bluuebunny/crossref_metadata_2025 huggingface dataset:
Bismuth zinc niobate pyrochlore dielectric thin films for capacitive applications
Authors: Ren Wei , Trolier-McKinstry Susan , Randall Clive A. , Shrout Thomas R.
Published: July 2002

Bi 2 O 3 –ZnO–Nb 2 O 5 pyrochlore thin films were prepared on platinum coated Si wafers using a metalorganic deposition process. The structures, morphologies, and dielectric properties of films with two compositions: (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 and Bi2(Zn1/3Nb2/3)2O7, were investigated. Thin films of (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 have a cubic pyrochlore phase when crystallized at 550 °C or higher. The crystal structure of Bi2(Zn1/3Nb2/3)2O7 thin films was dependent on the firing temperature; the films showed the cubic pyrochlore phase at temperatures below 650 °C, and a pseudo-orthorhombic pyrochlore structure at 750 °C. A mixture of cubic and pseudo-orthorhombic structures was found in thin films crystallized at 700 °C. (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 films fired at 750 °C had a dielectric constant of ∼150 and a negative temperature coefficient of capacitance of −400 ppm/°C. Bi2(Zn1/3Nb2/3)2O7 thin films fired at 750 °C had a smaller dielectric constant of ∼80 and a positive temperature coefficient of capacitance of 150 ppm/°C. The dielectric constants of the thin films are composition, structure, and firing temperature dependent. The loss tangents of both types of films were smaller than 0.008. Bias voltage dependence of dielectric constant showed that the cubic (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 films fired at 750 °C were tunable, while the pseudoorthorhombic Bi2(Zn1/3Nb2/3)2O7 films were nearly field independent. The relatively large dielectric constants, small loss tangents, controllable temperature coefficients of capacitance, and tunability of the dielectric constant suggests that Bi2O3–ZnO–Nb2O5 thin films have potential applications for integrated microwave components and decoupling capacitors.
Shear response of layered silicate nanocomposites
Authors: Krishnamoorti Ramanan , Ren Jiaxiang , Silva Adriana S.
Published: July 2002

The linear and nonlinear melt state viscoelastic properties for a series of layered silicate based intercalated polymer nanocomposites are studied to elucidate the role of highly anisotropic nanometer thick layers in altering the flow properties of such hybrids. The steady shear viscosities for the nanocomposites exhibit enhanced shear-thinning at all shear rates, with the viscosity at high shear rates being almost independent of silicate loading and comparable to that of the unfilled polymer. Further, the elasticity, as measured by the first normal stress difference, when compared at constant shear stress is surprisingly independent of the silicate loading and identical to that of the unfilled polymer. This unique combination of unfilled polymerlike viscosity and elasticity for these filled nanocomposites, is attributed to the ability of the highly-anisotropic layered silicates to be oriented in the flow direction and results in a minimal contribution by the silicate layers to both the viscosity and the elasticity of the hybrids.
Linewidths of excitonic luminescence transitions in AlGaN alloys
Authors: Coli Giuliano , Bajaj K. K. , Li J. , Lin J. Y. , Jiang H. X.
Published: July 2002

In this work, we present a study of the behavior of linewidths of excitonic photoluminescence transitions measured at 10 K in AlGaN alloys as a function of Al concentration. Samples we have investigated are grown by low-pressure metalorganic chemical vapor deposition on (0001) oriented sapphire substrates with low-temperature GaN buffer layers. The Al composition ranged from 0%–35%. We find that the values of the excitonic linewidth increase as a function of Al concentration and agree very well with those calculated using a model in which the broadening effect is assumed to be due to compositional disorder in semiconductor alloys. The values of the excitonic linewidths measured in our samples are considerably smaller than those reported recently, thus attesting to the high quality of our samples.
Epitaxial growth of Pb(Zr0.2Ti0.8)O3 on Si and its nanoscale piezoelectric properties
Authors: Lin A. , Hong X. , Wood V. , Verevkin A. A. , Ahn C. H. , McKee R. A. , Walker F. J. , Specht E. D.
Published: July 2002

We have demonstrated a route to epitaxial Pb(Zr0.2Ti0.8)O3 on (001) Si that exhibits a uniform piezoelectric response down to nanoscale levels through the utilization of an insulating, single-crystalline SrTiO3 transition layer. These structures, which were grown by a combination of molecular-beam epitaxy and off-axis magnetron sputtering, have a surface roughness of <5 Å, with piezoelectric microscopy measurements revealing a piezoelectric coefficient of ∼50 pm/V that is switchable down to sub-100-nm dimensions.
Tuning the optical properties of thiophene oligomers toward infrared emission: A theoretical study
Authors: Daminelli G. , Widany J. , Di Carlo A. , Lugli P.
Published: July 2002

The structural and electronic effects of various substituents on thiophene-based oligomers have been studied using a density-functional-based tight-binding scheme. Optical absorption spectra have been calculated applying the semiempirical Hartree–Fock-based ZINDO/S method. Electron withdrawing or donating substituents have been placed on the two terminal thiophene units in α, β, and γ positions with respect to the sulfur atom of the thiophene. The presence of substituents has in general only minor effects on the geometry of the parent molecules, but strong influence on the optical properties. Substitution in α and γ positions always leads to a redshift. The effect of β substitution is much weaker and electron withdrawing groups even cause a modest blueshift. The calculated shifts can be explained in terms of modifications of energy and shape of the frontier orbitals.
Direct current conduction in SiC powders
Authors: Mårtensson E. , Gäfvert U. , Lindefelt U.
Published: July 2002

