bluuebunny/crossref_metadata_2025 huggingface dataset:
Laser assisted associative desorption of N2 and CO from Ru(0001)
Authors: Diekhöner L. , Mortensen H. , Baurichter A. , Luntz A. C.
Published: July 2002

An experimental technique, laser assisted associative desorption (LAAD), is described for determining adiabatic barriers to activated dissociation at the gas-surface interface, as well as some aspects of the dynamics of associative desorption. The basis of this technique is to use a laser induced temperature jump (T-jump) at the surface to induce associative desorption and to measure the translational energy distribution of the desorbing molecules. The highest translational energies observed in desorption are a lower bound to the adiabatic barrier and the shapes of the translational energy distributions provide information on the dynamics. Implementation of the experimental technique is described in detail and unique advantages and possible limitations of the technique are discussed. The application of this technique to very high barrier surface processes is described; associative desorption of N2 from Ru(0001) and CO formed by C+O and C2+O on Ru(0001). N2 barriers to dissociation increases strongly with N coverage and co-adsorbed O, in good agreement with DFT calculations. No isotope effects are seen in the associative desorption, indicating that tunneling is not important. The full energy distributions suggest that very large energy loss to the lattice occurs after recombination at the high barrier and prior to N2 desorption into the gas phase. The mechanism for this remarkably large energy loss is not well understood, but is likely to be general for other high barrier associative desorption reactions. CO associatively desorbs nearly thermally from both C+O and C2+O associative reactions. It is argued that this is due to large energy loss for this system as well, followed by indirect scattering in the deep CO molecular well before final exit into the gas phase.
Characterization of ArnCl(−) clusters (n=2–15) using zero electron kinetic energy and partially discriminated threshold photodetachment spectroscopy
Authors: Lenzer Thomas , Yourshaw Ivan , Furlanetto Michael R. , Pivonka Nicholas L. , Neumark Daniel M.
Published: July 2002

Ar n Cl − clusters have been investigated by anion zero electron kinetic energy (ZEKE) and partially discriminated threshold photodetachment spectroscopy. The experiments yield size-dependent electron affinities (EAs) and electronic state splittings for the X, I, and II states accessed by photodetachment. Cluster minimum energy structures have been determined from calculations based on a “simulated annealing” approach employing our recently presented Ar–Cl(−) pair potentials from anion ZEKE spectroscopy [T. Lenzer, I. Yourshaw, M. R. Furlanetto, G. Reiser, and D. M. Neumark, J. Chem. Phys. 110, 9578 (1999)] and various nonadditive terms. The EAs calculated without many-body effects overestimate the experimental EAs by up to 1500 cm−1. Repulsive many-body induction in the anion clusters is found to be the dominant nonadditive effect. In addition, the attractive interaction between the chloride charge and the Ar2 exchange quadrupole is important. These findings are consistent with our earlier results for XenI−, ArnI−, and ArnBr− clusters and highlight again the necessity of an adequate implementation of many-body effects to describe the energetics of such systems. For ArnCl− clusters with n>12 we find some deviations between experimental and calculated (0 K) EA which can be explained by the population of less stable anion structures due to the finite temperatures of the clusters in our experiments. This results in lower EAs than predicted for the corresponding global minimum energy structures.
Formation of coupled quantum dots in single-wall carbon nanotubes
Authors: Ishibashi K. , Suzuki M. , Ida T. , Aoyagi Y.
Published: July 2002

The triple-barrier coupled quantum dots have been fabricated in individual single-wall carbon nanotubes by depositing a narrow SiO2 layer in between metallic source–drain contacts. The current–voltage characteristics at 4.2 K with different gate voltages before the SiO2 deposition have indicated the formation of a single quantum dot. After the SiO2 deposition, the irregular Coulomb diamonds and the negative differential conductance have been observed, which suggests the formation of coupled quantum dots.
Effect of impurity band conduction on the electrical characteristics of n-type CuInSe2
Authors: Essaleh L. , Wasim S. M. , Galibert J.
Published: July 2002

The Hall effect and electrical resistivity of n-type CuInSe2 single crystals are measured between 4.2 and 300 K. Using a single conduction band model, the variation of the electron concentration with temperature above 100 K is explained in terms of the thermal activation of a shallow donor. The density of states effective mass me*=0.09me of the electrons, the activation energy of the donors around 7 meV, their concentration, and the compensation ratio are estimated. The temperature dependence of the electron mobility in conduction band is analyzed by taking into account the scattering of the charge carriers by ionized impurities and acoustic and polar optical phonon modes. The adjustable parameters, thus obtained, are compared with those reported earlier. On the other hand, by considering the two-band model with electrons in both the conduction and impurity bands, the change in the Hall coefficient with temperature between 300 and 40 K is explained. It is found that at the temperature where the Hall coefficient is maximum, the mobility in the impurity band is about 20% as compared to its value in the conduction band. The width of the impurity band is found to increase with increasing impurity concentration and the electron mobility below 20 K is explained by considering the effect of Mott-type variable range hopping conduction.
N-induced vibrational modes in GaAsN and GaInAsN studied by resonant Raman scattering
Authors: Wagner J. , Geppert T. , Köhler K. , Ganser P. , Herres N.
Published: July 2002

