bluuebunny/crossref_metadata_2025 huggingface dataset:
An improved model for ternary nucleation of sulfuric acid–ammonia–water
Authors: Napari I. , Noppel M. , Vehkamäki H. , Kulmala M.
Published: July 2002

A revised homogeneous ternary nucleation model in H2O–H2SO4–NH3 vapors is presented. The model is based on a self-consistent version of classical nucleation theory with a rigorous treatment of nucleation kinetics. The calculation of equilibrium vapor pressures is completely revised and the effect of H2O–H2SO4 hydration is considered in detail. Compared to earlier models, the new model is able to predict nucleation rates over a wider range of temperatures and trace gas concentrations. A considerable dependence on relative humidity is found. The critical clusters corresponding to significant nucleation rates typically contain less than ten molecules and consist almost exclusively of H2SO4 and NH3.
Magnetic properties of CoFe2O4 nanoparticles synthesized through a block copolymer nanoreactor route
Authors: Ahmed Sufi R. , Ogale S. B. , Papaefthymiou Georgia C. , Ramesh Ramamoorthy , Kofinas Peter
Published: July 2002

The development of self-assembled magnetic CoFe2O4 nanoparticles within polymer matrices at room temperature is reported. Diblock copolymers consisting of poly (norbornene) and poly (norbornene-dicarboxcylic acid) (NOR/NORCOOH) were synthesized. The self-assembly of the mixed metal oxide within the NORCOOH block was achieved at room temperature by processing the copolymer nanocomposite using wet chemical methods. Morphology and magnetic properties were determined by superconducting quantum interference device magnetometry, transmission electron microscopy, wide angle x-ray diffraction, and Fe57 Mössbauer spectroscopy. The CoFe2O4 nanoparticles are uniformly dispersed within the polymer matrix, and have an average radius of 4.8±1.4 nm. The nanocomposite films are superparamagnetic at room temperature and ferrimagnetic at 5 K.
Time-resolved coherent anti-Stokes Raman scattering microscopy: Imaging based on Raman free induction decay
Authors: Volkmer Andreas , Book Lewis D. , Xie X. Sunney
Published: July 2002

A time-resolved coherent anti-Stokes Raman scattering (CARS) microscope allows three-dimensional imaging based on Raman free induction decay of molecular vibration with no requirement for labeling of the sample with natural or artificial fluorophores. A major benefit of the technique is the capability to completely remove nonresonant coherent background signal from the sample and the solvent, and thus increasing the detection sensitivity of CARS microscopy significantly.
Analytic second derivatives for the full coupled-cluster singles, doubles, and triples model: Nuclear magnetic shielding constants for BH, HF, CO, N2, N2O, and O3
Authors: Gauss Jürgen
Published: July 2002

An implementation of analytic second derivatives for the coupled-cluster singles, doubles, and triples (CCSDT) model is reported and applied to the calculation of nuclear magnetic shielding constants of BH, HF, CO, N2, N2O, and O3. The CCSDT calculations yield further evidence for the high accuracy provided by chemical shift calculations employing the coupled-cluster singles, doubles (CCSD) approach augmented by a perturbative triples correction [CCSD(T)].
Effects of three-body interactions on the structure and thermodynamics of liquid krypton
Authors: Jakse N. , Bomont J. M. , Bretonnet J. L.
Published: July 2002

Large-scale molecular dynamics simulations are performed to predict the structural and thermodynamic properties of liquid krypton using a potential energy function based on the two-body potential of Aziz and Slaman plus the triple-dipole Axilrod–Teller (AT) potential. By varying the strength of the AT potential we study the influence of three-body contribution beyond the triple-dipole dispersion. It is seen that the AT potential gives an overall good description of liquid Kr, though other contributions such as higher order three-body dispersion and exchange terms cannot be ignored.
Improving the thermal stability of organic light-emitting diodes by using a modified phthalocyanine layer
Authors: Mori Tomohiko , Mitsuoka Takuya , Ishii Masahiko , Fujikawa Hisayoshi , Taga Yasunori
Published: July 2002

Remarkable improvement in thermal stability has been demonstrated in an organic light-emitting diode (OLED) using a metal-free phthalocyanine (H2Pc)-doped copper phthalocyanine (CuPc) layer as a hole injection layer. Compared to an OLED using a CuPc layer, approximately twice the lifetime has been achieved in the OLED using the H2Pc-doped CuPc layer at a high temperature of 85 °C, operating under a constant current and starting at a luminance of 400 cd/m2. Atomic force microscopy measurements show that the dopant of H2Pc depresses the crystallization of a CuPc layer. It is suggested that the improved thermal stability of the OLED is attributable to that of the phthalocyanine layer in morphology.
A history of chemical oscillations and waves
Authors: Zhabotinsky Anatol M.
Published: July 2002

The history of the discovery and study of chemical oscillations and waves is presented from the very first accidental observations up to the systematic design of chemical oscillators. Special emphasis is devoted to the long-term debate over the possibility of pure chemical oscillations, i.e., concentration oscillations in homogeneous closed systems.
Practical implementation of nonlinear time series methods: The TISEAN package
Authors: Hegger Rainer , Kantz Holger , Schreiber Thomas
Published: July 2002

