bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulated and measured data-line parasitic capacitance of amorphous silicon large-area image sensor arrays
Authors: Mulato M. , Lu J. P. , Street R. A.
Published: July 2002

Calculations and measurements of parasitic capacitance in active matrix sensor arrays used for light and x-ray imaging are presented. We focus on arrays with continuous sensor layers and base the calculations on actual structures used for x-ray and light imaging. Different cross sections of the pixel allow the various components of the capacitance from the thin film transistor, the sensor, and metal crossovers to be determined by numerical two-dimensional solution of Poisson’s equation. The calculations give the total and the individual components of the parasitic capacitance in the data line, and allow us to evaluate their effect on electronic noise and imager sensitivity. The theoretical values are compared to measurements performed on arrays with 75 μm pixel pitch, showing agreement within 10%–20%. The numerical simulations are used to determine the optimized array configuration that can reduce the parasitic capacitance to ∼6 fF/pixel, which is only 15% of the present values. The capacitance is compared for direct and indirect x-ray detection imagers, using PbI2 and a-Si:H sensor layers, respectively.
Temporal evolution of the shock wave and hot core air in laser induced plasma
Authors: Sobral Hugo , Villagrán-Muniz Mayo , Navarro-González Rafael , Raga Alejandro C.
Published: July 2002

The temporal evolution of electric breakdown in air at atmospheric pressure by Nd:yttrium–aluminum–garnet Q-switched nanosecond laser pulses was studied from the nanosecond to the millisecond time scale by shadowgraphy and interferometry techniques. The results were modeled with a gasdynamic code with good agreement. It was possible to simultaneously model the whole evolution of the plasma, the shock wave, and the hot core air. The shock wave velocity was determined to be ⩾60 km s−1 at 20 ns. The plasma temperature was found to reach about 1.7×104 K at 1 μs and the hot core air temperature was determined to be <103 K at 100 μs. This letter presents an experimental work that extends the study of laser induced plasmas to millisecond time scales.
Thermodynamic aspects in a simplified model for the folding of two-stranded coiled-coils
Authors: Prolongo Silvia G. , Rubio Ana M. , Rey Antonio
Published: July 2002

We have investigated the thermodynamic properties of a simple model representing the thermal folding/unfolding transition of two-stranded coiled-coils. The transition temperature and the energy change for the process are analyzed in terms of the peptide concentration, using the standard properties and calculations involved in experimental work. The integration of the heat capacity curves provides realistic and correct results for the model, as it does the variation of the transition temperature with concentration. On the other hand, the van’t Hoff analysis of the equilibrium constant for the unfolding process produces apparently odd results. They can only be rationalized through a careful analysis of the reaction stoichiometry, according to the reference state defined for the very simple model interactions, and the definition of the unfolded state. This point is extensively discussed, for its possible implications in the correct analysis of this and other simulation models.
CuH and CuBr absorption measurements and consequences on copper seeding in a Cu–HBr laser medium
Authors: Girard Frédéric , Le Guyadec Erick , Delaporte Philippe
Published: July 2002

Molecular absorption measurements have been made across the tube radius and with respect to time in a Cu–HBr laser media. CuBr and CuH relative densities in their fundamental levels are found from spectra around 428 and 434 nm. CuBr density decreases from the wall to zero absorption on the axis with a steep gradient at 15 mm from the axis. Inversely, CuH is present on the axis and its absorption decreases toward the wall. Dissociation of the CuH molecules in different excited levels can explain the temporal and radial increase of the copper ground-state density in our operating conditions in a Cu–HBr laser.
Fractal crystallization and nonlinear V–I behavior of Au/Ge bilayer film
Authors: Chen Zhiwen , Zhang Shuyuan , Tan Shun , Hou Jianguo , Zhang Yuheng , Sekine Hisashi
Published: July 2002

The fractal formation and V–I behavior of the Au/Ge bilayer film have been investigated with a transmission electron microscope and by the two-probe configuration method. The experimental results suggest that the Au/Ge bilayer film, after the fractal crystallization, shows a nonlinear V–I behavior. This phenomenon was explained by the random tunneling junction model.
Relaxor behavior of La-modified (Pb0.75Ba0.25)(Zr0.70Ti0.30)O3 ceramics
Authors: Adamczyk M. , Ujma Z. , Hańderek J.
Published: July 2002

Strong influence of the lanthanum (La) admixture added to the lead–barium–zirconate–titanate (PBZT) ceramics of a chosen composition (Ba/Zr/Ti-25/70/30) on grain structure, electric conductivity, as well as dielectric and pyroelectric properties was confirmed. The La-modified ceramics exhibit classical relaxor ferroelectric behavior similar to other complex lead perovskites such as lead–lanthanum–zirconate–titanate-type ceramics. Additional anomalies in ε(T) curves in low frequency range were observed in the paraelectric phase for undoped ceramics. These anomalies and some disturbances in regularities typical for the relaxor ferroelectric behavior in the vicinity of diffuse ferroelectric–paraelectric phase transition in undoped PBZT ceramics are eliminated by the La admixture, which reduces the p-type electric conductivity by three orders of magnitude. An attempt at a qualitative explanation is given in the article.
Effect of Ga substitution on the magnetocrystalline anisotropy of arc-melted Sm2Co17−xGax (x=5,6) compounds
Authors: Liang B. , Kleinschroth I. , Kronmüller H.
Published: July 2002

