bluuebunny/crossref_metadata_2025 huggingface dataset:
Second-order stability analysis of the Vlasov–Poisson equations in the planar magnetron
Authors: Kaup D. J. , Roy Choudhury S. , Thomas G. E.
Published: July 2002

The second-order stability analysis of the Vlasov–Poisson equations for high-density, finite-temperature plasmas in the presence of inhomogeneous crossed fields and density gradients is carried out. The analysis is more general than earlier studies of high-beta inhomogeneous plasmas in that various approximations employed therein, such as the local approximation Jeffreys–Wentzel–Kramers–Brillouin, and low-frequency and small-wavelength restrictions, are not employed here. It is shown that the evolution of the particle guiding centers on the slow-time scale corresponds to an electron motion toward the anode at and above the diocotron resonance. This is in qualitative agreement with what has been observed during convective cell formation in recent particle simulations performed by Mission Research Corporation [Technical Digest—International Conference on Electron Devices (IEEE, New York, 1985), pp. 180–183]. The behavior predicted by the Vlasov–Poisson formalism is shown to be somewhat different from that obtained from the cold-fluid formalism, where the density profile for the latter evolved only at the positions of the diocotron and magnetron resonances.
Weak solutions of the three-dimensional vorticity equation with vortex singularities
Authors: Winckelmans G. , Leonard A.
Published: July 2002

The use of a modified scheme for the dynamics of vortex singularities is shown to lead to a weak solution of the three-dimensional inviscid incompressible vorticity equation.
Singular vortex systems and weak solutions of the Euler equations
Authors: Greengard Claude , Thomann Enrique
Published: July 2002

The relationship of point vortex and vorton (and related) systems to weak solutions of the Euler equations is discussed. It is observed that point vortices are more sensible models of fluid flow than are vortons because the latter cannot arise as nice limits of solenoidal vorticity fields.
The spreading of drops with intermolecular forces
Authors: Hocking L. M.
Published: July 2002

The spreading of a thin drop of fluid that partially wets a plane surface is calculated. Capillarity, slip, and intermolecular forces are included in the model, and it is shown that a complete solution is possible without having to exclude the vicinity of the contact line and without having to assume the dynamic behavior of the contact angle. An equation is found for the evolution of the drop radius; when the drop is not close to its equilibrium radius, the spreading law has the expected one-tenth power dependence on the radius, with a coefficient which is determined as a function of the intermolecular forces and the slip coefficient. The calculation is performed for small static contact angles and also for the limit when this angle is zero.
On secondary vortices in the cylinder wake
Authors: Brede M. , Eckelmann H. , Rockwell D.
Published: July 2002

The wake of a circular cylinder is investigated for Reynolds numbers between 160 and 500 by means of particle image velocimetry (PIV). For the first time cross-stream velocity fields are determined for two classes of secondary vortices (A-mode and B-mode). The circulation of the A-mode secondary vortices in this plane is approximately twice the circulation of the B-mode secondary vortices. The spanwise wavelength of the secondary vortices is four to five cylinder diameters for the A-mode and one diameter for the B-mode. The spatio-temporal development of the wake is analyzed by acquiring a time sequence of PIV images covering several Kármán periods. On the basis of the vorticity field, the A- and B-modes can be identified as topologically different vortex structures. Two vortex models are developed to explain the differences between these modes.
A theoretical analysis of the onset of surface-tension-driven convection in a horizontal liquid layer cooled suddenly from above
Authors: Kang Kyoung Hoon , Choi Chang Kyun
Published: July 2002

The onset of surface-tension-driven convection in a horizontal liquid layer cooled suddenly from above is analyzed by using linear stability theory. To obtain the critical condition to mark the onset of instability in the form of regular cellular motion the time domain is divided into two. With small time the basic temperature profile is strongly nonlinear and therefore the propagation theory we have developed is employed. Based on the propagation theory a new set of stability equations are derived and their scale analysis is discussed. It is found that the fluid layer is more stable with decreasing the Biot number and the Prandtl number. With large time the frozen-time model is applied. In this time domain the stability criteria are independent of the Prandtl number. By connecting the predictions from the above two models the overall stability criteria are constructed. The interesting role of the Biot number on the critical condition is discussed in detail.
Nonmagnetic particles in magnetic fluid: Reversal dynamics under rotating field
Authors: Kashevsky B. E.
Published: July 2002

The simultaneous action of magnetic and hydrodynamic forces responsible for the dynamic behavior of nonmagnetic particle assemblies within magnetic colloid (magnetic fluid) is investigated for the case of a pair of spherical particles in a rotating magnetic field. The antisymmetric viscous stresses that exist in magnetic fluid due to colloidal ferroparticle internal rotation are taken into account alongside particle–particle magnetic and hydrodynamic interactions. On each particle the internal rotation produces a couple acting in the direction opposite to the field rotation direction. The hydrodynamic interaction among rotating particles results in a global couple which acts to meet the rotating field and opposes the couple of magnetic particle–particle interaction. The antisymmetric stresses grow with field frequency, and at some frequency value the pair-field co-rotation gives way to the counter-rotation. It is shown that the pair motion is sensitive to the particle surface conditions, because of fast growth of lubricant forces between the approaching particles. The available experimental data are discussed.
Instabilities in plane Poiseuille flow due to the combined effects of stratification and viscosity
Authors: Ng B. S. , Reid W. H.
Published: July 2002

