  Despite considerable interest in layered transition metal dichalcogenides
(TMDs), such as MX2 with M = (Mo, W) and X = (S, Se, Te), the physical origin
of their topological nature is still poorly understood. In the conventional
view of topological phase transition (TPT), the non-trivial topology of
electron bands in TMDs is caused by the band inversion between metal d and
chalcogen p orbital bands, where the former is pulled down below the latter.
Here, we show that, in TMDs, the TPT is entirely different from the
conventional speculation. In particular, MS2 and MSe2 exhibits the opposite
behavior of TPT, such that the chalcogen p orbital band moves down below the
metal d orbital band. More interestingly, in MTe2, the band inversion occurs
between the metal d orbital bands. Our findings cast doubts on the common view
of TPT and provide clear guidelines for understanding the topological nature in
new topological materials to be discovered.
