bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of Root Exudates at Different Growth Stages of Ten Rice (Oryza sativa L.) Cultivars
Authors: Aulakh M. S. , Wassmann R. , Bueno C. , Kreuzwieser J. , Rennenberg H.
Published: July 2002

Abstract: Plant root exudates play important roles in the rhizosphere. We tested three media (nutrient solution, deionized water and CaSO4 solution) for three periods of time (2, 4 and 6 h) for collecting root exudates of soil‐grown rice plants. Nutrient culture solution created complications in the analyses of exudates for total organic C (TOC) by the wet digestion method and of organic acids by HPLC due to the interference by its components. Deionized water excluded such interference in analytical analyses but affected the turgor of root cells; roots of four widely different rice cultivars excreted 20 to 60 % more TOC in deionized water than in 0.01 M CaSO4. Furthermore, the proportion of carbohydrates in TOC was also enhanced. Calcium sulfate solution maintained the osmotic environment for root cells and did not interfere in analytical procedures. Collection for 2 h avoided under‐estimation of TOC and its components exuded by rice roots, which occurred during prolonged exposure. By placing plants in 0.01 M CaSO4 for 2 h, root exudates of soil‐grown traditional, tall rice cultivars (Dular, B40 and Intan), high‐yielding dwarf cultivars (IR72, IR52, IR64 and PSBRc 20), new plant type cultivars (IR65598 and IR65600) and a hybrid (Magat) were collected at seedling, panicle initiation, flowering and maturity and characterized for TOC and organic acids. The exudation rates were, in general, lowest at seedling stage, increased until flowering but decreased at maturity. Among organic acids, malic acid showed the highest concentration followed by tartaric, succinic, citric and lactic acids. With advancing plant growth, exudation of organic acids substituted exudation of sugars. Root and shoot biomass were positively correlated with carbon exudation suggesting that it is driven by plant biomass. As root exudates provide substrates for methanogenesis in rice fields, large variations in root exudation by cultivars and at different growth stages could greatly influence CH4 emissions. Therefore, the use of high‐yielding cultivars with lowest root excretions, for example IR65598 and IR65600, would mediate low exudate‐induced CH4 production. The screening of exciting rice cultivars and breeding of new cultivars with low exudation rates could offer an important option for mitigation of CH4 emission from rice agriculture to the atmosphere.
Behavior of Ga on Si(100) as studied by scanning tunneling microscopy
Authors: Nogami J. , Park Sang-il , Quate C. F.
Published: July 2002

The behavior of gallium on the Si(100) surface has been studied with scanning tunneling microscopy at low metal coverages. The Ga atoms are more mobile on Si(100) than on Si(111) under the same conditions. At less than 0.1 monolayer, the Ga atoms line up in rows parallel to the Si dimerization direction with a two unit cell periodicity. At higher metal densities, these rows are organized into areas of 3×2 two-dimensional order. The relevance of these results to studies of the initial stages of growth of GaAs on Si is discussed.
Threshold reduction through photon recycling in semiconductor lasers
Authors: Gigase Yves B. , Harder Christoph S. , Kesler Morris P. , Meier Heinz P. , Van Zeghbroeck Bart
Published: July 2002

The threshold pump power of an AlGaAs-GaAs ridge quantum well laser diode has been reduced by 42% by recycling the spontaneous emission. An integrated photodiode absorbs the spontaneous radiation emitted by the laser diode and converts it back into electrical power. The recycling of this power results in a reduction of the electrical power required to reach the lasing threshold.
Chemistry of fluorine in the oxidation of silicon
Authors: Kasi S. R. , Liehr M. , Cohen S.
Published: July 2002

The chemical environment of fluorine in the oxide layer of metal-oxide-semiconductor (MOS) structures has been examined by surface analytical spectroscopies. HF treatment of Si(100) results in subsurface SiF formation. Upon oxidation, oxyfluoride moieties are formed with a significant accumulation of fluorine throughout the oxide layer. These changes are correlated to the electrical integrity of MOS interfaces by performing Fowler–Nordheim electron injection studies.
High T
c superconductor-normal-superconductor step-edge junction dc SQUIDs with CaRuO3 as the normal metal
Authors: Lee Soon-Gul , Park Keunseop , Park Yong Ki , Park Jong-Chul
Published: July 2002

Superconductor-normal-superconductor (SNS) type step-edge junction dc SQUIDs were fabricated from YBa2Cu3O7−δ (YBCO) films with CaRuO3 as the normal metal by pulsed laser deposition and Ar+ beam patterning techniques. CaRuO3 films were grown epitaxially on YBCO and had a metallic conduction property. Current-voltage curves of the junctions showed a resistively shunted-junction behavior. Temperature dependence of the junction critical current was linear in agreement with the theory for junctions with the junction length shorter than twice of the normal coherence length (dN<2ξN). A well-behaving SQUID modulation signal was observed up to 52 K with a focusing parameter of 8.6.
Visible photoluminescence in Si+-implanted thermal oxide films on crystalline Si
Authors: Shimizu-Iwayama Tsutomu , Nakao Setsuo , Saitoh Kazuo
Published: July 2002

