Unusual Phonon Properties in Highly Anisotropic Two-dimensional Ta₂Ni₃Te₅

Symmetry, a fundamental aspect of quantum materials, is crucial in determining physical properties. Crystals with low symmetry often exhibit strong anisotropy in energy dispersion for various particles. Consequently, the electrical, optical, thermal, and phonon properties change in different directions. Layered Ta₂Ni₃Te₅ was proposed as second-order topological insulator due to double-band inversion, which provides an excellent platform for studying the interplay between topology and dimensionality. Additionally, Ta₂Ni₃Te₅ possesses strong anisotropic 1D crystal structure, delivering extra modulation to the unexplored exotic properties. Despite the intriguing features, Ta₂Ni₃Te₅ still needs in-depth investigation on its anisotropic electrical, optical, and phonon properties. We investigated the phonon properties of few-layer Ta₂Ni₃Te₅ flakes with Raman spectroscopy. 23 Raman modes, including 5 ultralow-frequency modes, have been observed. We assigned 18 modes to A_g vibrational symmetry and 5 modes to B_3g. Using angle-resolved polarized Raman spectroscopy, we observed that A_g modes exhibit unconventional multifold angle dependence. The temperature dependence of these Raman modes was also investigated. Surprisingly, in some Raman modes, the four-phonon process is more significant and dominant than the three-phonon process. We attribute these behaviors to the strong electron-photon and electron-phonon interactions induced by the highly anisotropic crystal structures.