Observation of Fermi arcs in the type-II Weyl semimetal candidate WTe2

We use ultrahigh resolution, tunable, vacuum ultraviolet laser angle-resolved photoemission spectroscopy (ARPES) to study the electronic properties of WTe2, a material that was predicted to be a type-II Weyl semimetal. The Weyl fermion states in WTe2 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 WTe2 confirming the theoretical predictions. This provides strong evidence for type-II Weyl semimetallic states in WTe2. 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. WTe2 therefore provides a tunable playground for studying exotic topological quantum effects.