Fermi arc electronic structure and Chern numbers in the type-II Weyl semimetal candidate MoxW1 -xTe2

It has recently been proposed that electronic band structures in crystals can give rise to a previously overlooked type of Weyl fermion, which violates Lorentz invariance and, consequently, is forbidden in particle physics. It was further predicted that Mo_xW_{1 -x}Te₂ may realize such a type-II Weyl fermion. Here, we first show theoretically that it is crucial to access the band structure above the Fermi level ɛ_F to show a Weyl semimetal in Mo_xW_{1 -x}Te₂ . Then, we study Mo_xW_{1 -x}Te₂ by pump-probe ARPES and we directly access the band structure >0.2 eV above ɛ_F in experiment. By comparing our results with ab initio calculations, we conclude that we directly observe the surface state containing the topological Fermi arc. We propose that a future study of Mo_xW_{1 -x}Te₂ by pump-probe ARPES may directly pinpoint the Fermi arc. Our work sets the stage for the experimental discovery of the first type-II Weyl semimetal in Mo_xW_{1 -x}Te₂ .