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

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 (MR). Here, we present experimental evidence that the observation of the chiral anomaly can be hindered by an effect called 'current jetting'. This effect also leads to a strong apparent negative longitudinal MR, 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 Weyl semimetal candidate materials NbP, NbAs, TaAs, and 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 magneticfield 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 negative longitudinal MR as evidence for the chiral anomaly in putative Weyl semimetals.

Weyl and Dirac semimetals in three-dimensional solids

“…Firstly, in materials like TaP, it appears to be (Arnold et al, 2016b) that E F is such that the electron and hole Fermi-surface pockets surrounding the W1 nodes contain a pair of Weyl nodes and hence the total Berry flux through the Fermi surface is zero. Secondly, it has been pointed out recently that many of these materials have a large enough transverse magnetoresistance to have the effect corrupted by the classic "current jetting" phenomenon in compensated semiconductors (Dos Reis et al, 2016;Yuan et al, 2016b) in which current becomes narrowly directed along the applied field due to a very large field induced transverse re-sistance. Historically, the observed effect was known as "anomalous longitudinal magnetoresistance" in materials like antimony and bismuth (Babiskin, 1957;Steele, 1955;Yoshida, 1976), but was later shown to arise from current inhomogeneity inside the sample from large transverse magnetoresistance.…”

Negative magnetoresistance due to conductivity fluctuations in films of the topological semimetal Cd3As2

“…S4 in the Supplemental Material [22]), so geometric current jetting, see Ref. [10], is likely not the origin of the negative longitudinal MR. To further exclude geometrical current jetting, additional measurements with completely straight Hall bars were conducted, which show similar behavior (see Fig. S5 [22]).…”

“…Recent observations [3][4][5][6][7][8][9] of longitudinal negative MR in Dirac and Weyl semimetals have therefore generated significant attention. While most studies consider the negative longitudinal MR to be evidence of the chiral anomaly, current jetting has been proposed as an alternative explanation [10,11]. Current jetting arises under inhomogeneous current injection into high-mobility materials that have a large conductivity anisotropy [12].…”

Dynamic Chiral Magnetic Effect and Faraday Rotation in Macroscopically Disordered Helical Metals

“…In this sense, the static CME has been observed via magnetotransport measurements in [35][36][37][38] (see Ref. [39,40] for further references and review of recent results), and via non-local voltage measurements in [41].…”