A charge-density-wave topological semimetal

Topological physics and strong electron–electron correlations in quantum materials are typically studied independently. However, there have been rapid recent developments in quantum materials in which topological phase transitions emerge when the single-particle band structure is modified by strong interactions. Here we demonstrate that the room-temperature phase of (TaSe₄)₂I is a Weyl semimetal with 24 pairs of Weyl nodes. Owing to its quasi-one-dimensional structure, (TaSe₄)₂I also hosts an established charge-density wave instability just below room temperature. We show that the charge-density wave in (TaSe₄)₂I couples the bulk Weyl points and opens a bandgap. The correlation-driven topological phase transition in (TaSe₄)₂I provides a route towards observing condensed-matter realizations of axion electrodynamics in the gapped regime, topological chiral response effects in the semimetallic phase, and represents an avenue for exploring the interplay of correlations and topology in a solid-state material.