Critical Conductivity Fluctuations of $\hbox {YBa}_{2}\hbox {Cu}_{2.985}\hbox {Fe}_{0.015}\hbox {O}_{7-\delta }$ Single Crystal

We have studied experimentally the in-plane conductivity fluctuations, Δσ(T), of a YBa₂(Cu_1-xFe_x)₃O_7-δ (x=0.005) single crystal. Different DC magnetic fields (H ≤ 500 Oe) and current densities (J ≤ 209 A/cm^-2) intensities were applied parallel to the CuO₂ (ab) planes at H||J||ab field-current configuration. Close to the superconducting transition, the data reveal the occurrence of a genuine critical regime, where the associated critical exponent is consistent with the predictions of the 3D-XY universality class with model-E dynamics. Decreasing further the temperature towards the transition temperature, T_C, our results systematically show very small exponent values that could be associated to the occurrence of a supercritical regime beyond the 3D-XY scaling. This regime can be interpreted as revealing an ultimate first order character of the superconducting transition driven by antiferromagnetic fluctuations related to the pseudogap phenomenon. The data show that the applied magnetic fields cause an increase in the average exponent related to the supercritical scaling. Unlike the magnetic field, the current density does not affect the supercritical regime. The results indicated that the trivalent Fe doping in YBa₂Cu₃O_7-δ improves the stability of the beyond 3D-XY conductivity fluctuation regime to H ≤ 500 Oe. Also, a further approach towards the first order transition behavior is observed.