1. National Laboratory of Solid State Microstructures, Nanjing University 1 , Nanjing 210093, China
2. Department of Materials Science and Engineering, Nanjing University 2 , Nanjing 210093, China
3. Department of Physics, Nanjing University 3 , Nanjing 210093, China
4. Collaborative Innovation Center of Advanced Microstructures, Nanjing University 4 , Nanjing 210093, China