bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of functionalized pentacene field-effect transistors and its logic gate application
Authors: Park Jin Gyu , Vasic Relja , Brooks James S. , Anthony John E.
Published: September 2006

Functionalized pentacene, 6,13-bis(tri-isopropylsilylethynyl)pentacene (TIPS-pentacene), field-effect transistors (FETs) were made by both thermal evaporation and solution deposition methods, and the mobility was measured as a function of temperature and intensity of incident illumination. The field-effect mobility (μFET) has a gate-voltage dependent activation energy. A nonmonotonic temperature dependence was observed at high gate voltage (VG<−30V) with an activation energy of Ea∼60–170meV, depending on the fabrication procedure. The gate-voltage dependent mobility and nonmonotonic temperature dependence indicate that shallow traps play important role in the transport of TIPS-pentacene films. The current in the saturation regime as well as the mobility increase upon light illumination in proportion to the light intensity, mainly due to the photoconductive response. Transistors with submicron channel length showed unsaturating current-voltage characteristics due to the short channel effect. Realization of simple circuits such as NOT (inverter), NOR, and NAND logic gates are demonstrated for thin film TIPS-pentacene transistors.
Evolution of Barkhausen jumps in annealed amorphous wire having vanishing values of magnetostriction
Authors: Badini Confalonieri G. A. , Davies H. A. , Gibbs M. R. J.
Published: September 2006

A detailed study of the dc hysteresis loops for current and furnace annealed amorphous wires with vanishing saturation magnetostriction constant, λs, is reported. Measurements were conducted on wires of nominal compositions (Co0.94Fe0.06)72.5Si12.5B15 and (Co0.93Fe0.07)72.5Si12.5B15 which had λs values of similar magnitude but opposite sign. The results revealed the establishment and evolution of Barkhausen jumps at different points on the major hysteresis loops. These are interpreted on the basis of the core-shell domain model, widely used in the study of amorphous magnetic wires. The control of these jumps is essential for applications of wires where magnetic noise may be an issue. The influence of annealing conditions on the coercivity and susceptibility of the wires is also reported.
Enhanced low field magnetoresistance of Fe3O4 nanosphere compact
Authors: Song P. Y. , Wang J. F. , Chen C. P. , Deng H. , Li Y. D.
Published: September 2006

Unusually large low field magnetoresistance (LFMR), ∼10%, at 300K has been observed with the sample of monodispersed Fe3O4 magnetite nanospheres, ∼200nm, compactly cold pressed and sintered at 800°C. A detailed analysis on the transport and magnetic measurements indicates that the electron conduction is dominated by the spin-dependent scattering or tunneling at the grain boundaries. At low temperatures, 140 and 100K near the Verwey transition, ∼115K, the LFMR (below 2kOe) does not show any sign of dependence on the transition and does not follow the variation of magnetization to reach the saturation region either. On the other hand, at 300K, the MR saturates fast with the magnetization below 2kOe. This temperature dependent property in LFMR is very likely attributed to the scattering or tunneling of the conduction electron passing through the grain boundary layer with spin disordered state.
The valence state of Yb ion in YbInAu2 compound at high pressure determined by x-ray diffraction and x-ray absorption near edge structure measurements
Authors: Fuse A. , Nakamoto G. , Ishimatsu N. , Kurisu M.
Published: September 2006

X-ray diffraction patterns and LIII-edge x-ray absorption near edge structure (XANES) spectra of YbInAu2 and LuInAu2 compounds have been measured at high pressure and room temperature using a diamond anvil cell and a synchrotron radiation at SPring-8. YbInAu2 is more compressible at pressures lower than 4GPa than above it; the evaluated bulk modulus by Birch-type equation of state is 54.7GPa which is one-half of that of the LuInAu2 (112.5GPa). The mean valence v¯ of Yb ion in YbInAu2 determined by the XANES measurement is an increasing function of pressure: 2.71(2) at normal pressure and 2.94(2) at 10GPa. The rate of increase in v¯ with pressure is two times larger at pressures below 4GPa than that above 4GPa. However, the v¯ is described by a linear increasing function of the lattice compression: v¯=v¯0+2.9∣ΔV∕V0∣ where v¯0 is 2.71. The extrapolation to the trivalent state gives the critical pressure of 13GPa.
Dot size dependence of magnetic properties in microfabricated L10-FePt (001) and L10-FePt (110) dot arrays
Authors: Seki T. , Shima T. , Yakushiji K. , Takanashi K. , Li G. Q. , Ishio S.
Published: September 2006

L 1 0 - Fe Pt (001) and L10-FePt (110) dot arrays with well-defined geometry were fabricated through the use of electron beam lithography and Ar ion etching. The lateral size of dots was varied in the range from 0.2×0.2to5×5μm2. Coercivity (Hc) for the perpendicularly magnetized FePt (001) dots increases with decreasing the dot size. In the case of the FePt (110) dots with in-plane magnetization, on the other hand, the dot size dependence of Hc is completely different from that for FePt (001) dots: Hc shows a slight decrease as the dot size decreases. After annealing at 600°C, the values of Hc for both FePt (001) and FePt (110) dots are remarkably enhanced although the dot size dependence shows similar behavior to that before annealing. The magnetization reversal for all the dots occurs through the nucleation of reversed domains and subsequent domain wall propagation.
Ion-beam characteristics of the metal cluster complex of Ir4(CO)12
Authors: Fujiwara Yukio , Kondou Kouji , Teranishi Yoshikazu , Nonaka Hidehiko , Fujimoto Toshiyuki , Kurokawa Akira , Ichimura Shingo , Tomita Mitsuhiro
Published: September 2006

