Effects of columnar defects on the superconducting properties of Y1Ba2Cu3O7−δ: Limits to the irreversibility line

A series of YBa2Cu3O7−δ(YBCO) epitaxial films and crystals were irradiated with energetic heavy ions (Sn30+ or Ag29+ at energy 0.58 GeV) to produce columnar defects of highly disordered material. The irreversibility lines, B∗(T), show enhancement due to the increased pinning. However, while B∗ does not exceed that observed in the highest-Jc as-grown epitaxial thin films, the values approach the proposed vortex melting line determined by a number of experiments on nearly defect-free single crystals. These results are interpreted in terms of the predicted upper limit of pinning energies in anisotropic superconductors, and the thermally-induced decoupling of vortex segments.