Columnar defects and relaxation in Bi-2212 single crystals dual analysis by the single-barrier model and the Hagen and Griessen inversion scheme

Abstract

The activation energies in Bi-2212 single crystals before irradiation and after different fluences of Pb ions are derived from magnetic relaxation measurements. It is now well known that such heavy ions create in high- T c superconductors amorphous latent tracks which can act as very efficient pinning centers. The relaxation measurements are analyzed successively by the classical single-barrier model and by the Hagen and Griessen model which considers a distribution of activation energies. In this experiment, the weak reliability of the former model is clearly found evidence for. On the contrary, the HG model is shown to reasonably account for the expected modification on the distribution induced by the irradiation. In particular, the emergence of a peak at high energies can be observed in the distribution of the irradiated samples. The discussion of the magnetic field and fluence dependences of this peak leads to an estimation of 200 meV for the basic interaction energy between track and vortex. This value implies that the basic depinning process from a track does not involve only one pancake but rather a part of a vortex line which can be seen as a stacking of several pancakes i.e. a kind of “american pancake”.