bluuebunny/crossref_metadata_2025 huggingface dataset:
Vortex rings generated by drops just coalescing with a pool
Authors: Shankar P. N. , Kumar Manoj
Published: July 2002

A detailed experimental study has been made of the formation and motion of vortex rings generated when drops of liquid are allowed to come into contact at zero velocity with a quiescent flat surface of the same liquid. In this case the vortex motion is driven principally by the drop surface energy. It is shown that in the case considered, the phenomenon depends on two dimensionless parameters alone, the reciprocal of a Bond number B−1 and a global Reynolds number Re where the velocity scale is based on the surface energy. Using a video camera, measurements were made, over the whole trajectory, of the ring velocity and diameter for a number of drop sizes and liquids. Vortex rings generated by small drops reach higher peak velocities but decelerate more rapidly than ones generated by large drops. The latter, however, may become turbulent before relaminarizing during deceleration. Although the most interesting finding of the investigation was the discovery of oscillations in the ring translational velocity and diameter during deceleration, the identification of the appropriate scaling laws and a suggestion regarding the early stages of vorticity generation should also be of interest.
Direct numerical simulation of rotating fluid flow in a closed cylinder
Authors: So/rensen Jens No/rkær , Christensen Erik Adler
Published: July 2002

Present numerical simulations of the transition scenario of a rotating fluid flow in a closed cylinder are presented, where the motion is created by a rotating lid. The numerical algorithm, which is based on a finite-difference discretization of the axisymmetric Navier-Stokes equations, is validated against experimental visualizations of both transient and stable periodic flows. The complexity of the flow problem is illuminated numerically by injecting flow tracers into the flow domain and following their evolution in time. The vortex dynamics appears as stretching, folding and squeezing of flow structures which wave along the contour of a central vortex core. The main purpose of the study is to clarify the mechanisms of the transition scenario and relate these to experiences known from other dynamical systems and bifurcation theory. The dynamical system was observed to exhibit up to three multiple solutions for the same Reynolds number, and to contain four discernible branches. The transition to strange attractor behavior was identified as a nontrivial Ruelle-Takens transition through a transient torus. The various solution branches of the rotating flow problem are illustrated by phase portraits and summarized on a frequency diagram.
Wave energy losses from a suspended container
Authors: Cooker M. J.
Published: July 2002

We model the incompressible, inviscid flow in a hydraulic jump in a closed horizontal tube. Downstream of the jump the flow fills the cross-section of the tube, and there the pressure is hydrostatic plus an additional pressure caused by the confines of the tube. We calculate the rate of energy loss by the jump and use this to account for the dramatic damping seen when an enclosed tube, partly-filled with water, swings freely like a pendulum.
A statistical formulation of the dynamic model
Authors: Germano M.
Published: July 2002

Two different large eddy simulations are statistically equivalent if they produce the same statistical representation. In this paper this basic definition is discussed in detail and a new formulation of the subgrid scale dynamic model that should force the statistical equivalence of different simulations at different resolution levels is proposed.
The Rayleigh–Taylor instability of viscous fluid layers
Authors: Elgowainy A. , Ashgriz N.
Published: July 2002

The effects of viscosity and surface tension on the nonlinear evolution of Rayleigh–Taylor instability of plane fluid layers are investigated. Full two-dimensional incompressible Navier–Stokes equations and exact boundary equations are solved simultaneously for a precise prediction of this phenomenon. An accurate flux line segment model (FLAIR) for fluid surface advection is employed for the interface reconstruction. The instability is characterized by three stages of development, which are defined based on the competition of the bubble and spike growth. This competition is responsible for the development of different spike and bubble morphologies and is decided based on geometrical factors, mainly the amplitude and wavelength of the initial perturbation, and on the fluid properties, mainly viscosity and surface tension. It is addressed and explained why the spike sometimes grows faster than the bubble, and vice versa. The cutoff and the most unstable wave numbers are identified numerically based on the Weber number. The effect of Weber and Reynolds numbers on the growth rate of instability and the role of viscosity in dragging the development of instability are also investigated.
Simulation of the Enskog equation à la Bird
Authors: Montanero José Marı́a , Santos Andrés
Published: July 2002

We present a modification of a recently proposed Monte Carlo simulation method [J. M. Montanero and A. Santos, Phys. Rev. E 54, 438 (1996)] to numerically solve the Enskog equation for a dense hard-sphere fluid. While in the original method momentum and energy are conserved by collisions only on average [as happens with Nanbu’s scheme of the direct simulation Monte Carlo (DSMC) method for solving the Boltzmann equation], the modified algorithm (that extends Bird’s null-time-counter version of the DSMC method) preserves exactly the conservation laws. Both methods are applied to fluids under shear for a wide range of densities. The agreement with the theoretical predictions for the shear viscosity and the viscometric coefficients is found to be excellent.
Turbulent cascades: Limitations and a statistical test of the lognormal hypothesis
Authors: Molchan G. M.
Published: July 2002

Multiplicative random cascade models were introduced in the 1970s to explain the intermittency of turbulent energy dissipation. The rigorous results in the multifractality of cascade measures recently derived by this author are used in two ways. (1) The statistical test for the Kolmogorov–Obukhov lognormal hypothesis (K62) is revised. Contrary to what is generally believed, we show that the K62 theoretical prediction is in good agreement with experimental data in the range (1,18) of the parameter p (the order of velocity structure functions). This revised conclusion was necessitated by violations in previous comparisons of the “ergodic hypothesis” for large p. (2) Physical limitations on cascade models are analyzed. We show that cascade measures demonstrate a strict dependence on the scaling parameter. This circumstance affects interpretations of statistics of multipliers, shows that the models used in practice are not really superior to others, and indicates the necessity to study cascades with a random scaling parameter.
Linear stability analysis of controlled Rayleigh-Bénard convection using shadowgraphic measurement
Authors: Howle Laurens E.
Published: July 2002

