Microwave response of high-transition-temperature superconducting thin films

The author has studied the microwave response of YBa2Cu3O7-δ, Bi-Sr-Ca-Cu-O, and Tl-Ba-Ca-Cu-O high transition temperature superconducting (HTS) thin films by performing power transmission measurements. These measurements were carried out in the temperature range of 300 to 20 K and at frequencies within the range of 30 to 40 GHz. Through these measurements he has determined the magnetic penetration depth (λ), the complex conductivity (σ* = σ1- jσ2), and the surface resistance (Rs). An estimate of the intrinsic penetration depth (λ ∼ 121 nm) for the dependence of λ. This value compares favorably with the best values reported so far (∼140 nm) in single crystals and high quality c-axis oriented thin films. Furthermore, it was observed that the technique is sensitive to the intrinsic anisotropy of λ in this superconductor. Values of λ are also reported for Bi-based and Tl-based thin films. He observed that for the three types of superconductors, both σ1 and σ2 increased when cooling the films below their transition temperature. This indicates that the temperature dependence of σ1 is not consistent with that expected from the two-fluid model, and also deviates from the values obtained using the Mattis-Bardeen equations based on the BCS theory. The analysis shows that, of the three types of HTS films studied, the YBa2Cu3O7-δ thin films, deposited by laser ablation and off-axis magnetron sputtering are the most promising for microwave applications