1. Pohl Institute of Solid State Physics, Tongji University, Shanghai 200092, P.R. China
2. Physics Department, Wuhan University, Wuhan 430072, P.R. China
3. Structure Research Laboratory, University of Science and Technology of China, Hefei 230026, P.R. China
4. National Lab. for Infrared Physics, Academia Sinica, Shanghai 200092, P.R. China
5. International Center for Material Physics, Shenyang 110015, P.R. China