bluuebunny/crossref_metadata_2025 huggingface dataset:
Color of red mercuric iodide at cryogenic temperatures
Authors: Hunt A. W. , Ciampi G. , Twamley B. , Vaccaro F. , Tjossem R. , Lynn K. G.
Published: July 2002

We investigated a red to yellow color change observed in α-HgI2 when cooled below 150 K. A phase transformation to β-HgI2, which has a yellow color, was ruled out by a variable temperature x-ray diffraction study. Instead the color change at cryogenic temperatures is caused by a shift in the transmission edge to shorter wavelengths which we attribute to a widening band gap at low temperatures. Using optical transmission spectroscopy the width of the band gap was measured between 10 and 330 K. At room temperature the gap was 2.11±0.03 eV, which is significantly smaller than the most recently published values of ∼2.3 eV. This smaller band gap was further verified by measuring the thermoelectric current at elevated temperatures.
Carrier density imaging of lateral epitaxially overgrown GaN using scanning confocal Raman microscopy
Authors: Matthews Manyalibo J. , Hsu J. W. P. , Gu Shulin , Kuech T. F.
Published: July 2002

GaN thin films, grown by the lateral epitaxial overgrowth (LEO) method, are studied by scanning confocal Raman microscopy. By measuring changes in coupled longitudinal-optical phonon–plasmon frequencies and using a standard harmonic oscillator dielectric function, detailed images of carrier density could be formed. Carrier concentrations are extremely high (∼1020 cm−3) immediately above SiOx mask layers and decrease abruptly when the SiOx mask are not directly exposed to the growth surface, implying that SiOx masks are the source of dopants. Images of intergrated E1 longitudinal-optical phonon intensities could be compared with free-carrier images and showed a clear anticorrelation throughout the LEO structure.
Investigation of the relativistic equivalent Hamiltonian in the LS coupling scheme
Authors: Karazija R. , Jonauskas V.
Published: July 2002

Armstrong’s method of relativistic description of atoms in LS coupling using the equivalent operator is developed. Its form in terms of standard unit operators acting within a space of one shell is obtained. The interpretation of separate terms of equivalent Breit operator is investigated in a general case of nonequivalent electrons. The relativistic Dirac–Fock equations for the level and term in LS coupling are derived. The equivalent operator is applied to obtain the averages of relativistic operators too.
Dissociation and suppressed ionization of H2O molecules embedded in He clusters: The role of the cluster as a cage
Authors: Kanaev A. V. , Museur L. , Laarmann T. , Monticone S. , Castex M. C. , von Haeften K. , Möller T.
Published: July 2002

Electronic structure and energy transfer in H2O doped HeN clusters (N≈104) is studied with photoexcitation in the spectral range of 140–40 nm (9–30 eV). The reaction dynamics is investigated by fluorescence of neutral OH* and H* and ionic H2O+* fragments. The rotational temperature of embedded water molecules has been estimated from the 124 nm line shape (3pa1 C 1B1←1b1 X̃ 1A1 transition). Two different temperatures (T1⩽5 K and T2≈30 K) have been found. We propose that the lower temperature (T1) is due to completely thermalized water molecules trapped inside helium clusters, while the warmer molecules (T2) are formed if they are first captured by helium clusters but then leave the clusters again. Predissociation of H2O with excitation below the ionization limit (λexc>100 nm) is found to be unaffected by the cluster environment. On the other hand, the ionization (λexc<100 nm) seems to be suppressed inside helium clusters in favor of the fragmentation into neutral products.
Femtosecond multiphoton ionization of free and aggregated NaI via Rydberg states
Authors: Stibbe Darian T. , Charron Eric , Suzor-Weiner Annick
Published: July 2002

Using a wave packet simulation on simplified potential curves, we investigate the vibrational dynamics of NaI in its first excited electronic state, both isolated and aggregated with a molecule of acetonitrile. The probe signal is obtained by resonant multiphoton ionization (REMPI) of the diatomic chromophore NaI via Rydberg states. The addition of acetonitrile is found to change greatly the ionization signal, suggesting strong modification of the reaction dynamics.
Intervalence transition in triarylamine mixed-valence systems: A time-dependent density functional theory study
Authors: Coropceanu V. , Malagoli M. , André J. M. , Brédas J. L.
Published: July 2002

A theoretical model is developed to describe the intervalence transitions in the mixed-valence N,N,N,′N′-tetrakis(4-methoxyphenyl)-1, 4-benzenediamine system. The excited-state energies and transition dipole moments are evaluated by means of time-dependent density functional theory while the shape of the lowest absorption band is analyzed in the framework of a dynamic vibronic model. Our results are discussed in relation with recent experimental data.
Polarization induced modification of thermal radiative properties of solid density plasmas produced by subpicosecond laser
Authors: Dorchies F. , Forget P. , Gallant P. , Jiang Z. , Kieffer J. C. , Pépin H. , Peyrusse O.
Published: July 2002