We conduct a linear stability analysis of Rayleigh-Bénard convection in an infinite horizontal layer with active control of the lower boundary heat flux. A simple linear proportional control loop uses a shadowgraph of the convection pattern to actively distribute the constant-mean lower boundary heat flux while the upper boundary is kept at a constant temperature. We find it possible to shift the convection threshold by a factor of approximately 3. This is a companion paper to our recent experimental work.
Görtler vortices in wall jet flow on a rotating cylinder
Authors: Matsson O. John E.
Published: July 2002

Hot-wire anemometry, smoke visualization and nonlinear calculations were used to study the wall jet on a cylinder with rotation. It was found that streamwise vortices are amplified in convex wall jet flow without rotation and that the maximum amplitude was higher than for the concave case. Furthermore, the transition region was located downstream compared with the concave wall jet. Rotation was found to destabilize the convex wall jet, i.e., the transition region appeared upstream compared with nonrotation.
A new approach to model and simulate numerically surface chemistry in rarefied flows
Authors: Choquet Isabelle
Published: July 2002

A new approach is proposed to model and simulate numerically surface chemistry within the frame of rarefied gas flows. It is developed to satisfy all together the following points: (i) describe the gas phase at the microscopic scale, as required in rarefied flows, (ii) describe the wall at the macroscopic scale, to avoid prohibitive computational costs and not only consider crystalline but also amorphous surfaces, (iii) reproduce on average macroscopic laws correlated with experimental results, and (iv) derive analytic models in a systematic and exact way. The problem is stated and solved in the general framework of a nonstatic flow in contact with a catalytic and nonporous surface, without ageing process. It is shown that the exact and systematic solution method based on the Laplace transform, introduced previously by the author to model collisions in the gas phase, can be extended to the present problem. The proposed approach is applied to Eley-Rideal and Langmuir-Hinshelwood recombination, assuming a coverage locally at equilibrium. The models are developed considering one atomic species and extended to the general case of several atomic species. Numerical calculations verify that the models derived in this way reproduce the experimentally observed behavior accurately.
Quasihydrodynamic description of ion acoustic waves in a collisional plasma
Authors: Bychenkov V. Yu. , Myatt J. , Rozmus W. , Tikhonchuk V. T.
Published: July 2002

A quasihydrodynamic description of ion acoustic waves (IAW) has been developed in the entire range of particle collisionality. It is shown that a frequency dependent 21-moment Grad closure to ion fluid equations and phenomenological delocalized electron heat transport formulation can be used for an accurate analytical description of ion acoustic wave dispersion and damping. This description is in good agreement with the results of Fokker–Planck simulations. The authors’ new model has been derived with the practical motivation of improving IAW description in large scale numerical modeling of ion wave instabilities. This theory has been also applied to the linear analysis of heat flux driven plasma instabilities. In particular, a new IAW instability in the regime of collisional electrons, which is induced by the density dependence of the electron thermal transport coefficient, has been found.
Theory of electric-field curvature effects on long-wavelength drift wave turbulence
Authors: Sidikman K. L. , Carreras B. A. , Diamond P. H. , Garcia L.
Published: July 2002

A simple model of long-wavelength drift waves is used to study the way in which a radial electric-field profile influences the growth and saturation of turbulence. For a fixed external field, the effect of curvature (∂2Er/∂r2) dominates that of shear (∂Er/∂r). In the linear regime, both affect the average k∥ at which ion damping occurs: shear by shifting the eigenmode off the resonant surface and curvature by changing the eigenmode width. Curvature damps more efficiently and also shifts the real frequency of the drift wave, changing the instability drive. In the nonlinear regime, radial trapping at large fluctuation levels limits the ability of an external electric-field profile to affect the spatial structure. Changes in damping are now less effective than the feedback between frequency shift and drive. The importance of the frequency shift caused by electric-field curvature in the presence of finite-amplitude fluctuations has been demonstrated by numerical calculations.
Relaxation of temperature and chemical composition in nonideal plasmas
Authors: Ohde Th. , Bonitz M. , Bornath Th. , Kremp D. , Schlanges M.
Published: July 2002

Rate equations for the coupled evolution of density and temperature in nonideal partially ionized plasmas are presented. The equations are derived from generalized quantum kinetic equations that include many-particle effects, such as screening, self-energy and lowering of the binding energy. The equations are solved for a three-component plasma. The influence of many-body effects on the density–temperature relaxation is demonstrated.
Random phase plate hot spots and their effect on stimulated Brillouin backscatter and self-focusing
Authors: Rose Harvey A.
Published: July 2002

