bluuebunny/crossref_metadata_2025 huggingface dataset:
Dissociative electron attachment to gas-phase 5-bromouracil
Authors: Abdoul-Carime H. , Huels M. A. , Brüning F. , Illenberger E. , Sanche L.
Published: July 2002

We report measurements of dissociative electron attachment (DEA) to gaseous 5-bromouracil (BrU) for incident electron energies between 0 and 16 eV. Low energy electron impact on BrU leads not only to the formation of a long lived parent anion BrU−, but also various anion fragments resulting from endo- and exo-cyclic bond ruptures, such as Br−, uracil-yl anions, i.e., (U-yl)−, OCN−, and a 68 amu anion tentatively attributed to H2C3NO−. The incident electron energy dependent signatures of either the Br− and (U-yl)− yields (at 0, 1.4, and 6 eV), or the OCN− and H2C3NO− yields (at 1.6 and 5.0 eV) suggests competing DEA channels for anion fragment formation. The production cross sections, at 0 eV incident electron energy, for BrU−, Br−, and (U-yl)− are estimated to be about 6×10−15, 6×10−14, and 1.0×10−15 cm2, respectively.
Preparation of air-stable, low recombination velocity Si(111) surfaces through alkyl termination
Authors: Royea William J. , Juang Agnes , Lewis Nathan S.
Published: July 2002

A two-step, chlorination/alkylation procedure has been used to convert the surface Si–H bonds on NH4F(aq)-etched (111)-oriented Si wafers into Si–alkyl bonds of the form Si–CnH2n+1 (n⩾1). The electrical properties of such functionalized surfaces were investigated under high-level and low-level injection conditions using a contactless rf apparatus. The charge carrier recombination velocities of the alkylated surfaces were <25 cm s−1 under high-level and low-level injection conditions, implying residual surface trap densities of <3×109 cm−2. Although the carrier recombination velocity of hydrogen-terminated Si(111) surfaces in contact with aqueous acids is <20 cm s−1, this surface deteriorates within 30 min in an air ambient, yielding a high surface recombination velocity. In contrast, methylated Si(111) surfaces exhibit low surface recombination velocities in air for more than 4 weeks. Low surface recombination velocities were also observed for Si surfaces that had been modified with longer alkyl chains.
Detection of atomic oxygen: Improvement of actinometry and comparison with laser spectroscopy
Authors: Katsch H. M. , Tewes A. , Quandt E. , Goehlich A. , Kawetzki T. , Döbele H. F.
Published: July 2002

The density of atomic oxygen in an oxygen rf discharge in the Gaseous Electronics Conference reference cell is determined from the intensity ratios of the argon λ=750 nm line (2p1–1s2) and the atomic oxygen lines λ=777 nm (5P–5S) and λ=844 nm (3P–3S). Laser induced fluorescence spectroscopy with two-photon excitation is applied to the oxygen plasma, and the results of both methods are compared. The improved actinometry is based on the calculation of electronic collisional excitation of the upper levels of these transitions. The required information on the electron energy distribution function is obtained from a model calculation of the discharge. Good agreement of the results are obtained, if excitations via dissociative channels and also quenching rates are accounted for.
Physics of oscillatory flow and its effect on the mass transfer and separation of species
Authors: Thomas Aaron M. , Narayanan R.
Published: July 2002

The mass transport of species in the presence of oscillating flow is calculated for a flow generated by an oscillating pressure drop. Calculations have been done for several binary systems where two dilute species are transported in the presence of a carrier. It is observed that there are multiple frequencies of the imposed oscillation at which the total mass transport of each species is the same. These frequencies, called crossover frequencies, are obtained by way of a graphical intersection when the total mass transport is plotted against the imposed frequency. Explanations on the occurrence of these crossover frequencies are put forth in terms of time constants due to the kinematic viscosity, diffusion coefficients, and the periodicity of the flow. Consequently, these explanations give an idea of the separation of species in pulsatile flow.
The hyperbolic–hypergeometric functions
Authors: Gasaneo G. , Colavecchia F. D. , Otranto S. , Garibotti C. R.
Published: July 2002

In this work we present a new function to represent the approximate solution of a system of three charged particles. This function is based on an extension to two variables of the confluent hypergeometric function 1F1 of Kummer and can be obtained using a method similar to that used by Appell and Kampé de Fériet. We analyze the general properties of the function such as integral representations, series expansions, and asymptotic limits. We also show that the proposed functions verify a relation similar to that satisfied by the exponential and trigonometric–hyperbolic ones. A generalization to n-dimension is also presented. The mathematical properties of the functions indicate that they are suitable to be included in computation of electronic emission in collision processes.
Emission spectroscopy, harmonic vibrational frequencies, and improved ground state structures of jet-cooled monochloro- and monobromosilylene (HSiCl and HSiBr)
Authors: Hostutler David A. , Ndiege Nicholas , Clouthier Dennis J. , Pauls Steven W.
Published: July 2002

