bluuebunny/crossref_metadata_2025 huggingface dataset:
Transient radiation responses of silica-based optical fibers: Influence of modified chemical-vapor deposition process parameters
Authors: Girard S. , Ouerdane Y. , Boukenter A. , Meunier J.-P.
Published: February 2006

We characterized the behaviors of eight prototype single-mode optical fibers, made by the modified chemical-vapor deposition process, under pulsed x-ray (∼1MeV) irradiation. For this purpose, we measured the time-dependent changes (10−6–102s) in the radiation-induced attenuation at 1.55 and 1.31μm after exposure to an x-ray pulse. By using a dedicated set of prototype germanosilicate fibers with carefully designed process parameters, we show the predominant impact on their vulnerability of the two codopants (germanium and phosphorus) incorporated in their claddings (∼0.3Wt%). Compared to these influences on the radiation-induced loss levels and recovery kinetics, the impacts of the preform deposition temperature and of the fiber drawing tension on the fiber radiation sensitivity are less important. However, our results show that lowering the standard preform deposition temperature from 2000 to 1600 °C and the drawing tension from 140 to 20 g slightly decreases the induced losses at both wavelengths. We propose some hypotheses on the radiation-induced defects and physical mechanisms at the origin of these influences.
Influence of the superfluid transition on the adsorption of thin helium films
Authors: Syvokon V. E.
Published: February 2006

An approach to the calculation of the chemical potential of a thin helium film in equilibrium with the vapor is proposed. In the calculation of the chemical potential the surface layer of helium atoms in contact with the vapor is treated as a two-dimensional system. The chemical potential of superfluid helium is approximated by the chemical potential of an ideal two-dimensional Bose gas, while the chemical potential of normal helium contains an additional, temperature-dependent term. This approach leads to the prediction that the thickness of the film will increase at the transition to the superfluid state. It is also predicted that a coexistence of normal and superfluid films of different thickness is possible under certain conditions.
Features of the reflection of a bulk TM electromagnetic wave on a nongyrotropic multiferroic slab
Authors: Savchenko A. S. , Tarasenko S. V. , Tarasenko T. N. , Primak K. N.
Published: February 2006

The anomalies arising in the reflection of a bulk TM wave from the surface of a slab of a nongyrotropic multiferroic on account of anisotropy of the magnetoelectric interaction are investigated for the example of a ferroelectric antiferromagnet in the collinear phase under the condition that the ferroelectric axis is perpendicular to the interface between the media.
Features of the Shubnikov–de Haas oscillations of the conductivity of a high-mobility two-dimensional hole gas in a SiGe∕Ge∕SiGe quantum well
Authors: Komnik Yu. F. , Berkutov I. B. , Andrievskiĭ V. V. , Mironov O. A. , Myronov M. , Leadley D. R.
Published: February 2006

The Shubnikov–de Haas oscillations in a two-dimensional hole gas in a quantum well of pure germanium in a SiGe∕Ge∕SiGe heterostructure with a hole concentration pH=5.68×1011cm2 and mobility μ=4.68×104cm2V−1s−1 are investigated in magnetic fields up to 15 T at temperatures from 40 mK to 4 K. The observed deviation from the known relation describing the conductivity oscillations in the Shubnikov–de Haas effect are explained by additional broadening of the Landau levels due to the existence of a nonuniform distribution of the concentration of charge carriers, and, accordingly, of their energy, in the plane of the two-dimensional gas. It is assumed that the latter is due to natural atomic-step variations of the well width. The effective hole mass (m*=0.112m0) is determined from the temperature dependence of the oscillation amplitude, and its dependence on magnetic field is used to determine the quantum scattering time and the value of the carrier concentration fluctuations.
One-dimensional and quasi-one-dimensional electron systems in nanochannels
Authors: Zueva T. I. , Kovdrya Yu. Z. , Sokolov S. S.
Published: February 2006

A study is made of the energy spectrum of electrons localized in cylindrical nanochannels coated by a thick layer of solid hydrogen or neon. The interaction potential of an electron with the surrounding matter is determined. An algorithm, which includes both analytic and numerical calculations, is devised for solving the wave equation. The dependence of the electron energy on the radius of the channel is investigated, and the electron wave functions are determined. The influence of a helium film on the energy state of the electron is considered. It is shown that at channel radii of the order of 50Å the localization of the electron inside the channel is energetically favorable.
Distribution of field-induced charges in C60 fullerite
Authors: Kuprievich V. A. , Kapitanchuk O. L. , Shramko O. V. , Kudritska Z. G.
Published: February 2006

The profile of injected charges in a C60-based field-effect transistor (FET) is considered. A simple scheme for calculations of the charge distribution between the 2D layers of C60 molecules is founded on a small magnitude of the interball electron hopping. Analytical solutions of the equations for the charge distributions are obtained in the limits of thick and thin crystals. The charge density is shown to drop exponentially with the crystal depth. The calculations predict the relative part of induced charges involved in the surface layer to be 3∕4 and 2∕3 in the cases of electron and hole injection, respectively.
Characterization of 6H-SiC surfaces after ion implantation and annealing using positron annihilation spectroscopy and atomic force microscopy
Authors: Brauer G. , Anwand W. , Skorupa W. , Brandstetter S. , Teichert C.
Published: February 2006