We describe the implementation of methods of nonlinear time series analysis which are based on the paradigm of deterministic chaos. A variety of algorithms for data representation, prediction, noise reduction, dimension and Lyapunov estimation, and nonlinearity testing are discussed with particular emphasis on issues of implementation and choice of parameters. Computer programs that implement the resulting strategies are publicly available as the TISEAN software package. The use of each algorithm will be illustrated with a typical application. As to the theoretical background, we will essentially give pointers to the literature.
Alignment of nematic liquid crystals by inhomogeneous surfaces
Authors: Ong Hiap Liew , Hurd Alan J. , Meyer Robert B.
Published: July 2002

Variable oblique alignment of nematic liquid crystals has been achieved on microscopically inhomogeneous surfaces. The surfaces consist of small patches favoring vertical (homeotropic) alignment surrounded by a matrix favoring a planar alignment. The construction of these surfaces employs randomly distributed microscopic metal islands formed by certain metals as vapor-deposited films. Larger scale periodic patterns were made as well to verify the techniques. The results are interpreted in terms of a continuum elasticity theory and azimuthal degeneracy is also discussed.
Strain-energy effects on dynamic fragmentation
Authors: Glenn L. A. , Chudnovsky A.
Published: July 2002

Grady’s model of the dynamic fragmentation process, in which the average fragment size is determined by balancing the local kinetic energy and the surface energy, is modified to include the stored elastic (strain) energy. The revised model predicts that the strain energy should dominate for brittle materials, with low fracture toughness and high fracture-initiation stress. This conclusion is not borne out, however, by limited experimental data on brittle steels, even when the kinetic-energy density is small compared with the strain-energy density.
Magnetization and magnetic anisotropy of R2Fe14B measured on single crystals
Authors: Hirosawa Satoshi , Matsuura Yutaka , Yamamoto Hitoshi , Fujimura Setsuo , Sagawa Masato , Yamauchi Hiroshi
Published: July 2002

The temperature dependence of the saturation magnetization and the magnetocrystalline anisotropy field have been measured on single-crystal samples of the R2Fe14B compounds for R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm from 4.2 K to the magnetic ordering temperatures. A spin reorientation transition of the Nd2Fe14B type has been found in Ho2Fe14B at 57.6 K in zero field. Another type of spin reorientation caused by anisotropy compensation between the Fe and the R sublattices exists in Er2Fe14B and Tm2Fe14B. The temperature dependence of the angle of the easy direction of magnetization from the c axis has been measured for R=Nd, Ho, Er, and Tm. The relation between the magnetocrystalline anisotropy and the sublattice magnetization is investigated by employing a simplified two-sublattice molecular field model.
Magnetization reversal, coercivity, and the process of thermomagnetic recording in thin films of amorphous rare earth–transition metal alloys
Authors: Mansuripur M.
Published: July 2002

A model is proposed for the mechanism of magnetization reversal in thin films of amorphous alloys with perpendicular magnetic anisotropy. Examples of these alloys are TbFe, GdCo, DyFe, and GdTbFeCo, which are currently under investigation as storage media for erasable optical recording applications. The model exhibits the observed behavior of the media such as nucleation and growth of reverse-magnetized domains under external magnetic fields; square hysteresis loops; temperature dependence of coercivity; formation and stability of domains under conditions of thermomagnetic recording; and incomplete erasure with insufficient applied fields.
Kozeny–Carman relations and image processing methods for estimating Darcy’s constant
Authors: Berryman James G. , Blair Stephen C.
Published: July 2002

A natural connection is demonstrated between Kozeny–Carman relations for porous media and the image processing techniques which have recently been applied to the problem of estimating the parameters in such relations. It is shown that the term in the Kozeny–Carman relation related to the specific surface area is best estimated from a smoothed version of the actual material surface. To measure this image specific surface, the magnification of a cross section of the porous material should be chosen so that a typical correlation length for the sample corresponds to a distance comparable to 100 discrete picture elements. Under these conditions, the assumptions typically made in the derivation of a Kozeny–Carman relation are entirely compatible with the resolution constraints imposed by digitizing the image. Thus, although the measured image specific surface may be considerably smaller in magnitude than the true specific surface area of the material (due to resolution constraints), this smaller value is nevertheless the required input to the Kozeny–Carman relation. The argument is based on a known comparison theorem relating the permeabilities of two porous materials which differ only by the addition (without rearrangement) of the solid to the more porous material.
Transmission line analogy of resonance tunneling phenomena: The generalized impedance concept
Authors: Khondker A. N. , Khan M. Rezwan , Anwar A. F. M.
Published: July 2002

This paper presents a simple yet accurate method for solving the Schrödinger equation to calculate the quantum mechanical transmission probability across arbitrary potential barriers. It is based on the concept of wave impedance analogous to transmission line theory. The quantum mechanical transmission probability in a resonant tunneling device can be easily calculated using this method.
Internal and near-surface electromagnetic fields for a spherical particle irradiated by a focused laser beam
Authors: Barton J. P. , Alexander D. R. , Schaub S. A.
Published: July 2002