Silicon carbide (SiC) powder is used in nonlinear field grading materials. The composite material, consisting of an insulating polymer matrix filled with the SiC-grains, is usually a percolated system with established conducting paths. In order to explain the properties, the electrical characteristic and conduction mechanisms of the SiC powder itself are of interest. SiC powders have been studied by current–voltage measurements and the influences of grain size and doping have been investigated. The macroscopic current characteristics of green and black SiC powders can be described by the transport mechanisms at the grain contacts, which can be modeled by Schottky-like barriers. The SiC is heavily doped and tunneling by field emission is the dominating conduction mechanism over the major part of the nonlinear voltage range. It is suggested that preavalanche multiplication influences the current at the highest voltages, especially for p-type black SiC.
Direct optimization of the atomic-orbital density matrix using the conjugate-gradient method with a multilevel preconditioner
Authors: Larsen Helena , Olsen Jeppe , Jørgensen Poul , Helgaker Trygve
Published: July 2002

Based on a recently proposed exponential parametrization of the one-electron atomic-orbital density matrix of a single-determinant wave function [Chem. Phys. Lett. 327, 397 (2000)], we present an implementation of the direct optimization of the atomic-orbital density matrix as an alternative to the diagonalization of the Fock–Kohn–Sham matrix when solving the Roothaan–Hall self-consistent field equations. The optimization of the density matrix is carried out by the conjugate-gradient method with a multilevel nondiagonal preconditioner and is well suited to linear scaling. Although a diagonal preconditioner may be sufficient for minimal basis sets and large highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gaps, a nondiagonal preconditioner is needed in more difficult cases—that is, for basis sets with polarization and diffuse functions and for systems with small HOMO–LUMO gaps. Redundancies of the exponential parametrization of the density matrix are handled by a projection technique, thereby avoiding singular equations in the optimization of the density matrix.
Comment on “A fallacious argument in the finite time thermodynamic concept of endoreversibility” [J. Appl. Phys. 83, 4561 (1998)]
Authors: Andresen Bjarne
Published: July 2002

In his paper D. P. Sekulic [J. Appl. Phys. 83, 4561 (1998)] advances a number of arguments about finite-time thermodynamics which call for refutation as well as comment. It provides an opportunity to put into print a clarification of just what finite-time thermodynamics is, what it is not, and how it is related to some of the other, parallel efforts in the treatment of irreversible systems and their performance. The two major points of the present comment are (i) finite-time thermodynamics is a general theory independent of the concept of endoreversibility, which was originally introduced only to generate simple examples, and (ii) the strict division between the system to be optimized and the given fixed surroundings is essential to a correct interpretation of the results. I will refrain from commenting on details in D. P. Sekulic [J. Appl. Phys. 83, 4561 (1998)] in order not to obscure these main points.
Crowdions in atomic cryocrystals and metals with fcc and bcc lattices
Authors: Natsik V. D. , Smirnov S. N. , Nazarenko E. I.
Published: July 2002

Numerical values are obtained for the parameters of the crystal-field potentials and crowdions (self-energy, effective mass, characteristic length) for Ar and Kr fcc cryocrystals, Cu and Al fcc metals, and α- and δ-Fe bcc metals. The calculations are performed assuming that the interatomic interaction in the crystals can be described by empirical Lenard–Jones and Morse pair potentials. A new algorithm is developed and used for calculating the crystal-field potentials. The algorithm is based on a representation of the crystal lattice as a collection of parallel atomic rows. An analytic expression in the form of a trigonometric series is obtained for the potential describing the interaction between an atom in a close-packed row and the crystal matrix. An explicit analytic expression is also obtained for the energy parameter characterizing the interatomic interaction inside a distinguished row. It is shown that the main condition for weak coupling between close-packed rows and the crystal matrix, admitting the existence of crowdions and the application of the long-wave approximation for describing crowdions, is satisfied in all crystals investigated.
Interfacial characteristics of a carbon nanotube–polystyrene composite system
Authors: Liao Kin , Li Sean
Published: July 2002

The performance of a composite material system is critically controlled by the interfacial characteristics of the reinforcement and the matrix material. Here we report a study on the interfacial characteristics of a carbon nanotube (CNT)-reinforced polystyrene (PS) composite system through molecular mechanics simulations and elasticity calculations. In the absence of atomic bonding between the reinforcement and the matrix material, it is found that the nonbond interactions consists of electrostatic and van der Waals interaction, deformation induced by these forces, as well as stress/deformation arising from mismatch in the coefficients of thermal expansion. All of these contribute to the interfacial stress transfer ability, the critical parameter controlling material performance. Results of a CNT pullout simulation suggests that the interfacial shear stress of the CNT–PS system is about 160 MPa, significantly higher than most carbon fiber reinforced polymer composite systems.
L
1
,(001)
-oriented FePt:B2O3 composite films for perpendicular recording
Authors: Yan M. L. , Zeng H. , Powers N. , Sellmyer D. J.
Published: July 2002

