1. Department of Materials ScienceFudan University Shanghai 200433 P. R. China
2. Institute of PhysicsChinese Academy of Sciences Beijing 100190 P. R. China
3. Center of Materials Science and Optoelectronics EngineeringUniversity of Chinese Academy of Sciences Beijing 100049 P. R. China
4. Songshan Lake Materials Laboratory Dongguan Guangdong 523808 P. R. China