The stability of stably stratified plane Poiseueille flow is considered in the case when the buoyancy frequency is given by N2(z)=z4. It is well known that the Tollmien–Schlichting mode of instability for this flow becomes completely stable as the Richardson number Ri↑0.222. For larger values of Ri, however, new modes of instability appear which are of the “center” or “fast” type. These new modes are due to the combined effects of stratification and viscosity which are strongly destabilizing and lead to a minimum critical Reynolds number of about 124. Detailed stability characteristics in the form of curves of marginal stability are presented for various values of the Richardson and Prandtl numbers.
The instability growth leading to a liquid sheet breakup
Authors: Lozano Antonio , Garcı́a-Olivares Antonio , Dopazo César
Published: July 2002

The instability growth leading to a liquid sheet breakup has been studied with the objective of improving the understanding of the fundamental mechanisms of atomization. A three-dimensional Lagrangian code based on vortex dynamics methods has been implemented to track the air/liquid interfaces treated as inviscid vortex sheets. The results of these numerical simulations indicate a possible explanation for the presence of transverse and longitudinal filaments observed in liquid sheet air-assisted atomization experiments.
Behavior of drops in contact with pool surfaces of different liquids
Authors: Monti R. , Savino R. , Lappa M. , Tempesta S.
Published: July 2002

The behavior of small liquid drops, hanging from a circular disk and approaching a pool surface of the same liquid at different temperatures, is studied experimentally and numerically. The experiments show that if isothermal conditions prevail the drop is immediately engulfed by the liquid. On the contrary, if the temperature of the drop is sufficiently larger or sufficiently smaller than the temperature of the liquid surface, this engulfment is prevented even if the drop is pressed on the liquid surface and “enters” the liquid pool. A number of experiments have been carried out on silicone oils (with different viscosities). At the same time the problem was studied numerically with the assumption that a thin air film is formed between the drop and the liquid bath surface, due to the entrainment of the surrounding air caused by the Marangoni flow; the pressure in the air film balances the pressure necessary to keep the drop submerged in the liquid bath. The critical temperature differences for the drop engulfment are determined experimentally. A parametric numerical analysis is performed to determine the influence of the viscosities of the liquids. The experimental findings are correlated by computed critical temperature differences. A sufficient agreement is found between numerical and experimental results.
Viscous fingering in a shear-thinning fluid
Authors: Lindner Anke , Bonn Daniel , Meunier Jacques
Published: July 2002

We study the Saffman–Taylor instability in a rectangular Hele-Shaw cell. The driven fluid is a dilute (or semidilute) polymer solution, with a viscosity that exhibits shear thinning. Other non-Newtonian properties such as elastic effects are negligible under the present experimental conditions; the system thus allows for separate investigation of the influence of shear thinning on the instability. The experiments show that, for weak shear-thinning, the results for the width of the fingers as a function of the capillary number collapse onto the universal curve for Newtonian fluids, provided the shear-thinning viscosity is used to calculate the capillary number. For stronger shear thinning, narrower fingers are found. The experiment allows also for a study of the applicability of Darcy’s law to shear thinning fluids. For Newtonian fluids, this law gives the finger velocity as a function of the pressure gradient. For weakly shear-thinning fluids, we find that an effective Darcy’s law, in which the constant viscosity is replaced by the shear-thinning viscosity, gives good agreement with the experiments. For stronger shear thinning, the predictions from the effective Darcy’s law deteriorate. Satisfactory agreement with experimental data can be obtained when using a “shear-thinning” Darcy’s law, which can be derived using a power law model for the shear rate dependence of the viscosity.
On the characteristics of a spark generated shock wave
Authors: Aithal S. M. , Subramaniam V. V.
Published: July 2002

Recent experiments involving shock waves propagating through weakly ionized plasmas have raised questions regarding interpretation of the experimental results. In aid of analyzing these experimental results, shock waves initiated by a simulated spark and their subsequent propagation in a cylindrical tube containing argon initially at 30 Torr and 300 K, have been analyzed numerically in this paper. Numerical solutions to the compressible Navier–Stokes equations are considered under the four conditions of induced flow (a) without wall friction, (b) with wall friction, (c) with wall friction and purely axial thermal gradients, and (d) with wall friction and both axial and radial thermal gradients. Although plasma processes have not been simulated, it is found that the effects of wall shear and thermal gradients alone are sufficient to explain most of the experimental observations. This work represents a first step in the analysis of this problem before plasma effects can be modeled.
Stability of oblique modulation of ion-acoustic waves in a multicomponent plasma
Authors: Mishra M. K. , Chhabra R. S. , Sharma S. R.
Published: July 2002

The stability of oblique modulation of ion-acoustic waves in a collisionless plasma consisting of two cold-ion species with different masses, concentrations, and charge states, and hot isothermal electrons is studied. Using the Krylov–Bogoliubov–Mitropolosky (KBM) perturbation technique, a nonlinear Schrödinger equation governing the slow modulation of the wave amplitude, is derived for the system. It is found that the presence of second-ion species significantly changes the instability domain in the k-φ plane. The effect of charge state, concentration, and mass of second-ion species on the modulational instability is discussed in detail. The predictions of the theory are found to be in good agreement with the experimental observations.
Langmuir turbulence in moderately magnetized space plasmas*
Authors: Newman D. L. , Goldman M. V. , Ergun R. E.
Published: July 2002

