bluuebunny/crossref_metadata_2025 huggingface dataset:
In search of zonal flows using cross-bispectrum analysis in the boundary plasma of the Hefei Tokamak-7
Authors: Xu G. S. , Wan B. N. , Song M.
Published: July 2002

Langmuir probes have been used to measure the electrostatic Reynolds stress and the floating potential fluctuation in the boundary plasma of the Hefei Tokamak-7 (HT-7) [J. Li, B. N. Wan, and J. S. Mao, Plasma Phys. Controlled Fusion 42, 135 (2000)]. The cross bispectrum of 〈ṼrṼθφ̃f〉 indicates the existence of difference-frequency nonlinear phase coupling and the generation of fluctuations near the geodesic acoustic mode frequency. The inverse cascade process might be linked to the generation of zonal flows by small-scale electrostatic drift-wave turbulence.
Density functional calculations on the structure of crystalline polyethylene under high pressures
Authors: Miao M. S. , Zhang M.-L. , Van Doren V. E. , Van Alsenoy C. , Martins José Luís
Published: July 2002

The geometrical structures of the crystalline polyethylene under several different external pressures up to 10 GPa are optimized by a pseudopotential plane wave density functional method. Both local density (LDA) and generalized gradient (GGA) approximations for exchange-correlation energy and potential are used. It is found that LDA heavily underestimate the geometry parameters under ambient pressure but GGA successfully correct them and get results in good agreements with the experimental geometry. The calculated GGA volume is about 94 Å3 in comparison with the x-ray scattering value of about 92 Å3 and the neutron scattering value of 88 Å3. The bulk and Young’s modulus are calculated by means of several different methods. The Young’s modulus along the chain ranges from about 350 to about 400 GPa which is in good agreement with the experimental results. But the bulk modulus is several times larger than those of experiments, indicating a different description of the interchain interactions by both LDA and GGA. The band structures are also calculated and their changes with the external pressure are discussed.
X-ray study of Nd0.5Sr0.5MnO3 manganite structure above and below the ferromagnetic metal–antiferromagnetic insulator spontaneous phase transition
Authors: Eremenko V. , Gnatchenko S. , Makedonska N. , Shabakayeva Yu. , Shvedun M. , Sirenko V. , Fink-Finowicki J. , Kamenev K. V. , Balakrishnan G. , Paul D. Mck
Published: July 2002

The crystal structure of the manganite Nd0.5Sr0.5MnO3 is studied at temperature T=300 and 77.3 K by means of an x-ray diffractometer. It is shown that the transition from the ferromagnetic metallic state to the antiferromagnetic insulating charge-ordered state is accompanied by a lowering of the symmetry of the structure from orthorhombic to monoclinic. The space-group symmetry of the orthorhombic and monoclinic phases is identified as Imma and P21/m, respectively. Twinning of the crystal and the formation of a twin domain structure with coherent boundaries in the (00l) crystallographic planes are found.
Shock Hugoniot compression curve for water up to 1 GPa by using a compressed gas gun
Authors: Nagayama K. , Mori Y. , Shimada K. , Nakahara M.
Published: July 2002

A shock Hugoniot compression curve for water has been measured up to less than 1 GPa. A plane and steady shock wave is produced in water by the flat plate impact of a projectile accelerated by a compressed gas gun. A new experimental procedure was proposed to detect the shock wave front sensitively, which makes it possible to measure shock Hugoniot in higher precision than the previous method. The present method is based on the very large pressure dependence of the refractive index of water upon compression. By using this method, the shock compression curve was determined within the precision of 2% to 3% of the estimated shock pressure. The precision is better than that of the previous data. It was confirmed that within the pressure range covered in this experiment, the shock-particle velocity Hugoniot can be described by a linear relation with a large slope. Shock Hugoniot states on the pressure–temperature plane were calculated by using the obtained Hugoniot data combined with the values of thermodynamical variables. Thermodynamic analysis of the shock compression process was developed to estimate the contributions of irreversible heating by shock compression.
Chemically limited reactions on a percolation cluster
Authors: Saxton Michael J.
Published: July 2002

Theoretical arguments and Monte Carlo results show that under certain circumstances, obstruction may increase the reaction rate of a binary reaction A+B→products. Near the percolation threshold, anomalous subdiffusion slows the initial encounter of the reactants, but it also slows the separation of the reactants after an unsuccessful collision. In a finite system, if the reaction probability P per collision is small enough, the recollision effect predominates and the reaction rate is enhanced slightly. As a result of this effect, as P is varied, the range of reaction rates on a percolation cluster is much smaller than the range for an unobstructed system. Furthermore, there is a crossover reaction probability PCR at which the reaction rate becomes practically independent of obstacle concentration for the entire range from the unobstructed system to the percolation threshold.
Electrical characteristics and thermal stability of n+ polycrystalline- Si/ZrO2/SiO2/Si metal–oxide–semiconductor capacitors
Authors: Lim Kwan-Yong , Park Dae-Gyu , Cho Heung-Jae , Kim Joong-Jung , Yang Jun-Mo , Choi II-Sang , Yeo In-Seok , Park Jin Won
Published: July 2002

