bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantification of polyimide carbonization after laser ablation
Authors: Raimondi F. , Abolhassani S. , Brütsch R. , Geiger F. , Lippert T. , Wambach J. , Wei J. , Wokaun A.
Published: July 2002

Polyimide was irradiated with a XeCl excimer laser (308 nm) and the ablated area and its surrounding were studied using transmission electron microscopy (TEM) and confocal Raman microscopy. Ring-like structures surrounding the ablated area were detected at all fluences. At fluences lower than  250 mJ/cm−2 the formation of conical structures was observed within the irradiated area. The width of the rings increases with fluence and only slightly with the number of pulses. The rings consist mainly of polycrystalline carbon with a relatively high bond angle disorder, with thickness decreasing radially from the crater edge. The thickness of the deposited carbon was determined from TEM analysis and calculated from the intensity ratios of Raman bands assigned to carbon and polyimide using a two layer model. Comparing the two results an estimate of the absorption coefficient of the deposited carbon could be obtained. On top of the cone structures carbon was detected with a higher degree of crystallinity and lower bond angle disorder as compared to the material deposited outside the crater. With energy dispersive x-ray analysis, calcium could be detected on top of the cones. Therefore, it can be assumed that the Ca impurities are causing the cone structures. The higher crystallinity of the carbon inside the irradiated area is probably due to a tempering-like process on top of the Ca compound which is heated upon laser irradiation or to a mixture of growth mechanisms similar to the ones suggested for the formation of carbon nanotubes on metal particles and carbon nanohorns without metal catalysis.
Three-dimensional instability of a multipolar vortex in a rotating flow
Authors: Le Dizès Stéphane
Published: July 2002

In this paper, the elliptic instability is generalized to account for Coriolis effects and higher order symmetries. We consider, in a frame rotating at the angular frequency Ω, a stationary vortex which is described near its center r=0 by the stream function written in polar coordinates Ψ=−(r2/2)+p(rn/n)cos(nθ), where the integer n is the order of the azimuthal symmetry, and p is a small positive parameter which measures the strength of the nonaxisymmetric field. Based on the Lifschitz and Hameiri [Phys. Fluids A 3, 2644–2651 (1991)] theory, the local stability analysis of the streamline Ψ=−1/2 is performed in the limit of small p. As for the elliptic instability [Bayly, Phys. Rev. Lett. 57, 2160–2163 (1986)], the instability is shown to be due to a parametric resonance of inertial waves when the inclination angle ξ of their wave vector with respect to the rotation axis takes a particular value given by cos ξ=±4/(n(1+Ω)). An explicit formula for the maximum growth rate of the inertial wave is obtained for arbitrary ξ, Ω, and n. As an immediate consequence, it is shown that a vortex core of relative vorticity Wr (assumed positive) is locally unstable if Ω<−(1+n/4)Wr/2 or Ω>(−1+n/4−p(n−1)/2)Wr/2. The predictive power of the local theory is demonstrated on several vortex examples by comparing the local stability predictions with global stability results. For both the Kirchhoff vortex and Moore and Saffman vortex, it is shown how global stability results can be derived from the local stability analysis using the dispersion relation of normal (Kelvin) modes. These results are compared to those obtained by global methods and a surprisingly good agreement is demonstrated. The local results are also applied to rotating Stuart vortices and compared to available numerical data.
Thomson scattering in a low-pressure argon mercury positive column
Authors: Bakker L. P. , Kroesen G. M. W.
Published: July 2002

The electron density and the electron temperature in a low-pressure argon mercury positive column are determined using Thomson scattering. Special attention has been given to the stray light reduction in the Thomson scattering setup. The results are obtained in a discharge tube with a 26 mm diam, 5 mbar of argon, a mercury pressure in between 0.14 and 1.7 Pa, and an electric current ranging from 200 to 400 mA. The systematic error in the electron density is 15%, the statistical error is 25%. The total error in the electron temperature is 10%–20%.
Coil size oscillatory packing in polymer solutions near a surface
Authors: van der Gucht J. , Besseling N. A. M. , van Male J. , Cohen Stuart M. A.
Published: July 2002

The theory developed by Scheutjens and Fleer to describe polymer adsorption and depletion is used to calculate the density profile of nonadsorbing polymers near a surface. The theory predicts damped oscillations in the segment density profile with a wavelength of about the coil size. As a consequence, the interaction energy between two surfaces immersed in a solution of nonadsorbing polymers is an oscillatory function of the separation distance, too. The decay length of the oscillations is proportional to the coil size and independent of the polymer concentration. The oscillations are associated with a liquid-like layering of polymer coils near the surface. An increase in concentration or chain length causes a decrease in the amplitude of the oscillations, because the stronger interpenetration of the coils suppresses inhomogeneities. In dilute solutions no oscillations are observed, because the decay length of the oscillations is smaller than the depletion correlation length, in analogy with the Fisher–Widom criterion in simple fluids. For polymer solutions it is found that the Fisher–Widom condition coincides with the overlap concentration. Our results show that the blob size is not the only relevant length scale above the overlap concentration. The size of the individual coils plays a role, too.
High-energy-resolution scintillator: Ce3+ activated LaCl3
Authors: van Loef E. V. D. , Dorenbos P. , van Eijk C. W. E. , Krämer K. , Güdel H. U.
Published: July 2002

