1. Beijing National Laboratory for Condensed Matter PhysicsInstitute of PhysicsChinese Academy of Sciences (CAS) Beijing 100190 P. R. China
2. School of Physical SciencesUniversity of Chinese Academy of Sciences Beijing 100049 China
3. School of Materials Science and EngineeringZhejing University Hangzhou 310027 P. R. China
4. Magdalene CollegeUniversity of Cambridge Cambridge CB3 0AG UK
5. Songshan Lake Materials Laboratory Dongguan 523808 China