1. School of Physics; Beihang University; Beijing 100191 China
2. School of Physics; Peking University; Beijing 100871 China
3. State Key Laboratory of Functional Materials for Informatics; Shanghai Institute of Microsystem and Information Technology; Chinese Academy of Sciences; Shanghai 200050 China
4. School of Physics and Department of Materials Science and Engineering; City University of Hong Kong; Kowloon 999077 Hong Kong China