bluuebunny/crossref_metadata_2025 huggingface dataset:
Relation between gas phase CN radical distributions, nitrogen incorporation, and growth rate in flame deposition of diamond
Authors: Stolk R. L. , van Herpen M. M. J. W. , ter Meulen J. J. , Schermer J. J.
Published: July 2002

Controlled amounts of nitrogen were added during oxyacetylene flame deposition of diamond to investigate the possible role of the CN radical in the effects of nitrogen addition. CN radical distributions were visualized using two-dimensional laser induced fluorescence (LIF) and compared with nitrogen incorporation into the layer and with the diamond growth rate, which were measured by means of cathodoluminescence (CL) and optical microscopy, respectively. For the studied range of nitrogen flows, it was found that the CN LIF signal in the center of the flame is linearly dependent on the added amount of nitrogen. Diamond deposition in the central region is mainly influenced by the deposition parameters, whereas deposition in the outer zone is largely determined by the interaction of the flame with the ambient; the annulus of enhanced growth is affected by both the deposition parameters and the ambient. By a simple consideration, in which the growth rate is separated in nitrogen dependent and independent contributions, an observed positional difference between CN LIF and growth rate maxima can be explained. Comparison of the CN LIF signal, the CL signal and the diamond deposition rate indicates that CN (or a closely related species) may be the species or one of the main species responsible for the effects of nitrogen addition during oxyacetylene flame deposition of diamond.
Experimental study of rotationally inelastic collisions of AlH(A 1Π) with Ar: State-to-state rate constants and Λ-doublet propensities
Authors: Nizamov Boris , Dagdigian Paul J.
Published: July 2002

A study of rotationally inelastic collisions of AlH(A 1Π) with Ar is reported. AlH was prepared by 193 nm photolysis of trimethylaluminum diluted in slowly flowing Ar at ∼0.5 Torr total pressure. Specific rotational, Λ-doublet levels of AlH(A 1Π,v=0) were prepared by pulsed dye laser excitation on rotational lines in the AlH A 1Π–X 1Σ+ (0,0) band, and final levels were detected by spectrally resolving the fluorescence emission. Bimolecular state-to-state and total removal rate constants were determined. The state-to-state rate constants display dramatic Λ-doublet propensities. In particular, for high initial rotational levels the ΔJ=−1 f→e Λ-doublet symmetry changing transitions had significantly larger rate constants than for ΔJ=−1 f→f Λ-doublet symmetry conserving transitions. These Λ-doublet changing transitions are facilitated by curve crossing between the effective potential energy curves correlating with different rotational asymptotes. The total removal rate constants decrease with increasing initial J, reflecting the increasing rotational spacings.
A density-functional approach to nucleation in micellar solutions
Authors: Talanquer V. , Oxtoby D. W.
Published: July 2002

We present a density-functional theory for binary mixtures of amphiphiles with solvent, placing particular emphasis on the structure and free energy of stable and metastable clusters of amphiphiles in dilute solution. These correspond to micelles (complexes of amphiphiles with the solvophobic groups clustered in the interior) and vesicles (spherical complexes with bilayer membranes of amphiphiles with solvent both inside and outside the membrane shell). Saddle points that connect these states are also explored to study the nucleation behavior of these systems. Our approach is applicable to both strong and weak amphiphiles, and can be readily extended to the full ternary phase equilibria characteristic of microemulsions.
Can a local mode picture account for vibration–torsion coupling? Ab initio test based on torsional variation of methyl stretching and bending frequencies in methanol
Authors: Xu Li-Hong
Published: July 2002

The torsional dependence of the CH3 stretching and bending modes of methanol has been explored in terms of a local mode internal coordinate picture [X. Wang and D. S. Perry, J. Chem. Phys. 109, 10795 (1998)]. First, the torsional variations of the small-amplitude vibrational frequencies along the mass weighted intrinsic reaction coordinate from the top to the bottom of the torsional potential barrier were calculated by means of ab initio frequency projection utilizing GAUSSIAN 98. The resulting curves for the three C–H stretch ab initio frequencies as functions of the torsional angle cannot be reproduced by the original 3×3 local mode model incorporating stretch–torsion and stretch–stretch couplings at lowest-order only, but are well-fitted if the model is extended to include higher-order coupling terms. For the CH-bending modes, with internal coordinates chosen to give a high degree of localization, bend–torsion and bend–bend coupling parameters were determined from the ab initio projected frequencies, and were then used to predict torsional tunneling splittings. Just as observed for the C–H stretch modes, the two higher-frequency asymmetric CH-bend modes are predicted to have inverted tunneling splittings with reduced amplitudes, while the splitting pattern for the lower frequency symmetric-bend mode is predicted to be normal.
Low-frequency waves and relaxation processes in semibounded and bounded plasma-like media
Authors: Shokri B.
Published: July 2002

In this article, the frequency spectrum and relaxation processes for volume and surface waves are investigated in semibounded plasmas and thin films of plasma-like media in the presence and absence of spatial dispersion. In doing so, two models are considered: (1) The model with mirror reflection of charge carriers from the boundary surface of plasma medium; (2) The model of a thin plasma layer which is confined by a strong magnetic field.
Initial results for an argon Z pinch using a double-shell gas puff
Authors: Sze H. , Coleman P. L. , Failor B. H. , Fisher A. , Levine J. S. , Song Y. , Waisman E. M. , Apruzese J. P. , Chong Y. K. , Davis J. , Cochran F. L. , Thornhill J. W. , Velikovich A. L. , Weber B. V. , Deeney C. , Coverdale C. A. , Schneider R.
Published: July 2002

