1. Ningbo Institute of Materials Technology and Engineering Chinese Academy of Sciences Ningbo 315201 China
2. Division of Materials Physics and Application Los Alamos National Laboratory Los Alamos NM 87545 USA
3. Department of Applied Physics The Hong Kong Polytechnic University Hung Hom Kowloon Hong Kong 999077 China
4. School of Physics and Electronics Hunan University Changsha 410082 China
5. Center of Materials Science and Optoelectronics Engineering University of Chinese Academy of Sciences Beijing 100049 China