Systematic slow positron implantation spectroscopy (SPIS) and atomic force microscopy studies of various 6H-SiC samples are presented to clarify the role of conductivity type, crystal quality, ion implantation (B+,Al+, and N+), and annealing (1.650 °C) in the formation of continuous long furrows running in one direction across the wafer surface. It is found that the observed changes in surface morphology are primarily the result of step bunching during thermal activation and thus occur regardless of conductivity type, crystal quality, and type of ion implantation. On terraces separating the step bunches, stripelike islands with a discrete height in the nanometer range have been observed which may have some link with the ion implantation chosen. SPIS results clearly indicate the formation of vacancy clusters in n-type material which are connected with the mobility of nitrogen in the samples at elevated temperatures. It is found that defect profiling by SPIS is not influenced by the changes in surface morphology observed due to annealing.
Optical properties of AlGaN∕GaN multiple quantum well structure by using a high-temperature AlN buffer on sapphire substrate
Authors: Bai J. , Wang T. , Comming P. , Parbrook P. J. , David J. P. R. , Cullis A. G.
Published: February 2006

We demonstrated highly improved optical properties of the AlGaN∕GaN multiple quantum well (MQW) structure grown on a GaN layer with significant reduction of dislocations achieved using an AlN buffer directly grown on a sapphire substrate at high temperature, in comparison with the MQW grown using a conventional two-step technique. An enhancement of room-temperature photoluminescence intensity by up to one order of magnitude has been observed, compared with the conventional MQW. Transmission electron microscopy measurements indicated that the dislocation density of the GaN grown using the AlN buffer technique was dramatically reduced. Furthermore, the significantly improved optical properties of this MQW using this AlN buffer technique were also demonstrated by increasing the thickness of the GaN layer underneath, whose crystal quality can be improved by increasing thickness, confirmed by detailed x-ray measurements. Since the technique can significantly improve the optical efficiency in the ultraviolet (UV) spectral region without involvement of any ex situ patterning process, it should be highlighted in the development of high performance UV light-emitting diodes, currently a highly regarded research area.
The motion of an internal solitary wave of depression over a fixed bottom boundary in a shallow, two-layer fluid
Authors: Carr M. , Davies P. A.
Published: February 2006

The motion of a large-amplitude internal solitary wave of depression over a fixed bottom boundary in a shallow, two-layer fluid is investigated experimentally. Measurements of the velocity fields close to the bottom boundary are presented to illustrate the generation of an unsteady boundary jet along the bed. The formation of the jet, the structural characteristics of which show striking similarities with those predicted by recent numerical model studies by Diamessis and Redekopp [J. Phys. Oceanogr. (in press)], is attributed to boundary layer separation in the adverse pressure gradient region of the wave-induced flow.
Scaling of the velocity fluctuations in turbulent channels up to Reτ=2003
Authors: Hoyas Sergio , Jiménez Javier
Published: February 2006

A new numerical simulation of a turbulent channel in a large box at Reτ=2003 is described and briefly compared with simulations at lower Reynolds numbers and with experiments. Some of the fluctuation intensities, especially the streamwise velocity, do not scale well in wall units, both near and away from the wall. Spectral analysis traces the near-wall scaling failure to the interaction of the logarithmic layer with the wall. The present statistics can be downloaded from http://torroja.dmt.upm.es/ftp/channels. Further ones will be added to the site as they become available.
Large-eddy simulation of a pure thermal plume under rotating conditions
Authors: Pham Minh Vuong , Plourde Frédéric , Kim Son Doan , Balachandar S.
Published: February 2006

The spatial development of turbulent thermal plumes under rotating conditions of the heat source has been investigated using large-eddy simulations. In order to analyze the rotation effect of the heat source in an otherwise quiescent environment, a range of rotation frequencies is considered in an intermediary Rossby number regime in which inertial and Coriolis forces strongly interact. Above a threshold Rossby number of about 1.82, the influence of heat source rotation is insignificant. Below the threshold heat source, rotation has a significant influence. In the region close to the heat source there is a strong interaction region between the plume development and heat source rotation. Radial transport across the plume is amplified and a mixing process through shear-layer development enhances engulfment of ambient fluid into the thermal plume. Farther above the plume the direct effect of source rotation decays and becomes insignificant. The plume continues to be wider, but this is due to the swirling effects imparted to the base of the plume at the heat source. Thus, after an initial enhancement of entrainment, the resulting flow field behaves as would a nonrotating plume driven by buoyancy, convection, and turbulent mixing.
Wide infrared and visible tunability from a Nd3+:Ba2NaNbO15 self-frequency-converter disordered laser crystal
Authors: Ródenas A. , Ramirez M. O. , Jaque D. , Garcia Solé J. , Kaminskii A. A.
Published: February 2006

We report on continuous-wave tunable laser light generation from a diode-pumped Nd3+-doped Ba2NaNb5O15 nonlinear crystal. The characteristic cationic disorder of this system leads to a F3∕24→I11∕24 broad emission band, allowing for continuous wavelength selection from 1055nm up to 1095nm in the infrared, and from 527nm up to 546nm in the green via self-frequency doubling. This broad tuning range makes Nd3+:Ba2NaNb5O15 a promising system for tunable frequency conversion as well as for short pulse generation.
Magnetic relaxation studies in organic-inorganic nanoclusters
Authors: Sharma Raksha , Suri Komilla , Tandon R. P. , Annapoorni S. , Lamba Subhalakshmi , Kumaraswami B. V.
Published: February 2006

