Event Details

(TaSe4)2I is a Weyl semimetal that undergoes charge density wave (CDW) ordering below 260K. It is thus a unique system to study the interplay of electronic topology with strong correlations. For example, recent magnetoresistance experiments indicated that (TaSe4)2I might host an axionic insulating phase in which the collective phase mode of the CDW acts as an axion field. Such an interpretation is currently under active debate. Taking inspiration from this debate, I will discuss two of our recent experiments using nonlinear light-matter interaction in the THz range to directly probe the dynamics of the collective modes of (TaSe4)2I. I will first show how upon transient photoexcitation at low temperatures, (TaSe4)2I strikingly emits coherent, narrow- band THz radiation. The frequency, polarization and temperature-dependence of the emitted radiation imply the existence of a phason that acquires mass by coupling to long-range Coulomb interactions. Our observations constitute the first direct evidence of the massive "Lee-Fukuyama" phason and highlight the potential applicability of fundamental collective modes of correlated materials as compact and robust sources of THz radiation. Second, I will show our recent results using THz pump, IR Kerr probe spectroscopy on (TaSe4)2I to highlight how THz-driven phonons can induce a sizeable dynamic optical Kerr rotation in this otherwise time-reversal invariant system. I will briefly discuss both these results in the context of the predicted axion electrodynamics in (TaSe4)2I.