bluuebunny/crossref_metadata_2025 huggingface dataset:
Impurity transport in edge plasmas with application to liquid walls
Authors: Rognlien T. D. , Rensink M. E.
Published: July 2002

The transport of impurity ions in a magnetically confined plasma is studied in the region between their origin at a material surface and the core plasma as defined by closed magnetic flux surfaces. The focus is physics understanding of the results of two-dimensional (2-D) transport modeling of the plasma and neutrals. A simple one-dimensional model is introduced to identify key processes and illustrate how such processes affect the core-edge impurity level. The 2-D simulation gives detailed results of scaling of the impurity level with parameters such as anomalous radial diffusion, hydrogen–plasma recycling, core power flux, and core-edge density. The results are obtained for a slab model of a tokamak, but by changing the magnetic connection length, scaling to other types of devices can be inferred. Lithium and fluorine impurities are considered explicitly as examples with low and moderate charge-state number, Z, for liquid wall materials; trends found for these cases provide guidance to the behavior of other impurities. The tolerable amount of impurity influx can be closely associated with the partial thermal collapse of the edge plasma. The results are used to provide a physics picture of previous results on the acceptable evaporative impurity flux from different types of liquid wall materials, and to show how these results can be expected to scale with parameters.
Three-dimensional magnetohydrodynamic simulations on the interaction of interplanetary density pulses with the bow shock
Authors: Park S. M. , Lee D.-Y. , Min K. W. , Seon J.
Published: July 2002

Three-dimensional magnetohydrodynamic (MHD) simulations are carried out in order to study the interaction of upstream density pulses with the planetary bow shock. In this work, three distinguished types of the density pulse have been simulated: (1) Pure density pulse with no variations in all other variables, (2) density pulse in fast/slow mode-like type, and (3) another pure density pulse with a much stronger peak. For the first case, where the peak density is two times the background value, its interaction with the bow shock results in various MHD waves and a shock in the magnetosheath, including the fast mode wave which evolves into a fast shock, the entropy wave, and the slow mode wave. For the second case, the amplitude of the transmitted density pulse in the magnetosheath is reduced overall as compared to that in the first case. This is due to the generation of the stronger fast mode wave in the magnetosheath, and the effect is more pronounced for the fast mode-like pulse than for the slow mode-like pulse. When the third type pulse is incident with the peak density being five times the background value, the magnetosheath is more strongly compressed, which is followed by an expansion due to the sunward repulsion and then a second compression again. This still results in similar MHD waves and a shock in the magnetosheath, but they are generated in manners well-distinguished from the previous two cases. Based on the results from all three cases, it is suggested that the density amplitude and the properties of its related waves/shock in the magnetosheath can appear quite different depending on the observing spacecraft’s position, and thus necessitating a multispacecraft measurement for a reliable monitor.
Kinetic theory of dusty plasmas. IV. Distribution and fluctuations of dust charges
Authors: Tsytovich V. N. , de Angelis U.
Published: July 2002

The recent results of the kinetic theory of dusty plasmas [V. N. Tsytovich and U. de Angelis, Phys. Plasmas 6, 1093 (1999)] are used to investigate the distribution and fluctuations of the dust charges in dusty plasmas. It is shown that the charge fluctuations induced by dust discreteness can be larger than the fluctuations due to electron and ion discreteness. A new Lorentz form for the regular part of the dust charge distribution is found.
Electronic coupling and coherences in disordered polymers: Femtosecond 2D-photon echo correlation spectroscopy, signatures of an excitonic two-segmental site system: A theoretical study
Authors: Szöcs V. , Pálszegi T. , Tortschanoff A. , Kauffmann H. F.
Published: July 2002

The two-dimensional (2D) temporal shape of the (degenerate) three-pulse photon-echo (3-PPE) polarization P(3)(t′,τ12,τ23=0) has been theoretically analyzed for a weakly interacting two-site system (TSS) in the presence of site inhomogeneous broadening. The TSS has been modeled in terms of two identical, energy-degenerate, excitonically coupled pairs of electronic states in the site-representation (i.e., two S0 and S1 states forming a 4-level system). The 2D time-domain signal S(t′,τ12,τ23=0) has been derived in a closed-form solution which allowed to formulate the analytical Fourier transform analog under sequential δ-pulse excitation. The resulting 2D PE signal in frequency–frequency space S(ωt′,ωτ12) is formed in the region of the resonant, optical carrier frequency by, generally, four peaks, arranged at the edges of a square with distances determined by the excitonic coupling. The two diagonal peaks are of one-exciton origin, while the two off-diagonal (cross) peaks involve two-exciton effects, too. The relative heights of the four peaks are in distinct relation with the mutual orientation of the site-localized transition dipoles, thus these 2D line shapes serving as a structural probe for resonantly interacting molecules. The shapes of the peaks reflect the ratio between homogeneous and inhomogeneous dephasings and can be used for structural studies of statically disordered systems of TSS. The semiclassical two-site model is intended to provide a preparatory platform for mimicking electronically coupled segmental pairs dispersed in energy and space along the strongly disordered main chain contour. The results of the model calculation are taken to discuss the potential and the limitations of 2D line shape echo-spectroscopy in the study of short-range coherences and electronic coupling in π-conjugated polymers.
Influence of dopant on the electronic structure of spiro-oligophenyl-based disordered organic semiconductors
Authors: Crispin A. , Crispin X. , Fahlman M. , dos Santos D. A. , Cornil J. , Johansson N. , Bauer J. , Weissörtel F. , Salbeck J. , Brédas J. L. , Salaneck W. R.
Published: July 2002

