bluuebunny/crossref_metadata_2025 huggingface dataset:
Superconducting properties and crystal structures of single-crystal niobium nitride thin films deposited at ambient substrate temperature
Authors: Wang Zhen , Kawakami Akira , Uzawa Yoshinori , Komiyama Bokuji
Published: July 2002

Single-crystal niobium nitride (NbN) thin films were fabricated at ambient substrate temperature so that photoresist lift-off techniques could be used in fabricating Josephson tunnel junctions. In this article, we describe the superconducting properties and crystal structure of the NbN films. Even though the substrates were not heated, the NbN films had excellent superconducting properties: a high Tc of 16 K, low normal-state resistivity (ρ20=62 μΩ cm), and residual resistivity ratios RRR=ρ300/ρ20 above one. The film structures, which were investigated by x-ray diffraction, electron diffraction patterns and transmission electron micrographs, show a single-phase orientation without columnar and granular structures. We have found that the superconducting properties depend on the lattice parameter, and the best films had a lattice parameter of 0.446 nm. NbN/AlN/Nb tunnel junctions were fabricated to measure the superconducting energy gap of the NbN films. We estimated the energy gap ΔNbN to be 2.6 meV and the magnetic penetration depth λNbN to be 176 nm.
High energy negative ions in a radio-frequency discharge
Authors: Zeuner Michael , Meichsner Jürgen , Rees J. Alan
Published: July 2002

We measured energy distributions of negative ions at the grounded electrode of an oxygen parallel-plate rf discharge. Negative ions are generated in the plasma sheath in front of the rf electrode, are accelerated away from the driven electrode, and can be detected at the grounded electrode. The maximum energy of the negative ions corresponds to the negative self-bias voltage of the rf electrode. Structures in the energy distribution reflect sheath properties and characteristics of the ion generation processes.
Site-selective chemical-vapor-deposition of submicron-wide conducting polypyrrole films: Morphological investigations with the scanning electron and the atomic force microscope
Authors: Cacialli F. , Bruschi P.
Published: July 2002

We report morphological investigations of polypyrrole thin films deposited by means of a self-aligning vapor phase technique onto glass, silicon and silicon dioxide substrates, coated with an oxidizing precursor. The variation of the deposition parameters allows the control of the film microstructure which can be fibrillar and strongly anisotropic or globular and tendentially isotropic. Patterning of the precursor by electron-beam lithography allows the production of submicron wide lines as shown by both the scanning electron microscope and the atomic force microscope.
X-ray photoelectron spectroscopy study on composition and structure of sol-gel derived PbTiO3 thin films
Authors: Lu C. J. , Kuang A. X. , Huang G. Y.
Published: July 2002

The surface states of sol-gel derived PbTiO3 thin films on Si substrates before and after Ar+ sputtering were studied by x-ray photoelectron spectroscopy (XPS). The results showed that there was no residual carbon or other impurity element in the films except some carbon surface contamination due to specimen handling or pumping oil. A large amount of absorbed oxygen was near the surfaces of the films. The chemical composition of the films was found to be stoichiometric, as proved by inductively coupled plasma results. The valence states of the ions indicated that the films were PbTiO3 with perovskite structure. The XPS spectra of the films after Ar+ sputtering for 10 min differed greatly from those of as-prepared films. This probably results from the preferred sputtering of lead atoms and the production of many new dangling bonds during Ar+ bombardment.
Investigation of K-shell emission from moderate-Z, low-η (-velocity), Z-pinch implosions
Authors: Thornhill J. W. , Whitney K. G. , Davis J. , Apruzese J. P.
Published: July 2002

Because of the large amount of kinetic energy and mass needed for efficient production of K-shell emission from moderate atomic number z-pinch plasmas (Z≳22), moderate Z experiments performed in the near future will likely take place at relatively low implosion velocities or low-η values, where η is defined as the ratio of the maximum kinetic energy per ion generated prior to stagnation to the minimum energy per ion, Emin, needed to instantly heat and to ionize a plasma into the K shell upon stagnation of the pinch. Since there has been no systematic theoretical or experimental investigations of K-shell yield scaling with mass and atomic number in the low-η regime η∼1–3, in anticipation of such future experiments, we report on the results of our theoretical investigation into this regime. In particular aluminum, argon, titanium, and krypton plasmas were studied using a 1D radiation hydrodynamics model that uses enhanced transport coefficients to phenomenologically attain the stagnation conditions of experimental z-pinches. The results of this study have identified the following important effects: low-η plasmas remain in an inefficient mass2 (or current4) scaling regime at much higher masses than predicted by our earlier work, which was based on η≳3.7 calculations. Thus, more energy than predicted by the earlier scaling laws will apparently be needed to obtain significant amounts of, for example, krypton K-shell emission. In addition, lags in ionization times become important increasingly with the Z of the plasma and adversely affect K-shell emission. The physics responsible for these time dependent effects and how they will influence the design of future experiments are discussed.
Study of oxygen chemisorption on the GaN(0001)-(1×1) surface
Authors: Bermudez V. M.
Published: July 2002