The scintillation properties of LaCl3 doped with 10% Ce3+ are presented. Under optical and gamma ray excitation, Ce3+ emission is observed to peak at 330 and 352 nm. The scintillation light output is 46 000±1000 photons/MeV at 662 keV. Forty percent is emitted with a decay time of 26 ns, 30% with 210 ns, and 30% with about 1000 ns. An energy resolution (full width at half maximum over the peak position) of 3.3±0.3% was observed for the 662 keV full absorption peak.
Linear scaling density matrix search based on sign matrices
Authors: Németh Károly , Scuseria Gustavo E.
Published: July 2002

This paper presents a new approach to the linear scaling evaluation of density matrices in electronic structure theory. The new approach is based on the iterative computation of a special matrix function, the sign of the matrix and its performance is compared to that of some other methods developed for similar purpose. One particular variant of the sign approach turned out to be very competitive with other linear scaling density matrix evaluation algorithms, in terms of computational time and accuracy. It is also shown that a special damping technique greatly improves the stability of self-consistent field (SCF) calculations when using density matrix purification and sign methods.
Effective adiabatic index in a wave structure of the solar wind: A case study for 17 December 1990
Authors: Pudovkin Mikhail I. , Lubchich Andris A. , Zaitseva Svetlana A. , Meister Claudia-Veronika , Besser Bruno P.
Published: July 2002

A wave structure observed in the solar wind on 17 December 1990, is discussed, and it is found that this structure is associated with a slow-mode magnetosonic wave. The parameters of the wave such as the direction of the wave vector, the wave frequency and the decrement of attenuation, are estimated. It is shown that the peculiarities of the plasma motion in the wave may provide conditions corresponding to an effective adiabatic index less than unity. The variations of the proton density and temperature are analyzed. It proved that for the entire period of the wave structure passing the Earth, the value of the adiabatic index was less than unity, which agrees with the model by Pudovkin et al. [J. Geophys. Res. 102, 27145 (1997); Astron. Nachr. 320, 87 (1999)].
A theoretical study of the electronic spectrum of water
Authors: Christiansen Ove , Nymand Thomas M. , Mikkelsen Kurt V.
Published: July 2002

The electronic spectrum of water is investigated using coupled cluster electronic structure methods. Vertical excitation energies are calculated for both gas phase H2O, various water clusters, as well as from models designed to simulate condensed phase effects. Four different approaches for describing condensed phase effects on the electronic transitions are investigated: continuum (a single water molecule embedded in a dielectric medium), discrete (water clusters), semidiscrete (a water pentamer cluster embedded in a dielectric medium), and intermolecular perturbation methods. The results are compared with experimental results. The solvent shift on the lowest state appears to be reasonably described by discrete and semidiscrete methods. It is very difficult to model the condensed-phase effects for the higher states of the pure liquid.
Electronic structure of the 4d transition metal carbides: Dispersed fluorescence spectroscopy of MoC, RuC, and PdC
Authors: DaBell Ryan S. , Meyer Raymond G. , Morse Michael D.
Published: July 2002

Dispersed fluorescence studies of the diatomic molecules MoC, RuC, and PdC are reported. New states identified in MoC and RuC are the […]2δ112σ1, 3,1Δ2 states and the […]2δ312σ1, 1Δ2 state, respectively. Five states are observed by dispersed fluorescence in PdC. The ground state is found to be […]2δ412σ2, 1Σ+, with the […]2δ412σ16π1, 3ΠΩ manifold of states lying about 2500 cm−1 above the ground state. The [17.9]Ω=1 state of PdC is also identified as […]2δ412σ113σ1, 3Σ+(Ω=1), corroborating recent results of resonant two photon ionization spectroscopy studies. The spin-orbit interactions of these molecules are analyzed to deduce the composition of the molecular orbitals, and comparisons are made to ab initio theory when possible. An examination of the trends in bond energy, bond length, and vibrational frequency among the 4d transition metal carbides is also provided.
Full quantum mechanical model for the charge distribution and the leakage currents in ultrathin metal–insulator–semiconductor capacitors
Authors: Magnus W. , Schoenmaker W.
Published: July 2002

A method is presented for the evaluation of the charge distribution and quantum-mechanical leakage currents in ultrathin metal–insulator–semiconductor gate stacks that may be composed of several layers of materials. The charge distribution due to the finite penetration depth inside the insulating material stack is also obtained. The method successfully applies the Breit–Wigner theory of nuclear decay to the confined carrier states in inversion layers and provides an alternative approach for the evaluation of the gate currents to that based on the Wentzel–Kramers–Brillouin approximation or Bardeen’s perturbative method. A comparison between experimental and simulated current–voltage characteristics has been carried out.
Coaxial probe and apparatus for measuring the dielectric spectra of high pressure liquids and supercritical fluid mixtures
Authors: Lee Sung B. , Smith Richard L. , Inomata Hiroshi , Arai Kunio
Published: July 2002

A probe and apparatus were developed for measuring the dielectric spectra (complex permittivity) of high pressure liquids and supercritical fluid mixtures. The probe consisted a 2.2 mm semirigid coaxial cable that was cut off flat and mounted into a high pressure tube. The apparatus for measuring complex permittivity consisted of the dielectric probe, cell, densimeter, piston for varying the system density at constant composition, and magnetic pump for agitation and recirculation, all of which were housed in a constant temperature air bath. The probe is simple, robust, inexpensive, and further, its design allows for quick connection to high pressure systems. Probe accuracy is estimated to be ±0.5 in ε′ and ±0.5 in ε″ from 200 MHz to 18 GHz based on replicate measurements of calibration and 2σ deviations over the interval. Dielectric spectra were measured over the 200 MHz–20 GHz range for methanol+carbon dioxide mixture at 323.2 K and a pressures up to 18 MPa.
Second-harmonic and sum-frequency imaging of organic nanocrystals with photon scanning tunneling microscope
Authors: Shen Yuzhen , Swiatkiewicz Jacek , Winiarz Jeff , Markowicz Przemyslaw , Prasad Paras N.
Published: July 2002

