP phases, and it was found that large TE power can be obtained in conjunction with high symmetry TiO6 octahedra. The largest dimensionless figure of merit (ZT), 0.14 at 1000K, was obtained in 5at.% Nb-doped SrO(SrTiO3)2.
Design and operating characteristics of a transient kinetic analysis catalysis reactor system employing in situ transmission Fourier transform infrared
Authors: Yang Yong , Disselkamp R. S. , Szanyi J. , Peden C. H. F. , Campbell C. T. , Goodwin J. G.
Published: October 2006

A novel apparatus for gas phase heterogeneous catalysis kinetics is described. The apparatus enables fast isotopic transient kinetic analysis (ITKA) to be performed in which both the gaseous and adsorbed species inside the catalytic reactor are monitored simultaneously with rapid-scan transmission Fourier transform infrared (FTIR), and its gaseous effluent can be monitored by mass spectroscopy during rapid switching of reagent gas streams. This enables a more powerful version of the well-known steady-state isotopic transient kinetic analysis (SSITKA) technique in which the vibrational spectra of the gas phase and adsorbed species are also probed: FTIR-SSITKA. Unique reactor characteristics include tungsten construction, liquid nitrogen cooling or heating (∼200–770K), pressures of 1.0–2.5atm, fast reactor disassembly and reassembly, and catalyst loading in a common volume. The FTIR data acquisition rate of this apparatus (3Hz) is tenfold faster than previously reported instruments. A 95% signal decay time of ∼3s for gas switching was measured. Very good temperature reproducibility and uniformity (<±3K) were observed by in situ rotational temperature analysis, which allows accurate calibration of the reactor thermocouple to the reactor gas temperature. Finally, FTIR-SSITKA capabilities are demonstrated for CO2 isotope switching over a γ-alumina sample at 75°C, which reveal an adsorbed carbonate species with an average surface residence time of τ=148±5s and a coverage of ∼2.5×1015moleculescm−2.
Particle transport and flow modification in planar temporally evolving laminar mixing layers. I. Particle transport under one-way coupling
Authors: Narayanan Chidambaram , Lakehal Djamel
Published: October 2006

Simulations of two-dimensional, particle-laden mixing layers were performed for particles with Stokes numbers of 0.3, 0.6, 1, and 2 under the assumption of one-way coupling using the Eulerian-Lagrangian method; two-way coupling is addressed in Part II. Analysis of interphase momentum transfer was performed in the Eulerian frame of reference by looking at the balance of fluid-phase mean momentum, mean kinetic energy, modal kinetic energy, and particle-phase mean momentum. The differences in the dominant mechanisms of vertical transport of streamwise momentum between the fluid and particle phases is clearly brought out. In the fluid phase, growth of the mixing layer is due to energy transfer from the mean flow to the unstable Kelvin-Helmholtz modes, and transport of mean momentum by these modes. In contrast, in the particle phase, the primary mechanism of vertical transport of streamwise momentum is convection due to the mean vertical velocity induced by the centrifuging of particles by the spanwise Kelvin-Helmholtz vortices. Although the drag force and the particle-phase modal stress play an important role in the early stages of the evolution of the mixing layer, their role is shown to decrease during the pairing process. After pairing, the particle-phase mean streamwise momentum balance is accounted for by the convection and drag force term. The particle-phase modal stress term is shown to be strongly connected to the fluid phase modal stress with a Stokes-number-dependent time lag in its evolution.
Automatic generation of force fields and property surfaces for use in variational vibrational calculations of anharmonic vibrational energies and zero-point vibrational averaged properties
Authors: Kongsted Jacob , Christiansen Ove
Published: October 2006

An automatic and general procedure for the calculation of geometrical derivatives of the energy and general property surfaces for molecular systems is developed and implemented. General expressions for an n-mode representation are derived, where the n-mode representation includes only the couplings between n or less degrees of freedom. The general expressions are specialized to derivative force fields and property surfaces, and a scheme for calculation of the numerical derivatives is implemented. The implementation is interfaced to electronic structure programs and may be used for both ground and excited electronic states. The implementation is done in the context of a vibrational structure program and can be used in combination with vibrational self-consistent field (VSCF), vibrational configuration interaction (VCI), vibrational Møller-Plesset, and vibrational coupled cluster calculations of anharmonic wave functions and calculation of vibrational averaged properties at the VSCF and VCI levels. Sample calculations are presented for fundamental vibrational energies and vibrationally averaged dipole moments and frequency dependent polarizabilities and hyperpolarizabilities of water and formaldehyde.
Path integral ground state study of finite-size systems: Application to small (parahydrogen)N (N=2–20) clusters
Authors: Cuervo Javier Eduardo , Roy Pierre-Nicholas
Published: October 2006

