bluuebunny/crossref_metadata_2025 huggingface dataset:
Noncommutative Bloch theory
Authors: Gruber Michael J.
Published: July 2002

For differential operators which are invariant under the action of an Abelian group Bloch theory is the preferred tool to analyze spectral properties. By shedding some new noncommutative light on this we motivate the introduction of a noncommutative Bloch theory for elliptic operators on Hilbert C*-modules. It relates properties of C*-algebras to spectral properties of module operators such as band structure, weak genericity of cantor spectra, and absence of discrete spectrum. It applies, e.g., to differential operators invariant under a projective group action, such as Schrödinger, Dirac, and Pauli operators with periodic magnetic field, as well as to discrete models, such as the almost Matthieu equation and the quantum pendulum.
Temperature dependence of fundamental band gaps in group IV, III–V, and II–VI materials via a two-oscillator model
Authors: Pässler R.
Published: July 2002

We perform numerical analyses of the temperature dependences of fundamental band gaps, Eg(T), and/or exciton peak positions, Egx(T), for a large variety of group IV, III–V, and II–VI (including wide band gap) materials using a two-oscillator model. This model assumes a fixation of the low-energy oscillator in the vicinity of the dominant TA peak, whereas the location of the high-energy oscillator is taken as an adjustable parameter depending on the relative weights of the contributions of short-wavelength LA and LO/TO phonons. The material-specific sets of empirical parameters have been estimated from fittings of measured E(T) dependences. The results indicate significant changes of the relative contributions of optical versus acoustical phonons from one material to the other. The degree of dispersion is found to increase significantly with contribution of low-energy acoustical phonons.
Characterization of the effective electrostriction coefficients in ferroelectric thin films
Authors: Kholkin A. L. , Akdogan E. K. , Safari A. , Chauvy P.-F. , Setter N.
Published: July 2002

Electromechanical properties of a number of ferroelectric films including PbZrxTi1−xO3(PZT), 0.9PbMg1/3Nb2/3O3–0.1PbTiO3(PMN-PT), and SrBi2Ta2O9(SBT) are investigated using laser interferometry combined with conventional dielectric measurements. Effective electrostriction coefficients of the films, Qeff, are determined using a linearized electrostriction equation that couples longitudinal piezoelectric coefficient, d33, with the polarization and dielectric constant. It is shown that, in PZT films, electrostriction coefficients slightly increase with applied electric field, reflecting the weak contribution of non-180° domains to piezoelectric properties. In contrast, in PMN-PT and SBT films electrostriction coefficients are field independent, indicating the intrinsic nature of the piezoelectric response. The experimental values of Qeff are significantly smaller than those of corresponding bulk materials due to substrate clamping and possible size effects. Electrostriction coefficients of PZT layers are shown to depend strongly on the composition and preferred orientation of the grains. In particular, Qeff of (100) textured rhombohedral films (x=0.7) is significantly greater than that of (111) layers. Thus large anisotropy of the electrostrictive coefficients is responsible for recently observed large piezoelectric coefficients of (100) textured PZT films. Effective electrostriction coefficients obtained by laser interferometry allow evaluation of the electromechanical properties of ferroelectric films based solely on the dielectric parameters and thus are very useful in the design and fabrication of microsensors and microactuators.
Shear-induced displacement of isotropic-nematic spinodals
Authors: Lenstra T. A. J. , Dogic Z. , Dhont J. K. G.
Published: July 2002

The shear dependent location of the isotropic-nematic spinodals in suspensions of bacteriophage fd is studied by means of time resolved birefringence experiments. The hysteresis in the birefringence on increasing and subsequently decreasing the shear-rate allows the determination of the location of points in the shear-rate vs concentration phase diagram between the isotropic-to-nematic and the nematic-to-isotropic spinodals. Experimental hysteresis curves are interpreted on the basis of an equation of motion for the orientational order parameter tensor, as derived from the N-particle Smoluchowski equation. The spinodals are found to shift to lower concentrations on increasing the shear-rate. Above a critical shear-rate, where shear forces dominate over thermodynamic forces, no spinodal instability could be detected.
Two-dimensional ultrafast infrared vibrational echo studies of solute–solvent interactions and dynamics
Authors: Thompson David E. , Merchant K. A. , Fayer M. D.
Published: July 2002

Two-dimensional spectrally resolved ultrafast infrared vibrational echo experiments were used to investigate the nature of solute–solvent interactions in solution. The experiments were performed on (acetylacetonato)dicarbonylrhodium(I) in dibutylphthalate at 150 K. The 2D spectra display features that reflect the 0–1 and 1–2 transitions and the combination band transition of the symmetric (S) and antisymmetric (A) CO stretching modes. Three oscillations in the data arise from the frequency difference between the S and A modes (quantum beats) and the S and A anharmonicities. The novel mechanism that gives rise to the anharmonic oscillations, which is distinct from that of a conventional quantum beat, is described. The frequency of the S/A mode-splitting quantum beats varies for different observation wavelengths across the 0–1 inhomogeneous lines. For either the S or A lines, as the wavelength of observation of the vibrational echo is moved to higher energy, the quantum beat frequency decreases. The change in frequency is related to the nature of the solute–solvent interactions (inhomogeneous broadening) of the S and A transitions. A simple analytical approach is used to determine how a solute vibrational oscillator is influenced by the solvent. Three models of solute–solvent interactions are considered in terms of CO local mode energies and coupling. In one, the transition energies in the S and A lines are anticorrelated either because the inhomogeneous broadening arises from variations in the local mode coupling or the local mode energies are anticorrelated. In the other two, the local mode energies are either correlated or uncorrelated. The results of the model calculations indicate that interactions with the solvent result in local mode frequencies that are strongly correlated.
Statistical analysis of early failures in electromigration
Authors: Gall M. , Capasso C. , Jawarani D. , Hernandez R. , Kawasaki H. , Ho P. S.
Published: July 2002