Near solid density plasmas have been obtained by the interaction of ultraintense (2×1018 W cm−2) clean laser pulses with targets composed of different thickness of Al on a Si substrate. The depth of the x-ray emission and spectral shapes were measured using x-ray spectroscopy, which simultaneously characterized the emitting plasma and the suprathermal electron distribution. Strong modifications of the plasmas thermal radiative properties have been observed, for the first time, by changing the laser polarization from S to P. This correlates with an increase of suprathermal electrons production.
Linearly modulated photoconductivity and linearly modulated optically stimulated luminescence measurements on Al2O3:C
Authors: Whitley V. H. , McKeever S. W. S.
Published: July 2002

A measurement technique is introduced which involves linearly modulating the power of the stimulation light while measuring the photoconductivity in Al2O3:C. This produces peaks in the measured photoconductivity instead of the typical exponential decay seen when using a constant power stimulation light. The technique, called linearly modulated photoconductivity (LM-PC), is the conductivity equivalent of linearly modulated optically stimulated luminescence (LM-OSL). Two peaks are seen in the LM-PC measurements that correspond to the same charge residing in the 450 and 600 K thermally stimulated conductivity peaks, respectively. Simultaneous measurements on both the LM-PC and LM-OSL reveal that the second peak in the LM-PC is not present in the LM-OSL. Deconvolutions are performed on the LM-OSL to determine the underlying components that make up the detected OSL emissions.
Electrical properties of electron irradiated thin polycrystalline CdSe layers
Authors: Antohe S. , Ion L. , Ruxandra V.
Published: July 2002

Electrical properties of nonirradiated and electron-irradiated thin layers of CdSe, sandwiched between two gold electrodes, were investigated. Thin films of CdSe, prepared by thermal-vacuum evaporation on glass substrate at a temperature of 220 °C, were subjected to two sessions of irradiation with 7 MeV electrons to the fluences of 2×1015 and 4×1015 e/cm2, respectively. The current–voltage characteristics, recorded at temperatures in the range 150–350 K, showed that the Ohm’s law is followed at low-applied voltages, in both nonirradiated and irradiated CdSe layers. In the range of high-applied voltages, the space-charge-limited current (SCLC), controlled by a Gaussian trap distribution, placed in the vicinity of the Fermi level, has been identified as the dominant conduction mechanism. An analysis in the frame of SCLC theory allowed us to obtain the parameters characterizing the trap distribution and their changes induced by electron irradiation.
Ab initio calculations of electric field effects on the g-tensor of a nitroxide radical
Authors: Ding Zhebo , Gullá Andrea F. , Budil David E.
Published: July 2002

The effects of applied electric fields on the magnetic parameters of a nitroxide spin label have been calculated using an intermediate level of Rayleigh–Schrödinger perturbation theory based on unrestricted Hartree–Fock ab initio calculations. The theory is tested for selected simple model organic radicals and gives electronic g-tensors that compare well with previous calculations and experimental values. The method is then applied to calculate effects of a local electrostatic field on the larger nitroxide radical, 2,2,5,5-tetramethyl-3,4-dehydro pyrrolidine-1-oxyl (TMDP) using a finite field approach. The method slightly underestimates the gx and gy of TMDP, but the predicted shifts in these quantities with field are in excellent agreement with recent experimental observations using high-frequency (220 GHz) electron paramagnetic resonance. Results are expressed in terms of the phenomenological formalism given earlier by Mims to characterize linear electric field effects in metal centers.
Quantum-mechanical dielectric model of the electron–ion collision frequency in strong laser fields
Authors: Kull H.-J. , Plagne L.
Published: July 2002

The electron–ion collision frequency in a strong laser field is calculated in the framework of the quantum Vlasov theory in first-order Born approximation. Using a Wigner representation of the density matrix, the collision frequency can be expressed in terms of the Lindhard dielectric function and a close correspondence between classical and quantum-mechanical approaches can be obtained. Asymptotic formulas for the high-frequency collision frequency in weak and strong electric fields are obtained and compared with complete numerical calculations. The basic strong-field behavior can be explained in terms of the cold plasma model.
Partition asymptotics from one-dimensional quantum entropy and energy currents
Authors: Blencowe Miles P. , Koshnick Nicholas C.
Published: July 2002

We give an alternative method to that of Hardy–Ramanujan–Rademacher to derive the leading exponential term in the asymptotic approximation to the partition function p(n,a), defined as the number of decompositions of a positive integer n into integer summands, with each summand appearing at most a times in a given decomposition. The derivation involves mapping to an equivalent physical problem concerning the quantum entropy and energy currents of particles flowing in a one-dimensional (1D) channel connecting thermal reservoirs, and which obey Gentile’s intermediate statistics with statistical parameter a. The method is also applied to partitions associated with Haldane’s fractional exclusion statistics.
Control of adhesion to the mask of epitaxial laterally overgrown GaAs layers
Authors: Zytkiewicz Z. R. , Domagała J. , Dobosz D.
Published: July 2002

