bluuebunny/crossref_metadata_2025 huggingface dataset:
A comparison of the perceptions of visually impaired children by their managing optometrists and specialist teachers
Authors: Carter Susan C. , Hill Adrian R.
Published: July 2002

The Warnock Report and the 1981 Education Act resulted in the majority of visually impaired (VI) children attending local mainstream schools with specialist support. Such children attend their closest low vision clinic for low vision aids (LVAs). The structure of these two services varies throughout the country and the communication between them is patchy.

A group of LVA practitioners and specialist teachers for the VI, were asked to rate a common group of VI children whom they had managed, against a list of 20 provided constructs. The results were compared using cluster analysis, showing some commonality in perception within the optometric and teaching groups, but wide variation in how the child was viewed between the two groups.

These results have implications for communication of information between the managing professional groups.
Gait Alterations Associated with Walking Impairment in People with Peripheral Arterial Disease with and without Intermittent Claudication
Authors: McDermott Mary McGrae , Ohlmiller Sara M. , Liu Kiang , Guralnik Jack M. , Martin Gary J. , Pearce William H. , Greenland Philip
Published: July 2002

OBJECTIVES: To describe gait alterations associated with impaired walking endurance in patients with and without lower‐extremity peripheral arterial disease (PAD) and determine whether the Caltrac accelerometer provides a valid measure of physical activity in PAD.

DESIGN: Cross‐sectional.

SETTING: Academic medical center.

PARTICIPANTS: PAD (n = 40) and non‐PAD patients (n = 22) from two Chicago hospitals.

MEASUREMENTS: Participants underwent measurement of the ankle brachial index (ABI), leg length, and 6‐minute walk. Steps per minute and step length were measured during the first and last 100 feet of the 6‐minute walk. Participants wore a Caltrac accelerometer, sensitive to vertical acceleration, during the 6‐minute walk and for 7 continuous days.

RESULTS: Five PAD participants (13%) and one non‐PAD participant (5%) ceased walking before the end of 6 minutes. Among the remaining participants, distance walked in 6 minutes was more highly related to walking velocity during the last 100 feet of the walk than walking velocity during the first 100 feet. ABI was associated significantly with cadence (20.77 steps/minute per unit ABI, P < .001) but not step length (10.12 centimeters/unit ABI, P = .08). ABI was associated significantly with 6‐minute walk distance (493 feet/unit ABI, P = .018), but this association disappeared completely after adjustment for step length and cadence. We found no difference in accelerometer scores between PAD and non‐PAD participants over a fixed distance of 800 feet (7.34 vs 7.17 activity units, P = .789). However, scores were significantly different after 7 days (730.8 vs 1,485.0 activity units, P = .003).

CONCLUSION: Walking performance in PAD patients who completed 6 minutes of walking was largely determined by a decline in walking velocity rather than slower initial walking velocity. ABI was more closely associated with cadence than step length. Future studies should assess the effect of exercise programs and revascularization on cadence and step length in PAD.
Insect Attracting Structures on Erodium Petals (Geraniaceae)
Authors: Aldasoro J. J. , Aedo C. , Navarro C.
Published: July 2002

Abstract: Two types of structures previously unrecorded in Erodium petals are investigated. Spherical hairs filled with liquid resembling nectar droplets are exclusive to an ibero‐mauritanic group of species included in Erodium subsect. Romana. Broad flat hairs which reflect light, shining as do nectar droplets, are restricted to most of the species included in Erodium sect. Malacoidea. Long, simple hairs in petals and sepals are involved in collection of nectar droplets. Some of them are arranged at the margin of the petal claw, just over the nectaries. Others are on the internal surface of sepals or on the upper surface of petals, serving apparently the same function. Their shape is aciculate or flattened. The nectar collected among the hairs forms shining spherical droplets, perceptible to insects. The glistening flat hairs and spheres shine in a similar way, probably mimicking nectar and attracting insects. Species with these special nectar‐like structures produce nectar in quantities that can be observed by the naked eye, suggesting that these structures increase the attraction efficiency of flowers. Some taxonomic and biogeographic consequences are also discussed.
Assimilate Transport in the Xylem Sap of Pedunculate Oak (Quercus robur) Saplings
Authors: Heizmann U. , Kreuzwieser J. , Schnitzler J.‐P. , Brüggemann N. , Rennenberg H.
Published: July 2002

Abstract: The rates of photosynthesis and transpiration, as well as the concentrations of organic compounds (total soluble non‐protein N compounds [TSNN], soluble carbohydrates), in the xylem sap were determined during two growth seasons in one‐year‐old Quercus robur saplings. From the data, the total C gain of the leaves, by both photosynthesis and the transpiration stream, was calculated. Large amounts of C were allocated to the leaves by the transpiration stream; depending on the time of day and the environmental conditions the portion of C originating from xylem transport amounted to 8 to 91% of total C delivery to the leaves. Particularly under conditions of reduced photosynthesis, e.g., during midday depression of photosynthesis, a high percentage of the total C delivery was provided to the leaves by the transpiration stream (83 to 91 %). Apparently, attack by phloem‐feeding aphids lowered the assimilate transport from roots to shoots; as a consequence the portion of C available to the leaves from xylem transport amounted to only 12 to 16 %. The most abundant organic compounds transported in the xylem sap were sugars (sucrose, glucose, fructose) with concentrations of ca. 50 to 500 μmol C ml‐1, whereas C from N compounds was of minor significance (3 to 20 μmol ml‐1 C). The results indicate a significant cycling of C in the plants because the daily transport of C with the transpiration stream exceeded the daily photosynthetic CO2 fixation in several cases. This cycling pool of C may sustain delivery of photosynthate to heterotrophic tissues, independent of short time fluctuations in photosynthetic CO2 fixation.
Ultrahigh efficiency microwave signal transmission using tandem-contact single quantum well GaAlAs lasers
Authors: Moore N. , Lau K. Y.
Published: July 2002

