Tl2Ba2CaCu2O8 films with very low microwave surface resistance up to 95 K

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Abstract
Very low surface resistance Tl2Ba2CaCu2O8 films of thickness 1.0 μm on (100) LaAlO3 were fabricated and characterized. The films are highly c‐axis oriented (x‐ray diffraction rocking curve full width half maximum of 0.68°) with good in‐plane epitaxy with respect to the substrate. The Tc of the films was 107.6±0.5 K as measured by ac magnetic susceptibility. The surface resistance at 10 GHz was measured with the parallel plate resonator technique and found to be 23±5 μΩ at 4.2 K, 130±20 μΩ at 77 K, and 300±60 μΩ at 95 K. Using a 20 GHz TE011 end‐wall replacement copper cavity, the surface resistance was found to be comparable to that of copper, about 18 mΩ, at 104±1 K. The surface resistance of the films is significantly lower than that of all films reported to date at temperatures above 70 K and only slightly higher than the best high‐temperature superconductor films reported to date at 4.2 K.