Strain commonly observed in layers grown by epitaxial lateral overgrowth (ELO) and arising from interaction of the layers with the mask underneath is studied. We show that GaAs ELO layers grown by liquid-phase epitaxy on SiO2-coated GaAs substrates are strain free if the laterally overgrown parts (“wings”) of the layers hang over and have no direct contact with the mask. In other cases, tilting of the wings can be efficiently tailored by controlling the ratio of vertical to lateral growth rates at the beginning of ELO growth. In particular, this has been achieved by growing GaAs ELO layers on SiO2-coated GaAs substrates with increasing density of dislocations. Then, the ratio of vertical to lateral growth rates at the beginning of the growth is increased which in turn leads to reduction of the adhesion-induced bending of the ELO wings, as we observe by high-resolution x-ray diffraction. In the limiting case of heavily dislocated substrates, namely, on GaAs-coated Si, the vertical growth of GaAs ELO is so fast that air-bridged structures without any wing adhesion to the SiO2 mask are obtained. Next, the same model is used to explain our earlier data on negligible bending of GaAs ELO layers on graphite-masked GaAs substrates. In this case, delayed start of lateral growth is caused by the change of the shape of the melt in the corner between the sidewall of the ELO layer and the mask when SiO2 was replaced by graphite film not wetted by the gallium melt.
Effects of cell parameters on the properties of hybrid twisted nematic displays
Authors: Kubono Atsushi , Kyokane Yozo , Akiyama Ryuichi , Tanaka Katsufumi
Published: July 2002

This article presents the results of an investigation of the electro-optical response for a hybrid twisted nematic (HTN) liquid crystal display. The device used both a hybrid alignment and a twisted director configuration. In comparison with the conventional twisted nematic mode, a faster response and a smaller operation voltage were obtained by using an appropriate amount of chiral dopant. The characteristics of the HTN cells were dependent on cell parameters such as the chemical nature of the polymer alignment layer, the cell thickness, the set twist angle, and the concentration of chiral dopant. Response times of 1.0 ms for turn-on and 16 ms for turn-off were achieved by the regulation of cell parameters. Computer simulations of the director configuration demonstrated that twist deformation plays an important role in the determination of the response characteristics.
Optical functions, phonon properties, and composition of InGaAsN single layers derived from far- and near-infrared spectroscopic ellipsometry
Authors: Leibiger G. , Gottschalch V. , Schubert M.
Published: July 2002

We study the optical properties of compressively strained InxGa1−xAs1−yNy (x<0.13, y<0.03) single layers for photon energies from 0.75 to 1.3 eV (near infrared), and for wave numbers from 100 to 600 cm−1 (far infrared) using spectroscopic ellipsometry. The intentionally undoped InGaAsN layers were grown pseudomorphically on top of undoped GaAs buffer layers deposited on Te-doped (001) GaAs substrates by metalorganic vapor-phase epitaxy. We provide parametric model functions for the dielectric function spectra of InGaAsN for both spectral ranges studied here. The InGaAsN layers show a two-mode phonon behavior in the spectral range from 100 to 600 cm−1. We detect the transverse GaAs- and GaN-sublattice phonon modes at wave numbers of about 267 and 470 cm−1, respectively. The polar strength f of the GaN sublattice resonance changes with nitrogen composition y and with the biaxial strain εxx resulting from the lattice mismatch between InGaAsN and GaAs. This effect is used to derive the nitrogen and indium content of the InGaAsN layers combining the observed f dependence with results from high-resolution double-crystal x-ray diffractometry and using Vegard’s law for the lattice constants and the elastic coefficients C11 and C12. The calculated nitrogen concentrations reflect growth properties such as increasing N incorporation in InGaAsN with decreasing growth temperature, with increasing concentration of nitrogen in the gas phase, and with decreasing indium concentration in InGaAsN.
Structural change in p-type porous silicon by thermal annealing
Authors: Ogata Yukio H. , Yoshimi Naoki , Yasuda Ryo , Tsuboi Takashi , Sakka Tetsuo , Otsuki Akira
Published: July 2002

The morphological change of p-type porous silicon during annealing has been investigated. The x-ray diffraction (XRD) pattern was composed of a sharp Bragg reflection peak and a diffuse scattering. The diffuse scattering is not related to the presence of the amorphous phase. The shape of the XRD pattern started to change at an annealing temperature as low as 400 °C, and the 2θ angle of the sharp peak varied at a temperature as low as 350 °C. These changes at low temperatures seem to be closely related to the desorption of hydrogen and the resultant change of the dangling bond density in porous silicon. The molecular orbital calculations also support the participation of dangling bonds in the structural reorganization in the surface region.
Conformational disorder in long polyenes
Authors: Wood Phillip , Samuel Ifor D. W. , Schrock Richard , Christensen Ronald L.
Published: July 2002

We have studied the temperature dependence of the absorption spectra of soluble, conjugated polymers of known chain length (∼100–1000 double bonds), synthesized by Schattenmann et al. [Macromolecules 29, 8990 (1996)] using living polymerization techniques. The polymer spectra show significant redshifts upon cooling from 300 to 80 K. To estimate the distributions of conjugated segments in these conformationally disordered samples, the spectra were fitted to a superposition of absorption spectra of conjugated segments of known lengths. These “experimental” distributions of conjugated segments then were compared to those predicted by theoretical models of conformational disorder. Although the low-temperature samples contain significant populations of long segments, the shortest polyene segments dominate all distributions. We compare these experimental results with distributions derived from the model of Yaliraki and Silbey [J. Chem. Phys. 104, 1245 (1996)]. Adjustment of relevant energy parameters gives good qualitative agreement between the experimental and theoretical distributions in long polyenes.
Phases and amplitudes of recurrences in autocorrelation function by a simple classical trajectory method
Authors: Žďánská Petra , Moiseyev Nimrod
Published: July 2002