Inorganic∕organic nanocomposites of iron oxide and polypyrrole, prepared by two different techniques, namely, simultaneous gelation∕polymerization and polymerization over core iron oxide particles, were investigated for their structural and magnetic properties. The presence of the polymer was confirmed by Fourier transform infrared spectroscopy. The magnetic properties were found to be very sensitive not only to the concentration of polymer but also to the method of preparation. ac susceptibility measurements performed in the temperature range of 77–300 K show the signature of superparamagnetic behavior in both the systems. The blocking temperature was found to be dependent on the cluster size and interaction effects. The frequency variation of the blocking temperature in the samples prepared by simultaneous gelation and polymerization seems to indicate weakly interacting magnetic clusters, whereas in the samples prepared by polymerization around the core oxide particles, a strongly interacting signature is seen. ac susceptibility as calculated by kinetic Monte Carlo simulations based on an interacting model for single domain particles shows a qualitative agreement with the trends observed in the experimental results.
Poly(N-vinylcarbazole) doped with a pyrazoloquinoline dye: A deep blue light-emitting composite for light-emitting diode applications
Authors: Luszczynska Beata , Dobruchowska Ewa , Glowacki Ireneusz , Ulanski Jacek , Jaiser Frank , Yang Xiaohui , Neher Dieter , Danel Andrzej
Published: February 2006

We investigated the spectral properties of light-emitting diodes based on a deep blue-emitting pyrazoloquinoline dye doped into a poly(N-vinylcarbazole)-based matrix. Even though the electroluminescence (EL) of the host is redshifted and broadened with respect to the emission of the dye, the EL spectrum becomes fully dominated by the dye emission at concentrations of ca. 2wt%. This is attributed to a competition of exciplex formation on the matrix and exciton formation on the dye.
Investigation of electromechanical properties in ferroelectric thin films using Monte Carlo simulation
Authors: Cao Hai-Xia , Lo Ven Cheong , Chung Winnie W. Y.
Published: February 2006

Electromechanical properties of ferroelectric thin films are investigated using a two-dimensional four-state Potts model and Monte Carlo simulation. In this model, the mechanical energy density induced by strains of individual cells is included in the system Hamiltonian, in addition to the contributions from dipole-dipole and electric-field-dipole couplings. Moreover, the dipole of each individual perovskite cell is aligned to one of the four mutually perpendicular directions. Four different states of dipole orientations can be defined. The deformation of each cell is associated with its dipole orientation, resulting in two different strain states. Polarization–electric-field hysteresis loops, butterfly loops for both transverse and longitudinal strains against electric field, as well as the phase-transition temperature under different stresses and anisotropy conditions are simulated. Results are comparable to the experimental measurements.
Source brightness and useful beam current of carbon nanotubes and other very small emitters
Authors: Kruit P. , Bezuijen M. , Barth J. E.
Published: February 2006

The potential application of carbon nanotubes as electron sources in electron microscopes is analyzed. The resolution and probe current that can be obtained from a carbon nanotube emitter in a low-voltage scanning electron microscope are calculated and compared to the state of the art using Schottky electron sources. Many analytical equations for probe-size versus probe-current relations in different parameter regimes are obtained. It is shown that for most carbon nanotube emitters, the gun lens aberrations are larger than the emitters’ virtual source size and thus restrict the microscope’s performance. The result is that the advantages of the higher brightness of nanotube emitters are limited unless the angular emission current is increased over present day values or the gun lens aberrations are decreased. For some nanotubes with a closed cap, it is known that the emitted electron beam is coherent over the full emission cone. We argue that for such emitters the parameter “brightness” becomes meaningless. The influence of phase variations in the electron wave front emitted from such a nanotube emitter on the focusing of the electron beam is analyzed.
X-ray investigation of nanostructured stain-etched porous silicon
Authors: Abramof P. G. , Beloto A. F. , Ueta A. Y. , Ferreira N. G.
Published: February 2006

The structure of porous silicon layers was accurately investigated by diverse x-ray methods. A series of samples with etching times varying from 1 to 10 min was produced by chemical etch using a HF∕HNO3-based solution assisted with NaNO2. The porosity determined from low-angle x-ray reflectivity spectra was found to fluctuate from 35% to 55% as the etch proceeds. Reciprocal space mapping around the (004) Si lattice point revealed that the Si crystallites are deformed due to a distribution of in-plane compressive strain caused by the neighboring pores, which leads to an expansion of the perpendicular lattice parameter. No signature of mosaicity was found. The perpendicular strain could be precisely determined by fitting the x-ray-diffraction spectra, measured in the triple-axis configuration, to a set of Voigt and Gaussian distributions. These strain distributions are certainly associated with the different population of crystallite sizes formed during the stain etching process. We were able here to determine the temporal evolution of the strain inside the nanostructured porous silicon.
Note on the memory force on a slightly eccentric fluid spheroid in unsteady creeping flows
Authors: Abbad M. , Souhar M. , Caballina O.
Published: February 2006

The transient Stokes equations are solved inside and outside a slightly eccentric fluid spheroid, which oscillates about its zero-mean position in an unbounded viscous medium. By using techniques of regular perturbations, the internal and the external flow fields are determined, and a first approximation of the hydrodynamic force is derived. In addition to the classical perturbed forces: the quasisteady drag, the added-mass force and the history force, the unsteady drag contains an additional history term due mainly to the eccentricity of the particle. For viscous spheroids, its kernel is found to be different from that of the history force acting on spherical droplets. So, we examine here the effects of the eccentricity and the viscosity ratio on its magnitude, its limits for solid and gaseous spheroids, and its asymptotic behaviors at low and high frequencies.
Molecular-dynamics simulation of argon nucleation from supersaturated vapor in the NVE ensemble
Authors: Kraska T.
Published: February 2006