The electronic structure of the propeller-shaped spiro-type molecules 2,2′, 7,7′-tetraphenyl-9,9′- spirobifluorene (spiro-quarterphenyl) and 2,2′, 7,7′-tetrakis(biphenyl-4-yl)-9,9-spirobifluorene (spiro-sexiphenyl) and its evolution upon doping with Li atoms or Na atoms have been studied by means of photoelectron spectroscopy. With lithium atoms as the dopant, two charges are stored on the same spiro branch in the form of bipolarons, for both spiro-quarterphenyl and spiro-sexiphenyl. In contrast, for doping with sodium atoms, the size of the counter ion makes it less energetically desirable to store two charges onto a single branch, and the charged species are polarons independent of the level of doping; this result is confirmed by optical absorption data.
Carbon nanotube/carbon fiber hybrid multiscale composites
Authors: Thostenson E. T. , Li W. Z. , Wang D. Z. , Ren Z. F. , Chou T. W.
Published: July 2002

Carbon nanotubes were grown directly on carbon fibers using chemical vapor deposition. When embedded in a polymer matrix, the change in length scale of carbon nanotubes relative to carbon fibers results in a multiscale composite, where individual carbon fibers are surrounded by a sheath of nanocomposite reinforcement. Single-fiber composites were fabricated to examine the influence of local nanotube reinforcement on load transfer at the fiber/matrix interface. Results of the single-fiber composite tests indicate that the nanocomposite reinforcement improves interfacial load transfer. Selective reinforcement by nanotubes at the fiber/matrix interface likely results in local stiffening of the polymer matrix near the fiber/matrix interface, thus, improving load transfer.
Small-signal analysis and particle-in-cell simulations of planar dielectric Cherenkov masers for use as high-frequency, moderate-power broadband amplifiers
Authors: Carlsten Bruce E.
Published: July 2002

A small-signal gain analysis of the planar dielectric Cherenkov maser is presented. The analysis results in a Pierce gain solution, with three traveling-wave modes. The analysis shows that the dielectric Cherenkov maser has a remarkable broadband tuning ability near cutoff, while maintaining reasonable gain rates. Numerical simulations verifying the small-signal gain results are presented, using a particle-in-cell code adapted specifically for planar traveling-wave tubes. An instantaneous bandwidth is numerically shown to be very large, and saturated efficiency for a nominal high-power design is shown to be in the range of standard untapered traveling-wave tubes.
Nanocages of layered BN: Super-high-pressure nanocells for formation of solid nitrogen
Authors: Golberg D. , Bando Y. , Sato T. , Grobert N. , Reyes-Reyes M. , Terrones H. , Terrones M.
Published: July 2002

We observed high N contents inside BN fullerenelike multilayered nanocages (ca. 30–100 nm) formed by reacting CNx nanotubes (x⩽0.1) and B2O3 in a N2 atmosphere at 1985–2113 K. High-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy, energy dispersion x-ray spectroscopy, electron diffraction analysis, spatially resolved energy-filtered TEM, and in situ TEM electron irradiation studies suggest that super-high pressures (of several GPa) are generated inside nanocages, responsible of crystallizing gaseous nitrogen trapped within the BN shells. In addition, high-pressure-induced phase transformation of graphiticlike BN into diamondlike cubic BN occurs inside the nanocages. Thus, BN nanocages play an unusual role of super-high-pressure nanocells.
Chemical equilibrium in supercritical fluids: Solvent effects on the dimerization equilibrium constant
Authors: Egorov S. A. , Rabani Eran
Published: July 2002

We study dimerization equilibrium between two dilute solutes in a supercritical solvent. Nonlocal hypernetted chain integral equation theory is employed to calculate the dimerization equilibrium constant for a wide range of solvent thermodynamic conditions and model potential parameters. The equilibrium constant displays a characteristic three-regime density dependence along near-critical isotherms, where it initially increases at low densities, decreases in the medium-density near-critical region, and increases again at high densities. The three regimes in the behavior of the equilibrium constant are discussed in terms of the energy-dominated low-density, desolvation-dominated medium-density, and entropy-dominated high-density regions.
Temperature dependence of the space–charge distribution in injection limited conjugated polymer structures
Authors: Feller F. , Rothe C. , Tammer M. , Geschke D. , Monkman A. P.
Published: July 2002

Temperature dependent pyroelectric measurements have been carried out on high barrier devices of conjugated polymers. At room temperature we find significant buildup of space charge at the metal/polymer interface as well as in the bulk despite the very low device current. At lower temperature (180 K) the charge profile is “frozen in” and shows nearly no bias dependence, while above room temperature the space charge seems to become more mobile and leads to broadening of the charge profile. By applying an injection model that accounts for charge backflow via interface recombination we demonstrate that the high amount of space charge detected in our films indicates the existence of deep traps in the surface region that can effectively collect charge from the electrode. In the bulk of the film we find space charge of different sign, which is assumed to be due to separation in the field of charge from intrinsic defect states. These bulk charges are also immobile at low temperatures, indicating that they correspond to electrons or holes that are transported by hopping or tunneling between traps.
Linear and nonlinear wave propagation in rarefied plasmas
Authors: Ballai I. , Erdélyi R. , Voitenko Y. , Goossens M.
Published: July 2002

