bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonalloyed ohmic contacts to n-GaAs by solid-phase epitaxy of Ge
Authors: Marshall E. D. , Zhang B. , Wang L. C. , Jiao P. F. , Chen W. X. , Sawada T. , Lau S. S. , Kavanagh K. L. , Kuech T. F.
Published: July 2002

A low resistance nonalloyed ohmic contact to n-GaAs is formed which utilizes the solid-phase epitaxy of Ge through PdGe. Discussion focuses on the conditions necessary to attain low specific contact resistivity (∼10−6 Ω cm2 on 1018 cm−3 n-GaAs) and on the interfacial morphology between the contact metallization and the GaAs substrate. MeV Rutherford backscattering spectrometry and channeling show the predominant reaction to be that of Pd with amorphous Ge to form PdGe followed by the solid-phase transport and epitaxial growth of Ge on 〈100〉 GaAs. Cross-sectional transmission electron microscopy and lattice imaging show a very limited initial Pd-GaAs reaction and a final interface which is planar and structurally abrupt to within atomic dimensions. The presence of excess Ge over that necessary for PdGe formation and the placement of Pd initially in contact with GaAs are required to result in the lowest contact resistivity. The experimental data suggest a replacement mechanism in which an n+-GaAs surface region is formed when Ge occupies excess Ga vacancies.
Two-magnon excitations in quantum spin chains
Authors: Liu Jian-Min , Bonner Jill C.
Published: July 2002

Quantum spin chains of higher spin are interesting in the case of both ferromagnets and antiferromagnets. They comprise a wide range of model types—Bethe–Ansatz integrable, nonintegrable, and with limited integrability. It is not well appreciated that the two-magnon (two-body) problems is always solvable, by Bethe–Ansatz and Green’s-function techniques, regardless of the integrability character of the entire model, with interesting implications in the context of solitons and bound solitons. Chubukov and Kveschenko have suggested using the two-magnon problem in a test for integrability and claim to find additional classes of integrable spin chains for s=1 and s= (3)/(2) . Here we investigate this situation.
Dielectric anisotropy of stretched poly(ethylene terephthalate) at microwave frequencies
Authors: Osaki Shigeyoshi
Published: July 2002

A new method for determining quickly and nondestructively the complex dielectric constant at microwave frequencies was applied to poly(ethylene terephthalate) (PET) films. The dielectric loss ε″ at 4.0 GHz decreased with increasing degree of crystallinity, indicating that it affects molecular motions in the amorphous region. From the angular dependence of ε″ for the uniaxially stretched PET film dielectric anisotropy was detected, showing ε″ to be larger in the transverse direction (TD) than in the machine direction (MD) or draw direction. The ratio of ε″ in the TD to MD increased with increasing draw ratio. The PET molecules in the amorphous region are considered to be oriented mainly in the MD. This anisotropy was considered to result from a change in dipole moment due to torsional motions of the ethylene glycol unit around the main chain in the amorphous region. Values of ε″max/ε″min were found to be useful as a measure of the degree of orientation, where ε″max and ε″min denote the maximum and minimum dielectric losses in the angular dependence, respectively. The amorphous orientation function f obtained from the angular dependence of ε″ at 4.0 GHz increases from 0 to 0.04 with increasing draw ratio from 1.0 to 3.6. It was shown that the dielectric anisotropy at microwave frequencies gave a probe for studying the molecular orientation in the amorphous region.
Optical properties of sputter-deposited ZnO:Al thin films
Authors: Jin Z.-C. , Hamberg I. , Granqvist C. G.
Published: July 2002

ZnO:Al coatings were prepared by rf magnetron sputtering of ZnO together with dc magnetron sputtering of Al onto rapidly revolving unheated substrates under weakly oxidizing conditions. Optimized films had ∼1% luminous absorptance, ∼85% thermal infrared reflectance, and ∼5×10−4 Ω cm electrical resistivity at a thickness of ∼0.3 μm. The Al content was ≲2 at. %, as determined by Rutherford backscattering spectrometry. Transmission electron microscopy and electron diffraction showed ∼50-nm average crystallite size and a hexagonal wurtzite structure. Spectrophotometric transmittance and reflectance were recorded in the 0.2–50-μm wavelength interval, and the complex dielectric function was evaluated by computation. The optical data were explained from an effective mass model for n-doped semiconductors. The Al atoms are singly ionized, and the associated electrons occupy the bottom of the conduction band as free-electron gas. The Al ions act as pointlike Coulomb scatterers and are screened by the electrons according to the random phase approximation or an extension thereof. The optical properties of ZnO:Al could be understood by considering the free electrons to be damped primarily by ionized impurity scattering. ZnO:Al films can have high luminous transmittance, high solar ultraviolet absorptance, low thermal infrared emittance, and high electrical conductance; hence, they are of large interest for energy-efficient windows.
Characterization of individual interfacial barriers in a ZnO varistor material
Authors: Olsson E. , Dunlop G. L.
Published: July 2002

