Anomalous scaling of ΔCversusTcin the Fe-based superconductors: the ${S}_{\pm }$-wave pairing state model

For a newly discovered iron-based high \(T_c\) superconductor LaFeAsO\(_{1-x}\)F\(_x\), we have constructed a minimal model, where inclusion of all the five Fe \(d\) bands is found to be necessary. Random-phase approximation is applied to the model to investigate the origin of superconductivity. We conclude that the multiple spin fluctuation modes arising from the nesting across the disconnected Fermi surfaces realize an extended s-wave pairing, while d-wave pairing can also be another candidate.

We argue that the newly discovered superconductivity in a nearly magnetic, Fe-based layered compound is unconventional and mediated by antiferromagnetic spin fluctuations, though different from the usual superexchange and specific to this compound. This resulting state is an example of extended s-wave pairing with a sign reversal of the order parameter between different Fermi surface sheets. The main role of doping in this scenario is to lower the density of states and suppress the pair-breaking ferromagnetic fluctuations.