Second-harmonic generation and sum-frequency generation with photon scanning tunneling microscopy and shear-force detection are used to map the nonlinear optical response and the surface topograph of N-(4-nitrophenyl)-(L)-prolinol crystals with a subdiffraction-limited resolution. The domain-size dependence of the spatial feature is obtained, which shows the local orientational distribution of the optical near field radiated by nonlinear nanocrystals and reveals the difference between nanoscopic and macroscopic second-order optical nonlinearities of molecular crystals.
Design and development of an interferometer with improved angular tolerance and its application to x–y theta measurement
Authors: Menq Chia-Hsiang , Zhang Ji-Hua , Shi Jian
Published: July 2002

Plane mirror interferometers are widely used in the measurement of x–y motions. Although large range planar motion can be measured, it suffers from very tight angular tolerance. In this article an interferometer system having improved angular range and linear resolution is developed. In the system, a tilted laser beam is incident to a plane mirror attached to a moving stage and an additional retroreflector and a plane mirror are placed off the stage to help return the measuring beam. In such a design both the resolution and angular tolerance are improved. Meanwhile, large travel ranges along both the x and y directions remain. The optical principle pertinent to the proposed optical configuration is explained and an optical path analysis is performed to establish the relation between the optical path change and the motion to be measured. Based on the established relation, design of system parameters and workspace analysis are also discussed. Experimental results including resolution test and scaling procedure are then demonstrated. Using the developed interferometer an x–y theta measurement scheme is introduced. In the proposed scheme, using three interferometers the rotation angle of an x–y theta stage can be obtained first and it can then be used to compensate errors of the measured linear motion.
Evaluation of the hydrostaticity of a helium-pressure medium with powder x-ray diffraction techniques
Authors: Takemura Kenichi
Published: July 2002

The hydrostaticity of a helium-pressure medium has been evaluated with powder x-ray diffraction techniques up to 77 GPa at room temperature. The relative change of d values and the broadening of diffraction peaks have been investigated for three cubic substances, CeO2, the high-pressure rocksalt phase of ZnO, and Au. I observed no evidence of nonhydrostaticity of the helium-pressure medium to at least 50 GPa. The powder x-ray diffraction method has been compared with the ruby fluorescence method in order to get a better understanding of nonhydrostatic stress conditions.
Transmission electron microscopy observation of deformation microstructure under spherical indentation in silicon
Authors: Bradby J. E. , Williams J. S. , Wong-Leung J. , Swain M. V. , Munroe P.
Published: July 2002

Spherical indentation of crystalline silicon has been studied using cross-sectional transmission electron microscopy (XTEM). Indentation loads were chosen below and above the yield point for silicon to investigate the modes of plastic deformation. Slip planes are visible in the XTEM micrographs in both indentation loads studied. A thin layer of polycrystalline material has been identified (indexed as Si-XII from diffraction patterns) on the low-load indentation. The higher-load indentation revealed a large region of amorphous silicon. The sequence of structural deformation by indentation in silicon has been observed with the initial deformation mechanism being slip until phase transformations can take place.
Experimental characterization of the dynamic failure behavior of mortar under impact loading
Authors: Grote D. L. , Park S. W. , Zhou M.
Published: July 2002

The failure behavior of mortar under dynamic impact loading is characterized through a series of plate impact experiments. The analysis focuses on the spall strength and the shear stress carrying capacity in different regions of the specimen under normal impact loading. Special attention is paid to the possible existence of a failure wave phenomenon that has been widely recognized as an important failure mechanism for glasses during plate impact. The experiments are designed to allow the strengths of the medium at locations behind and ahead of a possible failure wave front to be analyzed. The diagnostics used include velocity interferometer system for any reflector (VISAR) recording of the free surface velocities of targets and monitoring of the internal stresses via polyvinylidene fluoride (PVDF) gauges embedded in the specimen. Experiments conducted do not provide evidence for the existence of a failure wave phenomenon for mortar under plate normal impact loading. While the study suggests that a threshold impact stress must be exceeded for failure to occur, a clearly defined failure wave that propagates behind the loading wave is not observed. Instead, a gradual failure process that initiates upon the arrival of the loading wave and progresses thereafter is observed. This gradual failure process in mortar is in contrast to the well-defined failure front and complete loss of tensile strength associated with the failure wave phenomenon reported for glasses. The study also indicates that, under impact involving high levels of input stresses, attenuation of the loading wave occurs as it propagates through the failed medium.
Comparison of aluminum and sodium doped poly(vinylidene fluoride-trifluoroethylene) copolymers by x-ray photoemission spectroscopy
Authors: Xu B. , Choi Jaewu , Borca C. N. , Dowben P. A. , Sorokin A. V. , Palto S. P. , Petukhova N. N. , Yudin S. G.
Published: July 2002

The chemical interaction between the simple metals, aluminum and sodium, and crystalline copolymer thin films of vinylidene fluoride (70%) with trifluoroethylene (30%), has been studied using x-ray photoemission spectroscopy. Aluminum and sodium metalize the polymer differently and different binding sites for the two metals can be inferred from the corresponding core level shifts. Aluminum leads to enhanced screening of final photoemission states associated with the polymer, while sodium doping strongly influences the fluorine, but perturbs the carbon backbone only slightly.
Room-temperature copper etching based on a plasma–copper reaction
Authors: Kuo Yue , Lee Sangheon
Published: July 2002