The role of different types of interfaces in a ZnO varistor material has been investigated by directly measuring the current/voltage characteristics of individual interfacial junctions. The electrically characterized interfaces were subsequently studied by analytical transmission electron microscopy. It was shown that barriers to electrical conduction form during cooling from the sintering temperature. The ZnO/ZnO junctions exhibited breakdowns at either 3.2 or 3.6 V, where the higher breakdown was observed at boundaries which contained a thin (∼2 nm) intergranular amorphous Bi-rich film, while the breakdown at 3.2 V was found at boundaries without secondary phases but which contained intergranularly segregated Bi atoms. Junctions between ZnO and intergranular Bi2O3 exhibited asymmetric current/voltage characteristics with breakdowns at 3.2 V for one polarity of the applied voltage and either 0.4 or 0.9 V for the other polarity. No varistor behavior was observed at interfaces between ZnO and pyrochlore, Zn2Sb3Bi3O14. This gives rise to a significant number of ‘‘inactive’’ grain boundaries in the material.
Various types of nonbridging oxygen hole center in high-purity silica glass
Authors: Munekuni Shuji , Yamanaka Toshihisa , Shimogaichi Yasushi , Tohmon Ryoichi , Ohki Yoshimichi , Nagasawa Kaya , Hama Yoshimasa
Published: July 2002

Optical absorption measurements of the 2.0-eV band and photoluminescence measurements of the 1.9-eV emission, excited by various excitation bands, were carried out on high-purity silica glasses subjected to γ-ray irradiation. Two, and possibly three, different forms of nonbridging oxygen hole centers were deconvoluted from the results of the isochronal annealing experiments. The difference in the peak wavelength of the 2.0-eV absorption and 1.9-eV luminescence bands among various forms of nonbridging oxygen hole centers is reported.
Cathodoluminescence imaging of defects and impurities in diamond films grown by chemical vapor deposition
Authors: Graham R. J. , Moustakas T. D. , Disko M. M.
Published: July 2002

Defects and impurities in diamond films grown by chemical vapor deposition (CVD) were analyzed by high-resolution cathodoluminescence (CL) spectroscopy and imaging in transmission electron microscopy (TEM). The combination of CL and TEM makes it possible to correlate the film microstructure with the electronic structure due to defects. Broad CL bands observed at 428±1 nm (2.90±0.01 eV) and 551±1 nm (2.250±0.004 eV) are attributed to closely spaced and widely separated donor-acceptor (D-A) pairs, respectively. A narrow peak at 738.7±0.5 nm (1.679±0.001 eV) is attributed to interstitial silicon atom impurities. An additional wide band at 365±1 nm (3.40±0.01 eV) was not identified. The material was found to be type IIb (semiconducting) and varied in quality with the growth conditions. Impurities are evidently distributed nonuniformly on a submicrometer scale, and both highly faulted and defect-free grains were found to emit no visible CL. For the first time in CVD-grown diamond, band-A CL due to closely spaced D-A pairs was found to be directly correlated with dislocations. Widely spaced D-A pairs were more uniformly distributed throughout the film. The distribution of interstitial silicon impurities varied greatly from grain to grain, but was not correlated with any microstructure.
Magnetic and magneto-optical properties of rare-earth transition-metal alloys containing Dy, Ho, Fe, Co
Authors: Hansen P. , Klahn S. , Clausen C. , Much G. , Witter K.
Published: July 2002

Amorphous rare-earth transition-metal alloys of composition REl−xTMx with RE=Dy, Ho; TM=Fe,Co and 0<x<1 and Dyl−x(Fe,Co)x were prepared by evaporation. The saturation magnetization, uniaxial anisotropy, coercivity, and Faraday rotation were investigated as a function of composition and temperature. Also, the spectral variation of the Kerr rotation was measured. The magnetization data indicate a strong dispersion of the RE moments due to randomly oriented local crystal field axes. The strong turndown of the Curie temperature for the Fe-rich alloys suggests that an additional dispersion is present in the Fe subnetwork. The mean field theory was used to analyze the temperature variation of the magnetization yielding smaller TM spin values and exchange coupling constants as compared with those of the Gd and Tb analogs. The uniaxial anisotropy constant Ku for Dy-Co based alloys was found to vary with the square of the Dy subnetwork magnetization as predicted by the random single-ion theory. The anisotropy of Dy-Fe and Ho based alloys require additionally dipolar terms to account for the measured temperature dependence of Ku. The coercive field Hc follows a relation Hc ∼ K1.5u/Ms. The magneto-optical effects are primarily caused by the transition metals and therefore their compositional, temperature, and spectral dependence correspond to that of their Gd and Tb analogs. Optical recording experiments on Dy-FeCo disks yield good write and erase sensitivities and carrier-to-noise ratios up to 60 dB which are comparable to those of GdTb-Fe and Tb-FeCo disks.
Charging and flashover induced by surface polarization relaxation process
Authors: Blaise G. , Le Gressus C.
Published: July 2002

Electric charges trapping into a dielectric medium are described from polarons trapped into sites which are characteristic of the material structure (polaron trap). In terms of susceptibility, that means charges are trapped into sites whose susceptibility is lower than their surrounding environment. The charge trapping produces polarization of the medium. The energy of polarization per embedded charge is estimated to be equal to 5χ (χ being the susceptibility). The surface breakdown of the dielectric is attributed both to charge detrapping and to relaxation of energy of polarization, using the collective many-body process. The consequences of this interpretation are consistent with experiments on surface breakdown performed with a scanning electron microscope. Furthermore, considerations about the improvement of breakdown voltage for dielectric materials are introduced.
Shock response of pentaerythritol tetranitrate single crystals
Authors: Dick J. J. , Mulford R. N. , Spencer W. J. , Pettit D. R. , Garcia E. , Shaw D. C.
Published: July 2002