Clean, ordered GaN(0001)-(1×1) surfaces are prepared by sputtering with nitrogen ions followed by annealing in ultrahigh vacuum. The surfaces are subsequently exposed at room temperature to O2 and the chemisorption process studied using Auger, valence and core-level photoemission and electron energy loss spectroscopies, low-energy electron diffraction, and work function measurements. Saturation occurs at a coverage of Θox=0.4 ML and is accompanied by the removal of surface states near the band edges. The continued presence of a clear (1×1) diffraction pattern, together with other data, indicates a well-defined adsorption site, but the relative importance of Ga–O and N–O bonding remains undetermined. The realization that surface states exist near the valence-band maximum has led to a more accurate determination of the surface Fermi-level pinning position, and of dependent quantities, than given previously. Clean-surface data are also compared with those for surfaces prepared by in situ deposition of Ga metal followed by thermal desorption. No significant differences are seen, which suggests that nitrogen-ion sputtering and annealing is suitable for preparing clean, ordered GaN(0001)-(1×1) surfaces. The results for O chemisorption on atomically clean surfaces have been applied to evaluating the passivation of surfaces prepared by ex situ wet-chemical cleaning. The band bending is found to be ∼0.5 eV less than on atomically clean surfaces.
Evidence for multipolar charge distribution in falling water drops
Authors: Fornés José A. , Procopio Joaquim , Sartorelli José C.
Published: July 2002

A method for producing water drops with very small net charge (few fC) was employed in conjunction with a sensing device, in order to identify the electric signals produced by falling water drops. The sensing device consists of a wire loop hooked to a patch-clamp amplifier set into voltage-clamp mode. Water drops with very small net charge produce a negligible signal at the sensing device but when falling through a horizontal charged capacitor they produce well-defined and reproducible signals which are proportional to the capacitor voltage. The experimental curves can be fitted to the theoretical ones considering the drop as possessing a multipolar charge distribution. There is evidence that this distribution is due to the contribution of the field-oriented water molecules multipoles.
Characterization of thick lead zirconate titanate films fabricated using a new sol gel based process
Authors: Barrow D. A. , Petroff T. E. , Tandon R. P. , Sayer M.
Published: July 2002

Lead zirconate titanate (PZT) films 60 μm in thickness have been fabricated using a new sol gel based process. PZT powders are dispersed in a sol gel matrix to form a 0–3 ceramic/ceramic composite. The dielectric properties of these films have been studied as a function of powder concentration, frequency, and temperature. The characteristic Curie point is observed at 420 °C. The ferroelectric behavior measured in terms of the remanant polarization (Pr=35 μC/cm2) and coercive field (Ec=20 kV/cm) was an improvement over values quoted for thin PZT films but lower than that of bulk ceramic. The piezoelectric properties d33 (325 pC/N) and d31 (−80 pC/N) were comparable with those of the bulk ceramic.
High coercivity nanostructured networks
Authors: Barnard J. A. , Butera A. , Fujiwara H. , Inturi V. R. , Jarratt J. D. , Klemmer T. J. , Scharr T. W. , Weston J. L.
Published: July 2002

Films of Fe, Co, and Co35Fe65 alloy have been sputter deposited onto the surface of porous nanochannel alumina substrates producing nanostructured contiguous magnetic “networks.” Large room temperature coercivities have been measured which approximately scale with the bulk saturation magnetization of the material used. In the as-deposited state the largest coercivities (e.g., >1000 Oe for Co35Fe65) are observed in ∼15-nm-thick networks deposited on the smallest commercially available pore size substrates (those having an average pore diameter of ∼20 nm and a wall thickness of ∼15 nm). Preliminary studies of the effects of annealing indicate that coercivities can be substantially increased (by as much as ∼50%) with an appropriate post-deposition thermal treatment.
Three-dimensional scattering by an infinite homogeneous anisotropic circular cylinder: An analytical solution
Authors: Wu Xin Bao , Yasumoto Kiyotoshi
Published: July 2002

Closed form expressions for the electromagnetic fields in homogeneous anisotropic media are derived in terms of the circular cylindrical vector wave functions for isotropic media. An analytical solution is obtained for the problem of scattering by an infinite homogeneous anisotropic circular cylinder for an obliquely incident plane wave of an arbitrary linear polarization. Numerical implementation and convergence behavior of this solution is discussed. Numerical results for some special cases are compared with existing data. We also present numerical results for the more general anisotropic material case in both graphical and tabular forms. The formulation of this article can be easily generalized to the cases of more complex cases, such as other complex media cases whose eigenwave numbers and eigenwave vectors can be determined, layered structures cases, and multiscatterers cases.
Thermodynamic cycles in oscillating flow regenerators
Authors: Liang J.
Published: July 2002

A sinusoidal model is proposed to analyze the thermodynamic processes of gas parcels oscillating in a regenerator. The regenerator is considered equivalent to a bundle of capillary tubes with large thermal capacities. With perfect thermal contact between the gas and tube wall and the proper phase shift between velocity and pressure, the overall effect of a gas parcel oscillating in the capillary in one cycle is that it absorbs heat from one lengthwise part of the wall and delivers heat to the other part of the wall within its range of displacement. Each gas parcel working in the regenerator undergoes a thermodynamic cycle. When the work flow is in the same direction as the temperature gradient of the regenerator, it undergoes a heat engine cycle. Otherwise it undergoes a refrigeration cycle. The regenerator can work as a heat engine or as a refrigerator.
Photo-illumination hardening of C60 crystals
Authors: Tachibana M. , Sakuma H. , Kojima K.
Published: July 2002

