Flux line pinning by point defects in single crystalline high-T c superconductors

Abstract

The temperature and time dependence of the magnetization M of single crystalline high- T c YBa 2Cu 3O 7−δ and Bi 2Sr 2Ca Cu 2O 8+δ samples is analysed in order to determine critical magnetization currents j c as well as pinning potentials U. The evaluation of the experimental data within the framework of critical state and flux creep theories is performed under the assumption of a distribution of activation energies. These energy spectra, as well as the temperature dependence of the critical current density, turn out not to depend significantly on crystal orientations relative to the applied magnetic field. Flux bundle volumes V that are coherently moved in an activation process seem to be much smaller than the volume of a single flux line. From these results we conclude that the dominant pinning process in as-grown single crystalline high- T c superconductors is given by the interaction of flux line segments with point defects, located in the CuO planes. Furthermore, the energy spectra, resulting from evaluations considering first-order kinetics, are temperature independent and give a natural explanation for the increase of the effective potential U with temperature, if the barrier heights U are determined within a single barrier model.