bluuebunny/crossref_metadata_2025 huggingface dataset:
Ab initio potential energy surfaces for excited states of the NO2+ molecular ion and for the reaction of N+ with O2
Authors: Hirst David M.
Published: July 2002

Multi-reference configuration interaction calculations are presented for excited states of the NO2+ molecular ion. Bending potential energy curves are calculated for a number of low-lying states and dissociation to [NO+O]+ is considered for linear geometries and for a bond angle of 120°. The very fast dissociation of the b̃ 3A2 state can be understood in terms of a very shallow quasi-bound minimum on the potential energy surface whereas the potential well for the ã 3B2 state is considerably deeper. Potential energy curves relevant to the reaction N+(3P)+O2(X 3Σg−) are calculated. It is suggested that the reaction proceeds via a nonadiabatic transition in the entrance channel from the 2 1Σ+ surface to the 1 1Σ+ surface which correlates adiabatically with O(1D)+NO+(X 1Σ+). The observation of O(3P)+NO+(X 1Σ+) as a minor channel is interpreted in terms of a nonadiabatic transition, mediated by spin-orbit coupling, from the 1 1Σ+ surface to the 1 3Π surface in the exit channel.
On the anisotropic intermolecular potential of biaxial apolar solutes in nematic solvents: Monte Carlo predictions and experimental data
Authors: Celebre Giorgio
Published: July 2002

Recently, a new formulation has been proposed about a strictly short-range anisotropic potential acting on biaxial apolar particles dissolved in a uniaxial medium [Chem. Phys. Lett. 342, 375 (2001)], where the solute-solvent interactions are treated at a molecular level and the solute order parameters are calculated by making use of the Monte Carlo–Metropolis sampling scheme. In the present paper the cited model has been used for the study of 1,4-difluorobenzene, 1,4-dichlorobenzene, and 1,4-dibromobenzene molecules and the simulated order parameters have been compared with the H-NMR1 experimental data for the solutes in the nematic solvents ZLI1132 (a Merck commercial eutectic mixture of alkylcyclohexylcyanobenzenes and alkylcyclohexylcyanobiphenyl), EBBA [the N-(4-ethoxybenzylidene)-4′-n-butylaniline], and in the zero average electric field gradient nematic mixture 55 wt% ZLI1132+EBBA (the so-called “magic” mixture). The orientations predicted by the model match almost perfectly the experimental Saupe matrices of the molecules dissolved in the “magic” mixture: implications of this result are discussed in terms of nature of the interactions in the different nematic phases and reliability and effectiveness of the suggested intermolecular potential.
Intermolecular forces and conformational change upon crystallization: The case of phosphorobenzopyrane derivatives
Authors: Rybarczyk-Pirek Agnieszka J. , Zgierski Marek Z.
Published: July 2002

The possible conformations of two phosphorobenzopyrane derivatives (I and II) are investigated with the help of ab initio and density functional theory methods. Special attention is paid to the conformation around the phosphine group. It is demonstrated that while I possesses the same conformations in the gas and crystal phases, II assumes a conformation in a crystal that is not the lowest energy conformer in the isolated molecule. The effect is explained by the difference in the crystal packing of benzopyrane rings for the two compounds. The higher energy conformer of II is favored in a crystal since it avoids creation of strong repulsive interaction between neighboring molecules due to short interatomic distances.
Epitaxial growth of ferroelectric SrBi2Ta2O9 thin films of mixed (100) and (116) orientation on SrLaGaO4(110)
Authors: Lee Ho Nyung , Zakharov Dmitri N. , Senz Stephan , Pignolet Alain , Hesse Dietrich
Published: July 2002

Epitaxial ferroelectric SrBi2Ta2O9 (SBT) thin films with a well-defined crystallographic orientation state consisting of a mix of (100) and (116) orientations have been grown on (110)-oriented SrLaGaO4 (SLG) substrates by pulsed laser deposition. X-ray pole figure analyses revealed the presence of two epitaxial orientation relationships, viz. SBT(100)‖SLG(110); SBT[001]‖SLG[001] and SBT(116)‖SLG(110); SBT[1̄10]‖SLG[001]. By calculating the integrated intensity of certain x-ray diffraction peaks, it was established that the crystallinity and the in-plane orientation of the (100) and (116) orientation are maximized at a substrate temperature of 775 and 788 °C, respectively, and that the volume fraction of the (100) orientation at about 770 °C reached about 60%.
Effect of direct current bias field and alternating current excitation field on vibration amplitudes and resonance frequencies of a magnetostrictively actuated bimorph microresonator
Authors: Bourouina Tarik , Garnier Amalia , Fujita Hiroyuki
Published: July 2002

