1. National Laboratory of Solid State Microstructures Jiangsu Key Laboratory of Artificial Functional Materials College of Engineering and Applied Sciences Nanjing University Nanjing 210093 P. R. China
2. Collaborative Innovation Center of Advanced Microstructures Nanjing University Nanjing 210093 P. R. China
3. Suzhou Laboratory Suzhou 215125 P. R. China
4. Department of Physics University of Warwick Coventry CV4 7AL United Kingdom