  Recently, giant planar Hall effect originating from chiral anomaly has been
predicted in nonmagnetic Dirac/Weyl semimetals. ZrTe5 is considered to be an
intriguing Dirac semimetal at the boundary of weak topological insulators and
strong topological insulators, though this claim still remains controversial.
Here, we report the observation in ZrTe5 of the giant planar Hall resistivity
that shows two different magnetic-field dependences as predicted by theory and
a maximum at the Lifshitz transition temperature. We found that the giant
planar Hall resistivity fades out with decreasing the thickness of ZrTe5
nanoplates, which may be ascribed to the vanishing of the 3D nature of the
samples. In addition, we have observed a nontrivial Berry phase,
chiral-anomaly-induced negative longitudinal magnetoresistance, and a giant
in-plane anisotropic magnetoresistance in these ZrTe5 nanoplates. All the
experimental observations demonstrated coherently that ZrTe5 is a Dirac
semimetal.
