Three-dimensional correlated topological semimetals

The physics of topological materials is typically well described by the band theory of non-interacting fermions. In contrast, some of the most fascinating effects in condensed matter physics merely emerge from electron correlations. Here, we will present two recent examples of phases of matter, arising from the interplay of interaction and non-trivial topology: an axionic charge density wave and three-dimensional quantum Hall states. Our electrical transport measurements reveal a positive longitudinal magnetoconductance in the sliding mode of the quasi-one-dimensional CDW–Weyl semimetal (TaSe₄)₂I, a signature that is linked to the presence of an axionic phason. In addition, we report the observation of integer and fractional quantum Hall plateaus in the bulk semimetal HfTe₅.