The dynamic behavior of magnetomechanical bimorph microresonators is analyzed. These resonators are composed of a sputter-deposited TbDy(CoFe)2 thin film on silicon cantilevers. This film has giant magnetostrictive properties. It is made anisotropic by postdeposition annealing under magnetic field. We prepared two samples with different orientations of the easy axis: along the cantilever width axis and with an orientation of 22°, respectively. Both bending and torsion modes of vibration can be excited by proper combination of external magnetic field and the direction of the easy axis. Vibration amplitudes and resonance frequencies are studied in those modes. Several experimental conditions are investigated. The dependence of vibration amplitudes on a magnetic dc bias field applied either along the hard axis or along the easy axis is studied. The bending/torsion vibration amplitude ratio was shown to be tunable over a very wide range, offering interesting practical applications. In addition, the dependence of the resonance frequencies on both dc and ac fields applied along the hard axis is studied. For both vibration amplitudes and resonance frequencies, the experimental data are compared with a theoretical model and good agreement is found. The magnetomechanical resonator may be considered as a building block for more complex microelectromechanical systems using magnetostrictive actuation and as an alternative to more classical solutions based on electrostatic or electrothermal actuation.
Reversible control of silver nanoparticle generation and dissolution in soda-lime silicate glass through x-ray irradiation and heat treatment
Authors: Chen Sihai , Akai Tomoko , Kadono Kohei , Yazawa Tetsuo
Published: July 2002

Reversible generation and dissolution of 3 nm silver particles in a glass containing only silver ions are repeatedly achieved through a combination of x-ray radiation and heat treatment. X-ray irradiation and 400 °C heat treatment induce a yellow color, whereas heating at 500 °C can change the glass to colorless again. X-ray irradiation produces large amounts of silver atoms and defects, mainly nonbridging oxygen hole centers (NBOs), in glass. Heating at 400 °C accelerates the aggregation of silver atoms into forming silver particles. The NBOs can continuously oxidize the silver atoms of particles into silver ions at higher temperature, leading to dissolution of the silver particles.
Layered planar capacitor based on BaxSr1−xTiO3 with variable parameter x
Authors: Vendik O. G. , Zubko S. P. , Karmanenko S. F. , Nikol’ski M. A. , Isakov N. N. , Serenkov I. T. , Sakharov V. I.
Published: July 2002

The temperature dependence of a capacitance of a planar capacitor based on Ba1−xSr1−xTiO3 (BSTO) films was investigated and some anomalies were observed. It was experimentally found that the barium concentration decreased across the film thickness in the direction from film/substrate interface to the film surface. The barium concentration, x, of BSTO films grown on sapphire (r-cut) substrates changed from x=0.52 on the film/substrate interface to x=0.3 on the film surface. The investigated films should be considered as a multiphase composition with different phase transition temperatures. The phenomenological model of dielectric response of the film structure has been suggested, which served as a basis for introducing an effective dielectric permittivity of the layered film.
Current and voltage noise in WO3 nanoparticle films
Authors: Hoel A. , Vandamme L. K. J. , Kish L. B. , Olsson E.
Published: July 2002

Current and voltage noise measurements have been carried out on nanoparticle WO3 films. The fluctuation dissipation theorem holds, which indicates that the observed noise is an equilibrium phenomenon. Results on the thinnest films show that noise measurements can be used for quality assessment of nanocrystalline insulating films.
Molecular beam epitaxially grown n type Hg0.80Cd0.20Te(112)B using iodine
Authors: Schmidt J. , Ortner K. , Jensen J. E. , Becker C. R.
Published: July 2002

The molecular beam epitaxial growth and n type doping of Hg0.80Cd0.20Te(112)B have been investigated. The surface morphology has been determined via atomic force microscopy and is appreciably different from that of the (001) orientation; long ridges and trenches are observed whose height difference is considerably less than that of the nearly elliptically shaped mounds found in (001) alloys. The mobility has been optimized with regard to growth parameters such as the Hg/Te flux ratio resulting in a maximum value of 4.0±0.6×105 cm2/(V s) for an undoped sample at low temperatures. n type Hg0.80Cd0.20Te(112)B has been grown using iodine in the form of CdI2 as a dopant. The maximum electron concentration due to iodine doping has been determined after an Hg vacancy anneal to be 4.2±0.7×1018 cm−3. Typical mobilities at low temperatures are 4.0×104 and 1.0×105 cm2/(V s) for electron concentration levels of 1.0×1017 and 5.0×1016 cm−3, respectively. By means of secondary ion mass spectroscopy the iodine concentration has been determined and hence the degree of activation. The electron concentration is proportional to that of iodine with a 100% electrical activation for concentrations up to ≈1018 cm−3. At higher concentrations compensation is observed.
Physical phenomena in a coplanar macroscopic plasma display cell I. Infrared and visible emission
Authors: Ganter R. , Ouyang J. , Callegari Th. , Boeuf J. P.
Published: July 2002

The space and time variations of the light emission in a macroscopic plasma display panel (PDP) discharge cell have been studied with an image-intensified charge-coupled device. The discharge cell is similar to a real PDP cell with a coplanar electrode configuration. The cell dimensions are on the order of 100 times larger than those of a real PDP cell and the operating pressure is about 100 times smaller. Different xenon–neon gas mixtures have been investigated. Optical filters have been used to measure infrared emission from xenon excited states 823.1 and 828.0 nm, and visible emission from neon at 640.2 nm. The measurements show that the neon visible emission occurs only above the cathode while xenon infrared emission occurs above both cathode and anode. Standing striations can be observed above the anode. The strong xenon emission above anode indicates that this region is more efficient than the cathode region in terms of ultraviolet production. The measurements are in excellent qualitative agreement with similar measurements performed in real PDP cells. However the velocity of plasma spreading above the cathode in the macrocell is significantly larger than in a real PDP cell. The interpretation of this discrepancy is given in a companion paper (part II) [J. Appl. Phys. 91, 1000 (2002)] where the experimental results are compared with results from a fluid model of the plasma.
Stimulated Brillouin scattering of a short pulse laser in a self-induced plasma channel
Authors: Kar Satyabrata , Tripathi V. K. , Sawhney B. K.
Published: July 2002