The detection of early failures in electromigration (EM) and the complicated statistical nature of this important reliability phenomenon have been difficult issues to treat in the past. A satisfactory experimental approach for the detection and the statistical analysis of early failures has not yet been established. This is mainly due to the rare occurrence of early failures and difficulties in testing of large sample populations. Furthermore, experimental data on the EM behavior as a function of varying number of failure links are scarce. In this study, a technique utilizing large interconnect arrays in conjunction with the well-known Wheatstone Bridge is presented. Three types of structures with a varying number of Ti/TiN/Al(Cu)/TiN-based interconnects were used, starting from a small unit of five lines in parallel. A serial arrangement of this unit enabled testing of interconnect arrays encompassing 480 possible failure links. In addition, a Wheatstone Bridge-type wiring using four large arrays in each device enabled simultaneous testing of 1920 interconnects. In conjunction with a statistical deconvolution to the single interconnect level, the results indicate that the electromigration failure mechanism studied here follows perfect lognormal behavior down to the four sigma level. The statistical deconvolution procedure is described in detail. Over a temperature range from 155 to 200 °C, a total of more than 75 000 interconnects were tested. None of the samples have shown an indication of early, or alternate, failure mechanisms. The activation energy of the EM mechanism studied here, namely the Cu incubation time, was determined to be Q=1.08±0.05 eV. We surmise that interface diffusion of Cu along the Al(Cu) sidewalls and along the top and bottom refractory layers, coupled with grain boundary diffusion within the interconnects, constitutes the Cu incubation mechanism.
The rovibrational structure of the Ar–CO complex from a model interaction potential
Authors: Gianturco F. A. , Paesani F.
Published: July 2002

The full three-variable potential-energy surface for the Ar–CO complex, V(R,θ,rCO), has been calculated using a recently developed scheme which combines density-functional theory (DFT) with the long-range dispersion contributions obtained from perturbation theory. The two adiabatic surfaces given by integration of the full potential over the vibrational coordinate of CO have been then used to calculate the bound states of the van der Waals complex for both vCO=0 and vCO=1. Calculations of the wave functions and of the frequencies of various rotational and rovibrational transitions provide overall good agreement both with the experiments and with the results obtained using the most recent, and more computationally demanding, potential-energy surfaces for the title system.
High throughput fabrication of transition-metal-doped epitaxial ZnO thin films: A series of oxide-diluted magnetic semiconductors and their properties
Authors: Jin Zhengwu , Fukumura T. , Kawasaki M. , Ando K. , Saito H. , Sekiguchi T. , Yoo Y. Z. , Murakami M. , Matsumoto Y. , Hasegawa T. , Koinuma H.
Published: July 2002

Combinatorial laser molecular-beam epitaxy method was employed to fabricate epitaxial ZnO thin films doped with all the 3d transition metal (TM) ions in a high throughput fashion. The solubility behavior of TM ions was discussed from the viewpoints of the ionic radius and valence state. The magneto-optical responses coincident with absorption spectra were observed for Mn- and Co-doped samples. Cathodoluminescence spectra were studied for Cr-, Mn-, Fe-, and Co-doped samples, among which Cr-doped ZnO showed two sharp peaks at 2.97 eV and 3.71 eV, respectively, at the expense of the exciton emission peak of pure ZnO at 3.25 eV. Different magnetoresistance behavior was observed for the samples codoped with n-type carriers. Ferromagnetism was not observed for Cr- to Cu-doped samples down to 3 K.
The effect of lone pairs and electronegativity on the indirect nuclear spin–spin coupling constants in CH2X (X=CH2, NH, O, S): Ab initio calculations using optimized contracted basis sets
Authors: Provasi Patricio F. , Aucar Gustavo A. , Sauer Stephan P. A.
Published: July 2002

