作者
Ilya Belopolski,Su Xu,Y. Ishida,Xingchen Pan,Peng Yu,Daniel S. Sanchez,Hao Zheng,Madhab Neupane,Nasser Alidoust,Guoqing Chang,Tay Rong Chang,Yun Wu,Guang Bian,Shin Ming Huang,Chi‐Cheng Lee,Daixiang Mou,Lunan Huang,You Song,Baigeng Wang,Guanghou Wang,Yao‐Wen Yeh,Nan Yao,Julien Rault,Patrick Le Fèvre,F. Bertran,Horng Tay Jeng,Takeshi Kondo,Adam Kaminski,Hsin Lin,Zheng Liu,Fengqi Song,Shik Shin,M. Zahid Hasan
摘要
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.