Resolving the Quantum Origin of Anisotropic Raman Response in Quasi-1D Ta2Ni3Te5

Probing anisotropic electron–phonon interactions (EPIs) is fundamental to understanding low-dimensional quantum materials, yet isolating their intrinsic signatures remains a challenge due to the confounding influence of extrinsic effects. Here, we resolve this ambiguity in quasi-one-dimensional Ta2Ni3Te5 through a combination of angle-resolved polarized Raman spectroscopy and density functional perturbation theory (DFPT). Anomalous four-lobe angular dependences were observed in Ag phonon modes, which deviate from the real-valued Raman tensor prediction. Further, we explicitly highlight that, despite their distinct physical origins, light absorption and birefringence yield mathematically identical angular dependencies in Ta2Ni3Te5, rendering phenomenological fitting alone insufficient to identify the true physical mechanism. By employing a full quantum treatment incorporating detailed interaction mechanisms, we capture the qualitative characteristics and mode-to-mode diversity of the anomalous angular responses observed experimentally. The DFPT calculations further reveal mode-specific and momentum-resolved EPIs, providing microscopic support for anisotropic EPIs as the mechanism of anomalous Raman responses.