Aspects of electron–phonon interactions with strong forward scattering in FeSe Thin Films on SrTiO3substrates

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Abstract
Mono- and multilayer FeSe thin films grown on SrTiO 3 and BiTiO 3 substrates exhibit a greatly enhanced superconductivity over that found in bulk FeSe. A number of proposals have been advanced for the mechanism of this enhancement. One possibility is the introduction of a cross-interface electron–phonon (e–ph) interaction between the FeSe electrons and oxygen phonons in the substrates that is peaked in the forward scattering (small q ) direction due to the two-dimensional nature of the interface system. Motivated by this, we explore the consequences of such an interaction on the superconducting state and electronic structure of a two-dimensional system using Migdal–Eliashberg (ME) theory. This interaction produces not only deviations from the expectations of conventional phonon-mediated pairing but also replica structures in the spectral function and density of states, as probed by angle-resolved photoemission spectroscopy, scanning tunneling microscopy/spectroscopy, and quasiparticle interference imaging. We also discuss the applicability of ME theory for a situation where the e–ph interaction is peaked at small momentum transfer and in the FeSe/STO system.
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