bluuebunny/crossref_metadata_2025 huggingface dataset:
Long-term stability of a one-dimensional current-driven double layer
Authors: Hori Naoko , Yamamoto Takashi
Published: July 2002

Long-term (>an electron transit time over the system) stability of a one-dimensional current-driven double layer is studied by numerical experiments using particles. In these experiments, the potential difference across the system is self-consistently determined by the space charge distributions inside the system. Each boundary of the system supplies a nondrifting half-Maxwellian plasma. The current density is increased by increasing the number density of the source plasma at the injection (right) boundary. A double layer can be developed by injection of a sufficiently high current density. For a fixed level of current injection, plasmas carrying no current with various densities (n ̂0) are loaded on the left side of the system. Whether or not the generated double layer can maintain its potential drop for a long period depends on the density (n ̂0) relative to the initial density (n*0) near the injection boundary: (1) the double layer is found to grow when n ̂0=n*0; (2) the steady double layer is seen for a long period when n ̂0≳n*0; (3) the double layer is found to decay when n ̂0 is even higher than n*0. A new concept of the current polarizability Pc=J/n♯ is introduced for understanding these results, where J is the current density flowing through the double layer and n♯ is the plasma density at the injection front, i.e., the low-potential edge of the double layer. Here Pc represents the potential of negatively polarizing a plasma. The different long-term features of the double layers in results 1–3 above can be explained by analyzing physical processes to control the current polarizability Pc.
Nonlinear cascade processes in magnetized plasmas with temperature anisotropy
Authors: Liljeström M. , Weiland J.
Published: July 2002

A simplified nonlinear gyrokinetic formalism has been developed for general distribution functions and general magnetic perturbations. It has been found that the nonlinear coupling strength for low frequency electromagnetic modes in plasmas with anisotropic temperature is larger than for the isotropic case for small Larmor radius. A nonlinear cascade where the pump mode is the mode with intermediate parallel phase velocity has been found.
A collisional treatment of the trapped particle mode in multiregion mirror systems
Authors: Ramachandran H. , Lichtenberg A. J. , Lieberman M. A. , Sen A. K.
Published: July 2002

A method of analysis has been developed for treating the trapped particle mode in magnetic mirror systems in which three or more spatially distinct regions are present. The method applies for plasmas in which the electrons are sufficiently collisional that a Krook collision operator can be used. The ions have been treated as trapped within individual regions. The approximations allow the problem to be reduced to coupled algebraic equations, which are solved numerically. The numerical solutions are compared with asymptotic analytic solutions including known results. The results are applied to the multiple mirror experiment at Berkeley [Phys. Fluids 29, 1208 (1986)], which has three mirror cells of equal length but variable density, in which the central cell contains a potential barrier produced by a hot electron distribution.
Resistive evolution of plasmas with imperfect magnetic surfaces
Authors: Miller Guthrie
Published: July 2002

The theory of resistive evolution of plasmas is extended to include plasmas with imperfect magnetic surfaces, that is, plasmas having magnetic field lines that wander stochastically. In resistive evolution theory the plasma is assumed to be always in mechanical equilibrium and the evolution occurs by allowing the magnetic flux constraints determining the equilibrium to change, which happens on a resistive time scale. In this paper a formalism is created to describe magnetic field line structure in a general way in order to evaluate the field line integrals that are the magnetic flux constraints. The equations expressing evolution of these flux constraints, along with the plasma mechanical equilibrium equations, are a mathematical model that can be solved to determine the evolution of the plasma. An immediate result of this model is that, for plasmas with stochastic magnetic field lines, the current density has a filamentary structure, giving a self-consistent source of the stochasticity. A simple two-field-line model of the reversed field pinch is discussed.
Particle acceleration by electrostatic waves traveling perpendicular to nonuniform magnetic fields
Authors: Rath Sharadini , Kaw P. K.
Published: July 2002

A novel method has been proposed for indefinite nonstochastic acceleration of particles by electrostatic waves propagating normal to a magnetic field. Use is made of a magnetic field inhomogeneity to prevent the particle from detrapping from the electrostatic wave. Numerical plots of particle trajectories are presented.
The fluctuating wall-shear stress and the velocity field in the viscous sublayer
Authors: Alfredsson P. Henrik , Johansson Arne V. , Haritonidis Joseph H. , Eckelmann Helmut
Published: July 2002

The fluctuating wall-shear stress was measured with various types of hot-wire and hot-film sensors in turbulent boundary-layer and channel flows. The rms level of the streamwise wall-shear stress fluctuations was found to be 40% of the mean value, which was substantiated by measurements of the streamwise velocity fluctuations in the viscous sublayer. Heat transfer to the fluid via the probe substrate was found to give significant differences between the static and dynamic response for standard flush-mounted hot-film probes with air or oil as the flow medium, whereas measurements in water were shown to be essentially unaffected by this problem.
Nonlinear oscillations of pendant drops
Authors: Basaran Osman A. , DePaoli David W.
Published: July 2002

