Topological properties of few-layer WTe₂

Chen-Chia Hsu^1*, Yang-hao Chan¹

¹Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan

* Presenter:Chen-Chia Hsu, email:hsucc@as.edu.tw

Topological materials, which are characterized by topological invariants, represent a new paradigm of matter. Their robust, symmetry-protected surface states promise emerging technologies. Among them, Tungsten Ditelluride (WTe₂), a member of the transition metal dichalcogenides (TMDs), exhibits a remarkably rich spectrum of topological phases, ranging from the Type-II Weyl semimetal in bulk to the quantum spin Hall insulator (QSHI) state in the monolayer limit. Investigating how the topological property evolves with film thickness remains a crucial challenge. In this study, we employ density functional theory (DFT), utilizing both the DFT+U approach and the HSE06 hybrid functional, to systematically investigate the electronic structure and the evolution of topological properties in few-layer WTe₂ (1 to 3 layers) in their 1Td phase. Our calculations reveal a distinctive non-monotonic dependence of the topological phase on the film thickness. Specifically, the single-layer (1L) WTe₂ is confirmed to be topologically non-trivial with a Z₂ invariant of 1, hosting protected gapless edge states. Intriguingly, upon increasing the thickness to the bilayer (2L), the system undergoes a topological phase transition to a trivial insulator with Z₂ = 0. Furthermore, the topological state is re-established in the trilayer (3L) system, exhibiting a restored Z₂ invariant of 1. We detail the precise band structures of each system. The topological transitions are further verified through the calculation and characterization of the edge states. These findings not only provide crucial theoretical insights into the rich quantum phases of WTe2 thin films but also suggest a promising platform for engineering layer-controlled topological quantum devices.

Keywords: Topological insulator, Transition metal dichalcogenide, Layer dependence, Electronic structure, First-principles calculation