Recent observations are given for an argon double-shell gas puff imploded with up to 4 MA in 200 ns on the Double Eagle generator [G. B. Frazier et al., Digest of Technical Papers, Fourth IEEE Pulsed Power Conference (IEEE, Piscataway, NJ, 1983), p. 583]. Good K-shell x-ray output with good pinch quality was observed. A novel experimental procedure was used to selectively seed the inner or outer gas plenums with a chlorine tracer. The tracer data provide the first direct experimental evidence that the mass initially closest to the axis is the dominant contributor to the hot core of the radiating pinch.
Nonlinear stimulated Brillouin scattering in inhomogeneous plasmas
Authors: Maximov A. V. , Oppitz R. M. , Rozmus W. , Tikhonchuk V. T.
Published: July 2002

The combined effects of ion-acoustic nonlinearity and plasma inhomogeneity on a stimulated Brillouin scattering (SBS) are considered in the model of coupled Korteweg–de Vries (KdV) and electromagnetic wave equations. In the regime of linear ion-acoustic waves the results are in good agreement with the existing linear theories of SBS and demonstrate the regimes of both convective and absolute instabilities. New features of SBS saturation due to ion-acoustic nonlinearity and pump depletion in inhomogeneous plasmas are found. They include the scaling for saturated reflectivity, the transition from convective to absolute growth, and the temporal pulsations of the reflectivity coefficient. The autoresonance process is analyzed and its relevance to SBS for realistic plasma parameters is discussed.
An approximate short time Laplace transform inversion method
Authors: Plimak Lev , Pollak Eli
Published: July 2002

The “standard” numerical methods used for inverting the Laplace transform are based on a regularization of an exact inversion formula. They are very sensitive to noise in the Laplace transformed function. In this article we suggest a different strategy. The inversion formula we use is an approximate one, but it is stable with respect to noise. The new approximate expression is obtained from a short time expansion of the Bromwich inversion formula. We show that this approximate result can be significantly improved when iterated, while remaining stable with respect to noise. The iterated method is exact for the class of functions of type EmeaE. The method is applied to a harmonic model of the stilbene molecule, to a truncated exponent series, and to the flux–flux correlation function for the parabolic barrier. These examples demonstrate the utility of the method for application to problems of interest in molecular dynamics.
Raman free-electron laser with a coaxial wiggler
Authors: Farokhi B. , Maraghechi B. , Willett J. E.
Published: July 2002

A one-dimensional theory of the stimulated Raman scattering mechanism for a coaxial free-electron laser (FEL) is developed. The beam-frame FEL dispersion relation and a formula for the lab-frame spatial growth rate are derived. A numerical study of the growth rate for the coaxial wiggler is made and compared with that for the helical wiggler. Except for a part of the group II orbits, the growth rate is found to be less than the helical wiggler. Relativistic effects due to the transverse oscillation of electrons in the wiggler field prevent the FEL operation from approaching magnetoresonance. In the absence of these relativistic mass effects, the calculations show a magnetoresonance associated with the first spatial harmonic and a much narrower resonance at the third spatial harmonic.
Fluid equations for a dusty plasma with dust charge and mass distribution interacting with neutral dust through dust grain charging and secondary emission
Authors: Varma Ram K.
Published: July 2002

A set of fluid equations is obtained for a dusty plasma, with dust charge and mass distributions, interacting with ambient neutral dust through inelastic collision processes involving dust grain charging and discharging and secondary electron emission from the dust grains. These are obtained as appropriate moments of the Boltzmann–Vlasov equations for the positively and negatively charged dust components, neutral dust component, electrons, and ions. Also obtained are the charge and mass moments of these equations, which bring out a new term involving the charge dispersion in the equation for the phase space charge density. The inelastic collision process involving dust grain charging and discharging and secondary electron emission provides a coupling between the positive and negatively charged dust components and the neutral dust component. The source/sink terms in the equations of continuity, momentum balance, and heat equations then are the proper terms obtained systematically from the Boltzmann–Vlasov equations, as against some ad hoc formulations that have been used earlier. The set of equations so obtained should prove useful in analyzing the various dusty plasma phenomena: waves, instabilities, and nonlinear processes in space and astrophysical scenarios.
Dynamics of fluxons in narrow window junctions
Authors: Flytzanis N. , Lazarides N. , Chiginev A. , Kurin V. , Caputo J. G.
Published: July 2002

We numerically investigate the dynamics of a long window junction under the influence of a constant current bias, and intermediate damping in the junction area. The current voltage characteristics are calculated for various sizes of the passive region extension w′ and different thickness of the isolating layer in the idle region. The existence of the idle region influences the fluxon solutions, and consequently the I–V characteristics, causing the fluxons to increase their critical velocity above the Swihart velocity in the window and to radiate by way of the Cherenkov effect as they move in the junction. The fluxons become phase locked with the Cherenkov radiation, and this is reflected in the I–V curve as very strong resonances at relatively high velocities.
The 193 nm photodissociation of CH2=CClF
Authors: Lee Y. R. , Wang L. D. , Lee Y. T. , Lin S. M.
Published: July 2002

