Superconducting clusters and phase separation in Pr1+xBa2-xCu3O7+delta: A (CU)-C-63,65 nuclear quadrupole resonance and zero-field NMR study

We present a study on the effect of the Pr3+ substitution for Ba2+ in the P r1+xBa2-xCu3O7+delta system, where small superconducting fractions have bee n observed in some of the samples with x in the range 0.1<x<0.6. Copper nuc lear quadrupole resonance (NQR) and zero-field NMR are reported for the x=0 .3 sample Pr1.3Ba1.7Cu3O7+delta in which the largest superconducting fracti on has been detected below 90 K. Cu-63,Cu-65 NQR in this sample shows the p resence of a fraction of metallic copper in the CuO2 planes and confirms th at the substitution of Pr onto Ba sites leads to oxygen interchain sites O( 5) occupancy. The existence of two different magnetic Cu(2) sites is eviden ced for x=0.3 from the comparison of the Cu(2) zero-field NMR in the x=0 an d x=0.3 samples. The whole set of results is interpreted in the framework o f a phase separation mechanism in the CuO2 planes induced by the Ba/Pr subs titution. Three characteristic domains are involved, in which respectively localized holes are present, no holes are transferred, and mobile holes res ide. We present a model where the observed superconductivity is due to a se gregation of local defects in the structure which gives a distribution of c lusters containing mobile holes. A local weakening of the Pr-4f-O-2p hybrid ization by the Pr/Ba substitution is argued to explain the presence of thes e clusters.