Vibrational modes introduced by the incorporation of N into GaAs and GaInAs have been studied by Raman spectroscopy on samples grown by molecular-beam epitaxy using a rf nitrogen plasma source. When proceeding from GaAs1−xNx to Ga1−yInyAs1−xNx with x⩽0.04 and y⩽0.12, the nitrogen-induced vibrational mode near 470 cm−1 observed in GaAsN was found to broaden and to split into up to three components with one component at a frequency higher than that of the Ga–N mode in GaAsN. This observation shows that the incorporation of In into GaAsN strongly affects the local bonding of the N atoms by changing the local strain distributions as well as the formation of a significant fraction of In–N bonds. The resonant enhancement in the scattering cross section of the Ga–N vibrational mode, observed in low N-content GaAs1−xNx (x≈0.01) for incident photon energies matching the mostly N-related E+ transition at around 1.8 eV, was found to broaden significantly upon increasing N content as well as upon the addition of In to form GaInAsN.
Vibrational dynamics of solid molecular nitrogen to megabar pressures
Authors: Goncharov A. F. , Gregoryanz E. , Mao H.-K. , Hemley R. J.
Published: July 2002

We report the results of Raman and synchrotron infrared absorption measurements of several molecular phases of solid nitrogen to pressures above 100 GPa (300 K). Low-temperature vibrational spectra to 45 GPa are also presented. The changes in Raman and infrared spectra at 60 GPa and 300 K are interpreted as arising from the ε→ζ transition reported at low temperature. The character of splitting of the Raman vibron ν2 observed at 25 GPa and low temperatures differs from that previously reported, a difference that we ascribe to different experimental procedures employed and metastability of the low-temperature phase.
Coupled InP quantum-dot InGaP quantum well InP–InGaP–In(AlGa)P–InAlP heterostructure diode laser operation
Authors: Walter G. , Holonyak N. , Ryou J. H. , Dupuis R. D.
Published: July 2002

Data are presented showing that a p–n InP–In0.5Ga0.5P–In0.5(Al0.3Ga0.2)P–In0.5Al0.5P quantum-dot (QD) heterostructure diode, with an auxiliary ∼20 Å InGaP quantum well (QW) coupled via an In(AlGa)P barrier (∼20 Å) to the single layer of QDs to aid carrier collection, has a steeper current–voltage characteristic than the case of a similar diode with no auxiliary QW. The p–n InP+InGaP QD+QW diode is capable of 300 K visible-spectrum QD laser operation, while the single-layer InP QD diode (single QD layer) saturates at low current (≲1 mA) and does not exhibit stimulated emission.
Heuristic closures for numerical simulations of neoclassical tearing modes
Authors: Gianakon T. A. , Kruger S. E. , Hegna C. C.
Published: July 2002

Heuristic closures are presented for use in simulations of neoclassical modifications to magnetohydrodynamic phenomenon in tokamaks. The closures capture the dominant physics expected from linear and quasilinear neoclassical instability theory and are computationally easy to implement. Numerical results from the NIMROD [A. H. Glasser, C. R. Sovinec, R. A. Nebel et al., Plasma Phys. Controlled Fusion 41, A747 (1999)] code are shown which demonstrate poloidal flow damping, growth rate reduction due to the neoclassical enhancement of the polarization current, and generation of perturbed bootstrap currents and subsequent generation of a neoclassical tearing mode.
Absence of recombination of neighboring H atoms in highly purified solid parahydrogen: Electron spin resonance, electron-nuclear double resonance, and electron spin echo studies
Authors: Kumada Takayuki , Sakakibara Masahiro , Nagasaka Toshimitsu , Fukuta Hiroya , Kumagai Jun , Miyazaki Tetsuo
Published: July 2002

Diffusion and recombination of H atoms were studied in solid hydrogen containing ortho-H2 molecules at relative concentration Xo=0.001→0.75 using electron spin resonance (ESR), electron-nuclear double resonance (ENDOR), and electron spin echo (ESE) methods at around 4 K. When the rate-determining step for recombination is assumed to be the diffusion of H atoms, the rate constant for recombination at Xo⩾0.1 is consistent with the diffusion coefficient estimated from the analysis of ENDOR spectra and longitudinal spin relaxation behaviors. The recombination rate constant at Xo<0.1, however, is too slow to be explained using the diffusion coefficient estimated from longitudinal spin relaxation and forbidden spin-flip satellite transition studies. This result suggests that, even if one H atom finds another H in its immediate neighborhood, these H atoms do not react to form a H2 molecule at Xo≪0.1. The absence of recombination of H atoms is due to lack of the energy dispersion path required for the recombination of diatomic molecules. Since the absence of recombination becomes less significant at higher Xo, ortho-H2 molecules are found to play an important role in the energy dispersion which accompanies the recombination reaction.
Morphologies of ABC triblock copolymer thin films
Authors: Chen Hsuan-Yi , Fredrickson G. H.
Published: July 2002

We apply self-consistent field theory to confined films of symmetric ABC triblock copolymer melts, where the A and C blocks are equal in size and A-B, B-C, and A-C interactions are identical. We focus on the regime where, in bulk systems, B forms the majority domain and the minority A and C domains form cylinders or spheres. We show that the morphologies of confined films may have different symmetries from bulk melts, and the shape and orientation of the minority A and C domains can be manipulated by tuning the wall potentials and film thickness. Our calculations suggest a route for experimentally controlling complex nanometer-scale surface patterning.
Anomalous field enhancement from the superfocusing of surface plasmons at contacting silver surfaces
Authors: Lalayan A. A. , Bagdasaryan K. S. , Petrosyan P. G. , Nerkararyan Kh. V. , Ketterson J. B.
Published: July 2002