A multilayered deposition structure was developed for fabricating FePt:B2O3 films. We successfully obtained nanostructured FePt:B2O3 films with FePt grains aligned perpendicular to film plane by postannealing the as-deposited multilayers at 550 °C for 30 min. It was found that development of (001) texture depends strongly on the total film thickness, initial B2O3 layer thickness, and Fe concentration. Nearly perfect (001) orientation of FePt-ordered grains can be obtained in the films with small total film thickness, large initial B2O3 layer thickness, and slightly higher Fe concentration. Our results show that highly (001) oriented films with ordered fct phase have significant potential as perpendicular media for extremely high-density recording.
Needle-shaped silicon carbide nanowires: Synthesis and field electron emission properties
Authors: Wu Z. S. , Deng S. Z. , Xu N. S. , Chen Jian , Zhou J. , Chen Jun
Published: July 2002

Bunches of needle-shaped silicon carbide (SiC) nanowires were grown from commercially available SiC powders in thermal evaporation process and using iron as catalyst. Their structure and chemical composition were studied by Raman spectroscopy and high-resolution transmission electron microscopy. The powder of these nanowires may be easily dispersed, and was used to form samples of field electron emitters. The needle shape of individual nanowires is well-suited to field electron emission. Stable emission with current density of 30.8 mA/cm2 was observed at fields as low as 9.6 V/μm, and current density of up to 83 mA/cm2 was recorded.
Very-low-threshold photonic band-edge lasers from free-standing triangular photonic crystal slabs
Authors: Ryu Han-Youl , Kwon Soon-Hong , Lee Yong-Jae , Lee Yong-Hee , Kim Jeong-Soo
Published: July 2002

Photonic band-edge lasers are realized in the free-standing triangular photonic crystal slab with InGaAsP quantum wells at 80 K. Surface-emitting lasing actions are observed from the samples whose wavelengths correspond to the second or the third photonic band edge near the Γ point of the band structure, which is confirmed by the three-dimensional finite-different time-domain calculation. A very low threshold of 35 μW (incident pump power) is achieved from the laser operating near the third transverse-electric-mode band edge. This low threshold is benefited from low optical losses near the Γ point as well as reduced surface recombination carrier losses and enhanced material gain at low temperature. It is interesting to note that this two-dimensional distributed feedback lasing action is observed from a surprisingly small pump area of 6 μm in diameter.
Frequency self-upshifting of intense microwave radiation producing ionization in a thin gaseous layer
Authors: Bakunov M. I. , Bystrov A. M. , Gildenburg V. B.
Published: July 2002

The mechanism of frequency upshifting of an electromagnetic wave incident on a thin (compared to the wavelength) layer of a time-varying plasma, which was found recently within the linear parametric consideration [Phys. Plasmas 8, 2987 (2001)], is extended to the nonlinear case when ionization in the layer is produced by the incident wave itself due to breakdown process in the gas. The frequency self-upshifting is achieved for the oblique incidence of a powerful p-polarized wave and connected with the transient excitation of free plasma oscillations in the layer at the moment when the arising plasma density passes the critical value and, thus, plasma resonance in the layer occurs. The excited plasma oscillations, whose frequency follows growing plasma frequency, sustain a high value of the electric field in the layer even after the plasma frequency exceeds the frequency of the incident wave. This and the electron temperature inertia provide fast creation of a highly overdense plasma. Gradual radiation of the oscillations gives rise to the upshifted outgoing electromagnetic waves. The phenomenon is considered both analytically and numerically. Concrete estimations of the effect for a cm range microwave dicharge in the air of 0.1–1 Torr pressure are given.
Electrodeposition of patterned CdSe nanocrystal films using thermally charged nanocrystals
Authors: Islam Mohammad A. , Herman Irving P.
Published: July 2002

A dc electric field is used to attract charged CdSe nanocrystals in hexane to rapidly form very smooth, robust, large-area, several micron-thick films of equal thickness on both electrodes. This deposition on both electrodes implies there are both positively and negatively thermally charged dots, unlike conventional electrophoretic deposition. With patterned electrodes, controllable and locally selective assembly is achieved.
Quantitative analysis of heart rate variability
Authors: Kurths J. , Voss A. , Saparin P. , Witt A. , Kleiner H. J. , Wessel N.
Published: July 2002

In the modern industrialized countries every year several hundred thousands of people die due to sudden cardiac death. The individual risk for this sudden cardiac death cannot be defined precisely by common available, noninvasive diagnostic tools like Holter monitoring, highly amplified ECG and traditional linear analysis of heart rate variability (HRV). Therefore, we apply some rather unconventional methods of nonlinear dynamics to analyze the HRV. Especially, some complexity measures that are based on symbolic dynamics as well as a new measure, the renormalized entropy, detect some abnormalities in the HRV of several patients who have been classified in the low risk group by traditional methods. A combination of these complexity measures with the parameters in the frequency domain seems to be a promising way to get a more precise definition of the individual risk. These findings have to be validated by a representative number of patients.
Quantum coherence, evolution of the Wigner function, and transition from quantum to classical dynamics for a chaotic system
Authors: Kolovsky Andrey R.
Published: July 2002