The effect of light on the hardness of C60 crystals was investigated. The hardness increased after the crystals were illuminated with white light. Such a photo-illumination hardening was effective in a temperature range of 298–380 K. The photopolymerized C60 molecules near the crystal surface were responsible for the hardening. The anomalous temperature dependence of the hardness at temperatures higher than 300 K, which we previously have observed, is explained as due to the photo-illumination hardening.
Conceptual advantages and limitations of the Laguerre polynomial approach to analyze surface acoustic waves in semi-infinite substrates and multilayered structures
Authors: Lefebvre J. E. , Zhang V. , Gazalet J. , Gryba T.
Published: July 2002

Laguerre polynomial approach is used to calculate velocities, coupling coefficients, and field distributions of surface acoustic waves in both semi-infinite substrates and multilayered structures. Approximation used in boundary conditions for applying the Laguerre polynomial method is investigated. Effectiveness of the method to calculate the coupling coefficient is checked. Its capabilities with regard to the various types of waves are reviewed. It is shown that Laguerre polynomial method cannot be used to study leaky surface acoustic waves. For true surface acoustic waves it can, but for deep penetration, computing time becomes prohibitive. Laguerre polynomial method is interesting for true surface acoustic waves modes if decay length is of the order of one acoustic wavelength: velocities, coupling coefficient and, continuous field profiles are quite well returned. However, for multilayered structures, very dissimilar parameters of the constituent materials give rise to significant field level discontinuities at the interfaces. It is shown that the Laguerre polynomial method does not return, within reasonable computing time and required memory, these discontinuities of which multiple quantum well-based devices can take advantage.
Structural and optical investigation of InAsxP1−x/InP strained superlattices
Authors: Lamberti C. , Bordiga S. , Boscherini F. , Mobilio S. , Pascarelli S. , Gastaldi L. , Madella M. , Papuzza C. , Rigo C. , Soldani D. , Ferrari C. , Lazzarini L. , Salviati G.
Published: July 2002

We report a complete characterization of InAsxP1−x/InP (0.05<x<0.59) superlattices epitaxially grown by low pressure metalorganic chemical vapor deposition and by chemical beam epitaxy. Samples were obtained by both conventional growth procedures and by periodically exposing the just-grown InP surface to an AsH3 flux. Using the latter procedure, very thin InAsxP1−x/InP layers (10–20 Å) are obtained by P↔As substitutions effects. Arsenic composition of the so obtained layers depends both on AsH3 flux intensity and exposure times. Samples have been characterized by means of high resolution x-ray diffraction, high resolution transmission electron microscopy, 4 K photoluminescence, and extended x ray absorption fine structure spectroscopy. The combined use of high resolution x-ray diffraction and of 4 K photoluminescence, with related simulations, allows us to predict both InAsP composition and width, which are qualitatively confirmed by electron microscopy. Our study indicates that the effect of the formation of thin InAsP layers is due to the As incorporation onto the InP surface exposed to the As flux during the AsH3 exposure, rather than the residual As pressure in the growth chamber during InP growth. Arsenic K-edge extended x-ray absorption fine structure analysis shows that the first shell environment of As at these interfaces is similar to that found in bulk InAsxP1−x alloys of similar composition. In particular we measure an almost constant As–In bond length (within 0.02 Å), independent of As concentration; this confirms that epitaxy with InP is accompanied by local structural distortions, such as bond angle variations, which accommodate the nearly constant As–In bond length. In our investigation we characterize not only very high quality heterostructures but also samples showing serious interface problems such as nonplanarity and/or a consistent chemical spread along the growth axis. In the study presented here we thus propose a general method, based on several independent techniques, for the characterization of the interface quality of semiconductor superlattices.
Temperature dependencies of linewidths, positions, and line shifts of spectral transitions of trivalent neodymium ions in barium magnesium yttrium germanate laser host
Authors: Sardar Dhiraj K. , Stubblefield Steven C.
Published: July 2002

The spectral widths, positions, and shifts of the sharp emission lines for the R1→X5 and R2→Y3 inter-Stark transitions within the respective intermanifold transitions of F3/24→I9/24 and F3/24→I11/24 of Nd3+ ions in Ba0.25Mg2.75Y2Ge3O12 laser host have been investigated as a function of temperature. The linewidths of these transitions increased as the temperature of the host crystal was increased. The emission line for the R1→X5 transition shifted toward the shorter wavelength (blue shift), while the emission line for the R2→Y3 transition shifted toward the longer wavelength (red shift) with increasing temperature. The experimental results of the temperature-dependent linewidths and line shifts of these transitions are explained in light of the existing theory of the phonon-ion interaction in crystalline solids.
Simulation studies of electrorheological fluids under shear, compression, and elongation loading
Authors: Lukkarinen A. , Kaski K.
Published: July 2002

