Flux-creep and depinning from columnar defects in layered superconductors

Localization of vortex lines on columnar defects introduced in BSCCO:2212 crystals by heavy ion irradiation is supposed to enhance the critical current Jc to the depairing limit . In experiments, the increase of the magnetic irreversibility is not accompanied by suppression of flux creep, even at low temperatures. This impedes the determination of Jc. Using the Hall-array technique, we have determined the dependence of the energy barrier for flux creep U on current density J at various temperatures and applied magnetic fields. On the basis of the functional form of U(J) dependence, we identify three regimes of flux creep: from linear, Kim-Anderson like, at high currents, to logarithmic at intermediate currents, and power law at low currents. Critical currents determined from U(J) variations, lie orders of magnitude below the depairing current, indicating that the mechanism of creep is different from the half-loop nucleation predicted in the Bose-glass model.