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

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.

💡 Research Summary

In this work the authors investigate whether coherent phonons can non‑thermally drive a topological phase transition in the quasi‑one‑dimensional charge‑density‑wave (CDW) material (TaSe₄)₂I. Using density‑functional theory with the PBE functional and PAW potentials, they model the low‑temperature CDW phase with an orthorhombic F222 supercell containing 44 atoms. Phonon spectra are obtained via finite‑displacement calculations (PHONOPY), and spin‑orbit coupling is included throughout. The authors focus on zone‑center (Γ‑point) phonons, classifying them by irreducible representations of the D₂ point group. Of the 14 Raman‑active A‑symmetry modes, nine preserve the full crystal symmetry while strongly affecting the electronic structure.

The most important mode is the 2.51 THz Raman‑active amplitude mode A(18). Its eigenvector is dominated by Ta displacements along the chain direction, directly modulating the tetramerization that opens the CDW gap. Frozen‑phonon calculations show that as the normal‑mode coordinate Q increases, the direct Γ–Z gap shrinks from 0.32 eV at equilibrium to a residual 3 meV at Q≈2.5 Å√amu. At Q≈2.0 Å√amu the long and short Ta–Ta distances become nearly equal, indicating a transient restoration of the uniform‑chain geometry. Even before the gap fully closes, a Brillouin‑zone‑wide search reveals the emergence of Weyl nodes at generic k‑points; at Q≈2.5 Å√amu there are 24 pairs of Weyl points within 10 meV of the Fermi level. Thus, the CDW gap collapse is accompanied by a global Weyl‑semimetallic state without breaking any crystal symmetry.

The remaining eight Raman‑active A modes are dominated by Se motions. They also reduce the Γ–Z gap, but require substantially larger critical displacements (Qc≈4–7 Å√amu). The Se‑only component partially compensates the Ta‑driven gap reduction, explaining the higher Qc values. Consequently, while all nine modes can in principle drive the system into a Weyl phase, A(18) is the most efficient because it couples directly to the structural order parameter.

Beyond linear phonon‑electron coupling, the authors explore anharmonic phonon‑phonon interactions. They identify a low‑frequency infrared‑active mode B₃(7) at 1.14 THz that exhibits strong third‑order coupling with A(18). This suggests an indirect pathway: driving B₃(7) with a THz pump can nonlinearly amplify A(18), facilitating the CDW‑gap collapse even when direct Raman excitation of A(18) is experimentally challenging.

The paper concludes that (i) symmetry‑preserving Raman modes, especially the low‑frequency amplitude mode, provide a highly selective, nonthermal knob to melt the CDW gap and restore a Weyl semimetal; (ii) the emergence of Weyl nodes at generic k‑points demonstrates that lattice‑driven topological switching can occur without symmetry breaking; and (iii) anharmonic coupling offers a versatile multi‑mode control strategy. The authors discuss experimental implications for ultrafast pump‑probe and THz spectroscopy, proposing that coherent excitation of the identified phonons should enable real‑time manipulation of electronic topology in (TaSe₄)₂I and, by extension, in other quasi‑1D materials. This work thus provides both a microscopic mechanism and a practical roadmap for lattice‑engineered topological phase transitions.