bluuebunny/crossref_metadata_2025 huggingface dataset:
Compartmentation of Assimilate Fluxes in Leaves
Authors: Voitsekhovskaja Olga V. , Pakhomova Marina V. , Syutkina Anna V. , Gamalei Y. V. , Heber U.
Published: July 2002

Abstract: Sugar levels in the apoplast of assimilate exporting leaves were studied in two groups of plant species with contrasting structures of companion cells in minor veins. These species are termed either “symplastic” (with intermediary cells) or “apoplastic” (with transfer or ordinary cells). Sugars were measured in intercellular washing fluid after extracting the apoplast by an infiltration‐centrifugation technique. During the course of a day, sugar contents in the apoplast were, in general, lower in species with intermediary cells than in species with transfer or ordinary cells. In “symplastic” species, apoplastic sucrose concentrations were between 0.3 and 1 mM. In “apoplastic” species with transfer cells, they ranged between 2 and 6 mM. Apoplastic hexose contents were between 0.3 and 1 mM irrespective of presumed transport mode. “Symplastic” and “apoplastic” plants differed markedly in their response to a'translocation block. In “symplastic” plants, inhibition of assimilate export left apoplastic concentrations of sucrose and hexoses unchanged, whereas in “apoplastic” plants sugar levels increased, the maximal increase being observed with sucrose. In these plants, concentrations of sucrose were two to six times higher in the apoplast under export inhibition than in control leaves. The data suggest a different role of the leaf apoplast in the compartmentation and export of assimilates in the two plant groups under study.
Epitaxial growth of YBa2Cu3O7−x thin films on (110) SrTiO3 single crystals by activated reactive evaporation
Authors: Terashima T. , Bando Y. , Iijima K. , Yamamoto K. , Hirata K.
Published: July 2002

The orientation of YBa2Cu3O7−x thin films grown on (110) planes of SrTiO3 by activated reactive evaporation was investigated by means of reflection high-energy electron diffraction. The orientation of the films depended on the substrate temperature. The films with (110) planes parallel to the substrate surface grew in a narrow range of substrate temperatures around 530 °C, while the films with (103) planes parallel to the surface grew at temperatures above 600 °C. The change of the epitaxial orientation with the substrate temperature is discussed in terms of the temperature dependence of the lattice mismatch between YBa2Cu3O7−x and SrTiO3.
Submillimeter wave generation using Josephson junction arrays
Authors: Wan K. , Jain A. K. , Lukens J. E.
Published: July 2002

Arrays of phase-locked Josephson junctions have been developed and tested which deliver over 1 μW of power, via superconducting microstrip, to loads with resistances ranging from 20 to 60 Ω. The arrays, designed to operate at 350 GHz, typically are continuously tunable from 350 to 450 GHz or higher, the upper limit being set by the onset of loss in the lead alloy microstrip.
Theoretical analysis of timing jitter in gain-switched semiconductor lasers
Authors: Sano E. , Shinagawa M. , Takahashi R.
Published: July 2002

We investigate the timing jitter in gain-switched Fabry–Perot (FP) and distributed feedback (DFB) lasers based on the Langevin equations. Numerical results are in good agreement with measured ones. The present analysis demonstrates that the difference between the timing jitter in FP lasers and that in DFB lasers is caused by the difference in rapidity of turn-on.
Bi4Ca3Sr3Cu4Oy ceramic fibers from crystallization of glasses
Authors: Zheng Haixing , Hu Yi , Mackenzie J. D.
Published: July 2002

Bi4Ca3Sr3Cu4Oy glass fibers a few centimeters in length were prepared by melt spinning. After heat treatment at 830 °C, the fibers became superconducting. The x-ray diffraction pattern revealed that the heat-treated fibers have the 75 K phase as the major phase and Bi2 Sr2 CuO6 and CuO as minor phases. The resistivity measurement showed that the onset superconducting temperature of the fibers was ∼80 K.
Key role of oxygen at zinc oxide varistor grain boundaries
Authors: Stucki F. , Greuter F.
Published: July 2002

The electrical transport properties of zinc oxide varistors are correlated with the chemistry of their grain boundaries. An adsorbed layer of bismuth, about 5 Å thick, is necessary to create potential barriers at the grain boundaries. The height of these potential barriers depends sensitively on the excess amount of oxygen (∼1 monolayer) present at the interfaces between grains. This oxygen enrichment is influenced by thermal processing and by electrical degradation.
Lower plasma-induced damage in SiO2/Si at lower temperatures
Authors: Mizutani Tatsumi , Yunogami Takashi , Tsujimoto Kazunori
Published: July 2002

We found that the radiation damage induced in a SiO2/Si system during plasma processing depends strongly on the specimen temperature. The surfaces of the SiO2 have been exposed to a microwave plasma at different temperatures and the resultant damage has been evaluated by capacitance-voltage (C-V) measurements. The flatband voltage shift (ΔVFB) for the specimen exposed to the plasma at 126 K has been found to be only 1/3 of that at 300 K. In case of vacuum ultraviolet photon irradiation through a thin Al film, the ΔVFB for the irradiation at 126 K has been only 1/5 of that at 300 K. It is believed that this lower plasma-induced damage at the lower temperature is due to the small mobility of hole in SiO2 at lower temperatures. Plasma etching at low temperature has the advantage of low damage generation in the SiO2/Si structures.
Two-photon absorption peculiarities of potassium dihydrogen phosphate crystal at 216 nm
Authors: Gurzadyan G. G. , Ispiryan R. K.
Published: July 2002