A promising room-temperature copper etching process is described. The copper thin film can be etched into a vertical profile at a high rate using a parallel-plate reactor under mild conditions. The key factor for the success of this process is a copper swelling phenomenon from the plasma–copper reaction. The reaction product has been identified as a crystalline copper chloride. Key parameters that influence the reaction process and the final copper profile have been studied. In addition to the plasma phase chemistry, ion bombardment plays a critical role in the process. Although results reported in this letter are based on the chlorine plasma, similar results can be obtained with other halogen gases. This process is potentially important to the fabrication of advanced microelectronic, storage, display, and optical devices.
InAs/AlSb quantum-cascade light-emitting devices in the 3–5 μm wavelength region
Authors: Becker C. , Prevot I. , Marcadet X. , Vinter B. , Sirtori C.
Published: July 2002

Midinfrared (3.7–5.3 μm) electroluminescent devices based on a quantum-cascade (QC) design have been demonstrated using InAs/AlSb heterostructures, grown on GaSb substrates. The very high conduction band discontinuity (>2 eV) of this material system allows the design of QC devices at very short wavelengths. Well-resolved luminescence peaks were observed up to 300 K, with a full-width-at-half-maximum to peak wavelength ratio (Δλ/λ) of the order of 8%. The emission wavelengths are in good agreement with the results of our model. The emitted optical power is lower than that predicted, due to a nonoptimized electron injection into the active region.
Maximal mixing rate in turbulent stably stratified Couette flow
Authors: Caulfield C. P. , Kerswell R. R.
Published: July 2002

A rigorous upper bound on the long-time-averaged vertical buoyancy flux is derived from the Navier–Stokes equations for a Boussinesq fluid confined between two parallel horizontal plates a distance d apart, maintained at a constant statically stabilizing temperature difference ΔT and driven at a constant relative velocity ΔU. The upper bound on the volume and long-time-averaged vertical buoyancy flux B≔limt→∞1/t∫0t 〈ρu3〉g/ρ0 dt̃ is B⩽Bmax=(1−16√/Re)(ΔU)3/(64√d), where Re=ΔUd/ν and ρ0 is some reference density. Significantly, Bmax is independent of the bulk Richardson number of the flow and is achieved by an optimal solution with a mixing efficiency (or flux Richardson number) which approaches 0.5 as the Reynolds number becomes large. The time-averaged turbulent dissipation of kinetic energy and the time-averaged vertical buoyancy flux are then in equipartition for the optimizing flow.
Microcrystalline n-i-p tunnel junction in a-Si:H/a-Si:H tandem cells
Authors: Rubinelli F. A. , Rath J. K. , Schropp R. E. I.
Published: July 2002

The kinetics controlling the electrical transport inside the μc-Si tunnel-recombination junction (TRJ) of a-Si:H/a-Si:H tandem solar cells was studied in detail with computer simulations. Trap assisted recombination tunneling and Poole–Frenkel mechanisms were included in our analysis. Three different μc-Si tunnel junctions were investigated: (a) n-p, (b) n-oxide-p and (c) n-i-p. The highest theoretical efficiencies in a-Si:H/a-Si:H tandem cells were achieved with the n-i-p tunnel junction structure. The impact of the μc-Si effective masses, mobility gap, and mobilities in the tandem solar cell efficiency is also studied in this article. Several a-Si:H/a-Si:H tandem solar cells were made with the μc-Si tunnel configurations of types (b) and (c). In all of these samples one extra oxide layer was needed at the i-a-Si:H/n-μc-Si interface. Both tunnel junctions lead us to comparable experimental tandem solar cell efficiencies. When the n-i-p structure is implemented as TRJ in the a-Si:H/a-Si:H tandem solar cell, efficiencies sensitively depend upon the tunnel junction intrinsic layer thickness. The optimization of this thickness provides a more controlled way of maximizing the tandem solar cell efficiency. Illuminated J–V and QE characteristics were successfully fitted using computer modeling.
Birefringence evaluation of multimode multilayer AlGaAs/AlAs waveguides
Authors: Leo G. , Secondini M. , Morabito M. , De Rossi A. , Assanto G. , Fiore A. , Berger V. , Calligaro M. , Nagle J.
Published: July 2002

We investigate surface-emitting second harmonic generation as a powerful technique for the evaluation of birefringence in optical waveguides supporting several modes. This one-shot diagnostic is demonstrated at 1319 nm in multilayer AlGaAs waveguides, before and after the lateral oxidation of AlAs layers.
Structural studies of clusters in melt of FeAl compound
Authors: Hui Li , Feng Ding , Jinlan Wang , Xiufang Bian , Guanghou Wang
Published: July 2002

By means of constant pressure molecular dynamics simulation technique, a series of simulations of the Fe50Al50 alloy have been carried out. The atoms interact via semi-empirical n-body noncentral potential. The pair correlation functions and the pair analysis technique is applied to reveal the cluster evolution in the process of quick solidification. By using the bond orientation order parameters, we have measured both local and extended orientation symmetries for computer-generated models of dense liquid and glass. A lot of polyhedra in liquid systems, e.g., icosahedra, are also obtained. In order to test the reliance of the computational results, corresponding x-ray diffraction experiments have been performed on the material.
Shear-induced diffusion and rheology of noncolloidal suspensions: Time scales and particle displacements
Authors: Breedveld Victor , van den Ende Dirk , Jongschaap Robert , Mellema Jorrit
Published: July 2002

