1. Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), Department of Physics, Jilin University, Changchun 130012, China
2. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, China
3. Department of Applied Physics, Tohoku University, Sendai, Miyagi 980-8579, Japan
4. Institute of Microelectronics, Tsinghua University, Beijing 100084, China