bluuebunny/crossref_metadata_2025 huggingface dataset:
An explicit filtering method for large eddy simulation of compressible flows
Authors: Mathew Joseph , Lechner Richard , Foysi Holger , Sesterhenn Joern , Friedrich Rainer
Published: July 2003

A method for large eddy simulation (LES) is presented in which the sub-grid-scale modeling is achieved by filtering procedures alone. The procedure derives from a deconvolution model, and provides a mathematically consistent approximation of unresolved terms arising from any type of nonlinearity. The formal steps of primary filtering to obtain LES equations, approximate deconvolution to construct the subgrid model term and regularization are combined into an equivalent filter. This filter should be an almost perfect low pass filter below a cut-off wavenumber and then fall off smoothly. The procedure has been applied to a pressure-velocity-entropy formulation of the Navier–Stokes equations for compressible flow to perform LES of two fully developed, turbulent, supersonic channel flows and has been assessed by comparison against direct numerical simulation (DNS) data. Mach numbers are 1.5 and 3.0, and Reynolds numbers are 3000 and 6000, respectively. Effects of filter cut-off location, choice of differentiation scheme (a fifth-order compact upwind formula and a symmetric sixth-order compact formula were used), and grid refinement are examined. The effects are consistent with, and are readily understood by reference to, filtering characteristics of the differentiation and the LES filter. All simulations demonstrate a uniform convergence towards their respective DNS solutions.
Vortex breakdown in compressible flows in pipes
Authors: Herrada M. A. , Pérez-Saborid M. , Barrero A.
Published: July 2003

The effects of compressibility on vortex flows in pipes have been analyzed using both the axisymmetric Navier–Stokes (NS) equations and the quasi-cylindrical (QC) approximation. Numerical simulations of the full axisymmetric NS equations show that, for sufficiently large values of the Reynolds number, compressible flows present a multiplicity of steady-state solutions in a range of values of the swirl strength. The sensitivity of the vortex flow structure to the parameters of the problem: Mach number, Reynolds number, velocity profiles at the pipe entrance and pipe geometry has been also investigated. In particular, the critical swirl parameter necessary for the occurrence of vortex breakdown has been determined as a function of both the Mach number and the axial momentum of the flow at the pipe entrance. For large Reynolds number, the results of the QC approximation are found to be in good agreement with those of the full NS simulations. These results show that the upstream propagation of the sound waves seems to have a negligible influence on vortex breakdown and give support to the use of the QC approximation for the study of some aspects of the compressible swirling flows.
Integral space–time scales in turbulent wall flows
Authors: Quadrio Maurizio , Luchini Paolo
Published: July 2003

A direct numerical simulation of the Navier–Stokes equations is used to compute the space–time correlations of velocity fluctuations in a turbulent channel flow. By examining the autocorrelation R(ξ,τ) of the longitudinal wall shear-stress as a function of the streamwise and temporal separations, the effects of the limited extent of the computational domain when (artificial) periodic boundary conditions are used can be described and quantified. A time scale similar to the conventional integral scale but statistically related to the life time of the turbulent structures is computed from spatio-temporal data. The convection velocity, defined as the direction in the ξ,τ plane where the autocorrelations have their maximum at vanishingly small time delay, is computed as a function of the distance from the wall, and compared with the data available in the literature. Based on autocorrelations, the accuracy within which Taylor’s hypothesis is verified is quantitatively assessed. Last, the effect of the spatial discretization on the statistical characterization of wall turbulence is discussed.
Method based on polarized infrared spectroscopy for the determination of the spatial orientation of transition dipole moments of a ferroelectric liquid crystal
Authors: Zhao Jinggang , Ozaki Yukihiro
Published: July 2003

A method to determine the spatial orientation of the transition dipole moments for ferroelectric liquid-crystal (FLC) molecules in the surface-stabilized FLC states by use of both conventional and oblique polarized infrared spectroscopy is proposed. It is found that the core moiety is tilted by an angle of about 7° to the surface of the cell windows.
Comment on Cercignani’s second-order slip coefficient
Authors: Hadjiconstantinou Nicolas G.
Published: July 2003

Cercignani’s second-order slip model has been neglected over the years, perhaps due to Sreekanth’s claim that it cannot fit his experimental data. In this paper we show that Sreekanth’s claim was based on an incorrect interpretation of this model. We also show that Cercignani’s second-order slip model, when modified and used appropriately, is in good agreement with solutions of the Boltzmann equation for a hard-sphere gas for a wide range of rarefaction. Given its simplicity, we expect this model to be a valuable tool for describing isothermal micro- and nanoscale flows to the extent that the hard-sphere approximation is appropriate.
First normal stress difference and crystallization in a dense sheared granular fluid
Authors: Alam Meheboob , Luding Stefan
Published: July 2003

The first normal stress difference (N1) and the microstructure in a dense sheared granular fluid of smooth inelastic hard-disks are probed using event-driven simulations. While the anisotropy in the second moment of fluctuation velocity, which is a Burnett-order effect, is known to be the progenitor of normal stress differences in dilute granular fluids, we show here that the collisional anisotropies are responsible for the normal stress behavior in the dense limit. As in the elastic hard-sphere fluids, N1 remains positive (if the stress is defined in the compressive sense) for dilute and moderately dense flows, but becomes negative above a critical density, depending on the restitution coefficient. This sign-reversal of N1 occurs due to the microstructural reorganization of the particles, which can be correlated with a preferred value of the average collision angle θav=π/4±π/2 in the direction opposing the shear. We also report on the shear-induced crystal-formation, signaling the onset of fluid–solid coexistence in dense granular fluids. Different approaches to take into account the normal stress differences are discussed in the framework of the relaxation-type rheological models.
A triple deck model of ripple formation and evolution
Authors: Lagrée P.-Y.
Published: July 2003

