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

Abstract

Pairing symmetry in recently discovered Fe-based metallic superconductors AFe$_2$Se$_2$ (A = K, Rb, Cs) with high transition temperature $T_c \sim 40$ K is currently a subject of intensive debates. These systems contain only electron pockets, according to photoemission, and differ from the majority of Fe-based superconductors in which both electron and hole pockets are present. Both d-wave and s-wave pairing symmetries have been proposed for AFe$_2$Se$_2$, but a d-wave gap generally has nodes, while experiments clearly point to no-nodal behavior, and a conventional s-wave gap is inconsistent with the observation of the neutron resonance below $T_c$. We argue that current theories of pairing in such systems are incomplete and must include not only intra-pocket pairing condensate but also inter-pocket condensate made of fermions belonging to different electron pockets. We analyze the interplay between intra-pocket and inter-pocket pairing depending on the ellipticity of electron pockets and the strength of their hybridization and show that hybridization brings the system into a new $s^{+-}$ state, in which the gap changes sign between hybridized pockets. This state has the full gap and at the same time supports spin resonance, in agreement with the data. Near the boundary of $s^{+-}$ state we found a long-thought $s+id$ state which breaks time-reversal symmetry.