The shear-induced self-diffusion and rheology of concentrated suspensions of noncolloidal hard spheres have been studied experimentally. The combined results provide an interesting physical picture. The projection of the trajectories of individual particles on the vorticity (z)–velocity (x) plane were determined through particle tracking. The particle trajectories turned out to be very useful for gaining qualitative insight into the microscopic particle motion. However, the technique is less suitable to obtain quantitative information. For a quantitative analysis of the particle displacements we measured the evolution of the ensemble averaged displacements as a function of time. The statistical analysis revealed two diffusion regimes, where 〈ΔzΔz〉∼γ̇Δt. For large strain values (γ̇Δt>1) long-time self-diffusion was observed. The associated diffusion coefficient D̂∞ is in excellent agreement with literature data on shear-induced self-diffusion. On very short times (γ̇Δt≪1) a novel diffusive regime was discovered, characterized by a diffusion coefficient D̂0, which is significantly smaller than D̂∞ and grows monotonically with φ. D̂0 is detected on time scales on which the particle configuration is not changed significantly and thus it must represent the fluctuating motion of particles in the “cage” formed by their nearest neighbors. Finally, the rheology was studied with steady shear and oscillatory rheometry. The dynamic measurements in a controlled stress rheometer revealed that the viscoelastic response of the suspension is determined mainly by the amplitude of deformation. At small strain amplitudes γ0<1, the response is linear and a dynamic viscosity η′ is found, which is in excellent agreement with the high frequency limit η∞′ as reported in literature for colloidal hard sphere suspensions. Around γ0=1 the “cage” around a particle is deformed and a shear-induced microstructure is built. This leads to O(a) displacements of the particles and the viscoelastic response becomes strongly nonharmonic. Although the effect persists at large amplitudes, it becomes relatively small for γ0≫1. The microstructure is rearranged immediately after flow reversal and remains unchanged for the larger part of the period of oscillation. As a result a pseudolinear viscoelastic regime is found with a viscosity close to steady shear viscosity. Experiments show a correlation between the time scales controlling the D̂0/D̂∞ diffusive behavior and the ones controlling the shear-induced changes in particle configuration as probed by the rheological measurements.
Lattice dynamics and elasticity of silver thiogallate (AgGaS2) from ab initio calculations
Authors: Łażewski J. , Parlinski K.
Published: July 2002

Using first principle calculations within the local density approximation with either norm-conserving nonlocal or ultrasoft pseudo-potentials the structural parameters of AgGaS2 were calculated. The phonon dispersion relations were determined from Hellmann–Feynman forces with the direct method using 2×2×1 supercell. Obtained phonon frequencies at the Γ point were compared with Raman and infrared data, and they agree very well. The elastic constants derived from the stress–strain relationships agree with experiments too and show that this material is elastically quite anisotropic.
Structure and mechanical flexibility of carbon nanotube ribbons: An atomic-force microscopy study
Authors: Yu Min-Feng , Dyer Mark J. , Ruoff Rodney S.
Published: July 2002

Collapsed multiwalled carbon nanotube (MWCNT) sections and segments (carbon nanotube ribbons) on surfaces were observed by atomic-force microscopy. The collapsed form of these MWCNTs allowed the determination of the layer number and the inner and outer diameters for their uncollapsed form. The results show that such carbon nanotube ribbons are extremely flexible and readily conform to the underlying substrate morphology. The formation of the collapsed form of these nanotubes is consistent with recent theoretical analysis on the mechanics of nanotubes.
Giant magnetoresistance in ion beam deposited spin-valve films with specular enhancement
Authors: Sant S. , Mao M. , Kools J. , Koi K. , Iwasaki H. , Sahashi M.
Published: July 2002

Three different techniques, natural oxidation, remote plasma oxidation and low energy ion beam oxidation, have been proved to be equally effective in forming nano-oxide layers (NOLs) in spin-valve films for specular enhancement of giant magnetoresistance (GMR) effect. GMR values over 12% have been routinely obtained in spin-valve films with NOL, corresponding to a 30% specular enhancement over those without NOL. The consistency and robustness of the oxidation processes has been demonstrated by a very large GMR value ∼19% in a dual spin-valve film with the NOLs formed in both pinned layers, the oscillatory dependence of the interlayer coupling field on Cu layer thickness in specular enhanced spin-valve films and the uniform and repeatable film performance over 5 in. substrates.
External coupling efficiency in planar organic light-emitting devices
Authors: Lu M.-H. , Sturm J. C.
Published: July 2002

The external coupling efficiency in planar organic light-emitting devices is modeled based on a quantum mechanical microvavity theory and measured by examining both the far-field emission pattern and the edge emission of light trapped in the glass substrate. The external coupling efficiency is dependent upon the thickness of the indium–tin–oxide layer and the refractive index of the substrate. The coupling efficiency ranges from ∼24% to ∼52%, but in general it is much larger than the 18.9% expected from classical ray optics.
Nonlinear dispersion properties of subwavelength photonic crystals
Authors: Vijayalakshmi S. , Zhang Y. , Grebel H. , Yaglioglu Gul , Dorsinville R. , White C. W.
Published: July 2002

Nonlinear pulse compression and pulse broadening have been analyzed and demonstrated with a coherent array of submicron silica spheres embedded with silicon and germanium nanoparticles (nano within a nano).
Oblique stacking of three-dimensional dome islands in Ge/Si multilayers
Authors: Sutter P. , Mateeva-Sutter E. , Vescan L.
Published: July 2002