The photodissociation of CH2=CClF at 193 nm has been studied by product translational spectroscopy. A total of five primary dissociation channels was detected for the products of Cl (fast), HCl, HF, Cl (slow), and F in the decreasing order of relative yield. Product translational energy distributions have been measured for these reaction products. While a thermal-like distribution was detected for the slow Cl and F atoms, a nonthermal distribution was determined for the fast Cl atom and the molecular elimination of HCl and HF. Apart from the fast Cl atom with anisotropy parameter β=0.5, the remaining products recoil isotropically following photolysis with linearly polarized light. By analogy with the photodissociation of chloroethylenes, these products are found to originate from two potential energy surfaces. In addition to the primary dissociation channels, we have detected the secondary products of F+C2H2 from the internally excited α-fluorovinyl radical through a fast fragment isomerization.
Dielectric and ferroelectric properties of compositionally graded (Pb,La)TiO3 thin films on Pt/Ti/SiO2/Si substrates
Authors: Bao Dinghua , Mizutani Nobuyasu , Yao Xi , Zhang Liangying
Published: July 2002

Compositionally graded (Pb,La)TiO3 thin films were prepared on platinum-coated silicon substrates by a sol–gel technique. The crystalline orientation and surface morphology of the graded films were closely related to the deposition sequence of the film layer. The dielectric constants, for up-graded and down-graded films annealed at 600 °C for 60 min, were found to be 765 and 374, respectively. The compositionally graded films had large polarization offsets in hysteresis loops when driven by an alternating electric field. The magnitude of polarization offsets displayed a power-law dependence on the electric field, and the direction of the offsets depended on the direction of the composition gradient with respect to the substrate. The offset, 250 μC/cm2 at the driving electric field of 250 kV/cm, was obtained. These results showed that the sol–gel technique was a very promising route for the realization of compositionally graded ferroelectric thin films and the compositionally-graded (Pb,La)TiO3 thin films had excellent dielectric properties and abnormal ferroelectric properties which can be used in various microelectronic devices.
Erbium/ytterbium fluorescence based fiber optic temperature sensor system
Authors: Sun T. , Zhang Z. Y. , Grattan K. T. V.
Published: July 2002

Erbium/ytterbium co-doped fibers have been investigated in this work for the first time to determine their potential in thermometry applications based on the use of fluorescence decay. Several samples have been used and their performance characteristics determined. These have included studies of fiber annealing as well as thermal cycling tests and it was found that unlike other rare-earth ion doped fibers previously studied, such as Nd3+, Er3+, or Tm3+ doped fibers, the Er/Yb co-doped samples used here have shown a much smaller degree of thermal annealing when their thermal characteristics have been evaluated and compared. The response of the thermometer and the error in the measurement was found to be, at <±5 °C, within the stability of the oven used in the tests over a wide temperature range from 0 to 850 °C.
Semiclassical inelastic electron-ion collisional excitations in nonideal plasmas
Authors: Yoon Jung-Sik
Published: July 2002

Semiclassical electron-hydrogenic ion collisional excitation cross sections for 1s→2s and 1s→2p0 in nonideal plasmas are obtained. An effective pseudopotential model taking into account the plasma screening and collective effects is applied to describe the electron-ion interaction potential in a classical nonideal plasma. The semiclassical straight-line trajectory approximation is applied to the motion of the projectile electron in order to investigate the variation of the total inelastic scattering cross section as a function of the projectile energy, nonideal plasma parameter, and Debye length. It is found that the transition probabilities in ideal plasmas described by the Debye–Hückel potential model are greater than those in nonideal plasmas described by the pseudopotential model. The maximum position of the transition probability gets closer to the target core as the projectile energy increases. It is also found that the scaled cross section decreases when the nonideal plasma parameter (γ) is increased, i.e., the nonideality of plasmas reduces the electron-ion collision cross section.
A new formulation of the resistive magnetohydrodynamics stability problem for finite β toroidal plasmas
Authors: Galkin S. A. , Turnbull A. D. , Greene J. M. , Chu M. S.
Published: July 2002

A new approach to the study of linear resistive magnetohydrodynamics stability is described. The approach is based on the traditional toroidal plasma model where the plasma resistivity and mass effects are essential only in thin layers around resonance surfaces, whereas the outer plasma is ideal and inertia free. This leads to differential equations with singular points. A new technique to solve these equations is proposed and it is shown that it has superior numerical convergence and accuracy properties to previous methods. The new technique is generally applicable to other problems in which differential equations with singular points arise.
Vibrational spectra and electron detachment energy of the anionic water hexamer
Authors: Suh Seung Bum , Lee Han Myoung , Kim Jongseob , Lee Jin Yong , Kim Kwang S.
Published: July 2002

A number of experimental and theoretical studies have been carried out on the anionic water hexamer in the last decade. However, none of these studies have reported the adiabatic electron detachment energy. The present study employing extensive high-level ab initio calculations report the adiabatic electron detachment energy, which explains the unusual stability of the anionic water hexamer. This stability can be correlated to the unusually intense peak observed in the photoelectron-detachment spectra. It is also shown that our previously predicted pyramid structure reproduces the important characteristics of the experimental O–H vibrational spectra.
The prospects for electron Bernstein wave heating of spherical tokamaks
Authors: Cairns R. A. , Lashmore-Davies C. N.
Published: July 2002

