In quantum field theory, massless fermions must segregate into left- and right-handed populations that do not ever mix. Chiral symmetry (handedness) is a protected global symmetry. However, coupling of the fermion fields to a vector gauge field ruins the symmetry. The resulting axial current that flows is known as the chiral anomaly. The first example appeared in the theory of the ultrafast decay of neutral pions into photons (Adler Bell Jackiw anomaly). In the ensuing 5 decades, anomalies have appeared at every energy scale, from QCD to gravitation physics. In 1983, Nielsen and Ninomiya predicted that the chiral anomaly should be observable as well in bulk semimetals that feature protected 3D Dirac cones. The axial current appears as an enhanced conductance in parallel electric and magnetic fields. I will describe experiments on Na3Bi and GdPtBi which show the dramatic emergence of the anomaly when carriers are confined to the lowest Landau level. Tests to distinguish this quantum effect from (classical) artifacts caused by “current jetting” will be described.

Host: Joe Brill