Coherent transition induced by Sr doping on the YBa₂Cu₃O_7-δ single crystal magnetoconductivity

We have studied by magnetoresistivity measurements the influence of the low Sr chemical doping on the conductivity thermodynamic fluctuation regimes (TFR) of a YBa_1.98Sr_0.02Cu₃O_7-δ single crystal. The low-current and low-frequency resistivity, ρ(T,H) measurements were performed in a PPMS while DC magnetic fields up to 50kOe were simultaneously applied perpendicular to the ab plane of the single crystal and to the measurement current density. The dρ(T,H)/dT data shows that the superconducting transition of the sample occur is two stages that corresponds to the signature of a granular superconductor. At this scenario is possible to define two critical temperature transitions T_C(H) and T_C0(H) [T_C(H) > T_C0(H)]. At T_C(H) the superconductivity sets in within the grains and at T_C0(H) a coherent transition takes place and a long-range superconducting order takes place over the whole sample. Special attention is reserved to the paracoherent region [T_C0(H) ≤ T < T_C(H)] of the ρ(T,H) data. The field and the temperature dependence of the paracoherent TFR behavior is characterized by a power law represented by a critical exponent S₀ ≈ 3. This exponent is an indicative that the coherence transition of the sample belongs to the 3D-XY universality class with relevant disorder that probably is associated to the low Sr doping