Splitting of the pairing transition and coherence transition in R_1−xCe_xBa₂Cu₃O_7−δ(R = Y, Er, Ho and Gd)

Abstract

Systematic conductivity measurements in granular R1−xCexBa2Cu3O7−δ (R = Y, Er, Ho and Gd) superconductors are presented. The focus is the role of the cerium ion on the superconductor–normal transition. The results showed that the resistive transition proceeds in two stages, intragranular and intergranular, as seen by the temperature derivative of the resistivity near TC. Therefore, from the critical exponent analysis, we have also observed two close and sharp genuine superconducting transitions in all samples investigated. Two critical temperatures were identified, but only one is strongly affected by Ce doping. The splitting of the pairing transition was associated with Ce and related to the occurrence of a phase separation. R1−xCexBa2Cu3O7−δ can be regarded as a mixed crystalline solid solution consisting of two crystal unit cells: R-123 (R cell) and Ce-123 (Ce cell), where a two-dimensional (2D) site-percolation structure is built along the ab plane. The ionic size effect of rare earth element R on the superconductor–normal transition has been investigated and it was observed that the temperature splitting Δ varies between approximately 4.2 K for Ho to 12.1 K for Gd.