bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of premetallization surface treatment on the formation of Schottky Au-nGaN contacts
Authors: Maffeis T. G. G. , Simmonds M. C. , Clark S. A. , Peiro F. , Haines P. , Parbrook P. J.
Published: August 2002

The influence of premetallization surface preparation on the structural, chemical, and electrical properties of Au–nGaN interfaces has been investigated by x-ray photoemission spectroscopy (XPS), current-voltage measurement (I-V) and cross-section transmission electron microscopy (TEM). XPS analysis showed that the three GaN substrate treatments investigated i.e., ex situ hydrofluoric acid etch, in situ anneal in ultrahigh-vacuum (UHV), and in situ Ga reflux cleaning in UHV result in surfaces increasingly free of oxygen contamination. XPS and TEM characterization of Au–nGaN formed after the three premetallization surface treatments show that HF etching and UHV annealing produce abrupt, well-defined interfaces. Conversely, GaN substrate cleaning in a Ga flux results in Au/GaN intermixing. I-V characterization of Au–nGaN contacts yields a Schottky barrier height of 1.25 eV with a very low-ideality factor and very good contact uniformity for the premetallization UHV anneal, while the Ga reflux cleaning results in a much lower barrier (0.85 eV), with poor ideality and uniformity. I-V and XPS results suggest a high density of acceptor states at the surface, which is further enhanced by UHV annealing. These results are discussed in the context of current models of Schottky barrier formation.
Titanium dioxide thin-film growth on silicon (111) by chemical vapor deposition of titanium(IV) isopropoxide
Authors: Sandell A. , Anderson M. P. , Alfredsson Y. , Johansson M. K.-J. , Schnadt J. , Rensmo H. , Siegbahn H. , Uvdal P.
Published: August 2002

The initial stages of TiO2 growth on Si(111) under ultra-high vacuum conditions is studied using core level photoelectron spectroscopy, x-ray absorption spectroscopy, and scanning tunneling microscopy. The TiO2 film was formed by means of chemical vapor deposition of titanium(IV) isopropoxide at a sample temperature of 500 °C. The thickness and composition of the amorphous interface layer and its subsequent transition to crystalline anatase TiO2 are discussed. Three different stages are identified: In the initial stage (film thickness <10 Å), the oxygen atoms are coordinated mainly to Si atoms giving rise to Ti atoms with oxidation states lower than 4+. At this stage, a small amount of carbon (0.15 ML) is observed. The next stage (<25 Å) is best described as an amorphous TiSixOy compound in which the oxidation state of Ti is 4+ and the x and y values vary monotonically with the film thickness, from 2 to 0 and 4 to 2, respectively. Finally (>30 Å) a stoichiometric TiO2 layer starts to form. The TiO2 phase is anatase and the layer consists of particles ∼10 nm wide.
High-frequency impedance of driven superlattices
Authors: Jappsen A.-K. , Amann A. , Wacker A. , Schöll E. , Schomburg E.
Published: August 2002

The complex impedance of a semiconductor superlattice biased into the regime of negative differential conductivity and driven by an additional gigahertz ac voltage is computed. From a simulation of the nonlinear spatiotemporal dynamics of traveling field domains we obtain strong variations of the amplitude and phase of the impedance with increasing driving frequency. These serve as fingerprints of the underlying quasiperiodic or frequency locking behavior. An anomalous phase shift appears as a result of phase synchronization of the traveling domains. If the imaginary part of the impedance is compensated by an external inductor, both the frequency and the intensity of the oscillations strongly increase.
Frequency upconversion involving triads and quartets of ions in a Pr3+/Nd3+ codoped fluoroindate glass
Authors: Falcão-Filho Edilson L. , de Araújo Cid B. , Messaddeq Y.
Published: August 2002

Frequency upconversion (UC) processes involving energy transfer (ET) among Nd3+ and Pr3+ ions in a fluoroindate glass are reported. In a first experiment, the excitation of Pr3+ [transition H34→D32] and of Nd3+ [transition I49/2→(G27/2+G45/2)] was achieved with a dye laser operating in the 575–590 nm range. In a second experiment, the Nd3+ ions were excited with the second harmonic of a Nd: YAG laser at 532 nm. The ET processes leading to UC in both experiments were studied by monitoring the blue fluorescence decay at 480 nm due to the transition P30→H34 in Pr3+. In the more relevant UC process, quartets of ions (Nd–Nd–Pr–Pr) are excited due to absorption of three laser photons by two Nd3+ ions which transfer their energy to two Pr3+ ions. Each Pr3+ ion promoted to the P30 level decays to the ground state emitting one photon in the blue region. This conclusion was achieved investigating the dependence of the UC fluorescence intensity as a function of laser intensity, samples concentrations, and temporal behavior of the UC signal. Other UC processes involving nonisoionic groups of three ions are also reported.
Theory of clustered-cavity gyroklystron
Authors: Nusinovich G. S. , Antonsen T. M. , Guo H. , Granatstein V. L.
Published: August 2002