Laser hot spots, as determined by Random Phase Plate (RPP) hot spots, control the critical value of the average intensity, Ic, at which there is a rapid onset of stimulated scatter in the strongly damped convective regime of three wave parametric instabilities. For the case of stimulated Brillouin backscatter in a long scale length plasma, nascent hot spot ponderomotive self-focusing is shown to reduce the value of Ic in the regime of very strongly damped acoustic waves. RPP hot spots have two, intrinsically nonlinear, thresholds for ponderomotive self-focusing. Large intensity amplifications occur in the hot spot neighborhood when the hot spot power exceeds a certain critical power, Pc, which is independent of the optic’s f number, F. When the second, F-dependent, hot spot power threshold is exceeded, a filament emerges from the far side of the hot spot, whose extent grows erratically in time.
Passive cyclotron current drive efficiency for relativistic toroidal plasmas
Authors: Kasilov S. V. , Kernbichler W.
Published: July 2002

In this paper we present results of analytical and numerical studies of the passive cyclotron current drive efficiency in mildly relativistic toroidal plasmas. The problem of linearization and separation of the electron and photon balance equations becomes nontrivial for high-temperature plasmas (e.g., D–3He) with low electron pressure (βe<0.1) due to the increased effect of radiation friction. The conditions under which this separation is possible is derived in this paper. The linearized problem for the electron distribution is formulated in the form of a standard variational principle, which includes both Coulomb collisions and ‘‘collisions’’ due to cyclotron radiation. The reduced variational principle for the current drive efficiency (generalized Spitzer–Härm function) is derived, as well as its bounce-averaged form for toroidal plasmas. Finally, a convenient form of the passive cyclotron current drive efficiency is introduced, which can be used for a self-consistent modeling of passive cyclotron current generation in tokamaks with the help of fish-scale structures [see the companion paper, W. Kernbichler and S. Kasilov, Phys. Plasmas 3, 4128 (1996)].
Drift-orbit-driven flux and plasma viscosity in asymmetric toroidal plasmas
Authors: Shaing K. C.
Published: July 2002

It is shown, based on the kinetic definition of the drift-orbit-driven flux, that the drift-orbit-driven flux is related to either the toroidal viscosity, or the poloidal viscosity, or a combination of both. This result is the same as that obtained from the less direct moment equation approach [Phys. Fluids 26, 3315 (1983)].
Two-temperature relaxation in nonideal partially ionized plasmas
Authors: Ohde Th. , Bonitz M. , Bornath Th. , Kremp D. , Schlanges M.
Published: July 2002

Evolution equations for the coupled relaxation of densities and temperatures for the components in nonideal partially ionized plasmas are given. In these equations many-body effects, such as screening, self-energy, and lowering of the binding energy, are included. The coupled equations are solved numerically for a hydrogen plasma consisting of electrons, protons, and atoms. Impact ionization, three-body recombination, and elastic processes are taken into account. Thermal relaxation times are determined and the results are compared with those from the literature. The influence of many-body effects on the evolution process are discussed. In some cases, a significantly increased lifetime of the two-temperature regime is found.
A new form of the magnetohydrodynamic potential energy
Authors: Greene John M.
Published: July 2002

In this Letter a useful and instructive form of the magnetohydrodynamic potential energy is presented. In this form all but one term is positive definite, each term is self-adjoint, and it can be directly evaluated numerically in nonorthogonal coordinate systems. The product of the local shear and the current parallel to the magnetic field is identified as a significant parameter for current-driven instabilities.
Particle simulation study of collisionless driven reconnection in a sheared magnetic field
Authors: Horiuchi Ritoku , Sato Tetsuya
Published: July 2002

Nonlinear development of collisionless driven reconnection and the consequent energy conversion process between the field and particles in a sheared magnetic field are investigated by means of a two-and-one-half-dimensional particle simulation. Magnetic reconnection takes place in two steps irrespective of a longitudinal magnetic field, but the growth rate of the reconnection field varies in proportion to the E×B drift velocity at an input boundary. It is clearly observed that the triggering mechanism of collisionless driven reconnection for the fast growing phase changes from an electron meandering dominance in a weak longitudinal field to an electron inertia dominance in a strong field. The electron acceleration and heating take place in the reconnection area under the influence of reconnection electric field, while the electron energy is converted to the ion energy through the action of an electrostatic (ambipolar) field excited by magnetic compression in the downstream. It is also found that, in the presence of a longitudinal magnetic field, the electron acceleration by the reconnection field takes place effectively and the generated force-free current is maintained for a long period while forming an asymmetric spatial profile of current layer.
One-dimensional simulations of ultrashort intense laser pulses on solid-density targets
Authors: Lawson William S. , Rambo Peter W. , Larson David J.
Published: July 2002

Presented are simulations in one dimension of laser pulses with a pre-ionized thin target. The duration of the laser pulse is between 10 and 200 fs, the laser power is between 1018 and 1022 W/cm2, and the wavelength is 400 nm. The targets were foils of either aluminum at normal density, or in a few cases plastic (chemical formula CH) at a density of 1.1 g/cm3, and varied from 20 to 4000 nm in thickness. The simulation results show that at these intensities the absorption mechanism for a normally incident pulse changes to a collisionless mechanism first proposed by Kruer and Estabrook [W. L. Kruer and K. Estabrook, Phys. Fluids 28, 430 (1985)]. Use of a thin target ensures that the energy imparted to the electrons will be electrostatically transferred to the ions rather than transported for large distances into the target.
Computational study of laser imprint mitigation in foam-buffered inertial confinement fusion targets
Authors: Mason R. J. , Kopp R. A. , Vu H. X. , Wilson D. C. , Goldman S. R. , Watt R. G. , Dunne M. , Willi O.
Published: July 2002

