Evolution of topological phases in atomically thin WTe2 films

TL;DR

This study investigates how the topological electronic properties of atomically thin WTe2 films change with thickness, revealing non-monotonic phase transitions driven by layer-dependent band structure reconfigurations.

Contribution

It provides experimental ARPES data and first-principles calculations showing the evolution of topological phases in WTe2 from monolayer to bulk, including oscillations of the Z2 invariant.

Findings

Monolayer WTe2 exhibits a band gap associated with topological insulator behavior.

Adding layers suppresses the gap, leading to metallic states in three-layer films.

The topological Z2 invariant oscillates between 1 and 0 with increasing layers, indicating phase transitions.

Abstract

Topological materials ranging from topological insulators to semimetals host many novel quantum phenomena including quantum spin Hall effect and topological Fermi arcs. Transitions between these topological phases have attracted much research interest. We performed angle-resolved photoemission spectroscopy (ARPES) on WTe2 ranging from a monolayer to the bulk and reveal 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 the first-principles calculations. Topological Z2 invariant is shown to oscillate between 1 and 0 with the addition of layers, originating from the interlayer coupling-induced change in band crossing. The system evolves into a Weyl semimetal when the conduction and valence bands touch near the Fermi level and the topological nature is described by the Chern number. Our findings demonstrate the non-monotonic dependence of topological states on dimensionality and how layer-driven electronic band reconfiguration leads to phase transitions in solids.