We demonstrate, both theoretically and experimentally, that an anomalous field enhancement occurs in the vicinity of contact points between two plasmon-resonant surfaces. The theoretical model involves two spheres in contact, and the enhancement is shown to result from the localization of surface plasmon polaritons in the vicinity of the contact point brought on by a decrease of the wavelength and an enhancement of the field of plasmons propagating toward the tip. This model is applied to experiments involving silver films having mesoscopic surface protrusions which are irradiated by intense laser pulses: films in close contact (where the protrusions touch) suffer laser-induced damage which is absent in separately irradiated films. This is interpreted as arising from the additional field enhancement, relative to the well-known enhancement occurring at isolated silver surface protrusions, brought on by the contacts.
Shadowing effects on the microstructure of obliquely deposited films
Authors: Paritosh  , Srolovitz D. J.
Published: July 2002

Two spatial dimension front tracking simulations have been performed to study the growth of polycrystalline, faceted films from randomly oriented nuclei by varying the deposition angle of the incident flux during physical vapor deposition. The orientation of grain columns, the porosity, the crystallographic texture, and grain size are sensitive to the deposition angle. The origin of this effect is widely believed to be associated with shadowing. In order to isolate the effects of shadowing from other physical effects (such as surface diffusion, deposition species size, flux divergence, etc.), we have constructed a simulation where all of these effects are completely removed. These simulations demonstrate that while many of the observed structural properties of obliquely deposited films are controlled by shadowing, a few key properties cannot be attributed solely to shadowing.
Picosecond ultrasonics study of the vibrational modes of a nanostructure
Authors: Antonelli G. Andrew , Maris Humphrey J. , Malhotra Sandra G. , Harper James M. E.
Published: July 2002

We report experiments in which a subpicosecond pump light pulse is used to excite vibrations in a nanostructure consisting of a periodic array of copper wires embedded in a glass matrix on a silicon substrate. The motion of the wires after excitation is detected using a time-delayed probe light pulse. From the measured data, it is possible to determine the frequencies νn and damping rates Γn of a number of the normal modes of the structure. These modes have frequencies lying in the range 1–30 GHz. By comparison of the measured νn and Γn with the frequencies and damping rates calculated from a computer simulation of the vibrations of the nanostructure, we have been able to deduce the vibration patterns of six of the normal modes.
Exploration of stellarator configuration space with global search methods
Authors: Mynick H. E. , Pomphrey N. , Ethier S.
Published: July 2002

An exploration of stellarator configuration space z for quasi-axisymmetric stellarator (QAS) designs is discussed, using methods which provide a more global view of that space. To this end, a “differential evolution” search algorithm has been implemented in an existing stellarator optimizer, which is much less prone to become trapped in local, suboptimal minima of the cost function χ than the local search methods used previously. This search algorithm is complemented by mapping studies of χ over z aimed at gaining insight into the results of the automated searches. It is found that a wide range of the attractive QAS configurations previously found fall into a small number of classes, with each class corresponding to a basin of χ(z). Maps are developed on which these earlier stellarators can be placed, the relations among them seen, and understanding gained into the physics differences between them. It is also found that, while still large, the region of z space containing practically realizable QAS configurations is much smaller than earlier supposed.
Generation of MeV electrons and positrons with femtosecond pulses from a table-top laser system
Authors: Gahn C. , Tsakiris G. D. , Pretzler G. , Witte K. J. , Thirolf P. , Habs D. , Delfin C. , Wahlström C.-G.
Published: July 2002

In experiments, the feasibility was demonstrated of generating multi-MeV electrons in a form of a collimated beam utilizing a table-top laser system delivering 200 fs pulses with PL=1.2 TW and 10 Hz capability. The method uses the process of relativistic self-channeling in a high-density gas jet producing electron densities in the range of 3×1019–6×1020 cm−3. In a thorough investigation, angularly resolved and absolutely calibrated electron spectra were measured and their dependence on the plasma density, laser intensity, and gas medium was studied. For the optimum electron density of ne=2×1020 cm−3 the effective temperature of the electron energy distribution and the channel length exhibit a maximum of 5 MeV and 400 μm respectively. The laser-energyto-MeV-electron efficiency is estimated to be 5%. In a second step, utilizing the multi-MeV electron beam anti-particles, namely positrons, were successfully generated in a 2 mm Pb converter. The average intensity of this new source of positrons is estimated to be equivalent to a radioactivity of 2×108 Bq and it exhibits a very favorable scaling for higher laser intensities.
Two straightforward methods for the measurement of optical losses in planar waveguides
Authors: Ramponi Roberta , Osellame Roberto , Marangoni Marco
Published: July 2002

Two new straightforward methods for the evaluation of optical losses in planar waveguides are proposed and discussed. The first method exploits a single nonsliding isosceles prism and it allows the attenuation to be determined through the measurement of the power exiting the waveguide and the evaluation of the coupled power. It requires a very simple operation procedure, it allows a mode-selective determination of losses, and it presents a good accuracy provided that the sample is not too short. The second method uses end-fire coupling and it is based on the measurement of the output power together with the power back-reflected by the output face of the waveguide. The main advantage of the method is that it can be very accurate also for shorter waveguides and that its accuracy is to a high degree insensitive with respect to the optical depth of the waveguide. It provides better results in the case of high refractive index waveguides that give an intense back-reflected signal. Experimental results obtained with both methods on two different waveguides are presented.
Reduction of spike noise in perpendicular recording media by using MnIr antiferromagnetic films
Authors: Tanahashi Kiwamu , Kikukawa Atsushi , Shimizu Noboru , Hosoe Yuzuru
Published: July 2002