We have investigated the thermal stability of n+ polycrystalline-Si(poly-Si)/ZrO2(50–140 Å)/SiO2(7 Å)/p-Si metal–oxide–semiconductor (MOS) capacitors via electrical and material characterization. The ZrO2 gate dielectric was prepared by atomic layer chemical vapor deposition using ZrCl4 and H2O vapor. Capacitance–voltage hysteresis as small as ∼12 mV with the flatband voltage of −0.5 V and the interface trap density of ∼5×1010 cm−2 eV−1 were attained with activation anneal at 750 °C. A high level of gate leakage current was observed at the activation temperatures over 750 °C and attributed to the interfacial reaction of poly-Si and ZrO2 during the poly-Si deposition and the following high temperature anneal. Because of this, the ZrO2 gate dielectric is incompatible with the conventional poly-Si gate process. In the MOS capacitors having a smaller active area (<50×50 μm2), fortunately, the electrical degradation by further severe silicidation does not occur up to an 800 °C anneal in N2 for 30 min.
Crystalline phase and orientation control of manganese nitride grown on MgO(001) by molecular beam epitaxy
Authors: Yang Haiqiang , Al-Brithen Hamad , Trifan Eugen , Ingram David C. , Smith Arthur R.
Published: July 2002

The phase and orientation of manganese nitride grown on MgO(001) using molecular beam epitaxy are shown to be controllable by the manganese/nitrogen flux ratio as well as the substrate temperature. The most N-rich phase, θ phase (MnN), is obtained at very low Mn/N flux ratio. At increased Mn/N flux ratio, the next most N-rich phase, the η phase (Mn3N2), is obtained having its c axis normal to the surface plane. Further increasing the Mn/N flux ratio, the η phase (Mn3N2) having its c axis in the surface plane is obtained. Finally, the ε phase (Mn4N) is obtained at yet higher Mn/N flux ratio. The structural phase variation with Mn/N flux ratio is due to the kinetic control of the surface chemical composition, which determines the energetically most favorable phase. For a given Mn/N flux ratio, the phase is also found to be a function of the substrate temperature, with the less N-rich phase occurring at the higher substrate temperature. The change of phase with temperature is attributed to the change in the chemical composition resulting from the diffusion of N vacancies. Since the magnetic properties of MnxNy depend on the phase, the Mn/N flux ratio provides a way of directly controlling the magnetic properties. A phase diagram for molecular beam epitaxial growth is presented.
Surface enhanced Raman scattering from a single molecule adsorbed on a metal particle aggregate: A theoretical study
Authors: Corni S. , Tomasi J.
Published: July 2002

A methodology for the theoretical evaluation of Raman intensities for a molecule adsorbed on a metal particle aggregate, possibly in the presence of a solvent, is presented. The molecule is treated at the ab initio level, both in the evaluation of ground state electronic density and nuclear geometry, and in the evaluation of normal modes and Raman scattering factors. The whole metal aggregate, built by exploiting a Cluster–Cluster Aggregation (CCA) model, is described as an ensemble of polarizable dipoles. The metal particles nearest to the molecule are treated as a single complex shaped metal nanoparticle and the quasielectrostatic problem for the molecule–nanoparticle system is solved by using a Boundary Element Method (BEM). The solvent is modeled by using the Polarizable Continuum Model (PCM). Applications of the methodology to systems for which huge enhancements (1013–1014) of Raman intensities have been experimentally measured are given.
Epitaxial ferroelectric Pb(Zr, Ti)O3 thin films on Si using SrTiO3 template layers
Authors: Wang Y. , Ganpule C. , Liu B. T. , Li H. , Mori K. , Hill B. , Wuttig M. , Ramesh R. , Finder J. , Yu Z. , Droopad R. , Eisenbeiser K.
Published: July 2002

In this letter, we report on the integration of epitaxial ferroelectric Pb(Zr, Ti)O3 (PZT) thin films on Si [100] substrates using a SrTiO3 (STO) template layer and a conducting perovskite (La0.5Sr0.5)CoO3 electrode. X-ray diffraction studies reveal both in-plane and out-of-plane alignment of the heterostructure. The epitaxial films show extremely high remnant polarization as well as piezoelectric d33 coefficients compared to textured and untextured polycrystalline films.
A simple low-dissipation amplifier for cryogenic STM
Authors: Urazhdin S. , Tessmer S. H. , Ashoori R. C.
Published: July 2002

A current sensitive preamplifier designed for low-temperature scanning tunneling microscopy applications is presented. It combines the dc current measurement necessary for the feedback loop operation with a low noise ac measurement used for spectroscopy. The active device is a high electron mobility transistor which was chosen for its low input capacitance and excellent low-temperature performance. The power dissipation of the transistor can be kept at about 10 μW making it compatible with a variety of cryogenic systems. The ac current sensitivity is about 4 fA/√Hz at 4.2 K.
Orientations of phenyl sidegroups and liquid crystal molecules on a rubbed polystyrene surface
Authors: Oh-e Masahito , Hong Seok-Cheol , Shen Y. R.
Published: July 2002

Surface–specific sum–frequency vibrational spectroscopy and second-harmonic generation have been used to study the structure of a rubbed polystyrene (PS) surface and the orientation of 4′-n-pentyl-4-cyanobiphenyl (5CB) liquid crystal molecules on it. The results show that the phenyl sidegroups are well aligned by rubbing in the direction perpendicular to rubbing but tilt from the surface normal with a broad distribution. Although the PS surface is nonpolar, the 5CB molecules appear to adsorb on PS preferentially with the terminal cyano group facing the PS surface.
Laminar flow in a porous channel with large wall suction and a weakly oscillatory pressure
Authors: Jankowski Todd A. , Majdalani Joseph
Published: July 2002

