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 P. R. China
3. Department of Materials Science and Engineering; City University of Hong Kong Kowloon Tong Hong Kong SAR; Hong Kong SAR P. R. China