The ground state harmonic frequencies of gas phase H/DSi35Cl and H/DSi79Br have been determined by exciting single vibronic bands of the Ã 1A″–X̃ 1A′ electronic transition and recording the dispersed fluorescence. The jet-cooled radicals were produced in a pulsed discharge jet using H/DSiX3 (X=Cl or Br) precursors. The emission data were fitted to an anharmonic model and a normal coordinate analysis of the harmonic frequencies allowed the determination of five of the six force constants of each molecule. Using previously obtained v″=0 rotational constants and the improved force fields, average (rz) and estimated equilibrium (rez) structures were calculated for both monohalosilylenes. The validity of the force constants was evaluated by comparing calculated and observed zero-point inertial defects and by simulating the Franck–Condon profiles of the observed emission spectra in the harmonic approximation.
Centrifugal instabilities in a curved rectangular duct of small aspect ratio
Authors: Gauthier G. , Gondret P. , Thomé H. , Rabaud M.
Published: July 2002

We report experimental results on the stability of the flow occurring in a curved rectangular duct of small aspect ratio, where the centrifugational forces act along the largest dimension. The basic flow is three-dimensional as in a square or circular curved duct, and above a critical flow rate, streamwise vortices are observed. The threshold of the instability is found to be controlled by a Dean number.
Carrier localization and the origin of luminescence in cubic InGaN alloys
Authors: Kent P. R. C. , Zunger Alex
Published: July 2002

The electronic structure and optical properties of cubic (nonpiezoelectric) InGaN are investigated using large scale atomistic empirical pseudopotential calculations. We find that (i) strong hole localization exists even in the homogeneous random alloy, with a preferential localization along the [1,1,0] In–N–In–N–In chains, (ii) even modest sized (<50 Å) indium rich quantum dots provide substantial quantum confinement and readily reduce emission energies relative to the random alloy by 200–300 meV, depending on size and composition, consistent with current photoluminescence, microscopy, and Raman data. The dual effects of alloy hole localization and localization of electrons and hole at intrinsic quantum dots are responsible for the emission characteristics of current grown cubic InGaN alloys.
Resonant infrared pulsed-laser deposition of a sorbent chemoselective polymer
Authors: Bubb D. M , Horwitz J. S. , McGill R. A. , Chrisey D. B. , Papantonakis M. R. , Haglund R. F. , Toftmann B.
Published: July 2002

Fluoropolyol, a sorbent chemoselective polymer, has been deposited as a thin film by resonant infrared pulsed laser deposition using a free electron laser operating at 2.90 μm, a wavelength resonant with the hydroxl stretch. A comparison of the infrared absorbance of the deposited film and starting material shows no evidence of chemical decomposition in the film. Gel permeation chromatography shows that the film and starting material have nearly the same molar mass. The results are particularly applicable to the fabrication of chemical and biological sensors. The concept of resonant pulsed laser deposition using intrinsic vibrational modes may have wide applicability for organic materials.
Binding and ordering of C60 on Pd(110): Investigations at the local and mesoscopic scale
Authors: Weckesser J. , Cepek C. , Fasel R. , Barth J. V. , Baumberger F. , Greber T. , Kern K.
Published: July 2002

We present a comprehensive study on the binding and ordering of C60 on a Pd(110) surface employing scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD). Three well-ordered structures evolving at elevated temperatures were identified, with (∓2 ±5  7 −1), (4×5), (4×8) unit cells and a coverage of 1, 0.82 and 0.77 physical monolayers, respectively. The rearrangement of Pd substrate atoms turns out to play a crucial role in the evolution of thin C60 films. In STM images alternating bright and dark molecular rows are found for the regular structures. This height difference is attributed to a restructuring of the Pd substrate. Whereas the C60 molecules of the bright molecular rows are embedded in one layer deep microscopic pits, C60 accommodated in two layer deep pits account for the dark molecular rows. XPD results complete the understanding of the film structure. They reveal that the orientation of the C60 cages is unique in the regular structures; the molecules are facing with a 5–6 bond towards the substrate.
Interfacing relativistic and nonrelativistic methods. IV. One- and two-electron scalar approximations
Authors: Dyall Kenneth G.
Published: July 2002

Two approximations for the inclusion of scalar relativistic effects into otherwise nonrelativistic molecular self-consistent field calculations are described, which continue the sequence of approximations from previous work. In the first, the two-electron contributions are confined to those where each small component density is one-center. In the second, the two-electron contributions are approximated by using the bare Coulomb operator, an approach which parallels the Douglas–Kroll–Hess method. The results show that both approximations yield results close to the full two-electron scalar relativistic method, within 0.001 Å in bond lengths and 0.1 kcal/mol in dissociation energies. The approximations also give results which are very similar to those of the Douglas–Kroll–Hess method. The second approximation is similar in cost to the Douglas–Kroll–Hess method and only contains simple operators whose integrals are easy to evaluate.
Porosity characterization by beam-based three-photon positron annihilation spectroscopy
Authors: Petkov Mihail P. , Weber Marc H. , Lynn Kelvin G. , Rodbell Kenneth P.
Published: July 2002

We present a straightforward and fast positron annihilation spectroscopy (PAS) technique for measuring the 2 to 3 photon annihilation ratio of Ps (electron-positron) atoms (3γ PAS), utilized here for the nondestructive characterization of mesoporous (pore size >1 nm) dielectric films. Examples are given for ∼1-μm-thick foamed methyl-silsesquioxane (MSSQ) films, produced by mixing MSSQ (0–90 wt % fraction) with a sacrificial foaming agent (porogen). Probing these films as a function of depth allows one to monitor Ps escape from interconnected pores and to determine the threshold for pore interconnectivity to the film surface. A classical treatment of Ps diffusion is used to calculate the open and closed porosity fractions as a function of the initial porogen load.
Breakdown threshold and localized electron density in water induced by ultrashort laser pulses
Authors: Fan C. H. , Sun J. , Longtin J. P.
Published: July 2002

