1. State Key Laboratory of Materials Processing and Die & Mould Technology School of Materials Science and Engineering Huazhong University of Science and Technology (HUST) Wuhan 430074 P. R. China
2. Department of Applied Physics The Hong Kong Polytechnic University Kowloon Hong Kong P. R. China
3. School of Physics University of Electronic Science and Technology of China (UESTC) Chengdu 610054 P. R. China