Electromagnetic, atomic-structure and chemistry changes induced by Ca-doping of low-angle $YB{a}_{2}C{u}_3{O}_{7-\delta }$ grain boundaries

Abstract

Practical high temperature superconductors must be textured to minimize the reduction of the critical current density $J_{gb}$ at misoriented grain boundaries. Partial substitution of Ca for Y in $YB{a}_{2}C{u}_3{O}_{7-\delta }$ has shown significant improvement in $J_{gb}$ but the mechanisms are still not well understood. Here we report atomic-scale, structural and analytical electron microscopy combined with transport measurements on $7^{\circ }$ $[001]$-tilt $Y_{0.7}C{a}_{0.3}B{a}_{2}C{u}_3{O}_{7-\delta }$ and $YB{a}_{2}C{u}_3{O}_{7-\delta }$ grain boundaries, where the dislocation cores are well separated. We show that the enhanced carrier density, higher $J_{gb}$ 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 $C{a}^{2+}$ solute atmospheres in the strain and electric fields of the grain boundary and show that Ca doping expands the dislocation cores yet enhances $J_{gb}$ by improving the screening and local hole concentration.

19 pages, 6 Figures