An analytical theory of a new device configuration, a clustered-cavity gyroklystron, is developed. The device considered has two clusters of cavities: an input cluster and an output cluster. The results show that, by using a cluster cavity concept, the bandwidth of gyroklystrons can be enlarged significantly without sacrifice of gain or efficiency which may lead to the development of a new type of high power, moderate bandwidth millimeter-wave amplifier. The theory has also been used to analyze the effect of stagger tuning between cavity frequencies within a single cluster, as well as between different clusters on the bandwidth and gain of the device.
Kinetic simulation of a quasisteady state in collisionless ion temperature gradient driven turbulence
Authors: Watanabe Tomo-Hiko , Sugama Hideo
Published: August 2002

Existence of a quasisteady state with a mean transport flux in the collisionless ion temperature gradient driven turbulence has been confirmed by means of a direct numerical simulation of a basic kinetic equation for the perturbed ion velocity distribution function δf. The phase mixing generates fine-scale fluctuations of δf and leads to continuous growth of high-order moments which balances the transport flux. The phase relation between the temperature and the parallel heat flux is also examined and compared with a fluid closure model.
The nucleation behavior of supercooled water vapor in helium
Authors: Peeters P. , Gielis J. J. H. , van Dongen M. E. H.
Published: September 2002

The nucleation behavior of supersaturated water vapor in helium is experimentally investigated in the temperature range of 200–240 K. The experiments are performed using a pulse expansion wave tube. The experimental results show a sharp transition in the nucleation rates at 207 K. We suggest that the transition is due to the transition of vapor/liquid to vapor/solid nucleation (ordered with decreasing temperature). A qualitative theoretical explanation is given based on the classical nucleation theory and the surface energy of ice.
Complex absorbing potentials in the framework of electron propagator theory. I. General formalism
Authors: Santra Robin , Cederbaum Lorenz S.
Published: September 2002

Metastable electronic states, ubiquitous in electron–molecule scattering and in ionization processes, still pose a serious challenge to theory. We suggest to make use of electron propagators and present a many-particle theory that facilitates the direct computation of energy and decay width of electronic resonance states in molecular systems. A complex absorbing potential (CAP) is introduced to render resonance wave functions square-integrable and, in this way, representable in standard basis sets. We perform a perturbative analysis, explicitly taking into account all diagrams up to third order, and then employ the algebraic diagrammatic construction (ADC) scheme to derive a numerically efficient and practical method for computing resonance parameters. We call this new method CAP/ADC.
Elastic analysis of an inhomogeneous quantum dot in multilayered semiconductors using a boundary element method
Authors: Yang B. , Pan E.
Published: August 2002

In this work, we examine the elastostatic field due to a buried quantum dot (QD) in multilayered semiconductors using a boundary element method. Since the integral kernels employ a special Green’s function that satisfies the interfacial continuity and boundary conditions for a multilayered matrix, coupled with the conventional Kelvin-type Green’s function for the QD, the present method only requires discretization along the interface between the matrix and QD to solve the problem. With this method, the QD can be modeled in general as an inhomogeneity relative to the matrix. We have examined a practical semiconductor multilayer system of an InAs wetting/GaAs spacer with a buried cuboidal QD of either wetting or a spacer medium. The QD is correspondingly modeled by either the inhomogeneity or inclusion approach. Two crystallographic orientations of the spacer medium, GaAs(001) and GaAs(111), are considered. The analytical results have shown that these two approaches generally result in considerable differences in the prediction of the QD-induced elastic field. Also, different crystallographic orientation of a spacer medium can cause a characteristic change in the QD-induced field.
Effects of Zr substitution on phase transformation and thermoelectric properties of β-FeSi2
Authors: Ito Mikio , Nagai Hiroshi , Tahata Takashi , Katsuyama Shigeru , Majima Kazuhiko
Published: August 2002

Zr-substituted β-FeSi2 thermoelectric materials, Fe1−xZrxSi2, were synthesized by mechanical alloying (MA) and subsequent hot pressing, and the effects of Zr substitution on phase transformation and thermoelectric properties were investigated. The samples with x up to 0.06 were mostly composed of the Zr-doped β phase with small amounts of the ε phase and the ZrSi phase. In the case of the samples with an x value over 0.10, formation of a large amount of the α phase was observed in the x-ray diffraction patterns, and the β phase completely disappeared for the sample with x=0.30. The Seebeck coefficient below 900 K was markedly reduced by Zr substitution. However, Zr substitution was effective for enhancing the Seebeck coefficient above 900 K and for reducing the electrical resistivity over the entire temperature range. The temperature dependence of the Seebeck coefficient and electrical resistivity of the samples with high Zr content showed a metallic behavior, which was well consistent with the formation of a large amount of the metallic α phase. Although the thermal conductivity of the sample with high Zr content increased at high temperature range, the influence of Zr substitution was quite small as compared to that on the electrical resistivity. The figure of merit was significantly improved by Zr substitution, indicating that Zr substitution is quite effective for enhancing the thermoelectric performance of the β-FeSi2. The maximum figure of merit, 3.38×10−5/K, was obtained for the sample with x=0.24 at 1070 K, which is about 44 times larger than that of the sample without Zr.
Quantitative prediction of optical excitations in conjugated organic oligomers: A density functional theory study
Authors: Pogantsch A. , Heimel G. , Zojer E.
Published: September 2002