Whereas oscillations of free drops have been scrutinized for over a century, oscillations of supported (pendant or sessile) drops have only received limited attention to date. Here, the focus is on the axisymmetric, free oscillations of arbitrary amplitude of a viscous liquid drop of fixed volume V that is pendant from a solid rod of radius R and is surrounded by a dynamically inactive ambient gas. This nonlinear free boundary problem is solved by a method of lines using Galerkin/finite element analysis for discretization in space and an implicit, adaptive finite difference technique for discretization in time. The dynamics of such nonlinear oscillations are governed by four dimensionless groups: (1) a Reynolds number Re, (2) a gravitational Bond number G, (3) dimensionless drop volume V/R3 or some other measure of drop size, and (4) a measure of initial drop deformation a/b. In contrast to free drops whose frequencies of oscillation ω decrease as the amplitudes of their initial deformations increase, the change in frequency Δω of pendant drops with increasing initial deformation is drop size dependent. As the average linear size of pendant drops characterized by V1/3 becomes large compared to the rod radius, V1/3/R≫1, Δω falls as a/b rises, in accordance with results for free drops. The dynamics of very small drops, i.e., ones for which V1/3/R≪1, however, are profoundly affected by the presence of the solid rod. For such small drops, Δω rises as a/b rises, a remarkable fact. The results show that for drops of a given size, the frequency is insignificantly affected by viscosity over a wide of range of Reynolds numbers. However, when Re falls below a critical value, the nature of drop motion changes from underdamped oscillations to an aperiodic return to the rest state. Detailed examination of flow fields inside oscillating drops and decomposition of drop shapes into their linear modes supply further insights into the underlying physics. The effect of finite G in modifying the frequencies of oscillations and the rate at which they are damped is also investigated.
Flow regimes in model viscoelastic fluids in a circular couette system with independently rotating cylinders
Authors: Baumert Brandon Max , Muller Susan J.
Published: July 2002

Flow visualization of two highly elastic, nonshear-thinning polyisobutylene/polybutene fluids in the gap between concentric cylinders was performed over a range of shear rates and choices of relative cylinder rotations. The observed secondary flows are discussed in terms of destabilizing elastic and centrifugal forces. In the more viscous, more elastic fluid, instabilities are found to be independent of the choice of rotating cylinder and due entirely to elasticity. At the lowest shear rates examined, the first detectable secondary flows are steady counter-rotating vortices forming after a shearing time more than five orders of magnitude greater than the characteristic relaxation time of the fluid. At somewhat higher shear rates, a much more rapidly appearing oscillatory flow is observed to evolve into the steady vortex structure. In the less elastic fluid, the structure first detectable at the lowest shear rates is again steady vortices regardless of the choice of driving cylinder. At all shear rates examined, only elastic stationary vortices are observed in the absence of centrifugal destabilization (outer cylinder rotating). Secondary flows are significantly stronger in the presence of the centrifugal destabilization due to a rotating inner cylinder. Interaction of elasticity and centrifugal forces is found to generate a number of axially translating vortex structures, many of which are described here for the first time. At a shear rate more than five times the critical, another family of instability is observed which closely resembles a purely elastic instability observed by Baumert and Muller (1995). These experimental results are expected to provide a challenging test of numerical simulations of these viscoelastic flows.
Characterization of titanium laser-produced plasmas
Authors: Batha S. H. , Procassini R. J. , Hammel B. A. , Shepard T. D. , Drake R. P. , Bradley K. S. , Estabrook Kent , Hsieh E. J. , Keane C. J. , Montgomery D. S. , Phillion D. W.
Published: July 2002

The development of a plasma environment that is appropriate for the study of laser-plasma processes in laser-fusion plasma is reported. A material (titanium) with medium atomic number (Z) was used to provide x-ray measurements of radial and axial plasma symmetry as well as electron temperature. The electron density evolution was measured using stimulated scattering processes and odd half-harmonic generation from probe lasers of different wavelengths. The plasmas were created by two-sided irradiation of thin foils with 24 kJ of 351 nm laser light. When the peak electron density had decayed to about 4×1020 cm−3, the density profile was estimated to have a full width at half-maximum of 2 mm and the electron temperature was measured to be about 3 keV using K-shell spectroscopy. Two-dimensional computer simulations were found to reproduce some features of both electron density and temperature evolution.
The field and power absorption profiles in helicon plasma resonators
Authors: Cho Suwon
Published: July 2002

Analytic expressions are presented for electromagnetic waves excited by Boswell and Nagoya III type antennae [F. F. Chen, J. Vac. Sci. Technol. A 10, 1389 (1992)] in a uniform plasma column, and profiles of the wave field and power absorption are calculated. The Trivelpiece-Gould mode is found to be dominant causing the surface electron heating, while the linear dependence of the plasma density on the magnetic field is obeyed. In addition, the fast helicon wave approximation, with which the electron inertia is neglected, is examined in detail and its validity is discussed.
Shielding of resonant magnetic perturbations by rotation
Authors: Boozer Allen H.
Published: July 2002

Magnetic perturbations that drive islands are shielded from a rotating plasma by either a torque limit or an equilibrium limit. The physics of the shielding is given in a simple slab model along with the mechanism by which the shielding currents are maintained. Shielding currents in a channel of width Δ are damped by a resistance η/Δ with η the resistivity. If an island is shielded in a nearly ideal plasma, η/Δ=(μ0/π)VpAM2p with VpA the poloidal Alfvén speed, Mp≡v⊥/VpA, and v⊥ the speed of the cross-field flow at the resonance surface.
External kinks in plasmas with helical boundary deformation and net toroidal current
Authors: Ardelea A. , Cooper W. A.
Published: July 2002