The two-dimensional laminar quasisteady asymptotically simplified flow with mass transport of sediments is solved over an erodible bed in various laminar hydraulic regimes (infinite depth, finite depth subcritical or supercritical, nondisturbed boundary layer). Compared to the boundary layer thickness, the bump is supposed longer and thinner and the triple deck theory is used. Furthermore, the flow is linearized. Next, a simplified mass transport equation is obtained which includes the two following phenomena: there is a flux of erosion when the skin friction goes over a threshold value, and concentration of sediment in suspension is convected but falls at a constant settling velocity. It is shown that two ingredients (convection of the longitudinal flux or particles and advanced response of the skin friction to the bump changes) are necessary to produce (except in the supercritical regime which, in this flux convected model, is always stable) a band of amplified spatial frequencies. Furthermore, putting the effect of slope limitation makes long wave stable (in the infinite depth case). Examples of evolution in various regimes are presented, wave trains of ripples are created and merge in a unique bump. A very long time is required for this process. This coarsening appends except in the infinite depth case when the effect of slope limitation is turned on: in this case a train of several bumps fills the computation domain.
Anisotropic spectrum of homogeneous turbulent shear flow in a Lagrangian renormalized approximation
Authors: Yoshida Kyo , Ishihara Takashi , Kaneda Yukio
Published: July 2003

An analytical study of the anisotropic velocity correlation spectrum tensor in the inertial subrange of homogeneous turbulent shear flow is performed using a Lagrangian renormalized spectral closure approximation. The analysis shows that the spectrum in the asymptotic limit of infinitely large Reynolds numbers Re is determined by two nondimensional universal constants; theoretical estimates for the constants are provided. The anisotropic component of the spectrum at finite Re is more sensitive to large-scale turbulence structures than the isotropic component. A preliminary analysis of the effect of finite Re or the width of the inertial subrange is in qualitative agreement with direct numerical simulations.
Large-eddy simulation of high-Reynolds-number supersonic boundary layers using the approximate deconvolution model and a rescaling and recycling technique
Authors: Stolz S. , Adams N. A.
Published: July 2003

Spatially developing supersonic turbulent boundary layers at a Mach number of 2.5 and momentum-thickness Reynolds numbers at inflow of 4530 and 10 049, respectively, are computed with large-eddy simulation using the approximate deconvolution model [Stolz et al., Phys. Fluids 13, 2985 (2001)]. Turbulent inflow conditions are generated by rescaling the turbulent boundary layer at some distance downstream of inflow and reintroducing the rescaled mean profiles and fluctuation fields at inflow. This technique follows essentially that of Lund et al. [J. Comput. Phys. 140, 233 (1998)], albeit simplified and adapted for compressible flow. The simulations feature rather short spatial transient behavior and the results agree well with experimental data and theoretical correlations. The validity of assumptions of the strong Reynolds analogy is addressed.
Is the turbulent wind in convective flows driven by fluctuations?
Authors: Burr Ulrich , Kinzelbach Wolfgang , Tsinober Arkady
Published: July 2003

In this paper, a direct check is presented whether the turbulent wind in Rayleigh–Bénard convection is driven by turbulent Reynolds stresses, associated with the tilting plumes at the upper and the lower horizontal walls. This is done by evaluation of experimental data obtained from particle image velocimetry measurements in the centerplane of a cubic convection cell and two-dimensional solution of the Navier–Stokes equations in a square domain. Although, in both, there are regions of negative turbulent energy production P=−〈uiuj〉∂Ui/∂xj, meaning that, locally, energy is transferred from velocity fluctuations to the mean flow, the integral of turbulent energy production over the whole flow field is essentially positive. This implies that the turbulent wind is not driven by the turbulent Reynolds stresses. It is demonstrated from the numerical results that once the mean flow is established, the temperature of the fluid is larger at one side wall and smaller at the other and therefore, the mean flow is driven by the mean buoyant force at the side walls.
Spatially resolved scanning tunneling luminescence on self-assembled InGaAs/GaAs quantum dots
Authors: Jacobs S. E. J. , Kemerink M. , Koenraad P. M. , Hopkinson M. , Salemink H. W. M. , Wolter J. H.
Published: July 2003

Scanning-tunneling microscope induced luminescence at low temperature has been used to study the carrier injection into single self-assembled InGaAs/GaAs quantum dots. Electrons are injected from the tip into the dots, which are located in the intrinsic region of a p-i-n junction, and contain excess holes under typical operational conditions. Only a fraction (∼4%) of the dots is found to be optically active under local electrical excitation. Spatial dependent measurements indicate a highly nonhomogeneous electron diffusion towards the dots. By analyzing the spatial dependence of individual peaks in the measured spectra, the contributions of individual dots to the total, multidot spectrum can be disentangled.
On the disappearance of the lowest-order instability for steep gravity waves in finite depth
Authors: Francius Marc , Kharif Christian
Published: July 2003

This paper extends to very large steepness the results of the linear stability analysis of two-dimensional periodic gravity waves over a bottom of uniform depth. Both subharmonic and superharmonic instabilities are investigated. For the case considered herein, it is shown how the lowest-order instability (class I) completely disappears at a critical wave steepness. These results also confirm numerically a theorem connected with the appearance of superharmonic instabilities in finite depth.
A numerical study of the statistics of a two-dimensional Rayleigh–Taylor mixing layer
Authors: Clark Timothy T.
Published: July 2003

