bluuebunny/crossref_metadata_2025 huggingface dataset:
Time-resolved probing of femtosecond-laser-produced plasmas in transparent solids by electron thermal transport
Authors: Vu B.-T. V. , Landen O. L. , Szoke A.
Published: July 2002

This paper is a comprehensive report on both the experimental and theoretical aspects of the recently observed supersonic ionization front in solid density plasmas, produced on the surface of a transparent fused quartz target by 100 fs-laser-pulse irradiation. The ionization front is driven by electron thermal conduction into the bulk region of the target on a time scale of ∼1 ps, before any significant hydrodynamic expansion. The observed large and persistent increase in the reflectivity of the plasma–quartz region indicates that the plasma–quartz interface is steep and long lived (∼15 ps). The experiments include pump-probe schemes with the probe incident on either side of the target, to provide different but complementary information on the plasma evolution and energy transport mechanisms. A simple analytical model, modified from earlier nonlinear heat wave models, is proposed to account for the ionization front velocity and plasma evolution in both time and space. Calculations of the probe light reflectance after interacting with the plasma at the plasma–quartz region lead to a time history of the electron–ion collision frequency. The result shows the electron scattering length comparable with the average interionic spacing (∼3 Å), implying localization of thermal electrons.
Nonlinear magnetic reconnection with collisionless dissipation
Authors: Vekstein G. E. , Priest E. R.
Published: July 2002

An analytical model of two-dimensional collisionless reconnection in an X-type magnetic geometry is presented. The conversion of magnetic energy to the kinetic energy of accelerated ions takes place in the vicinity of the neutral line. The structure of this dissipation region and the magnetic energy release rate have been investigated both for linear and nonlinear regimes of collisionless reconnection. A simple model of global reconnection flow has been constructed, assuming an incompressible ideal magnetohydrodynamics approximation outside the dissipation region. The corresponding scaling for the external Mach number (Me) is found, which predicts a maximum reconnection rate Me=1/2R̃m −1/2, where R̃m≊(Le/λi)2≫1 is the effective magnetic Reynolds number for collisionless reconnection (Le is the global size of the system and λi is the ion inertial skin depth).
Edge-localized mode simulations in divertor tokamaks
Authors: Strauss H. R.
Published: July 2002

The three-dimensional nonlinear evolution of moderate wavelength pressure-driven edge-localized instabilities is calculated in divertor tokamaks. The evolution consists of several stages: linear growth, nonlinear saturation and turbulence, shear flow generation, pressure outflow to the outboard divertor, and finally, outflow to the inboard divertor. The shear flow generation appears to be an important factor in transporting pressure perturbations to the inboard side, and in causing pressure loss to the inboard divertor. The physical model consists of dissipative compressional reduced magnetohydrodynamics which includes the important effect of sound wave propagation. A novel numerical discretization, using a poloidal unstructured mesh and a staggered toroidal mesh, has been implemented on a parallel, distributed memory computer. Nonlinear, three-dimensional numerical computations include a separatrix X-point in the computational domain.
Growth rates in electromagnetically pumped free-electron lasers with axial guide magnetic field
Authors: Jaber I. , Demokan O.
Published: July 2002

The dispersion relations of transverse and longitudinal modes in an electromagnetically pumped free-electron laser with an axial guide field are derived exactly, for the one-dimensional case. The modes with positive and negative helicity are distinguished clearly. Analytic expressions for the growth rate, frequency, and wavelength of the modes, in resonance with the beam are obtained in terms of the system parameters. The dependence of the growth rate on these parameters is illustrated for a broad range of values.
Collisionless particle orbits in a torsatron with the complicated magnetic-field harmonic spectrum. II. Adiabatic invariants of particle motion
Authors: Smirnova M. S.
Published: July 2002

Charged particle orbits are investigated in a torsatron with the complicated magnetic-field harmonic spectrum where the distant, high-order harmonics (the harmonics of the toroidal-field ripple) are present. The investigation is carried out on the basis of the adiabatic invariants of particle motion. The validity of the analytical expressions for the adiabatic invariants derived in the previous works are examined for the case of the torsatron magnetic field perturbed by the toroidal-field ripple of the arbitrary value and periodicity. For a torsatron–heliotron [K. Uo, J. Phys. Soc. Jpn. 16, 1380 (1961)] where the TF ripple of the magnetic field of periodicity equals twice the number of the helical field periods, exact analytical expressions for the adiabatic invariants of motion are derived for all possible kinds of particle orbits.
Nonlinear magnetic chains associated with plasma flow
Authors: Vranješ J. , Jovanović D.
Published: July 2002

Nonlinear equations describing magnetic electron mode in a nonuniform, unmagnetized plasma, with a space-dependent flow are derived. A set of two coupled equations, for the self-generated magnetic field and electron temperature perturbation is obtained, and two kinds of solutions, in the form of a moving single and double chain of magnetic islands are found.
Ambipolar magnetic fluctuation-induced heat transport in toroidal devices
Authors: Terry P. W. , Fiksel G. , Ji H. , Almagri A. F. , Cekic M. , Den Hartog D. J. , Diamond P. H. , Prager S. C. , Sarff J. S. , Shen W. , Stoneking M. , Ware A. S.
Published: July 2002