We present a density functional theory (DFT) study on excitations in conjugated organic materials including oligomers of α-thiophenes, para-phenylenes, para-phenylenevinylenes and ladder-type para-phenylenes. Time-dependent DFT was applied to study optically allowed transitions within the singlet and triplet manifolds, establishing good agreement with experimental data. The splitting between the lowest lying singlet and triplet states is calculated and found to match available experimental data. Additionally, we have employed a Hartree–Fock configuration interaction scheme limited to single excitations for the optimization of excited state geometries in ladder-type para-phenylenes and used time-dependent DFT to calculate the transition energies for fluorescence. Also in this case we found excellent agreement with experimental data.
First principles determination of the photoelectron spectrum of LiH−
Authors: Chang Daniel T. , Reimann K. , Surratt G. , Gellene Gregory I. , Lin Ping , Lucchese Robert R.
Published: September 2002

The photoelectron spectra of LiH− and LiD− have been determined from a first principles theoretical treatment using techniques which very accurately reproduce the experimentally known electron affinity of Li and H, the equilibrium bond length and dissociation energy of LiH, and the vibrational levels of LiH (v=0–13) and LiD (v=0–8). A satisfactory simulation of the available experimental photoelectron spectra [J. Chem. Phys. 100, 1884 (1994)] can be accomplished by assuming a non-Boltzmann distribution of anion vibrational levels. A long-standing discrepancy between the experimentally determined (920±80 cm−1) and theoretically calculated (1176.1 cm−1) values for ωe of LiH− is resolved by a reassignment of the hot band transition region of the photoelectron spectrum.
Contact values of the radial distribution functions of additive hard-sphere mixtures in d dimensions: A new proposal
Authors: Santos Andrés , Yuste Santos B. , López de Haro Mariano
Published: September 2002

The contact values gij(σij) of the radial distribution functions of a d-dimensional mixture of (additive) hard spheres are considered. A “universality” assumption is put forward, according to which gij(σij)=G(η,zij), where G is a common function for all the mixtures of the same dimensionality, regardless of the number of components, η is the packing fraction of the mixture, and zij=(σiσj/σij)〈σd−1〉/〈σd〉 is a dimensionless parameter, 〈σn〉 being the nth moment of the diameter distribution. For d=3, this universality assumption holds for the contact values of the Percus–Yevick approximation, the scaled particle theory, and, consequently, the Boublík–Grundke–Henderson–Lee–Levesque approximation. Known exact consistency conditions are used to express G(η,0), G(η,1), and G(η,2) in terms of the radial distribution at contact of the one-component system. Two specific proposals consistent with the above-mentioned conditions (a quadratic form and a rational form) are made for the z dependence of G(η,z). For one-dimensional systems, the proposals for the contact values reduce to the exact result. Good agreement between the predictions of the proposals and available numerical results is found for d=2, 3, 4, and 5.
Vibrational spectroscopy of HOD in liquid D2O. I. Vibrational energy relaxation
Authors: Lawrence C. P. , Skinner J. L.
Published: September 2002

We present calculations of the vibrational lifetimes for the three fundamentals of HOD in liquid D2O. The calculations build on the work of Oxtoby and of Rey and Hynes, but also introduce some new ideas, the most important of which is a self-consistent renormalization scheme for determining the system and bath Hamiltonians for a given vibrational state of the HOD molecule. Our result for T1 for the OH stretch fundamental is 2.7 ps, which is about a factor of 3 larger than the experimental number. We suggest that including solvent vibrations may bring theory in closer agreement with experiment.
Kramers–Fokker–Planck equation for polyatomic molecules
Authors: Nagaoka Masataka , Okamoto Takuya , Maruyama Yutaka
Published: September 2002

Starting from the microscopic Hamiltonian of a whole solution system, we have derived a Kramers–Fokker–Planck (KFP) equation for polyatomic molecules by using Zubarev’s techinique of nonequilibrium statistical distribution function. For the purpose, a set of normal coordinates of a solute molecule is defined at a stationary point on the free energy surface. In the normal coordinate representation, the expression of the KFP equation includes not only the deformation originating in the Coriolis coupling but also the friction tensor inducing a new type of coupling. For demonstration, two examples are examined. First, under the approximation of adiabatic elimination of fast variables, a one-dimensional KFP equation is derived from the above KFP equation, which corresponds to the equation that Kramers first derived intuitively for one-dimensional chemical reaction model in solution. Second, assuming a model system which describes simultaneously a couple of vibrational and rotational motions, a two-dimensional KFP equation is solved numerically to investigate the role of the friction tensor. It is found that the off-diagonal friction coupling reduces the diagonal friction influence.
Fragment charge difference method for estimating donor–acceptor electronic coupling: Application to DNA π-stacks
Authors: Voityuk Alexander A. , Rösch Notker
Published: September 2002