Optical breakdown by ultrashort laser pulses in dielectrics presents an efficient method to deposit laser energy into materials that otherwise exhibit minimal absorption at low laser intensities. During optical breakdown, a high density of free electrons is formed in the material, which dominates energy absorption, and, in turn, the material removal rate during ultrafast laser-material processing. Classical models assume a spatially uniform electron population and constant laser intensity in the focal region, which results in time-dependent expressions only, i.e., the rate equations, to predict electron evolution induced by nanosecond and picosecond pulses. For femtosecond pulses, however, the small spatial extent of the pulse requires that the pulse propagation be considered, which results in an inhomogeneous plasma and localized electron formation during optical breakdown. In this work, a femtosecond breakdown model is combined with the classical rate equations to determine both time- and position-dependent electron density during femtosecond optical breakdown in water. The model exhibits good agreement when compared with experimental results. For other transparent or moderately absorbing dielectric media, the model also shows promise for determining the time- and position-dependent electron evolution induced by ultrashort laser pulses. Another interesting result is that the maximum electron density formed during femtosecond-laser-induced optical breakdown may exceed the conventional limit imposed by the plasma frequency.
Scanning the potential energy surface of iron clusters: A novel search strategy
Authors: Bobadova-Parvanova P. , Jackson K. A. , Srinivas S. , Horoi M. , Köhler C. , Seifert G.
Published: July 2002

A new methodology for finding the low-energy structures of transition metal clusters is developed. A two-step strategy of successive density functional tight binding (DFTB) and density functional theory (DFT) investigations is employed. The cluster configuration space is impartially searched for candidate ground-state structures using a new single-parent genetic algorithm [I. Rata et al., Phys. Rev. Lett. 85, 546 (2000)] combined with DFTB. Separate searches are conducted for different total spin states. The ten lowest energy structures for each spin state in DFTB are optimized further at a first-principles level in DFT, yielding the optimal structures and optimal spin states for the clusters. The methodology is applied to investigate the structures of Fe4, Fe7, Fe10, and Fe19 clusters. Our results demonstrate the applicability of DFTB as an efficient tool in generating the possible candidates for the ground state and higher energy structures of iron clusters. Trends in the physical properties of iron clusters are also studied by approximating the structures of iron clusters in the size range n=2–26 by Lennard-Jones-type structures. We find that the magnetic moment of the clusters remains in the vicinity of 3μB/atom over this entire size range.
Nonuniform thermal magnetization noise in thin films: Application to GMR heads
Authors: Safonov Vladimir L. , Bertram H. Neal
Published: July 2002

A general scheme is developed to analyze the effect of nonuniform thermal magnetization fluctuations in a thin film. The normal mode formalism is utilized to calculate random magnetization fluctuations. The magnetization noise is proportional to the temperature and inversely proportional to the film volume. The total noise power is the sum of normal mode spectral noises and mainly determined by spin-wave standing modes with an odd number of oscillations. The effect rapidly decreases with increasing mode number. An exact analytical calculation is presented for a two-cell model.
Properties of epitaxial chromium dioxide films grown by chemical vapor deposition using a liquid precursor
Authors: Anguelouch A. , Gupta A. , Xiao Gang , Miao G. X. , Abraham D. W. , Ingvarsson S. , Ji Y. , Chien C. L.
Published: July 2002

Epitaxial chromium dioxide (CrO2) films have been grown using chemical vapor deposition on (100) TiO2 substrate with chromyl chloride (CrO2Cl2) as a liquid precursor. The films are extremely smooth (rms roughness less than 4.6 Å for a 1000-Å-thick film) and have the largest spin polarization (P=98.4%) yet observed, as determined by point contact Andreev reflection. Magnetization switching properties of the films are close to those of a single-domain particle. Preliminary results on the in situ growth of exchange-biased CrO2/Cr2O3 multilayers are also reported. Although a bias field is observed, it is much smaller in comparison with the coercivity of the CrO2 film.
Surface anisotropy in ferromagnetic nanoparticles
Authors: Labaye Y. , Crisan O. , Berger L. , Greneche J. M. , Coey J. M. D.
Published: July 2002

The effect of surface anisotropy on the magnetic ground state of a ferromagnetic nanoparticle is investigated using atomic Monte Carlo simulation for spheres of radius R=6a and R=15a, where a is the interatomic spacing. It is found that the competition between surface and bulk magnetocrystalline anisotropy imposes a “throttled” spin structure where the spins of outer shells tend to orient normal to the surface while the core spins remain parallel to each other. For large values of surface anisotropy, the spins in sufficiently small particles become radially oriented either inward or outward in a “hedgehog” configuration with no net magnetization. Implications for FePt nanoparticles are discussed.
Morphological transitions of elastic domain structures in constrained layers
Authors: Slutsker J. , Artemev A. , Roytburd A. L.
Published: July 2002

The phase transformation in a constrained layer is the subject of this article. The formation and evolution of polydomain microstructure under external stress in the constrained layer are investigated by phase-field simulation and analytically using homogeneous approximation. As a result of simulation, it has been shown that the three-domain hierarchical structure can be formed in the epitaxial films. Under external stress there are two types of morphological transitions: from the three-domain structure to the two-domain one and from the hierarchical three-domain structure to the cellular three-domain structure. The results of phase-field simulation are compared with conclusions of homogenous theory and with available experimental data.
Initial growth stages of epitaxial BaTiO3 films on vicinal SrTiO3 (001) substrate surfaces
Authors: Visinoiu A. , Alexe M. , Lee H. N. , Zakharov D. N. , Pignolet A. , Hesse D. , Gösele U.
Published: July 2002