We have investigated visible photoluminescence excited by Ar ion laser (488 nm, 2.54 eV) at room temperature from Si+-implanted thermal oxide films grown on crystalline Si wafer, as-implanted and after subsequent annealing in vacuum. We found two types of visible luminescence bands similar to those of silica glasses; one band is observed in as-implanted specimens and disappears after heating to about 600 °C, and the other band is observed only after heating the specimens to about 1100 °C. Though the shapes of these luminescence spectra are different from those having been observed in Si+-implanted silica glass, the origins of these bands are the same as in silica glass. We discuss the similarities and the differences of luminescence bands in Si+-implanted silica glasses and thermal oxide films grown on crystalline Si.
Nanolithography with an atomic force microscope for integrated fabrication of quantum electronic devices
Authors: Wendel M. , Kühn S. , Lorenz H. , Kotthaus J. P. , Holland M.
Published: July 2002

We describe a novel technique using an atomic force microscope (AFM) for integrated nanometer-scale lithography on various mask materials such as photoresist or gold covering a mesa-etched GaAs-AlGaAs heterostructure at ambient conditions. The generated patterns can be transferred to the two-dimensional electron gas by wet chemical etching or by ion beam irradiation. We succeed in fabricating hole arrays with a periodicity down to 35 nm and a hole diameter of only a few nanometers. In magnetoresistance studies on so-called antidot devices with 95 nm period at T=4.2 K we can clearly observe commensurability oscillations, demonstrating the successful pattern transfer to the electron system. With the AFM we can also pattern lines of varying width and depth into prefabricated devices.
Current distribution measurement in a tokamak by FIR polarimetry (invited)
Authors: Soltwisch H.
Published: July 2002

The poloidal magnetic field distribution in a tokamak can be determined by probing the plasma with an array of FIR laser beams perpendicular to BT and measuring the Faraday rotation ψ of their plane of polarization. A concomitant detection of the phase shifts Δφ provides sufficient information to take the electron density profile properly into account. For the TEXTOR tokamak (R=1.75 m, a=0.5 m, BT =2 T) a nine-channel Véron-type interferometer has been upgraded to allow simultaneous measurements of Δφ and ψ with sensitivities of about 0.01×2π and 0.15° and time resolutions of 0.1 and 3 ms, respectively. The number of channels and their distribution are sufficient to reconstruct the local poloidal field and hence the current density profile with an accuracy of about ±5%–10% at half the plasma radius. For the central current density an accuracy of ±15%–20% is achieved by slowly moving the plasma across the probe beams and thereby multiplying the number of chords especially near the magnetic axis. For rapid qualitative information the current profile is approximated by j0 [1−(r/a)2]p, and the shape parameter p is obtained by real-time conversion of measured signals in an analog computing circuit.
Detection of mercury vapor using resonating microcantilevers
Authors: Thundat T. , Wachter E. A. , Sharp S. L. , Warmack R. J.
Published: July 2002

Oscillating silicon nitride microcantilevers coated with a thin gold film have been used to detect mercury vapor in air. Cantilever resonance frequency changes due to surface mass loading as a result of adsorption of mercury vapor. Furthermore, cantilever bending is also altered due to changes in surface stress induced by mercury adsorption on the gold overlayer. Both of these phenomena can be used to quantitatively detect adsorbed vapors with picogram mass resolution.
Design and testing of a high-temperature emissometer for porous and particulate dielectrics
Authors: Vader D. T. , Viskanta R. , Incropera F. P.
Published: July 2002

A new, high-temperature (500–1500 °C) emissometer has been designed, assembled, and tested. The apparatus is unique in that it may be used to measure the emittance of media that are semitransparent and show substantial surface roughness, internal voidage, and/or chemical inhomogeneities throughout the emitting volume. Data reduction procedures are developed and an analysis of systematic error sources and experimental uncertainty is presented. The emittance of a high-purity alumina was measured at several temperatures in the wavelength range from 2 to 14 μm. Results are compared to published values.
Photoluminescence imaging of porous silicon using a confocal scanning laser macroscope/microscope
Authors: Ribes A. C. , Damaskinos S. , Dixon A. E. , Carver G. E. , Peng C. , Fauchet P. M. , Sham T. K. , Coulthard I.
Published: July 2002

This letter describes a confocal scanning beam macroscope/microscope that can image specimens up to 7 cm in diameter using both photoluminescence and reflected light. The macroscope generates digital images (512×512 pixels) with a maximum 5 μm lateral resolution and 200 μm axial resolution in under 5 s, and in combination with a conventional confocal scanning laser microscope can provide quality control at a macroscopic/microscopic level for porous silicon specimens, wafers, detectors, and similar devices. This combination of instruments can also be used as a method for evaluating preparation parameters involved in the manufacture of porous silicon. Various confocal and nonconfocal photoluminescence and reflected-light images of porous silicon are shown using both a macroscope and a conventional confocal scanning laser microscope. A 3D profile of a porous silicon structure reconstructed from confocal slices is also shown.
Improvements of phase-modulated ellipsometry
Authors: Acher O. , Bigan E. , Drévillon B.
Published: July 2002