The possibility to conduct simulations of homogeneous nucleation of argon from a supersaturated vapor phase using a microcanonical or NVE ensemble is evaluated (NVE: number of particles N, volume V, and energy E are constant). In order to initiate a phase separation kinetic energy is removed from the system in one step which transfers the system into a supersaturated state. After this temperature jump the simulation is continued in a NVE ensemble. The simulations are performed for different initial-state points and different temperature jumps. The cluster formation and growth over the course of the adiabatic simulations are analyzed. The progression of the temperature being related to the cluster size in NVE systems is traced. Also the influence of the size of the simulation system is investigated. For a certain range of low supersaturation a dynamic coexistence between two states has been found. Furthermore, the obtained nucleation rates are correlated with two simple functions. By applying the nucleation theorems to these functions the size and excess energy of the critical cluster are estimated. The results are consistent with other theoretical data and experimental data available in the literature.
Phase transformation in sputter-deposited PdMn and PdPtMn thin films
Authors: Huang Mianliang , Austin Chang Y.
Published: February 2006

The phase transformations of PdMn and PdPtMn films were investigated using differential scanning calorimetry (DSC), x-ray diffraction (XRD), and transmission electron microscopy (TEM). The enthalpy for the fcc to L10 transformation in the PdMn and PdPtMn thin films has been measured by DSC as −5.4 and −7.6kJ∕molat., respectively. The fcc to L10 phase transformation was identified by XRD and TEM on as-deposited and annealed samples. The transition temperature for the PdPtMn is approximately 40 °C lower than that for PdMn. PdPtMn thin films have better corrosion resistance than PdMn.
Probing photonic crystals on silicon-on-insulator with Ge∕Si self-assembled islands as an internal source
Authors: Li X. , Boucaud P. , Checoury X. , Kermarrec O. , Campidelli Y. , Bensahel D.
Published: February 2006

We report the study of two-dimensional photonic crystals fabricated on silicon-on-insulator substrates. Ge∕Si self-assembled islands are embedded as an active internal optical source inside the photonic crystals. We present a detailed analysis of photonic crystal microcavities and waveguides using the room-temperature Ge∕Si island photoluminescence. The tunability of the microcavity resonant emission is demonstrated between 1.2 and 1.5μm. We show that the microcavity photoluminescence is weakly dependent on the temperature. The polarized transmission properties of W1 single-line defect waveguides are investigated using the photoluminescence as an internal source. The transmission spectra are correlated to those given by two-dimensional finite-difference time-domain calculations.
Investigation of indium distribution in InGaAs∕GaAs quantum dot stacks using high-resolution x-ray diffraction and Raman scattering
Authors: Mazur Yu. I. , Wang Zh. M. , Salamo G. J. , Strelchuk V. V. , Kladko V. P. , Machulin V. F. , Valakh M. Ya. , Manasreh M. O.
Published: February 2006

Using high-resolution x-ray diffraction (HRXRD), Raman scattering, photoluminescence, and atomic-force microcopy, we investigated InxGa1−xAs∕GaAs quantum dot (QD) stacks grown by molecular-beam epitaxy with nominal In contents of 0.30 and 0.35. The analysis of x-ray-diffraction rocking curves using symmetrical (004), asymmetrical (113), and quasiforbidden (002) reflections within the framework of dynamical theory allowed us to determine the average values of strains parallel and perpendicular to the growth direction. We also measured nonuniform In profiles in the InxGa1−xAs layers along the growth direction. This observation confirms the important role of surface segregation of In atoms and interdiffusion of Ga atoms from GaAs layers in the formation of InxGa1−xAs QDs. Both HRXRD and Raman scattering in InxGa1−xAs∕GaAs-stacked QD structures demonstrate that the InGaAs inserts in these structures can be modeled effectively as sublayers with two different compositions: sufficiently thick InxGa1−xAs sublayer with the In concentration lower than the nominal one, which includes the thin layer of InGaAs islands with the In concentration much higher than the nominal one.
Real-time tunability of chip-based light source enabled by microfluidic mixing
Authors: Bilenberg Brian , Rasmussen Torben , Balslev Søren , Kristensen Anders
Published: February 2006

We demonstrate real-time tunability of a chip-based liquid light source enabled by microfluidic mixing. The mixer and light source are fabricated in SU-8 which is suitable for integration in SU-8-based laboratory-on-a-chip microsystems. The tunability of the light source is achieved by changing the concentration of rhodamine 6G dye inside two integrated vertical resonators, since both the refractive index and the gain profile are influenced by the dye concentration. The effect on the refractive index and the gain profile of rhodamine 6G in ethanol is investigated and the continuous tuning of the laser output wavelength is demonstrated using an ethanolic rhodamine 6G solution of 2×10−2mol∕l mixed with pure ethanol. This yields rhodamine 6G concentrations from 5×10−3 to 1.5×10−2mol∕l inside the laser resonators and a wavelength change of 10 nm with a response time of 110 s.
Dynamic measurements of electrical conductivity in metastable intermolecular composites
Authors: Tasker Douglas G. , Asay Blaine W. , King James C. , Sanders V. Eric , Son Steven F.
Published: February 2006