Mechanical properties of electrorheological fluids under various dynamical loading conditions have been studied using a computer simulation model. The model assumes electrostatic point–dipole interaction between particles with or without multipolar corrections and the interaction with the base fluid due to viscous laminar flow is described with Stokes drag. Mechanical loading is introduced as constant rate shear, compression or elongation to a system of particles set initially to a single chain, a column of body centered tetragonal (BCT) unit cells, a thick BCT structure or to a structure grown with electric field from originally random configuration. Results show that the single chain structure has usually the highest relative strength. Electrorheological systems under compressive loading were found to transmit the largest force from one plate to another. Under elongation loading a thick BCT structure seemed surprisingly weak compared with the system under compression or shear. In addition, the response of a BCT structure to sinusoidally alternating shear or tensile straining has been studied. Under tensile loading it was found that the ability of the system to transfer force is much more dependent on oscillation frequency than under shear loading.
Photoluminescence of Eu3+ ion doped phosphate ceramics
Authors: Cao Zhicheng , Lee Burtrand I. , Samuels William D. , Exarhos Gregory J.
Published: July 2002

Phosphate ceramics doped with Eu3+ ions were synthesized through sol-gel process. The fluorescence spectra of Eu3+ as a function of firing temperature, dopant concentration, composition, and structure of the matrices were investigated. The gels synthesized by the reaction of P2O5 with tetraethoxy silane or titanium tetraethoxide (Ti(OEt)4) and EuCl3⋅6H2O as the dopant were fired in air from the temperature range 200–900 °C to form P–Si or P–Ti crystalline phases. The crystal structure was examined by powder x-ray diffractometry. Si5O(PO4)6 was the only crystalline phase for the P–Si system. The amount of doping Eu3+ varied from 1 up to 10 at %, and no obvious concentration quenching was observed in the concentration range. It was found that the phosphosilicates gave stronger fluorescence emission than the phosphotitanates. For both systems, the intensity of Eu3+ emission increased with higher firing temperatures. For the P–Si system, the emission intensity increased with greater amounts of phosphorus, while the emission intensity did not change with the variation of composition in the P–Ti system.
Transparent, metallo-dielectric, one-dimensional, photonic band-gap structures
Authors: Scalora M. , Bloemer M. J. , Pethel A. S. , Dowling J. P. , Bowden C. M. , Manka A. S.
Published: July 2002

We investigate numerically the properties of metallo-dielectric, one-dimensional, photonic band-gap structures. Our theory predicts that interference effects give rise to a new transparent metallic structure that permits the transmission of light over a tunable range of frequencies, for example, the ultraviolet, the visible, or the infrared wavelength range. The structure can be designed to block ultraviolet light, transmit in the visible range, and reflect all other electromagnetic waves of lower frequencies, from infrared to microwaves and beyond. The transparent metallic structure is composed of a stack of alternating layers of a metal and a dielectric material, such that the complex index of refraction alternates between a high and a low value. The structure remains transparent even if the total amount of metal is increased to hundreds of skin depths in net thickness.
The effect of radio frequency plasma processing reactor circuitry on plasma characteristics
Authors: Rauf Shahid , Kushner Mark J.
Published: July 2002

Past experiments have demonstrated that details of the external electrical circuitry can strongly influence the performance of radio frequency (rf) plasma processing reactors. Seemingly minor changes in the circuit, such as changing cable lengths, can lead to significantly different plasma characteristics. To investigate these couplings, a plasma equipment model has been developed which consists of a linked reactor simulation and a circuit model. In this hierarchy the results of the reactor simulation are periodically used in the circuit model to construct a simple representation of the plasma consisting of sheaths and resistors which are connected to the external circuit. Voltages (dc, fundamental and harmonics) and currents at all electrodes and reactor surfaces are computed, and are then employed as boundary conditions for the plasma reactor simulation. The models were used to investigate the effects of operating conditions, reactor geometry and stray coupling on the electrical characteristics of asymmetric capacitively coupled rf discharges. It was found that nonlinear sheaths lead to voltages and currents that have significant amplitudes at higher harmonics. As a consequence, external circuits that may appear identical to the plasma at the fundamental frequency may produce different plasma characteristics. Since plasmas are generally nonlinear and the combined plasma and circuit impedance is usually reactive, it was found that the voltage (or power) at the supply is not simply related to the voltages on electrode surfaces which generate the plasma.
Influence of pressures on the crystallization process of an amorphous Fe73.5Cu1Nb3Si13.5B9 alloy
Authors: Zhang X. Y. , Zhang F. X. , Zhang J. W. , Yu W. , Zhang M. , Zhao J. H. , Liu R. P. , Xu Y. F. , Wang W. K.
Published: July 2002

Amorphous Fe73.5Cu1Nb3Si13.5B9 alloys, prepared by a melt-spinning technique, were annealed at a temperature of 823 K under pressures in the range of 1–5 GPa and ambient pressure. The high pressure experiments were carried out in a belt-type pressure apparatus. The microstructure of the annealed alloys has been investigated by x-ray diffraction, electron diffraction, and transmission electron microscopy. Experimental results show that the initial crystalline phase in these annealed alloys is α-Fe solid solution (named as α-Fe phase below), and high pressure has a great influence on the crystallization process of the α-Fe phase. The grain size of the α-Fe phase decreases with the increase of pressure (P). The volume fraction of the α-Fe phase increases with increasing the pressure as the pressure is below 2 GPa, and then decreases (P>2 GPa). The mechanism for the effects of the high pressure on the crystallization process of amorphous Fe73.5Cu1Nb3Si13.5B9 alloy and latent applications of high-pressure annealing amorphous Fe73.5Cu1Nb3Si13.5B9 alloy have been discussed.
Wavelength dependence of the photoinduced indentation hardening in C60 crystals
Authors: Tachibana M. , Kojima K. , Sakuma H. , Komatsu T. , Sunakawa T.
Published: July 2002

