I will talk about recent experiments on the chiral anomaly in Dirac semimetals. In field theory, Dirac fermions of zero mass must segregate into left- and right-handed populations that do not ever mix. In this limit, chiral symmetry (handedness) is a global symmetry of the Lagrangian. However, quantum effects induced by coupling to a vector gauge field kill the symmetry. This is known as the chiral (or axial) anomaly. The first example appeared (1968) in the decay of neutral pions into 2 photons (the Adler-Bell-Jackiw anomaly). Since then, anomalies have been implicated in many important problems, e.g. the U(1)_A problem, renormalizability of the electroweak theory, and the fermion-doubling problem in lattice QCD. It is also directly related to the Atiyah-Singer index theorem. In 1983, Nielsen and Ninomiya predicted that the chiral anomaly should be observable as well in bulk semimetals that feature protected 3D Dirac cones. Breaking of time-reversal invariance in a magnetic field B converts the Dirac electrons in a semimetal to Weyl fermions. The chiral nature of the Weyl fermions become manifest in the lowest Landau level (in intense B). An electric field applied parallel to B shifts the left and right-moving branches to produce an axial current. I will describe experiments on Na₃Bi and GdPtBi which show the dramatic emergence of the anomaly, and focus on issues peculiar to the anomaly in crystals. I will describe a “litmus” test that sharply distinguishes this quantum effect from (classical) artifacts caused by “current jetting.”

Recording of Professor Ong's Talk: http://www.kaltura.com/tiny/4ztmk