Ferromagnetic inter-layer coupling in FeSe$_{1-x}$S$_{x}$ superconductors revealed by inelastic neutron scattering

TL;DR

This study reveals significant ferromagnetic inter-layer coupling in FeSe$_{1-x}$S$_{x}$ superconductors through inelastic neutron scattering, challenging the view of these materials as purely layered van der Waals systems.

Contribution

It uncovers ferromagnetic inter-layer coupling in FeSe$_{1-x}$S$_{x}$, providing new insights into their magnetic interactions and the nature of their superconductivity.

Findings

Inter-layer magnetic coupling affects excitations up to 15 meV.

The coupling is ferromagnetic, unlike in most unconventional superconductors.

Superconductivity is more robust in the two-dimensional limit.

Abstract

FeSe$_{1-x}$S$_{x}$ superconductors are commonly considered layered van der Waals materials with negligible inter-layer coupling. Here, using inelastic neutron scattering to study spin excitations in single-crystal samples, we reveal that the magnetic coupling between adjacent Fe layers is not only significant, as it affects excitations up to \textcolor{black}{15} meV, but also ferromagnetic in nature, making the system different from most unconventional superconductors including iron pnictides. Our observation provides a new standpoint to understand the absence of magnetic order in FeSe$_{1-x}$S$_{x}$. Since intercalating between the Fe layers is known to enhance superconductivity and suppress the inter-layer coupling, superconductivity appears to be a more robust phenomenon in the two-dimensional limit than antiferromagnetic order.