The photoinduced indentation hardening of C60 crystals was investigated in ultraviolet-visible light. The hardening strongly depended on the wavelength of the illumination light: the magnitude increased with increasing wavelength, and then reached a maximum at 700 nm. This indicated that red light is more effective than ultraviolet light in the hardening. The wavelength dependence of the hardening is discussed in terms of an interplay between the effective depth of the photoinduced polymerization and the penetration depth of the indenter.
Pressure as a probe of the glassy properties of compositionally disordered soft mode ferroelectrics: (Pb0.82La0.12)(Zr0.40Ti0.60)O3 (PLZT 12/40/60)
Authors: Samara G. A.
Published: July 2002

It is shown that hydrostatic pressure qualitatively changes the dielectric response of a lanthanum-modified, lead zirconate–titanate (PLZT 12/40/60). At 1 bar this material exhibits, on cooling, a very weak relaxor (R) response followed by a spontaneous R-to-normal ferroelectric (nFE) transition. Pressure induces crossover to full relaxor behavior and reveals the continuous evolution of the dynamics and energetics of the relaxation process. These results, which are similar to our earlier results on other disordered perovskites, appear to be general features of soft mode ferroelectrics with random site dipolar impurities or polar nanodomains. They can be understood in terms of a large decrease with pressure in the correlation radius among polar nanodomains—a unique property of soft ferroelectric mode systems. An analogy between the influences of increasing pressure and increasing La concentration on the dielectric response of PLZTs is demonstrated, but it is the pressure results that clarify the physics.
Structure, magnetic, and magneto-optical properties of MnBi films grown on quartz and (001)GaAs substrates
Authors: Harder K.-U. , Menzel D. , Widmer T. , Schoenes J.
Published: July 2002

MnBi films are prepared on quartz and on (001) orientated GaAs substrates by molecular beam epitaxy in ultrahigh vacuum environment. Both kinds of substrates are used simultaneously. The influence of the substrate material is investigated with respect to the structural, the magnetic, and magneto-optical properties of the MnBi films. By evaporating a 100 nm thick SiOx buffer layer the homogeneity of the composition is improved and a grain size of about 100 nm is achieved without adding other elements. In contrast to previous investigations, our measured magneto-optical Kerr rotation spectra show no Kerr rotation peak near 3.35 eV. These results confirm theoretical predictions whereupon this peak is attributed to oxygen which occupies interstitial sites in the regular MnBi lattice.
Analysis of the temperature evolution from the time resolved thermo-optical interferometric measurements with few Fabry–Pérot peaks
Authors: Pogany D. , Seliger N. , Gornik E. , Stoisiek M. , Lalinský T.
Published: July 2002

A mathematical method is proposed to calculate the temperature evolution in semiconductor layers from the measurements of Fabry–Pérot (FP) interferences in the reflected or transmitted intensity of a probing laser beam. The changes in the optical intensity are caused by the temperature induced changes in the refractive index and thermal expansion of the semiconductor layer. The method is particularly suitable in cases where the intensity curve exhibits few (at least two) FP intensity extrema. The unknown temperature evolution is obtained from a comparison of mathematical representations of the intensity–time and intensity–temperature dependences and using a symmetry property of the FP intensity–temperature function around the intensity extremum. Expressions for polynomial and exponential approximations of the temperature evolution are given together with empirical rules to maximize the accuracy of output parameters as thermal time constant, polynomial expansion coefficients, and temperature amplitudes. The applicability of the method is demonstrated by time resolved optical reflectivity measurements on semiconductor devices with the active layer forming a FP resonator: smart power devices prepared by silicon-on-insulator technology and power sensors fabricated on GaAs micromachined cantilevers. The temperature evolution in the former and latter devices is studied in the μs and ms time scale up to the temperature increase of 200 and 350 K, respectively. The relative error in both the extracted temperature evolution and time constants is about 15%.
Positron annihilation investigation of porous silicon heat treated to 1000 °C
Authors: Dannefaer S. , Wiebe C. , Kerr D.
Published: July 2002

Positron lifetime and Doppler broadening spectroscopies were applied to investigate a porous silicon film subjected to heat treatment in an argon atmosphere. Heating between 300 and 500 °C increased the mass of the film by 17% due to oxygen uptake and the concentration of open volume defects associated with the formation of an oxide layer on the silicon nanocrystallites increased by a factor of 3. Between 600 and 1000 °C their concentration decreased gradually to 1/2 the original concentration. Doppler broadening results indicate two distinct electron momentum distributions, one arising from open volume defects and one from pickoff annihilation of positronium at the pore walls caused by electrons with an unexpectedly narrow momentum distribution.
Remanent magnetization plots of fine particles with competing cubic and uniaxial anisotropies
Authors: Geshev J. , Mikhov M. , Schmidt J. E.
Published: July 2002

