Depth profiling of the microwave surface resistance of high-J_C GdBa₂Cu₃O_7-δ coated conductors grown using the RCE-DR process

We study depth profiling of the microwave surface resistance (R_S) of GdBa₂Cu₃O_7-δ (GdBCO) coated conductors (CCs) grown using the reactive co-evaporation by deposition and reaction (RCE-DR) method, a method enabling extremely high deposition rate. GdBCO CCs with the critical current (I_C) of more than 790 A cm^-1 at 77 K in self-field are used for the study. The R_S of the GdBCO CCs is measured at temperatures of 10-80 K using a 8.5 GHz TE₀₁₁-mode rutile resonator, which is compared with that of YBa₂Cu₃O_7-δ films and GdBCO films epitaxially grown on single crystal substrates. It turns out that there is significant inhomogeneity in the R_S over the thickness of the GdBCO layer, with the R_S value of the top part at 30 K being almost two times higher than the corresponding one of the bottom part. A transmission electron microscopy study reveals that Gd₂O₃ grains coexist with GdBCO grains with the average Gd₂O₃ grain sizes being ∼150 nm at the top and ∼100 nm at the bottom of the GdBCO layer. We relate the inhomogeneity in the R_S of the GdBCO layer with the positional dependence of the Gd₂O₃ grain size, for which effects of the dielectric losses from the Gd₂O₃ grains on the measured R_S of the GdBCO layer are considered. Our results imply that the critical current density, another important transport property of superconductors, could be inhomogeneous over the thickness of the GdBCO layer grown using the RCE-DR method.