The effect of Ga substitution on the magnetocrystalline anisotropy of Sm2Co17−xGax (x=5,6) compounds has been investigated by means of x-ray diffraction (XRD) and magnetic measurements. XRD patterns show that the samples studied here crystallize in the rhombohedral Th2Zn17-type structure. The Curie temperature for Sm2Co12Ga5 and Sm2Co11Ga6 is 367 and 166 K, respectively, according to thermal-magnetic curves. XRD measurement on the magnetically aligned sample shows that the room temperature easy magnetization direction of Sm2Co12Ga5 compound corresponds to the c axis. The spin-reorientation transition is observed at 32 K for Sm2Co11Ga6. The anisotropy constants K1 and K2 of Sm2Co17−xGax (x=5,6) were derived by fitting the magnetization recoil curve measured between 5 K and their Curie temperature. An anomalous jump in magnetization is found in the magnetization curve of Sm2Co12Ga5 compound. In order to investigate the temperature dependence of the critical field Hcrit in which the jumps of magnetization occur, hysteresis loops were measured in a field range of −90–90 kOe between 5 and 150 K. The critical field is found to shift rapidly to lower value with increasing temperature.
Differential calculuses on the quantized braided groups
Authors: Gao Ya-Jun , Gui Yuan-Xing
Published: July 2002

Braided differential calculuses on the quantized braided groups are constructed and their braided bialgebra (Hopf algebra) structures are demonstrated. These are a kind of generalization and unification of the differential calculuses on quantum groups, braided groups, quantum supergroups, etc., and contain the latter ones as special cases. Moreover, it is shown that some quantum differential (co)vector algebras are covariant under the braided “local” coactions of the obtained braided differential bialgebras (Hopf algebras). Some examples are also given.
Lattice deformation and interdiffusion of InAs quantum dots on GaAs(100)
Authors: Matsumura N. , Haga T. , Muto S. , Nakata Y. , Yokoyama N.
Published: July 2002

InAs quantum dots (QDs) on GaAs(100) grown by molecular-beam epitaxy were structurally characterized by ion channeling. Lattice deformation of the InAs QDs and diffusion of Ga atoms into InAs QDs were clearly observed to depend strongly on the InAs coverage. It was revealed that the diffusion is significantly enhanced when the InAs coverage is changed from 1.53 to 1.71 monolayer. During this change, lattice deformation was reduced while the average size (base diameter) of dots was decreased. These phenomena suggest that some growth process change occurred.
Effect of pressure on the microstructure of α-Fe/Sm2(Fe, Si)17Cx nanocomposite magnets
Authors: Zhang X. Y. , Zhang J. W. , Wang W. K.
Published: July 2002

The effect of pressure on the microstructure of α-Fe/Sm2(Fe, Si)17Cx nanocomposite magnets has been studied by annealing amorphous Sm8Fe85Si2C5 alloy under pressures up to 6 GPa at 923 K. The high-pressure experiments were carried out in a belt-type apparatus. Experimental results show that, with increasing pressure from normal pressure to 6 GPa, the grain size of the α-Fe and Sm2(Fe, Si)17Cx phases in the α-Fe/Sm2(Fe, Si)17Cx nanocomposites decreases, from 30.6 to 6.4 nm for the former and from 28.7 to 5.8 nm for the latter. The volume fraction of Sm2(Fe, Si)17Cx increases as pressure increases. The α-Fe/Sm2(Fe, Si)17Cx nanocomposite magnets prepared under 4 GPa have a significant increase in both coercivity and remanent magnetization as compared to those prepared under normal pressure.
Nuclear dynamics near conical intersections in the adiabatic representation: I. The effects of local topography on interstate transitions
Authors: Yarkony David R.
Published: July 2002

The local topography of a conical intersection can be represented by four parameters, readily determined from multireference configuration interaction wave functions, describing the pitch and tilt of the double cone. The time-dependent Schrödinger equation is solved in the vicinity of a conical intersection in the adiabatic basis using an approach tailored to this representation. It is shown that an adiabatic state treatment, which offers conceptual advantages is, in the appropriate set of internal coordinates, not qualitatively more difficult than the equivalent calculation in a diabatic basis. The present treatment is fully hermitian and takes full account of the geometric phase effect being, for example, gauge invariant (in the infinite basis limit) and could be used to develop a fully adiabatic description of nonadiabatic dynamics. The gauge invariant formulation provides interesting insights into the consequences of neglecting the geometric phase. The algorithm is used to study the effects of the double cone’s topography on the outcome of a nonadiabatic transition. Transitions from both the upper state to the lower state and from the lower state to upper state are considered for representative sets of conical parameters. The effects of the local topography on the outcome of nonadiabatic transitions can be dramatic.
Spontaneous compositional modulation in the AlGaN layers of a thick AlGaN/GaN multilayer structure
Authors: Levin I. , Robins L. H. , Vaudin M. D. , Tuchman J. A. , Lakin E. , Sherman M. J. , Ramer J.
Published: July 2002