Recent experiments have shown that low density foam layers can significantly mitigate the perturbing effects of beam nonuniformities affecting the acceleration of thin shells. This problem is studied parametrically with two-dimensional LASNEX [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 51 (1975)]. Foam-buffered targets are employed, consisting typically of 250 Å of gold, and 50 μm of 50 mg/cm3 C10H8O4 foam attached to a 10 μm foil. In simulation these were characteristically exposed to 1.2 ns, flat-topped green light pulses at 1.4×1014 W/cm2 intensity, bearing 30 μm lateral perturbations of up to 60% variation in intensity. Without the buffer layers the foils were severely disrupted by 1 ns. With buffering only minimal distortion was manifest at 3 ns. The smoothing is shown to derive principally from the high thermal conductivity of the heated foam. The simulation results imply that (1) the foam thickness should exceed the disturbance wavelength; (2) intensities exceeding 5×1013 W/cm2 are needed for assured stability beyond 2 ns; (3) longer foams at lower densities are needed for effective mitigation with shorter wavelength light; (4) the gold layer hastens conversion of the structured foam to a uniform plasma.
Heat flows through plasma sheaths
Authors: Takamura S. , Ye M. Y. , Kuwabara T. , Ohno N.
Published: July 2002

Plasma heat flow to material surfaces through sheaths is studied, taking several key physics factors into account. Electron emission from the surface, which breaks a thermal insulation of the sheath, is studied in both thermoelectron emission (TEE) and secondary electron emission (SEE), in which a correct expression under space charge limited condition is given for arbitrary sheath voltages. Nonlinear thermal bifurcation induced by electron emission is analyzed in the experiment and the theory. The local heat flow was found to be enhanced by a thermal contraction induced by cross-field potential variation in a plasma. An enhancement of SEE of hydrogen-absorbed graphite, and a suppression of SEE by the gyromotion of emitted electrons in obliquely incident magnetic field are identified. The effects of ion reflection on the surface and ponderomotive force are also discussed in terms of energy transmission factor δ. An anomaly of δ in detached recombining plasmas is discussed.
Comments on particle and energy balance in the edge plasma of Alcator C-Mod
Authors: Umansky M. V. , Krasheninnikov S. I. , LaBombard B. , Terry J. L.
Published: July 2002

Particle balance is examined in a large set of representative Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] diverted plasmas using measurements of neutral gas pressures and ionization source strengths in the main chamber, and parallel plasma flow into the divertor. It is inferred that plasma flow in the scrape-off layer (SOL) is dominated by transport to the main chamber walls rather than by flow into the divertor. It follows that (i) the effective diffusion coefficient for anomalous cross-field particle transport in the SOL must rapidly grow with distance from the magnetic separatrix to account for the measured density profiles, (ii) a significant fraction of the power from the core plasma can be convected rather than conducted into the SOL, and (iii) the neutral pressure at the outer midplane is governed more by cross-field plasma transport than by wall–plasma separation or divertor/limiter geometries.
Direct measurements of two-dimensional velocity profiles in direct current glow discharge dusty plasmas
Authors: Thomas Edward
Published: July 2002

The letter details the first application of particle image velocimetry (PIV) techniques to dusty plasmas. Using PIV, two-dimensional velocity profiles are measured in dusty plasmas of silica particles that are suspended in the anode spot of an argon dc glow discharge plasma. This letter discusses the initial results of these studies and the potential applications of PIV techniques to dusty plasma studies.
Laboratory studies of magnetic vortices. II. Helicity reversal during reflection of a magnetic vortex at a conducting boundary
Authors: Stenzel R. L. , Urrutia J. M. , Griskey M. C.
Published: July 2002

The reflection of a magnetic vortex from a conducting boundary is studied experimentally in a large laboratory plasma. The parameter regime is that of electron magnetohydrodynamics and the vortex consists of a spheromak-like magnetic field perturbation propagating in the whistler mode along a uniform background magnetic field. In this work we focus on the helicity properties of the vortex magnetic field, electron velocity, and vorticity. The reflection conserves magnetic energy but reverses the sign of all helicities. The change in topology arises from a self-consistent reversal of one linked vector field without involving helicity injection, reconnection, or dissipation processes. The breakdown of helicity conservation and the frozen-in concept is explained by the presence of a vacuum-like sheath at the plasma–boundary interface.
Electric field-induced plasma convection in tokamak divertors
Authors: Boedo J. A. , Schaffer M. J. , Maingi R. , Lasnier C. J.
Published: July 2002

Measurements of the electric fields, E in the DIII-D tokamak divertor region [J. C. Luxon and L. G. Davis, Fusion Technology 8, Part 2A, 441 (1985)] are quantitatively consistent with recent computational modeling establishing that E×BT circulation is the main cause of changes in divertor plasmas with the direction of the toroidal magnetic field, BT. Extensive two-dimensional measurements of plasma potential in the DIII-D tokamak divertor region are reported for the first time. The resulting E×BT/B2 drift particle flux is calculated for standard (ion ∇BT drift toward divertor X-point) and reversed BT direction and for low (L) and high (H) confinement modes. Perpendicular field strengths of up to E∼5 kV/m are observed at the separatrix between the divertor private region and the scrape-off layer (SOL). The E×BT drift, which reverses with reversal of BT, creates a poloidal circulation pattern in the divertor that convects 25%–40% of the total ion flow to the divertor target. The circulation strongly couples the various regions of the divertor and SOL and fuels the X-point region. An outward shift of the profiles is seen in reversed BT.
Fast collisionless reconnection in the whistler frequency range
Authors: Attico N. , Califano F. , Pegoraro F.
Published: July 2002