Beam-driven Langmuir turbulence is studied in two moderately magnetized (Ωe≊ωe) space-plasma regimes: regions of the lower solar corona and the Earth’s auroral ionosphere. The turbulence is modeled using modified Zakharov equations, which are employed in two-dimensional numerical simulations. For coronal parameters, highly anisotropic coherent wave packets form and collapse when Ωe<ωe. By contrast, the turbulence is phase incoherent when Ωe≳ωe, as a result of change in the topology of the Langmuir dispersion relation. In the auroral ionosphere, intense Langmuir waves (up to 500 mV/m) have been measured, in conjunction with field-aligned electron streams and nonthermal electron tails. Approximate agreement with high-time-resolution electric-field measurements, is found in the simulations. However, because of strong damping on nonthermal electrons, wave collapse is inhibited, irrespective of the ordering of Ωe and ωe.
Computer studies on plasmoid dynamics associated with the spontaneous fast reconnection mechanism
Authors: Ugai M.
Published: July 2002

The spontaneous fast reconnection model is applied to the plasmoid dynamics, and computer simulations are performed for a wide range of parameters. According to the fast reconnection development, the resulting strong (Alfvénic) plasma jet drives a large-scale plasmoid to swell and propagate. Once the plasmoid fully develops, the propagation speed Vp becomes almost constant. It is the J×B force, not the pressure-gradient force, that plays the dominant role on the plasmoid propagation. It is estimated that Vp∼0.8VA l0, where VA l0 is the Alfvén velocity measured in the magnetic field region ahead of the plasmoid. Along the backward-half plasmoid boundary a strong slow (almost switch-off) shock is formed, whereas along the forward-half boundary a rather weak slow shock stands. The plasma pressure is largely enhanced in the middle of the plasmoid, and the magnetic energy just outside the plasmoid is notably enhanced in the forward half because of the generator effect and is then distinctly reduced in the backward half by the strong motor effect. For the uniform resistivity model, any distinct plasmoid cannot be set up. It is argued that the spontaneous fast reconnection mechanism should be most applicable to catastrophic events observed in space plasmas.
Localized whistler eigenmodes in tokamaks
Authors: Kamelander G. , Kolesnichenko Ya. I.
Published: July 2002

The existence of radially well-localized whistler eigenmodes (WE) is shown. The location of WE modes may vary from the region close to the plasma edge to the central region, depending on the ratio of poloidal and toroidal wave numbers and on the plasma density profile.
Computer studies on the spontaneous fast reconnection mechanism in three dimensions
Authors: Ugai M. , Shimizu T.
Published: July 2002

The spontaneous evolution of fast reconnection is studied in three dimensions by extending (in the z direction) the previous two-dimensional model that considered only the x-y plane [M. Ugai, Phys. Fluids B 4, 2953 (1992)]. It is demonstrated that the reconnection development strongly depends on three-dimensional effects; only when the central current sheet is sufficiently long in the z direction, say more than a few times larger than the current sheet width, the fast reconnection mechanism fully develops by the self-consistent coupling between the global reconnection flow and the current-driven anomalous resistivity. In this case, the reconnection flow can grow so powerfully as to enhance the current density (the current-driven resistivity) locally near an X line; otherwise, such a vital reconnection flow cannot be caused. The resulting quasisteady fast reconnection mechanism is significantly confined in the z direction, where a strong (Alfvénic) plasma jet results from standing switch-off shocks; accordingly, a large-scale plasmoid is formed and propagates in the middle of the system. It is concluded that the well-known two-dimensional spontaneous fast reconnection model can reasonably be extended to three dimensions.
Magnetic electron drift vortex modes with the ion dynamics
Authors: Mirza Arshad M. , Shukla P. K.
Published: July 2002

A new dispersion relation for low-frequency magnetic electron drift modes (MEDV) in a nonuniform plasma is derived by incorporating the ion dynamics. It is shown that the ion dynamics introduces new classes of instabilities when the wave frequency either lies between the electron and ion plasmas frequencies or is close to the ion plasma frequency. The modified MEDV modes involving the ion motion can cause nonthermal particle as well as electron heat transports.
Quasipotential analysis for deriving the multidimensional Sagdeev potential equation in multicomponent plasma
Authors: Roychoudhury Rajkumar , Das G. C. , Sarma Jnanjyoti
Published: July 2002

The exact multidimensional Sagdeev potential is derived in a multicomponent plasma consisting of negative ions wherein a fraction of electrons is trapped in the potential well developed in the plasma. More precisely, the Sagdeev potential equation revisits the results stemming from the Kadomtsev–Petviashvili (K–P) equation deduceable by applying the reductive perturbation technique in plasma-acoustic wave dynamics. In the study we show that the multidimensional Sagdeev potential derived here yields the formation and propagation of solitons, as well as double layers in plasma, by using a new approach known as the tanh-method to solve out the soliton phenomena. It is seen that different ordering in φ, the electrical potential, yields different solitary wave solutions that agree with earlier observations.
Instabilities of Alfvén and magnetosonic waves in dusty cometary plasmas with an ion ring beam
Authors: Cramer N. F. , Verheest F. , Vladimirov S. V.
Published: July 2002

