THALLIUM-BASED HIGH-T-C FILMS WITH VERY-LOW SURFACE IMPEDANCE

Epitaxial Tl2Ba2Ca2Cu3Ox films with very low microwave losses have bee n prepared on LaAlO3 substrates by the two-step method: sputtering of TI free precursor films and post-annealing in a TI-oxide atmosphere in the temperature range from 1161 K to 1168 K. We found that the anneal ing temperature has a large influence on the film quality. The highest -quality Tl2Ba2Ca2Cu3Ox film, prepared at 1168 K, exhibits a surface r esistance of R(s) = 6.5 m Omega at 77 K and 1 m Omega at 4.2 K and 87 GHz, and a penetration depth lambda(O K) = 170 nm. Furthermore we have observed for the first time a decrease of R(s)(T) below about 30 K fo r several TBCCO films. The surface resistance measured at different fr equencies by means of two types of cavity measurement techniques exhib its a quadratic dependence on frequency at 77 K over the frequency ran ge from 17.8 GHz to 87 GHz.