1. Department of Physics, National Laboratory of Solid State Microstructures, Nanjing University, 22 Hankou Road, Nanjing 210093, People's Republic of China
2. Department of Physics, Shanghai University, 99 Shangda Road, Shanghai 200444, People's Republic of China
3. Collaborative Innovation Center of Advanced Microstructures, Nanjing University, 22 Hankou Road, Nanjing 210093, People's Republic of China