Theoretical expressions for the internal and external electromagnetic fields for an arbitrary electromagnetic beam incident upon a homogeneous spherical particle are derived, and numerical calculations based upon this theoretical development are presented. In particular, spatial distributions of the internal and near-surface electric field magnitude (source function) for a focused fundamental (TEM00 mode) Gaussian beam of 1.06 μm wavelength and 4 μm beam waist diameter incident upon a 5-μm-diam water droplet in air are presented as a function of the location of the beam focal point relative to the sphere center. The calculations indicate that the internal and near-surface electric field magnitude distribution can be strongly dependent upon relative focal point positioning and may differ significantly from the corresponding electric field magnitude distribution expected from plane-wave irradiation.
Raman determination of layer stresses and strains for heterostructures and its application to the cubic SiC/Si system
Authors: Feng Z. C. , Choyke W. J. , Powell J. A.
Published: July 2002

A set of formulas for Raman shifts of diamond and zinc-blende semiconductors under a generalized axial stress are derived. The Raman shifts under hydrostatic pressure, uniaxial and biaxial stresses are only the special cases of the general formulas. Raman-stress coefficients and mode parameters are introduced, by which the Raman shifts due to different kinds of applied stresses are related. By the analysis of known Raman data of cubic SiC (3C-SiC) under hydrostatic pressures, one of the Raman-stress coefficients is obtained and the existing elastic stiffness constants of 3C-SiC are optimized. Unreliable data in the literature are pointed out and a choice of the best available data is made (C11=5.4, C12=1.8, and C44=2.5×1012 dyn/cm2). A series of chemical-vapor-deposited 3C-SiC films on (100) Si with the film thicknesses between 4 and 17 μm are measured by Raman scattering. We propose a method for calculating the stress and strain in these SiC films. The stresses in 3C-SiC on Si are in the range of 0.3–1.1 GPa and strains are on the order of 0.1%-0.2%, which is a factor of 100 below the lattice mismatch between bulk 3C-SiC and Si. Further discussion of the release of strain due to misfit dislocations and the effect of the buffer layer on the film stress is given. Our expressions and the method of the measurement of the stress and strain in heterostructures are quite general and may be used for other systems.
Band-gap narrowing in highly doped n- and p-type GaAs studied by photoluminescence spectroscopy
Authors: Borghs G. , Bhattacharyya K. , Deneffe K. , Van Mieghem P. , Mertens R.
Published: July 2002

Band-gap narrowing of GaAs as a function of doping concentration has been measured using photoluminescence spectroscopy on samples grown by molecular beam epitaxy. Both n- (Si) and p- (Be) doped samples with concentrations varying from 3×1017 to 3×1018 cm−3 have been measured. The experimental results obtained from a line-shape analysis of the spectra taking tailing effects into account are in good agreement with recent theoretical calculations. A simple expression for the band-gap narrowing as a function of concentration for both n-and p-doped GaAs is given.
Impedance and modulus spectroscopy of semiconducting BaTiO3 showing positive temperature coefficient of resistance
Authors: Sinclair D. C. , West A. R.
Published: July 2002

Polycrystalline barium titanate that has been doped to give a positive temperature coefficient of resistance (PTCR) effect is an inhomogeneous material electrically. Analysis of ac impedance data using the complex impedance plane representation gives the dc resistance of PTCR ceramics. By additional use of the complex electric modulus formalism to analyze the same data, the inhomogeneous nature of the ceramics may be probed. This reveals the presence of two, sometimes three elements in the equivalent circuit. Grain-boundary and bulk effects may be distinguished from capacitance data: grain-boundary effects have temperature-independent capacitances, whereas bulk effects show a capacitance maximum at the Curie point and Curie–Weiss behavior above the Curie point. Both grain-boundary and bulk effects appear to contribute to the PTCR effect. These results reveal limitations in current theories of the PTCR effect.
Ellipsometric measurement of magneto-optical Kerr rotation at normal incidence
Authors: Chao Shiuh , Yeh Long-Jewel , Lo Tsong-Jen , Jeng Tzuan-Ren
Published: July 2002

Two simple methods were introduced for measuring the magneto-optical Kerr rotation when the light is incident upon the surface of the magneto-optical media at zero degree angle of incidence. The methods can readily be implemented on an existing ellipsometer.
Ion and chemical radical effects on the step coverage of plasma enhanced chemical vapor deposition tetraethylorthosilicate films
Authors: Chang C.-P. , Pai C. S. , Hsieh J. J.
Published: July 2002

We have compared the step coverage of plasma enhanced chemical vapor deposition tetraethylorthosilicate films of microwave downstream, high frequency radio frequency (rf), and low frequency rf depositions. The microwave-downstream deposition, characterized by bimolecular surface reactions, produces a conformal step coverage. The rf depositions with ion-induced surface reactions produce a low sidewall, high bottom coverage. The chemical radical and the ion effects on the step coverage are discussed.
Paramagnetic resonance and susceptibility of ilmenite, FeTiO3 crystal
Authors: McDonald P. F. , Parasiris A. , Pandey R. K. , Gries B. L. , Kirk W. P.
Published: July 2002