An intense short pulse laser with Gaussian radial profile of intensity propagating through homogeneous plasma depletes electrons from the axial region due to the radial ponderomotive force, leaving ions immobile. The resulting electron density profile and relativistic mass nonlinearity aid the laser to propagate self-guided, overcoming diffraction divergence with minimum spot size, few times of c/ωp, where c is the velocity of light in vacuum and ωp is the plasma frequency at equilibrium. The laser undergoes stimulated Brillouin backscattering in the self-induced plasma channel. As the ratio of the oscillatory electron velocity to the velocity of light, |v0|/c exceeds 0.1, the ion acoustic wave goes over to a reactive quasimode with large growth rate, comparable to ion plasma frequency. The nonlocal effects due to high intensity laser play a major role in the growth rate of the ion acoustic reactive quasimode. In a plasma of 5 KeV electron temperature, the largest value of the growth rate occurs at a plasma density n00≅0.6ncr, where ncr is the critical density.
Results and interpretation of high frequency experiments at 28 GHz in ECR ion sources, future prospects
Authors: Hitz D. , Girard A. , Melin G. , Gammino S. , Ciavola G. , Celona L.
Published: July 2002

For the needs of future heavy ion accelerators, electron cyclotron resonance ion sources (ECRISs) should be able to deliver higher intensities and higher charge states. The 1e mA level intensity has already been reached by room temperature ECRIS for medium charge states of light elements (O6+, Ar8+). However, such level of intensity for heavy elements (like Pb27+ for CERN/LHC and GSI) requires more powerful ECRIS with higher electron densities (up to 1013 cm−3). On the other hand, an optimized magnetic configuration system has to be used in order to obtain the suitable compromise between the electron confinement and the high flux ion losses. Before the design of the future “high intensity ECRIS,” experiments have been performed with the superconducting SERSE source both at 18 and 28 GHz. After an overview of major results recently obtained, some scaling laws will be presented. Our results show that much larger intensities and charges can be reached with ECRIS. Then, we will show how the next ECRIS generation will look like, based on the scaling laws derived in the above-mentioned experiments.
Radiation of relativistic electrons in a dusty plasma crystal: A novel approach to free electron lasers
Authors: Selemir D. V. , Dubinov A. E.
Published: July 2002

An electrostatic undulator based on a dusty plasma crystal with a period of ∼10 μm is proposed. The linear chain of dust grains is taken as an undulator model. The main mechanisms of radiation are discussed and analytic expressions for the radiation field are shown. The possible application in astrophysics is discussed.
Magnetohydrodynamic instability of a high magnetic shear layer with a finite curvature radius
Authors: Arshukova Irina L. , Erkaev Nikolai V. , Biernat Helfried K.
Published: July 2002

This article deals with the magnetohydrodynamic instability of a thin layer which is characterized by a high magnetic shear, a constant curvature radius, and a plasma velocity shear. The magnetic field and the plasma parameters are considered to be piecewise constant inside the layer and in the regions adjacent to the layer. The plasma parameters and the magnetic field are assumed to obey the ideal incompressible magnetohydrodynamics. Fourier analysis is used to calculate small perturbations of the magnetic field and plasma parameters near the layer in linear approximation. The instability growth rate is obtained as a function of different parameters: the magnetic shear angle, the velocity direction angle, the tangential plasma velocity, the layer thickness, the wave number, and the curvature radius. The resulting instability is a mixture of interchange and Kelvin–Helmholtz instabilities on a surface with nonzero curvature. For a fixed velocity shear and curvature radius, the instability growth has a maximum in the case of antiparallel magnetic fields (maximal magnetic shear). This growth rate is an increasing function of the tangential velocity component perpendicular to the magnetic field, and a decreasing function of the velocity component along the magnetic field. The instability is stronger for smaller curvature radius.
Toroidal ion temperature gradient driven instability in plasma with shear flow
Authors: Mikhailenko Vladimir S. , Weiland Jan
Published: July 2002

The temporal evolution of the toroidal ion temperature gradient driven (ηi) instability in plasma with homogeneous shear flow is studied directly without the use of spectral expansions in time. The regimes of weak flow shear, which corresponds to the period of the low-to-high (L–H) transition and the regime of strong flow shear which corresponds to the stage of the developed transport barriers, were studied separately. In the case of weak flow shear stabilization of the toroidal ηi instability is more a mathematical artifact. In contrast, the region of the strong flow shear is stable against the development of the toroidal ηi instability and is impenetrable for the anomalous transport conditioned by the toroidal ηi instability developed in the inner part of the plasma.
Electronic structure and optical properties of SrCu2O2
Authors: Ohta Hiromichi , Orita Masahiro , Hirano Masahiro , Yagi Iwao , Ueda Kazushige , Hosono Hideo
Published: July 2002

