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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]

The in situ process—laser ablation in combination with thermal evaporation of Tl2O—has turned out to be a preparation method for single-phase and epitaxial TlBa2Ca2Cu3O9 (1223) thin films with Tc values up to 109 K. It was found by several groups that a partial substitution of Tl by Bi simplifies the phase development of the 1223 compound in the usual two-step process. We have investigated the influence of the Bi doping on the in situ growth. X-ray measurements show that the films consisted mainly of the 1223 compound. In 300-nm thin films there was no evidence of a Bi amount in the crystal structure, but thinner films (80 nm) show a small amount of Bi. We concluded that Bi doping supports the phase development of the 1223 compound only in an early stage of the film growth. The Bi-doped films have higher Tc values up to 114 K, higher jc values up to 6 × 105 A/cm2 (77 K, 0 T), and lower surface resistances of 56 mΩ (77 K, 87 GHz) than the undoped films. [ABSTRACT FROM AUTHOR]

Crack-free thick YBa2Cu3O7 − x films are prepared on CeO2 buffered r-cut sapphire (2 inch in diameter) with thickness up to 700 nm, smooth surfaces (“peak-to-valley” roughness <10 nm), high critical currents ( JC > 2 MA/cm2 at 77 K and 0 T), and low microwave surface resistances ( Rs(77K) ≍.4mω and Rs(4.2K) ≍.110μω at 19.15GHz) comparable to the best values reported in the literature for YBCO films on structurally better matched substrates. These thick YBCO films were able to handle high microwave power corresponding to magnetic field amplitudes ( BHF) up to 54, 37, and 17.4 mT at 4.2, 50, and 77 K, respectively, which for the lower temperatures were limited by the available power of the 25-W HF amplifier. The high-power performance, which to our knowledge belongs to the best reported so far for unpatterned YBa2Cu3O7 − x films, was achieved without any degradation of the samples despite frequent thermal cycling. [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]