Electron Bernstein waves are analyzed as possible candidates for heating spherical tokamaks. An inhomogeneous plane slab model of the plasma with a sheared magnetic field is used to calculate the linear conversion of the ordinary mode (O-mode) to the extraordinary mode (X-mode). A formula for the fraction of the incident O-mode energy which is converted to the X-mode at the O-mode cutoff is derived. This fraction is then able to propagate to the upper hybrid resonance where it is converted to the electron Bernstein mode. The damping of electron Bernstein waves at the fourth harmonic resonance, corresponding to a 60 GHz source on the Mega Amp Spherical Tokamak MAST [A. C. Darke et al., Proceedings of the 16th Symposium on Fusion Energy, Champaign-Urbana, Illinois (IEEE, Piscataway, NJ, 1995, Vol. 2, p. 1456)], is computed. For comparison, results are also presented for a lower frequency source, close to the fundamental electron cyclotron resonance. Both the fundamental and the fourth harmonic are shown to be so strongly absorbing that the electron Bernstein wave would be totally absorbed in the outer regions of the resonance. This feature implies that electron Bernstein wave current drive (on- or off-axis) could be very efficient.
A combined crossed beam and ab initio investigation on the reaction of carbon species with C4H6 isomers. II. The dimethylacetylene molecule, H3CCCCH3(X1A1g)
Authors: Huang L. C. L. , Lee H. Y. , Mebel A. M. , Lin S. H. , Lee Y. T. , Kaiser R. I.
Published: July 2002

The reaction of ground state carbon atoms, C(3Pj), with dimethylacetylene, H3CCCCH3, was studied at three collision energies between 21.2 and 36.9 kJmol−1 employing the crossed molecular beam approach. Our experiments were combined with ab initio and RRKM calculations. It is found that the reaction is barrierless via a loose, early transition state located at the centrifugal barrier following indirect scattering dynamics through a complex. C(3Pj) attacks the π system of the dimethylacetylene molecule to form a dimethylcyclopropenylidene intermediate either in one step via an addition to C1 and C2 of the acetylenic bond or through an addition to only one carbon atom to give a short-lived cis/trans dimethylpropenediylidene intermediates followed by ring closure. The cyclic intermediate ring opens to a linear dimethylpropargylene radical which rotates almost parallel to the total angular momentum vector J. This complex fragments to atomic hydrogen and a linear 1-methylbutatrienyl radical, H2CCCCCH3(X2A″), via a tight exit transition state located about 18 kJmol−1 above the separated products. The experimentally determined exothermicity of 190±25 kJmol−1 is in strong agreement with our calculated data of 180±10 kJmol−1. The explicit verification of the carbon versus hydrogen exchange pathway together with the first identification of the H2CCCCCH3 radical represents a third pathway to form chain C5H5 radicals in the reactions of C(3Pj) with C4H6 isomers under single collision conditions. Previous experiments of atomic carbon with the 1,3-butadiene isomer verified the formation of 1- and 3-vinylpropargyl radicals, HCCCHC2H3(X2A″), and H2CCCC2H3(X2A″), respectively. In high-density environments such as combustion flames and circumstellar envelopes of carbon stars, these linear isomers can undergo collision-induced ring closure(s) and/or H atom migration(s) which can lead to the cyclopentadienyl radical. The latter is thought to be a crucial reactive intermediate in soot formation and possibly in the production of polycyclic aromatic hydrocarbon molecules in outflow of carbon stars. Likewise, a H atom catalyzed isomerization can interconvert the 3-vinylpropargyl and the 1-methylbutatrienyl radical.
Rotational symmetry control of electronic relaxation in ultracold gaseous molecules: Optical–optical double resonance probe of the forbidden T1→S intersystem crossing in jet-cooled thiophosgene, Cl2CS
Authors: Moule David C. , Judge Richard H. , Liu Haisheng , Lim Edward C.
Published: July 2002

Two-color optical–optical double resonance studies of the T1→S0 intersystem crossing in jet-cooled thiophosgene indicate that rotational symmetry strongly influences the efficiency of an intersystem crossing in symmetric molecules with low degrees of rotational excitation.
Energies of ultrarelativistic electrons produced by an oblique shock wave
Authors: Bessho Naoki , Ohsawa Yukiharu
Published: July 2002

Production of ultrarelativistic electrons in an oblique magnetosonic shock wave is studied theoretically and numerically. First, the structure of the oblique shock wave is analytically discussed on the basis of a relativistic, two-fluid model. Then, by use of the field strengths thus obtained, the maximum energy of accelerated electrons is calculated as a function of the propagation speed and angle of the shock wave. Next, a one-dimensional, relativistic, electromagnetic, particle simulation code is used to further investigate the shock propagation and associated electron acceleration. Lorentz factors of accelerated electrons can exceed 100. Detailed comparisons are made between the theoretically predicted field strengths and electron energy and those obtained by the simulations.
Linear scaling algorithm for the coordinate transformation problem of molecular geometry optimization
Authors: Németh Károly , Coulaud Olivier , Monard Gérald , Ángyán János G.
Published: July 2002

