1. National
Laboratory of Solid State Microstructures, Collaborative Innovation
Center of Advanced Microstructures, and College of Physics, Nanjing University, Nanjing 210093, P. R. China
2. National Laboratory of Solid State Microstructures, Collaborative
Innovation Center of Advanced Microstructures, and Department of Material
Science and Engineering, Nanjing University, Nanjing 210093, P. R. China
3. National Laboratory of Solid State Microstructures,
Collaborative Innovation Center of Advanced Microstructures, and School
of Electronic Science and Engineering, Nanjing University, Nanjing 210093, P. R. China