We report measurements of the dc and microwave properties of epitaxial thin films of Tl2CaBa2Cu2O8 on LaAlO3. ac magnetic susceptibility measurements yield critical temperatures at the transition midpoint Tc∼100 K and 10–90% transition widths of ∼1.0 K. We observed a critical current density of 1.06×106 A/cm2 at 77 K measured by dc transport. We measured a low-power surface resistance of Rs=0.2 mΩ at 9.55 GHz and 77 K. This is ∼50 times lower than oxygen-free high-conductivity (OFHC) Cu at the same frequency and temperature. At 90 K, RS rises to 0.4 mΩ, and at 95 K, RS=0.7 mΩ. We measured the microwave power dependence of RS at 77, 90, and 95 K. At high microwave field (≳30 G, 77 K) Rs=0.8 mΩ, more than ten times lower than OFHC Cu.
We have fabricated and measured 5 GHz microstrip resonators from a series of YBa2Cu3O7−δ thin films grown on LaAlO3(001) substrates by in situ laser ablation. We have studied the correlations between unloaded quality factor and various film properties, such as transition temperature, width of transition, critical current density, narrowness of x-ray rocking curve, sharpness of electron channeling pattern, and most important substrate temperature during growth. We found that in general, higher transition temperature, higher critical current density, sharper transition, sharper channeling pattern, and narrower x-ray rocking curve correlate positively with good microwave performance. The best quality factor exists for a narrow growth temperature window (around 800 °C). We also report the dependence of quality factor on device power for each film.
High-quality epitaxial Tl2Ba2CaCu2O8 superconducting thin films have been grown on NdGaO3(001) by a two step process. One film, annealed at 850°C, shows interdiffusion of Ga by secondary ion mass spectroscopy, resulting in an incomplete AC susceptibility transition at 80 K. For films annealed at lower temperature, excellent crystallinity and transport properties were obtained. Ground plane substitution of the film in a 5 GHz microstrip resonator indicated worse microwave performance than similar films on LaAlO3(001) substrates. The 2.3 GHz resonator made from the film on NdGaO3 gave significantly worse quality factor (Q) values (1850) than those made on LaAlO3.
High-quality YBa2Cu3O7-δ thin films have been grown on CaNdAlO4 (001) substrates by in situ laser ablation. The best transition temperature determined by AC susceptibility midpoint was 88.3 K with a critical current density of 1.0×106 A/cm2 at 77 K. The full width at half maximum of the X-ray rocking curve of the YBCO (004) peak was 0.33°. The film crystallinity was slightly worse than those grown on LaAlO3 and NdGaO3 judging from electron channeling pattern. A 2.3 GHz resonator made from a YBCO film on CaNdAlO4 exhibited a low power Q of 3,945 which is worse than films on LaAlO3 (Q∼18,000) and also lower than the expected value from loss tangent data.
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