Large high-purity single crystals of FeTiO3 with ilmenite structure have been grown from a stoichiometric melt of Fe2O3 and TiO2 under an inert atmosphere using the modified Czochralski technique. Susceptibility and x-band paramagnetic resonance studies have been performed. Susceptibility measurements indicate a Néel temperature ∼59 K. The paramagnetic resonance spectrum for magnetic field perpendicular to the crystal c axis consists of a portion of a single, very intense approximately Lorentzian absorption line with its peak at about 600 G and half width at half maximum almost 1200 G. The absorption extends to zero magnetic field. For magnetic field approximately parallel to the c axis, the paramagnetic absorption is much smaller and may be considered a superposition of two approximately Lorentzian line shapes. The magnetic resonance measurements indicate a weak temperature dependence and large angular anisotropy.
Raman investigations of the surface modes of the crystallites in CdS thin films grown by pulsed laser and thermal evaporation
Authors: Chuu D. S. , Dai C. M. , Hsieh W. F. , Tsai C. T.
Published: July 2002

CdS films have been grown on Corning glass by pulsed laser evaporation (PLE) and thermal evaporation (TE) techniques at a substrate temperature between room temperature and 250 °C. The quality of these films is investigated by resonance Raman scattering, x-ray diffraction and optical transmittance. Our results reveal that the Raman shifts of the surface phonon mode are observed with 300 and 297 cm−1 by PLE and TE techniques, respectively, and as many as four overtones are obtained by PLE method. The difference of Raman shift between these two techniques are caused by the discrepancy of crystallite sizes which is larger for PLE technique. The crystallite sizes are in the range of 200–500 Å in diameter. Highly oriented films have been grown by both of the techniques even when the substrate is at room temperature.
Local polar configurations in lead magnesium niobate relaxors
Authors: Viehland Dwight , Jang S. J. , Cross L. Eric , Wuttig Manfred
Published: July 2002

The field dependence of the dielectric response has been measured for lead magnesium niobate relaxors. The frequency dispersion of the temperature of the permittivity maximum, Tmax, was analyzed using the Vogel–Fulcher relationship. The field dependence of the permittivity at various temperatures was also modeled using a phenomenological expansion in the electric field. The activation energy and a softening of the dielectric response were found to be maximum under a small bias. The freezing temperature was found to be minimum near this same bias. These results are interpreted to mean that the moments of the polar clusters do not freeze in random orientations, but rather locally preferred configurations of moments are proposed to exist. Evidence is presented that the scale of these configurations is approximately 100 Å. At higher biases the relaxation mechanism was shut down; consequently, the nonlinear response was stiffened and Tmax increased. The nonlinear response was also found to be maximum near the freezing temperature.
Optimal heating and cooling strategies for heat exchanger design
Authors: Andresen Bjarne , Gordon J. M.
Published: July 2002

The optimal heating or cooling strategy that minimizes entropy production is derived for a simple class of common heat transfer processes that are constrained to proceed in a fixed, finite time. The empirical wisdom embodied in conventional single-pass counterflow heat exchanger design is examined in light of this solution. For judiciously selected system parameters, the counterflow heat exchanger can yield the optimal solution.
Influence of growth and microstructure of electrodeposited cadmium telluride films on the properties of n-CdS/p-CdTe thin-film solar cells
Authors: Das S. K. , Morris G. C.
Published: July 2002

Thin-film n-CdS/p-CdTe solar cells have been fabricated from electrodeposited CdS and CdTe layers. Cells made with crystalline CdTe films were 9.0%–11.5% efficiencies whereas those with CdTe of structural imperfection gave up to 6.8% efficiency. Cell efficiencies varied with growth conditions of CdTe depositions. Scanning electron micrographs and x-ray diffraction patterns showed that the CdTe deposits of relatively good cells had less grain boundaries and better crystallinity than less efficient cells. Capacitance measurements showed that the cells made with CdTe of structural imperfection had large number of interface states relative to those with crystalline CdTe films.
Formation and magnetic properties of nanocrystalline mechanically alloyed Fe-Co and Fe-Ni
Authors: Kuhrt C. , Schultz L.
Published: July 2002

Fe100−xCox and Fe100−xNix powders were prepared by mechanical alloying of the elements in a planetary ball mill. They were investigated with respect to phase formation and magnetic properties using x-ray diffraction and measurements of the saturation magnetization and the coercivity. In both systems, disordered solid solutions were formed by mechanical alloying as proved especially by the measurement of the saturation magnetization. Moreover, nonequilibrium microstructures were established consisting in a nanocrystalline state (average minimum grain sizes 20–30 nm) accompanied by the introduction of considerable atomic-level strain (root mean square strain up to 1%). The soft magnetic behavior of this material shows some features of rapidly quenched nanocrystalline ribbons, but very low coercivities are prevented by a predominant influence of strain via magnetoelastic interaction.
Raman investigation of the nonlinear optical phenomenon of polarization rotation in Ti:LiNbO3 channel waveguides
Authors: Ramabadran Uma B. , Jackson Howard E. , Boyd Joseph T.
Published: July 2002

The nonlinear phenomenon of polarization rotation in Ti:LiNbO3 optical channel waveguides was investigated using Raman microprobe microscopy. Polarization sensitive Raman selection rules permitted the identification of polarization rotation of the propagating transverse electric or transverse magnetic waveguide mode. From an analysis of the Raman scattered light collected orthogonal to the waveguide surface the threshold power necessary to initiate the process of mode conversion could be determined. In addition, the Raman microprobe was used to determine waveguide loss coefficients by collecting inelastically scattered light. The values obtained by this method were compared to those measured by collecting the elastically scattered light. A value for the asymmetric component, β15, of the photovoltaic tensor was calculated to be 7.8×10−13 A/W for the X-cut, Y-propagating and 3.8×10−14 A/W and 1.2×10−14 A/W for the rapid thermally annealed Z-cut, Y-propagating channel waveguides, respectively.
Fragmentation processes in reactive molecular ion beam etching
Authors: Hoffmann P. , Stoll H.-P. , Heinrich F.
Published: July 2002