The two-photon absorption coefficient of potassium dihydrogen phosphate at λ=216 nm was measured to be β=(6.0±0.5)10−10 cm/W. The presence of impurities is shown to increase nonlinear losses at very low intensities, I≊107 W/cm2.
Observation of high-pressure blast-wave decursors
Authors: Grun J. , Stamper J. , Manka C. , Resnick J. , Burris R. , Ripin B. H.
Published: July 2002

We observe, for the first time in a laboratory, the formation of decursors at intersections of planar surfaces with powerful blast waves. The blast waves, which have hundreds-kilobar overpressures, are created by rapid ablation of material heated with an intense laser beam.
Semiconducting polymer-buckminsterfullerene heterojunctions: Diodes, photodiodes, and photovoltaic cells
Authors: Sariciftci N. S. , Braun D. , Zhang C. , Srdanov V. I. , Heeger A. J. , Stucky G. , Wudl F.
Published: July 2002

The characterization of rectifying heterojunctions (diodes) fabricated from a semiconducting polymer, a soluble derivative of poly(phenylene-vinylene), and buckminsterfullerene, C60, are reported. Rectification ratios in the current versus voltage characteristics exceed 104. When illuminated, the devices exhibit a large photoresponse as a result of photoinduced electron transfer across the heterojunction interface from the semiconducting polymer (donor) onto C60 (acceptor). The photodiode and photovoltaic responses are characterized. Photoinduced electron transfer across the donor-accepted rectifying heterojunction offers potential for photodetector and for solar cell applications.
CsB3O5: A new nonlinear optical crystal
Authors: Wu Yicheng , Sasaki Takatomo , Nakai Sadao , Yokotani Atsushi , Tang Honggao , Chen Chuangtian
Published: July 2002

On the basis of the anionic group theory, a new nonlinear optical crystal cesium triborate, (CsB3O5, CBO) was discovered. Single crystals of CBO in centimeter size were grown from a stoichiometric melt. The transparent range of CBO covers from 170 to 3000 nm. The preliminary measurement of its second harmonic generation coefficient shows that its d14 is ∼0.468×d11 (β−BaB2O4). Its measured damage threshold is as high as 26 GW/cm2 at 1.053 μm, 1.0 ns.
Oriented nucleation and growth of diamond films on β-SiC and Si
Authors: Kohl R. , Wild C. , Herres N. , Koidl P. , Stoner B. R. , Glass J. T.
Published: July 2002

Oriented diamond nuclei prepared by bias-enhanced microwave plasma chemical vapor deposition on β-SiC and Si were characterized by x-ray texture diffractometry. In both cases, x-ray pole figures reveal an epitaxial relation between the orientation of diamond nuclei and the substrate. However, the angular spread of the nuclei orientation is rather large, amounting to 9°–13° (FWHM) in both polar and azimuthal directions. When growing thick diamond films on top of these already oriented diamond nuclei, the evolution of the orientational order depends critically on the growth conditions. In the case of 〈100〉 oriented nuclei, growth conditions which favor the formation of a 〈100〉 fiber texture can even improve the degree of orientational order, whereas other growth conditions result in a deterioration of the epitaxial relationship.
Behavior of porous silicon emission spectra during quenching by immersion in metal ion solutions
Authors: Andsager D. , Hilliard J. , Nayfeh M. H.
Published: July 2002

The photoluminescence emission of porous silicon was regularly measured while immersed in dilute metal ion solutions of Cu, Ag, and Au. The emission spectra show progressive quenching that advances from the blue edge towards the red edge of the emission band, causing a continuous shift in the band center and a narrowing of its width. Auger electron spectroscopy data show that the penetration of the metal adsorbate into the porous layer correlates with the degree of quenching of the photoluminescence. These results are interpreted as a progression of the quenching of the photoluminescence inward from the surface of the sample toward the bulk.
GaAs/AlGaAs microdisk lasers
Authors: Mohideen U. , Hobson W. S. , Pearton S. J. , Ren F. , Slusher R. E.
Published: July 2002

GaAs/AlGaAs microdisk lasers have been achieved using continuous optical pumping at 80 K. Surface passivation with a new sulfur/SiNx process is required in order to achieve steady-state lasing. Approximately 15% of the spontaneous emission is coupled into the lasing mode.
Dual-laser ablation for particulate-free film growth
Authors: Witanachchi S. , Ahmed K. , Sakthivel P. , Mukherjee P.
Published: July 2002

A novel dual laser ablation process that leads to particulate-free film growth is presented. A pulsed CO2 laser and an excimer (KrF) laser have been spatially overlapped on a Y2O3 target with a temporal delay between the pulses. The particulate density of the films grown by this method are at least three orders of magnitude smaller than the particulate density of a single excimer laser ablated film of similar thickness. In addition, a time-of-flight ion probe study indicates a sixfold enhancement of the plume species kinetic energies under dual-laser ablation. The degree of the plume excitation is observed to depend strongly on the delay between the laser pulses.
150 mW unsaturated output power at 3 μm from a single-mode-fiber erbium cascade laser
Authors: Pollnau M. , Ghisler Ch. , Bunea G. , Bunea M. , Lüthy W. , Weber H. P.
Published: July 2002