We introduced a NiFe/antiferromagnetic–MnIr bilayer or a NiFe/MnIr/NiFe trilayer below a CoTaZr soft magnetic underlayer in perpendicular recording media as a way of controlling the magnetic domain structure of the soft magnetic underlayer, and we investigated the effect of exchange biasing on the spike noise. Samples consisting of a layer structure—NiFe (5 nm thick)/MnIr (2.5–50 nm)/NiFe (5 nm)/CoTaZr (50–200 nm)—were sputter deposited on precoated glass disks. The samples were heated with a lamp heater and cooled in a magnetic field along the radial direction of the disk. Both uniaxial and unidirectional anisotropies were induced along the magnetic field when the thickness of the MnIr layer was more than 5 nm. The first NiFe layer promoted a fcc–MnIr (111) crystalline texture, while the second NiFe layer enhanced the value of exchange-bias field by about 20%. The exchange-bias field increased from 6 to 24 Oe as the CoTaZr-layer thickness decreased from 200 to 50 nm. Many spikes along the radial direction were observed for a 100-nm-thick CoTaZr single-layer film, while no remarkable spikes were observed for a NiFe/MnIr/NiFe/CoTaZr (100 nm) film. It was found that the NiFe/MnIr/NiFe trilayer restrained the formation of domain walls in the CoTaZr layer, thereby reducing the spike noise.
Valence one-electron and shake-up ionization bands of polycyclic aromatic hydrocarbons. II. Azulene, phenanthrene, pyrene, chrysene, triphenylene, and perylene
Authors: Deleuze Michael S.
Published: July 2002

An exhaustive investigation of the outer-valence ionization spectra of azulene, phenanthrene, pyrene, chrysene, triphenylene, and perylene is presented. The analysis is based on one-particle Green’s function calculations performed upon correlated [density-functional theory/Becke three-parameter Lee–Yang–Parr (DFT/B3LYP)] geometries using the third-order outer-valence Green’s function [OVGF] and algebraic-diagrammatic construction [ADC(3)] schemes, as well as basis sets of improving quality (6-31G, 6-31G*, cc-pVDZ). The ionization bands of polycyclic aromatic hydrocarbons show a great diversity, which reflects the strong impact of the molecular architecture on orbital energies. Despite the intricacy of ionization spectra and the extent of the shake-up contamination in the π- and σ-band systems, the ADC(3) results enable consistent insights into available ultraviolet photoelectron measurements, up to the inner-valence region. They also indirectly support recent assignments of low-lying π*←π doublet excitations in the electronic absorption spectrum of the pyrene and perylene cations by means of time-dependent density functional theory [J. Chem. Phys. 111, 8904 (1999)]. It has been found that OVGF pole strengths smaller than 0.85 very systematically foretell a breakdown of the orbital picture of ionization at the ADC(3) level.
Materials with a high secondary-electron yield for use in plasma displays
Authors: Vink T. J. , Balkenende A. R. , Verbeek R. G. F. A. , van Hal H. A. M. , de Zwart S. T.
Published: July 2002

Reduction of the firing voltage in plasma display panels calls for electrode coatings with a high secondary-electron yield. We have explored a range of materials that exhibit very low firing voltages, and a 50% reduction has been achieved relative to the best quality MgO. It is further shown that a high electron emission yield originates from both ion- and photon-induced processes, and is strongly influenced by the electronic structure of the material in terms of band gap and electron affinity.
Unidirectional bistability in semiconductor waveguide ring lasers
Authors: Sorel M. , Laybourn P. J. R. , Giuliani G. , Donati S.
Published: July 2002

Large-diameter ridge-guided semiconductor lasers weakly coupled to a straight output waveguide show unidirectional operation and directional bistability at currents up to about twice the threshold. The direction of lasing in the ring may be controlled by biasing contacts at either end of the coupled guide.
Anisotropic geophysical vortices
Authors: McWilliams James C. , Weiss Jeffrey B.
Published: July 2002

A survey is made of many types of coherent vortices in the Earth’s ocean and atmosphere. These vortices often occur with strong, environmentally induced anisotropy in their velocity and vorticity fields. We propose a definition of the essential characteristics of coherent vortices and formulate hypotheses concerning their dynamical role in complex, anisotropic fluid motions. Finally, we analyze numerical solutions both for uniformly rotating, stably stratified three-dimensional flow and for two-dimensional flow for the phenomena of enstrophy cascade and dissipation, intermittency, isotropy in the appropriate coordinate frame, coherent vortex emergence, vortex population dynamics, and approach to a nonturbulent end state.
Characterization of epitaxially grown GaAs on Si substrates with III-V compounds intermediate layers by metalorganic chemical vapor deposition
Authors: Soga Tetsuo , Hattori Shuzo , Sakai Shiro , Takeyasu Masanari , Umeno Masayoshi
Published: July 2002

