bluuebunny/crossref_metadata_2025 huggingface dataset:
Photopolymer holographic recording material parameter estimation using a nonlocal diffusion based model
Authors: Lawrence Justin R. , O’Neill Feidhlim T. , Sheridan John T.
Published: July 2002

It has recently been shown [Sheridan and Lawrence, J. Opt. Soc. Am. A 17, 1108 (2000)] that by introducing a nonlocal response function into the one-dimensional diffusion equation governing holographic grating formation in photopolymers, both high frequency and low frequency cutoffs in the spatial frequency response of photopolymer materials can be deduced. Starting with this result, analytic solutions to this equation are derived assuming a two harmonic expansion of the monomer concentration. These expressions are first compared to the more rigorous four harmonic numerical results, indicating the range of physical parameters for which they are accurate. They are then compared to the experimental diffraction intensity growth curves produced during the formation of sinusoidal gratings recorded with different spatial frequencies. Fitting these curves using the analytic expressions, various material parameters, including the monomer diffusion constant, D, and the nonlocal variance, σ, can be estimated. Trends in material behavior as the grating spatial frequency is changed are discussed.
Buoyant-thermocapillary instabilities in extended liquid layers subjected to a horizontal temperature gradient
Authors: Burguete J. , Mukolobwiez N. , Daviaud F. , Garnier N. , Chiffaudel A.
Published: July 2002

We report experiments on buoyant-thermocapillary instabilities in differentially heated liquid layers. The results are obtained for a fluid of Prandtl number 10 in a rectangular geometry with different aspect ratios. Depending on the height of liquid and on the aspect ratios, the two-dimensional basic flow destabilizes into oblique traveling waves or longitudinal stationary rolls, respectively, for small and large fluid heights. Temperature measurements and space–time recordings reveal the waves to correspond to the hydrothermal waves predicted by the linear stability analysis of Smith and Davis [J. Fluid Mech. 132, 119 (1983)]. Moreover, the transition between traveling and stationary modes agrees with the work by Mercier and Normand [Phys. Fluids 8, 1433 (1996)] even if the exact characteristics of longitudinal rolls differ from theoretical predictions. A discussion about the relevant nondimensional parameters is included. In the stability domain of the waves, two types of sources have been evidenced. For larger heights, the source is a line and generally evolves towards one end of the container leaving a single wave whereas for smaller heights, the source looks like a point and emits a circular wave which becomes almost planar farther from the source in both directions.
Observation of enhancement of superconductivity by an external electromagnetic field and of rf phase-slip lines in wide tin films
Authors: Agafonov A. B. , Zolochevkiı̆ I. V. , Khristenko E. V. , Dmitriev V. M.
Published: July 2002

The enhancement of superconductivity by an external electromagnetic field and rf phase-slip lines are observed in wide tin films. The temperature, frequency, and power dependences of the critical current are investigated and demonstrate the phenomena observed. It is shown that this class of nonequilibrium effects is common to both narrow and wide films.
A quantum mechanical method for calculating nonlinear optical properties of condensed phase molecules coupled to a molecular mechanics field: A quadratic multiconfigurational self-consistent-field/molecular mechanics response method
Authors: Poulsen Tina D. , Ogilby Peter R. , Mikkelsen Kurt V.
Published: July 2002

An approach for determining nonlinear optical properties within a quantum mechanics/molecular mechanics method is presented. The response equations in the multiconfigurational self-consistent-field/molecular mechanics approach that includes polarization effects are derived and implemented for second order in response theory. The method is employed to calculate frequency-dependent first hyperpolarizabilities and two-photon absorption properties for H2O in aqueous solution. The results are in close agreement with experimental measurements.
Interaction of Stokes boundary layer flow with a sound wave
Authors: Ramos Eduardo , Cuevas Sergio , Huelsz Guadalupe
Published: July 2002

We analyze the interaction of a standing sound wave with the flow generated by the oscillation of a plate in its own plane (Stokes second problem). The sound wave acts in the direction transverse to the plate and it is considered that the plate oscillation and the sound wave have the same frequency but a nonzero relative phase. The sound wave induces a modification of the axial velocity that consists of two parts, an oscillation with twice the frequency of the plate oscillation and a steady streaming that persists beyond the Stokes boundary layer, resulting in a double boundary layer structure. This mechanism for generating steady streaming differs from those studied previously in the literature. The relative phase of the two oscillatory motions determines the direction of the net flow. The direction of the steady streaming far away from the plate, coincides with the direction of the displacement of the plate at the moment of maximum compression and is proportional to the velocity of the plate at this moment.
Reduced magnetic moment per atom in small Ni and Co clusters embedded in AlN
Authors: Zanghi D. , Teodorescu C. M. , Petroff F. , Fischer H. , Bellouard C. , Clerc C. , Pélissier C. , Traverse A.
Published: July 2002

Nanoparticles of cobalt and nickel embedded in AlN matrix have been prepared by ion implantation or sputter deposition with average diameters ranging from 0.8 to 9 nm. The average magnetic moment per atom versus the average cluster diameter was measured either by x-ray magnetic circular dichroism and/or by SQUID magnetometry. The resulting values are always smaller than the bulk ones. An interpretation is given in terms of an interaction between the atoms located at the cluster surface and the AlN matrix, leading to a magnetically dead layer about 0.1 nm thick.
Photoluminescence frequency up-conversion in GaSe single crystals as studied by confocal microscopy
Authors: Fan Y. , Bauer M. , Kador L. , Allakhverdiev K. R. , Salaev E. Yu.
Published: July 2002