The total magnetic fluctuation-induced electron thermal flux has been determined in the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 131 (1991)] from the measured correlation of the heat flux along perturbed fields with the radial component of the perturbed field. In the edge region the total flux is convective and intrinsically ambipolar constrained, as evidenced by the magnitude of the thermal diffusivity, which is well approximated by the product of ion thermal velocity and the magnetic diffusivity. A self-consistent theory is formulated and shown to reproduce the experimental results, provided nonlinear charge aggregation in streaming electrons is accounted for in the theory. For general toroidal configurations, it is shown that ambipolar constrained transport applies when remote magnetic fluctuations (i.e., global modes resonant at distant rational surfaces) dominate the flux. Near locations where the dominant modes are resonant, the transport is nonambipolar. This agrees with the radial variation of diffusivity in MST. Expectations for the tokamak are also discussed.
Current drive by Alfvén waves in elongated cross-section tokamak
Authors: Tsypin V. S. , Elfimov A. G. , Nekrasov F. M. , de Azevedo C. A. , de Assis A. S.
Published: July 2002

The general approach to the Alfvén wave current drive problem in tokamaks with elongated transverse cross-sections was considered in this paper. Model approximations are used to describe circulating and trapped particle dynamics. This approach gives the accuracy of some percents. The expressions for the time-averaged longitudinal current and the radio-frequency currents have been obtained. They are supposed to be useful for a further analytical and computational solution of this problem. As an example, kinetic Alfvén waves are considered in this paper.
Observations of the plasma dynamics of a vacuum spark from its soft x-ray emission
Authors: Chuaqui H. , Favre M. , Saavedra R. , Wyndham E. S. , Choi P. , Dumitrescu-Zoita C. , Soto L. , Aliaga Rossel R. , Mitchell I. H.
Published: July 2002

Experimental observations of the plasma dynamics in a vacuum spark are presented which permit measurements of the electron density and temperature during a large part of the compression phase of the pinch. The vacuum spark is generated by a low-impedance pulse forming line with a maximum current in excess of 100 kA. The discharge is operated in the hybrid mode in a titanium plasma. A laser focused onto the cathode provides the preionizing source. Soft x-ray emission from the current sheath is observed with an x-ray framing camera well before maximum compression. These observations are compared with holographic interferograms, both showing the formation of an axisymmetric rhombic boundary to the plasma sheath. The temperature and density of both the sheath and the internal plasma are observed until the formation of hot spots at the time of maximum compression. The temperature evolution of the hot spots is presented, showing a repeatable behavior that depends on the axial position.
Cylindrical ideal magnetohydrodynamic equilibria with incompressible flows
Authors: Throumoulopoulos G. N. , Tasso H.
Published: July 2002

It is proved that (a) the solutions of the ideal magnetohydrodynamic equation, which describes the equilibrium states of a cylindrical plasma with purely poloidal flow and arbitrary cross-sectional shape [G. N. Throumoulopoulos and G. Pantis, Plasma Phys. Controlled Fusion 38, 1817 (1996)], are also valid for incompressible equilibrium flows with the axial velocity component being a free surface quantity and that (b) for the case of isothermal incompressible equilibria the magnetic surfaces necessarily have a circular cross-section.
Electron heating via mode converted ion Bernstein waves in the Alcator C-Mod tokamak
Authors: Bonoli P. T. , O’Shea P. , Brambilla M. , Golovato S. N. , Hubbard A. E. , Porkolab M. , Takase Y. , Boivin R. L. , Bombarda F. , Christensen C. , Fiore C. L. , Garnier D. , Goetz J. , Granetz R. , Greenwald M. , Horne S. F. , Hutchinson I. H. , Irby J. , Jablonski D. , LaBombard B. , Lipschultz B. , Marmar E. , May M. , Mazurenko A. , McCracken G. , Nachtrieb R. , Niemczewski A. , Ohkawa H. , Pappas D. A. , Reardon J. , Rice J. , Rost C. , Schachter J. , Snipes J. A. , Stek P. , Takase K. , Terry J. , Wang Y. , Watterson R. L. , Welch B. , Wolfe S. M.
Published: July 2002

Highly localized direct electron heating [full width at half-maximum (FWHM)≅0.2a] via mode converted ion Bernstein waves has been observed in the Alcator C-Mod Tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. Electron heating at or near the plasma center (r/a⩾0.3) has been observed in H(3He) discharges at B0=(6.0–6.5) T and ne(0)≅1.8×1020 m−3. [Here, the minority ion species is indicated parenthetically.] Off-axis heating (r/a⩾0.5) has also been observed in D(3He) plasmas at B0=7.9 T. The concentration of He3 in these experiments was in the range of n3He/ne≅(0.2–0.3) and the locations of the mode conversion layer and electron heating peak could be controlled by changing the He3 concentration or toroidal magnetic field (B0). The electron heating profiles were deduced using a rf modulation technique. Detailed comparisons with one-dimensional and toroidal full-wave models in the ion cyclotron range of frequencies have been carried out. One-dimensional full-wave code predictions were found to be in qualitative agreement with the experimental results. Toroidal full-wave calculations indicated the importance of volumetric and wave focusing effects in the interpretation of the experimental results.
Measurements on rotating ion cyclotron range of frequencies induced particle fluxes in axisymmetric mirror plasmas
Authors: Hatakeyama R. , Hershkowitz N. , Majeski R. , Wen Y. J. , Brouchous D. B. , Proberts P. , Breun R. A. , Roberts D. , Vukovic M. , Tanaka T.
Published: July 2002