Small-amplitude magnetohydrodynamic waves are studied in a dilute collisionless plasma with an anisotropic plasma pressure. The parallel and perpendicular pressure components are defined by two polytropic pressure laws. Previous results obtained within the framework of the Chew–Goldberger–Low double-adiabatic and double-isothermal models are recovered for specific values of the polytropic indices. The double-polytropic model can be considered as an extension of its single-polytropic counterpart model. Dispersion relations for the linear waves are derived and analyzed in the presence of pressure anisotropy. Particular cases of linear and nonlinear waves in a magnetic slab with double-polytropic plasma are investigated. The presence of the effective parallel and perpendicular sound speeds in the slab gives rise to a complicated interplay between kink and sausage modes supported by the slab. The behavior of weakly nonlinear waves in the slab are governed by the Benjamin–Ono equation. The soliton-like solution of this equation may be accelerated or retarded depending on the values of parallel and perpendicular sound speeds. The solitons are always accelerated in the magnetically dominated slab.
Carbon reactivation kinetics in the base of heterojunction GaInP–GaAs bipolar transistors
Authors: Mimila-Arroyo J. , Bland S. W. , Chevallier J.
Published: July 2002

The reactivation kinetics of carbon acceptors in the base region of GaInP/GaAs heterojunction bipolar transistors was studied. The reactivation was achieved by ex situ thermal annealing, through a multistage annealing experiment where the carrier concentration was monitored at each stage. Results indicate that carbon reactivation follows a first-order kinetics process in which the activation energy appears to be the sum of the energy needed to debond the hydrogen from the carbon–hydrogen complex, and the energy necessary to overcome the electrostatic junction barrier. The reactivation constant is thermally activated with an activation energy of 2.83 eV and an attempt frequency of 1.2×1013 s−1.
Selective dissociation of the stronger bond in HCN using an optical centrifuge
Authors: Hasbani R. , Ostojić B. , Bunker P. R. , Yu. Ivanov M.
Published: July 2002

Using the example of the HCN molecule, we study theoretically the possibility of selectively breaking the stronger bond in a triatomic molecule by rotationally accelerating it in an optical centrifuge using a combination of two oppositely chirped and counter-rotating strong laser fields. In our simulation the resultant field forces rotational acceleration of the HCN molecule to a point where the centrifugal force between the two heavy atoms (C and N) exceeds the strength of their (triple) bond. The effects of bending, rovibrational coupling, and the Coriolis force, which conspire to prevent the molecule from rotational dissociation into HC+N, can be efficiently counteracted by simple optimization of the frequency chirp.
Temperature and pressure dependence of the α-relaxation in polymethylphenylsiloxane
Authors: Paluch M. , Roland C. M. , Pawlus S.
Published: July 2002

The α-relaxation process in polymethylphenylsiloxane was studied over a broad temperature and pressure range by dielectric spectroscopy. In the vicinity of the glass temperature, the shape of the dielectric loss peak is independent of both temperature and pressure. The steepness index (fragility), describing the temperature dependence of the relaxation times, is also independent of pressure (and of molecular weight as well). Thus, the correlation between fragility and nonexponentiality of the relaxation function is maintained under conditions of high compression. The combined temperature and pressure dependences of the relaxation time conformed to the Avramov equation. This model offers a means to relate the relaxation behavior to thermodynamic properties of the material.
A theory of induced interaction between rotating particles in electrorheological fluids
Authors: Gu Guo-qing , Yu K. W. , Hui P. M.
Published: July 2002

Recent experiments on electrorheological effects provide evidence that the rotation of particles weakens the induced electric forces between dielectric particles. To study the induced electric force exerted on rotating particles phenomenologically, we establish a differential equation to describe the relaxation process of the surface polarized charge distribution on rotating particles in an electrorheological fluid. In the case of a uniformly rotating particle, a general solution to the relaxation equation for the polarized charge is obtained. The surface charge distribution of a two-particle system consisting of a particle at rest and a uniformly rotating particle is calculated. Based on the Green’s function approach, we evaluate the electric potential in both the host and the rotating particle. A formula relating the angular velocity of rotating particle to the induced electric force on the particle is then derived.
An improved chain extension algorithm and its application for various branched polymers
Authors: Shida Kazuhito , Ohno Kaoru , Kawazoe Yoshiyuki
Published: July 2002

Statistical properties of flexible polymers with controlled molecular weights (length of arms) and branching patterns (topology) has been attracting strong interest. Basically, the lattice enrichment algorithms are suitable for simulating this kind of subject because they allow direct estimation of the total configuration number. However, it is difficult to apply this kind of algorithm for polymers with highly complicated structures, namely the comb-polymers. One of the difficulties is that the efficiency of simulation is severely limited. In this paper, a previously reported improvement technique is revisited, enhanced, and combined with several new ideas for developing a modified algorithm, which can solve such a situation. The validation and performance evaluation of the modified algorithm is presented. The algorithm is then applied to small scale polymer combs. In particular, the value of the critical exponent related to the configuration number is estimated on these combs. The result obtained for an H-shaped polymer is in accordance with previous simulations and a theoretical prediction formula already proposed. Possibilities for the future extension of the algorithm are briefly discussed.
Comment on “Hydrogen bonding and stacking interactions of nucleic acid base pairs: A density-functional-theory treatment” [J. Chem. Phys. 114, 5149 (2001)]
Authors: Cybulski S. M. , Bledson T. M. , Toczyłowski R. R.
Published: July 2002