We report on an erbium cascade laser in a fluorozirconate fiber. Lasing on the transition 4I11/2→4I13/2 at 2.71 μm is supported by colasing on the transition 4S3/2→4I9/2 at 1.72 μm. This recycles the excitation that is lost via excited-state absorption and avoids the saturation of the output power. Threshold at 2.71 μm is 33 mW launched pump power at 791 nm. The measured slope efficiency of 22.6% is relatively close to the 29.1% stokes-efficiency limit. An output power of 158 mW is obtained, limited only by the 1.43 W power available from the Ti: sapphire pump laser. Output power is 15 and slope efficiency 2.5 times higher than reported in previous publications.
Comparison of surface roughness of polished silicon wafers measured by light scattering topography, soft-x-ray scattering, and atomic-force microscopy
Authors: Teichert C. , MacKay J. F. , Savage D. E. , Lagally M. G. , Brohl M. , Wagner P.
Published: July 2002

The surface roughness of silicon wafers after different stages of chemomechanical polishing was investigated by light scattering topography, soft-x-ray scattering, and atomic-force microscopy. Quantitative values of the rms roughness, the lateral correlation length, and the roughness exponent are extracted. The results suggest deviations from the ‘‘ideal’’ polishing process at large length scales.
Effects of Mn2+ distribution in Cu-modified ZnS on the concentration quenching of electroluminescence brightness
Authors: Yu Il , Senna Mamoru
Published: July 2002

Change in the distribution state of Mn2+ before and after annealing was examined by electron spin resonance (ESR) in order to elucidate the electroluminescence properties of Mn-doped ZnS modified by Cu. Formation of Mn2+ ion pairs and clusters, as determined from the hyperfine structure of ESR profiles, was revealed to be dominant for concentration quenching.
Bench top Fourier transform infrared based instrument for simultaneously measuring surface spectral emittance and temperature
Authors: Markham James R. , Kinsella Karen , Carangelo Robert M. , Brouillette Carl R. , Carangelo Martin D. , Best Philip E. , Solomon Peter R.
Published: July 2002

A bench top instrument has been constructed which allows for the measurements of radiance, directional-hemispherical reflection, and directional-hemispherical transmission from materials at elevated temperatures from 100 to over 2000 °C. The instrument measures these radiative properties over a wide spectral range, in the near- and mid-IR, from 12 500 to 500 cm−1 (0.8–20 μm). These measurements are then processed to determine the spectral emittance of the material and the temperature at the point of measurement. The instrument has applications for (1) industrial quality control of radiative properties of processed materials; (2) research and development of new materials; (3) temperature measurement by optical techniques in the near- and mid-IR; and (4) determination of heat transfer properties of materials. This article describes the instrument and its novel components, and presents measurement results for several materials.
Neutron focusing optic for submillimeter materials analysis
Authors: Xiao Q. F. , Chen H. , Sharov V. A. , Mildner D. F. R. , Downing R. G. , Gao N. , Gibson D. M.
Published: July 2002

A neutron lens constructed with polycapillary glass fibers is used to focus a 50×45 mm2 beam exiting a cold neutron guide onto a spot of 0.53 mm (full width at half maximum) with a current density gain of 80. The characteristics of the lens are presented. This lens is designed to enhance the detection limit and lateral resolution for prompt gamma activation analysis using cold neutron beams.
Heat capacity measurement of dielectric solids using a linear surface heater: Application to ferroelectrics
Authors: Lee Seung-Min , Kwun Sook-ll
Published: July 2002

A new ac calorimetry for dielectric solids was developed. A long and thin metal strip directly evaporated onto the surface of the sample is used as a heater and thermometer. Temperature oscillation of the heater is measured from the third harmonic voltage at the heater. Both the heat capacity and thermal conductivity of the sample are obtained by the frequency dependent phase and amplitude of the third harmonic voltage. Since heat capacity addenda and heat leaks in the measurement are extremely small, prior calibration is not required. The calorimeter is fully automated and has been tested on two ferroelectrics, KH2PO4 and triglycine sulfate, at temperatures above 77 K. The optimal frequency region for the calorimetry is also discussed.
Photoelectron imaging spectrometry: Principle and inversion method
Authors: Bordas C. , Paulig F. , Helm H. , Huestis D. L.
Published: July 2002

A new photoelectron spectrometer has recently been used to analyze the energy and spatial distribution of photoelectrons produced by multiphoton ionization of rare gases. It is based on the analysis of the image obtained by projecting the expanding electron cloud resulting from the ionization process onto a two-dimensional position sensitive detector by means of a static electric field. In this article, we present the principle of this imaging spectrometer and the relevant equations of motion of the charged particle in this device, together with an inversion method that allows us to obtain the energy and angular distribution of the electrons. We present here the inversion procedure relevant to the case where the electrostatic energy acquired in the static field is large as compared to the initial kinetic energy of the charged particles. A more general procedure relevant to any regime will be described in a following article.
Fluorescence lifetime sensor of copper ions in water
Authors: Birch D. J. S. , Rolinski O. J. , Hatrick D.
Published: July 2002

We demonstrate an optical method for the selective detection of Cu(II) ions in water using time-resolved fluorescence resonance energy transfer from the dye rhodamine 800 encapsulated in a sensor. In comparison to copper, quenching by other metal ions such as cobalt, nickel, and chromium is shown to be negligible. The experimental arrangement incorporates picosecond diode laser excitation and time-correlated single-photon counting for detection. Down to 5 mM of copper deposited on the sensor can be measured and a linear response is obtained up to at least 50 mM. A lower limit of detection for the sensor in the region of 10 ppb is shown to be readily achievable with good resolution.
Electron Penning trap for the generation of high density non-neutral plasmas
Authors: Schauer M. M. , Mitchell T. B. , Holzscheiter M. H. , Barnes D. C.
Published: July 2002