A periodic modulation with the wave vector parallel to the [0001] direction was observed in the AlGaN layers of a thick AlGaN/GaN multilayer heterostructure grown by metalorganic chemical vapor deposition. The modulation was attributed to a nearly sinusoidal spatial variation of the Al/Ga ratio with an average periodicity of about 3 nm. The observed periodicity was highly regular and incommensurate with the periodicity of the underlying lattice. The average Al fraction in the AlxGa1−xN layers was estimated to be x=0.115±0.010 and the peak-to-valley amplitude of the modulation was estimated to be Δx=0.075±0.016.
Preferential ion scattering from 6H-SiC: Identification of the substitutional site of the implanted Ga impurities
Authors: Satoh M. , Nakaike Y. , Kuriyama K.
Published: July 2002

The substitutional site of the implanted Ga impurity in (11̄00)-oriented 6H-SiC has been investigated using the preferential scattering effect of He ions. The Si and C monoatomic strings are preferentially observed by the angular scans across 〈11̄00〉 axial channels parallel to (112̄0) plane. Ga ions are implanted at an energy of 400 keV to a dose of 5×1015/cm2 through 200-nm-thick SiO2 at room temperature. In the samples annealed at 1200 °C for 30 min, the implanted Ga impurities in SiC are located preferentially on the Si sublattice sites rather than on the C sublattice sites in 6H-SiC.
Ultra-highly doped Si1−xGex(001):B gas-source molecular-beam epitaxy: Boron surface segregation and its effect on film growth kinetics
Authors: Kim H. , Glass G. , Desjardins P. , Greene J. E.
Published: July 2002

Si 1−x Ge x (001) layers doped with B concentrations CB between 2×1016 and 2×1021 cm−3 were grown on Si(001)2×1 at Ts=500–700 °C by gas-source molecular-beam epitaxy (GS-MBE) from Si2H6, Ge2H6, and B2H6. Secondary-ion mass spectrometry measurements of modulation-doped structures demonstrate that B doping has no effect on the Ge incorporation probability. Steady-state B and Ge surface coverages (θB and θGe) were determined as a function of CB using in situ isotopically tagged temperature-programmed desorption. Results for Si0.82Ge0.18 layers grown at Ts=500 °C show that θGe remains constant at 0.63 ML while the bulk B concentration increases linearly up to 4.6×1020 cm−3, corresponding to saturation coverage at θB,sat=0.5 ML, with the incident precursor flux ratio ξ=JB2H6/(JSi2H6+JGe2H6). B is incorporated into substitutional electrically active sites over this entire concentration range. At higher B concentrations, CB increases faster than ξ and there is a large decrease in the activated fraction of incorporated B. The B segregation enthalpy during Si0.82Ge0.18(001) growth is −0.42 eV, compared to −0.53 and −0.64 eV during Si(001):B and Ge(001):B GS-MBE, respectively. Measured segregation ratios rB=θB/xB, where xB is the bulk B fraction, range from 15 to 500 with a temperature dependence which is consistent with equilibrium segregation. Film deposition rates RSiGe(CB) decrease by up to a factor of 2 with increasing CB⩾5×1019 cm−3, due primarily to a B-segregation-induced decrease in the dangling bond density. The above results were used to develop a robust model for predicting the steady-state H coverage θH, θB, θGe, and RSiGe as a function of ξ and Ts.
Vibrational modes of c(4×2) CO–Pt(111) from first-principles calculations
Authors: Hassel M.
Published: July 2002

The vibrational modes of CO adsorbed on Pt(111) in the c(4×2) structure have been studied within the harmonic approximation, using density functional calculations. The characters, fundamental energies, and dipole activities have been determined for all modes. For top-adsorbed molecules, the static adsorbate–adsorbate interaction is found to induce energy splitting among frustrated lateral translational modes, which have previously been assumed to be degenerate, and a reassignment of previously measured vibrational energies to low-energy modes is proposed. For bridge-adsorbed molecules, the frustrated rotational fundamental transitions, which should be dipole forbidden from the local adsorption site symmetry, are found to be weakly dipole active.
Characterizing the local topography of conical intersections using orthogonality constrained parameters: Application to the internal conversion S1→S in HNCO
Authors: Yarkony David R.
Published: July 2002

Portions of the S1[1A″(2 1A)]–S0[1A′(1 1A)] seam of conical intersection relevant to the internal conversion S1→S0 of photoexcited isocyanic acid HNCO are analyzed. The topography of the potential energy surfaces, and the derivative coupling, in the vicinity of a conical intersection is described in terms of four conical parameters. These parameters are also used to obtain a local diabatic representation that removes the singularity in the derivative coupling. Continuity is achieved through the use of a recently described orthogonalization procedure. The conical parameters demonstrate that the double cones of concern are significantly tilted, which has important implications for the nuclear dynamics.
Saturation of normal-mode coupling in aluminum-oxide-aperture semiconductor nanocavities
Authors: Lee E. S. , Ell C. , Brick P. , Spiegelberg Ch. , Gibbs H. M. , Khitrova G. , Deppe D. G. , Huffaker D. L.
Published: July 2002

The photon density required for the saturation of normal-mode coupling in oxide-apertured nanocavities is measured to be 90 photons/μm2 by pump-probe experiments. The photon number is only 300 for a semiconductor nanocavity with a 2 μm diameter aluminum-oxide aperture, drastically reduced from 200 000 for a 50 μm waist on a planar microcavity.
Ar dilution effects on the elastic properties of hydrogenated amorphous hard-carbon films grown by plasma-enhanced chemical vapor deposition
Authors: Valentini L. , Kenny J. M. , Carlotti G. , Socino G. , Lozzi L. , Santucci S.
Published: July 2002

