MAGNETORESISTANCE AND CONDUCTIVITY FLUCTUATIONS IN (TL0.5PB0.5)SR2(CA1-XYX)CU2O7 SUPERCONDUCTORS

We have studied magnetoresistance R(H) of the (Tl0.5Pb0.5)Sr2(Ca1-xYx)Cu2O7 high-T, superconductors, with x = 0.1, 0.2, 0.3, 0.4 at T < T(c) in a magnetic field H up to 12 T. For H > 1T, the magnetoresistance can be successfully fitted by the Larkin two-dimensional superconducting fluctuation expression with only two adjustable parameters (A(T) and H(phi) (T) ) per each R vs H curve. This indicates that a large part of the resistive transition broadening in applied magnetic field is related to the giant conductivity fluctuations in these ceramic samples. The first scaling parameter A(T) is determined by the fluctuation amplitude. For x =0.1, the temperature dependence of A(T) follows the behaviour of the beta(T) function introduced by Larkin. For larger x, the deviation of A(T) from beta(T) is observed which may be attributed to an additional scattering caused by the Y3+ --> Ca2+ substitution. The second parameter H(phi), corresponding to the phase coherence breaking field, shows a good agreement with the the recent theoretical prediction by Reizer.