1. State
Key Laboratory of Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, P. R. China
2. Department
of Nuclear Science and Engineering and Department of Materials Science
and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
3. State Key Laboratory of Information Photonics and Optical Communications & School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, P. R. China
4. Collaborative Innovation Center of Quantum Matter, Beijing 100871, P. R. China