Observation of large topologically trivial Fermi arcs in the candidate type-II Weyl semimetal $\mathrm{WT}{\mathrm{e}}_2$

We report angle-resolved photoemission experiments resolving the distinct electronic structure of the inequivalent top and bottom (001) surfaces of $\mathrm{WT}{\mathrm{e}}_2$. On both surfaces, we identify a surface state that forms a large Fermi arc emerging out of the bulk electron pocket. Using surface electronic structure calculations, we show that these Fermi arcs are topologically trivial and that their existence is independent of the presence of type-II Weyl points in the bulk band structure. This implies that the observation of surface Fermi arcs alone does not allow the identification of $\mathrm{WT}{\mathrm{e}}_2$ as a topological Weyl semimetal. We further use the identification of the two different surfaces to clarify the number of Fermi surface sheets in $\mathrm{WT}{\mathrm{e}}_2$.

Figure 1

(a) Side view of the crystal structure of $\mathrm{WT}{\mathrm{e}}_2$. (b) Top and bottom view of the $\mathrm{WT}{\mathrm{e}}_2$ lattice. The + and − (U and D) symbols denote the Te (W) atoms that are further away and closer to the mean position. (c) and (d) ARPES Fermi surface of $\mathrm{WT}{\mathrm{e}}_2$ obtained with 6.01 eV excitation energy measured on two distinct surfaces parallel to the $ab$ plane denoted as type $A$ and type $B$, respectively. (e) and (f) Fermi surface contours extracted from measurements on samples type $A$ and $B$, respectively. Measurements obtained with different polarizations are shown as different symbols, shaded blue and green areas denote the electron and hole pockets, and the surface state SS1 is shown in red. (g) and (h) Fermi surface calculated for a semi-infinite crystal for the inequivalent top and bottom surfaces.

Figure 2

(a)–(c) Dispersion of the low-energy electronic states measured along the Γ-$X$ direction on a type-$A$ surface. (b)–(d) Same measurements on a type-$B$ surface. (e) and (f) Extracted dispersion of the bands from measurements with different excitation energies and polarizations for surfaces of type $A$ and $B$, respectively. The thick lines are a guide to the eyes. (g) Bulk band structure of $\mathrm{WT}{\mathrm{e}}_2$ along Γ-$X$ from density functional theory. (h)–(j) $k_z$ dispersion of the bulk electron pocket and surface state on a type-$B$ surface measured at $k_x=0.36\AA {}^{-1}$ and $k_x=0.3\AA {}^{-1}$, respectively. (i) Constant energy surface measured at $E_F$ on a type-$A$ surface.

Figure 3

(a), (b) Momentum-resolved (001) surface density of states calculated along the Γ-$X$ direction for the bottom surface of $\mathrm{WT}{\mathrm{e}}_2$. The lattice parameters at $T=113\mathrm{K}$ and room temperature are used in (a) and (b), respectively, resulting in the presence of eight and zero Weyl points. (c) Fermi surface calculation of the $\mathrm{WT}{\mathrm{e}}_2$ bottom surface. The top and bottom half correspond to the case of eight and zero Weyl points, respectively.