A two-dimensional Rayleigh–Taylor mixing layer was numerically simulated to determine the growth rate of the mixing layer for an ensemble of initial conditions. The numerical algorithm used is a recently developed lattice Boltzmann method for multiphase flow. Small variations in the initial conditions of the fluid interface are observed to yield large variations in the growth rate of the mixing-layer width. Consequently, an ensemble of simulations has been generated to provide a statistical basis for determining the mixing-layer growth rate. The results for this ensemble indicate that the mixing-layer attains an approximate state of self-similarity and yields a distribution of mixing-layer growth rates.
Reflective thermal lensing and optical measurement of thermal diffusivity in liquids
Authors: Comeau Daniel , Haché Alain , Melikechi Noureddine
Published: July 2003

Using a pump-probe laser technique, we analyze the relaxation dynamics of a thermal lens generated at the surface of metal films deposited on a glass substrate. When the film is in contact with a liquid and the film is periodically heated with an optical source, the dynamics of the thermal lens is found to be closely related to the thermal properties of its environment. As a result, reflective thermal lensing can be used to measure the thermal diffusivity of a variety of liquids placed in contact with the film to an accuracy better than 2%. Experimental results point to applications to the study of transient effects in fluids and immiscible liquids, and detection of trace of contaminants in solvents.
Lift forces on a cylindrical particle in plane Poiseuille flow of shear thinning fluids
Authors: Wang J. , Joseph D. D.
Published: July 2003

Lift forces on a cylindrical particle in plane Poiseuille flow of shear thinning fluids are investigated by direct numerical simulation. Previous works on this topic for Newtonian fluids show that the two-dimensional channel can be divided into alternating regions defined by the stability of the particle’s equilibrium. We observe stability regions with the same pattern in flows of shear thinning fluids and study the effects of shear thinning properties on the distribution of the stability regions. Joseph and Ocando [J. Fluid Mech. 454, 263 (2002)] analyzed the role of the slip velocity Us=Uf−Up and the angular slip velocity Ωs=Ωp−Ωf on migration and lift in plane Poiseuille flow of Newtonian fluids. They concluded that the discrepancy Ωs−Ωse, where Ωse is the angular slip velocity at equilibrium, changes sign across the equilibrium position. In this paper we verify that this conclusion holds in shear thinning fluids. Correlations for lift forces may be constructed by analogy with the classical lift formula L=CUΓ of aerodynamics and the proper analogs of U and Γ in the present context are Us and Ωs−Ωse. Using dimensionless parameters, the correlation is a power law near the wall and a linear relation (which can be taken as a power law with the power of one) near the centerline. The correlations are compared to analytical expressions for lift forces in the literature and we believe that the correlations capture the essence of the mechanism of the lift force. Our correlations for lift forces can be made completely explicit provided that the correlations relating Us and Ωs to prescribed parameters are obtained.
Laminar flow past a hemisphere
Authors: Kim Dongjoo , Choi Haecheon
Published: July 2003

Numerical simulations are conducted for laminar flow past a hemisphere up to Re=300 in order to investigate its characteristics. It is found that unlike steady flows significant differences exist between unsteady flows past a hemisphere and a sphere. First, in the case of time-periodic planar-symmetric flow, alternating vortex shedding takes place behind a hemisphere with zero mean lift force, while nonzero mean lift force occurs for flow over a sphere. Second, there exists a time-periodic asymmetric flow regime for flow past a hemisphere, but it does not exist for flow past a sphere. It is also found that for flow past a hemisphere, the onsets of axisymmetry breaking and unsteadiness take place at lower Reynolds numbers than those for flow past a sphere.
Gouy phase shift of single-cycle picosecond acoustic pulses
Authors: Holme N. C. R. , Daly B. C. , Myaing M. T. , Norris T. B.
Published: July 2003

Ultrafast laser pulses are used to generate single-cycle picosecond acoustic pulses in thin metal films on silicon. For small initial excitation spot sizes, propagation of the acoustic pulses across a 485-μm Si crystal leads to significant diffraction effects. The temporal reshaping of the acoustic wave form due to diffraction is investigated, and we demonstrate that the acoustic far field can be reached.
High quality atomic-layer-deposited ultrathin Si-nitride gate dielectrics with low density of interface and bulk traps
Authors: Nakajima Anri , Khosru Quazi D. M. , Yoshimoto Takashi , Kasai Tetsurou , Yokoyama Shin
Published: July 2003

Interface trap and bulk trap densities of atomic-layer-deposited (ALD) Si-nitride dielectrics have been evaluated by bipolar-voltage-pulse-induced current and electrical-stress-induced leakage-current measurements, respectively. In comparison with the conventional SiO2 dielectrics, significantly lower (∼1/3) interface trap density near the conduction band edge is observed in the ALD Si-nitride dielectrics. Moreover, the observed lower interface and bulk trap generations consistently explain the soft-breakdown-free phenomena observed in capacitors with the ALD Si-nitride gate dielectrics. Enhanced dielectric reliability and several other significant features have made the ALD Si-nitride gate dielectrics a front line candidate for extremely thin (equivalent oxide thickness ∼1 nm) gate dielectrics of sub-100-nm technology nodes.
Single-electron tunneling in InP nanowires
Authors: De Franceschi S. , van Dam J. A. , Bakkers E. P. A. M. , Feiner L. F. , Gurevich L. , Kouwenhoven L. P.
Published: July 2003