We describe an experiment to produce high density pure electron plasmas in a cryogenic Penning trap. The apparatus and its operation are described in detail. A brief summary of data acquired to date and its interpretation are given. Possible uses and future work are mentioned.
Large-area radio frequency plasma sources for fusion applications
Authors: Kraus W. , Speth E. , Feist J.-H. , Frank P. , Heinemann B. , Riedl R. , Trainham R. , Jacquot C.
Published: July 2002

Four powerful inductively coupled radio frequency (rf) plasma generators producing 90 A, 55 kV hydrogen beams were manufactured for the second injector of ASDEX Upgrade. The design of the source and the power supply was optimized based on the experience with a prototype source. During conditioning of the first two sources, it was demonstrated that power efficiency, transmission of the beam power and reliability meet all requirements. The start of the neutral beam injection with the second injector is scheduled for October 1997. Recently, also negative ions in rf sources are being investigated within a collaboration with CEA Cadarache. In this development project a positive ion prototype rf source is being utilized with a small extraction area (48 cm2) and a 700 G cm magnetic filter. Without cesium a current density of 4.5 mA/cm2 was achieved so far at elevated pressure (1.6 Pa), the H−-current density being proportional to the rf power. At medium pressure (0.6 Pa) the current density is lower approximately by a factor of 5, but preliminary results with cesium injection show a relative increase by almost the same factor in this pressure range. Probe measurements indicate electron temperatures of 2.5–4.5 eV close to the plasma grid. Attempts to improve the performance by better plasma confinement, different wall materials, and filter strengths are under way.
Free-space electro-optic sampling of terahertz beams
Authors: Wu Q. , Zhang X.-C.
Published: July 2002

Free-space electro-optic sampling is an alternative method for the characterization of freely propagating terahertz beams with subpicosecond temporal resolution. In contrast to resonant photoconductive dipole antennas, free-space electro-optic sampling via the linear electro-optic effect (Pockels effect) offers a flat frequency response over an ultrawide bandwidth and the potential for a simple cross-correlation signal of the terahertz and optical pulses.
Hohlraum symmetry measurements with surrogate solid targets (invited)
Authors: Glendinning S. G. , Amendt P. , Cline B. D. , Ehrlich R. B. , Hammel B. A. , Kalantar D. H. , Landen O. L. , Turner R. E. , Wallace R. J. , Weiland T. J. , Dague N. , Jadaud J.-P. , Bradley D. K. , Pien G. , Morse S.
Published: July 2002

Time-resolved radiographic imaging of low density, solid spherical surrogate targets has been used to provide a time-dependent measurement of drive pressure symmetry in cylindrical hohlraums on both the Nova and Omega lasers. The experiments replace the usual capsule at the center of a gold hohlraum with a sphere of SiO2 foam (ρ=0.3 g/cm3). The laser generates an x-ray drive inside the hohlraum which does not produce perfectly symmetric drive pressure on a spherical target, giving rise to a distorted shock traveling radially inward. The rarefaction behind the shock generated in this sphere produces a rapid rise in x-ray transmission which is easily detectable experimentally by radiography. The position of this feature may be determined to within a few microns in our experimental setup using a gated x-ray pinhole camera. Time-dependent control of drive symmetry in a hohlraum requires the ability to adjust the laser power as a function of both time and position along the hohlraum axis. We have implemented this control on the Omega laser by delaying sets of beams (“beam staggering”) and on the Nova laser (“beam phasing”) by delivering two independent pulse shapes on inner and outer halves of the Nova beams. The improvement in P2 pressure symmetry with this control as measured with the foam ball technique was found to be as much as a factor of 3, compared with deliberately mistuned drives. On the National Ignition Facility (NIF), we expect to use this technique as part of the effort to tune drive symmetry to achieve higher gain. We present results from these experiments from both facilities and comparisons with LASNEX simulations. In addition, we discuss the application of the foam ball technique to NIF targets.
Noncontact technique for measuring surface tension and viscosity of molten materials using high temperature electrostatic levitation
Authors: Rhim Won-Kyu , Ohsaka Kenichi , Paradis Paul-François , Spjut R. Erik
Published: July 2002

A new, noncontact technique is described which entails simultaneous measurements of the surface tension and the dynamic viscosity of molten materials. In this technique, four steps were performed to achieve the results: (1) a small sample of material was levitated and melted in a high vacuum using a high temperature electrostatic levitator, (2) the resonant oscillation of the drop was induced by applying a low level ac electric field pulse at the drop of resonance frequency, (3) the transient signals which followed the pulses were recorded, and (4) both the surface tension and the viscosity were extracted from the signal. The validity of this technique was demonstrated using a molten tin and a zirconium sample. In zirconium, the measurements could be extended to undercooled states by as much as 300 K. This technique may be used for both molten metallic alloys and semiconductors.
Annular Malmberg–Penning trap for studies of plasma confinement
Authors: Robertson Scott , Walch Bob
Published: July 2002

An annular Malmberg–Penning trap is described for studies of plasma confinement. A plasma of electrons is contained in the annular region between coaxial cylindrical conductors and is confined radially by an axial magnetic field and axially by an electrostatic field. An azimuthal magnetic field created by a current-carrying center conductor causes gradient, curvature, and additional electric drifts thus allowing new types of transport studies. An initial electron density of 106 cm−3 is obtained with axial and azimuthal fields of ∼10 mT and fill pressures of 10−5 to 10−4 Torr of helium or argon. The electric mobility drift arising from electron collisions with neutral gas determines the density decay time of ∼2 ms.
Nanobeam production with the multicusp ion source
Authors: Lee Y. , Ji Q. , Leung K. N. , Zahir N.
Published: July 2002

