•Ermile Gaganidze¹, Roland Heidinger¹, Juergen Halbritter¹ und Henrik Schneidewind^2
1Forschungszentrum Karlsruhe, IMF I, Postfach 3640, D-76021 Karlsruhe
²IPHT Jena, Winzerlaer Strasse 10, D-07745 Jena

Whereas YBa₂Cu₃O₇ epitaxial film quality has reached a stage where rf applications at T £ 80 K are carried through, higher operating temperatures or higher power handling are very much desired. Tl₂Ba₂CaCu₂O₈ is a perfect candidate by its T_C @ 100-105 K. Such films on MgO substrates are already successfully used in low power rf receiver filters. For high power applications, however, films on Al₂O₃ substrates are needed. We study the rf field dependence of Z_s of epitaxial large area (2-3 inch) Tl₂Ba₂CaCu₂O₈ films on sapphire substrates using a dielectric resonator with TE₀₁₁@8.5 GHz at 4.2 £ T £ 100 K and at 0.01 £ H_rf £ 10 mT. The thin Tl₂Ba₂CaCu₂O₈ films prepared by thallination of an amorphous Ba₂CaCu₂O_x precursor show already promising field dependencies dR_s µ H_rfⁿ µ dX_s(H_rf) with exponents n @ 1-2 allowing operation up to 5-7 mT. Bad spots seem to be related to dR_s µ H_rf with anomalous dependencies dR_s µ Ö{H_rf/(H_rf+H₀)} at m₀ H_rf £ 0.3 mT hinting flow of Josephson fluxons yielding at m₀ H_rf ³ 0.3 mT hysteresis losses with r = dX_s/dR_s @ 1. The lateral R_s-homogeneity of the films were analyzed with an open resonator at 145 GHz.