Hydrogenated amorphous carbon (a-C:H) films were deposited, at room temperature, from a CH4/Ar plasma produced by a radio frequency (rf) glow discharge system at 13.56 MHz, and different power values. X-ray reflectivity and Brillouin light scattering have been used to investigate the relationship between deposition conditions and elastic properties of hydrogenated amorphous carbon films. The composition of the initial gaseous mixture and the rf power input are shown to affect significantly both the mechanical and the microstructural properties of deposited films. As the fraction of methane in the feed gas is increased, both the film density and the phase velocity of the surface Rayleigh wave exhibit an enhancement, thus indicating the production of a harder material. On the other hand, x-ray reflectivity measurements suggest the occurrence of a higher degree of structural order in the sp2 lattice. Experimental findings are discussed in terms of the different chemical composition of the plasma.
Dielectric function models for describing the optical properties of hexagonal GaN
Authors: Djurišić Aleksandra B. , Li E. Herbert
Published: July 2002

Several different models have been employed for modeling the dielectric function of hexagonal GaN in the range from 1 to 10 eV. Models are compared in terms of number of parameters required, intricacy of model equations, and possibility of accurate estimation of important physical parameters, such as energies of critical points and exciton binding energies. Shortcomings and advantages of each model are discussed in detail. Excellent agreement with the experimental data for GaN has been achieved with three of the investigated models. It has also been shown that an assumption of adjustable broadening instead of a purely Lorentzian one improves the agreement with the experimental data and enables elimination of excessive absorption below the gap which is inherent to the models with Lorentzian broadening.
Role of the energy transfer in the optical properties of undoped and Er-doped interacting Si nanocrystals
Authors: Priolo Francesco , Franzò Giorgia , Pacifici Domenico , Vinciguerra Vincenzo , Iacona Fabio , Irrera Alessia
Published: July 2002

In this article the luminescence properties of Si nanocrystals (nc) formed by plasma enhanced chemical vapor deposition and their interaction with Er ions introduced by ion implantation are investigated in detail. Si nc with different size distributions and densities were produced and all show quite intense room temperature luminescence (PL) in the range 700–1100 nm. It is shown that the time-decay of the luminescence follows a stretched exponential function whose shape tends towards a single exponential for almost isolated nc. This suggests that stretched exponential decays are related to the energy transfer from smaller towards larger nc. Indeed, by comparing samples with similar nc size distributions, but with very different nc densities, it is demonstrated that the PL has a quite strong redshift in the high density case, demonstrating a clear energy redistribution within the sample. Excitation cross sections have been measured in all samples yielding a value of ∼1.8×10−16 cm2 for isolated nc excited with 2.54 eV photons. This effective excitation cross section is shown to increase by a factor of 4 in interacting nc as a result of the energy transfer within the sample. When Er ions are introduced in these samples a strong nc–Er interaction sets in and the energy is preferentially transferred from the nc to the Er ions. The nc-related luminescence is quenched and the Er-related luminescence at 1.54 μm appears. The effective excitation cross section of Er ions through Si nc has been determined to be ∼1.1×10−16 cm2. This number resembles the excitation cross section of nc themselves demonstrating that the coupling is extremely strong. Moreover, by increasing the Er content the effective excitation cross section is seen to increase. In the same concentration range the Er lifetime decreases demonstrating that “concentration quenching” effects, with the energy transferred among Er ions, are setting in. These Er–Er interactions are responsible for the effective increase of the cross section. However, since the increase in the cross section is related to a simultaneous decrease in lifetime the net effect for the luminescence efficiency is negative. The best Er content to take advantage of the sensitizer action of Si nc avoiding the detrimental Er–Er interactions has been determined to be ∼2×1020/cm3. These data are presented and their implications discussed.
Domain size determination of granular ferromagnetic systems with neutron depolarization
Authors: te Velthuis S. G. E. , van Dijk N. H. , Rekveldt M. Th. , Sietsma J. , van der Zwaag S.
Published: July 2002

The depolarization of a polarized neutron beam after transmission through a granular magnetic medium is related to the size and orientation of the ferromagnetic domains. Unfortunately, this relationship is simple only for a limited number of cases. In order to investigate the interpretation of neutron depolarization data, calculations for a three-dimensional model system of granular magnetic domains are presented. A comparison is made between the equations generally used to approximate the average domain size and the actual average domain size of the model system. Calculations are performed for several configurations of monodisperse spherical domains, as well as for a polydisperse distribution of nonspherical domains. While there is a good agreement between the actual domain size and that derived from the calculated neutron depolarization at low volume fractions of magnetic domains, at increasing volume fractions there is an increasing discrepancy. This discrepancy is partially due to magnetic correlations between domains and is sensitive to the magnetic configuration. Furthermore, the size distribution and the spatial distribution of the domains in the sample have a significant effect on the neutron depolarization data.
Nonspecular x-ray reflectivity study of roughness scaling in Si/Mo multilayers
Authors: Freitag J. M. , Clemens B. M.
Published: July 2002