A comparison of phenomenological features of plasmas is made with a special emphasis on radio-frequency induced transport, which are maintained when a set of two closely spaced dual half-turn antennas in a central cell of the Phaedrus-B axisymmetric tandem mirror [J. J. Browning et al., Phys. Fluids B 1, 1692 (1989)] is phased to excite electromagnetic fields in the ion cyclotron range of frequencies (ICRF) with m=−1 (rotating with ions) and m=+1 (rotating with electrons) azimuthal modes. Positive and negative electric currents are measured to flow axially to the end walls in the cases of m=−1 and m=+1 excitations, respectively. These parallel nonambipolar ion and electron fluxes are observed to be accompanied by azimuthal ion flows in the same directions as the antenna-excitation modes m. The phenomena are argued in terms of radial particle fluxes due to a nonambipolar transport mechanism [Hojo and Hatori, J. Phys. Soc. Jpn. 60, 2510 (1991); Hatakeyama et al., J. Phys. Soc. Jpn. 60, 2815 (1991), and Phys. Rev. E 52, 6664 (1995)], which are induced when azimuthally traveling ICRF waves are absorbed in the magnetized plasma column.
Thermal transport in collision dominated edge plasmas
Authors: Singh R. , Nordman H. , Anderson J. , Weiland J.
Published: July 2002

Effects of ion temperature gradients on strong ballooning drift resistive edge modes are considered. The ion temperature perturbations are found to have a significant influence on the stability properties. In particular, the ion temperature gradient leads to a finite-Larmor radius stabilization of the strongest mode with an ideal growth rate for lower heating power than that required for stabilization due to the poloidal sheared rotation.
Observation of the spatiotemporal dynamics of ionization wave mode transitions
Authors: Dinklage A. , Bruhn B. , Deutsch H. , Jonas P. , Koch B.-P. , Wilke C.
Published: July 2002

Experimental observation of transitions between discrete longitudinal wave modes in a dc neon glow discharge is reported. Experimental findings concerning the pattern selecting instability agree with observations of the Eckhaus instability in fluid systems. This is supported by theoretical considerations of a model equation (Complex Ginzburg–Landau equation) based on a fluid model.
Collisionless transport parallel to the magnetic field in a toroidal plasma
Authors: Hazeltine R. D.
Published: July 2002

Transport parallel to the magnetic field of a toroidal plasma confinement system is investigated through kinetic theory, with emphasis on the long mean-free path limit. The crucial differences between transport on rational and irrational (ergodic) magnetic surfaces is discussed in detail. A collisionless transport law, involving a nonlocal operator that accounts for toroidal topology, is derived for parallel heat conduction on irrational magnetic surfaces. In the rational surface case, perpendicular diffusion is included in the kinetic equation to avoid singularity; this allows a calculation of the width and amplitude of resonant temperature perturbations that will be excited by heat sources with sufficiently broad Fourier spectra.
The second adiabatic invariant and neoclassical transport in stellarators
Authors: Nemov V. V.
Published: July 2002

The second adiabatic invariant J∥ for locally trapped particles is used for calculating neoclassical transport in stellarators. A numerical method for calculating the variation of J∥ on a magnetic surface is developed, which allows one to make the calculations in an arbitrary stellarator magnetic field given in real-space coordinates without transforming the field to magnetic coordinates. Equations for neoclassical particle and energy fluxes for the 1/ν transport regime are derived, which are expressed through the variation of J∥. By making use of these method and equations, an evaluation of neoclassical transport properties is made for a real-space realization of quasihelical symmetry, which has been examined recently in magnetic coordinates.
Kinetic theory of dusty plasmas II. Dust–plasma particle collision integrals
Authors: Tsytovich V. N. , de Angelis U.
Published: July 2002

The recent results of the kinetic theory of dusty plasmas [V. N. Tsytovich and U. de Angelis, Phys. Plasmas 6, 1093 (1999)] are analyzed to assess the importance of new qualitative effects: Inelasticity of dust–plasma particle collisions, deviations from Debye screening and from the dust equilibrium charge in the interactions, the existence of a friction force on plasma particles, and their diffusion in energy. These collective effects depend on the plasma dielectric constant modified by dust charging effects as well as on two new types of dusty plasma responses, related to the fluctuations of plasma particle currents to dust particles. Analytic expressions for the responses are derived and numerical results are given to assess their importance and dependence on dusty plasma parameters. The rate of energy and momentum transfer from plasma particles to dust particles is calculated analytically and numerical results are presented.
On the gyrokinetic equilibrium
Authors: Qin H. , Tang W. M. , Rewoldt G. , Lee W. W.
Published: July 2002

Recent developments in gyrokinetic-magnetohydrodynamics (MHD) theory and in electromagnetic gyrokinetic particle simulations raise the question of consistency between the gyrokinetic model and the fluid model. Due to the special characteristics of the guiding center coordinates, it is a nontrivial exercise to show this consistency. In this paper it is shown, in a very general setting, that the gyrokinetic theory and the fluid equations do give an equivalent description of plasma equilibrium (∂/∂t=0). The fluid continuity equation and momentum equation for equilibrium plasmas are recovered entirely from the gyrokinetic theory. However, it was Spitzer who first realized the importance of consistency between guiding-center motion and fluid equations. In particular, he studied the “apparent paradoxical result” regarding the difference between perpendicular particle flow and guiding-center flow, which will be referred to as the Spitzer paradox in this paper. By recovering the fluid equations from the gyrokinetic theory, we automatically resolve the Spitzer paradox, whose essence is how the perpendicular current and flow are microscopically generated from particles’ guiding-center motion. The mathematical construction in the gyrokinetic theory which relates observable quantities in the laboratory frame to the distribution function in the guiding-center coordinates is consistent with Spitzer’s original physical picture, while today’s gyrokinetic-MHD theory covers a much wider range of problems in a much more general and quantitative way.
Adiabatic frequency shifting of a surface wave guided by a time-varying plasma structure
Authors: Bakunov M. I. , Gildenburg V. B. , Zhukov S. N. , Zharova N. A.
Published: July 2002

