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

Monolayer FeSe films grown on SrTiO(3) (STO) substrate show superconducting gap-opening temperatures ([Formula: see text] ) 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 [Formula: see text] 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 [Formula: see text] . Armed with these results, we return to the question “what makes the [Formula: see text] of monolayer FeSe on SrTiO(3) so high?” in the conclusion and discussions. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11434-016-1087-x) contains supplementary material, which is available to authorized users.