The indirect nuclear spin–spin coupling constants of C2H4, CH2NH, CH2O, and CH2S were investigated by means of correlated ab initio calculations at the level of the second order polarization propagator approximation (SOPPA) and the second order polarization propagator approximation with coupled cluster singles and doubles amplitudes—SOPPA(CCSD) using large basis sets, which are optimized for the calculation of coupling constants. It is found that at the self-consistent-field (SCF) level CH2NH and CH2S exhibit triplet instabilities whereas CH2CH2 and CH2O show triplet quasi-instabilities, which renders the SCF results meaningless. Our best results deviate between 0.3 and 2.7 Hz from the experimental values. We find that although the one-bond C–H and Y–H couplings as well as the two- and three-bond H–H couplings are dominated by the Fermi contact term, significant contributions of the orbital paramagnetic and sometimes even spin–dipolar terms are observed for the one-bond C–Y and two-bond C–H and Y–H coupling constants. Similarly the changes in the couplings caused by the electronegativity and the lone-pair of Y are mostly due to changes in the Fermi contact (all couplings) and the orbital paramagnetic contribution (C–Y and two-bond Y–H couplings). However, the trend in the changes are neither the same for both terms not for all couplings. In particular, the position of CH2S in the series varies indicating that either the electronegativity or the lone pairs are the dominating perturbation. Furthermore, small but optimized Gaussian basis sets for the calculation of indirect nuclear spin–spin coupling constants are presented. They were obtained by contraction of the s- and p-type basis functions for C, N, O, and S and of the s-type basis functions for H of the large uncontracted basis sets. Molecular orbital coefficients of self-consistent-field calculations on CH4, NH3, H2O, H2S, and H2 with the uncontracted basis sets were used as contraction coefficients. Applied in the calculation of all coupling constants in C2H4, CH2NH, CH2O, and CH2S the contraction leads to a maximum basis set error of ∼0.5 Hz.
Microcavities in polymeric photonic crystals
Authors: Sun Hong-Bo , Mizeikis Vygantas , Xu Ying , Juodkazis Saulius , Ye Jia-Yu , Matsuo Shigeki , Misawa Hiroaki
Published: July 2002

We report the fabrication and characteristics of planar microcavities in a log-pile photonic crystal structure formed using light-induced photopolymerization of resin. A planar defect was introduced into the middle of the log-pile structure as a single layer with every second rod missing. The existence of confined cavity states was confirmed experimentally and by numeric simulations. The cavity resonance found at the midgap wavelength λM∼4.0 μm had a quality factor of about 130.
Interaction of hydrogen with gallium vacancies in wurtzite GaN
Authors: Wright A. F.
Published: July 2002

First-principles techniques are used to investigate the interaction of hydrogen with gallium vacancies in wurtzite GaN. The calculations reveal that hydrogen can either compensate a vacancy by donating an electron to a vacancy acceptor level, or passivate the vacancy by forming a hydrogen-vacancy complex. A gallium vacancy can bind up to four hydrogen atoms, and hydrogen removal energies are computed as a function of the number of hydrogen atoms. Removal energies are found to depend strongly on Fermi level and complexes containing more than two hydrogen atoms are predicted to be unstable in n-type GaN. Hydrogen vibration frequencies are computed and compared with previously reported infrared absorption measurements for hydrogen-implanted GaN.
Formation mechanism of V defects in the InGaN/GaN multiple quantum wells grown on GaN layers with low threading dislocation density
Authors: Cho H. K. , Lee J. Y. , Yang G. M. , Kim C. S.
Published: July 2002

V-defect formation of the InxGa1−xN/GaN multiple quantum wells (MQWs) grown on GaN layers with different threading dislocation (TD) densities was investigated. From cross-sectional transmission electron microscopy, we found that all V defects are not always connected with TDs at their bottom. By increasing the indium composition in the InxGa1−xN well layer or decreasing the TD density of the thick GaN layer, many V defects are generated from the stacking mismatch boundaries induced by stacking faults which are formed within the MQW due to the strain relaxation. Also, TD density in the thick GaN layer affects not only the origin of V-defect formation but also the critical indium composition of the InxGa1−xN well on the formation of V defects.
Spectroscopic, collisional, and thermodynamic properties of the He–CO2 complex from an ab initio potential: Theoretical predictions and confrontation with the experimental data
Authors: Korona Tatiana , Moszynski Robert , Thibault Franck , Launay Jean-Michel , Bussery-Honvault Béatrice , Boissoles Joël , Wormer Paul E. S.
Published: July 2002

Symmetry-adapted perturbation theory has been applied to compute the intermolecular potential energy surface of the He–CO2 complex. The ab initio potential has a global minimum of εm=−50.38 cm−1 at Rm=5.81 bohr for the “T”-shaped geometry of the complex, and a local one of εm=−28.94 cm−1 at Rm=8.03 bohr for the linear He⋅⋅⋅O=C=O structure. The computed potential energy surface has been analytically fitted and used in converged variational calculations to generate bound rovibrational states of the He–CO2 complex and the infrared spectrum corresponding to the simultaneous excitation of the ν3 vibration and internal rotation in the CO2 subunit within the complex. The complex was shown to be a semirigid asymmetric top and the rovibrational energy levels could be classified with the asymmetric top quantum numbers. The computed frequencies of the infrared transitions in the ν4 band of the spectrum are in very good agreement with the high resolution experimental data of Weida et al. [J. Chem. Phys. 101, 8351 (1994)]. The energy levels corresponding to the ν5 bending mode of the complex have been used to compute the transition frequencies in the ν5 hot band of He–CO2. A tentative assignment of the transitions observed in the ν5 band with the quantum numbers of the asymmetric rotor is presented. As a further test of the ab initio potential we also report the pressure broadening coefficients of the R branch rotational lines of the ν3 spectrum of CO2 in a helium bath at various temperatures. Very good agreement is found with the wealth of experimental results for various rotational states of CO2 at different temperatures. Finally, we also tested the potential by computing the second virial coefficients at various temperatures. Again, the agreement between theory and experiment is satisfactory, showing that the ab initio potential can reproduce various physical properties of the complex.
The role of secondary instabilities in the stabilization of a nonpremixed lifted jet flame
Authors: Demare David , Baillot Françoise
Published: July 2002

