1. Department of Material Science, Sichuan University, Chengdu 610064, P. R. China
2. International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, P. R. China
3. Surface Physics Laboratory (National Key Lab), Fudan University, Shanghai 200433, P. R. China
4. Institute of Solid State Physics, Sichuan Normal University, Chengdu 610066, P. R. China