Mystery of the 175 cm$^{-1}$ Raman Mode in MnTe Altermagnet

TL;DR

The paper addresses a Raman peak around $175~\mathrm{cm}^{-1}$ in MnTe that challenges the conventional $E_{2g}$ phonon interpretation and a proposed symmetry-lowering leakage. It combines first-principles density functional theory, Placzek Raman activity calculations, and a $k\cdot p$ plasmon model to evaluate competing explanations. The results rule out symmetry-lowering leakage as the origin and show that a hole self-doped plasmon, with energies in the $\sim 170$–$320~\mathrm{cm}^{-1}$ window after screening, is a plausible alternative, with Raman activity expected in XX polarization. If validated experimentally, this finding links the Raman feature to electronic transport in MnTe and provides a new angle on hole-doping effects in altermagnetic MnTe.

Abstract

MnTe has recently attracted exceptional attention due to its well-established altermagnetism, prompting a thorough reexamination of its properties. In particular, it was found that a Raman-active excitation at ~175 cm$^{-1}$, routinely assigned to the E2g phonon, is incompatible with this interpretation. It was further hypothesized that this mode is a "leakage", due to symmetry lowering, of an otherwise forbidden phonon. Here, using first-principles calculations, we decisively rule out this hypothesis and propose an alternative interpretation that the "mystery mode" is an electronic excitation, i.e., a plasmon, enabled by hole self-doping. The resolution of this mystery will require additional experiments and shed new light on the nature of electronic transport in MnTe.

Figures (3)

• Figure 1: Symmetry lowering $B_{1u}$ mode applied to parent $P6_3/mmc$.
• Figure 2: Dielectric derivatives $d\varepsilon_{\alpha\beta}/dQ$ for the $A_1'$ Raman mode of MnTe ($P\bar{6}m2$, $D_{3h}$). (a) In-plane response $d\varepsilon_{xx}/dQ$. (b) Out-of-plane response $d\varepsilon_{zz}/dQ$. In each panel, the upper subplot shows the real and imaginary parts of $d\varepsilon_{\alpha\beta}/dQ$, while the lower subplot shows $|d\varepsilon_{\alpha\beta}/dQ|^2$, which is proportional to the Raman intensity. By hexagonal symmetry, $\varepsilon_{xx}=\varepsilon_{yy}$; the cross-polarization components vanish for the totally symmetric $A_1'$ mode, consistent with the experimental observation that this mode is active only in parallel (XX) polarization geometry. The out-of-plane ($zz$) response is significantly weaker than the in-plane components.
• Figure 3: Plasma frequencies $\omega_\parallel$ (blue circles) and $\omega_\perp$ (orange squares) as a function of the hole concentration $p$ at $T=150$ K, calculated assuming $\varepsilon=\varepsilon_0$.