The propagation of an electromagnetic wave along a planar waveguiding structure that contains gaseous/solid-state plasma slabs with time-varying free carrier density is considered in the case when the time scale of the density variation is much greater than the wave period (adiabatic approximation). Both ionization and deionization (recombination, attachment) processes are included. General relations describing frequency shifting and energy losses of a guided wave are derived for an arbitrary open/closed waveguiding structure. Relations between the energy of the wave and its frequency conserved during the process of plasma density variation (adiabatic invariants) are found. Detailed analyses are given for surface waves guided by the boundary of a time-varying plasma half-space and a plasma slab. Slow variations of plasma density are shown to be energetically more efficient than fast variations. A new effect of reflection of the wave envelope that occurs without reflection of the fast oscillating carrier is pointed out.
Effects of dust charging associated with an electron beam in the cathode sheath of glow discharges
Authors: Wang De-Zhen , Liu Jin-Yuan , Ma Teng-Cai
Published: July 2002

The charging of dust particles caused by the thermal electrons, the cold ions, and the electron beam released from a plane cathode, including secondary emission of dust in a collisionless cathode sheath of glow discharges, is investigated with a self-consistent theoretical model. Effects of the dust charging relevant to the electron beam including secondary emission of dust are important to the dust potential in the close cathode region. Collection of electrons from the electron beam by dust leads to a decline in the dust potential, but secondary emission of dust by the electron beam impact causes a rise in the dust potential. The dust potential can be negative in the whole sheath if an electron beam emitted from a cathode is strong enough.
Heating experiment of field-reversed configuration plasma by low-frequency magnetic pulse
Authors: Yamanaka Koji , Yoshimura Satoru , Kitano Katsuhisa , Okada Shigefumi , Goto Seiichi
Published: July 2002

An effective heating method and experimental verification for a field-reversed configuration (FRC) plasma by applying the low-frequency magnetic pulse is presented. The low-frequency magnetic field is applied by an antenna that consists of two single-turn coils, and the frequency of the magnetic field is lower than the ion cyclotron frequency at the separatrix. An increase of the plasma energy and a fluctuation of the internal magnetic field are simultaneously observed. The comparison of the total temperature and the ion temperature shows that the increase of the plasma energy is mostly due to the ion heating. The fluctuation is directly observed by the internal magnetic probe arrays. The analysis of the phase velocity along the equilibrium magnetic field implies that the Alfvén wave is excited and propagates.
Fast nonlinear optical response from proton-bombarded multiple quantum well structures
Authors: Silberberg Y. , Smith P. W. , Miller D. A. B. , Tell B. , Gossard A. C. , Wiegmann W.
Published: July 2002

Proton bombardment is shown to shorten the recovery time of the excitonic absorption in GaAs/GaAlAs multiple quantum well saturable absorbers. The response time can be reduced from 30 ns to 150 ps without affecting the absorption characteristics or the saturation energy.
Direct measurement of dilute dye solution quantum yield by photothermal laser heterodyne interferometry
Authors: Lee Wing-Kee , Güngör Ali , Ho Ping-Tong , Davis Christopher C.
Published: July 2002

Laser heterodyne measurements have been used to study the nonradiative energy loss of isolated rhodamine 6G molecules in methanol solutions with different potassium iodide quencher concentrations. Simultaneous measurement of both the relative fluorescent intensity and photothermal phase modulation induced by argon ion laser excitation allows absolute determination of both the quantum yield and the fraction of absorbed photo-energy converted to heat. At zero quencher concentration, their values were found to be 98.2% and 12%, respectively.
Measurement of silicon interstitial diffusivity
Authors: Griffin P. B. , Fahey P. M. , Plummer J. D. , Dutton R. W.
Published: July 2002

A conceptually simple experiment to measure silicon interstitial diffusivity is described. The structure uses a silicon wafer with buried layers deep in the bulk. Oxidation of the wafer surface generates interstitials that diffuse into the wafer and enhance the buried layer diffusion. The results show that the interstitial profiles are flat out to 40 μm for the times and temperatures investigated. We can thus conclude that the interstitials move significantly faster than previously thought.
Third order nonlinear optical interactions in thin films of poly-p-phenylenebenzobisthiazole polymer investigated by picosecond and subpicosecond degenerate four wave mixing
Authors: Rao D. Narayana , Swiatkiewicz Jacek , Chopra Pratibha , Ghoshal Suniti K. , Prasad Paras N.
Published: July 2002

The third order nonlinear optical susceptibility has been measured by degenerate four wave mixing in a 33-μm-thick biaxial film of a conjugated aromatic heterocyclic polymer, poly-p-phenylenebenzobisthiazole, commonly known as PBT, which has a very high mechanical strength due to its rigid rod conformation as well as environmental stability and a high laser damage threshold. For the first time, the response of the optical nonlinearity in the π-electron conjugated system has been experimentally verified to be in subpicoseconds. The value of χ(3) is found to be about an order of magnitude larger than that of CS2. The measurement at two different wavelengths suggests that they are nonresonant χ(3) values. The measured anisotropy of χ(3) as a function of angular orientation at two different sets of laser polarization is explained by using the tensor properties of χ(3) in an anisotropic medium.
Patterned aluminum growth via excimer laser activated metalorganic chemical vapor deposition
Authors: Higashi G. S. , Fleming C. G.
Published: July 2002

