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.