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