VORTEX LINES IN LAYERED SUPERCONDUCTORS .2. PINNING AND CRITICAL CURRENTS IN HIGH-TEMPERATURE SUPERCONDUCTORS

In this article, a qualitative survey is given on the various phenomen a which influence the critical current of high temperature superconduc tors. Critical current is defined as a property related to a non-zero electric field criterion, the level of which is fixed by experimental considerations, or efficiency requirements of applications. The presen tation is restricted to extrinsic intragranular critical current, whic h depends in a complex way on the interplay between the characteristic s of pinning centres and the properties of the vortex lattice. The dis cussion is focussed on the configuration B // c, which contains the ma in elements of this problem. Differences of behaviour between Y(123) a nd BSCCO (Bi(2212) or Bi(2223)) are analysed in the context of their r espective anisotropy factors. Possible regimes for pinning and creep a re discussed in various temperature domains. From critical current res ults, a strong pinning regime is found to occur in BSCCO, whereas the pinning strength in Y(123) is still an open question. The thermal decr ease of critical current allows a collective creep regime to appear in both materials, but at different temperature ranges. The disappearanc e of correlation effects near the irreversibility line results in a fa ll of the effective pinning energy. We show that in BSCCO, the effecti ve pinning energy deduced from experimental results is not in agreemen t with pinning by randomly dispersed oxygen vacancies. Finally, we sho rtly describe the microstructures which could allow a more efficient p inning in future materials.