The shock response of single crystals of the explosive pentaerythritol tetranitrate was studied from 3.4 to 19 GPa. Four orientations, 〈110〉, 〈001〉, 〈101〉, and 〈100〉 were studied. The shock Hugoniot and shock initiation sensitivities were measured in wedge tests. Intermediate velocity transitions were noted. A sensitivity anomaly near 4 GPa was discovered for 〈110〉 orientations. Chemiluminescent phenomena associated with this anomaly were studied using framing photography and uv/visible spectroscopy. Both plane-wave explosive and gas-gun impact experiments were done. Analysis of the steric hindrance occurring during edge dislocation motion in the shear flow associated with shock-induced uniaxial strain was done. The results corroborate the hypothesis that the orientation dependence of shock initiation sensitivity is due to the anisotropy of plastic flow, that is, the relative ease of slip in different orientations.
Behavior of charged and uncharged bubbles in dielectric liquids subjected to electric stress
Authors: Beroual A.
Published: July 2002

This paper is devoted to the calculation of the electric traction Pe acting at every point on the surface of a charged gas or liquid bubble immersed in an insulating liquid and subjected to an electric field between parallel-plate electrodes. It is shown that the deformation of a bubble charged or situated near the electrodes is not symmetric. This dissymmetry has been verified experimentally. The expression for Pe used in a previous work [C. G. Garton and Z. Krasucki, Proc. R. Soc. London Ser. A 280, 211 (1964)] represents a particular case (bubble uncharged or electrically neutral) of the one we have established. In this case, the shape of the distorted bubble is that of a prolate spheroid. This deformation of bubbles also depends on their concentration and positions in the electrode gap.
Transmission studies of explosive vaporization of a transparent liquid film on an opaque solid surface induced by excimer-laser-pulsed irradiation
Authors: Leung P. T. , Do Nhan , Klees Leander , Leung W. P. , Tong Frank , Lam Lui , Zapka Werner , Tam Andrew C.
Published: July 2002

The dynamics of the explosion of a liquid film by an ultraviolet excimer pulsed laser is studied experimentally on top of an amorphous silicon (a-Si) film deposited on fused quartz. In particular, the transient temperature as well as the nucleation of the superheated liquid at the interface for laser fluences below and above the spontaneous nucleation threshold are monitored using an optical transmission technique. Results for different pure and mixed liquids are presented for cases of different thicknesses of the a-Si film. Details for the calibration of the transmission signal to obtain the interface temperature are provided through the application of the one-dimensional diffusion model and the characteristic matrix theory using data obtained from previous steady-state experiments. The calibrated results are in reasonable agreement with the known data for superheated liquids in the literature as well as theoretical calculations. In addition, the delay times for spontaneous nucleation of the liquids are estimated to be ≲1 μs, generally comparable with those observed in droplet explosion experiments.
Deposition mechanism of hydrogenated hard-carbon films in a CH4 rf discharge plasma
Authors: Mutsukura Nobuki , Inoue Shin-ichi , Machi Yoshio
Published: July 2002

To examine the mechanism of film deposition in a planar rf CH4 discharge plasma, measurements were made of the spatial distributions of the deposition rate and optical emission intensity along the discharge axis between parallel electrodes. Optical-absorption properties of the deposited carbon films were also measured over both the infrared and visible regions. To measure the spatial deposition rates, the substrate surface was elevated from the cathode electrode with the use of quartz glass plates. It was found that the spatial properties of films, which were deposited in both the ion-sheath and bulk-plasma regions, differ markedly from each other. The carbon films obtained from within the ion-sheath region were found to be extremely hard, while those obtained in the bulk-plasma region were polymerlike soft films. This disparity was thought to be due to the difference in the kinetic energy of the ions bombarding the substrate surface; that is, the substrate surface potential could be changed by elevating the substrate surface. These results were incorporated in the discussion of the deposition mechanism, with emphasis on the contribution of ion bombardment to the film-deposition process. It was tentatively concluded that the film-deposition rate was predominantly dependent on the product of the ion kinetic energy and ion flux density that reached the substrate surface.
Reduction in the series resistance of the distributed Bragg reflector in vertical cavities by using quasi-graded superlattices at the heterointerfaces
Authors: Kurihara K. , Numai T. , Ogura I. , Yasuda A. , Sugimoto M. , Kasahara K.
Published: July 2002

Surface emitting optical devices with a vertical cavity have been investigated for applications in optical interconnections. To integrate these devices into a two-dimensional array, it is necessary to improve the conversion efficiency from electrical power to optical power. To meet this requirement, the series resistance of the distributed Bragg reflectors that form the vertical cavity must be reduced. This article demonstrates the reduction in the series resistance of the distributed Bragg reflector by introducing quasi-graded superlattices at the heterointerfaces. By using this structure, we obtain a low series resistance distributed Bragg reflector without compromising the high reflectivity. The mechanism of the reduction in the series resistance is studied and it is found that an increase in tunneling current leads to a decrease in the resistance. The dependence of tunneling current on doping concentration of the distributed Bragg reflector and the superlattice structure is also discussed.
Lines of interacting quantum-dot cells: A binary wire
Authors: Lent Craig S. , Tougaw P. Douglas
Published: July 2002

