Layer-dependent quantum cooperation of electron and hole states in the anomalous semimetal WTe2

Followed by the discovery of remarkable electronic and physical properties of two-dimensional (2D) materials such as graphene and topological insulators, transition metal dichalcogenides (TMDs), long of interest in material science, have received a renewed attention.

TMDs are a group of 2D layered materials with chemical formula MX2 (M is transition metal, and X can be S, Se, or Te). Depending on the composition and crystal structure, TMDs exhibit diverse physical properties, ranging from semiconductors (e.g., MoS2, WS2), semimetals (WTe2, TiSe2), pure metals (NbS2, VSe2), and also various low temperature phenomena such as metal-insulator transitions, superconductivity and charge density waves.

Dimensionality plays an important role in TMDs because of the presence of anisotropic bonding (the inter-layer interaction is mainly weak van der Walls type, while the intra-layer bonding between the atoms is strong and of covalent type).

The system of our focus is WTe2, a special member of TMD family that exhibits an additional structural distortion: the W atoms form zigzag chains along the crystallographic a-axis, rendering the material effectively a 1D system. Electronically, WTe2 is a semimetal, which exhibits an extremely large uniaxial positive magnetoresistance with no sign of saturation up to a magnetic field as high as 60 T. This titanic magnetoresistance has been attributed to the existence of perfect balance between the electron and hole charge carriers.

In our present work we explored the details and the evolution of electron and hole states as a function of material thickness by combing the results of spin and angle-resolved photoemission spectroscopy (ARPES), ab-initio calculations (Fig. 1) and high resolution surface topography (Fig. 2). The high resolution ARPES measurements were performed using the new VG-Scienta DA30 electron analyser with spin resolution at the APE beamline at Elettra. We found that the electron - hole balance, the key reason behind the non-saturating magnetoresistance in this system, is established only when we consider at least three Te-W-Te layers, and it is maintained in the bulk (Fig. 1). This consideration provides a fundamental input for future exploitation of WTe2 in devices and heterogeneous interfaces. As there are two heavy elements present in the system, spin orbit coupling (SOC) represents another important ingredient determining the details of the Fermi surface and relevant low-energy excitations. We obtained important insight about the spin texture and the role of SOC by means of spin-resolved ARPES measurements. Our STM results show that WTe2 crystal surface is of extremely high quality and low impurity concentration, with approximately one underlying (tie-fighter) defect per 3,000 atoms (Fig. 2). This extreme purity suggests that WTe2 could be a potential candidate to form an excitonic dielectric in the Abrikosov sense.

Figure 1) Evolution of band structure with number of layers. a) Crystal structure of WTe2 with the bulk and surface Brillouin zones and the slab model used in the theoretical calculations, consisting of 6 Te-W-Te layers. b-e) measured electronic band structure along the ΓX high symmetry direction (along the W chains). b), bulk electronic structures as calculated with SOC (red bands) and without SOC (blue bands); c) theoretical bands projected on the topmost WTe2 planes d,e) theoretical bands projected on second and third plane, respectively. In panels c) and e) blue arrows mark the positions of the theoretical electron and hole pockets, respectively. Panel f): the theoretical spectral function A(k,E).

Figure 2) Surface topography and structure of WTe2. (a) STM topographic image of (001) surface of WTe2 with several ‘tie-fighter’-like impurities. Inset: Zoom-in on an individual defect.



Ivana Vobornik