Increased percolation threshold for 100-K superconductivity in lead-substituted Bi-Sr-Ca-Cu-O thin films

In lead-substituted Bi-Sr-Ca-Cu-O thin films (1 {μ}m thickness), containing the (2:2:1:2) and the (2:2:2:3) phases in variable proportions, we observe that zero resistance above 100 K is obtained only if the proportion of the (2:2:2:3) phase exceeds {~}50%. This result contrasts with the situation found in the lead-free thin films, for which zero resistance above 100 K can be achieved with the (2:2:2:3) phase constituting less than 10% of the sample. This difference is explained on the basis of modifications induced by lead substitution in the structures and in the intergrowth of the considered phases, which determine a two-dimensional type of percolation for the conductivity of the Pb-substituted thin films, while the other system can be associated with a three-dimensional type of percolation, having a much lower threshold.