Evolution of Topological Phases in Atomically Thin WTe₂ Films

Topological materials, ranging from topological insulators to semimetals, host many novel quantum phenomena, including the quantum spin Hall effect and topological Fermi arcs. Transitions between these topological phases have attracted considerable research interest. We performed angle-resolved photoemission spectroscopy on WTe₂, from monolayer to bulk, and revealed the evolution of the electronic structure and the band gap. Notably, the gap observed in the monolayer system is suppressed in the three layers, where the film becomes metallic. Variations in the topological properties with thickness are demonstrated by first-principles calculations. The topological Z₂ invariant is shown to oscillate between 1 and 0 upon addition of layers, originating from interlayer coupling-induced changes in band crossing. The system evolves into a Weyl semimetal when the conduction and valence bands touch near the Fermi level, and its topological nature is characterized by the Chern number. Our findings demonstrate the nonmonotonic dependence of topological states on dimensionality and how layer-driven electronic band reconfiguration leads to phase transitions in solids.