The photoluminescence spectrum of melt-grown GaSe single crystals was investigated with a confocal Raman microscope equipped with a HeNe laser. Three luminescence bands of different intensity were observed, which are mainly located to the blue of the laser line. The luminescence signals show a quadratic dependence on excitation intensity. The effect is interpreted as second-harmonic generation in the strongly optically nonlinear material followed by the excitation of electrons into the conduction band and luminescence emission from direct-gap Wannier excitons. The relative intensities of the three luminescence peaks exhibit a spatial variation on the crystal surface, which was mapped with the confocal microscope. Possible explanations of this effect are discussed. In an external electric field the luminescence shows a strong increase and a quadratic redshift, whose magnitude is consistent with the Franz–Keldysh mechanism.
Controlled p-doping of zinc phthalocyanine by coevaporation with tetrafluorotetracyanoquinodimethane: A direct and inverse photoemission study
Authors: Gao Weiying , Kahn Antoine
Published: July 2002

P-doping of zinc phthalocyanine (ZnPc) with tetrafluorotetracyanoquinodimethane (F4-TCNQ) is investigated with ultraviolet and x-ray photoemission spectroscopy, inverse photoemission spectroscopy, and in situ current–voltage (I–V) measurements. The electron affinity of F4-TCNQ (5.24 eV) is found to be equal, within experimental error, to the ionization energy of ZnPc (5.28 eV), consistent with efficient host-to-dopant electron transfer. As a result, the Fermi level in doped ZnPc drops from near midgap to 0.18 eV above the leading edge of the highest occupied molecular orbital and a narrow space-charge layer (<32 Å) is formed at the interface with the Au substrate. In situ I–V measurements show a seven orders of magnitude doping-induced increase in hole current.
C–Cl bond fission, HCl elimination, and secondary radical decomposition in the 193 nm photodissociation of allyl chloride
Authors: Morton Melita L. , Butler Laurie J. , Stephenson Thomas A. , Qi Fei
Published: July 2002

The primary photodissociation dynamics of allyl chloride upon excitation at 193 nm is investigated in a crossed laser-molecular beam scattering apparatus. Tunable vacuum ultraviolet (VUV) photoionization of the products provides a unique ability to learn about the secondary reaction products of the nascent photoproducts formed. The data show evidence for four significant primary reaction channels: a previously unidentified low kinetic energy C–Cl bond fission channel producing unstable allyl radicals, an excited state C–Cl bond fission channel producing Cl atoms with high translational energy, an HCl elimination pathway releasing significant energy to product translation to HCl and its momentum-matched mass 40 partner, and an HCl elimination channel producing low kinetic energy HCl products and predominantly unstable mass 40 products. The measured branching of these primary reaction channels of [all C–Cl] : [fast C–Cl] : [slow C–Cl] : [fast HCl] : [slow HCl] : [all HCl] is 1.00: 0.971: 0.029: 0.291: 0.167: 0.458 (where fast refers to the high recoil kinetic energy channels). The high internal energy allyl radicals formed in the slow C–Cl fission pathway of allyl chloride further dissociate/isomerize, as do the unstable mass 40 products formed in the HCl elimination pathways, and these products are investigated. Photoionization efficiency (PIE) curves of the HCl product suggest that a three-centered elimination mechanism contributes significantly to an observed HCl elimination reaction.
Fiber optic extensometer for concrete deformation measurements
Authors: Yuan Libo , Zhou Li-min , Lau K. T. , Jin Wei , Demokan M. S.
Published: July 2002

A fiber optic extensometer based on a scanning white light Michelson interferometer is presented. The instrument employs a light emitting diode as the light source and a single mode fiber with predetermined gauge length as the extensometer sensor head. Light to and from the sensor head is transmitted through a single mode lead (i.e., in/out) fiber. The sensor performance is insensitive to the in/out fiber extensions. The fiber optic extensometer was applied to measure the compression and tension of concrete specimens. The measurement results compare well with that from a conventional extensometer.
Electrical and dielectric behavior of MgO doped Ba 0.7Sr0.3TiO 3 thin films on Al2 O3 substrate
Authors: Lee S. Y. , Tseng T. Y.
Published: July 2002

In this letter, we present the results of the fabrication and characterization of 5 mol % MgO doped Ba0.7Sr0.3TiO3 (BST) films grown on Pt/TiN/SiO2 coated on Al2O3 substrates using the rf magnetron sputtering technique. The dielectric and electrical properties of Ba0.7Sr0.3TiO3 thin film were found to improve obviously by means of MgO doping. The leakage current density of BST thin film decreased about 1 order of magnitude on MgO doping, while BST film with MgO doping had a higher dielectric constant than that without MgO doping. The dielectric constant of the films increased with increasing annealing temperature due to the consistent increase in grain size and crystallinity. The 750 °C annealed, 100 nm thick film indicated a high dielectric constant of 440 at 100 kHz and the lattice constant of 3.986 Å. The improvement of the electrical properties of BST films was associated with the reduced oxygen vacancies due to improved oxygenation of BST films in the presence of MgO. The MgO doped BST films exhibited a high tunability of 25% and dc resistivity of 6×1010 Ω cm at an applied electric field of 200 kV/cm. The time-dependent dielectric breakdown studies indicated that the films had a longer lifetime of over 10 yrs on operation at the electric field of 0.4 MV/cm which is better than undoped BST film.
High-energy (MeV) Al and B ion implantations into 4H-SiC and fabrication of pin diodes
Authors: Kimoto Tsunenobu , Miyamoto Nao , Schöner Adolf , Saitoh Akira , Matsunami Hiroyuki , Asano Katsunori , Sugawara Yoshitaka
Published: July 2002