A density-functional-theory (DFT) based treatment for calculating the interaction energies of nucleic acid base pairs proposed recently by Elstner et al. [J. Chem. Phys. 114, 5149 (2001)] is criticized. It is shown that dispersion energy that they evaluated amounts to approximately only 30% of the value obtained in rigorous ab initio calculations. The use of the second-order Mo/ller–Plesset perturbation theory results obtained with the 6-31G*(0.25) basis set as a benchmark for validating the DFT results is also criticized. It is suggested that the method of Elstner et al. requires much more stringent testing before it can be deemed to be “very accurate.”
Rhombic patterns: Broken hexagonal symmetry
Authors: Ouyang Qi , Gunaratne Gemunu H. , Swinney Harry L.
Published: July 2002

Landau–Ginzburg equations derived to conserve two-dimensional spatial symmetries lead to the prediction that rhombic arrays with characteristic angles slightly differ from 60° should form in many systems. Beyond the bifurcation from the uniform state to patterns, rhombic patterns are linearly stable for a band of angles near the 60° angle of regular hexagons. Experiments conducted on a reaction–diffusion system involving a chlorite–iodide–malonic acid reaction yield rhombic patterns in good accord with the theory.
Magneto-optical garnet films with high Faraday rotation and controlled coercivity
Authors: Gualtieri D. M. , Tumelty P. F.
Published: July 2002

Magneto-optical garnet films of (Bi, Y, Tm, Gd) 3(Fe, Ga)5O12 were grown epitaxially on (111)-oriented substrates of GGG. Film thickness was varied between 1.35 and 7.95 μm. Melt composition and growth conditions were chosen to ensure a film-substrate lattice constant mismatch of at least 0.5%, and the films exhibited a uniform, facetlike, defect structure as a consequence. These films had uniaxial anisotropy and were optically active along (111). The specific rotations of these films, as measured at 546.1 nm, were generally greater than 3 deg/μm, and the films exhibited a B-H loop hysteresis of optical rotation as a function of applied field. The value of the applied field necessary to switch the films between opposite states of saturation ranged from 36–267 Oe. The switching field and coercivity were found to vary with the thickness and magnetization of the films, thick films of highest magnetization having the lowest switching fields. Compositional analysis by x-ray fluorescence showed a significant increase in bismuth content (and hence the specific rotation) as the film lattice constant was increased just past the point of facet formation. As a consequence of their coercivity, the films retain the induction and net optical rotation of their last state upon removal of an applied field. Their hysteresis loop characteristics allow a gray-scale response, as averaged over areas large compared with the facet size, to pulsed fields. These properties are obtained from as-grown material, as distinguished from the properties of patterned films. These films may find application as display, optical computing, and data storage elements.
Random magnetism in amorphous rare-earth alloys (invited)
Authors: Sellmyer D. J. , Nafis S.
Published: July 2002

Several aspects of the magnetic transitions seen in rare-earth metallic glasses are discussed, particularly with reference to recent theoretical work. These include: (a) apparent double transitions observed in Gd glasses where exchange fluctuations are important, (b) evidence for a correlated speromagnetic state recently predicted by Chudnovsky and Serota, and (c) the analysis of a Tb glass with strong random anisotropy in terms of an Ising-type spin-glass transition.
Photoconductivity decay kinetics in silver bromide photographic films
Authors: Deri R. J. , Spoonhower J. P. , Hamilton J. F.
Published: July 2002

There is a component of the complex decay of the photoconductivity signal for silver bromide photographic materials that is intermediate in time constant and in temperature dependence between the initial electron trapping and the subsequent ionic processes. Previous attempts to explain this transient have not been totally satisfactory. We propose that it is a thermally activated lattice relaxation of the otherwise shallow coulombic electron states.
Optimal sample shape for internal friction measurements using a dual cantilevered beam
Authors: Baur J. , Kulik A.
Published: July 2002

An optimal shape for internal friction measurements using a dual cantilevered beam is described. The new shape is analyzed by a finite element method, in order (a) to optimize its parameters to reduce parasitic interactions with the sample holder, and (b) to calculate the stress distribution in the vibrating sample. The numerical analysis shows that the interactions between the sample and its holder are much weaker for the new shape than for the classical flexural vibration of a simple cantilevered beam. The predictions of the numerical analysis are compared to experimental results on a niobium sample at low temperature.
Deposition of hard carbon films by rf glow discharge method
Authors: Kobayashi Kenji , Mutsukura Nobuki , Machi Yoshio
Published: July 2002

