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 electron doped FeSe films, and (2) examine the effects of electron–phonon interaction between FeSe and STO as well as nematic fluctuations on T c . Armed with these results, we return to the question ‘‘what makes the T c of monolayer FeSe on SrTiO 3 so high?’’ in the conclusion and discussions.

EB00000002094K

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