1. Department of Physics, Carnegie Mellon University 1 , Pittsburgh, Pennsylvania 15213
2. Paul-Drude-Institut für Festkörperelektronik 2 , Hausvogteiplatz 5-7, 10117 Berlin, Germany
3. Center for Spintronics and Quantum Systems, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University 3 , Xi’an 710049, China
4. Department of Materials Science and Engineering, and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University 4 , University Park, Pennsylvania 16802