The organization of Ge “dome” islands in Ge/Si multilayers has been investigated by cross-sectional transmission electron microscopy. Ge domes are found to spontaneously arrange in oblique stacks, replicating at a well-defined angle from one bilayer to the next. The formation of oblique island stacks is governed by a complex interplay of surface strain, generated by the already buried islands, and surface curvature, caused by the inherent tendency of large domes to carve out material from the surrounding planar substrate.
The dilution wave in polymer crystallization is described by Fisher’s reaction-diffusion equation
Authors: Higgs Paul G. , Ungar Goran
Published: July 2002

Monodisperse long-chain alkanes such as C198H398 form lamellar crystals in both extended- and folded-chain forms. Folded-chain crystals are in a meta-stable equilibrium with polymer solution at a concentration CF. The crystal growth rate is virtually zero at this point, due to the self-poisoning phenomenon. If extended-chain crystallization is initiated from this state, a wave of crystallization proceeds through the solution, termed the dilution wave. The solution concentration falls as the wave passes, until a value CE is reached that is in equilibrium with the extended-chain crystal phase. We write down a reaction-diffusion equation to describe the dilution wave, and show that this is equivalent to Fisher’s equation, which has previously been used to describe many other traveling wave phenomena. Numerical solutions of the equation are used to show examples of the wave shape.
Application of reduced-order controller to turbulent flows for drag reduction
Authors: Lee Keun H. , Cortelezzi Luca , Kim John , Speyer Jason
Published: July 2002

A reduced-order linear feedback controller is designed and applied to turbulent channel flow for drag reduction. From the linearized two-dimensional Navier–Stokes equations a distributed feedback controller, which produces blowing/suction at the wall based on the measured turbulent streamwise wall-shear stress, is derived using model reduction techniques and linearquadratic-Gaussian/loop-transfer-recovery control synthesis. The quadratic cost criterion used for synthesis is composed of the streamwise wall-shear stress, which includes the control effort of blowing/suction. This distributed two-dimensional controller developed from a linear system theory is shown to reduce the skin friction by 10% in direct numerical simulations of a low-Reynolds number turbulent nonlinear channel flow. Spanwise shear-stress variation, not captured by the distributed two-dimensional controller, is suppressed by augmentation of a simple spanwise ad hoc control scheme. This augmented three-dimensional controller, which requires only the turbulent streamwise velocity gradient, results in a further reduction in the skin-friction drag. It is shown that the input power requirement is significantly less than the power saved by reduced drag. Other turbulence characteristics affected by these controllers are also discussed.
Transport properties of V–VI semiconducting thermoelectric BiSbTe alloy thin films and their application to micromodule Peltier devices
Authors: Boulouz A. , Chakraborty S. , Giani A. , Delannoy F. Pascal , Boyer A. , Schumann J.
Published: July 2002

Thin semiconducting thermoelectric films with narrow energy band gaps are considered to be very promising for future microdevice applications (sensors and generators). The polycrystalline BiSbTe alloys (V–VI semiconductors) are examples. In this report, the detailed temperature dependence of electrical resistivity [ρ(T)], n- and p-type carrier concentration [n(T) and p(T)], and Hall mobility [μ(T)] of n-type Bi2Te3, p-type Sb2Te3, and p-type (Bi1−xSbx)2Te3 (x=0.73 and 0.77) alloy films prepared by metalorganic chemical vapor deposition are presented in the range of 100–500 K. From the room temperature measurement of the Seebeck coefficient (α), the values of α for Bi2Te3, Sb2Te3, and (Bi1−xSbx)2Te3 with x=0.73 and 0.77 are found to be −220, +110, +240, and +210 μV/K, respectively, which are optimal in these types of film materials. The carrier concentration of these films at 300 K is found to be around (1019–1020) cm−3. The ρ(T) data show an exponential increase with increasing temperature irrespective of the carrier types. For the temperature dependence of the Hall mobility, the lattice contribution is found to be predominant for all the films. Also, we have fabricated a simple micromodule Peltier device (MMP) using the n-type Bi2Te3 and the p-type (Bi1−xSbx)2Te3 (x=0.77) films where a maximum cooling of 2.6 °C was obtained with a low input current of 2.5 mA.
Retardation of boron diffusion in silicon by defect engineering
Authors: Shao Lin , Lu Xinming , Wang Xuemei , Rusakova Irene , Liu Jiarui , Chu Wei-Kan
Published: July 2002

By judiciously placing vacancy and interstitial defects at different depths, we are able to enhance or retard boron diffusion. This opens up a new approach for the formation of shallow P+n junction in silicon. After preimplantation with 50 or 500 keV Si+ ions to produce a surface vacancy-rich region, we studied the diffusion of deposited B on predamaged samples with annealing between 900 and 1010 °C. Boron diffusion retardation was observed in both implantation conditions after low temperature annealing with enhancement occurring in a 50 keV implanted sample at high temperature. Choosing high energy implantation to separate vacancies and interstitials can reduce the boron diffusion significantly. Such suppression became more obvious with higher implant doses. A junction less than 10 nm deep (at 1×1017 cm−3 according to carrier concentration profiles) can be formed.
Photodissociation of D2O at 121.6 nm: A state-to-state dynamical picture
Authors: Harich Steven A. , Yang Xuefeng , Hwang Dennis W. H. , Lin Jim J. , Yang Xueming , Dixon Richard N.
Published: July 2002

