CaBaCuO Ultrathin Films and Junctions

We investigated transport properties of ultrathin CaBaCuO (CBCO) bridges, focusing on vortex dynamics, and Josephson bicrystal junctions. The CBCO films used are composed of a stacking sequence of two structural subunits having different functions, namely a Ba-based charge reservoir (CR) block and a Ca-based infinite layer (IL) superconducting block. SQUID microscopy investigations of these films provide Pearl lengths more than the bridge widths. Measuring current-voltage characteristics we found evidence that at high temperatures the vortices are thermally activated at the edge of the bridge and pushed in, causing dissipation and a power-law dependence of the voltage from the current. At lower temperatures, the experimental curves change their shape. We compared these data with the predicted behavior for vortex quantum tunneling and we found very good agreement. Moreover, we have realized Josephson junctions composed of only a few superconducting CuO₂ planes. The CBCO films used in these experiments are only 8 nm thick and have six superconducting CuO₂ planes. The simple structure of the GB allows a reliable estimation of the coupling along ab the planes of the CuO₂. We calculate for 24deg symmetric and asymmetric cases a critical current density per plane of about (1-3) kA/cm².