1. State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, P. R. China
2. School of Mechanical Engineering and Automation, Fuzhou University, Fuzhou 350108, P. R. China
3. School of Materials Science and Engineering, National Institute for Advanced Materials, Nankai University, Tianjin 300350, P. R. China
4. Institute of Applied Physics and Computational Mathematics, P.O. Box 8009
(28), Beijing 100088, P. R. China