The electronic structure of SrCu2O2, a wide gap (∼3.3 eV) p-type oxide semiconductor, was examined by photoelectron and optical spectroscopy. The spectroscopy results were compared with the energy band structure calculated by the local density approximation method to clarify the origins of p-type conductivity in this material. Despite the significant difference in band gap energy, the basic electronic structure around the band gap region was found to be quite similar to that of Cu2O. Thus, the 3d−4sp orbital of Cu+ ion is hybridized with the 2p orbital of ligand O2− ions due to the covalency of Cu–O bonds, to form states near the valence band maximum; the conduction band minimum is predominantly composed of the hybridized orbital of Cu 4sp and O 2p, forming the direct band gap at Γ point. A sharp absorption band observed near the fundamental absorption edge is likely attributable to an exciton. Although the corresponding exciton emission was not observed near the absorption edge, a blue-green emission band (Stokes shift of ∼1 eV) was observed at ∼2.47 eV. The emission is presumably attributable to intra-atomic transitions of Cu+, partially allowed by p-orbital mixing into s and d orbitals.
Simulating vapor–liquid nucleation of n-alkanes
Authors: Chen Bin , Siepmann J. Ilja , Oh Kwang J. , Klein Michael L.
Published: July 2002

A combination of the aggregation-volume-bias Monte Carlo algorithm, the configurational-bias Monte Carlo algorithm, and the umbrella sampling technique was applied to investigate homogeneous vapor–liquid nucleation in ethane, n-butane, and n-heptane. The simple transferable potentials for phase equilibria-united atom (TraPPE-UA) force field was used in this investigation. It was found that for the n-heptane case, the TraPPE-UA force field predicted a nucleation rate that is about three to four orders of magnitude higher than that measured by an upward thermal diffusion cloud chamber experiment. Comparison of the simulation results to the classical nucleation theory (CNT) shows that CNT consistently overestimates the barrier heights for all chain lengths investigated. The offset on the barrier heights was found nearly independent of the supersaturation for both ethane and n-butane, similar to a Lennard-Jones system previously studied. This also directly leads to a good agreement on the cluster sizes between the simulation and the CNT calculated from the nucleation theorem. For n-heptane, however, the offset was found to depend on the supersaturation. It appears that CNT predicts a slightly weaker dependence of the nucleation rate on supersaturation, which agrees with both density functional calculations and the experiments. Structural analysis demonstrates that the orientational order near the surface differs significantly between the critical nucleus and the bulk planar liquid–vapor interface for n-heptane systems, whereas the density in the interior of the critical nucleus is in good agreement with the bulk liquid density. The different surface ordering offers a microscopic explanation for the differences observed for n-heptane between the CNT on one side and experimental observations and simulations on the other side.
Erase band width evaluation by phase coherency
Authors: Li Shaoping , Zhu W. , Wang Lei , Li Jin , Liu Feng , Brown Daniel , Michel W. , Palmer D.
Published: July 2002

As the track density increases, the properties of the erase band and track edge become more and more important. In this study, we report on a pertinent erase band measurement method, based on a magnetic force microscope, to characterize the erase bands and track edge overwrite properties resulting from the write-head fringe field in a precise manner. The erase bands of written tracks were measured by using the proposed technique, the so-called phase coherency method, for both longitudinal recording tracks and perpendicular tracks in comparison to the amplitude technique.
Amorphous layer formation at the TaC/Cu interface in the Si/TaC/Cu metallization system
Authors: Laurila Tomi , Zeng Kejun , Kivilahti Jorma K. , Molarius Jyrki , Suni Ilkka
Published: July 2002

An amorphous Ta(O,C)x layer was found to form at the TaC/Cu interface in the Si/TaC/Cu metallization system. The formation of the layer was induced by oxygen trapped in the as-deposited films, since on the basis of thermodynamic evaluation of the ternary Ta–C–O system, the dissociation of the TaC layer and the formation of the Ta2O5 and graphite can be expected to occur during subsequent annealings in this case. However, as observed experimentally, the formation of the amorphous Ta(O,C)x preceded the formation of the stable tantalum oxide.
Training effect in ferro (F)/antiferromagnetic (AF) exchange coupled systems: Dependence on AF thickness
Authors: Zhang K. , Zhao T. , Fujiwara H.
Published: July 2002

Systematic investigation on the training effect in polycrystalline ferro (F)/antiferromagnetic (AF) exchange coupled systems using sputtered NiFe/IrMn bilayers has revealed that the type of the training effect changes from one type (Type I) to another (Type II) with increasing the AF layer thickness. In Type I the hysteresis loop in the pinned direction shrinks from both sides with the cycle of the applied field, while in Type II it shifts toward the positive pinned direction accompanied by loop shrinkage. This phenomenon can be understood as the stabilization of both AF grains contributing to the enhancement of coercivity and to the exchange biasing with cycles of applied field. This conclusion is based on our recently proposed extended Fulcomer and Charap’s model in which direct positive and negative exchange coupling between the AF grains is incorporated.
A theory of ferroelectric hysteresis with a superimposed stress
Authors: Li W. F. , Weng G. J.
Published: July 2002