The interfacial roughness and lateral correlation length of a series of Si/Mo multilayers with bilayer period 69 Å and number of bilayers ranging from 5 to 40 have been characterized by diffuse x-ray scattering. Superlattice peaks are preserved in offset radial scans indicating a high degree of conformality in the roughness. The lateral correlation length ξ increases with total film thickness h as ξ∼h0.55; however, the magnitude of the roughness is approximately 2 Å for all film thicknesses, in disagreement with scaling laws for self-affine growing surfaces. This observation suggests that interfaces retard the evolution of high-frequency roughness while replicating longer wavelength roughness from one layer to the next
Analytical model of the fluorescence fluctuation spectroscopy experiment
Authors: Novikov Eugene , Boens Noël
Published: July 2002

The mathematical formalism of generating functionals was adopted for the unified statistical description of a typical experiment in fluorescence fluctuation spectroscopy. The generating functionals formalism leads to a set of functional equations, linking the statistical characteristics of the excitation and detected photons via the characteristics of a molecular system. The statistical characteristics of the detected photons can then be obtained in a straightforward and consistent way. We particularly show how characteristics such as the photocount correlation function and the probability distribution of the number of photocounts per detection interval can be derived based on the developed theory. We further demonstrate that the probability density distribution of the first and single photocount time occurrences in the detection interval also can be used for probing molecular behavior. The presented approach is applicable to a broad range of experimental configurations (continuous and pulsed excitation, one-photon and two-photon laser excitation, scanning laser excitation) and molecular movements (diffusion in two and three dimensions, fluid flows).
Tautomerism of 6-thioxanthine in the gas and aqueous phases using AM1 and PM3 methods
Authors: Ünal Civcir Pervin
Published: July 2002

Heats of formation, entropies, Gibbs free energies, relative tautomerization energies, tautomeric equilibrium constants, relative proton affinities, dipole moments, and ionization potentials for the fourteen possible tautomers of 6-thioxanthine have been studied by using semiempirical AM1 and PM3 quantum-chemical calculations at the self-consistent field level, both in the gas and aqueous phases, with full geometry optimization. The conductor-like screening solvation model was employed for aqueous solution calculations. The calculations show that the two oxothione tautomers, TX (1,3,7) and TX (1,3,9), are the predominant species at room temperature in both phases, but the oxothione-N7(H) form is more stable than the oxothione-N9(H) form. The energy difference between TX (1,3,7) and TX (1,3,9) is predicted to be significantly lowered by the polar solution. The results are in good agreement with available experimental results. The entropy effect on the Gibbs free energy of the 6-thioxanthine base is very small and has practically no significance for the tautomeric equilibrium of the base. The enthalpic term is dominant in the determination of the equilibrium constant. The protonation of 6-thioxanthine in both phases occurs at the imidazole nitrogen (N7 or N9), then pyrimidine nitrogen (N1 or N3).
The effect of acid-base equilibria on the fractional charge and conformational properties of polyelectrolyte solutions
Authors: Shew Chwen-Yang , Yethiraj Arun
Published: July 2002

The effect of acid-base equilibria on the properties of polyelectrolyte solutions is investigated using the self-consistent polymer reference interaction site model theory with the random phase approximation. In the model, each monomer on the chain consists of charged and protonated sites, which contribute an electrostatic repulsion and a short-ranged attraction, respectively. The proportions of charged and protonated sites are governed by a chemical equilibrium constant. The polymer chains are highly charged and expanded when the strength of attraction is low. As the strength of the attraction is increased, the monomer fractional charge and chain size decrease in a continuous or discontinuous manner, depending on the equilibrium constant. For a fixed value of the strength of the attractive interaction, the monomer charge fraction and chain size display a nonmonotonic dependence on the concentration. The theory could provide a rationalization of some recent experimental measurements of the conformational and structural properties of DNA fragments and synthetic polyelectrolytes.
Ethanol separation from ethanol-water solution by ultrasonic atomization and its proposed mechanism based on parametric decay instability of capillary wave
Authors: Sato Masanori , Matsuura Kazuo , Fujii Toshitaka
Published: July 2002

We show the experimental data of selective ethanol separation from ethanol–water solution, using ultrasonic atomization. Pure ethanol could be obtained directly from a solution with several mol% ethanol–water solution at 10 °C. This result can be explained in terms of parametric decay instability of capillary wave, in which high localization and accumulation of acoustic energy occur, leading to ultrasonic atomization. That is, parametric decay instability condenses the energy of longitudinal waves in a highly localized surface area of the capillary wave, and causes ultrasonic atomization.
Filament instability and rotational tissue anisotropy: A numerical study using detailed cardiac models
Authors: Rappel Wouter-Jan
Published: July 2002

The role of cardiac tissue anisotropy in the breakup of vortex filaments is studied using two detailed cardiac models. In the Beeler–Reuter model, modified to produce stable spiral waves in two dimensions, we find that anisotropy can destabilize a vortex filament in a parallelepipedal slab of tissue. The mechanisms of the instability are similar to the ones reported in previous work on a simplified cardiac model by Fenton and Karma [Chaos 8, 20 (1998)]. In the Luo–Rudy model, also modified to produce stable spiral waves in two dimensions, we find that anisotropy does not destabilize filaments. A possible explanation for this model-dependent behavior based on spiral tip trajectories is offered.
Scanned-probe field-emission studies of vertically aligned carbon nanofibers
Authors: Merkulov Vladimir I. , Lowndes Douglas H. , Baylor Larry R.
Published: July 2002

