1. Hunan Key Laboratory of Super-Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, People's Republic of China
2. State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics and Collaborative Innovation Center of Quantum Matter, Tsinghua University, Beijing 100084, People's Republic of China
3. Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, USA