Abstract
Epitaxial Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub y/ and Tl(Ba,Sr)/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ thin films have been prepared on CeO/sub 2/ buffered sapphire substrate by an ex situ process with high reproducibility. Microstructure analysis of the surface of the Tl-2212 films showed well connected and smaller sized grains of 1 /spl mu/m or less. In the case of Tl-1223 film, plate-like crystals with pits and pin holes could be observed. The superconducting critical temperatures of both films are around 96 K. Critical current density measured by ac susceptibility technique is as high as 1 MA/cm/sup 2/ at 77 K. The microwave surface resistance of these films was measured over a wide range of temperatures by a HTS-sapphire-HTS resonator method at 38 GHz on films with area 10 mm /spl times/ 10 mm. The temperature dependence of surface resistance of Tl-2212 film follows very closely to that of the best YBCO films on MgO substrate, whereas the Tl-1223 exhibits slightly higher values resulting from an inferior surface morphology.
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