We use the path integral ground state method to study the energetic and structural properties of small para-H2 clusters of sizes ranging from 2to20molecules. A fourth order formula is used to approximate the short imaginary-time propagator and two interaction potentials are considered. Our results are compared to those of exact basis set calculations and other quantum Monte Carlo methods when available. We find that for all cluster sizes considered, our results show a lower ground state energy than literature values obtained by diffusion Monte Carlo and variational Monte Carlo. For the dimer and trimer, ground state energies are in good agreement with exact results obtained using the discrete variable representation. Structural properties are found to be insensitive to the choice of interaction potential. We explore the use of Pekeris coordinates to analyze the importance of linear arrangement in trimers and for trimers within clusters of larger size.
Effect of surface elasticity on the motion of a droplet in a viscous fluid
Authors: Felderhof B. U.
Published: October 2006

The motion of a droplet with adsorption layer in a viscous incompressible fluid is studied on the basis of the linearized Navier-Stokes equations. It is shown that dilatational elasticity of the layer has a strong effect on the decay of velocity after a sudden impulse. If the elasticity is sufficiently strong the droplet shows backtracking, i.e., during part of the time the velocity relaxation function becomes negative. The motion is independent of the surface shear modulus or surface shear viscosity. The friction coefficient of the droplet at zero frequency is the same as for a rigid sphere with stick boundary conditions, independent of the elasticity modulus.
Numerical evaluation of nuclide analysis of I129, Sr90, and Cs137 using bremsstrahlung high energy x ray generated by ultrashort pulse laser
Authors: Takashima Ryuta , Todoriki Masaru , Hasegawa Shuichi , Nemoto Koshichi , Kato Kazuyuki
Published: October 2006

We analyzed numerically the feasibility of nuclide analysis of spent fuels using an ultraintense, ultrashort laser. The (γ,n) reaction yields for each nuclide such as I129, Sr90, and Cs137 were calculated using the bremsstrahlung spectrum and the nuclear reaction cross sections. The dependence of the (γ,n) reaction yield on the irradiation time was derived. We also calculated the dependence of the nuclide ratio on the bremsstrahlung photon temperature. In order to investigate the influence of electron temperature and target thickness on the bremsstrahlung photon spectrum, the calculations were performed using the Monte Carlo code GEANT4. When gold was used as bremsstrahlung converter target at a temperature of the incident electrons of 30MeV, the target thickness providing maximum nuclear reaction yield was found to be 5mm.
Velocity distributions of Ba (S01, DJ3, D21, P13, and P11) and Ba+(P3∕22) produced by 1064nm pulsed laser ablation of barium in vacuum
Authors: Rossa Maximiliano , Rinaldi Carlos A. , Ferrero Juan C.
Published: October 2006

Various electronic states of Ba, from ground state up to 2.24eV (S01, DJ3, D21, P13, and P11) together with Ba+(P3∕22), were produced by 1064nm high-irradiance pulsed nanosecond laser ablation of Ba in vacuum. The velocity distribution for every species was obtained from time-of-flight measurements, using pulsed laser induced fluorescence or time-resolved optical emission spectroscopy, as applicable to each species. The distributions are bimodal, Maxwell-Boltzmann functions for S01, DJ3, and D21 and shifted Maxwell-Boltzmann for the rest of the states, with different peak velocities and average, hyperthermal translational temperatures. Possible mechanisms for the production of these velocity distributions are discussed.
Iodization of rf sputter induced disordered Ag thin films reveals volume plasmon-exciton “transition”
Authors: Bharathi Mohan D. , Sunandana C. S.
Published: October 2006

Quasiamorphous Ag films of thicknesses ranging from 5to30nm were prepared using rf magnetron sputtering technique and their controlled iodization was carried out for selected durations in the range of 15min–60h at room temperature. As deposited Ag and iodized films were characterized using x-ray diffraction (XRD), atomic force microscope (AFM), and optical absorption techniques. From XRD, γ and β+γ (mixed) phases of AgI nanoparticles have been observed for 5–10 and 20–30nm thick films, respectively. Lattice parameters (a and c) and average strain (ε) were calculated versus iodization time for γ and β-AgI nanoparticles. Uniform and nonuniform spherically shaped AgI nanoparticles (∼20–130nm) are realized through AFM for 5–10 and 20–30nm thick films. Optical absorption shows volume plasmons (classified as PR1) for short duration iodization, which “decay” upon further iodization to convert to Z1,2 and Z3 excitons at 420 and 330nm, respectively, in the manner of a metal-semiconductor/dielectric phase transition. Ag “colloidal” particles (classified as PR2) are formed for 5–10nm thick films and thereby control the γ phase—a significant and applicable effect attributed to critical film thickness. With increasing thickness, a surface strain field lifting the degeneracy of the valence band results in Z1,2 and Z3 exciton formation at room temperature. Blueshift in the exciton absorption with decreasing film thickness implies the progressive quantum confinement due to decrease in the particle size. A thickness induced phase transition from γ-AgI to β-AgI is discussed by means of x-ray diffraction and optical absorption studies.
The influence of incorporated nitrogen on the thermal stability of amorphous HfO2 and Hf silicate
Authors: Ikarashi Nobuyuki , Watanabe Koji , Masuzaki Koji , Nakagawa Takashi , Miyamura Makoto
Published: October 2006

