bluuebunny/crossref_metadata_2025 huggingface dataset:
Spin coating with slow evaporation
Authors: Lawrence C. J.
Published: July 2002

The spin coating of polymer solutions is discussed, with emphasis on the nature of the evaporation boundary condition. A dimensional analysis reveals that the appropriate condition is a constant small evaporation rate, which is determined by the mass transfer in the adjacent atmosphere. An error in previous work [Phys. Fluids 31, 2786 (1988)] is brought to light, and it is shown that the previous paper would be valid for rapid evaporation, such as might occur if the ambient condition were near vacuum. The transport of solvent in the liquid film is described by a boundary layer diffusion equation with large Peclet number P0. When the dimensionless evaporation rate ê is small, the diffusion equation has a self-similar solution in which the boundary layer thickness and polymer concentration increase in proportion to the decrease in film thickness. The final film thickness is determined by the boundary layer thickness, which is controlled by P0 and is independent of ê. Thus the final film thickness is found to be the same as for the case of rapid evaporation.
Examples of air-entraining flows
Authors: Og̃uz H. N. , Prosperetti A. , Lezzi A. M.
Published: July 2002

Four examples of air-entraining flows at the free surface of a liquid are briefly considered: (a) the transient impact of a jet, (b) the application of an excess pressure, (c) two counter-rotating vortices below the surface, and (d) a disturbance on a vortex sheet.
Global modes and high-energy particles in ignited plasmas
Authors: Coppi B. , Migliuolo S. , Pegoraro F. , Porcelli F.
Published: July 2002

Fusion-produced alpha particles may spontaneously enhance the stability of an ignited plasma against m=1 internal modes. Stable values of the poloidal beta of the thermal plasma component significantly in excess of the ideal magnetohydrodynamic threshold can be attained, provided the region where the magnetic helical parameter q is below unity is not too wide and the ignition temperature is not too high. A comprehensive analysis of the different instability regimes is presented, with special attention to so-called ‘‘fishbone oscillations’’ and their influence on the ignition energy balance. The theoretical predictions compare favorably with sawtooth suppression experiments in the JET tokamak [Phys. Rev. Lett. 60, 2148 (1988)] with high-power, ion-cyclotron resonant frequency heating, where energetic ions in the MeV range are produced.
Experimental study of multiple frequency effects in a free electron laser amplifier
Authors: Xu K. , Bekefi G.
Published: July 2002

Measurements are presented of the gain and phase of a free electron laser (FEL) amplifier in the presence of a second, perturbing electromagnetic wave of different frequency. The studies are carried out in the microwave range of 7–11 GHz using a mildly relativistic electron beam of ∼160 kV energy and a current of ∼1 A. The presence of the perturbing wave causes serious deterioration of the FEL performance.
Effect of ion drift on arbitrary-amplitude ion-acoustic solitary waves
Authors: Kuehl H. H. , Zhang C. Y.
Published: July 2002

The properties of arbitrary-amplitude solitary waves in a plasma consisting of drifting ions and nondrifting electrons are investigated. The conditions for the existence of solitary-wave solutions are determined. It is shown that electron inertia has the significant effect of restricting the regime of solitary waves to ion drift velocities that are less than the electron thermal speed. Because of this restriction relativistic effects are small.
Nonlinear laser–matter interaction processes in long-scale-length plasmas
Authors: Seka W. , Bahr R. E. , Short R. W. , Simon A. , Craxton R. S. , Montgomery D. S. , Rubenchik A. E.
Published: July 2002

This paper reports on nonlinear laser–plasma interaction experiments using long-scale-length plasmas produced by the 24-beam OMEGA laser system operating at 351 nm. The experiments were carried out with distributed phase plates (DPP’s) in all beams and with and without smoothing by spectral dispersion (SSD). Most of the beams were used to create a large preformed plasma, which had gradient scale lengths of ≤800 μm at electron densities below a quarter of the critical density nc and temperatures in excess of 1 keV. One of the beams, the ‘‘interaction beam,’’ was timed independently and tightly focused to intensities ∼1015 W/cm2. All beams had pulse durations of ∼0.6 nsec. The interaction processes studied were mainly Raman scattering and the two-plasmon decay (TPD) instability as evidenced by its characteristic 3/2-harmonic emission. Details of the Raman and 3/2-harmonic spectra are presented. Evidence was found for the TPD instability close to its Landau cutoff density at ∼ 0.2nc. Raman emission was narrow-band and observed only from densities < 0.2nc. For late timings of the interaction beam, the Raman emission appeared to originate from near the peak of the density profile, but for earlier timings it appeared to come from densities a factor of 2 below the calculated peak. Application of SSD affected the 3/2-harmonic emission only slightly, but it strongly reduced the Raman emission. A discussion is given of some models that attempt to explain these observations.
Three-dimensional toroidal equilibria and stability by a variational spectral method
Authors: Lao L. L. , Greene J. M. , Wang T. S. , Helton F. J. , Zawadzki E. M.
Published: July 2002

The characteristics of the partial differential equations describing three-dimensional toroidal magnetohydrodynamic equilibria with nested flux surfaces in inverse flux coordinates are derived and examined. The equilibrium equations are then variationally reduced to a truncated set of ordinary differential equations by decomposing the flux surface geometry into a spectral representation. The magnetic field lines on the flux surfaces are given in terms of a variable stream function to allow optimum choice of the angle coordinates over the flux surfaces and to simplify the treatment in the vicinity of a rational magnetic surface. Analytic properties of the spectral representation and moment equations are considered. Comparative calculations are performed numerically. The results agree well with those calculated using a standard three-dimensional equilibrium code, but the variational spectral method is substantially faster computationally. The Mercier stability criterion is given.
Velocity-space structure of runaway electrons
Authors: Fuchs V. , Cairns R. A. , Lashmore-Davies C. N. , Shoucri M. M.
Published: July 2002