Magnetization and remanent magnetization curves for noninteracting single-domain particles whose anisotropy is made up of a cubic magnetocrystalline and an uniaxial components were investigated. For a given cubic anisotropy, the saturation remanence, coercivity, remanence coercivity, and coercivity factor values have been obtained for several different uniaxial anisotropy directions. The corresponding δM plots have been constructed for initially thermal or ac demagnetized systems, and a great variety of their shapes has been obtained when the uniaxial anisotropy does not dominate. Coercivity factor values rather higher than those for pure uniaxial or cubic anisotropy particles have been obtained as well, even for already dominating uniaxial anisotropy for some uniaxial anisotropy directions. Thus, it has been shown that one may not safely draw conclusions about interactions from deviations from the zero line in the δM plots or coercivity factor values for particles with competing anisotropies without specific analysis for any particular case.
Morphology of Si nanowires synthesized by high-temperature laser ablation
Authors: Tang Y. H. , Zhang Y. F. , Wang N. , Lee C. S. , Han X. D. , Bello I. , Lee S. T.
Published: July 2002

Silicon nanowires have been synthesized by laser ablation of Si powder targets at 1200 °C. Transmission electron microscopy study showed that most Si nanowires had smooth surfaces and nearly the same diameter of about 16 nm. Beside the most abundant smooth-surface nanowires, four other forms of nanowires, named spring-shaped, fishbone-shaped, frog-egg-shaped, and necklace-shaped nanowires, were observed. The formation of nanowires into different shapes was explained by the two-step growth model based on the vapor–liquid–solid mechanism.
Electrical response during indentation of piezoelectric materials: A new method for material characterization
Authors: Sridhar S. , Giannakopoulos A. E. , Suresh S. , Ramamurty U.
Published: July 2002

The electrical responses of piezoelectric materials subjected to spherical microindentation are evaluated. Theoretical analysis based on normal indentation of a transversely isotropic, linear piezoelectric solid by a conducting steel sphere with zero potential bias is compared to experimental results. The materials considered are PZT-4 and (Ba0.917Ca0.083)TiO3. Effects of poling, poling direction, indentation velocity, and polarization loss due to annealing were investigated. All the basic trends predicted by the theory are confirmed by the experiments. The current induced into the indentor due to the polarized layer on the contact surface of the piezoelectric specimen increases with time as the contact area increases. Experimentally, observed power dependence of current to indentation velocity is close to the theoretical value of 1.5. The relation between induced current and indentation time is specific to the properties of the material. It is demonstrated that in addition to some of the material constants, the poling direction and the aging behavior of piezoelectric ceramics can be determined by spherical indentation.
Fe
57
Mössbauer spectroscopic investigation of complex magnetic structures in Ga, Sc, and In substituted M-type hexagonal ferrites
Authors: Clark T. M. , Evans B. J. , Thompson G. K. , Freeman S.
Published: July 2002

Investigation of preliminary evidence that Ga, Sc, and In influence the bulk and dynamic magnetic properties of hexagonal ferrites by means other than those supported in existing models has been undertaken. In SrFe12−xMxO19 (M=Ga, In) and BaFe12−xScxO19, the predominant 12k sublattice exhibits a remarkable splitting into two distinct subpatterns, 12k1 and 12k2: At x=0.3, hyperfine fields, Heff for 12k1 are 411, 408, and 405 kOe, respectively, and for 12k2 are 358, 339, and 311 kOe, respectively. The 12k1 hyperfine field values are virtually unchanged from that of the pure hexagonal ferrites and are independent of substitution level. Contrastingly, the abruptness of the drop in the 12k2 hyperfine field and its dependence on the nature of the substituting cation are remarkable. The relative intensity of the 12k2 component correlates with the concentration of nonmagnetic species on the 2b and 4f2 sites and with the magnetic anisotropy. Scandium seems to have a more profound influence on the magnetic structure and interactions than indium or gallium. Further, at technically significant substitution levels, Heff of the different sublattices exhibit broad and overlapping distributions of values far removed from their distinctiveness in the pure hexaferrites. Thus, the net magnetization of Ga, Sc, and In-doped hexaferrites results from a complex interplay of magnetic dilution on the 2b site, enhancement of the magnetization through substitutions on the 4f2 site, and a complex influence from the substitution-induced 12k2 sublattice.
Coercivity analysis in sputtered Sm–Co thin films
Authors: Prados C. , Hernando A. , Hadjipanayis G. C. , González J. M.
Published: July 2002

Hard nanocrystalline Sm–Co thin films having a thickness of 500 nm have been prepared by heat-treating magnetron sputtered amorphous samples deposited over a 300 nm Cr buffer layer onto room temperature water cooled Si substrates. The thermal treatments resulted in the nanocrystallization of the precursor films and in the development of room temperature coercivities covering a range going from a few kOe, measured in the samples treated at low temperature, up to some tens of kOe, observed in the case of the films annealed at high temperatures (the room temperature coercivity of the sample treated at 825 K was 26 kOe). Our analysis of the influence of the thermal treatments on the particularities of the reversal process was based on the measurement of the temperature dependence of the coercive force. It was carried out in the framework of the micromagnetic model, which allowed us to evaluate two parameters accounting for the local anisotropy reduction and the magnitude of the local dipolar fields, respectively. Our results show that the high temperature treatments result in a better degree of crystallization (lower local anisotropy deterioration). That crystallinity improvement is associated to an increase of the local dipolar interactions.
Simulation of magnetization reversal in polycrystalline patterned Co elements
Authors: Schrefl T. , Fidler J. , Kirk K. J. , Chapman J. N.
Published: July 2002

