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. The metallic and doped insulator approaches to high temperature superconductivity are essentially electronic ones, while in ferroelectrics atomic displacements play a key role. We show that pairing by contraction of in-plane Cu-O (or As-Fe) bonds, as proposed by the bond contraction pairing model, is prevented by the tensile strain generated by dislocations at grain boundaries. This explains why weak link behavior already sets in at low angle boundaries.