The region of velocity space is determined in which electron runaway occurs because of a dc electric field. Phase-space analysis of the relaxation equations describing test electrons, corroborated by two-dimensional (2-D) numerical integration of the Fokker–Planck equation, reveals that the Dreicer condition for runaway v2∥≥(2+Zi)Ec/E is only sufficient. A weaker condition v2∥≥(2+Zi)1/2Ec/E is established, and it is shown, in general, that runaway in velocity space only occurs for those electrons that are outside one of the separatrices of the relaxation equations. The scaling with v∥ of the parallel distribution function and of the perpendicular temperature is also derived.
A paradox concerning the extended Stokes series solution for the pressure drop in coiled pipes
Authors: Ramshankar R. , Sreenivasan K. R.
Published: July 2002

While the extended Stokes series (ESS) solution for the laminar pressure drop in coiled pipes predicts that the friction ratio (the ratio of pressure drop in the coiled pipe to that in the straight pipe of the same flow rate) varies asymptotically as one-fourth power of the Dean number, the existing experimental data largely support a square-root variation; previous boundary layer analyses also predict a square-root variation. The subject of this paper is an examination of this paradox. A detailed review shows that the existing set of experimental data can be grouped into various categories depending on the precise flow conditions. It is shown that none of the existing data satisfies the proper experimental conditions required by the ESS method. New experiments reveal that the pressure drop demanded by the ESS solution can actually be observed if they are expressly designed to satisfy the conditions demanded by the ESS solution. The implication is that the domains of initial conditions appropriate to the boundary layer and ESS solutions are quite different.
Effect of carrier gas pressure on condensation in a supersonic nozzle
Authors: Wyslouzil B. E. , Wilemski G. , Beals M. G. , Frish M. B.
Published: July 2002

Supersonic nozzle experiments were performed with a fixed water or ethanol vapor pressure and varying amounts of nitrogen to test the hypothesis that carrier gas pressure affects the onset of condensation. Such an effect might occur if nonisothermal nucleation were important under conditions of excess carrier gas in the atmospheric pressure range, as has been suggested by Ford and Clement [J. Phys. A 22, 4007 (1989)]. Although a small increase was observed in the condensation onset temperature as the stagnation pressure was reduced from 3 to 0.5 atm, these changes cannot be attributed to any nonisothermal effects. The pulsed nozzle experiments also exhibited two interesting anomalies: (1) the density profiles for the water and ethanol mixtures were shifted in opposite directions from the dry N2 profile; (2) a long transient period was required before the nozzle showed good pulse-to-pulse repeatability for condensible vapor mixtures. To theoretically simulate the observed onset behavior, calculations of nucleation and droplet growth in the nozzle were performed that took into account two principal effects of varying the carrier gas pressure: (1) the change in nozzle shape due to boundary layer effects and (2) the variation in the heat capacity of the flowing gas. Energy transfer limitations were neglected in calculating the nucleation rates. The trend of the calculated results matched that of the experimental results very well. Thus, heat capacity and boundary layer effects are sufficient to explain the experimental onset behavior without invoking energy transfer limited nucleation. The conclusions about the rate of nucleation are consistent with those obtained recently using an expansion cloud chamber, but are at odds with results from thermal diffusion cloud chamber measurements.
Ignition of aluminum droplets behind shock waves in water
Authors: Theofanous T. G. , Chen X. , Di Piazza P. , Epstein M. , Fauske H. K.
Published: July 2002

Gram quantities of molten aluminum droplets at temperatures up to 1973 K are forced to interact with water under sustained pressure pulses of up to 40.8 MPa in a hydrodynamic shock tube. Conditions are identified under which the thermal interaction (physical regime) develops into chemical ignition and total combustion events. This, and the absence of catastrophic breakup in the physical regime, raises some very interesting questions with regard to the sequence of events during large-scale aluminum–water explosions.
A new limiting form for steady periodic gravity waves with surface tension on deep water
Authors: Debiane M. , Kharif C.
Published: July 2002

The method developed by Longuet-Higgins [J. Inst. Math. Appl. 22, 261 (1978)] for the computation of pure gravity waves is extended to capillary-gravity waves in deep water. Surface tension provides an additional term in the identities between the Fourier coefficients in Stokes’ expansion. This term is then reduced to a simple function of the slope of the local tangent to the profile of the free surface. A set of nonlinear algebraic equations is derived and solved by using the Newton’s method. A new family of limiting profiles of steady gravity waves with surface tension is found.
(1+1)-dimensional turbulence
Authors: Benzi R. , Biferale L. , Tripiccione R. , Trovatore E.
Published: July 2002

A class of dynamical models of turbulence living on a one-dimensional dyadic-tree structure is introduced and studied. The models are obtained as a natural generalization of the popular GOY shell model of turbulence. These models are found to be chaotic and intermittent. They represent the first example of (1+1)-dimensional dynamical systems possessing non trivial multifractal properties. The dyadic structure allows us to study spatial and temporal fluctuations. Energy dissipation statistics and its scaling properties are studied. The refined Kolmogorov hypothesis is found to hold.
Dynamics of spontaneous spreading with evaporation on a deep fluid layer
Authors: Dussaud Anne D. , Troian Sandra M.
Published: July 2002