The onset of spontaneous magnetic reconnection in a collisionless plasma in the whistler frequency range is investigated from the analytical and from the numerical point of view in a two dimensional slab geometry. This is intended as a first step in the direction of simulating the nonlinear phase of kinetic reconnection using a single species Vlasov code. The evolution of the magnetic field lines is described in the framework of electron magnetohydrodynamics (EMHD). The linear growth rate, previously studied in the so-called constant-ψ approximation is determined for all values of the wavevector k. In the small k limit the (stabilizing) effect of the frequency term in the external region is included. The scaling laws of the growth rate with respect to the inertial skin depth are obtained and compared with those of standard inertial magnetohydrodynamic theory.
Light-induced defects in hydrogenated amorphous silicon observed by picosecond photoinduced absorption
Authors: Strait J. , Tauc J.
Published: July 2002

Phosphorus-doped and boron-doped hydrogenated amorphous silicon were studied by photoinduced absorption (PA) in the time range from 2 to 1800 ps. Prolonged light exposure (ℏω=2.0 eV) causes PA to decay more rapidly. The data are fit to a multiple trapping model. Minority carriers get trapped at deep defect states whose density increases with light exposure. These results are consistent with the interpretation that the light-induced defects in amorphous silicon are dangling bonds.
Enhanced thermal stability of single longitudinal mode coupled cavity lasers
Authors: Andrews John R.
Published: July 2002

Thermally induced changes in the length of the external resonator in a three-mirror coupled cavity laser can enhance the temperature range for single longitudinal mode operation. The enhancement occurs when the length of the external resonator is altered to track the change in the effective length of the laser diode, hence maintaining feedback on the same longitudinal mode. We observe stable single longitudinal mode operation for 15 °C with a planar external mirror and for as much as 25 °C with a confocal spherical mirror in a coupled cavity device incorporating an AlGaAs laser diode at constant current. This is a three to fivefold enhancement over what would be expected if the passive cavity were of fixed length. The data are consistent with thermal compensation of the external cavity length to match the thermal effects on the refractive index of the AlGaAs diode.
Geometry-defined electrical interconnection by a homogeneous medium
Authors: Etemad S. , Quan X. , Sanders N. A.
Published: July 2002

We describe a geometry-defined electrical interconnection medium based on a composite of small metal particles dispersed uniformly in an insulator. The volume fraction of metal particles, p, is just below the critical value pc, which corresponds to the percolation threshold. As a result, the conductance between two points drops sharply by several orders of magnitude as their separation increases beyond a well-defined short range coherence length ξ. We show how, by careful control of p, a homogeneous composite can provide isolated electrical interconnection paths, or ‘‘vias’’. These vias are simply defined by controlling their geometry; thus, they provide an easily processable, solderless means for vertical interconnection in a variety of high density packaging schemes.
Characteristics of CdTe grown on Si by low pressure metalorganic chemical vapor deposition
Authors: Chou Rey-Lin , Lin Min-Shyong , Chou Kan-Sen
Published: July 2002

Specular CdTe epitaxial layers have been grown on (100) Si substrates by the low pressure metalorganic chemical vapor deposition method with dimethylcadmium (DMCd) and diethyltelluride (DETe) source materials. Auger electron spectroscopy and scanning electron microscopy have been used to demonstrate the uniform composition, good surface morphology, and abrupt interface between the CdTe layer and the silicon substrate. The dominant sharp bound exciton related emission peak at 1.593 eV and the weak defect related extrinsic band at 1.483 eV were observed in the 14 K photoluminescence spectra, demonstrating that high quality CdTe epitaxial layers can be grown on Si at 375 °C with the DMCd/DETe mole ratio at approximate unity.
Optically pumped laser oscillation in the 1.6–1.8 μm region from Al0.4Ga0.6Sb/GaSb/Al0.4Ga0.6Sb double heterostructures grown by molecular beam heteroepitaxy on Si
Authors: van der Ziel J. P. , Malik R. J. , Walker J. F. , Mikulyak R. M.
Published: July 2002

Double heterostructures consisting of GaSb active layers with Al0.4Ga0.6Sb cladding layers were grown by molecular beam heteroepitaxy on Si substrates. The intrinsic ∼12% lattice mismatch between the GaSb and the Si at the growth temperature is largely taken up by a GaSb/AlSb superlattice. The larger thermal contraction of the GaSb layer relative to that of Si causes the GaSb layer to be under severe dilatory strain relative to the Si substrate at room temperature. Optically pumped laser emission ranging from 1.62 μm at 80 K to 1.82 μm at 350 K was observed. The threshold varied somewhat nonexponentially with temperature, with a change in slope at ∼250 K. The exponential threshold-temperature dependences at 80 and 300 K are T0=158 and 100 K, respectively, and are higher than previously reported for GaSb lasers. At 300 K the threshold corresponds to an effective current of 12 kA/cm2.
Narrowband grating resonator filters in InGaAsP/InP waveguides
Authors: Alferness R. C. , Joyner C. H. , Divino M. D. , Martyak M. J. R. , Buhl L. L.
Published: July 2002