Metastable intermolecular composite (MIC) materials are comprised of a mixture of oxidizer and fuel with particle sizes in the nanometer range. Dynamic electrical conductivity measurements have been performed on a reacting MIC material. Simultaneous optical measurements of the wavefront position have shown that the reaction and conduction fronts are coincident within 160μm. It has been observed that MICs, like high explosives, are insulators before reaction is initiated. Once reaction is induced, there is a conduction zone that corresponds with the reaction zone behind the reaction front. Unlike detonating high explosives (HEs) where the conductivity profile is represented by an initial peak followed by an exponential decay of conductivity, the MIC conductivity profile is a gradual, irregular ramp which increases from zero over many microseconds. This supports other studies that show the MIC reaction process to be significantly different from detonating HEs. Static measurements of conductivity of pressed MIC pellets suggest that the electrical conduction is associated with chemical reaction in the MIC and not compaction effects alone.
Characterization of densified fully stabilized nanometric zirconia by positron annihilation spectroscopy
Authors: Garay J. E. , Glade S. C. , Asoka-Kumar P. , Anselmi-Tamburini U. , Munir Z. A.
Published: February 2006

Fully stabilized nanometric zirconia samples with varying degrees of porosity and grain sizes were analyzed using the coincidence Doppler broadening mode of the positron annihilation spectroscopy (PAS). A decrease in the low-momentum fraction was observed and coincided with a decrease in porosity. In addition to pores, it is proposed that defects in the negatively charged grain-boundary space region act as positron trapping centers; their effectiveness decreases with an increase in grain size. It is shown that PAS is sensitive to small grain-size differences within the nanometric regime in these oxide materials.
Optical phase shift induced by intersubband transitions in multiple quantum well structures
Authors: Załużny M. , Nalewajko C.
Published: February 2006

We discuss, theoretically, employing formalism based on the transfer matrix method and the so-called sheet model, the linear and nonlinear (saturated) optical phase shifts associated with intersubband transitions in multiple quantum well (MQW) structures. A commonly used total internal reflection geometry is considered. Numerical simulations are performed for two-level GaAs∕AlxGa1−xAs MQWs. They show that the above-mentioned phase shift can be greatly modified by optical interference effects and radiative coupling between QWs particularly at the grazing angle of incidence. For the restricted range of parameters even the bistable behavior of the nonlinear phase shift and reflectivity are possible.
Rotational averaging and optimization of laser-induced population transfer in molecules
Authors: Van Leuven Piet , Persico Maurizio
Published: February 2006

The dynamics of a molecule subject to a short laser pulse is investigated, with focus on the averaging over initial rotational states and on the optimization of laser parameters for the efficient population transfer between vibrational and electronic states. A relation is established between final-state populations obtained with a fixed orientation and those based on a full treatment of the rotational degrees of freedom. In the short-pulse approximation, rotational averaging amounts to integrating the fixed molecule results over all orientations. The theory is applied to a variety of model systems and verified with numerical calculations using Gaussian pulses. We calculate target state populations with three procedures, optimizing the laser pulse for a fixed orientation without orientational averaging, averaging without changing the laser parameters, and reoptimizing the parameters after averaging. The analysis of the two-level system provides a reference for the order of magnitude of the effects of averaging. The three-level system brings out the relevant role of the geometry of polarization vectors and transition dipoles. The multiphoton excitation of a Morse oscillator shows the importance of taking into account the dependence of resonance frequencies on the laser intensity. Within a proton transfer model we discuss the results obtained with and without chirping and we show that “optimizing after averaging” can be as effective as choosing a more refined pulse shape.
Dissipative particle dynamics with attractive and repulsive particle-particle interactions
Authors: Liu Moubin , Meakin Paul , Huang Hai
Published: February 2006

In molecular dynamics simulations, a combination of short-range repulsive and long-range attractive interactions allows the behavior of gases, liquids, solids, and multiphase systems to be simulated. We demonstrate that dissipative particle dynamics (DPD) simulations with similar pairwise particle-particle interactions can also be used to simulate the dynamics of multiphase fluids. In these simulations, the positive, short-range, repulsive part of the interaction potentials were represented by polynomial spline functions such as those used as smoothing functions in smoothed particle hydrodynamics, and the negative long-range part of the interaction has the same form but a different range and amplitude. If a single spline function corresponding to a purely repulsive interaction is used, the DPD fluid is a gas, and we show that the Poiseuille flow of this gas can be described accurately by the Navier-Stokes equation at low Reynolds numbers. In a two-component system in which the purely repulsive interactions between different components are substantially larger than the purely repulsive intracomponent interactions, separation into two gas phases occurs, in agreement with results obtained using DPD simulations with standard repulsive particle-particle interactions. Finally, we show that a combination of short-range repulsive interactions and long-range attractive interactions can be used to simulate the behavior of liquid drops surrounded by a gas. Similar models can be used to simulate a wide range of processes such as multiphase fluid flow through fractures and porous media with complex geometries and wetting behaviors.
Electrical properties of epoxy/silver nanocomposites
Authors: Gonon P. , Boudefel A.
Published: February 2006