The photodissociation dynamics of D2O at 121.6 nm have been studied using the D atom Rydberg “tagging” time-of-flight technique. Product kinetic energy distributions and angular distributions have been measured. From these distributions, rovibronic distributions of the OD radical product and the rotational state-resolved angular anisotropy parameters have been determined. The dissociation energy D00 (D–OD) is determined to be 41912±20 cm−1. Alternations of population in the OD (X, v=0) rotational distribution, similar to those in the H2O photodissociation [S. A. Harich et al., J. Chem. Phys. 113, 10 073 (2000)] have also been observed, and thus are also attributed to dynamical interference between two conical intersection pathways. Very highly vibrationally excited OD (X) products (at least up to v=9) have also been observed, which is in accord with recent theoretical studies of H2O photodissociation by van Hemert et al. [J. Chem. Phys. 112, 5797 (2000)]. Evidence for the triple dissociation channel, O(3P)+2D, is also shown in this work. Branching ratios for the different product channels have been determined through simulations. Overall, the energy disposal among the products of D2O photodissociation at 121.6 nm is qualitatively similar to that for H2O, although the mass change leads to changes in the branching ratios.
Gas–liquid phase separation in charged colloidal systems
Authors: Reščič Jurij , Linse Per
Published: July 2002

A model system of charged spherical macroions and oppositely charged point counterions with a macroion-charge to counterion-charge ratio Zr=10 was investigated by employing a temperature and density scaling Monte Carlo simulation method. This approach allowed for a determination of the relative free energies among arbitrary number of thermodynamic states over a substantial region of the state space. Here, relative free energies were determined for the macroion volume fraction interval 0.04<φM<0.24 at different values of the electrostatic coupling parameter ΓII≡ZI2LB/RM, where ZI denotes the counterion charge, LB the Bjerrum length, and RM the macroion radius. The binodal curve separating the one-phase system and a gas-liquid two-phase region was determined near the critical point which was determined to φMcr=0.15 and ΓIIcr=2.6.
Epitaxial growth of (103)-oriented ferroelectric SrBi2Ta2O9 thin films on Si(100)
Authors: Lee Ho Nyung , Senz Stephan , Pignolet Alain , Hesse Dietrich
Published: July 2002

Non-c-axis-oriented ferroelectric SrBi2Ta2O9 (SBT) epitaxial thin films with (103) orientation have been grown by pulsed laser deposition on buffered Si(100) substrates. For the buffer layers, a heterostructure consisting of MgO(111)/YSZ(100)/Si(100) was applied to induce the growth of a (111)-oriented SrRuO3 (SRO) bottom electrode. X-ray diffraction θ–2θ and φ scans revealed well-defined orientation relationships, viz. SBT(103)∥SRO(111)∥MgO(111)∥YSZ(100)∥Si(100); SBT[010]∥SRO[01̄1]∥MgO[01̄1]∥YSZ〈001〉∥Si〈001〉. The ferroelectric measurements of the (103)-oriented SBT films showed a remanent polarization (Pr) of 5.2 μC/cm2 and a coercive field (Ec) of 76 kV/cm for a maximum applied electric field of 440 kV/cm.
Population dynamics advected by chaotic flows: A discrete-time map approach
Authors: López Cristóbal , Hernández-Garcı́a Emilio , Piro Oreste , Vulpiani Angelo , Zambianchi Enrico
Published: July 2002

A discrete-time model of reacting evolving fields, transported by a bidimensional chaotic fluid flow, is studied. Our approach is based on the use of a Lagrangian scheme where fluid particles are advected by a two-dimensional symplectic map possibly yielding Lagrangian chaos. Each fluid particle carries concentrations of active substances which evolve according to its own reaction dynamics. This evolution is also modeled in terms of maps. Motivated by the question, of relevance in marine ecology, of how a localized distribution of nutrients or preys affects the spatial structure of predators transported by a fluid flow, we study a specific model in which the population dynamics is given by a logistic map with space-dependent coefficient, and advection is given by the standard map. Fractal and random patterns in the Eulerian spatial concentration of predators are obtained under different conditions. Exploiting the analogies of this coupled-map (advection plus reaction) system with a random map, some features of these patterns are discussed.
High-power AlGaInN flip-chip light-emitting diodes
Authors: Wierer J. J. , Steigerwald D. A. , Krames M. R. , O’Shea J. J. , Ludowise M. J. , Christenson G. , Shen Y.-C. , Lowery C. , Martin P. S. , Subramanya S. , Götz W. , Gardner N. F. , Kern R. S. , Stockman S. A.
Published: July 2002

Data are presented on high-power AlGaInN flip-chip light-emitting diodes (FCLEDs). The FCLED is “flipped-over” or inverted compared to conventional AlGaInN light-emitting diodes (LEDs), and light is extracted through the transparent sapphire substrate. This avoids light absorption from the semitransparent metal contact in conventional epitaxial-up designs. The power FCLED has a large emitting area (∼0.70 mm2) and an optimized contacting scheme allowing high current (200–1000 mA, J∼30–143 A/cm2) operation with low forward voltages (∼2.8 V at 200 mA), and therefore higher power conversion (“wall-plug”) efficiencies. The improved extraction efficiency of the FCLED provides 1.6 times more light compared to top-emitting power LEDs and ten times more light than conventional small-area (∼0.07 mm2) LEDs. FCLEDs in the blue wavelength regime (∼435 nm peak) exhibit ∼21% external quantum efficiency and ∼20% wall-plug efficiency at 200 mA and with record light output powers of 400 mW at 1.0 A.
Ultrafast directional beam switching in coupled vertical-cavity surface-emitting lasers
Authors: Ning C. Z. , Goorjian P.
Published: July 2002