GaAs grown on Si substrate with AlP, AlGaP, GaP/GaAs0.5P0.5 superlattice, and GaAs0.5P0.5/GaAs superlattice was investigated by varying the structure of the intermediate layers between GaAs and Si by metalorganic chemical vapor deposition. It was found that (1) the insertion of AlP and AlGaP layers makes the crystallinity and the surface morphology better, (2) PL (photoluminescence) intensity with two superlattice layers is about one order of magnitude stronger than that without these layers, (3) the crack formation in the GaAs surface layer can be avoided by the strained superlattice layers, (4) the PL intensity has a maximum at about 20 nm for each layer thickness in the superlattices, and (5) the PL intensity increases and the carrier concentration decreases while increasing the thickness of the surface GaAs and saturates over 3 μm. The PL intensity of GaAs on Si substrates is about 80% of that grown on GaAs substrates.
Growth of crystalline zirconium dioxide films on silicon
Authors: Morita M. , Fukumoto H. , Imura T. , Osaka Y. , Ichihara M.
Published: July 2002

Zirconium dioxide (ZrO2) films have been grown on Si(100), Si(111), and SiO2/Si substrates heated at the range from room temperature to 800 °C by vacuum evaporation. X-ray diffraction and reflection high-energy electron diffraction observations reveal the epitaxial growth of tetragonal ZrO2(200) films on Si(100) substrates at 800 °C. The epitaxial imperfection is caused by preferentially oriented tetragonal ZrO2(002) grains. The crystalline system of ZrO2 films depends on the substrate temperature. The crystalline perfection and the orientation of tetragonal ZrO2 films grown at 800 °C depend on the substrate orientation.
Magnetic and Mössbauer study of coprecipitated Sr2Zn2Fe28O46 (SrZn-X) hexagonal ferrite
Authors: Leccabue F. , Albanese G. , Muzio O. Ares
Published: July 2002

Polycrystalline samples of Sr2Zn2Fe28O46 (SrZn-X) hexagonal ferrite have been prepared by the chemical coprecipitation method. The saturation magnetization and anisotropy field measured at room temperature were found to be 79.4 emu/g and 12.4 kOe, respectively. The Curie temperature is 710±5 K. The temperature dependence of the hyperfine magnetic fields at Fe-57 nuclei gives evidence that the Fe3+ ions in the ‘‘k’’ sublattice are magnetically unequivalent. Thermal magnetic, x-ray, and Mössbauer analyses performed on samples subjected to different thermal treatments made it possible to follow the main steps in the formation of the SrZn-X phase. The X phase is formed through a solid-state reaction between the M and Zn2-W intermediate phases at 1400 °C.
Segregation and transport coefficients of impurities at the Si/SiO2 interface
Authors: Sakamoto K. , Nishi K. , Ichikawa F. , Ushio S.
Published: July 2002

Segregation and transport coefficients of impurities at the Si/SiO2 interface have been studied. A brief model has been proposed which relates the two coefficients to the oxidation rate and the impurity concentrations at both sides of a Si/SiO2 interface. The model enables us to obtain the transport coefficients from the measured impurity profiles for the first time. The transport coefficients of arsenic and phosphorus are, for example, 0.12 and 5.0 μm/min at 1100 °C, respectively. The segregation coefficients can be also calculated from this model. They are more than 1000 for arsenic and phosphorus, and less than 1 for boron at 1000–1100 °C.
Time-dependent open-circuit voltage in CuInSe2/CdS solar cells: Theory and experiment
Authors: Ruberto Mark N. , Rothwarf Allen
Published: July 2002

The increase with illumination time of the open-circuit voltage of the CuInSe2/CdS solar cell has been studied as a function of illumination intensity, temperature, and preillumination bias voltage. The results indicate time constants on the order of minutes (40 to 500 s at room temperature), with an activation energy of ∼0.20 eV. The magnitude of the effect (ΔVoc ≤30 mV) and the time constants both decrease with increasing preillumination voltage bias, indicating that the effect is due to voltage rather than illumination. A model involving the tunneling of electrons trapped in deep states in the CdS, near the junction with the CuInSe2 layer to holes in the CuInSe2 valence band, seems to account for the magnitude, as well as the time and voltage dependence of the effect. The increased positive charge near the interface shifts the division of the diffusion voltage between the two sides of the junction, thereby increasing Voc. The fill factor and efficiency of the cell are also increased, while the short-circuit current is unaffected.
Structural and electrical properties of rf-sputtered amorphous barium titanate thin films
Authors: Sreenivas K. , Mansingh Abhai , Sayer M.
Published: July 2002

Structural and electrical properties of rf-sputtered amorphous BaTiO3 thin films grown on water-cooled substrates have been investigated. The dielectric and electrical properties have been studied for films grown under varying sputtering gas composition (Ar+O2 gas mixtures) as a function of film thickness, frequency, and temperature. As-grown films were amorphous in nature and highly transparent. Post-deposition annealing had no discernible effect either on film structure or dielectric properties, and there was no evidence of ferroelectricity. Films sputtered in pure argon showed a dielectric constant ε′∼12 with little dependence on frequency (0.1–100 kHz) over the temperature range 0–75 °C. The dielectric properties were a function of film thickness and the percentage of oxygen in the sputtering gas during growth. The thickness-dependent dielectric properties of amorphous BaTiO3 thin films on conducting glass substrates could be satisfactorily explained by a model based on the existence of electrode barriers. The high breakdown voltage (106 V/cm), charge storage capacity (3.1 μC/cm2), and frequency- and temperature-dependent dielectric properties of a-BaTiO3 films sputtered in pure argon show promise for application as insulating layers in thin-film electroluminescent display devices.
Evaluation of the coupling coefficient of a distributed feedback laser with residual facet reflectivity
Authors: Kihara K. , Soda H. , Ishikawa H. , Imai H.
Published: July 2002

