TlBaCaCuO-(2212) thin films on lanthanum aluminate and sapphire substrates for microwave filters

High-quality Tl2Ba2CaCu2O8 films for microwave filter application are prepared on both sides of 2 inch lanthanum aluminate substrates as well as on double-sided CeO2 coated r-cut sapphire substrates. The films have thicknesses as high as 1 µm in the case of lanthanum aluminate substrates. On CeO2 buffered sapphire substrates it is possible to prepare crack-free superconducting films as thick as 400 nm. For both substrate materials we reach high critical current densities of Jc>2×106 A cm-2 at 77 K. The microwave surface resistances Rs were measured to fall short of 100 µΩ at 5.6 GHz and 77 K in the low-field case, comparable to the best films made of YBa2Cu3O7-x material. The films were able to tolerate high microwave power levels corresponding to magnetic field amplitudes of BRF>2 mT. The critical temperature exceeds 102 K. This excellent performance is maintained even after several thermal cycles. The films were used to produce four-pole band-pass filters for the C-band which were operated without any detectable loss in their transfer characteristics up to temperatures of about 85 K. This is the first time that large area (2 inch) double-sided Tl2Ba2CaCu2O8 films have been prepared on sapphire with excellent critical current density in conjunction with low surface resistance.