Vortex-antivortex unbinding in oxygen-deficient YBa2Cu3O7−δ films

In-plane zero-magnetic-field current-voltage (I-V) characteristics of oxygen-deficient YBa2Cu3O7−δ films (7 − δ ∼6.5, 6.55, and 6.65) were thoroughly investigated. A good agreement with the quasi two-dimensional (2D) vortex-antivortex unbinding scenario was observed, similar to the behaviour of highly anisotropic Bi2Sr2CaCu2O8+δ films. The temperature variation of the I-V exponent allows the determination of the Berezinskii-Kosterlitz-Thouless transition temperature TKT at the superconducting Cu-O layer level, and the mean-field critical temperature Tc0. We found that both TKT and Tc0 increase with increasing doping, and Tc0 remains in the region of the electrical resistivity drop. These results do not support the vortex fluctuation scenario for the superconducting transition in underdoped cuprates.