  The nature of the normal state and the mechanism of superconductivity in two
families of high-temperature superconductors, cuprates and pnictides, remain a
matter of intense discussions. According to band-structure calculations,
confirmed by ARPES data, the electronic structure of pnictides differs
considerably from that of cuprates. However, in spite of these differences, it
looks like in both cases there exists some general and fairly "coarse"
mechanism independent of fine details of the band structure and responsible for
superconductive pairing in these materials. Here, we suggest a qualitative
model describing the ground state and the mechanism of superconductive pairing
in cuprates and pnictides that, in our opinion, can explain many of their
unusual properties.
