  A phase transition between topologically distinct insulating phases involves
closing and reopening of the bandgap. Close to this topological phase
transition, the bulk energy spectrum is characterized by a massive Dirac
dispersion, where the mass plays the role of bandgap. Here we report the
observation of a non-monotonic strain dependence of resistivity and negative
longitudinal magnetoresistance in ZrTe5, which is known to host massive Dirac
Fermions in the bulk. This non-monotonic strain dependence is consistent with
the closing and reopening of the bandgap at the Brillouin-zone center,
indicative of a topological phase transition. This observation suggests that
the topological state of ZrTe5 is highly sensitive to uniaxial stress. Our
study presents a promising platform for continuous in-situ control of
nontrivial topological properties of materials.