The effects of charged dust grains on the instabilities of electromagnetic waves in the solar wind plasma due to ring-beam cometary ion velocity distributions are considered. The waves are assumed to propagate parallel to the solar wind magnetic field. Collisionless cold plasma theory is used to derive the dispersion relations of linear waves in the solar wind, for a delta-function ring-beam velocity distribution of cometary water-group ions. The resonant and nonresonant instabilities are considered, and it is shown that the dust has its greatest effect on the nonresonant firehose instability at very long wavelength. The dust can cause an unstable dust-free firehose mode to be stable, and vice versa. It is also shown that as the frequency approaches the cometary ion cyclotron frequency, there is an upper limit to the wave number for the unstable firehose mode. For the left-hand polarized wave, the presence of dust increases this upper wave number limit, until the firehose mode merges with the resonant mode. The right-hand polarized firehose mode also has an upper wave number limit, which is almost independent of the presence of dust. It is shown that, although there is a high frequency resonant instability of the normal whistler mode induced by the ring beam, there is no corresponding instability of the dust-induced whistler-like mode at low frequencies.
Compressional ideal magnetohydrodynamics: Unstable global modes, stable spectra, and Alfvén eigenmodes in Wendelstein 7–X-type equilibria
Authors: Nührenberg Carolin
Published: July 2002

The code for the analysis of the stability of three-dimensional (3-D) equilibria (CAS3D) [C. Schwab, Phys. Fluids B 5, 3195 (1993)] now treats the full ideal magnetohydrodynamic (MHD) energy principle, i.e., it includes the fluid compression term and a physical kinetic energy, so that it allows for the computation of physical growth rates in fully 3-D MHD configurations. A study of the MHD stability properties of a configuration space representing the Wendelstein 7–X (W7–X) stellarator experiment [G. Grieger et al., Nucl. Fusion Suppl. (International Atomic Energy Agency, Vienna, 1991), Vol. 3, p. 525] shows that for unstable modes both the fluid compression and, as it was shown in previous work [C. Nührenberg, Phys. Plasmas 3, 2401 (1996)], the field compression vanish to a very good approximation. Points of marginal stability found for the, in principle, compressible modes and the, a priori, incompressible modes coincide. Also, this new code version has been used to study stable spectra and stable global perturbations in both tokamak and stellarator equilibria. The tokamak calculations recover all the typical properties of the stable MHD spectrum, e.g., the existence of spectral gaps and gap modes in the various continuum branches (sound and Alfvén); in the truly 3-D, low-shear, finite-β case similar results are obtained, exhibiting a strong interaction of sound and Alfvén branches in the lower part of the stable spectrum. The existence of helicity induced gaps and gap modes in 3-D cases could be demonstrated. The first results indicate that in these 3-D cases global, stable perturbations exist, but that they are damped, since they resonate with the continuum.
Laser-plasma interactions in long-scale-length plasmas under direct-drive National Ignition Facility conditions
Authors: Regan S. P. , Bradley D. K. , Chirokikh A. V. , Craxton R. S. , Meyerhofer D. D. , Seka W. , Short R. W. , Simon A. , Town R. P. J. , Yaakobi B. , Carroll J. J. , Drake R. P.
Published: July 2002

Laser-plasma interaction experiments have been carried out on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] under plasma conditions representative of the peak of a 1.5 MJ direct-drive laser pulse proposed for the National Ignition Facility (NIF). Plasmas have been formed by exploding 18–20 μm thick CH foils and by irradiating solid CH targets from one side, using up to 20 kJ of laser energy with phase plates installed on all beams. These plasmas and the NIF plasmas are predicted to have electron temperatures of 4 keV and density scale lengths close to 0.75 mm at the peak of the laser pulse. The electron temperature and density of the exploding-foil plasmas have been diagnosed using time-resolved x-ray spectroscopy and stimulated Raman scattering, respectively, and are consistent with predictions of the two-dimensional Eulerian hydrodynamics code SAGE [R. S. Craxton and R. L. McCrory, J. Appl. Phys. 56, 108 (1984)]. When the solid-target or exploding-foil plasmas were irradiated with an f/6 interaction beam at 1.5×1015 W/cm2, well above the NIF f/8 cluster intensity of ∼2×1014 W/cm2, stimulated Brillouin backscattering (SBS) was found to be completely inhibited. A conservative upper limit of direct-backscattered SRS was found to be ∼5% from the solid targets. SRS and SBS are thus unlikely to have a significant impact on target performance at the peak of the NIF direct-drive laser pulse.
Dynamics of spherical tokamak plasma on the internal reconnection event
Authors: Mizuguchi Naoki , Hayashi Takaya , Sato Tetsuya
Published: July 2002

Nonlinear magnetohydrodynamic (MHD) simulations are executed to investigate the dynamical behavior of the relaxation phenomenon observed in spherical tokamak (ST) plasma that is known as the Internal Reconnection Event (IRE). The simulation results successfully reproduce several key features of IRE, and the physical mechanisms are revealed. A sudden collapse of the pressure profile takes place as a result of a nonlinear time development of a pressure-driven instability. A magnetic reconnection induced between the internal and the external magnetic field is found to play a crucial role in determining the nature of the overall process, namely, the rapid expulsion of the plasma heat energy due to the pressure imbalance along the reconnected field lines, and the large distortion in the overall shape. The resultant deformations in overall shape of the plasma are in good agreement with the experimental observations.
Adiabatic expansion acceleration mechanism of superfast jets in the spontaneous fast magnetic reconnection model
Authors: Shimizu T. , Ugai M.
Published: July 2002