The behavior of linear arrays of cells composed of quantum dots is examined. Each cell holds two electrons and interacts Coulombically with neighboring cells. The electrons in the cell tend to align along one of two axes resulting in a cell ‘‘polarization’’ which can be used to encode binary information. The ground-state polarization of a cell is a highly nonlinear function of the polarization of its neighbors. The resulting bistable saturation can be used to transmit binary information along the line of cells, thus forming a binary wire.
Monte Carlo simulation of x-ray spectra in electron probe microanalysis: Comparison of continuum with experiment
Authors: Ding Z.-J. , Shimizu R. , Obori K.
Published: July 2002

A Monte Carlo simulation code has been developed to describe the x-ray generation in a specimen for electron probe microanalysis (EPMA), enabling x-ray spectra observed by EPMA to be reproduced theoretically. The Monte Carlo simulation is based on the use of the Mott cross section and Bethe stopping power equation in describing elastic and inelastic scattering processes, respectively. With respect to x-ray generation the Sommerfeld theory for bremsstrahlung radiation was described by equations of Kirkpatrick–Wiedmann and of Statham for bremsstrahlung cross section. The up-to-date compilation of mass absorption coefficient by Henke, Gullikson, and Davis [At. Data Nucl. Data Tables 54, 181 (1993)] is used to evaluate attenuation of x-ray intensity. To verify the present Monte Carlo simulation measurements of x-ray spectra for Cu and Rh targets have been performed for primary electron energies of 10–30 keV by energy-dispersive x-ray spectrometry of an electron probe microanalyzer. Excellent agreement between experimental spectra and calculated results has been confirmed in the cases investigated, leading to the conclusion that the Monte Carlo simulation technique will be a powerful tool for more comprehensive understanding of x-ray generation in EPMA specimens.
Maxwell–Wagner–Sillars relaxations in polystyrene–glass-bead composites
Authors: Perrier G. , Bergeret A.
Published: July 2002

The Maxwell–Wagner–Sillars (MWS) relaxation is studied for glass-bead–polystyrene (PS) composites having compositions from 0% to ∼50% in volume, in the range 20 Hz–1 MHz and for temperatures varying from room temperature to 280 °C. For the PS matrix, the complex permittivities and ac conductivities are within the limits of 3.3–3.7 and 2–3×10−9 Ω−1 cm−1, respectively, at 1 kHz in a 120–180 °C temperature interval. For the glass, these parameters vary between 20 and 300 for the complex permittivity, and between 9×10−9 and 4×10−8 Ω−1 cm−1 for the ac conductivity, in the same conditions. Good agreement is found between the calculated values of the maximum loss factor frequencies and the experimental ones. The relaxation found in the composite at temperatures above the α relaxation of the PS is ascribed to the trapping of conductive carriers at the interface between the glass spheres and the polymer matrix. This MWS relaxation is showed to have an activation energy of 20 kcal mol−1 and a relaxation time of 10−13.6 s. From the Argand representations it is observed that the MWS phenomenum deviates from a simple Debye-like process for composites having glass volume fractions from 12.5% upward.
Two-dimensional surface dopant profiling in silicon using scanning Kelvin probe microscopy
Authors: Henning Albert K. , Hochwitz Todd , Slinkman James , Never James , Hoffmann Steven , Kaszuba Phil , Daghlian Charles
Published: July 2002

A simultaneous combination of scanning Kelvin probe microscopy and scanning atomic force microscopy has been applied to the problem of profiling dopant concentrations in two dimensions in silicon microstructures. By measuring the electrochemical potential difference which minimizes the electrostatic force between probe tip and sample surface, the work-function difference between the tip and surface is estimated. To the extent that this work-function difference is a consequence of the dopant concentration at or near the sample surface, doping profiles are inferred from the measurement. Structures examined and presented here include contact holes, and the technologically significant lightly doped drain of a metal–oxide–silicon field-effect transistor. Using this methodology, one can distinguish relative changes in dopant concentration with lateral resolution less than 100 nm. Sample preparation is minimal, and measurement time is fast compared to other techniques. The measurements have been compared to predictions based on two- and three-dimensional process and device simulation tools. The comparisons show that the technique is sensitive to changes in dopant concentration, from ≊1015 to 1020 cm−3, of less than 10% at these size scales. Suggestions to resolve absolute dopant concentration are made.
Enhanced performance of polymer light-emitting diodes using high-surface area polyaniline network electrodes
Authors: Yang Y. , Westerweele E. , Zhang C. , Smith P. , Heeger A. J.
Published: July 2002

