1. State Key Laboratory of Artificial Microstructure and Mesoscopic Physics; School of Physics; Peking University; Beijing 100871 P. R. China
2. Collaborative Innovation Center of Quantum Matter; Beijing 100871 P. R. China
3. King Abdullah University of Science and Technology; Division of Physical Science and Engineering; Thuwal 23955-6900 Kingdom of Saudi Arabia
4. Department of Chemistry; National Tsing Hua University; Hsinchu 30013 Taiwan
5. Microsystem and Terahertz Research Center; China Academy of Engineering Physics (CAEP); Chengdu 610200 P. R. China