  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.
