1. Institute of Applied Physics and Computational Mathematics, Beijing 100088, China
2. Department of Engineering Physics, Tsinghua University, Beijing 100084, China
3. Department of Physics, Beijing Normal University, Beijing 100875, China
4. School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom
5. Department of Physics, Tsinghua University, Beijing 100084, China