A nonpremixed lifted jet flame is studied dynamically in the hysteresis zone. High-speed laser tomography images show clearly that, in the case of an organized jet, the flame is located on streamwise counter-rotating vortex filaments generated to secondary instabilities and ejected towards ambient air. Particle image velocimetry is used to evaluate the amplitude of the translational and rotational velocity of these filaments. The use of an acoustic field to force jet instabilities shows that the flame, following large filament ejections, moves back upstream very close to the nozzle without anchoring at it. The role of streamwise vortices in the stabilization mechanism of the lifted flame is confirmed by measurements obtained with a disordered jet, from a straight tube burner. From these results, it is proposed that secondary vortices at the flame base are sufficiently strong to create a premixed zone and to oppose flame propagation.
High performance feedback for fast scanning atomic force microscopes
Authors: Schitter G. , Menold P. , Knapp H. F. , Allgöwer F. , Stemmer A.
Published: July 2002

We identify the dynamics of an atomic force microscope (AFM) in order to design a feedback controller that enables faster image acquisition at reduced imaging error compared to the now generally employed proportional integral differential (PID) controllers. First, a force model for the tip–sample interaction in an AFM is used to show that the dynamic behavior of the cantilever working in contact mode can be neglected for control purposes due to the relatively small oscillation amplitude of the cantilever in response to a defined topography step. Consequently, the dynamic behavior of the AFM system can be reduced to the behavior of the piezoelectric scanner making the design of a model based controller for the AFM possible. Second, a black box identification of the scanner of a commercial AFM (Nanoscope IIIa, Digital Instruments) is performed using subspace methods. Identification yields a mathematical model of the scanner which allows us to design a new controller utilizing H∞ theory. Finally, this controller is implemented on an existing AFM and operated in contact mode. We demonstrate that such an H∞-controlled AFM system, while scanning at rates five times faster than conventional PID-controlled systems, operates with reduced measurement error and allows scanning at lower forces.
A class of nonconservative Lagrangian systems on Riemannian manifolds
Authors: Crampin M. , Sarlet W.
Published: July 2002

We generalize results of Rauch-Wojciechowski, Marciniak and Lundmark, concerning a class of nonconservative Lagrangian systems, from the Euclidean to the Riemannian case.
Exchange repulsion increases internal rotation floppiness
Authors: Pophristic Vojislava , Goodman Lionel
Published: July 2002

The influence of antisymmetrization (exchange repulsion) on the shape of internal rotation potential surfaces is investigated by comparing potential curves with and without antisymmetrization. The ab initio 6-311G(3df,2p) and 6-311G++(3df,2p) calculations show that exchange interaction, for the three ethane-family molecules (ethane, disilane, and methanol) investigated, increases the torsional amplitude and level splittings. An outcome of the exchange repulsion-triggered lowered torsional frequencies is increase in excited torsional level populations with consequent effect on thermodynamic and spectroscopic properties.
Experimental configuration for isentropic compression of solids using pulsed magnetic loading
Authors: Hall C. A. , Asay J. R. , Knudson M. D. , Stygar W. A. , Spielman R. B. , Pointon T. D. , Reisman D. B. , Toor A. , Cauble R. C.
Published: July 2002

A capability to produce quasi-isentropic compression of solids using pulsed magnetic loading on the Z accelerator has recently been developed and demonstrated [C. A. Hall, Phys. Plasmas 7, 2069 (2000)]. This technique allows planar, continuous compression of materials to stresses approaching 1.5 Mbar. In initial stages of development, the experimental configuration used a magnetically loaded material cup or disk as the sample of interest pressed into a conductor. This installation caused distortions that limited the ability to attach interferometer windows or other materials to the rear of the sample. In addition, magnetic pressure was not completely uniform over sample dimensions of interest. A new modular configuration is described that improves the uniformity of loading over the sample surface, allows materials to be easily attached to the magnetically loaded sample, and improves the quality of data obtained. Electromagnetic simulations of the magnetic field uniformity for this new configuration will also be presented. Comparisons between data on copper to ∼300 kbar using the old and new experimental configurations will also be made. Results indicate that to within experimental error, the configurations produce similar results in the pressure-volume plane.
High critical current density of MgB2 bulk superconductor doped with Ti and sintered at ambient pressure
Authors: Zhao Y. , Feng Y. , Cheng C. H. , Zhou L. , Wu Y. , Machi T. , Fudamoto Y. , Koshizuka N. , Murakami M.
Published: July 2002

Ti-doped MgB2 superconductors with different doping levels were prepared by solid-state reaction at ambient pressure. The density, diamagnetic signal, and Jc of the samples change significantly with the doping level, with the best result achieved at x=0.1. At 5 K, the Jc reaches 2×106 A/cm2 in the self-field and 5×104 A/cm2 in 5 T. At 20 K, the Jc is still as high as 1.3×106 A/cm2 in the self-field and 9.4×104 A/cm2 in 2 T. It is observed that partial melting occurs in the Ti-doped samples, resulting in an excellent grain connection and extremely high density. In addition, some fine particles (with sizes from 10 to 100 nm) of the second phases induced by Ti doping are distributed in the MgB2 matrix, and this may play an important role in flux pinning enhancement.
Charging effect in InAs self-assembled quantum dots
Authors: Wang T. H. , Li H. W. , Zhou J. M.
Published: July 2002