The spontaneous spreading of a thin volatile film along the surface of a deep fluid layer of higher surface tension provides a rapid and efficient transport mechanism for many technological applications. This spreading process is used, for example, as the carrier mechanism in the casting of biological and organic Langmuir–Blodgett films. We have investigated the dynamics of spontaneously spreading volatile films of different vapor pressures and spreading coefficients advancing over the surface of a deep water support. Laser shadowgraphy was used to visualize the entire surface of the film from the droplet source to the leading edge. This noninvasive technique, which is highly sensitive to the film surface curvature, clearly displays the location of several moving fronts. In this work we focus mainly on the details of the leading edge. Previous studies of the spreading dynamics of nonvolatile, immiscible thin films on a deep liquid layer have shown that the leading edge advances in time as t3/4 as predicted by laminar boundary layer theory. We have found that the leading edge of volatile, immiscible spreading films also advances as a power law in time, tα, where α∼1/2. Differences in the liquid vapor pressure or the spreading coefficient seem only to affect the speed of advance but not the value of the spreading exponent, which suggests the presence of a universal scaling law. Sideview laser shadowgraphs depicting the subsurface motion in the water reveal the presence of a single stretched convective roll right beneath the leading edge of the spreading film. This fluid circulation, likely caused by evaporation and subsequent surface cooling of the rapidly spreading film, resembles a propagating Rayleigh–Bénard convective roll. We propose that this sublayer rotational flow provides the additional dissipation responsible for the reduced spreading exponent.
Brinkman screening and the covariance of the fluid velocity in fixed beds
Authors: Koch Donald L. , Hill Reghan J. , Sangani Ashok S.
Published: July 2002

The phenomenon of Brinkman screening, whereby the fluid velocity disturbance produced by each particle in a fixed bed is attenuated by the forces that the fluid exerts on surrounding particles, plays a crucial role in limiting the range of velocity correlations in porous media and fixed beds. Koch and Brady [J. Fluid Mech. 154, 399 (1985)] showed theoretically that Brinkman screening leads to a finite hydrodynamic diffusion coefficient for fluid phase tracers in dilute fixed beds. In this Letter, we present the results of two simulation techniques (lattice-Boltzmann method and a multipole method) confirming the screening of the fluid velocity covariance that underlies the Koch and Brady theory.
Highly nonlinear ion-acoustic solitons in a warm magnetoplasma with drifting effect of electrons
Authors: Kalita B. C. , Bhatta R. P.
Published: July 2002

Ion-acoustic solitary waves in a warm, magnetoplasma under the initial drift motion of the electrons in the direction of the magnetic field, have been investigated. It has been established that both compressive and rarefactive solitons exist within the parametric domains. For higher (lower) values of the electron drift velocity ve′, it is demonstrated that the solitary wave is always subsonic (supersonic) near the lower regime of α (=Ti/Te, the ion to electron temperature ratio) and upper regime of α, respectively. It is of paramount importance to report that a higher and constant amplitude soliton region corresponding to low ion temperature and Mach number M exists for the upper ranged values of ve′ but to the contrary a smaller and constant amplitude soliton region exists corresponding to high ion temperature for the lower ranged values of ve′.
A study of mode conversion in an oxygen–hydrogen plasma
Authors: Johnson Jay R. , Chang Tom , Crew G. B.
Published: July 2002

Linear mode conversion is considered between the ion-cyclotron and magnetosonic branches in a multispecies plasma with parallel magnetic field gradients. The results are interpreted in terms of ion conic heating. The mode-conversion coefficients are solved using perturbation theory, a phase integral approach, and saddle-point theory. These results are compared with numerical calculations. The coefficients thus obtained demonstrate that substantial coupling occurs between the four propagating modes, and a definite absorption occurs. Such absorption corresponds to ion heating and is, under realistic circumstances, sufficient to explain the outflow and heating of ionospheric oxygen.
Computational modeling of neoclassical and resistive magnetohydrodynamic tearing modes in tokamaks
Authors: Gianakon T. A. , Hegna C. C. , Callen J. D.
Published: July 2002

Numerical studies of the nonlinear evolution of magnetohydrodynamic-type tearing modes in three-dimensional toroidal geometry with neoclassical effects are presented. The inclusion of neoclassical physics introduces an additional free-energy source for the nonlinear formation of magnetic islands through the effects of a bootstrap current in Ohm’s law. The neoclassical tearing mode is demonstrated to be destabilized in plasmas which are otherwise Δ′ stable, albeit once an island width threshold is exceeded. The plasma pressure dynamics and neoclassical tearing growth is shown to be sensitive to the choice of the ratio of the parallel to perpendicular diffusivity (χ∥/χ⊥). The study is completed with a demonstration and theoretical comparison of the threshold for single helicity neoclassical magnetohydrodynamic tearing modes, which is described based on parameter scans of the local pressure gradient, the ratio of perpendicular to parallel pressure diffusivities χ⊥/χ∥, and the magnitude of an initial seed magnetic perturbation.
Transverse electron susceptibility and the electromagnetic wave absorption in weakly collisional plasmas
Authors: Bychenkov V. Yu. , Tikhonchuk V. T. , Rozmus W.
Published: July 2002