The purpose of this communication is two-fold. We introduce the fragment charge difference (FCD) method to estimate the electron transfer matrix element HDA between a donor D and an acceptor A, and we apply this method to several aspects of hole transfer electronic couplings in π-stacks of DNA, including systems with several donor–acceptor sites. Within the two-state model, our scheme can be simplified to recover a convenient estimate of the electron transfer matrix element HDA=(1−Δq2)1/2(E2−E1)/2 based on the vertical excitation energy E2–E1 and the charge difference Δq between donor and acceptor. For systems with strong charge separation, Δq≳0.95, one should resort to the FCD method. As favorable feature, we demonstrate the stability of the FCD approach for systems which require an approach beyond the two-state model. On the basis of ab initio calculations of various DNA related systems, we compared three approaches for estimating the electronic coupling: the minimum splitting method, the generalized Mulliken–Hush (GMH) scheme, and the FCD approach. We studied the sensitivity of FCD and GMH couplings to the donor–acceptor energy gap and found both schemes to be quite robust; they are applicable also in cases where donor and acceptor states are off resonance. In the application to π-stacks of DNA, we demonstrated for the Watson–Crick pair dimer [(GC),(GC)] how structural changes considerably affect the coupling strength of electron hole transfer. For models of three Watson–Crick pairs, we showed that the two-state model significantly overestimates the hole transfer coupling whereas simultaneous treatment of several states leads to satisfactory results.
The dynamics of formation of HCl products from the reaction of Cl atoms with methanol, ethanol, and dimethyl ether
Authors: Rudić Svemir , Murray Craig , Ascenzi Daniela , Anderson Helen , Harvey Jeremy N. , Orr-Ewing Andrew J.
Published: September 2002

The dynamics of ground state Cl(2P3/2) atom reactions with methanol, methanol-d1, ethanol, and dimethyl ether have been studied both experimentally and theoretically. The reactions were photoinitiated by 355 nm photolysis of Cl2 to produce monoenergetic Cl(2P3/2) atoms that react with ground electronic state organic molecules under single collision conditions. The rotational quantum state population distributions of the nascent HCl(ν′) products were probed by 2+1 resonance-enhanced multiphoton ionization in a time-of-flight mass spectrometer. Nascent HCl(ν′=0) products from reaction of Cl atoms with methanol, methanol-d1 (CH3OD), ethanol, and dimethyl ether, at mean collision energies in the range of 5.6–6.7 kcal/mol, exhibit distributions of population over rotational levels that all peak at J′=3–5. The average rotational energies of the HCl(ν′=0) products for the respective reactions are 〈Erot〉=330±29, 300±24, 340±24, and 256±17 cm−1 (1σ uncertainties). Ab initio calculations were performed in order to examine the mechanisms of Cl atom abstraction of hydrogen from the alcohols and ether. Optimized geometrical structures and vibrational frequencies of molecular complexes and transition states on the reaction pathways were obtained at the MP2/6-311G(d,p) level and their energies were further refined at the G2 level of theory. Comparisons are drawn between the mechanisms and energetic pathways of the various reactions. The degree of rotational excitation of the HCl, which is significantly greater than for Cl atom abstraction of an H atom from alkanes, is attributed to a dipole–dipole interaction between the HCl and RCHOR′ (R, R′=H or CH3) moieties in the products’ region of the potential energy surface.
Potential surfaces and delocalization of excitons in dimers
Authors: Beenken W. J. D. , Dahlbom M. , Kjellberg P. , Pullerits T.
Published: September 2002

In the present work we will demonstrate that the nuclear dynamics have a strong influence on the delocalization of an exciton in a dimer, even if they do not effect the excitonic interaction. It will be shown that the internal nuclear conformation of the molecules forming the dimer depends critically on the delocalization of the exciton state in the dimer and vice versa. The resulting closed loop enforces a localization of the lower excitonic state, but, contrary to the commonly accepted view, a delocalization of the upper one. Qualitatively different time-evolution of the delocalization length for the lower and upper excitonic state will be shown. Besides, it will turn out that the nuclear motions inhibit a complete delocalization of the excitonic state in any case. To accomplish nuclear and exciton dynamics, the nonadiabatic coupling between the two excitonic states will be deduced. This causes a relaxation from the upper to the lower excitonic state, which limits the maximum reachable exciton delocalization.
Microscopic theory of some thermodynamic properties of the solid–vapor transition
Authors: Krukowski Stanislaw
Published: September 2002

A new microscopic model allowing us to calculate the entropy of evaporation is introduced. The entropy of evaporation, has been calculated for the variety of solid substances, such as metals, elemental and compound semiconductors. The entropy of evaporation, calculated for the solids with various lattice symmetries is in good agreement with the existing thermodynamic data. An electronic formulation of the enthalpy of evaporation of semiconducting solids will be given. This formulation will be used in the analysis of solid–vapor equilibrium conditions of elemental and III–V compound semiconductors. The criterion for metal-rich and nonmetal rich growth conditions will be formulated in terms of partial pressures of the constituents of compound semiconductors. The criterion will be used to describe the conditions of growth gallium nitride (GaN) and aluminum nitride (AlN) from their vapor constituents. The metal/nonmetal rich criterion explains success of the growth of AlN single crystals from Al and N2 vapor and disappointing results of the growth of GaN from the vapor constituents.
A scanning tunneling microscopy study of the reaction between hydrogen and oxygen to form water on Pd(111)
Authors: Mitsui T. , Rose M. K. , Fomin E. , Ogletree D. F. , Salmeron M.
Published: September 2002

The reaction between hydrogen and oxygen to form water on Pd(111) was investigated by scanning tunneling microscopy. The reaction was followed by observing the decreasing size of the oxygen islands in the presence of coadsorbed hydrogen. The observations indicate that the reaction sites are the Pd step edges and that the reaction rate is limited by formation of OH at these sites.
Magnetocaloric effect and temperature coefficient of resistance of La2/3(Ca,Pb)1/3MnO3
Authors: Sun Young , Salamon M. B. , Chun S. H.
Published: August 2002

