PERCOLATION AND FLUX CREEP IN HIGH-TEMPERATURE SUPERCONDUCTORS

Thermally activated magnetic flux creep in superconductors has been interpreted as a percolation of magnetic flux in a random pinning potential for which the resistivity and flux creep activation energy U have been calculated by the effective medium theory. We also present measurements of U at 4.2 K < T < 90 K and 0 < B < 12 T on grain-oriented YBa2Cu3O7-x for which U(T) increases with T as U(T, B) = U0(B) + A(B) T2 at low T with a maximum U(m)(B) at some T = T(m)(B). The percolation model and the obtained scaling of U(T, B) are in good agreement with our and other experimental data on magnetic relaxation in high-T(c) oxides. The effect of the topology of vortex percolation channels on U(T, B) obtained by resistive magnetic and noise measurements is discussed.