Microwave response of perfect YBa2Cu3O7_x thin films deposited on CeO2-buffered saphire : A probe for pairing symmetry

• Published in: Journal of superconductivity and novel magnetism 2007-01, Vol.20 (1), p.59-69
• Main Authors: PAN, V. M, KALENYUK, A. A, KASATKIN, A. L, IVANYUTA, O. M, MELKOV, G. A
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Pairing symmetries; Physics; Properties of type I and type II superconductors; Response to electromagnetic fields (nuclear magnetic resonance, surface impedance, etc.); Superconductivity; Theory and models of superconducting state
• ISSN: 1557-1939; 1557-1947
• DOI: 10.1007/s10948-006-0190-7

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 [-delta/T] with a small gap delta value (delta{approximately equal to} 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.