Abrupt formation of isolated superconducting droplets in heavily disordered cuprates

The effect of controlled disorder on the superconducting transition temperature, T sub(c), resistivity, and the magnitude of the field-cooled ac susceptibility (ACS) has been investigated for Ca and Zn substituted Y123 (Y sub(0.80)Ca sub(0.20)Ba sub(2)(Cu sub(1-y)Zn sub(y)) sub(3)O sub(7_ delta )) sintered compounds over a wide range of compositions. Ca was used to explore the overdoped side. The in-plane hole content, p, was changed by varying the oxygen deficiency ( delta ). Irrespective of the hole content, T sub(c) decreased almost linearly with Zn, though the rate of suppression was strongly p dependent. The magnitude of the low-field, field-cooled ACS, on the other hand, decreased abruptly and significantly for the heavily disordered (y > 0.05) sintered compounds when p was decreased. More strikingly, the qualitative features of the field dependent ACS signal became identical for sintered and powdered samples, indicating a complete absence of coupling among superconducting grains in heavily disordered (Y sub(0.80)Ca sub(0.20)Ba sub(2)(Cu sub(1-y)Zn sub(y)) sub(3)O sub(7_ delta )). A non-monotonic and pronounced enhancement of the residual resistivity for samples with y > 0.05 lends further support to this picture. The abruptness of this 'isolated superconducting droplet' behavior points toward possible interplay among various length scales. We discuss the implications of these findings in detail in this paper.