Excimer laser photolysis of organoaluminum adlayers has been used to catalytically activate the deposition of Al via thermal decomposition of triisobutylaluminum. The process exhibits good spatial selectivity and patterns with 4 μm resolution have been accurately reproduced. Patterned Al metallizations have been performed on Si, SiO2, Al2O3, and GaAs substrates and show promise for practical applications. Electrical measurements probing Al/substrate interface quality indicate that this technique may be suitable for the fabrication of rectifying contacts on GaAs.
Deep level transient spectroscopy of hole defects in bulk-grown p-GaAs using Schottky barrier diodes
Authors: Auret F. D. , Nel M.
Published: July 2002

Schottky barrier diodes were used to detect and study hole defects in bulk-grown p-type GaAs by deep level transient spectroscopy. Several defects with concentrations of 1013–1016/cm3 were studied. It was found that two of these defects, with electronic levels at Ev +0.42 eV and Ev +0.58 eV, have electronic properties that closely correspond to those of Cu- and Fe-related defects in GaAs. It is concluded that Schottky barrier diodes on p-GaAs can be very useful to detect and characterize typical metallic contaminants in GaAs.
Intensity profiles of short optical pulses via temporally reversed pulses
Authors: Montgomery Steven R. , Pederson Donald O. , Salamo Gregory J.
Published: July 2002

A new technique is reported for determining the intensity profile of optical pulses on a subpicosecond timescale. Several examples are presented in order to demonstrate the capability of the technique.
Optically pumped tunable mode-locked Si-doped GaAs laser
Authors: Valk B. , Call T. S. , Salour M. M. , Kopp W. , Morkoç H.
Published: July 2002

Mode-locked operation of Si-doped bulk GaAs in external cavity was achieved by synchronous pumping with a Kr+ laser at 647.1 nm. High beam quality and peak powers of up to 3.3 W are unique features of this laser. The spontaneous spectrum is narrower than those of dyes, allowing a stabilized single-frequency operation with fewer wavelength selective elements, while tunability over a range of 300 Å was achieved by varying the temperature.
Low-frequency noise in low 1/f noise dc SQUID’s
Authors: Foglietti V. , Gallagher W. J. , Ketchen M. B. , Kleinsasser A. W. , Koch R. H. , Raider S. I. , Sandstrom R. L.
Published: July 2002

We demonstrate that the low-frequency noise in our edge junction dc superconducting quantum interference devices, with a basic 1/f flux noise of 2×10−12 Φ20/Hz at 1 Hz, can all be accounted for in terms of junction critical current fluctuations. A novel modulation readout scheme is able to cancel the effect of junction critical current fluctuations and reduce our total noise to 1×10−12 Φ20/Hz at 0.1 Hz, a level that is three times lower than the lowest flux noise ever previously reported at this frequency.
Laser energy deposition at sapphire surfaces studied by pulsed photothermal deformation
Authors: Dreyfus R. W. , McDonald F. A. , von Gutfeld R. J.
Published: July 2002

A surface deformation technique measures the energy deposited in sapphire surfaces irradiated by focused excimer laser light. The absorbed energy creates ≲3 electron-hole pairs/oxygen atom, consistent with a photochemical etching mechanism.
Determination of critical layer thickness in InxGa1−xAs/GaAs heterostructures by x-ray diffraction
Authors: Orders P. J. , Usher B. F.
Published: July 2002

The critical thickness hc of strained InxGa1−xAs layers grown by molecular beam epitaxy on GaAs(100) substrates is determined by double-crystal x-ray diffraction for 0.07≤x≤0.25. The experimental results are in good agreement with critical thicknesses calculated from the energy balance model of R. People and J. C. Bean [Appl. Phys. Lett. 47, 322 (1985)] but differ from prior photoluminescence measurements of hc for this material. Beyond the critical thickness the transition from the strained to the relaxed state occurs more rapidly as the In concentration and hence the lattice mismatch increases.
Optical properties of AlxIn1−xP grown by organometallic vapor phase epitaxy
Authors: Bour D. P. , Shealy J. R. , Wicks G. W. , Schaff W. J.
Published: July 2002

AlxIn1−xP epilayers grown by organometallic vapor phase epitaxy are characterized by electroreflectance, photoluminescence, and Raman spectroscopy. Electroreflectance measurements of the E0 band gap show significant bowing of the direct band gap with composition. The compositional dependence of the direct gap is found to be EΓ(x)=1.35+1.83x+0.38x2 (eV). In the Raman scattering spectrum, the AlP-like longitudinal optical phonon mode is identified and its energy is measured as a function of composition.
New method of characterizing majority and minority carriers in semiconductors
Authors: Leslie-Pelecky D. L. , Seiler D. G. , Loloee M. R. , Littler C. L.
Published: July 2002

A novel characterization method using magnetoconductivity tensor components to determine the carrier concentration and mobility of majority and minority carriers is presented. Results are given for bulk n-HgCdTe (one carrier), liquid phase epitaxial n-HgCdTe (two carriers), and p-InSb (two or three carriers). Advantages of this method over the standard Hall coefficient analysis are discussed.
Dependence of the AlxGa1−xAs band edge on alloy composition based on the absolute measurement of x
Authors: Kuech T. F. , Wolford D. J. , Potemski R. , Bradley J. A. , Kelleher K. H. , Yan D. , Farrell J. Paul , Lesser P. M. S. , Pollak F. H.
Published: July 2002