This article gives several new insights on ellipsometers using photoelastic modulators. The assumption that the modulation has the form δ=δ0+𝒜 sin ωt is ruled out. In contrast, it is shown that the presence of higher harmonics in the modulation affects the measured signal. A new formalism is proposed to take this effect into account, and experimental evidences of its consistency and relevance are exposed. Using this multiple-harmonic model, ellipsometric measurements showed a dispersion four to five times less than using the conventional model. A new method is proposed to adjust the modulation amplitude during the measurements, by measuring the third harmonic of the signal. It is proven experimentally that this method actually improves the precision of phase-modulated ellipsometry. Other sources of errors are reviewed, such as multiple reflections in coherent light. A practical procedure to test whether a modulator is well adapted to ellipsometry is given.
A vertical/horizontal two-dimensional piezoelectric driven inertial slider micropositioner for cryogenic applications
Authors: Blackford B. L. , Jericho M. H. , Boudreau M. G.
Published: July 2002

We have developed a simple, two-dimensional piezoelectric tube device which uses the inertial slider method to achieve remote micropositioning in the vertical and horizontal directions. The motion of the slider car occurs with respect to a quartz rod which is attached to, and accelerated by the piezo tube. The vertical motion, against or with gravity, is achieved by activating the longitudinal mode of the piezo tube. The horizontal motion is a rotation about the quartz rod, achieved by activating the bending mode of the piezo tube. The device is very compact, works at cryogenic temperatures and is ultrahigh vacuum compatible. Step sizes from 10 to 3000 nm and speeds up to 0.2 mm/s are possible. A computer simulation model of the inertial slider process has been developed also, to investigate the effectiveness of various activating waveforms. It confirms the advantage of a cycloidal-like waveform recently reported in the literature. Also, the model predictions are consistent with the results obtained in the laboratory. A diagram of the analog circuit developed to produce a cycloidal-like waveform to drive the piezoelectric is included. We briefly describe an alternative design where the activating piezoelectric is attached to the slider car which moves vertically on a fixed quartz rod.
Flash pyrolysis nozzle for generation of radicals in a supersonic jet expansion
Authors: Kohn Daniel W. , Clauberg Horst , Chen Peter
Published: July 2002

A method for the high-temperature generation of reactive species in supersonic molecular beams has been developed. Pyrolysis temperatures (25–1800 °C) can be maintained for longer than 100 h for extended experiments. Short contact time (≊10 μs) of the seeded (<1 Torr partial pressure in 2 atm inert carrier gas) precursor molecules suppresses recombination of the product radicals. Number densities for radicals are estimated to be ≊1014 cm−3 at the sonic orifice with radical fluxes of ≊1016 s−1.
One-dimensional scanning of moisture in porous materials with NMR
Authors: Kopinga K. , Pel L.
Published: July 2002

A versatile and modular nuclear magnetic resonance (NMR) instrument is described that is particularly suited for the study of moisture transport in porous media such as various building materials in which moisture can give rise to several kinds of damages. Quantitative measurements of one-dimensional moisture profiles and their time evolution can be performed on cylindrical samples having a diameter up to 20 mm with a spatial resolution better than 1 mm. Water absorption and drying experiments on various building materials demonstrate that the instrument can also be used for materials containing relatively large amounts of magnetic impurities, which until now were hardly accessible by NMR techniques.
Cooling power of the dilution refrigerator with a perfect continuous counterflow heat exchanger
Authors: Takano Y.
Published: July 2002

The model of the perfect continuous counterflow heat exchanger introduced by Frossati et al. to describe the performance of dilution refrigerators is solved rigorously with the proper boundary condition. Unlike the original solution by Frossati et al., the present solution gives the refrigerator cooling power which is in good agreement with experimental data over the entire temperature range. The analysis of the cooling power using the present result allows more detailed evaluation of the refrigerator performance than has been possible with the analysis of the base temperature. An approximate expression for the cooling power is discussed which simplifies the prediction of the cooling rate of a large thermal load such as a copper nuclear-demagnetization stage.
A nonoptical tip–sample distance control method for near-field scanning optical microscopy using impedance changes in an electromechanical system
Authors: Hsu J. W. P. , Lee Mark , Deaver B. S.
Published: July 2002

We present a nonoptical shear-force feedback method to regulate tip–sample distance for near-field scanning optical microscopy. In the shear force setup, the dither piezo and the attached fiber tip form an electromechanical system, whose power dissipation on resonance is sensitive to the change in damping force as the tip approaches and interacts with the sample. At the frequencies of interest (∼10–100 kHz), the change in power dissipation is conveniently manifested as a change in the electrical impedance of the dither piezo. We demonstrate that tip–sample distance feedback control can be achieved by measuring this change in dither piezo impedance. The sensitivity is currently ∼0.5 Å. This new technique is compared to other methods currently used for distance control in near-field scanning optical microscopy.
A new gas purifier for ArF excimer lasers
Authors: Ito S. , Saito T. , Tada A.
Published: July 2002

We have developed a new gas purifier for ArF excimer lasers that can remove all the impurities from degraded ArF excimer laser gas. We found that cooled activated charcoal at temperatures below −50 °C effectively removes CF4, which is the preponderant impurity generated in ArF excimer laser gas mixtures. We also demonstrated that the new gas purifier can regenerate the ArF excimer laser gas up to its initial purity level after about 1×109 shots of operation.
A crusher for single particle testing
Authors: Verspui M. A. , de With G. , Dekkers E. C. A.
Published: July 2002

