Electron phonon coupling versus photoelectron energy loss at the origin of replica bands in photoemission of FeSe on SrTiO$_3$

TL;DR

This paper demonstrates that the replica bands observed in ARPES experiments on FeSe/SrTiO$_3$ are primarily caused by extrinsic photoelectron energy loss processes involving substrate phonons, not by intrinsic electron-phonon coupling.

Contribution

It provides a semi-classical dielectric theory-based analysis showing that photoelectron energy loss explains replica bands, challenging previous interpretations of electron-phonon coupling effects.

Findings

Replica bands are mainly due to extrinsic photoelectron energy loss.

Photoelectron energy loss can be accurately modeled using dielectric theory.

Intrinsic electron-phonon coupling is not the primary cause of replica bands.

Abstract

The recent observation of replica bands in single-layer FeSe/SrTiO$_3$ by angle-resolved photoemission spectroscopy (ARPES) has triggered intense discussions concerning the potential influence of the FeSe electrons coupling with substrate phonons on the superconducting transition temperature. Here we provide strong evidence that the replica bands observed in the single-layer FeSe/SrTiO$_3$ system and several other cases are largely due to the energy loss processes of the escaping photoelectron, resulted from the well-known strong coupling of external propagating electrons to Fuchs-Kliewer (F-K) surface phonons in ionic materials in general. The photoelectron energy loss in ARPES on single-layer FeSe/SrTiO$_3$ is calculated using the demonstrated successful semi-classical dielectric theory in describing low energy electron energy loss spectroscopy of ionic insulators. Our result shows that the observed replica bands are mostly a result of extrinsic photoelectron energy loss and not a result of the electron phonon interaction of the Fe d electrons with the substrate phonons. The strong enhancement of the superconducting transition temperature in these monolayers remains an open question.