A 1.8-cm-diam multicusp ion source to be used for focused ion beam applications has been tested for Xe, He, Ne, Ar, and Kr ions. The extractable ion and electron currents were measured. The extractable ion current is similar for all these ion species except for Ne+, but the extractable electron current behaves quite differently. The multicusp ion source will be used with a combined extractor–collimator electrode system that can provide a few hundred nA of Xe+ or Kr+ ions. Ion optics computation indicates that these beams can be further focused with an electrostatic column to a beam spot size of ∼100 nm.
Formation of threading defects in GaN wurtzite films grown on nonisomorphic substrates
Authors: Sverdlov B. N. , Martin G. A. , Morkoç H. , Smith David J.
Published: July 2002

Possible causes of a dense network of threading defects in epitaxial hexagonal GaN films grown on various substrates are discussed. We show that these defects originate at the substrate/film interface as the boundaries between differently stacked hexagonal domains, and are created by surface steps on substrates nonisomorphic with wurtzite GaN. We argue that these defects are inherent in the epitaxy of wurtzite films on nonisomorphic substrates. As a result, isomorphic substrates such as ZnO and GaN should be explored for wurtzite nitride growth. Possible effects of these defects on the properties of wurtzite nitrides are briefly discussed.
Electrical properties’ maxima in thin films of the lead zirconate–lead titanate solid solution system
Authors: Chen H. D. , Udayakumar K. R. , Gaskey C. J. , Cross L. E.
Published: July 2002

The piezoelectric strain coefficients have been measured as a function of composition for films in the PbZrO3–PbTiO3 (PZT) solid solution system, using a double-beam laser interferometry technique. This compositional dependence of piezoelectric, and the associated dielectric and ferroelectric properties for films 1 μm in thickness with varying Zr/Ti ratio, deposited on platinized silicon substrates using a modified sol-gel route, corresponds to data reported for undoped PZT ceramics with respect to the effective morphotropic phase boundary composition. Films with composition near the morphotropic phase boundary, Pb(Zr0.52Ti0.48)O3, show enhanced values of the longitudinal piezoelectric coefficient, 194 pC/N; dielectric permittivity, 1310; and remanent polarization, 36 μC/cm2.
Studies of MoS2 structures produced by electron irradiation
Authors: José-Yacamán M. , López H. , Santiago P. , Galván D. H. , Garzón I. L. , Reyes A.
Published: July 2002

In this letter, we report the study of MoS2 crystals after irradiation with a high dose of electrons. A pelletron accelerator was used to irradiate the sample with electrons at an energy of 0.5 MeV. Samples were examined in high resolution electron microscopy. Two main structures were observed: onion layers with fullerene like structure and others with their planes rotated with respect to each other by well-defined angles. We present a calculation showing that relative rotation of the sulphur layers by 3°, 5°, 8°, and 16° basically have the same average energy as the unrotated structure. Therefore rotations of the layers through those angles will be favored. Instability on the structure produced by irradiation will induce rotations on the structure.
Simultaneous blue and green upconversion lasing in a laser-diode-pumped Pr3+/Yb3+ doped fluoride fiber laser
Authors: Baney D. M. , Rankin G. , Chang Kok Wai
Published: July 2002

An upconversion laser operating simultaneously at visible wavelengths of 520 and 490 nm under infrared semiconductor pumping is demonstrated. The Pr3+/Yb3+doped ZBLAN fluorozirconate fiber laser yielded 1.4 mW total power at green and blue wavelengths with approximately 350 mW of incident pump light at a wavelength of 856 nm. Threshold launched powers of 55 and 85 mW were obtained for the wavelengths of 520 and 490 nm, respectively.
Near field optics: Snapshot of the field emitted by a nanosource using a photosensitive polymer
Authors: Davy Stéphane , Spajer Michel
Published: July 2002

We propose here a method for probe characterization based on lithography on a photosensitive polymer without film development. We record in near field conditions the light emitted by different kinds of probes in order to distinguish in situ the near field contribution from the far field contribution of the tip. This information, which cannot be deduced from far field diffraction, allows an estimation of the field confinement and of the expected resolution of the microscope.
Ramp-type junction parameter control by Ga doping of PrBa2Cu3O7−δ barriers
Authors: Verhoeven M. A. J. , Gerritsma G. J. , Rogalla H. , Golubov A. A.
Published: July 2002

We analyzed the transport of charge carriers across PrBa2Cu3−xGaxO7−δ (PBCGO) barriers as a function of barrier thickness, Ga-doping level, temperature, and bias voltage. It was found that by Ga doping, the normal state resistance (Rn) of the junctions was systematically increased, while the critical current (Ic) remained constant. We argue that pair transport takes place by direct tunneling, whereas the quasiparticles have access to channels formed by one or more localized states inside the barrier. By Ga doping the IcRn products were increased, up to 8 mV at 4.2 K for junctions with 8 nm thick PrBa2Cu2.6Ga0.4O7−δ barriers.
Growth of colossal magnetoresistance thin films on silicon
Authors: Trajanovic Z. , Kwon C. , Robson M. C. , Kim K.-C. , Rajeswari M. , Ramesh R. , Venkatesan T. , Lofland S. E. , Bhagat S. M. , Fork D.
Published: July 2002