In a reactive ion beam etching system, gas phase collision processes in the reaction chamber were identified from the energy distributions of positive ions originating from source plasmas with O2, SF6, and CF4 as feed gases. The ion energy distributions are determined by a quadrupole mass spectrometer for main beam energies below 500 eV at typical working pressures in the reaction chamber of 1–10×10−2 Pa. Besides near thermal ions a considerable amount of high energy fragmentation products were detected for a number of primary molecular ions. The relative intensities of these products compared to the parent ions suggest a non-negligible influence of gas phase dissociation processes on the etch or deposition characteristics of molecular ion beams and the resulting properties of surfaces treated under elevated working pressure conditions.
The influence of the strain-induced electric field on the charge distribution in GaN-AlN-GaN structure
Authors: Bykhovski Alexei , Gelmont Boris , Shur Michael
Published: July 2002

We show that strongly pronounced piezoelectric properties play a key role in GaN-AlN-GaN semiconductor-insulator-semiconductor (SIS) and related structures. In sufficiently thin AlN layers, the lattice constant mismatch is accommodated by internal strains rather than by the formation of misfit dislocations. These lattice-mismatch-induced strains generate polarization fields. We demonstrate that, in a GaN-AlN-GaN SIS structure with the growth axis along a (0001) crystallographic direction, the strain-induced electric fields can shift the flat band voltage and produce an accumulation region on one side and a depletion region on the other side of the AlN insulator. On which side of the insulator the accumulation region is produced depends on the type of atomic plane at the heterointerface (Ga or N). The surface charge density caused by the piezoeffect is on the order of 1012 cm−2. As a consequence of the asymmetry in the space charge distribution, the capacitance-voltage (C-V) characteristics of the SIS structure become asymmetrical. The asymmetrical shift of the C-V characteristics with respect to the origin is on the order of 1.5 V for a 30 Å AlN film. This asymmetry should vanish in a relaxed film. Hence, capacitance-voltage measurements of GaN-AlN-GaN SIS structures can be used for quantitative characterization as to the degree of AlN film relaxation, depending on the film thickness. This and related techniques should become important tools for the characterization of piezoelectric layered semiconductor films.
Stochastic resonance in a superconducting loop with a Josephson junction
Authors: Hibbs A. D. , Singsaas A. L. , Jacobs E. W. , Bulsara A. R. , Bekkedahl J. J. , Moss F.
Published: July 2002

A simple superconducting loop with a Josephson junction subject to a time-sinusoidal magnetic flux embedded in a noise background is considered. Cooperative effects, arising from the interplay between the noise and modulation are described; they manifest themselves in the response, measured as an output signal-to-noise ratio. In particular, it is shown that the response displays the stochastic resonance effect, wherein the output signal-to-noise ratio passes through a maximum at a critical value of the noise strength. A simple theory, based on the characterization of the superconducting quantum interference device as a bistable switching element, is seen to yield good qualitative agreement with the experimental results.
Sample-probe interactions in spectroscopy: Sampling microscopic property gradients
Authors: Lipkin Don M. , Clarke David R.
Published: July 2002

An analytical model is presented to describe the influence of sample-probe interactions on the shape of spectroscopic lines. By considering the discrete sampling of spatial property variations using a focused probe, the model predicts gradient-induced line broadening, as well as probe channeling near free surfaces. The magnitude of sample-probe interactions is governed by two system parameters: the probe size and the spatial property gradient. Furthermore, certain spatial heterogeneities are found to contribute appreciably to the signal and its associated line shape over distances several times the probe size. Experimental observations in fluorescence spectroscopy are used to evaluate the line-shape model. Spectra measured along an embedded ruby fiber under a residual stress gradient are in good agreement with those calculated using the model.
The low-temperature infrared optical functions of SrTiO3 determined by reflectance spectroscopy and spectroscopic ellipsometry
Authors: Kamarás K. , Barth K.-L. , Keilmann F. , Henn R. , Reedyk M. , Thomsen C. , Cardona M. , Kircher J. , Richards P. L. , Stehlé J.-L.
Published: July 2002

By combining reflectance spectroscopy and spectroscopic ellipsometry, the complex dielectric function of SrTiO3 in the frequency range 40–5000 cm−1 at 20, 100, 200, and 300 K has been determined. Using a factorized description, analytical expressions for the optical quantities were derived, giving excellent agreement with the experimental data. These can be used for two-layer fits of films on SrTiO3, e.g., of high-Tc superconductors. The fit parameters complement very well those found at higher temperatures.
Method based on the saturation approach law for monitoring the quality of texture in 3% Si–Fe
Authors: Birsan M. , Szpunar J. A.
Published: July 2002

