Preparation of Tl-2212 and Tl-1223 superconductor thin films and their microwave surface resistance

Epitaxial Tl₂Ba₂CaCu₂O_y and Tl(Ba,Sr)₂Ca₂Cu₃O_y thin films have been prepared on CeO₂ 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 μm 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/cm² 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 × 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.