The absolute determination of the Al concentration, x, in epitaxial layers of AlxGa1−xAs was carried out using a nuclear reaction technique. This technique utilizes the narrow resonances found in the 27Al( p,γ)Si28 reaction, together with Rutherford backscattering measurements, to obtain accurate values of the alloy composition. The AlxGa1−xAs band edge was measured on these samples through low-temperature photoluminescence (2 K) measurements. An improved value of the direct edge (Γ) on composition was determined to be EΓg =1.512 +1.455x(eV) within a ±0.3% limit. The direct-to-indirect transition was found to occur at an Al concentration of x≂0.37±0.015, lower than previously reported for He temperatures.
Focused ion beam induced fine patterns of organogold films
Authors: Ohmura Y. , Shiokawa T. , Toyoda K. , Namba S.
Published: July 2002

It has been found that a relatively low-dose focused ion beam irradiation makes an organogold film insoluble to its solvents (e.g., trichloroethylene), resulting in submicron pattern formation. Ion beam induced, low conductivity as with resistivities of (4–4000) MΩ/⧠ has been observed for more than 1×1015 ions/cm2 dose for a 0.65-μm-thick film. However, by heat treatment at 300 °C after low-dose ion beam patterning, gold patterns with resistivity of 3×10−4 Ω cm, which is about 100 times the bulk gold value, are obtained.
Space-charge buildup and bistability in resonant-tunneling double-barrier structures
Authors: Sheard F. W. , Toombs G. A.
Published: July 2002

Using the sequential theory of resonant tunneling, the dc current-voltage characteristic of a double-barrier structure is calculated, taking into account the effect of space charge in the quantum well. A region of current bistability is found over a voltage range which is determined by the maximum space charge and the capacitance of the structure. These parameters are directly related to the periodicity of magnetoquantum oscillations in the current.
Semi-insulating current blocking property simulations for buried heterostructure laser diodes
Authors: Asada S. , Sugou S. , Kasahara K. , Kato Y. , Kumashiro S.
Published: July 2002

A compound-semiconductor device simulator, in which deep levels in the semi-insulating layers can be taken into account, has been developed. By using this simulator, the electrical properties for the semi-insulating InP buried heterostructure laser diodes were investigated. The leakage current, without passing through the active region, was found to be small when the trap density in the semi-insulating InP layers is more than 3×1015 cm−3 and less than 1×1016 cm−3. This simulator will be a useful tool in predicting the semi-insulating properties of electrical and optical semiconductor devices.
Critical current enhancement in NbN/AlN multilayers
Authors: Murduck J. M. , Capone D. W. , Schuller Ivan K. , Foner S. , Ketterson J. B.
Published: July 2002

Measurements are presented of the critical current density, Jc, and parallel upper critical field, Hc2∥, for the refractory superconductor/insulator multilayer system NbN/AlN. A dramatic increase in Jc, together with an increase of Hc2∥, is observed with decreasing NbN layer thickness. The Jc enhancement arises from flux pinning in the AlN or at the NbN-AlN interface, not at defects in the NbN structure. The results indicate that the development of superconductor/insulator composites is a viable strategy to increase the critical current. Thermal stabilization which does not degrade the very high Jc values was also accomplished in a novel structure by addition of copper layers within the superlattice.
Polyimide on copper: The role of solvent in the formation of copper precipitates
Authors: Kowalczyk Steven P. , Kim Young-Ho , Walker G. F. , Kim J.
Published: July 2002

The interaction at the polyimide-copper (oxide) interface has been investigated by means of cross-sectional transmission electron microscopy. By using a solventless molecular beam epitaxy growth technique, the role of solvent in the aggregation and diffusion of copper-rich particles into the polyimide film and away from the interface was unambiguously demonstrated. The polyimide on copper growth sequence is contrasted to the copper on polyimide growth sequence, and the reasons for the differences between the two growth interfaces are discussed.
Amorphous SiC/Si three-color detector
Authors: Tsai Hsiung-Kuang , Lee Si-Chen
Published: July 2002

A hydrogenated amorphous SiC/hydrogenated amorphous Si heterojunction photodetector whose peak response could be voltage adjusted to three wavelengths, i.e., 480, 530, and 575 nm, by applying a small bias within ±2 V has been successfully fabricated. The basic principle is to use two back-to-back p-i-n junction diodes (or an n-i-p-i-n transistor) in which photons with wavelength λ<500 nm (blue) are mainly collected in the front a-SiC:H/a-Si:H heterojunction and the rest (green and red) are absorbed in the rear a-Si:H homojunction. To further distinguish the green from the red, two undoped a-Si:H layers, deposited at different conditions, were used in the rear homojunction to obtain two distinct collection regions. It is found that the required voltage to select one of the collection regions is less than 2 V. This detector shows a very high rejection ratio at various responses and thus is good for distinguishing the entire color spectrum.
Crystal structure of the macrocycle-forming thioesterase domain of the erythromycin polyketide synthase: Versatility from a unique substrate channel
Authors: Tsai Shiou-Chuan , Miercke Larry J. W. , Krucinski Jolanta , Gokhale Rajesh , Chen Julian C.-H. , Foster Paul G. , Cane David E. , Khosla Chaitan , Stroud Robert M.
Published: July 2002

