Microfocus Laser–Angle-Resolved Photoemission on Encapsulated Mono-, Bi-, and Few-Layer 1T′-WTe₂

Abstract

Two-dimensional crystals of semimetallic van der Waals materials hold much potential for the realization of novel phases, as exemplified by the recent discoveries of a polar metal in few layer 1T'-WTe 2 and of a quantum spin Hall state in monolayers of the same material. Understanding these phases is particularly challenging because little is known from experiment about the momentum space electronic structure of ultrathin crystals. Here, we report direct electronic structure measurements of exfoliated mono-bi-and few-layer 1T'-WTe 2 by laser-based micro-focus angle resolved photoemission. This is achieved by encapsulating with monolayer graphene a flake of WTe 2 comprising regions of different thickness. Our data support the recent identification of a quantum spin Hall state in monolayer 1T'-WTe 2 and reveal strong signatures of the broken inversion symmetry in the bilayer. We finally discuss the sensitivity of encapsulated samples to contaminants following exposure to ambient atmosphere.

Influence of lattice termination on the edge states of the quantum spin Hall insulator monolayer 1T′−WTe2

“…Most importantly, the model has an indirect energy gap of ∆E = 56 meV, compatible with experiments. 37,40,50 Moreover, close to the Fermi level, the dispersion of our model matches experimental results 40,50 by construction. An analysis of the parities ξ of the states at the four time-reversal invariant momenta Γ, X, Y , and M shows that the lowest conduction band and the lowest valence band in our model are inverted at Γ [see Fig.…”

“…The edges of typical samples used in experiments on 2D monolayer materials are highly irregular, i.e., they have cracks, steps or bumps. 42,50 Hence, the samples are no longer translationally invariant along their boundary, enabling backscattering of edge states when time-reversal symmetry is broken. We model a rough edge by the boundary following a random walk with a step appearing at every site with probability p (see Supplemental Material 47 for details).…”

“…Fully-relativistic DFT calculations, which take into account the spin degree of freedom of the electrons, however, have not been able to reproduce quantitatively the size of the energy gap and the dispersion of the valence band close to the Fermi level, as measured in scanning tunneling spectroscopy and ARPES experiments. 35,37,40,50 To overcome this issue, we choose to take an alternative route by combining scalar-relativistic DFT calculations with symmetry considerations and data from ARPES, thereby obtaining an accurate bulk model.…”

Terahertz response of monolayer and few-layer WTe_2 at the nanoscale

“…While the presence of a large gap of >60 meV in monolayer WTe2 is demonstrated by transport [3] and ARPES [12] measurements, the semiconductor versus semimetallic nature of the bilayer remains unclear. ARPES experiments on bi-layer WTe2 [12] indicate a vanishing, if not negative, gap (Fig. 4d and 4e).…”

“…Bi-layers exhibit a narrow gap (<10 meV) semiconducting behavior in transport measurements [3]. Angle resolved photoemission spectroscopy (ARPES), however, revealed that bi-layers could also be weakly semimetallic with a small negative gap [12]. A combination of inverted bands, strong spin-orbit coupling and low crystal symmetry makes few-layer WTe2 an ideal system for studying topological effects such as the nonlinear anomalous Hall effect [13] [14] [15] and various unusual photogalvanic effects [16] [17] [18].…”