Flux pinning in superconducting 123 materials

Mechanisms, features, and properties of strong flux pinning in superconducting 123 materials are reviewed. These include pinning by nonsuperconducting particles of 211 or 422 phase, regions with lower critical temperature (T-c) such as a region where Ba sites are substituted for with light rare-earth elements and an oxygen-deficient region, and radiation-induced columnar defects. In particular, the pinning by lower-T-c regions which seem to be associated with the well-known pronounced peak effect are focused upon. It is speculated that these regions work as strong repulsive pinning centres resulting from the kinetic energy of the order parameter under a significant proximity effect. This pinning mechanism is different from the usual attractive condensation energy interaction which occurs in 211 or 422 particles. A discussion will be given on the mechanism which brings about the peak effect. General guiding principles for the improvement of the flux pinning property will also be discussed.