The global ideal magnetohydrodynamic (MHD) stability of plasmas with a prescribed helical boundary deformation and nonvanishing toroidal current is investigated with respect to the (m,n) external modes with n=1,2,3, and m=n+1. L= 2, 3 and mixtures (of both) configurations were studied by systematically varying parameters such as the aspect ratio, the number of equilibrium field periods, the toroidal current density, and the pressure profiles. The study focuses mainly on the q<2 area. It is shown that a fixed helical boundary deformation can stabilize the (m,n) external modes with n=1,2,3, and m=n+1 at values of β≃1%–2%. These modes are unstable in the circular tokamak at the same value of β. If δ is a measure of the plasma boundary deformation, then windows of stability [δminδmax] may exist depending strongly on equilibrium parameters. The results are described in terms of stability zones in the (qaxis,qedge) plane.
New methods for diagnosing and controlling hohlraum drive asymmetry on Nova
Authors: Amendt Peter , Glendinning S. G. , Hammel B. A. , Landen O. L. , Murphy T. J. , Suter L. J. , Hatchett S. , Rosen M. D. , Lafitte S. , Desenne D. , Jadaud J. P.
Published: July 2002

A novel method to control lowest-order (P2) flux asymmetry in Nova cylindrical hohlraums [E. M. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)] with fixed laser beams is to use a pair of axial gold disks of varying radii to partially block the capsule view of the laser-entrance holes. Some advantages in using axial disks include the prospect for added drive on target, the potential for P4 control when used in tandem with laser pointing, and possibly reduced time-dependent P2(t) flux asymmetry swings at early time. Neutron-based diagnostics have provided some suggestion of increased drive, but a more direct measure of drive enhancement is with the use of backlit, low-density (0.3 g/cc) foam surrogate targets. In this scheme, an ablatively driven, inwardly propagating shock is imaged in time using backlighting from an irradiated Ti disk placed outside of the hohlraum. The benefit in using low-density surrogate targets is an amplified shock speed that enables easier detection of both average shock motion (drive) and distortion (flux asymmetry). Experiments and calculations are in excellent agreement over a nearly 10% enhancement in peak drive temperature in the presence of axial gold disks. Measurements of lower-order distortion, P2(t) and P4(t), versus time for several laser pointings (without axial disks) using this technique have also been carried out and show good agreement between experiment and simulations. Efforts to further control time-dependent flux asymmetry using multiple ring, beam-phasing techniques on Nova, as will be required for the National Ignition Facility [J. Lindl, Phys. Plasmas 2, 3933 (1995)], are under development. Current designs indicate an appreciable reduction in P2(t) is possible. Significant control of time-integrated P4 flux asymmetry with appreciable inner and outer ring separation also appears possible.
Arbitrary amplitude double layers in dusty plasma
Authors: Roychoudhury Rajkumar , Chatterjee Prasanta
Published: July 2002

Arbitrary amplitude double layers in a dusty plasma consisting of warm dust particles and two temperature isothermal ions are studied using Sagdeev’s pseudopotential technique. Regions of the existence of double layers are investigated both analytically and numerically. It is seen that when one considers two-temperature ions, both compressive and rarefactive solitons are found but only compressive double layers could be obtained. It is also shown that the result obtained in the small-amplitude limit conforms to the perturbation result obtained using the reduction perturbation technique.
High efficiency indium tin oxide/indium phosphide solar cells
Authors: Coutts T. J. , Naseem S.
Published: July 2002

Solar cells have been fabricated by rf sputter depositing indium tin oxide onto single crystal p-type indium phosphide. Four different substrate doping densities have been used but in all cases the dopant was zinc and the wafers were 〈100〉 oriented. The optimum doping density from the range studied was 3×1016 cm−3 and devices based on such substrates have yielded total area efficiencies up to 16.2% using the air mass 1.5 spectrum normalized to 100 mW cm−2, which correspond to active area efficiencies of 19.1%. A doping density less than the optimum yielded devices with excessive series resistance. Higher doping densities led to a marked loss of red response.
X-ray double crystal diffraction study of porous silicon
Authors: Young I. M. , Beale M. I. J. , Benjamin J. D.
Published: July 2002

Depending on the dopant concentration, two distinct types of porous silicon can be formed during the anodization of silicon in hydrofluoric acid. A range of samples of both types of porous silicon has been investigated using x-ray double crystal diffraction techniques. The crystal lattice of porous silicon is found to be tetragonally distorted. In the plane of the substrate, the interplanar spacing of the porous film is identical to that of the substrate but is increased in the direction normal to it. The increase is typically 700 ppm in the type of film formed on heavily doped silicon and 6000 ppm in that on lightly doped silicon. We propose that stresses, generated by the growth of a native oxide on the surface of the pores, are responsible for the observed increase in lattice parameter. The different interplanar spacings of the two types of film are related to the observed differences in their oxygen contents which are a consequence of their different surface area to volume ratios.
Resonant tunneling of holes in AlAs-GaAs-AlAs heterostructures
Authors: Mendez E. E. , Wang W. I. , Ricco B. , Esaki L.
Published: July 2002

We have observed resonant tunneling of holes through double-barrier AlAs-GaAs-AlAs structures sandwiched between p+-GaAs regions. The resonances appeared as negative resistance regions in the current-voltage characteristics perpendicular to the interface planes. A set of resonant structures was already visible at high temperatures (T≂250 K) while an additional set was observable only at low temperatures (T≤100 K). Although in principle the sets can be attributed to tunneling of light and heavy holes, respectively, an analysis of the number and relative voltage position of the resonances suggests that considerable band mixing occurs. The application of a strong magnetic field, parallel to the tunneling current, introduced shifts in the resonances which support that interpretation.
Location of positive charge trapped near the Si-SiO2 interface at low temperature
Authors: Chang S. T. , Lyon S. A.
Published: July 2002