A theory of plasma response to electromagnetic perturbations has been developed based on the full solution to the Fokker–Planck equation in high Z plasmas without electron–electron collisions. For the first time the transverse electron susceptibility has been calculated for the entire range of frequencies and wave numbers including the quantitative description of the weakly collisional regime where the wave frequency is comparable to the electron–ion collision frequency and the wave number is comparable to the inverse electron mean free path. The results have been compared to approximate expressions for the electron conductivity based on the Drude model showing discrepancy by a factor of few in regions where the spatial dispersion is important. The theory is applied to the calculation of laser light absorption in solid density plasmas.
Laser-produced carbon plasma expanding in vacuum, low pressure ambient gas and nonuniform magnetic field
Authors: Neogi A. , Thareja R. K.
Published: July 2002

Laser-produced carbon plasma expanding in vacuum, a low pressure ambient gas (5 mTorr and 100 mTorr) and a nonuniform magnetic field is studied using fast photography. The temporal behavior of various plasma parameters, like velocity, electron density, electron temperature and intensity, is reported. In 5 mTorr air, the plasma breaks into slow and fast components. In the magnetic field, oscillations are observed in the plasma parameters.
Hot plasma and nonlinear effects in inductive discharges
Authors: Godyak V. A. , Piejak R. B. , Alexandrovich B. M. , Kolobov V. I.
Published: July 2002

Recent advances in the physics of low pressure inductively coupled plasma (ICP) are reviewed where the interaction of an electromagnetic field with electrons is governed by electron thermal motion rather than electron-atom collisions. Direct measurement of the radio-frequency (rf) electric field and rf current density in the plasma made it possible to distinguish between collisional and stochastic (collisionless) electron heating. Several new phenomena such as a wave phase bifurcation, phase velocity reversal, a second current layer, and negative power absorption have been found and interpreted in terms of nonlocal electrodynamics. Nonlinear effects induced by the rf Lorentz force as well as modification of the spatial distribution of the plasma caused by ponderomotive forces have been demonstrated in ICP under discharge conditions typical for industrial applications. Classical works on the anomalous skin effect have been revived and successfully applied to describe these phenomena. The effects observed in these experiments with weakly ionized cold plasma are intrinsic to fusion and space plasmas.
Experimental investigation of the neutral sheet profile during magnetic reconnection
Authors: Yamada Masaaki , Ji Hantao , Hsu Scott , Carter Troy , Kulsrud Russell , Trintchouk Fedor
Published: July 2002

During magnetic reconnection, a “neutral sheet” current is induced, heating the plasma. The resultant plasma thermal pressure forms a stationary equilibrium with the opposing magnetic fields. The reconnection layer profile holds significant clues about the physical mechanisms which control reconnection. In the Magnetic Reconnection Experiment [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)], a quasi steady-state and axisymmetric neutral sheet profile has been measured precisely using a magnetic probe array with spatial resolution equal to one quarter of the ion gyro-radius. It was found that the reconnecting field profile fits well with a Harris-type profile [E. G. Harris, Il Nuovo Cimento 23, 115 (1962)], B(x)∼tanh(x/δ). This agreement is remarkable since the Harris theory does not take into account reconnection and associated electric fields and dissipation. An explanation for this agreement is presented. The sheet thickness δ is found to be ∼0.4 times the ion skin depth, which agrees with a generalized Harris theory incorporating nonisothermal electron and ion temperatures and finite electric field. The detailed study of additional local features of the reconnection region is also presented.
Vacuum ultraviolet laser emission from Nd+3:LaF3
Authors: Waynant Ronald W. , Klein Philipp H.
Published: July 2002

Laser emission at 172 nm has been produced by pumping a Nd:LaF3 crystal with incoherent Kr*2 radiation at 146 nm. The 5-ns pulse contained approximately 20–30 μJ of energy. Fluorescence measurements indicate potential for tuning from 170–175 nm, which should be observable with OH−-free crystals.
Room-temperature continuous-wave operation of an AlGaInP double heterostructure laser grown by atmospheric pressure metalorganic chemical vapor deposition
Authors: Ikeda Masao , Mori Yoshifumi , Sato Hiromitsu , Kaneko Kunio , Watanabe Naozo
Published: July 2002

Continuous-wave (cw) operation at temperatures up to 23 °C of an Al0.26Ga0.26In0.48P/Ga0.52In0.48P/ Al0.26Ga0.26In0.48P double heterostructure (DH) laser has been achieved for the first time. The threshold current was 160 mA at 20 °C for a device with a 10-μm-wide and 250-μm-long ion-implanted stripe geometry. The emission wavelength was 671 nm during cw operation at 10 °C. To reduce thermal resistance to a heat sink, a dually stacked structure made of a thin (∼0.3 μm) p-AlGaInP layer and a p-Al0.76Ga0.24As layer was used as a cladding layer. The DH wafer was grown by atmospheric pressure metalorganic chemical vapor deposition.
Tunneling through narrow-gap semiconductor barriers
Authors: Heremans J. , Partin D. L. , Dresselhaus P. D. , Lax B.
Published: July 2002