We show that the stop-band width of a distributed feedback (DFB) laser with both facets having antireflective coatings is affected by residual facet reflectivity. In the weakly coupled case the stop-band width of a DFB laser with residual facets having a power reflectivity of about 1.0% is narrower than that with no reflective facets. When the residual power reflectivity of the facets is less than 0.01%, the stop-band width is the same as that with no reflective facets. It is therefore essential to consider the effect of reflective facets to evaluate correctly the coupling coefficient.
Light-induced effects on doped- and undoped-hydrogenated amorphous silicon
Authors: Park Jin-Seok , Han Min-Koo , Lee Chung-Han
Published: July 2002

This article presents the light-induced effects on the electrical and optical properties of undoped- and doped-hydrogenated amorphous silicon films. The changes in the conductivities and the activation energies of various types of a-Si:H films due to the prolonged exposure to light have been characterized as a function of deposition conditions and illumination periods. We have also analyzed the variations of microstructure of a-Si:H film such as silicon–hydrogen bondings in the rocking and stretching modes utilizing infrared spectroscopy. From the experimental results, it is clear that doping effects must be crucial to the degradations of the fundamental properties of a-Si:H due to light-induced effects.
Detection of radio-frequency signals emitted by an arc discharge
Authors: Damas M. C. , Robiscoe R. T.
Published: July 2002

We analyze the nature of radiation signals detected from electrical arcing events originating on highly charged dielectric surfaces. Starting from a simple model of the arc current pulse, we calculate the electromagnetic radiation signal incident on a distant radio-frequency receiver. The receiver is tuned to some central frequency and operates in a given bandwidth. Because the receiver responds to the arc current derivative rather than the arc current itself, the detected radiation pulse profile can differ substantially from the current pulse profile, even when the receiver bandwidth is large compared to the reciprocal pulse width. This signal distortion significantly affects the interpretation of arc radiation data. To illustrate the point, we show that at high frequencies the detected pulse width carries very little information on the arc itself, and that even a scan of pulse width versus frequency may provide a misleading picture of possible arc dynamics. Finally, we argue that arc radiation data from radio-frequency measurements are most useful at low frequencies, when the receiver frequency and bandwidth are both comparable to the reciprocal arc duration.
A general model for fabrication processes of channel waveguides by ion exchange
Authors: Tervonen Ari
Published: July 2002

A general theoretical approach to the modeling of several different process steps in fabrication of ion-exchanged channel waveguides is presented. All of the commonly used fabrication processes are included, and the nature of the ion source and the effect of external electric voltage are taken into account. The model uses a numerical finite-difference calculation of diffusion in the glass with boundary conditions and electrical current distribution chosen according to the process being modeled. A single computer program includes the model, and can be used for simulating a fabrication process with different successive process steps. Several calculated examples are given, all with experimentally obtained parameters for Ag+ -Na+ exchange in Corning 0211 glass.
Phase locking of two strongly coupled vircators
Authors: Sze H. , Price D. , Harteneck B.
Published: July 2002

Phase locking of two high-power virtual cathode oscillators (vircators) has been achieved. The vircators interact directly by common connection through a short waveguide. Reproducible phase locking occurs for ∼45 ns. The time required to establish phase coherence is ≂10 ns. The peak power is ∼1.6 GW.
Magnetostriction measurement of amorphous wires by means of small-angle magnetization rotation
Authors: Yamasaki J. , Ohkubo Y. , Humphrey F. B.
Published: July 2002

The small-angle magnetization rotation method has been shown to be applicable for measuring the saturation magnetostriction of amorphous wires by using the circumferential field generated by an ac current through the wire. The method was used to measure compositional variation of magnetostriction of Fe-Co based amorphous wires. A saturation magnetostriction λs of 32.9×10−6 and −2.6×10−6 was found for Fe and Co based wires, respectively. The compositional variation of the saturation magnetostriction, λs, for (Fe1−xCox)75Si10B15 amorphous wires (X=0–0.8) decreases with increasing cobalt content and crosses zero around X=0.95 as in amorphous ribbons. The maximum experimental error of measurement was estimated to be about 5%.
Thickness dependence of the phase transition temperature in Ag2Se thin films
Authors: Das V. Damodara , Karunakaran D.
Published: July 2002

Thin films of silver selenide (Ag2Se) between thicknesses of about 700 and 2200 Å have been prepared on glass substrates at room temperature in a vacuum of 5×10−5 Torr. After vacuum annealing the films (at about 373 K for 3 h) electrical resistivity measurements on these films have been carried out in vacuum. From the increase in the rate of decrease of resistance with temperature, the phase transition temperatures (orthorhombic to body-centered cubic) of the different films have been located. It is found that the phase transition temperature of the thin films is a function of thickness increasing with a decrease in the thickness. This observation has been explained by a recently developed theory [V. Damodara Das and D. Karunakaran, J. Phys. Chem Solids 46, 551 (1985)] of phase transitions in thin films modified further. Also, an order-of-magnitude value of the difference in the function of specific surface and interfacial energies of the two phases has also been determined using the theory.
X-ray optics of doubly curved diffractors II
Authors: Wittry D. B. , Sun Songquan
Published: July 2002