The magnetic saturation of a polycrystalline material is achieved as a result of rotation of the magnetization vector toward the direction of the applied magnetic field, against the magnetocrystalline energy. The magnetization rotation process in a grain-oriented material depends on the values of anisotropy constants, on the distribution of grain orientations, and on the direction of magnetization. Based on the magnetization rotation process, a simple instrument was devised and tested to monitor the differences of texture strength in grain-oriented 3% Si–Fe laminates. The simplicity and robustness of this instrument are remarkable and it can easily work on line.
Energy handling capability of ZnO varistors
Authors: Bartkowiak M. , Comber M. G. , Mahan G. D.
Published: July 2002

Thermal and mechanical behavior of high power ZnO surge arresters under current pulses of various magnitude and duration is simulated. By solving heat transfer equations for a varistor disk with nonuniform electrical properties, we compute the time dependence of the temperature profile and the distribution of thermal stresses. The simulations identify failure processes and determine energy handling capability of varistor elements as a function of the applied current. The results are in good agreement with the experimental data and explain the energy handling dependence upon the surge intensity and duration.
Anode hole injection and trapping in silicon dioxide
Authors: DiMaria D. J. , Cartier E. , Buchanan D. A.
Published: July 2002

Hole injection into silicon dioxide films from the polycrystalline-silicon anode or from the anode/oxide interface is demonstrated to unequivocally occur for any case where electrons are present in the oxide conduction band and where the average electric field in the oxide exceeds 5 MV/cm (thick-film limit) or the voltage drop across the oxide layer is at least 8 V (thin-film limit). The hole generation is directly shown to be related to the appearance of hot electrons with kinetic energies greater than 5 eV in the oxide conduction band near the anode region. Monte Carlo simulations confirm that the electron energy distribution at the anode is the controlling variable and that hot hole injection occurs mostly over the anode/oxide energy barrier.
Dependence of the electro-optical properties of polymer dispersed liquid crystals on the photopolymerization process
Authors: LeGrange J. D. , Carter S. A. , Fuentes M. , Boo J. , Freeny A. E. , Cleveland W. , Miller T. M.
Published: July 2002

We have studied the dependence of the electro-optical properties of polymer dispersed liquid crystals (PDLC) on the ultraviolet (UV) cure of the solution of monomer and liquid crystal. The kinetics of UV polymerization and its effect on the morphology of the phase separated droplets of liquid crystal determine the switching voltage, response time, and luminance of the PDLC. Using a series of statistically designed experiments, we have mapped the dependence of these responses on the weight fraction of liquid crystal, the temperature of the cell during cure, and light intensity. Temperature and composition are strongly coupled parameters that influence switching voltage, luminance, and response times. Switching voltages are minimized at 4–5 V for an 8 μm cell gap over a large region of temperature-composition space. An abrupt transition line occurs through that space. On one side of the transition line, voltage increases linearly either as temperature increases or composition decreases, and on the other side of the line, voltage is constant. Analyses of decay times, the slower response time of the PDLC, show that the times peak along a line of points in temperature-composition space that is close to the transition line for increasing switching voltages. We present these results as contours on the same graphs and relate them to our understanding of the phase separation process in the PDLC mixture.
Properties of ferrites at low temperatures (invited)
Authors: Dionne Gerald F.
Published: July 2002

At cryogenic temperatures magnetic properties of ferrites change significantly from their values at room temperature, which has been the main regime for most device applications. Recently, microwave ferrite devices with superconducting microstrip circuits have been demonstrated at a temperature of 77 K with virtually no electrical conduction losses. Conventional ferrimagnetic garnet and spinel compositions, however, are not generally optimized for low temperatures and may require chemical redesign if the full potential of these devices is to be realized. Saturation magnetizations increase according to the Brillouin–Weiss function dependence that is characteristic of all ferromagnetic materials. Increased magnetocrystalline anisotropy and magnetostriction can have large effects on hysteresis loop squareness and coercive fields that are essential for stable phase shift and efficient switching. Rare-earth impurities and other ions with short spin-lattice relaxation times can cause increased microwave losses. In this article, the basic magnetochemistry pertaining to ferrites will be examined for adaptation of ferrite technology to cryogenic environments.
Two-powder Nd2Fe14B magnets with DyGa-addition
Authors: de Groot C. H. , Buschow K. H. J. , de Boer F. R. , de Kort Kees
Published: July 2002

Magnetic and structural properties of sintered NdFeB magnets with addition of various amounts of DyGa as a separate powder are studied and compared with single-powder magnets of the same nominal composition. When the magnets are annealed above 640 °C, the coercivity decreases strongly as compared to magnets annealed below 640 °C. Decomposition of the δ phase (Nd6Fe14−xGax) accompanies this drop. Measurements of coercivity as a function of temperature show that the decomposition of the δ phase does not increase the effective demagnetization factor, but enlarges either the magnetic inhomogeneous region at the grain surface or the exchange coupling between the grains. Line faults, which are present within the Nd2Fe14B grains when the magnet is annealed below 640 °C, do not act as pinning centers. Phase analysis and diffusion profiles of the Dy and Ga concentration within the grains are made using electron probe micro-analysis. This shows that Ga forms Nd3Ga2 and Nd6Fe12.7Ga1.3 as intergranular phases. Its concentration within the Φ phase is significantly lower than in the Φ phase of single-powder magnets. At 1090 °C, a diffusion coefficient for Ga of D=2.5±0.1×10−15 m2/s is found. Dy is shown only to be present in that part of the Nd2Fe14B grain which has precipitated from the liquid during sintering. At the boundary of the Dy-free center and the outer part of the grain, an increased Dy concentration is found.
Study of the directivity of laser generated ultrasound in solids
Authors: Yaping Wu , Dufang Shi , Yulong He
Published: July 2002