Field emission properties of dense and sparse “forests” of randomly placed, vertically aligned carbon nanofibers (VACNFs) were studied using a scanned probe with a small tip diameter of ∼1 μm. The probe was scanned in directions perpendicular and parallel to the sample plane, which allowed for measuring not only the emission turn-on field at fixed locations but also the emission site density over large surface areas. The results show that dense forests of VACNFs are not good field emitters as they require high extracting (turn-on) fields. This is attributed to the screening of the local electric field by the neighboring VACNFs. In contrast, sparse forests of VACNFs exhibit moderate-to-low turn-on fields as well as high emission site and current densities, and long emission lifetime, which makes them very promising for various field emission applications.
Crystal lattice of the cadmium alkanoate monolayer at the air/water interface investigated by polarization modulation infrared spectroscopy
Authors: Ren Yanzhi , Iimura Ken-ichi , Kato Teiji
Published: July 2002

The crystal lattice of an arachidic acid/Cd2+ (CdA) monolayer on the surface of aqueous CdCl2 subphase in a Langmuir trough was investigated in situ by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The spectra reveal a hexagonal packing of the hydrocarbon chains on the subphase of pH=6.2 and 293–283 K. Cooling from 283 to 280 K induces the hexagonal unit cell to transform to an orthorhombic one, as evidenced by the splitting of the CH2 scissoring mode. This transition is observed to be reversible with temperature variation within 293–274 K. On the subphase of pH=8.4, the crystal lattice of CdA is orthorhombic even at 293 K. The temperature-dependent crystal lattice transition of CdA is compared with the monotonic hexagonal subcell of a cadmium stearate monolayer within 293–274 K and the orthorhombic subcell of a cadmium behenate monolayer within 293–274 K.
Microwave characterization of filled polymers
Authors: Brosseau Christian , Quéffélec Patrick , Talbot Philippe
Published: July 2002

Interest in filled polymers has expanded in recent years as investigators have recognized the great flexibility allowed by these materials to suit particular properties such as electrical, mechanical, and/or coupling between these properties. This article describes the work undertaken to investigate the microwave response of two different types of samples: one with carbon black or silica particles embedded in a linear low-density polyethylene, and the other with carbon black particles or carbon fibers embedded in an epoxy resin. We report broad-band (30 MHz–14 GHz) measurements of the complex permittivity of these materials obtained by measuring the scattering parameters (S parameters) of a microstrip line loaded with a rectangular sample of the test material. The experimental results presented give access to data which can be rationalized in terms of a combination of Bruggeman’s self-consistent model with Jonscher’s phenomenological analysis. This analytical approach yields data that are in good correspondence with experimental data in terms of the concentration dependence of inclusions within the polymeric matrixes and demonstrates large practical capabilities for analyzing the electromagnetic properties of these materials at microwave frequencies because it allows one to make an explicit connection between these properties and the experimentally accessible parameters.
Force-free equilibria and reconnection of the magnetic field lines in collisionless plasma configurations
Authors: Bobrova N. A. , Bulanov S. V. , Sakai J. I. , Sugiyama D.
Published: July 2002

The plasma equilibrium in the force-free magnetic field in the framework of collisionless approximation is investigated. The equilibrium solution of the Vlasov–Maxwell equations, describing the distribution function of charged particles in the one-dimensional force-free magnetic field is obtained. It is shown that such a magnetic field can exist in plasma with anisotropic temperatures. Then, the tearing-mode instability of this configuration is investigated, and the results of 2D3V PIC (two coordinates and three velocity components particle-in-cell) simulations of the magnetic field lines reconnection during nonlinear development of the tearing-mode instability of the collisionless force-free equilibrium are presented.
Nonmetallic gaskets for ultrahigh pressure diamond-cell experiments
Authors: Solli Daniel , Jeanloz Raymond
Published: July 2002

We have developed freestanding, nonmetallic gaskets for use in ultrahigh-pressure experiments with the diamond-anvil cell. These gaskets consist of a thermal-epoxy binder mixed with an additional ∼10%–30% (by weight) of either an oxide (MgO), or a carbide (B4C) plus oxide filler. Experiments at pressures above 30–40 GPa show that these insulating-composite gaskets can play a significant role in ultrahigh-pressure research.
Magnetic-field measurements of current-carrying devices by force-sensitive magnetic-force microscopy with potential correction
Authors: Alvarez Tony , Kalinin Sergei V. , Bonnell Dawn A.
Published: July 2002

A scanning probe technique for current-carrying device imaging is proposed that combines magnetic-force microscopy with surface-potential nulling measurements. The device is ac biased at an off-resonant frequency and the current-induced magnetic field results in cantilever deflection which is detected by a lock-in amplifier. An ac bias at the resonant frequency is simultaneously applied to the tip and conventional scanning surface-potential microscopy feedback is used to match the tip and surface potentials. This multiple-modulation technique allows electrostatic and magnetic interactions to be distinguished and surface-potential and magnetic-force images to be collected simultaneously. The technique, which is referred to as potential-correction magnetic-force microscopy, produces force rather than force-gradient images as in conventional magnetic-force microscopy. Further prospects for potential-sensitive magnetic-force imaging are discussed.
Dielectrophoresis with application to boiling heat transfer in microgravity. II. Experimental investigation
Authors: Snyder Trevor J. , Chung Jacob N. , Schneider John B.
Published: July 2002

