Transformation of in-plane ρ(T) in YBa₂Cu₃O_7-δ at fixed oxygen content

This paper reveals the origin of a variation in magnitude and temperature dependence of the normal-state resistivity frequently observed in different YBa₂Cu₃O_7-δ (YBCO) single-crystal or thin-film samples with the same T_c. We investigated temperature dependence of resistivity in YBa₂Cu₃O_7-δ thin films with 7-δ between 6.80 and 6.95, which were subjected to annealing in argon at 400–420 K (120–140°C) or aging at room temperature in air. Before annealing (or aging) these films exhibited a nonlinear ρ_ab(T), with a flattening [superlinear ρ_ab(T)] below 220–230 K, similar to ρ_b(T) and ρ_ab(T) observed in untwinned and twinned YBCO crystals, respectively. For all films the annealing causes an increase of resistivity and a transformation of ρ_ab(T) from a nonlinear (superlinear) dependence towards a more linear one (less flattening). For films annealed in argon with 7-δ=6.90 the increase of resistivity is also associated with an increase in T_c. We proposed a model that provides an explanation of these phenomena in terms of thermally activated redistribution of residual O(5) oxygens in the chain layer of YBCO. A good agreement between the experimental data for ρ_ab(t,T), where t is the annealing time, and numerical calculations was obtained.