In contrast to the Petschek reconnection model, the plasma outflow jet in front of the plasmoid associated with the spontaneous fast reconnection model is found to exceed steadily the Alfvén velocity measured in the upstream magnetic field region. According to two-dimensional magnetohydrodynamic simulations, the final velocity of the plasma jet is observed to be superfast and can reach 1.4 times of the Alfvén velocity, which is maintained until the jet encounters a fast shock generated in front of the plasmoid. On the basis of the Rankine Hugoniot relation and the Bernoulli equation, it is theoretically found that the superfast plasma jet generated by slow shocks associated with the reconnection process is effectively accelerated beyond the Alfvén velocity by the adiabatic expansion of the plasma jet without any magnetic effect. In the plasma accelerations, the initial plasma acceleration caused in the slow shock is consistent with that of the Petschek reconnection model, but the subsequent plasma acceleration caused by the adiabatic expansion is not considered in his model. In association with the new acceleration mechanism, one pair of low-pressure regions emerges in the upstream magnetic field region. The generation of the low-pressure regions indicates that the significant adiabatic expansion results from the distortion of the surrounding magnetic field lines associated with the swelling plasmoid.
Elastic relaxation in transmission electron microscopy of strained-layer superlattices
Authors: Gibson J. M. , Hull R. , Bean J. C. , Treacy M. M. J.
Published: July 2002

We demonstrate that thin, cross-sectioned transmission electron microscopy samples from strained-layer superlattices undergo elastic relaxation such that the local lattice parameter modulation amplitude can be reduced by a large fraction. Relaxation is dependent on the ratio of the superlattice wavelength to the local sample thickness and is demonstrated experimentally for molecular beam expitaxially grown GexSi1−x superlattices both by selected-area diffraction and high-resolution electron microscopy. The results can be qualitatively explained by a simple linear elasticity theory model. Reports of anomalies in the elastic properties of semiconductor superlattices from electron microscopy can be resolved. Evidence is also presented for expected lattice plane bending due to relaxation which can cause strong diffraction contrast. This thin-sample ‘‘artefact’’ allows unexpectedly weak strain fields to be imaged and permits probing of local elastic properties of individual layers within a superlattice. Similar relaxation effects occur for more general, nonperiodic, elastically strained thin samples, such as the important case of single interfaces.
Abrikosov vortex memory
Authors: Miyahara Kazunori , Mukaida Masashi , Hohkawa Kohji
Published: July 2002

A nondestructive readout random access memory (RAM) cell based on the use of Abrikosov vortices in thin-film type-II superconductors is newly designed and experimentally tested. The cell occupies an area of 30×60 μm2 with a 5-μm design rule. Proper memory cell operation is achieved in the so-called 1, −1 mode using the shift saturation effect, which occurs on the shift value characteristics of the sense gate threshold curves as a function of the write currents. This is the first Abrikosov vortex RAM cell to operate properly. The write current level has been reduced one order of magnitude using a Pb/In/Au film co-evaporated at 90 K for the vortex storage region.
Composition dependence of Au/InxAl1−xAs Schottky barrier heights
Authors: Lin C. L. , Chu P. , Kellner A. L. , Wieder H. H. , Rezek Edward A.
Published: July 2002

The surface barrier heights φbn and room-temperature band gaps Eg of Si-doped InxAl1−xAs layers grown by molecular beam epitaxy on n-type (100) oriented InP substrates have been determined as a function of composition with capacitance versus voltage, internal photoemission, photoluminescence, and double-crystal x-ray rocking curve measurements for 0.45<x<0.55. The results indicate that Eg and φbn are linear functions of x; they also suggest that φbn (0.78)=0 and, for x>0.78, n-type surfaces might be accumulated and p-type surfaces are likely to be inverted.
Motion of small gold clusters in the electron microscope
Authors: Williams Peter
Published: July 2002

The rapid motion of small (∼1000 atom) gold clusters observed with atomic resolution in the electron microscope is argued to be the result of brief melting events, triggered by the Auger decay of an M-shell vacancy created in a gold atom within the cluster. Each molten episode lasts ∼10 ps, too short for noticeable evaporation. The observed frequency of motion events is shown to be consistent with ionization cross sections for the gold M shell and the electron current densities employed. It is argued from the lack of observable evaporation that the steady-state temperature of the clusters is ≤1100 K, well below the melting point. The model correctly predicts the cessation of motion at a particle diameter ∼8 nm, when a single Auger decay can no longer melt the cluster.
Ni-InP reaction: Formation of amorphous and crystalline ternary phases
Authors: Sands T. , Chang C. C. , Kaplan A. S. , Keramidas V. G. , Krishnan K. M. , Washburn J.
Published: July 2002

