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 Sciences & Engineering, Zhejiang University, Hangzhou 310058, China
3. Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, China
4. Department of Physics, Nanyang Technological University, Singapore 639798