Metal-semiconductor-metal diode devices are fabricated with InAs self-assembled quantum dots (QDs). Both hysteresis loops and staircases are observed in the I–V characteristics, which are analyzed by employing the ideality factor of the Schottky contacts. The charging and discharging of the InAs QDs induce the hysteresis loops, resonant tunneling through the energy levels of QDs gives rise to the staircases. The charging effect is found to be weakened by the resonant tunneling, and the resonant tunneling cannot be observed in the voltage range where the loops are most pronounced.
Triplet superconducting proximity effect in inhomogeneous magnetic materials
Authors: Kadigrobov A. , Shekhter R. I. , Jonson M.
Published: July 2002

We show that quantum spin fluctuations in inhomogeneous conducting ferromagnets drastically affect the Andreev reflection of electrons and holes at a ferromagnet–superconductor interface. As a result, a strong long-range proximity effect appears, associated with electron–hole spin triplet correlations and persisting on a length scale typical for nonmagnetic materials but anomalously large for ferromagnets. For applications, an important consequence is that this long-range proximity effect permits the creation of superconducting quantum interference devices with magnetic Josephson junctions of anomalously large length.
High-resolution laser spectroscopy of LiAr: Improved interaction potential and spin-rotation-coupling in the ground state X 2Σ+
Authors: Brühl Rüdiger , Zimmermann Dieter
Published: July 2002

The spin-rotation coupling constant γ has been determined to be −0.15(6)⋅10−3 cm−1 for vibrational levels v=0, 1, and 2 of the X 2Σ+ state of LiAr7 by means of an analysis of the high-resolution absorption spectrum due to the A←X transition in the 670–677 nm region. All rovibrational levels of the X 2Σ+ state have been observed, and improved values have been obtained for the spectroscopic parameters of vibration and rotation. In addition, the dispersed fluorescence A→X has been recorded by populating selectively a particular rovibrational level of the A state. Using all experimental data, an improved X 2Σ+ interaction potential has been determined for LiAr7, which is provided in the form of an analytical Hartree-Fock-Dispersion function.
Brown’s “fundamental theorem” revisited
Authors: Aharoni Amikam
Published: July 2002

Brown’s method of calculating an upper and a lower bound of the “critical size” for a single domain behavior is recalled, with a particular emphasis on the case of soft ferromagnets. The upper bound for a sphere is very slightly improved. Any further improvement, if at all needed for isolated particles of soft materials, should be done in the same direction. This improvement is used to demonstrate that this method can be extended to the case of interacting neighbors, but not to the study of very many grains, for which micromagnetics is inadequate and has to be superimposed by domain theory.
Anistropic hole formation in thin polymer films confined by walls
Authors: Suh K. Y. , Lee Hong H.
Published: July 2002

We report dewetting behavior of thermally annealed thin polymer strips of poly (styrene-b-butadiene-b-styrene) and polystyrene films that are laterally confined by polydimethylsiloxane walls on silicon substrate. Regularly spaced holes are initially nucleated on the polymer surface and then grow with the aid of the confining walls, resulting in the formation of distinctly observable, regularly spaced blocks of the strip. It is observed that the dependence of hole density on the film thickness severely deviates from the conventional capillary wave model. This anomaly may be related to the viscoleastic properties of the polymer film.
Electrical characteristics of a Dy-doped HfO2 gate dielectric
Authors: Lee Hyelan , Jeon Sanghun , Hwang Hyunsang
Published: July 2002

This letter describes a process for the preparation of high-quality Dy-doped HfO2 for use in gate dielectric applications. Compared with conventional undoped HfO2, the HfO2 gate dielectric which contains an optimum concentration of Dy exhibits the lowest leakage current and minimum effective oxide thickness. The lower electronegativity and larger atomic radii of Dy contribute to the improvement in leakage current for Dy-doped HfO2. The Dy-doped HfO2 is a promising gate dielectric layer for use in future high-k gate dielectric applications.
Spectroscopic properties of lead trimer (Pb3 and Pb3+): Potential energy surfaces, spin–orbit and Jahn–Teller effects
Authors: Balasubramanian K. , Majumdar D.
Published: July 2002

Spectroscopic properties of the low-lying electronic states of neutral, cationic, and anionic lead trimer (Pb3) are investigated. We have obtained the bending potential energy surfaces of several electronic states of Pb3 and Pb3+ both with and without spin–orbit coupling. These computations were carried out using high level techniques that included electron correlation effects and spin–orbit coupling simultaneously using a multireference relativistic configuration interaction (RCI) scheme in the double group, subsequent to complete active-space–multiconfiguration self-consistent-field (CAS–MCSCF) computations. We have computed the equilibrium geometries, vibrational frequencies, excitation energies, atomization energies, ionization potentials, and adiabatic electron affinities. Our computations facilitated the assignment of the anion photodetachment spectra of Pb3− and explained the “closed-shell singlet like” structures in the observed photodetachment spectra. Our computations show that spin–orbit coupling has a substantial impact on the geometries (bond angles changing up to 20°) and the potential energy surfaces, which exhibit multiple minima separated by barriers due to avoided crossings and substantial spin–orbit mixings. The interplay between the Jahn–Teller effect and spin–orbit coupling was considered, and it was shown that the Jahn–Teller coupling is quenched by spin–orbit effect for Pb3+, but for the neutral trimer the bending potential energy surface of the ground state exhibits multiple minima due to a combination of these effects. The spin–orbit effect was also shown to reduce the strength of the Pb–Pb bonding and the atomization energy of Pb3. Our computed atomization energy of Pb3 including spin–orbit coupling is 224 KJ/mole in full agreement with the experimental value of 224 KJ/mole. We have shown that the spin–orbit coupling enhances the stability of Pb3+ while it weakens Pb3 dramatically compared to lighter analogs such as Si3.
N
2
dissociative adsorption on Ru(0001): The role of energy loss
Authors: Diekhöner L. , Mortensen H. , Baurichter A. , Jensen E. , Petrunin V. V. , Luntz A. C.
Published: July 2002