The paper deals with dynamics of a quantum chaotic system under influence of an environment. The effect of an environment is known to destroy the quantum coherence and can convert the quantum dynamics of a system to classical. We use a semiclassical technique for studying the process of decoherence. The condition for transition from quantum to classical dynamics is obtained in general form and checked numerically for a particular chaotic system, known as quantum the standard map on a torus. The relevance of the obtained results to the problem of correspondence between quantum and classical mechanics is briefly discussed.
Mean-field analysis on the Macintosh
Authors: Alex Michael , Crain Jason
Published: July 2002

A brief overview of magnetic ordering in solids is presented and the basic principles of mean-field theory are outlined. An algorithm for determining the molecular field coefficients of ferrimagnetic materials having several magnetic sublattices is developed that reproduces the observed temperature dependence of magnetization. The method is then applied to several ferrimagnetic garnets having very different thermomagnetic properties. The calculations were performed on a Macintosh II computer running MATHEMATICA■.
Novel Fourier methods for boundary value problems
Authors: Cameron Seth A.
Published: July 2002

A novel Fourier technique for solving boundary value problems is introduced. The technique, called Fourier projection, is based on the geometric properties of vector calculus operations in reciprocal space. For demonstration, an algorithm based on Fourier projection for solving the biomagnetic volume current problem is presented.
Computer-assisted photometry using simulated annealing
Authors: Groisman G. , Parker J. R.
Published: July 2002

The computational technique known as simulated annealing, which traditionally has been used to solve problems in combinatorial optimization, can be quite effectively used to fit functions of many parameters to a set of data. This paper discusses the application of simulated annealing to the fitting of Moffat functions to crowded field star images. Overlapping star profiles can be fit simultaneously and missed stars can be located on the χ2 value of the fit, and the resulting functional model yields excellent photometric results. This method has been integrated into a photometry system that allows the user to select the accuracy of the results at the expense of time needed to perform the calculations.
Numerical tracking of small deviations from analytically known periodic orbits
Authors: Fairén Victor , Martin Pablo , Manuel Ferrándiz José
Published: July 2002

In this article we present in detail the G and Γ function expansions of the solution of a perturbed linear system of ordinary differential equations, and its implementation as the recursion relation for a one-step integrator. The two first terms of this expansion offer an exact solution to the unperturbed system, a fact which makes the corresponding error for the solution of the perturbed system proportional to the strength of the perturbation. This contrasts with what is offered by popular algorithms, which is a flat response of the error with the perturbation strength. Accordingly, the scheme presented here provides a substantial gain in accuracy when it comes to detailed monitoring of very small deviations from the unperturbed orbit. Finally, we brush over its generalization to linear multistep algorithms.
The inflating wormhole: a mathematica animation
Authors: Roman Thomas A.
Published: July 2002

It has been recently speculated that the laws of physics might allow an arbitrarily advanced civilization to construct traversable wormholes for interstellar travel. If submicroscopic traversable wormholes existed in the very early universe, then it is conceivable that an inflationary phase might have enlarged some of them to macroscopic size. In a recent paper the author explored this possibility, and in so doing found it useful to construct some visualizable (albeit simplified) models as an aid to intuition. To purpose of the current paper is to present the details of these computer simulations, and in the process introduce the reader to some techniques of simple graphing in mathematica. The models take the form of animated embedding diagrams of traversable wormholes in an inflationary universe.
GRPP, a scientific programming language preprocessor designed using lex and yacc
Authors: Blackburn James Kent
Published: July 2002

There have been many advances in the functionality and usefulness of high level computer languages since the early incarnations of FORTRAN. The recent trend towards structured and object oriented languages has given the scientific programmer very sophisticated tools for developing applications motivated by data, procedure, and reusability. However, in many areas of scientific and numerical software development, what is needed most is a programming language possessing the basic grammar familiar from analytical discussions of the field of study. The use of lex and yacc for developing specialized computer dialects or languages provide a powerful means of injecting the customary analytic notation and grammar into a scientific programming project. A general relativity preprocessor, grpp, is presented as such an application of lex and yacc to extend C style computer languages to handle tensor analysis in a manner consistent with that which is taught to students of general relativity, differential geometry, and tensor analysis. As an application demonstrating the usefulness of grpp, the fully relativistic four-dimensional trajectory of a test particle around a Kerr black hole is calculated using a very simple to follow, bug-free program taken straight out of any text on general relativity or differential geometry.
Computation of the Whittaker functions W
k,μ(z) with series expansions and Padé Approximants
Authors: de Izarra C. , Vallée O. , Picart J. , Tran Minh N.
Published: July 2002

Different mathematical representations of the Whittaker functions Wk,μ(z) are investigated, allowing to compute them numerically with high precision for various values of the argument z and of the parameters k and μ. Application of Padé Approximants with Wynn’s algorithm on an asymptotic series expansion appears to be a powerful numerical method. © 1995 American Institute of Physics.
Group theory (symmetries) in a computer game
Authors: Biswas Tarun
Published: July 2002

