1. Institute of Physics University of Chinese Academy of Science Chinese Academy of Science Beijing 100190 P. R. China
2. Beijing National Laboratory for Molecular Sciences State Key Laboratory of Rare Earth Materials Chemistry and Applications College of Chemistry and Molecular Engineering Peking University Beijing 10087 P. R. China
3. Empa Swiss Federal Laboratories for Materials Science and Technology Überlandstrasse 129 8600 Dübendorf Switzerland