This article presents a new algorithm to solve the coordinate transformation problem of molecular geometry optimization. The algorithm is very fast and its CPU time consumption scales linearly with the system size. It makes use of the locality of internal coordinates by efficient sparse matrix techniques. The new algorithm drastically reduces the time needed for coordinate transformations as demonstrated by test calculations on polyalanine and carbone nanotube systems: for a 2000 atom system it requires just seven seconds, instead of the hours consumed by traditional schemes.
A new type of rocket-borne neutral atom analyzer
Authors: Asamura K. , Mukai T. , Saito Y. , Kazama Y. , Machida S.
Published: July 2002

A new-type analyzer for measurement of energetic neutral atoms (ENAs) in an energy range of 4–40 keV is described. Incoming ENAs are ionized by electron stripping at passage of an ultrathin carbon foil. After post-acceleration (by 3 kV), the particles are guided to a time-of-flight (TOF) section over a wide energy-per-charge bandwidth by means of electrostatic deflection without any potential sweeping for electrodes. Then, their velocity is measured by the TOF technique, with which species can also be identified, because the particle energies are limited to a certain range by the electrostatic deflector and acceleration upon entering the TOF section. A unique feature in the present analyzer is in the rejection method of extreme ultraviolet (EUV) contamination. In contrast to conventional usage of serrated electrodes for EUV attenuation, one of the electrostatic deflection plates is machined to be so flat that EUV photons are guided to a photon trap regardless of wavelength. The TOF device can also be used in a coincidence mode for noise suppression. The present instrument was flown on a sounding rocket, and has successfully measured ENAs precipitating into the low-latitude upper atmosphere from the magnetosphere.
Size analysis of nanocrystals in semiconductor doped silicate glasses with anomalous small-angle x ray and Raman scattering
Authors: Irmer G. , Monecke J. , Verma P. , Goerigk G. , Herms M.
Published: July 2002

Semiconductor doped glasses containing CdSxSe1−x nanocrystallites embedded in a silicate glass matrix were investigated. The dimensions of the nanocrystallites are in the range of a few nanometers and vary as a function of a secondary heat treatment. The confinement of such quantum dots for elementary excitations depends strongly on their size. In order to obtain a mean particle size and the size distribution, anomalous small angle x-ray scattering (ASAXS) and low-frequency inelastic Raman scattering measurements were performed. The sizes and the size distributions were evaluated for samples of different mean crystallite radius and composition x. The results of Raman measurements agree well with those of ASAXS, if both the acoustic mode damping across the nanocrystallite–matrix interface and the particle size distribution are taken into account in the Raman band shape analysis. The concentration of nanocrystallites in the glass matrix was determined by using the technique of contrast variation. Scattering curves were recorded at three energies below but close to the K-absorption edge of selenium (12.66 keV) and at 9.64 keV for comparison, which is significantly below the absorption edge.
Defect structure of SiNx:H films and its evolution with annealing temperature
Authors: Martı́nez F. L. , del Prado A. , Mártil I. , Bravo D. , López F. J.
Published: July 2002

The structure of defects of SiNx:H films is investigated by electron-spin resonance. It is found that a relaxation process takes place at annealing temperatures below 600 °C for those compositions in which the nitrogen-to-silicon ratio is above the percolation threshold of the Si–Si bonds in the nitride lattice. The nature of this process is discussed and attributed to a thermally activated charge transfer between metastable defects. No such relaxation occurs in the films with a composition below the percolation threshold, possibly due to a positive correlation energy and a structural lack of flexibility. For higher annealing temperatures, an increase of the defect density is observed and associated with the thermal release of hydrogen.
Electrical characteristics of 25 nm Pr(ZrTi)O3 thin films grown on Si by metalorganic chemical vapor deposition
Authors: Lin C. H. , Friddle P. A. , Lu X. , Chen Haydn , Kim Young , Wu T. B.
Published: July 2002

Pb(Zr 0.5 Ti 0.5 )O 3 thin films 25 nm in thickness were grown on LaNiO3/Pt/Ti buffered Si substrates at 600 °C by metalorganic chemical vapor deposition. P–E studies showed a remanent polarization value of 8 μC/cm2 with a coercive field of 200 kV/cm. In polarization fatigue studies, these films only showed slight degradation in remanent polarization up to 4×108 cycles (±3 V oscillation) before breakdown. Moreover, the effect of space charge on the C–V behavior of these films was illustrated I–V characteristics of these films were also described.
The role of kinetics in the nucleation and void formation in copper films produced by chemical vapor deposition
Authors: Kröger R. , Eizenberg M. , Rabkin E. , Cong D. , Chen L.
Published: July 2002