Hard carbon films were produced by the rf glow discharge plasma decomposition of hydrocarbon gases diluted with H2 gas at relatively low temperature and low pressure. Optical emission spectra were measured during the deposition of the carbon films, and the correlation between the carbon film’s properties and the plasma’s condition was examined. From these results, H radicals were considered to contribute to the formation of hard carbon films. These were examined by transmission electron microscopy, and were found to be predominantly amorphous. The existence of diamond grains was confirmed in the amorphous phase. A relatively large diamond grain in the order of 10 μm was obtained under the conditions of a gas mixture ratio of 70% and a gas pressure of 5×10−3 Torr.
Infrared study of the kinetics of oxidation in porous amorphous silicon
Authors: Koropecki R. R. , Arce R.
Published: July 2002

Infrared spectroscopy has been applied to the study of the kinetics of oxidation in high-pressure dc-sputtered amorphous silicon. The different spectra obtained during the evolution of the oxidation are processed by factor analysis. Two oxidation mechanisms have been found. Their associated infrared spectra and time evolution can be explained by a model that proposes the existence of a two-level microstructure. The spectra associated with each one of the oxidation mechanisms seems to correspond mainly to SiO2, modified by the presence of surface modes.
Phase lock of a long Josephson junction to an external microwave source
Authors: Cirillo M. , Lloyd Frances L.
Published: July 2002

A long Josephson junction dc biased on a zero-field singularity and emitting radiation at microwave frequencies is irradiated with external microwave power. This power can be supplied either by a room-temperature oscillator or by another long junction. We find that the oscillations of the junction can coherently lock to the external signal for frequency intervals ranging from 500 kHz to 50 MHz. The dependence of the width of these intervals of coherence on the external microwave power is measured for the case in which the power is generated by a room-temperature oscillator.
Low-temperature heat capacity of linear-chain magnetic compounds CsNiCl3, RbNiCl3, and CsCuCl3
Authors: Collocott S. J. , Rayne J. A.
Published: July 2002

Heat-capacity measurements from 0.5 to 15 K have been made on single-crystal specimens of the linear-chain magnetic compounds CsNiCl3, RbNiCl3, and CsCuCl3. In all cases anomalies are observed at the temperature corresponding to the onset of three-dimensional magnetic order. The magnitude of the anomaly in RbNiCl3 varies slightly from specimen to specimen but shows no structure corresponding to the existence of spin frustration. At low temperatures all compounds show a T3 spin-wave contribution to the specific heat associated with a simple cubic antiferromagnet, even though they have a complex spiral-spin structure.
Engineering magnetic materials on the atomic scale (invited)
Authors: Arrott A. S. , Heinrich B. , Purcell S. T. , Cochran J. F. , Urquhart K. B.
Published: July 2002

Ultrahigh vacuum (UHV) systems and the use of atomic beams for deposition of atoms layer by layer combine to make possible the creation of new materials. The applications to metallic magnetism are gaining increasing attention. The building of sandwiches of magnetic and nonmagnetic layers should lead to increased understanding of the propagation of spin polarization through metals and the effects of finite thickness on the ground state properties and the thermodynamics of magnetic materials. The most important step in this process is in the first layer, i.e., the preparation of the substrate and the determination of the quality of the interface and of the overlayer. The techniques of surface science, e.g., residual gas analysis (RGA), reflection high energy electron diffraction (RHEED), Auger electron spectroscopy (AES), and x-ray photoemission spectroscopy (XPS) are essential for the characterization of the interface. Illustrations of these include our own work on body-centered-cubic Ni deposited epitaxially on (100)Fe with and without intermediate epitaxial Au layers and with and without Au cover layers, studied using ferromagnetic resonance (FMR). The engineering of new magnetic phases takes into account the tendency of increased atomic volume to enhance magnetic moments. Epitaxy can be used to create negative pressures. New phases of Mn have been formed using the (0001) surface of Ru as a template for expanded structures.
Magnetostriction and thermal expansion of RE2Fe14B
Authors: Ibarra M. R. , Algarabel P. A. , Alberdi A. , Bartolomé J. , del Moral A.
Published: July 2002

Magnetostriction and thermal expansion measurements have been carried out on polycrystalline RE2Fe14B permanent magnets (RE=Nd, Ho, Dy, and Y) between 4.2 K and room temperature in pulsed magnetic fields up to 15 T. Nd2Fe14B exhibits an anisotropic magnetostriction which amounts to λt=−308×10−6 at 4.2 K and −96×10−6 at 300 K at 15 T. In the whole series magnetostriction at low temperatures is dominated by the RE contribution. The volume magnetostriction has been also determined for all the compounds and increases with temperature, which is the behavior for Y2Fe14B. Linear thermal expansion has been measured for RE=Nd and Y in the range of 77–300 K, showing for Nd2Fe14B an anomaly at the spin reorientation temperature (TSR=115 K) and a large invar effect.
Dispersion compensation for femtosecond pulses using high-index prisms
Authors: Salin François , Brun Alain
Published: July 2002

We have compensated the residual chirp of the pulses emerging from the thick output coupler of our femtosecond dye laser with a sequence of four high-refraction index prisms. Using the same prisms, we show that it is possible to compensate the great amount of dispersion due to a femtosecond multistage amplifier leading to a compact and low-loss compensation system.
Quartz crystal microbalance measurements of absolute laser photodeposition rates: Application to 257-nm deposition from W(CO)6
Authors: Jackson Robert L. , Tyndall George W.
Published: July 2002