New molecular beam experiments on the dissociation probability S0 for N2 on Ru(0001) are presented. These are in general agreement with prior measurements and exhibit very unusual behavior; a very slow increase of S0 with incident kinetic energy E and the fact that S0 is still only ∼10−3 at incident energies considerably above the barrier. A simple dynamical model is developed to describe this unusual sticking behavior. The key aspect is that there is considerable energy loss Δ from E upon initial impact with the surface (principally to the lattice) and only E−Δ is then available to surmount the activation barrier in the exit channel. Using experimentally measured values of Δ from scattering experiments gives good qualitative agreement of this model with the measured S0. One implication of the strong energy loss is that there is an apparent violation of detailed balance when comparing only the reactive fluxes of activated adsorption and associative desorption.
Electron temperature measurement by a helium line intensity ratio method in helicon plasmas
Authors: Boivin R. F. , Kline J. L. , Scime E. E.
Published: July 2002

Electron temperature measurements in helicon plasmas are difficult. The presence of intense rf fields in the plasma complicates the interpretation of Langmuir probe measurements. Furthermore, the non-negligible ion temperature in the plasma considerably shortens the lifetime of conventional Langmuir probes. A spectroscopic technique based on the relative intensities of neutral helium lines is used to measure the electron temperature in the HELIX (Hot hELicon eXperiment) plasma [P. A. Keiter et al., Phys. Plasmas 4, 2741 (1997)]. This nonintrusive diagnostic is based on the fact that electron impact excitation rate coefficients for helium singlet and triplet states differ as a function of the electron temperature. The different aspects related to the validity of this technique to measure the electron temperature in rf generated plasmas are discussed in this paper. At low plasma density (ne⩽1011 cm−3), this diagnostic is believed to be very reliable since the population of the emitting level can be easily estimated with reasonable accuracy by assuming that all excitation originates from the ground state (steady-state corona model). At higher density, secondary processes (excitation transfer, excitation from metastable, cascading) become more important and a more complex collisional radiative model must be used to predict the electron temperature. In this work, different helium transitions are examined and a suitable transition pair is identified. For an electron temperature of 10 eV, the line ratio is measured as a function of plasma density and compared to values predicted by models. The measured line ratio function is in good agreement with theory and the data suggest that the excitation transfer is the dominant secondary process in high-density plasmas.
Electrical and structural properties of indium tin oxide films prepared by pulsed laser deposition
Authors: Izumi Hirokazu , Adurodija Frederick O. , Kaneyoshi Takahiro , Ishihara Tsuguo , Yoshioka Hideki , Motoyama Muneyuki
Published: July 2002

The relation between electrical and structural properties of indium tin oxide (ITO) films prepared by pulsed laser deposition with and without in situ laser irradiation is examined. The residual stresses of the films were estimated from x-ray diffraction patterns measured by grazing-incidence asymmetric Bragg and grazing-incidence x-ray diffraction geometries. For the films prepared without in situ irradiation, the residual stress depended on oxygen pressure (PO2) during deposition and had minimum around PO2 of 1.3 Pa, which coincided with the optimum PO2 for growing the lowest resistivity films. The resistivity was only slightly improved with an increase of substrate temperature (Ts) because a large residual stress was introduced. In contrast, the ITO films prepared with in situ laser irradiation showed very low resistivity (ρ<10−4 Ω cm) which can be attributed to the high crystallinity and low residual stress.
Characteristics of interface-engineered Josephson junctions using a YbBa2Cu3Oy counterelectrode layer
Authors: Katsuno H. , Inoue S. , Nagano T. , Yoshida J.
Published: July 2002

We have fabricated interface-engineered junctions with YbBa2Cu3Oy as the counterelectrode. The junctions fabricated on YBa2Cu3Oy base electrodes exhibited excellent Josephson characteristics with the 1σ-spread in Ic as low as 5.4% for 16 junctions with an average Ic of around 1 mA. We also confirmed that the 1σ in Ic correlates with the surface morphology of the base-electrode layer, indicating that further improvements in 1σ would be possible by advancing the thin-film growth technology.
Thirty years of surface plasma sources for efficient negative ion production
Authors: Dudnikov Vadim
Published: July 2002