The heterogeneous nucleation and subsequent growth of chemical vapor deposited copper using hexafluoroacetylacetonate-Cu(I)-trimethylvinylsilane on physical vapor deposited tantalum and chemical vapor deposited titanium nitride was studied by means of electron microscopy and atomic force microscopy. It was found that the nucleation densities are about two orders of magnitude higher on TiN than on Ta. This leads to an increased roughness of films deposited on Ta compared to those produced on TiN. Moreover, the Cu films on the Ta substrate show a large number of voids, whereas no such voids were observable in the Cu films deposited on top of TiN. A simplified model for the influence of gas-surface reaction and surface self-diffusion on the shape of the Cu grains was developed. This model, which is supported by the experimental data, shows that if the grain shape changes from spherical to nonspherical before coalescence with neighboring grains, voids occur. A critical grain size and nucleation density of about 150 nm and 5×1013 m−2, respectively, were calculated for the deposition conditions used in this work.
Electroless polyol deposition and magnetic properties of nanostructured Ni50Co50 films
Authors: Zhang J. , Chow G. M.
Published: July 2002

The deposition of NiCo films with a nominal composition of 50:50 by a single-step, electroless polyol method was investigated. The film thickness increased with increasing deposition time from 5 to 60 min. Measured film compositions showed Ni: Co stoichiometry for films deposited at 5 and 60 min: In the intermediate range of deposition time used, deviation from nominal precursor concentration to Co-rich compositions was observed as a result of change in deposition chemistry. Through-thickness concentration gradient conceivably existed for thicker films as diffusion for compositional homogenization would not be significant at the deposition temperature. The apparent film density increased as a result of coalescence of adjacent particles with increasing film thickness. The particles consisted of nanostructured crystallites. The crystallite size did not significantly change with increasing deposition time, indicating no grain growth had occurred. Nanoscale particles were also observed among the larger particles in the films. All the films showed in-plane magnetization anisotropy, whereas perpendicular magnetic saturation was only achieved for the film deposited at 60 min. The perpendicular coercivity was higher than in-plane coercivity. Possible explanations are suggested.
Structural properties of slightly off-stoichiometric homoepitaxial SrTixO3−δ thin films
Authors: Fuchs D. , Adam M. , Schweiss P. , Gerhold S. , Schuppler S. , Schneider R. , Obst B.
Published: July 2002

Ti-deficient SrTixO3−δ films, x<1, were grown on 〈100〉 oriented SrTiO3 single crystal substrates by radio frequency magnetron sputtering from stoichiometric targets. The Ti-deficiency was adjusted by the sputtering gas pressure. The Ti/Sr cation ratio, x, was determined by Rutherford backscattering and energy dispersive x-ray analysis in a scanning electron microscope. To obtain information on the Ti/O ratio, x-ray absorption spectroscopy was carried out as well. We investigated SrTixO3−δ films with x=0.98, 0.95, and 0.89. The epitaxial growth and lattice imperfections were characterized by x-ray diffraction, electron diffraction, and high resolution transmission electron microscopy. The films crystallized in a tetragonal structure with a maximum mosaic spread of about 0.1°. The c axis was oriented perpendicular to the substrate surface where the c-lattice parameter was increasing with decreasing x. For x>0.89, the Ti deficiency was primarily compensated by a change of the site occupation on the cation sublattices in combination with oxygen vacancies, i.e., the formation of SrTi and VO point defects, whereas for x<0.95 the intergrowth of homologs series of the Ruddlesden–Popper phases, Srn+1TinO3n+1, was observed. The dielectric properties of the films are briefly discussed in terms of (SrTiVO) defect complexes.
Schottky barrier height of MnSb(0001)/GaAs(111)B contacts: Influence of interface structure
Authors: Manago T. , Miyanishi S. , Akinaga H. , Van Roy W. , Roelfsema R. F. B. , Sato T. , Tamura E. , Yuasa S.
Published: July 2002

The Schottky barrier height (SBH) of MnSb(0001)/n-GaAs(111)B diodes was investigated in terms of current–voltage characteristics for three different GaAs surfaces, GaAs (19×19), GaAs (2×2), and sulfur passivated GaAs. We observed that the SBH and the ideality factor changed significantly depending on the GaAs surface structure prepared before the MnSb growth. The sulfur passivated sample was superior to the others in that it has a lower ideality factor and higher barrier. The SBH fell off linearly with increasing ideality factor n. The SBH of MnSb(0001)/n-GaAs(111)B was estimated to be 0.94 eV by extrapolating the linear relationship to n=1.
Characterization of radiation from a figure-8 undulator by a gas-scattering method
Authors: Tanaka Takashi , Oura Masaki , Ohashi Haruhiko , Goto Shunji , Suzuki Yoshio , Kitamura Hideo
Published: July 2002

The absolute flux, polarization properties, and spatial dependence of the intensity of figure-8-undulator radiation were measured by a gas-scattering method. Absolute-flux measurements were in good agreement with calculated values and the angular divergence measured at 6.6 keV was also close to expected value. Polarization measurements showed oscillations of the degree of linear polarization which varied with photon energy, indicating that both horizontal and vertical polarizations were included. In addition, the polarization plane was found to rotate when the vertical observation angle was changed. These polarization characteristics are not observed in other conventional-undulator radiation and thus unique to figure-8-undulator radiation.
Emission spectrum of a depleted neon–mercury positive column
Authors: Bakker L. P. , Kroesen G. M. W.
Published: July 2002

