Measuring Chern numbers above the Fermi level in the Type II Weyl semimetal Mo${}_x$W${}_{1-x}$Te${}_2$

Abstract

It has recently been proposed that electronic band structures in crystals 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${}_x$W${}_{1-x}$Te${}_2$ may realize such a Type II Weyl fermion. One crucial challenge is that the Weyl points in Mo${}_x$W${}_{1-x}$Te${}_2$ are predicted to lie above the Fermi level. Here, by studying a simple model for a Type II Weyl cone, we clarify the importance of accessing the unoccupied band structure to demonstrate that Mo${}_x$W${}_{1-x}$Te${}_2$ is a Weyl semimetal. Then, we use pump-probe angle-resolved photoemission spectroscopy (pump-probe ARPES) to directly observe the unoccupied band structure of Mo${}_x$W${}_{1-x}$Te${}_2$. For the first time, we directly access states $>0.2$ eV above the Fermi level. By comparing our results with $\text{ab initio}$ calculations, we conclude that we directly observe the surface state containing the topological Fermi arc. Our work opens the way to studying the unoccupied band structure as well as the time-domain relaxation dynamics of Mo${}_x$W${}_{1-x}$Te${}_2$ and related transition metal dichalcogenides.

arXiv commentIncorporates earlier results presented in arXiv:1512.09099, by the same authors

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