Identify the origin of the high-energy ζ excitation

Determine whether the ζ feature observed in Fe L3-edge resonant inelastic x-ray scattering from Li0.5Fe2.5O4 is a high-energy optical phonon mode rather than a two-magnon excitation.

Background

The paper examines magnetic excitations in the spinel ferrites Li0.5Fe2.5O4 (LFO) and Li0.5AlFe1.5O4 (LAFO) using Fe L3-edge resonant inelastic x-ray scattering (RIXS). In the LFO spectrum, the authors identify several features, including the ζ mode at approximately 245 meV and the δ mode at approximately 120 meV.

Because the ζ feature occurs at roughly twice the energy of the putative single-magnon δ feature, the authors suggest that it may be a two-magnon excitation. However, its assignment remains unresolved because it could instead be a high-energy optical mode. The paper restricts its subsequent analysis to the putative single-magnon δ mode and does not resolve the nature of ζ.

References

Mode ζ appears at an energy approximately twice that of δ, indicating it is potentially a two-magnon excitation similar to multi-magnon features reported in other magnetic insulators ; we do not presently rule out, however, that it is a high energy optical mode.

— Divergence between long- and short-wavelength magnon damping in spinel ferrites  (2609.19112 - Parzyck et al., 16 Sep 2026) in Results and discussion, paragraph introducing the ζ mode following Fig. 2