The laminar oscillatory flow inside a rectangular channel with wall suction is considered here. The scope is limited to large suction imposed uniformly along the permeable walls. Inside the channel, the onset of small amplitude pressure disturbances produces an oscillatory field that we wish to investigate. Based on the normalized pressure-wave amplitude, the conservation equations are linearized and split into leading-order (steady) and first-order (time-dependent) equations. The first-order set is subdivided into an acoustic, pressure-driven, wave equation, and a vortical, boundary-driven, viscous equation. For longitudinal pressure oscillations, both equations are written to the order of the wall suction Mach number. The resulting equations are then solved in an exact fashion. The novelty lies in the vortical response that reduces to a Weber equation following a Liouville–Green transformation. The emerging rotational solution is expressible in terms of confluent hypergeometric functions of the suction Reynolds number, Strouhal number, and spatial coordinates. The total solution is then constructed and found to coincide with the numerical solution of the linearized momentum equation. The oscillatory velocity exhibits similar characteristics to the exact Stokes profile for oscillations inside a long channel with hard walls. In particular, a thin rotational layer is observed in addition to the small velocity overshoot near the wall. Both depth and overshoot are nowhere near their values obtained by switching from mass extraction to mass addition. In contrast to former studies involving injection, the so-called acoustic boundary layer is found to depreciate when suction is increased or when viscosity is reduced. This response is similar to that of the Stokes layer over hard walls. Overall, the effect of increasing frequency is that of compressing the rotational layer near the wall.
Optical properties of epitaxial ZnMnTe and ZnMgTe films for a wide range of alloy compositions
Authors: Liu X. , Bindley U. , Sasaki Y. , Furdyna J. K.
Published: July 2002

Zn 1−x Mn x Te and Zn1−xMgxTe ternary wide-gap semiconductor alloys were grown by molecular beam epitaxy on (100) GaAs substrates over a wide range of compositions (0⩽x⩽0.75 and 0⩽x⩽0.67, respectively). Values of the band gap were measured by photoluminescence at 12 K, and by optical reflectivity at room temperature. The wavelength dependence of the indices of refraction n of these ternary systems was also measured for these alloys at wavelengths below their respective energy gaps. The measurements were performed using a combination of the prism coupler method and reflectivity. Compilation of these results allows us to establish a set of empirical parameters for the two alloy families, that can be used to calculate the index of refraction for an arbitrary alloy composition and arbitrary wavelength. It is interesting that the values of n show a surprisingly linear dependence on the corresponding energy gaps for both these alloy systems.
A Monte Carlo study of solutions of oppositely charged polyelectrolytes
Authors: Hayashi Yoshikatsu , Ullner Magnus , Linse Per
Published: July 2002

The formation of complexes appearing in solutions containing oppositely charged polyelectrolytes has been investigated by Monte Carlo simulations using two different models. The polyions are described as flexible chains of 20 connected charged hard spheres immersed in a homogenous dielectric background representing water. The small ions are either explicitly included or their effect described by using a screened Coulomb potential. The simulated solutions contained 10 positively charged polyions with 0, 2, or 5 negatively charged polyions and the respective counterions. Two different linear charge densities were considered, and structure factors, radial distribution functions, and polyion extensions were determined. A redistribution of positively charged polyions involving strong complexes formed between the oppositely charged polyions appeared as the number of negatively charged polyions was increased. The nature of the complexes was found to depend on the linear charge density of the chains. The simplified model involving the screened Coulomb potential gave qualitatively similar results as the model with explicit small ions. Finally, owing to the complex formation, the sampling in configurational space is nontrivial, and the efficiency of different trial moves was examined.
Anomalous dielectric relaxation in the context of the Debye model of noninertial rotational diffusion
Authors: Coffey W. T. , Kalmykov Yu. P. , Titov S. V.
Published: July 2002

The Debye theory of dielectric relaxation of an assembly of polar molecules is reformulated using a fractional noninertial Fokker–Planck equation for the purpose of extending that theory to explain anomalous dielectric relaxation. The fractional Fourier–Planck equation in question is a generalization of the Smoluchowski equation for the dynamics of Brownian particles in configuration space to include anomalous relaxation. It is shown that this model can reproduce nonexponential Cole–Cole-type anomalous dielectric relaxation behavior and that it reduces to the classical Debye model of rotational diffusion when the anomalous exponent is unity.
Porous silicon multilayer structures: A photonic band gap analysis
Authors: Lugo J. E. , Lopez H. A. , Chan S. , Fauchet P. M.
Published: July 2002

A photonic model for freshly anodized porous silicon multilayer structures is presented. The photonic structures are composed of alternating high and low dielectric function porous silicon layers. The model takes into account the presence of silicon dioxide and its lattice expansion in the porous structure. We work with oxidized structures and our results fit completely the experimentally measured optical shift.
Extension of the method of moments of coupled-cluster equations to excited states: The triples and quadruples corrections to the equation-of-motion coupled-cluster singles and doubles energies
Authors: Kowalski Karol , Piecuch Piotr
Published: July 2002