We show theoretically and experimentally that enhancements of up to 40 dB in the efficiency of optical transmission of microwave signals can be obtained by using a single quantum well laser transmitter whose contact is segmented into two sections, and modulation is applied to one of the sections. The improvement is dependent fundamentally on the relaxation oscillation frequency of the device, with the improvement factor dropping to around 15 dB at relaxation oscillation frequencies approaching 10 GHz.
Lateral mode control of an AlGaAs laser array in a Talbot cavity
Authors: Leger James R.
Published: July 2002

The lateral mode thresholds of an external cavity AlGaAs laser diode array are measured as a function of cavity length. It is found that certain cavity lengths induce the array to lase in either the fundamental or highest order lateral mode. The results are explained using the theory of Talbot self-imaging in fractional Talbot planes
Experimental evidence for nucleation during thin-film reactions
Authors: Coffey K. R. , Clevenger L. A. , Barmak K. , Rudman D. A. , Thompson C. V.
Published: July 2002

The reaction between solid layers to form a product phase has been studied using scanning calorimetry of multilayer Nb/Al and Ni/amorphous-Si thin films. The most striking feature for both materials systems is the occurrence of two maxima in the reaction rate during the formation of a single product phase, suggesting a two step growth process. A model has been developed in which the first step is taken to be the nucleation and two-dimensional growth to coalescence of the product phase, in the plane of the initial interface. The second step is taken to be the thickening of the product layer by growth perpendicular to the interface plane. The success of this simple model in describing the principal features of the experimental results on two different materials systems suggests that nucleation is an important aspect of phase formation and selection in these thin-film reactions.
Extended Jones matrix representation for the twisted nematic liquid-crystal display at oblique incidence
Authors: Lien A.
Published: July 2002

A simple and accurate extended Jones matrix representation for the twisted nematic liquid-crystal display at the oblique incidence was obtained for the first time. The results obtained by this extended Jones matrix representation are quite close to those obtained by the faster 4×4 matrix method with spectrum averaging to account for the nonzero bandwidth of the incident light. However, the computation time for the extended Jones matrix method is less than half that of the faster 4×4 matrix method without spectrum averaging. Furthermore, at normal incidence, this extended Jones matrix representation reduces to the ordinary Jones matrix representation.
Weak links and dc SQUIDS on artificial nonsymmetric grain boundaries in YBa2Cu3O7−δ
Authors: Ivanov Z. G. , Nilsson P. Å. , Winkler D. , Alarco J. A. , Claeson T. , Stepantsov E. A. , Tzalenchuk A. Ya.
Published: July 2002

We have studied weak links and dc–SQUIDs made from pulsed laser deposited YBa2Cu3O7−δ thin films on Y–ZrO2 bicrystal substrates. The transport properties of the weak links were studied as a function of the misorientation angle (θ) between the two halves of the bicrystal and an exponential dependence of the weak link critical current density was observed for angles up to 40° at 77 K. Josephson effects with clear microwave and magnetic field responses were observed. An optimum dc–SQUID performance at 77 K was obtained for θ=32°. At this temperature, we achieved a periodic magnetic field response with a modulation depth of 12 μV.
Combined shear force and near-field scanning optical microscopy
Authors: Betzig E. , Finn P. L. , Weiner J. S.
Published: July 2002

A distance regulation method has been developed to enhance the reliability, versatility, and ease of use of near-field scanning optical microscopy (NSOM). The method relies on the detection of shear forces between the end of a near-field probe and the sample of interest. The system can be used solely for distance regulation in NSOM, for simultaneous shear force and near-field imaging, or for shear force microscopy alone. In the latter case, uncoated optical fiber probes are found to yield images with consistently high resolution.
Novel geometrical effects observed in debris when polymers are laser sputtered
Authors: Miotello Antonio , Kelly Roger , Braren Bodil , Otis Charles E.
Published: July 2002

When polymers are sputtered with 248 or 308 nm laser pulses there are two generically different responses. The most straightforward is where the emitted particles expand outwards away from the target surface obeying, in so doing, the laws of one-dimensional adiabatic flow. The other is where the particles expand both outwards and sideways and, because a certain fraction recondenses on the target surface, there is a prominent deposit of debris lying around the bombarded spot. For spots with other than circular shape the debris show interesting symmetry in which there is rotation with respect to the spot. We show that this rotation occurs wholly through the laws of flow and we conclude, therefore, that the debris phenomenon is a purely gas-dynamic effect. It follows that the elimination of debris can be approached in gas-dynamic terms. We also show that the numerical extent of debris formation increases with the complexity of the particles involved, a result which suggests additional methods to control debris.
Er3+:YLiF4 continuous wave cascade laser operation at 1620 and 2810 nm at room temperature
Authors: Schmaul B. , Huber G. , Clausen R. , Chai B. , LiKamWa P. , Bass M.
Published: July 2002