For the investigation of particle failure in abrasive processes a single particle crusher has been developed. Basically the apparatus consists of two approaching diamond anvils between which a particle is positioned. Both the force and displacement can be either measured or controlled during an experiment. The force is determined by the current through the voice coil with a resolution of 0.5 mN. The vertical displacement of the lower anvil is measured by three inductive displacement transducers, each with a resolution of 0.1 μm. Single abrasive particles in the size range 10–500 μm can be used. The crushing process can be monitored through the upper anvil by a long distance microscope and recorded video. Preliminary experiments show that three different failure mechanisms can be distinguished: chipping, breaking, and fragmentation. By far the most dominant failure mechanism is chipping. The data of the crushing experiments are represented in a Weibull plot. The generally low value for the Weibull modulus indicates a large variability in strengths of the abrasive particles.
Radiation effects on heated optical fibers
Authors: Ramsey A. T. , Tighe W. , Bartolick J. , Morgan P. D.
Published: July 2002

Optical fibers are ubiquitous on today’s plasma devices, and will play a major role in future machines. However, radiation causes luminescence and transmission loss in fibers at troubling levels even on today’s machines when they operate in DT. We have evaluated these effects and studied the use of elevated operating temperatures to reduce them. Using high-purity UV grade silica-silica fibers at 400 °C reduces transmission loss by a factor of at least 100, but has little or no effect on radioluminescence. The radioluminescent spectrum appears to be Cerenkov radiation. The transmission loss spectrum depends on the fiber material and details of the manufacture. The mode structure of transmission loss is mainly simple path length attenuation, with a suggestion of internal reflection degradation.
A new optical method for measuring surface temperature at large incident probe angles
Authors: Lee A. S. , Norris P. M.
Published: July 2002

A novel thermoreflectance technique has been developed for noncontact temperature measurements using laser light incident at large angles on solid materials and devices. The method involves measuring the differential reflectance from a polarization modulated laser beam. The polarization differential reflectance technique is demonstrated on single-crystal Si wafers and on a polycrystalline carbon thin film over a temperature range of 20–60 °C. The method is shown to be an extremely sensitive temperature probe for near grazing angle measurements, which could be useful for monitoring the surface temperature of closely stacked silicon wafers used in batch processing in the microelectronics industry.
A high-accuracy, calibration-free technique for measuring the electrical conductivity of liquids
Authors: Schiefelbein Susan L. , Fried Naomi A. , Rhoads Kevin G. , Sadoway Donald R.
Published: July 2002

A high-accuracy, calibration-free technique to measure the electrical conductivity of liquids has been developed — the coaxial cylinders technique. Because the liquid under investigation comes in contact only with metal and not with anything dielectric, this technique enables the measurement of the electrical properties of liquids inaccessible by classical high-accuracy techniques. Two coaxial cylindrical electrodes are immersed in the liquid to an arbitrary initial depth, and ac impedance is measured over a wide range of frequency. This process is repeated at many immersions. The electrical conductivity is calculated from the change in measured conductance with immersion. This technique was validated in 1.0, 0.1, and 0.01 D KCl(aq) solutions at room temperature. Measured electrical conductivities were within ±0.5% of the standard reference values.
High-resolution x-ray spectrometer based on spherically bent crystals for investigations of femtosecond laser plasmas
Authors: Young B. K. F. , Osterheld A. L. , Price D. F. , Shepherd R. , Stewart R. E. , Faenov A. Ya. , Magunov A. I. , Pikuz T. A. , Skobelev I. Yu. , Flora F. , Bollanti S. , Di Lazzaro P. , Letardi T. , Grilli A. , Palladino L. , Reale A. , Scafati A. , Reale L.
Published: July 2002

Ultrashort-pulse, laser-produced plasmas have become very interesting laboratory sources to study spectroscopically due to their very high densities and temperatures, and the high laser-induced electromagnetic fields present. Typically, these plasmas are of very small volume and very low emissivity. Thus, studying these near point source plasmas requires advanced experimental techniques. We present a new spectrometer design called the focusing spectrometer with spatial resolution (FSSR-2D) based on a spherically bent crystal which provides simultaneous high spectral (λ/Δλ≈104) and spatial resolution (≈10 μm) as well as high luminosity (high collection efficiency). We described in detail the FSSR-2D case in which a small, near point source plasma is investigated. An estimate for the spectral and spatial resolution for the spectrometer is outlined based on geometric considerations. Using the FSSR-2D instrument, experimental data measured from both a 100 fs and a nanosecond pulse laser-produced plasma are presented.
Nanoindentation and contact stiffness measurement using force modulation with a capacitive load-displacement transducer
Authors: Asif S. A. Syed , Wahl K. J. , Colton R. J.
Published: July 2002

We have implemented a force modulation technique for nanoindentation using a three-plate capacitive load-displacement transducer. The stiffness sensitivity of the instrument is ∼0.1 N/m. We show that the sensitivity of this instrument is sufficient to detect long-range surface forces and to locate the surface of a specimen. The low spring mass (236 mg), spring constant (116 N/m), and damping coefficient (0.008 Ns/m) of the transducer allows measurement of the damping losses for nanoscale contacts. We present the experimental technique, important specimen mounting information, and system calibration for nanomechanical property measurement.
Fast imaging and fast force spectroscopy of single biopolymers with a new atomic force microscope designed for small cantilevers
Authors: Viani M. B. , Schäffer T. E. , Paloczi G. T. , Pietrasanta L. I. , Smith B. L. , Thompson J. B. , Richter M. , Rief M. , Gaub H. E. , Plaxco K. W. , Cleland A. N. , Hansma H. G. , Hansma P. K.
Published: July 2002