We report the first demonstration of efficient narrowband optical wavelength filters using InGaAsP/InP passive waveguide grating resonators. Filter bandwidths as narrow as 1 Å, centered about λ=1.55 μm with excess resonator loss as low as 1 dB, have been achieved.
Ultrafast all-optical gate with subpicosecond O
N and O
F
F response time
Authors: Hulin D. , Mysyrowicz A. , Antonetti A. , Migus A. , Masselink W. T. , Morkoç H. , Gibbs H. M. , Peyghambarian N.
Published: July 2002

An all-optical logic gate consisting of a GaAs-GaAlAs multiple quantum well structure inserted in a 1.3-μm-thick Fabry–Perot cavity is demonstrated to perform with subpicosecond on and off switching time. The use of a purely optical field effect allows for a recovery time as rapid as the switch-on time.
Stepwise monolayer growth of GaAs by switched laser metalorganic vapor phase epitaxy
Authors: Doi Atsutoshi , Aoyagi Yoshinobu , Namba Susumu
Published: July 2002

Growth of GaAs by repeated deposition of single atomic layers using the switched laser metalorganic vapor phase epitaxy technique is reported. Suspension of Ga deposition at 100% coverage is an essential part of the growth mechanism for stepwise epitaxy—the ideal atomic layer epitaxy. This is achieved by suppressing pyrolytic decomposition and favoring photocatalytic decomposition of trimethylgallium (TMG). A growth model for stepwise monolayer epitaxy is proposed which suggests that photocatalytic decomposition of TMG occurs only at surface As atoms and not at Ga atoms.
Epitaxial growth of silicon by photochemical vapor deposition at a very low temperature of 200 °C
Authors: Nishida Shoji , Shiimoto Tsunenori , Yamada Akira , Karasawa Shiro , Konagai Makoto , Takahashi Kiyoshi
Published: July 2002

A novel Si epitaxial growth technique using mercury-sensitized photochemical vapor deposition has been developed. Epitaxial thin films (300–8000 Å) were grown on (100)Si substrates at 100–300 °C from a gas mixture of Si2H6+SiH2F2+H2 by irradiation of a low pressure mercury lamp (1849,2537 Å). The growth rate, plotted as a function of the reciprocal substrate temperature, represented an activation energy which was found to be a small value of 0.18 eV. Observation of the surface structure by reflective high-energy electron diffraction showed fine (100) streak patterns for films grown at 200 °C as well as at 250 and 300 °C.
Photoluminescence characteristics of AlGaAs-GaAs single quantum wells grown by migration-enhanced epitaxy at 300 °C substrate temperature
Authors: Horikoshi Yoshiji , Kawashima Minoru , Yamaguchi Hiroshi
Published: July 2002

When Ga or Al atoms are evaporated on a clean GaAs surface in an As-free or a very low As pressure atmosphere, they are quite mobile and migrate very rapidly along the surface even at low temperatures. This characteristic is exploited for growing high-quality GaAs and AlGaAs layers at very low temperatures by alternately supplying Ga and/or Al atoms and As4 molecules to the GaAs substrate. Applying this method, AlGaAs-GaAs single quantum well structures with excellent photoluminescence characteristics are grown at 300 °C.
Observation of millimeter-wave oscillations from resonant tunneling diodes and some theoretical considerations of ultimate frequency limits
Authors: Sollner T. C. L. G. , Brown E. R. , Goodhue W. D. , Le H. Q.
Published: July 2002

Recent observations of oscillation frequencies up to 56 GHz in resonant tunneling structures are discussed in relation to calculations by several authors of the ultimate frequency limits of these devices. We find that calculations relying on the Wentzel–Kramers–Brillouin (WKB) approximation give limits well below the observed oscillation frequencies. Two other techniques for calculating the upper frequency limit were found to give more reasonable results. In one method we use the solution of the time-dependent Schrödinger equation obtained by Kundrotas and Dargys [Phys. Status Solidi B 134, 267 (1986)], while in the other we use the energy width of the transmission function for electrons through the double-barrier structure. This last technique is believed to be the most accurate since it is based on general results for the lifetime of any resonant state. It gives frequency limits on the order of 1 THz for two recently fabricated structures. It appears that the primary limitation of the oscillation frequency for double-barrier resonant tunneling diodes will be imposed by intrinsic device circuit parameters and by the transit time of the depletion layer rather than by time delays encountered in the double-barrier region.
Carrier trapping in single quantum wells with different confinement structures
Authors: Feldmann J. , Peter G. , Göbel E. O. , Leo K. , Polland H.-J. , Ploog K. , Fujiwara K. , Nakayama T.
Published: July 2002

The trapping efficiency and trapping dynamics of photoexcited carriers in GaAs/AlGaAs single quantum wells with different confinement structures are examined at low temperature by means of picosecond luminescence as well as photoluminescence excitation spectroscopy. The trapping efficiency is 100% only in graded-index separate confinement heterostructures with a linear band-gap profile. The lower trapping efficiency of other confinement structures is due to radiative and nonradiative recombination in the confinement layers.
Synthesis of diamond films in a rf induction thermal plasma
Authors: Matsumoto Seiichiro , Hino Mototsugu , Kobayashi Toyohiko
Published: July 2002

Microcrystals and microcrystalline films of diamond were prepared on molybdenum substrates in a thermal plasma which was produced by rf inductive heating in an argon-hydrogen-methane mixture gas under 1 atm pressure. The deposition rate amounted to 1 μm/min.
Theoretical investigation of hole transport in strained III-V semiconductors: Application to GaAs
Authors: Hinckley J. M. , Singh J.
Published: July 2002

