Experimental Tests of the Chiral Anomaly Magnetoresistance in the Dirac-Weyl Semimetals Na3Bi and GdPtBi

Abstract

In the Dirac-Weyl semimetal, the chiral anomaly appears as an “axial” current arising from charge pumping between the lowest (chiral) Landau levels of the Weyl nodes, when an electric field is applied parallel to a magnetic field $\mathbf{B}$. Evidence for the chiral anomaly was obtained from the longitudinal magnetoresistance (LMR) in ${\mathrm{Na}}_3\mathrm{Bi}$ and GdPtBi. However, current-jetting effects (focusing of the current density $\mathbf{J}$) have raised general concerns about LMR experiments. Here, we implement a litmus test that allows the intrinsic LMR in ${\mathrm{Na}}_3\mathrm{Bi}$ and GdPtBi to be sharply distinguished from pure current-jetting effects (in pure Bi). Current jetting enhances $J$ along the mid-line (spine) of the sample while decreasing it at the edge. We measure the distortion by comparing the local voltage drop at the spine (expressed as the resistance $R_{\text{spine}}$) with that at the edge ($R_{\text{edge}}$). In Bi, $R_{\text{spine}}$ sharply increases with $B$, but $R_{\text{edge}}$ decreases (jetting effects are dominant). However, in ${\mathrm{Na}}_3\mathrm{Bi}$ and GdPtBi, both $R_{\text{spine}}$ and $R_{\text{edge}}$ decrease (jetting effects are subdominant). A numerical simulation allows the jetting distortions to be removed entirely. We find that the intrinsic longitudinal resistivity ${\rho }_{xx}(B)$ in ${\mathrm{Na}}_3\mathrm{Bi}$ decreases by a factor of 10.9 between $B=0$ and 10 T. A second litmus test is obtained from the parametric plot of the planar angular magnetoresistance. These results considerably strengthen the evidence for the intrinsic nature of the chiral-anomaly-induced LMR. We briefly discuss how the squeeze test may be extended to test ${\mathrm{ZrTe}}_5$.