Non-ohmic I-V behaviour in granular YBa₂Cu₃O_7-δ and (Bi, Pb)₂Sr₂Ca₂Cu₃O₁₀ high-T_c superconductors

Current-voltage measurements performed on granular YBa₂Cu₃O_7-δ and (Bi, Pb)₂Sr₂Ca₂Cu₃O₁₀ high-T_c superconducting samples below their transition temperatures revealed a power law (V = αI^β) over a wide range of applied voltage. The exponent β decreases linearly with increasing temperature, deviating from linearity at temperature T_nl lower than T_c. Moreover the resistivity (ρ) exhibits a slope change in a log(ρ)-log(1 - T/T_c) plot at a temperature T* < T_c. We argue that T_nl and T* may reflect changes in the dynamics of the intergranular (or Josephson) vortices just below the superconducting transition temperatures.