We investigated the electrical properties of nanocomposites made of epoxy resin filled with 70-nm-sized silver particles. These composites were studied for the fabrication of integrated capacitors in electronics packaging. The dc conductivity was studied as a function of the filler concentration and as a function of temperature. We also studied the ac conductivity and the permittivity in the 10−1–105Hz range as a function of the filler concentration. Experimental properties were analyzed using standard percolation theories. The dc conductivity varies as (ϕ-ϕc)t, where ϕ is the filler concentration, ϕc is the percolation threshold, and t is the dc critical exponent. A very low percolation threshold is obtained (ϕc=1%) which is believed to be related to a segregated distribution of the fillers in the epoxy matrix. We also measured a very high dc critical exponent (t=5) probably related to the interparticle electrical contact. A universal scaling law is observed for σ(ω) and ε(ω). Above a cutoff frequency (ωc, which scales with the dc conductivity as ωc∼σdcq) the conductivity and the permittivity follow the universal power laws (σ∼ωu and ε∼ω−v) with critical exponents taking nonstandard values (q=0.83–0.98, u=0.79, and v=0.03).
Large thermoelectric figure of merit at high temperature in Czochralski-grown clathrate Ba8Ga16Ge30
Authors: Saramat A. , Svensson G. , Palmqvist A. E. C. , Stiewe C. , Mueller E. , Platzek D. , Williams S. G. K. , Rowe D. M. , Bryan J. D. , Stucky G. D.
Published: February 2006

The Czochralski method was used to grow a 46-mm-long crystal of the Ba8Ga16Ge30 clathrate, which was cut into disks that were evaluated for thermoelectric performance. The Seebeck coefficient and electrical and thermal conductivities all showed evidence of a transition from extrinsic to intrinsic behavior in the range of 600–900K. The corresponding figure of merit (ZT) was found to be a record high of 1.35 at 900K and with an extrapolated maximum of 1.63 at 1100K. This makes the Ba8Ga16Ge30 clathrate an exceptionally strong candidate for medium and high-temperature thermoelectric applications.
Electron mobility dependence on annealing temperature of W∕HfO2 gate stacks: The role of the interfacial layer
Authors: Callegari A. , Jamison P. , Neumayer D. , Feely F. Mc. , Shepard J. , Andreoni W. , Curioni A. , Pignedoli C.
Published: February 2006

Interfacial layers between the high-k dielectric and Si surface have a very important role to play in the achievement of high electron mobilities required in the next generations of high performance silicon technologies. W∕HfO2 gate stacks formed on SiO2∕SiON interfacial layers subjected to various process conditions were characterized by electrical measurements such as electron mobility, inversion layer thickness, and leakage reduction with respect to standard SiO2 technology. The required high electron mobilities were obtained only when the W∕HfO2 gate stack was annealed at high temperature. Electrical data and physical analysis of the stack suggest intermixing of the HfO2 with the interfacial layer and formation of a silicate layer which may lessen the effects of phonon scattering. Densification of the interfacial layer by spike annealing or interfacial layer stabilization by nitrogen plasma inhibits such reactions and lower electron mobilities were obtained. Also, no electrical performance advantages are seen by thinning the HfO2 down to 1.5nm.
Electromigration in Pb-free flip chip solder joints on flexible substrates
Authors: Nah J. W. , Ren Fei , Tu K. N. , Venk Sridharan , Camara Gabe
Published: February 2006

Flip chip on flexible substrate technology has attracted much attention in the electronics industry for portable applications because the reduction of packaging volume coupled with long-term reliability is a challenging issue. In this study, the electromigration behavior of flip chip Sn96Ag3.5Cu0.5 solder joints on flexible substrates has been investigated under a constant current density of 4×104A∕cm2 at room temperature. The in situ resistance change of the circuit during the electromigration test was measured and found to show stepwise changes before the final failure. Scanning electron microscopic images of the cross section of samples showed the existence of voids in the bulk of the solder joints before electromigration and the movement of these voids after electromigration. The void movement was matched with the resistance change during the electromigration test, and voids moved from within the bulk of solder bump to the cathode interface between Al interconnection and under bump metallurgy in the downward electron flow (from chip to substrate) joints. The mechanisms of electromigration-induced void migration and failure in the Pb-free flip chip on flexible substrates are discussed.
Coalescence aspects of cobalt nanoparticles during in situ high-temperature annealing
Authors: Palasantzas G. , Vystavel T. , Koch S. A. , De Hosson J. Th. M.
Published: February 2006

In this work we investigate the coalescence aspects of Co nanoparticles. It was observed that nanoparticles in contact with the substrate are relatively immobile, whereas those on top of other Co particles can rearrange themselves during high-temperature annealing and further coalesce. Indeed, similar size particles prior to coalescence come at close proximity forming an arc-shaped area, which leads to finite-size necking and thereafter to coalescence towards a single partially spherical particle. This is in contrast to the theoretical predictions where necking occurs following an initial pathway of a point contact. Moreover, it was shown that after necking a transient period of relatively fast coalescence occurs followed by a slower coalescence rate at constant speed towards a single particle with partial spherical shape. In addition, the coalescence is faster with decreasing particle size, where in the case of unequal size the smallest particle is mainly absorbed by an adjacent large one in an Ostwald ripening process.
Thermal stability of TiO2, ZrO2, or HfO2 on Si(100) by photoelectron emission microscopy
Authors: Zeman M. C. , Fulton C. C. , Lucovsky G. , Nemanich R. J. , Yang W.-C.
Published: February 2006

