Evidence for paramagnon-phonon coupling in the inelastic x-ray spectra of altermagnetic MnF₂

Altermagnets are a recently identified class of materials that exhibit a spin-split valence band without net magnetization. However, detecting the underlying multipolar order that drives this behavior is challenging. The paramagnon-polaron mode is a predicted key experimental signature of altermagnets theorized to emerge from a linear piezomagnetic coupling between lattice excitations and mixed multipolar order. To investigate this coupling, we performed inelastic x-ray scattering (IXS) experiments on altermagnetic candidate MnF₂ and searched for this hybrid mode. Near the Neel temperature, we observe an emergent low-energy mode in the IXS spectra indicative of acoustic phonon-spin mixing. Notably, this feature is observed in the Gamma→M reciprocal space direction but not along Gamma→X. I will discuss how these findings relate to predictions of d-wave altermagnetism.