We investigate pairing mechanism in multiband superconductors. To put our feet on firm ground, unbiased renormalization group analysis is carried out for iron-based superconductors. It is quite remarkable that, after integrating out quantum fluctuations, the renormalization-group flows agree exceedingly well with a mean-field Hamiltonian where interband pair hopping plays an essential role. Through interband pair hopping, electrons can overcome the repulsive interaction between them and form resonating Cooper pairs between different bands. Unlike the conventional superconductors, the pairing mechanism in multiband superconductors is resonating pair hopping between different bands, just like the resonating chemical bonds in benzene. The effective mean-field Hamiltonian spots a small parameter dictating the critical temperature and also explains how interband pair hopping always enahnces spin fluctuations at the nesting momentum connecting the Fermi surfaces. In short, no attractive glue is needed and resonating interband pair hopping is the key to Cooper pair formation in unconventional superconductors. Implications to cuprates and related issues are also discussed at the end.
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