Out‐of‐plane ionicity versus in‐plane covalency interplay in high‐ T c cuprates

It may be argued that the remarkable properties of the high‐temperature superconducting cuprates such as the insulator–metal transition (IMT) and the metal–superconductor transition (MST) originate from competition and interplay between the interlayer ionic interaction and the intralayer covalent bonds in these materials. It is proposed here that the microscopic order parameter is the local field estimated from the ionic polarization at the sub‐unit cell level, and it is demonstrated that it shows a strong temperature as well as chemical doping dependence. The out‐of‐plane ionicity induces an interlayer electron transfer that reduces the ionicity of the layers and leads to IMT, while the in‐plane covalency induces in‐plane intersite hole transfer that increases the out‐of‐plane ionicity. It is suggested that this competition leads to a local field catastrophe at a critical temperature T c that drives the compound to MST. The asymmetry of the free charge carrier density breaks locally the mirror reflection symmetry of the order parameter, leading to a pairing between the real current and the polarization current.