Light-emitting diodes (LEDs) of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) were fabricated using high surface area hole-injecting network electrodes made in situ from blends of polyaniline (PANI) in low molecular weight polyester resin. Due to the high interface area between the PANI network and the active luminescent layer and enhancement of the local electric field at the rough surface of the PANI network, charge carrier injection was significantly improved. LEDs using the PANI network electrode have lower device operating voltage, higher quantum efficiency, and significantly increased brightness. Polymer LEDs fabricated with the PANI-CSA network as anode and aluminum:lithium alloy (0.2% Li) as the cathode turn on at 1.7 V, and achieve a brightness in excess of 400 cd/m2 at 3 V with a measured external quantum efficiency of 2.23% photons/electron.
Electron irradiation induced outgrowths from quartz
Authors: Stevens Kalceff M. A. , Phillips M. R.
Published: July 2002

The irradiation of natural and ultrapure synthetic crystalline quartz by a stationary electron beam produces surface outgrowths, which have been analyzed using scanning electron microscopy, atomic force microscopy, cathodoluminescence spectroscopy, and microscopy. Oxygen enrichment of the quartz surface occurs due to electromigration resulting from the trapped charge induced electric field. It is proposed that the accumulated oxygen is incorporated into the quartz surface structure as peroxy linkages, the formation of which results in the permanent volume increase observed as amorphous outgrowths on crystalline quartz. A cathodoluminescence emission at 2.3 eV localized on the outgrowths, is attributed to an intrinsic process.
Relationship between electroluminescence and current transport in organic heterojunction light-emitting devices
Authors: Burrows P. E. , Shen Z. , Bulovic V. , McCarty D. M. , Forrest S. R. , Cronin J. A. , Thompson M. E.
Published: July 2002

We measure the current–voltage and electroluminescence characteristics of single-heterojunction, vacuum-deposited organic light-emitting devices (OLEDs) over a wide range of materials, temperatures, and currents. We find that the current is limited by a large density of traps with an exponential energy distribution below the lowest unoccupied molecular orbital. The characteristic trap depth is 0.15 eV. Furthermore, in metal–quinolate-based devices, electroluminescence originates from recombination of Frenkel excitons, and its temperature dependence is consistent with the excitons being formed by Coulombic relaxation of the trapped electrons with holes injected from the counter electrode. By semiempirical molecular orbital modeling, we find that the trap distribution obtained from the current–voltage characteristics is consistent with a distribution in the metal–quinolate molecular conformations which result in a continuous, exponential distribution of allowed states below the lowest unoccupied molecular orbital. We discuss the implications of the intrinsic relationship between electroluminescence and current transport in OLEDs for the optimization of efficiency and operating voltage in these devices.
Optical response in two-dimensional optical superlattices with Kerr nonlinearity
Authors: Wang Z. L. , Zhu Y. Y. , Xu N. , Ming N. B.
Published: July 2002

We present an analysis of the steady state optical response of two-dimensional optical superlattices with Kerr nonlinearity. We consider the effects of the modification of the refractive index modulation strengths in the superlattices and the angular deviation of incoming radiation from Bragg conditions. We find that under Bragg incidence the system displays optical bistability only when the modulation ratio exceeds a certain threshold. We also predict that optical bistability can disappear under non-Bragg incidence. Physical considerations are presented and discussed on how to optimize the chances for observing optical bistability in such a superlattice.
Fine structure analysis of near-edge electron energy loss spectra related to the compositional variation at the diffuse interfaces in FeGe multilayers
Authors: Suenaga K. , Thorel A. , Houdy Ph. , Colliex C.
Published: July 2002

Analysis of the Fe L23 edge fine structures in electron energy loss spectra is performed with nanometer-scale spatial resolution across the diffuse interfaces in FeGe multilayers, in which an interphase (with Ge penetration into Fe layers) has been found to be nonmagnetic. Results are correlated to the chemical composition of Ge and Fe, which is obtained simultaneously by mapping the relative weight of the Fe and Ge L23 signals, in order to investigate significant differences in the Fe L23 edge fine structures of magnetized iron and of demagnetized iron due to Ge penetration. As a function of position across the multilayer we find that the L3 peak width and the relative energy interval between the L2 and L3 white lines fluctuate in concert with the Ge concentration. Furthermore, if the total weight of the white lines does not seem to change significantly, we identify an increase in the white line ratio at the interface between Ge and Fe. These results are connected to the detailed changes, at the nanometer level, of electron properties such as d-band occupation and spin pairing.
Structure and magnetic properties of TbMn6−xAlxSn6 compounds
Authors: Hu Jifan , Yang Fuming , Wang Jianli , Pan Hongge , Zhao Ruwen , Tang Ning , Wang Yingang , Wang Zhenxi , Zhou Kaiwen , Zhong Xiaping , de Boer F. R.
Published: July 2002

In the present work, the influence of the substitution of Al for Mn on the structure and magnetic properties of TbMn6−xAlxSn6 compounds was investigated. With increasing Al concentration the lattice constants a, c, and the unit-cell volume V decrease monotonically. The TbMn6−xAlxSn6 compounds exhibit ferrimagnetic ordering. The magnetic ordering temperature first decreases with increasing Al content up to x=1.5, then increases with further increasing Al content. As for the unsubstituted material, it was found that this series of compounds exhibits a second magnetic transition at lower temperature that is attributed to spin reorientation. The spin-reorientation temperature and the magnetic anisotropy field first decrease with increasing Al content, go through a minimum at x=0.5, then increase with further increasing Al content. The saturation magnetization decreases monotonically with increasing Al content. Two metamagnetic transitions are found in TbMn6Sn6 compounds, one at 0.44 T, and the other at about 0.16 T. With increasing Al content only one metamagnetic transition remains.
Piezoelectric transducer embedded in a composite plate: Application to Lamb wave generation
Authors: Moulin Emmanuel , Assaad Jamal , Delebarre Christophe , Kaczmarek Henri , Balageas Daniel
Published: July 2002

