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  • 1
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Smooth, c-axis oriented, superconducting thin films of YBa2Cu3O7−x and DyBa2Cu3O7−x have been grown on (100) MgO by organometallic molecular-beam epitaxy. To our knowledge this is the first report of this technique for the fabrication of this class of high Tc superconductors. Y (or Dy) β-diketonate organometallic sources are utilized with Ba and Cu metallic sources. Films are grown in situ by codepositing from these sources in an ozone ambient with a substrate temperature of 700 °C. The use of an active oxygen source (ozone) is required for decomposition of the organometallic sources as well as for the attainment of superconducting films in situ. The films are characterized by Tc's as high as 85 K and Jc's of 1.2×106 A/cm2 at 12 K and 2.4×103 A/cm2 at 77 K. In addition to the four point Tc and Jc results, alternating-current susceptibility measurements are also reported. The composition of the films has been determined by Rutherford backscattering spectroscopy, Auger and secondary ion mass spectroscopy analysis, with the smoothness of the films characterized by scanning tunneling microscopy.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1365-2427
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: 1. Thirty small upland lakes in Cumbria, Wales, Scotland and Northern Ireland were each visited once during June and July 2000. From each lake, samples of surface sediment epilithon, macrophytes and total dissolved nitrogen (TDN) were collected for nitrogen stable isotope analysis. As part of a wider programme, samples were also collected for chemical analysis and bioassays.2. Considerable variation was found in δ15N values in all measured nitrogen compartments. Some regional variation was evident but was generally weak. Sediment and epilithon δ15N were positively correlated with δ15N of TDN, suggesting that baseline nitrogen isotope ratios influence those in some organic matter compartments in the lakes.3. Sediment δ15N was higher when inorganic nitrogen concentration in the water was low, possibly reflecting reduced isotope fractionation under these conditions. However, this was not the case for epilithon or macrophytes. Sediment δ15N values were also negatively related to annual nitrogen deposition.4. Sediment, epilithon and macrophyte δ15N values all showed significant relations to nutrient limitation in the lakes as determined by algal bioassays. We suggest that sediment δ15N might be developed as a simple integrating measure of the degree of nitrogen limitation in lakes.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1365-2427
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: SUMMARY 1. Thirty small upland lakes in Cumbria, Wales, Scotland and Northern Ireland were visited three times between April and August 2000. On each occasion water chemistry was measured and phytoplankton bioassays were performed in the laboratory to assess growth-rate and yield limitation by phosphorus and nitrogen. In addition, yield limitation of periphyton growth was investigated twice, in situ, using nutrient-diffusing substrata.2. Over the whole season the percentage frequency of P, N and co-limitation was 24, 13 and 63%, respectively, for phytoplankton rate limitation and 20, 22 and 58%, respectively, for phytoplankton yield limitation.3. A clear response of periphyton yield to nutrient additions was found in 75% of all cases and of these, co-limitation was most common (54%). Average percentage frequency for P and N limitation was 26 and 20%, respectively.4. Phytoplankton and periphyton showed seasonal changes in nutrient limitation within sites. In particular, co-limitation became progressively more common as the season progressed.5. The response of phytoplankton growth rate to ammonium and nitrate addition was identical, but ammonium was a slightly better source of nitrogen than nitrate for phytoplankton yield on 7% and for periphyton yield on 10% of the occasions. However, the magnitude of the effect was small.6. The concentration of dissolved inorganic nitrogen (DIN) and the molar ratio of DIN to total dissolved phosphorus (TDP), appeared to be the main environmental factors controlling the extent of nitrogen or phosphorus limitation at a given site. Nitrogen limitation was more likely than phosphorus limitation where the DIN was 〈6.5 mmol m−3 and the ratio of DIN : TDP was 〈53. Co-limitation was the most likely outcome at a DIN concentration 〈13 mmol m−3 and at a DIN : TDP molar ratio 〈250. Above these values phosphorus limitation was most likely.7. The relatively high frequency of nitrogen limitation and co-limitation at higher N : P ratios than previously reported, may result from the inability of nitrogen-fixing cyanobacteria to thrive in these upland lakes where pH and the concentration of phosphorus tended to be low and where flushing rates tended to be high.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 30 (1987), S. 2054-2062 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Some studies of forced and unforced plane turbulent mixing layers have been conducted using two experimental and two computational approaches. The present paper contains an overall comparison and discussion of the measured and computed results. The experimental results include flow-visualization data using the smoke-laser technique and mean flow and turbulence measurements obtained with hot X wires and a two-component laser-Doppler velocimeter (LDV). The mean flow and turbulence results indicate that the two experimental techniques agree reasonably well for this shear flow. Two-dimensional computations of the measured mixing layers have also been conducted in a coordinated effort; one method uses the inviscid discrete vortex technique for a spatially developing layer and the other is based on an approximation to the Reynolds-averaged Navier–Stokes equations. The vortex method was found to give excellent results for the forced mixing layer while the Reynolds-averaged computations, with a modified turbulence model, were particularly successful at capturing the near-field viscous behavior that included the splitter plate wake.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 3251-3255 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The isothermal decay of the second-order nonlinear optical susceptibility of a guest-host azo dye methacrylate film was measured by second-harmonic generation at a series of temperatures. The decay curves were fit to Williams–Watt–Kohlrausch stretched exponentials. The activation energies and pre-exponential factors were found to agree with those derived from nonisothermal electric-field-induced second-harmonic generation measurements on similar films. It is concluded that a common underlying process involving a distribution of local relaxations governs the film behavior.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 63 (1993), S. 1005-1007 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Epitaxial barriers of ferromagnetic SrRuO3 have been used to fabricate high Tc superconductor-normal-superconductor Josephson junctions in the edge junction geometry. At small barrier thicknesses the SrRuO3 junctions follow the behavior of nonferromagnetic but otherwise closely related CaRuO3 junctions. A rapid disappearance of critical current is observed when the barrier thickness is increased to 250 A(ring). Possible origins of such a large critical thickness are discussed.
    Type of Medium: Electronic Resource
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