For the first time cw, cascade lasing was demonstrated in 1% Er doped yttrium lithium fluoride (YLF) at room temperature at both 1620 and 2810 nm. In addition, cw lasing in Er[1%]:YLF at 1640 nm and in Er[5%]:YLF at 2810 nm at room temperature is reported for the first time in material of such low concentration.
Relationship among reflectance-difference spectroscopy, surface photoabsorption, and spectroellipsometry
Authors: Hingerl K. , Aspnes D. E. , Kamiya I. , Florez L. T.
Published: July 2002

From the reflectance expressions for a thin biaxial layer on an isotropic substrate we relate reflectance-difference spectroscopy (RDS), surface photoabsorption (SPA), and spectroellipsometry. Using these results and our recently acquired RD database, we determine surface reconstructions present during flow-modulated organometallic chemical vapor growth of epitaxial GaAs from SPA data that were published by others.
Electrical resistivity of nanocrystalline PbS grown in a polymer matrix
Authors: Mukherjee M. , Datta A. , Chakravorty D.
Published: July 2002

PbS particles of average diameters ranging from 9.9 to 18.0 nm have been synthesized within a polyacrylamide matrix. The percolative chains of these particles exhibit intrinsic semiconducting behavior at temperatures higher than 340 K. The estimated band gap for these particles are found to be much higher than that of bulk PbS being in the range 1.03 to 1.49 eV. The low-temperature conductivity of these composites appears to be controlled by an electron tunneling mechanism.
Strain characterization of CoSi2/n-Si0.9Ge0.1/p-Si heterostructures
Authors: Nur O. , Willander M. , Radamson H. H. , Sardela M. R. , Hansson G. V. , Petersson C. S. , Maex K.
Published: July 2002

We report on the structural characterization of CoSi2/n-Si0.9Ge0.1/p-Si heterostructures. Silicon molecular beam epitaxy is combined with Co sputtering to obtain these structures. The strain in the Si1−xGex is investigated after the formation of the CoSi2 by using high-resolution x-ray diffraction mapping in reciprocal space and cross-sectional transmission electron microscopy. The results show that in order to keep the strain in Si1−xGex unaffected, a sacrificial Si layer is needed. It was possible to obtain transistor action, but with low-current gain (β).
Growth of alternating 〈100〉/〈111〉-oriented II-VI regions for quasi-phase-matched nonlinear optical devices on GaAs substrates
Authors: Angell M. J. , Emerson R. M. , Hoyt J. L. , Gibbons J. F. , Eyres L. A. , Bortz M. L. , Fejer M. M.
Published: July 2002

We present a technique for fabricating laterally patterned 〈111〉 and 〈100〉-oriented regions of CdTe on GaAs by metalorganic chemical vapor deposition. Patterning of the crystal orientation is important for quasi-phase-matched nonlinear optical frequency conversion in semiconductor waveguides. Scanning electron micrographs and x-ray diffraction analysis are used to confirm the presence of 〈111〉/〈100〉 grating structures. The CdTe layer is shown to be a suitable template to pattern the orientation of subsequently grown wide-band-gap films of ZnSe and ZnTe.
Direct observation of potential distribution across Si/Si p-n junctions using off-axis electron holography
Authors: McCartney M. R. , Smith David J. , Hull Robert , Bean J. C. , Voelkl E. , Frost B.
Published: July 2002

Off-axis electron holography was used to observe the potential distribution across a 2×1018/cm3 p- and n-doped Si/Si p-n junction. With digital image recording and processing, and a novel method for thickness determination, we have successfully extracted two-dimensional maps of the depletion region potential. For a defect-free region, we measured relatively abrupt changes in potential in the range 1.0–1.5 V across lateral distances of 20–30 nm. Although influenced by instrumental and sample limitations, these values are reasonably consistent with expected Si junction parameters and thus establish the promise of this technique for measuring potential distributions across device junctions and interfaces.
Behavior and mechanism of step bunching during metalorganic vapor phase epitaxy of GaAs
Authors: Shinohara Masanori , Inoue Naohisa
Published: July 2002

The behavior of step bunching during GaAs growth for metalorganic vapor phase epitaxy is investigated by varying growth temperatures and substrate misorientation angles. It is found that monolayer step-flow growth changes to step-bunching growth when the misorientation angle, that is, the substrate surface step density, increases to a certain value, showing the existence of a critical step density. This critical value decreases with increasing growth temperature. The bunched terrace width does not depend on the misorientation angle, but it increases with growth temperature. We propose that the transition to step bunching from step-flow growth occurs when the concentration of Ga adatoms at the step edges is less than the equilibrium concentration. Various bunching characteristics are explained using this model.
Quantitative data processing of parallel recorded electron energy-loss spectra with low signal to background
Authors: Shuman Henry , Kruit Pieter
Published: July 2002