High-energy (MeV) implantation of Al+ or B+ into 4H-SiC epilayers has been investigated. A 3 μm deep pn junction was formed by multiple-step Al+ or B+ implantation with implantation energies up to 6.2 or 3.4 MeV, respectively. Rutherford backscattering channeling and cross-sectional transmission electron microscopy analyses have revealed residual damages in the implanted layers even after high-temperature annealing at 1600–1800 °C. Nevertheless, high electrical activation ratios over 90% have been achieved for both Al+- and B+-implanted layers by annealing at 1800 °C. Mesa pin diodes with a 15-μm-thick i layer formed by MeV implantation have exhibited high breakdown voltages of 2860–3080 V. The reverse characteristics of diodes have been substantially improved by increasing annealing temperature up to 1800 °C. The diode performance is discussed with the results of deep level analyses near the junctions.
Dihydrogen bonded phenol–borane-dimethylamine complex: An experimental and theoretical study
Authors: Naresh Patwari G. , Ebata Takayuki , Mikami Naohiko
Published: July 2002

Continuing with our earlier communication on the dihydrogen bonded phenol–borane-dimethylamine complex [J. Chem. Phys. 113, 9885 (2000)], we report here, the realistic structure of the said complex calculated using density functional theory at B3LYP/6-31++G(d,p) level. The agreement between the experimental and calculated vibrational spectrum for both the N–H and O–H stretching vibrations along with the low-frequency vibrations that appear in combination with O–H stretching, provides the basis for structural assignment. Analysis of the fate of B–H bonds and B–H stretching vibrations upon formation of dihydrogen bond reveals an anomalous behavior of average bond strengthening.
Three-dimensional position detection of optically trapped dielectric particles
Authors: Rohrbach Alexander , Stelzer Ernst H. K.
Published: July 2002

A theory is presented together with simulation results that describe three-dimensional position detection of a sphere located in a highly focused beam by back-focal plane interferometry. This technique exploits the interference of scattered and unscattered light, which is projected on a quadrant photodiode placed in the back-focal plane of a condenser lens. Due to the Gouy-phase shift inherent in focused beams, it is not only possible to determine the lateral but also the axial position of a spherical particle with nanometer accuracy. In this paper we describe the calculation of arbitrary focused electromagnetic fields, the Gouy phase shift, Mie scattering by focused beams and the resulting position signals using the angular momentum representation. The accuracy and the sensitivity of the detection system are investigated theoretically for various sphere parameters. Both accuracy and sensitivity depend on the incident light distribution as well as on the particle’s properties and position. It is further shown that the maximum capture angle of the detection lens influences the detector’s sensitivity in a nonlinear manner. Additionally, for optical trapping applications the influence of the laser power is taken into account and is considered through a noise analysis. For all investigated trapping conditions the reconstructed position deviates on average <1 nm laterally and <5 nm axially from the actual particle position.
A three-dimensional renormalization group bubble merger model for Rayleigh–Taylor mixing
Authors: Cheng Baolian , Glimm J. , Sharp D. H.
Published: July 2002

In this paper we formulate a model for the merger of bubbles at the edge of an unstable acceleration driven (Rayleigh–Taylor) mixing layer. Steady acceleration defines a self-similar mixing process, with a time-dependent inverse cascade of structures of increasing size. The time evolution is itself a renormalization group (RNG) evolution, and so the large time asymptotics define a RNG fixed point. We solve the model introduced here at this fixed point. The model predicts the growth rate of a Rayleigh–Taylor chaotic fluid mixing layer. The model has three main components: the velocity of a single bubble in this unstable flow regime, an envelope velocity, which describes collective excitations in the mixing region, and a merger process, which drives an inverse cascade, with a steady increase of bubble size. The present model differs from an earlier two-dimensional (2-D) merger model in several important ways. Beyond the extension of the model to three dimensions, the present model contains one phenomenological parameter, the variance of the bubble radii at fixed time. The model also predicts several experimental numbers: the bubble mixing rate, αb=hb/Agt2≈0.05–0.06, the mean bubble radius, and the bubble height separation at the time of merger. From these we also obtain the bubble height to the radius aspect ratio. Using the experimental results of Smeeton and Youngs (AWE Report No. O 35/87, 1987) to fix a value for the radius variance, we determine αb within the range of experimental uncertainty. We also obtain the experimental values for the bubble height to width aspect ratio in agreement with experimental values.
Structural instability of 4H–SiC polytype induced by n-type doping
Authors: Liu J. Q. , Chung H. J. , Kuhr T. , Li Q. , Skowronski M.
Published: July 2002