Thirty years ago, July 1, 1971, significant enhancement of negative ion emission from a gas discharge following a small admixture of cesium was observed for the first time. This observation became the basis for the development of the Surface Plasma Source (SPS) for efficient production of negative ions. In the SPS, negative ions are produced from the interaction of plasma particles with electrodes on which adsorbed cesium reduces the surface work function. Following this discovery, the emission current density of negative ions increased rapidly from j∼0.01 A/cm2 to 3.7 A/cm2 with a flat cathode and up to 8 A/cm2 with optimized geometrical focusing in a long pulse SPS and up to 1 A/cm2 for a dc SPS. Charge-exchange cooling decreases a negative ion temperature below 1 eV and increases the negative ion brightness by many orders to a level compatible with the best proton sources, B=j/T>1 A/cm2 eV. The intensity of negative ion beams was increased from mA to tens of amperes. Features of different SPS’s, negative ion beam formation, transportation, space-charge neutralization over neutralization and instability damping are considered. Practical aspects of SPS operation and high brightness beam production will be discussed.
Space charge compensation of low energy ion beam
Authors: Dudnikov V. , Dudnikov A.
Published: July 2002

Compensation of the space charge of high perveance, low energy ion beam in beam lines for ion implantation and isotope separation has been investigated. Space charge compensation and overcompensation by ions will be considered. Different mechanisms of the compensating particle formation such as ionization by the beam, secondary emission, electronegative gas admixture, and external plasma sources will be discussed. Advanced space charge compensation increases an intensity of low energy ion beam after analyzer magnet up to 3–4 times. Space charge compensation of positive ion beam by admixture of electronegative gases and damping of the beam instability will be discussed. Up to 6 mA of B+11 ions with energy 3 keV, 11 mA with 5 keV, and 18 mA with 10 keV have been transported through an analyzer magnet of a high current implanter with space charge neutralization by electronegative gases.
Sensitivity of Pt x-ray absorption near edge structure to the morphology of small Pt clusters
Authors: Ankudinov A. L. , Rehr J. J. , Low John J. , Bare Simon R.
Published: July 2002

A theoretical study of the sensitivity of Pt L3 x-ray absorption near edge structure (XANES) to the size and shape in small Ptn clusters is reported. Calculations, based on a full multiple scattering, self-consistent field, real-space Green’s function approach implemented in the ab initio FEFF8 code, show that XANES provides a characteristic signature of cluster shape. For example, the calculated white line intensity exhibits a large variation for small cluster sizes and geometry, but becomes independent of cluster size for large clusters. A strong polarization dependence of the white line is predicted for two-dimensional clusters. For three-dimensional clusters the polarization dependence is smaller, but can be used as a measure of the “flatness” of a cluster. A series of semirelativistic all-electron, full potential density functional calculations was also performed for several Ptn clusters. These calculations show the existence of intrinsic static disorder in these clusters due to nonisotropic shrinkage. There is an expansion along the (111) direction, and a contraction in the other directions. Additional calculations for a model Pt6 cluster in zeolite-LTL pore show that, except for additional broadening of the white line, all XANES features of the free clusters are qualitatively preserved.
Strong localization in InGaN layers with high In content grown by molecular-beam epitaxy
Authors: Naranjo F. B. , Sánchez-Garcı́a M. A. , Calle F. , Calleja E. , Jenichen B. , Ploog K. H.
Published: July 2002

The effect of the III/V ratio and growth temperature on the In incorporation has been studied in thick (>300 nm) InGaN layers, with In mole fractions from 19% to 37%, grown by molecular-beam epitaxy on sapphire and on GaN templates. Significant desorption of In occurs at growth temperatures above 550 °C. Symmetric and asymmetric reflections from high resolution X-ray diffraction reveals that the layers are not fully relaxed. A bowing parameter of 3.6 eV is calculated from optical absorption data, once corrected for strain-free band gap values. The increase of both, the absorption band-edge broadening and the photoluminescence full width at half maximum at room temperature with the In content, is discussed in terms of a strong In localization effect. This localization effect is further evidenced by the S-shaped temperature dependence of the emission energy.
Marked enhancement of 320–360 nm ultraviolet emission in quaternary InxAlyGa1−x−yN with In-segregation effect
Authors: Hirayama Hideki , Kinoshita Atsuhiro , Yamabi Takayoshi , Enomoto Yasushi , Hirata Akira , Araki Tsutomu , Nanishi Yasushi , Aoyagi Yoshinobu
Published: July 2002

We demonstrated room-temperature (RT) intense ultraviolet (UV) emission in the wavelength range of 315–370 nm from quaternary InxAlyGa1−x−yN alloys grown by metalorganic vapor-phase epitaxy. We found that the UV emission is considerably enhanced by the In-segregation effect upon introducing 2%–5% of In into AlGaN. The In incorporation in quaternary InxAlyGa1−x−yN is markedly enhanced with the increase of Al content when using a relatively high growth temperature (830–850 °C), resulting in efficient RT UV emission. Maximally efficient emission was obtained at around 330–360 nm from the fabricated quaternary InxAlyGa1−x−yN (x=2.0%–4.8%,y=12%–34%). The intensity of the 330 nm emission from quaternary In0.034Al0.12Ga0.85N was as strong as that of the 430 nm emission from In0.22Ga0.78N at RT. We clearly observed In segregation of submicron size from cathode luminescence images of quaternary InAlGaN films.
Constructing accurate potential energy surfaces for a diatomic molecule interacting with a solid surface: H2+Pt(111) and H2+Cu(100)
Authors: Olsen R. A. , Busnengo H. F. , Salin A. , Somers M. F. , Kroes G. J. , Baerends E. J.
Published: July 2002

