1. Department of Materials ScienceFudan University Shanghai 200433 P. R. China
2. Institute of PhysicsChinese Academy of Sciences Beijing 100190 P. R. China
3. Institute of PhysicsCenter of Materials Science and Optoelectronics EngineeringUniversity of Chinese Academy of Sciences Beijing 100049 P. R. China
4. Institute of PhysicsSongshan Lake Materials Laboratory Dongguan 523808 P. R. China
5. School of Materials Science and EngineeringUniversity of Shanghai for Science & Technology Shanghai 200093 P. R. China