Effect of Sn substitution on the para-conductivity of polycrystalline Cu_0.5Tl_0.5Ba₂Ca₂Cu_3-ySn_yO_10-δ superconductors

High quality electrical resistivity ρ (T) versus temperature data of as-prepared and O₂ -annealed Cu_0.5 Tl_0.5 Ba₂ Ca₂ Cu_3-y Sn_y O _10-δ (y=0, 0.25, 0.5, 0.75, 1.0, 1.25, and 1.5) superconductors has been studied for fluctuation-induced phenomena setting in at temperatures well above the critical temperature [T_c (R=0)]. The analysis of the data is done by using Aslamazov-Larkin (AL) and Lawrence-Doniach models for the excess conductivity. We have estimated several physical parameters, including coherence length, interplane coupling strength, exponents, and dimensionality of the fluctuations. The as-prepared and oxygen postannealed samples have shown a cross-over temperature associated with two distinct exponents and the excess conductivity data fits well with the two-dimensional and three-dimensional AL equations. The coherence length along the c-axis [ℰ_c (0)] and the interlayer coupling strength (J) are found to decrease with increased Sn doping. These values are increased after annealing the samples in oxygen atmosphere, which is most likely associated with the approach of carrier concentration in the conducting CuO₂ / SnO₂ planes to the optimum value. The T_c (R=0) and the peak temperature (T_P) as determined from the dρ /dT versus temperature plots are also found to decrease with increased Sn substitution, however, these temperatures are improved to higher values after annealing the samples in oxygen atmosphere. The decreased values of T_c (R=0) and T_P with increased Sn substitution in the as-prepared samples are most likely arising from the suppression of carrier's density promoted by the increased volume of unit cell and the localization of the carriers at the Sn⁴⁺ sites.