A theoretical study of the directivity of laser generated ultrasound in solids was presented. By solving thermoelastic wave equations, we obtained analytic expressions of three-dimensional ultrasound displacements in the far field. The results show that thermal diffusion, optical absorption, ultrasonic frequency, and the diameter of source all have effects on the directivity.
Colossal magnetoresistance in La1−xLixMnO3
Authors: Wang X. L. , Kennedy S. J. , Gehringer Peter , Lang Wolfgang , Liu H. K. , Dou S. X.
Published: July 2002

La 1−x Li x MnO 3 (x=0, 0.1, 0.2, and 0.3) were synthesized by using conventional solid state reaction and partial melting technique. The compounds were determined to be of rhombohedral structure by x-ray powder diffraction. For the x=0.2 and x=0.3 samples, room temperature neutron powder diffraction patterns were collected and analyzed with the Rietveld method. The analysis showed that the Li substitutes onto the La site in the rhombohedral perovskite lattice. Resistivities were measured between 4.2 K and room temperature in the magnetic fields of 0, 1, 2, 4, 8, and 13 T. A wide semiconductor-metal transition at temperatures between 160 and 210 K or between 200–230 K were observed for both samples with x=0.2 and 0.3, respectively. Large magnetoresistances above 20%–80% were achieved at fields from null to 13 T over a wide temperature range from the ferromagnetic transitions down to 4.2 K. However, the x=0 and 0.1 samples show a semiconducting behavior in resistivity measured between 77 and 300 K.
Contact analysis of elastic-plastic fractal surfaces
Authors: Yan W. , Komvopoulos K.
Published: July 2002

Rough surfaces are characterized by fractal geometry using a modified two-variable Weierstrass–Mandelbrot function. The developed algorithm yields three-dimensional fractal surface topographies representative of engineering rough surfaces. This surface model is incorporated into an elastic-plastic contact mechanics analysis of two approaching rough surfaces. Closed form solutions for the elastic and plastic components of the total normal force and real contact area are derived in terms of fractal parameters, material properties, and mean surface separation distance. The effects of surface topography parameters and material properties on the total deformation force are investigated by comparing results from two- and three-dimensional contact analyses and elastic and elastic-perfectly plastic material behaviors. For normal contact of elastic-perfectly plastic silica surfaces and range of surface interference examined, the interfacial force is predominantly elastic and the real contact area is approximately one percent of the apparent contact area or less, depending on the mean interfacial distance. The analysis can be easily modified to account for anisotropic fractal surfaces and different material behaviors.
Consequences of space dependence of effective mass in heterostructures
Authors: Dyakonov Michael , Shur Michael S.
Published: July 2002

The space dependence of the effective mass in heterostructures might lead to a strong interdependence of the longitudinal and transverse motion in systems with heterointerfaces, such as quantum wells. Such interdependence results in additional nonparabolicity of the longitudinal motion in quantum wells. At large energies, comparable to or exceeding the depth of a quantum well, the effective mass changes sign and the longitudinal two-dimensional spectrum terminates at a certain critical value of the longitudinal momentum. The space dependence of the effective mass also strongly increases the transmission through a potential barrier for electrons with large incidence angles. These effects should have important consequences for impact ionization and tunneling phenomena in GaN-based heterostructures.
Temperature dependence of the Raman spectra of carbon nanotubes
Authors: Huang Fumin , Yue Kwok To , Tan Pingheng , Zhang Shu-Lin , Shi Zujin , Zhou Xihuang , Gu Zhennan
Published: July 2002

We report on a temperature dependence of the frequency of all the major peaks in the Raman spectra of carbon nanotubes, using different excitation laser powers at the sample. The frequency decreases with increasing temperature for all peaks, and the shifts in Raman frequencies are linear in the temperature of the sample. In comparison, a similar dependence is found in active carbon, but no shift is observed for the highly ordered pyrolytic graphite within the same range of variation in laser power. A lowering of frequency at higher temperature implies an increase in the carbon–carbon distance at higher temperature. The relatively strong temperature dependence in carbon nanotubes and active carbon may be due to the enhanced increase in carbon–carbon distance. This enhancement may originate from the heavy defects and disorder in these materials.
Energy levels and symmetry of Ce3+ in fluoride and oxide crystals
Authors: Kodama Nobuhiro , Yamaga Mitsuo , Henderson Brian
Published: July 2002

