Microwave properties of YBa2Cu3O7−δ high-transition-temperature superconducting thin films measured by the power transmission method

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Abstract
The microwave response of YBa2Cu3O7−δ superconducting thin films has been studied by performing power transmission measurements. The measurements were made at frequencies within the 26.5–40.0‐GHz frequency range, and at temperatures from 20 to 300 K. The films were deposited on LaAlO3, MgO, yttria‐stabilized zirconia, and LaGaO3 substrates by laser ablation and dc off‐axis magnetron sputtering. From these measurements the complex conductivity σ* = σ1 − jσ2, the magnetic penetration depth λ, and the surface resistance Rs of the films have been determined. It was observed that both σ1 and σ2 increased when cooling the films below their transition temperature. This behavior disagreed with that expected from the two‐fluid model. In addition, it was observed that the temperature behavior of σ1 deviates from the predictions of the Bardeen–Cooper–Schrieffer theory. Values of λ have been obtained that are in good agreement with the best reported values for high quality c‐axis‐oriented thin films and single crystals (∼140 nm). The anisotropy of λ was determined by measuring this parameter in c‐ and a‐axis‐oriented films. An intrinsic penetration depth value (λ∼90±30 nm) has been estimated from the film thickness dependence of λ. Values of Rs for the YBa2Cu3O7−δ films were calculated and found to be comparable or lower than that of copper at temperatures below 80 K. These Rs values were consistent with those found on the same films using resonant‐cavity techniques.