  A qualitative model describing the ground state and the mechanism of
superconducting pairing in Cu- and Fe-based high-temperature superconductors
(HTSCs) is suggested. In this model, doping by localized charges (as well as
physical or chemical pressure) is supposed to be responsible for transition of
Cu- and Fe-based HTSC to new ground state common for both HTSC classes where
specific mechanism of superconductivity takes place. The resulting HTSC ground
state is strongly correlated insulator with not fully filled exciton-electronic
band, where the incoherent electron transport is impossible but coherent
superconducting transport is possible because the band is not fully occupied.
It is shown also that such electronic system is inherently predisposed to
superconductive pairing because each pair of nearest cations acts as a two-atom
negative-U center. The nature of Fermi arcs and mechanism of pseudogap are
considered. It is shown that both of these features result from d-wave pairing
and therefore have to be observed only in cuprates. We believe that the
considered ground state is common for various families of HTSCs including
cuprates, pnictides, selenides, bismutates and probably some other.