Small cantilevers allow for faster imaging and faster force spectroscopy of single biopolymers than previously possible because they have higher resonant frequencies and lower coefficients of viscous damping. We have used a new prototype atomic force microscope with small cantilevers to produce stable tapping-mode images (1 μm×1 μm) in liquid of DNA adsorbed onto mica in as little as 1.7 s per image. We have also used these cantilevers to observe the forced unfolding of individual titin molecules on a time scale an order of magnitude faster than previously reported. These experiments demonstrate that a new generation of atomic force microscopes using small cantilevers will enable us to study biological processes with greater time resolution. Furthermore, these instruments allow us to narrow the gap in time between results from force spectroscopy experiments and molecular dynamics calculations.
A method and device for measurement of a sedimentation constant of magnetorheological fluids
Authors: Gorodkin S. R. , Kordonski W. I. , Medvedeva E. V. , Novikova Z. A. , Shorey A. B. , Jacobs S. D.
Published: July 2002

An original procedure and device used for the estimation of a sedimentation constant of magnetorheological fluids have been developed. Experiments that vary the volume concentration and particle size of the dispersed phase, as well as the viscosity of the dispersion medium, show the flexibility of the proposed procedure. A procedure for using this device to estimate the yield stress of these fluids is also described. The results given by our device are shown to be in good agreement with flow curves generated by a commercial rheometer.
Distributions of single-carrier traps in GaAs/AlxGa1−xAs heterostructures
Authors: Sakamoto T. , Nakamura Y. , Nakamura K.
Published: July 2002

We study single-carrier traps in a GaAs/AlxGa1−xAs heterostructure by observing random telegraph signals (RTSs), which are caused by the traps having energy levels within a few kBT of the Fermi level. RTSs are observed in a split gate device while a narrow channel is shifted by independently controlling the voltage applied to each part of the split gate. This measurement reveals the variations of the energy levels of traps with the channel position. From these variations the locations and the energy distributions of the traps are demonstrated. The strength of the confinement potential around the trap is also discussed.
Photoluminescence in amorphous Si/SiO2 superlattices fabricated by magnetron sputtering
Authors: Sullivan Brian T. , Lockwood David J. , Labbé Henri J. , Lu Z.-H.
Published: July 2002

Amorphous Si/SiO2 superlattices with 100–525 periods and a 2–3 nm periodicity were deposited by radio frequency magnetron sputtering onto silicon and quartz substrates. All samples exhibited visible photoluminescence (PL) at room temperature and the PL peak wavelength shifted towards shorter wavelengths with decreasing Si layer thickness. For 425 and 525 period superlattices there was a strong modulation of the PL intensity and optical transmittance versus wavelength resulting from optical interference within the superlattice. The PL intensity increased dramatically after annealing the superlattices in air at 1100 °C.
Evidence for channel conduction in low misorientation angle [001] tilt YBa2Cu3O7−x bicrystal films
Authors: Heinig N. F. , Redwing R. D. , Tsu I Fei , Gurevich A. , Nordman J. E. , Babcock S. E. , Larbalestier D. C.
Published: July 2002

Extended voltage–current (V–I) characteristics of [001] tilt YBa2Cu3O7−x bicrystal films having misorientation angles, θ, of 3° to 20° showed a substantial and progressive change with θ, when measured in large fields. Transmission electron microscopy of the 10° bicrystal showed the grain boundary to contain edge dislocations separated by channels of relatively undisturbed lattice. A large drop in the intergrain irreversibility field, H*, occurred between 10° and 15°, and the characteristics became qualitatively different by 20°. Both the microscopy and the electromagnetic data support a heterogeneous description of the grain boundary, consisting of strongly coupled channels that close at a misorientation angle of around 15°.
A magnetically driven oscillating probe microscope for operation in liquids
Authors: Han Wenhai , Lindsay S. M. , Jing Tianwei
Published: July 2002

Oscillating probe atomic force microscopy in fluids is simplified when an oscillating force is applied directly to a magnetized cantilever using a solenoid. The response of the detector is simpler than that obtained with acoustic excitation. Reliable operation over a broad frequency range is achieved with excitation amplitudes of a few nm. This lower amplitude appears to facilitate imaging by means of small asperities on the tip. Images of a DNA plasmid bound weakly to mica in water have a full width of 5 nm when scanned with tips of a nominal macroscopic radius of curvature of 50 nm.
Dielectric properties of BaTiO3/SrTiO3 multilayered thin films prepared by pulsed laser deposition
Authors: Qu B. D. , Evstigneev M. , Johnson D. J. , Prince R. H.
Published: July 2002

BaTiO 3 /SrTiO 3 multilayered thin films were deposited on Si and Pt/Si substrates. X-ray diffraction clearly shows the formation of the superstructures. Phase transition properties were studied via dielectric measurements. Glassy behavior, characterized by a strong frequency dispersion of dielectric properties, was found in samples with a total thickness of 400 nm, while in samples with a total thickness of 800 nm, normal ferroelectric phase transitions with two dielectric peaks were observed. A preliminary interpretation assumes that size effects which frustrate long range ferroelectric ordering may lead to the relaxational behavior in BaTiO3/SrTiO3 superstructures.
Monolithically integrated surface and substrate emitting vertical cavity lasers for smart pixels
Authors: Bond Aaron E. , Dapkus P. Daniel
Published: July 2002

