VORTEX PINNING AND CREEP IN HIGH-TEMPERATURE SUPERCONDUCTORS WITH COLUMNAR DEFECTS

Columnar defects introduced by irradiation with very energetic heavy i ons are the most effective pinning centres for flux lines in high-temp erature superconductors. This correlated disorder generates large incr eases in the critical current densities and expansion of the irreversi ble regime in YBa2Cu3O7 and the various Bi- and Tl-based compounds. In YBa2Cu3O7 single crystals and thin films, the pinning enhancement is strongly angular-dependent, and maximizes when the applied magnetic fi eld is parallel to the amorphous latent tracks. In contrast, in the mu ch more anisotropic Bi- and Tl- based materials this unidirectional an isotropy is very small due to the quasi two-dimensional character of t he vortices. Some of the extensive experimental studies on this topic are reviewed. Measurements of the temperature, field and defect densit y dependence of the persistent currents and their time relaxation in Y BCO are presented. The analysis of these results based on recent theor etical models permits the identification of various pinning and creep regimes. Studies in other compounds, including technologically relevan t Bi-based tapes, are summarized. The influence of the angular dispers ion (splay) of the tracks on vortex pinning and creep is discussed.