In recent fields of condensed matter physics, the coupling between topological band structures and various degrees of freedom, such as lattice, charge, orbital, and spin, is highlighted. These understandings are particularly appealing for the development of energy-efficient functional devices, as they can be used to control the dissipation-less electronic transport in correlated topological materials. Identifying which degrees of freedom are coupled to a topological band is, however, a challenge. Investigating electrodynamics can overcome the difficulty because many physical phenomena can be decoupled in time.

(TaSe4)2I provides an excellent platform to explore topology, lattice, and charge degrees of freedom. It is a nonmagnetic Weyl semimetal which undergoes a charge-density-wave phase transition near 260 K on its structurally chiral lattice. The phase part of the CDW collective mode is expected to follow the axion electrodynamics of high-energy physics [1,2], provided the pairs of Weyl fermions are coupled by a CDW modulation. From the axionic band structure, exciting phenomena such as chiral magnetoelectric and quantized photogalvanic effects are proposed [3].

In this talk, we will first discuss the characteristics of topological Kramers-Weyl band structure and its presence in (TaSe4)2I, which was examined via laser-based angle-resolved photoemission (ARPES) with full control of light helicity [4]. Next, we will consider a way to probe the CDW collective modes through ultrafast optical setups for THz emission and THz-pump birefringence spectroscopies. The experimental results will be compared to the theoretically suggested axionic phase mode.

[1] J. Gooth et al., Nature 575, 315 (2019).
[2] W. Shi et al., Nat. Phys. 17, 381 (2021).
[3] D. M. Nenno et al., Nat. Rev. Phys. 2, 682 (2020).
[4] S. Kim et al., arXiv:2108.10874 (2021).