Based on the mechanism of domain switch, a micromechanics-based model is developed to calculate the hysteresis loop of ferroelectric ceramics under combined electromechanical loading. The development makes use of an extension of the Eshelby-type elastic inclusion problem to a heterogeneous electromechanically coupled ferroelectric medium with distribution of eigenstrain and eigenpolarization generated by domain switch. The ferroelectric ceramic at a generic state is considered to consist of the parent domain and the switched domains whose volume fraction fp, continues to evolve under an increasing electromechanical load. At a given level of applied stress and electric field, the volume fraction of the new domain is determined from a kinetic equation that is derived from consideration of the thermodynamic driving force caused by the reduction of Gibbs free energy and the resistance force associated with the domain wall movement. The theory developed is used first to simulate the hysteresis behavior of a PZT-51 without any superimposed stress, and then with the derived material constants, to independently predict the influence of a superimposed compression. It is found that, consistent with experimental observations, the hysteresis loops flatten out under the axial compression, but that the loops elongate under a transverse compression. Both the remanent polarization and coercive field also tend to decrease with the compressive force regardless of how the compression is applied.
Control of hole carrier density of polycrystalline Cu2O thin films by Si doping
Authors: Ishizuka S. , Kato S. , Okamoto Y. , Akimoto K.
Published: July 2002

The effects on the electrical properties of Si doping into Cu2O thin films deposited by reactive sputtering were studied. The hole density increased from 1×1015 to 1×1017 cm−3 with increasing Si content and the minimum resistivity obtained was 12 Ω cm. It was suggested that the electrically active acceptor with an activation energy of 0.19 eV was generated by Si doping. Infrared absorption measurements indicated the formation of silicate in Si-doped Cu2O. The mechanism for Si acting as an acceptor in Cu2O is discussed and modeled based on the silicate formation in Cu2O.
Effect of low-energy N2+ ion beam bombardment on silicate glass thin films studied by x-ray photoelectron spectroscopy
Authors: Garcı́a M. , Montero I. , Ripalda J. M. , Galán L.
Published: July 2002

Thin film borophosphosilicate glasses, SiOxByPz, have been obtained by chemical vapor deposition on (100) Si substrates. The interaction of low-energy N2+ ions with these films has been studied by x-ray photoelectron spectroscopy (XPS) and infrared spectroscopy. Determination of the chemical composition of the borophosphosilicate glasses was done by qualitative and quantitative XPS analysis. The differences in composition between the films before and after the bombardment have been explained in terms of the relative thermodynamic stability of the compounds present in the sample.
Effects of C content on the formation and magnetic properties of Nd2Fe14(BC)/α-Fe nanocomposite magnets
Authors: Wang Z. C. , Davies H. A. , Zhou S. Z.
Published: July 2002

The phase evolution, microstructure, and magnetic properties of Nd8Fe86B6−xCx (x=0, 2, 4, 5, 6) melt-spun ribbons were systematically studied as a function of C content. It was found that the addition of C decreases the glass-forming tendency of the as-spun ribbons significantly. A uniform nanoscale exchange coupled Nd2Fe14(BC)/α-Fe microstructure with an average grain size of 20–25 nm can be developed in the directly quenched ribbons with C contents up to 4 at. %. Further increase of C content to x=5 leads to, in the optimally quenched ribbons, the presence of an undesirable Nd2Fe17Cx phase in addition to the 2:14:1 and α-Fe phases, whereas the alloy ribbon containing 6 at. % C consists almost entirely of the soft magnetic Nd2Fe17Cx and α-Fe phases. Subsequent annealing induces a transformation of the 2:17:Cx phase to the 2:14:1 phase +α-Fe in the ribbons with x=5 and 6, resulting in the formation of a composite 2:14:1/α-Fe structure having relatively large crystallite sizes. Magnetic measurements revealed that, for the optimally processed samples, replacement of up to 4 at. % of B by C significantly increases the coercivity Hci, with only slight reduction in remanence Jr; an optimum coercivity of 542 kA/m was obtained in the Nd8Fe86B2C4 ribbon compared with 430 kA/m for the Nd8Fe86B6 ribbon. Excessive substitution of C (x>4) causes a drastic deterioration of both Hci and Jr due to the microstructural coarsening. Moreover, the Curie temperature of the 2:14:1 phase in the samples decreases progressively with increasing C content from 312 °C for x=0 to 270 °C for x=6.
Influence of Sb, Bi, Tl, and B on the incorporation of N in GaAs
Authors: Dimroth F. , Howard A. , Shurtleff J. K. , Stringfellow G. B.
Published: July 2002

GaAs:N is an interesting material for many devices due to its unique compositional variation of band gap. Small amounts of N lead to a strong decrease in band gap energy as well as lattice constant. The further addition of In or Sb leads to quaternary alloys with band gap energies below 1.4 eV lattice matched to GaAs. One drawback of these alloys is the low solubility of N in GaAs. Some success has been obtained using low growth temperatures and V/III ratios during organometallic vapor phase epitaxy to kinetically limit phase separation. This article describes mechanisms for N incorporation into the GaAs crystal during growth and shows how surfactants like Sb, Bi, and Tl, as well as B, affect N incorporation. A decrease of the N content in GaAs was found for Sb, Bi, and Tl, which can be explained using a simple Langmuir model with competitive adsorption. The surface morphology of the epitaxial layers and the influence of surfactants was analyzed using atomic force microscopy.
Magnetic force microscopy observation of sensitized Inconel 600
Authors: Takaya Shigeru , Suzuki Takayuki , Uchimoto Tetsuya , Miya Kenzo
Published: July 2002