Tetrairidium dodecacarbonyl, Ir4(CO)12, is a metal cluster complex which has a molecular weight of 1104.9. Using a metal-cluster-complex ion source, it has been demonstrated that stable ion beams of Ir4(CO)7+ were produced. Energy dependence of sputtering yield of silicon bombarded with Ir4(CO)7+ ions was investigated at a beam energy from 2to10keV at normal incidence. Experimental results showed that the sputtering yield varied substantially with beam energy. The sputtering yield at 10keV was higher than that with SF5+ or Ar+ ions by a factor of 3–24, whereas the sputtering yield at 3keV was lower than that with Ar+ ions. In the case of 2keV, deposition was found to occur. The substantial variation in the sputtering yields was examined using empirical equations for calculating sputtering yields. It was shown that the high sputtering yield at 10keV would be due to what is called “nonlinear effect” unique to complex-projectile bombardment. It was also indicated that the substantial variation in the sputtering yield would result from lower kinetic energies of each atom constituting the cluster ions. Further, the deposition was explained by considering changes in surface properties caused by the irradiation of the cluster ions.
Electronic transport mechanism for nonalloyed Ti-based Ohmic contacts to n-AlGaN
Authors: Jang Ja-Soon , Seong Tae-Yeon , Jeon Seong-Ran
Published: September 2006

We have investigated the carrier transport mechanism of nonalloyed Ti-based Ohmic contacts to n-AlGaN using current-voltage-temperature and the specific contact resistance-temperature relations. It is shown that the theoretical result obtained using the field emission mode through the shallow-donor-assisted tunneling model is in agreement with the experimental data, although there is a small deviation below 383K. The deviation is attributed to an increase of the self-compensation between donor-type and acceptorlike defects. This indicates that the electronic transport of the AlGaN contacts is sensitively dependent on Ga and oxygen/nitrogen related surface point defects.
Far-infrared upconversion imaging devices: Imaging characteristics and quantum efficiency
Authors: Wu L. K. , Shen W. Z.
Published: September 2006

We have carried out an investigation of imaging characteristics and quantum efficiency of far-infrared (FIR) semiconductor imaging devices. The realization of the FIR imaging employs the concept of photon frequency upconversion in GaAs homojunction interfacial work-function internal photoemission (HIWIP) FIR detectors integrated with GaAs∕AlGaAs near-infrared light-emitting diodes (LEDs). Satisfying images have been expected through the analysis of modulation transfer function of the system, where the FIR detector parameters play key roles in the low spatial frequency image contrast, while those of LEDs dominate in the high spatial frequency range. We have examined in detail the dependence of the quantum efficiency on the emitter layer number, thickness, doping concentration, and applied bias of the FIR detectors, as well as the active layer thickness and internal quantum efficiency of the LEDs. The present study has also yielded an optimal structure for the integrated HIWIP-LED FIR imaging devices.
Electrical and optical properties of Li-doped Ni–Si–O thin films
Authors: Zhao Rongjuan , Lin Yuan-Hua , Zhou Xisong , Li Ming , Nan Ce-Wen
Published: September 2006

The Li-doped Ni–Si–O thin films (LixNi0.90−xSi0.10O, x=0.0–0.05) have been prepared by a sol-gel spin-coating method on silicon and quartz glass substrates. Analysis of phase composition and microstructure indicates that the films consist of cubic NiO phase and amorphous phase as annealed at 700°C for 5min. With the Li-doping concentration increasing, the core-level photoemission spectra show that a loss of the double-peak structure of the Ni 2p3∕2 main line can be observed and current density of the films increases greatly, which should be ascribed to the increase of the hole carriers induced by the Li doping. Ultraviolet-visible absorption spectra indicate that the absorption edge of the films exhibits a redshift due to the related defects.
A first-principles model of birefringent porous silicon
Authors: Bonder Yuri , Wang Chumin
Published: September 2006

Optical properties of birefringent porous silicon (b-PSi) layers are studied by means of the density functional theory (DFT) within the local density approximation (LDA). A systematic study of crystalline silicon (c-Si) is performed in order to validate this DFT-LDA calculation of optical properties of semiconductors. In order to simulate b-PSi, elliptical columns of 1–4 atoms are removed from a c-Si supercell of 16 atoms in the [100] and [010] directions. The dangling bonds are saturated with hydrogen atoms. A geometry optimization is carried out to get the minimum energy configuration. The results of the refractive index (n) show an enhanced anisotropy and the difference Δn=n[11¯0]−n[001] agrees well with experimental data. In particular, measurements in p+ and p++ doped b-PSi samples are consistent with the results obtained in the limit cases of pore branches along the [001] direction and the perfectly straight pores, respectively.
A sensitive double quantum well infrared phototransistor
Authors: An Zhenghua , Ueda T. , Chen Jeng-Chung , Komiyama S. , Hirakawa K.
Published: September 2006