A general equation is derived for x-ray diffractors with arbitrary toroidally curved surface and planes when used with a point source. This equation, which gives the locus of points on a curved diffractor’s surface for a given deviation from the Bragg angle, is shown to yield the appropriate equations for all of the crystal diffractor geometries that were previously discussed by Wittry and Sun [J. Appl. Phys. 67, 1633 (1990)]. Two examples are given of the use of the general equation to design diffractors with improved collection solid angle over a range of Bragg angles. The x-ray collection solid angle and focusing properties of these two examples are given.
Temperature dependence of domain-wall coercive field in magnetic garnet films
Authors: Vértesy G. , Tomáš I. , Půst L. , Pačes J.
Published: July 2002

The coercive properties of magnetically uniaxial liquid-phase epitaxy garnet films were investigated between 10 K and the Néel temperature (TN≤500 K). Two independent methods, the results of which are nearly identical (magnetical response of oscillating domain walls and the method of coercive loops measured in a vibrating sample magnetometer), were used. Besides the usual domain-wall coercive field, Hdw, the critical coercive pressure, pdw, was also introduced as it describes in a direct way the interactions of the domain walls with the wall-pinning traps. Both Hdw and pdw were found to increase exponentially with decreasing temperature. Three different types of wall-pinning traps were identified in the sample and their strength, their rate of change with temperature, and their temperature range of activity were determined.
Experimental test of theories for the effective thermal conductivity of a dispersed composite
Authors: Kutcherov Vladimir , Håkansson Björn , Ross Russell G. , Bäckström Gunnar
Published: July 2002

The effective thermal conductivity of binary dispersed composites prepared from mixed and compacted powders of NaCl and low-density polyethylene (LDPE) was measured over the entire range of volume fraction at temperatures in the range 120–320 K together with pressures up to 1.73 GPa. Measurements were made using the transient hot-wire method. The ratio of thermal conductivities of NaCl and LDPE varied over the range 10–50 at the temperatures and pressures employed. Using our results (together with previous data for AgCl-LDPE composites), we tested theoretical predictions from both the effective-medium approximation and the real-space renormalization group approximation. The two approximations exhibited a comparable overall level of agreement with experiment and neither was successful in describing our data within experimental inaccuracy over the entire range of volume fraction. For both approximations, the extent to which agreement with experiment was achieved depended significantly on the volume fraction being considered.
Size effects appearing in the Raman spectra of polycrystalline diamonds
Authors: Namba Yoshikatsu , Heidarpour Ebrahim , Nakayama Masatoshi
Published: July 2002

Various spectra of Raman for different grain sizes of diamond powder have been studied. The spectra are also correlated to the results of transmission electron diffraction, transmission electron microscopy, scanning electron microscopy, and x-ray photoelectron spectroscopy. When the Raman peak intensity was decreased, the spectra broadened and shifted to a lower frequency in accordance with the decrease of the grain size. Finally, in the region of a few hundred angstroms, the detected signals were almost comparable to the noise level. While unchanged, lattice spacings for transmission electron diffraction patterns and constant peak heights for x-ray photoelectron spectroscopy analysis were observed for different grain sizes.
Effects of Al on the magnetic properties of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloys
Authors: Lim S. H. , Pi W. K. , Noh T. H. , Kim H. J. , Kang I. K.
Published: July 2002

The effects of Al on the soft magnetic properties of nanocrystalline Fe73.5−xAlxCu1Nb3Si13.5B9 alloy ribbons are investigated in the composition range of 0≤x≤1.0. The relative initial permeability at 1 kHz is found to increase by the addition of Al, and reaches the peak value at x=0.1. The coercivity decreases, rather significantly, with the Al content in the whole composition range investigated in the present work, the values of the coercivity being 12.5 mOe at x=0 and 9.3 mOe at x=1.0. The magnetic induction at an applied field of 10 Oe, however, decreases moderately by the introduction of Al, possibly due to the dilution effect. The improvement in the soft magnetic properties is considered to result from the reduction in the grain size of the α Fe-Si solid solution phase of the Al-added alloy ribbons, which has been observed by transmission electron microscopy. Another factor may be due to the decrease in the intrinsic magnetocrystalline anisotropy K1 as Al is added to the alloy.
Influence of Cu2+ ion on magnetic moment and Curie temperature of Zn2-X hexaferrites
Authors: Gu B. X.
Published: July 2002

Polycrystalline samples of Ba2Cu2xZn2−2xFe28O46 (0≤X≤1) X-type hexaferrites were prepared by the classical ceramic method. Low-temperature magnetic properties and Curie temperature were investigated. The substitution of Zn2+ ion with Cu2+ ion leads to a decrease of the magnetic moment and an increase of Curie temperature due to the preference of Cu2+ ions for the octahedral sites of spinel S blocks.
Effect of grain size of sputtered cerium-oxide films on their electrical and kinetic behavior at high temperatures
Authors: Lohwasser Werner , Gerblinger Josef , Lampe Uwe , Meixner Hans
Published: July 2002

Following an annealing process of several hours duration at a temperature of at least 1100 °C, reactively sputtered cerium-oxide films with film thicknesses ranging from 0.5 to 3 μm show a dependence of electric conductivity on oxygen partial pressure similar to that of polycrystalline bulk material within the temperature range studied (700 to 1000 °C). But films with comparatively small grain structures have specific electrical conductivities that are as much as an order of magnitude higher than those of large-grained structures, let alone bulk materials. This outcome justifies the supposition that the carrier transport in CeO2−x thin films occurs in a grain barrier layer within which electrons are enriched. This negative carrier enrichment layer may be due to a positive surface charge. An investigation of the interaction between oxygen vacancies of CeO2−x thin films and the oxygen of the environment showed that for layer thickness of from 1 to 3 μm and temperatures of 700–1000 °C, the reaction of the oxygen molecules at the surface is always the kinetics-governing step. Above 950 °C the transport reaction through the laminar boundary zone on the surface determines the reaction kinetics. Under these conditions the volume diffusion of oxygen vacancies in the thin film proceeds more rapidly than the surface reaction or the gas-phase transport of oxygen molecules.
How to isolate effects of spin-flip scattering on giant magnetoresistance in magnetic multilayers (invited)
Authors: Bass J. , Yang Q. , Lee S. F. , Holody P. , Loloee R. , Schroeder P. A. , Pratt W. P.
Published: July 2002

