Length-scale-dependent phase transition in Bi₂Sr₂CaCu₂O₈single crystals

Electrical transport measurements using a multi-terminal configuration are presented, which prove that in Bi2Sr2CaCu2O8 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.