An infrared phototransistor (∼14.5μm) on a GaAs∕AlGaAs double quantum well (QW) heterostructure is studied. A confined upper QW behaves as a photoactive gate to a conducting channel formed by the lower QW. By properly biasing the narrow gates for isolating the upper QW island, the lateral tunneling rate of cold electrons on upper QW can be tuned and hence the lifetime of photocarriers on the QW island can be controlled. Associated with this controllable lifetime, photoresponse takes a sharp maximum, which reaches as high as ∼103A∕W. Analysis in terms of a simple model suggests that the peak response originates from the interplay∕trade-off between the lifetime of photocarriers and the efficiency of photodetection process. The photodetection efficiency substantially varies as a consequence of large band bending induced by the 300K thermal background radiation. The long (approximately millisecond order) and controllable lifetime in our device paves the way for future development of photon counters in the long wavelength range. In addition, our device has a good compatibility with standard GaAs integrated circuit technology.
Analysis of tunable photonic crystal directional couplers
Authors: Kosmidou Elissavet P. , Kriezis Emmanouil E. , Tsiboukis Theodoros D.
Published: September 2006

Tunable directional coupler structures based on triangular photonic crystal lattices are investigated using the finite difference time domain method. The infiltration of nematic liquid crystal materials into the coupler waveguides allows for the control of its properties through the application of external static electric fields, which reorient the nematic director. Strong dynamical shifting of the dispersion curves, along with the decoupling frequencies and the coupling coefficient, is demonstrated. Such features render this class of couplers suitable for a range of applications. The coupling lengths accomplished are quite short even when the interaction region between the two coupler branches is widened. Furthermore, operation as a channel interleaver in wavelength division multiplexing systems is explored, and it is revealed that by proper selection of the geometrical parameters a constant channel separation of 0.8nm is achieved, with an overall length of a few hundreds of microns in wide frequency ranges.
Domain structure during magnetization reversal of PtMn∕CoFe exchange bias micropatterned lines
Authors: Liedke Maciej Oskar , Potzger Kay , Bothmer Anne H. , Fassbender Juergen , Hillebrands Burkard , Rickart Marc , Freitas Paulo P.
Published: September 2006

The magnetic domain configuration and the magnetization reversal behavior of micropatterned exchange bias elements were investigated by means of magnetic force microscopy. In addition to the unidirectional anisotropy the shape anisotropy determines the overall magnetization reversal behavior. In order to modify the ratio between both anisotropy contributions, the exchange bias field strength was reduced by means of 5keV He+ ion irradiation. For the as-prepared samples, a monodomain magnetization state with the magnetization direction aligned along the exchange bias field direction was found regardless of the element shape. After irradiation the unidirectional anisotropy contribution is reduced and hence the previously homogeneous magnetization state breaks up into small domains with 360° domain walls in between. The appearance of these domain walls, which was mainly observed for the descending branch of the magnetization reversal, is found to depend strongly on the structure width and orientation.
Internal plasma potential measurements of a Hall thruster using xenon and krypton propellant
Authors: Linnell Jesse A. , Gallimore Alec D.
Published: September 2006

For krypton to become a realistic option for Hall thruster operation, it is necessary to understand the performance gap between xenon and krypton and what can be done to reduce it. A floating emissive probe is used with the Plasmadynamics and Electric Propulsion Laboratory’s High-speed Axial Reciprocating Probe system to map the internal plasma potential structure of the NASA-173Mv1 Hall thruster [R. R. Hofer, R. S. Jankovsky, and A. D. Gallimore, J. Propulsion Power 22, 721 (2006); R. R. Hofer, R. S. Jankovsky, and A. D. Gallimore, J. Propulsion Power22, 732 (2006)] using xenon and krypton propellant. Measurements are taken for both propellants at discharge voltages of 500 and 600V. Electron temperatures and electric fields are also reported. The acceleration zone and equipotential lines are found to be strongly linked to the magnetic-field lines. The electrostatic plasma lens of the NASA-173Mv1 Hall thruster strongly focuses the xenon ions toward the center of the discharge channel, whereas the krypton ions are defocused. Krypton is also found to have a longer acceleration zone than the xenon cases. These results explain the large beam divergence observed with krypton operation. Krypton and xenon have similar maximum electron temperatures and similar lengths of the high electron temperature zone, although the high electron temperature zone is located farther downstream in the krypton case.
Plane Poiseuille-Couette problem in micro-electro-mechanical systems applications with gas-rarefaction effects
Authors: Cercignani Carlo , Lampis Maria , Lorenzani Silvia
Published: September 2006

Rarefied gas flows in micro-electro-mechanical systems (MEMS) devices, calculated from the linearized Bhatnagar-Gross-Krook model equation [P. L. Bhatnagar, E. P. Gross, and M. Krook, Phys. Rev. 94, 511 (1954)], are studied in a wide range of Knudsen numbers. Both plane Poiseuille and Couette flows are investigated numerically by extending a finite difference technique first introduced by Cercignani and Daneri [J. Appl. Phys. 34, 3509 (1963)]. Moreover, a variational approach, applied to the integrodifferential form of the linearized Boltzmann equation [C. Cercignani, J. Stat. Phys. 1, 297 (1969)], is used to solve in a unified manner the plane Poiseuille-Couette problem by means of the computation of only one functional. General boundary conditions of Maxwell’s type have been considered, assuming both symmetric and nonsymmetric molecular interaction between gas-solid interfaces, in order to take into account possible differences in the accommodation coefficients on the walls of MEMS devices. Based on the analysis presented in this paper, an accurate database valid in the entire Knudsen regime can be created for the Poiseuille-Couette problem, to be used in micromechanical applications.
Band structure and reflectivity of omnidirectional Si-based mirrors with a Gaussian profile refractive index
Authors: Arriaga J. , Saldaña X. I.
Published: September 2006

