1. Wenzhou Institute University of Chinese Academy of Sciences Wenzhou 325001 P.R. China
2. State Key Laboratory of Rare Earth Resource Utilization Changchun Institute of Applied Chemistry, Chinese Academy of Sciences Changchun 130022 P.R. China
3. School of Engineering and Applied Sciences & Department of Physics Harvard University 29 Oxford Street Cambridge MA 02138 USA
4. Beijing National Laboratory for Condensed Matter Physics Institute of Physics Chinese Academy of Sciences Beijing 100190 P.R. China
5. Department of Chemistry Tsinghua University Beijing 100084 P.R. China