A new technique has been developed to obtain the depth profile of trapped positive charge which has been introduced into the SiO2 layer in metal-oxide-silicon structures at low temperature. We use photon-assisted tunneling at a fixed photon energy to inject electrons from the Si into the SiO2. Trapped positive charge located closer to the interface than the point at which the electrons emerge into the oxide conduction band is not affected, while that farther away can capture electrons. From the shifts of the capacitance versus voltage curves at various biases we can profile the location of the trapped positive charge. We demonstrate the use of this technique with a comparison of the positive charge introduced by x irradiation and Fowler–Nordheim tunneling (high-field stress). For the x-ray irradiated samples, the positive charge is located considerably farther (20–35 Å) from the Si-SiO2 interface than for high-field stressed samples (<20 Å).
Ultralow-threshold graded-index separate-confinement single quantum well buried heterostructure (Al,Ga)As lasers with high reflectivity coatings
Authors: Derry Pamela L. , Yariv Amnon , Lau Kam Y. , Bar-Chaim Nadav , Lee Kevin , Rosenberg Jan
Published: July 2002

Unlike conventional semiconductor lasers, single quantum well lasers with high reflectively coatings have dramatically reduced threshold currents as a result of the smaller volume of the (active) quantum well region. A cw threshold current of 0.95 mA was obtained for a buried graded-index separate-confinement heterostructure single quantum well laser with facet reflectivities of ∼70%, a cavity length of 250 μm, and an active region stripe width of 1 μm.
Growth of high-quality GaAs using trimethylgallium and diethylarsine
Authors: Bhat R. , Koza M. A. , Skromme B. J.
Published: July 2002

In this letter we report the growth of high-quality gallium arsenide using trimethylgallium and diethylarsine. The epitaxial layers had excellent morphology, an n-type background free-carrier concentration as low as 3×1014 cm−3 and a liquid nitrogen temperature mobility as high as 64 600 cm2/V s. The low-temperature (2 K) photoluminescence spectrum has well-resolved excitonic peaks, confirming the high quality of the material.
Molecular beam epitaxy of GaAs/AlGaAs superlattice heterostructures on nonplanar substrates
Authors: Kapon E. , Tamargo M. C. , Hwang D. M.
Published: July 2002

GaAs/AlGaAs superlattice heterostructures with layer thicknesses ≲100 Å were grown by molecular beam epitaxy on nonplanar GaAs substrates. The resulting superlattices exhibit different periods, depending on the crystal plane on which they grow. Period variation of more than 50%, from 180 to 80 Å, was obtained for adjacent superlattice sections. The transition between regions of different periodicity was mostly smooth and occurred within lateral dimensions ≲100 Å. Our results suggest that molecular beam epitaxy of superlattice heterostructures on patterned substrates provides a method for obtaining controllable lateral variations in physical properties which depend on the superlattice period. In particular, by growing quantum well heterostructures on nonplanar substrates, it might be possible to utilize the strong dependence of the carrier confinement energy on the well thickness in order to achieve lateral carrier confinement.
Acoustic-jet plating of gold and copper at 7.5 MHz
Authors: von Gutfeld R. J. , Vigliotti D. R. , Wickramasinghe H. K.
Published: July 2002

We describe a new type of jet plating that uses a focused high-frequency sound field directed through the center of the jet. The acoustic jet results in an improved morphology for gold and copper depositions although plating rates are not enhanced. Four-point probe measurements of acoustic-jet plated copper lines show up to a 50% decrease in electrical resistivity compared to jet plated deposits.
Hard magnetic properties of R-Fe-Ti alloys
Authors: Hadjipanayis G. C. , Aly S. H. , Cheng Shu-Fan
Published: July 2002

Magnetic and structural properties of as-cast and melt-spun alloys of the RxFeyTiz system with R≡Sm, Nd and 3≤x≤15, 78≤y≤87, and 3≤z≤13 are presented. Two major anisotropic phases with approximate compositions (RTi0.9)15–17Fe83–85 and (R3Ti0.9)15–17Fe83–85 were found to have anisotropy fields greater than 18 kOe and Tc in the range of 150–320 °C. Room-temperature coercivities in crystallized ribbons, powders, and sintered magnets were found to be in the range 0.2–2.5 kOe.
Raman scattering measurements of decreased barrier heights in GaAs following surface chemical passivation
Authors: Farrow L. A. , Sandroff C. J. , Tamargo M. C.
Published: July 2002

We present Raman scattering data from GaAs samples whose surfaces had been treated with thin films of sodium sulfide nonahydride (Na2S⋅9H2O). Raman scattering provides a quantitative, contactless means of measuring the reduced barrier height associated with decreased density of GaAs surface states. For GaAs samples doped at levels of n≊1018 cm−3, the barrier height is reduced to 0.48±0.10 eV.
Formation of the interface between GaAs and Si: Implications for GaAs-on-Si heteroepitaxy
Authors: Bringans R. D. , Olmstead Marjorie A. , Uhrberg R. I. G. , Bachrach R. Z.
Published: July 2002