The authors present a way to monolithically integrate surface—and substrate—emitting vertical cavity surface emitting lasers (VCSELs) on a single substrate for use in smart pixel applications. Spatially selective oxidation is used to adjust the reflectivity of distributed Bragg reflectors to fabricate surface and substrate emitting VCSELs with threshold currents of 65–70 μA, far field FWHMs of 9°–16°, and slope efficiencies of 16%–18%. Threshold currents and far field angles for various aperture dimensions are measured and discussed.
Electrical characterization of GaN p-n junctions with and without threading dislocations
Authors: Kozodoy P. , Ibbetson J. P. , Marchand H. , Fini P. T. , Keller S. , Speck J. S. , DenBaars S. P. , Mishra U. K.
Published: July 2002

The effect of dislocations on the electrical characteristics of GaN p-n junctions has been examined through current–voltage measurements. Lateral epitaxial overgrowth (LEO) was used to produce areas of low dislocation density in close proximity to areas with the high dislocation density typical for growth on sapphire. A comparison of p-n diodes fabricated in each region reveals that reverse-bias leakage current is reduced by three orders of magnitude on LEO GaN. Temperature-dependent measurements on the LEO diodes indicate that the remaining leakage current in these devices is associated with a deep trap level.
Application of plasma columns to radiofrequency antennas
Authors: Borg Gerard G. , Harris Jeffrey H. , Miljak David G. , Martin Noel M.
Published: July 2002

Plasma offers a promising alternative to metal for a wide variety of radiofrequency antenna applications. In this letter we report measurements of efficiencies up to 50% and radiation patterns for plasma column antenna elements. It is demonstrated that the current distribution along the antenna can be controlled by the plasma density. Plasma columns can be used instead of metal elements in communications antennas.
Bulk periodically poled lithium niobate doped with Yb3+ ions: Growth and characterization
Authors: Capmany J. , Bermúdez V. , Diéguez E.
Published: July 2002

We have obtained Yb3+-doped bulk periodically poled LiNbO3 crystals grown by adding Yb2O3 to the congruent melt. The structures obtained have been characterized by scanning electron microscopy, x-ray fluorescence, optical absorption, and second-harmonic generation. About 100 nm of fundamental tunable quasi-phase-matched second-harmonic generation at the pm/V level is obtained in some of the samples.
Self-assembled InSb quantum dots grown on GaSb: A photoluminescence, magnetoluminescence, and atomic force microscopy study
Authors: Alphandéry E. , Nicholas R. J. , Mason N. J. , Zhang B. , Möck P. , Booker G. R.
Published: July 2002

We report a study of self-assembled quantum dots (QDs) of InSb embedded in a GaSb matrix grown by metalorganic vapor phase deposition. Growth temperatures and deposition times have been optimized for maximal photoluminescence peak intensities. Photoluminescence (PL), magneto-PL, and atomic force microscopy (AFM) have been performed to estimate the size of the QDs. The quantum dots luminesce in the midinfrared at around 0.73 eV. The application of magnetic fields up to 15 T both parallel and perpendicular to the growth direction enhanced the wetting layer and bulk PL intensity and enabled an estimate to be made of the QD height and widths as 2–4 and 20–30 nm, respectively. These sizes were confirmed by AFM.
Quantitative theory for laser-generated Lamb waves in orthotropic thin plates
Authors: Cheng J. C. , Zhang S. Y.
Published: July 2002

A quantitative theory for modeling the laser-generated transient ultrasonic Lamb waves propagating along arbitrary directions in orthotropic thin plates is presented by employing an expansion method of the generalized Lamb wave modes. The displacement is expressed by a summation of the symmetric and antisymmetric modes in the surface stress-free orthotropic plate, and it is particularly appropriate for wave form analyses of Lamb wave in thin plates because one needs only to evaluate a few of the lowest order modes. The transient wave forms are analyzed in the thermoelastic regime and the oil coating generation method for a transversely isotropic plate. The results show that the theory provides a quantitative analysis to characterize anisotropic properties and elastic stiffness properties of the orthotropic plates by the laser-generated Lamb wave detection.
Nanomachining of mesoscopic electronic devices using an atomic force microscope
Authors: Schumacher H. W. , Keyser U. F. , Zeitler U. , Haug R. J. , Eberl K.
Published: July 2002

An atomic force microscope (AFM) is used to locally deplete the two-dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure. The depletion is induced by repeated mechanical scribing of the surface layers of the heterostructure using the AFM tip. Measuring the room-temperature resistance across the scribed lines during fabrication provides in situ control of the depletion of the 2DEG. Variation of the room-temperature resistance of such lines tunes their low-temperature characteristics from tunneling up to insulating behavior. Using this technique, an in-plane-gate transistor and a single-electron transistor were fabricated.
Microwave properties of metallic nanowires
Authors: Goglio G. , Pignard S. , Radulescu A. , Piraux L. , Huynen I. , Vanhoenacker D. , Vander Vorst A.
Published: July 2002

