1. National Laboratory of Solid State Microstructures and Department of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P.R. China
2. School of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333001, P.R. China
3. Department of Physics, Hong Kong Baptist University, Kowloon, Hong Kong SAR, P.R. China