A quartz crystal microbalance has been used to measure absolute rates of laser-induced photochemical deposition processes in real time. A cw UV laser (257 nm) focused at normal incidence onto the microbalance crystal surface is used to induce deposition by photodecomposition of an organometallic vapor. An equation is derived to obtain the absolute mass of the deposit. The method is demonstrated for photodeposition from W(CO)6.
Misfit dislocation structure at a Si/SixGe1−x strained-layer interface
Authors: Rajan Krishna , Denhoff Mike
Published: July 2002

The misfit dislocation structure at a Si/Si0.75Ge0.25 strained-layer interface has been characterized by transmission electron microscopy. Through weak-beam imaging it is found that partial dislocation in the form of extended nodes exist in the misfit dislocation network. The density of nodes as observed by microscopy compares favorably with the estimate of the density of charged interface states derived from capacitance-voltage measurements.
New rapid thermal annealing for GaAs digital integrated circuits
Authors: Tamura Akiyoshi , Uenoyama Takeshi , Nishii Katsunori , Inoue Kaoru , Onuma Takeshi
Published: July 2002

A new rapid thermal annealing (RTA) method that uses GaAs guard rings has been developed. A new temperature monitoring method is also described. Generations of slip lines on 2-in.-diam GaAs wafers annealed in different kinds of RTA arrangements were investigated by x-ray transmission topography. The use of three GaAs guard rings has been found to be very effective in reducing slip lines. The temperature dependence of activation and uniformity of annealing characteristics for a selective Si-implanted 2-in.-diam GaAs wafer at 100 keV with a dose of 5×1012 cm−2 were evaluated by drain saturation current (Idss) distribution of gateless field-effect transistors (FETs) over the wafer. The best uniformity, as well as the highest activation, was obtained by RTA at 920 °C for 15 s. The activation energy of 1.47 eV for the average value of Idss (∼(Idss)) was obtained. By using this RTA method, GaAs digital integrated circuits (ICs), dual-modulus prescalers, have been successfully fabricated with high yield for the first time. This RTA method is very promising for GaAs digital IC processing.
Are the high T
c superconducting materials bulk superconductors or grain boundary percolating network superconductors? (abstract)
Authors: Garcia N. , Vieira S. , Baro A. M. , Tornero J. , Vazquez L. , Gomez J. , Aguilo A. , Bourgeal S. , Buendia A. , Hortal M. , Lopez de la Torre M. A. , Ramos M. A. , Villar R. , Rao K. V. , Chen D-X. , Nogues J. , Karpe N.
Published: July 2002

We have studied the physical properties of high Tc Y-Ba-Cu-Ox superconducting materials with levitation, ac susceptibility, macroscopic resistivity, resistivity as measured by scanning tunneling microscopy (STM) and Hall effect. Levitation experiments show that the powder of the as-prepared material does not levitate at liquid nitrogen while pellets and powders that have been heated above 450 °C do levitate (are superconducting). These experiments seem to indicate that clustering and intimate contact of fine grains are necessary for levitating. The ac susceptibility experiments show that diamagnetism is extremely sensitive to pellet density. The higher the density and the smaller the field amplitude the less diamagnetic is the system. This is interpreted as evidence for a surface, not a bulk effect. Resistivity at 17 °C measured with macroscopic contacts is of the order of 10 Ω cm. When measured with microscopic STM contacts, a clear semiconducting behavior is observed. This observation does not preclude grain boundary regions with metallic conductivity. We conjecture that the bulk of the grain is semiconducting with a conducting percolative network of grain boundaries that is the source of the superconductivity. We propose a model of a superconductor-metal-superconductor percolative network based on the excitonic model of Allender, Bray, and Bardeen to explain the high values of Tc. Hall measurements show large effective masses in agreement with an excitonic model.
Charge transfer adsorption in silicon vapor-phase epitaxial growth
Authors: Ishitani A. , Takada T. , Ohshita Y.
Published: July 2002

Silicon vapor-phase epitaxial growth with SiH2Cl2 is theoretically studied. The optimized geometries and total energies of the species, generated from SiH2Cl2, are calculated by using an ab initio molecular orbital method. The charge transfer of the interaction between a silicon surface and SiCl2 is considered. Based on the computational result that SiCl−2 has the lower total energy that SiCl2, a new adsorption mechanism, named charge transfer adsorption, is proposed. By using this charge transfer adsorption followed by the surface reaction at the hollow bridge site, the epitaxial growths on the silicon (001), (111), and (110) surfaces are discussed. The epitaxial growths take place in different ways for these three surfaces because of the specific locations of the hollow bridge sites.
Co-Mo thin films with an artificially layered structure
Authors: Sato Noboru
Published: July 2002

Cobalt-molybdenum (Co-Mo) thin films deposited into an alternating multilayer structure by a dc-magnetron sputtering method have a compositionally modulated structure. Films with thicker Co layers than Mo layers have an artificial superlattice hcp structure with the c-axis normal to the film plane. Films with reverse thicknesses of Co and Mo have an artifical superlattice bcc structure with the 〈110〉 axis normal to the film plane. Films are ferromagnetic with an easy magnetization direction in the film plane. The Co layers in contact with Mo layer have reduced magnetization. The paramagnetism or nonmagnetism is observed in films with a small periodicity composed of thinner Co layers than Mo layers. The small magnetization is originated from both low Curie temperature and decrease in the magnetization due to lattice deformation of Co.
Magnetization of Ni-Si, Co-Si, and Fe-Si layered systems
Authors: Foiles C. L. , Slaughter J. M.
Published: July 2002