The magnetocaloric effect of a La2/3(Ca,Pb)1/3MnO3 single crystal was examined. From the measurements of isothermal magnetization and heat capacity around the Curie temperature, we have calculated the maximum isothermal magnetic entropy change as ∼7.5 J/kg K and the adiabatic temperature changes as ∼5.6 K, near room temperature for a 7 T magnetic field variation. Besides, the sharp drop of resistivity at the insulator–metal transition associated with the magnetic transition gives rise to a large temperature coefficient of resistance (TCR). For the La0.7Ca0.3MnO3 and La2/3(Ca,Pb)1/3MnO3 single crystal, the peak of TCR is ∼50%/K at 218 K and ∼10%/K at 290 K, respectively.
High quality chemical vapor deposition diamond growth on iron and stainless steel substrates
Authors: Nakamura Eri , Hirakuri Kenji K. , Ohyama Manabu , Friedbacher Gernot , Mutsukura Nobuki
Published: August 2002

Due to the catalytic effect and the rapid diffusion coefficients of carbon species into iron-based materials such as iron and 18-8 stainless steel [18% chrome (Cr) and 8% nickel (Ni)], it is very difficult to produce diamond grains on such substrates. However, diamond growth on iron-based materials is extremely important for mechanical and electrical applications, since these materials are widely used in industrial field and fundamental science. In our previous study, diamond nucleation and subsequent growth have been precisely controlled by the residence time of the source gas, which is an essential parameter. Here, we have carried out diamond growth on iron-based materials using the hot-filament chemical vapor deposition technique with varying residence times. At low residence times, diamond grains with practically useful growth rate are grown. The growth rate of diamond grains on stainless steel substrates was a factor of about 10 greater than that on a regular silicon substrate at optimum conditions. At optimized conditions, diamond growths with high crystalline quality on stainless steel substrates were confirmed by Raman spectroscopy and scanning electron microscopy. The full width of half maximums of the Raman peaks for diamonds grown in this study are comparable to the ones of natural diamonds.
Electron trapping nonuniformity in high-pressure-Bridgman-grown CdZnTe
Authors: Amman Mark , Lee Julie S. , Luke Paul N.
Published: August 2002

Gamma-ray spectroscopy is a valuable tool of science and technology. Many applications for this tool are in need of a detector technology capable of achieving excellent energy resolution and efficient detection while operating at room temperature. Detectors based on the material cadmium zinc telluride (CdZnTe) could potentially meet this need if certain material deficiencies are addressed. The coplanar-grid as well as other electron-only detection techniques are effective in overcoming some of the material problems of CdZnTe and, consequently, have led to efficient gamma-ray detectors with good energy resolution while operating at room temperature. At the present time, the performance of these detectors is limited mainly by the degree of uniformity in electron generation and transport. Despite recent progress in the growth of CdZnTe material, small variations in these properties remain a barrier to the widespread success of such detectors. Alpha-particle response characterization of CdZnTe crystals fabricated into simple planar detectors provides an effective tool to accurately study such variations. We have used a finely collimated alpha source to produce two-dimensional maps of detector response. For a number of crystals, a clear correlation has been observed between their alpha response maps and the distribution of tellurium inclusions inside the crystals. An analysis of the induced charge signals indicates that regions of enhanced electron trapping are associated with the inclusions, and that these regions extend beyond the physical size of the inclusions. Such regions introduce nonuniform electron trapping in the material that then degrades the spectroscopic performance of the material as a gamma-ray detector.
Quantitative electron microprobe analysis of aluminum, copper, and gold thin films on silicon substrates
Authors: Yasuda Masaaki , Yamauchi Shunji , Kawata Hiroaki , Murata Kenji
Published: August 2002

The thicknesses of aluminum, copper, and gold thin films on silicon substrates have been measured with an electron probe microanalyzer. k ratio versus thickness calibration curves are obtained by a Monte Carlo simulation of electron scattering. The simulation results based on two energy loss models are compared. One is the continuous slowing down approximation model and another is the hybrid model for the discrete and the continuous energy loss processes. Inner shell ionizations and free electron excitations are selected out for the discrete process in the hybrid model. In both models the Mott cross section and the Gryzinski cross section are used for elastic collisions and ionizations, respectively. The exact film/substrate boundary condition is considered. The characteristic and continuum fluorescence corrections are also included in the simulation. The simulation results agree well with experimental ones measured with a quartz oscillator. Effects of the introduction of the discrete energy loss process and the fluorescence correction are discussed in comparison between simulation and experiment.
A hierarchy of coupled maps
Authors: Balmforth Neil J. , Provenzale Antonello , Sassi Roberto
Published: August 2002

A large number of logistic maps are coupled together as a mathematical metaphor for complex natural systems with hierarchical organization. The elementary maps are first collected into globally coupled lattices. These lattices are then coupled together in a hierarchical way to form a system with many degrees of freedom. We summarize the behavior of the individual blocks, and then explore the dynamics of the hierarchy. We offer some ideas that guide our understanding of this type of system.
Insulator surface charging and dissipation during plasma immersion ion implantation using a thin conductive surface film
Authors: Oates T. W. H. , Bilek M. M. M.
Published: August 2002