Spontaneous formation of stacking faults in heavily nitrogen-doped 4H-polytype silicon carbide crystals have been observed by transmission electron microscopy (TEM). Faults were present in as-grown boules and additional faults were generated by annealing in argon at 1150 °C. All faults had identical structure consisting of six layers stacked in a cubic sequence as determined by high-resolution TEM, and were interpreted as a result of two Shockley partial dislocations gliding on two neighboring basal planes of SiC. It is argued that the energy of faulted 4H silicon carbide is lower than the energy of perfect heavily doped (n>1×1019 cm−3) crystal at typical processing temperatures, thus providing a driving force for transformation.
Direct Monte Carlo simulations of the equilibrium properties of n-pentane liquid–vapor interface
Authors: Goujon Florent , Malfreyt Patrice , Boutin Anne , Fuchs Alain H.
Published: July 2002

Direct MC calculations have been carried out to study the liquid–vapor equilibrium of n-pentane. We have used the local long range correction to the configurational energy within the Metropolis scheme and an algorithm allowing us to select randomly with equal probability two different maximum displacements. The thermal equilibrium conditions are checked by calculating the profiles of the configurational temperature in the vapor and liquid phases. We also check the mechanical equilibrium by calculating the profiles of the normal and tangential pressure components. The normal pressure profile is constant through the interface and in both phases on the conditions that the two parts of the long range corrections to the normal pressure area included in the calculations. The critical densities and temperatures are well predicted and the vapor pressures agree satisfactory with the experimental values within larger statistical fluctuations. The long range corrections to the surface tension are calculated using various analytical forms and are included in the computation to give values which are in good agreement with those available in the literature. Finally, the surface tension is computed from 299 K to 425 K for two system sizes and three cutoff values.
Quantum size effects in PbSe quantum wells
Authors: Rogacheva E. I. , Tavrina T. V. , Nashchekina O. N. , Grigorov S. N. , Nasedkin K. A. , Dresselhaus M. S. , Cronin S. B.
Published: July 2002

In PbSe epitaxial thin films grown by thermal evaporation on KCl(001) substrates and covered with an EuS protective layer, oscillatory dependences of the galvanomagnetic and thermoelectric properties (electrical conductivity σ, the Hall coefficient RH, charge carrier mobility μ, and the Seebeck coefficient S) on the PbSe layer thickness d (3<d<200 nm) were observed at room temperature. Oscillations of the transport properties are associated with quantum size effects due to electron confinement in the PbSe quantum wells.
State-to-state energy transfer of NH(X 3Σ−,v=0,J,N) in collisions with He and N2
Authors: Rinnenthal Jan Leo , Gericke Karl-Heinz
Published: July 2002

State-to-state rotational energy transfer of ground state NH(X 3Σ−,v=0,J,N) in collisions with He and N2 is studied. A complete inversion between the metastable NH(a 1Δ) state and the NH(X 3Σ−) state is generated via the photodissociation of hydrazoic acid at a wavelength of 266 nm. Single state NH(X 3Σ−,v=0,J,N) is generated by applying the stimulated emission pumping technique using the strongly forbidden NH(a 1Δ→X 3Σ−) intercombination transition around 794 nm. The ground state NH(X 3Σ−,v=0,J,N) distribution is probed with respect to all quantum states using laser induced fluorescence varying delay times and pressures. The collision induced energy transfer between the different rotational and spin levels is extensively studied and two comprehensive sets of rate constants for vibrationally elastic and rotationally inelastic collisions with He and N2 as collision partners are given which include the effect of multiple collisions. We find propensities for (ΔN=0,Δi=±1) and (ΔN=±1,Δi=0) transitions where N represents the quantum state for nuclear rotation and i represents the index of the spin component Fi. The rotational relaxation for N2 as a collision partner occurs on the average three times faster than the rotational relaxation with He as a collision partner. The energy dependence of the transition efficiency for only the nuclear rotational quantum number N obeys an energy-gap law for both He and N2.
Self-consistent scattering theory of transport and output characteristics of quantum cascade lasers
Authors: Indjin D. , Harrison P. , Kelsall R. W. , Ikonić Z.
Published: July 2002

Electron transport in GaAs/AlGaAs quantum cascade lasers operating in midinfrared is calculated self–consistently using an intersubband scattering model. Subband populations and carrier transition rates are calculated and all relevant electron-LO phonon and electron–electron scatterings between injector/collector, active region, and continuum resonance levels are included. The calculated carrier lifetimes and subband populations are then used to evaluate scattering current densities, injection efficiencies, and carrier backflow into the active region for a range of operating temperatures. From the calculated modal gain versus total current density dependencies the output characteristics, in particular the gain coefficient and threshold current, are extracted. For the original GaAs/Al0.33Ga0.67As quantum cascade structure [C. Sirtori et al., Appl. Phys. Lett. 73, 3486 (1998)] these are found to be g=11.3 cm/kA and Jth=6±1 kA/cm2 (at T=77 K), and g=7.9 cm/kA and Jth=10±1 kA/cm2 (at T=200 K), in good agreement with the experiment. Calculations shows that threshold cannot be achieved in this structure at T=300 K, due to the small gain coefficient and the gain saturation effect, also in agreement with experimental findings. The model thus promises to be a powerful tool for the prediction and optimization of new, improved quantum cascade structures.
Strained gallium nitride nanowires
Authors: Seo Hee Won , Bae Seung Yong , Park Jeunghee , Yang Hyunik , Park Kwang Soo , Kim Sangsig
Published: July 2002

