Investigation of Ybco Step-Edge Josephson Junctions

We report on the superconducting transport properties of YBa2Cu307Josephson junctions fabricated by pulsed laser deposition on steep steps in epitaxial substrates. The steps were prepared by Ar-ion milling. The YBa2Cu307 thin films were patterned either by Ar-ion milling or an inhibit process. The current-voltage characteristics of step-edge junctions (SEJ) fit approximately the RSJ (Stewart-McCumber) model up to temperatures of 85K. The temperature dependence of the characteristic voltage can be explained by a SNS-type model. Simulations of Shapiro steps were performed. The simulations, the form of the I(V) curves, the IC(H) curves and the dc-SQUID quantization properties all suggest that a SEJ consists of two weak links in series formed by the grain boundaries at the lower and upper edges of the steps. This conclusion is in good agreement with HREM results, which show that the upper and lower grain boundaries at the step edge are different. Each of these weak links is a parallel array of Josephson junctions with different current densities.