Many superconductor applications rely on the ability to pin vortex lattices. This ability depends on structural defects to pin individual vortices, but vortex-vortex interactions often play an important role in pinning the other vortices. Experimental progress on this complex problem can be made by introducing random arrays of well-defined pinning centers and studying the flux dynamics for a range of vortex densities (i.e., fields). The results of such studies in epitaxial Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub y/ films containing ion tracks show the importance of vortex-vortex interactions. As an example, the effective pinning field of the defects rises to many times the ion-dose field for temperatures well below T/sub c/.

The effects of columnar defects on electrical transport measurements in Tl2Ba2CaCu2O8 epitaxial films were found to persist up to magnetic fields of at least forty times the vortex matching field, that is, the field at which the vortex density corresponds to the density of columnar defects. This demonstrates the importance of interactions between vortices and is in contrast to experiments that suggest that the matching field has sharp significance.