We are able to grow high quality La0.67Sr0.33MnO3(LSMO) colossal magnetoresistive (CMR) thin films on Y-stabilized zirconia (YSZ) buffered (100) Si substrates using a Bi4Ti3O12 texturing and lattice matching layer. The CMR films have very high structural perfection and show excellent transport and ferromagnetic properties, including the almost full saturation magnetization values and narrow ferromagnetic resonance peaks (15 Oe at 290 K). The lattice matching template/buffer layer approach is suitable for the high quality CMR films on Si. A close correlation between the magnetic hysteresis loop and the field dependence of MR is observed at lower temperatures.
Coupled-cavity resonant photodetectors for high-performance wavelength demultiplexing applications
Authors: Hu S. Y. , Hegblom E. R. , Coldren L. A.
Published: July 2002

A new coupled-cavity resonant photodetector structure is studied to provide a flat-topped passband with steep skirts in the photoresponse. The idea is to add a low-loss cavity that is optically coupled to the absorptive cavity via an intermediate mirror. Experimental reflection spectra confirm this coupling effect. Modeling for an InGaAs/GaAs/AlGaAs design reveals that this new coupled-cavity structure will be able to show a 22 dB channel rejection ratio even when the optical channel spacing is as narrow as 2 nm. In addition, the design and processing tolerances for cavity-mode adjustment are also addressed.
Smoothing of YBa2Cu3O7−δ films by ion cluster beam bombardment
Authors: Chu W. K. , Li Y. P. , Liu J. R. , Wu J. Z. , Tidrow S. C. , Toyoda N. , Matsuo J. , Yamada I.
Published: July 2002

Smoothing high-temperature superconductor (HTS) surfaces, especially HTS thin-film surfaces, is crucial for HTS thin-film device processing. In this letter, we describe a method to planarize the surface of a YBa2Cu3O7−δ HTS film down to a smoothness with a standard deviation of 1 nm or better. The method includes first smoothing the HTS surface by ion cluster beam bombardment, followed by annealing in oxygen ambient to regrow the damaged surface layer. Additional YBCO layers can be grown epitaxially on the treated surface, even without removing the top surface layer, which contained some residual damage after annealing. This method can be integrated into HTS circuit fabrication as a key step of planarization.
Theory of the (12_10) prismatic stacking fault in GaN
Authors: Northrup John E.
Published: July 2002

The atomic and electronic structure for a (12_10) prismatic stacking fault in GaN is calculated from first principles. The fault vector is 1/2[101_1]. Such a fault is believed to occur when GaN is grown on sapphire or SiC, and may originate when basal plane stacking faults fold onto the prism plane. We find that the boundary is heavily reconstructed, but that the atoms remain fourfold coordinated, with no wrong bonds or dangling bonds present. Despite the existence of distorted four-membered rings of bonds at the boundary, the fault does not induce deep states inside the gap. The calculated stacking fault energy is 72 meV/Å2.
Phonons in a strained hexagonal GaN–AlN superlattice
Authors: Gleize J. , Demangeot F. , Frandon J. , Renucci M. A. , Widmann F. , Daudin B.
Published: July 2002

A GaN (6.3 nm)-AlN (5.1 nm) superlattice, grown by molecular beam epitaxy on a sapphire substrate and an AlN buffer layer, has been studied by means of micro-Raman spectroscopy. Most of the observed features have been identified and assigned to optical phonons of the superlattice layers. The average biaxial strain in GaN layers has been deduced from the detailed analysis of the frequency shift observed on the phonon lines. Additional measurements on the bevelled sample clearly suggest the significant increase of this strain for decreasing distances from the interface with the buffer layer.
On the origin of strong visible photoluminescence in a Ge/porous Si structure
Authors: Wu X. L. , Siu G. G. , Gu Y. , Tang N. , Gao T. , Bao X. M.
Published: July 2002

We have studied the origin of strong visible photoluminescence (PL) in a Ge/porous Si (PS) structure in terms of infrared spectroscopy and electron spin resonance (ESR). Spectral analyses indicate that the enhanced PL cannot arise from both the quantum confinement on Ge nanocrystals embedded in the pores and the chemical compound of Ge, O, and H at the surface of the porous Si formed during Ge deposition. The experimental result from ESR strongly suggests that optical transitions in the oxygen-related defect centers (nonbridging oxygen hole centers) at the interface between PS and the Ge layer are responsible for the enhanced PL.
Role of lower valent substituent-oxygen vacancy complexes in polarization pinning in potassium-modified lead zirconate titanate
Authors: Tan Qi , Li Jianxing , Viehland Dwight
Published: July 2002

The dependence of polarization switching on thermal histories has been investigated for K1+-modified lead zirconate titanate (PZT) ceramics by Sawyer–Tower polarization methods. It was found that double-loop-like polarization characteristics in the aged condition could be reversed to normal single loop polarization characteristics by quenching specimens from above the ferroelectric phase transition temperature. However, the P–E curves reversed back to double-loop-like characteristics after reaging specimens at elevated temperatures. Excess oxygen vacancies in La3+-modified PZT were not found to result in double-loop-like polarization hysteresis, whereas excess oxygen vacancies in K1+-modified PZT did. These results provide evidence for role of K1+-oxygen vacancy complexes in polarization pinning.
Large supercooled liquid region and phase separation in the Zr–Ti–Ni–Cu–Be bulk metallic glasses
Authors: Hays C. C. , Kim C. P. , Johnson W. L.
Published: July 2002