Results of photoemission core-level spectroscopy measurements for coverages of around one monolayer of As, Ga, and GaAs on Si substrates are presented. The interfaces were formed on on-axis Si(100) and Si(111) substrates using molecular beam epitaxy techniques. The bonding between As and the substrate surface leaves the As atoms fully coordinated and thus extremely unreactive. This causes the GaAs films to form islands at average coverages of less than one monolayer. The surface between the islands is found to be terminated by a single atomic layer of As. Use of a Ga predeposition technique shows evidence of decreasing the area between islands. Results for As interaction with stepped Si(100) surfaces and the implications for avoidance of antiphase domain boundaries are discussed.
Homogeneous gain saturation in 1.5 μm InGaAsP traveling-wave semiconductor laser amplifiers
Authors: Mukai Takaaki , Inoue Kyo , Saitoh Tadashi
Published: July 2002

Spectral line broadening in semiconductor lasers is studied experimentally through the gain saturation characteristics of 1.5 μm InGaAsP traveling-wave laser amplifiers. Wide signal gain spectrum under the injection of an intense saturating signal is measured using a weak probe signal. The saturated signal gain spectrum is found to coincide exactly with the unsaturated spectrum under a less biased condition, thus verifying that the semiconductor laser gain saturates homogeneously over the entire gain spectrum. Cross-saturation characteristics between the two signal channels having identical input powers are also investigated and found to be in good agreement with theoretical calculations based on the homogeneous gain model. The degree of gain saturation is confirmed to be uniquely determined by the total output power from both channels.
Investigation of triethylarsenic as a replacement for arsine in the metalorganic chemical vapor deposition of GaAs
Authors: Lum R. M. , Klingert J. K. , Wynn A. S. , Lamont M. G.
Published: July 2002

GaAs growth experiments have been performed with triethylarsenic (TEAs) to investigate its potential as a replacement for arsine, and to compare the effects on film properties of substituting ethyl for methyl groups in alkyl arsenic sources used in metalorganic chemical vapor deposition. Films were deposited over a wide range of growth conditions (Tg=550–750 °C, V/III=2–13), and data were obtained on film electrical and optical properties and variations in growth rate. Growth with TEAs yielded films with good surface morphology, low background doping levels (<1015 cm−3) and 77 K mobilities of 13 000 cm2/V s. Although this represents a considerable improvement over films grown with trimethylarsenic, film properties still appear to be limited by unacceptably high levels of carbon incorporation. Experiments using triethylgallium as the group III source in place of trimethylgallium resulted in substantially reduced and nonuniform growth due to prereaction at the reactor walls.
Photovoltaic detection of infrared light in a GaAs/AlGaAs superlattice
Authors: Kastalsky A. , Duffield T. , Allen S. J. , Harbison J.
Published: July 2002

We report the observation of photovoltaic infrared photodetection in an n-type GaAs/AlGaAs superlattice within the optical range 3.6–6.2 μm. A built-in graded AlGaAs barrier (∼0.2 eV) provides charge polarization in the sample by allowing the optically excited electrons in the second miniband of the superlattice to diffuse over this barrier. The optical polarization of the infrared signal is consistent with the selection rules applicable to the superlattice. The infrared photoresponse results from first-to-second miniband as well as shallow donor-to-second miniband photoexcitations within the superlattice. Donor-to-second miniband photoexcitation is dominant at low temperatures (<25 K).
Near-ideal low threshold behavior in (111) oriented GaAs/AlGaAs quantum well lasers
Authors: Hayakawa T. , Suyama T. , Takahashi K. , Kondo M. , Yamamoto S. , Hijikata T.
Published: July 2002

Fundamental characteristics of (111) oriented GaAs/AlGaAs graded-index separate-confinement-heterostructure single quantum well lasers have been compared with conventional (100) oriented lasers. In particular, the threshold current density Jth of (111) oriented lasers does not change with the well width Lz in the range of Lz=30–100 Å, which corresponds to an ideal extreme. The lowest Jth of 145 A/cm2 together with a high characteristic temperature T0 of 186 K in the threshold-temperature dependence has been achieved for an Lz of 40 Å and a cavity length of 490 μm. The dependence of T0 on Lz showed that T0 is maximum at Lz∼60 Å for both (111) and (100) oriented lasers.
Linear 1×2 directional coupler for electromagnetic field detection
Authors: Howerton Marta McWright , Bulmer Catherine H. , Burns William K.
Published: July 2002

A 1×2 directional coupler which can be used as an integrated optical electromagnetic field detector is described. Modulators have been formed from titanium-indiffused lithium niobate waveguides operating at the 1.3 μm wavelength. Linear dynamic ranges of 74.3 dB with 190 μW light and 76.3 dB with 760 μW light were observed for separate devices. Voltage sensitivity measurements are in good agreement with theoretical predictions. A 3.1 μV rms sensitivity was obtained for a device with a 10 mm electrode length.
Electrodispersive multiple quantum well modulator
Authors: Lee Y. H. , Jewell J. L. , Walker S. J. , Tu C. W. , Harbison J. P. , Florez L. T.
Published: July 2002

Quantum-confined Stark effect is combined with a Fabry–Perot resonance to build a multiple quantum well electro-optic modulator. The structure consists of GaAs/AlGaAs quantum wells between two epitaxial AlAs/AlGaAs dielectric multilayer mirrors, all grown by molecular beam epitaxy. The modulator uses refractive index changes induced by applied electric fields. In reflection mode of operation, the modulator demonstrates >5:1 contrast ratio and >50% absolute maximum reflectivity with 17 V applied.
Characteristics of glycine receptors expressed by embryonic rat brain mRNAs
Authors: García-Alcocer Guadalupe , García-Colunga Jesús , Martínez-Torres Ataúlfo , Miledi Ricardo
Published: July 2002

