Entangled Orbital Triplet Pairs in Iron-Based Superconductors

A key question in high temperature iron-based superconductivity is the mechanism by which the paired electrons minimize their strong mutual Coulomb repulsion. While electronically paired superconductors generally avoid the Coulomb interaction through the formation of nodal, higher angular momentum pairs, iron based superconductors appear to form singlet s-wave (s$^{\pm}$) pairs. By taking the orbital degrees of freedom of the iron atoms into account, here we argue that the s$^{\pm}$ state in these materials possesses internal d-wave structure, in which a relative d-wave ($L=2$) motion of the pairs entangles with the ($I=2$) internal angular momenta of the d-orbitals to form a low spin $J=L+I=0$ singlet. We discuss how the recent observation of a nodal gap with octahedral structure in KFe$_{2}$As$_{2}$ can be understood as a high spin ($J=L+I=4$) configuration of the orbital and isospin angular momenta; the observed pressure-induced phase transition into a fully gapped state can then interpreted as a high-to-low spin phase transition of the Cooper pairs.