CDW Gap Collapse and Weyl State Restoration in (TaSe4)2I 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 (TaSe4)2I 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 2.51 THz CDW amplitude mode A(18) directly weakens the Ta-chain tetramerization, approaching a transient restoration of the uniform-chain geometry. It is also the most efficient mode owing to its low frequency and a relatively small critical displacement dominated by Ta motions. 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, we explore nonlinear phonon-phonon interactions and find that the low-frequency infrared-active mode B3(7) (1.14 THz) exhibits strong anharmonic coupling with A(18), providing an indirect pathway to drive the system toward a Weyl-semimetallic regime. Our results provide predictive insight for ultrafast pump-probe experiments and present a generalizable framework for lattice-driven topological switching in quasi-one-dimensional quantum materials.