Discovery of a new type of topological Weyl fermion semimetal state in Mo x W 1-x Te 2 .

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 Mox W1-x Te2 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 Mox W1-x Te2 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 Mox W1-x Te2 is a Type II Weyl semimetal. We also find that certain Weyl points are at the Fermi level, making Mox W1-x Te2 a promising platform for transport and optics experiments on Weyl semimetals.