1. Department
of Physics and Jiangsu
Key Laboratory of Thin Films, Soochow University, Suzhou 215006, P. R. China
2. Center for Statistical and Theoretical
Condensed Matter Physics and Department of Physics, Zhejiang Normal University, Jinhua 321004, P. R. China
3. National Laboratory of Solid State
Microstructure and Department of Physics, Nanjing University, Nanjing 210093, P. R. China