Loss of interplane correlation in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ single crystals

By means of dc magnetization and the ac response of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_6$ single crystals it is shown that at the dc irreversibility line the vortex system has no long-range order in the c direction. We find an energy dissipation peak at 7 Hz for interplane current that takes place at a temperature well below the irreversibility line. In this sense, the irreversibility line marks the temperature where quasi-two-dimensional vortices are depinned. The experimental data clearly show the different nature of two dissipation peaks in the susceptibility: one related to the interplane currents and the other associated with the intraplane ones.