The aim of this paper is to show that Lamb waves may be effectively generated using piezoelectric transducers embedded inside a composite plate, for nondestructive evaluation and health monitoring applications. A cylindrical transducer embedded in a composite host plate is considered. The electrical impedance of the transducer alone in vacuum and then of the embedded transducer, which allows the identification of the resonance modes, have been obtained by the finite element method (FEM). Moreover, the displacement fields in the plate, which allow the identification of the types of Lamb waves, have been computed at the resonance frequencies. Comparison between the FEM results and the Lamb wave dispersion curves of the host material are in good agreement. Experimental results (electrical impedance, frequency response, and phase velocities) concerning a composite plate specimen containing the same piezoelectric transducer inside it are shown. A good agreement is generally obtained between numerical and experimental results. In addition, it has been shown that the radial mode of the embedded transducer, which has a high coupling coefficient (around 50%), can be used to generate S0 Lamb waves.
Monte Carlo study of electron transport in SiC
Authors: Mickevičius Rimvydas , Zhao Jian H.
Published: July 2002

Temperature- and electric field-dependent electron transport in 3C–, 4H–, and 6H–SiC has been calculated by the Monte Carlo technique. Due to the freezeout of deep donor levels the role of ionized impurity scattering in 6H–SiC is suppressed and the role of phonon scattering is enhanced, compared to 3C– and 4H–SiC. There are indications of impurity band formation for impurity concentrations exceeding 1019 cm−3. It is found that ionized impurity scattering along with the deep donor ionization is responsible for the temperature dependence of mobility anisotropy ratio. Electron effective masses and electron-phonon coupling constants have been deduced from the comparison of Monte Carlo simulation results with available experimental data on low-field electron mobility. The extracted model parameters are used for high-field electron transport simulations. The calculated velocity-field dependencies agree with experimental results. The saturation velocities in all three polytypes are close, but the transient velocity overshoot at high electric field steps is much more pronounced in 3C–SiC.
Interfacial intermixing and arsenic incorporation in thin InP barriers embedded in In0.53Ga0.47As
Authors: Wagner J. , Peter M. , Winkler K. , Bachem K.-H.
Published: July 2002

In 0.53 Ga 0.47 As/InP/In 0.53 Ga 0.47 As heterostructures with InP barrier widths between 2 and 20 nm, grown by metal–organic chemical-vapor deposition (MOCVD), were analyzed with respect to interfacial intermixing and As incorporation in the InP. Raman scattering in conjunction with spectroscopic ellipsometry revealed the formation of intermixed (InGa)(AsP) interface layers with a width of about 2 nm. A second effect to be distinguished from interfacial intermixing was detected by the same experimental techniques, namely, the incorporation of As into the InP at an almost constant concentration for InP layer thicknesses up to at least 20 nm. The cross incorporation of As into the InP was attributed to memory effects in the MOCVD growth system.
Enhancement of electro-optical performance in nematic emulsion films
Authors: De Filpo G. , Lanzo J. , Nicoletta F. P. , Chidichimo G.
Published: July 2002

We have found new monomers which are able to give stable nematic and isotropic emulsions with micron-sized droplets. A poor contrast ratio in optical transmittance is obtained when thin electro-optical films of nematic emulsions are prepared with only one monomer. Matching between a monomer matrix refractive index and that of a liquid crystal, and a consequent improvement of the electro-optical performance of emulsion films have been obtained by mixing such monomers. The films thus prepared show opacities less than 1% in the absence of an applied field and a transmittances increase up to 85% by applying a 0.2 V μm−1 electric field at 1 kHz. The long time stability, the high contrast ratio, and the low driving fields suggest that nematic emulsion films are promising systems for electro-optical applications.
Zinc self-diffusion, electrical properties, and defect structure of undoped, single crystal zinc oxide
Authors: Tomlins Gregory W. , Routbort Jules L. , Mason Thomas O.
Published: July 2002

Zinc self-diffusion was measured in single crystal zinc oxide using nonradioactive Zn70 as the tracer isotope and secondary ion mass spectrometry for data collection. Crystal mass was closely monitored to measure ZnO evaporation. Diffusion coefficients were isotropic with an activation energy of 372 kJ/mol. Zinc self-diffusion is most likely controlled by a vacancy mechanism. Electrical property measurements exhibit a plateau in conductivity at intermediate pO2 with an increase in reducing atmospheres. An analysis of the defect structure is presented that indicates that oxygen vacancies are probably the intrinsic ionic defects responsible for n-type conductivity in reducing atmospheres.
A Monte Carlo method for determining free-energy differences and transition state theory rate constants
Authors: Voter Arthur F.
Published: July 2002

