  The nature and mechanism of superconductivity in the extremely electron-doped
FeSe based superconductors continues to be a matter of debate. In these
systems, the hole-like band has moved below the Fermi energy, and various
spin-fluctuation theories involving pairing between states near the electron
Fermi surface and states of this incipient band have been proposed. Here, using
a dynamic cluster quantum Monte Carlo calculation for a bilayer Hubbard model
we show that the pairing in these systems can be understood in terms of an
effective retarded attractive interaction between electrons near the electron
Fermi surface.