The objective of this paper is to analyze experimentally the feasibility of utilizing the dielectrophoretic (DEP) force to sustain boiling in space where the gravity-driven buoyancy force is absent. First, a bubble trajectory experiment is present to determine the magnitude of the DEP force produced at the edge of two diverging-plate electrodes to the highly nonuniform electric field. The results showed that the DEP forces measured experimentally are equal to those predicted in Part I to within 10%–15%. Second, the bubble detachment diameter and frequency of bubbles generated with a gold-film, single-bubble heater were measured in microgravity so that the masking effect of gravity could be eliminated. In particular, a comparison is made between the DEP forces produced with a relatively high-electric-field gradient at the edge of two flat-plate electrodes to the DEP forces produced with the relatively low electric-field gradient between two diverging-plate electrodes. It was discovered that the bubble detachment diameter and frequency could be reduced to a single curve by plotting the data against effective gravity ratio [g(b,e)] due to the dielectrophoretic forces. In this way, the normalized bubble diameter at the time of breakoff from the heated surface was found to vary inversely as the effective gravity ratio to the 1/2 power.
Influence of heteroepitaxy on the width and frequency of the E2 (high)-phonon line in GaN studied by Raman spectroscopy
Authors: Giehler M. , Ramsteiner M. , Waltereit P. , Brandt O. , Ploog K. H. , Obloh H.
Published: July 2002

Wurtzite GaN layers are commonly grown heteroepitaxially on 6H–SiC or Al2O3 substrates, because of the lack of lattice-matched substrates. We study the influence of these substrates mainly on the E2(high)-phonon Raman line by temperature dependent Raman spectroscopy. We find that the line broadening with sample heating is predominantly caused by intrinsic phonon–phonon scattering in GaN. The small three-phonon contribution as well as the small intrinsic linewidth at low temperature are due to the rather low two-phonon density of states at the E2(high)-phonon energy. Substrates with large lattice mismatch cause inhomogeneous strain and defects in the layers, which lead to a large, temperature independent, line broadening. We show that the temperature shift of the E2(high)-phonon frequency is dominated by the GaN lattice expansion. The lattice of epilayers is strongly modified by the thermal in-plane expansion of the substrate. The degree of relaxation at the growth temperature is reflected by deviation of the E2(high)-line from the intrinsic phonon frequency.
Robustness of the bistable behavior of a biological signaling feedback loop
Authors: Bhalla Upinder S. , Iyengar Ravi
Published: July 2002

Biological signaling networks comprised of cellular components including signaling proteins and small molecule messengers control the many cell function in responses to various extracellular and intracellular signals including hormone and neurotransmitter inputs, and genetic events. Many signaling pathways have motifs familiar to electronics and control theory design. Feedback loops are among the most common of these. Using experimentally derived parameters, we modeled a positive feedback loop in signaling pathways used by growth factors to trigger cell proliferation. This feedback loop is bistable under physiological conditions, although the system can move to a monostable state as well. We find that bistability persists under a wide range of regulatory conditions, even when core enzymes in the feedback loop deviate from physiological values. We did not observe any other phenomena in the core feedback loop, but the addition of a delayed inhibitory feedback was able to generate oscillations under rather extreme parameter conditions. Such oscillations may not be of physiological relevance. We propose that the kinetic properties of this feedback loop have evolved to support bistability and flexibility in going between bistable and monostable modes, while simultaneously being very refractory to oscillatory states.
Alfvén wave heating and current drive
Authors: Elfimov A. G.
Published: July 2002

Progress in the theory of Alfvén wave (AW) heating, current drive, and plasma flow, which may be relevant for improved confinement scenarios in tokamaks, is discussed. The effect of poloidal mode coupling on the power deposition of AW eigenmodes is investigated. Resonant absorption of the coupled side bands causes a broad power deposition close to the plasma boundary which can surpass the power deposition of the main global AW at the plasma center. A small population of impurities, such as carbon, in a hydrogen plasma can strongly modify the dispersion of the global AW, the AW continuum and the wave dissipation profile. The ponderomotive forces produced by rf fields are expressed as a sum of a gradient (radial derivative) part and of a wave momentum transfer force found to be proportional to wave dissipation. Finally, the rf force is balanced by the friction (or neo-classical viscosity) force to calculate relevant current drive and plasma specie flows. Estimates of AW power requirements to drive substantial heating, current, and poloidal flow are made for plasmas in the Tokamak Chauffage Alfvén wave experiment in Brazil (TCABR) [L. Ruchko et al., Nucl. Fusion 30, 503 (1996)]. The first results of Alfvén wave heating and current drive experiments with a low level of wave dissipation in the TCABR are discussed.
Near-field scanning microwave probe based on a dielectric resonator
Authors: Abu-Teir M. , Golosovsky M. , Davidov D. , Frenkel A. , Goldberger H.
Published: July 2002

