Out-of-plane Ionicity versus In-plane Covalency Interplay in High-Tc Superconducting Oxides

Abstract: It seems that the remarkable properties of the high temperature superconducting oxides, especially the Insulator-Metal Transition (IMT) and the Metal-Superconductor Transition (MST) both originate from the competition (interplay) between ionic versus in-plane covalence nature of bonds in these materials. As a result of this competition, the microscopic order parameter, that is firmly identified to be the local field estimated from the ionic polarization at the sub-unit level (one half of the unit cell), shows a strong temperature as well as chemical doping dependence. While the out-of-plane ionicity is responsible for the interlayer charge transfer (electrons) that reduces it leading to IMT, the in-plane covalency is responsible for the in-plane intersite transfer of charge (holes) that increases the ionicity and leads to MST. This interplay of charge transfer, driven by the out-of-plane ionicity and the in-plane covalency, leads at the critical temperature Tc to a local field avalanche (ionic polarization catastrophe at the sub-unit cell level) that triggers an out-of-plane correlation between the local polarization fields at the unit cell and drives the compound into the superconducting state. The asymmetry of the free charge carrier density breaks locally the inversion symmetry of the order parameter leading to an unconventional pairing mechanism characterized by the CPT transformation where C is the charge conjugation symmetry, P is the parity symmetry and T is the time reversal symmetry. It is established that the out-of-plane ionicity versus in-plane covalency is a necessary condition for the occurrence of both IMT and MST. The underlined microscopic mechanism provides a route to a possible unified description of high-Tc superconductivity.