Defects, defect ordering, structural coherence and superconductivity in the 123 copper oxides

We discuss the influence of oxygen defects and defect ordering on superconductivity in 123 compounds. Because of the complexity of the high-temperature phase diagram, the synthesis history can significantly influence the properties. Room-temperature annealing experiments show that defect ordering on a short length scale can have a remarkable affect on superconductivity. We propose that structural coherence in the CuO₂ planes, which is present only if there is orthorhombic symmetry on at least a short length scale, is a requirement for superconductivity in these compounds.