We investigated the thermal stability of a N-incorporated amorphous Hf silicate film in terms of Hf diffusion in the film using high-angle annular-dark-field scanning transmission electron microscopy. We first examined HfxSi1−xO2 (x=0.5,1.0) films with and without N incorporation. Our analysis showed that N incorporation (15at.% of N) into the Hf0.5Si0.5O2 film significantly suppressed chemical component separation during annealing at 1000°C. In contrast, clear separation of Hf-rich and Hf-poor (SiO2-rich) regions occurred in the Hf0.5Si0.5O2 film without N incorporation. In addition, HfO2 crystalline particle formation was observed in the HfO2 films with and without N incorporation (25at.% of N). These results strongly suggest that Si–N bonding in the N-incorporated Hf0.5Si0.5O2 film, rather than Hf–N chemical bonding, is the main cause of the suppression of the chemical component separation and HfO2 crystallization. Second, we examined Hf diffusion in a SiO2 film with and without N incorporation and found that the N incorporation significantly reduced the Hf diffusion. We therefore infer that the suppression of the chemical component separation in the N-incorporated Hf silicate film can be explained in terms of the suppression of Hf diffusion by the Si–O, N network in the N-incorporated Hf silicate film.
Superconducting-quantum-interference-device array magnetometers with directly coupled pickup loop and serial flux dams
Authors: Wu Chiu-Hsien , Yang Hong-Chang , Chen Ji-Cheng , Chen Kuen-Lin , Chen M. J. , Jeng J. T. , Horng Herng-Er
Published: October 2006

In this work, we studied the engineering of high-transition-temperature superconductor Josephson junctions and superconducting quantum interference device (SQUID) by using step-edge or the bicrystal grain-boundary technologies. Serial Josephson junctions and bare SQUID array reveal high quality device characteristics. A high-Tc SQUID magnetometer exhibiting magnetic field sensitivity of 33fT∕Hz1∕2 in the white regime and 80fT∕Hz1∕2 at 1Hz was demonstrated by incorporating the flux dams and serial SQUID into the pickup loop of magnetometer. Furthermore, we demonstrate the opening of the flux dams by applying an external magnetic field to induce a current higher than the critical current of the serial flux dams. We show that the serial flux dams effectively suppress the low frequency 1∕f-like noises.
Stability of ion implanted single-walled carbon nanotubes: Thermogravimetric and Raman analysis
Authors: Adhikari Ananta R. , Huang Mengbing , Bakhru Hassaram , Vajtai Robert , Ryu Chang Y. , Ajayan Pulickel M.
Published: October 2006

In this work, the effect of different ions (hydrogen, helium, and neon) implanted on single-walled carbon nanotube (SWNT) is being analyzed using thermogravimetric analysis (TGA), Raman scattering, and x-ray photoelectron spectroscopy (XPS). The TGA result shows that the temperature for maximum decomposition rate (Tmax) increases at relatively low doses, i.e., by about 30°C after hydrogen ion implantation (at the ion dose of 1015cm−2), 17°C after helium ion implantation (at the ion dose of 1013cm−2), and contributes no significant enhancement after neon implantation for all doses. The increase of Tmax indicates that small mass ion can be utilized to improve the thermal-oxidative stability of SWNTs. Raman scattering and XPS were used to monitor the lattice damage from ion implantation and chemical bonding states of the materials. The results indicated the material rigidity for low doses of hydrogen and helium, while the application of higher doses of neon caused the material to transform towards amorphous carbon (a-C).
Microscopic and macroscopic thermal contact resistances of pressed mechanical contacts
Authors: Prasher Ravi S. , Phelan Patrick E.
Published: October 2006

Contact heat transfer at mechanical pressed contacts between two materials is very important in many applications. There are two types of thermal contact resistance at the interface of two solids. One of them is due to the constriction of heat flow lines at the interface, commonly known as thermal contact resistance, and the engineering literature has extensively dealt with this macroscopic phenomenon. The other type of constriction resistance is instead microscopic in nature. If the characteristic dimension of the constriction becomes comparable to the mean free path of the heat carriers (i.e., electrons and phonons), then there is a ballistic component to the constriction resistance. For different materials on the two sides, thermal boundary resistance due to phonon acoustic mismatch and to electron-phonon interaction in the case of metals becomes important. Here a unified model is developed which bridges the gap between the macroscopic constriction resistance and the microscopic contact resistance for pressed mechanical contacts. The model predictions are in good agreement with experimental data for a mechanically pressed Cu∕Si interface.
Three-dimensional electronic properties of multiple vertically stacked InAs∕GaAs self-assembled quantum dots
Authors: Kim J. H. , Woo J. T. , Kim T. W. , Yoo K. H. , Lee Y. T.
Published: October 2006

The microstructural properties and the shape of an InAs∕GaAs array grown by molecular beam epitaxy were studied using transmission electron microscopy (TEM) measurements, and the interband transitions were investigated by using temperature-dependent photoluminescence (PL) measurements. The shape of the InAs quantum dots (QDs) on the basis of the cross-sectional bright-field TEM image was modeled to be a convex-plane lens. The electronic subband energies and the wave functions were numerically calculated by using a three-dimensional finite-difference method, taking into account strain effects. The excitonic peaks corresponding to interband transitions from the ground electronic subband to the ground heavy-hole band (E1-HH1) in the multiple-stacked QDs, as determined from the PL spectra, were in reasonable agreement with the (E1-HH1) interband transition energies obtained from the results of the numerical calculations.
Annealing induced phase transformations in amorphous As2S3 films
Authors: Wang R. P. , Madden S. J. , Zha C. J. , Rode A. V. , Luther-Davies B.
Published: October 2006

