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

High-resolution electrical resistivity ρ(T) data of as-prepared and O2-annealed Cu0.5Tl0.5Ba2Ca1.5M1.5Cu4O12−δ (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 O2-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(ε)<−3.76 and −4.76<ln(ε)<−1.63 in the Mg- and Be-doped Cu0.5Tl0.5Ba2Ca1.5M1.5Cu4O12-δ (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 (Tc) varies from 99 to 116 K and the transition widths (ΔTc), 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.