The chiral anomaly in the Dirac semimetal Na3Bi and the half-Heusler GdPtBi*

The Dirac semimetal Na₃Bi has two protected 3D Dirac nodes. In a magnetic field B, each node splits into two Weyl nodes. The lowest Weyl Landau levels are chiral with velocity v||B determined by the chirality χ. Alignment of the electric field E with B violates conservation of the axial charge density. The resulting “axial” current J⁵ leads to a large, negative, longitudinal magnetoresistance (MR) known as the “chiral anomaly.” This is the analog of the process π⁰→2γ in the physics of pion decay. The negative MR has been observed in Na₃Bi and in GdPtBi (where field-induced band crossings create twin Weyl nodes). Here we report new test results that distinguish the chiral anomaly MR from spurious current-jetting effects. In high-mobility semimetals, the "squeezing" of the current density into a narrow beam || B increases dissipation along the beam axis (“the spine”), but lowers it at the edge of the sample. In Bi, we indeed observe a voltage (difference) that increases rapidly with B along the spine, but decreases along the edge. By contrast, in both Na₃Bi and GdPtBi, both voltage differences decrease. The chiral anomaly causes the spine voltage to decrease despite the increased dissipation. In a second test based on the planar angular MR, we find that the parametric plots of the angular MR voltages V_yx vs. V_xx define concentric orbits in the chiral anomaly systems. In Bi, however, the orbits expand in a strongly skewed fashion. The difference brings out a crucial difference in the conductance anisotropy. The separation of the Weyl nodes in Na₃Bi also creates anomalies in the Hall signal in weak B.