Abstract
Monolayer FeSe films grown on SrTiO3 (STO) substrate show superconducting gap-opening temperatures (Tc) 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 Tc 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 Tc. Armed with these results, we return to the question “what makes the Tc of monolayer FeSe on SrTiO3 so high?” in the conclusion and discussions.
Highlights
Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.F
A definitive answer to ‘‘why Tc is so high in monolayer FeSe on SrTiO3?’’ requires one to (1) determine the intrinsic pairing mechanisms which is primarily responsible to Cooper pairing in heavily electron doped FeSe-based high temperature superconductors, and (2) pin down the effects of substrate
Regarding (2), our results show that small momentum transfer electron–phonon scattering enhances superconductivity regardless of whether it is triggered by the spin or orbital fluctuations, lend support to the phonon enhancement mechanism discussed in Refs. [15, 16]
Summary
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. Concerning the intrinsic pairing mechanism there are two main candidates: the spin [18,19,20,21,22,23] and orbital [24] fluctuations mediated pairing These proposals are based on approximations that are often not controlled in the presence of strong correlations. It turns out that the fermiology, namely the existence of two separate electron Fermi pockets, of (FeSe)1=STO allows the simulation to be free of the fermion minus sign problem This enables us to perform approximation-free unbiased study of the intrinsic electronic pairing mechanisms, namely, the antiferromagnetic (AFM) and antiferro-orbital (AFO) fluctuation mediated pairing.
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