Intermodulation distortion and Josephson vortices in YBCO bicrystal grain boundaries

The properties of grain boundaries at microwave-frequencies in the high-T_c materials are of interest for both practical applications and basic physics. We report the measurements of intermodulation distortion (IMD) in engineered grain boundaries in YBCO films fabricated on sapphire bicrystals with misorientation angles of 0, 2, 5, 7.5, 10 and 24°. The measurements indicate that low-angle grain boundaries are not the source of IMD in epitaxial films. We also report the microwave response of YBCO 24° bicrystal grain boundaries to small dc magnetic fields, from 0 to 10 Oe. Peaks in the microwave loss as high as 5 times the baseline values at certain dc magnetic fields are observed. The losses result from individual Josephson vortices penetrating into the grain-boundary which is modeled as a long Josephson junction. Excellent quantitative agreement between the experimental data and the model has been obtained for the case of the increasing field