The pairing in moderately doped Fe-pnictides and Fe-chalcogenides is generally understood as being due to magnetically enhanced interaction between hole and electron pockets.
Recently, superconductivity has been observed in AFe2Se2 (A = K, Rb, Cs), which contain only electron pockets, and in KFe2As2, which contains only hole pockets.
In this talk I l review different (and sometimes conflicting) scenarios for the pairing in these systems and propose my own one. I argue that the pairing condensate in systems with only electron pockets necessary contains an inter-pocket component, made of fermions belonging to different electron pockets.
I analyze the interplay between intra-pocket and inter-pocket pairing, depending on the ellipticity of the electron pockets and the strength of their hybridization,
and show that, with increasing hybridization, the system undergoes a transition from a d-wave state to a novel s+- state, in which the gap changes sign between hybridized pockets.
This s+- state has the full gap and at the same time supports spin resonance, in agreement with the experimental data.
Near the boundary between d and s+- states the system develops s+id state which breaks time-reversal symmetry.
For systems with only hole pockets, I present arguments for an s+- state, in which the gap changes sign between hole pockets. I show that this state is qualitatively different from a "conventional" s+- state
in which the gap on hole pockets has the same sign. I further show that the transition from one s-wave state to the other involves highly unusual s+is state, which again breaks time reversal symmetry.