Measurements and modeling of microwave impedance of high-Tc grain boundaries

We present measurements and modeling of the microwave-frequency (rf) power-dependent impedance of YBCO thin-film grain-boundaries, The measurements were performed using a stripline-resonator technique that separates the characteristics of the grain boundary from the rest of the film. The devices were fabricated on sapphire bicrystal substrates with misorientation angles varying from 2 to 24 degrees and on single crystals, Measurements of the de I-T: characteristics were also performed. The rf measurements are presented as a function of rf current (0.1 mA to 1 A) and temperature (6 to 70 K). Grain-boundary angles of 10 and 24 degrees show Josephson-junction behavior in the rf measurements, but at angles of 5 degrees or less the rf impedance is indistinguishable from films grown on single-crystal substrates in contrast to de measurements that show reduced critical current for the 5 degrees grain boundary. The measured impedance of the large angles is compared with a long-junction model which fits the measured data qualitatively. The measurements and modeling show the creation, annihilation, and motion of Josephson vortices by rf currents which produce the observed nonlinearities In the impedance. We also present measurements of second harmonic generation.