The initial growth mechanism of epitaxial BaTiO3 films is studied by combined application of atomic force microscopy, cross sectional high-resolution transmission electron microscopy, and x-ray diffraction. Epitaxial BaTiO3 thin films were grown by pulsed laser deposition on vicinal Nb-doped SrTiO3 (SrTiO3:Nb) (001) substrates with well-defined terraces. X-ray diffraction and cross sectional high-resolution transmission electron microscopy investigations revealed well-defined epitaxial films and a sharp interface between BaTiO3 films and SrTiO3:Nb substrates. The layer-then-island (Stranski–Krastanov mode) growth mechanism observed by analyzing the morphology of a sequence of films with increasing amount of deposited material has been confirmed by microstructure investigations.
Enlargement of omnidirectional total reflection frequency range in one-dimensional photonic crystals by using photonic heterostructures
Authors: Wang Xin , Hu Xinhua , Li Yizhou , Jia Wulin , Xu Chun , Liu Xiaohan , Zi Jian
Published: July 2002

We show theoretically that the omnidirectional total reflection frequency range of a multilayer dielectric reflector can be substantially enlarged as desired by using photonic heterostructures. This photonic heterostructure consists of different one-dimensional (1D) photonic crystals. The constituent 1D photonic crystals have to be properly chosen such that their omnidirectional photonic band gaps of the adjacent photonic crystals overlap each other.
Theory and examples of the inverse Frobenius–Perron problem for complete chaotic maps
Authors: Pingel D. , Schmelcher P. , Diakonos F. K.
Published: July 2002

The general solution of the inverse Frobenius–Perron problem considering the construction of a fully chaotic dynamical system with given invariant density is obtained for the class of one-dimensional unimodal complete chaotic maps. Some interesting connections between this general solution and the special approach via conjugation transformations are illuminated. The developed method is applied to obtain a class of maps having as invariant density the two-parametric beta-probability density function. Varying the parameters of the density a rich variety of dynamics is observed. Observables like autocorrelation functions, power spectra, and Liapunov exponents are calculated for representatives of this family of maps and some theoretical predictions concerning the decay of correlations are tested.
Crystal field analysis of the 3d
N ions at low symmetry sites including the ‘‘imaginary’’ terms
Authors: Chang Y. M. , Rudowicz C. , Yeung Y. Y.
Published: July 2002

The crystal field (CF) package applicable to orthorhombic or higher symmetry has been extended to enable calculations of energy levels and ligand field states for 3dN ions in crystals at arbitrary symmetry sites by taking into account the contributions of the ‘‘imaginary’’ CF terms. The full Hamiltonian matrix, including the electrostatic terms, Trees correction, the spin–orbit interaction, and the ligand field terms, is diagonalized within the whole 3dN configuration. Several techniques based on the transformation properties of the crystal field Hamiltonian have been devised to test the package for internal consistency. The package is especially suitable for analysis of the low symmetry effects for transition metal ions at sites with the tetragonal and trigonal symmetry involving the imaginary CF terms as well as monoclinic and triclinic symmetry. It can be useful in analyzing the electron paramagnetic resonance spectra of 3dN ions and in correlating the ligand field parameters with the spin Hamiltonian parameters.
A two-level system as a model for a photovoltaic solar cell
Authors: Baruch P.
Published: July 2002

The operation of a photovoltaic solar cell is discussed with a quantum two-level system as a model. A detailed-balance calculation is carried out, from which the parameters of the converter, illuminated by radiation from a black body, are exactly obtained in different geometries, taking into account radiative recombination processes. It is shown that in a 4π geometry (source fully surrounding the converter) with total radiative recombination, the thermodynamic efficiency is equal to the Carnot efficiency at zero current (open circuit): the converter behaves as an ideal thermal engine, fully reversible when delivering no power (the practical efficiency is evidently zero). The reversibility is ensured by the complete exchange of photons between the source and the converter. The current-voltage relation is obtained in all cases, and it is shown that the two-level system follows the ideal diode equation. The calculation of the thermodynamic efficiency is generalized to an energy band system (real semiconductor) with radiative recombination and is shown to be maximum at open circuit, but lower than the Carnot efficiency because of irreversibilities induced by the thermalization of carriers. The effective source temperature concept is discussed. It is shown to be valid for a two-level system, but has less physical meaning for a two-band system.
Point defects in Mg-doped lithium niobate
Authors: Sweeney K. L. , Halliburton L. E. , Bryan D. A. , Rice R. R. , Gerson Robert , Tomaschke H. E.
Published: July 2002

