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

TL;DR

This paper investigates an anomalous Raman mode in MnTe, concluding it is an electronic plasmon related to hole doping, challenging previous phonon-based interpretations and advancing understanding of MnTe's electronic properties.

Contribution

The study provides a first-principles analysis that identifies the 175 cm$^{-1}$ Raman mode as a plasmon, offering a new perspective on MnTe's electronic excitations.

Findings

The 175 cm$^{-1}$ mode is not a phonon but an electronic plasmon.

Symmetry lowering does not explain the mode as previously hypothesized.

Hole self-doping enables the electronic excitation observed in Raman spectra.

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.