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 fieldB. Evidence for the chiral anomaly was obtained from the longitudinal magnetoresistance (LMR) inNa3Biand GdPtBi. However, current-jetting effects (focusing of the current densityJ) have raised general concerns about LMR experiments. Here, we implement a litmus test that allows the intrinsic LMR inNa3Biand GdPtBi to be sharply distinguished from pure current-jetting effects (in pure Bi). Current jetting enhancesJalong 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 resistanceRspine) with that at the edge (Redge). In Bi,Rspinesharply increases withB, butRedgedecreases (jetting effects are dominant). However, inNa3Biand GdPtBi, bothRspineandRedgedecrease (jetting effects are subdominant). A numerical simulation allows the jetting distortions to be removed entirely. We find that the intrinsic longitudinal resistivityρxx(B)inNa3Bidecreases by a factor of 10.9 betweenB=0and 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 testZrTe5.

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Plain Language Summary

In certain metals called Dirac-Weyl semimetals, the electrons behave as if they have zero mass. A fundamental result of quantum field theory is that all zero-mass particles must segregate into equal left- and right-handed populations (when viewed head-on, the former spin clockwise while the latter spin anticlockwise). However, these populations mix in the presence of parallel electric and magnetic fields. Why this effect, known as the chiral anomaly, occurs has long fascinated physicists. Here, we describe a test to distinguish evidence for this anomaly from a distinct classical behavior that has a similar experimental signature.

Physicists have detected the chiral anomaly in the Dirac-Weyl semimetalsNa3Biand GdPtBi as a fivefold-to-tenfold enhancement of the electrical conductance in applied parallel electric and magnetic fields at cryogenic temperatures. However, measurements of the resistance of a metal in a parallel magnetic field (magnetoresistance) are bedeviled by a classical effect called current jetting, which causes a strongly inhomogeneous flow pattern. To exclude these artifacts, we devised a test in which the resistance is simultaneously measured in two regions: along the midline, where the local current is strongest, and on an edge, where it is weakest.

From the test results, we confirm that the magnetoresistance inNa3Biand GdPtBi is intrinsic—it arises from the chiral anomaly. The test also yields the intrinsic profile of the chiral anomaly resistance versus field. By contrast, the test shows that magnetoresistance in pure bismuth (which should not display chiral anomaly effects) is entirely caused by current jetting.