Optical absorption, electron spin resonance (ESR), thermoluminescence, and x-ray-induced luminescence techniques have been used to characterize point defects in LiNbO3. A series of crystals with different magnesium-doping levels and Li/Nb ratios were investigated, and the defects were introduced either by x-ray irradiation at low temperatures (10–85 K) or by reduction at high temperature in a vacuum. The samples may be classified into two groups, according to their response to radiation and reduction. A threshold (i.e., a sharp change in behavior due to a small change in composition) marks the change from one type of response to the other. The concentrations of both magnesium and lithium affect the threshold level. An electron trap, represented by an optical absorption band peaking at 1200 nm and a broad ESR spectrum centered at gc=1.82, is observed in several of the heavily-doped crystals after irradiation. A corresponding hole trap produced during the same irradiation has an optical absorption peak near 500 nm and an ESR line at gc=2.03. An intense thermoluminescence peak, obtained after x-ray irradiation at 15 K, occurs at 70 K in this latter group of crystals and correlates with the thermal decay of the electron and hole traps just described. Following vacuum reduction at 1000 °C, these heavily-doped crystals exhibit an optical absorption spectrum that can be decomposed into two bands peaking near 760 and 1200 nm, and a broad ESR spectrum with gc=1.82. The 1200-nm band and ESR signal are associated with an electron trap (identical to the one produced during the irradiations). This electron trap is suggested to be a Mg+ complex.
Neutron-spin-echo study of the reentrant spin glass EuxSr1−xS
Authors: Shapiro S. M. , Maletta H. , Mezei F.
Published: July 2002

Neutron-spin-echo (NSE) experiments were performed on polycrystalline samples of EuxSr1−xS for x=0.4 and 0.54 in the temperature range of 1.2<T<10 K and for 0.036<Q<0.18 Å−1. The x=0.4 sample exhibits a paramagnetic (PM) to spin-glass (SG) transition near Tg ∼2 K. In the x=0.54 sample, large ferromagnetic correlations develop below 5 K and a SG state appears at lower temperatures. In the NSE experiment, the spin-spin correlation function S(Q,t) is measured directly for times between 0.03<t<5 nsec. At low temperatures, both materials exhibit a weak Q dependence in the dynamics and the spins are essentially frozen over the time range explored. On heating the x=0.4 sample, the spins start to fluctuate more rapidly, but no dramatic change occurs around Tg. On heating the x=0.54 sample, S(Q,t) decreases rapidly with time. Near 5 K, S(Q,t) is exponential (e−Γt) with Γ being strongly Q dependent. Measurements of the depolarization of the scattered beam confirms the absence of true long-range ferromagnetic order below Tc.
Grain growth study in aluminum films and electromigration implications
Authors: Iyer S. S. , Wong C. Y.
Published: July 2002

Studies of grain structure, size, distribution, and geometric arrangement are gaining importance especially from the point of view of thin-film interconnects for micron and submicron semiconductor technologies. We have studied the effect of annealing on the grain growth characteristics of pure Al films. As-deposited films exhibit a uniform grain size distribution between 500 and 800 Å. With heat treatment, the average grain size increases with the largest grains exhibiting a parabolic growth with time, limited to a size in the neighborhood of 1 μ. Smaller grains, however, cannot be eliminated and about 30% of the grains are less than 2500 Å across. The grain boundaries of the smaller grains have a higher oxygen content than the rest of the film. Lines defined by liftoff exhibit a characteristic grain boundary seam along the length of the line. The implications of the above study in terms of processibility and reliability with respect to electromigration are also discussed.
Diffusion theory of the low-pressure positive column with two-step ionization from a metastable state. I
Authors: Romeiras F. J. , Ferreira C. M.
Published: July 2002

We present a self-consistent theory of a low-pressure positive column with two-step ionization from a metastable state in which the coupling between the continuity equations for the electrons and the metastables is appropriately taken into account. The corresponding boundary value problem is solved numerically for both cylindrical and planar geometries and vanishing electron and metastable densities at the boundary, for a wide range of the two parameters on which the problem is shown to depend. Curves of the electron and metastable density profiles, of the eigenvalues, and of conveniently defined generalized diffusion lengths for both types of particles are presented. We also show that the eigenvalues together with Boltzmann calculations of the electron rate coefficients and transport parameters provide similarity laws for a steady-state positive column with radius R: ratio of the electric field to the gas density, E/N vs NR for I/R=const and ratio of the metastable density to the gas density, nM(0)/N vs I/R for NR=const, where I is the discharge current. An application to argon is discussed.
Use of eigenfunctions for solving radiation transfer in anisotropically scattering, plane-parallel media
Authors: Thynell S. T. , Özişik M. N.
Published: July 2002

Radiation transfer in an absorbing, emitting, gray, anisotropically scattering, plane-parallel medium is solved by using the naturally occurring eigenfunctions, and expressions for the forward and backward radiation intensities, the incident radiation, and forward and backward radiation heat fluxes are presented. To illustrate the method of solution, the situation involving an externally incident isotropic radiation at the boundary surface x=0 and no energy sources in the medium is considered for the cases of four different scattering phase functions. It is shown that the convergence to the exact results is fast, and that lower order approximations are accurate. In addition, the present method of solution has an excellent potential for generalization to problems of plane-parallel media without azimuthal symmetry and to problems in cylindrical and spherical geometries.
An effective dipole model for predicting band offsets in semiconductor heterojunctions
Authors: Ruan Ying-Chao , Ching W. Y.
Published: July 2002

A simple model to predict the band offsets in semiconductor heterojunctions is presented. The model is based on the formation of an effective dipole induced by the penetration of bulk band states into a quantum barrier in the neighboring semiconductor. Application of the model to a large number of heterojunctions gives band offset values in good agreement with experiment.
Hydrogen-induced amorphization and its effect on magnetic properties of the Laves-phase GdFe2 compound
Authors: Aoki K. , Nagano M. , Yanagitani A. , Masumoto T.
Published: July 2002

