  It was observed recently (K. Shigekawa et al, PNAS 116, 2470 (2019)) that
while monolayer iron chalcigenide FeSe on 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 (60mev) vibrations longitudinal optical (LO) mode. The
mode propagates mainly in the TiO2 layer adjacent to the monolayer (and
genrally present also in similar highly polarized ionic crystals like BaTiO3;
rutile, anatase). It has stronger electron - phonon coupling to electron gas in
FeSe than a well known (100mev) harder LO mode. It is shown that while (taking
into account screened Coulomb repulsion efects) 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 efective mass in the
later 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
reducinng the central peak in electron-phonon interaction. The theory is
extended to other ionic insulating substrates.
