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

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.

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• Quasiparticle interaction originating from Bogoliubov Fermi Surfaces under pressure in 18%-S substituted FeSe studied via NMR
• Three-dimensional spin susceptibility in Ba${}_{0.75}$K${}_{0.25}$Fe${}_2$As${}_2$: Out-of-plane modulation revealed by neutron spectroscopy and theoretical modeling