Computer games can lay early seeds of mathematical concepts in children. The game discussed here is a computer puzzle that allows the movement of pieces through exchanges as well as in-place rotations of 180° about the x, y, or z axis. As a result, the player becomes comfortable with some nontrivial aspects of the permutation and rotation groups. © 1998 American Institute of Physics.
New formula for 9-j symbols and their direct calculation
Authors: Wei Liqiang
Published: July 2002

An algebraic expression for 9-j symbols in the form of a summation of the products of binomial coefficients is obtained. An algorithm is also devised to calculate these binomial coefficients recursively. This avoids the evaluation of factorials of integers, which is the main source of overflow in calculation of coupling coefficients for large angular momenta. Thus, the new formula permits accurate calculation of 9-j symbols. In addition, it has higher symmetry and involves only a twofold summation. Therefore, a direct approach for accurate and efficient calculation of 9-j symbols for very large angular momenta is thereby established. © 1998 American Institute of Physics.
Explicit sixth-order Bessel and Neumann fitted method for the numerical solution of the Schrödinger equation
Authors: Simos T. E.
Published: July 2002

An explicit sixth-algebraic-order method for the numerical solution of the Schrödinger equation for a neutral particle is developed. The new formula considered contains free parameters that are defined in order to integrate the spherical Bessel and Neumann functions exactly. Based on the new method and a method of Simos we obtained a variable-step algorithm. The results produced, based on the numerical solution of the radial Schrödinger equation, indicate that this new approach is more efficient than other well-known ones. © 1998 American Institute of Physics.
Propagation of ion acoustic waves close to a plasma boundary
Authors: Schott L.
Published: July 2002

It is observed that the electron component of an ion acoustic wave vanishes at a point in the transitional layer between the plasma and the ion sheath at an extended plane boundary. For the first time we present qualitative evidence that the ion component of the wave propagates beyond this point.
Capillary instabilities in thin films. I. Energetics
Authors: Srolovitz D. J. , Safran S. A.
Published: July 2002

A stability theory is presented which describes the conditions under which thin films rupture. It is found that holes in the film will either grow or shrink, depending on whether their initial radius is larger or smaller than a critical value. If the holes grow large enough, they impinge to form islands; the size of which are determined by the surface energies. The formation of grooves where the grain boundary meets the free surface is a potential source of holes which can lead to film rupture. Equilibrium grain boundary groove depths are calculated for finite grain sizes. Comparison of groove depth and film thickness yields microstructural conditions for film rupture. In addition, pits which form at grain boundary vertices, where three grains meet, are another source of film instability.
New vector finite elements for three-dimensional magnetic field computation
Authors: Barton M. L. , Cendes Z. J.
Published: July 2002

Finite-element vector potential solutions of three-dimensional magnetic field problems are usually obtained by approximating each component of the vector potential by a separate set of scalar finite-element approximation functions and by imposing continuity conditions between elements on all three components. This procedure is equivalent to imposing continuity of both the normal and the tangential components of the vector potential. We show in this paper that this procedure is too restrictive: While continuity of the tangential component of the vector potential is required, continuity of the normal components is not essential in the variational formulation. We introduce a new type of vector finite-element approximation function that has the property that it interpolates not to point values of each component of vector potential, but rather to the tangential projection of the vector potential on each edge of tetrahedral finite elements. With the new basis functions, continuity of the normal component of the vector potential is provided only approximately by means of the natural interface conditions inherent in the variational procedure. This results in a more efficient procedure for the solution of three-dimensional magnetostatic field problems than is obtained by enforcing normal component continuity exactly.
Effect of point defects on the phonon thermal conductivity of bcc iron
Authors: Williams R. K. , Graves R. S. , Weaver F. J. , Yarbrough D. W.
Published: July 2002

The effects of low concentrations of seven different substitutional impurities on the phonon thermal conductivity of iron were determined at 300 K. Of the seven solutes, Ni caused the smallest reduction and W the greatest. The data were analyzed on the basis of Callaway’s model, and the resulting impurity scattering was compared with theory. The data show that phonon scattering from the local strain field associated with point defects is smaller than predicted from theory, but not negligible. The results are consistent with theoretical formulas that show that mass difference and lattice-strain scattering can either reinforce or cancel.
A time dependent regular solution model for the thermal evaporation of an Al-Mg alloy
Authors: Romig A. D.
Published: July 2002

A series of Al-6.27 at. % Mg alloys were thermally evaporated in a vacuum at 1910 K. The length of time during which the alloy was molten and was evaporating was varied from very short times to a length of time sufficient for complete evaporation of the alloy. The thickness and average composition of the deposited films were determined with thin-film x-ray microanalysis in the analytical electron microscope. A solution/flux model was developed to simulate the evaporation process. The model treated the liquid Al-Mg alloy as a regular solution using experimentally determined Raoultian activity coefficients. The evaporative flux was calculated according to the expression of Langmuir. The solution and flux equations were numerically integrated with respect to time to accommodate changes in mass and liquid alloy composition as the molten alloy evaporated. The agreement between the model and the experimental data (evaporation rate, rate of composition change in the molten alloy, film thickening rate, and average film composition) was excellent. The experimental data and the results of the solution/flux model show that the Mg is evaporated from the melt very quickly. The time required for Mg depletion is approximately 3% of the time required for total evaporation of the alloy. The solution/flux model calculates an average activation energy for evaporation of 72 400±4000 cal/mol over the temperature range 1800–2400 K, which is in good agreement with the thermodynamic enthalpy change for vaporization of 72 300±2000 cal/mol at 2110 K.
Magnetic structure of bcc and fcc manganese
Authors: Brener N. E. , Fuster G. , Callaway J. , Fry J. L. , Zhao Y. Z.
Published: July 2002