Amorphous arsenic sulphide (As2S3) films prepared by ultrafast pulsed laser deposition have been vacuum annealed at a range of different temperatures. Measurements of the glass transition temperature indicate that a crystallization process initiates at annealing temperatures around 170°C. In combination with Raman scattering analysis, we conclude that phase separation is intrinsic for our as-deposited films. During annealing two sorts of phase transformation are identified: one between different amorphous polymorphs, and another from the amorphous to a crystalline state. We point out a correlation between these two types of transformation and two characteristic time scales identified from measurements of the relaxation of the refractive index, and explain the Arrhenius and non-Arrhenius behaviors leading to the observed temporal characteristics.
Unimolecular dissociation of the propargyl radical intermediate of the CH+C2H2 and C+C2H3 reactions
Authors: McCunn Laura R. , FitzPatrick Benjamin L. , Krisch Maria J. , Butler Laurie J. , Liang Chi-Wei , Lin Jim J.
Published: October 2006

This paper examines the unimolecular dissociation of propargyl (HCCCH2) radicals over a range of internal energies to probe the CH+HCCH and C+C2H3 bimolecular reactions from the radical intermediate to products. The propargyl radical was produced by 157nm photolysis of propargyl chloride in crossed laser-molecular beam scattering experiments. The H-loss and H2 elimination channels of the nascent propargyl radicals were observed. Detection of stable propargyl radicals gave an experimental determination of 71.5 (+5∕−10)kcal∕mol as the lowest barrier to dissociation of the radical. This barrier is significantly lower than predictions for the lowest barrier to the radical’s dissociation and also lower than calculated overall reaction enthalpies. Products from both H2+HCCC and H+C3H2 channels were detected at energies lower than what has been theoretically predicted. An HCl elimination channel and a minor C–H fission channel were also observed in the photolysis of propargyl chloride.
Steady flows of a laterally heated ferrofluid layer: Influence of inclined strong magnetic field and gravity level
Authors: Hennenberg M. , Weyssow B. , Slavtchev S. , Desaive Th. , Scheid B.
Published: October 2006

A horizontal ferrofluid layer is submitted to a lateral heating and to a strong oblique magnetic field. The problem, combining the momentum and heat balance equations with the Maxwell equations, introduces two Rayleigh numbers, Ra the gravitational one and Ram the magnetic one, to represent the buoyancy and the Kelvin forces, which induce motion, versus the momentum viscous diffusion and heat diffusion. Whatever the inclination of the magnetic field, the steady solution of the problem is presented as a power series of a small parameter ϵH measuring the ratio of variation of the magnetization across the layer divided by the magnitude of the external imposed field. For cases of physical relevance, comparisons between analytical and numerical studies have lead to a major statement: in the strong field region (ϵH≪1) the zero order solution is the product of the Birikh solution that corresponds to the usual Newtonian fluid submitted to a lateral gradient, multiplied by a modulating factor accounting for inclination and both Rayleigh numbers. Physically, this simplified solution is valid for microgravity conditions where the magnetic field competes enough with microgravity effects to invert the laminar flow and thus suppress the motion for two specific values of the inclination angle.
Extraction of Cu diffusivities in dielectric materials by numerical calculation and capacitance-voltage measurement
Authors: Kim Ki-Su , Joo Young-Chang , Kim Ki-Bum , Kwon Jang-Yeon
Published: October 2006

A rigorous method of obtaining the Cu diffusivities in various SiO2-based dielectric materials is proposed. The diffusion profile of Cu ions in a dielectric material is first simulated and the resulting flatband voltage shift (ΔVFB) is compared with the experimental results obtained by C-V measurements after bias-temperature stressing (BTS). The evolution of the Cu concentration in dielectric materials is evaluated using a one-dimensional finite differential method with two unknown parameters, the diffusivity, and the maximum solid solubility of Cu ions in the dielectric material. C-V measurements are conducted at 1MHz to measure the ΔVFB value of Cu∕SiO2∕Si capacitors before and after BTS at an electric field of +1.0MV∕cm and in the temperature range between 200 and 275°C. With this process, the Cu diffusivities in thermally grown SiO2, oxynitride and SiO2 deposited by plasma-enhanced chemical vapor deposition, and methyl-doped SiO2 are found to be 2.22×10−3exp(−1.54eV∕kT), 3.09×10−5exp(−1.34eV∕kT), 2.59×10−5exp(−1.18eV∕kT), and 6.07×10−9exp(−0.71eV∕kT), respectively.
Influence of laser intensity on absorption line broadening in laser absorption spectroscopy
Authors: Matsui Makoto , Komurasaki Kimiya , Ogawa Satoshi , Arakawa Yoshihiro
Published: October 2006

