  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.