An application of magnetic force microscopes (MFM) is proposed to the characterization of sensitized Inconel 600 alloy with submicron order accuracy. The magnetic microstructures of Inconel 600 sensitized at 620 °C for various heat treatment periods were observed by means of a MFM. MFM images show semicontinuous morphologies of the magnetizations near and at grain boundaries. The absolute value of the MFM signal, the phase shift between the oscillation of the cantilever and the piezoelectric actuator, at the grain boundary increases at first and then decreases with increasing heat treatment time, while the full width at half maximum about the MFM signals across the grain boundary increases continuously. These results coincide with the fact that the chromium content decreases at first due to production of the chromium carbide and then increase slightly due to the change in the activities of carbon and chromium near the grain boundaries. This method is useful because MFMs enable very high spatial resolution observations with minimal sample preparation and relatively easy operations.
Temperature dependence of switching fields of single 3 nm cobalt nanoparticles
Authors: Thirion C. , Wernsdorfer W. , Jamet M. , Dupuis V. , Mélinon P. , Pérez A. , Mailly D.
Published: July 2002

We study the temperature dependence of the magnetization switching fields of single nanoparticles with an improved micro-superconducting quantum interference device (SQUID) technique, that allows us to perform measurements independently of the critical superconducting temperature of the SQUID. A simplified Néel–Brown model that describes the magnetization reversal by thermal activation over a single anisotropy barrier is compared to measurements on a 3 nm cobalt nanoparticle embedded in a niobium matrix.
Many-spin calculation of tunneling splittings in Mn12 magnetic molecules
Authors: Raedt H. A. De , Hams A. H. , Dobrovitski V. V. , Al-Saqer M. , Katsnelson M. I. , Harmon B. N.
Published: July 2002

We calculate the tunneling splittings in a Mn12 magnetic molecule taking into account its internal many-spin structure. We discuss the precision and reliability of these calculations and show that restricting the basis (limiting the number of excitations taken into account) may lead to significant error (orders of magnitude) in the resulting tunneling splittings for the lowest energy levels, so that an intuitive picture of different decoupled energy scales does not hold in this case.
Structural properties and stability of Zr and Ti germanosilicides formed by rapid thermal annealing
Authors: Aubry-Fortuna V. , Chaix-Pluchery O. , Fortuna F. , Hernandez C. , Campidelli Y. , Bensahel D.
Published: July 2002

Because of their good ohmic and rectifying properties, silicides are routinely used in Si technology. This approach has been recently extended to the novel devices produced using Si1−xGex alloys. Here, we study the Zr and Ti germanosilicides produced in the low thermal budget contact formation during Si/Si1−xGex heterodevice processing. Phase formation was monitored by combining a range of spectrometries with electron microscopy and x-ray diffraction techniques, while sheet resistance measurements allowed correlation of phase formation with film conductance. After completion of the reaction, the final crystalline phase was either C49–Zr(Si1−yGey)2 in the entire Ge composition (x) range, or C54–Ti(Si1−yGey)2 in the Ge composition range 0–0.47. In the Zr–Si–Ge system, the C49–Zr(Si1−yGey)2 formation temperature (Tf) decreases as x increases, and films formed at this temperature are continuous. Excess heating (above Tf) produces islanded films with embedded grains. A most significant feature of the results was that no Ge segregation was detected at any annealing temperature and that the Ge content in the C49 phase (y) remained equal to x for all x. This is in contrast to results on the C54–Ti(Si1−yGey)2 films, which were discontinuous when x>0.10, and in which Ge segregation occurred in the form of Ge-rich SiGe decorations separating the germanosilicide grains. The Ge content in the final C54 phase (y) was always lower than the value of x in the initial SiGe alloy, and the measured sheet resistance of the corresponding contacts was large. Our results indicate that the alloys formed between Zr and Si1−xGex are good candidates as stable contacts on Si1−xGex, and hence that Zr should be preferred for contacting in Ge-rich SiGe-based applications.
Enhanced tunneling in GaN/InGaN multi-quantum-well heterojunction diodes after short-term injection annealing
Authors: Polyakov A. Y. , Smirnov N. B. , Govorkov A. V. , Kim J. , Luo B. , Mehandru R. , Ren F. , Lee K. P. , Pearton S. J. , Osinsky A. V. , Norris P. E.
Published: July 2002

Multi-quantum-well GaN/InGaN heterojunction diodes prepared by metalorganic chemical vapor deposition on sapphire showed effects of strong tunneling in their I–V characteristics. The space charge region was shown to be located in the GaN/InGaN superlattice (SL). The injection of moderately high forward currents through the structure for several hours enhanced the overall tunneling through the structure and facilitated faster tunneling between the layers in the GaN/InGaN SL. These results may have relevance to the aging characteristics of light-emitting diodes under bias.
Power spectrum of passive scalars in two dimensional chaotic flows
Authors: Yuan Guo-Cheng , Nam Keeyeol , Antonsen Thomas M. , Ott Edward , Guzdar Parvez N.
Published: July 2002

In this paper the power spectrum of passive scalars transported in two dimensional chaotic fluid flows is studied theoretically. Using a wave-packet method introduced by Antonsen et al., several model flows are investigated, and the fact that the power spectrum has the k−1-scaling predicted by Batchelor is confirmed. It is also observed that increased intermittency of the stretching tends to make the roll-off of the power spectrum at the high k end of the k−1 scaling range more gradual. These results are discussed in light of recent experiments where a k−1 scaling range was not observed.
Parallel Monte Carlo simulations of epitaxial growth
Authors: Haider Niaz , Khaddaj Souheil A. , Wilby Mark R. , Vvedensky Dimitri D.
Published: July 2002

