TEMPERATURE-DEPENDENCE OF THE MICROWAVE SURFACE-RESISTANCE OF YBCO THIN-FILMS FROM MICROSTRIP RESONATOR MEASUREMENTS

The surface resistance of YBa2Cu3O7-x (YBCO) thin films were obtained from measuring the Q-factor of inverted microstrip resonators at 6-7 GHz over the temperature range of 16-80 K. The inverted microstrip which included a sapphire spacer between the YBCO line and a YBCO ground plane has lower dielectric loss than that of regular microstrip since sapphire has a much lower loss tangent than that of the film substrate. This enables the determination of R(s) to a much higher sensitivity. We have examined good quality epitaxial YBCO films fabricated by several techniques; ion beam sputtering, pulsed-laser deposition, and off-axis magnetron sputtering. Most films have T(c)'s of 85-90 K and J(c) of 0.5-2 x 10(6) A/cm2 at 77 K. The loaded Q-factors were 2500-4800 at 77K increasing up to 77 000 at 16 K, which to our knowledge, is the highest Q measured for a high-T(c) microwave planar resonator. We have found that the surface resistances of the YBCO films of comparable T(c) and J(c) have similar values at relatively high temperatures (T > 70 K) even though these films were grown by different techniques. The surfaces resistance, scaled to 10 GHz was in the range 0.44-0.7 OMEGA-m at 77 K and as low as 37 mu-OMEGA at 16 K.