Results of calorimetric, differential thermal analysis, and structural measurements are presented for a series of bulk metallic glass forming compositions in the Zr–Ti–Cu–Ni–Be alloy system. The calorimetric data for five alloys, prepared along the tie line between phase separating and nonphase separating compositions, show that the transition from phase separating to nonphase separating behavior is smooth. The bulk glasses near the center of the tie line exhibit large supercooled liquid regions: ΔT≈135 K, the largest known for a bulk metallic glass.
Metallic nanowires created by biopolymer masking
Authors: Fritzsche Wolfgang , Böhm Konrad J. , Unger Eberhard , Köhler J. Michael
Published: July 2002

Nanometer-scale structures are of great interest as potential building blocks for future electronic devices. One significant challenge is the electrical connection of such blocks by nanowires. We present here the use of filamentous biopolymer structures (microtubules) as a template for the creation of metallic nanowires. The microtubules served as a mask in an etching step of a thin metal layer. This approach results in formation of metallic paths (wires) of well-defined dimensions, with a shape determined by the biological template. Electrical measurements confirmed the realization of conductive metallic nanowires.
Low-loss Al-free waveguides for unipolar semiconductor lasers
Authors: Sirtori C. , Kruck P. , Barbieri S. , Page H. , Nagle J. , Beck M. , Faist J. , Oesterle U.
Published: July 2002

A promising waveguide design for midinfrared (λ=5–20 μm) unipolar semiconductor lasers is proposed and demonstrated in (Al)GaAs quantum cascade structures. In the latter, the active region is embedded between two GaAs layers, with an appropriate doping profile which allows optical confinement, with low waveguide losses and optimal heat dissipation. Low internal cavity losses of 20 cm−1 have been measured using different techniques for lasers with emission wavelength at ∼9 μm. At 77 K, these devices have peak output power in excess of 550 mW and threshold current of 4.7 kA/cm2.
Photoinduced oxide film formation on n-type GaN surfaces using alkaline solutions
Authors: Rotter T. , Mistele D. , Stemmer J. , Fedler F. , Aderhold J. , Graul J. , Schwegler V. , Kirchner C. , Kamp M. , Heuken M.
Published: July 2002

We report on photoassisted wet chemical formation of thin oxide films on n-GaN layers in potassium hydroxide based electrolytes at room temperature. The kinetics of the oxide formation and dissolution were examined via photocurrent transients. The tendency of the photocurrent to level out during photoelectrochemical etching experiments is associated with a quasiequilibrium state at the semiconductor/electrolyte interface. Homogeneous oxide films were grown in weak alkaline solutions (11<pH<13) under potentionstatic control with oxidation rates of up to 250 nm/h and characterized by Auger electron spectroscopy. Consequences on wet photochemical etch strategies are discussed.
Direct evidence of spontaneous quantum dot formation in a thick InGaN epilayer
Authors: Nistor L. , Bender H. , Vantomme A. , Wu M. F. , Van Landuyt J. , O’Donnell K. P. , Martin R. , Jacobs K. , Moerman I.
Published: July 2002

We report a direct observation of quantum dots formed spontaneously in a thick InGaN epilayer by high resolution transmission electron microscopy. Investigation of a (280 nm thick) In0.22Ga0.78N single layer, emitting in the blue/green spectral region, reveals quantum dots with estimated sizes in the range of 1.5–3 nm. Such sizes are in very good agreement with calculations based on the luminescence spectra of this specimen.
Magnetic and optical properties of ionic ferrofluids based on nickel ferrite nanoparticles
Authors: Hasmonay E. , Depeyrot J. , Sousa M. H. , Tourinho F. A. , Bacri J.-C. , Perzynski R. , Raikher Yu. L. , Rosenman I.
Published: July 2002

New ionic ferrofluids containing NiFe2O4 nanoparticles of size ⩽10 nm are investigated. The crystalline structure of the particles is probed by transmission electron microscopy and x-ray scattering. Static magnetization and field-induced birefringence measurements are performed on three samples differing by particle volume fraction. Cross analyzing of the results of those two types of macroscopic tests completely rejects a simple single-domain particle model but readily supports the two-component scheme of a particle as consisting of a core with a uniform magnetization and a surface layer of comparable thickness stowed with a spin-glass-like arrangement.
Electrowetting-based actuation of liquid droplets for microfluidic applications
Authors: Pollack Michael G. , Fair Richard B. , Shenderov Alexander D.
Published: July 2002

A microactuator for rapid manipulation of discrete microdroplets is presented. Microactuation is accomplished by direct electrical control of the surface tension through two sets of opposing planar electrodes fabricated on glass. A prototype device consisting of a linear array of seven electrodes at 1.5 mm pitch was fabricated and tested. Droplets (0.7–1.0 μl) of 100 mM KCl solution were successfully transferred between adjacent electrodes at voltages of 40–80 V. Repeatable transport of droplets at electrode switching rates of up to 20 Hz and average velocities of 30 mm/s have been demonstrated. This speed represents a nearly 100-fold increase over previously demonstrated electrical methods for the transport of droplets on solid surfaces.
Dielectric dispersion and critical behavior in relaxor ferroelectric Pb(Mg1/3Nb2/3)O3–PbTiO3
Authors: Bokov A. A. , Ye Z.-G.
Published: July 2002

