Electromagnetic, atomic-structure and chemistry changes induced by Ca-doping of low-angle YBa₂Cu₃O₇₋δ grain boundaries

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Practical high temperature superconductors must be textured to minimize the reduction of the critical current density Jgb at misoriented grain boundaries. Partial substitution of Ca for Y in YBa₂Cu₃O₇₋δ has shown significant improvement in Jgb but the mechanisms are still not well understood. Here we report atomic-scale, structural and analytical electron microscopy combined with transport measurements on 7∘ [001]-tilt Y₀.₇Ca₀.₃Ba₂Cu₃O₇₋δ and YBa₂Cu₃O₇₋δ grain boundaries, where the dislocation cores are well separated. We show that the enhanced carrier density, higher Jgb and weaker superconductivity depression at the Ca-doped boundary result from a strong, non-monotonic Ca segregation and structural rearrangements on a scale of 1 nm near the dislocation cores. We propose a model of the formation of Ca²⁺ solute atmospheres in the strain and electric fields of the grain boundary and show that Ca doping expands the dislocation cores yet enhances Jgb by improving the screening and local hole concentration.