Gallium nitride nanowires were synthesized on silicon substrates by chemical vapor deposition using the reaction of gallium and gallium nitride mixture with ammonia. Iron nanoparticles were used as catalysts. The diameter of nanowires is uniform as 25 nm and the lengths are 20–40 μm. The nanowires have single crystalline wurtzite structure with a few stacking faults. A careful examination into x-ray diffraction and Raman scattering data revealed that the separations of the neighboring lattice planes along the growth direction are shorter than those of bulk gallium nitride. The nanowires would experience biaxial compressive stresses in the inward radial direction and the induced tensile uniaxial stresses in the growth direction. The shifts of the band gap due to the stresses have been estimated using the experimental data, showing that the reduction of the band gap due to the tensile stresses can occur more significantly than the increase due to the compressive stresses. The temperature-dependent photoluminescence (PL) of the nanowires exhibit a strong broad band in the energy range of 2.9–3.6 eV. The PL could originate from the recombination of bound excitons. The strong room-temperature PL would be in line with the existence of strains inside the nanowires. The peak appears at the lower energy than that of the epilayer, which is consistent with the decrease of the band gap predicted from the x-ray diffraction and Raman data. The various strengths of stress may result in the widely distributed PL energy position.
Lagrangian coherent structures from approximate velocity data
Authors: Haller G.
Published: July 2002

This paper examines whether hyperbolic Lagrangian structures—such as stable and unstable manifolds—found in model velocity data represent reliable predictions for mixing in the true fluid velocity field. The error between the model and the true velocity field may result from velocity interpolation, extrapolation, measurement imprecisions, or any other deterministic source. We find that even large velocity errors lead to reliable predictions on Lagrangian coherent structures, as long as the errors remain small in a special time-weighted norm. More specifically, we show how model predictions from the Okubo–Weiss criterion or from finite-time Lyapunov exponents can be validated. We also estimate how close the true Lagrangian coherent structures are to those predicted by models.
Quantifying symmetric chaos
Authors: Johnson M. A. , Meisel L. V.
Published: July 2002

Nonlinear systems exhibit features which cannot be understood, controlled, or exploited without addressing their behavior in a nonlinear form. Symmetric chaotic mappings, which were described by Field and Golubitsky [Comput. Phys. 4, 470 (1990)], comprise an interesting subclass of nonlinear systems. Field and Golubitsky described qualitative techniques for the characterization of the attractors for these mappings. Here we present qualitative and quantitative descriptions of the attractors of a number of symmetric chaotic mappings, which undergo a change of symmetry from Dn to Zn as a parameter ω is varied.
Laplace and Poisson equation solution by relax3d
Authors: Houtman H. , Jones F. W. , Kost C. J.
Published: July 2002

The program RELAX3D is useful for the solution of the three-dimensional, second-order, elliptic partial differential equation, ∇2V(x,y,z)=F(x,y,z), which includes Laplace’s equation and Poisson’s equation. It uses the finite-difference algorithm known as symmetric successive over-relaxation. After a brief survey of other solution techniques, the program is described in detail and several examples of its actual use are given. The program is interactive and accurately solves for V according to the function F and the arbitrary boundary conditions, both supplied via the user-encoded subroutine ‘‘BND.’’ The functions V and F are defined on grids of two or three dimensions, in Cartesian, cylindrical, or polar coordinates. To improve the efficiency of the repetitive five-point or seven-point relaxation iterations on the grid, conventional fortran array addressing is replaced by a fast hard-coded addressing method. In the inner loop where the program spends most of its time, references to the V and F arrays are accomplished using only one addition per address. Detailed derivations are provided for 2D and 3D Cartesian relaxation formulas, with error terms of 𝒪 (h4), 𝒪 (h6), and 𝒪 (h8); we report a set of test computations which verify these formulas. RELAX3D provides alphanumeric printer output with input boundaries clearly displayed, and high-resolution printer or plotter contour maps of any desired slices through the solution region.
Reflection and propagation of waves in one-dimensional quasi-periodic structures
Authors: Allodi G. , Coïsson R.
Published: July 2002

A software package has been developed, with C compiled functions used in a Matlab environment, in order to model optical quasi-periodic structures such as distributed feedback laser resonators, filters, and gratings for dispersion compensation. This ‘‘Bragg’’ package also accounts for grating chirp, tapering, gain or losses, and optical nonlinearity. © 1996 American Institute of Physics.
Time decay of the remanent magnetization in spin glasses (invited)
Authors: Chamberlin Ralph V.
Published: July 2002

We begin with a brief history of magnetoviscosity measurements in materials. We then show that the relaxation of the thermoremanent magnetization in spin glasses is accurately characterized, over several decades in time, by the stretched exponential: σTRM=σ0 exp[−C(ωt)1−n/(1−n)]. The time-stretch exponent (n) is given as a function of temperature for both 2.6% AgMn+0.46% Sb and 1.0% CuMn; dissimilarities in the temperature dependence of n may be due to the different anisotropy energy of the samples. We emphasize the experimental fact that a field-cooled sample is not in equilibrium and show that the time development of the field-cooled state may be characterized as an exponential decrease of the relaxation frequency with increasing wait time: ω=ω0 exp(−tw/t0). We briefly outline three general theories which have the stretched-exponential time dependence, but no theory can yet explain all of the observed behavior in spin glasses. We conclude by showing that the remanent magnetization in a ferromagnet may also have the stretched-exponential time dependence.
Ionic conductivity in solid, crosslinked dimethylsiloxane-ethylene oxide copolymer networks containing sodium
Authors: Adamić K. J. , Greenbaum S. G. , Wintersgill M. C. , Fontanella J. J.
Published: July 2002