The influence of laser intensity on absorption line broadening was investigated. Laser absorption spectroscopy was applied to low-pressure plasma, and the translational temperature deduced from the Doppler width was found to increase with laser intensity; this was in contrast to the conventional laser theory. Consequently, the dependency of absorption saturation on the Doppler frequency was considered. The predicted variation in broadening width with laser intensity showed a good agreement with the measurement. In addition, we obtained a correction factor for the temperature measurement at high laser intensity.
Treelike branched structures formed in dewetting thin films of a binary solution
Authors: Govor L. V. , Reiter G. , Bauer G. H. , Parisi J.
Published: October 2006

The authors present evidence for treelike patterns which developed during solvent evaporation from a phase separated bilayer resulting from a binary polymer solution spin coated onto a solid substrate. Initially, a bilayer structure containing a poly(isobutyl methacrylate) (BMA) solution layer on top of a nitrocellulose (NC) solution layer. During subsequent solvent evaporation, the top BMA solution layer becomes unstable and transforms into short ridges. Finally, solvent evaporation from the NC solution layer connects the BMA ridges to treelike patterns.
Influence of shear stress fluctuation on bed particle mobility
Authors: Cheng Nian-Sheng
Published: October 2006

Whether or not a sediment particle is entrained from a channel bed is associated with both average bed shear stress and shear stress fluctuation, the latter being flow-dependent and also related to bed irregularities. In the first part of this study, a preliminary analysis of possible fluctuations induced by bed roughness is presented for the case of an immobile plane bed comprised of unisized sediments. The result shows that the roughness-induced variation is generally comparable to that associated with near-bed turbulence, and both variations can be approximated as log-normal in terms of probability density distribution. The bed particle mobility is then analyzed by considering the effects of shear stress fluctuations. The relevant computations demonstrate that with increasing shear stress fluctuations, the probability of the mobility of a bed particle may be enhanced or weakened. For the case of low sediment entrainment, the probability is increased by turbulence. However, the probability may be reduced by the shear stress fluctuation if the average bed shear stress becomes relatively higher. This study shows that the critical condition for initial sediment motion could be overestimated if the Shields diagram is applied for the condition of flows with high turbulence intensities.
Nanoprobes for near-field optical microscopy manufactured by substitute-sheath etching and hollow cathode sputtering
Authors: Chaigneau M. , Ollivier G. , Minea T. , Louarn G.
Published: October 2006

This article reports a new approach for probe manufacturing, which is the key component in scanning near-field optical microscope (SNOM). The wet-etching process, to create the tip at the apex of a tapered fiber, has been optimized. Typical tip features are short tapers, large cone angles (30°), and very small diameters (<50nm). Next process steps are performed in an original arrangement of plasma device, based on a modified hollow cathode discharge. It is used for both, to remove the dust particles or the etching residues from the tip surface and to coat the tapered region with a metallic ultrathin and compact film. To complete the probe’s fabrication, the tips are opened by dry electrolytic erosion. These probes have been successfully tested for SNOM applications.
Fabrication, assembly, and electrical characterization of CuO nanofibers
Authors: Wu Hui , Lin Dandan , Pan Wei
Published: October 2006

The authors report on the assembly and electrical properties of ultralong copper oxide (CuO) nanofibers prepared by electrospinning method. The prepared CuO nanofibers were polycrystalline, with diameter of ∼60nm and length over 100μm. By a specially designed fiber collector, single CuO nanofiber field-effect transistor (FET) was assembled directly. Electrical transport measurement was conducted on the nanowire FET device, showing that CuO nanofibers are intrinsic p-type semiconductor. Electrospinning is thus proposed as a simple and low-cost method of fabricating one-dimensional semiconductive ceramic FETs.
Self-organized critical gating of ion channels: On the origin of long-term memory in dwell time series
Authors: Brazhe A. R. , Maksimov G. V.
Published: October 2006

We present the model of an ion channel, operating in a regime of self-organized criticality. It is suggested that complex cooperative dynamics takes place in the protein and the overall tension of it facilitates an open or closed state of the rigid gates in the pore-making domain. For the first time multifractal spectra of ion channel dynamics are presented. Our model well reproduces the multifractal properties of ion channel dwell time series and provides an insight on the origin of the long-term correlations in these series.
Studies of improved electron confinement in low density L-mode National Spherical Torus Experiment discharges
Authors: Stutman D. , Finkenthal M. , Tritz K. , Redi M. H. , Kaye S. M. , Bell M. G. , Bell R. E. , LeBlanc B. P. , Hill K. W. , Medley S. S. , Menard J. E. , Rewoldt G. , Wang W. X. , Synakowski E. J. , Levinton F. , Kubota S. , Bourdelle C. , Dorland W. , Team The NSTX
Published: October 2006