A Monte Carlo method has been developed and applied to study the anisotropic transport of holes in unstrained and strained bulk III-V compound semiconductors. In this letter, we present the results for the prototypical GaAs, T=300 K material system. We find that the hole mobility can be significantly increased by the presence of biaxial compressive strain in the system. This arises from strain-induced modifications in the densities of states and the overlap functions and from a separation of the heavy and light hole bands at k=0 which decreases the heavy to light hole interband scattering. For a 1.5% biaxial compressive strain, the hole mobilities are increased by up to a factor of 2 over the unstrained values. This improvement is sustained up to the highest field in our simulations which was 20 kV/cm.
Diffusion of atomic silicon in gallium arsenide
Authors: Schubert E. F. , Stark J. B. , Chiu T. H. , Tell B.
Published: July 2002

Silicon impurities with an initial Dirac-delta-function-like distribution profile are diffused into GaAs using rapid thermal annealing. The diffusion of atomic Si is determined by a novel method of comparing experimental capacitance-voltage profiles with a corresponding self-consistent profile calculation. Capacitance-voltage profiles broaden from 30 to 137 Å upon rapid thermal annealing at 1000 °C for 5 s. The diffusion coefficient and the activation energy of atomic Si diffusion in GaAs are determined to be D0=4×10−4 cm2/s and Ea=2.45 eV, respectively. The diffusion coefficient is two orders of magnitude smaller as compared to Si-pair diffusion in GaAs.
Identification of motifs in cholera toxin A1 polypeptide that are required for its interaction with human ADP-ribosylation factor 6 in a bacterial two-hybrid system
Authors: Jobling Michael G. , Holmes Randall K.
Published: July 2002

The latent ADP-ribosyltransferase activity of cholera toxin (CT)
that is activated after proteolytic nicking and reduction is associated
with the CT A1 subunit (CTA1) polypeptide. This activity is stimulated
in vitro
by interaction with eukaryotic proteins termed
ADP-ribosylation factors (ARFs). We analyzed this interaction in a
modified bacterial two-hybrid system in which the T18 and T25 fragments
of the catalytic domain of
Bordetella pertussis
adenylate cyclase were fused to CTA1 and human ARF6 polypeptides,
respectively. Direct interaction between the CTA1 and ARF6 domains in
these hybrid proteins reconstituted the adenylate cyclase activity and
permitted cAMP-dependent signal transduction in an
Escherichia
coli
reporter system. We constructed improved vectors and
reporter strains for this system, and we isolated variants of CTA1 that
showed greatly decreased ability to interact with ARF6. Amino acid
substitutions in these CTA1 variants were widely separated in the
primary sequence but were contiguous in the three-dimensional structure
of CT. These residues, which begin to define the ARF interaction motif
of CTA1, are partially buried in the crystal structure of CT holotoxin,
suggesting that a change in the conformation of CTA1 enables it to bind
to ARF. Variant CTA polypeptides containing these substitutions
assembled into holotoxin as well as wild-type CTA, but the variant
holotoxins showed greatly reduced enterotoxicity. These findings
suggest functional interaction between CTA1 and ARF is required for
maximal toxicity of CT
in vivo
.
Role of backbone solvation in determining thermodynamic β propensities of the amino acids
Authors: Avbelj Franc , Baldwin Robert L.
Published: July 2002

There is a paradox concerning the β propensities of the amino acids: the amino acids with the highest β propensities such as valine and isoleucine have the highest tendency to desolvate the peptide backbone, which should result in a loss of stability. Nevertheless, backbone solvation, calculated as electrostatic solvation free energy (ESF), is highly correlated with mutant stability in the zinc-finger system studied by Kim and Berg [Kim, C. A. & Berg, J. M. (1993)
Nature (London)
362, 267–270], and valine and isoleucine are among the most stabilizing amino acids. This inverse correlation between stability and ESF can be explained, because the mutant ESF differences in the unfolded protein are larger than in the native protein. Consequently, mutations such as Ala to Val destabilize the unfolded form more than the native protein. By comparing mutant ΔESF values in isolated β-strands versus β-sheets, we conclude that amino acids with high β propensities should exert their stabilizing effects at early stages in folding. This deduction agrees with the studies by Clarke and coworkers [Lorch, M., Mason, J. M., Clarke, A. R. & Parker, M. J. (1999)
Biochemistry
38, 1377–1385, and Lorch, M., Mason, J. M., Sessions, R. B. & Clarke, A. R. (2000)
Biochemistry
39, 3480–3485] of the thermodynamics of folding of the β-sheet protein CD2.d1.
Crystal structure of the C-terminal domain of the RAP74 subunit of human transcription factor IIF
Authors: Kamada Katsuhiko , De Angelis Jacqueline , Roeder Robert G. , Burley Stephen K.
Published: July 2002

The x-ray structure of a C-terminal fragment of the RAP74 subunit
of human transcription factor (TF) IIF has been determined at 1.02-Å
resolution. The α/β structure is strikingly similar to the
globular domain of linker histone H5 and the DNA-binding domain of
hepatocyte nuclear factor 3γ (HNF-3γ), making it a winged-helix
protein. The surface electrostatic properties of this compact domain
differ significantly from those of
bona fide
winged-helix transcription factors (HNF-3γ and RFX1) and from the
winged-helix domains found within the RAP30 subunit of TFIIF and the
β subunit of TFIIE. RAP74 has been shown to interact with the
TFIIF-associated C-terminal domain phosphatase FCP1, and a putative
phosphatase binding site has been identified within the RAP74
winged-helix domain.
Expression of CD40 identifies a unique pathogenic T cell population in type 1 diabetes
Authors: Wagner David H. , Vaitaitis Gisela , Sanderson Richard , Poulin Michelle , Dobbs Cathleen , Haskins Kathryn
Published: July 2002