The thermal stability of thin films (3nm) of transition-metal (TM) oxides (TiO2, ZrO2, and HfO2) grown on ultrathin (∼0.5nm) SiO2 buffer layers on Si(100) surfaces was investigated with ultraviolet photoelectron emission microscopy (UV-PEEM). The decomposition of the TM oxides was observed in the PEEM during ultrahigh-vacuum annealing at temperatures of ∼870, ∼900, and ∼1000°C for the TiO2, ZrO2, and HfO2, respectively. Following the decomposition reaction, atomic force microscopy measurements of the annealed surfaces revealed a high density of islands in the decomposed regions. The degradation of the TM oxide films is attributed to a reaction occurring at defects at the TM oxide/SiO2∕Si interfaces, which forms SiO species. Once a portion of the interfacial SiO2 layer is desorbed as a result of this reaction, Si from the substrate can diffuse into contact with the TM oxide layer, resulting in the formation of a TM silicide and the evolution of SiO. This process continues until the entire TM oxide layer is consumed and only silicide islands remain.
Characterization of field-effect transistors with La2Hf2O7 and HfO2 gate dielectric layers deposited by molecular-beam epitaxy
Authors: Rittersma Z. M. , Hooker J. C. , Vellianitis G. , Locquet J.-P. , Marchiori C. , Sousa M. , Fompeyrine J. , Pantisano L. , Deweerd W. , Schram T. , Rosmeulen M. , De Gendt S. , Dimoulas A.
Published: February 2006

The electrical properties of La2Hf2O7 (LHO) and HfO2 (HO) high-k dielectric layers deposited by molecular-beam epitaxy are reported. Capacitors and transistors with LHO and HO gate dielectric layers and TaN metal gate electrodes deposited using physical vapor deposition were fabricated. The (SiO2) equivalent oxide thickness (EOT), the electrical oxide thickness in inversion toxinv, and the gate leakage current density (Jg) were determined on large area metal-oxide-semiconductor capacitors and metal-oxide-semiconductor field-effect transistors. HO layers with a physical thickness tph=30Å showed an EOT (toxinv) of 15.1Å (19.3Å) with Jg=8.1×10−6A∕cm2 at Vg=Vfb−1V. LHO layers deposited on SiON showed a minimum EOT (toxinv) of 18.7Å (25Å) with Jg=4×10−8A∕cm2 at Vg=Vfb−1V. The (effective) electron mobility at high E field for LHO layers was observed for a 40-Å-thick LHO layer deposited on Si with μeff=147cm2∕Vs at E=1MV∕cm. For a 30-Å-thick HO layer at identical field, μeff=170cm2∕Vs was found. LHO layers deposited on SiON interface exhibited 5%–10% higher electron mobility at high E field than identical layers deposited on Si. Further, both low E field and high E field mobilities decreased for thicker high-k layers, indicating remote charge scattering from both the interface and the bulk of the high-k dielectric. It was observed that LHO layers showed strongly reduced electron trapping in preexisting bulk defects as compared to HO layers. At E=1MV∕cm (corresponding to Vg=VT+0.6V), the trapped charges per area Ntr were Ntr<5×1010∕cm2 for the LHO layers and Ntr>5×1011∕cm2 for the HO layers. These results show that low-leakage Hf-based gate dielectric layers with low defect density can be obtained by alloying with La.
Influence of interface spin-flip scattering on spin accumulation and spin currents in magnetic multilayers with collinear magnetizations
Authors: Wawrzyniak M. , Gmitra M. , Barnaś J.
Published: February 2006

The macroscopic description of spin-dependent electronic transport in magnetic-layered structures is extended by including the effects due to spin-flip scattering at the interfaces between magnetic and nonmagnetic layers. Such processes lead to spin-memory losses at the interfaces and therefore play a significant role in the giant magnetoresistance and current-induced magnetic switching phenomena. Interface spin-flip scattering significantly modifies the distribution of spin currents and spin accumulation in the vicinity of the interface. Two semi-infinite systems in direct contact, as well as magnetic/nonmagnetic (F∕N) sandwiches and superlattices, are considered for both parallel and antiparallel magnetic configurations.
Atomic stereophotograph of intercalation compound Fe1∕3NbS2
Authors: Guo F. Z. , Matsushita T. , Kobayashi K. , Matsui F. , Kato Y. , Daimon H. , Koyano M. , Yamamura Y. , Tsuji T. , Saitoh Y.
Published: February 2006

Stereoatomscope was used to study the atomic arrangements of intercalation compound Fe1∕3NbS2. The three-dimensional atomic arrangements around different kinds of atoms (Nb and Fe) are visualized by taking the photoelectron angular distribution (PEAD) patterns at clockwise and counterclockwise circularly polarized lights. Atomic distances between the emitters and the scatterers are obtained from the PEAD patterns by measuring the rotation angles of the forward focusing peaks. The applications of stereoatomscope to intercalation compound show the possibility to build an ultimate microscope for scientist.
Reduction of the cantilever hydrodynamic damping near a surface by ion-beam milling
Authors: Maali A. , Cohen-Bouhacina T. , Jai C. , Hurth C. , Boisgard R. , Aimé J. P. , Mariolle D. , Bertin F.
Published: February 2006

