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Temperature dependences of the critical current density and the pinning potential near the irreversibility temperature in YBa2Cu3O7- delta

In order to clarify the mechanism of the irreversibility line in high-Tc oxide superconductors, we measured temperature dependences of the critical current density, Jc, and the averaged pinning potential, Up, using an ac inductive method and investigated the flux motion in the pinning potential well in the vicinity of the irreversibility temperature, Tirr. From our experimental results, it has been found that Up decreases with increasing temperature and becomes nearly equal to the thermal activation energy, kB T, at Tirr. This result suggests that the irreversibility line is the thermally induced depinning line.