Magnetization measurements have been performed, using a SQUID magnetometer, on multilayered X-Si films where X is Fe, Co, or Ni. The samples were produced by dc sputtering with the thickness of the Si layers fixed at a value between 30 and 70 Å and the thickness of the X layers varying from 8 to 30 Å. These nominal preparation thicknesses were verified by x-ray diffraction studies. We find that at 5 K the saturation magnetization decreases rapidly with decreasing thickness with a loss of ferromagnetism by layer thicknesses 8 and 14 Å for Fe and Co, respectively. A Ni-Si sample with a Ni thickness of 30 Å was not ferromagnetic. Comparison of measurements made with the applied field parallel and perpendicular to the layers indicates no magnetic aniostropy other than the expected shape anisotropy. Our data are compared with data by Wong and co-workers for Fe-V and data by Kazama and co-workers for amorphous Fe(C)-Si.
A specific model for domain-wall nucleation in thin-film Permalloy microelements (invited)
Authors: Smith Neil
Published: July 2002

A particular mechanism for domain-wall nucleation in thin (350 Å) Permalloy films of typical geometry (7×140 μm) used for magnetoresistive (MR) sensors is studied in detail. A numerical micromagnetic model is described which can theoretically predict an observed domain-wall process (including domain-wall creation, destruction, and intermediate wall motions and magnetization distributions) without imposition of prior assumptions concerning domain-wall existence or artificial nucleation mechanisms. Calculated results for the nucleation and annihilation fields of a parallel pair of Néel domain walls responsible for observed Barkhausen noise in an MR sensor yield good overall agreement with experimental magnetoresistive measurements. An analysis of the energetics of the wall nucleation/annihilation process is provided. Bitter pattern measurements confirming the proposed domain-wall mechanism are also included.
New silicon-related deep broadband luminescence emission in Al0.3Ga0.7As epitaxial layers
Authors: Souza P. , Rao E. V. K. , Alexandre F. , Gauneau M.
Published: July 2002

Luminescence spectra of variously silicon-doped Al0.3Ga0.7As (Al0.3Ga0.7As: Si) single layers grown by molecular-beam epitaxy were investigated as a function of silicon effusion-cell temperature. A correlation between silicon incorporation as a complex involving SiAs and the existence of a deep broadband emission is suggested. To achieve this, in addition to photoluminescence, Hall effect, capacitance, and secondary ion mass spectroscopy measurements were also performed.
Mass transport during laser chemical vapor deposition
Authors: Copley Stephen M.
Published: July 2002

Two analyses of steady-state mass transport during laser chemical vapor deposition (LCVD) are compared for a hemispherical source/sink geometry. The first considers only diffusion while the second includes diffusion and also convection due to the mole number change of vapor species in the reaction, which is modeled as the radial outflow of an incompressible gas from a point source. The results of the two analyses differ markedly for high deposition fluxes and large radii. For example, in the LCVD of Ni from Ni(CO4), reactant partial pressures at the hemispherical surface predicted by the diffusion plus convection analysis are less than those predicted by the diffusion analysis because the inward diffusion of the reactant species is hindered by the convective outflow of the product species. The diffusion analysis predicts an increase in total pressure at the hemispherical surface, which normally is relieved by the neglected convective flow. Bounds for transport controlled deposition and for deposition where convection can be safely neglected are specified. The diffusion plus convection analysis is generalized to cover a surface distribution of sources/sinks, and can be employed to predict local partial pressures in steady-state LCVD.
Magnetization curling in perpendicular iron particle arrays (alumite media)
Authors: Huysmans G. T. A. , Lodder J. C. , Wakui J
Published: July 2002

Alumite has been investigated by measuring the hysteresis loops for different field directions to determine the influence of the particle interactions in a perpendicular oriented particle medium on the angular dependence of the coercivity and the hysteresis loss. Alumite is an ideal model material for this purpose because the particle size and particle distance are well defined and can be varied independently. It is shown that the coercivity of the particle array can be identified with the nucleation field of one single particle for perpendicular fields. It is clear from the radius dependence of the coercivity that the reversal mode is the curling mode. The angular dependence of the coercivity does not agree with the well-known theoretical curve for one particle, which is caused by the particle interactions. The same can be said about the angular dependence of the hysteresis loss. Calculations of hysteresis loops of particle arrays with interacting particles show the influence of these interactions on the angular dependence of the coercivity.
Analysis of valence shell electronic excitations in silicon and its refractory compounds using electron energy loss microspectroscopy
Authors: Skiff W. M. , Carpenter R. W. , Lin S. H.
Published: July 2002

Valence shell electronic excitations in silicon, and its carbide, nitride, and oxide are studied using electron energy loss microspectroscopy in a transmission electron microscope, at 1-eV resolution with a 100-keV electron beam. This so-called ‘‘low-loss’’ region of the spectrum is normalized to scattering cross section per valence electron for each compound, and a large, sequential, variation in the cross section across the series of compounds is reported. The low-loss cross section is calculated using the Born approximation by including single-electron transitions in the target. Both band-to-band and band-to-continuum transitions are included, by using extended Hückel band calculations to obtain the valence shell crystal states, using the electron gas model for the ionization states, and using the random phase approximation to obtain differential cross sections. The calculations yield semiquantitative results, and allow for a chemical interpretation of the experimental results. The relation of the present model to that for plasmon excitation is also briefly discussed.
X-ray reflectivity and transmission electron microscopy studies on thin and ultrathin W/C and W/Si multilayers structures
Authors: Vidal B. A. , Marfaing J. C.
Published: July 2002

