1. Department of Physics and Institute of Advanced Materials; Hong Kong Baptist University; Kowloon Tong Hong Kong SAR P. R. China
2. School of Physics; State Key Laboratory of Crystal Materials; Shandong University; Jinan Shandong 250100 P. R. China
3. Department of Physics and Materials Science; City University of Hong Kong; Kowloon Tong Hong Kong SAR P. R. China