In this work, we evaluate the influence of the cantilever width on the hydrodynamic drag force. To do so, we present an experimental analysis of the thermal motion in air and liquid of a commercial and modified by focused ion-beam (FIB) milling silicon nitride cantilevers. From the thermal noise spectrum, we extract the damping for different cantilever-sample distances. We show that the hydrodynamic force due to the drag can be reduced by almost an order of magnitude when reducing the cantilever width. With the FIB modification (milling) one can still use conventional atomic force microscope heads with a significant reduction of the hydrodynamic forces.
Fabrication of induced two-dimensional hole systems on (311)A GaAs
Authors: Clarke W. R. , Micolich A. P. , Hamilton A. R. , Simmons M. Y. , Muraki K. , Hirayama Y.
Published: February 2006

We demonstrate a method for fabricating induced two-dimensional hole devices in (311)A GaAs. The method uses a metallic p+-GaAs capping layer as an in situ top gate that pins the Fermi energy close to the valence band, thereby allowing very small gate biases to be used to induce a two-dimensional hole system at a AlGaAs∕GaAs interface. We present transport data from devices with different levels of background impurities. Modeling the mobility as a function of hole density gives a quantitative measure of the level of disorder and indicates that these systems can be used for a systematic study of the effects of disorder in strongly interacting low-dimensional systems.
Pulsatile flow and mass transport past a circular cylinder
Authors: Zierenberg Jennifer R. , Fujioka Hideki , Suresh Vinod , Bartlett Robert H. , Hirschl Ronald B. , Grotberg James B.
Published: February 2006

The mass transport of a pulsatile free-stream flow past a single circular cylinder is investigated as a building block for an artificial lung device. The free stream far from the cylinder is represented by a time-periodic (sinusoidal) component superimposed on a steady velocity. The dimensionless parameters of interest are the steady Reynolds number (Re), Womersley parameter (α), sinusoidal amplitude (A), and the Schmidt number (Sc). The ranges investigated in this study are 5⩽Re⩽40, 0.25⩽α⩽4, 0.25⩽A⩽0.75, and Sc=1000. A pair of vortices downstream of the cylinder is observed in almost all cases investigated. These vortices oscillate in size and strength as α and A are varied. For α<αc, where αc=0.005A−1.13Re1.33, the vortex is always attached to the cylinder (persistent); while for α>αc, the vortex is attached to the cylinder only during part of a time cycle (intermittent). The time-averaged Sherwood number, Sh̿, is found to be largely influenced by the steady Reynolds number, increasing approximately as Re1∕2. For α=0.25, Sh̿ is less than the steady (α=0, A=0) value and decreases with increasing A. For α=2 and α=4, Sh̿ is greater than the steady value and increases with increasing A. These qualitatively opposite effects of pulsatility are discussed in terms of quasisteady versus unsteady transport. The maximum increase over steady transport due to pulsatility varies between 14.4% and 20.9% for Re=10-40, α=4, and A=0.75.
Low Schottky barrier to etched p-GaN using regrown AlInGaN and InGaN contact layer
Authors: Hsueh Kuang-Po , Hsin Yue-Ming , Sheu Jinn-Kong
Published: February 2006

The p-type AlInGaN and InGaN contact layers were regrown on the etched p-GaN to study the Ni∕Au contact current-voltage (I-V) characteristics. The thickness of the contact layer was 100nm and regrown by metalorganic chemical vapor deposition. By using the regrown contact layer on etched p-GaN, Schottky barrier height (SBH) from the I-V characterization was reduced. The SBH of 0.65eV from the contact to the etched p-GaN was reduced to 0.56eV and 0.58eV, respectively, after the AlInGaN and InGaN contact layers were formed. In addition to the I-V characterization of Ni∕Au contacts, surface morphology and x-ray analysis were studied.
Spin-lattice coupling in the ferrimagnetic semiconductor FeCr2S4 probed by surface acoustic waves
Authors: Müller C. , Zestrea V. , Tsurkan V. , Horn S. , Tidecks R. , Wixforth A.
Published: February 2006

Using surface acoustic waves, the elastomagnetic coupling could be studied in thin single-crystalline plates of the ferrimagnetic semiconductor FeCr2S4 by measuring the attenuation and the frequency tracking in the temperature range of 4.2–200K. The data clearly display the anomalies found in low-field magnetization measurements.
Effect of contact properties on current transport in metal/molecule/GaAs devices
Authors: Lodha Saurabh , Carpenter Patrick , Janes David B.
Published: February 2006

Previous reports on metal/molecule/semiconductor (MMS) devices have investigated the effects of molecular species, including dipoles, but have not considered the semiconductor contact properties in detail. In this paper we report on a study of the effects of variations in the semiconductor contact on the conduction properties of MMS devices. Metal/molecule/gallium arsenide (GaAs) devices were fabricated using various semiconductor contact layers, electrically characterized versus temperature and analyzed using an electrostatic model. The various semiconductor contacts included heavily doped n-type and p-type GaAs layers, as well as n-doped and p-doped surface layers of low-temperature-grown GaAs (LTG:GaAs), which provide a high density of midgap defect states near the semiconductor surface. The impact of changing the work function of the top metal contact has also been studied. An electrostatic model that incorporates information on the molecular dipole moment, defect states in GaAs surface layers, and the work function of the metal contact, has been developed in order to understand the band diagrams corresponding to the various device types, and to explain the current-voltage behavior observed in the devices. It is shown by controlling the properties of the semiconductor contact that the device characteristics can be tuned from being dominated by the GaAs barrier (heavily doped n-type GaAs) to being dominated by the molecular states that are strongly coupled to the GaAs contact (LTG:GaAs and heavily doped p-type GaAs).