Ultrathin and thin bilayers of W coupled with C and Si have been deposited by rf magnetron sputtering. Behavior of the multilayer structures (MLS) has been analyzed through two combined techniques: x-ray reflectivity and electron microscopy study and conventional (TEM) and high-resolution transmission (HRTEM). The experimental results we present provide average informations on the reflective characteristics of the mirrors in agreement with theory, comparative values of parameters (thickness and interfacial roughness of the bilayers), as well as structural characteristics of the stackings. We focused on ultrathin W/Si MLS with a bilayer thickness of 1.5 nm that present excellent regularity with effective roughness less than 3 Å. They offer reflective qualities to be used as reflectors with an angle 2θ>6° in the first order for the medium x-ray range.
The role of small-impact-parameter electron-loss processes on ion temperatures measured by active-beam plasma diagnostics
Authors: Donné A. J. H. , de Heer F. J. , Olson R. E.
Published: July 2002

The shape of the energy distribution of scattered particles in an active-beam scattering experiment can be influenced considerably by the presence of highly charged impurity ions in the plasma. In this work it is shown theoretically that multiply charged impurities have only a small effect on the scattering distribution at low energies (<10 keV/amu) and at high energies (>75 keV/amu). The effect of the impurities is the most pronounced for energies close to 50 keV/amu. The results of the calculations are in good agreement with experimental results at various energies. Extrapolation to higher beam energies leads to the conclusion that the active-beam scattering diagnostics can be applied also in future fusion devices for measuring the local ion temperature.
Defects due to nonstoichiometric growth in semi-insulating GaAs and their effects on Si implantation activation efficiency
Authors: Zhao Jian H. , Fang Zhaoqiang , Shan Lei , Schlesinger T. E. , Milnes A. G.
Published: July 2002

The effects of starting semi-insulating GaAs conditions on Si-implantation activation efficiency have been studied by capacitance deep level transient spectroscopy, 4.2-K photoluminescence, and thermally stimulated current spectroscopy. Experimental evidence shows that samples with good Si activation efficiency have higher EL2 trap concentration, compared with samples with a lower activation efficiency, and a higher density of trap T2 at Ev +0.49 eV believed to be the native defect AsGa++. For samples with a lower activation efficiency, a deep trap T3 attributed to either VAs or GaAs −VGa appears at Ev +0.44 eV with a concentration larger than that of T2. A PL emission at 1.44 eV due to GaAs or its related complex is observed in the samples with a lower activation efficiency, but not in samples with good activation efficiency. Comparison between samples grown under Ga-rich or As-rich conditions and samples that show different activation efficiencies confirms that growth stoichiometric conditions largely determine the Si implantation activation efficiency.
Structural characterization of Ta/Al multilayer films
Authors: Jiang S. S. , Hu A. , Chen H. , Liu W. , Zhang Y. X. , Qiu Y. , Feng D.
Published: July 2002

Ta/Al multilayer films with modulation wavelength 4.6 and 9.5 nm were fabricated by magnetron sputtering. The structure of the multilayer was studied by x-ray diffraction. The combination of Ta and Al forms a metallic superlattice with Ta(110) and Al(111) textures. The x-ray diffraction patterns were simulated using the model for the modulated composition multilayer. The coherence length perpendicular to the film is about 100 nm. The composition modulation is approximately a trapezoidal wave. The thickness of the transition layer is about 1.00 nm for the multilayer with a modulation wavelength of 4.6 nm. The fluctuation of atomic planes in each period is about 5%.
Photoacoustic investigation of iodine-doped polystyrene
Authors: Torres-Filho A. , Leite N. F. , Miranda L. C. M. , Cella N. , Vargas H.
Published: July 2002

The kinetics of iodine doping of atactic polystyrene films is investigated using photoacoustic spectroscopy. The changes in the photoacoustically measured physical properties, such as nonradiative relaxation time and thermal diffusivity, are present as a function of the doping time. The results show strong evidence that an order-disorder transition is taking place as a function of the doping time. The suggested order-disorder transition is also evident in the dielectric constant measurements of the doped films.
Interference effects probed by photoreflectance spectroscopy
Authors: Huang D. , Mui D. , Morkoç H.
Published: July 2002

We report on oscillations in the photoreflectance (PR) spectra in the energy range below the band gap of GaAs on samples containing GaAs/AlGaAs quantum wells and the other multilayers grown on Si-doped GaAs substrates. The PR spectra with the quantum well chemically removed continue to show these oscillations, indicating that they are not related to the quantum well. These oscillations, which are probed by the modulation technique, are attributed to the interference effect of two light beams reflected from different interfaces of the sample. These spectra are calculated and good agreement with experimental data is obtained. We find that the interference effect distorts the line shape of the PR features associated with quantum wells and band edge of bulk semiconductors, especially at the low-energy side. Therefore, this effect must be taken into account during analysis. As an application, this interference effect can be used to accurately measure epilayer thicknesses.