The phonon mechanism explanation of the superconductivity dichotomy between FeSe and FeS monolayers on STO and other substrates

Abstract

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.

References in corpus (16)

• Near-degeneracy of several pairing channels in multiorbital models for the Fe-pnictides
• Structure determination and coexistence of superconductivity and antiferromagnetic order in (Li0.8Fe0.2)OHFeSe
• Superconductivity at 32 K in single crystal Rb${}_{0.78}$Fe${}_2$Se${}_{1.78}$}
• On the Remarkable Superconductivity of FeSe and its Close Cousins
• Universal Substrate Effect on the Superconductivity of FeSe Monolayer Films
• New iron-based arsenide oxides (Fe2As2)(Sr4M2O6)(M = Sc, Cr)
• Topological Phases in the Single-Layer FeSe
• Interface high-temperature superconductivity
• Evolution of spin excitations from bulk to monolayer FeSe
• Dichotomy of superconductivity between monolayer FeS and FeSe
• Lattice Dynamics of Ultrathin FeSe Films on SrTiO3
• Eliashberg theory for spin-fluctuations mediated superconductivity -- Application to bulk and monolayer FeSe
• The electron-phonon interaction with forward scattering peak is a relevant approach to high Tc superconductivity in FeSe films on SrTiO3 and TiO2
• Multichannel superconductivity of monolayer FeSe on SrTiO${}_3$: Interplay of spin fluctuations and electron-phonon interaction
• Enhanced superconductivity in FeSe/SrTiO${}_3$ from the combination of forward scattering phonons and spin fluctuations
• Origin of the Electronic Structure in Single-Layer FeSe/SrTiO3 Films