Axionic charge-density wave in the Weyl semimetal (TaSe4)2I

Abstract

An axion insulator is a correlated topological phase, predicted to arise from the formation of a charge-density wave in a Weyl semimetal^1,2. The accompanying sliding mode in the charge-density-wave phase, the phason, is an axion^3,4. It is expected to cause anomalous magneto-electric transport effects. However, this axionic charge density wave has so far eluded experimental detection. In this paper, we report the observation of a large, positive contribution to the magneto-conductance in the sliding mode of the charge-density wave Weyl semimetal (TaSe₄)₂I for collinear electric and magnetic fields (E||B). The positive contribution to the magneto-conductance originates from the anomalous axionic contribution of the chiral anomaly to the phason current, and is locked to the parallel alignment of E and B. By rotating B, we show that the angular dependence of the magneto-conductance is consistent with the anomalous transport of an axionic charge-density wave.