Electron transport is rapid in most National Spherical Torus Experiment, M. Ono et al., Nucl. Fusion 40, 557 (2000) beam heated plasmas. A regime of improved electron confinement is nevertheless observed in low density L-mode (“low-confinement”) discharges heated by early beam injection. Experiments were performed in this regime to study the role of the current profile on thermal transport. Variations in the magnetic shear profile were produced by changing the current ramp rate and onset of neutral beam heating. An increased electron temperature gradient and local minimum in the electron thermal diffusivity were observed at early times in plasmas with the fastest current ramp and earliest beam injection. In addition, an increased ion temperature gradient associated with a region of reduced ion transport is observed at slightly larger radii. Ultrasoft x-ray measurements of double-tearing magnetohydrodynamic activity, together with current diffusion calculations, point to the existence of negative magnetic shear in the core of these plasmas. Discharges with slower current ramp and delayed beam onset, which are estimated to have more monotonic q-profiles, do not exhibit regions of reduced transport. The results are discussed in the light of the initial linear microstability assessment of these plasmas, which suggests that the growth rate of all instabilities, including microtearing modes, can be reduced by negative or low magnetic shear in the temperature gradient region. Several puzzles arising from the present experiments are also highlighted.
Focused-ion-beam fabricated vertical fiber couplers on silicon-on-insulator waveguides
Authors: Schrauwen Jonathan , Van Thourhout Dries , Baets Roel
Published: October 2006

Focused-ion-beam technology is a versatile micro- and nanostructuring tool, in which material is sputtered locally by a finely focused ion beam. But for photonic applications this process has the inherent inconvenience of generating optical losses due to ion induced damage. Process optimization is needed to reduce these losses. In this letter the authors present the focused-ion-beam fabrication of shallow gratings in silicon-on-insulator waveguides for the coupling of light to a vertically positioned fiber. Using Al2O3 as mask and I2 as selective etchant this resulted in a 24% fiber-to-chip coupling efficiency, which is comparable to similar devices fabricated with conventional etching techniques.
Mirror tunnel microscope
Authors: Ozcan Aydogan , Bilenca Alberto , Bouma Brett E. , Tearney Guillermo J.
Published: October 2006

A microscope paradigm, which the authors term the “mirror tunnel microscope” (MTM), is described. The MTM uses a low numerical aperture (NA) lens together with parallel mirrors positioned between the lens plane and the object plane to provide a relatively simple means for digital wide-field microscopy. In MTM, the mirror tunnel acts as a spatial periodic-bandpass filter, which creates low-resolution, bandpassed versions of the object function in the image plane. However, each low-resolution image formed by the MTM carries a unique band of spatial frequencies. Coherent addition of the phase and amplitude of the spatial frequency information contained in each of these low-resolution images enhances the effective NA of the lens without decreasing field of view. To demonstrate the proof of principle, they have utilized a two-mirror MTM to reconstruct an image of a pinhole. Along the axis perpendicular to the mirrors, the image was reconstructed with higher resolution, commensurate with an effective fivefold NA increase. Initial results indicate that MTM is a promising method for wide-field digital microscopy.
Gain structure optimization of vertical external cavity surface emitting laser at 920nm
Authors: Yoo Jaeryung , Kim Kisung , Lee Sangmoon , Lim Seongjin , Kim Gibum , Kim Junyoun , Cho Soohaeng , Lee Junho , Kim Taek , Park Yongjo
Published: October 2006

The authors have optimized a resonant gain structure of a 920nm vertical external cavity surface emitting laser. They found that long saturated carrier lifetime in shallow quantum well (QW) under a high injection level restricted the laser performance. An insertion of nonabsorbing layer in the middle of barrier layers with multi-QWs was very effective to reduce the saturated carrier lifetime and, therefore, to enhance the laser performance. With the optimized gain structure, which had ten periods of triple InGaAs QWs with Al0.3Ga0.7As nonabsorbing layers in the middle of GaAs barriers, they achieved 4.9W output power at 10°C.
An ignored but most favorable channel for NCO+C2H2 reaction
Authors: Xie Hong-bin , Wang Jian , Zhang Shao-wen , Ding Yi-hong , Sun Chia-chung
Published: October 2006

The NCO+C2H2 reaction has been considered as a prototype for understanding the chemical reactivity of the isocyanate radical towards unsaturated hydrocarbons in fuel-rich combustion. It has also been proposed to provide an effective route for formation of oxazole-containing compounds in organic synthesis, and might have potential applications in interstellar processes. Unfortunately, this reaction has met mechanistic controversy both between experiments and between experiments and theoretical calculations. In this paper, detailed theoretical investigations at the Becke’s three parameter Lee-Yang-Parr-B3LYP∕6-31G(d), B3LYP∕6-311++G(d,p), quadratic configuration interaction with single and double excitations QCISD∕6-31G(d), and Gaussian-3 levels are performed for the NCO+C2H2 reaction, covering various entrance, isomerization, and decomposition channels. Also, the highly cost-expensive coupled-cluster theory including single and double excitations and perturbative inclusion of triple excitations CCSD(T)/aug-cc-pVTZ single-point energy calculation is performed for the geometries obtained at the Becke’s three parameter Lee-Yang-Parr-B3LYP∕6-311++G(d,p) level. A previously ignored yet most favorable channel via a four-membered ring intermediate with allyl radical character is found. However, formation of P3 H+HCCNCO and the five-membered ring channel predicted by previous experimental and theoretical studies is kinetically much less competitive. With the new channel, master equation rate constant calculations over a wide range of temperatures (298–1500K) and pressures (10–560Torr) show that the predicted total rate constants exhibit a positive-temperature dependence and no distinct pressure dependence effect. This is in qualitative agreement with available experimental results. Under the experimental conditions, the predicted values are about 50% lower than the latest experimental results. Also, the branching ratio variations of the fragments P2 HCN+HCCO and P5 OCCHCN+H as well as the intermediates L1 HCHCNCO, r4 cCHCHNC–O, and L5 NCHCHCO are discussed with respect to the temperature and pressure. Future experimental reinvestigations are strongly desired to test the newly predicted channel for the model NCO+C2H2 reaction. Implications of the present results in various fields are discussed.
Intermittent transport in the scrape-off layer of the SINP tokamak
Authors: Saha S. K. , Chowdhury S.
Published: October 2006