A study was made of glycine (Gly) and γ-aminobutyric acid (GABA)
receptors expressed in
Xenopus
oocytes injected with rat
mRNAs isolated from the encephalon, midbrain, and brainstem of
18-day-old rat embryos. In oocytes injected with encephalon, midbrain,
or brainstem mRNAs, the Gly-current amplitudes (membrane current
elicited by Gly; 1 mM Gly) were respectively 115 ± 35, 346
± 28, and 389 ± 22 nA, whereas the GABA-currents (1 mM GABA)
were all ≤40 nA. Moreover, the Gly-currents desensitized faster in
oocytes injected with encephalon or brainstem mRNAs. The
EC
50
for Gly was 611 ± 77 μM for encephalon,
661 ± 28 μM for midbrain, and 506 ± 18 μM for brainstem
mRNA-injected oocytes, and the corresponding Hill coefficients were all
≈2. Strychnine inhibited all of the Gly-currents, with an
IC
50
of 56 ± 3 nM for encephalon, 97 ± 4 nM for
midbrain, and 72 ± 4 nM for brainstem mRNAs. During repetitive
Gly applications, the Gly-currents were potentiated by 1.6-fold for
encephalon, 2.1-fold for midbrain, and 1.3-fold for brainstem
RNA-injected oocytes. Raising the extracellular Ca
2+
concentration significantly increased the Gly-currents in oocytes
injected with midbrain and brainstem mRNAs. Reverse transcription–PCR
studies showed differences in the Gly receptor (GlyR) α-subunits
expressed, whereas the β-subunit was present in all three types of
mRNA. These results indicate differential expression of GlyR mRNAs in
the brain areas examined, and these mRNAs lead to the expression of
GlyRs that have different properties. The modulation of GlyRs by
Ca
2+
could play important functions during brain
development.
Molecular phenotyping for analyzing subtle genetic effects in mice: Application to an angiotensinogen gene titration
Authors: Kim Hyung-Suk , Lee Gene , John Simon W. M. , Maeda Nobuyo , Smithies Oliver
Published: July 2002

The angiotensinogen M235T polymorphism in humans is linked to differential expression of the human angiotensinogen gene (
AGT
) gene and hypertension, but the homeostatic responses resulting from this polymorphism are not known. We therefore investigated how mice respond to five genetically determined levels of mouse angiotensinogen gene (
Agt
) expression covering the range associated with the M235T variants. By using high-throughput molecular phenotyping, tissue RNAs were assayed for expression of 10 genes important in hypertension. Significant positive and negative responses occurred in both sexes as
Agt
expression increased twofold, including a three-fold increase in aldosterone synthase expression in adrenal gland, and a two-fold decrease in renin expression in kidney. In males, cardiac expression of the precursor of atrial natriuretic peptide B and of adrenomedullin also increased approximately twofold. The relative expression of all genes studied except
Agt
differed significantly in the two sexes, and several unexpected relationships were encountered. A highly significant correlation between renal expression of the angiotensin type 1a receptor and kallikrein, independent of
Agt
genotype, is present in females (
P
< 0.0001) but not males (
P
= 0.4). The correlation between blood pressure (BP) and liver
Agt
expression
within
the five
Agt
genotypes is significant in females (
P
= 0.0005) but not in males (
P
= 0.2), whereas correlation of BP with differences
between
the genotypes is less in females (
P
= 0.06) than in males (
P
= 0.001). The marked gender differences in gene expression in wild-type mice and the changes induced by moderate alterations in
Agt
expression and BP emphasize the need to look for similar differences in humans.
Inositol hexakisphosphate promotes dynamin I- mediated endocytosis
Authors: Høy Marianne , Efanov Alexander M. , Bertorello Alejandro M. , Zaitsev Sergei V. , Olsen Hervør L. , Bokvist Krister , Leibiger Barbara , Leibiger Ingo B. , Zwiller Jean , Berggren Per-Olof , Gromada Jesper
Published: July 2002

Membrane homeostasis is maintained by exocytosis and endocytosis. The molecular mechanisms regulating the interplay between these two processes are not clear. Inositol hexakisphosphate (InsP
6
) is under metabolic control and serves as a signal in the pancreatic β cell stimulus-secretion coupling by increasing Ca
2+
-channel activity and insulin exocytosis. We now show that InsP
6
also promotes dynamin I-mediated endocytosis in the pancreatic β cell. This effect of InsP
6
depends on calcineurin-induced dephosphorylation and is accounted for by both activation of protein kinase C and inhibition of the phosphoinositide phosphatase synaptojanin and thereby formation of phosphatidylinositol 4,5-bisphosphate. In regulating both exocytosis and endocytosis, InsP
6
thus may have an essential integral role in membrane trafficking.
Inactivation of the acid labile subunit gene in mice results in mild retardation of postnatal growth despite profound disruptions in the circulating insulin-like growth factor system
Authors: Ueki Iori , Ooi Guck T. , Tremblay Michel L. , Hurst Kelley R. , Bach Leon A. , Boisclair Yves R.
Published: July 2002

