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 YBa 2 Cu 3 O 7- x for which U ( T ) increases with T as U ( T, B ) = U 0 ( B ) + A ( B ) T 2 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.