As the first structural elucidation of a modular
polyketide synthase (PKS) domain, the crystal structure of the
macrocycle-forming thioesterase (TE) domain from the
6-deoxyerythronolide B synthase (DEBS) was solved by a combination of
multiple isomorphous replacement and multiwavelength anomalous
dispersion and refined to an
R
factor of 24.1% to
2.8-Å resolution. Its overall tertiary architecture belongs to the
α/β-hydrolase family, with two unusual features
unprecedented in this family: a hydrophobic leucine-rich dimer
interface and a substrate channel that passes through the entire
protein. The active site triad, comprised of Asp-169, His-259, and
Ser-142, is located in the middle of the substrate channel, suggesting
the passage of the substrate through the protein. Modeling indicates
that the active site can accommodate and orient the
6-deoxyerythronolide B precursor uniquely, while at the same time
shielding the active site from external water and catalyzing
cyclization by macrolactone formation. The geometry and organization of
functional groups explain the observed substrate specificity of this TE
and offer strategies for engineering macrocycle biosynthesis. Docking
of a homology model of the upstream acyl carrier protein (ACP6) against
the TE suggests that the 2-fold axis of the TE dimer may also be the
axis of symmetry that determines the arrangement of domains in the
entire DEBS. Sequence conservation suggests that all TEs from modular
polyketide synthases have a similar fold, dimer 2-fold axis, and
substrate channel geometry.
Identification of the endogenous smooth muscle myosin phosphatase-associated kinase
Authors: MacDonald Justin A. , Borman Meredith A. , Murányi Andrea , Somlyo Avril V. , Hartshorne David J. , Haystead Timothy A. J.
Published: July 2002

Ca
2+
sensitization of smooth muscle contraction
involves inhibition of myosin light chain phosphatase (SMPP-1M) and
enhanced myosin light chain phosphorylation. Inhibition of SMPP-1M is
modulated through phosphorylation of the myosin targeting subunit
(MYPT1) by either Rho-associated kinase (ROK) or an unknown
SMPP-1M-associated kinase. Activated ROK is predominantly
membrane-associated and its putative substrate, SMPP-1M, is mainly
myofibrillar-associated. This raises a conundrum about the mechanism of
interaction between these enzymes. We present ZIP-like kinase,
identified by “mixed-peptide” Edman sequencing after affinity
purification, as the previously unidentified SMPP-1M-associated kinase.
ZIP-like kinase was shown to associate with MYPT1 and phosphorylate the
inhibitory site in intact smooth muscle. Phosphorylation of ZIP-like
kinase was associated with an increase in kinase activity during
carbachol stimulation, suggesting that the enzyme may be a terminal
member of a Ca
2+
sensitizing kinase cascade.
Keeping Mars warm with new super greenhouse gases
Authors: Gerstell M. F. , Francisco J. S. , Yung Y. L. , Boxe C. , Aaltonee E. T.
Published: July 2002

Our selection of new super greenhouse gases to fill a putative
“window” in a future Martian atmosphere relies on
quantum-mechanical calculations. Our study indicates that if Mars could
somehow acquire an Earth-like atmospheric composition and surface
pressure, then an Earth-like temperature could be sustained by a
mixture of five to seven fluorine compounds. Martian mining
requirements for replenishing the fluorine could be comparable to
current terrestrial extraction.
Substratal idiothetic navigation of rats is impaired by removal or devaluation of extramaze and intramaze cues
Authors: Stuchlik A. , Fenton A. A. , Bures J.
Published: July 2002

The spatial orientation of vertebrates is implemented by two
complementary mechanisms: allothesis, processing the information about
spatial relationships between the animal and perceptible landmarks, and
idiothesis, processing the substratal and inertial information produced
by the animal's active or passive movement through the environment.
Both systems allow the animal to compute its position with respect to
perceptible landmarks and to the already traversed portion of the path.
In the present study, we examined the properties of substratal
idiothesis deprived of relevant exteroceptive information. Rats
searching for food pellets in an arena formed by a movable inner disk
and a peripheral immobile belt were trained in darkness to avoid a
60° sector; rats that entered this sector received a mild foot shock.
The punished sector was defined in the substratal idiothetic frame, and
the rats had to determine the location of the shock sector with the use
of substratal idiothesis only, because all putative intramaze cues were
made irrelevant by angular displacements of the disk relative to the
belt. Striking impairment of place avoidance by this “shuffling
procedure” indicates that effective substratal idiothesis must be
updated by exteroceptive intramaze cues.
Molecular identification of the insect adipokinetic hormone receptors
Authors: Staubli Frank , Jørgensen Thomas J. D. , Cazzamali Giuseppe , Williamson Michael , Lenz Camilla , Søndergaard Leif , Roepstorff Peter , Grimmelikhuijzen Cornelis J. P.
Published: July 2002

The insect adipokinetic hormones (AKHs) are a large family of peptide hormones that are involved in the mobilization of sugar and lipids from the insect fat body during energy-requiring activities such as flight and locomotion, but that also contribute to hemolymph sugar homeostasis. Here, we have identified the first insect AKH receptors, namely those from the fruitfly
Drosophila melanogaster
and the silkworm
Bombyx mori
. These results represent a breakthrough for insect molecular endocrinology, because it will lead to the cloning of all AKH receptors from all model insects used in AKH research, and, therefore, to a better understanding of AKH heterogeneity and actions. Interestingly, the insect AKH receptors are structurally and evolutionarily related to the gonadotropin-releasing hormone receptors from vertebrates.
Local gene transfer of tissue factor pathway inhibitor regulates intimal hyperplasia in atherosclerotic arteries
Authors: Zoldhelyi Pierre , Chen Zhi-Qiang , Shelat Harnath S. , McNatt Janice M. , Willerson James T.
Published: July 2002