Using the transfer matrix method we calculate the band structure for a one-dimensional photonic crystal consisting of alternating layers of two dielectric materials A and B with refractive indices nA and nB, respectively. The refractive index of layer A is constant and the refractive index for layer B varies according to the envelope of a Gaussian function. We find that under certain circumstances it is possible to obtain a 100% reflectivity for both polarizations (TE and TM) and any value of the incident angle of the electromagnetic waves. The interval of maximum reflectivity coincides with the photonic band gap of the structure. By an adequate selection of the parameters forming the structure, it is possible to tune the interval of frequencies with maximum reflectivity. This could be used in the fabrication of the so-called omnidirectional mirrors.
Gas-cluster targets for femtosecond laser interaction: Modeling and optimization
Authors: Boldarev A. S. , Gasilov V. A. , Faenov A. Ya. , Fukuda Y. , Yamakawa K.
Published: September 2006

A novel mathematical model for the investigations of a cluster formation process in a gas jet is presented, which enables us to obtain the detailed description of the spatial and temporal distributions of all cluster target parameters. In this model, a cluster target is considered as a two-phase medium, consisting of the continuous gas phase and the discrete condensed phase (clusters). The detailed nozzle geometry is also taken into account in this model. In order to confirm the advantage of the present model over a conventional model, a considerable amount of numerical computations has been carried out and the results are compared with the data obtained from Hagena’s theory [Rev. Sci. Instrum. 63, 2374 (1992)]. Based on the developed modeling, a three-staged nozzle, which cannot be modeled using the conventional model, is designed for the purpose of producing a sufficient amount of micron-sized clusters. The generation of unprecedented amount of keV x rays from the laser-cluster interaction experiments with this nozzle and their accurate intensity dependences on various experimental parameters support the adequateness of the nozzle design.
Dynamic proportional-integral-differential controller for high-speed atomic force microscopy
Authors: Kodera Noriyuki , Sakashita Mitsuru , Ando Toshio
Published: September 2006

In tapping mode atomic force microscopy, the cantilever tip intermittently taps the sample as the tip scans over the surface. This mode is suitable for imaging fragile samples such as biological macromolecules, because vertical oscillation of the cantilever reduces lateral forces between the tip and sample. However, the tapping force (vertical force) is not necessarily weak enough for delicate samples, particularly for biomolecular systems containing weak inter- or intramolecular interactions. Light tapping requires an amplitude set point (i.e., a constant cantilever amplitude to be maintained during scanning) to be set very close to its free oscillation amplitude. However, this requirement does not reconcile with fast scans, because, with such a set point, the tip may easily be removed from the surface completely. This article presents two devices to overcome this difficulty; a new feedback controller (named as “dynamic proportional-integral-differential controller”) and a compensator for drift in the cantilever-excitation efficiency. Together with other devices optimized for fast scan, these devices enable high-speed imaging of fragile samples.
Precision and accuracy of thermal calibration of atomic force microscopy cantilevers
Authors: Matei G. A. , Thoreson E. J. , Pratt J. R. , Newell D. B. , Burnham N. A.
Published: September 2006

To have confidence in force measurements made with atomic force microscopes (AFMs), the spring constant of the AFM cantilevers should be known with good precision and accuracy, a topic not yet thoroughly treated in the literature. In this study, we compared the stiffnesses of uncoated tipless uniform rectangular silicon cantilevers among thermal, loading, and geometric calibration methods; loading was done against an artifact from the National Institute of Standards and Technology (NIST). The artifact was calibrated at NIST using forces that were traceable to the International System of units. The precision and accuracy of the thermal method were found to be 5% and 10%, respectively. Force measurements taken with different cantilevers can now be meaningfully compared.
Comparison of hybrid Hall thruster model to experimental measurements
Authors: Scharfe Michelle K. , Gascon Nicolas , Cappelli Mark A. , Fernandez Eduardo
Published: September 2006

A two-dimensional hybrid particle-in-cell numerical model has been constructed in the radial-axial plane with the intent of examining the physics governing Hall thruster operation. The electrons are treated as a magnetized quasi-one-dimensional fluid and the ions are treated as collisionless, unmagnetized discrete particles. The anomalously high electron conductivity experimentally observed in Hall thrusters is accounted for using experimental measurements of electron mobility in the Stanford Hall Thruster. While an experimental mobility results in improved simulation of electron temperature and electric potential relative to a Bohm-type model, results suggest that energy losses due to electron wall interactions may also be an important factor in accurately simulating plasma properties. Using a simplified electron wall damping model modified to produce general agreement with experimental measurements, an evaluation is made of differing treatments of electron mobility, background gas, neutral wall interactions, and charge exchange collisions. Although background gas results in two populations of neutrals, the increased neutral density has little effect on other plasma properties. Diffuse neutral wall interactions are in better agreement with experimental measurements than specular scattering. Also, charge exchange collisions result in an increase in average neutral velocity of 11% and a decrease in average ion velocity of 4% near the exit plane. The momentum exchange that occurs during charge exchange collisions is found to be negligible.
Investigation of Ohmic mechanism for chlorine-treated p-type GaN using x-ray photoelectron spectroscopy
Authors: Chen Po-Sung , Lee Ching-Ting
Published: September 2006