Tunneling probability calculations through the gap of narrow-gap semiconductors reveal that a large increase in current density near zero bias can be expected when electrons tunnel near the top of the valence band, or conversely holes near the bottom of the conduction band. This effect is obtained by extending the nonparabolic relativistic dispersion relation into the gap to describe the imaginary electron wave vector k which is used in the Wentzel–Kramers–Brillouin calculation. The effect is not limited to that case because it is due to the fact that at the band edges the wave vector goes to zero, or to a Brillouin zone edge k0 value. A double heterojunction device is proposed in Pb1−zSnzTe to illustrate this property, and the tunneling current through it fully modeled. Under a bias voltage applied to the semiconductor, electrons are forced away from the valence-band edge. The tunneling probability is then decreased, because k is larger and a negative resistance is expected.
Low-temperature elastic properties of the permanent magnet compound Nd2Fe14B
Authors: Dadon D. , Dariel M. P. , Gefen Y. , Klimker H. , Rosen M.
Published: July 2002

The velocity of longitudinal and transverse sound waves was measured between 77 and 300 K and used to determine the Young modulus, the shear modulus, and the adiabatic compressibility of the polycrystalline, strong permanent magnet compound, Nb2Fe14B. The spin reorientation occurring near 150 K appears as an anomaly in the temperature dependence of the elastic moduli. The ΔE effect is of the order of 0.5% at 77 K.
Containerless undercooling of bulk Fe-Ni melts
Authors: Willnecker R. , Herlach D. M. , Feuerbacher B.
Published: July 2002

Bulk samples of Fe1−xNix, with x covering the whole concentration range 0≤x≤1, have been undercooled in a containerless state by electromagnetic levitation, yielding undercooling levels not achieved to date by any other technique. Comparison with a theoretical approximation indicates that, despite the large undercoolings attained, the limit of homogeneous nucleation is not yet reached. The solidification of the undercooled melts is initiated rather by heterogeneous surface nucleation originating from surface oxidation processes.
Effect of gain nonlinearities on period doubling and chaos in directly modulated semiconductor lasers
Authors: Agrawal Govind P.
Published: July 2002

The mode gain in semiconductor lasers decreases with an increase in power due to nonlinear processes such as spectral hole burning. When these small nonlinearities are included in the single-mode rate equations, it is found that they can eliminate the previously predicted sequence of period-doubling bifurcations leading to chaos in directly modulated semiconductor lasers. For InGaAsP lasers used in optical communication systems, the nonlinear effects are strong enough that the possibility of chaotic behavior should be of no concern. The results also show that gain nonlinearities should be included when gain switching is used for the generation of picosecond pulses.
Optical spectroscopy of ultrasmall structures etched from quantum wells
Authors: Kash K. , Scherer A. , Worlock J. M. , Craighead H. G. , Tamargo M. C.
Published: July 2002

We have studied the photoluminescence and photoexcitation spectra of ultrasmall structures, of approximately 500 Å in dimension, which we refer to as quantum ribbons and quantum disks. These are fabricated from GaAs-AlGaAs quantum wells grown by molecular beam epitaxy and patterned by electron beam lithography. Contrary to our expectation, photoluminescence from these structures is extremely efficient. The excitation spectra of the two types of small structures differ greatly from each other and from that of the as-grown quantum wells. These differences may be a result of the confinement of the carriers in these small structures.
Determination of bonding in amorphous carbon films: A quantitative comparison of core-electron energy-loss spectroscopy and 13C nuclear magnetic resonance spectroscopy
Authors: Jarman R. H. , Ray G. J. , Standley R. W. , Zajac G. W.
Published: July 2002

The direct determination of carbon–carbon bonding in amorphous carbon thin films has been performed by core-electron energy-loss spectroscopy and cross-polarization magic-angle spinning 13C nuclear magnetic resonance (NMR) for a series of varying band-gap amorphous carbon films (1.1–3.0 eV). The amorphous carbon films were prepared using rf plasma discharge with either capacitive or inductive coupling. The core-electron energy-loss spectroscopy is shown to be quantitative by measuring the relative number of unoccupied π* and σ* states available to the C 1s core electron. Determination of the percentage of sp2 and sp3 bonded carbon atoms follows from the measured ratio of π* and σ* states. If unity cross section for the C 1s to π* and σ* core losses is assumed, the agreement with high-resolution 13C NMR determination of the sp2 and sp3 content is very encouraging. The low band-gap material (1.1 eV) has the largest sp2 content (55–60%) which decreases with increasing band gap (3.0 eV, 12–19%). The orientation dependence of the intensities of the π* and σ* loss features is not as strong as found in highly oriented pyrolytic graphite. The observation of the orientation dependence suggests that there is a tendency for the π* orbitals to be parallel to the substrate.
Fabrication and characterization of optical waveguides in KTiOPO4
Authors: Bierlein John D. , Ferretti August , Brixner Lothar H. , Hsu William Y.
Published: July 2002

Planar and channel optical waveguides have been fabricated in KTiOPO4 (KTP) using ion exchange processes. The exchange kinetics are diffusion limited and depend strongly on composition and surface orientation. Increases in surface refractive index of up to 0.23 and waveguide thicknesses of up to 15 μm have been observed. A simple waveguide modulator has been fabricated which shows electro-optic r coefficients in the waveguide region to be similar to bulk KTP.
Catalytic growth rate enhancement of electron beam deposited iron films
Authors: Kunz R. R. , Mayer T. M.
Published: July 2002

