Non-ohmic resistance characteristic of inhomogeneous structure in Pr-doped cuprate Nd-123

Electrical characteristics of the Pr-doped cuprate Nd-123 (NdPr-123) have been investigated in reference to an inhomogeneous structure consisting of Nd-123 and Pr-123 phases. The superconductor-insulator (S-I) transition takes place there when Pr-concentration x exceeds a threshold point xc ∼ 0.32. At x just below the threshold point xc, the resistance has been found to become highly non-ohmic, appearing as an increasing function of the current I. In contrast, at x much higher than xc, the resistance turns into a decreasing function of I. The non-ohmic resistance having such dual aspects was observed also for another series of samples of NdPr-123 that was prepared in a different way and had a much lower threshold xc ∼ 0.14. For this degraded samples, the resistance becomes non-ohmic more easily, indicating that the structural inhomogeneity is closely related to the non-ohmic behaviour. The non-ohmic resistance can be explained in terms of the current distribution governed by a percolation process pertain to the inhomogeneous structure consisting of unit cells of Nd -123 and Pr-123