CDW Gap Collapse and Weyl State Restoration in Ta$_2$Se$_8$I via Coherent Phonons: A First-Principles Study

Abstract

Coherent phonon excitation offers a nonthermal route to control quantum phases of condensed matter. In this work, we employ first-principles calculations to investigate the phonon landscape of (TaSe$_4$)$_2$I in its charge-density-wave (CDW) phase. We identify nine symmetry-preserving Raman-active modes that can suppress the $\Gamma$-Z direct gap to the meV scale and render the system globally gapless by generating Weyl nodes at generic k points. Among them, the 2.52 THz CDW amplitude mode A(18) directly weakens the Ta-chain tetramerization, approaching a transient restoration of the uniform-chain geometry, while its low frequency and relatively small, Ta-dominated displacement make it the most efficient mode. Other Raman modes, dominated by Se vibrations, require significantly larger displacements to reach the Weyl-semimetallic regime and are generally less effective than A(18) at reducing the Ta-chain tetramerization. Furthermore, among the IR-active modes considered, the low-frequency B$_3$(7) mode (1.15 THz) provides the most favorable channel for nonlinear modulation of the A(18) potential. Although the coupling favors displacement toward the CDW-suppressing direction, the estimated indirect response remains far below that required for strong gap suppression. Direct excitation of A(18) therefore remains the most effective pathway to the Weyl-semimetallic regime.