Interpocket pairing and gap symmetry in Fe-based superconductors with only electron pockets

We analyze the pairing symmetry in Fe-based superconductors AFe ₂Se ₂ (A=K, Rb, Cs) which contain only electron pockets. We argue that the pairing condensate in such systems contains not only intrapocket component but also interpocket component, made of fermions belonging to different electron pockets. We analyze the interplay between intrapocket and interpocket pairing, depending on the ellipticity of electron pockets and the strength of their hybridization. We show that with increasing hybridization, the system undergoes a transition from a d-wave state to an 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 data. Near the boundary between d and s ^+- states, we found a long-sought s+id state which breaks time-reversal symmetry.