The performance of a multielement array detector for measuring small spectral features superimposed on a large background has been examined. In order to have the sensitivity of detection limited only by counting statistics, two factors have to be critically considered: the nonuniform spatial response of the array detector, and the possible addition of noise by the collection system. For the special case discussed here, energy-loss spectroscopy of 100-keV electrons, the added noise was small. The detective quantum efficiency (DQE) was measured to be DQE=0.87. Several techniques reducing the effect of the nonuniform response were tested: normalizing the spectra with an experimentally measured response curve, electronic differentiation to reduce the background, dynamic gain correction, and two methods of experimentally averaging the spatial response by measuring a sequence of repositioned spectra. Preservation of the statistical information present in a representative energy-loss spectrum is shown to be feasible.
Imaging radio frequency electron-spin-resonance spectrometer with high resolution and sensitivity for i
n v
i
v
o measurements
Authors: Halpern Howard J. , Spencer David P. , van Polen Jerry , Bowman Michael K. , Nelson Alan C. , Dowey Elizabeth M. , Teicher Beverly A.
Published: July 2002

We report the development of a novel radio frequency electron-spin-resonance spectrometer designed to provide measurements with high molar sensitivity and resolution in vivo. Radio frequency (250 MHz) is chosen to obtain good penetration in animal tissue and large aqueous samples with modest sacrifice of sensitivity. The spectrometer has a lumped component resonator and operates in continuous-wave mode. The spectrometer is capable of two-dimensional imaging, and with a modest addition should be capable of three-dimensional imaging. We demonstrate 3-mm spatial resolution for DPPH samples. For 10-mℓ samples of aqueous nitroxide, we demonstrate sensitivity (normalized to spectral width of 1 G) to 3×10−8-M concentrations and spectral resolution of 0.1 G. Spectra from nitroxide spin label injected into a live mouse are shown.
Application of two-photon-excited laser-induced fluorescence to atomic hydrogen measurements in the edge region of high-temperature plasmas
Authors: Kajiwara Toshinori , Shinkawa Tatsuya , Uchino Kiichiro , Masuda Mitsuharu , Muraoka Katsunori , Okada Tatsuo , Maeda Mitsuo , Sudo Sigeru , Obiki Tokuhiro
Published: July 2002

A two-photon-excited laser-induced fluorescence (LIF) was applied to measurement of atomic hydrogen densities in the edge region of high-temperature plasmas of Heliotron E for the first time. In this application, fluorescence signals were observed coaxially to the laser beam. This was possible because, in two-photon-excitation LIF, the visible fluorescence is localized at the laser focus and stray light can be completely eliminated by optical spatial filters, allowing much easier optical access than in previous LIF experiments. The densities at the initiation of the discharge and at the quasisteady state of the discharge were determined to be 3×1016 and 1×1016 m−3, respectively within an error of a factor of 2. Optimization to maximize the signal-to-noise ratio of the observation and resulting detection limit were discussed and confirmed the above results.
Extrapolation procedures in Mott electron polarimetry
Authors: Gay T. J. , Khakoo M. A. , Brand J. A. , Furst J. E. , Meyer W. V. , Wijayaratna W. M. K. P. , Dunning F. B.
Published: July 2002

In standard Mott electron polarimetry using thin gold film targets, extrapolation procedures must be used to reduce the experimentally measured asymmetries A to the values they would have for scattering from single atoms. These extrapolations involve the dependence of A on either the gold film thickness t or the maximum detected electron energy loss in the target ΔE. Using a concentric cylindrical-electrode Mott polarimeter, we have studied and compared these two types of extrapolations over the electron energy range 20–100 keV. The potential systematic errors which can result from such procedures are analyzed in detail, particularly with regard to the use of various fitting functions in thickness extrapolations, and the failure of perfect energy-loss discrimination to yield accurate polarizations when thick foils are used. A critical discussion of previous work on this subject is presented.
High-resolution focused ion beams
Authors: Orloff Jon
Published: July 2002

The technology of high-resolution focused ion beams has advanced dramatically in the past 15 years as focusing systems have evolved from laboratory instruments producing minuscule current densities to high current density tools which have sparked an important new process: direct micromachining at the micrometer level. This development has been due primarily to the exploitation of field emission ion sources and in particular the liquid-metal ion source. Originally developed in the early 1960’s as a byproduct of the development of electrostatic rocket engines, the liquid-metal ion source was adapted for focused beam work in the late 1970’s, when it was demonstrated that submicrometer focused ion beams could be produced with current densities greater than 1 A cm−2. Ions can be produced with liquid-metal ion sources from elements including Al, As, Au, B, Be, Bi, Cs, Cu, Ga, Ge, Fe, In, Li, P, Pb, Pd, Si, Sn, and Zn. In the past decade, focused ion beam systems with liquid-metal ion sources have had a significant impact on the semiconductor industry as they were applied to new and greatly improved methods of failure analysis, as well as circuit repair and modification, in situ processing, and lithographic mask repair. This article discusses field emission ion sources, high-resolution ion focusing systems, and means for analyzing their performance. A number of technologically interesting and useful applications are also discussed.
A variable pressure/temperature scanning tunneling microscope for surface science and catalysis studies
Authors: McIntyre B. J. , Salmeron M. , Somorjai G. A.
Published: July 2002

