Ionic size effect due to a nonclassical percolation process on the superconductor-insulator transition in R1−xPrxBa2Cu3O7−δ (R=rare earth)

Abstract

The ionic size effect of rare earth R on the superconductor-insulator (S-I) transition in Pr-doped cuprates R1−xPrxBa2Cu3O7−δ (RPr−123) has been quantitatively investigated in terms of a nonclassical percolation process which includes two kinds of virtual percolation thresholds x1 and x2 originating from the classical percolation thresholds xpc1 and xpc2. Such virtual thresholds take values different from those of the classical ones, and have been experimentally observed for YPr-123. The nonclassical percolation process is based on a mechanism where the crystal unit cell of R−123 changes when adjoining unit cells of Pr-123 into a different cell that behaves as if it were a Pr-123 cell. This cell conversion needs an assist from the 4f(Pr)−2p(O) orbital hybridization, and creates the virtual threshold x2 corresponding to the S-I transition point. The cell conversion advances more easily as the ionic size of R increases, giving rise to the size effect. The model gives quantitative results for the S-I transition depending on the ionic size in agreement with experimental observations for a variety of doped cuprates RPr−123. This nonclassical percolation process involving the cell conversion will also change the effective area of the antiferromagnetic ordering of the magnetic moments of Pr3+.

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