Revisiting the symmetry and optical phonons of altermagnetic α -MnTe

Using infrared and Raman spectroscopies combined with high-resolution x-ray diffraction, we address several controversial aspects of altermagnetic α -MnTe. We show that mechanical stress applied to crystals of this material causes a drastic broadening of Bragg peaks that conceals signatures of additional phases present in the sample. Indeed, spatially resolved Raman spectroscopy reveals that the modes around 175 cm − 1 often reported in α -MnTe are not reproducible across different positions and samples and originate from the secondary phase of MnTe 2 . By combining spectroscopic probes with calculations, we establish the IR-active optical phonon of α -MnTe around 155 cm − 1 ( E 1 u ) and the Raman-active optical phonon around 100 cm − 1 ( E 2 g ) at room temperature. Two intense Raman modes around 120 and 140 cm − 1 are shown to be intrinsic, even though they can not be assigned to Γ -point optical phonons. These modes couple to magnetic order in α -MnTe and also to the transient reflectivity, resulting in coherent oscillations. Both sixfold rotation symmetry and inversion symmetry are preserved in bulk α -MnTe within our experimental resolution.