To investigate the function and mechanism of oxidation, the surface of the chlorine-treated p-type GaN semiconductor was analyzed using x-ray photoelectron spectroscopy. The chlorinated surface treatment was performed by electrolyzing HCl chemical solution to generate HClO, which in turn could be used to oxidize the p-type GaN. The chlorinated surface treatment enhances the formation of GaOx on the GaN surface and removing GaOx layer from the surface thereafter leads to the creation of additional Ga vacancies. Consequently, more holes are generated as a result of the generated Ga vacancies. Therefore, a relatively higher Ohmic performance with a specific contact resistance of 6.1×10−6Ωcm2 can be obtained for Ni∕Au metal contact subsequently patterned on the chlorine-treated p-type GaN via the enhanced formation of GaOx.
Carbon segregation as a strain relaxation mechanism in thin germanium-carbon layers deposited directly on silicon
Authors: Garcia-Gutierrez D. I. , José-Yacamán M. , Lu Shifeng , Kelly D. Q. , Banerjee S. K.
Published: September 2006

We report experimental evidence for the segregation and preferential localization of C atoms at the surface and substrate interfaces in thin Ge1−xCx films deposited directly on Si (100). The results are interpreted in the context of C segregation providing a mechanism for strain relaxation. Four different experimental techniques, including energy-dispersive spectroscopy, electron energy loss spectroscopy (EELS), energy-filtering transmission electron microscopy, and secondary ion mass spectrometry, support our claims. The EELS analyses showed that the C bonding near the Ge1−xCx∕Si substrate interface presented a higher sp3 character than in the central region or at the surface. Two interpretations are given for this observation; one is that structural relaxation occurs when C atoms occupy substitutional sites in the Ge crystal closer to the Ge1−xCx∕Si substrate interface; the other is that the higher sp3 character of the C atoms might be an indication that C-containing tetrahedral interstitial complexes form at the interface (chemical relaxation).
Alternating high-voltage biasing for terahertz large-area photoconductive emitters
Authors: Turton David A. , Welsh Gregor H. , Carey John J. , Reid Gavin D. , Beddard Godfrey S. , Wynne Klaas
Published: September 2006

High-voltage biasing is necessary for efficient generation of terahertz radiation using large-area photoconductive emitters and for electric-field-oriented charge-transfer studies. Coherent detection of terahertz pulses allows ac biasing to be the basis of modulation for lock-in detection. Biasing emitters with an ac field also removes the need for a complete conduction path. The experimental advantages of this approach along with a simple resonant method of generating the high-voltage bias applicable to higher-repetition-rate (up to a few hundred kilohertz) regeneratively amplified systems are described.
Curie-Weiss law in thin-film ferroelectrics
Authors: Wang Biao , Woo C. H.
Published: September 2006

The stationary self-polarization field of a thin film in an open circuit is analytically solved for temperatures near the para-/ferroelectric transformation within the Ginzburg-Landau theory. For second-order ferroelectrics, or first-order ferroelectrics with a sufficiently large elastic self-energy of the transformation strain, the solution is real and stable, from which the corresponding electric susceptibility of the film can be derived. A Curie-Weiss-type relation of the permittivity is obtained for both the supercritical and subcritical temperature regimes near the transition. In the paraelectric state, the Curie parameter of the thin film is found to be independent of its thickness, whereas in the ferroelectric state, its magnitude decreases rapidly with decreasing film thickness.
Large eddy simulation of particle settling in the ocean mixed layer
Authors: Noh Y. , Kang I. S. , Herold M. , Raasch S.
Published: September 2006

The settling process of suspended particles in the turbulent flow was investigated by analyzing the Lagrangian motion of a large number of particles in the ocean mixed layer simulated by large eddy simulation (LES), focusing on the role of Langmuir circulation (LC). At the beginning, particles released near the surface are swept down rapidly following the downward jets in the presence of LC, contrary to the case of the horizontally uniform downward propagation in the absence of it. The strong vertical mixing by LC keeps the particle concentration almost uniform after the initial period in the presence of LC, while its mean concentration continues to decrease through particle settling. Under the influence of turbulence in the ocean mixed layer, the particle settling velocity W is always smaller than ws, and W∕ws decreases with decreasing ws∕u∗, where ws is the terminal velocity of a particle in the still fluid and u∗ is the frictional velocity. The presence of LC causes a further decrease of W ultimately, and the difference between the cases with and without LC is the largest at ws∕u∗∼1. The analysis of LES data reveals that particles spend more time in upward flows and that more particles tend to accumulate in the high vorticity region in the presence of LC. It suggests that particle trapping is more likely when the length scale of vortices is larger, which may lead to the decrease of W in the presence of LC. It was also found that the preferential concentration occurs away from the regions of high horizontal vorticity and divergence when ws∕u∗∼1.
Rheology of particle suspensions with low to moderate fluid inertia at finite particle inertia
Authors: Verberg Rolf , Koch Donald L.
Published: September 2006