Plasma structures, localized in the two-dimensional poloidal cross section propagating radially and poloidally, have been detected in the scrape-off layer of the SINP tokamak. Structures with densities both higher and lower than the density of the background plasma are present, with electron temperatures higher and lower, respectively, than that of the background plasma. These are responsible for the large and sporadic bursts of particle flux to the wall, amounting to 30%–40% of the total cross-field flux. These intermittent structures also cause the probability distribution function of the fluctuations to deviate from the Gaussian behavior, especially away from the limiter.
Derivation of time-dependent two-dimensional velocity field maps for plasma turbulence studies
Authors: Munsat T. , Zweben S. J.
Published: October 2006

Time-resolved two-dimensional (2D) velocity maps have been derived for fluctuation measurements in the edge plasma of the National Spherical Torus Experiment (NSTX). The maps have been derived from time sequences of 2D images recorded with the gas puff imaging diagnostic. A hybrid technique combining optical flow and local pattern matching has been implemented to overcome the individual limitations of each when used with data of limited temporal and/or spatial resolution. Local flow velocities of up to ∼8km∕s and average poloidal flow velocities of up to ∼5km∕s are found. Results are compared to previous velocity extraction techniques and NSTX results.
Development and validation of an experimental setup for the biaxial fatigue testing of metal thin films
Authors: Eve S. , Huber N. , Kraft O. , Last A. , Rabus D. , Schlagenhof M.
Published: October 2006

In this article, we describe a new experimental setup which has been developed to investigate the fatigue behavior of thin metal films on polymer substrates as used in micro-optical components. The setup is based on the ring-on-ring test and includes an optical in situ failure detection system. The method is validated for thin Au films by comparing the in situ results with microscopical analyses using scanning electron and atomic force microscopies.
Performance of carbon nanotube-dispersed thin-film transistors
Authors: Kumar S. , Blanchet G. B. , Hybertsen M. S. , Murthy J. Y. , Alam M. A.
Published: October 2006

A numerical technique that relies on modifying the organic semiconducting host with metallic carbon nanotubes (CNTs) to increase the transconductance or, equivalently, reduce effective channel length (Leff) has recently been proposed. The authors use an extensive set of experimental data to analyze the performance of these transistors using the theory of heterogeneous two-dimensional percolating networks of metal-semiconducting CNTs embedded in the organic host. Their analysis (i) reproduces experimental characteristics, (ii) shows that Leff scales as a power law of CNT-doping density (ρ), (iii) illustrates the importance of an active subpercolating network of semiconducting CNTs in an organic host, and (iv) establishes the upper limit of transistor count for an integrated circuit based on this technology as a function of ρ, on current (Ion), and circuit-failure probability (F).
Pressure sensing by flexible, organic, field effect transistors
Authors: Manunza I. , Sulis A. , Bonfiglio A.
Published: October 2006

A mechanical sensor based on a pentacene field effect transistor has been fabricated. The pressure dependence of the output current has been investigated by applying a mechanical stimulus by means of a pressurized air flow. Experimental results show a reversible current dependence on pressure. Data analysis suggests that variations of threshold voltage, mobility and contact resistance are responsible for current variations. Thanks to the flexibility of the substrate and the low cost of the technology, this device opens the way for flexible mechanical sensors that can be used in a variety of innovative applications such as e-textiles and robotic interfaces.
Multiferroic epitaxial Pb(Fe1∕2Nb1∕2)O3 thin films: A relaxor ferroelectric/weak ferromagnet with a variable structure
Authors: Yan Li , Li Jiefang , Suchicital Carlos , Viehland D.
Published: October 2006

The authors report the structural, ferroelectric, and ferromagnetic properties of Pb(Fe1∕2Nb1∕2)O3 epitaxial thin layers grown on (001), (110), and (111) SrTiO3 substrates by pulsed-laser deposition; films were of sufficient resistivity to enable high-field P-E measurements. Findings are as follows: epitaxial strain results in (i) a dramatic increase in the spontaneous polarization Ps; (ii) a lattice structure that is dependent on substrate orientation; (iii) a slim-loop P-E response and relaxor ferroelectric characteristics in the dielectric constant, both of which are nearly independent of crystallographic orientation; and (iv) a weak ferromagnetic moment, which is dependent on epitaxial mismatch.
Growth and photoluminescence studies of Al-rich AlN∕AlxGa1−xN quantum wells
Authors: Al Tahtamouni T. M. , Nepal N. , Lin J. Y. , Jiang H. X. , Chow W. W.
Published: October 2006