Tissue factor (TF), the initiator of blood coagulation and
thrombosis, is up-regulated after vascular injury and in
atherosclerotic states. Systemic administration of recombinant TF
pathway inhibitor (TFPI) has been reported to decrease intimal
hyperplasia after vascular injury and also to suppress systemic
mechanisms of blood coagulation and thrombosis. Here we report that, in
heritable hyperlipidemic Watanabe rabbits, adenoviral gene transfer of
TFPI to balloon-injured atherosclerotic arteries reduced the extent of
intimal hyperplasia by 43% (
P
< 0.05) compared
with a control vector used at identical titer (1 ×
10
10
plaque-forming units/ml). Platelet aggregation and
coagulation studies performed 7 days after local gene transfer of TFPI
failed to show any impairment in systemic hemostasis. At time of
sacrifice, 4 weeks after vascular injury, the 10
Ad-TFPI
treated carotid arteries were free of thrombi, whereas two
control-treated arteries were occluded (
P
, not
significant). These findings suggest that TFPI overexpressed in
atherosclerotic arteries can regulate hyperplastic response to injury
in the absence of changes in the hemostatic system, establishing a role
for local TF regulation as target for gene transfer-based
antirestenosis therapies.
Chiral discrimination in cyclodextrin complexes of amino acid derivatives: β-cyclodextrin/
N
-acetyl-
l
-phenylalanine and
N
-acetyl-
d
-phenylalanine complexes
Authors: Alexander Jennifer M. , Clark Joanna L. , Brett Tom J. , Stezowski John J.
Published: July 2002

In a systematic study of molecular recognition of amino acid derivatives in solid-state β-cyclodextrin (β-CD) complexes, we have determined crystal structures for complexes of β-cyclodextrin/
N
-acetyl-
l
-phenylalanine at 298 and 20 K and for
N
-acetyl-
d
-phenylalanine at 298 K. The crystal structures for the
N
-acetyl-
l
-phenylalanine complex present disordered inclusion complexes for which the distribution of guest molecules at room temperature is not resolvable; however, they can be located with considerable confidence at low temperature. In contrast, the complex with
N
-acetyl-
d
-phenylalanine is well ordered at room temperature. The latter complex presents an example of a complex in this series in which a water molecule is included deeply in the hydrophobic torus of the extended dimer host. In an effort to understand the mechanisms of molecular recognition giving rise to the dramatic differences in crystallographic order in these crystal structures, we have examined the intermolecular interactions in detail and have examined insertion of the enantiomer of the
d
-complex into the chiral β-CD complex crystal lattice.
In vitroevolution of a T cell receptor with high affinity for peptide/MHC
Authors: Holler Phillip D. , Holman Philmore O. , Shusta Eric V. , O'Herrin Sean , Wittrup K. Dane , Kranz David M.
Published: July 2002

T cell receptors (TCRs) exhibit genetic and structural diversity similar to antibodies, but they have binding affinities that are several orders of magnitude lower. It has been suggested that TCRs undergo selectionin vivoto maintain lower affinities. Here, we show that there is not an inherent genetic or structural limitation on higher affinity. Higher-affinity TCR variants were generated in the absence ofin vivoselective pressures by using yeast display and selection from a library of Vα CDR3 mutants. Selected mutants had greater than 100-fold higher affinity (KD≈ 9 nM) for the peptide/MHC ligand while retaining a high degree of peptide specificity. Among the high-affinity TCR mutants, a strong preference was found for CDR3α that contained Pro or Gly residues. Finally, unlike the wild-type TCR, a soluble monomeric form of a high-affinity TCR was capable of directly detecting peptide/MHC complexes on antigen-presenting cells. These findings prove that affinity maturation of TCRs is possible and suggest a strategy for engineering TCRs that can be used in targeting specific peptide/MHC complexes for diagnostic and therapeutic purposes.
S
-nitrosothiol repletion by an inhaled gas regulates pulmonary function
Authors: Moya Martin P. , Gow Andrew J. , McMahon Timothy J. , Toone Eric J. , Cheifetz Ira M. , Goldberg Ronald N. , Stamler Jonathan S.
Published: July 2002

NO synthases are widely distributed in the lung and are extensively
involved in the control of airway and vascular homeostasis. It is
recognized, however, that the O
2
-rich environment of the
lung may predispose NO toward toxicity. These Janus faces of NO are
manifest in recent clinical trials with inhaled NO gas, which has shown
therapeutic benefit in some patient populations but increased morbidity
in others. In the airways and circulation of humans, most NO
bioactivity is packaged in the form of
S
-nitrosothiols
(SNOs), which are relatively resistant to toxic reactions with
O
2
/O

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. This finding has led to the
proposition that channeling of NO into SNOs may provide a natural
defense against lung toxicity. The means to selectively manipulate the
SNO pool, however, has not been previously possible. Here we report on
a gas,
O
-nitrosoethanol (ENO), which does not react with
O
2
or release NO and which markedly increases the
concentration of indigenous species of SNO within airway lining fluid.
Inhalation of ENO provided immediate relief from hypoxic pulmonary
vasoconstriction without affecting systemic hemodynamics. Further, in a
porcine model of lung injury, there was no rebound in cardiopulmonary
hemodynamics or fall in oxygenation on stopping the drug (as seen with
NO gas), and additionally ENO protected against a decline in cardiac
output. Our data suggest that SNOs within the lung serve in matching
ventilation to perfusion, and can be manipulated for therapeutic gain.
Thus, ENO may be of particular benefit to patients with pulmonary
hypertension, hypoxemia, and/or right heart failure, and may offer a
new therapeutic approach in disorders such as asthma and cystic
fibrosis, where the airways may be depleted of SNOs.