The influence of hydrogenation on the structural change and magnetic properties of the Laves-phase GdFe2 compound has been investigated. Crystalline c-GdFe2 absorbs 4.4 hydrogen atoms per formula unit without any change in the crystal structure below 423 K. On the other hand, GdFe2 transforms to amorphous a-GdFe2H3.6 by hydrogenation from 423 to 523 K, where the decomposition of GdFe2 into GdH2 and α-Fe is prevented. The same amount of hydrogen (3.6 atoms per formula unit) dissolves in amorphous alloys produced by both hydrogenation and rapid quenching. Hydrogenation sharply reduces the Curie temperature of the crystalline alloy from 818 to 107 K, in contrast to the Curie temperature of 443 K for the hydrogen-induced amorphous alloy. The temperature dependence of the magnetization of the hydrogen-induced amorphous alloy is identical to that of the hydrogenated rapidly quenched amorphous alloy, suggesting similarity of their structures. The magnetization of the hydrogen-induced amorphous alloy shows a broad peak around 300 K. Above 300 K the magnetization of the amorphous alloys is largely uninfluenced by hydrogen. Below the peak it is reduced by hydrogenation.
Progress of the amorphization reaction during mechanical alloying in Fe-Zr
Authors: Hellstern E. , Schultz L.
Published: July 2002

The amorphization process in mechanically alloyed Fe-Zr powders has been investigated by optical and scanning electron microscopy, x-ray diffraction, differential scanning calorimetry, and saturation magnetization measurements. The transformation from the initially layered composite to the amorphous alloy occurs between 2 and 16 h milling time. Longer milling leads to a homogenization within the particles. Prior to amorphization the internal stress within the powder particles strongly increases.
Analysis and assessment of electromagnetic ring expansion as a high-strain-rate test
Authors: Gourdin William H.
Published: July 2002

The rapid expansion of electromagnetically launched thin metal rings is analyzed in detail, with particular emphasis on the determination of the expansion speed, currents, and specimen temperature as a function of launch voltage. Electrodynamic calculations for copper, tin, and lead rings are presented and compared with typical experimental results. Currents derived from Rogowski-probe records and expansion speeds measured with a velocity interferometer (VISAR) are found to be in excellent agreement with these calculations. Currents are large, of the order of tens of kiloamperes, and heating of the specimen can be significant. It is found that, for good conductors such as copper, the maximum useful strain rate will be limited by the Joule heating of the specimen. For materials with resistivities and strengths substantially larger than those of copper, direct expansion of a specimen ring is difficult. Use of a composite ring, in which a high-conductivity driver is used to launch a low-conductivity specimen, is discussed as a means of overcoming this limitation.
Study on catalytic chemical vapor deposition method to prepare hydrogenated amorphous silicon
Authors: Matsumura Hideki
Published: July 2002

A new type of thermal chemical vapor deposition (CVD) method is presented. In the method, material gases are decomposed by catalytic or pyrolytic reaction with a heated catalyzer, so that films can be deposited at temperatures less than 300 °C without any plasma or photochemical excitation, and the method is particularly called ‘‘Catalytic-CVD.’’ Hydrogenated amorphous silicon films are deposited by this method, and the deposition mechanism is also investigated. It is found that device-quality amorphous silicon films can be obtained and that inactive species, which are generated at the catalyzer and transported without gas-phase reactions, are key species to make a high-quality film by this method.
Raman and resistivity investigations of carbon overcoats of thin-film media: Correlations with tribological properties
Authors: Marchon B. , Heiman N. , Khan M. R. , Lautie A. , Ager J. W. , Veirs D. K.
Published: July 2002

Tribological measurements have been performed on a series of carbon films deposited on untextured 95-mm magnetic disks by magnetron sputtering. Raman spectra and resistivity data of these films have been correlated to friction and wear performance. Tribochemical wear, as estimated by the rate of frictional buildup, is correlated to the G-band position in the Raman spectrum. Abrasive wear is best correlated to the I(D)/I(G) intensity ratio of the D and G bands, as well as to electrical resistivity. Interpretation of these data using structural models of amorphous carbon films is discussed.
Longitudinal electron diffusion coefficients in gases: Noble gases
Authors: Pack J. L. , Voshall R. E. , Phelps A. V. , Kline L. E.
Published: July 2002

Values of the ratio of the longitudinal diffusion coefficient to mobility DL/μ for electrons in He, Ar, Kr, and Xe are derived from current waveforms obtained during earlier measurements of electron mobility. The electric field to gas density ratios E/N cover the wide range of 10−3 to 20 Td, thereby bridging previous experiments at low E/N to recent experiments at high E/N. Here 1 Td=1×10−21 V m2. The corresponding DL/μ values range from 0.0066 eV for thermal electrons at 77 K to 10 eV. In addition to the well-known peak in DL/μ for Ar at E/N between 0.01 and 0.1 Td caused by the Ramsauer minimum in the momentum transfer cross section, we find previously unreported low-energy peaks in DL/μ vs E/N in Kr and Xe and previously unreported pronounced leveling-off in DL/μ at E/N≳8 Td in Ar, Kr, and Xe. Calculations of transport coefficients using numerical solutions of the Boltzmann equation and cross section sets in the literature give good agreement with experiment from E/N producing thermal electrons up to average energies ≊10 eV and E/N up to 100 Td, the upper limit of our calculations. The leveling off of DL/μ at high E/N is caused by inelastic collisions.
Interaction force detection in scanning probe microscopy: Methods and applications
Authors: Dürig U. , Züger O. , Stalder A.
Published: July 2002