We present a new Monte Carlo procedure for determining the Helmholtz free-energy difference between two systems that are separated in configuration space. Unlike most standard approaches, no integration over intermediate potentials is required. A Metropolis walk is performed for each system, and the average Metropolis acceptance probability for a hypothetical step along a probe vector into the other system is accumulated. Either classical or quantum free energies may be computed, and the procedure is also ideally suited for evaluating generalized transition state theory rate constants. As an application we determine the relative free energies of three configurations of a tungsten dimer on the W(110) surface.
Application of the density matrix method to multiphoton ionization of molecules
Authors: Boeglin A. , Fain B. , Lin S. H.
Published: July 2002

In this paper we shall apply the density matrix method to study the one-photon ionization and two-photon ionization of molecules. Fano’s results for one-photon autoionization will be reproduced by this formalism. It will be shown that the existence of autoionization states will interfere with the determination of ionization thresholds of molecules by using the photoionization method if the autoionization state is located near the ionization threshold. It will also be shown that the density matrix method can treat two-color photoionization spectroscopy and the excited state lifetime measurement by multiphoton ionization method.
Angular distributions of H2 thermal desorption: Coverage dependence on Ni(111)
Authors: Russell J. N. , Chorkendorff I. , Lanzillotto A.-M. , Alvey M. D. , Yates J. T.
Published: July 2002

Temperature programmed desorption measurements for H2/Ni(111) yield two desorption states, β1(T=290 K) and β2(T=370 K) for saturation H coverage. The two states are found to have distinctly different angular distributions. β2-H2 desorption is strongly focused along the surface normal, while β1-H2 desorbs diffusely. The angular distribution of desorbing β1-H2 is very close to that for the CO/Ni(111) system which has a cos θ angular distribution. The different angular distributions can be explained by a model involving a coverage-dependent location of an activation energy barrier on the potential energy surface which describes the interaction of hydrogen with Ni(111).
T
1 barrier height, S1–T1 intersystem crossing rate, and S0 radical dissociation threshold for H2CO, D2CO, and HDCO
Authors: Chuang Mei-Chen , Foltz Mary F. , Moore C. Bradley
Published: July 2002

Vacuum UV laser-induced fluorescence has been used to detect the atomic products in the photofragmentation of H2CO, D2CO, and HDCO. The dissociation is shown to occur on both the ground (S0) and the first triplet (T1) potential surfaces. The T1 exit barrier height (2.9–6.0 kcal/mol) and the S1→T1 intersystem crossing rate near the T1 barrier top (4×107 s−1) have been determined experimentally. The threshold energy for the radical dissociation channel on the S0 surface, 86.57±0.16 kcal/mol, has also been measured accurately by rovibronically resolved photolysis. The best values for the heat of formation (9.99±0.19 kcal/mol) and the C–H bond enthalpy (15.69±0.19 kcal/mol) of HCO radical have been derived from the measured threshold energies as well.
Theoretical studies of fine-structure effects and long-range forces: Potential-energy surfaces and reactivity of O(3P)+OH(2Π)
Authors: Graff M. M. , Wagner A. F.
Published: July 2002

The role of fine structure in reactions without barriers in the potential-energy surface is examined in general, and calculations are carried out for the specific case of O+OH→H+O2. The long-range Hamiltonian, including electrostatic (dipole–quadrupole and quadrupole–quadrupole) and spin-oribt interactions, is expressed in the asymptotic (separated species) basis for the 18 doubly degenerate states correlating to ground-state reactants O(3P2,1,0)+OH(2Π3/2,1/2). Adiabatic potential-energy surfaces are determined by diagonalization of the long-range Hamiltonian. The adiabaticity of the reaction has been analyzed using general considerations about nonadiabatic processes and confirmed by direct integration of the coupled equations. The half collision through the coupling region is found to be predominantly adiabatic for the state correlating to reaction. Single-surface reaction cross sections and rate constants have been obtained using the adiabatic capture, infinite-order sudden approximation method. Our results indicate that the reaction is probably fast even at very low temperatures. The effect of reagent rotation on the reaction cross section is also discussed.
The rheological behavior of concentrated colloidal dispersions
Authors: Brady John F.
Published: July 2002

A simple model for the rheological behavior of concentrated colloidal dispersions is developed. For a suspension of Brownian hard spheres there are two contributions to the macroscopic stress: a hydrodynamic and a Brownian stress. For small departures from equilibrium, the hydrodynamic contribution is purely dissipative and gives the high-frequency dynamic viscosity. The Brownian contribution has both dissipative and elastic parts and is responsible for the viscoelastic behavior of colloidal dispersions. An evolution equation for the pair-distribution function is developed and from it a simple scaling relation is derived for the viscoelastic response. The Brownian stress is shown to be proportional to the equilibrium radial-distribution function at contact, g(2;φ), divided by the short-time self-diffusivity, D0s(φ), both evaluated at the volume fraction φ of interest. This scaling predicts that the Brownian stress diverges at random close packing, φm, with an exponent of −2, that is, η′0 ∼ η(1 − φ/φm)−2, where η′0 is the steady shear viscosity of the dispersion and η is the viscosity of the suspending fluid.