We describe the design and construction of a scanning tunneling microscope (STM), which is contained in a reactor cell and allows in situ operation throughout a wide range of pressures and temperatures. This STM is capable of imaging in pressures ranging from ultrahigh vacuum up to several atmospheres. Samples can also be easily moved in and out of the STM cell in a small separate vacuum transfer chamber for external characterization and treatment. The equilibration time and stability of the microscope after temperature changes was determined at atmospheric pressures, by monitoring the evolution of atomically resolved images of highly oriented pyrolytic graphite at temperatures ranging from 300 to 425 K. We have also examined the stability when flowing gases are used instead of stationary pressures. In preliminary experiments using Pt(111) and (110) crystals, we have observed the effect of atmospheres of H2 and O2 on the structure of these surfaces.
Improved capillary optics applied to microbeam x-ray fluorescence: Resolution and sensitivity
Authors: Attaelmanan A. , Voglis P. , Rindby A. , Larsson S. , Engström P.
Published: July 2002

A new and improved type of x-ray capillary optics unit (ellipsoidal), is developed, tested, and characterized using a table-top x-ray microbeam setup. The beam profiles of the x-ray beams generated by the 7.5 μm ellipsoidal capillary were constructed from ‘‘knife-edge’’ scans across the beam. The detection limits for various elements were determined, and the spatial resolution, when the beam was used in scanning mode, was demonstrated.
In situ synchrotron radiation small-angle x-ray scattering study of the kinetics of growth of CdTe nanocrystals in borosilicate glass
Authors: Craievich A. F. , Alves O. L. , Barbosa L. C.
Published: July 2002

A number of isolator–semiconductor nanocrystal composites exhibit quantum confinement effects and nonlinear optical properties. In this work, the formation and growth of CdTe and CdTe0.9S0.1 nanocrystals immersed in a borosilicate glass host matrix were studied by small-angle x-ray scattering using synchrotron radiation during in situ annealing in the 560–580 °C temperature range. The values of the average radii of the CdTe nanocrystals determined by using Guinier plots for different annealing times (20–30 Å) agree with those obtained from optical absorption spectroscopy measurements. The nanocrystal size distribution depends on the thermal history and composition of the samples. The existence of other structural heterogeneities having an average size of several hundred ångstroms was detected.
An electron imaging approach to soft-x-ray transmission spectromicroscopy
Authors: De Stasio Gelsomina , Lorusso G. F. , Droubay T. , Kohli M. , Muralt P. , Perfetti P. , Margaritondo G. , Kelly T. F. , Tonner B. P.
Published: July 2002

We tested a new soft-x-ray transmission spectromiscropy technique on the Aladdin storage ring at the Wisconsin Synchrotron Radiation Center. Transmitted x rays were converted with a photocathode into photoelectrons, which were subsequently electron-optically processed by an x-ray secondary electron-emission microscope producing submicron-resolution images. Test images demonstrated the excellent contrast due to the chemical differences between silicon features and a silicon nitride substrate. We also obtained x-ray transmission versus photon energy curves for microscopic specimen areas.
Scanning near-field optical microscopy in reflection mode imaging in liquid
Authors: Keller Thomas Hans , Rayment Trevor , Klenerman David , Stephenson Robert J.
Published: July 2002

A combined atomic force microscope (AFM) and scanning near-field optical microscope (SNOM) has been constructed to obtain images under liquids. In this combined AFM-SNOM an inverted ac mode configuration is used where the AFM cantilever is driven by exciting acoustic modes of the liquid cell. Optical images of latex spheres on a cantilever have been obtained under perfluorononane, butanol, and water with a lateral resolution of 100 nm. Fluorescence images of latex spheres have been obtained under perfluorononane. The ability to image under liquids opens up a wide range of applications of near field optical microscopy in chemistry and biology.
The “Reactor STM”: A scanning tunneling microscope for investigation of catalytic surfaces at semi-industrial reaction conditions
Authors: Rasmussen P. B. , Hendriksen B. L. M. , Zeijlemaker H. , Ficke H. G. , Frenken J. W. M.
Published: July 2002

An instrument is introduced that combines a scanning tunneling microscope (STM) and a small volume (300 μl) flow reactor, for the in situ study of catalytic surfaces at semi-industrial conditions. The tip is led through a gold membrane separating the reactor from the rest of the STM. We show (1) that the scanning through the membrane is possible with no noticeable effect of the membrane; (2) that it is possible to image surfaces in a gas flow of 10 ml/min, and that the imaging is unaffected by moderate changes in the flow conditions during the scanning; and (3) that imaging of atomic steps is possible on a Co(0001) surface in a reactive environment at room temperature and at elevated temperatures. Furthermore, it is observed, that the step mobility on Co(0001) is strongly enhanced in the presence of H2.
Damage threshold for ion-beam induced graphitization of diamond
Authors: Uzan-Saguy C. , Cytermann C. , Brener R. , Richter V. , Shaanan M. , Kalish R.
Published: July 2002

The critical dose for graphitization of diamond as a result of ion implantation induced damage (boron and arsenic) and subsequent thermal annealing is determined by combining secondary ion mass spectroscopy measurements, chemical etching of the graphitized layer, and TRIM simulations. Li ions are implanted as a deep marker to accurately determine the position of the graphite/diamond interface. The damage density threshold, beyond which graphitization occurs upon annealing, is found to be 1022 vacancies/cm3. This value is checked against published data and is shown to be of general nature, independent of ion species or implantation energy.
Coherent detection of freely propagating terahertz radiation by electro-optic sampling
Authors: Nahata Ajay , Auston David H. , Heinz Tony F. , Wu Chengjiu
Published: July 2002