Fundamental aspects of interaction force detection and force microscopy are discussed. A formalism is developed for studying the dynamics of experimental setups in general terms. The analysis focuses on stability criteria and resonant properties of the force sensor. The latter are important for measuring interaction force gradients. Experimental techniques used for interaction force detection are examined in detail. Finally, experimental results are presented that demonstrate the potential of combining atomic scale interaction force detection with scanning tunneling microscopy.
Effects of lattice mismatch and thermal annealing on deep traps and interface states in Ga0.92In0.08As(n+)/GaAs(p) heterojunctions
Authors: Huang Z. C. , Wie C. R. , Johnstone D. K. , Stutz C. E. , Evans K. R.
Published: July 2002

The effects of the lattice-mismatch-induced defects on deep level traps in Ga0.92In0.08As(n+)/GaAs(p) heterojunction diodes have been studied by means of various deep level transient spectroscopy techniques and the frequency-dependent capacitance-voltage (C-V-f) characteristics. Three hole traps at 0.58, 0.42, and 0.27 eV were observed. We attribute the 0.42 eV trap to Cu impurity, the 0.58 eV trap to VGa or Fe, and the 0.27 eV trap to a complex associated with the 0.42 and 0.58 eV traps. Depth profiles of these hole traps in the GaAs side were measured in different lattice-mismatched samples. The depth profile data near the interface and from deep inside the bulk show evidence of impurity gettering by the mismatched interface. We also found that the concentrations of these traps were reduced by rapid thermal annealing. A U-shaped energy distribution of the interface states was obtained from the C-V-f measurements. For an in-plane mismatch greater than 0.25%, the interface state density shows no obvious dependence on the in-plane lattice mismatch, while at smaller mismatch the interface state density increases with increasing mismatch. The interface state density was on the order of 1011 cm−2.
A radial finite model of thermal lens spectrometry and the influence of sample radius upon the validity of the radial infinite model
Authors: Shen Jun , Snook Richard D.
Published: July 2002

Conventional thermal lens models unrealistically assume that the sample has an infinite radius. Taking realistic radial sample boundary conditions into account, a cw laser-induced mode-mismatched dual-beam thermal lens model is presented. Comparison of these two kinds of models has been made, which gives the quantitatively necessary and sufficient condition for the validity of the radial-infinite treatment on the sample: The sample radius should be larger than 2.5 times both excitation beam and probe beam radii in the sample.
Magnetic and optical properties of γ-Fe2O3 nanocrystals
Authors: Vassiliou John K. , Mehrotra Vivek , Russell Michael W. , Giannelis Emmanuel P. , McMichael R. D. , Shull R. D. , Ziolo Ronald F.
Published: July 2002

γ-Fe2O3 nanocrystals have been synthesized in a polymer matrix by an ion exchange and precipitation reaction. The particles are crystalline with a mean diameter of 85 Å. The unit cell volume is 1735.2 Å3 corresponding to a unit cell compression of 0.35%. Magnetization and susceptibility data from experiment and computer simulations indicate that the system is superparamagnetic. The calculated magnetic anisotropy for the particles is two orders of magnitude larger than that of bulk γ-Fe2O3 . The optical absorption edge is red shifted with respect to that of an epitaxially grown single-crystal film of γ-Fe2O3 . The red shift is attributed to lattice strain in the small particles.
Monte Carlo simulations of the transport of laser-ablated atoms in a diluted gas
Authors: Kools J. C. S.
Published: July 2002

The interaction between gas clouds, created by laser ablation, and a diluted gas background is studied by means of Monte Carlo simulations. More specifically, the effect of elastic collisions between ablated atoms and background gas atoms on the kinetic energy and spatial distributions of atoms arriving at the substrate is investigated. The pressure regime under study is typically the range 0.1–200 mTorr. Thermalization is observed in a pressure range that varies over two orders of magnitude, dependent on the initial energy of the atoms and the ratio of the background gas and ablated atom molecular masses. During the first stages of thermalization, forward focusing is found to occur.
Optical and electron spin resonance spectroscopy of Ti3+ and Ti4+ in Al2O3
Authors: Yamaga M. , Yosida T. , Hara S. , Kodama N. , Henderson B.
Published: July 2002

The absorption spectrum of Ti3+:Al2O3 crystals grown in a reducing atmosphere consists of the main blue-green absorption band and a weak infrared band. This infrared absorption decreases in intensity on annealing the as-grown samples in a reducing atmosphere. The analysis of electron spin resonance spectra of the as-grown samples, in which the infrared absorption band is observed, indicates that the center associated with the band is a cluster involving Ti3+ and Ti4+ ions with a neighboring, charge compensating Al3+ vacancy. The coexistence of a Ti4+ ion and an Al3+ vacancy in the neighborhood of the Ti3+ ion weakens the crystal field at this ion more than a single Ti4+ ion, giving rise to a red shift of the Ti3+ absorption.
Magnetism and magnetization structure of c-axis oriented single-crystal cobalt films grown epitaxially on rigid underlayers
Authors: Krishnan Kannan M. , Takeuchi T. , Donnet D. M. , Tanahashi K.
Published: July 2002

