Strong $(\pi,0)$ spin fluctuations in $\beta$-FeSe observed by neutron spectroscopy

TL;DR

This study uses neutron spectroscopy to reveal strong, dispersive $( ext{pi},0)$ spin fluctuations in $eta$-FeSe, indicating proximity to antiferromagnetic instability without significant temperature dependence across the structural transition.

Contribution

It provides direct experimental evidence of $( ext{pi},0)$ spin fluctuations in $eta$-FeSe, highlighting its closeness to antiferromagnetic order and independence from structural phase changes.

Findings

Strong dispersive paramagnetic fluctuations at $( ext{pi},0)$

Fluctuations extend beyond 80 meV in energy

No temperature dependence from $T_c$ to 104 K

Abstract

We have performed powder inelastic neutron scattering measurements on the unconventional superconductor $\beta$-FeSe ($T_{\rm c} \simeq 8\,\mathrm{K}$). The spectra reveal highly dispersive paramagnetic fluctuations emerging from the square-lattice wave vector $(\pi,0)$ extending beyond 80 meV in energy. Measurements as a function of temperature at an energy of $\sim 13\,\mathrm{meV}$ did not show any variation from $T_{\rm c}$ to $104\,\mathrm{K}$. The results show that FeSe is close to an instability towards $(\pi,0)$ antiferromagnetism characteristic of the parent phases of the high-$T_{\rm c}$ iron arsenide superconductors, and that the iron paramagnetic moment is neither affected by the orthorhombic-to-tetragonal structural transition at $T_{\rm s} \simeq 90\,\mathrm{K}$ nor does it undergo a change in spin state over the temperature range studied.