Interpocket Pairing and Gap Symmetry in Fe-Based Superconductors with Only Electron Pockets

Abstract

We analyze the pairing symmetry in Fe-based superconductors AFe2Se2 (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.

The orbital characters of low-energy electronic structure in iron-chalcogenide superconductor K x Fe2−y Se2

“…One of the two bands is dominated by the d xz and d yz orbitals, while the other one is dominated by the d xy orbital. The weak interactions between these two Fermi cylinders suggest that even if the superconducting order parameters on them have opposite signs, gap nodes would be absent [30,31]. Our results provide a foundation for the understanding and realistic modeling of K x Fe 2−y Se 2 .…”

Multiband Superconductivity in KFe2As2: Evidence for One Isotropic and Several Lilliputian Energy Gaps

“…This hypothetical change from s-to d-wave symmetry as a function of doping is allowed theoretically via an intermediate s + id state that breaks time-reversal symmetry. [10][11][12][13] On the other hand, laser ARPES 14 have revealed the existence of accidental line nodes on only one of the zone-centered pockets in KFe 2 As 2 , which is only compatible with a nodal s± state. * E-mail: frederic.hardy@kit.edu † Present address: Department of Low Temperature Physics, Charles UniHowever, none of these methods are bulk probes of the superconducting state.…”

Probing the pairing symmetry in the over-doped Fe-based superconductorBa0.35Rb0.65Fe2As2as a function of hydrostatic pressure

“…Recently, we have shown unambiguous evidence for the appearance of SC nodes in optimally-doped Ba 1−x Rb x Fe 2 As 2 upon applied pressure, consistent with a change from a nodeless s +− -wave state to a d-wave state 39 . Interestingly, the theoretical calculations [41][42][43][44][45][46][47] as well as Raman experiments 48,49 revealed a sub-dominant d-wave state close in energy to the dominant s +− state. It seems that pressure affects this intricate balance, and tip the balance in favor of the d-wave state.…”