A set of AlN∕AlxGa1−xN (x∼0.65) quantum wells (QWs) with well width Lw varying from 1to3nm has been grown by metal organic chemical vapor deposition. Low temperature photoluminescence (PL) spectroscopy has been employed to study the Lw dependence of the PL spectral peak position, emission efficiency, and linewidth. These results have shown that these AlN∕AlGaN QW structures exhibit polarization fields of ∼4MV∕cm. Due to effects of quantum confinement and polarization fields, AlN∕AlGaN QWs with Lw between 2 and 2.5nm exhibit the highest quantum efficiency. The dependence of the emission linewidth on Lw yielded a linear relationship. The implications of our results on deep ultraviolet optoelectronic device applications are also discussed.
Single-mode submonolayer quantum-dot vertical-cavity surface-emitting lasers with high modulation bandwidth
Authors: Hopfer F. , Mutig A. , Kuntz M. , Fiol G. , Bimberg D. , Ledentsov N. N. , Shchukin V. A. , Mikhrin S. S. , Livshits D. L. , Krestnikov I. L. , Kovsh A. R. , Zakharov N. D. , Werner P.
Published: October 2006

Single-mode vertical-cavity surface-emitting lasers based on dense arrays of stacked submonolayer grown InGaAs quantum dots, emitting near 980nm, demonstrate a modulation bandwidth of 10.5GHz. A low threshold current of 170μA, high differential efficiency of 0.53W∕A, and high modulation current efficiency factor of 14GHz∕mA are realized from a 1μm oxide aperture single-mode device with a side mode suppression ratio of >40dB and peak output power of >1mW. The lasers are also suitable for high temperature operation.
Simulating temperature effects in the growth of tetrahedral amorphous carbon: The importance of infrequent events
Authors: Marks N. A. , Cover M. F. , Kocer C.
Published: October 2006

This thin-film deposition study of tetrahedral amorphous carbon shows that including infrequent processes on the millisecond scale substantially improves the accuracy of molecular dynamics simulations. Elevated temperature between energetic impacts is used to activate processes which are typically ignored. In agreement with experiment, the simulations show an abrupt transition in which diamondlike carbon transforms into vertically oriented graphitic sheets. The simulations also highlight the importance of infrequent events in combination with energetic impact. In the absence of the latter, the transition temperature is significantly higher, in good correlation with experiment.
Mechanisms of dislocation reduction in an Al0.98Ga0.02N layer grown using a porous AlN buffer
Authors: Bai J. , Wang T. , Parbrook P. J. , Cullis A. G.
Published: October 2006

Greatly improved optical properties of Al0.55Ga0.45N∕Al0.65Ga0.35N multiple quantum well (MQW) structures grown on the low dislocation density Al0.98Ga0.02N layer grown using a porous AlN buffer have been observed by photoluminescence measurement, in comparison with a MQW grown on a normal AlN layer. The mechanism for the dislocation reduction has been explored by transmission electron microscopy. Due to an attraction towards lateral growth fronts, the threading dislocations in the AlN layer bend during the overgrowth, but mainly by small angles, which is different from the 90° bending generally observed in classic epitaxial lateral overgrowth method. The dislocation reduction is realized through two main pathways: enhanced dislocation annihilation due to the small-angle line bending and the termination of dislocations at local free surfaces introduced by the nanoscale voids, produced around the interface between the Al0.98Ga0.02N and AlN layers.
Simultaneous growth of Ni5Ge3 and NiGe by reaction of Ni film with Ge
Authors: Nemouchi F. , Mangelinck D. , Bergman C. , Clugnet G. , Gas P. , Lábár J. L.
Published: October 2006

The reaction between nanometric Ni films and Ge is analyzed using isothermal x-ray diffraction measurements and transmission electron microscopy. It is found that NiGe is formed during deposition at room temperature. The metal rich phase that grows during heat treatment has been clearly identified to be Ni5Ge3. The simultaneous growths of Ni5Ge3 and NiGe have been observed on amorphous and polycrystalline germanium. This is in contrast with the usual sequential growth reported in thin films.
Visible quantum cutting through downconversion in green-emitting K2GdF5:Tb3+ phosphors
Authors: Lee Te-Ju , Luo Li-Yang , Diau Eric Wei-Guang , Chen Teng-Ming , Cheng Bing-Ming , Tung Chien-Yueh
Published: October 2006

Visible quantum cutting under excitations at 212 and 172nm in a green-emitting phosphor K2GdF5:Tb3+ (11%) via a downconversion mechanism is investigated. The authors measured the vacuum ultraviolet (VUV) excitation and emission spectra and proposed mechanisms to rationalize the quantum-cutting effect. One short-UV or one VUV photon absorbed by Tb3+ is split into multiple visible photons emitted by Tb3+ through cross relaxation and direct energy transfer. Calculations indicate an optimal quantum efficiency as great as 189% for this phosphor.