The interference between time-dependent recurrences in the quantum autocorrelation function is eliminated by carrying out orthogonal transformations in the time-energy domain. The time-dependent phases and amplitudes of the individual recurrences are compared with the results obtained from simple classical trajectory calculations. Using classical trajectories we calculate a two-dimensional survival probability which is defined in the time and energy domain. The two-dimensional survival probability provides the phase and enables to distinguish between overlapping recurrences. Remarkable agreement between the quantum and classical results is obtained for the initial Gaussian wave packet which is preferentially located either in the regular or in the chaotic regimes in the classical phase space of the Pullen–Edmonds Hamiltonian (nonlinearly coupled two harmonic oscillators). A novel method which enables to determine the molecular potential energy surfaces from a measured absorption or emission spectra is proposed. The method employs the matching of Wigner transforms of individual quantum recurrences with the two-dimensional classical survival probability.
Edge pedestal deterioration in tokamak high-mode discharges
Authors: Stacey Weston M.
Published: July 2002

A model for calculating edge gradients in density and temperature from transport and atomic physics considerations is combined with a model for calculating the enhancement of edge transport by thermal instabilities, the onset and strength of which in turn depend on edge gradients and atomic physics. A minimum threshold value of the nonradiative power flux passing through the edge that is required to suppress transport enhancing thermal instabilities is established. When the power flux through the edge is less than this minimum threshold value, the model predicts a deterioration of the edge gradients as the neutral concentration, the impurity concentration and/or the impurity radiation emissivity in the edge plasma increase, which is consistent with experimental observations. Model problem calculations are presented to illustrate the predicted pedestal deterioration for representative experimental conditions.
Argon predissociation and electron autodetachment spectroscopy of size-selected CH3NO2−⋅Arn clusters
Authors: Weber J. M. , Robertson W. H. , Johnson M. A.
Published: July 2002

Photodetachment spectra of CH3NO2−⋅Arn clusters in the mid-IR are dominated by three strong resonances. These are assigned to autodetaching (AD) C–H stretching vibrational transitions in the valence (as opposed to dipole-bound) form of the molecular anion on the basis of a H/D isotopic substitution study and their solvation dependence. The AD resonances disappear promptly upon addition of the third argon atom, while the resonant structure appears in the action spectrum for formation of CH3NO2− photoproducts for n⩾2. The strong argon solvation dependence of the photoproducts is traced to the rapidly changing endoergicity of the electron loss channel due to the differential solvation behavior of the valence anion relative to the neutral. We discuss a statistical limit for this competition, and introduce an intramolecular vibrational energy redistribution mediated AD mechanism unique to polyatomic anions.
Ideal n=3 phase Ca4Mn3O10 of Ruddlesden–Popper series obtained using high pressure and high temperature
Authors: Yu R. C. , Li S. Y. , Zhu J. L. , Li F. Y. , Zhang Z. , Jin C. Q. , Voigt-Martin I. G.
Published: July 2002

An ideal n=3 member of Ruddlesden–Popper (RP) phase Ca4Mn3O10 was obtained by solid-state reaction under high pressure. This phase has been studied by transmission electron microscopy, convergent beam electron microscopy, high-resolution transmission electron microscopy, and electron energy loss spectroscopy. The lattice parameters are derived as a=b=0.37 nm and c=2.69 nm with a space group of I4/mmm, which is the space group of ideal RP phase. The advantages of high pressure sintering are discussed.
Effect of trapped ions on shielding of a charged dust grain in a plasma
Authors: Tskhakaya D. D. , Shukla P. K. , Stenflo L.
Published: July 2002

The trapped ion effects on the shielding of a charged dust grain in a plasma are examined. It is shown that ions with low angular momentum cannot form a trapped cloud in a potential well. Therefore, it is necessary to examine the possibility of producing trapped ion clouds with large angular momentum which can shield the grains.
Global time asymmetry as a consequence of a wave packets theorem
Authors: Castagnino Mario A. , Gueron Jorge , Ordóñez Adolfo R.
Published: July 2002

When t→∞ any wave packet in the Liouvillian representation of the density matrices becomes a Hardy class function from below. This fact, in the global frame of the Reichenbach diagram, is used to explain the observed global time asymmetry of the universe.
Destabilization of dislocation dipole at high velocity
Authors: Wang Jian , Woo C. H. , Huang Hanchen
Published: July 2002

Under mechanical deformation of very high strain rates, it is expected that dislocations may move very fast (being subsonic, transonic, or even supersonic). As a result, two dislocations may approach each other at high velocities. Our earlier analyses of linear elasticity indicate that a dipole is destabilized when the approaching velocity is high enough—even below the speed of sound. In this letter, using the molecular dynamics method, we demonstrate that a dipole indeed can be destabilized above a critical velocity, and stable below that.
Angle operators on weighted Bergman spaces
Authors: Dubin D. A. , Hennings M. A.
Published: July 2002