The preparation of an ion-conducting elastomeric solid based on a dimethylsiloxane-ethylene oxide copolymer complexed with a sodium salt is described. 23Na nuclear magnetic resonance measurements reveal the presence of both bound and mobile sodium species throughout the temperature range −120 to 100 °C. Electrical conductivity measurements over a similar temperature range are found to be consistent with the configurational entropy model for transport, with a T0 parameter about 50 °C below the ‘‘central’’ glass transition temperature Tg.
A molecular effect of hydrogen trapping in the implantation of molecular hydrogen ions into aluminum and aluminum oxide
Authors: Imanishi N. , Fukumura T. , Miyamoto S. , Iwasaki M.
Published: July 2002

Molecular effects of hydrogen trapping are examined for the implantation of H+2 and H+3 molecular ions into aluminum and aluminum oxide. It was found that the H+3 bombardment enhances trapping efficiencies more than H+2 by a factor of 2.7 for Al and 4.3 for Al2O3. These results indicate the importance of the interaction between hydrogen and vacancy clusters.
Metal deposition at 532 nm using a laser transfer technique
Authors: Bohandy J. , Kim B. F. , Adrian F. J. , Jette A. N.
Published: July 2002

The laser-induced forward transfer technique in which material is ablatively transferred from a thin film to a target substrate by a pulsed excimer laser has been extended to 532 nm using a frequency-doubled YAG laser. Cu and Ag have been deposited on fused silica substrates using microscope objectives for focusing, resulting in reductions in feature size over that obtainable with the multimode excimer laser. The photothermal deposition process has been modeled using the one-dimensional thermal diffusion equation, including a moving solid-melt boundary, with good agreement between theoretical and experimental results.
Heavy fermion behavior in CePtSi and CeRuSi
Authors: Rebelsky L. , Reilly K. , Horn S. , Borges H. , Thompson J. D. , Willis J. O. , Aikin R. , Caspari R. , Bredl C. D.
Published: July 2002

We present results of specific heat, electrical resistance, and magnetic susceptibility measurements on the heavy fermion system CePtSi and compare it to CeRuSi. Both compounds show a low-temperature maximum in cp/T, usually associated with the formation of a coherent heavy fermion ground state. An additional feature in cp/T is observed for CePtSi and is attributed to cooperative magnetism, an interpretation confirmed by magnetic measurements. A comparison with other heavy fermion systems, especially stoichiometric and nonstoichiometric CeCu2Si2, is performed.
Common mode choke cores using the new Fe-based alloys composed of ultrafine grain structure
Authors: Yoshizawa Y. , Yamauchi K. , Yamane T. , Sugihara H.
Published: July 2002

Common mode choke cores using the new Fe-based alloys called ‘‘FINEMET’’ composed of ultrafine grain structure were developed. These new cores show high attenuation to high-voltage pulse noises and good attenuation characteristics over a wide frequency range. Accordingly, the new cores are expected to be applied in many applications as line filters for electronic devices.
A comprehensive kinetic model of the electron-beam-excited xenon chloride laser
Authors: Johnson Thomas H. , Cartland Harry E. , Genoni Thomas C. , Hunter Allen M.
Published: July 2002

A new kinetics model capable of simulating performance of electron-beam-pumped xenon chloride lasers over the full range of experimental evidence is presented. The model comprises 202 chemical processes employing 41 species. Its operation is described and the full set of rate equations given. Calculations of stimulated emission and absorption cross sections for XeCl are presented, and simulations of various lasing results are shown in the context of explicating dominant processes. Major kinetics issues are examined, particularly those leading to the model’s rates for vibrational excitation of HCl and for electron quenching of the excited excimer molecule.
Ellipsometric measurement of the optical constants of solid and molten aluminum and copper at λ=10.6 μm
Authors: Brückner M. , Schäfer J. H. , Uhlenbusch J.
Published: July 2002

The index of refraction, the extinction coefficient, and the absorptance were measured under ultrahigh vacuum conditions in a temperature range from 295 to 1000 K for aluminum and from 295 to 1400 K for copper for the CO2 laser wavelength at λ=10.6 μm. A photometric infrared ellipsometer was applied. The absorptance of the clean metallic surfaces jumps discontinuously from 3% to 7% in the case of aluminum and from 2% to 6% in the case of copper at the melting point. Calculations of the optical constants basing on the model of free electrons taking account of the effective mass and the anomalous skin effect describe the measurements for the solid state very well, but are less accurate for the molten state.
Crystallinity and Schottky diode characteristics of GaAs grown on Si by metalorganic chemical vapor deposition
Authors: Egawa T. , Nozaki S. , Noto N. , Soga T. , Jimbo T. , Umeno M.
Published: July 2002

