High-temperature superconductivity in single unit cell layer FeSe due to soft phonons in the interface layer of the SrTiO3 substrate

Using a microscopic model of lattice vibrations in the STO(001) substrate, an additional ωs=50 meV longitudinal optical (LO) interface mode is identified. The soft mode propagating mainly in the first TiO2 layer ("O chains") has stronger electron-phonon coupling to electron gas in FeSe than a well-known ωh=100 meV hard mode. The coupling constant, critical temperature, and replica band are calculated. Although there exists a forward in the electron-phonon scattering peak, it is clearly not as sharp as assumed in recent theories (delta-function-like). The critical temperature is obtained by solution of the gap equation and agrees with the observed one. The corresponding electron phonon coupling constant λ=0.23. The quasiparticle normal state "satellite" in spectral weight is broad and its peak appears at frequency much higher than ωs consistent with observations usually associated with ωh. Possible relation of the transversal counterpart of the surface LO soft mode with known phonons is discussed.