The recently proposed extension of the method of moments of coupled-cluster equations (MMCC) to excited states via the equation-of-motion coupled-cluster (EOMCC) formalism [K. Kowalski and P. Piecuch, J. Chem. Phys. 115, 2966 (2001)] is developed further. A new approximate variant of the excited-state MMCC theory, termed the MMCC(2,4) method, is proposed and tested. In the MMCC(2,4) method, relatively simple noniterative corrections due to triples and quadruples are added to the excited-state energies obtained in the standard EOMCCSD (EOMCC singles and doubles) calculations. The performance of the MMCC(2,4) approach is illustrated by the results of calculations for the excited states of N2, C2, and CH+. The MMCC(2,4) energies are compared with the results of the MMCC(2,3) calculations, in which noniterative corrections due to triples only are added to the EOMCCSD energies, and with the results of other EOMCC calculations, including various EOMCC triples schemes.
Atomistic molecular dynamics simulation of diffusion in binary liquid n-alkane mixtures
Authors: Harmandaris V. A. , Angelopoulou D. , Mavrantzas V. G. , Theodorou D. N.
Published: July 2002

Well relaxed atomistic configurations of binary liquid mixtures of n-alkanes, obtained via a new Monte Carlo simulation algorithm [Zervopoulou et al., J. Chem. Phys. 115, 2860 (2001)], have been subjected to detailed molecular dynamics simulations in the canonical ensemble. Four different binary systems have been simulated (C5–C78 at T=474 K, C10–C78 at T=458 K, and C12–C60 at T=403.5 and 473.5 K). Results are presented for the diffusion properties of these mixtures over a range of concentrations of the solvent (lighter component). The self-diffusion coefficients of the n-alkanes, calculated directly from the simulations, are reported and compared with the predictions of two theories: the detailed free volume theory proposed by Vrentas and Duda based on the availability of free volume in the blends, and a combined Rouse diffusant and chain-end free volume theory proposed by Bueche and von Meerwall et al. A direct comparison with recently obtained experimental data [von Meerwall et al., J. Chem. Phys. 111, 750 (1999)] is also presented.
Raman scattering in a LiNiPO4 single crystal
Authors: Fomin V. I. , Gnezdilov V. P. , Kurnosov V. S. , Peschanskii A. V. , Yeremenko A. V. , Schmid H. , Rivera J.-P. , Gentil S.
Published: July 2002

The complete Raman spectra of a single crystal of LiNiPO4 for a wide temperature range are reported. Of the 36 Raman-active modes predicted by group theory, 33 have been detected. The spectra are successfully analyzed in terms of internal modes of the (PO4)3− group and external modes. Multiphonon Raman scattering is also discussed. Low-frequency lines, observed in the antiferromagnetic phase, are assigned to magnon scattering and are discussed briefly.
A portable ultrahigh vacuum apparatus for the production and in situ characterization of clusters and cluster-assembled materials
Authors: Barborini E. , Siviero F. , Vinati S. , Lenardi C. , Piseri P. , Milani P.
Published: July 2002

We present and discuss the design and operation of a compact ultrahigh vacuum compatible apparatus for the production and deposition of supersonic cluster beams. The apparatus is equipped with a pulsed microplasma cluster source capable of providing supersonic beams of high stability and intensity. The cluster mass distribution can be analyzed by a two-stage time-of-flight mass spectrometer. The transportability and versatility of the apparatus make the system well suited to perform in situ studies on both gas phase clusters and cluster assembled materials using different characterization facilities. The performances of the system have been tested by scanning tunneling microscopy and photoemission spectroscopy experiments on cluster-assembled carbon films.
Laboratory surface astrophysics experiment
Authors: Fraser Helen J. , Collings Mark P. , McCoustra Martin R. S.
Published: July 2002

In this article we describe the design and construction of a laboratory astrophysics experiment that recreates the harsh conditions of the Interstellar Medium (ISM) and is used to study the heterogeneous chemistry that occurs there. The Nottingham Surface Astrophysics Experiment is used to determine, empirically, accurately, and usually for the first time, key physical and chemical constants that are vital for modeling and understanding the ISM. It has been designed specifically to investigate gas–solid interactions under interstellar conditions. The pressure regime is ideally matched to molecular densities in dusty disks in protostellar or protoplanetary regions. The ultrahigh vacuum system is routinely capable of obtaining pressures that are only three orders of magnitude above those in the ISM, with similar relative concentrations of the two most abundant gases in such regions, H2 and CO, and an absence of any other major gas components. A short introduction describes the astronomical motivation behind this experiment. In Sec. II we then give details of the design, construction, and calibration of each component of the experiment. The cryostat system has far exceeded design expectations, and reaches temperatures between 7 and 500 K. This is comparable with the ISM, where dust temperatures from 10 K have been observed. Line-of-sight mass spectrometry, reflection absorption infrared spectroscopy, and quartz crystal microbalance mass measurements were combined into a single instrument for the first time. The instrument was carefully calibrated, and its control and data acquisition system was developed to ensure that experimental parameters are recorded as accurately as possible. In Sec. III we present some of the experimental results from this system that have not been published elsewhere. The results presented here demonstrate that the system can be used to determine desorption enthalpies, ΔdesH, bonding systems, and sticking probabilities between a variety of gases and ices common to the ISM. This instrument will greatly facilitate our understanding of surface processes that occur in the ISM, and allow us to investigate “mimic” ISM systems in a controlled environment. In this article we illustrate that laboratory surface astrophysics is an exciting and emerging area of research, and this instrument in particular will have a major impact through its contributions to both surface science and astronomy.
Formation and evolution of scroll waves in photosensitive excitable media
Authors: Amemiya Takashi , Kettunen Petteri , Kádár Sándor , Yamaguchi Tomohiko , Showalter Kenneth
Published: July 2002