Under certain discharge conditions, radial cataphoresis causes significant mercury depletion in a low-pressure neon–mercury positive column. This depletion can result in the addition of neon radiation to the emission spectrum of the column. The addition of neon radiation can be used to change the color of fluorescent lamps. In order to investigate the radial cataphoresis process, we performed spatially resolved emission measurements. For the relevant spectral lines of mercury and neon, the emission coefficient is determined, along with the density of the upper state of the corresponding transition. Absorption measurements are performed to check the amount of self-absorption of the spectral lines. We present emission and density profiles for various discharge conditions. The obtained results can be understood using an approximate description of the radial cataphoresis process.
Optical characterization of hydrogenated silicon thin films using interference technique
Authors: Globus Tatiana , Ganguly Gautam , Roca i Cabarrocas Pere
Published: July 2002

This work introduces an application of an “interference spectroscopy technique” (IST) for determination of absorption coefficient and refractive index spectra of amorphous silicon (a-Si:H) and related thin film materials. The technique is based on computer analysis of measurements of optical transmission and specular reflection (T & R) of thin films (including the films on substrates) over a wide range of the incident photon energies (0.5–2.8 eV) using carefully controlled spectrometer conditions. IST is used to investigate the absorption spectrum in the sub-gap energy range (0.8–1.6 eV) of intrinsic and phosphorous-doped a-Si:H, “polymorphous-Si:H,” and microcrystalline silicon films. The enhanced sensitivity of the technique over conventional analysis of T & R data results from utilization of interference to obtain absorption coefficient values at the maxima of transmission. The factors limiting the accuracy of the calculated absorption coefficient are discussed in detail. Measurement on films of thickness ranging from 0.1 to 5 μm identifies that the sub-gap absorption in these films arises from the bulk rather than the surface. A set of samples prepared under widely different conditions that appear to have overlapping (α=20 cm−1) sub-gap absorption spectra measured using photo-thermal deflection spectroscopy (PDS), reveal significant differences (α=10 to 100 cm−1) using IST. Changes (factor of 2) in sub-gap absorption spectra due to light soaking are also clearly observable using IST.
Self-diffusion studies of N15 in amorphous Si3BC4.3N2 ceramics with ion implantation and secondary ion mass spectrometry
Authors: Schmidt H. , Borchardt G. , Weber S. , Scherrer S. , Baumann H. , Müller A. , Bill J.
Published: July 2002

Nitrogen self-diffusion studies in amorphous precursor-derived Si3BC4.3N2 ceramics were carried out, using ion implanted stable N15 isotopes as tracers and secondary ion mass spectrometry for depth profiling. The analysis of the diffusion profiles in the range of 1500–1700 °C did not show the typical Gaussian broadening of the implantation profiles. Instead, we observed the occurrence of a high concentration region where the width of the implantation profile is nearly unchanged due to implantation damage, and a low concentration region where diffusion occurs. The experimentally determined diffusivities obey an Arrhenius behavior with an activation enthalpy of about H=7 eV and a pre-exponential factor D0 in the order of 5 m2/s which indicates a diffusion mechanism via vacancy-like defects.
Atomic-layer expulsion in nanoindentations on an ionic single crystal
Authors: Terán Arce P. F. M. , Riera G. Andreu , Gorostiza P. , Sanz F.
Published: July 2002

Utilizing atomic-force microscopy, we have performed and characterized nanoindentations on a MgO(100) surface with depths varying between one monatomic layer and tens of nm. Our results show that plastic deformation is indicated by discrete events in the indentation curve which are associated with the number of atomic layers being expelled by the tip penetrating the surface. An estimation of the energy required to expel MgO ions from a monatomic deep cavity correlates well with our data. The critical shear strength was evaluated and lies in the same order of magnitude as the theoretical value.
Anomalous particle pinch in tokamaks
Authors: Miskane F. , Garbet X. , Dezairi A. , Saifaoui D.
Published: July 2002

The diffusion coefficient in phase space usually varies with the particle energy. A consequence is the dependence of the fluid particle flux on the temperature gradient. If the diffusion coefficient in phase space decreases with the energy in the bulk of the thermal distribution function, the particle thermodiffusion coefficient which links the particle flux to the temperature gradient is negative. This is a possible explanation for the inward particle pinch that is observed in tokamaks. A quasilinear theory shows that such a thermodiffusion is generic for a tokamak electrostatic turbulence at low frequency. This effect adds to the particle flux associated with the radial gradient of magnetic field. This behavior is illustrated with a perturbed electric potential, for which the trajectories of charged particle guiding centers are calculated. The diffusion coefficient of particles is computed and compared to the quasilinear theory, which predicts a divergence at low velocity. It is shown that at low velocity, the actual diffusion coefficient increases, but remains lower than the quasilinear value. Nevertheless, this differential diffusion between cold and fast particles leads to an inward flux of particles.
Transmission electron microscopy analysis of crystallographic transition from fcc to fct on PtMn spin valves
Authors: Maesaka Akihiro , Ishii Satoru , Okabe Akihiiko
Published: July 2002

