  The topological materials have attracted much attention recently. While
three-dimensional topological insulators are becoming abundant, two-dimensional
topological insulators remain rare, particularly in natural materials. ZrTe5
has host a long-standing puzzle on its anomalous transport properties; its
underlying origin remains elusive. Lately, ZrTe5 has ignited renewed interest
because it is predicted that single-layer ZrTe5 is a two-dimensional
topological insulator and there is possibly a topological phase transition in
bulk ZrTe5. However, the topological nature of ZrTe5 is under debate as some
experiments point to its being a three-dimensional or quasi-two-dimensional
Dirac semimetal. Here we report high-resolution laser-based angle-resolved
photoemission measurements on ZrTe5. The electronic property of ZrTe5 is
dominated by two branches of nearly-linear-dispersion bands at the Brillouin
zone center. These two bands are separated by an energy gap that decreases with
decreasing temperature but persists down to the lowest temperature we measured
(~2 K). The overall electronic structure exhibits a dramatic temperature
dependence; it evolves from a p-type semimetal with a hole-like Fermi pocket at
high temperature, to a semiconductor around ~135 K where its resistivity
exhibits a peak, to an n-type semimetal with an electron-like Fermi pocket at
low temperature. These results indicate a clear electronic evidence of the
temperature-induced Lifshitz transition in ZrTe5. They provide a natural
understanding on the underlying origin of the resistivity anomaly at ~135 K and
its associated reversal of the charge carrier type. Our observations also
provide key information on deciphering the topological nature of ZrTe5 and
possible temperature-induced topological phase transition.

Materials Science;Strongly Correlated Electrons;Superconductivity