We report on the fabrication and electrical characterization of field-effect devices based on wire-shaped InP crystals grown from Au catalyst particles by a vapor–liquid–solid process. Our InP wires are n-type doped with diameters in the 40–55-nm range and lengths of several micrometers. After being deposited on an oxidized Si substrate, wires are contacted individually via e-beam fabricated Ti/Al electrodes. We obtain contact resistances as low as ∼10 kΩ, with minor temperature dependence. The distance between the electrodes varies between 0.2 and 2 μm. The electron density in the wires is changed with a back gate. Low-temperature transport measurements show Coulomb-blockade behavior with single-electron charging energies of ∼1 meV. We also demonstrate energy quantization resulting from the confinement in the wire.
Dc-transport properties of ferromagnetic (Ga,Mn)As semiconductors
Authors: Jungwirth T. , Sinova Jairo , Wang K. Y. , Edmonds K. W. , Campion R. P. , Gallagher B. L. , Foxon C. T. , Niu Qian , MacDonald A. H.
Published: July 2003

We study the dc transport properties of (Ga,Mn)As diluted magnetic semiconductors with Mn concentration varying from 1.5% to 8%. Both diagonal and Hall components of the conductivity tensor are strongly sensitive to the magnetic state of these semiconductors. Transport data obtained at low temperatures are discussed theoretically within a model of band-hole quasiparticles with a finite spectral width due to elastic scattering from Mn and compensating defects. The theoretical results are in good agreement with measured anomalous Hall effect and anisotropic longitudinal magnetoresistance data.
Analysis of transport properties of tetrahertz quantum cascade lasers
Authors: Callebaut Hans , Kumar Sushil , Williams Benjamin S. , Hu Qing , Reno John L.
Published: July 2003

We present a self-consistent modeling of a 3.4-THz intersubband laser device. An ensemble Monte Carlo simulation, including both carrier–carrier and carrier-phonon scattering, is used to predict current density, population inversion, gain, and electron temperature. However, these two scattering mechanisms alone appear to be insufficient to explain the observed current density. In addition, the insufficient scattering yields a gain that is slightly higher than inferred from experiments. This suggests the presence of a non-negligible scattering mechanism which is unaccounted for in the present calculations.
Intense vortex pinning enhanced by semicrystalline defect traps in self-aligned nanostructured MgB2
Authors: Li S. , White T. , Laursen K. , Tan T. T. , Sun C. Q. , Dong Z. L. , Li Y. , Zho S. H. , Horvat J. , Dou S. X.
Published: July 2003

In this work, we report the discovery of a vortex pinning source: semicrystalline defect wells in self-aligned nanostructured MgB2. It is demonstrated that these aperiodic regions trap numerous crystal defects migrating along nanodomain boundaries during self-alignment and act as intense vortex pinning centers that significantly enhance the high-field performance of MgB2. This suggests that the density of trapped defects in the wells is much greater than that found in other vortex pinning sources.
Suppression of the Richtmyer–Meshkov instability in the presence of a magnetic field
Authors: Samtaney Ravi
Published: July 2003

We present numerical evidence from two dimensional simulations that the growth of the Richtmyer–Meshkov instability is suppressed in the presence of a magnetic field. A bifurcation occurs during the refraction of the incident shock on the density interface which transports baroclinically generated vorticity away from the interface to a pair of slow or intermediate magnetosonic shocks. Consequently, the density interface is devoid of vorticity and its growth and associated mixing is completely suppressed.
Silicon-based field-effect-transistor cantilever for surface potential mapping
Authors: Suh Moon Suhk , Choi J. H. , Kuk Young , Jung J.
Published: July 2003

A silicon-based scanning probe with a field effect transistor (FET) has been developed. The FET is integrated onto an atomic force microscope cantilever with a sharpened tip. The commonly used complementary-metal–oxide–semiconductor process has been employed to construct the FET using a silicon-on-insulator wafer. The probe is used to measure a surface potential with a resolution of <300 nm when determined by the edge of patterned SiO2 islands. The probe can be also used to detect local properties on semiconductor surfaces, such as isolated charge distributions on a surface or at subsurface.
Self-assembled subnanolayers as interfacial adhesion enhancers and diffusion barriers for integrated circuits
Authors: Ramanath G. , Cui G. , Ganesan P. G. , Guo X. , Ellis A. V. , Stukowski M. , Vijayamohanan K. , Doppelt P. , Lane M.
Published: July 2003

Preserving the structural and functional integrity of interfaces and inhibiting deleterious chemical interactions are critical for realizing devices with sub-50 nm thin films and nanoscale units. Here, we demonstrate that ∼0.7-nm-thick self-assembled monolayers (SAMs) comprising mercapto-propyl-tri-methoxy-silane (MPTMS) molecules enhance adhesion and inhibit Cu diffusion at Cu/SiO2 structures used in device metallization. Cu/SAM/SiO2/Si(001) structures show three times higher interface debond energy compared to Cu/SiO2 interfaces due to a strong chemical interaction between Cu and S termini of the MPTMS SAMs. This interaction immobilizes Cu at the Cu/SAM interface and results in a factor-of-4 increase in Cu-diffusion-induced failure times compared with that for structures without SAMs.
Low resistance high reflectance contacts to p-GaN using oxidized Ni/Au and Al or Ag
Authors: Hibbard D. L. , Jung S. P. , Wang C. , Ullery D. , Zhao Y. S. , Lee H. P. , So W. , Liu H.
Published: July 2003