Among the multiplication operators on weighted Bergman Hilbert spaces are those where the multiplying function (operator symbol) depends only on the angular polar coordinate in the unit disk: we call these “angle operators.” As these Hilbert spaces carry a CCR representation unitarily equivalent to the Schrödinger representation, angle operators are associated with quantum phase in the same way as are Toeplitz operators, for example. We determine the matrix elements of the angle operators with respect to the natural orthonormal basis on each of these spaces, and also with respect to the appropriate family of coherent states. By using a method of comparison with the corresponding results for Toeplitz operators, asymptotic expressions for the expectations and variances in these two families of states are obtained for the angle operators whose symbols are the polar angle function and its two complex exponentials. Notable is the fact that the asymptotic limit of the variance of the polar angle operator in the natural basis family is π2/3, which many authors take to be a requirement for a quantum phase operator.
The structure of the solution obtained with Reynolds-stress-transport models at the free-stream edges of turbulent flows
Authors: Cazalbou J.-B. , Chassaing P.
Published: July 2002

The behavior of Reynolds-stress-transport models at the free-stream edges of turbulent flows is investigated. Current turbulent-diffusion models are found to produce propagative (possibly weak) solutions of the same type as those reported earlier by Cazalbou, Spalart, and Bradshaw [Phys. Fluids 6, 1797 (1994)] for two-equation models. As in the latter study, an analysis is presented that provides qualitative information on the flow structure predicted near the edge if a condition on the values of the diffusion constants is satisfied. In this case, the solution appears to be fairly insensitive to the residual free-stream turbulence levels needed with conventional numerical methods. The main specific result is that, depending on the diffusion model, the propagative solution can force turbulence toward definite and rather extreme anisotropy states at the edge (one- or two-component limit). This is not the case with the model of Daly and Harlow [Phys. Fluids 13, 2634 (1970)]; it may be one of the reasons why this “old” scheme is still the most widely used, even in recent Reynolds-stress-transport models. In addition, the analysis helps us to interpret some difficulties encountered in computing even very simple flows with Lumley’s pressure-diffusion model [Adv. Appl. Mech. 18, 123 (1978)]. A new realizability condition, according to which the diffusion model should not globally become “anti-diffusive,” is introduced, and a recalibration of Lumley’s model satisfying this condition is performed using information drawn from the analysis.
Theory of phase separation kinetics in polymer–liquid crystal systems
Authors: Muratov C. B. , E Weinan
Published: July 2002

We introduce a kinetic model describing the phase separation in the mixture of long rod-like molecules and long chain-like molecules. The model uses the angular distribution function for the orientations of the rods as a dynamical variable. The energetics is based on the nonlocal Onsager theory for the rods combined with a nonlocal extension of the Flory–Huggins theory. The kinetics explicitly takes into account the preferential diffusion along the rods. We computed the phase diagrams in this model and found a number of transitions leading to phase separation. We also performed numerical simulations of the phase separation kinetics and studied the resulting morphologies.
Chiral fluctuations in achiral systems
Authors: Harris Robert A.
Published: July 2002

“Chiral fluctuations” are defined, and their relation to “dynamic chirality” is discussed. Simple experiments to measure chiral fluctuations are proposed. The unique aspects of these measurements for systems such as atomic clusters and gases are outlined.
Large eddy simulation of the effects of mild swirl on the near field of a round free jet
Authors: McIlwain S. , Pollard A.
Published: July 2002

The near fields of turbulent round free jets with swirl numbers of S=0.0 and 0.24 are modeled with large eddy simulations that use a dynamic Smagorinsky subgrid scale model. The time-averaged results are in accordance with other numerical and experimental data. Instantaneous plots of vorticity are used to examine the evolution of and interaction between coherent structures. Ring structures aligned with the plane normal to the flow form downstream of the jet shear layer and collide with the streamwise braid structures. The resulting interaction cause the rings to break apart into smaller, less organized turbulence structures. The addition of swirl increases the number of streamwise braids, which enhances the breakdown mechanism of the rings. The results suggest that the increased entrainment observed in swirling flows is due to the action of the braids rather than the rings.
Characterization of nanometer As-clusters in low-temperature grown GaAs by transient reflectivity measurements
Authors: Korn T. , Franke-Wiekhorst A. , Schnüll S. , Wilke I.
Published: July 2002

We report a systematic experimental study of the transient reflectivity of low-temperature grown GaAs as a function of excitation wavelength, excitation density, and lattice temperature. We observe that the reflectivity decay is temperature independent for excitation energies between 20 and 70 meV above the band gap of GaAs. Under this condition the reflectivity increases linearly with excitation density and is in very good quantitative agreement with the Drude model. Subsequently, we present a model which allows the extraction of the diameter and density of As clusters in low-temperature grown GaAs from the reflectivity decay.
Integrable cases of gravitating static isothermal fluid spheres
Authors: Ivanov B. V.
Published: July 2002

It is shown that different approaches toward the solution of the Einstein equations for a static spherically symmetric perfect fluid with a γ-law equation of state lead to an Abel differential equation of the second kind. Its only integrable cases at present are flat space–time, de Sitter solution and its Buchdahl transform, Einstein static universe, and the Klein–Tolman solution.
Governing parameter for electromigration damage in the polycrystalline line covered with a passivation layer
Authors: Sasagawa Kazuhiko , Hasegawa Masataka , Saka Masumi , Abé Hiroyuki
Published: July 2002