Plasma immersion ion implantation of insulating materials is inherently problematic due to charge accumulation on the insulator surface. Surface charge can be removed by the application of an ultrathin conductive film, which is essentially transparent to the incident ions. The minimum thickness of the film is determined by its capability to effectively conduct away the implanted charge. We present a model for charge accumulation on insulators during plasma immersion ion implantation and use this to study the plasma sheath width and voltage, with and without an ultrathin metal film. Charge accumulation occurs more quickly when the plasma has a directed velocity greater than the Bohm velocity, which is the case for a cathodic arc plasma. We show that for both cases the effectiveness of plasma immersion ion implantation is improved with the application of an ultrathin conductive film.
Polarized ultraviolet Raman spectroscopy of β-Si3N4
Authors: Vogelgesang R. , Grimsditch M. , Wallace J. S.
Published: August 2002

We have studied single crystals of β-Si3N4 with polarization-resolved Raman spectroscopy in order to unambiguously obtain the symmetry character of zone center optical phonon modes. By applying ultraviolet excitation radiation, approaching resonance condition, we have overcome the usual difficulties with overwhelming fluorescence obscuring Raman spectra of β-Si3N4 in the visible. Superior extinction ratios have been achieved and the application of group theory allows us to assign ten out of eleven Raman-active modes in excellent correlation with recent theoretical results. The probable nature of the eleventh mode is discussed.
High-efficiency optical emission, detection, and coupling using silicon diodes
Authors: Zhao J. , Green M. A. , Wang A.
Published: August 2002

This article reports band-edge light emission and detection using silicon diodes. A bulk crystalline silicon light-emitting diode (LED) demonstrating close to 1% power efficiency at room temperature is combined with a similar diode used as a detector, that collects the light emitted with a high quantum collection efficiency of 33%, to produce a silicon-to-silicon optically coupled system that demonstrates 0.18% coupling quantum efficiency. The silicon LED demonstrates similarly high performance at very low power levels, where it has higher power efficiency than a high efficiency GaAlAs LED.
Classical trajectory calculations for collision-energy/electron-energy resolved two-dimensional Penning ionization electron spectra of N2, CO, and CH3CN with metastable He*(2 3S) atoms
Authors: Yamazaki Masakazu , Maeda Satoshi , Kishimoto Naoki , Ohno Koichi
Published: September 2002

Collision-energy/electron-energy resolved two-dimensional Penning ionization electron spectra (2D-PIES) of N2, CO, and CH3CN with metastable He*(2 3S) atoms are measured, and classical trajectory calculations with anisotropic entrance and exit potential energy surfaces are performed for these systems. Numerical qualities of the entrance potential surfaces are decisively important to understand the collisional ionization dynamics as well as to reproduce observed 2D-PIES, whereas the exit potential surfaces are less sensitive to the collisional ionization dynamics and the electron spectra except for special cases in which a deep potential well is relevant in the entrance potential surface. Ab initio calculations of both entrance and exit potentials as well as ionization widths are found to be reliable in obtaining their anisotropy and radial dependence with good quantitative accuracy.
Electric-field imaging using optical techniques
Authors: Yang Xi , Wood Lowell T. , Miller John H.
Published: August 2002

We describe an optical technique using divergent laser beams and sample scanning for measuring electric-field distributions in the spaces between interdigital electrodes. We present experimental results using a LiNbO3 sample and compare them to calculated electric-field distributions. The basic structure of the fields and the locations of the maxima show excellent agreement between experiment and theory.
Temperature dependence of the complex effective piezoelectric coefficient of ferroelectric 0-3 composites
Authors: Wong C. K. , Poon Y. M. , Shin F. G.
Published: August 2002

Temperature dependence of the complex effective piezoelectric coefficient d31* for a ferroelectric 0-3 composite of small ceramic volume fraction has been studied. Theoretical predictions are based on our previously derived explicit expression of d31 for a dilute dispersion of spherical particles in a continuous matrix [C. K. Wong, Y. M. Poon, and F. G. Shin, Ferroelectrics 264, 39 (2001); J. Appl. Phys. 90, 4690 (2001)]. Comparison is made with the well-known Furukawa’s model and their experimental measurements on a lead zirconate titanate (PZT)/epoxy composite with 13 vol % PZT [T. Furukawa, K. Fujino, and E. Fukada, Jpn. J. Appl. Phys. 15, 2119 (1976)], covering a wide temperature range from −140 to +140 °C. The real part and the imaginary part of the effective piezoelectric coefficient for the composite are investigated separately. Predictions for the real part of d31* agree well with the observed values for temperatures larger than 60 °C, but are larger than the observed values for lower temperatures, while predictions for the imaginary part of d31* give fairly good agreement with the experimental data throughout the temperature range.
Electric charge measurement on a single microparticle using thermodynamic analysis of electrostatic forces
Authors: Wada Ken-ichiro , Sasaki Keiji , Masuhara Hiroshi
Published: August 2002

A method is proposed for analyzing the electric charge at the shear plane of a single microparticle. A particle dispersed in solution is three-dimensionally trapped with a focused laser beam, and an electrostatic field is applied with microelectrodes. The thermal motion of the particle is sequentially measured with a quadrant photodiode, such that the potential energy exerted on the particle can be estimated by thermodynamic analysis. A potential profile of an electrostatic force is obtained as the energy difference between two potential curves observed with and without the electrostatic field. The strength of the force exerted on a 4.5 μm polystyrene latex particle in 5×10−4 M NaCl solution was determined to be 1.6×10−13 N, which corresponded to the net charge of 5.8×10−17 C.
Spin-injection device based on EuS magnetic tunnel barriers
Authors: Filip A. T. , LeClair P. , Smits C. J. P. , Kohlhepp J. T. , Swagten H. J. M. , Koopmans B. , de Jonge W. J. M.
Published: August 2002