We report on a high reflectance low resistance multilayer contact to p-GaN composed of a thin oxidized Ni/Au bilayer overcoated with a thick Al or Ag layer and then capped by Ni/Au. Measurements on 500 μm diameter light emitting diode-like test structures show an operating voltage below 4 V at 20 mA that is comparable to identical devices fabricated with a conventional Ni/Au contact. Back surface light emission is about 70% greater than that from devices with the Ni/Au contact due to lower light absorption by the Al or Ag. Performance for both Al and Ag based contacts is stable at temperatures of up to around 100 °C.
Three-beam interference method for measuring very thin films
Authors: Leizerson I. , Lipson S. G.
Published: July 2003

We present an experimental method which allows the real time measurement of the thickness of a thin film on a transparent substrate in an imaging mode. Imaging ellipsometry cannot be used for such problems because of reflections from the backsurface of the substrate. Its application to the measurement of water films down to 30 Å on mica is described. The sensitivity is found to be comparable to that of imaging ellipsometry. We investigate the relationship between thickness resolution and spatial resolution and give an optional modification of the method which can be applied to opaque substrates.
Shape memory effect in nanoindentation of nickel–titanium thin films
Authors: Shaw G. A. , Stone D. S. , Johnson A. D. , Ellis A. B. , Crone W. C.
Published: July 2003

In this study, a series of nanoindentations was made on NiTi shape memory alloy thin films at millinewton loads with a Berkovich indenter. Mapping of the indentation topography using atomic force microscopy reveals direct evidence that the thermally induced martensitic transformation of these films allows for partial indent recovery on the nanoscale. Indeed, recovery is nearly complete at indentation depths of less than 100 nm. A hemispherical cavity model is presented to predict an upper limit to shape memory recovery of sharp indentations.
Optical properties of annealed CdTe self-assembled quantum dots
Authors: Mackowski S. , Smith L. M. , Jackson H. E. , Heiss W. , Kossut J. , Karczewski G.
Published: July 2003

We report on the influence of postgrowth thermal annealing on the optical properties of CdTe/ZnTe self-assembled quantum dots (SAQDs). Ultrasharp emission lines in the micro-photoluminescence spectra demonstrate the continued presence of quantum dots (QDs) after annealing. Upon annealing, the emission from CdTe QDs shifts toward higher energies with a narrowing of the photoluminescence line of QDs ensemble. In addition, the decay time of the excitonic recombination decreases from 300 ps to 200 ps. These results indicate that interdiffusion of Zn and Cd into and out of the QDs leads to an increase of the average dot size and simultaneously a decrease in the confining potentials of the QD. The absence of a wetting layer makes the II–VI CdTe SAQDs much more sensitive to the thermal treatment than the III–V InAs-based dots.
Imaging the strain fields resulting from laser micromachining of semiconductors
Authors: Borowiec A. , Bruce D. M. , Cassidy Daniel T. , Haugen H. K.
Published: July 2003

The residual strain fields resulting from laser micromachining of grooves in indium phosphide with femtosecond and nanosecond light pulses are analyzed using a spatially resolved degree-of-polarization photoluminescence technique. Significant differences in the geometry of the strain patterns are observed in grooves machined by femtosecond and nanosecond pulses. For the specific conditions investigated, the sign of the degree of polarization signal is opposite in the two cases indicating that areas under tension in femtosecond machined samples are under compression in nanosecond machined samples and visa versa. The experimental data are compared with results from a finite element model.
Assembly of mm-scale macrobridges with carbon nanotube bundles
Authors: Cao Anyuan , Ajayan P. M. , Ramanath G.
Published: July 2003

We report a chemical vapor deposition method for in situ bridging of mm-scale metal-contact patterns with bundles of multiwalled carbon nanotubes. The nanotube bundles synthesized from a hexane–ferrocene–thiophene mixture have a diameter of <50 μm and lengths up to millimeters, typically consisting of tens to hundreds of aligned nanotubes. These bundles are transported to the downstream end of the furnace, where they are captured by relief patterns of metal-contact tips. We can control the orientation and length of the nanotube bridges by preorganizing the metal tips to receive the bundles. This method is amenable to both scaling up, e.g., to create large-area arrays of nanotubes with contact electrodes, as well as scaling down, e.g., to bridge closely spaced contact structures.
Tris-(8-hydroxyquinoline) aluminum nanoparticles prepared by vapor condensation
Authors: Chiu J.-J. , Wang W.-S. , Kei C.-C. , Perng T.-P.
Published: July 2003

Tris-(8-hydroxyquinoline) aluminum (AlQ3) spherical nanoparticles of the average size varying from 50 to 500 nm were synthesized by vapor condensation. The surface of the nanoparticles is quite sleek and smooth like that of pearls. The x-ray diffraction patterns reveal that the nanoparticles have an amorphous structure. The chemical bonding of AlQ3 is preserved in the nanoparticles even after evaporation at 410 °C. The photoluminescence spectra of the nanoparticles show a broadened peak varying from 4500 to 7000 Å, with the maximum intensity at about 5380 Å. The maximum intensity increases as the particle size decreases, owing to the large specific surface area.
Imaging of terahertz radiation using a Rydberg atom photocathode
Authors: Gürtler A. , Meijer A. S. , van der Zande W. J.
Published: July 2003

