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- T-c oxide superconductors, we measured temperature dependences of the critical current density, J(c), and the averaged pinning potential, U- p, using an ac inductive method and investigated the flux motion in th e pinning potential well in the vicinity of the irreversibility temper ature, T-irr. From our experimental results, it has been found that U- p decreases with increasing temperature and becomes nearly equal to th e thermal activation energy, k(B)T, at T-irr. This result suggests tha t the irreversibility line is the thermally induced depinning line.