We report on the microwave properties of arrays of parallel magnetic nanowires constituted of nickel, cobalt, or Ni/Fe alloy embedded in nanoporous track-etched polymer membranes. The experiments consist of transmission measurements carried out on microwave stripline structures using a magnetically loaded membrane as the substrate. Measurements were performed at frequencies ranging from 100 MHz to 40 GHz and under static magnetic fields up to 5.6 kOe applied along the wires axis. Resonance phenomena have been observed in the magnitude of the complex transmission coefficient at frequencies which depend on the nature of the material and applied static magnetic field. Results are consistent with those expected for a ferromagnetic resonance (FMR) experiment and the observed behaviors are analyzed in the framework of the classical FMR theory.
Laser gain and threshold properties in compressive-strained and lattice-matched GaInNAs/GaAs quantum wells
Authors: Chow W. W. , Jones E. D. , Modine N. A. , Allerman A. A. , Kurtz S. R.
Published: July 2002

The optical gain spectra for compressive-strained and lattice-matched GaInNAs/GaAs quantum wells are computed using a microscopic laser theory. From these spectra, the peak gain and carrier radiative decay rate as functions of carrier density are determined. These dependences allow the study of lasing threshold current density for different GaInNAs/GaAs laser structures.
Room-temperature continuous-wave operation of a single-layered 1.3 μm quantum dot laser
Authors: Park Gyoungwon , Shchekin Oleg B. , Csutak Sebastion , Huffaker Diana L. , Deppe Dennis G.
Published: July 2002

Room-temperature continuous-wave operation of a 1.3 μm quantum dot laser is reported. The threshold current for a single layer active region with p–up mounting is only 4.1 mA with a threshold current density of 45 A/cm2. The minimum room temperature threshold current density is 25 A/cm2 for pulsed operation. Cryogenic and temperature dependent measurements are performed on broad-area lasers fabricated from the same active material. At 4 K the broad-area threshold current density for uncoated facets is 6 A/cm2.
Observation of deep traps responsible for current collapse in GaN metal–semiconductor field-effect transistors
Authors: Klein P. B. , Freitas J. A. , Binari S. C. , Wickenden A. E.
Published: July 2002

Deep traps responsible for current collapse phenomena in GaN metal–semiconductor field-effect transistors have been detected using a spectroscopic technique that employs the optical reversibility of current collapse to determine the photoionization spectra of the traps involved. In the n-channel device investigated, the two electron traps observed were found to be very deep and strongly coupled to the lattice. Photoionization thresholds for these traps were determined at 1.8 and at 2.85 eV. Both also appear to be the same traps recently associated with persistent photoconductivity effects in GaN.
In situ imaging and spectroscopy of single-wall carbon nanotube synthesis by laser vaporization
Authors: Puretzky A. A. , Geohegan D. B. , Fan X. , Pennycook S. J.
Published: July 2002

The synthesis of single-wall carbon nanotubes by Nd:YAG laser vaporization of a graphite/(Ni, Co) target is investigated by laser-induced luminescence imaging and spectroscopy of Co atoms, C2 and C3 molecules, and clusters at 1000 °C in flowing 500 Torr Ar. These laser-induced emission images under typical synthesis conditions show that the plume of vaporized material is segregated and confined within a vortex ring which maintains a ∼1 cm3 volume for several seconds. Using time-resolved spectroscopy and spectroscopic imaging, the time for conversion of atomic and molecular species to clusters was measured for both carbon (200 μs) and cobalt (2 ms). This rapid conversion of carbon to nanoparticles, combined with transmission electron microscopy analysis of the collected deposits, indicate that nanotube growth occurs over several seconds in a plume of mixed nanoparticles. By adjusting the time spent by the plume within the high-temperature zone using these in situ diagnostics, single-walled nanotubes of controlled length were grown at an estimated rate of 0.2 μm/s.
Spatially localized dynamic properties of individual interfaces in semiconducting oxides
Authors: Huey Bryan D. , Bonnell Dawn A.
Published: July 2002

Local electronic property variations at individual interfaces have been determined using scanning surface potential microscopy, a variant of atomic force microscopy in conjunction with locally applied electric fields. Micropatterning is used to isolate individual interfaces and position contacts so that biases can be controlled locally. Positional variations in the voltage dependent interface charge and density of states in polycrystalline zinc oxide are determined from surface potential imaging.
Characterizing the magnetic anisotropy constant of spinel cobalt ferrite nanoparticles
Authors: Rondinone Adam J. , Samia Anna C. S. , Zhang Z. John
Published: July 2002

Superconducting quantum interference device (SQUID) magnetometry and Mössbauer spectroscopy have been combined to determine the magnetic anisotropy constant K of spinel CoFe2O4 nanoparticles. The anisotropy constant is 2.23×106 erg/cm3 for CoFe2O4 nanoparticles with a mean diameter of 8.5 nm. A relaxation-time constant of τ0=1.90×10−14 s has also been obtained. The magnetic anisotropy distribution function of the nanoparticles has been extracted from the temperature dependence of remanence decay in SQUID measurements. The population percentage of the superparamagnetically relaxed nanoparticles at each temperature can be calculated from this anisotropy distribution function. Such a population calculation from SQUID measurements can also fit the results from Mössbauer spectroscopic measurements even though these two measuring methods are fundamentally distinct. Such a successful fit implies that the anisotropy distribution function from our SQUID measurement is truly an intrinsic feature of the nanoparticles.
Piezoresistive effect in metal–semiconductor–metal structures on p-type GaN
Authors: Gaska R. , Shur M. S. , Bykhovski A. D. , Yang J. W. , Khan M. A. , Kaminski V. V. , Soloviov S. M.
Published: July 2002

