Experimental signatures of the chiral anomaly in Dirac-Weyl semimetals

The chiral anomaly, originally studied in pion decay, leads to related effects in Dirac and Weyl semimetals. This Review surveys recent experiments that address the appearance of the anomaly in parallel electric and magnetic fields. In condensed matter, the chiral anomaly describes the conversion of left-moving Dirac-Weyl fermions to right-moving ones in parallel electric and magnetic fields. The resulting axial current leads to an unusual negative longitudinal magnetoresistance (LMR). Five years ago, the discovery of Dirac and Weyl semimetals led to many experiments investigating this phenomenon. In this Review, we critically assess LMR experiments in the Dirac-Weyl semimetals Na3Bi, GdPtBi, ZrTe5, Cd3As2 and TaAs, which have shown signatures of the chiral anomaly, and discuss possible current-jetting artefacts. The focus is on Dirac and Weyl nodes that are rigorously symmetry protected. Other experiments, such as non-local transport, thermopower, thermal conductivity and optical pump-probe response, are also reviewed. Looking ahead, we anticipate what can be gleaned from improved LMR experiments and new experiments on the thermal conductivity and optical response. An expanded purview of the chiral anomaly is provided in the Supplementary Information.