1. State Key Laboratory of High Performance Ceramics and Superfine Microstructure; Shanghai Institute of Ceramics; Chinese Academy of Sciences; Shanghai 200050 China
2. Department of Physics; Beijing Normal University; Beijing 100875 China
3. Department of Applied Physics; The Hong Kong Polytechnic University; Hong Kong China
4. Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics; University of Science and Technology of China; Hefei 230026 China