Self-consistent band-structure calculations are used to obtain the ferromagnetic moment as a function of lattice constant for bcc and fcc Mn. The ferromagnetic moment of bcc Mn is found to change discontinuously from a small to a large value as the lattice constant increases, while the fcc moment is found to change discontinuously from zero to a large value with increasing atomic volume. In the bcc case, the transition occurs inside a narrow double moment region in which the low spin and high spin states coexist. Information from magnetic susceptibility calculations on bcc and fcc Mn is used to predict whether the ground state is ferromagnetic or antiferromagnetic in certain ranges of lattice constant.
Modeling of velocity effects in eddy current applications
Authors: Ida Nathan
Published: July 2002

The effects of velocity on moving sources are encountered in many practical eddy current applications. In many instances these effects are ignored either because velocities are relatively low, because of our inability to quantify these effects, or for purposes of simplifying the solution. There are, however, a number of important applications in which this cannot be done and full account of velocity must be taken. Some obvious applications are magnetic recording, magnetic braking, and nondestructive testing. This work presents a finite element formulation for eddy current problems that takes into account the relative movement of sources. Results are presented indicating that velocity effects are significant at high velocities, and are important for correct signal interpretation. The effect of velocity on nondestructive testing signals is investigated and shown to display significant deviation from static behavior. Because of the form of the governing equations, spurious, nonphysical solutions may be generated. These are eliminated by two separate methods. One involves refinement of the finite element mesh and the second, upwinding of the finite elements.
The virtual cathode as a transient double sheath
Authors: Intrator T. , Cho M. H. , Wang E. Y. , Hershkowitz N. , Diebold D. , DeKock J.
Published: July 2002

The two-dimensional plasma potential measurements are given of a space-charge dominated double sheath near a hot cathode. Laboratory data show that a virtual cathode is a self-consistent solution only for a transient cathode-plasma system. Slow charge exchange ions get trapped in the potential dip that forms the virtual cathode and eventually destroy it.
Imaging of the grain-to-grain epitaxy in NiFe/FeMn thin-film couples
Authors: Hwang Cherngye , Geiss Roy H. , Howard J. Kent
Published: July 2002

It is known that a metastable crystalline thin-film structure can often be stabilized by choosing a suitable substrate. For single-crystal thin films, this pseudomorphic growth has been attributed to the stabilization of the metastable phase by epitaxial growth. However, in the polycrystalline structure, the mechanism is less clear. In this study, we have observed through plane-view and cross-section transmission electron microscopy (TEM) that the metastable fcc FeMn phase was stabilized by the underlaying NiFe layer. The continuous moiré fringes across the FeMn/NiFe interface as revealed by the cross-section TEM image indicate the existence of the epitaxial relationship between FeMn and NiFe. The epitaxial strain involved has also been found to be confined locally within each grain and was accommodated by the coherent strain across the interface. Consistent with the epitaxial strain observation, it is also found that this metastable fcc FeMn phase exists at the interface of NiFe and FeMn, and away from this interface, the stable A12 cubic FeMn phase is formed. Microdiffraction study showed that the epitaxially grown FeMn lattice was rotated through a small angle relative to the NiFe lattice. This rotation suggests that, in addition to the conventional lattice-dilation strain-epitaxy mechanism observed in single crystals with small misfit, there may be a rotation-epitaxy mechanism in the polycrystalline thin-film epitaxial structure.
Defect formation in silicon at a mask edge during crystallization of an amorphous implantation layer
Authors: Cerva H. , Küsters K.-H.
Published: July 2002

Implantation defects under a mask edge were studied by cross-sectional transmission electron microscopy. An arsenic implantation with a dose of 5×1015 cm−2 as used for source/drain implantations in metal-oxide-semiconductor transistor structures completely amorphizes a 63-nm-thick silicon surface layer and produces a sharply curved amorphous/crystalline interface under the mask edge. Annealing at 900 °C results in the formation of vacancy-type dislocation half-loops or microtwinning on {111} planes under the mask edge. Partial crystallization of the amorphous layer at 500 °C revealed a notch in the amorphous/crystalline interface which is shown to be due to the different epitaxial regrowth rates on the various lattice planes. On further annealing, the above-mentioned defects are generated when the crystallization fronts on both sides of the notch join.
Effects of trimethylantimonide/triethylgallium ratios on epilayer properties of gallium antimonide grown by low-pressure metalorganic chemical vapor deposition
Authors: Juang F. S. , Su Y. K. , Li N. Y. , Gan K. J.
Published: July 2002