Submicron (<0.25 μm) wide lines of iron have been deposited by low-energy (0.5–3.0 keV) electron beam induced decomposition of iron pentacarbonyl. Selective area thermal decomposition of iron pentacarbonyl has also been demonstrated. It was found that under certain conditions, the thermal decomposition only occurred on areas where a thin iron film had been previously deposited by electron stimulated decomposition. At 125 °C, the measured thermal decomposition probability was roughly 900 times greater on the deposited iron than previously reported data show for thermal decomposition on bare silicon. Anomalously high deposition yields of 15 to 50 iron atoms per electron were measured during electron stimulated decomposition.
Subthreshold model of a polycrystalline silicon thin-film field-effect transistor
Authors: Faughnan Brian
Published: July 2002

A model is presented for the subthreshold current versus gate voltage of a polycrystalline silicon thin-film field-effect transistor. It utilizes the experimentally observed exponential density of states of polycrystalline silicon grain boundaries and is based on an earlier model of M. Shur and M. Hack [J. Appl. Phys. 55, 3831 (1984)] which they applied to hydrogenated amorphous silicon. Experimental subthreshold curves are presented along with the corresponding curves predicted by the model. In addition current activation data are shown to fit the model. The primary fitting parameter is the density of states at the valence band.
Experimental study on the correlation between thermal-wave signals and dopant profiles for silicon-implanted GaAs
Authors: Uchitomi Naotaka , Mikami Hitoshi , Toyoda Nobuyuki , Nii Riro
Published: July 2002

The correlation between the contour maps of thermal-wave signals and those of carrier concentrations in GaAs substrates was investigated to understand which portion of an implanted profile mainly corresponds to the generation of thermal-wave signals. It was found that the thermal-wave signals on a Si-implanted GaAs wafer showed good correlation with the peak concentration in the carrier profile obtained from capacitance-voltage (C-V) measurements. This method is useful for the threshold voltage monitoring of GaAs metal-semiconductor field-effect transistors with a thin implanted channel layer, which cannot be characterized by conventional C-V measurement.
Heterologous regulation of trafficking and signaling of G protein-coupled receptors: β-Arrestin-dependent interactions between neurokinin receptors
Authors: Schmidlin Fabien , Déry Olivier , Bunnett Nigel W. , Grady Eileen F.
Published: July 2002

Cells express multiple G protein-coupled receptors that are simultaneously or sequentially activated by agonists. The consequences of activating one receptor on signaling and trafficking of another receptor are unknown. We examined the effects of selective activation of the neurokinin 1 receptor (NK1R) on signaling and trafficking of the NK3R and
vice versa
. Selective agonists of NK1R and NK3R induced membrane translocation of β-arrestins (β-ARRs). Dominant negative β-ARR
319–418
inhibited endocytosis of NK1R and NK3R. Whereas an NK1R agonist caused sequestration of NK1R with β-ARR in the same endosomes, thereby depleting them from the cytosol, β-ARRs did not prominently sequester with the activated NK3R and rapidly returned to the cytosol. In cells coexpressing both receptors, prior activation of the NK1R inhibited endocytosis and homologous desensitization of the NK3R, which was dose-dependently reversed by overexpression of β-ARR1. Similar results were obtained in enteric neurons that naturally coexpress the NK1R and NK3R. In contrast, activation of the NK3R did not affect NK1R endocytosis or desensitization. Thus, the high-affinity and prolonged interaction of the NK1R with β-ARRs depletes β-ARRs from the cytosol and limits their role in desensitization and endocytosis of the NK3R. Because β-ARRs are critical for desensitization, endocytosis, and mitogenic signaling of many receptors, this sequestration is likely to have important and widespread implications.
Selective association of the methyl-CpG binding protein MBD2 with the silent
p14/p16
locus in human neoplasia
Authors: Magdinier Frédérique , Wolffe Alan P.
Published: July 2002

DNA methylation of tumor suppressor genes is a common feature of
human cancer. The cyclin-dependent kinase inhibitor gene
p16/Ink4A
is hypermethylated in a wide range of
malignant tissues and the
p14/ARF
gene located 20 kb
upstream on chromosome 9p21 is also methylated in carcinomas.
p14/ARF
(ARF, alternative reading frame) does not
inhibit the activities of cyclins or cyclin-dependent kinase complexes;
however, the importance of the two gene products in the etiology of
cancer resides in their involvement in two major cell cycle regulatory
pathways: p53 and the retinoblastoma protein, Rb, respectively.
Distinct first exons driven from separate promoters are spliced onto
the common exons 2 and 3 and the resulting proteins are translated in
different reading frames. Both genes are expressed in normal cells but
can be alternatively or coordinately silenced when their CpG islands
are hypermethylated. Herein, we examined the presence of methyl-CpG
binding proteins associated with aberrantly methylated promoters, the
distribution of acetylated histones H3 and H4 by chromatin
immunoprecipitation assays, and the effect of chemical treatment with
5-aza-2′-deoxycytidine (5aza-dC) and trichostatin A on gene induction
in colon cell lines by quantitative reverse transcriptase–PCR. We
observed that the methyl-CpG binding protein MBD2 is targeted to
methylated regulatory regions and excludes the acetylated histones H3
and H4, resulting in a localized inactive chromatin configuration. When
methylated, the genes can be induced by 5aza-dC but the combined action
of 5aza-dC and trichostatin A results in robust gene expression. Thus,
methyl-CpG binding proteins and histone deacetylases appear to
cooperate
in vivo
, with a dominant effect of DNA
methylation toward histone acetylation, and repress expression of tumor
suppressor genes hypermethylated in cancers.
An
Arabidopsis
gene encoding an α-xylosyltransferase involved in xyloglucan biosynthesis
Authors: Faik Ahmed , Price Nicholas J. , Raikhel Natasha V. , Keegstra Kenneth
Published: July 2002