We report on a photocathode for far-infrared terahertz (THz) pulses based on ionization of gas-phase Rydberg atoms. We demonstrate the cathode by showing measurements of the beam profile of an unfocused THz beam in the far field of the emitter using an open ion optics and a contact print image of a spatial mask using THz pulse energies of less than 10 nJ.
Effects of outflow from the surface of a sphere on drag, shear lift, and scalar diffusion
Authors: Kurose Ryoichi , Makino Hisao , Komori Satoru , Nakamura Mariko , Akamatsu Fumiteru , Katsuki Masashi
Published: July 2003

A three-dimensional numerical simulation of an isothermal flow past a solid sphere with outflow in a linear shear flow is performed to investigate the effects of the outflow on drag and shear lift. In addition, the effects of the outflow and the fluid shear on diffusion and reaction of reactant from the surface of the sphere are also discussed. The results show that the outflow reduces the drag, and, in the linear shear flow, acts to push the sphere to the lower fluid velocity side and promote the negative lift for the high particle Reynolds numbers. The diffusion and reaction of the reactant from the surface of the sphere are strongly affected by the outflow and the fluid shear because these factors cause the deformation of vortices appearing behind the sphere.
On three-dimensional quasiperiodic Floquet instabilities of two-dimensional bluff body wakes
Authors: Blackburn H. M. , Lopez J. M.
Published: July 2003

Previous studies dealing with Floquet secondary stability analysis of the wakes of circular and square cross-section cylinders have shown that there are two synchronous instability modes, with long (mode A) and short (mode B) spanwise wavelengths. At intermediate wavelengths another mode arises, which reaches criticality at Reynolds numbers higher than modes A or B. Here we concentrate on these intermediate-wave number modes for the wakes of circular and square cylinders. It is found that in both cases these modes possess complex-conjugate pair Floquet multipliers, and can be combined to produce either standing or traveling waves. Both these states are quasiperiodic.
Optical properties and electronic structure of rock-salt ZnO under pressure
Authors: Segura A. , Sans J. A. , Manjón F. J. , Muñoz A. , Herrera-Cabrera M. J.
Published: July 2003

This letter reports on the pressure dependence of the optical absorption edge of ZnO in the rock-salt phase, up to 20 GPa. Both vapor-phase monocrystals and pulsed-laser-deposition thin films on mica have been investigated. Rock-salt ZnO is shown to be an indirect semiconductor with a band gap of 2.45±0.15 eV, whose pressure coefficient is very small. At higher photon energies, a direct transition is observed (4.6 eV at 10 GPa), with a positive pressure coefficient (around 40±3 meV/GPa between 5 and 19 GPa). These results are interpreted on the basis of first-principles electronic band structure calculations.
Laser-induced plasmas in micromachined fused silica cavities
Authors: Zeng Xianzhong , Mao Samuel S. , Liu Chunyi , Mao Xianglei , Greif Ralph , Russo Richard E.
Published: July 2003

Cavity formation is a frequent result in many laser ablation applications. Although most theoretical investigations have been devoted to laser ablation on a flat surface, the development of a laser plasma inside a cavity is of both fundamental as well as practical significance. In this study, the temperature and electron number density of laser-induced plasmas in fused silica cavities were determined using spectroscopic methods. The effects of cavity aspect ratio on plasma properties were investigated. The temperature and electron number density of laser-induced plasma were measured to be much higher and to decrease faster for a plasma inside a cavity than on the flat surface. Cavity wall influences on the plasma expansion are discussed.
Fabrication of wurtzite ZnS nanobelts via simple thermal evaporation
Authors: Li Quan , Wang Chunrui
Published: July 2003

Mass production of uniform wurtzite-ZnS nanobelts is achieved by a simple thermal evaporation method using Au as the catalyst. The as-synthesized ZnS nanobelts are single crystalline, usually several tens of microns in length and several hundreds of nanometers in width. Most of the nanobelts grow along [01 1̄ 0] direction. Stacking faults are commonly observed in these nanobelts. The room-temperature cathodoluminescence spectrum of such nanobelts reveals three peaks, which may be ascribed to surface states, defects, and impurity-induced emissions, and is consistent with the nanobelt microstructure. The growth mechanism of the nanobelts is discussed.
Stacking-fault-induced intermediate structure in bismuth titanate
Authors: Yoneda Y. , Mizuki J. , Katayama R. , Yagi K. , Terauchi H. , Hamazaki S. , Takashige M.
Published: July 2003

We observed an intermediate structure during the recrystallization process from the amorphous state of Bi4Ti3O12 prepared by rapid quenching. The intermediate structure which appears during the recrystallization process consists of two phases; one is pyrochlore Bi2Ti2O7 phase and the other is a stacking-fault-induced structure under the excessive Bi condition. The microstructure of the stacking-fault-induced structure was investigated by synchrotron x-ray diffraction. In the case of a large number of Bi2O2, some are inserted between the pseudoperovskite layers of Bi4Ti3O12, and a nonstoichiometric Bi2WO6-like structure is stabilized.
Magnetization configuration and switching behavior of submicron NiFe elements: Pac-man shape
Authors: Park M. H. , Hong Y. K. , Gee S. H. , Erickson D. W. , Choi B. C.
Published: July 2003

Two types of submicron permalloy element, namely Pac-man, were investigated by a magnetic force microscope for magnetization configuration and switching behaviors. Two distinct domain configurations, bidomain for Pac-man type I and single domain for Pac-man type II, were observed in arrays of Pac-man elements. The domain configuration depends on the slot angle for the Pac-man type I, but is independent of the slot angle for the Pac-man type II. Array of Pac-man elements with a slot angle of 180° shows the highest switching field and the narrowest switching field distribution, as compared to rectangular and hexagonal elements of the same overall dimensions.
Optical activity in subwavelength-period arrays of chiral metallic particles
Authors: Vallius Tuomas , Jefimovs Konstantins , Turunen Jari , Vahimaa Pasi , Svirko Yuri
Published: July 2003

