Topological insulators in the 1T’ phase of MX₂ (M=Mo,W; X=S,Se,Te) revisited

Monolayers of Mo and W based transition metal dichalcogenides (TMD) in the 1T’ phase are known to be topological insulators [1]. This has been understood within the Kane-Mele model and is associated with the level inversion between the anion p states and the transition metal d states. However, what is surprising is that this happens across the entire series varying the anion from S to Se to Te for both Mo and W based systems. Examining various families of topological insulators, one finds that while one member of the series would be close to the point of inversion, it is not expected for every member to be close to the point of inversion and therefore become a topological insulator. In this work, we study the 1H , 1T and 1T’ phases of the Mo and W based TMD extensively. We carry out ab-initio electronic structure calculations for each member of the TMD family. To quantify the electronic structure, we map the ab-initio band structure onto a tight binding model. We show that while the level inversion is not specific to the 1T’ phase, and is also present in the other phases, the reason for
realizing a topological insulator in certain instances is elucidated.

[1] X. Qian, J. Liu, L. Feu and J. Li, Science 346, 1344 (2016).