1. Department of Materials ScienceFudan University Shanghai 200433 P. R. China
2. Institute of PhysicsChinese Academy of Sciences Beijing 100190 P. R. China
3. Shanghai Synchrotron Radiation FacilityZhangjiang LaboratoryShanghai Advanced Research InstituteChinese Academy of Sciences Shanghai 201204 P. R. China
4. Songshan Lake Materials Laboratory Dongguan Guangdong 523808 P. R. China