Experimental and computational studies of the formation and evolution of scroll waves in three-dimensional excitable media are presented. Scroll waves are initiated in the photosensitive Belousov–Zhabotinsky reaction by perturbing traveling waves transverse to their direction of propagation. Scroll rings are generated by perturbing circular waves, which expand or contract depending on the strength of an imposed excitability gradient and its direction relative to the rotational direction of the scroll wave.
MacDose: A simulation for understanding radiological physics
Authors: Hobbie Russell K.
Published: July 2002

MacDose simulates the interaction of photons in matter in order to help students understand several concepts that are important in radiological physics. Its operation and the techniques used for the simulation are described, along with examples of the simulations.
Estimation of global errors in the numerical solution of ordinary differential equations
Authors: Brunini A. , Zadunaisky P. E.
Published: July 2002

The aim of this work is to present a new method to estimate errors propagated in the numerical integration of ordinary differential equations. The method has proved to be very efficient in many numerical applications, and its main advantages are shown in a set of examples concerning some cases of the gravitational N-body problem.
A variable-step procedure for the numerical integration of the one-dimensional Schrödinger equation
Authors: Simos T. E.
Published: July 2002

Two P-stable methods with phase lag of order 10 and 12 are developed in this paper for the numerical integration of the one-dimensional Schrödinger equation. Based on a new local error, the local phase-lag error (LPLE), a new variable-step procedure is produced. Numerical results indicate that the new variable-step method is more efficient than the similar well-known variable-step procedure of Raptis and Cash [Comput. Phys. Commun. 1985, 36, 113–119 (1985)].
Approximate atomic structure calculations using a microcomputer
Authors: Payne M. G. , Edwards Mark
Published: July 2002

The principles behind several microcomputer programs that can be used to calculate a wide variety of atomic parameters that are useful in laser spectroscopy are discussed. The programs make use of optimized central potentials due to Talman. Among the calculations demonstrated here are the calculation of oscillator strengths for transitions, photoionization cross sections, energy eigenvalues, and two-photon Rabi frequencies. Since a program is provided to calculate the Green’s function for the radial Schrödinger equation, other calculations involving multiple summations over intermediate states are also possible.
Vibrational mode analysis using maximum likelihood and maximum entropy
Authors: Redondo Antonio , Sinha Dipen N.
Published: July 2002

A simple algorithm is presented that uses the maximum likelihood and maximum entropy approaches to determine the vibrational modes of elastic bodies. This method assumes that the vibrational frequencies have been previously determined, but the modes to which they correspond are unknown. Although the method is illustrated through the analysis of simulated vibrational modes for a flat rectangular plate, it has broad applicability to any experimental technique in which spectral frequencies can be associated to specific modes by means of a mathematical model.
Lattice simulations of biological membranes
Authors: Tobochnik Jan , Gould Harvey
Published: July 2002

The cooperative effects of a collection of phospholipid layers were considered. The model of phospholipid layers was described and simulated using Monte Carlo method. The most important result is that the main phase transition is only weakly first-order with strong fluctuations near the transition. Simulation also provide information on the spatial correlation of the chain states and the geometry of chains. (AIP) © 1996 American Institute of Physics.
Distributed run-time visualization and solution steering of parallel flow simulations
Authors: Williams S. A. , Sawley M. L. , Cobut D.
Published: July 2002

A run-time visualization and solution-steering tool, TPview, has been developed to overcome difficulties commonly encountered with large-scale numerical simulations on high-performance parallel computer systems. TPview provides the interface between the parallel computer system and the visualization software running on a remote workstation. During the computation cycle, data distributed across multiple processors are periodically sent via sockets to the workstation for rendering. In addition, internal parameters of the simulation code can be modified using a user-defined graphical interface, enabling remote guiding of the solution procedure. It is shown that such a run-time visualization and solution-steering tool can be implemented with minimal effort from the programmer and end user. Examples are presented of the use of TPview in conjunction with a parallel multiblock flow solver and a commercial visualization package. These examples demonstrate its ability to provide greater insights for the analysis of complex flow simulations. © 1998 American Institute of Physics.
Surface flashover of solid insulators in atmospheric air and in vacuum
Authors: Pillai A. Sivathanu , Hackam Reuben
Published: July 2002

The surface flashover of Teflon, plexiglass, quartz, Pyrex glass, Macor glass-ceramic, and sapphire solid insulators has been measured in vacuum (∼10−8 Torr, ∼10−6 Pa) and in atmospheric air using dc, ac (60 Hz), and 1.2/50-μsec lightning impulse voltages. The dependence of the flashover voltage on the following parameters is investigated: (1) spacer material, (2) diameter of the spacer, (3) spacer length, (4) number of spacers stacked in series, (5) air pressure in the range 10−6–105 Pa, (6) electrode material, (7) spark conditioning, and (8) the external resistance in series with the gap. At a fixed insulator length the flashover voltage decreases with increasing spacer diameter. The withstand voltage of spacers stacked in series increases with increasing the number of spacers. The dc flashover voltage of different insulating materials is theoretically calculated in vacuum as a function of the length of the insulator and compared with the experimentally obtained results. Good agreement is obtained.
Effects of γ irradiation on double-heterostructure AlGaAs light-emitting diodes: Opposing emission and absorption edge wavelength shifts
Authors: Hava S. , Lam R.
Published: July 2002

