Revisiting the symmetry and optical phonons of altermagnetic $α$-MnTe

Abstract

Using infrared (IR) and Raman spectroscopies combined with high-resolution x-ray diffraction, we address several controversial aspects of altermagnetic $α$-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 $α$-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 $α$-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 $Γ$-point optical phonons. These modes couple to magnetic order in $α$-MnTe and also to the transient reflectivity resulting in coherent oscillations. Both 6-fold rotation symmetry and inversion symmetry are preserved in $α$-MnTe within our experimental resolution.

Figures (7)

• Figure 1: High-resolution XRD results for $\alpha$-MnTe. (a,b) Temperature dependence of the lattice parameters shows a magnetoelastic effect at $T_N$. (c) Temperature dependence of the atomic displacement parameters $U_{\rm iso}$, the lines are guides for the eye. (d) 101 Bragg peak measured on samples with different level of grinding, increasing from 1 to 4: non-ground polycristalline sample (1), sample gently ground for 30 sec (2), sample gently ground for 3 min (3), crushed and thoroughly ground single crystal (4). (e) Magnified view of the base of the same 101 Bragg peak reveals tiny impurity peaks of MnTe$_2$ and Te that are observable in a gently ground sample only. (f) 203 Bragg peak in sample 2 measured at 100 K and 330 K shows persistent hexagonal symmetry below $T_N$. (g) Rietveld refinement for the XRD data collected at 150 K (sample 2). Tick marks show the Bragg peak positions for $\alpha$-MnTe.
• Figure 2: (a) Representative Raman spectra for two $\alpha$-MnTe crystals. The inset shows the crystal S1, where two different regions, POS1 and POS2, can be distinguished. S1-POS2 is where we observe the 175 and 180 cm$^{-1}$ modes in addition to the regular modes of $\alpha$-MnTe. (b) Background-subtracted spectra measured on S1-POS1 with two different laser powers. The 175 cm$^{-1}$ mode along with several higher-energy modes appear when higher laser power is used.
• Figure 3: (a) Background-subtracted temperature-dependent Raman spectra measured at S1-POS2. Asterisks label the MnTe$_2$-related features that can be identified via their temperature dependence. (b) Polarization-dependent spectra show that the 175 cm$^{-1}$ mode disappears in cross-polarization, whereas the 180 cm$^{-1}$ mode weakens in the co-polarization configuration, thus confirming the assignment to the $A_g$ and $T_g$ modes of MnTe$_2$, respectively. (c,d) Temperature-dependent resonance frequencies of the MnTe$_2$-related modes. The magnon mode in (c) shows an order parameter-like behavior (solid line) with the critical temperature of $\sim$90 K, which is very close to the Néel temperature of 87 K in MnTe$_2$. Both $A_g$ and $T_g$ modes show weak anomalies around this temperature, while they do not exhibit any abrupt changes at 307 K, the critical temperature for $\alpha$-MnTe.
• Figure 4: (a) Temperature-dependent Raman spectra measured on the sample S2. The inset shows temperature dependence of the electrical resistivity measured on both crystals. (b-e) Resonance frequencies and scattering rates of the modes R1--R4. All modes are affected by the magnetic ordering, and small anomalies are observed across. The additional mode at $\sim$190 cm$^{-1}$ appearing at low temperatures is tentatively assigned to 2E$_{2g}$.
• Figure 5: Room-temperature transient reflectivity spectrum of $\alpha$-MnTe (sample S2). Short delay times reveal coherent phonon oscillations. The Fourier-transformed signal shown in the inset displays two frequencies that correspond to the R3 and R4 Raman modes, respectively.