Abstract
Microwave surface impedance, Z s(T), of epitaxial YBCO thin films deposited on CeO2-buffered sapphire substrates, was measured at several discrete frequencies within the range 5–134 GHz by use of coplanar resonator and end-plate cavity resonator techniques. The main features of obtained experimental results are as follows: (i) surface resistance R s(T) at low temperatures obeys the exponential law: R s(T) = R res+R 0⋅exp [−δ/T] with a small gap δ value (δ≈ 0.7 T c); (ii) the most perfect quasi-single-crystalline films reveal a distinct two-peak structure of R s(T) dependence, which is not observable in films with a less ordered crystal structure. These features are believed to reveal some intrinsic electron properties of such films, namely: (i) mixed (d+is) type symmetry of electron pairing, and (ii) dominant role of extended c-oriented defects (e.g., edge dislocation arrays or twin planes) in quasiparticles scattering for the most perfect films, which demonstrate the two-peak anomalous R s(T) behavior.
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