1. Yangtze Delta Region Institute of University of Electronic Science and Technology of China, Huzhou 313000, P. R. China
2. Center for Materials Surface Science, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China
3. Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, P. R. China
4. School of Physics, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China