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1 1 1 I H I G H L I G H T S 2 0 2 2

Fermi surface chirality in a low symmetry TaSe2 monosheet

Surface X-ray diffraction was used to show that the symmetry of a single TaSe2 monosheet grown by an interface reaction between Ta and the (0001) surface of Bi2Se3 is reduced from D3h to C3v, resulting from a vertical atomic shift of the tantalum atom within the hexagonal prism.

Topological materials are characterised by the topologic surface state (TSS) in which the spin is locked to the momentum. They have been shown to be very efficient in converting charge current to spin current. Recently, two- dimensional van der Waals materials such as heavy metal (Mo, W, Pt) containing transition metal dichalcogenides (TMDCs) characterised by a non-trivial electronic structure have found increasing interest as spin source materials [1,2].

By contrast, bulk 2H-TaSe2 has a trivial electronic structure. It crystallises in the space group P63/mmc (point group D6h with trigonal-prismatic coordination around tantalum by selenium). Spin-orbit coupling (SOC) lifts the spin- degeneracy of the bands, inducing a spin-polarisation that pins the electron spins to the out-of-plane direction ( Ising- SOC ) [3]. In the ultra-thin film limit, TMDC films have been generally assumed to be rigid units with bulk-like point group symmetry.

This study shows that this assumption is not justified, leading to significant modifications of the electronic structure. It is characterised by a chirality of the states at the Fermi surface (FS). A TaSe2 monosheet was prepared by exploiting an interface reaction after Ta deposition on the (0001) surface of Bi2Se3 induced by annealing at about 480°C. Surface X-ray diffraction (SXRD) experiments were carried out in situ at beamline BM25 by collecting the intensity distribution along the qz direction in reciprocal space. Figure 104a shows the structural model derived by fitting the calculated intensities to the experimental ones (Figure 104b). In analogy to the bulk structure, the TaSe2 monosheet crystallises in the hexagonal (H) form. In the monosheet, the central Ta atom relaxes downward from the centre of the prism at z=0.5 lattice units (l.u.) to z=0.42 l.u., thereby leading to a reduction of the point group symmetry from D3h to C3v.

As a result of the symmetry lowering in the monosheet, which is due to the lack of the horizontal mirror plane, the electronic states at the FS acquire an in-plane component. This is shown in Figure 105a, demonstrating the spin- and momentum-resolved photoemission map at the FS (Synchrotron Elettra, Italy) in comparison with the calculated map in Figure 105b. The H-TaSe2 monosheet related states at the FS have an in-plane component of

Fig. 104: a) Structural model of the TaSe2 monosheet derived from SXRD experiments. Numbers indicate

distances in picometers, bulk values are given in brackets. The central Ta atom is displaced from the centre (z=0.5) in the bulk to z=0.42 in the monosheet. b) Example of the intensity distribution along qz of the (01) reflection.

Symbols and line correspond to experimental and calculated intensities for the structural model shown in (a).

Fig. 105: a) Experimental spin-resolved photoemission momentum map of the H-TaSe2 monosheet on Bi2Se3 at the FS compared with calculations in which the position

of the tantalum atom is located at z = 0.43. b) The colour code quantifies the degree of the in-plane spin polarisation

along the y-axis as given by the scale bar on the right.