Undoped GaSb epitaxial layers were grown on (100) GaSb substrates by low-pressure metalorganic chemical vapor deposition. The trimethylantimonide/triethylgallium mole fraction (V/III) ratios were varied at a growth temperature of 600 °C and growth pressure of 100 Torr. It was found that the layer morphologies were strongly dependent on V/III ratios. The mirrorlike surface can be easily obtained under the V/III ratio in the range of 6–8. The growth rate was about 1.75 μm/h. The epitaxial layers were characterized by photoluminescence(PL) measurements and electron diffraction patterns. The bound-exciton peaks and strong acceptor band peak in the PL spectra were observed from the sample grown under a V/III ratio of 6.84. PL peak intensity was found to be a function of the V/III ratios. I-V characteristics of the p-n diodes fabricated on the sample of undoped GaSb/GaSb:Te was measured. The undoped acceptor carrier concentration and mobility were 9.47×1016 cm−3 (300 K), 1.60×1016 cm−3 (77 K), 275.3 cm2 /V s (300 K), and 491.6 cm2/V s (77 K), respectively.
Computation of electron diffraction patterns in Lorentz electron microscopy of thin magnetic films
Authors: Mansuripur M.
Published: July 2002

The vector potential field for a thin magnetic film with arbitrary pattern of magnetization is calculated using fast Fourier transforms. The vector potential is used to compute the phase modulation imparted to the electron beam in Lorentz electron microscopy. Calculated phase patterns and the corresponding intensity distributions for several magnetic configurations of practical interest are described.
Effect of oxygen pressure on the synthesis of YBa2Cu3O7−x thin films by post-deposition annealing
Authors: Feenstra R. , Lindemer T. B. , Budai J. D. , Galloway M. D.
Published: July 2002

The effect of ambient oxygen pressure on the synthesis of epitaxial YBa2Cu3O7−x films on (100) SrTiO3 substrates by post-deposition annealing of amorphous precursor films was studied for oxygen partial pressures pO2 between 1.0 and 8.0×10−5 atm and annealing temperatures between 890 and 650 °C. A pO2−1/T diagram containing recent literature data regarding YBa2Cu3O7−x oxygen stoichiometry, phase stability, and liquid-phase formation was compiled to provide guidance for the selection and interpretation of annealing conditions. The results evidence a strong dependency of growth properties on the oxygen pressure with enhanced c-oriented epitaxy at lower pO2 values. A particularly interesting result is the formation of predominantly c-oriented films at 740 °C and pO2=2.6×10−4 atm (0.2 Torr). Similar to YBa2Cu3O7−x films produced by in situ laser ablation at the same temperature and oxygen pressure, the films exhibited low ion channeling yields (χmin<0.1) and a dense (smooth) surface morphology, while critical currents at 77 K were well in excess of 1 MA/cm2. From the observed systematic variation of structural film properties with synthesis conditions, annealing lines were derived indicating (T-pO2) combinations for either c- or a-oriented epitaxial growth. A comparison is made between these lines and synthesis conditions for in situ film growth as compiled recently by Hammond and Bormann [Physica C 162–169, 703 (1989)].
Force constant and effective mass of 90° domain walls in ferroelectric ceramics
Authors: Arlt G. , Pertsev N. A.
Published: July 2002

Domain wall contributions to the dielectric, piezoelectric, and elastic properties of tetragonal ferroelectric ceramics, as discussed extensively in the past, are calculated. A simple model shows that the motion of 90° domain walls causes a shear deformation and an approximately homogeneous electric field in the grain. The elastic and electric field energies involved allow the calculation of the force constant for the domain wall displacement by external fields. The displacements agree with experimental results. In a moderate electric field the displacement is a small fraction of the lattice cell only. By averaging over the orientational distributions of all grains the contributions of the 90° domain walls to the material properties are calculated for unpolarized and for polarized ceramics and agree with experimental results. The effective mass, which has to be attributed to the domain walls is the mass of the whole grain reduced by the factor S0 (spontaneous deformation), is independent of the domain width. The resonance frequency of the domain walls therefore is lower than the elastic resonance frequency of the grain.
Radiative and conductive heat transfer in porous media: Estimation of the effective thermal conductivity
Authors: Helte Andreas
Published: July 2002

The effective thermal conductivity κeff of porous media has been studied when both radiation and conduction is present. Expressions for κeff of a solid containing dispersed, equisized, disconnected spherical pores are derived within an extended effective medium theory. When no radiation is present, the results agree very well with exact results for an fcc lattice of spheres. The effective medium approach is also used to calculate the transverse effective thermal conductivity of a solid containing aligned, infinitely long, equisized, disconnected, cylindrical pores. When no radiation is present, the result is in excellent agreement with exact results for a hexagonal array of cylinders. In the direction parallel to the cylinder axes, the effective thermal conductivity is found exactly. An estimation of κeff for a system of random, overlapping, solid spheres in a gas matrix is presented.
Chalcopyrite/defect chalcopyrite heterojunctions on the basis of CuInSe2
Authors: Schmid D. , Ruckh M. , Grunwald F. , Schock H. W.
Published: July 2002

