PINNING ENERGY IN HTSC AND ITS INFLUENCE ON ELECTRIC AND MAGNETIC-PROPERTIES

We investigated the voltage-current (V-1) characteristics in polycryst alline, melt textured an epitaxially grown Bi-2212 superconductors, in Bi-2223 melt textured Ag tapes and in Y-123 films. In all samples the V-I curves are well described by a power law behaviour V(is-proportio nal-to) I(P) over many orders of magnitude in the voltage V. With incr easing magnetic field and temperature the exponent p decreases with 1/ B from a maximum at p = 10 -20 down to the ohmic limit p = 1. In a dou ble logarithmic plot the V-I lines extrapolate to a common hinge point . These results are interpreted within the frame of thermally activate d flux creep. From the measurements on thin films and bulk of Bi2Sr2Ca Cu2Ox we derive an expression for an effective activation energy U(eff )(B,T,j) = 60 meV/B (1-T/T(c))5/2 In j0/j. We show that a number of ex perimental observations are in accordance with this explicit result. I n particular (a) exponential decay of the critical current density wit h field, (b) nonlogarithmic flux creep and (c) the irreversibility fie ld B(irr)(T) emerge as consequences of the specific functional depende nce of U(eff).