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  • 1
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The performance of photon detectors based on superconducting tunnel junctions are related to their current - voltage (I-V) curve characteristics and, ultimately, to the quality of the thin tunnel barriers (of order 1 nm) which separate the two superconducting thin films. Both the optimization of the spectroscopic performance of these detectors and the development of a reproducible and high yield fabrication route, require a better understanding of barrier quality and growth techniques. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) provide valuable tools for the investigation of the barrier region and for the control of the quality of the different thin films and related interfaces. In this paper, the results of a TEM and AFM evaluation of Nb-Al-AlOx-Nb tunnel junctions are reported, together with their interpretation on the basis of the I-V curve performance at low temperature (T≥0.3 K). Thickness disuniformities of the Al plus AlOx overlayer and evidence of barrier defects have been found, which may place constraints on the spectroscopic performance of such devices. Through the use of TEM it has also been possible to confirm the epitaxial nature of the Nb base electrode. The junction counter electrode however appears to be polycrystalline, with a columnar morphology and an average grain width of 40 nm. The overall structure of the various layers may well place constraints on the tunneling characteristics of the device. © 1995 American Institute of Physics.
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 8384-8390 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Superlattice structures of alternately undoped and [Si]=1×1019 cm−3 doped GaAs have been grown by molecular beam epitaxy at a substrate temperature of 250 °C. X-ray diffraction profiles give an average value of lattice constant in the structures in between the values measured in single, nominally undoped and [Si]=1×1019 cm−3 doped epilayers grown under identical conditions. Transmission electron microscopy dark-field (004) beam images of the as-grown structures contain bands of strain contrast and hybrid diffraction/imaging reveals periodic variations in the position of the higher-order Laue zone lines in the superlattice structures. We believe the results signify a modulation of the lattice constant along the growth direction: the lattice constant being smaller in the doped regions than in the undoped regions due to a reduction of excess As concentration at this high doping level. On annealing, dense bands of large As precipitates appear in the undoped regions with bands of fewer, smaller precipitates in the doped regions. Hall-effect measurements on the as-grown structures yield electron concentrations comparable with those measured in heavily Si-doped, low-temperature grown single epilayers, and the mobilities are indicative of band conduction in the doped regions of the superlattices. © 1996 American Institute of Physics.
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  • 3
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Epitaxial Ga2Se3 layers were grown on GaP (100) and GaAs (100) by metal–organic chemical vapor deposition and the heterovalent exchange reaction, respectively. Measurements of the sample reflectance were carried out in the spectral range from 70 to 50 000 cm−1 (∼10 meV−6.2 eV). The dielectric functions in the far infrared were determined from the reflectance measurements and are dominated by strong phonon features of the substrate and the layers. Substrate related multiphonon absorbances and Fabry–Perot interference dominate the mid infrared range. The spectra in the visible spectral range reveal Fabry–Perot interferences up to 2.6 eV indicating a fundamental band gap energy in the blue spectral range in contrast to the previously reported lower value of 2 eV. Further electronic transition energies were observed at 3.9, 4.7, and 5.0 eV. © 1996 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 69 (1998), S. 3938-3941 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We describe here experiments evaluating the performance of solenoidal radio frequency probes having submillimeter dimensions (microcoils) as detectors for liquid nuclear magnetic resonance (NMR) in very low-homogeneity (100 ppm/cm) magnetic fields. Performance is based on the measured H2O linewidth. A series of solenoidal microcoils having sample volumes 8, 53, and 593 nl were filled with distilled H2O and evaluated for smallest obtainable unshimmed NMR spectral linewidths in a vertical bore superconducting magnet, stabilized at 5.9 T (1H frequency=250 MHz). The smallest microcoil (472 μm diameter) gave a smallest H2O linewidth of 525 Hz, 25 times smaller than that from a standard 5.7 mm probe. Linewidth increased approximately as the square root of sample volume. For comparison, shimmed H2O linewidths using the same microcoils in a high-homogeneity (0.1 ppm/cm) NMR magnet were also measured. Shimmed linewidths in the high-homogeneity magnet were two orders of magnitude smaller and exhibited a similar dependence on volume. The results demonstrate that by using microcoils the volume over which the polarizing magnetic field must meet a specified homogeneity can be significantly reduced, which would be advantageous for smaller, less expensive NMR systems. © 1998 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 55 (1989), S. 1258-1260 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Superconducting Tl2CaBa2Cu2Oy thin films have been successfully grown on the single-crystal sapphire (11¯02) substrate by using the metalorganic chemical vapor deposition technique. The growth rate of the films was about 5 μm/h. The as-deposited film was post-annealed in a partially sealed ceramic crucible in the presence of a Tl2Ca2Ba2Cu3Oy pellet to achieve the superconducting phase. The x-ray data show strong diffraction from the Tl2CaBa2Cu2Oy superconducting phase in addition to the trace amounts of Ca2CuO3 and BaCO3. Superconducting transition temperatures with onset above 100 K and zero resistance at 94 K can be obtained by further heat treatment at 500 °C in oxygen.
