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Origin of Weak-Link Behavior of Grain Boundaries in Superconducting Cuprates and Pnictides

Superconducting cuprates and pnictides composed of CuO2 or AsFe planes respectively with intercalated insulating layers, are at the crossroads of three families of crystalline solids: metals, doped Mott insulators, and ferroelectrics. In the latter atomic displacements play a key role. Both the metallic and the doped insulator approaches to high temperature superconductivity are essentially electronic ones and do not directly involve the lattice. By contrast, in a recently proposed Bond Contraction Pairing (BCP) model, contraction of in-plane Cu-O (or As-Fe) bonds plays an essential role in the pairing mechanism. Here we apply it to low angle grain boundaries and show that their reduced critical current is due to tensile deformation generated by dislocations. The model also explains why interface misfit dislocations, which can result in a dead layer in the case of ferro-electrics, may improve vortex pinning in the cuprates.