Single crystal samples of Ce3+-doped fluorides [LiCaAlF6 (LiCAF), LiYF4 (LYF), NaCaYF6 (NCYF)], and oxides [Ca2Al2SiO7 (CASM), CaYAlO4 (CYA)] have been grown in inert and reducing atmospheres, with the intent of studying the effects of symmetry and ligand coordination on the dopant energy levels. The optical absorption spectra of Ce3+ in these crystals consist, at most, of five overlapping bands. The luminescence is also a broadband due to strong electron–phonon interaction in the 5d excited state. The energies of the lowest absorption band and the luminescence band decrease along the sequence LiCAF, LYF, NCYF, CASM, and CYA. This trend may be explained by the nephelauxetic effect and lowering symmetry of the anion-coordinate polyhedra of Ce3+ in these crystals. The lowest energy peak in the absorption spectrum of Ce3+ observed in CASM is coincident with the third harmonic (355 nm) of a Q-switched pulsed YAG laser.
Hysteretic internal fields and critical currents in polycrystalline superconductors
Authors: Kunchur M. N. , Askew T. R.
Published: July 2002

The transport critical current Jc in a polycrystalline superconductor is a hysteretic function of applied magnetic field H0 due to flux trapping by grains. This effect has been observed by several groups and attempts have been made to calculate the intergranular field Hi as a function of the applied H0 in terms of an effective geometrical demagnetization factor D. In general a first-principles calculation of D is very difficult, and furthermore, D is not constant but is itself a hysteretic function of H0. We develop a self-consistent scheme to extract the D and Hi directly from the Jc(H0) data itself. Our model exploits the fact that there are two field ranges for which the demagnetizing field is a simple function of H0. At low virgin fields, in the Meissner state, the susceptibility χv≈−1/4π is well defined, leading to a multiplicative correction: Hi=H0/(1+D4πχv). For fields that have returned from high values, a complete critical state is established and M is well defined—although χv is unknown. This leads to an additive correction: Hi=H0−D4πM for the total field. By matching Jc data in increasing and decreasing fields that satisfy these special cases, it is possible to extract the relevant parameters of the problem without detailed knowledge of the demagnetization geometry. We use this model to analyze data measured on sintered YBa2Cu3O7 superconductive rods.
The NaCl pressure standard
Authors: Brown J. Michael
Published: July 2002

A revised pressure scale for NaCl is proposed as an update for the 30-year-old work of Decker. An alternative approach to the analysis is utilized in conjunction with more recent data. The zero-Kelvin compression curve is parameterized using local basis functions (splines) and constrained by accurate pressure-volume-temperature data. Thermal pressures are estimated within a quasiharmonic framework using a volume-dependent Grüneisen parameter and the Debye thermal energy. In the pressure regime extending to 5 GPa uncertainties in pressure (based on measured volumes) are estimated to be less than 1%. Uncertainty increases to 1.5% at 10 GPa and 3% at 25 GPa. The largest contribution to systematic uncertainty at the highest pressures is the lack of knowledge of the volume dependence of the Grüneisen parameter. Misfit of other calculated thermodynamic properties with respect to data is relatively small. On the basis of the current analysis, pressures determined using the older Decker calibration are low. Along the 300 K isotherm, apparent errors in the Decker scale are as large as −3% (−0.3 GPa at 10 GPa, −0.47 GPa near 18 GPa, and −0.37 GPa at 25 GPa). At higher temperatures the apparent errors are smaller. At 1100 K and 20 GPa the error is −0.2 GPa.
Infrared spectroscopic analysis of the Si/SiO2 interface structure of thermally oxidized silicon
Authors: Queeney K. T. , Weldon M. K. , Chang J. P. , Chabal Y. J. , Gurevich A. B. , Sapjeta J. , Opila R. L.
Published: July 2002

The nature of the silicon oxide transition region in the vicinity of the Si/SiO2 interface is probed by infrared and x-ray photoelectron spectroscopies. The layer-by-layer composition of the interface is evaluated by uniformly thinning thermal oxide films from 31 Å down to 6 Å. We find that the thickness dependence of the frequencies of the transverse optical and longitudinal optical phonons of the oxide film cannot be reconciled by consideration of simple homogeneous processes such as image charge effects or stress near the interface. Rather, by applying the Bruggeman effective medium approximation, we show that film inhomogeneity in the form of substoichiometric silicon oxide species accounts for the observed spectral changes as the interface is approached. The presence of such substoichiometric oxide species is supported by the thickness dependence of the integrated Si suboxide signal in companion x-ray photoelectron spectra.
Transport properties of polypyrrole–ferric oxide conducting nanocomposites
Authors: Gangopadhyay R. , De A. , Das S.
Published: July 2002

Conducting nanocomposite samples were prepared combining colloidal ferric oxide particles with conducting polypyrrole. Three composite samples (prepared keeping colloidal content fixed and varying the content of the conducting polypyrrole) and the pure polymer were used in the present investigation. Temperature dependent dc and ac conductivity and thermoelectric power for the samples have been measured. The dc conductivity results were analyzed by Mott’s variable range hopping mechanism. The variation of ac conductivity with the frequency shows very little change in total conductivity up to a critical frequency, followed by a sudden jump with discontinuity and then increases monotonically following a power law. The frequency exponent decreases with temperature as predicted by the correlated barrier hopping theory. Above 50 K the ac component of the conductivity increases almost linearly as predicted by the quantum mechanical tunneling model. It is found that all the features of ac conductivity cannot be reconciled into an existing single theory. The thermoelectric power is positive, low, and varies linearly with temperature, indicating a metallic character and the presence of polarons and/or bipolarons as the cationic charge carriers in the composites. The overall nature of the S(T) curves suggests that in addition to a contribution from hopping a linear metallike component is also active for the thermopower.