We propose a spin-valve device consisting of a nonmagnetic semiconductor quantum well, sandwiched between ferromagnetic semiconductor layers that act as barriers. The total conductance through such a trilayer depends on the relative magnetization of the two ferromagnetic-barrier layers which act as “spin filters.” With respect to practical realization, EuS/PbS heterostructures may be a suitable candidate. The magnetoresistance should exceed 100% for a wide range of the thicknesses of both the quantum well and the ferromagnetic barriers. From a fundamental physics point of view, the device may not only give insight into the spin lifetimes of the nonmagnetic layer, but the strong spin accumulation taking place in the quantum well may lead to novel optical and nuclear magnetic resonance properties.
A primary mechanism for spiral wave meandering
Authors: Otani Niels F.
Published: August 2002

The stability and dynamics of spiral wave meandering were studied by examining the behavior of small perturbations to a steadily rotating action potential wave. The disturbances responsible for meandering were found to be generated through an interaction between the unstable local linear dynamics characteristic of the action potential trailing edge near the core and perturbations existing in the region immediately behind this edge. Significantly, for the cases studied, neither wavefront curvature nor head–tail interactions were involved in this process. Study of the generation mechanism using a series of representative mathematical models and computer experiments led to the prediction that the following features of rotating action potentials render them more susceptible to meandering: (1) proximity of the wave tip to the center of rotation, (2) wider action potential leading and trailing edges, and (3) slower wave rotation speeds. Variation of basic tissue properties, including firing threshold potentials and excitability above threshold, affected these properties, and those of the perturbation dynamics, in several ways, producing both stabilizing and destabilizing effects. The nature of the involvement of various tissue and membrane electrical properties is therefore complex, affecting several factors relevant to meandering at once.
Introduction: Mapping and control of complex cardiac arrhythmias
Authors: Christini David J. , Glass Leon
Published: August 2002

This paper serves as an introduction to the Focus Issue on mapping and control of complex cardiac arrhythmias. We first introduce basic concepts of cardiac electrophysiology and describe the main clinical methods being used to treat arrhythmia. We then provide a brief summary of the main themes contained in the articles in this Focus Issue. In recent years there have been important advances in the ability to map the spread of excitation in intact hearts and in laboratory settings. This work has been combined with simulations that use increasingly realistic geometry and physiology. Waves of excitation and contraction in the heart do not always propagate with constant velocity but are often subject to instabilities that may lead to fluctuations in velocity and cycle time. Such instabilities are often treated best in the context of simple one- or two-dimensional geometries. An understanding of the mechanisms of propagation and wave stability is leading to the implementation of different stimulation protocols in an effort to modify or eliminate abnormal rhythms.
Multiple mechanisms of spiral wave breakup in a model of cardiac electrical activity
Authors: Fenton Flavio H. , Cherry Elizabeth M. , Hastings Harold M. , Evans Steven J.
Published: August 2002

It has become widely accepted that the most dangerous cardiac arrhythmias are due to reentrant waves, i.e., electrical wave(s) that recirculate repeatedly throughout the tissue at a higher frequency than the waves produced by the heart’s natural pacemaker (sinoatrial node). However, the complicated structure of cardiac tissue, as well as the complex ionic currents in the cell, have made it extremely difficult to pinpoint the detailed dynamics of these life-threatening reentrant arrhythmias. A simplified ionic model of the cardiac action potential (AP), which can be fitted to a wide variety of experimentally and numerically obtained mesoscopic characteristics of cardiac tissue such as AP shape and restitution of AP duration and conduction velocity, is used to explain many different mechanisms of spiral wave breakup which in principle can occur in cardiac tissue. Some, but not all, of these mechanisms have been observed before using other models; therefore, the purpose of this paper is to demonstrate them using just one framework model and to explain the different parameter regimes or physiological properties necessary for each mechanism (such as high or low excitability, corresponding to normal or ischemic tissue, spiral tip trajectory types, and tissue structures such as rotational anisotropy and periodic boundary conditions). Each mechanism is compared with data from other ionic models or experiments to illustrate that they are not model-specific phenomena. Movies showing all the breakup mechanisms are available at http://arrhythmia.hofstra.edu/breakup and at ftp://ftp.aip.org/epaps/chaos/E-CHAOEH-12-039203/INDEX.html. The fact that many different breakup mechanisms exist has important implications for antiarrhythmic drug design and for comparisons of fibrillation experiments using different species, electromechanical uncoupling drugs, and initiation protocols.
Doping and thickness effects on dielectric properties and subswitching behavior of lead titanate thin films
Authors: Iakovlev S. , Solterbeck C.-H. , Es-Souni M.
Published: August 2002

