Speaker
Description
Mn$_3$Si$_2$Te$_6$ single crystals were first synthesized in 1985 [1], however, few studies were carried out on this compound since. It was only recently that the attention has shifted to them, mainly through the comparisons with quasi-two-dimensional materials, specifically CrSiTe$_3$. Layered magnetic van der Waals materials have lately received widespread attention due to their relevnce for spintronics, magneto-electronics and data storage.
Mn$_3$Si$_2$Te$_6$ crystalizes in a trigonal $P \overline{3} 1c$ crystal structure (No. 163 space group) [2]. First principle calculations suggested a competition between ferrimagnetic ground state and three additional magnetic configurations, originating from antiferromagnetic exchange for the three nearest Mn-Mn pairs [2]. Here we present a first principle study with the focus on the phonon properties [3]. We compare our computational results with experimental Raman scattering of Mn$_3$Si$_2$Te$_6$ single crystals. Eighteen Raman-active modes are identified, fourteen of which are assigned according to the trigonal symmetry. Five A$_{1g}$ modes and nine E$_g$ modes are observed and assigned according to the $P \overline{3} 1c$ symmetry group. Four additional peaks to the ones ascribed to the $P \overline{3} 1c$ symmetry group and obeying the A$_{1g}$ selection rules, are attributed to overtones. A pronounced asymmetry of the A$^5_{1g}$ phonon mode is evidenced at 100K and 300 K. The unconventional temperature evolution of the A$^5_{1g}$ Raman mode reveals three successive, possibly magnetic, phase transitions that are expected to have significant impact on the strength of the spin-phonon interaction in Mn$_3$Si$_2$Te$_6$. These are suggested to be caused by the competition between the various magnetic states, which are close in energy.
This study provides a comprehensive insight into the lattice properties of the considered system and shows arguments for the emergence of competing short-range magnetic phases in Mn$_3$Si$_2$Te$_6$.
The calculations are based on the density functional theory formalism as implemented in the Vienna Ab-initio Simulation Package (VASP) [4-7], with the plane wave basis truncated at a kinetic energy of 520 eV, using the Perdew-Burke-Ernzehof (PBE) exchange-correlation functional [8] and the projector augmented wave (PAW) method [9,10]. The Monkhorst and Pack scheme of the k point sampling is employed to integrate over the first Brillouin zone with 12×12×10 at the Γ-centered grid. The convergence criteria for energy and force have been set to 10$^{−6}$ eV and 0.001 eVÅ$^{−1}$, respectively. The DFT-D2 method of Grimme is employed for van der Waals corrections [11]. The vibrational modes are calculated applying the density functional perturbation theory implemented in VASP and Phonopy [12].
