Influence of strain-driven segregation in low-angle grain boundaries on critical current density of Y_0.9Nd_0.1Ba₂Cu₃O_7-d

Low-angle grain boundaries (GBs) constitute the most important current-limiting mechanism in the operation of biaxially textured YBa₂Cu₃O_7−d (YBCO)-coated conductors. Ca doping of YBCO is known to improve the critical current density J_c across the GB because of carrier doping by anisovalent Ca²⁺ substitution for Y³⁺ and the strain relief induced by Ca segregation at the GB cores; however, the reduction of the superconducting critical temperature T_c accompanying such doping is a marked drawback. Here we study the substitution of isovalent Nd³⁺ for Y³⁺ again using strain-driven segregation, in this case Nd³⁺, to improve J_c without incurring significant T_c reduction. Transport characteristics of low-angle GBs of 10% Nd-doped YBCO, Y_0.9Nd_0.1Ba₂Cu₃O_7−d, grown on single crystal and 6^◦ and 9^◦ [001] tilt symmetric bicrystal SrTiO₃ substrates are reported. It was found that J_c across the 6^◦ GB recovers to the intra-grain J_c value in the 10% Nd-doped YBCO, while the 9^◦ GB shows a modest J_c enhancement compared to the pure YBCO 9^◦ GB without a significant T_c reduction. It is shown that the transparency of the GB could be enhanced without a large T_c reduction by the isovalent substitution of rare-earth ions, suggesting new opportunities for cation segregation engineering in YBCO by isovalent rare-earth substitution.