Three previously unreported ternary phases of the type NixInP have been observed to be the primary reaction products of the Ni/InP reaction. The first phase, amorphous NixInP(x≂2.7), forms at the Ni/InP interface by a solid-state amorphization process at low temperatures (T≲200 °C). Amorphous NixInP crystallizes at ∼300 °C to form a hexagonal NixInP phase (a0=0.412 nm and c0=0.483 nm) with a similar composition. A third ternary phase with nominal composition Ni2InP (monoclinic, a0=0.681 nm, b0=0.529 nm, c0=1.280 nm, and β=95°) nucleates at higher temperatures and is the final reaction product. This final phase is stable at temperatures up to 500 °C in samples capped with SiO2.
Characterization of cesium diffusion in silicon dioxide films using backscattering spectrometry
Authors: Fishbein Bruce J. , Plummer James D.
Published: July 2002

The diffusion of ion-implanted cesium in thermally grown SiO2 has been examined in the temperature range 700–1000 °C using Rutherford backscattering spectrometry (RBS). Silicon samples were oxidized and implanted with 5×1014 cm−2 133Cs at 145 keV and then annealed in nitrogen using a rapid thermal system for times ranging from several seconds to several hours. Cesium concentration profiles were measured using RBS for each anneal time and temperature. By observing the decay in the peak cesium concentration as a function of anneal time, a diffusivity of DCs =0.50 exp(−2.9eV/kT) cm2/s has been obtained. Other diffusion effects including cesium pile-up at the oxide interfaces and outdiffusion of cesium into the ambient have been studied qualitatively.
Temperature dependence of viscosity of molten GaAs by an oscillating cup method
Authors: Kakimoto Koichi , Hibiya Taketoshi
Published: July 2002

A viscosity measurement system for molten semiconductors has been established by adopting an oscillating cup method. The temperature dependence of viscosity for molten GaAs in the temperature range from near the melting point up to 1480 °C was obtained. The viscosity and activation energy of molten GaAs showed a remarkable increase in the vicinity of the melting point (1238 °C). The viscosity of molten GaAs decreases with an increase in temperature above 1320 °C, with an activation energy of about 0.28 eV.
Nonlinear coupling of waveguide modes
Authors: Silberberg Yaron , Stegeman George I.
Published: July 2002

We show that the nonlinear coupling between two waveguide modes can lead to both power exchange and nonlinear phase shifting between the modes. New applications of nonlinear phase shifting to all-optical devices are described.
Carrier lifetime versus ion-implantation dose in silicon on sapphire
Authors: Doany F. E. , Grischkowsky D. , Chi C.-C.
Published: July 2002

We have measured the dependence of the free-carrier lifetime on O+ ion-implantation dose in silicon-on-sapphire. At low implant doses, the carrier trapping rate increased linearly with the trap density introduced by ion implantation. At doses above 3×1014 cm−2 the measured carrier lifetime reached a limit of 600 fs.
Direct imaging of 13-Å-diam Au clusters using scanning tunneling microscopy
Authors: Baró A. M. , Bartolome A. , Vazquez L. , García N. , Reifenberger R. , Choi E. , Andres R. P.
Published: July 2002

Controlled size clusters of Au with a diameter of 13 Å, prepared using a multiple expansion cluster source, have been supported on highly oriented pyrolytic graphite and observed with a scanning tunneling microscope. A reliable constant-current signature for a metallic cluster supported on a graphite substrate is identified. Images of the supported Au clusters are found to exhibit a narrow size distribution and a diameter which is in close agreement with the diameter predicted from conditions in the multiple expansion cluster source.
1.3-μm light-emitting diode from silicon electron irradiated at its damage threshold
Authors: Canham L. T. , Barraclough K. G. , Robbins D. J.
Published: July 2002

We report 77-K electroluminescence from an irradiated carbon-rich silicon diode that has an internal quantum efficiency more than 103 times higher than that of band-to-band recombination in an unirradiated, but otherwise identical diode. This is achieved by creating optically active Cs-SiI-Cs complexes with room-temperature electron bombardment at an energy between the displacement thresholds for single vacancy and divacancy formation. Under these irradiation conditions, it is possible to create a high concentration of radiative defects without gross degradation of the diode’s electrical characteristics. The technique could provide very large scale integration-compatible silicon light-emitting diodes for 1.3–1.6 μm all-silicon integrated optics.
Generation of divacancies in tin-doped silicon
Authors: Svensson B. G. , Svensson J. , Lindström J. L. , Davies G. , Corbett J. W.
Published: July 2002

Czochralski-grown tin-doped silicon samples have been irradiated by 2 MeV electrons at room temperature. The concentration of divacancies is studied as a function of bombardment dose, and as a function of temperature during a subsequent isochronal annealing. The results are compared with that for a control sample and the role of direct generation versus vacancy-vacancy pairing for divacancy formation is discussed.
Hole photoionization cross sections of EL2 in GaAs
Authors: Silverberg P. , Omling P. , Samuelson L.
Published: July 2002

The spectral dependence of the hole photoionization cross section σ0p of EL2 in GaAs has been determined in absolute numbers at T=78 and 295 K. From simultaneous measurements of the electron photoionization cross section σ0n, accurate values of the photon energies and the cross sections at which σ0n=σ0p could be obtained. These data are of importance for rapid and accurate determination of concentration and charge states of EL2 in GaAs, e.g., in wafer mapping applications.
High-resolution capacitance measurement and potentiometry by force microscopy
Authors: Martin Yves , Abraham David W. , Wickramasinghe H. Kumar
Published: July 2002