Gamma ray irradiation of double-heterostructure high-radiance LED’s utilizing junction current confinement results in a shift of emission spectrum to shorter wavelengths with increased linewidth, while the absorption edge is seen to shift to longer wavelengths. A novel hypothesis is presented to explain changes in electrical and optical properties based on diffusion of Al from layers of higher concentration to areas of lower concentration, as a result of the γ irradiation. This causes forbidden energy gaps and refractive indices of the various layers to become more uniform. This hypothesis is supported by all the various changes observed in electrical and optical properties.
Magnetic and morphological study of BaZn2Fe16O27 hexagonal ferrite prepared by chemical coprecipitation method
Authors: Leccabue F. , Panizzieri R. , Salviati G. , Albanese G. , Sanchez Llamazares J. L.
Published: July 2002

Polycrystalline samples of the BaZn2Fe16O27 (BaZn2-W) ferrite prepared by the chemical coprecipitation method have been examined by measuring the magnetic properties (saturation magnetization, coercive and anisotropy field, Curie temperature), thermomagnetic analysis, Mössbauer spectroscopy, and scanning electron microscopy. The correlations between the magnetic properties, the grain morphology, and the relative amounts of the various phases present in samples subjected to different heat treatments are discussed. The samples prepared by optimal heat treatment show a saturation magnetization value σs =77.7 emu/g and an anisotropy magnetic field Ha =12.5 kOe. The magnetic coercive field value is low (iHc ∼150 Oe); this may be related to the large-grain dimension (≥12 μm) of the material obtained. The Curie temperatures measured for the BaZn2-W phase vary from 370 to 410 °C depending on the heat treatment of the samples.
Occurrence of long-range helical spin ordering in Dy-Y multilayers (invited)
Authors: Rhyne J. J. , Erwin R. W. , Borchers J. , Sinha S. , Salamon M. B. , Du R. , Flynn C. P.
Published: July 2002

The magnetic ordering of highly perfect single-crystal multilayer films of alternate layers of magnetic Dy and nonmagnetic Y prepared by molecular beam epitaxy has been studied by neutron diffraction. Results on a series of films with Dy thicknesses of approximately 16 atomic planes (≊45 Å) and Y thicknesses ranging from 10 to 22 planes have confirmed the existence of long-range helimagnetic ordering of the Dy 4f spins which is propagated through the intervening Y layers in phase coherence. The propagation vectors in both Dy and Y layers have been calculated from the wave vector of the magnetic satellites and the intensity of the bilayer harmonics. The propagation vector for Dy decreases continuously with temperature, while that in the Y is temperature independent and equal to 0.31 Å−1. The nature of the ordering and the noninteger multiple of π for the phase change of the propagation wave vector across the Y, suggests that the mechanism of long-range coupling is a conduction band spin-density wave in both Y and Dy stabilized by the 4f spins of the Dy. No intrinsic ferromagnetic transition is observed due to the clamping effect of the Y layers on the Dy magnetostriction. The application of a field along basal plane directions destroys the helical order and produces a ferromagnetic state with all spins aligned along the field direction. The moment in the induced ferromagnetic state is 10 μB, while that calculated for the zero-field helical state ranges from 7.5μB to 9.5μB for different samples, suggesting a possible static disorder of some Dy spins superimposed on the intrinsic long-range helical state.
A comparative study of phase stability and film morphology in thin-film M/GaAs systems (M=Co, Rh, Ir, Ni, Pd, and Pt)
Authors: Sands T. , Keramidas V. G. , Yu K. M. , Washburn J. , Krishnan K.
Published: July 2002

To attain reproducible and stable contacts to compound semiconductor devices, it is necessary to achieve thermodynamically stable phases after the reaction of metals with the compound semiconductor. In this study, the final phases produced by the reactions between GaAs and thin metal films of Co, Rh, Ir, Ni, Pd, and Pt have been investigated. They are identified as MGa for M=Co, Rh, Ni, Pd, and Pt, monoarsenides of Co and Ni, diarsenides of Rh, Ir, Pd, and Pt, and Ir3Ga5. These phases, if deposited directly onto GaAs, will produce thermally stable contacts. In addition to the identification of these stable phases, analyses of the products of thin-film M/GaAs reactions by transmission electron microscopy, x-ray diffraction, and Rutherford backscattering spectrometry reveal the distribution, grain size, and crystallographic texture of these end phases. Trends in these observations across the six metal/GaAs reactions studied are explained by considering the effects of bond strength and the relative diffusivities of the reacting species. The role of film thickness in determining the final phases and the phase distribution is also considered.
Mechanism of crack formation in glass after high-power laser pulse irradiation
Authors: Kanemitsu Yoshihiko , Tanaka Yuichi
Published: July 2002

We report the observation of crack formation in glass surfaces after high-power laser pulse irradiation. The damage in the irradiated region was caused by laser ablation and plasma formation, but cracks and liquid drops also appeared outside the irradiated region over a distance of many millimeters. These cracks and liquid drops became clearly visible at 12 h or more after laser irradiation in the surface of both soda-lime glass and borosilicate crown glass. In contrast, such a delayed formation was not observed in fused silica glass. It is considered that the delayed crack formation is caused by chemical interactions of the components of the glass with atmospheric water vapor at microcracks in the surface induced by laser-driven shock waves.
Degradation of tin-doped indium-oxide film in hydrogen and argon plasma
Authors: Banerjee Ratnabali , Ray Swati , Basu N. , Batabyal A. K. , Barua A. K.
Published: July 2002