The lattice Boltzmann method is applied to simulate the rheology of particle suspensions with low-to-moderate fluid inertia and a wide range of particle inertia. The viscous dissipation in a suspension of particles with a Maxwellian velocity distribution is shown to have a linear dependence on the Reynolds number similar to the Ergun correlation for the drag in a fixed bed of particles. Dynamic simulations of the flow of a suspension of elastic particles being sheared between two rough walls are used to determine the range of Reynolds and Stokes numbers for which a treatment of a homogeneous, sheared suspension based on kinetic theory and the simulated viscous dissipation can accurately describe the particle-phase kinetic energy and effective viscosity of the suspension. The dependence of the particle-phase slip velocity and the depletion of particles near the boundaries on the particle volume fraction and Stokes number is determined.
Streamline patterns and their bifurcations near a wall with Navier slip boundary conditions
Authors: Tophøj Laust , Møller Søren , Brøns Morten
Published: September 2006

We consider the two-dimensional topology of streamlines near a surface where the Navier slip boundary condition applies. Using transformations to bring the streamfunction in a simple normal form, we obtain bifurcation diagrams of streamline patterns under variation of one or two external parameters. Topologically, these are identical with the ones previously found for no-slip surfaces. We use the theory to analyze the Stokes flow inside a circle, and show how it can be used to predict new bifurcation phenomena.
An experimental analysis of the linear vibration of axisymmetric liquid bridges
Authors: Ferrera C. , Cabezas M. G. , Montanero J. M.
Published: September 2006

The linear vibration of axisymmetric liquid bridges of millimetric size was analyzed experimentally. The equilibrium shape and the evolution of the interface position were recorded by a high-speed video camera. To calculate the surface tension, the Theoretical Image Fitting Analysis method was adapted to process images of liquid bridges at equilibrium. An image processing method was developed to obtain both the position of the solid supports delimiting the liquid bridge and the interface deformation as a function of time. The former allows one to accurately measure the inertial force acting on the liquid bridge. The interface deformation was determined at the subpixel level, so that vibrations of very small amplitude could be analyzed. The results are compared with various theoretical approaches in the linear regime.
Effective slip and friction reduction in nanograted superhydrophobic microchannels
Authors: Choi Chang-Hwan , Ulmanella Umberto , Kim Joonwon , Ho Chih-Ming , Kim Chang-Jin
Published: September 2006

Enabled by a technology to fabricate well-defined nanogrates over a large area (2×2cm2), we report the effect of such a surface, in both hydrophilic and hydrophobic conditions, on liquid slip and the corresponding friction reduction in microchannels. The grates are designed to be dense (∼230nm pitch) but deep (∼500nm) in order to sustain a large amount of air in the troughs when the grates are hydrophobic, even under pressurized liquid flow conditions (e.g., more than 1bar). A noticeable slip (i.e., slip length of 100–200nm, corresponding to 20%–30% reduction of pressure drop in a ∼3μm high channel) is observed for water flowing parallel over the hydrophobic nanogrates; this is believed to be an “effective” slip generated by the nanostrips of air in the grate troughs under the liquid. The effective slip is clearer and larger in flows parallel to the nanograting patterns than in transverse, suggesting that the nanograted superhydrophobic surfaces would not only reduce friction in liquid flows under pressure but also enable directional control of the slip. This paper is the first to use nanoscale grating patterns and to measure their effect on liquid flows in microchannels.
A complementary mild-slope equation derived using higher-order depth function for waves obliquely propagating on sloping bottom
Authors: Hsu Tai-Wen , Lin Ta-Yuan , Wen Chih-Chung , Ou Shan-Hwei
Published: September 2006

In this paper a complementary mild-slope equation (CMSE) is derived in order to investigate the transformation of progressive waves obliquely propagating over the sloping bottom more realistically. We introduce a new depth function which includes the wave refraction and the influence of the bottom slope α, perturbed to the second-order in the integral equation. A new depth-integrated mild-slope equation is derived, by using the above mentioned depth function, to model a time-harmonic motion of small amplitude waves in varying water depth. The simulated results reveal that the proposed model provides a significant improvement in the calculation of the wavenumber and the group velocity at different bottom slopes. With the increasing bottom slope, the discrepancies in the reflection coefficient of Bragg scattering between the analytical solution and the one calculated from the conventional mild-slope equation (MSE) and the modified MSE (MMSE) are seen to steadily increase. The group velocity of the waves, when compared with the conventional MSE and MMSE, also shows its dependence on the bottom slope and wave propagating angle. The present model is observed to be quite efficient in taking into account the effect of steeper bottom slope.
On the development of three-dimensional vortex breakdown in cylindrical regions
Authors: Herrada M. A. , Fernandez-Feria R.
Published: September 2006

