Terahertz (THz) induced optical activity in the charge-density wave (CDW) Weyl semimetal (TaSe₄)₂I

(TaSe₄)₂I is a nonmagnetic Weyl semimetal that undergoes a charge density wave (CDW) transition near T_C~260 K. Its CDW phase mode (phason) has been investigated with transport experiments for narrow band noise phenomena and has recently been highlighted as a putative axion candidate that can couple to an electromagnetic field through the term θ E·B. The ultrafast dynamics of the collective modes of (TaSe₄)₂I, both amplitude and phase, can provide a basis to understand and explore correlated topological physics in the time-domain.

In this work, we use ultrafast THz-pump near-infrared-probe spectroscopy to observe coherent oscillations in the complex refractive index of the low-temperature CDW phase of (TaSe₄)₂I. The frequencies of these modes coincide with past reports of CDW amplitude-related Raman-active modes. Analysis of the line shape of the modes for as a function of the driving THz field-strengths and temperature will be discussed in the context of the effect of the amplitude modes on the electronic structure of (TaSe₄)₂I.