We propose a strategy to performing ultrafast directional beam switching using two coupled vertical-cavity surface-emitting lasers (VCSELs). The proposed strategy is demonstrated for two VCSELs of 5.6 μm in diameter placed about 1 μm apart from the edges, showing a switching speed of 42 GHz with a maximum far-field angle span of about 10°.
Opal photonic crystals infiltrated with chalcogenide glasses
Authors: Astratov V. N. , Adawi A. M. , Skolnick M. S. , Tikhomirov V. K. , Lyubin V. , Lidzey D. G. , Ariu M. , Reynolds A. L.
Published: July 2002

Composite opal structures for nonlinear applications are obtained by infiltration with chalcogenide glasses As2S3 and AsSe by precipitation from solution. Analysis of spatially resolved optical spectra reveals that the glass aggregates into submillimeter areas inside the opal. These areas exhibit large shifts in the optical stop bands by up to 80 nm, and by comparison with modelling are shown to have uniform glass filling factors of opal pores up to 40%. Characterization of the domain structure of the opals prior to infiltration by large area angle-resolved spectroscopy is an important step in the analysis of the properties of the infiltrated regions.
Insulator–metal transition in Coulomb blockade nanostructures
Authors: Aslam M. , Mulla I. S. , Vijayamohanan K.
Published: July 2002

The transition from insulating to metallic behavior is observed at low temperatures for the protected nanocluster arrays of Cu, Ag, and Au. The disappearance of the Kubo gap at low temperature in these systems, where the interparticle spacing (0.5–1 nm) is less than the nanocluster dimensions (10–15 nm), is explained to effect the transition due to strongly coupled charge fluctuations and cluster vibrations. For the same interparticle separation of Cu, Ag, and Au clusters the transition temperature is found to vary significantly, possibly due to the difference in localization arising from the lack of precise size distribution.
Enhanced performance of pulse driven small area polyfluorene light emitting diodes
Authors: Wilkinson C. I. , Lidzey D. G. , Palilis L. C. , Fletcher R. B. , Martin S. J. , Wang X. H. , Bradley D. D. C.
Published: July 2002

We have investigated the performance of conjugated polymer light emitting diodes (LEDs) as their active areas are reduced to dimensions typical for pixellated displays. We find that by reducing the active diameter of the device from our standard size (1.5×3 mm) to 50 μm, the current density that can be sustained before LED failure increases by a factor of 9. This increase in current density is mirrored by similar increases in maximum LED brightness. We propose that such effects occur due to improved thermal management in small area devices. Using pulsed driving schemes, devices with a diameter of 50 μm sustained current densities of 5.1 MA m−2, and have emitted electroluminescence with a brightness of 6.5 Mcd m−2.
Electrical characterization and metallurgical analysis of Pd-containing multilayer contacts on GaN
Authors: Chor E. F. , Zhang D. , Gong H. , Chen G. L. , Liew T. Y. F.
Published: July 2002

Ti(250 Å)/Al(2200 Å)/Pd(600 Å)/Au(1600 Å) contact on aqua-regia surface treated n-GaN (Si∼1.0×1019 cm−3) has yielded a minimum specific contact resistance (ρc) of 4.21×10−8 Ω cm2, achieved after thermal annealing at 500 °C for 8 min. This is superior to the most common n-GaN contact, Ti/Al, which has also been studied in the current work and produced a lowest ρc of 4.63×10−6 Ω cm2, obtained after annealing at 700 °C for 2 min. The long-term thermal stability analysis has also revealed that the Ti/Al/Pd/Au contact is more stable than Ti/Al on n-GaN. The projected mean time to 50% increase in ρc(μ50) at 150 °C for the former is 1.22×1012 h, which is higher by about three orders of magnitude than that of the latter at 3.54×109 h. Pd(200 Å)/Ni(300 Å)/Au(2000 Å) contact on boiling aqua-regia surface treated p-GaN (Mg∼1.0×1018 cm−3) has also been investigated and demonstrated a reasonable ohmic behavior with a ρc of 5.03×10−4 Ω cm2 after thermal annealing at 450 °C for 2 min. However, its thermal stability is mediocre with μ50 at 150 °C of only 1.49×103 h.
The potential energy surface and vibrational structure of C3H−
Authors: Lakin Nicholas M. , Hochlaf Majdi , Chambaud Gilberte , Rosmus Pavel
Published: July 2002

A six-dimensional potential energy surface (PES) for the singlet electronic ground state of C3H− has been generated by electronic structure calculations using the coupled cluster CCSD(T) approach. Two potential minima are located: the global one, corresponding to an aromatic, cyclic structure (1A1), and a local one, lying about 0.56 eV higher in energy, with a trans chain structure (1A′). Both minima are found to be separated by a relatively high barrier on the singlet surface but the isomerization process can also proceed via a coupling between the singlet and triplet PESs with a lower barrier. Variational calculations for the vibrational levels (J=0) up to 3000 cm−1 were carried out for both isomers, taking into account the full dimensionality of the problem. The present results allow a clear distinction between the singlet isomers by infrared experiments. The structure of the vibrational and rotational stacks in both isomers is very different. For some vibrational overtones and combination levels inverse anharmonicity was found. The high density of the vibrational levels particularly in the chain structure suggests the existence of strong anharmonic resonances. In the bent isomer, the excited in-plane ν5 and out-of-plane ν6 modes fall into near-degenerate clusters of levels.