We report on a chirality-induced polarization effect in a planar subwavelength metallic nanograting. We demonstrate that the grating rotates the polarization at normal incidence. Because of the fourfold rotation symmetry, the effect does not depend on the incident beam polarization, but resembles optical activity in isotropic media. We use rigorous diffraction theory to show that polarization effects in the zeroth diffraction order take place in the presence of waveguide resonances with subwavelength-period arrays of chiral metallic particles.
Measurements and calculations of the sound attenuation by a phononic band gap structure suitable for an insulating partition application
Authors: Goffaux C. , Maseri F. , Vasseur J. O. , Djafari-Rouhani B. , Lambin Ph.
Published: July 2003

We report a strong attenuation of sound waves in a phononic band gap crystal of steel rods arranged in air. We investigate the attenuation level reached in the gaps for finite thickness samples. We show that a bypassing of sound waves by the sides of the sample are responsible for the observed saturation of the attenuation level in the gaps. Beyond this effect, we demonstrate the improvement of the acoustic insulation performance brought about by the periodicity of the crystal in comparison with the so-called mass law.
Formation and magnetic properties of Fe–Pt alloy clusters by plasma-gas condensation
Authors: Peng D. L. , Hihara T. , Sumiyama K.
Published: July 2003

Size-monodispersed FexPt1−x alloy clusters were synthesized using a plasma-gas-condensation technique which employs two separate elemental sputtering sources and a growth chamber. The composition of the alloy clusters was controlled by adjusting the ratio of the applied sputtering power. We found that high-temperature disordered fcc–FexPt1−x clusters whose mean diameters of 6–9 nm depend on the Ar gas flow ratio were formed for a wide average composition range (x≈0.3–0.7), and the lattice constant of as-doposited clusters increases almost linearly with decreasing x, being extrapolated to the value of pure Pt metal. For Fe49Pt51 cluster-assembled films, high coercivity (8.8 kOe) was obtained by annealing at 600 °C within 10 min due to improved chemical ordering, although as-deposited cluster-assembled films have lower blocking temperatures than room temperature, and show a small coercivity value (∼25 Oe) at room temperature due to intercluster magnetic interaction.
Kinetic surface segregation and the evolution of nanostructures
Authors: Tersoff J.
Published: July 2003

As nonplanar structures evolve via surface diffusion, the difference in diffusivity of the alloy components leads to kinetic surface segregation. This drastically affects the rate of shape evolution, and for nanoscale structures, also the final composition distribution. This is illustrated for a classic problem, the smoothing of a surface ripple. In contrast to the single-component case, alloy evolution is generally faster during growth than during annealing. Moreover, evolution at the nanoscale is generally faster than expected from extrapolation of macroscopic behavior.
Contact resonances in voltage-modulated force microscopy
Authors: Harnagea C. , Alexe M. , Hesse D. , Pignolet A.
Published: July 2003

A study of the frequency dependence of the signal in piezoresponse scanning force microscopy of ferroelectric materials has been performed. It is found that, for soft cantilevers, the signal is governed by the cantilever elastic properties. Both ferroelectric-electromechanical and electrostatic interaction contributions to the overall signal were found to depend on the frequency of the testing voltage. Indications for optimal measurement regimes are given.
Structural and ferroelectric studies of Bi3.44La0.56Ti3O12 films
Authors: Tomar M. S. , Melgarejo R. E. , Hidalgo A. , Mazumder S. B. , Katiyar R. S.
Published: July 2003

Bi 4−x La x Ti 3 O 12 materials were synthesized by the sol-gel process and thin films were prepared on Pt (i.e., Pt/TiO2/SiO2/Si) substrate by spin coating. Structural properties of the films were examined by x-ray diffraction and Raman spectroscopy. Dielectric and ferroelectric response was studied for 0.63-μm-thick Bi3.44La0.56Ti3O12 films. Butterfly dielectric behavior and remnant polarization of up to Pr=41 μC/cm2 has been achieved. These films also showed fatigue free response up to 109 switching cycles. On the basis of these studies, large polarization in Bi3.44La0.56Ti3O12 films is attributed to dipole formation which may tilt TiO6 octahedra to Bi2O2 interlayer.
Effects of film polarities on InN growth by molecular-beam epitaxy
Authors: Xu K. , Yoshikawa A.
Published: July 2003

Effects of the film polarity on InN growth were investigated in molecular-beam epitaxy (MBE). It was found that N-polarity InN could be grown at higher temperatures than In-polarity one. For the In-polarity films, which were grown on Ga-polar GaN template, the highest growth temperature was limited below 500 °C, and the surface morphology and crystal quality tended to be poor mainly because of the tolerated low growth temperature. While for the N-polarity InN films, which were grown on MBE-grown N-polar GaN, the growth temperature could be as high as 600 °C. The step-flow-like growth morphology was achieved for the InN films grown with N polarity at 580 °C. The resulting full widths of half maximum of x-ray rocking curve around InN (002) and (102) reflections were about 200–250 and 950–1100 arc sec, respectively. The photoluminescence of the InN films peaked at 0.697 eV. The recording Hall mobility of InN film grown in N polarity is 1400 cm2/V s with a background carrier concentration of 1.56×1018 cm−3 at room temperature. For both-polarity films, we found N-rich condition was necessary for the stable InN growth.
Strain profiling of HfO2/Si(001) interface with high-resolution Rutherford backscattering spectroscopy
Authors: Nakajima K. , Joumori S. , Suzuki M. , Kimura K. , Osipowicz T. , Tok K. L. , Zheng J. Z. , See A. , Zhang B. C.
Published: July 2003