We demonstrate the usefulness and high sensitivity of the atomic force microscope (AFM) for imaging surface dielectric properties and for potentiometry through the detection of electrostatic forces. Electric forces as small as 10−10 N have been measured, corresponding to a capacitance of 10−19 farad. The sensitivity of our AFM should ultimately allow us to detect capacitances as low as 8×10−22 F. The method enables us to detect the presence of dielectric material over Si, and to measure the voltage in a p-n junction with submicron spatial resolution.
Electric field control of optical second-harmonic generation in a quantum well
Authors: Tsang L. , Ahn D. , Chuang S. L.
Published: July 2002

We present theoretical predictions of the second-harmonic susceptibility due to an intersubband transition within the conduction band of a quantum well in an external applied electric field. The asymmetry of the quantum well due to the electric field accounts for the nonvanishing of the second-order susceptibilities. It is shown that for moderate values of an applied electric field of 10–70 kV/cm, the second-harmonic susceptibility is generally 10–100 times larger than that of bulk GaAs. Furthermore, this procedure of second-harmonic generation can be controlled by an external modulating voltage.
Electron-spin-resonance study of defects in plasma-enhanced chemical vapor deposited silicon nitride
Authors: Jousse D. , Kanicki J. , Krick D. T. , Lenahan P. M.
Published: July 2002

Silicon nitride films with low defect densities can be prepared by plasma-enhanced chemical vapor deposition with ammonia-to-silane ratios adjusted to obtain N-rich materials. An electron-spin-resonance signal with g value close to 2.002 is reported for such materials, and the defect is identified as a Si atom coordinated to three N atoms as observed earlier in high-temperature chemical vapor deposited silicon nitride. Densities below 1016 cm−3 are measured for substrate temperatures above 350 °C for the first time. The distribution of defects is uniform through the film thickness. A surface defect density of 1012 cm−2 has also been found in films deposited at 250 °C.
Latent sensitivity to Fas-mediated apoptosis after CD40 ligation may explain activity of CD154 gene therapy in chronic lymphocytic leukemia
Authors: Chu Peter , Deforce Dieter , Pedersen Irene M. , Kim Youngsoo , Kitada Shinichi , Reed John C. , Kipps Thomas J.
Published: July 2002

Patients with chronic lymphocytic leukemia (CLL) treated with adenovirus (Ad)-CD154 (CD40L) gene therapy experience reductions in leukemia cell counts and lymph node size associated with induction of the death receptor Fas (CD95). CD4 T cell lines can induce apoptosis of CD40-activated CLL cells via a CD95 ligand (CD95-L)-dependent mechanism. To examine whether CD95-L was sufficient to induce cytolysis of CD40-activated CLL cells, we used Chinese hamster ovary cells transfected with CD95-L as cytotoxic effector cells. CD40-activated CLL cells were initially resistant to CD95-mediated apoptosis despite high-level expression of CD95. However, after 72 h, CLL cells from seven of seven patients became increasingly sensitive to CD95-mediated apoptosis. This sensitivity correlated with a progressive decline in Flice-inhibitory protein (FLIP), which was induced within 24 h of CD40 ligation. Down-regulation of FLIP with an antisense oligonucleotide or a pharmacologic agent, however, was not sufficient to render CLL cells sensitive to CD95-mediated apoptosis in the 24–72 h after CD40 activation. Although the levels of pro-Caspase-8 appeared sufficient, inadequate levels of Fas-associated death domain protein (FADD) and DAP3 may preclude assembly of the death-inducing signaling complex. Seventy-two hours after CD40 ligation, sensitivity to CD95 and a progressive increase in FADD and DAP3 were associated with the acquired ability of FADD and FLIP to coimmunoprecipitate with the death-inducing signaling complex after CD95 ligation. Collectively, these studies reveal that CD40 ligation on CLL B cells induces a programmed series of events in which the cells initially are protected and then sensitized to CD95-mediated apoptosis through shifts in the balance of the anti- and proapoptotic proteins FLIP and FADD.
Ability of the hydrophobic FGF and basic TAT peptides to promote cellular uptake of recombinant Cre recombinase: A tool for efficient genetic engineering of mammalian genomes
Authors: Peitz Michael , Pfannkuche Kurt , Rajewsky Klaus , Edenhofer Frank
Published: July 2002

Conditional mutagenesis is a powerful tool to analyze gene functions in mammalian cells. The site-specific recombinase Cre can be used to recombine loxP-modified alleles under temporal and spatial control. However, the efficient delivery of biologically active Cre recombinase to living cells represents a limiting factor. In this study we compared the potential of a hydrophobic peptide modified from Kaposi fibroblast growth factor with a basic peptide derived from HIV-TAT to promote cellular uptake of recombinant Cre. We present the production and characterization of a Cre protein that enters mammalian cells and subsequently performs recombination with high efficiency in a time- and concentration-dependent manner. Histidine-tagged Cre recombinase transduced inefficiently unless fused to a nuclear localization signal (NLS). Fusion of NLS-Cre to the fibroblast growth factor transduction peptide did not improve the transducibility, whereas fusion with the TAT peptide significantly enhanced cellular uptake and subsequent recombination. More than 95% recombination efficiency in fibroblast cells, as well as murine embryonic stem cells, was achieved after His-TAT-NLS-Cre transduction. Efficient recombination could also be obtained in primary splenocytesex vivo. We expect that application of His-TAT-NLS-Cre, which can be produced readily in large quantities from a bacterial source, will expand our abilities to manipulate mammalian genomes.
Focal loss of the glutamate transporter EAAT2 in a transgenic rat model of SOD1 mutant-mediated amyotrophic lateral sclerosis (ALS)
Authors: Howland David S. , Liu Jian , She Yijin , Goad Beth , Maragakis Nicholas J. , Kim Benjamin , Erickson Jamie , Kulik John , DeVito Lisa , Psaltis George , DeGennaro Louis J. , Cleveland Don W. , Rothstein Jeffrey D.
Published: July 2002