The application of a distributed memory MIMD (multiple-instruction multiple-data) computer architecture to simulate growth during molecular-beam epitaxy is discussed. The stochastic nature of the problem and the spatial distribution of the lattice onto an array of processors leads to the development of some new ideas and constructs to ensure the system follows the correct dynamical evolution. Performance statistics for the parallel implementation and comparisons of morphological indicators with the sequential model are presented. © 1995 American Institute of Physics.
Crystal structure of low cristobalite at 10, 293, and 473 K: Variation of framework geometry with temperature
Authors: Pluth J. J. , Smith J. V. , Faber J.
Published: July 2002

The crystal structure of low cristobalite (SiO2) was refined at 10, 293, and 473 K from time-of-flight neutron powder diffraction data. One of the Si-O distances and one Si-O-Si angle change considerably: 10 K, 1.602(1) and 1.617(1) Å, 144.7(1)°; 473 K, 1.605(2) and 1.590(2) Å, 148.4(1)°. The angular distortion of the SiO4 tetrahedron is greater at 473 K than at the lower temperatures. Root-mean-square displacements of Si and O are greater for cristobalite than for quartz and coesite at room temperature. The negative correlation between Si-O distance and Si-O-Si angle for cristobalite is similar to that for quartz held at a higher temperature (∼350 K). Any adjustment of the Si-O distance for riding motion between Si and O atoms would reduce the variation of Si-O distance with temperature and with Si-O-Si angle. Furthermore, adjustment of the Si-O-Si angle for rocking of the oxygen atom out of the Si-O-Si plane could improve the correlation between secant (Si-O-Si) and the isotropic chemical shift for 29Si nuclear magnetic resonance in silica polymorphs. The temperature variation of the crystal structure of low cristobalite cannot be modeled by simple tilting of rigid tetrahedra.
Efficiency of surface interdigital transducers on anisotropic media
Authors: Planat M. , Schiavone P.
Published: July 2002

Excitation and detection of surface acoustic waves by means of interdigital transducers are studied here in the case of general anisotropic media with low piezoelectric coupling. Relations between mechanical wave amplitude and applied and detected voltage are established. The electromechanical coupling coefficient is calculated from the equivalent electrical circuit of the transducer for different quartz-crystal orientations. Experimental results are given for different quartz-crystal cuts, in agreement with the theoretical values.
Simple model for the time dependence of the periodically excited crossed-beam thermal lens
Authors: Weimer Wayne A. , Dovichi Norman J.
Published: July 2002

A time-dependent model is presented for the periodically excited crossed-beam thermal lens. In order to obtain good agreement between the theory and data for chopped excitation, it is necessary to consider the finite time required to block and unblock the pump beam. Unfortunately, a closed form expression has not been developed which considers the excitation waveform for a chopped Gaussian beam. On the other hand, a useful approximation has been developed based on a trapezoidal excitation function. This model is verified with respect to time, absorbance, alignment, and thermal diffusivity.
Theoretical study of hole transport in ZnSe
Authors: Ruda H. E.
Published: July 2002

Hole mobilities in ZnSe were calculated by solving the Boltzmann transport equation by the variational method, with all major scattering mechanisms included. Valence-band state symmetries were accounted for, both intra- and interband scattering terms for light and heavy holes were included, and generalized Fermi–Dirac statistics were used throughout. Screening of polar optical phonon and ionized impurity scatterings was also included. The contributions of all component scattering mechanisms to the total hole mobility as a function of hole concentration at 300 and 77 K, have been calculated for the first time. The influence of the compensation ratio, and the concentration of ionized impurities on both the hole mobility and resistivity has also been calculated for the first time; these results are shown to facilitate rapid and direct evaluation of electronic materials quality. In addition, the theoretical model was used to calculate the temperature dependence of the hole mobility in ZnSe, which proved to be in excellent agreement with available experimental data, confirming the choice of scattering parameters. Ultimate limits for the hole mobility in ZnSe at 300 and 77 K were calculated to be about 110 and 2000 cm2/V s, respectively, given zero compensation. Finally a derivation of the heavy-hole effective mass in ZnSe is presented, which yields a value 0.60 m0.
Photo-field effect in amorphous silicon thin-film transistors
Authors: van Berkel C. , Powell M. J.
Published: July 2002

Amorphous silicon thin-film transistors show a marked photosensitivity, with the illumination producing an increase in the off-current and a negative shift of the threshold voltage. Here we present the results of a complete theory of this photo-field effect, which includes the steady state flux of electrons and holes perpendicular to the source-drain current path. We show that illumination always produces a change in the band bending and a consequent redistribution of the space charge. The theory gives an excellent agreement with the experimental results using a simple model for the density of states in the amorphous silicon.
Case-II diffusion in polymers. II. Steady-state front motion
Authors: Hui C.-Y. , Wu K.-C. , Lasky Ronald C. , Kramer Edward J.
Published: July 2002