A governing parameter, called AFDgen*, which reflects electromigration damage in a polycrystalline line covered with a passivation layer is proposed. The formulation is based on the parameter AFDgen previously introduced in our studies. With the help of AFDgen we can calculate the atomic flux divergence due to electromigration by considering two-dimensional distributions of current density and temperature and also by simply considering the microstructure of polycrystalline lines and bamboo lines. AFDgen has been identified as a governing parameter for electromigration damage in unpassivated polycrystalline lines and bamboo lines through experimental verification. As the first step in the development of a practical and universal prediction method for electromigration damage, we treated metal lines not covered with a passivation layer. On the other hand, metal lines used in packaged silicon integrated circuits are covered with passivation. Electromigration induces a mechanical stress (atomic density) gradient in such lines. This gradient plays an important role in the mechanism of electromigration damage. The new parameter proposed here, AFDgen*, includes the effect of the atomic density gradient. We develop also an AFDgen*-based method for determination of film characteristics. This method is applied to both covered and uncovered metal lines made of the same Al film. The film characteristics of both line types are obtained experimentally. Based on a discussion about the validity of the obtained characteristic constants, we were finally able to conclude that the AFDgen* parameter and the proposed method for deriving film characteristics are useful.
Formation of current-vortex filaments
Authors: Yatsuyanagi Y. , Ebisuzaki T. , Hatori T. , Kato T.
Published: July 2002

Time evolution of a system where electric current and vorticity coexist is demonstrated by two-dimensional magnetohydrodynamic simulations. Such a system has two characteristics. The first one is that once the electric current and the vorticity share the same position, they tend to move simultaneously, staying overlapped with each other. The second one is that the electric current and the vorticity evolve to create more overlapping regions where they coexist, even if their initial spatial distributions are not the same. During the evolution, the distributions of the electric current and the vorticity are fragmented, and the current-vortex filaments are formed. Once the current-vortex filaments are formed, they survive stably. This is due to the strong correlation between the electric current and the vorticity.
Incipient air entrainment in a translating axisymmetric plunging laminar jet
Authors: Chirichella D. , Gomez Ledesma R. , Kiger K. T. , Duncan J. H.
Published: July 2002

Air entrainment due to a translating axisymmetric laminar water jet plunging into an otherwise quiescent pool of water was studied experimentally. The jet impact diameter, Dj, and velocity, Vj, ranged from 0.33 to 1.21 cm and 104 to 365 cm/s, respectively. For all of these jet flow conditions, the water surface around the jet impact site of the stationary jet is smooth and no air is entrained. When the jet is moving horizontally with velocity Vt, a depression of the free surface forms directly downstream of the jet. In any set of experiments with constant Vj and Dj, as Vt is increased, the depth and streamwise length of this depression increases and a cusp forms at the bottom of its upstream edge. Air entrainment first occurs in the form of discrete small bubbles with diameters of about 0.05 cm that are injected from the cusp when the Froude number (Fr=Vt/gDj, where g is the acceleration of gravity) exceeds a critical value of about 1.4 for Vj/Vt>5. At higher values of Vt, a separate mode of entrainment occurs where large pockets of air are injected from the cusp into the flow in the form of intermittent bursts. At still higher values of Vt, the high-entrainment-rate condition occurs continuously. High-speed flow visualization movies of the entrainment process show that in all entrainment regimes, the bubbles enter the water when vortices from the jet shear layer pass over the leading-edge cusp of the depression and that these bubbles are initially trapped within the vortices. The boundaries between the three entrainment regimes are determined and presented on a plot of Froude number versus velocity ratio (Vj/Vt).
Nonaxisymmetric correction factors for the bounce-averaged electron Fokker–Planck equation
Authors: Ju M. , Basiuk V. , Peysson Y.
Published: July 2002

The bounce-averaging procedure for the electron Fokker–Planck equation is extended to the case of nonaxisymmetric geometry, in order to calculate consistently electron ripple-induced losses in tokamak during rf heating and current drive experiments. New explicit expressions for the bounce-averaged coefficients are developed for circulating electrons but also for banana electrons. While the effect of magnetic ripple is fairly negligible for circulating electrons, nonaxisymmetric corrections become significantly larger for banana electrons as compared to the axisymmetric case.
Comparison of hard magnetic electrodes for magneto-electronics by magnetic force microscopy
Authors: Brückl H. , Schmalhonst J. , Boeve H. , Gieres G. , Wecker J.
Published: July 2002