The crystallinity of GaAs grown on Si with various intermediate layers by metalorganic chemical vapor deposition and characteristics of Schottky diodes fabricated on the grown GaAs/Si have been studied. The GaAs/Si with an Al0.55Ga0.45P intermediate layer has both good crystallinity, x-ray full width at half maximum of 188 arc sec, and a smooth surface. The Schottky diode fabricated on the GaAs/Si with an Al0.55Ga0.45P intermediate layer also shows good forward and reverse current-voltage characteristics with an ideality factor of 1.06, as good as for an n-type GaAs substrate. However, a relatively large leakage current is observed under reverse and small forward bias. This leakage current is caused by generation and recombination through the centers related to dislocations in the GaAs/Si.
Transport of high-current electron beams in a racetrack betatron
Authors: Humphries S. , Terry R. L. , Frauenglass A.
Published: July 2002

This paper reports theoretical and experimental work on the transport of space-charge-dominated relativistic electron beams in a racetrack betatron with cusp focusing. High-efficiency propagation of 550-keV, 280-A beams through a complex optical system consisting of two 180° arcs with 36 focusing cells and a straight section was observed. More than 90% of the injected beam was captured and transported through the 7-m system. The experiments resolved several recent problems of beam injection and trapping in the machine and showed that electron beam transport at the space-charge limit is possible in a curved cusp array despite strong axial variations of focusing forces. We encountered no problems with the transitions between curved and straight focusing sections. The experiments demonstrated good beam containment in a cusp array with mixed focusing cell geometries as long as the axially averaged focusing force was the same for all cells. The ability to use long cells is a critical feasibility issue for other planned injection experiments into a closed racetrack geometry.
Metal/water chemical reaction coupled to a pulsed electrical discharge
Authors: Lee W. M.
Published: July 2002

Capacitor-stored energy is transferred to a wire conductor surrounded by a mixture of a reactive metal powder and water. The overload current pulse quickly explodes the small wire conductor, initiating a chemical reaction in the mixture. The discharge after the explosion is made through a highly condensed state undergoing chemical reaction (producing both heat and hydrogen gas). In this type of discharge the heat and gas from the reaction help the reacting medium maintain some conductance necessary for effective dissipation of the electrical energy. The coupling of the discharge to the reaction is found to be crucial to sustain the reaction.
Influence of photon recycling on lifetime and diffusion coefficient in GaAs
Authors: Renaud Ph. , Raymond F. , Bensaïd B. , Vèrié C.
Published: July 2002

The continuity equation of minority carriers is solved with the inclusion of photon recycling. A new development technique is used to provide a simple evaluation of effective lifetime and diffusion coefficients. Results are found to be in agreement with already published methods in a wide range of sample geometries. This provides a reliable determination method for radiative and nonradiative lifetimes through a single time-resolved photoluminescence experiment on GaAs structures such as solar cells.
Very intense source of flash x rays
Authors: Sanford T. W. L. , Halbleib J. A. , Poukey J. W. , Welch D. R. , McAtee W. H. , Mock R. C.
Published: July 2002

This article describes the design of a very intense source of flash x rays using the HERMES III electron accelerator that delivers a measured peak dose and peak dose rate of (370±58) krad(CaF2) and (3.5±0.7)×1013 rad(CaF2)/s, respectively, over a useful area of 80 cm2, without target destruction. Measured electron and photon source distributions agree with those generated from a numerical model. The quality of the agreement gives credibility to both and validates the main assumptions of the model.
Optimized performance of quantum well intersubband infrared detectors: Photovoltaic versus photoconductive operation
Authors: Schneider H.
Published: July 2002

A recently demonstrated transport mechanism giving rise to novel photovoltaic multiquantum well intersubband infrared detectors is analyzed theoretically. It is shown that optimized photovoltaic devices have detectivities that can be even larger than those of photoconductive ones. In addition, they have a high dynamic range and are particularly suited for the large photon fluxes occurring in the 8–12 μm regime.
The enhanced Stark effects of coupled quantum wells and their application to tunable IR photodetectors
Authors: Huang Yimin , Lien Chenhsin , Lei Tan-Fu
Published: July 2002

A tunable asymmetric coupled quantum well far-infrared photodetector is proposed in this paper. The basic asymmetric coupled quantum wells are composed of two quantum wells separated by a thin barrier. In this way, the electron in each well interacts strongly with other electrons to achieve a large Stark tuning effect. The eigenenergies and the wave functions of the quantum-well structures are solved by the self-consistent method, and the effect of the exchange interaction on the ground-state subband has also been taken into account. The absorption coefficient is evaluated by the density-of-states formalism. Based on theoretical calculations, tuning ranges from 8.2 to 11.3 μm and 7.8 to 10.5 μm are predicted for the proposed asymmetric coupled-quantum-well structure and high-low coupled-quantum-well structure, respectively. This tuning capability is achieved by varying the applied electric field in the 20–90-kV/cm range.
Solar-cell collection efficiency and its variation with voltage
Authors: Liu X. X. , Sites J. R.
Published: July 2002

Internal collection efficiency of solar cells is calculated as a function of depletion width, minority-carrier diffusion length, solar spectrum, and absorption coefficient. Particular attention is given to the variation of depletion width with voltage, which may reduce collection efficiency by a few percent at operating voltages and may be misinterpreted as a shunting effect. Calculations of collection-efficiency magnitude and voltage dependence, plus comparisons with experiment, are presented for several types of solar cells. Primary emphasis is placed on polycrystalline thin-film cells with relatively short diffusion lengths and hence potentially more pronounced collection efficiency effects.
Temperature dependent Raman scattering in KTiOPO4 and KTiOAsO4 single crystals
Authors: Tu Chi-Shun , Guo A. R. , Tao Ruiwu , Katiyar R. S. , Guo Ruyan , Bhalla A. S.
Published: July 2002

