1. Department of Physics, Tsinghua University, Beijing 100084, P. R. China
2. Key Laboratory of Atomic and Molecular Nanosciences, Tsinghua University, Beijing 100084, P. R. China
3. School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, P. R. China
4. Tsinghua National Laboratory for Information Science and Technology, Tsinghua University, Beijing 100084, P. R. China