We report the demonstration of an electro-optic sampling technique that allows for the detection of freely propagating terahertz radiation. Coherent sampling is performed in a poled polymer device that is physically separated from the emitter. The poling electrodes in the sampling element are found to have an integrating effect on the incident terahertz field. The shot noise limited minimum detectable field in the polymer is 100 (mV/cm)/√Hz. We discuss methods by which the sensitivity may be significantly enhanced.
Monoclinic phase of boron nitride appearing during the hexagonal cubic phase transition at high pressure and high temperature
Authors: Horiuchi Shigeo , He Lian-Long , Onoda Mitsuko , Akaishi Minoru
Published: July 2002

Fine structures appearing on the phase transition from h (hexagonal) to c (cubic) boron nitride under high pressure (7.7 GPa) and high temperature (1800–2150 °C) are examined by high-resolution transmission electron microscopy. A prominent contraction of the interplanar spacing between sp2 sheets from 3.33 to 3.10 Å in so-called ‘‘compressed h-BN’’ is attributable to a monoclinic lattice distortion of the residual h-BN, which originates from the difference in the compressibility as well as the thermal expansion between adjoining h- and c-BN grains. The parameters of the monoclinic unit cell are am=4.33, bm=2.50, cm=3.1–3.3 Å, and β=92–95°. Thin plates of h-BN are often folded and the folding also causes the monoclinic structure. The sheet sequence of r (rhombohedral)-BN locally appears when the strong volume shrinkage occurs due to the formation of a c-BN grain. Nanoscale twins appear in resulting c-BN grains, as long as they are small, and w (wurzite)-BN is sometimes included in them.
London moment for heavy-fermion superconductors
Authors: Sanzari Martin A. , Cui H. L. , Karwacki Francis
Published: July 2002

Measurements of the London magnetic moment generated in rotating heavy-fermion supercon- ductors are presented. The magnitude of the London moment is found to be identical to that of conventional superconductor, as well as that of high-Tc superconductors, within the experimental resolution of 5%–10%.
Deformation behavior of Zr65Al10Ni10Cu15 glassy alloy with wide supercooled liquid region
Authors: Kawamura Yoshihito , Shibata Tsutomu , Inoue Akihisa , Masumoto Tsuyoshi
Published: July 2002

We report the stress-strain behavior for a Zr65Al10Ni10Cu15 (at.%) glassy alloy with a significant supercooled liquid region of 105 K. The transition temperature from inhomogeneous deformation to a homogeneous one increases with increasing strain rate. The alloy in the supercooled liquid state, however, exhibited the homogeneous deformation even at higher strain rates above 0.5 s−1. The strength was also dependent on the strain rate and independent of temperature in the inhomogeneous deformation region, and was sensitive to both strain rate and temperature in the homogeneous region. The stress-strain curves in the homogeneous deformation mode were accompanied by a stress overshoot and its height increases with increasing temperature and strain rate.
Transmission electron holography of silicon nanospheres with surface oxide layers
Authors: Wang Y. C. , Chou T. M. , Libera M. , Kelly T. F.
Published: July 2002

Phase images of 20–30-nm-diam silicon spheres were collected by holographic methods in a field-emission transmission electron microscope. The spherical geometry enables the effect of specimen thickness on the electron-wave phase to be separated from the intrinsic Si electron-optical refractive effects allowing a determination of the mean inner potential Φ0. This work finds Φ0=11.9±0.9 V characterizing amorphous Si and 12.1±1.3 V characterizing crystalline Si. The phase images can resolve a 2-nm-thick native oxide layer and give Φ0 for SiO2=10.1±0.6 V. The phase data can quickly recognize a surface layer, and the effect of a surface layer on the determination of the bulk mean potential can be minimized.
The role of the dielectric loss of dispersed material in the electrorheological effect
Authors: Hao Tian
Published: July 2002

A set of electrorheological (ER) suspensions composed of solid materials with different dielectric losses and dielectric constants were examined by means of dielectric and rheological methods. We found that the dielectric loss of dispersed material, which was only slightly stressed in previous works, plays a considerable role in ER response; a large dielectric loss value is necessary for a clear ER effect. Two processes, the turning of randomly distributed microparticles toward the direction of an applied external electric field, probed recently by an x-ray diffraction method, and a coalescing of ordered microparticles, widely observed in ER fluids under a microscope, are suggested for understanding the ER mechanism. The first process is dominated by the dielectric loss of dispersed material, and the second one by the dielectric constant. Our result can be used to understand why certain materials of high dielectric constant but low dielectric loss would have shown an obvious ER effect according to the polarization and the conduction models, yet only displays weak or even no detectable ER effect experimentally. Our findings help to further understand the ER mechanism and also offer a clear implication on how to design high performance ER fluids.
Direct observation of the core structures of threading dislocations in GaN
Authors: Xin Y. , Pennycook S. J. , Browning N. D. , Nellist P. D. , Sivananthan S. , Omnès F. , Beaumont B. , Faurie J. P. , Gibart P.
Published: July 2002