Dielectric and ferroelectric properties of sol-gel derived PbTiO3 and (Pb,Er)TiO3 films with thickness in the range from 134 to 420 nm have been investigated. It is demonstrated that 1% Er doping improves the shape of the ferroelectric hysteresis loop probably due to compensation of p-type conductivity. The dielectric constant is shown to be linearly dependent on the amplitude of the measuring signal. It is shown that the Rayleigh equation can be used to discriminate between reversible and irreversible contributions to the dielectric properties of the films. Both reversible and irreversible Rayleigh components were found to be affected by donor doping and film thickness. Introducing 1% of Er is thought to counteract domain wall pinning on “dead layer” near the film–electrode interface.
Controlling the crystallographic orientation and the axis of magnetic anisotropy in L1 FePt films
Authors: Chen J. S. , Lim B. C. , Wang J. P.
Published: August 2002

L 1 0 ordered FePt films with (200) or (001) preferred orientations and longitudinal or perpendicular anisotropy have been prepared by dc magnetron sputtering. Different base pressures for sputtering were controlled by air feeding. The fct-FePt film exhibits (200) preferred orientation and has longitudinal anisotropy at the base pressure of 4×10−6 Torr. As the base pressure is improved (below 9×10−7 Torr), the fct-FePt films exhibit (001) preferred orientation and has perpendicular anisotropy. When the CrRu underlayer thickness decreases from 80 to 30 nm, an improved fct-FePt (001) orientation is obtained. The atmospheric constituents such as oxygen and nitrogen play an important role in controlling the crystallographic orientation and axis of magnetic anisotropy.
Synthesis and properties of c-axis oriented epitaxial MgB2 thin films
Authors: Bu S. D. , Kim D. M. , Choi J. H. , Giencke J. , Hellstrom E. E. , Larbalestier D. C. , Patnaik S. , Cooley L. , Eom C. B. , Lettieri J. , Schlom D. G. , Tian W. , Pan X. Q.
Published: August 2002

We report the growth and properties of epitaxial MgB2 thin films on (0001) Al2O3 substrates. The MgB2 thin films were prepared by depositing boron films via radio-frequency (rf) magnetron sputtering, followed by a postdeposition anneal at 850 °C in magnesium vapor. X-ray diffraction and cross-sectional transmission electron microscopy reveal that the epitaxial MgB2 films are oriented with their c-axis normal to the (0001) Al2O3 substrate with a 30° rotation in the (0001) plane with respect to the substrate. The critical temperature was found to be 35 K and the anisotropy ratio, Hc2∥/Hc2⊥, was about 3 at 25 K. The critical current densities at 4.2 and 20 K (at 1 T perpendicular magnetic field) are 5×106 and 1×106 A/cm2, respectively. The controlled growth of epitaxial MgB2 thin films opens a new avenue in both understanding superconductivity in MgB2 and technological applications.
Reduced electromigration of Cu wires by surface coating
Authors: Hu C.-K. , Gignac L. , Rosenberg R. , Liniger E. , Rubino J. , Sambucetti C. , Domenicucci A. , Chen X. , Stamper A. K.
Published: August 2002

Electromigration in on-chip Cu interconnections with a selective electroless metal coating, CoWP, CoSnP, or Pd, on the top surface of Cu damascene lines has been investigated. The 10–20 nm thick metal cap significantly improves electromigration lifetime by providing protection against interface diffusion of Cu which has been the leading contributor to metal line failure by electromigration.
Optical and reduced band gap in n- and p-type GaN and AlN
Authors: Persson C. , Sernelius Bo E. , Ferreira da Silva A. , Araújo C. Moysés , Ahuja R. , Johansson B.
Published: August 2002

We present a full band calculation of the doping-induced energy shifts of the conduction-band minimum and the valence-band maximum for n- and p-type GaN and AlN. Both wurtzite and zinc-blende structures have been considered. The resulting optical and reduced band-gap energies are presented as functions of the ionized impurity concentration in the heavily doped regime. The computational method is based on a zero-temperature Green’s function formalism within the random phase approximation and with the local-field correction of Hubbard. The calculation goes beyond the spherical approximation of the energy bands by using energy dispersions and overlap integrals from a first-principle, full-potential band-structure calculation. Inclusion of the spin-orbit interaction is crucial for describing the uppermost valence bands properly, and we show that the nonparabolicity of the valence bands influences the energy shifts strongly, especially the shift of the optical band gap. With the full band structure, we can explain the results of photoluminescence measurements by Yoshikawa et al. [J. Appl. Phys. 86, 4400 (1999)].
Luminescence quenching in thermally-treated barium magnesium aluminate phosphor
Authors: Sohn Kee-Sun , Kim Sang Sub , Park Hee Dong
Published: August 2002

BaMgAl 10 O 17 : Eu 2+ (BAM) phosphors used for plasma display panels (PDP) are compelled to be exposed to an oxidizing environment at about 500 °C, which is currently unavoidable in the actual manufacturing process of PDP. We investigated the mechanism of the luminance degradation of BAM caused by the annealing at 500 °C, using photoluminescence (PL), decay measurement, and synchrotron light source x-ray absorption and diffraction measurements. The annealing treatment altered the valence state of Eu ions, whereas no new Eu compounds were detected. By estimating the exact fraction of divalent Eu ions and by comparing it with the luminance data, we found that more than 30% degradation of luminance was induced at the expense of only a few percent of divalent Eu. This finding led us to suggest that the origin of the dramatic decrease in PL intensity is not due to the valence state change but due to the local structure change surrounding the Eu2+ ions.