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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 64 (1988), S. 5800-5802 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Nonresonant microwave (MW) absorption is observed at the surface of high-Tc superconducting oxides in the presence of a static magnetic field and a scanning field. Nonlinear signals are obtained that show absorption troughs at the endpoints of the field scan superimposed on a hysteretic variation in absorption with field. The model of Portis et al. accounts quantitatively for the detailed shape of the observed signals. It enables the direct measurement of a critical field H*=(4π/c)Jcλ which depends on the critical current Jc and the static field penetration depth λ. In addition, the ratio λMW/λ is deduced from the shape of the troughs, with λMW the penetration depth of the MW fields. Typical values are H*(approximately-equal-to)5 μT and λMW(approximately-equal-to)3λ for a sintered YBaCuO sample at 77 K, and nearly the same H* but λMW(approximately-equal-to)λ at 4.2 K. Since the observed shapes also depend on the previous slow changes of the applied field, these investigations account for the memory effect observed with the usual resonance techniques.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 6276-6282 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Understanding the coupling mechanism between SQUID (superconducting quantum interference device) loop and resonator in high-frequency SQUIDs is essential for the application of these devices in sensitive magnetometers or controllable microwave devices. We have investigated, therefore, two kinds of planar microstrip tank circuit resonators: (1) S-shaped λ/2-resonators with direct or indirect coupling to the SQUID loop with optional flux-focussing pads and (2) conventional rf-washer-SQUID structures for which we have found resonant modes of the washer providing a coupling to the SQUID loop. A lumped element representation of SQUID loop and resonator is presented in order to describe the effective SQUID-resonator coupling in case of direct or indirect coupling. According to this approach, the square of the effective coupling constant k in both cases is equal to the ratio of the magnetic energy stored in the SQUID loop to that stored in the resonator. For the indirect coupled S-shaped λ/2-resonators, k can be described in terms of a geometry-defined mutual inductance. The predictions of the lumped element description are compared with simulations using a standard full-wave analysis computer program in order to quantify the effect of the different layouts on microwave current distribution and on k. A method for the determination of k by simulation is described, leading to useful design rules for the optimization of high-frequency SQUIDs. The simulation results will also be compared with experimental data. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 4581-4587 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The temperature and the dc magnetic field dependence of the effective surface impedance Zs of epitaxial YBa2Cu3O7−x thin films on [001] tilt SrTiO3 bicrystals with tilt angles of 36.8° and 24° have been investigated at 87 GHz. The effects of the grain boundaries become increasingly important with decreasing characteristic voltage IcRn and increasing unit areal normal resistance RnA. The boundaries can consistently be described with a resistively shunted Josephson transmission line model taking into account the effect of finite film thickness. Thermal or magnetic loading of the junction leads to additional losses, whereas the effective microwave penetration depth exhibits a specific extremal behavior. From the magnetic response of Zs at 4.2 K, the junction lower critical fields as well as the values of IcRn and RnA can be deduced. The data agree well with dc transport measurements.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Measurements of hydrogen loss and luminescence as a function of annealing temperature in porous silicon suggest that luminescence is attributable to electron-hole recombination in SiOx surface layers with an intensity that is dependent upon the surface hydrogen content. The luminescence is composed of three Gaussian bands similar to those found in amorphous SiO2. X-ray photoelectron spectroscopy and scanning electron microscopy show porous silicon has SiOx on the surface, which is comprised of many particles of about 10 nm size. Collectively, the data strongly support the previously proposed quantum confinement/luminescence center model. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Tl-based superconducting thick films deposited on large-area (11.4 cm2) Ag alloy (Consil 995) substrates have been characterized by surface resistance measurements yielding values of 4 and 14 mΩ at 4 and 77 K, respectively, as measured in an 18 GHz Cu cavity; corresponding Cu values are 8 and 21 mΩ. Utilizing a Nb cavity, the dependence of surface resistance on microwave power was also measured. The surface resistance begins to rise in microwave magnetic fields of 1–10 Oe and saturates in 20–60 Oe, the exact values depending on the degree of c-axis texturing of the polycrystalline films. Enhanced texturing sharpens the high-frequency transition into the superconducting state and also weakens the dependence of surface resistance on microwave magnetic field. These results are significant for high temperature superconductor applications such as microwave cavities.