Realization of the axial anomaly in a quasi-two-dimensional metal

The axial anomaly leads to the violation of separate number conservation laws for left- and right-handed massless chiral (or Weyl-) fermions. For a certain class of gapless semiconductors, for which the low-energy band structure is described in terms of Weyl-fermions, the application of a magnetic field parallel to the electrical current is predicted to induce a large suppression of the electrical resistivity. To date, there is no concrete experimental realization of a Weyl semi-metal or unambiguous evidence for this field-induced phenomenon. Here, we report the observation of a very large negative magnetoresistance in the extremely clean quasi-two-dimensional metal PdCoO$_2$. Our experimental study provides strong support for a scenario where this unconventional response results from the axial anomaly of field-induced quasi-one-dimensional conduction channels. The observation of this effect in PdCoO$_2$ demonstrates that the axial anomaly is a general feature of the longitudinal magnetotransport of clean and weakly correlated three-dimensional metals.