Here we present the first direct observation of the atomic structure of threading dislocation cores in hexagonal GaN. Using atomic-resolution Z-contrast imaging, dislocations with edge character are found to exhibit an eight-fold ring core. The central column in the core of a pure edge dislocation has the same configuration as one row of dimers on the {10-10} surface. Following recent theoretical work, it is proposed that edge dislocations do not have deep defect states in the band gap, and do not contribute to cathodoluminescence dislocation contrast. On the other hand, both mixed and pure screw dislocations are found to have a full core, and full screw dislocation cores were calculated to have states in the gap.
Investigations of SiO2/n-GaN and Si3N4/n-GaN insulator–semiconductor interfaces with low interface state density
Authors: Arulkumaran S. , Egawa T. , Ishikawa H. , Jimbo T. , Umeno M.
Published: July 2002

The electrical properties of electron beam (EB) evaporated silicon dioxide (SiO2)/n-GaN, plasma enhanced chemical vapor deposited (PECVD) SiO2/n-GaN, and PECVD silicon nitride (Si3N4)/n-GaN interfaces were investigated using high frequency capacitance–voltage measurements. Compositions of the deposited insulating layers (SiO2 and Si3N4) were analyzed using x-ray photoelectron spectroscopy. Metal-insulator-semiconductor structures were fabricated on the metalorganic chemical vapor deposition grown n-type GaN layers using EB, PECVD grown SiO2 and PECVD grown Si3N4 layers. Minimum interface state density (2.5×1011 eV−1 cm−2) has been observed in the PECVD grown SiO2/n-GaN interface when it was compared with EB evaporated SiO2/n-GaN interface (5.3×1011 eV−1 cm−2) and PECVD Si3N4/n-GaN interface (6.5×1011 eV−1 cm−2). The interface state density (Nf) depends on the composition of deposited insulating layers.
High temperature excitonic stimulated emission from ZnO epitaxial layers
Authors: Bagnall D. M. , Chen Y. F. , Zhu Z. , Yao T. , Shen M. Y. , Goto T.
Published: July 2002

The emission spectrum of high quality ZnO epilayers is studied from room temperature up to 550 K. At room temperature and low excitation power a single emission peak is observed which may be identified with the free exciton from its peak energy and dependence on temperature. However, when excitation intensities exceed 400 kW cm−2 a sharp peak emerges at lower energy which we attribute to exciton-exciton scattering. At higher excitation intensities (>800 kW cm−2) a second stimulated emission peak emerges at even lower energies: we attribute this peak to be stimulated emission of an electron hole plasma. Similar features are observed for all temperatures up to 550 K.
Self-limiting oxidation of copper
Authors: Yang J. C. , Kolasa B. , Gibson J. M. , Yeadon M.
Published: July 2002

The classical theory of Cabrera–Mott describes passivation film formation on metals, where they predicted that this film grows as a uniform layer due to a field-enhanced ionic transport mechanism. Here we present experimental evidence, based on in situ transmission electron microscopy of copper oxidation, that the passivation film nucleates and grows as oxide islands, not as a uniform layer. We propose an alternative phenomenological theory to describe passivation film formation, based on island growth followed by coalescence.
Structural disorder induced in hydrogenated amorphous silicon by light soaking
Authors: Gibson J. M. , Treacy M. M. J. , Voyles P. M. , Jin H-C. , Abelson J. R.
Published: July 2002

We show, using variable coherence transmission electron microscopy, that light soaking of amorphous hydrogenated silicon thin films leads to structural changes. We speculate that the structural changes are associated with instability in the as-deposited material. We suggest that improved immunity to Staebler–Wronski degradation could be achieved by a less-ordered material which is closer to the ideal continuous random network.
Grain boundary barrier breakdown in niobium donor doped strontium titanate using in situ electron holography
Authors: Johnson Kevin D. , Dravid Vinayak P.
Published: July 2002

The electrical activity of numerous electroceramics originates with space charge potential across internal interfaces. Space charge formation and the resultant potential barrier at interfaces are believed to be responsible for many interesting and useful properties of electroceramics, ranging from nonlinear current–voltage characteristics to enhanced dielectric properties. Direct current electrical measurements of individual grain boundaries in Nb donor doped SrTiO3 bicrystals reveal a highly resistive and nonlinear behavior compared to single crystals. The origin of this nonlinear resistance has been examined with electron holography, observing both static and dynamic attributes of the internal potential. In the static case with no applied current, the grain boundary potential barrier height was measured to be about 0.45 V. During the application of a high current, this potential barrier was suppressed, presenting the first direct real space evidence for breakdown of an internal grain boundary barrier.
Infrared vision using uncooled micro-optomechanical camera
Authors: Perazzo T. , Mao M. , Kwon O. , Majumdar A. , Varesi J. B. , Norton P.
Published: July 2002

This letter presents the design, fabrication, and imaging results of an uncooled infrared (IR) camera that contains a focal plane array of bimaterial microcantilever sensors, and an optical readout technique that measures cantilever deflections in the nanometer range to directly project a visible image of the IR scene on the human eye or a visible camera. The results suggest that objects at temperatures as low as 100 °C can be imaged with the best noise-equivalent temperature difference (NEΔT) in the range of 10 K. It is estimated that further improvements that are currently being pursued can improve NEΔT to about 50 mK.
Optical method for two-dimensional ultrasonic detection
Authors: Paltauf G. , Schmidt-Kloiber H. , Köstli K. P. , Frenz M.
Published: July 2002

