Spin-isotropic continuum of spin excitations in antiferromagnetically ordered Fe$_{1.07}$Te

TL;DR

This study reveals that Fe$_{1.07}$Te exhibits a spin-isotropic continuum of excitations above 20 meV, indicating quasi-degenerate magnetic states, which may be relevant for understanding unconventional superconductivity.

Contribution

It demonstrates the presence of an isotropic spin excitation continuum in Fe$_{1.07}$Te, suggesting quasi-degenerate magnetic ground states beyond simple spin wave descriptions.

Findings

Low energy excitations are transverse spin waves.

Magnetic scattering above 20 meV is isotropic and persists up to 50 meV.

Continuum may arise from quasi-degenerate magnetic states.

Abstract

Unconventional superconductivity typically emerges in the presence of quasi-degenerate ground states, and the associated intense fluctuations are likely responsible for generating the superconducting state. Here we use polarized neutron scattering to study the spin space anisotropy of spin excitations in Fe$_{1.07}$Te exhibiting bicollinear antiferromagnetic (AF) order, the parent compound of FeTe$_{1-x}$Se$_x$ superconductors. We confirm that the low energy spin excitations are transverse spin waves, consistent with a local-moment origin of the bicollinear AF order. While the ordered moments lie in the $ab$-plane in Fe$_{1.07}$Te, it takes less energy for them to fluctuate out-of-plane, similar to BaFe$_2$As$_2$ and NaFeAs. At energies above $E\gtrsim20$ meV, we find magnetic scattering to be dominated by an isotropic continuum that persists up to at least 50 meV. Although the isotropic spin excitations cannot be ascribed to spin waves from a long-range ordered local moment antiferromagnet, the continuum can result from the bicollinear magnetic order ground state of Fe$_{1.07}$Te being quasi-degenerate with plaquette magnetic order.