Phonon mechanism explanation of the superconductivity dichotomy between FeSe and FeS monolayers on SrTiO3 and other substrates

It was observed recently [Shigekawa, PNAS 116, 2470 (2019)PNASA60027-842410.1073/pnas.1912836116] that while monolayer iron chalcigenide FeSe on a SrTiO3 (STO) substrate has a very high critical temperature, its chemical and structural twin material FeS/STO has a very low Tc, if any. To explain this, the substrate interfacial phonon model of superconductivity in iron chalcogenides is further developed. The main glue is the oxygen ion ωs=60meV vibrations longitudinal optical (LO) mode. The mode propagates mainly in the TiO2 layer adjacent to the monolayer (and also generally present in similar highly polarized ionic crystals like BaTiO3, rutile, and anatase). It has stronger electron-phonon coupling to electron gas in FeSe than a well-known ωh=100meV harder LO mode. It is shown that while (taking into account screened Coulomb repulsion effects) the critical temperature of FeSe on STO and TiO2 is above 65K, it becomes less than 5K for FeS due to two factors suppressing the electron-phonon coupling. The effective mass in the latter is twice smaller and, in addition, the distance between the electron gas in FeSe to the vibrating substrate oxygen atoms is 15% smaller than in FeS, reducing the central peak in electron-phonon interaction. The theory is extended to other ionic insulating substrates.