Topological States and Electronic Structure of Layered 2M-WSe₂

The 2M phase of transitional metal dichalcogenides are an underexplored family of materials that have recent indicators of their topological significance. Here, we investigate the electronic structure and Fermi surface of 2M-WSe₂ in both computation and experiment, finding evidence of topological states and a Lifshitz transition. Shubnikov-de Haas oscillations combined with first-principles calculations reveal multiple Fermi pockets, including one with a nontrivial Berry phase. This evidence, combined with topological Z₂ invariant calculation, indicates that bulk 2M-WSe₂ is a weak topological insulator. A drastic change of electron carrier density as a function of temperature is found, indicating a Lifshitz transition. The layer-dependence and electric field-dependence of these Fermi pockets and topological properties are also explored, suggesting that 2M-WSe₂ provides a promising platform for rich topological phases in the two-dimensional limit.