We consider front formation and steady-state front motion in a one-dimensional polymer system undergoing case-II diffusion. The polymer system approximates a polymer sheet whose thickness is very small compared with its lateral dimensions. The osmotic pressure of Thomas and Windle (TW) is used in the theoretical analysis. The transient problem of front formation is formulated. It is found that the original coupled system of partial differential equations proposed by TW can be reduced to one equation. An exact solution of this equation for a diffusion front moving with a velocity V is presented. The solution allows us to predict the dependence of the steady-state velocity on material parameters and the equilibrium concentration of penetrant outside the sheet. The concentration and pressure profile ahead of the moving front is obtained. We also show that the TW Model predicts the existence of a Fickian tail ahead of the steadily moving front. Conditions for the dominance of the Fickian tail are determined. The predicitions of our theoretical analysis are then compared with concentration profiles of iodohexane diffusing into polystyrene determined from Rutherford backscattering spectrometry.
Evidence in rare-earth (R)–transition metal (M) intermetallics for a systematic dependence of R-M exchange interactions on the nature of the R atom
Authors: Belorizky E. , Fremy M. A. , Gavigan J. P. , Givord D. , Li H. S.
Published: July 2002

In rare earth (R)–transition metal (M) compounds, large R-M magnetic interactions can occur, which give rise to higher values of the ordering temperature (TC,TN) for compounds with magnetic R elements than for compounds with R nonmagnetic, i.e., La, Lu, Y. Due to the localized character of the 4f shell, these R-M interactions are indirect, mediated by the 5d, 6s conduction electrons. The highest value of the ordering temperature is obtained for Gd compounds, and as a first approximation it is reasonable to write the interaction energy as ER-M=−nRMMSRMSM, where MSR and MSM are the rare-earth and transition metal spin moments, respectively. The molecular field coefficient nRM is generally assumed to be a constant throughout a given series, owing to the similarities of band structure for all R elements. In this paper, the molecular field coefficient nRM has been obtained for a number of series of rare earth-transition metal compounds. The analysis reveals that nRM is not a constant going across a given series but decreases by a factor of 2. The observed variation in nRM is shown to be related to the variation of the exchange interactions between 4f and 5d electrons which are larger for light rare-earth elements since the difference between the spatial extent of the 4f and 5d electrons is reduced. As a general rule the observed enhancement of magnetic interactions for lightest rare earths in all rare-earth metallic systems can be attributed to the same phenomenon.
Strongly enhanced 2D magnetism at surfaces and interfaces (invited)
Authors: Freeman A. J. , Fu C. L.
Published: July 2002

The study of magnetism in low-dimensional systems has entered a new phase thanks to (i) the advent of sophisticated synthesis and characterization techniques and (ii) the development of highly precise theoretical methods. We describe recent developments and applications of an all-electron total energy local spin density approach for determining the structural, electronic, and magnetic properties of surfaces, overlayers and interfaces, and sandwiches. Particular emphasis is placed, and results are given, on these structures involving transition metals (V, Cr, and Fe) on noble metals (Cu, Ag, and Au), simple metals (Al), and a nonmagnetic transition metal (W). Magnetic hyperfine fields are given for some Fe systems since conversion electron Mössbauer spectroscopy now permits detailed layer-by-layer tests of the theoretical predictions.
Effect of plastic deformation on electronic properties of GaAs
Authors: Skowronski M. , Lagowski J. , Milshtein M. , Kang C. H. , Dabkowski F. P. , Hennel A. , Gatos H. C.
Published: July 2002

A systematic study of plastically deformed (in compression) GaAs was carried out employing deep-level spectroscopies, optical absorption, and electronic transport measurements. Deformation-induced changes in the free-carrier concentration, the mobility, and occupation of deep levels were associated with a deep acceptor defect. Changes of the optical absorption in deformed samples were found to be due to a localized stress field of dislocations rather than transitions via localized levels. No evidence was found of any meaningful increase (>2×1015 cm−3) of the concentration of EL2 or other midgap donors for deformation up to about 3%. Thus, it is evident that the enhancement of the electron paramagnetic resonance signal of the arsenic antisite AsGa in deformed semi-insulating GaAs must be due to the increased ionization of AsGa rather than the generation of new antisite defects.
Characterization of haze-forming precipitates in silicon
Authors: Seibt M. , Graff K.
Published: July 2002

The defects leading to haze formation after diffusion of the transition metals cobalt, nickel, copper, and palladium have been studied by means of transmission electron microscopy and preferential etching. In all cases crystalline metal silicide particles have been observed. Both cobalt and nickel form disilicide particles of different morphologies in the surface regions of silicon wafers, whereas copper and palladium form metal-rich silicide particles causing a supersaturation of Si self-interstitials or undersaturation of vacancies and leading to the formation of extrinsic dislocation loops.
The reduction of dislocations in oxygen implanted silicon-on-insulator layers by sequential implantation and annealing
Authors: Hill Dale , Fraundorf Phil , Fraundorf Gail
Published: July 2002

Reduction of the dislocation density in silicon-on-insulator layers produced by oxygen implantation has been achieved by sequentially implanting and annealing the wafers with oxygen doses less than a critical value of about 4×1017/cm2 at 150 keV between heat treatments. Anneals of 17 h duration at 1250 °C were employed. Dislocation densities of less than 103/cm2 were obtained by etching studies on thin epitaxial layers grown on these wafers, in comparison to values around 109/cm2 with single doses greater than the critical value. Since the dislocations in the epitaxial layer arise from those in the implanted silicon overlayer, we expect a six order of magnitude decrease in dislocations at the top of the overlayer as well. The absence of threading dislocations in transmission electron microscope cross sections and plan views reinforces this expectation.