Frequency dependence of the depinning and irreversibility lines in BSCCO

Penetration of AC magnetic field in Bi₂Sr₂CaCu₂O_δ single crystals was studied using the magnetooptic technique. It was found that the apparent depinning line shifts toward higher (H, T) values with increasing field frequency. At higher frequencies irreversibility in magnetization is controlled by pinning but not by the geometrical or surface barrier. Comparison with the melting line has shown that the depinning line lies completely in the vortex liquid region. Moreover, at least in low DC fields, the vortex liquid does not become unpinned up to the critical temperature of superconductor.