Revisiting the symmetry and optical phonons of altermagnetic $\alpha$-MnTe

TL;DR

This study clarifies the optical phonon modes of altermagnetic $ ext{MnTe}$, resolves previous controversies, and demonstrates the coupling of certain phonons to magnetic order, using combined spectroscopic and computational methods.

Contribution

The paper provides a comprehensive analysis of the optical phonons in $ ext{MnTe}$, identifying intrinsic modes and their coupling to magnetic order, resolving prior ambiguities caused by secondary phases and stress effects.

Findings

IR-active optical phonon around 155 cm$^{-1}$ identified

Raman-active optical phonon around 100 cm$^{-1}$ established

Coupling of specific phonons to magnetic order demonstrated

Abstract

Using infrared (IR) and Raman spectroscopies combined with high-resolution x-ray diffraction, we address several controversial aspects of altermagnetic $\alpha$-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 $\alpha$-MnTe are not reproducible across different positions and samples and originate from the secondary phase of MnTe$_2$. By combining spectroscopic probes with ab initio calculations, we establish the IR-active optical phonon of $\alpha$-MnTe around 155 cm$^{-1}$ ($E_{1u}$) and the Raman-active optical phonon around 100 cm$^{-1}$ ($E_{2g}$) 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 $\Gamma$-point optical phonons. These modes couple to magnetic order in $\alpha$-MnTe and also to the transient reflectivity resulting in coherent oscillations. Both 6-fold rotation symmetry and inversion symmetry are preserved in $\alpha$-MnTe within our experimental resolution.