Both the scaling law and the predicted magnitude are in excellent accord with experiment. For viscoelastic response, the theory predicts that the proper time scale is a2/D0s, where a is the particle radius, and, when appropriately scaled, the form of the viscoelastic response is a universal function for all volume fractions, again in agreement with experiment. In the presence of interparticle forces there is an additional contribution to the stress analogous to the Brownian stress. When the length scale characterizing the interparticle forces is comparable to the particle size, the theory predicts that there is only a quantitative contribution from the interparticle forces to the stress; the qualitative behavior, particularly the singular scaling of the viscosity and the form of the viscoelastic response, remains unchanged from the Brownian case. For strongly repulsive interparticle forces characterized by a length scale b (≫a), however, the theory predicts that the viscosity diverges at the random close packing volume fraction, φbm, based on the length scale b, with a weaker exponent of −1. The viscoelastic response now occurs on the time scale b2/D0s(φ), but is of the same form as for Brownian dispersions.
Determination of surface diffusion coefficients by Monte Carlo methods: Comparison of fluctuation and Kubo–Green methods
Authors: Uebing Christian , Gomer Robert
Published: July 2002

A comparison of the Kubo–Green and fluctuation methods for determining surface diffusion coefficients of adsorbates was carried out by Monte Carlo simulations on a square lattice for various adsorbate–adsorbate nearest-and-next-nearest neighbor interactions. Even in ordered regions of the phase diagrams both methods agree, when the system has been fully equilibrated and nearly so if it has not, provided that the ‘‘local’’ mean square fluctuation is used in the Kubo–Green method for the latter situation. Both the mean square fluctuations and D were found to be extremely sensitive to even small deviations from equilibrium near θ=0.5 below Tc. It was also found that the tracer and jump diffusion coefficients are nearly identical under all conditions. The present work was made possible by the enormous computing power of the Jülich Intel Paragon computer, which allowed both diffusion and equilibration simulations to be carried out simultaneously in the canonical ensemble. Thus the discrepancy between the two methods found at low temperatures in our previous work, where mean square fluctuations were found in the grand canonical ensemble and diffusion in the canonical ensemble, is resolved.
Quantum reactive scattering with a deep well: Time-dependent calculation for H+O2 reaction and bound state characterization for HO2
Authors: Zhang Dong H. , Zhang John Z. H.
Published: July 2002

We show in this paper a time-dependent (TD) quantum wave packet calculation for the combustion reaction H+O2 using the DMBE IV (double many-body expansion) potential energy surface which has a deep well and supports long-lived resonances. The reaction probabilities from the initial states of H+O2(3Σ−g) (v=0–3, j=1) for total angular momentum J=0 are obtained for scattering energies from threshold up to 2.5 eV, which show numerous resonance features. Our results show that, by carrying out the wave packet propagation to several picoseconds, one can resolve essentially all the resonance features for this reaction. The present TD results are in good agreement with other time-independent calculations. A particular advantage of the time-dependent approach to this reaction is that resonance structures—strong energy dependence of the reaction probability—can be mapped out in a single wave packet propagation without having to repeat scattering calculations for hundreds of energies. We also report calculations of some low-lying vibrational energies of the hydroperoxyl radical HO2(2A″) and their spectroscopic assignments. The vibrational frequencies of HO2(2A″) on the DMBE IV potential energy surface are lower than experimental values, indicating the need to further improve the accuracy of the potential energy surface.
Neural network models of potential energy surfaces
Authors: Blank Thomas B. , Brown Steven D. , Calhoun August W. , Doren Douglas J.
Published: July 2002

Neural networks provide an efficient, general interpolation method for nonlinear functions of several variables. This paper describes the use of feed-forward neural networks to model global properties of potential energy surfaces from information available at a limited number of configurations. As an initial demonstration of the method, several fits are made to data derived from an empirical potential model of CO adsorbed on Ni(111). The data are error-free and geometries are selected from uniform grids of two and three dimensions. The neural network model predicts the potential to within a few hundredths of a kcal/mole at arbitrary geometries. The accuracy and efficiency of the neural network in practical calculations are demonstrated in quantum transition state theory rate calculations for surface diffusion of CO/Ni(111) using a Monte Carlo/path integral method. The network model is much faster to evaluate than the original potential from which it is derived. As a more complex test of the method, the interaction potential of H2 with the Si(100)-2×1 surface is determined as a function of 12 degrees of freedom from energies calculated with the local density functional method at 750 geometries. The training examples are not uniformly spaced and they depend weakly on variables not included in the fit. The neural net model predicts the potential at geometries outside the training set with a mean absolute deviation of 2.1 kcal/mole.
Internal rotational barriers of ClOOCl
Authors: Francisco J. S.
Published: July 2002

The rotational barrier for ClOOCl has been determined. The barrier for the cis and trans configurations is 8.8 kcal mol−1 and 4.9 kcal mol−1, respectively. This is in disagreement with the experimental result of Birk et al. [J. Chem. Phys. 91, 6588 (1989)] and the discrepancy is discussed.
A molecular theory of flow alignment and tumbling in sheared nematic liquid crystals
Authors: Archer L. A. , Larson R. G.
Published: July 2002

A molecular theory is presented for the dynamics of small-molecule nematic liquid crystals in shear flow. The flow-alignment angle is predicted to depend on the aspect ratio of the liquid crystal molecules, as well as the second and fourth moments, P2 and P4 of the distribution of molecules about the nematic director. This theory also predicts a transition from flow alignment to tumbling well away from any nematic-smectic-A phase transitions.