Evidence of non-linear to linear transformation of the normal state resistivity in YBCO at constant oxygen content

We studied the origin of variation in the magnitude and temperature dependence of the normal state resistivity frequently observed in different Y Ba₂Cu₃O_7-δ (YBCO) single crystal or thin film samples with the same T_C We investigated the temperature dependence of resistivity in YBCO thin films, which were subjected to annealing in argon at 120 - 140°C or aging at room temperature in air. Before annealing (or aging) these films exhibited a non-linear in-plane resistivity ρ_ab(T), with upward deviation (flattening) in ρ_ab(T) from the linear high temperature behavior below 220-230 K. For all films the annealing causes an increase of resistivity and a transformation of ρ_ab(T) from a non-linear dependence towards a more linear one (less flattening). Our proposed model provides explanation of these phenomena based on the thermally activated redistribution of residual O(5) oxygen in the chain-layer of YBCO without any change in the overall oxygen content within the sample. The results show that a perfectly linear ρ(T) can be observed in YBCO of oxygen content 7 - δ as low as 6.8. The agreement between the our data for ρ_ab(T, t) and the proposed model was very good.