Two-dimensional detection of ultrasonic waves is based on pressure-induced changes of optical reflectance at a glass–liquid interface, imaged with a time-gated video camera. The method is used to record optoacoustic waves generated after irradiation of optically absorbing targets with 6 ns long laser pulses. Measurements of absolute pressure values with high temporal and spatial resolution (in the range of 10 ns and 10 μm, respectively) is demonstrated. The sensitivity is varied between 0.19% and 0.81% gray level modulation per bar. The detector plane is optically transparent, making it possible to irradiate the sample through the detector without disturbing the acoustic measurement. Two-dimensional recording of ultrasonic waves is ideally suited for the analysis of acoustic emission from small sources and for optoacoustic imaging of optical absorption differences in an opaque material.
Nanotube composite carbon fibers
Authors: Andrews R. , Jacques D. , Rao A. M. , Rantell T. , Derbyshire F. , Chen Y. , Chen J. , Haddon R. C.
Published: July 2002

Single walled carbon nanotubes (SWNTs) were dispersed in isotropic petroleum pitch matrices to form nanotube composite carbon fibers with enhanced mechanical and electrical properties. We find that the tensile strength, modulus, and electrical conductivity of a pitch composite fiber with 5 wt % loading of purified SWNTs are enhanced by ∼90%, ∼150%, and 340% respectively, as compared to the corresponding values in unmodified isotropic pitch fibers. These results serve to highlight the potential that exits for developing a spectrum of material properties through the selection of the matrix, nanotube dispersion, alignment, and interfacial bonding.
Tunnel conductance as a probe of spin polarization decay in Cu dusted Co/Al2O3/Co tunnel junctions
Authors: LeClair P. , Swagten H. J. M. , Kohlhepp J. T. , de Jonge W. J. M.
Published: July 2002

Tunneling magnetoresistance (TMR), dynamic resistance and bias dependence measurements were performed on Co/Al2O3/Co magnetic tunnel junctions with a thin Cu layer inserted at either the Co/Al2O3 (“bottom”) or Al2O3/Co (“top”) interfaces. Careful comparative analysis allows detailed growth characteristics to be elucidated, as well as providing information on the underlying mechanisms behind spin polarized transport in these structures. Conductance for top dusted junctions is indicative of parallel Co/Al2O3/Co and Co/Al2O3/Cu junctions, consistent with three-dimensional growth of Co and Cu on Al2O3, while conductance for bottom dusted junctions show novel behavior dissimilar to either type of junction. The bias dependence of the TMR, surprisingly, is unaffected by either type of dusting.
Infrared absorption peak due to Ta=O bonds in Ta2O5 thin films
Authors: Ono Haruhiko , Koyanagi Ken-ichi
Published: July 2002

Ta 2 O 5 films deposited on Si substrates were investigated using transmission Fourier-transform infrared spectroscopy. We found a new absorption peak at 2340 cm−1 that can be characterized as a stretching vibration mode due to Ta=O bonds in the films. This peak appeared following annealing in O2 ambient, but not in N2 ambient. It was located at 2335 cm−1 in amorphous Ta2O5 films and shifted to 2340 cm−1 after crystallization by annealing at over 700 °C. The bonds associated with the peak were homogeneously distributed in the film. We demonstrated that Ta2O5 films can include strong double bonds between Ta and O (Ta=O) in the structure, independent of whether they are crystalline or amorphous.
Band structure nonlocal pseudopotential calculation of the III-nitride wurtzite phase materials system. Part II. Ternary alloys AlxGa1−xN, InxGa1−xN, and InxAl1−xN
Authors: Goano Michele , Bellotti Enrico , Ghillino Enrico , Garetto Carlo , Ghione Giovanni , Brennan Kevin F.
Published: July 2002

This work presents detailed information on the band structures of the III-nitride wurtzite ternary alloys, computed through the virtual crystal approximation approach. The key ingredient of this study is the set of realistic atomic effective potentials described in Part I of the present work, dedicated to the constituent binary compounds. The model relies on the linear interpolation of the structural parameters and of the local and nonlocal effective potentials: no further empirical corrections are included. The dependence on the mole fraction is computed for the energy gaps at all the high-symmetry points, the valence-band width, and the electron effective masses in the valleys relevant for carrier-transport simulation.
Characterization of the natural barriers of intergranular tunnel junctions: Cr2O3 surface layers on CrO2 nanoparticles
Authors: Dai Jianbiao , Tang Jinke , Xu Huiping , Spinu Leonard , Wang Wendong , Wang Kaiying , Kumbhar Amar , Li Min , Diebold Ulrike
Published: July 2002

Cold-pressed powder compacts of CrO2 show large negative magnetoresistance (MR) due to intergranular tunneling. Powder compacts made from needle-shaped nanoparticles exhibit MR of about 28% at 5 K. Temperature dependence of the resistivity indicates that the Coulomb blockade intergranular tunneling is responsible for the conductance at low temperature. In this letter we report direct observation and characterization of the microstructure of the intergranular tunnel barriers, using transmission electron microscopy, x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). A very thin native oxide layer with a thickness of 1–3 nm on the surface of CrO2 powders has been observed. The composition and crystal structure of this surface layer has been determined to be Cr2O3 by XPS and XRD. The dense and uniform Cr2O3 surface layers play an ideal role of tunnel barriers in the CrO2 powder compacts.