The longitudinal (LO) and transverse (TO) A1 vibrational modes have been measured between 30 and 1200 cm−1 as a function of temperature (30–1240 K) for both KTiOPO4 (KTP) and KTiOAsO4 (KTA). KTP and KTA exhibit an obviously different Raman spectra in the frequency region 400–650 cm−1 (with respect to KTA). This middle-frequency difference is attributed to the substitution ions in XO4 group (X=P or As) modifing the force constant of crystal. The relative intensities of the low-frequency bands increase dramatically with increasing temperature due to high mobility of K+ ion. There is no typical soft-mode like behavior in the measured frequency range. A higher symmetric structure taking place above Tc has been confirmed by the disappearance of the A1g(LO) stretching modes of TiO6 group. Comparison of each frequency belonging to the symmetry A1, A2, B1, and B2 measured along the [110] phonon direction shows complex difference. The vibrational frequencies of various symmetries were also obtained.
Molecular-beam epitaxy of beryllium-chalcogenide-based thin films and quantum-well structures
Authors: Waag A. , Fischer F. , Lugauer H. J. , Litz Th. , Laubender J. , Lunz U. , Zehnder U. , Ossau W. , Gerhardt T. , Möller M. , Landwehr G.
Published: July 2002

A variety of BeMgZnSe–ZnSe- as well as BeTe-based quantum-well structures has been fabri- cated and investigated. BeTe buffer layers improve the growth start on GaAs substrates drasti- cally compared to ZnSe/GaAs. The valence-band offset between BeTe and ZnSe has been determined to be 0.9 eV (type II). Due to the high-lying valence band of BeTe, a BeTe–ZnSe pseudograding can be used for an efficient electrical contact between p-ZnSe and p-GaAs. BeMgZnSe quaternary thin-film structures have reproducibly been grown with high struc- tural quality, and rocking curve widths below 20 arcsec could be reached. Quantum-well structures show a high photoluminescence intensity even at room temperature.
Mathematical modeling of melting during laser materials processing
Authors: Xie J. , Kar A.
Published: July 2002

Melting is encountered in almost all laser materials processing. This article deals with a one-dimensional heat conduction problem to investigate the melting rate during laser materials processing. The problem is solved approximately to obtain a correlation among melt depth, power density, and laser irradiation time. Based on this correlation, the dynamics of melting, a relationship between the melt depth and power density and an average melting velocity are expressed by simple analytic formulas. These expressions are further simplified for high power densities (I⩾109 W/m2). The times to reach the melting and boiling temperatures at the surface of the workpiece are also calculated.
Characterization of Permalloy thin films electrodeposited on Si(111) surfaces
Authors: Gao L. J. , Ma P. , Novogradecz K. M. , Norton P. R.
Published: July 2002

The electrochemical conditions for the direct electroplating of Ni–Fe Permalloy (19% Fe, 81% Ni) thin films onto Si(111) surfaces were determined. The film composition, structure, and magnetic properties were examined by various techniques involving Auger electron spectroscopy depth profiles analysis, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and magneto-optic Kerr effect measurements. The Permalloy films consisted of 10–50-nm-diam Ni/Fe nanocrystallites. It was found that the coercivity decreased with increased film thickness. Moderate annealing of the films below 200 °C released stress in the films and led to a decrease in coercivity. Interfacial diffusion occurred after the annealing of films above 300 °C. The Ni–Fe films exhibited bulk properties after the completion of electrodeposition of a buffer layer of thickness >20 nm; below this thickness, the films were discontinuous and consisted of separated clusters or islands on the Si surface.
Theory of hole initiated impact ionization in bulk zincblende and wurtzite GaN
Authors: Oğuzman Ismail H. , Bellotti Enrico , Brennan Kevin F. , Kolnı́k Ján , Wang Rongping , Ruden P. Paul
Published: July 2002

In this article, the first calculations of hole initiated interband impact ionization in bulk zincblende and wurtzite phase GaN are presented. The calculations are made using an ensemble Monte Carlo simulation including the full details of all of the relevant valence bands, derived from an empirical pseudopotential approach, for each crystal type. The model also includes numerically generated hole initiated impact ionization transition rates, calculated based on the pseudopotential band structure. The calculations predict that both the average hole energies and ionization coefficients are substantially higher in the zincblende phase than in the wurtzite phase. This difference is attributed to the higher valence band effective masses and equivalently higher effective density of states found in the wurtzite polytype. Furthermore, the hole ionization coefficient is found to be comparable to the previously calculated electron ionization coefficient in zincblende GaN at an applied electric field strength of 3 MV/cm. In the wurtzite phase, the electron and hole impact ionization coefficients are predicted to be similar at high electric fields, but at lower fields, the hole ionization rate appears to be greater.
Absorption coefficient of 4H silicon carbide from 3900 to 3250 Å
Authors: Sridhara S. G. , Devaty R. P. , Choyke W. J.
Published: July 2002

We report the values of the absorption coefficient of 4H SiC at room temperature, in the wavelength range from 3900 to 3350 Å and at 3250 Å. By using the known shift in the band gap with temperature, we also present an estimate of the absorption coefficient of 4H SiC at 2 K.