Excess-conductivity analysis of Mg- and Be-doped polycrystalline Cu0.5Tl0.5Ba2Ca1.5M1.5Cu4O12-δ (M=0, Be, Mg) superconductors

High-resolution electrical resistivity rho(T) data of as-prepared and O-2-annealed Cu0.5Tl0.5Ba2Ca1.5M1.5Cu4O12-delta (M=0, Be, Mg) superconductor samples have been taken for investigating critically the superconducting fluctuations. Using the Aslamazov-Larkin (AL) and Lawrence-Doniach (LD) models of excess conductivity, several physical parameters of the models have been estimated. In all the as-prepared and oxygen postannealed samples one crossover temperature and two distinct exponents have been observed. The excess conductivity data of the as-prepared as well as O-2-annealed undoped sample seem to fit well two-dimensional (2D), three-dimensional (3D) AL equations, but for Mg- and Be-doped and their respective oxygen annealed samples do not seem to fit well with 2DAL and 3DAL equations with one distinct crossover temperature. We observed two more critical regions: -4.86 < ln(epsilon)<-3.76 and -4.76 < ln(epsilon)<-1.63 in the Mg- and Be-doped Cu0.5Tl0.5Ba2Ca1.5M1.5Cu4O12-delta (M=0, Be, Mg) samples, giving exponents of -0.37 and -0.79, respectively. The interlayer coupling strength J is found to increase with increased Mg or Be content and oxygen postannealing. The critical temperatures (T-c) varies from 99 to 116 K and the transition widths (Delta T-c), estimated from dp/dT(T) plots by using the full width at half maximum of the peaks, come out to be lying between 1.91 and 6.28 K for all samples, indicating a sharp transition.