The degradation of tin-doped indium-oxide (ITO) films in glow-discharge plasmas of hydrogen and argon have been investigated. Parameters which have been varied for the study include the temperature of ITO under ion bombardment, the rf power density, the time of exposure to plasma, and the gas flow rate. After bombardment, scanning electron micrograph observation, measurement of sheet resistance, transmittance and reflectance, and Auger analysis have been carried out to decide the extent of degradation. Magnetron-sputtered ITO films have been found to be more resistant to ion bombardment damage compared to electron-beam evaporated films. The degradation of ITO under the plasma of the reducing species such as hydrogen has been found to take place at lower temperature and power density compared to argon plasma.
Domain wall pinning versus nucleation of reversed domains in R-Fe-B magnets (invited)
Authors: Hadjipanayis George C. , Kim A.
Published: July 2002

The validity of nucleation and domain wall pinning models has been examined in R-Fe-B magnets by correlating the hard magnetic properties with the microstructure and magnetic domain structure. The nucleation model can be used to explain the magnetic properties of R-Fe-B magnets at low temperatures, but it fails at high temperatures. On the other hand, domain wall pinning can explain the magnetic properties over a much wider temperature range (up to the Curie temperature). The magnetic and structural data suggest that the coercivities (Hc) of powders and stoichiometric melt-spun ribbons are due to ‘‘strong pinning’’ by a random distribution of pins while the Hc of sintered magnets and ribbons with off-stoichiometric composition are due to ‘‘localized pinning’’ at grain boundaries.
Ion implantation, a method for fabricating light guides in polymers
Authors: Kulish J. R. , Franke H. , Singh Amarjit , Lessard Roger A. , Knystautas Emile J.
Published: July 2002

Li+ and N+ ions were implanted into aliphatic polymethylmethacrylate (PMMA), polyvinylalcohol (PVA), and aromatic polyimide (PI) polycarbonate (PC) polymers in the energy range of 100–130 keV. Planar optical waveguides guiding between one and three modes were formed. For low implantation doses (≤ 1014 ions/cm2), total waveguide loss values at λ=633 nm were found to be less than 2 dB/cm. The changes in the refractive index were found to be very large (Δn≥0.05) in the case of PMMA and PVA. We interpret this change in refractive index as being due to the formation of aromatic compounds in the regions of electronic scattering.
Properties of CoSi2 formed on (001) Si
Authors: Ommen A. H. van , Bulle-Lieuwma C. W. T. , Langereis C.
Published: July 2002

Properties of CoSi2 films formed on (001) Si have been investigated for silicide films formed by solid-state reaction of either evaporated or sputtered Co with (001) Si substrates. It turns out that CoSi2 films formed from evaporated Co have a systematically higher resistivity than films formed from sputtered Co, which can be attributed to contaminants that were present in the evaporated metal film. Silicide films formed from evaporated Co develop a strong (110) texture, which becomes more pronounced with increasing annealing temperature. Microstructural analyses revealed that this is due to epitaxial growth of (110) CoSi2 upon (001) Si. This is a surprising phenomenon as the epitaxy of (001) CoSi2 on (001) Si would be expected in view of the similarity of the crystal structure and the small lattice mismatch of CoSi2 and Si. The expected epitaxy of (001) CoSi2 on (001) Si is also present but for a smaller fraction of the silicide grains. The film stress has been analyzed in terms of thermal and intrinsic stress. From the thermal stress contribution, a value of 140 GPa was calculated for the biaxial elastic constant of CoSi2. Using this value, it was also possible to interpret the intrinsic stress in terms of the density change upon silicide formation. Transmission electron microscopy and x-ray diffraction revealed that CoSi2 can not only grow epitaxially on (001) Si in a (001) and a (110) orientation but that there is also epitaxy close to these orientations with tilts of around 10° relative to the substrate normal. Simple geometrical calculations show that there is a good match for both (001) and (110) CoSi2 on (001) Si and for a slight tilt from these orientations, all in very good agreement with the observations.
The epitaxial growth of microprotrusions on field emitter tips
Authors: Schwoebel P. R.
Published: July 2002

One precursor to cathode initiated vacuum arc breakdown is the growth of microprotrusions on the surface of the cathode. Ion bombardment activated surface atom transport in the presence of a large applied electric field has been associated with microprotrusion growth. Previous low-temperature cathode bombardment investigations have been extended into a cathode temperature regime of ∼0.2Tm–0.4Tm, where Tm is the cathode melting point. At cathode temperatures of ∼0.3Tm, helium ion cathode bombardment initiates microprotrusion growth which is epitaxial and confined to particular crystal planes. This form of epitaxial growth is partially associated with the thermally activated component of surface transport present at ∼0.3Tm.
Characteristics of electron-beam-excited Xe*2 at low pressures as a vacuum ultraviolet source
Authors: Eckstrom D. J. , Nakano H. H. , Lorents D. C. , Rothem T. , Betts J. A. , Lainhart M. E. , Dakin D. A. , Maenchen J. E.
Published: July 2002

