# Phonon linewidth due to electron-phonon interactions with strong forward scattering in FeSe thin films on oxide substrates

## TL;DR

This paper investigates how strong forward-scattering electron-phonon interactions in FeSe thin films affect phonon linewidths, revealing that Coulomb screening mitigates broadening and challenging expectations of anomalous phonon behavior.

## Contribution

It demonstrates that Coulombic screening reduces phonon linewidth broadening caused by forward-scattering electron-phonon interactions in FeSe interfaces.

## Key findings

- Strong forward scattering causes phonon linewidth broadening.
- Coulomb screening significantly reduces phonon broadening.
- Anomalously broad phonon lines are unlikely despite strong forward e-ph interactions.

## Abstract

The discovery of an enhanced superconducting transition temperature $T_c$ in monolayers of FeSe grown on several oxide substrates has opened a new route to high-$T_c$ superconductivity through interface engineering. One proposal for the origin of the observed enhancement is an electron-phonon (e-ph) interaction across the interface that peaked at small momentum transfers. In this paper, we examine the implications of such a coupling on the phononic properties of the system. We show that a strong forward scattering leads to a sizable broadening of phonon lineshape, which may result in charge instabilities at long-wavelengths. However, we further find that the inclusion of Coulombic screening significantly reduces the phonon broadening. Our results show that one might not expect anomalously broad phonon linewidths in the FeSe interface systems, despite the fact that the e-ph interaction has a strong peak in the forward scattering (small $q$) direction.