We report a near-field microwave microscopy based on a novel scanning probe—a long and narrow slot microfabricated on the convex surface of the dielectric resonator. The probe is mounted in the cylindrical waveguide. Tunable coupling to the probe is effectuated through the variable air gap. The whole probe is very compact, has a coaxial input, operates at 25–30 GHz, has a spatial resolution of 1–10 μm and, most important, has a low impedance of ∼20 Ω. This allows us to use it for characterization of metallic layers with high conductivity, in particular, thickness mapping.
Mechanism for the high reactivity of CO oxidation on a ruthenium–oxide
Authors: Liu Zhi-Pan , Hu P. , Alavi Ali
Published: July 2002

A long-standing puzzle in heterogeneous catalysis, CO oxidation on a Ru(0001) surface under high oxygen pressure, was clarified recently by Over et al. [Science 287, 1474 (2000)], who obtained experimental evidence that the active phase is RuO2(110) when the reaction is operated at an elevated temperature. To find microscopic details of the reaction and to understand the high reactivity of RuO2(110), we performed density functional theory calculations for CO oxidation on RuO2(110). The transition state is located and the reaction barrier is determined to be 1.15 eV, being considerably lower than that on Ru(0001). The high reactivity of RuO2(110) for CO oxidation is analyzed.
Temperature dependence of the critical current in high-Tc superconductors with low-angle boundaries between crystalline blocks
Authors: Pashitskiı̆ É. A. , Vakaryuk V. I. , Ryabchenko S. M. , Fedotov Yu. V.
Published: July 2002

A model for the limiting of the critical current in rather perfect high-Tc superconducting crystals and epitaxial films with a block structure with small angles of misorientation θ of the crystalline blocks is considered for the case when the distance d between edge dislocations along the boundary between blocks is greater than the coherence length ξ(T). It is shown that under these conditions the transparency of low-angle boundaries for the superconducting current carriers near the critical temperature Tc is practically independent of θ and T. As a result, the only factor governing the temperature dependence of the critical current density jc(T) remains the depairing current j0(T)∝(1−T/Tc)3/2. Near Tc, when ξ(T)>d, a transition from the dependence jc(T)∼(1−T/Tc)3/2 to a dependence jc(T)∼(1−T/Tc)2 occurs. This behavior of jc(T) is in good agreement with the results of experimental measurements of the critical currents in thin epitaxial films of YBa2Cu3O7−δ.
Monte Carlo estimation of neoclassical transport for the TJ-II stellarator
Authors: Tribaldos V.
Published: July 2002

The neoclassical transport properties of TJ-II stellarator [C. Alejaldre et al., Fusion Technol. 13, 521 (1988)] are studied with the monoenergetic Monte Carlo technique. A compromise between the number of modes and particles and the required computing time to obtain reliable estimates, from the computational point of view, of the monoenergetic diffusion coefficients is shown to be of one thousand particles and one hundred harmonics, because of the rich magnetic-field structure of TJ-II. Although, these requirements are probably too demanding in making the transport estimations. The data base containing the normalized monoenergetic diffusion coefficient for several radial positions, radial electric fields and collisionalities have been fitted using a neural network. This fit reduces the number of points necessary in the data base and allows a smooth interpolation and extrapolation to perform the convolutions of the monoenergetic coefficients with the Maxwellian. For two different typical TJ-II discharges the ambipolar radial electric field, and the neoclassical particle and heat fluxes are presented both showing rather large positive radial electric fields at the plasma core and small negative fields at the edge. The neoclassical particle and energy confinement time are in surprisingly good agreement with the experimental energy balance analysis and the international stellarator scaling. Although no satisfactory explanation is available yet the large neoclassical diffusion caused by the complex ripple structure of TJ-II magnetic field may be an important ingredient.
Ballistic current transport studies of ferromagnetic multilayer films and tunnel junctions (invited)
Authors: Rippard W. H. , Perrella A. C. , Buhrman R. A.
Published: July 2002

Three applications of ballistic electron microscopy are used to study, with nanometer-scale resolution, the magnetic and electronic properties of magnetic multilayer thin films and tunnel junctions. First, the capabilities of ballistic electron magnetic microscopy are demonstrated through an investigation of the switching behavior of continuous Ni80Fe20/Cu/Co trilayer films in the presence of an applied magnetic field. Next, the ballistic, hot-electron transport properties of Co films and multilayers formed by thermal evaporation and magnetron sputtering are compared, a comparison which reveals significant differences in the ballistic transmissivity of thin film multilayers formed by the two techniques. Finally, the electronic properties of thin aluminum oxide tunnel junctions formed by thermal evaporation and sputter deposition are investigated. Here the ballistic electron microscopy studies yield a direct measurement of the barrier height of the aluminum oxide barriers, a result that is invariant over a wide range of oxidation conditions.
On the theory of an Omegatron with asymmetric three-dimensional electric fields: Ion trajectories and resonance peak shapes
Authors: Amerl P. V. , Krouse H. R. , Fichtner H.
Published: July 2002

The theory and modeling of an Omegatron, an instrument exploiting the cyclotron resonance principle for various laboratory measurements, is revisited. A general model for the ion motion in the instrument is developed to compute expected resonance peak shapes. In order to numerically integrate the equation of ion motion, the actual three-dimensional electric field in which the ions move is computed. With this model, the influence of various parameters on the shape of the resonance peaks is studied, and the theoretical findings are compared with experimental results.