The performance of Xe*2 as a 172-nm fluorescence or laser source when pumped by a low-current, long-pulse electron beam was determined. The fluorescence efficiency of Xe*2 is near the theoretical limit of ∼50% at modest pressures over a range of pump rates up to 106 W/cm. The laser efficiency is limited to values <1% by a very strong medium absorption that is probably due to Xe*2 photoionization. Laser performance is further degraded by early pulse termination that appears related to mirror degradation. An improved kinetics and extraction code was developed to model the performance of the Xe*2 system. A key component of the model is a more detailed treatment of the interactions between secondary electrons and excited atomic and molecular xenon states. Rates for these processes were derived as described herein. With this model, good absolute agreement was obtained between experiments and calculated parameters at pressures as low as 0.5 atm.
Cherenkov-type sharp energy dissipation associated with kinks (solitons) moving harmonically in the atmosphere of paraelastic interstitial atoms
Authors: Ogurtani Tarik Ömer
Published: July 2002

A geometric kink (soliton) moving rigidly and harmonically in the cloud of paraelastic interstitials showing hypo-Snoek and hyper-Snoek viscosity regimes, is described by a second order ordinary differential equation with a nonlinear Debye-type dissipative drag term. The computer simulations on this system show not only the existence of the dissipative resonance behavior with zero Q−1 factor but also the occurrence of the unusually sharp (quasi-quantum jumps) energy dissipations, like the Cherenkov radiation, when the kink velocity exceeds the Snoek jump velocity of interstitials at high external driving force amplitudes, in certain circumstances.
Liquid junctions for characterization of electronic materials. IV. Impedance spectroscopy of reactive ion-etched Si
Authors: Fantini M. C. A. , Shen Wu-Mian , Tomkiewicz Micha , Gambino J. P.
Published: July 2002

Impedance spectroscopy was used to analyze the dielectric properties of reactive ion-etched silicon/liquid junction interfaces for five different plasmas: CHF3 /Ar, CF4 , CClF3 /H2 , CClF3 /H2 +O2 ash, and O2 ash. The results were interpreted in terms of equivalent circuits, which are basically the damaged layer constituents. These elements dominate the impedance spectra and their contributions arise from two different regions inside the treated substrates: a residue overlayer and a Si-damaged layer. We have estimated the thickness of these regions. Except for the CClF3 /H2 treatment, the photoresponse of the cells is very poor. A comparison between these results and those obtained for samples not submitted to the reactive ion etching (RIE) processes in the same electrochemical cell will be presented. The effect of wet etchings, in buffered HF, 2M KOH, and a photoetching in 0.5% HF solutions, on the performance of the photoelectrochemical cells will be discussed. The results indicate that the CHF3 /Ar RIE promotes a deeper damage, compared with the CF4 RIE, and it cannot be removed by the chemical treatments that were used. The observed shifts of the flat-band potential, shown by the RIE-damaged substrates, are associated with the accumulation of positive charges at the semiconductor interface. The O2 ash treatment partially recovers the surface damage by removing these charges but is responsible for the formation of an insulating overlayer.
A new analytic model for amorphous silicon thin-film transistors
Authors: Shur Michael , Hack Michael , Shaw John G.
Published: July 2002

We present a new theory describing current-voltage characteristics of amorphous silicon thin-film transistors. We calculate the output conductance in saturation by considering channel shortening effects caused by the space-charge-limited current in the pinch-off region. In this model the drain current is expressed through the free-carrier concentration at the source side of the channel. This allows us to obtain an accurate description of the different operating regimes of a thin-film transistor using one equation that accounts for the dependence of the free-carrier concentration in the channel for different regimes. Our model is in good agreement both with experimental data and the results of our two-dimensional computer simulation. This approach allows one to account for different distributions of localized states in the energy gap. The model has also been developed to be incorporated into a circuit simulator and used for computer-aided design of amorphous silicon integrated circuits.
Photoelectron spectroscopic studies on a silicon interface with Bi2Sr2CaCu2BO8+δ high T
c superconductor
Authors: Kulkarni Pramada , Mahamuni Shailaja , Chandrachood M. , Mulla I. S. , Sinha A. P. B. , Nigavekar A. S. , Kulkarni S. K.
Published: July 2002

X-ray and ultraviolet photoelectron spectroscopies have been used to investigate the interaction between silicon and Bi2Sr2CaCu2O8+δ high Tc superconducting material. For low coverages, silicon adatoms disrupt CuO bonds and SrO bonds to form a complex Sr-Si-O phase. This interlayer efficiently prevents further reaction between silicon and the Bi2Sr2CaCu2O8+δ superconductor.
The nonlinear optical characteristics of a LiB3O5 crystal
Authors: Lin Shujie , Sun Zhaoyang , Wu Bochang , Chen Chuangtian
Published: July 2002

The nonlinear optical coefficients of lithium triborate LiB3O5 (LBO) have been determined by the method of Maker fringes at λ=1.079 μm. It is found that these coefficients relative to d36 for potassium dihydrogen phosphate are d31=∓2.51 (l±0.09), d32=±2.69 (l±0.12), d33=±0.15 (l±0.10), d15=∓2.30 (l±0.22), and d24=±2.52 (l±0.20). The phase-matching conditions, effective second-harmonic generation (SHG) coefficients, walk-off angles, and acceptance angles for SHG at λ=1.079 μm are also investigated. The figures of merit of LBO in different cases are then compared with those of potassium dihydrogen phosphate (KDP), β-barium metaborate (BBO), and potassium titanyl phosphate (KTP) crystals. All these figures combined with the damage thresholds of the crystals show that an LBO crystal may operate better than KDP and BBO crystals for frequency conversions of a tightly focused Nd:YAlO3 (YAP) or Nd:Y3Al5O12 (YAG) laser and also a dye laser.