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  • 1995-1999  (2)
  • 1990-1994  (2)
  • 1
    Digitale Medien
    Digitale Medien
    Low-resistivity epitaxial YBa2Cu3O7 thin films with improved microstructure and reduced microwave losses (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 5572-5578 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Epitaxial thin films of YBa2Cu3O7 have been prepared on SrTiO3 and LaAlO3 substrates by a high-pressure planar dc-sputtering technique. By covering the substrate heater with a frame of polycrystalline YBa2Cu3O7 substantial improvements of the YBa2Cu3O7 film properties were achieved. These are characterized by dc-resistivity values ρ(T) of less than 50 μΩ cm at 100 K and ρ(300 K)/ρ(100 K) values of up to 3.9. Significant deviations from the usual linear ρ(T) behavior were found. Critical temperatures above 90 K, resistive transition widths down to 0.3 K, and critical current densities of about 5 × 106 A/cm2 at 77 K confirm the high quality of the films. As indicated by Rutherford backscattering and high-resolution transmission electron microscopy the films exhibit a microstructure characterized by a reduced density of lattice defects. However, lattice-coherent precipitates with a diameter of about 5–10 nm were observed. As an outstanding feature the films exhibit, besides the initial steep falloff at Tc, a further gradual decrease of the microwave surface resistance at 87 GHz below 50 K by at least one order of magnitude. These results are very promising for millimeter-wave applications of epitaxial YBa2Cu3O7 thin films.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.350535
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  • 2
    Digitale Medien
    Digitale Medien
    Microwave surface resistance of epitaxial YBa2Cu3O7 thin films at 18.7 GHz measured by a dielectric resonator technique (1992)
    Springer
    Journal of superconductivity 5 (1992), S. 195-201 
    Details
    ISSN: 1572-9605
    Schlagwort(e): High-temperature superconductivity ; microwave cavities ; sample surface impedance
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Elektrotechnik, Elektronik, Nachrichtentechnik , Physik
    Notizen: Abstract We used a dielectric resonator technique for highly sensitive measurements of the temperature dependence of the microwave surface resistanceR s of 1×1 cm2 superconducting films at 18.7 GHz. It consists of a sapphire disc positioned on the film under investigation within a copper cavity which is acting as a radiation shield. In the TE01δ oscillation mode the highly reproducible quality factor of about 105 results in a sensitivity of ±50μΩ forR s measurements. The temperature dependence ofR s can be measured up to values as high as 1 Ω. We have investigated several YBa2Cu3O7 thin films prepared by high oxygen pressure d.c. sputtering on LaAlO3 and NdGaO3. Our best films exhibit a pronounced nonlinear behavior of the d.c. resistivityρ(T) withρ(300K)/ρ(100K) values of about 3.7. Those films show, besides the initial fall-off just belowT c , a further strong decrease ofR s at low temperatures. This was observed both at 18.7 GHz and 87 GHz, as measured by a conventional cavity end plate replacement technique. ForT≪Tc/2 these films exhibit an exp (−αT c/T) dependence ofR s withα-values around 0.4. These observations may be explained by a superconducting energy gap with 2Δ/kT c≈0.8 for charge carriers localized in the CuO chains for YBa2Cu3O7.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00618067
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  • 3
    Digitale Medien
    Digitale Medien
    Surface impedance of epitaxial YBCO thin films — Comparison of experimental data with a d-wave theory of superconductivity (1996)
    Springer
    Journal of low temperature physics 105 (1996), S. 503-508 
    Details
    ISSN: 1573-7357
    Schlagwort(e): 05.70 Ln ; 05.70 Jk ; 64
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract The surface impedance of two 350nm thick YBCO films was measured for temperatures between 4.2K and 150K in a copper cavity at 87 GHz. Both films, one grown by electron beam evaporation on MgO, the other one by high oxygen-pressure dc sputtering from a stoichiometric target on LaAlO3, provide critical temperatures of about 91K, low residual surface resistances of Rs(4.2K) 〈 1mΩ and low specific resistivities in the normal state of ϱ(100K) 〈 85μΩcm. The experimental data obtained on these two films are compared to a d-wave model of superconductivity which incorporates elastic and inelastic scattering. Good agreement between theory and the experimental results for both the surface resistance and the penetration depth in the whole temperature range is achieved for scattering phase shifts near 0.4π and order parameter amplitudes in the range of 2Δ0(0)/kBTc = 6.0 – 7.5 without subtracting an extrinsic residual surface resistance.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00768435
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  • 4
    Digitale Medien
    Digitale Medien
    Pair and Quasiparticle States of YBa2Cu3O7−x Films Deduced from the Surface Impedance and a Comparison with Nb3Sn (1999)
    Springer
    Journal of superconductivity 12 (1999), S. 129-138 
    Details
    ISSN: 1572-9605
    Schlagwort(e): Surface impedance ; residual resistance ; energy gap ; two-band model ; magnetic scattering
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Elektrotechnik, Elektronik, Nachrichtentechnik , Physik
    Notizen: Abstract Surface impedance data at 19 and 87 GHz of high-quality epitaxial YBCO films on different substrates are compared with data for Nb3Sn films on sapphire in terms of pair and quasiparticle (qp) transport. Surface resistance R s and penetration depth λ of YBCO are strongly affected by temperature dependent qp scattering, which is depressed in films with enhanced lattice strain. All films showed a comparable residual resistance R res(19 GHz)∼90 μΩ constituting a qp reservoir which is likely to be caused by the electronic configuration and by impurities. Subtracting R res from R s (T) revealed activated behavior with a reduced energy gap Δ0/k B T c∼0.9 for a film on sapphire, but power-law behavior for the other films. The penetration depth did not reveal power-law dependences at T≤0.5 ·T c, but was consistent with a reduced energy gap of 0.45 for a film on MgO. The increase of λ(T) at T≥0.5 · T c was related to qp scattering, which also caused an extremal conductivity σ1(T). A shoulder in λ(T) at T=(0.6–0.7) · T c confirmed evidence for the existence of two superconducting bands. The magnetic-field induced recovery of λ(B) of various YBCO films hinted for an important role of magnetic scattering. The results are in contradiction to a d-wave symmetry of the order parameter, at least for the chain band.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1007750426204
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