  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.
