1. College of Applied Mathematics and Physics, Beijing University of Technology, Beijing 100022, P. R. China
2. Quantum Materials Laboratory, Beijing University of Technology, Beijing 100022, P. R. China
3. Department of Molecular Science and Technology, Ajou University, Suwon 442-749, Korea
4. Department of Applied Physics, Chongqing University, Chongqing 400044, P. R. China