Microsomal membranes catalyze the formation of xyloglucan from UDP-Glc and UDP-Xyl by cooperative action of α-xylosyltransferase and β-glucan synthase activities. Here we report that etiolated pea microsomes contain an α-xylosyltransferase that catalyzes the transfer of xylose from UDP-[
14
C]xylose onto β(1,4)-linked glucan chains. The solubilized enzyme had the capacity to transfer xylosyl residues onto cello-oligosaccharides having 5 or more glucose residues. Analysis of the data from these biochemical assays led to the identification of a group of
Arabidopsis
genes and the hypothesis that one or more members of this group may encode α-xylosyltransferases involved in xyloglucan biosynthesis. To evaluate this hypothesis, the candidate genes were expressed in
Pichia pastoris
and their activities measured with the biochemical assay described above. One of the candidate genes showed cello-oligosaccharide-dependent xylosyltransferase activity. Characterization of the radiolabeled products obtained with cellopentaose as acceptor indicated that the pea and the
Arabidopsis
enzymes transfer the
14
C-labeled xylose mainly to the second glucose residue from the nonreducing end. Enzymatic digestion of these radiolabeled products produced results that would be expected if the xylose was attached in an α(1,6)-linkage to the glucan chain. We conclude that this
Arabidopsis
gene encodes an α-xylosyltransferase activity involved in xyloglucan biosynthesis.
Excitatory versus inhibitory GABA as a divergence point in steroid-mediated sexual differentiation of the brain
Authors: Auger Anthony P. , Perrot-Sinal Tara S. , McCarthy Margaret M.
Published: July 2002

Whereas adult sex differences in brain morphology and behavior result from developmental exposure to steroid hormones, the mechanism by which steroids differentiate the brain is unknown. Studies to date have described subtle sex differences in levels of proteins and neurotransmitters during brain development, but these have lacked explanatory power for the profound sex differences induced by steroids. We report here a major divergence in the response to injection of the γ-aminobutyric acid type A (GABAA) agonist, muscimol, in newborn male and female rats. In females, muscimol treatment primarily decreased the phosphorylation of cAMP response element binding protein (CREB) within the hypothalamus and the CA1 region of the hippocampus. In contrast, muscimol increased the phosphorylation of CREB in males within these same brain regions. Within the arcuate nucleus, muscimol treatment increased the phosphorylation of CREB in both females and males. Thus, the response to GABA can be excitatory or inhibitory on signal-transduction pathways that alter CREB phosphorylation depending on the sex and the region in developing brain. This divergence in response to GABA allows for a previously unknown form of steroid-mediated neuronal plasticity and may be an initial step in establishing sexually dimorphic signal-transduction pathways in developing brain.
Polyphosphate kinase is essential for biofilm development, quorum sensing, and virulence of
Pseudomonas aeruginosa
Authors: Rashid M. Harunur , Rumbaugh Kendra , Passador Luciano , Davies David G. , Hamood Abdul N. , Iglewski Barbara H. , Kornberg Arthur
Published: July 2002

The human opportunistic pathogen
Pseudomonas aeruginosa
causes a variety of infections in immunocompromised hosts and in individuals with cystic fibrosis. A knockout mutation in the polyphosphate kinase (
ppk)
gene, encoding PPK responsible for the synthesis of inorganic polyphosphate from ATP, renders
P. aeruginosa
cells unable to form a thick and differentiated biofilm. The mutant is aberrant in quorum sensing and responses in that production of the quorum-sensing controlled virulence factors elastase and rhamnolipid are severely reduced. In a burned-mouse pathogenesis model, the virulence of the mutant is greatly reduced with severe defects in the colonization of mouse tissues. The conservation of PPK among many bacterial pathogens and its absence in eukaryotes suggest that PPK might be an attractive target for antimicrobial drugs.
An empirical explanation of color contrast
Authors: Lotto R. Beau , Purves Dale
Published: July 2002

For reasons not well understood, the color of a surface can appear
quite different when placed in different chromatic surrounds. Here we
explore the possibility that these color contrast effects are generated
according to what the same or similar stimuli have turned out to
signify in the past about the physical relationships between
reflectance, illumination, and the spectral returns they produce. This
hypothesis was evaluated by (
i
) comparing the physical
relationships of reflectances, illuminants, and spectral returns with
the perceptual phenomenology of color contrast and (
ii
)
testing whether perceptions of color contrast are predictably changed
by altering the probabilities of the possible sources of the stimulus.
The results we describe are consistent with a wholly empirical
explanation of color contrast effects.
Phosphorylation of serines 635 and 645 of human Rad17 is cell cycle regulated and is required for G
1
/S checkpoint activation in response to DNA damage
Authors: Post Sean , Weng Yi-Chinn , Cimprich Karlene , Chen Lan Bo , Xu Yang , Lee Eva Y.-H. P.
Published: July 2002

