Quantum Monte Carlo study of pairing in systems with incipient bands*

The nature and mechanism of superconductivity in the extremely electron-doped iron-chalcogenides continues to be a matter of wide debate. In these systems, the hole-like band has dropped below the Fermi energy, challenging the leading spin-fluctuation based theory of pairing that relies on the presence and nesting properties of both electron- and hole-Fermi surface pockets. Here, we use dynamic cluster quantum Monte Carlo calculations of a bilayer Hubbard model to gain insight into this problem. These calculations show that s^+- pairing remains robust and dominant over d-wave pairing, even when the hole band does not cross the Fermi energy. The gap on the submerged band has the opposite sign and larger magnitude than the gap on the electron band. Consistent with previous phenomenological calculations, this demonstrates that an incipient band can have a decisive effect on the pairing symmetry of the Fermi surface states.