Monolayer FeSe films grown on SrTiO 3 (STO)
substrate show superconducting gap-opening temperatures
(T c ) which are almost an order of magnitude higher than
those of the bulk FeSe and are highest among all known
Fe-based superconductors. Angle-resolved photoemission
spectroscopy observed ‘‘replica bands’’ suggesting the
importance of the interaction between FeSe electrons and
STO phonons. These facts rejuvenated the quest for T c
enhancement mechanisms in iron-based, especially iron-
chalcogenide, superconductors. Here, we perform the first
numerically-exact sign-problem-free quantum Monte Carlo
simulations to iron-based superconductors. We (1) study
the electronic pairing mechanism intrinsic to heavily

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• What makes the T c of monolayer FeSe on SrTiO 3 so high: a sign-problem-free quantum Monte Carlo study