Both spin valve sensor elements and magnetic tunnel junctions usually consist of a magnetically fixed hard magnetic layer and a soft magnetic counter electrode which can rotate freely in an external driving field. In order to yield reliable sensor signals, the hard magnetic electrode should be magnetically rigid against small fields, i.e., must not be influenced during switching of the sensing electrode. Furthermore, in order to provide a homogeneous switching behavior, the electrodes should show uniformity in their magnetic properties, especially if scaled and patterned down to sub-micron lateral sizes. In this paper, magnetic force microscopy is used to compare the magnetic properties of different hard magnetic layer stacks commonly used in magneto-electronic device technology, i.e. an artificial antiferrimagnet (CoFe/Ru/CoFe and Co/Cu/Co) and exchange biased stacks (MnIr/CoFe/Ru/CoFe). The domain pattern of a polycrystalline artificial ferrimagnet (AFi) shows a strong magnetization ripple, i.e., local variations of the magnetization directions, which are maintained in patterned submicron elements. These statistical local fluctuations can be quantitatively correlated with a reduction of the tunneling magnetoresistance (TMR) of 3%–5% in the minor loop. In addition for small area junctions, the ripple can cause different switching fields in neighboring elements. An additional exchange biasing by a thin natural antiferromagnet is shown to rigidly pin the local magnetization fluctuations of the AFi. The process used for the preparation of this system, however, creates 360° domain walls, which again deteriorate the TMR signal.
Influence of electron injection on performance of GaN photodetectors
Authors: Chernyak Leonid , Schulte Alfons , Osinsky Andrei , Graff John , Schubert E. Fred
Published: July 2002

It is demonstrated that short-time (up to 1200 s) electron injection into the p-region of GaN p–n junction, as a result of forward bias application, leads to a long-term multifold enhancement of the device peak responsivity as well as to a spectral broadening of the photoresponse. The effect is found to persist for several days and is related to an increased minority carrier diffusion length in the p region, due to an injected electron trapping on deep levels associated with Mg acceptors.
Irreversible magnetostriction and magnetization of superconducting 2H-NbSe2 single crystals in a peak-effect regime
Authors: Eremenko V. V. , Sirenko V. A. , Shabakayeva Yu. A. , Schleser R. , Gammel P. L.
Published: July 2002

Magnetostriction measurements in the mixed state of superconducting 2H-NbSe2 single crystals under in-plane magnetic fields 0–12 T have revealed a peak on the magnetostriction versus magnetic field dependences in the vicinity of the upper critical field Hc2. The peak value of the longitudinal magnetostriction is higher by more than an order of magnitude in comparison with that of the transverse magnetostriction when measured along the hexagonal axis. Analysis of the measured field dependences of the magnetostriction and magnetization of 2H-NbSe2 allows one to relate the observed peculiarities of magnetostriction with the loss of order in the lattice of Abrikosov vortices, which occurs by a first-order phase transition.
Current-directionality-induced giant absorption dichroism in III–V semiconductors and its potential for polarization control in vertical cavity surface-emitting lasers
Authors: Ryvkin B. S. , Avrutin E. A. , Walker A. C.
Published: July 2002

We theoretically analyze the effect of current-directionality induced dichroism of fundamental absorption in III–V semiconductors with a high nonequilibrium carrier density. Band filling is shown to provide giant values of relative absorption difference for two orthogonal polarizations near the transparency point. Self-adjustment of the transparency point to the incident photon energy due to absorption saturation at high incident optical powers leads to large absorption dichroism in a broad spectral range. The situation we model is relevant to polarization modulation in vertical-cavity surface-emitting laser diodes. A specialized laser construction to achieve this purpose is proposed.
Design variants of modular permanent magnet brushless machine
Authors: Ede Jason D. , Atallah Kais , Howe David
Published: July 2002

The article describes an analytical technique for determining all possible slot-number and pole-number combinations, of modular permanent magnet brushless machines. It is shown that a large number of design variants exist. Furthermore, typical performance parameters, such as back-emf and cogging torque wave forms, for selected fault-tolerant designs are presented.
Raman sidescatter in numerical models of short pulse laser plasma interactions
Authors: Gordon D. F. , Hafizi B. , Hubbard R. F. , Sprangle P.
Published: July 2002

Certain reduced models of a plasma are subject to a phenomenon known as the ultraviolet catastrophe, whereby the Raman growth rate diverges with increasing transverse wave number. Numerical solutions that make use of such models cannot account for large angle scattering. Several of these models are evaluated in terms of their ability to reproduce the growth rates given by the two-dimensional Raman dispersion relation. The quasistatic approximation causes a much more severe ultraviolet catastrophe than does the paraxial approximation alone. Removal of the quasistatic approximation eliminates the ultraviolet catastrophe provided nonparaxial terms are retained. Fully explicit particle-in-cell simulations show that large angle scattering leads to a fine structure which perturbs the pump wave and heats the plasma.
Surface evolution of ultrahigh vacuum magnetron sputter deposited amorphous SiO2 thin films
Authors: Li B. Q. , Kojima I. , Zuo J. M.
Published: July 2002

The surface morphology evolution of amorphous SiO2 thin films deposited by ultrahigh vacuum radio-frequency magnetron sputtering was studied by atomic force microscopy. The results show that: (1) the surface roughness of the deposited films reduces with increased substrate temperatures; (2) the surface roughness increases with higher deposition pressures; and (3) there is a roughening transition at the critical thickness of ∼ 90 nm for the substrate temperature of 713 K. The results also show that the surface roughness at the early stages of growth evolves according to a power law. Further growth beyond the critical thickness leads to a sharp increase in roughness. The experimental results are compared with the previous theoretical and experimental studies on surface evolution during sputter deposition, and discussed in terms of the competition between surface diffusion and shadowing.
Shear reduction of collisional transport: Experiments and theory
Authors: Driscoll C. F. , Anderegg F. , Dubin D. H. E. , Jin D.-Z. , Kriesel J. M. , Hollmann E. M. , O’Neil T. M.
Published: July 2002