By applying a corrugation-reducing procedure we have interpolated the six-dimensional (6D) potential energy surfaces for the H2/Pt(111) and H2/Cu(100) systems from data obtained by density functional theory (DFT) calculations. We have compared interpolated values with a large number of DFT results not used in the basis for the interpolation and we have obtained an average error below 20 meV and a maximum error of about 30 meV in the regions important for dissociative adsorption. Near the surface the corrugation-reducing procedure gives excellent results using only data from high-symmetry sites. However, we show that to reach the above mentioned accuracy level far from the surface, it is necessary to include information from at least one low-symmetry site. Care has been taken to demonstrate the quality of the interpolation along all degrees of freedom in different regions of the configuration space. The strengths of the method are shown together with the aspects requiring careful handling. A comparison with a direct interpolation method is also made.
CoPt and FePt nanowires by electrodeposition
Authors: Huang Y. H. , Okumura H. , Hadjipanayis G. C. , Weller D.
Published: July 2002

In this study we have fabricated by electrodeposition CoPt(FePt) nanowires embedded inside an array of empty holes in anodized aluminum disks. By adjusting the current density and solution composition, one can control the composition of the film to 50:50 in order to obtain the high anisotropy CoPt and FePt face centered tetragonal (L10) phases. The as-made films are magnetically soft. Magnetic hardening is developed after annealing at 700 °C with coercivity typically in the range of 3–6 kOe. The scanning electron microscopy and transmission electron microscopy studies showed that nanowire structured CoPt and FePt with near stoichiometric composition inside an array of holes which are about 25–100 nm in diameter. A preferred perpendicular anisotropy is observed in the CoPt nanowires.
Turbulent flow over water waves in the presence of stratification
Authors: Sullivan Peter P. , McWilliams James C.
Published: July 2002

Direct numerical simulation is used to investigate stratified turbulent flow over a series of prescribed moving water waves at a bulk Reynolds number Re=8000 and waveslope ak=0.1. Unstable, neutral, and stable stratifications are considered for a range of wave phase speeds c. Stratification is shown to significantly alter the mean vertical profiles of velocity and temperature, turbulence variances, wave-induced flow fields, and surface form stress. For the range of conditions considered, the surface form stress (drag) and flow patterns (critical-layer height and streamlines) are well correlated with the friction velocity u*, which therefore contains the essential information about stratification influences. Nonseparated sheltering [Belcher and Hunt, Annu. Rev. Fluid. Mech. 30, 507 (1998)], which determines the drag in neutral flow over stationary topography, is modified by stratification and the movement of the underlying waves. The variation of the form stress with phase speed is correlated with the movement of the critical layer above the surface. Compared to neutral flow at a given phase speed, the flow patterns with unstable stratification are similar to the flow patterns over slower moving waves while stable stratification results in flow patterns typical of faster moving waves. This behavior is qualitatively captured by the wave age parameter c/u*. The wave-induced temperature field responds to the wave-induced velocity fields by forming positive and negative patches over the wave crests and troughs, respectively, with the resulting wave-induced heat flux as much as 15% of the total surface heat flux. Estimates of wave growth from the DNS are in reasonable agreement with field observations and laboratory experiments, and they are larger than predictions from high Reynolds-number, second-order closure models for c/u*<10. For c/u*>10, the present calculations predict less negative form stress (or less damping) of the waves compared to second-order closure models.
Sol–gel glass distributed feedback waveguide laser
Authors: Zhu Xiao-lei , Lo Dennis
Published: July 2002

Titania–silica glass films doped with Rhodamine 6G dye were fabricated using the low-temperature sol–gel technique. Films of thickness varying from 0.7 to 1.1 μm and of refractive index from 1.61 to 1.54 were obtained by controlling the revolution speed of spin coating and by changing the ratio of the sol–gel precursors. Tunable laser action was observed for the sol–gel glass films on glass or quartz substrates when a dynamic gain modulation was optically generated. Lasing with multiple guiding modes was achieved at a film thickness of 1.1 μm. The effect of optical confinement was studied by varying the difference of refractive indices between the sol–gel film and the substrate.
Perturbed ion traps: A generalization of the three-dimensional Hénon–Heiles problem
Authors: Lanchares V. , Pascual A. I. , Palacián J. , Yanguas P. , Salas J. P.
Published: July 2002

This paper presents an analytical study of an axially symmetric perturbation of the Penning trap. This system is modeled as a generalization of the three-dimensional (3D) Hénon–Heiles potential. Thus, the same techniques which succeeded in the study of the 3D Hénon–Heiles system apply here. The departure Hamiltonian is three dimensional, although it possesses an axial symmetry. This property, together with an averaging process, is used to reduce the original system to an integrable one. We study the flow of the reduced Hamiltonian: equilibria, bifurcations, and stability, extracting thereafter the relevant information about the dynamics of the original problem.
Multicanonical ensemble with Nosé–Hoover molecular dynamics simulation
Authors: Jang Soonmin , Pak Youngshang , Shin Seokmin
Published: July 2002

We demonstrate that molecular dynamics simulations using the force scaling method with a Nosé–Hoover-chain thermostat are capable of generating multicanonical ensembles. The frequency distribution of the Nosé–Hoover-chain is broad enough to handle the energy dependent force scaling factor over a wide potential energy range, when three independent Nosé–Hoover thermostats corresponding to the three orthogonal directions are attached to each particle. The performance of this method has been tested by reproducing various equilibrium properties of one-dimensional model potential, an Ar13 cluster, and a flexible water model.