1. Department of Physics, College of Science, Hohai University, Nanjing 210098, P. R. China
2. Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, P. R. China
3. Nanjing National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China
4. Ames Laboratory, US DOE, Ames, Iowa 50011, United States