A new model for the formation of heterojunctions in polycrystalline CuInSe2 thin films on the basis of surface analysis experiments is presented. In situ photoemission measurements of CuInSe2 clearly show the existence of an In-rich n-type surface layer on samples relevant for solar-cell devices. Furthermore, this layer has been identified as an ordered vacancy compound (OVC) with a band gap of about 1.3 eV. The previous model of the CuInSe2/CdS solar cell with a p-n heterojunction between p-type CuInSe2 and n-type CdS is replaced by the model of a chalcopyrite/defect chalcopyrite heterojunction between p-type bulk CuInSe2 and the In-rich n-type OVC. The existence of this junction was proven directly by evaporating an ohmic metal contact onto the surface n-type layer and measuring the spectral quantum efficiency and electron-beam-induced current of this device. The band offsets of CuInSe2-based devices have been determined.
Observation of a charge limit for semiconductor photocathodes
Authors: Woods M. , Clendenin J. , Frisch J. , Kulikov A. , Saez P. , Schultz D. , Turner J. , Witte K. , Zolotorev M.
Published: July 2002

The Stanford Linear Accelerator Center is currently operating with a photocathode electron gun (PEG) to produce polarized electrons for its experimental program. Bunch intensities of up to 1011 electrons within 2 ns (8 A) are required from the electron gun. Operation of PEG has demonstrated a charge limit phenomenon, whereby the charge that can be extracted from the gun with an intense laser beam saturates at significantly less than 1011 electrons (the expected space-charge-limited charge) when the photocathode quantum efficiency is low. Studies of this charge limit phenomenon observed with a GaAs photocathode are reported.
Origins of colossal magnetoresistance in perovskite-type manganese oxides (invited)
Authors: Tokura Y. , Tomioka Y. , Kuwahara H. , Asamitsu A. , Moritomo Y. , Kasai M.
Published: July 2002

Phenomena of colossal magnetoresistance (MR) or magnetic field induced insulator–metal (I–M) transitions have been investigated for single crystals of perovskite-type manganese oxides with controlled carrier density and one-electron bandwidth. In addition to the canonical MR behavior near the Curie temperature, the first-order phase transition accompanying several orders of magnitude change in resistivity has been observed under an external magnetic field for many of the composition-controlled crystals as an intrinsic bulk phenomenon. It was proved by the systematic experimental investigations that the field-induced destruction of the charge-ordered state accompanying the lattice structural as well as metamagnetic transition is a major origin of such a colossal MR. Versatile MR phenomena and I–M phase diagrams in the T–H plane are presented with their interpretation.
Crystallographic tilting in lattice-mismatched heteroepitaxy: A Dodson–Tsao relaxation approach
Authors: Riesz Ferenc
Published: July 2002

Asymmetric strain relaxation and the concomitant misorientation (tilt) of the lattice planes of the epitaxial layer in lattice-mismatched heterostructures is analyzed theoretically. The kinetic relaxation model of Dodson and Tsao [Appl. Phys. Lett. 51, 1325 (1987); 52, 852(E) (1988)] is extended to the growth on vicinal surfaces. We calculate the equilibrium tilt values as well as the evolution of tilt during strain relaxation as a function of material parameters, initial defect densities, and substrate miscut angle. Literature data are interpreted within the framework of the model.
Optical switching in polymer gels
Authors: Suzuki A. , Ishii T. , Maruyama Y.
Published: July 2002

Soft materials have long been sought after for use in devices such as actuators, artificial muscles, separators, switches, sensors, memories, and so forth. We developed a soft, optically transparent material using polymer gels that can not only be activated by visible light (switched on) but also deactivated (switched off) by altering the local environment using three different means: pH, temperature, and light. This copolymer gel is a covalently cross-linked network of N-isopropylacrylamide, sodium acrylate, and a chromophore, which is found to undergo phase transitions exhibiting large hysteresis in the degree of swelling in response to pH, temperature, and light. In each system, between the transitions for swelling and shrinking, the gel can show either a swollen or a collapsed state, which can be selected according to the history of the variables. It has been established that a thermoresponsive gel with chromophore exhibits a local volume phase transition upon illumination with visible light. By making use of this phenomenon, we have successfully controlled the phase in which a gel exists with visible light: Without light illumination the gel stays in the swollen state. Upon illumination beyond a threshold intensity, however, a volume transition is locally induced, thereby forming a material in which both phases coexist stably for at least several hours after the light source has been removed. The phenomenological stability of the material in the coexistence state is discussed on the basis of the Landau theory.
Thermal diffusivity measurement of single fibers by an ac calorimetric method
Authors: Yamane Tsuneyuki , Katayama Shin-ichiro , Todoki Minoru , Hatta Ichiro
Published: July 2002

An ac calorimetric method is applied to the direct measurement of thermal diffusivity of single fibers. Theoretical analysis is presented, taking into account the two-dimensional ac thermal wave for cylindrical systems and the effect of time delay at the temperature detecting system. Experimental tests are performed using nickel and stainless steel wires about 20 μm in diameter. The thermal conductivity of fibers about 10 μm in diameter of the advanced materials, polyacrylonitrile-based carbon, TORAYCA, and aramide, Kevlar, is determined. The relationship between the thermal conductivity and electric conductivity and the crystalline size of these fibers is discussed. For composite materials made of carbon fibers, the observed thermal conductivity agrees with the thermal conductivity estimated using the values determined by the present method.