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

We present a study on the effect of the Pr3+ substitution for Ba2+ in the Pr1+xBa2-xCu3O7+delta system, where small superconducting fractions have been observed in some of the samples with x in the range 0.1<x<0.6. Copper nuclear quadrupole resonance (NQR) and zero-field NMR are reported for the x=0.3 sample Pr1.3Ba1.7Cu3O7+delta in which the largest superconducting fraction has been detected below 90 K. Cu-63,Cu-65 NQR in this sample shows the presence of a fraction of metallic copper in the CuO2 planes and confirms that 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 evidenced for x=0.3 from the comparison of the Cu(2) zero-field NMR in the x=0 and x=0.3 samples. The whole set of results is interpreted in the framework of a phase separation mechanism in the CuO2 planes induced by the Ba/Pr substitution. Three characteristic domains are involved, in which respectively localized holes are present, no holes are transferred, and mobile holes reside. We present a model where the observed superconductivity is due to a segregation of local defects in the structure which gives a distribution of clusters containing mobile holes. A local weakening of the Pr-4f-O-2p hybridization by the Pr/Ba substitution is argued to explain the presence of these clusters.