Figure 1
Illustration: Alan Stonebraker

Figure 1: An artist’s rendering of the hole (c) and electron (d) pockets on a Fe-pnictide Fermi surface (FS). With moderately sized U and JHund found by Yang et al. [3] the main dynamical feature of Fe pnictides is the nesting tendency between these pockets at wave vector M=(π,π). The particle-hole interband repulsion W induces SDW while its particle-particle counterpart G2 favors a s± interband superconductor. Such superconductor comes in two varieties: in the type-A case both c and d pockets are superconducting on their own, with G2 providing an additional boost. The intrinsic type-B interband superconductivity is entirely due to G2 and has only a single order parameter Ψ, in contrast to the type-A.