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  • 6
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 54 (1989), S. 380-382 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Highly textured single phase superconducting YBa2Cu3O7−x films have been successfully grown on the yttria-stabilized zirconia (100) substrates by using the metalorganic chemical vapor deposition technique. The as-deposited films grown at 650 °C were homogeneous mixtures of the related metal oxides and carbonates. Subsequent thermal annealing under oxygen flow yielded single phase superconducting films whose thickness corresponded to the deposition rates of approximately 10 μm/h. After the post-annealing the films deposited on the yttria-stabilized zirconia substrates exhibited a highly textured x-ray pattern with c axis perpendicular to the substrate surface. These films show an onset superconducting transition temperature of 93 K with the resistance becoming zero at 84 K.
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  • 7
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 64 (1994), S. 707-709 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Gallium arsenide doped with erbium has been grown by molecular beam epitaxy. At growth temperatures in the range 540–605 °C, and with arsenic to gallium flux ratios of 2 and more, the erbium forms uniform crystalline microprecipitates of ErAs when the concentration exceeds 7×1017 cm−3. The diameter can be varied in the range 11–21 A(ring) by altering the growth temperature. Reducing the arsenic to gallium flux ratio to close to stoichiometry changes the growth mode to one yielding quantum wires aligned in the growth direction. Subtle changes in growth conditions lead to bifurcated structures, which we refer to as quantum trees.
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Plant pathology 41 (1992), S. 0 
    ISSN: 1365-3059
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: The effect of disease on growth and yield of two barley cultivars sown at different times and in different years was investigated in New Zealand. The data were used to develop yield-loss models based on both disease severity (measured as green leaf area) and yield target (estimated by duration of crop growth). Disease influenced yield differently in Triumph and Sonya barley (spring and winter types, respectively), and yield components were affected to different degrees. Empirical yield-loss models for individual cultivars, sowing dates and seasons had different slope values, and the best models were based on measurements of disease at different growth stages. Combined models were less significant and explained less variation in yield than the individual models. Models which included the duration of crop growth as an estimate of yield target improved the fit to the data. Empirical models were specific to cultivar, sowing date and season, suggesting that they were not applicable in the varying conditions tested. The inclusion of estimated yield target improved the general applicability of models, and provided a method of using models in crops sown at different times and in different seasons, without measuring yield target directly.
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Plant pathology 41 (1992), S. 0 
    ISSN: 1365-3059
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: The effect of disease in two cultivars of barley, sown at different times in two seasons, on the relative importance of stored carbohydrate reserves and current photosynthesis for grain filling was assessed. Three methods of measuring stem reserve contributions to grain filling are reviewed and compared. Disease reduced stem dry weight and the amount of stored carbohydrate in most situations. In contrast, the total amount of stored carbohydrates used for grain filling was often increased by disease. The magnitude of these effects varied with the method used for estimation, and was also different in the crops sown at different times and in different seasons. The estimates of stem reserve contributions to grain filling ranged up to a maximum of 50% in some cases. At least 10 t/ha of reserve material was retranslocated in the healthy 1984 crop studied using 14C pulse feeding, and up to 0.3 t/ha more was utilized in a diseased crop. The effect of disease on the storage and utilization of stem reserves depended on the time of epidemic development, its duration, and the yield potential of the crop. This suggests that crops could be characterized as those which are very sensitive to disease during grain filling, with low stem reserves or high yield potential, and those with lower sensitivity, with more stem reserves or lower yield potential. Such interacting factors could be incorporated in future plant and yield-loss mechanistic models.
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Journal of materials science 7 (1996), S. 305-308 
    ISSN: 1573-482X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Cadmium-doped calcium sulphide has great potential as a broadband light source in both powder and thin film electroluminescent devices. We now report the first structural investigation of polycrystalline cadmium-doped calcium sulphide using a combination of transmission electron microscopy (TEM) with microanalysis, and X-ray diffraction. We show that the crystals formed by direct reaction of CaS and CdS under excess sulphur do not yield an even distribution of cadmium. We also show that the crystals are heavily dislocated with defect structures typical of rock-salt structures. We show that TEM can be successfully applied to the study of these moisture-sensitive materials without the formation of artefacts and that TEM should be a valuable tool for studying their thin film structures.
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