Raman scattering study of two-dimensional magnetic van der Waals compound VI3 Project supported by the Science Fund from the Ministry of Science and Technology of China (Grant Nos. 2017YFA0302904 and 2016YFA0300504), the National Natural Science Foundation of China (Grant Nos. 11774419, U1932215, 11774423, and 11822412), the Fundamental Research Funds for the Central Universities, China, and the Research Funds of Renmin University of China (RUC) (Grant Nos. 15XNLQ07, 18XNLG14, and 19XNLG17)

The layered magnetic van der Waals materials have generated tremendous interest due to their potential applications and importance in fundamental research. Previous x-ray diffraction (XRD) studies on the magnetic van der Waals compound VI3, revealed a structural transition above the magnetic transition but output controversial analysis on symmetry. In this paper we carried out polarized Raman scattering measurements on VI3 from 10 K to 300 K, with focus on the two Ag phonon modes at ∼ 71.1 cm−1 and 128.4 cm−1. Our careful symmetry analysis based on the angle-dependent spectra demonstrates that the crystal symmetry can be well described by C2h rather than D3d both above and below structural phase transition. We further performed temperature-dependent Raman experiments to study the magnetism in VI3. Fano asymmetry and anomalous linewidth drop of two Ag phonon modes at low temperatures, point to a significant spin-phonon coupling. This is also supported by the softening of 71.1-cm−1 mode above the magnetic transition. The study provides the fundamental information on lattice dynamics and clarifies the symmetry in VI3. And spin-phonon coupling existing in a wide temperature range revealed here may be meaningful in applications.