Fluctuation-enhanced conductivity in the Sb-doped Bi-Pb-Sr-Ca-Cu-O superconducting system

Fluctuations above the transition temperature in the Sb-doped 2:2:2:3 Bi-Pb-Sr-Ca-Cu-O compounds have been studied. The excess conductivity above ${\mathit{T}}_{\mathit{c}}$ was determined from both the normal metal and the Anderson-Zou fits to the normal-state resistivity. With use of Aslamazov-Larkin theory, the dimensionality of the materials was determined. The value of the critical exponent (λ=-1) obtained from the experimental excess conductivity clearly indicates that the superconductivity in the Sb-doped (Bi,Pb)-based system is of a two-dimensional nature. From the fluctuation-enhanced conductivity, a coherence length of 7.9±0.8 Å was obtained for the samples, in good agreement with values obtained from magnetization curves of polycrystalline Bi-Sr-Ca-Cu-O superconductors. The fit of the Anderson-Zou model indicates that, although the crystallites in the polycrystalline samples are randomly oriented, the resistivity in the a-b plane dominates in the equivalent parallel resistor network.