Three-dimensional and axisymmetric numerical simulations of the incompressible Navier-Stokes equations have been conducted to study the appearance and development of vortex breakdown in a family of columnar vortex flows in a straight pipe without wall friction. The numerical simulations show that the basic form of breakdown is axisymmetric, and a transition to helical breakdown modes is shown to be caused by a sufficiently large pocket of absolute instability inside the original axisymmetric “bubble” of recirculating flow. Depending on the values of the Reynolds and swirl parameters, two distinct unstable modes corresponding to azimuthal wave numbers n=+1 and n=+2 have been found to yield a helical or a double-helical breakdown mode, respectively. By means of a simple linear spatial stability analysis carried out in the sections of the pipe where the basic axisymmetric flow presents reverse flow, we have identified the frequencies and the dominant azimuthal wave numbers observed in the three-dimensional simulations.
Extending the collisional fluid equations into the long mean-free-path regime in toroidal plasmas. III. Parallel heat conduction
Authors: Shaing K. C.
Published: September 2006

It is illustrated that plasma transport processes in the direction of the magnetic field are local in the vicinity of the magnetic island in the long mean-free-path regime where the collisionality parameter ν* is larger than 10−2, and the width of the island is about 3% of the minor radius or smaller. This is because the plasma temperature variation on the magnetic surface that results from the magnetic reconnection is gentle. Both the electron and the ion parallel transport fluxes including parallel heat flow in the banana regime where ν*<1 are calculated using a model Coulomb collision operator that conserves momentum.
Inhomogeneity scale lengths in a magnetized, low-temperature, collisionless, Q-machine plasma column containing perpendicular-velocity shear
Authors: Reynolds E. W. , Koepke M. E. , Carroll J. J. , Shinohara S.
Published: September 2006

Radial inhomogeneity scale lengths for radial electric field, ion density, and magnetic-field-aligned (parallel) electron-drift velocity have been measured and interpreted in magnetized, low-temperature, collisionless plasma. The effect of a narrow layer of inhomogeneity in these parameters on the excitation of electrostatic ion-cyclotron waves is investigated. When the ion Larmor radius ρi is on the order of, or larger than, the half-width at half-maximum σr{Er} of the Gaussian-like, radially localized, radial electric-field profile Er(r), the radial profile of the azimuthal ion rotation velocity, measured using laser-induced fluorescence (LIF), has a peak that, because of finite-Larmor-radius effects, is significantly lower than the peak of the combined radial profile of the E×B and diamagnetic drift velocities. Results of an experimentally validated test-particle simulation are presented and applied using experimentally relevant electric-field profiles. Two experimental configurations are explored for which the ions enter into the electric field at different rates. In one configuration, the ions experience an effectively adiabatic increase in electric-field strength. In the other configuration, the increase in electric-field strength is effectively instantaneous. The simulation reproduces both the main features of the radial profile of LIF-measured ion flow and the observed density depletion in regions of relatively high plasma potential for experimental conditions in which no waves were observed. The density depletion is interpreted as resulting from the finite-Larmor-radius ion orbits in the presence of an inhomogeneous electric field with radial scale length σr{Er}≈ρi.
Influence of silane coupling agent on morphology and dielectric property in BaTiO3/polyvinylidene fluoride composites
Authors: Dang Zhi-Min , Wang Hai-Yan , Xu Hai-Ping
Published: September 2006

Surface of BaTiO3 particle was chemically modified using silane coupling agent (KH550) in order to improve its compatibility with polyvinylidene fluoride (PVDF) matrix polymer, and therefore, expectable microstructure and dielectric property of the BaTiO3/PVDF composites were acquired. Infrared spectra reveal an obvious interaction between BaTiO3 and PVDF induced by the addition of KH550 coupling agent, and the interaction was also confirmed by the observation of morphology of fractured surface of the BaTiO3/PVDF composite when the concentration of KH550 is around 1.0wt%. Crystal lattice parameter of BaTiO3 in the composite was also changed because of the interaction. Finally, increased dielectric constant in the PVDF matrix composite with BaTiO3 treated by 1.0wt% KH550 was found.
Compaction and dilation rate dependence of stresses in gas-fluidized beds
Authors: Moon Sung Joon , Kevrekidis I. G. , Sundaresan S.
Published: September 2006

A particle dynamics-based hybrid model, consisting of monodisperse spherical solid particles and volume-averaged gas hydrodynamics, is used to study traveling planar waves (one-dimensional traveling waves) of voids formed in gas-fluidized beds of narrow cross-sectional areas. Through ensemble-averaging in a co-traveling frame, we compute solid phase continuum variables (local volume fraction, average velocity, stress tensor, and granular temperature) across the waves, and examine the relations among them. We probe the consistency between such computationally obtained relations and constitutive models in the kinetic theory for granular materials which are widely used in the two-fluid modeling approach to fluidized beds. We demonstrate that solid phase continuum variables exhibit appreciable “path dependence,” which is not captured by the commonly used kinetic theory-based models. We show that this path dependence is associated with the large rates of dilation and compaction that occur in the wave. We also examine the relations among solid phase continuum variables in beds of cohesive particles, which yield the same path dependence. Our results both for beds of cohesive and noncohesive particles suggest that path-dependent constitutive models need to be developed.
Effects of vortex filaments on the velocity of tracers and heavy particles in turbulence
Authors: Bec Jérémie , Biferale Luca , Cencini Massimo , Lanotte Alessandra S. , Toschi Federico
Published: September 2006