c-axis oriented, single-crystal, Co films were grown on mica using either Ti or Ru underlayers and exhibited the following crystallographic orientation relationship (00.1)mica ∥ (00.1)underlayer ∥ (00.1)Co, and [11.0]mica ∥ [10.0]underlayer ∥ [10.0]Co. Fundamental magnetic properties of the films were measured: Hc∼4 kA/m, Bs∼1.95 T, and Ku∼5×105 J/m3. The as-grown magnetization structure of thinner (t≤30 nm) films shows mainly 180° domain walls with a uniform distribution of crossties, while thicker (t=60 nm) samples show stripe domains with a width of 75 nm. In spite of their excellent c-axis orientation (FWHMCo(002)∼1.5°), the effective anisotropy of these films is in-plane but with a small perpendicular magnetization component. These features in the hysteresis behavior are consistent with a simple noninteracting particle model incorporating the shape anisotropy of the small sized grains. For thinner samples the validity of this model has been verified by annealing experiments.
Molecular slip at the solid-liquid interface of an acoustic-wave sensor
Authors: Ferrante Francesco , Kipling Arlin L. , Thompson Michael
Published: July 2002

Molecular slip is expected to occur at the interface between the surface of a bulk-acoustic-wave quartz chemical sensor and a liquid with which it is in contact. A new property of the interface, called the interfacial slip parameter, accounts for molecular slip. It is introduced in the theory of the sensor-liquid system by generalizing the no-slip boundary condition. The interfacial slip parameter is a complex-valued quantity which is defined as the displacement of a particle of liquid in contact with the sensor surface divided by the displacement of a particle on the surface of the sensor. The theoretical expression for the impedance of the sensor is derived in terms of the interfacial slip parameter. The impedance of the sensor is measured by the network analysis method for hydrophilic and hydrophobic surfaces in contact with water-glycerol solutions. The experimental values of impedance are fitted to the theory by nonlinear regression analysis to find the interfacial slip parameter. A mechanical model of molecular slip is devised which explains the variation of the interfacial slip parameter with the viscosity of the liquid. The results indicate that there is some slip in low-viscosity liquids, near the viscosity of water, and very little slip in liquids of high viscosity, near that of glycerol.
Magnetic and crystal structure of the novel compound Nd3Fe29−xTix
Authors: Hu Z. , Yelon W. B.
Published: July 2002

The structure of the compound previously reported as Nd2Fe19−xTix has been solved by powder neutron diffraction, which reveals a monoclinic cell and a stoichiometry of Nd3Fe29−xTix (x=1.24) and two formula units per unit cell. This low symmetry, and the large number of crystallographically unique sites (17), lead to a wide range of Fe—Fe bond lengths (from 2.36 to 3.01 Å) in a nearly continuous band. The phase forms through the replacement of two-fifths of the rare earths in the RFe5 phase by Fe-Fe dumbbells. The magnetic moments at room temperature lie along the monoclinic a axis with an average iron moment of 1.05 μB, while the magnetic moments at 12.5 K lie in the a-b plane with an average iron moment of about 1.36 μB.
Crystallographic orientation of epitaxial BaTiO3 films: The role of thermal-expansion mismatch with the substrate
Authors: Srikant V. , Tarsa E. J. , Clarke D. R. , Speck J. S.
Published: July 2002

Expitaxial ferroelectric BaTiO3 thin films have been grown on (001) MgO and MgO-buffered (001) GaAs substrates by pulsed laser deposition to explore the effect of substrate lattice parameter. X-ray-diffraction studies showed that the BaTiO3 films on both MgO single-crystal substrates and MgO-buffered (001) GaAs substrates have a cube-on-cube epitaxy; however, for the BaTiO3 films grown on MgO the spacing of the planes parallel to the substrate was close to the c-axis dimension of the unconstrained tetragonal phase, whereas the BaTiO3 films on MgO/GaAs exhibited a spacing closer to the a-axis dimension of the unconstrained tetragonal phase. The cube-on-cube epitaxy was maintained through the heterostructures even when thin epitaxial intermediate buffer layers of SrTiO3 and La0.5Sr0.5CoO3 were used. The intermediate layers had no effect on the position of the BaTiO3 peak in θ-2θ scans. Together, these observations indicate that, for the materials combinations studied, it is the thermal-expansion mismatch between the film and the underlying substrate that determines the crystallographic orientation of the BaTiO3 film. Preliminary measurements indicate that the BaTiO3 films are ‘‘weakly’’ ferroelectric.
High efficiency AlGaAs/Si monolithic tandem solar cell grown by metalorganic chemical vapor deposition
Authors: Soga T. , Kato T. , Yang M. , Umeno M. , Jimbo T.
Published: July 2002

The improvements of the AlGaAs solar cell grown on the Si substrate and the AlGaAs/Si tandem solar cell by metalorganic chemical vapor deposition have been investigated. The active-area conversion efficiency of the Al0.1Ga0.9As solar cell on the Si substrate as high as 12.9% has been obtained by improving the growth sequence and adopting an Al compositionally graded band emitter layer. A high efficiency monolithic AlGaAs/Si tandem solar cell with the active-area conversion efficiency of 19.9% and 20.6% (AM0 and 1 sun at 27 °C) under two-terminal and four-terminal configurations, respectively, is demonstrated.