Normal-State Hourglass Dispersion of the Spin Excitations in FeSe$_{x}$Te$_{1-x}$

TL;DR

This study reveals that in FeSe$_{x}$Te$_{1-x}$, incommensurate spin excitations form an hourglass shape in the normal state, independent of superconductivity, suggesting a non-superconducting origin related to Fermi surface nesting.

Contribution

It demonstrates that hourglass spin dispersion occurs in both superconducting and non-superconducting FeSeTe, indicating it's not directly linked to superconductivity and can be explained by Fermi surface nesting effects.

Findings

Hourglass dispersion observed in both SC and NSC FeSeTe.

Incommensurate spin excitations are not solely associated with superconductivity.

Results support a Fermi surface nesting model with low Fermi velocities.

Abstract

We use cold neutron spectroscopy to study the low-energy spin excitations of superconducting (SC) FeSe$_{0.4}$Te$_{0.6}$ and essentially non-superconducting (NSC) FeSe$_{0.45}$Te$_{0.55}$. In contrast to BaFe$_{2-x}$(Co,Ni)$_{x}$As$_2$, where the low-energy spin excitations are commensurate both in the SC and normal state, the normal-state spin excitations in SC FeSe$_{0.4}$Te$_{0.6}$ are incommensurate and show an hourglass dispersion near the resonance energy. Since similar hourglass dispersion is also found in the NSC FeSe$_{0.45}$Te$_{0.55}$, we argue that the observed incommensurate spin excitations in FeSe$_{1-x}$Te$_{x}$ are not directly associated with superconductivity. Instead, the results can be understood within a picture of Fermi surface nesting assuming extremely low Fermi velocities and spin-orbital coupling.