Insulin-like growth factors (IGFs) I and II are important regulators of cell proliferation and differentiation. After birth, plasma IGFs, representing mostly liver-derived IGFs, circulate in ternary complexes of 150 kDa consisting of one molecule each of IGF, IGF-binding protein (IGFBP) 3, and an acid labile subunit (ALS). Onset of ALS synthesis after birth is the primary factor driving the formation of ternary complexes. Capture of IGFs by ALS is thought to allow the development of a plasma reservoir without negative effects such as hypoglycemia and cell proliferation. To evaluate the importance of ALS and ternary complexes, we have created mice in which the
ALS
gene has been inactivated. The mutation was inherited in a Mendelian manner, without any effects on survival rates and birth weights. A growth deficit was observed in null mice after 3 weeks of life and reached 13% by 10 weeks. This modest phenotype was observed despite reductions of 62 and 88% in the concentrations of plasma IGF-I and IGFBP-3, respectively. Increased turnover accounted for these reductions because indices of synthesis in liver and kidney were not decreased. Surprisingly, absence of ALS did not affect glucose and insulin homeostasis. Therefore, ALS is required for postnatal accumulation of IGF-I and IGFBP-3 but, consistent with findings supporting a predominant role for locally produced IGF-I, is not critical for growth. This model should be useful to determine whether presence of ALS is needed for other actions of liver-derived IGF-I and for maintenance of homeostasis in presence of high circulating levels of IGF-II.
Diverse developmental programs of
Xenopus laevis
metamorphosis are inhibited by a dominant negative thyroid hormone receptor
Authors: Schreiber Alexander M. , Das Biswajit , Huang Haochu , Marsh-Armstrong Nicholas , Brown Donald D.
Published: July 2002

Metamorphosis of anuran tadpoles is controlled by thyroid hormone (TH). Here we demonstrate that transgenic
Xenopus
laevis
tadpoles expressing a dominant negative form of TH receptor-α are resistant to a wide variety of the metamorphic changes induced by TH. This result confirms that TH receptors mediate both early and late developmental programs of metamorphosis as diverse as growth in the brain, limb buds, nose and Meckel's cartilage, remodeling of the intestine, and death and resorption of the gills and tail.
Caveolin-1 down-regulates inducible nitric oxide synthase via the proteasome pathway in human colon carcinoma cells
Authors: Felley-Bosco Emanuela , Bender Florent C. , Courjault-Gautier Françoise , Bron Claude , Quest Andrew F. G.
Published: July 2002

To investigate whether caveolin-1 (cav-1) may modulate inducible
nitric oxide synthase (iNOS) function in intact cells, the human
intestinal carcinoma cell lines HT29 and DLD1 that have low
endogenous cav-1 levels were transfected with cav-1 cDNA.
In nontransfected cells, iNOS mRNA and protein levels were increased by
the addition of a mix of cytokines. Ectopic expression of cav-1 in both
cell lines correlated with significantly decreased iNOS activity and
protein levels. This effect was linked to a posttranscriptional
mechanism involving enhanced iNOS protein degradation by the proteasome
pathway, because (
i
) induction of iNOS mRNA by cytokines
was not affected and (
ii
) iNOS protein levels increased
in the presence of the proteasome inhibitors
N-
acetyl-Leu-Leu-Norleucinal and lactacystin. In
addition, a small amount of iNOS was found to cofractionate with cav-1
in Triton X-100-insoluble membrane fractions where also iNOS
degradation was apparent. As has been described for endothelial and
neuronal NOS isoenzymes, direct binding between cav-1 and human iNOS
was detected
in vitro
. Taken together, these results
suggest that cav-1 promotes iNOS presence in detergent-insoluble
membrane fractions and degradation there via the proteasome pathway.
A functional peptide encoded in the Escherichia coli 23S rRNA.
Authors: Tenson T , DeBlasio A , Mankin A
Published: July 2002

A pentapeptide open reading frame equipped with a canonical ribosome-binding site is present in the Escherichia coli 23S rRNA. Overexpression of 23S rRNA fragments containing the mini-gene renders cells resistant to the ribosome-inhibiting antibiotic erythromycin. Mutations that change either the initiator or stop codons of the peptide mini-gene result in the loss of erythromycin resistance. Nonsense mutations in the mini-gene also abolish erythromycin resistance, which can be restored in the presence of the suppressor tRNA, thus proving that expression of the rRNA-encoded peptide is essential for the resistance phenotype. The ribosome appears to be the likely target of action of the rRNA-encoded pentapeptide, because in vitro translation of the peptide mini-gene decreases the inhibitory action of erythromycin on cell-free protein synthesis. Thus, the new mechanism of drug resistance reveals that in addition to the structural and functional role of rRNA in the ribosome, it may also have a peptide-coding function.
Expression and characterization of glycogen synthase kinase-3 mutants and their effect on glycogen synthase activity in intact cells.
Authors: Eldar-Finkelman H , Argast G M , Foord O , Fischer E H , Krebs E G
Published: July 2002