Strain depth profiles in HfO2 (3 nm)/Si(001) prepared by atomic-layer chemical vapor deposition have been measured using high-resolution Rutherford backscattering spectroscopy in combination with a channeling technique. It is found that the Si lattice is compressed in the vertical direction around the interface. The observed maximum strain is about 1% at the interface and the strained region extends down to ∼3 nm from the interface.
Short-wavelength intersubband absorption in strain compensated InGaAs/AlAs quantum well structures grown on InP
Authors: Georgiev N. , Dekorsy T. , Eichhorn F. , Helm M. , Semtsiv M. P. , Masselink W. T.
Published: July 2003

We have studied intersubband absorption in strain compensated InxGa1−xAs/AlAs/InyAl1−yAs multiple quantum wells and superlattices grown on InP. X-ray diffraction shows that the layers are pseudomorphically strained and exhibit slight compositional grading of the interfaces. Owing to the high AlAs barriers, the intersubband absorption can be tailored to wavelengths shorter than 2 μm. In some samples, a small, but non-negligible absorption is also observed with s-polarized light.
Three-dimensional photonic-crystal emitter for thermal photovoltaic power generation
Authors: Lin S. Y. , Moreno J. , Fleming J. G.
Published: July 2003

A three-dimensional tungsten photonic crystal is experimentally realized with a complete photonic band gap at wavelengths λ⩾3 μm. At an effective temperature of 〈T〉∼1535 K, the photonic crystal exhibits a sharp emission at λ∼1.5 μm and is promising for thermal photovoltaic (TPV) power generation. Based on the spectral radiance, a proper length scaling and a planar TPV model calculation, an optical-to-electric conversion efficiency of ∼34% and electrical power of ∼14 W/cm2 is theoretically possible.
Ion flux nonuniformities in large-area high-frequency capacitive discharges
Authors: Perret A. , Chabert P. , Booth J.-P. , Jolly J. , Guillon J. , Auvray Ph.
Published: July 2003

Strong nonuniformities of plasma production are expected in capacitive discharges if the excitation wavelength becomes comparable to the reactor size (standing-wave effect) and/or if the plasma skin depth becomes comparable to the plate separation (skin effect) [M. A. Lieberman et al., Plasma Sources Sci. Technol. 11, 283 (2002)]. Ion flux uniformity measurements were carried out in a large-area square (40 cm×40 cm) capacitive discharge driven at frequencies between 13.56 MHz and 81.36 MHz in argon gas at 150 mTorr. At 13.56 MHz, the ion flux was uniform to ±5%. At 60 MHz (and above) and at low rf power, the standing-wave effect was seen (maximum of the ion flux at the center), in good quantitative agreement with theory. At higher rf power, maxima of the ion flux were observed at the edges, due either to the skin effect or to other edge effects.
Photonic bandpass filter for 1550 nm fabricated by femtosecond direct laser ablation
Authors: Li Ming , Mori Kiyotaka , Ishizuka Makoto , Liu Xinbing , Sugimoto Yoshimasa , Ikeda Naoki , Asakawa Kiyoshi
Published: July 2003

A bandpass filter, based on the one-dimensional photonic band-gap crystal concept, was fabricated using an ultrafast laser. As predicted by theory, a high-transmission window appeared in the band gap. The photonic crystal operates in the 1550 nm region. A high machining accuracy of ∼15 nm was achieved during our investigation, which is the key for the operability of the photonic device.
Electrical activity of nitrogen acceptors in ZnO films grown by metalorganic vapor phase epitaxy
Authors: Rommeluère J. F. , Svob L. , Jomard F. , Mimila-Arroyo J. , Lusson A. , Sallet V. , Marfaing Y.
Published: July 2003

The electrical activity of nitrogen as an acceptor in ZnO has been investigated in two ways. First, nitrogen was introduced by means of diallylamine during metalorganic vapor phase epitaxy (MOVPE) yielding incorporation of nitrogen in the range 1016–1021 cm−3. This led to significant compensation of the natural donors with a minimum electron concentration of 5×1014 cm−3. Second, diffusion of nitrogen was carried out on undoped MOVPE layers under high pressure conditions stemming from the decomposition of NH4NO3. Conversion to p-type conductivity was observed in a systematic way with measured hole concentrations up to 6.5×1017 cm−3.
Correlation between the ordered structure and the valence-band splitting in highly strained CdxZn1−xTe epilayers
Authors: Kim T. W. , Kwack K. D. , Park J. G. , Lee H. S. , Lee J. Y. , Jang M. S. , Park H. L.
Published: July 2003

Selected-area electron diffraction pattern (SADP) results showed two sets of {1/2 1/2 1/2} superstructure reflections with symmetrical intensities along the [110] axis, and the corresponding high-resolution transmission electron microscopy images indicated a doublet periodicity in the contrast of the {111} lattice planes. Photoluminescence spectra from highly strained CdxZn1−xTe/GaAs heterostructures showed that the valence-band splitting into the heavy hole and the light hole bands occurred as the Cd mole fraction was increased. The valence-band splitting is strongly correlated to the CuPtB-type ordered structure in highly strained heterostructures.