Thermal depinning of flux lines in superconductors

The thermal depinning of flux lines on the irreversibility line is investigated from a viewpoint of thermodynamics using the coherent potential approximation theory, a kind of statistical theory for flux pinning. The effect of thermal activation is approximately introduced by making the pinning potential shallow following the theoretical scheme by Yamafuji et al. It is shown that the thermal depinning is the transition of the second order. Since the vortex glass-liquid transition, which is considered to be identical with the thermal depinning, is originally predicted to be the second-order transition, the present theoretical result is reasonable. The present result also suggests that the irreversibility line observed at higher temperatures and at lower fields than the critical point for weakly pinned superconductors is also the transition line of the second order. In addition, it is shown that the degree of disorder of the flux line lattice decreases fast, suggesting a quick recovery of the order of the lattice with elevating temperature above the critical temperature.