Juvenile diabetes (type 1) is an autoimmune disease in which CD4+T cells play a major role in pathogenesis characterized by insulitis and β cell destruction leading to clinical hyperglycemia. To date, no marker for autoimmune T cells has been described, although it was previously demonstrated that autoimmune mice have a large population of CD4+cells that express CD40. We show here that established, diabetogenic T cell clones of either the Th1 or Th2 phenotype are CD40-positive, whereas nondiabetogenic clones are CD40-negative. CD40 functionally signals T cell clones, inducing rapid activation of the transcription factor NFκB. We show that autoimmune diabetes-prone nonobese diabetic mice have high levels of CD40+CD4+T cells in the thymus, spleen, and importantly, in the pancreas. Finally, as demonstrated by adoptive transfers, CD4+CD40+cells infiltrate the pancreatic islets causing β-cell degranulation and ultimately diabetes.
Supramolecular chemistry of dendrimers with functional cores
Authors: Diederich François , Felber Béatrice
Published: July 2002

Dendritic microenvironments are analogous to local environments created within protein superstructures. Correspondingly, properties of functional cores such as molecular recognition and catalytic activity are profoundly influenced by the surrounding dendritic branches.
Some properties of human neuronal α7 nicotinic acetylcholine receptors fused to the green fluorescent protein
Authors: Palma Eleonora , Mileo Anna M. , Martínez-Torres Ataúlfo , Eusebi Fabrizio , Miledi Ricardo
Published: July 2002

The functional properties and cellular localization of the human neuronal α7 nicotinic acetylcholine (AcCho) receptor (α7 AcChoR) and its L248T mutated (
mut
) form were investigated by expressing them alone or as gene fusions with the enhanced version of the green fluorescent protein (GFP).
Xenopus
oocytes injected with wild-type (
wt
),
mut
α7, or the chimeric subunit cDNAs expressed receptors that gated membrane currents when exposed to AcCho. As already known, AcCho currents generated by
wt
α7 receptors decay much faster than those elicited by the
mut
α7 receptors. Unexpectedly, the fusion of GFP to the
wt
and mutated α7 receptors led to opposite results: the AcCho-current decay of the
wt
receptors became slower, whereas that of the mutated receptors was accelerated. Furthermore, repetitive applications of AcCho led to a considerable “run-down” of the AcCho currents generated by
mut
α7-GFP receptors, whereas those of the
wt
α7-GFP receptors remained stable or increased in amplitude. The AcCho-current run-down of
mut
α7-
GFP
oocytes was accompanied by a marked decrease of α-bungarotoxin binding activity. Fluorescence, caused by the chimeric receptors expressed, was seen over the whole oocyte surface but was more intense and abundant in the animal hemisphere, whereas it was much weaker in the vegetal hemisphere. We conclude that fusion of GFP to
wt
α7 and
mut
α7 receptors provides powerful tools to study the distribution and function of α7 receptors. We also conclude that fused genes do not necessarily recapitulate all of the properties of the original receptors. This fact must be borne close in mind whenever reporter genes are attached to proteins.
Characterization of mutant tobacco mosaic virus coat protein that interferes with virus cell-to-cell movement
Authors: Bendahmane Mohammed , Szécsi Judit , Chen Iju , Berg R. Howard , Beachy Roger N.
Published: July 2002

Expression of tobacco mosaic virus (TMV) coat protein (CP) in plants confers resistance to infection by TMV and related tobamoviruses. Certain mutants of the CP (CP
T42W
) provide much greater levels of resistance than wild-type (wt) CP. In the present work, infection induced by RNA transcripts of TMV clones that contain wt CP or mutant CP
T42W
fused to the green fluorescent protein (GFP) (TMV-CP:GFP, TMV-CP
T42W
:GFP) and clones harboring TMV movement protein (MP):GFP were followed in nontransgenic and transgenic tobacco BY-2 protoplasts and
Nicotiana tabaccum
Xanthi-nn plants that express wt CP or CP
T42W
. On nontransgenic and wt CP transgenic plants, TMV-CP:GFP produced expanding, highly fluorescent disk-shaped areas. On plants expressing CP
T42W
, infection by TMV-CP:GFP or TMV-MP:GFP-CP produced infection sites of smaller size that were characterized by low fluorescence, reflecting reduced levels of virus spread and reduced accumulation of both CP:GFP and MP:GFP. TMV-CP
T42W
:GFP failed to produce visible infection sites on nontransgenic plants, yet produced normal infection sites on MP-transgenic plants that produce MP. TMV infection of transgenic BY-CP
T42W
protoplasts resulted in very low levels of MP accumulation, whereas on BY-CP protoplasts (containing wt CP), infection produced higher levels of MP than in nontransgenic BY-2 cells. The results suggest that wt CP has a positive effect on the production of MP, whereas the CP
T42W
has a negative effect on MP accumulation and/or function. This effect results in very high levels of resistance to TMV infection in plants containing CP
T42W
. This report shows that the CP of a plant virus regulates production of the MP, and that a mutant CP interferes with MP accumulation and cell-to-cell movement of infection.