The Lagrangian statistics of heavy particles and of fluid tracers transported by a fully developed turbulent flow are investigated by means of high resolution direct numerical simulations. The Lagrangian velocity structure functions are measured in a time range spanning about three decades, from a tenth of the Kolmogorov time scale, τη, up to a few large-scale eddy turnover times. Strong evidence is obtained that fluid tracer statistics are contaminated in the time range τ∊[1:10]τη by a bottleneck effect due to vortex filament. This effect is found to be significantly reduced for heavy particles which are expelled from vortices by inertia. These findings help in clarifying the results of a recent study by H. Xu et al. [Phys. Rev. Lett. 96, 024503 (2006)], where differences between experimental and numerical results on scaling properties of fluid tracers were reported.
Stability and evolution of one-dimensional relativistic solitons on the ion time scale
Authors: Lehmann G. , Laedke E. W. , Spatschek K. H.
Published: September 2006

The one-dimensional dynamics of trapped relativistic electromagnetic radiation, which appears during laser plasma interaction, is investigated within a relativistic fluid-Maxwell model. The modifications of plasma density due to trapped laser fields are considered for linear as well as circular polarizations. It is shown that standing (V=0) solitons are stable on the electron time scale. However, the stability region does not agree with the prediction from the Vakhitov-Kolokolov criterion. Ions always drive the standing solitons unstable, irrespective of the polarization. The stability of moving (0<V<c) solitons, which have been obtained in the literature as stationary solutions of the fluid-Maxwell equations including ion dynamics, is demonstrated. The problem of soliton generation is addressed. The time evolution of the so called post-solitons, which are generated behind a broad laser pulse propagating in underdense plasma, is analyzed. The effect of finite electron and ion temperatures is briefly discussed.
Influence of grain boundary scattering on the electrical properties of platinum nanofilms
Authors: Zhang Q. G. , Zhang X. , Cao B. Y. , Fujii M. , Takahashi K. , Ikuta T.
Published: September 2006

The electrical conductivity and temperature coefficient of resistance of polycrystalline platinum nanofilms have been investigated experimentally and theoretically. The results show that these electrical properties have been greatly reduced mainly by grain boundary scattering. By applying the theory of Mayadas and co-workers [Appl. Phys. Lett. 14, 345 (1969); Phys. Rev. B 1, 1382 (1970)] to predict the electrical conductivity and temperature coefficient of resistance with the same reflection coefficient, however, obvious discrepancies have been found. These discrepancies indicate that Drude’s relation for bulk metals cannot be applied directly in the nanosized grain interior of polycrystalline metallic films.
Vapor-liquid equilibrium of hexadecapolar fluids from a perturbation-based equation of state
Authors: Gámez Francisco , Lago Santiago , del Río Fernando , Benavides Ana Laura
Published: September 2006

In this work a numerically tractable expression for the interaction potential between two point hexadecapoles with octahedral symmetry and a molecular-based equation of state derived by perturbation theory for hexadecapolar fluids are presented. The polar system is modeled by square-well particles with a point hexadecapole with octahedral symmetry at their centers. This equation of state is analytical in the state variables and in the potential parameters and allows us to study the effects of the hexadecapolar moment strength on the thermodynamic properties and liquid-vapor phase diagram. The equation presented here is applied to the thermodynamics of sulfur hexafluoride and gives very good predictions for the saturation pressures and the vapor-liquid phase diagram.
Radiation effects on switching kinetics of three-dimensional ferroelectric capacitor arrays
Authors: Courtade L. , Muller Ch. , Andreoli G. , Turquat Ch. , Goux L. , Wouters D. J.
Published: September 2006

Effects of x-ray irradiation combined with either bias voltage or bipolar electrical cycling were analyzed on three-dimensional ferroelectric capacitor arrays developed for highly reliable ferroelectric random access memories. Using dedicated experimental setup enabling in situ measurements, switching kinetics were followed over several hours of exposure. Polarization changes and voltage shifts were interpreted in considering interactions between trapping of photoinduced charges and ferroelectric domain structure, which depends upon the capacitor geometry and external field. In memorylike arrays in “written” or “writing” states, high dose of x rays accelerates fatiguelike (polarization reduction) and/or imprintlike (voltage shift) mechanisms that may perturb normal memory operations.
Charge trapping in polymer transistors probed by terahertz spectroscopy and scanning probe potentiometry
Authors: Lloyd-Hughes J. , Richards T. , Sirringhaus H. , Castro-Camus E. , Herz L. M. , Johnston M. B.
Published: September 2006

Terahertz time-domain spectroscopy and scanning probe potentiometry were used to investigate charge trapping in polymer field-effect transistors fabricated on a silicon gate. The hole density in the transistor channel was determined from the reduction in the transmitted terahertz radiation under an applied gate voltage. Prolonged device operation creates an exponential decay in the differential terahertz transmission, compatible with an increase in the density of trapped holes in the polymer channel. Taken in combination with scanning probe potentionmetry measurements, these results indicate that device degradation is largely a consequence of hole trapping, rather than of changes to the mobility of free holes in the polymer.
Thermal instability caused by plasma-wall coupling
Authors: Krasheninnikov S. I. , Soboleva T. K.
Published: September 2006

Both analytical study and numerical modeling show that the interactions of plasma with the first wall saturated with hydrogen can cause thermal instability resulting in massive desorption of gas from the wall. In a tokamak it can lead to the formation of multifaceted asymmetric radiation from the edge (MARFE), detachment, or even disruption.