The influence of the polycrystalline microstructure on the switching mechanisms of acicular shaped Co elements was investigated using finite element micromagnetics. The Gilbert equation of motion with a Gilbert damping constant α=1 was solved using a semi-implicit time integration scheme. The elements were 200×40 nm2 and 25 nm thick. The grain size is approximately 8 nm, leading to edge irregularities of the same size. The competitive effects of the shape anisotropy and the random, magnetocrystalline anisotropy lead to a magnetization ripple structure with a wavelength of about 100 nm for zero applied field. With increasing applied field, the magnetization ripple becomes more pronounced. At an applied field of 95 kA/m a vortex, originally formed near sharp edge irregularities, moves into the width of the element. The vortex reaches the opposite edge after about one ns. Then a transverse domain structure of a head to head domain wall forms and the reversed domain expands through the entire particle. The magnitude of magnetostatic interaction field for an array of elements with 50 nm gaps was found to be in the range of 8–20 kA/m. The calculated switching field and the magnetostatic interaction field agree well with in situ magnetization experiments in a transmission electron microscope.
Micro-Raman characterization of crystallinity of laser-recrystallized silicon films on SiO2 insulators
Authors: Mizoguchi K. , Nakashima S. , Sugiura Y. , Harima H.
Published: July 2002

The crystallinity of laser-recrystallized Si films on insulators (SOI films) was characterized by micro-Raman imaging. A small-angle bevel made by angle lapping of the SOI film was used to probe the structure at different depths. The Raman signals that varied with the position along the tilt show that interference of both the incident and scattered light induced in the angle-lapped specimens results in periodic enhancements in the intensity as a function of film thickness. An analysis of the fringe patterns in the Raman images provides us with depth profiles of strain and structural disorders with a high depth resolution on a scale of a few tens of nanometers. When moving away from the interface between the silicon film and the insulator, the peak frequency of the polycrystalline silicon band shifts to the lower frequency side and the band width becomes smaller. The depth profile of the Raman feature shows that the defect density is high in the region near the interface of Si and SiO2, and that the stress increases toward the top surface of the silicon film. Moreover, a partial relaxation of the stress occurs near the interface region due to the generation of a high density of defects.
Lorentz transmission electron microscope study of ferromagnetic domain walls in SrRuO3: Statics, dynamics, and crystal structure correlation
Authors: Marshall A. F. , Klein L. , Dodge J. S. , Ahn C. H. , Reiner J. W. , Mieville L. , Antagonazza L. , Kapitulnik A. , Geballe T. H. , Beasley M. R.
Published: July 2002

The magnetic microstructure of SrRuO3 thin films is studied using Lorentz transmission electron microscopy (TEM). The zero-field-cooled magnetic stripe structure shows one-to-one correlation with the crystal domain structure and is used to identify the uniaxial magnetic anisotropy of these films, consistent with results on single domain films. The anisotropy is shown to be primarily magnetocrystalline in nature with the b axis as the easy axis. Lorentz TEM also yields quantitative information about the domain structure and domain wall resistivity, and allows for in situ observation of domain wall formation and dynamic response to an applied field.
Electric field induced surface modification of Au
Authors: Mayer T. M. , Houston J. E. , Franklin G. E. , Erchak A. A. , Michalske T. A.
Published: July 2002

We discuss the role of localized high electric fields in the modification of Au surfaces with a W probe using the interfacial force microscope. Upon bringing a probe close to a Au surface, we measure both the interfacial force and the field emission current as a function of separation with a constant potential of 100 V between tip and sample. The current initially increases exponentially as the separation decreases. However, at a distance of less than ∼500 Å, the current rises sharply as the surface begins to distort and rapidly close the gap. Retraction of the tip before contact is made reveals the formation of a mound on the surface. We propose a simple model, in which the localized high electric field under the tip assists the production of mobile Au adatoms by detachment from surface steps, and a radial field gradient causes a net flux of atoms toward the tip by surface diffusion. These processes give rise to an unstable surface deformation which, if left unchecked, results in a destructive mechanical contact. We discuss our findings with respect to earlier work using voltage pulses in the scanning tunneling microscope as a means of nanofabrication.
Pressure and temperature effects on optical transitions in cubic GaN
Authors: Liu Z. X. , Goñi A. R. , Syassen K. , Siegle H. , Thomsen C. , Schöttker B. , As D. J. , Schikora D.
Published: July 2002

Pressure and temperature effects on optical transitions in cubic GaN grown on a GaAs substrate have been studied by photoluminescence (PL) spectroscopy at hydrostatic pressures up to 9 GPa (10 K) and as a function of temperature (10–300 K) at ambient pressure. The dominant emissions at 10 K and ambient pressure are assigned to the bound-exciton transition (zero-phonon line), the donor-acceptor-pair (DAP) emission, and, tentatively, to the first three LO-phonon replicas of the bound exciton. These PL features shift to higher energy with increasing pressure. The pressure coefficients indicate that the observed recombination processes involve states which are closely related to the band edges. Temperature-induced evolutions from bound to free-exciton (FE) transition and DAP emission to free-to-bound transition are resolved. The binding energies of the FE and donor and acceptor levels in cubic GaN have been determined from the temperature and power-density dependence of the PL emission energies.
Thermally induced changes in the optical properties of SiNx:H films deposited by the electron cyclotron resonance plasma method
Authors: Martı́nez F. L. , del Prado A. , Mártil I. , González-Dı́az G. , Selle B. , Sieber I.
Published: July 2002