Experiments and theory on collisional diffusion and viscosity in quiescent single-species plasmas demonstrate enhanced transport in the two-dimensional (2D) bounce-averaged regime, limited by shear in the plasma rotation. For long plasma columns, the measured diffusion agrees quantitatively with recent theories of three-dimensional long-range E×B drift collisions, and is substantially larger than predicted for classical velocity-scattering collisions. For short plasmas, diffusion is observed to be enhanced by Nb, the number of times a thermal particle bounces axially before being separated by shear. Equivalently, recent theory in the 2D bounce-averaged regime shows how diffusion decreases with increasing shear, generalizing the zero-shear perspective which gives Bohm diffusion. Viscosity is similarly enhanced in the 2D regime, but there is presently only qualitative agreement with theory. These results apply to both non-neutral and neutral plasmas, and provide the first rigorous analysis of shear reduction of transport in a paradigmatic system.
Single-electron transistors operating at room temperature, fabricated utilizing nanocrystals created by focused-ion beam
Authors: Kim T. W. , Choo D. C. , Shim J. H. , Kang S. O.
Published: July 2002

A focused-ion-beam (FIB) technique utilizing both lithographic and nanoparticle formation processes has been introduced to fabricate a single-electron transistor (SET) that can operate at room temperature. The results for the drain current as a function of the gate voltage at different source voltages at room temperature clearly show Coulomb oscillations indicative of Coulomb-blockade effects. These results indicate that SETs operating at room temperature, fabricated utilizing the FIB technique, hold promise for potential applications in ultra-high-density memory devices.
Modeling and simulation design of advanced Cu alloy interconnects
Authors: Liu Chun-Li
Published: July 2002

We use modeling and simulation tools to determine the beneficial additives or dopants to Cu interconnect. We have designed a virtual simulation procedure to cover several important aspects in screening a potential dopant to Cu with the assumption that grain-boundary (GB) diffusion is dominant for Cu electromigration performance. The procedure investigates dopant segregation to GB, bulk diffusion, dopant and Cu self-diffusion at the GB, and the effect of the dopant’s presence on Cu diffusion at the GB. Defect formation and migration energies as well as activation energies were calculated using the state of the art ab initio method. Two primary mechanisms for a dopant to be effective were identified, namely, dopant blocking and dopant dragging mechanisms. For dopant blocking mechanism the desired dopants occupy the GB interstitial sites and block the fast diffusion pathway for Cu. In the case where Cu atoms occupy the GB interstitial sites, the desired dopants segregate to the nearby substitutional sites and drag the fast diffusing Cu. Early experimental results have confirmed model prediction for several dopants identified so far. The mean time to failure has increased more than 60% with a dopant concentration as low as 0.01 at. % in Cu and the resistivity increase can be controlled below 15% compared to undoped Cu. We demonstrate that modeling and simulation have become valuable alternatives to experiment for design of advanced materials systems for technology research and development.
Pulsed electrical breakdown of a void-filled dielectric
Authors: Anderson R. A. , Lagasse R. R. , Schroeder J. L.
Published: July 2002

We report breakdown strengths in a void-filled dielectric material, epoxy containing 48 vol % hollow glass microballoon filler, which is stressed with unipolar voltage pulses of the order of 10 μs duration. The microballoon voids had mean diameters of approximately 40 μm and contained SO2 gas at roughly 30% atmospheric pressure. This void-filled material displays good dielectric strength (of the order of 100 kV mm−1) under these short-pulse test conditions. Results from a variety of electrode geometries are reported, including arrangements in which the electric stress is highly nonuniform. Conventional breakdown criteria based on mean or peak electric stress do not account for these data. A statistics-based predictive breakdown model is developed, in which the dielectric is divided into independent, microballoon-sized “discharge cells” and the spontaneous discharge of a single cell is presumed to launch full breakdown of the composite. We obtain two empirical parameters, the mean and standard deviation of the spontaneous discharge field, by fitting breakdown data from two electrode geometries having roughly uniform fields but with greatly differing volumes of electrically stressed material. This model accounts for many aspects of our data, including the inherent statistical scatter and the dependence on the stressed volume, and it provides informative predictions with electrode geometries giving highly nonuniform fields. Issues related to computational spatial resolution and cutoff distance are also discussed.
Local existence proofs for the boundary value problem for static spherically symmetric Einstein–Yang–Mills fields with compact gauge groups
Authors: Oliynyk Todd A. , Künzle H. P.
Published: July 2002

We prove local existence and uniqueness of static spherically symmetric solutions of the Einstein–Yang–Mills (EYM) equations for an arbitrary compact semisimple gauge group in the so-called regular case. By this we mean the equations obtained when the rotation group acts on the principal bundle on which the Yang–Mills connection takes its values in a particularly simple way (the only one ever considered in the literature). The boundary value problem that results for possible asymptotically flat soliton or black hole solutions is very singular and just establishing that local power series solutions exist at the center and asymptotic solutions at infinity amounts to a nontrivial algebraic problem. We discuss the possible field equations obtained for different group actions and solve the algebraic problem on how the local solutions depend on initial data at the center and at infinity.