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

TL;DR

This study uses pump-probe ARPES to directly observe unoccupied electronic states above the Fermi level in Mo$_x$W$_{1-x}$Te$_2$, providing evidence for its classification as a Type II Weyl semimetal with topological Fermi arcs.

Contribution

It demonstrates the first direct measurement of unoccupied band structures in Mo$_x$W$_{1-x}$Te$_2$ using pump-probe ARPES, confirming the presence of topological surface states.

Findings

Direct observation of states > 0.2 eV above Fermi level

Identification of surface states with topological Fermi arcs

Validation of Mo$_x$W$_{1-x}$Te$_2$ as a Type II Weyl semimetal

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 $\textit{ab initio}$ 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.