Paramagnetic phase of MnTe$_2$: complete one phonon Raman spectrum

Overview & Synopsis

In this study we present a complete first-order Raman scattering spectrum of paramagnetic phase of MnTe$_2$- a material, where new type of spin texture induced by noncoplanar antiferromagnetic ordering below the Néel temperature ($T_N \approx 90$ K) gives rise to unconventional scenario of magnetism. Basic properties of all five phonon modes ($A_g$, $E_g$ and three $T_g$) predicted by the group theory have been successfully approached through polarization, excitation and temperature dependent measurements. In particular, the ever-missing phonon mode of $T_g$ symmetry has been unambiguously resolved in the room temperature Raman spectrum at approximately 107 cm$^{-1}$, just 1.5 cm$^{-1}$ higher in frequency compared to the $E_g$ mode. Density functional theory calculations reproduce the observed Raman-active phonon spectrum and support the assignment of all five first-order Raman modes, including the near-degeneracy of the $E_g$ and one of the $T_g$ vibrations. The calculations further reveal a dense manifold of low-energy antiferromagnetic spin configurations, separated from the ground state by only a few meV per formula unit. Furthermore, strong signals of inelastically scattered light from the paramagnetic (PM) phase of MnTe$_2$ were acquired with long wavelength excitations, i.e. 633 nm and 785 nm, revealing the $2A_g$ overtone at approx. 360 cm$^{-1}$. The enhanced absorption of the light in the near infrared range could be a putative reason behind the anomalous angular dependence of scattered intensity of the highest frequency $T_g$ phonon mode (183 cm$^{-1}$) measured in PM MnTe$_2$ with parallelly aligned polarizers. Finally, upon cooling just below the $T_N$, the measured properties of phonons remained basically unperturbed by the incipient long-range antiferromagnetic ordering.