I: 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. In [Shi*, Wieder*, Meyerheim*, et al., Nat. Phys. (2021)], we demonstrate that the room-temperature phase of (TaSe4)₂I is a Weyl semimetal with 24 pairs of Weyl nodes. Owing to its quasi-1D structure, (TaSe4)₂I hosts an established CDW instability just below room temperature. Using X-ray diffraction, angle-resolved photoemission spectroscopy, and first-principles calculations, we find that the CDW in (TaSe4)₂I couples the bulk Weyl points and opens a band gap. 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.