Abstract
Superconducting samples of type Y3Ba5Cu8O18-delta substituted by both Nd3+ and Ca2+ ions were prepared by a solid-state reaction technique. These substitutions affect the phase formation, lattice parameters, grains formations and superconducting transition temperature T-c. The change in these parameters is mainly due to the partial substitution of Ba2+ by Ca2+ ions rather than the partial substitution of Y3+ by Nd3+ ions. The analysis of the electrical resistivity rho(T) versus temperature above T-c was carried out using Aslamazov-Larkin (AL) and Lawrence-Doniach (LD) models. Different fluctuation regions starting from high temperature up to T-c were estimated, and denoted by short-wave (sw), two-dimensional (2D), three-dimensional (3D) and critical (cr) fluctuations. A distinct crossover temperature (T2D-3D = T-LD) from 2D to 3D, in the mean field region (MFR), is shifted to lower temperature by increasing substitutions content. The superconducting parameters such as zero temperature coherence length along the c-axis xi(c)(0), inter-layer coupling strength nu, Fermi velocity V-F of the carriers and Fermi energy E-F were calculated as a function of substitution content.