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 c an be successfully fitted by the Larkin two-dimensional superconductin g 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 re lated to the giant conductivity fluctuations in these ceramic samples. The first scaling parameter A(T) is determined by the fluctuation amp litude. For x =0.1, the temperature dependence of A(T) follows the beh aviour 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 breakin g field, shows a good agreement with the the recent theoretical predic tion by Reizer.