Topology of the CDW insulator phase in Ta2Se8I

Determine the topological classification of the charge density wave (CDW) insulator phase in Ta2Se8I (also known as (TaSe4)2I), specifying whether the low-temperature CDW-induced insulating state realizes an axion insulator or a distinct topological phase, by establishing its bulk and boundary topological characteristics.

Background

Ta2Se8I is a quasi-one-dimensional material where a charge density wave (CDW) transition gaps a Weyl semimetal state. Prior theoretical proposals suggested the possibility of an axion insulator when a Weyl semimetal is gapped by a CDW, but the applicability to Ta2Se8I has been debated, particularly in chiral crystals where paired Weyl nodes lie at different energies.

The paper reports scanning tunneling microscopy and spectroscopy that reveal a large CDW gap (~550 meV), absence of a time-reversal-protected surface state within this gap, and the presence of a localized in-gap edge mode that is correlated with the CDW wavevector and exhibits energy-dependent phase shifts. These observations challenge the assumption of an axion insulator ground state and indicate a potentially different topological nature for the CDW phase.

Against this backdrop, the authors explicitly note that the topology of the CDW insulator phase in Ta2Se8I is not yet determined and remains debated, motivating a precise resolution of its topological classification.