Nature of the 175-180 cm${}^{-1}$ Raman Feature in Altermagnetic $\alpha$-MnTe

Abstract

The 175 cm${}^{-1}$ Raman mode in $\alpha$-MnTe has long been assigned to the material's only symmetry-allowed phonon ($E_{2g}$), yet first-principles calculations consistently place this mode below 100 cm${}^{-1}$. Recent works have offered three competing explanations for the discrepancy: a weak symmetry-lowering ($D_{6h}\to D_{3h}$) phonon, an electronic plasmon arising from intrinsic hole self-doping, or an extrinsic MnTe${}_2$ impurity signature. Here we resolve this controversy using stoichiometry-controlled molecular beam epitaxy (MBE). We deliberately vary the fraction of pyrite-type MnTe${}_2$ secondary phase relative to the $\alpha$-MnTe matrix. Across this series, the 175 cm${}^{-1}$ mode tracks the presence of MnTe${}_2$ essentially one-to-one -- present wherever MnTe${}_2$ is detectable, absent in stoichiometric, single-phase $\alpha$-MnTe at nominal LASER powers. When LASER power is ramped up, an additional mode emerges in MnTe as well positioned close to $\approx$ 175 cm${}^{-1}$; our detailed temperatures dependendent Raman analysis identifies the LASER power scorched region close to a distorted MnTe phase, confirmed via transmission electron microscopy. This direct, growth-controlled correlation identifies the mode as an extrinsic MnTe${}_2$ impurity signature or as a distorted MnTe feature, rather than an intrinsic phonon or plasmon feature, providing a practical diagnostic for phase purity in MBE-grown $\alpha$-MnTe and clarifying which Raman features can be reliably attributed to the intrinsic altermagnetic phase.