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The increase in the microwave surface resistance Rs of high Tc superconductors at elevated microwave power levels is reported for both oriented and unoriented Tl-based films as a function of rf magnetic field at 820 MHz and 18 GHz. The application of dc magnetic fields produces qualitatively similar increases in Rs and in the surface reactance Xs. The increase in Rs with dc field is shown to arise from simple decoupling of grains by intergranular magnetic flux. The increase in Rs with microwave power, on the other hand, is a consequence of hysteretic intergranular processes.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An increase by over three orders of magnitude from 0.58 mΩ to 1.17 Ω was observed at 4.0 K in the 3 GHz microwave surface resistance of ceramic YBa2Cu3O7−δ following exposure to neutrons. The transport resistivity of an unirradiated pellet was linear in temperature down to Tc with a room-temperature value of 2.13 mΩ cm and a resistivity extrapolated to 4.0 K of 0.53 mΩ cm. Following irradiation, the resistivity rose with decreasing temperature from a room-temperature value of 1.5 Ω cm to a maximum at around 45 K with little change evident at Tc . These results, in conjunction with eddy current, susceptibility, iodometric titration, and thermally stimulated luminescence measurements on irradiated and unirradiated samples, collectively suggest that the effect of neutron irradiation has been to decrease intergranular coupling without the development of an insulating phase.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    Journal of superconductivity 3 (1990), S. 297-304 
    ISSN: 1572-9605
    Keywords: High-temperature superconductivity ; microwave cavities ; sample surface impedance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract A description of cavities used in the study of the microwave properties of the high-temperature superconductors is followed by a lumped-circuit analysis of the coupling of transmission lines and resonators. The frequency dependence of the reflected and transmitted microwave power and the character of transient cavity response are analyzed. Techniques are discussed for the introduction of samples of the high-temperature superconductors into microwave cavities. Following a discussion of sample surface impedance and sample geometry factor, the connection between surface resistance and cavityQ is examined as well as the connection between cavity frequency shift and surface reactance. Measurement techniques that utilize reflected or transmitted power or transient response are described.
    Type of Medium: Electronic Resource
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  • 9
    ISSN: 1572-9605
    Keywords: High-temperature superconductivity ; microwave surface impedance ; Tl-based superconducting films ; metallic substrates ; field dependence of surface resistance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Microwave surface resistance (R s ) measurements on large-area (11.4 cm2) Tl-based films deposited onto Ag (Consil 995) substrates have been made at a frequency of 18 GHz as a function of temperature. Deposition onto unoriented Ag substrates yields unoriented films characterized byR s values of 8.2 and 33.6 mΩ at 10.6 and 77 K, respectively. In contrast, similar deposition onto oriented Ag substrates yields oriented films with values of 12.6 and 14.6 mΩ at 11.2 and 77 K, respectively; corresponding Cu values are 9 and 21 mΩ. Additionally, it is found that the dependence ofR s on microwave surface magnetic fieldH s is weaker for the oriented films. These results suggest that the effect of orienting thec-axis in Tl films is twofold: (1) the high-frequency superconducting transition is made considerably sharper, resulting in a lowerR s value at 77 K, and (2) the rate of increase inR s with appliedH s is reduced. Both effects have significant ramifications for the potential application of these materials to high-frequency accelerating cavities.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Journal of superconductivity 5 (1992), S. 319-330 
    ISSN: 1572-9605
    Keywords: HTC superconductors ; microwave power effects
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Extensive studies have been performed in a number of laboratories of the microwave properties of granular and partially granular superconductors at elevated levels of microwave power. The power-induced surface impedance initially increases at a rate that is quadratic in the microwave field. At intermediate power levels the surface resistance increases linearly with the microwave field, finally saturating in microwave fields between 10 and 100 Oe. The induced surface resistance is generally found to increase at low power with the square of the frequency and at high power directly with the frequency. A transmission-line analysis of the penetration of intergranular microwave vortices, pinned by defects, leads to results in qualitative agreement with these observations.
    Type of Medium: Electronic Resource
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