We report on a strong piezoresistive effect in metal–semiconductor–metal structures fabricated on p-type GaN. The maximum measured gauge factor was 260, which is nearly two times larger than for piezoresistive silicon transducers. We attribute this large sensitivity to applied strain to the combination of two mechanisms: (i) a high piezoresistance of bulk p-GaN and (ii) a strong piezoresistive effect in a Schottky contact on p-GaN. The obtained results demonstrate that GaN-based structures can be suitable for stress/pressure sensor applications.
Polarization effects, surface states, and the source of electrons in AlGaN/GaN heterostructure field effect transistors
Authors: Ibbetson J. P. , Fini P. T. , Ness K. D. , DenBaars S. P. , Speck J. S. , Mishra U. K.
Published: July 2002

The origin of the two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructure field effect transistors is examined theoretically and experimentally. Based on an analysis of the electrostatics, surface states are identified as an important source of electrons. The role of the polarization-induced dipole is also clarified. Experimental Hall data for nominally undoped Al0.34Ga0.66N/GaN structures indicate that ∼1.65 eV surface donors are the actual source of the electrons in the 2DEG, which forms only when the barrier thickness exceeds 35 Å.
Structural evolution of ZnO/sapphire(001) heteroepitaxy studied by real time synchrotron x-ray scattering
Authors: Park Sang Il , Cho Tae Sik , Doh Seok Joo , Lee Jong Lam , Je Jung Ho
Published: July 2002

The structural evolution during heteroepitaxial growth of ZnO/sapphire(001) by radio-frequency magnetron sputtering has been studied using real-time synchrotron x-ray scattering. The two-dimensional (2D) ZnO(002) layers grown in the initial stage are highly strained and well aligned to the substrate having a mosaic distribution of 0.01° full width at half maximum (FWHM), in sharp contrast to the reported transition 2D layers grown by molecular-beam epitaxy. With increasing film thickness, the lattice strain is relieved and the poorly aligned (1.25° FWHM) three-dimensional (3D) islands are nucleated on the 2D layers. We attribute the 2D–3D transition to the release of the strain energy stored in the film due to the film/substrate lattice mismatch.
Ultrafast intersubband relaxation (⩽150 fs) in AlGaN/GaN multiple quantum wells
Authors: Iizuka Norio , Kaneko Kei , Suzuki Nobuo , Asano Takashi , Noda Susumu , Wada Osamu
Published: July 2002

The ultrafast intersubband relaxation in GaN quantum wells has been verified. Al0.65Ga0.35N/GaN multiple quantum wells, with as many as 200 wells, were grown by optimizing the barrier thickness and introducing GaN intermediate layers. The intersubband absorption is sufficiently strong for the relaxation time to be measured. A pump–probe measurement is performed to investigate the relaxation. An ultrashort relaxation time of less than 150 fs is obtained at a wavelength of 4.5 μm. The transient time is shorter than that of InGaAs quantum wells by approximately an order of magnitude. This result is promising for realizing ultrafast optical switches.
Computer simulation studies of aqueous sodium chloride solutions at 298 K and 683 K
Authors: Koneshan S. , Rasaiah Jayendran C.
Published: July 2002

We have carried out molecular dynamics simulations of NaCl solutions at room temperature (298 K) and at a supercritical temperature of 683 K using discrete simple point charge (SPC or SPC/E) molecular models for the water solvent. The solvent densities were 0.997 g cm−3 at 298 K and 0.35 g cm−3 and 0.175 g cm−3 at 683 K. The ion–ion and ion–solvent distribution functions were calculated and compared with corresponding functions for a continuum model of the solvent also determined by computer simulation. Our studies confirm the presence of significant amounts of ion pairing and clustering at supercritical conditions as seen in visualizations of the equilibrium configurations of the solution. However, the degree of pairing and clustering of ions in supercritical solutions is significantly different for discrete and continuum representations of the solvent. Simulations of a 1 molal solution of NaCl at 683 K, using a discrete molecular model for the solvent at a density of 0.35 g cm−3, show the presence of a single megacluster of 10 sodium and chloride ions in a system of 555 water molecules. Three smaller clusters containing positive and negative charges are observed at 683 K when the electrolyte concentration is reduced to 0.5 molal at a solvent density of 0.35 g cm−3 and also at a lower solvent density of 0.175 g cm−3. Molecular dynamics simulations of the velocity auto correlation functions of Na+ and Cl− ions have distinct forms related to the cluster to which the ion belongs. The diffusion coefficients of Na+ and Cl− ions, at infinite dilution, are larger at 683 K than at 298 K, and decrease with increasing electrolyte concentration. They are nearly equal to each other in the one molal solution at 683 K, which may correspond to a supersaturated solution in which the large cluster of sodium and chloride ions moves as an entity over an observed lifetime greater than 200 ps.
The architecture of fibronectin at surfaces
Authors: Guemouri L. , Ogier J. , Zekhnini Z. , Ramsden J. J.
Published: July 2002

The adsorption of the protein fibronectin onto smooth metal oxide surfaces has been monitored in situ and in real time at both low and high ionic strengths using optical waveguide lightmode spectroscopy (OWLS). The kinetics of the evolution of the adsorbed layer thickness, its packing density, and of the total amount deposited were analyzed and used to deduce that at low ionic strength the protein has a compact conformation prone to lateral clustering at the surface, and at high ionic strength it is in a random extended conformation.