  By employing angle-resolved photoemission spectroscopy combined with
first-principles calculations, we performed a systematic investigation on the
electronic structure of LaBi, which exhibits extremely large magnetoresistance
(XMR), and is theoretically predicted to possess band anticrossing with
nontrivial topological properties. Here, the observations of the Fermi-surface
topology and band dispersions are similar to previous studies on LaSb [Phys.
Rev. Lett. 117, 127204 (2016)], a topologically trivial XMR semimetal, except
the existence of a band inversion along the $\Gamma$-$X$ direction, with one
massless and one gapped Dirac-like surface state at the $X$ and $\Gamma$
points, respectively. The odd number of massless Dirac cones suggests that LaBi
is analogous to the time-reversal $Z_2$ nontrivial topological insulator. These
findings open up a new series for exploring novel topological states and
investigating their evolution from the perspective of topological phase
transition within the family of rare-earth monopnictides.
