  Electrical transport measurements using a multiterminal configuration are
presented, which prove that in BSCCO single crystals near the transition
temperature in zero external magnetic field the secondary voltage is induced by
thermally activated vortex loop unbinding. The phase transition between the
bound and unbound states of the vortex loops was found to be below the
temperature where the phase coherence of the superconducting order parameter
extends over the whole volume of the sample. We show experimentally that 3D/2D
phase transition in vortex dimensionality is a length-scale-dependent layer
decoupling process and takes place simultaneously with the 3D/2D phase
transition in superconductivity at the same temperature.
