Aspects of electron-phonon interactions with strong forward scattering in FeSe Thin Films on SrTiO$_3$ substrates

TL;DR

This paper investigates how a forward-scattering electron-phonon interaction influences superconductivity and electronic structure in FeSe thin films on SrTiO3, revealing unique spectral features and discussing theoretical applicability.

Contribution

It models the effects of a peaked forward-scattering electron-phonon interaction on superconductivity in FeSe/STO using Migdal-Eliashberg theory, highlighting spectral signatures and theoretical considerations.

Findings

Forward-scattering interaction causes spectral replica features.

Enhanced superconductivity linked to interface electron-phonon coupling.

Discussion on Migdal-Eliashberg theory's applicability in this context.

Abstract

Mono- and multilayer FeSe thin films grown on SrTiO$_\mathrm{3}$ and BiTiO$_\mathrm{3}$ 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 ${\bf q}$) 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 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 quasi-particle interference imaging. We also discuss the applicability of Migdal-Eliashberg theory for a situation where the \ep interaction is peaked at small momentum transfer and in the FeSe/STO system.