CDW Gap Collapse and Weyl State Restoration in (TaSe4)2I via Coherent Phonons: A First-Principles Study

TL;DR

This study uses first-principles calculations to show how coherent phonons can induce a transition from a charge-density-wave state to a Weyl semimetal in (TaSe4)2I by suppressing the CDW gap and restoring the uniform chain structure.

Contribution

It identifies specific Raman-active phonon modes capable of driving topological phase transitions in (TaSe4)2I, providing a predictive framework for ultrafast control of quantum phases.

Findings

The 2.51 THz A(18) mode can suppress the CDW gap and restore the Weyl state.

Certain phonon modes can induce topological transitions with relatively small atomic displacements.

Strong anharmonic coupling between modes offers an indirect pathway to Weyl semimetallic behavior.

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.