The structural changes during the crystallographic transition from fcc to fct of antiferromagnetic PtMn layers with postdeposition annealing, in spin-valve multilayers constructed by Ta/PtMn/CoFe/Cu/CoFe/NiFe/Ta/Si, were investigated using transmission electron microscopy and energy dispersive x-ray spectroscopy. We determined that the crystallographic orientation of the PtMn layers after postdeposition annealing depends on the initial size of the columnar grains in as-deposited spin valves. For spin valves with an initial grain size of 10 nm, the fcc→fct transition of PtMn layers was completed with the crystallographic orientation on the (111) texture. In contrast, spin valves with an initial grain size of 50 nm brought about a crystallographic reconstruction of the PtMn layer due to the tensile stress from neighboring grains during the fcc→fct transition. This results in a transformation from a fcc(111) to a fct(101) texture and a recombination of grains to form giant grains of 300–500 nm in the PtMn layer. This dynamic reconstruction promotes the interlayer diffusion of constituent atoms in spin-valve multilayers, leading to an increase in the interlayer coupling field between the pinned and free layers (Hin) and a promotion of thermal degradation of magnetoresistance (ΔR).
Two-dimensional simulation of an oxy-acetylene torch diamond reactor with a detailed gas-phase and surface mechanism
Authors: Okkerse M. , Kleijn C. R. , van den Akker H. E. A. , de Croon M. H. J. M. , Marin G. B.
Published: July 2002

A two-dimensional model is presented for the hydrodynamics and chemistry of an oxy-acetylene torch reactor for chemical vapor deposition of diamond, and it is validated against spectroscopy and growth rate data from the literature. The model combines the laminar equations for flow, heat, and mass transfer with combustion and deposition chemistries, and includes multicomponent diffusion and thermodiffusion. A two-step solution approach is used. In the first step, a lumped chemistry model is used to calculate the flame shape, temperatures and hydrodynamics. In the second step, a detailed, 27 species / 119 elementary reactions gas phase chemistry model and a 41 species / 67 elementary reactions surface chemistry model are used to calculate radicals and intermediates concentrations in the gas phase and at the surface, as well as growth rates. Important experimental trends are predicted correctly, but there are some discrepancies. The main problem lies in the use of the Miller–Melius hydrocarbon combustion mechanism for rich oxy-acetylene flames. [J. A. Miller and C. F. Melius, Combustion and Flame 91, 21 (1992)]. Despite this problem, some aspects of the diamond growth process are clarified. It is demonstrated that gas-phase diffusion limitations play a minor role in the diamond growth process, which is determined by surface kinetics. Except for atomic hydrogen, gas phase diffusion is also of minor importance for the transport of species in and behind the flame front. Finally, it is shown that penetration of nitrogen from the ambient air into the flame cannot explain the observed changes at the center of the diamond films as reported in the literature.
Electrical discharge properties in dielectric-loaded diodes
Authors: Uhm Han S.
Published: July 2002

Properties of the electrical discharge in a dielectric-loaded diode are investigated. An analytical expression of the saturated voltage φs in the gap between the dielectric materials is obtained in terms of parameter ξ related to the rise time of the applied voltage. It is shown that the saturated gap voltage approaches unity very closely, initiating an electrical breakdown, when parameter ξ increases to a large value (typically 1000). This corresponds to the usual high-pressure discharge. On the other hand, if abundant triggering electrons are provided by radiation or by any other means, the saturated gap voltage is considerably less than unity and therefore electrical breakdown in the gap never occurs.
Some aspects of shock-like nonlinear acoustic waves in magnetized dusty plasma
Authors: Das G. C. , Sarma J. , Roychoudhury Rajkumar
Published: July 2002

A nonlinear wave equation is derived in a magnetized plasma, comprising dust charged grains, to study the existence and propagation of solitary wave. The Sagdeev pseudopotential technique is applied to obtain the nonlinear wave equation in the dusty plasma. In contrast to the usual technique, the tanh-method, or to say the hyperbolic-method, to the nonlinear wave equation is used to derive the formation of soliton and its shock-like nature, as well. The nonlinear waves in dusty plasma, which could be of a greater interest in astrophysical plasmas, are also presented here to reflect significant advanced information on solitons.
Terahertz quantum cascade structures: Intra- versus interwell transition
Authors: Ulrich J. , Zobl R. , Schrenk W. , Strasser G. , Unterrainer K. , Gornik E.
Published: July 2002

Terahertz quantum cascade structures (emission around 20 meV) based on intra- and interwell transitions have been compared in magnetotransport and intersubband-electroluminescence experiments. The interwell transition exhibits a Stark shift of up to 6 meV. We also observe a smaller energy shift of the intrawell transition which is attributed to the tunnel splitting, when the initial and final subbands mix with those of the injectors. The electric field induced narrowing of the emission line and our band structure calculations support this interpretation. The quantum efficiency of the electroluminescence is of similar magnitude for the two transitions, despite their large difference in the spatial subband overlap.
Influence of electric field on interwell tunneling rate in quasi two dimensional organic quantum wells
Authors: Donovan K. J. , Elliott J. E. , Jeong I. S. , Scott K. , Wilson E. G.
Published: July 2002

The tunneling rate of photocreated charge carriers between layers in Langmuir–Blodgett multilayer structures is measured indirectly using the novel technique of bimolecular recombination quenching. The tunneling rate is demonstrated to be dependent upon the applied electrostatic potential difference between the layers. This dependence is explored in light of the Marcus theory of charge transfer. That theory was developed to describe redox reactions where the driving force is supplied by a chemical potential difference between two chemically different parts of a more complex system. In the current work the electrostatic potential replaces the chemical potential as the driving potential. The field dependence of the exciton dissociation probability is also determined.