A critical behavior of dielectric susceptibility has been discovered in relaxor ferroelectric 0.75Pb(Mg1/3Nb2/3)O3–0.25PbTiO3. The real part of permittivity ε′ was found to follow the relation ε′−ε∞=χ′∝(T−T0)−2 at T>T0, and ε′∝(TC−T)−1 at T<TC, where T0≈TC<Tm; Tm is the temperature of the diffused ε′(T) maximum as is typical of relaxors; ε∞ is the frequency-independent parameter, and χ′ is the susceptibility depending on frequency via the “universal” relaxation law χ′∝fn−1. The susceptibility χ′ is suggested to originate from the polarization of polar nanoclusters.
Stability of thermal ions confined by rigid-rotor electron clouds in Penning fusion systems
Authors: Chacón L. , Barnes D. C.
Published: July 2002

In the Penning fusion device, a spherical cloud of electrons, confined in a Penning-type trap, creates the ion-confining electrostatic well. Calculations performed with a bounce-averaged Fokker–Planck model have suggested that highest fusion energy gains in Penning fusion systems occur when the ion population is predominantly Maxwellian. However, the question arises about the stability of the electron cloud in the presence of thermal ions. Here, a stability analysis of rigid-rotor electrons confining thermal ions is performed. The results indicate that such configuration is absolutely stable to electrostatic perturbations for arbitrary rigid-rotor electron distributions.
Preferential solvation in supercritical fluids: An integral equation study
Authors: Egorov S. A.
Published: July 2002

A statistical mechanical study of preferential solvation in supercritical fluids is presented. The study is focused on attractive mixtures, which are characterized by local density and composition enhancement in the vicinity of the solute molecule. The solute–solvent radial distribution functions are obtained from two versions of the integral equation formalism, both of which are designed for treating inhomogeneous systems. Model calculations are performed for a ternary Lennard-Jones mixture composed of a dilute solute and two solvent species, which are identical except for their interaction strength with the solute. It is shown that theoretical results agree well with Monte Carlo simulations in reproducing local solvent density and composition augmentation induced by the solute.
Benchmark variational coupled cluster doubles results
Authors: Van Voorhis Troy , Head-Gordon Martin
Published: July 2002

We present the first application of the Rayleigh–Ritz variational procedure to the coupled cluster doubles trial function. The variational approach is applied to the potential surface of H4, the double dissociation of water and the dissociation of N2, and the results are compared to standard coupled cluster doubles calculations. It is found that the variational approach gives a greatly improved description of strongly correlated systems, where the standard approach is known to fail. Some examination of the basis set dependence of the results is presented.
Strain relaxation in AlGaN under tensile plane stress
Authors: Einfeldt S. , Kirchner V. , Heinke H. , Dießelberg M. , Figge S. , Vogeler K. , Hommel D.
Published: July 2002

Relaxation of tensile strain in AlxGa1−xN layers of different compositions epitaxially grown on GaN/sapphire is investigated. Extended crack channels along 〈211¯0〉 directions are formed if the aluminum content exceeds a critical value, which decreases with increasing layer thickness. This process is found to limit the average strain energy density to a maximum value of 4 J/m2. By calculating the stress distribution between cracks and the strain energy release rate for crack propagation, the relaxed strain as measured by x-ray diffraction is correlated to the crack density, and the onsets of crack channeling and layer decohesion are fitted to a fracture toughness of 9 J/m2. Moreover, the crack opening at the surface is found to linearly increase with the stress. Annealing of samples above the growth temperature introduces additional tensile stress due to the mismatch in thermal expansion coefficients between the layer and substrate. This stress is shown to relieve not only by the formation of additional cracks but also by the extension of cracks into the GaN layer and a thermal activated change in the defect structure.
Topological analysis of the electron localization function applied to the study of the [1,3] sigmatropic shift of fluorine in 3-fluorpropene
Authors: Chamorro Eduardo , Santos Juan C. , Gómez Badhin , Contreras Renato , Fuentealba Patricio
Published: July 2002

The [1,3] sigmatropic shift of the fluorine atom in the 3-fluorpropene system has been studied from the topological analysis of the electron localization function (ELF). All calculations have been carried out at the B3PW91/6-311g** level of theory. The allowed suprafacial pathway through a concerted four-membered Cs transition state (TS) has been characterized in terms of an ion pair structure which shows the halogen atom bearing −0.6e. The ionic nature of the TS is supported by the absence of any polisynaptic basin connecting the fluorine and the allyl fragments in this stationary point. Finally, the evolution of the bonding characteristics along the intrinsic reaction coordinate reaction path has been analyzed in detail in terms of the basin properties of the ELF function.
Reaction dynamics of S(1D)+H2/D2 on a new ab initio potential surface
Authors: Zyubin Alexander S. , Mebel Alexander M. , Der Chao Sheng , Skodje Rex T.
Published: July 2002

A new ab initio potential energy surface is generated for the chemical reaction, S(1D)+H2. The quantum chemistry calculations were carried out at the multi-reference configuration interaction (MRCI) level with multi-configuration self-consistent field (MCSCF) reference wave functions. The 1A′, 2A′, 3A′, 1A″, and 2A″ singlet surfaces were computed on a uniform spatial grid of over 2000 points to simulate the full reaction pathway. The results indicate a barrierless insertion pathway along the T-shaped geometry and an 8 kcal/mol barrier to abstraction along the collinear geometry. The lowest surface was fit to a smooth analytical function form based on the reproducing kernel Hilbert space approach and a Carter–Murrell-type expansion. The dynamics of the S(1D)+H2/D2 reactions were simulated using the quasi-classical trajectory method. The results are generally consistent with an insertion mechanism mediated through capture dynamics in the entrance channel followed by the statistical decay of a long-lived complex. Comparison to recent molecular beam experiments shows agreement in the broad pattern of results but also exhibits significant differences in the more finely resolved quantities.