A technique is described for isolating effects of finite spin diffusion lengths on the magnetoresistance (MR) of magnetic multilayers at low temperatures by measuring the MR with the current perpendicular to the layer planes. Applying this technique to relatively concentrated Co/AgMn and Co/CuMn multilayers yields large effects, generally consistent with expectations from a theory by Valet and Fert [Phys. Rev. B 48, 7099 (1993)]. However, some complicating issues still need to be resolved.
Effect of annealing on the giant magnetoresistance of sputtered Co/Cu multilayers
Authors: Zhang H. , Cochrane R. W. , Huai Y. , Mao Ming , Bian X. , Muir W. B.
Published: July 2002

Co/Cu multilayers with the form of glass/Cu 50 Å[Co11.5 Å/Cu(tCu)]30/Cu50 Å, prepared by rf triode sputtering, exhibit oscillatory magnetoresistance (MR), as a function of the Cu layer thickness with a period of ≂12 Å and maximum MR values of 36.8% and 22.3% at 77 and 295 K, respectively. In order to study the effect of annealing on the structural and magnetic properties, five samples with Cu thicknesses between 9 and 34 Å were heat treated at temperatures up to 300 °C and analyzed by low- and high-angle x-ray diffraction, MR, and mageto-optic Kerr effect measurements. Annealing at moderate temperatures for the samples with Cu thicknesses around the second and third MR peaks leads to an initial increase in the MR. In contrast, annealing causes only a monotonic MR decrease for the sample at the first peak with a Cu thickness of 9 Å.
Magnetostriction and magnetic properties of iron-cobalt alloys multilayered with silver
Authors: Lafford Tamzin A. , Gibbs M. R. J. , Zuberek R. , Shearwood C.
Published: July 2002

Commercially available Fe-Co alloys with Co contents of 50 and 24 wt. % have been rf sputtered to produce multilayers with noble metal (Ag or Cu) interlayers. Series with fixed Fe-Co or noble metal thicknesses have been produced, as have plain Fe-Co films. In all cases, the coercivities of the multilayers are significantly reduced from those exhibited by the plain alloy films. For example, a film of the alloy containing 50 wt. % Co had Hc=6.9 kA m−1, whereas a multilayer with 12 nm Fe-Co layers and 2 nm Ag layers had Hc=480 A m−1. The saturation magnetostriction is also enhanced, in some cases increasing by 60% over the expected bulk polycrystalline alloy value. For example, the literature λs value for a Fe-Co alloy containing 24% Co is about 28 ppm; in a multilayer with 12 nm alloy layers and 8 nm Ag layers, λs=45 ppm. The combination of high magnetostriction and low coercivity makes such materials as these attractive for their potential as stress sensor and actuation transducer elements.
Fractals in annealed Ge-Au/Au bilayer films
Authors: Ba Long , Zen Jianlin , Zhang Shuyuan , Wu Ziqin
Published: July 2002

Ge-22 at. %Au/Au bilayer films annealed at various temperatures have been observed by transmission electron microscopy. The as-deposited bilayer film is composed of crystalline Ge,Au and metastable Ge0.4Au0.6 phases. The Au grain size is nonuniform, and no amorphous Ge is observed. During annealing at 60–350 °C, patterns with several wide branches are generated that are different from the dense branching fractals induced by crystallization of amorphous Ge in a-Ge/Au films. The fractal dimension of the pattern (measured by the box-counting method) increases with the annealing temperature. The agglomeration of small Au grains to coarse Au grains in the Ge-Au/Au bilayer film may be the mechanism for the fractal structure formation.
Magnetic properties of very high permeability, low coercivity, and high electrical resistivity in Fe87Zr7B5Ag1 amorphous alloy
Authors: Kim B. G. , Song J. S. , Kim H. S. , Oh Y. W.
Published: July 2002

The magnetic properties of an Fe87Zr7B5Ag1 (at. %) amorphous alloy, which contains an additional insoluble element (Ag) in a small amount, have been investigated as a function of annealing temperatures in order to know its potential applicability as a core material used at high-frequency. As a result, a new excellent soft magnetic material with very high resistivity was developed. The amorphous alloy annealed at a relatively low temperature of Ta=300 °C exhibited a very high initial permeability μi of 146 000 at 1 kHz and 2 mOe, resistivity ρRT of 4.5 μΩ m, and very low coercivity Hc of 20 mOe, respectively. The values obtained are the best ones among various kinds of Fe-based soft amorphous materials reported up to now. Furthermore, the amorphous ribbon heat treated at Ta=300 °C still retained good ductility enough to endure 180° bending, which is very important from the viewpoint of processing for mass production. The phenomenon of good soft magnetic properties presumably arises from the homogenous formation of very fine α-Fe clusters in an amorphous matrix, which can be deduced from the increase of resistivity, and the reduction of magnetostriction caused by the dissolution of Zr and B into the α-Fe cluster.