1. Department of Physics and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education; Wuhan University; Wuhan 430072 China
2. Department of Materials Science and Engineering; National Chiao Tung University; Hsin-chu 30010 Taiwan
3. Beijing National Laboratory for Molecular Sciences; Key Laboratory of Organic Solids; Institute of Chemistry; Chinese Academy of Science; Beijing 100190 China
4. Center for Advanced Quantum Studies; Department of Physics; Beijing Normal University; Beijing 100875 China
5. Physics Department; Xiamen University; Xiamen 361005 China
6. Department of Physics and Materials Science; City University of Hong Kong; Tat Chee Avenue Kowloon Hong Kong SAR China