CHI''-PEAKS, FLUX-PINNING, AND THE IRREVERSIBILITY LINE IN HIGH-TEMPERATURE SUPERCONDUCTORS

Physical processes that produce various peaks in the adsorption component chi'' of the AC susceptibility of high-temperature superconductors are described. It is shown that for good YBa2Cu3O7-x crystals twinning planes are the main source of flux pinning and the primary determinant of the reported irreversibility line (IL) position in the H-T plane. In other situations, particularly ones involving weaker pinning, the apparent IL is found to lie much lower in the H-T plane. Ambiguous results from experiments designed to produce pinning sites by irradiation are discussed in the context to this new observation.