Observation of Fermi arcs in the type-II Weyl semimetal candidate ${\mathrm{WTe}}_2$

We use ultrahigh resolution, tunable, vacuum ultraviolet laser angle-resolved photoemission spectroscopy (ARPES) to study the electronic properties of ${\mathrm{WTe}}_2$, a material that was predicted to be a type-II Weyl semimetal. The Weyl fermion states in ${\mathrm{WTe}}_2$ were proposed to emerge at the crossing points of electron and hole pockets, and Fermi arcs connecting electron and hole pockets would be visible in the spectral function on (001) surface. Here we report the observation of such Fermi arcs in ${\mathrm{WTe}}_2$ confirming the theoretical predictions. This provides strong evidence for type-II Weyl semimetallic states in ${\mathrm{WTe}}_2$. We also find that trivial and topological domains coexist on the same surface of the sample due to the presence of inhomogeneous strain detected by scanning electron microscopy data. This is in agreement with the theoretical prediction that strain can drive this system from topological Weyl to trivial semimetal. ${\mathrm{WTe}}_2$ therefore provides a tunable playground for studying exotic topological quantum effects.

Figure 1

Two types of Fermi surface plot and band dispersion measured at photon energy of 6.7 eV. (a) Schematic drawing of the crystal structure of single unit cell layer with two possible cleaving sites marked by arrows. Note that these are not equivalent to “N” and “T” data types. (b) Optical image of the cleaved sample with three measured regions marked by circles. (c)–(e) Data measured in red, green, and blue regions marked in (b), respectively. (f) Type “N” Fermi surface plot of ARPES intensity integrated within 10 meV about the chemical potential measured at $T=40\mathrm{K}$. (g) Band dispersion along white dashed line cut in (f). (h) Type “T” Fermi surface plot of ARPES intensity integrated within 10 meV about the chemical potential measured at $T=16\mathrm{K}$. (l)–(n) Band dispersions along cuts #1–#6 in (h). Red dashed lines in (i) mark the electron pocket and two left branches of the hole bands.

Figure 2

Fermi surface plot and band dispersion measured at $T=16\mathrm{K}$ and photon energy of 6.7 eV. (a) Fermi surface plot of ARPES intensity integrated within 5 meV about the chemical potential. The red dashed line marks the contour of the surface state (SS) and the black dashed lines mark the contour of the two bulk hole pockets. (b)–(j) Band dispersion along cuts #1–#9. Dashed lines in (b) mark the electron pocket and the two hole pockets. The white arrows point to the location of the SS. The red arrows in (j) point to the locations of the two hole bands crossing Fermi level.

Figure 3

Fermi surface plot and band dispersion measured at $T=160\mathrm{K}$ and photon energy of 6.7 eV. (a) Fermi surface plot of ARPES intensity integrated within 10 meV about the chemical potential. The red dashed line marks the contour of the SS and the black dashed line marks the contour of the bulk electron pocket. (b)–(i) Band dispersion along cuts #1–#8. The white arrows point to the location of the SS and the bulk state (BS).

Figure 4

Fermi surface plot measured at $T=40\mathrm{K}$. (a)–(c) Fermi surface plot of ARPES intensity integrated within 10 meV about the chemical potential measured at photon energies of 6.7, 6.36, and 6.05 eV, respectively. (d) Fermi surface contour extracted from peak positions of momentum dispersion curves. (e) Scanning electron microscopy (SEM) image of cleaved sample surface. The red arrow points to the flat area close to the sample center, while green arrows point to areas away from the center where buckling and thus strain is present.