(TaSe4)2I is a Weyl semimetal that undergoes charge density wave (CDW) ordering below 260 K. It is thus a unique system to study the interplay of topological effects and strong correlations. For example, (TaSe4)2I has been shown to host an axionic insulating phase in which the collective phase mode of the CDW acts as an axion field. Taking inspiration from this result, I will discuss our recent experiments using nonlinear light-matter interaction in the THz range to directly probe the collective modes of (TaSe4)2I. In particular, I will provide evidence for the optical phase mode of this Weyl-CDW and report how strong THz pulses can excite intrinsic collective modes which appear to induce a sizeable dynamic optical Kerr rotation in this otherwise time-reversal invariant system.
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