  An axion insulator is a correlated topological phase, predicted to arise from
the formation of a charge density wave in a Weyl semimetal. The accompanying
sliding mode in the charge density wave phase, the phason, is an axion. 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.
