On the search for the chiral anomaly in Weyl semimetals: The negative longitudinal magnetoresistance

TL;DR

This study investigates the negative longitudinal magnetoresistance in Weyl semimetals, revealing that current jetting effects can mimic the chiral anomaly signal, emphasizing the need for careful interpretation of experimental results.

Contribution

It demonstrates that inhomogeneous current distribution effects can produce false signals of the chiral anomaly in Weyl semimetals, challenging previous interpretations.

Findings

Current jetting causes apparent negative magnetoresistance.

Potential distribution is highly distorted by inhomogeneous current injection.

Simulations align with experimental data, highlighting measurement artifacts.

Abstract

Recently, the existence of massless chiral (Weyl) fermions has been postulated in a class of semi-metals with a non-trivial energy dispersion.These materials are now commonly dubbed Weyl semi-metals (WSM).One predicted property of Weyl fermions is the chiral or Adler-Bell-Jackiw anomaly, a chirality imbalance in the presence of parallel magnetic and electric fields. In WSM, it is expected to induce a negative longitudinal magnetoresistance (NMR), the chiral magnetic effect.Here, we present experimental evidence that the observation of the chiral magnetic effect can be hindered by an effect called "current jetting". This effect also leads to a strong apparent NMR, but it is characterized by a highly non-uniform current distribution inside the sample. It appears in materials possessing a large field-induced anisotropy of the resistivity tensor, such as almost compensated high-mobility semimetals due to the orbital effect.In case of a non-homogeneous current injection, the potential distribution is strongly distorted in the sample.As a consequence, an experimentally measured potential difference is not proportional to the intrinsic resistance.Our results on the MR of the WSM candidate materials NbP, NbAs, TaAs, TaP exhibit distinct signatures of an inhomogeneous current distribution, such as a field-induced "zero resistance' and a strong dependence of the `measured resistance" on the position, shape, and type of the voltage and current contacts on the sample. A misalignment between the current and the magnetic-field directions can even induce a "negative resistance". Finite-element simulations of the potential distribution inside the sample, using typical resistance anisotropies, are in good agreement with the experimental findings. Our study demonstrates that great care must be taken before interpreting measurements of a NMR as evidence for the chiral anomaly in putative Weyl semimetals.