Iron-based superconductors were discovered seven years ago, in 2008. This short review summarizes what we learned about these materials over the last seven years, what are open questions, and what new physics we expect to extract from studies of this new class of high-temperature superconductors.
We consider the evolution of d-wave pairing, mediated by nearly critical spin fluctuations, with the coupling strength. We show that the onset temperature for pairing, T*, smoothly evolves between weak and strong coupling, passing through a broad max
imum at intermediate coupling. At strong coupling, T* is of order the magnetic exchange energy J. We argue that for all couplings, pairing is confined to the vicinity of the Fermi surface. We also find that thermal spin fluctuations only modestly reduce T*, even at criticality, but they substantially smooth the gap anisotropy. The latter evolves with coupling, being the largest at weak coupling.