ATR [ataxia–telangiectasia-mutated (ATM)- and Rad3-related] is a protein kinase required for both DNA damage-induced cell cycle checkpoint responses and the DNA replication checkpoint that prevents mitosis before the completion of DNA synthesis. Although ATM and ATR kinases share many substrates, the different phenotypes of ATM- and ATR-deficient mice indicate that these kinases are not functionally redundant. Here we demonstrate that ATR but not ATM phosphorylates the human Rad17 (hRad17) checkpoint protein on Ser
635
and Ser
645
in vitro
. In undamaged synchronized human cells, these two sites were phosphorylated in late G
1
, S, and G
2
/M, but not in early–mid G
1
. Treatment of cells with genotoxic stress induced phosphorylation of hRad17 in cells in early–mid G
1
. Expression of kinase-inactive ATR resulted in reduced phosphorylation of these residues, but these same serine residues were phosphorylated in ionizing radiation (IR)-treated ATM-deficient human cell lines. IR-induced phosphorylation of hRad17 was also observed in ATM-deficient tissues, but induction of Ser
645
was not optimal. Expression of a hRad17 mutant, with both serine residues changed to alanine, abolished IR-induced activation of the G
1
/S checkpoint in MCF-7 cells. These results suggest ATR and hRad17 are essential components of a DNA damage response pathway in mammalian cells.
Sex-ratio segregation distortion associated with reproductive isolation inDrosophila
Authors: Tao Yun , Hartl Daniel L. , Laurie Cathy C.
Published: July 2002

Sex-ratio distortion is the most common form of non-Mendelian segregation observed in natural populations. It may occur even more frequently than direct observations suggest, because the dysgenic population consequences of a biased sex ratio are expected to result in the rapid evolution of suppressors, resulting in suppressed or “cryptic” segregation distortion. Here we report evidence for cryptic sex-ratio distortion that was discovered by introgressing segments of the genome ofDrosophila mauritianainto the genome ofDrosophila simulans. The autosomal suppressor of sex-ratio distortion, which is also associated with a reduction in hybrid male fertility, has been genetically localized to a region smaller than 80-kb pairs in chromosome 3.
Coevolution of transcriptional and allosteric regulation at the chorismate metabolic branch point of
Saccharomyces cerevisiae
Authors: Krappmann Sven , Lipscomb William N. , Braus Gerhard H.
Published: July 2002

Control of transcription and enzyme activities are two interwoven
regulatory systems essential for the function of a metabolic node.
Saccharomyces cerevisiae
strains differing in enzyme
activities at the chorismate branch point of aromatic amino acid
biosynthesis were constructed by recombinant DNA technology. Expression
of an allosterically unregulated, constitutively activated chorismate
mutase encoded by the
ARO7
T226I
(
ARO7
c
) allele depleted the chorismate pool.
The resulting tryptophan limitation caused growth defects, which could
be counteracted only by transcriptional induction of
TRP2
encoding the competing enzyme anthranilate
synthase.
ARO7
expression is not transcriptionally
regulated by amino acids. Transcriptional activation of the
ARO7
c
allele led to stronger growth
retardation upon tryptophan limitation. The same effect was achieved by
removing the competing enzyme anthranilate synthase, which is encoded
by the
TRP2
gene, from the transcriptional control. The
allelic situation of
ARO7
c
being under
general control instead of
TRP2
resulted in severe
growth defects when cells were starved for tryptophan. In conclusion,
the specific regulatory pattern acting on enzymatic activities at the
first metabolic node of aromatic amino acid biosynthesis is necessary
to maintain proper flux distribution. Therefore, the evolution of the
sophisticated allosteric regulation of yeast chorismate mutase requires
as prerequisite (
i
) that the encoding
ARO7
gene is not transcriptionally regulated, whereas
(
ii
) the transcription of the competing
feedback-regulated anthranilate synthase-encoding gene is controlled by
availability of amino acids.
Construction and characterization of an azurin analog for the purple copper site in cytochrome c oxidase.
Authors: Hay M , Richards J H , Lu Y
Published: July 2002

A protein analog of a purple copper center has been constructed from a recombinant blue copper protein (Pseudomonas aeruginosa azurin) by replacing the loop containing the three ligands to the blue copper center with the corresponding loop of the CuA center in cytochrome c oxidase (COX) from Paracoccus denitrificans. The electronic absorption in the UV and visible region (UV-vis) and electron paramagnetic resonance (EPR) spectra of this analog are remarkably similar to those of the native CuA center in COX from Paracoccus denitrificans. The above spectra can be obtained upon addition of a mixture of Cu2+ and Cu+. Addition of Cu2+ only results in a UV-vis spectrum consisting of absorptions from both a purple copper center and a blue copper center. This spectrum can be converted to the spectrum of a pure purple copper by a prolonged incubation in the air, or by addition of excess ascorbate. The azurin mutant reported here is an example of an engineered purple copper center with the A480/A530 ratio greater than 1 and with no detectable hyperfines, similar to those of the CuA sites in COX of bovine heart and of Paracoccus denitrificans.
Quantitative long-term imaging of the functional representation of a whisker in rat barrel cortex.
Authors: Masino S A , Frostig R D
Published: July 2002

In this study, we implement chronic optical imaging of intrinsic signals in rat barrel cortex and repeatedly quantify the functional representation of a single whisker over time. The success of chronic imaging for more than 1 month enabled an evaluation of the normal dynamic range of this sensory representation. In individual animals for a period of several weeks, we found that: (i) the average spatial extent of the quantified functional representation of whisker C2 is surprisingly large--1.71 mm2 (area at half-height); (ii) the location of the functional representation is consistent; and (iii) there are ongoing but nonsystematic changes in spatiotemporal characteristics such as the size, shape, and response amplitude of the functional representation. These results support a modified description of the functional organization of barrel cortex, where although a precisely located module corresponds to a specific whisker, this module is dynamic, large, and overlaps considerably with the modules of many other whiskers.