We analyze the effect of thermal processes on the optical properties (refractive index, optical gap, Tauc coefficient, and Urbach energy) of SiNx:H films. Films with three different nitrogen to silicon ratios (x=0.97, x=1.43, and x=1.55, respectively) were deposited by a chemical vapor deposition technique assisted by an electron cyclotron resonance generated plasma. After deposition they were subjected to rapid thermal annealing at temperatures ranging from 300 °C to 1050 °C. We found that the percolation threshold for Si–Si bonds (at x=1.1) separates films with different response to thermal treatments. The changes of the Tauc coefficient and the Urbach energy at moderate annealing temperatures indicate a structural relaxation of the network for the films with x above the percolation threshold, while at higher temperatures the trends are inverted. In the case of x below the percolation limit the inversion point is not observed. These trends are well correlated with the width of the Si–N infrared stretching absorption band. Additionally the samples with as-grown x=1.43 show a good correlation between the Urbach energy and the density of unpaired spins in silicon dangling bonds.
Coherence-enhanced synchrotron radiology: Refraction versus diffraction mechanisms
Authors: Hwu Y. , Hsieh H. H. , Lu M. J. , Tsai W. L. , Lin H. M. , Goh W. C. , Lai B. , Je J. H. , Kim C. K. , Noh D. Y. , Youn H. S. , Tromba G. , Margaritondo G.
Published: July 2002

Tests performed in different regimes reveal the interplay of two edge-enhancement mechanisms in radiological images taken with coherent synchrotron light. The relative weight of the two mechanisms, related to refraction and to Fresnel edge diffraction, can be changed in a controlled way. This makes it possible to obtain different images of the same object with complementary information.
High-quality off-stoichiometric YBa2Cu3O7−δ films produced by diffusion-assisted preferential laser ablation
Authors: Huijbregtse J. M. , Dam B. , Rector J. H. , Griessen R.
Published: July 2002

The fluence dependence of the composition of pulsed-laser deposited YBa2Cu3O7−δ films is investigated and interpreted in terms of laser-induced target modification. Both target degradation (at fluence J<1.0 J/cm2) and diffusion-assisted preferential ablation (1.0<J<1.3 J/cm2) are found to be responsible for nonstoichiometric transfer. A one-dimensional, moving-boundary diffusion model is developed to describe diffusion-assisted preferential ablation. This model predicts stoichiometric transfer at large ablation rates. Indeed, for J≫1.3 J/cm2 stoichiometric deposition is found, resulting in precipitate-free films. However, slightly off-stoichiometric films, deposited in the diffusion-assisted preferential ablation regime, exhibit the best superconducting properties (Tc=91.0 K, ΔTc=0.4 K) and can be produced with a remarkably high reproducibility.
Photoluminescence of CuGaS2 epitaxial layers grown by metalorganic vapor phase epitaxy
Authors: Shirakata Sho , Chichibu Shigefusa
Published: July 2002

High quality epitaxial layers of CuGaS2 have been grown on GaAs and GaP substrates by low-pressure metalorganic vapor phase epitaxy using cycropentadienylcoppertriethylphosphine, normal-tripropylgallium, and ditertiallybutyl sulphide precursors. Photoluminescence (PL) studies at 8 K have been carried out, and exciton-related PL peaks at 2.489 and 2.477 eV have been observed in addition to PL peaks at 2.43 (free-to-bound), 2.39 (donor-acceptor pair), and broad band at 1.65 eV. Temperature and excitation intensity dependences of the PL peaks have been studied in detail, and PL properties are compared with those for the bulk CuGaS2 crystals grown by the chemical vapor transport (CVT) method. PL spectra for nearly stoichiometric CuGaS2 epilayers exhibited the intense free exciton-related PL peak over the entire temperature range (8–300 K), which are in contrast to PLs dominated by defect-related deep peaks for Ga-rich or Cu-rich epilayers and the impurity related near-band edge PL for the CVT bulk crystal. These PL results show that the nearly stoichiometric CuGaS2 epilayers grown in this study are the highest quality reported so far.
Interlayer coupling within individual submicron magnetic elements
Authors: Smith David J. , Dunin-Borkowski R. E. , McCartney M. R. , Kardynal B. , Scheinfein M. R.
Published: July 2002

The interlayer coupling and magnetization reversal of patterned, submicron Co/Au/Ni nanostructures, shaped as diamonds, ellipses, and rectangles, have been investigated using off-axis electron holography and micromagnetic simulations. Antiferromagnetic coupling between the ferromagnetic layers, attributed to the strong Co demagnetization field, was visualized directly. Simulated hysteresis loops overall showed reasonable agreement with the experimental results. Local structural imperfections may be responsible for small discrepancies between the observed magnetization states of the patterned elements and the simulations.