Discovery of a new type of topological Weyl fermion semimetal state in MoxW1−xTe2

The recent discovery of a Weyl semimetal in TaAs offers the first Weyl fermion observed in nature and dramatically broadens the classification of topological phases. However, in TaAs it has proven challenging to study the rich transport phenomena arising from emergent Weyl fermions. The series Mo x W 1− x Te 2 are inversion-breaking, layered, tunable semimetals already under study as a promising platform for new electronics and recently proposed to host Type II, or strongly Lorentz-violating, Weyl fermions. Here we report the discovery of a Weyl semimetal in Mo x W 1− x Te 2 at x =25%. We use pump-probe angle-resolved photoemission spectroscopy (pump-probe ARPES) to directly observe a topological Fermi arc above the Fermi level, demonstrating a Weyl semimetal. The excellent agreement with calculation suggests that Mo x W 1− x Te 2 is a Type II Weyl semimetal. We also find that certain Weyl points are at the Fermi level, making Mo x W 1− x Te 2 a promising platform for transport and optics experiments on Weyl semimetals. A Type II Weyl fermion semimetal has been predicted in Mo x W 1− x Te 2 , but it awaits experimental evidence. Here, Belopolski et al . observe a topological Fermi arc in Mo x W 1− x Te 2 , showing it originates from a Type II Weyl fermion and offering a new platform to study novel transport phenomena in Weyl semimetals.