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The temperature dependent surface impedance Z s ( T ) of single-crystalline YBa 2 Cu 3 O 7-x (YBCO) was analyzed within the two-fluid model in terms of the fraction of paired charge carriers, f s ( T ), and of the quasiparticle scattering time . The usual approach was extended by considering a temperature dependent effective quasiparticle mass m * ( T ), which results from a strong electron-phonon interaction. This effect must not be neglected in the description of high-temperature superconductors due to the large ratio of T c to the Debye temperature T D. The temperature dependence of the penetration depth, , of high-quality YBCO crystals and films could be described with an electron-phonon coupling constant , and using f s ( T ) = 1- ( T / T c ) 2.8 as an approximation of the BCS theory. Different trial phonon spectra were encountered in terms of their ability to reproduce the -data. The scattering time was described by the Bloch-Grüneisen formalism with T D =460 K. Assuming an Einstein spectrum with , a residual resistivity and a fraction of unpaired quasiparticles at T = 0 K yielded a surprisingly good agreement of the model with Z s ( T )-data measured at 87 GHz with a high-quality epitaxial YBCO film between T =4 K and T c. While an exact reproduction of the surface impedance asks for a rigorous theoretical computation, our analysis demonstrates that strong electron-phonon coupling is relevant for discussing the unconventional transport properties of YBCO. [ABSTRACT FROM AUTHOR]

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 Rs 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 Rres(19 GHz)∼90 μΩ constituting a qp reservoir which is likely to be caused by the electronic configuration and by impurities. Subtracting Rres from Rs (T) revealed activated behavior with a reduced energy gap Δ0/ kB Tc∼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 · Tc, 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 · Tc was related to qp scattering, which also caused an extremal conductivity σ1( T). A shoulder in λ( T) at T=(0.6–0.7) · Tc 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. [ABSTRACT FROM AUTHOR]

An anomalous magnetic field-effect has been observed in the microwave surface impedance of epitaxial YBa2Cu3O7−x films on LaAlo3 substrates. The application of dc and microwave magnetic fields up to 20 mT caused a significant decrease of microwave surface reactance X s (≤3%) and resistance R s (≤20%) at 8.5 GHz, 77 K and at 87 GHz, 4.2 K. The field effects on X s and R s s were found to correlate within the framework of the two-fluid model. The observed effect is attributed to the presence of magnetic impurities in the superconducting films, giving rise to pair breaking. Alignment of the impurity spins suppresses the spin-flip scattering rate and, as a result, leads to a recovery of microwave superconductivity. [ABSTRACT FROM AUTHOR]

The conditions for the design of compact high-power HTS filters are deduced from basic considerations, and related to the microwave properties of the HTS films and the substrates. Potential mechanisms limiting the linear microwave response of HTS films are analyzed systematically. Magnetic and thermal field breakdowns are found to confine the power handling capability to comparable field levels. While magnetic limitation is related to flux penetration at the lower critical field Bc1, thermally induced quenches occur dependent on the presence of defects and on the thermal conductivity of the substrate. The detrimental role of film inhomogeneities for the linear microwave response is confirmed experimentally for microwave fields at 19 GHz and for dc fields at 87 GHz. [ABSTRACT FROM AUTHOR]

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