In these studies we expressed and characterized wild-type (WT) GSK-3 (glycogen synthase kinase-3) and its mutants, and examined their physiological effect on glycogen synthase activity. The GSK-3 mutants included mutation at serine-9 either to alanine (S9A) or glutamic acid (S9E) and an inactive mutant, K85,86MA. Expression of WT and the various mutants in a cell-free system indicated that S9A and S9E exhibit increased kinase activity as compared with WT. Subsequently, 293 cells were transiently transfected with WT GSK-3 and mutants. Cells expressing the S9A mutant exhibited higher kinase activity (2.6-fold of control cells) as compared with cells expressing WT and S9E (1.8- and 2.0-fold, respectively, of control cells). Combined, these results suggest serine-9 as a key regulatory site of GSK-3 inactivation, and indicate that glutamic acid cannot mimic the function of the phosphorylated residue. The GSK-3-expressing cell system enabled us to examine whether GSK-3 can induce changes in the endogenous glycogen synthase activity. A decrease in glycogen synthase activity (50%) was observed in cells expressing the S9A mutant. Similarly, glycogen synthase activity was suppressed in cells expressing WT and the S9E mutant (20-30%, respectively). These studies indicate that activation of GSK-3 is sufficient to inhibit glycogen synthase in intact cells, and provide evidence supporting a physiological role for GSK-3 in regulating glycogen synthase and glycogen metabolism.
Nuclease-resistant ribozymes decrease stromelysin mRNA levels in rabbit synovium following exogenous delivery to the knee joint.
Authors: Flory C M , Pavco P A , Jarvis T C , Lesch M E , Wincott F E , Beigelman L , Hunt S W , Schrier D J
Published: July 2002

Catalytic RNA molecules, or ribozymes, have generated significant interest as potential therapeutic agents for controlling gene expression. Although ribozymes have been shown to work in vitro and in cellular assays, there are no reports that demonstrate the efficacy of synthetic, stabilized ribozymes delivered in vivo. We are currently utilizing the rabbit model of interleukin 1-induced arthritis to assess the localization, stability, and efficacy of exogenous antistromelysin hammerhead ribozymes. The matrix metalloproteinase stromelysin is believed to be a key mediator in arthritic diseases. It seems likely therefore that inhibiting stromelysin would be a valid therapeutic approach for arthritis. We found that following intraarticular administration ribozymes were taken up by cells in the synovial lining, were stable in the synovium, and reduced synovial interleukin 1 alpha-induced stromelysin mRNA. This effect was demonstrated with ribozymes containing various chemical modifications that impart nuclease resistance and that recognize several distinct sites on the message. Catalytically inactive ribozymes were ineffective, thus suggesting a cleavage-mediated mechanism of action. These results suggest that ribozymes may be useful in the treatment of arthritic diseases characterized by dysregulation of metalloproteinase expression.
Covalent HLA-B27/peptide complex induced by specific recognition of an aziridine mimic of arginine.
Authors: Weiss G A , Valentekovich R J , Collins E J , Garboczi D N , Lane W S , Schreiber S L , Wiley D C
Published: July 2002

The class I major histocompatibility complex (MHC) glycoprotein HLA-B27 binds short peptides containing arginine at peptide position 2 (P2). The HLA-B27/peptide complex is recognized by T cells both as part of the development of the repertoire of T cells in the cellular immune system and during activation of cytotoxic T cells. Based on the three-dimensional structure of HLA-B27, we have synthesized a ligand with an aziridine-containing side chain designed to mimic arginine and to bind covalently in the arginine-specific P2 pocket of HLA-B27. Using tryptic digestion followed by mass spectrometry and amino acid sequencing, the aziridine-containing ligand is shown to alkylate specifically cysteine 67 of HLA-B27. Neither free cysteine in solution nor an exposed cysteine on a class II MHC molecule can be alkylated, showing that specific recognition between the anchor side-chain pocket of an MHC class I protein and the designed ligand (propinquity) is necessary to induce the selective covalent reaction with the MHC class I molecule.
Random circular permutation of genes and expressed polypeptide chains: application of the method to the catalytic chains of aspartate transcarbamoylase.
Authors: Graf R , Schachman H K
Published: July 2002

Recent studies on proteins whose N and C termini are in close proximity have demonstrated that folding of polypeptide chains and assembly of oligomers can be accomplished with circularly permuted chains. As yet no methodical study has been conducted to determine how extensively new termini can be introduced and where such termini cannot be tolerated. We have devised a procedure to generate random circular permutations of the catalytic chains of Escherichia coli aspartate transcarbamoylase (ATCase; EC 2.1.3.2) and to select clones that produce active or stable holoenzyme containing permuted chains. A tandem gene construct was made, based on the desired linkage between amino acid residues in the C- and N-terminal regions of the polypeptide chain, and this DNA was treated with a suitable restriction enzyme to yield a fragment containing the rearranged coding sequence for the chain. Circularization achieved with DNA ligase, followed by linearization at random with DNase I, and incorporation of the linearized, repaired, blunt-ended, rearranged genes into a suitable plasmid permitted the expression of randomly permuted polypeptide chains. The plasmid with appropriate stop codons also contained pyrI, the gene encoding the regulatory chain of ATCase. Colonies expressing detectable amounts of ATCase-like molecules containing permuted catalytic chains were identified by an immunoblot technique or by their ability to grow in the absence of pyrimidines in the growth medium. Sequencing of positive clones revealed a variety of novel circular permutations. Some had N and C termini within helices of the wild-type enzyme as well as deletions and insertions. Permutations were concentrated in the C-terminal domain and only few were detected in the N-terminal domain. The technique, which is adaptable generally to proteins whose N and C termini are near each other, can be of value in relating in vivo folding of nascent, growing polypeptide chains to in vitro renaturation of complete chains and determining the role of protein sequence in folding kinetics.