  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 $YBa_2Cu_3O_{7-\delta}$ 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
$Y_{0.7}Ca_{0.3}Ba_2Cu_3O_{7-\delta}$ and $YBa_2Cu_3O_{7-\delta}$ 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 $Ca^{2+}$ 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.