Transgenic overexpression of Cu+2/Zn+2superoxide dismutase 1 (SOD1) harboring an amyotrophic lateral sclerosis (ALS)-linked familial genetic mutation (SOD1G93A) in a Sprague–Dawley rat results in ALS-like motor neuron disease. Motor neuron disease in these rats depended on high levels of mutant SOD1 expression, increasing from 8-fold over endogenous SOD1 in the spinal cord of young presymptomatic rats to 16-fold in end-stage animals. Disease onset in these rats was early, ≈115 days, and disease progression was very rapid thereafter with affected rats reaching end stage on average within 11 days. Pathological abnormalities included vacuoles initially in the lumbar spinal cord and subsequently in more cervical areas, along with inclusion bodies that stained for SOD1, Hsp70, neurofilaments, and ubiquitin. Vacuolization and gliosis were evident before clinical onset of disease and before motor neuron death in the spinal cord and brainstem. Focal loss of the EAAT2 glutamate transporter in the ventral horn of the spinal cord coincided with gliosis, but appeared before motor neuron/axon degeneration. At end-stage disease, gliosis increased and EAAT2 loss in the ventral horn exceeded 90%, suggesting a role for this protein in the events leading to cell death in ALS. These transgenic rats provide a valuable resource to pursue experimentation and therapeutic development, currently difficult or impossible to perform with existing ALS transgenic mice.
IL-18 binding protein increases spontaneous and IL-1-induced prostaglandin production via inhibition of IFN-γ
Authors: Reznikov Leonid L. , Kim Soo-Hyun , Westcott Jay Y. , Frishman Jordan , Fantuzzi Giamila , Novick Daniela , Rubinstein Menachem , Dinarello Charles A.
Published: July 2002

IL-18 shares with IL-1 the same family of receptors and several identical signal transduction pathways. Because of these similarities, IL-18 was investigated for its ability to induce prostaglandin E2(PGE2) synthesis in human peripheral blood mononuclear cells (PBMC), a prominent, proinflammatory property of IL-1. IL-18 was highly active in PBMC by inducing the synthesis of the chemokine IL-8; however, no induction of PGE2synthesis nor cyclooxygenase type-2 gene expression was observed in PBMC stimulated with IL-18. In the same cultures, IL-1β induced a 12-fold increase in PGE2. Although IL-1β-induced IL-8 synthesis was augmented 3-fold by IL-18, IL-18 suppressed IL-1β-induced PGE2production by 40%. The suppressive effect of IL-18 on PGE2production was mediated by interferon (IFN)-γ because anti-human IFN-γ-antibody prevented IL-18-induced reduction in PGE2. Consistent with these observations, IL-12, a known inducer of IFN-γ, augmented IL-1β-induced IFN-γ but suppressed IL-1β-induced PGE2by 75%. IL-18 binding protein (IL-18BP) is a naturally occurring and specific inhibitor of IL-18. When recombinant IL-18BP was added to PBMC cultures, unexpectedly, spontaneous PGE2production increased. PGE2production was also increased by the addition of IL-18BP to PBMC stimulated with either IL-1β or IL-12 and also in whole blood cultures stimulated withStaphylococcus epidermidis. These studies demonstrate that IL-18BP decreases endogenous IL-18 activity by reducing IFN-γ-mediated responses.
Ligand binding to somatostatin receptors induces receptor-specific oligomer formation in live cells
Authors: Patel Ramesh C. , Kumar Ujendra , Lamb Don C. , Eid John S. , Rocheville Magalie , Grant Michael , Rani Aruna , Hazlett Theodore , Patel Shutish C. , Gratton Enrico , Patel Yogesh C.
Published: July 2002

Heptahelical receptors (HHRs) are generally thought to function as monomeric entities. Several HHRs such as somatostatin receptors (SSTRs), however, form homo- and heterooligomers when activated by ligand binding. By using dual fluorescent ligands simultaneously applied to live cells monotransfected with SSTR5 (R5) or SSTR1 (R1), or cotransfected with R5 and R1, we have analyzed the ligand receptor stoichiometry and aggregation states for the three receptor systems by fluorescence resonance energy transfer and fluorescence correlation spectroscopy. Both homo- and heterooligomeric receptors are occupied by two ligand molecules. We find that monomeric, homooligomeric, and heterooligomeric receptor species occur in the same cell cotransfected with two SSTRs, and that oligomerization of SSTRs is regulated by ligand binding by a selective process that is restricted to some (R5) but not other (R1) SSTR subtypes. We propose that induction by ligand of different oligomeric states of SSTRs represents a unique mechanism for generating signaling specificity not only within the SSTR family but more generally in the HHR family.